SYNTHESIS AND CHARACTERIZATION OF HYDROPHILIC ......Synthesis and Characterization of...
Transcript of SYNTHESIS AND CHARACTERIZATION OF HYDROPHILIC ......Synthesis and Characterization of...
SYNTHESIS AND CHARACTERIZATION OF HYDROPHILIC-HYDROPHOBIC
DISULFONATED POLY(ARYLENE ETHER SULFONE)-DECAFLUORO BIPHENYL
BASED POLY(ARYLENE ETHER) MULTIBLOCK COPOLYMERS FOR PROTON
EXCHANGE MEMBRANES (PEMS)
by
Xiang Yu
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 S. Riffle Dr. John G. Dillard
Dr. Richey M. Davis Dr. Scott W. Case
January 25, 2008 Blacksburg, VA
Key words: Fuel cells, Proton exchange membranes, Multiblock copolymers, Fluorinated
copolymers, Poly(arylene ether sulfone)s, Morphology, Nanophase separation
Copyright 2008, Xiang Yu
Synthesis and Characterization of Hydrophilic-Hydrophobic Disulfonated Poly(Arylene
Ether Sulfone)-Decafluoro Biphenyl Based Poly(Arylene Ether) Multiblock Copolymers for
Proton Exchange Membranes (PEMs)
Xiang Yu
Abstract
Hydrophilic/hydrophobic block copolymers as proton exchange membranes (PEMs) has become
an emerging area of research in recent years. Three series of hydrophilic/hydrophobic,
fluorinated/sulfonated multiblock copolymers were synthesized and characterized in this thesis.
These copolymers were obtained through moderate temperature (~100°C) coupling reactions,
which minimize the ether-ether interchanges between hydrophobic and hydrophilic telechelic
oligomers via a nucleophilic aromatic substitution mechanism. The hydrophilic blocks were
based on the nucleophilic step polymerization of 3,3’-disulfonated, 4,4’-dichlorodiphenyl sulfone
with an excess 4,4’-biphenol to afford phenoxide endgroups. The hydrophobic (fluorinated)
blocks were largely based on decafluoro biphenyl (excess) and various bisphenols. The
copolymers were obtained in high molecular weights and were solvent cast into tough
membranes, which had nanophase separated hydrophilic and hydrophobic regions. The
performance and structure-property relationships of these materials were studied and compared
to random copolymer systems. NMR results supported that the multiblock sequence had been
achieved. They displayed superior proton conductivity, due to the ionic proton conducting
channels formed through the self-assembly of the sulfonated blocks. The nano-phase separated
morphologies of the copolymer membranes were studied and confirmed by atomic force
microscopy. Through control of a variety of parameters, including ion exchange capacity and
sequence lengths, performances as high, or even higher than those of the state-of-the-art PEM,
Nafion, were achieved.
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Acknowledgments
I would like to express my sincere gratitude to my advisor, Prof. James E. McGrath, for his
guidance, encouragement and inspiration throughout my Ph.D. career. I am extremely lucky to
have been his student, and to have benefited from his knowledge, experience, and exceptional
personality. I would also like to thank the other members of my advisory committee, Dr. Judy S.
Riffle, Dr. John G. Dillard, Dr. Richey M. Davis and Dr. Scott Case, for their great support.
My work could not have been accomplished without the help of the people in our research group.
I particularly want to thank Mr. Abhishek Roy for fuel-cell related testing, Ms. Juan Yang for
molecular weight characterizations, Dr. Anand Badami for morphological characterizations, and
Ms. Ozma Lane for mechanical and dynamic mechanical analyses. Dr. William Harrison, Dr.
Brian Einsla, Dr. Melinda Hill, and Dr. Kent Wiles are all acknowledged for familiarizing me
with chemicals, equipments, procedures and basic research techniques in the lab. I have also
benefited much from valuable discussions with my lab mates including Dr. Yanxiang Li, Dr.
Guangyu Fan, Dr. Zhongbiao Zhang, Dr. Hang Wang, Mr. Harry Lee, Mr. Yu Chen, Ms. Rachael
Hopp, and Ms. Natalie Arnett.
I am also very grateful to the staff in the Macromolecules and Interfaces Institute, particularly
Mrs. Laurie Good, Mrs. Millie Ryan, and Mrs. Angie Flynn—all of whom have been of
significant assistance with all the little details that go hand-in-hand with being a graduate student, iv
and who helped make this department such a great environment in which to work.
Finally, I would like to thank my fiancé, Yi Hou, for her love and understanding, and for being so
patient, and my parents for their love and support through all these years.
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Table of Contents
ABSTRACT................................................................................................................................... ii ACKNOWLEDGMENTS ........................................................................................................... iv LIST OF FIGURES ..................................................................................................................... ix LIST OF TABLES....................................................................................................................... xv INTRODUCTION......................................................................................................................... 1 CHAPTER 1. LITERATURE REVIEW .................................................................................... 3 1.1. INTRODUCTION........................................................................................................................ 3 1.1.1. FUNDAMENTALS OF PROTON EXCHANGE MEMBRANES (PEMS).............................................. 3 1.1.2. BASIC CRITERIA FOR A PEM................................................................................................... 5 1.1.3. PEMS BASED ON HYDROPHOBIC-HYDROPHILIC BLOCK COPOLYMERS .................................... 7 1. 2. PEMS BASED ON HIGH PERFORMANCE ENGINEERING MATERIALS ...................................... 8 1.2.1. SYNTHESIS OF COPOLYMER BACKBONES ................................................................................ 8 1.2.1.1. Poly(arylene ether)s ........................................................................................................... 9 1.2.1.2. Aromatic Poly(imide)s ..................................................................................................... 13 1.2.1.3. Aromatic 5-membered-ring heterocyclic polymers ......................................................... 15 1.2.1.4. Poly(p-phenylene) derivatives ......................................................................................... 17 1.2.2. FABRICATION OF PEMS: INTRODUCTION OF PROTON-CONDUCTING MOIETIES...................... 19 1.2.2.1. Post sulfonation of poly(arylene ether)s .......................................................................... 19 1.2.2.2. Direct copolymerization of sulfonated monomers: preparation of poly(arylene ether) random copolymers....................................................................................................................... 20 1.2.2.3. Sulfonated poly(imide)s: hydrolytic stability issues........................................................ 24 1.2.2.4. Sulfonated poly(benzimidazole)s, poly(benzoxazole)s and poly(benzthiazole)s............ 26 1.2.2.5. Sulfonation of poly(2,5-benzophenone)s......................................................................... 30 1.3. FLUORINATED AROMATIC HIGH PERFORMANCE COPOLYMERS FOR PEMS ........................ 32 1.3.1. MODERATELY FLUORINATED COPOLYMERS CONTAINING HEXAFLUOROISOPROPYLIDENE UNITS
....................................................................................................................................................... 32 1.3.2. HIGHLY FLUORINATED POLY(ARYLENE ETHER)S CONTAINING PERFLUOROPHENYLENE UNITS
....................................................................................................................................................... 34 1.4. HYDROPHILIC-HYDROPHOBIC BLOCK COPOLYMER SYSTEMS AS PEMS............................. 41 1.4.1. BLOCK COPOLYMER PEMS BASED ON SULFONATED STYRENICS AND HYDROGENATED
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POLYDIENES ................................................................................................................................... 42 1.4.2. MULTIBLOCK COPOLYMERS SYNTHESIZED BY STEP OR CONDENSATION POLYMERIZATION.... 47 1.4.2.1. Partially aromatic multiblock copolymers ....................................................................... 47 1.4.2.2. Wholly aromatic multiblock copolymers......................................................................... 51 1.4.3. COMPARISONS BETWEEN RANDOM AND BLOCK COPOLYMER PEMS ..................................... 65 CHAPTER 2. SYNTHESIS AND CHARACTERIZATION OF BISAF-BPSH HYDROPHOBIC-HYDROPHILIC MULTIBLOCK COPOLYMERS ............................... 75 2.1. EXPERIMENTAL ..................................................................................................................... 76 2.1.1.. SOLVENTS ........................................................................................................................... 76 2.1.2. MONOMERS ......................................................................................................................... 77 2.1.3. MONOMER SYNTHESIS......................................................................................................... 79 2.1.4: POLYMER SYNTHESIS ........................................................................................................... 80 2.1.5. NMR SPECTROSCOPY, GEL PERMEATION CHROMATOGRAPHY, INTRINSIC VISCOSITY AND
ATOMIC FORCE MICROSCOPY CHARACTERIZATION ....................................................................... 82 2.1.6. CHARACTERIZATION OF FUEL CELL RELATED PROPERTIES .................................................. 83 2.2. RESULTS AND DISCUSSION..................................................................................................... 86 2.2.1. SYNTHESIS AND CHARACTERIZATION ................................................................................... 86 2.2.1.1. Synthesis of fluorinated Oligomers ................................................................................. 86 2.2.1.2. Synthesis of fully disulfonated hydrophilic oligomers .................................................... 90 2.2.1.3. Synthesis of Multiblock Copolymers............................................................................... 93 2.2.1.4. Fundamental characterizations of BisAF-BPSH multiblock copolymers ....................... 94 2.2.2. FUEL CELL RELATED CHARACTERIZATIONS OF MULTIBLOCK COPOLYMERS...................... 100 2.2.2.1. Proton conductivity under fully hydrated conditions..................................................... 100 2.2.2.2. Proton conductivity under partially hydrated conditions............................................... 104 2.2.2.3. Diffusion coefficients..................................................................................................... 107 2.2.2.4. Methanol Permeability................................................................................................... 108 CHAPTER 3. SYNTHESIS AND CHARACTERIZATION OF 6FBISAF-BPSH MULTIBLOCK COPOLYMERS.............................................................................................111 3.1. EXPERIMENTAL ....................................................................................................................111 3.1.1. MATERIALS. ........................................................................................................................111 3.1.2. POLYMER SYNTHESIS......................................................................................................... 112 3.1.3. POLYMER ISOLATION AND CHARACTERIZATION .................................................................. 114 3.2. RESULTS AND DISCUSSION ....................................................................................................115
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3.2.1. POLYMER SYNTHESIS AND CHARACTERIZATION ................................................................. 115 3.2.2. FUNDAMENTAL CHARACTERIZATIONS……………………………………………………………..118 3.2.3. CHARACTERIZATION OF FUEL CELL RELATED PROPERTIES .................................................. 126 CHAPTER 4. SYNTHESIS AND CHARACTERIZATION OF BISSF-BPSH MULTIBLOCK COPOLYMERS............................................................................................ 132 4.1. EXPERIMENTAL ................................................................................................................... 133 4.1.1. MATERIALS ........................................................................................................................ 133 4.2. RESULTS AND DISCUSSION ................................................................................................... 133 4.2.1. POLYMER SYNTHESIS AND CHARACTERIZATION ................................................................. 133 4.2.1.1. Synthesis of fluorinated oligomers ................................................................................ 133 4.2.1.2. Synthesis of Fully disulfonated hydrophilic oligomers ................................................. 139 4.2.1.3. Synthesis of BisSF-BPSH multiblock copolymers........................................................ 141 4.2.1.4. Characterizations of molecular weights of BisSF-BPSH copolymers........................... 144 4.2.1.5. Thermal analysis ............................................................................................................ 152 4.2.1.6. Mechanical Properties.................................................................................................... 154 4.2.1.7. Surface morphological features ..................................................................................... 155 4.2.2. STUDY OF FUEL CELL-RELATED PROPERTIES....................................................................... 158 4.2.2.1. Effects of block lengths on proton conductivity and water uptake................................ 158 4.2.2.2. Effects of composition (IEC) on proton conductivity and water uptake ....................... 162 4.2.2.3. Effects of hydrophobic block length (hydrophobic/hydrophilic block length ratio) ..... 165 4.2.2.4. Swelling-deswelling properties of multiblock copolymers ........................................... 168 4.2.3. SOME COMPARISONS OF BISAF-BPSH, 6FBISAF-BPSH AND BISSF-BPSH MULTIBLOCK
COPOLYMERS................................................................................................................................ 173 CHAPTER 5. CONCLUSIONS............................................................................................... 176 REFERENCES.......................................................................................................................... 180
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List of Figures Figure 1.1.1. Electrochemical reactions for a PEMFC and DMFC......................................... 3
Figure 1.1.2. Membrane electrode assembly in a proton exchange fuel cell membrane......... 4
Figure 1.1.3. Proposed structure of Nafion®............................................................................ 5
Figure1.2.1. Some possible structures of poly(arylene ether)s................................................ 9
Figure 1.2.2. Mechanism of SNAr nucleophilic aromatic substitution .................................. 10
Figure 1.2.3. Synthesis of bisphenol-A polysulfones ............................................................ 11
Figure 1.2.4. Mechanism of K2CO3-catalyzed synthesis of Bisphenol-A polysulfones........ 12
Figure 1.2.5. Synthesis of poly(arylene ether ketone)s via the ketimine precursor method.. 13
Figure 1.2.6. Synthesis of poly(arylene ether ketone)s from bulky substituted bisphenol.... 13
Figure 1.2.7. Two-stage synthesis of poly(imide)s ................................................................ 14
Figure 1.2.8. One-stage synthesis of poly(imide)s with improved solubility........................ 15
Figure 1.2.9. General scheme for the synthesis of (a) poly(benzimidazole)s, (b) poly(benzoxazole)s and (c) poly(benzthiazole)s ........................................................... 16
Figure 1.2.10. Synthesis of PBI by melt polymerization....................................................... 16
Figure 1.2.11. Synthesis of substituted PPPs via Ni(0) coupling .......................................... 17
Figure 1.2.12. Synthesis of PPP alternating copolymers via Suzuki coupling ...................... 18
Figure 1.2.13. Synthesis and polymerization of 2,5-dichlorobenzophenone monomers....... 18
Figure 1.2.14. Examples of post-sulfonated poly(arylene ether sulfone)s and poly(arylene ether ketone)s ................................................................................................................. 19
Figure 1.2.15. Synthesis of 3,3’-disulfonated 4,4’-dichlorodiphenyl sulfone (SDCDPS) .... 21
Figure 1.2.16. Synthesis of BPSH-xx random copolymers ................................................... 22
Figure 1.2.17. Sodium 5,5'-carbonylbis(2-fluorobenzenesulfonate) (SDFBP) ..................... 23
Figure 1.2.18. Structures of sulfonated poly(arylene ether ketone) random copolymers studied............................................................................................................................ 24
Figure 1.2.19. Structures of (a) phthalic and (b) naphthalenic imide units............................ 25
Figure 1.2.20. Synthesis of naphthalenic sulfonated poly(imide) random copolymers......... 25
Figure 1.2.21. Structures of (a)sulfonated and (b)unsulfonated diamine monomers used .... 26
Figure 1.2.22. Synthesis of sulfonated heterocyclic homopolymers ..................................... 27
Figure 1.2.23. Synthesis of sulfonated poly(benzimidazole) random copolymers................ 27
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Figure 1.2.24. Synthesis of sulfonated poly(benzimidazole) random copolymers................ 28
Figure 1.2.25. Sulfonation of (a)PBI and (b)ABPBI ............................................................. 29
Figure 1.2.26. Proposed structures of sulfonated poly(2,5-benzophenone)s......................... 31
Figure 1.2.27. Sulfonation of poly(4-fluoro-2,5-benzophenone)s......................................... 31
Figure 1.3.1. Synthesis of 6F-BPA ........................................................................................ 32
Figure 1.3.2. Structures of partially sulfonated, partially fluorinated poly(arylene ether sulfone)s......................................................................................................................... 33
Figure 1.3.3. Partially sulfonated, partially fluorinated poly(arylene ether sulfone benzonitrile)s.................................................................................................................. 34
Figure 1.3.4. Synthesis of poly(arylene ether)s containing perfluorophenylene units .......... 35
Figure 1.3.5. Synthesis of perfluorinated aromatic compounds by saturation-aromatization 36
Figure 1.3.6. Synthesis of perfluorinated aromatic compounds by nucleophilic aromatic substitution..................................................................................................................... 36
Figure 1.3.7. Reactions of ortho-position fluorine leading to branching and/or gelation ..... 37
Figure 1.3.8. Direct synthesis of sulfonated perfluorinated poly(arylene ether) random copolymers..................................................................................................................... 38
Figure 1.3.9. Post sulfonation of fluorinated poly(arylene ether)s ........................................ 39
Figure 1.3.10. Synthesis and sulfonation of fluorinated poly(arylene ether sulfone) random copolymers..................................................................................................................... 40
Figure 1.3.11. Synthesis and sulfonation of fluorinated poly(arylene ether sulfone) random copolymers..................................................................................................................... 41
Figure 1.4.1. Structure of S-SEBS block copolymers ........................................................... 43
Figure 1.4.2. Modification of S-SEBS surface with plasma treatment followed by hydrolysis........................................................................................................................................ 44
Figure 1.4.3. Structure of S-SIBS block copolymers............................................................. 45
Figure 1.4.4. Synthesis of partially sulfonated (PVDF-ran-PHFP)-b-PS copolymers .......... 46
Figure 1.4.5. Synthesis of sulfonated PAES-PB multiblock copolymers .............................. 48
Figure 1.4.6. Synthesis of sulfonated PAES-PVDF multiblock copolymers......................... 49
Figure 1.4.7. Epoxidation of sulfonated PAES-PB multiblock copolymers.......................... 50
Figure 1.4.8. Synthesis of sulfonated PPP-PAES multiblock copolymers ............................ 52
Figure 1.4.9. Synthesis of sulfonated poly(arylene ether sulfone) multiblock copolymers .. 54
Figure 1.4.10. Synthesis of sulfonated poly(arylene ether ketone) multiblock copolymers.. 54
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Figure 1.4.11. Synthesis of sulfonated poly(arylene ether ketone) multiblock copolymers.. 56
Figure 1.4.12. Synthesis of (a) phenoxide-teminal; (b) Cl-terminal; (c) NH2-endcapped BPS-100 oligomers ........................................................................................................ 57
Figure 1.4.13. Synthesis of PPP-BPSH100 multiblock copolymers ..................................... 58
Figure 1.4.14. Synthesis of polyimide-BPS100 multiblock copolymers............................... 59
Figure 1.4.15. Synthesis of Perfluoroarylene ether-BPS100 multiblock copolymers ........... 60
Figure 1.4.16. BisAF-BPS100 multiblock copolymers ......................................................... 61
Figure 1.4.17. Synthesis of fluorine-terminal hydrophobic oligomers.................................. 63
Figure 1.4.18. Questionable synthesis of poly(arylene ether sulfone) multiblock copolymers........................................................................................................................................ 64
Figure 1.4.19. Synthesis of sulfonated PAES-PAEK multiblock copolymers....................... 65
Figure 1.4.20. (a) structure of S-SE “pseudo-random” copolymers. (b) proton conductivity vs. water content for S-SEBS, S-SE and Nafion PEMs................................................. 67
Figure 1.4.21. Structures of (a) poly(ether sulfone) and (b) poly(ether ketone) random copolymers..................................................................................................................... 67
Figure 1.4.22. Proton conductivity vs. RH plots for Nafion 117, poly(ether sulfone) random copolymers (HQSH 30), and poly(ether ketone) random copolymers (PB-diketone 50)........................................................................................................................................ 69
Figure 1.4.23. Proton conductivity vs. RH plots for Nafion 117 and BisAF-BPSH multiblock copolymers..................................................................................................................... 70
Figure 1.4.24. Synthesis of BisSF-BPSH multiblock copolymers ........................................ 70
Figure 1.4.25. Proton conductivity vs.RH plots for BisSF-BPSH multiblock copolymers, Nafion 112 and BPSH-35 random copolymers.............................................................. 71
Figure 1.4.26. Tapping mode AFM images of BPSH-xx random copolymer membranes: (a).BPSH-30; (b).BPSH-35; (c).BPSH-40; (d).BPSH-45.............................................. 72
Figure 1.4.27. Tapping mode AFM phase images of BPSH-PI multiblock copolymer membranes with different block lengths: (a).5K:5K; (b).10K:10K; (c).15K:15K. ....... 72
Figure 1.4.28. Proton conductivity vs.RH plots for BPSH-PI multiblock copolymers, Nafion 112 and BPSH-35 random copolymers.......................................................................... 73
Figure 2.1. Structures of fluorinated-sulfonated, hydrophobic-hydrophilic multiblock copolymers..................................................................................................................... 75
Figure 2.2. Pulse sequence schematic for PGSE NMR experiments..................................... 85
Figure 2.3. Synthesis of BisAF oligomers ............................................................................. 86
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Figure 2.4. 1H NMR spectrum of a BisAF oligomer ............................................................. 87
Figure 2.5. 19F NMR spectra of a BisAF oligomer................................................................ 87
Figure 2.6. logη vs. logMn plot for BisAF oligomers ............................................................ 88
Figure 2.7. Reaction of decafluorobiphenyl with various bisphenol monomers ................... 90
Figure 2.8. Synthesis of fully sulfonated BPS-100 oligomers............................................... 91
Figure 2.9. 1H NMR spectrum of a BPS-100 oligomer ......................................................... 92
Figure 2.10. Synthesis of BisAF-BPSH multiblock copolymers........................................... 93
Figure 2.11. 1H NMR spectrum of a BisAF-BPSH multiblock copolymer........................... 95
Figure 2.12. 19F NMR spectrum of a BisAF-BPSH multiblock copolymer .......................... 96
Figure 2.13. Monitoring of multiblock copolymer synthesis using 1H NMR spectra: (a) BPS-100 oligomer prior to the reaction; (b) 12 h; (c) 20 h; (d) 36 h............................. 97
Figure 2.14. 1H NMR spectra of BisAF-BPSH multiblock copolymers showing the linkages between blocks............................................................................................................... 98
Figure 2.15. Tapping mode AFM phase images of BisAF-BPSH multiblock copolymers with different block lengths ................................................................................................. 100
Figure 2.16. Structures of partially disulfonated random copolymers. (a) BPSH; (b) HQSH; (c) poly(ether ketone) B and PB series ........................................................................ 101
Figure 2.17. Proton conductivity at 30oC in liquid water for partially disulfonated random copolymers plotted against IEC................................................................................... 102
Figure 2.18. Proton conductivity under fully hydrated conductions for BisAF-BPSH copolymers as a function of temperature ..................................................................... 104
Figure 2.19. Proton conductivity vs. RH plots for Nafion 117, poly(ether sulfone) random copolymers (HQSH-30), and poly(ether ketone) random copolymers (PB-50) .......... 105
Figure 2.20. Activation energy of proton transport for HQSH-30 random copolymers as a function of relative humidity: proton transport barrier increases as RH decreases ..... 106
Figure 2.21. Proton conductivity vs. RH plots for Nafion 117and BisAF-BPSH multiblock copolymers................................................................................................................... 107
Figure 2.22. Self-diffusion coefficient for water as a function of IEC for random and block copolymers and Nafion ................................................................................................ 108
Figure 2.23. Methanol permeability at 80 oC as a function of IEC for random and block copolymer membranes and Nafion .............................................................................. 110
Figure 3.1. Synthesis of 6FBisAF oligomers....................................................................... 115
Figure 3.2. Synthesis of BPS-75 hydrophilic oligomers...................................................... 116
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Figure 3.3. Synthesis of 6FBisAF-BPSH100 multiblock copolymers ................................ 117
Figure 3.4. Synthesis of 6FBisAF-BPSH75 multiblock copolymers .................................. 117
Figure 3.5. 19F NMR spectrum of a 6FBisAF oligomer ...................................................... 118
Figure 3.6. 19F NMR spectrum of a 6FBisAF oligomer (aromatic region) showing endgroups in detail......................................................................................................................... 119
Figure 3.7. logη vs. logMn plot for 6FBisAF oligomers ...................................................... 120
Figure 3.8. 1H NMR spectrum of a BPS-75 oligomer ......................................................... 120
Figure 3.9. 1H NMR spectrum of a 6FBisAF-BPSH100 multiblock copolymer................. 121
Figure 3.10. 1H NMR spectra of a partially disulfonated BPS oligomer and the corresponding 6FBisAF-BPS83 multiblock copolymer .............................................. 122
Figure 3.11. DSC trace of a 6FBisAF-BPSH100 (9K:9K) multiblock copolymer.............. 125
Figure 3.12. Tapping mode AFM images for a 6FBisAF-BPSH100 (15K:10K) multiblock copolymer .................................................................................................................... 129
Figure 3.13. Tapping mode AFM images for a 6FBisAF-BPSH75 (15K:9K) multiblock copolymer .................................................................................................................... 129
Figure 3.14. Proton conductivity as a function of RH for 6FBisAF-BPSH multiblock copolymers................................................................................................................... 130
Figure 4.1. Synthesis of BisSF telechelic oligomers ........................................................... 134
Figure 4.2. Reaction at para- positions leading to branching .............................................. 134
Figure 4.3. Synthesis of fluorinated poly(ether sulfone) under mild conditions ................. 135
Figure 4.4. Evolution of intrinsic viscosity for BisSF (17K) oligomer synthesis as a function of reaction time ............................................................................................................ 136
Figure 4.5. 19F NMR of a BisSF telechelic oligomer .......................................................... 137
Figure 4.6. logη vs. logMn plot for BisSF oligomers ........................................................... 138
Figure 4.7. 19F NMR plots of BisSF oligomers with Mn of 1) 5K; 2) 10K; 3) 17K; 4) 25K139
Figure 4.8. Synthesis of BisSF-BPSH multiblock copolymers ........................................... 141
Figure 4.9. 19F NMR spectra of a BisSF-BPSH multiblock copolymer .............................. 143
Figure 4.10. 1H NMR spectra of a BisSF-BPSH multiblock copolymer............................. 143
Figure 4.11. 1H NMR spectra of BisSF-BPSH copolymers with increasing block lengths 144
Figure 4.12. Intrinsic viscosity as a function of block lengths for BisSF-BPSH multiblock copolymers (IEC=1.3) ................................................................................................. 147
Figure 4.13. Schematic plots showing the decrease of endgroup concentration as a function
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of time for the syntheses of multiblock copolymers with (a) low block lengths; (b) high block lengths ................................................................................................................ 147
Figure 4.14. Intrinsic viscosity as a function of block lengths for BisSF-BPSH multiblock copolymers (IEC=1.1) ................................................................................................. 149
Figure 4.15. Intrinsic viscosity as a function of block lengths for BisSF-BPSH multiblock copolymers (IEC=1.5) ................................................................................................. 151
Figure 4.16. DSC trace of a BisSF-BPSH (17K-12K) multiblock copolymer .................... 153
Figure 4.17. TGA traces of BisSF-BPSH (17K-12K) multiblock copolymers.................... 153
Figure 4.18. Stress-strain curves for BisSF-BPSH (17K-12K) (IEC=1.5) copolymers ...... 155
Figure 4.19. Tapping mode AFM height (left) and phase (right) images for BisSF-BPSH (5K-5K), (7K-7K), (17K-12K) and (25K-16K) multiblock copolymer membranes... 157
Figure 4.20. Proton conductivity for BisSF-BPSH copolymers having an IEC of 1.3........ 158
Figure 4.21. Water uptake as a function of Block lengths for BisSF-BPSH (IEC=1.3) copolymers................................................................................................................... 159
Figure 4.22. Water uptake as a function of disulfonation degree for BPSH random copolymers................................................................................................................... 160
Figure 4.23. Proton conductivity at 80 oC as a function of relative humidity for BisSF-BPSH (IEC=1.3) copolymers ................................................................................................. 161
Figure 4.24. Proton conductivity vs. RH: the effect of IEC on the performance of BisSF-BPSH (17K-12K) copolymers .......................................................................... 163
Figure 4.25. Proton conductivity vs. RH: the effect of IEC on the performance of BisSF-BPSH (25K-16K) copolymers .......................................................................... 165
Figure 4.26. Proton conductivity under partially hydrated conditions for BisSF-BPSH (25K-16K) (IEC=1.5) and BisSF-BPSH (15K-15K) (IEC=1.5) ................................. 166
Figure 4.27. Degrees of swelling in x, y and z directions for different copolymer membranes...................................................................................................................................... 169
Figure 4.28. Multiblock copolymers with long blocks or higher IEC show higher z-direction swelling........................................................................................................................ 171
Figure 4.29. Imaginary cross-sectional view of BisSF-BPSH (17K-12K) copolymer membranes ................................................................................................................... 172
Figure 4.30. Comparison of TGA traces for three series of multiblock copolymers........... 175
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List of Tables
Table 2.1. Molecular weight characterizations of BisAF oligomers ..................................... 88
Table 2.2. Some characterizations of BisAF-BPSH multiblock copolymers ........................ 99
Table 2.3. IEC, water uptake and proton conductivity for partially disulfonated random copolymers................................................................................................................... 102
Table 2.4. IEC, water uptake and liquid water proton conductivity for BisAF-BPSH multiblock copolymers................................................................................................. 103
Table 3.1. Molecular weight characterizations of 6FBisAF oligomers ............................... 119
Table 3.2. Comparison of target IEC with experimental values for 6FBisAF-BPSH100 multiblock copolymers................................................................................................. 123
Table 3.3. Solubility of oligomers in DMSO at room temperature ..................................... 123
Table 3.4. Proton conductivity (liquid water) and water uptake for 6FBisAF-BPSH100 multiblock copolymer membranes............................................................................... 127
Table 3.5. Proton conductivity (liquid water) and water uptake for 6FBisAF-BPSH75 and 6FBisAF-BPSH83 multiblock copolymer membranes................................................ 128
Table 4.1. Molecular weight characterizations of BisSF oligomers .................................... 138
Table 4.2. Characterization of BisSF-BPSH copolymers with 1.3 IEC (Series A) ............. 145
Table 4.3. Characterization of BisSF-BPSH copolymers with 1.1 IEC (Series B) ............. 148
Table 4.4. Characterizations of BisSF-BPSH copolymers with 1.5 IEC (Series C)............ 150
Table 4.5. Effects of IEC on the mechanical properties of BisSF-BPSH (17K-12K) multiblock copolymers................................................................................................. 155
Table 4.6. The effect of IEC on the properties of BisSF-BPSH copolymers....................... 162
Table 4.7. Comparison between BisSF-BPSH multiblock copolymers having different hydrophobic block lengths........................................................................................... 166
Table 4.8. Comparison between BisSF-BPSH multiblock copolymers having different hydrophobic block lengths........................................................................................... 167
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Introduction
The proton exchange membrane (PEM) is the key component in a proton exchange membrane
fuel cell (PEMFC). Nafion® and comparable perfluorosulfonic acid-based membranes are
currently the state-of-the-art PEMs, but suffer from shortcomings such as high permeability, cost,
and limited operating temperatures.
These drawbacks have collectively sparked an interest in developing novel copolymers as
alternative PEMs, particularly in the last decade or so. Some hydrophilic-hydrophobic block
copolymer membranes, in particular, have shown great promise as potential candidates for PEMs.
Therefore, the goal of this Ph.D. research was to synthesize and characterize multiblock
(segmented) copolymers containing sulfonated and fluorinated blocks.
In Chapter 1, the literature review, research on the synthesis of partially sulfonated high
performance copolymer membranes as alternative PEMs, has been outlined. The main focus is
on wholly aromatic engineering copolymer systems, such as poly(arylene ether)s, poly(imides),
poly(benzimidazole)s and poly(p-phenylene) derivatives. Next, recent progress on nanophase
separated block copolymers of varying structures, which were synthesized by a variety of
methods, is reviewed, and their advantages over random copolymers are discussed.
Specifically, my research involved the investigation of three series of hydrophilic/hydrophobic,
fluorinated/sulfonated multiblock copolymers, which had the same structures in the hydrophilic
1
blocks, but which differed slightly in terms of the hydrophobic (fluorinated) blocks. The results
are shown and discussed in Chapters 2-4, respectively. Their performance and structure-property
relationships were studied and compared to random copolymer systems. These copolymers
displayed superior proton conductivity, due to the ionic proton conducting channels formed
through the self-assembly of the sulfonated blocks. At the end of Chapter 4, the three series are
briefly compared and contrasted. This chapter also includes a discussion of how to develop a
material with good performance and water sorption capabilities. This study concludes with a
summary chapter.
The author is a synthetic chemist who was in charge of the synthesis, structural characterization,
thermal analysis, etc. of the copolymers. Here I would like to again sincerely thank Abhishek
Roy, Juan Yang, Anand Badami, Ozma Lane, and Mark Flynn for their advice and help with
respect to electrochemical testing, molecular weight characterization, morphological studies,
mechanical testing, etc.
2
Chapter 1. Literature Review
1.1. Introduction
1.1.1. Fundamentals of proton exchange membranes (PEMs)
Fuel cells are electrochemical devices that convert chemical energy directly into electrical
energy.1, 2 Proton exchange membrane fuel cells (PEMFCs) have shown promise as alternative
automotive and stationary power sources.2, 3 In a PEMFC, hydrogen is the fuel, oxygen is the
oxidant, and water is produced as the only by-product. A direct methanol fuel cell (DMFC),
which uses dilute methanol as the fuel, is the portable power version of the PEMFC. The basic
electrochemical reactions for PEMFC and DMFC are summarized in Figure 1.1.1.
Anode: 2H2 4H+ + 4e-
Cathode: 4H+ + O2 + 4e- 2H2O
Overall: 2H2 + O2 2H2O
Anode: CH3OH + H2O CO2 + 6H+ + 6e-
Cathode: 3/2 O2 + 6H+ + 6e- 3H2O
Overall: CH3OH + H2O + 3/2 O2 CO2 + 3H2O
DMFC
PEMFC
Figure 1.1.1. Electrochemical reactions for a PEMFC and DMFC
PEMFCs operate through a membrane electrode assembly (MEA), the basic structure of which is
shown in Figure 1.1.2.1, 3 It is composed of an anode, cathode, and a proton exchange membrane
(PEM) sandwiched in between. The PEM, which is the electrolyte that transfers protons from the
3
anode to the cathode, is the key component of the system. It also serves as a separator to prevent
mixing of the fuel and oxygen.2
Figure 1.1.2. Membrane electrode assembly in a proton exchange fuel cell membrane
The current state-of-the-art PEMs are perfluorosulfonic acid membranes such as Nafion®,
developed by Dupont in the late 1960s. The reported structure of Nafion®, which is shown in
Figure 1.1.3, is based on a crystallizable tetrafluoroethylene backbone and contains pendant side
chains of perfluorinated vinyl ethers terminated by perfluorosulfonic acid groups. Nafion® is
believed to be synthesized by free radical polymerization. The sulfonated comonomer cannot
easily self-propagate and thus its sequence length should not be more than one. The highly acidic
perfluorosulfonic acid groups impart high proton conductivity under both fully hydrated and
partially hydrated conditions, while the semicrystalline backbone provides excellent chemical
and electrochemical stability. However, Nafion® and other perfluorosulfonated PEMs suffer from
disadvantages including high cost, limited operating temperature (80°C), and high fuel
permeability (in DMFCs).4, 5 Unfortunately,, the use of thicker membrane in DMFC applications
in order to reduce methanol permeability results in resistance losses of the cell.
4
CF2 CF2 CF CF2
OCF2CF
O(CF2)2SO3H
CF3
x y
Figure 1.1.3. Proposed structure of Nafion®
Therefore, researchers have attempted to develop alternative proton exchange membranes that
can withstand the harsh fuel cell operating conditions that severely compromise their
performance.6, 7 However,it has been a challenge to achieve performance levels comparable to
perfluorosulfonic acid PEMs—especially while keeping costs low. In general, a successful PEM
should have high proton conductivity, low electronic conductivity, good mechanical strength,
high oxidative and hydrolytic stability, low fuel permeability, ease of fabrication into MEA, and
controlled swelling-deswelling behavior as a function of relative humidity.3
1.1.2. Basic criteria for a PEM
Among the most important properties of a PEM are proton conductivity and water uptake, both
of which are closely related to the concentration of ion-conducting units in the membrane. This
is typically known as its ion exchange capacity (IEC). Los Alamos National Laboratory (LANL)
developed a simple method for determining the proton conductivity of PEMs using
electrochemical impedance spectroscopy.8 The conductivity is measured in the plane, because
measuring it normal to the plane is difficult due to large interfacial resistances.9 Water uptake is
most frequently reported in mass percent. Since most proton exchange membranes rely on water
to facilitate proton transport, a PEM must undergo the required water uptake to perform 5
efficiently. Despite the fact that proton conductivity generally increases with water uptake,
excessive water uptake may cause the membrane to lose its mechanical strength in the
water-swollen state. In addition, in the membrane electrode assembly, too much swelling will
lead to stress in the swelling-deswelling cycles in a fuel cell environment and the membrane
and/or MEA may fail. For H2-O2 fuel cell applications, the water uptake generally should not
exceed 50 weight %, and ideally it should be in the 20~30 weight % range.
Due to harsh fuel cell operating conditions, oxidative stability and hydrolytic stability are critical
to the long-term durability of a fuel cell membrane. PEMs based on partially aliphatic backbones,
such as polystyrene, are often subjected to oxidative degradation and thus are generally utilized
in low temperature fuel cells, such as those intended for use in portable power devices.10-12 In
contrast, polymers with high performance aromatic backbones, such as poly (arylene ether)s and
polyimides13-16, are generally much more oxidatively stable. However, certain functional groups
in these systems may be susceptible to hydrolytic degradation, as will be discussed later.
Like other polymeric materials, a PEM must have sufficient mechanical strength, both in the dry
state and water-swollen state, to survive the stress of electrode attachment. The membrane must
also be tough and flexible. Therefore, high molecular weight is always desired to enhance
intermolecular forces and chain entanglements. In contrast, weak and brittle membranes are often
formed from low molecular weight materials and/or result from the degradation of the polymer.
In addition to molecular weight, a membrane’s chemical structure also affects its mechanical
properties. For instance, even substituted poly(1,4-phenylene) derivatives are incapable of 6
forming flexible membranes due to their extremely rigid rod-like chains.17 As will be shown later,
some PEMs have been synthesized by block-copolymerizing poly(1,4-phenylene) derivatives
with other species to improve their film-forming ability.18
Of course, a PEM must be capable of being fabricated into a membrane electrode assembly
(MEA). Therefore, associated research efforts have investigated membrane-electrode interfaces.
Not only must the membrane itself possess long-term stability, but it must also be compatible
with the electrodes, which usually contain highly fluorinated, Nafion-based binders (catalyst
layer).19 Thus, PEMs based on partially fluorinated copolymer backbones have been an emerging
area of research.
1.1.3. PEMs based on hydrophobic-hydrophilic block copolymers
Block copolymers are macromolecules made up of two or more usually multiphase blocks that
are chemically conjoined in the same chain. Unlike random copolymers, in which the monomers
are arranged statistically, in block copolymers different chemical components exist in ordered
sequences. Therefore, they have the potential to display interesting physico-chemical properties.
Various morphological features of block copolymers can be obtained by tailoring the chemical
composition, molecular weight, and/or volume fraction of the blocks.20
Hydrophobic/hydrophilic (amphiphilic) block copolymers can be obtained when one or more of
the blocks is water-soluble, or is fully or partially modified with hydrophilic functional groups.
Due to their phase behavior, these materials can be utilized for a variety of purposes, including as
7
biomaterials, as stabilizers in suspensions and emulsions, for pharmaceutical applications, and in
the synthesis of advanced materials, adhesives, and coatings.21-23
The development of hydrophobic-sulfonated partially ionic block copolymers as PEMs has been
of great interest. These materials contain sequences of sulfonated and nonsulfonated segments
which result in interesting structural and morphological features.24-26 Therefore, various block
copolymer ionomers containing sulfonic acid groups have been synthesized. The systematic
evaluation of their potential as PEMs has, however, largely been ignored. A review of some
recent research on the development of PEMs based on low-cost, high-performance engineering
materials for use as alternatives to perfluorosulfonic acid-based PEMs—with particular emphasis
on hydrophilic-hydrophobic block copolymer membranes—will be provided. At the end of this
overview, current progress on the comparative properties of random and block copolymers (and
especially the advantages of the latter) will be discussed.
1. 2. PEMs based on high performance engineering materials
1.2.1. Synthesis of copolymer backbones
High performance engineering thermoplastics, based on wholly aromatic polymers and
copolymers, are important in a wide variety of applications including automotive, structural, and
microelectronic components. In particular, poly(arylene ether)-based ionomers are a family of
promising candidates for novel PEMs due to their low cost, high glass transition temperatures,
good mechanical properties, excellent oxidative and hydrolytic stability, as well as the ease by
8
which protein conducting moieties can be incorporated. 27 Furthermore, their structures can be
easily modified by varying the linkages between the phenyl rings. As shown in Figure 1.2.1, the
X variable, which can be either a sulfone group, a ketone group, or a phenyl phosphine oxide
group, determines whether the polymer will be a poly(arylene ether sulfone), a poly(arylene
ether ketone), or a poly(arylene ether phosphine oxide). There is also flexibility in the Y variable,
which in principle can be a bond, a sulfone group, an isopropylidene linkage, etc.
1.2.1.1. Poly(arylene ether)s
nO X O Y
S
O
O
C
O P
O
, , C
CH3
CH3
C
CF3
CF3
S
O
O
P
O
, , ,X = Y = a bond,
Figure1.2.1. Some possible structures of poly(arylene ether)s
Although Friedel-Crafts electrophilic sulfonylation and acylation reactions have been used to
synthesize poly(arylene ether sulfone)s and poly(arylene ether ketone)s, respectively, the most
practical method for preparing poly(arylene ether)s is via nucleophilic aromatic substitution.28-31
Figure 1.2.2 shows the generalized mechanism for an SNAr nucleophilic aromatic substitution. In
the first step, which is the rate-determining step, the carbon atom of the activated C-X bond is
attacked by the nucleophile, and a resonance-stabilized Meisenheimer complex is formed. The
leaving group, X, departs in the second step.
9
Y X Nu-+slow Y
X
NuMeisenheimer complex
fast Y Nu + X-
Figure 1.2.2. Mechanism of SNAr nucleophilic aromatic substitution
The synthesis of poly(arylene ether)s is usually achieved via the step polymerization of a
dihalide monomer and a bisphenol monomer, with a base producing the active phenolate. The
bisphenol is converted into the phenoxide ion, which acts as the nucleophile, after which the
carbon atom adjacent to the halogen is attacked by the phenoxide ion to form an ether linkage.
Since the sulfone, ketone and phosphine oxide groups are all electron-withdrawing groups, they
stabilize the Meisenhheimer complex and facilitate the reaction. For this reason, all three
families of polymers are readily synthesized in this way. For example, with respect to the
catalysis of sodium hydroxide, 4,4’dichlorodiphenyl sulfone (DCDPS) and bisphenol-A can be
polymerized to afford bisphenol-A polysulfone (Udel®) (Figure 1.2.3).28, 31 Even though
difluoride monomers are more reactive than their dichloride counterparts, the latter are more
commonly used due to their lower costs.
10
+S
O
O
Cl Cl C
CH3
CH3
HO OH
NaOH Chlorobenzene, DMSO(dry)160oC / 1 h
C
CH3
CH3
O OS
O
O n
Figure 1.2.3. Synthesis of bisphenol-A polysulfones
The use of a strong base catalyst like sodium hydroxide ensures rapid polymerization at
relatively low temperatures, but requires the addition of an exact stoichiometric amount of the
base. Moreover, the diphenolate must be soluble. The use of weak bases (e.g., potassium
carbonate) to synthesize phenolates has been proposed and studied by McGrath et al.32, 33 The
proposed mechanism for K2CO3-catalyzed Udel® synthesis is shown in Figure 1.2.4. The
reaction mixture must be carefully dehydrated using an azeotroping agent while the phenolate is
formed. A high reaction temperature and polar aprotic solvents, such as dimethyl sulfoxide
(DMSO), N-methyl pyrrolidone (NMP) and N,N-dimethyl acetamide (DMAc), are usually
required. The polymerization is believed to be second order, so the rate of polymerization can be
improved by increasing monomer concentrations, but only to the extent that the solution does not
become too viscous during polymer formation. As a rule, reaction mixtures with 15~20% g/mL
(0.3~0.5 mol/L) monomer concentrations are used.
11
+S
O
O
Cl Cl C
CH3
CH3
HO OK
C
CH3
CH3
O OS
O
O n
C
CH3
CH3
HO OHK2CO3
Aprotic solvent/toluene C
CH3
CH3
HO OK + KHCO3
C
CH3
CH3
O OHS
O
O
Cl
+ nH2O + nCO2 + nKCl
KHCO3
Figure 1.2.4. Mechanism of K2CO3-catalyzed synthesis of Bisphenol-A polysulfones
The mechanism of poly(arylene ether ketone) synthesis is similar to that for poly(sulfone)s.
However, the direct synthesis of high molecular weight poly(ether ketone)s from dihalide
benzophenone monomers and bisphenol monomers such as biphenol and hydroquinone can be
challenging, primarily because the resulting polymers tend to be semicrystalline and display poor
solubility.34 High reaction temperatures are therefore needed, but can give rise to undesirable
side reactions.35 An alternative is the “soluble precursor” method, in which certain functional
groups are introduced to suppress crystallinity and improve solubility. For instance, poly(arylene
ether ketone)s have been synthesized using poly(ether ether ketimine) as the precursor,36, 37 as
shown in Figure 1.2.5. These can then be hydrolyzed under acidic conditions to yield the
semi-crystalline ketone counterpart. Precursors with bulky substituents such as t-butyl groups on
the bisphenol unit have also been reported (Figure 1.2.6).38, 39 The poly(ether ketone) is obtained
by removing the t-butyl group using a Lewis acid catalyst.
12
HO OH C
N
F F+ C
N
OOn
K2CO3
NMP, 160oC
HCl, R.T. COO
O
n
Figure 1.2.5. Synthesis of poly(arylene ether ketone)s via the ketimine precursor method
HO OH C
O
F F+ C
O
OOn
K2CO3
DMSO, 170oC
CF3SO3H, R.T. COO
O
n
Figure 1.2.6. Synthesis of poly(arylene ether ketone)s from bulky substituted bisphenol
1.2.1.2. Aromatic Poly(imide)s
Due to their excellent thermal, chemical and mechanical properties, aromatic poly(imide)s have
been important advanced materials ever since they were first introduced by DuPont. Aromatic
polyimides can be synthesized via the step growth polymerization of diamine monomers with
dianhydride monomers.40, 41 One factor limiting their processability and resulting applications is
that they tend to be insoluble in polar organic solvents. Therefore, the synthesis of aromatic
poly(imide)s requires two distinct stages, as shown in Figure 1.2.7. In the first step a poly(amic
acid) is generated at a relatively low temperature, which is soluble and can be made into a film,
for example. The second stage involves the ring closure of the poly(amic acid) to form the
13
ultimate polyimide structure. This is done by subjecting the poly(amic acid) to high temperatures
(200~300oC).
NN
O
O O
O
O
H2N O NH2 + O N
OO
N
O
HO OH
O
H H
n
DMAc
Heatcyclodehydration
O N
OO
N
O
HO OH
O
H H
n n
OO
O
O O
O
(a)
(b)
Figure 1.2.7. Two-stage synthesis of poly(imide)s
During the imidization stage, the high Tg of the system often leads to low mobility of the chains.
Therefore, the type of conformation that favors a cyclodehydration reaction may be hindered. As
a result, quantitative imidization is often hard to achieve. Moreover, the hydrolytic degradation
of the residual amic acid units can cause chain scission. To improve both the solubility and
processability of aromatic poly(imide)s, researchers have introduced bulky groups and/or
flexible linkages, and have also utilized monomers containing meta linkages.42, 43 This allows the
polymerization to be carried out under moderate conditions in one stage. One such example is
the production of Ultem® by GE (Figure 1.2.8).
14
C
CH3
CH3
OOOO
O
O
O
O
H2N N 2H+
C
CH3
CH3
OONN
O
O
O
On
o-dichlorobenzene180oC
Figure 1.2.8. One-stage synthesis of poly(imide)s with improved solubility
1.2.1.3. Aromatic 5-membered-ring heterocyclic polymers
Aromatic heterocyclic polymers such as poly(benzimidazole)s, poly(benzoxazole)s and
poly(benzthiazole)s are well known to have excellent thermal/chemical stability, as well as good
mechanical properties. As shown in Figure 1.2.9, they are produced from the condensation
reaction of a dicarboxyl (or phenyl ester) with a tetramine, a bis-o-aminophenol, or a
bis-o-aminothiophenol, shown sequentially.44-48 Although fabrication of proton exchange
membranes based on these materials has been attempted, particular attention has focused on the
potential applications of phosphoric acid-doped poly[2,2’(m-phenylene)-5,5’bibenzimidazole]
(PBI) membranes. 49-53
15
n+HOOC COOH
H2N
NH2
OH
HO
HOOC COOHH2N
NH2
SH
HS
O
N O
N
S
N S
N+
n
(b)
(c)
n+HOOC COOH
H2N
NH2
NH2
H2N N
N N
N
H
H
(a)
Figure 1.2.9. General scheme for the synthesis of (a) poly(benzimidazole)s, (b) poly(benzoxazole)s and
(c) poly(benzthiazole)s
The conventional way to synthesize PBI and other poly(benzimidazole)s is via the melt
polymerization of a tetramine with the diphenyl ester of a diacid (Figure 1.2.10). Solution
polymerizations of tetramines with dicarboxylates or diacids in solvents such as polyphosphoric
acid have also been developed. These synthetic techniques can be conducted under less harsh
conditions, but also afford completely soluble polymers.54
H2N
H2N
NH2
NH2
C CO O
O O
N
NN
N
H Hn
+
1. 290oC2. 390oC
Figure 1.2.10. Synthesis of PBI by melt polymerization
16
1.2.1.4. Poly(p-phenylene) derivatives
Poly(p-phenylene)s (PPP) derivatives are important high-performance engineering materials that
have received significant attention—particularly due to their excellent mechanical properties and
thermal oxidative stability.55-57 The synthesis, processing and characterization of unsubstituted
PPPs are all challenging due to the intractability of the rod-like chains. Therefore, a wide variety
of lateral substituents has been introduced into the PPP main chains to improve its solubility. One
of the most widely used methods for preparing substituted PPPs is by the Ni(0)-catalyzed
coupling of dihalide monomers.58-61 In addition to homopolymers, statistical copolymers can also
be synthesized by the copolymerization of substituted and unsubstituted monomers (Figure
1.2.11).
Cl Cl
R
n
RNiCl2, Zn, PPh3
Figure 1.2.11. Synthesis of substituted PPPs via Ni(0) coupling
The palladium-catalyzed cross coupling of aromatic bromides with aromatic boronic acids has
been reported (Suzuki coupling).62, 63 Such reactions are suitable for synthesizing homopolymers
or copolymers bearing alternating arrays of substituted and unsubstituted phenyl rings (Figure
1.2.12).
17
Br Br
R
R
Pd(0), Na2CO3(HO)2B B(OH)2+
Br Br
R
R
B BO
OO
O+
R
R
n
R
Rn
Pd(0), NaHCO3
Figure 1.2.12. Synthesis of PPP alternating copolymers via Suzuki coupling
Substituted poly(2,5-benzophenone)s are an important family of thermal-oxidatively stable PPP
derivatives. The pendent phenyl rings not only enhance the solubility of the polymers, but also
facilitate further modification with various functional groups. The monomers are synthesized by
the Friedel-Crafts acylation of substituted benzene with 2,5-dichlorobenzoyl chloride (Figure
1.2.13).17, 59, 64
Cl Cl
COOH
Cl Cl
COCl
X
Cl Cl
C O
X
AlCl3
SOCl2 C O
X
n
NiCl2, Zn, PPh3
NMP, 80oC
X = H, X, O,
Figure 1.2.13. Synthesis and polymerization of 2,5-dichlorobenzophenone monomers
18
1.2.2. Fabrication of PEMs: introduction of proton-conducting moieties
1.2.2.1. Post sulfonation of poly(arylene ether)s
The most widely used proton-conducting moiety is the sulfonic acid group, primarily because of
its availability, high acidity, and ease of introduction into the polymer backbone. Sulfonated
copolymers using either poly(arylene ether sulfone)s or poly(arylene ether ketone)s are most
commonly obtained via the electrophilic sulfonation of the polymers’ aromatic rings (post
sulfonation).65-70 Common sulfonating agents include concentrated sulfuric acid, fuming
sulfuric acid, sulfur trioxide, chlorosulfonic acid, etc. Electrophilic substitution reactions are
favored by electron-donating substituents on the phenyl ring. Therefore, post sulfonations of
poly(arylene ether)s are generally believed to take place on the activated phenyl rings, rather
than on the phenyl rings directly attached to the deactivating sulfone groups or ketone groups
(Figure 1.2.14). In addition, usually no more than one sulfonic acid can be introduced to each
repeat unit.65
C
CH3
CH3
O S
O
O
O
SO3H
O C
O
O
SO3H(a) (b)
Figure 1.2.14. Examples of post-sulfonated poly(arylene ether sulfone)s and poly(arylene ether ketone)s
19
The choice of sulfonating agent and reaction conditions has been found to significantly influence
the properties of the modified polymer.66-68 Moreover, although higher degrees of sulfonation
have been achieved using stronger sulfonating agents such as fuming sulfuric acid and
chlorosulfonic acid, they also result in unwanted side reactions that can lead to the degradation of
the polymer chains. In contrast, the use of mild sulfonating agent, such as concentrated sulfuric
acid or trimethylsilylchlorosulfonate, has been studied with little or no polymer degradation
reported.67, 68 However, such reactions usually involve longer reaction times and low sulfonation
efficiencies. Therefore, no reliable correlations have been established between the amount of
sulfonating agent used, reaction time, and the degree of sulfonation. In addition, the selection of
solvents or solvating agents has also been shown to be important. Even though a homogeneous
reaction medium is considered to be critical for obtaining high sulfonation efficiency,
reproducibility is generally not good.69
1.2.2.2. Direct copolymerization of sulfonated monomers: preparation of poly(arylene ether)
random copolymers
Another method for generating sulfonated copolymers involves the copolymerization of
sulfonated and unsulfonated monomers to form random copolymers. This method has advantages
relative to post-modification with regard to control of the degree and location of sulfonation.
Also, the acidity of the sulfonic acid group may be improved because this methodology
facilitates its introduction onto the more electron-deficient phenyl rings, i.e., those connected to
the sulfone or ketone groups.
20
Based on a method proposed by Ueda et al.,70 McGrath and coworkers71 sulfonated
4,4’-dichlorodiphenyl sulfone (DCDPS) to synthesize 3,3’-disulfonated 4,4’-dichlorodiphenyl
sulfone (SDCDPS) (Figure 1.2.15). They also investigated the copolymerizations of bisphenols
with SDCDPS and DCDPS.13, 14, 72
S
O
O
Cl ClSO3/H2SO4
110oCS
O
O
Cl Cl
HO3S
SO3H
S
O
O
Cl Cl
NaO3S
SO3Na
NaOH
pH 6~7
Figure 1.2.15. Synthesis of 3,3’-disulfonated 4,4’-dichlorodiphenyl sulfone (SDCDPS)
As shown in Figure 1.2.16, a series of so-called BPSH-xx random copolymers were synthesized
by polymerizing 4,4’-biphenol, DCDPS, and SDCDPS, where xx represents the degree of
disulfonation. We have shown that this variable can be precisely controlled by the molar feed
ratio of sulfonated to unsulfonated monomer. Generally, proton conductivity and water uptake
increase almost linearly with the degree of disulfonation. However, when x exceeds 50%,
water uptake increases dramatically and the membrane swells in water like a hydrogel. Based on
these results, Kim et al. proposed that the ”percolation limit” was exceeded at that point.19, 73
Among the various copolymers we tested, BPSH35 was found to have the best combination of
proton conductivity and water uptake; consequently, it is thought to be one of the most promising
alternatives to the perfluorosulfonic-acid PEMs.
21
HO OH S
O
O
Cl Cl
NaO3S
SO3Na
O OO O S
O
O
S
O
O
KO3S
SO3K
K2CO3NMP
Toluene
150oC/4h190oC/18~36h
+ +
1-x x
O OO O S
O
O
S
O
O
HO3S
SO3H1-x x
S
O
O
Cl Cl
H2SO4
Figure 1.2.16. Synthesis of BPSH-xx random copolymers
As illustrated in Figure 1.2.16, once the potassium salt forms of the copolymers are synthesized,
they are converted into the sulfonic acid form through treatment with sulfuric acid. We
determined that the conditions of the acidification reaction strongly influence the morphology of
the membrane. Typically, a so-called “Method 2” is used,74 in which the membrane is boiled in
0.5M H2SO4 for 2h, then boiled in deionized water for 2h. This contrasts to “Method 1,” where
the polymer film is immersed in 1.5M H2SO4 at room temperature for 24h, then in deionized
water for 24h. “Method 2” results in higher proton conductivity and water uptake, and more
distinct ionic-hydrophobic microphase separation as indicated by AFM phase images.19, 73
The synthesis of sulfonated poly(ether ketone) random copolymers via the direct polymerization
of sulfonated monomers has also been studied. Wang et al. was the first to report the synthesis of
the disulfonated difluoride ketone monomer, sodium 5,5'-carbonylbis(2-fluorobenzenesulfonate)
22
(SDFBP) (Figure 1.2.17),75-77 by sulfonating 4,4’-difluorobenzophenone (DFBP) using a
procedure similar to the synthesis of SDCDPS. The sulfonated and unsulfonated monomers were
copolymerized with bisphenol A to afford sulfonated poly(arylene ether ketone) copolymers. As
reported, these copolymers were amorphous and their solubility was significantly influenced by
the degree of sulfonation.
C
O
F F
NaO3S
SO3Na
Figure 1.2.17. Sodium 5,5'-carbonylbis(2-fluorobenzenesulfonate) (SDFBP)
A variety of sulfonated and unsulfonated difluoride ketone monomers were copolymerized with
different bisphenols to prepare PAEK-based PEMs (Figure 1.2.18).78-80 The changes in
electrochemical properties associated with the degree of disulfonation were similar to those
observed for the poly(arylene ether sulfone) copolymer membranes. However, the PAEK-based
PEMs generally displayed lower proton conductivity at comparable IEC or water uptake values.
23
O X O Y O X O Zm n
C
CF3
CF3
S
F3C
CF3
C
O
C
O
C
O
C
O
C
O
C
OHSO3 SO3H
C
O
C
OHSO3 SO3H
C
O
C
OHSO3 SO3H
X =
Y =
Z =
, ,
, ,
Figure 1.2.18. Structures of sulfonated poly(arylene ether ketone) random copolymers studied
1.2.2.3. Sulfonated poly(imide)s: hydrolytic stability issues
Due to the instability of imide groups under post sulfonation conditions, sulfonated aromatic
poly(imide)s have primarily been synthesized by directly polymerizing sulfonated diamine
monomers. However, phthalic poly(imide)s with five-membered rings have been found to
degrade quickly and become brittle in fuel cell environments. This is attributed to the
hydrolysis of the imide structure under acidic conditions, which leads to chain scission.
Conversely, naphthalenic poly(imide)s are generally considered to be much more stable16, 81
(Figure 1.2.19), even though studies on model compounds have questioned the hydrolytic
stability of these materials. Research on polyimide-based PEMs is ongoing.82
24
NN
O
O O
O
NN
O
O
O
O(a) (b)
Figure 1.2.19. Structures of (a) phthalic and (b) naphthalenic imide units
The most frequently used dianhydride monomer is 1,4,5,8-tetracarboxylic dianhydride (NDA),
which has been copolymerized with sulfonated and unsulfonated diamines to synthesize a variety
of partially-sulfonated poly(imide)s, an example of which is shown in Figure 1.2.20.16 The
sulfonated diamine is usually converted to the triamine salt to improve solubility. After
polymerization, acid form copolymers are obtained by treatment in dilute sulfuric acid.
NH2H2N OH2N NH2
SO3NH(Et)3
(Et)3HNO3S
OO
O
O
O
O
NN
O
O
O
O
NN
O
O
O
O
SO3NH(Et)3
(Et)3HNO3S
Om n
+ +
NDA sulfonated diamine unsulfonated diamine
Figure 1.2.20. Synthesis of naphthalenic sulfonated poly(imide) random copolymers
The structures of the sulfonated and unsulfonated diamines do not have to be the same for
copolymer synthesis. In fact, they were found to be closely related to the solubility and
hydrolytic stability of the copolymer.16, 83-86 As a general rule, the solubility of a copolymer in an 25
organic solvent can be improved by using unsulfonated diamines containing flexible linkages,
such as phenyl-ether bonds, or bulky groups.15 In contrast, introducing such groups into
sulfonated diamine units has been found to lead to better stability in water. Figure 1.2.21 shows
the sulfonated and unsulfonated diamine monomers that have been used in this study.
H2N NH2
SO3H
HO3S
H2N NH2
HO3S SO3H
NH2
HO3S
H2N
SO3H
O
H2N O C
CF3
CF3
O NH2
HO3S
SO3H
H2N O S
O
O
O NH2
HO3S
SO3H
H2N NH2 NH2H2N O
H2N NH2
H2N NH2
H2N O S
O
O
O NH2
(a)
(b)
Figure 1.2.21. Structures of (a)sulfonated and (b)unsulfonated diamine monomers used
1.2.2.4. Sulfonated poly(benzimidazole)s, poly(benzoxazole)s and poly(benzthiazole)s
Sulfonated poly(benzimidazole), poly(benzoxazole) and poly(benzothiazole) homopolymers
have been synthesized by the direct polymerizations of sulfonated diacid monomers (Figure
1.2.22).87-90
26
Cl-+H3N SH
HS NH3+Cl-
HOOC COOH
SO3H
H2N NH2
H2N NH2
H2N NH2
HO OH
+
+
+
HOOC COOH
SO3H
HOOC COOH
SO3H
N
O
O
N
SO3H
n
N
S
S
N
SO3H
n
N
N
N
N
SO3H
H
Hn
nn
n
n n
n
Figure 1.2.22. Synthesis of sulfonated heterocyclic homopolymers
Qing et al.91, 92 reported the synthesis of partially sulfonated poly(benzimidazole) random
copolymers by the copolymerization of sulfonated and unsulfonated diacid monomers, as shown
in Figures 1.2.23 and 1.2.24. The PEM properties of these materials have not yet been studied in
detail.
+ +
PPA
COOH
HOOC
SO3H
SO3H
C
CF3
CF3
HOOC COOH
H2N
H2N
NH2
NH2
N
N
N
N
H
H
SO3H
SO3H
N
N
N
N
H
H
C
CF3
CF3m n
Figure 1.2.23. Synthesis of sulfonated poly(benzimidazole) random copolymers
27
S
O
O
HOOC COOH
H2N
H2N
NH2
NH2
m n
N
N
N
N
H
H
S
O
O
N
N
N
N
H
H
SO3H
+ +
SO3H
HOOC COOH
PPA
Figure 1.2.24. Synthesis of sulfonated poly(benzimidazole) random copolymers
The post-sulfonation of poly(benzimidazole)s have also been reported. Both PBI (produced from
A-A and B-B monomers) and AB-PBI copolymers can be sulfonated by immersing the
membrane in sulfuric acid, then heating it to about 400oC.93-95 The proposed mechanism is
shown in Figure 1.2.25. It was found that a sulfonation degree of about 0.6 mole H2SO4 per
repeat unit was usually achieved, regardless of the concentration of the sulfuric acid. However,
these materials showed low proton conductivity, presumably due to the protonation of the
nitrogen in the imidazolium ring. Therefore, while these membranes are not suitable for use as
PEMs themselves, they have been found to be useful in fabricating phosphoric acid-doped
PEMs.
28
N
N
N
N
H
H
H
+
HSO4-
N
N
N
N
H
H
H
+
-O3S
H2SO4
n
n
N
N
H
Hn
+
HSO4-
N
N
H
Hn
+
-O3S
H2SO4N
NH
n
N
NN
N
H Hn
(a)
(b)
Figure 1.2.25. Sulfonation of (a)PBI and (b)ABPBI
When a poly(benzimidazole) membrane is imbibed in phosphoric acid, the basic polymer
absorbs up to 75% of the acid. These acid-doped BPI and ABPBI membranes have sparked the
interest of many research groups as potential PEM candidates.49, 51-53 These membranes do not
require water for proton conduction, can tolerate working temperature as high as 200oC, and are
impermeable to gases and methanol. Their conductivity was found to increase with an increase in
the doping level (acid content).49, 52, 53 One of the problems for systems with higher acid contents
is the issue of acid molecule loss over time. In comparison to the membranes we prepared by
soaking them in acid, membranes cast directly from a solution of PBI and phosphoric acid are
thought to possess better long-term durability.93 This method, however, is only applicable to
polymers that do not precipitate in the presence of phosphoric acid.
29
Sulfonated poly(benzimidazole)s prepared by post sulfonation have also been used as hosts for
phosphoric acid doping.87, 94 Sulfonated polymers that have been subjected to the same acid
doping conditions are reported to have much higher conductivity than their unsulfonated
counterparts. While some researchers have attributed this to the possibility that the presence of
sulfonic acid groups in the polymer facilitates phosphoric acid doping,94 others have suggested
that excess sulfonic acid groups themselves might also contribute to proton conduction.87
1.2.2.5. Sulfonation of poly(2,5-benzophenone)s
The fabrication of potential PEMs obtained via the sulfonation of substituted (Figure 1.2.26, a,b)
and unsubstituted (Figure 1.2.26, c) poly(2,5-benzophenone)s has been studied.17, 18, 96, 97 The
sulfonation reactions of poly(4-phenoxybenzoyl-1,4-phenylene) (Figure 1.2.26, a) and
poly(4-phenyl-2,5-benzophenone) (Figure 1.2.26, b) are generally believed to take place on the
pendent phenyl rings, which have the highest electron density. For poly(2,5-benzophenone)
(Figure 1.2.26, c), however, it was argued that the phenyl rings in the main chains, rather than the
pendant ones, are the ones subjected to sulfonation.98 The synthesis of sulfonated
poly(4-phenoxybenzoyl-1,4-phenylene)s was also achieved by the condensation reaction of
poly(4-fluoro-2,5-benzophenone) with 4-hydroxybenzenesulfonic acid (Figure 1.2.27).
30
C O
n
SO3H
C O
O
n
C O
n
C O
n
SO3H
SO3H
(a) (b) (c) (d)
SO3H
Figure 1.2.26. Proposed structures of sulfonated poly(2,5-benzophenone)s
C O
F
n
NMP/Toluene
K2CO3, 180oCHO SO3Na+
H2SO4
C O
O
SO3H
n
Figure 1.2.27. Sulfonation of poly(4-fluoro-2,5-benzophenone)s
The main obstacle to applying sulfonated PPP derivatives as PEMs is their poor
membrane-forming characteristics. Even though pendent substituents generally enhance
solubility, the polymer chains were still so rigid that in most cases we could only synthesize
brittle films with poor mechanical strength. The synthesis of block copolymers bearing
substituted PPP blocks and flexible poly(arylene ether) blocks have been attempted to improve
the film-forming capabilities of PPPs. Unsulfonated PPP-poly(arylene ether ketone) multiblock
copolymers were synthesized by Sheares et al.59, 64 McGrath et al. developed PPP-poly(arylene 31
ether sulfone) multiblock copolymers containing either sulfonated PPP or sulfonated
poly(arylene ether sulfone) segments.18, 99 The synthesis of hydrophilic-hydrophobic multiblock
copolymers will be discussed in greater detail in Chapter 4.
1.3. Fluorinated aromatic high performance copolymers for PEMs
1.3.1. Moderately fluorinated copolymers containing hexafluoroisopropylidene units
Fluorinated copolymers are well known for their thermal and chemical stability and mechanical
strength. Therefore, the use of partially fluorinated copolymers as backbones for PEMs has
been widely studied.13, 15, 100-104 For PEM applications, introducing some level of fluorine content
has two advantages. Firstly, the water uptake of the membrane can be reduced due to the high
hydrophobicity of the fluorine units. Secondly, the membrane may be more compatible with
Nafion-based electrodes in comparison to hydrocarbon-based systems, which is likely to lead to
higher fuel cell performance and better MEA stability.
OH
F3C C CF3
O
+CF3SO3H HO C
CF3
CF3
OH2
Figure 1.3.1. Synthesis of 6F-BPA
As shown earlier, 4,4’-hexafluoroisopropylidene diphenol (6F-BPA) as an alternative to
non-fluorinated bisphenols has been widely used in synthesizing partially sulfonated
poly(arylene ether)s, primarily for the purpose of introduce hexafluoroisopropylidene linkages.
Similar to the synthesis of bisphenol-A, 6F-BPA can be prepared via the condensation reaction of 32
hexafluoroacetone with phenol (Figure 1.3.1).105 Using 6F-BPA, biphenol, and bisphenol-A as
the bisphenol monomers, Harrison et al.13 synthesized partially fluorinated and non-fluorinated,
partially disulfonated poly(arylene ether sulfone)s, as shown in Figure 1.3.2 (a). TGA studies
showed that the fluorinated copolymers had higher thermal stability. At similar degrees of
disulfonation, the partially fluorinated membranes displayed lower water uptake, but similar
proton conductivity.
OC
CF3
CF3
O R OO C
CF3
CF3
S
O
O
HO3S
SO3H1-x x
S
O
O
(a) R = (b) R =
CN
Figure 1.3.2. Structures of partially sulfonated, partially fluorinated poly(arylene ether sulfone)s
Partially fluorinated nitrile-functional poly(arylene ether sulfone)s with varying degrees of
disulfonation were also synthesized (Figure 1.3.2, b).101 We then compared these copolymer
membranes to our BPSH-xx membranes and observed similar improvements in stability and
water uptake. A more systematic study was undertaken by Sankir et al., who synthesized partially
disulfonated poly(arylene ether benzonitrile)s with varying degrees of fluorination (Figure
1.3.3).100 Although a substantial decrease in water uptake was observed as the content of
hexafluoroisopropylidene units increased, the membranes displayed only a minor loss in
conductivity. Thermal oxidative stability was increased as well with the increase in fluorination.
The 5% weight loss temperature reached 400oC for the >75% fluorinated copolymers.
33
O
CN
RO O S
O
O
HO3S
SO3H
RO1-x x
C
CF3
CF3
R = controlled amount of and
Figure 1.3.3. Partially sulfonated, partially fluorinated poly(arylene ether sulfone benzonitrile)s
A major disadvantage of hexafluoroisopropylidene-containing copolymers is that 6F-BPA has
lower reactivity toward activated aryl dichloride monomers in comparison to the
hydrocarbon-based bisphenol monomers, which we attributed to the electron-withdrawing nature
of the hexafluoroisopropylidene linkage. It has, therefore, been difficult to produce high
molecular weight copolymers—although the use of longer polymerization times, higher
temperatures, and higher reaction solution concentrations have been somewhat useful in
overcoming this challenge.
1.3.2. Highly fluorinated poly(arylene ether)s containing perfluorophenylene units
Poly(arylene ether)s bearing perfluorophenylene groups have been synthesized from bisphenol
and perfluorinated difluoride monomers via step growth polymerizations, as shown in Figure
1.3.4. These polymers were initially developed as optical wave guide materials for potential
telecommunication applications 106-110. The vibration absorption overtone of C-H bonds can be
decreased by substituting the hydrogen with fluorine so that the optical propagation losses can be
34
reduced. The most widely used perfluorinated monomer is decafluorobiphenyl, in which two
perfluorophenyl rings are directly attached to each other. Because of decafluorobiphenyl’s rigid
structure, the other monomer (bisphenol) used in the polymerization must have a flexible linkage
between the two phenyl rings, such as an isopropylidene or a sulfone group. If a rigid bisphenol
(e.g., hydroquinone or 4,4’-biphenol) is used, insoluble, it is thought that semicrystalline
materials will precipitate out of the solution at the start of the reaction, thereby inhibiting the
formation of a polymer.
+XF F
F F
F F
F F
F F
YHO OH
YO OX
F F
F F
F F
F Fn
X = a bond, Y =S
O
O
C
O
S ,,, ,, C
CH3
CH3
C
CF3
CF3
S S
O
O
DMAc, K2CO3
Figure 1.3.4. Synthesis of poly(arylene ether)s containing perfluorophenylene units
Decafluorobiphenyl and other perfluorinated aromatic compounds have been synthesized by a
saturation-rearomatization method.111-113 As shown in Figure 1.3.5, the perchlorinated precursors
are treated with VF5 or BrF3-SbF5 to form halogenated cycloalkanes, which are then
dechloronated by zinc. Using cobalt trifluoride as the fluorinating agent, the fluorination of
unsubstituted aromatic compounds has also been reported.111
35
Cl Cl Cl Cl
Cl
ClClCl
Cl
Cl BrFCl BrFCl
F
F F
F F
F
F F
F F
BrF3-SbF5
CFCl2CF2Cl
Zn powderdioxane
Cl Cl
Cl
Cl
ClCl
Cl
Cl
F F
F
F
FF
F
F
BrFCl BrFCl
Cl
Cl
Cl
Cl
Cl
Cl
F
F
F
F
F
F
ClF
BrF3-SbF5 Zn
VF5 Zn
C6H3F9, etcKOHCoF3
F
F
F
F
F
F
Figure 1.3.5. Synthesis of perfluorinated aromatic compounds by saturation-aromatization
Perfluorinated compounds can also be obtained by treating perchlorinated counterparts with KF
via a nucleophilic aromatic substitution mechanism (Figure 1.3.6).111-113
Cl Cl Cl Cl
Cl
ClClCl
Cl
Cl
F
F F
F F
F
F F
F F
Cl
Cl
Cl
Cl
Cl
Cl
F
F
F
F
F
FKF
KFSulfolane, 230oC
400oC
Figure 1.3.6. Synthesis of perfluorinated aromatic compounds by nucleophilic aromatic substitution
36
O
F F
F F
F
O F
F
O C
CH3
CH3
OC
CH3
CH3
CH3C
CH3
Figure 1.3.7. Reactions of ortho-position fluorine leading to branching and/or gelation
Due to the large number of fluorine substituents on the phenyl rings, the C-F bonds at the
para-positions are highly activated and the monomer is much more reactive toward the attack of
phenolate ions than other dihalide monomers. Thus, high molecular weight can be achieved at
much lower temperatures (100~120oC) and within shorter reaction times (4~5 h).106, 114, 115
Actually, these polymerizations must be conducted at lower temperatures because the fluorines at
ortho-positions are only slightly less reactive than those at para-positions, which may also
participate in the reaction.109, 116 This undesirable side reaction seems to become even more
pronounced with increasing reaction temperature, and may produce branched or even crosslinked
structures (Figure 1.3.7).116
Decafluorobiphenyl has been polymerized with various bisphenols routinely in DMAc at 120oC
and no gel formation has been reported.106, 114, 115 More reactive monomers such as
decafluorobenzophenone and bis(pentafluorophenyl)sulfone, however, are thought to have
poorer selectivity and have been reported to produce insoluble microgels when polymerized
under similar conditions.108, 117, 118 Zhou and coworkers109, 117 developed mild reaction conditions
37
to polymerize decafluorobenzophenone and bis(pentafluorophenyl)sulfone linearly with 6F-BPA,
where molecular sieves were used during the vapor phase to dehydrate the system.
Polymerizations were conducted at lower temperatures and were completed within 1h.
Kim et al. reported the direct copolymerization of 2,8-dihydroxynaphthalene-6-sulfonated
sodium salt (2,8-DHNS-6) with 6F-BPA and decafluorobiphenyl to synthesize sulfonated random
copolymers for use as PEMs (Figure 1.3.8).104 The degree of sulfonation ranged from 20 to 80
mol%. Since the copolymers had only one sulfonic acid group per repeat unit, they showed low
water uptake and proton conductivity compared to Nafion 117. High molecular weight
copolymers with higher degrees of sulfonation could not be obtained due to the restricted
mobility of the bulky, rigid DHNS monomer.
+F
F F
F F
F
F F
F F
CHO OH
CF3
CF3
O
F F
F F
F F
F F
O
F F
F F
CO O
F F
F F
CF3
CF3
NaSO3
m n
OH
OH
NaSO3
+
K2CO3DMSO
Figure 1.3.8. Direct synthesis of sulfonated perfluorinated poly(arylene ether) random copolymers
Due to limited availability of sulfonated bisphenols, sulfonated copolymers containing
perfluorophenylene units have been mostly synthesized by post-sulfonation methods. Lee et al.114
studied the sulfonation of two series of highly fluorinated poly(arylene ether)s by fuming sulfuric
38
acid (Figure 1.3.9). The IEC values were controlled by varying the amount of sulfonic acid
and/or the reaction time. The copolymer membranes showed higher water uptake and lower
conductivity than Nafion 117 at similar IEC values. Nevertheless, while the more highly
fluorinated series, i.e. copolymers containing hexafluoroisopropylidene units (Figure 1.3.9, a),
displayed much lower water uptake than the relatively lower fluorinated series (Figure 1.3.9, b),
they also achieved comparable proton conductivity. It is likely that the electron-negative
hexafluoroisopropylidene groups both increased the hydrophobicity and enhanced the acidity of
the sulfonic acid groups.
F F
F F
CO O
F F
F F
CF3
CF3
F F
F F
CO O
F F
F F
CF3
CF3
F F
F F
CO O
F F
F F
CH3
CH3
F F
F F
CO O
F F
F F
CH3
CH3
Fuming H2SO4
Chloroform
Fuming H2SO4
Chloroform(a)
(b)
n
n
n
n
SO3H SO3H
SO3HSO3H
Figure 1.3.9. Post sulfonation of fluorinated poly(arylene ether)s
Some bulky bisphenol monomers, although rigid, have also been used for the synthesis of high
molecular weight fluorinated copolymers. The good solubility of these polymers may be due to
the increased inter-chain spacing, which suppresses crystallinity. Hay et al.102 synthesized
unsulfonated fluorinated random copolymers (Figures 1.3.10 and 1.3.11). The bisphenols were
isocynate-masked to improve the monomers’ solubility in the reaction medium. The copolymers
were then sulfonated using chlorosulfonic acid to yield the sulfonated materials. Because the
other possible sulfonation locations were either sterically hindered or masked by methyl groups,
39
it was proposed that the sulfonation only occurred at positions para to the pendant phenyl rings
and thus was quantitative. This selective sulfonation technique may be superior to conventional
direct copolymerization methods because it yields random copolymer ionomers, while enabling
the degree of sulfonation to be easily controlled by the molar feed ratio of the comonomers.
PrNHCOO OCONHPr SPrNHCOO OCONHPr
O
O
F
F F
F F
F
F F
F F
+ +
SO O
O
O
O O
F F
F F
F F
F F
F F
F F
F F
F Fm n
SO O
O
O
O O
F F
F F
F F
F F
F F
F F
F F
F F
SO3HHO3S
SO3HHO3S
m n
K2CO3DMAc
1. ClSO3H, CH2Cl22. KOH, DMSO3. HCl
Figure 1.3.10. Synthesis and sulfonation of fluorinated poly(arylene ether sulfone) random copolymers
40
PrNHCOO OCONHPr PrNHCOO OCONHPr
O OO O
F F
F F
F F
F F
F F
F F
F F
F Fm n
O OO O
F F
F F
F F
F F
F F
F F
F F
F F
SO3H SO3H
SO3H
m n
F
F F
F F
F
F F
F F
1. ClSO3H, CH2Cl22. KOH, DMSO3. HCl
K2CO3DMAc
+ +
Figure 1.3.11. Synthesis and sulfonation of fluorinated poly(arylene ether sulfone) random copolymers
1.4. Hydrophilic-hydrophobic block copolymer systems as PEMs
The proton conductivity of PEMs based on sulfonated random copolymers has been shown to be
strongly dependent on humidity. Although the conductivity of BPSH and other random
copolymer membranes under fully hydrated conditions (i.e. in liquid water) is comparable to that
of Nafion®, it tends to decrease markedly as the level of hydration decreases.26 This trend may be
because a substantial number of water molecules are required to establish connectivity among
the randomly distributed sulfonic acid groups. Conversely, Nafion® is a modestly crystalline
copolymer which may be above its Tg at room temperature. It has also been reported to show a
nanophase separated morphology in the dry state, featuring ionic clusters that are interconnected
by narrow ionic channels.4 The challenge, therefore, lies in developing alternative PEMs which
41
feature associated ionic domains at even low hydration levels. Nanophase separated
hydrophilic/hydrophobic block copolymer ionomers are desirable for this purpose. In contrast to
what occurs in random copolymers, in block copolymer membranes the ionic groups are
selectively incorporated into one or more blocks and may exist in ordered sequences. Continuous
proton conducting channels may thus be formed even at low hydration levels. It is postulated that
high proton conductivity can therefore be sustained under partially hydrated conditions.
1.4.1. Block copolymer PEMs based on sulfonated styrenics and hydrogenated polydienes
Polystyrene-based triblock copolymer PEMs have been developed from commercially available
Kraton G as low-cost alternatives to Nafion®, and some of them have been commercialized. The
most widely studied materials are sulfonated styrene-ethylene/butylenes-styrene (S-SEBS)
membranes developed by Dais Analytics (Figure 1.4.1).24, 119 The unsulfonated precursor, SEBS,
is a well-known commercial polymer, which has been synthesized by the sequential anionic
polymerization of styrene and butadiene, followed by the hydrogenation of the unsaturated
double bonds resulting from both the 1,2- and 1,4-addition of butadiene.11 The polystyrene
blocks are partially sulfonated to produce S-SEBS. The properties of a Dais S-SEBS PEM with
18% polystyrene and 55 mol% sulfonation of the PS blocks were studied by Wnek et al.24, 119
The membrane showed a proton conductivity of 0.085 S/cm when fully hydrated, as compared to
0.079 S/cm for Nafion 117. Its wt% water absorption (water uptake), however, was four times
higher than that of Nafion. X-ray and neutron scattering studies of the wet membrane indicated
co-continuous hydrophobic and hydrophilic domains.
42
In addition to commercially available S-SEBS membranes, several research groups have also
synthesized S-SEBS samples via the sulfonation of commercially-available SEBS block
copolymers that contain about 20~30 mol% styrene.120-124 The sulfonation reagents used include
chlorosulfonic acid, acetyl sulfate, etc., and membranes with up to 47 mol% sulfonation of the
PS block were studied. Generally, both the liquid water proton conductivity and water uptake
were found to increase with the increased level of sulfonation for these ionomer membranes.
CH2 CH CH2 CH CH2 CH2 CH2 CH
CH2
CH3
CH2 CH CH2 CH
SO3H SO3H
m n m
Figure 1.4.1. Structure of S-SEBS block copolymers
Similar to Nafion, the S-SEBS membranes showed high methanol permeability, which makes
them undesirable for DMFC applications. Won et al.125 studied S-SEBS membranes that were
modified by the addition of a thin layer, which was introduced to the top of the membrane by
plasma treatment in the presence of maleic anhydride (Figure 1.4.2). This modification resulted
in a decrease in methanol crossover, as well as an undesirable loss of proton conductivity.
However, hydrolysis of the anhydride layer produced carboxylic acid groups which partially
recovered the diminished proton conductivity.
43
Figure 1.4.2. Modification of S-SEBS surface with plasma treatment followed by hydrolysis
Another important type of styrene-based block ionomers for PEMs is sulfonated
poly(styrene-isobutylene-styrene) (S-SIBS) (Figure 1.4.3).10, 12, 25, 126 These copolymers possess a
completely saturated mid-block, which thus eliminates the need for post-polymerization
hydrogenation. Like S-SEBS membranes, they are obtained via the partial sulfonation of the
polystyrene blocks of commercially-available SIBS triblock copolymers. It is not clear how the
commercial precursors are synthesized, but some authors have reported the synthesis of SIBS
copolymers by living cationic polymerization at -80°C using a
1,3-di(2-chloro-2-propyl)-5-tert-butylbenzene (t-Bu-DCC)-TiCl4 initiating system.127
44
CH2 CH CH2 CH CH2 C
CH3
CH3
SO3H
CH2 CH CH2 CH
SO3H
m n m
Figure 1.4.3. Structure of S-SIBS block copolymers
Elabd et al.12, 25 studied the electrochemical properties of S-SIBS membranes with IECs of up to
1.0. Sulfonation of the block copolymer was achieved using acetyl sulfate in methyl chloride,
after which proton conductivity and methanol permeability were measured and compared to
those of Nafion 117. Interestingly, very different property characteristics were observed in the
plane of the membrane as compared to normal to the plane. Specifically, the selectivities of the
membranes (i.e. proton conductivity/methanol permeability) were higher than those of Nafion
117 at both high and low temperatures when conductivity was measured in the plane. When it
was measured normal to the plane, however, the selectivities were much lower, leading to much
poorer DMFC performance than Nafion. This difference was explained in terms of a lamellar
morphology with a preferred orientation in the plane, as revealed by small angle X-ray scattering
(SAXS) studies. Elabd el al. also synthesized S-SIBS copolymers with high levels of sulfonation
(up to 82 mol% sulfonation and 2.04 IEC).10 These highly sulfonated membranes were found to
be elastic hydrogels that swelled dramatically in water.
Shi et al.128, 129 described the synthesis of partially fluorinated styrenic block ionomer membranes
where the hydrophobic blocks were the random copolymers of vinylidene difluoride (VDF) and
45
hexafluoropropylene (HFP). The block copolymers were prepared via the atom transfer radical
polymerization of styrene initiated by Cl-terminal poly(VDF-co-HFP) macroinitiators, which
were synthesized using an emulsion polymerization of VDF and HFP in the presence of CHCl3
(Figure 1.4.4). The copolymers were partially sulfonated by acetyl sulfate to generate ionomers
bearing up to ~50 mol% sulfonation. The membranes showed comparable proton conductivity
and water uptake to those of S-SEBS PEMs. The conductivity increased with the degree of
sulfonation (DS) of the PS block, but reached a maximum at a DS of 40 mol%. Although the
membrane with 49 mol% DS had 388% water uptake, its conductivity similar to the membrane
with 40 mol% DS. This was likely the result of the percolation limit being exceeded and the
membrane becoming a hydrogel. A similar trend (maximum conductivity at a certain IEC) has
been observed for other PEMs.
CF2 CH2 CF2 CF CF3+CHCl3
chain transferpolymerization
H CH2 CF2 CF2 CF
CF3
CCl3x yyx
styreneCuCl/bpy
H CH2 CF2 CF2 CF
CF3
CCl2 CH2 CH Clyx
H CH2 CF2 CF2 CF
CF3
CH2 CH CH2 CH
SO3H
yx
CH3COOSO3H
Figure 1.4.4. Synthesis of partially sulfonated (PVDF-ran-PHFP)-b-PS copolymers
46
1.4.2. Multiblock copolymers synthesized by step or condensation polymerization
Unlike aliphatic block copolymers based on styrenic and other vinyl monomers, the synthesis of
block copolymers having thermally stable aromatic moieties cannot be achieved by chain-growth
living copolymerization. Instead, telechelic oligomers with appropriate end groups and molecular
weights can be copolymerized by step-growth condensation polymerization to form multiblock,
or segmented, copolymers. The wholly aromatic oligomers themselves also have to be prepared
through condensation polymerizations. First, some multiblock copolymers containing aliphatic
blocks will be reviewed.
1.4.2.1. Partially aromatic multiblock copolymers
Zhang et al.130, 131 synthesized a series of multiblock copolymers containing poly(arylene ether
sulfone) (PAES) and partially sulfonated polybutadiene (PB) blocks. As shown in Figure 1.4.5,
the copolymers were obtained by coupling the telechelic PEAS and PB oligomers. Commercially
available carboxyl-terminated polybutadiene was treated with SOCl2 to give the Cl-terminal
oligomer. The PAES oligomers were obtained by the base catalyzed condensation polymerization
of 4,4’-dichlorodiphenyl sulfone (DCDPS) with 4,4’-isopropylidenediphenol (Bisphenol-A),
which had previously been reported by McGrath et al.18 The products were then reacted with
aminophenol to afford the amino-terminated oligomers (PAES-NH2). A linkage was established
between the two blocks by reacting the endgroups. The PB block was then partially sulfonated
by acetyl sulfate. The flexibility of the sulfonated PB blocks was thought to facilitate the
47
aggregation of the sulfonic acid groups into ion-rich channels, whereas the relatively rigid PAES
blocks provided mechanical strength. However, copolymers with only low degrees of sulfonation
(up to 11.5 mol% of PB) were synthesized with ion exchange capacities of only up to 0.62; their
proton conductivity at 25°C was no more than 0.03 S/cm. The water uptake values, on the other
hand, were unacceptably high considering the low IEC and proton conductivity values.
HO C
CH3
CH3
OH Cl S Cl
O
O
NMP/Toluene K2CO3160oC, 4h
190oC, 16h
+
O C OCH3
CH3
S
O
O
S
O
O
Cl Cl
HO NH250oC
O OCH3
CH3
S
O
O
S
O
O
O OH2N NH2n
O OCH3
CH3
S
O
O
S
O
O
O OHN NH
O
O
n m
CH3COOSO3H 75oC
NH
O
O
PAES NH PAES NH
O
O
OH
SO3H
NHm1 m2
n
ClCl
O
O
m
Figure 1.4.5. Synthesis of sulfonated PAES-PB multiblock copolymers
Yang et al.132, 133 investigated sulfonated PAES-PVDF multiblock ionomer membranes (Figure
1.4.6). The а,ω-dibromo PVDF oligomers were prepared via the radical telomerization of
VDF, using di-t-butylperoxide as the initiator and 1,2-dibromotetrafluoroethylene as the telogen.
48
The copolymers were synthesized by coupling hydroxy-terminated PAES oligomers with PVDF
telechelics. Partial sulfonation of the PAES block was carried out using (CH3)3SiSO3Cl. The IEC
of the sulfonated copolymers ranged from 0.78 to 2.18, and both proton conductivity and water
uptake increased with IEC. Partially sulfonated PAES homopolymers were also synthesized and
compared with the block copolymer PEMs. At low IEC, the latter were found to have higher
proton conductivity than the homopolymers. Because the degree of sulfonation of the polymers
was not clearly defined, and no information was available with respect to the block length or
weight ratio of the block copolymers, it was difficult to determine proton conductivity. Moreover,
the block copolymers displayed low molecular weight and it was unclear whether or not tough
membranes had been obtained.
HO C
CH3
CH3
OH Cl S Cl
O
O
DMAc/Toluene K2CO3 160oC, 16h
+
O S OO
OC C
CH3
CH3
HO OHCH3
CH3
Br CF2 CF2 CH2 CF2 Brm
m
NaH, RT 6h
O S OO
OC C
CH3
CH3
O OCH3
CH3
CF2 CF2 CH2 CF2n
1. (CH3)3SiSO3Cl2. CH3ONa3. HCl
mO S O
O
OC C
CH3
CH3
O OCH3
CH3
CF2 CF2 CH2 CF2n
SO3H
n
Figure 1.4.6. Synthesis of sulfonated PAES-PVDF multiblock copolymers
49
The major disadvantage of proton exchange membranes based on wholly aliphatic or partially
aliphatic backbones is their poor oxidative stability compared to wholly aromatic systems. This
makes them useful only in portable devices at low temperatures and low power densities. To
improve the thermal oxidative stability of their PAES-PB multiblock copolymers, Zhang et al.134
modified the membranes via the epoxidation of the unsulfonated butadiene units (Figure 1.4.7),
which was conducted using a mixture of formic acid and hydrogen peroxide. The modified
copolymer membranes showed only slight improvements in thermal stability, as suggested by the
higher degradation temperature as shown by thermal gravimetric analysis. The proton
conductivity of the membranes was more or less unaffected by the epoxidation. However, the
modification increased both water uptake and the swelling ratio in water.
NH
O
O
PAES NH PAES NH
O
O
OH
SO3H
NHm1 m2
NH
O
O
PAES NH PAES NH
O
O
OH
SO3H
NHO m1 m2
HCOOH
Figure 1.4.7. Epoxidation of sulfonated PAES-PB multiblock copolymers
50
1.4.2.2. Wholly aromatic multiblock copolymers
For the past few years there has been growing interest in developing block copolymer ionomers
having thermally and chemically stable aromatic backbones. As mentioned above, such
multiblock copolymers can be synthesized via the step growth polymerizations of hydrophobic
and hydrophilic telechelic oligomers. The copolymerization between the oligomers takes place
by coupling the end groups via a nucleophilic aromatic substitution mechanism, which is usually
carried out in polar solvents such as N-methyl-2-pyrrolidinone (NMP) and
N,N-dimethylacetamide (DMAc) in the presence of a salt.
Ghassemi et al.18 synthesized multiblock ionomers composed of poly(arylene ether sulfone) and
substituted poly(p-phenylene) blocks. As shown in Figure 1.4.8,
poly(4’-phenyl-2,5-benzophenone) oligomers, precursors for the hydrophilic blocks, were
prepared by a Ni(0) catalytic coupling of 2,5-dichloro-4’-phenylbenzophenone.
4-chloro-4’-fluorobenzophenone, which can participate in the reaction only on the chlorine end,
was used as the end-capper to afford fluorine-terminated telechelic oligomers. These oligomers
were sulfonated by H2SO4 to introduce one sulfonic acid group to each repeat unit. The
sulfonated oligomers were then coupled with phenoxide-terminated PAES oligomers to form
multiblock copolymers.
51
O
Cl Cl
O
Cl F+NiCl2, Zn, PPh3, bpy
NMP, 80oC
O
F
O
F
O
nn
O
F
O
F
O
n
SO3-Na+
H2SO4 50oC
NaCl
O O S O O
O
O m +
HO OH Cl S Cl
O
O+
DMActoluene K2CO3
DMActoluene K2CO3
O O
O O S O O
O
O
O
n m
SO3-Na+
hydrophobic oligomer hydrophilic oligomer
Figure 1.4.8. Synthesis of sulfonated PPP-PAES multiblock copolymers
The copolymers have been reported to be capable of forming strong and flexible membranes.
Our GPC analysis, however, showed that they had relatively low molecular weights (Mn<17,300
g/mol). The potassium salt forms of the copolymer membranes were converted to their acid
forms through treatment with H2SO4 at room temperature. Although their IEC values were
comparable to that of Nafion 117, these multiblock systems showed lower proton conductivity at
room temperature in liquid water.
As mentioned earlier, introducing sulfonic acid groups by post-sulfonation has some
disadvantages. These include unwanted side reactions, poor control in the reaction sites, and/or
the degree of sulfonation in the product. In contrast, direct copolymerizations of sulfonated
52
monomers offer a high quantitative degree of sulfonation, precise control of the sulfonation site,
and favorable ion exchange capacity. In a study by Taeger et al.,135 sulfonated poly(arylene ether
sulfone) telechelics, synthesized from the direct polymerization of hydroquinone 2-potassium
sulfate with bis(4-fluorophenyl)sulfone (DFDPS), were used as hydrophilic oligomers for the
preparation of multiblock copolymers for use as PEMs (Figure 1.4.9). The proton conductivity
and water uptake of these copolymer membranes were both higher than what Ghassemi et al.
obtained in their studies, i.e. they exhibited low proton conductivity at IECs (weight-based)
similar to Nafion. To compare the properties of block copolymers with those of random
copolymers with the same composition, random copolymers were also prepared from the
copolymerizations of DFDPS with sulfonated and unsulfonated hydroquinone. No significant
differences in either water uptake or proton conductivity were observed between the random and
multiblock copolymers having the same IEC. However, the multiblock copolymers showed
higher proton diffusion coefficients than the random copolymers with comparable IECs.
Therefore, the existence of a particular block sequence length might have played a role in the
establishment of nano-sized, ion-rich channels. Unfortunately, no further characterization studies,
which might have included assessing proton conductivity measurements at reduced hydration
levels, were undertaken.
53
F S O O S F
O
O
O
O
SO3-K+
nO O S O O
O
O m+
DMAcK2CO3
50oC
mS O O S
O
O
O
O
SO3-K+
O O S O O
O
On
hydrophilic oligomer hydrophobic oligomer
Figure 1.4.9. Synthesis of sulfonated poly(arylene ether sulfone) multiblock copolymers
The synthesis and characterization of a series of poly(arylene ether ketone) multiblock
copolymers were carried out by Shin et al.136 As shown in Figure 1.4.10,
1,3-bis-(4-fluorobenzoyl)benzene was polymerized with bisphenol-A (in excess) to form the
phenoxide-terminated hydrophobic oligomers. The hydrophilic oligomer can be prepared by the
direct polymerization of hydroquinone 2-potassium with 4,4’-difluorobenzophenone. However,
because a common solvent for both oligomers could not be found, the multiblock
copolymerizations were carried out in such a way that the hydrophilic blocks were generated
in-situ. Thus, it is questionable whether long sequences of sulfonated units had really been
formed in the copolymer.
n
180oC
C C
OO
F F
CH3
CH3
HO OH+
NMP/benzeneK2CO3
CH3
CH3
O O C C
OO CH3
CH3
O O + C
O
F F HO OH
SO3-K+
+
180oCDMSO/benzeneK2CO3
mn
CH3
CH3
O O C C
OO CH3
CH3
O O C
O
O
SO3-K+
C
O
O
hydrophobic oligomer
monomers used to produce hydrophilic oligomers in-situ
Figure 1.4.10. Synthesis of sulfonated poly(arylene ether ketone) multiblock copolymers
54
To improve the copolymers’ solubility in the film-casting solvent, they were treated with sulfonyl
chloride to convert the sulfonic salts into the SO2Cl form. The cast membranes were hydrolyzed
in a NaOH solution and then acidified with 1N HCl to regenerate the sulfonic acid groups. One
of these block copolymers displayed high conductivity and low water uptake (0.081 S/cm and
15%, respectively) at room temperature. However, the data presented in such a way that no clear
trend could be observed in the molecular weight of the hydrophobic and hydrophilic blocks,
which seemed to have been randomly selected. Further systematic studies on the influence of
block length was not performed for this study.
A similar approach was taken by Na et al. in the synthesis of multiblock copolymers containing
sulfonated poly(ether ketone) blocks (Figure 1.4.11). Here, the 3,3’-5,5’-
tetramethyl-4,4’-biphenol monomer was presumably used to suppress the crystallinity of the
poly(ether ketone) oligomers and copolymers. Again, the hydrophilic oligomers were formed in
situ, which led to poor control of the copolymers’ structure and composition. The resulting
copolymers had lower intrinsic viscosities than the corresponding random copolymers, and the
IEC decreased with increasing block length. Nevertheless, the proton conductivity and water
uptake values, which both increased as a function of the targeted block length, do suggest the
existence of sulfonated blocks, although no other proof was manifest. Due to their low IEC,
however, the membranes generally displayed lower proton conductivity compared to Nafion.
55
+
DMSO, K2CO3, 170oC
C
O
OF O C
O
F
H3C
H3C
CH3
CH3n
F C
O
F
SO3Na
NaO3S
HO OH
H3C
H3C
CH3
CH3
C
O
O O
H3C
H3C
CH3
CH3
b C
O
O
H3C C 3
H3C C 3
O
SO3Na
NaO3S
H
Hn m
Figure 1.4.11. Synthesis of sulfonated poly(arylene ether ketone) multiblock copolymers.
The McGrath research group has recently made significant progress in developing multiblock
copolymer membranes that feature our so-called “BPS-100 hydrophilic blocks,” i.e.
biphenol-based 100% disulfonated poly(arylene ether sulfone)s.17, 26, 99, 137-143 These BPS-100
telechelic oligomers have been synthesized by the polymerization between
3,3’-disulfonated-4,4’-dichlorodiphenylsulfone (SDCDPS) and 4,4’-biphenol (BP). As
mentioned earlier, such direct polymerization of sulfonated monomers enables precise and
consistent control of the degree of sulfonation. Furthermore, the use of disulfonated monomers
ensures that each repeat unit contains two sulfonic acid groups. In comparison to Taeger and
Shin’s work,135, 136 where only one ionic group was introduced to each repeat unit, this increased
population of proton conducting sites in the hydrophilic block may be superior in terms of the
56
formation of proton conducting channels. Both stoichiometry imbalance (one monomer in excess)
and end-capper chemistry have been used by McGrath et al. to prepare oligomers with the
desired molecular weights and endgroups (Figure 1.4.12).
HO OH Cl S Cl
SO3Na
NaO3S
O
O+
O O S O O
O
O
SO3M
MO3S
n
biphenol SDCDPS
(a)
Cl S O O
O
O
SO3M
MO3S
S Cl
SO3Na
NaO3S
O
O n(b)
nO S O O
O
O
SO3M
MO3S
S O
SO3Na
NaO3S
O
OH2N NH2 (c)
excessbiphenol
excessSDCDPS
HO NH2
end-capping
Figure 1.4.12. Synthesis of (a) phenoxide-teminal; (b) Cl-terminal; (c) NH2-endcapped BPS-100
oligomers
The McGrath Group has also attempted to synthesize hydrophobic/hydrophilic multiblock
copolymers by copolymerizing these oligomers with various hydrophobic telechelic groups.
Wang et al.99, 138 prepared substituted poly(p-phenylene)s, i.e. poly(4’-phenyl-2,5-benzophenone)
oligomers with fluorine terminal groups and varying molecular weights, and coupled them with
phenoxide-terminated BPS-100 oligomers (Figure 1.4.12, a) in an attempt to synthesize
multiblock copolymers (Figure 1.4.13). Using a procedure similar to that reported by Ghassemi
et al., substituted poly(p-phenylene) oligomers were obtained by the Ni(0) catalytic coupling of
2,5-dichloro-4’-phenylbenzophenone. 4-Chloro-4’-fluorobenzophenone was monofunctional in
57
this reaction and therefore was used as the end-capper to control the molecular weight and
endgroups. However, block copolymerization between the hydrophobic and hydrophilic
oligomers could not be achieved in a consistently reproducible way. The copolymer membranes
were often brittle and had poor mechanical strength. We attributed this problem to our inability
to control the stoichiometry, which might have originated from the difficulty in obtaining
quantitatively end-capped poly(4’-phenyl-2,5-benzophenone) oligomers. Another possible cause
might be that although the copolymers were only partially composed of poly(p-phenylene)
segments, their rigid, rod-like components dominated the copolymer structure, resulting in the
poor flexibility of the membranes.
C
O
F C
O
Fm
O
O O S O O
O
O
SO3M
MO3S
n
DMAc/TolueneK2CO3
165oC
+
C
OC
O
O O S O O
O
O
SO3M
MO3S
m
O
n
Figure 1.4.13. Synthesis of PPP-BPSH100 multiblock copolymers
In a study by Lee et al.,142-144 amine-terminated BPS-100 telechelics (Figure 1.4.12,c) were
copolymerized with polyimide oligomers to make multiblock copolymers. As shown in Figure
1.4.14, the NDA-terminated polyimide oligomers were prepared by the polymerization of
58
1,4,5,8-naphthalene tetracarboxylic dianhydride (NDA) with
bis[4-(3-aminophenoxy)phenyl]sulfone] (DADPS) in m-cresol. Because these hydrophobic
oligomers were insoluble in NMP, the block copolymerizations were also carried out in m-cresol.
However, only low conversion was observed, resulting in copolymer membranes with low
proton conductivity, high water uptake and poor mechanical strength. Based on these results, we
concluded that m-cresol was not a good solvent for the BPS-100 oligomers. Indeed,
copolymerizations using an m-cresol/NMP solvent mix yielded multiblock copolymers with
much higher molecular weight. These copolymer membranes displayed high proton conductivity
and atomic force micrographs showed an orientated nanophase separated morphology.142, 143, 145
180oC
O O
O O
O O
O S O
O
O
H2N NH2+
m-cresol
nO S O O
O
O
SO3M
MO3S
S O
SO3Na
NaO3S
O
OH2N NH2
O S O
O
O
N
O O
O O
NO
O O
O O
N O S O
O
O
N O
O O
O Om
180oCm-cresol +
Polyimide b BPS-100 multiblock copolymer
NDADADPS
Polyimide oligomer
BPS-100 oligomer
Figure 1.4.14. Synthesis of polyimide-BPS100 multiblock copolymers
Multiblock copolymers composed of perfluoro poly(arylene ether) blocks and
fluorine-terminated hydrophobic blocks were synthesized by Ghassemi et al.146 The hydrophobic
perfluoro oligomers were synthesized by the polymerization of decafluorobiphenyl and 6F-BPA
(Figure 1.4.15). Due to the strong electron-withdrawing effect imposed by the fluorine
59
substituents, the C-F bonds at the 4,4’- positions were very electron-deficient and highly
activated toward nucleophilic aromatic substitution. This allowed the copolymerization to be
carried out at much lower temperatures compared to those used for the less activated dihalide
monomers such as DCDPS (~100°C vs. ~180°C). The block copolymerization of perfluoro
oligomers and hydrophilic oligomers was also conducted at a similar temperature range.
However, our initial attempts to synthesize multiblock copolymers yielded high-viscosity,
gel-like mixtures, and the water uptake values of these membranes were unacceptably high. This
problem might have been due to the fact that perfluoro oligomers have reduced solubility in
DMSO, making it a poor solvent for block copolymers as well. Also, as mentioned earlier, the
fluorine groups at the ortho- positions may have participated in the copolymerization, leading to
branched or even crosslinked structures.
F
F F
F F
F
F
F
FF HO OH
CF3
CF3
DMAc / benzeneK2CO3
120oC
F
F F
F F
F
F
F
F O O
CF3
CF3
F
F F
F F
F
F
F
Fm
+
O O S O O
O
O
SO3M
MO3S
n
n
O S O O
O
O
SO3M
MO3S
O
F
F F
F F
F
F
F
O O
CF3
CF3
F
F F
F F
F
F
Fm
DMSO / benzeneK2CO3
110oC
Figure 1.4.15. Synthesis of Perfluoroarylene ether-BPS100 multiblock copolymers
60
Using slightly modified procedures, in which the block copolymerizations were conducted in
NMP instead of DMSO and at lower reaction temperatures (95~110°C instead of 120°C), we
successfully synthesized multiblock copolymer membranes with block lengths up to 5 kg/mol.137
While the proton conductivity of the membranes was comparable to that of Nafion®, the water
uptake values were generally still higher than 100%.
Based on Ghassemi’s work, Yu et al.26, 139 reported the synthesis and characterization of a series
of BisAF-BPS100 copolymers for use as PEMs, as shown in Figure 1.4.16. The only difference
in the structures is that Bisphenol-A was used in the place of 6F-BPA. With the hydrophobic
block still heavily fluorinated, the interaction parameter, Х, between the hydrophobic and
hydrophilic blocks was still high enough to promote nanophase separation.
n
O S O O
O
O
SO3M
MO3S
O
F
F F
F F
F
F
F
O O
CH3
CH3
F
F F
F F
F
F
Fm
Figure 1.4.16. BisAF-BPS100 multiblock copolymers
A number of properties for the copolymers with block lengths ranging from 3 to 8 kg/mol are
listed in Table 1.1. In general, the copolymers displayed high proton conductivity in liquid water.
Moreover, they showed much lower water uptake than the fluorinated multiblock copolymers
synthesized by Ghassemi et al., even though they possessed lower fluorine contents per repeat
unit. Conductivity measurements were also carried out under partially hydrated conditions and
the results will be discussed later.
61
Table 1.1. Properties of BisAF-BPSH100 multiblock copolymers
Polymer Block Lengthsa
(g/mol)
Molar
Feed
Ratio
IECc
Water
Uptake
(mass %)
Proton
Conductivityd
(mS/cm)
BisAF-BPSH-1 3000:3000 1 : 0.93 1.6 71 130
BisAF-BPSH-2 5000:5000 1 : 0.82 1.4 58 104
BisAF-BPSH-3 8000:8000 1 : 0.70 1.1 42 90
a: Block lengths are expressed in the form hydrophobic:hydrophilic
b: The hydrophobic:hydrophilic molar charge ratio in the reaction
c: Measured from 1H NMR
d: Measured at 30oC in liquid water
Due to the high cost of the perfluorinated monomer, decafluorobiphenyl, it would be practical to
reduce its use in multiblock copolymer synthesis. In recent studies, Lee et al.147 used
decafluorobiphenyl as an end-capping agent to produce fluoro-terminal hydrophobic
poly(arylene ether sulfone) oligomers, as opposed to polymerizing it with a bisphenol monomer
(Figure 1.4.17). A three-fold excess of decafluorobiphenyl was used to ensure the quantitative
end-capping of the oligomers. These end-capped oligomers, bearing the same endgroups as the
highly fluorinated oligomers, were equally reactive and readily polymerized with BPS-100
oligomers to obtain the largely hydrocarbon-based, high molecular weight multiblock
copolymers.
62
MO O S O OM
O
On
F
F F
F F
F
F
F
FF (excess)
O O S O O
O
O
F
F F
F F
F
F
F
F
F
F F
F F
F
F
F
Fn
+
Figure 1.4.17. Synthesis of fluorine-terminal hydrophobic oligomers
It is worth mentioning that at high temperatures and in the presence of salts such as K2CO3,
ether-ether interchange reactions can occur and, in the synthesis of multiblock copolymers, may
randomize the block sequences. Nevertheless, in ways that aren’t entirely clear to us, such side
reactions may have been minimized or avoided, as documented in studies by Wang, Lee and
Ghassemi. 137, 138, 142 We surmise that either low polymerization temperatures were used, or the
polymerizations took place via a mechanism that ruled out the possibility of an ether-ether
interchange. Conversely, if both the hydrophobic and hydrophilic oligomers had low-reactivity
chain-ends, then coupling reactions via nucleophilic aromatic substitution to synthesize block
copolymers may have been difficult (Figure 1.4.18).
63
MO O S O OM
O
O
SO3M
MO3S
Cl S O O
O
OS C
O
O
l
+
BPS-00 BPS-100
x
Sulfonated-unsulfonatedpoly(arylene ether sulfone) multiblock copolymer
m
n
randomization?
Figure 1.4.18. Questionable synthesis of poly(arylene ether sulfone) multiblock copolymers
Poly(arylene ether ketone)s or poly(arylene ether phosphine oxide)s, however, are thought to be
less susceptible to ether-ether interchanges. Li et al. reported some preliminary studies of
copolymers containing poly(arylene ether ketone) hydrophobic blocks (Figure 1.4.19).141 A
multiblock copolymer with 4K:4K block lengths was obtained, and its block sequences were
evidenced by 13C NMR. However, higher block length copolymers were not reported,
presumably due to the difficulties in achieving high conversion in the coupling reactions between
the less reactive hydrophilic and hydrophobic oligomers.
64
MO O S O OM
O
O
SO3M
MO3S
+
m
C
O
O C
CF3
CF3
O S O O
O
O
SO3M
MO3S
NMP, 180-190oC
m nx
C
O
O C
CF3
CF3
OF C
O
Fn
Figure 1.4.19. Synthesis of sulfonated PAES-PAEK multiblock copolymers
1.4.3. Comparisons between random and block copolymer PEMs
As mentioned earlier, in contrast to random copolymer ionomers—where ionic groups are
randomly distributed along the polymer chain—block ionomers contain highly ionic sequences
which may aggregate to form ion-rich channels, thereby facilitating the transport of protons. This
is particularly important when the level of hydration is low. For random copolymers, at a certain
temperature the diffusion coefficient of protons (D) should only be related to the level of
hydration. For block ionomers, however, the sequence length may also play a role. Although
only a few studies have explored this field, the advantages of using block copolymer membranes
as PEMs have been demonstrated by comparing the diffusion coefficient and/or proton
conductivity of block ionomer membranes to those of random systems under partially hydrated
conditions.
65
Serpico et al.119 studied proton conductivity under partially hydrated conditions of several types
of proton exchange membranes, including Nafion 117, a Dais Analytic S-SEBS block copolymer,
and a sulfonated styrene-ethylene “pseudo-random” copolymer (S-SE), also developed by Dais
Analytic (Figure.1.4.20, a). On the one hand, ethylene-styrene copolymer membranes were
synthesized by metallocene-catalyzed polymerizations in such a way that no two styrene repeat
units were adjacent to each other. The sulfonated forms, therefore, had every sulfonated styrene
unit separated from each other by at least one ethylene repeat unit. On the other hand, in the
S-SEBS copolymers the styrene units exist within the blocks. Therefore, many of the sulfonated
styrene units should be adjacent to each other. Figure 1.4.20 (b) shows the plots of proton
conductivity vs. water content (humidity) for the S-SEBS, S-SE, and Nafion 117 membranes. All
three PEMs displayed similar proton conductivity at high levels of hydration. With the decrease
in water content, however, the S-SE membrane showed a much more rapid decrease compared to
the Nafion 117. This suggests that the proton conducting channels in the S-SE membranes were
poorly developed (in contrast to Nafion 117) and that protons could transport readily through the
membrane only at high hydration levels. The dependence of proton conductivity on humidity as
observed for the S-SEBS membranes was stronger than that of Nafion 117, but lower than
observed for the S-SE membranes. Therefore, the ion-rich channels may be more established in
block copolymers than in the “pseudo-random” copolymers due to the difference in the sequence
length of the sulfonated units.
66
(a) SO3H S 3HO
x x
(b)
Figure 1.4.20. (a) structure of S-SE “pseudo-random” copolymers. (b) proton conductivity vs. water
content for S-SEBS, S-SE and Nafion PEMs
Roy et al.26 investigated the transport properties of highly fluorinated block copolymers
synthesized by Yu and coworkers. Because no highly fluorinated random copolymers were
available for comparison, two series of poly(ether sulfone) and poly(ether ketone) random
copolymer membranes were studied. Their structures are shown in Figure 1.4.21.
1-xxO O S
O
O
SO3H
HO3S
O O S
O
O
1-xxCF3
CF3
O O C
SO3H
HO3S
OCF3
CF3
O Y
O
(a)
(b)
Figure 1.4.21. Structures of (a) poly(ether sulfone) and (b) poly(ether ketone) random copolymers
67
The plots of proton conductivity vs. relative humidity (RH) for the random and block
copolymers, as well as Nafion 117, are displayed in Figures 1.4.22 and 1.4.23. As shown in
Figure 1.4.22, the conductivity of both random copolymer membranes is comparable to that of
Nafion 117 at high RH. As expected, with a decrease in RH, both decrease more rapidly as
compared to Nafion. The proton conductivity of the multiblock copolymers (Figure 1.4.23),
however, increased with increasing sequence length. Moreover, the conductivity of the
copolymer with the highest block length (8K:8K) was analogous to that of Nafion 117 at all RH
levels tested—in spite of the fact that the 8K:8K copolymer had the lowest IEC and water uptake
values among all the multiblock copolymers. Similar trends were observed for the multiblock
copolymers containing polyimide hydrophobic blocks, as well as the ones containing
fluoro-end-capped hydrophobic blocks, which were synthesized by Lee and coworkers. These
copolymers, however, displayed lower conductivity at similar IEC and/or water uptake values.
The high performance of the BisAF-BPSH copolymers may be attributed to their high content of
highly hydrophobic fluorinated units, which may have resulted in more distinct nanophase
separation. Although these studies were performed using different copolymers under a variety of
experimental conditions, these results suggest that among all the block copolymers we tested, the
ionomers have the most potential for use as alternative PEMs
68
1.0E-01
1.0E+00
1.0E+01
1.0E+02
1.0E+03
20 30 40 50 60 70 80 90 100Relative Humidity (%)
Prot
on c
ondu
ctiv
ity (m
S/cm
)
1 .Nafion117
2. HQSH 30
3. PB-diketone 50
1
2
3
Figure 1.4.22. Proton conductivity vs. RH plots for Nafion 117, poly(ether sulfone) random copolymers
(HQSH 30), and poly(ether ketone) random copolymers (PB-diketone 50)
Furthermore, the polystyrene blocks in the S-SEBS and other styrene-based block systems were
only partially sulfonated, and the sulfonated poly(arylene ether) block copolymers prepared by
Taeger and Shin135, 136 had only one ionic group in each repeat unit of the hydrophilic blocks. The
BPSH-100-based multiblock copolymers, therefore, may be superior in that their hydrophilic
blocks are fully sulfonated and possess two sulfonic acid groups in each repeat unit. As a result,
ion-rich channels may be formed more easily due to highly concentrated ionic groups, thereby
enhancing proton conductivity, as demonstrated in Figure 1.4.23.
69
1.0E+00
1.0E+01
1.0E+02
1.0E+03
20 40 60 80 100Relative Humidity (%)
Prot
on c
ondu
ctiv
ity (m
S/cm
)
1.BisAF-BPSH(8:8)K
2.Nafion117
3.BisAF-BPSH(5:5)K
4.BisAF-BPSH(3:3)K
1
3 2
4
Figure 1.4.23. Proton conductivity vs. RH plots for Nafion 117 and BisAF-BPSH multiblock
copolymers
F
F F
F F
F
F
F
F O S O
F
F F
F F
F
F
F
FO
O
m
n
O S O OMO
O
SO3M
MO3S
MO
+
m
O S O O
O
O
SO3M
MO3S
O
F
F F
F F
F
F
F
O S O
F
F F
F F
F
F
F
O
O n
x
NMP
K2CO3
100 ~ 110oC
4~5 days
Figure 1.4.24. Synthesis of BisSF-BPSH multiblock copolymers
70
Another variation of the BisAF-BPSH multiblock copolymers are the BisSF-BPSH copolymers,
synthesized by Yu et al. (Figure 1.4.24).140 A similar trend was observed for their proton
conductivity vs. RH performance; i.e., conductivity increased as a function of block length, as
illustrated in Figure 1.4.25.
Figure 1.4.25. Proton conductivity vs.RH plots for BisSF-BPSH multiblock copolymers, Nafion 112 and
BPSH-35 random copolymers
The high performance of multiblock copolymer membranes are believed to have morphological
origins. Figure 1.4.26 shows tapping mode AFM images of partially disulfonated BPSH-xx
random copolymers, with the degree of disulfonation ranging from 30 to 45 mol%. As shown,
the increase in the degree of sulfonation is accompanied by an increase in the size of the
hydrophilic (darker) domains. At high degrees of sulfonation, the hydrophilic regions begin to
display some connectivity. In general, however, all four images show a dispersed morphology,
which is typical of random copolymers.
71
Figure 1.4.26. Tapping mode AFM images of BPSH-xx random copolymer membranes: (a).BPSH-30;
(a) (b)
(c) (d)
500 nm
(a) (b)
(c) (d)
(a) (b)
(c) (d)
500 nm
(b).BPSH-35; (c).BPSH-40; (d).BPSH-45.
(a) (b) (c)
400 nm
(a) (b) (c)
400 nm
Figure 1.4.27. Tapping mode AFM phase images of BPSH-PI multiblock copolymer membranes with
different block lengths: (a).5K:5K; (b).10K:10K; (c).15K:15K.
72
Block copolymers have been reported to show a lamellar or cylindrical-type morphology under
optimized volume fraction and interaction parameters. Most studies, however, have only
investigated copolymers with aliphatic backbones synthesized by living polymerization. Recently,
however, the work by Lee et al. demonstrated that the same morphological features can be
observed in block copolymers with aromatic backbones synthesized via step growth
copolymerization. Figure 1.4.25 shows AFM phase images of BPSH-PI multiblock copolymer
membranes.142, 143 Here, in contrast to the BPSH random copolymers, all images show nanoscale
phase separation. More importantly, as block lengths increased from 5 to 15 Kg/mol, the
hydrophilic regions gradually transformed from dispersed domains to co-continuous channels.
This morphological trend is shown in the performance curves of these membranes under partially
hydrated conditions, wherein proton conductivity increased with increases in block length
(Figure 1.4.28).142, 143
Figure 1.4.28. Proton conductivity vs.RH plots for BPSH-PI multiblock copolymers, Nafion 112 and
BPSH-35 random copolymers
73
.
In summary, although Nafion® is still referred to as the state-of-the-art proton exchange
membrane, new synthetic techniques and a growing understanding of complex structure-property
relationships have resulted in the development of alternative hydrocarbon-based membranes,
which feature optimized combinations of proton conductivity, water sorption, and long-term
durability. Partially sulfonated poly(arylene ether) random copolymers, especially those obtained
through the direct polymerization of sulfonated comonomers, have shown comparable proton
conductivity to Nafion® under fully hydrated conditions, even though their performance tends to
degrade faster than Nafion® as the water content decreases. Although high conductivity at low
humidity, as seen in the block copolymer membranes, is particularly encouraging, the challenge
remains in how to control water uptake and swelling-deswelling without sacrificing performance.
74
Chapter 2. Synthesis and characterization of BisAF-BPSH
hydrophobic-hydrophilic multiblock copolymers
We have synthesized multiblock (segmented) copolymers via coupling reactions of hydrophobic
and hydrophilic telechelic oligomers. Such materials contain fully disulfonated poly(arylene
ether sulfone) (BPSH-100) blocks and highly fluorinated poly(arylene ether) blocks. The
generalized structure of the copolymers studied is shown in Figure 2.1, where X can be an
isopropylidene group, a hexafluoroisopropylidene group, or a sulfone group. Accordingly, the
multiblock copolymers are termed BisAF-BPSH, 6FBisAF-BPSH, and BisSF-BPSH,
respectively. In the following sections, the synthesis and characterization of these materials will
be illustrated in detail and the influence of their structures on resulting properties will be
discussed.
X = CCH3
CH3
CCF3
CF3
SO2X = X = , BisAF-BPSH100 series; , 6FBisAF-BPSH100 series; , BisSF-BPSH100 series
m
O S O O
O
O
SO3H
HO3S
O
F
F F
F F
F
F
F
O X O
F
F F
F F
F
F
Fn
x
(a) (b) (c)
Figure 2.1. Structures of fluorinated-sulfonated, hydrophobic-hydrophilic multiblock copolymers
75
2.1. Experimental
2.1.1.. Solvents
N-Methyl-2-Pyrrolidone (NMP)
N O
CH3 Source: Fisher Scientific Molecular weight: 99.13 g/mol Purification: Dried over CaH2 for 12 h and then distilled at 120 oC under distilled pressure and stored over molecular sieves.
N,N-dimethylacetamide (DMAc)
CH3 C
O
NCH3
CH3
Source: Ficher Scientific Molecular weight: 89.13 g/mol Purification: Dried over CaH2 for 12 h and then distilled at 80 oC under distilled pressure and stored over molecular sieves.
Toluene
CH3
Source: Fisher Scientific Molecular weight: 92.14 g/mol Purification: Used as received.
76
Isopropyl alcohol
CH3CHCH3
OH
Source: Fisher Scientific Molecular weight: 60.01 g/mol Purification: Used as received.
Methanol
CH3OH
Source: Fisher Scientific Molecular weight: 32.04 g/mol Purification: Used as received.
Fuming Sulfuric Acid
H2SO4 SO3
Source: Aldrich Molecular weight: 98.08 g/mol Purification: Used as received.
2.1.2. Monomers
4.4’-Biphenol
HO OH
Source: Molecular weight: 186.21 g/mol Melting point: 282-284 oC Purification: Dried under vacuum at 60 oC for 24 h.
77
4,4-Dichlorodiphenyl Sulfone (DCDPS)
S
O
O
Cl Cl
Source: Solvay Molecular weight: 287.13 g/mol Melting point: 145-147 oC Purification: For sulfonated monomer synthesis, it was used as received.
4,4’-isopropylidenediphenol (Bisphenol A)
C
CH3
CH3
HO OH
Source: Aldrich Molecular weight: 228.29 g/mol Melting point: 152-153 oC Purification: Recrystallized from a 25% (w/v) solution in toluene, and then dried under vacuum at 60 oC for 24 h.
Decafluorobiphenyl
F
F
F
F
F
F F
F
FF
Source: Aldrich Molecular weight: 334.11 g/mol Melting point: Purification: Dried under vacuum at 50 oC for 24 h before use.
78
Potassium Carbonate
K2CO3
Source: Aldrich Molecular weight: 138.21 g/mol Purification: Dried under vacuum at 60oC for 12 h before use.
2.1.3. Monomer Synthesis
3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS)
S
O
O
Cl Cl
NaO3S S 3NaO
To a 100 mL round bottom flask equipped with a magnetic stirrer and a nitrogen inlet was added
30 g of DCDPS and 60 mL of fuming sulfuric acid (30% SO3). The mixture was stirred until
DCDPS dissolved and the solution was reacted at 110 oC for 6 h. The reaction was cooled to
room temperature and added to 500 mL of ice-water. Then NaCl was added to salt out the white
powder-like product, which was filtered and redissolved into 400mL of deionized water. The
solution was treated with 2N NaOH aqueous solution to a pH of 6, and then NaCl was added to
salt out the sulfonated monomer again. The product was filtered and recrystallized twice from
methanol.
Molecular weight: 491.24 g/mol
Purification: Dried under vacuum at 160 oC for 3 days before use.
79
2.1.4: Polymer synthesis
Highly Fluorinated Hydrophobic Oligomers (BisAF)
F
F F
F F
F
F
F
F O C O
F
F F
F F
F
F
F
FCH3
CH3n
The synthesis of a fluorine terminated BisAF oligomer with 3 Kg/mol molecular weight, for
example, was carried out as follows: Bisphenol-A (1.174 g, 5.142 mmol) was added to a
three-necked round bottom flask equipped with a mechanical stirrer, condenser, nitrogen inlet,
and a Dean-Stark trap. DMAc (10 mL) was added and the mixture was stirred until dissolved.
K2CO3 (1.183 g, 7.20 mmol) and toluene (5 mL) were added and the system was dehydrated at
150 oC. Then the reaction bath was cooled to 50 oC and decafluorobiphenyl (2.046 g, 6.124
mmol) was added. The polymerization was allowed to proceed at 110 oC for 5 h. The reaction
mixture was isolated by precipitation into a H2O/methanol (50/50 v/v) mixture. The white
polymer was washed with H2O and methanol, and then dried under vacuum at 80 oC before use.
Fully Disulfonated BPS-100 oligomers
MO O S O OM
O
O
SO3M
MO3Sn
The procedures for the synthesis of 3 Kg/mol phenoxide terminated BPS-100 oligomers are as
follows: Biphenol (0.412 g, 2.213 mmol), SDCDPS (0.912 g, 1.856 mmol), and NMP (10 mL)
80
were charged to a three-necked round bottom flask. The mixture was stirred until dissolved, then
K2CO3 (0.430 g, 3.12 mmol) and toluene (5 mL) were added. The system was dehydrated at 150
oC for 4 h and then the temperature was raised to 190 oC with controlled removal of toluene. The
polymerization was allowed to proceed at 190 oC for at least 30 h. 1H NMR was run to study the
molecular weight of the oligomers. Later, the BisAF oligomer was added to the same flask to
originate the formation of multiblock copolymers. Before that, a small BPS-100 sample was
taken from the reaction mixture and precipitated from isopropal alcohol for NMR use.
BisAF-BPSH Hydrophobic-Hydrophilic Multiblock Copolymers
m
O S O O
SO3M
MO3S
O
F
F F
F F
F
F
F
O C O
F
F F
F F
F
F
F
O
O
CH3
CH3n
x
To the flask of BPS-100 oligomers under synthesis was added the corresponding BisAF
oligomers to carry out the synthesis of a multiblock copolymer. The reaction was cooled to 80oC,
and the BisAF oligomer (1.050 g, 0.350 mmol) was dissolved in NMP and slowly added to the
flask. The coupling reactions were run at 95 oC , which was gradually increased to 110 oC, for up
to 6 days, until there was no significant change in viscosity if allowed to proceed for another 12
h. The reaction mixture was then precipitated into isopropal alcohol to obtain a brownish fibrous
polymer. The product was washed twice in deionized water at 60 oC, then washed twice in
acetone. It was then dried and redissolved in NMP to afford a 5% (w/v) solution. The solution
was cast onto a glass substrate and dried with a IR lamp at ~35 oC to obtain a polymer membrane.
81
The membrane was then dried under vacuum at 110 oC for 24 h. The salt form membrane
(SO3-M+) was converted to its acid form (SO3H) by boiling in deionized water for 2 h, and was
then boiled in deionized water for 2 h. It was then stored in water until it was used for
measurements.
2.1.5. NMR Spectroscopy, Gel Permeation Chromatography, Intrinsic Viscosity and Atomic
Force Microscopy Characterization
1H and 19F NMR analysis were conducted on a Varian Unity 400 spectrometer. The spectra of
BPS-100 oligomers and BisAF-BPSH multiblock copolymers were obtained from a 10%
solution (w/v) in a DMSO.d6 solution at room temperature. The spectra of BisAF hydrophobic
oligomers were obtained from a solution in CDCl3. Gel permeation chromatography (GPC)
experiments for BisAF-BPSH copolymers were performed on a liquid chromatograph equipped
with a Waters 1515 isocratic HPLC pump, Waters Autosampler, Waters 2414 refractive index
detector, and Viscotek 270 RALLS/ viscometric dual detector. NMP (containing 0.05M LiBr)
was used as the mobile phase. For BisAF oligomers THF was used as the solvent. 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.
Molecular weights were determined from universal calibration plot using polystyrene as a
standard. Intrinsic viscosities were determined in 0.05M LiBr NMP at 25 oC using a Cannon
Ubbelholde viscometer. Atomic force microscopy characterization (AFM) images were taken
82
using Digital Instruments Dimension 3000 with a microfabricated cantilever. The force constant
was 40 N/m.
2.1.6. Characterization of Fuel Cell Related Properties
Proton Conductivity
Proton conductivity at 30 oC at full hydration (in liquid water) was determined in a window cell
geometry using a Solartron 1252 + 1287 Impedance/Gain-Phase Analyzer over the frequency
range of 10 Hz to 1 MHz following procedures reported in the literature. In determining proton
conductivity in liquid water, membranes were equilibrated at 30 oC in DI water for 24 h prior to
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 24 h before each measurement.
Water Uptake
The water uptake of all membranes was determined gravimetrically. First, the membranes were
soaked in water at 30 °C for 2 days after acidification. Wet membranes were removed from the
liquid water, blotted dry to remove surface droplets, and quickly weighed. The membranes were
then dried at 120 oC under vacuum for at least 24 h and weighed again. The water uptake of the
membranes was calculated according to Equation 2.1 where massdry and masswet refer to the mass
of the dry membrane and the wet membrane, respectively.
83
100mass
massmassuptake%water
dry
drywet ×−
= ……..(2.1)
The hydration number (λ), namely, 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 Equation 2.2, where MWH2O is the molecular weight of water (18.01 g/mol) and IEC is the ion
exchange capacity of the dry copolymer in equivalents per gram.
dry
OHdrywet
massIEC)/MWmass(mass
λ 2
×
−= ……..(2.2)
Pulsed-Field Gradient Spin Echo Nuclear Magnetic Resonance
The self-diffusion coefficient of water was measured using a Varian Inova 400 MHz (for protons)
nuclear magnetic resonance spectrometer with a 60 G/cm gradient diffusion probe. A total of 16
points were collected across the range of gradient strength and the signal-to-noise ratio was
enhanced by coadding 4 scans. The standard stimulated echo NMR pulse sequence is shown in
Figure 2.2.
84
Figure 2.2. Pulse sequence schematic for PGSE NMR experiments
Measurements were conducted by observing the echo signal intensity (A) as a function of
gradient strength. The diffusion coefficient (D) was determined by fitting the data to Equation
2.3, where A is the NMR signal intensity (A) as a function of gradient strength, γ is the
gyromagnetic ratio (26,752 rad G-1 s-1 for protons), δ is length of the gradient pulse, ∆ is the time
between gradient pulse.
)]3/(exp[)()( 222 δδγ −∆−= gg DoAA ……..(2.3)
Membrane samples of approximately 5 mm × 15 mm × 150 µm were equilibrated in liquid water
for at least 24 h. The samples were removed from the liquid water, blotted to remove droplets,
quickly inserted into the NMR tube, and immediately measured over a span of about 5 min.
Measurements were repeated by reimmersing the sample in DI water, waiting at least 30 min,
and then repeating the transfer and measurement process. Separate measurements were collected
with different times between the gradient pulses.
85
2.2. Results and Discussion
2.2.1. Synthesis and characterization
2.2.1.1. Synthesis of fluorinated Oligomers
The synthetic scheme for the synthesis of BisAF oligomers is shown in Figure 2.3.
Decafluorobiphenyl was used in excess to obtain fluorine terminated oligomers as well as to
control the molecular weight. Figure 2.4 shows the 1H NMR spectrum of a BisAF oligomer. Its
19F NMR spectrum is shown in Figure 2.5.
F
F F
F F
F
F
F
FF HO C OHCH3
CH3
DMAc / cyclohexaneK2CO3
110oC, 5h
F
F F
F F
F
F
F
F O C O
F
F F
F F
F
F
F
FCH3
CH3n
+
(excess)
Figure 2.3. Synthesis of BisAF oligomers
86
F
F F
F F
F
F
F
F O O
CH3
CH3
F
F F
F F
F
F
F
Fn
a
a
b b ccF
F F
F F
F
F
F
F O O
CH3
CH3
F
F F
F F
F
F
F
Fn
a
a
b b cc
Figure 2.4. 1H NMR spectrum of a BisAF oligomer
F
F F
F F
F
F
F
F O O
CH3
CH3
F
F F
F F
F
F
F
Fn
a b b a a b c d
eF
F F
F F
F
F
F
F O O
CH3
CH3
F
F F
F F
F
F
F
Fn
a b b a a b c d
e
Figure 2.5. 19F NMR spectra of a BisAF oligomer
87
The peaks due to the main chain, as well as to the endgroup fluorine moieties, were labeled and
their integrals were used to calculate the oligomers’ experimental Mn. GPC was also used to
verify the molecular weights of the oligomers having target Mn values of 3, 4, 5 and 8 Kg/mol.
Figure 2.6 displays the plot of logη ~ log (Mn) for these oligomers. The experimental and
theoretical Mn values for the oligomers are listed in Table 2.1.
Table 2.1. Molecular weight characterizations of BisAF oligomers
Target Mn Mn from 19F NMR Mn from GPC Intrinsic Viscosity
(dL/g)
3K 3.2K 4.4K 0.08
5K 5.5K 5.2K 0.13
8K 8.3K 8.5K 0.17
-1.2
-1.1
-1
-0.9
-0.8
-0.7
-0.6
3.4 3.5 3.6 3.7 3.8 3.9 4
log(M n)
log(IV)
Figure 2.6. logη vs. logMn plot for BisAF oligomers
88
We have found that not all bisphenol monomers could be polymerized with decafluorobiphenyl
(Figure 2.7). Polymerizations of biphenol with the perfluoro monomer were attempted in DMAc
or NMP 110 oC, but a white powder would precipitate out of the solution in 30 min. Our DSC
experiments using these materials showed a trace with a large endotherm at ~350 oC and a glass
transition at 180 oC. This indicates that the polymerization product was semicrystalline and
precipitated out at low molecular weight due to its poor solubility. The same results were
observed for polymerizations of hydroquinone or 4,4’-dihydroxybenzophenone with
decafluorobiphenyl. The possible crystallinity of the polymers may be attributed to the
presumable planar structures of all these monomers. Conversely, when bisphenol monomers
containing flexible groups in between the phenyl rings (i.e. Bisphenol-A, 6F Bisphenol-A, and
Bisphenol-S) were used, the polymerizations invariably proceeded readily and were complete
within 4~5 h at 110 oC.
89
HO OH
HO S OHO
O
HO C OHO
F
F F
F F
F
F
F
FF
Precipitated
Precipitated
Polymerized successfully
HO C OHCH3
CH3
HO C OHCF3
CF3
OHHO
Precipitated
Polymerized successfully
Polymerized successfully
Figure 2.7. Reaction of decafluorobiphenyl with various bisphenol monomers
2.2.1.2. Synthesis of fully disulfonated hydrophilic oligomers
As in the synthesis of partially disulfonated BPSH-xx random copolymers, the direct
polymerization of the sulfonated monomer, SDCDPS, was used in the synthesis of the fully
disulfonated polymers. Biphenol was polymerized with SDCDPS as shown in Figure 2.8. Excess
biphenol was used to control the molecular weight in order to produce phenoxide-terminated
oligomers.
90
HO OH Cl S Cl
SO3Na
NaO3S
O
O
NMP/TolueneK2CO3
150oC, 4h190oC, 30h
MO O S O OM
O
O
SO3M
MO3Sn
+
(excess)
Figure 2.8. Synthesis of fully sulfonated BPS-100 oligomers
The 1H NMR spectrum of an oligomer is shown in Figure 2.9. The integral of the peaks due to
the endgroups and the main chain were used to estimate the experimental Mn. In the early
attempts, however, the oligomers thus obtained usually showed lower measured molecular
weights than the values predicted based on the Carothers equation:
r-1r1Dp
+=
Where Dp is the degree of polymerization and r is the molar feed ratio of the monomers. It is
now generally accepted that this discrepancy is due to residual NaCl left in the synthesis of
SDCDPS, and, to a much less extent, to moisture absorbed by the monomer during weighing, etc.
The SDCDPS/biphenol stoichiometry (r) is therefore overestimated, as is the product's molecular
weight.
91
MO O S O OM
O
O
SO3M
MO3Sn
a ab b c d
e
e
d c
fghi
Figure 2.9. 1H NMR spectrum of a BPS-100 oligomer
In this work, the problem was solved by using a convenient engineering approach. We
calculated the "effective" molar ratio of SDCDPS to biphenol from the Mn, then adjusted the feed
ratio accordingly, i.e. added more SDCDPS than calculated for the next experiment. Normally,
the use of an extra 1~1.5 mol % of SDCDPS was found to be sufficient to compensate for the
monomer's impurity. Still, it was of fundamental importance to measure the purity of the
monomer. Studies of NaCl content using UV-Vis spectroscopy are underway in our laboratory.
The purity of the solvent also seemed to affect the polymerization. As the NMP aged, the color of
the reaction solution became darker. Moreover, the difference between experimental and target
molecular weights tended to become greater, and even more SDCDPS was needed to maintain
the desired stoichiometry. This mechanism is yet unknown, but we postulated that, although
sealed, the NMP have have absorbed oxygen slowly from the air which caused its oxidative
decomposition under the polymerization conditions. There is no evidence, however, of the
chemical integrity of the oligomer being affect by the decomposition of the solvent.
92
2.2.1.3. Synthesis of Multiblock Copolymers
The synthesis of BisAF-BPSH multiblock copolymers is shown in Figure 2.10. The reaction was
carried out in the same flask, directly after the BPS-100 oligomer synthesis was complete. This
was done in order to simplify experimental procedures, since additional steps would have been
needed to remove the salt from the isolated BPS-100 oligomer for weighing purpose. Also, the
complexities associated with the uncertainties in stoichiometry caused by the isolation of both
oligomers was therefore reduced.
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
95~110oC
NMP K2CO3
Figure 2.10. Synthesis of BisAF-BPSH multiblock copolymers
The viscosity of the reaction solution increased slowly during the polymerization. In general,
the reactions were deemed to be complete and generally stopped if there were no significant
increases in viscosity after they were allowed to run for another ~12 h. It should be noted that
reactions tended to proceed more slowly for higher molecular weight oligomers, ranging from 24
93
h for BisAF-BPSH (3K:3K) to 6 days for BisAF-BPSH (8K:8K). These longer reaction times
might have resulted from the differences in solubility of the hydrophobic and hydrophilic
oligomers in NMP. Specifically, we speculated that the solubility differences might have led to
some degree of phase separation, causing the chain-ends to migrate to the
hydrophobic-hydrophilic interfaces in order to meet and react with each other. Nevertheless, the
isolated products were completely soluble in DMAc, NMP or DMSO without any gelation
observed. Thus, the reactions were thought to have resulted in linear copolymers, and
contributions from side reactions associated with pendent fluorine moieties along the chains were
disregarded.
2.2.1.4. Fundamental characterizations of BisAF-BPSH multiblock copolymers
An 1H NMR spectrum for a BisAF-BPSH (8K:8K) copolymer is shown in Figure 2.11. The
integral of the peaks due to hydrophilic and hydrophobic moieties were utilized to calculate the
experimental IEC values of the materials, which were found to agree well with theoretical
values.
94
e
d
bf
a,g
c
12000 12500 13000 13500 14000 14500 15000 pts
mO S O
O
O
SO3M
MO3S
OCH3
CH3F
F F
F F
F
F
F
On
a ab b c d e f fgg
x
e
d
bf
a,g
c
12000 12500 13000 13500 14000 14500 15000 pts
mO S O
O
O
SO3M
MO3S
OCH3
CH3F
F F
F F
F
F
F
On
a ab b c d e f fgg
x
Figure 2.11. 1H NMR spectrum of a BisAF-BPSH multiblock copolymer
An 19F NMR spectrum of the copolymer is shown in Figure 2.12. Only the peaks due to the main
chain can be observed in both 1H and 19F spectra, thus indicating the high conversion of the
reaction.
95
i
h
-140 -145 -150 -155 PPM
mO S O
O
O
SO3M
MO3S
OCH3
CH3F
F F
F F
F
F
F
On
x
h i i h
i
h
-140 -145 -150 -155 PPM
mO S O
O
O
SO3M
MO3S
OCH3
CH3F
F F
F F
F
F
F
On
x
h i i h
Figure 2.12. 19F NMR spectrum of a BisAF-BPSH multiblock copolymer
The synthesis of the BisAF-BPSH (4K:4K) copolymer was monitored using 1H NMR
spectroscopy. Figure 2.13 shows the spectra of samples taken at different stages of the coupling
reaction (b ~ c), as well as that of the 4K BPS-100 oligomer prior to the reaction (a). It can be
seen that the small peaks corresponding to the oligomer’s endgroups are the largest in the spectra
of the BPS-100 oligomer, then gradually become smaller as the reaction proceeds, and eventually
disappear. This largely confirms the pathway of the polymerization, which occurs through the
reactions between phenoxide and fluorine endgroups.
96
8.2 8.0 7.8 7.6 7.4 7.2 7.0 6.8 PPM
(a)
(b)
(c)
(d)
8.2 8.0 7.8 7.6 7.4 7.2 7.0 6.8 PPM
(a)
(b)
(c)
(d)
Figure 2.13. Monitoring of multiblock copolymer synthesis using 1H NMR spectra: (a) BPS-100
oligomer prior to the reaction; (b) 12 h; (c) 20 h; (d) 36 h.
1H NMR spectra of the BisAF-BPSH multiblock copolymers with block lengths of 3K, 4K and
8K are overlaid in Figure 2.14. Here, the small peaks at around 7.3 ppm, though partially
overlapping with the other peaks, were thought to be due to one of the four types of protons at
the linkages between blocks. As shown in this figure, with the intensity of the other peaks more
or less normalized, the peak for the 4K:4K copolymer is slightly smaller than that of the 3K:3K
copolymer. In comparing the 4K:4K to the 8K:8K copolymers, however, the intensity of the peak
significantly decreased. It would seem, therefore, that for the multiblock copolymers, the higher
the block length, the smaller the number of linkages between hydrophobic and hydrophilic
blocks. This is consistent with observations from NMR spectra that verify the existence of block
sequences in these materials. 97
7.8 7.6 7.4 7.2 7.0 PPM
m
O S O O
O
O
SO3M
MO3S
O
F
F F
F F
F
F
F
O C O
F
F F
F F
F
F
F
CH3
CHe n
x
Possible causes for the peak
4K:4K
3K:3K
8K:8K7.8 7.6 7.4 7.2 7.0 PPM
m
O S O O
O
O
SO3M
MO3S
O
F
F F
F F
F
F
F
O C O
F
F F
F F
F
F
F
CH3
CHe n
x
Possible causes for the peak
4K:4K
3K:3K
8K:8K
Figure 2.14. 1H NMR spectra of BisAF-BPSH multiblock copolymers showing the linkages between
blocks
Characterization data for a series of the multiblock copolymers are listed in Table 2.2. It is worth
noting that the copolymers have different IEC values due to differences in the molar charge
ratios for the hydrophilic:hydrophobic oligomers. Although the feed ratios are generally lower
than 1:1, all the polymerizations still yielded materials with high intrinsic viscosities that are
capable of being cast into tough membranes. Thus, unlike polymerizations of smaller monomers,
in coupling reactions of telechelic oligomers, a perfect stoichiometry is not necessary. Therefore,
the molar feed ratio (which does not have to be 1:1) can be adjusted to control a copolymer’s
IEC. This type of approach was more systematically used in the synthesis of BisSF-BPSH
multiblock copolymers, which will be discussed in more detail in a subsequent chapter.
98
Table 2.2. Some characterizations of BisAF-BPSH multiblock copolymers
Copolymer Block Lengthsa (g/mol)
Molar Feed Ratiob IECc Intrinsic Viscosity
(dL/g)d
BisAF-BPSH-1 3000:3000 1 : 0.93 1.6 0.60
BisAF-BPSH-2 4000:4000 1 : 0.87 1.5 0.64
BisAF-BPSH-3 5000:5000 1 : 0.82 1.4 0.54
BisAF-BPSH-4 8000:8000 1 : 0.70 1.1 0.53
a. Block lengths are expressed in the form hydrophobic:hydrophilic b. Molar ratios are expressed in the form hydrophobic:hydrophilic c. Measured from 1H NMR d. Measured at 25 oC in 0.05M LiBr NMP solution
Tapping mode atomic force microscopy (AFM) phase images for the copolymers with 3K, 4K
and 5K molecular weights are displayed in Figure 2.15. In these images, the dark regions are
believed to be due to the hydrophilic domains (BPSH), whereas the bright regions correspond to
the hydrophobic ones (BisAF). Although we noted the absence of an oriented morphology, we
observed from the phase images that the phase separation between the hydrophilic and
hydrophobic regions was sharper as block lengths increased. Furthermore, copolymers with
longer blocks featured more connectivity among the hydrophilic domains. In other words, there
tended to be increasing numbers of ionic channels in the membranes, which are thought be
important to proton conductivity at low humidity, as block lengths increased—even though this
was accompanied by a decrease in IEC. Thus, this study demonstrated that the morphological
features of these copolymers can be controlled by synthesis. As will be discussed below, by
controlling morphology, we were also able to control how these materials performed under fuel
cell conditions.
99
100 nm 100 nm 100 nm
3K:3K 4K:4K 5K:5K
100 nm 100 nm 100 nm
3K:3K 4K:4K 5K:5K
Figure 2.15. Tapping mode AFM phase images of BisAF-BPSH multiblock copolymers with different
block lengths
2.2.2. Fuel Cell Related Characterizations of Multiblock Copolymers
2.2.2.1. Proton conductivity under fully hydrated conditions
It has been reported in the literature that the proton conductivity of sulfonated polymers is a
function of both water uptake and the ion exchange capacity. In addition to IEC and water
sorption on a mass basis, the hydration number (λ) is widely used to compare membranes with
different polymer backbone structures. In this portion of the research, the properties of our
BisAF-BPSH multiblock copolymers will be compared to those of several series of poly(arylene
ether sulfone) and poly(arylene ether ketone) random copolymers, i.e. BPSH, HQSH, PB and B
series, as shown in Figure 2.16.
100
1-xxO O S
O
O
SO3H
HO3S
O O S
O
O
1-xxCF3
CF3
O O Y
SO3H
HO3S
OCF3
CF3
O Y
(a)
(b)
O S
O
O
SO3H
HO3S
O O S
O
OO
x 1-x
(c)
Y = C
OC
O
C
Oor(B series)
HQSH series
(PB series)
BPSH series
Figure 2.16. Structures of partially disulfonated random copolymers. (a) BPSH; (b) HQSH; (c) poly(ether
ketone) B and PB series
Previous studies in our laboratory indicated that proton conductivity scales linearly with IEC for
the BPSH random copolymer membranes under fully hydrated conditions. Here, similar tends
were observed for HQSH, PB, and B series, as shown in Table 2.3. and Figure 2.17.
The increase in conductivity can be explained in terms of an increase in the concentration of
protons and a corresponding increase in water uptake. It has been reported that water uptake for
these random copolymers also increases with increases in the ion exchange capacity.
101
Table 2.3. IEC, water uptake and proton conductivity for partially disulfonated random
copolymers
Copolymer IEC
(mequiv/g)
Water Uptake
(λ)
Proton Conductivity at 30oC in
liquid water (mS/cm)
HQSH-30 1.6 29 100
HQSH-25 1.4 24 80
HQSH-20 1.2 14 70
PB-50 1.4 26 71
PB-40 1.2 12 40
PB-30 0.9 9 10
B-50 1.7 32 90
B-40 1.4 18 73
B-30 1.1 13 23
Figure 2.17. Proton conductivity at 30oC in liquid water for partially disulfonated random copolymers
plotted against IEC
102
The proton conductivity and water uptake for BisAF-BPSH multiblock copolymers are listed in
Table 2.4. Similar to the case of random copolymers, it can be observed that the proton
conductivity for the multiblock copolymers under fully hydrated conditions is generally a
function of the IEC. Although both proton conductivity and water uptake decrease with the
decrease in IEC, there does not seem to be a significant effect of block length on these properties.
Table 2.4. IEC, water uptake and liquid water proton conductivity for BisAF-BPSH multiblock
copolymers
Copolymer Block Lengths
(g/mol)
IEC
(mequiv/g)
Water
Uptake
Proton Conductivity at
30oC (mS/cm)
BisAF-BPSH-1 3K:3K 1.6 71% 130
BisAF-BPSH-3 5K:5K 1.4 58% 104
BisAF-BPSH-4 8K:8K 1.1 42% 90
Thus, under fully hydrated conditions, proton conductivity is determined mainly by the ion
exchange capacity of the membrane, regardless of the polymer architecture or the sequence
length of sulfonic acid groups.
Proton conductivity for the BisAF-BPSH copolymers was also measured at various temperatures
under fully hydrated conditions, and the results are shown in Figure 2.18. Again, within the
entire temperature range, proton conductivity increased with increasing IEC values.
103
0
0.05
0.1
0.15
0.2
0.25
0 20 40 60 80 100
Temperature (deg C)
Prot
onic
con
dutiv
ity(S
/cm
)
1Xu12 Xu23. Xu4
1
2
3
Figure 2.18. Proton conductivity under fully hydrated conductions for BisAF-BPSH copolymers as a
function of temperature
2.2.2.2. Proton conductivity under partially hydrated conditions
Up to this point, we have only discussed proton conductivity under fully hydrated conditions,
with the concentration of ionic groups identified as the primary factor influencing this important
variable. Figure 2.19 shows proton conductivity as a function of relative humidity (RH) for
HQSH-30, PB-50, and Nafion 117. It can be seen that below 50% RH, the proton conductivity
decreases significantly for the random copolymers. Thus, under partially hydrated conditions,
proton conductivity for random copolymer membranes is strongly dependent on the hydration
level. This observation is consistent with the decrease in activation energy for proton transport
with an increase in RH, as shown in Figure 2.20 for the HQSH-30 random copolymers. For
Nafion, however, the decrease in conductivity as RH decreases is not as great. We attributed this
104
to the unique chemical structure of Nafion, which consists of highly flexible side chain
hydrophilic sulfonic acid groups and a hydrophobic fluorinated backbone. These chemical
attributes are believed to promote strong nanophase separation between hydrophilic and
hydrophobic domains, as well as facilitate proton transport among the interconnected hydrophilic
domains—even at low hydration levels.
1.0E-01
1.0E+00
1.0E+01
1.0E+02
1.0E+03
20 30 40 50 60 70 80 90 100Relative Humidity (%)
Prot
on c
ondu
ctiv
ity (m
S/cm
)
1 .Nafion117
2. HQSH 30
3. PB-diketone 50
1
2
3
Figure 2.19. Proton conductivity vs. RH plots for Nafion 117, poly(ether sulfone) random copolymers
(HQSH-30), and poly(ether ketone) random copolymers (PB-50)
105
Figure 2.20. Activation energy of proton transport for HQSH-30 random copolymers as a function of
relative humidity: proton transport barrier increases as RH decreases
The proton conductivity of BisAF-BPSH multiblock copolymers and Nafion is plotted against
RH in Figure 2.21. Here, it can be seen that the conductivity of the block copolymers increased
with increasing block length. The 8K-8K sample displayed the best performance—in fact,
comparable to that of Nafion—despite the fact that it had the lowest IEC and water uptake values
in the series, This contrasts to performance under fully hydrated conditions where IEC, rather
than block length, dominated conductivity. This suggests that with an increase in block length,
the extent of nanophase separation and the connectivity between the hydrophilic domains
increase, which then decreases the barrier for proton transport.
106
1.0E+00
1.0E+01
1.0E+02
1.0E+03
20 40 60 80 100Relative Humidity (%)
Prot
on c
ondu
ctiv
ity (m
S/cm
)
1.BisAF-BPSH(8:8)K
2.Nafion117
3.BisAF-BPSH(5:5)K
4.BisAF-BPSH(3:3)K
1
3 2
4
Figure 2.21. Proton conductivity vs. RH plots for Nafion 117and BisAF-BPSH multiblock copolymers
2.2.2.3. Diffusion coefficients
The self-diffusion coefficient of water was measured under fully hydrated conditions by PGSE
NMR. The obtained values at different IECs for the HQSH random copolymers, as well as for
the BisAF-BPSH multiblock copolymers, are shown in Figure 2.22. As expected, for the random
copolymers the diffusion coefficient increased as a function of the IEC. The diffusion
coefficients for the block copolymers, however, increased with an increase in block length,
irrespective of IEC values. This clearly illustrates the importance of connectivity between the
hydrophilic domains with respect to transport properties.
107
3K:3K
5K:5K
8K:8K
3K:3K
5K:5K
8K:8K
Figure 2.22. Self-diffusion coefficient for water as a function of IEC for random and block copolymers
and Nafion
2.2.2.4. Methanol Permeability
In a direct methanol fuel cell (DMFC), methanol—rather than hydrogen—is used as the fuel. The
use of a DMFC, however, means that not only is there a tradeoff between proton conductivity
and water uptake, but also between the transport properties of proton and methanol, since both
depend to some extent on the diffusion of water molecules. One of the major disadvantages of
using Nafion in a DMFC is the material’s high methanol permeability. Conversely, BPSH and
other random copolymer membranes have generally shown lower methanol permeability
compared to Nafion.
108
Methanol permeability (at 80oC) values as a functional of IEC for Nafion 117, HQSH-xx,
B-ketone-xx and BisAF-BPSH multiblock copolymers are shown in Figure 2.23. As in the case
of proton conductivity, the methanol permeability of random copolymers increased with an
increase in IEC, which was also seen for the multiblock copolymers. However, the block
copolymers were designed in such a way that the sample with the lowest IEC had the longest
hydrophobic and hydrophilic blocks. Thus, while the low-IEC random copolymers may suffer
from low conductivity, this would not be a problem for the BisAF-BPSH (8k:8K) multiblock
copolymers. This membrane, despite its low IEC, showed high proton conductivity under
partially hydrated conditions and a high water self-diffusion coefficient, as shown earlier. This
suggests that the transport of methanol may have a different mechanism than that of water in
hydrophilic-hydrophobic block copolymers, in that both high proton conductivity and low
methanol crossover can be achieved by controlling the morphology. The connectivity among the
long hydrophilic sequences ensures a better pathway for the diffusion of water and protons,
whereas the low methanol permeability we observed may have resulted from the methanol
transport being significantly hampered by the long hydrophobic blocks.
109
Figure 2.23. Methanol permeability at 80 oC as a function of IEC for random and block copolymer
membranes and Nafion
110
Chapter 3. Synthesis and Characterization of 6FBisAF-BPSH Multiblock
Copolymers
3.1. Experimental
The materials and procedures used to synthesize 6FBisAF-BPSH multiblock copolymers are
analogous to those of BisAF-BPSH copolymers, which were discussed earlier. Therefore, only
those that have not been previously described will be discussed here.
3.1.1. Materials.
4,4’-(Hexafluoroisopropylidene) diphenol (6F-Bisphenol A, 6F-BPA)
C
CF3
CF3
HO OH
Source: Ciba
Molecular weight: 336.33 g/mol
Purification: The as-received, slightly pink-colored monomer was purified by sublimation,
yielding a white powder. The product was dried under vacuum at room temperature for at least
24 h.
111
3,3’-Disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS)
S
O
O
Cl Cl
NaO3S S 3NaO
SDCDPS was synthesized and purified as described earlier.
3.1.2. Polymer Synthesis
6F-BisAF hydrophobic oligomers
nC O
F
F F
F F
F
F
F
FCF3
CF3
O
F
F
F
F
F F
F F
The synthesis of a 6FBisAF oligomer with 5 Kg/mol molecular weight, for example, was carried
out as follows: 6F-BPA (1.147 g, 3.411 mmol) was added to a three-neck round bottom flask
equipped with a mechanical stirrer, a condenser, a nitrogen inlet and a Dean-Stark trap. DMAc
(10 mL) was added to the flask and the mixture was dissolved. Then K2CO3 (0.644 g, 4.669
mmol) was added, followed by 5 mL of toluene. The reaction bath was heated to 150 oC and kept
at this temperature for 2 h to dehydrate the system. The reaction was cooled to 50 oC and
decafluorobiphenyl (1.200 g, 3.591 mmol) was added. The bath temperature was raised to 110 oC
and the reaction was allowed to proceed at this temperature for 5 h. The mixture was precipitated
into 200 mL of water/methanol (50/50 v/v) and rinsed with water and methanol. The precipitated
polymer was dried under vacuum at 100 oC.
112
BPS-100 hydrophilic oligomers
MO O S O OM
O
O
SO3M
MO3Sn
BPSH-100 oligomers with various molecular weights were synthesized by the same procedures
described earlier.
Partially disulfonated hydrophilic oligomers (BPS-75 and BPS-83)
MO O S
O
O
SO3M
MO3S
O O S
O
OO OM
x 1-x
To study the effect of the degree of disulfonation on the properties of the multiblock copolymers,
some partially disulfonated BPS oligomers—specifically, BPS-75 and BPS-83—were
synthesized and used later in the preparation of multiblock copolymers. The procedures we used
to synthesize a BPS-75 oligomer with 8 Kg/mol molecular weight are as follows: A three-neck
round bottom flask, equipped with a mechanical stirrer, a condenser, a nitrogen inlet and a
Dean-Stark trap, was charged with biphenol (0.27 g, 1.450 mmol), SDCDPS (0.513 g, 1.044
mmol), DCDPS (0.100 g, 0.348 mmol), and 10 mL of NMP. The mixture was dissolved, then
K2CO3 (0.26 g, 1.885 mmol) and 5 mL of toluene was added. The reaction bath was heated to
150 oC to dehydrate the system. The bath temperature was then slowly raised to 190 oC by the
controlled removal of toluene. The polymerization was allowed to proceed at this temperature for
30 h, and the resulting oligomer was used in the block copolymer synthesis without isolation.
113
6FBisAF-BPSH100 multiblock copolymers containing fully disulfonated hydrophilic blocks
m
O S O O
O
O
SO3H
HO3S
O
F
F F
F F
F
F
F
O O
CF3
CF3
F
F F
F F
F
F
Fn
x
The 6FBisAF-BPSH100 multiblock copolymers were synthesized by coupling reactions of
6FBisAF and BPS100 oligomers, using similar procedures described earlier for BisAF-BPSH
series.
6FBisAF-BPSH75 and 6FBisAF-BPSH83 multiblock copolymers containing partially disulfonated hydrophilic blocks
n
O O S
O
O
SO3H
HO3S
O O S
O
OO
O
F
F F
F F
F
F
F
O O
CF3
CF3
F
F F
F F
F
F
F
0.75 0.25
x
The 6FBisAF-BPSH75 and 6FBisAF-BPSH83 multiblock copolymers were synthesized by
coupling reactions of 6FBisAF and the partially disulfonated BPS oligomers, using similar
procedures described earlier for the BisAF-BPSH series.
3.1.3. Polymer isolation and characterization
The multiblock copolymers synthesized were purified and characterized using the same
procedure described earlier for the BisAF-BPSH series.
114
3.2. Results and discussion
3.2.1. Polymer synthesis and characterization
The synthetic scheme for the 6FBisAF oligomers is shown in Figure 3.1. The procedures for the
synthesis of the 6FBisAF oligomers are the same as those used for the BisAF oligomers. It was
found, however, that the polymerization would not proceed well when NMP instead of DMAc
was used as the solvent. The synthetic scheme for BPS-75 oligomers is shown in Figure 3.2 as an
example of the partially disulfonated BPS oligomers. The same procedures used to synthesize the
BPSH random copolymers were used for the BPS-75 hydrophilic oligomers, except that biphenol
was used in excess to control molecular weight.
F
F F
F F
F
F
F
FF HO C OHCF3
CF3
DMAc / cyclohexaneK2CO3
110oC, 5h
F
F F
F F
F
F
F
F O C O
F
F F
F F
F
F
F
FCF3
CF3n
+
(excess)
Figure 3.1. Synthesis of 6FBisAF oligomers
115
HO OH Cl S Cl
SO3Na
NaO3S
O
O
NMP/TolueneK2CO3
150oC, 4h
190oC, 30h
MO O SO
O
SO3M
MO3S
O O SO
OO OM
+
(excess)
0.75 0.25
Cl S Cl
O
O
+
Figure 3.2. Synthesis of BPS-75 hydrophilic oligomers
The block copolymerizations (i.e. coupling reactions) between 6FBisAF and BPS100 oligomers
were slower than those shown earlier with the BisAF oligomers. Two types of oligomers were
used as the hydrophilic block: BPS-100 and the partially disulfonated oligomers, BPS-75 and
BPS-83. This was done in order to investigate the effect of the hydrophilic block’s sulfonation
degree on the properties of the multiblock copolymers. The schemes for the synthesis of
6FBisAF-BPSH100 and 6FBisAF-BPSH75 are shown in Figures 3.3 and 3.4, respectively. When
isolated from isopropanol, the resulting 6FBisAF-BPSH copolymers formed weaker fibers
compared to the BisAF-BPSH series copolymers, although they generally showed acceptable
intrinsic viscosity values.
116
F
F F
F F
F
F
F
F O C O
F
F F
F F
F
F
F
FCF3
CF3
m
n
O S O OMO
O
SO3M
MO3S
MO
+
mO S O O
O
O
SO3M
MO3S
O
F
F F
F F
F
F
F
O C O
F
F F
F F
F
F
F
CF3
CF3n
x
NMP
K2CO3
105 ~ 115oC
4~5 days
Figure 3.3. Synthesis of 6FBisAF-BPSH100 multiblock copolymers
MO O S
O
O
SO3M
MO3S
O O S
O
OO OM
0.75 0.25
F
F F
F F
F
F
F
F O O
CF3
CF3
F
F F
F F
F
F
F
F+n
O O SO
O
SO3M
MO3S
O O SO
OO O
F
F F
F F
F
F
F
O O
CF3
CF3
F
F F
F F
F
F
F
0.75 0.25
n
K2CO3, NMP
105~115oC , 4~5 days
Figure 3.4. Synthesis of 6FBisAF-BPSH75 multiblock copolymers
117
3.2.2. Fundamental characterizations
The 19F NMR spectrum of a 6FBisAF hydrophobic oligomer is shown in Figure 3.5. A spectrum
of the aromatic region, emphasizing the fluorine endgroups, is shown in Figure 3.6. Again, the
integral of peaks due to the main chain and endgroups were used to obtain the experimental Mn.
The calculated and experimental molecular weights for the oligomers, as well as intrinsic
viscosity values (IV), are listed in Table 3.1. Figure 3.7 shows the plot of lnη as a function of
lgMn.
c
ab
-60 -80 -100 -120 -140 -160 PPM
nC OCF3
CF3
O
F
F
F
F
F F
F F
abbac
c
ab
-60 -80 -100 -120 -140 -160 PPM
nC OCF3
CF3
O
F
F
F
F
F F
F F
abbac
Figure 3.5. 19F NMR spectrum of a 6FBisAF oligomer
118
ab
c de
-135 -140 -145 -150 -155 -160 PPM
n
abba c
nC O
F
F F
F F
F
F
F
FCF3
CF3
O
F
F
F
F
F F
F F
d
e
a b
ab
c de
-135 -140 -145 -150 -155 -160 PPM
n
abba c
nC O
F
F F
F F
F
F
F
FCF3
CF3
O
F
F
F
F
F F
F F
d
e
a b
Figure 3.6. 19F NMR spectrum of a 6FBisAF oligomer (aromatic region) showing endgroups in detail
Table 3.1. Molecular weight characterizations of 6FBisAF oligomers
Target Mn
(g/mol
Mn from 19F NMR
(g/mol)
Mn from GPC
(g/mol)
Intrinsic Viscosity
(dL/g)
6K 6.4K 5.6K 0.12
9K 9.5K 10.1K 0.20
12K 13.0K 13.3K 0.23
15 15.9 15.8 0.27
119
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
3.7 3.8 3.9 4 4.1 4.2 4.3
log(M n)
Log(IV)
Figure 3.7. logη vs. logMn plot for 6FBisAF oligomers
MO O SO
O
SO3M
MO3S
O O SO
OO OM
0.75 0.25
a b b a c d e i h g f
MO O SO
O
SO3M
MO3S
O O SO
OO OM
0.75 0.25
a b b a c d e i h g f
k l l k m n n m k l l k m n n m
Figure 3.8. 1H NMR spectrum of a BPS-75 oligomer
120
The 1H NMR spectrum of a BPS-75 oligomer is shown in Figure 3.8, where all the peaks are
assigned. Figure 3.9 shows the 1H spectrum of a 6FBisAF-BPSH100 multiblock copolymer.
Figure 3.10 shows the spectra of a 6FBisAF-BPSH83 copolymer, as well as the corresponding
BPS-83 oligomer. The peaks due to the oligomer’s endgroups were not observed in the spectrum
of the copolymer, suggesting high conversion of the coupling reaction.
mO S O
O
O
SO3M
MO3S
O
CF3
CF3
F
F F
F F
F
F
F
On
a ab b c
c
d
de
e f fgg
mO S O
O
O
SO3M
MO3S
O
CF3
CF3
F
F F
F F
F
F
F
On
a ab b c
c
d
de
e f fgg
Figure 3.9. 1H NMR spectrum of a 6FBisAF-BPSH100 multiblock copolymer
121
MO O SO
O
SO3M
MO3S
O O SO
OO OM
0.83 0.17
a b b a c d e k l l k m n n m i h g f
n
F
F F
F F
F
F
F
O OCF3
CF3F
F F
F F
F
F
F
BPSH83
O P P O
MO O SO
O
SO3M
MO3S
O O SO
OO OM
0.83 0.17
a b b a c d e k l l k m n n m i h g f
n
F
F F
F F
F
F
F
O OCF3
CF3F
F F
F F
F
F
F
BPSH83
O P P O
Figure 3.10. 1H NMR spectra of a partially disulfonated BPS oligomer and the corresponding
6FBisAF-BPS83 multiblock copolymer
From Figures 3.9 and 3.10, it can be observed that the two peaks that were anticipated for the
6FBisAF blocks merged into one broad peak in these spectra. Therefore, it is possible that the
copolymers might not have formed true solutions in the NMR solvent, DMSO-d6, but rather
tended to form micelle-like aggregates. Furthermore, as shown in Table 3.2, the copolymers’
experimental IEC, obtained from 1H NMR, did not agree with either theoretical value. The
titration values, on the other hand, showed reasonably good agreement with the target IEC.
122
Table 3.2. Comparison of target IEC with experimental values for 6FBisAF-BPSH100
multiblock copolymers
Copolymer Target IEC IEC from 1H NMR IEC from Titration
6FBisAF-BPSH100 (6K-5K) 1.77 1.84 ---
6FBisAF-BPSH100 (12K-12K) 1.46 2.10 1.37
6FBisAF-BPSH100 (9K-9K) 1.32 1.97 1.39
6FBisAF-BPSH100 (12K-7K) 1.25 1.74 1.26
6FBisAF-BPSH100 (12K-11K) 1.2 1.70 1.15
6FBisAF-BPSH100 (9K-8K) 1.2 1.77 1.24
Such large discrepancies cannot be justified by experimental uncertainties. The reason is not
clear, but some qualitative hypotheses could be suggested on the basis of the solution properties
of the copolymers. In fact, the various hydrophobic oligomers including BisAF, 6FBisAF, and
BisSF (which will be discussed in the following chapter), have different solubility in DMSO, as
shown in Table 3.3.
Table 3.3. Solubility of oligomers in DMSO at room temperature
Hydrophobic Hydrophilic
BisAF 6FBisAF BisSF BPS-100 BPS-83 BPS-75
Yes No Yes Yes Yes Yes
123
The insolubility of the 6FBisAF block was probably due to a decrease in polarity, caused by its
much higher fluorine content. The 6FBisAF-BPSH block copolymer chains could, therefore,
have very different conformations in DMSO, compared to those of BisAF-BPSH and
BisSF-BPSH. This may be the reason why experimental IEC values from NMR were not
consistent with the target ones.
Although the copolymerizations were conducted in NMP, in which both 6FBisAF and BPSH-100
oligomers are soluble, there could be finite differences between their solubility. This, in addition
to the increased difference in hydrophobicity of the blocks due to the hexafluoroisopropylidene
group, may partially explain the longer reaction times for the 6FBisAF-BPSH series compared to
the BisAF-BPSH series. We also propose that there could be more pronounced separation
between the hydrophobic and hydrophilic phases, thereby inhibiting the oligomers from
migrating toward and reacting with each other.
Figure 3.11 shows the DSC trace of a 6FBisAF-BPSH100 multiblock copolymer. Two transitions,
one at around 195 oC and the other 245 oC, can be clearly observed, which we assigned to the
glass transitions for the hydrophobic and hydrophilic oligomers, respectively. Such obvious dual
Tgs are only occasionally seen in block ionomers because the molecular motion tends to be
hindered by electrostatic interactions among ionic groups. The two separate Tgs were never
clearly identified in the DSC traces for the BisAF-BPSH and BisSF-BPSH systems, where only
one single Tg was observed. Again, this is probably due to the more distinct phase separation 124
between the 6FBisAF and BPSH-100 components, i.e. the higher χ parameter caused by the
larger differences in their hydrophobicity, in contrast to the BisAF-BPSH and BisSF-BPSH
systems. As we shall see later, the 6FBisAF-BPSH series copolymers displayed very different
proton conductivity and water uptake than the other two series—possibly due to their peculiar
morphological features.
Figure 3.11. DSC trace of a 6FBisAF-BPSH100 (9K:9K) multiblock copolymer
125
3.2.3. Characterization of fuel cell related properties
The proton conductivity and water uptake values for 6FBisSF-BPSH100 copolymers are listed in
Table 3.4. The proton conductivity values at 30 oC in liquid water were generally lower
compared to the BisAF-BPSH and BisSF-BPSH series, and decreased with the decrease of IEC,
regardless of block length. The water uptake values, conversely, were surprising high given the
low IEC and conductivity values. The high water may indicate a percolated hydrophilic
(sulfonated) nanophase throughout the membrane, possibly due to the very high hydrophobicity
of the fluorinated blocks. Although a certain degree of water sorption is required for good proton
conductivity, excessive water uptake may adversely affect the conductivity. In other words, high
water uptake results in high volume swelling of the membrane, so the concentration of sulfonic
acid groups is greatly lowered, leading to low proton conductivity. The only exception is the
15K:10K sample, which featured low IEC values and long hydrophilic block length. This
copolymer displayed much lower water uptake yet still reasonably good proton conductivity. We
attributed this to the fact that the long hydrophobic blocks may have served as a barrier and
prevented the formation of a percolated morphology.
126
Table 3.4. Proton conductivity (liquid water) and water uptake for 6FBisAF-BPSH100
multiblock copolymer membranes
Copolymer IEC (Target) Proton Conductivity at 30 oC (S/cm) Water Uptake
6FBisAF-BPSH100 (6K:5K) 1.77 0.13 150%
6FBisAF-BPSH100 (12K:12K) 1.46 0.09 260%
6FBisAF-BPSH100 (9K:9K) 1.32 0.09 124%
6FBisAF-BPSH100 (12K:7K) 1.25 0.08 69%
6FBisAF-BPSH100 (12K:11K) 1.15 0.065 50%
6FBisAF-BPSH100 (9K:9K) 1.20 - 78%
6FBisAF-BPSH100 (15K:10K) 1.20 0.09 42%
Table 3.5 shows data for the copolymers having partially sulfonated BPSH blocks; the
6FBisAF-BPSH100 (15K:10K) sample is shown for comparison. They all possessed 15 Kg/mol
fluorinated blocks and increasing BPSH block lengths. The 6FBisSF-BPSH-100 (15K:10K) is
also listed for comparison. Although the copolymers had hydrophilic blocks with lower degrees
of sulfonation (as well as lower IECs), they showed much higher water uptake compared to the
BPSH-100-based block copolymers. Thus, they are unlikely candidates for PEMFC applications.
The reason for the high water uptakes in these materials, although not clear, could lie in their
more complex morphology. 127
Table 3.5. Proton conductivity (liquid water) and water uptake for 6FBisAF-BPSH75 and
6FBisAF-BPSH83 multiblock copolymer membranes
Copolymer IEC (Target) Proton Conductivity at 30 oC (S/cm) Water Uptake
6FBisAF-BPSH75 (15K:8K) 1.09 0.07 72%
6FBisAF-BPSH75 (15K:10K) 1.17 0.09 87%
6FBisAF-BPSH75 (15K:20K) 1.27 0.08 113%
6FBisAF-BPSH83 (15K:10K) 1.15 0.08 89%
6FBisAF-BPSH100 (15K:10K) 1.20 0.09 42%
Figures 3.12 and 3.13 display tapping mode AFM images for the 6FBisSF-BPSH-100 (15K:10K)
and 6FBisSF-BPSH-75 (15K:10K) samples. In the phase image in Figure 3.12, the copolymer
with fully sulfonated BPSH blocks shows sharp nanophase separation between distinct
hydrophobic and hydrophilic domains. The morphology for the latter sample, shown in Figure
3.13, is not well defined. Since there are non-sulfonated moieties in the BPSH blocks, there
might exist phase separation between the sulfonated and non-sulfonated potions in the
hydrophilic regions, in addition to the phase separation between 6FBisSF and BPSH domains.
128
Height Phase
500 nm
Height Phase
500 nm
Figure 3.12. Tapping mode AFM images for a 6FBisAF-BPSH100 (15K:10K) multiblock copolymer
Height Phase
500 nm
Height Phase
500 nm
Figure 3.13. Tapping mode AFM images for a 6FBisAF-BPSH75 (15K:9K) multiblock copolymer
129
The proton conductivity at 80 oC under partially-hydrated conditions for the 6FBisSF-BPSH
copolymers is shown in Figure 3.14. Surprisingly, all the 6FBisSF-BPSH100 samples showed
very similar conductivity as a function of RH, which was comparable to that of Nafion 117,
regardless of their IEC and block lengths. This may be attributed to the possible percolated
morphology in all these membranes, but further evidence from X-ray or neutron scattering
experiments would be helpful in determining the reason. In contrast, the 6FBisSF-BPSH75
membranes (not shown) displayed much lower performance under partially hydrated conditions.
This is consistent with its very different morphological features as shown by AFM (discussed
earlier).
0.1
1
10
100
1000
20 30 40 50 60 70 80 90 100Relative Humidity
Pro
ton
Con
duct
ivity
(mS
/cm
)
15-9
12-11
12-7
Nafion 112
Figure 3.14. Proton conductivity as a function of RH for 6FBisAF-BPSH multiblock copolymers
130
In summary, the 6FBisSF-BPSH100 copolymer membranes generally showed higher water
uptake, which was reduced to a satisfactory level when a 15 Kg/mol fluorinated block was used.
The 6FBisSF-BPSH100 (15K:10K) copolymers, displaying higher proton conductivity under
partially hydrated conditions and lower water uptake, have been shown to be excellent
candidates for use in proton exchange membrane. However, the copolymers with partially
sulfonated BPSH blocks displayed lower proton conductivity and higher water uptake, making
them undesirable as PEMs.
131
Chapter 4. Synthesis and characterization of BisSF-BPSH multiblock
copolymers
Although BisAF-BPSH and 6FBisAF-BPSH copolymers have been successfully synthesized and
have been shown to display high performance under partially hydrated conditions, both materials
have drawbacks. While the former have questionable hydrolytic and thermal stability, the latter
suffer from excessive water sorption and swelling, making them less useful as proton exchange
membranes. In this section, we discuss a BisSF-BPSH series featuring fluorinated poly(arylene
ether sulfone) hydrophobic blocks. This type of copolymer should have much better thermal and
hydrolytic stability than BisAF-BPSH series. Moreover, since they don’t have the extremely
hydrophobic 6FBisAF blocks, the undesirable morphology of the 6FBisAF-BPSH samples may
be avoided. Furthermore, this approach is more economically viable because the
4,4’-dihydroxydiphenyl sulfone (bisphenol-S) monomer, which is polymerized with
decafluorobiphenyl, is very inexpensive and readily produced from phenol and sulfuric acid.
For this series of copolymers, we synthesized and characterized the largest number of samples,
with variations in both ion content and block length, and their structure-property relationships
will be discussed in a more systematic way.
132
4.1. Experimental
4.1.1. Materials
4,4’-Dihydroxydiphenyl sulfone (bisphenol-S)
S
O
O
HO OH
Source: Aldrich
Molecular weight: 250.27 g/mol
Purification: Dried under vacuum at 60 oC for 24 h.
4.2. Results and discussion
4.2.1. Polymer synthesis and characterization
4.2.1.1. Synthesis of fluorinated oligomers
The synthetic scheme for BisSF hydrophobic oligomers is shown in Figure 4.1, which was fairly
straightforward. However, the polymerization of decafluorobiphenyl with bisphenol monomers
can be complicated due to the high reactivity of the decafluorobiphenyl. The fluorine moieties at
its para positions are presumably only slightly more reactive than the ones at the ortho positions,
so careful control of the polymerization temperature was needed to avoid branching or
crosslinking (Figure 4.2). For even more reactive perfluoro monomers, such as
133
bis(pentafluorophenyl) sulfone and decafluorobenzophenone, it was even more difficult to obtain
linear polymers, and novel dehydration methods have been attempted to enable the use of even
lower reaction temperatures (Figure 4.3)
F
F F
F F
F
F
F
FF HO S OHO
O
DMAc / cyclohexaneK2CO3
110oC, 5h
F
F F
F F
F
F
F
F O S O
F
F F
F F
F
F
F
FO
On
+
(excess)
Figure 4.1. Synthesis of BisSF telechelic oligomers
O
F F
F F
F
O F
F
O SOS
S
O
O
O
O
O
O
Figure 4.2. Reaction at para- positions leading to branching
134
C
CH3
CH3
HO OH S
O
O
F
F
F F
F F F
F
F F
+
DMAc/benzene/THFK2CO3Molecular sieves 83 oC
C
CH3
CH3
O O S
O
O
F
F F
F F F
F Fn
Figure 4.3. Synthesis of fluorinated poly(ether sulfone) under mild conditions
We found, however, that the polymerization would produce a variety of results depending on
monomer stoichiometry. In the synthesis of all the fluorine-terminal BisSF oligomers, where
excess decafluorobiphenyl was used, the reactions were allowed to proceed for 5 h. However,
as shown in Figure 4.4, a high IV was attained within 2 h. No gel formation was observed during
the reactions, and NMR and GPC results ruled out the existence of branched or crosslinked
structures. However, when the other monomer, 4,4'-dihydroxydiphenyl sulfone, was used in
excess (phenoxide-terminal BisSF oligomer synthesis) under identical conditions, the gelation
would invariably occur within 1.5~2 h. This time range roughly coincides with the time needed
to achieve maximum IV of the linear chains. Thus, one may reasonably infer that bisphenol
monomers and phenoxide chain ends first tend to react preferentially with the para- fluorine
groups until they are depleted (i.e. when the linear polymerization has completed), after which
135
the free phenoxide endgroups remaining at the chain ends can react with the fluorine moieties
along the chains, leading to crosslinking.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 1 2 3 4 5 6
Polymerization Time (h)
Intrinsic Viscosity (dL/g)
Figure 4.4. Evolution of intrinsic viscosity for BisSF (17K) oligomer synthesis as a function of reaction
time
Stoichiometry may, therefore, be an alternative parameter one can adjust to control side reactions.
To verify this point, two control polymerizations were conducted using a 1:1 stoichiometry. One
of them resulted in a linear polymer with an IV of 0.86 dL/g, whereas gelation was observed in
the other. The slight inconsistency in weight, causing either monomer to be in excess relative to
the other, may have contributed to the unpredictability of these reactions. To reproducibly
synthesize high molecular weight linear polymers, a slight excess of the fluorinated monomer
may be used to avoid the existence of free phenoxide groups. Fortunately, only the
fluorine-terminated oligomers are of interest here, although phenoxide-terminated oligomers may
also be successfully synthesized through very careful control of reaction temperature and/or
time. 136
The 19F NMR spectrum of a BisSF oligomer is shown in Figure 4.5. The Mn values obtained
from GPC and 19F NMR, as well as the target values, are listed in Table 4.1. Figure 4.6 shows log
η plotted against ln(Mn) for the BisSF oligomers, wherein a linear relationship was observed.
F
F F
F F
F
F
F
F O S O
F
F F
F F
F
F
F
FO
On
a b b a a b c d
e
Figure 4.5. 19F NMR of a BisSF telechelic oligomer
137
Table 4.1. Molecular weight characterizations of BisSF oligomers
Target Mn
(g/mol
Mn from 19F NMR
(g/mol)
Mn from GPC
(g/mol)
Intrinsic Viscosity
(dL/g)
5K 5.3K 5.6K 0.15
10K 10.7K 10.9K 0.22
17K 16.8K 17.6K 0.34
25K 26.3 25.9K 0.43
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5
log(M n)
log(IV)
Figure 4.6. logη vs. logMn plot for BisSF oligomers
The 19F NMR spectra of a series of BisSF oligomers, with number average molecular weights
ranging from 5K to 25K, are displayed in Figure 4.7. As expected, the peaks corresponding to the
endgroups become smaller as the molecular weight increases.
138
a
e d
-150 -152 -154 -156 -158 -160 PPM
F
F F
F F
F
F
F
F O S O
F
F F
F F
F
F
F
FO
On
a a a d
e
1)
2)
3)
4)
a
e d
-150 -152 -154 -156 -158 -160 PPM
F
F F
F F
F
F
F
F O S O
F
F F
F F
F
F
F
FO
On
a a a d
e
1)
2)
3)
4)
Figure 4.7. 19F NMR plots of BisSF oligomers with Mn of 1) 5K; 2) 10K; 3) 17K; 4) 25K
4.2.1.2. Synthesis of Fully disulfonated hydrophilic oligomers
Fully disulfonated BPS-100 oligomers were synthesized by the nucleophilic aromatic
substitution polymerization of SDCDPS with biphenol. In our early attempts, however, the
oligomers obtained in this way usually showed lower measured molecular weights than the
values predicted based on the Carothers equation. It is now generally accepted that this
discrepancy is due to the presence of residual NaCl from the synthesis of SDCDPS, and, to a
much lesser extent, to moisture absorbed by the monomer during weighing, etc. The
139
SDCDPS/biphenol stoichiometry is therefore overestimated, which has corresponded to an
overestimation of the product's molecular weight. This problem was solved in our study by
taking a convenient engineering approach, namely, calculating the "effective" molar ratio of
SDCDPS to biphenol from the Mn, then adjusting the feed ratio accordingly, i.e. adding more
SDCDPS than calculated for the next experiment. Normally, the use of an extra 1~1.5 mol % of
SDCDPS was found to be sufficient to compensate for the monomer's impurity. Still, it is of
fundamental importance to measure the purity of the monomer. Studies of NaCl content using
UV-Vis spectroscopy are underway in our laboratory.
The purity of the solvent also seemed to affect the polymerization. As the NMP aged, the color of
the reaction solution became darker. Moreover, the difference between experimental and target
molecular weights tended to become greater, requiring even more SDCDPS to maintain the
desired stoichiometry. The reason for this phenomenon is yet unknown, but it is postulated that,
although sealed, NMP may slowly absorb oxygen from the air which causes its oxidative
decomposition under polymerization conditions. There is no evidence, however, of the chemical
integrity of the oligomer being affect by the decomposition of the solvent.
140
4.2.1.3. Synthesis of BisSF-BPSH multiblock copolymers
As illustrated in Figure 4.8, the multiblock copolymers were synthesized via a sequential
addition method. In other words, the selected BisSF oligomer was added to the same flask after
the BPS-100 oligomer synthesis was complete. The BPS-100 oligomers were not isolated before
the block copolymerization because additional steps would have been needed to remove the
residual salt. Furthermore, oligomers with very low molecular weights may not precipitate out
and this would have affected the stoichiometry
F
F F
F F
F
F
F
F O S O
F
F F
F F
F
F
F
FO
O
m
n
O S O OM
O
O
SO3M
MO3S
MO
+
m
O S O O
O
O
SO3M
MO3S
O
F
F F
F F
F
F
F
O S O
F
F F
F F
F
F
F
O
O n
x
NMP
K2CO3
100 ~ 110oC
4~5 days
Figure 4.8. Synthesis of BisSF-BPSH multiblock copolymers
In fact, in our work a perfect 1:1 stoichiometry of BisSF and BPS-100 oligomers was never
utilized. This enabled us to independently control the three parameters of a multiblock
copolymer: hydrophilic block length, hydrophobic block length, and ion exchange capacity
(IEC). In some studies, a 1:1 stoichiometry has typically been used in coupling reactions, and
control of IEC was usually achieved by changing the ratios of hydrophilic/hydrophobic block
141
lengths. For instance, a 10K-5K (hydrophobic-hydrophilic) copolymer, which has shorter
hydrophilic blocks than a 10K-10K specimen, obviously has a lower ion exchange capacity than
the latter copolymer. The same goal can be achieved by raising the hydrophobic block length to
make it a 20K-10K specimen. Thus, as IEC values change, the length of either block will also no
longer be constant. The study of the properties of multiblock copolymer membranes as a function
of ion exchange capacity is, therefore, complicated.
Previously, we demonstrated that a 1:1 stoichiometry was not necessary to synthesize block
copolymers via oligomeric coupling. This is based on the fact that because the repeat units
(blocks) have molecular weights of at least a few thousand g/mol, the degree of polymerization
(coupling) does not have to be very high to achieve a substantial total molecular weight.
Furthermore, the IEC can be controlled by simply changing the stoichiometry (molar feed ratio)
of the oligomers, while maintaining the length of the blocks. Two series of copolymers were
synthesized, each of which had three different IEC values. Within each IEC value, the
copolymers were varied with respect to block length. For simplicity, the hydrophobic block
length was altered simultaneously as the hydrophilic block became longer, and the former was
the same or slighter longer than the latter. As will be shown, however, when other parameters
were kept constant, the only variable that impacted the properties of the membrane was
hydrophobic block length.
142
19F and 1H NMR spectra of a multiblock copolymer are shown in Figures 4.9 and 4.10,
respectively. Here, the peaks due to the endgroups in the case of the BisSF and BPS100
oligomers were not observed; they either disappeared (Figure 4.9) or shifted (Figure 4.10). Thus,
a high conversion coupling reaction was thought to have been achieved. In Figure 4.10, the small
peak at about 7.3 ppm was tentatively assigned to the junction between the fluorinated and
sulfonated blocks.
mO S O
O
O
SO3M
MO3S
S O
F
F F
F F
F
F
F
OO
O n
a b b a
mO S O
O
O
SO3M
MO3S
S O
F
F F
F F
F
F
F
OO
O n
a b b a
Figure 4.9. 19F NMR spectra of a BisSF-BPSH multiblock copolymer
mO S O
O
O
SO3M
MO3S
S O
F
F F
F F
F
F
F
OO
O n
a ab b c d e f fgg
x
mO S O
O
O
SO3M
MO3S
S O
F
F F
F F
F
F
F
OO
O n
a ab b c d e f fgg
x
fg
fg
Figure 4.10. 1H NMR spectra of a BisSF-BPSH multiblock copolymer
143
An 1H NMR spectra of a series of BisSF-BPSH copolymers with increasing block lengths are
overlaid in Figure 4.11. With the other peaks normalized, it can be observed that, as block length
increased, the intensity of the 7.3 ppm peak gradually became smaller. This is consistent with the
fact that, for multiblock copolymers, the longer the segments—the smaller the number of
linkages in each copolymer chain. Therefore, this finding indicates that the hydrophilic and
hydrophobic sequences were preserved and not randomized.
Figure 4.11. 1H NMR spectra of BisSF-BPSH copolymers with increasing block lengths
4.2.1.4. Characterizations of molecular weights of BisSF-BPSH copolymers
Table 4.2 lists the characterization properties of a series of multiblock copolymers having a target
IEC of 1.3 mmeq/mol (Series A). On the one hand, the hydrophobic/hydrophilic weight charge
ratios, which determines IEC, were all the same. The molar charge ratios, on the other hand,
would depend on the hydrophobic and hydrophilic block lengths. Equation 4.1 illustrates the
relationships among the theoretical molecular weight, feed ratio, and block length.
144
2ic)](hydrophilic)(hydrophob[
r-1r1al)(theoretic nn
nMMM +
×+
= ……..(4.1)
Where Mn (theoretical) is the theoretical total molecular weight of the multiblock copolymer, r is
the hydrophilic/hydrophobic molar feed ratio, and Mn(hydrophobic) and Mn(hydrophilic)
represent the hydrophobic and hydrophilic block lengths, respectively. As expected, the
molecular weight (intrinsic viscosity) generally increased as block length increased.
Table 4.2. Characterization of BisSF-BPSH copolymers with 1.3 IEC (Series A)
Copolymer BisSF-BPSH
(xK:yK)a
Weight Feed Ratiob
Molar Feed Ratioc Target IEC IEC from
1H NMR η
(dL/g)
5K:5K 1 : 0.59 1 : 0.59 1.30 1.31 0.64
9K:7K 1 : 0.59 1 : 0.76 1.30 1.20 0.85
17K:12K 1 : 0.59 1 : 0.84 1.30 1.32 1.00
25K:16K 1 : 0.59 1 : 0.79 1.30 1.29 1.03
28K:20K 1 : 0.59 1 : 0.83 1.30 1.33 1.04
a. x and y represent hydrophobic and hydrophilic block lengths, respectively b. Hydrophobic : hydrophilic weight feed ratio c. Hydrophobic : hydrophilic molar feed ratio
The copolymers’ IV values are graphically displayed in Figure 4.12., where it can be seen that as
block lengths increased, the increase in IV gradually slowed down. Apparently, the average
degree of polymerization appears to be lower for copolymers with higher block lengths. In other
words, with an increase in block length, each block copolymer chain tends to incorporate fewer
145
blocks. This trend can be explained as follows: It is hard to achieve quantitative conversion in
such reactions, possibly due to the solution’s high viscosity near the end of the polymerization,
as well as the low concentration of reactive endgroups. If the initial endgroup concentrations are
lower in reactions of longer oligomers—and the residual concentration at the end of the
polymerization presumably shows little variation from reaction to reaction—then lower
endgroup conversion would be observed in the synthesis of longer block copolymers. This is
schematically illustrated in Figure 4.13. This would lead to lower degrees of polymerization as
governed by the Carothers equation, which takes monomer conversion (p) into account:
2rp-r1r1Xn
++
= ………4.2
2ic)](hydrophilic)(hydrophob[
2rp-r1r1al)(theoretic nn
nMM
M+
×++
= ……..4.3
According to Equation 4.2, the degree of polymerization nX decreases as p decreases. As
shown in Equation 4.3, the contributions from the molecular weights of the repeat units and
endgroup conversions may, to some extent, have cancelled each other out.
146
0
0.2
0.4
0.6
0.8
1
1.2
5K:5K 9K:7K 17K:12K 25K:16K 28K:20K
Figure 4.12. Intrinsic viscosity as a function of block lengths for BisSF-BPSH multiblock copolymers
(IEC=1.3)
End
-gro
up c
once
ntra
tion
End
-gro
up c
once
ntra
tion
Time Time
Highconversion
Lowconversion
(a) (b)
End
-gro
up c
once
ntra
tion
End
-gro
up c
once
ntra
tion
Time Time
Highconversion
Lowconversion
(a) (b)
Figure 4.13. Schematic plots showing the decrease of endgroup concentration as a function of time for
the syntheses of multiblock copolymers with (a) low block lengths; (b) high block lengths
147
The above discussion is highly qualitative and idealized. First, since no reliable molecular weight
data can be obtained from GPC for these block ionomers, intrinsic viscosity may be the only
parameter one can use to assess molecular weight. Secondly, it is impossible to know whether as
many functional groups as possible have reacted at the end of the polymerization. Finally, r, the
molar feed ratio, varies from reaction to reaction, and even small variations can cause big
changes in stoichiometry. Yet, the molar feed ratio may still be of some practical importance in
controlling molecular weight control during block copolymer synthesis.
The properties of another series of copolymers with 1.1 mmeq/mol IEC (Series B) are shown in
Table 4.3. In contrast to Series A, the feed ratios used in this series were much lower in order to
achieve lower IEC values. It can be inferred from the IV values that the copolymers still display
acceptable molecular weights despite the use of such low molar feed ratios.
Table 4.3. Characterization of BisSF-BPSH copolymers with 1.1 IEC (Series B)
Copolymer
BisSF-BPSH
(xK:yK)a
Weight
Feed Ratiob
Molar Feed
Ratioc Target IEC
IEC from 1H NMR
η
(dL/g)
17K:12K 1 : 0.45 1 : 0.64 1.10 1.06 0.78
25K:16K 1 : 0.45 1 : 0.65 1.10 1.08 0.87
28K:20K 1 : 0.45 1 : 0.63 1.10 1.04 0.95
a. x and y represent hydrophobic and hydrophilic block lengths respectively b. Hydrophobic : hydrophilic weight feed ratio c. Hydrophobic : hydrophilic molar feed ratio
148
A plot of intrinsic viscosity vs. block length for BisSF-BPSH multiblock copolymers is shown in
Figure 4.14, which clearly indicates a more linear relationship. Due to the fact that the
hydrophobic oligomers were all used in great excess, high endgroup conversion may have been
achieved without the endgroups being depleted. In other words, there would always be a large
number of residual fluorine endgroups remaining in the system, even at high oligomer
conversion. Therefore, even when low molar feed ratios were used, the target total molecular
weights would remain low, such that the low viscosity of the reaction solution would allow the
polymerizations to proceed until completion. And indeed, the molecular weights of this series of
copolymers show a trend that is consistent with predicted values.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
17K:12K 25K:16K 28K:20K
Figure 4.14. Intrinsic viscosity as a function of block lengths for BisSF-BPSH multiblock copolymers
(IEC=1.1)
Finally, the characterization of copolymers with 1.5 mmeq/mol IEC (Series C) are listed in Table
4.4. For this series of copolymers, in order to obtain high IEC values the
149
hydrophilic/hydrophobic molar feed ratios were kept quite high and close to a 1:1 ratio. The
intrinsic viscosity values for these copolymers, as expected, were lower than the ones in the other
two series.
Table 4.4. Characterizations of BisSF-BPSH copolymers with 1.5 IEC (Series C)
Copolymer BisSF-BPSH
(xK:yK)a
Weight Feed Ratiob
Molar Feed Ratioc Target IEC IEC from
1H NMR η
(dL/g)
7K:5K 1 : 0.72 1 : 1.01 1.50 1.51 1.01
17K:12K 1 : 0.72 1 : 1.02 1.50 1.46 1.12
25K:16K 1 : 0.72 1 : 1.04 1.50 1.47 1.09
28K:20K 1 : 0.72 1 : 1.01 1.50 1.44 1.08
a. x and y represent hydrophobic and hydrophilic block lengths respectively b. Hydrophobic : hydrophilic weight feed ratio c. Hydrophobic : hydrophilic molar feed ratio
Figure 4.15 compares intrinsic viscosity vs. block length for this series of copolymers. Here, the
copolymers all had similar intrinsic viscosity values, regardless of the block length, i.e., the
longer block length materials did not benefit from the high MW of the repeat units. This is due to
the fact that the synthesis of these copolymers is of a type in which quantitative conversion can
never be achieved. The coupling reactions generally stop when the concentration of reactive
endgroups falls below a certain level, and the endgroup concentration is inversely proportional to
the total molecular weight, no matter what the block lengths are.
150
0
0.2
0.4
0.6
0.8
1
7K:5K 17K:12K 25K:16K 28K:20K
Figure 4.15. Intrinsic viscosity as a function of block lengths for BisSF-BPSH multiblock copolymers
(IEC=1.5)
Two conclusions may be drawn from this discussion. First, the molecular weights of multiblock
copolymers, which have been synthesized by block copolymerizations of telechelic oligomers,
are semi-quantitatively predictable from the Carothers equation, but only when the blocks are
short, or when low molar feed ratios are used. For reactions of oligomers with high molecular
weights and/or near perfect feed ratios, the experimental molecular weights tend to deviate from
the predicted values, presumably due to low endgroup conversion. Secondly, since similar
molecular weights are achieved at both high and low feed ratios, there is no advantage in using
high molar ratios, especially for copolymers with high block lengths. This fact, as well as the fact
that acceptable molecular weights were obtained at very low feed ratios (Table 4.3), justifies
molar ratio as another parameter that can be adjusted with some success to control IEC without
adversely affecting molecular weight.
151
4.2.1.5. Thermal analysis
The DSC trace of the BisSF-BPSH (17K-12K) multiblock copolymer is shown in Figure 4.16.
The copolymer was tested in the salt (K+) form because the acid-form copolymer membranes are
known to be thermally and oxidatively unstable above about 240 oC. A sharp transition was
observed at 205 oC and was assigned to the Tg of BisSF blocks. Another very broad transition,
covering a temperature range of 230-270 oC, was linked to the relaxation of the BPSH ionic
blocks. The electrostatic interactions among sulfonic groups in this system probably compound
the intermolecular forces, resulting in the broad thermal transition. Nevertheless, the observation
of the clear Tg for fluorinated blocks indicates the existence of nanophase separated block
structures.
The TGA traces of BisSF-BPSH (17K-12K) copolymers, whose IECs range from 1.1 to 1.5 are
shown in Figure 4.17. Such copolymers generally display a three-step weight loss: desulfonation
(~270 oC), degradation of BPSH blocks (~490 oC), and degradation of BisSF blocks (~550 oC).
The three copolymers in Figure 4.17 displayed very similar weight loss behaviors, which differed
slightly in terms of percentage weight within the range of 270 ~ 490 oC. As expected, this
suggests different degrees of desulfonation, caused by differences in the ion content of the
copolymers (IEC). Similar trends have been observed for BPSH-xx random copolymers.
152
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
160 180 200 220 240 260 280 300
Hea
t Flo
w (W
/g)
Temp (oC)
Tg of BisSF blocks
BPSH blocks
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
160 180 200 220 240 260 280 300
Hea
t Flo
w (W
/g)
Temp (oC)
Tg of BisSF blocks
BPSH blocks
Figure 4.16. DSC trace of a BisSF-BPSH (17K-12K) multiblock copolymer
-20
0
20
40
60
80
100
120
0 100 200 300 400 500 600 700
1. BisSF-BPSH (17K-12K) IEC=1.12. BisSF-BPSH (17K-12K) IEC=1.33. BisSF-BPSH (17K-12K) IEC=1.5
Wei
ght %
Temp (oC)
1
23
Figure 4.17. TGA traces of BisSF-BPSH (17K-12K) multiblock copolymers
153
4.2.1.6. Mechanical Properties
Stress-strain test results for the BisSF-BPSH (17K-12K) (IEC=1.5) copolymer membranes are
illustrated in Figure 4.18. These materials show the typical mechanical response of glassy
amorphous polymers. Copolymer membranes having the same block length—but an IEC of
1.1—were also tested, and Table 4.5 lists the moduli, tensile strength, and elongation-at-break
results for the two samples. The low-IEC copolymer had lower molecular weight, but according
to Table 4.5, it displayed a higher modulus than the high-IEC sample. The two membranes,
however, had very similar tensile strengths. The molecular weight, therefore, has no significant
effect on the moduli of the copolymers. The differences in moduli may have resulted from the
differences in their composition. In addition, the material having an IEC of 1.1 had a higher
content of fluorinated (hard) segments and fewer BPSH (soft) segments. In general, BPSH
blocks are relatively soft because they readily absorb moisture, which acts as a plasticizer. The
only negative effect of low-IEC (low molecular weight) on the copolymer’s mechanical
properties was lower ductility, as reflected in its lower elongation-at-break values compared to
the high-IEC materials.
154
Elongation
Stre
ss (M
pa)
0
10
20
30
40
50
60
0% 10% 20% 30% 40% 50%
Elongation
Stre
ss (M
pa)
0
10
20
30
40
50
60
0% 10% 20% 30% 40% 50%
Figure 4.18. Stress-strain curves for BisSF-BPSH (17K-12K) (IEC=1.5) copolymers
Table 4.5. Effects of IEC on the mechanical properties of BisSF-BPSH (17K-12K) multiblock
copolymers
IEC 1.1 1.5
Modulus (MPa) 1191 1716
Tensile Strength (MPa) 50.4 48.5
Elongation-at-Break (%) 29.4 44.9
4.2.1.7. Surface morphological features
Block copolymers are well known to form a nanophase separated morphology. The
tapping-mode atomic force micrographs of four BisAF-BPSH multiblock copolymers, with
increasing block lengths, are shown in Figure 4.19. Here, the bright regions are due to the
fluorinated hydrophobic component, while the dark regions represent the hydrophilic, sulfonated
155
domains. From the phase images it can be seen that, with an increase in block length, the
nanophase separation between hydrophobic and hydrophilic domains becomes sharper and,
moreover, the hydrophilic domains tend to become increasingly connected. For the copolymer
with the highest block lengths, i.e. BisSF-BPSH (25:16), co-continuous hydrophilic domains can
be observed. This increased connectivity with an increase in sequence length has been identified
for other multiblock copolymer systems as well. In fact, it is thought to be responsible for the
increase in proton conductivity with increasing block length under partially hydrated conditions
(to be documented in a later publication).
156
100 nm
100 nm
100 nm
100 nm
5K-5K
17K-12K
25K-16K
7K-7K
100 nm
100 nm
100 nm
100 nm
100 nm
100 nm
100 nm
100 nm
5K-5K
17K-12K
25K-16K
7K-7K
Figure 4.19. Tapping mode AFM height (left) and phase (right) images for BisSF-BPSH (5K-5K),
(7K-7K), (17K-12K) and (25K-16K) multiblock copolymer membranes
157
4.2.2. Study of fuel cell-related properties
4.2.2.1. Effects of block lengths on proton conductivity and water uptake
The proton conductivity for selected BisSF-BPSH copolymers is shown in Figure 20. These
samples all have a nominal IEC of 1.3 and display gradually increasing block lengths. As shown,
the proton conductivity at 30 oC in liquid water gradually increased with IEC, but reached a
maximum of 0.13 S/cm for the 17K:12K sample.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
Nafion112 BPSH-35 7K:5K 10K:8K 15K:10K 17K:12K 25K:16K
Proton Conductivity (S/cm)
Figure 4.20. Proton conductivity for BisSF-BPSH copolymers having an IEC of 1.3
The water uptake values for this series of copolymers are shown in Figure 4.21. The data are
shown as a function of the hydrophilic (BPSH) block length. When it was below 16K,
corresponding to the BisSF-BPSH (25K-16K) copolymer, the water uptake gradually increased
up to around 40 wt%, which was well within the acceptable range for the membrane’s
dimensional stability and long-term durability. When the hydrophilic block length was further
158
increased to 20K, corresponding to a BisSF-BPSH (28K-20K) copolymer, the water uptake
experienced a sharp increase to over 130 wt%. At this level, the membrane loses all of its
mechanical strength in the water-swollen state and becomes impractical for use as a PEM.
0
20
40
60
80
100
120
140
160
0 10 20
Hydrophilic block length (Kg/mol)
Wat
er u
ptak
e (w
t%)
30
Figure 4.21. Water uptake as a function of Block lengths for BisSF-BPSH (IEC=1.3) copolymers
A plot of water uptake vs. disulfonation degree for BPSH random copolymers is shown in Figure
4.22. A similar trend was observed here; namely, the water uptake increased almost linearly as a
function of the degree of disulfonation and then rose abruptly to over 180 wt%. This is
commonly referred to as the “phase inversion” (i.e. percolated hydrophilic and isolated
hydrophobic domains) or the point at which the percolation limit has been exceeded. The high
water uptake for BisSF-BPSH (28K-20K) copolymers (Figure 4.21) can therefore be viewed as
the block copolymer equivalent of the percolation limit. Thus, we have found that the same
159
phenomenon that occurs in random copolymers as a result of excessive IEC, can also occur in
multiblock copolymers as a result of excessive block length. It is also clear that when an IEC of
1.3 is utilized, the maximum block length that can be used without losing dimensional stability is
25K-16K.
0
20
40
60
80
100
120
140
160
180
200
0 20 40 60
mol% Disulfonation
Wat
er u
ptak
e (w
t%)
80
0
20
40
60
80
100
120
140
160
180
200
0 20 40 60
mol% Disulfonation
Wat
er u
ptak
e (w
t%)
80
Figure 4.22. Water uptake as a function of disulfonation degree for BPSH random copolymers
The performance characteristics of a multiblock copolymer under partially-hydrated conditions is
extremely important for a PEM. Figure 4.23 shows proton conductivity at 80 oC as a function of
relative humidity (RH) for a series of BisSF-BPSH multiblock copolymers. The data for Nafion
112 and BPSH-35 random copolymer membranes are shown for comparison. As shown, the
conductivity for BPSH-35 decreased very rapidly as RH decreased. The performance of the
block copolymer membranes, however, was much higher and consistently increased with
increases in block length. This is consistent with the more developed nanophase separated
160
morphology illustrated in our AFM characterization studies (Figure 4.19). As our results
indicated, however, Nafion 112 was still the best material we evaluated, outperforming the
BisSF-BPSH (25K-16K) copolymers. Since the 25K-16K materials were the highest block
length copolymers we used (while maintaining reasonable water uptake), the BisSF-BPSH
(25K-16K) materials may represent the upper limit of performance improvements by means of
altering (hydrophilic) sequence lengths.
0.01
0.1
1
10
100
1000
20 30 40 50 60 70 80 90 100
(1) Nafion112(2) 25K-16K(3) 15K-10K(4) 10K-8K(5) 5K-5K(6) BPSH-35 Random
Proton Conductivity (mS/c
m)
Relative Humidity (%)
1
2
34
5
6
Figure 4.23. Proton conductivity at 80 oC as a function of relative humidity for BisSF-BPSH (IEC=1.3)
copolymers
Thus, the BisSF-BPSH series copolymers generally showed high liquid-water proton
conductivity (up to 0.13 S/cm), but at similar IECs and block lengths, their performances under
partially hydrated conditions generally seemed to be lower than those of the BisAF-BPSH and
161
6FBisAF-BPSH systems. This indicates a lower extent of nanophase separation between the
BisSF and BPSH blocks. In other words, the presence of the sulfone groups in the BisSF
blocks may cause some similarity between the hydrophobic and hydrophilic blocks, thus
discouraging them from phase separating.
4.2.2.2. Effects of composition (IEC) on proton conductivity and water uptake
Another parameter that can be tailored in multiblock copolymers, in addition to block length, is
the ion exchange capacity. With the goal of achieving higher performance at low RH,
BisSF-BPSH copolymer membranes having higher IEC values were synthesized and tested.
Table 4.6 shows some characterizations of two series of copolymers: BisSF-BPSH (17K-12K)
and BisSF-BPSH (25K-16K). The IEC values of each series varied from 1.1 to 1.5. As shown,
when the IEC was increased from 1.1 to 1.3, significant changes were observed in both liquid
water conductivity (30 oC) and water uptake. However, there seemed to be little effect on IEC as
documented by the fact that it only increased from 1.3 to 1.5.
Table 4.6. The effect of IEC on the properties of BisSF-BPSH copolymers
Block Lengths (g/mol)
IEC Molar Feed Ratio
Intrinsic Viscosity
(dL/g)
Water Uptake Proton Conductivity
at 30oC (S/cm)
17K-12K 1.1 1 : 0.64 0.78 25% 0.08 17K-12K 1.3 1 : 0.84 1.00 42% 0.13 17K-12K 1.5 1 : 1.02 1.02 43% 0.13 25K-16K 1.1 1 : 0.65 0.95 23% 0.08 25K-16K 1.3 1 : 0.84 1.03 44% 0.13 25K-16K 1.5 1 : 1.04 1.04 50% 0.14
162
The effects of IEC on the performance of copolymers under partially hydrated conditions have
also been studied. Figure 4.24 shows the proton conductivity at 80 oC as a function of RH for
both BisSF-BPSH (17-12) copolymers and Nafion 112. Here, in contrast to the case of fully
hydrated conditions, performance significantly increased as IEC increased from 1.3 to 1.5.
Moreover, the copolymer with an IEC of 1.5 outperformed the Nafion 112. Thus, what could not
be achieved by increasing block length was accomplished by changing the IEC. More
importantly, good performance was obtained without sacrificing durability, since the increase in
IEC did not result in any changes in water uptake.
0.1
1
10
100
1000
20 30 40 50 60 70 80 90 100
(1) 17K-12K (IEC=1.5)(2) 17K-12K (IEC=1.3)(3) 17K-12K (IEC=1.1)(4) Nafion112
Proton Conductivity (mS/cm)
Relative Humidity (%)
1
2
3
4
Figure 4.24. Proton conductivity vs. RH: the effect of IEC on the performance of BisSF-BPSH (17K-12K)
copolymers
163
Figure 4.25 shows the same data for the BisSF-BPSH (25K-16K) copolymers. Again, the
increase in IEC resulted in higher performance. It is worth noting that the performance of the
17K-12K samples exhibited a greater dependence on IEC than did the 25K-16K copolymers,
possibly because the longer block samples displayed a better established morphology, regardless
of ion content. However, it is not clear why the 25K-16K (IEC=1.5) sample performed poorly in
comparison to the 17K-12K (IEC=1.5) sample, even though the former has longer blocks. This
might have been due to the fact that both the hydrophobic (BisSF) and hydrophilic (BPSH)
blocks are higher in the 25K-16K (IEC=1.5) sample. As will be discussed later, while longer
hydrophilic (BPSH) blocks favor higher performance, an increase in hydrophobic block length
tends to lower a membrane’s transport properties. These are contradicting variables and in most
cases, and the hydrophilic block length effect seems to dominate when BisSF and BPSH block
lengths are raised simultaneously. For the 25K-16K copolymer (IEC=1.5), however, it is thought
that the BisSF block is so long that its influence is dominant over the BPSH block.
164
0.1
1
10
100
1000
20 30 40 50 60 70 80 90 100
(1) 25K-16K (IEC=1.5)(2) 25K-16K (IEC=1.3)(3) 25K-16K (IEC=1.1)P
roton Conductivity (mS/cm)
Relative Humidity (%)
1
2
3
Figure 4.25. Proton conductivity vs. RH: the effect of IEC on the performance of BisSF-BPSH (25K-16K)
copolymers
4.2.2.3. Effects of hydrophobic block length (hydrophobic/hydrophilic block length ratio)
Up to this point, the hydrophobic blocks of most of the copolymers we have studied have been
longer than the hydrophilic blocks for the same copolymer—and have increased as the latter
have increased. Therefore, the discrete effects of only the hydrophobic block on a copolymer’s
properties have not been studied. The properties of a BisSF-BPSH (15K-15K) copolymer, with
an IEC of 1.5, are listed in Table 4.7. The BisSF-BPSH (25K-16K) sample is also shown for
comparison. In contrast to the 25K-16K sample, the 15K-15K copolymer, with its shorter
hydrophobic blocks, displayed slightly higher proton conductivity at 30 oC in liquid water and
significantly higher water uptake (78% vs. 42%). We surmised that this sample probably has a 165
percolated morphology simlar to the 28K:20K (IEC=1.3) material. As shown in Figure 4.26, the
BisSF-BPSH (15K-15K) materials exhibited significantly higher performance than the
BisSF-BPSH (25K-16K) copolymers under partially hydrated conditions.
Table 4.7. Comparison between BisSF-BPSH multiblock copolymers having different
hydrophobic block lengths
Block Lengths
(g/mol)
IEC Proton Conductivity
at 30 oC (S/cm)
Water Uptake
15K:15K 1.5 0.14 78%
25K:16K 1.5 0.13 50%
1
10
100
1000
20 30 40 50 60 70 80 90 100
(1) BisSF-BPSH (25K-16K)(2) BisSF-BPSH (15K-15K)
Pro
ton
Con
duct
ivity
(mS
/cm
)
Relative Humidity (%)
2
1
Figure 4.26. Proton conductivity under partially hydrated conditions for BisSF-BPSH (25K-16K)
(IEC=1.5) and BisSF-BPSH (15K-15K) (IEC=1.5)
166
As another example, Table 4.8 shows the properties of BisSF-BPSH (25K:12K) and
BisSF-BPSH (17K:12K) multiblock copolymers with IEC values of 1.3. As shown, these two
materials differed only in their hydrophobic block lengths. Again, the (25K:12K) copolymers
showed much lower water uptake and proton conductivity than the 17K:12: materials. Thus, the
molecular weight of the hydrophobic block alone does have a substantial effect on a copolymer’s
properties.
Table 4.8. Comparison between BisSF-BPSH multiblock copolymers having different
hydrophobic block lengths
Block Lengths
(g/mol)
IEC Proton Conductivity
at 30 oC (S/cm)
Water Uptake
25K:12K 1.3 0.04 23%
17K:12K 1.3 0.13 50%
In summary, given a certain structure of the hydrophobic and hydrophilic blocks, the
performance of a hydrophilic/hydrophobic multiblock copolymer with a certain chemical
structure is a function of three parameters: hydrophilic block length, IEC, and hydrophobic block
length. New synthetic methodologies have enabled us to tailor a material for optimal
performance by controlling all three parameters independently. Accordingly, within a certain
range the proton conductivity of a copolymer at low hydration levels can increase when any of
the following occurs:
167
The hydrophilic block length increases;
Or the hydrophobic block length decreases;
Or the IEC increases.
However, an increase in conductivity performance would generally be accompanied by an
increase in water uptake. But since conductivity and water sorption can change by different
degrees depending on nature of the copolymer, it is critical to determine the optimum balance
between these two variables. Thus far, the BisSF-BPSH (17K-12K) (IEC=1.5) copolymers
have displayed the highest performance/water sorption selectivity of all the multiblock
copolymers we tested based on our careful control of composition and sequence length.
4.2.2.4. Swelling-deswelling properties of multiblock copolymers
The weight-based water sorption of a PEM is an important parameter in determining its
mechanical strength in the water swollen state. However, we have recently been studying the
volume-based water swelling ratio, which may be more relevant to the long-term stability of a
membrane electrode assembly (MEA). When a membrane undergoes significant swelling in
water, considerable stress is created at the interface between the membrane and the electrodes,
causing fatigue of the PEM during the wet-dry cycles. These swelling-deswelling cycles can
eventually to the delamination or failure of the membrane. We have found, however, that the
BisSF-BPSH multiblock copolymer membranes display attractive swelling-deswelling behavior,
making them promising as alternative PEMs.
168
The volume-based swelling ratios for the copolymers were measured in x, y and z axes. Figure
4.27 shows the results for BisSF-BPSH (17:12) (IEC=1.3), BisSF-BPSH (5K:5K) (IEC=1.3)
copolymers, BPSH-35 random copolymer membranes, and Nafion 117. The BPSH-35
copolymer displayed an isotropic swelling behavior, that is, it had similar degrees of swelling in
all x, y and z directions. Nafion, however, exhibited different swelling in different directions.
Specifically, the increase in thickness (z axis) was greater than in x-y plane. This anisotropic
swelling behavior has its origin in Nafion’s unique nanophase separated morphology.
0
5
10
15
20
25
30
1 2 3 4
X
Y
Z
BPSH-35 Nafion112 5K-5K 17K-12K0
5
10
15
20
25
30
1 2 3 4
X
Y
Z
BPSH-35 Nafion112 5K-5K 17K-12K
Figure 4.27. Degrees of swelling in x, y and z directions for different copolymer membranes
The block copolymers with different block lengths showed very different swelling behaviors due
to differences in their morphology. The BisSF-BPSH (5K-5K) sample had slightly more in-plane
swelling than through-plane swelling. The copolymers with long hydrophobic and hydrophilic
blocks, however, showed different swelling patterns. For example, the 17K-12K sample
demonstrated tremendously higher swelling in the z axis than in the x-y plane. This is exactly
169
what is desired for a PEM, because the stability of the membrane-electrode interfaces should be
mostly dependent on the in-plane swelling, and should not be strongly affected by any increase
in the membrane thickness. This strong anisotropic swelling has been found for other types of
hydrophilic-hydrophobic multiblock copolymers, which we believe will result in significant
improvements in the MEA’s long term durability, making these materials promising candidates
as PEMs. Some propose that Nafion, with its sulfonic acid group-bearing side chains, possesses
some block copolymer-like characteristics. The argument is supported by the fact that Nafion
shows a swelling behavior that falls somewhat in between random and block copolymers.
The swelling-deswelling properties of BisSF-BPSH (17K-12K) and BisSF-BPSH (25K-16K)
copolymers are shown in Figure 4.28. Each series contains two copolymers having IECs of 1.3
and 1.5, respectively. As shown in Table 4.6, although the four copolymers possess very similar
weight-based water sorption levels, in this graph one can observe that the z-direction swelling
degree for the multiblock copolymers increases as the block length or IEC is increased. This can
be ascribed to the higher proportion of “free water” (freezable water) in a system with a more
developed nanophase separated morphology due to higher block lengths or IEC.
170
BPSH-350
10
20
30
40
50
60
70
Swel
ling
Rat
io (%
)X
Y
Z
Nafion112 17K-12KIEC=1.3
17K-12KIEC=1.5
25K-16KIEC=1.3
25K-16KIEC=1.5
BPSH-350
10
20
30
40
50
60
70
Swel
ling
Rat
io (%
)X
Y
Z
Nafion112 17K-12KIEC=1.3
17K-12KIEC=1.5
25K-16KIEC=1.3
25K-16KIEC=1.5
Figure 4.28. Multiblock copolymers with long blocks or higher IEC show higher z-direction swelling
It is worth noting here that while the swelling degree in the z direction increased as the block
lengths and/or IEC increased, these parameters had no significant effect on swelling in x and y
directions. Thus, if such swelling behavior persists for copolymers having even higher block
lengths and/or IEC (at least to a certain extent), then one may be able to obtain copolymer
membranes that show higher performance—but that still possess dimensional stability. That said,
there must be a limit to this approach. For example, the BisSF-BPSH (28K-20K) (IEC=1.3)
and BisSF-BPSH (15K-15K) (IEC=1.5) samples demonstrated completely different water
sorption and swelling properties compared to other materials in the series as a result of
qualitative, significant changes in morphology.
171
Although we are uncertain as to why the copolymer membranes tend to swell in the z direction,
with instruments currently available it is impossible to characterize the cross-sectional
morphology of a water-swollen membrane. However, we propose a possible explanation for
the z-direction swelling, as follows. During the casting of a membrane, the block copolymer
chains may be orientated parallel to the membrane plane, but there is no orientation in the x-y
plane. The hydrophilic blocks may be brought together vertically through self-assembly, thereby
creating the hydrophilic domains (Figure 4.29). When the membrane is wet, the ionic domains
swell vertically as water molecules enter the intermolecular spaces, causing an increase in the
membrane’s thickness. Along the x-y plane, however, intermolecular forces and chain
entanglements preclude any significant degree of parallel expansion due to lack of polymer chain
orientation. The reason why the BisSF-BPSH (5K-5K) copolymer demonstrated largely isotropic
swelling may have been due to the less sharp nanophase separation (and thus the absence of
self-assembly) associated with the short sequence lengths in this material.
BisSF block BPSH block
z
BisSF block BPSH block
z
Figure 4.29. Imaginary cross-sectional view of BisSF-BPSH (17K-12K) copolymer membranes
172
If this somewhat idealized scenario is true, then it turns out that the multiblock copolymer PEMs
have another great advantage: the ionic, hydrophilic domains are preferentially orientated in the z
direction. This would cause the through-plane conductivity, which dominates fuel cell
performance, to be much higher than the in-plane conductivity. Therefore, the block copolymer
PEM approach is promising in terms of both performance (in a real-world fuel cell testing
environment) and durability.
4.2.3. Some comparisons of BisAF-BPSH, 6FBisAF-BPSH and BisSF-BPSH multiblock
copolymers
All three series of copolymer membranes studied thus far, BisAF-BPSH (Series 1),
6FBisAF-BPSH (Series 2) and BisSF-BPSH (Series 3), have shown superior performance over
random copolymers under both fully- and partially hydrated conditions. However, both Series 1
and Series 2 multiblock copolymers suffer from disadvantages that prevent them from being
seriously considered for fuel cell applications.
For example, with respect to the Series 1 materials (BisAF-BPSH), the hydrolytic stability of the
isopropylidene units has been questioned. We postulated that by catalyzing the sulfonic acid
groups, a reverse reaction of bisphenol-A synthesis would occur in water at elevated
temperatures, leading to chain scission. However, a BisAF-BPSH membrane that was kept in
water at 80 oC for up to 8 days did not show any decrease in intrinsic viscosity—nor did NMR
173
studies reveal any structural changes. Therefore, the concentration of the sulfonic acid groups is
probably not high enough to catalyze the degradation.
Instead, it may be that thermal oxidative stability is more important. To make a membrane
electrode assembly, the copolymer membrane must undergo a “hot-pressing” procedure at 220 oC
for 10 min to obtain good interfaces between the membrane and the electrodes. After
hot-pressing the BisAF-BPSH copolymer, it would become dark-colored and brittle; whereas
samples from the other two series did not show any visible change. Thus, although the
BisAF-BPSH copolymers may be durable in the fuel cell operation environment, they are
currently unable to survive essential processing procedures due to the existence of thermally
unstable isopropylidene groups—thereby making them impractical.
The BisSF-BPSH copolymers (Series 3), therefore, are the most promising candidates as PEMs.
They do not suffer from thermal oxidative stability issues associated with Series 1 copolymers,
and they do not show the excessive water sorption and swelling-deswelling behaviors that were
observed in the Series 2 membranes. Moreover, the 4,4’-dihydroxydiphenol (Bisphenol-S)
monomer is very inexpensive, which makes this approach also economically more attractive than
Series 1 and 2.
The TGA traces for the copolymers from all three series are shown and compared in Figure 4.30.
As expected, the BisAF-BPSH copolymer displayed much more rapid weight loss than the other
174
two series. Also, the BisSF-BPSH and 6FBisAF-BPSH copolymers showed very similar weight
loss profiles, with the former just as thermally stable as the latter.
-20
0
20
40
60
80
100
120
0 100 200 300 400 500 600 700
1. BisAF-BPSH 8K-8K2. BisSF-BPSH 17K-12K3. 6FBisAF-BPSH 12K-9K
Wei
ght %
Temp (oC)
12
3
Figure 4.30. Comparison of TGA traces for three series of multiblock copolymers
175
Chapter 5. Conclusions
Hydrophilic-hydrophobic, sulfonated-fluorinated multiblock copolymers, were obtained through
moderate temperature (100°C) coupling reactions between fluorinated and fully disulfonated
telechelic oligomers. Three series of hydrophilic-hydrophobic copolymers, BisAF-BPSH (Series
1), 6FBisAF-BPSH (Series 2), and BisSF-BPSH (Series 3), were studied. Their structures were
similar except for the molecular structure of the bisphenol unit in the hydrophobic fluorinated
blocks: isopropylidene for Series 1, hexafluoroisopropylidene for Series 2, and a sulfone group
for Series 3. The highly reactive fluorine endgroups in the hydrophobic oligomers enabled the
coupling reactions to be run at low temperatures (100 ~ 115 oC) at reasonable rates, in order to
minimize the possibility of ether-ether interchange. The composition (ion exchange capacity) of
these copolymers could be conveniently controlled through molar charge ratios of
hydrophilic/hydrophobic telechelic oligomers and verified by 1H NMR studies or titration. The
existence of block sequences in these materials was supported by 1H NMR results, where the
distinct peak associated with the junction of the hydrophobic and hydrophilic blocks could be
observed. Nanophase separated morphologies have been confirmed by atomic force microscopy
studies, and the high molecular weights were confirmed by intrinsic viscosity and indirectly by
mechanical testing results.
All three series of copolymer membranes studied have shown superior performance over random
copolymers under both fully and partially hydrated conditions, which is probably related to
easier water diffusion paths through the co-continuous hydrophobic phase. However, both 176
BisAF-BPSH (Series 1) and 6FBisAF-BPSH (Series 2) copolymers suffer from disadvantages
that make them less suitable as PEMs. The former may be thermally and/or hydrolytically
unstable under fuel cell conditions due to the presence of aliphatic isopropylidene groups; while
the latter ones—although much more stable—show excessive water sorption and swelling that
may reduce their mechanical and dimensional stability. However, the BisSF-BPSH (Series 3)
copolymers are the most promising candidates as PEMs: they possess a wholly aromatic
backbone, which makes them as stable as Series 2 polymers, and they also show controllable
water sorption and swelling properties. The low cost of the 4,4’-dihydroxydiphenol (Bisphenol-S)
monomer also contributes to their economical advantages.
A major discovery was made in the synthesis of all the multiblock copolymers
studied. Specifically, the molar feed ratios of hydrophilic/hydrophobic telechelic moderate
molecular weight oligomers in the coupling reactions do not have to be close to 1:1 and can be as
low as 0.6:1. Another important outcome of this study was the development of an effective,
novel synthetic methodology that was systematically used in the synthesis of the BisSF-BPSH
(Series 3) copolymers. Because the segment repeat units (blocks) in these copolymers had
molecular weights of at least 5Kg/mol, only a low degree of polymerization (coupling) was
needed to achieve a high total molecular weight. Therefore, the hydrophobic block length,
hydrophilic block length, and IEC can be altered independently to some degree to tailor the
properties of copolymer membranes.
177
The electrochemical properties of the BisSF-BPSH copolymer membranes were found to be
affected by their composition (IEC) and sequence lengths. While proton conductivity and water
uptake are both favored by higher IEC and longer hydrophilic (sulfonated) block lengths,
increasing the hydrophobic (fluorinated) block length was found to adversely affect these
properties. The effects of all three parameters on the performance of the membranes were studied
independently. Through careful control of block length and IEC, the optimal trade-off between
performance (proton conductivity under partially hydrated conditions) and water uptake was
obtained with a BisSF-BPSH (17K-12K) (IEC=1.5) copolymer. Although BisSF-BPSH
copolymers generally demonstrated reduced performance compared to Series 1 and Series 2
copolymers at similar IEC and block lengths, these materials were the only ones in the series that
were able to outperform the commercial product, Nafion 112, while still maintaining low water
uptake (~40 wt%) well within the acceptable range.
The BisSF-BPSH multiblock copolymers also displayed interesting swelling-deswelling
behaviors. In contrast to sulfonated poly(ether sulfone) random copolymers, which swell
isotropically in water, these block copolymer membranes swelled much less in the x-y plane than
in the z direction (through-plane). This should significantly reduce the stress at the interface
between the membrane and the electrode, which can result from a significant amount of in-plane
swelling-deswelling during wet-dry cycles. The long-term durability of the MEAs, therefore,
may be greatly improved.
178
Although these sulfonated-fluorinated multiblock copolymers offer improved performance
compared to random copolymer membranes, their practical application may be limited because
the decafluorobiphenyl monomer needed to produce them is not yet commercially available.
However, the high reactivity of the fluorine endgroups is essential because the coupling reactions
must be conducted at low temperatures to avoid an ether-ether interchange. Research in our
group has recently focused on an alternative type of multiblock copolymer, where the
hydrophobic oligomers are mostly hydrocarbon-based and bear fluorinated moieties only at the
chain-ends. In principle, these oligomers have the same reactivity as the fully fluorinated ones,
enabling multiblock copolymers to be obtained through low-temperature coupling reactions as
well.
179
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