The Influence of Aromatic Disulfonated Random and Block ......Exchange Membranes for Fuel Cells...

The Influence of Aromatic Disulfonated Random and Block Copolymers’ Molecular Weight, Composition,

and Microstructure on the Properties of Proton Exchange Membranes for Fuel Cells

Yanxiang Li

Dissertation submitted to the faculty of 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. Larry Taylor

Dr. John G. Dillard

Dr. Richey M Davis

August 31, 2007 Blacksburg, Virginia

Keywords: proton exchange membrane, fuel cell, disulfonated copolymers, molecular weight, random, multiblock, morphology, poly(arylene

ether sulfone), poly(arylene ether ketone)

Copyright 2007, Yanxiang Li

The Influence of Aromatic Disulfonated Random and Block Copolymers’ Molecular Weight, Composition,

and Microstructure on the Properties of Proton Exchange Membranes for Fuel Cells

Yanxiang Li

(ABSTRACT)

The purity of the disulfonated monomer, such as 3,3’-disulfonated-4,4’-

dichlorodiphenyl sulfone (SDCDPS), was very important for obtaining high molecular

weight copolymers and accurate control of the oligomer’s molecular weight. A novel

method to characterize the purity of disulfonated monomer, SDCDPS, was developed by

using UV-visible spectroscopy. This allowed for utiliziation of the crude SDCDPS

directly in the copolymerization to save money, energy, and time.

Three series of tert-butylphenyl terminated disulfonated poly(arylene ether sulfone)

copolymers (BPSH35, 6FSH35, and 6FSH48) with controlled molecular weights（ nM ）,

20 to 50 kg·mol-1, were successfully prepared by the direct copolymerization method. The

molecular weight of the copolymer was controlled by a monofunctional monomer tert-

butylphenyl, and characterized by the combination of 1H NMR spectra and modified

intrinsic viscosity measurements in NMP with 0.05 M LiBr, which was added to suppress

the polyelectrolyte effect. The mechanical properties of the membranes, such as the

modulus, strength and elongation at break, were improved by increasing the molecular

weights, but water uptake and proton conductivities found insensitive to copolymers’

molecular weights.

iii

Three series of disulfonated poly(arylene ether ketone) random copolymers have

been synthesized and comparatively studied, according to their different chemical

structures, for use as proton exchange membranes. The copolymers containing more

flexible molecular structures had higher water uptake and proton conductivity than the

rigid structures at the same ion exchange capacity. This may be due to the more flexible

chemical structures being able to form better phase separated morphology and higher

hydration levels.

A new hydrophobic-hydrophilic multiblock copolymer has been successfully

synthesized based on the careful coupling of a fluorine terminated poly(arylene ether

ketone) (6FK) hydrophobic oligomer and a phenoxide terminated disulfonated

poly(arylene ether sulfone) (BPSH) hydrophilic oligomer. AFM images and the water

diffusion coefficient results confirmed that the multiblock copolymer formed better proton

transport channels. This multiblock copolymer showed comparable proton conductivity

and fuel cell performance to the Nafion® control and had much better proton transport

properties than random ketone copolymers under partially hydrated conditions. This

suggested that the multiblock copolymers are promising candidates for proton exchange

membranes especially for applications at high temperatures and low relative humidity.

iv

Dedicated to my loving husband, Jun and dear son Dachuan for their understanding, support, and love

v

ACKNOWLEDGEMENTS

I would first like to give my sincere gratitude to my advisor, Dr. James E. McGrath,

for providing me with the opportunity of being his student, which I feel really lucky and

proud of. I especially thank him for his guidance, encouragement, and patience over my

graduate years. I have greatly benefited not only from his breadth and depth of knowledge,

but also from his wonderful personality, which will continue to be a great source of

inspiration for me. I would also like to thank the members of my advisory committee, Dr.

Judy S. Riffle, Dr. John G. Dillard, Dr. Richey M Davis, and Dr. Larry Taylor for their

valuable suggestions and support. A special thanks goes to Dr. Judy. S. Riffle for her

excellent classes, entitled Presentation Skills and Written Skills. I also appreciate Dr.

Richey M Davis very much for his advice and helpful discussions.

I would like to give a big thanks to our wonderful secretarial ladys, Mrs. Laurie

Good, Mrs. Millie Ryan, and Mrs. Angie Flynn, for their kindness and tremendous

assistance during my stay at Virginia Tech.

Many thanks go to my colleagues in our great research group for their selfless

assistance and friendship: Ahbishek Roy, Anand S. Badami, Juan Yang, Hang Wang, Dr.

William Harrison, Dr. Melinda Einsla, Dr. Brian Einsla, Dr. Charles Tchatchoua, Dr.

Thekkekara Mukundan, Dr. Kent Wiles, Dr. Zhongbiao Zhang, Natalie Arnet, Xiang Yu,

Haeseung Lee, Rachael VanHouten, Ozma Lane, and Dr. Guangyu Fang.

I especially wish to express my acknowledgments to Tom Glass, Anand S.

Badami, Juan Yang, Ahbishek Roy, and Ozma Lane, for their assistance in NMR, AFM,

GPC, and PEM properties measurements. A special thanks also goes to Mrs. Lauie Good,

vi

Dr. Melinda Einsla, Rachael VanHouten, Anand S. Badami, and Ozma Lane for helping

me with preprints, manuscripts, and presentations. I am really grateful to all of them.

Fianally, I would like to thank all my family members. My parents, Mr. Shuhe Li

and Mrs. Gonghua Xu, My sister Yanhong Li, and my brother Zhigang Li are always there

to support me. Many thanks are given to my parents-in-law, Mr. Tingxi Yan and Mrs

Ruiping Hou, family of my sister-in-law Hong Yan, Gang Zhou, and my lovely niece

Tianyi Zhou for their help in taking care of my son, which makes it possible for me to

focus on my Ph.D. study. I sincerely thank my husband and my best friend, Jun Yan, for

his love, understanding, patience, and encouragement, without which I would not have

succeeded this far. Most importantly, I would like to thank our dear son, Dachuan Yan.

His bright smile was always the source of my energy, and he was there to remind me often

of what was most important in life.

vii

ATTRIBUTION

Several colleagues and coworkers aided in the writing and research behind several

of the chapters of this dissertation. A brief description of their contributions is included

here.

Prof. James E. McGrath is the primary advisor and committee chair. Prof.

McGrath gave the author tremendous help and guidance on this work.

Rachael VanHouton helped the author with some UV-vis measurements in

chapter 3.

Andrew Brink provided crude SDCDPS samples in chapter 3.

Dr. Feng Wang contributed to the discussion of the synthesis of controlled

molecular weight copolymers in chapter 4

Juan Yang helped the author with GPC and IV measurements and discussion in

chapter 4, 5, and 7.

Dan Liu, Prof. Scott Case, and Prof. Jack Lesco contributed to chapter 4 in

mechanical measurements.

Abhishek Roy aided in the discussion and measurements of proton exchange

membrane properties in chapter 4, 5, 6, and 7.

Anand Badami helped with atom force microscopy measurements in chapter 5, 6,

and 7.

Dr. Melinda Einsla contributed to chapter 6 in discussing the synthesis of

poly(arylene ether ketone)s and chapter 7 in fuel cell performance measurements.

Ozma Lane helped with the proton conductivity measurements in chapter 5.

Dr. Thekkekara Mukundan contributed to the discussion of poly(arylene ether

ketone) synthesis in chapter 6.

Stuart Dunn aided in the measurement of PEM properties in chapter 7.

viii

TABLE OF CONTENTS

ABSTRACT ....................................................................................................................... ii

ACKNOWLEDGEMENTS.................................................................................................v

ATTRIBUTION ............................................................................................................... vii

TABLE OF CONTENTS ................................................................................................ viii

LIST OF FIGURES......................................................................................................... xiii

LIST OF TABLES......................................................................................................... xviii

Chapter 1. Research Significance and Objectives ..............................................................1

Chapter 2. Literature Review...............................................................................................3

2.1. Proton Exchange Membrane Fuel Cells (PEMFCs).....................................................3

2.1.1. Fuel Cell Introduction........................................................................................3

2.1.2. Types of Fuel Cells............................................................................................5

2.1.3. Proton Exchange Membrane Fuel Cells (PEMFCs) .........................................6

2.1.3.1. Hydrogen/Air PEM Fuel Cells…………………………………………6

2.1.3.2. Direct Methanol Fuel Cells (DMFCs)………………………………….8

2.1.3.3. Requirements for PEM Materials………………………………………9

2.1.3.4. Laboratory Evaluation of PEM Materials…………………………….10

2.2. Nafion® and the Other Perfluoropolymers .................................................................13

2.3. Polystyrene Type PEM Materials...............................................................................16

2.4. Partially- or Non-Fluorinated Copolymers with Aromatic Backbones ......................21

2.4.1. Poly(Arylene Ethers) .......................................................................................22

2.4.1.1. Synthetic Routes of Poly(Arylene Ethers)……………………………23

2.4.1.2. Molecular Weight Control and Characterization……………………..30

2.4.1.3. “Post” vs. “Direct” Sulfonation Methods…………………………….33

2.4.1.4. Sulfonated Poly(Arylene Ether Sulfone)s and Poly(Arylene Ether

Ketone)s via the Post Modification Method…………………………………..37

2.4.1.5. Direct Sulfonation of Poly(Arylene Ether Sulfone)s and Poly(Arylene

Ether Ketone)s………………………………………………………………...45

2.4.1.6. Other Poly(Arylene Ethers)…………………………………………..50

2.4.2. Sulfonated Polyimides (SPIs)………………………………………………..54

ix

2.4.3. Polyphosphazene Ionomers for PEMs.............................................................62

2.5. Other Novel Approaches to Improve PEM Properties ...............................................65

2.5.1. Controlling Morphology Using Block and Multiblock Copolymers...............66

2.5.2. Organic/Inorganic Composite PEMs...............................................................71

2.5.3. Polymer Blends................................................................................................75

References .................................................................................................................77

Chapter 3. Purity Characterization of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl Sulfone

(SDCDPS) Monomer by UV-visible Spectroscopy ..........................................................87

3.1. Abstract.......................................................................................................................88

3.2. Introduction ................................................................................................................89

3.3. Experimental...............................................................................................................93

3.3.1. Materials ..........................................................................................................93

3.3.2. Synthesis Procedures .......................................................................................93

3.3.2.1. Synthesis and Purification of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl

Sulfone (SDCDPS) monomer………………………………………………….93

3.3.2.2. Synthesis of Disulfonated Poly(Arylene Ether Sulfone) (BPSH) Model

Copolymers…………………………………………………………………… 94

3.3.3. Characterization...............................................................................................94

3.3.3.1. Monomer and Copolymer Characterization…………………………..94

3.3.3.2. Procedure for SDCDPS Monomer Purity Characterization by UV-

Visible Spectroscopy…………………………………………………………..95

3.4. Results and Discussion ...............................................................................................96

3.5. Conclusions ..............................................................................................................108

3.6. References ................................................................................................................109

Chapter 4. Synthesis & Characterization of Controlled Molecular Weight Disulfonated

Poly(Arylene Ether Sulfone) Copolymers and Their Applications to Proton Exchange

Membranes ......................................................................................................................111

4.1. Abstract.....................................................................................................................112

4.2. Introduction ..............................................................................................................113

4.3. Experimental.............................................................................................................115

4.3.1. Materials ........................................................................................................115

x

4.3.2. Synthesis of Disulfonated Poly(Arylene Ether Sulfone) Copolymers with

Controlled Molecular Weight..................................................................................116

4.3.3. Membrane Preparation ..................................................................................117

4.3.4. Characterization.............................................................................................117

4.4. Results and Discussion .............................................................................................119

4.4.1. Synthesis and Characterization of Copolymers.............................................119

4.4.2. Membrane Characterization ..........................................................................131

4.5. Conclusions ..............................................................................................................136

4.6. Acknowledgements ..................................................................................................136

4.7. References ................................................................................................................137

Chapter 5. Partially Fluorinated Disulfonated Poly(Arylene Ether Sulfone) Copolymers

with Controlled Molecular Weights for Proton Exchange Membranes ..........................139

5.1. Abstract.....................................................................................................................140

5.2. Introduction ..............................................................................................................141

5.3. Experimental.............................................................................................................145

5.3.1. Materials ........................................................................................................145

5.3.2. Copolymerization ..........................................................................................145

5.3.3. Membrane Preparation and Acidification......................................................146

5.3.4. Characterization…………………………………………………………….147


5.4.1. Synthesis and Characterization of Copolymers.............................................148

5.4.2. Characterization of Membranes.....................................................................156

5.5. Conclusions ..............................................................................................................160


5.7. References ................................................................................................................161

Chapter 6. Comparative Investigation of Three Series of Poly(Arylene Ether Ketone)

Copolymers for Proton Exchange Membrane Fuel Cells................................................163

6.1. Abstract.....................................................................................................................164

6.2. Introduction ..............................................................................................................165

6.3. Experimental.............................................................................................................169

6.3.1. Materials ........................................................................................................169

xi

6.3.2. Synthesis of the Disodium Salt of Comonomers...........................................169

6.3.3. Synthesis of Disulfonated Poly(Arylene Ether Ketone) Copolymers (B, PB and

MB) Based on Three Types of Ketone Monomers..................................................170


6.3.5. Characterization.............................................................................................171


6.4.1. Synthesis and Characterization of Disulfonated Monomers and Copolymers173

6.4.2. Morphology Characterization of the Membranes..........................................181

6.4.3. Characterization of Membrane Properties.....................................................183

6.5. Conclusions ..............................................................................................................193


6.7. References ................................................................................................................194

Chapter 7. Synthesis and Characterization of Partially Fluorinated Hydrophobic -

Hydrophilic Multiblock Copolymers Containing Sulfonate Groups for Proton Exchange

Membrane........................................................................................................................196

7.1. Abstract.....................................................................................................................197

7.2. Introduction ..............................................................................................................198

7.3. Experimental.............................................................................................................202

7.3.1. Materials ........................................................................................................202

7.3.2. Synthesis of Fluorine Terminated Hydrophobic Oligomers..........................202

7.3.3. Synthesis of Multiblock Copolymers ............................................................203

7.3.4. Characterization of Oligomers and Multiblock Copolymers ........................204


7.3.6. Characterization of Membranes.....................................................................204

7.3.6. 1 Morphology Characterization by Atomic Force Microscopy (AFM).204

7.3.6.2. Ion Exchange Capacity (IEC) and Conductivity…………………….205

7.3.6.3. Water Uptake and Water Self-diffusion Coefficients……………….205

7.3.6.4. MEA Fabrication and Fuel Cell Testing…………………………….206

7.4. Results and Discussions............................................................................................207

7.4.1. Synthesis and Characterization of Oligomer and Multiblock Copolymer ....207

7.4.2. Morphology of Membranes ...........................................................................215

xii

7.4.3. Characterization of PEM Properties ..............................................................218

7.4.4. Fuel Cell Performance ...................................................................................226

7.5. Conclusions ..............................................................................................................228

7.6. Acknowledgments ....................................................................................................229

7.7. References ................................................................................................................230

Chapter 8. Overall Conclusions.......................................................................................232

xiii

LIST OF FIGURES

Figure 2.1. Schematic of a PEM Fuel Cell Operation .................................................. 7

Figure 2.2. Chemical Structure of Nafion® ............................................................... 14

Figure 2.3. Structure of Dais Styrene–Ethylene/Butylene–Styrene (SEBS) Triblock

Copolymer Electrolyte........................................................................................ 17

Figure 2.4. Molecular Structure of Ballard Advanced Materials Corp’s BAM3G .... 18

Figure 2.5. Synthetic Scheme of Polystyrene-g-Poly(sodium styrenesulfonate) ....... 20

Figure 2.6. Chemical Reactions of PSEBS Photocrosslinking Using Benzonphenone

as Initiator. ......................................................................................................... 21

Figure 2.7. Several Possible Poly(arylene ether) Chemical Structures ................... 23

Figure 2.8. Nucleophilic Aromatic Substitution Mechanism..................................... 24

Figure 2.9． The Electrophilic Substitution Mechanism........................................... 29

Figure 2.10. Synthesis of Tert-Butylphenyl Terminated Poly(Arylene Ether Sulfone)

Copolymers Containing 35 mol% Disulfonated Repeat Unit. ........................... 32

Figure 2.11. Stress-strain Curves of BPSH35 Copolymers (Thin Films) as a Function

of Molecular Weight........................................................................................... 33

Figure 2.12. Direct Copolymerization of sulfonated Monomers versus Post

Sulfonation.......................................................................................................... 35

Figure 2.13. Synthesis of 3,3’-Disulfonated 4,4’-dichloro-diphenyl Sulfone in Its

Sodium Salt Form............................................................................................... 36

Figure 2.14. Direct Copolymerization of Wholly Aromatic Sulfonated Poly(arylene

ether sulfone) “BPSH-xx,” where xx is the ratio of sulfonated to unsulfonated

activated halide. .................................................................................................. 37

Figure 2.15. Sulfonated Poly(ether sulfone) (Udel®) and Poly(ether ether ketone)

(Victrex®)............................................................................................................ 38

Figure 2.16. Synthesis of Poly(arylene ether) Ionomers Containing Sulfofluorenyl

Groups via a Post-modification Method............................................................. 41

Figure 2.17. Synthesis of Sulfonated Poly(ether sulfone) Udel® PSU via the

Metalation Route ................................................................................................ 42

xiv

Figure 2.18. Crosslinking via the Metalation Route................................................... 43

Figure 2.19. Influence of the Dgree of Sulfonation on the Water Uptake of BPSH

Copolymers......................................................................................................... 48

Figure 2.20. Four Investigated Bisphenol Structures ................................................. 49

Figure 2.21. Several investigated sulfonated ketone or diketone structures .............. 50

Figure 2.22. Structure of Mono-Sulfonated BFPPO .................................................. 52

Figure 2.23. Post Modification of PPES and PPEK................................................... 53

Figure 2.24. Direct Synthesis of SPPEKs and SPPESs.............................................. 54

Figure 2.25. Chemical Structure of Sulfonated Polyimides Containing Fluorenyl

Groups ................................................................................................................ 57

Figure 2.26 Sulfonated Diamines .............................................................................. 59

Figure 2.27. Synthesis of Disulfonated Polyimide Copolymers ................................ 61

Figure 2.28. The Reaction Scheme for Sulfonation of Polyphosphazene with SO3 .. 63

Figure 2. 29. Direct Sulfonation of Polyphosphazenes by the Noncovalent Protection

Method................................................................................................................ 65

Figure 2.30. Synthetic Scheme of BisAF-BPSH Series of Multiblock Copolymers . 69

Figure 2.31. Proton Conductivity vs. Relative Humidity for “BisAF-BPSH” Series of

Multiblock Copolymers and Nafion® 117 .......................................................... 70

Figure 2.32 Chemical Structure of PPP/BPS Multiblock Copolymer........................ 71

Figure 2.33. Schematic View of the Increased Pathways of Composite Membrane.. 74

Figure 3.1. Synthetic Scheme of Disulfonated Monomers with Several Different

Structures ............................................................................................................ 92

Figure 3.2. UV-Visible Spectra of SDCDPS Dilute Solutions Using Different

Solvents ............................................................................................................ 100

Figure 3.3. Effect of the Number of Recrystallization Times on the Absorbance at the

Same Concentration Values (After Two Times Recrystallization, SDCDPS Still

Contains 2.6% ±1% salt) ................................................................................ 101

Figure 3.4. The UV-Vis Absorbances of SDCDPS Solutions with Different

Concentrations were Used to Develop the Calibration Curve.......................... 102

xv

Figure 3.5. Calibration Curve Used to Develop the Beer’s Law Slope. The Left

Graph Shows the Deviation at High Concentrations. The Right graph is the

Linear Calibration Curve at Low Concentrations............................................. 103

Figure 3.6. Effect of Drying Time and Storage Time on the Absorbance at the Same

Concentrations .................................................................................................. 104

Figure 3.7. Comparison of the Absorbance of Pure and Crude Samples of SDCDPS.

(The Crude Sample was Provided by Hydrosize Inc.) ..................................... 105

Figure 3.8. 1H NMR of Poly(Arylene Ether Sulfone) Copolymers (BPSH-40) was

Used to Determine the Degree of Sulfonation.................................................. 106


Copolymers Containing 35 mole % Disulfonate Repeat Unit.......................... 125

Figure 4.2. The Copolymer Structures and Degree of Sulfonation were Determined

by 1H NMR Spectra in the Aromatic Region (BPS35-50 Copolymer). ........... 126

Figure 4.3. Molecular Weights can be Calculated from the Relative 1H NMR

Integrals of the Tert-Butyl Endgroups and the Aromatic Resonances (BPS35-50

Copolymer in DMSO-d6).................................................................................. 127

Figure 4.4. Correlations of Reduced (▲) and Inherent (■) Viscosities with

Copolymer Concentration of BPS35-Control in Pure NMP (3a), and NMP

Containing 0.05 M LiBr (3b)............................................................................ 128

Figure 4.5. Relationship Between Log(Intrinsic Viscosity) and Log(Mn) for BPS-35

Copolymers....................................................................................................... 130

Figure 4.6. Stress-strain Curves of BPSH35 Copolymers (Thin Films) as Function of

Molecular Weight. ............................................................................................ 134

Figure 5.1. Disulfonated Copolymer Structures with Biphenol (BPSH) or 6F

Bisphenol A (6FSH) Units in the Backbones................................................... 144

Figure 5.2. Synthesis of Tert-butylphenyl Terminated Partially Fluorinated

Poly(Arylene Ether Sulfone) Containing Sulfonic Acid Groups (x = 0.35 or 0.48)

.......................................................................................................................... 151

Figure 5.3. 1H NMR Spectrum of 6FS35-50 Copolymer (Aromatic Region).......... 152

xvi

Figure 5.4. The Molecular Weight of the Copolymer can be Calculated from the

Relative 1H NMR Integrals of the Tert-butyl Endgroups and the Aromatic

Resonances. (6FS35-50 Copolymer in DMSO-d6)........................................... 153

Figure 5.5. Relationship Between Log(Intrinsic Viscosity) and log(Mn) for 6FS35 and

6FS48 Copolymers ........................................................................................... 155

Figure 5.6. Morphology Characterization of the BPSH35 and 6FSH35 Series of

Copolymers with Different Molecular Weights by AFM. ............................... 159

Figure 6.1. Three Disulfonated Ketone-Type Comonomer Structures .................... 168

Figure 6.2. 1H NMR Spectrum of SMBFB Disulfonated Comonomer.................... 176

Figure 6.3. Synthetic Scheme of Three Series of Disulfonated Poly(Arylene Ether

Ketone) Copolymers......................................................................................... 177

Figure 6.4. 1H NMR was Used to Calculate the Degree of Sulfonation of the

Copolymers (MB-30) ....................................................................................... 179

Figure 6.5. IR Spectra of MB Series Copolymers.................................................... 180

Figure 6.6. AFM Image of Copolymer Membranes: (a) B-30, (b) PB-40, (c) MB-40,

(d) B-40, (e) PB-50, (f) MB-50. The IEC value of the left group copolymers is

around 1.1-1.2 meq·g-1, and the right groups is around 1.4 meq·g-1................. 182

Figure 6.7. Influence of IEC and Copolymer Structure on Water Uptake of the

Membranes (Acid Form) in Liquid Water at Room Temperature.................... 186

Figure 6.8. Proton Conductivity vs. IEC of Three Ketone Type Copolymers in Liquid

Water at Room Temperature ............................................................................ 187

Figure 6.9. Proton Conductivity in Liquid Water Tends to Depend on Hydration

Number (RT) .................................................................................................... 188

Figure 6.10. Effect of Temperature on Protonic Conductivity in Liquid Water ...... 189

Figure 6.11. Influence of Copolymer Composition on Water Sorption of the B Series

as a Function of Humidity ................................................................................ 190

Figure 6.12. Influence of Copolymer Composition and Temperature on Methanol

Permeability...................................................................................................... 191

Figure 7.1. Copolymer Chemical Structures Studied in This Work (a) B-ketone-xx

and PB-diketone-xx Copolymers, (b) 6FK-BPSH Multiblock Copolymer...... 201

xvii

Figure 7.2. Synthesis of Fluorine Terminated Poly(Arylene Ether Ketone) (6FK)

Hydrophobic Oligomer..................................................................................... 210

Figure 7.3. Molecular Weight of 6FK Hydrophobic Oligomer Can be Calculated

from the 19F NMR Spectrum (6FK Oligomer with Target MW 4 kg·mol-1 in

CDCl3) .............................................................................................................. 211

Figure 7.4. Synthesis of 6FK-BPSH Multiblock Copolymers via Two-step Sechnique

.......................................................................................................................... 212

Figure 7.5. 1H NMR Spectra of BPS Hydrophilic Oligomer (Top), and Multiblock

6FK-BPS Copolymer (Bottom) ........................................................................ 213

Figure 7.6. 13C NMR Spectra of Random (Top) and 6FK-BPS Multiblock (Bottom)

Copolymers....................................................................................................... 214

Figure 7.7. Tapping Mode Atomic Force Microscopy Images: (a) Phase Image and (b)

Height Image of a 4k-4k 6FK-BPSH Multiblock Copolymer Film Acidified by

Method 1 at 30 °C, (c) Phase Image and (d) Height Image of the Same Film

Acidified by Method 2 at 100 °C, (e) Phase Image and (f) Height Image of a

Sulfonated Poly(Arylene Ether Ketone) Random Copolymer Film (B-30)

Acidified by Method 2. Image size: 500 nm; z Ranges: (a) 4°, (c) 12°, (e) 8°, All

Height Ranges: 10 nm. ..................................................................................... 217

Figure 7.8. Retention of Water as a Function of Water Activity is Enhanced for the

Block Copolymer.............................................................................................. 222

Figure 7.9. Proton Conductivity as a Function of Temperature for Multiblock 6FK-

BPSH (4:4)k, B ketone-30, and Nafion® 112 ( The numbers in the box are

activation energy, kJ·mol-1) .............................................................................. 223

Figure 7.10. The Block Copolymer Has a Much Higher Self-diffusion Coefficient of

Water (Multiblock Copolymer Has Similar IEC Value to the PB-40 and B-30

Random Copolymers)....................................................................................... 224

Figure 7.11. Comparison of Conductivity vs. RH for 6FK-BPSH (4:4)k Multiblock,

PB-diketone-50 Random Copolymers, and Nafion® 112................................. 225

Figure 7.12. Hydrogen-air Fuel Cell Performance of 6FK-BPSH (4:4)k and Nafion®

at 80 ℃ under Fully Humidified Conditions.................................................... 227

xviii

LIST OF TABLES

Table 2.1. Different Fuel Cell Types………………………………………………...5

Table 2.2. Several Poly(arylene ether ketone) Structures…………………………..45

Table 3.1. Characterization of the Model BPS Copolymers ...................................107

Table 4.1. Characterization of BPS35 copolymers..................................................129

Table 4.2. The Results of Water Swelling and Conductivity Test for BPSH35

Copolymers with Controlled Molecular Weights in Liquid Water at 30 oC ...133

Table 4.3. The Tensile Properties of BPSH35 Copolymers (thin films) as Function of

Molecular Weight. ...........................................................................................135

Table 5.1. Characterization of 6FS35 and 6FS48 Series Copolymers ....................154

Table 5.2. Water Uptake and Conductivity Characterization for 6FSH35 and 6FS48

Copolymers with Controlled Molecular Weights in Liquid Water at 30 oC ...158

Table 6.1. Characterization of B/PB/MB Series of Copolymers with Different Degree

of Sulfonation ..................................................................................................178

Table 6.2. Comparisons of Thin Film Protonic Conductivity in Liquid Water to That

of MEA............................................................................................................192

Table 7.1. Characterization of Multiblock, Random Copolymers and Nafion®Control

.........................................................................................................................221

1

Chapter 1. Research Significance and Objectives

Environmentally-friendly renewable energy sources are becoming more and more

desirable because of two main reasons: the depletion of natural energy resources and

pollution caused by the use of fossil fuels. Fuel cells are becoming increasingly important

because they are capable of converting chemical energy directly into electrical energy.

Proton exchange membrane (PEM) fuel cells have several advantages over other energy

sources, e.g., gasoline engines, such as their high efficiency and very low pollutant

emissions. The fuel used in PEM fuel cells can be either hydrogen or methanol. Because

the only byproduct of a hydrogen/air fuel cell is water, it has become widely known as a

source of “green energy.” The growing list of applications for PEM fuel cells, such as

stationary power, automobiles, and small electronic devices, has created great interest not

only in academia, but also from researchers in government and industry.

The proton exchange membrane is the key component of a PEM fuel cell. It

serves as the barrier for fuel and transports protons from the anode to the cathode to

generate power. Despite their potential, limitations have existed in the current state-of-art

perfluorosulfonic acid Nafion® materials. These limitations have primarily included high

cost and low performance, especially with respect to long term durability. Therefore,

novel PEM materials with improved properties and lower cost have been important areas

of research for more than two decades. In particular, wholly aromatic disulfonated

poly(arylene ether) copolymers, including poly(arylene ether sulfone)s and poly(arylene

ether ketone)s, have displayed promise for use in PEM fuel cells and merit further

investigation.

2

This research investigated hydrocarbon type wholly aromatic poly(arylene ether)

copolymers containing sulfonic acid groups for PEM fuel cells. The overall objectives

were to synthesize and characterize sulfonated random and multiblock copolymers for

PEMs, and to investigate the copolymer molecular weight, composition, and

microstructure of the proton exchange membranes for use in fuel cells. Specifically, this

study accomplished the following goals:

1. Developed purity characterization method for the disulfonated monomer

SDCDPS by UV-visible spectroscopy, which was important for high

molecular weigh copolymer synthesis via direct step growth copolymerization,

especially for mass production;

2. Synthesized controlled molecular weight poly(aryelene ether sulfone) random

copolymers, and investigated the effect of molecular weight on the PEM

properties;

3. Synthesized three series of disulfonated poly(arylene ether ketone) random

copolymers, and studied the chemical structure effects of the ketone

copolymers on the PEM properties;

4. Synthesized novel multiblock copolymers containing ionic groups to improve

membrane morphology by forming cocontinuous hydrophilic-hydrophobic

phase separation, which may enhance membrane properties—especially for

applications at high temperature and low relative humidities.

3

Chapter 2. Literature Review

This dissertation addresses the effects of copolymer composition, molecular

weight, and microstructure on the properties of proton exchange membranes (PEMs).

Fuel cell concepts and related background will be discussed, followed by a review of the

current state-of-the-art of membrane materials. A review of partially or non-fluorinated

aromatic copolymers containing sulfonic acid groups encompass the major focus of this

chapter, although it will also include a brief review of perfluorinated copolymers and

other novel approaches in PEM research.

2.1. Proton Exchange Membrane Fuel Cells (PEMFCs)

2.1.1. Fuel Cell Introduction1,2, 3

The rapid depletion of non-renewable traditional energy sources due to increasing

demands, as well as the pollution created by fossil fuels are two major driving forces for

developing more efficient and reliable fuel cell technologies. Fuel cells are considered an

excellent renewable and environmentally friendly alternative energy resource.

Furthermore, they have higher electrical efficiencies compared to conventional engines.

The initial concept for fuel cells was developed more than 150 years ago by

William Grove, a British physicist.1 However, little practical use was made of this

technology until the 1960s, when the National Aeronautics and Space Administration

(NASA) turned to fuel cells for its space program. The space shuttle, for example, uses

fuel cells to generate electric power and drinking water. Fuel cells are now being

4

investigated for motor vehicles, power plants, and as replacement batteries for laptop

computers and other electronic equipment.

A fuel cell is a device that converts the chemical energy of a fuel and an oxidant

directly into electricity. In principle, a fuel cell operates like a battery. Unlike a battery

however, a fuel cell does not run down or require recharging. It will produce electricity

and heat as long as fuel and an oxidizer are supplied. Accordingly, fuel cell systems have

the potential to solve challenging problems associated with currently available battery

systems, namely their insufficient energy at a given weight (specific energy density) or

volume (volumetric energy density). Moreover, while the leading battery technologies are

reaching practical limits of their energy storage capabilities, commercial fuel cells are

still in their infancy. Furthermore, since hydrogen/air fuel cells operate without a thermal

cycle, they offer high energy efficiency and can virtually eliminate air pollution and do

not require the use of emission control devices as in conventional energy conversion.

Fuel cell construction generally consists of a fuel electrode (anode) and an oxidant

electrode (cathode) separated by an ion-conducting membrane. Oxygen passes over one

electrode, and fuel (generally hydrogen or methanol) passes over the other, generating

electricity, water, and heat. Using pure hydrogen fuel, fuel cells only produce water, thus

eliminating all emissions usually caused by the production of electricity. Despite the fact

that there is certainly potential for renewable energy from wind, solar and hydroelectric

power, these sources are not well suited to handle the electrical base load due to their

irregular availability. The combination of these various sources to produce hydrogen in

co-operation with fuel cells may well be a viable option for future power generation.

5

2.1.2. Types of Fuel Cells3, 4

Fuel cells are generally categorized by their electrolyte, namely, the material

sandwiched between the two electrodes. The electolyte’s characteristics determine the

optimal operating temperature and the fuel used to generate electricity, and each features

specific benefits and shortcomings. Some of the essential characteristics of all types of

fuel cells are summarized in Table 2.1. 3

Table 2.1. Different Fuel Cell Types

Fuel Cell Type Electrolyte Fuel Operating

Temperature Efficiency Application

Proton Exchange

Membrane (PEM)

solid polymer

membrane

H2(pure or reformed)

60-120oC 35–60%

Portable, transportation

Direct Methanol (DMFC)

solid polymer

membrane CH3OH 60-120oC

35–40% Portable,

transportation

Alkaline (AFC)

potassium hydroxide(8-

12 N) H2 50-250 oC 50–70%

Space

Phosphoric Acid

(PAFC)

Phosphoric Acid (85%-

100%)

H2 (reformed)

160-220 oC 35–50%

Power Generation,

cogeneration, transportation

Molten Carbonate (MCFC)

Molten Carbonates

H2 and CO

reformed and CH4

600-800 oC 40–55%

Power generation,

transportation

Solid Oxide

(SOFC)

Solide Oxide

H2 and CO

reformed and CH4

800-1000 oC 45–60%

Power generation,

cogeneration

6

2.1.3. Proton Exchange Membrane Fuel Cells (PEMFCs) 5, 6

Proton Exchange Membrane Fuel Cells (PEMFCs) are believed to hold the most

promise for eventually replacing gasoline and diesel internal combustion engines. As

noted above, they were first used in the 1960s for the NASA Gemini program. However,

PEMFCs are currently being developed and demonstrated for systems ranging from 5kW

to 250kW3. PEM fuel cells use a solid polymer membrane (a thin plastic film) as the

electrolyte. This polymer is permeable to protons when it is saturated with water, but it

does not conduct electrons. The fuels for the PEMFC can be hydrogen (hydrogen/air fuel

cell) or methanol (direct methanol fuel cell).

2.1.3.1. Hydrogen/Air PEM Fuel Cells

Figure 2.1 shows how the PEM fuel cell works. For a hydrogen/ air PEM fuel cell,

the fuel is hydrogen and the charge carrier is the hydrogen ion (proton). At the anode, a

platinum catalyst splits the hydrogen into protons (hydrogen ions) and negatively charged

electrons. The hydrogen ions permeate across the electrolyte to the cathode while the

electrons flow through an external circuit and produce electric power. Oxygen, usually in

the form of air, is supplied to the cathode and combines with the electrons and the

hydrogen ions to produce water. The reactions at the electrodes are as follows:

7

Anode Reactions: 2H2 => 4H+ + 4e-

Cathode Reactions: O2 + 4H+ + 4e- => 2 H2O

Overall Cell Reactions: 2H2 + O2 => 2 H2O

Figure 2.1. Schematic of a PEM Fuel Cell Operation

Compared to other types of fuel cells, PEMFCs generate more power per given

volume or weight. This high-power density capability makes them compact and

lightweight. In addition, the operating temperature is less than 100ºC, which allows rapid

start-up. These traits, as well as the ability to rapidly change power output, are some of

the characteristics that make PEMFCs excellent candidates for automotive power

applications.

Other important advantages result from the electrolyte being a solid material

instead of a liquid. For example, sealing the anode and the cathode gases is simpler with a

solid electrolyte; therefore, it is less expensive to manufacture. The solid electrolyte is

8

also more immune to difficulties associated with orientation and has fewer problems with

corrosion, compared to many of the other electrolytes, thus leading to a longer cell and

stack life.

One of the disadvantages of the PEMFC for some applications, however, is that

the operating temperature is low. Temperatures near 100ºC are not high enough to

perform useful cogeneration. Also, since the electrolyte is required to be saturated with

water to operate optimally, careful control of the moisture of the anode and cathode

streams is important.

2.1.3.2. Direct Methanol Fuel Cells (DMFCs)

Direct-methanol fuel cells, or DMFCs, are a subcategory of proton exchange

membrane fuel cells where the fuel (methanol) is not reformed, but is instead fed directly

to the fuel cell. A DMFC is similar to a hydrogen/air PEM fuel cell in that the electrolyte

is a polymer and the charge carrier is the hydrogen ion (proton). However, the liquid

methanol (CH3OH) is oxidized in the presence of water at the anode generating CO2,

hydrogen ions and the electrons that travel through the external circuit as the electric

output of the fuel cell. The hydrogen ions travel through the electrolyte membrane and

react with oxygen from the air and the electrons from the external circuit to form water at

the cathode, completing the circuit.

Anode Reaction: CH3OH + H2O => CO2 + 6H+ + 6e-

Cathode Reaction: 3/2 O2 + 6 H+ + 6e- => 3 H2O

Overall Cell Reaction: CH3OH + 3/2 O2 => CO2 + 2 H2O

9

Initially developed in the early 1990s, DMFCs were not embraced because of

their low efficiency and power density, as well as other problems. Improvements in

catalysts and other recent developments have increased power density 20-fold and the

efficiency may eventually reach 40%.

These cells have been tested over the temperature range of about 60 ºC-120 ºC.

This relatively low operating temperature and no requirement for a fuel reformer make

the DMFC an excellent candidate for very small to mid-sized applications, such as

cellular phones and other consumer products, up to automobile power plants.

One of the drawbacks of DMFCs is that the low-temperature oxidation of

methanol to hydrogen ions and carbon dioxide requires a more active catalyst, which

typically means a larger quantity of expensive platinum catalyst is required than in

conventional PEMFCs. This increased cost is, however, expected to be more than

outweighed by the convenience of using a liquid fuel and the ability to function without a

reforming unit.

2.1.3.3. Requirements for PEM Materials7

Polymeric membranes play a crucial role during electricity generation in

hydrogen/air and direct methanol proton-exchange membrane (PEM) fuel cells. The

membrane in such devices performs two roles: it separates the positive and negative

electrodes and provides a conduit for ion (proton) movement between the electrodes.

PEMs are made from high performance polymers containing ionic groups (sulfonic acid

or phosphoric acid) to produce protonic conductivity under hydrated conditions.

10

Consequently, there is considerable research and development around the world to

develop new membrane materials with tailored physical and transport properties.

The general required properties of an ion-exchange membrane for use in a PEM

fuel cell include high protonic conductivity, low electronic conductivity, low

permeability to fuel and oxidants, low water transport through diffusion and electro-

osmosis, oxidative and hydrolytic stability, good mechanical properties both in the dry

and hydrated state, low cost, and ease of fabrication into membrane electrode assemblies

(MEAs).

The overall properties of the membrane need to balance all these requirements.

For example, the improvement of protonic conductivity can be achieved by increasing the

ion exchange capacity (IEC), which will be detrimental to mechanical strength due to an

increase in water sorption. Nevertheless, these requirements can serve as screening tests

for novel membrane materials, after which the qualified material must be fabricated into a

well-bonded, robust membrane electrode assembly (MEA). Thus, the ease of MEA

fabrication and the resulting properties of the MEA are also critical. The membrane

electrode assembly consists of the proton conducting membrane which is bonded on

either side to the anode and cathode. One major objective of MEA fabrication is to

promote good interfacial adhesion between the electrodes and the membrane, since they

often have dissimilar chemical structures and properties.

2.1.3.4. Laboratory Evaluation of PEM Materials

Many techniques are employed to characterize the properties of PEM materials in

the lab. Here, several fundamental but important parameters are introduced, which are

11

used in the first stage as initial screening tests for PEM materials. Further details on the

instruments and techniques employed will be provided in later chapters.

Ion-exchange capacity (IEC) or Degree of Ionic Content

The ion-exhange capacity (IEC), which is a measure of the ionic content of the

polymer materials, is defined as the molar equivalents of ion conductor per mass of dry

membrane. IEC is typically expressed in units of milliequivalents per gram (meq·g-1) of

polymer. Sometimes, equivalent weight (EW) is used to express this ionomer property.

EW is the inverse of the IEC (EW =1000/IEC) with the unit of grams of polymer per

equivalent of ionic groups. The theoretical IEC or degree of the ionic content of a

polymer can be designed and controlled during polymer synthesis, which will be

discussed in the experimental chapter. However, the actual IEC value of a membrane is

usually experimentally determined by potentiometric titration of the acid groups with

base or determined by 1H NMR. IEC affects both conductivity and water uptake, so the

conductivity and water uptake can be controlled by varying the ionic content of the

membrane. The overall effects of increasing IEC should be considered for the reason that

too many ionic groups will definitely increase the conductivity, but will also cause the

membrane to swell excessively with water, which compromises mechanical integrity and

durability.

Water Uptake

PEM fuel cells are operated under humid environments, because the water is

needed as the mobile phase to facilitate proton conductivity. However, too much water

12

swelling will affect the mechanical properties of the membrane by acting as a plasticizer,

lowering the Tg and modulus of the membrane. Careful control of water uptake is critical

for reducing adverse effects.7 Thus, one of the challenges in the future is to modify the

chemistry of PEMs to obtain significant protonic conductivity at low hydration levels.

Multiblock copolymers show potential in this area due to their enhanced ability for

phase-separation.8 The water content of a membrane is expressed either by water uptake

or by the lambda value (λ), which is defined as the number of water molecules absorbed

per acid site as calculated using the following equations:

Water Uptake (%) = [(Wwet-Wdry) / Wdry] * 100

λ = 1000 * [(Wwet-Wdry) / Wdry] / ( 18 * IEC)

Where Wwet and Wdry are the weight of the membrane under hydrated and dry conditions.

Protonic Conductivity

Protonic conductivity is a critical parameter in evaluating PEM materials. It is a

function of many variables, such as temperature, IEC, humidity, and water uptake. A

successful PEM should exhibit a careful balance of maximum protonic conductivity with

minimum water swelling in order to maintain its mechanical properties. Proton

conductivity may be measured by either a through-plane measurement or an in-plane

measurement. Although the through-plane measurement is analogous to the true fuel cell

condition where the protons are transported through the plane of the membrane, it is more

difficult experimentally and usually has larger error than the in-plane measurements

because the material between the test electrodes is so thin that small changes in

13

conductivity are hard to detect due to small measured membrane resistances. Under this

circumstance, the interfacial resistances may play a more significant role. Thus, the most

often used method is a facile in-plane measurement, which was devised at Los Alamos

National Labs (LANL). In this geometry, the resistances measured are much greater than

those of the through-plane measurement, and the experimental errors are relatively lower,

so the result is more precise.

Methanol Permeability

Methanol permeability is a critical transport property when considering a new

proton exchange membrane for use in liquid-fed direct methanol fuel cells. Methanol

that is unoxidized at the anode can “cross over” through the membrane and be oxidized at

the cathode. This methanol short circuit decreases the fuel efficiency of the system,

lowers the cell voltage by causing a mixed potential at the cathode, and increases cell

heating. In general, methanol crossover through the membrane in a membrane electrode

assembly scales with the feed concentration of methanol. This is why most active DMFC

systems are supplied with low methanol feed concentrations (1M or less).8

New PEMs can be screened for their potential as DMFC membranes by

measuring their methanol permeability and comparing with that of a Nafion® membrane,

which is often considered unsuitable for use in DMFCs due to its high methanol

permeability.

2.2. Nafion® and the Other Perfluoropolymers

Perfluoropolymer ionomers have been known since the late 1960s, when the

14

Nafion® ionomer was developed by the DuPont Company and employed as the polymer

electrolyte in a GE fuel cell designed for NASA spacecraft missions9. Today, it remains

the most common commercially available membrane used in PEM fuel cells. Nafion® is a

perflurosulfonic acid polymer membrane consisting of a Teflon-like backbone (~87% in

Nafion® 1100) with side chains terminated with –SO3H groups, as shown in Figure 2.2

Figure 2.2. Chemical Structure of Nafion®7

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

The IEC or EW of Nafion® membranes can be modified by changing the ratio of

the two types of repeat units (x and y values) and is reflected in the commercial name.

For example, the commercial name Nafion® 117 means that the membrane equivalent

weight is 1100 EW (IEC = 0.91 meq·g-1) and its thickness is 7 mil (1 mil is equal to 25.4

microns).

Other commercially available perfluoroionomers include Flemion® produced by

Asahi Glass and Aciplex-S® produced by Asahi Chemical. Among the three major types,

the DuPont product is considered to be superior because of its high proton conductivity,

good chemical stability, and mechanical strength.10 In the mid-1980s, the Dow Chemical

company also developed a material with a shorter side–chain than those of Nafion® and

the other perfluorosulfonates. Though higher power-generating capability in fuel cell was

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

15

demonstrated using the Dow ionomer, no commercialization of these very promising

experimental membranes followed. The complexity of the Dow process for the synthesis

of the base functional monomer used for the production of the shorter side–chain ionomer

was possibly one of the reasons for the abandonment of this interesting development.9

Innumerable papers investigating the properties of Nafion® have been published.

The data available include fuel cell performance, transport characteristics (of water, gases,

and protons), swelling and solubility properties, mechanical, viscoelastic and thermal

behavior, morphology and structure, etc. Nafion® is therefore by far the most extensively

used and studied ionomer for fuel cell applications.9 From these numerous studies, three

distinct phases in the morphology of Nafion® have been confirmed. The first region is a

hydrophobic semicrystalline region primarily made up of the backbone chains. The

backbone provides structural stability to the membrane and prevents it from dissolving in

water. The second region is a largely empty amorphous region that consists of side chains

and some sulfonic acid groups. The final region consists of clusters of the hydrophilic

sulfonic acid groups which are responsible for conducting protons across the

membrane.11

Based on these structural characteristics, Nafion® is so far the best candidate

membrane with many advantages, including high protonic conductivity (~ 0.1 S·cm-1

with IEC = 0.91 meq·g-1), moderate swelling in water, exceptional oxidative and

chemical stability, good mechanical properties due to the semicrystalline morphology,

and long term stability.12 However, some drawbacks to Nafion® limit its applications, like

its high cost ($800/m2 for ~100 µm thick membranes)13, loss of membrane performance at

temperatures above 100 °C, and high methanol permeability in DMFCs,2 relatively low

16

mechanical strength at higher temperature and moderate glass transition temperature.7

These deficiencies of the Nafion® membrane have limited its practical application,

especially at high temperature and in DMFCs; therefore, the development of competitive

and less expensive PEM materials that will overcome these problems is important. As a

result, a large number of novel non-or partially fluorinated copolymers have been

developed and their properties investigated.

2.3. Polystyrene Type PEM Materials

Since the 1960s, a variety of polystyrene sulfonic acid membrane systems have

been evaluated. In fact, the first PEM fuel cell employed in the Gemini program was a

crosslinked polystyrene sulfonic acid (PSSA).14 The major issue found for polystyrene

sulfonic acid membranes is that they have poor oxidative stability. The presence of

hydrogen peroxide at the membrane-catalyst interface results in a free radical oxidation

of the aliphatic backbone that severely limits the life of the cell above 60 oC. For instance,

at 50-60 oC, the useful lifetime of a PSSA in a fuel cell was measured in thousands of

hours, while at 80 oC its life time was only 100 hours.15 Although the initial polystyrene

membrane showed poor fuel cell performance, the modified polystyrene type membrane

materials still attract the interest of many researchers. This is because the styrenic

monomers are widely available, easy to modify, and the polymers are easily synthesized

via conventional free-radical and other polymerization techniques.

One of the representatives is the tri-block copolymer produced by DAIS-Analytic

Corporation (Fig. 2.3). The membrane, which is based on commercially available

styrene–ethylene/butylene–styrene (SEBS) triblock copolymers, contains styrenic blocks,

which are subsequently sulfonated. DAIS membranes are reported to be much less

17

expensive to produce than Nafion®. However, as mentioned above, the main drawback in

employing hydrocarbon-based materials is their poor chemical stability compared to

perfluorinated or partially perfluorinated membranes due to the lower C–H bond

dissociation enthalpy. For this reason, DAIS membranes are suitable for portable fuel cell

power sources of 1 kW or less, with operating temperatures below 60 ◦C. Another

drawback of sulfonated SEBS (sSEBS) membranes is their high methanol transport in

DMFCs. 16

Figure 2.3. Structure of Dais Styrene–Ethylene/Butylene–Styrene (SEBS) Triblock

Copolymer Electrolyte.16

Several sulfonation methods have been reported for the SEBS triblock copolymer.

Ehrenberg, et al.17 used a sulfur trioxide/triethylphosphate sulfonating complex solution

to sulfonate the SEBS triblock copolymers which were dissolved in a

dichloroethane/cyclohexane solvent mixture at low temperature between -5 oC and 0 oC.

The resulting PEM had conductivities of 0.07-0.1 S·cm-1 when fully hydrated. However,

there is not much information on the extent of sulfonation of the styrene moieties in the

blocks.18,19 Another method employed acetyl sulfate, which was prepared from the

reaction of acetic anhydride with sulfuric acid, as the sulfonating reagent. The solvent

18

was 1,2-dichloroethane and the reaction temperature was 50 oC. The sulfonation degree

of purified SSEBS was evaluated quantitatively by elemental analysis. The SSEBS

membranes were prepared by a solution casting method. The proton conductivity and

methanol permeability increased abruptly when the sulfonation degree of the polystyrene

end blocks exceeded 15 mol%, and proton conductivity equivalent to that of Nafion® was

obtained for SSEBS with 34 mol% sulfonation，but the methanol permeability of

SSEBS was decreased by more than half compared with Nafion®.20

Ballard Power developed another important vinyl perfluorosulfonate system,

which has the trade name BAM3G (Fig. 2.4).21, 22

Figure 2.4. Molecular Structure of Ballard Advanced Materials Corp’s BAM3G21

This third generation Ballard Advanced Material membrane is composed of a

perfluorosulfonic acid system that uses α,β,β-trifluorostyrene monomers with different

pendent groups on the phenyl ring. The unsulfonated copolymer was synthesized first and

then it was post-sulfonated using reagents such as chlorosulfonic acid or a sulfur trioxide

complex. It has been reported that the backbone fluorination will mitigate hydroperoxide

19

formation, which results in long-term stability of over 100,000 hours, and also good

proton conductivity (~0.08 S·cm-1). However, this kind of membrane is expensive

probably due to the high cost of the monomer used, and the mechanical properties are

still unknown due to few related reports on this system.

In order to improve the properties of styrene-type PEMs, some novel synthetic

methods or techniques were developed, such as grafting23-25 and crosslinking.26, 27

Holdcroft et al.23 reported the synthesis of polystyrene with grafted poly(sodium styrene

sulfonate) via stable free radical polymerization (Fig. 2.5). These grafted copolymers

displayed excellent proton conductivities up to ~0.24 S·cm-1, although the oxidative

degradation of the backbone is still a problem for practical use. Recently, Chen Li et al.26

also reported a photo-crosslinking method to improve the mechanical strength and to

decrease the swelling of fuel cell membranes (Fig. 2.6). The process starts with photo

excitation of the benzophenone to generate radicals. The radicals take a tertiary or

secondary hydrogen atom from the polymer chain, making a radical center on the

polymer. The radicals attack another polymer chain forming a cross-link. According to

their results, the cross-linked PSEBS had lower water swelling, lower proton conductivity,

and higher chemical stability than uncrosslinked PSEBS because the cross-linking may

have resulted in less and smaller hydrophilic channels for water absorption and proton

mobility.

20

Figure 2.5. Synthetic Scheme of Polystyrene-g-Poly(sodium styrenesulfonate)23

21

Figure 2.6. Chemical Reactions of PSEBS Photocrosslinking Using Benzonphenone

as Initiator. 26

2.4. Partially- or Non-Fluorinated Copolymers with Aromatic Backbones

Aromatic polymers show excellent properties, such as good mechanical strength,

low cost, ease of processing, and chemical and thermal stability even at elevated

temperatures, so they are promising PEM candidates to replace the state-of-the-art

Nafion® membrane. Research on these polymers has specifically emphasized these three

targets: (1) reducing the PEM material cost for mass production; (2) lowering the

membrane methanol permeability for DMFCs; (3) developing membrane materials that

can be used at high temperature (>100 oC).

Increased fuel cell operating temperature is attractive for a number of reasons,

22

including (1) improved tolerance of the electrodes to carbon monoxide, which enables the

use of hydrogen produced by reforming of natural gas, methanol or gasoline; (2)

simplification of the cooling system; (3) possible use of cogenerated heat; (4) increased

proton conductivity; and (5) in DMFC, improved kinetics of the methanol oxidation

reaction at the anode.28 However, membranes such as Nafion® lose water which is

necessary to produce protonic conductivity at elevated temperature. Devising systems

that can conduct protons with little or no water is perhaps the greatest challenge for new

membrane materials.7

In recent years, a variety of new aromatic ionomers have been prepared and

characterized as membrane candidates for PEM fuel cells, for example, sulfonated

poly(arylene ether sulfones), sulfonated poly(arylene ether ketones), sulfonated

poly(arylene ether phosphine oxide)s, sulfonated polyimides, sulfonated

polyphosphazenes, and so on. These materials show some promise with respect to

conductivity, stability, methanol crossover, and water transport, and will be reviewed in

the following sections.

2.4.1. Poly(Arylene Ethers)

Poly(arylene ethers) have an attractive combination of chemical, physical and

mechanical properties that have made them an important class of engineering

thermoplastics.7,29-33 The basic repeat units in this family of copolymers consist of phenyl

rings linked together by ether groups and other groups like sulfones, ketone, or

arylphosphine oxide linkages (Fig. 2.7). The aromatic ether part provides chain flexibility,

thereby imparting good impact strength and toughness. The sulfone or ketone groups tend

to attract electrons from the phenyl rings and to enhance the resonance of the ether bond.

23

This results in good thermal, hydrolytic, and oxidative stability. Due to these well known

properties, wholly aromatic poly(arylene ether)s have attracted much attention for use in

PEMs with introduction of active proton exhange sites to the structure.

Figure 2.7. Several Possible Poly(arylene ether) Chemical Structures 7

2.4.1.1. Synthetic Routes of Poly(Arylene Ethers)

Several different synthetic methods for preparing poly(arylene ethers) have been

reported. The major ones include nucleophilic aromatic substitution, Friedel-Crafts

electophilic substitution, the Ullman reaction, and metal coupling reactions. Nucleophilic

aromatic substitution, which is the focus of this research, will be described in greater

detail here, and the others will be introduced briefly.

Nucleophilic Aromatic Substitution Polymerization: Since it was developed in

1967 by Johnson and coworkers,34 nucleophilic aromatic substitution polymerization

became more and more important and it is currently the most common route to synthesize

X Y X Z

n

X = O, S

Y = a bond, C

CH3

CH3

, C

CF3

CF3

, S

O

O,

P

O

Z = S

O

O

,C

O P

O

,

24

polymers like poly(arylene ethers). In its original form this route involved the

nucleophilic displacement of activated dihalo aryl derivatives by bisphenol salts based on

a strong base such as sodium hydroxide to yield high molecular weight polymer. Several

years later Clendinning et al.35 reported that potassium carbonate or bicarbonate could be

used in these reactions instead of sodium hydroxide. McGrath and coworkers36, 37 were

the first to systematically study the use of the weak base K2CO3 to obtain phenolate salts.

This type of polymerization has been investigated in the intervening years, and the

reaction mechanisms and conditions leading to most of the common poly(ary1 ethers)

(e.g. polysulfones and poly(ether ketones)) are rather well understood.38

Figure 2.8. Nucleophilic Aromatic Substitution Mechanism

The nucleophilic displacement of a halogen from an activated aryl halide system

25

occurs in a two-step addition-elimination reaction (SNAr) as shown in Figure 2.8. The

nucleophile adds to the electron-deficient aryl halide, forming a negatively charged

Meisenheimer complex, from which the halide is eliminated, leading to the formation of

an aryl-ether linkage. The activating group present in the aryl halide serves two purposes.

The group must be an electron-withdrawing moiety, which decreases the electron density

at the site of the reaction, and secondly, its presence must lower the energy of the

transition state for the reaction by stabilizing the anionic intermediate formed. These

SNAr reactions only proceed if the electron-withdrawing substituent is located either in

ortho or para positions relative to the halide. The most commonly employed activating

groups in these reactions have been sulfones, ketones, and more recently, phosphine

oxides, which are all strongly electron withdrawing substituents.39 Although these

strongly electron withdrawing groups are preferred in the SNAr reaction, it has been

demonstrated recently that some other weakly electron withdrawing functional groups

can also activate aryl fluorides toward nucleophilic aromatic substitution. For example,

certain heterocycles as well as other functional groups (e.g. perfluoroalkyl groups, azines,

acetylenes, etc.) can effectively activate aryl fluorides toward SNAr reactions and many

of these groups have been successfully used in the preparation of the corresponding

poly(ary1 ethers).40-42 The reactivity of the activated halides can be estimated by

measuring 1H, 13C and 19F NMR chemical shifts. 1H NMR chemical shift data from the

protons ortho or para to the electron-withdrawing group can be used to determine the

reactivity of the monomer indirectly.43 13C NMR and 19F NMR can be used to probe the

chemical shift at the actual site of the nucleophilic reaction. In general, lower chemical

shifts correlate with lower monomer reactivity. The reactivity of a number of aryl

26

fluoride monomers used in nucleophilic aromatic substitution polymerization was

explored by Carter,39 and he reported that a compound may be appropriate for

nucleophilic displacement if the 13C chemical shift of an activated fluoride ranges from

164.5 to 166.2 ppm in CDCl3.

The two-step mechanism is supported by the isolation of many Meisenheimer

salts. Evidence for a rate-determining first step comes from the observation that

fluoroaromatics undergo nucleophilic substitution much more rapidly than their iodo–

counterparts, despite the fact that I– is a better nucleophile than F–. This is due to fluorine

being more inductively electron withdrawing than iodine, reducing electron density on

the aromatic ring and enhancing the rate of nucleophilic attack. The high-energy

Meisenheimer intermediate is stabilized by resonance, resulting in higher electron density

at the ortho– and para– positions. Therefore, the reactivity of halogen leaving groups

follows F >> Cl > Br > I, which is the opposite of normal SN2 reactions.44 Although

fluoroaromatics such as 4,4’-difluorodiphenyl sulfone (DFDPS) are much more reactive,

the chloroaromatics are preferred in practical applications for economic reasons.

In addition to the strength of the activating group and the electronegativity of the

leaving group, many other factors will affect the kinetics of the SNAr reaction, such as the

nucleophilicity of the attacking nucleophile, the nature of the solvent, the reaction

temperature, and other experimental conditions.

The reaction rate increases with increasing strength of the nucleophile. The

overall approximate order of nucleophilicity is:

ArS- > RO- > R2NH > ArO- > OH- > ArNH2 > NH3 >I- > Br- > Cl- > H2O > ROH.45

Thus the phenolate is formed from the phenol type monomer before

27

polymerization by addition of either a strong base (NaOH) or a weak base (K2CO3). The

earliest strong base route works most of the time at high temperature to yield high

molecular weight in a short period of time. However, many salts such as the

hydroquinone or biphenol salt are so insoluble that they do not work well. Furthermore, a

stoichiometric amount of base used for the reaction is critical to obtain high molecular

weight polymers. Moreover, excess strong base may undesirably hydrolyze the dihalides

to afford deactivated diphenolates which upset the stoichiometry. Due to these

disadvantages, an alternate method was developed which involved utilizing anhydrous

potassium carbonate as the base. This method is advantageous in that K2CO3 can be used

in excess without the occurrence of any side reactions.34, 35 K2CO3 was found to be better

than Na2CO3 due to its relative stronger basicity and higher solubility in the reaction

medium.46 In addition, to obtain high molecular weight, water generated during this step

should be removed from the system to avoid hydrolyzing the activated substrate, since

hydrolysis reduces the reaction rate and upsets the stoichiometry of the monomers. To

remove the water, an azeotroping agent, such as toluene and xylene, is commonly used.

Aprotic polar solvents, such as dimethyl sulfoxide (DMSO), N,N-dimethyl

acetamide (DMAc), N,N-dimethyl formamide (DMF), N-methyl pyrrolidone (NMP), and

cyclohexylpyrrolidone (CHP) are most often used in the reaction. The solvent should

provide good solubility for both the monomers and the polymer product. Otherwise, the

stoichiometry will be offset by some precipitation. Thus under some circumstances, very

high reaction temperature and boiling point solvents, such as sulfolane, diphenyl sulfone

(DPS) have to be used due to the poor reactivity of the monomers or poor solubility of

the resulting, possibly semi-crystalline polymers, as in the PEEK systems.

28

In addition to the factors emphasized above, reaction conditions (such as

temperature and inert reaction environment) are also important for a successful SNAr

polymerization.

Friedel-Crafts Electrophonic Substitution: This is another important route to

synthesize Poly(arylene ethers). The mechanism involves a two-stage reaction, as shown

in Figure 2.9. The first step is the attack of the electrophile on the π electrons of the

aromatic benzene ring, generating a positively charged benzenonium intermediate, which

is the rate-limiting step of the reaction. The second fast step is the loss of a proton to

restore the aromaticity of the ring, yielding a substituted benzene ring. Benzene is a poor

electron source compared to alkenes and therefore requires a catalyst to initiate the

reaction. Several catalysts, such as AlCl3, AlBr3, FeCl3, SbCl5, and BF3 have been

utilized in Friedel-Crafts reactions.47

29

The formation of electrophile:

AlCl3 + SO

O

ArCl AlCl4 + SO

O

Ar+-

Stage one:

SO

OAr+

Stage two:

AlCl

Cl

Cl

Cl

-SO2ArH S Ar

O

O

AlCl3HCl

Figure 2.9． The Electrophilic Substitution Mechanism

The Ullman Reaction: The distinct advantage of this synthetic method is that the

non-activated aromatic halides can be polymerized, which is not possible by the normal

activated nucleophilic aromatic substitution route. The reactivity of the leaving group

follows the order I > Br > Cl > F, which is opposite to that observed for the classical

SNAr because the rate determining step is breaking the aryl halide bond. Therefore, iodine

or bromine is preferred under the Ullman reaction conditions. Ullman coupling of

bisphenols and dibromoarylenes using a copper catalyst results in high molecular weight

poly(arylene ether)s. However, poor reproducibility, the need for brominated monomers,

and the difficulty of removing copper salts are major disadvantages of this reaction.48-50

Metal coupling reaction: A relatively new approach developed by Colon and

30

Kelsey, nickel-(0)-catalyzed coupling has proven to be a powerful synthetic method for

the formation of carbon-carbon aryl bonds. A zero valent nickel-triphenylphosphine

complex is used as the catalyst prepared from nickel chloride and zinc metal. In this route,

polymers with biphenyl units can be made from monomers that contain only one phenyl

ring.51, 52 The reaction conditions tolerate many functionalities with the only known

exceptions being protic, nitro, and amine-containing substituents. A wide range of

polymeric materials have been synthesized from inexpensive arylene chlorides and

mesylates as well as the more costly bromide, iodide, and triflate derivatives. The major

advantage is that the reaction can occur under very mild conditions (temp: 60 to 80 oC).

Furthermore, both activated and nonactivated halide monomers can be used. 53-56

2.4.1.2. Molecular Weight Control and Characterization

The molecular weight of polymers synthesized by step-growth polycondensation

can be controlled by two methods. Firstly, by upsetting the mole ratio of the two

difunctional monomers to break up the 1:1 stoichiometry, one can easily control the

molecular weight. The second method involves addition of a stoichiometric amount of a

monofunctional comonomer to endcap the polymer chain. Both methods can obtain

specific designed molecular weight polymers by following the modified Carother’s

equation.29

Control of molecular weight is very important, especially for PEM materials,

because molecular weight is a fundamental parameter affecting the mechanical behavior

of polymers. Proton exchange membranes with good mechanical properties in both the

dry and hydrated states are critical to successful MEA fabrication and long-term

31

durability in a fuel cell device. The membrane must be able to withstand the stresses of

both electrode processing and attachment and must also be mechanically robust enough

to endure startup and shutdown of the fuel cell with repeated

swelling/drying/heating/cooling of the membrane. On the other hand, the molecular

weight may also have some influence on other PEM properties like water uptake and

conductivity. Despite the large body of research on this topic, there is little in the PEM

literature describing molecular weights of candidate materials, even including Nafion®.7

Recently, McGrath research group began to work in this area. Wang et al.

synthesized controlled molecular weight (Mn) poly(arylene ether sulfone)s (Mn from 20

to 40 kg·mol-1) by offsetting stoichiometry with a t-butylphenyl endcapping reagent.57

The t-butylphenyl concentrations relative to the polymer backbone were characterized by

1H NMR to calculate the molecular weight of the copolymers. They provided intrinsic

viscosity (IV) data for these copolymers, and found that the intrinsic viscosities were not

comparable to those of non-sulfonated polymers, since the polymer electrolyte chains

interact via the sulfonate groups. Li et al.58 lately reported further research based on the

previous results. They synthesized a series of controlled molecular weight (from 20 to 70

kg·mol-1), poly(arylene ether sulfone) copolymers containing 35 mol% disulfonated

monomer per repeat unit (Fig. 2.10). The molecular weight characterization combined 1H

NMR analysis of end groups and modified intrinsic viscosity measurements, which used

NMP with 0.05 M LiBr as a solvent. The small amount of salt effectively suppressed the

polyelectrolyte effect, allowing improved characterization of the ion containing

materials.59

32

HO OHSClO

O

BPDCDPSSO3Na

CH3

CH3CH3HO

S ClCl

O

O

TB

SDCDPS

K2CO3

SO

O*

DMAc/Toluene~ 160 oC, Reflux 4 h175 oC, 24 h

+

+

NaO3S

O O SO

OO O * S

O

OO

CH3

CH3CH3

CH3

CH3

H3C Ox

1-x n

KO3S SO3K

Cl

x = 0.35; BPS35, Target Mn: 20, 30, 40 and 50 kg·mol-1


Copolymers Containing 35 mol% Disulfonated Repeat Unit.58

It was determined that mechanical properties (Fig. 2.11) and water uptake of the

material are dependent on the molecular weight of the copolymer, possibly related to

chain entanglement issues. The primary results showed that molecular weight also has

some influence on the proton conductivity, but this needs to be further confirmed and the

research is still ongoing.

33

45

47

49

51

53

55

57

59

0 20 40 60 80

Strain, %

Stre

ss, M

Pa

1, 202, 303, 404, 505, Control-70

12

43

5Mn (kg · mol-1)

45

47

49

51

53

55

57

59

0 20 40 60 80

Strain, %

Stre

ss, M

Pa

1, 202, 303, 404, 505, Control-70

12

43

5Mn (kg · mol-1)

Figure 2.11. Stress-strain Curves of BPSH35 Copolymers (Thin Films) as a Function

of Molecular Weight.58

Another purpose of controlling the molecular weight is to prepare reactive

oligomers by end-capping the polymer chain with functional groups. These oligomers can

be used in the synthesis of block copolymers or for modifying polymer networks. This

will be discussed in a later section.

2.4.1.3. “Post” vs. “Direct” Sulfonation Methods

Generally, two methods have been used to introduce active proton exchange sites

(here referred to as sulfonic acid groups) to the polymers for use as PEM materials. These

include a post-modification approach and direct copolymerization of sulfonated

monomers.

34

The post sulfonation method employs electrophilic aromatic sulfonation to

sulfonate the commercially available polymer，thus the electron-donating substituents on

the aromatic ring will favor the sulfonation reaction, whereas electron-withdrawing

substituents will not.60 This mechanism dictates that the sulfonation reaction occurs more

likely on the aromatic ring activated by the electron-donating ether link instead of the one

deactivated by the electron-attracting ketone or sulfone group (Fig. 2.12). Many kinds of

sulfonating reagents have been investigated including concentrated sulfuric acid, fuming

sulfuric acid, chlorosulfonic acid, sulfur trioxide or complexes. The obvious advantage of

this postmodification reaction is that the polymers are commercially available, but the

disadvantages include difficulty in controlling the degree and location of

functionalization, the possibility of side reactions, and degradation of the polymer

backbone.

35

Figure 2.12. Direct Copolymerization of sulfonated Monomers versus Post

Sulfonation

The direct method is a relatively new approach, which involves the synthesis of a

disulfonated monomer, followed by copolymerization of the sulfonated monomer with

unsulfonated monomers to obtain a polymer containing ionic groups. The first report of

the required sulfonated monomer was from Robeson and Matzner,61 who obtained a

composition of matter patent, which primarily was of interest for its flame retarding

properties. More recently, Ueda et al.62 reported the sulfonation of 4,4’- dichlorodiphenyl

sulfone and provided general procedures for its purification and characterization.

Professor McGrath’s research group at Virginia Tech modified the procedure for

disulfonation of the monomer, shown in Figure 2.13. Sulfonated poly(arylene ether

•Post sulfonation occurs on the most reactive, but least stable, position•High electron density leads to relatively easy desulfonation

•Monomer sulfonation on the deactivated position•Enhanced stability due to low electron density

O O S

O

OHO3S SO3H

n

Activated

O O S

O

OSO3HSO3H

n

Deactivated

36

sulfone) copolymers were then synthesized via direct copolymerization in any

composition desired (Fig. 2.14). This direct method overcomes some disadvantages of

postmodification, including: (1) the degree of sulfonation can be precisely controlled by

varying the sulfonated and unsulfonated monomer ratio, (2) the location of the sulfonic

acid group is on the deactivated sites of the repeat units, thus enhancing the stability and

increasing the proton conductivity due to the electron-withdrawing sulfone or ketone

groups, and (3) the possible side reactions such as cross-linking can be reduced or

avoided, which may result in better thermal stability and mechanical properties.7, 63, 64

Because of its practicality and efficiency, the direct sulfonation method attracts much

attention and has become more and more important in recent years.

SCl ClO

OSCl ClO

O

SO3HHO3S

SCl ClO

O

SO3NaNaO3S

SO3 (28%)

110 oC

NaCl H2O NaOH NaCl

PH = 6~7

Figure 2.13. Synthesis of 3,3’-Disulfonated 4,4’-dichloro-diphenyl Sulfone in Its

Sodium Salt Form63

37

Figure 2.14. Direct Copolymerization of Wholly Aromatic Sulfonated Poly(arylene

ether sulfone) “BPSH-xx,” where xx is the ratio of sulfonated to unsulfonated

activated halide.

2.4.1.4. Sulfonated Poly(Arylene Ether Sulfone)s and Poly(Arylene Ether Ketone)s via the Post Modification Method

Modification of poly(arylene ethers) by addition of sulfonic acid using various

reagents has been investigated extensively.28 Among many structures of poly(arylene

ether)s, two major types attract the most attention. The first is the commercially available

poly(ether sulfone) (PSU Udel®) , and the second is the commercially available wholly

aromatic poly(ether ether ketone) (PEEK Victrex®).

O O SO

O

SO3Na

NaO3S

O O SO

O

O O SO

O

SO3H

HO3S

O O SO

O

SO

OCl Cl HO OH S

O

OCl Cl

SO3Na

NaO3S

NMP/Toluene/K2CO3~ 160°C reflux 4 hrs190°C 16 hours

H+

y

y x

xx

x

1-x

1-x

38

O O C

O

**

SPEEKHO3S

CO

CH3

CH3

O* S

HO3S O

O

*

SPSU

n

n

Figure 2.15. Sulfonated Poly(ether sulfone) (Udel®) and Poly(ether ether ketone)

(Victrex®)

Chlorosulfonic acid was probably the first reagent used to sulfonate the

bisphenol-A-based poly(ether sulfone), Udel®.65, 66 The primary purpose was to produce a

sulfonated poly(arylene ether sulfone) for application in desalination via reverse osmosis.

The reaction was conducted at room temperature and the sulfonation level was controlled

by the reaction time. However, the chlorosulfonic acid may be capable of cleaving the

bisphenol A polysulfone partially at the isopropylidene link, or it might undergo

branching and crosslinking reactions by converting the intermediate sulfonic acid group

into a partially branched or crosslinked sulfone unit. An alternative milder route was

employed to sulfonate bisphenol A polysulfone,67 in which a complex of SO3 and triethyl

phosphate (TEP) with a molar ratio of 2:1 was used to sulfonate the polymer at room

temperature. This mild sulfonation treatment could minimize or even eliminate possible

side reactions such as branching and crosslinking, so it has been employed by some other

researchers to obtain sulfonated PES.68, 69 In 1993, Nolte et al.70 sulfonated the

poly(arylene ether sulfone) Udel® by chlorotrimethylsilyl sulfonate, which was generated

39

in-situ by reacting chlorosulfonic acid with trimethyl chlorosilane in 1,2-dichloroethane

at room temperature. Reaction of trimethylsilylchloro-sulfonate with PSU gives a

silylsulfonated polysulfone from which the trimethylsilyl moieties can be cleaved to give

the acid form of the alkaline sulfonate.

As described above, three different sulfonating agents have generally been used in

sulfonating poly(ether sulfone)s: chlorsulfonic acid (ClSO3H),

trimethylsilylchlorsulfonate ((CH3)3SiSO3Cl), and a complex of SO3 with

triethylphosphate (PO(OCH2CH3)3). Because the last one is not recommended due to its

high toxicity, Genova-Dimitrova et al.71 recently designed experiments to compare the

other two sulfonating agents. They found that ClSO3H, a strong sulfonating agent,

required the addition of a small amount of dimethylformamide (DMF) to prevent

precipitation and generate a homogenous reaction medium. But the reaction mixture

remained perfectly homogeneous with (CH3)3SiSO3Cl. The viscometric measurements

and mechanical stress–strain tests results showed that ClSO3H provoked chain cleavages,

whereas the mild sulfonating agent (CH3)3SiSO3Cl induced neither chain cleavage nor

branching. Although there are also some disadvantages related to the milder acid route,

for example, the longer reaction time needed and the lower sulfonation efficiency, the

advantages of avoiding polymer degradation and side reactions probably outweigh these

drawbacks.

The post-modification of other polysulfones has also been reported. Harrison et

al.72 reported the synthesis of high molecular weight poly(arylene ether sulfone)s by

polycondensation of bisphenol AF or biphenol with dichlorodiphenylsulfone, and then

40

sulfonated them by both a strong acid (ClSO3H) and a mild acid ((CH3)3SiSO3Cl). The

similar results implied that the mild acid route may only be necessary for sulfonation of

polymers with aliphatic groups to prevent degradation. Recently, a novel parent high

molecular weight poly(arylene ether) copolymer containing fluorenyl group was

synthesized via nucleophilic aromatic substitution reaction, and then post sulfonated with

chlorosulfonic acid in CH2Cl2.73 The sulfonic acid groups were introduced only at a

specific position on the fluorenyl groups as shown in Figure 2.16. The resulting ionomer

showed high proton conductivity (0.2 S.cm-1) and excellent hydrolytic stability under

harsh hydrolytic conditions (140 °C and 100% RH). The membranes were highly

mechanically stable at 85 °C and 93% RH and keep their strength even at 120 °C.

Hydrogen and oxygen permeability of this ionomer membrane was much lower than that

of Nafion® 112 under a wide range of conditions (40-120 °C and 0-90% RH).74 The

authors claimed that these properties of the membrane are superior to other hydrocarbon-

based ionomers for fuel cell applications.

41

O SO

OO

n

O SO

OO

n

ClSO3H

2/x(HO3S)

(SO3H)x/2

Figure 2.16. Synthesis of Poly(arylene ether) Ionomers Containing Sulfofluorenyl

Groups via a Post-modification Method73

As mentioned before, the difficulty in controlling the sulfonation location is one

of the major problems with the post-modification method. The electrophilic substitution

reactions introduce the sulfonic acid group at the activated site ortho to the aromatic ether

bond. This leads to a less stable product which is more susceptible to desulfonation than

those where sulfonation is directed to the electron-deficient ring of the repeat unit. Kerres

et al.75 have developed a novel sulfonation method that proceeds via a metalation-

sulfination-oxidation procedure to sulfonate poly(ether su1fone) (Udel®), in which the

sulfonation occurs at a position ortho to the sulfone group ( Fig. 2.17). Four steps are

involved in the procedure: (1) lithiation of the polymer at temperatures from -50 to -80°C

under argon, (2) gassing of the lithiated polymer with SO3, (3) oxidation of the formed

polymeric sulfinate with H2O2, NaOC1, or KMnO4, (4) ion-exchange of the lithium salt

42

of the sulfonic acid in aqueous HC1. The choice of the oxidant in the third step is

important according to the paper, because the proper oxidant will prevent loss of IEC due

to the desulfonation and will increase the efficiency of the oxidation. KMnO4 is suitable

for oxidizing poly(ethersulfonsufinate)s at high to medium degrees of sulfonation (ca.

3.3-1.9 meq·g-1) and minimize the loss of IEC, whereas H2O2 is better for low degrees of

sulfonation ( <1.5 meq·g-1). This synthetic method was also employed in crosslinking

procedures for the sulfonated PSU membranes to enhance the chemical and thermal

stability (Fig. 2.18).76, 77 Although this method can control the sulfonation location and

theoretically work with all polymers which can be lithiated, the complicated steps still

hinder its scale-up.

C

O O

CH3H3C SO O

C

O O

CH3H3C SO O Li

C

O O

CH3H3C SO O

C

O O

CH3H3C SO O S

OLiOSO3H

n-BuLiTHF, -65oC

SO2-65 oC

1. H2O2/OH

2. H+/H2O

Figure 2.17. Synthesis of Sulfonated Poly(ether sulfone) Udel® PSU via the

Metalation Route75

43

SO

OO C

CH3

CH3O

n

SO

OO C

CH3

CH3O

n

LiO2S

LiO2S

SO

OO C

CH3

CH3O

n

SO

OO C

CH3

CH3O

n

O2S

O2S

DMAc80-120 oC-LiI

Figure 2.18. Crosslinking via the Metalation Route76

Another interesting approach to controlling the degree and location of sulfonation

has been reported by Al-Omran and Rose.78 In this paper, several poly(arylene ether

sulfone)s were prepared with 4,4’-dichlorodiphenylsulfone, hydroquinone, and

durohydroquinone via the classic nucleophilic aromatic substitution, followed by

sulfonation using sulfuric acid. The idea is that the sulfonation will only occur on the

hydroquinone residue since there are no aromatic hydrogens on the durohydrquinone

structure. Thus, by varying the molar ratios of hydroquinone to durohydroquinone, the

degree of sulfonation could theoretically be controlled. However, 1H NMR spectra

revealed that the results were not as expected - some sulfonation also occured on the

phenyl ring meta to the sulfone linkage probably due to electron accession from the

methyl groups transmitted via ether linkages to the reaction site on adjacent phenylene

ether sulfone rings.

Post-sulfonation of commercially available poly(ether ether ketone) (Victrex®

PEEK) has been investigated extensively for PEM fuel cells.79-83 PEEK is a

semicrystalline polymer and is not easily dissolved in organic solvents. By introducing

the sulfonic acid to the polymer backbone, the solubility increases due to a decrease in

44

crystallinity.84 Sulfonation of PEEK has been carried out using concentrated sulfuric acid,

oleum or chlorosulfonic acid. 95–98% concentrated sulfuric acid is a common choice to

avoid polymer degradation and cross-linking reactions, which occur for sulfonation with

oleum or chlorosulfonic acid.85 At room temperature with concentrated sulfuric acid used

as the solvent, there is at most one SO3H group attached to each repeat unit.79 The

sulfonation substitution reaction is a second-order reaction; the reverse reaction is

neglected for high acid concentrations.86 The IEC or degree of sulfonation can be

controlled by changing the reaction time, temperature and the acid concentration.

Polymers with a sulfonation range of 30-100% are achieved without apparent degradation

or crosslinking reactions, but the truly random copolymer at sulfonation levels less than

30% is difficult to obtain due to the heterogeneous reaction medium.86, 87

Except PEEK, other poly(arylene ether ketone)s, such as PEK and PEKK (Table

2.2), have also been investigated by post sulfonation. These structures are potentially

interesting because the oxidative stability of PAEKs increases with increasing K/E ratio

of the repeating unit. However, because the electron-withdrawing effect of the ketone

groups deactivated the aromatic ring, sulfonation becomes more difficult and more

reactive sulfonation reagents are required at high K/E values. For example, Swier et al.88

sulfonated PEKK using a mixture of concentrated and fuming sulfuric acids. They found

that unlike sulfonation of PEEK where the reaction occurs during the dissolution of the

polymer in concentrated sulfuric acid, PEKK polymer is not sulfonated during the initial

dissolution in concentrated sulfuric acid, but requires the subsequent addition of fuming

sulfuric acid. For excess SO3, the sulfonation kinetics exhibits a pseudo first-order

dependence on the concentration of the reactive aromatic rings between the ether and

45

ketone groups. Recently, a new modified structure of SPEEK, poly(oxa-p-phenylene-3,3-

phtalido-p-phenyleneoxa-p- phenileneoxy-phenylene) (PEEK-WC), has been investigated

as a proton conductive material by the post –sulfonation route.89

Table 2.2. Several Poly(arylene ether ketone) Structures88

Polymer Repeat Unit Ketone/ether ratio

PEEK O O C

O

0.5

PEK O C

O

1.0

PEKK O C C

O O

2.0

2.4.1.5. Direct Sulfonation of Poly(Arylene Ether Sulfone)s and Poly(Arylene Ether Ketone)s

As described previously, the dihalide monomers can be sulfonated first and then

copolymerized with the unsulfonated dihalide and bisphenol monomers directly to

synthesize sulfonated copolymers. This direct sulfonation route makes it possible to

better control the location and the degree of sulfonation, thus enhancing the thermal

stability, mechanical properties, and even increasing the acidity without degradation.

Although the route was developed earlier, it was first employed for preparing PEM

materials by the McGrath group at Virginia Tech.63 Wang et al.63, 64 modified the

previous procedure and synthesize 3,3’-disulfonated 4,4’-dichlorodiphenylsulfone

46

(SDCDPS) in high yield (~ 80%). Subsequently, the random wholly aromatic

disulfonated copolymers (BPSH) were synthesized by copolymerizing this disulfonated

monomer with 4,4’-dichlorodipheylsulfone (DCDPS) and 4,4’-biphenol via the classic

nucleophilic substitution reaction. Higher temperature and longer reaction time may be

needed to reach high molecular weight due to the sterically decreased activity of the

sulfonated dihalide monomer. The properties of this BPSH copolymer have been

investigated extensively for its potential application as a PEM candidate including

protonic conductivity, water uptake, thermal stability, mechanical properties, morphology,

and more.63, 64, 90-93

The degree of sulfonation (IEC) of the copolymer, which contributes to the

PEM’s electrochemical performance, can be precisely controlled by varying the molar

ratio of SDCDPS and DCDPS monomers. The actual IEC values of the resulting

copolymers were calculated from 1H NMR spectra or measured by nonaqueous

potentiometric titrations. FT-IR and TGA can also qualitatively confirm the results. The

experimental IEC values matched well with the targeted IEC values, this suggests that the

method is reproducible. Although the IEC is a key factor in affecting conductivity and

water uptake of the membrane, it is also found that the acidification method has a distinct

effect on these properties.90 Either a room temperature acidification method (1.5M H2SO4,

Method 1) or a boiling acidification method (0.5M H2SO4, Method 2) was used to

convert the sulfonate salt copolymers to the sulfonic acid form. Both acidification

methods did not change the initial degree of sulfonation, but 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

47

the membranes acidified by method 1. This effect may be attributed to the morphological

changes that occur during the acidification, as observed by atomic force microscopy

(AFM). The samples treated by method 2 had larger hydrophilic domains with more

phase continuity than the samples treated by method 1. Futhermore, the conductivity and

water uptake are also a function of temperature and relatively humidity. High protonic

conductivity is preferred in fuel cell applications and this can be simply achieved by

incorporating more sulfonic acid moieties. However, the water uptake of the BPSH

copolymers increased dramatically when the degree of sulfonation was greater than 50%

(Fig. 2.19). Combined with the DSC and AFM results, it was concluded that the BPSH

system reaches a percolation limit at about 50 mol% of the disulfonated monomer, above

which a hydrogel was formed which was not usable in a PEM fuel cell. These results

indicate that the protonic conductivity must be balanced with the water swelling and

mechanical properties of the membrane in these random copolymers.

48

Figure 2.19. Influence of the Dgree of Sulfonation on the Water Uptake of BPSH

Copolymers64

Harrison et al.94 have studied the influence of the bisphenol structure on the direct

synthesis of sulfonated poly(arylene ether)s. Four bisphenols (Fig. 2.20) were

copolymerized with SDCDPS and DCDPS to synthesize disulfonated poly(arylene ether

sulfone)s. The IEC values of the copolymers change according to the bisphenol used even

at the same degree of sulfonation, for example, the IEC of hydroquinone based

copolymer is higher than others at the same degree of disulfonation due to the smaller

molecular weight of hydroquinone residue in the repeat unit. But the general trend was

the same - the thin film properties of these copolymers scaled with the IEC values.

Bisphenol-A is a very inexpensive and reactive monomer, but the aliphatic groups may

suffer some degradation under the harsh fuel cell environment. The partially fluorinated

49

monomer, bisphenol-AF, has attracted much attention recently because it is thought that

the fluorine rich surface of the membrane will be more compatible with electrodes that

contain Nafion® and may produce more durable MEAs. The more hydrophobic

membrane surface may also reduce the swelling and enhance the stability.7, 95

Figure 2.20. Four Investigated Bisphenol Structures94

Disulfonated poly(arylene ether ketone)s can also be prepared via the direct

polymerization method similar to the poly(arylene ether sulfone)s. The disulfonated

monomer sodium 5,5’-carbonylbis(2-fluorobenzenesulfonate) was first used in direct

polymerization reported by Wang.96, 97 After that, several other diketone monomers (Fig.

2.21) were sulfonated and copolymerized with various bisphenols.98-101 The bisphenol-

AF-based poly(arylene ether ketone)s reported by Li101 show comparable properties to

the BPSH system at the same IEC. Other results showed that the ketone type copolymers

have lower methanol permeability than Nafion®, and therefore may be more suitable for

50

DMFC.98, 102

XF F

NaO3S SO3Na

X = CO

CO

CO

CO

CO

, , or

Figure 2.21. Several investigated sulfonated ketone or diketone structures

2.4.1.6. Other Poly(Arylene Ethers)

Besides poly(arylene ether ketone)s and poly(arylene ether sulfone)s, there are

several other poly(arylene ethers) which have also been synthesized and will be described

in this section.

Wiles et al. directly copolymerized poly(arylene sulfide sulfone) disulfonated

copolymers from disodium SDFDPS, DFDPS, and 4,4’-thiobisbenzenethiol. Compared

with their chlorinated analogs, the more active difluoro monomers increase the reaction

rate and the ease of obtaining high molecular weight copolymers. The membrane

properties showed the same trends as the disulfonated poly(arylene ether sulfone) (BPSH)

series copolymers.103, 104

Sulfonated poly(arylene ether nitriles) attract some attention due to their excellent

thermal and chemical properties.105-107 It was thought that the strongly polar nitrile groups

on the aromatic backbone would contribute some special properties to the membranes.

For example, the enhanced interaction ability of nitrile groups with other polar groups

could facilitate the preparation of composite membranes doped with inorganic particles,

51

promote the adhesion of the membrane to the electrodes, and even reduce the water

swelling. Sumner et al.105 first synthesized disulfonated poly(arylene ether nitrile)s via

direct step copolymerization of 4,4’-(hexafluoroisopropylidene)diphenol, 2,6-

dichlorobenzonitrile, and SDCDPS. The membranes showed similar proton conductivity

to previously synthesized BPSH copolymers and Nation 117 at the same IEC values, but

the water uptake was much lower, probably due to some interaction between the aromatic

nitrile and sulfonic acid moieties and also the more hydrophobic fluorine-rich surface.

Kim et al.106 then investigated the properties of this copolymer with 35% degree of

sulfonation in a DMFC. Their results showed that the methanol crossover is

approximately 2-fold lower than that Nafion®, but similar to nonfluorinated analogs

(BPSH-40). Furthermore, greatly improved DMFC performance compared to the Nafion®

and BPSH-40 under the same test condition suggests that the interfacial effects are very

important. Zhang et al.107 reported another similar poly(arylene ether nitrile) using a

disulfonated ketone monomer instead of the disulfonated sulfone, and studied composite

membranes doped with heteropolyacids (HPAs). HPA is an attracting inorganic additive

because of its high proton conductivity and thermal stability, but it is soluble in water.

The polar nitrile group presumably can retain more HPA in the membrane through

intermolecular interactions. In this paper, the composite membranes showed lower water

sorption but higher proton conductivity compared to the pure membrane.

Sulfonated poly(arylene ether phosphine oxide) is another copolymer possessing

polar groups along the main chain. The phosphine oxide functional moiety may also

serve as a compatibilizer with other materials. Sulfonated 4,4’-bis(fluorophenyl)phenyl

phosphine oxide) (SBFPPO) monomer was prepared by modifying the unsulfonated

52

monomer with fuming sulfuric acid.108 Although a small percentage of di- and tri-

sulfonated compounds were generated, monosulfonated monomer (Fig. 2.22) was the

major product and could be isolated. Wholly aromatic sulfonated copolymers

synthesized from SBFPPO, BFPPO, and 4,4’-biphenol showed high thermal stability.

However, the protonic conductivity was relatively low for two reasons. One is that only

one sulfonic acid group per repeat unit can be introduced to the 4,4’-

bis(fluorophenyl)phenyl phosphine oxide), and another reason may be due to the

observed hydrogen bonding between the pendent sulfonic acid group and the phosphine

oxide moiety. Recently, sulfonated poly(arylene ether phosphine oxide sulfone)

terpolymers have been prepared with SBFPPO, BFPPO, SDCDPS, and 4,4’-biphenol.109

The primary results of the phosphine oxide-containing copolymer doped with HPA

suggest that HPA retention from the composite membranes is significantly improved as a

result of hydrogen bonding.110

Figure 2.22. Structure of Mono-Sulfonated BFPPO

Poly(phthalazinone ether sulfone) (PPES) and poly(phthalazinone ether ketone)

(PPEK) are new high performance polymers with properties such as excellent chemical

and oxidative resistance, mechanical strength, good thermal stability and very high glass

transition temperatures.111 Thus they are considered to be promising candidates for PEM

53

fuel cells. Gao et al.111-113 and others114 introduced the ion exchange sites to the polymer

chain by both the post-modification method and direct copolymerization of the sulfonated

monomers. The post modifications (Fig. 2.23) were conducted at room temperature using

mixtures of 95–98% concentrated sulfuric acid and 27–33% fuming sulfuric acid. The

purpose of using the mixed acid is to promote the sulfonation efficiency and reduce the

degradation of the polymer during the sulfonation, which would be difficult to achieve

with any single acid. Moreover, by changing the acid ratios, the degree of sulfonation can

be controlled.

N N

O

X

ON N

O

X

O

SO3HSulfonation

X = C

O, S

O

O

Figure 2.23. Post Modification of PPES and PPEK111

Sulfonated PPES and PPEK were also synthesized by direct copolymerization of

4-(4-hydroxyphenyl)-1(2H)-phthalazinone with disulfonated activated dihalide sulfone or

ketone monomers as shown in Figure 2.24.113 Compared with the post-sulfonated

membranes, SPPEKH showed less temperature dependence of proton conductivity, and

SPPESH showed higher conductivity. From the results reported by several researchers,114,

115 it appears that the major advantage of these directly synthesized SPPES and SPPEK

54

over either the post-sulfonated products or other sulfonated poly(arylene ethers) is their

much lower water swelling, which originates from intermolecular hydrogen bonds. It was

also found that SPPEKs have lower methanol permeability than Nafion® in DMFC.116

Figure 2.24. Direct Synthesis of SPPEKs and SPPESs113

2.4.2. Sulfonated Polyimides (SPIs)

Polyimides are high-performance macromolecules which are usually obtained via

polycondensation of aromatic and/or alicylic dianhydride and diamine structures. They

exhibit excellent mechanical properties, as well as good chemical and long-term thermal

stability.117, 118 Sulfonated polyimides as potential PEM candidates, including five-

membered ring phthalic polyimides and six-membered ring naphthalenic polyimides,

have been investigated extensively. The introduction of ionic groups to the polymer

backbone can also be achieved by direct copolymerization of the pre-sulfonated diamine

55

monomers.

Five-membered ring sulfonated polyimides represent the first-generation PEMs,

but were unsuitable for the fuel cell working conditions since chain scission under the

strong acid environment causes materials to degrade quickly and the membrane to

become brittle.119 Therefore, the more hydrolytically stable six-membered ring sulfonated

polyimides have attracted more attention recently. This hydrolytic stability has been

investigated by Genies using model compounds of the sulfonic acid-containing phthalic

imide (Model A) and the naphthalenic imide (Model B).119 13C NMR and IR spectra were

utilized to monitor the intensity of peaks during the aging of two model compounds in

distilled water at 80 oC. It was found that the Model A compound was modified after 1 h

at 80 oC and hydrolyzed completely after 10 h, whereas the Model B stability reached

120 h under the same conditions. Moreover, the naphthalenic imide formed an

equilibrium with its products after a period of time and this limited the hydrolysis to

about 12%. More recent results reported by Jiang,120 who studied the hydrolytic stability

with phthalic sulfonated polyimides and naphthalenic sulfonated polyimides, confirmed

that the six-membered ring polyimides are much more hydrolytically stable than the five-

membered ring anologs. This effect can be explained by the fact that six-membered ring

polyimides have far lower ring strain, positive charges on the carbonyl carbon atoms and

higher orders of the corresponding bonds than five-membered ring polyimides.121

One of the earliest six-membered ring sulfonated polyimides was synthesized by

Mercier and coworkers based on 4,4’-diamino-biphenyl 2,2’-disulfonic acid (BDSA) (a

commercially available sulfonated diamine), 4,4’- oxydianiline (ODA) and 1,4,5,8-

naphthalene tetracarboxylic dianhydride (NTDA).122 Although the primary results

56

showed that naphthalenic SPIs are promising materials for PEMFCs, their solubility was

not good except in chlorophenol. It is known that by introducing ether linkages into the

main chain or bulky groups as substituent to the polyimides, the solubility will be

improved. Thus the new naphthalenic copolyimides from BDSA and non-sulfonated aryl

ether diamines were synthesized.122, 123 For example, the naphthalenic copolyimides

obtained from BDSA, NTDA, and the bis[(4-aminophenyl-oxy)methyl] 2,2-propane

(APMP, a non-sulfonated diamine) are soluble in N-methyl pyrrolidone (NMP) and have

good mechanical properties as well as high ionic conductivity (14.4×10−2 Scm−1 at 80

◦C).123

The Litt group tried to introduce bulky nonsulfonated diamines, such as 4,4’-(9-

fluorenylidene dianiline) (FDA), into the polymer backbone to improve the properties.124,

125 The idea is that by introducing the bulky diamine, a more open structure will be

formed instead of regular close parallel packing of the backbones, which will lead to

larger free volume and confine more water even at elevated temperatures. The hope was

that this may result in higher conductivity and application at higher temperatures. The

bulky group effects have also been investigated by the Watanabe group recently.126, 127

They claimed that the highest ever proton conductivity (1.67 Scm-1 at 120 °C) reported

for a PEM has been obtained with the sulfonated polyimide copolymers containing the

bulky fluorenyl groups (Fig. 2.25).126 Although these bulky polyimides produced higher

conductivites than Nafion®, the hydrolytic stability of these membranes is still a problem.

Watanabe et al. also synthesized partially fluorinated SPIs by incorporating the fluorine-

containing non-sulfonated diamine in order to take advantage of the benefits both of

hydrocarbon and perfluorinated ionomers. The results show that the oxidative stability of

57

these partially fluorinated SPIs is effectively improved, but no further information on the

hydrolytic stability was reported.128

NN

O

O

O

O SO3H

HO3S

NN

O

O

O

O

x

100 - x

SPIH - x (x = 0 - 60)

Figure 2.25. Chemical Structure of Sulfonated Polyimides Containing Fluorenyl

Groups126

The structure of the sulfonated diamine is another important factor that will affect

the properties of the polyimide membranes. Since commercially available sulfonated

diamines are limited and much work has been done on BDSA-based copolyimides as

described above, syntheses of various new sulfonated diamine monomers are very

important for studying the “structure-property” relationships. Much work has been done

in this area by Okamoto et al.129-135 Figure 2.26 shows the structures of the sulfonated

diamines synthesized by the Okamoto group as well as the commercial BDSA.

Compared with the commercially available BDSA sulfonated diamine, the

synthesized ones have either ether linkages or bulky groups. Some of them are of the

main-chain type where the sulfonic acid groups are directly bonded to the polymer

backbone and others are of the side-chain type where the sulfonic acid groups are

attached to the side chains. The variety of the structures makes it possible to investigate

the effects of the structure on the properties comprehensively, especially on the water

stability. The authors suggested that compared with BDSA-based SPIs, the copolyimides

58

based on more flexible sufonated diamines, such as the ODADS-based polyimides, will

enhance the hydrolytic stability greatly but display similar proton conductivity at similar

IEC, and the BAPFDS-based polyimide membranes showed much better water stability

because the rigid structure can be offset by the highly basic nature of BAPFDS. This

means that the basicity of the sulfonated diamine moieties will have positive effect on the

water stability of the polyimide membrane. Moreover, the side-chain type SPI

membranes exhibit much better water stability than the main–chain type SPI membranes

and other aromatic sulfonated polymer membranes, SPIs with lower IEC values will have

better water stability, and the stability of random copolyimides is higher than that of

block or sequenced ones.

59

H2N NH2

SO3H

HO3S 4,4’-diamino-2,2’-biphenyl disulfonic acid (BDSA)

NH2H2N

HO3S SO3H

9,9’-bis(4-aminophenyl)fluorine-2,7-disulfonic acid (BAPFDS)

OH2N

SO3H

NH2

HO3S 4,4’- Diaminodiphenylether-2,2’-diaulfonic acid (ODADS)

NH2

H2N

HO3S(H2C)3O

(2’,4’-diaminophenoxy)propane sulfonic acid

H2N O

HO3S

C

CF3

CF3

O

SO3H

NH2

2,2’-bis[4-(4-aminophenoxy)phenyl] hexafluoropropane disulfonic acid (BAHFDS)

H2N O

HO3S

O

SO3H

NH2

4,4’-bis(4-amino-phenoxy) biphenyl-3,3’-disulfonic acid (BAPBDS)

H2N NH2

O(CH2)3SO3H

O(CH2)3SO3H 2,2’-bis(3-sulfo-propoxy)benzidine (2,2’-BSPB)

or 3,3’- bis(3-sulfo-propoxy)benzidine (3,3’-BSPB)

Figure 2.26 Sulfonated Diamines

60

The McGrath research group has also developed two novel sulfonated diamines.

The first one is 3-sulfo-4’,4’’-bis(3-aminophenoxy) triphenyl phosphine oxide sodium

salt, which was used in the synthesis of a five-membered ring sulfonated polyimide.136

Because of the well known poor water stability of the five-membered ring SPIs and only

one ionic site on the monomer, which will limit the polymer’s conductivity, there was no

further investigation on this sulfonated monomer. Einsla et al137, 138 synthesized another

sulfonated diamine, 3,3’-disulfonic acid-bis[4-(3-aminophenoxy)phenyl] sulfone (SA-

DADPS), which has flexible ether and sulfone linkages to improve the copolymer’s

solubility. This SA-DADPS monomer was then copolymerized with NTDA and three

nonsulfonated diamines, bis[4-(3-aminophenoxy)phenyl] sulfone (m-BAPS), 4,4’-

oxydianiline (ODA), and 1,3-phenylenediamine (m-PDA), to synthesize three series of

high molecular weight disulfonated SPIs (Fig. 2.27). The degree of sulfonation was

controlled by varying the stoichiometric ratio of sulfonated diamine to several

nonsulfonated diamines. These naphthalenic polyimides were prepared by a one-pot

high-temperature polycondensation reacton in m-cresol. Due to the low reactivity of the

six-membered ring anhydride, a catalyst was necessary. Benzoic acid catalyst was added

in the first step, which was believed to promote formation of the trans-isoimide. Then a

basic catalyst, isoquinoline, was added to convert the trans-isoimide to a naphthalimide.

The copolyimides were soluble in NMP and tough films were obtained. Two series of

SPIs were selected to further study the structure-property relationships. The authors

indicated that influence of the structure of the nonsulfonated diamine on the proton

conductivity and water sorption were small at similar IEC, whereas they had a large

influence on the water stability at 80 oC. Although the copolyimide membranes based on

61

the nonsulfonated diamine m-BAPS displayed better water stability, their hydrolytic

stability was still much lower than Nafion® or analogous poly(arylene ether)s. McGrath

et al.7 also indicated that it is desirable to use SPI membranes with low methanol

permeation in room temperature DMFCs instead of hydrogen/air PEM fuel cells due to

their relatively short-term water stability.

H2N O SO

OO NH2

SO3HHO3S

H2N O SO

OO NH2

SO3NH(Et)3(Et)3HNO3S

OO

O O

OO

X = m-BAPS, ODA, or m-PDA

NN

O O

OO

O SO

OO

SO3NH(Et)3(Et)3HNO3S

NN

O O

OO

X

Y 1-Y X

4hrTEAm-Cresol

180 oC1) Benzoic acid, 9 hr2) Isoquinoline, 9hr

Figure 2.27. Synthesis of Disulfonated Polyimide Copolymers137

62

2.4.3. Polyphosphazene Ionomers for PEMs

Polyphosphazenes are semiorganic polymers with a backbone composed of

alternating phosphorus and nitrogen atoms and organic side groups chosen for a

particular application. The resistance to both thermal and chemical degradation and the

ease of chemically attaching various side chains for ion-exchange sites and polymer

crosslinking onto the -P=N- polymer backbone has made them attractive for alternative

PEM materials.139, 140 Principally, two general approaches can be used for the preparation

of sulfonated polyphosphazenes (sPPZs). In the first, an aryl oxide, alkoxide or arylamine

that already bears a terminal sulfonic acid or sulfonate group, replaces the chlorine atoms

in poly(dichlorophosphazene). This method can be viewed as analogous to direct

sulfonation. The second approach, which is a post-modification method, involves the

synthesis of phosphazenes with unsubstituted aryloxy side groups, followed by

sulfonation of these side groups. The sulfonating reagents used include SO3, concentrated

and fuming sulfuric acid, and chlorosulfonic acid.28

Gleria and coworkers141 first reported the sulfonation of

poly[aryloxyphosphazenes] by SO3. From 1996, Wycisk and Pintauro142 began to use the

same technique to sufonate poly[aryloxyphosphazene] for ion exchange membrane

purposes (Fig. 2.28). Poly[(3-methylphenoxy)(phenoxy)phosphazene], poly[(4-

methylphenoxy)(phenoxy)phosphazene], and the corresponding ethyl-substituted

polymers were sulfonated with SO3 in dichloroethane. In sulfonation, the skeletal

nitrogens were attacked to form ≡N→ SO 3 complexes in the first stage, followed by the

arenesulfonation taking place on the methylphenoxy, rather than the phenoxy, side group.

63

The authors found that the methylphenoxy polyphosphazene could be sulfonated easily

using SO3 to a high ion-exchange capacity (up to 2.0 mmol·g-1) with no detectable

polymer degradation, whereas the ethylphenoxy polyphosphazenes undergo severe

polymer degradation. The two sulfonated poly[(methylphenoxy)(phenoxy)phosphazene]

membranes had good mechanical properties and represented an attractive combination of

high IEC, acceptable swelling, and low resistivity, and the poly[(3-

methylphenoxyXphen- oxy)phosphazene] was found to be the best starting material.

Photocrosslinking initiated by benzophenone has been carried out with various

methylphenoxy-, ethylphenoxy-, and isopropylphenoxy-substituted phosphazenes. These

experiments showed that methylphenoxy side chains were the most effective for UV

photocrosslinking of dry films (via a hydrogen abstraction mechanism). The crosslinked

membranes swelled less than Nafion® 117 in both water and methanol without sacrificing

the protonic conductivity compared to non-crosslinked membranes. Moreover, the

methanol permeabilities of the crosslinked membranes were very low (<1.2 x 10-7 cm2·s-

1).143, 144

R

O

PN

On

+ 3SO3

R

O

PN

On

SO3H

SO3H

SO3

Figure 2.28. The Reaction Scheme for Sulfonation of Polyphosphazene with SO3

142

64

Allcock et al.139 sulfonated various aryloxy- and arylaminophosphazenes with

either sulfuric acid (concentrated or fuming) or chlorosulfonic acid. Sulfuric acid

produced sulfonate polyphosphazenenes substituted at the para-position and various

sulfonation degrees were obtained. However, significant chain degradation was observed,

especially with fuming sulfuric acid. The use of chlorosulfonic acid as a sulfonating agent

led to crosslinked insoluble products.

These post-sulfonations have similar disadvantages to post modification of other

copolymers as described earlier. For example, the reactions introduce significant

irregularities in the polymer structure, suffer from severe heterogeneity of the reaction

mixture, and allow for little or no control over the position and degree of sulfonation.

Therefore, direct sulfonation may offer a better route. However, the initial tests on the

direct replacement of chlorine atoms of poly(dichlorophosphazene) (PDCP) with 4-

hydroxybenzenesulfonic acid yielded macromolecular products. The results showed that

the sodium sulfonate group has the ability to react with PDCP first to generate an

unstable substitution product, which led to polymer degradation. Recently, Andrianov et

al.145 developed a novel route to direct sulfonation of polyphosphazenes by using

“noncovalent protection” of the sulfonic acid functionalities (Fig. 2.29). They added a

hydrophobic ammonium ion, such as the dimethyldipalmitylammonium ion, to suppress

the reactivity of the arylsulfonate. Sodium benzenesulfonate was converted to a

hydrophobic ammonium salt, which had no reactivity against PDCP and could be easily

removed after the completion of the reaction. The high molecular weight sulfonated

polyphosphazenes were then synthesized by this method without noticeable degradation

of the polymer backbones.

65

P

Cl

Cl

Nn

PDCP

SS

O

O

O

O

ONR'2R''2R'2R''2NO

DPSA P Nn

O

O

SO3H

SO3H

PDSA

P Nn

O

R

SO3HSS

O

O

O

O

ONR'2R''2R'2R''2NO

NaOR

Mixed Substituent Copolymers

Figure 2. 29. Direct Sulfonation of Polyphosphazenes by the Noncovalent Protection

Method145

2.5. Other Novel Approaches to Improve PEM Properties

Besides the synthesis of new functionalized proton-conducting copolymers, many

other methods have been investigated to improve the PEM properties. For example,

modification of the membranes by acid- or base-doping, using polymer blends, better

understanding and control of polymer microstructure, development of organic/ inorganic

composite systems, crosslinking, grafting of a functional group, or incorporating other

ionic sites like phosphonic acids have all been explored. In this section, several of these

novel approaches will be briefly reviewed.

66

2.5.1. Controlling Morphology Using Block and Multiblock Copolymers

Better understanding and control of copolymer morphology is important for

improving the fuel cell performance. It is well known that Nafion® has highly phase-

separated hydrophilic and hydrophobic domains. The hydrophilic sulfonated groups

interconnected through electrostatic interactions to form ion channels for water and ion

transportation. This unique morphology can explain why the perfluorosulfonate ionomer

has high proton conductivity, even with lower ion exchange capacity and relatively low

water content.146, 147 Thus the morphology of membranes made from the aromatic random

copolymers, such as BPSH copolymers, has been studied64, 90, 148, 149 and compared with

Nafion®. It has been noted that the formation of the continuous hydrophilic domain

strongly depends on the degree of sulfonation and hydrothermal treatment of the

membrane. For BPSH copolymers acidified by method 1 (the membranes have lower

water uptake compared to method 2), the co-continuous phase was observed (AFM)

when the degree of sulfonation was greater than 50%, which is called the percolation

threshold. Below this degree, the morphology forms a closed domain structure, where

isolated hydrophilic domains are surrounded by a hydrophobic matrix. When method 2

acidification was used, the percolation limit decreased to 35% sulfonation degree.

Although increasing ionic concentrations may help form continuous hydrophilic domains

and lead to higher protonic conductivity, the high water uptake values at high degrees of

sulfonation result in a reduction in the mechanical strength, which limits membrane

applicability.

The challenge here is to modify the chemistry of the polymers to obtain

significant protonic conductivity at low hydration levels. Nanophase separated ion-

67

containing block copolymers may offer a possibility.8 Block copolymer ionomers have

hydrophilic blocks (which contain ionic groups such as sulfonic acid) and hydrophobic

blocks. The hydrophilic blocks provide the protonic conductivity and the hydrophobic

blocks offer good thermal and mechanical properties. Phase separation is driven by

chemical incompatibility between the different blocks. The morphology of the block

copolymer (cylinders, spheres, lamellae, etc.) can be controlled by tailoring the chemical

composition, molecular weight, and volume fraction of each of the blocks.150 For

example, lamellar domains can be achieved by using a 50:50 volume fraction of the two

blocks in the copolymer composition. This lamellar morphology makes it possible for the

ionic groups on the hydrophilic block to self-assemble into a co-continuous phase, which

may facilitate high protonic conductivity even at lower degrees of sulfonation and low

water contents.

Considerable research has been done on sulfonated block copolymers, and most

of it has focused focus on sulfonated polystyrene-based triblock copolymers, such as

sulfonated polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene copolymers

(SSEBS).16, 17 Although these copolymers have shown acceptable proton conductivity

and fuel cell performance, the poor chemical stability of the aliphatic backbone limits

their application especially at higher temperatures (> 60 oC). Recently, block or

multiblock copolymers with wholly aromatic backbones have been synthesized by the

McGrath research group151-157 and others.158-160 One of the most interesting candidates

developed by the McGrath group is a series of multiblock ionomers combining highly

fluorinated hydrophobic blocks and 100% sulfonated BPS hydrophilic blocks (Fig.

2.30).152 These multiblock copolymers were synthesized by a two-step process: first the

68

hydrophobic and hydrophilic telechelic macromonomers were prepared with the desired

molecular weights and appropriate end groups, then multiblock copolymers were

obtained through a coupling reaction between the end groups of the two macromonomers.

A significant advantage of this multiblock synthesis is that the possible ether-ether

interchange side reaction, which is well known to occur in pure poly(arylene ether)s, may

be avoided under the mild reaction conditions due to the much higher reactivity of the

activated difluoride monomer. The protonic conductivity vs. relative humidity for the

block copolymers with different block lengths and Nafion® 117 (Fig. 2.31) shows that the

block copolymers with longer block lengths have higher protonic conductivity under

partially hydrated conditions, which may be due to the better nanophase separation. The

“BisAF-BPSH” sample with hydrophobic and hydrophilic block lengths of 8,000 g·mol-1

each has similar or higher proton conductivity compared to Nafion® 117 at all RH values.

This and other results, such as higher water diffusion coefficients (enhanced transport)

than those of the random copolymers, suggests that the multiblock copolymers have good

potential for applicability in low humidity environments.8 Considering that the aliphatic

groups on the bisphenol A monomer may affect the overall oxidative stability, a partially

fluorinated monomer bisphenol-6FA was used to replace BPA in a similar series of

multiblock copolymers.153 Detailed results for the partially fluorinated analogs will be

reported later.

69

F

F F

F F

F

F

F

F O O

CH3

CH3

F

F F

F F

F

F

F

F

m

n

O S O OM

O

O

SO3M

MO3S

MO

m

+

O S O

O

O

SO3M

MO3S

O

CH3

CH3

F

F F

F F

F

F

F

On

Figure 2.30. Synthetic Scheme of BisAF-BPSH Series of Multiblock Copolymers152

70

Figure 2.31. Proton Conductivity vs. Relative Humidity for “BisAF-BPSH” Series

of Multiblock Copolymers and Nafion® 1178

(Reprinted with permission of John Wiley & Sons, Inc., copyright 2006)

Another interesting multiblock copolymer developed in the McGrath research

group used poly(p-phenylene) as the hydrophobic block, which offers excellent thermal

and mechanical properties, and 100% sulfonated BPS as the hydrophilic block (Fig. 2.32).

Ether-ether interchange can also be avoided because there is no ether group in the

hydrophobic poly(p-pheylene) backbone. The properties of this multiblock copolymer are

under investigation.151

71

Figure 2.32. Chemical Structure of PPP/BPS Multiblock Copolymer151

Although early results for multiblock copolymers are promising, the difficulties

encountered in the synthesis of these materials present an obstacle for researchers. For

example, the synthesis of high molecular weight multiblock copolymers is not facile due

to the difficulty in controlling the stoichiometry of the oligomers, and poor

reproducibility. Despite these obstacles, multiblock copolymers continue to capture the

attention of researchers due to the promise of controlling properties through tailored

microstructures.

2.5.2. Organic/Inorganic Composite PEMs

High-temperature proton exchange membranes are very interesting since the

overall cell efficiency can be greatly improved by the accompanying reduction in CO

poisoning of the catalyst and the enhanced kinetics of the fuel oxidation at temperatures

over 100 oC. However, purely polymeric PEMs, such as Nafion®, show poor fuel cell

performance above 100 oC because of the low proton conductivity at low relative

humidity and the poor mechanical properties. Incorporating suitable inorganic fillers into

the ionomer matrix seem to be a promising way to solve these problems and many reports

O SO

O

SO3H

HO3S

O

C O

CO

CO

bn m

Hydrophobic Hydrophilic

72

show that the inorganic fillers can enhance the mechanical properties and water retention

at high temperatures. The incorporation of inorganic particles may also increase the

tortuousity of the pathways for methanol molecules, which can help lower the methanol

crossover in DMFCs.

Several excellent reviews have been published regarding composite

membranes.161, 162 The major inorganic fillers such as heteropolyacids (HPAs), layered

metal phosphates or phosphonates, and metal oxides have been used，and the polymer

matrix has focused on perfluorinated polymers (Nafion®) or nonfluorinated PEEK

copolymers. Properties such as ionic conductivity, water uptake, tensile strength, and

thermal behavior have been systematically investigated. The composite membranes can

be macro-, micro-, or nano-composites depending on the size of inorganic fillers and they

can be fabricated by (1) dispersion filler particles in an ionomer solution followed by

casting, or (2) growth of the filler particles within a preformed membrane or in an

ionomer solution (in-situ method).

Crystalline HPAs are a class of inorganic fillers with high proton conductivity in

their hydrated forms. Although they can not be used alone as a solid electrolyte due to

their high solubility in water, experimental results show that well-dispersed HPA in

sulfonated polymer membranes can improve the protonic conductivity at temperatures

above 100 oC. Among many forms of HPAs, phosphotungstic acid, silicotungstic acid,

and phosphomolybdic acid are the most common. For example, Kim et al.163 fabricated

phosphotungstic acid/BPSH composite membranes with different disulfonation levels of

the polymer matrix via solution blending. The results showed that incorporation of HPA

73

into the sulfonated copolymer significantly reduced the water swelling behavior, without

influencing the proton conductivity at room temperature, whereas the composite

membrane exhibited greater proton conductivity in the temperature range from 100–130

oC. FTIR band shifts suggest that there were interactions between the sulfonic acid and

HPA particles via hydrogen bonding and this may prevent HPA from leaching out of the

membrane. The extraction of HPA depended on the degree of sulfonation but was always

lower than Nafion®. The long-term stability needs to be further confirmed.

Extraction of HPA particles from the membrane is an important issue for the

practical use of the composite membranes, especially for long-term stability. It has been

proposed that incorporation of some polar groups, such as nitrile or phosphine oxide

functional groups, in the polymer matrix may help to prevent HPA loss.7 Zhang et al.164

synthesized sulfonated poly(arylene ether ketone)s containing aromatic nitriles (SPAENK)

and blended them with HPA. The cyano groups on the aromatic rings may serve as a

compatilizer with HPA through polar interactions, but there is not enough evidence to

confirm this hypothesis.

BPSH copolymers have been blended with another inorganic filler - zirconium

hydrogen phosphate by Hill et al.165 The in-situ method was used because the inorganic

filler is insoluble. The water-swollen acid-form BPSH membranes were immersed in

ZrOCl2 solutions at 80 oC. The Zr4+ ions in the hydrophilic portion of the membrane then

can precipitate to form zirconium hydrogen phosphate after immersion in a 1M H3PO4

solution. The composite films had good thermal stability and excellent retention of

zirconium phosphate after water treatment at 120 oC for 100 h. Although the composite

membranes exhibited lower proton conductivity than the pure BPSH membrane at room

74

temperature, the presence of the inorganic particles led to an improvement in high-

temperature conductivity. For example, fully hydrated membranes (40 mol%

disulfonation) with 38 wt% zirconium phosphate had a conductivity of 0.06 S·cm-1 at

room temperature and linearly increased up to 0.13 S·cm-1 in water vapor at 130 oC,

whereas the pure copolymer which had a conductivity of 0.07 S·cm-1 at room temperature

only reached a conductivity of 0.09 S·cm-1 at 130 oC. These results suggest that the

composite membranes are promising for high temperature fuel cell applications.

In the last several years, many organic/inorganic composites have also been

investigated for DMFC. The incorporation of inorganic components such as SiO2, ZrO2,

HPA and metal phosphates has been successfully used to control the methanol

permeability and the proton conductivity.166-168 One major reason that the methanol

permeability was reduced in the composite membranes is that the presence of inorganic

particles increases the tortuous pathways that molecules encounter during permeation as

shown in Figure 2.33.

Polymeric membrane

Inorganic fillers

Diffusing SpeciesPolymeric membrane

Inorganic fillers

Diffusing Species

Figure 2.33. Schematic View of the Increased Pathways of Composite Membrane

75

2.5.3. Polymer Blends

Physically blending different polymers takes advantage of existing materials and

is a straightforward way to make new PEMs. The properties of polymer blends can be

tailored for fuel cell application by varying the components and their compositions. This

method has been found to improve important PEM properties, including water uptake,

proton conductivity, mechanical strength, and methanol crossover. However, the

incompatibility between two different polymers, which can lead to phase separation, is a

major factor that affects the final material properties. Kerres et al. have investigated

extensively the effects of various interactions between two polymer chains, such as ionic

interaction, hydrogen bonding, and dipole-dipole interaction, on the polymer

miscibility.169-172

The van der Waals and dipole-dipole forces between the polymer chains turned

out to be too weak to obtain good polymer blends. For example, simple blending of the

sulfonated PSU and unsulfonated PSU Udel® led to a heterogeneous morphology, too

much swelling, and even dissolution of sPSU at high temperature. Although the hydrogen

bonds formed between sPEEK and the weak base polyamide or polyetherimide will

improve the polymer blend properties including proton conductivity and glass transition

temperature, the high water swelling at elevated temperature, the partial phase separation

and the poor hydrolytic stability in acidic environments suggest that hydrogen bonding

alone is not enough.173

Therefore, a stronger interaction is needed. It was found that the interaction forces

between the acidic and strongly basic blend components, including electrostatic

interactions and hydrogen bonding can serve this purpose. These ionically crosslinked

76

acid-base blends are prepared by mixing a sulfonated polymer with a polymeric N-base,

followed by acid washing to reprotonate the acidic component. A large number of acid-

base blend membranes with various properties have been prepared by this method. For

example, sPEEK Victrex or sPSU Udel® as the acidic component have been blended with

the basic polymers poly(4-vinylpyridine), poly(benzimidazole), poly(ethyleneimine) PEI,

and a self-developed PSU- ortho-sulfone diamine. The membranes showed good proton

conductivity at an IEC of 1 and excellent thermally stability.170

However, some acid-base blends still suffer from the major disadvantages of

increased water swelling and instable mechanical properties at temperatures over 70-90

oC, where the hydrogen bridges and electrostatic interactions break in aqueous

enviorments. In this case, covalently crosslinked blends may offer some advantages.173

77

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190, 191. 142. Wycisk, R.; Pintauro, P. N., J. Membr. Sci. 1996, 119, 155. 143. Graves, R.; Pintauro, P. N., J. Appl. Polym. Sci. 1998, 68, 827. 144. Guo, Q.; Pintauro, P. N.; Tang, H.; O'Connor, S., J. Membr. Sci. 1999, 154, 175. 145. Andrianov, A. K.; Marin, A.; Chen, J.; Sargent, J.; Corbett, N., Macromolecules

2004, 37, 4075-4080. 146. Heitner-Wirguin, C., J. Membr. Sci. 1996, 120, 1. 147. Won, J.; Hye, H. P.; Kim, Y. J., Macromolecules 2003, 36, 3228. 148. Kim, Y. S.; Dong, L.; Hickner, M. A.; Pivovar, B. S.; McGrath, J. E., Polymer 2003,

44, 5729-5736. 149. Kim, Y. S.; A, M.; Hickner; Dong, L.; Pivovar, B. S.; McGrath, J. E., J. Membr. Sci.

2004, 243, 317. 150. Noshay, A.; McGrath, J. E., "Block Copolymers: Overview and Critical Survey."

Academic Press: New York, January 1977. 151. Wang, H.; McGrath, J. E., Preprs Symp. ACS, Div. Fuel Chem. 2005, 50(2), 581. 152. Yu, X.; Roy, A.; McGrath, J. E., 2005, 50(2), 577. 153. Harrison, W.; Ghassemi, H.; Tom A., J. Z.; McGrath, J. E. WO Patent 2005053060,

2005. 154. Ghassemi, H.; Harrison, W.; Zawodzinski, T. A. J.; McGrath, J. E., ACS Polym.

Preprs. 2004, 45(1), 68. 155. Lee, H.-S.; Einsla, B.; McGrath, J. E., Preprs Symp. ACS, Div. Fuel Chem. 2005,

50(2), 579. 156. Wang, F.; Kim, Y.; Hickner, M.; Zawodzinski, T. A., ACS Polym. Mat.: Sci. & Eng.

(PMSE) 2001, 85, 517. 157. Ghassemi, H.; Ndip, G.; McGrath, J. E., Polymer 2004, 45, 5855.

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158. Ghassemi, H.; Zawodzinski, T. A. J. Preprs Symp. ACS, Div. Fuel Chem. 2005, 50(2), 531.

159. Shin, C. K.; Maiera, G.; Andreaus, B.; Scherer, G. u. G., J. Membr. Sci. 2004, 245,

147. 160. Taeger, A.; Vogel, C.; Lehmann, D.; Lenk, W.; Schlenstedt, K.; Meier-Haack, J.,

Macromol. Symp. 2004, 210, 175. 161. Alberti, G.; Casciola, M., Annu. Rev. Mater. Res. 2003, 33, 129. 162. Savadogo, O., J. Power Sources 2004, 127, 135. 163. Kim, Y. S.; Wang, F.; Hickner, M.; Zawodzinski, T. A.; McGrath, J. E., J. Membr.

Sci. 2003, 212, 263. 164. Zhang, H.; Pang, J. h.; Wang, D.; Li, A.; Li, X.; Jiang, Z., J. Membr. Sci. 2005, 264,

56. 165. Hill, M. L.; Kim, Y. S.; Einsla, B. R.; Harrison, W. L.; Wang, F.; Hickner, M. A.;

McGrath, J. E., J. Membr. Sci. 2006. 166. Nunes, S. P.; Ruffmann, B.; Rikowski, E.; Vetter, S.; Richau, K., J. Membr. Sci.

2002, 203, 215.. 167. Ruffmann, B.; Silva, H.; Schulte, B.; Nunes, S. P., Solid State Ionics 2003, 269, 162. 168. Ponce, M. L.; Prado, L.; Ruffmann, B.; Richau, K.; Mohr, R.; Nunes, S. P., J.

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87

Chapter 3. Purity Characterization of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl Sulfone (SDCDPS) Monomer by UV-visible Spectroscopy

Yanxiang Li1, Rachael VanHouten1, Andrew E.Brink2, and James E. McGrath1*

1Macromolecular Science and Engineering & Macromolecules and Interfaces Institute Virginia Polytechnic Institute and State University Blacksburg, VA 24061

2Hydrosize Technologies Inc., Raleigh, North Carolina

88

3.1. Abstract

The purity of the disulfonated monomer, 3,3’-disulfonated-4,4’- dichlorodiphenyl

sulfone (SDCDPS), is very important for obtaining high molecular weight disulfonated

poly(arylene ether sulfone) copolymers, which are promising candidates for proton

exchange membrane (PEM) fuel cells. For the commercialization purpose, direct use of

crude SDCDPS monomer with known purity in the copolymerization will save much

money, energy and time caused by the traditional recrystallization purification process. In

this paper, a novel method to characterize the purity of crude disulfonated monomer,

SDCDPS, has been developed by using UV-visible spectroscopy. The purity of the crude

comonomer was determined from the Beer’s Law plot developed using a pure SDCDPS

sample. The model poly(arylene ether sulfone) copolymers, based on this crude SDCDPS

monomer, 4,4’-dichlorodiphenyl sulfone (DCDPS), and biphenol, were successfully

synthesized. The molecular weight obtained from gel permeation chromatography (GPC)

(Mn> 40 kg·mol-1) was high enough to allow tough films for PEMs to be cast. This

confirmed that the purity characterization method was relatively accurate and applicable,

especially for mass production. The storage time and drying time of SDCDPS were also

studied using Beer’s Law.

Keywords: Disulfonated Monomer; Purity Characterization; UV-Visible Spectroscopy;

Beer’s Law; Direct Copolymerization; Proton Exchange Membrane Fuel Cells

89

3.2. Introduction

A large number of novel ion-containing copolymers have been extensively

investigated for proton exchange membranes (PEMs) to overcome the drawbacks of the

currently commercialized perfluorosulfonic acid Nafion®.1-3 Sulfonated poly(arylene

ether) materials, such as poly(arylene ether ketone)s and poly(arylene ether sulfone)s, are

attractive for use in PEMs because of their well known oxidative and hydrolytic stability

under a fuel cell’s harsh conditions.4 Introduction of the sulfonic acid groups to the

polymer backbone has been achieved by either post sulfonation of commercially

available copolymers or direct copolymerization of sulfonated monomers. 1 It has been

widely admitted that the direct copolymerization method has advantages over the post

modification method, including its easy control of the position and degree of sulfonation,

high acidity, and the ease of minimizing side reactions. Recently, using the direct

copolymerization method to synthesize sulfonated copolymers has also extended its

applications in other areas such as reverse osmosis water purification,5, 6 and polymeric

transducers.7

The disulfonated monomers, which were used in the direct copolymerizations,

were usually synthesized via electrophilic substitution by fuming sulfuric acid. The

monomer 3,3’-disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) was a typical

choice. SDCDPS was first reported by Robeson and Matzner8 in a patent for its flame

retarding properties and was subsequently studied by Ueda et al.9 McGrath’s group10, 11

modified its purification and characterization procedure and was first to directly

copolymerize SDCDPS with 4,4’-dichlorodiphenyl sulfone (DCDPS) and 4,4’-biphenol

90

to synthesize poly(arylene ether sulfone)s for use as proton exchange membranes.

Thereafter, several different disulfonated monomer structures were designed and

synthesized using a similar procedure for the same purpose (Fig. 3.1).12-14

The purity of the disulfonated monomer is very important in obtaining high

molecular weight copolymers using step-growth copolymerization. A systematic study of

the synthesis and characterization of the SDCDPS monomer ensured that the possible

monosulfonated and starting material DCDPS impurities could be avoided when

following the standard procedure.15 The only impurity that remained in the product was

sodium chloride, which was used in excess to salt out the crude sulfonated monomer.

Traditionally, the sodium chloride was removed by recrystallization of the crude product

with a mixture of isopropanol and water. This was conducted two to three times to

increase monomer purity. This recrystallization method was effective in lab scale

experiments but not economical for mass production because it substantially decreased

the monomer yield and wasted solvent, time, and energy.

In this paper, a novel method to characterize the purity of crude disulfonated

monomer, SDCDPS, was developed using UV-visible spectroscopy. A Beer’s Law plot

was developed by first measuring the absorbance of several pure SDCDPS /methanol

dilute solutions with known concentrations, then plotting the absorbance vs.

concentrations of these solutions. The purity of the crude SDCDPS was then easily

determined from the Beer’s Law plot. Because the SDCDPS monomer is sensitive to

moisture, the storage time and drying time were also studied. Poly(arylene ether sulfone)

model copolymers with 35% and 40% degree of sulfonation were synthesized using the

crude SDCDPS monomer with determined purity to confirm the accuracy and

91

applicability of this characterization method. Gel permeation chromatography (GPC)

results showed that copolymers with molecular weights higher than 40 kg·mol-1 were

obtained, which was high enough to form tough membranes for PEM fuel cells.16 The

purpose of this study is to provide an accurate and practical way to characterize the purity

of the novel sulfonated monomer SDCDPS, especially for mass production purposes.

This method could also be used to characterize other similar disulfonated monomer

structures like those illustrated in Figure 3.1.

92

Y X YSO3 (30%)

110 oCNaCl H2O

NaOH

PH = 6-7

NaClY X Y

SO3NaNaO3S

Y = Cl or F X = SO

OCO

C COO

C CO O

, , , or

Figure 3.1. Synthetic Scheme of Disulfonated Monomers with Several Different

Structures

93

3.3. Experimental

3.3.1. Materials

High purity 4,4’-dichlorodiphenyl sulfone (DCDPS) monomer was kindly

provided by Solvay Advanced Polymers Inc. Fuming sulfuric acid with 27-33 wt% of

sulfur trioxide (SO3) was purchased from Aldrich and used as received. Crude 3,3’ –

disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) was provided by Hydrosize Inc.

Monomer grade, high purity 4,4’- biphenol (BP) was obtained from Eastman Chemical.

All the monomers were well-dried in a vacuum oven before copolymerization. The

solvent dimethylacetamide (DMAc) was vacuum-distilled from calcium hydride onto

molecular sieves and stored under nitrogen. Potassium carbonate was dried in vacuo at

120 oC before use. Toluene, methanol, isopropanol, sodium chloride, and sodium

hydroxide pellets were obtained from Aldrich and used as received.

3.3.2. Synthesis Procedures

3.3.2.1. Synthesis and Purification of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl

Sulfone (SDCDPS) monomer

The synthesis of the disulfonated monomer, SDCDPS, followed the synthetic

procedure reported in the literature.10,11,15 The reaction conditions were chosen to make

sure that the monosulfonated and the original DCDPS impurities were avoided. DCDPS

(30 g) and fuming sulfuric acid (60 mL, 30% SO3) (molar ratio was 1:3.3) were added to

a 250 mL three-necked flask equipped with an overhead mechanical stirrer, nitrogen inlet

and condenser. The reaction was performed at 110 oC for 6-7 h. Isolation of the product

94

was achieved using a standard process (Fig. 3.1): salt out by sodium chloride, neutralized

with 10 N sodium hydroxide, and salt out again. The crude SDCDPS was purified by

recrystallization with a mixture of deionized water and IPA (3/7, v/v) to remove the

sodium chloride, which was the only impurity. Recrystallization was repeated up to six

times such that no absorption intensity change was observed in the UV-visible spectrum

when performed at the same solution concentration. The SDCDPS monomer was dried

under vacuum at 160 oC for at least two days.

3.3.2.2. Synthesis of Disulfonated Poly(Arylene Ether Sulfone) (BPSH) Model

Copolymers

The crude monomer SDCDPS obtained from Hydrosize Inc. was directly used to

synthesize the disulfonated poly(arylene ether sulfone) model copolymers. The purity

determined from the Beer’s Law plot was 82.5% ± 1% following the procedure described

in section 3.3.3.2. Poly(arylene ether sulfone) copolymers with 35 mol% (BPSH30) and

40 mol% (BPSH40) degree of sulfonation were synthesized by copolymerization of

SDCDPS, DCDPS, and BP monomers. As an example, the following monomer feed was

used for synthesis of BPSH35: 5.000 g crude SDCDPS (8.397 mmol), 3.617 g DCDPS

(12.595 mmol), 3.909 g BP (20.99mmol). The synthetic procedure was the same as

reported earlier10,11.

3.3.3. Characterization

3.3.3.1. Monomer and Copolymer Characterization

95

1H NMR analyses were conducted with a Varian Unity 400 NMR spectrometer to

confirm the chemical structures of both the SDCDPS monomer and the disulfonated

copolymers. Spectra were obtained using DMSOd6 as the solvent. Intrinsic viscosities

(I.V.) and the molecular weights of the copolymers were characterized by gel permeation

chromatography (GPC) using polystyrene as a standard. GPC experiments were

performed on a liquid chromatograph equipped with a Waters 1515 isocratic HPLC pump,

Waters Autosampler, Waters 2414 refractive index detector and Viscotek 270 dual

detector. 0.05 M LiBr/NMP 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.

3.3.3.2. Procedure for SDCDPS Monomer Purity Characterization by UV-Visible

Spectroscopy

Purity characterizations of the SDCDPS monomer were carried out using a

Shimadzu Model UV-1601 UV-visible spectrometer. The first step was to determine the

molar absorptivity (ε) of SDCDPS monomer in Beer’s Law: A = εbc, in which A is the

absorbance measured from UV-visible spectrum, c is the dilute solution concentration

(mol·L-1), and b is the path length of the sample cell (1 cm). The procedure was as

follows: pure SDCDPS monomer was obtained by purification of crude sample as

described earlier using IPA/DI H2O as the recrystallization solvent mixture. This

recrystallization was performed up to six times until the UV-visible absorption had no

change when measured at the same concentration. The completely dried, pure SDCDPS

monomer (61.6 mg) was dissolved in methanol in a 100 mL volumetric flask to prepare

solution (1) with concentration: 1.254 X 10-3 mol·L-1. Exact volumes in the range of 2-10

96

mL of solution (1) were transferred to a 250 mL volumetric flask and then diluted with

methanol to prepare dilute solutions with various concentrations. UV-Visible absorbance

data generated from these dilute solutions were used to develop the Beer’s Law

calibration curve, which was a straight line in the low absorbance range (< 1.5). The

slope of this straight line was the molar absorptivity (ε) in the equation based on Beer’s

Law. Once the Beer’s Law plot was determined, the purity of the crude SDCDPS sample

was easily obtained by measuring the UV-visible absorbance of the crude sample solution

with a specific concentration that fell in the linear relationship range of the calibration

curve. Each measurement was repeated at least three times, and the average values were

used.

3.4. Results and Discussion

An electrophilic substitution reaction was employed to synthesize the disulfonated

monomer, SDCDPS, by using 30% fuming sulfuric acid as the sulfonation agent.

Possible impurities that could have resulted when synthesizing SDCDPS were the

monosulfonated byproduct, starting materials, and sodium chloride. The monosulfonated

byproduct and residual starting monomer (DCDPS) impurities were effectively avoided

by following the standard synthesis conditions. The absence of these impurities was

confirmed by proper chemical shifts and integrations of the proton NMR peaks. However,

sodium chloride was used in excess to salt out the SDCDPS from the water and thus was

the only impurity that needed to be removed from the crude product. Several

recrystallizations from an IPA/H2O solvent mixture were usually used to purify the

97

SDCDPS monomer. This procedure worked well in lab-scale experiments, but it

substantially decreased the product yield because of the high solubility of SDCDPS in

water. It also wasted time, solvent, and energy, especially when being mass produced. If

one can obtain the accurate purity of the sulfonated monomer, it is desirable to use the

crude SDCDPS directly in the copolymerization because sodium chloride has no

influence on the reaction. UV-visible spectroscopy is a sensitive instrument in

quantitatively determining the concentration of a solution using Beer’s Law if the molar

absorptivity of the sample is available. The SDCDPS monomer dilute solution had two

absorption peaks in the UV-Visible range (wavelength: 210nm and 254nm), which

allowed the use of this technique to determine the monomer’s purity. For consistency, the

peak at 210 nm was used for all following measurements.

The choice of the solvent was the first consideration because it would affect both

the peak position and intensity of the UV-Vis spectra. Three good solvents for SDCDPS

(water, methanol and DMAc) were tested, and the UV-visible spectra were obtained (Fig.

3.2.). SDCDPS sample in DMAc did not generate a good spectrum for unknown reasons.

Good spectra were obtained when water or methanol were used as the solvent. Because it

was found that methanol was easier to use in the solution preparation process to obtain

more accurate results, methanol was chosen as the solvent for all measurements.

To develop the Beer’s Law plot, a very pure SDCDPS sample was required.

SDCDPS was purified by recrystallization from an IPA/H20 (7/3, v/v) solvent mixture,

and the process was repeated up to six times. The purity of the SDCDPS was monitored

by using UV-visible spectra. By keeping the sample concentrations of all measurements

the same, it could be determined that the purity was unchanged after the third

98

recrystallization because the absorbance was no longer changing (Fig. 3.3). Although the

first recrystallization removed most of the salt, at least two more recrystallizations were

required to remove all the salt. The pure SDCDPS (after six times recrystallization) was

used to prepare solution (1) with concentration 1.254 X 10-3 mol·L-1, and then exact

volumes (2~10 mL) of solution (1) were transferred to 250 mL volumetric flasks and

diluted with methanol to prepare dilute solutions with various concentrations. The

absorbance of the solutions scaled with the concentrations linearly as shown in Figure 3.4.

These UV-visible absorbance data (peak at 210 nm) were used to develop the Beer’s Law

calibration curve (Fig 3.5.). It showed that when the absorbance was higher than 1.5, the

curve deviated from the linear relationship (Fig 3.5 left). The nonlinearity may be caused

by many reasons, for example, deviations in absorptivity coefficients at high

concentrations due to electrostatic interactions between molecules in close proximity,

scattering of light due to particulates in the sample, and changes in refractive index at

high analyte concentration, etc.16 The linear portion was plotted (Fig 3.5 right), and the

standard calibration curve for SDCDPS monomer was obtained by averaging three

measurements. The equation of the straight line was averaged to be: Y = 51081X –

0.0229. The measurement error was ±0.7%.

Once the Beer’s Law plot was established, the purity of the crude monomer was

determined by measuring the absorbance of crude sample solution with a known

concentration as described in the experimental part. Since SDCDPS is very susceptible to

moisture uptake, the drying time and storage time were also studied. The fresh, pure

SDCDPS was first dried in a vacuum oven at 160 oC, and the purity was measured after

24, 48, and 72 h. The purity was also measured after 15 days storage in a desiccator. It is

99

shown in Figure 3.6 that the SDCDPS needed to be dried at least 48 h to completely

remove the water, and it was suggested to dry SDCDPS prior to copolymerization

because it absorbed around 4.0% water after 15 days in a desiccator.

The purity of crude SDCDPS monomer synthesized by Hydrosize Inc. was

determined using the Beer’s Law plot and used directly in copolymerization with DCDPS

and biphenol. Figure 3.7 shows the comparison of the absorbance between the pure and

the crude SDCDPS sample at the same concentration. It should be noted that because the

sodium chloride salt was not evenly distributed in the sample, thorough blending of the

crude sample was very important in obtaining accurate results. The purity of the crude

SDCDPS was calculated to be 82.5% ±1%. The model poly(arylene ether sulfone)

copolymers (BPSH) were characterized by GPC and intrinsic viscosity. Results are listed

in Table 3.1. The molecular weights for both polymers with 35% and 40% (mole percent)

degree of sulfonation exceeded 40 kg·mol-1, which were high enough to cast tough films

for PEM fuel cells17. The degree of sulfonation calculated from the 1H NMR spectra

matched very well with the theoretical values (Fig. 3.8). These results confirmed that the

UV-visible characterization method for determining the purity of crude SDCDPS is

relatively accurate and applicable, especially for the mass production of copolymers.

100

H2O

methanol

DMAc

Wavelength (nm)

Abs

orba

nce

H2O

methanol

DMAc

Wavelength (nm)

Abs

orba

nce

Figure 3.2. UV-Visible Spectra of SDCDPS Dilute Solutions Using Different Solvents

101

Two times

Recryst. 3, 5, 6 times, Overlay

Wavelength (nm)

Abs

orba

nce

Two times


Two times


Wavelength (nm)

Abs

orba

nce

Figure 3.3. Effect of the Number of Recrystallization Times on the Absorbance at

the Same Concentration Values (After Two Times Recrystallization, SDCDPS Still

Contains 2.6% ±1% salt)

102

Concentration increase

Wavelength (nm)

Abs

orba

nce

Concentration increase

Wavelength (nm)

Abs

orba

nce

Figure 3.4. The UV-Vis Absorbances of SDCDPS Solutions with Different

Concentrations were Used to Develop the Calibration Curve

103

0

0.5

1

1.5

2

2.5

3

0 1E-05 2E-05 3E-05 4E-05 5E-05 6E-05 7E-05

Concentration (Mol/L)

Abso

rbance

y = 50956x - 0.0216

R2 = 0.9994

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

2.50E-05

3.00E-05

Concentration (Mol/L)Absorbance

0

0.5

1

1.5

2

2.5

3

0 1E-05 2E-05 3E-05 4E-05 5E-05 6E-05 7E-05

Concentration (Mol/L)

Abso

rbance

y = 50956x - 0.0216

R2 = 0.9994

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

2.50E-05

3.00E-05

Concentration (Mol/L)Absorbance

Figure 3.5. Calibration Curve Used to Develop the Beer’s Law Slope. The Left

Graph Shows the Deviation at High Concentrations. The Right graph is the Linear

Calibration Curve at Low Concentrations

104

Wavelength (nm)

Abs

orba

nce

72hr, 48hr, 24hr, in desiccator 15days

Wavelength (nm)

Abs

orba

nce

Wavelength (nm)

Abs

orba

nce

72hr, 48hr, 24hr, in desiccator 15days72hr, 48hr, 24hr, in desiccator 15days

Figure 3.6. Effect of Drying Time and Storage Time on the Absorbance at the Same

Concentrations

105

Wavelength (nm)

Abs

orba

nce

Pure

Unrecrystallized

Wavelength (nm)

Abs

orba

nce

Pure

Unrecrystallized

Wavelength (nm)

Abs

orba

nce

Pure

Unrecrystallized

Figure 3.7. Comparison of the Absorbance of Pure and Crude Samples of SDCDPS.

(The Crude Sample was Provided by Hydrosize Inc.)

106

S* OO

OO S O

O

OO *

x 1-x n

SO3NaNaO3S

fe

g

g

f e

b

b

S* OO

OO S O

O

OO *

x 1-x n

SO3NaNaO3S

fe

g

g

f e

b

b

Degree of sulfonation (%) =( )[ ]

( )[ ] %0.394/2/3/

2/3/=

+++

++

bgfe

gfe

HHHHHHH

Figure 3.8. 1H NMR of Poly(Arylene Ether Sulfone) Copolymers (BPSH-40) was

Used to Determine the Degree of Sulfonation16

107

Table 3.1. Characterization of the Model BPS Copolymers

GPC Results Copolymers SDCDPS purity (%) Mn

(kg·mol-1)Mw

(kg·mol-1)I.V.

(dL·g-1)

Degree of Sulfonation By

1H NMR (%)

BPS35

82.5 45.7 78.9 0.61 35.1

BPS40 82.5 41.9 72.2 0.69 39.0

108

3.5. Conclusions

A novel characterization method for determining the purity of the disulfonated

monomer SDCDPS has been developed by using UV-Visible spectroscopy. Pure

SDCDPS recrystallized from IPA/H2O was used to establish a Beer’s Law plot, which

was then used to determine the purity of the crude product. The results also showed that

the SDCDPS needed to be dried in a vacuum oven at 160 oC for at least 48 h to

completely remove the water. Since the SDCDPS absorbed small amounts of moisture

after storage in a desiccator for several days, it was suggested to dry the SDCDPS

directly before the copolymerization. The model poly(arylene ether sulfone) copolymers

were synthesized by direct copolymerization of the crude SDCDPS with known purity,

DCDPS and BP. The relatively high molecular weights of the copolymers confirmed that

this characterization method was applicable to accurately determine the purity and

directly use the crude SDCDPS without purification process, which can save money, time

and energy. This is especially attractive for the mass production of the copolymers.

109

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McGrath, J. E., Macromol. Symp. 2001, 175, 387. 11. Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.; McGrath, J. E., J. Membr.

Sci. 2002, 197, 231. 12. Li, Y.; Mukundan, T.; Harrison, W.; Hill, M.; Sankir, M.; Yang, J.; McGrath, J. E.,

ACS Preprs. Div. Fuel Chem., 2004, 49(2). 13. Li, X.; Zhao, C.; Lu, H.; Wang, Z.; Na, H., Polymer, 2005, 46, 5820. 14. Xing P.; Robertson G. P.; Guiver, M. D. Mikhailenko, S. D.; Kaliaguine, S., Polymer,

2005, 3257. 15. Sankir M.; Bhanu V. A.; Harrison W. L.; Ghassemi H.; Wiles K. B.; Glass T. E.;

Brink A. E.; Brink M. H.; McGrath J. E., J. Appl. Polym. Sci., 2006,100, 4595

110

16. http://chemistry.hull.ac.uk/lectures/adw/06523- 3%20Molecular%20Spectroscopy%20UV-Vis.pdf

17. Li Y.; Wang F., Yang J.; Liu D.; Roy A.; Case S.; Lesco J.; McGrath, J. E. Polymer,

2006, 47, 4210.

111

Chapter 4. Synthesis & Characterization of Controlled Molecular Weight Disulfonated Poly(Arylene Ether Sulfone) Copolymers and Their Applications to Proton Exchange Membranes Taken from: Yanxiang Li1, Feng Wang2, Juan Yang1, Dan Liu1, Abhishek Roy1，Scott Case3, Jack Lesko3, and James E. McGrath1,* 1Macromolecular Science and Engineering & Macromolecules and Interfaces Institute Virginia Polytechnic Institute and State University Blacksburg, VA 24061 2 PPG Industries Inc., 440 College Park Drive, Monroeville, PA 15146 3 Engineering Science and Mechanics Virginia Polytechnic Institute and State University Blacksburg, VA 24061 Polymer, 2006, 47, 4210-4217 Reprinted with permission from Elsevier, copyright (2006)

112

4.1. Abstract

Tert-butylphenyl terminated disulfonated poly(arylene ether sulfone) copolymers

with controlled molecular weight （s Mn）, 20 to 50 kg·mol-1, were successfully prepared

by direct copolymerization of the two activated halides, biphenol and the endcapper, 4-

tert-butylphenol. The high molecular weight copolymer (molecular weight over 80

kg·mol-1) was also synthesized with 1:1 stoichiometry without an endcapping reagent.

The chemical compositions and the molecular weights of the endcapped copolymers were

characterized by their 1H NMR spectra utilizing the 18 unique protons at the chain ends.

Modified intrinsic viscosity measurements in 0.05 M LiBr/NMP solution further

correlated well with NMR results. Combining the endcapping chemistry with proton

NMR end group analysis and intrinsic viscosity measurements, one can demonstrate a

powerful tool for characterizing molecular weight of sulfonated poly(arylene ether

sulfone) random copolymers. This enables one to further investigate the influence of

molecular weight on several critical parameters important for proton exchange

membranes, including water uptake, in-plane protonic conductivity and selected

mechanical properties. These are briefly discussed herein and will be more fully

described in subsequent publications.

Keywords: Disulfonated Poly(Arylene Ether Sulfone) Copolymer, Controlled Molecular

Weight, Proton Exchange Membrane Fuel Cells.

113

4.2. Introduction

Proton exchange membrane (PEM) fuel cells have attracted much attention in

recent years as promising green energy device. One of the key components for the PEM

fuel cell is the polymeric electrolyte membrane, which serves as the barrier for fuels and

the electrolyte for transporting protons from the anode to the cathode. According to the

type of the fuel used, there are two kinds of PEM fuel cells: Hydrogen/air fuel cells and

direct methanol fuel cells (DMFCs). The perfluorinated sulfonic acid copolymers such as

DuPont’s Nafion®, are promising PEM materials due to their good mechanical, thermal

and chemical stability as well as good protonic conductivity at lower temperatures (<80

oC). However, the high methanol crossover, the reduction in conductivity at higher

temperature and the cost are the major drawbacks that limit their commercial

application.1,2,3 Therefore, to develop the alternative membrane materials that will

overcome these drawbacks is important.

Many families of polymers with differing chemical structures and various

strategies for incorporation of sulfonic acid groups have been explored as PEM

materials.3 Sulfonated poly(arylene ether sulfone)s are good candidates due to their good

acid and thermal oxidative stabilities, high glass transition temperatures and excellent

mechanical strengths.4 Sulfonated poly(arylene ether sulfone)s have been prepared via

polymer modification route, where sulfonate groups were achieved on polymer chain by

sulfonating agents, such as concentrated sulfuric acid or sulfur trioxide.5,6 The McGrath

has reported synthesis of poly(arylene ether sulfone) copolymers by directly

copolymerizing sulfonated monomers. This procedure is more preferable relative to post

114

modification method because of its easy control of the degree of sulfonation, high acidity,

and the ease of side reactions related to the post polymer modification technique.7-9 Other

copolymers synthesized in the McGrath group by direct copolymerization for PEMs have

included: sulfonated poly(arylene ether phosphine oxide),10 poly(arylene ether

ketone)s,11,12 poly(phenyl sulfide sulfone)s,13 substituted polyphenylenes,14 poly(arylene

ether) copolymers containing aromatic nitriles,15 and naphthalene based polyimides. 16

Molecular weight is a fundamental parameter affecting all mechanical behavior of

polymers as is well known. Most reports of new proton exchange membrane materials

have included information on ion content expressed either by the equivalent weight (EW,

g·mole-1) or by the ion exchange capacity (IEC, meq·g-1), protonic conductivity, and

water uptake. Despite the large body of research on this topic, there is almost nothing in

the PEM literature describing molecular weights of candidate materials, even including

Nafion®!3 F. Wang et al.17 previously synthesized controlled molecular weight (Mn)

poly(arylene ether sulfone) (Mn from 20 to 40 kg·mol-1) by offsetting stoichiometry with

a t-butylphenyl endcapping reagent. The t-butylphenyl concentrations relative to the

polymer backbone were characterized by proton NMR to calculate the molecular weight

of the copolymers. They provided intrinsic viscosity (IV) data for these copolymers, and

they found that the intrinsic viscosities were not comparable to those of non-sulfonated

polymers, since the polymer electrolyte chains interact via sulfonate groups. The

McGrath group has begun to utilize NMP with 0.05M LiBr to measure the intrinsic

viscosity. The small amount of salt effectively suppressed the polyelectrolyte effect

allowing improved characterization of the ion containing materials.

115

The overall aim of this research is to establish molecular weight vs mechanical and

electrical property correlations for sulfonated poly(arylene ether sulfone) copolymers that

could be used as proton exchange membranes in fuel cells. In this paper, poly(arylene

ether sulfone) copolymers with 35 mole % disulfonated monomer repeat unit were

successfully synthesized via direct copolymerization method.8, 9 The number average

molecular weights of the copolymers were controlled from 20 to 50 kg·mol-1 by a tert-

butylphenol endcapping reagent and were characterized by the combination of proton

NMR and intrinsic viscosity. The intrinsic viscosities were measured using NMP as

solvent with 0.05 M lithium bromide to break up the ion group aggregation. The linear

correlation of Log Mn with Log intrinsic viscosity showed that this method can provide

more accurate molecular weight information. On the basis of this, the effects of the

molecular weight on the properties of proton exchange membranes, such as water

swelling, protonic conductivity, and mechanical properties were investigated.

4.3. Experimental

4.3.1. Materials

Highly purified 4, 4’-dichlorodiphenyl sulfone (DCDPS) and biphenol (BP) were

kindly provided by Solvay Advanced Polymers and Eastman Chemical, respectively.

They were well dried in vacuo before polymerization but otherwise were used as received.

The 4-tert-butylphenol (TB) endcapper was purchased from Aldrich and was purified by

sublimation. The 3, 3’-disulfonated 4, 4’-dichlorodiphenyl sulfone (SDCDPS) was

synthesized as reported earlier.8 The solvent N, N-Dimethylacetamide (DMAc, Fisher)

116

was vacuum-distilled from calcium hydride onto molecular sieves and stored under

nitrogen before use. Potassium carbonate was dried in vacuo before copolymerization.

Toluene and methanol were obtained from Aldrich and were used as received.

4.3.2. Synthesis of Disulfonated Poly(Arylene Ether Sulfone) Copolymers with

Controlled Molecular Weight

The aromatic nucleophilic step growth copolymerization was conducted in a 3-

neck flask equipped with a mechanical stirrer, nitrogen inlet and a Dean Stark trap. One

typical polymerization for a controlled molecular weight of 40 kg·mol-1 (BPS35-40)

copolymer was as follows: The 4,4'-biphenol (5.000 g, 26.866 mmol), 4,4'-

dichlorodiphenyl sulfone (5.075g, 17.671 mmol), 3,3’-disulfonated 4,4’-dichlorodiphenyl

sulfone (4.674 g, 9.515 mmol) and 4-tert-butylphenol (0.096 g, 0.641 mmol) were added

to the flask, followed by 1.15 equivalent of potassium carbonate. Dry DMAc was

introduced to afford about a 20% solids concentration and toluene was used as an

azeotropic agent. The reaction mixture was heated under reflux at 160 ºC for 4 h, which

stripped off most of the toluene to dehydrate the system. Finally, the bath temperature

was raised slowly to 175 ºC for 24 h, which caused the DMAc to reflux. The viscous

solution was cooled to room temperature, and then diluted with DMAc to form about a

20% copolymer solution. The copolymer was isolated by precipitation in deionized water,

filtered and dried in a vacuum oven at 120 oC for 24 h. Dried polymer was ground into

powder and then washed extensively with methanol and deionized water several times to

completely remove salt and any potential residual endcapping reagent, and finally

vacuum dried at 120 ºC for 24 h. The molecular weights of the copolymers were

117

controlled by varying the ratio of monofunctional monomer TB to difunctional monomer

4, 4’-biphenol. The copolymers synthesized were designiated as BPS35-xx (salt form) or

BPSH35-xx (acid form), where 35 means that all copolymers were contained 35% (mole

%) of disulfonated repeat units, and xx represents the target molecular weight was xx

kg·mol-1.

4.3.3. Membrane Preparation

The salt form copolymers were dissolved in DMAc (5~10% w/v) at room

temperature. The solutions were first filtered with 0.45 µm syringe filters, and then cast

onto clean glass substrates. The films were carefully dried with infrared heat at gradually

increasing temperatures (up to ~ 60 oC). The membranes were removed from the glass

plates by submersion in water and then were dried in vacuo at 120 oC for at least 24 h.

The salt form membranes were completely converted into their acid forms by

boiling the membranes in 0.5 M sulfuric acid for 2 h, followed by boiling in deionized

water for another 2 h. The acid form membranes were washed with deionized water

completely and then stored in fresh deionized water at room temperature.


1H NMR spectra were conducted with a Varian Unity 400 NMR spectrometer in

DMSO-d6. Intrinsic viscosities (IV) were determined in NMP with or without 0.05 M

LiBr at 25 ºC using an Ubbelohde viscometer.

The water uptake was obtained by measuring the difference in the weight between

dry and fully hydrated membranes. The sample films were equilibrated in deionized

118

water at room temperature for at least 48 h. Then the membranes were dried in the

vacuum oven at 110 oC for 24 h. Weights of wet and dry membranes were measured. The

ratio of weight gain to the original membrane weight was taken as the water uptake (WU)

according to equation (1).

%100×−

=dry

drywet

WWW

WU …………….(1)

where Wwet and Wdry are the masses of wet and dried samples, respectively.

Proton conductivity at 30°C at full hydration (in liquid water) was determined in

a window cell geometry18 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

response19. The conductivity of the membrane can be calculated from the measured

resistance and the geometry of the cell according to equation (2):

AZl'

=σ …………..(2)

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.

In determining proton conductivity in liquid water, membranes were equilibrated

at 30 °C in DI water for 24 h prior to the testing.

Mechanical tensile tests were performed using an Instron 4468 Unversal Testing

Machine at room temperature and 40% relative humidity, with a crosshead displacement

speed of 5 mm·min-1. The gauge lengths were all set to 40 mm. Pneumatic grips were

119

employed with a pressure of 207 kPa. The specimens with thickness around 50 µm and

size of 60 mm × 12 mm were used for testing. For each film, four replicates were tested.

Since the stress-strain curves of the specimens that lasted the longest under stretching

represent the real mechanical behavior of the films better (i.e., the specimen did not fail

by macroscopic defects), the curve with the largest elongation at break was chosen to plot

in Figure 4.5. Thermogravimetric analysis was performed in air with a heating rate of 5

oC·min-1 to determine the water contents of the test specimens, which were were about

10%.


4.4.1. Synthesis and Characterization of Copolymers

The tert-butylphenyl terminated BPS35 series copolymers were successfully

synthesized by the aromatic nucleophilic substitution reaction of 3, 3’-disulfonated 4, 4’-

dichlorodiphenyl sulfone, 4, 4’-dichlorodiphenyl sulfone, 4, 4’-biphenol and 4-tert-

butylphenol in DMAc, which contained toluene as an azeotropic agent to dehydrate the

system (Fig. 4.1). The monofunctional monomer, 4-tert-butylphenol, was used as the

endcapping reagent, in which the phenol functional group has similar reactivity as

biphenol. The mole ratio of the tert-butylphenol monomer to the difunctional

comonomers was varied to control the stoichiometry in accordance with the modified

Carother’s equation.20 The molecular weights were controlled from 20 to 50 kg·mol-1.

For all copolymers, the mole ratio of SDCDPS to DCDPS was fixed to 3.5/6.5. The

copolymers were first isolated by precipitation of the reaction solutions in stirred

deionized water and dried in an oven. Then the dried crude copolymers were ground into

120

powders and washed extensively with methanol and deionized water to remove any

possible residual endcapping agent and salt. It is very important that all the inorganic

salts involved in the condensation process, and any possible residue starting monomers

need to be removed as complete as possible, since any of these impurities in the final

products would interfere the characterization of viscosity and molecular weight by NMR

as discussed in the following paragraphs.

For comparison, a BPS35-control copolymer with high molecular weight was

synthesized with 1:1 stoichiometry and no endcapping reagent.

The 1H NMR spectra were used to identify the molecular structure of the

copolymers and to confirm the degree of sulfonation. The peak assignments of the

aromatic region of BPS35-50 (Fig. 4.2) confirm the anticipated chemical structure. The

degree of sulfonation was determined from the integral ratios of proton peaks e, f, g, and

b. The chemical shifts for the three protons (e, f and g) attached to the sulfonated unit are

7.0, 7.7 and 8.3 ppm, respectively, while the peak at 7.8 ppm corresponds to the proton b

attached on the non-sulfonated unit. The mole content of sulfonated unit in BPS35-50

copolymer chain is 34.6% based on the integrals of b proton to the average of e, f and g

protons. The calculation could be described by the equation (3):

Degree of sulfonation (%) = ( )[ ]

( )[ ] %6.344/2/3/

2/3/=

+++

++

bgfe

gfe

HHHHHHH

……….(3)

Table 4.1 lists the contents of sulfonated units in the series of copolymers. All the values

are in good agreement with the ratio of feed monomers (SDCDPS/DCDPS – 35/65),

121

which suggests that all the starting monomers were successfully incorporated into the

copolymer chains.

The molecular weights of the copolymers were calculated from the relative 1H

NMR integrals of the tert-butyl endgroups and the aromatic resonances. For example,

Figure 4.3 shows the proton NMR spectrum of a copolymer with target molecular weight

50 kg ·mol-1 (BPS35-50), in which methyl protons were observed at 1.2 ppm. The

presence of the tert-butyl peak in BPS35-50 proton NMR confirms that tert-butylphenol

was chemically attached at the ends of copolymer, and only the chemical bonded TB

groups are useful in quantitatively calculating the molecular weight of one polymer chain

based on its 1H-NMR spectrum. While there are 18 methyl protons on the two tert-butyl

endgroups of one polymer chain, two sources contribute to the aromatic protons: the

sulfonated or non-sulfonated aromatic repeat units in the interior of the polymer chain,

and the terminal phenyl rings. There are 16 and 14 phenyl protons for non-sulfonated and

sulfonated units, respectively. Since the experimental degree of sulfonation for BPS35-50

was calculated to be 34.6% by proton NMR, the average protons per repeat unit is

16*0.654 + 14*0.346 = 15.3, and the total number of phenyl protons from the interior

repeat units is 15.3n, where n is the number of the average polymer chain repeat units.

Assuming 100% conversion of all monomers in the condensation polymerization, two

types of terminal groups existed in these copolymers, as shown in Scheme 1. One

terminal group (left side) is a simple 4-tert-butylphenol residue, which contains 4 phenyl

protons Another terminal group (right side) is a residue of 4-tert-butylphenol, which

contains 4 phenyl protons, and is attached with either a disulfonated monomer (34.6%

possibility) or non-sulfonated dihalide monomer (65.4% possibility), so the total phenyl

122

protons at this end is 4 + (8 * 0.654 + 6 * 0.346) = 11.3. Therefore the total average

phenyl protons from two terminal groups are 11.3 + 4 = 15.3, and the total phenyl protons

in one polymer chain is 15.3n + 15.3. The number ratio of aromatic to methyl protons

equals to the integration ratio of the two types protons in 1H-NMR spectrum, so the

molecular weight of copolymer was calculated from the equation (4).

299.168

183.153.15=

+×n Aromatic protonsMethyl protons2

99.16818

3.153.15=

+×n Aromatic protonsMethyl protons ………(4)

Where n is the number of repeat units, which was calculated to be 98.41. Accordingly the

average molecular weight (Mn) of the BPS35-50 copolymer was calculated to be: Mn =

98.41×481. 7 (g·mol-1) + 585.4 (g·mol-1) = 47,989 g·mol-1, where 481.7 g·mol-1 is the

average molecular weight per repeat unit, and 585.4 g·mol-1 is the molar mass of the two

endgroups. Molecular weights of 20 to 40 kg·mol-1 copolymers were prepared and

characterized using the similar technique (Table 4.1). It is obvious that the experimental

molecular weights are in close agreement with the targeted values, which on the other

hand confirmed the close 100% conversion of all the monomers.

Viscosity is one of the most important parameters in charactering polymer property.

Simple dilute solution viscosity measurements are widely used in polymer science, but

have not been well developed for ion-containing PEMs. One possible reason is that by

comparing neutral and charged macromolecules, neutral systems retain their random coil

conformation down to very low concentrations. In contrast, it is well known that as one

dilutes a charged macromolecule the so called “polyelectrolyte effect” appears. This

123

effect produces a more extended chain showing higher dilute solution viscosities as the

concentration is reduced, largely due to charge repulsion.21

It is also well known within the biomembrane community that dilute solution

viscosities of polyelectrolytes should be measured in the presence of a low molar mass

salt, which is able to screen the charges. It is recognized that the optimum salt

concentration might depend upon the chemical structures, molar mass range, charged

densities, etc. We have established that as little as 0.05 M LiBr allows one to obtain the

correct intrinsic viscosity values (Fig. 4.4).

Both reduced and inherent viscosities of BPS35-control were measured in NMP

with or without LiBr in the polymer concentration range of 0.15 to 1.0 g·dL-1.

Correlations of viscosities and concentration are plotted in Figure 4.3a (without LiBr) and

4.33b (with 0.05M LiBr). In the case of polymer solution without LiBr, both reduced and

inherent viscosities increase as polymer concentration decreases, consistent with the idea

of the polyelectrolyte effect. However, the introduction of lithium bromide into polymer

solution made the viscosities (reduced and inherent)-concentration relationships of

BPS35-control very similar to a non-charged polymer solution, as shown in Figure 4.3b.

The conventional extrapolation of reduced and inherent viscosities to the infinite dilute

solution gives us the relatively accurate intrinsic viscosity of BPS35-control (1.04 dL·g-1),

which is much lower than the value measured in pure NMP (2.79 dL·g-1). As summarized

in Table 4.1, intrinsic viscosities of all copolymers agree quite well with their designed

and measured molecular weights. The intrinsic viscosity measurements of the copolymers

could be compared to one another since the only difference in polymer structure is the

endgroups, which are less than 1% by weight.

124

A fairly good linear relationship of Log(Mn) and Log(IV) for 4 tert-butylphenol

endcapped copolymers is plotted in Figure 4.5. This relationship provides us an

alternative but powerful tool in characterizing molecular weights of polymers with

similar chemical composition. For example, the molecular weight of BPS35-control

copolymer was estimated from the above linear relationship to be 83.1 kg·mol-1.

Moreover, this technique can be used for other PEMs as well, such as sulfonated

poly(arylene ether ketone)s, polyimides and more.

125

HO OHSClO

O

BPDCDPSSO3Na

CH3

CH3CH3HO

S ClCl

O

O

TB

SDCDPSK2CO3

SO

O*

DMAc/Toluene~ 160 oC, Reflux 4 h175 oC, 24 h

+

+

NaO3S

O O SO

OO O * S

O

OO

CH3

CH3CH3

CH3

CH3

H3C Ox

1-x n

KO3S SO3K

Cl

x = 0.35; BPS35, Target Mn: 20, 30, 40 and 50 kg·mol-1


Copolymers Containing 35 mole % Disulfonate Repeat Unit.

126

SO

OO O S

O

OO O *

x 1-x

KO3S SO3K

e f

g

h i a b c d

g

b

f

d, i a, c, h

e

SO

OO O S

O

OO O *

x 1-x

KO3S SO3K

e f

g

h i a b c dSO

OO O S

O

OO O *

x 1-x

KO3S SO3K

e f

g

h i a b c d

g

b

f

d, i a, c, h

e

Figure 4.2. The Copolymer Structures and Degree of Sulfonation were Determined

by 1H NMR Spectra in the Aromatic Region (BPS35-50 Copolymer).

127

H2O

DMSO

Aromatic Protons

Methyl Protons

H2O

DMSO

Aromatic Protons

Methyl Protons

Figure 4.3. Molecular Weights can be Calculated from the Relative 1H NMR

Integrals of the Tert-Butyl Endgroups and the Aromatic Resonances (BPS35-50

Copolymer in DMSO-d6).

128

y = -1.1575x + 3.2424

y = -1.243x + 2.3294

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1 1.2Concentration (g·dL-1)

Vis

cosi

ty (d

L·g-1

)

In pure NMP In 0.05M LiBr/NMP solution

y = -0.1651x + 1.0394

y = 0.359x + 1.0466

0.5

0.8

1.1

1.4

1.7

2

0 0.2 0.4 0.6 0.8 1Concentration (g·dL-1)

Vis

cosi

ty (d

L·g-1

)

y = -1.1575x + 3.2424

y = -1.243x + 2.3294

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1 1.2Concentration (g·dL-1)

Vis

cosi

ty (d

L·g-1

)

In pure NMP In 0.05M LiBr/NMP solution

y = -0.1651x + 1.0394

y = 0.359x + 1.0466

0.5

0.8

1.1

1.4

1.7

2

0 0.2 0.4 0.6 0.8 1Concentration (g·dL-1)

Vis

cosi

ty (d

L·g-1

)

(3a) (3b)

Figure 4.4. Correlations of Reduced (▲) and Inherent (■) Viscosities with

Copolymer Concentration of BPS35-Control in Pure NMP (3a), and NMP

Containing 0.05 M LiBr (3b).

129

Table 4.1. Characterization of BPS35 copolymers

SDCDPS (%) Target Mn (kg · mol-1)

n by NMR (kg · mol-1)

IVa (dL · g-1)

Target Experimental (by NMR)

20 19.9 0.43 35 33.9 30 28.8 0.48 35 34.1 40 38.1 0.63 35 34.7 50 48.0 0.74 35 34.6

Controlb 83.1c 1.04d 35 34.2 a Intrinsic viscosities were determined in 0.05 M LiBr/NMP solution at 25 °C. b Control copolymer was prepared with 1:1 stoichiometry without tert-butylphenol. c Mn of the control copolymer was derived from the log(I.V.)-log(Mn) plot. d Intrinsic viscosity of the control copolymer without LiBr was 2.79 dL·g-1.

130

y = 0.6457x - 3.1623R2 = 0.9464

-0.45

-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

4.2 4.3 4.4 4.5 4.6 4.7

Log[Mn, (g.mol -1)]

Log[

I.V.,(

dL.g-1

)]

Figure 4.5. Relationship Between Log(Intrinsic Viscosity) and Log(Mn) for BPS-35

Copolymers

131

4.4.2. Membrane Characterization

These controlled molecular weight copolymers enabled ones to examine the

influence of molecular weight on several different critical parameters important for

proton exchange membranes. Sulfonated copolymers tend to phase separate in

hydrophilic and hydrophobic domain morphology. Water resides in these hydrophilic

domains and plays a critical role in proton transport22. Kim, et. al.23 reported increasing

water uptake and proton conductivity with increasing degree of disulfonation for BPSH

copolymers. High proton conductivity is desirable but high water uptake results in

excessive swelling and poor dimensional stability. Understanding the influence of

molecular weight on water uptake and conductivity for the copolymer at a particular

degree of disulfonation will lead in optimizing the overall fuel cell performance. In Table

4.2, the water uptake and conductivity of BPSH35-xx copolymers in liquid water at room

temperature is provided. The water uptake decreased modestly as the molecular weight of

BPSH35-xx increased from 20 to 80 kg·mol-1. But, at this point it is difficult to correlate

the influence of molecular weight for the copolymers on proton transport under fully

hydrated conditions. The small deviations in the conductivity values are well within the

experimental error range of 10%. However, determination of proton conductivity under

partially hydrated conditions for these copolymers with varying molecular weight is

ongoing. This will give a better understanding about the influence of molecular weight on

proton transport.

The mechanical properties of PEMs, such as fatigue resistance, are very important for

the development of non-fluoroniated PEMs operated at both room and elevated

temperatures. Although the intermediate degree of sulfonation (35 mol %) was chosen to

132

keep relatively low water uptake and maintain membranes’ mechanical strength, it is also

believed that mechanical properties of PEMs should be enhanced by high molecular

weight. Stress-strain curves showed the modulus, strength, and elongation to break of

PEM films were significantly influenced by the molecular weight. These materials were

measured under ambient conditions with 40% relative humidity to investigate the effect

of molecular weight (Fig. 4.6 and Table 4.3). It can be seen that Young’s modulus, yield

stress/strain and elongation at break were all affected by the sample molecular weight.

Among these parameters, the elongation at break varied the most with molecular weight,

increasing from approximately 15% for BPSH35-20 to 78% for BPSH35-50. This

behavior was attributed to more chain entanglements at higher molecular weights. It is

reasonable to expect that high molecular weight could be important for preventing

pinhole formation, even at elevated temperatures, and the molecular weight may also

improve fatigue resistance and long term stability, which will be investigated in the future.

133

Table 4.2. The Results of Water Swelling and Conductivity Test for BPSH35

Copolymers with Controlled Molecular Weights in Liquid Water at 30 oC

Target Mn (kg · mol-1)

IEC* (meq · g-1)

Water uptake (%)

Conductivity (S·cm-1)

20 1.49 40 0.070 30 1.50 43 0.080 40 1.52 42 0.081 50 1.52 38 0.080

Control 1.50 36 0.077 * IEC = (1000/MWrepeat unit)* Degree of Sulfonation *2 (–SO3H), where degree of sulfonation was determined by 1H NMR.

134

Figure 4.6. Stress-strain Curves of BPSH35 Copolymers (Thin Films) as Function of

Molecular Weight.

45

47

49

51

53

55

57

59

0 20 40 60 80

Strain, %

Stre

ss, M

Pa

1, 202, 303, 404, 505, Contro

12

43

5Mn (kg · mol-1

45

47

49

51

53

55

57

59

0 20 40 60 80

Strain, %

Stre

ss, M

Pa

1, 202, 303, 404, 505, Contro

12

43

5Mn (kg · mol-1 Mn (kg·mol-1)

135

Table 4.3. The Tensile Properties of BPSH35 Copolymers (thin films) as Function of

Molecular Weight.

Mn (kg · mol-1)

Modulus (GPa)

Yield Strain (%)

Yield Stress(MPa)

Strength*

(MPa) Elongation at Break * (%)

20 1.46 ± 0.28 3.04 ± 0.38 39.6 ± 7.7 52.0 15.7 30 1.08 ± 0.23 4.34 ± 0.21 38.0 ± 8.4 52.1 32.3 40 1.36 ± 0.24 4.83 ± 0.28 47.9 ± 7.7 57.7 63.4 50 1.53 ± 0.27 4.23 ± 0.69 44.1 ± 10.2 59.0 78.7

Control 1.92 ± 0.30 3.68 ± 0.79 58.1 ± 7.1 66.3 48.8 * For strength and elongation at break data, the values are from the longest stress-strain curves.

136

4.5. Conclusions

A series of controlled molecular weight, poly(arylene ether sulfone) copolymers

containing 35 mole % disulfonated monomer per repeat unit were synthesized and

characterized by 1H NMR, intrinsic viscosity, water uptake, proton conductivity, and

mechanical property. A small amount of lithium bromide (0.05 M) in NMP can

effectively suppress the “polyelectrolyte effect” appearing in measuring the intrinsic

viscosity of a charged macromolecule, which allowed obtaining more accurate data than

previously used simple dilute solution viscosity measurements. Combing 1H NMR

analysis of end groups and intrinsic viscosity measurements, it can be conclude that the

molecular weights of the synthesized copolymers were from 20 to 50 kg·mol-1, which are

much closed to the designed values. The effects of molecular weights on the properties of

proton exchange membranes were also studied. It was found that with increasing the

molecular weights, the water uptake decreased modestly. The molecular weight had no

obvious influence on proton conductivity under fully hydrated conditions. Furthermore,

the mechanical properties of the membrane, such as the modulus strength and elongation

at break were improved by increasing the molecular weight as well. The characterizations

of conductivity, water uptake, and mechanical properties provide us some very useful

guidelines in designing sulfonated polymers as PEM.

4.6. Acknowledgements

The authors would like to thank the National Science Foundation Partnership for

Innovation Program (EHR- 0332648), and the Department of Energy (contract # DE-

FC36-01G011086) for the financial support that funded this research.

137

4.7. References

1. Thomas, S.; Zalbowitz, M. Fuel Cells: Green Power; Los Alamos National Laboratory: Los Alamos, NM, 1999

2. Inzelt, G.; Pineri, M.; Schultze, J.W.; Vorotyntsev, M.A., Electrochim. Acta, 2000; 45,

2403. 3. Hickner, M.A.; Ghassemi, H.; Kim, Y.S.; Einsla, B.R.; and McGrath J.E., Chem. Rev.,

2004; 104, 4587 4. Wang, S.; McGrath, J.E. Step Polymerization; Rogers, M.; Long, T. E., Eds.; Wiley:

New York, 2003 5. Noshay, A.; Robeson, L.M. J. of Appl. Polym. Sci.,1976; 20, 1885 6. Johnson, B.C.; Yilgor, I.; Tran, C.; Iqubal, M.; Wightman, J.P.; Lloyd, D.R.; McGrath,

J.E. J. Polym. Sci. Part A: Polym. Chem. 1984; 22, 721 7. Wang, F.; Ji, Q.; Harrison, W.; Mecham, J.B.; Formato, R.; Kovar, R.; Osenar, P.; and

McGrath, J.E., Polym. Preprs., 2000; 41(1), 237 8. Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.; Zawodzinski, T.; and

McGrath, J.E. Macroml. Symp., 2001; 175, 387. 9. Wang, F.; Hickner, M.; Kim, Y.S.; Zawodzinski, T.; and McGrath, J.E., J. Membr. Sci,

2002; 197, 231. 10. Wang, F.; Mecham, J.; Harrison, W.; and McGrath, J.E., Polym. Mater.: Sci. & Engi.

2001, 84, 913 11. Wang, F.; Chen, T.; and Xu, J. Macrmol. Chem. and Phys. 1998; 199, 1421 12. Wang, F.; Li, J.; Chen, T.; and Xu, J., Polymer, 1999, 40, 795 13. Wang, F.; Mecham, J.; Harrison, W.; and McGrath, J.E. Polym.Preprs.,2000; 41(2),

1401 14. Ghassemi, H.; McGrath, J.E. Polym. Preprs. 2002; 43, 1021. 15. Sumner, M.J.; Harrison, W.L.; Weyers, R.M.; Kim, Y.S.; McGrath, J.E; Riffle, J.S.;

Brink, A.; Brink, M.H., J. Membr. Sci 2004, 239, 199. 16. Einsla, B.R.; Hong, Y.T.; Kim, Y.S.; Wang, F.; Gunduz, N.; and McGrath, J.E. J

Polym Sci, Part A: Polym. Chem., 2004, 42, 862.

138

17. Wang, F; Glass, T.; Li, X.; Hickner, M.; Kim, Y.S.; and McGrath, J.E., Polym. Preprs.

2002, 43(1), 492. 18. Zawodzinski, T.A.; Neeman, M.; Sillerud, L.O.; Gottesfeld, S., J. Phys. Chem. 1991,

95, 6040. 19. Springer, T. E.; Zawodzinski, T. A.; Wilson, M. S.; Gottesfeld, S., J. Electrochem.

Soc. 1996, 143, 587. 20. Jurek, M. J.; McGrath, J.E. Polymer, 1989, 30, 978. 21. Schmidt, M., Polyelectrolytes with Defined Molecular Archetecture, Advanced in

Polymer Sciences, 2004. 22. Kreuer, K. D., Solid State Ionics, 2000, 136, 149. 23. Kim, Y. S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y. T.; Harrison, W.;

Zawodzinski, T. A.; McGrath, J. E., J. Polym. Sci. Part B: Polym. Phys., 2003, 41, 2816.

139

Chapter 5. Partially Fluorinated Disulfonated Poly(Arylene Ether Sulfone) Copolymers with Controlled Molecular Weights for Proton Exchange Membranes Yanxiang Li, Juan Yang, Anand Badami, Abhishek Roy, Ozma Lane, and James E McGrath Macromolecular Science and Engineering & Macromolecules and Interfaces Institute Virginia Polytechnic Institute and State University Blacksburg, VA 24061

140

5.1. Abstract

Partially fluorinated disulfonated poly(arylene ether sulfone) copolymers based on

a hexafluoro bisphenol A monomer were investigated as proton exchange membranes

(PEMs). The introduction of fluorine groups into the polymer backbone is thought to

potentially increase the durability of the membrane electrode assembly (MEA). Two

series of controlled molecular weight, partially fluorinated disulfonated poly(arylene

ether sulfone) copolymers (6FSH) with acid content fixed at 35 mol% and 48 mol% were

successfully prepared via direct step growth copolymerization. The acid contents were

chosen to effectively compare with the previously reported results of biphenol-based

polysulfones (BPSH). For each sulfonation level, a series of copolymers with molecular

weights ranging from 20 to 50 kg·mol-1 were synthesized. The molecular weight was

controlled by addition of a monofunctional monomer, tert-butylphenol, together with

offsetting the stoichiometry of the feed comonomer ratios. The experimental molecular

weights were characterized by a combination of 1H NMR end group analysis and

modified intrinsic viscosity measurements in 0.05 M LiBr/NMP. Atomic force

microscopy (AFM) was used to characterize the membrane morphology. The influence of

molecular weight on water uptake and proton conductivity were studied and compared

with the BPSH copolymer results.

Keywords: Partially Fluorinated Copolymers, Disulfonated Poly(Arylene Ether Sulfone)s,

Controlled Molecular Weight, Proton Exchange Membrane Fuel Cells.

141

5.2. Introduction

The electrochemical and mechanical properties of sulfonated copolymers such as

Nafion®, sulfonated polyethersulfones, etc. have become increasingly important as their

potential application in proton exchange membrane fuel cells (PEMFC) becomes more

evident.1,2 Molecular weight is a fundamental parameter affecting all mechanical

properties of polymers. Although most reports of new proton exchange membrane

materials have included information on ion content (EW or IEC), protonic conductivity,

and water uptake, there is little in the PEM literature describing molecular weights of

candidate materials, especially for Nafion®.3 Thus, it is meaningful to clarify this issue

for the effective comparison of membrane properties.

Sulfonated poly(arylene ether sulfone) copolymers are good candidates for PEMs

due to their good acid and thermal oxidative stabilities, high glass transition temperatures

and excellent mechanical strength.4,5 This family of copolymers has been investigated

thoroughly by McGrath’s research group for both hydrogen/air and direct methanol fuel

cells.6-10 The representative BPSH copolymers (Fig. 5.1) have shown comparable or even

better PEM properties to the state-of-art perfluorosulfonic acid Nafion®. However, when

fabricated into membrane electrode assemblies (MEAs), delamination and high

interfacial resistance of the BPSH membranes caused by the incompatibility between the

membrane and the Nafion-bonded electrodes lowered their fuel cell performance and

decreased the MEA’s long-term stability. Long-term stability of MEAs is one of the most

critical requirements for the successful large-scale production and application of proton

exchange membrane fuel cell technology. Two strategies have been suggested to address

142

this problem: using a hydrocarbon copolymer to replace the Nafion® binder in the

catalyst layer, making the electrodes more similar to the membrane, and introduction of

some fluorine component into the copolymer backbone to promote the compatibility

between the existing Nafion® electrodes and the membrane.11, 12

Poly(arylene ether sulfone)s based on the partially fluorinated 6F bisphenol A

monomer are of interest because they may improve certain properties of PEMs. It has

been proposed that they can provide a more hydrophobic membrane surface that may

lower the water uptake, and the fluorine-rich surface may be more compatible with

electrodes that contain Nafion® and may result in more durable MEAs, which will display

lower interfacial losses. The McGrath research group has reported some promising

properties on these partially fluorinated copolymers.12,13 Previous results showed that the

molecular weight for the BPSH copolymers had an effect on the membrane mechanical

properties, but little effect on the water uptake and proton conductivity was found.14,15 A

correlation of molecular weight and PEM properties would also be valuable for the

promising partially fluorinated copolymers.

In this paper, partially fluorinated disulfonated poly(arylene ether sulfone) copolymers

(6FSH, Fig. 5.1) with controlled number average molecular weights ( nM ) from 20 to 50

kg·mol-1 were successfully synthesized for two series with acid content (or degree of

disulfonation) fixed at 35 mol% and 48 mol%. These acid contents were chosen to

effectively compare with the previously reported results of biphenol-based polysulfones

(BPSH). nM values of the copolymers were controlled by addition of a monofunctional

monomer, tert-butylphenol, and were characterized by 1H NMR and modified intrinsic

viscosity measurements. NMP with 0.05 M LiBr has been used as a solvent for intrinsic

143

viscosity measurements of ion-containing copolymers instead of simple dilute solution

viscosity measurements in pure solvents. The small amount of salt effectively suppressed

the polyelectrolyte effect, allowing improved characterization of the ion-containing

materials.3 The aim of this study was to confirm the molecular weight effect on

membrane properties using the partially fluorinated 6FSH copolymer structures, and also

to examine the effect of molecular structure to some extent.

144

O

1-x n

SO

OO

HO3S SO3H

O SO

OO

x

O

1-x n

SO

OO

HO3S SO3H

C

CF3

CF3

O SO

OO

xC

CF3

CF3

BPSH Copolymer

6FSH Copolymer

Figure 5.1. Disulfonated Copolymer Structures with Biphenol (BPSH) or 6F

Bisphenol A (6FSH) Units in the Backbones

145

5.3. Experimental

5.3.1. Materials

High purity 4,4’-difluorodiphenylsulfone (DFDPS) was purchased from Aldrich.

Monomer grade 4,4’-hexafluoroisopropylidenediphenol (6F-BPA) was provided by

DuPont. Both were well dried in a vacuum oven before use but otherwise used as

received. The 4-tert-butylphenol (TB) endcapper was purchased from Aldrich and was

purified by sublimation. The disulfonated monomer 3,3’-disulfonated 4,4’-

difluorodiphenylsulfone (SDFDPS) was synthesized following a literature procedure.6,16

The solvent N,N-dimethylacetamide (DMAc, Fisher) was vacuum-distilled from calcium

hydride onto molecular sieves. Potassium carbonate was dried in vacuo before use.

Toluene and methanol were obtained from Aldrich and used as received.

5.3.2. Copolymerization

The step-growth copolymerization employed a modified procedure from that

reported previously.15 A typical copolymerization for a controlled molecular weight of 40

kg·mol-1 copolymer with 35 mol% disulfonated comonomer was described as follows:

6F-BPA (5.000 g, 14.871 mmol), DFDPS (2.496 g, 9.818 mmol), SDFDPS (2.423 g,

5.287 mmol), and TB (0.071 g, 0.469 mmol) were added to a three-neck flask equipped

with mechanical stirrer, nitrogen inlet and a Dean-Stark trap. Potassium carbonate (1.15

equivalents) and dry DMAc were introduced to afford a 20% solids concentration.

Toluene (DMAc/Toluene = 2/1) was used as an azeotroping agent. The reaction mixture

was heated under reflux at 160 oC for 4 h to dehydrate the system, followed by complete

146

removal of the toluene. Then, the bath temperature was raised slowly to 175 oC for 48 h,

which caused the DMAc to reflux. The solution became viscous and was cooled to room

temperature, then diluted with DMAc to form about a 20% copolymer solution. The

copolymer was isolated by precipitation in deionized water, filtered, and dried in a

vacuum oven for 24 h at 120 oC. The crude dry copolymer was then ground to a powder

and washed with ethanol and deionized water extensively to remove any free endcapping

agent and salt. The copolymer was finally vacuum dried at 120 oC for 24 h. The resulting

copolymers were designated as 6FS35-xx (salt form) or 6FSH35-xx (acid form), and

6FS48-xx (salt form) or 6FSH48-xx (acid form), where numbers 35 and 48 represent the

mole percent of disulfonated monomer in the copolymer structure (or degree of

sulfonation), and xx represents the target number average molecular weight (kg·mol-1).

The molecular weight was controlled from 20 to 50 kg·mol-1 by varying the amount of

monofunctional monomer 4-tert-butylphenol and the ratio of the difunctional monomers.

The control copolymers, which were synthesized using a 1:1 stoichometry of the

difunctional monomers without endcapper, were also synthesized for comparison.

5.3.3. Membrane Preparation and Acidification

Films of the copolymers were cast from 5~10 wt% solutions in DMAc on a glass

plate under an IR light and carefully dried in a vacuum oven at 120 oC for at least 24 h.

The salt-form films were then transformed into the acid form by boiling the films in 0.5

M H2SO4 for 2 h to convert the pendant sulfonate salt groups into free acid groups. The

residual sulfuric acid was removed by boiling the films in deionized water for another 2 h.

The acid-form membranes were then stored in fresh deionized water at room temperature.

147


Chemical structures and acid contents of all the copolymers were characterized by

1H NMR spectra conducted with a Varian Unity 400 NMR spectrometer. The solvent was

DMSO-d6. Intrinsic viscosity (IV) measurements were carried out in NMP with 0.05 M

LiBr at 25 ºC using an Ubbelohde viscometer. Each solution for viscometry was freshly

prepared and was kept at 25 ºC for 10 min prior to the measurement.

To obtain water uptake values, the sample films were equilibrated in deionized

water at room temperature for at least 48 h. The wet membranes were blotted dry and

immediately weighed. Then the membranes were dried in the vacuum oven at 110 oC for

24 h, and weighed again. Water uptake was calculated as the ratio of the difference

between wet and dry membrane weight divided by dry membrane weight and expressed

as a weight percent.

Proton conductivity at 30 °C at full hydration (in liquid water) was determined in

a window cell geometry17 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.18 In determining proton conductivity in liquid water, membranes were

equilibrated at 30 °C in deionized water for 24 h prior to the testing.

Atomic force microscopy (AFM) images were obtained using a Digital

Instruments MultiMode scanning probe microscope with a NanoScope IVa controller

(Veeco Instruments, Santa Barbara, CA) in the tapping mode. A silicon probe (Veeco)

with an end radius of <10 nm and a force constant of 5 N·m-1 was used to image samples.

148

Samples were equilibrated at 30% relative humidity for at least 12 h before being imaged

immediately at room temperature and approximately 15-20% relative humidity.


5.4.1. Synthesis and Characterization of Copolymers

Figure 5.2 shows the reaction scheme for the direct step-growth copolymerization

of tert-butylphenyl-terminated partially fluorinated poly(arylene ether sulfone)s

containing sulfonic acid groups. These copolymers were based on the monomers DFDPS,

SDFDPS, 6F-BPA, and the endcapper TB. The mole percent of disulfonated repeat units

was fixed theoretically at 35% (6FSH35) and 48 % (6FSH48) for the two series,

respectively. These acid contents were chosen to compare the results with the previously

reported BPSH35 series controlled molecular weight results with either the same degree

of sulfonation (6FSH35) or the same ion exchange capacity (6FSH48 series, IEC: 1.50

meq·g-1). The monofunctional monomer, tert-butylphenol, was used as the endcapping

reagent to control the copolymer molecular weight from 20 to 50 kg·mol-1 for both

degrees of sulfonation. Since the 6F-BPA monomer has low reactivity due to the strong

electronegativity of -CF3 groups, the reaction required 48 h or longer reaction times,

which is much longer than the 24 h required for the BPSH system. Also, the more

reactive fluorine functional monomers DFDPS and SDFDPS were used instead of

chlorine functional monomers DCDPS and SDCDPS to compensate for the low reactivity

of 6F-BPA. The crude dry copolymers were ground to a powder and washed extensively

with ethanol and deionized water to remove any possible free endcapping reagent and

residual salt to obtain accurate NMR integration values for molecular weight (Mn)

149

calculation. For comparison, high molecular weight copolymers (6FS35-control and

6FS48-control) were synthesized with 1:1 stoichiometry without an endcapping reagent.

The aromatic region of the 1H NMR spectrum indicated that the expected

chemical composition was indeed obtained (Fig. 5.3 shows the spectrum for 6FS35-50 as

an example). The degree of disulfonation was calculated based on the integration values

of k, and (a, b) proton peaks. Peak k was assigned to the proton attached on the

sulfonated units, while the peaks (a, b) were protons on the unsulfonated units. Since

there were two k protons and eight (a, b) protons per repeat unit, the mole percent of

sulfonated unit for 6FS35-50 was calculated to be 32.7% according to the following

equation:

Degree of sulfonation = %7.322/8/)(

2/=

++ kba

k

HHHH

The degrees of sulfonation for all copolymers were calculated using the same method

(Table 5.1). The calculated values were slightly lower than the theoretical values due to

the low monomer reactivity of 6F-BPA and SDFDPS.

The experimental molecular weights of the copolymers were calculated from the

relative 1H NMR integrals of the tert-butyl endgroups and the aromatic resonances

following the same procedure described in the literature.15 1H NMR spectra of 6FS35-50

are shown in Figure 5.4 as an example. There were 18 methyl protons on the two end

tert-butyl groups, the average aromatic protons per repeat unit was 15.3, and 15.3

aromatic protons were attached on the terminal phenyl rings. Thus, the following

equation was used to calculate the number of repeat units:

(15.3n +15.3)/18 = 136.94/2.00

150

Where n is the number of repeat units, which was calculated to be 79.6. Accordingly the

number average molecular weight (Mn) of 6FS35-50 was calculated to be:

Mn = 79.6 × 621.4 g·mol-1 + 585.4 g·mol-1 =5.0 × 103 g·mol-1

where the average molecular weight per repeat unit is 621.4 g·mol-1 and the molar mass

of the endgroups is 585.4 g·mol-1. All the molecular weights calculated are listed in Table

5.1. It was reassuring that the experimental molecular weights were in agreement with the

target values.

Intrinsic viscosities were measured using the solvent NMP with 0.05 M LiBr at

room temperature. The small amount of LiBr effectively suppress the polyelectrolyte

effect caused by the interactions of the ionic groups in the dilute solution.15, 19 The

intrinsic viscosity values matched the molecular weight trend very well. The log(Mn) vs.

log(IV) plots are shown for both series of copolymers in Figure 5.5. The good linear

relationship confirmed the experimental molecular weight results. Also, the plots were

used to determine the molecular weight of control copolymers without the endcapper.

Compared with the BPSH copolymers at similar molecular weight, the intrinsic viscosites

of the 6FSH copolymers were lower due to the polymer and solvent interactions.

151

CHOSFO

O

6F bisphenol ADFDPS

SO3Na

CH3

CH3CH3HOS FF

O

O

TBSDFDPS

K2CO3DMAc/Toluene~ 160 oC, Reflux 4 h175 oC, 48 h

+

+NaO3S

O * S

O

OO

CH3

CH3CH3

1-x n

F OH

CF3

CF3

SO

O* O

CH3

CH3

H3C O

KO3S SO3K

C

CF3

CF3

O SO

OO

xC

CF3

CF3

O * SO

OO

CH3

CH3CH3

1-x n

SO

O* O

CH3

CH3

H3C O

HO3S SO3H

C

CF3

CF3

O SO

OO

xC

CF3

CF3

H+

Figure 5.2. Synthesis of Tert-butylphenyl Terminated Partially Fluorinated

Poly(Arylene Ether Sulfone) Containing Sulfonic Acid Groups (x = 0.35 or 0.48)

152

O SO

OC

CF3

CF3

O O SO

OC

CF3

CF3

O*

SO3KKO3S

*

x 1-x

abcdefghk

j i

k

d

i

e

f, g, j

a, b

h, e

O SO

OC

CF3

CF3

O O SO

OC

CF3

CF3

O*

SO3KKO3S

*

x 1-x

abcdefghk

j i

k

d

i

e

f, g, j

a, b

h, e

Figure 5.3. 1H NMR Spectrum of 6FS35-50 Copolymer (Aromatic Region)

153

Figure 5.4. The Molecular Weight of the Copolymer can be Calculated from the

Relative 1H NMR Integrals of the Tert-butyl Endgroups and the Aromatic

Resonances. (6FS35-50 Copolymer in DMSO-d6)

154

Table 5.1. Characterization of 6FS35 and 6FS48 Series Copolymers

Degree of Sulfonationb (%) Copolymer

Mn by NMR (kg·mol-1)

IVa

(dL·g-1) Target Calculated

6FS35-20K 19.9 0.23 35 32.5 6FS35-30K 27.5 0.30 35 33.7

6FS35-40K 37.8 0.44 35 34.1

6FS35-50K 49.5 0.53 35 32.7

6FS35-control 56.0c 0.60 35 31.7

6FS48-30K 33.9 0.28 48 46.0

6FS48-40K 47.4 0.42 48 47.0

6FS48-50K 55.9 0.46 48 46.3

6FS48-control 74.8c 0.64 48 46.3

a Intrinsic viscosities were determined in 0.05 M LiBr/NMP solution at 25 oC. b Mole % of sulfonated monomer in repeat unit c The molecular weights of the control copolymers were determined from the Log(I.V.)

vs. Log (Mn) plots

155

y = 0.939x - 4.677

R2 = 0.9937

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

4.2 4.3 4.4 4.5 4.6 4.7 4.8

Log [Mn, g.mol-1]

Log [IV, dL.g

-1]

y = 1.0251x - 5.19

R2 = 0.9902

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

4.5 4.6 4.7 4.8 4.9

Log [Mn, g.mol-1]

Log [IV, dL.g

-1]

6FS35 Series 6FS48 Series

y = 0.939x - 4.677

R2 = 0.9937

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

4.2 4.3 4.4 4.5 4.6 4.7 4.8

Log [Mn, g.mol-1]

Log [IV, dL.g

-1]

y = 1.0251x - 5.19

R2 = 0.9902

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

4.5 4.6 4.7 4.8 4.9

Log [Mn, g.mol-1]

Log [IV, dL.g

-1]

6FS35 Series 6FS48 Series

Figure 5.5. Relationship Between Log(Intrinsic Viscosity) and log(Mn) for 6FS35

and 6FS48 Copolymers

156

5.4.2. Characterization of Membranes

Water uptake and proton conductivity are two of the most important properties for

PEM materials. For the same chemical structure under fully hydrated conditions, these

two parameters depend mainly on the ion exchange capacity (IEC). It has been reported

that the proton conductivities of BPSH copolymers had a linear relationship with IEC,

while water uptake increased with the increase of IEC before the percolation limit.20

When the IEC was higher than the percolation limit, the membrane absorbed too much

water to maintain good mechanical strength. For practical applications, relatively high

proton conductivity with low water uptake is always the target for developing new PEM

materials. Introducing the fluorine component to the polymer backbone was a strategy to

make the membrane more hydrophobic and decrease the water swelling.

The water uptakes and proton conductivities of 6FSH35 and 6FSH48 copolymer

membranes have been measured under fully hydrated conditions at room temperature

(Table 5.2). It was found that the molecular weight had little effect on these two

properties, which agreed with the previously reported results for BPSH35.15 Compared

with the BPSH35 series of copolymers, at the same degree of sulfonation, 6FSH35

copolymers had lower water uptake and conductivity, but at the same IEC values,

6FSH48 copolymers had higher water uptake and conductivity. This difference was the

result of the different chemical structures.

The microphase-separated morphology is an important feature in determining

PEM performance. IEC and hydrothermal treatment are two major factors that will affect

the membrane morphology. In this paper, all the membranes were acidified at boiling

temperature to achieve the best phase separated morphology.20 AFM showed that the

157

molecular weight had no apparent effect on the membrane morphology within each series,

because all of the copolymers have similar IEC values within the same series (Fig. 5.6).

158

Table 5.2. Water Uptake and Conductivity Characterization for 6FSH35 and 6FS48

Copolymers with Controlled Molecular Weights in Liquid Water at 30 oC

Copolymers IECa

(meq·g-1) Water uptake

(%) Conductivity

(S·cm-1)

6FS35-20k 1.02 27.6 NA* 6FS35-30k 1.06 26.4 0.060 6FS35-40k 1.07 27.3 0.050 6FS35-50k 1.06 27.6 0.055

6FS35-Control 1.05 23.4 0.050 6FS48-20k 1.48 - - 6FS48-30k 1.50 - - 6FS48-40k 1.50 145 0.095 6FS48-50k 1.48 132 0.090

6FS48-Control 1.48 118 0.100 a. The ion exchange capacity (IEC) values were calculated based on the experimental

degree of sulfonation using the equation: IEC = (1000/MWrepeat unit)* Degree of Sulfonation *2 (–SO3H),

159

100 nm

28.8 kg/mol

100 nm

48.0 kg/mol

100 nm

83.1 kg/mola

100 nm

27.5 kg/mol

100 nm

49.5 kg/mol

100 nm

~70 kg/mol

BPSH series

6FSH series

100 nm

28.8 kg/mol

100 nm100 nm

28.8 kg/mol

100 nm

48.0 kg/mol

100 nm100 nm

48.0 kg/mol

100 nm

83.1 kg/mola

100 nm100 nm

83.1 kg/mola

100 nm

27.5 kg/mol

100 nm100 nm100 nm

27.5 kg/mol

100 nm

49.5 kg/mol

100 nm100 nm100 nm

49.5 kg/mol

100 nm

~70 kg/mol

100 nm100 nm100 nm

~70 kg/mol

BPSH series

6FSH series Figure 5.6. Morphology Characterization of the BPSH35 and 6FSH35 Series of

Copolymers with Different Molecular Weights by AFM.

160

5.5. Conclusions

Two series of partially fluorinated poly(arylene ether sulfone) copolymers with 35

mol% and 48 mol% acid contents were successfully prepared via direct step-growth

polymerization. The molecular weights of both series of copolymers were controlled

from 20 to 50 kg·mol-1 by addition of the monofunctional monomer tert-butylphenol,

together with offsetting the stoichiometry of the feed comonomer ratios. 1H NMR

combined with modified intrinsic viscosity measurements was used to characterize the

experimental molecular weights. The improved intrinsic viscosity measurement method

in 0.05 M LiBr/NMP allows more accurate results due to the suppression of the

polyelectrolyte effect. The experimental values were in close agreement with the

theoretical values. The morphology of copolymer membranes with different molecular

weights was characterized by AFM, which suggested that the morphology is a function of

the degree of sulfonation, irrespective of the molecular weight. This result further

confirmed that the molecular weight has little effect on the properties of proton exchange

membranes, such as water swelling and conductivity, although it did affect the

membrane’s mechanical properties.


The author appreciated the support of this research by the Department of Energy

(contract # DE-FC36-01G011086) and UTC Fuel Cells (contract #PO 3651).

161

5.7. References 1. Savadogo O. J., New Mater Electrochem. Syst., 1998, 1, 47 2. Kim Y. S.; Dong L.; Hickner M.A.; Pivovar B.S.; McGrath J.E., Polymer, 2003, 44,

5729 3. Hickner M.A.; Ghassemi H.; Kim Y.S.; Einsla B.R.; and McGrath J.E., Chem. Rev.,

2004, 104, 4587 4. Dumais J.J; Cholli A.L.; Jelinski L.W.; Hedrick J.L.; and McGrath J.E.,

Macromolecules, 1986, 19,1884 5. Wang S.; McGrath J. E., in Step Polymerization, M. Rogers, T. E. Long, Wiley, Eds.

2002 6. Wang F.; Hickner M.; Ji Q.; Harrison W.; Mecham J.; Zawodzinski T.; McGrath J. E.,

Macromol. Symp. 2001, 175 (1), 387. 7. Harrison W.L.; Wang F.; Mecham J.; Bhanu V.A.; Hill M.; Kim Y.S.; and McGrath

J.E., J. Polym. Sci. Part A: Polym. Chem., 2003, 41, 2264. 8. Kim Y.S.; Wang F.; Hickner M.; Zawodzinski T.A.; McGrath J.E., J. Membr. Sci.

2003, 212 (1-2), 263. 9. Wang F.; Hickner M; Kim Y. S.; Zawodzinski T. A.; and McGrath J. E., J. Membr.

Sci. 2002, 197, 231. 10. Harrison W. L.; Hickner M. A.; Kim Y. S.; and McGrath J. E., Fuel Cells, 2005, 5(2),

201 11. Zhang L; Ma C.; and Mukerjee S., Electrochem. Acta. 2003 48, 1845. 12. Wiles K. B.; Dissertation, Virginia Tech, 2005, 311 13. Kim Y. S.; Einsla B.; Sankir M.; Harrison W.; Pivovar B. S., Polymer, 2006, 47,

4026 14. Wang F.; Glass T.; Li X., ; Hickner M.; Kim Y.S.; McGrath J.E., Polym. Preprs.,

2002, 43(1), 492 15. Li Y.; Wang F., Yang J.; Liu D.; Roy A.; Case S.; Lesco J.; McGrath, J. E. Polymer,

2006, 47, 4210. 16. Sankir M.; Bhanu V. A.; Harrison W. L.; Ghassemi H.; Wiles K. B.; Glass T. E.;

Brink A. E.; Brink M. H.; McGrath J. E., J. Appli. Polym. Sci., 2006,100, 4595.

162

17. Zawodzinski, T.A.; Neeman, M.; Sillerud, L.O.; Gottesfeld, S., J. Phys. Chem. 1991,

95, 6040. 18. Springer, T. E.; Zawodzinski, T. A.; Wilson, M. S.; Gottesfeld, S., J. Electrochem.

Soci. 1996, 143, 587. 19. Yang J., Li Y.; Wang H.; Hill M.; Yu X.; Wiles K. B.; Lee H.; McGrath J. E., Preprs.,

ACS, Fuel Divi., 2005, 50(2), 701 20. Kim Y. S.; Wang F.; Hickner M.; MaCartney S.; Hong Y. T.; Zawodzinski T.A.;

McGrath, J. E., J. Polymer Sci. Part B: Polym. Phys. 2003, 41, 2816

163

Chapter 6. Comparative Investigation of Three Series of Poly(Arylene Ether Ketone) Copolymers for Proton Exchange Membrane Fuel Cells Yanxiang Li, Abhishek Roy, Anand Badami, Melinda Einsla, Thekkekara Mukundan, and James E. McGrath* Macromolecular Science and Engineering & Macromolecules and Interfaces Institute Virginia Polytechnic Institute and State University Blacksburg, VA 24061

164

6.1. Abstract

This paper has comparatively investigated three series of disulfonated

poly(arylene ether ketone) random copolymers for proton exchange membrane fuel cells,

which are becoming increasingly important as alternative energy sources with high

efficiency and no pollution for stationary, automobile and portable power. These

copolymers were synthesized based on 4,4’-hexafluoroisopropylidenediphenol (6F-BPA),

one of the three ketone-type monomers [4,4’-difluorobenzophenone (DFBP), 1,4-bis (p-

fluorobenzoyl) benzene (PBFB), and 1,3-bis(p-fluorobenzoyl) benzene (MBFB)], and

their corresponding disulfonated comonomers. The synthesis followed the direct

copolymerization method and the sulfonic acid content of each polymer was controlled

by varying the feed ratio of the non-sulfonated and sulfonated dihalide monomers. The

membrane properties such as water uptake, proton conductivity, and morphology were

studied and compared among these three series. It was found that the copolymers

containing more flexible molecular structures had better phase separation, higher water

uptake and proton conductivity than the copolymers with rigid structures at the same ion

exchange capacity (IEC). The primary methanol permeabilities of copolymers with

mono-ketone units showed lower values than Nafion®.

Keywords: Disulfonated Poly(Arylene Ether Ketone) Copolymers; Proton Exchange

Membrane; Fuel Cells; Direct Copolymerization

165

6.2. Introduction

It is well known that the applicable proton exchange membrane (PEM) materials

for both hydrogen/air fuel cell and direct methanol fuel cell (DMFC) systems should

possess the following characteristics: high protonic and low electronic conductivity, low

permeability to fuel and oxidant, low water uptake and water transport, oxidative and

hydrolytic stability, good mechanical properties, low cost, and capability for fabrication

into membrane electrode assemblies (MEAs).1 To substitute the current commercialized

perfluorosulfonic acid Nafion® type PEM materials which are limited by their loss of

conductivity at high temperatures, high methanol permeability in DMFCs, and high cost,

many strategies have been explored, including synthesis of new functionalized proton

conducting polymers, better understanding and control of polymer microstructure,

polymer blends, and composite membranes.2

Sulfonated poly(arylene ether ketone)s are a family of important candidates for

PEMs in fuel cells. These materials have excellent physical properties, including high

modulus, toughness, and good thermal and chemical resistance.3, 4 Generally, the sulfonic

acid groups have been introduced to the polymer backbone via two methods: post

modification of the commercially available polymers by sulfonating agent or directly

copolymerizing sulfonated comonomers. Direct copolymerization of sulfonated

comonomers has been proven to be better than the post sulfonation method, in that it

allows for precise control the degree of sulfonation and avoids several side reactions.5, 6

These ion containing copolymers tend to phase separate into hydrophobic and

hydrophilic domains. It was believed that the phase-separated morphology of these ionic

166

materials played a dominant role in the hydration and conductivity of membranes.7, 8

Research in the McGrath group9-10 showed that the better phase separation improved

several properties of PEMs, including water swelling and proton conductivity, especially

at lower relative humidity. The effect of polymer molecular structure on the membrane

properties is worthy of investigation, since it will provide useful information for the

molecular design of future PEM materials. It was reported that for different chemical

structures, copolymers with higher hydration numbers had higher proton conductivity,

irrespective of the sulfonic acid content. The hydration number of the membrane, which

is the mass-based water uptake, is directly related to the polymer chemical structure.

Therefore, this value can be used to compare membranes of different polymer

architectures.9

In this paper, three disulfonated ketone-type comonomers were synthesized, and

the chemical structures are shown in Figure 6.1. These comonomers were subjected to

direct step growth copolymerization with their corresponding unsulfonated monomers

and 4,4’-hexafluoroisopropylidenediphenol (6F-BPA) to obtain three series of

copolymers containing sulfonic acid groups: B series with mono ketone units, and PB and

MB series with para diketone and meta diketone units, respectively. Previously,

McGrath’s group had reported the primary results of B and PB series copolymers for

PEMs.11 More and detailed results will be shown and discussed here. In this work, the

consistent membrane preparation and acidification methods for all the membranes were

used, and followed the high temperature acidification method, which was proven to result

in better phase separated morphology than the room temperature method.12 The goals of

this research are to explore the novel poly(arylene ether ketone) PEM materials, and give

167

some insight into how the chemical structures affect the membrane morphology and the

PEM properties.

In this research, three ketone monomer structures were chosen to examine the

structure-property relationships of the PEM membranes under consistent conditions. It

was found that the membranes with mono ketone (B series) and para diketone (PB series)

rigid structures had similar water uptake and proton conductivity at similar ion exchange

capacity (IEC) values, while the meta diketone (MB series) structures had higher values

in both water uptake and proton conductivity at the same IEC. This may be due to two

reasons: one potential explanation is that the higher flexibility of meta ketone linkages

resulted in easier aggregation of the hydrophilic and hydrophobic domains, which was

also reflected in the membrane morphology characterized by atomic force microscopy

(AFM); another reason may be caused by the higher hydration level (or bigger hydration

number) of the MB series copolymers structures at the same IEC values. The partially

fluorinated monomer 6F-BPA was used here to decrease the crystallinity of the ketone

copolymers and thus improve their solubility. Furthermore, the methanol permeability

and MEA proton conductivities of the B series copolymers were also studied and

compared with Nafion®.

168

F CO

F

NaO3S SO3Na

F CO

NaO3S

F

SO3Na

CO

F

SO3Na

CO

F CO

NaO3S

SDFBP

SPBFB

SMBFB

Figure 6.1. Three Disulfonated Ketone-Type Comonomer Structures

169

6.3. Experimental

6.3.1. Materials

4,4’-Hexafluoroisopropylidenediphenol (6F-BPA), received from Ciba, was

purified by sublimation. The ketone-type monomers 4,4’-difluorobenzophenone (DFBP),

1,4-bis(p-fluorobenzoyl)benzene (PBFB), and 1,3-bis(p-fluorobenzoyl)benzene (MBFB)

were purchased from Aldrich and used as received. Disulfonated derivatized

comonomers, 3,3’-disodium sulfonate-4,4’-difluorobenzophenone (SDFBP), 1,4-bis(3-

sodium sulfonate-4-fluorobenzoyl)benzene (SPBFB), and 1,3-bis(3-sodium sulfonate-4-

fluorobenzoyl)benzene (SMBFB), were synthesized in house. All these monomers were

well dried under vacuum prior to use. The solvent N-methyl-2-pyrrolidinone (NMP,

Fisher) was vacuum-distilled from calcium hydride onto molecular sieves. Potassium

carbonate (Aldrich) was dried in vacuo before use. Toluene, sodium chloride, fuming

sulfuric acid (30% free SO3), and isopropanol were obtained from Aldrich and used as

received. Nafion® 117 was obtained from ElectroChem.

6.3.2. Synthesis of the Disodium Salt of Comonomers

Disulfonated derivatized comonomers (SDFBP, SPBFB, SMBFB, Fig. 6.1) were

synthesized according to a modified literature method.13, 14 For example, SDFBP was

synthesized as follows: DFBP (30 g, 0.14 mol) was dissolved in 60 mL of 30% fuming

sulfuric acid in a 100-mL, three-necked flask equipped with a mechanical stirrer and a

nitrogen inlet/outlet. The solution was heated to 120 oC for 6 h to produce a

homogeneous solution. Then it was cooled to room temperature, and poured into 450 mL

170

of ice water. Next, NaCl (110.0 g) was added which produced a white solid. The

precipitate was filtered and re-dissolved in 300 mL of deionized water. The solution was

treated with 2 N NaOH to a pH of 6~7 and diluted with deionized water to a total volume

of 500 mL. Then, NaCl (110.0 g) was again added to salt out the sodium-form sulfonated

monomer. The crude product was recrystallized twice from a mixture of isopropyl

alcohol and deionized water (3/1 v/v). The white powder-like disulfonated comonomers

were dried in a vacuum oven at 160 oC for two days.

6.3.3. Synthesis of Disulfonated Poly(Arylene Ether Ketone) Copolymers (B, PB and

MB) Based on Three Types of Ketone Monomers

The copolymerization procedures for these three polyketone series were similar.

The B-xx series copolymers were base on mono ketone monomers DFBP, SDFBP, and

6F-BPA, PB-xx series copolymers were based on para diketone monomers PBFB,

SPBFB, and 6F-BPA, while the MB-xx series copolymers were based on meta diketone

monomers MBFB, SMBFB, and 6F-BPA. In each series, the number xx represents the

theoretical molar percentage of disulfonated repeat units. A typical polymerization for

PBFB-30 was described as follows: 6F-BPA (3.3623 g, 10mmol), PBFB (2.2562 g, 7

mmol), and SPBFB (1.5792 g, 3 mmol) were added to a 3-neck flask equipped with

mechanical stirrer, nitrogen inlet and a Dean-Stark trap. Next, 1.15 equivalent of

potassium carbonate and NMP were introduced to afford a 33% solids concentration.

Toluene (NMP/Toluene=3/4, v/v) was used as an azeotroping agent. The reaction mixture

was heated under reflux at 150 oC for 4 h to dehydrate the system and remove most of the

toluene. Next, the temperature was raised slowly to 175 oC. The solution became viscous

171

after about 7 h and was subsequently cooled to room temperature and diluted with NMP.

The product was then isolated by addition to stirred deionized water. The precipitated

fibrous copolymer was heated around 50 oC in deionized water overnight, then filtered

and vacuum dried at 120 oC for 24 h.


Copolymer (1.0 g) was dissolved in DMAc (20 mL). The solutions were first

filtered with 0.45-µm Teflon syringe filters, and then cast onto clean glass substrates. The

film was carefully dried with infrared heat at gradually increasing temperature (up to ~ 60

oC) under a nitrogen atmosphere. Transparent, flexible films were lifted by immersing

them in deionized water. The sodium-form membranes were converted into the acid form

by boiling in 0.5 N H2SO4 for 2 h, followed by boiling in deionized water for 2 h. The

acid-form films were stored in fresh deionized water.


1H NMR spectra were obtained with a Varian 400 MHz spectrometer using

DMSO-d6 as a solvent. FTIR spectra were measured with a Nicolet Impact 400 FT-IR

spectrometer with thin homogenous films. The thin films were cast from dilute DMAc

solutions and completely dried in an oven. Intrinsic viscosity (IV) measurements were

obtained in NMP with 0.05 M LiBr as a solvent at 25 oC using a Cannon Ubbelohde

viscometer. Number average molecular weights (Mn) of copolymers were determined by

gel permeation chromatography based on polystyrene standards.

172



(Veeco Instruments, Santa Barbara, CA) in tapping mode. A silicon probe (Veeco) with

an end radius of <10 nm and a force constant of 5 N·m-1 was used to image samples.

Samples were equilibrated at 30% RH for at least 12 h before being imaged immediately

at room temperature and approximately 15-33% RH.

Membrane water uptake was determined by a simple weight difference approach:

The sample films were equilibrated in deionized water at room temperature for at least 48

h. The membranes were then dried in the vacuum oven at 110 oC for 24 h. Weights of wet

and dry membranes were measured. The ratio of weight gain to initial film weight was

expressed as % water uptake. The hydration number (λ), the number of water molecules

absorbed per sulfonic acid, can be calculated from the mass water uptake and the ion

content of the dry copolymer as shown in the following equation:

λ = [(masswet - massdry)/ OHMW2

]/IEC *massdry

where OHMW2

is the molecular weight of water (18.01 g·mol-1) and IEC is the ion

exchange capacity of the dry copolymer in equivalents per gram.

Protonic conductivity at 30 °C under full hydration (in liquid water) was

determined using a Solartron 1260 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

173

according to equation: AZl

'=σ , where σ is the protonic 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. In determining protonic conductivity in liquid water,

membranes were equilibrated at 30 °C in deionized water for 24 h prior to the testing.

Methanol permeability of the membranes was determined by measuring the

crossover current in a DMFC at open circuit. The measurement was performed in an

identical manner to Ren et al.16


6.4.1. Synthesis and Characterization of Disulfonated Monomers and Copolymers

The sulfonation agent, 30% fuming sulfuric acid, was used to synthesize the

disulfonated comonomers SDFBP，SPBFB，and SMBFB via electrophilic aromatic

substitution. It was found that the sulfonation of the two diketone monomers (SPBFB and

SMBFB) required longer reaction time (16 h) than the mono ketone monomer (6~7 h) at

the same reaction temperature (120 oC). This was due to the two carbonyl electron

withdrawing groups of the diketone monomers, which lowered the electron density on the

benzene ring, and thus decreased the reactivity in the electrophilic aromatic substitution

reactions. Although the reaction temperature for the synthesis of mono ketone SDFBP

and para di-ketone SPBFB could be varied between 110 oC and 150 oC without changes

in the product, it was necessary to control the temperature below 120 oC for the meta

diketone monomer (SMBFB) synthesis. It was noted that the MBFB monomer was easily

oxidized when the reaction temperature was raised higher than 120 oC, as evidenced by

174

the small impurity peaks in the 1H NMR spectra. All these disulfonated comonomer

structures were characterized and confirmed by 1H NMR spectra. Figure 6.2 showed a

representative 1H NMR spectrum of SMBFB comonomer.

Three series of poly(arylene ether ketone) copolymers with high molecular

weights were synthesized via a step-condensation process (Fig. 6.3). Table 6.1 lists the

1H NMR, IV, IEC, and proton conductivity data for these copolymers. It was not

surprising that the two diketone copolymers (PB and MB series) were much easier to

synthesize and obtain relatively higher molecular weights than the mono ketones (B

series), because the fluorine functional groups in these two series were activated by the

two carbonyl electron-withdrawing groups, and were more easily attacked by the

nucleophiles. The high molecular weights were confirmed by the high intrinsic viscosity

values, which were measured via the modified procedure using NMP as the solvent with

0.05 M LiBr salt to suppress the polyelectrolyte effect.16, 17 These copolymers were

amorphous and soluble in polar aprotic solvents due to the flexible

hexafluoroisopropylidene linkages and the introduction of the sulfonic acid groups.

Copolymers with target acid content (or degree of sulfonation) of 20 to 50 mol%

were prepared by adjusting the sulfonated vs. unsulfonated monomer feed ratio. 1H NMR

was used to identify the copolymer structures and calculate the experimental degree of

sulfonation. For example, the 1H NMR integrations for the protons attached to the

sulfonated unit (peak “i”, 8.3 ppm) and the overlapping peaks from protons on both the

sulfonated and unsulfonated units (peak “b+f”, 7.35 ppm) were used to calculate the

degree of sulfonation for the MB-30 copolymer (Fig. 6.4). The mole percent of

175

sulfonated units in the MB copolymer was calculated according to the proton number

relationship as described in the following equation:

Degree of sulfonation % =

The experimental degree of sulfonation of all copolymers was determined using the

above method (Table 6.1). These experimental values agreed with the theoretical target

values very well. IEC is an important parameter which can affect other membrane

properties like morphology, water uptake and protonic conductivity. The IEC values of

all copolymers were calculated from the corresponding degree of sulfonation (Table 6.1).

Due to the bigger molecular structures of the diketone monomers, the PB and MB series

copolymers had the same IEC values at the same degree of sulfonation, but lower than

the B series copolymers. For example, PB-50 and MB-50 copolymers both had IEC

values around 1.40 meq·g-1, and this value was lower than B-50, but similar to B-40.

The IR spectra qualitatively confirmed the functional groups of the synthesized

copolymers for all series. The FTIR spectrum of the MB series of copolymers is shown in

Figure 6.5. Peaks at 1030 cm-1 and 1087 cm-1 correspond to symmetric and asymmetric

stretching of the sodium sulfonate groups. These two peaks increased with increasing

degree of sulfonation as expected.

%1004/)(

2/×

+ fbi

176

Figure 6.2. 1H NMR Spectrum of SMBFB Disulfonated Comonomer

c

CO

F

NaO3S

CO

F

SO3Nac

ba

d

ef

e

d

bf

a

177

CCF3

CF3O

F F CCF3

CF3OHHO

O CCF3

CF3O

F F

SO3NaNaO3S

SO3MMO3SO

X X+ +

X Xn 1-n

Toluene, NMP, K2CO3150 oC, reflux 4 h175 oC, 7 h

CO

CO

CO

"B" "PB"

X = , or CO

CO

"MB"

Figure 6.3. Synthetic Scheme of Three Series of Disulfonated Poly(Arylene Ether

Ketone) Copolymers

178

Table 6.1. Characterization of B/PB/MB Series of Copolymers with Different Degree

of Sulfonation

Copolymers %sulfonation

by 1H NMR

I.V.(dL·g-1)a IEC(meq·g-1) Proton Conductivity

(S·cm-1) (30 oC in H2O)

B-20 18 0.51 0.72 -

B-30 29 0.50 1.05 0.021

B-40 40 0.66 1.38 0.071

B-50 47 0.45 1.70 0.092

PB-20 22 1.06 0.62 -

PB-30 29 0.62 0.90 0.010

PB-40 40 1.16 1.17 0.038

PB-50 48 1.43 1.43 0.076

MB-20 19 1.12 0.62 0.010

MB-30 28 0.69 0.90 0.035

MB-40 39 0.73 1.17 0.060

MB-50 50 0.74 1.43 0.085

a. Intrinsic Viscosity (I. V.) was measured in 0.05 M LiBr/NMP at 25 oC

179

i

b, f

a, c, e, g

d, j, j’k, k’m,m’

CO

CO

O* C

CF3

CF3

O * C

CF3

O CO

CO

*

SO3NaNaO3S

CF3x 1-x

a b b a cc dd

e

e ff

g h

ik k'

j j'm m'

i

b, f

a, c, e, g

d, j, j’k, k’m,m’

CO

CO

O* C

CF3

CF3

O * C

CF3

O CO

CO

*

SO3NaNaO3S

CF3x 1-x

a b b a cc dd

e

e ff

g h

ik k'

j j'm m'

Figure 6.4. 1H NMR was Used to Calculate the Degree of Sulfonation of the

Copolymers (MB-30)

180

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

950 1000 1000 1100 1200 cm-1

Abs

orba

nce

MB-50

MB-40

MB-30MB-20

MB-00

S=O asym. str.

S=O sym. str.

Wave Number (cm-1)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

950 1000 1000 1100 1200 cm-1

Abs

orba

nce

MB-50

MB-40

MB-30MB-20

MB-00

S=O asym. str.

S=O sym. str.

Wave Number (cm-1)

Figure 6.5. IR Spectra of MB Series Copolymers

181

6.4.2. Morphology Characterization of the Membranes

There is much evidence that the morphology of PEMs affects their transport

properties.9,12,18 Sharper phase separation means that the hydrophilic transport channels

are more well-defined, which usually results in improved proton conduction properties.

The membrane morphology can be affected by many factors, such as the chemical

structure, IEC value, water uptake, membrane preparation conditions, and the

acidification method, etc. Previous research reported12 that copolymers with higher IEC

values had better phase separation, while at the same IEC value the high temperature

acidification method improved the phase separation compared to the room temperature

acidification method. In this research, three series of copolymer membranes were

prepared under the same conditions and all acidified using the high temperature method.

The morphology of the membranes was characterized by tapping mode AFM to

investigate the effect of chemical structure on phase separation.

The AFM images of two groups of membranes with similar IEC values were

compared in Figure 6.6. The left group of copolymers (B-30, PB-40, and MB-40) had

lower IEC values around 1.1~1.2 meq·g-1, while the right group of copolymers (B-40,

PB-50, and MB-50) had higher IEC values around 1.4 meq·g-1. It was obvious that for the

same chemical structures, better phase separation was observed for the copolymers with

higher IEC. But at the same IEC value, the meta diketone MB series showed sharper

phase separation and bigger phase domain than the mono ketone and para diketone B and

PB series. This improved morphology for the MB series membranes may be caused by

the more flexible meta carbonyl linkages, which allowed for more aggregation of the

hydrophilic domains during membrane formation.

182

(a) 100 nm

(b) 100 nm

(c) 100 nm

(d) 100 nm

(e) 100 nm

(f) 100 nm

(a) 100 nm

(b) 100 nm

(c) 100 nm

(a) 100 nm(a) 100 nm

(b) 100 nm(b) 100 nm

(c) 100 nm(c) 100 nm

(d) 100 nm(d) 100 nm

(e) 100 nm(e) 100 nm

(f) 100 nm(f) 100 nm

Figure 6.6. AFM Image of Copolymer Membranes: (a) B-30, (b) PB-40, (c) MB-40,

(d) B-40, (e) PB-50, (f) MB-50. The IEC value of the left group copolymers is around

1.1-1.2 meq·g-1, and the right groups is around 1.4 meq·g-1

183

6.4.3. Characterization of Membrane Properties

As discussed earlier, MB series copolymers tend to have better hydrophobic and

hydrophilic phase separation than the other two series at similar IEC. It is known that the

phase separation is a function of copolymer chemical structure and is critical for both

proton and water transport properties. In order to understand the effect of the chemical

structure on the membrane properties, the water uptake and proton conductivities of B,

PB and MB copolymers were measured and compared under fully hydrated conditions.

It has been widely reported that both the proton conductivity and water uptake of

sulfonated materials increase with increasing IEC. Figure 6.7 and Figure 6.8 show the

same trends within these three series of copolymers. The difference in the structure of

these three copolymers resulted in different properties. The MB series showed more

water absorption and higher proton conductivity compared with the other two series at

similar IEC. It is likely that the two carbonyl groups linked on the meta positions of the

benzene ring resulted in less ordered packing and thus larger free volume between

polymer chains, in which more water molecules could be confined, and this could also

lead to the higher hydration numbers. The hydration number is the water uptake on a

mass basis and it is more related to the polymer backbone architectures. Research has

shown that the proton conductivity at fully hydrated conditions for the copolymers with

similar acidity was a linear function of hydration number.9 Figure 6.9 shows proton

conductivity as a function of hydration number for all copolymers. The hydration number

is more important than IEC in terms of proton conductivity. For example, MB-30 had

much lower IEC value than PB-40, but they had similar proton conductivities due to the

similar hydration numbers.

184

Temperature also affected the proton conductivity for all the copolymers. The

proton conductivities of the PB copolymers as a function of temperature are given in

Figure 6.10. The proton conductivities increased with temperature and with the degree of

sulfonation or IEC, and this was the general trend for the B and MB series as well. The

proton conductivities of these random copolymers under partially hydrated conditions

were measured. It was found that the proton conductivities decreased dramatically at low

relative humidities even for the MB series. This phenomenon can be at least partially

explained by the water uptake at various humidity conditions for the B series of

copolymers (Fig. 6.11). When the relative humidity was lower than 80%, the water

uptake decreased prominently, which resulted in an isolated morphology, and hindered

proton transport.

One of the major disadvantages of Nafion® is its high methanol permeability. The

methanol permeability of the B series of copolymers increased with increasing

temperature and IEC, but all of the copolymers in this series had lower methanol

permeability than Nafion® 117 in the temperature range from 30 to 80 °C (Fig, 6.12). The

methanol permeabilities of B-40 and B-30 were much lower than Nafion® and increased

only modestly with temperature compared to the higher IEC B-50 and Nafion®, which

demonstrated the application potential in DMFC of the B-30 and B-40 copolymers.

Differences in the proton conductivity between liquid water and in-MEA can

reflect the interfacial loss or the compatability of the MEA, which is important in

practical applications. The relatively small losses for B series copolymers (Table 6.2)

mean that these copolymers were compatible with the Nafion-bonded electrodes and can

185

be fabricated into robust MEAs, although the long term durability has not yet been

determined.

186

Figure 6.7. Influence of IEC and Copolymer Structure on Water Uptake of the

Membranes (Acid Form) in Liquid Water at Room Temperature

0

20

40

60

80

100

120

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

IEC (meq/g)

Wat

er U

ptak

e (m

ass%

)

MB ketonePB ketoneB ketone

187

0

10

20

30

40

50

60

70

80

90

100

0.4 0.9 1.4

IEC (meq/g)

Pro

ton

cond

uciv

ity (m

S/cm

)


MB-30PB-40

B-30

0

10

20

30

40

50

60

70

80

90

100

0.4 0.9 1.4

IEC (meq/g)

Pro

ton

cond

uciv

ity (m

S/cm

)


MB-30PB-40

B-30

Figure 6.8. Proton Conductivity vs. IEC of Three Ketone Type Copolymers in

Liquid Water at Room Temperature

188

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50

Hydration Number

Prot

on c

ondu

civi

ty (m

S/cm

)

MB ketonePB ketoneB ketonePB-40

MB-30

B-30

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50

Hydration Number

Prot

on c

ondu

civi

ty (m

S/cm

)

MB ketonePB ketoneB ketonePB-40

MB-30

B-30

Figure 6.9. Proton Conductivity in Liquid Water Tends to Depend on Hydration

Number (RT)

189

2

2.5

3

3.5

4

4.5

5

5.5

6

2.8 2.9 3 3.1 3.2 3.3 3.41000/T (oK-1)

Ln σ

(mS/

cm)

1. PB-diketone 50

2. PB-diketone 40

3. PB-diketone 30

1

2

3

2

2.5

3

3.5

4

4.5

5

5.5

6

2.8 2.9 3 3.1 3.2 3.3 3.41000/T (oK-1)

Ln σ

(mS/

cm)

1. PB-diketone 50

2. PB-diketone 40

3. PB-diketone 30

1

2

3

Figure 6.10. Effect of Temperature on Protonic Conductivity in Liquid Water

190

Figure 6.11. Influence of Copolymer Composition on Water Sorption of the B Series

as a Function of Humidity

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 20 40 60 80 100 120

Relative humidity (%)

Wat

er u

ptak

e (m

ass

%) 1, B-20

2, B-303, B-404, B-50

1

2

3

4

191

Figure 6.12. Influence of Copolymer Composition and Temperature on Methanol

Permeability

0

1

2

3

4

5

6

30 60 80Temperature (oC)

Met

hano

l Per

mea

bilit

y x

10-6

(cm

2 /s)

B-50B-40B-30Nafion 117

192

Table 6.2. Comparisons of Thin Film Protonic Conductivity in Liquid Water to

That of MEA

Sample Protonic conductivity in

liquid water (S·cm-1) @80 0C

Protonic conductivity in MEA (S·cm-1)

@80 0C

Percent decrease

(%)

Methanol Permeability

(E-06 cm2·s-1) @ 80 °C

B-50 0.15 0.14 7 4.79 B-40 0.13 - - 1.99 B-30 0.04 0.03 25 1.48

193

6.5. Conclusions

Three series of poly(arylene ether ketone) copolymers based on 6F-BPA, three

sulfonated ketone-type comonomers and their corresponding unsulfonated monomers

have been synthesized. The degree of sulfonation was controlled from 20 to 50 mol%.

The intrinsic viscosity measured with a modified method in 0.05 M LiBr/NMP confirmed

that the molecular weights of the copolymers were high enough to form tough, flexible

films. The membrane properties of these three series of copolymers have been

comparatively studied. Copolymers containing a meta diketone unit (MB series) showed

higher water uptake and proton conductivities compared with the mono ketone (B series)

and para diketone (PB series) at similar IEC values. This may be caused by the more

flexible MB chemical structures which resulted in better phase separation and higher

hydration levels. The B series copolymers also showed lower methanol permeability

compared to Nafion® and low interfacial loss in MEA measurements.


The author would like to acknowledge the Department of Energy for funding

(contract # DE-FC36-01G011086).

194

6.7. References

1. Hickner, M.A.; Ghassemi, H.; Kim, Y.S.; Einsla, B.R.; and McGrath, J.E.; Chem. Rev., 2004, 104, 4587.

2. Tchicaya-Bouckary, L.; Jones, D.J.; and Roziere. J., Fuel Cells 2002, 2, No. 1, 40. 3. Kroschwitz, J. I., Eds; Encyclopedia of Polym. Sci. and Eng. 1988, 12, 313. 4. Kreuer, K.D., Solid State Ionics 1997, 97 (1–4), 1. 5. Liu, S.; Wang, F.; Chen, T.; Macromol. Rapid Commun. 2001, 22, 579. 6. Harrison, W. L.; Wang, F.; Mecham, J. B.; Bhanu, V. A.; Hill, M.; Kim, Y. S.;

McGrath, J. E.; J. Polym. sci.: Part A: Polym. Chem., 2003, 41,2264. 7. Kreuer, K.D.; Solid State Ionics, 2000, 149, 136. 8. Kim, Y.S.; Dong, L.; Hickner, M.A.; Pivovar, B.S.; McGrath, J.E., Polymer, 2002, 44,

5729. 9. Roy, A.; Hickner, M.A.; Yu, X.; Li, Y.; Glass, T.E.; McGrath, J.E.; J. Polym. Sci.,

Part B: Polym.Phys. 2006, 44, 2226. 10. Li, Y.; Roy, A.; Badami, A. S.; Hill, M.; Yang, J.; Dunn, S; McGrath, J. E., J. Power

Sources, 2007, In press. 11. Li, Y.; Mukundan, T.; Harrison, W.; Hill, M.; Sankir, M.; Yang, J.; McGrath, J. E.;

Prepris Symp. ACS Div. Fuel Chem., 2004, 49(2), 536. 12. Kim, Y.S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y.T.; Harrison, W.;

Zawodzinski, T.A.; McGrath, J.E., J. Polym. Sci., Part B: Poly. Phys., 2003, 41, 2816. 13. Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.; Zawodzinski, T. A.;

McGrath, J. E., Macromol Symp, 2001, 175, 387. 14. Sankir, M.; Bhanu, V. A.; Harrison, W. L.; Ghassemi, H.; Wiles, K. B.; Glass, T. E.;

Brink, A. E.; Brink, M. H.; McGrath, J. E., J. Appl. Polym. Sci., 2006, 100, 4595 15. Ren, X. M.; Springer, T. E.; Zawodzinski, T. A.; Gottesfeld, S., J. Electrochem. Soc.,

2000, 147, 466. 16. Li, Y.; Wang, F.; Yang, J.; Liu, D.; Roy, A.; Case, S.; Lesko, J.; McGrath, J.

E., Polymer, 2006, 47, 4210.

195

17. Yang, J.; Li, Y.; Wang, H.; Hill, M.; Yu, X.; Wiles, K.B.; Lee, H.; McGrath, J.E., Preprs. Symp. ACS, Div. Fuel Chem., 2005, 50(2), 701

18. Badami, A.S.; Lee, H.S.; Li, Y.; Roy, A.; Wang, H.; McGrath, J. E., Preprs. Symp.

ACS, Div. Fuel Chem., 2006,51(2), 612.

196

Chapter 7. Synthesis and Characterization of Partially Fluorinated Hydrophobic - Hydrophilic Multiblock Copolymers Containing Sulfonate Groups for Proton Exchange Membrane

Taken From:

Yanxiang Li1, Abhishek Roy1, Anand S. Badami1, Melinda Einsla2, Juan Yang1, Stuart

Dunn1, James E. McGrath1*

1Macromolecular Science and Engineering & Macromolecules and Interfaces Institute

Virginia Polytechnic Institute and State University

Blacksburg, VA 24061, USA

2MPA-11: Sensors and Electrochemical Devices, Los Alamos National Laboratory, Los

Alamos, NM 87544, USA

Journal of Power Sources, 2007, 172, 30-38

Reprinted with permission from Elsevier, copyright (2007)

197

7.1. Abstract

A new hydrophobic-hydrophilic multiblock copolymer has been successfully

synthesized based on the careful coupling of a fluorine terminated poly(arylene ether

ketone) (6FK) hydrophobic oligomer and a phenoxide terminated disulfonated

poly(arylene ether sulfone) (BPSH) hydrophilic oligomer. 19F and 1H NMR spectra were

used to characterize the oligomers’ molecular weights and multiblock copolymer’s

structure. The comparison of the multiblock copolymer 13C NMR spectrum with that of

the random copolymer showed that the transetherfication side reaction was minimized in

this synthesis. The morphologies of membranes were investigated by tapping mode

atomic force microscopy (AFM), which showed that the multiblock membrane acidified

by the high temperature method has sharp phase separation. Membrane properties like

protonic conductivity, water uptake, and self-diffusion coefficient of water as a function

of temperature and relative humidity (RH) were characterized for the multiblock

copolymer and compared with ketone type random copolymers at similar ion exchange

capacity value and Nafion® controls. The multiblock copolymers are promising

candidates for proton exchange membranes especially for applications at high

temperatures and low relative humidity.

Keywords: Proton Exchange Membrane Fuel Cell, Poly(Arylene Ether Ketone),

Poly(Arylene Ether Sulfone), Multiblock Copolymer, Morphology

198

7.2. Introduction

Proton exchange membrane (PEM) materials have attracted much attention due to

the environmentally friendly nature of PEM fuel cells and their potential applications in

automobiles, stationary power, and small electronics.1,2 Currently nearly all commercially

available membranes are based on copolymers containing perfluorosulfonic acid groups

such as DuPont’s Nafion®. Nafion®-type materials have exceptional oxidative and

chemical stability as well as high protonic conductivity, which are critical to PEM fuel

cells. But they have limitations like low performance at high temperature due to a low

hydrated Tg value, high methanol permeability in direct methanol fuel cells, and high

cost.2,3 Therefore, many polymeric materials with ionic groups have been explored as

alternative PEM candidates, such as poly(arylene ether)s,4-8 polyimides,9,10 poly(arylene

sulfide sulfone)s,11 substituted polyphenylenes,12 etc. The wholly aromatic partially-

disulfonated poly(arylene ether sulfone) (BPSH) random copolymer developed in the

McGrath group6 is a potential PEM candidate due to its good acid and thermal oxidative

stabilities, high glass transition temperature and excellent mechanical strength.13 For

example, with 35 mol percent degree of sulfonation, the BPSH copolymer has excellent

oxidative stability as shown in open circuit test. The test was conducted at 100 oC under

H2/O2 environment at 25% RH. BPSH outperformed the benchmarked material Nafion®

and was stable up to 300 h. This inherent stability was attributed to the extremely low

oxygen permeability (10 times lower than Nafion®).14

Most of the copolymers developed are random or “statistical” copolymers

because monomer units are connected irregularly and the sulfonic acid groups are also

199

randomly distributed along the copolymer chain. These randomly located ionic groups

will lead to isolated morphological domains especially at a low hydration level, which

limit the transport properties. While under fully hydrated conditions, water assisted

percolated morphology ensures connectivity between the hydrophilic domains. As a

result, the random copolymers show satisfactory performance under fully hydrated

conditions, but they will lose the performance dramatically at low relative humidity. The

big challenge here is how to modify the chemistry of the polymers to obtain significant

proton conductivity at low hydration levels which will make the PEM fuel cell more

applicable under ambient environments. Recent research results showed that the

hydrophobic-hydrophilic block copolymers with tailored chemical structure of the

polymer backbone may achieve this goal.15

Multiblock thermally stable copolymers are interesting because the morphology

of the copolymer membrane can be better controlled by varying the two sequences length

in the multiblock structures.16 In hydrophobic-hydrophilic multiblock copolymers, the

ionic groups located within the hydrophilic blocks provide protonic conductivity, and the

hydrophobic blocks offer good mechanical strength. It is especially interesting when the

molecular weight ratio of the two blocks is about 1: 1, since a hydrophilic cocontinuous

phase may form under this condition, which may form associated hydrophilic domains

even under low hydration levels. This may greatly facilitate transport and the proton

conductance. Furthermore, the microphase separation in multiblock copolymers may be

helpful in controlling the water swelling and generating copolymers which have good

conductivity even at low relative humidity. The McGrath group17,22 has recently focused

on this topic and developed several series of thermally stable multiblock copolymers to

200

investigate both composition and chemical structure effects on PEM properties. It was

found that the cocontinuous phase can form at certain oligomer block lengths according

to atomic force microscopy (AFM) images, and under partially hydrated conditions the

block copolymers showed much improved proton conductivity over the random

copolymers.23,24 The block copolymers also showed higher self-diffusion coefficients of

water over the Nafion® control and random copolymers, suggesting a lower

morphological barrier to transport.15,23,24

In this paper, a novel multiblock copolymer [Fig. 7.1 (b)] has been successfully

synthesized based on a fluorine terminated hydrophobic poly(arylene ether ketone) (6FK)

oligomer and a phenoxide terminated hydrophilic poly(arylene ether sulfone) (BPSH)

oligomer. Keeping the chemical backbone similar to BPSH, one may hope to get the

same or even better oxidative stability. The comparison of 13C NMR spectra between the

multiblock copolymer and the random copolymer shows that the ether-ether interchange

side reaction, which may result in a randomized copolymer chain, has been minimized in

this copolymerization. The multiblock copolymer can form a tough film. Membrane

properties of this multiblock copolymer like morphology, proton conductivity, and water

uptake were characterized and compared with Nafion® control and ketone random

copolymers [Fig. 7.1 (a)]. This paper aims to understand that how the differences

between multiblock and random copolymer morphologies affect water and proton

transport. This will be instructional in designing better membranes for improved fuel cell

performance.

201

OCF3

CF3O Y O

CF3

CF3O Y

SO3HHO3S

X 1-X

Y = CO

("B")

or C CO O

( " PB' )

SO

OOO m

SO3HHO3S

CO

O CCF3

O

CF3n X

(a)

(b)

Figure 7.1. Copolymer Chemical Structures Studied in This Work (a) B-ketone-xx

and PB-diketone-xx Copolymers, (b) 6FK-BPSH Multiblock Copolymer

202

7.3. Experimental

7.3.1. Materials

4,4’-Hexafluoroisopropylidenediphenol (6F-BPA), received from Ciba, was

purified by sublimation. 4,4’-Difluorobenzophenone (DFBP) was purchased from

Aldrich, and biphenol (BP) was kindly provided by Eastman Chemical. They were used

as received. The ionic comonomer 3,3’-disulfonated 4,4’-dichlorodiphenyl sulfone

(SDCDPS) was synthesized as reported earlier.8 All these monomers were well-dried in a

vacuum oven before polymerization. The solvents N-methyl-2-pyrrolidinone (NMP) and

dimethylacetamide (DMAc) were vacuum-distilled from calcium hydride onto molecular

sieves. Potassium carbonate was dried in vacuo before use. Toluene, ethanol, and

isopropanol were obtained from Aldrich and used as received. B ketone-xx and PB

diketone-xx series random copolymers were synthesized in house,8,15 where xx refers to

the degree of sulfonation. Nafion® 112 and Nafion® 1135 were obtained from

ElectroChem.

7.3.2. Synthesis of Fluorine Terminated Hydrophobic Oligomers

Fluorine terminated poly(arylene ether ketone) oligomers (6FK) with target

molecular weights were synthesized via step growth polymerization. For example, the

4 kg·mol-1 6FK oligomer was prepared in a three-neck 100 mL flask with DFBP (2.580 g,

11.82 mmol) and 6F BPA (3.500 g, 10.41 mmol) dissolved in 20 mL DMAc, 1.65 g

potassium carbonate was added, and toluene (10 mL) was used as an azeotropic agent.

The reaction temperature was first set to 150 oC to dehydrate the system for about 4 h,

203

and the toluene was removed completely. The oil bath temperature was then raised to 175

oC for 16 h. The oligomer solution was cooled to room temperature and filtered to

remove most of the salt, then precipitated in IPA. The oligomers were collected by

filtration and washed with DI water and ethanol thoroughly to remove residual salt and

monomer residues. The resulting oligomers were dried in vacuo at 100 oC for at least

24 h.

7.3.3. Synthesis of Multiblock Copolymers

The step growth copolymerization employed a two-step procedure for the

multiblock copolymer synthesis. A copolymerization of a 4k-4k multiblock copolymer is

described as follows: First, phenoxide terminated disulfonated poly(arylene ether sulfone)

(BPSH) with target molecular weight 4 kg·mol-1 was synthesized by charging biphenol

(1.776 g, 9.54 mmol) and 3,3’-disulfonated 4,4’-dichlorodiphenyl sulfone (4.044 g, 8.23

mmol) to a three-neck 100 mL flask equipped with mechanical stirrer, nitrogen inlet and

a Dean Stark trap. Potassium carbonate (1.15 equivalents) and dry NMP were introduced

to afford 20% solid concentration. Toluene (NMP/Toluene = 2/1 v/v) was used as an

azeotropic agent. The reaction mixture was heated under reflux at 150 oC for 4 h to

remove water. Then, the bath temperature was raised slowly to 190 oC for 16 h. The

oligomer solution was cooled to 160 oC for the next step reaction without isolation. In the

second step, 6FK/NMP solution was added dropwise to the BPSH system. Then the

temperature was raised again to 190 oC for 2 days. The copolymer was isolated by

precipitation in IPA and deionized water (1:1), filtered, and dried in a vacuum oven for

24 h at 120 oC.

204

7.3.4. Characterization of Oligomers and Multiblock Copolymers

The 19F, 1H and 13C NMR spectra were conducted with a Varian Unity 400

NMR spectrometer. Solvent CDCl 3 was used for the hydrophobic oligomers, and DMSO-

d6 was used for the hydrophilic oligomers and multiblock copolymers. Intrinsic

viscosities (IV) were determined in NMP at 25 oC using an Ubbelohde viscometer for

ionic copolymers with 0.05 M LiBr in the NMP solvent to suppress the polyelectrolyte

effect.25


The salt form copolymers were redissolved in DMAc to afford transparent 5

wt% solutions, which were then cast onto clean glass substrates. The films were slowly

dried for 48 h with infrared heat at gradually increasing temperatures, and then dried

under vacuum at 110 oC for 2 days. Two methods can be employed to convert the sodium

salt form membranes to their acid form.26 In Method 1, the membranes were immersed in

1.5 M sulfuric acid solution at 30 oC for 24 h followed by immersion in deionized water

at 30 oC for 24 h. Method 2 involved boiling the membranes in 0.5 M H2SO4 for 2 h, and

then boiling in deionized water for another 2 h to remove any residual acid. Membranes

were stored in deionized water after the acidification until they were used for

measurements.

7.3.6. Characterization of Membranes

7.3.6. 1 Morphology Characterization by Atomic Force Microscopy (AFM)

205



(Veeco Instruments, Santa Barbara, CA) in tapping mode. A silicon probe (Veeco) with

an end radius of <10 nm and a force constant of 5 N·m-1 was used to image samples.

Samples were equilibrated at 30% RH for at least 12 h before being imaged immediately

at room temperature and approximately 15-33% RH.

7.3.6.2. Ion Exchange Capacity (IEC) and Conductivity

Sulfonic acid concentration in the copolymers (IEC, mequiv·g-1) was

quantitatively determined by titration. The acid form membrane was immersed in 50~60

mL DI water with 1 M sodium sulfate. The solution was stirred overnight to allow the

protons to exchange with sodium completely. The solution was then titrated with 0.01 M

sodium hydroxide solution in which phenolphthalein was used as an indicator.

Proton conductivity was determined in a window cell geometry27 using a

Solartron 1252+1287 Impedance/Gain-Phase Analyzer over the frequency range of 10 Hz

to 1 MHz following the procedure reported in the literature.28 In determining proton

conductivity in liquid water, membranes were equilibrated at 30 oC in DI water for 24 h

prior to the testing. The temperature range chosen for calculation of activation energy for

proton transport was from 30 to 80 oC. For determining proton conductivity under

partially hydrated conditions, membranes were equilibrated in a humidity-temperature

oven (ESPEC, SH-240) at the specified RH and 80 oC for 6 h before each measurements.

7.3.6.3. Water Uptake and Water Self-diffusion Coefficients

206

The water uptake of the membranes was determined by measuring the

difference in the weight between dry and fully hydrated membranes. The sample films

were equilibrated in deionized water at room temperature for at least 48 h. The

membranes were dried in the vacuum oven at 110 oC for 24 h. Weights of wet and dry

membranes were measured. The water uptake was calculated as follows: water uptake %

= [(masswet - massdry)/ massdry] * 100%, where massdry and masswet refer to the mass of the

wet membrane and the mass of the dry membrane, respectively.

The hydration number (λ), number of water molecules absorbed per sulfonic

acid, can be calculated from the mass water uptake and the ion content of the dry

copolymer as shown in the equation: λ = [(masswet - massdry)/ OHMW2

]/IEC *massdry,

where OHMW2

is the molecular weight of water (18.01 g·mol-1) and IEC is the ion

exchange capacity of the dry copolymer in equivalents per gram.

Water self-diffusion coefficients were measured in a Varian Inova 400 MHz (for

protons) nuclear magnetic resonance spectrometer with a 30 G·cm-1 gradient diffusion

probe as described in the literature.15,29

7.3.6.4. MEA Fabrication and Fuel Cell Testing

Membrane electrode assemblies (MEAs) were prepared from protonated

membranes and standard unsupported Pt catalyst inks by procedures developed at Los

Alamos National Laboratory.30 The catalyst loading was approximately 6 mg·cm-2 on

both the anode and the cathode. The polymer binder in the catalyst layers was Nafion®,

and the active cell area was 5 cm2. Gas diffusion layers (GDLs) were comprised of

carbon cloth from E-TEK. The cell temperature was maintained at 80 °C and humidified

207

hydrogen (200 Sccm) and air (500 Sccm) were supplied to the anode and cathode,

respectively.

7.4. Results and Discussions

7.4.1. Synthesis and Characterization of Oligomer and Multiblock Copolymer

Figure 7.2 shows the synthesis of fluorine terminated 6FK hydrophobic

oligomers. The molecular weights and the fluorine endgroups were controlled by using

excess DFBP monomer. The theoretical molecular weights were calculated according to

the Carothers equation. 19F NMR spectrum (Fig. 7.3) was used to determine the resulting

oligomer molecular weight. For the 19F NMR integrals of the two peaks, one is attributed

to the fluorine in the chain (-63.7 ppm, integration value: 225.91), another one is the

endgroup aromatic fluorine peak (-107.6 ppm, integration value: 10), and the two kinds

of fluorine have the number ratio of 6n to 2, therefore the Mn can be calculated by the

equation: 6n/2 = 225.91/10, where n is the number of repeat unit and was calculated to be

7.53. Accordingly the actual number average molecular weight (Mn) of oligomer was

calculated to be 4089 g·mol-1, which is very close to the target value (4 kg·mol-1).

The multiblock copolymer was synthesized via a two-step technique (Fig. 7.4).

First, the phenoxide terminated disulfonated poly(arylene ether sulfone) (BPSH)

hydrophilic oligomer with target molecular weight 4 kg·mol-1 was synthesized using

biphenol and SDCDPS in NMP, following the similar polycondensation method as the

hydrophobic oligomer. After 16 h reaction, the temperature was lowered to 160 oC

without isolation. Then the 6FK (4 kg·mol-1) oligomer dissolved in NMP was added

208

dropwise to the BPSH system in about 1 h. Finally the temperature was raised to 190 oC

for 2 days. The completion of the reaction can be monitored by 19F and 1H NMR during

the reaction. It was found the endgroup fluorine peak was gone after 2 days reaction (not

shown) and in the 1H NMR the small proton peaks close to the hydroxyl group in BPSH

oligomer [Fig. 7.5 (top), peak h, g, i, f,] also disappeared. The 1H NMR spectrum [Fig.

7.5 (bottom)] showed all proton peaks from both hydrophilic and hydrophobic segments

as assigned, which confirmed the success of the coupling reaction.

In this coupling reaction, a concern is that the ether-ether chain interchange side

reaction will occur, which may result in the randomized copolymer chain. For

comparison, a random copolymer possessing the same chemical composition as the

multiblock copolymer was synthesized by the one-step copolymerization of SDCDPS,

BP, 6F-BPA, and DFBP. Because the crystalline segments comprised of BP and DFBP

would precipitate out of the reaction solution and upset the stoichiometry, the high

molecular weight random copolymer was difficult to obtain. However, the comparison of

13C NMR spectra between low molecular weight random and the multiblock copolymers

still provided enough information of the sequence connection. As shown in Figure 7.6,

the carbons in random copolymer all have multiple peaks, suggesting the irregular

connection of the repeating sequence. In contrast, the carbons in the multiblock

copolymer showed strong single peaks, which confirmed the multiblock structure31.

All of the above results indicated that a multiblock 6FK-BPS (4:4)k copolymer

with approximately 4 kg·mol-1 for each segmental lengths has been successfully

synthesized with high intrinsic viscosity. The block copolymer can be dissolved in

209

dipolar aprotic solvents like NMP and DMAc. Tough films were prepared by solution

casting from 5 % DMAc solution.

210

C

O

FF HO C OH

CF3

CF3

+

K2CO3 /DMAc / Toluene

150 oC 4h

175 oC 16h

C

O

F C O

CF3

CF3

O C

O

Fn

Figure 7.2. Synthesis of Fluorine Terminated Poly(Arylene Ether Ketone) (6FK)

Hydrophobic Oligomer

211

Figure 7.3. Molecular Weight of 6FK Hydrophobic Oligomer Can be Calculated

from the 19F NMR Spectrum (6FK Oligomer with Target MW 4 kg·mol-1 in CDCl3)

212

F F

SO

OMO OO m

SO3MMO3S- +

-+

-+

SO

OOO

SO3MMO3S- +-+

Add dropwise at 160 oCThen 190 oC, 48 h

HO OH SO

ClO

SO3HHO3S

Cl+

K2CO3/NMP/Toluene150 oC 4h190 oC 16h

/NMP6FK

OM- +

mCO

O CCF3

O

CF3n

Figure 7.4. Synthesis of 6FK-BPSH Multiblock Copolymers via Two-step Sechnique

213

Figure 7.5. 1H NMR Spectra of BPS Hydrophilic Oligomer (Top), and Multiblock

6FK-BPS Copolymer (Bottom)

214

Random Copolymer

Multiblock Copolymer

Random Copolymer

Multiblock Copolymer

Figure 7.6. 13C NMR Spectra of Random (Top) and 6FK-BPS Multiblock (Bottom)

Copolymers

215

7.4.2. Morphology of Membranes

The multiblock 6FK-BPS (4:4)k copolymer membrane was acidified to assess the

effect of acidification upon its phase separation. Film samples of the multiblock

copolymer were acidified in sulfuric acid either by “Method 1” at 30 °C or by “Method

2” at 100 °C26. Films were imaged by tapping mode atomic force microscopy (AFM)

after acidification [Fig. 7.7(a)-(d)] When the phase images for acidification by Method 1

[Fig. 7.7 (a)] and Method 2 [Fig. 7.7(c)] are compared, two observations can be made.

First, the connectivity between the hydrophilic ionic domains (which appear darker) is

greater following acidification by Method 2. Second, acidification by Method 2 results in

a sharper contrast between the ionic domains and the hydrophobic non-ionic domains

(which appear brighter). These observations suggest that there is a greater degree of

phase separation in the multiblock copolymer following acidification by Method 2.

These results are consistent with previous results for phase separation of sulfonated

poly(arylene ether sulfone) random copolymers acidified by both methods26,32 and those

subjected to different hydrothermal treatments5. Height image micrographs [Fig. 7.7

(b),(d)] obtained concurrently with the phase images for the same area suggest that

acidification temperature may increase the difference between the highest and lowest

topographical features of the film. It is speculated that the hydrothermal treatment of

“Method 2” acidification imparts greater segmental mobility to the polymer chains within

the film compared to “Method 1” because the water depresses the glass transition

temperature of the polymer while it is concurrently heated at elevated temperature.

While temperature-induced topographic differences were not reported for poly(arylene

ether sulfone) random copolymer membranes5, it is possible that this observation may be

216

a result of two factors. The first is the increased flexibility of ketone linkages compared

to that of sulfone linkages. The second and possibly more important factor is that the

multiblock structure of this copolymer may allow more phase separation to occur during

acidification than a random copolymer structure could.

To confirm this latter hypothesis, a sulfonated poly(arylene ether ketone)

random copolymer film was acidified by Method 2 to evaluate the differences in phase

separation between random and multiblock copolymers with sulfonated poly(arylene

ether ketone) components. AFM phase images (Fig. 7.7c,e) and height images (Fig.

7.7d,f) of the two copolymers indicate that phase separation is sharper for the multiblock

copolymer than for the random copolymer, supporting the hypothesis that multiblock

structure may contribute to increased differences in topography. These results are

understandable given that the length of the ion-containing blocks in these multiblock

copolymers is longer than the length of an ion-containing comonomer in the random

copolymers. Consequently, the ionic groups should be located closer to each other in the

multiblock copolymer when ionic group aggregation occurs during acidification,

facilitating phase separation and resulting in larger domains and greater differences in

topography.

217

100nm

100nm

100nm

(a)

(c)

(e)

(b)

(d)

(f)

100nm100nm

100nm100nm

100nm100nm

(a)

(c)

(e)

(b)

(d)

(f)

Figure 7.7. Tapping Mode Atomic Force Microscopy Images: (a) Phase Image and

(b) Height Image of a 4k-4k 6FK-BPSH Multiblock Copolymer Film Acidified by

Method 1 at 30 °C, (c) Phase Image and (d) Height Image of the Same Film

Acidified by Method 2 at 100 °C, (e) Phase Image and (f) Height Image of a

Sulfonated Poly(Arylene Ether Ketone) Random Copolymer Film (B-30) Acidified

by Method 2. Image size: 500 nm; z Ranges: (a) 4°, (c) 12°, (e) 8°, All Height Ranges:

10 nm.

218

7.4.3. Characterization of PEM Properties

As discussed in section 7.4.2.1., ion containing polymers tend to phase separate

into hydrophobic and hydrophilic domain like morphology. The extent of phase

separation is critical for both proton and water transport. It is known that in block

copolymers, the sequence lengths play an important role in phase separation. In order to

understand the importance of block copolymer morphology on transport properties, B and

PB random copolymers with similar chemical structures and IECs were used as controls.

Table 7.1 lists the various properties for the random, block and Nafion®

copolymers. At similar IECs, a significant increase in proton conductivity was observed

for the 6FK-BPSH (4:4)k multiblock copolymer over the random B-30 and PB-40

copolymers. A similar trend in water uptake is also observed. The sharpness of phase

separation seems to increase both proton and water transport. To investigate in details,

water uptake was studied over a wide range of water activities and so as proton

conductivity.

Figure 7.8 represents the plots of hydration number as a function of water

activity for 6FK-BPSH (4:4)k multiblock, Nafion® and B ketone-30 copolymers. The

multiblock showed much higher water uptake at all hydration levels. In contrast to the

random copolymer, both the multiblock and Nafion® showed a sudden increase after 0.85

water activity. Kreuer et al33 reported similar observation for Nafion®. The dielectric

constant of the water in the membrane at higher water activities was found close to bulk

or free water. Earlier studies have demonstrated the importance of the presence of free

water on the transport properties. At low water activities, the phase separated morphology

of the block copolymers tends to hold up more water than the random and Nafion®. This

219

may be important when addressing the proton conductivity under partially hydrated

conditions.

The temperature dependency on proton transport was determined over the

temperature range of 30-80 oC for the copolymers studied (Fig. 7.9). Both the block and

Nafion® copolymers showed higher proton conductivities over the temperature range.

The slope of the graphs can be related to the activation energy for proton transport. It

follows that the multiblock has the lowest activation energy. The well phase separated

block copolymer morphology may increase the extent of connectivity between the

hydrophilic domains and lowers the activation energy for transport.

Self-diffusion coefficient of water gives a better understanding on the

importance of connectivity on water or proton transport. Higher value indicates well

phase separated morphology. Also under partially hydrated conditions, self-diffusion

coefficient scales with proton diffusion coefficient for Nafion®. 27,33 Hence a clear

understanding about the influence of block lengths on self-diffusion coefficient of water

is needed. Figure 7.10 shows the self-diffusion coefficients of water measured at 25 oC

for the copolymers studied. Although the ion exchange capacity and chemical

composition of the multiblock copolymer is similar to that of random, a change in

sequence length distribution increases the self-diffusion coefficient value significantly.

This reflects the importance of morphology and is consistent with the activation energy

results.

Studying proton conductivity as a function of relative humidity illustrated the

effect of morphology on proton conductivity under partially hydrated conditions (Fig.

7.11). For the random copolymer, PB-diketone 50, proton conductivity drops

220

significantly at lower RH values. Random copolymers show decent performance under

fully hydrated conditions since there are sufficient water molecules to provide proton

transport through water molecules in a scattered morphology; however, they lack the

connectivity among sulfonic acid groups for proton transport under partially hydrated

conditions. Conversely, with the multiblock copolymers, the performance under partially

hydrated conditions was very much comparable to Nafion®. The presence of long, co-

continuous channels improved the proton transport along the sulfonic acid groups and

water molecules.

221

Table 7.1. Characterization of Multiblock, Random Copolymers and

Nafion®Control

Copolymers IECa (meq·g-1)

Water Uptake (%)

Proton Conductivityb (S·cm-1)

I.V.c (dL·g-1)

Nafion® 112 0.9 22 0.08 -

PB-50 1.4 66 0.08 1.4

PB-40 1.2 26 0.04 1.2

B-30 1.1 25 0.02 0.5

6FK-BPSH(4:4)k 1.2 53 0.08 0.7

a. IEC values for the copolymers (except Nafion®) were measured by titration b. Proton conductivities were measured in liquid water at 30 ℃ c. Intrinsic viscosities (I.V.) were measured in NMP with 0.05 M LiBr

222

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

2

4

6

8

10

12

14

16

18

20

22

24

Hyd

ratio

n N

umbe

r (λ)

Water Activity (aw)

(1) 6Fk-BPSH (4:4)k (2) Nafion 112 (3) B ketone 30

Figure 7.8. Retention of Water as a Function of Water Activity is Enhanced for the

Block Copolymer

223

2.8 2.9 3.0 3.1 3.2 3.3 3.4

3.0

3.5

4.0

4.5

5.0

10.5

7.9

13.0 Pro

ton

Con

duct

ivity

(mS/

cm)

Ln (σ

/ m

S cm

-1)

1000/T (K-1)

(1) 6Fk-BPSH (4:4)k (2) Nafion 112 (3) B ketone-30

90 80 70 60 50 40 30 20

Temperature ( oC)

17

2020

29

37

46

5555

78

102

125

148148

Pro

ton

Con

duct

ivity

(mS

·cm

-1)

2.8 2.9 3.0 3.1 3.2 3.3 3.4

3.0

3.5

4.0

4.5

5.0

10.5

7.9

13.0 Pro

ton

Con

duct

ivity

(mS/

cm)

Ln (σ

/ m

S cm

-1)

1000/T (K-1)

(1) 6Fk-BPSH (4:4)k (2) Nafion 112 (3) B ketone-30

90 80 70 60 50 40 30 20

Temperature ( oC)

17

2020

29

37

46

5555

78

102

125

148148

Pro

ton

Con

duct

ivity

(mS

·cm

-1)

Figure 7.9. Proton Conductivity as a Function of Temperature for Multiblock 6FK-

BPSH (4:4)k, B ketone-30, and Nafion® 112 ( The numbers in the box are activation

energy, kJ·mol-1)

224

6FK-BPSH(4:4)k Nafion 112 PB diketone-40 B ketone-300

1

2

3

4

5

6

7

8

Sel

f diff

usio

n co

effic

ient

of w

ater

(10-6

/cm

2 )S

elf d

iffud

ion

Coe

ffici

ent o

f Wat

er (1

0-6·cm

-2)

6FK-BPSH(4:4)k Nafion 112 PB diketone-40 B ketone-300

1

2

3

4

5

6

7

8

Sel

f diff

usio

n co

effic

ient

of w

ater

(10-6

/cm

2 )S

elf d

iffud

ion

Coe

ffici

ent o

f Wat

er (1

0-6·cm

-2)

Figure 7.10. The Block Copolymer Has a Much Higher Self-diffusion Coefficient of

Water (Multiblock Copolymer Has Similar IEC Value to the PB-40 and B-30

Random Copolymers)

225

20 30 40 50 60 70 80 90 100 110

1

10

100

Prot

on c

ondu

ctiv

ity (m

S/cm

)

Relative Humidity (%)

(1) Nafion 112 (2) 6FK-BPSH(4:4)k (3) PB-diketone 50

12 3

20 30 40 50 60 70 80 90 100 110

1

10

100

Prot

on c

ondu

ctiv

ity (m

S/cm

)



12 3

Pro

ton

cond

uctiv

ity (m

S·c

m-1

)

20 30 40 50 60 70 80 90 100 110

1

10

100

Prot

on c

ondu

ctiv

ity (m

S/cm

)



12 3

20 30 40 50 60 70 80 90 100 110

1

10

100

Prot

on c

ondu

ctiv

ity (m

S/cm

)



12 3

Pro

ton

cond

uctiv

ity (m

S·c

m-1

)Pr

oton

Con

duct

ivity

(mS·

cm-1)

20 30 40 50 60 70 80 90 100 110

1

10

100

Prot

on c

ondu

ctiv

ity (m

S/cm

)



12 3

20 30 40 50 60 70 80 90 100 110

1

10

100

Prot

on c

ondu

ctiv

ity (m

S/cm

)



12 3

Pro

ton

cond

uctiv

ity (m

S·c

m-1

)

20 30 40 50 60 70 80 90 100 110

1

10

100

Prot

on c

ondu

ctiv

ity (m

S/cm

)



12 3

20 30 40 50 60 70 80 90 100 110

1

10

100

Prot

on c

ondu

ctiv

ity (m

S/cm

)



12 3

Pro

ton

cond

uctiv

ity (m

S·c

m-1

)Pr

oton

Con

duct

ivity

(mS·

cm-1)

Figure 7.11. Comparison of Conductivity vs. RH for 6FK-BPSH (4:4)k Multiblock,

PB-diketone-50 Random Copolymers, and Nafion® 112

226

7.4.4. Fuel Cell Performance

Evaluation of this multiblock copolymer also included fuel cell performance

testing. All tests were conducted at Los Alamos National Laboratory at an elevation of

approximately 7000 feet. A 6FK-BPS (4:4)k multiblock membrane was evaluated and

compared with a commercial Nafion® 1135 membrane under full humidification of the

inlet gases (Fig. 7.12). The novel multiblock copolymer showed very promising fuel cell

performance, which was similar to Nafion® under these conditions. The high frequency

resistance (HFR) of the Nafion® membrane was 0.072 Ω-cm2, while that of the 6FK-BPS

(4:4)k was 0.087 Ω-cm2. Since the conductivity of the membranes is very similar, the

difference in HFR is thought to be due at least partially to the resistance at the interface

between the membrane and the electrodes34. This incompatibility might be due to the

difference in water uptake between the novel multiblock copolymer and the Nafion®-

based electrode layers. Performance might be further improved by replacement of the

Nafion® binder in the anode and cathode with one more similar to the membrane.

227

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Current Density (A/cm2)

Vol

tage

(V)

1: 6FK-4-BPSH-4 (1 mil)2: N1135

12


Vol

tage

(V)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Vol

tage

(V)

1: 6FK-4-BPSH-4 (1 mil)2: N1135

12


Vol

tage

(V)

Current Density (A·cm-1)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Vol

tage

(V)

1: 6FK-4-BPSH-4 (1 mil)2: N1135

12


Vol

tage

(V)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Vol

tage

(V)

1: 6FK-4-BPSH-4 (1 mil)2: N1135

12


Vol

tage

(V)

Current Density (A·cm-1)Current Density (A·cm-2)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Vol

tage

(V)

1: 6FK-4-BPSH-4 (1 mil)2: N1135

12


Vol

tage

(V)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Vol

tage

(V)

1: 6FK-4-BPSH-4 (1 mil)2: N1135

12


Vol

tage

(V)

Current Density (A·cm-1)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Vol

tage

(V)

1: 6FK-4-BPSH-4 (1 mil)2: N1135

12


Vol

tage

(V)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Vol

tage

(V)

1: 6FK-4-BPSH-4 (1 mil)2: N1135

12


Vol

tage

(V)

Current Density (A·cm-1)Current Density (A·cm-2) Figure 7.12. Hydrogen-air Fuel Cell Performance of 6FK-BPSH (4:4)k and Nafion®

at 80 ℃ under Fully Humidified Conditions.

228

7.5. Conclusions

A new hydrophobic-hydrophilic multiblock 6FK-BPSH (4:4)k copolymer was

successfully synthesized via a two-step polycondensation method. 19F and 1H NMR

spectra were used to characterize the oligomers’ molecular weight and multiblock

copolymer’s structure. 13C NMR was a powerful tool to confirm the sequence connection

in the block copolymer. Good film-forming material with high conductivity was obtained.

Morphologies of the membranes characterized with tapping mode atomic force

microscopy showed that the high temperature acidification method can improve the phase

separation. The results were consistent with previous studies on the BPSH random

copolymer. The AFM images also indicated that the phase separation is sharper for the

multiblock than for the random copolymer.

This well phase separated block copolymer morphology increases the extent of

connectivity between the hydrophilic domains and improves the PEM properties. It was

found that, compared with the random ketone type copolymer with the similar IEC value,

the multiblock copolymer has higher protonic conductivity, lower activation energy, and

much improved self-diffusion coefficient of water. It can keep more free water than the

random copolymer, which results in much improved protonic conductivity under partially

hydrated conditions. The multiblock copolymer also showed very promising fuel cell

performance, which was comparable to Nafion®. Larger block lengths are currently being

investigated.

229

7.6. Acknowledgments

The author would like to acknowledge the Department of Energy (contract #DE-

FG36-06G016038), National Science Foundation (contract #EHR-0090556), and Nissan

Motor Company for their support of this project.

230

7.7. References 1. Thomas, S.; Zalbowitz, M., Fuel Cells: Green Power, Los Alamos National

Laboratory: Los Alamos, NM, 1999. 2. Winter, M.; Brodd, R. J., Chem. Rev. 2004, 104, 4245. 3. Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. , Chem. Rev.

2004,104, 4587. 4. Wang, F.; Hickner, M.A.; Ji, Q.; Harrison, W.L.; Mecham, J.F.; Zawodzinski, T.A.;

McGrath, J.E., Macromol. Symp., 2001, 175, 387. 5. Kim, Y.S.; Dong, L.; Hickner, M.A.; Pivovar, B.S.; McGrath, J.E., Polymer, 2002,

44 ,5729. 6. Wang, F.; Hickner, M.; Kim, Y.S.; Zawodzinski, T.A.; McGrath, J.E., J. Membr. Sci.

2002,197, 231. 7. Harrison, W. L.; Wang, F.; Mecham, J. B.; Bhanu, V. A.; Hill, M.; Kim, Y. S.;

McGrath, J. E., J. Polym. Sci. Part A: Polym. Chem., 2003, 41, 2264. 8. Li, Y.; Mukundan, T.; Harrison, W.; Hill, M.; Sankir, M.; Yang, J.; McGrath, J. E.,

Prepr. Symp. Am Chem Soc Div Fuel Chem, 2004, 49(2), 536. 9. Einsla, B.R.; Hong, Y.T.; Kim, Y.S.; Wang, F.; Gunduz, N.; McGrath, J.E., J. Polym.

Sci., Part A: Polym. Chem., 2004, 42, 862. 10. Einsla, B.R.; Kim, Y.S.; Hickner, M.A.; Hong, Y.T.; Hill, M.L.; Pivovar, B.S.;

McGrath, J.E., J. Memb. Sci., 2005, 255, 141. 11. Wiles, K. B.; Wang, F.; McGrath, J. E.; J. Polym.Sci., Part A: Polym. Chem., 2005,

43, 2964. 12. Ghassemi, H.; McGrath, J.E., ACS Polym. Prepr., 2002, 43, 1021. 13. Wang, S.; McGrath, J.E.; In Step Polymerization. M. Rogers, T. E.Long, ed., Wiley:

New York: 2003 14. 2005 DOE hydrogen program review May 23-26, Arlington, VA. 15. Roy, A.; Hickner, M.A.; Yu, X.; Li, Y.; Glass, T.E.; McGrath, J.E., J. Polym. Sci.,

Part B: Polym.Phys., 2006, 44, 2226. 16. Badami, A.S.; Lee, H.S.; Li, Y.; Roy, A.; Wang, H.; McGrath, J. E., Prepr. Symp.

Am Chem Soc Div Fuel Chem, 2006,51(2), 612.

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17. Ghassemi, H.; Ndip, G.; McGrath, J. E., Polymer, 2004, 45, 5855. 18. Ghassemi, H.; Zawodzinski, T.; McGrath, J. E., Polymer, 2006, 47, 4132. 19. Yu, X.; Roy, A.; McGrath, J. E., ACS PMSE Prepr., 2006, 95, 141. 20. Lee, H. S.; Roy, A.; Badami, A. S.; McGrath, J. E., ACS PMSE Prepr., 2006, 95, 210. 21. Wang, H.; Badami, A.S.; Roy, A.; McGrath, J. E., ACS PMSE Prepr., 2006, 95, 202. 22. Li, Y.; Roy, A.; Badami, A.S.; Yang, J.; McGrath, J.E., Prepri. Symp. Am Chem Soc

Div Fuel Chem, 2006, 51(2), 682. 23. Roy, A.; Yu, X.; Badami, A. S.; McGrath, J.E., ACS PMSE prepr., 2006, 94, 169. 24. Roy, A.; Lee, H.S.; Badami, A.S.; Yu, X.; Li, Y.; Glass, T.E.; McGrath, J.E. Prepr.

Symp. Am Chem Soc Div Fuel Chem, 2006, 51(2), 660. 25. Li, Y.; Wang, F.; Yang, J.; Liu, D.; Roy, A.; Case, S.; Lesko, J.; McGrath, J. E.,

Polymer, 2006, 47, 4210. 26. Kim, Y.S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y.T.; Harrison, W.;

Zawodzinski, T.A.; McGrath, J.E., J. Polym. Sci., Part B: Poly. Phys. 2003, 41, 2816 27. Zawodzinski, T. A.; Neeman, M.; Sillerud, L. O.; Gottesfeld, S., J. Phys. Chem.,

1991, 95, 6040. 28. Springer, T. E.; Zawodzinski, T. A.; Wilson, M. S.; Gottesfeld, S., J. Electrochem.

Soc., 1996, 143, 587. 29. Stejskal, E. O.; Tanner, J. E., J.chem.Phys, 1965, 288. 30. Thomas, S.C.; Ren, X.; Gottesfeld, S.; Zelenay, P., Electrochim. Acta, 2002, 47, 3741. 31. Wang, F.; Kim, Y.S.; Hickner, M.; Zawodzinski, T.A.; McGrath, J.E., ACS PMSE

Prepr., 2001, 85, 517. 32. Kim, Y.S.; Hickner, M.A.; Dong, L.M.; Pivovar, B. S.; McGrath, J.E., J. Membr. Sci.,

2004, 243, 317. 33. Kreuer, K. D., Solid State Ionics, 2000,136, 149. 34. Kim, Y.S.; McGrath, J.E.; Pivovar, B.S., Abstracts of the 206th Electrochem. Soci.

Meeting, 2004, 2004-02, 1471.

232

Chapter 8. Overall Conclusions Poly(arylene ether) copolymers containing sulfonic acid groups have

demonstrated the characteristics for potential application in proton exchange membrane

(PEM) fuel cells. The objectives of this research were to synthesize novel random and

multiblock disulfonated poly(arylene ether) copolymers via the direct copolymerization

method, and systematically investigate the effects of copolymer molecular weight,

chemical composition, and microstructure on the properties of PEMs. The investigated

PEM properties included ion exchange capacity, water uptake, proton conductivity, water

self-diffusion coefficient, methanol permeability, and fuel cell performance.

Direct copolymerization of the disulfonated monomers required high monomer

purity in obtaining high molecular weight copolymers for PEMs. A novel

characterization method for determining the purity of the disulfonated monomer

SDCDPS has been developed by using UV-Visible spectroscopy. Pure SDCDPS

recrystallized from IPA/H2O was used to establish a Beer’s Law plot, which was then

used to determine the purity of the crude product. The model poly(arylene ether sulfone)

copolymers were synthesized by direct copolymerization of the crude SDCDPS with

known purity, DCDPS, and BP. The relatively high molecular weights of the copolymers

confirmed that this characterization method was applicable to accurately determine the

purity and directly use the crude SDCDPS without purification process, which can save

money, time and energy. This is especially attractive for the mass production of the

copolymers. The results also showed that the SDCDPS needed to be dried in a vacuum

oven at 160 oC for at least 48 h to completely remove the water. Since the SDCDPS

233

absorbed small amounts of moisture after storage in a desiccator for several days, it was

suggested to dry the SDCDPS directly before the copolymerization.

The effects of copolymer molecular weight on PEM properties have been

systematically studied by synthesizing three series of controlled molecular weight

copolymers: BPSH35, partially fluorinated 6FSH35, and 6FSH48. BPSH35 copolymers

have the same degree of sulfonation as 6FSH35 copolymers, but have the same ion

exchange capacity values as the 6FSH48 copolymers. The molecular weights were

controlled from 20 to 50 kg·mol-1 using the monofunctional endcapper t-butyl phenol.

High molecular weight copolymers with 1:1 stoichiometry were also synthesized for the

comparison purpose. The molecular weight of the ionic copolymer was characterized by

a combination of 1H NMR endgroup analysis and modified intrinsic viscosity

measurements. Small amount of lithium bromide (0.05 M) in NMP was used to

effectively suppress the “polyelectrolyte effect” appearing in measuring the intrinsic

viscosity of a charged macromolecule, which allowed obtaining more accurate data than

previously used simple dilute solution viscosity measurements. Effects of molecular

weights on the properties of proton exchange membranes were studied. It was found that

with increasing the molecular weights, water uptake decreased modestly. But the

molecular weight was found to have no obvious influence on proton conductivity under

fully hydrated conditions. Furthermore, the mechanical properties of the BPS35

membranes, such as the modulus strength and elongation at break were improved by

increasing the molecular weight as well. Morphologies of copolymer membranes with

different molecular weights for BPSH35 and 6FSH35 series were characterized by AFM

images, which suggested that the morphology is a function of the degree of sulfonation,

234

irrespective of the molecular weight. Compared with BPSH35 series, 6FSH 35

copolymers with the same degree of sulfonation had lower water uptake and proton

conductivities, while 6FSH48 copolymers with the same IEC value had higher water

uptake and proton conductivities. These differences resulted from the different chemical

structures.

The chemical structure effects on the PEM properties were further investigated by

using three series of poly(arylene ether ketone) copolymers based on 6F-BPA, three

sulfonated ketone-type comonomers and their corresponding unsulfonated monomers.

The degree of sulfonation was controlled from 20 to 50 mol%. The intrinsic viscosity

measured with a modified method in 0.05 M LiBr/NMP confirmed that the molecular

weights of the copolymers were high enough to form tough, flexible films. The

membrane properties of these three series of copolymers have been comparatively

studied. Copolymers containing a meta diketone unit (MB series) showed higher water

uptake and proton conductivities compared with the mono ketone (B series) and para

diketone (PB series) at similar IEC values. This may be caused by the more flexible MB

chemical structures which resulted in better phase separation and higher hydration levels.

The B series copolymers also showed lower methanol permeability compared to Nafion®

and low interfacial loss in MEA measurements.

The use of multiblock copolymers is a new strategy to improve the PEM

properties over the random copolymers, especially under partially hydrated conditions. A

new hydrophobic-hydrophilic multiblock 6FK-BPSH (4:4)k copolymer was successfully

synthesized via a two-step polycondensation method. 19F and 1H NMR spectra were used

to characterize the oligomers’ molecular weight and multiblock copolymer’s structure.

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13C NMR was a powerful tool to confirm the sequence connection in the block copolymer.

Good film-forming material with high conductivity was obtained. Morphologies of the

membranes characterized with tapping mode atomic force microscopy showed that the

high temperature acidification method can improve the phase separation. The results were

consistent with previous studies on the BPSH random copolymer. AFM images also

indicated that the phase separation is sharper for the multiblock than for the random

copolymer. This well phase separated block copolymer morphology increases the extent

of connectivity between the hydrophilic domains and improves the PEM properties. It

was found that, compared with the random ketone type copolymer with the similar IEC

value, the multiblock copolymer has higher protonic conductivity, lower activation

energy, and much improved self-diffusion coefficient of water. It can keep more free

water than the random copolymer, which results in much improved protonic conductivity

under partially hydrated conditions. The multiblock copolymer also showed very

promising fuel cell performance, which was comparable to Nafion®.

The Influence of Aromatic Disulfonated Random and Block ......Exchange Membranes for Fuel Cells...

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Transcript of The Influence of Aromatic Disulfonated Random and Block ......Exchange Membranes for Fuel Cells...