SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK ......terminated disulfonated poly (arylene ether...

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SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK COPOLYMERS FOR PROTON EXCHANGE MEMBRANE FUEL CELLS (PEMFC) by Hang Wang Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Macromolecular Science and Engineering Dr. James E. McGrath, Chairman Dr. Judy Riffle Dr. Timothy Long Dr. John G. Dillard Dr. Richey M Davis Defense date: December 15 th 2006 Blacksburg, Virginia Keywords: Proton Exchange Membranes, Fuel Cells, Multiblock Copolymers, Morphology, Poly(p-phenylene), Poly(2,5-benzophenone), Ni (0)-catalyzed coupling reaction, Disulfonated Poly(arylene ether sulfones)

Transcript of SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK ......terminated disulfonated poly (arylene ether...

Page 1: SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK ......terminated disulfonated poly (arylene ether sulfone)s (hydrophilic) by nucleophilic aromatic substitution to form novel hydrophilic-hydrophobic

SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK COPOLYMERS FOR PROTON EXCHANGE MEMBRANE

FUEL CELLS (PEMFC)

by

Hang Wang

Dissertation submitted to the Faculty of the

Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY in

Macromolecular Science and Engineering

Dr. James E. McGrath, Chairman Dr. Judy Riffle

Dr. Timothy Long Dr. John G. Dillard Dr. Richey M Davis

Defense date: December 15th 2006 Blacksburg, Virginia

Keywords: Proton Exchange Membranes, Fuel Cells, Multiblock Copolymers,

Morphology, Poly(p-phenylene), Poly(2,5-benzophenone), Ni (0)-catalyzed coupling reaction, Disulfonated Poly(arylene ether sulfones)

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SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK COPOLYMERS FOR PROTON EXCHANGE MEMBRANE

FUEL CELLS (PEMFC)

by

Hang Wang

Committee Chairman: Dr. James E. McGrath

Macromolecules and Interfaces Institute

ABSTRACT

Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are

promising proton exchange membrane (PEM) materials due to their ability to form

various morphological structures which enhance transport.

Four arylene chlorides monomers (2,5-Dichlorobenzophenone and its derivatives)

were first successfully synthesized from aluminum chloride-catalyzed, Friedel-Crafts

acylation of benzene and various aromatic compounds with 2,5-dichlorobenzoyl chloride.

These monomers were then polymerized via Ni (0)-catalyzed coupling reaction to form

various high molecular weight substituted poly(2,5-benzophenone)s. Great care must be

taken to achieve anhydrous and inert conditions during the reaction.

A series of poly(2,5-benzophenone) activated aryl fluoride telechelic oligomers

with different block molecular weights were then successfully synthesized by Ni (0)-

catalyzed coupling of 2,5-dichloro-benzophenone and the end-capping agent 4-chloro-4'-

fluorobenzophenone or 4-chlorophenly-4’-fluorophenyl sulfone. The molecular weights

of these oligomers were readily controlled by altering the amount of end-capping agent.

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These telechelic oligomers (hydrophobic) were then copolymerized with phenoxide

terminated disulfonated poly (arylene ether sulfone)s (hydrophilic) by nucleophilic

aromatic substitution to form novel hydrophilic-hydrophobic multiblock copolymers.

A series of novel multiblock copolymers with number average block lengths

ranging from 3,000 to 10,000 g/mol were successfully synthesized. Two separate Tgs

were observed via DSC in the transparent multiblock copolymer films when each block

length was longer than 6,000 g/mol (6k). Tapping mode atomic force microscopy (AFM)

also showed clear nanophase separation between the hydrophilic and hydrophobic

domains and the influence of block length, as one increased from 6k to 10k. Transparent

and creasable films were solvent-cast and exhibited good proton conductivity and low

water uptake.

These PAES-PBP multiblock copolymers also showed much less relative

humidity (RH) dependence than random sulfonated aromatic copolymers BPSH 35 in

proton conductivity, with values that were almost the same as Nafion with decreasing

RHs. This phenomenon lies in the fact that this multiblock copolymer possesses a unique

co-continuous nanophase separated morphology, as confirmed by AFM and DSC data.

Since this unique co-continuous morphology (interconnected channels and networks)

dramatically facilitates the proton transport (increase the diffusion coefficient of water),

improved proton conductivity under partially hydrated conditions becomes feasible.

These multiblock copolymers are therefore considered to be very promising candidates

for high temperature proton exchange membranes in fuel cells.

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Dedicated to my parents, Peiqi Zhang and Zaisong Wang for their never-ending love, support and encouragement

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Acknowledgements

I would like to take this opportunity to express my sincere gratitude to my

research advisor, Dr. James E. McGrath, for his encouragement, guidance and inspiration

throughout my graduate career. His knowledge, patience, and generosity toward his

students make him an exceptional advisor and person. I have exceedingly benefited from

his vast knowledge, lasting enthusiasm, and exceptional personality. I would also like to

thank the members of my advisory committee, Dr. Timothy E. Long, Dr. John G. Dillard,

Dr. Dr. Richey M Davis and Dr. Judy S. Riffle for their support.

The staffs in the Macromolecules and Interfaces Institute as well as in the

Chemistry Department have been very helpful in my success. Mrs. Laurie Good, Mrs.

Millie Ryan, and Mrs. Angie Flynn have been of tremendous assistance in all the little

details that really make the department such a great place to work.

I would like to especially acknowledge Mr. Tom Glass (NMR), Mr. Anand S.

Badami (Atom Force Microscopy), Juan Yang (GPC) and Mr. Ahbishek Roy (Proton

Conductivity Measurement) for their valuable assistance.

Many valuable discussions with my lab mates and post-doctoral researchers in the

McGrath research group contributed to my success. For this, I thank: Yanxiang Li, Hae-

seung Lee, Dr. William Harrison, Dr. Melinda Hill, Dr. Brian Einsla, Dr.Kent Wiles, Dr.

Guangyu Fang, Natalie Arnet, Xiang Yu, Rachael Hopp, Dr. Zhongbiao Zhang and Dr.

Hossein Ghassemi. I am grateful to each of them for their patience and for sharing their

knowledge with me.

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My friends, Ran Miao, Danny Hsu, Qi Guo, Yiqun Zhang and Suolong Ni have

been a very important part of my life and I hope that our friendships will continue

throughout the rest of our lives.

My best friend and fiancé, Juanjuan Han has always been there for me. I could

not have made it without her love and understanding. I would also like to thank my

entire family for their love and assistance that have aided me to this point of my life.

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SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK COPOLYMERS FOR PROTON EXCHANGE

MEMBRANE FUEL CELLS (PEMFC)

Table of Contents

Chapter 1............................................................................................................................. 1 1 Literature Review ....................................................................................................... 1 1.1 Introduction............................................................................................................. 1

1.1.1 What is a Fuel Cell?.................................................................................... 2 1.1.2 Proton Exchange Membrane Fuel Cells (PEMFC)..................................... 4 1.1.3 Proton Exchange Membranes ..................................................................... 7 1.1.4 Important Parameters .................................................................................. 9

1.1.4.1 Proton Conductivity ................................................................................ 9 1.1.4.2 Water uptake ......................................................................................... 10 1.1.4.3 Ion Exchange Capacity--IEC ................................................................ 11 1.1.4.4 Methanol Permeability.......................................................................... 12 1.1.4.5 Electro-Osmotic Drag ........................................................................... 12

1.2 Perfluorinated Copolymers for PEM .................................................................... 13 1.3 Non-Fluorinated Polymer Materials for PEM ...................................................... 18

1.3.1 Polybenzimidazole.................................................................................... 20 1.3.1.1 Acid Doping.......................................................................................... 20 1.3.1.2 Direct Sulfonation of Polymeric Backbone .......................................... 21 1.3.1.3 Synthesis from Sulfonated Monomer ................................................... 22 1.3.1.4 Grafting of a Functional Group............................................................. 22

1.3.2 Poly(phenylquinoxalines) ......................................................................... 23 1.3.3 Poly(phenylene oxides)............................................................................. 24 1.3.4 Poly(phenylene)s and Derivatives ............................................................ 26 1.3.5 Poly(phenylene sulfide) ............................................................................ 28 1.3.6 Poly(arylene ether sulfones) ..................................................................... 29

1.3.6.1 Post Sulfonated Copolymers................................................................. 30 1.3.6.2 Synthesis of copolymers from sulfonated monomers........................... 33 1.3.6.3 Processing and Morphological Issues................................................... 37

1.3.7 Poly(arylene ether ketone) Copolymers.................................................... 39 1.3.8 Poly(phosphazene)s .................................................................................. 45

1.3.8.1 Post Sulfonated Copolymers................................................................. 45 1.3.9 Polyimides................................................................................................. 46

1.4 Block Copolymers for PEM.................................................................................. 51 1.4.1 Copolymer Architecture............................................................................ 52 1.4.2 Nanophase Separation in Block Copolymers ........................................... 53 1.4.3 Di-Block and Tri-Block Copolymers for PEM......................................... 56 1.4.4 Multiblock copolymers for PEM .............................................................. 60

1.4.4.1 Poly(arylene ether sulfone) Multiblock Copolymers............................ 60 1.4.4.2 Polyimide Multiblock Copolymers....................................................... 66

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1.5 Multiblock Copolymers Based on Substituted Poly(phenylene)s ............................ 69 1.5.1 Ni (0) -Catalyzed Coupling Polymerization Reactions ............................... 70 1.5.2 Poly(p-phenylene) (PPP) and Derivatives ................................................... 72 1.5.3 Multiblock Copolymers Based on Substituted Poly(p-phenylene)s ............ 76

Chapter 2........................................................................................................................... 81 2 EXPERIMENTAL................................................................................................ 81 2.1 Solvent Purification .............................................................................................. 81

2.1.1 N,N-Dimethylacetamide (DMAc) ............................................................ 81 2.1.2 N-Methyl-2-Pyrrolidone or 1-Methyl-2-Pyrrolidone (NMP) ................... 82 2.1.3 Fuming Sulfuric Acid ............................................................................... 82 2.1.4 Toluene ..................................................................................................... 83 2.1.5 Ethanol or Ethyl Alcohol .......................................................................... 83 2.1.6 Methanol or Methyl Alcohol .................................................................... 84 2.1.7 Isopropyl Alcohol or 2-Propanol or Isopropanol...................................... 84 2.1.8 Tetrahydrofuran (THF) ............................................................................. 85 2.1.9 Hydrochloride Acid (HCl) ........................................................................ 85 2.1.10 Benzene..................................................................................................... 85 2.1.11 Cyclohexane.............................................................................................. 86 2.1.12 Acetic Anhydride ...................................................................................... 86

2.2 Reagents and Purification of Monomers............................................................... 87 2.2.1 2,2’-bis(4-hydroxyphenol)propane (Bisphenol A) or (Bis A) .................. 87 2.2.2 4,4’-Biphenol (BP).................................................................................... 87 2.2.3 4,4'-(hexafluoroisopropylidene) diphenol (6F-Bisphenol A) ................... 88 2.2.4 4,4’-Dichlorodiphenyl sulfone (DCDPS) ................................................. 89 2.2.5 Potassium Carbonate (Anhydrous) ........................................................... 89 2.2.6 Chlorosulfonic acid................................................................................... 90 2.2.7 Chlorotrimethylsilane ............................................................................... 90 2.2.8 2,2’-Bipyridyl (bipy)................................................................................. 91 2.2.9 Triphenylphosphine (PPh3) ...................................................................... 91 2.2.10 Dichlorobis(triphenylphosphine)nickel-(II).............................................. 92 2.2.11 Nickel(II) Chloride (NiCl2) ...................................................................... 92 2.2.12 Zn Powder................................................................................................. 92 2.2.13 2,5-dichlorobenzoic acid........................................................................... 93 2.2.14 4-Fluorobenzoyl chloride.......................................................................... 93 2.2.15 4-chlorophenly-4’-fluorophenyl sulfone................................................... 94 2.2.16 Thionyl chloride (SOCl2).......................................................................... 94 2.2.17 Chlorobenzene .......................................................................................... 95 2.2.18 Fluorobenzene........................................................................................... 95 2.2.19 Biphenyl.................................................................................................... 96 2.2.20 Diphenyl ether........................................................................................... 96 2.2.21 Aluminum Chloride (AlCl3) ..................................................................... 97

2.3 Monomer Synthesis .............................................................................................. 98 2.3.1 2,5-Dichlorobenzophenone....................................................................... 98 2.3.2 2,5-Dichloro-4’-fluorobenzophenone ..................................................... 100 2.3.3 2,5-Dichloro-4’-phenylbenzophenone .................................................... 101 2.3.4 2,5-Dichloro-4’-oxyphenylbenzophenone.............................................. 102

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2.3.5 4-Chloro-4'-fluorobenzophenone............................................................ 103 2.3.6 Sodium Salt of 3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) 105

2.4 Synthesis of Homopolymers from Substituted Poly(phenylene)s via Ni(0)-catalyzed Coupling Reaction .......................................................................................................... 107

2.4.1 Poly(2,5-benzophenone) ......................................................................... 107 2.4.2 Poly(4’-fluoro-2,5-benzophenone) ......................................................... 107 2.4.3 Poly(4’-phenyl-2,5-benzophenone) ........................................................ 108 2.4.4 Poly(4’-oxyphenyl-2,5-benzophenone) .................................................. 109

2.5 Telechelic Oligomer Synthesis ............................................................................... 110 2.5.1 Synthesis of Telechelic Poly(2,5-benzophenone) (PBP) Oligomers ...... 110 2.5.2 Synthesis of Telechelic Poly(4’-oxyphenyl-2,5-benzophenone) (PPBP) Oligomers................................................................................................................ 110 2.5.3 Synthesis of Telechelic Disulfonated Poly(arylene ether sulfone) (PAES) Oligomers................................................................................................................ 111

2.6 Synthesis of Multiblock Copolymers...................................................................... 112 2.6.1 Synthesis of PAES-PBP Multiblock Copolymers .................................. 112 2.6.2 Synthesis of PPAES-PBP Multiblock Copolymers ................................ 112

2.7 Characterization ...................................................................................................... 113 2.7.1 Nuclear Magnetic Resonance (NMR) Spectroscopy .............................. 113 2.7.2 Fourier Transform Infrared (FTIR) Spectroscopy .................................. 113 2.7.3 Gel Permeation Chromatography (GPC) ................................................ 114 2.7.4 Intrinsic Viscosity Determinations ([ ])  ............................................... 114 2.7.5 Thermogravimetric Analysis (TGA)....................................................... 115 2.7.6 Differential Scanning Calorimetry (DSC) .............................................. 115 2.7.7 Elemental Analysis ................................................................................. 116 2.7.8 Non-Aqueous Potentiometric Titration................................................... 116 2.7.9 Film Preparation...................................................................................... 117 2.7.10 Water Uptake .......................................................................................... 117 2.7.11 Conductivity Measurements ................................................................... 118 2.7.12 Ion Exchange Capacity (IEC) ................................................................. 119 2.7.13 UV-visible Absorption Spectroscopy (UV-vis)...................................... 119 2.7.14 Atomic Force Microscopy (AFM).......................................................... 121

Chapter 3......................................................................................................................... 122 3 Results and Discussion ........................................................................................... 122 3.1 Introduction............................................................................................................. 122 3.2 Synthesis of Substituted Poly(p-phenylene)s Homopolymers................................ 126

3.2.1 Monomer Synthesis ................................................................................ 128 3.2.2 Polymer Synthesis................................................................................... 131 3.2.3 Polymer Characterization........................................................................ 135

3.2.3.1 1H and 13C NMR Spectroscopy......................................................... 136 3.2.3.2 Fourier Transform Infrared Spectroscopy (FTIR) .............................. 138 3.2.3.3 Intrinsic Viscosity [η] (IV) ................................................................. 139 3.2.3.4 Gel Permeation Chromatography (GPC) ............................................ 141 3.2.3.5 Differential Scanning Calorimetry (DSC) .......................................... 142 3.2.3.6 Thermogravimetric Analysis (TGA)................................................... 143

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3.2.3.7 Ultraviolet-visible Spectroscopy (UV/ VIS)....................................... 145 3.3 Disulfonated Poly (arylene ether sulfone) Copolymers (BPSH series) .................. 148

3.3.1 Synthesis of Disulfonated Poly (arylene ether sulfone) Copolymers (BPSH) 148 3.3.2 Characterization of BPSH copolymers ................................................... 150

3.4 Multiblock Copolymers .......................................................................................... 155 3.4.1 Synthesis of poly (2,5-benzophenone) (PBP) telechelic oligomers........ 156 3.4.2 Synthesis of disulfonated poly(arylene ether sulfone) (PAES) telechelic oligomers................................................................................................................. 166 3.4.3 Synthesis of PAES-PBP Multiblock Copolymers .................................. 172 3.4.4 Characterization of Multiblock Copolymers .......................................... 173

3.4.4.1 1H and 13C NMR Spectroscopy......................................................... 173 3.4.4.2 Fourier Transform Infrared Spectroscopy (FT-IR)............................. 176 3.4.4.3 Gel Permeation Chromatography (GPC) and Intrinsic Viscosity [η] (IV) 178 3.4.4.4 Differential Scanning Calorimetry (DSC) .......................................... 181 3.4.4.5 Thermogravimetric Analysis (TGA)................................................... 183 3.4.4.6 Atomic force microscopy (AFM) ....................................................... 186 3.4.4.7 X-ray Photoelectron Spectroscopy (XPS) .......................................... 188 3.4.4.8 Proton conductivity and water uptake................................................. 189

Chapter 4......................................................................................................................... 195 4 Multiblock Copolymers of Poly (2,5-benzophenone) and Disulfonated Poly (arylene ether sulfone) for Proton Exchange Membranes. I. Synthesis and Characterization………………………………………………………………………...195 Abstract………………………………………………………...……………………….196 INTRODUCTION .......................................................................................................... 197 EXPERIMENTAL.......................................................................................................... 200

Materials. ................................................................................................................ 200 Characterization ...................................................................................................... 200 Water Uptake, Ion Exchange Capacity and Proton Conductivity........................... 202 Monomer synthesis ................................................................................................. 203 Synthesis of telechelic poly(2,5-benzophenone) (PBP) oligomers......................... 203 Synthesis of PAES telechelic oligomers and PAES-PBP multiblock copolymers. 204 Membrane preparation ............................................................................................ 205

RESULTS AND DISCUSSION………………………………………………………..206 Synthesis of Monomers and Oligomers.................................................................. 206 Synthesis of PAES-PBP Multiblock Copolymers .................................................. 214 Morphology............................................................................................................. 216 Ion exchange capacity, Water uptake and Proton conductivity.............................. 218

CONCLUSIONS............................................................................................................. 220 ACKNOWLEDGEMENTS............................................................................................ 220 Chapter 5......................................................................................................................... 221 5 Summary and Conclusions ................................................................................. 221 Chapter 6......................................................................................................................... 224 6 Suggested Future Research ................................................................................. 224 Reference: ....................................................................................................................... 227

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Table of Figures

Figure 1 Membrane electrode assembly7............................................................................ 6 Figure 2 Fuel cell stack made up of flow field plates (or bipolar plates) and MEAs

(shown in the insert) 7 ................................................................................................. 7 Figure 3Typical cell geometry for an in-plane proton conductivity measurement........... 10 Figure 4 Chemical Structure of Nafion ......................................................................... 14 Figure 5 Conductivity of Nafion 117 membranes at the indicated relative humidities for

temperature increasing from 80 to 160oC 37. ............................................................ 16 Figure 6 Development of proton-conducting membranes by the modification of

thermostable polymers.41 .......................................................................................... 19 Figure 7 Synthesis route to sulfonated (stabilized) polybenzimidazole46......................... 20 Figure 8 Synthesis route to benzylsulfonated polybenzimidazole via polyanion

formation68 ................................................................................................................ 23 Figure 9 Sulfonated poly(phenyl quinoxaline) ................................................................. 24 Figure 10 (a) Sulfonated poly(2,6-disubstituted-1,4-phenylene oxide); (b) reaction

scheme for the sulfonation of poly(3–bromo-2,6-diphenyl-4-phenylene oxide)...... 25 Figure 11 Chemical structures of PEEK, PPBP, and their sulfonated polymers76 ........... 27 Figure 12 Diels-Alder Polymerization of 1,4-Bis(2,4,5-

triphenylcyclopentadienone)benzene and di(ethynyl)benzene78 .............................. 27 Figure 13 Synthesis of poly(phenylenesulfide sulfonic acid) via a poly(sulfonium)

cation.81 ..................................................................................................................... 29 Figure 14 (a) Poly(arylene ether sulfone); (b) Sulfonic Acid Group in Post-Sulfonation

(Activated Ring); (c) Sulfonic Acid Group in Direct Copolymerization (Deactivated Ring) ......................................................................................................................... 30

Figure 15 Metalation Route to Sulfonated Polysulfones85 ............................................... 32 Figure 16 Chemical Structure of Sulfophenylated Polysulfones86 ................................... 33 Figure 17 Direct polymerization synthesis of random sulfonated poly(arylene ether

sulfone) copolymers 89. ............................................................................................. 34 Figure 18 Atomic Force Micrographs of BPSH-40 and Nafion 117 ................................ 35 Figure 19 Influence of the degree of sulfonation on the water uptake of sulfonated

poly(arylene ether sulfone) copolymers.95................................................................ 36 Figure 20 TM-AFM phase image of BPSH-45 in (a) Regime1, (b) Regime2, and (c)

Regime3; Hydro-thermal treatment temperatures of (a), (b), and (c) were 30, 80, and 120 oC, respectively, phase angle: 30o Scan size: 500 nm; darker features represent the hydrophilic phase while lighter features represent the hydrophobic phase102 .... 38

Figure 21 Structures of representative membranes of the poly(ether ketone) family....... 40 Figure 22 (a) Conductivity, at 100oC, as a function of relative humidity for Nafion 117

and S-PEEK-2.48 membranes. The conductivity of g-zirconium sulfophenylphosphonate is reported for comparison; (b) Conductivity of Nafion 117 and S-PEEK-2.48 at 75% r.h. for temperature increasing from 80 to 160oC. The conductivity of S-PEEK-1.6 is reported for comparison. 37 .................................... 41

Figure 23 Schematic representation of the microstructures of NAFION and a sulfonated polyetherketone, illustrating the less pronounced hydrophobic/hydrophilic separation of the latter. 109......................................................................................... 42

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Figure 24 Synthesis of sulfonated poly(aryl ether ether ketone ketone)s111 ..................... 43 Figure 25 Synthesis of highly fluorinated poly(arylene ether) copolymers containing

sulfonic acid groups .................................................................................................. 44 Figure 26 The repeat unit of poly[bis(3-methylphenoxy)phosphazene].......................... 46 Figure 27 Structures of phthalic model A and naphthalenic model B imides.124 ............. 47 Figure 28 Synthesis of sequenced sulfonated copolyimides124......................................... 49 Figure 29 Naphthalenic sulfonated polyimides ............................................................... 50 Figure 30 Illustration of copolymer architectures: III: di-block; IV: tri-block; V: multi-

block; VI: graded block; VII: four arm star block. ................................................... 52 Figure 31 Morphological phase diagram for PS-PI diblock copolymers ......................... 55 Figure 32 Preparation of Fluorine-Containing Block Copolymers by Chain Transfer

Emulsion Polymerization and ATRP 155................................................................... 58 Figure 33 TEM of partially sulfonated P(VDF-co-HFP)-b-PS block copolymer (IEC =

0.89 meq/g) ............................................................................................................... 58 Figure 34 Chemical Structure of Sulfonated SEBS Block Copolymer ............................ 59 Figure 35 Synthetic scheme of Sulfonated–fluorinated poly(arylene ether) multiblocks 61 Figure 36 AFM taping mode phase images of a Sulfonated–fluorinated poly(arylene ether)

multiblock ................................................................................................................. 62 Figure 37 Synthesis of SPSF-b-PVDF.............................................................................. 63 Figure 38 Dependence of proton conductivity on IEC for SPSF (diamond) and SPSF-b-

PVDF (square) membranes. (30 oC, 95% relative humidity).................................... 64 Figure 39 TEM micrographs of polymer membranes. (a): SPSF (IEC: 1.55); (b): SPSF-b-

PVDF (IEC:1.62); (c): SPFSF (IEC: 0.83); (d): SPSF-b-PVDF (IEC: 0.78) ........... 65 Figure 40 Synthesis of sequenced sulfonated naphthalenic polyimide block copolymers

.................................................................................................................................. .67 Figure 41 Ionic conductivity vs. ionic block length for BDSA/NTDA/ODA copolymers

with X/Y = 30/70. ..................................................................................................... 68 Figure 42 Reaction Mechanism Proposed by Colon for the Ni(0)-Catalyzed

Polymerization of Arylene Chlorides ....................................................................... 70 Figure 43 Alkyl-substituted Poly(2,5-benzophenone)s .................................................... 73 Figure 44 Structures of 3-benzoyl-2,5-dichlorothiophene (M1), 2,5-dichloro-3-(2’-

thiophenecarbonyl)thiophene (M2), 3-benzenesulfonyl-2,5-dichlorothiophene (M3), 2,5-dichlorobenzophenone (M4), and 2-benzenesulfonyl-1,4-dichlorobenzene (M5).................................................................................................................................... 76

Figure 45 Synthesis of poly(4’-methyl-2,5-benzophenone) telechelics. .......................... 77 Figure 46 Synthesis of Hydrophilic-Hydrophobic Multiblock copolymers164 ................. 79 Figure 47 Synthesis scheme of substituted 2,5-dichlorobenzophenone monomers ......... 99 Figure 48 Synthesis scheme for 4-Chloro-4'-fluorobenzophenone ................................ 104 Figure 49 Synthesis scheme for 3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone

(SDCDPS) and its salt form.................................................................................... 106 Figure 50 Schematic of a four-point membrane proton conductivity cell. ..................... 119 Figure 51 Chemical structure of disulfonated poly(arylene ether sulfone) copolymers

where the letter x in abbreviation refers to the sulfonation percentage of a disulfonated monomer. ........................................................................................... 123

Figure 52 Synthesis of substituted 2,5-Dichlorobenzophenones by Friedel-Crafts Reactions................................................................................................................. 129

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Figure 53 Proton NMR spectrum of 2,5-dichlorobenzophenone.................................... 130 Figure 54 Proton NMR spectrum of 2,5-dichloro-4’-phenyl-benzophenone ................. 130 Figure 55 Proton NMR spectrum of 2,5-dichloro-4’-oxyphenyl-benzophenone ........... 131 Figure 56 Synthesis of substituted poly(2,5-benzophenone)s by Ni (0)-catalyzed coupling

reaction.................................................................................................................... 134 Figure 57 Proton NMR of Poly (2,5-benzophenone) in CDCl3...................................... 135 Figure 58 13C NMR of Poly (2,5-benzophenone) in CDCl3 ........................................... 136 Figure 59 Poly (2,5-benzophenone)s structures.............................................................. 137 Figure 60 FTIR spectrum of poly (2,5-benzophenone)s................................................. 138 Figure 61 Substituted poly (2,5-benzophenone)s P1-P4 structures ................................ 141 Figure 62 TGA thermogram for poly(2,5-benzophenenone) in N2 ................................ 145 Figure 63 UV-vis spectrum of poly(2,5-benzophenone) in CHCl3 ................................ 147 Figure 64 Synthesis of “BPSH” series random copolymers via direct copolymerization

................................................................................................................................. 149 Figure 65 Proton NMR spectrum of BPSH 35 in d6-DMSO.......................................... 151 Figure 66 Synthetic scheme of fluorine terminated poly(2,5-benzophenone) oligomers.

................................................................................................................................. 157 Figure 67 13C NMR spectrum of a fluorine terminated poly(2,5-benzophenone) oligomer

in CDCl3 ................................................................................................................. 158 Figure 68 19 F NMR of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3

................................................................................................................................. 159 Figure 69 Double logarithmic plot of [η] against Mn of fluorine terminated poly(2,5-

benzophenone) oligomers ....................................................................................... 160 Figure 70 UV-vis spectra of a series of poly(2,5-benzophenone) oligomers ................. 162 Figure 71 Synthetic scheme of fluorophenyl sulfone terminated PBP oligomers .......... 164 Figure 72 19 F NMR of a fluorophenyl sulfone-terminated and fluorophenyl ketone-

terminated PBP telechelic oligomers in CDCl3 ...................................................... 165 Figure 73 Proton NMR of a fluorophenyl sulfone-terminated PBP oligomer with target

Mn of 6,000 g/mol .................................................................................................. 166 Figure 74 Synthetic scheme for hydroxy terminated disulfonated poly(arylene ether

sulfone) oligomers .................................................................................................. 167 Figure 75 2D-COSY spectrum (COrrelation SpectroscopY) of a phenol terminated

disulfonated poly(arylene ether sulfone) oligomer ................................................. 168 Figure 76 Proton NMR spectrum of a phenol terminated disulfonated poly(arylene ether

sulfone) oligomer .................................................................................................... 169 Figure 77 13 C NMR spectrum of a phenol terminated disulfonated poly(arylene ether

sulfone) oligomer .................................................................................................... 170 Figure 78 Synthetic scheme of a PAES-PBP hydrophobic-hydrophilic multiblock

copolymer ............................................................................................................... 172 Figure 79 Proton NMR of PAES-PBP hydrophobic-hydrophilic multiblock copolymer

with block lengths of approximately 6000 g/mol ................................................... 174 Figure 80 13C NMR spectra of a PAES-PBP multiblock copolymer (a), a PBP oligomer

(b) and a PAES oligomer (c)................................................................................... 175 Figure 81 FT-IR spectrum of a PAES-PBP multiblock copolymer................................ 177 Figure 82 Comparison of neutral and ion-containing polymers with different ion

concentration........................................................................................................... 179

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Figure 83 Effect of ion strength on the shape of a polyelectrolyte molecular in solution................................................................................................................................. 180

Figure 84 DSC thermogram of PAES-PBP hydrophobic-hydrophilic multiblock copolymer with block lengths of approximately 6000 g/mol showing two Tgs………………………………………………………………………….……...182

Figure 85 TGA thermogram of a PAES-PBP multiblock copolymer, a BPS 40 random copolymer and a PBP homopolymer………………………………….…………..185

Figure 86 Tapping mode AFM phase images of (a) a PAES-6K-PBP-6K multiblock copolymer, and (b) a PAES-10K-PBP-10K multiblock copolymer with block lengths of approximately 6000 and 10,000 g/mol, respectively.(Scan sizes are 500 nm; Z ranges for phase images are 10 o)………………………………..…………188

Figure 87 Comparison of water uptake of BPSH 35, Nafion, and PAES-PBP multiblock copolymers……………………………………………………………..………….191

Figure 88 Influence of relative humidity on proton conductivity for Nafion 117, BPSH 35 and multiblock copolymer at 80 oC……………………………………………….193

Figure 89 13C NMR spectrum of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3. ……………………………………………………………..…………..208

Figure 90 Double logarithmic plot of [η] against Mn of fluorine terminated poly(2,5-benzophenone) oligomers. ………………………………………………………..210

Figure 91 Proton NMR spectrum of a phenol terminated disulfonated poly(arylene ether sulfone) oligomer. ……………………………………………………………..….213

Figure 92 DSC thermogram of PPP-PAES hydrophobic-hydrophilic multiblock copolymer with block lengths of approximately 6000 g/mol showing two Tgs…..217

Figure 93 Tapping mode AFM phase images of (a) a PAES-6K-PBP-6K multiblock copolymer, and (b) a PAES-10K-PBP-10K multiblock copolymer with block lengths of approximately 6000 and 10,000 g/mol, respectively.(Scan sizes are 500 nm; Z ranges for phase images are 10 o) . …………………………………………218

Figure 94 Synthetic scheme of PAES-PBP segmented copolymers……………………225

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Table of Schemes

Scheme 1 Synthesis of fluorine terminated poly(2,5-benzophenone) oligomers………207 Scheme 2 Synthesis of hydroxy terminated disulfonated poly(arylene ether sulfone)

oligomers………………………………………………………………………….212 Scheme 3 Synthesis of a PBP-PAES hydrophobic-hydrophilic multiblock

copolymer…………………………………………………………………………215

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Table of Tables

Table 1 Properties of substituted poly(2,5-benzophenone)s……………………………140 Table 2 Degree of sulfonations and intrinsic viscosities of BPSH series of disulfonated

copolymers………………………………………………………………………...152 Table 3 Influence of degree of disulfonation on several properties of BPSH

copolymers..……………………………………………………………………….153 Table 4 Characterization of fluorine terminated poly(2,5-benzophenone)

oligomers………………………………………………………………………….161 Table 5 Summary of UV-vis λmax of a series of poly(2,5-benzophenone)

oligomers………………………………………………………………………….163 Table 6 Properties of hydroxy terminated poly(arylene ether sulfone) oligomers……..171 Table 7 Molecular Weight Data of PAES-PBP Multiblock Copolymers……………....181 Table 8 DSC and TGA results for PAES-PBP multiblock copolymers………………..184 Table 9 XPS data of a PAES-PBP multiblock copolymer……………………………..189 Table 10 Characterization of PPP-PAES hydrophobic-hydrophilic multiblock

copolymers………………………………………………………………………...192 Table 11 Characterization of fluorine terminated poly(2,5-benzophenone)

oligomers………………………………………………………………………….211 Table 12 Properties of hydroxy terminated poly(arylene ether sulfone) oligomers……213 Table 13 Characterization of PPP-PAES hydrophobic-hydrophilic multiblock

copolymer…………………………………………………………………………215

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Chapter 1

1 Literature Review

1.1 Introduction

The whole world is facing a rapid depletion of natural resources and serious

global environmental problems.1 Renewable and environmentally friendly energy

sources will be essential for an ever-changing and populous planet. Solar, hydro, and

wind power systems have been employed to complement current electric power sources.

Arguably, the most attractive alternative energy sources are fuel cells. Hydrogen may be

used to produce energy by combining with oxygen in a fuel cell. Hydrogen is the

cleanest, most sustainable and renewable energy carrier.

Fuel cells are viable, renewable, and environmentally friendly energy sources

since the only byproduct is water. 2-5 The recent success of fuel-cell-powered

demonstration vehicles using the proton exchange membrane fuel cell (PEMFC)

developed by Ballard Power Systems, DaimlerChrysler, GM, Nissan and many others,

suggests that fuel cells are beginning finally to come of age.

This literature review will focus on the synthesis of polymeric materials that have

attached ion-conducting groups for proton exchange membranes. First of all, fuel cell

and the basics of proton exchange membrane will be introduced. Then, state-of-the-art

Nafion® and its commercially available perfluorosulfonic acid relatives will be discussed.

The large amount of recent literature dealing with non-fluorinated polymer materials for

PEMFC will be reviewed. Finally, synthetic strategies for obtaining block or multiblock

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copolymers with controlled molecular structure, which afford nanophase separation of

ionic and hydrophobic domains, will be reviewed. This research thesis is directed

towards whether multiblock copolymers for proton exchange membrane can generate

superior membranes.

1.1.1 What is a Fuel Cell?

As early as 1839, William Grove discovered the basic operating principle of fuel

cells by reversing water electrolysis to generate electricity from hydrogen and oxygen.6

The principle that he discovered remains unchanged today.

A fuel cell is an electrochemical “device” that continuously converts chemical

energy into electric energy (and some heat) for as long as fuel and oxidant are supplied.

Fuel cells therefore bear similarities both to batteries, with which they share the

electrochemical nature of the power generation process, and to engines which — unlike

batteries — will work continuously consuming a fuel of some sort.

Unlike engines or batteries, a fuel cell does not need recharging, it operates

quietly and efficiently, and — when hydrogen is used as fuel — it generates only power

and water. Thus, it is a so-called zero emission engine. Thermodynamically, the most

striking difference is that thermal engines are limited by the Carnot efficiency while fuel

cells are not.

Fuel cells are expected to eventually come into widespread commercial use

through three main applications: transportation, stationary power generation, and portable

applications. Fuel cells are in fact rather different for each of these three sectors.

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In the transportation sector, fuel cells are probably the most serious contenders to

compete with internal combustion engines (ICEs). They are highly efficient because they

are electrochemical rather than thermal engines. Hence, they can help to reduce the

consumption of primary energy and the emission of CO2. What makes hydrogen fuel

cells most attractive for transport applications is the fact that they emit zero or ultralow

emissions. This is what mainly inspired automotive companies and other fuel cell

developers in the 1980s and 1990s to start developing fuel-cell-powered cars and buses.

Stationary power generation is viewed as the leading market for fuel cell

technology other than buses. The reduction of CO2 emissions is an important argument

for the use of fuel cells in small stationary power systems, particularly in combined heat

and power generation (CHP). In fact, fuel cells are currently the only practical engines

for micro-CHP systems in the domestic environment (5–10 kW). The higher capital

investment for a CHP system would be offset against savings in domestic energy supplies

and — in more remote locations — against power distribution cost and complexity. In the

50- to 500-kW range, CHP systems will have to compete with spark or compression

ignition engines modified to run on natural gas. So far, several hundred 200-kW

phosphoric acid fuel cell plants manufactured by UTC now have UTC fuel cells and have

been installed worldwide.

The portable market is less well defined, but Plug Power also sells stationary units

for quiet fuel cell power generation may be in the 1-kW portable range and possibly, as

ancillary supply in cars, so-called auxiliary power units (APUs). They are much lighter

and have higher power density than batteries. The term “portable fuel cells” often

includes grid-independent applications such as camping, yachting, and traffic monitoring.

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In addition, the choice of fuel is not the only way in which these applications vary.

Different fuel cells based on H2 or methanol may be needed for each sub-sector in the

portable market.

1.1.2 Proton Exchange Membrane Fuel Cells (PEMFC)

Principally, five fuel cell systems are considered to be significant, each with its

distinct electrochemical reaction and operation requirements. They are generally

classified by the electrolyte used: phosphoric acid fuel cells (PAFC), molten carbonate

fuel cells (MCFC), solid oxide fuel cells (SOFC), alkaline fuel cells (AFC), and proton

exchange membrane fuel cells (PEMFC). The proton exchange membrane fuel cell,

PEMFC, stands out because of its simplicity and high power density, which make it the

only fuel cell type currently being considered for powering passenger cars. The PEMFC

is also being developed for stationary and portable power generation.

The smallest building block of a PEM fuel cell is the membrane electrode

assembly or MEA, as shown in Figure 1. Consisting of two electrodes, anode and

cathode, and the polymer electrolyte, it essentially forms the basic fuel cell. Thin gas-

porous electrodes featuring noble metal (usually Pt and its alloy) layers (several microns

to several tens of microns) on either side of the membrane contain all the necessary

electrocatalysis, which drives the electrochemical power generation process.

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The electrochemical reactions of proton exchange membrane fuel cells using

hydrogen and oxygen gas are:

Anode: H2 → 2H+ + 2e- Equation 1 Cathode: ½ O2 + 2H+ + 2e- → H2O Equation 2

Overall: H2 + ½ O2 → H2O Equation 3

Theory predicts a maximum of 1.23 voltage could be generated per single fuel

cell.6 The gas diffusion layer or electrode substrate (or electrode backing material) at the

anode allows hydrogen to reach the reactive zone within the electrode. Upon reacting,

hydrated protons migrate through the ion conducting membrane, and electrons are

conducted through the substrate layer and, ultimately, to the electric terminals of the

multi fuel cell stack. The anode substrate therefore has to be gas porous as well as

electronically conducting. Not all of the chemical energy supplied to the MEA by the

reactants is converted into electric power and heat will also be generated somewhere

inside the MEA. Hence, the gas porous substrate also acts as a heat conductor to remove

heat from the reactive zones of the MEA.

At the cathode, the functions of the substrate become even more complex.

Product water is formed at the cathode. Should this water exit from the electrode in

liquid form (as it usually does if the reactants are saturated with water vapor), there is a

risk of liquid blocking the pores within the substrate and, consequently, gas access to the

reactive zone, which produces flooding. This poses a serious performance problem

because for economic reasons the oxidant used in most practical applications is not pure

oxygen but air. Therefore, 80% of the gas present within the cathode is inert — with all

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associated boundary and stagnant layer problems. Fuel cell operation will therefore result

in a depletion of oxygen towards the active cathode catalyst.

Figure 1 Membrane Electrode Assembly7

The membrane acts as a proton conductor. This requires the membrane to be well

humidified because the proton conduction process relies on the hydration of the

membrane for transport. As a consequence, an additional water flux from anode to

cathode is present and is associated with the migration of protons (electro-osmotic drag).

Since this will eventually lead to a depletion of water from the anode interface of the

membrane, humidity is often provided with the anode gas by pre-humidifying the

reactant.

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Figure 2 Fuel cell stack made up of flow field plates (or bipolar plates) and MEAs (shown in the

insert) 7

The MEA is typically located between a pair of current collector plates with

machined flow fields for distributing fuel and oxidant to the anode and cathode,

respectively, as shown in Figure 2. A water jacket for cooling is often placed at the back

of each reactant flow field followed by currently a metallic current collector plate. The

cell can also contain a humidification section for the reactant gases, which are kept close

to their saturation level in order to prevent dehydration of the membrane electrolyte.

1.1.3 Proton Exchange Membranes

The key component in a proton exchange membrane fuel cell (PEMFC), the

proton exchange membrane (PEM), performs two basic, essential functions in PEMFCs:

(1) a separator to prevent mixing of the fuel (i.e., hydrogen gas, methanol, etc.) and the

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oxidant (i.e., pure oxygen or air), and (2) an electrolyte for transporting protons from the

anode to the cathode 8.

Desirable properties of the proton exchange membrane in fuel cells include 9:

(1) High proton conductivity;

(2) Low electronic conductivity;

(3) Low permeability to fuel and oxidants;

(4) Low water transport through diffusion and electro-osmosis;

(5) Oxidative and hydrolytic stability;

(6) Low swelling stresses;

(7) Good mechanical properties both in the dry and hydrated state;

(8) Affordable cost;

(9) Capability of fabrication into membrane electrode assemblies (MEAs).

In addition, proton exchange membranes must be fabricated into a membrane

electrode assembly (MEA) as outlined in Figure 1. Therefore, the ease of membrane

electrode assembly fabrication and the resulting properties of the MEA must be

considered. Current work in the area of fabricating MEAs from novel polymeric

membranes has focused on the electrode-membrane interface and the problems of having

dissimilar ion conducting copolymers in the membrane and the electrode.10, 11 Moreover,

ion conducting polymers that are compatible for use in the catalyst layer, in concert with

novel polymer membranes are also an emerging area of research.

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1.1.4 Important Parameters

Several important parameters need to be defined before beginning detailed

discussion of various proton exchange membrane candidates. Those parameters are

extremely useful when it comes to comparison between different polymeric materials.

1.1.4.1 Proton Conductivity

How does one measure proton conductivity? Scientists at Los Alamos National

Labs (LANL) have devised a facile method for determining the conductivity of proton

exchange membranes using electrochemical impedance spectroscopy and a simple cell

that allows equilibration in a variety of environments.12 This method measures protonic

conductivity in the plane of the membrane as opposed to through the plane (as in a fuel

cell), and thus works well as an initial screening test. Through-plane conductivity

measurements13-15 are considered more difficult experimentally than in-plane

measurements because the measured membrane resistances are small in this geometry

and interfacial resistances may play a very significant role.

Figure 3 Typical cell geometry for an in-plane proton conductivity measurement.

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While the through-plane technique is more analogous to actual proton conduction

in a fuel cell, the resistance across a thin membrane is usually too small to be measured

accurately. For this reason, the in-plane technique is more commonly used. A typical cell

for in-plane proton conductivity measurements is shown in Figure 3. In this case, the cell

geometry was chosen to minimize the resistance of the cell so that the measured

resistance was due primarily to the membrane resistance.16

1.1.4.2 Water uptake

Water uptake is another important parameter in determining the ultimate

performance of PEM materials. For most current conducting polymeric materials, water

is needed as the mobile phase to facilitate proton conductivity. Absorbed water also

affects the mechanical properties of the membrane by acting partially as a plasticizer,

which may lower the Tg and modulus of the membrane. Also, swelling and degradation

of the mechanical properties of the membrane may become serious problems in humid

environments or during cyclic operations. Water uptake is usually reported as a mass

fraction, mass percent, or a lambda value, where lambda (λ) equals the number of water

molecules absorbed per acid site.

There has been extensive research on types of water in the area of hydrogel, and

the three types of water are generally described as nonfreezing water (Tg ~ 130 oC17, 18),

freezable loosely bound water, and free water. 18 Nonfreezing water is strongly

associated with the acid sites on the polymer chain, and causes depression of the polymer

Tg (plasticization), but does not participate in the melting at 0 °C. Freezable loosely

bound water is loosely associated with the copolymer. Freezable loosely bound water has

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a broad melting centered around 0 °C. Free water is not associated with the polymer

chain, and behaves like bulk water. It shows a sharp melting endothermic at 0 °C in DSC.

1.1.4.3 Ion Exchange Capacity--IEC

Both conductivity and water uptake rely heavily on the concentration of ion

conducting units in the polymer membrane. The ion content is characterized by the

molar equivalents of ion conductor per mass of dry membrane and is expressed as

equivalent weight (EW) with units of grams of polymer per equivalent or ion exchange

capacity (IEC) with units of milli-equivalents per gram (meq/g or mmol/g) of polymer

(EW = 1000/IEC). (Usually expressed on a weight basis)

IEC is a very important property because it allows for a more accurate

comparison of materials than does the degree of sulfonation. IEC can be determined

from NMR by integrating known signals due to the sulfonated and non-sulfonated repeat

units. However, titration of the acidic groups can often be a more precise method of

determining IEC. Titration of sulfonic acid groups is typically accomplished by one of

two methods. The first is a potentiometric titration of the polymer in an organic solvent,

such as dimethyl acetamide, with a soluble base, such as tetramethyl ammonium

hydroxide. When this method is used, a “blank” titration of the solvent should be

performed. The second common method is aqueous titration with sodium hydroxide. To

use this method, the acidic sites on the polymer must first be dissociated using a weak

base such as sodium sulfate. This method can sometimes result in lower-than-theoretical

IEC values, possibly due to incomplete dissociation of the sulfonic acid sites. While it is

desirable to maximize the conductivity of the membrane by increasing its ion content

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(decreasing equivalent weight), excessively swelling with water and decreased

mechanical integrity and durability must be considered.

1.1.4.4 Methanol Permeability

Methanol permeability of the membrane is an important factor in Direct Methanol

Fuel Cells (DMFCs) because it leads to undesirable transport of methanol across the

membrane. This has two important consequences: 1) the fuel is wasted instead of being

usefully oxidized at the anode, and 2) the oxidation of methanol at the cathode results in a

backward flux of protons and electrons, and lowers the overall efficiency of the cell.

The diffusion of methanol across a membrane can be measured by a diffusion

experiment in which the membrane separates two compartments: one rich in methanol

and the other rich in water. This method is fairly simple; the main equipment

requirements are a methanol pump, a refractive index detector, and a desktop computer.

In a working fuel cell, the movement of protons and water through the membrane

complicates the measurement of methanol permeation, but in-situ measurements of

methanol crossover current are possible with fuel cell hardware.10

1.1.4.5 Electro-Osmotic Drag

When protons are transported across the PEM, they are usually hydrated by a

certain number of water molecules. The number of water molecules per proton is defined

as the electroosmotic drag coefficient.19, 20 Electro-osmotic drag is an important factor in

PEMFCs because as protons are conducted across the membrane, water is transported

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from the anode to the cathode. The amount of water in the electrodes has a large

influence on the electrode performance. If there is too much water, flooding will occur

on the cathode side, which results in decreased gas diffusion to the reactive sites. If there

is too little water, the electrode will become too dry and the proton conductivity will be

restricted. The electro-osmotic drag coefficient can be measured during fuel cell

operation.21 The electro-osmotic drag has also been correlated to the states of water in the

membrane.

1.2 Perfluorinated Copolymers for PEM

Perfluorinated sulfonic acid copolymers are the most widely studied and applied

materials in proton exchange membrane (PEM) fuel cell investigations. The

perfluorinated copolymers have demonstrated the most superior properties for PEM up to

now.22, 23 Perfluorinated copolymers (PFSA) are well known for their exceptionally high

chemical and thermal stabilities. Several studies have confirmed the chemical stability of

perfluorinated polymers at less than 100 oC in high humidity environment, including

strong bases, strong oxidizing and reducing agents. These copolymers are chemically

stable against the oxidative conditions of a fuel cell due to the strong carbon-fluorine

bonds24, which are approximately 4 kcal stronger than aliphatic carbon-hydrogen bonds25.

The development of perfluorinated polymers increased confidence in the application of

PEMFCs after the early limit use of crosslinked sulfonated polystyrene (essentially ion

exchange resins).

Nafion® is produced and marketed by E.I. DuPont de Nemours and was

developed for electrochemical chlorine production and for space exploration

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applications.26 These fully fluorinated electrolytes have been the focus of several books

and reviews in PEMFC literature.27-29 Nafion is a free radical initiated random

copolymer of a crystallizable hydrophobic tetrafluoroethylene (TFE) backbone sequence

(~ 87 moles %) with a comonomer which ultimately has pendant side chains of

perfluorinated vinyl ethers terminated by perfluorosulfonic acid groups. The proposed

structure of Nafion® is shown in Figure 4.

Figure 4. Chemical Structure of Nafion®

**x and y represent molar compositions and do not imply a sequence length.

Nafion 1100 EW in thicknesses of 2, 5, and 7 mils (1 mil equals 25.4 microns)

(Nafion 112, 115, and 117, respectively) is currently widely available. This equivalent

weight provides high protonic conductivity and moderate swelling in water, which seems

to suit many current applications and research efforts. Modest retention of a

semicrystalline morphology at this composition is no doubt important to Nafion’s

mechanical strength. The thinner membranes are generally applied to hydrogen/air

applications to minimize Ohmic losses, while thicker membranes are employed for direct

methanol fuel cells (DMFC) to reduce methanol crossover in direct methanol fuel cell

(DMFC) systems.

Unsaturated perfluoroalkyl sulfonyl fluoride and their derivatives are believed to

be the starting comonomers for preparing perfluorosulfonic membranes by DuPont.

Nafion is prepared via the copolymerization of variable amounts of the unsaturated

CF2 CF2 CF CF2

OCF2 CF O(CF2)2 SO3-H+

CF3

x y

z

nCF2 CF2 CF CF2

OCF2 CF O(CF2)2 SO3-H+

CF3

x y

z

n

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perfluoroalkyl sulfonyl fluoride with tetrafluoroethylene.30-35 There have been no

detailed literature reports of Nafion’s synthesis and processing, but it is generally

believed that the copolymer is then extruded in the melt processible sulfonyl fluoride

form into a membrane, which is later converted from the sulfonyl fluoride via base

catalyzed hydrolysis and acidification to the sulfonic acid functionality. It seems unlikely

that the sulfonyl fluoride containing precursor unit in the copolymer would self propagate.

Thus, the length of the comonomer sequence (y) is likely only one unit.

These membranes exhibit a protonic conductivity as high as 0.1 S⋅cm-1 in liquid

water under fully hydrated conditions. For a membrane thickness of, 175µm (Nafion

117), this conductivity corresponds to a real resistance of 0.2 ohm (Ω)⋅cm2, i.e., a voltage

loss of about 150mV at a practical current density of 750 mA⋅cm-2. Under relative low

current density, a lifetime of over 60,000 hours under fuel cell conditions (80% relative

humidity at 80oC) has been reported with commercial Nafion membranes 36.

However, Nafion membranes are limited in their temperature range of operation

to around 80oC. Figure 5 shows the protonic conductivity of Nafion 117 membranes at

the indicated relative humidities for temperature increasing from 80 to 160oC. At

different relative humidities there was a dramatic decrease in conductivity when

temperatures reached certain values. Besides, the decrease was shifted to higher

temperature when lowered the relative humidity. Furthermore, it was found that the

conductivity decrease was not reversible since the initial value was not recovered when

the temperature was again decreased below 110 oC. Alberti et al 37 believe this is due to

the excessive swelling of the polymer, which leads to the decrease of the effective contact

area with the electrode. However, the Virginia Tech group perhaps was the first one to

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point out that the glass transition temperatures (Tg) or at least of α-relaxations in Nafion

hydrated Nafion 117 membrane were actually reached and thus some dramatic

morphological changes happened and ruined the transport of protons. 37 (Water is well

known as a plasticizer to lower the glass transition temperature of an ion containing

polymer.) As a matter of fact, a major loss of conductivity of hydrated Nafion 117

membrane shifted to higher temperature (from around 120 oC to 160 oC) as relative

humidity decreased from 100% to 34%.37

Figure 5 Conductivity of Nafion 117 membranes at the indicated relative humidities for temperature

increasing from 80 to 160oC 37.

Recently, Moore et al. 29, 38 report the existence of α and β relaxations in Nafion.

The β-relaxation is often related to chemical characteristics of the molecular structure,

like reorientational or vibrational motions of the highly flexible side chain with

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perfluorosulfonic acid groups. The α relaxation, on the other hand, denotes the complete

disruption of the ionic domains. More recently, the glass transition temperature (β-

relaxation temperature) of pure H+ form Nafion was reported to be approximately -20 oC

by dynamic mechanical analysis (DMA) data.29, 39 More importantly, the presence of α

and β relaxations in Nafion is good evidence of nanophase separation in this ion

containing polymer which has the highly hydrophobic tetrafluoroethylene backbone.

Other drawbacks include low conductivity at low water contents, relative low

mechanical strength, poor barrier properties to methanol, allowing methanol crossover

from the anode to the cathode in a DMFC, high osmotic drag, which makes water

management difficult at high current densities. These features are also common to other

perfluorinated membranes, such as those that are or were produced by Dow, Asahi Glass,

Asahi Chemical and 3M.40 Finally, the high cost of Nafion is currently incommensurate

with potential mass markets such as vehicles.

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1.3 Non-Fluorinated Polymer Materials for PEM

Consequently, much effort has focused on the development of alternative proton

exchange membranes for PEM fuel cells and in DMFC, in particular with the aim of

increasing the temperature of operation of the fuel cell. An increase in temperature is

attractive for a number of reasons: (a) improved tolerance of the electrodes to carbon

monoxide, which enables the use of hydrogen produced by reforming of natural gas,

methanol or gasoline; (b) simplification of the cooling system; (c) possible use of

cogenerated heat; (d ) increased proton conductivity; and (e) in DMFC, improved kinetics

of the methanol oxidation reaction at the anode, and at the cathode.

However, an increase in temperature also has some restraints. The majority of

proton-conducting polymer materials proton conduction is water-assisted, and they

exhibit the highest proton conductivity when fully hydrated. This requires humidification

of gas feeds before entering the fuel cell and the application of pressure to maintain

adequate relative humidity.

There are few non-fluorinated membrane materials appropriate for fuel cell

application at temperatures above 80oC. These polymers are normally made up of

polyaromatic or polyheterocyclic repeat units, and examples include polysulfones (PSU),

poly(ether sulfone) (PES), poly(ether ketone)s (PEK), poly(phenyl quinoxaline) (PPQ),

and polybenzimidazole (PBI). Developed for high-temperature applications, the thermal

stability of these types of polymers is well documented. However, they are electrically

insulating until modified. Several methods for the preparation of thermally stable proton-

conducting polymers have been developed that include acid or base doping of a

thermostable polymer, direct sulfonation of a polymer backbone, grafting of a sulfonated

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or phosphonated functional group on to a polymer main chain, graft polymerization

followed by sulfonation of the graft component, and total synthesis from monomer

building blocks (Figure 6). These methods will be reviewed in details with respect to

various polymeric materials. In addition, the proton-conducting properties may be

entirely conferred or enhanced by the addition of an inorganic proton conductor41; see the

review by Casciola and Alberti42 for details. The approach chosen depends on the

particular properties and chemical reactivity of the polymer concerned. Recent progress

in the area of non-fluorinated proton conducting membranes includes a detailed review

by Rozière & Jones41 and a survey early Savadogo43.

Figure 6. Development of proton-conducting membranes by the modification of thermostable

polymers.41

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1.3.1 Polybenzimidazole

Aromatic polybenzimidazoles are well known as highly thermostable materials

with glass transition temperatures as high as 430°C.44 Due to its commercial availability,

the most widely used one is poly[2,2”-(m-phenylene)-5,5’-bibenzimidazole] or PBI as

shown in Figure 7. 45

Figure 7. Synthesis route to sulfonated (stabilized) polybenzimidazole46

1.3.1.1 Acid Doping

PBI has very low proton conductivity (∼10-7 S cm-1) because it is able to take up

small amounts of water47, 48. Nevertheless, after treatment with sulfuric acid or especially

phosphoric acid, its conductivity can increase significantly. The original acid uptake

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was 5 mol H3PO4 per PBI repeat unit, far beyond the quantity for other systems. This

treatment is also called as “Doping” 49-51. The reason for this dramatic difference lies in

the fact that PBI readily forms complexes with organic and inorganic acids due to its

basicity. The properties of such doped membranes and their application in PEM fuel

cells have been studied extensively since 199449, 52, 53,54,55-57culminating in the production

by Celanese Ventures of MEAs based on phosphoric acid-doped PBI. These membranes

have some distinct features such as low permeability to methanol and low electroosmotic

drag, which is extremely suitable for the direct methanol fuel cell and high-temperature

operation58. Various acid and different methods are used in the formation of PBI-acid

complexes and, eventually, a homogeneous polymer electrolyte is formed by dissolution

of the acid in the PBI matrix. The conductivity depends on the level of doping, which is

often expressed as the number of H3PO4 molecules per PBI matrix. For membranes with

4-5 H3PO4/PBI, the conductivity is >10-3 S cm-1 at 25°C and >3×10-2 S cm-1 at 190°C52.

More recently, Benicewicz et al, 59 reported a sol-gel process which allows very high

levels of H3PO4 to be achieved by direct casting of the PPA(poly phosphoric acid)-

polymerization solution, without isolation or re-dissolving the polymers. The phosphoric

acid (PA)-doped polybenzimidazole (PBI) films produced in this way can be operated in

a fuel cell at temperatures above 150 °C without any feed gas humidification or pressure

requirements for much more than 1000 h.

1.3.1.2 Direct Sulfonation of Polymeric Backbone

Sulfonated polybenzimidazole can be made by heating sulfuric acid –doped PBI

at 450-500°C for a short period of time (Figure 7), leading to levels of substitution up to

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0.6 sulfonic acid groups/PBI46,60, 61. However this kind of treatment seems to bring no

benefits at all. The conductivity is barely higher than that of non-substituted PBI and the

membrane can not be processed further because solubility in solvent like

dimethylacetamide is lost. The thermally initiated sulfonation leads to crosslinking,

either by strong hydrogen bonding or through sulfone linkages, or degradation.

1.3.1.3 Synthesis from Sulfonated Monomer

Sulfonated polybenzimidazole can also be prepared from pre-sulfonated monomer

units, for example, by the polycondensation of 2-sulfoterephthalic acid with 1,2,4,5-

tetraaminobenzene tetrahydrochloride at 190 °C 62, 63 . But the number of pendant

sulfonic acid groups should be controlled in a low range to avoid solubility in water.

1.3.1.4 Grafting of a Functional Group

Another approach of making sulfonated polybenzimidazole involves a hydrogen

abstraction with LiH, followed by reaction of the PBI polyanion with a functionalized

grafting species, which includes both sulfonated and carboxymethyl groups (Figure 8). It

is actually a functional group grafting reaction.64-67 The advantage of this method is that

the chemical stability of PBI can be improved by introducing less reactive groups directly

into the imidazole ring68. Furthermore, the degree of sulfonation can be controlled by

limiting the number of –NH sites ionized or limiting the ratio of the sulfonated side chain

group to PBI. Control of the number and location of ionic groups is critical to a

systematic study of the PEM properties.

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Figure 8. Synthesis route to benzylsulfonated polybenzimidazole via polyanion formation68

As the degree of sulfonation increases, solubility and textural properties of PBI

improve significantly. The hydration number is ∼7 water molecules per sulfonic group,

which is much lower than 12 of Nafion. The conductivity was measured at 25°C in

aqueous phosphoric acid and the membranes were allowed to equilibrate for 8 h before

measurement. Compared with the low conductivity of non-grafted PBI (∼10-5 S cm-1),

grafted PBI displays fairly high conductivity in the range of 3×10-3 to 2 ×10-2 S cm-1.68

1.3.2 Poly(phenylquinoxalines)

Poly(phenylquinoxaline) or PPQ as shown in Figure 9, can be sulfonated as a cast

film by briefly treating sulfuric acid-doped membranes at high temperature69, just like

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immersion/thermal treatment for polybenzimidazoles above. In contrast to low

conductivity of polybenzimidazoles, sPPQ exhibits proton conductivity up to 0.1 S cm-1.

Figure 9. Sulfonated poly(phenyl quinoxaline)

Sulfonation of PPQs was carried out usually by dissolving PPQ in chlorosulfonic

acid and the site of sulfonation is ortho to the electron donating ether linkage 70. The

performance of MEAs prepared using sPPQs in a hydrogen/air fuel cell at a current

density of 0.538 A cm-2 at 70 °C is in the range of 0.50 V to 0.66V.69 However, the

average MEA lifetime was only 350h, which is possibly due to increased permeability

and/or membrane embrittlement. In fact, no details of MEA fabrication have been

published.

1.3.3 Poly(phenylene oxides)

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is an important material for the

molding and preparation of membranes because its excellent film and membrane-forming

properties, as well as good thermal and chemical stability.

Sulfonation of PPO was accomplished either in a chloroform solution with

chlorosulfonic acid 71, or in a 1,2-dichlorethane 72. Then the sulfonated product was

precipitated after the addition of a certain amount of chlorosulfonic acid and could

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subsequently be easily isolated. The SPPO has shown good thermal stability and

resistance against aqueous solutions of strong acids and bases and oxidation agents. 73-

75(Note benzylic methyl oxidizes easily)

Sulfonated poly(2,6-diphenyl-4-phenylene oxide) or P3O (Figure 10a) is made by

oxidative coupling of 2,6-diphenylphenol followed by directly sulfonation by

chlorosulfonic acid. Ion exchange capacity (IEC) values were high (2.22 meq g-1) but

mechanical properties are poor and life time was limited to 500h, owing to a reported

combination of physical and chemical degradation.71 Bromide or cyanide was introduced

to the central aryl group, as shown in Figure 10b aiming to deactivate or improve

chemical stability of P3O. However, the fuel cell lifetimes were still 450-500h.

(a) (b)

Figure 10. (a) Sulfonated poly(2,6-disubstituted-1,4-phenylene oxide); (b) reaction scheme

for the sulfonation of poly(3–bromo-2,6-diphenyl-4-phenylene oxide).

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1.3.4 Poly(phenylene)s and Derivatives

Poly(p-phenylene) (PPP) and its derivatives are a promising class of high-

performance polymers because of their excellent thermal and mechanical properties.

The sulfonated poly(p-phenylene) derivatives were synthesized76 and later by

Ghassemi and McGrath77 via Ni (0)-catalyzed coupling polymerization. Concentrated or

fuming sulfuric acid was utilized at room temperature to introduce sulfonic acid moieties

to the aromatic side group. Although the sulfonated polymers were not good film

formers, they exhibited high proton conductivity in the range of 0.06-0.11 S⋅cm-1. To

obtain good films, approaches such as polymer blending and multiblock copolymers were

also studied.

Sulfonation of poly(4-phenoxybenzoyl-1,4-phenylene) (PPBP) was also

reported76 and sulfuric acid was utilized to minimize degradation of the polymer by

chlorosulfonic acid or fuming sulfuric acid. At same IEC, sulfonated PPBP showed

much higher and more stable proton conductivity than similar sulfonated PEEK (Figure

11).

Recently, ionomeric poly(phenylene) prepared by Diels-Alder polymerization

first reported by Harris and Stille78, was reported by Fujimoto et al.79 Post-sulfonation of

this high molecular weight and thermochemically stable poly(phenylene) with

chlorosulfonic acid resulted in homogeneous polyelectrolytes with controllable ion

content (IEC = 0.98-2.2 mequiv/g). Fuel cell relevant properties such as high proton

conductivity (123 mS/cm), chemical/thermal stability, and film toughness suggest that

this polyelectrolyte material shows promise as a potential candidate for PEMFCs.

However, economic and availability of the monomer may be questionable.

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Figure 11 Chemical structures of PEEK, PPBP, and their sulfonated polymers76

Figure 12 Diels-Alder Polymerization of 1,4-Bis(2,4,5-triphenylcyclopentadienone)benzene and

di(ethynyl)benzene78

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The proton transport properties of a series of sulfonated poly(phenylene)s were

found to strongly correlate to the ion exchange capacity of the polymer.80 In general,

these materials have minimal methanol crossover while maintaining high proton

conductivity, which is necessary for efficient operation of fuel cells powered by liquid

fuels. Proton conductivity in addition to methanol permeability were compared to Nafion

as a function of ion exchange capacity and it was found that the transport in Nafion

membranes was much higher than that in the sulfonated poly(phenylene)s for a given ion

exchange capacity.

1.3.5 Poly(phenylene sulfide)

Poly(phenylene sulfide) or PPS can be directly sulfonated by concentrated

sulfuric acid (Figure 13)81. A polysulfonium cation in 10% SO3/H2SO4 can give a higher

degree of sulfonation and sulfonated PPS can reach proton conductivity of 2.5×10 -3 S/cm.

However, the system became water soluble at a degree of sulfonation > 30%. Here, the

strong electron withdrawing property of the sulfonation group in the main chain

suppresses the crosslinking reaction and promotes sulfonation at 120 °C. The poly

(phenylene sulfide sulfonic acid) is formed after demethylation and acidification

treatment.

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Figure 13 Synthesis of poly(phenylenesulfide sulfonic acid) via a poly(sulfonium) cation.81

1.3.6 Poly(arylene ether sulfones)

Poly(arylene ether sulfone)s (PES) are known for their excellent thermal and

mechanical properties (Figure 14).82 The basic repeat units in this family of polymers

consist of phenyl rings separated by alternate ether and sulfone (-SO2-) linkages. The

aromatic ether (bisphenol) part provides flexibility, whereas the sulfone group is stable

with respect to oxidation and reduction. The sulfonation of poly (arylene ether sulfone)s

can be accomplished either by simply sulfonation of polymer ( or so called “post-

sulfonation”) or direct copolymerization of sulfonated comonomer. One big difference

between these two methods is that the sites of sulfonic acid in post-sulfonation and direct

copolymerization, as shown in Figure 14 b and c.

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S

O

O

O

n

S

O

O

OXO

n

S

O

O

OXO

n

SO3H

SO3H

HO3S

Activated Ring

Deactivated Ring

(a)

(b)

(c)

Figure 14 (a) Poly(arylene ether sulfone); (b) Sulfonic Acid Group in Post-Sulfonation (Activated

Ring); (c) Sulfonic Acid Group in Direct Copolymerization (Deactivated Ring)

1.3.6.1 Post Sulfonated Copolymers

Poly (arylene ether sulfone)s can be directly sulfonated by various sulfonation

reagents and these modification reactions have been investigated intensively. 83,84

Noshay and Robeson83 developed a mild sulfonation procedure for the commercially

available bisphenol-A-based poly(ether sulfone) and since then various sulfonating agent

such as chlorosulfonic acid and sulfur trioxide-triethyl phosphate complex have been

employed. A systematic study of the effect of sulfonating agent was undertaken by

Genova-Dimitrova et al. 84 They found chlorosulfonic acid, a strong sulfonating agent,

yielded an inhomogeneous reaction, while mild trimethylsilyl-chlorosulfonate sulfonating

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agent gave homogeneous reactions. Besides, chlorosulfonic acid also induced chain

cleavages during sulfonation as indicated by viscometric measurements whereas no

polymer degradation or crosslinking was observed with the milder

trimethylsilylchlorosulfonate. However, the degree of sulfonation was limited to 0.85

(85% of the monomer units sulfonated) under short reaction times when the mild

sulfonating agent was employed. Despite its relative low efficiency, the mild sulfonating

agent is still favored because the properties of the resultant sulfonated polymers were

more predictable.

All the above electrophilic substituent reactions locate the sulfonic acid group at

the most activated site ortho to the aromatic ether bond in the bisphenol A part of the

molecule. Ortho site to the sulfone bond in the sulfone unit is also possible for

sulfonation by a series of reactions, as shown in Figure 15, including metalation,

sulfonation by SO2 gas and oxidation.

Kerres 85, et al. invented this method and it turned out the choice of oxidant to

convert sulfonic acid is critical in this synthesis. Because the oxidation step is

accompanied by a loss in ion exchange capacity IEC and this is probably due to the

splitting-off of the sulfonated group during the oxidation process and subsequent

substitution by H. However, by carefully selecting the best suitable oxidation conditions

the loss in IEC can be minimized. Hydrogen peroxide was found to be the best oxidant

due to its good accessibility to all the ionic groups in the polymer, thus minimizing

undesired crosslinking reactions.

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Figure 15 Metalation Route to Sulfonated Polysulfones85

This method has some advantages over ordinary post-sulfonation procedures: 1) it

can be applied to any polymer that can be lithiated, such as poly (2,6-dimethyl-

paraphenylene ether) (PPO), poly(styrene), poly(vinylthiophene), and

poly(methylphenylphosphazene); 2) the sulfonic acid group is inserted into the more

hydrolysis-stable part of the molecule; 3) it is ecologically less harmful than many

common sulfonation procedures. Therefore, it is an attractive way for controlled

sulfonation for a variety of polymers although it is more expensive.

In fact, this method has been extended to attach a pendant sulfonated phenyl

group on polysulfones via ketone links, as shown in Figure 16. One may think some

phase separation and interesting morphology phenomenal may take place here by the

pendant attachment of the ionic group.

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Figure 16 Chemical Structure of Sulfophenylated Polysulfones86

1.3.6.2 Synthesis of copolymers from sulfonated monomers

As mentioned before, post-sulfonation has several problems: 1) lack of precise

control over the degree and location of functionalization;2) the possibility of side

reactions, and 3) the possibility of degradation of the polymer backbone.

On the other hand, greater tailoring ability of the polymer can be achieved via

direct synthesis from monomer functionalized with sulfonic groups. Control of position,

number and distribution of sulfonic groups along the polymer backbone could provide

more thermo and hydrolytically stable sulfonated copolymers. Furthermore,

microstructure and important PEM membrane parameters such as conductivity, degree of

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swelling can be easily tuned by modification of monomers. The difference between sites

of sulfonic acid in post-sulfonation and direct copolymerization are shown in Figure 14.

McGrath’s group in Virginia Tech used one modified procedure to make poly

(aryl ether sulfone)s from sulfonated monomers16, 87-93. As shown in Figure 17, the

nucleophilic substitution condensation polymerization of 4,4’-dichlorodiphenylsulfone,

4,4’-biphenol, and 3,3’-disulfonate-4,4’-dichlorodiphenylsulfone leads to a random

sulfonated poly(arylene ether sulfone) with two sulfonic acid groups per repeat unit, and

in which the sulfonic acid groups are sited on the deactivated sulfone-linked rings.

Figure 17 Direct polymerization synthesis of random sulfonated poly(arylene ether sulfone)

copolymers 89.

The weak base potassium carbonate, K2CO3, instead of a strong base was utilized

here to avoid weighing an exactly stoichiometric amount of strong base and undesirably

hydrolyzing the dihalides to afford deactivated diphenolates94, which would upset the

stoichiometry. Potassium carbonate has high solubility in the reaction medium and the

precise amount of weak base is not extremely critical for achieving high molecular

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weight, as long as it is in excess. Therefore, this approach makes it fairly easy to control

the molecular weights and the endgroups.

Figure 18 Atomic Force Micrographs of BPSH-40 and Nafion 117

An atomic force microscopy (AFM) tapping mode comparison of the 40%

copolymer with Nafion is shown in Figure 18. The resultant random sulfonated

copolymers displayed a hydrophilic/hydrophobic phase separated morphology that varied

depending on the degree of disulfonation.

The conductivity and water uptake of this series of copolymers also increased

with disulfonation. However, a semi continuous hydrophilic phase was generated and the

membranes swelled dramatically if the degree of disulfonation reached 60 mole %,

resulting in a hydrogel that can not be used as a proton exchange membrane (Figure 19).

1 1 µµmm 700 nm700 nm

Phase Image of BPSH-40 dark regions are sulfonic acid + water(softer portion)

Phase Image of Nafion 117

1 1 µµmm 700 nm700 nm

Phase Image of BPSH-40 dark regions are sulfonic acid + water(softer portion)

Phase Image of Nafion 117

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Therefore, to obtain a useful proton exchange membrane, one must balance ionic

conductivity with other physical properties.

Figure 19 Influence of the degree of sulfonation on the water uptake of sulfonated

poly(arylene ether sulfone) copolymers.95

BPSH random copolymers exhibited much lower methanol permeability than

Nafion. For example, even though Nafion and BPSH-30 (a poly(arylene ether sulfone)

copolymer with 30 mole% disulfonation) have similar water uptake values, the methanol

permeability of Nafion is much higher (167 x 10-8 cm2/s compared to 36 x 10-8 cm2/s for

BPSH-30).95-97 Recent research has shown that this may be due to a difference in the

“types” of water that are present in each of the membranes. 17, 98 BPSH-30 has a higher

fraction of tightly bound water and a lower fraction of free water than Nafion.

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1.3.6.3 Processing and Morphological Issues

Processing is a critical factor that affects the proton conductivity and other

electrochemical properties of PEM.99, 100 Kim and McGrath et al.95, 101 studied the effect

of acidification treatment and morphological stability of sulfonated poly (arylene ether

sulfone) copolymer in great detail. Two methods were developed to study how

acidification treatment would influence the proton conductivity and water absorption:

either immersing the solvent-cast membrane in sulfuric acid at 30 °C for 24 h and

washing with water at 30 °C for 24 h (method 1) or immersion in sulfuric acid at 100 °C

for 2 h followed by similar water treatment at 100 °C for 2 h (method 2). The fully

hydrated BPSH membranes treated by method 2 exhibited higher proton conductivity,

greater water absorption, and less temperature dependence on proton conductivity as

compared with the membranes acidified at 30 °C. In contrast, the conductivity and water

absorption of a control perfluorosulfonic acid copolymer (Nafion1135) were invariant

with treatment temperature; however, the conductivity of the Nafion membranes at

elevated temperature was strongly dependent on heating rate or temperature.

Tapping-mode atomic force microscope results also demonstrated that all of the

membranes exposed to high-temperature conditions underwent an irreversible change of

the ionic domain microstructure, resulting in a conductivity decrease. This indicates that

it is likely that observed morphological change in these membranes is at least partially

responsible for dehydration, as was previously hypothesized.101

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Figure 20 TM-AFM phase image of BPSH-45 in (a) Regime1, (b) Regime2, and (c)

Regime3; Hydro-thermal treatment temperatures of (a), (b), and (c) were 30, 80, and 120

oC, respectively, phase angle: 30o Scan size: 500 nm; darker features represent the

hydrophilic phase while lighter features represent the hydrophobic phase102

Furthermore, Kim102 et al. suggested that the sulfonated poly(arylene ether

sulfone) copolymers have three morphological regimes, which are determined by the

copolymer composition and the hydrothermal treatment temperature of the membrane.

Regime1 has hydrophilic domains that are isolated (closed structure) and both water

uptake and proton conductivity was almost constant with treatment temperature. In

Regime2 hydrophilic domains show increased connectivity (open structure) and water

uptake and proton conductivity monotonically increased. In Regime3 the

hydrophilic/hydrophobic phase domain structure was no longer well defined

(morphology changed and viscoelastic relaxation occurred) and water uptake greatly

increased while the proton conductivity decreased. The TM-AFM phase images of

BPSH-45 in three Regimes are shown in Figure 20.

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1.3.7 Poly(arylene ether ketone) Copolymers

The poly(ether ketone)s are a family of polyarylenes linked through varying

sequences of ether(E) and ketone (K) units to give ether-rich: PEEK and PEEKK, or

ketone-rich semi-crystalline thermoplastic polymers: PEK and PEKEKK (Figure 21).

The sulfonated poly (ether arylene) family of polymers has probably been more broadly

and extensively studied in recent years than any other systems. For the most readily

commercially available Victrex PEEK, the ether groups offers the ortho position with the

highest reactivity among the four equivalent sites on the hydroquinone unit. Ortho-ether

substitution by sulfonic acid groups can be carried out in concentrated sulfuric acid or

oleum, the extent of sulfonation being a function of the reaction time and temperature and

SO3 concentration. 103,104, 105,106, 107

sPEEK starts to lose weight between 240 and 300°C,76, 105 depending on the rate

of heating and, to a lesser extent, the degree of sulfonation. The sulfur content of the

residue recovered after heating sPEEK at 400°C decreased by 90%, indicating that

thermal decomposition begins by desulfonation76. The Tg increases from 150°C (PEEK)

to around 230°C in sPEEK of 60% sulfonation.

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Figure 21 Structures of representative membranes of the poly(ether ketone) family.

The proton conductivity of sPEEK depends not only on the degree of sulfonation,

ambient relative humidity and temperature, but also on the thermal history of a given

membrane.76, 108 For example, for the membranes pre-treated in boiling water for 4 h

prior to measurement, the conductivity showed weak temperature dependence whereas

that of a non-treated membrane increased by more than a factor 10 over the same range 76.

Alberti 37 found that the conductivity of non-treated sPEEK membranes shows greater

dependence on the relative humidity than does that of Nafion (Figure 22 a); However, the

differences in conductivity of sPEEK and Nafion 117 level out by either high relative

humidity values or an increasing temperature. Measurements made as a function of

temperature at a relative humidity of 75% (Figure 22 b) show that highly sulfonated

PEEK displays the same conductivity as Nafion at 160°C, whereas that of sPEEK at

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lower degree of sulfonation 60% is an order of magnitude lower under these conditions.

These data confirmed that conductivity at medium temperatures and high relative

humidity values essentially depends on the concentration of the sulfonic groups while the

effect of the acid strength is evident only at low relative humidity values and vanishes at

75% relative humidity for a temperature of 155°C. These results can be explained by the

fact that interaction between polymer chains decreases with increasing temperature,

favoring greater hydration of the polymer at a given value of the relative humidity.

(a) (b)

Figure 22 (a) Conductivity, at 100oC, as a function of relative humidity for Nafion 117 and

S-PEEK-2.48 membranes. The conductivity of g-zirconium sulfophenylphosphonate is

reported for comparison; (b) Conductivity of Nafion 117 and S-PEEK-2.48 at 75% r.h. for

temperature increasing from 80 to 160oC. The conductivity of S-PEEK-1.6 is reported for

comparison. 37

Kreuer et al.109 reported detailed studies on the transport properties and the

swelling behavior of different sulfonated polyetherketones and compared them with

Nafion. Small angle X-ray scattering (SAXS) results indicate a smaller characteristic

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separation length with a wider distribution and a larger internal interface between the

hydrophobic and hydrophilic domain for the hydrated sulfonated polyetherketone. As

schematically illustrated in Figure 23, the water filled channels in sulfonated PEEKK are

narrower compared to those in Nafion. They are less separated and more branched with

more dead-end “pockets”. These features correspond to the larger

hydrophilic/hydrophobic interface and, therefore, also to a larger average separation of

neighboring sulfonic acid functional groups.

Figure 23 Schematic representation of the microstructures of NAFION and a sulfonated

polyetherketone, illustrating the less pronounced hydrophobic/hydrophilic separation of

the latter. 109

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Synthesis of sulfonated poly(aryl ether ether ketone ketone)s statistical

copolymers was reported by Liu & Guiver, as shown in Figure 24.110, 111 Membranes

prepared from the present sulfonated poly(ether ether ketone ketone) copolymers

containing the hexafluoroisopropylidene moiety (SPEEKK-6F) and copolymers

containing the pendant 3,5-ditrifluoromethylphenyl moiety (SPEEKK-6FP) had low

water uptakes and showed proton conductivities >0.01 S/cm at room temperature. 111

Figure 24 Synthesis of sulfonated poly(aryl ether ether ketone ketone)s111

Polymer blends based on sulfonated poly(ether ether ketone) (SPEEK) and

polysulfone bearing benzimidazole side groups have also been prepared by Guiver et

al.112-114 The benzimidazole group tethered to the polysulfone backbone acts as a

medium through the basic nitrogen for transfer of protons between the sulfonic acid

groups of SPEEK. The blend membrane exhibits better performance in PEMFC at 90 oC

and 100 oC compared to the pure SPEEK and Nafion 115 membranes.112 The polymers

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bearing pendant benzimidazole groups may offer a promising strategy to develop new

membranes that can operate at higher temperatures and low relative humidity.

Guiver115 also reported the synthesis of highly fluorinated poly(arylene ether)s

containing sulfonic acid groups meta to ether linkage for PEMFC. They used potassium

carbonate mediated nucleophilic polycondensation reactions of commercially available

monomers: 2,8-dihydroxynaphthalene-6-sulfonated salt (2,8-DHNS-6),

hexafluorobisphenol A (6F-BPA), and decafluorobiphenyl (DFBP) in dimethylsulfoxide

(DMSO) (Figure 25). The proton conductivities of acid form membrane increased with

sulfonation degree and reached 0.103 S/cm at 90 C. The methanol permeabilities ranged

from 7.4×10−7 to 2.2×10−9 cm/s at 30 C and were much lower than commercial

perfluorinated sulfonated ionomer (Nafion).

Figure 25 Synthesis of highly fluorinated poly(arylene ether) copolymers containing sulfonic acid

groups

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1.3.8 Poly(phosphazene)s

Sulfonated poly(phosphazene)s may be PEM candidates because of their well-

known chemical and thermal stability116. The variety of different polymer compositions

through the nature of the side chains on the P = N- backbone and polymer crosslinking

onto the -P=N- polymer backbone makes them especially good in controlling the ionic

groups as well as water uptake.

1.3.8.1 Post Sulfonated Copolymers

Allcock et al.116 studied the sulfonation process of polyphosphazenes and

sulfonating agents, including SO3, and concentrated and fuming sulfuric and

chlorosulfonic acids. Additionally, they also describe the sulfonation of

aminophosphazenes with 1,3-propanesulfone.117 Although these materials may not be

useful as PEMs, this study demonstrated a novel technique for synthesizing sulfonated

polyphosphazene materials that may provide more control over sulfonated product than

the strong sulfonating agents approach. However, the glass transition temperatures of

sulfonated polyphosphazenes are low (-10°C) and therefore polymer blends or

crosslinking is necessary.

With respect to crosslinking, Pintauro et al.118 reported proton-exchange

membranes fabricated from poly[bis(3-methylphenoxy)phosphazene] (Figure 26) by first

sulfonating the base polymer with SO3 and then solution-casting thin films. Polymer

crosslinking was carried out by dissolving 15 mol% benzophenone photoinitiator in the

membrane casting solution and then exposing the resulting films after solvent

evaporation to UV light.

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The methanol diffusivity in the crosslinked polyphosphazene was very low (1.6--

8.5×10-8 cm2/s). Surprisingly, the proton conductivity did not differ between the

crosslinked and non-crosslinked specimens even though their water uptake was different.

Crosslinking usually would accompany reduced conductivity and brittleness in the dry

state. This result indicates that it is possible to reduce water uptake while at the same

time maintain proton conductivity.

Figure 26 The repeat unit of poly[bis(3-methylphenoxy)phosphazene]

1.3.9 Polyimides

Sulfonated polyimides are known for their thermal stability and therefore have

been prepared as proton exchange membranes in fuel cells.119-121 However, their poor

hydrolytic stability is a significant problem. Sulfonated five-membered phthalic

polyimides quickly degrade hydrolytically and chain scission occurs rapidly in water at

80°C.122 Since the six-membered ring of the naphthalic polyimide is much more stable to

hydrolysis, this chemical structure is somewhat better suited for PEM fuel cell

applications.

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The nature of hydrolysis associated with the sulfonic acid group in phthalic and

naphthalic polyimides has been studied by Genies et al.123, 124, using model compounds

along with IR and NMR spectroscopy. Model compounds of the sulfonic acid containing

phthalic imide (model A) and the sulfonic acid containing naphthalic imide (model B) as

shown in Figure 27, were prepared by a one-step high temperature condensation in m-

cresol. The NMR and IR spectroscopic analyses of model compounds, before and after

aging in water, at temperature representative of fuel cell working temperature, indicated

structural modifications according to imide ring structure. The phthalic model stability

was inferior to 1 h at 80°C whereas the confirmed naphthalenic model one was superior

to 100 h in the same conditions. Moreover, limited hydrolysis of naphthalenic imide

cycle (12%) was also shown. This critical fraction is probably enough to cause some

degradation of polymeric materials.

Figure 27 Structures of phthalic model A and naphthalenic model B imides.124

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Various random and sequenced sulfonated polyimides from naphthalene-1,4,5,8-

tetracarboxylic dianhydride (NTCD) or 4,4-oxydiphthalic anhydride (OPDA), 4,4’-

diaminobiphenyl-2,2’-disulfonic acid (BDSA), and common non-sulfonated diamine

monomers such as oxydianiline (ODA), were synthesized in several laboratories125, 126.

The incorporation of diamine comonomers adjusts properties such as flexibility and water

uptake by pushing apart the rigid rod backbone of the polymer, thereby creating free

volume for water. This leads to higher water uptakes and therefore higher conductivity,

especially at low humidity.

A synthetic method of two-stage polycondensation of BDSA-NTCD-ODA to

form naphthalenic sulfonated polyimide (sPI) copolymers was first found by Mercier et

al.124 (Figure 28) The molar ratio of BDSA-NTCD in the first stage of polycondensation

gives the length of the ionic sequence, while the subsequent introduction of ODA and the

remaining dianhydride spaces the ionic blocks with hydrophobic sequences. These latter

sequences have some residual flexibility, whereas the phenylene bonds in the sulfonated

moiety confer a glassy character. The degree of sulfonation can be precisely controlled

by regulating the molar ratio of BDA and the unsulfonated diamine, which is 4,4’-

oxydianiliine (ODA) in SPI.

Polymers of various equivalent weight and different ratios (x/y) of sulfonated (x)

to neutral (y) diamine (Figure 29) have been prepared that can be cast into membranes

from m-cresol. 127 The length of the block sequence for a constant equivalent weight has

greater impact on water uptake and conduction properties than does a variation of the

equivalent weight for polymers with the same block length. For membranes swollen in

liquid water, the hydration number increases with the length of the ionic block sequence,

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but the conductivity decreases (from 1.8 ⋅ 10-2 to 4 ⋅ 10-3 S cm-1 for sPI of equivalent

weight 793 g mol-1 when the length of the ionic block increases from 3 to 9 units). 127

This observation seems in contrast with the general idea that conductivity increases with

the hydration number and suggests that the conductivity also depends on microstructural

changes accompanying the increase in block length.

Figure 28 Synthesis of sequenced sulfonated copolyimides124

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Figure 29 Naphthalenic sulfonated polyimides

Some polyimide copolymer membranes containing bulky and/or angled

comonomers128-132 have been reported with higher proton conductivities than Nafion,

possibly by increasing the free volume available for water. The larger comonomers

prevent close packing of backbones and result in a more open structure. X-ray diffraction

showed that polymer interchain spacings are higher by ∼0.1-0.4 Å for sulfonated

polyimides than those for the corresponding BDSA-NTCD homopolymers. With more

space between the polymeric chains, more free volume is available for water and which,

therefore, leads to higher water uptake and higher conductivity. Recently, Einsla133, 134 et

al also reported some detailed results on naphthalene dianhydride based polyimide

copolymers. At relatively high ion exchange capacities, the proton conductivities of the

polyimides in water at 30 oC were equivalent to Nafion 1135. Unfortunately, the best

hydrolytic stability achieved was still much lower than Nafion or analogous poly(arylene

ether)s.

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1.4 Block Copolymers for PEM

In this section reviews synthetic strategies for obtaining block or multiblock

copolymers with controlled molecular structure, which afford nanophase separation of

ionic and hydrophobic domains. Strategies to control the molecular structure of polymers

that bear ionic and hydrophobic segments, and on the influence of phase separation on

selected properties pertinent to fuel cell performance will be highlighted.

Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are

promising materials that can be used as alternate PEMs. This is particularly due to their

ability to form unique morphologies, such as: the spherical, cylinder, and lamellar shape.

It is well known that the membrane morphology is important for the PEM fuel cell

performance, and depends strongly on the water content, and on the type concentration

and distribution of the acidic moieties.27 The unique morphologies of multiblock

copolymers may play an important role in providing good proton transport at low water

contents and high temperatures.135 Compared with random copolymers, multiblock

copolymers are also expected to provide less water swelling and better thermal and

mechanical properties.98, 136, 137

Before detailing the various architectures of proton conducting polymers reported

in the literature, it is necessary to first reviews some basics of copolymer architecture and

nanophase separation in block copolymers

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1.4.1 Copolymer Architecture

It is unlikely that PEMs will be fabricated from homopolymers of ion bearing

monomers because the ionic exchange capacity (IEC) required will be too high, leading

to problems in processing and dimensional stability in the presence of water.

Copolymers are likely to be more common, i.e. those polymers containing two or more

different monomers. Copolymers can be divided into several classes by the arrangement

of repeating units in the polymer chain, as shown in Figure 30. Random copolymers have

repeating units distributed randomly. Strictly speaking, the majority of random

copolymers reported are not truly random, but “statistical”, which includes all

copolymers for which the sequential distribution of the repeating units follows a specific

statistical law depending on the reactivity of specific monomers and the method of

synthesis. 138 Alternating copolymers possess alternating repeat units along the chain.

Figure 30 Illustration of copolymer architectures: III: di-block; IV: tri-block; V: multi-block; VI:

graded block; VII: four arm star block.

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Block copolymers are linear polymers which possess one or more uninterrupted

sequences of each repeat unit. Block copolymers can be further sub-categorized as di-

block, tri-blocks, multiblocks, graded blocks and star blocks. Graft copolymers may be

considered as a special case of block copolymers. They are identified as having a graft

chain of a different repeat unit to the main chain.

1.4.2 Nanophase Separation in Block Copolymers

Block copolymers consist of chemically distinct polymer chains covalently linked

to form a single molecule. Owing to their mutual repulsion, dissimilar blocks tend to

segregate into different domains, but because A and B blocks are chemically joined

macroscopic phase separation is prevented. Instead, the system forms a microstructure

with an extensive amount of internal interface between the A- and B-rich regions.139

Although interfacial tension drives the system towards large domain sizes, this is

countered by tension in the stretched copolymers. The actual structure the system form is

governed by the detailed balance between the tension in the A blocks and that in the B

blocks.140 Various microdomain structures are achieved, depending on relative volume

ratio between blocks and chain architecture as well as the persistence lengths of the

respective blocks.141

Leibler142 reported important theoretical studies in the 1980s on the nanophase

separation in block copolymers, expanding earlier work by Molau143. The essential

conclusion is that, at equilibrium, the state of the system is determined by only two

relevant parameters: the polymer chain composition f and the product χN. χ is the Flory-

Huggins interaction parameter and N is the number of monomers along the copolymer

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backbone. The composition of the A-type segments f =NA/N and N=NA+NB, where NA

and NB are degrees of polymerization of A-type and B-type blocks.

For a symmetric (50/50 vol%) copolymer, f = 0.5, the transition from a disordered

state to one possessing long-range order occurs when χN = (χN)c ≈ 10.5 in the absences of

critical fluctuation. (In polymer melts fluctuation effects are strongly reduced as a result

of screening, due to the interactions of chains with a large number of other chains.144 )

When χN >>(χN)c ≈ 10.5, a strongly segregated symmetric copolymer orders into a

lamellar morphology of alternation A and B blocks. This prediction should also be valid

in multiblock copolymers as long as 1:1 stoichiometry is achieved. Therefore, to obtain a

desired co-continuous microstructure, one must either increase the Flory-Huggins

interaction parameter χ between two monomers or increase the degree of polymerization

(the number of monomers along the copolymer backbone).

The Flory-Huggins interaction parameter χ is a fairly complicated parameter

which contains enthalpy and entropy contributions and is also dependent upon polymer

concentration φ2. Usually it can be expressed in following equation:139, 145

Equation 4

where β1 is sometimes called "the lattice constant of entropic origin"; V1 is the

molar volume of the solvent; δ1 and δ2 are solubility parameters of two monomers.

Therefore, χ is roughly dependent reversely on temperature (1/T, T in Kevins). 146

It is well-established that di- or tri-block copolymers may assemble into a variety

of morphological structures, including spheres, cylinders and lamellae, depending on the

ratio of the molecular weight of the blocks.138, 145 A phase diagram for a series of

poly(styrene-b-isoprene) (PS-PI) diblock copolymers is presented in Figure 31 where χN

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represents the enthalpic-entropic balance (χ is the Flory-Huggins interaction parameter

and N is the number of monomers along the copolymer backbone).147, 148 As the volume

fraction of polyisoprene (fPI) is changed there is a corresponding change in morphology

of the block copolymer.

The above provides a brief example of how block composition influences the

morphology of the copolymer. However, the effect of sample preparation-casting,

molding, etc.—on the resulting morphology cannot be understated. It is well known that

a morphology observed at low temperature may significantly change upon annealing. In

addition, different morphologies for the same material,149 can be observed in response to

shear forces, solvent, rates of solvent evaporation and film thickness.150

Figure 31 Morphological phase diagram for PS-PI diblock copolymers

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1.4.3 Di-Block and Tri-Block Copolymers for PEM

While random copolymerization is often an effective means of controlling the

bulk characteristics of a polymer system, block copolymerization potentially offers a

higher level of molecular control. Block copolymers display a nanophase-separated

morphology in which the physicochemical properties of individual block components can

be realized in a single polymer structure.

The most studied, most understood class of block copolymers is the di-block. A

proton conducting analog of a di-block would possess a length of ionic and non-ionic

polymer. Preferentially, the length of the blocks would be controlled in such a way that

structure-morphology relationships could be established both theoretically and

experimentally. However, few papers on the subject are published. This is probably due

to the tendency of ionic block structures being soluble in water. In the case of sulfonic

acid or similar ionic polymers there is a strong tendency for dissolution in water, which is

accentuated by the ability of the polymers to form micelles and other aggregated

structures that expose the ionic regions to the aqueous solution. In principle, the extent of

solubility can be diminished if the non-ionic block is highly hydrophobic, such as a

fluoropolymer—this would also promote phase separation in films.

Main chain fluoro-block copolymers are rare because fluorine-containing

monomers cannot readily be polymerized by living ionic polymerization151 or pseudo-

living radical polymerization.152 One approach to prepare these block copolymers is to

first synthesize halogen-terminated low molecular weight fluoropolymers by means of

telomerization. These low molecular weight fluoropolymers are then used to initiate

atom transfer radical polymerization (ATRP) of a non-fluorinated monomer.

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For example, telomerization of vinylidene difluoride in the presence of

BrCF2CF2Br provides α,ω-dibrominated poly(vinylidene difluoride) (PVDF), which has

been subsequently used to initiate the ATRP of styrene to form PS-b-PVDF-b-PS triblock

copolymers. 153 PVDF, terminated with trichloromethyl groups, and prepared by

telomerization of VDF, has been used to initiate the ATRP of styrene, methyl

methacrylate, methyl acrylate and butyl acrylate, to form various diblock copolymers. 153,

154 However, the drawback of the telomerization approach is the low molecular weight of

the fluoropolymer segments produced (2,500 g/mol), which are too small, in comparison

to the non-fluorinated segments for the block copolymers to take on fluoropolymer

characteristics.

To address this, chain transfer polymerization of fluoromonomers has been used

to obtain higher molecular weight, halogen-terminated fluoropolymers. 155 For example,

as shown in Figure 32, trichloromethyl-terminated copolymers of vinylidene difluoride

(VDF) and hexafluoropropylene (HFP), possessing molecular weights up to 25 000 g/mol,

were obtained by emulsion polymerization in the presence of chloroform and used to

initiate the ATRP of styrene (St) and methyl methacrylate (MMA) to form a series of

P(VDF-co-HFP)-b-PS and P(VDF-co-HFP)-b-PMMA block copolymers.155

The chain length of both fluorinated block and PS block can be adjusted, and the

polystyrene block sulfonated with acetyl sulfate to provide P(VDF-co-HFP)-b-

poly(styrenesulfonic acid) (P(VDF-co-HFP)-b-PSSA). The IEC of these novel ion-

containing polymers was controlled by two strategies: adjusting the length of the

sulfonated polystyrene block by ATRP or varying the degree of sulfonation of the

polystyrene segments. The conductivity of block copolymer membranes lies in the same

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order of magnitude of Nafion for a similar IEC.156 Although only preliminary data is

provided, TEM micrographs (Figure 33) clearly show the presence of a network of ionic

channels domains having widths ~ 10 nm. 157

Figure 32 Preparation of Fluorine-Containing Block Copolymers by Chain Transfer Emulsion

Polymerization and ATRP 155

Figure 33 TEM of partially sulfonated P(VDF-co-HFP)-b-PS block copolymer (IEC = 0.89 meq/g)

Wnek158-161 reported sulfonated styrene/ethylene-butylene /styrene (S-SEBS)

triblock copolymers (Figure 34) as a PEM for use in hydrogen fuel cells. These triblock

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copolymers (Dais Analytic’s PEMs) are based on well-known commercial block

copolymers of the styrene-ethylene/butylene-styrene family (Kraton). To synthesize

these sulfonated PEMs, the unsulfonated polymer is dissolved in a

dichloroethane/cyclohexane solvent mixture. The sulfur trioxide/triethylphosphate

sulfonating complex in solution is then added and allowed to react at temperatures

between –5°C and 0°C. PEM can be solvent cast from lower alcohols such as n-propanol

to afford an elastomeric hydrogel with conductivities of 0.07 - 0.1 S/cm when fully

hydrated.161 For the process described above, the unsulfonated block copolymer could

have a number average molecular weight of about 50,000 g/mol with a styrene content of

20 to 35 weight % of the triblock copolymer. Transmission electron microscopy (TEM)

of cast films (50% mole of the styrene units sulfonated, stained with RuO4) reveals

distinct, phase-separated structures which indicate a lamellar structure with thickness of

approximately 25nm.162

Figure 34 Chemical Structure of Sulfonated SEBS Block Copolymer

The main drawback of employing hydrocarbon-based materials is their much

poorer oxidative stability compared to perfluorinated or partially perfluorinated

membranes due to the partially aliphatic character.163 For this reason, Dais membranes

are aimed at portable fuel cell power sources of 1 kW or less, for which operating

temperatures are less than 60°C.

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1.4.4 Multiblock copolymers for PEM

1.4.4.1 Poly(arylene ether sulfone) Multiblock Copolymers

Ghassemi et al.77, 136, 164 reported new multiblock copolymers containing

perfluorinated poly(arylene ether) as the hydrophobic segment and highly sulfonated

poly(arylene ether sulfone) as hydrophilic segment, with the aim of providing polymeric

materials with a highly phase-separated morphology.

As shown in Figure 35, sulfonated–fluorinated poly(arylene ether) multiblocks

(MBs) 3 were synthesized by nucleophilic aromatic substitution of highly activated

fluorine terminated telechelics 2 made from decafluorobiphenyl with 4,4’-

(hexafluoroisopropylidene)diphenol and hydroxyl-terminated telechelics 1 made from

4,4’-biphenol and 3,3’-disulfonated-4,4’-dichlorodiphenylsulfone.

An increase sulfonated block size in the copolymer resulted in enhanced

membrane ion exchange capacity and proton conductivity. AFM images (Figure 36) of

these sulfonated–fluorinated poly(arylene ether) multiblocks revealed a very well defined

phase separation, which may explain their higher proton conductivities compared to the

random copolymers. In addition, multiblock copolymers exhibited higher proton

conductivities than Nafion at low relative humidity. This may be attributed to the

existence of nano-structure morphology forming sulfonated hydrophilic domains

surrounded by fluorinated hydrophobic segments as supported by AFM data.

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Figure 35 Synthetic scheme of Sulfonated–fluorinated poly(arylene ether) multiblocks

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Figure 36 AFM taping mode phase images of a Sulfonated–fluorinated poly(arylene ether)

multiblock

However, these sulfonated–fluorinated poly(arylene ether) multiblock copolymers

suffer from extensive water swelling.

Poly(arylene ether sulfone) block copolymers have been prepared based on the

polycondensation of α,ω-dihydroxy bisphenol A polysulfone precursors (Mn = 1800,

4900, 9500 g/mol) with α,ω-dibromo polyvinylidene fluoride (Mn = 1200 g/mol). 165

These polymers have been post-sulfonated to yield a series of block copolymers

containing sulfonated bisphenol A polysulfone and poly(vinylidene fluoride), SPSF-b-

PVDF, as shown in Figure 37. GPC (polystyrene base) showed the number average

molecular weights of these block copolymers were pretty low, in the range of 10,000 to

20,000 g/mol.

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Figure 37 Synthesis of SPSF-b-PVDF

Membranes are compared with sulfonated bisphenol A polysulfone

homopolymers (SPSF) to examine the effect of the fluoropolymer blocks on membrane

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morphology and proton conductivity.166 The difference in proton conductivity between

SPSF and SPSF-b-PVDF polymers is illustrated in Figure 38.

In the low IEC regime, SPSF-b-PVDF block copolymer exhibits proton

conductivity up to four times higher than the corresponding homopolymers. As IEC is

increased the conductivity of the homopolymer and block copolymer is increased, but the

difference in conductivity between the two series diminishes. At the highest IEC

examined, the effect of the fluoropolymer block is negligible. Water contents and λ

values ([H2O]/[SO3H]) are similar for both polymers at a given IEC. The enhancement in

conductivity of the block copolymers is therefore judged to be due to the presence of the

fluoropolymer block which promotes the formation of ionic aggregates and an ionic

network. For higher IEC membranes, where both the concentration of acidic sites and λ

values are much higher, and the network of ions more fully formed, the presence of the

relatively small fluoropolymer segments has little effect on conductivity.

Figure 38 Dependence of proton conductivity on IEC for SPSF (diamond) and SPSF-b-PVDF (square)

membranes. (30 oC, 95% relative humidity)

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Figure 39 TEM micrographs of polymer membranes. (a): SPSF (IEC: 1.55); (b): SPSF-b-PVDF

(IEC:1.62); (c): SPFSF (IEC: 0.83); (d): SPSF-b-PVDF (IEC: 0.78)

Transmission electron microscopy (TEM) analysis of SPSF and SPSF-b-PVDF

membranes (Figure 39) show the presence of ionic aggregates. The size of the

aggregates is smaller in the block copolymer (~ 7 nm vs. ~ 11 nm) for the high IEC

copolymers. Ionic aggregates are also observed for low IEC copolymers (Figure 39 c and

Figure 39 d) but in addition to small aggregates the block copolymer possesses larger

regions of ionic aggregation, similar to those reported for polymer blends of sulfonated

polymers. It appears that these 50—200 nm size domains are the result of gross phase

separation of ionic and non-ionic regions. However, it should be noted that the TEMs are

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measured under vacuum with the membrane in its dehydrated state, while conductivites

are measured in their hydrated state. Further work is need to determine why these

morphologies form and if they are the reason for the observed enhanced conductivity for

low IEC block copolymers. One thing that is need to point out is that, the molecular

weight of the PVDF block is relatively small, ~ 1200 g/mol, thus further refinement of

morphology and proton conductivity might be observed if longer blocks of fluoropolymer

were incorporated.

1.4.4.2 Polyimide Multiblock Copolymers

Multiblock copolymers have also been of interest in the design of tailored

sulfonated polyimides. Sequenced copolymers of phthalic polyimide have been prepared

via a two step condensation polymerization of aromatic diamines and dianhydrides. 126, 167

The average length of the ionic sequence is controlled by the molar ratios of monomers.

The morphology and the conducting properties of these phthalic polyimide membranes

are dependent on the distribution of ionic groups along the polymer chain. A small-angle

neutron scattering (SANS) study of the membrane was performed to determine if phase

separation between the ionic domains and the polymer matrix exists. SANS

measurements indicate the intensity of the ionomer peak is larger by a factor of 100 and

located at a lower q value for the sulfonated phthalic polyimide when compared to Nafion.

This suggests the ionic domains are larger in the polyimide. This result is reportedly due

to the block character of the sulfonated phthalic polyimide polymers. However, it is yet

to be determined how the size of ionic domains effects proton conductivity or other

properties related to fuel cell performance.

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Sulfonated phthalic polyimides are sensitive to hydrolysis and possess a short

lifetime under typical fuel cell operating conditions. This has led to research on

sulfonated naphthalenic polyimides, which have a demonstrated lifetime of 3,000

hours.168, 169 Sequenced naphthalenic polyimides may be synthesized via a similar

process as for phthalic polyimides as shown in Figure 40. 123, 170-174

Figure 40 Synthesis of sequenced sulfonated naphthalenic polyimide block copolymers

Proton conductivity of naphthalenic polyimide block copolymers varies with ionic

sequence length as shown in Figure 41 . Proton conductivity does not linearly relate to

water uptake in the polymers, which is not in agreement with the general observation that

the ionic conductivity of ion exchange membranes increases with water content but,

rather, may be due to the concentration of protons in the hydrated membranes.

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Figure 41 Ionic conductivity vs. ionic block length for BDSA/NTDA/ODA copolymers with X/Y =

30/70.

Of significance is that the observation that sequenced sulfonated naphthalenic

polyimides exhibit a higher conductivity than randomly sulfonated naphthalenic

polyimides. Both λ and conductivity values are systematically lower for random

polyimides. However, the difference in conductivity between sequenced and random

polymers decreases as IEC increases as observed in SPSF-b-PVDF block copolymers.123

In similar work, sequenced copolymers of naphthalene-1,4,5,8-tetracarboxylic

dianhydride (NTDA), 4,4-oxydianiline (ODA), and 9,9-bis(4-aminophenyl)fluorine-2,7-

disulfonic acid (BAPFDS), in which the ionic block length is 2, are reported to exhibit

higher proton conductivity than random copolymers. Increased phase separation is

suggested as an explanation. 172

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1.5 Multiblock Copolymers Based on Substituted

Poly(phenylene)s

The goal of this research is to develop a series of hydrophilic-hydrophobic

nanophase-separated multiblock copolymers for proton exchange membranes. Recall

that two blocks have to be very incompatible (or Flory interaction parameter χ has to be

high) and block length of each block (number of monomers along the polymer chain) has

to be large enough to develop phase-separated lamellar structures. One must carefully

choose appropriate monomers and meet the requirement χN >>(χN)c ≈ 10.5.142

Keeping that in mind, telechelic substituted poly(phenylene) (PBP) oligomers

were chosen as hydrophobic blocks and telechelic sulfonated poly(arylene ether sulfone)

(PAES) oligomers as hydrophilic blocks. They are very incompatible since one is made

of hydrophobic hydrocarbon while the other is completely hydrophilic and therefore there

is a fairly high χ between these two monomers. In addition, PBP and PAES are quite

different in terms of chemical structures: PBP has very rigid polymer chain while the

PAES backbone is much more flexible because of the arylene-ether bonds. The N should

be very high when a high molecular weight copolymer is formed. Another requirement is

polymer chain composition f ≈ 0.5, which means that the ratio between two segments

should be close to 1:1. This should also be feasible to achieve.

Before going into the detail of multiblock synthesis, the method of preparing

substituted poly(p-phenylene) via Ni (0)-catalyzed coupling reaction is reviewed.

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1.5.1 Ni (0) -Catalyzed Coupling Polymerization

Reactions

The coupling of aryl halides using the Ni (0)-catalyst system was first reported by

Colon and Kelsey175, 176 to quantitatively convert chlorobenzene to biphenyl. Using a

catalytic amount of nickel, triphenylphosphine, and excess zinc in a dry, dipolar aprotic

solvent at 60-80°C, chlorobenzene is coupled quantitatively in a few minutes. The

efficiency of this method lies in the fact that excess reducing metals (normally zinc) play

a crucial role, which allows biaryls to be formed rapidly in excellent yields from

chlorobenzene under mild conditions. In fact, rather than simply acting as an agent for

regenerating nickel(0), zinc is crucially involved in the formation of the active

intermediate, which is inaccessible from aryl chlorides without zinc.

Figure 42 Reaction Mechanism Proposed by Colon for the Ni(0)-Catalyzed Polymerization of

Arylene Chlorides

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The catalytic cycle proposed by Colon and Kelsey contains three steps (Figure 42).

175, 177 These steps were oxidative addition of Ni (0) across the aryl chloride bond (step 1),

reduction resulting in an arylnickel (I) species (step 2), and oxidative addition of the

arylnickel (I) species to a second aryl chloride (step 3). The first two steps were shown to

be accelerated by the presence of an electron-withdrawing substituent on the aryl chloride.

In the third step, the presence of an electron-withdrawing group ortho to the Ni-C bond

would be expected to increase the stability of the electron-rich ArNi(I)L3 (where L= PPh3

or L2 = bipy) complex and thus slow the oxidative addition reaction of this complex with

a second aryl chloride. Colon and Kelsey proposed that early in the reaction (conversions

less than 80%) the reduction of the ArNi(II)ClL2 (step 2) was the rate-limiting step. As

the concentration of ArCl approached that of the nickel species, the oxidative addition of

ArNi-(I)L3 (step 3) became rate limiting. The degree of polymerization is related to the

conversion through [Xn] = 1/(1-ρ) ([Xn] = average degree of polymerization, ρ =

conversion). Oligomers of 10 repeat units equated to a 90% conversion of the aryl

chloride. At this concentration, the oxidative addition of ArCl to the ArNi(I)L3 would be

rate limiting.

Since Colon’s pioneering work, Sheares177-181 and Percec 182, 183 have extended

this reaction to polymerization, giving high-performance materials. In general, the Ni

(0)-catalyst system is fairly sensitive to atmosphere, solvent, and the functional group in

the starting material. For a successful Ni (0) coupling reaction, an inert atmosphere must

be used and the aprotic solvent should be dried thoroughly to suppress reduction of aryl

chlorides to arene. Besides, a small amount of 2, 2’-bipyridine can effectively suppress

some side reactions such as aryl transfer. For aryl chlorides, electron-withdrawing or

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weakly electron-donating substituent gave high yields of biaryl. Functional groups such

as nitro groups and acidic substituents would either completely inhibit the reaction or

cause aryl chlorides to be reduced to arene. Aryl bromides and iodides need a longer

reaction time than those for aryl.175 A number of nickel salts can be used to generate the

catalyst, but nickel chloride and bromide were most effective. The hydrated nickel salts

led to substantial reduction. Although many metals have reduction potentials lower than

that of nickel, only zinc, magnesium, and manganese gave high yields of coupled

products from chlorobenzene. Triarylphosphines were the best ligands and other ligands

normally gave much slower reactions and lower yields.175

1.5.2 Poly(p-phenylene) (PPP) and Derivatives

Poly(p-phenylene) (PPP) and its derivatives are a promising class of high-

performance polymers because of their excellent thermal and mechanical properties.178,

184,185 However, high molecular weight PPPs derivatives are difficult to synthesize

mainly due to the poor solubility of the growing rigid-rod chains during polymerization.

This problem has been decreased by introducing pendant groups to the phenyl rings

which improve the solubility of PPP, while retaining many of the most useful

characteristics. Appropriately substituted PPPs are soluble and yet exhibit good thermal

and mechanical properties. Thus, poly(2,5-benzophenone) (PBP) is known for its high

degree of polymerization, good solubility in dipolar aprotic solvents, and excellent

thermal and mechanical properties.77, 183, 186

High molecular weight PBP shows a glass transition temperature of 206 oC and

5% weight loss temperatures in nitrogen and air of 526 and 520 oC, respectively. 177 In

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addition, the tensile modulus of PBP was from two to four times greater than other high-

performance isotropic thermoplastics including polysulfone, poly(ether imide),

poly(ether ether ketone), poly(phenylene sulfide), and thermoplastic

polybenzimidazole.187 PBPs can also be processed by compression molding and

extrusion and are now commercially available under the trade name Parmax from Solvay

Advanced Polymer Inc.

Figure 43 Alkyl-substituted Poly(2,5-benzophenone)s

Sheares184 and Quirk185 independently successfully synthesized poly(2,5-

benzophenone)s and alkyl-substituted poly(2,5-benzophenone)s via Ni (0)-catalyzed

coupling reaction, shown in Figure 43. The resulting polymers are soluble and show no

evidence of crystallinity by DSC. Number average molecular weights are in the range of

9.2 ×103–11.7 ×103 g/mol by multiple angle laser light scattering (MALLS). These

polymers exhibit high thermal stability with glass transition temperatures ranging from

173 to 225°C and weight loss occurring above 450°C in nitrogen and 430°C in air.

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The regiochemistry of the nickel-catalyzed polymerization of 2,5-

dichlorobenzophenone in the presence of excess zinc and triphenylphosphine with (PBP-

B) and without added coligand (PBP-A), 2,2’-bipyridine has been examined by Quirk188

and Wang and coworkers.189 Based on 13C NMR study of model dimers, Quirk

concluded that PBP-B contained fewer H–T units (71%) than PBP-A (88%).188 However,

PBP-B exhibits a longer UV-vis maximum wavelength, indicating of longer conjugation

lengths along the polymer backbone. Quirk argued what determines conjugation length

may not be the percentage of H–T units, but the absence of a significant number of

conjugation-interrupting head-to-head units. These H–H groups appear to be absent in

PBP-B, but to be present in PBP-A. This is the reason for the decreased conjugation

lengths in PBP-A, and for the increased conjugation lengths in PBP-B.

Wang and coworkers189 used a chemical method (controlled “intramolecular”

cyclization converting 2,2’-dibenzoylbiphenyl moieties into phenanthrene units)

combined with spectroscopic analysis (Raman spectroscopy) to study the regiochemistry

of PBP. The resulting polymers were extremely insoluble. They also concluded that

PBP-B prepared in the presence of 2,2’-bipyridyl had fewer H-T units than PBP-A.

More recently, Hagberg and Sheares177 studied the effects of solvent and

monomer structure on the Ni(0)-catalyzed polymerization with various monomers, as

monomer structure shown in Figure 44. They found solvent and the monomer structure

were shown to be key considerations in the Ni (0) polymerization of arylene dichloride

monomers containing electron-withdrawing substituents. Polymerization in DMAc

resulted in side reactions, such as reduction of the carbonyl groups, which limited the

materials utility. On the contrary, through the use of THF as solvent, high molecular

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weight poly[3-(2’-thiophenecarbonyl)-2,5-thiophene] and poly(2,5-benzophenone) were

synthesized with no side reactions being observed.

Poly(2,5-benzophenone) of 58 ×103 g/mol ([η] = 1.15 dL/g) with a Tg of 180 °C

and a 10% weight loss temperature in nitrogen of 576 °C was synthesized. In addition to

higher molecular weights than previously reported179, 190, no reduction of the carbonyl

was observed, even without the addition of TPO. (Polymerization in amide solvents led

to reduction of nearly 15% of the carbonyl functionalities. This limitation was overcome

by simply oxidizing the material post-polymerization or by the addition of small

quantities of triphenylphosphine oxide (TPO) to the reaction mixture.) This result

eliminated the need for oxidation reactions post-polymerization. The polymer was

soluble in solvents such as chloroform, THF, and DMAc. Thin films of the polymer were

cast from chloroform. Although not highly creasable, the films were flexible and showed

much better overall filmforming properties than the polymers synthesized previously in

amide solvents.

Analysis of the results by NMR and consideration of the mechanism showed that

in Ni (0)-catalyzed polymerization there is a window of electron withdrawing ability for

which functionalities such as carbonyl-containing pendants meet the criteria and greatly

accelerate the reaction. Increasing the electron-withdrawing ability by substitution with a

sulfone slows the reaction due to the stabilization of a reactive intermediate.177

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Figure 44 Structures of 3-benzoyl-2,5-dichlorothiophene (M1), 2,5-dichloro-3-(2’-

thiophenecarbonyl)thiophene (M2), 3-benzenesulfonyl-2,5-dichlorothiophene (M3), 2,5-

dichlorobenzophenone (M4), and 2-benzenesulfonyl-1,4-dichlorobenzene (M5).

1.5.3 Multiblock Copolymers Based on Substituted

Poly(p-phenylene)s

Multiblock copolymers have also been of interest in the design of tailored poly(p-

phenylene)s. Sheares et al. 180 synthesized rigid-rod poly(4’-methyl-2,5-benzophenone)

macromonomers (telechelic oligomers) by Ni(0) catalytic coupling of 2,5-dichloro-4’-

methylbenzophenone and end-capping agent 4-chloro-4’-fluorobenzophenone (Figure 45).

Since aryl fluorides do not participate in the Ni(0)-catalyzed coupling reaction, aryl

fluorides activated by an electron-withdrawing group can be used as reactive sites for

nucleophilic aromatic substitution in poly(p-phenylene)s. Because of the step-growth

nature of the Ni (0)-coupling polymerization and relatively high rates of reaction, an aryl

chloride monomer with a functionality of 1 can be introduced into the reaction mixture to

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end cap poly(2,5-benzophenone)s. End capping of poly(2,5-benzophenone)s with 4-

chloro-4’-fluorobenzophenone will result in PPP chains that contain aromatic fluoride

end groups activated toward nucleophilic aromatic substitution. Furthermore, the

molecular weights of these polymers can be readily controlled by altering the amount of

4’-chloro-4-fluorobenzophenone added to the reaction mixture.

Substitution of the macromonomer end groups was determined to be nearly

quantitative by 1H NMR and gel permeation chromatography. The macromonomers

(telechelic oligomers) produced were proved to be labile to nucleophilic aromatic

substitution.

Figure 45 Synthesis of poly(4’-methyl-2,5-benzophenone) telechelics.

A similar approach was applied in the synthesis of hydrophilic-hydrophobic

multiblock copolymers of sulfonated poly (4’-phenyl-2,5-benzophenzone) and

poly(arylene ether sulfone) (as shown in Figure 46) by Ghassemi and McGrath164. Rigid-

rod poly(4’-phenyl-2,5-benzophenzone) telechelics were first synthesized by Ni (0)-

catalyzed coupling of 2,5-dichloro-4’-phenylbenzophenone and the end capping agent 4-

chloro-4’fluorobenzophenone. After sulfonating at 50°C with concentrated sulfuric acid,

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the fluoroketone activated sulfonated poly(4’-phenyl-2,5-benzophenzone) oligomers

(Mn=3.05×103 g/mol) was copolymerized with hydroxyl terminated biphenol base

polyarylethersulfone (Mn=4.98×103 g/mol) to afford an alternating multiblock sulfonated

copolymer that formed flexible transparent films. These sulfonated multiblock

copolymers exhibited proton conductivity up to 0.036 S⋅cm -1 with IEC value varying

from 0.70 meq/g to 1.20 meq/g. While these values are lower than the control

commercial membrane Nafion® 1135 (IEC 0.91 meq/g and proton conductivity 0.12

S⋅cm -1), they are considered promising and further composition are being investigated.

DSC only showed one Tg value for sulfonated multiblocks, indicating the segments may

not be of sufficient length to develop two Tg values. Therefore, a study on longer

segments sulfonated multiblocks is needed.

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ClCl

C O

Cl CO

FNMP, 80oC, 4h

C O OOFF+

NiCl2, Zn, PPh3, Bipy

m

NaCl

C O OOFF

SO3M

H2SO4, 50oC, 24h

SClO

OCl OHHO

SO

OOOHO OH

1

2

3

2 + 3NMAc/Toluene, K2CO3

+ NMP/Toluene, K2CO3

n

SO

OOOO O

C O OO

SO3M

A B

5

H2SO4 (1.5M), 25oC

SO

OOOO O

C O OO

SO3H

A B6

Figure 46 Synthesis of Hydrophilic-Hydrophobic Multiblock copolymers164

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McGrath research group at Virginia Tech has been focusing on poly(aryl ether

sulfone) random copolymers for PEM for several years. The technique and procedure of

making hydrophilic blocks--synthesizing controlled molecular weight sulfonated

poly(aryl ether sulfone) is pretty mature in our lab. Therefore, it is feasible to control the

length of both hydrophobic and hydrophilic blocks during copolymerization and

synthesize multiblock copolymers via nucleophilic aromatic substitution reaction. By

varying the block length and chemical composition, one may expect some interesting

morphology phenomenon that controls the degree of connectivity between ionic domains

as well as water swelling and mechanical properties.

In summary, phase-separated hydrophilic-hydrophobic multiblock copolymers are

promising PEM materials due to their ability to provide good conductivity at low water

content. Other advantages include less water swelling and better mechanical and thermal

properties. So, more detailed research is needed on this topic.

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Chapter 2

2 EXPERIMENTAL

2.1 Solvent Purification

2.1.1 N,N-Dimethylacetamide (DMAc)

H3C C N

CH3

CH3O

Source: Aldrich Chemical

Boiling Point: 164.5-166 oC

Density: 0.937 g/mL

Molecular Weight: 87.12 g/mol

Purification: DMAc was dried over calcium hydride (CaH2) or

phosphorus pentoxide (P2O5) for at least 12 hours. DMAc

was distilled under reduced pressure (10 mmHg / 60 oC)

and stored over molecular sieves under nitrogen.

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2.1.2 N-Methyl-2-Pyrrolidone or 1-Methyl-2-Pyrrolidone (NMP)

N CH3

O

Source: Aldrich Chemical

Boiling Point: 202 oC

Density: 1.028 g/mL

Molecular Weight: 99.13 g/mol

Purification: NMP was dried over calcium hydride (CaH2) for at least 12

hours then distilled under reduced pressure (10 mmHg /80

oC) and stored over molecular sieves under nitrogen.

2.1.3 Fuming Sulfuric Acid

H2SO4· xSO3

Source: Aldrich Chemical

Molecular Weight: 98.08 g/mol

Density: 1.925g/mL

Purification: Sulfuric acid containing oleum (27-30% SO3) was used as

received. Film was used to wrap and seal the bottle after

each use.

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2.1.4 Toluene

Source: Aldrich Chemical

Boiling Point: 110-111 oC

Density: 0.865 g/mL

Molecular Weight: 92.14 g/mol

Purification: Toluene was transferred into 250 mL round-bottom flasks

and stored over molecular sieves under nitrogen.

2.1.5 Ethanol or Ethyl Alcohol

CH3CH2OH

Source: Aldrich Chemical

Boiling Point: 78 oC

Density: 0.789 g/mL

Molecular Weight: 46.07 g/mol

Purification: Absolute (99%) ethanol was used as a coagulating and / or

recrystallization solvent and used without any purification.

CH3

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2.1.6 Methanol or Methyl Alcohol

CH3OH

Source: Fisher

Boiling Point: 64.7 oC

Density: 0.791 g/mL

Molecular Weight: 32.04 g/mol

Purification: Methanol was used as a coagulating and / or

recrystallization solvent and was used without any

purification.

2.1.7 Isopropyl Alcohol or 2-Propanol or Isopropanol

(CH3)2CHOH

Source: Fisher

Boiling Point: 82 oC

Density: 0.785 g/mL

Molecular Weight: 60.01 g/mol

Purification: Isopropanol was used as a coagulating and / or as a

recrystallization solvent and used without any purification.

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2.1.8 Tetrahydrofuran (THF)

C4H8O

Source: Aldrich Chemical

Boiling Point: 65-67 oC

Density: 0.889 g/mL

Molecular Weight: 72.11 g/mol

Purification: Tetrahydrofuran (THF) was fresh distilled from sodium and

benzophenone before using.

2.1.9 Hydrochloride Acid (HCl)

Source: Aldrich Chemical

Boiling Point: > 100 oC

Density: 1.2 g/mL

Molecular Weight: 36.46 g/mol

Purification: A.C.S. reagent grade hydrochloride acid (HCl) was used as

received.

2.1.10 Benzene

C6H6

Source: Aldrich Chemical

Boiling Point: 80 oC

Density: 0.874 g/mL

Molecular Weight: 78.11 g/mol

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86

Purification: Anhydrous 99.8% benzene was used and stored under

nitrogen and used without any purification

2.1.11 Cyclohexane

C6H12

Source: Aldrich Chemical

Boiling Point: 80.7 oC

Density: 0.779 g/mL

Molecular Weight: 36.46 g/mol

Purification: A.C.S. reagent grade cyclohexane was used as

recrystallization solvent and used without any purification.

2.1.12 Acetic Anhydride

(CH3CO)2O

Source: Aldrich Chemical

Boiling Point: 138-140 oC

Density: 1.08 g/mL

Molecular Weight: 102.09 g/mol

Purification: A.C.S. reagent grade acetic anhydride was used as

recrystallization solvent and used without any purification.

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2.2 Reagents and Purification of Monomers

2.2.1 2,2’-bis(4-hydroxyphenol)propane (Bisphenol A) or (Bis A)

Source: Dow Chemical

Molecular Weight: 228.29 g/mol

Melting Point: 152-153 oC

Purification: Bisphenol A was recrystallized from a 25% (w/v) solution

of toluene. After refluxing for several hours, the solution

was allowed to cool to room temperature. The milky white

crystals were collected by filtration and dried in a vacuum

for over 12 hours at 60 oC, then at least 12 hours at 90 oC.

2.2.2 4,4’-Biphenol (BP)

Source: BP-Amoco

Molecular Weight: 186.21 g/mol

HO C

CH3

CH3

OH

OHHO

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Melting Point: 108-110 oC

Purification: Monomer grade biphenol was used as received. Prior to

use, it was dried in a vacuum oven for at least 12 hours at

100 oC. Biphenol can be recrystallized from deoxygenated

acetone or toluene.

2.2.3 4,4'-(hexafluoroisopropylidene) diphenol (6F-Bisphenol A)

Source: DuPont Chemical

Molecular Weight: 336.33 g/mol

Melting Point: 160-162 oC

Purification: 6F bisphenol A (sometimes called bisphenol AF) was

received as a slightly colored (pink) powder. It was first

dissolved in warm acetic acid and precipitated with

deionized water. The precipitated powder was then filtered

and recrystallized from refluxing toluene yielding off-white

crystals. The collected crystals were dried in a vacuum

oven for at least 12 hours at 120 oC. 6F can also be

purified by sublimation.

HO C

CF3

CF3

OH

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2.2.4 4,4’-Dichlorodiphenyl sulfone (DCDPS)

Source: Solvay Advanced Polymer

Molecular Weight: 287.13 g/mol

Melting Point: 145-147 oC

Purification: Monomer grade dichlorodiphenyl sulfone was used as

received. Prior to use, it was dried in a vacuum oven for at

least 12 hours at 100 oC. DCDPS can be recrystallized

from toluene and yields large crystals that are pulverized

prior to drying.

2.2.5 Potassium Carbonate (Anhydrous)

K2CO3

Source: Aldrich

Molecular Weight: 138.21 g/mol

Melting Point: 891 oC

Purification: Potassium carbonate was dried under vacuum at 140 oC for

at least 12 hours prior to use.

S

O

O

Cl Cl

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2.2.6 Chlorosulfonic acid

ClSO3H

Source: Aldrich

Molecular Weight: 116.52 g/mol

Density: 1.753 g/mL

Purification: Chlorosulfonic acid was used as received. Film was used

to wrap and seal moisture from the contents. The container

was stored in a desiccator once opened.

2.2.7 Chlorotrimethylsilane

Source: Aldrich

Boling Point: 57-59 oC

Density: 0.85 g/mL

Molecular Weight: 108.64 g/mol

Purification: Chlorotrimethylsilane (98%) was used as received. A

positive pressure of nitrogen was used when syringing the

reagent from the sealed container. Chlorotrimethylsilane

was stored in a desiccator once opened.

Cl Si

CH3

CH3

CH3

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2.2.8 2,2’-Bipyridyl (bipy)

N N

C10H8N2

Source: Aldrich

Molecular Weight: 156.18 g/mol

Melting Point: 70-73 oC

Purification: 2,2’-Bipyridyl (bipy) (> 99%) was purified by

recrystallization from ethanol and dried at 50 oC

under vacuum for 12h.

2.2.9 Triphenylphosphine (PPh3)

P

(C6H5)3P

Source: Aldrich

Molecular Weight: 262.29 g/mol

Melting Point: 79-81 oC

Purification: Triphenylphosphine (PPh3) (97%)was purified by

recrystallization from cyclohexane and dried at 60

oC under vacuum for 12h.

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2.2.10 Dichlorobis(triphenylphosphine)nickel-(II)

Ni ClCl

Ph3P

Ph3P

[(C6H5)3P]2NiCl2

Source: Aldrich

Molecular Weight: 654.2 g/mol

Purification: Dichlorobis(triphenylphosphine)nickel-(II) (> 99.5%)was

used as received without further purification and stored

under nitrogen.

2.2.11 Nickel(II) Chloride (NiCl2)

Source: Aldrich

Molecular Weight: 129.6 g/mol

Purification: Anhydrous nickel(II) chloride was dried under vacuum at

120 oC for 24h and kept in the oven to avoid moisture.

2.2.12 Zn Powder

Source: Aldrich

Molecular Weight: 65.39 g/mol

Purification: Zn powder was washed with acetic anhydride, filtered,

washed with dry Et2O, and dried under vacuum at 150 °C

for at least 48h.

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93

2.2.13 2,5-dichlorobenzoic acid

COOH

Cl Cl

Source: Aldrich

Molecular Weight: 191.01 g/mol

Melting Point: 151-153 oC

Purification: 2,5-dichlorobenzoic acid (97%) was used to

prepare dichloro monomers for Ni (0)-catalyzed

coupling reaction and used without further

purification.

2.2.14 4-Fluorobenzoyl chloride

CO

FCl

Source: Aldrich

Molecular Weight: 158.56 g/mol

Melting Point: 10-12 oC

Density: 1.342 g/mL

Purification: 4-Fluorobenzoyl chloride (98%) was stored in desiccator and used

without further purification

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2.2.15 4-chlorophenly-4’-fluorophenyl sulfone

Cl SO

FO

Source: Aldrich

Molecular Weight: 270.71 g/mol

Melting Point: 60-62 oC

Density: 1.302 g/mL

Purification: 4-Chlorophenyl-4’-fluorophenyl sulfone (99%) was used without

further purification

2.2.16 Thionyl chloride (SOCl2)

Source: Aldrich

Molecular Weight: 118.97 g/mol

Boiling Point: 79 oC

Melting Point: -105 oC

Density: 1.631 g/mL

Purification: Thionyl chloride (> 99%) was used without

further purification.

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95

2.2.17 Chlorobenzene

Cl

Source: Aldrich

Molecular Weight: 112.56 g/mol

Boiling Point: 132 oC

Density: 1.106 g/mL

Purification: Chlorobenzene (> 99.5%) was used without further

purification

2.2.18 Fluorobenzene

F

Source: Aldrich

Molecular Weight: 96.10 g/mol

Boiling Point: 85 oC

Density: 1.024 g/mL

Purification: Fluorobenzene (> 99%) was used without further

purification

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96

2.2.19 Biphenyl

Source: Aldrich

Molecular Weight: 154.21 g/mol

Boiling Point: 255 oC

Melting Point: 68-70 oC

Purification: Biphenyl (99.5%) was used without further purification

2.2.20 Diphenyl ether

O

Source: Aldrich

Molecular Weight: 170.21 g/mol

Boiling Point: 259 oC

Melting Point: 25-27 oC

Density: 1.073 g/mL

Purification: Biphenyl ether (> 99%) was used without further

purification

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2.2.21 Aluminum Chloride (AlCl3)

Source: Aldrich

Molecular Weight: 133.34 g/mol

Melting Point: 190 oC

Purification: Anhydrous aluminum chloride was handled under

hood and was used without further purification.

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98

2.3 Monomer Synthesis

2.3.1 2,5-Dichlorobenzophenone

C O

Cl Cl

Source: Synthesized in house

Molecular Weight: 251.11 g/mol

Melting Point: 88-89 oC.

Procedure: Thionyl chloride (21.5 mL, 0.2945 mol) and 2,5-dichlorobenzoic

acid (45.0 g, 0.2356 mol) were added together in 500 mL 3-neck

flask and stirred at 63 oC for 12 h. Excessive thionyl chloride was

removed under reduced pressure by gradually increasing the

temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%)

was obtained by distillation at 76-79 oC (5-6 mmHg).

2,5-Dichlorobenzoyl chloride (34 g, 0.16 mol) was added dropwise

via an addition funnel to a suspension of AlCl3 (24 g, 0.18 mol) in

80 mL of dry benzene (0.89 mol) in a 250-mL flask fitted with a

reflux condenser followed by heating at 63 oC for 12 h. The

reaction mixture was quenched by pouring onto acidified, ground

ice and then extracted with toluene. The organic layer was washed

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99

with water, aqueous NaHCO3, and water and then dried with

anhydrous Na2S04. After removal of the toluene, the resulting

yellow-red residue was first recrystallized from ethanol/water

(90/10, v/v) and then from hexane/toluene (50-80 vol % hexane) to

give white crystals. (yield = 89%)

ClCl

COOH

SOCl2

ClCl

CO

ClR

AlCl3

ClCl

CO

R

ClCl

CO

Cl

O

R

H

+63oC

12h

Distilled under vacum

+

(dropwise)

63oC 12h

Quenced with ice

extracted with toluene

Recrystallization

ethnol/water=90/10

Recrystallization

hexane/toluene = 50/50White Crystals (90%)

Light yellow liquid (99%)

(slightly excess)

(slightly excess)

Friedel-Crafts Acylation

(in excess)

1

2

3

F

4

Figure 47 Synthesis scheme of substituted 2,5-dichlorobenzophenone monomers

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100

2.3.2 2,5-Dichloro-4’-fluorobenzophenone

C O

Cl Cl

F

Source: Synthesized in house

Molecular Weight: 269.10 g/mol

Melting Point: 87-88 oC.

Procedure: Thionyl chloride (21.5 mL, 0.2945 mol) and 2,5-dichlorobenzoic

acid (45.0 g, 0.2356 mol) were added together in 500 mL 3-neck

flask and stirred at 63 oC for 12 h. Excessive thionyl chloride was

removed under reduced pressure by gradually increasing the

temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%)

was obtained by distillation at 76-79 oC (5-6 mmHg).

2,5-Dichlorobenzoyl chloride (34 g, 0.16 mol) was added dropwise

via an addition funnel to a suspension of AlCl3 (24 g, 0.18 mol) in

84 mL of fluorobenzene (0.89 mol) in a 250-mL flask fitted with a

reflux condenser followed by heating at 63 oC for 12 h. The

reaction mixture was quenched by pouring onto acidified, ground

ice and then extracted with toluene. The organic layer was washed

with water, aqueous NaHCO3, and water and then dried with

anhydrous Na2S04. After removal of the toluene, the resulting

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101

yellow-red residue was first recrystallized from ethanol/water

(90/10, v/v) and then from hexane/toluene (50-80 vol % hexane) to

give white crystals. (yield = 85%)

2.3.3 2,5-Dichloro-4’-phenylbenzophenone

C O

Cl Cl

Source: Synthesized in house

Molecular Weight: 327.20 g/mol

Melting Point: 126-127 oC.

Procedure: Thionyl chloride (21.5 mL, 0.2945 mol) and 2,5-dichlorobenzoic

acid (45.0 g, 0.2356 mol) were added together in 500 mL 3-neck

flask and stirred at 63 oC for 12 h. Excessive thionyl chloride was

removed under reduced pressure by gradually increasing the

temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%)

was obtained by distillation at 76-79 oC (5-6 mmHg).

2,5-Dichlorobenzoyl chloride (34 g, 0.16 mol) was added dropwise

via an addition funnel to a suspension of AlCl3 (24 g, 0.18 mol)

and biphenyl (28 g, 0.18 mol) in 100 mL of cyclohexane in a 250-

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102

mL flask fitted with a reflux condenser followed by heating at 80

oC for 12 h. The reaction mixture was quenched by pouring onto

acidified, ground ice and then extracted with toluene. The organic

layer was washed with water, aqueous NaHCO3, and water and

then dried with anhydrous Na2S04. After removal of the toluene,

the resulting yellow-red residue was first recrystallized from

ethanol/water (90/10, v/v) and then from hexane/toluene (50-80

vol % hexane) to give white crystals. (yield = 80%)

2.3.4 2,5-Dichloro-4’-oxyphenylbenzophenone

C O

Cl Cl

O

Source: Synthesized in house

Molecular Weight: 343.20 g/mol

Melting Point: 97 oC.

Procedure: Thionyl chloride (21.5 mL, 0.2945 mol) and 2,5-dichlorobenzoic

acid (45.0 g, 0.2356 mol) were added together in 500 mL 3-neck

flask and stirred at 63 oC for 12 h. Excessive thionyl chloride was

removed under reduced pressure by gradually increasing the

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103

temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%)

was obtained by distillation at 76-79 oC (5-6 mmHg).

2,5-Dichlorobenzoyl chloride (34 g, 0.16 mol) was added dropwise

via an addition funnel to a suspension of AlCl3 (24 g, 0.18 mol)

and biphenyl ether (30.6 g, 0.18 mol) in 100 mL of cyclohexane in

a 250-mL flask fitted with a reflux condenser followed by heating

at 80 oC for 12 h. The reaction mixture was quenched by pouring

onto acidified, ground ice and then extracted with toluene. The

organic layer was washed with water, aqueous NaHCO3, and water

and then dried with anhydrous Na2S04. After removal of the

toluene, the resulting yellow-red residue was first recrystallized

from ethanol/water (90/10, v/v) and then from hexane/toluene (50-

80 vol % hexane) to give white crystals. (yield = 82%)

2.3.5 4-Chloro-4'-fluorobenzophenone

CO

FCl

Source: Synthesized in house

Molecular Weight: 234.65 g/mol

Melting Point: 97 oC.

Procedure: A 1L three-necked, round-bottomed flash was equipped with a

mechanical stirrer and p-chlorobenzoyl chloride (86.19 g, 0.496

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104

mol) and fluorobenzene (400 mL). Iron (III) chloride (104.33 g,

0.640 mol) was then added to the reaction mixture through a solid

additional funnel. The reaction was stirred at 5-10 oC for 1.5 h,

then at 50-60 oC for an additional 3 h. The orange-yellow reaction

mixture was quenched with water, and the product was extracted

with chloroform. The chloroform extracts were dried over sodium

sulfate. The solvent was evaporated in vacuum and the residue

was distilled under vacuum to give the desired product as solid.

The final product was then recrystallized from ethanol to give a

yield of 79%.

Cl C ClO

+ FAlCl3 Cl C

OF

Figure 48 Synthesis scheme for 4-Chloro-4'-fluorobenzophenone

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105

2.3.6 Sodium Salt of 3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone

(SDCDPS)

SO

O

SO3Na

NaO3S

ClCl

Source: Synthesized in house

Molecular Weight: 491.25 g/mol

Melting Point: > 350 oC.

Procedure: The procedure utilized was slightly modified from previously

reported procedures. To a 100-mL, three necked flask equipped

with a mechanical stirrer and a nitrogen inlet/outlet was added 28.7

g of DCDPS dissolved in 60mL 30% fuming sulfuric acid. The

solution was heated to 110 oC for 6 hours. The reaction mixture

was allowed to cool to room temperature and added into 400ml of

ice-water. Next, 180 g of NaCl was added and, subsequently, the

disodium salt of disulfonated dichlorodiphenyl sulfone precipitated

as a white powder. The latter was filtered and re-dissolved in 400

mL of cold deionized water, treated by 2N NaOH aqueous solution

to a pH of 6~7, and finally an excess amount of NaCl was added

again to salt out the sodium form of the sulfonated monomer. The

crude product was filtered and recrystallized from a mixture of

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106

alcohol (methanol or isopropanol) and deionized water (9/1, v/v),

producing white needle-like crystals. Yield: 87%.

Figure 49 Synthesis scheme for 3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) and its

salt form.

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107

2.4 Synthesis of Homopolymers from Substituted

Poly(phenylene)s via Ni(0)-catalyzed Coupling

Reaction

2.4.1 Poly(2,5-benzophenone)

A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.0470 g, 1.6

mmol), PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g,

1.6 mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated

under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2

followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe

through the rubber septum to initiate the reaction, which became deep red after 10

minutes. Then, 2,5-dichlorobenzophenone (4.0178 g, 16 mmol) was charged under N2

flow. The mixture was stirred and heated at 65 oC for 24 h. After cooling to room

temperature, the reaction mixture was added into concentrated aqueous HCl/methanol

(40/60 v/v) solution and stirred overnight. The resulting precipitate was collected by

filtration and washed thoroughly with 10% sodium bicarbonate solution and deionized

water. After drying in a vacuum oven at 100 oC overnight, the light yellow polymer was

isolated with a yield of 95%.

2.4.2 Poly(4’-fluoro-2,5-benzophenone)

A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),

PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6

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108

mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated

under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2

followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe

through the rubber septum to initiate the reaction, which became deep red after 10

minutes. Then, 2,5-dichloro-4’-fluorobenzophenone (4.3056 g, 16 mmol) was charged

under N2 flow. The mixture was stirred and heated at 65 oC for 24 h. After cooling to

room temperature, the reaction mixture was added into concentrated aqueous

HCl/methanol (40/60 v/v) solution and stirred overnight. The resulting precipitate was

collected by filtration and washed thoroughly with 10% sodium bicarbonate solution and

deionized water. After drying in a vacuum oven at 100 oC overnight, the light yellow

polymer was isolated with a yield of 99%.

2.4.3 Poly(4’-phenyl-2,5-benzophenone)

A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),

PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6

mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated

under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2

followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe

through the rubber septum to initiate the reaction, which became deep red after 10

minutes. Then, 2,5-dichloro-4’-phenylbenzophenone (5.2352 g, 16 mmol) was charged

under N2 flow. The mixture was stirred and heated at 65 oC for 24 h. After cooling to

room temperature, the reaction mixture was added into concentrated aqueous

HCl/methanol (40/60 v/v) solution and stirred overnight. The resulting precipitate was

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109

collected by filtration and washed thoroughly with 10% sodium bicarbonate solution and

deionized water. After drying in a vacuum oven at 100 oC overnight, the light yellow

polymer was isolated with a yield of 92%.

2.4.4 Poly(4’-oxyphenyl-2,5-benzophenone)

A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),

PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6

mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated

under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2

followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe

through the rubber septum to initiate the reaction, which became deep red after 10

minutes. Then, 2,5-dichloro-4’-oxyphenylbenzophenone (5.4912 g, 16 mmol) was

charged under N2 flow. The mixture was stirred and heated at 65 oC for 24 h. After

cooling to room temperature, the reaction mixture was added into concentrated aqueous

HCl/methanol (40/60 v/v) solution and stirred overnight. The resulting precipitate was

collected by filtration and washed thoroughly with 10% sodium bicarbonate solution and

deionized water. After drying in a vacuum oven at 100 oC overnight, the light yellow

polymer was isolated with a yield of 98%.

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110

2.5 Telechelic Oligomer Synthesis

2.5.1 Synthesis of Telechelic Poly(2,5-benzophenone) (PBP)

Oligomers

A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),

PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6

mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated

under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2

followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe

through the rubber septum to initiate the reaction, which became deep red after 10

minutes. Then, 2,5-dichlorobenzophenone (3.7667 g, 15 mmol) and the end-capping

agent 4-chloro-4’-fluorobenzophenone (0.2346 g, 1 mmol) were charged under N2 flow.

The mixture was stirred and heated at 65 oC for 24 h. After cooling to room temperature,

the reaction mixture was added into concentrated aqueous HCl/methanol (40/60 v/v)

solution and stirred overnight. The resulting precipitate was collected by filtration and

washed thoroughly with 10% sodium bicarbonate solution and deionized water. After

drying in a vacuum oven at 100 oC overnight, the light yellow functional oligomer (or

polymer) was isolated with a yield of 95%.

2.5.2 Synthesis of Telechelic Poly(4’-oxyphenyl-2,5-benzophenone)

(PPBP) Oligomers

A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),

PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6

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111

mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated

under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2

followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe

through the rubber septum to initiate the reaction, which became deep red after 10

minutes. Then, 2,5-dichloro-4’-oxyphenylbenzophenone (5.1480 g, 15 mmol) and the

end-capping agent 4-chloro-4’-fluorobenzophenone (0.2346 g, 1 mmol) were charged

under N2 flow. The mixture was stirred and heated at 65 oC for 24 h. After cooling to

room temperature, the reaction mixture was added into concentrated aqueous

HCl/methanol (40/60 v/v) solution and stirred overnight. The resulting precipitate was

collected by filtration and washed thoroughly with 10% sodium bicarbonate solution and

deionized water. After drying in a vacuum oven at 100 oC overnight, the light yellow

functional oligomer was isolated with a yield of 98%.

2.5.3 Synthesis of Telechelic Disulfonated Poly(arylene ether sulfone)

(PAES) Oligomers

The aromatic nucleophilic reaction was conducted in a 3-neck flask equipped with

a mechanical stirrer, nitrogen inlet and a Dean-Stark trap. In a typical polymerization to

prepare 5,000 g/mol oligomer, biphenol (0.3742 g, 2 mmol), SDCDPS (0.8725 g, 1.776

mmol) and potassium carbonate (0.3179 g, 2.3 mmol) were added to the flask. Dry NMP

(10 mL) and toluene(5 mL) were added as the solvents. The reaction mixture was heated

under reflux at 150 oC for 4 h, which stripped off most of the toluene to dehydrate the

system. The temperature was then raised slowly to 190 oC for 36 h. The viscous

polymer solution was cooled to room temperature, diluted with NMP and filtered to

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112

remove most of the salts. The oligomer was isolated by precipitation in isopropanol.

After drying in a vacuum oven at 100 oC overnight, the white functional oligomer was

isolated with a yield of 99%.

2.6 Synthesis of Multiblock Copolymers

2.6.1 Synthesis of PAES-PBP Multiblock Copolymers

The viscous solution of poly(arylene ether sulfone) (PAES) oligomers prepared

above was first cooled to room temperature. Poly(2,5-benzophenone) telechelic oligomer

(1.1171 g, 0.223 mmol), potassium carbonate (0.069 g, 0.5 mmol), dry NMP (5 mL) and

toluene (10 mL) were added into the same reaction flask under nitrogen. The reaction

mixture was heated to 150 oC and refluxed for 4 h. The toluene was removed to

dehydrate the system and the temperature was raised to 190 oC for another 48 h. The

viscous polymer solution was cooled to room temperature, diluted with NMP and filtered

to remove most of the salts. The block copolymer was isolated by precipitation in

isopropanol. It was washed extensively with deionized water and chloroform and dried at

120 oC under vacuum for 24 h.

2.6.2 Synthesis of PPAES-PBP Multiblock Copolymers

The viscous solution of poly(arylene ether sulfone) (PAES) oligomers prepared

above was first cooled to room temperature. Poly(4’-oxyphenyl-2,5-benzophenone)

telechelic oligomer (1.1171 g, 0.223 mmol), potassium carbonate (0.069 g, 0.5 mmol),

dry NMP (5 mL) and toluene (10 mL) were added into the same reaction flask under

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nitrogen. The reaction mixture was heated to 150 oC and refluxed for 4 h. The toluene

was removed to dehydrate the system and the temperature was raised to 190 oC for

another 48 h. The viscous polymer solution was cooled to room temperature, diluted

with NMP and filtered to remove most of the salts. The block copolymer was isolated by

precipitation in isopropanol. It was washed extensively with deionized water and

chloroform and dried at 120 oC under vacuum for 24 h.

2.7 Characterization

2.7.1 Nuclear Magnetic Resonance (NMR) Spectroscopy

Proton (1H), Carbon (13C) and Fluorine (19 F) Nuclear Magnetic Resonance were

used to confirm the chemical composition of the monomers and polymers in this research.

Samples were dissolved in appropriate deuterated solvents (DMSO-d6, CDCl3, or D2O),

at a typical concentration of 5% (0.05g / 1 mL). NMR spectra were obtained on a Varian

Unity Spectrometer operating at 400 MHz. Quantitive Carbon (13C) NMR was

conducted with a concentration of at least 10% (0.3 g/3 mL) and longer T1 (> 6s)

relaxation time.

2.7.2 Fourier Transform Infrared (FTIR) Spectroscopy

FTIR was utilized to confirm the functional groups of synthesized homo-and

copolymers. Measurements were conducted on a Nicolet Impact 400 FTIR Spectrometer

using thin solution-cast polymer films. The typical thickness of the films is around 10

µm.

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2.7.3 Gel Permeation Chromatography (GPC)

GPC measurements were used to determine molecular weight and molecular

weight distributions of synthesized polymers and oligomers. GPC experiments were

performed on a liquid chromatograph equipped with a Waters 1515 isocratic HPLC pump,

Waters Autosampler, Waters HR5-HR4-HR3 column, Waters 2414 refractive index

detector and Viscotek 270 RALLS/viscometric dual detector. NMP (containing 0.05 M

LiBr) was used as the mobile phase. The addition of LiBr is well known to shield the

polyions from intramolecular expansion for ion-containing multiblock polymers. The

column temperature was maintained at 60 oC because of the viscous nature of NMP.

Both the mobile phase solvent and sample solution were filtered before introduction to

the GPC system.

2.7.4 Intrinsic Viscosity Determinations ([η])

Intrinsic viscosity measurements were carried out with a Cannon Ubbelholde

viscometer in NMP (containing 0.05 M LiBr) as solvent at 25 oC. The addition of LiBr is

well known to shield the polyions from intramolecular expansion for ion-containing

multiblock polymers. The measurements of intrinsic viscosities of ion-containing

multiblock copolymers as well as oligomers provide valuable information on the size of a

polymer molecule in solution and shield some light on the molecular weight. Intrinsic

viscosities are very important in this dissertation since GPC can not be effectively used

for ion-containing multiblock copolymers. This is because ion-containing multiblock

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copolymers typically form micelles-like structures in solvents such as NMP and DMAc,

leading to non-true solution. Therefore, gel permeation chromatography (GPC) can not

provide reasonable values of molecular weights for this series ion-containing multiblock

copolymers.

2.7.5 Thermogravimetric Analysis (TGA)

Thermogravimetric analyses (TGA) were performed on a TA Instrument TGA Q-

500 thermogravimetric analyzer in either air or nitrogen to assess the thermal and thermo-

oxidative stability of multiblock copolymers as well as various oligomers. All the

samples were first vacuum dried and kept in the TGA furnace at 150 oC in a nitrogen

atmosphere for 30 min to remove water before TGA characterization. The typical

heating rate was 10 oC/min in nitrogen or air.

2.7.6 Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) was conducted using TA Instrument

DSC Q-1000 at a heating rate of 10 oC/min, under a stream of nitrogen to determine the

glass transition temperatures (Tg) of multiblock copolymers as well as various oligomers

and melting points (Tm) of various monomers. Second heat Tg values were reported as

the midpoints of the change in the slopes of the baselines.

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2.7.7 Elemental Analysis

Galbraith Laboratories (TN) performed elemental analysis of all synthesized

monomers. The atomic compositional percentages of carbon, hydrogen, oxygen, sulfur

and sodium were obtained.

2.7.8 Non-Aqueous Potentiometric Titration

Non-aqueous potentiometric titrations were employed to determine the milli-

equivalent (meq) weight of sulfonic acid groups in sulfonated poly(arylene ether

sulfone)s. This value was then used to experimentally determine the Ion-Exchange

Capacity (IEC) of the sulfonated copolymers containing pendant sulfonic acid groups.

The ion-exchange capacity is defined as the milli-equivalents of reactive –SO3H sites per

gram of polymer and have units, therefore, of meq/g. The measured IEC values are

compared to the theoretical or calculated IEC value based on the moles of sulfonated

dihalide monomer charged to the reaction flask. Potentiometric titrations were performed

using a MCI GTOS Automatic Titrator equipped with a standard calomel electrode and a

reference electrode.

A typical titration is conducted as follows: Tetramethyl ammonium hydroxide

(TMAH) was utilized as titrant (~ 0.02 N) for the titration of sulfonic acid groups in the

polymer. The titrant was standardized against dry potassium hydrogen phthalate (KHP)

immediately prior to titrating. Sulfonated polymers were dried at ~120 oC before being

weighed and dissolving in dimethyl acetamide (DMAc). The end-point was detected as

the maximum of the first derivative for the potential versus the used volume of titrant.

The end point was then used to calculate the IEC (meq/g) of the sulfonated membrane.

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The reported experimentally calculated IECs were those of the average of at least three

titrated samples.

2.7.9 Film Preparation

Films were prepared by casting solutions of sulfonated copolymers dissolved in

DMAc on clean glass substrates. The concentration of the polymer solution was varied

from 5 – 15% (w/v) which allowed some control over the thickness of the films. Polymer

solutions were filtered through a disposable syringe and disc filters (0.45 µm) prior to

casting to remove particulates or gels. Evaporation of DMAc was accomplished via an

infrared lamp in an inert (N2) environment with gradual increasing lamp intensity over 24

hours. The films were then placed in a vacuum oven at 120 oC for approximately 24

hours and 150 oC for four hours.

2.7.10 Water Uptake

The membranes were vacuum-dried at 100 oC for 24 h, weighed and immersed in

deionized water at room temperature for 24 h to measure the water uptake. The wet

membranes were wiped dry and quickly weighed again. The water uptake of the

membranes is reported in weight percent as follows:

100uptakewater ×−

=dry

drywet

WWW

Equation 5

where Wwet and Wdry are the weights of the wet and dry membranes, respectively.

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2.7.11 Conductivity Measurements

Proton conductivity at 30 °C at full hydration (in liquid water) was determined in

a window cell geometry using a Solartron 1252+1287 Impedance/Gain-Phase Analyzer

over the frequency range of 10 Hz - 1 MHz. The cell geometry was chosen to ensure that

the membrane resistance dominated the response of the system. The resistance of the

film was taken at the frequency which produced the minimum imaginary response. The

conductivity of the membrane can be calculated from the measured resistance and the

geometry of the cell according to:

AZl'

=σ Equation 6

where σ is the proton conductivity, l is the length between the electrodes, A is the

cross sectional area available for proton transport, and Z’ is the real impedance response.

For determining proton conductivity in liquid water, membranes were equilibrated at 30

°C in deionized water for 24 h prior to the testing. For determining proton conductivity

under partially hydrated conditions, membranes were equilibrated in a relative humidity

chamber (ESPEC SH-240) at the specified relative humidity (RH) and 80 °C for 24 h

before measurement.

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Figure 50 Schematic of a four-point membrane proton conductivity cell.

2.7.12 Ion Exchange Capacity (IEC)

Ion Exchange Capacity (IEC) was determined by aqueous potentiometric

titrations using an MCI Automatic Titrator Model GT-05. The membranes were soaked

in deionized water containing sodium sulfate (1 M solution) for 24 h at room temperature

to form the salt form polymer and an acid form, water soluble compound (i.e., sodium

hydrogen sulfate). The solutions were titrated by standard sodium hydroxide solution

(0.01 M) to quantitatively determine sulfonic acid concentration in the sulfonated

polymers in terms of ion exchange capacity (IEC, mequiv/g).

2.7.13 UV-visible Absorption Spectroscopy (UV-vis)

Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry

(UV/ VIS) involves the spectroscopy of photons and spectrophotometry. It uses light in

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the visible and adjacent near ultraviolet (UV) and near infrared (NIR) ranges. In this

region of energy space molecules undergo electronic transitions. UV/Vis spectroscopy is

routinely used in the quantitative determination of solutions of transition metal ions and

highly conjugated organic compounds.

The method is most often used in a quantitative way to determine concentrations

of an absorbing species in solution, using the Beer-Lambert law:

− Equation 7

where A is the measured absorbance, I0 is the intensity of the incident light at a

given wavelength, I is the transmitted intensity, L the pathlength through the sample, and

c the concentration of the absorbing species. For each species and wavelength, ε is a

constant known as the molar absorptivity or extinction coefficient. This constant is a

fundamental molecular property in a given solvent, at a particular temperature and

pressure, and has units of 1 / M * cm or often AU / M * cm. The absorbance and

extinction ε are sometimes defined in terms of the natural logarithm instead of the base-

10 logarithm.

The Beer-Lambert law states that the absorbance of a solution is due to the

solution's concentration. Thus UV/VIS spectroscopy can be used to determine the

concentration of a solution. It is necessary to know how quickly the absorbance changes

with concentration. This can be taken from references (tables of molar extinction

coefficients), or more accurately, determined from a calibration curve.

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An ultraviolet-visible spectrum is essentially a graph of light absorbance versus

wavelength in a range of ultraviolet or visible regions. Such a spectrum can often be

produced by a more sophisticated spectrophotometer. Wavelength is often represented by

the symbol λ. Similarly, for a given substance, a standard graph of extinction coefficient ε

vs. wavelength λ may be made or used if one is already available. Such a standard graph

would be effectively "concentration-corrected" and thus independent of concentration.

For the given substance, the wavelength at which maximum absorption in the spectrum

occurs is called λmax.

UV-visible Absorption Spectroscopy (UV-vis) were conducted using a Shimadzu

Model UV-1601 UV–visible spectrometer. The samples were dissolved in methanol or

THF to maintain several ppm solutions and absorbance data were generated.

2.7.14 Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) images were obtained using a Digital

Instruments MultiMode scanning probe microscope with a NanoScope IVa controller

(Veeco) in tapping mode. A silicon probe (Veeco) with an end radius of <10 nm and a

force constant of 5 N/m was used to image samples. The ratio of amplitudes used in the

feedback control was adjusted to a constant value for most samples (the ratio of

amplitudes is 0.83 of the free air amplitude for both the PAES-6k-PBP-6k and PAES-

10k-PBP-10k samples). Samples were dried under vacuum at 60 °C for 3 h and then

equilibrated at 30% relative humidity for at least 12 h before being imaged immediately

at room temperature in a relative humidity of approximately 15-20%.

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Chapter 3

3 Results and Discussion

3.1 Introduction

We are currently interested in the synthesis of potentially economical, and highly

thermooxidatively stable polymers as candidates for proton-exchange membranes (PEMs)

in fuel cells. PEMFCs have potential as energy conversion devices for both

transportation, mobile and stationary applications.191 The primary demands on the

hydrated PEM are high proton conductivity ( ≥ 0.1 S cm-1), low fuel and O2 permeability,

limited swelling in water and high chemical, thermal and mechanical stability.192

Conventional PEMFCs typically operate with Nafion® membranes29, which offer quite

good performance below 90 oC. However, today there is a strong need for PEMs capable

of sustained operation up to 120 oC to offer many benefits including increased resistance

to fuel impurities, most notably carbon monoxide (CO), fast electrode kinetics, and

simplified water/thermal management. Unfortunately, the proton conductivity of Nafion

suffers greatly at temperatures above 90 oC due to the depressed hydrated α-relaxation or

too restrained transport.18, 29 Also, the barrier properties of this membrane are poor when

methanol is used as fuel. These factors, in addition to the high cost of Nafion, have

encouraged extensive research on alternative PEM materials. 9, 91, 136

Our group at Virginia Tech has been focusing on synthesis, characterization and

performance of disulfonated poly (arylene ether sulfone) copolymers from the sodium

form of 3,3’-disulfonated-4,4’-dichlorophenyl sulfone, 4,4’-dichlorophenyl sulfone and

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4,4’-biphenol via direct step-growth polycondensation. There wholly aromatic biphenol-

bases copolymers are oxidatively stable, which is critical for long term performance

under harsh conditions within fuel cells. The incorporation of different functional groups

generates structurally distinct sulfonated copolymers and allows qualitative and

quantitive structure-properties relationships.193-195 Additionally, the incorporation of polar

groups, such as phenyl phosphine oxides and phenyl nitriles, may aid in preparation and

improve stability of filled composite (e.g., inorganic heter-polyacids).196-198

Figure 51 Chemical structure of disulfonated poly(arylene ether sulfone) copolymers where the letter

x in abbreviation refers to the sulfonation percentage of a disulfonated monomer.

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Structure–property–performance relationships of directly copolymerized

poly(arylene ether sufone) were investigated by Kim et al. 195 The polysulfone membrane

incorporated with hexafluoro bisphenol A (6F) showed decreased water uptake compared

to non-fluorinated polysulfones (BP) while conductivity increased at a similar IEC,

attributed to a greater degree of phase separation. The polysulfone membranes

incorporated with polar groups such as benzonitrile and PPO, on the other hand, showed

decreased water uptake, conductivity and methanol permeability. Increased conductivity

of the fluorine incorporated system improved fuel cell performance by reducing cell

resistance, while polar group incorporation adversely impacted the fuel cell performance

by lowering conductivity.

As mentioned before in Chapter 1, the ideal PEM materials would show good

proton conductivity at high temperature and low relative humidities, controlled swelling

and deswelling, and exhibit good oxidative stability and mechanical properties. Although

we have obtain very good results by using disulfonated poly (arylene ether sulfone)

random copolymers (BPSH series) in DMFC fuel cells (mainly because of very low

methanol permeability), the performance in H2/air fuel cells is poor compared with

Nafion, especially under low relative humidities. For the random copolymers, we found

the proton conductivity decrease drastically with a decrease in hydration level.98

Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are

promising PEM materials, particularly due to their ability to form unique morphologies.

It is well known that the membrane morphology is important for the PEM fuel cell

performance, and depends strongly on the water content, and on the type concentration

and distribution of the acidic moieties.27 The unique morphologies of multiblock

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copolymers may play an important role in providing good proton transport at low water

contents and high temperatures. 135

In order to synthesize nanophase separated hydrophobic-hydrophilic multiblock

copolymers, we decided to choose telechelic substituted poly(phenylene) (PBP)

oligomers as hydrophobic blocks and telechelic sulfonated poly(arylene ether sulfone)

(PAES) oligomers as hydrophilic blocks. This is mainly because PBP and PAES are very

incompatible since one is made of hydrophobic hydro carbon while the other is

completely hydrophilic and therefore fairly high χ between these two monomers. In

addition, PBP and PAES are quite different in terms of chemical structures: PBP has very

rigid polymer chain while PAES backbone is much more flexible because of the arylene-

ether bonds. As a result, PAES-PBP multiblock copolymer would develop a continuous,

nanophase separated, hydrophobic-hydrophilic morphology, which would enhance the

proton transport and maybe even control the extensive swelling problem.

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3.2 Synthesis of Substituted Poly(p-phenylene)s

Homopolymers

The coupling of bis(aryl halide)s is one of the most recent approaches for the

development of high performance materials. The synthetic procedure was first reported

by Colon et al.175 for synthesis of biphenyl from chlorobenzene. They reported that a

Ni(0)-catalyzed reaction would quantitatively convert chlorobenzene to biphenyl at mild

temperatures and short reaction times. Colon et al.176 have demonstrated a method for

synthesizing poly (arylene ether sulfone)s by a catalytic process involving the coupling of

bis(aryl chlorides) with a Ni(0) complex. Ueda et al. have also investigated the Ni(0)-

catalyzed coupling reaction to make poly(arylene ether sulfone)s199 as well as several

poly(arylene ether ketone)s.200

Our interests in high-performance polymers via nickel-catalyzed coupling has led

us to various poly(p-phenylene) (PPP) derivatives. These polymers are currently

receiving considerable interest because of their high thermal stability and potential in

numerous thermally robust materials including composites, lubricant additives, and

thermoset precursors. However, because the material is quite rigid, it is difficult to

synthesize and to form into useful products. Several steps have been taken to improve

the solubility of PPP while retaining the useful characteristics.201, 202 Appropriately

substituted materials are soluble and yet exhibit thermal and mechanical properties

comparable to the unsubstituted polymer. Percec et al. 183, 203 have synthesized a variety

of soluble polyphenylenes by the Ni(0)-catalyzed coupling of bis(aryl mesylates). The

resulting polymers are high molecular weight, soluble, and thermally stable.

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There have been several reports on the synthesis of substituted polyphenylenes

utilizing techniques other than the Ni(0)-catalyzed reaction. Schluter et al. 204-207 have

described the palladium-catalyzed polymerization of alkyl-substituted poly(p-phenylene)

derivatives. Tour et al.208, 209 have also investigated the functionalization of brominated

poly(p-phenylene) with several alkynes with the goal of producing thermoset precursors.

Novak has reported on the synthesis of water-soluble poly(p-phenylenes) via the

palladium-mediated, Suzuki cross-coupling of aryl halides and arylboronic acids.210-212

Our efforts are focused on the Ni(0)-catalyzed coupling of various substituted 2,5-

dichlorobenzophenones. DeSimone et al. have previously reported on the synthesis of

poly(2,5-benzophenone) from 2,5-dichlorobenzophenone utilizing this catalyst sytem.178

Maxdem Inc. (now Solvay Advanced Polymer Inc.) has also reported the synthesis of

poly(2,5-benzophenone) in a similar manner but with different glass transition and

thermal decomposition temperatures. poly(2,5-benzophenone)s can also be processed by

compression molding and extrusion and are now commercially available under the trade

name Parmax from Solvay Advanced Polymer Inc.

Wang and Quirk185 have reported that higher glass transition temperatures, which

result from a more stereoregular head-to-tail geometry in the polymer backbone, are

obtained when 2,2’dipyridyl is used as a coligand in the reaction. The versatility and

ease of the monomer synthesis led us to study poly (2,5-benzophenone) materials for

potential applications such as conducting polymers and gas separation membranes.

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3.2.1 Monomer Synthesis

2,5-Dichlorobenzophenone and its derivatives were synthesized by aluminum

chloride-catalyzed, Friedel-Crafts acylation of benzene and other aromatic compounds

with 2,5-dichlorobenzoyl chloride as shown in Figure 52. In the case of acylation of

biphenyl and diphenylether, addition of cyclohexane as solvent was necessary in order to

dissolve the aromatic compound and form a suspension with aluminum chloride.

Excessive thionyl chloride was removed under reduced pressure by gradually

increasing the temperature; light yellow 2,5-dichlorobenzoyl chloride (90-95%) was

obtained by distillation at 76-79 oC (5-6 mmHg). The dark-colored acid chloride could

also be directly used from the first reaction for the second step, by simply degassing

under vacuum to remove excess thionyl chloride. One thing needs to be addressed here is

that 2,5-dichlorobenzoyl chloride should be added dropwise (slowly) to the suspension of

AlCl3 and dry benzene or other aromatic compounds since Friedel-Crafts acylation is a

extremely exothermic reaction. The color of the suspension changed quickly to deep

green and then to black color at the end.

After heating at 63 oC for 12h, the reaction mixture was quenched by pouring

onto acidified, ground ice to neutralize all residual AlCl3. Then the organic/water

mixture was extracted with toluene for at least three times. The organic layer was

washed with water, aqueous NaHCO3, and water and then dried with anhydrous Na2S04

to remove all residual water. After removal of the toluene, the resulting yellow-red

residue was first recrystallized from ethanol/water (90/10, v/v) and then from

hexane/toluene (50-80 vol % hexane). Extra care should be taken in recrystallization step

to minimize the loss of final product. After recrystallization, white needle crystals were

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obtained. The yield of the final product was about 84% based on the acid when acid

chloride was used without purification. If purified (distillation) acid chloride (light

yellow colored liquid) was used, the overall yields were 90-95%.

Figure 52 Synthesis of substituted 2,5-Dichlorobenzophenones by Friedel-Crafts Reactions

The recrystallized monomers were analyzed by proton NMR (Figure 53, Figure

54 and Figure 55) with d-chloroform as solvent to confirm the structure and purity.

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Figure 53 Proton NMR spectrum of 2,5-dichlorobenzophenone

Figure 54 Proton NMR spectrum of 2,5-dichloro-4’-phenyl-benzophenone

1.00 1.02 1.00

3.02

8.0 7.8 7.6 7.4 7.2 PPM

2.10 2.10

1.02

2.09 1.90

1.00

7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 PPM

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Figure 55 Proton NMR spectrum of 2,5-dichloro-4’-oxyphenyl-benzophenone

3.2.2 Polymer Synthesis

Colon and Kelsey175 have reported detailed general procedures for the efficient

synthesis of biaryls from aryl chlorides utilizing a coupling reagent composed of a

mixture of a catalytic amount of an anhydrous nickel salt and triphenylphosphine in the

presence of an excess of a reducing metal (Zn, Mg, or Mn). Variables investigated

included the effects of the reducing metal, added coligands, halide salts, and substituents

on the aromatic ring. From the results of Kaeriyama and co-workers213 and the

requirements for high molecular weight, step-growth (condensation) polymerization, it

was apparent that the use of high-purity, dry reagents were required for the preparation of

high molecular weight, substituted poly- (p-phenylene)s.

Colon and Kelsey175, 176 also observed that the nickel-catalyzed coupling reaction

of aryl chlorides in the presence of zinc is accelerated dramatically by addition of 1 equiv

2.04

4.07

1.00 0.96 1.92 2.01

8.0 7.8 7.6 7.4 7.2 7.0 6.8PPM

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of 2,2'-bipyridyl (BPY). It was also reported that 2,2'-bipyridyl effectively suppresses the

principal side reaction of phenyl transfer from triphenylphosphine which would limit the

polymer molecular weight. The effect of 2,2'-bipyridyl on the polymerization of 2,5-

dichlorobenzophenone was investigated by Quirk185 using the same conditions as

described previously, i.e., NiCl2, Zn, and 4-7 equiv of triphenylphosphine, with the

addition of 1 mol equiv of 2,2'-bipyridyl relative to nickel. The red-brown color of the

catalyst formed within 5 min after addition of solvent and at a temperature of 40 oC.

Another difference observed for polymerizations in the presence of bipyridyl was the fact

that gel formation was observed after approximately 40 min at 70 oC ([η] = 0.94 dL/g).

The gel disappeared upon increasing the temperature to 130 oC; however, the molecular

weight did not increase significantly upon raising the temperature and continued heating

([η] = 0.94 dL/g).

More recently, Hagberg and Sheares177 studied the effects of solvent and

monomer structure on the Ni(0)-catalyzed polymerization with various monomers. They

found solvent and the monomer structure were shown to be key considerations in the Ni

(0) polymerization of arylene dichloride monomers containing electron-withdrawing

substituents. Polymerization in DMAc or DMF resulted in side reactions, such as

degradation of reduction of the carbonyl groups, which limited the materials utility. On

the contrary, through the use of THF as solvent, high molecular weight poly(2,5-

benzophenone) were synthesized with no side reactions being observed.

Poly(2,5-benzophenone) of 58 ×103 g/mol ([η] = 1.15 dL/g) with a Tg of 180 °C

and a 10% weight loss temperature in nitrogen of 576 °C was synthesized. In addition to

higher molecular weights than previously reported179, 190, no reduction of the carbonyl

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was observed, even without the addition of TPO. (Polymerization in amide solvents led

to reduction of nearly 15% of the carbonyl functionalities. This limitation was overcome

by simply oxidizing the material post-polymerization or by the addition of small

quantities of triphenylphosphine oxide (TPO) to the reaction mixture.) This result

eliminated the need for oxidation reactions post-polymerization. The polymer was

soluble in solvents such as chloroform, THF, and DMAc. Thin films of the polymer were

cast from chloroform. Although not highly creasable, the films were flexible and showed

much better overall filmforming properties than the polymers synthesized previously in

amide solvents.

Analysis of the results by NMR and consideration of the mechanism showed that

in Ni (0)-catalyzed polymerization there is a window of electron withdrawing ability for

which functionalities such as carbonyl-containing pendants meet the criteria and greatly

accelerate the reaction. Increasing the electron-withdrawing ability by substitution with a

sulfone slows the reaction due to the stabilization of a reactive intermediate.177

Therefore, we chosen THF as solvent in Ni (0)-catalyzed polymerization with the

catalyst mixture as: NiCl2(PPh3)2 (0.1 equiv), PPh3 (0.2 equiv) , Zn (3.1 equiv), 2,2’-

bipyridyl (0.1 equiv) and dichloride monomer ( 1.0 equiv), as shown in Figure 56.214 The

Ni (0)-catalyzed polymerization was carried out under oxygen and water free atmosphere.

After charging the catalyst mixture and dry THF, the solution first became brown and

deep green and at last deep-red. The red color is a good indication of formation of low

valent arylnickel (I) complexes, which are responsible for rapid coupling of aryl

chlorides.175

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ClCl

C O C O

m

R R

R= OH, F,,

1 mole

NiCl2(PPh3)2(0.1), Zn (3.1)

PPh3(0.2), 2,2'-Dipyridine(0.1), THF

Figure 56 Synthesis of substituted poly(2,5-benzophenone)s by Ni (0)-catalyzed coupling reaction

Then, the dichloride monomers were charged under N2 flow. The mixture was

stirred and heated at 65 oC for 4h to 12 h. The polymerization proceeded until the

viscosity increased to a point which the reaction could no longer be stirred efficiently.

After cooling to room temperature, the reaction mixture was added into concentrated

aqueous HCl/methanol (40/60 v/v) solution and stirred overnight to remove all residual

Zinc powder. The resulting precipitate was collected by filtration and washed thoroughly

with 10% sodium bicarbonate solution and deionized water. After drying in a vacuum

oven at 100 oC overnight, the light yellow polymer was isolated with a yield of 95%.

The polymers are insoluble in organic solvents such as hexanes, toluene, and

acetone. However, the polymers are soluble in solvents such as chloroform,

tetrahydrofuran, DMAc and NMP.

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3.2.3 Polymer Characterization

1H and 13C NMR and FTIR were employed to provide structural and

compositional characterizations of the poly(2,5-benzophone) and its derivatives

synthesized from Ni(0)-catalyzed coupling polymerization. UV-visible spectrum was

carried out to shed more light on the level of conjugation in this series of substituted

poly(p-phenylene)s. Thermogravimetric analysis (TGA) and differential scanning

calorimetry (DSC) were used to explore the thermal behavior of these rigid rod poly(p-

phenylene)s.

Figure 57 Proton NMR of Poly (2,5-benzophenone) in CDCl3

8.5 8.0 7.5 7.0 6.5 6.0 PPM

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Figure 58 13C NMR of Poly (2,5-benzophenone) in CDCl3

3.2.3.1 1H and 13C NMR Spectroscopy

Proton NMR spectroscopy has been repeatedly employed to provide structural

confirmation of the polymers.215 Compared proton NMR of monomer 2,5-

dichlorobenzophenone, the biggest difference in the polymer (Poly (2,5-benzophenone))

spectrum is the chemical shift of three protons on the dichloride benzene rings from 7.3-

7.5 ppm to high field 6.5-7.2 ppm due to the coupling of aryl chlorides. Before

polymerization, these three protons are adjunct to chlorides, which is electron-

withdrawing group. However, after coupling polymerization, the three protons are

nearby other benzene rings (electron-donating group) since all chlorides are reacted and

the benzene rings are connected to each other to form a rigid rod polymer chain. As a

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consequence, the chemical shift of these three protons changes from low field (7.3-7.5

ppm) to high field (6.5-7.2 ppm).

Because the 2,5-dichlorobenzophenone monomers are not symmetric, there is a

possibility of several region-isomers in the polymer backbone. 13C-NMR was utilized to

elucidate the isomerism of the synthesized polymers. The 13C-NMR of poly (2,5-

benzophenone) is shown in Figure 58. This polymer shows only two peaks in the

carbonyl region of the spectrum (196-198 ppm) with the main peak at 197.6 ppm, while

polymer prepared in the presence of 2,2'-bipyridyl shows an additional distinct peak at

196.3 ppm.185 Therefore, this polymer seems to generate carbonyl groups which are in a

more regular environment, i.e., in fewer different regiochemical environments, than the

polymer prepared in the absence of 2,2'-bipyridyl.

Figure 59 shows the different regiochemical regularities in the placement of the

lateral benzoyl groups along the polymer backbone. This polymer seems to have a more

regular structure, i.e., more head-to-tail structures (1) and fewer head-to-head structures

(2) and tail-to tail structures (3).

Figure 59 Poly (2,5-benzophenone)s Structures

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3.2.3.2 Fourier Transform Infrared Spectroscopy (FTIR)

Fourier transform infrared spectroscopy is a powerful tool used to characterize the

functional groups in a material.216 It basically shows at which wavelengths the sample

absorbs the IR, and allows an interpretation of which bonds are present. This technique

works almost exclusively on covalent bonds, and as such is of most use in organic

chemistry.217 Figure 60 shows the FTIR spectrum of poly (2,5-benzophenone) and the

big peak at 1670 cm-1 is assigned to the C=O stretching frequency from the lateral

carbonyl substitution along the poly(p-phenylene)s chain.

Figure 60 FTIR spectrum of poly (2,5-benzophenone)s

1670

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3.2.3.3 Intrinsic Viscosity [η] (IV)

The intrinsic viscosity measured in a specific solvent is related to the molecular

weight, M, by the Mark-Houwink equation.218

αη vKM=][ Equation 8

where K and α are Mark-Houwink constants that depend upon the type of polymer,

solvent, and the temperature of the viscosity determinations. Exponent α is a function of

polymer geometry, and varies from 0.5 to 2.0. α contains information about the shape of

the molecules219:

• a = 1/2 ⇒ “bad” solvent, theta condition, a tighter configuration of polymer

chain, intrinsic viscosity is low

• 0.5 < a < 0.8 ⇒ “good” solvent, fairly loosely extended polymer chain, intrinsic

viscosity is high.

• a > 0.8 ⇒ stiff chain

If K and α are known, molecular weight can be derived from intrinsic viscosity.

However, for a newly synthesized polymer, Mark–Houwink constants may not be

available in the literature. In this case, these constants can be determined experimentally

by measuring the intrinsic viscosities of several polymer samples for which the molecular

weight has been determined by an independent method (i.e. osmotic pressure or light

scattering) provided that samples tested have sharp molecular weight distribution.220, 221

A plot of the log[η] vs logM (Mark-Houwink plot) usually gives a straight line. The

slope of this line is the α value and the intercept is equal to the log of the K value.

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As shown in Table 1, the intrinsic viscosities of this series of substituted poly(2,5-

benzophenone)s are in the range of 0.85 to 0.96 dl/g, indicating fairly high molecular

weight polymers have been prepared by Ni (0) catalyzed coupling reactions.214 For this

series of poly(p-phenylene)s, however, the α value should be at least close to 0.8 because

of rod-like stiff chain in solvents.222, 223 In other words, the intrinsic viscosity of poly(p-

phenylene)s would appear to be higher than normal coil-like polymers even if they may

have the same molecular weight.

Table 1 Properties of substituted poly(2,5-benzophenone)s

Polymer Mn

a

(×103 g/mol)

Mw a

(×103 g/mol) PDI a

Intrinsic

Viscosity b

(dL/g)

Tg c

(oC)

TGA

(5% weight loss)

N2 (oC) d

P1 36.5 66.8 1.83 0.96 190 535

P2 25.2 52.1 2.07 0.87 188 520

P3 26.8 69.8 2.60 0.91 197 528

P4 23.2 53.4 2.30 0.85 202 506

a Determined by GPC with THF as solvent at 30 oC (polystyrene standard)

b Determined in THF at 30 oC

c Determined by DSC during second run with heating rate of 10 oC/min

d Obtained by TGA at heating rate of 10 oC/min in N2

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C O

n

C O

n

F

C O

n

C O

n

O

P1 P2 P3 P4

Figure 61 Substituted poly (2,5-benzophenone)s P1-P4 structures

3.2.3.4 Gel Permeation Chromatography (GPC)

Unlike other modes of chromatography, gel permeation or size exclusion

chromatography (GPC or SEC) is an entropically controlled separation technique in

which molecules are separated on the basis of hydrodynamic volume or size. With

proper column calibration or by the use of molecular-weight-sensitive detectors, such as

light scattering, viscometry, or mass spectrometry, the molecular weight distribution

(MWD) and the statistical molecular weight averages can be obtained readily.224-226

In order to utilize GPC method to determine the molecular weight of a polymer,

one must make a good choice of solvent, temperature, sample concentration, column

selection, detector selection and molecular weight calibration method. Among these

factors, solvent is probably the most critical factor that influences the accuracy of GPC

measurement since partially soluble sample can introduce many problems, which might

be multiplied because of high column pressure. It is also important to dissolve the

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sample at appropriate temperature and for sufficiently long before injecting in order to

allow the macromolecular coils to fully swell in the solvent or to break down any

aggregates.224 In some cases, the addition of electrolytes can be required to achieve

disaggregation.227 This issue will be addressed in details later when we have to deal with

ion-containing copolymers.

THF was used as solvent for GPC characterization and the results are summarized

in Table 1. This series of poly (2,5-benzophenone)s exhibit number-average molecular

weight varying from 23.2 × 103 g/mol to 36.5 × 103 g/mol. However, in the case of

poly(p-phenylene)s, it is worthy to point out that GPC can only give a rough estimation

rather than exact molecular weight due to the nature of rigid-rod polymer chain. The

deviation from the expected value of 2.0 is consistent with a broad range of

polydispersities previously reported for similar materials made via Ni(0)-catalyzed

coupling polymerization.184, 185, 196 The origin of varying polydispersities is not clear yet.

3.2.3.5 Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) is a technique for measuring the energy

necessary to establish a nearly zero temperature difference between a substance and an

inert reference material, as the two specimens are subjected to identical temperature

regimes in an environment heated or cooled at a controlled rate.228

DSC is one important characterization technique utilized for determining the glass

transition temperature Tg and the crystalline melting transition temperature for polymers.

The crystalline-melting transition temperature (Tm) a polymer is the melting temperature

of the semi-crystalline regions or domains within the polymer.229 The glass transition

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temperature (Tg) of a polymer is the temperature at which backbone segments in the

amorphous domains of the polymer attain sufficient thermal energy to move in a

coordinated manner. In the region of the glass transition temperature, the polymer will

change from a glassy material to a rubbery material. 230, 231

There are many factors that affect the glass transition of a polymer. Some of

these factors include polymer structure, molecular symmetry, molecular weight,

structural rigidity, and the presence of secondary forces.232 Differential scanning

calorimetry (2nd Heat) analysis was used to characterize the thermal transition for this

series of poly(p-phenylene)s synthesized in this research. A heating rate of 10 oC/min

was used on polymer films of typical weights of 5-10 mg.

These polymers exhibit no evidence of crystallinity by DSC, probably due to

existence of region-isomers as discussed previously, resulting in soluble and processible

polymers. These polymers exhibit fairly high Tgs owing to their wholly aromatic

backbones and pendant groups. Another interesting phenomenon is that as one increase

molecular bulkiness from P1 to P4, Tg constantly goes up from 188 to 202 oC because of

the hindered internal rotation by the larger pendant groups.

3.2.3.6 Thermogravimetric Analysis (TGA)

Thermal Gravimetric Analysis (TGA) is a simple analytical technique that

measures the weight loss (or weight gain) of a material as a function of temperature.233,

234 As materials are heated, they can loose weight from a simple process such as drying,

or from chemical reactions that liberate gasses. Some materials can gain weight by

reacting with the atmosphere in the testing environment.

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The thermal and thermo-oxidative stability as a function of weight loss of this

series of poly(p-phenylene)s were investigated by thermogravimetric analysis. All the

samples were pre-heated at 150oC for 30 minutes in TGA furnace to remove trace solvent

and moisture. Then, the dynamic TGA experiments were run from 50 to 700 oC, at a

heating rate of 10oC/min under nitrogen.

These polymers exhibit high thermal stability as is expected for rigid rod poly(p-

phenylene)s with 5% weight loss temperature varies from 520 to 538 oC in nitrogen. The

high thermal stabilities of these polymers were no doubt derived from their fully aromatic

structure extending through the backbone and pendent groups. P1 exhibits the highest

thermal stability with a 5% weight loss temperature of 538 oC in nitrogen with 60% of the

polymer remaining at 600 oC. Polymer P4 had lower 5% weight loss temperature

compared to others, which may have been due to the presence of relatively less thermally

stable phenyl ether pendent group.

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Figure 62 TGA thermogram for poly(2,5-benzophenenone) in N2

3.2.3.7 Ultraviolet-visible Spectroscopy (UV/ VIS)

Ultraviolet-visible spectroscopy (UV/ VIS) uses light in the visible and adjacent

near ultraviolet (UV) and near infrared (NIR) ranges. In this region of energy space

molecules undergo electronic transitions.235

UV/Vis spectroscopy is routinely used in the quantitative determination of

solutions of transition metal ions and highly conjugated organic compounds. Organic

compounds, especially those with a high degree of conjugation, absorb light in the UV or

visible regions of the electromagnetic spectrum.236 The solvents for these determinations

are often water for water soluble compounds, or ethanol for organic-soluble compounds.

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Note organic solvents may have significant UV absorption and not all solvents are

suitable for use in UV spectroscopy. Ethanol absorbs very weakly at most wavelengths.

Samples for UV/Vis spectrophotometry are most often liquids are typically placed

in a transparent cell, known as a cuvette. Cuvettes are typically rectangular in shape,

made of high quality quartz, commonly with an internal width of 1 cm. (This width

becomes the path length, L, in the Beer-Lambert law.) An ultraviolet-visible spectrum is

essentially a graph of light absorbance versus wavelength in a range of ultraviolet or

visible regions. Wavelength is often represented by the symbol λ. For the given

substance, the wavelength at which maximum absorption in the spectrum occurs is called

λmax. 236

It is well known that poly(p-phenylene)s absorb light in the UV or visible regions

due to their highly conjugated phenyl rings. Figure 63 shows UV-vis spectrum of

poly(2,5-benzophene) in CHCl3 and the absorption maximum λmax is at 352 nm, which is

good agreement with values for high molecular weight poly(2,5-benzophene) in

literature.178, 185 In addition, it has been well-established that the wavelength of

maximum absorbance in the p-phenylene series increases (bathochromic shifts) as the

number of conjugated phenyl rings increases. However, this does not necessarily mean

that the higher the λmax, the higher the molecular weight because larger conjugated

systems does not ensure longer polymer chain. This is mainly due to the fact that region-

isomers exist in this polymer backbone, which break up some conjugated phenyl rings.

This is also the reason why these poly(p-phenylene)s are not crystalline.

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Figure 63 UV-vis spectrum of poly(2,5-benzophenone) in CHCl3

Poly(2,5-benzophenone)

-0.4

0

0.4

0.8

1.2

1.6

2

200 250 300 350 400 450 500

Wavelength (nm)

Abs

orba

ncee

U)

352 nm

256 nm

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3.3 Disulfonated Poly (arylene ether sulfone) Copolymers

(BPSH series)

Poly(arylene ether sulfone)s are a promising class of proton exchange membranes

due to their excellent thermal and chemical stability and high glass transition

temperatures.237 High proton conductivity can be achieved through post-sulfonation of

poly(arylene ether) materials, but this most often results in very high water sorption or

even water solubility. In addition, post-sulfonation lacks good control of ionic groups’

distribution along the polymer chain.

Previously, our group has reported the synthesis and properties of several directly

polymerized disulfonated poly (arylene ether sulfone) random copolymers, which exhibit

high proton conductivity and excellent stability, and also reduced methanol permeability

and a higher upper limit use temperature compared to Nafion.10, 16, 18, 89, 102

3.3.1 Synthesis of Disulfonated Poly (arylene ether sulfone)

Copolymers (BPSH)

Figure 64 shows the synthesis of poly (arylene ether sulfone) random copolymers

via using biphenol with various molar stoichiometric amounts of dichlorodiphenyl

sulfone (DCDPS) and sulfonated dichlorodiphenyl sulfone (SDCDPS). The respective

molar amount of the sodium salt of 3,3’-disulfonated 4,4’-dichlorodiphenyl sulfone

(SDCDPS) was increased in increments of ten mole % while, in order to maintain

stoichiometric balance, the dichlorodiphenyl sulfone (DCDPS) molar amount was

correspondingly decreased. Series of sulfonated copolymerizations were attempted using

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0 to 60 mol% sulfonated comonomer. This series of polymers are called BPSH-xx,

where BP stands for biphenol, S is for sulfonated, and H denotes the proton form of the

acid where xx represents the degree of disulfonation.

A typical reaction, using similar polymerization conditions as the control

polymers, is given for biphenol containing 40 mol% sulfonated dichlorodiphenyl sulfone

and 60 mol% unsulfonated sulfone monomer. First, 1.86 g (10 mmol) biphenol, 1.1487 g

(4 mmol) 4,4’-dichlorodiphenyl sulfone (DCDPS), 2.9476 g (6 mmol) disodium-3,3’-

Figure 64 Synthesis of “BPSH” series random copolymers via direct copolymerization

disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) and 1.15 equivalents (1.6 g) of

potassium carbonate were added to a 3-neck flask fitted with a mechanical stirrer, a

nitrogen inlet and a Dean Stark trap. Dry N,N-dimethylacetamide (DMAc) (~30 mL)

was introduced to afford a 20% solids concentration, and toluene (12 mL) was used as an

azeotroping agent. The reaction mixture was refluxed at 150 ºC for 4 hours to remove

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most of the toluene and dehydrate the system. Next, the reaction temperature was raised

slowly to 175 ºC for 48 hours. The viscous solution obtained was cooled and diluted

with DMAc to allow easier filtering. The reaction product was filtered to remove most

of the salts, and isolated by addition to stirred deionized water. The disulfonated

copolymers were dried under vacuum at 120 ºC for 24 hours after being washed several

times with deionized water.

The sulfonated copolymers were isolated from deionized water as swollen fibers.

The amount of swelling in the copolymers was a direct function of the degree of

sulfonation, based on the amount of sulfonated dichlorodiphenyl sulfone utilized. For

each sulfonated copolymer, films were cast on glass substrates by re-dissolving

copolymers as described in the experimental section.

In order to be used as proton exchange membranes in fuel cells, these BPS series

copolymers must be in their free-acid forms--BPSH. To convert the films into their acid-

form, they were typically boiled in dilute 0.5 M H2SO4 for 2h, followed by boiling in

deionized water for another 2h. We have published papers about the methods of

acidification for the copolymer.101 Boiling in dilute 0.5 M H2SO4 for 2h is sufficient to

fully acidify the copolymer, but does not increase the level of sulfonation and that was

proved by NMR before and after acidification.

3.3.2 Characterization of BPSH copolymers

Proton NMR is a powerful tool to identify and characterize BPSH series of

sulfonated copolymers. Integration and appropriate analysis of known reference protons

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of the copolymer allowed for determination of the relative composition of the BPSH

series copolymers. Figure 65 shows a representative 1H NMR spectrum of the

copolymer BPSH-35. As shown in Figure 65, the protons adjacent to the sulfonated

group derived from the disulfonated dihalide monomer in the copolymer were well

separated from the other aromatic protons (~8.25 ppm). The degree of sulfonation is

determined by comparing integrals of the aromatic protons of the sulfonated sulfone

(SDCDPS) moiety a relative to those of unsulfonated sulfone (DCDPS) g. The

calculation for degree of sulfonation is provided for BPSH-35 copolymer. The

calculation concluded that 34.2 mol% of 3,3’-disulfonated-4,4’-dichlorodiphenyl sulfone

(SDCDPS) was incorporated into the random copolymer, which agrees well with target

value 35%. Similar calculations done for other BPSH series sulfonated copolymer are

tabulated on Table 2.

Figure 65 Proton NMR spectrum of BPSH 35 in d6-DMSO

7.92

2.00

8.5 8.0 7.5 7.0 6.5 PPM

c

g

b

e,i d,f,h

a

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%2.34%100

42

2n sulfonatio of percentage Mole =×+

=ga

a

Equation 9

Intrinsic viscosities of BPSH series sulfonated copolymers were measured in

NMP with 0.05 M LiBr and listed in Table 2. All BPSH sulfonated copolymers show

high viscosities, varying from 0.7 to 0.9 dL/g. Recall that the intrinsic viscosity

measured in a specific solvent is related to the molecular weight, M, by the Mark-

Houwink equation.

αη vKM=][ Equation 8

Table 2 Degree of sulfonations and intrinsic viscosities of BPSH series of disulfonated copolymers

BPSH

series

DCDPS/SDCDPS

(mole ratio)

Degree of

sulfonation

(100%) a

Degree of

sulfonation

(100%) b

Intrinsic

viscosity [η] c

(dL/g)

BPSH-0 10/0 0 0 0.69

BPSH-10 9/1 10 9.6 0.77

BPSH-20 8/2 20 19.5 0.80

BPSH-35 6.5/3.5 35 34.2 0.92

BPSH-40 6/4 40 38.9 0.88

a Target value

b Experimental values determined by proton NMR

c NMP with 0.05M LiBr as solvent at 25 oC

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Table 3 Influence of degree of disulfonation on several properties of BPSH copolymers

BPSH

series

Degree of sulfonation

(100%) a

Ion Exchange

Capacity b

(mequiv./g)

Water uptake

(wt. %)

Proton

Conductivity c

(S/cm)

BPSH-10 9.6 0.41 5 0.01

BPSH-20 19.5 0.88 12 0.03

BPSH-35 34.2 1.40 40 0.08

BPSH-40 38.9 1.50 45 0.09

a Experimental values determined by proton NMR

b Determined by back-titration of acid groups

c Measured in liquid water at 30 oC

However, common characterization of molecular weight in ion containing

systems, such as BPSH sulfonated copolymers, is complicated by the presence of ionic

groups (sulfonated groups) attached to the polymer backbone where ion-ion interactions

affect the characteristic size of the macromolecule in a solvent. This ion effect on chain

size is often termed as the polyelectrolyte effect. The addition of a strong electrolyte

(such as LiBr) to the solvent is helpful in suppressing the polyelectrolyte effect to allow

characterization of the ion containing materials. Therefore, intrinsic viscosity measured

in solvent of NMP with 0.05 M LiBr can be treated as reasonable data, and can thus be

compared with normal polymers without any ion groups.

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This series of BPSH sulfonated random copolymers exhibit good conductivity

(0.08-0.09 S/cm) when the sulfonation degree is higher than 35%. As shown in Table 3,

the conductivity and water uptake of this series of copolymers increase with disulfonation

(or with increasing IEC). However, as we report previously, a semi continuous

hydrophilic phase was observed and the membranes swelled dramatically--once the

degree of disulfonation reached 60 mole %, resulting in a hydrogel that can not be used

as a proton exchange membrane. Therefore, to obtain a decent proton exchange

membrane, we here limit the ionic exchange capacity (IEC) to around 1.5 mequiv/g, or in

other words, to limit the sulfonation degree to around 35%, to balance the proton

conductivity with other physical properties.

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3.4 Multiblock Copolymers

Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are

promising PEM materials, particularly due to their ability to form unique morphologies.

It is well known that the membrane morphology is important for the PEM fuel cell

performance, and depends strongly on the water content, and on the type concentration

and distribution of the acidic moieties.27 The unique morphologies of multiblock

copolymers may play an important role in providing good proton transport at low water

contents and high temperatures. 135

This chapter reports the end-capping of 2,5-dichlorobenzophenone with 4-chloro-

4’-fluorobenzophenone via Ni (0)-catalyzed coupling reaction to produce difunctional

poly (2,5-benzophenone) (PBP) oligomers with aryl fluoride endgroups, which are labile

to subsequent nucleophilic aromatic substitution. These functionalized PBP

(hydrophobic) telechelic oligomers were then combined with phenoxide terminated

disulfonated wholly aromatic biphenyl poly(arylene ether sulfone) oligomers (hydrophilic)

to produce high molecular weight multiblock copolymers capable of forming flexible

films. Randomization by ether–ether interchange reaction was obviously avoided.

Morphology studies on these multiblock copolymers as well as its relationship to proton

conductivity and water uptake have been initiated and are discussed.

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156

3.4.1 Synthesis of poly (2,5-benzophenone) (PBP) telechelic

oligomers

As reviewed in Chapter 3.1 and 3.2, the 2,5-dichlorobenzophenone can react

readily via Ni(0)-catalyzed coupling to form isomeric high molecular weight poly(2,5-

benzophenone) (PBP) with excellent thermal properties.

Molecular weight and end-group functionality of step-growth polymers, such as

poly (aryl ether sulfone)s, can be controlled by the addition of a monofunctional end-

capping agent. Because of the step-growth nature of the Ni(0)-coupling polymerization

and relatively high rates of reaction, an aryl chloride monomer with a functionality of 1

can be introduced into the reaction mixture to end cap poly(2,5-benzophenone)s.175 Since

aryl fluorides do not participate in the Ni(0)-catalyzed coupling reaction, end capping of

poly(2,5-benzophenone)s with 4-chloro-4’-fluorobenzophenone will result in PBP chains

that contain aromatic fluoride end groups.180 These aryl fluorides activated by electron-

withdrawing group (carbonyl group) can be used as reactive sites for nucleophilic

aromatic substitution block copolymerization. Furthermore, the molecular weights of

these oligomers are controlled by altering the amount of 4-chloro-4’-fluorobenzophenone.

The end-capping agent 4-chloro-4’-fluorobenzophenone was synthesized by an

iron (III) chloride-catalyzed Friedel–Crafts acylation of chlorobenzene with 4-

fluorobenzoyl chloride. Sheares et al.180 and Ghassemi et al.164 had previously

demonstrated the use of this end-capping agent in the synthesis of substituted poly(2,5-

benzophenone) oligomers.

The synthesis of fluorine-terminated PBP telechelic oligomers, shown in Figure

66, was conducted using Ni (0)-catalyzed coupling method from 2,5-

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157

dichlorobenzophenone and a controlled amount of the end-capping agent 4-chloro-4’-

fluorobenzophenone. The monomer and end-capping agent were added quickly under

nitrogen flow to the polymerization flask containing a mixture of nickel chloride, 2, 2’-

bypyridyl and triphenylphosphine in THF.

ClCl

C O

Cl CO

F

THF, 60oC, 12h

C OOO

FF

+NiCl2(PPh3)2, Zn, PPh3, Bpy

n

Figure 66 Synthetic scheme of fluorine terminated poly(2,5-benzophenone) oligomers

The molecular weights of PBP oligomers were controlled by varying the end-

capping agent stoichiometry. Note here although proton NMR has a much higher

sensitivity than 13C NMR (because the major isotope of carbon, the 12C isotope, has a

spin quantum number of zero and is not magnetically active) 238, there are so many

aromatic protons overlapping together, making it impossible to identify the end units and

determine the number average molecular weight Mn. Therefore, quantitive 13C NMR

spectra were used to determine the degree of polymerization of the PBP telechelics.

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158

The peaks at 194.6 ppm and in the range of 198.3–196.2 ppm are assigned to

carbonyl moieties in the end-capping agent c and in the main chain d, respectively

(Figure 67). The integrals of peaks 194.6 ppm and in the range of 198.3–196.2 ppm are

compared to calculate the degrees of polymerization of the PBP telechelics. As

summarized in Table 4, the experimental values from 13C NMR agree well with the

target values. Other prominent features in the 13C NMR spectra are the signals from the

ipso carbon a attached to the fluoride and its adjacent carbon b from the end-capping

agent. The ipso carbon a is assigned as a doublet at 165.0 ppm with a coupling constant

of 255.1 Hz. The adjacent carbon b signal is clearly visible as a doublet at 115.3 ppm

with a coupling constant of 23.1 Hz.

Figure 67 13C NMR spectrum of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3

d c a

b

C O

CO

CO

FF

n

ab

c

d

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159

Figure 68 19 F NMR of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3

19 F NMR was conducted to confirm the functionality of fluorine at each end of

poly(2,5-benzophenone) oligomers. As shown in Figure 68, only one peak shows up at

106.2-106.4ppm, which agrees well with literature value of 106.7-106.9ppm.164

As expected, intrinsic viscosities η of this series PBP oligomers increase with

molecular weight (Table 4). Intrinsic viscosity η can be related to molecular weight by

the Mark-Houwink-Sakurada equation

αη vKM=][ Equation 8

where Mv is viscosity average molecular weight; K and α are the Mark-Houwink

constant and the Mark-Houwink-Sakurada exponent. The plot of log [η] with log Mn ()

gave an approximately straight line and the Mark-Houwink-Sakurada exponent α of

these rigid PBP oligomers was found to be 0.83. The data confirm that the molecular

weight control of this series of PBP oligomers was successful.

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160

The thermal properties of these low molecular weight PBP telechelics were found

to be very robust. A steady increase in the glass transition temperature Tg was observed

from 151 to 187 °C, as molecular weight increased from 2200 to 9400 g/mol (Table 4).

The highest Tg at 187 °C is close to that found for the non-end-capped homopolymer,

namely 190 °C.

Figure 69 Double logarithmic plot of [η] against Mn of fluorine terminated poly(2,5-benzophenone)

oligomers

y = 0.8253x - 3.5863

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1

Log Mn

Log [η]

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161

Table 4 Characterization of fluorine terminated poly(2,5-benzophenone) oligomers

Polymer

Molar Fraction

of End-capping

Agent

Mn a

(g/mol)

Mn b

(g/mol)

Intrinsic

Viscosity c

[η] (dl/g)

Tg d

(oC)

1 0.20 2200 1980 0.16 151

2 0.10 4000 3800 0.21 163

3 0.07 5600 5500 0.32 167

4 0.05 7600 7000 0.46 176

5 0.04 9400 -- 0.48 187

6 0.00 -- -- 0.96 190

a Target molecular weight

b Determined from 13C NMR

c NMP as solvent at 25 oC

d Determined by DSC

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162

Figure 70 UV-vis spectra of a series of poly(2,5-benzophenone) oligomers

Figure 70 shows the UV-vis spectra of a series of poly(2,5-benzophenone)

oligomers with different molecular weight, varying from 2,200 to 9,400 g/mol. Recall

that when an atom or molecule absorbs energy, electrons are promoted from their ground

state to an excited state. As one might expect, they all exhibit similar absorptions at two

wavelength regions: one is around 256 nm, which is assigned to the jump of π→π*; the

other is in the neighborhood of 350 nm, which is corresponded to the promotion of n→π.

It has been well-established that the wavelength of maximum absorbance in the p-

phenylene series increases (bathochromic shifts) as the number of conjugated phenyl

rings increases.

-0.5

0

0.5

1

1.5

2

200 250 300 350 400

7.6k

control

2.2k

9.4k

5.6k

4.0k

Wavelength (nm)

Absorbanc

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163

However, it seems that the wavelength of maximum absorbance λmax does not

correlate very well with molecular weight for this series of poly(2,5-benzophenone)

oligomers. (Table 5) This is probably because the regiochemical irregularity (Head to

Head and Tail to Tail) disrupts the conjugation along the polymer backbone. Thus, there

is no clear trend in the wavelength of maximum absorbance as molecular weight

increases. Nevertheless, for the sample PBP-control (uncapped polymer), the absorption

maximum λmax is at 352 nm, which is match well with the values for high molecular

weight poly(2,5-benzophene) in literature.178, 185

Table 5 Summary of UV-vis λmax of a series of poly(2,5-benzophenone) oligomers

Samples λ1 (nm) λmax2 (nm)

PBP-2.2k 256 346

PBP-4.0k 256 344

PBP-5.6k 256 342

PBP-7.6k 256 346

PBP-9.4k 256 350

PBP-control 254 352

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164

Figure 71 Synthetic scheme of fluorophenyl sulfone terminated PBP oligomers

Another endcapping agent can be used in synthesizing fluorine terminated PBP

oligomers is 4-chlorophenyl-4’-fluorophenyl sulfone, as shown in Figure 71. Because

sulfone is a stronger electron-withdrawing group than ketone, it would promote

nucleophilic substitution reaction in the next coupling reaction. Thus, 4-chloroph enly-4’-

fluorophenyl sulfone is a better endcapping agent than 4-chloro-4’-fluorobenzophenone.

PBP telechelic oligomers endcapped with fluorophenyl sulfone group were also prepared

in the same way as described above. The molecular weights of PBP oligomers were

controlled by varying the end-capping agent stoichiometry.

Figure 72 shows the 19F NMR spectra of a fluorophenyl sulfone-terminated and

fluorophenyl ketone-terminated PBP telechelic oligomers. This again confirms the

functionality of fluorine at each end of PBP oligomers. Additionally, the fluorine peak

ClCl

C O

Cl SO

F

THF, 60oC, 12h

C O

SO

SO

FF

+NiCl2(PPh3)2, Zn, PPh3, Bpy

n

a bO

OO

Mn = 5k to 10k

2,5-dichlorobenzophenone 4-chlorophenly-4’-fluorophenyl sulfone

ClCl

C O

Cl SO

F

THF, 60oC, 12h

C O

SO

SO

FF

+NiCl2(PPh3)2, Zn, PPh3, Bpy

n

a bO

OO

Mn = 5k to 10k

2,5-dichlorobenzophenone 4-chlorophenly-4’-fluorophenyl sulfone

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165

shifts from -106 ppm in ketone-terminated PBP to -104 ppm in sulfone-terminated PBP,

which is presumably due to the stronger electron-withdrawing nature of sulfone.

Figure 72 19 F NMR of a fluorophenyl sulfone-terminated and fluorophenyl ketone-terminated PBP

telechelic oligomers in CDCl3

Proton NMR was used to determine the number average molecular weight of

these fluorophenyl sulfone-terminated PBP oligomers. Compared with 13C NMR, proton

NMR has a much higher sensitivity because the major isotope of carbon, the 12C isotope,

has a spin quantum number of zero and is not magnetically active 238. Thus, proton NMR

offers a much better way than 13C NMR to characterize number average molecular

weight. Figure 73 shows proton NMR of a fluorophenyl sulfone-terminated PBP

oligomer with target Mn of 6,000 g/mol. Number-average molecular weight Mn was

determined by comparing integrals of the aromatic protons of the phenyl sulfone moiety

in the terminal group a,b relative to the rest protons in repeat units.

100 -102 -104 -106 -108 PPM

F19 OBS ERVEST ANDARD PARAM ET ERS

F19 OBS ERVEST ANDARD PARAM ET ERS

Fluorophenyl sulfone-terminated

Fluorophenyl ketone-terminated -106 ppm

-104 ppm

100 -102 -104 -106 -108 PPM

F19 OBS ERVEST ANDARD PARAM ET ERS

F19 OBS ERVEST ANDARD PARAM ET ERS

Fluorophenyl sulfone-terminated

Fluorophenyl ketone-terminated -106 ppm

-104 ppm

Page 182: SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK ......terminated disulfonated poly (arylene ether sulfone)s (hydrophilic) by nucleophilic aromatic substitution to form novel hydrophilic-hydrophobic

166

For example:

g/mol 5700 272280192

unit end ofweight Molecular 2unit)repeat ofweight Molecular (n) unitsrepeat ofNumber (

g/mol 272unit end ofweight Molecular g/mol 801unitrepeat ofweight Molecular

29n 240 48n protonsrest of intergel 8, Hb and Ha of Intergel

=×+×=

×+×=

==

=⇒=+==

nM

Figure 73 Proton NMR of a fluorophenyl sulfone-terminated PBP oligomer with target Mn of 6,000

g/mol

3.4.2 Synthesis of disulfonated poly(arylene ether sulfone) (PAES)

telechelic oligomers

Hydroxy (phenoxide) terminated sulfonated poly(arylene ether sulfone) (PAES)

telechelics were synthesized (Figure 74) as hydrophilic blocks in multiblock copolymers.

8.00 4.43

240.31

8.0 7.5 7.0 6.5 6.0 PPM

a,b

C O

SO

SO

FF

n OO

a b c d

c

d 8.00 4.43

240.31

8.0 7.5 7.0 6.5 6.0 PPM

a,b

C O

SO

SO

FF

n OO

a b c d

c

d

a,b

C O

SO

SO

FF

n OO

a b c dC O

SO

SO

FF

n OO

a b c d

c

d

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167

The 4,4’-biphenol monomer was used in excess base on the total amount of disulfonated

4,4’-dichlorodiphenylsulfone (SDCDPS) to produce telechelics with the target number

average molecular weight of 3000, 6000 and 10,000 g/mol.

SClO

OCl

+NaO3S

SO3Na+

OHHO

SO

OMO3S

SO3M

OOH OH

BPS100

(in excess)

K2CO3

NMP/Toluene Reflux 150oC for 4hrs190oC for 36 hrs under N2

n

+

O

endgroups were acidified byacetic acid

Figure 74 Synthetic scheme for hydroxy terminated disulfonated poly(arylene ether sulfone)

oligomers

In order to use proton NMR to determine the number average molecular weight

Mn of this series of telechelic oligomers, Two-dimensional COSY (COrrelation

SpectroscopY) experiment was first performed to confirm the protons assignments in

proton NMR, as shown in Figure 75. It is well known that 2-D COSY NMR determines

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168

the connectivity of a molecule by determining which protons are spin-spin coupled.239, 240

Usually, COSY experiment gives information regarding three-bond couplings (from a

proton to its carbon, to the adjacent carbon, then to that carbon's proton). Off-diagonal

peaks denote splitting between protons on adjacent carbons.240, 241 Thus, the protons are

assigned as in Figure 76.

Figure 75 2D-COSY spectrum (COrrelation SpectroscopY) of a phenol terminated disulfonated

poly(arylene ether sulfone) oligomer

85 80 75 70 65

8.5

8.0

7.5

7.0

PP

M In

dire

ct D

imen

sion

1

g

fa

i d

b

h c

g

f

ai

d

b

e

c

85 80 75 70 65

8.5

8.0

7.5

7.0

PP

M In

dire

ct D

imen

sion

1

g

fa

i d

b

h c

85 80 75 70 65

8.5

8.0

7.5

7.0

PP

M In

dire

ct D

imen

sion

1

g

fa

i d

b

h c

g

f

ai

d

b

e

c

Page 185: SYNTHESIS AND CHARACTERIZATION OF MULTIBLOCK ......terminated disulfonated poly (arylene ether sulfone)s (hydrophilic) by nucleophilic aromatic substitution to form novel hydrophilic-hydrophobic

169

Figure 76 Proton NMR spectrum of a phenol terminated disulfonated poly(arylene ether sulfone)

oligomer

Number-average molecular weight Mn was determined by comparing integrals of

the aromatic protons of the biphenol moiety in the terminal group c relative to those in

repeat units a. Peaks at 7.71 ppm and 6.84 ppm are assigned to c and a, respectively.

For example:

g/mol 5900 1806379.0

unit end ofweight Molecular unit)repeat ofweight Molecular (n) unitsrepeat ofNumber (g/mol 180unit end ofweight Molecular

g/mol 637unitrepeat ofweight Molecular

0.9aproton of Intergralcproton of Intergraln unitsrepeat ofNumber

=+×=

+×==

=

===

Mn

HaHc

OO S

O

O

OHH

n

SO3K

KO3S

b a a b e f

g

g

f e

h i d cj

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170

Figure 77 13 C NMR spectrum of a phenol terminated disulfonated poly(arylene ether sulfone)

oligomer

13 C NMR (Figure 77) was also conducted to confirm the structural of this series

of phenol terminated disulfonated poly(arylene ether sulfone) oligomers. By comparing

the carbon peak in repeat units (for example C-1 at 157 ppm) with carbon peak in end

units (for example C-11 at 156 ppm), one can calculate the degree of polymerization and

thus the number-average molecular weight Mn. However, carbon NMR is much less

sensitive than proton NMR since the major isotope of carbon, the 12C isotope, has a spin

quantum number of zero and is not magnetically active. Only the less common 13C

isotope present naturally at 1.1% abundance is magnetically active with a spin quantum

number of 1/2 much like a proton. Therefore, only the few carbon-13 nuclei present

resonate in the magnetic field, resulting in reduced sensitivity. 238 For this reason, we

SO

O

SO3M

OOM OM

MO3S

O1

2 34

n

56 7

8

910

1112

C-4C-10

C-7

C-9

C-6 C-3

C-2

C-5 C-1 C-8

C-11 C-12

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171

used much more sensitive proton NMR to determine the number-average molecular

weight Mn.

As shown in Table 6, for this series of PAES oligomers, Mn determined by proton

NMR agree well with the target values. Besides, the intrinsic viscosity [η] increases from

0.15 to 0.35 dL/g as number-average molecular weight Mn increases from 3,000 to 10,000

g/mol. These results indicate that the synthesis of sulfonated poly(arylene ether sulfone)

(PAES) telechelic oligomers was successful and number-average molecular weight Mn

can be readily controlled by upsetting the stoichiometry of polycondensation.

Table 6 Properties of hydroxy terminated poly(arylene ether sulfone) oligomers

Polymer

Molar Excess of

Biphenol

Monomer (%)

Mn a

(g/mol)

Mn b

(g/mol)

Intrinsic

Viscosity c

[η] (dl/g)

1 22.6 3000 3100 0.15

2 10.9 6000 5900 0.24

3 6.40 10,000 9900 0.35

a Target molecular weight

b Determined from proton NMR

c NMP with 0.05M LiBr as solvent at 25 oC

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172

3.4.3 Synthesis of PAES-PBP Multiblock Copolymers

In general, one of the drawbacks of poly(2,5-benzophenone) (PBP) is their

inability to form flexible films. On the other hand, PAES forms excellent films but lacks

the high modulus and strength of PBP. To develop more flexible, film-forming PBP-base

materials, hydroxy terminated sulfonated PAES telechelics (hydrophilic) prepared above

were copolymerized with fluorine terminated PBP telechelics (hydrophobic) via

nucleophilic aromatic substitution (Figure 78).89

K2CO3

OOFF

C O

PBP

SO

OO

MO3S

SO3M

OO OKK

BPS100

SO

OO

MO3S

SO3M

OO OOO

C O

m

NMP/Toluene

150oC, 4h190oC, 48h

n+

x n

Hydrophilic Hydrophobic

+ KFy

Figure 78 Synthetic scheme of a PAES-PBP hydrophobic-hydrophilic multiblock copolymer

The stoichiometry of the copolymerization was based on a 1:1 molar ratio

between functional end groups of hydrophilic (PAES)/hydrophobic (PBP) oligomers

prepared above. Randomization, which is to be expected in the high temperature

poly(arylene ether) multiblock copolymers forming step, was avoided since there is no

ether–ether interchange possible in the PBP blocks. By varying the block length of

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173

hydrophilic (PAES) /hydrophobic (PBP) telechelics from 3,000 to 10,000 g/mol, three

multiblock copolymers: PAES-3k-PBP-3k, PAES-6k-PBP-6k and PAES-10k-PBP-10k

were synthesized.

3.4.4 Characterization of Multiblock Copolymers

1H and 13C NMR and FTIR were employed to provide structural and

compositional characterizations of the PAES-PBP multiblock copolymers synthesized

from the coupling reaction. Thermogravimetric analysis (TGA) and differential

scanning calorimetry (DSC) were used to explore the thermal behavior of these novel

multiblock copolymers. Gel permeation chromatograph (GPC) was utilized to shed more

light on molecular weight as well as molecular distribution of these multiblock

copolymers. Atomic force microscopy (AFM) was carried out to explore the surface

morphology of the hydrophilic-hydrophobic multiblock copolymers. X-ray

Photoelectron Spectroscopy (XPS) experiment was performed to give qualitative and

quantitative elemental analysis for the surface of multiblock copolymer films. Proton

conductivity, water uptake and ion change capacity (IEC) of these multiblock copolymers

were also measured to examine their potentials to use as proton exchange membrane in

fuel cell.

3.4.4.1 1H and 13C NMR Spectroscopy

Proton NMR spectroscopy has been repeatedly employed to provide structural

confirmation of the synthesized polymers.215 Figure 79 shows the proton NMR spectrum

of PAES-6k-PBP-6k multiblock copolymer with block lengths of approximately 6,000

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174

g/mol. Compared with proton NMR spectra of PBP and PEAS oligomers (Figure 76), it

is very clear that this spectrum contains the characteristic proton peaks from both PBP

and PEAS oligomers, indicating good incorporation of both oligomers. For instance, the

two broad peaks at 7.5 ppm and 7.3 ppm belong to the aromatic protons in PBP’s

phenylene backbone; while the unique peak at 8.3 ppm comes from the aromatic protons

in the repeat unit of polysulfone (PAES).

Figure 79 Proton NMR of PAES-PBP hydrophobic-hydrophilic multiblock copolymer with block

lengths of approximately 6000 g/mol

In addition, the proton peak at 6.8 ppm, which is assigned to aromatic protons of

the biphenol moiety in the terminal group in PAES oligomers (refer to Figure 76),

completely disappears in the multiblock copolymer spectrum. This peak is supposed to

reflect the connecting units between two blocks and the disappearance of this peak

strongly suggest that high molecular weight multiblock copolymers were successfully

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175

synthesized. In other words, the proton peaks in connecting units are so small compared

with protons in each blocks that one can not observe them any more.

Figure 80 13C NMR spectra of a PAES-PBP multiblock copolymer (a), a PBP oligomer (b) and a

PAES oligomer (c).

Figure 80 exhibit the comparison of 13C NMR of a PAES-PBP multiblock

copolymer (a), a PBP oligomer (b) and a PAES oligomer (c). As expected, the

multiblock copolymer spectrum shares characteristic peaks of both PBP and PAES

oligomers. For example, the peaks at 195-197 ppm, a unique peak for carbonyl groups

in the PBP lateral substituent, were also observed in the spectra of multiblock copolymers;

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176

on the other hand, the characteristic peaks of PAES at 159 ppm and 158 ppm also can be

found in the spectra of multiblock copolymers. This again confirms that both PBP and

PAES oligomers have been incorporated into multiblock copolymers.

3.4.4.2 Fourier Transform Infrared Spectroscopy (FT-IR)

Fourier transform infrared spectroscopy is a powerful tool used to characterize the

functional groups in a material. Usually, the mid- infrared (approx. 4000-400 cm-1) is

more useful in studying the fundamental vibrations and associated rotational-vibational

structure. FT-IR instrument measure all wavelengths at once and a transmittance or

absorbance spectrum may be plotted. It basically shows at which wavelengths the sample

absorbs the IR, and allows an interpretation of which bonds are present. This technique

works almost exclusively on covalent bonds, and as such is of most use in organic

chemistry.

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177

Figure 81 FT-IR spectrum of a PAES-PBP multiblock copolymer

FT-IR experiment was conducted to confirm the chemical structure of PAES-PBP

multiblock copolymer (Figure 81), especially shed more light on the functional groups.

As one can observed from the spectrum, a strong peak at 1666 cm-1 are corresponded to

lateral carbonyl groups in PBP blocks; while two peaks at 1027 and 1096 cm-1 (1030 and

1098 in literature89) clearly come from a vibrational symmetric and asymmetric

stretching of the sulfonic acid groups in PAES segments. This result confirms that

PAES-PBP multiblock copolymers contain both PBP and PAES blocks and multiblock

copolymers were successfully synthesized.

1006

10271096

1666

1164

1006

10271096

1666

1164

1006

10271096

1666

1164

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3.4.4.3 Gel Permeation Chromatography (GPC) and Intrinsic Viscosity

[η] (IV)

Gel Permeation Chromatography (GPC) is a powerful, fast and economic way to

determine MW of polymers in which molecules are separated on the basis of

hydrodynamic volume or size.242 With proper column calibration or by the use of

molecular-weight-sensitive detectors, such as light scattering, viscometry, or mass

spectrometry, the molecular weight distribution (MWD) and the statistical molecular

weight averages can be obtained readily. 224-226

However, characterization of MW in ion containing systems (in this case

hydrophilic-hydrophobic multiblock copolymers) is complicated by the presence of ionic

groups attached to the polymer backbone. Ion-containing polymers are usually termed as

“polyelectrolytes”,227 which generally describe any macromolecules which can dissociate

into highly charged polymeric molecules upon being placed in water or other ionizing

solvent irrespective of their ion content.243, 244 As shown in Figure 82, it is well known

that a polyelectrolyte strongly extend with dilution due to the mutual repulsion of the

charges on the polymer backbone.227 Therefore, it is especially important to allow the

macromolecular coils to fully swell in the solvent or to break down any aggregates in

GPC measurements.

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Figure 82 Comparison of neutral and ion-containing polymers with different ion concentration.

When the ionic strength of the solution is increased, a polyelectrolyte tends to

become more coiled due to the screening effects of polymer charges by the excessive

presence of smaller salt counterions in solution as shown in Figure 83. 245 This

characteristic behavior of polyelectrolyte was often explained by counterion condensation

and chain expansion due to intramolecular repulsion.245 Therefore, the conformation of

polyelectrolyte chain is strongly influenced by the strength of electrostatic interaction,

which may be moderated by the addition of small-molar-mass salt. It has been

established experimentally that the larger the amount of the added salt, the closer the

macromolecular behavior of the polyelectrolyte solutions approaches that of solutions of

conventional, uncharged macromolecules.246-249 However, high concentrations of salt

may also lead to clog of the columns and irreproducible results.250 Plus, limited solubility

of the salt in solvent is another big problem.

c < c* c = c* c* < c < c** c = c**

Neutral macromolecules keep the coil conformation, even below the overlap concentration.

Charged macromolecules strongly extend with dilution due to the mutual repulsion of the charges on the polymer backbone.

c < c* c = c* c* < c < c** c = c**

Neutral macromolecules keep the coil conformation, even below the overlap concentration.

Charged macromolecules strongly extend with dilution due to the mutual repulsion of the charges on the polymer backbone.

c < c* c = c* c* < c < c** c = c**

Neutral macromolecules keep the coil conformation, even below the overlap concentration.

Charged macromolecules strongly extend with dilution due to the mutual repulsion of the charges on the polymer backbone.

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Figure 83 Effect of ion strength on the shape of a polyelectrolyte molecular in solution

In view of the above discussion, we chose NMP with 0.05 M LiBr as a solvent

both in GPC and Intrinsic viscosity measurements for this series of hydrophilic-

hydrophobic multiblock copolymers. LiBr is good salt with very good solubility in NMP

and 0.05 M concentration is proved to be good enough to suppress the “polyelectrolyte

effect” but not easily cause the clog of the columns in GPC instrument. 251

These multiblock copolymers, as shown in Table 7, exhibit Mn and Mw in the

range of 36,000 to 66,000 g/mol. Additionally, they all showed reasonable high intrinsic

viscosity [η] from 0.68 to 0.82 dL/g in NMP (0.05 M LiBr). Furthermore, they can all be

solvent-cast into transparent and creasable films, which can then be used in proton

conductivity and water uptake measurements. Therefore, both GPC and IV data again

confirm that high molecular weight multiblock copolymers were successfully synthesized.

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Table 7 Molecular Weight Data of PAES-PBP Multiblock Copolymers

No.

Hydrophilic

Block

Length

Hydrophobic

Block

Length

Mn

(× 103 g/mol)

Mw

(× 103 g/mol)

Intrinsic

Viscosity

(dL/g)

PDI

1 3,000 3,000 36.5 62.8 0.68 1.72

2 6,000 6,000 38.5 61.4 0.72 1.60

3 10,000 10,000 56.9 65.9 0.82 1.16

3.4.4.4 Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) is a technique for measuring the energy

necessary to establish a nearly zero temperature difference between a substance and an

inert reference material, as the two specimens are subjected to identical temperature

regimes in an environment heated or cooled at a controlled rate.

DSC is one important characterization technique utilized for determining the glass

transition temperature Tg and the crystalline melting transition temperature Tm for

polymers.220

As is well known, multiblock copolymers consist of chemically distinct blocks

covalently linked to form a single molecule. Owing to mutual repulsion between

dissimilar blocks, they tend to segregate into separate nanophases. However, because

each block is chemically joined, macroscopic phase separation is prevented. Instead,

nanophase separation takes place. 138 As a result, two separate glass transition

temperatures (Tgs) may be observed by methods such as DSC.

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Figure 84 DSC thermogram of PAES-PBP hydrophobic-hydrophilic multiblock copolymer with

block lengths of approximately 6000 g/mol showing two Tgs

Figure 84 shows the DSC thermogram of a PAES-6k-PBP-6k multiblock

copolymer with block lengths of approximately 6000 g/mol. Two glass transition

temperatures (Tgs) were observed: one is 174 oC, which assigned to the Tg of hydrophobic

PBP segment; the other is around 224oC, which corresponded to the Tg of hydrophilic

disulfonated PAES block. This suggests nanophase separation happened due to the

immiscibility of the PBP and PAES blocks. This is expected since PBP and PAES are

quite different in terms of chemical structures: PBP has very rigid polymer chain while

PAES backbone is much more flexible because of the arylene-ether bonds. This two Tgs

phenomenon was also observed in a PAES-10k-PBP-10k sample (Tgs: 180oC and 225oC)

Temperature (oC)

Heat Flow

(W/g)

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but only one Tg (225oC) was found in a PAES-3k-PBP-3k multiblock copolymer (Table

8). This is expected since longer blocks should increase the immiscibility between

dissimilar blocks, favoring the nanophase separation in the multiblock systems. 138

Recall that the theory of nano-phase separation has been developed by Leibler et

al. 142 in the 1980s and the essential conclusion is that, at equilibrium, the state of the

system is determined by only two relevant parameters: the polymer chain composition f

and the product χN. The composition of the A-type segments f =NA/N and N=NA+NB,

where NA and NB are degrees of polymerization of A-type and B-type blocks. χ is the

Flory-Huggins interaction parameter and N is the number of monomers along the

copolymer backbone. For a symmetric (50/50 vol%) copolymer, f = 0.5, the transition

from a disordered state to one possessing long-range order occurs when χN = (χN)c ≈ 10.5

in the absences of critical fluctuation. 144 However, Flory-Huggins interaction parameter

χ is a fairly complicated parameter and the χ value of poly(arylene ether sulfone) is still

unknown.

Nevertheless, two separate Tgs were observed via DSC for multiblock copolymers

when the block length was longer than 6,000 g/mol. This strongly suggests that nano-

phase separation take place due to the immiscibility of two blocks.

3.4.4.5 Thermogravimetric Analysis (TGA)

Thermal Gravimetric Analysis (TGA) is a simple analytical technique that

measures the weight loss (or weight gain) of a material as a function of temperature. As

materials are heated, they can loose weight from a simple process such as drying, or from

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chemical reactions that liberate gasses. Some materials can gain weight by reacting with

the atmosphere in the testing environment. 233, 234

Table 8 DSC and TGA results for PAES-PBP multiblock copolymers

No.

Hydrophilic

Block

Length

Hydrophobic

Block

Length

Tgsa

(oC)

5% weight

loss (air)b

(oC)

5% weight

loss (N2)b

(oC)

1 3,000 3,000 225 413 440

2 6,000 6,000 174, 224 445 465

3 10,000 10,000 180, 225 459 486

a Determined by DSC during second run with heating rate of 10 oC/min

b Obtained by TGA at heating rate of 10 oC/min

The thermal and thermo-oxidative stability as a function of weight loss of this

series of PAES-PBP multiblock copolymers (salt form) were investigated by

thermogravimetric analysis (TGA). All the samples were pre-heated at 150oC for 30

minutes in TGA furnace to remove trace solvent and moisture. Then, the dynamic TGA

experiments were run from 50 to 700 oC, at a heating rate of 10oC/min under nitrogen or

air.

These multiblock copolymers (salt form) exhibit high thermal stability with 5%

weight loss temperature varies from 440 to 486 oC in nitrogen and 413 to 459 oC. This is

probably due to the excellent thermostability of both the wholly aromatic PBP blocks and

PAES segments.

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Figure 85 TGA thermogram of a PAES-PBP multiblock copolymer, a BPS 40 random copolymer and

a PBP homopolymer

Figure 85 shows the comparison of TGA thermogram of a PAES-PBP multiblock

copolymer, a BPS 40 random copolymer and a PBP homopolymer. Usually, block

copolymers offer no advantages over comparable homopolymers or random copolymers

in terms of thermal and thermal-oxidative stability. This is because a macromolecule is

only as stable as its most sensitive bond. It can be clearly seen that these multiblock

copolymers (salt form) show comparable thermal stability with BPS random copolymers

and PBP homopolymers. Thus, to some extent, this indicates that high molecular weight

Multiblock

BPS random

PBP homopolymer

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PAES-PBP multiblock copolymers were successfully synthesized and the linkage units

are pretty thermal stable.

3.4.4.6 Atomic force microscopy (AFM)

The atomic force microscope (AFM) system has evolved into a useful tool for

direct measurements of micro-structural parameters and unraveling the intermolecular

forces at nanoscale level with atomic-resolution characterization. 252, 253

Typically, these micro-cantilever systems are operated in three open-loop modes;

non-contact mode, contact mode, and tapping mode.253 In order to probe electric,

magnetic, and/or atomic forces of a selected sample, the non-contact mode is utilized by

moving the cantilever slightly away from the sample surface and oscillating the cantilever

at or near its natural resonance frequency. Alternatively, the contact mode acquires

sample attributes by monitoring interaction forces while the cantilever tip remains in

contact with the target sample.

The tapping mode of operation combines qualities of both the contact and non-

contact modes by gleaning sample data and oscillating the cantilever tip at or near its

natural resonance frequency while allowing the cantilever tip to impact the target sample

for a minimal amount of time. As a matter of fact, the technique of tapping mode AFM

(T-AFM) is a key advance in AFM technology. This technique allows high resolution

imaging of soft samples that are difficult to examine using the contact AFM technique. It

overcomes problems such as friction and adhesion that are usually associated with

conventional AFM imaging systems.253

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Compared with electron microscopes such as TEM and SEM, T-AFM provides a

true three-dimensional surface profile. Additionally, samples viewed by AFM do not

require any special treatments (such as metal/carbon coatings) that would irreversibly

change or damage the sample. While an electron microscope needs an expensive vacuum

environment for proper operation, most AFM modes can work perfectly well in ambient

air or even a liquid environment.254

Tapping mode AFM images were obtained under ambient conditions on a 500 nm

× 500 nm size scale to investigate nanophase separation for multiblock copolymers of

PAES-6k-PBP-6k and PAES-10k-PBP-10k (Figure 86). The dark regions of the phase

images are assigned to the softer blocks, which represent the hydrophilic ionic groups

(sulfonic acid) containing small amounts of water. The light regions are related to hard

segments, derived from the hydrophobic hydrocarbon units. It can be clearly seen that

for the PAES-6k-PBP-6k multiblock sample, the dark regions (hydrophilic domains) are

connected to each other and form channels and networks that extend through the whole

sample surface; while light regions (hydrophobic domains) are divided into small

domains, which have diameters of 20 to 40 nm. This nanophase separation phenomenon

develops further in the PAES-10k-PBP-10k sample, where even the light regions

(hydrophobic domains) start to aggregate too, forming bigger and longer hydrophobic

continuous phases. This again confirms that nanophase separation took place in these

hydrophilic-hydrophobic multiblock copolymers and is considered significant in terms of

the transport properties.135

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Figure 86 Tapping mode AFM phase images of (a) a PAES-6K-PBP-6K multiblock copolymer, and

(b) a PAES-10K-PBP-10K multiblock copolymer with block lengths of approximately 6000 and

10,000 g/mol, respectively.(Scan sizes are 500 nm; Z ranges for phase images are 10 o)

3.4.4.7 X-ray Photoelectron Spectroscopy (XPS)

X-ray Photoelectron Spectroscopy (XPS) is a surface chemical analysis technique

that can be used to analyze the chemistry of the surface of a material. In fact, XPS is

arguably the most useful surface analysis technique because of the excellent qualitative

and quantitative elemental analysis it provides. In 1967, it was established that energy

analyzed electrons, photo-emitted during the irradiation of a solid sample by

monochromatic x-rays, exhibited sharp peaks corresponding to the binding energy of

core-level electrons. These binding energies can be used to identify the chemical

constituents of materials to a depth of 50 Å, and can distinguish between oxidation states

of the elements analyzed.

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189

Since these hydrophilic-hydrophobic multiblock copolymers were cast on clean

glass to form films, one may wonder if there is any element enrichment on either side of

the films. Thus, XPS was utilized to examine both the air side and glass side of the films.

Table 9 summarizes the XPS data of a PAES-10k-PBP-10k multiblock copolymer.

Basically, atomic % of sulfur only reflects the PAES hydrophilic blocks enrichment at the

surface because of its only source is from the sulfones and sulfuric acids. However,

atomic % values at each side are quite close to each other, meaning no significant

element difference between air and glass sides of the films. This result also suggests that

since there is no real difference between each side of the membrane, it does not matter

which side to put in when these films are made into MEA to test in real fuel cell

instrument.

Table 9 XPS data of a PAES-PBP multiblock copolymer

C (1s)

(Atomic %)

O (1s)

(Atomic %)

S (2p)

(Atomic %)

Air-side 86.5 12.7 0.9

Glass-side 88.1 11.1 0.8

3.4.4.8 Proton conductivity and water uptake

The hydrophilic-hydrophobic nanophase separation morphology is considered to

be particularly good for PEM materials because it may increase the water self diffusion

coefficient, which may facilitate proton transportation at even low humidities.

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190

Recently, Roy and McGrath 135 investigated of the effect of chemical composition,

morphology, and ion exchange capacity (IEC) on the transport properties of proton

conducting membranes. The self diffusion coefficient of water was measured by pulsed-

field gradient spin echo nuclear magnetic resonance (PGSE NMR) technique.255-261 They

found that both proton conductivity and self diffusion coefficient of water increased with

an increase in block length and irrespective of the IEC values. This clearly emphasizes

the importance of connectivity between the hydrophilic domains on the transport

properties.

Since protons are able to migrate through these continuous hydrophilic channels

through the membrane with less hindrance by hydrophobic domains, theoretically,

smaller amounts of water (lower relative humidities) would be required to assist the

proton transport. Moreover, extensive swelling, which is very normal for most ion

containing hydrocarbon random copolymers, can be minimized.

This series of multiblock copolymers showed surprising low water uptake values

of only 7%~10% (Figure 87). These values are not only lower than most ion-containing

random copolymers such as poly(arylene ether sulfone)s (BPSH-series, where BP stands

for biphenol, S is for sulfonated, and H denotes the proton form of the acid)89, but even

lower than highly hydrophobic perfluorinated copolymers like Nafion®. The reason

behind this interesting phenomenon is not yet clear, but one possible explanation is the

rigid hydrophobic phase may be important.

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191

38

21

7 7

10

0

5

10

15

20

25

30

35

40

Wat

er u

ptak

e %

BPSH 35 Nafion 112 Multiblock(PAES-3k-PBP-3k)

Multiblock(PAES-6k-PBP-6k)

Multiblock(PAES-10k-PBP-10k)

BPSH 35Nafion 112 Multiblock (PAES-3k-PBP-3k) Multiblock (PAES-6k-PBP-6k) Multiblock (PAES-10k-PBP-10k)

Figure 87 Comparison of water uptake of BPSH 35, Nafion, and PAES-PBP multiblock copolymers

The proton conductivities of these multiblock copolymer membrane samples were

measured under full hydrated condition in water at room temperature. The acid form

membranes showed conductivity of 0.03, 0.04 and 0.06 S/cm, for PAES-3k-PBP-3k (IEC

0.95 meq/g), PAES-6k-PBP-6k (IEC 1.05 meq/g), PAES-10k-PBP-10k (IEC 1.10 meq/g),

respectively (Table 10). These values are considered to be good considering low IEC and

very low uptake. Thus, these multiblock copolymers are promising PEM materials for

fuel cells. However, for all three multiblock copolymers the experimental IEC values

were lower than the theoretical values (1.57 meq/g) calculated for an ideal alternating

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192

multiblock, indicating that there was somewhat less than quantitative incorporation of

sulfonated telechelics.

Table 10 Characterization of PPP-PAES hydrophobic-hydrophilic multiblock copolymers

Multiblock

copolymers

Hydrophilic

block

length

Hydrophobic

block

length

Intrinsic

viscosity

(dl/g)a

Ion

exchange

capacity

(meq/g)b

Ion

exchange

capacity

(meq/g)c

Water

uptake

Proton

conductivity

(mS/cm)d

1 3000 3000 0.68 1.57 0.95 7% 30

2 6000 6000 0.72 1.57 1.05 7% 40

3 10,000 10,000 0.82 1.57 1.10 10% 60

a NMP with 0.05M LiBr as solvent at 25 oC

b. Target value

c Determined by back-titration of acid groups

d Measured in liquid water at 30 oC

Proton exchange membrane materials that enable higher temperature operation

would be preferred in vehicle applications because it demands smaller radiator, higher

CO tolerance and faster cathode reaction kinetics. For a higher temperature system to be

feasible, the membrane must have improved proton conductivity at low relative humidity

(RH) vs. current materials. For determining proton conductivity under partially hydrated

conditions, membranes were equilibrated in a relative humidity chamber (ESPEC SH-240)

at the specified relative humidity (RH) and 80 °C for 24 h before measurement.

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193

0.0001

0.001

0.01

0.1

1

30 40 50 70 90 100

Relative Humidity (%)

Prot

on C

ondu

ctiv

ity (S

/cm

)

Nafion 117BPSH 35Multiblock

Figure 88 Influence of relative humidity on proton conductivity for Nafion 117, BPSH 35 and

multiblock copolymer at 80 oC.

Figure 88 exhibits the plot of proton conductivities vs relative humidity (RH) for

three different PEM materials: Nafion 117, BPSH 35 and a PAES-6k-PBP-6k multiblock

copolymer. Three materials all show similar proton conductivity at high RH regions.

However, below 50% RH, the proton conductivity decreases significantly for the random

sulfonated aromatic copolymers—BPSH 35. This is mainly due to the discontinuous

morphological structure that limits the proton transport under partially hydrated

conditions.

For Nafion, the extent of decrease in conductivity at low RHs is not as severe as

that measured for random copolymers. This is related to the unique chemical structure of

the Nafion, which consists of highly flexible side chain hydrophilic sulfonic acid groups

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194

and a hydrophobic fluorinated flexible backbone. These chemical features apparently

promote strong nano-phase separation between distinctly hydrophilic and hydrophobic

domains even in random ionomers and allow proton transport between the interconnected

hydrophilic domains even at low hydration levels.

For PAES-6k-PBP-6k multiblock copolymer, proton conductivity shows much

less RH dependence than random sulfonated aromatic copolymers BPSH 35, almost same

or less RH dependence as Nafion with decreasing RHs. This phenomenon lies in the fact

that this multiblock copolymer possesses a co-continuous nanophase separated

morphology, as confirmed previously by AFM and DSC data. Since this unique co-

continuous morphology (interconnected channels and networks) dramatically facilitate

the proton transport (increase the diffusion coefficient of water), improved proton

conductivity under partially hydrated conditions becomes feasible.

However, the proton conductivities of these multiblock copolymers are still not as

good as Nafion, probably because the experimental IEC values are still pretty low, just

around 1.0 meq/g. Since the water uptake of this series of multiblock copolymers are

very low, we can push the IEC to a higher value to further increase the proton

conductivity. Therefore, future studies in multiblock copolymer systems with higher IEC

systems (1.5-2.0 meq/g), longer block length (15k-20k) and higher molecular weight are

needed.

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195

Chapter 4

Taken from Journal of Polymer Science, Part A: Polymer Chemistry, Accepted, Oct 2006

4 Multiblock Copolymers of Poly (2,5-benzophenone)

and Disulfonated Poly (arylene ether sulfone) for

Proton Exchange Membranes. I. Synthesis and

Characterization

Hang Wang, Anand S. Badami, Abhishek Roy, James E. McGrath∗

Macromolecules and Interfaces Institute, Virginia Polytechnic Institute and State

University, Blacksburg, VA 24061

∗ Corresponding author. Tel.: 540-2315976; fax: 540-2318517.

E-mail address: [email protected] (J.E. McGrath).

Attribution: Mr. Anand S. Badami did a great job on Atom Force Microscopy

experiment and Mr. Ahbishek Roy kindly helped me on proton conductivity

measurements. Professor James E. McGrath is my advisor and he gave me tremendous

help and guidance on this work.

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196

ABSTRACT: Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are

promising PEM materials due to their ability to form various morphological structures

which enhance transport. A series of poly(2,5-benzophenone) activated aryl fluoride

telechelic oligomers with different block molecular weights were successfully

synthesized by Ni (0)- catalyzed coupling of 2,5-dichloro-benzophenone and the end-

capping agent 4-chloro-4'-fluorobenzophenone. These telechelic oligomers (hydrophobic)

were then copolymerized with phenoxide terminated disulfonated poly (arylene ether

sulfone)s (hydrophilic) by nucleophilic aromatic substitution to form hydrophilic-

hydrophobic multiblock copolymers. High molecular weight multiblock copolymers

with number average block lengths ranging from 3,000 to 10,000 g/mol were successfully

synthesized. Two separate Tgs were observed via DSC in the transparent multiblock

copolymer films when each block length was longer than 6,000 g/mol (6k). Tapping

mode atomic force microscopy (AFM) also showed clear nanophase separation between

the hydrophilic and hydrophobic domains and the influence of block length, as one

increased from 6k to 10k. Transparent and creasable films were solvent-cast and

exhibited moderate proton conductivity and low water uptake. These copolymers are

promising candidates for high temperature proton exchange membranes in fuel cells,

which will be reported separately in part II of this series.

Keywords: proton exchange membrane; fuel cell; multiblock copolymers; phase

separation; morphology; poly(arylene ether sulfone); poly (p-phenylene) derivatives

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INTRODUCTION

Poly(p-phenylene)s (PPPs) and especially their derivatives continue to receive

much attention due to their excellent thermal, mechanical, and electrical properties.178,

184,185 However, high molecular weight PPPs derivatives are difficult to synthesize

mainly due to the poor solubility of the growing rigid-rod chains during polymerization.

This problem has been decreased by introducing pendent groups to the phenyl rings

which improve the solubility of PPP, while retaining many of the most useful

characteristics. Appropriately substituted PPPs are soluble and yet exhibit thermal and

mechanical properties approaching the unsubstituted polymer. Thus, poly(2,5-

benzophenone) (PBP) is known for its high degree of polymerization, good solubility in

dipolar aprotic solvents, and excellent thermal and mechanical properties.77, 183, 186 High

molecular weight PBP shows a glass transition temperature of 206 oC and 5% weight loss

temperatures in nitrogen and air of 526 and 520 oC, respectively. 5 In addition, the tensile

modulus of PBP was shown to be from two to four times greater than other high-

performance isotropic thermoplastics including polysulfone, poly(ether imide),

poly(ether ether ketone), poly(phenylene sulfide), and thermoplastic

polybenzimidazole.187 PBPs can also be processed by compression molding and

extrusion and are now commercially available under the trade name Parmax from Solvay

Advance Polymer Inc.

High molecular weight PBPs can be synthesized by the nickel-catalyzed

polymerization of 2,5-dichlorobenzophenone using the coupling method of Colon and

Kelsey.175 However, the inability to form good flexible films is a still major drawback of

most PPPs. Even amorphous, high molecular weight poly(2,5-benzophenone) suffers

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198

from this problem, possibly because of its stiff chain. The inability to form flexible,

creasable films from these stiff chains remains a major barrier in the testing and

development of PPP-based proton-exchange and gas separation membranes and in other

applications.

One alternative is to use a rigid-rod PBP difunctional telechelic oligomer,

combined with a flexible, random coil, thermally stable telechelic oligomer, to produce

thermally stable multiblock copolymers. This methodology was reported earlier by

Ghassemi 164 and Sheares 180 and flexible and transparent films have been formed from

these PBP based multiblock copolymers.

We are currently interested in the synthesis of potentially economical, and highly

thermooxidatively stable polymers as candidates for proton-exchange membranes (PEMs)

in fuel cells. PEMFCs have potential as energy conversion devices for both

transportation, mobile and stationary applications.191 The primary demands on the

hydrated PEM are high proton conductivity (≥0.1 S cm-1), low fuel and O2 permeability,

limited swelling in water and high chemical, thermal and mechanical stability.192

Conventional PEMFCs typically operate with Nafion® membranes29, which offer quite

good performance below 90 oC. However, today there is a strong need for PEMs capable

of sustained operation up to 120 oC to offer many benefits including increased resistance

to fuel impurities, most notably carbon monoxide (CO), fast electrode kinetics, and

simplified water/thermal management. Unfortunately, the proton conductivity of Nafion

suffers greatly at temperatures above 90 oC due to the depressed hydrated α-relaxation or

too restrained transport.18, 29 Also, the barrier properties of this membrane are poor when

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methanol is used as fuel. These factors, in addition to the high cost of Nafion, have

encouraged extensive research on alternative PEM materials. 9, 91, 136, 262, 263

Nanophase-separated hydrophilic-hydrophobic multiblock copolymers are

promising PEM materials, particularly due to their ability to form unique morphologies.

It is well known that the membrane morphology is important for the PEM fuel cell

performance, and depends strongly on the water content, and on the type concentration

and distribution of the acidic moieties.27 The unique morphologies of multiblock

copolymers may play an important role in providing good proton transport at low water

contents and high temperatures. 135

This paper reports the end-capping of 2,5-dichlorobenzophenone with 4-chloro-

4’-fluorobenzophenone via Ni (0)-catalyzed coupling reaction to produce difunctional

PBP oligomers with aryl fluoride endgroups, which are labile to subsequent nucleophilic

aromatic substitution. These functionalized PBP (hydrophobic) telechelic oligomers

were then combined with phenoxide terminated disulfonated wholly aromatic biphenyl

poly(arylene ether sulfone) oligomers (hydrophilic) to produce high molecular weight

multiblock copolymers capable of forming flexible films. Randomization by ether–ether

interchange reaction was obviously avoided. Morphology studies on these multiblock

copolymers as well as its relationship to proton conductivity and water uptake have been

initiated and are discussed.

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EXPERIMENTAL

Materials

All reagents were purchased from Aldrich and used without further purification

unless otherwise noted. N,N-Dimethylacetamide (DMAc) and N-methyl-2-pyrrolidione

(NMP) were dried over calcium hydride, distilled under vacuum and stored under

nitrogen before use. Tetrahydrofuran (THF) was distilled from sodium and benzophenone.

Then 2,2’-bipyridyl (bipy) and triphenylphosphine were purified by recrystallization from

ethanol and cyclohexane, respectively. Dichlorobis(triphenylphosphine)nickel-(II) was

stored under N2. Zn powder was washed with acetic anhydride, filtered, washed with dry

Et2O, and dried under vacuum at 150 °C. 4,4’-Dichlorodiphenylsulfone (DCDPS) and

4,4’-biphenol were obtained from Solvay Advanced Polymers and Eastman Chemical,

respectively. The 3,3’-disulfonate-4,4’-dichlorodiphenylsulfone (SDCDPS) was

synthesized from DCDPS according to a modified literature method,264 which has been

further developed in our laboratory.88 The 2,5-dichlorobenzophenone was prepared

according to a procedure reported in the literature.77

Characterization

1H and 13C NMR spectra were recorded on a Varian 400 spectrometer, using

CDCl3 and DMSO-d6 as solvents. Glass transition temperatures (Tg) were determined by

differential scanning calorimetry, using a TA Instrument DSC Q-1000 at a heating rate of

10 oC/min, under a stream of nitrogen. Second heat Tg values were reported as the

midpoints of the change in the slopes of the baselines. Thermogravimetric analyses (TGA)

were performed on a TA Instrument TGA Q-500 thermogravimetric analyzer. All the

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samples were first vacuum dried and kept in the TGA furnace at 150 oC in a nitrogen

atmosphere for 30 min to remove water before TGA characterization. The typical

heating rate was 10 oC/min in nitrogen. Atomic force microscopy (AFM) images were

obtained using a Digital Instruments MultiMode scanning probe microscope with a

NanoScope IVa controller (Veeco) in tapping mode. A silicon probe (Veeco) with an

end radius of <10 nm and a force constant of 5 N/m was used to image samples. The

ratio of amplitudes used in the feedback control was adjusted to 0.83 of the free air

amplitude for both the PAES-6k-PBP-6k and PAES-10k-PBP-10k samples. Samples

were dried under vacuum at 60 °C for 3 h and then equilibrated at 30% relative humidity

for at least 12 h before being imaged immediately at room temperature in a relative

humidity of approximately 15-20%.

Intrinsic viscosity measurements were carried out with a Cannon Ubbelholde

viscometer in NMP (containing 0.05 M LiBr) as solvent at 25 oC. The addition of LiBr is

well known to shield the polyions from intramolecular expansion. GPC experiments

were performed on a liquid chromatograph equipped with a Waters 1515 isocratic HPLC

pump, Waters Autosampler, Waters HR5-HR4-HR3 column, Waters 2414 refractive

index detector and Viscotek 270 right angle laser light scattering (RALLS)/viscometric

dual detector. NMP (containing 0.05 M LiBr) was used as the mobile phase. The column

temperature was maintained at 60 oC because of the viscous nature of NMP. Both the

mobile phase solvent and sample solution were filtered before introduction to the GPC

system.

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Water Uptake, Ion Exchange Capacity and Proton Conductivity

The membranes were vacuum-dried at 100 oC for 24 h, weighed and immersed in

deionized water at room temperature for 24 h to measure the water uptake. The wet

membranes were wiped dry and quickly weighed again. The water uptake of the

membranes is reported in weight percent as follows:

100uptakewater ×−

=dry

drywet

WWW

1 where Wwet and Wdry are the weights of the wet and dry membranes, respectively.

Ion Exchange Capacity (IEC) was determined by aqueous potentiometric

titrations using an MCI Automatic Titrator Model GT-05. The membranes were soaked

in deionized water containing sodium sulfate (1 M solution) for 24 h at room temperature

to form the salt form polymer and an acid form, water soluble compound (i.e., sodium

hydrogen sulfate). The solutions were titrated by standard sodium hydroxide solution

(0.01 M) to quantitatively determine sulfonic acid concentration in the sulfonated

polymers in terms of ion exchange capacity (IEC, mequiv/g).

Proton conductivity at 30 °C at full hydration (in liquid water) was determined in

a window cell geometry12 using a Solartron 1252+1287 Impedance/Gain-Phase Analyzer

over the frequency range of 10 Hz - 1 MHz. The cell geometry was chosen to ensure that

the membrane resistance dominated the response of the system. The resistance of the

film was taken at the frequency which produced the minimum imaginary response.265

The conductivity of the membrane can be calculated from the measured resistance and

the geometry of the cell according to:

AZl'

=σ 2

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203

where σ is the proton conductivity, l is the length between the electrodes, A is the cross

sectional area available for proton transport, and Z’ is the real impedance response.

For determining proton conductivity in liquid water, membranes were equilibrated at 30

°C in deionized water for 24 h prior to the testing.

Monomer synthesis

The 2,5-dichlorobenzophenone was prepared by Friedel–Crafts acylation of

benzene with 2,5-dichlorobenzoyl chloride and the yield was 80%. The monomer was

purified by recrystallization twice from ethanol/water (90/10, v/v) and dried under

vacuum at 50 oC for 24 h. mp 88–89 oC 1H NMR δ 7.80 (m, 2H), 7.62 (ddd, 1H), 7.48

(m, 2H), 7.42 (m, 2H), 7.36 (m, 1H); 13C NMR δ 193.6 (C=O), 139.9 (C1), 135,8 (C1’),

134.1 (C4), 132.9 (C2), 131.3 and 131.1 (C3, C6), 130.1 (C2’), 129.6 (C5), 128.9 (C4’),

128.8 (C3’).

Synthesis of telechelic poly(2,5-benzophenone) (PBP) oligomers

A 100-mL Schlenk flask was first charged with NiCl2(PPh3)2 (1.047 g, 1.6 mmol),

PPh3 (0.8387 g, 3.2 mmol) , Zn (3.2438 g, 49.6 mmol), 2,2’-bipyridyl (0.2504 g, 1.6

mmol) and a magnetic stir bar. The flask was sealed with a rubber septum, evacuated

under flame for 10 minutes, and placed under an N2 atmosphere by filling with N2

followed by three evacuation-fill cycles. Dry THF (10 mL) was added via syringe

through the rubber septum to initiate the reaction, which became deep red after 10

minutes. Then, 2,5-dichlorobenzophenone (3.7667 g, 15 mmol) and the end-capping

agent 4-chloro-4’-fluorobenzophenone (0.2346 g, 1 mmol) were charged under N2 flow.

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204

The mixture was stirred and heated at 65 oC for 24 h. After cooling to room temperature,

the reaction mixture was added into concentrated aqueous HCl/methanol (40/60 v/v)

solution and stirred overnight. The resulting precipitate was collected by filtration and

washed thoroughly with 10% sodium bicarbonate solution and deionized water. After

drying in a vacuum oven at 100 oC overnight, the light yellow functional oligomer (or

polymer) was isolated with a yield of 95%.

Synthesis of PAES telechelic oligomers and PAES-PBP multiblock copolymers

The aromatic nucleophilic reaction was conducted in a 3-neck flask equipped with

a mechanical stirrer, nitrogen inlet and a Dean-Stark trap. In a typical polymerization to

prepare 5,000 g/mol oligomer, biphenol (0.3742 g, 2 mmol), SDCDPS (0.8725 g, 1.776

mmol) and potassium carbonate (0.3179 g, 2.3 mmol) were added to the flask. Dry

NMP(10 mL) and toluene(5 mL) were added as the solvents. The reaction mixture was

heated under reflux at 150 oC for 4 h, which stripped off most of the toluene to dehydrate

the system. The temperature was then raised slowly to 190 oC for 36 h. The viscous

solution obtained was cooled to room temperature. Poly(2,5-benzophenone) telechelic

oligomer (1.1171 g, 0.223 mmol), potassium carbonate (0.069 g, 0.5 mmol), dry NMP (5

mL) and toluene (10 mL) were added into the same reaction flask under nitrogen. The

reaction mixture was heated to 150 oC and refluxed for 4 h. The toluene was removed to

dehydrate the system and the temperature was raised to 190 oC for another 48 h. The

viscous polymer solution was cooled to room temperature, diluted with NMP and filtered

to remove most of the salts. The block copolymer was isolated by precipitation in

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205

isopropanol. It was washed extensively with deionized water and chloroform and dried at

120 oC under vacuum for 24 h.

Membrane preparation

Membranes were prepared by dissolving the salt form copolymers in DMAc (10%

w/v). The solution was then filtered through a 0.45 µm TEFLON syringe filter and cast

onto a clean glass substrate. The membranes were slowly dried with a heat IR lamp for

24-48 h and then under vacuum at 100 °C for 24 h. The copolymer membranes in their

salt form were then converted to a corresponding acid form by boiling in 0.5 M sulfuric

acid solution for 2 h followed by immersion in boiling deionized water for 2 h.

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RESULTS AND DISCUSSION

Synthesis of Monomers and Oligomers

The monomer 2,5-dichlorobenzophenone was synthesized by an aluminum

chloride catalyzed Friedel–Crafts acylation of benzene with 2,5-dichlorobenzoyl

chloride.180 2,5-Dichlorobenzoyl chloride was prepared from 2,5-dichlorobenzoic acid

and thionyl chloride and was purified by fractional vacuum distillation. The monomer

2,5-dichlorobenzophenone structure was confirmed by 1H and 13C NMR.

The 2,5-dichlorobenzophenone can react readily via Ni(0)-catalyzed coupling to

form isomeric high molecular weight poly(2,5-benzophenone) (PBP) with excellent

thermal properties. Since aryl fluorides do not participate in the Ni(0)-catalyzed coupling

reaction, end capping of poly(2,5-benzophenone)s with 4-chloro-4’-fluorobenzophenone

will result in PBP chains that contain aromatic fluoride end groups.180 These aryl

fluorides activated by electron-withdrawing group (carbonyl group) can be used as

reactive sites for nucleophilic aromatic substitution block copolymerization. Furthermore,

the molecular weights of these oligomers are controlled by altering the amount of 4-

chloro-4’-fluorobenzophenone.

The end-capping agent 4-chloro-4’-fluorobenzophenone was synthesized by an

iron (III) chloride-catalyzed Friedel–Crafts acylation of chlorobenzene with 4-

fluorobenzoyl chloride. Sheares et al.180 and Ghassemi et al.164 had previously

demonstrated the use of this end-capping agent in the synthesis of substituted poly(2,5-

benzophenone) oligomers.

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Scheme 1 Synthesis of fluorine terminated poly(2,5-benzophenone) oligomers

ClCl

C O

Cl CO

F

THF, 60oC, 12h

C OOO

FF

+NiCl2(PPh3)2, Zn, PPh3, Bpy

n

The synthesis of fluorine-terminated PBP telechelic oligomers, shown in Scheme

1, was conducted using Ni (0)-catalyzed coupling method from 2,5-

dichlorobenzophenone and a controlled amount of the end-capping agent 4-chloro-4’-

fluorobenzophenone. The monomer and end-capping agent were added quickly under

nitrogen flow to the polymerization flask containing a mixture of nickel chloride, 2, 2’-

bypyridyl and triphenylphosphine in THF. Previously, Sheares et al. 177 found the use of

THF rather than amide solvents (DMF and NMP) yielded higher molecular weight

polymers with better film-forming properties and no reduction of the carbonyl groups

during the Ni(0)-catalyzed coupling reaction.

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208

Figure 89 13C NMR spectrum of a fluorine terminated poly(2,5-benzophenone) oligomer in CDCl3

The molecular weights of PBP oligomers were controlled by varying the end-

capping agent stoichiometry. 13C NMR spectra were used to determine the degree of

polymerization of the PBP telechelics. The peaks at 194.6 ppm and in the range of

198.3–196.2 ppm are assigned to carbonyl moieties in the end-capping agent c and in the

main chain d, respectively (Figure 89). The integrals of peaks 194.6 ppm and in the

range of 198.3–196.2 ppm are compared to calculate the degrees of polymerization of the

PBP telechelics. As summarized in Table 11, the experimental values from 13C NMR

agree well with the target values. Other prominent features in the 13C NMR spectra are

the signals from the ipso carbon a attached to the fluoride and its adjacent carbon b from

the end-capping agent. The ipso carbon a is assigned as a doublet at 165.0 ppm with a

d c a

b

C O

CO

CO

FF

n

ab

c

d

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209

coupling constant of 255.1 Hz. The adjacent carbon b signal is clearly visible as a doublet

at 115.3 ppm with a coupling constant of 23.1 Hz.

As expected, intrinsic viscosities η of this series PBP oligomers increase with

molecular weight (Table 11). Intrinsic viscosity η can be related to molecular weight by

the Mark-Houwink-Sakurada equation

[η] = KMva 

where Mv is viscosity average molecular weight; K and α are the Mark-Houwink

constant and the Mark-Houwink-Sakurada exponent. The plot of log [η] with log Mn

(Figure 90) gave an approximately straight line and the Mark-Houwink-Sakurada

exponent α of these rigid PBP oligomers was found to be 0.83. The data confirm that

the molecular weight control of this series of PBP oligomers was successful.

The thermal properties of these low molecular weight PBP telechelics were found

to be very robust. A steady increase in the glass transition temperature Tg was observed

from 151 to 187 °C, as molecular weight increased from 2200 to 9400 g/mol (Table 11).

The highest Tg at 187 °C is close to that found for the non-end-capped homopolymer,

namely 190 °C.

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210

y = 0.8253x - 3.5863

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1

Log Mn

Log [η]

Figure 90 Double logarithmic plot of [η] against Mn of fluorine terminated poly(2,5-benzophenone)

oligomers

Hydroxy (phenoxide) terminated sulfonated poly(arylene ether sulfone) (PAES)

telechelics were synthesized (Scheme 2) as hydrophilic blocks in multiblock copolymers.

The 4,4’-biphenol monomer was used in excess base on the total amount of disulfonated

4,4’-dichlorodiphenylsulfone (SDCDPS) to produce telechelics with the target number

average molecular weight of 3000, 6000 and 10,000 g/mol.

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211

Table 11 Characterization of fluorine terminated poly(2,5-benzophenone) oligomers

Polymer

Molar Fraction

of End-capping

Agent

Mn a

(g/mol)

Mn b

(g/mol)

Intrinsic

Viscosity c

[η] (dl/g)

Tg d

(oC)

1 0.20 2200 1980 0.16 151

2 0.10 4000 3800 0.21 163

3 0.07 5600 5500 0.32 167

4 0.05 7600 7000 0.46 176

5 0.04 9400 -- 0.48 187

6 0.00 -- -- 0.96 190

a Target molecular weight

b Determined from 13C NMR

c NMP as solvent at 25 oC

d Determined by DSC

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212

SClO

OCl

+NaO3S

SO3Na+

OHHO

SO

OMO3S

SO3M

OOH OH

BPS100

(in excess)

K2CO3

NMP/Toluene Reflux 150oC for 4hrs190oC for 36 hrs under N2

n

+

O

endgroups were acidified byacetic acid

Scheme 2 Synthesis of hydroxy terminated disulfonated poly(arylene ether sulfone) oligomers

1H NMR (Figure 91) was employed to determine number-average molecular

weight Mn by comparing integrals of the aromatic protons of the biphenol moiety in the

terminal group c relative to those in repeat units a. Peaks at 7.71 ppm and 6.84 ppm are

assigned to c and a, respectively. As shown in Table 12, for this series of PAES

oligomers, Mn determined by proton NMR agree well with the target values.

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213

OO S

O

O

OHH

n

SO3K

KO3S

b a a b e f

g

g

f e

h i d cj

Figure 91 Proton NMR spectrum of a phenol terminated disulfonated poly(arylene ether sulfone)

oligomer

Table 12 Properties of hydroxy terminated poly(arylene ether sulfone) oligomers

Polymer

Molar Excess of

Biphenol

Monomer (%)

Mn a

(g/mol)

Mn b

(g/mol)

Intrinsic

Viscosity c

[η] (dl/g)

1 22.6 3000 3100 0.15

2 10.9 6000 5900 0.24

3 6.40 10,000 9900 0.35

a Target molecular weight

b Determined from proton NMR

c NMP with 0.05M LiBr as solvent at 25 oC

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214

Synthesis of PAES-PBP Multiblock Copolymers

In general, one of the drawbacks of poly(2,5-benzophenone) (PBP) is their

inability to form flexible films. On the other hand, PAES forms excellent films but lacks

the high modulus and strength of PBP. To develop more flexible, film-forming PBP-base

materials, hydroxy terminated sulfonated PAES telechelics (hydrophilic) prepared above

were copolymerized with fluorine terminated PBP telechelics (hydrophobic) via

nucleophilic aromatic substitution (Scheme 3).89 The stoichiometry of the

copolymerization was based on a 1:1 molar ratio between functional end groups of

hydrophilic (PAES)/hydrophobic (PBP) oligomers prepared above. Randomization,

which is to be expected in the high temperature poly(arylene ether) multiblock

copolymers forming step, was avoided since there is no ether–ether interchange possible

in the PBP blocks. By varying the block length of hydrophilic (PAES) /hydrophobic

(PBP) telechelics from 3,000 to 10,000 g/mol, three multiblock copolymers: PAES-3k-

PBP-3k, PAES-6k-PBP-6k and PAES-10k-PBP-10k were synthesized. They all showed

reasonable high intrinsic viscosity η from 0.68 to 0.82 dl/g in NMP (0.05 M LiBr) and

formed transparent, creasable films. All these results strongly indicate high molecular

weight PAES-PPP multiblock copolymers were successfully synthesized.

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215

K2CO3

OOFF

C O

PBP

SO

OO

MO3S

SO3M

OO OKK

BPS100

SO

OO

MO3S

SO3M

OO OOO

C O

m

NMP/Toluene

150oC, 4h190oC, 48h

n+

x n

Hydrophilic Hydrophobic

+ KFy

Scheme 3 Synthesis of a PBP-PAES hydrophobic-hydrophilic multiblock copolymer

Table 13 Characterization of PPP-PAES hydrophobic-hydrophilic multiblock copolymer

Multiblock

copolymers

Hydrophilic

block

length

Hydrophobic

block

length

Intrinsic

viscosity

(dl/g)a

Ion

exchange

capacity

(meq/g)b

Ion

exchange

capacity

(meq/g)c

Water

uptake

Proton

conductivity

(mS/cm)d

1 3000 3000 0.68 1.57 0.95 7% 30

2 6000 6000 0.72 1.57 1.05 7% 40

3 10,000 10,000 0.82 1.57 1.10 10% 60

a NMP with 0.05M LiBr as solvent at 25 oC

b. Target value

c Determined by back-titration of acid groups

d Measured in liquid water at 30 oC

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216

Morphology

As is well known, multiblock copolymers consist of chemically distinct blocks

covalently linked to form a single molecule. Owing to mutual repulsion between

dissimilar blocks, they tend to segregate into separate nanophases. However, because

each block is chemically joined, macroscopic phase separation is prevented. Instead,

nano or nanophase separation takes place. 138 As a result, two separate glass transition

temperatures (Tgs) may be observed by methods such as DSC. Figure 92 shows the DSC

thermogram of a PAES-6k-PBP-6k multiblock copolymer with block lengths of

approximately 6000 g/mol. Two glass transition temperatures (Tgs) were observed: one

is 174 oC, which assigned to the Tg of hydrophobic PBP segment; the other is around

224oC, which corresponded to the Tg of hydrophilic disulfonated PAES block. This

suggests nanophase separation happened due to the immiscibility of the PBP and PAES

blocks. This is expected since PBP and PAES are quite different in terms of chemical

structures: PBP has very rigid polymer chain while PAES backbone is much more

flexible because of the arylene-ether bonds. This two Tgs phenomenon was also observed

in a PAES-10k-PBP-10k sample (Tgs: 180oC and 225oC) but only one Tg (225oC) was

found in a PAES-3k-PBP-3k multiblock copolymer. This is expected since longer blocks

should increase the immiscibility between dissimilar blocks, favoring the nanophase

separation in the multiblock systems. 138

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217

Temperature (oC)

Heat Flow

(W/g)

Figure 92 DSC thermogram of PPP-PAES hydrophobic-hydrophilic multiblock copolymer with

block lengths of approximately 6000 g/mol showing two Tgs

Tapping mode AFM images were obtained under ambient conditions on a 500 nm

× 500 nm size scale to investigate nanophase separation for multiblock copolymers of

PAES-6k-PBP-6k and PAES-10k-PBP-10k (Figure 93). The dark regions of the phase

images are assigned to the softer blocks, which represent the hydrophilic ionic groups

(sulfonic acid) containing small amounts of water. The light regions are related to hard

segments, derived from the hydrophobic hydrocarbon units. It can be clearly seen that

for the PAES-6k-PBP-6k multiblock sample, the dark regions (hydrophilic domains) are

connected to each other and form channels and networks that extend through the whole

sample surface; while light regions (hydrophobic domains) are divided into small

domains, which have diameters of 20 to 40 nm. This nanophase separation phenomenon

develops further in the PAES-10k-PBP-10k sample, where even the light regions

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(hydrophobic domains) start to aggregate too, forming bigger and longer hydrophobic

continuous phases. This again confirms that nanophase separation took place in these

hydrophilic-hydrophobic multiblock copolymers and is considered significant in terms of

the transport properties.135

Figure 93 Tapping mode AFM phase images of (a) a PAES-6K-PBP-6K multiblock copolymer, and

(b) a PAES-10K-PBP-10K multiblock copolymer with block lengths of approximately 6000 and

10,000 g/mol, respectively.(Scan sizes are 500 nm; Z ranges for phase images are 10 o)

Ion exchange capacity, Water uptake and Proton conductivity

The hydrophilic-hydrophobic nanophase separation morphology is considered to

be particularly good for PEM materials because it may increase the water self diffusion

coefficient, which may facilitate proton transportation at even low humidities.135 Since

protons are able to migrate through these continuous hydrophilic channels through the

membrane with less hindrance by hydrophobic domains, theoretically, smaller amounts

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219

of water (lower relative humidities) would be required to assist the proton transport.

Moreover, extensive swelling, which is very normal for most ion containing hydrocarbon

random copolymers, can be minimized. This series of multiblock copolymers showed

surprising low water uptake values of only 7%~10% (Table 13). These values are not

only lower than most ion-containing random copolymers such as poly(arylene ether

sulfone)s (BPSH-series, where BP stands for biphenol, S is for sulfonated, and H denotes

the proton form of the acid)89, but even lower than highly hydrophobic perfluorinated

copolymers like Nafion®. The reason behind this interesting phenomenon is not yet clear,

but one possible explanation is the rigid hydrophobic phase may be important.

The proton conductivities of these multiblock copolymer membrane samples were

measured under full hydrated condition in water at room temperature. The acid form

membranes showed conductivity of 0.03, 0.04 and 0.06 S/cm, for PAES-3k-PBP-3k (IEC

0.95 meq/g), PAES-6k-PBP-6k (IEC 1.05 meq/g), PAES-10k-PBP-10k (IEC 1.10 meq/g),

respectively. These values are considered to be good considering low IEC and very low

water uptake. Thus, these multiblock copolymers are promising PEM materials for fuel

cells. However, for all three multiblock copolymers the experimental IEC values were

lower than the theoretical values (1.57 meq/g) calculated for an ideal alternating

multiblock, indicating that there was somewhat less than quantitative incorporation of

sulfonated telechelics. Future studies will investigate higher IEC systems, longer block

length and higher molecular weight.

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CONCLUSIONS

Multiblock copolymers derived from fluorine-terminated poly(2,5-benzophenone),

as the hydrophobic blocks, and hydroxyl functional sulfonated poly(arylene ether sulfone)

as hydrophilic blocks were successfully prepared by nucleophilic step copolymerization.

Transparent and creasable films were successfully solvent-cast from these PAES-PBP

multiblock copolymers. Two separate Tgs were observed via DSC for multiblock

copolymers when the block length was longer than 6,000 g/mol. Tapping mode AFM

studies also showed clear nanophase separation between hydrophilic and hydrophobic

domains. This series of PAES-PBP multiblock copolymers showed moderate

conductivities up to 0.06 S/cm with very low water uptake of 7-10%. Therefore, they are

considered to be promising PEM materials in fuel cells.

ACKNOWLEDGEMENTS

The authors would like to thank the National Science Foundation “Partnership for

Innovation” Program (HER-0090556) and the Department of Energy (DE-FG36-

06G016038) for support of this research effort.

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Chapter 5

5 Summary and Conclusions

This research focused on synthesis and characterization of poly (2,5-

benzophenone) (PBP) and Disulfonated Poly (arylene ether sulfone) (PAES) multiblock

copolymers for Proton Exchange Membranes in fuel cells. The main objective of this

research was to synthesize nanophase-separated multiblock copolymers containing

hydrophilic and hydrophobic blocks and study the effect of chemical composition and

block length on morphology, water uptake and proton conductivity. The thermal

stabilities of these multiblock copolymers were also evaluated.

Four arylene chlorides monomers (2,5-Dichlorobenzophenone and its derivatives)

were first synthesized from aluminum chloride-catalyzed, Friedel-Crafts acylation of

benzene and other aromatic compounds with 2,5-dichlorobenzoyl chloride. All four

monomers were further purified to yield high purity monomers in high yields. The

benzoyl pendant group is known to increase the solubility of the wholly aromatic poly(p-

phenylene) material, while maintaining the outstanding thermal and mechanical

properties of the conjugated backbone.

These monomers were then polymerized via Ni (0)-catalyzed coupling reaction to

form various substituted poly(2,5-benzophenone)s. It was very efficient to use dry

tetrahydrofuran (THF) as solvent with the catalyst mixture as: NiCl2(PPh3)2 (0.1 equiv),

PPh3 (0.2 equiv) , Zn (3.1 equiv), 2,2’-bipyridyl (0.1 equiv) and dichloride monomer ( 1.0

equiv) in Ni (0)-catalyzed polymerization. As a result, a series of amorphous substituted

poly(2,5-benzophenone)s with intrinsic viscosity as high as 0.96 dL/g were successfully

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222

synthesized and they all showed excellent thermal stabilities. Great care must be taken to

minimize the content of water and oxygen during the reaction.

Then, a series of poly(2,5-benzophenone) activated aryl fluoride telechelic

oligomers with different block molecular weights were successfully synthesized by Ni

(0)- catalyzed coupling of 2,5-dichloro-benzophenone and the end-capping agent 4-

chloro-4'-fluorobenzophenone. The molecular weights of these oligomers are controlled

by altering the amount of 4-chloro-4’-fluorobenzophenone. These telechelic oligomers

(hydrophobic) were then copolymerized with phenoxide terminated disulfonated poly

(arylene ether sulfone)s (hydrophilic) by nucleophilic aromatic substitution to form

hydrophilic-hydrophobic multiblock copolymers.

High molecular weight multiblock copolymers with number average block

lengths ranging from 3,000 to 10,000 g/mol were successfully synthesized. Two separate

Tgs were observed via DSC in the transparent multiblock copolymer films when each

block length was longer than 6,000 g/mol (6k). Tapping mode atomic force microscopy

(AFM) also showed clear nanophase separation between the hydrophilic and hydrophobic

domains and the influence of block length, as one increased from 6k to 10k. Transparent

and creasable films were solvent-cast and exhibited moderate proton conductivity and

low water uptake. These PAES-PBP multiblock copolymers also showed much less

relative humidity (RH) dependence than random sulfonated aromatic copolymers BPSH

35 in proton conductivity, almost the same as Nafion with decreasing RHs. This

phenomenon lies in the fact that this multiblock copolymer possesses a co-continuous

nanophase separated morphology, as confirmed previously by AFM and DSC data. Since

this unique co-continuous morphology (interconnected channels and networks)

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223

dramatically facilitate the proton transport (increase the diffusion coefficient of water),

improved proton conductivity under partially hydrated conditions becomes feasible.

These multiblock copolymers are therefore considered to be promising candidates for

high temperature proton exchange membranes in fuel cells.

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224

Chapter 6

6 Suggested Future Research

Results from previous chapters demonstrated that nano-phase separated

hydrophilic-hydrophobic multiblock copolymers are promising candidates for high

temperature PEMFCs. They exhibit low water absorption and much less relative

humidity (RH) dependence than random sulfonated aromatic copolymers BPSH 35 in

proton conductivity, almost the same as Nafion with decreasing RHs. This phenomenon

lies in the fact that this multiblock copolymer possesses a co-continuous nanophase

separated morphology, which dramatically facilitates the proton transport (increase the

diffusion coefficient of water) under partially hydrated conditions.

However, the proton conductivities of these multiblock copolymers are still not as

good as Nafion, probably because the experimental IEC values are still pretty low (1.0

meq/g). Therefore, future studies in multiblock copolymer systems with higher IEC

systems (1.5-2.0 meq/g), longer block length (15k-20k) and higher molecular weight are

highly recommended.

Another approach of synthesizing segmented multiblock copolymers is to utilize

two monomers and one oligomer to prepare segmented copolymers. That is, instead of

using one PAES telechelic oligomer and one PBP telechelic oligomer, one can use two

monomers, BP and SDCDPS, directly react with a PBP telechelic oligomer to synthesize

segmented copolymers (Figure 94). This method has been used widely in preparing

polyurethane segmented copolymers.266, 267 The advantage of this method is that it not

only cuts one step of preparing PAES a telechelic oligomer, but also makes it much easier

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225

to synthesize high molecular weight segmented copolymers since stoichiometry is not

very critical for this method. Recall that in the synthesis of multiblock copolymers one

must keep the stoichiometry of each telechelic oligomers to be close to 1:1 to ensure high

molecular weight multiblock copolymer. This requirement is not that serious here in the

segmented copolymer synthesis because the concentration of functionality in the

telechelic oligomer is very small compared with two monomers. Therefore, controlling

the stoichiometry is much easier and simpler than in the synthesis of multiblock

copolymer.

Figure 94 Synthetic scheme of PAES-PBP segmented copolymers

The resultant segmented copolymers should have the similar nanophase

separation behavior as multiblock copolymers. Thus, one might expect that segmented

copolymer membrane would as well have good properties such as low water absorption

OHHO

OO

FF

C O

PBP Oligomers

m

K2CO3DMAc/Toluene Reflux 150oC for 4 h

SO

OO

MO3S

SO3M

OO OOO

C O

n m

S

O

ONaO3S

SO3Na

Cl Cl

+

165 o C72 h

Segmented Copolymers

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226

and low relative humidity (RH) dependence on conductivity. This segmented copolymer

research has been initiated in McGrath group at Virginia Tech and we have prepared a

few segmented copolymer samples. Further studies should continue to test whether this

approach would be better than multiblock copolymer method.

Recently, Zhang et al.268 reported using sulfonated poly(p-phenylene)s (SPPs) as

PEM and proton conductivity as high as 0.34 S/cm was reached at 120 oC. Sulfonated

poly(p-phenylene)s were directly synthesized by Ni(0) catalytic coupling of sodium 3-

(2,5-dichlorobenzoyl) benzenesulfonate and 2,5-dichlorobenzophenone (same monomer

we used in hydrophobic blocks). Although no molecular weight data was reported, the

sulfonated copolymers displayed excellent film forming abilities and mechanical

properties due to the increased interaction between chains induced by the --SO3H groups.

Significant hydrophilic/hydrophobic microphase-separated structures were observed for

these membranes, which were favorable for water keeping, proton transport and

limitations of swelling. This is very interesting and more research on synthesis of

sulfonated poly(p-phenylene)s direct from sulfonated dichloride monomers by Ni (0)

catalyzed coupling reaction are also needed.

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