The Influence of Aromatic Disulfonated Random and Block Copolymers’ Molecular Weight, Composition,
and Microstructure on the Properties of Proton Exchange Membranes for Fuel Cells
Yanxiang Li
Dissertation submitted to the faculty of Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY In
Macromolecular Science and Engineering
Dr. James E. McGrath, Chairman
Dr. Judy Riffle
Dr. Larry Taylor
Dr. John G. Dillard
Dr. Richey M Davis
August 31, 2007 Blacksburg, Virginia
Keywords: proton exchange membrane, fuel cell, disulfonated copolymers, molecular weight, random, multiblock, morphology, poly(arylene
ether sulfone), poly(arylene ether ketone)
Copyright 2007, Yanxiang Li
The Influence of Aromatic Disulfonated Random and Block Copolymers’ Molecular Weight, Composition,
and Microstructure on the Properties of Proton Exchange Membranes for Fuel Cells
Yanxiang Li
(ABSTRACT)
The purity of the disulfonated monomer, such as 3,3’-disulfonated-4,4’-
dichlorodiphenyl sulfone (SDCDPS), was very important for obtaining high molecular
weight copolymers and accurate control of the oligomer’s molecular weight. A novel
method to characterize the purity of disulfonated monomer, SDCDPS, was developed by
using UV-visible spectroscopy. This allowed for utiliziation of the crude SDCDPS
directly in the copolymerization to save money, energy, and time.
Three series of tert-butylphenyl terminated disulfonated poly(arylene ether sulfone)
copolymers (BPSH35, 6FSH35, and 6FSH48) with controlled molecular weights( nM ),
20 to 50 kg·mol-1, were successfully prepared by the direct copolymerization method. The
molecular weight of the copolymer was controlled by a monofunctional monomer tert-
butylphenyl, and characterized by the combination of 1H NMR spectra and modified
intrinsic viscosity measurements in NMP with 0.05 M LiBr, which was added to suppress
the polyelectrolyte effect. The mechanical properties of the membranes, such as the
modulus, strength and elongation at break, were improved by increasing the molecular
weights, but water uptake and proton conductivities found insensitive to copolymers’
molecular weights.
iii
Three series of disulfonated poly(arylene ether ketone) random copolymers have
been synthesized and comparatively studied, according to their different chemical
structures, for use as proton exchange membranes. The copolymers containing more
flexible molecular structures had higher water uptake and proton conductivity than the
rigid structures at the same ion exchange capacity. This may be due to the more flexible
chemical structures being able to form better phase separated morphology and higher
hydration levels.
A new hydrophobic-hydrophilic multiblock copolymer has been successfully
synthesized based on the careful coupling of a fluorine terminated poly(arylene ether
ketone) (6FK) hydrophobic oligomer and a phenoxide terminated disulfonated
poly(arylene ether sulfone) (BPSH) hydrophilic oligomer. AFM images and the water
diffusion coefficient results confirmed that the multiblock copolymer formed better proton
transport channels. This multiblock copolymer showed comparable proton conductivity
and fuel cell performance to the Nafion® control and had much better proton transport
properties than random ketone copolymers under partially hydrated conditions. This
suggested that the multiblock copolymers are promising candidates for proton exchange
membranes especially for applications at high temperatures and low relative humidity.
iv
Dedicated to my loving husband, Jun and dear son Dachuan for their understanding, support, and love
v
ACKNOWLEDGEMENTS
I would first like to give my sincere gratitude to my advisor, Dr. James E. McGrath,
for providing me with the opportunity of being his student, which I feel really lucky and
proud of. I especially thank him for his guidance, encouragement, and patience over my
graduate years. I have greatly benefited not only from his breadth and depth of knowledge,
but also from his wonderful personality, which will continue to be a great source of
inspiration for me. I would also like to thank the members of my advisory committee, Dr.
Judy S. Riffle, Dr. John G. Dillard, Dr. Richey M Davis, and Dr. Larry Taylor for their
valuable suggestions and support. A special thanks goes to Dr. Judy. S. Riffle for her
excellent classes, entitled Presentation Skills and Written Skills. I also appreciate Dr.
Richey M Davis very much for his advice and helpful discussions.
I would like to give a big thanks to our wonderful secretarial ladys, Mrs. Laurie
Good, Mrs. Millie Ryan, and Mrs. Angie Flynn, for their kindness and tremendous
assistance during my stay at Virginia Tech.
Many thanks go to my colleagues in our great research group for their selfless
assistance and friendship: Ahbishek Roy, Anand S. Badami, Juan Yang, Hang Wang, Dr.
William Harrison, Dr. Melinda Einsla, Dr. Brian Einsla, Dr. Charles Tchatchoua, Dr.
Thekkekara Mukundan, Dr. Kent Wiles, Dr. Zhongbiao Zhang, Natalie Arnet, Xiang Yu,
Haeseung Lee, Rachael VanHouten, Ozma Lane, and Dr. Guangyu Fang.
I especially wish to express my acknowledgments to Tom Glass, Anand S.
Badami, Juan Yang, Ahbishek Roy, and Ozma Lane, for their assistance in NMR, AFM,
GPC, and PEM properties measurements. A special thanks also goes to Mrs. Lauie Good,
vi
Dr. Melinda Einsla, Rachael VanHouten, Anand S. Badami, and Ozma Lane for helping
me with preprints, manuscripts, and presentations. I am really grateful to all of them.
Fianally, I would like to thank all my family members. My parents, Mr. Shuhe Li
and Mrs. Gonghua Xu, My sister Yanhong Li, and my brother Zhigang Li are always there
to support me. Many thanks are given to my parents-in-law, Mr. Tingxi Yan and Mrs
Ruiping Hou, family of my sister-in-law Hong Yan, Gang Zhou, and my lovely niece
Tianyi Zhou for their help in taking care of my son, which makes it possible for me to
focus on my Ph.D. study. I sincerely thank my husband and my best friend, Jun Yan, for
his love, understanding, patience, and encouragement, without which I would not have
succeeded this far. Most importantly, I would like to thank our dear son, Dachuan Yan.
His bright smile was always the source of my energy, and he was there to remind me often
of what was most important in life.
vii
ATTRIBUTION
Several colleagues and coworkers aided in the writing and research behind several
of the chapters of this dissertation. A brief description of their contributions is included
here.
Prof. James E. McGrath is the primary advisor and committee chair. Prof.
McGrath gave the author tremendous help and guidance on this work.
Rachael VanHouton helped the author with some UV-vis measurements in
chapter 3.
Andrew Brink provided crude SDCDPS samples in chapter 3.
Dr. Feng Wang contributed to the discussion of the synthesis of controlled
molecular weight copolymers in chapter 4
Juan Yang helped the author with GPC and IV measurements and discussion in
chapter 4, 5, and 7.
Dan Liu, Prof. Scott Case, and Prof. Jack Lesco contributed to chapter 4 in
mechanical measurements.
Abhishek Roy aided in the discussion and measurements of proton exchange
membrane properties in chapter 4, 5, 6, and 7.
Anand Badami helped with atom force microscopy measurements in chapter 5, 6,
and 7.
Dr. Melinda Einsla contributed to chapter 6 in discussing the synthesis of
poly(arylene ether ketone)s and chapter 7 in fuel cell performance measurements.
Ozma Lane helped with the proton conductivity measurements in chapter 5.
Dr. Thekkekara Mukundan contributed to the discussion of poly(arylene ether
ketone) synthesis in chapter 6.
Stuart Dunn aided in the measurement of PEM properties in chapter 7.
viii
TABLE OF CONTENTS
ABSTRACT ....................................................................................................................... ii
ACKNOWLEDGEMENTS.................................................................................................v
ATTRIBUTION ............................................................................................................... vii
TABLE OF CONTENTS ................................................................................................ viii
LIST OF FIGURES......................................................................................................... xiii
LIST OF TABLES......................................................................................................... xviii
Chapter 1. Research Significance and Objectives ..............................................................1
Chapter 2. Literature Review...............................................................................................3
2.1. Proton Exchange Membrane Fuel Cells (PEMFCs).....................................................3
2.1.1. Fuel Cell Introduction........................................................................................3
2.1.2. Types of Fuel Cells............................................................................................5
2.1.3. Proton Exchange Membrane Fuel Cells (PEMFCs) .........................................6
2.1.3.1. Hydrogen/Air PEM Fuel Cells…………………………………………6
2.1.3.2. Direct Methanol Fuel Cells (DMFCs)………………………………….8
2.1.3.3. Requirements for PEM Materials………………………………………9
2.1.3.4. Laboratory Evaluation of PEM Materials…………………………….10
2.2. Nafion® and the Other Perfluoropolymers .................................................................13
2.3. Polystyrene Type PEM Materials...............................................................................16
2.4. Partially- or Non-Fluorinated Copolymers with Aromatic Backbones ......................21
2.4.1. Poly(Arylene Ethers) .......................................................................................22
2.4.1.1. Synthetic Routes of Poly(Arylene Ethers)……………………………23
2.4.1.2. Molecular Weight Control and Characterization……………………..30
2.4.1.3. “Post” vs. “Direct” Sulfonation Methods…………………………….33
2.4.1.4. Sulfonated Poly(Arylene Ether Sulfone)s and Poly(Arylene Ether
Ketone)s via the Post Modification Method…………………………………..37
2.4.1.5. Direct Sulfonation of Poly(Arylene Ether Sulfone)s and Poly(Arylene
Ether Ketone)s………………………………………………………………...45
2.4.1.6. Other Poly(Arylene Ethers)…………………………………………..50
2.4.2. Sulfonated Polyimides (SPIs)………………………………………………..54
ix
2.4.3. Polyphosphazene Ionomers for PEMs.............................................................62
2.5. Other Novel Approaches to Improve PEM Properties ...............................................65
2.5.1. Controlling Morphology Using Block and Multiblock Copolymers...............66
2.5.2. Organic/Inorganic Composite PEMs...............................................................71
2.5.3. Polymer Blends................................................................................................75
References .................................................................................................................77
Chapter 3. Purity Characterization of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl Sulfone
(SDCDPS) Monomer by UV-visible Spectroscopy ..........................................................87
3.1. Abstract.......................................................................................................................88
3.2. Introduction ................................................................................................................89
3.3. Experimental...............................................................................................................93
3.3.1. Materials ..........................................................................................................93
3.3.2. Synthesis Procedures .......................................................................................93
3.3.2.1. Synthesis and Purification of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl
Sulfone (SDCDPS) monomer………………………………………………….93
3.3.2.2. Synthesis of Disulfonated Poly(Arylene Ether Sulfone) (BPSH) Model
Copolymers…………………………………………………………………… 94
3.3.3. Characterization...............................................................................................94
3.3.3.1. Monomer and Copolymer Characterization…………………………..94
3.3.3.2. Procedure for SDCDPS Monomer Purity Characterization by UV-
Visible Spectroscopy…………………………………………………………..95
3.4. Results and Discussion ...............................................................................................96
3.5. Conclusions ..............................................................................................................108
3.6. References ................................................................................................................109
Chapter 4. Synthesis & Characterization of Controlled Molecular Weight Disulfonated
Poly(Arylene Ether Sulfone) Copolymers and Their Applications to Proton Exchange
Membranes ......................................................................................................................111
4.1. Abstract.....................................................................................................................112
4.2. Introduction ..............................................................................................................113
4.3. Experimental.............................................................................................................115
4.3.1. Materials ........................................................................................................115
x
4.3.2. Synthesis of Disulfonated Poly(Arylene Ether Sulfone) Copolymers with
Controlled Molecular Weight..................................................................................116
4.3.3. Membrane Preparation ..................................................................................117
4.3.4. Characterization.............................................................................................117
4.4. Results and Discussion .............................................................................................119
4.4.1. Synthesis and Characterization of Copolymers.............................................119
4.4.2. Membrane Characterization ..........................................................................131
4.5. Conclusions ..............................................................................................................136
4.6. Acknowledgements ..................................................................................................136
4.7. References ................................................................................................................137
Chapter 5. Partially Fluorinated Disulfonated Poly(Arylene Ether Sulfone) Copolymers
with Controlled Molecular Weights for Proton Exchange Membranes ..........................139
5.1. Abstract.....................................................................................................................140
5.2. Introduction ..............................................................................................................141
5.3. Experimental.............................................................................................................145
5.3.1. Materials ........................................................................................................145
5.3.2. Copolymerization ..........................................................................................145
5.3.3. Membrane Preparation and Acidification......................................................146
5.3.4. Characterization…………………………………………………………….147
5.4. Results and Discussion .............................................................................................148
5.4.1. Synthesis and Characterization of Copolymers.............................................148
5.4.2. Characterization of Membranes.....................................................................156
5.5. Conclusions ..............................................................................................................160
5.6. Acknowledgements ..................................................................................................160
5.7. References ................................................................................................................161
Chapter 6. Comparative Investigation of Three Series of Poly(Arylene Ether Ketone)
Copolymers for Proton Exchange Membrane Fuel Cells................................................163
6.1. Abstract.....................................................................................................................164
6.2. Introduction ..............................................................................................................165
6.3. Experimental.............................................................................................................169
6.3.1. Materials ........................................................................................................169
xi
6.3.2. Synthesis of the Disodium Salt of Comonomers...........................................169
6.3.3. Synthesis of Disulfonated Poly(Arylene Ether Ketone) Copolymers (B, PB and
MB) Based on Three Types of Ketone Monomers..................................................170
6.3.4. Membrane Preparation and Acidification......................................................171
6.3.5. Characterization.............................................................................................171
6.4. Results and Discussion .............................................................................................173
6.4.1. Synthesis and Characterization of Disulfonated Monomers and Copolymers173
6.4.2. Morphology Characterization of the Membranes..........................................181
6.4.3. Characterization of Membrane Properties.....................................................183
6.5. Conclusions ..............................................................................................................193
6.6. Acknowledgements ..................................................................................................193
6.7. References ................................................................................................................194
Chapter 7. Synthesis and Characterization of Partially Fluorinated Hydrophobic -
Hydrophilic Multiblock Copolymers Containing Sulfonate Groups for Proton Exchange
Membrane........................................................................................................................196
7.1. Abstract.....................................................................................................................197
7.2. Introduction ..............................................................................................................198
7.3. Experimental.............................................................................................................202
7.3.1. Materials ........................................................................................................202
7.3.2. Synthesis of Fluorine Terminated Hydrophobic Oligomers..........................202
7.3.3. Synthesis of Multiblock Copolymers ............................................................203
7.3.4. Characterization of Oligomers and Multiblock Copolymers ........................204
7.3.5. Membrane Preparation and Acidification......................................................204
7.3.6. Characterization of Membranes.....................................................................204
7.3.6. 1 Morphology Characterization by Atomic Force Microscopy (AFM).204
7.3.6.2. Ion Exchange Capacity (IEC) and Conductivity…………………….205
7.3.6.3. Water Uptake and Water Self-diffusion Coefficients……………….205
7.3.6.4. MEA Fabrication and Fuel Cell Testing…………………………….206
7.4. Results and Discussions............................................................................................207
7.4.1. Synthesis and Characterization of Oligomer and Multiblock Copolymer ....207
7.4.2. Morphology of Membranes ...........................................................................215
xii
7.4.3. Characterization of PEM Properties ..............................................................218
7.4.4. Fuel Cell Performance ...................................................................................226
7.5. Conclusions ..............................................................................................................228
7.6. Acknowledgments ....................................................................................................229
7.7. References ................................................................................................................230
Chapter 8. Overall Conclusions.......................................................................................232
xiii
LIST OF FIGURES
Figure 2.1. Schematic of a PEM Fuel Cell Operation .................................................. 7
Figure 2.2. Chemical Structure of Nafion® ............................................................... 14
Figure 2.3. Structure of Dais Styrene–Ethylene/Butylene–Styrene (SEBS) Triblock
Copolymer Electrolyte........................................................................................ 17
Figure 2.4. Molecular Structure of Ballard Advanced Materials Corp’s BAM3G .... 18
Figure 2.5. Synthetic Scheme of Polystyrene-g-Poly(sodium styrenesulfonate) ....... 20
Figure 2.6. Chemical Reactions of PSEBS Photocrosslinking Using Benzonphenone
as Initiator. ......................................................................................................... 21
Figure 2.7. Several Possible Poly(arylene ether) Chemical Structures ................... 23
Figure 2.8. Nucleophilic Aromatic Substitution Mechanism..................................... 24
Figure 2.9. The Electrophilic Substitution Mechanism........................................... 29
Figure 2.10. Synthesis of Tert-Butylphenyl Terminated Poly(Arylene Ether Sulfone)
Copolymers Containing 35 mol% Disulfonated Repeat Unit. ........................... 32
Figure 2.11. Stress-strain Curves of BPSH35 Copolymers (Thin Films) as a Function
of Molecular Weight........................................................................................... 33
Figure 2.12. Direct Copolymerization of sulfonated Monomers versus Post
Sulfonation.......................................................................................................... 35
Figure 2.13. Synthesis of 3,3’-Disulfonated 4,4’-dichloro-diphenyl Sulfone in Its
Sodium Salt Form............................................................................................... 36
Figure 2.14. Direct Copolymerization of Wholly Aromatic Sulfonated Poly(arylene
ether sulfone) “BPSH-xx,” where xx is the ratio of sulfonated to unsulfonated
activated halide. .................................................................................................. 37
Figure 2.15. Sulfonated Poly(ether sulfone) (Udel®) and Poly(ether ether ketone)
(Victrex®)............................................................................................................ 38
Figure 2.16. Synthesis of Poly(arylene ether) Ionomers Containing Sulfofluorenyl
Groups via a Post-modification Method............................................................. 41
Figure 2.17. Synthesis of Sulfonated Poly(ether sulfone) Udel® PSU via the
Metalation Route ................................................................................................ 42
xiv
Figure 2.18. Crosslinking via the Metalation Route................................................... 43
Figure 2.19. Influence of the Dgree of Sulfonation on the Water Uptake of BPSH
Copolymers......................................................................................................... 48
Figure 2.20. Four Investigated Bisphenol Structures ................................................. 49
Figure 2.21. Several investigated sulfonated ketone or diketone structures .............. 50
Figure 2.22. Structure of Mono-Sulfonated BFPPO .................................................. 52
Figure 2.23. Post Modification of PPES and PPEK................................................... 53
Figure 2.24. Direct Synthesis of SPPEKs and SPPESs.............................................. 54
Figure 2.25. Chemical Structure of Sulfonated Polyimides Containing Fluorenyl
Groups ................................................................................................................ 57
Figure 2.26 Sulfonated Diamines .............................................................................. 59
Figure 2.27. Synthesis of Disulfonated Polyimide Copolymers ................................ 61
Figure 2.28. The Reaction Scheme for Sulfonation of Polyphosphazene with SO3 .. 63
Figure 2. 29. Direct Sulfonation of Polyphosphazenes by the Noncovalent Protection
Method................................................................................................................ 65
Figure 2.30. Synthetic Scheme of BisAF-BPSH Series of Multiblock Copolymers . 69
Figure 2.31. Proton Conductivity vs. Relative Humidity for “BisAF-BPSH” Series of
Multiblock Copolymers and Nafion® 117 .......................................................... 70
Figure 2.32 Chemical Structure of PPP/BPS Multiblock Copolymer........................ 71
Figure 2.33. Schematic View of the Increased Pathways of Composite Membrane.. 74
Figure 3.1. Synthetic Scheme of Disulfonated Monomers with Several Different
Structures ............................................................................................................ 92
Figure 3.2. UV-Visible Spectra of SDCDPS Dilute Solutions Using Different
Solvents ............................................................................................................ 100
Figure 3.3. Effect of the Number of Recrystallization Times on the Absorbance at the
Same Concentration Values (After Two Times Recrystallization, SDCDPS Still
Contains 2.6% ±1% salt) ................................................................................ 101
Figure 3.4. The UV-Vis Absorbances of SDCDPS Solutions with Different
Concentrations were Used to Develop the Calibration Curve.......................... 102
xv
Figure 3.5. Calibration Curve Used to Develop the Beer’s Law Slope. The Left
Graph Shows the Deviation at High Concentrations. The Right graph is the
Linear Calibration Curve at Low Concentrations............................................. 103
Figure 3.6. Effect of Drying Time and Storage Time on the Absorbance at the Same
Concentrations .................................................................................................. 104
Figure 3.7. Comparison of the Absorbance of Pure and Crude Samples of SDCDPS.
(The Crude Sample was Provided by Hydrosize Inc.) ..................................... 105
Figure 3.8. 1H NMR of Poly(Arylene Ether Sulfone) Copolymers (BPSH-40) was
Used to Determine the Degree of Sulfonation.................................................. 106
Figure 4.1. Synthesis of Tert-Butylphenyl Terminated Poly(Arylene Ether Sulfone)
Copolymers Containing 35 mole % Disulfonate Repeat Unit.......................... 125
Figure 4.2. The Copolymer Structures and Degree of Sulfonation were Determined
by 1H NMR Spectra in the Aromatic Region (BPS35-50 Copolymer). ........... 126
Figure 4.3. Molecular Weights can be Calculated from the Relative 1H NMR
Integrals of the Tert-Butyl Endgroups and the Aromatic Resonances (BPS35-50
Copolymer in DMSO-d6).................................................................................. 127
Figure 4.4. Correlations of Reduced (▲) and Inherent (■) Viscosities with
Copolymer Concentration of BPS35-Control in Pure NMP (3a), and NMP
Containing 0.05 M LiBr (3b)............................................................................ 128
Figure 4.5. Relationship Between Log(Intrinsic Viscosity) and Log(Mn) for BPS-35
Copolymers....................................................................................................... 130
Figure 4.6. Stress-strain Curves of BPSH35 Copolymers (Thin Films) as Function of
Molecular Weight. ............................................................................................ 134
Figure 5.1. Disulfonated Copolymer Structures with Biphenol (BPSH) or 6F
Bisphenol A (6FSH) Units in the Backbones................................................... 144
Figure 5.2. Synthesis of Tert-butylphenyl Terminated Partially Fluorinated
Poly(Arylene Ether Sulfone) Containing Sulfonic Acid Groups (x = 0.35 or 0.48)
.......................................................................................................................... 151
Figure 5.3. 1H NMR Spectrum of 6FS35-50 Copolymer (Aromatic Region).......... 152
xvi
Figure 5.4. The Molecular Weight of the Copolymer can be Calculated from the
Relative 1H NMR Integrals of the Tert-butyl Endgroups and the Aromatic
Resonances. (6FS35-50 Copolymer in DMSO-d6)........................................... 153
Figure 5.5. Relationship Between Log(Intrinsic Viscosity) and log(Mn) for 6FS35 and
6FS48 Copolymers ........................................................................................... 155
Figure 5.6. Morphology Characterization of the BPSH35 and 6FSH35 Series of
Copolymers with Different Molecular Weights by AFM. ............................... 159
Figure 6.1. Three Disulfonated Ketone-Type Comonomer Structures .................... 168
Figure 6.2. 1H NMR Spectrum of SMBFB Disulfonated Comonomer.................... 176
Figure 6.3. Synthetic Scheme of Three Series of Disulfonated Poly(Arylene Ether
Ketone) Copolymers......................................................................................... 177
Figure 6.4. 1H NMR was Used to Calculate the Degree of Sulfonation of the
Copolymers (MB-30) ....................................................................................... 179
Figure 6.5. IR Spectra of MB Series Copolymers.................................................... 180
Figure 6.6. AFM Image of Copolymer Membranes: (a) B-30, (b) PB-40, (c) MB-40,
(d) B-40, (e) PB-50, (f) MB-50. The IEC value of the left group copolymers is
around 1.1-1.2 meq·g-1, and the right groups is around 1.4 meq·g-1................. 182
Figure 6.7. Influence of IEC and Copolymer Structure on Water Uptake of the
Membranes (Acid Form) in Liquid Water at Room Temperature.................... 186
Figure 6.8. Proton Conductivity vs. IEC of Three Ketone Type Copolymers in Liquid
Water at Room Temperature ............................................................................ 187
Figure 6.9. Proton Conductivity in Liquid Water Tends to Depend on Hydration
Number (RT) .................................................................................................... 188
Figure 6.10. Effect of Temperature on Protonic Conductivity in Liquid Water ...... 189
Figure 6.11. Influence of Copolymer Composition on Water Sorption of the B Series
as a Function of Humidity ................................................................................ 190
Figure 6.12. Influence of Copolymer Composition and Temperature on Methanol
Permeability...................................................................................................... 191
Figure 7.1. Copolymer Chemical Structures Studied in This Work (a) B-ketone-xx
and PB-diketone-xx Copolymers, (b) 6FK-BPSH Multiblock Copolymer...... 201
xvii
Figure 7.2. Synthesis of Fluorine Terminated Poly(Arylene Ether Ketone) (6FK)
Hydrophobic Oligomer..................................................................................... 210
Figure 7.3. Molecular Weight of 6FK Hydrophobic Oligomer Can be Calculated
from the 19F NMR Spectrum (6FK Oligomer with Target MW 4 kg·mol-1 in
CDCl3) .............................................................................................................. 211
Figure 7.4. Synthesis of 6FK-BPSH Multiblock Copolymers via Two-step Sechnique
.......................................................................................................................... 212
Figure 7.5. 1H NMR Spectra of BPS Hydrophilic Oligomer (Top), and Multiblock
6FK-BPS Copolymer (Bottom) ........................................................................ 213
Figure 7.6. 13C NMR Spectra of Random (Top) and 6FK-BPS Multiblock (Bottom)
Copolymers....................................................................................................... 214
Figure 7.7. Tapping Mode Atomic Force Microscopy Images: (a) Phase Image and (b)
Height Image of a 4k-4k 6FK-BPSH Multiblock Copolymer Film Acidified by
Method 1 at 30 °C, (c) Phase Image and (d) Height Image of the Same Film
Acidified by Method 2 at 100 °C, (e) Phase Image and (f) Height Image of a
Sulfonated Poly(Arylene Ether Ketone) Random Copolymer Film (B-30)
Acidified by Method 2. Image size: 500 nm; z Ranges: (a) 4°, (c) 12°, (e) 8°, All
Height Ranges: 10 nm. ..................................................................................... 217
Figure 7.8. Retention of Water as a Function of Water Activity is Enhanced for the
Block Copolymer.............................................................................................. 222
Figure 7.9. Proton Conductivity as a Function of Temperature for Multiblock 6FK-
BPSH (4:4)k, B ketone-30, and Nafion® 112 ( The numbers in the box are
activation energy, kJ·mol-1) .............................................................................. 223
Figure 7.10. The Block Copolymer Has a Much Higher Self-diffusion Coefficient of
Water (Multiblock Copolymer Has Similar IEC Value to the PB-40 and B-30
Random Copolymers)....................................................................................... 224
Figure 7.11. Comparison of Conductivity vs. RH for 6FK-BPSH (4:4)k Multiblock,
PB-diketone-50 Random Copolymers, and Nafion® 112................................. 225
Figure 7.12. Hydrogen-air Fuel Cell Performance of 6FK-BPSH (4:4)k and Nafion®
at 80 ℃ under Fully Humidified Conditions.................................................... 227
xviii
LIST OF TABLES
Table 2.1. Different Fuel Cell Types………………………………………………...5
Table 2.2. Several Poly(arylene ether ketone) Structures…………………………..45
Table 3.1. Characterization of the Model BPS Copolymers ...................................107
Table 4.1. Characterization of BPS35 copolymers..................................................129
Table 4.2. The Results of Water Swelling and Conductivity Test for BPSH35
Copolymers with Controlled Molecular Weights in Liquid Water at 30 oC ...133
Table 4.3. The Tensile Properties of BPSH35 Copolymers (thin films) as Function of
Molecular Weight. ...........................................................................................135
Table 5.1. Characterization of 6FS35 and 6FS48 Series Copolymers ....................154
Table 5.2. Water Uptake and Conductivity Characterization for 6FSH35 and 6FS48
Copolymers with Controlled Molecular Weights in Liquid Water at 30 oC ...158
Table 6.1. Characterization of B/PB/MB Series of Copolymers with Different Degree
of Sulfonation ..................................................................................................178
Table 6.2. Comparisons of Thin Film Protonic Conductivity in Liquid Water to That
of MEA............................................................................................................192
Table 7.1. Characterization of Multiblock, Random Copolymers and Nafion®Control
.........................................................................................................................221
1
Chapter 1. Research Significance and Objectives
Environmentally-friendly renewable energy sources are becoming more and more
desirable because of two main reasons: the depletion of natural energy resources and
pollution caused by the use of fossil fuels. Fuel cells are becoming increasingly important
because they are capable of converting chemical energy directly into electrical energy.
Proton exchange membrane (PEM) fuel cells have several advantages over other energy
sources, e.g., gasoline engines, such as their high efficiency and very low pollutant
emissions. The fuel used in PEM fuel cells can be either hydrogen or methanol. Because
the only byproduct of a hydrogen/air fuel cell is water, it has become widely known as a
source of “green energy.” The growing list of applications for PEM fuel cells, such as
stationary power, automobiles, and small electronic devices, has created great interest not
only in academia, but also from researchers in government and industry.
The proton exchange membrane is the key component of a PEM fuel cell. It
serves as the barrier for fuel and transports protons from the anode to the cathode to
generate power. Despite their potential, limitations have existed in the current state-of-art
perfluorosulfonic acid Nafion® materials. These limitations have primarily included high
cost and low performance, especially with respect to long term durability. Therefore,
novel PEM materials with improved properties and lower cost have been important areas
of research for more than two decades. In particular, wholly aromatic disulfonated
poly(arylene ether) copolymers, including poly(arylene ether sulfone)s and poly(arylene
ether ketone)s, have displayed promise for use in PEM fuel cells and merit further
investigation.
2
This research investigated hydrocarbon type wholly aromatic poly(arylene ether)
copolymers containing sulfonic acid groups for PEM fuel cells. The overall objectives
were to synthesize and characterize sulfonated random and multiblock copolymers for
PEMs, and to investigate the copolymer molecular weight, composition, and
microstructure of the proton exchange membranes for use in fuel cells. Specifically, this
study accomplished the following goals:
1. Developed purity characterization method for the disulfonated monomer
SDCDPS by UV-visible spectroscopy, which was important for high
molecular weigh copolymer synthesis via direct step growth copolymerization,
especially for mass production;
2. Synthesized controlled molecular weight poly(aryelene ether sulfone) random
copolymers, and investigated the effect of molecular weight on the PEM
properties;
3. Synthesized three series of disulfonated poly(arylene ether ketone) random
copolymers, and studied the chemical structure effects of the ketone
copolymers on the PEM properties;
4. Synthesized novel multiblock copolymers containing ionic groups to improve
membrane morphology by forming cocontinuous hydrophilic-hydrophobic
phase separation, which may enhance membrane properties—especially for
applications at high temperature and low relative humidities.
3
Chapter 2. Literature Review
This dissertation addresses the effects of copolymer composition, molecular
weight, and microstructure on the properties of proton exchange membranes (PEMs).
Fuel cell concepts and related background will be discussed, followed by a review of the
current state-of-the-art of membrane materials. A review of partially or non-fluorinated
aromatic copolymers containing sulfonic acid groups encompass the major focus of this
chapter, although it will also include a brief review of perfluorinated copolymers and
other novel approaches in PEM research.
2.1. Proton Exchange Membrane Fuel Cells (PEMFCs)
2.1.1. Fuel Cell Introduction1,2, 3
The rapid depletion of non-renewable traditional energy sources due to increasing
demands, as well as the pollution created by fossil fuels are two major driving forces for
developing more efficient and reliable fuel cell technologies. Fuel cells are considered an
excellent renewable and environmentally friendly alternative energy resource.
Furthermore, they have higher electrical efficiencies compared to conventional engines.
The initial concept for fuel cells was developed more than 150 years ago by
William Grove, a British physicist.1 However, little practical use was made of this
technology until the 1960s, when the National Aeronautics and Space Administration
(NASA) turned to fuel cells for its space program. The space shuttle, for example, uses
fuel cells to generate electric power and drinking water. Fuel cells are now being
4
investigated for motor vehicles, power plants, and as replacement batteries for laptop
computers and other electronic equipment.
A fuel cell is a device that converts the chemical energy of a fuel and an oxidant
directly into electricity. In principle, a fuel cell operates like a battery. Unlike a battery
however, a fuel cell does not run down or require recharging. It will produce electricity
and heat as long as fuel and an oxidizer are supplied. Accordingly, fuel cell systems have
the potential to solve challenging problems associated with currently available battery
systems, namely their insufficient energy at a given weight (specific energy density) or
volume (volumetric energy density). Moreover, while the leading battery technologies are
reaching practical limits of their energy storage capabilities, commercial fuel cells are
still in their infancy. Furthermore, since hydrogen/air fuel cells operate without a thermal
cycle, they offer high energy efficiency and can virtually eliminate air pollution and do
not require the use of emission control devices as in conventional energy conversion.
Fuel cell construction generally consists of a fuel electrode (anode) and an oxidant
electrode (cathode) separated by an ion-conducting membrane. Oxygen passes over one
electrode, and fuel (generally hydrogen or methanol) passes over the other, generating
electricity, water, and heat. Using pure hydrogen fuel, fuel cells only produce water, thus
eliminating all emissions usually caused by the production of electricity. Despite the fact
that there is certainly potential for renewable energy from wind, solar and hydroelectric
power, these sources are not well suited to handle the electrical base load due to their
irregular availability. The combination of these various sources to produce hydrogen in
co-operation with fuel cells may well be a viable option for future power generation.
5
2.1.2. Types of Fuel Cells3, 4
Fuel cells are generally categorized by their electrolyte, namely, the material
sandwiched between the two electrodes. The electolyte’s characteristics determine the
optimal operating temperature and the fuel used to generate electricity, and each features
specific benefits and shortcomings. Some of the essential characteristics of all types of
fuel cells are summarized in Table 2.1. 3
Table 2.1. Different Fuel Cell Types
Fuel Cell Type Electrolyte Fuel Operating
Temperature Efficiency Application
Proton Exchange
Membrane (PEM)
solid polymer
membrane
H2(pure or reformed)
60-120oC 35–60%
Portable, transportation
Direct Methanol (DMFC)
solid polymer
membrane CH3OH 60-120oC
35–40% Portable,
transportation
Alkaline (AFC)
potassium hydroxide(8-
12 N) H2 50-250 oC 50–70%
Space
Phosphoric Acid
(PAFC)
Phosphoric Acid (85%-
100%)
H2 (reformed)
160-220 oC 35–50%
Power Generation,
cogeneration, transportation
Molten Carbonate (MCFC)
Molten Carbonates
H2 and CO
reformed and CH4
600-800 oC 40–55%
Power generation,
transportation
Solid Oxide
(SOFC)
Solide Oxide
H2 and CO
reformed and CH4
800-1000 oC 45–60%
Power generation,
cogeneration
6
2.1.3. Proton Exchange Membrane Fuel Cells (PEMFCs) 5, 6
Proton Exchange Membrane Fuel Cells (PEMFCs) are believed to hold the most
promise for eventually replacing gasoline and diesel internal combustion engines. As
noted above, they were first used in the 1960s for the NASA Gemini program. However,
PEMFCs are currently being developed and demonstrated for systems ranging from 5kW
to 250kW3. PEM fuel cells use a solid polymer membrane (a thin plastic film) as the
electrolyte. This polymer is permeable to protons when it is saturated with water, but it
does not conduct electrons. The fuels for the PEMFC can be hydrogen (hydrogen/air fuel
cell) or methanol (direct methanol fuel cell).
2.1.3.1. Hydrogen/Air PEM Fuel Cells
Figure 2.1 shows how the PEM fuel cell works. For a hydrogen/ air PEM fuel cell,
the fuel is hydrogen and the charge carrier is the hydrogen ion (proton). At the anode, a
platinum catalyst splits the hydrogen into protons (hydrogen ions) and negatively charged
electrons. The hydrogen ions permeate across the electrolyte to the cathode while the
electrons flow through an external circuit and produce electric power. Oxygen, usually in
the form of air, is supplied to the cathode and combines with the electrons and the
hydrogen ions to produce water. The reactions at the electrodes are as follows:
7
Anode Reactions: 2H2 => 4H+ + 4e-
Cathode Reactions: O2 + 4H+ + 4e- => 2 H2O
Overall Cell Reactions: 2H2 + O2 => 2 H2O
Figure 2.1. Schematic of a PEM Fuel Cell Operation
Compared to other types of fuel cells, PEMFCs generate more power per given
volume or weight. This high-power density capability makes them compact and
lightweight. In addition, the operating temperature is less than 100ºC, which allows rapid
start-up. These traits, as well as the ability to rapidly change power output, are some of
the characteristics that make PEMFCs excellent candidates for automotive power
applications.
Other important advantages result from the electrolyte being a solid material
instead of a liquid. For example, sealing the anode and the cathode gases is simpler with a
solid electrolyte; therefore, it is less expensive to manufacture. The solid electrolyte is
8
also more immune to difficulties associated with orientation and has fewer problems with
corrosion, compared to many of the other electrolytes, thus leading to a longer cell and
stack life.
One of the disadvantages of the PEMFC for some applications, however, is that
the operating temperature is low. Temperatures near 100ºC are not high enough to
perform useful cogeneration. Also, since the electrolyte is required to be saturated with
water to operate optimally, careful control of the moisture of the anode and cathode
streams is important.
2.1.3.2. Direct Methanol Fuel Cells (DMFCs)
Direct-methanol fuel cells, or DMFCs, are a subcategory of proton exchange
membrane fuel cells where the fuel (methanol) is not reformed, but is instead fed directly
to the fuel cell. A DMFC is similar to a hydrogen/air PEM fuel cell in that the electrolyte
is a polymer and the charge carrier is the hydrogen ion (proton). However, the liquid
methanol (CH3OH) is oxidized in the presence of water at the anode generating CO2,
hydrogen ions and the electrons that travel through the external circuit as the electric
output of the fuel cell. The hydrogen ions travel through the electrolyte membrane and
react with oxygen from the air and the electrons from the external circuit to form water at
the cathode, completing the circuit.
Anode Reaction: CH3OH + H2O => CO2 + 6H+ + 6e-
Cathode Reaction: 3/2 O2 + 6 H+ + 6e- => 3 H2O
Overall Cell Reaction: CH3OH + 3/2 O2 => CO2 + 2 H2O
9
Initially developed in the early 1990s, DMFCs were not embraced because of
their low efficiency and power density, as well as other problems. Improvements in
catalysts and other recent developments have increased power density 20-fold and the
efficiency may eventually reach 40%.
These cells have been tested over the temperature range of about 60 ºC-120 ºC.
This relatively low operating temperature and no requirement for a fuel reformer make
the DMFC an excellent candidate for very small to mid-sized applications, such as
cellular phones and other consumer products, up to automobile power plants.
One of the drawbacks of DMFCs is that the low-temperature oxidation of
methanol to hydrogen ions and carbon dioxide requires a more active catalyst, which
typically means a larger quantity of expensive platinum catalyst is required than in
conventional PEMFCs. This increased cost is, however, expected to be more than
outweighed by the convenience of using a liquid fuel and the ability to function without a
reforming unit.
2.1.3.3. Requirements for PEM Materials7
Polymeric membranes play a crucial role during electricity generation in
hydrogen/air and direct methanol proton-exchange membrane (PEM) fuel cells. The
membrane in such devices performs two roles: it separates the positive and negative
electrodes and provides a conduit for ion (proton) movement between the electrodes.
PEMs are made from high performance polymers containing ionic groups (sulfonic acid
or phosphoric acid) to produce protonic conductivity under hydrated conditions.
10
Consequently, there is considerable research and development around the world to
develop new membrane materials with tailored physical and transport properties.
The general required properties of an ion-exchange membrane for use in a PEM
fuel cell include high protonic conductivity, low electronic conductivity, low
permeability to fuel and oxidants, low water transport through diffusion and electro-
osmosis, oxidative and hydrolytic stability, good mechanical properties both in the dry
and hydrated state, low cost, and ease of fabrication into membrane electrode assemblies
(MEAs).
The overall properties of the membrane need to balance all these requirements.
For example, the improvement of protonic conductivity can be achieved by increasing the
ion exchange capacity (IEC), which will be detrimental to mechanical strength due to an
increase in water sorption. Nevertheless, these requirements can serve as screening tests
for novel membrane materials, after which the qualified material must be fabricated into a
well-bonded, robust membrane electrode assembly (MEA). Thus, the ease of MEA
fabrication and the resulting properties of the MEA are also critical. The membrane
electrode assembly consists of the proton conducting membrane which is bonded on
either side to the anode and cathode. One major objective of MEA fabrication is to
promote good interfacial adhesion between the electrodes and the membrane, since they
often have dissimilar chemical structures and properties.
2.1.3.4. Laboratory Evaluation of PEM Materials
Many techniques are employed to characterize the properties of PEM materials in
the lab. Here, several fundamental but important parameters are introduced, which are
11
used in the first stage as initial screening tests for PEM materials. Further details on the
instruments and techniques employed will be provided in later chapters.
Ion-exchange capacity (IEC) or Degree of Ionic Content
The ion-exhange capacity (IEC), which is a measure of the ionic content of the
polymer materials, is defined as the molar equivalents of ion conductor per mass of dry
membrane. IEC is typically expressed in units of milliequivalents per gram (meq·g-1) of
polymer. Sometimes, equivalent weight (EW) is used to express this ionomer property.
EW is the inverse of the IEC (EW =1000/IEC) with the unit of grams of polymer per
equivalent of ionic groups. The theoretical IEC or degree of the ionic content of a
polymer can be designed and controlled during polymer synthesis, which will be
discussed in the experimental chapter. However, the actual IEC value of a membrane is
usually experimentally determined by potentiometric titration of the acid groups with
base or determined by 1H NMR. IEC affects both conductivity and water uptake, so the
conductivity and water uptake can be controlled by varying the ionic content of the
membrane. The overall effects of increasing IEC should be considered for the reason that
too many ionic groups will definitely increase the conductivity, but will also cause the
membrane to swell excessively with water, which compromises mechanical integrity and
durability.
Water Uptake
PEM fuel cells are operated under humid environments, because the water is
needed as the mobile phase to facilitate proton conductivity. However, too much water
12
swelling will affect the mechanical properties of the membrane by acting as a plasticizer,
lowering the Tg and modulus of the membrane. Careful control of water uptake is critical
for reducing adverse effects.7 Thus, one of the challenges in the future is to modify the
chemistry of PEMs to obtain significant protonic conductivity at low hydration levels.
Multiblock copolymers show potential in this area due to their enhanced ability for
phase-separation.8 The water content of a membrane is expressed either by water uptake
or by the lambda value (λ), which is defined as the number of water molecules absorbed
per acid site as calculated using the following equations:
Water Uptake (%) = [(Wwet-Wdry) / Wdry] * 100
λ = 1000 * [(Wwet-Wdry) / Wdry] / ( 18 * IEC)
Where Wwet and Wdry are the weight of the membrane under hydrated and dry conditions.
Protonic Conductivity
Protonic conductivity is a critical parameter in evaluating PEM materials. It is a
function of many variables, such as temperature, IEC, humidity, and water uptake. A
successful PEM should exhibit a careful balance of maximum protonic conductivity with
minimum water swelling in order to maintain its mechanical properties. Proton
conductivity may be measured by either a through-plane measurement or an in-plane
measurement. Although the through-plane measurement is analogous to the true fuel cell
condition where the protons are transported through the plane of the membrane, it is more
difficult experimentally and usually has larger error than the in-plane measurements
because the material between the test electrodes is so thin that small changes in
13
conductivity are hard to detect due to small measured membrane resistances. Under this
circumstance, the interfacial resistances may play a more significant role. Thus, the most
often used method is a facile in-plane measurement, which was devised at Los Alamos
National Labs (LANL). In this geometry, the resistances measured are much greater than
those of the through-plane measurement, and the experimental errors are relatively lower,
so the result is more precise.
Methanol Permeability
Methanol permeability is a critical transport property when considering a new
proton exchange membrane for use in liquid-fed direct methanol fuel cells. Methanol
that is unoxidized at the anode can “cross over” through the membrane and be oxidized at
the cathode. This methanol short circuit decreases the fuel efficiency of the system,
lowers the cell voltage by causing a mixed potential at the cathode, and increases cell
heating. In general, methanol crossover through the membrane in a membrane electrode
assembly scales with the feed concentration of methanol. This is why most active DMFC
systems are supplied with low methanol feed concentrations (1M or less).8
New PEMs can be screened for their potential as DMFC membranes by
measuring their methanol permeability and comparing with that of a Nafion® membrane,
which is often considered unsuitable for use in DMFCs due to its high methanol
permeability.
2.2. Nafion® and the Other Perfluoropolymers
Perfluoropolymer ionomers have been known since the late 1960s, when the
14
Nafion® ionomer was developed by the DuPont Company and employed as the polymer
electrolyte in a GE fuel cell designed for NASA spacecraft missions9. Today, it remains
the most common commercially available membrane used in PEM fuel cells. Nafion® is a
perflurosulfonic acid polymer membrane consisting of a Teflon-like backbone (~87% in
Nafion® 1100) with side chains terminated with –SO3H groups, as shown in Figure 2.2
Figure 2.2. Chemical Structure of Nafion®7
**x and y represent molar compositions and do not imply a sequence length.
The IEC or EW of Nafion® membranes can be modified by changing the ratio of
the two types of repeat units (x and y values) and is reflected in the commercial name.
For example, the commercial name Nafion® 117 means that the membrane equivalent
weight is 1100 EW (IEC = 0.91 meq·g-1) and its thickness is 7 mil (1 mil is equal to 25.4
microns).
Other commercially available perfluoroionomers include Flemion® produced by
Asahi Glass and Aciplex-S® produced by Asahi Chemical. Among the three major types,
the DuPont product is considered to be superior because of its high proton conductivity,
good chemical stability, and mechanical strength.10 In the mid-1980s, the Dow Chemical
company also developed a material with a shorter side–chain than those of Nafion® and
the other perfluorosulfonates. Though higher power-generating capability in fuel cell was
CF2 CF2 CF CF2
OCF2 CF O(CF2)2 SO3-H+
CF3
x y
z
nCF2 CF2 CF CF2
OCF2 CF O(CF2)2 SO3-H+
CF3
x y
z
n
15
demonstrated using the Dow ionomer, no commercialization of these very promising
experimental membranes followed. The complexity of the Dow process for the synthesis
of the base functional monomer used for the production of the shorter side–chain ionomer
was possibly one of the reasons for the abandonment of this interesting development.9
Innumerable papers investigating the properties of Nafion® have been published.
The data available include fuel cell performance, transport characteristics (of water, gases,
and protons), swelling and solubility properties, mechanical, viscoelastic and thermal
behavior, morphology and structure, etc. Nafion® is therefore by far the most extensively
used and studied ionomer for fuel cell applications.9 From these numerous studies, three
distinct phases in the morphology of Nafion® have been confirmed. The first region is a
hydrophobic semicrystalline region primarily made up of the backbone chains. The
backbone provides structural stability to the membrane and prevents it from dissolving in
water. The second region is a largely empty amorphous region that consists of side chains
and some sulfonic acid groups. The final region consists of clusters of the hydrophilic
sulfonic acid groups which are responsible for conducting protons across the
membrane.11
Based on these structural characteristics, Nafion® is so far the best candidate
membrane with many advantages, including high protonic conductivity (~ 0.1 S·cm-1
with IEC = 0.91 meq·g-1), moderate swelling in water, exceptional oxidative and
chemical stability, good mechanical properties due to the semicrystalline morphology,
and long term stability.12 However, some drawbacks to Nafion® limit its applications, like
its high cost ($800/m2 for ~100 µm thick membranes)13, loss of membrane performance at
temperatures above 100 °C, and high methanol permeability in DMFCs,2 relatively low
16
mechanical strength at higher temperature and moderate glass transition temperature.7
These deficiencies of the Nafion® membrane have limited its practical application,
especially at high temperature and in DMFCs; therefore, the development of competitive
and less expensive PEM materials that will overcome these problems is important. As a
result, a large number of novel non-or partially fluorinated copolymers have been
developed and their properties investigated.
2.3. Polystyrene Type PEM Materials
Since the 1960s, a variety of polystyrene sulfonic acid membrane systems have
been evaluated. In fact, the first PEM fuel cell employed in the Gemini program was a
crosslinked polystyrene sulfonic acid (PSSA).14 The major issue found for polystyrene
sulfonic acid membranes is that they have poor oxidative stability. The presence of
hydrogen peroxide at the membrane-catalyst interface results in a free radical oxidation
of the aliphatic backbone that severely limits the life of the cell above 60 oC. For instance,
at 50-60 oC, the useful lifetime of a PSSA in a fuel cell was measured in thousands of
hours, while at 80 oC its life time was only 100 hours.15 Although the initial polystyrene
membrane showed poor fuel cell performance, the modified polystyrene type membrane
materials still attract the interest of many researchers. This is because the styrenic
monomers are widely available, easy to modify, and the polymers are easily synthesized
via conventional free-radical and other polymerization techniques.
One of the representatives is the tri-block copolymer produced by DAIS-Analytic
Corporation (Fig. 2.3). The membrane, which is based on commercially available
styrene–ethylene/butylene–styrene (SEBS) triblock copolymers, contains styrenic blocks,
which are subsequently sulfonated. DAIS membranes are reported to be much less
17
expensive to produce than Nafion®. However, as mentioned above, the main drawback in
employing hydrocarbon-based materials is their poor chemical stability compared to
perfluorinated or partially perfluorinated membranes due to the lower C–H bond
dissociation enthalpy. For this reason, DAIS membranes are suitable for portable fuel cell
power sources of 1 kW or less, with operating temperatures below 60 ◦C. Another
drawback of sulfonated SEBS (sSEBS) membranes is their high methanol transport in
DMFCs. 16
Figure 2.3. Structure of Dais Styrene–Ethylene/Butylene–Styrene (SEBS) Triblock
Copolymer Electrolyte.16
Several sulfonation methods have been reported for the SEBS triblock copolymer.
Ehrenberg, et al.17 used a sulfur trioxide/triethylphosphate sulfonating complex solution
to sulfonate the SEBS triblock copolymers which were dissolved in a
dichloroethane/cyclohexane solvent mixture at low temperature between -5 oC and 0 oC.
The resulting PEM had conductivities of 0.07-0.1 S·cm-1 when fully hydrated. However,
there is not much information on the extent of sulfonation of the styrene moieties in the
blocks.18,19 Another method employed acetyl sulfate, which was prepared from the
reaction of acetic anhydride with sulfuric acid, as the sulfonating reagent. The solvent
18
was 1,2-dichloroethane and the reaction temperature was 50 oC. The sulfonation degree
of purified SSEBS was evaluated quantitatively by elemental analysis. The SSEBS
membranes were prepared by a solution casting method. The proton conductivity and
methanol permeability increased abruptly when the sulfonation degree of the polystyrene
end blocks exceeded 15 mol%, and proton conductivity equivalent to that of Nafion® was
obtained for SSEBS with 34 mol% sulfonation,but the methanol permeability of
SSEBS was decreased by more than half compared with Nafion®.20
Ballard Power developed another important vinyl perfluorosulfonate system,
which has the trade name BAM3G (Fig. 2.4).21, 22
Figure 2.4. Molecular Structure of Ballard Advanced Materials Corp’s BAM3G21
This third generation Ballard Advanced Material membrane is composed of a
perfluorosulfonic acid system that uses α,β,β-trifluorostyrene monomers with different
pendent groups on the phenyl ring. The unsulfonated copolymer was synthesized first and
then it was post-sulfonated using reagents such as chlorosulfonic acid or a sulfur trioxide
complex. It has been reported that the backbone fluorination will mitigate hydroperoxide
19
formation, which results in long-term stability of over 100,000 hours, and also good
proton conductivity (~0.08 S·cm-1). However, this kind of membrane is expensive
probably due to the high cost of the monomer used, and the mechanical properties are
still unknown due to few related reports on this system.
In order to improve the properties of styrene-type PEMs, some novel synthetic
methods or techniques were developed, such as grafting23-25 and crosslinking.26, 27
Holdcroft et al.23 reported the synthesis of polystyrene with grafted poly(sodium styrene
sulfonate) via stable free radical polymerization (Fig. 2.5). These grafted copolymers
displayed excellent proton conductivities up to ~0.24 S·cm-1, although the oxidative
degradation of the backbone is still a problem for practical use. Recently, Chen Li et al.26
also reported a photo-crosslinking method to improve the mechanical strength and to
decrease the swelling of fuel cell membranes (Fig. 2.6). The process starts with photo
excitation of the benzophenone to generate radicals. The radicals take a tertiary or
secondary hydrogen atom from the polymer chain, making a radical center on the
polymer. The radicals attack another polymer chain forming a cross-link. According to
their results, the cross-linked PSEBS had lower water swelling, lower proton conductivity,
and higher chemical stability than uncrosslinked PSEBS because the cross-linking may
have resulted in less and smaller hydrophilic channels for water absorption and proton
mobility.
20
Figure 2.5. Synthetic Scheme of Polystyrene-g-Poly(sodium styrenesulfonate)23
21
Figure 2.6. Chemical Reactions of PSEBS Photocrosslinking Using Benzonphenone
as Initiator. 26
2.4. Partially- or Non-Fluorinated Copolymers with Aromatic Backbones
Aromatic polymers show excellent properties, such as good mechanical strength,
low cost, ease of processing, and chemical and thermal stability even at elevated
temperatures, so they are promising PEM candidates to replace the state-of-the-art
Nafion® membrane. Research on these polymers has specifically emphasized these three
targets: (1) reducing the PEM material cost for mass production; (2) lowering the
membrane methanol permeability for DMFCs; (3) developing membrane materials that
can be used at high temperature (>100 oC).
Increased fuel cell operating temperature is attractive for a number of reasons,
22
including (1) improved tolerance of the electrodes to carbon monoxide, which enables the
use of hydrogen produced by reforming of natural gas, methanol or gasoline; (2)
simplification of the cooling system; (3) possible use of cogenerated heat; (4) increased
proton conductivity; and (5) in DMFC, improved kinetics of the methanol oxidation
reaction at the anode.28 However, membranes such as Nafion® lose water which is
necessary to produce protonic conductivity at elevated temperature. Devising systems
that can conduct protons with little or no water is perhaps the greatest challenge for new
membrane materials.7
In recent years, a variety of new aromatic ionomers have been prepared and
characterized as membrane candidates for PEM fuel cells, for example, sulfonated
poly(arylene ether sulfones), sulfonated poly(arylene ether ketones), sulfonated
poly(arylene ether phosphine oxide)s, sulfonated polyimides, sulfonated
polyphosphazenes, and so on. These materials show some promise with respect to
conductivity, stability, methanol crossover, and water transport, and will be reviewed in
the following sections.
2.4.1. Poly(Arylene Ethers)
Poly(arylene ethers) have an attractive combination of chemical, physical and
mechanical properties that have made them an important class of engineering
thermoplastics.7,29-33 The basic repeat units in this family of copolymers consist of phenyl
rings linked together by ether groups and other groups like sulfones, ketone, or
arylphosphine oxide linkages (Fig. 2.7). The aromatic ether part provides chain flexibility,
thereby imparting good impact strength and toughness. The sulfone or ketone groups tend
to attract electrons from the phenyl rings and to enhance the resonance of the ether bond.
23
This results in good thermal, hydrolytic, and oxidative stability. Due to these well known
properties, wholly aromatic poly(arylene ether)s have attracted much attention for use in
PEMs with introduction of active proton exhange sites to the structure.
Figure 2.7. Several Possible Poly(arylene ether) Chemical Structures 7
2.4.1.1. Synthetic Routes of Poly(Arylene Ethers)
Several different synthetic methods for preparing poly(arylene ethers) have been
reported. The major ones include nucleophilic aromatic substitution, Friedel-Crafts
electophilic substitution, the Ullman reaction, and metal coupling reactions. Nucleophilic
aromatic substitution, which is the focus of this research, will be described in greater
detail here, and the others will be introduced briefly.
Nucleophilic Aromatic Substitution Polymerization: Since it was developed in
1967 by Johnson and coworkers,34 nucleophilic aromatic substitution polymerization
became more and more important and it is currently the most common route to synthesize
X Y X Z
n
X = O, S
Y = a bond, C
CH3
CH3
, C
CF3
CF3
, S
O
O,
P
O
Z = S
O
O
,C
O P
O
,
24
polymers like poly(arylene ethers). In its original form this route involved the
nucleophilic displacement of activated dihalo aryl derivatives by bisphenol salts based on
a strong base such as sodium hydroxide to yield high molecular weight polymer. Several
years later Clendinning et al.35 reported that potassium carbonate or bicarbonate could be
used in these reactions instead of sodium hydroxide. McGrath and coworkers36, 37 were
the first to systematically study the use of the weak base K2CO3 to obtain phenolate salts.
This type of polymerization has been investigated in the intervening years, and the
reaction mechanisms and conditions leading to most of the common poly(ary1 ethers)
(e.g. polysulfones and poly(ether ketones)) are rather well understood.38
Figure 2.8. Nucleophilic Aromatic Substitution Mechanism
The nucleophilic displacement of a halogen from an activated aryl halide system
25
occurs in a two-step addition-elimination reaction (SNAr) as shown in Figure 2.8. The
nucleophile adds to the electron-deficient aryl halide, forming a negatively charged
Meisenheimer complex, from which the halide is eliminated, leading to the formation of
an aryl-ether linkage. The activating group present in the aryl halide serves two purposes.
The group must be an electron-withdrawing moiety, which decreases the electron density
at the site of the reaction, and secondly, its presence must lower the energy of the
transition state for the reaction by stabilizing the anionic intermediate formed. These
SNAr reactions only proceed if the electron-withdrawing substituent is located either in
ortho or para positions relative to the halide. The most commonly employed activating
groups in these reactions have been sulfones, ketones, and more recently, phosphine
oxides, which are all strongly electron withdrawing substituents.39 Although these
strongly electron withdrawing groups are preferred in the SNAr reaction, it has been
demonstrated recently that some other weakly electron withdrawing functional groups
can also activate aryl fluorides toward nucleophilic aromatic substitution. For example,
certain heterocycles as well as other functional groups (e.g. perfluoroalkyl groups, azines,
acetylenes, etc.) can effectively activate aryl fluorides toward SNAr reactions and many
of these groups have been successfully used in the preparation of the corresponding
poly(ary1 ethers).40-42 The reactivity of the activated halides can be estimated by
measuring 1H, 13C and 19F NMR chemical shifts. 1H NMR chemical shift data from the
protons ortho or para to the electron-withdrawing group can be used to determine the
reactivity of the monomer indirectly.43 13C NMR and 19F NMR can be used to probe the
chemical shift at the actual site of the nucleophilic reaction. In general, lower chemical
shifts correlate with lower monomer reactivity. The reactivity of a number of aryl
26
fluoride monomers used in nucleophilic aromatic substitution polymerization was
explored by Carter,39 and he reported that a compound may be appropriate for
nucleophilic displacement if the 13C chemical shift of an activated fluoride ranges from
164.5 to 166.2 ppm in CDCl3.
The two-step mechanism is supported by the isolation of many Meisenheimer
salts. Evidence for a rate-determining first step comes from the observation that
fluoroaromatics undergo nucleophilic substitution much more rapidly than their iodo–
counterparts, despite the fact that I– is a better nucleophile than F–. This is due to fluorine
being more inductively electron withdrawing than iodine, reducing electron density on
the aromatic ring and enhancing the rate of nucleophilic attack. The high-energy
Meisenheimer intermediate is stabilized by resonance, resulting in higher electron density
at the ortho– and para– positions. Therefore, the reactivity of halogen leaving groups
follows F >> Cl > Br > I, which is the opposite of normal SN2 reactions.44 Although
fluoroaromatics such as 4,4’-difluorodiphenyl sulfone (DFDPS) are much more reactive,
the chloroaromatics are preferred in practical applications for economic reasons.
In addition to the strength of the activating group and the electronegativity of the
leaving group, many other factors will affect the kinetics of the SNAr reaction, such as the
nucleophilicity of the attacking nucleophile, the nature of the solvent, the reaction
temperature, and other experimental conditions.
The reaction rate increases with increasing strength of the nucleophile. The
overall approximate order of nucleophilicity is:
ArS- > RO- > R2NH > ArO- > OH- > ArNH2 > NH3 >I- > Br- > Cl- > H2O > ROH.45
Thus the phenolate is formed from the phenol type monomer before
27
polymerization by addition of either a strong base (NaOH) or a weak base (K2CO3). The
earliest strong base route works most of the time at high temperature to yield high
molecular weight in a short period of time. However, many salts such as the
hydroquinone or biphenol salt are so insoluble that they do not work well. Furthermore, a
stoichiometric amount of base used for the reaction is critical to obtain high molecular
weight polymers. Moreover, excess strong base may undesirably hydrolyze the dihalides
to afford deactivated diphenolates which upset the stoichiometry. Due to these
disadvantages, an alternate method was developed which involved utilizing anhydrous
potassium carbonate as the base. This method is advantageous in that K2CO3 can be used
in excess without the occurrence of any side reactions.34, 35 K2CO3 was found to be better
than Na2CO3 due to its relative stronger basicity and higher solubility in the reaction
medium.46 In addition, to obtain high molecular weight, water generated during this step
should be removed from the system to avoid hydrolyzing the activated substrate, since
hydrolysis reduces the reaction rate and upsets the stoichiometry of the monomers. To
remove the water, an azeotroping agent, such as toluene and xylene, is commonly used.
Aprotic polar solvents, such as dimethyl sulfoxide (DMSO), N,N-dimethyl
acetamide (DMAc), N,N-dimethyl formamide (DMF), N-methyl pyrrolidone (NMP), and
cyclohexylpyrrolidone (CHP) are most often used in the reaction. The solvent should
provide good solubility for both the monomers and the polymer product. Otherwise, the
stoichiometry will be offset by some precipitation. Thus under some circumstances, very
high reaction temperature and boiling point solvents, such as sulfolane, diphenyl sulfone
(DPS) have to be used due to the poor reactivity of the monomers or poor solubility of
the resulting, possibly semi-crystalline polymers, as in the PEEK systems.
28
In addition to the factors emphasized above, reaction conditions (such as
temperature and inert reaction environment) are also important for a successful SNAr
polymerization.
Friedel-Crafts Electrophonic Substitution: This is another important route to
synthesize Poly(arylene ethers). The mechanism involves a two-stage reaction, as shown
in Figure 2.9. The first step is the attack of the electrophile on the π electrons of the
aromatic benzene ring, generating a positively charged benzenonium intermediate, which
is the rate-limiting step of the reaction. The second fast step is the loss of a proton to
restore the aromaticity of the ring, yielding a substituted benzene ring. Benzene is a poor
electron source compared to alkenes and therefore requires a catalyst to initiate the
reaction. Several catalysts, such as AlCl3, AlBr3, FeCl3, SbCl5, and BF3 have been
utilized in Friedel-Crafts reactions.47
29
The formation of electrophile:
AlCl3 + SO
O
ArCl AlCl4 + SO
O
Ar+-
Stage one:
SO
OAr+
Stage two:
AlCl
Cl
Cl
Cl
-SO2ArH S Ar
O
O
AlCl3HCl
Figure 2.9. The Electrophilic Substitution Mechanism
The Ullman Reaction: The distinct advantage of this synthetic method is that the
non-activated aromatic halides can be polymerized, which is not possible by the normal
activated nucleophilic aromatic substitution route. The reactivity of the leaving group
follows the order I > Br > Cl > F, which is opposite to that observed for the classical
SNAr because the rate determining step is breaking the aryl halide bond. Therefore, iodine
or bromine is preferred under the Ullman reaction conditions. Ullman coupling of
bisphenols and dibromoarylenes using a copper catalyst results in high molecular weight
poly(arylene ether)s. However, poor reproducibility, the need for brominated monomers,
and the difficulty of removing copper salts are major disadvantages of this reaction.48-50
Metal coupling reaction: A relatively new approach developed by Colon and
30
Kelsey, nickel-(0)-catalyzed coupling has proven to be a powerful synthetic method for
the formation of carbon-carbon aryl bonds. A zero valent nickel-triphenylphosphine
complex is used as the catalyst prepared from nickel chloride and zinc metal. In this route,
polymers with biphenyl units can be made from monomers that contain only one phenyl
ring.51, 52 The reaction conditions tolerate many functionalities with the only known
exceptions being protic, nitro, and amine-containing substituents. A wide range of
polymeric materials have been synthesized from inexpensive arylene chlorides and
mesylates as well as the more costly bromide, iodide, and triflate derivatives. The major
advantage is that the reaction can occur under very mild conditions (temp: 60 to 80 oC).
Furthermore, both activated and nonactivated halide monomers can be used. 53-56
2.4.1.2. Molecular Weight Control and Characterization
The molecular weight of polymers synthesized by step-growth polycondensation
can be controlled by two methods. Firstly, by upsetting the mole ratio of the two
difunctional monomers to break up the 1:1 stoichiometry, one can easily control the
molecular weight. The second method involves addition of a stoichiometric amount of a
monofunctional comonomer to endcap the polymer chain. Both methods can obtain
specific designed molecular weight polymers by following the modified Carother’s
equation.29
Control of molecular weight is very important, especially for PEM materials,
because molecular weight is a fundamental parameter affecting the mechanical behavior
of polymers. Proton exchange membranes with good mechanical properties in both the
dry and hydrated states are critical to successful MEA fabrication and long-term
31
durability in a fuel cell device. The membrane must be able to withstand the stresses of
both electrode processing and attachment and must also be mechanically robust enough
to endure startup and shutdown of the fuel cell with repeated
swelling/drying/heating/cooling of the membrane. On the other hand, the molecular
weight may also have some influence on other PEM properties like water uptake and
conductivity. Despite the large body of research on this topic, there is little in the PEM
literature describing molecular weights of candidate materials, even including Nafion®.7
Recently, McGrath research group began to work in this area. Wang et al.
synthesized controlled molecular weight (Mn) poly(arylene ether sulfone)s (Mn from 20
to 40 kg·mol-1) by offsetting stoichiometry with a t-butylphenyl endcapping reagent.57
The t-butylphenyl concentrations relative to the polymer backbone were characterized by
1H NMR to calculate the molecular weight of the copolymers. They provided intrinsic
viscosity (IV) data for these copolymers, and found that the intrinsic viscosities were not
comparable to those of non-sulfonated polymers, since the polymer electrolyte chains
interact via the sulfonate groups. Li et al.58 lately reported further research based on the
previous results. They synthesized a series of controlled molecular weight (from 20 to 70
kg·mol-1), poly(arylene ether sulfone) copolymers containing 35 mol% disulfonated
monomer per repeat unit (Fig. 2.10). The molecular weight characterization combined 1H
NMR analysis of end groups and modified intrinsic viscosity measurements, which used
NMP with 0.05 M LiBr as a solvent. The small amount of salt effectively suppressed the
polyelectrolyte effect, allowing improved characterization of the ion containing
materials.59
32
HO OHSClO
O
BPDCDPSSO3Na
CH3
CH3CH3HO
S ClCl
O
O
TB
SDCDPS
K2CO3
SO
O*
DMAc/Toluene~ 160 oC, Reflux 4 h175 oC, 24 h
+
+
NaO3S
O O SO
OO O * S
O
OO
CH3
CH3CH3
CH3
CH3
H3C Ox
1-x n
KO3S SO3K
Cl
x = 0.35; BPS35, Target Mn: 20, 30, 40 and 50 kg·mol-1
Figure 2.10. Synthesis of Tert-Butylphenyl Terminated Poly(Arylene Ether Sulfone)
Copolymers Containing 35 mol% Disulfonated Repeat Unit.58
It was determined that mechanical properties (Fig. 2.11) and water uptake of the
material are dependent on the molecular weight of the copolymer, possibly related to
chain entanglement issues. The primary results showed that molecular weight also has
some influence on the proton conductivity, but this needs to be further confirmed and the
research is still ongoing.
33
45
47
49
51
53
55
57
59
0 20 40 60 80
Strain, %
Stre
ss, M
Pa
1, 202, 303, 404, 505, Control-70
12
43
5Mn (kg · mol-1)
45
47
49
51
53
55
57
59
0 20 40 60 80
Strain, %
Stre
ss, M
Pa
1, 202, 303, 404, 505, Control-70
12
43
5Mn (kg · mol-1)
Figure 2.11. Stress-strain Curves of BPSH35 Copolymers (Thin Films) as a Function
of Molecular Weight.58
Another purpose of controlling the molecular weight is to prepare reactive
oligomers by end-capping the polymer chain with functional groups. These oligomers can
be used in the synthesis of block copolymers or for modifying polymer networks. This
will be discussed in a later section.
2.4.1.3. “Post” vs. “Direct” Sulfonation Methods
Generally, two methods have been used to introduce active proton exchange sites
(here referred to as sulfonic acid groups) to the polymers for use as PEM materials. These
include a post-modification approach and direct copolymerization of sulfonated
monomers.
34
The post sulfonation method employs electrophilic aromatic sulfonation to
sulfonate the commercially available polymer,thus the electron-donating substituents on
the aromatic ring will favor the sulfonation reaction, whereas electron-withdrawing
substituents will not.60 This mechanism dictates that the sulfonation reaction occurs more
likely on the aromatic ring activated by the electron-donating ether link instead of the one
deactivated by the electron-attracting ketone or sulfone group (Fig. 2.12). Many kinds of
sulfonating reagents have been investigated including concentrated sulfuric acid, fuming
sulfuric acid, chlorosulfonic acid, sulfur trioxide or complexes. The obvious advantage of
this postmodification reaction is that the polymers are commercially available, but the
disadvantages include difficulty in controlling the degree and location of
functionalization, the possibility of side reactions, and degradation of the polymer
backbone.
35
Figure 2.12. Direct Copolymerization of sulfonated Monomers versus Post
Sulfonation
The direct method is a relatively new approach, which involves the synthesis of a
disulfonated monomer, followed by copolymerization of the sulfonated monomer with
unsulfonated monomers to obtain a polymer containing ionic groups. The first report of
the required sulfonated monomer was from Robeson and Matzner,61 who obtained a
composition of matter patent, which primarily was of interest for its flame retarding
properties. More recently, Ueda et al.62 reported the sulfonation of 4,4’- dichlorodiphenyl
sulfone and provided general procedures for its purification and characterization.
Professor McGrath’s research group at Virginia Tech modified the procedure for
disulfonation of the monomer, shown in Figure 2.13. Sulfonated poly(arylene ether
•Post sulfonation occurs on the most reactive, but least stable, position•High electron density leads to relatively easy desulfonation
•Monomer sulfonation on the deactivated position•Enhanced stability due to low electron density
O O S
O
OHO3S SO3H
n
Activated
O O S
O
OSO3HSO3H
n
Deactivated
36
sulfone) copolymers were then synthesized via direct copolymerization in any
composition desired (Fig. 2.14). This direct method overcomes some disadvantages of
postmodification, including: (1) the degree of sulfonation can be precisely controlled by
varying the sulfonated and unsulfonated monomer ratio, (2) the location of the sulfonic
acid group is on the deactivated sites of the repeat units, thus enhancing the stability and
increasing the proton conductivity due to the electron-withdrawing sulfone or ketone
groups, and (3) the possible side reactions such as cross-linking can be reduced or
avoided, which may result in better thermal stability and mechanical properties.7, 63, 64
Because of its practicality and efficiency, the direct sulfonation method attracts much
attention and has become more and more important in recent years.
SCl ClO
OSCl ClO
O
SO3HHO3S
SCl ClO
O
SO3NaNaO3S
SO3 (28%)
110 oC
NaCl H2O NaOH NaCl
PH = 6~7
Figure 2.13. Synthesis of 3,3’-Disulfonated 4,4’-dichloro-diphenyl Sulfone in Its
Sodium Salt Form63
37
Figure 2.14. Direct Copolymerization of Wholly Aromatic Sulfonated Poly(arylene
ether sulfone) “BPSH-xx,” where xx is the ratio of sulfonated to unsulfonated
activated halide.
2.4.1.4. Sulfonated Poly(Arylene Ether Sulfone)s and Poly(Arylene Ether Ketone)s via the Post Modification Method
Modification of poly(arylene ethers) by addition of sulfonic acid using various
reagents has been investigated extensively.28 Among many structures of poly(arylene
ether)s, two major types attract the most attention. The first is the commercially available
poly(ether sulfone) (PSU Udel®) , and the second is the commercially available wholly
aromatic poly(ether ether ketone) (PEEK Victrex®).
O O SO
O
SO3Na
NaO3S
O O SO
O
O O SO
O
SO3H
HO3S
O O SO
O
SO
OCl Cl HO OH S
O
OCl Cl
SO3Na
NaO3S
NMP/Toluene/K2CO3~ 160°C reflux 4 hrs190°C 16 hours
H+
y
y x
xx
x
1-x
1-x
38
O O C
O
**
SPEEKHO3S
CO
CH3
CH3
O* S
HO3S O
O
*
SPSU
n
n
Figure 2.15. Sulfonated Poly(ether sulfone) (Udel®) and Poly(ether ether ketone)
(Victrex®)
Chlorosulfonic acid was probably the first reagent used to sulfonate the
bisphenol-A-based poly(ether sulfone), Udel®.65, 66 The primary purpose was to produce a
sulfonated poly(arylene ether sulfone) for application in desalination via reverse osmosis.
The reaction was conducted at room temperature and the sulfonation level was controlled
by the reaction time. However, the chlorosulfonic acid may be capable of cleaving the
bisphenol A polysulfone partially at the isopropylidene link, or it might undergo
branching and crosslinking reactions by converting the intermediate sulfonic acid group
into a partially branched or crosslinked sulfone unit. An alternative milder route was
employed to sulfonate bisphenol A polysulfone,67 in which a complex of SO3 and triethyl
phosphate (TEP) with a molar ratio of 2:1 was used to sulfonate the polymer at room
temperature. This mild sulfonation treatment could minimize or even eliminate possible
side reactions such as branching and crosslinking, so it has been employed by some other
researchers to obtain sulfonated PES.68, 69 In 1993, Nolte et al.70 sulfonated the
poly(arylene ether sulfone) Udel® by chlorotrimethylsilyl sulfonate, which was generated
39
in-situ by reacting chlorosulfonic acid with trimethyl chlorosilane in 1,2-dichloroethane
at room temperature. Reaction of trimethylsilylchloro-sulfonate with PSU gives a
silylsulfonated polysulfone from which the trimethylsilyl moieties can be cleaved to give
the acid form of the alkaline sulfonate.
As described above, three different sulfonating agents have generally been used in
sulfonating poly(ether sulfone)s: chlorsulfonic acid (ClSO3H),
trimethylsilylchlorsulfonate ((CH3)3SiSO3Cl), and a complex of SO3 with
triethylphosphate (PO(OCH2CH3)3). Because the last one is not recommended due to its
high toxicity, Genova-Dimitrova et al.71 recently designed experiments to compare the
other two sulfonating agents. They found that ClSO3H, a strong sulfonating agent,
required the addition of a small amount of dimethylformamide (DMF) to prevent
precipitation and generate a homogenous reaction medium. But the reaction mixture
remained perfectly homogeneous with (CH3)3SiSO3Cl. The viscometric measurements
and mechanical stress–strain tests results showed that ClSO3H provoked chain cleavages,
whereas the mild sulfonating agent (CH3)3SiSO3Cl induced neither chain cleavage nor
branching. Although there are also some disadvantages related to the milder acid route,
for example, the longer reaction time needed and the lower sulfonation efficiency, the
advantages of avoiding polymer degradation and side reactions probably outweigh these
drawbacks.
The post-modification of other polysulfones has also been reported. Harrison et
al.72 reported the synthesis of high molecular weight poly(arylene ether sulfone)s by
polycondensation of bisphenol AF or biphenol with dichlorodiphenylsulfone, and then
40
sulfonated them by both a strong acid (ClSO3H) and a mild acid ((CH3)3SiSO3Cl). The
similar results implied that the mild acid route may only be necessary for sulfonation of
polymers with aliphatic groups to prevent degradation. Recently, a novel parent high
molecular weight poly(arylene ether) copolymer containing fluorenyl group was
synthesized via nucleophilic aromatic substitution reaction, and then post sulfonated with
chlorosulfonic acid in CH2Cl2.73 The sulfonic acid groups were introduced only at a
specific position on the fluorenyl groups as shown in Figure 2.16. The resulting ionomer
showed high proton conductivity (0.2 S.cm-1) and excellent hydrolytic stability under
harsh hydrolytic conditions (140 °C and 100% RH). The membranes were highly
mechanically stable at 85 °C and 93% RH and keep their strength even at 120 °C.
Hydrogen and oxygen permeability of this ionomer membrane was much lower than that
of Nafion® 112 under a wide range of conditions (40-120 °C and 0-90% RH).74 The
authors claimed that these properties of the membrane are superior to other hydrocarbon-
based ionomers for fuel cell applications.
41
O SO
OO
n
O SO
OO
n
ClSO3H
2/x(HO3S)
(SO3H)x/2
Figure 2.16. Synthesis of Poly(arylene ether) Ionomers Containing Sulfofluorenyl
Groups via a Post-modification Method73
As mentioned before, the difficulty in controlling the sulfonation location is one
of the major problems with the post-modification method. The electrophilic substitution
reactions introduce the sulfonic acid group at the activated site ortho to the aromatic ether
bond. This leads to a less stable product which is more susceptible to desulfonation than
those where sulfonation is directed to the electron-deficient ring of the repeat unit. Kerres
et al.75 have developed a novel sulfonation method that proceeds via a metalation-
sulfination-oxidation procedure to sulfonate poly(ether su1fone) (Udel®), in which the
sulfonation occurs at a position ortho to the sulfone group ( Fig. 2.17). Four steps are
involved in the procedure: (1) lithiation of the polymer at temperatures from -50 to -80°C
under argon, (2) gassing of the lithiated polymer with SO3, (3) oxidation of the formed
polymeric sulfinate with H2O2, NaOC1, or KMnO4, (4) ion-exchange of the lithium salt
42
of the sulfonic acid in aqueous HC1. The choice of the oxidant in the third step is
important according to the paper, because the proper oxidant will prevent loss of IEC due
to the desulfonation and will increase the efficiency of the oxidation. KMnO4 is suitable
for oxidizing poly(ethersulfonsufinate)s at high to medium degrees of sulfonation (ca.
3.3-1.9 meq·g-1) and minimize the loss of IEC, whereas H2O2 is better for low degrees of
sulfonation ( <1.5 meq·g-1). This synthetic method was also employed in crosslinking
procedures for the sulfonated PSU membranes to enhance the chemical and thermal
stability (Fig. 2.18).76, 77 Although this method can control the sulfonation location and
theoretically work with all polymers which can be lithiated, the complicated steps still
hinder its scale-up.
C
O O
CH3H3C SO O
C
O O
CH3H3C SO O Li
C
O O
CH3H3C SO O
C
O O
CH3H3C SO O S
OLiOSO3H
n-BuLiTHF, -65oC
SO2-65 oC
1. H2O2/OH
2. H+/H2O
Figure 2.17. Synthesis of Sulfonated Poly(ether sulfone) Udel® PSU via the
Metalation Route75
43
SO
OO C
CH3
CH3O
n
SO
OO C
CH3
CH3O
n
LiO2S
LiO2S
SO
OO C
CH3
CH3O
n
SO
OO C
CH3
CH3O
n
O2S
O2S
DMAc80-120 oC-LiI
Figure 2.18. Crosslinking via the Metalation Route76
Another interesting approach to controlling the degree and location of sulfonation
has been reported by Al-Omran and Rose.78 In this paper, several poly(arylene ether
sulfone)s were prepared with 4,4’-dichlorodiphenylsulfone, hydroquinone, and
durohydroquinone via the classic nucleophilic aromatic substitution, followed by
sulfonation using sulfuric acid. The idea is that the sulfonation will only occur on the
hydroquinone residue since there are no aromatic hydrogens on the durohydrquinone
structure. Thus, by varying the molar ratios of hydroquinone to durohydroquinone, the
degree of sulfonation could theoretically be controlled. However, 1H NMR spectra
revealed that the results were not as expected - some sulfonation also occured on the
phenyl ring meta to the sulfone linkage probably due to electron accession from the
methyl groups transmitted via ether linkages to the reaction site on adjacent phenylene
ether sulfone rings.
Post-sulfonation of commercially available poly(ether ether ketone) (Victrex®
PEEK) has been investigated extensively for PEM fuel cells.79-83 PEEK is a
semicrystalline polymer and is not easily dissolved in organic solvents. By introducing
the sulfonic acid to the polymer backbone, the solubility increases due to a decrease in
44
crystallinity.84 Sulfonation of PEEK has been carried out using concentrated sulfuric acid,
oleum or chlorosulfonic acid. 95–98% concentrated sulfuric acid is a common choice to
avoid polymer degradation and cross-linking reactions, which occur for sulfonation with
oleum or chlorosulfonic acid.85 At room temperature with concentrated sulfuric acid used
as the solvent, there is at most one SO3H group attached to each repeat unit.79 The
sulfonation substitution reaction is a second-order reaction; the reverse reaction is
neglected for high acid concentrations.86 The IEC or degree of sulfonation can be
controlled by changing the reaction time, temperature and the acid concentration.
Polymers with a sulfonation range of 30-100% are achieved without apparent degradation
or crosslinking reactions, but the truly random copolymer at sulfonation levels less than
30% is difficult to obtain due to the heterogeneous reaction medium.86, 87
Except PEEK, other poly(arylene ether ketone)s, such as PEK and PEKK (Table
2.2), have also been investigated by post sulfonation. These structures are potentially
interesting because the oxidative stability of PAEKs increases with increasing K/E ratio
of the repeating unit. However, because the electron-withdrawing effect of the ketone
groups deactivated the aromatic ring, sulfonation becomes more difficult and more
reactive sulfonation reagents are required at high K/E values. For example, Swier et al.88
sulfonated PEKK using a mixture of concentrated and fuming sulfuric acids. They found
that unlike sulfonation of PEEK where the reaction occurs during the dissolution of the
polymer in concentrated sulfuric acid, PEKK polymer is not sulfonated during the initial
dissolution in concentrated sulfuric acid, but requires the subsequent addition of fuming
sulfuric acid. For excess SO3, the sulfonation kinetics exhibits a pseudo first-order
dependence on the concentration of the reactive aromatic rings between the ether and
45
ketone groups. Recently, a new modified structure of SPEEK, poly(oxa-p-phenylene-3,3-
phtalido-p-phenyleneoxa-p- phenileneoxy-phenylene) (PEEK-WC), has been investigated
as a proton conductive material by the post –sulfonation route.89
Table 2.2. Several Poly(arylene ether ketone) Structures88
Polymer Repeat Unit Ketone/ether ratio
PEEK O O C
O
0.5
PEK O C
O
1.0
PEKK O C C
O O
2.0
2.4.1.5. Direct Sulfonation of Poly(Arylene Ether Sulfone)s and Poly(Arylene Ether Ketone)s
As described previously, the dihalide monomers can be sulfonated first and then
copolymerized with the unsulfonated dihalide and bisphenol monomers directly to
synthesize sulfonated copolymers. This direct sulfonation route makes it possible to
better control the location and the degree of sulfonation, thus enhancing the thermal
stability, mechanical properties, and even increasing the acidity without degradation.
Although the route was developed earlier, it was first employed for preparing PEM
materials by the McGrath group at Virginia Tech.63 Wang et al.63, 64 modified the
previous procedure and synthesize 3,3’-disulfonated 4,4’-dichlorodiphenylsulfone
46
(SDCDPS) in high yield (~ 80%). Subsequently, the random wholly aromatic
disulfonated copolymers (BPSH) were synthesized by copolymerizing this disulfonated
monomer with 4,4’-dichlorodipheylsulfone (DCDPS) and 4,4’-biphenol via the classic
nucleophilic substitution reaction. Higher temperature and longer reaction time may be
needed to reach high molecular weight due to the sterically decreased activity of the
sulfonated dihalide monomer. The properties of this BPSH copolymer have been
investigated extensively for its potential application as a PEM candidate including
protonic conductivity, water uptake, thermal stability, mechanical properties, morphology,
and more.63, 64, 90-93
The degree of sulfonation (IEC) of the copolymer, which contributes to the
PEM’s electrochemical performance, can be precisely controlled by varying the molar
ratio of SDCDPS and DCDPS monomers. The actual IEC values of the resulting
copolymers were calculated from 1H NMR spectra or measured by nonaqueous
potentiometric titrations. FT-IR and TGA can also qualitatively confirm the results. The
experimental IEC values matched well with the targeted IEC values, this suggests that the
method is reproducible. Although the IEC is a key factor in affecting conductivity and
water uptake of the membrane, it is also found that the acidification method has a distinct
effect on these properties.90 Either a room temperature acidification method (1.5M H2SO4,
Method 1) or a boiling acidification method (0.5M H2SO4, Method 2) was used to
convert the sulfonate salt copolymers to the sulfonic acid form. Both acidification
methods did not change the initial degree of sulfonation, but the fully hydrated BPSH
membranes treated by method 2 exhibited higher proton conductivity, greater water
absorption, and less temperature dependence on proton conductivity as compared with
47
the membranes acidified by method 1. This effect may be attributed to the morphological
changes that occur during the acidification, as observed by atomic force microscopy
(AFM). The samples treated by method 2 had larger hydrophilic domains with more
phase continuity than the samples treated by method 1. Futhermore, the conductivity and
water uptake are also a function of temperature and relatively humidity. High protonic
conductivity is preferred in fuel cell applications and this can be simply achieved by
incorporating more sulfonic acid moieties. However, the water uptake of the BPSH
copolymers increased dramatically when the degree of sulfonation was greater than 50%
(Fig. 2.19). Combined with the DSC and AFM results, it was concluded that the BPSH
system reaches a percolation limit at about 50 mol% of the disulfonated monomer, above
which a hydrogel was formed which was not usable in a PEM fuel cell. These results
indicate that the protonic conductivity must be balanced with the water swelling and
mechanical properties of the membrane in these random copolymers.
48
Figure 2.19. Influence of the Dgree of Sulfonation on the Water Uptake of BPSH
Copolymers64
Harrison et al.94 have studied the influence of the bisphenol structure on the direct
synthesis of sulfonated poly(arylene ether)s. Four bisphenols (Fig. 2.20) were
copolymerized with SDCDPS and DCDPS to synthesize disulfonated poly(arylene ether
sulfone)s. The IEC values of the copolymers change according to the bisphenol used even
at the same degree of sulfonation, for example, the IEC of hydroquinone based
copolymer is higher than others at the same degree of disulfonation due to the smaller
molecular weight of hydroquinone residue in the repeat unit. But the general trend was
the same - the thin film properties of these copolymers scaled with the IEC values.
Bisphenol-A is a very inexpensive and reactive monomer, but the aliphatic groups may
suffer some degradation under the harsh fuel cell environment. The partially fluorinated
49
monomer, bisphenol-AF, has attracted much attention recently because it is thought that
the fluorine rich surface of the membrane will be more compatible with electrodes that
contain Nafion® and may produce more durable MEAs. The more hydrophobic
membrane surface may also reduce the swelling and enhance the stability.7, 95
Figure 2.20. Four Investigated Bisphenol Structures94
Disulfonated poly(arylene ether ketone)s can also be prepared via the direct
polymerization method similar to the poly(arylene ether sulfone)s. The disulfonated
monomer sodium 5,5’-carbonylbis(2-fluorobenzenesulfonate) was first used in direct
polymerization reported by Wang.96, 97 After that, several other diketone monomers (Fig.
2.21) were sulfonated and copolymerized with various bisphenols.98-101 The bisphenol-
AF-based poly(arylene ether ketone)s reported by Li101 show comparable properties to
the BPSH system at the same IEC. Other results showed that the ketone type copolymers
have lower methanol permeability than Nafion®, and therefore may be more suitable for
50
DMFC.98, 102
XF F
NaO3S SO3Na
X = CO
CO
CO
CO
CO
, , or
Figure 2.21. Several investigated sulfonated ketone or diketone structures
2.4.1.6. Other Poly(Arylene Ethers)
Besides poly(arylene ether ketone)s and poly(arylene ether sulfone)s, there are
several other poly(arylene ethers) which have also been synthesized and will be described
in this section.
Wiles et al. directly copolymerized poly(arylene sulfide sulfone) disulfonated
copolymers from disodium SDFDPS, DFDPS, and 4,4’-thiobisbenzenethiol. Compared
with their chlorinated analogs, the more active difluoro monomers increase the reaction
rate and the ease of obtaining high molecular weight copolymers. The membrane
properties showed the same trends as the disulfonated poly(arylene ether sulfone) (BPSH)
series copolymers.103, 104
Sulfonated poly(arylene ether nitriles) attract some attention due to their excellent
thermal and chemical properties.105-107 It was thought that the strongly polar nitrile groups
on the aromatic backbone would contribute some special properties to the membranes.
For example, the enhanced interaction ability of nitrile groups with other polar groups
could facilitate the preparation of composite membranes doped with inorganic particles,
51
promote the adhesion of the membrane to the electrodes, and even reduce the water
swelling. Sumner et al.105 first synthesized disulfonated poly(arylene ether nitrile)s via
direct step copolymerization of 4,4’-(hexafluoroisopropylidene)diphenol, 2,6-
dichlorobenzonitrile, and SDCDPS. The membranes showed similar proton conductivity
to previously synthesized BPSH copolymers and Nation 117 at the same IEC values, but
the water uptake was much lower, probably due to some interaction between the aromatic
nitrile and sulfonic acid moieties and also the more hydrophobic fluorine-rich surface.
Kim et al.106 then investigated the properties of this copolymer with 35% degree of
sulfonation in a DMFC. Their results showed that the methanol crossover is
approximately 2-fold lower than that Nafion®, but similar to nonfluorinated analogs
(BPSH-40). Furthermore, greatly improved DMFC performance compared to the Nafion®
and BPSH-40 under the same test condition suggests that the interfacial effects are very
important. Zhang et al.107 reported another similar poly(arylene ether nitrile) using a
disulfonated ketone monomer instead of the disulfonated sulfone, and studied composite
membranes doped with heteropolyacids (HPAs). HPA is an attracting inorganic additive
because of its high proton conductivity and thermal stability, but it is soluble in water.
The polar nitrile group presumably can retain more HPA in the membrane through
intermolecular interactions. In this paper, the composite membranes showed lower water
sorption but higher proton conductivity compared to the pure membrane.
Sulfonated poly(arylene ether phosphine oxide) is another copolymer possessing
polar groups along the main chain. The phosphine oxide functional moiety may also
serve as a compatibilizer with other materials. Sulfonated 4,4’-bis(fluorophenyl)phenyl
phosphine oxide) (SBFPPO) monomer was prepared by modifying the unsulfonated
52
monomer with fuming sulfuric acid.108 Although a small percentage of di- and tri-
sulfonated compounds were generated, monosulfonated monomer (Fig. 2.22) was the
major product and could be isolated. Wholly aromatic sulfonated copolymers
synthesized from SBFPPO, BFPPO, and 4,4’-biphenol showed high thermal stability.
However, the protonic conductivity was relatively low for two reasons. One is that only
one sulfonic acid group per repeat unit can be introduced to the 4,4’-
bis(fluorophenyl)phenyl phosphine oxide), and another reason may be due to the
observed hydrogen bonding between the pendent sulfonic acid group and the phosphine
oxide moiety. Recently, sulfonated poly(arylene ether phosphine oxide sulfone)
terpolymers have been prepared with SBFPPO, BFPPO, SDCDPS, and 4,4’-biphenol.109
The primary results of the phosphine oxide-containing copolymer doped with HPA
suggest that HPA retention from the composite membranes is significantly improved as a
result of hydrogen bonding.110
Figure 2.22. Structure of Mono-Sulfonated BFPPO
Poly(phthalazinone ether sulfone) (PPES) and poly(phthalazinone ether ketone)
(PPEK) are new high performance polymers with properties such as excellent chemical
and oxidative resistance, mechanical strength, good thermal stability and very high glass
transition temperatures.111 Thus they are considered to be promising candidates for PEM
53
fuel cells. Gao et al.111-113 and others114 introduced the ion exchange sites to the polymer
chain by both the post-modification method and direct copolymerization of the sulfonated
monomers. The post modifications (Fig. 2.23) were conducted at room temperature using
mixtures of 95–98% concentrated sulfuric acid and 27–33% fuming sulfuric acid. The
purpose of using the mixed acid is to promote the sulfonation efficiency and reduce the
degradation of the polymer during the sulfonation, which would be difficult to achieve
with any single acid. Moreover, by changing the acid ratios, the degree of sulfonation can
be controlled.
N N
O
X
ON N
O
X
O
SO3HSulfonation
X = C
O, S
O
O
Figure 2.23. Post Modification of PPES and PPEK111
Sulfonated PPES and PPEK were also synthesized by direct copolymerization of
4-(4-hydroxyphenyl)-1(2H)-phthalazinone with disulfonated activated dihalide sulfone or
ketone monomers as shown in Figure 2.24.113 Compared with the post-sulfonated
membranes, SPPEKH showed less temperature dependence of proton conductivity, and
SPPESH showed higher conductivity. From the results reported by several researchers,114,
115 it appears that the major advantage of these directly synthesized SPPES and SPPEK
54
over either the post-sulfonated products or other sulfonated poly(arylene ethers) is their
much lower water swelling, which originates from intermolecular hydrogen bonds. It was
also found that SPPEKs have lower methanol permeability than Nafion® in DMFC.116
Figure 2.24. Direct Synthesis of SPPEKs and SPPESs113
2.4.2. Sulfonated Polyimides (SPIs)
Polyimides are high-performance macromolecules which are usually obtained via
polycondensation of aromatic and/or alicylic dianhydride and diamine structures. They
exhibit excellent mechanical properties, as well as good chemical and long-term thermal
stability.117, 118 Sulfonated polyimides as potential PEM candidates, including five-
membered ring phthalic polyimides and six-membered ring naphthalenic polyimides,
have been investigated extensively. The introduction of ionic groups to the polymer
backbone can also be achieved by direct copolymerization of the pre-sulfonated diamine
55
monomers.
Five-membered ring sulfonated polyimides represent the first-generation PEMs,
but were unsuitable for the fuel cell working conditions since chain scission under the
strong acid environment causes materials to degrade quickly and the membrane to
become brittle.119 Therefore, the more hydrolytically stable six-membered ring sulfonated
polyimides have attracted more attention recently. This hydrolytic stability has been
investigated by Genies using model compounds of the sulfonic acid-containing phthalic
imide (Model A) and the naphthalenic imide (Model B).119 13C NMR and IR spectra were
utilized to monitor the intensity of peaks during the aging of two model compounds in
distilled water at 80 oC. It was found that the Model A compound was modified after 1 h
at 80 oC and hydrolyzed completely after 10 h, whereas the Model B stability reached
120 h under the same conditions. Moreover, the naphthalenic imide formed an
equilibrium with its products after a period of time and this limited the hydrolysis to
about 12%. More recent results reported by Jiang,120 who studied the hydrolytic stability
with phthalic sulfonated polyimides and naphthalenic sulfonated polyimides, confirmed
that the six-membered ring polyimides are much more hydrolytically stable than the five-
membered ring anologs. This effect can be explained by the fact that six-membered ring
polyimides have far lower ring strain, positive charges on the carbonyl carbon atoms and
higher orders of the corresponding bonds than five-membered ring polyimides.121
One of the earliest six-membered ring sulfonated polyimides was synthesized by
Mercier and coworkers based on 4,4’-diamino-biphenyl 2,2’-disulfonic acid (BDSA) (a
commercially available sulfonated diamine), 4,4’- oxydianiline (ODA) and 1,4,5,8-
naphthalene tetracarboxylic dianhydride (NTDA).122 Although the primary results
56
showed that naphthalenic SPIs are promising materials for PEMFCs, their solubility was
not good except in chlorophenol. It is known that by introducing ether linkages into the
main chain or bulky groups as substituent to the polyimides, the solubility will be
improved. Thus the new naphthalenic copolyimides from BDSA and non-sulfonated aryl
ether diamines were synthesized.122, 123 For example, the naphthalenic copolyimides
obtained from BDSA, NTDA, and the bis[(4-aminophenyl-oxy)methyl] 2,2-propane
(APMP, a non-sulfonated diamine) are soluble in N-methyl pyrrolidone (NMP) and have
good mechanical properties as well as high ionic conductivity (14.4×10−2 Scm−1 at 80
◦C).123
The Litt group tried to introduce bulky nonsulfonated diamines, such as 4,4’-(9-
fluorenylidene dianiline) (FDA), into the polymer backbone to improve the properties.124,
125 The idea is that by introducing the bulky diamine, a more open structure will be
formed instead of regular close parallel packing of the backbones, which will lead to
larger free volume and confine more water even at elevated temperatures. The hope was
that this may result in higher conductivity and application at higher temperatures. The
bulky group effects have also been investigated by the Watanabe group recently.126, 127
They claimed that the highest ever proton conductivity (1.67 Scm-1 at 120 °C) reported
for a PEM has been obtained with the sulfonated polyimide copolymers containing the
bulky fluorenyl groups (Fig. 2.25).126 Although these bulky polyimides produced higher
conductivites than Nafion®, the hydrolytic stability of these membranes is still a problem.
Watanabe et al. also synthesized partially fluorinated SPIs by incorporating the fluorine-
containing non-sulfonated diamine in order to take advantage of the benefits both of
hydrocarbon and perfluorinated ionomers. The results show that the oxidative stability of
57
these partially fluorinated SPIs is effectively improved, but no further information on the
hydrolytic stability was reported.128
NN
O
O
O
O SO3H
HO3S
NN
O
O
O
O
x
100 - x
SPIH - x (x = 0 - 60)
Figure 2.25. Chemical Structure of Sulfonated Polyimides Containing Fluorenyl
Groups126
The structure of the sulfonated diamine is another important factor that will affect
the properties of the polyimide membranes. Since commercially available sulfonated
diamines are limited and much work has been done on BDSA-based copolyimides as
described above, syntheses of various new sulfonated diamine monomers are very
important for studying the “structure-property” relationships. Much work has been done
in this area by Okamoto et al.129-135 Figure 2.26 shows the structures of the sulfonated
diamines synthesized by the Okamoto group as well as the commercial BDSA.
Compared with the commercially available BDSA sulfonated diamine, the
synthesized ones have either ether linkages or bulky groups. Some of them are of the
main-chain type where the sulfonic acid groups are directly bonded to the polymer
backbone and others are of the side-chain type where the sulfonic acid groups are
attached to the side chains. The variety of the structures makes it possible to investigate
the effects of the structure on the properties comprehensively, especially on the water
stability. The authors suggested that compared with BDSA-based SPIs, the copolyimides
58
based on more flexible sufonated diamines, such as the ODADS-based polyimides, will
enhance the hydrolytic stability greatly but display similar proton conductivity at similar
IEC, and the BAPFDS-based polyimide membranes showed much better water stability
because the rigid structure can be offset by the highly basic nature of BAPFDS. This
means that the basicity of the sulfonated diamine moieties will have positive effect on the
water stability of the polyimide membrane. Moreover, the side-chain type SPI
membranes exhibit much better water stability than the main–chain type SPI membranes
and other aromatic sulfonated polymer membranes, SPIs with lower IEC values will have
better water stability, and the stability of random copolyimides is higher than that of
block or sequenced ones.
59
H2N NH2
SO3H
HO3S 4,4’-diamino-2,2’-biphenyl disulfonic acid (BDSA)
NH2H2N
HO3S SO3H
9,9’-bis(4-aminophenyl)fluorine-2,7-disulfonic acid (BAPFDS)
OH2N
SO3H
NH2
HO3S 4,4’- Diaminodiphenylether-2,2’-diaulfonic acid (ODADS)
NH2
H2N
HO3S(H2C)3O
(2’,4’-diaminophenoxy)propane sulfonic acid
H2N O
HO3S
C
CF3
CF3
O
SO3H
NH2
2,2’-bis[4-(4-aminophenoxy)phenyl] hexafluoropropane disulfonic acid (BAHFDS)
H2N O
HO3S
O
SO3H
NH2
4,4’-bis(4-amino-phenoxy) biphenyl-3,3’-disulfonic acid (BAPBDS)
H2N NH2
O(CH2)3SO3H
O(CH2)3SO3H 2,2’-bis(3-sulfo-propoxy)benzidine (2,2’-BSPB)
or 3,3’- bis(3-sulfo-propoxy)benzidine (3,3’-BSPB)
Figure 2.26 Sulfonated Diamines
60
The McGrath research group has also developed two novel sulfonated diamines.
The first one is 3-sulfo-4’,4’’-bis(3-aminophenoxy) triphenyl phosphine oxide sodium
salt, which was used in the synthesis of a five-membered ring sulfonated polyimide.136
Because of the well known poor water stability of the five-membered ring SPIs and only
one ionic site on the monomer, which will limit the polymer’s conductivity, there was no
further investigation on this sulfonated monomer. Einsla et al137, 138 synthesized another
sulfonated diamine, 3,3’-disulfonic acid-bis[4-(3-aminophenoxy)phenyl] sulfone (SA-
DADPS), which has flexible ether and sulfone linkages to improve the copolymer’s
solubility. This SA-DADPS monomer was then copolymerized with NTDA and three
nonsulfonated diamines, bis[4-(3-aminophenoxy)phenyl] sulfone (m-BAPS), 4,4’-
oxydianiline (ODA), and 1,3-phenylenediamine (m-PDA), to synthesize three series of
high molecular weight disulfonated SPIs (Fig. 2.27). The degree of sulfonation was
controlled by varying the stoichiometric ratio of sulfonated diamine to several
nonsulfonated diamines. These naphthalenic polyimides were prepared by a one-pot
high-temperature polycondensation reacton in m-cresol. Due to the low reactivity of the
six-membered ring anhydride, a catalyst was necessary. Benzoic acid catalyst was added
in the first step, which was believed to promote formation of the trans-isoimide. Then a
basic catalyst, isoquinoline, was added to convert the trans-isoimide to a naphthalimide.
The copolyimides were soluble in NMP and tough films were obtained. Two series of
SPIs were selected to further study the structure-property relationships. The authors
indicated that influence of the structure of the nonsulfonated diamine on the proton
conductivity and water sorption were small at similar IEC, whereas they had a large
influence on the water stability at 80 oC. Although the copolyimide membranes based on
61
the nonsulfonated diamine m-BAPS displayed better water stability, their hydrolytic
stability was still much lower than Nafion® or analogous poly(arylene ether)s. McGrath
et al.7 also indicated that it is desirable to use SPI membranes with low methanol
permeation in room temperature DMFCs instead of hydrogen/air PEM fuel cells due to
their relatively short-term water stability.
H2N O SO
OO NH2
SO3HHO3S
H2N O SO
OO NH2
SO3NH(Et)3(Et)3HNO3S
OO
O O
OO
X = m-BAPS, ODA, or m-PDA
NN
O O
OO
O SO
OO
SO3NH(Et)3(Et)3HNO3S
NN
O O
OO
X
Y 1-Y X
4hrTEAm-Cresol
180 oC1) Benzoic acid, 9 hr2) Isoquinoline, 9hr
Figure 2.27. Synthesis of Disulfonated Polyimide Copolymers137
62
2.4.3. Polyphosphazene Ionomers for PEMs
Polyphosphazenes are semiorganic polymers with a backbone composed of
alternating phosphorus and nitrogen atoms and organic side groups chosen for a
particular application. The resistance to both thermal and chemical degradation and the
ease of chemically attaching various side chains for ion-exchange sites and polymer
crosslinking onto the -P=N- polymer backbone has made them attractive for alternative
PEM materials.139, 140 Principally, two general approaches can be used for the preparation
of sulfonated polyphosphazenes (sPPZs). In the first, an aryl oxide, alkoxide or arylamine
that already bears a terminal sulfonic acid or sulfonate group, replaces the chlorine atoms
in poly(dichlorophosphazene). This method can be viewed as analogous to direct
sulfonation. The second approach, which is a post-modification method, involves the
synthesis of phosphazenes with unsubstituted aryloxy side groups, followed by
sulfonation of these side groups. The sulfonating reagents used include SO3, concentrated
and fuming sulfuric acid, and chlorosulfonic acid.28
Gleria and coworkers141 first reported the sulfonation of
poly[aryloxyphosphazenes] by SO3. From 1996, Wycisk and Pintauro142 began to use the
same technique to sufonate poly[aryloxyphosphazene] for ion exchange membrane
purposes (Fig. 2.28). Poly[(3-methylphenoxy)(phenoxy)phosphazene], poly[(4-
methylphenoxy)(phenoxy)phosphazene], and the corresponding ethyl-substituted
polymers were sulfonated with SO3 in dichloroethane. In sulfonation, the skeletal
nitrogens were attacked to form ≡N→ SO 3 complexes in the first stage, followed by the
arenesulfonation taking place on the methylphenoxy, rather than the phenoxy, side group.
63
The authors found that the methylphenoxy polyphosphazene could be sulfonated easily
using SO3 to a high ion-exchange capacity (up to 2.0 mmol·g-1) with no detectable
polymer degradation, whereas the ethylphenoxy polyphosphazenes undergo severe
polymer degradation. The two sulfonated poly[(methylphenoxy)(phenoxy)phosphazene]
membranes had good mechanical properties and represented an attractive combination of
high IEC, acceptable swelling, and low resistivity, and the poly[(3-
methylphenoxyXphen- oxy)phosphazene] was found to be the best starting material.
Photocrosslinking initiated by benzophenone has been carried out with various
methylphenoxy-, ethylphenoxy-, and isopropylphenoxy-substituted phosphazenes. These
experiments showed that methylphenoxy side chains were the most effective for UV
photocrosslinking of dry films (via a hydrogen abstraction mechanism). The crosslinked
membranes swelled less than Nafion® 117 in both water and methanol without sacrificing
the protonic conductivity compared to non-crosslinked membranes. Moreover, the
methanol permeabilities of the crosslinked membranes were very low (<1.2 x 10-7 cm2·s-
1).143, 144
R
O
PN
On
+ 3SO3
R
O
PN
On
SO3H
SO3H
SO3
Figure 2.28. The Reaction Scheme for Sulfonation of Polyphosphazene with SO3
142
64
Allcock et al.139 sulfonated various aryloxy- and arylaminophosphazenes with
either sulfuric acid (concentrated or fuming) or chlorosulfonic acid. Sulfuric acid
produced sulfonate polyphosphazenenes substituted at the para-position and various
sulfonation degrees were obtained. However, significant chain degradation was observed,
especially with fuming sulfuric acid. The use of chlorosulfonic acid as a sulfonating agent
led to crosslinked insoluble products.
These post-sulfonations have similar disadvantages to post modification of other
copolymers as described earlier. For example, the reactions introduce significant
irregularities in the polymer structure, suffer from severe heterogeneity of the reaction
mixture, and allow for little or no control over the position and degree of sulfonation.
Therefore, direct sulfonation may offer a better route. However, the initial tests on the
direct replacement of chlorine atoms of poly(dichlorophosphazene) (PDCP) with 4-
hydroxybenzenesulfonic acid yielded macromolecular products. The results showed that
the sodium sulfonate group has the ability to react with PDCP first to generate an
unstable substitution product, which led to polymer degradation. Recently, Andrianov et
al.145 developed a novel route to direct sulfonation of polyphosphazenes by using
“noncovalent protection” of the sulfonic acid functionalities (Fig. 2.29). They added a
hydrophobic ammonium ion, such as the dimethyldipalmitylammonium ion, to suppress
the reactivity of the arylsulfonate. Sodium benzenesulfonate was converted to a
hydrophobic ammonium salt, which had no reactivity against PDCP and could be easily
removed after the completion of the reaction. The high molecular weight sulfonated
polyphosphazenes were then synthesized by this method without noticeable degradation
of the polymer backbones.
65
P
Cl
Cl
Nn
PDCP
SS
O
O
O
O
ONR'2R''2R'2R''2NO
DPSA P Nn
O
O
SO3H
SO3H
PDSA
P Nn
O
R
SO3HSS
O
O
O
O
ONR'2R''2R'2R''2NO
NaOR
Mixed Substituent Copolymers
Figure 2. 29. Direct Sulfonation of Polyphosphazenes by the Noncovalent Protection
Method145
2.5. Other Novel Approaches to Improve PEM Properties
Besides the synthesis of new functionalized proton-conducting copolymers, many
other methods have been investigated to improve the PEM properties. For example,
modification of the membranes by acid- or base-doping, using polymer blends, better
understanding and control of polymer microstructure, development of organic/ inorganic
composite systems, crosslinking, grafting of a functional group, or incorporating other
ionic sites like phosphonic acids have all been explored. In this section, several of these
novel approaches will be briefly reviewed.
66
2.5.1. Controlling Morphology Using Block and Multiblock Copolymers
Better understanding and control of copolymer morphology is important for
improving the fuel cell performance. It is well known that Nafion® has highly phase-
separated hydrophilic and hydrophobic domains. The hydrophilic sulfonated groups
interconnected through electrostatic interactions to form ion channels for water and ion
transportation. This unique morphology can explain why the perfluorosulfonate ionomer
has high proton conductivity, even with lower ion exchange capacity and relatively low
water content.146, 147 Thus the morphology of membranes made from the aromatic random
copolymers, such as BPSH copolymers, has been studied64, 90, 148, 149 and compared with
Nafion®. It has been noted that the formation of the continuous hydrophilic domain
strongly depends on the degree of sulfonation and hydrothermal treatment of the
membrane. For BPSH copolymers acidified by method 1 (the membranes have lower
water uptake compared to method 2), the co-continuous phase was observed (AFM)
when the degree of sulfonation was greater than 50%, which is called the percolation
threshold. Below this degree, the morphology forms a closed domain structure, where
isolated hydrophilic domains are surrounded by a hydrophobic matrix. When method 2
acidification was used, the percolation limit decreased to 35% sulfonation degree.
Although increasing ionic concentrations may help form continuous hydrophilic domains
and lead to higher protonic conductivity, the high water uptake values at high degrees of
sulfonation result in a reduction in the mechanical strength, which limits membrane
applicability.
The challenge here is to modify the chemistry of the polymers to obtain
significant protonic conductivity at low hydration levels. Nanophase separated ion-
67
containing block copolymers may offer a possibility.8 Block copolymer ionomers have
hydrophilic blocks (which contain ionic groups such as sulfonic acid) and hydrophobic
blocks. The hydrophilic blocks provide the protonic conductivity and the hydrophobic
blocks offer good thermal and mechanical properties. Phase separation is driven by
chemical incompatibility between the different blocks. The morphology of the block
copolymer (cylinders, spheres, lamellae, etc.) can be controlled by tailoring the chemical
composition, molecular weight, and volume fraction of each of the blocks.150 For
example, lamellar domains can be achieved by using a 50:50 volume fraction of the two
blocks in the copolymer composition. This lamellar morphology makes it possible for the
ionic groups on the hydrophilic block to self-assemble into a co-continuous phase, which
may facilitate high protonic conductivity even at lower degrees of sulfonation and low
water contents.
Considerable research has been done on sulfonated block copolymers, and most
of it has focused focus on sulfonated polystyrene-based triblock copolymers, such as
sulfonated polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene copolymers
(SSEBS).16, 17 Although these copolymers have shown acceptable proton conductivity
and fuel cell performance, the poor chemical stability of the aliphatic backbone limits
their application especially at higher temperatures (> 60 oC). Recently, block or
multiblock copolymers with wholly aromatic backbones have been synthesized by the
McGrath research group151-157 and others.158-160 One of the most interesting candidates
developed by the McGrath group is a series of multiblock ionomers combining highly
fluorinated hydrophobic blocks and 100% sulfonated BPS hydrophilic blocks (Fig.
2.30).152 These multiblock copolymers were synthesized by a two-step process: first the
68
hydrophobic and hydrophilic telechelic macromonomers were prepared with the desired
molecular weights and appropriate end groups, then multiblock copolymers were
obtained through a coupling reaction between the end groups of the two macromonomers.
A significant advantage of this multiblock synthesis is that the possible ether-ether
interchange side reaction, which is well known to occur in pure poly(arylene ether)s, may
be avoided under the mild reaction conditions due to the much higher reactivity of the
activated difluoride monomer. The protonic conductivity vs. relative humidity for the
block copolymers with different block lengths and Nafion® 117 (Fig. 2.31) shows that the
block copolymers with longer block lengths have higher protonic conductivity under
partially hydrated conditions, which may be due to the better nanophase separation. The
“BisAF-BPSH” sample with hydrophobic and hydrophilic block lengths of 8,000 g·mol-1
each has similar or higher proton conductivity compared to Nafion® 117 at all RH values.
This and other results, such as higher water diffusion coefficients (enhanced transport)
than those of the random copolymers, suggests that the multiblock copolymers have good
potential for applicability in low humidity environments.8 Considering that the aliphatic
groups on the bisphenol A monomer may affect the overall oxidative stability, a partially
fluorinated monomer bisphenol-6FA was used to replace BPA in a similar series of
multiblock copolymers.153 Detailed results for the partially fluorinated analogs will be
reported later.
69
F
F F
F F
F
F
F
F O O
CH3
CH3
F
F F
F F
F
F
F
F
m
n
O S O OM
O
O
SO3M
MO3S
MO
m
+
O S O
O
O
SO3M
MO3S
O
CH3
CH3
F
F F
F F
F
F
F
On
Figure 2.30. Synthetic Scheme of BisAF-BPSH Series of Multiblock Copolymers152
70
Figure 2.31. Proton Conductivity vs. Relative Humidity for “BisAF-BPSH” Series
of Multiblock Copolymers and Nafion® 1178
(Reprinted with permission of John Wiley & Sons, Inc., copyright 2006)
Another interesting multiblock copolymer developed in the McGrath research
group used poly(p-phenylene) as the hydrophobic block, which offers excellent thermal
and mechanical properties, and 100% sulfonated BPS as the hydrophilic block (Fig. 2.32).
Ether-ether interchange can also be avoided because there is no ether group in the
hydrophobic poly(p-pheylene) backbone. The properties of this multiblock copolymer are
under investigation.151
71
Figure 2.32. Chemical Structure of PPP/BPS Multiblock Copolymer151
Although early results for multiblock copolymers are promising, the difficulties
encountered in the synthesis of these materials present an obstacle for researchers. For
example, the synthesis of high molecular weight multiblock copolymers is not facile due
to the difficulty in controlling the stoichiometry of the oligomers, and poor
reproducibility. Despite these obstacles, multiblock copolymers continue to capture the
attention of researchers due to the promise of controlling properties through tailored
microstructures.
2.5.2. Organic/Inorganic Composite PEMs
High-temperature proton exchange membranes are very interesting since the
overall cell efficiency can be greatly improved by the accompanying reduction in CO
poisoning of the catalyst and the enhanced kinetics of the fuel oxidation at temperatures
over 100 oC. However, purely polymeric PEMs, such as Nafion®, show poor fuel cell
performance above 100 oC because of the low proton conductivity at low relative
humidity and the poor mechanical properties. Incorporating suitable inorganic fillers into
the ionomer matrix seem to be a promising way to solve these problems and many reports
O SO
O
SO3H
HO3S
O
C O
CO
CO
bn m
Hydrophobic Hydrophilic
72
show that the inorganic fillers can enhance the mechanical properties and water retention
at high temperatures. The incorporation of inorganic particles may also increase the
tortuousity of the pathways for methanol molecules, which can help lower the methanol
crossover in DMFCs.
Several excellent reviews have been published regarding composite
membranes.161, 162 The major inorganic fillers such as heteropolyacids (HPAs), layered
metal phosphates or phosphonates, and metal oxides have been used,and the polymer
matrix has focused on perfluorinated polymers (Nafion®) or nonfluorinated PEEK
copolymers. Properties such as ionic conductivity, water uptake, tensile strength, and
thermal behavior have been systematically investigated. The composite membranes can
be macro-, micro-, or nano-composites depending on the size of inorganic fillers and they
can be fabricated by (1) dispersion filler particles in an ionomer solution followed by
casting, or (2) growth of the filler particles within a preformed membrane or in an
ionomer solution (in-situ method).
Crystalline HPAs are a class of inorganic fillers with high proton conductivity in
their hydrated forms. Although they can not be used alone as a solid electrolyte due to
their high solubility in water, experimental results show that well-dispersed HPA in
sulfonated polymer membranes can improve the protonic conductivity at temperatures
above 100 oC. Among many forms of HPAs, phosphotungstic acid, silicotungstic acid,
and phosphomolybdic acid are the most common. For example, Kim et al.163 fabricated
phosphotungstic acid/BPSH composite membranes with different disulfonation levels of
the polymer matrix via solution blending. The results showed that incorporation of HPA
73
into the sulfonated copolymer significantly reduced the water swelling behavior, without
influencing the proton conductivity at room temperature, whereas the composite
membrane exhibited greater proton conductivity in the temperature range from 100–130
oC. FTIR band shifts suggest that there were interactions between the sulfonic acid and
HPA particles via hydrogen bonding and this may prevent HPA from leaching out of the
membrane. The extraction of HPA depended on the degree of sulfonation but was always
lower than Nafion®. The long-term stability needs to be further confirmed.
Extraction of HPA particles from the membrane is an important issue for the
practical use of the composite membranes, especially for long-term stability. It has been
proposed that incorporation of some polar groups, such as nitrile or phosphine oxide
functional groups, in the polymer matrix may help to prevent HPA loss.7 Zhang et al.164
synthesized sulfonated poly(arylene ether ketone)s containing aromatic nitriles (SPAENK)
and blended them with HPA. The cyano groups on the aromatic rings may serve as a
compatilizer with HPA through polar interactions, but there is not enough evidence to
confirm this hypothesis.
BPSH copolymers have been blended with another inorganic filler - zirconium
hydrogen phosphate by Hill et al.165 The in-situ method was used because the inorganic
filler is insoluble. The water-swollen acid-form BPSH membranes were immersed in
ZrOCl2 solutions at 80 oC. The Zr4+ ions in the hydrophilic portion of the membrane then
can precipitate to form zirconium hydrogen phosphate after immersion in a 1M H3PO4
solution. The composite films had good thermal stability and excellent retention of
zirconium phosphate after water treatment at 120 oC for 100 h. Although the composite
membranes exhibited lower proton conductivity than the pure BPSH membrane at room
74
temperature, the presence of the inorganic particles led to an improvement in high-
temperature conductivity. For example, fully hydrated membranes (40 mol%
disulfonation) with 38 wt% zirconium phosphate had a conductivity of 0.06 S·cm-1 at
room temperature and linearly increased up to 0.13 S·cm-1 in water vapor at 130 oC,
whereas the pure copolymer which had a conductivity of 0.07 S·cm-1 at room temperature
only reached a conductivity of 0.09 S·cm-1 at 130 oC. These results suggest that the
composite membranes are promising for high temperature fuel cell applications.
In the last several years, many organic/inorganic composites have also been
investigated for DMFC. The incorporation of inorganic components such as SiO2, ZrO2,
HPA and metal phosphates has been successfully used to control the methanol
permeability and the proton conductivity.166-168 One major reason that the methanol
permeability was reduced in the composite membranes is that the presence of inorganic
particles increases the tortuous pathways that molecules encounter during permeation as
shown in Figure 2.33.
Polymeric membrane
Inorganic fillers
Diffusing SpeciesPolymeric membrane
Inorganic fillers
Diffusing Species
Figure 2.33. Schematic View of the Increased Pathways of Composite Membrane
75
2.5.3. Polymer Blends
Physically blending different polymers takes advantage of existing materials and
is a straightforward way to make new PEMs. The properties of polymer blends can be
tailored for fuel cell application by varying the components and their compositions. This
method has been found to improve important PEM properties, including water uptake,
proton conductivity, mechanical strength, and methanol crossover. However, the
incompatibility between two different polymers, which can lead to phase separation, is a
major factor that affects the final material properties. Kerres et al. have investigated
extensively the effects of various interactions between two polymer chains, such as ionic
interaction, hydrogen bonding, and dipole-dipole interaction, on the polymer
miscibility.169-172
The van der Waals and dipole-dipole forces between the polymer chains turned
out to be too weak to obtain good polymer blends. For example, simple blending of the
sulfonated PSU and unsulfonated PSU Udel® led to a heterogeneous morphology, too
much swelling, and even dissolution of sPSU at high temperature. Although the hydrogen
bonds formed between sPEEK and the weak base polyamide or polyetherimide will
improve the polymer blend properties including proton conductivity and glass transition
temperature, the high water swelling at elevated temperature, the partial phase separation
and the poor hydrolytic stability in acidic environments suggest that hydrogen bonding
alone is not enough.173
Therefore, a stronger interaction is needed. It was found that the interaction forces
between the acidic and strongly basic blend components, including electrostatic
interactions and hydrogen bonding can serve this purpose. These ionically crosslinked
76
acid-base blends are prepared by mixing a sulfonated polymer with a polymeric N-base,
followed by acid washing to reprotonate the acidic component. A large number of acid-
base blend membranes with various properties have been prepared by this method. For
example, sPEEK Victrex or sPSU Udel® as the acidic component have been blended with
the basic polymers poly(4-vinylpyridine), poly(benzimidazole), poly(ethyleneimine) PEI,
and a self-developed PSU- ortho-sulfone diamine. The membranes showed good proton
conductivity at an IEC of 1 and excellent thermally stability.170
However, some acid-base blends still suffer from the major disadvantages of
increased water swelling and instable mechanical properties at temperatures over 70-90
oC, where the hydrogen bridges and electrostatic interactions break in aqueous
enviorments. In this case, covalently crosslinked blends may offer some advantages.173
77
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140. Blonsky, P. N.; Shriver, D. F.; Austin, P.; Allcock, H. R., J. Am. Chem. Soc. 1984, 106, 6854.
141. Montoneri, E.; Gleria, M.; Ricca, G.; Pappalardo, G. C., Makromol. Chem. 1989,
190, 191. 142. Wycisk, R.; Pintauro, P. N., J. Membr. Sci. 1996, 119, 155. 143. Graves, R.; Pintauro, P. N., J. Appl. Polym. Sci. 1998, 68, 827. 144. Guo, Q.; Pintauro, P. N.; Tang, H.; O'Connor, S., J. Membr. Sci. 1999, 154, 175. 145. Andrianov, A. K.; Marin, A.; Chen, J.; Sargent, J.; Corbett, N., Macromolecules
2004, 37, 4075-4080. 146. Heitner-Wirguin, C., J. Membr. Sci. 1996, 120, 1. 147. Won, J.; Hye, H. P.; Kim, Y. J., Macromolecules 2003, 36, 3228. 148. Kim, Y. S.; Dong, L.; Hickner, M. A.; Pivovar, B. S.; McGrath, J. E., Polymer 2003,
44, 5729-5736. 149. Kim, Y. S.; A, M.; Hickner; Dong, L.; Pivovar, B. S.; McGrath, J. E., J. Membr. Sci.
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Academic Press: New York, January 1977. 151. Wang, H.; McGrath, J. E., Preprs Symp. ACS, Div. Fuel Chem. 2005, 50(2), 581. 152. Yu, X.; Roy, A.; McGrath, J. E., 2005, 50(2), 577. 153. Harrison, W.; Ghassemi, H.; Tom A., J. Z.; McGrath, J. E. WO Patent 2005053060,
2005. 154. Ghassemi, H.; Harrison, W.; Zawodzinski, T. A. J.; McGrath, J. E., ACS Polym.
Preprs. 2004, 45(1), 68. 155. Lee, H.-S.; Einsla, B.; McGrath, J. E., Preprs Symp. ACS, Div. Fuel Chem. 2005,
50(2), 579. 156. Wang, F.; Kim, Y.; Hickner, M.; Zawodzinski, T. A., ACS Polym. Mat.: Sci. & Eng.
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158. Ghassemi, H.; Zawodzinski, T. A. J. Preprs Symp. ACS, Div. Fuel Chem. 2005, 50(2), 531.
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Macromol. Symp. 2004, 210, 175. 161. Alberti, G.; Casciola, M., Annu. Rev. Mater. Res. 2003, 33, 129. 162. Savadogo, O., J. Power Sources 2004, 127, 135. 163. Kim, Y. S.; Wang, F.; Hickner, M.; Zawodzinski, T. A.; McGrath, J. E., J. Membr.
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87
Chapter 3. Purity Characterization of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl Sulfone (SDCDPS) Monomer by UV-visible Spectroscopy
Yanxiang Li1, Rachael VanHouten1, Andrew E.Brink2, and James E. McGrath1*
1Macromolecular Science and Engineering & Macromolecules and Interfaces Institute Virginia Polytechnic Institute and State University Blacksburg, VA 24061
2Hydrosize Technologies Inc., Raleigh, North Carolina
88
3.1. Abstract
The purity of the disulfonated monomer, 3,3’-disulfonated-4,4’- dichlorodiphenyl
sulfone (SDCDPS), is very important for obtaining high molecular weight disulfonated
poly(arylene ether sulfone) copolymers, which are promising candidates for proton
exchange membrane (PEM) fuel cells. For the commercialization purpose, direct use of
crude SDCDPS monomer with known purity in the copolymerization will save much
money, energy and time caused by the traditional recrystallization purification process. In
this paper, a novel method to characterize the purity of crude disulfonated monomer,
SDCDPS, has been developed by using UV-visible spectroscopy. The purity of the crude
comonomer was determined from the Beer’s Law plot developed using a pure SDCDPS
sample. The model poly(arylene ether sulfone) copolymers, based on this crude SDCDPS
monomer, 4,4’-dichlorodiphenyl sulfone (DCDPS), and biphenol, were successfully
synthesized. The molecular weight obtained from gel permeation chromatography (GPC)
(Mn> 40 kg·mol-1) was high enough to allow tough films for PEMs to be cast. This
confirmed that the purity characterization method was relatively accurate and applicable,
especially for mass production. The storage time and drying time of SDCDPS were also
studied using Beer’s Law.
Keywords: Disulfonated Monomer; Purity Characterization; UV-Visible Spectroscopy;
Beer’s Law; Direct Copolymerization; Proton Exchange Membrane Fuel Cells
89
3.2. Introduction
A large number of novel ion-containing copolymers have been extensively
investigated for proton exchange membranes (PEMs) to overcome the drawbacks of the
currently commercialized perfluorosulfonic acid Nafion®.1-3 Sulfonated poly(arylene
ether) materials, such as poly(arylene ether ketone)s and poly(arylene ether sulfone)s, are
attractive for use in PEMs because of their well known oxidative and hydrolytic stability
under a fuel cell’s harsh conditions.4 Introduction of the sulfonic acid groups to the
polymer backbone has been achieved by either post sulfonation of commercially
available copolymers or direct copolymerization of sulfonated monomers. 1 It has been
widely admitted that the direct copolymerization method has advantages over the post
modification method, including its easy control of the position and degree of sulfonation,
high acidity, and the ease of minimizing side reactions. Recently, using the direct
copolymerization method to synthesize sulfonated copolymers has also extended its
applications in other areas such as reverse osmosis water purification,5, 6 and polymeric
transducers.7
The disulfonated monomers, which were used in the direct copolymerizations,
were usually synthesized via electrophilic substitution by fuming sulfuric acid. The
monomer 3,3’-disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) was a typical
choice. SDCDPS was first reported by Robeson and Matzner8 in a patent for its flame
retarding properties and was subsequently studied by Ueda et al.9 McGrath’s group10, 11
modified its purification and characterization procedure and was first to directly
copolymerize SDCDPS with 4,4’-dichlorodiphenyl sulfone (DCDPS) and 4,4’-biphenol
90
to synthesize poly(arylene ether sulfone)s for use as proton exchange membranes.
Thereafter, several different disulfonated monomer structures were designed and
synthesized using a similar procedure for the same purpose (Fig. 3.1).12-14
The purity of the disulfonated monomer is very important in obtaining high
molecular weight copolymers using step-growth copolymerization. A systematic study of
the synthesis and characterization of the SDCDPS monomer ensured that the possible
monosulfonated and starting material DCDPS impurities could be avoided when
following the standard procedure.15 The only impurity that remained in the product was
sodium chloride, which was used in excess to salt out the crude sulfonated monomer.
Traditionally, the sodium chloride was removed by recrystallization of the crude product
with a mixture of isopropanol and water. This was conducted two to three times to
increase monomer purity. This recrystallization method was effective in lab scale
experiments but not economical for mass production because it substantially decreased
the monomer yield and wasted solvent, time, and energy.
In this paper, a novel method to characterize the purity of crude disulfonated
monomer, SDCDPS, was developed using UV-visible spectroscopy. A Beer’s Law plot
was developed by first measuring the absorbance of several pure SDCDPS /methanol
dilute solutions with known concentrations, then plotting the absorbance vs.
concentrations of these solutions. The purity of the crude SDCDPS was then easily
determined from the Beer’s Law plot. Because the SDCDPS monomer is sensitive to
moisture, the storage time and drying time were also studied. Poly(arylene ether sulfone)
model copolymers with 35% and 40% degree of sulfonation were synthesized using the
crude SDCDPS monomer with determined purity to confirm the accuracy and
91
applicability of this characterization method. Gel permeation chromatography (GPC)
results showed that copolymers with molecular weights higher than 40 kg·mol-1 were
obtained, which was high enough to form tough membranes for PEM fuel cells.16 The
purpose of this study is to provide an accurate and practical way to characterize the purity
of the novel sulfonated monomer SDCDPS, especially for mass production purposes.
This method could also be used to characterize other similar disulfonated monomer
structures like those illustrated in Figure 3.1.
92
Y X YSO3 (30%)
110 oCNaCl H2O
NaOH
PH = 6-7
NaClY X Y
SO3NaNaO3S
Y = Cl or F X = SO
OCO
C COO
C CO O
, , , or
Figure 3.1. Synthetic Scheme of Disulfonated Monomers with Several Different
Structures
93
3.3. Experimental
3.3.1. Materials
High purity 4,4’-dichlorodiphenyl sulfone (DCDPS) monomer was kindly
provided by Solvay Advanced Polymers Inc. Fuming sulfuric acid with 27-33 wt% of
sulfur trioxide (SO3) was purchased from Aldrich and used as received. Crude 3,3’ –
disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) was provided by Hydrosize Inc.
Monomer grade, high purity 4,4’- biphenol (BP) was obtained from Eastman Chemical.
All the monomers were well-dried in a vacuum oven before copolymerization. The
solvent dimethylacetamide (DMAc) was vacuum-distilled from calcium hydride onto
molecular sieves and stored under nitrogen. Potassium carbonate was dried in vacuo at
120 oC before use. Toluene, methanol, isopropanol, sodium chloride, and sodium
hydroxide pellets were obtained from Aldrich and used as received.
3.3.2. Synthesis Procedures
3.3.2.1. Synthesis and Purification of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl
Sulfone (SDCDPS) monomer
The synthesis of the disulfonated monomer, SDCDPS, followed the synthetic
procedure reported in the literature.10,11,15 The reaction conditions were chosen to make
sure that the monosulfonated and the original DCDPS impurities were avoided. DCDPS
(30 g) and fuming sulfuric acid (60 mL, 30% SO3) (molar ratio was 1:3.3) were added to
a 250 mL three-necked flask equipped with an overhead mechanical stirrer, nitrogen inlet
and condenser. The reaction was performed at 110 oC for 6-7 h. Isolation of the product
94
was achieved using a standard process (Fig. 3.1): salt out by sodium chloride, neutralized
with 10 N sodium hydroxide, and salt out again. The crude SDCDPS was purified by
recrystallization with a mixture of deionized water and IPA (3/7, v/v) to remove the
sodium chloride, which was the only impurity. Recrystallization was repeated up to six
times such that no absorption intensity change was observed in the UV-visible spectrum
when performed at the same solution concentration. The SDCDPS monomer was dried
under vacuum at 160 oC for at least two days.
3.3.2.2. Synthesis of Disulfonated Poly(Arylene Ether Sulfone) (BPSH) Model
Copolymers
The crude monomer SDCDPS obtained from Hydrosize Inc. was directly used to
synthesize the disulfonated poly(arylene ether sulfone) model copolymers. The purity
determined from the Beer’s Law plot was 82.5% ± 1% following the procedure described
in section 3.3.3.2. Poly(arylene ether sulfone) copolymers with 35 mol% (BPSH30) and
40 mol% (BPSH40) degree of sulfonation were synthesized by copolymerization of
SDCDPS, DCDPS, and BP monomers. As an example, the following monomer feed was
used for synthesis of BPSH35: 5.000 g crude SDCDPS (8.397 mmol), 3.617 g DCDPS
(12.595 mmol), 3.909 g BP (20.99mmol). The synthetic procedure was the same as
reported earlier10,11.
3.3.3. Characterization
3.3.3.1. Monomer and Copolymer Characterization
95
1H NMR analyses were conducted with a Varian Unity 400 NMR spectrometer to
confirm the chemical structures of both the SDCDPS monomer and the disulfonated
copolymers. Spectra were obtained using DMSOd6 as the solvent. Intrinsic viscosities
(I.V.) and the molecular weights of the copolymers were characterized by gel permeation
chromatography (GPC) using polystyrene as a standard. GPC experiments were
performed on a liquid chromatograph equipped with a Waters 1515 isocratic HPLC pump,
Waters Autosampler, Waters 2414 refractive index detector and Viscotek 270 dual
detector. 0.05 M LiBr/NMP was used as the mobile phase. The column temperature was
maintained at 60 oC because of the viscous nature of NMP. Both the mobile phase solvent
and sample solution were filtered before introduction to the GPC system.
3.3.3.2. Procedure for SDCDPS Monomer Purity Characterization by UV-Visible
Spectroscopy
Purity characterizations of the SDCDPS monomer were carried out using a
Shimadzu Model UV-1601 UV-visible spectrometer. The first step was to determine the
molar absorptivity (ε) of SDCDPS monomer in Beer’s Law: A = εbc, in which A is the
absorbance measured from UV-visible spectrum, c is the dilute solution concentration
(mol·L-1), and b is the path length of the sample cell (1 cm). The procedure was as
follows: pure SDCDPS monomer was obtained by purification of crude sample as
described earlier using IPA/DI H2O as the recrystallization solvent mixture. This
recrystallization was performed up to six times until the UV-visible absorption had no
change when measured at the same concentration. The completely dried, pure SDCDPS
monomer (61.6 mg) was dissolved in methanol in a 100 mL volumetric flask to prepare
solution (1) with concentration: 1.254 X 10-3 mol·L-1. Exact volumes in the range of 2-10
96
mL of solution (1) were transferred to a 250 mL volumetric flask and then diluted with
methanol to prepare dilute solutions with various concentrations. UV-Visible absorbance
data generated from these dilute solutions were used to develop the Beer’s Law
calibration curve, which was a straight line in the low absorbance range (< 1.5). The
slope of this straight line was the molar absorptivity (ε) in the equation based on Beer’s
Law. Once the Beer’s Law plot was determined, the purity of the crude SDCDPS sample
was easily obtained by measuring the UV-visible absorbance of the crude sample solution
with a specific concentration that fell in the linear relationship range of the calibration
curve. Each measurement was repeated at least three times, and the average values were
used.
3.4. Results and Discussion
An electrophilic substitution reaction was employed to synthesize the disulfonated
monomer, SDCDPS, by using 30% fuming sulfuric acid as the sulfonation agent.
Possible impurities that could have resulted when synthesizing SDCDPS were the
monosulfonated byproduct, starting materials, and sodium chloride. The monosulfonated
byproduct and residual starting monomer (DCDPS) impurities were effectively avoided
by following the standard synthesis conditions. The absence of these impurities was
confirmed by proper chemical shifts and integrations of the proton NMR peaks. However,
sodium chloride was used in excess to salt out the SDCDPS from the water and thus was
the only impurity that needed to be removed from the crude product. Several
recrystallizations from an IPA/H2O solvent mixture were usually used to purify the
97
SDCDPS monomer. This procedure worked well in lab-scale experiments, but it
substantially decreased the product yield because of the high solubility of SDCDPS in
water. It also wasted time, solvent, and energy, especially when being mass produced. If
one can obtain the accurate purity of the sulfonated monomer, it is desirable to use the
crude SDCDPS directly in the copolymerization because sodium chloride has no
influence on the reaction. UV-visible spectroscopy is a sensitive instrument in
quantitatively determining the concentration of a solution using Beer’s Law if the molar
absorptivity of the sample is available. The SDCDPS monomer dilute solution had two
absorption peaks in the UV-Visible range (wavelength: 210nm and 254nm), which
allowed the use of this technique to determine the monomer’s purity. For consistency, the
peak at 210 nm was used for all following measurements.
The choice of the solvent was the first consideration because it would affect both
the peak position and intensity of the UV-Vis spectra. Three good solvents for SDCDPS
(water, methanol and DMAc) were tested, and the UV-visible spectra were obtained (Fig.
3.2.). SDCDPS sample in DMAc did not generate a good spectrum for unknown reasons.
Good spectra were obtained when water or methanol were used as the solvent. Because it
was found that methanol was easier to use in the solution preparation process to obtain
more accurate results, methanol was chosen as the solvent for all measurements.
To develop the Beer’s Law plot, a very pure SDCDPS sample was required.
SDCDPS was purified by recrystallization from an IPA/H20 (7/3, v/v) solvent mixture,
and the process was repeated up to six times. The purity of the SDCDPS was monitored
by using UV-visible spectra. By keeping the sample concentrations of all measurements
the same, it could be determined that the purity was unchanged after the third
98
recrystallization because the absorbance was no longer changing (Fig. 3.3). Although the
first recrystallization removed most of the salt, at least two more recrystallizations were
required to remove all the salt. The pure SDCDPS (after six times recrystallization) was
used to prepare solution (1) with concentration 1.254 X 10-3 mol·L-1, and then exact
volumes (2~10 mL) of solution (1) were transferred to 250 mL volumetric flasks and
diluted with methanol to prepare dilute solutions with various concentrations. The
absorbance of the solutions scaled with the concentrations linearly as shown in Figure 3.4.
These UV-visible absorbance data (peak at 210 nm) were used to develop the Beer’s Law
calibration curve (Fig 3.5.). It showed that when the absorbance was higher than 1.5, the
curve deviated from the linear relationship (Fig 3.5 left). The nonlinearity may be caused
by many reasons, for example, deviations in absorptivity coefficients at high
concentrations due to electrostatic interactions between molecules in close proximity,
scattering of light due to particulates in the sample, and changes in refractive index at
high analyte concentration, etc.16 The linear portion was plotted (Fig 3.5 right), and the
standard calibration curve for SDCDPS monomer was obtained by averaging three
measurements. The equation of the straight line was averaged to be: Y = 51081X –
0.0229. The measurement error was ±0.7%.
Once the Beer’s Law plot was established, the purity of the crude monomer was
determined by measuring the absorbance of crude sample solution with a known
concentration as described in the experimental part. Since SDCDPS is very susceptible to
moisture uptake, the drying time and storage time were also studied. The fresh, pure
SDCDPS was first dried in a vacuum oven at 160 oC, and the purity was measured after
24, 48, and 72 h. The purity was also measured after 15 days storage in a desiccator. It is
99
shown in Figure 3.6 that the SDCDPS needed to be dried at least 48 h to completely
remove the water, and it was suggested to dry SDCDPS prior to copolymerization
because it absorbed around 4.0% water after 15 days in a desiccator.
The purity of crude SDCDPS monomer synthesized by Hydrosize Inc. was
determined using the Beer’s Law plot and used directly in copolymerization with DCDPS
and biphenol. Figure 3.7 shows the comparison of the absorbance between the pure and
the crude SDCDPS sample at the same concentration. It should be noted that because the
sodium chloride salt was not evenly distributed in the sample, thorough blending of the
crude sample was very important in obtaining accurate results. The purity of the crude
SDCDPS was calculated to be 82.5% ±1%. The model poly(arylene ether sulfone)
copolymers (BPSH) were characterized by GPC and intrinsic viscosity. Results are listed
in Table 3.1. The molecular weights for both polymers with 35% and 40% (mole percent)
degree of sulfonation exceeded 40 kg·mol-1, which were high enough to cast tough films
for PEM fuel cells17. The degree of sulfonation calculated from the 1H NMR spectra
matched very well with the theoretical values (Fig. 3.8). These results confirmed that the
UV-visible characterization method for determining the purity of crude SDCDPS is
relatively accurate and applicable, especially for the mass production of copolymers.
100
H2O
methanol
DMAc
Wavelength (nm)
Abs
orba
nce
H2O
methanol
DMAc
Wavelength (nm)
Abs
orba
nce
Figure 3.2. UV-Visible Spectra of SDCDPS Dilute Solutions Using Different Solvents
101
Two times
Recryst. 3, 5, 6 times, Overlay
Wavelength (nm)
Abs
orba
nce
Two times
Recryst. 3, 5, 6 times, Overlay
Two times
Recryst. 3, 5, 6 times, Overlay
Wavelength (nm)
Abs
orba
nce
Figure 3.3. Effect of the Number of Recrystallization Times on the Absorbance at
the Same Concentration Values (After Two Times Recrystallization, SDCDPS Still
Contains 2.6% ±1% salt)
102
Concentration increase
Wavelength (nm)
Abs
orba
nce
Concentration increase
Wavelength (nm)
Abs
orba
nce
Figure 3.4. The UV-Vis Absorbances of SDCDPS Solutions with Different
Concentrations were Used to Develop the Calibration Curve
103
0
0.5
1
1.5
2
2.5
3
0 1E-05 2E-05 3E-05 4E-05 5E-05 6E-05 7E-05
Concentration (Mol/L)
Abso
rbance
y = 50956x - 0.0216
R2 = 0.9994
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0.00E+00
5.00E-06
1.00E-05
1.50E-05
2.00E-05
2.50E-05
3.00E-05
Concentration (Mol/L)Absorbance
0
0.5
1
1.5
2
2.5
3
0 1E-05 2E-05 3E-05 4E-05 5E-05 6E-05 7E-05
Concentration (Mol/L)
Abso
rbance
y = 50956x - 0.0216
R2 = 0.9994
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0.00E+00
5.00E-06
1.00E-05
1.50E-05
2.00E-05
2.50E-05
3.00E-05
Concentration (Mol/L)Absorbance
Figure 3.5. Calibration Curve Used to Develop the Beer’s Law Slope. The Left
Graph Shows the Deviation at High Concentrations. The Right graph is the Linear
Calibration Curve at Low Concentrations
104
Wavelength (nm)
Abs
orba
nce
72hr, 48hr, 24hr, in desiccator 15days
Wavelength (nm)
Abs
orba
nce
Wavelength (nm)
Abs
orba
nce
72hr, 48hr, 24hr, in desiccator 15days72hr, 48hr, 24hr, in desiccator 15days
Figure 3.6. Effect of Drying Time and Storage Time on the Absorbance at the Same
Concentrations
105
Wavelength (nm)
Abs
orba
nce
Pure
Unrecrystallized
Wavelength (nm)
Abs
orba
nce
Pure
Unrecrystallized
Wavelength (nm)
Abs
orba
nce
Pure
Unrecrystallized
Figure 3.7. Comparison of the Absorbance of Pure and Crude Samples of SDCDPS.
(The Crude Sample was Provided by Hydrosize Inc.)
106
S* OO
OO S O
O
OO *
x 1-x n
SO3NaNaO3S
fe
g
g
f e
b
b
S* OO
OO S O
O
OO *
x 1-x n
SO3NaNaO3S
fe
g
g
f e
b
b
Degree of sulfonation (%) =( )[ ]
( )[ ] %0.394/2/3/
2/3/=
+++
++
bgfe
gfe
HHHHHHH
Figure 3.8. 1H NMR of Poly(Arylene Ether Sulfone) Copolymers (BPSH-40) was
Used to Determine the Degree of Sulfonation16
107
Table 3.1. Characterization of the Model BPS Copolymers
GPC Results Copolymers SDCDPS purity (%) Mn
(kg·mol-1)Mw
(kg·mol-1)I.V.
(dL·g-1)
Degree of Sulfonation By
1H NMR (%)
BPS35
82.5 45.7 78.9 0.61 35.1
BPS40 82.5 41.9 72.2 0.69 39.0
108
3.5. Conclusions
A novel characterization method for determining the purity of the disulfonated
monomer SDCDPS has been developed by using UV-Visible spectroscopy. Pure
SDCDPS recrystallized from IPA/H2O was used to establish a Beer’s Law plot, which
was then used to determine the purity of the crude product. The results also showed that
the SDCDPS needed to be dried in a vacuum oven at 160 oC for at least 48 h to
completely remove the water. Since the SDCDPS absorbed small amounts of moisture
after storage in a desiccator for several days, it was suggested to dry the SDCDPS
directly before the copolymerization. The model poly(arylene ether sulfone) copolymers
were synthesized by direct copolymerization of the crude SDCDPS with known purity,
DCDPS and BP. The relatively high molecular weights of the copolymers confirmed that
this characterization method was applicable to accurately determine the purity and
directly use the crude SDCDPS without purification process, which can save money, time
and energy. This is especially attractive for the mass production of the copolymers.
109
3.6. References 1. Hickner M, Ghassemi H, Kim YS, Einsla B, McGrath JE. Chem. Rev., 2004, 104,
4587. 2. Kerres J. A., J. Membr. Sci., 2001, 185, 3 3. Roziere J, Jones D. J. Ann. Rev. Mater. Res., 2003, 33, 503. 4. Wang, S.; McGrath, J. E., Synthesis of Poly(arylene ether)s. In Synthetic Methods in
Step Growth Polymers, Rogers, M.; Long, T. E., ed. John Wiley and Sons: N.Y, 2003; 327.
5. Zhang, Z.; Fan, G.; Sankir, M.; Park, H. B.; Freeman, B. D.; McGrath, J.E., PMSE
Preprs., 2006, 95, 887. 6. Park, H. B.; Freeman, B. D.; Zhang, Z..; Fan, G.; Sankir, M.; McGrath, J. E., PMSE
Preprs, 2006, 95, 889. 7. Wiles, K. B., Ph.D Thesis, Virginia Tech, 2005 8. Robeson, L. M.; Matzner, M. Flame retardant polyarylate compositions. US Patent
4,380,598, 1983. 9. Ueda, M.; Toyota, H.; Ochi, T.; Sugiyama, J.; K. Yonetake; Masuko, T.; Teramoto,
T., J. Polym. Sci., Polym. Chem. Ed. 1993, 31, 853. 10. Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.; Zawodzinski, T. A.;
McGrath, J. E., Macromol. Symp. 2001, 175, 387. 11. Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.; McGrath, J. E., J. Membr.
Sci. 2002, 197, 231. 12. Li, Y.; Mukundan, T.; Harrison, W.; Hill, M.; Sankir, M.; Yang, J.; McGrath, J. E.,
ACS Preprs. Div. Fuel Chem., 2004, 49(2). 13. Li, X.; Zhao, C.; Lu, H.; Wang, Z.; Na, H., Polymer, 2005, 46, 5820. 14. Xing P.; Robertson G. P.; Guiver, M. D. Mikhailenko, S. D.; Kaliaguine, S., Polymer,
2005, 3257. 15. Sankir M.; Bhanu V. A.; Harrison W. L.; Ghassemi H.; Wiles K. B.; Glass T. E.;
Brink A. E.; Brink M. H.; McGrath J. E., J. Appl. Polym. Sci., 2006,100, 4595
110
16. http://chemistry.hull.ac.uk/lectures/adw/06523- 3%20Molecular%20Spectroscopy%20UV-Vis.pdf
17. Li Y.; Wang F., Yang J.; Liu D.; Roy A.; Case S.; Lesco J.; McGrath, J. E. Polymer,
2006, 47, 4210.
111
Chapter 4. Synthesis & Characterization of Controlled Molecular Weight Disulfonated Poly(Arylene Ether Sulfone) Copolymers and Their Applications to Proton Exchange Membranes Taken from: Yanxiang Li1, Feng Wang2, Juan Yang1, Dan Liu1, Abhishek Roy1,Scott Case3, Jack Lesko3, and James E. McGrath1,* 1Macromolecular Science and Engineering & Macromolecules and Interfaces Institute Virginia Polytechnic Institute and State University Blacksburg, VA 24061 2 PPG Industries Inc., 440 College Park Drive, Monroeville, PA 15146 3 Engineering Science and Mechanics Virginia Polytechnic Institute and State University Blacksburg, VA 24061 Polymer, 2006, 47, 4210-4217 Reprinted with permission from Elsevier, copyright (2006)
112
4.1. Abstract
Tert-butylphenyl terminated disulfonated poly(arylene ether sulfone) copolymers
with controlled molecular weight (s Mn), 20 to 50 kg·mol-1, were successfully prepared
by direct copolymerization of the two activated halides, biphenol and the endcapper, 4-
tert-butylphenol. The high molecular weight copolymer (molecular weight over 80
kg·mol-1) was also synthesized with 1:1 stoichiometry without an endcapping reagent.
The chemical compositions and the molecular weights of the endcapped copolymers were
characterized by their 1H NMR spectra utilizing the 18 unique protons at the chain ends.
Modified intrinsic viscosity measurements in 0.05 M LiBr/NMP solution further
correlated well with NMR results. Combining the endcapping chemistry with proton
NMR end group analysis and intrinsic viscosity measurements, one can demonstrate a
powerful tool for characterizing molecular weight of sulfonated poly(arylene ether
sulfone) random copolymers. This enables one to further investigate the influence of
molecular weight on several critical parameters important for proton exchange
membranes, including water uptake, in-plane protonic conductivity and selected
mechanical properties. These are briefly discussed herein and will be more fully
described in subsequent publications.
Keywords: Disulfonated Poly(Arylene Ether Sulfone) Copolymer, Controlled Molecular
Weight, Proton Exchange Membrane Fuel Cells.
113
4.2. Introduction
Proton exchange membrane (PEM) fuel cells have attracted much attention in
recent years as promising green energy device. One of the key components for the PEM
fuel cell is the polymeric electrolyte membrane, which serves as the barrier for fuels and
the electrolyte for transporting protons from the anode to the cathode. According to the
type of the fuel used, there are two kinds of PEM fuel cells: Hydrogen/air fuel cells and
direct methanol fuel cells (DMFCs). The perfluorinated sulfonic acid copolymers such as
DuPont’s Nafion®, are promising PEM materials due to their good mechanical, thermal
and chemical stability as well as good protonic conductivity at lower temperatures (<80
oC). However, the high methanol crossover, the reduction in conductivity at higher
temperature and the cost are the major drawbacks that limit their commercial
application.1,2,3 Therefore, to develop the alternative membrane materials that will
overcome these drawbacks is important.
Many families of polymers with differing chemical structures and various
strategies for incorporation of sulfonic acid groups have been explored as PEM
materials.3 Sulfonated poly(arylene ether sulfone)s are good candidates due to their good
acid and thermal oxidative stabilities, high glass transition temperatures and excellent
mechanical strengths.4 Sulfonated poly(arylene ether sulfone)s have been prepared via
polymer modification route, where sulfonate groups were achieved on polymer chain by
sulfonating agents, such as concentrated sulfuric acid or sulfur trioxide.5,6 The McGrath
has reported synthesis of poly(arylene ether sulfone) copolymers by directly
copolymerizing sulfonated monomers. This procedure is more preferable relative to post
114
modification method because of its easy control of the degree of sulfonation, high acidity,
and the ease of side reactions related to the post polymer modification technique.7-9 Other
copolymers synthesized in the McGrath group by direct copolymerization for PEMs have
included: sulfonated poly(arylene ether phosphine oxide),10 poly(arylene ether
ketone)s,11,12 poly(phenyl sulfide sulfone)s,13 substituted polyphenylenes,14 poly(arylene
ether) copolymers containing aromatic nitriles,15 and naphthalene based polyimides. 16
Molecular weight is a fundamental parameter affecting all mechanical behavior of
polymers as is well known. Most reports of new proton exchange membrane materials
have included information on ion content expressed either by the equivalent weight (EW,
g·mole-1) or by the ion exchange capacity (IEC, meq·g-1), protonic conductivity, and
water uptake. Despite the large body of research on this topic, there is almost nothing in
the PEM literature describing molecular weights of candidate materials, even including
Nafion®!3 F. Wang et al.17 previously synthesized controlled molecular weight (Mn)
poly(arylene ether sulfone) (Mn from 20 to 40 kg·mol-1) by offsetting stoichiometry with
a t-butylphenyl endcapping reagent. The t-butylphenyl concentrations relative to the
polymer backbone were characterized by proton NMR to calculate the molecular weight
of the copolymers. They provided intrinsic viscosity (IV) data for these copolymers, and
they found that the intrinsic viscosities were not comparable to those of non-sulfonated
polymers, since the polymer electrolyte chains interact via sulfonate groups. The
McGrath group has begun to utilize NMP with 0.05M LiBr to measure the intrinsic
viscosity. The small amount of salt effectively suppressed the polyelectrolyte effect
allowing improved characterization of the ion containing materials.
115
The overall aim of this research is to establish molecular weight vs mechanical and
electrical property correlations for sulfonated poly(arylene ether sulfone) copolymers that
could be used as proton exchange membranes in fuel cells. In this paper, poly(arylene
ether sulfone) copolymers with 35 mole % disulfonated monomer repeat unit were
successfully synthesized via direct copolymerization method.8, 9 The number average
molecular weights of the copolymers were controlled from 20 to 50 kg·mol-1 by a tert-
butylphenol endcapping reagent and were characterized by the combination of proton
NMR and intrinsic viscosity. The intrinsic viscosities were measured using NMP as
solvent with 0.05 M lithium bromide to break up the ion group aggregation. The linear
correlation of Log Mn with Log intrinsic viscosity showed that this method can provide
more accurate molecular weight information. On the basis of this, the effects of the
molecular weight on the properties of proton exchange membranes, such as water
swelling, protonic conductivity, and mechanical properties were investigated.
4.3. Experimental
4.3.1. Materials
Highly purified 4, 4’-dichlorodiphenyl sulfone (DCDPS) and biphenol (BP) were
kindly provided by Solvay Advanced Polymers and Eastman Chemical, respectively.
They were well dried in vacuo before polymerization but otherwise were used as received.
The 4-tert-butylphenol (TB) endcapper was purchased from Aldrich and was purified by
sublimation. The 3, 3’-disulfonated 4, 4’-dichlorodiphenyl sulfone (SDCDPS) was
synthesized as reported earlier.8 The solvent N, N-Dimethylacetamide (DMAc, Fisher)
116
was vacuum-distilled from calcium hydride onto molecular sieves and stored under
nitrogen before use. Potassium carbonate was dried in vacuo before copolymerization.
Toluene and methanol were obtained from Aldrich and were used as received.
4.3.2. Synthesis of Disulfonated Poly(Arylene Ether Sulfone) Copolymers with
Controlled Molecular Weight
The aromatic nucleophilic step growth copolymerization was conducted in a 3-
neck flask equipped with a mechanical stirrer, nitrogen inlet and a Dean Stark trap. One
typical polymerization for a controlled molecular weight of 40 kg·mol-1 (BPS35-40)
copolymer was as follows: The 4,4'-biphenol (5.000 g, 26.866 mmol), 4,4'-
dichlorodiphenyl sulfone (5.075g, 17.671 mmol), 3,3’-disulfonated 4,4’-dichlorodiphenyl
sulfone (4.674 g, 9.515 mmol) and 4-tert-butylphenol (0.096 g, 0.641 mmol) were added
to the flask, followed by 1.15 equivalent of potassium carbonate. Dry DMAc was
introduced to afford about a 20% solids concentration and toluene was used as an
azeotropic agent. The reaction mixture was heated under reflux at 160 ºC for 4 h, which
stripped off most of the toluene to dehydrate the system. Finally, the bath temperature
was raised slowly to 175 ºC for 24 h, which caused the DMAc to reflux. The viscous
solution was cooled to room temperature, and then diluted with DMAc to form about a
20% copolymer solution. The copolymer was isolated by precipitation in deionized water,
filtered and dried in a vacuum oven at 120 oC for 24 h. Dried polymer was ground into
powder and then washed extensively with methanol and deionized water several times to
completely remove salt and any potential residual endcapping reagent, and finally
vacuum dried at 120 ºC for 24 h. The molecular weights of the copolymers were
117
controlled by varying the ratio of monofunctional monomer TB to difunctional monomer
4, 4’-biphenol. The copolymers synthesized were designiated as BPS35-xx (salt form) or
BPSH35-xx (acid form), where 35 means that all copolymers were contained 35% (mole
%) of disulfonated repeat units, and xx represents the target molecular weight was xx
kg·mol-1.
4.3.3. Membrane Preparation
The salt form copolymers were dissolved in DMAc (5~10% w/v) at room
temperature. The solutions were first filtered with 0.45 µm syringe filters, and then cast
onto clean glass substrates. The films were carefully dried with infrared heat at gradually
increasing temperatures (up to ~ 60 oC). The membranes were removed from the glass
plates by submersion in water and then were dried in vacuo at 120 oC for at least 24 h.
The salt form membranes were completely converted into their acid forms by
boiling the membranes in 0.5 M sulfuric acid for 2 h, followed by boiling in deionized
water for another 2 h. The acid form membranes were washed with deionized water
completely and then stored in fresh deionized water at room temperature.
4.3.4. Characterization
1H NMR spectra were conducted with a Varian Unity 400 NMR spectrometer in
DMSO-d6. Intrinsic viscosities (IV) were determined in NMP with or without 0.05 M
LiBr at 25 ºC using an Ubbelohde viscometer.
The water uptake was obtained by measuring the difference in the weight between
dry and fully hydrated membranes. The sample films were equilibrated in deionized
118
water at room temperature for at least 48 h. Then the membranes were dried in the
vacuum oven at 110 oC for 24 h. Weights of wet and dry membranes were measured. The
ratio of weight gain to the original membrane weight was taken as the water uptake (WU)
according to equation (1).
%100×−
=dry
drywet
WWW
WU …………….(1)
where Wwet and Wdry are the masses of wet and dried samples, respectively.
Proton conductivity at 30°C at full hydration (in liquid water) was determined in
a window cell geometry18 using a Solartron (1252 + 1287) Impedance/Gain-Phase
Analyzer over the frequency range of 10 Hz - 1 MHz. The cell geometry was chosen to
ensure that the membrane resistance dominated the response of the system. The
resistance of the film was taken at the frequency which produced the minimum imaginary
response19. The conductivity of the membrane can be calculated from the measured
resistance and the geometry of the cell according to equation (2):
AZl'
=σ …………..(2)
where σ is the proton conductivity, l is the length between the electrodes, A is the cross
sectional area available for proton transport, and Z’ is the real impedance response.
In determining proton conductivity in liquid water, membranes were equilibrated
at 30 °C in DI water for 24 h prior to the testing.
Mechanical tensile tests were performed using an Instron 4468 Unversal Testing
Machine at room temperature and 40% relative humidity, with a crosshead displacement
speed of 5 mm·min-1. The gauge lengths were all set to 40 mm. Pneumatic grips were
119
employed with a pressure of 207 kPa. The specimens with thickness around 50 µm and
size of 60 mm × 12 mm were used for testing. For each film, four replicates were tested.
Since the stress-strain curves of the specimens that lasted the longest under stretching
represent the real mechanical behavior of the films better (i.e., the specimen did not fail
by macroscopic defects), the curve with the largest elongation at break was chosen to plot
in Figure 4.5. Thermogravimetric analysis was performed in air with a heating rate of 5
oC·min-1 to determine the water contents of the test specimens, which were were about
10%.
4.4. Results and Discussion
4.4.1. Synthesis and Characterization of Copolymers
The tert-butylphenyl terminated BPS35 series copolymers were successfully
synthesized by the aromatic nucleophilic substitution reaction of 3, 3’-disulfonated 4, 4’-
dichlorodiphenyl sulfone, 4, 4’-dichlorodiphenyl sulfone, 4, 4’-biphenol and 4-tert-
butylphenol in DMAc, which contained toluene as an azeotropic agent to dehydrate the
system (Fig. 4.1). The monofunctional monomer, 4-tert-butylphenol, was used as the
endcapping reagent, in which the phenol functional group has similar reactivity as
biphenol. The mole ratio of the tert-butylphenol monomer to the difunctional
comonomers was varied to control the stoichiometry in accordance with the modified
Carother’s equation.20 The molecular weights were controlled from 20 to 50 kg·mol-1.
For all copolymers, the mole ratio of SDCDPS to DCDPS was fixed to 3.5/6.5. The
copolymers were first isolated by precipitation of the reaction solutions in stirred
deionized water and dried in an oven. Then the dried crude copolymers were ground into
120
powders and washed extensively with methanol and deionized water to remove any
possible residual endcapping agent and salt. It is very important that all the inorganic
salts involved in the condensation process, and any possible residue starting monomers
need to be removed as complete as possible, since any of these impurities in the final
products would interfere the characterization of viscosity and molecular weight by NMR
as discussed in the following paragraphs.
For comparison, a BPS35-control copolymer with high molecular weight was
synthesized with 1:1 stoichiometry and no endcapping reagent.
The 1H NMR spectra were used to identify the molecular structure of the
copolymers and to confirm the degree of sulfonation. The peak assignments of the
aromatic region of BPS35-50 (Fig. 4.2) confirm the anticipated chemical structure. The
degree of sulfonation was determined from the integral ratios of proton peaks e, f, g, and
b. The chemical shifts for the three protons (e, f and g) attached to the sulfonated unit are
7.0, 7.7 and 8.3 ppm, respectively, while the peak at 7.8 ppm corresponds to the proton b
attached on the non-sulfonated unit. The mole content of sulfonated unit in BPS35-50
copolymer chain is 34.6% based on the integrals of b proton to the average of e, f and g
protons. The calculation could be described by the equation (3):
Degree of sulfonation (%) = ( )[ ]
( )[ ] %6.344/2/3/
2/3/=
+++
++
bgfe
gfe
HHHHHHH
……….(3)
Table 4.1 lists the contents of sulfonated units in the series of copolymers. All the values
are in good agreement with the ratio of feed monomers (SDCDPS/DCDPS – 35/65),
121
which suggests that all the starting monomers were successfully incorporated into the
copolymer chains.
The molecular weights of the copolymers were calculated from the relative 1H
NMR integrals of the tert-butyl endgroups and the aromatic resonances. For example,
Figure 4.3 shows the proton NMR spectrum of a copolymer with target molecular weight
50 kg ·mol-1 (BPS35-50), in which methyl protons were observed at 1.2 ppm. The
presence of the tert-butyl peak in BPS35-50 proton NMR confirms that tert-butylphenol
was chemically attached at the ends of copolymer, and only the chemical bonded TB
groups are useful in quantitatively calculating the molecular weight of one polymer chain
based on its 1H-NMR spectrum. While there are 18 methyl protons on the two tert-butyl
endgroups of one polymer chain, two sources contribute to the aromatic protons: the
sulfonated or non-sulfonated aromatic repeat units in the interior of the polymer chain,
and the terminal phenyl rings. There are 16 and 14 phenyl protons for non-sulfonated and
sulfonated units, respectively. Since the experimental degree of sulfonation for BPS35-50
was calculated to be 34.6% by proton NMR, the average protons per repeat unit is
16*0.654 + 14*0.346 = 15.3, and the total number of phenyl protons from the interior
repeat units is 15.3n, where n is the number of the average polymer chain repeat units.
Assuming 100% conversion of all monomers in the condensation polymerization, two
types of terminal groups existed in these copolymers, as shown in Scheme 1. One
terminal group (left side) is a simple 4-tert-butylphenol residue, which contains 4 phenyl
protons Another terminal group (right side) is a residue of 4-tert-butylphenol, which
contains 4 phenyl protons, and is attached with either a disulfonated monomer (34.6%
possibility) or non-sulfonated dihalide monomer (65.4% possibility), so the total phenyl
122
protons at this end is 4 + (8 * 0.654 + 6 * 0.346) = 11.3. Therefore the total average
phenyl protons from two terminal groups are 11.3 + 4 = 15.3, and the total phenyl protons
in one polymer chain is 15.3n + 15.3. The number ratio of aromatic to methyl protons
equals to the integration ratio of the two types protons in 1H-NMR spectrum, so the
molecular weight of copolymer was calculated from the equation (4).
299.168
183.153.15=
+×n Aromatic protonsMethyl protons2
99.16818
3.153.15=
+×n Aromatic protonsMethyl protons ………(4)
Where n is the number of repeat units, which was calculated to be 98.41. Accordingly the
average molecular weight (Mn) of the BPS35-50 copolymer was calculated to be: Mn =
98.41×481. 7 (g·mol-1) + 585.4 (g·mol-1) = 47,989 g·mol-1, where 481.7 g·mol-1 is the
average molecular weight per repeat unit, and 585.4 g·mol-1 is the molar mass of the two
endgroups. Molecular weights of 20 to 40 kg·mol-1 copolymers were prepared and
characterized using the similar technique (Table 4.1). It is obvious that the experimental
molecular weights are in close agreement with the targeted values, which on the other
hand confirmed the close 100% conversion of all the monomers.
Viscosity is one of the most important parameters in charactering polymer property.
Simple dilute solution viscosity measurements are widely used in polymer science, but
have not been well developed for ion-containing PEMs. One possible reason is that by
comparing neutral and charged macromolecules, neutral systems retain their random coil
conformation down to very low concentrations. In contrast, it is well known that as one
dilutes a charged macromolecule the so called “polyelectrolyte effect” appears. This
123
effect produces a more extended chain showing higher dilute solution viscosities as the
concentration is reduced, largely due to charge repulsion.21
It is also well known within the biomembrane community that dilute solution
viscosities of polyelectrolytes should be measured in the presence of a low molar mass
salt, which is able to screen the charges. It is recognized that the optimum salt
concentration might depend upon the chemical structures, molar mass range, charged
densities, etc. We have established that as little as 0.05 M LiBr allows one to obtain the
correct intrinsic viscosity values (Fig. 4.4).
Both reduced and inherent viscosities of BPS35-control were measured in NMP
with or without LiBr in the polymer concentration range of 0.15 to 1.0 g·dL-1.
Correlations of viscosities and concentration are plotted in Figure 4.3a (without LiBr) and
4.33b (with 0.05M LiBr). In the case of polymer solution without LiBr, both reduced and
inherent viscosities increase as polymer concentration decreases, consistent with the idea
of the polyelectrolyte effect. However, the introduction of lithium bromide into polymer
solution made the viscosities (reduced and inherent)-concentration relationships of
BPS35-control very similar to a non-charged polymer solution, as shown in Figure 4.3b.
The conventional extrapolation of reduced and inherent viscosities to the infinite dilute
solution gives us the relatively accurate intrinsic viscosity of BPS35-control (1.04 dL·g-1),
which is much lower than the value measured in pure NMP (2.79 dL·g-1). As summarized
in Table 4.1, intrinsic viscosities of all copolymers agree quite well with their designed
and measured molecular weights. The intrinsic viscosity measurements of the copolymers
could be compared to one another since the only difference in polymer structure is the
endgroups, which are less than 1% by weight.
124
A fairly good linear relationship of Log(Mn) and Log(IV) for 4 tert-butylphenol
endcapped copolymers is plotted in Figure 4.5. This relationship provides us an
alternative but powerful tool in characterizing molecular weights of polymers with
similar chemical composition. For example, the molecular weight of BPS35-control
copolymer was estimated from the above linear relationship to be 83.1 kg·mol-1.
Moreover, this technique can be used for other PEMs as well, such as sulfonated
poly(arylene ether ketone)s, polyimides and more.
125
HO OHSClO
O
BPDCDPSSO3Na
CH3
CH3CH3HO
S ClCl
O
O
TB
SDCDPSK2CO3
SO
O*
DMAc/Toluene~ 160 oC, Reflux 4 h175 oC, 24 h
+
+
NaO3S
O O SO
OO O * S
O
OO
CH3
CH3CH3
CH3
CH3
H3C Ox
1-x n
KO3S SO3K
Cl
x = 0.35; BPS35, Target Mn: 20, 30, 40 and 50 kg·mol-1
Figure 4.1. Synthesis of Tert-Butylphenyl Terminated Poly(Arylene Ether Sulfone)
Copolymers Containing 35 mole % Disulfonate Repeat Unit.
126
SO
OO O S
O
OO O *
x 1-x
KO3S SO3K
e f
g
h i a b c d
g
b
f
d, i a, c, h
e
SO
OO O S
O
OO O *
x 1-x
KO3S SO3K
e f
g
h i a b c dSO
OO O S
O
OO O *
x 1-x
KO3S SO3K
e f
g
h i a b c d
g
b
f
d, i a, c, h
e
Figure 4.2. The Copolymer Structures and Degree of Sulfonation were Determined
by 1H NMR Spectra in the Aromatic Region (BPS35-50 Copolymer).
127
H2O
DMSO
Aromatic Protons
Methyl Protons
H2O
DMSO
Aromatic Protons
Methyl Protons
Figure 4.3. Molecular Weights can be Calculated from the Relative 1H NMR
Integrals of the Tert-Butyl Endgroups and the Aromatic Resonances (BPS35-50
Copolymer in DMSO-d6).
128
y = -1.1575x + 3.2424
y = -1.243x + 2.3294
0
0.5
1
1.5
2
2.5
3
3.5
0 0.2 0.4 0.6 0.8 1 1.2Concentration (g·dL-1)
Vis
cosi
ty (d
L·g-1
)
In pure NMP In 0.05M LiBr/NMP solution
y = -0.1651x + 1.0394
y = 0.359x + 1.0466
0.5
0.8
1.1
1.4
1.7
2
0 0.2 0.4 0.6 0.8 1Concentration (g·dL-1)
Vis
cosi
ty (d
L·g-1
)
y = -1.1575x + 3.2424
y = -1.243x + 2.3294
0
0.5
1
1.5
2
2.5
3
3.5
0 0.2 0.4 0.6 0.8 1 1.2Concentration (g·dL-1)
Vis
cosi
ty (d
L·g-1
)
In pure NMP In 0.05M LiBr/NMP solution
y = -0.1651x + 1.0394
y = 0.359x + 1.0466
0.5
0.8
1.1
1.4
1.7
2
0 0.2 0.4 0.6 0.8 1Concentration (g·dL-1)
Vis
cosi
ty (d
L·g-1
)
(3a) (3b)
Figure 4.4. Correlations of Reduced (▲) and Inherent (■) Viscosities with
Copolymer Concentration of BPS35-Control in Pure NMP (3a), and NMP
Containing 0.05 M LiBr (3b).
129
Table 4.1. Characterization of BPS35 copolymers
SDCDPS (%) Target Mn (kg · mol-1)
n by NMR (kg · mol-1)
IVa (dL · g-1)
Target Experimental (by NMR)
20 19.9 0.43 35 33.9 30 28.8 0.48 35 34.1 40 38.1 0.63 35 34.7 50 48.0 0.74 35 34.6
Controlb 83.1c 1.04d 35 34.2 a Intrinsic viscosities were determined in 0.05 M LiBr/NMP solution at 25 °C. b Control copolymer was prepared with 1:1 stoichiometry without tert-butylphenol. c Mn of the control copolymer was derived from the log(I.V.)-log(Mn) plot. d Intrinsic viscosity of the control copolymer without LiBr was 2.79 dL·g-1.
130
y = 0.6457x - 3.1623R2 = 0.9464
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
4.2 4.3 4.4 4.5 4.6 4.7
Log[Mn, (g.mol -1)]
Log[
I.V.,(
dL.g-1
)]
Figure 4.5. Relationship Between Log(Intrinsic Viscosity) and Log(Mn) for BPS-35
Copolymers
131
4.4.2. Membrane Characterization
These controlled molecular weight copolymers enabled ones to examine the
influence of molecular weight on several different critical parameters important for
proton exchange membranes. Sulfonated copolymers tend to phase separate in
hydrophilic and hydrophobic domain morphology. Water resides in these hydrophilic
domains and plays a critical role in proton transport22. Kim, et. al.23 reported increasing
water uptake and proton conductivity with increasing degree of disulfonation for BPSH
copolymers. High proton conductivity is desirable but high water uptake results in
excessive swelling and poor dimensional stability. Understanding the influence of
molecular weight on water uptake and conductivity for the copolymer at a particular
degree of disulfonation will lead in optimizing the overall fuel cell performance. In Table
4.2, the water uptake and conductivity of BPSH35-xx copolymers in liquid water at room
temperature is provided. The water uptake decreased modestly as the molecular weight of
BPSH35-xx increased from 20 to 80 kg·mol-1. But, at this point it is difficult to correlate
the influence of molecular weight for the copolymers on proton transport under fully
hydrated conditions. The small deviations in the conductivity values are well within the
experimental error range of 10%. However, determination of proton conductivity under
partially hydrated conditions for these copolymers with varying molecular weight is
ongoing. This will give a better understanding about the influence of molecular weight on
proton transport.
The mechanical properties of PEMs, such as fatigue resistance, are very important for
the development of non-fluoroniated PEMs operated at both room and elevated
temperatures. Although the intermediate degree of sulfonation (35 mol %) was chosen to
132
keep relatively low water uptake and maintain membranes’ mechanical strength, it is also
believed that mechanical properties of PEMs should be enhanced by high molecular
weight. Stress-strain curves showed the modulus, strength, and elongation to break of
PEM films were significantly influenced by the molecular weight. These materials were
measured under ambient conditions with 40% relative humidity to investigate the effect
of molecular weight (Fig. 4.6 and Table 4.3). It can be seen that Young’s modulus, yield
stress/strain and elongation at break were all affected by the sample molecular weight.
Among these parameters, the elongation at break varied the most with molecular weight,
increasing from approximately 15% for BPSH35-20 to 78% for BPSH35-50. This
behavior was attributed to more chain entanglements at higher molecular weights. It is
reasonable to expect that high molecular weight could be important for preventing
pinhole formation, even at elevated temperatures, and the molecular weight may also
improve fatigue resistance and long term stability, which will be investigated in the future.
133
Table 4.2. The Results of Water Swelling and Conductivity Test for BPSH35
Copolymers with Controlled Molecular Weights in Liquid Water at 30 oC
Target Mn (kg · mol-1)
IEC* (meq · g-1)
Water uptake (%)
Conductivity (S·cm-1)
20 1.49 40 0.070 30 1.50 43 0.080 40 1.52 42 0.081 50 1.52 38 0.080
Control 1.50 36 0.077 * IEC = (1000/MWrepeat unit)* Degree of Sulfonation *2 (–SO3H), where degree of sulfonation was determined by 1H NMR.
134
Figure 4.6. Stress-strain Curves of BPSH35 Copolymers (Thin Films) as Function of
Molecular Weight.
45
47
49
51
53
55
57
59
0 20 40 60 80
Strain, %
Stre
ss, M
Pa
1, 202, 303, 404, 505, Contro
12
43
5Mn (kg · mol-1
45
47
49
51
53
55
57
59
0 20 40 60 80
Strain, %
Stre
ss, M
Pa
1, 202, 303, 404, 505, Contro
12
43
5Mn (kg · mol-1 Mn (kg·mol-1)
135
Table 4.3. The Tensile Properties of BPSH35 Copolymers (thin films) as Function of
Molecular Weight.
Mn (kg · mol-1)
Modulus (GPa)
Yield Strain (%)
Yield Stress(MPa)
Strength*
(MPa) Elongation at Break * (%)
20 1.46 ± 0.28 3.04 ± 0.38 39.6 ± 7.7 52.0 15.7 30 1.08 ± 0.23 4.34 ± 0.21 38.0 ± 8.4 52.1 32.3 40 1.36 ± 0.24 4.83 ± 0.28 47.9 ± 7.7 57.7 63.4 50 1.53 ± 0.27 4.23 ± 0.69 44.1 ± 10.2 59.0 78.7
Control 1.92 ± 0.30 3.68 ± 0.79 58.1 ± 7.1 66.3 48.8 * For strength and elongation at break data, the values are from the longest stress-strain curves.
136
4.5. Conclusions
A series of controlled molecular weight, poly(arylene ether sulfone) copolymers
containing 35 mole % disulfonated monomer per repeat unit were synthesized and
characterized by 1H NMR, intrinsic viscosity, water uptake, proton conductivity, and
mechanical property. A small amount of lithium bromide (0.05 M) in NMP can
effectively suppress the “polyelectrolyte effect” appearing in measuring the intrinsic
viscosity of a charged macromolecule, which allowed obtaining more accurate data than
previously used simple dilute solution viscosity measurements. Combing 1H NMR
analysis of end groups and intrinsic viscosity measurements, it can be conclude that the
molecular weights of the synthesized copolymers were from 20 to 50 kg·mol-1, which are
much closed to the designed values. The effects of molecular weights on the properties of
proton exchange membranes were also studied. It was found that with increasing the
molecular weights, the water uptake decreased modestly. The molecular weight had no
obvious influence on proton conductivity under fully hydrated conditions. Furthermore,
the mechanical properties of the membrane, such as the modulus strength and elongation
at break were improved by increasing the molecular weight as well. The characterizations
of conductivity, water uptake, and mechanical properties provide us some very useful
guidelines in designing sulfonated polymers as PEM.
4.6. Acknowledgements
The authors would like to thank the National Science Foundation Partnership for
Innovation Program (EHR- 0332648), and the Department of Energy (contract # DE-
FC36-01G011086) for the financial support that funded this research.
137
4.7. References
1. Thomas, S.; Zalbowitz, M. Fuel Cells: Green Power; Los Alamos National Laboratory: Los Alamos, NM, 1999
2. Inzelt, G.; Pineri, M.; Schultze, J.W.; Vorotyntsev, M.A., Electrochim. Acta, 2000; 45,
2403. 3. Hickner, M.A.; Ghassemi, H.; Kim, Y.S.; Einsla, B.R.; and McGrath J.E., Chem. Rev.,
2004; 104, 4587 4. Wang, S.; McGrath, J.E. Step Polymerization; Rogers, M.; Long, T. E., Eds.; Wiley:
New York, 2003 5. Noshay, A.; Robeson, L.M. J. of Appl. Polym. Sci.,1976; 20, 1885 6. Johnson, B.C.; Yilgor, I.; Tran, C.; Iqubal, M.; Wightman, J.P.; Lloyd, D.R.; McGrath,
J.E. J. Polym. Sci. Part A: Polym. Chem. 1984; 22, 721 7. Wang, F.; Ji, Q.; Harrison, W.; Mecham, J.B.; Formato, R.; Kovar, R.; Osenar, P.; and
McGrath, J.E., Polym. Preprs., 2000; 41(1), 237 8. Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.; Zawodzinski, T.; and
McGrath, J.E. Macroml. Symp., 2001; 175, 387. 9. Wang, F.; Hickner, M.; Kim, Y.S.; Zawodzinski, T.; and McGrath, J.E., J. Membr. Sci,
2002; 197, 231. 10. Wang, F.; Mecham, J.; Harrison, W.; and McGrath, J.E., Polym. Mater.: Sci. & Engi.
2001, 84, 913 11. Wang, F.; Chen, T.; and Xu, J. Macrmol. Chem. and Phys. 1998; 199, 1421 12. Wang, F.; Li, J.; Chen, T.; and Xu, J., Polymer, 1999, 40, 795 13. Wang, F.; Mecham, J.; Harrison, W.; and McGrath, J.E. Polym.Preprs.,2000; 41(2),
1401 14. Ghassemi, H.; McGrath, J.E. Polym. Preprs. 2002; 43, 1021. 15. Sumner, M.J.; Harrison, W.L.; Weyers, R.M.; Kim, Y.S.; McGrath, J.E; Riffle, J.S.;
Brink, A.; Brink, M.H., J. Membr. Sci 2004, 239, 199. 16. Einsla, B.R.; Hong, Y.T.; Kim, Y.S.; Wang, F.; Gunduz, N.; and McGrath, J.E. J
Polym Sci, Part A: Polym. Chem., 2004, 42, 862.
138
17. Wang, F; Glass, T.; Li, X.; Hickner, M.; Kim, Y.S.; and McGrath, J.E., Polym. Preprs.
2002, 43(1), 492. 18. Zawodzinski, T.A.; Neeman, M.; Sillerud, L.O.; Gottesfeld, S., J. Phys. Chem. 1991,
95, 6040. 19. Springer, T. E.; Zawodzinski, T. A.; Wilson, M. S.; Gottesfeld, S., J. Electrochem.
Soc. 1996, 143, 587. 20. Jurek, M. J.; McGrath, J.E. Polymer, 1989, 30, 978. 21. Schmidt, M., Polyelectrolytes with Defined Molecular Archetecture, Advanced in
Polymer Sciences, 2004. 22. Kreuer, K. D., Solid State Ionics, 2000, 136, 149. 23. Kim, Y. S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y. T.; Harrison, W.;
Zawodzinski, T. A.; McGrath, J. E., J. Polym. Sci. Part B: Polym. Phys., 2003, 41, 2816.
139
Chapter 5. Partially Fluorinated Disulfonated Poly(Arylene Ether Sulfone) Copolymers with Controlled Molecular Weights for Proton Exchange Membranes Yanxiang Li, Juan Yang, Anand Badami, Abhishek Roy, Ozma Lane, and James E McGrath Macromolecular Science and Engineering & Macromolecules and Interfaces Institute Virginia Polytechnic Institute and State University Blacksburg, VA 24061
140
5.1. Abstract
Partially fluorinated disulfonated poly(arylene ether sulfone) copolymers based on
a hexafluoro bisphenol A monomer were investigated as proton exchange membranes
(PEMs). The introduction of fluorine groups into the polymer backbone is thought to
potentially increase the durability of the membrane electrode assembly (MEA). Two
series of controlled molecular weight, partially fluorinated disulfonated poly(arylene
ether sulfone) copolymers (6FSH) with acid content fixed at 35 mol% and 48 mol% were
successfully prepared via direct step growth copolymerization. The acid contents were
chosen to effectively compare with the previously reported results of biphenol-based
polysulfones (BPSH). For each sulfonation level, a series of copolymers with molecular
weights ranging from 20 to 50 kg·mol-1 were synthesized. The molecular weight was
controlled by addition of a monofunctional monomer, tert-butylphenol, together with
offsetting the stoichiometry of the feed comonomer ratios. The experimental molecular
weights were characterized by a combination of 1H NMR end group analysis and
modified intrinsic viscosity measurements in 0.05 M LiBr/NMP. Atomic force
microscopy (AFM) was used to characterize the membrane morphology. The influence of
molecular weight on water uptake and proton conductivity were studied and compared
with the BPSH copolymer results.
Keywords: Partially Fluorinated Copolymers, Disulfonated Poly(Arylene Ether Sulfone)s,
Controlled Molecular Weight, Proton Exchange Membrane Fuel Cells.
141
5.2. Introduction
The electrochemical and mechanical properties of sulfonated copolymers such as
Nafion®, sulfonated polyethersulfones, etc. have become increasingly important as their
potential application in proton exchange membrane fuel cells (PEMFC) becomes more
evident.1,2 Molecular weight is a fundamental parameter affecting all mechanical
properties of polymers. Although most reports of new proton exchange membrane
materials have included information on ion content (EW or IEC), protonic conductivity,
and water uptake, there is little in the PEM literature describing molecular weights of
candidate materials, especially for Nafion®.3 Thus, it is meaningful to clarify this issue
for the effective comparison of membrane properties.
Sulfonated poly(arylene ether sulfone) copolymers are good candidates for PEMs
due to their good acid and thermal oxidative stabilities, high glass transition temperatures
and excellent mechanical strength.4,5 This family of copolymers has been investigated
thoroughly by McGrath’s research group for both hydrogen/air and direct methanol fuel
cells.6-10 The representative BPSH copolymers (Fig. 5.1) have shown comparable or even
better PEM properties to the state-of-art perfluorosulfonic acid Nafion®. However, when
fabricated into membrane electrode assemblies (MEAs), delamination and high
interfacial resistance of the BPSH membranes caused by the incompatibility between the
membrane and the Nafion-bonded electrodes lowered their fuel cell performance and
decreased the MEA’s long-term stability. Long-term stability of MEAs is one of the most
critical requirements for the successful large-scale production and application of proton
exchange membrane fuel cell technology. Two strategies have been suggested to address
142
this problem: using a hydrocarbon copolymer to replace the Nafion® binder in the
catalyst layer, making the electrodes more similar to the membrane, and introduction of
some fluorine component into the copolymer backbone to promote the compatibility
between the existing Nafion® electrodes and the membrane.11, 12
Poly(arylene ether sulfone)s based on the partially fluorinated 6F bisphenol A
monomer are of interest because they may improve certain properties of PEMs. It has
been proposed that they can provide a more hydrophobic membrane surface that may
lower the water uptake, and the fluorine-rich surface may be more compatible with
electrodes that contain Nafion® and may result in more durable MEAs, which will display
lower interfacial losses. The McGrath research group has reported some promising
properties on these partially fluorinated copolymers.12,13 Previous results showed that the
molecular weight for the BPSH copolymers had an effect on the membrane mechanical
properties, but little effect on the water uptake and proton conductivity was found.14,15 A
correlation of molecular weight and PEM properties would also be valuable for the
promising partially fluorinated copolymers.
In this paper, partially fluorinated disulfonated poly(arylene ether sulfone) copolymers
(6FSH, Fig. 5.1) with controlled number average molecular weights ( nM ) from 20 to 50
kg·mol-1 were successfully synthesized for two series with acid content (or degree of
disulfonation) fixed at 35 mol% and 48 mol%. These acid contents were chosen to
effectively compare with the previously reported results of biphenol-based polysulfones
(BPSH). nM values of the copolymers were controlled by addition of a monofunctional
monomer, tert-butylphenol, and were characterized by 1H NMR and modified intrinsic
viscosity measurements. NMP with 0.05 M LiBr has been used as a solvent for intrinsic
143
viscosity measurements of ion-containing copolymers instead of simple dilute solution
viscosity measurements in pure solvents. The small amount of salt effectively suppressed
the polyelectrolyte effect, allowing improved characterization of the ion-containing
materials.3 The aim of this study was to confirm the molecular weight effect on
membrane properties using the partially fluorinated 6FSH copolymer structures, and also
to examine the effect of molecular structure to some extent.
144
O
1-x n
SO
OO
HO3S SO3H
O SO
OO
x
O
1-x n
SO
OO
HO3S SO3H
C
CF3
CF3
O SO
OO
xC
CF3
CF3
BPSH Copolymer
6FSH Copolymer
Figure 5.1. Disulfonated Copolymer Structures with Biphenol (BPSH) or 6F
Bisphenol A (6FSH) Units in the Backbones
145
5.3. Experimental
5.3.1. Materials
High purity 4,4’-difluorodiphenylsulfone (DFDPS) was purchased from Aldrich.
Monomer grade 4,4’-hexafluoroisopropylidenediphenol (6F-BPA) was provided by
DuPont. Both were well dried in a vacuum oven before use but otherwise used as
received. The 4-tert-butylphenol (TB) endcapper was purchased from Aldrich and was
purified by sublimation. The disulfonated monomer 3,3’-disulfonated 4,4’-
difluorodiphenylsulfone (SDFDPS) was synthesized following a literature procedure.6,16
The solvent N,N-dimethylacetamide (DMAc, Fisher) was vacuum-distilled from calcium
hydride onto molecular sieves. Potassium carbonate was dried in vacuo before use.
Toluene and methanol were obtained from Aldrich and used as received.
5.3.2. Copolymerization
The step-growth copolymerization employed a modified procedure from that
reported previously.15 A typical copolymerization for a controlled molecular weight of 40
kg·mol-1 copolymer with 35 mol% disulfonated comonomer was described as follows:
6F-BPA (5.000 g, 14.871 mmol), DFDPS (2.496 g, 9.818 mmol), SDFDPS (2.423 g,
5.287 mmol), and TB (0.071 g, 0.469 mmol) were added to a three-neck flask equipped
with mechanical stirrer, nitrogen inlet and a Dean-Stark trap. Potassium carbonate (1.15
equivalents) and dry DMAc were introduced to afford a 20% solids concentration.
Toluene (DMAc/Toluene = 2/1) was used as an azeotroping agent. The reaction mixture
was heated under reflux at 160 oC for 4 h to dehydrate the system, followed by complete
146
removal of the toluene. Then, the bath temperature was raised slowly to 175 oC for 48 h,
which caused the DMAc to reflux. The solution became viscous and was cooled to room
temperature, then diluted with DMAc to form about a 20% copolymer solution. The
copolymer was isolated by precipitation in deionized water, filtered, and dried in a
vacuum oven for 24 h at 120 oC. The crude dry copolymer was then ground to a powder
and washed with ethanol and deionized water extensively to remove any free endcapping
agent and salt. The copolymer was finally vacuum dried at 120 oC for 24 h. The resulting
copolymers were designated as 6FS35-xx (salt form) or 6FSH35-xx (acid form), and
6FS48-xx (salt form) or 6FSH48-xx (acid form), where numbers 35 and 48 represent the
mole percent of disulfonated monomer in the copolymer structure (or degree of
sulfonation), and xx represents the target number average molecular weight (kg·mol-1).
The molecular weight was controlled from 20 to 50 kg·mol-1 by varying the amount of
monofunctional monomer 4-tert-butylphenol and the ratio of the difunctional monomers.
The control copolymers, which were synthesized using a 1:1 stoichometry of the
difunctional monomers without endcapper, were also synthesized for comparison.
5.3.3. Membrane Preparation and Acidification
Films of the copolymers were cast from 5~10 wt% solutions in DMAc on a glass
plate under an IR light and carefully dried in a vacuum oven at 120 oC for at least 24 h.
The salt-form films were then transformed into the acid form by boiling the films in 0.5
M H2SO4 for 2 h to convert the pendant sulfonate salt groups into free acid groups. The
residual sulfuric acid was removed by boiling the films in deionized water for another 2 h.
The acid-form membranes were then stored in fresh deionized water at room temperature.
147
5.3.4. Characterization
Chemical structures and acid contents of all the copolymers were characterized by
1H NMR spectra conducted with a Varian Unity 400 NMR spectrometer. The solvent was
DMSO-d6. Intrinsic viscosity (IV) measurements were carried out in NMP with 0.05 M
LiBr at 25 ºC using an Ubbelohde viscometer. Each solution for viscometry was freshly
prepared and was kept at 25 ºC for 10 min prior to the measurement.
To obtain water uptake values, the sample films were equilibrated in deionized
water at room temperature for at least 48 h. The wet membranes were blotted dry and
immediately weighed. Then the membranes were dried in the vacuum oven at 110 oC for
24 h, and weighed again. Water uptake was calculated as the ratio of the difference
between wet and dry membrane weight divided by dry membrane weight and expressed
as a weight percent.
Proton conductivity at 30 °C at full hydration (in liquid water) was determined in
a window cell geometry17 using a Solartron (1252 + 1287) Impedance/Gain-Phase
Analyzer over the frequency range of 10 Hz - 1 MHz. The cell geometry was chosen to
ensure that the membrane resistance dominated the response of the system. The
resistance of the film was taken at the frequency which produced the minimum imaginary
response.18 In determining proton conductivity in liquid water, membranes were
equilibrated at 30 °C in deionized water for 24 h prior to the testing.
Atomic force microscopy (AFM) images were obtained using a Digital
Instruments MultiMode scanning probe microscope with a NanoScope IVa controller
(Veeco Instruments, Santa Barbara, CA) in the tapping mode. A silicon probe (Veeco)
with an end radius of <10 nm and a force constant of 5 N·m-1 was used to image samples.
148
Samples were equilibrated at 30% relative humidity for at least 12 h before being imaged
immediately at room temperature and approximately 15-20% relative humidity.
5.4. Results and Discussion
5.4.1. Synthesis and Characterization of Copolymers
Figure 5.2 shows the reaction scheme for the direct step-growth copolymerization
of tert-butylphenyl-terminated partially fluorinated poly(arylene ether sulfone)s
containing sulfonic acid groups. These copolymers were based on the monomers DFDPS,
SDFDPS, 6F-BPA, and the endcapper TB. The mole percent of disulfonated repeat units
was fixed theoretically at 35% (6FSH35) and 48 % (6FSH48) for the two series,
respectively. These acid contents were chosen to compare the results with the previously
reported BPSH35 series controlled molecular weight results with either the same degree
of sulfonation (6FSH35) or the same ion exchange capacity (6FSH48 series, IEC: 1.50
meq·g-1). The monofunctional monomer, tert-butylphenol, was used as the endcapping
reagent to control the copolymer molecular weight from 20 to 50 kg·mol-1 for both
degrees of sulfonation. Since the 6F-BPA monomer has low reactivity due to the strong
electronegativity of -CF3 groups, the reaction required 48 h or longer reaction times,
which is much longer than the 24 h required for the BPSH system. Also, the more
reactive fluorine functional monomers DFDPS and SDFDPS were used instead of
chlorine functional monomers DCDPS and SDCDPS to compensate for the low reactivity
of 6F-BPA. The crude dry copolymers were ground to a powder and washed extensively
with ethanol and deionized water to remove any possible free endcapping reagent and
residual salt to obtain accurate NMR integration values for molecular weight (Mn)
149
calculation. For comparison, high molecular weight copolymers (6FS35-control and
6FS48-control) were synthesized with 1:1 stoichiometry without an endcapping reagent.
The aromatic region of the 1H NMR spectrum indicated that the expected
chemical composition was indeed obtained (Fig. 5.3 shows the spectrum for 6FS35-50 as
an example). The degree of disulfonation was calculated based on the integration values
of k, and (a, b) proton peaks. Peak k was assigned to the proton attached on the
sulfonated units, while the peaks (a, b) were protons on the unsulfonated units. Since
there were two k protons and eight (a, b) protons per repeat unit, the mole percent of
sulfonated unit for 6FS35-50 was calculated to be 32.7% according to the following
equation:
Degree of sulfonation = %7.322/8/)(
2/=
++ kba
k
HHHH
The degrees of sulfonation for all copolymers were calculated using the same method
(Table 5.1). The calculated values were slightly lower than the theoretical values due to
the low monomer reactivity of 6F-BPA and SDFDPS.
The experimental molecular weights of the copolymers were calculated from the
relative 1H NMR integrals of the tert-butyl endgroups and the aromatic resonances
following the same procedure described in the literature.15 1H NMR spectra of 6FS35-50
are shown in Figure 5.4 as an example. There were 18 methyl protons on the two end
tert-butyl groups, the average aromatic protons per repeat unit was 15.3, and 15.3
aromatic protons were attached on the terminal phenyl rings. Thus, the following
equation was used to calculate the number of repeat units:
(15.3n +15.3)/18 = 136.94/2.00
150
Where n is the number of repeat units, which was calculated to be 79.6. Accordingly the
number average molecular weight (Mn) of 6FS35-50 was calculated to be:
Mn = 79.6 × 621.4 g·mol-1 + 585.4 g·mol-1 =5.0 × 103 g·mol-1
where the average molecular weight per repeat unit is 621.4 g·mol-1 and the molar mass
of the endgroups is 585.4 g·mol-1. All the molecular weights calculated are listed in Table
5.1. It was reassuring that the experimental molecular weights were in agreement with the
target values.
Intrinsic viscosities were measured using the solvent NMP with 0.05 M LiBr at
room temperature. The small amount of LiBr effectively suppress the polyelectrolyte
effect caused by the interactions of the ionic groups in the dilute solution.15, 19 The
intrinsic viscosity values matched the molecular weight trend very well. The log(Mn) vs.
log(IV) plots are shown for both series of copolymers in Figure 5.5. The good linear
relationship confirmed the experimental molecular weight results. Also, the plots were
used to determine the molecular weight of control copolymers without the endcapper.
Compared with the BPSH copolymers at similar molecular weight, the intrinsic viscosites
of the 6FSH copolymers were lower due to the polymer and solvent interactions.
151
CHOSFO
O
6F bisphenol ADFDPS
SO3Na
CH3
CH3CH3HOS FF
O
O
TBSDFDPS
K2CO3DMAc/Toluene~ 160 oC, Reflux 4 h175 oC, 48 h
+
+NaO3S
O * S
O
OO
CH3
CH3CH3
1-x n
F OH
CF3
CF3
SO
O* O
CH3
CH3
H3C O
KO3S SO3K
C
CF3
CF3
O SO
OO
xC
CF3
CF3
O * SO
OO
CH3
CH3CH3
1-x n
SO
O* O
CH3
CH3
H3C O
HO3S SO3H
C
CF3
CF3
O SO
OO
xC
CF3
CF3
H+
Figure 5.2. Synthesis of Tert-butylphenyl Terminated Partially Fluorinated
Poly(Arylene Ether Sulfone) Containing Sulfonic Acid Groups (x = 0.35 or 0.48)
152
O SO
OC
CF3
CF3
O O SO
OC
CF3
CF3
O*
SO3KKO3S
*
x 1-x
abcdefghk
j i
k
d
i
e
f, g, j
a, b
h, e
O SO
OC
CF3
CF3
O O SO
OC
CF3
CF3
O*
SO3KKO3S
*
x 1-x
abcdefghk
j i
k
d
i
e
f, g, j
a, b
h, e
Figure 5.3. 1H NMR Spectrum of 6FS35-50 Copolymer (Aromatic Region)
153
Figure 5.4. The Molecular Weight of the Copolymer can be Calculated from the
Relative 1H NMR Integrals of the Tert-butyl Endgroups and the Aromatic
Resonances. (6FS35-50 Copolymer in DMSO-d6)
154
Table 5.1. Characterization of 6FS35 and 6FS48 Series Copolymers
Degree of Sulfonationb (%) Copolymer
Mn by NMR (kg·mol-1)
IVa
(dL·g-1) Target Calculated
6FS35-20K 19.9 0.23 35 32.5 6FS35-30K 27.5 0.30 35 33.7
6FS35-40K 37.8 0.44 35 34.1
6FS35-50K 49.5 0.53 35 32.7
6FS35-control 56.0c 0.60 35 31.7
6FS48-30K 33.9 0.28 48 46.0
6FS48-40K 47.4 0.42 48 47.0
6FS48-50K 55.9 0.46 48 46.3
6FS48-control 74.8c 0.64 48 46.3
a Intrinsic viscosities were determined in 0.05 M LiBr/NMP solution at 25 oC. b Mole % of sulfonated monomer in repeat unit c The molecular weights of the control copolymers were determined from the Log(I.V.)
vs. Log (Mn) plots
155
y = 0.939x - 4.677
R2 = 0.9937
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
4.2 4.3 4.4 4.5 4.6 4.7 4.8
Log [Mn, g.mol-1]
Log [IV, dL.g
-1]
y = 1.0251x - 5.19
R2 = 0.9902
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
4.5 4.6 4.7 4.8 4.9
Log [Mn, g.mol-1]
Log [IV, dL.g
-1]
6FS35 Series 6FS48 Series
y = 0.939x - 4.677
R2 = 0.9937
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
4.2 4.3 4.4 4.5 4.6 4.7 4.8
Log [Mn, g.mol-1]
Log [IV, dL.g
-1]
y = 1.0251x - 5.19
R2 = 0.9902
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
4.5 4.6 4.7 4.8 4.9
Log [Mn, g.mol-1]
Log [IV, dL.g
-1]
6FS35 Series 6FS48 Series
Figure 5.5. Relationship Between Log(Intrinsic Viscosity) and log(Mn) for 6FS35
and 6FS48 Copolymers
156
5.4.2. Characterization of Membranes
Water uptake and proton conductivity are two of the most important properties for
PEM materials. For the same chemical structure under fully hydrated conditions, these
two parameters depend mainly on the ion exchange capacity (IEC). It has been reported
that the proton conductivities of BPSH copolymers had a linear relationship with IEC,
while water uptake increased with the increase of IEC before the percolation limit.20
When the IEC was higher than the percolation limit, the membrane absorbed too much
water to maintain good mechanical strength. For practical applications, relatively high
proton conductivity with low water uptake is always the target for developing new PEM
materials. Introducing the fluorine component to the polymer backbone was a strategy to
make the membrane more hydrophobic and decrease the water swelling.
The water uptakes and proton conductivities of 6FSH35 and 6FSH48 copolymer
membranes have been measured under fully hydrated conditions at room temperature
(Table 5.2). It was found that the molecular weight had little effect on these two
properties, which agreed with the previously reported results for BPSH35.15 Compared
with the BPSH35 series of copolymers, at the same degree of sulfonation, 6FSH35
copolymers had lower water uptake and conductivity, but at the same IEC values,
6FSH48 copolymers had higher water uptake and conductivity. This difference was the
result of the different chemical structures.
The microphase-separated morphology is an important feature in determining
PEM performance. IEC and hydrothermal treatment are two major factors that will affect
the membrane morphology. In this paper, all the membranes were acidified at boiling
temperature to achieve the best phase separated morphology.20 AFM showed that the
157
molecular weight had no apparent effect on the membrane morphology within each series,
because all of the copolymers have similar IEC values within the same series (Fig. 5.6).
158
Table 5.2. Water Uptake and Conductivity Characterization for 6FSH35 and 6FS48
Copolymers with Controlled Molecular Weights in Liquid Water at 30 oC
Copolymers IECa
(meq·g-1) Water uptake
(%) Conductivity
(S·cm-1)
6FS35-20k 1.02 27.6 NA* 6FS35-30k 1.06 26.4 0.060 6FS35-40k 1.07 27.3 0.050 6FS35-50k 1.06 27.6 0.055
6FS35-Control 1.05 23.4 0.050 6FS48-20k 1.48 - - 6FS48-30k 1.50 - - 6FS48-40k 1.50 145 0.095 6FS48-50k 1.48 132 0.090
6FS48-Control 1.48 118 0.100 a. The ion exchange capacity (IEC) values were calculated based on the experimental
degree of sulfonation using the equation: IEC = (1000/MWrepeat unit)* Degree of Sulfonation *2 (–SO3H),
159
100 nm
28.8 kg/mol
100 nm
48.0 kg/mol
100 nm
83.1 kg/mola
100 nm
27.5 kg/mol
100 nm
49.5 kg/mol
100 nm
~70 kg/mol
BPSH series
6FSH series
100 nm
28.8 kg/mol
100 nm100 nm
28.8 kg/mol
100 nm
48.0 kg/mol
100 nm100 nm
48.0 kg/mol
100 nm
83.1 kg/mola
100 nm100 nm
83.1 kg/mola
100 nm
27.5 kg/mol
100 nm100 nm100 nm
27.5 kg/mol
100 nm
49.5 kg/mol
100 nm100 nm100 nm
49.5 kg/mol
100 nm
~70 kg/mol
100 nm100 nm100 nm
~70 kg/mol
BPSH series
6FSH series Figure 5.6. Morphology Characterization of the BPSH35 and 6FSH35 Series of
Copolymers with Different Molecular Weights by AFM.
160
5.5. Conclusions
Two series of partially fluorinated poly(arylene ether sulfone) copolymers with 35
mol% and 48 mol% acid contents were successfully prepared via direct step-growth
polymerization. The molecular weights of both series of copolymers were controlled
from 20 to 50 kg·mol-1 by addition of the monofunctional monomer tert-butylphenol,
together with offsetting the stoichiometry of the feed comonomer ratios. 1H NMR
combined with modified intrinsic viscosity measurements was used to characterize the
experimental molecular weights. The improved intrinsic viscosity measurement method
in 0.05 M LiBr/NMP allows more accurate results due to the suppression of the
polyelectrolyte effect. The experimental values were in close agreement with the
theoretical values. The morphology of copolymer membranes with different molecular
weights was characterized by AFM, which suggested that the morphology is a function of
the degree of sulfonation, irrespective of the molecular weight. This result further
confirmed that the molecular weight has little effect on the properties of proton exchange
membranes, such as water swelling and conductivity, although it did affect the
membrane’s mechanical properties.
5.6. Acknowledgements
The author appreciated the support of this research by the Department of Energy
(contract # DE-FC36-01G011086) and UTC Fuel Cells (contract #PO 3651).
161
5.7. References 1. Savadogo O. J., New Mater Electrochem. Syst., 1998, 1, 47 2. Kim Y. S.; Dong L.; Hickner M.A.; Pivovar B.S.; McGrath J.E., Polymer, 2003, 44,
5729 3. Hickner M.A.; Ghassemi H.; Kim Y.S.; Einsla B.R.; and McGrath J.E., Chem. Rev.,
2004, 104, 4587 4. Dumais J.J; Cholli A.L.; Jelinski L.W.; Hedrick J.L.; and McGrath J.E.,
Macromolecules, 1986, 19,1884 5. Wang S.; McGrath J. E., in Step Polymerization, M. Rogers, T. E. Long, Wiley, Eds.
2002 6. Wang F.; Hickner M.; Ji Q.; Harrison W.; Mecham J.; Zawodzinski T.; McGrath J. E.,
Macromol. Symp. 2001, 175 (1), 387. 7. Harrison W.L.; Wang F.; Mecham J.; Bhanu V.A.; Hill M.; Kim Y.S.; and McGrath
J.E., J. Polym. Sci. Part A: Polym. Chem., 2003, 41, 2264. 8. Kim Y.S.; Wang F.; Hickner M.; Zawodzinski T.A.; McGrath J.E., J. Membr. Sci.
2003, 212 (1-2), 263. 9. Wang F.; Hickner M; Kim Y. S.; Zawodzinski T. A.; and McGrath J. E., J. Membr.
Sci. 2002, 197, 231. 10. Harrison W. L.; Hickner M. A.; Kim Y. S.; and McGrath J. E., Fuel Cells, 2005, 5(2),
201 11. Zhang L; Ma C.; and Mukerjee S., Electrochem. Acta. 2003 48, 1845. 12. Wiles K. B.; Dissertation, Virginia Tech, 2005, 311 13. Kim Y. S.; Einsla B.; Sankir M.; Harrison W.; Pivovar B. S., Polymer, 2006, 47,
4026 14. Wang F.; Glass T.; Li X., ; Hickner M.; Kim Y.S.; McGrath J.E., Polym. Preprs.,
2002, 43(1), 492 15. Li Y.; Wang F., Yang J.; Liu D.; Roy A.; Case S.; Lesco J.; McGrath, J. E. Polymer,
2006, 47, 4210. 16. Sankir M.; Bhanu V. A.; Harrison W. L.; Ghassemi H.; Wiles K. B.; Glass T. E.;
Brink A. E.; Brink M. H.; McGrath J. E., J. Appli. Polym. Sci., 2006,100, 4595.
162
17. Zawodzinski, T.A.; Neeman, M.; Sillerud, L.O.; Gottesfeld, S., J. Phys. Chem. 1991,
95, 6040. 18. Springer, T. E.; Zawodzinski, T. A.; Wilson, M. S.; Gottesfeld, S., J. Electrochem.
Soci. 1996, 143, 587. 19. Yang J., Li Y.; Wang H.; Hill M.; Yu X.; Wiles K. B.; Lee H.; McGrath J. E., Preprs.,
ACS, Fuel Divi., 2005, 50(2), 701 20. Kim Y. S.; Wang F.; Hickner M.; MaCartney S.; Hong Y. T.; Zawodzinski T.A.;
McGrath, J. E., J. Polymer Sci. Part B: Polym. Phys. 2003, 41, 2816
163
Chapter 6. Comparative Investigation of Three Series of Poly(Arylene Ether Ketone) Copolymers for Proton Exchange Membrane Fuel Cells Yanxiang Li, Abhishek Roy, Anand Badami, Melinda Einsla, Thekkekara Mukundan, and James E. McGrath* Macromolecular Science and Engineering & Macromolecules and Interfaces Institute Virginia Polytechnic Institute and State University Blacksburg, VA 24061
164
6.1. Abstract
This paper has comparatively investigated three series of disulfonated
poly(arylene ether ketone) random copolymers for proton exchange membrane fuel cells,
which are becoming increasingly important as alternative energy sources with high
efficiency and no pollution for stationary, automobile and portable power. These
copolymers were synthesized based on 4,4’-hexafluoroisopropylidenediphenol (6F-BPA),
one of the three ketone-type monomers [4,4’-difluorobenzophenone (DFBP), 1,4-bis (p-
fluorobenzoyl) benzene (PBFB), and 1,3-bis(p-fluorobenzoyl) benzene (MBFB)], and
their corresponding disulfonated comonomers. The synthesis followed the direct
copolymerization method and the sulfonic acid content of each polymer was controlled
by varying the feed ratio of the non-sulfonated and sulfonated dihalide monomers. The
membrane properties such as water uptake, proton conductivity, and morphology were
studied and compared among these three series. It was found that the copolymers
containing more flexible molecular structures had better phase separation, higher water
uptake and proton conductivity than the copolymers with rigid structures at the same ion
exchange capacity (IEC). The primary methanol permeabilities of copolymers with
mono-ketone units showed lower values than Nafion®.
Keywords: Disulfonated Poly(Arylene Ether Ketone) Copolymers; Proton Exchange
Membrane; Fuel Cells; Direct Copolymerization
165
6.2. Introduction
It is well known that the applicable proton exchange membrane (PEM) materials
for both hydrogen/air fuel cell and direct methanol fuel cell (DMFC) systems should
possess the following characteristics: high protonic and low electronic conductivity, low
permeability to fuel and oxidant, low water uptake and water transport, oxidative and
hydrolytic stability, good mechanical properties, low cost, and capability for fabrication
into membrane electrode assemblies (MEAs).1 To substitute the current commercialized
perfluorosulfonic acid Nafion® type PEM materials which are limited by their loss of
conductivity at high temperatures, high methanol permeability in DMFCs, and high cost,
many strategies have been explored, including synthesis of new functionalized proton
conducting polymers, better understanding and control of polymer microstructure,
polymer blends, and composite membranes.2
Sulfonated poly(arylene ether ketone)s are a family of important candidates for
PEMs in fuel cells. These materials have excellent physical properties, including high
modulus, toughness, and good thermal and chemical resistance.3, 4 Generally, the sulfonic
acid groups have been introduced to the polymer backbone via two methods: post
modification of the commercially available polymers by sulfonating agent or directly
copolymerizing sulfonated comonomers. Direct copolymerization of sulfonated
comonomers has been proven to be better than the post sulfonation method, in that it
allows for precise control the degree of sulfonation and avoids several side reactions.5, 6
These ion containing copolymers tend to phase separate into hydrophobic and
hydrophilic domains. It was believed that the phase-separated morphology of these ionic
166
materials played a dominant role in the hydration and conductivity of membranes.7, 8
Research in the McGrath group9-10 showed that the better phase separation improved
several properties of PEMs, including water swelling and proton conductivity, especially
at lower relative humidity. The effect of polymer molecular structure on the membrane
properties is worthy of investigation, since it will provide useful information for the
molecular design of future PEM materials. It was reported that for different chemical
structures, copolymers with higher hydration numbers had higher proton conductivity,
irrespective of the sulfonic acid content. The hydration number of the membrane, which
is the mass-based water uptake, is directly related to the polymer chemical structure.
Therefore, this value can be used to compare membranes of different polymer
architectures.9
In this paper, three disulfonated ketone-type comonomers were synthesized, and
the chemical structures are shown in Figure 6.1. These comonomers were subjected to
direct step growth copolymerization with their corresponding unsulfonated monomers
and 4,4’-hexafluoroisopropylidenediphenol (6F-BPA) to obtain three series of
copolymers containing sulfonic acid groups: B series with mono ketone units, and PB and
MB series with para diketone and meta diketone units, respectively. Previously,
McGrath’s group had reported the primary results of B and PB series copolymers for
PEMs.11 More and detailed results will be shown and discussed here. In this work, the
consistent membrane preparation and acidification methods for all the membranes were
used, and followed the high temperature acidification method, which was proven to result
in better phase separated morphology than the room temperature method.12 The goals of
this research are to explore the novel poly(arylene ether ketone) PEM materials, and give
167
some insight into how the chemical structures affect the membrane morphology and the
PEM properties.
In this research, three ketone monomer structures were chosen to examine the
structure-property relationships of the PEM membranes under consistent conditions. It
was found that the membranes with mono ketone (B series) and para diketone (PB series)
rigid structures had similar water uptake and proton conductivity at similar ion exchange
capacity (IEC) values, while the meta diketone (MB series) structures had higher values
in both water uptake and proton conductivity at the same IEC. This may be due to two
reasons: one potential explanation is that the higher flexibility of meta ketone linkages
resulted in easier aggregation of the hydrophilic and hydrophobic domains, which was
also reflected in the membrane morphology characterized by atomic force microscopy
(AFM); another reason may be caused by the higher hydration level (or bigger hydration
number) of the MB series copolymers structures at the same IEC values. The partially
fluorinated monomer 6F-BPA was used here to decrease the crystallinity of the ketone
copolymers and thus improve their solubility. Furthermore, the methanol permeability
and MEA proton conductivities of the B series copolymers were also studied and
compared with Nafion®.
168
F CO
F
NaO3S SO3Na
F CO
NaO3S
F
SO3Na
CO
F
SO3Na
CO
F CO
NaO3S
SDFBP
SPBFB
SMBFB
Figure 6.1. Three Disulfonated Ketone-Type Comonomer Structures
169
6.3. Experimental
6.3.1. Materials
4,4’-Hexafluoroisopropylidenediphenol (6F-BPA), received from Ciba, was
purified by sublimation. The ketone-type monomers 4,4’-difluorobenzophenone (DFBP),
1,4-bis(p-fluorobenzoyl)benzene (PBFB), and 1,3-bis(p-fluorobenzoyl)benzene (MBFB)
were purchased from Aldrich and used as received. Disulfonated derivatized
comonomers, 3,3’-disodium sulfonate-4,4’-difluorobenzophenone (SDFBP), 1,4-bis(3-
sodium sulfonate-4-fluorobenzoyl)benzene (SPBFB), and 1,3-bis(3-sodium sulfonate-4-
fluorobenzoyl)benzene (SMBFB), were synthesized in house. All these monomers were
well dried under vacuum prior to use. The solvent N-methyl-2-pyrrolidinone (NMP,
Fisher) was vacuum-distilled from calcium hydride onto molecular sieves. Potassium
carbonate (Aldrich) was dried in vacuo before use. Toluene, sodium chloride, fuming
sulfuric acid (30% free SO3), and isopropanol were obtained from Aldrich and used as
received. Nafion® 117 was obtained from ElectroChem.
6.3.2. Synthesis of the Disodium Salt of Comonomers
Disulfonated derivatized comonomers (SDFBP, SPBFB, SMBFB, Fig. 6.1) were
synthesized according to a modified literature method.13, 14 For example, SDFBP was
synthesized as follows: DFBP (30 g, 0.14 mol) was dissolved in 60 mL of 30% fuming
sulfuric acid in a 100-mL, three-necked flask equipped with a mechanical stirrer and a
nitrogen inlet/outlet. The solution was heated to 120 oC for 6 h to produce a
homogeneous solution. Then it was cooled to room temperature, and poured into 450 mL
170
of ice water. Next, NaCl (110.0 g) was added which produced a white solid. The
precipitate was filtered and re-dissolved in 300 mL of deionized water. The solution was
treated with 2 N NaOH to a pH of 6~7 and diluted with deionized water to a total volume
of 500 mL. Then, NaCl (110.0 g) was again added to salt out the sodium-form sulfonated
monomer. The crude product was recrystallized twice from a mixture of isopropyl
alcohol and deionized water (3/1 v/v). The white powder-like disulfonated comonomers
were dried in a vacuum oven at 160 oC for two days.
6.3.3. Synthesis of Disulfonated Poly(Arylene Ether Ketone) Copolymers (B, PB and
MB) Based on Three Types of Ketone Monomers
The copolymerization procedures for these three polyketone series were similar.
The B-xx series copolymers were base on mono ketone monomers DFBP, SDFBP, and
6F-BPA, PB-xx series copolymers were based on para diketone monomers PBFB,
SPBFB, and 6F-BPA, while the MB-xx series copolymers were based on meta diketone
monomers MBFB, SMBFB, and 6F-BPA. In each series, the number xx represents the
theoretical molar percentage of disulfonated repeat units. A typical polymerization for
PBFB-30 was described as follows: 6F-BPA (3.3623 g, 10mmol), PBFB (2.2562 g, 7
mmol), and SPBFB (1.5792 g, 3 mmol) were added to a 3-neck flask equipped with
mechanical stirrer, nitrogen inlet and a Dean-Stark trap. Next, 1.15 equivalent of
potassium carbonate and NMP were introduced to afford a 33% solids concentration.
Toluene (NMP/Toluene=3/4, v/v) was used as an azeotroping agent. The reaction mixture
was heated under reflux at 150 oC for 4 h to dehydrate the system and remove most of the
toluene. Next, the temperature was raised slowly to 175 oC. The solution became viscous
171
after about 7 h and was subsequently cooled to room temperature and diluted with NMP.
The product was then isolated by addition to stirred deionized water. The precipitated
fibrous copolymer was heated around 50 oC in deionized water overnight, then filtered
and vacuum dried at 120 oC for 24 h.
6.3.4. Membrane Preparation and Acidification
Copolymer (1.0 g) was dissolved in DMAc (20 mL). The solutions were first
filtered with 0.45-µm Teflon syringe filters, and then cast onto clean glass substrates. The
film was carefully dried with infrared heat at gradually increasing temperature (up to ~ 60
oC) under a nitrogen atmosphere. Transparent, flexible films were lifted by immersing
them in deionized water. The sodium-form membranes were converted into the acid form
by boiling in 0.5 N H2SO4 for 2 h, followed by boiling in deionized water for 2 h. The
acid-form films were stored in fresh deionized water.
6.3.5. Characterization
1H NMR spectra were obtained with a Varian 400 MHz spectrometer using
DMSO-d6 as a solvent. FTIR spectra were measured with a Nicolet Impact 400 FT-IR
spectrometer with thin homogenous films. The thin films were cast from dilute DMAc
solutions and completely dried in an oven. Intrinsic viscosity (IV) measurements were
obtained in NMP with 0.05 M LiBr as a solvent at 25 oC using a Cannon Ubbelohde
viscometer. Number average molecular weights (Mn) of copolymers were determined by
gel permeation chromatography based on polystyrene standards.
172
Atomic force microscopy (AFM) images were obtained using a Digital
Instruments MultiMode scanning probe microscope with a NanoScope IVa controller
(Veeco Instruments, Santa Barbara, CA) in tapping mode. A silicon probe (Veeco) with
an end radius of <10 nm and a force constant of 5 N·m-1 was used to image samples.
Samples were equilibrated at 30% RH for at least 12 h before being imaged immediately
at room temperature and approximately 15-33% RH.
Membrane water uptake was determined by a simple weight difference approach:
The sample films were equilibrated in deionized water at room temperature for at least 48
h. The membranes were then dried in the vacuum oven at 110 oC for 24 h. Weights of wet
and dry membranes were measured. The ratio of weight gain to initial film weight was
expressed as % water uptake. The hydration number (λ), the number of water molecules
absorbed per sulfonic acid, can be calculated from the mass water uptake and the ion
content of the dry copolymer as shown in the following equation:
λ = [(masswet - massdry)/ OHMW2
]/IEC *massdry
where OHMW2
is the molecular weight of water (18.01 g·mol-1) and IEC is the ion
exchange capacity of the dry copolymer in equivalents per gram.
Protonic conductivity at 30 °C under full hydration (in liquid water) was
determined using a Solartron 1260 Impedance/Gain-Phase Analyzer over the frequency
range of 10 Hz - 1 MHz. The cell geometry was chosen to ensure that the membrane
resistance dominated the response of the system. The resistance of the film was taken at
the frequency which produced the minimum imaginary response. The conductivity of the
membrane can be calculated from the measured resistance and the geometry of the cell
173
according to equation: AZl
'=σ , where σ is the protonic conductivity, l is the length
between the electrodes, A is the cross sectional area available for proton transport, and Z’
is the real impedance response. In determining protonic conductivity in liquid water,
membranes were equilibrated at 30 °C in deionized water for 24 h prior to the testing.
Methanol permeability of the membranes was determined by measuring the
crossover current in a DMFC at open circuit. The measurement was performed in an
identical manner to Ren et al.16
6.4. Results and Discussion
6.4.1. Synthesis and Characterization of Disulfonated Monomers and Copolymers
The sulfonation agent, 30% fuming sulfuric acid, was used to synthesize the
disulfonated comonomers SDFBP,SPBFB,and SMBFB via electrophilic aromatic
substitution. It was found that the sulfonation of the two diketone monomers (SPBFB and
SMBFB) required longer reaction time (16 h) than the mono ketone monomer (6~7 h) at
the same reaction temperature (120 oC). This was due to the two carbonyl electron
withdrawing groups of the diketone monomers, which lowered the electron density on the
benzene ring, and thus decreased the reactivity in the electrophilic aromatic substitution
reactions. Although the reaction temperature for the synthesis of mono ketone SDFBP
and para di-ketone SPBFB could be varied between 110 oC and 150 oC without changes
in the product, it was necessary to control the temperature below 120 oC for the meta
diketone monomer (SMBFB) synthesis. It was noted that the MBFB monomer was easily
oxidized when the reaction temperature was raised higher than 120 oC, as evidenced by
174
the small impurity peaks in the 1H NMR spectra. All these disulfonated comonomer
structures were characterized and confirmed by 1H NMR spectra. Figure 6.2 showed a
representative 1H NMR spectrum of SMBFB comonomer.
Three series of poly(arylene ether ketone) copolymers with high molecular
weights were synthesized via a step-condensation process (Fig. 6.3). Table 6.1 lists the
1H NMR, IV, IEC, and proton conductivity data for these copolymers. It was not
surprising that the two diketone copolymers (PB and MB series) were much easier to
synthesize and obtain relatively higher molecular weights than the mono ketones (B
series), because the fluorine functional groups in these two series were activated by the
two carbonyl electron-withdrawing groups, and were more easily attacked by the
nucleophiles. The high molecular weights were confirmed by the high intrinsic viscosity
values, which were measured via the modified procedure using NMP as the solvent with
0.05 M LiBr salt to suppress the polyelectrolyte effect.16, 17 These copolymers were
amorphous and soluble in polar aprotic solvents due to the flexible
hexafluoroisopropylidene linkages and the introduction of the sulfonic acid groups.
Copolymers with target acid content (or degree of sulfonation) of 20 to 50 mol%
were prepared by adjusting the sulfonated vs. unsulfonated monomer feed ratio. 1H NMR
was used to identify the copolymer structures and calculate the experimental degree of
sulfonation. For example, the 1H NMR integrations for the protons attached to the
sulfonated unit (peak “i”, 8.3 ppm) and the overlapping peaks from protons on both the
sulfonated and unsulfonated units (peak “b+f”, 7.35 ppm) were used to calculate the
degree of sulfonation for the MB-30 copolymer (Fig. 6.4). The mole percent of
175
sulfonated units in the MB copolymer was calculated according to the proton number
relationship as described in the following equation:
Degree of sulfonation % =
The experimental degree of sulfonation of all copolymers was determined using the
above method (Table 6.1). These experimental values agreed with the theoretical target
values very well. IEC is an important parameter which can affect other membrane
properties like morphology, water uptake and protonic conductivity. The IEC values of
all copolymers were calculated from the corresponding degree of sulfonation (Table 6.1).
Due to the bigger molecular structures of the diketone monomers, the PB and MB series
copolymers had the same IEC values at the same degree of sulfonation, but lower than
the B series copolymers. For example, PB-50 and MB-50 copolymers both had IEC
values around 1.40 meq·g-1, and this value was lower than B-50, but similar to B-40.
The IR spectra qualitatively confirmed the functional groups of the synthesized
copolymers for all series. The FTIR spectrum of the MB series of copolymers is shown in
Figure 6.5. Peaks at 1030 cm-1 and 1087 cm-1 correspond to symmetric and asymmetric
stretching of the sodium sulfonate groups. These two peaks increased with increasing
degree of sulfonation as expected.
%1004/)(
2/×
+ fbi
176
Figure 6.2. 1H NMR Spectrum of SMBFB Disulfonated Comonomer
c
CO
F
NaO3S
CO
F
SO3Nac
ba
d
ef
e
d
bf
a
177
CCF3
CF3O
F F CCF3
CF3OHHO
O CCF3
CF3O
F F
SO3NaNaO3S
SO3MMO3SO
X X+ +
X Xn 1-n
Toluene, NMP, K2CO3150 oC, reflux 4 h175 oC, 7 h
CO
CO
CO
"B" "PB"
X = , or CO
CO
"MB"
Figure 6.3. Synthetic Scheme of Three Series of Disulfonated Poly(Arylene Ether
Ketone) Copolymers
178
Table 6.1. Characterization of B/PB/MB Series of Copolymers with Different Degree
of Sulfonation
Copolymers %sulfonation
by 1H NMR
I.V.(dL·g-1)a IEC(meq·g-1) Proton Conductivity
(S·cm-1) (30 oC in H2O)
B-20 18 0.51 0.72 -
B-30 29 0.50 1.05 0.021
B-40 40 0.66 1.38 0.071
B-50 47 0.45 1.70 0.092
PB-20 22 1.06 0.62 -
PB-30 29 0.62 0.90 0.010
PB-40 40 1.16 1.17 0.038
PB-50 48 1.43 1.43 0.076
MB-20 19 1.12 0.62 0.010
MB-30 28 0.69 0.90 0.035
MB-40 39 0.73 1.17 0.060
MB-50 50 0.74 1.43 0.085
a. Intrinsic Viscosity (I. V.) was measured in 0.05 M LiBr/NMP at 25 oC
179
i
b, f
a, c, e, g
d, j, j’k, k’m,m’
CO
CO
O* C
CF3
CF3
O * C
CF3
O CO
CO
*
SO3NaNaO3S
CF3x 1-x
a b b a cc dd
e
e ff
g h
ik k'
j j'm m'
i
b, f
a, c, e, g
d, j, j’k, k’m,m’
CO
CO
O* C
CF3
CF3
O * C
CF3
O CO
CO
*
SO3NaNaO3S
CF3x 1-x
a b b a cc dd
e
e ff
g h
ik k'
j j'm m'
Figure 6.4. 1H NMR was Used to Calculate the Degree of Sulfonation of the
Copolymers (MB-30)
180
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
950 1000 1000 1100 1200 cm-1
Abs
orba
nce
MB-50
MB-40
MB-30MB-20
MB-00
S=O asym. str.
S=O sym. str.
Wave Number (cm-1)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
950 1000 1000 1100 1200 cm-1
Abs
orba
nce
MB-50
MB-40
MB-30MB-20
MB-00
S=O asym. str.
S=O sym. str.
Wave Number (cm-1)
Figure 6.5. IR Spectra of MB Series Copolymers
181
6.4.2. Morphology Characterization of the Membranes
There is much evidence that the morphology of PEMs affects their transport
properties.9,12,18 Sharper phase separation means that the hydrophilic transport channels
are more well-defined, which usually results in improved proton conduction properties.
The membrane morphology can be affected by many factors, such as the chemical
structure, IEC value, water uptake, membrane preparation conditions, and the
acidification method, etc. Previous research reported12 that copolymers with higher IEC
values had better phase separation, while at the same IEC value the high temperature
acidification method improved the phase separation compared to the room temperature
acidification method. In this research, three series of copolymer membranes were
prepared under the same conditions and all acidified using the high temperature method.
The morphology of the membranes was characterized by tapping mode AFM to
investigate the effect of chemical structure on phase separation.
The AFM images of two groups of membranes with similar IEC values were
compared in Figure 6.6. The left group of copolymers (B-30, PB-40, and MB-40) had
lower IEC values around 1.1~1.2 meq·g-1, while the right group of copolymers (B-40,
PB-50, and MB-50) had higher IEC values around 1.4 meq·g-1. It was obvious that for the
same chemical structures, better phase separation was observed for the copolymers with
higher IEC. But at the same IEC value, the meta diketone MB series showed sharper
phase separation and bigger phase domain than the mono ketone and para diketone B and
PB series. This improved morphology for the MB series membranes may be caused by
the more flexible meta carbonyl linkages, which allowed for more aggregation of the
hydrophilic domains during membrane formation.
182
(a) 100 nm
(b) 100 nm
(c) 100 nm
(d) 100 nm
(e) 100 nm
(f) 100 nm
(a) 100 nm
(b) 100 nm
(c) 100 nm
(a) 100 nm(a) 100 nm
(b) 100 nm(b) 100 nm
(c) 100 nm(c) 100 nm
(d) 100 nm(d) 100 nm
(e) 100 nm(e) 100 nm
(f) 100 nm(f) 100 nm
Figure 6.6. AFM Image of Copolymer Membranes: (a) B-30, (b) PB-40, (c) MB-40,
(d) B-40, (e) PB-50, (f) MB-50. The IEC value of the left group copolymers is around
1.1-1.2 meq·g-1, and the right groups is around 1.4 meq·g-1
183
6.4.3. Characterization of Membrane Properties
As discussed earlier, MB series copolymers tend to have better hydrophobic and
hydrophilic phase separation than the other two series at similar IEC. It is known that the
phase separation is a function of copolymer chemical structure and is critical for both
proton and water transport properties. In order to understand the effect of the chemical
structure on the membrane properties, the water uptake and proton conductivities of B,
PB and MB copolymers were measured and compared under fully hydrated conditions.
It has been widely reported that both the proton conductivity and water uptake of
sulfonated materials increase with increasing IEC. Figure 6.7 and Figure 6.8 show the
same trends within these three series of copolymers. The difference in the structure of
these three copolymers resulted in different properties. The MB series showed more
water absorption and higher proton conductivity compared with the other two series at
similar IEC. It is likely that the two carbonyl groups linked on the meta positions of the
benzene ring resulted in less ordered packing and thus larger free volume between
polymer chains, in which more water molecules could be confined, and this could also
lead to the higher hydration numbers. The hydration number is the water uptake on a
mass basis and it is more related to the polymer backbone architectures. Research has
shown that the proton conductivity at fully hydrated conditions for the copolymers with
similar acidity was a linear function of hydration number.9 Figure 6.9 shows proton
conductivity as a function of hydration number for all copolymers. The hydration number
is more important than IEC in terms of proton conductivity. For example, MB-30 had
much lower IEC value than PB-40, but they had similar proton conductivities due to the
similar hydration numbers.
184
Temperature also affected the proton conductivity for all the copolymers. The
proton conductivities of the PB copolymers as a function of temperature are given in
Figure 6.10. The proton conductivities increased with temperature and with the degree of
sulfonation or IEC, and this was the general trend for the B and MB series as well. The
proton conductivities of these random copolymers under partially hydrated conditions
were measured. It was found that the proton conductivities decreased dramatically at low
relative humidities even for the MB series. This phenomenon can be at least partially
explained by the water uptake at various humidity conditions for the B series of
copolymers (Fig. 6.11). When the relative humidity was lower than 80%, the water
uptake decreased prominently, which resulted in an isolated morphology, and hindered
proton transport.
One of the major disadvantages of Nafion® is its high methanol permeability. The
methanol permeability of the B series of copolymers increased with increasing
temperature and IEC, but all of the copolymers in this series had lower methanol
permeability than Nafion® 117 in the temperature range from 30 to 80 °C (Fig, 6.12). The
methanol permeabilities of B-40 and B-30 were much lower than Nafion® and increased
only modestly with temperature compared to the higher IEC B-50 and Nafion®, which
demonstrated the application potential in DMFC of the B-30 and B-40 copolymers.
Differences in the proton conductivity between liquid water and in-MEA can
reflect the interfacial loss or the compatability of the MEA, which is important in
practical applications. The relatively small losses for B series copolymers (Table 6.2)
mean that these copolymers were compatible with the Nafion-bonded electrodes and can
185
be fabricated into robust MEAs, although the long term durability has not yet been
determined.
186
Figure 6.7. Influence of IEC and Copolymer Structure on Water Uptake of the
Membranes (Acid Form) in Liquid Water at Room Temperature
0
20
40
60
80
100
120
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
IEC (meq/g)
Wat
er U
ptak
e (m
ass%
)
MB ketonePB ketoneB ketone
187
0
10
20
30
40
50
60
70
80
90
100
0.4 0.9 1.4
IEC (meq/g)
Pro
ton
cond
uciv
ity (m
S/cm
)
MB ketonePB ketoneB ketone
MB-30PB-40
B-30
0
10
20
30
40
50
60
70
80
90
100
0.4 0.9 1.4
IEC (meq/g)
Pro
ton
cond
uciv
ity (m
S/cm
)
MB ketonePB ketoneB ketone
MB-30PB-40
B-30
Figure 6.8. Proton Conductivity vs. IEC of Three Ketone Type Copolymers in
Liquid Water at Room Temperature
188
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50
Hydration Number
Prot
on c
ondu
civi
ty (m
S/cm
)
MB ketonePB ketoneB ketonePB-40
MB-30
B-30
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50
Hydration Number
Prot
on c
ondu
civi
ty (m
S/cm
)
MB ketonePB ketoneB ketonePB-40
MB-30
B-30
Figure 6.9. Proton Conductivity in Liquid Water Tends to Depend on Hydration
Number (RT)
189
2
2.5
3
3.5
4
4.5
5
5.5
6
2.8 2.9 3 3.1 3.2 3.3 3.41000/T (oK-1)
Ln σ
(mS/
cm)
1. PB-diketone 50
2. PB-diketone 40
3. PB-diketone 30
1
2
3
2
2.5
3
3.5
4
4.5
5
5.5
6
2.8 2.9 3 3.1 3.2 3.3 3.41000/T (oK-1)
Ln σ
(mS/
cm)
1. PB-diketone 50
2. PB-diketone 40
3. PB-diketone 30
1
2
3
Figure 6.10. Effect of Temperature on Protonic Conductivity in Liquid Water
190
Figure 6.11. Influence of Copolymer Composition on Water Sorption of the B Series
as a Function of Humidity
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 20 40 60 80 100 120
Relative humidity (%)
Wat
er u
ptak
e (m
ass
%) 1, B-20
2, B-303, B-404, B-50
1
2
3
4
191
Figure 6.12. Influence of Copolymer Composition and Temperature on Methanol
Permeability
0
1
2
3
4
5
6
30 60 80Temperature (oC)
Met
hano
l Per
mea
bilit
y x
10-6
(cm
2 /s)
B-50B-40B-30Nafion 117
192
Table 6.2. Comparisons of Thin Film Protonic Conductivity in Liquid Water to
That of MEA
Sample Protonic conductivity in
liquid water (S·cm-1) @80 0C
Protonic conductivity in MEA (S·cm-1)
@80 0C
Percent decrease
(%)
Methanol Permeability
(E-06 cm2·s-1) @ 80 °C
B-50 0.15 0.14 7 4.79 B-40 0.13 - - 1.99 B-30 0.04 0.03 25 1.48
193
6.5. Conclusions
Three series of poly(arylene ether ketone) copolymers based on 6F-BPA, three
sulfonated ketone-type comonomers and their corresponding unsulfonated monomers
have been synthesized. The degree of sulfonation was controlled from 20 to 50 mol%.
The intrinsic viscosity measured with a modified method in 0.05 M LiBr/NMP confirmed
that the molecular weights of the copolymers were high enough to form tough, flexible
films. The membrane properties of these three series of copolymers have been
comparatively studied. Copolymers containing a meta diketone unit (MB series) showed
higher water uptake and proton conductivities compared with the mono ketone (B series)
and para diketone (PB series) at similar IEC values. This may be caused by the more
flexible MB chemical structures which resulted in better phase separation and higher
hydration levels. The B series copolymers also showed lower methanol permeability
compared to Nafion® and low interfacial loss in MEA measurements.
6.6. Acknowledgements
The author would like to acknowledge the Department of Energy for funding
(contract # DE-FC36-01G011086).
194
6.7. References
1. Hickner, M.A.; Ghassemi, H.; Kim, Y.S.; Einsla, B.R.; and McGrath, J.E.; Chem. Rev., 2004, 104, 4587.
2. Tchicaya-Bouckary, L.; Jones, D.J.; and Roziere. J., Fuel Cells 2002, 2, No. 1, 40. 3. Kroschwitz, J. I., Eds; Encyclopedia of Polym. Sci. and Eng. 1988, 12, 313. 4. Kreuer, K.D., Solid State Ionics 1997, 97 (1–4), 1. 5. Liu, S.; Wang, F.; Chen, T.; Macromol. Rapid Commun. 2001, 22, 579. 6. Harrison, W. L.; Wang, F.; Mecham, J. B.; Bhanu, V. A.; Hill, M.; Kim, Y. S.;
McGrath, J. E.; J. Polym. sci.: Part A: Polym. Chem., 2003, 41,2264. 7. Kreuer, K.D.; Solid State Ionics, 2000, 149, 136. 8. Kim, Y.S.; Dong, L.; Hickner, M.A.; Pivovar, B.S.; McGrath, J.E., Polymer, 2002, 44,
5729. 9. Roy, A.; Hickner, M.A.; Yu, X.; Li, Y.; Glass, T.E.; McGrath, J.E.; J. Polym. Sci.,
Part B: Polym.Phys. 2006, 44, 2226. 10. Li, Y.; Roy, A.; Badami, A. S.; Hill, M.; Yang, J.; Dunn, S; McGrath, J. E., J. Power
Sources, 2007, In press. 11. Li, Y.; Mukundan, T.; Harrison, W.; Hill, M.; Sankir, M.; Yang, J.; McGrath, J. E.;
Prepris Symp. ACS Div. Fuel Chem., 2004, 49(2), 536. 12. Kim, Y.S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y.T.; Harrison, W.;
Zawodzinski, T.A.; McGrath, J.E., J. Polym. Sci., Part B: Poly. Phys., 2003, 41, 2816. 13. Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.; Zawodzinski, T. A.;
McGrath, J. E., Macromol Symp, 2001, 175, 387. 14. Sankir, M.; Bhanu, V. A.; Harrison, W. L.; Ghassemi, H.; Wiles, K. B.; Glass, T. E.;
Brink, A. E.; Brink, M. H.; McGrath, J. E., J. Appl. Polym. Sci., 2006, 100, 4595 15. Ren, X. M.; Springer, T. E.; Zawodzinski, T. A.; Gottesfeld, S., J. Electrochem. Soc.,
2000, 147, 466. 16. Li, Y.; Wang, F.; Yang, J.; Liu, D.; Roy, A.; Case, S.; Lesko, J.; McGrath, J.
E., Polymer, 2006, 47, 4210.
195
17. Yang, J.; Li, Y.; Wang, H.; Hill, M.; Yu, X.; Wiles, K.B.; Lee, H.; McGrath, J.E., Preprs. Symp. ACS, Div. Fuel Chem., 2005, 50(2), 701
18. Badami, A.S.; Lee, H.S.; Li, Y.; Roy, A.; Wang, H.; McGrath, J. E., Preprs. Symp.
ACS, Div. Fuel Chem., 2006,51(2), 612.
196
Chapter 7. Synthesis and Characterization of Partially Fluorinated Hydrophobic - Hydrophilic Multiblock Copolymers Containing Sulfonate Groups for Proton Exchange Membrane
Taken From:
Yanxiang Li1, Abhishek Roy1, Anand S. Badami1, Melinda Einsla2, Juan Yang1, Stuart
Dunn1, James E. McGrath1*
1Macromolecular Science and Engineering & Macromolecules and Interfaces Institute
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061, USA
2MPA-11: Sensors and Electrochemical Devices, Los Alamos National Laboratory, Los
Alamos, NM 87544, USA
Journal of Power Sources, 2007, 172, 30-38
Reprinted with permission from Elsevier, copyright (2007)
197
7.1. Abstract
A new hydrophobic-hydrophilic multiblock copolymer has been successfully
synthesized based on the careful coupling of a fluorine terminated poly(arylene ether
ketone) (6FK) hydrophobic oligomer and a phenoxide terminated disulfonated
poly(arylene ether sulfone) (BPSH) hydrophilic oligomer. 19F and 1H NMR spectra were
used to characterize the oligomers’ molecular weights and multiblock copolymer’s
structure. The comparison of the multiblock copolymer 13C NMR spectrum with that of
the random copolymer showed that the transetherfication side reaction was minimized in
this synthesis. The morphologies of membranes were investigated by tapping mode
atomic force microscopy (AFM), which showed that the multiblock membrane acidified
by the high temperature method has sharp phase separation. Membrane properties like
protonic conductivity, water uptake, and self-diffusion coefficient of water as a function
of temperature and relative humidity (RH) were characterized for the multiblock
copolymer and compared with ketone type random copolymers at similar ion exchange
capacity value and Nafion® controls. The multiblock copolymers are promising
candidates for proton exchange membranes especially for applications at high
temperatures and low relative humidity.
Keywords: Proton Exchange Membrane Fuel Cell, Poly(Arylene Ether Ketone),
Poly(Arylene Ether Sulfone), Multiblock Copolymer, Morphology
198
7.2. Introduction
Proton exchange membrane (PEM) materials have attracted much attention due to
the environmentally friendly nature of PEM fuel cells and their potential applications in
automobiles, stationary power, and small electronics.1,2 Currently nearly all commercially
available membranes are based on copolymers containing perfluorosulfonic acid groups
such as DuPont’s Nafion®. Nafion®-type materials have exceptional oxidative and
chemical stability as well as high protonic conductivity, which are critical to PEM fuel
cells. But they have limitations like low performance at high temperature due to a low
hydrated Tg value, high methanol permeability in direct methanol fuel cells, and high
cost.2,3 Therefore, many polymeric materials with ionic groups have been explored as
alternative PEM candidates, such as poly(arylene ether)s,4-8 polyimides,9,10 poly(arylene
sulfide sulfone)s,11 substituted polyphenylenes,12 etc. The wholly aromatic partially-
disulfonated poly(arylene ether sulfone) (BPSH) random copolymer developed in the
McGrath group6 is a potential PEM candidate due to its good acid and thermal oxidative
stabilities, high glass transition temperature and excellent mechanical strength.13 For
example, with 35 mol percent degree of sulfonation, the BPSH copolymer has excellent
oxidative stability as shown in open circuit test. The test was conducted at 100 oC under
H2/O2 environment at 25% RH. BPSH outperformed the benchmarked material Nafion®
and was stable up to 300 h. This inherent stability was attributed to the extremely low
oxygen permeability (10 times lower than Nafion®).14
Most of the copolymers developed are random or “statistical” copolymers
because monomer units are connected irregularly and the sulfonic acid groups are also
199
randomly distributed along the copolymer chain. These randomly located ionic groups
will lead to isolated morphological domains especially at a low hydration level, which
limit the transport properties. While under fully hydrated conditions, water assisted
percolated morphology ensures connectivity between the hydrophilic domains. As a
result, the random copolymers show satisfactory performance under fully hydrated
conditions, but they will lose the performance dramatically at low relative humidity. The
big challenge here is how to modify the chemistry of the polymers to obtain significant
proton conductivity at low hydration levels which will make the PEM fuel cell more
applicable under ambient environments. Recent research results showed that the
hydrophobic-hydrophilic block copolymers with tailored chemical structure of the
polymer backbone may achieve this goal.15
Multiblock thermally stable copolymers are interesting because the morphology
of the copolymer membrane can be better controlled by varying the two sequences length
in the multiblock structures.16 In hydrophobic-hydrophilic multiblock copolymers, the
ionic groups located within the hydrophilic blocks provide protonic conductivity, and the
hydrophobic blocks offer good mechanical strength. It is especially interesting when the
molecular weight ratio of the two blocks is about 1: 1, since a hydrophilic cocontinuous
phase may form under this condition, which may form associated hydrophilic domains
even under low hydration levels. This may greatly facilitate transport and the proton
conductance. Furthermore, the microphase separation in multiblock copolymers may be
helpful in controlling the water swelling and generating copolymers which have good
conductivity even at low relative humidity. The McGrath group17,22 has recently focused
on this topic and developed several series of thermally stable multiblock copolymers to
200
investigate both composition and chemical structure effects on PEM properties. It was
found that the cocontinuous phase can form at certain oligomer block lengths according
to atomic force microscopy (AFM) images, and under partially hydrated conditions the
block copolymers showed much improved proton conductivity over the random
copolymers.23,24 The block copolymers also showed higher self-diffusion coefficients of
water over the Nafion® control and random copolymers, suggesting a lower
morphological barrier to transport.15,23,24
In this paper, a novel multiblock copolymer [Fig. 7.1 (b)] has been successfully
synthesized based on a fluorine terminated hydrophobic poly(arylene ether ketone) (6FK)
oligomer and a phenoxide terminated hydrophilic poly(arylene ether sulfone) (BPSH)
oligomer. Keeping the chemical backbone similar to BPSH, one may hope to get the
same or even better oxidative stability. The comparison of 13C NMR spectra between the
multiblock copolymer and the random copolymer shows that the ether-ether interchange
side reaction, which may result in a randomized copolymer chain, has been minimized in
this copolymerization. The multiblock copolymer can form a tough film. Membrane
properties of this multiblock copolymer like morphology, proton conductivity, and water
uptake were characterized and compared with Nafion® control and ketone random
copolymers [Fig. 7.1 (a)]. This paper aims to understand that how the differences
between multiblock and random copolymer morphologies affect water and proton
transport. This will be instructional in designing better membranes for improved fuel cell
performance.
201
OCF3
CF3O Y O
CF3
CF3O Y
SO3HHO3S
X 1-X
Y = CO
("B")
or C CO O
( " PB' )
SO
OOO m
SO3HHO3S
CO
O CCF3
O
CF3n X
(a)
(b)
Figure 7.1. Copolymer Chemical Structures Studied in This Work (a) B-ketone-xx
and PB-diketone-xx Copolymers, (b) 6FK-BPSH Multiblock Copolymer
202
7.3. Experimental
7.3.1. Materials
4,4’-Hexafluoroisopropylidenediphenol (6F-BPA), received from Ciba, was
purified by sublimation. 4,4’-Difluorobenzophenone (DFBP) was purchased from
Aldrich, and biphenol (BP) was kindly provided by Eastman Chemical. They were used
as received. The ionic comonomer 3,3’-disulfonated 4,4’-dichlorodiphenyl sulfone
(SDCDPS) was synthesized as reported earlier.8 All these monomers were well-dried in a
vacuum oven before polymerization. The solvents N-methyl-2-pyrrolidinone (NMP) and
dimethylacetamide (DMAc) were vacuum-distilled from calcium hydride onto molecular
sieves. Potassium carbonate was dried in vacuo before use. Toluene, ethanol, and
isopropanol were obtained from Aldrich and used as received. B ketone-xx and PB
diketone-xx series random copolymers were synthesized in house,8,15 where xx refers to
the degree of sulfonation. Nafion® 112 and Nafion® 1135 were obtained from
ElectroChem.
7.3.2. Synthesis of Fluorine Terminated Hydrophobic Oligomers
Fluorine terminated poly(arylene ether ketone) oligomers (6FK) with target
molecular weights were synthesized via step growth polymerization. For example, the
4 kg·mol-1 6FK oligomer was prepared in a three-neck 100 mL flask with DFBP (2.580 g,
11.82 mmol) and 6F BPA (3.500 g, 10.41 mmol) dissolved in 20 mL DMAc, 1.65 g
potassium carbonate was added, and toluene (10 mL) was used as an azeotropic agent.
The reaction temperature was first set to 150 oC to dehydrate the system for about 4 h,
203
and the toluene was removed completely. The oil bath temperature was then raised to 175
oC for 16 h. The oligomer solution was cooled to room temperature and filtered to
remove most of the salt, then precipitated in IPA. The oligomers were collected by
filtration and washed with DI water and ethanol thoroughly to remove residual salt and
monomer residues. The resulting oligomers were dried in vacuo at 100 oC for at least
24 h.
7.3.3. Synthesis of Multiblock Copolymers
The step growth copolymerization employed a two-step procedure for the
multiblock copolymer synthesis. A copolymerization of a 4k-4k multiblock copolymer is
described as follows: First, phenoxide terminated disulfonated poly(arylene ether sulfone)
(BPSH) with target molecular weight 4 kg·mol-1 was synthesized by charging biphenol
(1.776 g, 9.54 mmol) and 3,3’-disulfonated 4,4’-dichlorodiphenyl sulfone (4.044 g, 8.23
mmol) to a three-neck 100 mL flask equipped with mechanical stirrer, nitrogen inlet and
a Dean Stark trap. Potassium carbonate (1.15 equivalents) and dry NMP were introduced
to afford 20% solid concentration. Toluene (NMP/Toluene = 2/1 v/v) was used as an
azeotropic agent. The reaction mixture was heated under reflux at 150 oC for 4 h to
remove water. Then, the bath temperature was raised slowly to 190 oC for 16 h. The
oligomer solution was cooled to 160 oC for the next step reaction without isolation. In the
second step, 6FK/NMP solution was added dropwise to the BPSH system. Then the
temperature was raised again to 190 oC for 2 days. The copolymer was isolated by
precipitation in IPA and deionized water (1:1), filtered, and dried in a vacuum oven for
24 h at 120 oC.
204
7.3.4. Characterization of Oligomers and Multiblock Copolymers
The 19F, 1H and 13C NMR spectra were conducted with a Varian Unity 400
NMR spectrometer. Solvent CDCl 3 was used for the hydrophobic oligomers, and DMSO-
d6 was used for the hydrophilic oligomers and multiblock copolymers. Intrinsic
viscosities (IV) were determined in NMP at 25 oC using an Ubbelohde viscometer for
ionic copolymers with 0.05 M LiBr in the NMP solvent to suppress the polyelectrolyte
effect.25
7.3.5. Membrane Preparation and Acidification
The salt form copolymers were redissolved in DMAc to afford transparent 5
wt% solutions, which were then cast onto clean glass substrates. The films were slowly
dried for 48 h with infrared heat at gradually increasing temperatures, and then dried
under vacuum at 110 oC for 2 days. Two methods can be employed to convert the sodium
salt form membranes to their acid form.26 In Method 1, the membranes were immersed in
1.5 M sulfuric acid solution at 30 oC for 24 h followed by immersion in deionized water
at 30 oC for 24 h. Method 2 involved boiling the membranes in 0.5 M H2SO4 for 2 h, and
then boiling in deionized water for another 2 h to remove any residual acid. Membranes
were stored in deionized water after the acidification until they were used for
measurements.
7.3.6. Characterization of Membranes
7.3.6. 1 Morphology Characterization by Atomic Force Microscopy (AFM)
205
Atomic force microscopy (AFM) images were obtained using a Digital
Instruments MultiMode scanning probe microscope with a NanoScope IVa controller
(Veeco Instruments, Santa Barbara, CA) in tapping mode. A silicon probe (Veeco) with
an end radius of <10 nm and a force constant of 5 N·m-1 was used to image samples.
Samples were equilibrated at 30% RH for at least 12 h before being imaged immediately
at room temperature and approximately 15-33% RH.
7.3.6.2. Ion Exchange Capacity (IEC) and Conductivity
Sulfonic acid concentration in the copolymers (IEC, mequiv·g-1) was
quantitatively determined by titration. The acid form membrane was immersed in 50~60
mL DI water with 1 M sodium sulfate. The solution was stirred overnight to allow the
protons to exchange with sodium completely. The solution was then titrated with 0.01 M
sodium hydroxide solution in which phenolphthalein was used as an indicator.
Proton conductivity was determined in a window cell geometry27 using a
Solartron 1252+1287 Impedance/Gain-Phase Analyzer over the frequency range of 10 Hz
to 1 MHz following the procedure reported in the literature.28 In determining proton
conductivity in liquid water, membranes were equilibrated at 30 oC in DI water for 24 h
prior to the testing. The temperature range chosen for calculation of activation energy for
proton transport was from 30 to 80 oC. For determining proton conductivity under
partially hydrated conditions, membranes were equilibrated in a humidity-temperature
oven (ESPEC, SH-240) at the specified RH and 80 oC for 6 h before each measurements.
7.3.6.3. Water Uptake and Water Self-diffusion Coefficients
206
The water uptake of the membranes was determined by measuring the
difference in the weight between dry and fully hydrated membranes. The sample films
were equilibrated in deionized water at room temperature for at least 48 h. The
membranes were dried in the vacuum oven at 110 oC for 24 h. Weights of wet and dry
membranes were measured. The water uptake was calculated as follows: water uptake %
= [(masswet - massdry)/ massdry] * 100%, where massdry and masswet refer to the mass of the
wet membrane and the mass of the dry membrane, respectively.
The hydration number (λ), number of water molecules absorbed per sulfonic
acid, can be calculated from the mass water uptake and the ion content of the dry
copolymer as shown in the equation: λ = [(masswet - massdry)/ OHMW2
]/IEC *massdry,
where OHMW2
is the molecular weight of water (18.01 g·mol-1) and IEC is the ion
exchange capacity of the dry copolymer in equivalents per gram.
Water self-diffusion coefficients were measured in a Varian Inova 400 MHz (for
protons) nuclear magnetic resonance spectrometer with a 30 G·cm-1 gradient diffusion
probe as described in the literature.15,29
7.3.6.4. MEA Fabrication and Fuel Cell Testing
Membrane electrode assemblies (MEAs) were prepared from protonated
membranes and standard unsupported Pt catalyst inks by procedures developed at Los
Alamos National Laboratory.30 The catalyst loading was approximately 6 mg·cm-2 on
both the anode and the cathode. The polymer binder in the catalyst layers was Nafion®,
and the active cell area was 5 cm2. Gas diffusion layers (GDLs) were comprised of
carbon cloth from E-TEK. The cell temperature was maintained at 80 °C and humidified
207
hydrogen (200 Sccm) and air (500 Sccm) were supplied to the anode and cathode,
respectively.
7.4. Results and Discussions
7.4.1. Synthesis and Characterization of Oligomer and Multiblock Copolymer
Figure 7.2 shows the synthesis of fluorine terminated 6FK hydrophobic
oligomers. The molecular weights and the fluorine endgroups were controlled by using
excess DFBP monomer. The theoretical molecular weights were calculated according to
the Carothers equation. 19F NMR spectrum (Fig. 7.3) was used to determine the resulting
oligomer molecular weight. For the 19F NMR integrals of the two peaks, one is attributed
to the fluorine in the chain (-63.7 ppm, integration value: 225.91), another one is the
endgroup aromatic fluorine peak (-107.6 ppm, integration value: 10), and the two kinds
of fluorine have the number ratio of 6n to 2, therefore the Mn can be calculated by the
equation: 6n/2 = 225.91/10, where n is the number of repeat unit and was calculated to be
7.53. Accordingly the actual number average molecular weight (Mn) of oligomer was
calculated to be 4089 g·mol-1, which is very close to the target value (4 kg·mol-1).
The multiblock copolymer was synthesized via a two-step technique (Fig. 7.4).
First, the phenoxide terminated disulfonated poly(arylene ether sulfone) (BPSH)
hydrophilic oligomer with target molecular weight 4 kg·mol-1 was synthesized using
biphenol and SDCDPS in NMP, following the similar polycondensation method as the
hydrophobic oligomer. After 16 h reaction, the temperature was lowered to 160 oC
without isolation. Then the 6FK (4 kg·mol-1) oligomer dissolved in NMP was added
208
dropwise to the BPSH system in about 1 h. Finally the temperature was raised to 190 oC
for 2 days. The completion of the reaction can be monitored by 19F and 1H NMR during
the reaction. It was found the endgroup fluorine peak was gone after 2 days reaction (not
shown) and in the 1H NMR the small proton peaks close to the hydroxyl group in BPSH
oligomer [Fig. 7.5 (top), peak h, g, i, f,] also disappeared. The 1H NMR spectrum [Fig.
7.5 (bottom)] showed all proton peaks from both hydrophilic and hydrophobic segments
as assigned, which confirmed the success of the coupling reaction.
In this coupling reaction, a concern is that the ether-ether chain interchange side
reaction will occur, which may result in the randomized copolymer chain. For
comparison, a random copolymer possessing the same chemical composition as the
multiblock copolymer was synthesized by the one-step copolymerization of SDCDPS,
BP, 6F-BPA, and DFBP. Because the crystalline segments comprised of BP and DFBP
would precipitate out of the reaction solution and upset the stoichiometry, the high
molecular weight random copolymer was difficult to obtain. However, the comparison of
13C NMR spectra between low molecular weight random and the multiblock copolymers
still provided enough information of the sequence connection. As shown in Figure 7.6,
the carbons in random copolymer all have multiple peaks, suggesting the irregular
connection of the repeating sequence. In contrast, the carbons in the multiblock
copolymer showed strong single peaks, which confirmed the multiblock structure31.
All of the above results indicated that a multiblock 6FK-BPS (4:4)k copolymer
with approximately 4 kg·mol-1 for each segmental lengths has been successfully
synthesized with high intrinsic viscosity. The block copolymer can be dissolved in
209
dipolar aprotic solvents like NMP and DMAc. Tough films were prepared by solution
casting from 5 % DMAc solution.
210
C
O
FF HO C OH
CF3
CF3
+
K2CO3 /DMAc / Toluene
150 oC 4h
175 oC 16h
C
O
F C O
CF3
CF3
O C
O
Fn
Figure 7.2. Synthesis of Fluorine Terminated Poly(Arylene Ether Ketone) (6FK)
Hydrophobic Oligomer
211
Figure 7.3. Molecular Weight of 6FK Hydrophobic Oligomer Can be Calculated
from the 19F NMR Spectrum (6FK Oligomer with Target MW 4 kg·mol-1 in CDCl3)
212
F F
SO
OMO OO m
SO3MMO3S- +
-+
-+
SO
OOO
SO3MMO3S- +-+
Add dropwise at 160 oCThen 190 oC, 48 h
HO OH SO
ClO
SO3HHO3S
Cl+
K2CO3/NMP/Toluene150 oC 4h190 oC 16h
/NMP6FK
OM- +
mCO
O CCF3
O
CF3n
Figure 7.4. Synthesis of 6FK-BPSH Multiblock Copolymers via Two-step Sechnique
213
Figure 7.5. 1H NMR Spectra of BPS Hydrophilic Oligomer (Top), and Multiblock
6FK-BPS Copolymer (Bottom)
214
Random Copolymer
Multiblock Copolymer
Random Copolymer
Multiblock Copolymer
Figure 7.6. 13C NMR Spectra of Random (Top) and 6FK-BPS Multiblock (Bottom)
Copolymers
215
7.4.2. Morphology of Membranes
The multiblock 6FK-BPS (4:4)k copolymer membrane was acidified to assess the
effect of acidification upon its phase separation. Film samples of the multiblock
copolymer were acidified in sulfuric acid either by “Method 1” at 30 °C or by “Method
2” at 100 °C26. Films were imaged by tapping mode atomic force microscopy (AFM)
after acidification [Fig. 7.7(a)-(d)] When the phase images for acidification by Method 1
[Fig. 7.7 (a)] and Method 2 [Fig. 7.7(c)] are compared, two observations can be made.
First, the connectivity between the hydrophilic ionic domains (which appear darker) is
greater following acidification by Method 2. Second, acidification by Method 2 results in
a sharper contrast between the ionic domains and the hydrophobic non-ionic domains
(which appear brighter). These observations suggest that there is a greater degree of
phase separation in the multiblock copolymer following acidification by Method 2.
These results are consistent with previous results for phase separation of sulfonated
poly(arylene ether sulfone) random copolymers acidified by both methods26,32 and those
subjected to different hydrothermal treatments5. Height image micrographs [Fig. 7.7
(b),(d)] obtained concurrently with the phase images for the same area suggest that
acidification temperature may increase the difference between the highest and lowest
topographical features of the film. It is speculated that the hydrothermal treatment of
“Method 2” acidification imparts greater segmental mobility to the polymer chains within
the film compared to “Method 1” because the water depresses the glass transition
temperature of the polymer while it is concurrently heated at elevated temperature.
While temperature-induced topographic differences were not reported for poly(arylene
ether sulfone) random copolymer membranes5, it is possible that this observation may be
216
a result of two factors. The first is the increased flexibility of ketone linkages compared
to that of sulfone linkages. The second and possibly more important factor is that the
multiblock structure of this copolymer may allow more phase separation to occur during
acidification than a random copolymer structure could.
To confirm this latter hypothesis, a sulfonated poly(arylene ether ketone)
random copolymer film was acidified by Method 2 to evaluate the differences in phase
separation between random and multiblock copolymers with sulfonated poly(arylene
ether ketone) components. AFM phase images (Fig. 7.7c,e) and height images (Fig.
7.7d,f) of the two copolymers indicate that phase separation is sharper for the multiblock
copolymer than for the random copolymer, supporting the hypothesis that multiblock
structure may contribute to increased differences in topography. These results are
understandable given that the length of the ion-containing blocks in these multiblock
copolymers is longer than the length of an ion-containing comonomer in the random
copolymers. Consequently, the ionic groups should be located closer to each other in the
multiblock copolymer when ionic group aggregation occurs during acidification,
facilitating phase separation and resulting in larger domains and greater differences in
topography.
217
100nm
100nm
100nm
(a)
(c)
(e)
(b)
(d)
(f)
100nm100nm
100nm100nm
100nm100nm
(a)
(c)
(e)
(b)
(d)
(f)
Figure 7.7. Tapping Mode Atomic Force Microscopy Images: (a) Phase Image and
(b) Height Image of a 4k-4k 6FK-BPSH Multiblock Copolymer Film Acidified by
Method 1 at 30 °C, (c) Phase Image and (d) Height Image of the Same Film
Acidified by Method 2 at 100 °C, (e) Phase Image and (f) Height Image of a
Sulfonated Poly(Arylene Ether Ketone) Random Copolymer Film (B-30) Acidified
by Method 2. Image size: 500 nm; z Ranges: (a) 4°, (c) 12°, (e) 8°, All Height Ranges:
10 nm.
218
7.4.3. Characterization of PEM Properties
As discussed in section 7.4.2.1., ion containing polymers tend to phase separate
into hydrophobic and hydrophilic domain like morphology. The extent of phase
separation is critical for both proton and water transport. It is known that in block
copolymers, the sequence lengths play an important role in phase separation. In order to
understand the importance of block copolymer morphology on transport properties, B and
PB random copolymers with similar chemical structures and IECs were used as controls.
Table 7.1 lists the various properties for the random, block and Nafion®
copolymers. At similar IECs, a significant increase in proton conductivity was observed
for the 6FK-BPSH (4:4)k multiblock copolymer over the random B-30 and PB-40
copolymers. A similar trend in water uptake is also observed. The sharpness of phase
separation seems to increase both proton and water transport. To investigate in details,
water uptake was studied over a wide range of water activities and so as proton
conductivity.
Figure 7.8 represents the plots of hydration number as a function of water
activity for 6FK-BPSH (4:4)k multiblock, Nafion® and B ketone-30 copolymers. The
multiblock showed much higher water uptake at all hydration levels. In contrast to the
random copolymer, both the multiblock and Nafion® showed a sudden increase after 0.85
water activity. Kreuer et al33 reported similar observation for Nafion®. The dielectric
constant of the water in the membrane at higher water activities was found close to bulk
or free water. Earlier studies have demonstrated the importance of the presence of free
water on the transport properties. At low water activities, the phase separated morphology
of the block copolymers tends to hold up more water than the random and Nafion®. This
219
may be important when addressing the proton conductivity under partially hydrated
conditions.
The temperature dependency on proton transport was determined over the
temperature range of 30-80 oC for the copolymers studied (Fig. 7.9). Both the block and
Nafion® copolymers showed higher proton conductivities over the temperature range.
The slope of the graphs can be related to the activation energy for proton transport. It
follows that the multiblock has the lowest activation energy. The well phase separated
block copolymer morphology may increase the extent of connectivity between the
hydrophilic domains and lowers the activation energy for transport.
Self-diffusion coefficient of water gives a better understanding on the
importance of connectivity on water or proton transport. Higher value indicates well
phase separated morphology. Also under partially hydrated conditions, self-diffusion
coefficient scales with proton diffusion coefficient for Nafion®. 27,33 Hence a clear
understanding about the influence of block lengths on self-diffusion coefficient of water
is needed. Figure 7.10 shows the self-diffusion coefficients of water measured at 25 oC
for the copolymers studied. Although the ion exchange capacity and chemical
composition of the multiblock copolymer is similar to that of random, a change in
sequence length distribution increases the self-diffusion coefficient value significantly.
This reflects the importance of morphology and is consistent with the activation energy
results.
Studying proton conductivity as a function of relative humidity illustrated the
effect of morphology on proton conductivity under partially hydrated conditions (Fig.
7.11). For the random copolymer, PB-diketone 50, proton conductivity drops
220
significantly at lower RH values. Random copolymers show decent performance under
fully hydrated conditions since there are sufficient water molecules to provide proton
transport through water molecules in a scattered morphology; however, they lack the
connectivity among sulfonic acid groups for proton transport under partially hydrated
conditions. Conversely, with the multiblock copolymers, the performance under partially
hydrated conditions was very much comparable to Nafion®. The presence of long, co-
continuous channels improved the proton transport along the sulfonic acid groups and
water molecules.
221
Table 7.1. Characterization of Multiblock, Random Copolymers and
Nafion®Control
Copolymers IECa (meq·g-1)
Water Uptake (%)
Proton Conductivityb (S·cm-1)
I.V.c (dL·g-1)
Nafion® 112 0.9 22 0.08 -
PB-50 1.4 66 0.08 1.4
PB-40 1.2 26 0.04 1.2
B-30 1.1 25 0.02 0.5
6FK-BPSH(4:4)k 1.2 53 0.08 0.7
a. IEC values for the copolymers (except Nafion®) were measured by titration b. Proton conductivities were measured in liquid water at 30 ℃ c. Intrinsic viscosities (I.V.) were measured in NMP with 0.05 M LiBr
222
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00
2
4
6
8
10
12
14
16
18
20
22
24
Hyd
ratio
n N
umbe
r (λ)
Water Activity (aw)
(1) 6Fk-BPSH (4:4)k (2) Nafion 112 (3) B ketone 30
Figure 7.8. Retention of Water as a Function of Water Activity is Enhanced for the
Block Copolymer
223
2.8 2.9 3.0 3.1 3.2 3.3 3.4
3.0
3.5
4.0
4.5
5.0
10.5
7.9
13.0 Pro
ton
Con
duct
ivity
(mS/
cm)
Ln (σ
/ m
S cm
-1)
1000/T (K-1)
(1) 6Fk-BPSH (4:4)k (2) Nafion 112 (3) B ketone-30
90 80 70 60 50 40 30 20
Temperature ( oC)
17
2020
29
37
46
5555
78
102
125
148148
Pro
ton
Con
duct
ivity
(mS
·cm
-1)
2.8 2.9 3.0 3.1 3.2 3.3 3.4
3.0
3.5
4.0
4.5
5.0
10.5
7.9
13.0 Pro
ton
Con
duct
ivity
(mS/
cm)
Ln (σ
/ m
S cm
-1)
1000/T (K-1)
(1) 6Fk-BPSH (4:4)k (2) Nafion 112 (3) B ketone-30
90 80 70 60 50 40 30 20
Temperature ( oC)
17
2020
29
37
46
5555
78
102
125
148148
Pro
ton
Con
duct
ivity
(mS
·cm
-1)
Figure 7.9. Proton Conductivity as a Function of Temperature for Multiblock 6FK-
BPSH (4:4)k, B ketone-30, and Nafion® 112 ( The numbers in the box are activation
energy, kJ·mol-1)
224
6FK-BPSH(4:4)k Nafion 112 PB diketone-40 B ketone-300
1
2
3
4
5
6
7
8
Sel
f diff
usio
n co
effic
ient
of w
ater
(10-6
/cm
2 )S
elf d
iffud
ion
Coe
ffici
ent o
f Wat
er (1
0-6·cm
-2)
6FK-BPSH(4:4)k Nafion 112 PB diketone-40 B ketone-300
1
2
3
4
5
6
7
8
Sel
f diff
usio
n co
effic
ient
of w
ater
(10-6
/cm
2 )S
elf d
iffud
ion
Coe
ffici
ent o
f Wat
er (1
0-6·cm
-2)
Figure 7.10. The Block Copolymer Has a Much Higher Self-diffusion Coefficient of
Water (Multiblock Copolymer Has Similar IEC Value to the PB-40 and B-30
Random Copolymers)
225
20 30 40 50 60 70 80 90 100 110
1
10
100
Prot
on c
ondu
ctiv
ity (m
S/cm
)
Relative Humidity (%)
(1) Nafion 112 (2) 6FK-BPSH(4:4)k (3) PB-diketone 50
12 3
20 30 40 50 60 70 80 90 100 110
1
10
100
Prot
on c
ondu
ctiv
ity (m
S/cm
)
Relative Humidity (%)
(1) Nafion 112 (2) 6FK-BPSH(4:4)k (3) PB-diketone 50
12 3
Pro
ton
cond
uctiv
ity (m
S·c
m-1
)
20 30 40 50 60 70 80 90 100 110
1
10
100
Prot
on c
ondu
ctiv
ity (m
S/cm
)
Relative Humidity (%)
(1) Nafion 112 (2) 6FK-BPSH(4:4)k (3) PB-diketone 50
12 3
20 30 40 50 60 70 80 90 100 110
1
10
100
Prot
on c
ondu
ctiv
ity (m
S/cm
)
Relative Humidity (%)
(1) Nafion 112 (2) 6FK-BPSH(4:4)k (3) PB-diketone 50
12 3
Pro
ton
cond
uctiv
ity (m
S·c
m-1
)Pr
oton
Con
duct
ivity
(mS·
cm-1)
20 30 40 50 60 70 80 90 100 110
1
10
100
Prot
on c
ondu
ctiv
ity (m
S/cm
)
Relative Humidity (%)
(1) Nafion 112 (2) 6FK-BPSH(4:4)k (3) PB-diketone 50
12 3
20 30 40 50 60 70 80 90 100 110
1
10
100
Prot
on c
ondu
ctiv
ity (m
S/cm
)
Relative Humidity (%)
(1) Nafion 112 (2) 6FK-BPSH(4:4)k (3) PB-diketone 50
12 3
Pro
ton
cond
uctiv
ity (m
S·c
m-1
)
20 30 40 50 60 70 80 90 100 110
1
10
100
Prot
on c
ondu
ctiv
ity (m
S/cm
)
Relative Humidity (%)
(1) Nafion 112 (2) 6FK-BPSH(4:4)k (3) PB-diketone 50
12 3
20 30 40 50 60 70 80 90 100 110
1
10
100
Prot
on c
ondu
ctiv
ity (m
S/cm
)
Relative Humidity (%)
(1) Nafion 112 (2) 6FK-BPSH(4:4)k (3) PB-diketone 50
12 3
Pro
ton
cond
uctiv
ity (m
S·c
m-1
)Pr
oton
Con
duct
ivity
(mS·
cm-1)
Figure 7.11. Comparison of Conductivity vs. RH for 6FK-BPSH (4:4)k Multiblock,
PB-diketone-50 Random Copolymers, and Nafion® 112
226
7.4.4. Fuel Cell Performance
Evaluation of this multiblock copolymer also included fuel cell performance
testing. All tests were conducted at Los Alamos National Laboratory at an elevation of
approximately 7000 feet. A 6FK-BPS (4:4)k multiblock membrane was evaluated and
compared with a commercial Nafion® 1135 membrane under full humidification of the
inlet gases (Fig. 7.12). The novel multiblock copolymer showed very promising fuel cell
performance, which was similar to Nafion® under these conditions. The high frequency
resistance (HFR) of the Nafion® membrane was 0.072 Ω-cm2, while that of the 6FK-BPS
(4:4)k was 0.087 Ω-cm2. Since the conductivity of the membranes is very similar, the
difference in HFR is thought to be due at least partially to the resistance at the interface
between the membrane and the electrodes34. This incompatibility might be due to the
difference in water uptake between the novel multiblock copolymer and the Nafion®-
based electrode layers. Performance might be further improved by replacement of the
Nafion® binder in the anode and cathode with one more similar to the membrane.
227
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Current Density (A/cm2)
Vol
tage
(V)
1: 6FK-4-BPSH-4 (1 mil)2: N1135
12
Current Density (A/cm2)
Vol
tage
(V)
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Current Density (A/cm2)
Vol
tage
(V)
1: 6FK-4-BPSH-4 (1 mil)2: N1135
12
Current Density (A/cm2)
Vol
tage
(V)
Current Density (A·cm-1)
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Current Density (A/cm2)
Vol
tage
(V)
1: 6FK-4-BPSH-4 (1 mil)2: N1135
12
Current Density (A/cm2)
Vol
tage
(V)
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Current Density (A/cm2)
Vol
tage
(V)
1: 6FK-4-BPSH-4 (1 mil)2: N1135
12
Current Density (A/cm2)
Vol
tage
(V)
Current Density (A·cm-1)Current Density (A·cm-2)
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Current Density (A/cm2)
Vol
tage
(V)
1: 6FK-4-BPSH-4 (1 mil)2: N1135
12
Current Density (A/cm2)
Vol
tage
(V)
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Current Density (A/cm2)
Vol
tage
(V)
1: 6FK-4-BPSH-4 (1 mil)2: N1135
12
Current Density (A/cm2)
Vol
tage
(V)
Current Density (A·cm-1)
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Current Density (A/cm2)
Vol
tage
(V)
1: 6FK-4-BPSH-4 (1 mil)2: N1135
12
Current Density (A/cm2)
Vol
tage
(V)
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Current Density (A/cm2)
Vol
tage
(V)
1: 6FK-4-BPSH-4 (1 mil)2: N1135
12
Current Density (A/cm2)
Vol
tage
(V)
Current Density (A·cm-1)Current Density (A·cm-2) Figure 7.12. Hydrogen-air Fuel Cell Performance of 6FK-BPSH (4:4)k and Nafion®
at 80 ℃ under Fully Humidified Conditions.
228
7.5. Conclusions
A new hydrophobic-hydrophilic multiblock 6FK-BPSH (4:4)k copolymer was
successfully synthesized via a two-step polycondensation method. 19F and 1H NMR
spectra were used to characterize the oligomers’ molecular weight and multiblock
copolymer’s structure. 13C NMR was a powerful tool to confirm the sequence connection
in the block copolymer. Good film-forming material with high conductivity was obtained.
Morphologies of the membranes characterized with tapping mode atomic force
microscopy showed that the high temperature acidification method can improve the phase
separation. The results were consistent with previous studies on the BPSH random
copolymer. The AFM images also indicated that the phase separation is sharper for the
multiblock than for the random copolymer.
This well phase separated block copolymer morphology increases the extent of
connectivity between the hydrophilic domains and improves the PEM properties. It was
found that, compared with the random ketone type copolymer with the similar IEC value,
the multiblock copolymer has higher protonic conductivity, lower activation energy, and
much improved self-diffusion coefficient of water. It can keep more free water than the
random copolymer, which results in much improved protonic conductivity under partially
hydrated conditions. The multiblock copolymer also showed very promising fuel cell
performance, which was comparable to Nafion®. Larger block lengths are currently being
investigated.
229
7.6. Acknowledgments
The author would like to acknowledge the Department of Energy (contract #DE-
FG36-06G016038), National Science Foundation (contract #EHR-0090556), and Nissan
Motor Company for their support of this project.
230
7.7. References 1. Thomas, S.; Zalbowitz, M., Fuel Cells: Green Power, Los Alamos National
Laboratory: Los Alamos, NM, 1999. 2. Winter, M.; Brodd, R. J., Chem. Rev. 2004, 104, 4245. 3. Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. , Chem. Rev.
2004,104, 4587. 4. Wang, F.; Hickner, M.A.; Ji, Q.; Harrison, W.L.; Mecham, J.F.; Zawodzinski, T.A.;
McGrath, J.E., Macromol. Symp., 2001, 175, 387. 5. Kim, Y.S.; Dong, L.; Hickner, M.A.; Pivovar, B.S.; McGrath, J.E., Polymer, 2002,
44 ,5729. 6. Wang, F.; Hickner, M.; Kim, Y.S.; Zawodzinski, T.A.; McGrath, J.E., J. Membr. Sci.
2002,197, 231. 7. Harrison, W. L.; Wang, F.; Mecham, J. B.; Bhanu, V. A.; Hill, M.; Kim, Y. S.;
McGrath, J. E., J. Polym. Sci. Part A: Polym. Chem., 2003, 41, 2264. 8. Li, Y.; Mukundan, T.; Harrison, W.; Hill, M.; Sankir, M.; Yang, J.; McGrath, J. E.,
Prepr. Symp. Am Chem Soc Div Fuel Chem, 2004, 49(2), 536. 9. Einsla, B.R.; Hong, Y.T.; Kim, Y.S.; Wang, F.; Gunduz, N.; McGrath, J.E., J. Polym.
Sci., Part A: Polym. Chem., 2004, 42, 862. 10. Einsla, B.R.; Kim, Y.S.; Hickner, M.A.; Hong, Y.T.; Hill, M.L.; Pivovar, B.S.;
McGrath, J.E., J. Memb. Sci., 2005, 255, 141. 11. Wiles, K. B.; Wang, F.; McGrath, J. E.; J. Polym.Sci., Part A: Polym. Chem., 2005,
43, 2964. 12. Ghassemi, H.; McGrath, J.E., ACS Polym. Prepr., 2002, 43, 1021. 13. Wang, S.; McGrath, J.E.; In Step Polymerization. M. Rogers, T. E.Long, ed., Wiley:
New York: 2003 14. 2005 DOE hydrogen program review May 23-26, Arlington, VA. 15. Roy, A.; Hickner, M.A.; Yu, X.; Li, Y.; Glass, T.E.; McGrath, J.E., J. Polym. Sci.,
Part B: Polym.Phys., 2006, 44, 2226. 16. Badami, A.S.; Lee, H.S.; Li, Y.; Roy, A.; Wang, H.; McGrath, J. E., Prepr. Symp.
Am Chem Soc Div Fuel Chem, 2006,51(2), 612.
231
17. Ghassemi, H.; Ndip, G.; McGrath, J. E., Polymer, 2004, 45, 5855. 18. Ghassemi, H.; Zawodzinski, T.; McGrath, J. E., Polymer, 2006, 47, 4132. 19. Yu, X.; Roy, A.; McGrath, J. E., ACS PMSE Prepr., 2006, 95, 141. 20. Lee, H. S.; Roy, A.; Badami, A. S.; McGrath, J. E., ACS PMSE Prepr., 2006, 95, 210. 21. Wang, H.; Badami, A.S.; Roy, A.; McGrath, J. E., ACS PMSE Prepr., 2006, 95, 202. 22. Li, Y.; Roy, A.; Badami, A.S.; Yang, J.; McGrath, J.E., Prepri. Symp. Am Chem Soc
Div Fuel Chem, 2006, 51(2), 682. 23. Roy, A.; Yu, X.; Badami, A. S.; McGrath, J.E., ACS PMSE prepr., 2006, 94, 169. 24. Roy, A.; Lee, H.S.; Badami, A.S.; Yu, X.; Li, Y.; Glass, T.E.; McGrath, J.E. Prepr.
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Polymer, 2006, 47, 4210. 26. Kim, Y.S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y.T.; Harrison, W.;
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Meeting, 2004, 2004-02, 1471.
232
Chapter 8. Overall Conclusions Poly(arylene ether) copolymers containing sulfonic acid groups have
demonstrated the characteristics for potential application in proton exchange membrane
(PEM) fuel cells. The objectives of this research were to synthesize novel random and
multiblock disulfonated poly(arylene ether) copolymers via the direct copolymerization
method, and systematically investigate the effects of copolymer molecular weight,
chemical composition, and microstructure on the properties of PEMs. The investigated
PEM properties included ion exchange capacity, water uptake, proton conductivity, water
self-diffusion coefficient, methanol permeability, and fuel cell performance.
Direct copolymerization of the disulfonated monomers required high monomer
purity in obtaining high molecular weight copolymers for PEMs. A novel
characterization method for determining the purity of the disulfonated monomer
SDCDPS has been developed by using UV-Visible spectroscopy. Pure SDCDPS
recrystallized from IPA/H2O was used to establish a Beer’s Law plot, which was then
used to determine the purity of the crude product. The model poly(arylene ether sulfone)
copolymers were synthesized by direct copolymerization of the crude SDCDPS with
known purity, DCDPS, and BP. The relatively high molecular weights of the copolymers
confirmed that this characterization method was applicable to accurately determine the
purity and directly use the crude SDCDPS without purification process, which can save
money, time and energy. This is especially attractive for the mass production of the
copolymers. The results also showed that the SDCDPS needed to be dried in a vacuum
oven at 160 oC for at least 48 h to completely remove the water. Since the SDCDPS
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absorbed small amounts of moisture after storage in a desiccator for several days, it was
suggested to dry the SDCDPS directly before the copolymerization.
The effects of copolymer molecular weight on PEM properties have been
systematically studied by synthesizing three series of controlled molecular weight
copolymers: BPSH35, partially fluorinated 6FSH35, and 6FSH48. BPSH35 copolymers
have the same degree of sulfonation as 6FSH35 copolymers, but have the same ion
exchange capacity values as the 6FSH48 copolymers. The molecular weights were
controlled from 20 to 50 kg·mol-1 using the monofunctional endcapper t-butyl phenol.
High molecular weight copolymers with 1:1 stoichiometry were also synthesized for the
comparison purpose. The molecular weight of the ionic copolymer was characterized by
a combination of 1H NMR endgroup analysis and modified intrinsic viscosity
measurements. Small amount of lithium bromide (0.05 M) in NMP was used to
effectively suppress the “polyelectrolyte effect” appearing in measuring the intrinsic
viscosity of a charged macromolecule, which allowed obtaining more accurate data than
previously used simple dilute solution viscosity measurements. Effects of molecular
weights on the properties of proton exchange membranes were studied. It was found that
with increasing the molecular weights, water uptake decreased modestly. But the
molecular weight was found to have no obvious influence on proton conductivity under
fully hydrated conditions. Furthermore, the mechanical properties of the BPS35
membranes, such as the modulus strength and elongation at break were improved by
increasing the molecular weight as well. Morphologies of copolymer membranes with
different molecular weights for BPSH35 and 6FSH35 series were characterized by AFM
images, which suggested that the morphology is a function of the degree of sulfonation,
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irrespective of the molecular weight. Compared with BPSH35 series, 6FSH 35
copolymers with the same degree of sulfonation had lower water uptake and proton
conductivities, while 6FSH48 copolymers with the same IEC value had higher water
uptake and proton conductivities. These differences resulted from the different chemical
structures.
The chemical structure effects on the PEM properties were further investigated by
using three series of poly(arylene ether ketone) copolymers based on 6F-BPA, three
sulfonated ketone-type comonomers and their corresponding unsulfonated monomers.
The degree of sulfonation was controlled from 20 to 50 mol%. The intrinsic viscosity
measured with a modified method in 0.05 M LiBr/NMP confirmed that the molecular
weights of the copolymers were high enough to form tough, flexible films. The
membrane properties of these three series of copolymers have been comparatively
studied. Copolymers containing a meta diketone unit (MB series) showed higher water
uptake and proton conductivities compared with the mono ketone (B series) and para
diketone (PB series) at similar IEC values. This may be caused by the more flexible MB
chemical structures which resulted in better phase separation and higher hydration levels.
The B series copolymers also showed lower methanol permeability compared to Nafion®
and low interfacial loss in MEA measurements.
The use of multiblock copolymers is a new strategy to improve the PEM
properties over the random copolymers, especially under partially hydrated conditions. A
new hydrophobic-hydrophilic multiblock 6FK-BPSH (4:4)k copolymer was successfully
synthesized via a two-step polycondensation method. 19F and 1H NMR spectra were used
to characterize the oligomers’ molecular weight and multiblock copolymer’s structure.
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13C NMR was a powerful tool to confirm the sequence connection in the block copolymer.
Good film-forming material with high conductivity was obtained. Morphologies of the
membranes characterized with tapping mode atomic force microscopy showed that the
high temperature acidification method can improve the phase separation. The results were
consistent with previous studies on the BPSH random copolymer. AFM images also
indicated that the phase separation is sharper for the multiblock than for the random
copolymer. This well phase separated block copolymer morphology increases the extent
of connectivity between the hydrophilic domains and improves the PEM properties. It
was found that, compared with the random ketone type copolymer with the similar IEC
value, the multiblock copolymer has higher protonic conductivity, lower activation
energy, and much improved self-diffusion coefficient of water. It can keep more free
water than the random copolymer, which results in much improved protonic conductivity
under partially hydrated conditions. The multiblock copolymer also showed very
promising fuel cell performance, which was comparable to Nafion®.
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