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Progress in Polymer Science 39 (2014) 164– 198

Contents lists available at ScienceDirect

Progress in Polymer Science

j ourna l ho me pag e: www.elsev ier .com/ locate /ppolysc i

ecent progress in fluoropolymers for membranes

haoliang Cuia, Enrico Drioli a,b,∗, Young Moo Leea,c,∗

WCU Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of KoreaResearch Institute on Membrane Technology, ITM-CNR, Via Pietro Bucci 17/C, Rende 87036, ItalySchool of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea

a r t i c l e i n f o

rticle history:eceived 21 November 2012eceived in revised form 9 July 2013ccepted 16 July 2013vailable online 26 July 2013

eywords:luoropolymerembrane

a b s t r a c t

Fluoropolymers usually have high mechanical strength and excellent chemical stability andthus have been employed for the last several decades as materials for membrane separationprocesses, typically in water purification and energy applications such as microfiltration(MF), ultrafiltration (UF), fuel cells, battery separators, etc. Recently, new membrane oper-ations such as membrane distillation (MD), membrane crystallization (MCr), membraneemulsification (ME) and membrane contactors (MC) have become popular. A much strongerunderstanding of the basic properties of the materials utilized is required for adopting theappropriate membrane and for finding the relationship among material properties, mem-

embrane preparationembrane modification

brane morphology and the transport phenomena in the membranes. This review presentsthe structures and properties of fluoropolymers for membranes, the preparation and mod-ification methods of fluoropolymer membranes, and their applications. Recent progressin the development of novel fluoropolymers for membranes and their fabrication and

modification methods are reported as well.

© 2013 Elsevier Ltd. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1661.1. Membrane technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1661.2. Fluoropolymers for membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1661.3. Fluoropolymer membrane preparations and modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

2. Poly(vinylidene fluoride) (PVDF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1672.1. Structure of PVDF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

2.1.1. Crystalline forms of PVDF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1702.1.2. Crystallization of PVDF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

2.2. Properties of PVDF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1712.3. PVDF membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

2.3.1. PVDF membrane preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1712.3.2. PVDF membrane modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

2.4. PVDF copolymer membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

2.4.1. Poly(vinylidene fluoride-co-tetrafluoroethylene) 2.4.2. Poly(vinylidene fluoride-co-hexafluoropropene) (2.4.3. Poly(vinylidene fluoride-co-chlorotrifluoroethyle2.4.4. Poly(vinylidene fluoride)-graft-poly-(styrene sulf

∗ Corresponding authors.E-mail addresses: [email protected] (E. Drioli), [email protected] (Y.M. Lee

079-6700/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.progpolymsci.2013.07.008

(P(VDF-co-TFE)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175P(VDF-co-HFP)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176ne) (P(VDF-co-CTFE)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178onic acid) (PVDF-g-PSSA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

).

dx.doi.org/10.1016/j.progpolymsci.2013.07.008http://www.sciencedirect.com/science/journal/00796700http://www.elsevier.com/locate/ppolyscihttp://crossmark.crossref.org/dialog/?doi=10.1016/j.progpolymsci.2013.07.008&domain=pdfmailto:[email protected]:[email protected]/10.1016/j.progpolymsci.2013.07.008

Z. Cui et al. / Progress in Polymer Science 39 (2014) 164– 198 165

2.4.5. Poly(vinylidene fluoride–trifluoroethylene) (P(VDF-TrFE)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1813. Poly(tetrafluoroethylene) (PTFE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

3.1. Structure of PTFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1813.2. Properties of PTFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1823.3. PTFE membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

3.3.1. Preparation of PTFE membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1823.3.2. Modification of PTFE membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

3.4. PTFE copolymer membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1853.4.1. Perfluorosulfonic acid (PFSA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1853.4.2. Poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1883.4.3. Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1883.4.4. Poly(ethylene-alt-tetrafluoroethylene) (ETFE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

4. Poly(ethylene chlorotrifluoroethylene) (ECTFE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1904.1. Structure of ECTFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1904.2. Properties of ECTFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1904.3. ECTFE membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

5. Other fluoropolymers for membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1916. Conclusions and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

Nomenclature

AAEM alkaline anion-exchange membraneAAm acrylamideAGMD air-gap membrane distillationATRP atom transfer radical polymerizationCGE capillary gel electrophoresisCTF tri-ethyl citrateEC ethylene carbonateDBP dibutyl phthalateDEP diethyl phthalateDHP dihexyl phthalateDMAc N,N-dimethyl actamideDMF N,N-dimethyl formamideDMFC direct methanol fuel cellDMP dimethyl phthalateDMSO dimethyl sulfoxideDOP dioctyl phthalateDPC diphenyl carbonateDPK diphenyl ketoneEAA ethyl acetoacetateECTFE poly(ethylene chlorotrifluoroethylene)ETFE poly(ethylene-alt-tetrafluoroethylene)FEP poly(tetrafluoroethylene-co-

hexafluoropropylene)FO forward osmosisGMA glycidyl methacrylateGTA glycerin triacetateHFP hexafluoropropeneHMPA hexamethyl phosphoramideLSC long-side-chainMC membrane contactorMCr membrane crystallizationMD membrane distillationME membrane emulsificationMF microfiltration

MGA membrane gas absorptionMW molecular weightNaSS sodium 4-syrenesulfonatedNF nanofiltrationNMP N-methyl-2-pyrrolidoneNIPS non-solvent induced phase separationPC propylene carbonatePCTFE poly(chlorotrifluoroethylene)PEG poly(ethylene glycol)PEGMA poly(ethylene glycol) methacrylatePEM proton-exchange membranePEMFC proton-exchange membrane fuel cellPFA poly(tetrafluoroethylene-co-

perfluoropropyl vinyl ether)PFE poly(fluorenyl ether)PFSA perfluorosulfonic acidPGC propylene glycol carbonatePHEA poly(hydroxyl ethyl acrylate)PIMs polymer inclusion membranesPMMA poly(methyl methacrylate)PSSA poly(styrene sulfonic acid)PTFE poly(tetrafluoroethylene)PV pervaporationPVA poly(vinyl alcohol)PVAc poly(vinyl acetate)PVDF poly(vinylidene fluoride)P(VDF-co-CTFE) poly(vinylidene fluoride-co-

chlorotrifluoroethylene)P(VDF-co-HFP) poly(vinylidene fluoride-co-

hexafluoropropene)P(VDF-g-PSSA) poly(vinylidene fluoride)-graft-

(styrene sulfonic acid)P(VDF-co-TFE) poly(vinylidene fluoride-co-

tetrafluoroethylene)

166 Z. Cui et al. / Progress in Polymer Sc

P(VDF-TrFE) poly(vinylidene fluoride-trifluoroethylene)

PVF poly(vinyl fluoride)PVP poly(vinyl pyrrolidone)RAFT reversible addition-fragmentation chain

transferRH relative humidityRO reverse osmosisSDS sodium dodecyl sulfateSFVE sulfonyl fluoride vinyl etherSSC short-side-chainTEP triethyl phosphateTFE tetrafluoroethyleneTHF tetrahydrofuranTMP trimethyl phosphateTMU tetramethylureaTIPS thermally induced phase separationTTD 2,2,4-trifluoro-5-trifluoromethoxy-1,3-

dioxoleUF ultrafiltrationVBC vinylbenzul chlorideVDF vinylidene fluoride

1

1

pwaattnnpwemiiascupinompnp

gt

VIPS vapor induced phase separation

. Introduction

.1. Membrane technology

Water scarcity and energy consumption cause globalroblems even in regions currently considered asater- and/or energy-rich. As the world population

nd economies expand, problems with water shortagesnd energy consumption are increasing and are expectedo grow worse in the coming decades [1]. Membraneechnology has become an important separation tech-ology over the past few decades and is a promisingovel way to cope with these two problems thanks to itsrincipal advantages such as relatively low energy use,orking without the addition of chemicals, ease of use,

nvironmental friendliness and well-understood processethods [2]. Since the first commercial membrane was

nvented in the 1960s, many kinds of polymeric andnorganic materials have been developed scientificallynd commercially for membrane fabrication. With theignificant progress in membrane materials, many pro-esses including reverse osmosis (RO), nanofiltration (NF),ltrafiltration (UF), microfiltration (MF), electrodialysis,ervaporation (PV), etc. have already been widely used

n different applications. In recent years, some innovativeew processes like membrane contactors (MC), forwardsmosis (FO), catalytic membrane reactors, and fuel cellembranes have been widely investigated and have strong

otential for application in the process industry [3]. Theseew processes require novel membranes, consequently

romoting the development of new membrane materials.

For application in membranes, polymers need to haveood film-forming ability, high mechanical, chemical andhermal stability, and a good balance of permeability and

ience 39 (2014) 164– 198

selectivity. In addition to these attributes, membranesshould have highly hydrophilic characteristic when theyare used for MF/UF with aqueous solutions, but shouldbe very hydrophobic when they are used for membranedistillation (MD). Membranes need to have high gas per-meability and selectivity when they are employed for gasseparation. When they are used for energy applicationssuch as fuel cells, membranes should have high protonexchange capacity. During the past several decades, mostcontributions have been made on how to fabricate morehydrophilic membranes due to the prosperous research onMF/UF. However, recently, more attention has been paid tonew membrane processes, especially MD, which requiresmore hydrophobic membranes [4]. The main principals andrequirements of fluoropolymer membranes used in differ-ent processes are shown in Table 1. More details can befound in the literature.

1.2. Fluoropolymers for membranes

Fluoropolymers constitute a unique class of materi-als with a combination of interesting properties that hasattracted significant attention from material researchersover the past few decades [18,19]. Generally, these poly-mers have high thermal stability, improved chemicalresistance, and lower surface tension because of the lowpolarizability and the strong electronegativity of the flu-orine atom, its small van der Waals radius (1.32 Å), andthe strong C-F bond (485 kJ mol−1). Due to these outstand-ing properties, fluoropolymers are an excellent choice forvarious applications [19].

It has been observed that some fluoropolymers exhibitmany desirable properties for a much wider range of appli-cations in membranes. Recently, many fluorinated poly-mers and copolymers have been widely employed to fab-ricate membranes, especially with the development of thethermally induced phase separation (TIPS) method, whichcan be applied to polymers that are not soluble in mostcommon solvents at room temperature. The important flu-oropolymers for membrane operations are listed in Table 2,and their chemical structures are presented in Table 3.

1.3. Fluoropolymer membrane preparations andmodifications

Several methods have been studied and employed in thefabrication of fluoropolymer and copolymer membranes.These include phase inversion, electro-spinning, sintering,stretching, track etching, etc. To date, most commercialfluoropolymer membranes are produced via phase sep-aration methods mainly because of their simplicity andflexible production scales. Phase inversion can be describedas a demixing process whereby the initially homogeneouspolymer solution is transformed in a controlled mannerfrom a liquid to a solid state. Recently, electro-spinning wasemployed to fabricate PVDF nano-fiber membranes, whichis easier to make hydrophobic membranes for the MD pro-

cess [7]. Table 4 summarizes the main preparation methodsof fluoropolymers.

Post-treatment is a useful method to improve themembrane structure and properties. Stretching is usually


Table 1Representative membrane processes and requirements for membrane materials.

Membrane process General mechanism Main properties Reference

MF/UF Pressure-driven, liquid passes throughthe membrane pores

Hydrophilic used in aqueous systems, whilehydrophobic used in oil systems

[5]

MD Thermally-driven, water vapor passesthrough the membrane pores

Hydrophobic, high porosity [6,7]

Membrane crystallization (MCr) Thermally-driven, vapor passesthrough the membrane pores

Hydrophobic used for hydrophilic (aqueous)crystallizing solutions, while hydrophilic usedfor oleophilic solutions

[8]

Membrane emulsification (ME) Pressure-driven, contentious phasepassed through the membrane pores

Hydrophobic used for producing O/Wemulsions, while hydrophilic used forproducing W/O emulsions

[6]

Osmotic distillation Vapor pressure-driven, vapor diffusesthrough the membranes

Hydrophobic typically [6]

PV Concentration-driven, vapor passesthrough the membranes

Hydrophilic for dehydration of organicsolvents or organic mixtures; hydrophobic forremoval of organic solvents or volatile organiccompounds (VOCs) from water;organophilicfor organic/organic separation

[9,10]

Proton-exchange membrane (PEM) Proton transports in membranes High proton conductivity, mechanical,chemical and thermal stability, good barrierproperties for gas and methanol

[11–13]

Membrane separator for Li-ion battery Transport ionic charge carriers andprevent electric contact betweenanode and cathode electrodes

High ionic conductivity and good barrier forelectron

[14]

Gas separation membrane Pressure-driven High diffusivity and/or high solubility topermeate gases

[15,16]

en, gasne pore

Membrane gas absorption (MGA) Concentration gradient-drivpasses through the membra

employed to enhance the pore size and porosity. Heat-pressing is useful in enhancing the mechanical strength ofnano-fiber membranes.

There are two strategies to obtain a certain featurefor fluoropolymer membrane application. The first is todevelop new materials and optimize the formation con-ditions and parameters to fabricate membranes with the

required properties; another is to modify the existingmembranes to make them more suitable for a certainprocess, especially in water applications which usuallyrequire hydrophilicity considering that pure fluoropolymer

Table 2Fluoropolymers for membrane operations.

Polymer

Poly(vinylidene fluoride) (PVDF) homopolymer PVDF copolymer Poly(vinylidene fluoride-co-tetrafluoroethylene) (P

Poly(vinylidene fluoride-co-hexafluoropropene) (P(

Poly(vinylidene fluoride-co-chlorotrifluoroethylenePoly(vinylidene fluoride)-graft-poly-(styrene sulfonPoly(vinylidene fluoride-trifluoroethylene) (P(VDF-

Poly(tetrafluoroethylene) (PTFE) homopolymer PTFE copolymer Perfluorosulfonic acid (PFSA)

Poly(tetrafluoroethylene-co-perfluoropropyl vinyl ePoly(tetrafluoroethylene-co-hexafluoropropylene) Poly(ethylene-alt-tetrafluoroethylene) (ETFE)

Poly(ethylene chlorotrifluoroethylene) (ECTFE) Other fluoropolymers Poly(chlorotrifluoroethylene) (PCTFE)

Poly(vinyl fluoride) (PVF) Poly(fluorenyl ether) (PFE) Hyflon® AD, Teflon® AF, Cytop®

sHydrophobic [17]

membranes are usually hydrophobic [22]. Surface coating,surface grafting, blending, and filling are major ways tomodify the properties of fluorinated polymeric membranes(Table 5).

2. Poly(vinylidene fluoride) (PVDF)

Because of its combination of good properties and pro-cessability, PVDF has been widely used as a membranematerial. Remarkable progress has been made in the fabri-cation of PVDF membranes with high performance.

Membrane process

MF, UF, MD, MCr, ME, PV(VDF-co-TFE)) MF/UF, MDVDF-co-HFP)) MF/UF, MD, PV, fuel cell, lithium ion

battery) P(VDF-co-CTFE) MF/UF, NF, MD, PV, fuel cellic acid) PVDF-g-PSSA Fuel cellTrFE)) Lithium ion battery, tissue regeneration

MD, MC, PV, MGAFuel cell, lithium ion battery, chlor-alkaliindustry

ther) (PFA) Fuel cell(FEP) MD, fuel cell

Fuel cell

PV, has potential in MD, MC and MF/UF

Fuel cellFuel cellGas separation, have potential in MD, MC

168 Z. Cui et al. / Progress in Polymer Science 39 (2014) 164– 198

Table 3Chemical structures of main fluoropolymers for membranes.

Polymer Chemical structure

PVDF homopolymer CH2 CF2n

PVDF copolymer P(VDF-co-TFE)CH2 CF2

mCF2 CF2

n

P(VDF-co-HFP)CH2 CF2

mCF2 CF

nCF3

P(VDF-co-CTFE)CH2 CF2

mCF2 CF

nCl

PVDF-g-PSSA

CF2 CH2 mCF2 CH CF2 CH2 CFn

CH3

CH2 CH yCH2 CH3

CH2

CH

CH2

CH3

x

HO3S

SO3H

P(VDF-TrFE) CF2 CH2 CF2mCHF

n

PTFE homopolymerCF2 CF2

n

PTFE copolymer PFSA

CF2 CF2m

CF2 CFn

O

CF2

CFF3C

O CF2 SO3Hx

y Nafion® 117 x≥1, y= 2, m=5 -13 .5, n=1000 Flemio n® x=0,1; y =1-5 Aciplex® x=0,3; y=2 -5, m=1 .5-14 Dow membrane x=0, y =2, m=3 .6-10 Hyflon® Ion H x=0, y =2

Aquivion® PFSAx=0, y =2

(Gene ral che mical structures of PFSA iono mer [3, 18, 21] )

PFACF2 CF2 CF2

mCF

O CF2

n

CF2 CF3


Table 3 (Continued)

Polymer Chemical structure

FEPCF2 CF2 CF2

mCF

CF3

n

ETFECF2 CF2 CH2

mCH2

n

ECTFE CH2 CH2 C

Cl

F

CF2 n

Other fluoropolymers Hyflon® ADCF C

O

CF2

O

CF2

OF3C

CF2

m

n

Teflon® AF

CF CF

O

C

O

CF2 CF2m n

F3C CF3

Cytop®CF CF

O

CF2

CF2

CF2 CF2n

PCTFECF2 CF2

nCl

PVF CF2 CHF n

Table 4Preparation methods of fluoropolymer membranes.

Preparation method Mechanism

Phase inversionNon-solvent induced phase separation (NIPS) Phase separation is induced by the exchange of the solvent in polymer solution with

the non-solvent from the coagulation bath.Thermally induced phase separation (TIPS) Phase separation is induced by the temperature change.Vapor induced phase separation (VIPS) Phase separation is induced by penetration of non-solvent vapor into the solution.

Solution-casting Polymer solution is cast and evaporated.Casting-freezing Polymer solution is cast and frozen at a temperature lower than solvent’s freezing

point.Electro-spinning Polymer solution is injected through the nozzle onto the electrode or collector to

prepare nano-fibres.Sintering Polymer films are sintered.Stretching Polymer melts are subject to orientation.Track etching Dense polymer films are exposed to radiation to create holes for permeation.


Table 5Important modification methods of fluoropolymer membranes.

Modification method Mechanism

Surface coating A layer with specific features is coated onto the fluoropolymer membrane surface.Surface grafting Chains with specific features are grafted to the fluoropolymer membrane surface through covalent boding interaction.Blending Polymers or inorganic particles are blended with fluoropolymer during the solution preparation.Pore filing Inorganic particles or liquid are filled into the fluoropolymer membrane pores.Sputtering Polymer or inorganic particles are sputtered onto the fluoropolymer membrane surface.

Table 6Properties of PVDF with different polymorphs[26,27].

�-phase �-phase �-phase �-phase �-phase

Crystal system Monoclinic Orthorhombic MonoclinicPolarity Nonpolar Polar Polar PolarSpace group P21/c(C 52h ) Cm2m(C

142v ) C121(C

32 )

Lattice constants a = 4.96Å a = 8.58Å a = 8.66Åb = 9.64Å b = 4.91Å b = 4.93Åc(f.a.) = 4.62Å c(f.a.)a = 2.56Å c(f.a.) = 2.58Åˇ = 90 ◦ ̌ = 97 ◦

Number of chains per lattice 2 2 2Molecular conformation TGTG’ TTT TTTGTTTG’ TGTG’ TTTGTTTG’Density, Obsd at 30 ◦C (g/ml) 1.77 1.81 1.80Calcd (X-ray) (g/ml) 1.93 1.97 1.94

−1 470, 511, 600, 840, 431, 512, 776, 795, 812, 833,

840, 1233,18.5, 19.2, 20.46

2

caidheticr

2

ofifpImcTowf

p

s(it

FTIR peak (cm ) 408, 532, 612, 766, 795,855, 976, 1182, 1400

445,1270

Peak of 2� of X-ray diffraction (◦) 17.66, 18.30, 19.90, 26.56 20.2

.1. Structure of PVDF

Homopolymers of PVDF are semicrystalline and longhain macromolecules which contain 59.4 wt.% fluorinend 3 wt.% hydrogen [23] (chemical structure is shownn Table 3). They typically have a crystallinity of 35–70%,epending on the preparation and thermal mechanicalistory. Molecular weight, molecular weight distribution,xtent of irregularities along the polymer chain, crys-allinity and the crystalline form are the major factorsnfluencing the properties of PVDF. The high level ofrystallinity yields PVDF stiffness, toughness, and creep-esistance.

.1.1. Crystalline forms of PVDFIt has been well established that the crystalline phase

f PVDF has three different molecular conformations andve distinct crystal polymorphs. The three molecular con-

ormations are TGTG′, TTT and TTTGTTTG′. The crystalolymorphs are named � (phase II), � (phase I), � (phase

II), �, and �, among which the �-, �- and �-phases are theost frequent [24]. The �- and �-phases have the same

onformation of TGTG’, the �-phase has a conformation ofTT, and the �- and �-phases have the same conformationf TTTGTTTG′ (Fig. 1). The �-form is kinetically favorable,hile the �-form is the most thermodynamically stable

orm.The parameters of the PVDF molecules with different

olymorphs are summarized in Table 6.The polymorphism of PVDF is directly related to the

lightly larger van der Waals radius of the fluorine atom1.35 Å) versus that of the hydrogen one (1.20 Å). In fact,t is a rare characteristic to have these five types of crys-alline phases in PVDF for synthetic homopolymers [24].

Fig. 1. Three polymorphs of PVDF where the black, white, and grayspheres represent carbon, hydrogen, and fluorine atoms, respectively [25].(a) �-form; (b) �-form; (c) �-form.

lymer S
Z. Cui et al. / Progress in Po
For the �-phase, the polymeric chains are nonpolar, andthe hydrogen and fluorine atoms alternate in a regularway on both sides of the chain, producing a helix-typestructure (Fig. 1). For the �-phase with zigzag (all-trans)conformation, the orientation of the strong dipole in the-CH2CF2- base units along the polymer chain gives the flu-oropolymer its piezo-, pyrro- and ferro-electric properties[24]. The �-phase has an intermediate polar conformation(TTTGTTTG′) [28]. These different crystalline phase struc-tures have already been characterized [24,26].

2.1.2. Crystallization of PVDFSince different polymorphs exhibit various properties,

significant efforts have been made to obtain a specific poly-morph under reproducible, well-controlled conditions.PVDF crystals with a desired polymorph can be formedfrom melts, non-solvents, and temperature change.

The �-phase can be obtained directly from melt, solvent,or by thermal method. The �-phase is usually obtainedfrom a homopolymer including additives [29], from solu-tion under appropriate conditions [30], or annealing.Indirect phase transition methods, which can increase thefraction of the �-phase during processing, include tensiledeformation and, applying a strong electric field, cryogenicmechanical milling [31], and uniaxial compression defor-mation of the �-phase. In all these methods for preparingthe �-phase, solution deposition, especially solution cast-ing, is a convenient way to prepare �-phase PVDF filmsunder controlled conditions. The �-phase is only obtainedduring crystallization either from the melt or from solutionat temperatures close to Tm of the �-phase or by anneal-ing an originally �-phase sample at these temperatures[30].

Although there have been many attempts to explain themechanism of PVDF film formation with different crys-talline phases, the development of the different phases,especially from solution casting, has not been clarified.Because of the myriad conditions and parameters affect-ing the formation of distinct phases in PVDF, fine control isstill lacking. Most authors have concentrated on tempera-ture as the single parameter that exerts control, but it hasbeen recently found that evaporation rate may play an evenmore important role [32,33], related to the boiling points ofsolvents and the temperature, concentration, and thicknessof the samples.

It has been found that �-phase PVDF has some specificproperties such as polarity and higher mechanical strengthcompared with �-phase. It would be interesting to studythe effect of polymorphism on transport properties. Dif-ferent performances are expected for PVDF membraneswith different polymorphs in porous membrane processes,as well as fuel cell and lithium ion battery membraneprocesses. Recently, piezo-electric PVDF membranes havebeen fabricated by electrically poling the PVDF polymorphinto �-phase [34]. The poled PVDF membranes underwentvibration when alternating current signals were applied to

the membranes, so that the membranes possessed anti-fouling properties in a certain extent. However, there is noreport on the direct relationship between polymorphismand permeation behavior of PVDF membranes.

cience 39 (2014) 164– 198 171

2.2. Properties of PVDF

PVDF is inert to various solvents, oils, and acids andshows low permeability to gases and liquids. The glass tran-sition (Tg) and melting temperatures (Tm) of the amorphousand crystalline PVDF regions are in the ranges of −40 to−30 ◦C and 155–192 ◦C, respectively. The molecular weightand the number of chain defects have an effect on the Tg andTm. Amorphous PVDF regions have a density of 1.68 g cm−3,� and � polymorphs have a density of 1.92 and 1.93 g cm−3,respectively, while that of a � polymorph is 1.97g cm−3. Thus, the typical density of commercial products isin the range of 1.75 to 1.78 g cm−3, reflecting a crystallinitydegree around 40%. The melt density of a PVDF homopoly-mer is ca. 1.45–1.48 g cm−3 at 230 ◦C and 1.0 bar [19].

Because of its excellent combination of properties andprocessability, PVDF is available in a wide range of meltviscosities as powders and pellets to fulfill typical fabri-cation requirements. All common extrusion and moldingtechniques can be used to process PVDF into shapes.Typical molding temperatures in the cylinder and noz-zle are 180–240 ◦C for injection types and 50–90 ◦C formolds. Recently, PVDF with long chain branches has beensynthesized using potassium persulfate and n-butyl-4,4-bis(tert-butylperoxy) valerate [35]. These PVDFs exhibitsimilar behavior and overall rheological properties andshould find new applications in melt processing, whichmight be beneficial for PVDF membrane preparations andapplications.

2.3. PVDF membranes

PVDF membranes are widely employed in the processindustry and have been used in UF/MF, MD, PV [36] andother processes. They have also been adopted in energyapplications such as in the membranes of fuel cells andthe separators of lithium ion batteries [37,38]. A new inter-esting use involves making artificial muscles using blendsmade of PVDF [39]. In the past ten years, publication num-bers related to PVDF and its copolymer membranes in theWeb of Science increased dramatically (Fig. 2), displayingthe active research in this field.

2.3.1. PVDF membrane preparationThe main preparation methods of PVDF porous mem-

branes are NIPS and TIPS. For the NIPS process, PVDF isdissolved in a proper solvent close to room temperature,and a non-solvent is employed to induce phase separa-tion. For the TIPS process, PVDF is melt blended withdiluents at a high temperature, and phase separation isinduced by decreasing the temperature. For both of thesetwo methods, selecting suitable solvents/diluents is veryimportant for the morphologies, structures, properties, andperformances of the resulting PVDF membranes. Phase dia-grams have been employed as an effective way to predictcrystallization behavior during PVDF membrane prepara-tion using different solvents/diluents. The phase separation

mechanisms of TIPS and NIPS processes are quite differentfrom each other, and a unique structure is formed fromeach process. The main factors affecting PVDF membraneswere discussed in a recent review [40].

172 Z. Cui et al. / Progress in Polymer Sc

1996 1998 2000 2002 2004 2006 2008 2010 2012

20

40

60

80

100

Pu

blicatio

n n

um

be

r

Yea r

Fig. 2. Publication numbers related to PVDF and its copolymer mem-bflu

sfcsbcmT

receive more and more attention for preparing PVDF nano-

ranes in the Web of Science. Searched using keywords ‘(poly(vinylideneuoride) membrane) or (PVDF membrane) or (P VDF membrane)’ in title,ntil Dec. 31st, 2012.

The NIPS method is easier to realize if the polymer isoluble in a suitable solvent, while the TIPS method canabricate fluoropolymer membranes with higher mechani-al strength and narrower pore size distribution. The phaseeparation of a PVDF solution with diluent and solvent cane induced by either lowering the temperature via TIPS or

ounter diffusion with non-solvent via NIPS. Then the for-ation of PVDF membranes via a combination of NIPS and

IPS can be realized. This method has great potential for

Fig. 3. SEMs of the PVDF membranes prepared by the wet-inv

ience 39 (2014) 164– 198

fabricating PVDF membranes with structures and proper-ties combining the advantages of TIPS and NIPS.

Another novel method to fabricate PVDF membranes isVIPS, which has been studied in order to preferably controlthe formation process. In this process, a polymer solu-tion is cast or spun into a desired shape and exposed toa vapor atmosphere where the vapor phase consists of anon-solvent. Phase separation of the polymer solution isinduced by the penetration of non-solvent vapor into thesolution [41,42] (Fig. 3). This method is more efficient increating highly porous PVDF membranes with pore sizesin the micron and sub-micron ranges.

A simple and effective freezing method has beenexplored to fabricate PVDF microporous membranes withhigh water fluxes and excellent mechanical properties[43,44] (Fig. 4). The polymer solution was cast onto aglass plate, and the solidification of casting solutionswas achieved by a freezing method. The frozen solventwas exchanged, and PVDF microporous membranes wereformed. This method can yield PVDF membranes with someof the characteristics typically induced by the TIPS methodwhile avoiding high temperatures. The mechanical prop-erties of PVDF membranes were dramatically enhancedby this freezing method. Dimethyl sulfoxide (DMSO) wasselected as a solvent; its melting point of 18.4 ◦C wasthe reason selected for this freeze method due to ease offreezing.

Recently, an electro-spinning process has begun to

fiber membranes [45–47] (Fig. 5), which was found to havea contact angle much higher (130 ◦C) than that of the con-ventional phase inversion membrane (83 ◦C). With this

ersion method (a, b) and the VIPS method (c, d) [42].


d botto
Fig. 4. SEM micrographs of the top surface (left), cross-section (middle) anmethod [43].
hydrophobicity, PVDF nano-fiber membrane was used indesalination by air-gap membrane distillation (AGMD) [45]and relatively performed high permeate flux, NaCl rejectionand long-term operation. In addition, the Columbic force inthis process may promote the �-phase, which is beneficialfor use in ionic membranes [48].

Applicable solvents for the NIPS process [40] are N,N-

dimethyl acetamide (DMAc), N,N-dimethyl formamide(DMF), DMSO, hexamethyl phosphoramide (HMPA), N-methyl-2-pyrrolidone (NMP), tetramethylurea (TMU),triethyl phosphate (TEP), trimethyl phosphate (TMP),

Fig. 5. SEM images of a PVDF na

m surface (right) morphologies of PVDF membranes created by the freeze

acetone (Ac), and tetrahydrofuran (THF), while the applica-ble diluents for the TIPS process [49] are dimethyl phthalate(DMP), diethyl phthalate (DEP), DBP, dihexyl phthalate(DHP), glycerin triacetate (GTA), ethyl acetoacetate (EAA),propylene glycol carbonate (PGC), diphenyl ketone (DPK),diphenyl carbonate (DPC), cyclohexanone, camphor, andbutyrolactone. Most of the above solvents/diluents are

toxic, although DMSO and GTA are relatively less toxic.Since many membrane processes are employed withthe objective of environmentally improving the processindustry, toxic solvents/diluents reduce their contributions

nofibre membrane [45].


S PEGMAo ation; (bA erizatio

tri

2

yaigPmtii

2Psmisdaaita

cheme 1. Schematic illustration of the preparation process of PVDF-g-zone surface activation followed by thermal-induced radical polymerizTRP; (c) low pressure plasma treatment for plasma-induced graft-polym

o environmental protection. Thus, finding more envi-onmentally friendly solvents/diluents is becoming anmportant topic in the membrane preparation field.

.3.2. PVDF membrane modificationTo enhance the properties of PVDF membranes or to

ield new characteristics, some modification methods suchs surface coating, surface grafting, blending, and pore fill-ng have been investigated and used. Surface coating andrafting and pore filling can greatly modify the properties ofVDF membranes, but a disadvantage is that the benefits ofodification might be counteracted by unexpected resis-

ance. Blending is a simple method and usually does notnduce additional resistance, thus it possesses the possibil-ty of commercialization compared with other methods.

.3.2.1. Surface modification. Surface modifications ofVDF membranes mainly include surface coating andurface grafting. Most efforts regarding surface coatingodifications are attributed to changing or enhanc-

ng the hydrophilicity, hydrophobicity, charge or ionictrength, or proton conductivity of PVDF membranesue to the increasing requirements of water treatmentnd energy saving [40,50–52]. Chemical bonding [53]

nd solid-vapor interfacial cross-linking [22,54] werentroduced to enhance the stability of a coated layer onhe PVDF membrane surface. After alkaline modification,

PVDF membrane surface can be coated with hydrophilic

UF membranes via surface-initiated radical graft copolymerization: (a)) ozone treatment and bromide activation followed by surface-initiatedn [68].

macromolecules such as poly(vinyl pyrrolidone) (PVP),which possesses the lactam structure, allowing it tointeract with hydroxyl groups [55]. Chitosan has also beencoated both on the membrane surface and on pore wallsthrough the combined chitosan solution flow and surfacemethod [56]. Additionally, a polyurethane layer has beencoated onto PVDF membranes for gas separation [57].

To combat the instability issue of the surface coat-ing modification method, grafting polymerization ofmonomers has been developed to immobilize the func-tional chains, brushes, or layers onto the membrane surfacethrough covalent bonding interactions [58]. In contrast tothe physical surface coating method, long-term chemicalstability of grafted chains was offered by covalent attach-ment of the graft chains [59,60]. This can be achievedby irradiation, plasma, or ‘living’/controlled polymeriza-tion with a single monomer or with a mixture of twoor more monomers [61]. ‘Grafting to’ and ‘grafting from’are two main ways of surface grafting [59,62]. Nitroxide-mediated living free radical polymerization [63], reversibleaddition-fragmentation chain transfer (RAFT) polymeriza-tion [64,65], and atom transfer radical polymerization(ATRP) [66,67] are key strategies to prepare these mem-branes. Sulfonation is also a potential method to modify

the PVDF membrane surface, making it hydrophilic [50].

Hydrophilic poly(ethylene glycol) methacrylate(PEGMA) polymer can be grafted onto the PVDF mem-brane surface through different modification approaches


GMA an
Fig. 6. Illustration of the grafting structures of (a) brush-like PE
of surface-initiated radical graft copolymerization [68](Scheme 1). Thermal-induced radical polymerizationand ATRP are expected to produce brush-like PEGMAstructures, and low-pressure plasma-induced graft-polymerization is expected to induce network-like PEGMAstructures (Fig. 6). It was found that the hydration capac-ity of water molecules inside the grafting structure ofnetwork-like PEGMA is greater than that of brush-likePEGMA, but the grafting structure of brush-like PEGMAleads to lower protein adsorption and better antifoulingfor BSA filtration than that of network-like PEGMA on thePVDF membrane surface.

In order to avoid the changes in pore size and dis-tribution that accompany surface coating and grafting,hydrophilic PVDF membranes can be formed using mod-ified polymer materials by a pre-irradiation-induced graftpolymerization technique [69]. This method presents a wayto modify the surface of hollow fiber membranes usingrelatively simple steps.

2.3.2.2. Pore filling. The pore filling method is a simple wayto modify PVDF membranes by filling inorganic particles orliquids into PVDF membrane pores and can be employedto modify the properties of PVDF membranes, inducingpH-sensitive membranes [70,71], and prepare PVDF mem-branes used in fuel cells [72] and lithium ion batteries [73].

2.3.2.3. Blending. Blending is a simple and attractivemethod to modify fluoropolymer membranes. Polymers[74–79] and inorganic particles (especially nanoparticles)[80,81] have been used to make blending membranes

d (b) network-like PEGMA on the PVDF membrane surface [68].

to improve a membrane’s hydrophilicity/hydrophobicity,proton conductivity, ionic conductivity, porosity andmechanical strength. It is very interesting that amphiphiliccopolymers can yield hydrophilicity in a PVDF membranein a single process with membrane preparation [76,82](Fig. 7). Generally, the synthesis of amphiphilic copolymerscan be achieved by general radical polymerization, thermalgraft copolymerization, ATRP, or RAFT.

2.3.2.4. Copolymerization. Thermo-sensitive [83,84]and/or pH-sensitive [71,85,86], and electrolyte-sensitive[87], redox-sensitive [88] membranes and membranesused in fuel cells [89] can be fabricated using the designedPVDF copolymers.

2.4. PVDF copolymer membranes

Although PVDF has been widely employed in membraneprocesses, many copolymers of PVDF have been developedto obtain more specific properties for the requirements ofnewly emerged membrane processes. Properties such ascrystallinity, melting point, glass transition temperature,stability, elasticity, permeability, and chemical reactiv-ity may be changed as a result of copolymerization [23].P(VDF-co-TFE), P(VDF-co-HFP), P(VDF-co-CTFE), PVDF-g-PSSA and P(VDF-TrFE) are important PVDF copolymers inmembrane processes.

2.4.1. Poly(vinylidene fluoride-co-tetrafluoroethylene)(P(VDF-co-TFE))

P(VDF-co-TFE) (chemical structure is shown in Table 3)has a good mechanical nature and can be dissolved in


Fig. 7. Sketch of the presence of the amphiphilic polymers in the blend membranes. (a) Blending of PVDF and a tri-block copolymer of (polyethyleneoxide)–(polypropylene oxide)–(polyethylene oxide) (EPTBP), (b) blending of PVDF and a comb-like copolymer of polysiloxane with polyethylene oxidea d a hyp(

tmccPuoogtfwgTaflt

aaa

nd polypropylene oxide side chains (ACPS) and (c) blending of PVDF anHPE-g-MPEG) [76].

he usual solvents, revealing a potential for fabrication oficroporous membranes through the phase inversion pro-

ess. The hydrophobicity yields a membrane capacity thatan be used in the MD process. Flat-sheet microporous(VDF-co-TFE) membranes for MD have been preparedsing the phase inversion process [90]. The morphologiesf the membranes are shown in Fig. 8. The cross-sectionf the P(VDF-co-TFE) membrane from LiCl exhibited a fin-er structure, with a length of several microns, reaching upo the membrane surface, and there was no obvious dif-erence between the top and bottom surfaces. Comparedith PVDF membranes, P(VDF-co-TFE) membranes display

ood mechanical performance and high hydrophobicity.he membrane was successfully applied in the MD processnd maintained hydrophobicity with a higher permeateux than the PVDF membrane during the membrane dis-illation process.

Recently, P(VDF-co-TFE) has also been used to fabricate series of novel polymer inclusion membranes (PIMs) andpplied in Cr(VI) transport [91]. The membranes showed

high permeability coefficient and better selectivity for

erbranched star copolymer of polyester-g-methoxyl polyethylene glycol

Cr(VI) transport using bifunctional ionic liquid extractantas carriers with ionic liquid plasticizer. They may be usedto remove the metal ions and make contribution to theenvironmental protection.

2.4.2. Poly(vinylidene fluoride-co-hexafluoropropene)(P(VDF-co-HFP))

The first VDF/HFP copolymer was produced by E.I. duPont de Nemours & Co., under the Viton® trademark in1957, and it can be either a thermoplastic or elastomeraccording to its HFP content [92,93] (chemical structureis shown in Table 3).

This type of copolymer is expected to have broadapplications because properties of the homopolymer havebeen modified by the incorporation of an amorphousphase of hexafluoropropylene (HFP) into the main con-stituent VDF blocks. An increase in the fluorine content

because of the HFP group results in a more hydropho-bic substance than PVDF so that PVDF-co-HFP materialhas been used to make microporous membranes intendedfor use as membrane contactors for pervaporation [94]


membrn [90].

Fig. 8. Microphotographs taken with the SEM method of a P(VDF-co-TFE)(1) LiCl, (2) LiClO4·3H2O; T: top surface, B: bottom surface, C: cross-sectio

and membrane distillation [95]. It is especially suitablefor organophilic pervaporation separations because ofthe lower crystallinity and higher free volume. Addition-ally, the amorphous domains (HFP) can trap more liquidelectrolytes, and the crystalline regions (VDF) can helpmaintain mechanical integrity of the films, so that PVDF-co-HFP is regarded as a promising material for the membranesof fuel cells and polymer electrolytes of rechargeablelithium ion batteries [96,97].

Both flat sheet [95] and hollow fiber [98,99] PVDF-co-HFP membranes have been fabricated via a liquidextraction/activation method [100] and phase inversionmethods [95]. Supercritical CO2 has also been employed asthe non-solvent for the recovery of solvent during phaseinversion [101]. Interestingly, electro-spinning has beenfound to be a novel method employed to fabricate PVDF-co-HFP membranes for lithium ion batteries [102] because thelarge surface area and pore structure of a nanofibrous net-work membrane contribute to high electrolyte uptake andexcellent ionic conductivity [103]. The fact that the electro-

spinning method is favorable for inducing �-phase PVDFmight give a good result for use in lithium ion batteries.Recently, a template-leaching method was used to preparemicroporous P(VDF-co-HFP) membranes [104].

ane prepared from 12 wt% polymer solutions with an additive of 4.6 wt%.

2.4.2.1. P(VDF-co-HFP) membranes for water and organicseparation. P(VDF-co-HFP) membranes fabricated via thephase inversion process showed clear finger-like pores[95] (Fig. 9), and was tested in MD process. A strongerhydrophobic nature compared to that of PVDF membraneswas revealed by the long-term higher rejection coefficientand contact angle of a P(VDF-co-HFP) membrane to dis-tilled water.

Additives such as PVP [98], LiCl, and glycerol [99] havebeen used to modify the properties of P(VDF-co-HFP) hol-low fiber membranes. According to the phase diagram, theaddition of LiCl or glycerol reduces the thermodynamic sta-bility of the dope solution in the reaction with water. Thiscan cause the resultant membranes to exhibit a narrowpore size distribution and enhance the pure water flux. Theaddition of LiCl also enhanced the mechanical strength ofP(VDF-co-HFP) membranes.

Thanks to its high hydrophobicity and low crystallinity,P(VDF-co-HFP) has also been employed as the membranematerial for pervaporative separating ethylacetate (EtAc)

in aqueous solution [105].

2.4.2.2. P(VDF-co-HFP) membranes for energy applications.The simple phase separation process can also be employed


.6) mem

ttoeeLtptcms

fpmtsT1a1pPbt

pp((T

Fig. 9. SEM photographs of the cross-section of the P(VDF-co-HFP) (F2

o prepare P(VDF-co-HFP) membranes for lithium ion bat-eries, and a highly porous sponge-like structure has beenbtained [106] (Fig. 10). The ionic conductivity of thelectrolyte is 1.76 × 10−3 S cm−1 at room temperature, itslectrochemical oxidation stability can be up to 4.7 V (vs.i/Li+), and the apparent activation energy for transporta-ion of ions is 10.35 kJ mol−1, suggesting that this kind oforous gel polymer electrolyte has great potential for prac-ical applications. In addition, strategies such as surfaceoating, grafting, embedding, and blending are promisingethods to enhance the ionic conductivity and mechanical

trength [107,108].P(VDF-co-HFP) copolymer can be grafted with sul-

onated aromatic agents for application in fuel cells asroton exchange membranes [109]. Diamines and aro-atic containing-amines have been used for grafting of

hese membranes, which can then be functionalized byulfonic acid groups to obtain proton conductivity [110].hough the conductivities were lower than that of Nafion17, they increased with the ion exchange capacity. Inddition, the decomposition temperature increased to70 ◦C under severe oxidative conditions. Grafting witharasulfonic phenolate is another alternative to modify(VDF-co-HFP) copolymers by modifying the copolymersearing a sulfonic acid group, which also show some pro-onic conductivity [111].

Since blending is a simple method to modify theroperties of polymeric membranes, polymers such as

oly(methyl methacrylate) (PMMA), poly(vinyl acetate)PVAc), and ethylene carbonate (EC)/propylene carbonatePC) and inorganic particles like ceramics (Al2O3, SiO2 andiO2) and lithium ions (LiBF4, LiPF6, LiAsF6, LiClO4) can

Fig. 10. (a) Surface and (b) cross-section morpholo

brane prepared from (A) 9.1 wt%, (B) 16.7 wt% and (C) 26.5 wt% [95].

be added to the polymer matrix to increase the mechan-ical stability and/or conductivity [102,108,112–116].Due to the compatibility of PMMA with both P(VDF-co-HFP) and N-butyl-N′-methyl-imidiazolium hexafluo-rophosphate (BMIPF6), the plasticizer BMIPF6 has beentrapped in situ, thus improving the electrochemical stabil-ity of the resultant SPE [117]. The addition of CTFE to theP(VDF-co-HFP) copolymer has the effect of increasing themelting point and the amorphous content [118].

2.4.3. Poly(vinylidenefluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE))

P(VDF-co-CTFE) copolymers are widely used in indus-try (chemical structure is shown in Table 3). The contentof CTFE is a crucial factor influencing the properties ofthese copolymers. The glass transition temperature rangesbetween −40 ◦C (Tg of PVDF) and 45 ◦C (Tg of PCTFE). P(VDF-co-CTFE) copolymers containing small amounts of VDF aresemicrystalline copolymers with a hexagonal structure,while those composed of 25–70 mol% VDF are amorphous.VDF percentages higher than 70% lead to thermoplasticswith a monoclinic crystalline structure. These copolymersare called flexible PVDF [23].

2.4.3.1. P(VDF-co-CTFE) membranes for water and organicliquid treatment. Hydrophobicity limits P(VDF-co-CTFE)membranes in the areas of drinking and wastewater treat-ment, for which MF/UF membranes are usually employed.

Fortunately, because of the pseudo-block structure and theincorporation of CTFE units, P(VDF-co-CTFE) is possible tobe grafted via ATRP in a mild reaction condition with-out decreasing the high mechanical, thermal, and chemical

gies of a porous polymer membrane [106]..

lymer Science 39 (2014) 164– 198 179

C

CH3

H2C

CO CH2 CH2 O CH3m

Catalysst/LigandCF2 CH2 x

CF2 CF y

CH3C

CH2

CO CH2 CH2 O CH3m

zCl

ATRP

P(VDF-co-CTFE)PEGMA

P(VDF-co-CTFE)-g-PEGMA

CF2 CH2 xCF2 CF y

Cl

+

Z. Cui et al. / Progress in Po

stabilities. ATRP of unsaturated monomers such as PEGMAand styrene can be selectively initiated by the secondarychlorines of CTFE units. In the meantime, intactness can beremained and the chlorines for atom transfer can be acti-vated by the VDF units [66]. A series of proton conductingcomb copolymer consisting of P(VDF-co-CTFE) backboneand PSSA side chains have been synthesized using ATRP[67,119] and then converted to membranes via a cast-ing and solvent evaporation technique [67,120]. Using aP(VDF-co-CTFE) UF membrane as the substrate, the P(VDF-co-CTFE)-g-PSSA has also been employed as a top layer tofabricate a composite NF membrane [121]. The integrationof PSSA enhanced both the rejection and the flux of thecomposite membranes.

Corresponding amphiphilic copolymers can be synthe-sized by grafting poly(hydroxy ethyl acrylate) [67,119], 4-vinylpyridine [122,123], and poly(oxyethylene methacry-late) [121] to P(VDF-co-CTFE) main chains, and thesecopolymers can be fabricated into membranes by coating orblending. Adopting ATRP polymerization, P(VDF-co-CTFE)membranes can be modified directly [124] (Scheme 2). Anegatively charged graft copolymer of P(VDF-co-CTFE) canbe synthesized for composite NF membranes [125].

2.4.3.2. P(VDF-co-CTFE) membranes for energy applications.PSSA chains can also be grafted from TiO2 nanoparticles bythe introduction of chlorine atoms via the copper-mediatedsurface-initiated ATRP technique. The copolymerized poly-mer can be fabricated as membranes for high temperaturefuel cells [120] (Scheme 3), which exhibit good thermal sta-bility up to 280 ◦C. Zeolite particles strongly interact with

the sulfonic group of a PSSA chain and enhance the thermalstability up to 300 ◦C [126].

Another way to graft PSSA onto the P(VDF-co-CTFE) backbone has been developed following grafting

Scheme 3. Preparation of P(VDF-co-CTFE)-g-PSSA membranes [120]. (a) The reactATRP; (b) Schematic illustration for the preparation of nano-composite membranenanoparticles.

Scheme 2. Schematic illustration of the processes for the modification ofP(VDF-co-CTFE) by PEGMA via ATRP [124].

cross-linkable chains of poly(hydroxyl ethyl acrylate)(PHEA) via ATRP, and amphiphilic P(VDF-co-CTFE)-g-P(HEA-co-SSA) graft copolymers were prepared [127].

P(VDF-co-CTFE) can also be grafted with glycidylmethacrylate (GMA) via ATRP, thermally cross-linking the–OH of the graft copolymer and the -COOH of sulfosuc-cinic acid (SA). These structures can then be developed intoproton conductive grafted/cross-linked membranes [128].

2.4.4. Poly(vinylidene fluoride)-graft-poly-(styrenesulfonic acid) (PVDF-g-PSSA)

PVDF-g-PSSA graft copolymers present better chancesfor a DMFC, because they have the advantages of cheap-ness and low permeability to methanol than Nafion® [20].

A PVDF-g-PSSA membrane prepared by a solution-graftingmethod showed high proton conductivity and low vana-dium ion permeability [129]. Formation of proton transportpathways or ionic transport channels is one of the promis-

ion scheme for the graft polymerization of SSA from TiO2 nanoparticles vias consisting of P(VDF-co-CTFE)-g-PSSA grafted copolymer and TiO2-PSSA


CH2 CF2 n

O3/O2CH CF2

O OH

CF2 CH

O

CF2 CH

O

x y

SO3Na

CH CH2

+

CH2 CF2 CF2 CHa b

O CH2 CH

SO3Na

c

PVDF Ozone pretreated PVDF

NaSS

PVDF-g-PSSNa (LG)

a

CH2 CF2 n

O3/O2CH CF2

O OH

CF2 CH

O

CF2 CH

O

x y

+CH2 CF2 CF2 CHa b

O CH2 CH

d

PVDF Ozone pretreated PVDF

BIEA

PVDF-g-PBIEA

H2C CH

C O

O

H2C CH2 O C

O

C

CH3

Br

CH3

C

O

O

CH2 CH2 O C

O

C

CH3

CH3

Br

bipyridine (bpy), CuBr

CH2 CF2 CF2 CHa b

O CH2 CH

d

PVDF-g-PBIEA-ar-PSSNa (AG)

C

O

O

CH2 CH2 O C

O

C

CH3

CH3

CH2 CH Br

SO3Na

e

b

ers wi

iehbhtwehpbawio

Scheme 4. Synthetic schemes of PVDF-g-PSSA copolym

ng approaches to achieve PEMs with relatively low ionxchange capacities and high proton conductivities, whichas been pursued by many researchers [130]. This cane realized by synthesizing a graft copolymer with aydrophobic main chain which favors the formation of con-inuous ionic networks [131], and hydrophilic side chainsith sulfonic acid groups which are usually flexible and

nhance the phase separation between the hydrophilic andydrophobic domains [132]. Non-conductive hydrophobicolymer chains can be excluded from the ionic domainsy the phase separation, and the proton conductivities

re expected to be improved. Comb-shaped copolymersith either linear or branched side chains have signif-

cant advantages for fuel cells [133–135]. Aggregationsf sulfonic acid groups into hydrophilic clusters form

th (a) linear or (b) arborescent PSSA side chains [137].

proton transport pathways and consequently increase theirproton conductivities [134,136]. PVDF-g-PSSA copolymerswith either linear or arborescent PSSA side chains havebeen synthesized and prepared to be PEMs in DMFCs show-ing a higher selectivity [137] (Scheme 4). The agglomerateand large sulfonic acid domains were favored to form bythe presence of arborescent PSSA side chains. An increasein proton conductivity and a reduction in methanol per-meability were achieved and, consequently the selectivitywas enhanced.

Blending inorganic particles such as Al2O3 into a

PVDF-g-PSSA copolymer is another way to yield the mem-branes with higher proton conductivity, water-uptake andlower oxygen solubility compared to Nafion® membranes[138].


of 5/95 P
Fig. 11. SEM microphotographs of the surface and cross-section (a and b) by solvent evaporation at room temperature [141].
2.4.5. Poly(vinylidene fluoride–trifluoroethylene)(P(VDF-TrFE))

All portions of trifluoroethylene (TrFE) can be copoly-merized using VDF, leading to random semicrystallinethermoplastic copolymers [23]. As mentioned in Section2.1, PVDF has to be treated by methods such as mechan-ical stretching or poling to induce a net dipole in thematerials (�-phase), but PVDF-TrFE does not need thesetreatments to achieve a dipole-containing crystal structurethat provides permanent polarization in the polymer [139].It might be a promising base material for manipulating cellbehavior, proliferation in a three-dimensional matrix, andpotential tissue engineering applications [139].

The solvent-cast technique can be used to prepareP(VDF-TrFE) microporous membrane separators forlithium ion batteries [140], and a well-organized porousstructure can be produced via isothermal solvent evapo-ration [141] (Fig. 11). Additionally, electro-spinning canprepare P(VDF-TrFE) copolymer membranes that com-bine the advantages of low dielectric constant and highflexibility [142].

In one of the tissue engineering applications, namedguided tissue regeneration, a membrane plays an impor-tant role in isolating periodontal defects of the gingivalconnective and epithelial tissues and achieving the regen-eration of bone, periodontal ligament, and cementum fromtheir own cells. An ideal material should possess ade-

quate electromechanical properties and biocompatibilityfor favoring regeneration of periodontal tissues. Comparedwith the conventional expanded PTFE (ePTFE), compositemembrane of poly(vinylidene-trifluoroethylene)/barium

(VDF-TrFE)/DMF and (c and d) 20/80 P(VDF-TrFE)/DMF samples obtained

titanate (P(VDF-TrFE)/BT) presented a good in vitro biocom-patibility [143].

In order to avoid using any kind of solvents, which areusually toxic to human, membranes of PVDF and P(VDF-TrFE) have been fabricated using starch and latex of naturalrubber as additives [144]. The natural rubber presentselastomeric properties that allow one to tune the thermo-mechanical properties of the blends and the starch can beabsorbed by the human body. Both additives are biocom-patible and biodegradable materials, which is an importantissue considering their applications as biomaterials.

3. Poly(tetrafluoroethylene) (PTFE)

PTFE, discovered by Roy Plunkett in 1938 in the labora-tories of E.I. du Pont de Nemours & Co., was commercializedas Teflon® in 1950 and became the most significantfluoropolymer. It constituted 60 wt% of the world fluo-ropolymer consumption and reached 181.5 × 103 metrictons in 2008 [145]. Compared to PP and PVDF, PTFE hasthe lowest surface energy, which induces the highesthydrophobicity. Furthermore, the exceptional combinationof properties such as outstanding thermal resistance, goodchemical stability, and low surface friction makes PTFE theprime material for MD, MC, and MGA.

3.1. Structure of PTFE

Molecules of PTFE are very long and unbranched so thatthe virgin polymer produced by polymerization is highlycrystalline with a degree of crystallinity from 92% to 98%

1 lymer Sc

(aibc

3

0ibshoashrc

d[sot

3

bpepacobvbs(amfms

vp

3

asicmmcp

82 Z. Cui et al. / Progress in Po

chemical structure is shown in Table 3). The CF2 groupsre along the polymer chain, and the whole chain twistsnto a helix as in a length of rope; the carbon chain twistsecause the large fluorine atoms prohibit a planar zigzagonformation.

.2. Properties of PTFE

The density of 100% crystalline PTFE is 2.347 g cm−3 at◦C, but it decreases to 2.302 g cm−3 at 25 ◦C; an approx-

mately 1% decrease is caused by the transition at 19 ◦Cecause the helical conformation of the molecules uncoillightly when the temperature increases. On the otherand, the density of amorphous PTFE is 2.00 g cm−3 with-ut the effect of the transition at 19 ◦C [19]. The fluorinetoms surround the carbon backbone like a protectiveheath, yielding PTFE with outstanding properties such asigh melting, high thermal stability, very good chemicalesistance, low coefficient of friction, and a low dielectriconstant.

One of the disadvantages of PTFE is that the materialoes not flow easily even above its melting point 327 ◦C19], and it is impossible to dissolve PTFE in any sub-tance at room temperature. The high molecular weightf standard PTFE, which is in the range of 1 to 5 × 106, ishe main reason for the extremely high melt viscosity.

.3. PTFE membranes

Since PTFE exhibits high chemical and thermal sta-ilities, it has strong potential to be used in membranerocesses. PTFE is very hydrophobic, which is very ben-ficial for membrane processes in which vapor or gasasses through the membrane. PTFE membranes have beendopted and investigated in MD, ME, PV, and MGA pro-esses [146]. Of the commercially available membranes,nly PTFE has a sufficiently high contact angle to notecome wet or degraded on contact with the amine sol-ents used in MGA [147]. Recently, PTFE membranes haveeen discovered to be excellent materials for collectingodium dodecyl sulfate (SDS)–capillary gel electrophoresisCGE)-separated proteins [148]. They can also be used asn effective substrate material to reinforce sulfonated PEMembranes or battery membranes to improve their per-

ormance [149–152]. PTFE also can be treated with alkalietals to improve the wettability and, consequently, adhe-

ion to other substrates.Although a PTFE membrane has many advantages, it is

ery expensive. Thus, it is also employed to sputter otherolymers to enhance hydrophobicity [153].

.3.1. Preparation of PTFE membranesAs mentioned in Section 3.2, the PTFE homopolymer has

very high melting temperature and is not soluble in anyolvent at room temperature. Fabrication of PTFE productss difficult because its poor processing properties prohibitommon phase inversion or the melt spinning method from

anufacturing a porous membrane [154]. At present, com-ercial PTFE membranes are usually produced through

omplicated extrusion, rolling and stretching or sinteringrocedures.

ience 39 (2014) 164– 198

Many patents disclosed methods in which a non-sintered PTFE resin blend containing a liquid lubricant wasextruded at a high temperature. The extruded material wasdrawn by uniaxial or biaxial stretching and heated to 327 ◦Cor greater to form a PTFE resin porous membrane. How-ever, the performance of the membranes produced by thesemethods is inferior because of poor controllability of theporous structure and inferior membrane formability. PTFEfilm with slit yarns can be formed by adding some auxiliaryto the PTFE fine particles and extruding the paste througha slit die into film. The film is slit into flat yarns after sin-tering at an elevated temperature to fuse the PTFE particlesand to eliminate the auxiliary.

Efforts have been made to improve the preparationprocesses of PTFE membranes. PTFE with low MW, andhence low melt viscosity, known as PTFE micropowder,prepared mainly by irradiation of high-MW PTFE and char-acterized by a MW of 104–105 g mol−1, is available butexhibits brittleness and is not melt processable by itselfsince the resultant product has no practical strength. PTFEwith medium viscosity has been reported to permit ordi-nary melt processing of PTFE into parts with sufficientmechanical strength for practical applications [155]. Inorder to create PTFE with intermediate melt viscosity, PTFEgrades of high and low viscosities were blended, producingpolymers with pseudobimodal molar mass distributions.

A method has been reported to produce PTFE hollowfiber membranes in which PTFE emulsion was used toform a spinning dope. The dope then was spun into hol-low fiber by the conventional dry-wet spinning method.Subsequently, a PTFE hollow fiber membrane was finallyobtained after removing the sodium alginate by sintering[156].

Recently, poly(vinyl alcohol) (PVA) was blended witha PTFE emulsion in a prepared spinning solution [157].The spinning solution was then electrospun into compos-ite nanofibers, whereby a PTFE porous membrane wasobtained after sintering. The PTFE membranes maintainedboth good thermal stability and strong hydrophobicity[158]. Unique ‘worm-like’ structures, which are the crystalsof PTFE, were observed in the membranes with differ-ent PTFE/PVA mass ratios (Fig. 12), and the growth of the‘worm-like’ crystals as the PTFE/PVA mass ratio increasedimproved the hydrophobicity and mechanical strength ofthe PTFE membrane [159].

In addition, a novel fiber manufacturing process directlyfrom PTFE emulsion has been developed and is much sim-pler than the complicated processes described above [160].PTFE thick strand was directly prepared from emulsion inwater without auxiliary (Fig. 13), and the thinning of thestrand was carried out by a tensile die-drawing process ata low temperature (∼90 ◦C). The structure development ofthe strand was performed by a similar tensile die-drawingprocess at an elevated temperature (∼380 ◦C). This methodhas the potential to be employed for PTFE hollow fibermembrane preparation.

3.3.2. Modification of PTFE membranesAlthough PTFE is an attractive material in mem-

brane applications due to its superior chemical resistance,good thermal stability, and high mechanical strength, its


Fig. 12. FESEM images of the composite films with different PTFE/PVA mass ratios: (a) left 1:1, right larger magnification, (b) left 2:1, right largermagnification, (c) left 4:1, right larger magnification [159].

Fig. 13. A PTFE strand (a) and its inner structure (b) [160].


ic polym

humfomsUetar[

pf4mfasflo

tccaSsbpo

me

Scheme 5. Reaction route of grafting hydrophil

ydrophobicity is a problem when PTFE membranes aresed in MF/UF and dehydration by pervaporation. Thus,odification is necessary to make a PTFE membrane sur-

ace hydrophilic, which can be achieved by pre-adsorptionf proteins, dip-coating with a protective hydrophilic poly-er, and surface grafting of hydrophilic segments. For

urface grafting, energetic treatments via plasma, ozone,V, or ion-beam are required because the high C–F bondnergy means that PTFE is not susceptible to chemical reac-ions. In addition to enhancing the hydrophilicity, PTFElso can be modified to improve its mechanical properties,adiation resistance, and optical properties by cross-linking161].

N-vinylpyrrolidone, 4-vinylpyridine, and sulfonatedolystyrene have been grafted onto PTFE membranesor many years. Later, acrylamide (AAm) and sodium-styrenesulfonated (NaSS) were used to modify PTFEembranes with hydrogen plasma/ozone treatment and

ollowed surface-initiating grafting polymerization [162],s shown in Scheme 5. The modified PTFE membraneshowed high permselectivity and moderate permeationuxes in pervaporative dehydration on some organic aque-us solutions.

Then, in order to improve their performance, chi-osan/PTFE composite membranes were fabricated byasting �-(glycidyloxypropyl) trimethoxysilane (GPTMS)-ontaining chitosan solution onto a poly(styrene sulfuriccid) grafted ePTFE film surface [163], as shown incheme 6. The membranes showed high interlayer peeltrength and significant improvements in membrane sta-ility in organic aqueous solutions and exhibited greaterformances in dehydration by pervaporation in various

rganic aqueous solutions.

The ester group in poly(ethylene glycol) (PEG)-basedaterial is another ideal choice for surface functional moi-

ty with anti-fouling characteristics. A simple one-pot

er chains onto PTFE membrane surfaces [162].

preparation approach to introduce PEG-based segmentsonto an ePTFE membrane surface has been developed.Covalent immobilization of a PEG-based species can beachieved with their monomers or macromonomers in solu-tion following an energetic treatment such as plasma [164].

Well-defined comb copolymer brushes can be graftedonto the surface of PTFE film using a combination ofsurface-initiated RAFT polymerization and ATRP [165](Scheme 7). Glycidyl methacrylate (GMA) has been graft-copolymerized on Ar plasma-pretreated PTFE film viasurface-initiated RAFT polymerization. The epoxy groupsof poly(glycidyl methacrylate) (PGMA) brushes were sub-sequently used for the coupling of ATRP initiators via areaction with 2-bromo-2-methylpropionic acid (BMPA).Comb copolymer brushes on PTFE films were subsequentlyprepared via surface-initiated ATRP of two hydrophilicvinyl monomers, including PEGMA and NaSS.

Ozone treatment has been used for the surface modi-fication of PTFE film [166] (Scheme 8). First, a hydrogenplasma treatment was applied to PTFE films to incorporatesome C–H groups onto the film surface. Therefore, the PTFEsurface modified by hydrogen plasma possesses a hydro-carbon surface and PTFE bulk characteristics to becomesusceptible to ozone treatment like other polymers. Theincorporated hydrocarbons could be converted to alkyl per-oxide and hydro peroxide groups after ozone treatment.Surface initiated radical polymerization and ATRP are thenperformed to incorporate linear and arborescent macro-molecular structures on PTFE film surfaces. PTFE filmsexhibiting a small water contact angle of 21◦ and very highsurface hydrophilicity were obtained.

A novel approach to introduce specific chemical groups

onto a PTFE film surface has been established [167]. Withthe incorporation of 2-bromoisobutyryl bromide, used asan ATRP initiator, the surface-initiated ATRP of sodiumstyrene sulfate was performed. The experimental results


f chitos
Scheme 6. Preparation and structure o
demonstrate the ability to build up controlled polymerchains on PTFE film surfaces, which could not be achievedthrough conventional plasma or other modification meth-ods.

Recently, a PTFE membrane has been modified by blend-ing nanoscale CaCO3 particles into PTFE emulsion duringthe PTFE membrane preparation process by using PVA asthe fiber forming polymer [168]. Interfacial microvoidswere created when the hybrid membranes were drawnin boiling water. Moreover, the membrane hydrophobic-ity improved because of the increasing membrane surfaceroughness. The membrane porosity and pure water fluxincreased as the CaCO3 content and membrane draw ratiosincreased.

Polyaniline/gold (PANI/Au) has been successfullydeposited on the surface of a PTFE membrane by a onestep process via a simple diffusing interfacial reaction[169]. The membrane has the potential to be used in harshmembrane reaction systems and does not pollute thereaction system.

3.4. PTFE copolymer membranes

Since copolymerization is a promising way to createnovel properties, many interesting PTFE copolymers havebeen developed. PFSA, PFA, FEP, and ETFE are importantcommercial copolymers used in membrane processes.

3.4.1. Perfluorosulfonic acid (PFSA)3.4.1.1. PFSA copolymers and membranes. PFSA has a back-bone of PTFE and a branch of sulfonyl fluoride vinyl ether

an/PTFE composite membranes [163].

(SFVE) [3,18,21] (chemical structure is shown in Table 3).The backbone structure of PTFE provides the membranewith physical strength, and the presence of sulfonic acidpromotes water uptake, enabling the PFSA membrane to bea good protonic conductor. Protons become mobile whendissociated from the sulfonic acid groups in an aqueousenvironment. As a result, these membranes found theirapplication in the chlor-alkali industry and fuel cells includ-ing PEMFC and DMFC, showing good ionic conductivity aswell as chemical stability.

PFSA polymers such as Nafion® from du Pont, Flemion®

from Asahi Glass, Aciplex® from Asahi Chemical, andGore-Select® from Gore and Associates are called long-side-chain (LSC) polymers because they contain a longpendant group which imparts the ionic functionality [18].Membranes based on these materials, mainly Nafion®,are costly primarily due to their complex fabrication pro-cess [170]. In spite of their chemical resistance, oxidativestability, and good proton conductivity, Nafion® mem-branes have some disadvantages: the conductivity ofNafion® decreases rapidly with decreasing relative humid-ity (RH); methanol permeability through Nafion® is veryhigh; and the material is prohibitively expensive. How-ever, after decades, Nafion® membrane is still the standardmembranes for fuel cells. Preparing asymmetric [171]or composite [172] membranes, blending with low-pricepolymers such as PVDF [173] have been investigated to

reduce the price and methanol crossover of Nafion® mem-branes.

A type of short-side-chain (SSC) polymer, whichexhibits better performance and durability than Nafion®


S retreatma of comb

iSbwhhfmimbbSr

cheme 7. Schematic diagram illustrating the processes of Ar plasma pttachment of the ATRP initiators to the PGMA brushes, and preparation

n several cases, has been developed by Dow andolvay Specialty Polymers with brands of the Dow mem-rane and Hyflon® Ion, respectively [174,175]. Comparedith LSC-based membranes, SSC-based membranes showigher conductivity, better mechanical properties, and aigher ionic glass-transition temperature (up to 160 ◦C

or SSC-based membranes, but up to 110 ◦C for LSC-basedembranes), which is a necessary condition for operat-

ng the cell at a higher temperature without damaging theembrane [176–178]. The Dow membrane was abandoned

ecause of the complexity of the synthesis process of thease functional monomer used for the production of theSC ionomer. Fortunately, recent research on SSC perfluo-inated ionomers made a progress on a much simpler route

ent of the PTFE surface, surface-initiated RAFT polymerization of GMA, copolymer brushes on the PTFE surface [165].

for the synthesis of the base monomer developed by SolvaySpecialty Polymers (Scheme 9), and the Hyflon® Ion basedmembranes were fabricated.

The significantly higher elastic modulus may be thereason why SSC PFSA ionomer membranes show superiorperformance compared with LSC PFSA ionomer mem-branes. The protonic charge carriers can be increased, andthe hydrophobic/hydrophilic separation can be reduced byforming membranes with higher ion exchange capacity(lower equivalent weight) due to their better mechanical

properties [180].

Amorphous PFSA ionomers can impregnate micro-porous substrates, leading to ultra-high wettabilitymembranes exhibiting high water flux properties [181]


gen plas
Scheme 8. A typical scheme of a hydro
since sulfonic pendant groups supply hydrophilicity. Thesemembranes can be fruitfully applied to the filtration ofwater-based solutions in a variety of applications, espe-cially where aggressive reactants require a fluoropolymersubstrate membrane. PFSA ionomer membranes have alsobeen used as a separator in lithium ion batteries [182], air-drying membrane for medical systems [183], and artificialmuscles [39].

3.4.1.2. PFSA membrane preparations and modifications.PFSA ionomer membranes can be produced througha variety of processes conferring the final membranewith different characteristics because they are soluble influorinated solvents [3]. Typically, commercial dense mem-branes are produced by extrusion and casting methods.

The drawbacks mentioned in section 3.3.1.1 haveprompted an extensive effort to modify existing Nafion®

membranes and to improve its properties. Considerableefforts have been made to enhance the conductivity at lowrelative humidity and reduce the crossover of methanol indirect methanol fuel cell (DMFC) applications. Surface coat-ing and blending methods have been employed to reduce

the cost or improve its properties.

Novel supported liquid membranes have beenprepared by using Nafion® and Hyflon® membranesand impregnating 1-butyl-3-methylimidazolium

CF2 CF2CF2 CF2

O SO2

CsF /F2FO CF2

ClFC CFCl

ClCF2 CFCl O CF2 CF2 SO

Scheme 9. Synthesis of the SSC sulfonylfluoridevin

ma pretreated PTFE film surface [166].

bis(trifluoromethylsulfonyl)imide (BMI-BTSI) [184].The Hyflon/IL membrane shows an exponential trend inconductivity with increasing temperature as well as excel-lent thermal stability, making it more suitable for hightemperature electrochemical applications in anhydrousconditions.

Preparing asymmetric [171] or composite [172] struc-tures is a promising method to enhance the performanceand reduce the cost of PFSA ionomer membranes for PEM.Only a small amount of polymeric additive is required forsurface coating of the membrane [185,186]. The futuredirection of research and development efforts should befocused on the development of new methods of membranesurface coating and surface modification.

The most widely employed approach to modify Nafion®

membranes is the addition of inorganic additives toNafion® membranes to yield organic/inorganic compositemembranes [187]. Zirconium phosphates, heteropolyacids,hydrogen sulfates, and silica have been employed to blendwith Nafion®.

Blending high-price Nafion® and low-price PVDF poly-mers is also a promising method to produce relatively

low-price membranes with competitive proton conduc-tivity and a much lower methanol crossover for fuel cells[188]. However, a hydrophobic PVDF component does notallow water molecules to hydrate the membrane surface so

CF2 SO2F

2F CF2 CF O CF2 CF2 SO2F

ylether monomer for the Hyflon® Ion [179].


of irrad

tp

3e

btsaamfm

bbymosaa

3(

wbr[[

r(sSftu

fmbc

Scheme 10. Preparation process

hat modification is needed to increase water uptake androton conductivity [173].

.4.2. Poly(tetrafluoroethylene-co-perfluoropropyl vinylther) (PFA)

PFA (chemical structure is shown in Table 3) appears toe a promising material for membranes in fuel cell applica-ions due to their good combination of physico-chemical,tructural, and thermal properties [189]. PFA films have

significantly higher radiation resistance, stable radicals,nd efficient grafting yields upon reaction with variousonomers. Thus, a high irradiation dose can be applied

or membrane preparation without affecting their inherentechanical properties [190].Vinylbenzyl chloride (VBC), a versatile monomer, can

e copolymerized onto pre-irradiated PFA film, followedy quaternary-ammonium-functionalization and hydrox-lation [191] to prepare an alkaline anion-exchangeembrane (AAEM) (Scheme 10). A maximum conductivity

f 0.05 S cm−1 at 60 ◦C and maximum power density of theingle cell up to 16 mW cm−2 demonstrate the promisingpplication of this kind of AAEM for use in a direct alcohollkaline anion-exchange membrane fuel cell (DA3EMFC).

.4.3. Poly(tetrafluoroethylene-co-hexafluoropropylene)FEP)

FEP is a random copolymer of tetrafluoroethylene (TFE)ith HFP (chemical structure is shown in Table 3). It has

etter processability than PTFE, making it easier to fab-icate membranes for MD due to its high hydrophobicity192] and for fuel cells after being grafted and sulfonated193,194].

The melt spinning method has been employed to fab-icate FEP hollow fiber membranes with dioctyl phthalateDOP) as the diluent and a composite powder (composed ofoluble nano-sized KCl particles and insoluble nano-sizediO2 particles) as the pore-creating agent [192]. Macrovoid-ree and sponge-like structures can clearly be observed inhe cross section image. The membrane has potential to besed in the MD process.

Like PFA, a high irradiation dose can be applied to FEP

or membrane preparation without affecting the inherent

echanical properties, making it possible to prepare mem-ranes for fuel cells [193,194]. Additionally, cross-linkingan be employed to enhance the durability [195].

iation grafted membranes [191].

FEP can be grafted with VBC to synthesize AAEMmembranes followed by quaternization and alkalization[196,197]. Relatively high ionic conductivity has beenobtained but is still inferior to that of Nafion® membranes.

By blending cross-linked PTFE (RX-PTFE) and FEP, fuelcell membranes with a network structure can be fabricatedby radiation-grafting with reactive styrene monomersusing electron beam irradiation (EB) under nitrogen atroom temperat

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