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Synthesis and Characterization of Hydrophobic-Hydrophilic Segmented and Multiblock Copolymers for Proton Exchange Membrane and
Reverse Osmosis Applications
Rachael A. VanHouten
Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State
University in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSPHY
In
Macromolecular Science and Engineering
James E. McGrath, Chair
Judy S. Riffle
John G. Dillard
Richey M. Davis
Scott W. Case
December 1, 2009
Blacksburg, Virginia
Keywords: multiblock copolymer, segmented copolymer, disulfonated poly(arylene ether
sulfone)s, proton exchange membrane, reverse osmosis membrane
Synthesis and Characterization of Hydrophilic-Hydrophobic Segmented
and Multiblock Copolymers for Proton Exchange Membrane and
Reverse Osmosis Applications
Rachael A. VanHouten
ABSTRACT
This thesis research focused on the synthesis and characterization of disulfonated
poly(arylene ether sulfone) hydrophilic-hydrophobic segmented and multiblock
copolymers for application as proton exchange membranes (PEMs) in fuel cells or as
reverse osmosis (RO) membranes for water desalination. The first objective was to
demonstrate that synthesizing blocky copolymers using a one oligomer, two monomer
segmented copolymerization afforded copolymers with similar properties to those which
used a previous approach of coupling two preformed oligomers. A 4,4’-biphenol based
hydrophilic block of disulfonated poly(arylene ether sulfone) oligomer of controlled
number average molecular weight (Mn) with phenoxide reactive end groups was first
synthesized and isolated. It was then reacted with a calculated amount of hydrophobic
monomers, forming that block in-situ. Copolymer and membrane properties, such as
intrinsic viscosity, tensile strength, water uptake, and proton conductivity, were
consistent with those of multiblock copolymers synthesized via the oligomer-oligomer
approach.
The segmented polymerization technique was then used to synthesize a variety of
other copolymers for PEM applications. The well known bisphenol phenolphthalein was
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explored as a comonomer for either the hydrophilic and hydrophobic blocks of the
copolymer. Membrane properties were explored as a function of block length for both
series of copolymers. Both series showed that as block length increases, proton
conductivity increases across the entire range of relative humidity (30-100%), as does,
water uptake. This was consistent with earlier research which showed that the water self-
diffusion coefficient scaled with block length. Copolymers produced with
phenolphthalein had higher tensile strength, but lower ultimate elongation than the 4,4’-
biphenol based copolymers.
Multiblock copolymers were also synthesized and characterized to assess their
feasibility as RO membranes. A new series of multiblock copolymers was synthesized
by coupling hydrophilic disulfonated poly(arylene ether sulfone) (BisAS100) oligomer
with hydrophobic unsulfonated poly(arylene ether sulfone) (BisAS0) oligomer. Both
oligomers were derived using 4,4´-isopropylidenediphenol (Bis-A) as the bisphenol.
Phenoxide-terminated BisAS100 was end-capped with decafluorobiphenyl and reacted at
relatively low temperatures (~ 100 oC) with phenoxide-terminated BisAS0. Basic
properties were characterized as a function of block length. The initial membrane
characterization suggested these copolymers may be suitable candidates for reverse
osmosis applications, and water and salt permeability testing should be conducted to
determine desalination properties. The latter measurements are being conducted at the
University of Texas, Austin and will be reported separately.
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Acknowledgements
I would like to think my advisor, Dr. James E. McGrath, for his continued
guidance throughout my graduate education. His willingness to discuss my research and
help me overcome problems was invaluable for my research. I would also like to thank
my committee members, Drs. Judy S. Riffle, John G. Dillard, Richey M. Davis, and Scott
W. Case for their time and research suggestions.
I am appreciative of all the discussions and support my labmates from past and
present have provided me throughout the years. A special thanks to Dr. Harry Lee, Yu
Chen, Ozma Lane, Dr. Ruilan Guo, Dr. Gwangsu Byun, Dr. Xiang Yu, Dr. Yanxiang Li,
Dr. Abhishek Roy, Dr. Mou Paul, Dr. Gwangyu Fan, and Drs. Melinda and Brian Einsla
for their dedication to help me synthesize better polymers and write better papers. I am
grateful to the staff of the MACR program and MII for helping me with various tasks
throughout my time at Virginia Tech: Laurie, Millie, Angie, Tammy Jo, and Mary Jane.
I am thankful for the love and continued support of my mom, Betty Zeller, and
siblings, Stacy, Katie, and Alex. They have motivated me, encouraged me, and prayed
for me throughout my academic career. I am thankful to everyone at Main Street Baptist
Church and all my friends who have become my extended family away from home. I
would never have made it this far without constant encouragement, support, and prayers
throughout the past 5 ½ years from all my family and friends.
I am indebted to my loving husband and best friend, Desmond, for everything he
has been for me throughout my graduate career. He never gave up hope that I could get
this far and provided all the support he could to help me get here. No one else came close
to the understanding and patience he provided me through the long haul of graduate
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school. I am excited for the new addition to our family, baby Adeline, and looking
forward to seeing what else our future has in store.
Ultimately, I’m thankful to God. Without Him, I would be lost. He guided and
directed me daily. I owe all my blessings to Him.
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ATTRIBUTION
Several colleagues facilitated the research described in the chapters included in this
dissertation. Their contributions are described below.
James E. McGrath is the author’s academic advisor and committee chair. He provided
support and guidance on all of the work.
Ozma Lane aided in proton conductivity measurements for chapter 2 and 3.
Desmond VanHouten assisted with thermal analysis and tensile testing for chapters 2, 3,
4, and 5 and helped with proton conductivity measurements for chapters 3 and 4.
Myoungbae Lee obtained transmission electron microscopy images for chapter 5.
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TABLE OF CONTENTS
TABLE OF FIGURES....................................................................................................... xi TABLE OF TABLES ...................................................................................................... xiv 1 Literature Review……………........................................................................................ 1 1.1 Ionomers ..................................................................................................................1 1.2 Fuel Cells .................................................................................................................1 1.3 PEM Fuel Cells ........................................................................................................2 1.4 Materials Used for PEMs.........................................................................................3
1.4.1 Nafion® ...................................................................................................... 4 1.4.2 Poly(arylene ether) Copolymers ................................................................. 6
1.4.2.1 Synthesis ................................................................................................. 6 1.4.2.1.1 Nucleophilic Aromatic Substitution (SNAR) .................................... 6 1.4.2.1.2 Electrophilic Aromatic Substitution ................................................. 9 1.4.2.1.3 Ullmann Reaction ........................................................................... 10
1.4.2.2 Post-Sulfonation Modification.............................................................. 11 1.4.2.2.1 Post-Sulfonated Poly(arylene ether sulfone) Copolymers.............. 12 1.4.2.2.2 Post-Sulfonated Poly(arylene ether ketone) Copolymers ............... 14
1.4.3 Directly Polymerized Sulfonated Monomers to form Sulfonated Poly(arylene ether) Random Copolymers.................................................................17
1.4.3.1 Poly(arylene ether)s Containing Disulfonated Sulfone Monomers...... 17 1.4.3.2 Poly(arylene ether)s Containing Disulfonated Ketone Monomers....... 21 1.4.3.3 Poly(arylene ether)s Containing Sulfonated Naphthalene Monomers.. 23 1.4.3.4 Poly(arylene ether)s Containing Other Sulfonated Monomers............. 24
1.4.4 Block Copolymers .................................................................................... 25 1.4.4.1 Diblock and Triblock Copolymers........................................................ 26 1.4.4.2 Multiblock Copolymers ........................................................................ 31
1.4.4.2.1 Multiblocks Containing Aliphatic and Aromatic Blocks................ 31 1.4.4.2.2 Aromatic Multiblock Copolymers .................................................. 33
1.4.5 Segmented Copolymers ............................................................................49 1.4.5.1 Poly(arylene ether ketone) segmented copolymers .............................. 50 1.4.5.2 Poly(arylene ether sulfone) segmented copolymers ............................. 54
1.5 Water Desalination.................................................................................................56 1.6 Reverse Osmosis....................................................................................................57 1.7 Types of Membranes for Reverse Osmosis ...........................................................58 1.8 Materials for Reverse Osmosis Membranes ..........................................................60
1.8.1 Cellulose Membranes................................................................................ 60 1.8.2 Non-Cellulosic Membranes ...................................................................... 61 1.8.3 Sulfonated Aromatic Polymers ................................................................. 64
1.9 Research Objectives...............................................................................................67 2 Synthesis of Segmented Hydrophobic-Hydrophilic, Fluorinated-Sulfonated Block Copolymers for Use as Proton Exchange Membranes ..................................................... 79 2.1 Introduction............................................................................................................79 2.2 Experimental Section .............................................................................................83
2.2.1 Materials ................................................................................................... 83
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2.2.2 Synthesis of Phenoxide-Terminated Hydrophilic Blocks (BPS-100)....... 83 2.2.3 Synthesis of BisSF-BPSH100 Segmented Copolymers ........................... 84 2.2.4 Synthesis of BisSF-BPSH100 Multiblock Copolymer Controls .............. 85
2.2.4.1 Synthesis of Fluorine-Terminated Hydrophobic Blocks (BisSF) ........ 85 2.2.4.2 Synthesis of Phenoxide-Terminated Hydrophilic Blocks (BPS-100) .. 85 2.2.4.3 Synthesis of BisSF-BPS100 Multiblock Copolymers .......................... 86
2.2.5 Characterization of Copolymers ............................................................... 86 2.2.6 Membrane preparation .............................................................................. 87 2.2.7 Determination of water uptake and dimensional swelling........................ 87 2.2.8 Measurement of proton conductivity ........................................................ 88 2.2.9 Tensile testing ........................................................................................... 89
2.3 Results and Discussion ..........................................................................................89 2.3.1 Synthesis of Hydrophilic Oligomers......................................................... 89 2.3.2 Synthesis of BisSF-BPSH100 Segmented Copolymers ........................... 92 2.3.3 Synthesis of BisSF-BPSH100 Multiblock Copolymer Controls .............. 95 2.3.4 Comparison of BisSF-BPSH100 Segmented and Multiblock Copolymer Properties .................................................................................................................. 96
2.4 Conclusions..........................................................................................................100 3 Synthesis and Characterization of Highly Fluorinated-Disulfonated Hydrophobic-Hydrophilic Segmented Copolymers Containing Various Bisphenols for Use as Proton Exchange Membranes..................................................................................................... 105 3.1 Introduction..........................................................................................................106 3.2 Experimental ........................................................................................................108
3.2.1 Materials ................................................................................................. 108 3.2.2 Synthesis of Phenoxide-Terminated Hydrophilic Blocks (BPS-100)..... 108 3.2.3 Synthesis of segmented copolymer with simultaneous formation of hydrophobic segments ............................................................................................ 109 3.2.4 Membrane Preparation............................................................................110 3.2.5 Characterization ...................................................................................... 110 3.2.6 Determination of water uptake and dimensional swelling...................... 111 3.2.7 Measurement of proton conductivity ...................................................... 112 3.2.8 Dynamic Mechanical Analysis ............................................................... 112 3.2.9 Thermal Gravimetric Analysis................................................................ 113 3.2.10 Tensile testing ......................................................................................... 113
3.3 Results and Discussion ........................................................................................113 3.3.1 Synthesis of PhS-BPS100 Segmented Copolymers................................ 113 3.3.2 Comparison of PhF-BPSH100 and BisSF-BPSH100 Segmented Copolymer Properties ............................................................................................. 116
4 Synthesis and Characterization of Hydrophobic-Hydrophilic Segmented Copolymers with Unequal Hydrophobic and Hydrophilic Block Lengths for Use as Proton Exchange Membranes………………………................................................................................ 126 4.1 Introduction..........................................................................................................126 4.2 Experimental ........................................................................................................128
4.2.1 Materials ................................................................................................. 128 4.2.2 Synthesis of Phenoxide-Terminated Hydrophilic Oligomers
(PhS-100) ................................................................................................ 129
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4.2.3 Synthesis of segmented copolymer with simultaneous formation of hydrophobic segments ............................................................................ 130
4.2.4 Membrane Preparation............................................................................130 4.2.5 Characterization ...................................................................................... 131 4.2.6 Determination of water uptake and dimensional swelling...................... 131 4.2.7 Measurement of proton conductivity ...................................................... 132 4.2.8 Dynamic Mechanical Analysis ............................................................... 133 4.2.9 Tensile testing ......................................................................................... 133
4.3 Results and Discussion ........................................................................................133 4.3.1 Synthesis of Phenoxide-Terminated Disulfonated Hydrophilic Oligomer
Derived from Phenolphthalein................................................................ 133 4.3.2 Synthesis of BisSF-PhS Segmented Copolymer..................................... 136 4.3.3 Characterization of BisSF-PhSH100 Segmented Copolymer Properties 139
4.4 Conclusions..........................................................................................................144 5 Synthesis and Characterization of Multiblock Copolymers Derived from Bisphenol-A for Application as Reverse Osmosis Membranes ........................................................... 147 5.1 Introduction..........................................................................................................148 5.2 Experimental Section ...........................................................................................152
5.2.1 Materials ................................................................................................. 152 5.2.2 Synthesis of Phenoxide-Terminated Hydrophobic Oligomers
(BisAS0)…… ......................................................................................... 152 5.2.3 Synthesis of Phenoxide-Terminated Hydrophilic Oligomers
(BisAS100)……………………………………………….…………….153 5.2.4 Endcapping of Phenoxide-Terminated Hydrophilic Oligomers with
DFBP…................................................................................................... 154 5.2.5 Synthesis of Hydrophilic-Hydrophobic BisAS100-BisAS0 Multiblock
Copolymers ............................................................................................. 154 5.2.6 Characterization of Copolymers ............................................................. 154 5.2.7 Membrane preparation ............................................................................155 5.2.8 Determination of Ion Exchange Capacity (IEC)..................................... 156 5.2.9 Determination of water uptake and dimensional swelling...................... 156 5.2.10 Transmission Electron Spectroscopy (TEM).......................................... 157 5.2.11 Tensile testing ......................................................................................... 158 5.2.12 Dynamic Mechanical Analysis ............................................................... 158 5.2.13 Differential Scanning Calorimetry.......................................................... 158 5.2.14 Thermal Gravimetric Analysis................................................................ 159 5.2.15 Static Chlorine Exposure ........................................................................ 159
5.3 Results and Discussion ........................................................................................159 5.3.1 Synthesis of Phenoxide-Terminated Hydrophobic (BisAS0) and
Hydrophilic (BisAS100) Oligomers ....................................................... 159 5.3.2 Endcapping of Phenoxide-Terminated Hydrophilic Oligomers with
DFBP……............................................................................................... 164 5.3.3 Synthesis of Hydrophilic-Hydrophobic BisAS100-BisAS0 Multiblock
Copolymers ............................................................................................. 166 5.3.4 Membrane Characterization of BisAS100-BisAS0 Multiblock
Copolymers.. ........................................................................................... 168
x
5.4 Conclusions..........................................................................................................178 6 Overall Conclusions............................................................................................ 182 7 Future Research .................................................................................................. 184
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TABLE OF FIGURES
Figure 1.1. Electrochemistry for PEMFC. ......................................................................... 3 Figure 1.2. Electrochemistry for DMFC............................................................................ 3 Figure 1.3. Chemical structure of Nafion®. ....................................................................... 5 Figure 1.4. Synthesis of poly(arylene ether sulfone) by nucleophilic aromatic substitution.............................................................................................................................................. 7 Figure 1.5. Side reactions due to water or excess base in SNAR polymerizations. ............ 9 Figure 1.6. Synthesis of poly(arylene ether sulfone) by electrophilic aromatic substituion............................................................................................................................................ 10 Figure 1.7. Synthesis of poly(arylene ether sulfone) by a modified Ullmann Reaction., . 11 Figure 1.8. Examples of post-sulfonated poly(arylene ether sulfone)s. Sulfonic acid group is located in the activated position (ortho to the ether group). (a) Sulfonated poly(ethersulfone) (b) sulfonated polysulfone (c) hexafluorinated sulfonated polysulfone............................................................................................................................................ 12 Figure 1.9. Crosslinking scheme for post-sulfonated UDEL using 1,1’-carbonyldiimidazole (CDI) and diamine as the crosslinking agents. (a) Sulfonic acid groups are activated by CDI (b) N-sulfonylimidazoles are converted to sulfonamides. .. 16 Figure 1.10. Synthesis of monomer grade SDCDPS. .......................................................18 Figure 1.11. Direct polymerization of SDCDPS, DCDPS, and 4,4’-biphenol to form BPSH-35 random copolymer. ........................................................................................... 19 Figure 1.12. Disulfonated poly(arylene ether) random copolymers containing different aryl linkages. ..................................................................................................................... 20 Figure 1.13. Disulfonated poly(arylene ether ketone) random copolymers using (a-b) 5,5’-carbonylbis(2-fluorobenzene-sulfonate) or (c) 1,4-bis(3-sodium sulfonate-4-fluorobenzoyl)benzene as the sulfonated comonomers....................................................23 Figure 1.14. Sulfonated naphthalene diol monomers. (a) 2,7-dihydroxynaphthalene-3,6-sulfonate disodium salt (b) 2,8-dihydroxynaphthalene-6-sulfonate sodium salt and (c) 2,3-dihydroxynaphthalene-6-sulfonate sodium salt.......................................................... 24 Figure 1.15. S-SEBS triblock copolymer. ........................................................................ 26 Figure 1.16. S-SIBS triblock copolymer........................................................................... 28 Figure 1.17. S-HPBS diblock copolymer. ........................................................................ 29 Figure 1.18. P(VDF-co-HFP)-b-SPS diblock copolymer. ................................................ 31 Figure 1.19. Synthesis of PAES-b-SPB multiblock copolymer. ..................................... 32 Figure 1.20. Synthesis of BPS-100:PEPO multiblock copolymer.................................... 35 Figure 1.21. Synthesis of various sulfonated-fluorinated multiblock copolymers. .......... 36 Figure 1.22. BPSH-100:BPS-00 multiblock copolymer with (a) DFBP and (b) HFB linking groups. .................................................................................................................. 41 Figure 1.23. Poly(arylene ether sulfone) multiblock copolymers synthesized using a DFBP coupling agent containing (a) BPS-00 or (b) 6FS hydrophobic oligomer. ............ 42 Figure 1.24. BPSH-100:6FK multiblock copolymer. .......................................................43 Figure 1.25. Synthesis of BPS-100:polyimide multiblock copolymer. ............................ 45 Figure 1.26. Synthesis of BPS-00:SPPP multiblock copolymer. ..................................... 47 Figure 1.27. BPSH-100:PBP multiblock copolymer. .......................................................48
xii
Figure 1.28. Formation of polyurethane segmented copolymer with a diol-based carbamate hard segment.................................................................................................... 50 Figure 1.29. Synthesis of poly(arylene ether ketone) segmented copolymers. ................ 52 Figure 1.30. Synthesis of hydrophobic block with subsequent synthesis of poly(arylene ether ketone) segmented copolymer.,................................................................................ 53 Figure 1.31. Synthesis of segmented block copolymer (BisSF-BPSH100) with simultaneous formation of hydrophobic block (BisSF).................................................... 55 Figure 1.32. World Desalination Capacity by Process, as of June 1999. Membrane processes: reverse osmosis (RO) and electrodialysis (ED); Thermal processes: multistage flash distillation (MSF), multi-effect distillation (ME), and vapor compression (VC) .... 57 Figure 1.33. Schematic of reverse osmosis....................................................................... 58 Figure 1.34. Structures of aromatic (a) polyamide-hydrazine and (b) polyamide copolymers........................................................................................................................ 62 Figure 1.35. Crosslinked fully aromatic polymer. ........................................................... 63 Figure 2.1. Phenoxide-terminated BPS-100 with controlled molecular weight .............. 90 Figure 2.2. 1H NMR spectrum of BPS-100 oligomer...................................................... 91 Figure 2.3. Log (Mn) vs. log (I.V.) for the hydrophilic oligomers................................... 92 Figure 2.4. BisSF-BPSH100 segmented copolymer........................................................93 Figure 2.5. (a) 1H and (b) 19F NMR spectra for BisSF-BPS100 segmented copolymer... 94 Figure 2.6. 13C NMR spectra for BisSF-BPS100 multiblock and segmented copolymers........................................................................................................................................... 95 Figure 2.7. BisSF-BPSH100 multiblock copolymer........................................................ 96 Figure 2.8. Comparison of dimensional swelling data for segmented, multiblock, and random copolymers........................................................................................................... 99 Figure 2.9. Comparison of proton conductivity under partially hydrated conditions for segmented and multiblock copolymers with increasing block length ............................ 100 Figure 3.1. General synthetic scheme for highly fluorinated:disulfonated segmented copolymers...................................................................................................................... 114 Figure 3.2. (a) 1H and (b) 19F NMR spectra for PhF-BPS100 segmented copolymer.... 115 Figure 3.3. 13C NMR spectrum for PhF-BPS100 segmented copolymer and BPS-35 random copolymer .......................................................................................................... 116 Figure 3.4. Comparison of proton conductivity under partially hydrated conditions for BisSF-BPSH100 and PhF-BPSH100 segmented copolymers with increasing block length......................................................................................................................................... 118 Figure 3.5. Comparison of dimensional swelling data for BisSF-BPSH100 and PhF-BPSH100 segmented and BPSH35 random copolymers................................................ 119 Figure 3.6. Thermal gravimetric analysis plots for BisSF-BPSH100 and PhF-BPSH100 copolymers in air............................................................................................................. 120 Figure 3.7. DMA plots for a) BisSF-BPSH100 and b)PhF-BPSH100 segmented copolymers. In a) and b) the closed symbols represent the storage modulus and the open symbols represent the tan delta. ...................................................................................... 121 Figure 4.1. PhS100 phenoxide-terminated hydrophilic oligomers ................................. 135 Figure 4.2. 1H NMR spectrum of PhS100 oligomer....................................................... 135 Figure 4.3. BisSF-PhS100 segmented copolymer .........................................................137 Figure 4.4. Representative (a) 1H and (b) 19F NMR spectra for BisSF-PhS100 segmented copolymer ....................................................................................................................... 138
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Figure 4.5. 13C NMR spectrum for BisSF-PhS100 segmented copolymer..................... 139 Figure 4.6. Comparison of dimensional swelling data for segmented copolymers ....... 140 Figure 4.7. Proton conductivity under partially hydrated conditions for BisSF-PhSH100 segmented copolymers with increasing block length .....................................................141 Figure 4.8. DMA plots for BisSF-PhS100 multiblock copolymers. The solid lines represent the storage modulus and the dashed lines represent the tan δ. ........................ 142 Figure 4.9. Stress vs. Strain curves for BisSF-PhSH100 segmented copolymers .......... 143 Figure 5.1. Phenoxide-terminated BisAS0 with controlled molecular weight .............. 161 Figure 5.2. Aromatic region of a 1H NMR spectrum of BisAS0 oligomer ................... 161 Figure 5.3. Phenoxide-terminated BisAS100 with controlled molecular weight .......... 162 Figure 5.4 2D-COSY spectrum of BisAS100 oligomer ................................................. 162 Figure 5.5. Aromatic regions of a 1H NMR spectrum of BisAS100 oligomer before end-capping reaction .............................................................................................................. 163 Figure 5.6. Log (I.V.) vs. log (Mn) for the hydrophobic and hydrophilic oligomers..... 164 Figure 5.7. DFBP end-capping of phenoxide-terminated BiSA100 oligomer............... 165 Figure 5.8. Aromatic region of a 1H NMR spectrum of BisAS100 endcapped with DFBP......................................................................................................................................... 166 Figure 5.9. Coupling reaction of hydrophilic and hydrophobic oligomers.................... 167 Figure 5.10. Aromatic region of a 1H NMR spectrum for BisAS100-BisAS0 multiblock copolymer ....................................................................................................................... 167 Figure 5.11. Portions of 13C NMR spectra for (a) BisAS100-BisAS0 multiblock and (b) BisAS32 random copolymers ......................................................................................... 168 Figure 5.12. Water uptake (wt%) as a function of block length for BisAS100-BisAS0 multiblock copolymers.................................................................................................... 170 Figure 5.13. Comparison of dimensional swelling data for random and multiblock copolymers...................................................................................................................... 171 Figure 5.14. TEM images of 8k8k and 12k12k BisAS100-BisAS0 multiblock copolymers. (The bright white spot in the middle of the images is a camera artifact.) . 172 Figure 5.15. DMA plot of BisAS100-BisAS0 10k10k multiblock copolymer (black) and BisAS32 random copolymer (grey). Solid lines represent the storage modulus and dashed lines represent tan δ of the copolymers............................................................... 173 Figure 5.16. Thermograms for BisAS100-BisAS0 multiblock copolymers and BisAS32 random copolymer .......................................................................................................... 174 Figure 5.17. Thermal gravimetric analysis plot of BisAS32 random and BisAS100-BisAS0 multiblock copolymers ...................................................................................... 175 Figure 5.18. Stress-strain plots for BisAS copolymers.................................................. 176 Figure 5.19. 1H NMR spectra comparing copolymers before and after exposure to 500 ppm NaOCl for 24 h (pH of 4.5-5.0) (BisAS100-BisAS0 8k8k multiblock copolymer (a) before and (b) after exposure, BisAS32 random copolymer (c) before and (d) after exposure)......................................................................................................................... 177
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TABLE OF TABLES
Table 2.1. Characterization of Hydrophilic Telechelic Oligomers.................................. 91 Table 2.2. Characterization of Hydrophilic and Hydrophobic Telechelic Oligomers for BisSF-BPSH100 Multiblock Copolymers ........................................................................ 96 Table 2.3. Characterization of Segmented and Multiblock BisSF-BPSH100 Copolymers........................................................................................................................................... 97 Table 2.4. Comparison of Tensile Properties for Segmented and Multiblock Copolymers........................................................................................................................................... 98 Table 3.1. Characterization of BisSF-BPSH100 and PhF-BPS100 Segmented Copolymers......................................................................................................................................... 117 Table 3.2. Tensile Properties of BisSF-BPSH100 and PhF-BPSH100 Segmented Copolymers ..................................................................................................................... 122 Table 4.1. Target and Experimental Mn for PhS100 Oligomers.................................... 136 Table 4.2. Characterization of BisSF-PhSH100 Segmented Copolymer ....................... 140 Table 4.3. Tensile Properties of BisSF-PhS Segmented Copolymers ............................ 143 Table 5.1. Characterization of Hydrophobic and Hydrophilic Telechelic Oligomers ... 163 Table 5.2. Characterization of BisAS100-BisAS0 Multiblock Copolymers ................. 170 Table 5.3. Tensile Properties of BisAS Copolymers ...................................................... 176
1
1 Literature Review
1.1 Ionomers
Copolymers which contain ionic groups throughout the polymer backbone have
been termed ionomers.1 The interactions that result from the ionic groups strongly
influence the structure and properties of these copolymer systems.2 Ionomers, employed
as ion-exchange membranes, have found many applications in electro-membrane
processes and separation and purification processes.3 They have the ability to separate
ions and can be used to recover desirable ions from solution, remove unwanted ions, or as
a transportation medium. Examples of these applications include reverse osmosis,
nanofiltration, ultrafiltration, microfiltration, pervaporation, electrodialysis, fuel cell
applications, and membrane based sensors.
This review focuses on membrane applications in fuel cells and reverse osmosis.
Copolymer development for membrane materials for both of these areas is discussed
including the role disulfonated poly(arylene ether sulfone) copolymers play.
1.2 Fuel Cells
Fuel cells are electrochemical energy conversion devices which convert chemical
energy into electrical energy via redox reactions at the cathode and anode.4 An
electrolyte is present to facilitate ion transfer. Fuel cells have been utilized as energy
devices since their early applications in the Gemini and Apollo space programs.
Currently, they are being explored as alternative energy devices in applications such as
stationary power sources, automobiles, and portable electronics.
2
Fuel cells possess many desirable characteristics which have stimulated research
in this area.4 They offer an environmentally friendly alternative over conventional
systems because they produce far less emissions and are more efficient. This reduces
fossil fuel consumption and greenhouse gas emissions. They operate at a high energy
density. Because their operation is quiet and safe, they can be located close to the
application site.
There are five major classifications of fuel cells, which are categorized by the
type of electrolyte: alkaline fuel cells (AFC); polymer electrolyte membrane, or proton
exchange membrane, fuel cells (PEMFC); phosphoric acid fuel cells (PAFC); molten
carbonate fuel cells (MCFC); and solid oxide fuel cells (SOFC). The first three types are
classified as low temperature fuel cells, operating at or below temperatures of 200 oC;
whereas, the latter two are considered high temperature fuel cells and operate at
temperatures above 450 oC. The focus of this literature review will be on the synthesis of
polymers to be utilized as the proton exchange membrane in PEMFC applications.
1.3 PEM Fuel Cells
PEMFCs are subcategorized according to the type of fuel. The first type utilizes
hydrogen and oxygen as the fuel and is often referred to as PEMFCs, or hydrogen/air fuel
cells. Although the hydrogen can be obtained from natural gas or gasoline, pure
hydrogen is the optimal source because it could be produced by the electrolysis of water
using solar energy, which would eliminate the need for fossil fuels.5 The oxygen does
not have to be pure and can be obtained from air. PEMFCs are most applicable for
stationary power and automotive applications. The second type is direct methanol fuel
3
cells (DMFC), which utilize a dilute methanol solution as the fuel source. These have
applications for powering portable electronic devices.
The PEM is in the center of the fuel cell with an anode and cathode on either side.
These three components combine to form the membrane electrode assembly (MEA).
Hydrogen (H2) (or methanol in the case of DMFC) is oxidized into protons and electrons
at the anode. The protons pass through the PEM and the electrons form a current through
an external circuit. Oxygen enters the cathode, where it is reduced and combined with
the protons and electrons. Water is the only byproduct of this reaction when hydrogen
and oxygen are used as fuel. The electrochemical redox reactions for a PEMFC and
DMFC are provided below in Figure 1.1 and Figure 1.2, respectively.
Anode: −+ += eHH 442 2
Cathode: OHeHO 22 244 =++ −+
Overall: OHHO 222 22 =+
Figure 1.1. Electrochemistry for PEMFC.
Anode: −+ ++=+ eHCOOHOHCH 66223
Cathode: OHeHO 2223 366 =++ −+
Overall: OHCOOOHOHCH 22223
23 3+=++
Figure 1.2. Electrochemistry for DMFC.
1.4 Materials Used for PEMs
The PEM is such an important component of both PEMFCs and DMFCs, and
significant effort has gone into investigating new materials for use in these applications.
There are important criteria which need to be considered when evaluating materials for
4
PEM applications.6,7 PEMs should have high protonic conductivity and low electronic
conductivity. Low water transport through the membrane is necessary to prevent
“flooding” at the cathode. The membrane must be oxidatively and hydrolytically stable
and display good mechanical properties across broad temperature ranges (-40 to 120 oC)8
and humidity levels (20 to 100%)8. The cost and capability to fabricate membranes into
MEAs for fuel cells must be feasible. Low permeability to fuel and oxidants is required.
Finally, minimization of swelling-deswelling due to the cycling of water must be
considered.
1.4.1 Nafion®
Nafion® is the DuPont trade name for a perfluorosulfonic acid membrane, which
is the current ion exchange polymer being used in almost all commercial PEM
applications. It has been manufactured by DuPont since the late 1960s and has
application as a permselective separator in chlor-alkali electrolyzers.9 Nafion® is
produced via a free radical copolymerization of tetrafluoroethylene (TFE) comonomer
and perfluorinated vinyl ether comonomer bearing perfluorosulfonic acid (or their
precursor) groups.
The exact comonomer composition of the copolymer has not been disclosed.
Theoretically, different compositions can be obtained by altering the comonomer ratio (x
and y in Figure 1.3). The equivalent weight (EW) of Nafion® indicates the level of
sulfonation and refers to the moles of sulfonic acid group per gram of copolymer in acid
form.10 Until recently Nafion 1100 has been the most widely available material in
thicknesses of 2, 5, 7, and 10 mils, which corresponds to Nafion 112, 115, 117, and 1110,
respectively. This form of Nafion® is obtained by extrusion, which is in the –SO2F form
5
during processing and later converted to the salt or acid form. A more recent addition to
the Nafion® family is a dispersion cast film which is available in 1 and 2 mil thicknesses,
NRE211 and NRE212, respectively.
CF2 CF2 CF CF2
OCF2 CF O(CF2)2 SO3-H+
CF3
x y
z
n
Figure 1.3. Chemical structure of Nafion®.
Nafion® possesses many desirable properties, which is why it is currently the
state-of-the-art PEM.6 Nafion® provides excellent proton conductivity, ranging from
0.009 to 0.12 cm/S at 80 oC from 34-100% relative humidity (RH).11 It has good
chemical resistance due to a highly fluorinated partially crystalline backbone. The
modest amount of crystallinity retained by this copolymer during extrusion imparts
mechanical strength.
However, new materials are needed because Nafion® has several restrictions.6,9
Nafion® membranes have low conductivity at high temperatures, which limits them to
operation below 80 oC. They display high methanol permeability, which decreases
performance in DMFC applications. Nafion® is costly as well, making fuel cell vehicles
too expensive for most consumers. Because all perfluorinated membranes have similar
restrictions, hydrocarbon membranes are being explored for use in PEMFC and DMFC
applications.
6
1.4.2 Poly(arylene ether) Copolymers
1.4.2.1 Synthesis
Poly(arylene ether)s include poly(arylene ether sulfone)s, poly(arylene ether
ketone)s, poly(ether imide)s, and poly(phenylene ether)s. Poly(arylene ether)s possess
excellent properties, including chemical, mechanical, hydrolytic, and thermal stability,
making them good candidates for proton exchange membranes when sulfonated.12 There
are several ways to synthesize poly(arylene ether)s, including nucleophilic aromatic
substitution and electrophilic aromatic substitution. The Ullmann polymerization can be
used as well.12
1.4.2.1.1 Nucleophilic Aromatic Substitution (SNAR)
Poly(arylene ether)s were first synthesized in high molecular weight by Farnham
and Johnson12 for Union Carbide Corporation in the early 1960s using nucleophilic
aromatic substitution. They synthesized a polysulfone using Bisphenol A and 4,4’-
dichlorodiphenyl sulfone and commercially produced it under the tradename Udel®.
SNAR is still the method used today in commercial production now by Solvay Advanced
Polymers.
Proper reaction conditions must be used in order for poly(arylene ether)s to be
successfully synthesized using an SNAR reaction. High molecular weight and optimum
polymer properties can only be achieved when monomer choice, reaction stoichiometry,
and the minimization of side reactions are controlled precisely. A generalized SNAR
reaction is depicted in Figure 1.4.
7
OCH3
OCH3
+K2CO3 orAqueous NaOH
Dipolar aprotic solventAzeotroping agent
+ 2MCl
- -OHCH3
OHCH3
*CH3
CH3
OO* SO
Oy
SO
O
Cl ClOCH3
OCH3
+K2CO3 orAqueous NaOH
Dipolar aprotic solventAzeotroping agent
+ 2MCl
- -OHCH3
OHCH3
*CH3
CH3
OO* SO
Oy
SO
O
Cl Cl
Figure 1.4. Synthesis of poly(arylene ether sulfone) by nucleophilic aromatic substitution.
Monomer choice plays an important part in nucleophilic aromatic substitution
step polymerizations. The reactivity of the aromatic dihalide is affected by both the
electron-withdrawing nature of the substituents that are para or ortho to the halogen and
reactivity of the halogen itself. The dihalide must contain an electron-withdrawing group
which is ortho or para to the halogen being displaced. The reactivity of the dihalide
increases as the electron-withdrawing group becomes stronger (-NO2 ~ -SO2 > -C=O > -
N=N-). Dihalides which stabilize the reaction intermediate by forming a Meisenheimer
complex, are more reactive; thus, F>>Cl>Br, I.12 For a SNAR to occur, the bisphenol
must first be converted to a bisphenolate salt.13 Cotter12 reports K2CO3 as the preferred
base when using the carbonate process because it is more soluble than Na2CO3 and
affords a more reactive potassium phenoxide. Aqueous NaOH or KOH, often referred to
as the aqueous caustic method, have been used, but this method requires solubility of the
bisphenolate, a precise amount of base to prevent degradation of reactive halide groups,
as well as careful removal of the water associated with the base.14 Bisphenols should be
chosen based on the electron-donating power of the substituents para or ortho to the
8
reactive site. Bisphenols that have electron-withdrawing groups, or are more acidic, are
less reactive. It is important that the bisphenolate salts remain thermally stable
throughout the polymerization. Fortunately, the less reactive bisphenols containing
electron-withdrawing groups are thermally stable at higher temperatures, so higher
temperatures (> 165 oC) could be used to increase the reactivity.12 In order to achieve
high molecular weight, monomers must be of high purity and be added to the reaction
using a one to one stoichiometry.
Dipolar aprotic solvents are necessary for these reactions. They increase the
nucleophilicity of the bisphenolate when compared to protic solvents. The alkali
bisphenolate, aromatic dihalide, and the growing polymer chain are all soluble in these
solvents. Sulfolane, dimethylacetamide (DMAc), and N-methyl pyrrolidone (NMP) have
all been reported for the carbonate method and are chosen depending upon the boiling
point of the solvent and the desired reaction temperature;14 whereas, dimethyl sulfoxide
(DMSO) and diphenyl sulfone have been used for the aqueous caustic method.12,14
The water and oxygen content in the reactions should be minimized to reduce side
reactions. Moisture or alcohol contamination can prevent high molecular weight
polymers from forming. Water reacts with the activated bisphenolate salt to form caustic.
Caustic can then react with the activated dihalide which ultimately results in an upset in
monomer stoichiometry (Figure 1.5). The caustic can also react with the forming
polymer chain at the activated ether linkages resulting in chain cleavage. It is important
to distill all solvents prior to use and store over molecular sieves to prevent water from
entering the system. Water that is formed during the reaction or that may be introduced,
as in the aqueous caustic method, can be eliminated by azeotroping with an appropriate
9
solvent. Examples include toluene, cyclohexane, and chlorobenzene. An inert
atmosphere must be maintained to prevent oxidation of the bisphenolate salt, which
inhibits the formation of high molecular weight polymer and causes polymer
discoloration. This can be achieved by performing the reaction under constant nitrogen
or argon flow.
S
O
O
ClCl + 2NaOH S
O
O
ONaCl + NaCl + H2O
Figure 1.5. Side reactions due to water or excess base in SNAR polymerizations.12
1.4.2.1.2 Electrophilic Aromatic Substitution
Several commercial polymers have been synthesized using electrophilic aromatic
substitution (Figure 1.6). AstrelTM 330, which is a poly(aryl ether sulfone), was
synthesized by 3M starting in the early 1960s using this process. The Raychem
Corporation produced a poly(aryl ether ketone) under the name StilanTM in the early
1970s. Both polymers are no longer made due to problems with this synthetic method.
Defects, including ortho linkages in the polymer backbone and branch formation, make
this method less advantageous for industry because melt processability and mechanical
properties are hindered. Also, large amounts of Lewis acid are needed for electrophilic
reactions making this method costly.12
10
SO2Cl + O SO2Cl
FeCl3C6H5NO2
SO2O SO2* *x
y
SO2Cl + O SO2Cl
FeCl3C6H5NO2
SO2O SO2* *x
y
Figure 1.6. Synthesis of poly(arylene ether sulfone) by electrophilic aromatic substituion.12
1.4.2.1.3 Ullmann Reaction
Poly(arylene ether)s have been synthesized to high molecular weight using a
modified version of the Ullmann reaction.12,15,16 It is useful for applications when the
dihalide does not contain an electron-withdrawing group ortho or para to the halogen
being displaced. Therefore, poly(arylene ether)s with no sulfone or ketone linkages can
be synthesized. In this reaction, a copper catalyst is used, which coordinates with the pi
system of the aromatic halide, allowing the halogen to be cleaved from the carbon
(Figure 1.7).12,15 The order of halogen replacement (I>Br>Cl>F) is opposite that of
activated dihalide systems. The more reactive dihalides are more expensive, this is not
ideal.
11
NaO
CH3
ONa
CH3
Br Br
CH3
CH3
OO* *n
+
Cu2Cl2, pyridine200 oC
+ 2 NaBr
NaO
CH3
ONa
CH3
Br Br
CH3
CH3
OO* *n
+
Cu2Cl2, pyridine200 oC
+ 2 NaBr
Figure 1.7. Synthesis of poly(arylene ether sulfone) by a modified Ullmann Reaction.12,15
1.4.2.2 Post-Sulfonation Modification
Post-sulfonation is widely used to convert aromatic polymers, such as
poly(arylene ether)s, to sulfonated ionomers. It has remained an attractive avenue for
PEM material development because poly(arylene ether)s possess many of the desired
properties needed for an alternative PEM material and are commercially available. Post-
sulfonation proceeds via an electrophilic aromatic substitution reaction, which can be
achieved using a variety of sulfonating agents including concentrated sulfuric acid,
fuming sulfuric acid, chlorosulfonic acid,17 and various complexes of sulfur trioxide.18
The directing effects of the polymer backbone substituents determine where the
sulfonation will take place (Figure 1.8). Because post-sulfonation places the sulfonic
acid moieties in the activated positions, they are more susceptible to desulfonation.19
12
1.4.2.2.1 Post-Sulfonated Poly(arylene ether sulfone) Copolymers
The post-sulfonation of poly(arylene ether sulfone)s has been explored by many
researchers.18,20,21,22 As noted earlier, many methods of post-sulfonation have been
utilized for this process. Although most of the post-sulfonation methods yield ionomers
with similar properties, which largely depend on the degree of sulfonation, there are
advantages and drawbacks among the methods.
S
O
OSO3H
O* *n
O ** S O
O
O
CH3
CH3
HO3S
n
O *
CF3
CF3
* S O
O
O
SO3H n
(a)
(b)
(c)
S
O
OSO3H
O* *n
O ** S O
O
O
CH3
CH3
HO3S
n
O *
CF3
CF3
* S O
O
O
SO3H n
S
O
OSO3H
O* *n
O ** S O
O
O
CH3
CH3
HO3S
n
O *
CF3
CF3
* S O
O
O
SO3H n
(a)
(b)
(c)
Figure 1.8. Examples of post-sulfonated poly(arylene ether sulfone)s. Sulfonic acid group is located in the activated position (ortho to the ether group). (a) Sulfonated
poly(ethersulfone) (b) sulfonated polysulfone (c) hexafluorinated sulfonated polysulfone.19
In early work, Noshay and Robeson18 post-sulfonated a commercially available
bisphenol A-based poly(arylene ether sulfone) (PSF) using a mild sulfur trioxide-triethyl
phosphate complex. By varying the ratio of SO3 to PSF, the degree of sulfonation was
controlled from 0.1-1.0 SO3 groups per repeat unit, corresponding to ion exchange capacities
13
of 0.3-2.2 meq/g, respectively. Both the salt and acid from of these polymers exhibited an
increase in Tg when compared to the parent polymer, which scaled with the degree of
sulfonation. However, the difference in Tg was not as pronounced in the acid form as it was
in the salt form. Water uptake increased with increasing degree of sulfonation, ranging from
5.0% at 0.1 SO3Na/PSF to 61.4% at 1.0 SO3Na/PSF.
Genova-Dimitrova et al.21 also post-sulfonated the commercially available PSF
copolymer using two different techniques. A comparison was made between post-
sulfonation with chlorosulfonic acid and with trimethylsilylchlorosulfonate (TMSCS).
When chlorosulfonic acid was used as the sulfonating agent, a small amount of
dimethylformamide had to be added to maintain homogeneity during the reaction. This
ensured the polymer was uniformly sulfonated. This method of sulfonation resulted in a
decreased intrinsic viscosity (IV) when compared to the parent polymer, which implies
main chain cleavage during sulfonation. In contrast, when TMSCS was used as the
sulfonating agent, no heterogeneity was observed in the reaction mixture and the IV
results indicated no backbone cleavage. SPSF sulfonated with TMSCS displayed higher
elongation at break when subjected to mechanical stress-strain tests under a tensile load.
Both methods failed to produce polymer with a degree of sulfonation comparable to the
theoretical predictions. A degree of sulfonation of 1.35 SO3 groups per repeat unit was
achieved but further conversion could not be achieved regardless of the length of the
reaction time. Polymers with high IEC and conductivity values could be synthesized.
However, the polymers swelled, even partially dissolving in water at 80 oC, making them
unlikely candidates for PEMs.
14
1.4.2.2.2 Post-Sulfonated Poly(arylene ether ketone) Copolymers
Poly(ether ether ketone)s (PEEK) have also been used extensively to make post-
sulfonated polymers.23,24,25,26,27,28,29 Several problems arise when PEEK is post-
sulfonated. Unlike poly(aryl ether sulfone)s, PEEK dissolves in few solvents because of
its semi-crystalline nature. Some of the early sulfonation work resulted from the desire to
find a solvent to characterize these polymers. Strong acids were used as the solvent.29
However, dissolution and sulfonation of the polymer happened concurrently in strong
acids. Therefore, low levels of random sulfonation were hard to achieve because of the
heterogeneity of the polymer solution during sulfonation. Sulfonation levels as high as
30% could be reached before a homogeneous solution was formed.23,27 Various degrees
of crosslinking and degradation have been reported when a sulfur trioxide/triethyl
phosphate complex28 or chlorosulfonic acid29 were used as the sulfonating agent.
Bailly et al.23 studied the post-sulfonation of PEEK copolymer using two
sulfonation techniques. Various ratios of methanesulfonic acid (MSA) and sulfuric acid
were used as the first sulfonating agent and concentrated sulfuric acid was the second.
The former reaction medium allowed for dissolution and sulfonation to occur separately
because MSA was able to dissolve the polymer without sulfonating it. Although this
medium could be used to produce randomly sulfonated PEEK (SPEEK) with low levels
of sulfonation (5-40 mol%), it would not be useful for sulfonation levels greater than that
because the ratio of MSA to sulfuric acid becomes impractical. A sulfuric acid
concentration of 96.4% as the sulfonating agent resulted in sulfonation of 25-70 mol%.
Although the samples were not characterized in the acid form, SPEEK samples in the
sodium form displayed an increase in Tg as the sulfonation degree increased, much like
15
the SPSF samples. The addition of sodium sulfonate decreased the crystallinity, which
helped to increase the solubility of these copolymers in organic solvents.
Several groups have further studied the post-sulfonation of PEEK using sulfuric
acid, with the focus on characterizing SPEEK for use as a PEM in fuel cell
applications.24,26 SPEEK samples with percent sulfonation of 30–97%, which
corresponds to IEC values of 0.5 to 1.55 meq/g, were achieved. Sample preparation and
pretreatment methods used to prepare the films varied between the research groups, as
did the testing conditions. The highest conductivity measured for SPEEK was 0.11 S/cm.
This was measured for two different samples, one having 96% sulfonation26 and one 60%
sulfonation.24 The former was tested under fully hydrated conditions at 25 oC, while the
later was tested at 100% RH at 150 oC, 6.1 atm. The water uptake of the samples
obtained approached 100% at high sulfonation levels. In some cases, this prevented the
copolymers from being analyzed because their dimensional changes made the
conductivity measurements unreliable.
In order to combat the increased water swelling in sulfonated poly(arylene ether)
copolymers which possess high IEC values, several groups have proposed crosslinking
the membranes to suppress the swelling, while still maintaining high conductivity.20,30
Nolte et al.20 formed crosslinked membranes from post-sulfonated poly(arylene ether
sulfone)s (commercially available UDEL® P-1700) using 1,1’-carbonyldiimidazol and
diamine as the crosslinking agents (Figure 1.9). Because bis-(4-amino-phenyl)-sulfone is
not as reactive as its aliphatic counterparts, the ionomer and crosslinking agents could be
mixed, cast, and then cured at elevated temperatures, which afforded a sulfonated
16
crosslinked membrane. About a 50% decrease in swelling was observed for crosslinked
membranes, while still maintaining acceptable conductivity levels.
SPolymer
O
O
OH NN
NN
O
SPolymer
O
O
NN
SPolymer
O
O
O CO2H2N
N+ + + +
+
SPolymer
O
O
NN
NH2 R NH2 SPolymer
O
O
NH
R NH
S Polymer
O
O
NHN
+ + 2
(a)
(b)
SPolymer
O
O
OH NN
NN
O
SPolymer
O
O
NN
SPolymer
O
O
O CO2H2N
N+ + + +
+
SPolymer
O
O
NN
NH2 R NH2 SPolymer
O
O
NH
R NH
S Polymer
O
O
NHN
+ + 2
SPolymer
O
O
OH NN
NN
O
SPolymer
O
O
NN
SPolymer
O
O
O CO2H2N
N+ + + +
+SPolymer
O
O
OH NN
NN
O
SPolymer
O
O
NN
SPolymer
O
O
O CO2H2N
N+ + + +
++
SPolymer
O
O
NN
NH2 R NH2 SPolymer
O
O
NH
R NH
S Polymer
O
O
NHN
+ + 2
(a)
(b)
Figure 1.9. Crosslinking scheme for post-sulfonated UDEL using 1,1’-carbonyldiimidazole (CDI) and diamine as the crosslinking agents. (a) Sulfonic acid
groups are activated by CDI (b) N-sulfonylimidazoles are converted to sulfonamides.20
Kerres et al.30 crosslinked post-sulfonated UDEL using methods for post-
sulfonating and crosslinking which differed from previous studies. First, UDEL was
post-sulfonated via a metalation procedure.31 This multi-step procedure resulted in a
post-sulfonated UDEL where the sulfonic acid was placed in the deactivated position of
the UDEL backbone (ortho to the sulfone group). The reader is referred to the original
work for a detailed description of this post-sulfonation process.31 The polymer could be
crosslinked by first oxidizing a sulfinated polymer to form a partially
sulfonated/sulfinated polymer, followed by crosslinking between the sulfinate groups
using diiodobutane as an S-alkylation crosslinking agent. Crosslinking the polymer
decreased the water swelling but it also reduced the IEC. This method does not seem
feasible for large scale production due to the many steps needed to form the crosslinked
polymer.
17
1.4.3 Directly Polymerized Sulfonated Monomers to form Sulfonated Poly(arylene
ether) Random Copolymers
Among the limitations of post-sulfonation modification are the ability to fully
control the degree and location of sulfonation, as well as the ability to form a truly
random copolymer. Direct polymerization of sulfonated monomers not only allows
precise levels of sulfonation to be obtained, but also affords a statistical random
distribution of sulfonic acid moieties in the polymer backbone. The position of
sulfonation can be directed using this technique. Unlike in most post-sulfonation
modification reactions, where the sulfonic acid groups are placed in the activated
positions, acid groups could be placed in the more stable, more acidic, deactivated
positions. The degree of sulfonation can be increased to two sulfonic acids per repeat
unit, yielding a polymer with a higher IEC, and potentially higher conductivity, when
compared to a post-modified polymer.
1.4.3.1 Poly(arylene ether)s Containing Disulfonated Sulfone Monomers
To obtain a sulfonated copolymer via a direct polymerization route, a sulfonated
monomer is required. Although originally reported for its flame retardant applications,32
3,3’-disulfonated-4,4’-dichlorodiphenyl sulfone (SDCDPS) has been used extensively for
the synthesis of sulfonated polymers. Ueda et al.33 later described the synthesis of this
polymer-grade monomer and carried out several nucleophilic aromatic substitution
copolymerizations with SDCDPS, 4,4’-dichlorodiphenylsulfone (DCDPS), and various
bisphenols to produce hydrophilic polymers with varying degrees of sulfonation. The
purification and characterization of SDCDPS was further refined by the McGrath group
(Figure 1.10).34,35,36
18
S
O
O
ClCl
S
O
O
ClCl
SO3Na
NaO3S
dichlorodiphenyl sulfone(DCDPS)
disulfonated dichlorodiphenyl sulfone(SDCDPS)
SO3 (28%)
110 oC6 h
NaCl NaOHH2O NaCl
pH = 6-7
SDCDPS was recrystalized in H2O/IPA to produce polymer grade monomer
S
O
O
ClCl
SO3H
HO3S
S
O
O
ClCl
S
O
O
ClCl
SO3Na
NaO3S
dichlorodiphenyl sulfone(DCDPS)
disulfonated dichlorodiphenyl sulfone(SDCDPS)
SO3 (28%)
110 oC6 h
SO3 (28%)
110 oC6 h
NaCl NaOHH2O NaCl
pH = 6-7
NaCl NaOHH2O NaCl
pH = 6-7
SDCDPS was recrystalized in H2O/IPA to produce polymer grade monomer
S
O
O
ClCl
SO3H
HO3S
Figure 1.10. Synthesis of monomer grade SDCDPS.
The McGrath group further explored the use of this monomer in proton exchange
membranes for fuel cells; an especially promising series of copolymers was obtained when
biphenol was used (Figure 1.11).34,37 These polymers were synthesized with varying degrees
of sulfonation, ranging from zero to two sulfonic acid groups per repeat unit, by adjusting the
ratio of sulfonated to nonsulfonated monomer, while maintaining an overall one to one
stoichiometry of dihalide to biphenol. The polymer was coined BPSH-XX because it
contains a backbone based on 4,4’-biphenol and sulfonated and nonsulfonated
dichlorodiphenol sulfone monomers. The H refers to the polymer in acid form, and XX
denotes its molar percent of disulfonic acid unit.
19
O S
O
O
O
NaO3S
SO3Na
* O S
O
O
O *n
O S
O
O
O
HO3S
SO3H
* O S
O
O
O *n
OHOH S
O
ONaO3S
SO3Na
Cl ClS
O
O
Cl Cl
0.35 0.65
0.35 0.65
+ (0.65) + (0.35)
NMP/tolueneK2CO3
~160 oC, 4 h190 oC, 16 h
Acidification
BPS-35
BPSH-35
O S
O
O
O
NaO3S
SO3Na
* O S
O
O
O *n
O S
O
O
O
HO3S
SO3H
* O S
O
O
O *n
OHOH S
O
ONaO3S
SO3Na
Cl ClS
O
O
Cl Cl
0.35 0.65
0.35 0.65
+ (0.65) + (0.35)
NMP/tolueneK2CO3
~160 oC, 4 h190 oC, 16 h
Acidification
BPS-35
BPSH-35
Figure 1.11. Direct polymerization of SDCDPS, DCDPS, and 4,4’-biphenol to form
BPSH-35 random copolymer.
These disulfonated copolymers exhibited better characteristics than those
observed in post-sulfonated poly(arylene ether sulfone)s.34,37 The disulfonated polymers
showed no signs of side reactions, such as crosslinking. The sulfonic acid groups were
placed on the deactivated position of the polymer backbone (meta to the sulfone group
and ortho to the ether linkage). Thermogravimetric analysis (TGA) showed that
desulfonation of BPSH-60 began to occur above 300 oC, when tested in acid form under a
nitrogen atmosphere. Desulfonation did not occur until higher temperatures for polymers
with lower degrees of sulfonation. These results indicate that the deactivated position
may be more stable than the activated position in post-sulfonated systems. Differential
scanning calorimetry (DSC), atomic force microscopy (AFM), and water uptake values
for these films revealed an interconnected hydrophilic phase developed when the
polymers contained high levels of sulfonation (≥ 50%). Although the conductivity of
20
BPSH-60 (0.17 S/cm in liquid water at 30 oC) exceeded Nafion 1135 (0.12 S/cm under
the same conditions), the water uptake (150 %) of this polymer appears too high,
hindering the mechanical stability in PEM applications.
Many other disulfonated poly(arylene ether) random copolymers have been made
by directly copolymerizing SDCDPS with various bisphenols and dihalides (Figure 1.12).
X =
Y =
CH3
CH3
CF3
CF3
S
O
O
O
P
OCN
O S
O
O
O
HO3S
SO3H
On
x
X X O Y
(1-x)
X =
Y =
CH3
CH3
CF3
CF3
S
O
O
O
P
OCN
O S
O
O
O
HO3S
SO3H
On
x
X X O Y
(1-x)
Figure 1.12. Disulfonated poly(arylene ether) random copolymers containing different aryl linkages.
The properties of the polymer can be greatly altered by changing the bisphenol
used in the synthesis.38 Because the molecular weights of the bisphenols vary, IEC
values for the corresponding polymers differ. The use of hydroquinone (HQ) produced
polymers which had high IEC values at lower levels of sulfonation, whereas 4,4’-
hexafluoroisopropylidenediphenol (6F-BPA) produced polymers with lower IEC values
at higher levels of sulfonation. Although water uptake was affected by the IEC of the
21
copolymer, the hydrophobicity of the bisphenol appeared to impact water uptake as well.
When comparing copolymers with roughly 1.5 meq/g IEC, polymers containing 6F-BPA
monomer had the lowest water uptake (34 w/w%). The thermal stability of the
copolymers was dependant on the bisphenol structure. Polymers containing bisphenol-A
(BPA) displayed a 5% weight loss at 490 oC compared to a 5% weight loss at 520 oC for
HQ and biphenol-based copolymers. This is most likely due to the aliphatic C-H bonds
in the isopropylidene unit of BPA. The lower thermal stability could make disulfonated
BPA-based copolymers less likely candidates for fuel cell applications.
Different nonsulfonated dihalide structures have been used in combination with
SDCDPS to afford polymers with varying properties. Disulfonated poly(arylene ether)
random copolymers have been synthesized using 2,6-dichlorobenzonitrile (DCBN) along
with SDCDPS as the activated halide.39 These were reacted with 6F-BPA monomer to
afford polymers with 5 – 55% sulfonation. Although IEC values for these polymers were
higher than the analogous copolymers utilizing no DCBN monomer, water uptake values
were lower for polymers with 20-35 % disulfonation. This could be due to an increase in
overall fluorine content but has not been further investigated.
1.4.3.2 Poly(arylene ether)s Containing Disulfonated Ketone Monomers
Direct synthesis of sulfonated poly(arylene ether ketone) (SPAEK) random
copolymers from disulfonated ketone comonomer and nonsulfonated aromatic monomers
offers many advantages over post-sulfonation methods. As discussed previously, post-
sulfonation of PAEKs occurs heterogeneously due to their low solubility in acidic
medium. The direct polymerization of disulfonated ketones allows for sulfonation to
occur homogeneously throughout the polymer, which could result in more random
22
dispersion of the sulfonate groups. Also, higher degrees of sulfonation can be achieved
by increasing the stoichiometric amount of sulfonated monomer used. Sulfonation levels
greater than 1.0 sodium sulfonate group per repeat unit can be achieved.
Disulfonated ketone dihalides, such as sodium 5,5’-carbonylbis(2-fluorobenzene-
sulfonate) 40,41, 42,43 and 1,4-bis(3-sodium sulfonate-4-fluorobenzoyl)benzene,44 have been
utilized in the synthesis of various SPAEK copolymers (Figure 1.13). Partially
fluorinated disulfonated ketone polymers containing (3,5-ditrifluorometheyl)phenyl-
hydroquinone (6FP) (Figure 1.14c) showed suitable water uptake and proton
conductivity. Polymers containing 6FP comonomer with 50% disulfonation had water
uptake of 29% (80 oC) and proton conductivity of 1.0 x 10-1 S/cm (80 oC in liquid
water).44 It was proposed that the bulkiness of the 6FP pendent group increased the free
volume between the polymer chains, which increased the water uptake. However, this
led to a higher conductivity for these polymers at elevated temperatures.
23
O
CF3
CF3
O
OO
O
CF3
CF3
O
OSO3H
SO3H
n
n
CF3
CF3
CH3
CH3
X=
O X* O
SO3H
HO3S
O X O *
O
n
O
n
O
O
(a)
CF3
CF3
OO
*
O
O
O
YO* O Y On
n
SO3H
HO3S
Y=
(1-x)
(1-x)
(1-x)
X
X
X
(b)
(c)
O
CF3
CF3
O
OO
O
CF3
CF3
O
OSO3H
SO3H
n
n
CF3
CF3
CH3
CH3
X=
O X* O
SO3H
HO3S
O X O *
O
n
O
n
O
O
(a)
CF3
CF3
OO
*
O
O
O
YO* O Y On
n
SO3H
HO3S
Y=
(1-x)
(1-x)
(1-x)
X
X
X
(b)
(c)
Figure 1.13. Disulfonated poly(arylene ether ketone) random copolymers using (a-b) 5,5’-carbonylbis(2-fluorobenzene-sulfonate) or (c) 1,4-bis(3-sodium sulfonate-4-
fluorobenzoyl)benzene as the sulfonated comonomers.
1.4.3.3 Poly(arylene ether)s Containing Sulfonated Naphthalene Monomers
Several sulfonated naphthalene diol monomers have been used to synthesize
sulfonated poly(arylene ether)s (Figure 1.14).45,46,47 The sulfonated naphthalene
monomers were introduced to serve two purposes: increased dimensional stability and
improved interconnectivity. The enhancement of the polymer depended upon the
orientation of the naphthalene unit in the polymer backbone. It was proposed that
monomers (a) and (b) in Figure 1.14 could impart rigidity to the polymer chain,
improving the dimensional stability and mechanical properties under hydrated
24
conditions.45,47 If monomer (c) were introduced into a polymer backbone, the sulfonic
acid groups would be located on a pendant side chain which could created a more
interconnected hydrophilic network, leading to higher conductivity.46,47
OHOH
SO3NaNaO3S
OH
SO3Na
OH
(a) (b) (c)
OH OH
SO3Na
OHOH
SO3NaNaO3S
OH
SO3Na
OH
(a) (b) (c)
OH OH
SO3Na
Figure 1.14. Sulfonated naphthalene diol monomers. (a) 2,7-dihydroxynaphthalene-3,6-sulfonate disodium salt (b) 2,8-dihydroxynaphthalene-6-sulfonate sodium salt
and (c) 2,3-dihydroxynaphthalene-6-sulfonate sodium salt.
These monomers were reacted with 4,4’-biphenol or hydroquinone (to control the
degree of sulfonation) and either 1,3-(bis-fluorobenzoyl)-benzene (BFBB) or 2,6-
difluorobenzonitrile (DFBN) was used as the activated dihalide. When monomers (a) or
(b) were used as the diol, swelling in liquid water was less when compared to monomer
(c) if compared at similar equivalent weights.47 Polymers synthesized with monomer (c)
and DFBN had comparable conductivity and similar water uptake to Nafion®.
1.4.3.4 Poly(arylene ether)s Containing Other Sulfonated Monomers
Other sulfonated comonomers have been utilized to afford poly(arylene ether)
copolymers for use as PEM materials. Sulfonated bis(4-fluorophenyl)phenyl phosphine
25
oxide (SBFPPO)48 and sulfonated hydroquinone (SHQ)49 have been used to synthesize
sulfonated poly(arylene ether phosphine oxide)s and poly(arylene ether ketone)s,
respectively. Although conductivity values are not reported in literature, these
copolymers may not compare well to Nafion®. Because these comonomers are only
monosulfonated, they produce polymers with lower IEC values. Therefore, degrees of
sulfonation well over 50% are required to produce polymers with a similar IEC value to
BPSH-35 (1.53 meq/g).
1.4.4 Block Copolymers
Recent efforts have been directed towards the synthesis of hydrophilic-
hydrophobic block copolymer ionomers for use as PEMs.50 Block copolymers contain
two or more types of polymer, with dissimilar backbone chemistries, which are
chemically bonded within the same chain. Phase separation occurs between the two
polymers, as in blended polymer systems. However, because the two types of polymers
are chemically linked, only micro- or nanophase separation occurs.51
Block copolymers become desirable candidates for PEMs if one of the blocks
contains a partially or fully ionic backbone. This hydrophilic ionic block provides high
protonic conductivity while the hydrophobic block supplies mechanical stability to the
system and may reduce the swelling of the hydrophilic block. It is proposed that ion-rich
channels form when the hydrophobic and hydrophilic domains of block copolymers
nanophase separate, allowing for higher conductivity even under partially hydrated
conditions.52
There are several types of block copolymers, including diblocks, triblocks, and
multiblock copolymers. Chain growth polymerization can be used to synthesize all three
26
types of copolymers. In addition, step-growth polymerization can be used for successful
synthesis of multiblocks. 51 To date, several types of block copolymers have been
synthesized for use as PEMs.
1.4.4.1 Diblock and Triblock Copolymers
Sulfonated polystyrene is used as the hydrophilic block of many di- and tri-block
copolymers for several reasons. Because polystyrene is synthesized via controlled radical
polymerization, the molecular weight of the hydrophilic block can be controlled easily.
The degree of sulfonation can be controlled using this method.53 Also, styrene-based
block copolymers are commercially available and can be converted to ionomers using
post-sulfonation methods.
Sulfonated styrene-ethylene-butylene-styrene triblock (S-SEBS) copolymer
membranes have been studied for their use as PEMs for fuel cell applications (Figure
1.15). 54,55,56,57,58 A nanophase separated morphology is observed in these polymers even
when low percentages (30%) of polystyrene are present.56
CH
CH2
CH2
CH
CH2
SO3H
* CH2
CH2
CH
CH2
CH3
CH
CH2
CH2
CH
SO3H
* x
y
n
m
n
Figure 1.15. S-SEBS triblock copolymer.
27
Both commercially sulfonated membranes, produced by companies such as
DAIS-Analytical Corp, and membranes made by post-sulfonating SEBS, have been
studied. Post sulfonation of SEBS is commonly carried out using acetyl sulfate55,56,58 but
has been achieved using chlorosulfonic acid57. Membranes with varying degrees of
sulfonation have been studied. Conductivities have been measured on the order of 10-1
S/cm when the samples are fully hydrated.59 However, these films correspond to high
degrees of sulfonation (55 mol%) and have had substantial water uptake (400%). The
samples also swell considerably in methanol, making them unlikely candidates for
DMFC applications.
Several modification methods have been explored to make S-SEBS membranes
more suitable for PEM applications. Plasma surface treatments have been conducted
which utilize maleic anhydride to introduce succinic anhydride groups to the surface of
the S-SEBS films.54 However, this layer reduced the permeability of both methanol and
protons. When the layer was subsequently hydrolyzed and acidified, forming carboxylic
acid sites to facilitate proton conductivity, proton conductivity showed more recovery
than methanol permeability.
Fillers, such as silica gel, mesoporous silica nanoparticles (SBA-15), and
sepiolite, have been functionalized with phenylsulfonic acid groups and subsequently
added to S-SEBS block copolymers in order to improve membrane properties.58 The
conductivity of films impregnated with 10% functionalized SBA-15 exceeded that of the
neat polymers and Nafion® (80 oC and 100% RH). It was suggested that the hydrophilic
functionalized phenylsulfonic fillers were dispersed throughout the hydrophobic and
hydrophilic domains, which increased proton transport. However, at RH levels below
28
70%, a sharp decrease in conductivity was observed, possibly to due to a disruption in the
ionic paths. The addition of fillers decreased water uptake from 150 (neat S-SEBS) to
104% (S-SEBS with 10% functionalized SBA).
Sulfonated poly(styrene-isobutylene-styrene) (S-SIBS) has been another series of
triblock copolymers explored for their use as PEM materials (Figure 1.16).60,61,62,63 Post
sulfonation of SIBS using acetyl sulfate provided membranes with degrees of sulfonation
ranging from 4 to 82 mol% in the polystyrene block. This corresponds to IEC values from
0.11 to 2.04 meq/g, for a polymer which is composed of 30 wt% polystyrene. However, at
high levels of sulfonation, the reaction becomes significantly less efficient. The reaction
decreases from 60% efficiency at 13 mol% sulfonation to 12% efficiency at 82 mol%
sulfonation.62
CH
CH2
CH2
CH
CH2
SO3H
* x
y
n
m
CH
CH2
CH2
CH
SO3H
*x
y
m
CH3
CH3
Figure 1.16. S-SIBS triblock copolymer.
As expected, both conductivity and water uptake increased as the IEC values
increased in S-SIBS membranes. When Nafion 117 and S-SIBS were compared at similar
IEC values (~0.9 meq/g), Nafion® outperforms the block copolymer 10:1 (0.027
S/cm:0.0026 S/cm; when measuring conductivity through-plane using a two-electrode cell in
liquid water). However, if the IEC of S-SIBS is increased to 2.04 meq/g, conductivity
29
increases to 0.076 S/cm, which is three times that of Nafion® 117.63,64 Although the polymer
never became soluble, water uptake increased from 24.9% to 348% when the IEC increased
from 0.97 to 2.04 meq/g, making it undesirable for a DMFC application. The authors
suggest at low RH levels, these membranes could have potential in hydrogen/air applications,
but no evidence was presented to support this statement.
Mokrini et al.65,66 studied sulfonated hydrogenated poly(butadiene-styrene) (S-
HPBS) diblock copolymers (Figure 1.17). Much like S-SEBS, this ionomer can be
produced by post-modification of the commercially available polymer, poly(butadiene-
styrene) diblock copolymers. First the butadiene portions are hydrogenated, forming a
random block composed of butylene and ethylene units. This is followed by sulfonation
of the polystyrene block, using acetyl sulfate as the sulfonating agent.
CH2
CH
CH2
CH2
CH
CH2
CH2
CH
SO3H
*x
y
r
s
m
n
*
CH2
CH3
Figure 1.17. S-HPBS diblock copolymer.
Their initial S-HPBS work did not seem very promising because the level of
sulfonation did not reach targeted values. Four different sulfonation levels were targeted
(5-40 mol% of sulfonated styrene) and only the reaction which targeted 40 mol% of
sulfonated styrene produced an adequate level of sulfonation (15 mol% of sulfonated
styrene as determined by titration).66
30
The polymers were blended with small amounts (10-30%) of non-sulfonated
polymers (HPBS and polypropylene (PP)) so the polymers could be processed more
easily. This led to a reduction in conductivity, which was already inferior to Nafion®, in
all but one of the samples tested. The neat polymer had a conductivity of 8.10 x 10 -3
S/cm in liquid water at 50 oC. When tested under the same conditions, the polymer
blended with 10% HPBS had a conductivity which was slightly increased (9.92 x 10-3
S/cm). However, blending with 10% PP decreased the conductivity to 5.07 x 10-3 S/cm.65
Sulfonated poly[(vinylidene difluoride-co-hexafluoropropylene)-b-styrene]
(P(VDF-co-HFP)-b-SPS) block copolymers have been studied as PEM materials.53
These polymers were synthesized using atom transfer radical polymerization (ATRP) and
were comprised of 31% polystyrene units. The polystyrene was subsequently sulfonated
using acetyl sulfate as the acidifying agent (Figure 1.18). Degrees of sulfonation from 12
to 100 were achieved; however, due to their solubility in water, samples with 100%
sulfonation were not tested. Both conductivity and water uptake increased as the degree
of sulfonation increased. At levels over 47% sulfonation, the water uptake of the
samples was greater than 100%, which rendered the membranes mechanically unstable.
Despite obtaining a conductivity of 0.076 S/cm for a sample which was 49% sulfonated,
its water uptake of 388% was very undesirable.
31
CH2
CF2 CF2 CF
CF3
CH
CH2
CH2
CH
SO3H
*x
y
l
m
m
n
*
Figure 1.18. P(VDF-co-HFP)-b-SPS diblock copolymer.
1.4.4.2 Multiblock Copolymers
1.4.4.2.1 Multiblocks Containing Aliphatic and Aromatic Block s
Zhang et al.67,68 studied a multiblock system (PAES-b-SPB) which contained
hydrophobic poly(arylene ether sulfone) (PAES) blocks and sulfonated polybutadiene
(PB) hydrophilic blocks. The authors proposed, that the rigid aromatic blocks would
provide better mechanical properties and thermal stability; whereas, the flexible aliphatic
blocks would improve flexibility of the sulfonic acid groups, which could increase
conductivity.
To obtain this sulfonated multiblock copolymer, amino-terminated PAES blocks
were coupled to acidylated carboxyl-terminated polybutadiene. The polybutadiene
blocks were acidified to varying degrees using acetyl sulfate. These sulfonating
conditions did not sulfonate the aromatic rings of the PAES (Figure 1.19).
32
CH3COOSO3H
+
THF, 75 oC refluxing 12 hPrecipitated by NaOH
OHOHCH3
CH3
S ClClO
O
NH2OH
ONH2NH2OSO
O
OO
CH3
CH3
SO
O n
S ClOO
OO
CH3
CH3
SClO
O n
+
NMP/Toluene/K2CO3150 oC Refluxing 20 h
CH2
CH
CH
CH2
CC
OO
ClClm
NH
CH2
CH
CH
CH2
CC
OO
NH m
O OSOO
OO
CH3
CH3
SO
O n
p
+
+
C
O
NH
O OSOO
OO
CH3
CH3
SO
O n
p
CH2
CH
CH
CH2
x N
HCH2
CH
CH
CH2
C
O
OH SO3H
y
m
CH3COOSO3H
+
THF, 75 oC refluxing 12 hPrecipitated by NaOH
OHOHCH3
CH3
S ClClO
O
NH2OH
ONH2NH2OSO
O
OO
CH3
CH3
SO
O n
S ClOO
OO
CH3
CH3
SClO
O n
+
NMP/Toluene/K2CO3150 oC Refluxing 20 h
CH2
CH
CH
CH2
CC
OO
ClClm
NH
CH2
CH
CH
CH2
CC
OO
NH m
O OSOO
OO
CH3
CH3
SO
O n
p
+
+
C
O
NH
O OSOO
OO
CH3
CH3
SO
O n
p
CH2
CH
CH
CH2
x N
HCH2
CH
CH
CH2
C
O
OH SO3H
y
m
Figure 1.19. Synthesis of PAES-b-SPB multiblock copolymer.68
The membranes were characterized for possible use as PEM. Only low degrees of
sulfonation were possible using this technique (<12%). Therefore, the IECs were very
low, ranging from 0.107 to 0.624 meq/g. An increase in IEC resulted in increased
conductivity and water uptake. However, due to the low IEC values, the highest
33
conductivity achieved was 0.0302 S/cm (25 oC in liquid water). A significant water
uptake was observed for this membrane (62%) when considering the low IEC value.
Incorporation of PB into the block copolymer resulted in Tg values well below room
temperature, which increased with increasing sulfonation content (-37.7 to -4.5 oC).
1.4.4.2.2 Aromatic Multiblock Copolymers
Multiblock copolymers containing aromatic backbones are desirable candidates
for PEM applications. Unlike block copolymers containing a sulfonated polystyrene
hydrophilic block, which are susceptible to degradation at high temperatures and have
poor oxidative stability, multiblock copolymers containing aromatic blocks possess the
same thermal, chemical, and mechanical stability as their random copolymer counterparts
discussed previously. However, unlike the random copolymers, the highly ordered
sequencing in the polymer backbone allows the hydrophilic and hydrophobic blocks to
nanophase separate, which allows for optimization of desired PEM characteristics, such
as proton conductivity, water uptake, selectivity, and permeability.
Aromatic multiblock copolymers are synthesized via nucleophilic aromatic
substitution reactions.51 Almost all copolymers discussed in the following sections were
synthesized using small variations of the same basic procedure. Hydrophobic and
hydrophilic oligomers were synthesized to a desired molecular weight, using a derivation
of Carothers equation to determine the offset in stoichiometry (r) (equation 2.1). Because
these were step growth reactions utilizing A-A and B-B type monomers and the targeted
Mns were low, the number-average degree of polymerization, nX , is equal to (2n +1),
where n is equal to the number of repeat units.
34
( )( )
1
1
n
n
Xr
X
−=
+ 2.1
The two blocks were synthesized with mutually reactive end groups. Often one
block was terminated with di-hydroxy groups and one with dihalide functionality.
However, many other functionalities have been used, including, but not limited to,
amines, anhydrides, and thiols. The blocks were then be reacted together to form a
multiblock copolymer.
The same overall procedures and principles discussed in section 1.4.2.1.1,
regarding reaction conditions, are necessary for the successful synthesis of mulitblock
copolymers. However, some leniencies exist in the coupling of oligomers to form
multiblock copolymers, which will be presented in later discussions.
1.4.4.2.2.1 Multiblock systems with BPSH Hydrophilic Blocks
The McGrath group71,72,73,74,75,76,79 and some others50,80 have synthesized
multiblock copolymers using BPSH hydrophilic blocks. It is advantageous to use
directly synthesized oligomers containing disulfonated monomers as the hydrophilic
portion because higher IECs can be achieved when compared to post-sulfonated
polystyrene. The reaction of BPSH oligomer with many suitable hydrophobic oligomers
has been studied. Most researchers75,79,81 have utilized the fully disulfonated BPSH-100
oligomer. However, some work has been conducted using lower degrees of
sulfonation.77 By changing the volume fraction of blocks, block length, and the
interaction parameter of the hydrophilic and hydrophobic blocks, the extent of nanophase
separation can be altered.69,70
35
Early work was done utilizing hydroxyl terminated poly(arylene ether phosphine
oxide) (PEPO) as the hydrophobic block.71 First, the hydroxyl terminated PEPO was
synthesized and isolated. This was then coupled to chloro terminated BPS-100 via a
nucleophilic aromatic substitution reaction (Figure 1.20). Although high temperature
(190 oC; 24 h) was used for the coupling reaction, 13C NMR data confirmed that ether-
ether interchange had not occurred. The peaks for the multi block copolymer appeared as
single peaks, signifying the chemical environment was the same for any given carbon.
This occurred because of the highly ordered bonding sequence in the polymer backbone.
However, the corresponding random copolymer displayed doublets, indicating multiple
chemical environments for the same carbon atom, which is due to randomization in the
bonding of carbons in the backbone. These multiblock copolymers (hydrophobic:
hydrophilic block lengths of 5k:5k) outperformed their random copolymer counterparts in
proton conductivity and had lower water uptake, when tested at room temperature.
O P
O
OOH OHn
+S
O
O
SO3Na
NaO3S
O O S
O
O
SO3Na
NaO3S
ClCl n
NMP/toluene/K2CO3146 oC, 4 h190 oC, 16 h
O P
O
O* O S
O
O
SO3Na
NaO3S
O O S
O
O
SO3Na
NaO3S
On
n
p
O P
O
OOH OHn
+S
O
O
SO3Na
NaO3S
O O S
O
O
SO3Na
NaO3S
ClCl n
NMP/toluene/K2CO3146 oC, 4 h190 oC, 16 h
O P
O
O* O S
O
O
SO3Na
NaO3S
O O S
O
O
SO3Na
NaO3S
On
n
p
Figure 1.20. Synthesis of BPS-100:PEPO multiblock copolymer.
36
Much research has been devoted to studying sulfonated-fluorinated poly(arylene
ether) multiblock copolymers as candidates for PEM materials.72, 73, 74,75,76 These
polymers are synthesized by coupling hydroxyl terminated BPS-100 hydrophilic blocks
with highly fluorinated hydrophobic blocks (Figure 1.21). An excess of
decafluorobiphenyl (DFBP) has been reacted with various bisphenols, including 6F-BPA,
4,4’-isopropylidenediphenol (BPA), and bis(4-hydroxyphenyl) (Bis-S), to form the
fluorine terminated hydrophobic oligomer.
OOF ARYLn
FFFF
F F F F F F F F
F
FFFF
+
*
F F F F
FFFF
OARYLOOO S OO
OSO3Na
NaO3S
On
*
F F F F
FFFF
m
p
NMP90-110 oC
O S OO
OSO3Na
NaO3S
O-+Mn M+-O O-M++M-O
where ARYL = CF3
CF3
CH3
CH3
SO
O
OOF ARYLn
FFFF
F F F F F F F F
F
FFFF
+
*
F F F F
FFFF
OARYLOOO S OO
OSO3Na
NaO3S
On
*
F F F F
FFFF
m
p
NMP90-110 oC
O S OO
OSO3Na
NaO3S
O-+Mn M+-O O-M++M-O
where ARYL = CF3
CF3
CH3
CH3
SO
O
Figure 1.21. Synthesis of various sulfonated-fluorinated multiblock copolymers.
The use of DFBP monomer serves two purposes in these reactions. First,
terminating the hydrophobic block with the highly reactive DFBP monomer allows the
use of low reaction temperatures (90-110 oC) for the coupling of the oligomers, which
greatly reduces the risk of ether-ether interchange reactions. Secondly, the use of this
monomer results in a highly fluorinated oligomer which is very hydrophobic. This
37
increased hydrophobicity may promote sharper nanophase separation from the
hydrophilic block.
Ghassemi et al.72,73 first prepared this sulfonated-fluorinated poly(arylene ether)
multiblock copolymer using 6F-BPA as the bisphenol in the hydrophobic block.
Multiblock copolymers with various combinations of hydrophilic and hydrophobic block
lengths were synthesized. The molecular weights of the hydrophilic and hydrophobic
block lengths varied from 2k to 5k.
Membrane properties were affected by hydrophilic and hydrophobic block length
combinations, which ranged from 2k:2k to 5k:5k. There was no correlation between
conductivity and block length, when equal block lengths were used for the hydrophilic
and hydrophobic blocks. Values between 0.12-0.16 S/cm (liquid water, 30 oC) were
reported for equal block length multiblock copolymers. When the hydrophilic block
length was shorter than the hydrophobic block (3k:5k), conductivity decreased to 0.08
S/cm. The opposite effect was seen when the hydrophilic block length was longer than
the hydrophobic block (5k:2.8k), which had a reported conductivity value of 0.32 S/cm.
Similar trends were observed for water uptake values. Water uptake for multiblock
copolymers with equal block lengths ranged from 110-150%. Whereas, the 3k:5k
multiblock only had 40% water uptake. By offsetting the block lengths to 5k:2.8k, the
water uptake increased to 470%.
The 5k:5k system showed promising conductivity results in low RH conditions
when compared to Nafion 112. At 60% RH and below, the 5k:5k multiblock copolymer
exhibited higher proton conductivity than the Nafion 112 standard. Tapping mode AFM
images of multiblock copolymer with 5k:5k block length indicated defined nanophase
38
separation of the two domains when compared to Nafion® and the random copolymer,
BPSH-40, which could explain the enhanced performance.
Yu et al.74,77 studied this multiblock copolymer system further by investigating
copolymers with longer block lengths and various degrees of disulfonation in the
hydrophilic block, ranging from 75 to 100% disulfonation. Multiblock copolymers
containing BPSH-100 as the hydrophilic block were synthesized which had the same IEC
values and hydrophilic block lengths. As expected, these polymers demonstrated a
decrease in water uptake as the hydrophobic block length was increased. However,
proton conductivity in liquid water was preserved. When utilizing hydrophilic blocks
with varying degrees of sulfonation, hydrophilic and hydrophobic block lengths were
held constant. Unfortunately, a decrease in hydrophilicity did not lead to a decrease in
water uptake. Copolymers containing lower degrees of disulfonation (75 and 83%),
displayed higher water uptake values than those synthesized using a BPSH-100
hydrophilic block. The higher degree of hydrophilicity in the multiblocks containing
BPSH-100 most likely leads to increased nanophase separation.
A variation of this polymer was also studied, which used BPA monomer in the
hydrophobic block.75 Polymers with similar block lengths were studied, ranging from
3.5k:3k to 5k:5k. Direct comparisons cannot be made between the two systems utilizing
BPA and 6F-BPA as monomers because polymers of identical hydrophilic-hydrophobic
block lengths and IEC values are not available for both copolymers. Although the BPA
based system reported similar conductivity to the 6F-BPA system (0.10-0.13 S/cm),
polymers synthesized using BPA in the hydrophobic block appeared to have significantly
lower water uptake when compared to the 6F-BPA counterparts discussed above. Water
39
uptake values of 42 and 71% were reported for BPA containing multiblock copolymers,
despite having hydrophilic block lengths longer than hydrophobic block lengths (4k:3.5k
and 3.5k:3k, respectively) and higher IEC values than the 6F-BPA systems (~1.6 vs. ~1.5
meq/g).72
A third variation of this multiblock copolymer, utilizing Bis-S monomer in the
hydrophobic, was studied.76,78 In this research, Yu et al. utilized the ability to offset the
stoichiometry of the hydrophilic and hydrophobic oligomers to control the IEC of the
multiblock copolymers. It was realized that, when coupling two oligomers, the
stoichiometric balance of that reaction is more forgiving than a conventional step-growth
reaction performed with low molecular weight monomers. When coupling two
oligomers, the oligomers were already of substantial molecular weight, so the reaction
could tolerate a lower degree of polymerization. An offset in stoichiometry was utilized,
which still afforded high molecular weight multiblock copolymer, as indicated by IV
data.
Because the IEC of the multiblock copolymer series was controlled, a thorough
study of the effect block length has on membrane properties was possible. A controlled
excess of hydrophobic block was utilized in the reactions to maintain an IEC of 1.3
meq/g for the series of polymers. Equal hydrophilic-hydrophobic block lengths ranging
from 5k:5k to 20k:20k were targeted. Several polymers with unequal block lengths were
studied as well, all having longer hydrophobic block length than hydrophilic.
This series of multiblock copolymers displayed desirable membrane qualities,
making them possible candidates for PEMs. Conductivity in liquid water at 30 oC was as
least 0.10 S/cm for all copolymers studied and reached 0.15 S/cm for both the 15k:15k
40
and 12k:17k systems. Water uptake was desirable because it was at or below 78% for the
entire series, which is beneficial for maintaining mechanical stability. Synthesizing
multiblock copolymers with a longer hydrophobic block length than hydrophilic reduced
the water uptake, without compromising conductivity, when tested in liquid water at 30
oC and at 95% RH at 80 oC. However, as RH decreased, this trend did not continue. The
15k:15k multiblock copolymer displayed higher conductivity than the 12k:17k at all RH
values below 95%, when tested at 80 oC. The conductivity of this copolymer was higher
than Nafion® at all RH values, most likely due to an increase in nanophase separation.
Lee et al. 79 synthesized two series of multiblock copolymers utilizing BPSH-100
for the hydrophilic oligomer and BPS-00 as the hydrophobic oligomer. These block
compositions were chosen because of the chemical similarities to BPSH random
copolymers, which exhibit very good thermal, chemical, mechanical, and oxidative
stability. After the hydrophobic block was synthesized, small amounts of DFBP or
hexafluorobenzene (HFB) were used to end-cap these oligomers. However, unlike
previous work using these highly fluorinated monomers, only small amounts had to be
utilized, which minimized the high cost of these materials. End-capping with these
monomers provided highly reactive fluorine terminated oligomers, which could be
reacted with phenoxide terminated BPS-100 oligomers at low reaction temperatures,
minimizing the likelihood of ether-ether interchange. The resulting multiblock
copolymers are depicted in Figure 1.22.
41
O S O O
O
OSO3H
HO3S
F F
FFn
* O O S O
O
O
Om
F F
FF
O *p
O S O O
O
OSO3H
HO3S
F F F F
FFFF
O O S O
O
O
O
F F F F
FFFF
On
m
p
(a)
(b)O S O O
O
OSO3H
HO3S
F F
FFn
* O O S O
O
O
Om
F F
FF
O *p
O S O O
O
OSO3H
HO3S
F F F F
FFFF
O O S O
O
O
O
F F F F
FFFF
On
m
p
(a)
(b)
Figure 1.22. BPSH-100:BPS-00 multiblock copolymer with (a) DFBP and (b) HFB linking groups.
The effect of block length on various membrane properties, including proton
conductivity, water uptake, and water swelling, was examined.793 Polymers with similar
IEC values were synthesized by using a 1:1 stoichiometry to couple hydrophobic and
hydrophilic oligomers with equal block lengths. Systems with block lengths ranging
from 3k:3k to 15k:15k were studied. As block length increased, proton conductivity and
water uptake increased. These multiblock copolymers displayed anisotropic swelling,
compared to the isotropic swelling of NRE211 and BPSH-35 random copolymer. In-
plane swelling remained nearly constant as block length increased. Through-plane
swelling increased with an increase in block length. The anisotropy observed may be
advantageous in MEA fabrication because water swelling-deswelling mechanical failures
could be avoided.
The effect of IEC on membrane properties was studied as well.79 Copolymers
with higher IEC values were obtained by coupling hydrophilic oligomers to hydrophobic
oligomers with shorter block length using a 1:1 stoichiometry. An increase in
conductivity was observed as the IEC increased. Liquid water conductivity as high as
0.16 S/cm was observed for the BPSH-100:BPS-00 10k:5k containing DFBP linkages.
42
A similar study was completed by Nakabayashi et al.80 in which hydroxyl
terminated hydrophilic blocks and hydrophobic blocks were coupled using a DFBP
monomer as a highly reactive chain extender (Figure 1.23). BPSH-100 was used as the
hydrophilic block in all reactions. BPS-00 or a partially fluorinated poly(arylene ether
sulfone) (6FS) copolymer were used as hydrophobic blocks. Using a stoichiometric
amount of DFBP as a chain extender, instead of an excess amount as an endcapping
agent, reduces the cost to synthesize this polymer. Because both oligomers are end
capped with hydroxyl end groups and then chain extend using DFBP, the IEC of the
multiblock copolymer can be controlled by adjusting the molar feed ratio of hydrophobic
and hydrophilic oligomer.
O S O O
O
OSO3H
HO3S
F F F F
FFFF
O O S O
O
O
O
F F F F
FFFF
On
m
* *p
O S O O
O
OSO3H
HO3S
F F F F
FFFF
O O S O
O
On
m
* O
F F F F
FFFF
O *CF3
CF3
CF3
CF3
p
(a)
(b)
O S O O
O
OSO3H
HO3S
F F F F
FFFF
O O S O
O
O
O
F F F F
FFFF
On
m
* *p
O S O O
O
OSO3H
HO3S
F F F F
FFFF
O O S O
O
On
m
* O
F F F F
FFFF
O *CF3
CF3
CF3
CF3
p
(a)
(b)
Figure 1.23. Poly(arylene ether sulfone) multiblock copolymers synthesized using a
DFBP coupling agent containing (a) BPS-00 or (b) 6FS hydrophobic oligomer.
The two multiblock copolymers were compared to each other and a comparable
random copolymer.80 Although two comparable IEC values (~1.7 and 2.1 meq/g) were
achieved for both chemical compositions, different block lengths were chosen, making
comparisons between the two systems difficult. The authors note that the copolymer
using 6FS as the hydrophobic oligomer had lower water uptake values than copolymers
containing the BPS-00 hydrophobic oligomer. However, the copolymers containing 6FS
43
had longer hydrophobic block length than the copolymers containing BPS-00
hydrophobic blocks (8000 vs. 6500 g/mol). Regardless of the chemical composition or
block length, the multiblock copolymers had higher conductivities across the entire RH
range than BPSH-40 random copolymer. However, the conductivities of all the
multiblocks were lower than that for the Nafion 117 standard at both 50 and 80% RH.
Li et al. 81 synthesized a multiblock copolymer using BPSH-100 as the
hydrophilic block and partially fluorinated poly(arylene ether ketone) oligomer as the
hydrophobic block (Figure 1.24). A multiblock copolymer with hydrophilic:hydrophobic
block lengths of 4k:4k was synthesized and compared to a random copolymer prepared
from the same monomers. 13C NMR was used to confirm that ether-ether interchange
had not occurred in this reaction, despite the high reaction temperatures (190 oC).
O S O O
O
O
O
O
O
CF3
CF3
O *CF3
CF3
* n
n
p
SO3H
HO3S
Figure 1.24. BPSH-100:6FK multiblock copolymer.
This BPSH-100:6FK multiblock copolymer was compared to an analogous
random copolymer to evaluate how differences in sequence length affects water and
proton transport. Tapping mode AFM images of the multiblock copolymer showed that
the longer sequence lengths of the multiblock copolymer resulted in a sharper nanophase
separated morphology than the short sequences in the random copolymer counterparts.
This enhanced nanophase separation resulted in higher liquid water proton conductivity.
An increase in water uptake was observed, regardless of the hydration level. It appears
44
that the increase in water uptake at low levels of hydration allowed for better conductivity
performance at low RH values. At all RH levels, the multiblock copolymer was very
comparable to Nafion 112. This system displayed promising hydrogen-air fuel cell
performance (80 oC, fully humidified), which was comparable to the Nafion 1135
voltage-current curve.
Multiblock copolymers containing BPSH-100 hydrophilic blocks and polyimide
hydrophobic blocks have been studied.82,83 Polyimide oligomers were chosen as the
hydrophobic block because they can provide chemical resistance, low permeability, thermal
stability, and mechanical strength to the PEM, which are all desirable qualities for fuel cell
applications. A six-membered ring polyimide is required for fuel cell applications because
they are more hydrolytically stable than the five-membered rings under acidic conditions.84
A mixed solvent system of NMP and m-cresol was necessary to successfully couple the two
oligomers because BPSH-100 was not soluble in m-cresol, which is necessary for a
successful 6-membered ring imidization reaction (Figure 1.25).85,86
45
NO
O
OO
O
NN
O
OO
O
O S O
O
O
ON
O
OO
O
O S O
O
On
+
S
O
O
SO3Na
NaO3S
O O S
O
O
SO3Na
NaO3S
OO
NH2NH2
n
Benzoic acidNMP (80 oC, 4h)m-cresol180 oC, 12hIsoquinoline180 oC, 12h
NN
O
OO
O
NN
O
OO
O
O S O
O
On
*
S
O
O
SO3Na
NaO3S
O O S
O
O
SO3Na
NaO3S
OOn
*
p
NO
O
OO
O
NN
O
OO
O
O S O
O
O
ON
O
OO
O
O S O
O
On
+
S
O
O
SO3Na
NaO3S
O O S
O
O
SO3Na
NaO3S
OO
NH2NH2
n
Benzoic acidNMP (80 oC, 4h)m-cresol180 oC, 12hIsoquinoline180 oC, 12h
NN
O
OO
O
NN
O
OO
O
O S O
O
On
*
S
O
O
SO3Na
NaO3S
O O S
O
O
SO3Na
NaO3S
OOn
*
p
Figure 1.25. Synthesis of BPS-100:polyimide multiblock copolymer.
Block length played a very important role in the properties of this multiblock
system. AFM showed a defined nanophase separated morphology for the entire series.82
As block length was increased from 5k:5k to 20k:20k, the connectivity of the hydrophilic
regions dramatically increased. As the hydrophilic channel becomes more connected,
water can move more freely through the sample, which explains the increased
conductivity, water uptake, and self-diffusion coefficient of water as block length
increased in this system. The enhanced connectivity promoted higher proton
conductivity at lower RH because the proton transport was better facilitated.
46
Later, block lengths were offset to afford polymers with varying IEC values.83
Copolymers with different hydrophobic and hydrophilic lengths were synthesized to vary
the IEC from 0.62 to 2.58 S/cm. Overall, water uptake and proton conductivity increased
as IEC increased. However, polymers with high IECs (>1.9 S/cm) were not mechanically
stable enough to obtain conductivity or water uptake values.
1.4.4.2.2.2 Multiblock Copolymers Containing Substituted Poly(p-phenylene)s
Substituted poly(p-phenylene)s are desirable candidates for PEM materials
because of their excellent thermal and mechanical properties. However, high molecular
weight is difficult to achieve because the polymer’s rigidity decreases solubility during
polymer formation. Several studies have taken advantage of the desirable characteristics
of this polymer family by coupling low molecular weight poly(p-phenylene) oligomers
with poly(arylene ether sulfone) oligomers. The coupling of these two oligomers is
advantageous because possible ether-ether interchanges are avoided. Unlike multiblock
systems in which the oligomers are synthesized using nucleophilic aromatic substitution
reactions, poly(p-phenylene)s and their derivatives are not susceptible to randomization.87
Ghassemi et al.88,89 synthesized multiblock copolymers coupling fluorine
terminated sulfonated poly(4’-phenyl-2,5-benzophenone) (PPP) hydrophilic oligomers
with hydroxyl- terminated BPS-00 hydrophobic blocks. First, fluorine-terminated PPP
oligomers with controlled molecular weight were synthesized using a nickel-catalyzed
polymerization. A controlled amount of 4-chloro-4’-fluorobenzophenone was used as the
endcapping monomer. Only the chlorine group reacted, producing a fluorine endcapped
oligomer. The PPP oligomer was then post-sulfonated with concentrated sulfuric acid to
varying degrees of sulfonation: 75, 95, and 100%. After synthesis of the hydroxyl
47
terminated BPS-00 hydrophobic oligomer, the fluorine and hydroxyl end groups of the
two blocks were coupled via a nucleophilic aromatic substitution reaction (Figure 1.26).
ClCl
O
Cl F
O
+O
F F
O O
n
NiCl2, Zn, Triphenylphosphine,
2,2’-bypyridyl
NMP, 80 oC, 4h
O
F F
O O
n
NaO3S
H2SO450 oC, 2-48 h
O S O
O
O
OHm
OH +
DMAc/Toluene/K2CO3150 oC, 4h160 oC, 16h
O S O
O
O
O
OO O
m
*O* n
p
NaO3S
OHOH + Cl S Cl
O
O
NMP/toluene/K2CO3150 oC, 4h175 oC, 16h190 oC, 1h
ClCl
O
Cl F
O
+O
F F
O O
n
NiCl2, Zn, Triphenylphosphine,
2,2’-bypyridyl
NMP, 80 oC, 4h
O
F F
O O
n
NaO3S
H2SO450 oC, 2-48 h
O S O
O
O
OHm
OH +
DMAc/Toluene/K2CO3150 oC, 4h160 oC, 16h
O S O
O
O
O
OO O
m
*O* n
p
NaO3S
OHOH + Cl S Cl
O
O
NMP/toluene/K2CO3150 oC, 4h175 oC, 16h190 oC, 1h
Figure 1.26. Synthesis of BPS-00:SPPP multiblock copolymer.
BPS-00:SPPP multiblock copolymers were characterized to determine if they
would be potential candidates for PEM fuel cell applications. Coupling sulfonated PPP
with BPS-00 to form a multiblock copolymer provided some desirable results. The
ability to cast flexible, ductile films was reported,89 which is not attainable when casting
sulfonated PPP homopolymer.90 However, even the highest proton conductivity
48
achieved, 0.036 S/cm for a 10k:6k hydrophilic:hydrophobic block copolymer, was
inferior to Nafion 1135 control. Synthesizing copolymers with higher block lengths or
with higher hydrophilic content could increase the conductivity. Unfortunately there is a
limit on how far the hydrophilic:hydrophobic block length ratio can be offset. A polymer
with block lengths of 16k:12k was already mechanically unstable, which did not allow
for further characterization.
Later, Wang et al.87 synthesized multiblock copolymers utilizing poly(2,5-
benzophenone) (PBP) as the hydrophobic block and BPSH-100 as the hydrophilic block
(Figure 1.27). Utilizing BPSH-100 as the hydrophilic oligomer eliminated the need to
post-sulfonate the PBP oligomer. The degree of sulfonation could be precisely controlled
through the stoichiometric ratio of the monomers, in this case 100% sulfonation.
Hydrophilic:hydrophobic block lengths of 3k:3k, 6k:6k, and 10k:10k were studied.
O S O
O
O
O
OO O
m
*O* n
p
SO3H
HO3S
Figure 1.27. BPSH-100:PBP multiblock copolymer.
The phase separation in these copolymers was studied and related to membrane
properties as a function of block length. Two Tgs were observed for the 6k:6k and
10k:10k multiblock copolymers, which indicated that nanophase separation had occurred.
As expected, only one Tg was observed for the 3k:3k multiblock copolymer because the
49
shorter block lengths reduced the extent of nanophase separation. Tapping mode AFM
was used to observe the nanophase separation. The hydrophilic portions became more
connected with increasing block length. The water uptake for these multiblock
copolymers was very low, ranging from only 7 to 10%. A definitive explanation for why
this was so low was not presented; however, it was suggested that it may be a result of
the extremely rigid hydrophobic phase. The proton conductivity of these polymers
(liquid water, 30 oC) ranged from 0.03 to 0.06 S/cm and increased with block length.
1.4.5 Segmented Copolymers
Recently, segmented copolymers have been synthesized for use as PEMs in fuel
cell applications.91,92,93, 94 In this technique, one of the blocks is synthesized initially.
This block is then combined stoichiometrically with appropriate monomers, forming the
other block in-situ while the overall copolymer is being formed.51 Although most authors
refer to the subsequent copolymers as block or multiblock copolymers, it is important to
distinguish between these two polymerization techniques, which result in similar
polymers.
This method is a viable synthetic technique for several reasons. Using the
segmented technique, multiblock copolymers can be synthesized in a shorter amount of
time because there is no need to synthesize both oligomers separately and then couple
them together.51,94 The segmented technique can be used to synthesize unique polymers
that cannot be synthesized using previously discussed methods. Because the polymer is
produced from the coupling of two monomers and one oligomer, it may be easier to find
a common solvent than when two oligomers are used.91 This avoids polymer-polymer
incompatibilities between the two oligomers.51
50
Polyurethanes, which are one of the oldest categories of block copolymers, are
produced commercially using a segmented synthetic technique.51 In the case of
polyurethanes, the soft polyol is used as the starting oligomer. This is then endcapped with
excess difunctional isocyanate and further reacted with a glycol or diamine to form the
carbamate or urea hard segment, respectively (Figure 1.28).
HO OH
CO
ONH
RNCO CO
O NH
R N C O
NRNCO C O+
O O C NH
R NH
C
O O
n
HO R' OH
(excess)
(glycol)
(capped polyol)
(soft polyol)
Soft segment Hard Segment
HO OH
CO
ONH
RNCO CO
O NH
R N C O
NRNCO C O+
O O C NH
R NH
C
O O
n
HO R' OH
(excess)
(glycol)
(capped polyol)
(soft polyol)
Soft segment Hard Segment
Figure 1.28. Formation of polyurethane segmented copolymer with a diol-based carbamate hard segment.51
1.4.5.1 Poly(arylene ether ketone) segmented copolymers
Shin et al. synthesized various poly(arylene ether ketone) segmented copolymers
for use as PEMs.91 The authors chose to use the segmented polymerization method when
their initial attempts to form multiblock copolymer by separately synthesizing and
isolating both the hydrophobic and hydrophilic oligomers with a subsequent coupling
reaction failed because a suitable common solvent for both blocks could not be found.
51
Using the segmented technique, the phenoxide terminated hydrophobic block was
successfully synthesized, using the Carothers equation to offset the stoichiometry.
Desired molar masses of 2400 to 9900 g/mol were achieved. After isolation, the
hydrophobic block was reacted with the appropriate amounts of comonomers to
synthesize the hydrophilic segment while coupling it to the hydrophobic block. A molar
ratio of 1:k:k+1 of phenol-terminated telechelic hydrophobic block:bisphenol:dihalide
was used to complete the reaction, where k is the degree of polymerization for the
hydrophilic block (Figure 1.29). The theoretical molar mass of the hydrophilic segments
ranged from 3500 to 7200 g/mol and was altered in the copolymer by altering the value
for k.
Although several monomer combinations were utilized to provide polymers with
different molecular architectures, few reached high conversion. Segmented copolymers
containing SHQ exhibited the best conversion (Figure 1.29). For those that did not reach
high conversion, it was speculated that during the reaction low molecular weight
hydrophilic segments form that never coupled to hydrophobic block or only coupled to
small amounts of hydrophobic segment. These uncoupled and predominately hydrophilic
segments were washed away during subsequent work-up procedures. This was evidenced
when comparing experimental and theoretical IEC values. On average, the experimental
IEC values were only 55% of the calculated value. Even the most successful copolymers
had IEC values of only 80% of the calculated values.
52
OH O
O O
O OHn
OHOH
SO3K O
FF+(k) (k+1)
DMSO/Benzene/K2CO3142 oC 4h182 oC 24 h
OO
O
O O
OO O O
SO3K
Ok
**n
p
OH O
O O
O OHn
OHOH
SO3K O
FF+(k) (k+1)
DMSO/Benzene/K2CO3142 oC 4h182 oC 24 h
OO
O
O O
OO O O
SO3K
Ok
**n
p
Figure 1.29. Synthesis of poly(arylene ether ketone) segmented copolymers.91
The block lengths chosen appear to be arbitrary, making it difficult to detect
correlations between block length and conductivity or water uptake. No evidence of
increased nanophase separation due to increasing block length was presented. Although
TEM and SEM images were shown, they did not clearly indicate a nanophase separated
morphology for any of the polymers. The authors suggested91 that this may be because
the films were cast and dried at a temperature below the Tg of the polymer, which may
have prevented nanophase separation. Highly organized morphologies, as detected by
TEM and AFM, have been demonstrated by others even when using casting temperatures
well below the Tg of the polymers.81,82,83,87,95 The conformation and mobility of the
copolymer chains in the casting solvent may be a more important factor.
Zhao et al. explored the synthesis of poly(arylene ether ketone) segmented
copolymers as well.92,93 Their synthetic methods were similar to those described
previously. However, two changes were made, which they proposed would increase the
53
molecular weight of the segmented copolymer. A one-pot, two stage synthetic process
was used. First, the hydrophobic block was synthesized, followed by the addition of the
monomers which would form the hydrophilic segment while coupling to the hydrophobic
end groups (Figure 1.30). They suggested that isolation of the hydrophobic block with
subsequent addition of the monomers which form the hydrophilic segments impeded the
production of high molecular weight polymer.92 They prepared their hydrophobic block
with fluorine end groups because of the higher reactivity when compared to hydrophobic
groups terminated with phenol groups, which were used by Shin et al.91 Regardless of
the changes made in synthetic technique, titrated IEC values were still, on average, 38%
below theoretical values, indicating loss of hydrophilic segements.93
F
O
F OH OH
CH3
CH3
CH3
CH3
O
O
FF
O
O
CH3
CH3
CH3
CH3
n
(n+1) + n
DMSO/toluene/K2CO3140 oC 4h170 oC 6h
+ OH OH
CH3
CH3
CH3
CH3
+ F
O
F
SO3Na
NaO3S
DMSO (20% solids)/toluene/K2CO3140 oC 4h170 oC 6h
O
OO
O
CH3
CH3
CH3
CH3
O
CH3
CH3
CH3
CH3
O
O
NaO3S
SO3Na
O
CH3
CH3
CH3
CH3
Ok
p
n
k(k+1)
F
O
F OH OH
CH3
CH3
CH3
CH3
O
O
FF
O
O
CH3
CH3
CH3
CH3
n
(n+1) + n
DMSO/toluene/K2CO3140 oC 4h170 oC 6h
+ OH OH
CH3
CH3
CH3
CH3
+ F
O
F
SO3Na
NaO3S
DMSO (20% solids)/toluene/K2CO3140 oC 4h170 oC 6h
O
OO
O
CH3
CH3
CH3
CH3
O
CH3
CH3
CH3
CH3
O
O
NaO3S
SO3Na
O
CH3
CH3
CH3
CH3
Ok
p
n
k(k+1)
Figure 1.30. Synthesis of hydrophobic block with subsequent synthesis of poly(arylene ether ketone) segmented copolymer.92,93
One major problem associated with the segmented copolymerization technique is
ether-ether interchange. Because of the high reaction temperatures (>170 oC) used in the
54
synthesis of both segmented systems, it is important to show that ether-ether interchange
did not occur in these reactions. Neither account provided direct evidence that this
phenomenon was prevented.91,92,93
Indirect evidence was presented which suggested that the randomization of the
copolymer backbone, resulting from ether-ether interchange, had not occurred. Shin et
al. noted that the segmented copolymers were not soluble in boiling water,91 whereas,
their random copolymer counterparts were soluble in water at room temperature,96
suggesting this was evidence of nanophase separation, which would only occur if the
backbone still contained order. Zhao et al. noted differences in small angle X-ray
scattering (SAXS) when comparing their segmented copolymers and random copolymers,
which indicated phase separation had occurred in the segmented copolymers and not the
random counterparts.92
1.4.5.2 Poly(arylene ether sulfone) segmented copolymers
VanHouten et al.94 synthesized segmented copolymers with a fully sulfonated
hydrophilic block and highly fluorinated hydrophobic segments. These polymers were
compared to polymers made using a multiblock synthetic method reported by Yu et al.76
Simultaneously coupling the hydrophobic segments and hydrophilic block was an
alternate procedure for synthesizing the block copolymer, and it eliminated the need to
synthesize and isolate a separate hydrophobic block before coupling it to the hydrophilic
block, which was utilized previously.
Two precautions were taken to avoid ether-ether interchange. Because of the
decreased reactivity of SDCDPS, the phenoxide-terminated hydrophilic oligomer was
synthesized first, using SDCDPS and BP as the monomers. An excess molar ratio of
55
BP:SDCDPS was used to control the molecular weight. After isolation, the hydrophilic
oligomer was reacted with DFBP and Bis-S monomers in a nucleophilic aromatic
substitution reaction to form a segmented block copolymer (Figure 1.31). Choosing the
highly reactive DFBP as the dihalide for the hydrophobic segments allowed for low
reaction temperatures to be used (90 oC), which eliminated ether-ether interchange during
the coupling reaction. The stoichiometry was controlled such that the DFBP and Bis-S
monomers formed the hydrophobic segments of the copolymer, while also reacting with
the phenoxide-terminated hydrophilic oligomer. The segmented copolymers were
synthesized with equal hydrophilic and hydrophobic molecular weights. The block
lengths ranged from 3000 g/m to 16000 g/mol. The IV data confirmed that high
molecular weight polymer was achieved using this synthetic method. The ability to cast
tough films indicated high molecular weight polymer. When comparing polymers with
similar IEC values, water uptake increased as the block lengths increased, which is
attributed to an increase in the nanophase separated morphology.
OKO S O
O
OSO3K
KO3S
OKn
F
FFFF
F
F F F F
OH S OH
O
O
K2CO3Cyclohexane/NMP4 hrs @ 85 oC
add 36-70 hrs @ 90 oC
OO S O
O
OSO3K
KO3S
n
FFFF
F F F F
FFFF
F F F F
O S O
O
O
Om
x
OKO S O
O
OSO3K
KO3S
OKn
F
FFFF
F
F F F F
OH S OH
O
O
K2CO3Cyclohexane/NMP4 hrs @ 85 oC
K2CO3Cyclohexane/NMP4 hrs @ 85 oC
add 36-70 hrs @ 90 oC36-70 hrs @ 90 oC
OO S O
O
OSO3K
KO3S
n
FFFF
F F F F
FFFF
F F F F
O S O
O
O
Om
x
Figure 1.31. Synthesis of segmented block copolymer (BisSF-BPSH100) with simultaneous formation of hydrophobic block (BisSF).
56
1.5 Water Desalination
Water shortages are a growing concern across the world. Demand continues to
grow for a way to provide fresh water for an estimated 41% of the world that lives in
water-stressed areas.97 Oceans hold 97% of the earth’s water. 98 The quest for an
economically viable way to obtain fresh water from salt water continues because of its
large abundance. The variety of technical processes designed to remove salt from water
is termed “desalination”.
There are two major categories in which desalination of water can be achieved.
These include thermal and membrane processes.98,99 Thermal processes require salt
water to be heated and then condensed in various ways and stages, mimicking the natural
hydrologic cycle. The condensate, which is free of salt, is collected. Thermal processes
include multiple effect distillation, multistage flash distillation, and vapor compression
distillation. Membrane processes remove salt from water by selectively permitting or
prohibiting the passage of certain ions. These processes include electrodyalysis and
electrodyalysis reversal and reverse osmosis.
The two most commonly used methods of those listed are multistage flash
distillation (MSF) and reverse osmosis (RO) (Figure 1.32). As of 2002, MSF systems
accounted for 44% of installed or contracted desalination processes.98 MSF distillation
can effectively produce high quality fresh water from seawater, reducing salt
concentrations of 60,000 to 70,000 ppm total dissolved solids to less than 10 ppm.
However, this method is very expensive because of the large energy requirements to
evaporate and condense the water in multistage distillations. RO is a process used to
desalinate water using a semi-permeable membrane. Plants utilizing RO technology are
57
becoming more popular because the process requires less energy than evaporation, often
10 times less energy is required to produce fresh water using RO technology versus
thermal distillation.97 It can also remove microorganisms and organic contamination in
addition to salt.100 Recent developments in RO technology have allowed new membrane
capacity to surpass the annual additions to the distillation capacity.
44% (MSF)40% (RO)
4% (ME)4% (VC)
3% (other)6% (ED)
44% (MSF)40% (RO)
4% (ME)4% (VC)
3% (other)6% (ED)
Figure 1.32. World Desalination Capacity by Process, as of June 1999. Membrane processes: reverse osmosis (RO) and electrodialysis (ED); Thermal processes: multistage flash distillation (MSF), multi-effect distillation (ME), and vapor
compression (VC)
1.6 Reverse Osmosis
Osmosis occurs when two solutions of varying concentrations are separated by a
semipermeable membrane. Solute from the less concentrated side will pass through the
58
membrane to the more concentrated side in order to form an equilibrium between the two
solutions. This process creates a pressure called osmotic pressure.
Reverse osmosis is a technology used to separate the salt and other impurities
from sea water and brackish water to create fresh water. A semipermeable membrane is
placed between salt water and fresh water, similar to osmosis. However, pressure is
applied to the salt water to overcome the osmotic pressure. This causes the water from
the salt-water side to pass through the membrane to the fresh water side, leaving a more
concentrated salt-water stream behind (Figure 1.33).
Semipermeablemembrane
Fresh water
Salt water
Applied pressure to overcome
osmotic pressure
Semipermeablemembrane
Fresh water
Salt water
Applied pressure to overcome
osmotic pressure
Figure 1.33. Schematic of reverse osmosis.
1.7 Types of Membranes for Reverse Osmosis
There are currently two major categories of membranes types for reverse osmosis,
which include asymmetric membranes and thin film composite membranes. Both will be
59
discussed further in section 1.8 when the chemistry of typical materials is addressed;
however, a brief overview of their physical make-up will be provided here.
Asymmetric membranes are produced using a phase inversion process.101
Asymmetric membranes for RO application were first produced by Loeb and
Sourirajan.102 Membranes were fabricated by evenly casting solutions of cellulose
acetate in acetone onto a glass plate with a doctor blade. After a brief exposure to air, the
membranes were immersed in ice water. The membrane which resulted was chemically
homogeneous but physically asymmetric.103 A dense skin, which served as the selective
layer, formed on the top of the membrane, which is typically 0.2 µm thick.105 A porous
structure formed underneath, typically 100 µm thick, which served as the thin membrane
support.
Thin film composites (TFCs) are made in a two step process.101 A thick, porous,
non-selective layer of polymer is formed on a reinforcing fabric. This is then coated with
a very thin layer of polymer to serve as the selective membrane. The two layers are
almost always comprised of two chemically different species.
TFCs offer several advantages over asymmetric membranes. Because the layers
are formed separately from different polymers, the properties of each layer can be
tailored specifically for its purpose. For example, the porous support can be altered to
have mechanical integrity and resist compression, while the thin barrier allows for high
flux and salt rejection. Other benefits to TFCs are the polymer chemistries which can be
explored. The formation of asymmetric membranes limits polymers to be soluble and
able to be phase inverted. This eliminates candidates such as crosslinked systems.
60
1.8 Materials for Reverse Osmosis Membranes
The semi-permeable membrane is a critical component for an RO system.
Criteria for ideal RO membranes have been identified.104,105 They must be highly
permeable to water (high flux) while maintaining high salt rejection. Resistance to
microbiological attack and fouling by colloidal and suspended material, chemical
stability, and tolerance to chlorine and other oxidants maximizes membrane life. They
require mechanical integrity that is not affected by exposure to high pressures (up to 1200
psig) or high temperatures (25-90 oC). Easy formation of thin films or hollow fibers is
necessary to reduce operation cost.
1.8.1 Cellulose Membranes
Osmosis has been understood for over a century; however, the use of artificial
membranes to purify water by RO was not possible until the late 1950s. Until this time,
appropriate polymeric materials did not exist which could withstand the pressures and
chemicals required for RO processes, while still maintaining high flux and salt rejection.
Cellulose acetate (CA) membranes were the first to be utilized for RO applications. Reid
and Breton discovered that some compositions of cellulose acetate were able to provide
reasonable fluxes and permeabilities. Loeb and Sourirajan were able to substantially
increase the flux by fabrication of asymmetric membranes instead of homogeneous
ones.106 Performance of the asymmetric CA membranes could be enhanced by annealing
the membranes in water at temperatures up to 90 oC.103,105 Once annealed, an increase in
salt rejection was observed; however, decreased water flux resulted. Water flux
continued to decline with no improvements in salt rejection if membranes were annealed
at temperatures higher than 90 oC.
61
Although asymmetric cellulose acetate membranes were studied for decades after
their initial discovery,106,107 there were several drawbacks to these membranes. They are
susceptible to creep-induced compaction and biological attacks.108 Cellulose acetate
begins to degrade via hydrolysis under elevated temperatures and variable pH ranges. To
minimize degradation, the RO process has to take place at low temperatures (0 oC to 30
0C) and a pH of 4-6.5. 104 This puts constraints on the types of cleaning agents that can be
used to keep the membranes in working condition (void of organic and colloid
deposits).106
1.8.2 Non-Cellulosic Membranes
The exploration of other polymeric materials began when advancements being
made on cellulose acetate membranes became restricted. Aromatic polyamides and
polyamide-hydrazide copolymers were utilized to fabricate asymmetric membranes.109
Example aromatic polyamide-hydrazine and polyamide copolymers were synthesized by
reacting terephthaloyl chloride with p-aminobenzhydrazine or 1,3-bis(3-
aminobenzamide)-benzene in dimethylacetamide at 10 oC or -20 oC, respectively. The
structures of these copolymers can bee seen in Figure 1.34. Asymmetric membranes
were formed from both copolymers. The polyamide-hydrazide membrane required
annealing, similar to cellulose acetate membranes, to boost performance, whereas, the
polyamide displayed high selectivity as cast. These copolymers showed high salt
rejection, up to 99.8% at 600 psi with a 5000 ppm NaCl feed for the polyamide and 98%
for the polyamide-hydrazide.
62
C NH
NH
C
O O
C *
O
NH
*n
(a)
NH
* C NH
NH
C NH
C C *
O O O O
n (b)
C NH
NH
C
O O
C *
O
NH
*n
(a)
NH
* C NH
NH
C NH
C C *
O O O O
n (b)
Figure 1.34. Structures of aromatic (a) polyamide-hydrazine and (b) polyamide copolymers
TFCs began to be fabricated from aromatic polyureas and polyamides via
interfacial polymerization.101 Cadotte performed interfacial polymerizations to form a
thin, barrier layer of polyurea by reacting polyethylenimine and toluene diisocyanate on
the surface of a water saturated microporous polysulfone sheet. Greater than 99% salt
rejection was observed for this membrane, while maintaining a flux of 18 gfd (1500 psig,
3.5% synthetic seawater). This membrane became known as the NS-100, which was the
first noncellulosic composite reverse osmosis membrane. Polyamide membranes were
fabricated in a similar manner by reacting polyethylenimine with isophthaloyl chloride.
These membranes maintained higher flux but had slightly lower salt rejection than the
polyurea counterparts.
Many other membranes were made from various types of aromatic polyamide
copolymers beginning in the early 1970s.106 Overall polyamide membranes are able to
outperform those based on cellulose acetate. They do not degrade by hydrolysis
reactions, can operate over a wide pH range (4-10), and can withstand higher operating
temperatures (120 oF). They also possess better mechanical properties than cellulose
63
acetate membranes. The major drawback to aromatic polyamide membranes is their
inability to tolerate free chlorine110, which is added to the water to kill bacteria.104 This
requires the water to be dechlorinated before it can come in contact with the membrane.
Another drawback of aromatic polyamide membranes is the high pressure required to
push the water through. Biofouling is also an area of concern.97
In the 1990s, RO membranes comprised of aromatic polyamides were still being
produced, along with membranes base on several other different polymers.107 In order to
improve upon the properties of RO membranes, manufacturers, such as Filmtech (now
Dow Water Solutions), Toray, and Nitto Denko, produced crosslinked thin film
composites of aromatic polyamide (Figure 1.35).107 RO membranes made from
crosslinked aryl-alkyl polyamide/polyurea (UOP, Hydranautics, Nitto Denko, DuPont),
crosslinked polypiperazineamides (Toray), and crosslinked polyether (Toray) were also
produced. Manufacturers were also exploring polyacrylonitrile (Sumitomo),
polybenzimidazolone (Teijin), and sulfonated polysulfones (DSI, Millipore, Nitto Denko)
for RO membrane materials.
NH
C C NH
NH
C C *NH
*
O O O O
C
NH
O
NH
COOH
x
y N
HC C N
HNH
C C *NH
*
O O O O
C
NH
O
NH
COOH
x
y
Figure 1.35. Crosslinked fully aromatic polymer.
64
Currently, RO filtration units contain aromatic polyamide thin film composites as
their semi permeable membrane (Dow-FilmTec, GE-Osmonics, Nitto Denko-
Hydranautics, etc.)111 Despite some of their inadequacies, aromatic polyamide
membranes are still the state of the art membranes because they are able to reach a 99.9%
salt rejection rate, while still maintaining a reasonable flux.97,112
1.8.3 Sulfonated Aromatic Polymers
Sulfonated aromatic polymers have also been explored for use as RO membranes
since the 1970s. Research began with the exploration of sulfonated poly(phenylene
oxide) and sulfonated polyfurane membranes and progressed to sulfonated
polysulfones.101 Sulfonated membranes maintain a low permeability to salts because the
sulfonate ions allow the anions in the salt to be repelled. Allegrezza et al.113,114 reported
that RO modules utilizing sulfonated polysulfone membranes exhibited high tolerance to
chlorine because they lack the oxidizable amide links present in polyamide membranes.
The sulfonated polysulfone RO modules could also withstand a wide pH range (4-11),
were resistant to fouling, and could be operated at high flux for long periods of time.
Although sulfonated polysulfones had desirable properties, they were synthesized using
post-sulfonation modification procedures,6,13115,116,117,118 which have many drawbacks.
Among the limitations of post-sulfonation modification are the ability to fully control the
degree and location of sulfonation, as well as, side reactions and chain-degradation.9
Over the past decade, research efforts in the McGrath group have been focused on
the direct synthesis of disulfonated poly(arylene ether) random
copolymers.119,120,121,122,123,124 These copolymers were synthesized by a nucleophilic
aromatic substitution reaction of a disulfonated dihalide (3,3’-disulfonated-4,4’-
65
dichlorodiphenylsulfone, SDCDPS), unsulfonated dihalide, and bisphenol to afford
random copolymers, with predetermined degrees of disulfonation based on the
stoichiometric ratio of sulfonated to unsulfonated dihalide. Copolymers with degrees of
sulfonation ranging from zero to 100% disulfonation have been achieved. These
copolymers have excellent oxidative, hydrolytic, and mechanical stability, as well as,
good film forming properties. Disulfonated poly(arylene ether sulfone) random
copolymers derived from SDCDPS, 4,4’-dichlorodiphenylsulfone (DCDPS), and 4,4’-
biphenol (coined BPSxx, where xx represents the degree of sulfonation) have been shown
to have high chlorine tolerance across a broad pH range (4-10).125 Exposure to protein
water or oil/water emulsions resulted in minimal fouling.126 Salt rejection and water
permeability for this type of membrane were correlated to the degree of disulfonation.
Overall, copolymers with higher ion content (BPS40) displayed higher fluxes and lower
salt rejection than copolymers with lower ion content (BPS20).18,127 However, water flux
and salt rejection were also influenced by the structure of the bisphenol used to
synthesize the copolymer and whether the copolymer was in salt or acid form.
Additional synthetic variations have been suggested, which could tailor the
properties of disulfonated poly(arylene ether) copolymers further, making them more
suited for RO applications.112,128 Among these has been crosslinking random copolymers
in order to enhance salt rejection without hindering the flux. Paul et al.112 synthesized
50% disulfonated poly(arylene ether sulfone) random copolymers derived from 4,4’-
biphenol, which had controlled number-average molecular weight (Mn) and reactive
phenoxide end groups. These were used to crosslink the copolymer with tetraglycidyl
bis(p-aminophenyl)methane. Membranes which were cured for 90 minutes had a 97.2%
66
salt rejected compared to 73.4% for BPS-50 uncrosslinked copolymer. Only modest
decreases were observed in water permeability.
67
1.9 Research Objectives
The first objective of this research was to assess if a segmented synthesis
technique, which is simpler in concept than the current technique, could be effectively
used to produce “blocky” ionic copolymers. Chapter 2 describes the synthesis of a
segmented multiblock copolymer comprised of a disulfonated poly(arylene ether sulfone)
hydrophilic block and highly fluorinated poly(arylene ether sulfone) hydrophobic block.
The properties of this copolymer are compared to a multiblock which used a previous
synthetic approach of coupling two preformed oligomers.
The segmented method was studied further using the well known bisphenol
phenolphthalein as a comonomer in either the hydrophobic (chapter 3) or hydrophilic
(chapter 4) block. It is proposed that phenolphthalein may improve proton conductivity
at lower relative humidity because the bulkiness of the monomer increases free volume in
copolymer. The synthesis of segmented copolymers with unequal hydrophobic and
hydrophilic block lengths is also examined in chapter 4.
The final objective was to synthesize novel hydrophilic-hydrophobic multiblock
copolymers derived from Bisphenol-A for potential use as reverse osmosis membranes.
Chapter 5 describes the synthesis of a novel series of poly(arylene ether sulfone)s which
utilized Bisphenol-A as the comonomer in both the hydrophobic and hydrophilic blocks.
68
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85 Sek, D.; Wanic, A.; Schab-Balcerzak, E. Novel approach to the mechanism of the high-temperature formation of naphthalimides. Polymer 1993, 34(11), 2440-2442. 86 Einsla, B.R.; Hong, Y.-T.; Kim, Y.S.; Wang, F.; Gunduz, N.; McGrath, J.E. Sulfonated Naphthalene Dianhydride Based Polyimide Copolymers for Proton-Exchange-Membrane Fuel Cells. I. Monomer and Copolymer Synthesis. J. Polym. Sci. Pol. Chem. 2004, 42, 862-874. 87 Wang, H.; Badami, A.S.; Roy, A.; McGrath, J.E. Multiblock Copolymers of Poly(2,5-benzophenone and Disulfonated Poly(arylene ether sulfone) for Proton-Exchange Membranes. I. Synthesis and Characterization. J. Polym. Sci. Pol. Chem. 2007, 45, 284-294. 88 Ghassemi, H.; Ndip, G.; McGrath, J.E. New multiblock copolymers of sulfonated poly(4’-phenyl-2,5-benzophenone) and poly(arylene ether sulfone) for proton exchange membranes. Polym. Prepr. 2003, 44(1), 814-815. 89 Ghassemi, H.; Ndip, G.; McGrath J.E. New multiblock copolymers of sulfonated poly(4’-phenyl-2,5-benzophenone) and poly(arylene ether sulfone) for proton exchange membranes. II. Polymer 2004, 45, 5855-5862. 90 Ghassemi H.; McGrath , J.E. Synthesis and properties of new sulfonated poly( p-phenylene) derivatives for proton exchange membranes. I. Polymer 2004, 45(17), 5847-5854. 91 Shin, C.K.; Maier, G.; Andreaus, B.; Scherer, G.G. Block copolymer ionomers for ion conductive membranes. J. Membr. Sci. 2004, 245, 147-161. 92 Zhao, C.; Li, X.; Wang, Z.; Dou, Z.; Zhong, S.; Na, H. Synthesis of block sulfonated poly(ether ether ketone)s (S-PEEKs) materials for proton exchange membrane. J. Membr. Sci. 2006, 280, 643-650. 93 Zhao, C.; Lin, H.; Shao, K.; Li, X.; Ni, H.; Wang, Z.; Na, H. Block sulfonated poly(ether ether ketone)s (SPEEK) ionomers with high ion-exchange capacities for proton exchange membranes. J. Power Sources 2006, 162, 1003-1009. 94 VanHouten, R.A.; Lane, O.; McGrath, J.E. Synthesis of Segmented Hydrophobic:Hydrophilic, Fluorinated:Disulfonated Block Copolymers for use as Proton Exchange Membranes. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2008, 53(2), 765-766. 95 Badami, A.S. PhD Dissertation, Virginia Polytechnic Institute and State University, 2007.
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96 Shin, C.K.; Maier, G.; Scherer, G.G. Acid functionalized poly(arylene ether)s for proton-conducting membranes. J. Mebran. Sci. 2004, 245, 163-173. 97 Service, R.F. Desalination Freshens Up Science 2006, 313, 1088-1090. 98 Gleick, P.H. In The World's Water 2001-2002: The Biennial Report on Freshwater Resources, Island Press: Washington, D.C., 2000. 99 Van der Bruggen, B.; Vandecasteele, C. Distillation vs. membrane filtration: overview of process evolutions in seawater desalination. Desalination, 2002, 143, 207-218. 100 Gleick, P.H.; Cooley, H.; Wolff, G.H., With a Grain of Salt: An Update on Seawater Desalination. In The World's Water 2006-2007: The Biennial Report on Freshwater Resources, Island Press: Washington, D.C., 2006. 101 Petersen, R.J. Composite reverse osmosis and nanofiltration membranes. J. Membr. Sci. 1993, 83, 81-150. 102 Loeb, S, Sourirajan, S. Advances in Chemistry Series 1963, 38, 117-132. 103 Glater, J. The early history of reverse osmosis membrane development. Desalination, 1998, 117, 297-309. 104 Amjad, Z., Ed. Reverse Osmosis: Membrane Technology, Water Chemistry, and Industrial Applications; Van Nostrand Reinhold: New York, 1993. 105 Lonsdale, H. K., Podall, H. E., Ed., Reverse Osmosis Membrane Research; Plenum Press: NewYork-London, 1972. 106 Gill, W.N. Review of Reverse Osmosis Membranes and Transport Models Chem. En. Commun. 1981, 12, 279-363. 107 Kurihara, M.; Himeshima, Y. The major developments of the evolving reverse osmosis membranes and ultrafiltration membranes. Polym. J. 1991, 23(5), 513.-520. 108 Lloyd, D.R.; Gerlowski, L.E.; Sunderland, C.D.; Wightman, J.P.; McGrath, J.E.; Igbal, M; Kang, Y. Poly(aryl ether) Membranes for Reverse Osmosis. In: Turbak, Albin F., editor. Synthetic Membranes: Volume 1 Desalination. Washington D.C.: American Chemical Society, 1981. 109 McKinney Jr., R. Properties of aromatic polyamide and polyamide-hydrazide membranes. In: Lonsdale, H. K., Podall, H. E., Eds., Reverse Osmosis Membrane Research. Plenum Press: NewYork-London, 1972. 110 Avlonitis, S.; Hanbury, W.T.; Hodgkiess, T. Chlorine Degradation of Aromatic Polyamides Desalination 1992, 85, 321-334.
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111 Zhang, Z.; Fan, G.; Sankir, M.; Roy, A.; Li, Y.; Park, H.B., Freeman, B.D.; McGrath, J.E. Disulfonated Directly Copolymerized Poly(arylene ether) Random Copolymers: Applications to Chlorine Resistant Reverse Osmosis (RO) or Nanofiltration (NF) Membranes—Part 1 Synthesis San Francisco ACS Meeting, September 2006. 112 Paul, M.; Park, H.B.; Freeman, B.D.; Roy, A.; McGrath, J.E.; Riffle, J.S. Synthesis and crosslinking of partially disulfonated poly(arylene ether sulfone) random copolymers as candidates for chlorine resistant reverse osmosis membranes Polymer 2008, 49, 2243-2252. 113 Allegrezza, Jr., A.E.; Parekh, B.S.; Parise, P.L.; Swiniarski, E.J.; White, J.L. Chlorine Resistant Polysulfone Reverse Osmosis Modules. Desalination, 1987, 64, 285-304. 114 Parise, P.L.; Allegrezza Jr.; A.E.; Parekh, B.S. Super hi-flux CP® chlorine-resistant reverse osmosis modules. Ultrapure Water, 1987, 4(7), 54-65. 115 Johnson, B. C.; Yilgor, I.; Tran, C.; Iqbal, M. Whightman, J. P.; Lloyd, D. R.; McGrath, J. E. Synthesis and Characterization of Sulfonated Poly(arylene ether sulfone)s. J. Polym. Sci.: Polym. Chem. Ed. 1984, 22, 721-737. 116 Lloyd, D.R.; Gerlowski, L.E.; Sunderland, C.D.; Wightman, J.P.; McGrath, J.E.; Iqbal, M.; Kang, K. Poly(aryl ether) Membranes for Reverse Osmosis. In Synthetic Membranes; Turbank, F.T., Eds.; ACS Symposium Series No. 153, American Chemical Society:Washington, D.C., 1981; 1, 327-350. 117 Drzewinski, M.; Macknight, W. J. Structure and properties of sulfonated polysulfone ionomers J. Appl. Polym. Sci. 1985, 30, 4753 – 4770. 118 Quentin, J.P. Sulfonated Polyarylether Sulfones, U.S. 3,709,841, Rhone-Poulenc, January 9, 1973. 119 Hickner, M.A.; Ghassemi, H.; Kim. Y.S.; Einsla, B.R.; McGrath, J.E. Alternative Polymer Systems for Proton Exchange Membranes (PEMs). Chem. Rev. 2004, 104, 4587-4612. 120 Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.; Zawodzinski, T.A.; McGrath, J.E. Synthesis of Highly Sulfonated Poly(arylene ether sulfone) Random (Statistical) Copolymers Via Direct Polymerization. Macromol. Symp. 2001, 175, 387-395. 121 Wang, F.; Hickner, M.; Kim, Y.S.; Zawodzinski, T.A.; McGrath, J.E. Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes. J. Membr. Sci. 2002, 197, 231-242. 122 Harrison, W.L.; Wang, F.; Mecham, J.B.; Bhanu, V.A.; Hill, M.; Kim, Y.S.; McGrath, J.E. Influence of the Bisphenol Structure on the Direct Synthesis of Sulfonated
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2 Synthesis of Segmented Hydrophobic-Hydrophilic, Fluorinated-Sulfonated
Block Copolymers for Use as Proton Exchange Membranes
Rachael A. VanHouten, Ozma R. Lane, Desmond J. VanHouten, James E. McGrath*
Macromolecular Science and Engineering, Macromolecular and Interfaces Institute
Virginia Tech, Blacksburg, VA 24061 *[email protected]
Abstract
A series of hydrophobic:hydrophilic segmented poly(arylene ether sulfone) copolymers
were synthesized and characterized for potential use as proton exchange membranes in
fuel cell applications. A hydrophilic oligomer- two monomer reaction approach was used
to synthesize the segmented copolymers, containing highly fluorinated hydrophobic
segments and 100% disulfonated hydrophilic blocks, via a nucleophilic aromatic
substitution step polymerization reaction. This approach afforded high molecular weight,
transparent, and ductile copolymers. At comparable ion exchange capacities, water
uptake increased with block length, suggesting that the extent of nanophase separation
was a function of block length. This favorably influenced conductivity behavior at
reduced relative humidity.
2.1 Introduction
Over the past few decades ion exchange polymers, or “ionomers”, have been of
growing interest and this literature has brought about a better understanding of how they
function and their structure-property relationships.1,2 Their application as proton
exchange membranes (PEMs) in fuel cells has also grown due to environmental concern
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and energy source limitations.3 Fuel cells provide a promising alternative energy source
for automobiles, portable power, and stationary power generation. Most proton exchange
membrane fuel cells use the chemical energy generated by the reaction of hydrogen and
oxygen to create electrical energy with ideally the only by-product being water. One of
the key components of a fuel cell is the proton exchange membrane.4 Nafion®, which is
a perfluorosulfonic acid membrane, represents the current state of the art ion exchange
polymer being used in PEM applications.5 However, new materials are needed because
Nafion® is expensive, has somewhat high permeability to fuels such as hydrogen,
oxygen, and methanol, low conductivity at high temperatures, and is difficult to process.6
For these reasons, higher performance membranes are needed.
Previous efforts in our laboratory5,7,8 have focused on the direct copolymerization
of disulfonated monomers to form random disulfonated poly(arylene ether) copolymers.
Disulfonated poly(arylene ether) copolymers provide excellent thermal, chemical, and
mechanical stability for fuel cell applications and perform well in fully hydrated
conditions. However, at low relative humidity (RH), these polymers exhibit lower
conductivity compared to Nafion®.3
More recently, efforts have been focused on the formation of poly(arylene ether)
multiblock copolymers containing perfectly alternating hydrophilic and hydrophobic
segments.9,10,11,12,13 Ion-rich channels have been shown to form when the hydrophobic
and hydrophilic domains of these multiblock copolymers nano-phase separate, allowing
for higher conductivity even under partially hydrated conditions.14,15 Nakabayashi et al.16
synthesized randomly coupled multiblock copolymers using decafluorobiphenyl to chain
extend phenoxide-terminated hydrophobic and hydrophilic oligomers. These copolymers
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also exhibited higher conductivity values under partially hydrated conditions when
compared to a random copolymer with a similar ion exchange capacity.
One of the most promising systems of multiblocks studied at Virginia Tech (VT)
is based on a highly fluorinated hydrophobic block (BisSF) and a fully disulfonated
hydrophilic block (BPSH100).9,10 Multiblock copolymers with varying block lengths and
ion exchange capacities were synthesized and displayed higher conductivities at lower
RH compared to the BPSH-35 random copolymer. At higher molecular weight block
lengths, this multiblock copolymer showed enhanced conductivity over the entire RH
range when compared to Nafion® 112.
Segmented copolymers have also been synthesized for use as PEMs in fuel cell
applications.17,18,19 In this technique, one of the blocks is synthesized with difunctional
end groups and then combined stoichiometrically with appropriate monomers, forming
the other block in-situ while the overall copolymer is being formed.20 This method is an
attractive synthetic technique for several reasons. Using the segmented technique,
multiblock copolymers can be synthesized more easily in a shorter amount of time
because there is no need to synthesize both oligomers separately and then couple them
together.20 The segmented technique can be used to synthesize unique copolymers that
cannot be synthesized by coupling preformed hydrophobic and hydrophilic oligomers.
Because it is produced from the coupling of two monomers and one oligomer, it may be
easier to find a common solvent than when two oligomers are used.91 This avoids the
often observed polymer-polymer incompatibilities between the two oligomers that
complicate copolymer synthesis.20,21
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However, precautions need to be taken when utilizing this synthetic method to
form copolymers which maintain their ordered chemical sequencing. One major issue
associated with the segmented copolymerization technique could be ether-ether
interchange22, which can occur under nucleophilic step polymerization conditions and
which will disrupt the block sequences. Segmented poly(arylene ether ketone)
copolymers91,92,93 have been reported in the literature. High reaction temperatures (>170
oC) were necessary to synthesize these polymers, which, under nucleophilic conditions,
runs the risk of producing ether-ether interchange reactions, resulting in a randomization
of the copolymer.
In this paper, we discuss the synthesis of BisSF-BPSH100 segmented copolymers.
Phenoxide terminated hydrophilic blocks (BPS100) were reacted under mild conditions
with highly reactive decafluorobiphenyl and bis(4-hydroxyphenyl)sulfone (Bis-S)
monomers to form a segmented copolymer containing highly fluorinated hydrophobic
segments. The decafluorobiphenyl allows the reaction to proceed at relatively low
temperatures (< 110 oC), which minimizes or prevents ether-ether interchange reactions
from occurring. The properties of these segmented copolymers have been compared to
multiblock copolymers that have been synthesized using an oligomer-oligomer approach,
the previously reported synthetic method.9,10 The effect of block length on copolymer
properties, such as proton conductivity, water uptake, and dimensional swelling will also
be discussed and is being further explored.
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2.2 Experimental Section
2.2.1 Materials
Decafluorobiphenyl (DFBP) was obtained from Matrix Scientific and dried under
vacuum at room temperature overnight. Bis(4-hydroxyphenyl)sulfone (Bis-S) was
purchased from Alfa Aesar and dried under vacuum at 60 oC for 24 h before use.
Monomer grade 4,4’-biphenol (BP) was obtained from ChrisKev Company, Inc. and
dried at 60 oC for 24 h under vacuum before use. 4,4’-Dichlorodiphenylsulfone
(DCDPS) was kindly provided by Solvay Advanced Polymers and used as received to
synthesize 3,3’-disulfonated-4,4’-dichlorodiphenylsulfone (SDCDPS) according to a
procedure reported elsewhere,23,24 which was a refinement of a previously published
procedure by Ueda et al.25 SDCDPS was dried under vacuum at 160 oC for 48 h before
use. N,N-Dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP, Aldrich) were
vacuum-distilled from calcium hydride onto molecular sieves and stored under nitrogen
before use. Potassium carbonate (K2CO3, Aldrich) was dried under vacuum at 120 oC
overnight before use. Toluene, cyclohexane, acetone, and isopropyl alcohol (IPA) were
obtained from Aldrich and used as received. Concentrated sulfuric acid (H2SO4) was
obtained from VWR and used to make a 0.5 M aqueous solution.
2.2.2 Synthesis of Phenoxide-Terminated Hydrophilic Blocks (BPS-100)
Phenoxide-terminated hydrophilic blocks were synthesized using a previously
published procedure.13 The targeted molecular weights of the blocks ranged from 3000
to 9000 g/mol. In a typical procedure for an Mn of 3000 g/mol, the following conditions
were utilized. BP (4.7322 g, 25.41 mmol), SDCDPS (10.2764 g, 20.92 mmol), and
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DMAc (75 mL) were added to a three-neck, round-bottom flask, equipped with
mechanical stirrer, Dean-Stark trap, condenser, and N2 inlet. The reaction bath was set to
85 oC, and the monomers were allowed to dissolve. K2CO3 (4.039 g, 29.23 mmol) and
toluene (38 mL) were added to the flask. The temperature of the bath was increased to
155 oC, and the reaction was allowed to azeotrope water for 4 h. Toluene was removed
from the system by increasing the bath temperature to 180 oC. The reaction was allowed
to proceed for 96 h. After cooling, the solution was filtered to remove salts and
precipitated into acetone. The resulting oligomer was dried at 110 oC for at least 24 h
under vacuum and had an Mn of 3300 g/mol determined by end group analysis using 1H
NMR.
2.2.3 Synthesis of BisSF-BPSH100 Segmented Copolymers
A sample copolymerization procedure was as follows: a three-neck, round-bottom
flask, equipped with mechanical stirrer, Dean-Stark trap, condenser, and N2 inlet was
loaded with BPS-100 (Mn equal to 3300 g/mol, 4.0418 g, 1.225 mmol), Bis-S (1.6700 g,
6.673 mmol), and NMP (29 mL). After dissolution of reactants, K2CO3 (1.255 g, 9.081
mmol) and cyclohexane (5 mL) were added to the reaction solution. The reaction bath
was heated to 110 oC, and the reaction was allowed to azeotrope water for 4 h. The
cyclohexane was then drained from the system, and the bath temperature was lowered to
85 oC. DFBP (2.6391 g, 7.899 mmol) and NMP (13 mL) were added to the reaction flask
and the temperature was raised to 90 oC where it was maintained for 36 h. The viscous
solution was cooled and precipitated into IPA (1 L). The product was filtered and
washed in deionized water at 60 oC for 12 h and acetone for 12 h. It was dried under
vacuum at 110 oC for 24 h before casting into films.
85
2.2.4 Synthesis of BisSF-BPSH100 Multiblock Copolymer Controls
Multiblock copolymer controls were synthesized according to a previously
published method.9,10 Some modifications were made to the procedure and are reflected
below.
2.2.4.1 Synthesis of Fluorine-Terminated Hydrophobic Blocks (BisSF) (2)
Fluorine-terminated hydrophobic blocks were synthesized with targeted
molecular weights ranging from 3000 to 9000 g/mol. For an Mn of 3000 g/mol, the
following synthetic procedure was utilized. Bis-S (2.5045 g, 10.01 mmol), DFBP
(4.0263 g, 12.05 mmol), and DMAc (38 mL) were added to a three-neck, round-bottom
flask, equipped with mechanical stirrer, Dean-Stark trap, condenser, and N2 inlet. The
reaction bath was heated to 50 oC. After dissolution of the monomers, K2CO3 (1.798 g,
13.01 mmol) and cyclohexane (7 mL) were added to the reaction flask. The bath
temperature was increased to 110 oC over 30 min. The reaction was allowed to proceed
for 5 h at 110 oC. After cooling, the reaction was filtered to remove salts and precipitated
into a solution of methanol:water (1:1 v:v, 1 L). The oligomer was washed for 12 h in DI
water and dried at 90 oC for 24 h under vacuum before further use. It had an Mn of 3200
g/mol determined by end group analysis using 19F NMR.
2.2.4.2 Synthesis of Phenoxide-Terminated Hydrophilic Blocks (BPS-100) (1)
Phenoxide-terminated hydrophilic blocks were synthesized with targeted
molecular weights ranging from 3000 to 9000 g/mol. To obtain a targeted molecular
weight of 3000 g/mol, the following reaction procedure was utilized. BP (0.7939 g,
4.263 mmol), SDCDPS (1.7466 g, 3.555 mmol), and NMP (20 mL) were added to a
86
three-neck, round-bottom flask, equipped with mechanical stirrer, Dean-Stark trap,
condenser, and N2 inlet. The reaction bath was heated to 85 oC. Upon dissolution of the
monomers, K2CO3 (0.766 g, 5.543 mmol) and toluene (10 mL) were added to the flask.
The bath temperature was increased to 155 oC, and the reaction was allowed to azeotrope
for 4 h. Toluene was removed from the system and the reaction bath was increased to
190 oC for 36 h. The bath temperature was lowered to 85 oC for the following reaction.
This product was not isolated and was used directly in the synthesis of BisSF-BPS100
multiblock copolymer described below. The resulting oligomer had a Mn of 3300 g/mol
determined by end group analysis using 1H NMR.
2.2.4.3 Synthesis of BisSF-BPS100 Multiblock Copolymers
Oligomer (2) (Mn equal to 3200 g/mol, 2.0244 g, 0.6135 mmol) was added to (1)
and NMP (16 mL) was used to facilitate the addition of the oligomer and maintain a 14%
w/v reaction solution. The bath temperature was increased to 90 oC, and the reaction was
allowed to proceed for 36 h. The resulting viscous solution was precipitated into IPA
(500 mL) to form fibrous strands. The product was filtered and washed in deionized
water at 60 oC for 12 h and acetone for 12 h. It was dried under vacuum at 110 oC for 24
h before casting into films.
2.2.5 Characterization of Copolymers
1H and 19F NMR analyses were performed on a Varian INOVA 400 spectrometer.
13C NMR analyses were performed on a Varian Unity 400 MHz spectrometer. Spectra
were obtained from a 10% solution (w/v) of sample in DMSOd6 and run at ambient
temperatures. Intrinsic viscosities of the segmented and multiblock copolymers were
87
determined using size exclusion chromatography (SEC) or gel permeation
chromatography (GPC). SEC 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 and sample solution were filtered before
introduction to the SEC system. Further solution characterization procedures have been
described.26
2.2.6 Membrane preparation
Membranes were cast from a 7% w/v solution of polymer in DMAc onto a clean
glass plate. Solvent was removed using an IR lamp. The lamp intensity was held at 30-
35 oC for 24 h and then raised to 35-40 oC for an additional 24 h. It was dried under
vacuum at 110 oC for 24 h. The film was removed from the glass plate by submersion in
water and acidified in boiling 0.5 M H2SO4 for 2 h, followed by 2 h in boiling deionized
water as described earlier.27
2.2.7 Determination of water uptake and dimensional swelling
The water uptake for all membranes was determined gravimetrically. Acidified
membranes were equilibrated in liquid water at room temperature for 24 h. Wet
membranes were removed from the liquid water, blotted dry to remove excess water, and
quickly weighed. They were then dried at 110 oC under vacuum for 24 h and reweighed.
Water uptake was calculated according to Equation 2.1 where massdry and masswet refer to
88
the mass of the dry and wet membranes, respectively. An average of three samples was
used for each measurement.
( )wet dry
dry
mass masswater uptake% 100
mass
−= × 2.1
Percent swelling of the membranes was determined in the in-plane (x and y) and through-
plane (z) directions. Wet measurements were performed after equilibrating membranes
in liquid water for 24 h at room temperature. Membranes were then dried in a convection
oven at 80 oC for 2 h and measured again. Wet and dry measurements in the x and y
direction were performed by sandwiching the membrane between layers of polyethelene
and two glass plates and measuring with a ruler (mm). Wet and dry measurements in the
z direction were performed using a micrometer. Typical sample size was 2.5 x 2.5 cm
squares when wet. Percent swelling was reported for three directions and calculated
according to Equation 2.2 where lengthwet,i and lengthdry,i refer to the length (where i
represents the x, y, or z direction) of the dry and wet membrane, respectively.
( )wet,i dry,ii
dry,i
length lengthpercent swelling 100
length
−= × 2.2
2.2.8 Measurement of proton conductivity
Proton conductivity at 30 oC in liquid water was determined in a window cell
geometry28 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.29
In determining proton conductivity in liquid water, membranes were equilibrated at 30 oC
in DI water for 24 h prior to testing. Proton conductivity under partially hydrated
conditions was performed at 80 oC. Membranes were equilibrated at 80% RH for 8 h in a
89
humidity-temperature oven (ESPEC, SH-240). The thickness of the film was measured
using a micrometer. Membranes were allowed to equilibrate at 95% RH and at each
additional specified RH value for 4 h before each measurement. Thickness
measurements were performed at the lowest RH which was reached.
2.2.9 Tensile testing
Uniaxial load tests were performed using an Instron 5500R universal testing
machine equipped with a 200-lb load cell at room temperature and 44-54% relative
humidity (RH). Crosshead displacement speed was 5 mm/min and gauge lengths were set
to 25 mm. A dogbone die was used to punch specimens 50 mm long with a minimum
width of 4 mm. Prior to testing, specimens were dried under vacuum at 110 oC for at least
24 h and then equilibrated at 44% RH and 30 oC. All specimens were mounted in
manually tightened grips. Approximate tensile moduli for each specimen were calculated
based on the stress and elongation values for the specimen at the first data point at or
above 2% elongation.
2.3 Results and Discussion
2.3.1 Synthesis of Hydrophilic Oligomers
Phenoxide-terminated, fully disulfonated poly(arylene ether sulfone) hydrophilic
oligomers (BPS100) were synthesized via a nucleophilic aromatic substitution reaction
(Figure 5.1). A small molar excess of BP to SDCDPS was used to control the molecular
weight of the oligomers, targeting number-average molecular weights (Mn) of 3, 5, or 9
kg/mol. Proton NMR was used to confirm the oligomers were phenoxide-terminated and
to determine the Mn of the oligomers using end-group analysis (Figure 2.2). Protons
90
from the terminal BP were assigned to the peaks at 6.8, 7.05, 7.4, and 7.55 ppm, whereas,
protons from the BP in the middle of the backbone resulted in peaks at 7.1 and 7.65 ppm.
By comparing the integration value ratios of end-group peaks to main chain peaks, Mn
was determined. Theoretical and experimental Mn values are summarized in Table 2.1,
along with intrinsic viscosity (I.V.) values measured by SEC. An increase in I.V. was
observed as Mn of the oligomers increased. A log-log plot of Mn versus I.V. had a linear
relationship, exhibiting a strong correlation between I.V. and Mn (Figure 2.3).
K2CO3
Toluene/DMAc4 h @ 155 oC96 h @ 190 oC
+OHOH Cl S Cl
O
OSO3Na
NaO3S
OKO S O
O
OSO3K
KO3S
OKn
K2CO3
Toluene/DMAc4 h @ 155 oC96 h @ 190 oC
+OHOH Cl S Cl
O
OSO3Na
NaO3S
OKO S O
O
OSO3K
KO3S
OKn
Figure 2.1. Phenoxide-terminated BPS-100 with controlled molecular weight
91
OKO S O
O
OSO3K
KO3S
OKn
h b a
e
eg
g
ii d ch
f
f
OKO S O
O
OSO3K
KO3S
OKn
h b a
e
eg
g
ii d ch
f
f
OKO S O
O
OSO3K
KO3S
OKn
h b a
e
eg
g
ii d ch
f
f
Figure 2.2. 1H NMR spectrum of BPS-100 oligomer
Table 2.1. Characterization of Hydrophilic Telechelic Oligomers
a. Calculated from 1H NMRb. GPC results of oligomer in salt form performed in NMP with 0.05 M LiBr27
Target Experimentala
3000 2900 0.14
5000 4900 0.20
9000 9200 0.29
Molecular Weight(g/mol) I.V.b
(dL/g)
a. Calculated from 1H NMRb. GPC results of oligomer in salt form performed in NMP with 0.05 M LiBr27
Target Experimentala
3000 2900 0.14
5000 4900 0.20
9000 9200 0.29
Molecular Weight(g/mol) I.V.b
(dL/g)
92
y = 0.6082x - 2.9494
R2 = 0.9999
-0.9
-0.85
-0.8
-0.75
-0.7
-0.65
-0.6
-0.55
-0.5
3.4 3.6 3.8 4
Log (M n)
Log
( I.V
.)
Log(I.V)=0.61[Log(Mn)]-2.9y = 0.6082x - 2.9494
R2 = 0.9999
-0.9
-0.85
-0.8
-0.75
-0.7
-0.65
-0.6
-0.55
-0.5
3.4 3.6 3.8 4
Log (M n)
Log
( I.V
.)
Log(I.V)=0.61[Log(Mn)]-2.9
Figure 2.3. Log (Mn) vs. log (I.V.) for the hydrophilic oligomers
2.3.2 Synthesis of BisSF-BPSH100 Segmented Copolymers
The segmented copolymer was formed by reacting phenoxide-terminated
hydrophilic oligomer with DFBP and Bis-S monomers in a step growth polymerization
(Figure 2.4). Simultaneous formation of the hydrophobic segments and the block
copolymer eliminated the need to synthesize and isolate a separate hydrophobic block
before coupling it to the hydrophilic block. The stoichiometry was controlled such that
the DFBP and Bis-S monomers formed the hydrophobic segments of the copolymer,
while also reacting with the phenoxide-terminated hydrophilic oligomer. The molecular
weights of the hydrophobic segments were targeted to be 3, 5, or 9 kg/mol so copolymers
with equal hydrophobic and hydrophilic block lengths would result. To achieve high
molecular weight, it was important to ensure that the overall ratio of phenoxide to para–F
end-groups was 1:1. An excess of phenoxide groups was disadvantageous because once
the para-fluorines were consumed, the ortho-fluorines on the DFBP could react with the
93
excess phenoxide groups, resulting in a crosslinked network. However, if an insufficient
amount of phenoxide groups were present, high molecular weight copolymer was not
achieved.
K2CO3
Cyclohexane/NMP4 h @ 85 oC
Additionof
36-70 h @ 90 oC
(DFBP)FFFF
F F F F
FF
S
O
O
OHOHOKO S O
O
OSO3K
KO3S
OKn
FFFF
F F F F
FFFF
F F F F
O S O
O
O
OO S O
O
OSO3K
KO3S
On
m
x
K2CO3
Cyclohexane/NMP4 h @ 85 oC
K2CO3
Cyclohexane/NMP4 h @ 85 oC
Additionof
36-70 h @ 90 oC36-70 h @ 90 oC
(DFBP)FFFF
F F F F
FF
S
O
O
OHOHOKO S O
O
OSO3K
KO3S
OKn
FFFF
F F F F
FFFF
F F F F
O S O
O
O
OO S O
O
OSO3K
KO3S
On
m
x
Figure 2.4. BisSF-BPSH100 segmented copolymer
Representative 1H and 19F NMR spectra of the segmented copolymer are shown in
Figure 2.5. Both spectra indicate successful formation of the hydrophobic block with
successful coupling to the hydrophilic block. There were no peaks due to end-groups in
either spectrum, which signifies successful segmented copolymer formation. The peak at
7.3 ppm has been assigned to the protons of the BP in the linking unit.9
94
hi
(b)
(a)
FFFF
F F F F
FFFF
F F F F
O S OO
OOO S O
O
OSO3K
KO3S
On
m
x
a b c
e
d f g
h iFFFF
F F F F
FFFF
F F F F
O S OO
OOO S O
O
OSO3K
KO3S
On
m
x
a b c
e
d f g
h i
hi
(b)
(a)
FFFF
F F F F
FFFF
F F F F
O S OO
OOO S O
O
OSO3K
KO3S
On
m
x
a b c
e
d f g
h iFFFF
F F F F
FFFF
F F F F
O S OO
OOO S O
O
OSO3K
KO3S
On
m
x
a b c
e
d f g
h i
Figure 2.5. (a) 1H and (b) 19F NMR spectra for BisSF-BPS100 segmented copolymer
The highly reactive DFBP monomer allowed for mild reaction temperatures (90-
110 oC) to be used during synthesis, which eliminated randomization by ether-ether
interchange. Monomer sequencing is highly ordered in block copolymers. Therefore,
every carbon has only one possible chemical environment. Random copolymers develop
a larger number of short sequences during the copolymerization. This causes different
chemical environments for similar carbons, which results in splitting of the 13C NMR
peaks. If ether-ether interchange occurred during a segmented copolymerization, a
scrambling in the backbone would occur. This would be evidenced by peak splitting
similar to a random copolymers. The singlets in the 13C NMR spectrum of a segmented
copolymer were the same as for a multiblock copolymer with a similar composition,
which strongly supports that ether-ether interchange had been avoided (Figure 2.6).
95
Segmented 5K5K
Multiblock 5K5K
Segmented 5K5K
Multiblock 5K5K
Figure 2.6. 13C NMR spectra for BisSF-BPS100 multiblock and segmented copolymers
2.3.3 Synthesis of BisSF-BPSH100 Multiblock Copolymer Controls
BisSF-BPSH100 multiblock copolymer controls were synthesized as described in
literature.9,10 Phenoxide-terminated, BPS100 oligomers were synthesized using a slight
molar excess of BP to SDCDPS to control Mn. Hydrophobic oligomers (Bis-SF) were
synthesized using a slight molar excess of DFBP to Bis-S to achieve fluorine-terminated
hydrophobic oligomers with controlled Mn. Both hydrophilic and hydrophobic block lengths
were targeted at 3, 5, or 9 kg/mol. Experimental Mns were determined by end group analysis
from 1H and 19F NMR for BPS100 and BisSF oligomers, respectively, and agreed well with
the target values (Table 2.2). BPS100 and BisSF oligomers were coupled together using a
1:1 molar ratio to form multiblock copolymers with equal hydrophilic and hydrophobic block
lengths. The overall synthetic procedure is depicted in Figure 2.7.
96
Table 2.2. Characterization of Hydrophilic and Hydrophobic Telechelic Oligomers for BisSF-BPSH100 Multiblock Copolymers
a. Calculated from 1H NMRb. Calculated from 19F NMR
BPSH100
Experimentala
3000 3300
5000 5000
9000 9200
3200
6100
9300
ExperimentalbBisSF
Molecular Weight (Mn) (g/mol)
Target
a. Calculated from 1H NMRb. Calculated from 19F NMR
BPSH100
Experimentala
3000 3300
5000 5000
9000 9200
3200
6100
9300
ExperimentalbBisSF
Molecular Weight (Mn) (g/mol)
Target
K2CO3
Toluene/NMP4 h @ 150 oC36 h @ 190 oC
+ +
K2CO3
Cyclohexane/DMAc5 h @ 110 oC
+
NMP90-110 oC for 12-48 h
FFFF
F F F F
FF S
O
O
OHOHOHOH Cl S Cl
O
OSO3Na
NaO3S
OKO S O
O
OSO3K
KO3S
OKn
FFFF
F F F F
FFFF
F F F F
O S O
O
OF m
F
FFFF
F F F F
FFFF
F F F F
O S O
O
O
OO S O
O
OSO3K
KO3S
On
m
x
K2CO3
Toluene/NMP4 h @ 150 oC36 h @ 190 oC
+ +
K2CO3
Cyclohexane/DMAc5 h @ 110 oC
+
NMP90-110 oC for 12-48 h
FFFF
F F F F
FF S
O
O
OHOHOHOH Cl S Cl
O
OSO3Na
NaO3S
OKO S O
O
OSO3K
KO3S
OKn
FFFF
F F F F
FFFF
F F F F
O S O
O
OF m
F
FFFF
F F F F
FFFF
F F F F
O S O
O
O
OO S O
O
OSO3K
KO3S
On
m
x
Figure 2.7. BisSF-BPSH100 multiblock copolymer
2.3.4 Comparison of BisSF-BPSH100 Segmented and Multiblock Copolymer
Properties
The main objective of this research was to assess the feasibility of using a
segmented synthetic technique to produce copolymers with blocky hydrophilic and
97
hydrophobic segments. Multiblock copolymer controls with known blocky structures
were synthesized for comparative purposes. Select properties of both systems are
summarized in Table 2.3. Experimental IEC values, calculated from 1H NMR, were in
good agreement with theoretical values, confirming that the hydrophobic segments were
systematically incorporated into the copolymer backbones. The I.V. data confirmed that
high molecular weight polymers were achieved using both synthetic methods. The
ability to cast tough films also indicated high molecular weight polymers. When
comparing polymers with similar IEC values, water uptake increased as the block lengths
increased due to the development of a more defined phase separated morphology.
Tensile properties were also comparable for polymeric membranes prepared from both
synthetic techniques (Table 2.4).
Table 2.3. Characterization of Segmented and Multiblock BisSF-BPSH100 Copolymers
a. Calculated from experimental loading:IEC = (g of Hydrophilic *IEC of BPSH100)/(g of Hydrophobic + g of Hydrophilic)
a. Calculated from 1H NMRb. Performed in liquid water at 30 oC
I.V. Conductivityc Water Uptake
Theoreticala Experimentalb (dL/g) (S/cm) (%)3K:3K 1.6 1.8 0.63 0.10 62
Segmented 5K:5K 1.6 1.5 0.50 0.11 519K:9K 1.7 1.5 0.82 0.15 74
3K:3K 1.8 1.8 1.07 0.13 73Multiblock 5K:5K 1.6 1.6 0.89 0.13 78
9K:9K 1.7 1.7 0.84 0.13 101
IEC
(meq/g)
Polymer
a. Calculated from experimental loading:IEC = (g of Hydrophilic *IEC of BPSH100)/(g of Hydrophobic + g of Hydrophilic)
a. Calculated from 1H NMRb. Performed in liquid water at 30 oC
I.V. Conductivityc Water Uptake
Theoreticala Experimentalb (dL/g) (S/cm) (%)3K:3K 1.6 1.8 0.63 0.10 62
Segmented 5K:5K 1.6 1.5 0.50 0.11 519K:9K 1.7 1.5 0.82 0.15 74
3K:3K 1.8 1.8 1.07 0.13 73Multiblock 5K:5K 1.6 1.6 0.89 0.13 78
9K:9K 1.7 1.7 0.84 0.13 101
IEC
(meq/g)
Polymer
98
Table 2.4. Comparison of Tensile Properties for Segmented and Multiblock Copolymers
Modulus Tensile Strength % Elongation Max. Elongation
MPa MPa % %3K3K 1500 ±160 42 ± 5 43 ± 27 74
Segmented 5K5K 1470 ±120 39 ± 2 16 ± 5 219K9K 1510 ± 80 46 ± 4 24 ± 16 47
3K3K 1380 ± 200 42 ± 3 43 ± 15 55Multiblock 5K5K 1450 ± 150 41 ± 4 47 ± 21 71
9K9K 1390 ± 200 43 ± 3 22 ± 4 26
Copolymer
Dimensional swelling was performed on both series of copolymers. The results
for the segmented and multiblock copolymers were also compared to a random
copolymer (BPSH35) in Figure 2.8. The segmented and multiblock copolymers
exhibited anisotropic swelling in contrast to the isotropic swelling of the random
copolymer. Overall, swelling through the plane (z-direction) increased with an increase
in block length, while in-plane swelling (x- and y-directions) stayed the same or
decreased. This is likely indicative of the well-ordered morphology that develops as
block length increases. Membrane electrode failure, due to changes in humidity levels
(swelling and deswelling cycling), may be minimized with a reduction of in-plane
swelling.
99
0
10
20
30
40
50
60
70
80
90
BPSH35 3K3K 3K3K 5K5K 5K5K 9K9K 9K9K
% S
wel
ling
x y z
Segmented Segmented SegmentedMultiblock Multiblock Multiblock
z
Xy
Random0
10
20
30
40
50
60
70
80
90
BPSH35 3K3K 3K3K 5K5K 5K5K 9K9K 9K9K
% S
wel
ling
x y z
Segmented Segmented SegmentedMultiblock Multiblock Multiblock
z
Xy
Random Segmented Segmented SegmentedMultiblock Multiblock Multiblock
z
Xyz
Xy
Random
Figure 2.8. Comparison of dimensional swelling data for segmented, multiblock, and random copolymers
The effect of block length on proton conductivity under partially hydrated
conditions was assessed for both systems. A plot of proton conductivity versus relative
humidity is shown in Figure 2.9. The segmented and multiblock copolymers had similar
proton conductivities across the entire RH range when comparing copolymers with
equivalent block length compositions. It can be seen that conductivity increased over the
entire RH range as block length increased. This indicates that increasing the block length
of the hydrophilic and hydrophobic segments increases the connectivity in the
hydrophilic channels, regardless of the polymerization technique that was utilized.
100
20 30 40 50 60 70 80 90 1000.1
1
10
100
P
roto
n C
ondu
ctiv
ity (
mS
/cm
)
Relative Humidity (%)
Segmented 3K3K Multiblock 3K3K Segmented 5K5K Multiblock 5K5K Segmented 9K9K Multiblock 9K9K Nafion 112
Figure 2.9. Comparison of proton conductivity under partially hydrated conditions
for segmented and multiblock copolymers with increasing block length
2.4 Conclusions
Segmented copolymers containing highly fluorinated hydrophobic blocks and
100% disulfonated hydrophilic blocks have been successfully synthesized using an
oligomer- two monomer approach. The utilization of low temperature reactions
eliminated randomization by ether-ether interchange, evidenced by 13C NMR. The
properties of these copolymers were in good agreement with the properties of multiblock
copolymers synthesized using a more cumbersome oligomer-oligomer approach.9,10 An
increase in water uptake with increasing block length indicated the formation of well
connected channels as block length increased. Increased conductivity over the entire RH
range was also evidence that a connected hydrophilic morphology developed with
increased block length.
101
Acknowledgement. The authors would like to acknowledge the Department of
Energy (DE-FG36-06G016038) and NSF IGERT (DGE-0114346) for funding.
102
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23 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. Synthesis and Characterization of 3,3’-Disulfonated-4,4’-dichlorodiphenyl Sulfone (SDCDPS) Monomer for Proton Exchange Membranes (PEM) in Fuel Cell Applications J. Appl. Polym. Sci. 2006, 100, 4595-4602.
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24 Li, Y.; VanHouten, R.; Brink, A.; McGrath, J.E. Purity Characterization of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl Sulfone (SDCDPS) Monomer by UV-visible Spectroscopy. Polymer, 2008, 49, 3014-3019. 25 Ueda, M.; Toyota, H.; Ouchi, T.; Sugiyama, J.I.; Yonetake, K.; Masuko, T.; Teramoto, T. Synthesis and Characterization of Aromatic Poly(ether Sulfone)s Containing Pendant Sodium Sulfonate Groups. J. Polym. Sci.: Part A: Polym. Chem. 1993, 31, 853-858. 26 Yang, J.; Li, Y.; Roy, A.; McGrath J.E. Viscometric behavior of disulfonated poly(arylene ether sulfone) random copolymers used for proton exchange membranes. Polymer, 2008, 49(24), 5300-5306. 27 Kim, Y. S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y. T.; Harrison, W.; Zawodzinski, T. A.; McGrath, J. E. Effect of Acidification Treatment and Morphological Stability of Sulfonate Poly(arylene ether sulfone) Copolymer Proton Exchange Membranes for Fuel Cell Use Above 100 °C. J. Polym. Sci.: Part B: Polym. Phys. 2003, 41, 2816-2828. 28 Zawodzinski, T. A.; Neeman, M.; Sillerud, L. O.; Gottesfeld, S. Determination of water diffusion coefficients in perfluorosulfonate ionomeric membranes J. Phys. Chem. 1991, 95, 6040. 29 Springer, T. E.; Zawodzinski, T. A.; Wilson, M. S.; Gottesfeld, S. Characterization of polymer electrolyte fuel cells using ac impedance spectroscopy J. Electrochem. Soc. 1996, 143, 587.
105
3 Synthesis and Characterization of Highly Fluorinated-Disulfonated
Hydrophobic-Hydrophilic Segmented Copolymers Containing Various
Bisphenols for Use as Proton Exchange Membranes
Rachael A. VanHouten, Desmond J. VanHouten, Ozma Lane, James E. McGrath*
Macromolecular Science and Engineering Macromolecules and Interfaces Institute Virginia Tech, Blacksburg, VA 24061
Abstract
A new series of disulfonated hydrophobic: hydrophilic segmented poly(arylene ether
sulfone) copolymers was synthesized and characterized for potential use as proton
exchange membranes in fuel cell applications. Copolymers comprised of 100%
disulfonated hydrophilic segments and hydrophobic segments derived from the bisphenol
phenolphthalein and decafluorobiphenyl were synthesized using an oligomer-two
monomer approach via a nucleophilic aromatic substitution reaction. The properties of
segmented copolymers derived from phenolphthalein were compared to copolymers
previous synthesized using 4,4’-sulfonyldiphenol (bisphenol-S) to determine the effect
the bisphenol had on conductivity, tensile properties, and thermal behavior of the
membrane. An increase in tensile modulus, strength, and glass transition temperature
was observed for the segmented copolymer derived from phenolphthalein due to the
greater rigidity of the phenolphthalein compared to bisphenol-S. Water uptake for the
two systems increased as block length increased. Proton conductivity also increased
across the entire range of relative humidity for both series. However, copolymers
106
containing bisphenol-S displayed higher overall conductivity when scaled to block
length.
3.1 Introduction
Disulfonated poly(arylene ether sulfone) (PAES) random copolymers have shown
promise for use as materials for proton exchange membranes (PEMs).1,2,3 They produce
membranes which are chemically, thermally, and mechanically stable. When used as
PEMs at moderate temperatures and high relative humidity (RH), they exhibit
conductivities comparable to that of Nafion®, the benchmark polymer used for PEMs at
this time.4,5,6 However, at low RH and higher temperatures, the conductivity of random
disulfonated PAES decreases, and this has been attributed to loss of connectivity in the
hydrophilic domains.
The conductivity at low RH had been improved with hydrophobic-hydrophilic
multiblock copolymers. In particular, our group has focused on systematically
controlling the volume fraction of blocks, the block length and ion exchange capacities
IEC). Synthesis of the multiblock copolymers was achieved by coupling telechelic
wholly aromatic 4,4’-biphenol based disulfonated poly(arylene ether) hydrophilic
oligomers with several fluorinated, nonfluorinated, and polyimide hydrophobic
oligomers.7,8,9 Ion-rich channels have been shown to form by atomic force microscopy
(AFM) and transmission electron microscopy (TEM) when the hydrophobic and
hydrophilic domains of these multiblock copolymers are designed to nano-phase
separate. This has been shown to increase the water self diffusion coefficient in the
hydrophilic phase, which allows for higher conductivity even under partially hydrated
conditions.10 By changing the volume fraction of blocks, block length, and the
107
interaction parameter of the hydrophilic and hydrophobic blocks, the extent of nano-
phase separation can be altered.11,12
This paper is concerned with producing the multiblock copolymers via what has
been suggested to be termed a segmented technique.13 For example, segmented
copolymers have been synthesized using a preformed oligomer which is then directly
reacted with one A-B or two A-A and B-B monomers. For the present system, a
hydrophilic block of disulfonated poly(arylene ether sulfone) oligomer with phenoxide
reactive end groups was first synthesized and isolated (in principle, this might not be
required). It was then reacted with a calculated amount of hydrophobic monomers,
forming that block in-situ. Using the segmented technique, multiblock copolymers were
synthesized in a shorter amount of time because there was no need to synthesize both
oligomers separately before coupling.
One approach for altering the hydrophobic segments of the copolymer is to
employ various bisphenols in the copolymerization. Previous studies13,14 focused on the
use of bisphenol-S as the comonomer in the hydrophobic segments. This bisphenol,
which is economically viable, affords hydrolytically stable amorphous soluble
copolymers, which have manageable water uptake.9,15 This paper describes a series of
segmented copolymers using phenolphthalein as a comonomer in the hydrophobic
segments. Phenolphthalein was chosen as an alternate comonomer because its bulky
nature may increase the free volume of the copolymer,16 possibly allowing for higher
conductivity at lower relative humidity.17 The monomer rigidity may also enhance
mechanical strength. This paper describes the synthesis of segmented copolymers
utilizing phenolphthalein in the hydrophobic segments. The characteristics of these
108
segmented copolymers will also be compared to those of our previously synthesized
segmented copolymers derived from bisphenol-S.
3.2 Experimental
3.2.1 Materials
Decafluorobiphenyl (DFBP) was obtained from Matrix Scientific and dried under
vacuum at room temperature overnight. Bisphenol-S (Bis-S) was purchased from Alfa
Aesar and dried under vacuum at 60 oC for 24 h before use. Monomer grade 4,4’-
biphenol (BP) was obtained from ChrisKev Company, Inc. and dried at 60 oC for 24 h
under vacuum before use. 4,4’-dichlorodiphenylsulfone (DCDPS) was kindly provided
by Solvay Advanced Polymers and used as received to synthesize 3,3’-disulfonated-4,4’-
dichlorodiphenylsulfone (SDCDPS) according to a procedure reported elsewhere.18,19,20
Phenolphthalein was purchased from Sigma Aldrich and was recrystallized from ethanol
and water. The phenolphthalein was dried under vacuum at 90 oC for 24 h prior to use.
N,N-dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP) (Aldrich) were
vacuum-distilled from calcium hydride onto molecular sieves and stored under nitrogen
before use. Potassium carbonate (K2CO3) was obtained from Aldrich and dried under
vacuum at 120 oC overnight before use. Toluene, cyclohexane, and isopropyl alcohol
were obtained from Aldrich and used as received.
3.2.2 Synthesis of Phenoxide-Terminated Hydrophilic Blocks (BPS-100)
Phenoxide-terminated hydrophilic blocks were synthesized using a previously
published procedure from our laboratory.9 The targeted molecular weights of the blocks
ranged from 5000 to 13000 g/mol. In a typical procedure for an Mn of 5000 g/mol, the
109
following conditions were utilized. BP (4.2789 g, 22.98 mmol), SDCDPS (10.1116 g,
20.58 mmol), and DMAc (72 mL) were added to a three-neck, round-bottom flask,
equipped with mechanical stirrer, Dean-Stark trap, condenser, and N2 inlet. The reaction
bath was set to 85 oC, and the monomers were allowed to dissolve. K2CO3 (3.652 g,
26.43 mmol) and toluene (36 mL) were added to the flask. The temperature of the bath
was increased to 155 oC, and the reaction was allowed to azeotrope water for 4 h.
Toluene was removed from the system by increasing the bath temperature to 180 oC. The
reaction was allowed to proceed for 96 h. After cooling, the solution was filtered to
remove salts and precipitated into acetone. The resulting oligomer was dried at 110 oC
for at least 24 h under vacuum and had an Mn of 4700 g/mol determined by end group
analysis using 1H NMR.
3.2.3 Synthesis of segmented copolymer with simultaneous formation of
hydrophobic segments
The segmented copolymer was synthesized using DFBP and either Bis-S or Ph
monomers to form the hydrophobic block. For a Ph containing segmented copolymer (PhF-
BPSH100) (5K:5K): a 3-neck, round bottom flask, equipped with mechanical stirrer, Dean-
Stark trap, condenser, and N2 inlet was loaded with BPS-100 (5K; 3.1010 g, 0.6164 mmol),
Ph (1.4949 g, 4.6961 mmol), and NMP (31 mL). After dissolution of reactants, K2CO3
(0.844 g, 6.109 mmol) and cyclohexane (6 mL) were added to the reaction solution. The
reaction bath was heated to 110 oC and allowed to azeotrope for 4 h. The cyclohexane was
drained from the system and the bath temperature was lowered to 85 oC. DFBP (1.7750 g,
5.3125 mmol) and NMP (12 mL) were added to the reaction flask. The bath temperature was
raised to 90 oC and the reaction was allowed to proceed for 40 h. The reaction was cooled
110
and precipitated into isopropyl alcohol (1 L). The product was filtered and washed in
deionized water at 60 oC for 12 h and acetone for 12 h. It was dried under vacuum at 110 oC
for 24 h before casting (Figure 3.1). The Bis-S containing segmented copolymers (BisSF-
BPSH100) were synthesized in a similar manner.14
3.2.4 Membrane Preparation
Membranes were cast from a 6 w/v% solution of polymer in DMAc onto a clean
glass plate. Solvent was removed using an IR lamp. The lamp intensity was held at 30-
35 oC for 24 h and then raised to 35-40 oC for an additional 24 h. The film was dried
under vacuum at 110 oC for 24 h. The film was removed from the glass plate by
submersion in water and acidified in boiling 0.5 M H2SO4 for 2 h, followed by 2 h in
boiling deionized water.
3.2.5 Characterization
1H, 19F, and 13C NMR analyses were performed on a Varian Unity 400 MHz
spectrometer. 1H and 19F NMR spectra were obtained from a 1% solution (w/v) of
sample in DMSOd6. 13C NMR spectra were obtained from a 10% solution (w/v) of
sample in DMSOd6. All were run at ambient temperatures. Intrinsic viscosities of the
segmented copolymers were determined using universal calibration size exclusion
chromatography (SEC), (also known as gel permeation chromatography (GPC)). The
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
111
the mobile phase and sample solution were filtered before introduction to the GPC
system. This procedure has been described in detail.21
3.2.6 Determination of water uptake and dimensional swelling
The water uptake for all membranes was determined gravimetrically. Acidified
membranes were equilibrated in liquid water at room temperature for 24 h. Wet
membranes were removed from the liquid water, blotted dry to remove excess water, and
quickly weighed. Membranes were dried at 110 oC under vacuum for 24 h and weighed
again. Water uptake was calculated according to Equation 3.1 where massdry and masswet
refer to the mass of the dry and wet membranes, respectively. An average of three
samples was used for each measurement.
( )wet dry
dry
mass masswater uptake% 100
mass
−= × 3.1
Percent swelling of the membranes was determined in the in-plane (x and y) and through-
plane (z) directions. Wet measurements were performed after equilibrating membranes
in liquid water for 24 h at room temperature. Membranes were then dried in a convection
oven at 80 oC for 2 h and measured again. Wet and dry measurements in the x and y
direction were performed by sandwiching the membrane between layers of polyethelene
and two glass plates and measuring with a ruler (mm). Wet and dry measurements in the
z direction were performed using a micrometer. Typical sample size was 2.5 x 2.5 cm
squares when wet. Percent swelling was reported for three directions and calculated
112
according to Equation 3.2 where lengthwet,i and lengthdry,i refer to the length (where i
represents the x, y, or z direction) of the dry and wet membrane, respectively.
( )wet,i dry,ii
dry,i
length lengthpercent swelling 100
length
−= × 3.2
3.2.7 Measurement of proton conductivity
Proton conductivity at 30 oC in liquid water was determined in a window cell
geometry22 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.23
In determining proton conductivity in liquid water, membranes were equilibrated at 30 oC
in DI water for 24 h prior to the testing. Proton conductivity under partially hydrated
conditions was performed at 80 oC. Membranes were equilibrated at 80% RH for 8 h in a
humidity-temperature oven (ESPEC, SH-240). The thickness of the film was measured
with a micrometer. Membranes were allowed to equilibrate at 95% RH and each
additional specified RH value for 4 h before each measurement. Thickness
measurements were performed at the lowest RH which was reached.
3.2.8 Dynamic Mechanical Analysis
Dynamic mechanical analysis (DMA) was performed using a TA Instruments
2890 Dynamic Mechanical Analyzer. Salt-form rectangular membrane films measuring
0.35 mm x 4 mm x 25 mm were used for the test in order to observe the Tg before
degradation of the sulfonic acid groups begins. Multi-frequency tension tests were
conducted on the membranes, with an amplitude of 25 µm and a pre-load force of 0.025
N.
113
3.2.9 Thermal Gravimetric Analysis
Thermal gravimetric analysis (TGA) was performed using a TGA Q500 (TA
Instruments) on the membrane specimens to determine the thermal stability of the
copolymers. The samples were dried isothermally in the TGA at 150 oC for 20 min to
remove any residual moisture. The samples were then equilibrated at 30 oC and run at a
heating rate of 10 oC/min. in an air atmosphere.
3.2.10 Tensile testing
The tensile properties of the membranes were measured using an Instron 5500R
equipped with a 200 lb load cell at room temperature and 44-54% RH and a rate of 5
mm/min. Membrane samples were dried under vacuum at 110 oC for 24 h. A dogbone
die measuring 50 mm in length and 4 mm in width was used to stamp out 5 samples for
each membrane. The dogbone samples were then conditioned in a humidity chamber at
44% RH for 48 h prior to testing.
3.3 Results and Discussion
3.3.1 Synthesis of PhS-BPS100 Segmented Copolymers
The phenoxide-terminated hydrophilic blocks were synthesized via a well
understood step growth polymerization. A small molar access of BP to SDCDPS
monomer was used to synthesize oligomers with controlled molecular weight; number-
average molecular weights (Mn) of 5, 7, 10, and 13 kg/mol were targeted. Proton NMR
was used to confirm that the oligomers were phenoxide endcapped and to simultaneously
determine the Mn by end group analysis. Further details of this reaction and Mn
determination are available.14
114
The segmented copolymers were synthesized using an oligomer-two monomer
reaction approach as described earlier. Phenoxide-terminated 100% disulfonated
poly(arylene ether sulfone) oligomers (BPS100) were reacted with Ph and DFPB
monomers in a nucleophilic aromatic substitution reaction to afford the segmented
copolymer (PhF-BPS100) (Figure 3.1). The stoichiometric ratio of DFPB to phenoxide
end groups, from either Ph or BPS100, remained 1:1 for the copolymer series. Whereas,
the stoichiometric ratio of DFBP to Ph was controlled to target block lengths for the
hydrophobic portions (PhF) that were equal to the preformed hydrophilic block lengths.
Since DFBP is highly reactive, low reaction temperatures (90-105 oC) could be used for
the coupling reaction, which minimized ether-ether interchange.
OKO S O
O
OSO3K
KO3S
OKn
F
FFFF
F
F F F F
K2CO3
Cyclohexane/NMP4 hrs @ 110 oC
add 18-85 hrs @ 90-105 oC
Boiling H2SO4 (0.5 M), 2hBoiling H2O, 2h
(Phenolphthalein)
(DFBP)
O
O
OHOH
FFFF
F F F F
O
O
OO
F F F F
OO S O
O
OSO3H
HO3S
On
FFFF
m
x
OKO S O
O
OSO3K
KO3S
OKn
F
FFFF
F
F F F F
K2CO3
Cyclohexane/NMP4 hrs @ 110 oC
add 18-85 hrs @ 90-105 oC
Boiling H2SO4 (0.5 M), 2hBoiling H2O, 2h
(Phenolphthalein)
(DFBP)
O
O
OHOHOKO S O
O
OSO3K
KO3S
OKn
F
FFFF
F
F F F F
K2CO3
Cyclohexane/NMP4 hrs @ 110 oC
K2CO3
Cyclohexane/NMP4 hrs @ 110 oC
add 18-85 hrs @ 90-105 oC18-85 hrs @ 90-105 oC
Boiling H2SO4 (0.5 M), 2hBoiling H2O, 2h
(Phenolphthalein)
(DFBP)
O
O
OHOH
FFFF
F F F F
O
O
OO
F F F F
OO S O
O
OSO3H
HO3S
On
FFFF
m
x
Figure 3.1. General synthetic scheme for highly fluorinated:disulfonated segmented
copolymers
Representative 1H and 19F NMR spectra for the PhF-BPS100 series are shown in
115
Figure 3.2. The absence of peaks at 6.8, 7.05, 7.4, and 7.55 ppm in the 1H NMR signified
a successful coupling reaction had occurred. These peaks would be present if unreacted
hydrophilic oligomer remained due to protons from the terminal BP. There were also no
additional fluorine peaks in the 19F NMR spectrum. The two peaks present were assigned
to fluorines in the chain.
FFFF
F F F F
O
O
OO
F F F F
OO S O
O
O SO3K
KO3S
On
FFFF
m
x
edc
ed
bac
gf
hh
gf
ba
i j
i j
(a)
(b)
FFFF
F F F F
O
O
OO
F F F F
OO S O
O
O SO3K
KO3S
On
FFFF
m
x
edc
ed
bac
gf
hh
gf
ba
i j
i j
FFFF
F F F F
O
O
OO
F F F F
OO S O
O
O SO3K
KO3S
On
FFFF
m
x
edc
ed
bac
gf
hh
gf
ba
i j
i j
(a)
(b)
Figure 3.2. (a) 1H and (b) 19F NMR spectra for PhF-BPS100 segmented copolymer
Carbon NMR spectra for representative PhF-BPS100 segmented copolymer and a
35% disulfonated poly(arylene ether sulfone) random copolymer (BPS-35) are shown in
Figure 3.3. The sharp singlets in the segmented copolymer spectrum suggest the blocky
structure of the copolymer was maintained. Randomization of the backbone would result
in peak splitting similar to that shown in the BPS-35 random copolymer spectrum.
116
PhF-BPS100 7k7k
BPS-35
PhF-BPS100 7k7k
BPS-35
Figure 3.3. 13C NMR spectrum for PhF-BPS100 segmented copolymer and BPS-35
random copolymer
3.3.2 Comparison of PhF-BPSH100 and BisSF-BPSH100 Segmented Copolymer
Properties
The objective for synthesizing the PhF-BPS100 copolymer series was twofold.
Firstly, it allowed further investigation of the segmented synthetic procedure using a
different bisphenol to derive the hydrophobic segments. Secondly, it allowed for
comparisons to be made to an initial series of segmented copolymers14 to determine how
the structure of the bisphenol affects the properties of the copolymer. Properties of the
BisSF-BPSH100 and PhF-BPSH100 segmented copolymers are summarized in Table
3.1. The intrinsic viscosity (I.V.) data confirmed that high molecular weight polymer
was achieved using this synthetic method. Tough ductile transparent films were also cast
from the copolymers, indicating that high molecular weight was achieved. For the
BisSF-BPSH100 segmented copolymer, the water uptake increased and was interpreted
as reflecting the sharper nano-phase separation that occurred as block length increased.
117
Both the BisSF-BPSH100 and PhF-BPSH100 segmented copolymers yielded membranes
with manageable water uptake.
Table 3.1. Characterization of BisSF-BPSH100 and PhF-BPS100 Segmented Copolymers
a Calculated from experimental loading: IEC = (g of Hydrophilic *IEC of BPSH100)/(g of Hydrophobic + g of Hydrophilic)
b IEC was calculated according to 1H NMRc Intrinsic viscosity measured by GPCd Measured in liquid water at 30°Ce Water uptake was calculated through [(Wwet-Wdry) / Wdry] x 100%
Water
Uptakee
Theoreticala Experimentalb %BisSF:BPSH100
3K3K1.6 1.8 0.63 0.10 62
BisSF:BPSH100 5K5K
1.6 1.5 0.50 0.11 51
BisSF:BPSH100 9K9K
1.7 1.5 0.82 0.15 74
PhF:BPSH100 5K5K
1.7 1.7 0.48 0.11 42
PhF:BPSH100 7K7K
1.8 1.8 0.58 0.14 73
PhF:BPSH100 13K13K 1.7 1.7 0.46 0.11 73
IEC
(meq/g)Conductivity
d S/cmIV c
(dL/g)
The conductivity of the BisSF-BPSH100 and PhF-BPSH100 segmented
copolymers as a function of relative humidity is shown in Figure 3.4. At 95 %RH, both
systems of segmented copolymers yielded higher performance than Nafion®. However,
as the relative humidity was decreased the conductivity of the systems fell below
Nafion®. In both the BisSF-BPSH100 and PhF-BPSH100 segmented copolymers, the
conductivity increased with an increase in block length over the entire RH range. The
118
conductivity of the BisSF-BPSH100 segmented copolymers was greater than the PhF-
BPSH100, when comparing similar block lengths despite the PhF-BPSH100 segmented
copolymers having higher IECs.
20 30 40 50 60 70 80 90 1000.1
1
10
100
0.1
1
10
100
Con
duct
ivity
(m
S/c
m)
Relative Humidity (%)
BisSF-BPSH100 3K3K BisSF-BPSH100 5K5K BisSF-BPSH100 9K9K PhF-BPSH100 5K5K PhF-BPSH100 7K7K PhF-BPSH100 13K13K Nafion 112
Figure 3.4. Comparison of proton conductivity under partially hydrated conditions
for BisSF-BPSH100 and PhF-BPSH100 segmented copolymers with increasing block length
Dimensional swelling of the segmented membranes is shown in
Figure 3.5. Both the BisSF-BPSH100 and PhF-BPSH100 segmented copolymers
exhibited anisotropic swelling, with the greatest swelling being in the z-direction. This
was in contrast to the isotropic swelling displayed by random copolymers (BPSH-35).
Low swelling in the x,y directions is considered to put less in-plane stress on membrane
electrode assemblies in fuel cell applications and is therefore thought to be more
119
desirable. The swelling as a function of block length can also be observed. In the PhF-
BPSH100 system, as the block length increased, the z-direction swelling increased while
the x and y swelling decreased or stayed the same. An increase in block length allows
co-continuous hydrophilic pathways to form, which leads to increased swelling in the z-
direction. The BisSF-BPSH100 systems also exhibited a similar trend, with the
exception of the 3K3K copolymer. The differences observed in the 3K3K copolymer
were attributed to the significant differences in IEC of the copolymers in the system.
0
10
20
30
40
50
60
70
BPSH35 3K3K 5K5K 9K9K 5K5K 7K7K 13K13K
Sw
ellin
g (%
)
x y zz
Xy
BisSF-BPSH100 PhF-BPSH100
0
10
20
30
40
50
60
70
BPSH35 3K3K 5K5K 9K9K 5K5K 7K7K 13K13K
Sw
ellin
g (%
)
x y zz
Xy
BisSF-BPSH100 PhF-BPSH100
z
Xyz
Xy
BisSF-BPSH100 PhF-BPSH100
Figure 3.5. Comparison of dimensional swelling data for BisSF-BPSH100 and PhF-
BPSH100 segmented and BPSH35 random copolymers
The thermal stability of the BisSF-BPSH100 and PhF-BPSH100 segmented
copolymers can be seen in Figure 3.6. Two degradation temperatures were observed in
120
the TGA plots for both the BisSF-BPSH100 and PhF-BPSH100 segmented copolymers.
The initial weight loss at 250 oC was attributed to the degradation of the sulfonic acid
moieties in the segmented copolymers. The weight loss at 450 oC was attributed to the
degradation of the main chain of the segmented copolymers. The molecular weight of
the segments in the copolymers in this study did not affect the thermal stability. Despite
the differences in the chemical structures of the BisSF-BPSH100 and the PhF-BPSH100
segmented copolymers, no appreciable difference was observed in the weight loss
behavior in these dynamic TGA experiments.
0 100 200 300 400 500 6000
20
40
60
80
100
Wei
ght [
%]
Temperature [oC]
PhF-BPSH100 5K5K (1) PhF-BPSH100 7K7K (2) PhF-BPSH100 13K13K (3)
0 100 200 300 400 500 6000
20
40
60
80
100
Wei
ght [
%]
Temperature [oC]
BisSF-BPSH100 3K3K BisSF-BPSH100 5K5K BisSF-BPSH100 9K9K
*An isothermal drying run was conducted at 150 oC for 20 minutes prior to testing. Samples were heated at a rate of 10 oC/min in the presence of air.
0 100 200 300 400 500 6000
20
40
60
80
100
Wei
ght [
%]
Temperature [oC]
PhF-BPSH100 5K5K (1) PhF-BPSH100 7K7K (2) PhF-BPSH100 13K13K (3)
0 100 200 300 400 500 6000
20
40
60
80
100
Wei
ght [
%]
Temperature [oC]
BisSF-BPSH100 3K3K BisSF-BPSH100 5K5K BisSF-BPSH100 9K9K
*An isothermal drying run was conducted at 150 oC for 20 minutes prior to testing. Samples were heated at a rate of 10 oC/min in the presence of air.
Figure 3.6. Thermal gravimetric analysis plots for BisSF-BPSH100 and PhF-
BPSH100 copolymers in air
DMA analyses were performed on copolymers in the salt form since the acid-
form copolymer membranes are known to be thermally and oxidatively unstable above
about 250 oC (Figure 3.7). Both series of segmented copolymers exhibited higher glass
transition temperatures with an increase in IEC, which is expected and is attributed to the
121
greater amount of sulfonate ionic interactions. It is also shown from the DMA results
that the PhF-BPS100 had a higher Tg than the BisSF-BPS100 segmented copolymers.
This could be attributed to the increased rigidity in the phenolphthalein as compared to
the Bis-S monomer.
0 50 100 150 200 250 300102
103
104
10-2
10-1
Sto
rage
Mod
ulus
[MP
a]
Temperature [oC]
PhF-BPS100 5K5KPhF-BPS100 7K7KPhF-BPS100 13K13K
Tan δ
0 50 100 150 200 250 300102
103
104
10-2
10-1
100
Sto
rage
Mod
ulus
[MP
a]
Temperature [oC]
BisSF-BPS100 3K3KBisSF-BPS100 5K5KBisSF-BPS100 9K9K
Tan δ
*Tests run at a heating rate of 5 oC/min in an air atmosphere
(a) (b)
Figure 3.7. DMA plots for a) BisSF-BPSH100 and b)PhF-BPSH100 segmented copolymers. In a) and b) the closed symbols represent the storage modulus and the
open symbols represent the tan delta.
The tensile properties of the membranes are shown in Table 3.2. The PhF-
BPSH100 segmented copolymers exhibited significantly greater tensile moduli and
strength than the BisSF-BPSH100 segmented copolymers. The increase in both the
tensile moduli and strength may reflect the greater rigidity of the phenolphthalein. This
greater rigidity also decreased the elongation of the PhF-BPSH100 segmented
copolymers. However, both segmented copolymers yielded tough films.
122
Table 3.2. Tensile Properties of BisSF-BPSH100 and PhF-BPSH100 Segmented Copolymers
Modulus Tensile Strength % Elongation Max. Elongation
MPa MPa % %BisSF:BPS100
3K3K1500 ±160 42 ± 5 43 ± 27 74
BisSF:BPS100 5K5K
1470 ±120 39 ± 2 16 ± 5 21
BisSF:BPS100 9K9K
1510 ± 80 46 ± 4 24 ± 16 47
PhF:BPS100 5K5K
1970 ± 220 74 ± 5 9 ± 3 15
PhF:BPS100 7K7K
1800 ± 110 69 ± 6 16 ± 5 22
PHF:BPS100 13K13K
1650 ± 50 50 ± 3 7 ± 3 12
3.4 Conclusions
Segmented copolymers containing highly fluorinated hydrophobic blocks and
100% disulfonated hydrophilic blocks have been successfully synthesized using an
oligomer-monomer approach. Tough membranes were produced from BisSF-BPSH100
and PhF-BPSH100 segmented copolymers. The greater rigidity of the phenolphthalein
led to an increase in tensile modulus, strength, and Tg of the PhF-BPSH100 segmented
copolymer series. Further experiments are ongoing to assess whether utilization of the
phenolphthalein in the hydrophilic phase will behave differently.
Acknowledgment. The authors would like to acknowledge the Department of
Energy for funding under DE-FG36-06G016038.
123
References
1 Wang, F.; Hickner, M.; Kim, Y.S.; Zawodzinski, T.A.; McGrath, J.E. Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes. J. Membr. Sci. 2002, 197, 231-242. 2 Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.; Zawodzinski, T.A.; McGrath, J.E. Synthesis of Highly Sulfonated Poly(arylene ether sulfone) Random (Statistical) Copolymers Via Direct Polymerization. Macromol. Symp. 2001, 175, 387-395. 3 Hickner, M.A.; Ghassemi, H.; Kim. Y.S.; Einsla, B.R.; McGrath, J.E. Alternative Polymer Systems for Proton Exchange Membranes (PEMs). Chem. Rev. 2004, 104, 4587-4612. 4 Kim, Y.S.; Dong, L.; Hickner, M.A.; Pivovar, B.S.; McGrath, J.E. Processing induced morphological development in hydrated sulfonated poly(arylene ether sulfone) copolymer membranes. Polymer 2003, 44, 5729-5736. 5 Kim, Y. S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y. T.; Harrison, W.; Zawodzinski, T. A.; McGrath, J. E. Effect of Acidification Treatment and Morphological Stability of Sulfonate Poly(arylene ether sulfone) Copolymer Proton Exchange Membranes for Fuel Cell Use Above 100 °C. J. Polym. Sci.: Part B: Polym. Phys. 2003, 41, 2816-2828. 6 Kim, Y.S.; Sumner, M.J.; Harrison, W.L.; Riffle, J.S.; McGrath, J.E.; Pivovar, B.S. Direct Methanol Fuel Cell Performance of Disulfonated Poly(arylene ether benzonitrile) Copolymers. J. Electrochem. Soc. 2004, 151, A2150-A2156. 7 Yu, X.; Roy, A.; Dunn, S.; Yang, J.; McGrath, J.E. Synthesis and Characterization of Sulfonated-Fluorinated, Hydrophilic-Hydrophobic Multiblock Copolymers for Proton Exchange Membranes. Macromol. Symp. 2006, 245-245, 439-449. 8 Wang, H.; Badami, A.S.; Roy, A.; McGrath, J.E. Multiblock Copolymers of Poly(2,5-benzophenone and Disulfonated Poly(arylene ether sulfone) for Proton-Exchange Membranes. I. Synthesis and Characterization. J. Polym. Sci. Pol. Chem. 2007, 45, 284-294. 9 Lee, H.; Roy, A.; Lane, O.; Dunn, S.; McGrath, J. E. Segmented Sulfonated Poly(arylene ether sulfone)-b-Polyimide Copolymers for Proton Exchange Membrane Fuel Cells. I. Copolymer Synthesis and Fundamental Properties J. Polym. Sci. Part A. 2007, 45(21), 4879-4890. 10 Roy, A.; Hickner, M.A.; Yu, X.; Li. Y.; Glass, T.E.; McGrath, J.E. Influence of Chemical Composition and Sequence Length on the Transport Properties of Proton Exchange Membranes. J. Polym. Sci. Part B, 2006, 44, 2226-2239.
124
11 Leibler, L. Theory of Microphase Separation in Block Copolymers. Macromolecules 1968, 13(6), 1602-1617. 12 Matsen, M.W.; Bates, F.S. Origins of Complex Self-Assembly in Block Copolymers. Macromolecules 1996, 29, 7641-7644. 13 VanHouten, R.A.; Lane, O.; McGrath, J.E. Synthesis of Segmented Hydrophobic:Hydrophilic, Fluorinated:Sulfonated Block Copolymers for Use as Proton Exchange Membranes. Prep. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2008, 53(2), 765-766.
14 VanHouten, Rachael A.; Lane, Ozma. R.; VanHouten, Desmond J.; McGrath, James E. Synthesis of segmented hydrophobic:hydrophilic, fluorinated:sulfonated block copolymers for use as proton exchange membranes. Macromolecules 2009, Submitted. 15 Yu, X.; Roy, A.; Dunn, S.; Badami, A. S.; Yang, J.; Good, A. S.; McGrath, J. E. Synthesis and Characterization of Sulfonated-Fluorinated, Hydrophilic-Hydrophobic Multiblock Copolymers for Proton Exchange Membranes J. Polym. Sci. Part A: Polym. Chem., 2009, 47, 1038–1051. 16 Wang, Zhonggang; Chen, Tianlu; Xu, Jiping. Gas permeabilities of cardo polyoxyarylene membranes. Journal of Applied Polymer Science 2002, 83(4), 791-801. 17 Miyatake, Kenji; Zhou, Hua; Uchida, Hiroyuki; Watanabe, Masahiro. Highly proton conductive polyimide electrolytes containing fluorenyl groups. Chem Commun 2003, 3, 368. 18 Ueda, M.; Toyota, H.; Ouchi, T.; Sugiyama, J.I.; Yonetake, K.; Masuko, T.; Teramoto, T. Synthesis and Characterization of Aromatic Poly(ether Sulfone)s Containing Pendant Sodium Sulfonate Groups. J. Polym. Sci.: Part A: Polym. Chem. 1993, 31, 853-858. 19 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. Synthesis and Characterization of 3,3’-Disulfonated-4,4’-dichlorodiphenyl Sulfone (SDCDPS) Monomer for Proton Exchange Membranes (PEM) in Fuel Cell Applications J. Appl. Polym. Sci. 2006, 100, 4595-4602. 20 Li, Y.; VanHouten, R.; Brink, A.; McGrath, J.E. Purity Characterization of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl Sulfone (SDCDPS) Monomer by UV-visible Spectroscopy. Polymer, 2008, 49, 3014-3019. 21 Yang, J.; Li, Y.; Roy, A.; McGrath J.E. Viscometric behavior of disulfonated poly(arylene ether sulfone) random copolymers used for proton exchange membranes. Polymer, 2008, 49(24), 5300-5306.
125
22 Zawodzinski, T. A.; Neeman, M.; Sillerud, L. O.; Gottesfeld, S. Determination of water diffusion coefficients in perfluorosulfonate ionomeric membranes J. Phys. Chem. 1991, 95, 6040. 23 Springer, T. E.; Zawodzinski, T. A.; Wilson, M. S.; Gottesfeld, S. Characterization of polymer electrolyte fuel cells using ac impedance spectroscopy J. Electrochem. Soc. 1996, 143, 587.
126
4 Synthesis and Characterization of Hydrophobic-Hydrophilic Segmented
Copolymers with Unequal Hydrophobic and Hydrophilic Block Lengths for Use
as Proton Exchange Membranes
Rachael A. VanHouten, Desmond J. VanHouten, James E. McGrath*
Macromolecular Science and Engineering Macromolecules and Interfaces Institute Virginia Tech, Blacksburg, VA 24061
Abstract
A series of segmented copolymers was synthesized for use as proton exchange
membranes. They were comprised of 100% disulfonated poly(arylene ether sulfone)
hydrophilic blocks derived from phenolphthalein and highly fluorinated hydrophobic
blocks. Ion exchange capacity was controlled by synthesizing copolymers with shorter
hydrophobic blocks when compared to the hydrophilic blocks. Transparent, tough,
ductile films could be cast from the copolymer. As block length increased, water uptake
increased, as did proton conductivity over the entire RH range.
4.1 Introduction
One of the current issues for proton exchange membrane fuel cells (PEMFCs) as
viable energy producers is the rapid decrease in conductivity they exhibit at low relative
humidity (RH) values. The synthesis of hydrophobic-hydrophilic segmented and
multiblock copolymers has allowed for increased proton conductivity at low RH values
because of the well connected channels that form in the hydrophilic blocks.1,2 However,
further improvements have been deemed necessary.3
127
Comonomer choice is one way to enhance the properties of these membranes.
Altering the chemistry of the hydrophilic block could lead to increased proton
conductivity at low RH values. Previous studies4,5,6 for segmented and multiblock
copolymers focused on the use of 4,4’-biphenol (BP) and 3,3’-disulfonated-4,4’-
dichlorodiphenylsulfone (SDCDPS) as the comonomers in the hydrophilic segments.
This paper describes a series of segmented copolymers using the well known bisphenol
phenolphthalein as a comonomer in the hydrophilic segments. As a continuation of
chapter 3, phenolphthalein was chosen as an alternate comonomer because its bulky
nature may increase the free volume of the copolymer,7 possibly allowing for higher
conductivity at lower relative humidity.8 This may be more advantageous when used in
the hydrophilic blocks rather than the hydrophobic blocks because water retention is
needed in the hydrophilic channels in order to increase proton conductivity at low RH.
Mechanical strength enhancements are also expected due to the increases in tensile
strength that were observed when phenolphthalein was used in the hydrophobic block.
When altering the chemistry of the copolymer backbone, it is important to
consider the ion exchange capacity (IEC) that will result from monomer choice.
Monomers with higher molecular weights will produce hydrophilic blocks with lower
IEC values. There are several approaches available to adjust the IECs of multiblock
copolymers. Monomer choice can alter the IEC of the hydrophilic block. By choosing
monomers with lower molecular weights, the overall IEC can be increased, such as using
hydroquinone as the bisphenol.9 The stoichiometry between the hydrophobic and
hydrophilic oligomers can be offset to afford copolymers with varying hydrophilic to
hydrophobic volume fractions.1,10 The IEC of the copolymer can be increased by off-
128
setting the number-average molecular weight of the hydrophilic and hydrophobic blocks
to increase the over hydrophilic content.6,11
Because we are interested in using phenolphthalein as the bisphenol in the
hydrophilic block, the IEC of the hydrophilic block is fixed, producing a copolymer with
a lower IEC value when compared to 4,4’-biphenol. The segmented copolymerization
technique does not allow for anything other than a 1:1 molar ratio of phenoxide to halide
end-groups because the hydrophobic block is being formed in-situ. Therefore, the only
way to increase the IEC is to synthesize copolymers with longer hydrophilic block
lengths than hydrophobic block lengths.
This paper describes the synthesis of segmented copolymers utilizing
phenolphthalein in the hydrophilic segments. Unequal block lengths were targeted to
achieve copolymers with high IEC values. The effect of block length on copolymer
properties, such as proton conductivity, water uptake, and dimensional swelling will also
be discussed.
4.2 Experimental
4.2.1 Materials
Decafluorobiphenyl (DFBP) was obtained from Matrix Scientific and dried under
vacuum at room temperature overnight. Bis(4-hydroxyphenyl)sulfone (Bis-S) was
purchased from Alfa Aesar and dried under vacuum at 60 oC for 24 h before use.
SDCDPS was synthesized by Akron Polymer Systems according to a procedure reported
elsewhere,12,13 which was a refinement of a previously published procedure by Ueda et
al.14 Phenolphthalein (Ph) was purchased from Sigma Aldrich and was recrystallized
from ethanol and water. The Ph was dried under vacuum at 90 oC for 24 h prior to use.
129
N-methyl-2-pyrrolidone (NMP) (Aldrich) was vacuum-distilled from calcium hydride
onto molecular sieves and stored under nitrogen before use. N,N-dimethylacetamide
(DMAc) was obtained from Aldrich and used as received. Potassium carbonate (K2CO3)
was obtained from Aldrich and dried under vacuum at 120 oC overnight before use.
Toluene, cyclohexane, and isopropyl alcohol were obtained from Aldrich and used as
received.
4.2.2 Synthesis of Phenoxide-Terminated Hydrophilic Oligomers (PhS-100)
A series of phenoxide-terminated, disulfonated poly(arylene ether sulfone)s
oligomers derived from phenolphthalein was synthesized, with targeted number average
molecular weights (Mn) ranging from 7000 to 17000 g/mol. In a typical procedure for a
Mn of 7000 g/mol, the following conditions were utilized. Ph (4.8324 g, 15.18 mmol),
SDCDPS (6.7570 g, 13.75 mmol), and NMP (58 mL) were added to a three-neck, round-
bottom flask, equipped with mechanical stirrer, Dean-Stark trap, condenser, and N2 inlet.
The reaction bath was set to 85 oC, and the monomers were allowed to dissolve. K2CO3
(2.413 g, 17.5 mmol) and toluene (29 mL) were added to the flask. The temperature of
the bath was increased to 155 oC, and the reaction was allowed to azeotrope water for 4 h.
Toluene was removed from the system by increasing the bath temperature to 190 oC. The
reaction was allowed to proceed for 48 h. After cooling, the solution was filtered to
remove salts and precipitated into acetone. The resulting oligomer was dried at 110 oC
for at least 24 h under vacuum and had an Mn of 6500 g/mol determined by end group
analysis using 1H NMR.
130
4.2.3 Synthesis of segmented copolymer with simultaneous formation of
hydrophobic segments
The segmented copolymer was synthesized using DFBP and Bis-S monomers to form
the hydrophobic segments. A sample copolymerization procedure was as follows: a 3-neck,
round bottom flask, equipped with mechanical stirrer, Dean-Stark trap, condenser, and N2
inlet was loaded with PhS-100 (6.5K; 3.0176 g, 0.4350 mmol), Bis-S (0.8335 g, 3.330
mmol), and NMP (19 mL). After dissolution of reactants, K2CO3 (0.598 g, 4.33 mmol) and
cyclohexane (4 mL) were added to the reaction solution. The reaction bath was heated to
110 oC and allowed to azeotrope for 4 h. The cyclohexane was drained from the system and
the bath temperature was lowered to 85 oC. DFBP (1.2575 g, 3.764 mmol) and NMP (6 mL)
were added to the reaction flask. The bath temperature was raised to 95 oC and the reaction
was allowed to proceed for 12 h. The temperature was increased to 100 oC for 4 h and 110
oC for an addition 3 h until a viscous reaction solution resulted. The reaction was cooled and
precipitated into isopropyl alcohol (1 L). The product was filtered and washed in deionized
water at 60 oC for 12 h and acetone for 12 h. It was dried under vacuum at 110 oC for 24 h
before casting (Figure 4.3).
4.2.4 Membrane Preparation
Membranes were cast from a 6 w/v% solution of polymer in DMAc onto a clean
glass plate. Solvent was removed using an IR lamp. The lamp intensity was held at 30-
35 oC for 24 h and then raised to 35-40 oC for an additional 24 h. The film was dried
under vacuum at 110 oC for 24 h. The film was removed from the glass plate by
submersion in water and acidified in boiling 0.5 M H2SO4 for 2 h, followed by 2 h in
boiling deionized water.
131
4.2.5 Characterization
1H, 19F, and 13C NMR analyses were performed on a Varian Unity 400 MHz
spectrometer. 1H and 19F NMR spectra were obtained from a 1% solution (w/v) of
sample in DMSOd6. 13C NMR spectra were obtained from a 10% solution (w/v) of
sample in DMSOd6. All were run at ambient temperatures. Intrinsic viscosities of the
segmented copolymers were determined using size exclusion chromatography (SEC).
SEC 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 and sample solution were filtered before introduction to the GPC
system. Further solution characterization procedures have been described.15
4.2.6 Determination of water uptake and dimensional swelling
The water uptake for all membranes was determined gravimetrically. Acidified
membranes were equilibrated in liquid water at room temperature for 24 h. Wet
membranes were removed from the liquid water, blotted dry to remove excess water, and
quickly weighed. Membranes were dried at 110 oC under vacuum for 24 h and weighed
again. Water uptake was calculated according to equation 4.1 where massdry and masswet
refer to the mass of the dry and wet membranes, respectively. An average of three
samples was used for each measurement.
( )wet dry
dry
mass masswater uptake% 100
mass
−= × 4.1
132
Percent swelling of the membranes was determined in the in-plane (x and y) and through-
plane (z) directions. Wet measurements were performed after equilibrating membranes
in liquid water for 24 h at room temperature. Membranes were then dried in a convection
oven at 80 oC for 2 h and measured again. Wet and dry measurements in the x and y
direction were performed by sandwiching the membrane between layers of polyethelene
and two glass plates and measuring with a ruler (mm). Wet and dry measurements in the
z direction were performed using a micrometer. Typical sample size was 2.5 x 2.5 cm
squares when wet. Percent swelling was reported for three directions and calculated
according to equation 4.2, where lengthwet,i and lengthdry,i refer to the length (where i
represents the x, y, or z direction) of the dry and wet membrane, respectively.
( )wet,i dry,ii
dry,i
length lengthpercent swelling 100
length
−= × 4.2
4.2.7 Measurement of proton conductivity
Proton conductivity at 30 oC in liquid water was determined in a window cell
geometry16 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.17
In determining proton conductivity in liquid water, membranes were equilibrated at 30 oC
in DI water for 24 h prior to the testing. Proton conductivity under partially hydrated
conditions was performed at 80 oC. Membranes were equilibrated at 80% RH for 8 h in a
humidity-temperature oven (ESPEC, SH-240). The thickness of the film was measured.
Membranes were allowed to equilibrate at 95% RH and each additional specified RH
133
value for 4 h before each measurement. Thickness measurements were performed at the
lowest RH which was reached.
4.2.8 Dynamic Mechanical Analysis
Dynamic mechanical analysis (DMA) was performed using a TA Instruments
2890 Dynamic Mechanical Analyzer. Salt-form rectangular membrane films measuring
0.35 mm x 4 mm x 25 mm were used for the test. Multi-frequency tension tests were
conducted on the membranes, with an amplitude of 25 µm and a pre-load force of 0.025
N in a nitrogen atmosphere.
4.2.9 Tensile testing
The tensile properties of the membranes were measured using an Instron 5500R
equipped with a 200 lb load cell at room temperature and 44-54% RH and a rate of 5
mm/min. Membrane samples were dried under vacuum at 110 oC for 24 h. A dogbone
die measuring 50 mm in length and 4 mm in width was used to stamp out 5 samples for
each membrane. The dogbone samples were then conditioned in a humidity chamber at
44% RH for 24-48 h.
4.3 Results and Discussion
4.3.1 Synthesis of Phenoxide-Terminated Disulfonated Hydrophilic Oligomer
Derived from Phenolphthalein
Telechelic hydrophilic oligomer was synthesized via a nucleophilic aromatic
substitution reaction (Figure 4.1). A small molar excess of Ph to SDCDPS was used to
obtain phenoxide-terminated copolymer. The number-average molecular weight (Mn) of
134
the copolymer were controlled by offsetting the molar ratio of SDCDPS to Ph using a
derivation of Carothers equation to determine the offset (r) (equation 4.3). Because this
was a step growth reaction utilizing A-A and B-B type monomers and the targeted Mns
were low, the number-average degree of polymerization, nX , is equal to (2n +1), where n
is equal to the number of repeat units.
( )( )
1
1
n
n
Xr
X
−=
+ 4.3
Mns were targeted at 7, 10, 13, and 17 kg/mol. Proton NMR was used to confirm the
blocks were phenoxide-terminated and to determine the Mns of the copolymers. Protons
due to a terminal Ph moiety resulted in a doublet at 6.75 ppm. The number of repeat
units, n, was determined from the ratio of the integration of a peak resulting from main
chain protons (such as “e” in Figure 4.2) to the integration of the end-group peak. Mns of
the oligomers were calculated according to equation 4.4, where MRU is the molecular
weight of the repeat unit and MEG is the molecular weight of the end-group.
( )n RU EGM nM M= + 4.4
The experimental Mns determined using 1H NMR matched closely with the targeted
values (Table 4.1).
135
Cl S Cl
O
OSO3Na
NaO3S
K2CO3Toluene/NMP4 hrs @ 155 oC48 hrs @ 190 oC
+
O
O
OHOH
O S O
O
O SO3Na
NaO3SO
O
KO n
O
O
OK
Cl S Cl
O
OSO3Na
NaO3S
K2CO3Toluene/NMP4 hrs @ 155 oC48 hrs @ 190 oC
+
O
O
OHOH
O S O
O
O SO3Na
NaO3SO
O
KO n
O
O
OK
Figure 4.1. PhS100 phenoxide-terminated hydrophilic oligomers
O S O
O
O SO3Na
NaO3SO
O
KO n
O
O
OK
a
b
b
a
c
c
d e f
f
ed
a’
a’
17.97 4.00
O S O
O
O SO3Na
NaO3SO
O
KO n
O
O
OK
a
b
b
a
c
c
d e f
f
ed
a’
a’
17.97 4.00
Figure 4.2. 1H NMR spectrum of PhS100 oligomer
136
Table 4.1. Target and Experimental Mn for PhS100 Oligomers
a. Calculated from 1H NMR
Target Experimentala
7000 650010000 1040013000 1360017000 16800
Molecular Weight(g/mol)
4.3.2 Synthesis of BisSF-PhS Segmented Copolymer
Hydrophilic, PhS oligomers were reacted with DFBP and Bis-S monomers to
form four BisSF-PhS segmented copolymers via a nucleophilic aromatic substitution
reaction (Figure 4.3). This series of polymers will be referred to as BisSF-PhS100 or
BisSF-PhSH100 segmented copolymers to differentiate between copolymers in salt or
acid forms, respectively. Specific copolymers within the series are identified by the Mns
of the oligomers used in the syntheses, i.e. a copolymer with targeted and 9 kg/mol
hydrophobic segments and 13 kg/mol hydrophilic blocks is called 9k13k. These
copolymers were synthesized using a similar method to that described in chapters 3 and
4. The ratio of DFBP to Bis-S monomer was controlled using the Carothers equation to
obtain blocks with desired Mns (often referred to as block length throughout this
discussion) for the hydrophobic segments, which were shorter than the hydrophilic block
lengths. The overall stoichiometric ratio of DFBP to phenolic end groups (from the Bis-S
monomer or PhS100 oligomer) was maintained at 1:1.
137
The targeted ion exchange capacity (IEC) for the series of copolymers was 1.7
meq/g. This ion exchange capacity was targeted so that the copolymers could be more
closely compared to the BisSF-BPSH100 and PhF-BPSH100 copolymers described in
chapter 3 and 4, respectively. Copolymers with unequal hydrophilic and hydrophobic
block lengths needed to be synthesized in order to achieve a comparable IEC to the
aforementioned copolymers because PhS100 oligomers have a lower IEC value than
BPS100 due to the increased molecular weight of the repeat unit. Synthesizing a
copolymer with equal block lengths would have resulted in a theoretical IEC value of
~1.4 meq/g.
F
FFFF
F
F F F F
K2CO3Cyclohexane/NMP4 hrs @ 110 oC
add 15-30 hrs @ 90-110 oC
(DFBP)
O S O
O
O SO3Na
NaO3SO
O
KO n
O
O
OK OH S OH
O
O
(Bis-S)
FFFF
F F F F
OO
F F F F
O
FFm
O
O
O S O
O
O SO3K
KO3SO
O
O n
x
F
S
O
OF
F
FFFF
F
F F F F
K2CO3Cyclohexane/NMP4 hrs @ 110 oC
K2CO3Cyclohexane/NMP4 hrs @ 110 oC
add 15-30 hrs @ 90-110 oC15-30 hrs @ 90-110 oC
(DFBP)
O S O
O
O SO3Na
NaO3SO
O
KO n
O
O
OK OH S OH
O
O
(Bis-S)
FFFF
F F F F
OO
F F F F
O
FFm
O
O
O S O
O
O SO3K
KO3SO
O
O n
x
F
S
O
OF
Figure 4.3. BisSF-PhS100 segmented copolymer
138
In order to confirm the success of the reaction and chemical structure and
sequencing of the segmented copolymers, nuclear magnetic resonance (NMR) was
performed for several nuclides, including 1H, 19F, and 13C. Peaks from protons in both
the hydrophobic and hydrophilic block were able to be assigned (Figure 4.4a). Also the
absence of a peak at 6.75 ppm indicated that no unreacted hydrophilic block remained.
Peaks from residual DFBP monomer were absent from the 19F NMR spectrum (Figure
4.4b). The peaks at -138.2 and -153.8 ppm were assigned to the fluorine in the backbone
of the hydrophobic block. Figure 4.5 shows a portion of a 13C NMR spectrum for a
9k:13k segmented copolymer. The sharp singlets result from the blocky structure in the
copolymer backbone. The shorter sequencing in a random copolymer results in doublets
in the 13C NMR spectrum.
FFFF
F F F F
OO
F F F F
O
FFm
O
O
O S O
O
O SO3K
KO3SO
O
O n
x
F
S
O
OF
feba d
c
g h
cf
e
a
d
bh g
i j
i j
(a)
(b)
FFFF
F F F F
OO
F F F F
O
FFm
O
O
O S O
O
O SO3K
KO3SO
O
O n
x
F
S
O
OF
feba d
c
g h
cf
e
a
d
bh g
i j
i j
(a)
(b)
Figure 4.4. Representative (a) 1H and (b) 19F NMR spectra for BisSF-PhS100 segmented copolymer
139
162 160 158 156 PPM
Figure 4.5. 13C NMR spectrum for BisSF-PhS100 segmented copolymer
4.3.3 Characterization of BisSF-PhSH100 Segmented Copolymer Properties
BisSF-PhSH100 segmented copolymer membranes were characterized for their
application as proton exchange membranes. Transparent, tough, ductile films were able to be
cast from this series of copolymers. Table 4.2 summarizes selected copolymer properties of
the series. The experimental IEC agreed closely with the targeted value (~1.7 meq/g) for the
system. The I.V. of the copolymers suggested high molecular weight copolymer was
synthesized. Conductivity in liquid water (30 oC) was not dependent on block length.
However, water uptake was affected by block length. Copolymers with longer block lengths
sorbed more water than copolymers with shorter block lengths, which has been ascribed to
the formation of co-continuous hydrophilic channels that form with increasing block length.2
This was also observed in dimensional water sorption tests. Figure 4.6 shows water swelling
in the x, y, and z directions. As the block length of the copolymer increases, the z-directional
swelling increases, while only small changes were observed in the x and y directions.
140
Table 4.2. Characterization of BisSF-PhSH100 Segmented Copolymer
a. Calculated from 1H NMR; theoretical IEC for the series is 1.7 meq/gb. GPC performed in 0.05 M LiBr/NMP at 60 oCc. Performed in liquid water at 30 oC
4.5k:7k 0.95 137 706.5k:10k 0.97 131 779k:13k 1.08 132 12311k:17k 0.85 136 1091.6
1.6
1.61.7
Block Mn
(BisSF:PhS)I.V.b
(dL/g)
Water Uptake
(%)Conductivityc
(mS/cm)
Experimental
IECa (meq/g)
0
10
20
30
40
50
60
70
4.5k7k 6.5k10k 9k13k 11k17k
Sw
ellin
g (%
)
x y z
z
X
yz
X
y
0
10
20
30
40
50
60
70
4.5k7k 6.5k10k 9k13k 11k17k
Sw
ellin
g (%
)
x y z
z
X
yz
X
y
Figure 4.6. Comparison of dimensional swelling data for segmented copolymers
Proton conductivity was measured as a function of relative humidity for this series
of segmented copolymers (Figure 4.7). Proton conductivity across the entire RH range
was dependent on the block length of the copolymers. The copolymers with shorter
block lengths had conductivity values that decreased very quickly as RH decreased,
141
whereas, copolymers with longer block lengths maintained proton conductivity values
which were comparable to Nafion® 112 when the RH was maintained at 70% or above.
Most likely, there was a large increase in the connectedness of the hydrophilic pathways
when the hydrophilic block lengths increased from 10 kg/mol to 13 kg/mol. Increasing
the hydrophilic block length to 17 kg/mol did not seem to improve the proton
conductivity further.
20 30 40 50 60 70 80 90 100
0.1
1
10
100
Con
duct
ivity
(m
S/c
m)
Relative Humidity (%)
4.5k7k 6.5k10k 9k13k 11k17k Nafion 112
Figure 4.7. Proton conductivity under partially hydrated conditions for BisSF-PhSH100 segmented copolymers with increasing block length
DMA was used to determine the thermal transitions of the copolymers. All of the
copolymers exhibited similar transitions to one another, regardless of the molecular
weight of the blocks. Two transitions were observed, one at 200 oC and one at 240 oC,
142
which are attributed to the Tgs of the hydrophobic and hydrophilic blocks, respectively.
The 6.5k10k membrane exhibited a slightly higher upper thermal transition, 250 oC,
which is due to the slight increase in IEC as compared to the other membranes.
Membranes that have a higher IEC have a greater concentration of sulfonate groups,
which can lead to more ionic interactions, thus increasing in the Tg.
0 50 100 150 200 250 300102
103
104
10-2
10-1
100
Sto
rage
Mod
ulus
(M
Pa)
Temperature (oC)
4.5k7k 6.5k10k 9k13k 11k17k
Tan δ
Figure 4.8. DMA plots for BisSF-PhS100 multiblock copolymers. The solid lines
represent the storage modulus and the dashed lines represent the tan δ.
The tensile properties of the membranes were determined and are shown in Table
4.3 and Figure 4.9. All of the membranes had a tensile strength near 60 MPa. However,
the elongation for this block copolymer system was between six and 18%. The low
elongation was attributed to the rigid nature of the phenolphthalein monomer and the
morphology of the nanophase separation and was not necessarily indicative of the
copolymers having a low molecular weight. From Table 4.2, it can be seen that 4.5k7k
143
and 6.5k10k have almost the same I.V., indicating similar molecular weights, but have
significantly different elongations.
Table 4.3. Tensile Properties of BisSF-PhS Segmented Copolymers
Tensile Strength std dev Elongation std dev
BisSF-PhSH100 (MPa) (MPa) (%) (%)4.5K7K 58 1 18 66.5K10K 58 1 6 19K13K 60 2 11 711K17K 58 1 6 2
0
10
20
30
40
50
60
70
0 5 10 15 20
Tensile Strain (%)
Ten
sile
Str
ess
(MP
a)
4.5k7k
6.5k10k
9k13k
11k17k
1234
1
2 3
4
0
10
20
30
40
50
60
70
0 5 10 15 20
Tensile Strain (%)
Ten
sile
Str
ess
(MP
a)
4.5k7k
6.5k10k
9k13k
11k17k
1234
1
2 3
4
Figure 4.9. Stress vs. Strain curves for BisSF-PhSH100 segmented copolymers
144
4.4 Conclusions
A series of BisSF-PhS100 segmented copolymers was synthesized which were
comprised of 100% disulfonated poly(arylene ether sulfone) hydrophilic blocks derived
from phenolphthalein and high fluorinated hydrophobic blocks. Shorter hydrophobic
blocks were targeted when compared to the hydrophilic blocks to maintain high IEC
values. Transparent, tough, ductile films could be cast from the copolymer. Water
uptake and proton conductivity over the entire RH range, increased as the block length of
the copolymers was increased. The phenolphthalein in the hydrophilic block increased
the tensile strength when compared to segmented copolymers which utilize BPSH100 as
the hydrophilic block (see Chapter 2). However, a decrease in ultimate elongation was
also observed.
Acknowledgment. The authors would like to acknowledge the Department of
Energy for funding under DE-FG36-06G016038.
145
References
1 Yu, X.; Roy, A.; Dunn, S.; Badami, A. S.; Yang, J.; Good, A. S.; McGrath, J. E. Synthesis and Characterization of Sulfonated-Fluorinated, Hydrophilic-Hydrophobic Multiblock Copolymers for Proton Exchange Membranes J. Polym. Sci. Part A: Polym. Chem., 2009, 47, 1038–1051. 2 Roy, Abhishek; Lee, Hae-Seung; McGrath, James E. Hydrophilic–hydrophobic multiblock copolymers based on poly(arylene ether sulfone)s as novel proton exchange membranes – Part B Polymer 2008, 49, 5037-5044. 3 Program's Multi-Year Research: Fuel Cells. Department of Energy, 2007. 4 VanHouten, Rachael A.; Lane, Ozma. R.; VanHouten, Desmond J.; McGrath, James E. Synthesis of segmented hydrophobic:hydrophilic, fluorinated:sulfonated block copolymers for use as proton exchange membranes. Macromolecules 2009, Submitted. 5 Yu, X.; Roy, A.; Dunn, S.; Badami, A. S.; Yang, J.; Good, A. S.; McGrath, J. E. Synthesis and Characterization of Sulfonated-Fluorinated, Hydrophilic-Hydrophobic Multiblock Copolymers for Proton Exchange Membranes J. Polym. Sci. Part A: Polym. Chem., 2009, 47, 1038–1051. 6 Lee, H.-S.; Roy, A.; Lane, O.; Dunn, S.; McGrath, J.E. Hydrophilic-hydrophobic multiblock copolymers based on poly(arylene ether sulfone) via low-temperature coupling reactions for proton exchange membrane fuel cells. Polymer 2008, 49, 715-723. 7 Wang, Zhonggang; Chen, Tianlu; Xu, Jiping. Gas permeabilities of cardo polyoxyarylene membranes. Journal of Applied Polymer Science 2002, 83(4), 791-801. 8 Miyatake, Kenji; Zhou, Hua; Uchida, Hiroyuki; Watanabe, Masahiro. Highly proton conductive polyimide electrolytes containing fluorenyl groups. Chem Commun 2003, 3, 368. 9 Lee, Hae-Seung; Lane, Ozma; McGrath, James E. Development of Multiblock Copolymers with Novel Hydroquinone-Based Hydrophilic Blocks for Proton Exchange Membrane (PEM) Applications J. Power Sources, Accepted, 2009. 10 Yu, X.; Roy, A.; Dunn, S.; Yang, J.; McGrath, J.E. Synthesis and Characterization of Sulfonated-Fluorinated, Hydrophilic-Hydrophobic Multiblock Copolymers for Proton Exchange Membranes. Macromol. Symp. 2006, 245-245, 439-449. 11 Lee, H.; Roy, A.; Lane, O.; Dunn, S.; McGrath, J. E. Segmented Sulfonated Poly(arylene ether sulfone)-b-Polyimide Copolymers for Proton Exchange Membrane Fuel Cells. I. Copolymer Synthesis and Fundamental Properties J. Polym. Sci. Part A. 2007, 45(21), 4879-4890.
146
12 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. Synthesis and Characterization of 3,3’-Disulfonated-4,4’-dichlorodiphenyl Sulfone (SDCDPS) Monomer for Proton Exchange Membranes (PEM) in Fuel Cell Applications J. Appl. Polym. Sci. 2006, 100, 4595-4602. 13 Li, Y.; VanHouten, R.; Brink, A.; McGrath, J.E. Purity Characterization of 3,3’-Disulfonated-4,4’-Dichlorodiphenyl Sulfone (SDCDPS) Monomer by UV-visible Spectroscopy. Polymer, 2008, 49, 3014-3019. 14 Ueda, M.; Toyota, H.; Ouchi, T.; Sugiyama, J.I.; Yonetake, K.; Masuko, T.; Teramoto, T. Synthesis and Characterization of Aromatic Poly(ether Sulfone)s Containing Pendant Sodium Sulfonate Groups. J. Polym. Sci.: Part A: Polym. Chem. 1993, 31, 853-858. 15 Yang, J.; Li, Y.; Roy, A.; McGrath J.E. Viscometric behavior of disulfonated poly(arylene ether sulfone) random copolymers used for proton exchange membranes. Polymer, 2008, 49(24), 5300-5306. 16 Zawodzinski, T. A.; Neeman, M.; Sillerud, L. O.; Gottesfeld, S. Determination of water diffusion coefficients in perfluorosulfonate ionomeric membranes J. Phys. Chem. 1991, 95, 6040. 17 Springer, T. E.; Zawodzinski, T. A.; Wilson, M. S.; Gottesfeld, S. Characterization of polymer electrolyte fuel cells using ac impedance spectroscopy J. Electrochem. Soc. 1996, 143, 587.
147
5 Synthesis and Characterization of Multiblock Copolymers Derived from
Bisphenol-A for Application as Reverse Osmosis Membranes
Rachael A. VanHouten, Desmond J. VanHouten, Myoungbae Lee, James E. McGrath*
Macromolecular Science and Engineering, Macromolecular and Interfaces Institute
Virginia Tech, Blacksburg, VA 24061 *[email protected]
Abstract
A new series of multiblock copolymers was synthesized by coupling hydrophilic
disulfonated poly(arylene ether sulfone) (BisAS100) oligomer with hydrophobic
unsulfonated poly(arylene ether sulfone) (BisAS0) oligomer. Both oligomers were
derived using 4,4´-isopropylidenediphenol (Bis-A) as the bisphenol. Phenoxide-
terminated BisAS100 was end-capped with decafluorobiphenyl and reacted with
phenoxide-terminated BisAS0 at low temperatures. Basic membranes properties were
characterized as a function of block length. The copolymers were cast into tough, ductile
films. Water sorption tests showed that water uptake increased with increasing block
length, despite the copolymers having similar IECs. Transmission electron microscopy
was used to confirm that a nanophase separated morphology developed for multiblock
copolymers with 8k8k and 12k12k block lengths. Thermal gravimetric analysis was used
to determine the thermal stability to be above 250 oC. The copolymers appeared to have
high chlorine tolerance after exposure to chlorine, indicating these membranes could be
possible candidates for desalinating and purifying water. The initial membrane
characterization suggested these copolymers may be suitable candidates for reverse
148
osmosis applications, and water and salt permeability testing should be conducted to
determine desalination properties.
5.1 Introduction
Water shortages are a growing concern across the world. Demand continues to
grow for a way to provide fresh water for an estimated 41% of the world that lives in
water-stressed areas.1 One way to produce fresh water is to remove the salt from sea or
brackish water using a desalination process. Desalination can be achieved by thermal
processes (evaporation/distillation) or membrane separation processes, such as reverse
osmosis (RO).2 Traditionally, facilities have used evaporation to produce potable water.
Although still used in many parts of the world today, high energy costs associated with
this technique render it unaffordable to many of the areas suffering from water
shortages.1 Plants utilizing RO technology, which began in the 1970s, are becoming
more popular because the process requires less energy than evaporation. It can also
remove microorganisms and organic contamination in addition to salt. 2
Over the last four decades, research has been ongoing to find membrane materials
that would perform better than current RO membranes.3 Most commercial membranes
are made of polyamide. The major drawback to aromatic polyamide membranes is their
inability to tolerate free chlorine, which is used as a disinfectant and bacteriacide for
water treatment.4,5 This requires the water to be dechlorinated before it can come in
contact with the membrane. Other drawbacks of aromatic polyamide membranes are the
high pressure required to push the water through (low flux) and biofouling1.
Sulfonated aromatic polymers have also been explored for use as RO membranes
since the 1970s. Research began with the exploration of sulfonated poly(phenylene
149
oxide) and sulfonated polyfurane membranes and progressed to sulfonated polysulfones.3
Sulfonated membranes maintain a low permeability to salts because the sulfonate ions
allow the anions in the salt to be repelled. Allegrezza et al.6,7 reported that RO modules
utilizing sulfonated polysulfone membranes exhibited high tolerance to chlorine because
they lack the oxidizable amide links present in polyamide membranes. The sulfonated
polysulfone RO modules could also withstand a wide pH range (4-11), were resistant to
fouling, and could be operated at high flux for long periods of time. Although sulfonated
polysulfones had desirable properties, they were synthesized using post-sulfonation
modification procedures,6,8,9,10,11 which have many drawbacks. Among the limitations of
post-sulfonation modification are the ability to fully control the degree and location of
sulfonation, as well as, side reactions and chain-degradation.9
Over the past decade, research efforts in the McGrath group have been focused on
the direct synthesis of disulfonated poly(arylene ether) random copolymers.12,13,14,15,16,17
These copolymers were synthesized by a nucleophilic aromatic substitution reaction of a
disulfonated dihalide (3,3’-disulfonated-4,4’-dichlorodiphenylsulfone, SDCDPS),
unsulfonated dihalide, and bisphenol to afford random copolymers, with predetermined
degrees of disulfonation based on the stoichiometric ratio of sulfonated to unsulfonated
dihalide. Copolymers with degrees of sulfonation ranging from zero to 100%
disulfonation have been achieved. These copolymers have excellent oxidative,
hydrolytic, and mechanical stability, as well as, good film forming properties.
Disulfonated poly(arylene ether sulfone) random copolymers derived from SDCDPS,
4,4’-dichlorodiphenylsulfone (DCDPS), and 4,4’-biphenol (coined BPSxx, where xx
represents the degree of sulfonation) have been shown to have high chlorine tolerance
150
across a broad pH range (4-10).18 Exposure to protein water or oil/water emulsions
resulted in minimal fouling.19 Salt rejection and water permeability for this type of
membrane were correlated to the degree of disulfonation. Overall, copolymers with
higher ion content (BPS40) displayed higher fluxes and lower salt rejection than
copolymers with lower ion content (BPS20).18,20 However, water flux and salt rejection
were also influenced by the structure of the bisphenol used to synthesize the copolymer
and whether the copolymer was in salt or acid form.
Additional synthetic variations have been suggested, which could tailor the
properties of disulfonated poly(arylene ether) copolymers further, making them more
suited for RO applications.21,22 Among these has been crosslinking random copolymers
in order to enhance salt rejection without hindering the flux. Paul et al.22 synthesized
50% disulfonated poly(arylene ether sulfone) random copolymers derived from 4,4’-
biphenol, which had controlled number-average molecular weight (Mn) and reactive
phenoxide end groups. These were used to crosslink the copolymer with tetraglycidyl
bis(p-aminophenyl)methane. Membranes which were cured for 90 minutes had a 97.2%
salt rejected compared to 73.4% for BPS-50 uncrosslinked copolymer. Only modest
decreases were observed in water permeability.
Synthesizing multiblock copolymers comprised of hydrophilic and hydrophobic
blocks also allows for further tailoring of disulfonated poly(arylene ether) copolymer
systems.23,24 Block copolymers contain two or more types of polymer, with dissimilar
backbone chemistries, which are chemically bonded within the same chain. Phase
separation occurs between the two polymers, as in blended polymer systems. However,
because the two types of polymers are chemically linked, only micro- or nanophase
151
separation occurs.25, Block copolymers become desirable candidates for RO membranes
if one of the blocks contains a partially or fully ionic backbone, such as a disulfonated
poly(arylene ether) copolymer. This hydrophilic ionic block could provide high water
flux while the hydrophobic block supplies mechanical stability to the system. It is
proposed that ion-rich channels form when the hydrophobic and hydrophilic domains of
block copolymers nanophase separate, allowing for co-continuous channels of
hydrophobic and/or hydrophilic blocks to form.26 This could be advantageous in the RO
separation process.
This paper focuses on the synthesis of a series of hydrophilic-hydrophobic
multiblock copolymers which utilize 4,4´-isopropylidenediphenol (Bis-A) as the
bisphenol. Phenoxide-terminated hydrophobic oligomers were synthesized in a step
growth polymerization using DCDPS and Bis-A. Phenoxide-terminated hydrophilic
oligomers were synthesized similarly using SDCDPS and Bis-A. These were then
reacted with an excess of highly reactive decafluorobiphenyl (DFBP) to afford fluorine-
terminated hydrophilic oligomers. Hydrophobic and hydrophilic oligomers were coupled
together to afford multiblock copolymers with the number-average molecular weight of
the block lengths ranging from 4 kg/mol to 12 kg/mol. A disulfonated poly(arylene ether
sulfone) random copolymer derived from Bis-A with 32% disulfonation was also
synthesized (BisAS32). The membrane properties of these multiblocks were assessed as
a function of block length. Assessments were made as to the viability of these multiblock
copolymers as RO membranes.
152
5.2 Experimental Section
5.2.1 Materials
Monomer grade Bis-A and DCDPS were kindly provided by Solvay Advanced
Polymers and dried under vacuum at 60 oC for 24 h before used. SDCDPS was
synthesized by Akron Polymer Systems according to a procedure reported elsewhere,27,28
which was a refinement of a previously published procedure by Ueda et al.29 SDCDPS
was dried under vacuum at 160 oC for 48 h before use. DFBP was obtained from Matrix
Scientific and dried under vacuum at room temperature overnight. N,N-
Dimethylacetamide (DMAc, Aldrich) was vacuum-distilled from calcium hydride onto
molecular sieves and stored under nitrogen before use. Potassium carbonate (K2CO3,
Aldrich) was dried under vacuum at 120 oC overnight before use. Toluene, cyclohexane,
acetone, and isopropyl alcohol (IPA) were obtained from Aldrich and used as received.
Concentrated sulfuric acid (H2SO4) was obtained from VWR and used to make a 0.5 M
aqueous solution.
5.2.2 Synthesis of Phenoxide-Terminated Hydrophobic Oligomers (BisAS0) (1)
Phenoxide-terminated hydrophobic oligomers were synthesized with targeted
number-average molecular weights (Mn) ranging from 4000 to 12000 g/mol. In a typical
procedure for a Mn of 6000 g/mol, the following conditions were utilized. Bis-A (4.9621
g, 22.127 mmol), DCDPS (5.9076 g, 20.573 mmol), and DMAc (54 mL) were added to a
three-neck, round-bottom flask, equipped with mechanical stirrer, Dean-Stark trap,
condenser, and N2 inlet. The reaction bath was set to 85 oC, and the monomers were
allowed to dissolve. K2CO3 (3.567 g, 25.81 mmol) and toluene (27 mL) were added to
153
the flask. The temperature of the bath was increased to 155 oC, and the reaction was
allowed to azeotrope water for 4 h. Toluene was removed from the system by increasing
the bath temperature to 180 oC. The reaction was allowed to proceed for 48 h. After
cooling, the reaction was filtered to remove salts and precipitated into a solution of
methanol:water (1:1 v:v, 2 L). The oligomer was washed for 12 h in DI water at 60 oC
and 12 h in methanol and then dried at 90 oC for 24 h under vacuum before further use. It
had a Mn of 5900 g/mol determined by end group analysis using 1H NMR.
5.2.3 Synthesis of Phenoxide-Terminated Hydrophilic Oligomers (BisAS100) (2)
Phenoxide-terminated hydrophilic oligomers were synthesized with targeted Mn
ranging from 4000 to 12000 g/mol. In a typical procedure for a Mn of 6000 g/mol, the
following conditions were utilized. Bis-A (4.4532 g, 19.857 mmol), SDCDPS (8.7783 g,
17.869 mmol), and DMAc (66 mL) were added to a three-neck, round-bottom flask,
equipped with mechanical stirrer, Dean-Stark trap, condenser, and N2 inlet. The reaction
bath was set to 85 oC, and the monomers were allowed to dissolve. K2CO3 (3.156 g,
22.84 mmol) and toluene (33 mL) were added to the flask. The temperature of the bath
was increased to 155 oC, and the reaction was allowed to azeotrope water for 4 h.
Toluene was removed from the system by increasing the bath temperature to 180 oC. The
reaction was allowed to proceed for 96 h. The reaction bath was cooled to 80 oC and a
small aliquot was removed to perform SEC and 1H NMR analysis before proceeding to
the end-capping reaction. The resulting oligomer had a Mn of 6700 g/mol determined by
end group analysis using 1H NMR.
154
5.2.4 Endcapping of Phenoxide-Terminated Hydrophilic Oligomers with DFBP (3)
DFBP (3.6057 g, 10.792 mmol) was added to (2) using DMAc (18 mL). The bath
temperature was increased to 105 oC for 17 h. After cooling, the reaction was filtered to
remove salts and precipitated into acetone (2 L). The oligomer was washed 3 times with
acetone to remove excess DFBP. It was dried at 110 oC for 48 h under vacuum before
further use.
5.2.5 Synthesis of Hydrophilic-Hydrophobic BisAS100-BisAS0 Multiblock
Copolymers
Oligomer (1) (Mn of 6700 g/mol, 3.0988 g, 0.5274 mmol) and DMAc (17 mL)
were added to a three-neck, round-bottom flask, equipped with mechanical stirrer, Dean-
Stark trap, condenser, and N2 inlet. The reaction bath was set to 85 oC, and the oligomer
was allowed to dissolve. K2CO3 (0.292 g, 2.113 mmol) and cyclohexane (5 mL) were
added to the flask. The temperature of the bath was increased to 110 oC, and the reaction
was allowed to azeotrope water for 4 h. Oligomer (3) (3.8519g, 0.5274 mmol) was added
to the reaction using DMAc (17 mL) to keep the reaction at 20% solids and the bath
temperature was increased to 125 oC for 30 h. The resulting viscous solution was
precipitated into IPA (700 mL) to form fibrous strands. The product was filtered and
washed in deionized water at 60 oC for 12 h and chloroform for 12 h. It was dried under
vacuum at 110 oC for 24 h before casting into films.
5.2.6 Characterization of Copolymers
1H and 13C NMR analyses were performed on a Varian Unity 400 MHz
spectrometer using 1% and 10% solutions (w/v), respectively, of sample in deuterated
155
solvent and run at ambient temperatures. Spectra for hydrophilic oligomers and
multiblock copolymer were obtained using DMSOd6 and spectra for hydrophobic
oligomers were obtained using CDCl3. Intrinsic viscosities of the hydrophobic
oligomers, hydrophilic oligomers, and multiblock copolymers were determined using size
exclusion chromatography (SEC). SEC experiments for the hydrophilic oligomers and
multiblock copolymers were performed on a liquid chromatograph equipped with a
Waters 1515 isocratic HPLC pump, Waters Autosampler, Waters 2414 refractive index
detector and Viscotek 270 right angle laser light scattering (RALLS)/viscometric 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
and sample solution were filtered before introduction to the SEC system. Further
solution characterization procedures have been described.30 SEC experiments for the
hydrophobic oligomers were performed on an Alliance Waters 2690 separations module
with a Viscotek T60A dual viscosity detector and laser refractometer equipped with a
Waters HR 0.5 + HR 2 + HR 3 + HR 4 styragel column set. SEC data were collected in
chloroform at 30 °C.
5.2.7 Membrane preparation
Membranes were formed by casting a 6% w/v solution of polymer in DMAc,
filtered through a 0.45 µm PTFE syringe filter, onto a clean glass plate. Solvent was
removed using an IR lamp. The lamp intensity was held at 30-35 oC for 24 h and then
raised to 35-40 oC for an additional 24 h. It was dried under vacuum at 110 oC for 24 h.
The film was removed from the glass plate by submersion in water.
156
5.2.8 Determination of Ion Exchange Capacity (IEC)
The IEC of the copolymers was determined by titrating acidified membranes
using a Schott TitroLine Alfa Plus TA20 automated titrator with an Elaktrolyt N6480
electrode. Acidified membranes were obtained by boiling in 0.5 M H2SO4 for 2 h and
then washing in boiling deionized water for 2 h. Membranes were equilibrated in fresh
deionized water for at least 48 h before titrations were completed to remove residual
sulfuric acid. Acidified membranes were dried under vacuum for 24 h at 110 oC and the
dry weight was obtained. Each membrane was placed in approx. 25 mL of 0.075 M
Na2SO4 solution for 24 h with stirring to allow the sodium from the solution to exchange
with the protons in the membrane. The resulting solutions were titrated with standardized
0.0075 M NaOH solution. IEC was determined as the mmol of NaOH divided by the
weight obtained for the dry sample. An average of 3 membrane samples was used for
each copolymer.
5.2.9 Determination of water uptake and dimensional swelling
The water uptake for all membranes was determined gravimetrically.
Membranes were equilibrated in liquid water at room temperature for 24 h. Wet
membranes were removed from the liquid water, blotted dry to remove excess water, and
quickly weighed. They were then dried at 110 oC under vacuum for 24 h and reweighed.
Water uptake was calculated according to equation 5.1, where massdry and masswet refer
to the mass of the dry and wet membranes, respectively. An average of three samples
was used for each measurement.
157
( )wet dry
dry
mass masswater uptake% 100
mass
−= × 5.1
Percent swelling of the membranes was determined in the in-plane (x and y) and through-
plane (z) directions. Wet measurements were performed after equilibrating membranes
in liquid water for 24 h at room temperature. Membranes were then dried in a convection
oven at 80 oC for 2 h and measured again. Wet and dry measurements in the x and y
direction were performed by sandwiching the membrane between layers of polyethelene
and two glass plates and measuring with a ruler (mm). Wet and dry measurements in the
z direction were performed using a micrometer. Typical sample size was 2.5 x 2.5 cm
squares when wet. Percent swelling was reported for three directions and calculated
according to equation 5.2 where lengthwet,i and lengthdry,i refer to the length (where i
represents the x, y, or z direction) of the dry and wet membrane, respectively.
( )wet,i dry,ii
dry,i
length lengthpercent swelling 100
length
−= × 5.2
5.2.10 Transmission Electron Spectroscopy (TEM)
Hydrogen ions in the hydrophilic blocks were replaced with cesium ions by
immersing the membrane samples overnight in an aqueous solution with an excess of
CsOH, which resulted in appropriate enhancement of electron density contrast between
the hydrophilic and hydrophobic blocks. The membrane samples were rinsed with DI
water and dried. The dried samples were embedded in epoxy and ultramicrotomed into
70 ~ 100 nm thick pieces with a diamond knife. Transmission electron micrographs were
attained by operating a Philips EM 420 Transmission Electron Microscope with a
tungsten filament at an accelerating voltage of 100 kV.
158
5.2.11 Tensile testing
Uniaxial load tests were performed using an Instron 5500R universal testing
machine equipped with a 200-lb load cell at room temperature and 44-54% relative
humidity (RH). Crosshead displacement speed was 5 mm/min and gauge lengths were
set to 25 mm. A dogbone die was used to punch specimens 50 mm long with a minimum
width of 4 mm. Prior to testing, specimens were dried under vacuum at 110 oC for at
least 24 h and then equilibrated at 44% RH and 30 oC. All specimens were mounted in
manually tightened grips. Approximate tensile moduli for each specimen were calculated
based on the stress and elongation values for the specimen at the first data point at or
above 2% elongation.
5.2.12 Dynamic Mechanical Analysis
Dynamic mechanical analysis (DMA) was performed using a TA Instruments
2890 Dynamic Mechanical Analyzer. Salt-form rectangular membrane films measuring
0.35 mm x 4 mm x 25 mm were used for the test. Multi-frequency tension tests were
conducted on the membranes, with an amplitude of 25 µm and a pre-load force of 0.025
N in a nitrogen atmosphere.
5.2.13 Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) was performed on a DSC Q1000 (TA
Instruments) using aluminum hermetic pans to determine the glass transition (Tg) of the
copolymers. Film samples in salt-form were run in nitrogen at a rate of 10 oC/min. The
second heat was reported.
159
5.2.14 Thermal Gravimetric Analysis
Thermal gravimetric analysis (TGA) was performed using a TGA Q500 (TA
Instruments) on the membrane specimens to determine the thermal stability of the
copolymers. The samples were dried isothermally in the TGA at 150 oC for 20 min to
remove any residual moisture. The samples were then equilibrated at 30 oC and run at a
heating rate of 10 oC/min in an air atmosphere.
5.2.15 Static Chlorine Exposure
Membranes and copolymer powder were placed in 100 mL of a 500 ppm solution
of NaOCl in deionized (DI) water. The pH of the solution was adjusted to 4.5-5.0 with
HCl. The samples were placed on an orbital shaker for 24 h. Samples were filtered and
thoroughly rinsed with DI water. Proton NMR spectra were obtained before and after
exposure to the chlorine on powder copolymer sample.
5.3 Results and Discussion
5.3.1 Synthesis of Phenoxide-Terminated Hydrophobic (BisAS0) and Hydrophilic
(BisAS100) Oligomers
Phenoxide-terminated, unsulfonated poly(arylene ether sulfone) hydrophobic oligomers
(BisAS0) and fully disulfonated hydrophilic oligomers (BisAS100) and were synthesized
via a nucleophilic aromatic substitution reaction (Figure 5.1 and Figure 5.3, respectively).
A small molar excess of Bis-A to SDCDPS or DCDPS was used to control the molecular
weight of the oligomers, targeting Mns of 4, 6, 8, 10, or 12 kg/mol. Proton NMR was
used to confirm that both series of oligomers were phenoxide-terminated and
simultaneously determine the Mns of the oligomers using end-group analysis. To aide in
160
the assignment of the peaks from 1H NMR, two-dimensional homonuclear correlation
spectroscopy (2-D COSY) experiments were performed. COSY experiments allow spin-
coupled pairs of nuclei to be determined.31 Figure 5.4 depicts a 1H-1H COSY of
BisAS100 oligomer with a Mn of 4 kg/mol. Splitting between protons on adjacent
carbons was determined by examining off-diagonal peaks. Based on the pairing made
using COSY experiments, proper peak assignments were made for BisAS100 oligomer
(Figure 5.5). The terminal protons due to a Bis-A unit at the end of a chain were assigned
to peaks at 6.65 and 6.75 ppm for the hydrophilic and hydrophobic blocks, respectively
(Figure 5.2 and Figure 5.5). Whereas, aromatic protons from a Bis-A unit in the middle
of the chain resulted in peaks at 6.95 and 7.25 ppm for the hydrophilic and 7.22 and 7.0
ppm for the hydrophobic oligomers. By comparing the integration value ratios of main
chain peaks to end-group peaks, Mn was determined. Theoretical and experimental Mn
values are summarized in Table 5.1, along with intrinsic viscosity (I.V.) values measured
by SEC. An increase in I.V. was observed as Mn of the oligomers increased. Log-log
plots of Mn versus intrinsic viscosity for both copolymer series had a linear relationship,
indicating the expected strong correlation between I.V. and Mn for BisAS100 and BisAS0
oligomers (Figure 5.6).
161
K2CO3Toluene/DMAc4 hrs @ 155 oC48 hrs @ 180 oC
+ OH
CH3
CH3
OHCl S Cl
O
O
OK
CH3
CH3
O S O
O
O
CH3
CH3
KO m
K2CO3Toluene/DMAc4 hrs @ 155 oC48 hrs @ 180 oC
+ OH
CH3
CH3
OHCl S Cl
O
O
OK
CH3
CH3
O S O
O
O
CH3
CH3
KO m
Figure 5.1. Phenoxide-terminated BisAS0 with controlled molecular weight
OK
CH3
CH3
O S O
O
O
CH3
CH3
KO m
a b c d a’
a’
d cab
OK
CH3
CH3
O S O
O
O
CH3
CH3
KO m
a b c d a’
a’
d cab
Figure 5.2. Aromatic region of a 1H NMR spectrum of BisAS0 oligomer
162
K2CO3Toluene/DMAc4 hrs @ 155 oC96 hrs @ 180 oC
+
OK
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
KO n
Cl S Cl
O
O SO3Na
NaO3S
OH
CH3
CH3
OH
K2CO3Toluene/DMAc4 hrs @ 155 oC96 hrs @ 180 oC
+
OK
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
KO n
Cl S Cl
O
O SO3Na
NaO3S
OH
CH3
CH3
OH
Figure 5.3. Phenoxide-terminated BisAS100 with controlled molecular weight
ed
bg
ac f ih
ed
bg
a cf
ih
f1 (ppm)
f2 (
ppm
)
ed
bg
ac f ih
ed
bg
a cf
ih
ed
bg
ac f ih
ed
bg
a cf
ih
ed
bg
a cf
ih
f1 (ppm)
f2 (
ppm
)
Figure 5.4 2D-COSY spectrum of BisAS100 oligomer
163
OKO S O
O
O SO3K
KO3S
KO n
c
ab
d
e
i
dca b e f g h i
fhg
OKO S O
O
O SO3K
KO3S
KO n
c
ab
d
e
i
dca b e f g h i
fhg
Figure 5.5. Aromatic regions of a 1H NMR spectrum of BisAS100 oligomer before
end-capping reaction
Table 5.1. Characterization of Hydrophobic and Hydrophilic Telechelic Oligomers
a. Calculated from end group analysis using1H NMRb. SEC results of oligomer in salt form performed in NMP w/0.05 M LiBrc. SEC results of oligomer performed in chloroform
Targeted Mn (g/mol)
Actual Mna
(g/mol)IV b
(dL/g)Actual Mn
a
(g/mol)IV c
(dL/g)
4000 4388 0.26 4624 0.106000 6686 0.31 5876 0.128000 7241 0.34 8168 0.1510000 10464 0.40 10679 0.1612000 12279 0.47 11721 0.19
Hydrophilic Hydrophobic
164
-1.1
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
3.6 3.7 3.8 3.9 4 4.1
Log(M n)
Log(
IV)
BisAS100 Oligomers
BisAS0 Oligomers
Log(IV)=0.57[Log(Mn)]-2.68
Log(IV)=0.66[Log(Mn)]-3.40
-1.1
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
3.6 3.7 3.8 3.9 4 4.1
Log(M n)
Log(
IV)
BisAS100 Oligomers
BisAS0 Oligomers
Log(IV)=0.57[Log(Mn)]-2.68
Log(IV)=0.66[Log(Mn)]-3.40
Figure 5.6. Log (I.V.) vs. log (Mn) for the hydrophobic and hydrophilic oligomers
5.3.2 Endcapping of Phenoxide-Terminated Hydrophilic Oligomers with DFBP
In order to complete the coupling reaction between BisAS0 and BisAS100 to
form a multiblock copolymer, both oligomers could not be phenoxide-terminated. Lee et
al.32 demonstrated that performing an end-capping reaction on a hydrophobic oligomer
with a 200% molar excess of DFPB or hexafluorobenzene effectively produced fluorine-
terminated oligomer. In theory, other activated halides could be used for this reaction.
However, the highly fluorinated monomers afforded oligomers with high reactivity. The
production of a highly reactive end-group facilitates the subsequent coupling reaction
between the two oligomers. Here, DFBP was used to end-cap the hydrophilic BisAS100
oligomer (Figure 5.7). End-capping was performed at a low reaction temperature (105
oC) due to the high reactivity of DFPB. Based on Lee’s previous work, a 6:1 ratio of
DFBP to BisAS100 was utilized to prevent chain extension from occurring. Proton NMR
165
was used to observe the disappearance of end-group peaks due to protons on the
phenoxide end-groups (Figure 5.8), indicating the oligomer was successfully end-capped.
We chose to modify the end-groups of the hydrophilic oligomer (over the hydrophobic
oligomer) because a phenoxide-terminated BisAS100 oligomer is less reactive than a
similarly terminated BisAS0 oligomer. End-capping the former with DFBP was thought
to provide a greater enhancement to the reactivity for the coupling reaction. Also,
phenoxide-terminated BisAS0 has a higher compatibility with cyclohexane, which was
used to azeotrope water in the subsequent coupling reaction, than phenoxide-terminated
BisAS100. Leaving BisAS0 in phenoxide form prevented the oligomer from crashing
out of solution when the cyclohexane was added, allowing for a more effective removal
of water.
OK
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
KO n
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n F
F
F
F
F
FF
F F
F
F
F
F
FF
F F
F
K2CO3cyclohexane/DMAc4 hrs @ 110 oC17 hrs @ 105 oC
F F
F
F
F
F
FF
F F
200% molar
excess
OK
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
KO n
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n F
F
F
F
F
FF
F F
F
F
F
F
FF
F F
F
K2CO3cyclohexane/DMAc4 hrs @ 110 oC17 hrs @ 105 oC
F F
F
F
F
F
FF
F F
200% molar
excess
Figure 5.7. DFBP end-capping of phenoxide-terminated BiSA100 oligomer
166
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n F
F
F
F
F
FF
F F
F
F
F
F
FF
F F
F O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n F
F
F
F
F
FF
F F
F
F
F
F
FF
F F
F
Figure 5.8. Aromatic region of a 1H NMR spectrum of BisAS100 endcapped with
DFBP
5.3.3 Synthesis of Hydrophilic-Hydrophobic BisAS100-BisAS0 Multiblock
Copolymers
Fluorine-terminated, fully disulfonated poly(arylene ether sulfone) hydrophilic
oligomers were coupled to phenoxide-terminated, unsulfonated poly(arylene ether
sulfone) hydrophobic oligomers via a nucleophilic aromatic substitution reaction (Figure
5.9). This series of copolymers will be referred to as BisAS100-BisAS0 multiblock
copolymers. Specific copolymers within the series are identified by the Mns of the
oligomers used in the syntheses, i.e. a copolymer with 4 kg/mol hydrophobic and
hydrophilic blocks is called 4k4k. Multiblock copolymers which had equal Mn for the
BisAS100 and BisAS0 blocks were synthesized using a 1:1 stoichiometry. The aromatic
region of a representative 1H NMR spectrum for a BisAS100-BisAS0 multiblock
copolymer is shown in Figure 5.10. The spectrum indicates successful formation of
multiblock copolymer, as peaks from both hydrophilic and hydrophobic aromatic protons
are present. Completion of the reaction was evidenced by the disappearance of peaks due
to end-group protons which would have resulted if unreacted BisAS0 oligomer remained.
167
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n
F
F
F
F
FF
F F
F
F
F
F
FF
F F
O O
CH3
CH3
O S O
O
O
CH3
CH3
m
x
OK
CH3
CH3
O S O
O
O
CH3
CH3
KO m
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n F
F
F
F
F
FF
F F
F
F
F
F
FF
F F
F 24-48 hrs @ 125 oC
K2CO3cyclohexane/DMAc4 hrs @ 110 oC
Addition of hydrophilic BisAS100 oligomer
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n
F
F
F
F
FF
F F
F
F
F
F
FF
F F
O O
CH3
CH3
O S O
O
O
CH3
CH3
m
x
OK
CH3
CH3
O S O
O
O
CH3
CH3
KO m
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n F
F
F
F
F
FF
F F
F
F
F
F
FF
F F
F 24-48 hrs @ 125 oC
K2CO3cyclohexane/DMAc4 hrs @ 110 oC
Addition of hydrophilic BisAS100 oligomer
Figure 5.9. Coupling reaction of hydrophilic and hydrophobic oligomers
O
CH3
CH3
O S O
O
O
CH3
CH3
m
x
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n
F
F
F
F
FF
F F
F
F
F
F
FF
F F
O
a b c d
e
f g h i
e d ca
bf,hg
i
O
CH3
CH3
O S O
O
O
CH3
CH3
m
x
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n
F
F
F
F
FF
F F
F
F
F
F
FF
F F
O
O
CH3
CH3
O S O
O
O
CH3
CH3
m
x
O
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n
F
F
F
F
FF
F F
F
F
F
F
FF
F F
OO
CH3
CH3
O S O
O
O SO3K
KO3SCH3
CH3
O n
F
F
F
F
FF
F F
F
F
F
F
FF
F F
O
a b c d
e
f g h i
e d ca
bf,hg
i
Figure 5.10. Aromatic region of a 1H NMR spectrum for BisAS100-BisAS0 multiblock copolymer
The highly reactive DFBP end-groups on the hydrophilic blocks facilitated the
use of low reaction temperatures (125 oC). Low reaction temperatures are one way to
prevent well known ether-ether interchange reactions33 from occurring. Carbon NMR
168
was used to monitor the possible presence of randomization in the multiblock backbone,
which would arise if ether-ether interchange had occurred. Figure 5.11 shows 13C NMR
spectra of a BisAS100-BisAS0 multiblock copolymer with block Mns of 6 kg/mol and a
BisAS32 random copolymer. The high sequenced backbone of the multiblock copolymer
results in the formation of sharp singlets; whereas, the shorter monomer sequences of the
random copolymer, results in doublets.
(a)
(b)
(a)
(b)
Figure 5.11. Portions of 13C NMR spectra for (a) BisAS100-BisAS0 multiblock and (b) BisAS32 random copolymers
5.3.4 Membrane Characterization of BisAS100-BisAS0 Multiblock Copolymers
Criteria for ideal RO membranes have been identified.34,35 They must be highly
permeable to water (high flux) while maintaining high salt rejection. Resistance to
microbiological attack and fouling by colloidal and suspended material, chemical
stability, and tolerance to chlorine and other oxidants maximizes membrane life. They
require mechanical integrity that is not affected by exposure to high pressures (up to 1200
psig) or high temperatures (25-90 oC). Easy formation of thin films or hollow fibers is
necessary to reduce operation cost.
169
Basic membrane properties for BisAS100-BisAS0 multiblock copolymers were
evaluated to decide if RO testing is warranted. Table 5.2 summarizes some basic
copolymer properties for this series of copolymers. The IEC values for the multiblock
copolymers were obtained by titration and were slightly lower than the theoretical value
of ~1.5 meq/g. However, they were fairly consistent throughout the series and the value
matches closely with that of BisAS32, which makes comparing and contrasting other data
for this series easier. Tough, transparent, flexible films were formed from this series of
copolymers. The copolymers had high I.V. values which indicate high molecular weight
polymer had been formed.
Water sorption plays an important role in RO processes. Park et at.21 showed that
water permeability increased and salt rejection decreased as water uptake increased for
random copolymers synthesized with 4,4’-biphenol. The water uptake values were
dependent on the IEC (degree of sulfonation) of the copolymers. The multiblock copolymers
discussed here had a fixed IEC value. Instead, the changes seen in water uptake were a
function of block length (Figure 5.12). This may affect the trends observed between water
sorbtion and water permeability or salt rejection. Directional swelling may also play a role in
water flux and salt rejection.
Figure 5.13 compares dimensional swelling of random and multiblock copolymers in
(x and y) and through (z) the plane. BisAS32 random copolymer and 12k12k multiblock
copolymer had nearly isotropic swelling. Whereas, 4k4k through 10k10k copolymers had
greater swelling in the z direction. The way the water distributes itself in the copolymer may
affect how salt is rejected.
170
Table 5.2. Characterization of BisAS100-BisAS0 Multiblock Copolymers
a. Calculated from Titrationb. Intrinsic viscosity SEC results of polymer in salt form
performed in NMP w/0.05 M LiBrc. [(mass wet – mass dry)/(mass dry)] x 100
IEC (meq/g)
Copolymer Exp.a
4k4k 1.2 1.82 236k6k 1.3 1.31 348k8k 1.4 1.65 41
10k10k 1.2 1.77 5112k12k 1.2 1.99 59
BisAS32 random
1.3 1.35 17
Water
Uptakec IVb
(dL/g)
R2 = 0.9925
0
10
20
30
40
50
60
70
2 4 6 8 10 12 14
Block Length (kg/mol)
Wat
er U
ptak
e (%
)
Figure 5.12. Water uptake (wt%) as a function of block length for BisAS100-BisAS0 multiblock copolymers
171
0
5
10
15
20
25
30
35
BisAS32 1.3 meq/g
4k4k 1.2 meq/g
6k6k 1.3 meq/g
8k8k 1.4 meq/g
10k10k 1.2 meq/g
12k12k 1.2 meq/g
Sw
ellin
g (%
)
x y z
z
X
y
0
5
10
15
20
25
30
35
BisAS32 1.3 meq/g
4k4k 1.2 meq/g
6k6k 1.3 meq/g
8k8k 1.4 meq/g
10k10k 1.2 meq/g
12k12k 1.2 meq/g
Sw
ellin
g (%
)
x y z
z
X
yz
X
y
Figure 5.13. Comparison of dimensional swelling data for random and multiblock
copolymers
Figure 5.14 compares the nanostructures of 8k8k and 12k12k multiblock
copolymers. Nanophase separation between the hydrophilic (black) and hydrophobic
(grey) domains was evident in both copolymers. The hydrophilic and hydrophobic
pathways that formed in the 12k12k copolymer appeared to be co-continuous. The
hydrophobic pathways in the 8k8k copolymer were highly connected, whereas, the
hydrophilic pathways were shorter ranged. In some places complete segregation of
hydrophilic domain was observed. Increased block length of the copolymers, results in
better hydrophilic channel formation.
172
AKDA
KD AKD
AKDA
KD
Figure 5.14. TEM images of 8k8k and 12k12k BisAS100-BisAS0 multiblock copolymers. (The bright white spot in the middle of the images is a camera artifact.)
The glass transition temperature of the copolymers was determined using DMA.
BisAS100-BisAS0 10k10k was chosen as a representative plot of the multiblock
copolymers and is compared to BisAS32 random copolymer in Figure 5.15. A distinct
transition was observed between 200 and 210 oC from the DMA for all of the block
copolymers, which was attributed to chain relaxation in hydrophobic block. A plateau
was observed after the initial decrease in the storage modulus. The presence of the
sulfonate groups in the hydrophilic blocks led to ionic aggregation, which resulted in a
higher thermal transition for the hydrophilic block as compared to the hydrophobic block.
The exact temperature of the transition of the hydrophilic block could not be obtained
because degradation of the block copolymers occurs at temperatures lower than the
transition temperature, as is shown in the TGA plot (Figure 5.17). Since the random
copolymer had much smaller domains of the hydrophobic and hydrophilic regions, a
single thermal transition is observed in the DMA at 275 oC.
173
0 50 100 150 200 250 300 350102
103
104
10-3
10-2
10-1
100
Sto
rage
Mod
ulus
[MP
a]
Temperature [oC]
Tan δ
Figure 5.15. DMA plot of BisAS100-BisAS0 10k10k multiblock copolymer (black) and BisAS32 random copolymer (grey). Solid lines represent the storage modulus
and dashed lines represent tan δ of the copolymers.
DSC was also used to observe the thermal transitions in the random and
multiblock copolymers, and thermograms are shown in Figure 5.16. Multiblock
copolymers with the longest block lengths, 10k10k and 12k12k, exhibited two Tgs. The
Tg at 190 oC is attributed to the hydrophobic block and the Tg at 270 oC was attributed to
the hydrophilic block. As the Mn of block decreases, the presence of two Tgs was harder
to discern in the thermograms. The Tg of the random copolymer, BisAS32, was hard to
determine using the DSC.
174
50 100 150 200 250 300
Temperature ( oC)
Hea
t Flo
w (
Exo
dow
n)
6k6k8k8k10k10k12k12kBisAS32
Figure 5.16. Thermograms for BisAS100-BisAS0 multiblock copolymers and BisAS32 random copolymer
The results of the TGA are shown in Figure 5.17. The TGA was conducted in an
air atmosphere to assess the oxidative stability of the copolymers. It can be seen that all
of the block copolymers behave similarly and are oxidatively stable up to 375 oC. Two
distinct weight loss regions are also observed, one at 275 oC and the second at 375 oC.
The initial weight loss at 275 oC is attributed to the loss of the sulfonate groups on the
hydrophilic blocks. The main chain degradation leads to the weight loss at 375 oC.
These temperatures were well above temperatures use in RO processes.
175
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500 600
Temperature ( oC)
Wei
ght (
%)
4k4k
6k6k
8k8k
10k10k
12k12k
BisAS32
Figure 5.17. Thermal gravimetric analysis plot of BisAS32 random and BisAS100-
BisAS0 multiblock copolymers
The tensile properties of the random and multiblock copolymers are summarized
in Table 5.3 and stress-strain plots are shown in Figure 5.18. All of the membranes
synthesized had a tensile strength near 50 MPa. The multiblock copolymers exhibited a
slightly higher tensile strength than the BisAS32 random copolymer. However, the
elongation of the multiblock copolymers was less than the BisAS32 random copolymer.
Despite the differences between the random and multiblock copolymers, all of the
membranes produced were tough and ductile.
176
Table 5.3. Tensile Properties of BisAS Copolymers
Copolymer Tensile Strength std dev % Elongation std devBlock Length (MPa) (MPa) (%) (%)
4k4k 51 1 14 26k6k 46 4 47 258k8k 53 1 31 15
10k10k 49 1 33 812k12k 49 1 17 3
BisAS32 Random 45 3 63 22
0
10
20
30
40
50
60
0 20 40 60 80
Tensile Strain (%)
Ten
sile
Str
ess
(MP
a)
4k4k6k6k8k8k10k10k12k12kBisAS32
123456
6
2
5
3
1
4
0
10
20
30
40
50
60
0 20 40 60 80
Tensile Strain (%)
Ten
sile
Str
ess
(MP
a)
4k4k6k6k8k8k10k10k12k12kBisAS32
123456
123456
6
2
5
3
1
4
Figure 5.18. Stress-strain plots for BisAS copolymers
It is advantageous for copolymers being used in RO applications to have chlorine
resistance. Chlorine is used as a disinfectant and a bactericide throughout the water
177
treatment process. Currently, sea water is chlorinated to remove algae in order to prevent
RO membranes from fouling.104 The water is then dechlorinated because polyamide
membranes are susceptible to chlorine degradation. The water requires re-chlorination to
kill bacteria so it can be used as drinking water. The development of a membrane which
would not require the dechlorination and rechlorination steps could save money by
decreasing time and costly pre- and post-treatment processes.
Chlorine exposure tests were conducted on this series of membranes to ensure
chlorine tolerance before RO testing is commenced. Membranes were soaked in a 500
ppm solution of NaOCl in water for 24 h. The pH of the solution was adjusted to 4.5-5.0
with HCl. Proton NMR spectra were obtained before and after exposure to the chlorine.
Proton NMR spectra for a sample multiblock and random copolymer are shown in Figure
5.19. No changes were observed after exposure to chlorine indicating acceptable chlorine
tolerance.
(c)
(d)
(a)
(b)
(c)
(d)
(a)
(b)
(a)
(b)
Figure 5.19. 1H NMR spectra comparing copolymers before and after exposure to
500 ppm NaOCl for 24 h (pH of 4.5-5.0) (BisAS100-BisAS0 8k8k multiblock copolymer (a) before and (b) after exposure, BisAS32 random copolymer (c) before
and (d) after exposure)
178
5.4 Conclusions
A series of hydrophilic-hydrophobic poly(arylene ether sulfone) multiblock
copolymers which utilize Bis-A as the bisphenol were synthesized. The 100%
disulfonated hydrophilic oligomers were end-capped with DFBP to facilitate coupling
with phenoxide-terminated hydrophobic oligomers at low temperatures. Copolymers
with equal hydrophilic and hydrophobic block lengths were achieved, ranging from 4k4k
to 12k12k. The copolymers were cast into tough, ductile films. Water sorption was
measured gravimetrically and dimensionally. Both showed that water uptake increases
with increasing block length, despite the copolymers having similar IECs. This was due
to the formation of longer co-continuous hydrophilic pathways that develop within the
copolymer as block length increased. TEM was used to confirm that a nanophase
separated morphology resulted for multiblock copolymers with 8k8k and 12k12k block
lengths. Static exposure to chlorine resulted in no degradation, which indicated these
membranes have high chlorine tolerance making them possible candidates for
desalinating and purifying water. The water would not require the dechlorination steps
used in current desalination units. These copolymers have adequate thermal and
mechanical stability as evidenced by TGA and tensile testing, respectively, to justify
further RO testing. Further characterization is underway to determine if these
membranes are suitable for RO applications.
Acknowledgement. The authors would like to acknowledge Dow FilmTec for
funding.
179
References
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6 Overall Conclusions
A segmented synthesis technique was used to produce ionomers for use as proton
exchange membranes for fuel cell applications. In previous research, multiblock
copolymers were produced by separately synthesizing the hydrophobic and hydrophilic
oligomers with different telechelic functionality, followed by a coupling reaction between
the two oligomers. While the membranes formed from previous copolymers exhibited
good properties, the synthesis was time consuming. In the segmented approach, a
phenoxide-terminated hydrophilic block was first synthesized. The dihalide and
bisphenol comonomers used to produce the hydrophobic block were then reacted with the
hydrophilic oligomers so the coupling reaction proceeded in tandem with the
hydrophobic block formation. By using highly reactive decafluorobiphenyl as the
dihalide, low reaction temperatures (< 105 oC) could be used, which reduced ether-ether
interchange reactions. This helped ensure the formation of a blocky hydrophobic-
hydrophilic structure throughout the copolymer backbone. This technique was proven
successful by comparing the properties of segmented BisSF-BPS100 copolymers with
BisSF-BPS100 multiblock copolymers having the same block length compositions. Both
synthetic techniques produced copolymers with similar properties.
The segmented approach to synthesizing ionomers was then extended to PhF-
BPS100 and BisSF-PhS100 copolymers. These systems of ionomers produced ductile
membranes that were able to be characterized for use in fuel cell applications. The use of
the segmented technique to produce three different systems of ionomers demonstrated
that it is a suitable technique to produce copolymers with a blocky structure.
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While the segmented copolymers produced in this study did not yield membranes
with better conductivity than Nafion® over the entire RH range, the research produced a
better understanding of how the bisphenol affects copolymer properties. Segmented
copolymers containing phenolphthalein as the bisphenol yielded copolymers with greater
tensile strength due to the enhanced rigidity of the phenolphthalein as compared to
Bisphenol-S or 4,4’-biphenol. The research also gave greater insight into the importance
of the hydrophilic and hydrophobic block length on the membrane properties. Block
length was proven to have a greater impact on the conductivity and water uptake than the
ion exchange capacity of the copolymers.
In this research, multiblock copolymers were also produced for potential use as
reverse osmosis applications. Bisphenol-A was chosen as the bisphenol in the multiblock
synthesis due to the monomer cost. Phenoxide-terminated hydrophobic and hydrophilic
oligomers were initially synthesized. Decafluorobiphenyl was used to end-cap the
hydrophilic oligomers, converting the phenoxide-terminated copolymer to fluorine-
terminated copolymer. This functionality facilitated the use of low temperatures (< 125
oC) for the subsequent coupling reaction with the phenoxide-terminated hydrophobic
oligomer. The system of multiblock copolymers afforded ductile membranes. The
membranes were shown to be resistant to chlorine degradation, which can play an
important role in reverse osmosis applications and the future economics of water
desalination.
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7 Future Research
The next step in assessing the BisAS multiblock copolymers as RO membranes is
to evaluate the salt rejection and water permeability. This is currently being conducted at
University Texas-Austin in conjunction with Prof. Freeman. Based on the results
obtained from these studies several changes can be made to the copolymer to tailor the
properties.
In the current study, copolymers with an IEC of ~1.3 meq/g were synthesized.
Salt rejection and water permeability can be altered by changing the IEC of the
copolymer. This could be done by synthesizing copolymers with unequal hydrophobic
and hydrophilic block lengths. Converting the copolymers into acid form may also
change the membrane properties. It has been shown that the boiling procedure that is
used to convert membranes from salt to acid form alters the morphology of the
copolymers.1 This morphology change could alter the salt rejection and water
permeability of the copolymer even if the backbone chemistry was maintained.
Finally, if the series of copolymers were converted to acid from, it could be tested
for fuel cell applications as well.
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References
1 Kim, Y. S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y. T.; Harrison, W.; Zawodzinski, T. A.; McGrath, J. E. Effect of Acidification Treatment and Morphological Stability of Sulfonate Poly(arylene ether sulfone) Copolymer Proton Exchange Membranes for Fuel Cell Use Above 100 °C. J. Polym. Sci.: Part B: Polym. Phys. 2003, 41, 2816-2828.