A NEW CLASS OF POLYELECTROLYTE;
POLY(p-PHENYLENE DISULFONIC ACIDS)
by
JUNWON KANG
Submitted in partial fulfillment of the requirements
For the degree of Doctor of Philosophy
Dissertation Adviser: Dr. Morton H. Litt
Department of Macromolecular Science and Engineering
CASE WESTERN RESERVE UNIVERSITY
January, 2008
CASE WESTERN RESERVE UNIVERSITY
SCHOOL OF GRADUATE STUDIES
We hereby approve the dissertation of
______
candidate for the Ph.D. degree *.
(signed)______(chair of the committee)
______
______
______
______
______
(date) ______
*We also certify that written approval has been obtained for any proprietary material contained therein.
Copyright © 2008 by Junwon Kang
All rights reserved
This dissertation is dedicated to Sunglim, Haejin and Jason,
and to my parents and parents-in-law,
for their unconditional support, love and patience.
I appreciated Samsung SDI for providing great opportunity
to study in the state and financial support. Table of contents
Chapter 1. Introduction ………………………………………………… …………..1
1.1. Introduction: Basics of Fuel cells …………………………………………..……1
1.2. Nafion® and other perfluorosulfonated polymers ……………………..……….6
1.3. Sulfonated aromatic condensation polymers (ACPs) …………….….………12
1.3.1. Postsulfonation of aromatic condensation polymers………..………..13
1.3.2. Synthesis of sulfonated aromatic condensation polymers based on
sulfonated monomers……………………………………………..…….15
1.3.2.1. Sulfonated poly(ether ether ketone) (S-PEEK)…………..…...15
1.3.2.2. Sulfonated poly(imides) (SPIs) ……….………………….…….17
1.4. Concept of the structural approach used in the synthesis of poly(p-phenylene
sulfonic acids)…………...... 26
1.5. Overview of the dissertation…………………………………………..…………27
1.6. References……………………………………………………………….……….28
I Chapter 2. Design considerations and synthesis of poly(p-phenylene-2,5- disulfonic acid) (PPDSA) and characterization of its intrinsic and membrane properties: data, results and discussion………………………………………...28
2.1. Overview for chapter 2……………..……………………………….………..….28
2.2. Introduction……………………..…………………………………………………34
2.2.1. New approach to a highly sulfonated monomer:
Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid (DBBDSA) ...... 34
2.2.2. Synthetic routes towards poly(p-phenylene) (PPP)…...………………..41
2.2.2.1. Existing experimental methods for PPP synthesis...... ……….41
2.2.2.2. Synthetic route towards poly(p-phenylenes-2,5-disulfonic acid)
(PPDSA): Ullmann coupling…………………………..…………46
2.3. Experimental procedures………………………………………...………………50
2.3.1. Materials……………………………………………………………..………50
2.3.2. Characterization techniques for DBBDSA monomer…………...………50
2.3.3. Characterization techniques for PPDSA………...... 51
II 2.3.3.1. NMR spectroscopy…………………………………………………51
2.3.3.2. Rheology measurements……………………………...…………..51
2.3.3.3. GPC (Gel permeation chromatography)…………………….…..51
2.3.3.4. Viscosity measurements………………….……………………….52
2.3.3.5. Water uptake………………………………………………………..55
2.3.3.5.1. Water content evaluation………..………………………55
2.3.3.5.2. Lambda (λ) measurement……………...……………….57
2.3.3.6. Differential Scanning Calorimetry (DSC)……………..………….58
2.3.3.7. Thermogravimetric Analysis (TGA)………………...…………….59
2.3.3.8. Dimensional change with water uptake………………………….59
2.3.3.9. Proton conductivity measurements………………………...…….60
2.3.3.10. Wide angle X-ray diffraction (WAXD)…………………………..65
2.3.3.11. 2D X-ray diffraction……………………………………………….69
2.3.3.12. Optical Polarizing Microscope (OPM)……………….………….69
2.3.3.13. Dynamic Mechanical Analysis (DMA)………………….……….70
2.3.4. Synthetic procedures for 1,4-dibromo-2,5-benzenedisulfonic acid
(DBBDSA)...... 71
III 2.3.4.1. Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid, disodium
salt (DBBDSA-Na)…..……………………………………………..71
2.3.4.2. Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid, dibenzyl-
trimethylammonium (BTMA) salt (DBBDSA-BTMA).. ………….73
2.3.4.3. Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid, dilithium
salt (DBBDSA-Li)……………….....………………………….….74
2.3.4.4. Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid,
ditetrabutyl-phosphonium (TBP) salt (DBBDSA-TBP).…...……74
2.3.5. Synthetic procedures for poly(p-phenylene-2,5-disulfonic acid)
(PPDSA) synthesis: Ullmann coupling reaction………………….....….76
2.4. Results……………………………………………………………….……..……..78
2.4.1. Synthesis of DBBDSA………….………………….…....……………….78
2.4.1.1. Reaction conditions for DBBDSA synthesis...….……………..78
2.4.1.2. Characterizations of DBBDSA…….………..……………..……80
2.4.2. Synthesis for PPDSA……………………………………………...……..86
2.4.2.1. Polymerization conditions for PPDSA……………………..…..86
2.4.2.2. Characterizations of PPDSA…………..…………..……………87
IV 2.4.2.2.1. NMR spectra for PPDSA: 1H and 14C NMR spectra
…………………………………………………………………………87
2.4.2.2.2. Rheological properties………………………..…………90
2.4.2.2.3. PPDSA Viscosity results……………………….………..91
2.4.2.2.3.1. Effect of shear rate………………………………92
2.4.2.2.3.2. Effect of salt concentration on viscosity……….93
2.4.2.2.3.3. Effect of solvent………………………………….94
2.4.2.2.3.4. Effect of cationic species………………………..95
2.4.2.2.3.5. Effect of polymer molecular weight………...... 96
2.4.2.2.3. GPC (Gel Permeation Chromatography)...... …………97
2.4.2.2.5. Water uptake……………………………………………101
2.4.2.2.5.1. Experimental set-up……………………………101
2.4.2.2.5.2. Evaluation of water uptake and lambda (λ)….103
2.4.2.2.6. Dimensional changes with water uptake…………….106
2.4.2.2.7. Differential Scanning Calorimetry (DSC)……...……..109
2.4.2.2.8. Thermogravimetric analysis (TGA)……………….…..112
V 2.4.2.2.9. Wide angle X-ray diffraction (WAXD) ………………..113
2.4.2.2.9.1. WAXD sample preparation method...... …..113
2.4.2.2.9.2. WAXD diffractogram of PPDSA……………….115
2.4.2.2.10. 2D X-ray diffraction…………………………….……..122
2.4.2.2.11. Optical polarizing microscope……………….………125
2.4.2.2.12. Proton conductivity measurements…………………133
2.4.2.2.12.1. Theoretical basis……………….……………..133
2.4.2.2.12.2 Effect of the plastic plate used in cell…….....139
2.4.2.2.12.3 Effect of molecular weight on the proton
conductivity………….………………………………...140
2.4.2.2.12.4 Effect of the casting direction on
conductivity…...... 142
2.4.2.2.13. Mechanical property: Dynamic Mechanical Analysis
(DMA)…………………………………….…………….145
2.5. Discussion……………………………………………………….……………....147
2.5.1. Reaction conditions for DBBDSA synthesis………….………………..147
2.5.2. Polymerization conditions for PPDSA synthesis………….….….…….151
2.5.2.1. Proposed Ullmann coupling mechanism………….…..…………154
VI 2.5.3. PPDSA solution properties……………………….…………..………….156
2.5.3.1. Rheological properties and Solution OPM image…………..…..156
2.5.3.2. Viscosities of PPDSA………………………………………..……..160
2.5.3.2.1. Abnormal behavior in reduced viscosity of PPDSA………160
2.5.3.2.2. Shear thinning of PPDSA……………………………..….….163
2.5.3.2.3. Effect of salt concentration on viscosity……………..……..164
2.5.3.2.4. Effect of solvent: 0.1M LiBr-DMF solution vs. 0.1M LiBr-
DMF/NMP (33/67,v/v)…………………………….……………..165
2.5.3.2.5. Effect of cationic species……………………………….…...165
2.5.3.2.6. Modified Huggins equation for rigid rod polymer model....166
2.5.4. NMR spectra of PPDSA………………………………………………….173
2.5.4.1. 1H-NMR spectra of PPDSA………………………………...……173
2.5.4.2. 13C-NMR spectrum of PPDSA…………………………..………180
2.5.5. Determination of relative molecular weight of PPDSA using GPC…..182
2.5.6. Water retention of PPDSA film at different relative humidities…….…183
2.5.6.1. Lambda of PPDSA………………………………………………..183
2.5.6.2. State of water in PPDSA……………………………...………….185
VII 2.5.7. The presence of frozen-in free volume in PPDSA…………………….191
2.5.7.1. Macroscopic studies…………………………………….………..191
2.5.7.1.1. Dimensional change of PPDSA film at different
humidities……………………………………………….………191
2.5.7.1.2. Density of the absorbed water in PPDSA at different
humidities………………………………………………..……...194
2.5.7.2. Microscopic studies………………………………………...…….202
2.5.7.2.1. Analysis of X-ray diffractogram of PPDSA …………….202
2.5.7.2.2. Model study: Hexagonal packing model……………….209
2.5.7.2.3. The frozen-in free volume in PPDSA……………….….221
2.5.8. Effect of water contents on proton conductivity of PPDSA……….…..226
2.5.8.1. Activation energy for proton transport in PPDSA………….…..230
2.5.8.2. Corrected proton conductivity…………………………….……..237
2.5.8.3. Intrinsic conductivity: Proton conductivity in aqueous phase...241
2.5.9. Effect of water contents on mechanical property………………….…..246
2.5.10. Thermal stability of PPDSA……………………………….……………247
2.6. Conclusions………………………….………………………………….……….248
2.7. References……………………………………………………………..………..252
VIII Chapter 3. General conclusions and future work………………………….…262
3.1. General conclusions…………………………………………………..………..262
3.2. Future work…………………………………………………………………..….266
3.3. References………………………………………………………………………269
Bibliography…………………………...………………………………………………270
IX List of tables
Table 1.1. Various Fuel Cell systems categorized by the electrolyte used………3
Table 1.2. Technical protonic conductivity target for PEMFCs’ membranes….….5
Table 1.3. Various commercial polymer electrolyte membranes……………….…8
Table 2.1. Bulb volume, capillary radius and flow times for four viscometers used
in viscosity measurements……………………………………………...54
Table 2.2. Lithium chloride concentrations to achieve a given relative humidity..56
Table 2.3. Parameters for the AC impedance measurements……………………64
Table 2.4. Setting parameters for WAXD measurements…………………………65
Table 2.5. Sulfonation conditions and yields………………………………………..79
Table 2.6. Polymerization conditions for PPDSA using Ullmann coupling………86
Table 2.7. GPC test conditions for PPDSA (lot 1), disodium salt.………………..98
Table 2.8. Elution time and relative molecular weights of PPDSA (lot 1), disodium
salt under different test conditions……………………………………100
Table 2.9. Results of drying conditions test using PPDSA film (lot 2)………….101
Table 2.10. Equivalent weight of dried PPDSA films (lot 2)……………………..102
X Table 2.11. Water uptake and lambda evaluation for PPDSA (lot 2)…………...104
Table 2.12. Dimensional changes of PPDSA (lot 2) at different humidities…....107
Table 2.13. WAXD test conditions for studies to select sealing materials……..114
Table 2.14. WAXD peak data of PPDSA (lot 2)…………………………………...118
Table 2.15 Deconvolution results of d spacing of peaks……………...…………121
Table 2.16. d spacing from 2D X-ray spectra of PPDSA (lot 2) at different
humidities………………………………………………………………..122
Table 2.17. Proton conductivities of PPDSA films from lots 1, 2, and 3 at different
conditions………………………………………………………………..140
Table 2.18. The membrane conductivities of PPDSA films of lots 2 and 3 at
different temperatures and humidities………….…………………….143
Table 2.19. Reduced viscosities of PPDSA, diprotonated form…………………154
Table 2.20. Deconvolution results of 1H-NMR spectra from different lots……...177
Table 2.21. Area ratios of peaks in G5 (7.5~7.73 ppm) to peaks in G1, G2, G3
and G4 (8.0~7.73 ppm) and the number average degree of
polymerization of different lots………………………………………...179
Table 2.22. Chemical shifts of peaks in the 13C-NMR spectra of monomer
(DBBDSA-Li, Figure 2.8) and its polymer (PPDSA, Figure 2.18)…180
XI Table 2.23. Dimensional changes of PPDSA (lot 2) at different humidities……193
Table 2.24. Weight and dimensions of PPDSA (lot 2) at 0 %RH……………….195
Table 2.25. Weight changes of PPDSA (lot 2) at different humidities………….196
Table 2.26. Molar volume (a) and weight (b) changes of PPDSA (lot 2) at
different humidities………………….………………………………….197
Table 2.27. Statistical results of fitting line and exponential curve for plots in
Figure 2.63………………………………………………………………199
Table 2.28. Measured and calculated molar volume changes of PPDSA (lot 2) at
different humidities…………………………………………...... …….200
Table 2.29. Densities of the absorbed water in PPDSA (lot 2) at different
humidities………………………………………………………………..201
Table 2.30. The d spacing changes of PPDSA (lot 2) depending on lambda…203
Table 2.31. Comparison of d spacings calculated using WAXD data with that
calculated using 2D X-ray spectra…………………...……………….203
Table.2.32. Ratio of molar volume changes and (d spacing)2 of PPDSA (lot 2) at
different humidities……………………………………………………..213
Table 2.33. Statistical results of slopes and intercepts in Figure 2.73………….217
Table 2.34. Statistical results of slopes and intercepts in Figure 2.74………….220
XII Table 2.35. Curve fitting results for parameters in Equation 2.31………………222
Table 2.36. Changes of (d spacing)2 with lambda………………………………. 222
Table 2.37. The frozen-in free volume in PPDSA at λ=0…………...……………225
Table 2.38. Proton conductivities of PPDSA films and Nafion 117 at of different
lot at different conditions……………………………………………….228
Table 2.39. ln[σ (conductivity)] for PPDSA films (lots 2 (a) and 3, high molecular
weight polymer (b)) at different temperatures:………………………234
Table 2.40. Calculated Ea (activation energy) for PPDSA films (lots 2 and 3) at
different humidities…………………………………….……………….235
Table 2.41. Corrected conductivities for PPDSA film (lot 2) at 25°C and different
humidities……………………………………………………………….239
Table 2.42. Conductivities in aqueous phase for PPDSA film (lot 2) at 25°C and
different humidities…………………………………………………….243
XIII List of schemes
Scheme 2.1. Preparation of poly(phenylene sulfide sulfonic acid)…………….…37
Scheme 2.2. Relationship of X, Y, Z to casting direction.)…………………….…..60
Scheme 2.3. Schematic of assemblies of PPDSA film/electrodes for in-plane
conductivity measurements…………………………………………..63
Scheme 2.4. Schematic of different cell assemblies…………………………..….64
Scheme 2.5. Preparation of sealed WAXD sample…………………………….…67
Scheme 2.6. WAXD experimental setup……………………………………………68
Scheme 2.7. Equivalent circuit for an AC impedance conductivity
measurement………………………………………………………….133
Scheme 2.8. Synthesis reaction scheme for DBBDSA, disodium salt (DBBDSA-
Na)……………………………………………………………………..147
XIV List of figures
Figure 1.1. Schematic of hydrogen fuel cell………………………………………....1
Figure 1.2. Chemical structure of perfluorinated polymer electrolyte membranes
…………………………………………………………………………..…8
Figure 1.3. The cluster network model……………………………………………...10
Figure 1.4. Chemical structures of sulfonated aromatic condensation polymers
…………………………………………………………………………..14
Figure 1.5. Synthesis of directly copolymerized wholly aromatic sulfonated
poly(arylene ether sulfone) (BPSH)……………………………………15
Figure 1.6. Synthesis of imide compounds (model A and B)……………………..18
Figure 1.7. Synthesis of sulfonated polyimides based on BDA, ODA and NDA
………………………………………………………………………...….19
Figure 1.8. Sulfonated diamines used in the synthesis of sulfonated polyimides
………………………………………………………………………...….21
Figure 1.9. Structural approach for the synthesis of rigid rod liquid crystalline
polyimides………………………………………………………………...23
Figure 1.10. Room temperature proton conductivity for sulfonated polyimide
Copolymers ………………………………………………………….….24
XV Figure 2.1. TGA curves of poly(phenylene sulfide sulfonic acid)…...... 38
Figure 2.2. Relative humidity dependence of the conductivity (ó) of poly-
(phenylene sulfide sulfonic acid) (m = 2.0) and Nafion® 117…...... 39
Figure 2.3. Temperature dependence of the conductivity of of fully hydrated
poly(phenylene sulfide sulfonic acid). m =1.2 (○) and m = 2.0 (●)....40
Figure 2.4. Synthetic routes toward PPP’s……………………………………...….42
Figure 2.5. Nickel catalyzed synthesis of PPP’s with magnesium (a) and with
Zinc (b)…………………………………………………………………….44
Figure 2.6. Palladium catalyzed aryl-aryl cross-coupling synthesis of PPP’s…..45
1 Figure 2.7. H-NMR spectrum (300 MHz) of DBBDSA, disodium salt in D2O….81
13 Figure 2.8. C-NMR spectrum (300 MHz) of DBBDSA, disodium salt in D2O…81
Figure 2.9. FT-IR spectrum of DBBDSA, disodium salt in a KBr pellet………….82
1 Figure 2.10. H-NMR spectrum (600 MHz) of DBBDSA, dilithium salt in D2O…82
13 Figure 2.11. C-NMR spectrum (600 MHz) of DBBDSA, dilithium salt in D2O...83
Figure 2.12. FT-IR spectrum of DBBDSA, dilithium salt in a KBr pellet…………83
1 Figure 2.13. H-NMR spectrum (600 MHz) of DBBDSA, diBTMA salt in D2O….84
Figure 2.14. FT-IR spectrum of DBBDSA, diBTMA salt in a KBr pellet…………84
XVI 1 Figure 2.15. H-NMR spectrum (300 MHz) of DBBDSA, diTBP salt in DMSO-d6.
…………………………………………………………………………….85
Figure 2.16. FT-IR spectrum of DBBDSA, diTBP salt in a KBr pellet……………85
Figure 2.17. 1H-NMR spectra (from 600MHz except lot 1(300MHz)) of PPDSA,
disodium salt of lots 1, 2, and 3 (high and low molecular weight
polymer) in D2O at 25°C (concentration of PPDSA: 4.4 g/dL)….…..88
Figure 2.18. 13C-NMR spectrum (600MHz) of PPDSA, diprotonated form (lot 2)
in D2O at 25°C………………...... ……………89
Figure 2.19. Rheograms for aqueous PPDSA (lot 3, high molecular weight
polymer) solutions at different concentrations and shear rates……..90
Figure 2.20. Reduced viscosities as a function of concentration for different salt
forms of PPDSA (lots 1, 2 and 3) in different solvents at 35°C..……91
Figure 2.21. Reduced viscosities of PPDSA, diprotonated (lot 3, high molecular
weight polymer) in D.I. water at 35°C using different viscometers...92
Figure 2.22. Effect of salt concentration on reduced viscosities of PPDSA (lot 2)
in DMF……...... …..93
Figure 2.23. Effect of solvent on the reduced viscosity of PPDSA (lot 2)…...... 94
XVII Figure 2.24. Effect of PPDSA counterion on reduced viscosities of PPDSA
(lot 1)…...... 95
Figure 2.25. Comparison of the reduced viscosities of lots 1, 2 and 3,
diprotonated PPDSA in D.I. water at 35°C using 0C C453 viscometer
...... 96
Figure 2.26. GPC chromatogram of DBBDSA-Li in DMF:………...... …….97
Figure 2.27. GPC chromatograms of PPDSA (lot 1), disodium salt under different
elution conditions………...... …...... …99
Figure 2.28. Tests to determine the minimum storage time in controlled humidity
chamber for water uptake test……………...... …….103
Figure 2.29. Lambda (λ) of PPDSA films as a function of relative humidity. Lot 2
PPDSA had higher molecular weight than lot 1…………….………105
Figure 2.30. Plots of dimensional changes of PPDSA film (lot 2) vs. relative
humidity………………...... ………..108
Figure 2.31. DSC thermograms for melting of a) bulk water and b) absorbed
water in equilibrated PPDSA films (lot 3, high molecular weight
polymer) at different humidities………...... ….110
XVIII Figure 2.32. DSC thermograms for vaporization of a) bulk water and b) absorbed
water in equilibrated PPDSA films (lot 3, high molecular weight
polymer) at different humidities……………………...... ……111
Figure 2.33. TGA thermogram of PPDSA film (lot 2)………………...... ….112
Figure 2.34. WAXD diffractograms of materials studied for sealing………....…114
Figure 2.35. WAXD diffractograms in transmission mode of PPDSA (lot 2)..…119
Figure 2.36. WAXD diffractograms in reflection mode of PPDSA (lot 2)………120
Figure 2.37. 2D X-ray diffraction images of PPDSA (lot 2) at different relative
humidities…………...... …….123
Figure 2.38. Plot of meridional and equatorial d spacing calculated from 2D X-ray
photographs vs. relative humidity,…………………...... 124
Figure 2.39. OPM images of PPDSA (lot 2) under cross-polarized light:…...…125
Figure 2.40. OPM images of PPDSA films (lot 3, high molecular weight polymer)
under cross-polarized light……...... …………131
Figure 2.41. OPM image of PPDSA (lot 3, high molecular weight polymer
aqueous solution (38.51 g/dL) (X100) under cross-polarized
light…...... ….132
XIX Figure 2.42. Example of AC impedance measurement results for parallel-cut film
at 50°C and 50%RH with 50mV AC amplitude:……...... ………..135
Figure 2.43. Complex plots for PPDSA films (lot 2) obtained from 4-probe
impedance measurement at different humidities (15, 35, 50 and
75%RH) and at different temperatures (room temperature, 50 and
75°C)……………...... ……..138
Figure 2.44. Conductivity at different relative humidities: Effect of the plastic plate
used in the cell assembly ……………...... ……..139
Figure 2.45. Proton conductivities of PPDSA film from different lots at different
relative humidities and temperatures……………...... ………..141
Figure 2.46. Effect of the casting direction vs. measuring direction on
conductivity. a) lot 2, and b) lot 3 (high molecular weight
polymer)…………………...... ….144
Figure 2.47. Stress-strain curves of humidified PPDSA film (lot 3, high molecular
weight polymer) at 15 and 35%RH at room temperature…………..146
Figure 2.48. Proposed Ullmann coupling reaction mechanism for PPDSA…...155
XX Figure 2.49. Shear rate dependent viscosity of isotropic (---) and an.isotropic (—)
solutions of poly(p-phenylene terephthalamide in sulfuric acid at
60°C…...... 156
Figure 2.49. Typical three regions of flow behavior of LCP solution……….…..158
Figure 2.50. Viscosities plots for PBPDSA at different salt concentrations…...162
Figure 2.52. Viscosity behavior of nematic p-azoxyanisole in tubes surface
treated for perpendicular orientation………...... 164
0.5 0.5 Figure 2.53. Plot of ηsp/C to C of aqueous PPDSA, solution (lots 1, 2 and 3)
at 35°C……...... …………..169
Figure 2.54. Effect of PPDSA counterion on reduced viscosities of PPDSA (lot 1)
0.5 0.5 in plot of ηsp/C vs. C ……...... ………….170
Figure 2.55. Effect of solvent on the reduced viscosity of PPDSA in plot of
0.5 0.5 ηsp/C vs. C …………...... ……………170
Figure 2.56. Effect of salt concentration on the reduced viscosity of PPDSA
0.5 0.5 solution in plot of ηsp/C vs. C …………...... ……………171
Figure 2.57. Deconvoluted 1H-NMR spectra of PPDSA, disodium salt of lots 1, 2,
and 3 (high and low molecular weight) in D2O at 25°C (concentration
of PPDSA: 4.4 g/dL)……………...... ……174
XXI Figure 2.58. Comparison of chemical shifts of peaks in the 13C-NMR spectra of
monomer (DBBDSA-Li) and its polymer (PPDSA)…………....…….181
Figure 2.59. Lambda (λ) of PPDSA films and Nafion 117 as a function of relative
humidity…...... 184
Figure 2.60. Schematic of proton transport mechanisms…...... 185
Figure 2.61. Schematic of water gradient in reduction and oxidation in Fuel
Cell...... 187
Figure 2.62. Pressure DSC thermograms for (a) BPSH-40 and (b) Nafion-1135
as a function of water content……...... …….…………..188
Figure 2.63. Plots molar weight (a) and molar volume (b) vs. lambda of PPDSA
(lot 2) using data in Table 2.26……...... …………198
Figure 2.64. Plot of the measured molar volume changse vs. the calculated
molar volume changes of PPDSA (lot 2) at different humidities.….200
Figure 2.65. Plot of the molar weight change vs. the molar volume change of
PPDSA (lot 2)………………...... ………….………..201
Figure 2.66. Plot of d spacing vs. lambda for peak A at different humidities (0 to
75%RH)………...... ………………….……………..204
XXII Figure 2.67. Comparison of d spacings calculated from the 2D X-ray spectra with
the d spacing from WAXD data (in transmission mode) at different
humidities (0 to 50%RH)…………...... ……………..205
Figure 2.68. Schematic for a) peak C1, and b) peak C2………...... ……208
Figure 2.69. a) The chemical structure of a sulfonated poly(p-phenylene), b)
Model of the hexagonal column made by three strands of sulfonated
poly(p-phenylene)……...... ………………………209
Figure 2.70. Schematic of the proposed hexagonal packing structure in solid
PPDSA………………...... ………….210
Figure 2.71. Molecular structure of PPDSA………………………………………212
Figure 2.72. Plot of the ratio of molar volume change of PPDSA (lot 2) vs. (ratio
of d spacing change)2 at different humidities…………...... ………214
Figure 2.73. Plots of the ratio of d-spacing at different humidities (from 15%RH to
75%RH) as a function of lambda……………………………………..216
Figure 2.74. Plots of (d spacing)2 vs. lambda for PPDSA (lot 2) at different
humidities...... 219
Figure 2.75. Plots of (d spacing)2 of PPDSA (lot 2) vs. lambda…………..…….223
XXIII Figure 2.76. Proton conductivities of PPDSA film of different lot at different
conditions:……………………………………………………….………229
Figure 2.77. The ln[σ (conductivity)] plot for PPDSA strips (lot 2) as a function of
temperature (1/T)…………………………………………..…………..232
Figure 2.78. The ln[σ (conductivity)] plot for PPDSA strips (lot 3, high molecular
weight polymer) as a function of temperature (1/T)………..……….233
Figure 2.79. Activation energy in PPDSA films (lots 2 and 3) as a function of
relative humidity…………………………………………..…………….236
Figure 2.80. Corrected and uncorrected conductivities of PPDSA film (lot 2)
using a) parallel and (b) perpendicular cut films to the casting
direction………………………………………………………………….240
Figure 2.81. Comparison with the intrinsic conductivities and the measured
conductivities for PPDSA film (lot 2) with a) parallel and (b)
perpendicular cut films to the casting direction………….…………..244
Figure 2.82. The intrinsic conductivities of PPDSA (lot 2), Nafion 117 and
PBPDSA…………………………………………………………………245
XXIV A New class of Polyelectrolyte: Poly (p-phenylene disulfonic acids)
Abstract
by
JUNWON KANG
A water soluble polyelectrolyte, PPDSA [poly(p-phenylene-2,5-disulfonic acid)], was synthesized using Ullmann coupling between DBBDSA (1,4-dibromo-
2,5-benzenedisulfonic acid). For the design of PPDSA, two concepts were adopted: highly sulfonated and rigid rod liquid crystalline polymer. It has a high ionic exchange capacity, 8.46meq/g, and the positions of the sulfonic acids on its backbone were well controlled.
Its solution properties were studied using viscosity, rheometric measurements, and optical polarizing microscopy. Due to the rigid rod structure, its reduced viscosities increased at low concentration under all conditions.
PPDSA solutions showed shear rate dependent viscosity, and concentrated solutions were lyotropic with a typical nematic structure using crossed polarization.
PPDSA 1H-NMR spectra had many overlapping peaks that were analyzed by deconvolution. The highest number average degree of polymerization was
142. But, 1H-NMR spectra are not fully understood due to the local organization
XXV in solution. However, 13C-NMR showed a linear para-coupled PPDSA structure with only three 13C resonances.
PPDSA membrane properties were characterized as a function of temperature and relative humidity. Its water contents per acid group were about two water molecules more than that of Nafion between 15 and 75%RH at room temperature. DSC results showed that at least 8.45 H2O per sulfonic acid were strongly bound in the polymer.
The PPDSA hexagonally packed nematic liquid crystal polydomain structure with frozen-in free volume is a possible reason for the high water retention. This was suggested based on WAXD, 2D X-ray and dimensional changes. The inter-chain distance increased from ~8 to ~11Å as humidity increased. The structure consists of nano-size channels lined with sulfonic acids.
The free volume in the polymer was shown by the fact that the absorbed water had a density of about 1.23g/cc.
Its conductivities were extremely high at low humidities, ~0.01 S/cm at
15%RH and room temperature, and ~0.1 S/cm at 15%RH and 75°C. PPDSA with more than 4.5 water molecules per sulfonic acid had higher conductivity than
Nafion. PPDSA is very thermally stable (TGA decomposition temperature: ~
304°C at 10°C/min). Stress-strain measurements showed that the film had reasonable elongation for a linear rigid rod polymer.
XXVI Chapter 1
Chapter 1. Introduction
1.1. Introduction: Basics of Fuel cells
The successful conversion of chemical energy into electrical energy in a primitive fuel cell was first demonstrated1 over 160 years ago. Fuel cells are receiving considerable attention these days because of their high efficiency, high power density and pollution free fuel utilization.2 However, in spite of the attractive system efficiencies and environmental benefits associated with fuel cell technology, it has proved difficult to develop commercially viable industrial products. These problems have often been associated with the lack of appropriate materials or manufacturing routes that would enable the cost of electricity per kWh to compete with the existing technology, as outlined in a recent survey.3
Figure 1.1. Schematic of hydrogen fuel cell.
1 Chapter 1
A fuel cell is an electrochemical energy conversion device that produces
electricity, water and heat using fuel (hydrogen) and oxygen in the air. The basic
operating principle of a hydrogen fuel cell can be summarized as follows (see in
Figure 1.1): hydrogen (a fuel) flows into the fuel cell system on the anode side, platinum catalyst promotes the separation of the hydrogen gas into electrons and
protons. The protons, which pass through the electrolyte, combine with oxygen
and electrons at the cathode side, and produce water. The electrons, which
cannot pass through the electrolyte, flow from the anode to the cathode through
an external circuit containing an electric load which consumes the power
generated by the cell. Water is the only product of the oxidation and reduction
reaction at each electrode. The general electrochemical reactions occurring at
the electrodes are as follows:
+ - at anode: H2 → 2H +2e (the oxidation reaction);
+ - at cathode: 0.5 O2 + 2H +2e → H2O (the reduction reaction);
overall: H2 + 0.5 O2 → H2O + Eelectric + Eheat
where Eelectric and Eheat are the electrical energy and the heat energy, respectively.
The types of fuel cells under active development, and their features, are
summarized in Table 1.1. The nature of the electrolyte determines key properties,
particularly the operating temperature of the fuel cells. Five different types of fuel
cells can be categorized by their electrolyte materials: alkaline fuel cells (AFCs),
polymer electrolyte fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs),
2 Chapter 1 molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs). Each of these fuel cells operates at a different temperature range and was developed for particular applications.
Fuel cell Operating Electrolyte tempera- Application Advantages Disadvantages Type ture (°C)
Solid electrolyte Low temperature Polymer Solid Electric utility, reduces corrosion and requires expensive 60- Electrolyte organic Portable power, management catalysts. High 100 (PEMFC) polymer Transportation problems. Low temp. sensitivity to fuel Quick start-up. impurities
Aqueous solution of Cathode reaction Expensive removal of Alkaline potassium 90- Military, faster in alkaline CO from fuel and air (AFC) hydroxide 100 Space electrolyte- so high 2 streams required soaked in performance a matrix
Liquid Up to 85% efficiency in Phosphoric phosphoric Pt catalyst. Low current 175- Electric utility, co-generation of acid acid and power. Large 200 Transportation electricity and heat. (PAFC) soaked in size/weight Impure H as fuel a matrix 2
Liquid solution of lithium, High temperature High temperatures Molten sodium promotes higher 600- enhances corrosion and carbonate and/or Electric utility efficiency and flexibility 1000 breakdown of cell (MCFC) potassium to use more types of components carbonates fuels and catalysts soaked in a matrix
High temperature Solid advantages (see High temperature Solid oxide 600- zirconium Electric utility MCFC). Solid enhances breakdown of (SOFC) 1000 oxide electrolyte advantages cell components (see PEMFC)
Table 1.1. Various fuel cell systems categorized by the electrolyte used.
3 Chapter 1
PEMFCs employing a solid polymer electrolyte were first deployed for the
Gemini space program in the early 1960s. But these cells were too expensive
and had too short a lifetime for real applications due to the oxidative degradation
of their sulfonated polystyrene-divinylbenzene copolymer membranes. Nafion®, a widely available DuPont product, has served as the PEM platform of choice for the development of highly active catalyst layers, gas diffusion layer optimization, component design, and system demonstration. However, as PEMFCs have approached commercialization and the far reaching applications of fuel cell are explored, alternative PEM materials which provide advantages in terms of system performance, cost, and operational flexibility have been needed.
In recent years, polymer electrolyte fuel cells (PEMFCs) have received considerable attention because of their high efficiency, high power density and relatively low operating temperatures for mobile and stationary engines.2, 4-6 But, the major issue for use as a viable power source is the development of materials with low cost and high performance.6-8 Of particular interest to material scientists
and engineers in fuel cell research are the new polymer electrolyte membranes
that have improved performance, durability and reliability of PEMFCs.
PEMFCs are being developed for different applications (stationary, mobile
and portable); each application has its own operating condition and material
requirement. However, common critical themes should be considered for all high
performance proton exchange membrane; high protonic conductivity, low
electronic conductivity, low permeability to fuel and oxidant, low water transport
through diffusion and electro-osmosis, oxidative and hydrolytic stability, good
4 Chapter 1 mechanical properties in both the dry and hydrated states, cost and capability for fabrication into membrane electrode assemblies (MEAs). The protonic conductivity targets currently established by U.S. Department of energy are given in Table 1.2.9
2005 Characteristics Unit 2010 2015 status Relative humidity %RH 100 <10 <10
Operating temp. °C <80 ≤120 ≤120 Membrane conductivity at inlet water vapor partial pressure and a) Operating temperature S/cm 0.10 0.10 0.10
b) 20°C S/cm 0.07 0.07 0.07
c) -20°C S/cm 0.01 0.01 0.01
Table 1.2. Technical protonic conductivity target for PEMFCs’ membranes.9
5 Chapter 1
1.2. Nafion® and other perfluorosulfonated polymers
Nafion® is a DuPont product that was developed in the late 1960’s. Its
poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical
stability, which is important in fuel cell applications. Nafion® is a free radical
initiated copolymer with crystallizable hydrophobic tetrafluoroethylene (TFE)
backbone sequences and a unit that contain a pendant side chain of
perfluorinated vinyl ethers with a terminal sulfonic acid. The basic perfluorinated
chains of such polymers possess high chemical and thermal stability, while the
side chains have super acid groups. The ion content can be varied by changing
the ratio of the two components (the TFE and sulfonated monomer).
Perfluorinated electrolyte membranes with the general formula (Figure 1.2) are
also widely used.
Commercial Nafion® which has relatively high proton conductivity and moderate swelling in water, is a copolymer containing an equivalent repeat unit molecular weight of 1100 (~87 mol% of TFE) and is supplied in films with dry thicknesses of 175μm, 125μm and 50μm, designated as Nafion® 117, Nafion®
115 and Nafion® 112, respectively. This designation is derived from a membrane’s equivalent weight divided by 100 (first 2 digits) and its thickness expressed in thousandths of an inch, mils (third digit). These films are prepared
by the extrusion of the polymeric sulfonyl fluoride precursor which is then
hydrolyzed with base and acidified to the final perfluorosulfonic acid form. Modest retention of semicrystalline morphology is crucial to both mechanical strength and proton conductivity.10 The thinner membranes are generally preferred for
6 Chapter 1
hydrogen/air applications to minimize ohmic losses, while thicker membranes are
employed for direct methanol fuel cells (DMFC’s) to reduce crossover. Like many
other fluoropolymers, Nafion® is quite resistant to chemical attack, but it is its
strong perfluorosulfonic acid groups that confer most of its desirable properties
as a proton exchange membrane. Fine dispersions (sometimes incorrectly called
solutions) can be generated using alcohol/water treatments.11 Such dispersions
are commonly used in the preparation of interfaces between catalyst electrodes
and proton exchange membrane. Films prepared from these dispersions show very different properties from those of the extruded films. The success of Nafion®,
whose main use is as an ion separation membrane in industrial chlor-alkali
electrochemical cells, as a PEM has stimulated the development of other
perfluorosulfonated polymers by Asahi Glass Company, Asahi Chemical
Company and Dow Chemical Company. The structures and equivalent weights of these membranes are listed in Table 1.3.7 The Dow membrane has a shorter side
chain than that of Nafion® and the other perfluorosulfonated polymers.12 All of the
perfluorosulfonic acid membranes are expensive and have the same
disadvantages as Nafion®, which are low conductivity at low water content, and
relatively low mechanical strength at higher temperature.
7 Chapter 1
CF2 CF2 CF CF2 x y
OCF2 CF O CF2 SO3H m n CF 3
Figure 1.2. Chemical structure of perfluorinated polymer electrolyte membranes.
Equivalent weight Membrane x y m n (g/[SO3H])
Nafion 6 – 10 1 1 2 1000 – 1200
Flemion23 3 – 10 1 0.1 1 – 5 1000
Aciplex-S 1.5 – 14 1 0.3 2 – 5 1000 – 1200 Dow 3 - 10 1 0 2 800 -850 membrane
Table 1.3. Various commercial polymer electrolyte membranes: the structure of each polymer was based on the general formula in Figure 1.2. x, y, m, and n are the number of repeating unit in Figure 1.2.
8 Chapter 1
The high conductivity and water uptake of these materials are attributed
to their solid state morphology, microphase separated hydrophobic (fluorocarbon rich) and hydrophilic (sulfonic acid rich) domains. The widely accepted “reverse- micelle-like” cluster network model, proposed by Hsu and Gierke13, 14, postulated
that the fluorocarbon rich domains confer mechanical sturdiness to the
membrane, while proton conduction occurs exclusively within the sulfonic acid
rich hydrated domains. In an ionic cluster, water molecules are found in the core,
and sulfonate groups are bound near the interface, extending into the water
phase. Neighboring clusters are presumably interconnected through channels
that determine the transport properties of the ions and water (Figure 1.3, a)). A
swelling mechanism was proposed, involving a structural reorganization, which is
characterized by a cluster number decrease and an inter-cluster distance
increase. A conversion from isolated cluster to percolation structure occurs during
swelling and from percolation to isolated cluster structure in the dehydration
process (Figure 1.3, b)).13, 15 For instance, a percolation structure (Figure 1.3,
b1)), which has well-connected cluster morphology, has higher ionic-exchange
and water-diffusion rates than the isolated cluster structure (Figure 1. 3, b2)).16
For Nafion® at very low water levels, the inter-cluster channels may disappear,
and the clusters tend to be isolated from each other. The extent of cluster
structure has a marked influence on the physical properties of Nafion®, such as its ion exchange ability and rheological responses.
9 Chapter 1
(a)
(b1) (b2)
Dehydrating
Water swelling
Figure 1.3. The cluster network model: a) “reverse-micelle-like” cluster network model.13 b) Schematic dynamic swelling and dehydration model for Nafion-Na at low water level. (+,-) denote ionic groups. (b1) represents the percolation structure, with interconnected clusters, and (b2) represents the structure with isolated clusters. (b1) and (b2) have the same specific surface.13, 15
The supporting evidence for the percolation structure for Nafion® comes from small-angle X-ray scattering (SAXS) studies, which also provided an estimate for the cluster diameter of ca. 40 Å and a channel diameter of ca. 10 Å
(Figure 1.3.(b)).15 Obviously, the cluster size and degree of interconnection are
determined by water content and change during the water swelling and
dehydration processes.17, 18
10 Chapter 1
All fluorinated membranes in their fully hydrated state, including Nafion®
-2 -1 have water uptakes above 15 H2O/SO3H and proton conductivities of 10 to 10
S/cm. However, their conductivity decreases dramatically above 80ºC due to loss of absorbed water, which promotes a decrease of interconnection between neighboring sulfonic acid rich domains within the polymer membrane, see Figure
1.3, b2). This major drawback, along with the perfluorosulfonic acids’ high cost and polluting manufacturing processes, has driven the development of new polymer electrolytes. The goal of these research efforts is to develop materials with higher proton conductivities, better mechanical properties and dimensional stability than for Nafion® and the other similar polymers have, especially at
temperatures above 80ºC. Most of these efforts, strive to emulate the
morphology found in Nafion®, since many scientists in the field believe that the microphase separated systems are the best way to balance conductivity and mechanical properties.6, 7, 8, 19
11 Chapter 1
1.3. Sulfonated aromatic condensation polymers (ACPs)
Nafion® and other perfluorosulfonated membranes have been widely
used for fuel cell studies, but several shortcomings remain; high cost, low
conductivity at low humidity, etc. A promising way to overcome these problems
involves preparation of membranes based on aromatic condensation polymers
(ACPs). ACPs have some attractive advantages. They are cheaper than
perfluorinated polymers. They contain polar groups that have high water uptakes
over a wide temperature range, and ACP decomposition can be raised to high
temperature by proper molecular design. A number of papers have reviewed the
possibility of ACPs for proton exchange electrolytes.7
Sulfonated aromatic condensation polymers have been proposed by two ways: postsulfonation of aromatic condensation polymers and synthesis of sulfonated aromatic condensation polymers based on sulfonated monomers. In following sections (sections 1.3.1 and 1.3.2) these approaches will be discussed.
12 Chapter 1
1.3.1. Postsulfonation of aromatic condensation polymers
The simplest and most widely used methods for the synthesis of
sulfonated ACPs involves sulfonation of different classes of polymers, such as
poly(1,4-phenylenes),20, 21 poly(p-xylylene),22 poly(1,4-oxyphenylene),23 poly(ether ether ketone) (PEEK),24-26 poly(arylene ether sulfone),27, 28 poly(phenylene sulfide),29 poly(phenylquinoxalines),30 poly(benzimidazoles)31 and some other ACPs. The structures of sulfonated poly(4-phenoxybenzoyl-1,4- phenylene) (S-PPBP), poly(p-xylene) (S-PPX), poly(phenylene sulfide) (S-PPS), poly(phenylene oxide) (S-PPO), poly(ether ether ketone) (S-PEEK), poly(ether ether sulfone) (S-PEES) arylsulfonated poly(benzyimidazole) (S-PBI) and sulfonated poly(phenylquinoxaline) (S-PPQ) are shown in Figure 1.4.
ACPs are sulfonated using standard sulfonation agents.32, 33 In particular,
PEEK can be sulfonated with concentrated sulfuric acid,34 chlorosulfonic acid,35
35, 36 SO3 (either pure or taken in a mixture), a mixture of methanesulfonic acid with concentrated sulfuric acid,37 and acetyl sulfate.38, 39 But direct sulfonation
has some limitations. First, sulfonation with chlorosulfonic or fuming sulfuric acid
is sometimes accompanied by polymer degradation. Second, direct sulfonation
can not be used for preparation of randomly sulfonated copolymers at sulfonation
levels of less than 30%, since sulfonation in sulfuric acid occurs under
heterogeneous conditions. Last, sulfonation of PEEK in concentrated sulfuric
acid at room temperature is accompanied by incorporation of no more than one
sulfonic acid group into each repeat unit of the polymer.26, 35, 38
13 Chapter 1
SO3H
O HO3S
S O CH2 CH2 n n n CO (HO S) (HO3S)m 3 m S-PPX S-PPS S-PPO n
S-PPBP
O O OCO OSO O n n SO3H SO3H S-PEEK S-PEES
N N
SO3H SO3H N N N N O N N n SO H SO H 3 3 n S-PPQ S-PBI
Figure 1.4 . Chemical structures of sulfonated aromatic condensation polymers.
14 Chapter 1
1.3.2. Synthesis of sulfonated aromatic condensation polymers based on
sulfonated monomers
1.3.2.1. Sulfonated poly(ether ether ketone) (S-PEEK)
Postsulfonation reactions are usually restricted due to their lack of precise
control over the degree and location of functionalization, plus the possibility of
side reactions.
The first report of the required sulfonated monomer, in 198340, was as a
component in a flame retardant polymer. More recently, Ueda and co-workers41 reported the sulfonation of 4,4’-dichlorodiphenyl sulfone and provided general procedures for its purification and characterization. McGrath’s group has modified this procedure to achieve disulfonation of the monomer and used this compound to produce a series of sulfonated poly(arylene ether sulfone) copolymers following the scheme shown in Figure 1.5.42, 43
NaO3SSO3Na O O Cl S Cl Cl S Cl HO OH O O
K2CO3 150C/ 4hr Toluene 190C/ 18-36hr NMP KO3SSO3K O O S O O S O O
O x O y n H2SO4
HO3SSO3H O O S O O S O O
O x O y n
Figure 1.5. Synthesis of directly copolymerized wholly aromatic sulfonated poly(arylene ether sulfone) (BPSH).
15 Chapter 1
Copolymerizations were carried out using the potassium salt form of the 3,3’-
disulfonated-4,4’-dichlorodiphenyl sulfone. These random copolymers (BPSH)
displayed a hydrophilic/hydrophobic phase separated morphology that varied
depending on the degree of sulfonation and resembles very closely the
morphology found for Nafion® films.6 The conductivity and water uptake of these materials increased with disulfonation. However, once the fraction of sulfonated units reached 60 mol %, a semicontinuous hydrophilic phase was observed and the membranes swelled dramatically, forming a hydrogel that could not be used as a proton exchange membrane.6 These results indicate that protonic
conductivity, water swelling and mechanical properties in a copolymer must be
considered at the same time. As is the case for the unsulfonated polymers, many
variations are possible in the direct synthesis of sulfonated poly(arylene ether
sulfone)s thanks to the diversity of accessible monomers.
The BPSH copolymers seem to be a promising lower cost alternative to
the perfluorinated Nafion-like materials, since some of them exhibit comparable
proton conductivity and lower water uptake than Nafion®. A combination of these
properties may increase the dimensional stability of the PEM’s, (i.e. less swelling
than the swelling observed for Nafion® at comparable relative humidities) while
preserving acceptable proton conductivity values.
Although attractive, these materials suffer from the same inherent
drawback observed for their perfluorinated counterparts, they dehydrate
significantly when the relative humidity decreases. This behavior, although
unfortunate, is not completely unexpected, since is has been observed that the
16 Chapter 1
BPSH copolymers exhibit a phase separated morphology quite similar to the one
observed for the perfluorosulfonated polymers. It is, therefore, understandable
that, just as happens in Nafion®, the connectivity between the hydrophilic
domains within the membrane decreases sharply as the relative humidity
decreases producing an equally sharp decrease in the proton conductivity.
The results from the extensive work related to BPSH copolymers suggest that, in order to produce new PEM’s with improved proton conductivity and dimensional stability at low relative humidities, research efforts may need to focus on exploring not only new materials, but also new polymer morphologies.
1.3.2.2. Sulfonated poly(imides) (SPIs)
Aromatic five and six–membered ring sulfonated polyimides, one class of
high performance materials, have been studied to produce better and cheaper
proton exchange membranes. However, when sulfonated phthalic polyimides
(five-membered ring polyimides) were used, rapid hydrolytic degradation
occurred (hydrolysis leads to chain scission and decrease in molecular weight).
Naphthalenic polyimides (six-membered ring polyimides) were found to be more
stable in the highly acidic fuel cell environment.44, 45 Since the six-membered
naphthalenic polyimide is much more stable to hydrolysis, this structure may be
better suited for PEM fuel cell applications although its long term stability still
needs to be improved and systematically characterized.
Genies and coworkers45 used model compounds to examine the nature of
the hydrolysis mechanism associated with the sulfonic acid in five and six-
17 Chapter 1
membered ring polyimides. Model compounds of the sulfonic acid containing
phthalimide (model A) and the sulfonic acid containing naphthalenic imide (model
B) were prepared via a one step high temperature condensation in m-cresol, see
Figure 1.6.
O O O O SO3 Et3NH O O
O O + 2 NH2 N N +2H2O m-cresol O O HO3S Et3N Et3NH O3S O O Model A
O O O O SO3 Et3NH
+2HO O O +N2 H2 N N 2 m-cresol O O O O HO3S Et3N Et3NHO3S
Model B
Figure 1.6. Synthesis of imide compounds (model A and B).
13C-NMR was used to quantitatively determine the amount of imide, amic
acid and diacid as a function of time during the aging process of model A.13C-
NMR showed that the model A NMR spectrum changed after the solution was kept at 80°C for 1 hour, and the carbon signals associated with the starting compound disappeared after aging for 10 hours. In addition, during the first 2 hours of aging, amic acid formed preferentially to the diacid. This is intriguing, since the amic acid should easily hydrolyze to the diacid in water. In contrast, the spectrum of model B showed structural changes only after 120 hour of aging at
80ºC. After 120 hours, two small doublets appeared in the 1H-NMR spectrum and
several additional peaks were visible in the 13C-NMR spectrum. Finally, using
18 Chapter 1
NMR and IR spectroscopy it was determined that the hydrolysis products were an imide/carboxylic acid and an imide/anhydride. The relative intensity of carboxylic acid reached a constant value after 1200 hours at 80ºC. The authors suggest that equilibrium between model B imide and its hydrolysis products was established, and limited the hydrolysis to about 12%. This critical fraction is likely enough to cause a significant decrease in the molecular weight and therefore the mechanical properties of the resulting materials
Genies et.al.46 designed a synthetic method to produce random and block sulfonated copolyimides (SPIs). The synthetic procedure for their most studied copolymer is illustrated in Figure 1.7.
HO3S Et3NH O3S
Et3N H2N NH2 H2N NH2 4hr/ RT m-cresol SO3H Et3NH SO3
O O
80C/ 4hr 180C/ 15hr O O benzoic acid O O
O O
O O 80C/ 4hr 180C/ 15hr O O
O H2N NH2
Et3NH O3S O O O O O N N N N
Et3NH SO3 O O O O n m
Figure 1.7. Synthesis of sulfonated polyimides based on BDA, ODA and NDA.
19 Chapter 1
The first step of the synthesis involved preparation of short sequences of
4,4’-diamino-2,2’-biphenyl disulfonic acid (BDA) condensed with 1,4,5,8-
naphthalene tetracarboxylic dianhydride (NDA). An adjusted ratio of these two
monomers allows one to create different block lengths of the sulfonated
sequence. In the second polymerization step, the degree of sulfonation was
precisely controlled by regulating the molar ratio of BDA and the unsulfonated
diamine, which was 4,4’-oxydianiline (ODA) in their SPI. The degree of
sulfonation is an important factor controlling swelling.47
Preliminary investigations suggested that six-membered ring polyimides
had some promise as PEM’s; however, their poor solubility limited membrane
formation and subsequent use in fuel cells. The SPI shown in Figure 1.7 is only
soluble in chlorophenol. It was observed that the structure of the unsulfonated
diamine added in the second step of the polymer synthesis had a large effect on
improving the solubility of the resulting compounds and several bulky or kinked
unsulfonated diamines were tested.46
The preparation of sulfonated polyimides with different ion exchange
capacities and sulfonated block lengths was also considered. The solubility of polyimides can be improved by introducing a bulky group into the polymer backbone and controlling the microstructure of the polymer chain (random vs. blocky). For a given polymer structure, the water uptake increases with the ionic content, but the number of water molecules per sulfonic acid (λ) remains constant, suggesting that water is mainly located in the hydrophilic domains. It was found that both λ and proton conductivity are systematically lower for random than for
20 Chapter 1 block copolymers.
Recently, Okamoto et al. reported some SPIs using more basic, flexible or kinked sulfonated diamines instead of the commercially available 4,4’-diamino-
2,2’-biphenyl disulfonic acid (BDA).48-51 The chemical structures of these new diamines are shown in Figure 1.8.
HO3S 4,4’-diamino-2,2’-biphenyl disulfonic acid H2N NH2 (BDA) SO3H
H2N NH2
9,9'-Bis(4-aminophenyl)fluorene-2,7-
HO3S SO3H disulfonic acid (BAPFDS)
HO3S 4,4'-diaminodiphenylether H2N ONH2 -2,2'-disulfonic acid (ODADS) SO3H
HO3S NH2 2,2-bis[4-(4-aminophenoxy) CF3 phenyl]hexafluoropropane O C O disulfonic acid (BAPHFDS) CF3 H2N SO3H
Figure 1.8. Sulfonated diamines used in the synthesis of sulfonated polyimides.
A comparison of the hydrolytic stability of several six membered imide
SPI’s has been reported.49 The membranes were placed in distilled water at 80ºC
21 Chapter 1
until a loss of mechanical properties was observed. Improvements in membrane stability were observed for polymers with lower degrees of sulfonation (lower ion exchange capacity, IEC) and for random copolymers, as opposed to block or sequenced copolymers. Moreover, the flexibility of the sulfonated diamine in the polymer structure was found to play an important role in water stability. By simply changing the sulfonated diamine from the rigid BDA to the more flexible ODADS, the stability in water improved significantly. On the other hand, copolymers made from BAPFDS, a rigid and bulky sulfonated diamine, showed stabilities similar to that of the ODADS series at comparable ion exchange capacities. The authors suggest that the polyimides derived from BAPFDS should display higher hydrolytic stability due to the higher basicity of the sulfonated diamine.50 A more basic diamine would increase the electron density of the imide ring, making the hydrolysis reaction less favorable and thus increasing the water stability of the polymer.
Although the SPI’s synthesized by the Okamoto group showed an
improvement in proton conductivity, there have been no proposed structural
explanations for the differences in proton conductivities. The main contribution of
the work from Okamoto’s group was the observation that increasing the basicity
of the diamine and the flexibility of the polymer chains rendered the polyimides more hydrolytically stable.
22 Chapter 1
A fundamentally different approach to the use of sulfonated polyimides as
PEM’s was proposed by Litt and coworkers in 1999.52-54 Their idea consisted in
using rigid-rod nematic liquid crystalline polyimides, where a small amount of
bulky or angled comonomers was used to generate nano-size non-collapsible channels lined with sulfonic acid groups. These nano-size channels increased water absorption, and thus the proton conductivity of the polyimide membranes, especially at low relative humidities and high temperatures. Figure 1.9 illustrates the structural approach for copolymer synthesis with some of the bulky and angled comonomers.
Complete collapse when dry.
Bulky groups keep chains apart.
O
Small linear Bulky Angled comonomer comonomer comonomer
Figure 1.9. Structural approach for the synthesis of rigid rod liquid crystalline polyimides.
23 Chapter 1
Figure 1.10. Room temperature proton conductivity for sulfonated polyimide copolymers.56
The resulting conductivity and mechanical properties of these
membranes are very encouraging, the membranes containing 5% of the
hexaphenylbenzene units (bulky comonomer, blue in Figure 1.9) showed a
proton conductivity of 0.8 S/cm at room temperature and 100% RH while
maintaining a conductivity >0.001S/cm at room temperature and 15%RH. The high values of proton conductivity observed at low relative humidities (18 times larger than the value for Nafion®) were attributed to the “fixed” free volume present in the membrane as a result of the rigid nature of the polymer molecules, see Figure 1.9. The inter-chain spacing was found to increase as the bulkiness of the comonomer increased. The authors demonstrated that they had achieved a solid state structure inherently different from that commonly found in other polymers being studied as candidates for use as PEM’s. The materials produced
24 Chapter 1 by the Litt group seem to organize in such a way that the membranes exhibit non-collapsible continuous hydrophilic domains that promote high proton conductivity at low relative humidities. Although the structural approach produced very promising results, these membranes suffered from the same shortcomings common to all sulfonated polyimides, they hydrolyzed slowly. Attempts to increase the hydrolytic stability by using thermally and physically cross-linked membranes show only modest improvements.
25 Chapter 1
1.4. Concept of the structural approach used in the synthesis of poly(p-
phenylene sulfonic acids)
We decided to adopt the concept proposed previously by Litt group where
liquid crystalline rigid rod polymers were used in order to produce a nano-porous structure with stable free volume available to water (see section 1.3).52-54
The original structures based on this idea produced membranes with
significantly high proton conductivity values, but unfortunately turned out to be
hydrolytically unstable. However, the results encouraged us to extend this idea to
make the hydrolytic stable polymers. The polymer of choice to replace the
sulfonated polyimide backbone was poly(p-phenylene sulfonic acid). This
material could be synthesized using different transition metal mediated coupling
reactions, and was expected to produce liquid crystalline, thermally and
chemically stable membranes. The general approach for this thesis stems from
the application of the previously developed structural concept, the use of rigid rod
liquid crystalline sulfonated fully aromatic polymers to generate highly conductive
PEM’s.
26 Chapter 1
1.5. Overview of the dissertation
We propose the use of poly(p-phenylene-2,5-disulfonic acid) (PPDSA) to
generate a highly conductive proton exchange membrane for fuel cell
applications. Specifically, we have synthesized a new type of highly sulfonated
monomer and its homopolymer. The resulting polymer exhibits high IEC (ion
exchange capacity in meq/g), high proton conductivity (still relative high at low humidity), high water uptake, high water retention and a unique solid state
structure having nano-size channels. The new homopolymer overcomes some of
shortcoming of Nafion®.
The dissertation starts with Chapter 2, describing the design and
synthesis towards poly(p-phenylene-2,5-disulfonic acid) (PPDSA) and its
characterizations for intrinsic and membrane properties. This chapter will show
results and discuss the synthesis the monomer (DBBDSA, 1,4-dibromo-2,5-
benzenedisulfonic acid) and its polymer. Because the polymer properties are
related each other, first all results about intrinsic and membrane properties will be
shown and then will be discussed. Also, membrane properties will be discussed
and compared with Nafion in terms of water content.
Chapter 3 deals with general conclusions and future work. General
conclusions summarize the advantages and disadvantages of PPDSA. Future
work proposes possible ways to reduce or eliminate its shortcomings. The
synthesis of copolymers based on original monomer is suggested.
27 Chapter 1
1.6. References
1. Grove, W. R., Phil. Mag. Ser. 1839, 3(14), 127–130.
2. Zalbowitz, M., S. Thomas, Fuel Cells: Green Power, 1999, U.S. Department of
Energy.
3. B. C. H. Steele, J. Mater. Sci., 2001, 36, 1053–1068.
4. Carrette L., Friedrich K. A., Stimming U., Fuel Cells, 2001, 1(1), 5-39.
5. Dunn, S. Peterson, J. A., Micropower, the next Electrical Era, 2000, World
Watch Institute, Washington D.C.
6. Hickner, A.M., Ghassemi H., Kim, Y. S., Einsla B. R., McGrath J. E., Chem.
Rev., 2004, 104, 4587-4612.
7. Rikukawa R. M., Sanui K., Prog. Polym. Sci., 2000, 1463-1502.
8. Rusanov A. L., Likhatchev D. Y., Mullen K., Russian Chemical Reviews, 2002,
71(9), 761-774.
9. 2007 Multi-Year Research, Development and Demonstration Plan, U.S.
Department of Energy, 2007.
10. S. C. Yeo, A. Eisenberg, J. Appl. Polym. Sci., 1977, 21, 875-98.
11. Grot W. G., U.S. Patent, 1984, 4, 433, 082.
12. M. R. Tant, K. P. Darst, K. D. Lee, C. W. Martin, ACS Sym. Ser. 1989, 395,
370.
13. T. D. Gierke, W. Y. Hsu, ACS Symp. Ser., 180, American Chemical Society,
Washington, DC, 1982.
14. T. D. Gierke, W. Y. Hsu, Macromolecules, 1982, 15, 101-105.
15. G. Gebel, Polymer, 2000, 41, 5829.
28 Chapter 1
16. C. E. Bunker, H. W. Rollins, B. J. Ma, J. Photochem. Photobiol. A, 1999, 126,
71.
17. J. A. Elliott, S. Hanna, A. M. S. Elliott, G. E. Cooley, Polymer, 2000, 42, 2251-
2253.
18. G. Gebel, J. Lambard, Macromolecules, 1997, 30, 7914-7920.
19. Heitner-Wirguin, C. J. Membr. Sci., 1996, 120, 1-33.
20. Z. Qi, P. G. Pickup, Chem. Commun., 1998, 1, 15.
21. A. D, child, J. R. Reynolds, Macromolecules, 1994, 27, 1975.
22. V. A. Sochilin, A. V. Pdbalk, V. I. Semenov, M. A. Sevast’yanov, I. E. Kardash,
Vysokomol. Sodein., Ser. A, 1993, 35, 1480.
23. A. J. Chalk, A. S. Hay, J. Polm. Sci., A, 1982, 7, 5843.
24. H. Kobayashi, H. Tomita, H. Moriyama, A. Kobayashi, T. Watanabe, J. Am.
Cehm. Soc., 1994, 116, 3153.
25. M. I. Litter, C. S. Marvel, J. Polym. Sci., Polym. Chem. Ed., 1985, 23, 2205.
26. T. Ogawa, C. S. Marvel, J. Polym. Sci. Polym. Chem. Ed., 1985, 23, 1231.
27. C. Mottet, A. Revillon, P. Le Perchec, M. E. Lauro, A. Guyot, Polym. Bull.,
1982, 8, 511.
28. A. Noshay, L. M. Robeson, J. Appl. Polym. Sci., 1976, 20, 1885.
29. Z. Qi, M. C. Lefebre, P. G. pickup, J. Electrochem., 1998, 459, 9.
30. N. M.Belomoina, A. L. Rusanov, Vysokomol. Sodein., Ser. B, 1996, 38, 355.
31. U.S. patent, 4 634 530.
32. X. L. Wei, Y. Z. Wang, S. M. Long, C Bobeczko, A. J. Epstein, J. Am. Chem.
Soc., 1996, 118, 2545.
29 Chapter 1
33. E. E. Gilbert, in Sulfonation and Related Reactions (New York: Wiley-
Interscience, 1965, p. 78.
34. X. Jin, M. T. Bishop, T. S. Ellis, F. E. Karasz, Br. Polym. J., 1985, 17, 4.
35. J. Lee, C. S. Marvel, J. Polym. Sci., Polym. Chem. Ed., 1984, 22, 295.
36. B. C. Johnson, I. Ylgor, M. Iqbal, J. P. Wrightman, D. R. Lloyd, J. E. McGrath,
J. Polym. Sci., Polym. Chem. Ed., 1984, 22, 72.
37. C. Bially, D. Williams, F. E. Karasz, W. J. MacKnight, Polymer, 1987, 28, 1009.
38. W. A. Thaler, J. Polym. Sci., 1982, 20, 875.
39. W. A. Thaler, Macromolecules, 1983, 16, 623.
40. Robeson, L. M., Matzner, M., US Patent, 1983, US4,380,598.
41. Ueda M., Toyota H., Ochi T., Sugiyama J., Yonetake K., Masuko T., Teramoto
T., J. Polym. Sci., Polym. Chem. Ed., 1993, 31, 853.
42. Wang F., Hickner M., Kim Y. S., Zawodzinski T. A., McGrath J. E., J. Membr.
Sci., 2002, 197, 231.
43. Wang F., Hickner M., Ji Q., Harrison W., Mecham J.,Zawodzinski T. A.,
McGrath, J. E., Macromol. Symp., 2001, 175, 387.
44. Savadogo O., J. New Mater. Electrochem. Syst., 1998, 1, 47.
45. Genies C., Mercier R., Sillion B., Petiaud R., Cornet N., Gebel G., Pineri M.,
Polymer, 2001, 42, 5097-5101.
46. Genies C., Mercier R., Sillion B., Cornet N., Gebel G., Pineri M., Polymer,
2001, 42, 359.)
47. Cornet N., Diat O., Gebel G., Jousse F., Marsacq D., Mercier R., Pineri M., J.
New Mater. Electrochem. Syst., 2000, 3, 33.
30 Chapter 1
48. Zhou W., Watari T., Kita H., Okamoto K.-I., Chem. Lett., 2002, 5, 534.
49. Fang J., Guo X., Harada S., Watari T., Tanaka K., Kita H., Okamoto, K.-I.
Macromolecules, 2002, 35, 9022.
50. Guo X., Fang J., Watari T., Tanaka K., Kita H., Okamoto, K.-I.,
Macromolecules, 2002, 35, 6707.
51. Yin Y., Fang J., Cui Y., Tanaka K., Kita H., Okamoto, K.-I., Polymer, 2003, 44,
4509.
52. Zhang, Y., Litt M., Savinell R. F., Wainright J. S., Polym. Prepr., 1999, 40, 480.
53. Zhang Y., Litt M., Savinell R. F., Wainright J. S., Vendramint J., Polym. Prepr.,
2000, 41, 1561.
54. Zhang Y., Ph.D. Thesis, Case Western Reserve University, 2001, p151.
31 Chapter 2
Chapter 2. Design considerations and synthesis of poly(p-phenylene-2,5-
disulfonic acid) (PPDSA) and characterization of its intrinsic and membrane properties: data, results and discussion
2.1. Overview for chapter 2
This chapter consists of six sections; 1) Overview 2) Introduction, 3)
Experimental procedures, 4) Results, 5) Discussion, and 6) Conclusions. In this section, all contents in this chapter will be briefly reviewed. The monomer and polymer structural design, their characterization, polymer intrinsic properties, and membrane properties are related each other and are difficult to discuss separately. Thus, the experimental procedures and results sections will mainly deal with procedures and experimental data. In the discussion section, results are analyzed and the meaning of them for fuel cell applications is discussed.
The introduction section will discuss two concepts for the design of a new polyelectrolyte; a highly sulfonated and a rigid rod liquid crystalline polymer system. A background about aromatic condensation polymers will also be given
for supporting the new concepts.
The experimental procedures section will describe the monomer and
polymer synthetic procedures, characterization techniques and preparation
methods to analyze results. Especially, the characterizations of the membrane
properties under different environmental conditions (humidity and temperature)
will be shown.
32 Chapter 2
The raw data measured using the characterization techniques will be
reported in the results section, and these will be analyzed and discussed
systematically in the discussion section. Intrinsic polymer properties will be
discussed in terms of a rigid rod liquid crystalline polyelectrolyte polymer and the
membrane properties will be evaluated and compared with Nafion and aromatic
sulfonic acid polymers under simplified fuel cell operation conditions.
Last section, Conclusions, will summarize important experimental and analyzed results.
33 Chapter 2
2.2. Introduction
2.2.1. New approach to a highly sulfonated monomer:
Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid (DBBDSA)
Proton conduction in solid polymers has attracted the attention of many
researchers because of applications in electrical engineering. Recent interest has
been mainly stimulated by the potential use of such materials as separators in batteries, fuel cell electrolytes, and chemical sensors. Perfluorosulfonic acid
ionomer (Nafion®) membranes have been used for this purpose due to their
efficient proton conduction, permselectivity, and long term thermal and chemical
stability.1-4 Nafion conductivity reaches 102 mS/cm in the fully hydrated protonic
form, but decreases with temperature above the boiling temperature of water
because of water loss and structural5 and morphological change6. Thermostable
polymers with good water affinity and reliability at higher temperatures are
desired for solid polymer electrolyte fuel cells. One approach for an alternate
material is to introduce of ionic groups onto aromatic polymers; they are well-
known as thermally stable organic materials. Much effort has been expended to
functionalize poly(1,4-phenylene) (PPP)7, poly(oxy-1,4-phenyleneoxy-1,4-
phenylenecarbonyl-1,4-phenylene) (PEEK)8, poly(oxy-1,4-phenylene) (PPO)9, and other engineering plastics10. The resulting aromatic polymers have several
advantages; 1) They are cheaper than perfluorinated polymers, and some of
them are commercially available, 2) Aromatic polymers containing polar groups
have high water uptake over a wide temperature range, 3) Decomposition of
34 Chapter 2
aromatic polymers can be suppressed to a great extent by proper molecular
design.5, 11-13
Aromatic polymers can be sulfonated using concentrated sulfuric acid14, chlorosulfonic acid15, fuming sulfuric acid16-18 or acetyl sulfate19. The harsh
sulfonation conditions that are sometimes required can produce chemical
degradation and cross-linking. Bishop et. al.20 have reported that the rate of
sulfonation of PEEK can be controlled by the reaction time and the acid
concentration, providing a range of sulfonation of 30~100% without chemical or
cross-linking reactions21. However, direct sulfonation fails to produce truly
random copolymers due to the inhomogeneity of the reaction mixture. Because of the electrophilic nature to the sulfonation reaction, the structure of the aromatic polymer determines and limits the specific sites where sulfonic acids will be inserted. The specific location of sulfonic acids in the polymer repeat unit can affect the hydrolytic stability of the final polymers and the overall properties of the resulting membranes22.
23-26 Several attempts to sulfonate PPS by SO3 have been reported.
However, all the products suffered from a low degree of sulfonation (usually no more than 0.5 sulfonic acid groups per phenylene) and the formation of ladder or cross-linked structures due to the stringent reaction conditions.
Therefore, in this dissertation, a sulfonated monomer (1,4-dibromo-2,5- benzenedisulfonic acid) was synthesized and polymerized. This approach is possibly the only way to make highly sulfonated poly(p-phenylenes) with sulfonic
35 Chapter 2
acids in specific positions. Also, by control of the sulfonation positions, the resulting sulfonated polymer might have unique features in its overall properties.
Before starting this approach, the properties of highly sulfonated polymers
were reviewed and compared with those of polymers having low degree of
sulfonation. Miyatake et. al.27, 28 developed a successful route to high molecular
weight PPS [poly(phenylene sulfide)] containing sulfonium cations. Normally, a
high degree of sulfonation is difficult to achieve because of polymer insolubility
and side reactions such as cyclic sulfone formation and interpolymer cross-
linking at higher temperatures. The polysulfonium salt (I in Scheme 2.1),
prepared by polymerization of 4-(methylsulfinyl)diphenyl sulfide, was sulfonated
using a commercial oleum (10% SO3 - H2SO4) at 80°C (Scheme 2.1). The
sulfonated polysulfonium salt (II) was then converted to poly(phenylene sulfide
sulfonic acid sodium salt) (III) through demethylation by reaction with aqueous
NaCl solution. Poly(phenylene sulfide sulfonic acid) (PPS) (IV) was made by
acidifying compound III. The degree of sulfonation per repeat unit (m) increased
with the sulfonation temperature; m = 1.4 (at 80°C) and 2.0 (at 120°C). Higher
temperatures (140°C) resulted in a decrease in the degree of sulfonation (m =
1.7) because a cross-linking reaction took place through sulfone formation. The
final PPS was a pale brown powder soluble in water (m > 0.3) and methanol (m <
1.1).
36 Chapter 2
SO3/H2SO4 SS SS
CH3 n CH3 n SO H SO H 3 m 3 m
I II
NaCl aq. H+ S S NaOH 2n 2n SO Na SO H 3 m 3 m
III IV
Scheme 2.1. Preparation of poly(phenylene sulfide sulfonic acid) (PPS).27
The effect of high degree of sulfonation (m=2.0) was studied. The polymer’s thermal stability and proton conductivity were compared with those of
PPS with a low degree of sulfonation. PPS (m=2.0) had better thermal stability
(decomposition temperature) than less sulfonated PPS. In the thermogravimetric analysis (TGA) (Figure 2.1), PPS (m=0.6) showed a 31% weight loss between
140 and 380°C, which corresponds to the loss of the sulfonic acid group. Further weight loss, above 445°C, is attributed to polymer degradation. On the other hand, the starting decomposition temperature of highly sulfonated PPS (m=2.0) was 265°C, 125°C higher than PPS (m=0.6) and 75°C higher than a perfluorosulfonic acid ionomer (Nafion). This is due to the stronger C-S bond strength in PPS (m=2.0) due to two electron-withdrawing sulfonic acid substituents on each phenyl ring.29 The initial weight loss of PPS (m=2.0)
between 265 and 380°C was only 13%, which corresponds to the loss of two
37 Chapter 2
water molecules per repeat unit. This result suggested that the decomposition begins with the cyclization of the sulfonic acid groups to form sulfones.
Figure 2.1. TGA curves of poly(phenylene sulfide sulfonic acid) (heating rate: 20 °C/min).27
Proton conductivities (ó) of PPS (m=2.0) were measured at different relativity humidities. As shown in Figure 2.2, PPS with m = 2.0 had a conductivity of about 10-5 S/cm at 30%RH, which increased to 2 X 10-2 S/cm at 94%RH and
room temperature. The conductivities of PPS with m = 2.0 were higher than that of PPS with m = 1.2 (about 10-6 S/cm at 94%RH in Figure 2.3), but lower than
Nafion 117 (7.5 X 10-2 S/cm at 100%RH and room temperature)30. However,
Lambda (λ, the number of absorbed water molecules per sulfonic acid group) of
PPS (m=2.0) is 10.3. That lambda is higher than that of Nafion (ca. 5 H2O per sulfonic acid group for a Nafion 1200EW using vapor equilibrium technique).1
Although they did not have an explanation for the low conductivity of PPS with m
38 Chapter 2
= 2.0 compared with Nafion in spite of high water affinity, they claimed that its low equivalent weight (134 EW) and high concentration of sulfonic acid groups were the main reasons for the resulting proton conductivities and water uptakes.
Nafion 117
PPS (m=2.0)
Figure 2.2. Relative humidity dependence of the conductivity (ó) of poly- (phenylene sulfide sulfonic acid) (m = 2.0)28 and Nafion® 11730.
The temperature dependence of conductivity for PPS with m=1.2 and 2.0 showed that the conductivity increases with the degree of sulfonation (m) of PPS due to an increase in the carrier concentration (Figure 2.3). The conductivities in the temperature range below 80°C, approximately followed the Arrhenius-type dependence with an activation energy, Ea = 17kJ/mole for m = 2.0 (λ = 10.3) and
117 kJ/mole for m = 1.2. The activation energy for PPS with m=2.0 is comparable to that of Nafion (λ = 5: Ea = 19kJ/mole). It is noteworthy that PPS with m=2.0 has very high thermal stability up to 180°C, with proton conductivities on the order of 10-2 S/cm (the highest conductivity is 4.5 X 10-2 S/cm at 80°C). These results showed that a highly sulfonated polymer can be used as a high
39 Chapter 2
temperature PEMFC membrane. They claimed that the degree of sulfonation and
higher carrier concentration might also effectively retain the absorbed water in
the polymer above 100°C due to its high polarity. However, they did not present
information about the state of water in PPS at low and high temperature, and
describe a solid state structure for PPS (m=2.0) at different relative humidities to
explain a mechanism for strong binding of water molecules. Even though they did
not have this information, it can be expected that many water molecules are
strongly bound to the sulfonic acids and those are not freezable.
m = 2.0
m = 1.2
Figure 2.3. Temperature dependence of the conductivity of fully hydrated poly(phenylene sulfide sulfonic acid) . m =1.2 (○) and m = 2.0 (●). For the impedance data, Pt electrodes and wet pellet samples were used.28
40 Chapter 2
Based on the high conductivity and good thermal stability of
poly(phenylene sulfide sulfonic acid) due to its high degree of sulfonation, other
polymers with high degrees of sulfonation could also have unique solid state
structures that generate high conductivity at different relative humidities, good
thermal stability and good water affinity.
2.2.2. Synthetic routes towards poly(p-phenylene) (PPP).
2.2.2.1. Existing experimental methods for PPP synthesis.
Poly(p-phenylene) (PPP) has attracted much interest since it can act as
an excellent organic conductor upon doping.31-34 The conductivity of doped PPP
has reached beyond the semiconducting and into the conducting region, with
values of 500 S/cm. The synthesis of poly(p-phenylene) (PPP)35 has received a great amount of attention due to the rigid rod character of the resulting polymer backbone, high thermal stability of the neutral polymers, and ability of the π system to be redox doped to high levels of electrical conductivity. As an electroactive polymer, the research on PPP served as an impetus for the study of a broad variety of aromatic polymers. The direct synthesis of PPP from benzene has been accomplished using a variety of methods; however, most routes lead to highly insoluble materials containing irregular structures which are quite difficult to characterize. Besides the polymer insolubility made the unambiguous characterization of their structures impossible.
41 Chapter 2
The potential for using PPPs as building blocks for highly conductive and
thermal stable polymers has stimulated the development of many synthetic
methods, producing a wide variety of chemical structures.36-38 The first report of a linear rigid rod PPP synthesis was published in 1959 by Marvel7. In the early
1960’s, a second synthesis was reported by Kovacic and coworkers8. Marvel discussed on the synthesis of a precursor, poly(5,6-dibromo-1,4-cyclohex-2-ene), which could be thermally converted into the desired PPP. Kovacic’s approach involved the oxidative coupling of benzene using aluminum (III) chloride as a
Lewis acid catalyst and copper (II) chloride as the oxidant, see Figure 2.4.
a) H H R +2HBr HH H H n Br Br Br Br n
b) AlCl3
CuCl2 n
Figure 2.4. Synthetic routes toward PPP’s. a) reported by Marvel7, b) reported by Kovacic8.
The PPP’s produced by these routes contained a substantial amount of
defects due to meta- and ortho- linkages, cross-linking, and in the case of the
precursor synthesis, incomplete elimination. A milder and less defect prone route
to PPP’s used the transition metal catalyzed coupling of difunctionalized benzene
derivatives. Yamamoto et. al.9, 10 reported the reaction of 1,4-dibromobenzene with magnesium in the presence of low valent nickel catalysts in 1978 (Figure
42 Chapter 2
2.5,a)). The resulting PPP was an exclusively para-coupled material with degrees
of polymerization ranging from 5 to 15 repeat units. The relatively low molecular
weight was ascribed to the insolubility of the polymer, which precipitated from the reaction mixture, and could not participate in further polymerization. In order to circumvent this problem, the authors used 1,4-dibromobenzene derivatives containing solubilizing n- alkyl chains in the 2 and 5 positions as the monomer.
The isolated PPP had a maximum degree of polymerization of 15 and Yamamoto attributed the difficulties in achieving higher molecular weights to possible de- halogenation reactions and offset stoichiometric ratios. To optimize the nickel coupling reaction, Percec et. al.39, 40 proposed a reaction scheme that used zinc
powder with catalytic amounts of nickel(II) to couple dihalo- or bismesylate-
substituted benzene derivatives (Figure 2.5,b)). The reported degrees of
polymerization, as calculated from PS calibrated GPC chromatogram, ranged
from 6 to 43 depending on the specific experimental conditions and monomer structure. Further optimization of the reaction conditions and the monomer
structures raised the degree of polymerization to 101.41 The highest molecular
weights were obtained using monomers with mesylate as the leaving group and
with electron withdrawing groups linked ortho to the carbon undergoing
substitution. The only drawback of this synthetic method was its poor
compatibility with many nucleophilic functional groups, which narrowed its
applicability. Recently, a possible solution to this problem was proposed by
Sheares et. al..42 Sheares’ group used the nucleophilic substitution of the fluorine
atoms in the pendant aromatic ring of the carboxylic ester in their polymers to
43 Chapter 2
attach aryl ether and secondary and tertiary amine motifs to the pendant aromatic ring of the rigid rod PPP’s.
a) R1 R1 Mg Br Br (R1 = H, Alkyl) Ni(0) n R1 R1
R b) R2 2 X = Cl, Br, OMs Ni(II) O X X O O Zn, Base R2 = n OF OAlkyl OAryl
Figure 2.5. Nickel catalyzed synthesis of PPP’s with magnesium9,10 (a) and with Zinc39, 40, 42 (b).
An alternate approach to the synthesis of high molecular weight PPP’s
that was compatible with a wide range of functional groups was first proposed in
1989 by Wegner, et. al.43. The authors used palladium (0) catalyzed Suzuki
hetero aryl-aryl coupling of dibromo-benzene derivatives with phenyl diboronic
acids to produce relatively high molecular weight PPP’s. This method has become the most common synthetic route to produce polyphenylenes containing
a wide variety of functional groups and polymer architectures.36-38, 44 However, in
order to produce unambiguously linear polyphenylenes by Wegner’s method,
ortho-substituted triaryl phosphines were required to minimize metal-ligand
exchange reactions that affect the final polymer structure.45 Novak suggested,
based on 31P-NMR measurements, that the use of ortho-substituted triphenyl
phosphines limited the metal-ligand exchange reaction to about 1 in every 400
repeat units, which can be considered negligible for most practical purposes.46
44 Chapter 2
The palladium aryl-aryl heterocoupling reaction scheme used to produce polyphenylenes is depicted in Figure 2.6.
1) 1eq. n-BuLi R1 R1 R 2) B(OCH3)3 1 3) HCl Pd(0)L4 Br Br Br B(OH)2 Ni(0) Base n R 1 R1 R1
(R1 = H, alkyl) 1) 2eq. n-BuLi 2) B(OCH3)3 3) HCl
R1 R2 R1 R2
Pd(0)L4 (OH)2B B(OH)2 + Br Br Base n
R1 R1
(R2 = H, alkyl, aryl, O-alkyl, O-aryl, COOH, SO3-aryl)
Figure 2.6. Palladium catalyzed aryl-aryl cross-coupling synthesis of PPP’s.
45 Chapter 2
2.2.2.2. Synthetic route towards poly(p-phenylenes-2,5-disulfonic acid)
(PPDSA): Ullmann coupling.
The incorporation of sulfonic acids into the polymer backbone of the rigid polyphenylenes (PPP’s) limits the useful synthetic methods. The nickel catalyzed aryl-aryl coupling reactions can undergo side reactions in the presence of sulfonic acid esters, since these can also act as leaving groups upon attack by the nickel complex. The use of sulfonic acid inorganic salts (sodium or lithium
salt) may prove useful if a common solvent for all the participating species could
be found, but there are no reports on the use of this approach so far. Palladium
catalyzed Suzuki cross-coupling reactions have been successfully applied
towards the synthesis of sulfonated PPP via coupling and subsequent hydrolysis
of its corresponding sulfonic esters.47-49 This method limits the mole fraction of
sulfonic acids that can be incorporated into the polymer backbone to a maximum
of 0.66; no one has reported the synthesis of a sulfonated aryl diboronic acid.
Most Suzuki polycondensations (SPC’s)50 use the commonly employed tetrakis
triphenyl phosphine Pd(0) system for their syntheses, which could produce
undesired metal-ligand reactions, altering the expected rigid rod structure of the
resulting polymers. The SPC approach is relatively expensive and its cost has deterred researchers from testing sulfonated polyphenylenes made using this
route as PEM’s.
In an attempt to develop a low cost polymerization method applicable to the large scale synthesis of PEM’s, we decided to explore the possibility of using a well studied transition metal-mediated aryl halide coupling reaction, known as
46 Chapter 2
Ullmann coupling. Activated copper powder has been used as an aryl-aryl
coupling reagent and this method is relatively inexpensive. In addition to the cost
aspect, Ullmann coupling worked well in the condensation reactions of sulfonated
aryl monomers (such as, 4,4’-dibromo-2,2’-biphenyl bis-(benzyltrimethyl-
ammonium)-disulfonate, which was previously polymerized and studied in our
lab)51.
Ullmann coupling was initially reported in 190152 and has since been
employed by chemists to generate C-C bonds between two aromatic rings.
Typically, two equivalents of aryl halide are reacted with an excess of finely
divided copper at high temperature (>200ºC) to form a biaryl and copper halide.
The procedure variables have been optimized and were comprehensively
reviewed several decades ago53, 54 and notable improvements have been made
through the last century. Polar aprotic solvents like dimethylformamide (DMF)
and dimethylacetamide (DMAC) enable the use of lower temperatures and
smaller amounts of copper powder.55 The use of an activated form of copper powder, made by the reduction of copper (I) iodide allows the reaction to be carried out at even lower temperatures (~85ºC) with improved yields. Electron
withdrawing groups on the aryl halide, such as nitro and carboxymethyl,
especially in the position ortho to the halogen atom, provide an activating effect
on the coupling reaction. On the other hand, the presence of substituents which
could provide alternative reaction sites, such as amino, hydroxyl and free
carboxyl groups, greatly limits or prevents the reaction. Furthermore, bulky
groups ortho to the halogen inhibit the coupling reaction. The major limitation of
47 Chapter 2
the Ullmann reaction is its extreme sensitivity to the specific electronic
environment around the carbon linked to the halide. Subtle changes in chemical structure significantly change the reactivity of the compound towards coupling.
This ultimately makes the synthesis of asymmetric biaryls, and thus copolymers,
challenging. In spite of its well known disadvantages, the simplicity of the
required experimental conditions and its relatively low cost made it worthwhile to
test this approach for the synthesis of sulfonated PPPs.
Previous results (Sergio Granados-Focil in our lab)51 proved that the
position of the sulfonic acid relative to the halide is the key factor determining the
reactivity of the compound towards the coupling. In his thesis, a significant
increase in the viscosities and film forming properties of the resulting polymers was achieved by using a biphenyl monomer with the sulfonic acids ortho to the
halide rather than meta to the halide. His best results were obtained with coupling
the bis(benzyltrimethylammonium) salt of 4,4’-dibromo-3,3’-biphenyl disulfonic
acid in NMP (N-methyl pyrrolidinone). That was the firstly reported detailed study
of the use of the Ullmann coupling reaction to produce poly(p-phenylene)s.56, 57
Based on the previous results and a new concept for making highly sulfonated polymer systems, different disalt forms (lithium, benzyltrimethylammonium (BTMA) or tetrabutylphosphonium (TBP)) of a new monomer (1,4-dibromo-2,5-benzenedisulfonic acid, DBBDSA) were synthesized, and under the best conditions highly sulfonated poly(p-phenylene) was obtained via Ullmann coupling. A detailed study of the properties of this sulfonated poly(p-
48 Chapter 2
phenylene) [poly(p-phenylene-2,5-disulfonic acid), PPDSA] will be reported and discussed in this chapter.
49 Chapter 2
2.3. Experimental procedures
2.3.1. Materials
All reagents except 15% oleum (the concentration of SO3 gas = 13~17%,
purchased from Alfa Aesar) and the cation exchange resin, Rexyn 101
(2.05~2.2meq./g exchange capacity, purchased from Fisher) were purchased
from Aldrich Chemical Co. N-methyl pyrrolidinone (99%) and dimethylformamide
(DMF) were stirred overnight with calcium hydride, vacuum distilled and
degassed with Ar gas prior to use. The remaining solvents and reagents were
used without further purification. Copper bronze powder (for organic synthesis)
was activated according to a previously reported procedure58 and was used
immediately after preparation.
2.3.2. Characterization techniques for DBBDSA monomer
1H-and 13C-NMR spectra of the monomers having different salts were obtained using a Varian Gemini 300MHz or a Varian Inova 600 MHz spectrometer using D2O and DMSO-d6. FT-IR spectra of KBr pellets were
recorded on a BOMEM Arid Zone FTIR spectrometer. The monomer melting
point was measured using DSC (Differential Scanning Calorimeter) at a 10°C/min
rate under N2 gas.
50 Chapter 2
2.3.3. Characterization techniques for PPDSA
2.3.3.1. NMR spectroscopy
1H-and 13C-NMR spectra of the polymers, diprotonated form and disodium salt in D2O were obtained using a Varian Gemini 300MHz or a Varian
Inova 600 MHz spectrometer. The 1H-NMR spectra were deconvoluted using the
ACD Labs : curve processing module (version 9.05).
2.3.3.2. Rheology measurements
Rheological studies were performed on a Physica MCR501 Rheometer
(Anton Paar Company). Measurements were made using cone and plate geometry (CP25-1-SN4514) with a diameter of 25 mm and a gap of 53um between the cone and plate. Viscosities of samples were measured at shear rates ranging from 1.0 X 10-3 to 100s-1. The test temperature was 20 ± 2°C.
Samples were prepared by dissolving diprotonated PPDSA in D.I. water. Different sample concentrations were made by dilution in stages from the highest concentration of aqueous polymer solution (38.5 g/dL) to 0.03 g/dL.
2.3.3.3. GPC (Gel permeation chromatography)
Solvent (DMF or 0.01M LiBr in DMF) was pumped from a solvent reservoir by a Waters 510 pump at 0.7 mL/min through a column system consisting of a guard column and two main columns (Waters HR-5E DMF and
HR-4E DMF). The sample injection was made through a GPC valve with an
51 Chapter 2 injection loop (200uL) fitted between the pump and the column. The eluted sample passed first through a UV detector (Waters 996 Photodiode Array
Detector), and then to a refractive index detector (Waters 2414 Refractive Index
Detector) before it was collected. The column chamber temperature was controlled at 35 or 150°C depending on test conditions, and the refractive index cell temperature was set at 50°C. A calibration curve was obtained using polystyrene standards (PSS ReadyCal Polystyrene standard kit (SDK-600),
Polymer Standard Service-USA Company) to calculated a relative molecular weight of polymer.
GPC samples with the desired concentrations (1.0 g/dL or 0.125 g/dL) were prepared by dissolving the polymer in a DMF/D.I. water mixture (v/v, 67/33).
Before injection, the solution was filtered through a PTFE membrane filter
(0.45um).
2.3.3.4. Viscosity measurements
Reduced viscosities of dilute solutions of PPDSA (dilithium, disodium, diTBP (tetrabutyl-phophonium) and the diprotonated forms) in D.I. water, DMF, and DMF/NMP (v/v, 33/67) with various salt concentrations (0.3M LiCl, or 0.1, 0.5,
1.0M LiBr) were measured using Ubbelohde type viscometers.
Dilute polymer solutions at concentrations between 0.7g/dL and 1.0g/dL were prepared by dissolution of PPDSA films after drying at 90°C for 1 day.
Measurements of the solution flow time were extrapolated, as reduced viscosity, to zero concentration to hopefully obtain the intrinsic viscosity. The PPDSA
52 Chapter 2
solution was diluted by adding the same solvent or solution which was used to
make the polymer solution. By selecting the capillary width, the time (t) needed
for the solvent to flow through the capillary tube was adjusted to be above 4 mins
(except viscometers 1C C282 and 1 J659). Prior to measurement, all the
solutions were filtered through a 0.45μm pore diameter PTFE membrane filter.
Flow times were measured at least three times (accuracy of ±0.1 s) for each concentration.
For viscosity measurements in the presence of salts (LiCl or LiBr), the
procedure was modified as follows. A stock salt solution was prepared to act as the solvent for the polymer, and to perform all the subsequent dilutions required for the multi-point measurement. This process ensured that the salt concentration, and therefore the ionic strength, remained constant throughout the viscosity measurement (iso-ionic condition).
The shear rate (τ in sec-1) for the viscometers was calculated using
Equation 2.1
4Q τ = (Equation 2.1) πR3
where Q is the flow rate in mL/sec and R is the capillary radius in cm. The bulb volumes, capillary radii and flow times for four viscometers used in these measurements are shown in Table 2.1
53 Chapter 2
Efflux time (sec) Shear rate at 35°C (sec-1) at 35°C
Capillary Bulb Viscometer radius volume model # 0.3M 0.3M (cm) (mL) LiCl D.I. LiCl D.I. aqueous water aqueous water solution solution
0C C453 0.018 2 266.07 255.89 1642 1707 0C C434 0.018 2 288.80 1513 1C C282 0.0385 4 25.72 25.25 3472 3537 1 J659 0.029 4 77.46 2697
Table 2.1. Bulb volumes, capillary radii and flow times for four viscometers used in viscosity measurements. All viscometer was manufactured by Cannon Company and viscometer model #s were given in each viscometer certificate. Specification errors in capillary radii and bulb volumes were ±2% and ±5%, respectively, from Cannon Company specification using ASTM D446.
54 Chapter 2
2.3.3.5. Water uptake
2.3.3.5.1. Water content evaluation.
The water content of the polymer films was evaluated as a function of relative humidity using two techniques: weight increase and sulfonic acid titration.
The difference in weight of the polymer films between the dry and humidified states was quantified using several strips of film of about 200μm thickness, 3mm long and 2mm wide. The weighing bottles (with a ground glass joint and cap) were placed in an oven at 120ºC for 24 hours. After drying, the bottles were taken out of the oven and quickly put in desiccators containing dried molecular sieves
(4Å) for cooling. The weights of the weighing bottle and cap were measured before putting dried film into the bottle. After the film was vacuum dried at 90°C for 24 hours, the dried film was weighed in the pre-weighed capped weighing bottle. Weighing bottles containing polymer films were opened and placed in different %RH chambers containing lithium chloride solutions providing controlled relative humidities ranging from 11% to 75%RH. The LiCl solution preparation followed the protocol reported previously.59 The specific concentrations of LiCl in water used to achieve a given relative humidity are given in Table 2.2.
55 Chapter 2
Relative humidity(%) Concentration of LiCl Concentration of LiCl at 25°C (mol /kg of solvent) (g /kg of solvent)
11 > 18 Saturated solution. 15 17 720.64 35 11 466.29 50 8 339.12 75 5 211.95
Table 2.2. Lithium chloride concentrations to achieve a given relative humidity.
Weighing bottles containing polymer films were equilibrated for 24 hours.
After the films had reached equilibrium, they were sealed with the pre-weighed cap and their wet weight determined. The water uptakes at different humidities
were calculated using Equation 2.2.
()W −W Water uptake = 2 1 ×100 (Equation 2.2) W1
where W1 is the weight of the dried polymer film and W2 is the weight of
humidified polymer film after equilibration.
56 Chapter 2
2.3.3.5.2. Lambda (λ) measurement
Sodium chloride and sodium hydroxide aqueous solution used in titration
were made with D.I. water boiled to remove CO2. Sodium hydroxide solution was standardized as follows; 2.0 mL of potassium hydrogen phthalate aqueous solution (0.103M) and phenolphthalein ethanol solution (2 to 3 drops) as an end-
point indicator were put into a beaker. Sodium hydroxide aqueous solution (about
0.01M) was placed in a 50mL burette and the potassium hydrogen phthalate
solution was titrated drop by drop with stirring until a permanent light pink was
shown. The molar concentration of the sodium hydroxide solution was calculated
from the volume of sodium hydroxide solution used. This standardization was
carried out before and after the sulfonic acid titration.
The sulfonic acid titration was performed using films that had been
equilibrated at controlled relative humidities in the same manner as described for
the water uptake test. The weights of dried (W1’) and humidified (W2’) films were
recorded using the same procedures as in the water uptake test, and 5ml of 2M
aqueous sodium chloride was added to each bottle. The resulting solution was
titrated with the standardized sodium hydroxide solution using phenolphthalein as
the end-point indicator. The concentration of sulfonic acid was calculated from
the titration volume. The weight of absorbed water was the difference between
the dried and humidified film weights. Using those data, lambda (λ), the number
of water molecules per acid group in the membrane, at different humidities was calculated using Equation 2.3.
57 Chapter 2
()W '−W ' /18 Lambda (λ) = 2 1 (Equation 2.3) [SO3 H]
’ where (W2 –W1’) is the weight difference between the dried and humidified films,
’ the weight of absorbed water, and (W2 –W1’)/18 is the moles of water absorbed by the polymer film. [SO3H] is the moles of sulfonic acid determined from the
titration with the standardized sodium hydroxide solution.
2.3.3.6. Differential Scanning Calorimetry (DSC)
DSC measurements were performed using a Mettler Toledo STARe
system, DSC (822e/700) with a HAAKE EK90/MT cooling accessory and a
TS0800GC1 N2 flow control controller. Standard aluminum crucibles, 40ul, with
pin and lid (part # 00027331) were used. To hydrate the PPDSA films, about 5-10
mg of a previously dried film was placed on a glass slide in a controlled relative
humidity chamber (15 to 75%RH) at room temperature for about 24 hours. The
films was then transferred immediately to an aluminum pan and hermetically
sealed. The sealed DSC crucibles used for high temperature scanning had a hole
in the lid made by inserting a 23 gauge needle through the lid before running the
experiment, while those used at low temperature did not have a hole. Data were
collected at 10 °C/min for scans at low temperature (-50 to 60 °C) and scans at
high temperature (25 to 300 °C). As controls, a well-dried PPDSA sample
(0 %RH) and bulk water (D.I. water, filtered through a 0.45um PTFE membrane) were tested in the same way.
58 Chapter 2
2.3.3.7. Thermogravimetric Analysis (TGA).
TGA experiments were performed using TA-instruments TGA analyzer
2950. Heating rates of 10ºC/min were used. All experiments were performed under N2 atmosphere (60mL/min).
A dried sample was prepared by vacuum drying at 90ºC for 1 day. Then, the sample was put into the TGA pan and heated from 25 to 800ºC (platinium pan, ramp 10ºC/min).
2.3.3.8. Dimensional change with water uptake
The dimensional change of a polymer film was measured as a function of relative humidity. Initial dimensions (length, width and thickness) of the polymer films were measured using calipers (length and width) and a micrometer
(thickness) after vacuum drying for 1 day at 90°C. Polymer films were stored in different relative humidity controlled chambers (as described in the water uptake test) for 24 hours and the dimensional changes were measured by calipers
(length and width) and optical microscope (thickness). The X-axis is perpendicular and the Y-axis is parallel to the casting direction. The Z-axis is the thickness direction (Scheme 2.2).
59 Chapter 2
Scheme 2.2. Relationship of X, Y, Z to casting direction. X and Y axes are perpendicular and parallel, respectively to the casting direction. Z axis is the thickness of film.
2.3.3.9. Proton conductivity measurements
Proton conductivity was measured using the AC impedance method in a
4-probe configuration. Two outer probes supply current to the cell, while the two
inner electrodes measure the potential drop. PPDSA films were cut into
approximately 3 cm by 0.3 to 0.4 cm strips. In order to study the possible
orientation of the PPDSA molecules due to casting shear, strips were cut parallel
and perpendicular to the casting direction. All the films were vacuum dried at
90°C for 1 day, and the thickness and width of the polymer films were measured
before assembling the conductivity cells.
The configuration of the polymer/electrode assembly for the in-plane
conductivity measurements is shown in Scheme 2.3. Graphite paper (Tonnen
Company, Japan) was chosen as the electrode material and stainless steel
alligator clips were used to fix the film to a solid support (a plastic plate) and act
as the electrical contact points (Scheme 2.3, a) and b)). The two inner electrodes were spaced approximately 5 to 10 mm apart, the exact distance was measured
60 Chapter 2
by calipers after the sample preparation, and the average distance between the
graphite papers edges was recorded (Scheme 2.3, a)).
The alligator clips were soldered to copper wire (diameter: 0.8~0.9 mm) for better contact (the resistance from the end of copper wire to the alligator clip
was about 8 to 9 mΩ) (Scheme 2.3, b)). The other ends of the copper wires were
put through the cap of the plastic bottle, and the gap between the holes and the
wires were sealed with epoxy chosen to resist the attack of humid air. The length
of the copper wires protruding from the plastic cap is shown in Scheme 2.3, c).
Constant humidity conditions were maintained during the test using this
procedure.
Plastic bottles (Nalgene®, 250mL) containing various concentrations of
lithium chloride solutions were used to control the environmental relative
humidity. The LiCl solutions at different concentrations were prepared as shown
in Table 2.2.
The PBPDSA strip/electrode assemblies were enclosed in Nalgene® plastic bottles with defined relative humidities (15% to 75%RH), using aqueous lithium chloride solutions. About 50 ml of a selected aqueous lithium chloride solution were placed at the bottom of a bottle. The plastic bottles were placed in an oven for “high” temperature conductivity measurements at 50 and 75°C.
To measure the effect of the plastic plate on conductivity, two types of cell
assemblies were used. Scheme 2.4, a) is the film-on-plate cell assembly used in
all measurements, while (b) is a film-through-plate cell assembly. The plate in
Scheme 2.4, b) has a rectangular hole and the PPDSA strip was placed so the
61 Chapter 2
ends were on opposite side of the plastic plate. Most of the film that was part of
the voltage drop measurement was exposed on both sides.
When a four-point probe is used to measure the membrane resistance,
the proton conductivity can be calculated using Equation 2.4, where σ refers to
proton conductivity, S/cm; K is the resistance, obtained from the impedance
measurement at high frequency, Ω; L is the distance between the center two electrodes in cm; W is the width of the sample in cm and T is the thickness of the
polymer film in cm. The value calculated from the dimensions of the film, L/ (W*T)
is typically known as the cell constant.
L /()W ×T σ = (Equation 2.4) K
AC impedance measurements between 2 Hz and 20 KHz were taken using a Solatron SI 1287(electrochemical interface) and Solatron SI 1260
(Impedance/Gain-phase analysis). Copper mini-grabbers (Pomona, model 3782-
12) were used to connect the cell to the impedance equipment. The detailed
parameters of the impedance measurements are listed in Table 2.3. The results
were collected by Z-view and analyzed by Z-plot software.
62 Chapter 2
Graphite paper PPDSA film T W
L a) Plastic plate
10 mm
Cu wire 30 mm Copper wire
Plastic cap Alligator clip
20 mm
Graphite paper Plastic plate Alligator clip PPDSA film b) c)
Cu wire
Nalgene® plastic bottle
~50mL LiCl aq. solution d)
Scheme 2.3. Schematic of assemblies of PPDSA film/electrodes for in-plane conductivity measurements. a) Dimensions to be measured for calculation of cell constant; L, W and T are the average distances between the two inner graphite papers, the average width of the film, and the average thickness of the film, respectively, b) Typical cell Assembly (the alligator clips were connected with copper wire by soldering), c) Dimensions of the wiring in the assembled cell (when impedance was measured, testing copper grabbers were connected in the upper 10 mm region), d) final configuration for conductivity measurements, between room temperature and 75°C, using plastic bottles as environmental control chambers.
63 Chapter 2
Alligator Clip PPDSA film a)
Graphite paper Plastic plate
b) Alligator Clip PPDSA film Plastic plate
Graphite paper A hole on the plastic plate
Scheme 2.4. Schematic of different cell assemblies: a) a film-on-plate cell assembly, and b) a film-through-plate assembly.
Mode Sweep frequency mode AC Amplitude (mV) 25 and 50 mV Frequency sweep range 20 KHz (initial) to 2 Hz (final)
Table 2.3. Parameters for the AC impedance measurements.
64 Chapter 2
2.3.3.10. Wide angle X-ray diffraction (WAXD)
For the WAXD measurements, linear θ/2θ X-ray intensity scans were recorded using a Rigaku diffractometer with CuKα radiation (1.542Å) with a long fine focus mode. The setting values for each parameter are listed in Table 2.4.
Vacuum was not used during the scan; the relative humidity in the PPDSA films was kept at a fixed value for each scan using the procedure described below.
Parameters Setting values Start angle (°) 0.2 Stop angle (°) 35 Power 30kV / 30mA Sampling width (°) 0.1 Scanning speed (°/min) 0.1 Div. slit (mm) 2.0 Div. H. L. slit (mm) 5.0 Rec. slit (mm) Open Sct. Slit (mm or °) Open
Table 2.4. Setting parameters for WAXD measurements. Div. slit and Div. H. L. slit are a height and width of slit, respectively. Rec. slit and Sct. Slit are a recording slit and scattering slit, respectively.
For measurements of the orientation of polymer chains, specimens were prepared by cutting films parallel and perpendicular to the casting direction. The
PPDSA films, after vacuum drying at 90°C for 1 day, were equilibrated on PVC film-covered glass slides at 11, 15, 35, 50 and 75%RH in a closed chamber for more than 8 hours. The humidity was controlled by using different concentrations of LiCl aqueous solutions as shown in Table 2.2. To maintain the humidity of film
65 Chapter 2 during the experiment (Scheme 2.5), the films were then covered with a second sheet of PVC (film thickness: ~16um) and sealed by pressing area around the film with fingers wearing gloves. The sealed WAXD sample was fixed on the sample holder with tape. WAXD spectra were taken in the reflection and transmission modes for both orientations of cut films to compare the intensities and peak positions. The beam directions and sample positions are described in
Scheme 2.6.
Pure PPDSA diffraction profiles were obtained by subtraction of the diffraction spectrum of the two sheets of PVC from the obtained spectrum; a baseline correction was made and the curves were deconvoluted using the ACD
Labs, curve processing software (version 9.05). The d spacings were calculated using Bragg’s law (Equation 2.5)
λ d spacing = (Equation 2.5.) 2sinθ where 2θ was obtained from the peak positions in the deconvoluted curves; λ is
1.542 Å for CuKα radiation. The standard deviation of the d spacing changes with relative humidities, ±deviations in 2θ for all deconvoluted peaks were calculated using a half of FWHH (full width at half height), and those in Å were calculated from the maximum and minimum of θ for each peak, using Bragg’s law.
66 Chapter 2
Step 1 Dried PPDSA film on a PVC-covered glass
Controlled humidity chamber
Dried PPDSA film on Step 2 the PVC covered glass
LiCl aqueous solution
PVC film
Step 3 Humidified PPDSA film
Scheme 2.5. Preparation of sealed WAXD sample. Step 1) the dried PPDSA film was placed on the PVC-covered glass. Step 2) equilibration in humidity chamber for more than 8hours. Step 3) the humidified PPDSA film was sealed with a second PVC film.
67 Chapter 2
a) Transmission mode
PPDSA film
b) Reflection mode
PPDSA film
Scheme 2.6. WAXD experimental setup. Blue and red lines show the X-ray beam directions.
68 Chapter 2
2.3.3.11. 2D X-ray diffraction
2D X-ray diffraction spectra were recorded at room temperature on Kodak
Direct Exposure X-ray film (DEF5) using a Searle toroidal X-ray camera and Ni-
filtered CuKα radiation. Vacuum was not applied because it could dehydrate the
film. The PPDSA films at different relative humidities were prepared using the
same procedure as for the WAXD experiments. They were mounted on the X-ray
diffraction frame using double-sided tape. CaF2 was used as an internal standard.
The exposure time was about 24 hours. The lack of vacuum meant that air scattering was recorded. After developing the image, d spacing values of the polymer sample peaks were calculated from the peak diameter using the 2θ
value (28.31°) of CaF2 as a reference.
2.3.3.12. Optical Polarizing Microscope (OPM)
The PPDSA films for OPM were prepared using a process similar to that
for the WAXD sample preparation. The film was prepared by vacuum drying at
90°C for 1 day. The dried film was put on a dried glass slide and sealed with PVC film to protect from air humidity. The humidified samples were prepared as follows; 1) The dried films were put on glass slide and equilibrated in different relative humidity chambers for 1 day, 2) The humidified films were sealed with
PVC film and the images were recorded. Solution samples were prepared as follows; 1) One drop of polymer solution was placed on a glass slide, 2) The drop
was covered with a cover glass (~100um) and an image was recorded. OPM
images were recorded using an Olympus polarized light microscope equipped
with a CCD camera.
69 Chapter 2
2.3.3.13. Dynamic Mechanical Analysis (DMA)
All mechanical tests were performed using films approximately 3 cm. long
and 2~5 mm wide, with thicknesses of about 200μm. Stress-strain measurements
were made using a TA Instruments Q800 dynamic mechanical analyzer under a
N2 atmosphere. For the stress-strain test, a controlled force mode was applied as a linear ramp from 1.0N/m to 18N. Sample preparation at different humidities was the same as for the other tests.
70 Chapter 2
2.3.4. Synthetic procedures for 1,4-dibromo-2,5-benzenedisulfonic acid
(DBBDSA)
2.3.4.1. Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid, disodium salt
(DBBDSA-Na)
In a 500mL one neck flask containing an egg-shaped spin bar were charged 1,4-dibromobenzene (98%, 32.02g, 133 mmole) and 15% oleum (99mL, the concentration of SO3 gas was 13~17%). A condenser was fitted to the flask
and the reaction mixture was covered with Ar gas. The reaction mixture was stirred and heated in a bath at 220~230°C for 24hours. The reaction solution was cooled to room temperature and slowly poured into about 1L crushed ice. The brownish acidic aqueous solution was warmed to room temperature and undisolved solid was removed by filteration (less than 0.1 g). Sodium carbonate
(about 60 g) was added portion wise to the filterate to convert it to the sodium salt form. The solution was condensed to about 400mL; brownish solids salted out after 1 day at room temperature and were filtered using a sintered glass funnel. The salted-out solid, a crude mixture of 1,4-dibromo-2,5- benzenedisulfonic acid, disodium salt (para substituted form) and 1,4-dibromo-
2,6-benzenedisulfonic acid, disodium salt (meta substituted form) was dissolved in D.I. water, and neutralized with sodium carbonate to pH 7 by checking with pH paper. This avoided acid decomposition of the organic solvent that was used in next step. The neutral solution was evaporated by rotaevaporator and the obtained solid was dried under vacuum at 90°C for 2 days. The solid was stirred in DMF at room temperature for 1 day to separate sodium sulfate (by-product in
71 Chapter 2
neutralization), and filtered. The DMF was evaporated and the solid was vacuum
dried at 90°C for 2 days. To get pure 1,4-dibromo-2,5-benzenedisulfonic acid, disodium salt, the solid was extracted with ethanol using Soxhlet extraction. The pure 1,4-dibromo-2,5-dibenzene-sulfonic acid, disodium salt is much less soluble than 1,4-dibromo-2,6-benzenedisulfonic acid, disodium salt, and it remained in the thimble. The extraction process was monitored using 1H- and 13C-NMR to
decide the extraction time. After extraction for 1~2 days, solids in the thimble were vacuum dried at 90°C for 1 day and examined using 1H- and 13C-NMR. The
extraction process was repeated until the only desired product was remained in
the thimble. Finally, the desired, 1,4-dibromo-2,5-benzene-disulfonic acid,
disodium salt (DBBDSA-Na) was obtained after vacuum drying at 90°C for 1 day.
1 13 Yield was 38% (22.0 g). H-NMR (D2O): δ=8.14 ppm (s, 2H); C-NMR (D2O):
δ=118.3 (C-Br), 135.2 (C-H), 144.9 ppm (C-SO3H); FT-IR (KBr pellet) 3088
(aromatic C-H stretching), 1434 (C-C ring stretching), 1313 (in-plane ring
bending), 1223 (asymmetric stretching of SO2), 1080 (symmetric stretching of
SO2), 908 (C-H out-of-plane deformation for p-substituted benzene), 675 (out-of-
plane ring bending), 656 (C-Br stretching) cm-1. The melting temperature of 1,4-
dibromo-2,5-dibenzene-sulfonic acid, disodium salt (DBBDSA-Na) was 243°C,
measured by DSC.
72 Chapter 2
2.3.4.2. Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid, dibenzyl-
trimethylammonium (BTMA) salt (DBBDSA-BTMA)
DBBDSA-Na (6.47 g, 14.7 mmole) was dissolved in 60 mL D.I. water and
ion-exchanged to BTMA salt form by passing through a BTMA loaded cationic exchange resin. The collected aqueous solution was evaporated and dried under
vacuum at 70°C for 1 day. The final DBBDSA-BTMA was recrystallized from D.I. water at room temperature. The white solid was filtered using a Büchner funnel
1 and dried under vacuum at 70°C for 1 day. Yield was 90%. H-NMR (D2O):
δ=8.14 ppm (s, 2H), δ=7.40~7.47 ppm (m, 10H), δ=4.35 ppm (s, 4H), δ=2.97 ppm (s, 18H); FT-IR (KBr pellet): 3033 (C-H stretching in benzene), 2981
(aliphatic C-H stretching), 1304 (in plane ring bending), 1230 (asymmetric stretching of SO2), 1066 (symmetric stretching of SO2), 894 (C-H out-of-plane
deformation for p-substituted benzene), 663 (out-of-plane ring bending), 651 (C-
Br stretching) cm-1. The melting temperature of DBBDSA-BTMA was 198°C,
measured by DSC.
73 Chapter 2
2.3.4.3. Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid, dilithium salt
(DBBDSA-Li)
DBBDSA-BTMA was dissolved in D.I. water and ion-exchanged to the acid form and the collected aqueous acidic solution was titrated with aqueous
LiOH solution to pH 7 while stirring. The water was evaporated and the solid was
1 vacuum dried at 90°C for 1 day. Yield was 99%. H-NMR (D2O): δ=8.14 ppm (s,
13 1H); C-NMR (D2O): δ=118.3 (C-Br), 135.2 (C-H), 144.8 ppm (C-SO3H); FT-IR
(KBr pellet): 3103 (aromatic C-H stretching), 1438 (C-C ring stretching), 1317
(in-plane bending), 1209 (asymmetric stretching of SO2), 1081 (symmetric stretching of SO2), 898 (C-H out-of-plane deformation for p-substituted benzene),
673 (out-of-plane ring bending), 640 (C-Br stretching) cm-1. A melting temperature was not detected; it decomposed about 270°C.
2.3.4.4. Synthesis of 1,4-dibromo-2,5-benzenedisulfonic acid, ditetrabutyl- phosphonium (TBP) salt (DBBDSA-TBP)
DBBDSA-BTMA was dissolved in D.I. water and ion-exchanged to the acid form. The collected aqueous acidic solution was titrated with aqueous tetrabutylphosphonium hydroxide solution (40 wt%) to pH 7 while stirring.
DBBDSA-TBP started to precipitate from the aqueous solution at about pH 4~5.
After reaching pH 7, the white solid was filtered using a Büchner funnel, washed with THF (tetrahydrofuran), and dried under vacuum at 90°C for 1 day. Yield:
1 98%. H-NMR (DMSO-d6, 300MHz): δ=7.80 ppm (s, 2H), δ=1.92~2.02 ppm (m,
8H), δ=1.16~1.30 ppm (m, 16H), δ=0.71 ppm (t, 12H, J=12Hz); FT-IR (KBr
74 Chapter 2
pellet): 3095 (aromatic C-H stretching), 2958, 2939 and 2875 (aliphatic C-H stretching), 1470(-CH2- vibration and -CH3 deformation), 1304 (in plane ring
bending), 1223 (asymmetric stretching of SO2), 1064 (symmetric stretching of
SO2), 925 (C-H out-of-plane deformation for p-substituted benzene), 657 (out-of-
plane ring bending), 646 (C-Br stretching) cm-1. The melting temperature was
168°C.
75 Chapter 2
2.3.5. Synthetic procedures for poly(p-phenylene-2,5-disulfonic acid)
(PPDSA): Ullmann coupling reaction
6 g of DBBDSA-Li (14.71 mmole), dried at 100°C for 2 days under vacuum, was placed in a 500ml 3-necked round bottom flask and a condenser and rubber septa were fitted to the flask. During activation of the copper powder, the entire system was purged with dried Ar gas for about 30 mins. 9 g of freshly prepared activated copper powder (141.64 mmole) was transferred to the reaction flask and the entire system was kept under vacuum (about 10-3 mm Hg)
for 1 hour after 2 cycles of Ar gas purging and vacuum evacuation. After
releasing vacuum by Ar gas purging, a glass mechanical stirring rod with a Teflon
paddle and a lubricated Trubore glass joint were fitted to the flask under Ar gas purging, and 300ml of freshly distilled and degassed DMF was added to the flask
under Ar gas flow using a double-tipped needle. The monomer was allowed to
dissolve at around 70°C. A stirring speed of about 45rpm was used in this part; it
was then set to 100 rpm and the temperature was raised to 135ºC. After 7 days,
the reaction mixture was allowed to cool to room temperature under Ar gas
purging. The mixture of unreacted copper and precipitated polymer was filtered.
Greenish white low molecular weight polymer was precipitated from the
concentrated DMF solution by adding ~300mL ethanol; the precipitate was
dissolved in D.I. water and the solution was passed through an acidic cationic exchange resin column to protonate the sulfonic acid groups. The collected
aqueous polymer solution was titrated with aqueous NaOH solution to pH 7 by
checking with pH paper. Low molecular weight disodium salt polymer was
76 Chapter 2
obtained as a precipitate after the solution was poured into ethanol (about 10
times the volume of aqueous solution). The yield of low molecular weight polymer
was about 33%. The fraction insoluble in DMF was dissolved in D.I. water
(~600mL); the aqueous solution was separated from the remaining solid by
centrifugation and concentrated to ~50mL. Strings of high molecular weight
polymer formed when the aqueous solution was poured in acetone (~500mL, 2
times). The disodium salt was ion-exchanged to the acid form. It was then titrated
with aqueous NaOH solution to pH 7, and poured into ethanol (about 10 times to
volume of aqueous solution) to precipitate the polymer. The yield of high
molecular weight poly(p-phenylene-2,5-disulfonic acid) (PPDSA) was about 55%.
The polymer structure was confirmed using 1H- and 13C-NMR. 13C NMR
(600MHz, in D2O): δ=130.62 (C-C in 1 and 4 position of aryl), δ=136.62 (C-H in 3
1 and 6 position of aryl), δ=141.88 (C-SO3H in 2 and 5 position of aryl). The H-
NMR spectrum of the polymer will be shown in Results section and discussed in
the Discussion section. Polymer films were made by casting from aqueous
solution on a silanized glass plate and evaporating the water.
77 Chapter 2
2.4. Results
2.4.1. Synthesis of DBBDSA
2.4.1.1. Reaction conditions for DBBDSA synthesis
Several sulfonation conditions for DBBDSA are listed in Table 2.5. Yields
were calculated using the weight of the two compounds isolated after Soxhlet
extraction. The desired monomer is the para-substituted material (DBBDSA-Na,
1,4-dibromo-2,5-dibenzene-sulfonic acid, disodium salt). The disulfonation of
1,4-dibromobenzene (DBB) was studied by changing three experimental factors;
reaction temperature, reaction time and molar ratio of [SO3 (g)]/[DBB]. The
highest yield using best reaction conditions was 38%
To select the best polymerization conditions, several salts of the DBBDSA
monomer were made from DBBDSA-Na using cation exchange, with high yield:
DBBDSA-Li (dilithium slat), -BTMA (dibenzyltrimethylammonium salt) and –TBP
(ditetrabutylphosphonium salt).
78 Chapter 2
Yield of p - Yield of m- Reaction Reaction 15% Lot DBB DBB [SO3] substituted substituted Temp. Time Oleum # (g) (mmole) /[DBB] compound compound (°C) (hours) (mmole) (%) (%)
1 120 72 1.93 8 291 5.3 10% 51%
2 180 24 1.93 8 291 5.3 19% 56%
3 225 24 4.09 17 621 5.6 28% 25%
4 225 10 4.09 17 388 3.5 34% 48%
5 225 24 7.94 33 776 3.5 37% 30%
6 225 24 32.02 133 3103 3.5 29% 39%
7 225 24 32.02 133 2191 2.5 38% 30%
8 225 24 64.04 266 4383 2.5 32% 43%
9 225 24 32.02 133 1920 2.2 35% 22%
Table 2.5. Sulfonation conditions and yields: DBB and 15% oleum are 1,4- dibromo-benzene and fuming sulfuric acid (SO3 gas content was about 15%), respectively. The p-and m-substituted DBB are 1,4-dibromo-2,5-dibenzene- sulfonic acid, disodium salt and 1,4-dibromo-2,6-benzenedisulfonic acid, disodium salt, respectively. The reaction temperature was the bath temperature.
79 Chapter 2
2.4.1.2. Characterizations of DBBDSA
The chemical structures of the monomer salts were characterized using
1H- and 13C-NMR and FT-IR. As the monomer structures had perfect symmetry; there were only one kind of proton and three kinds of carbon on the dibromobenzene ring. Monomers have one proton peak in their spectra
(DBBDSA-Na (Figure 2.7), -Li (Figure 2.10), BTMA (Figure 2.13) and -TBP
(Figure 2.15). There are three peaks in the each of the 13C-NMR spectra of
DBBDSA-Na (Figure 2.8) and DBBDSA-Li (Figure 2.11). They have the same
chemical shifts. FT-IR spectra for all the monomers (Figures 2.9, 2.12, 2.14, and
2.16) showed the characteristic SO2 stretching peak; an asymmetric stretching of
-1 -1 SO2 at 1223 cm in the reported range of 1209 to 1230 cm and a symmetric
-1 -1 stretching of SO2 1080 cm in the reported range of 1060 to 1081cm . The characteristic C-H deformation in para-substituted benzene, 908 cm-1 for the monomer, was within the 894~ 925cm-1 range. Based on these results, the
chemical structures of the salt forms of DBBDSA are confirmed.
80 Chapter 2
8.14 4.63
SO Na a 3 a Br Br a H2O Na O3S
11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1.0 ppm (f1)
1 Figure 2.7. H-NMR spectrum (300 MHz) of DBBDSA, disodium salt in D2O. 3 3 144.9 135.2 118.31
SO3 Na b a Br Br c b Na O3S
c a
200 150 100 50 0 ppm (t1)
13 Figure 2.8. C-NMR spectrum (300 MHz) of DBBDSA, disodium salt in D2O.
81 Chapter 2
0.8 Sym. Stretching 0.7 of SO2 C-Br stretching 0.6 Asymmetric stretching of SO2 0.5
0.4
Absorbance 0.3 Aromatic C-H stretching 0.2
0.1
0 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 Wave number (cm-1)
Figure 2.9. FT-IR spectrum of DBBDSA, disodium salt in a KBr pellet.
8.14 4.63 SO Li a 3
Br Br a a Li O3S
H2O
11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1.0 ppm (t1)
1 Figure 2.10. H-NMR spectrum (600 MHz) of DBBDSA, dilithium salt in D2O.
82 Chapter 2 144.8 135.2 118.3
b SO3 Li b a Br Br c
Li O3S
a c
230 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 ppm (t1)
13 Figure 2.11. C-NMR spectrum (600 MHz) of DBBDSA, dilithium salt in D2O.
2 Sym. Stretching of SO2
1.5 Asymmetric stretching C-Br of SO2 stretching
1 Absorbance Aromatic C-H stretching 0.5
0 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600
Wavenumber(cm-1)
Figure 2.12. FT-IR spectrum of DBBDSA, dilithium salt in a KBr pellet.
83 Chapter 2 8.14 7.47 7.45 7.44 7.41 7.40 4.70 4.35 2.97
b c
SO N d a 3 d Br Br
O3S N
c b a 0.09 0.51 0.22 1.00
9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 ppm (t1)
1 Figure 2.13. H-NMR spectrum (600 MHz) of DBBDSA, diBTMA salt in D2O.
2.5 C-Br Sym. Stretching stretching 2 of SO2 Asymmetric 1.5 stretching of SO2 Aromatic C-H 1 stretching Absorbance Aliphatic C-H 0.5 stretching
0 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600
Wavenumber(cm-1)
Figure 2.14. FT-IR spectrum of DBBDSA, diBTMA salt in a KBr pellet.
84 Chapter 2 7.80 3.14 2.30 2.02 2.00 1.98 1.97 1.96 1.92 1.30 1.27 1.21 1.19 1.16 0.73 0.71 0.69
b c P d c SO3 a d Br Br
O3S
P c b a 0.08 0.67 1.36 1.00
12.0 11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1.0 -2.0 ppm (t1)
1 Figure 2.15. H-NMR spectrum (300 MHz) of DBBDSA, diTBP salt in DMSO-d6.
1.8 Sym. Asymmetric 1.6 Stretching stretching of SO2 of SO2 1.4 Aliphatic C-H
1.2 stretching C-Br stretching 1 Aromatic C-H 0.8 stretching -CH2-, CH3
Absorbance vibration 0.6
0.4
0.2
0 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 Wavenumber(cm-1)
Figure 2.16. FT-IR spectrum of DBBDSA, diTBP salt in a KBr pellet.
85 Chapter 2
2.4.2. Synthesis for PPDSA
2.4.2.1. Polymerization conditions for PPDSA
The Ullmann coupling reaction conditions for PPDSA are listed in Table
2.6. A pretest lot was designed to test the compatibility of conterions on the
Ullmann coupling of the DBBDSA monomer. To make high molecular weight
polymer, different conditions were used in the polymerization.
Conct. Salt form Reaction of Reaction Lot of Solvent temp. reaction Type of stirring time DBBDSA (°C) system (mole/L)
pretest BTMA NMP 130 20 hours 0.10 magnetic
Lot 1 BTMA NMP 135 31 hours 0.18 mechanical
Lot 2 TBP NMP 135 7 days 0.22 mechanical
Lot 3 Li DMF 135 7 days 0.05 mechanical
Table 2.6. Polymerization conditions for PPDSA using Ullmann coupling. BTMA and TBP are benzyltrimethylammonium and tetrabutylphosphonium salts, respectively. The stirring speed was 100 rpm for all lots.
Polymers from lots 1 and 2 precipitated from the reaction solvent during
polymerization. A high molecular weight polymer from lot 3 was collected from an
fraction insoluble in cold DMF (yield: 55%) and a lower molecular weight polymer
fraction was obtained from the DMF solution (yield: 33%). Viscosities of the
collected polymers from lots 1, 2 and 3 were measured and their reduced
viscosities are listed in viscosity results section.
86 Chapter 2
2.4.2.2. Characterization of PPDSA
2.4.2.2.1. NMR spectra for PPDSA: 1H and 14C NMR spectra
1H-NMR spectra of PPDSA, disodium salt form from different lots are
shown in Figure 2.17. This overlapped pattern did not match with our
expectations. Several peaks between 7~8 ppm are closely overlapped in all spectra. Peaks integrations of are not meaningful due to the strong overlaps.
However, peaks in each spectra can be divided into 5 groups; G1 (8.0~8.2 ppm),
G2 (7.94~8.0 ppm), G3 (7.85~7.94 ppm), G4 (7.70~7.85 ppm) and G5
(7.50~7.73 ppm). The relative small peaks in G5 can be seen in all spectra and
are clearly separated from the other peaks.
13 C-NMR spectra of diprotonated PPDSA (lot 2) in D2O is shown in Figure
2.18. The PPDSA spectrum has three major peaks (130.6, 136.6, 141.9 ppm);
this exactly matches with the expected chemical structure of polymer.
87 Chapter 2 8.02 8.01 8.00 7.98 7.97 7.95 7.88 7.84 7.83 7.81 7.80 7.78 7.70 7.65 7.61 7.55
Lot 1 8.21 5.22 3.63 15.24 0.35 1.00 0.09 0.16
8.04 8.03 8.00 7.99 7.99 7.98 7.92 7.91 7.89 7.88 7.85 7.84 7.83 7.80 7.79 7.65
Lot 2 12.46 2.29 11.63 5.40 1.00 8.04 8.02 8.00 7.99 7.98 7.90 7.89 7.88 7.86 7.83 7.82 7.80 7.79 7.78 7.65
Lot 3, low mol wt. 33.75 4.74 14.10 12.65 1.00
8.04 8.03 8.00 7.99 7.98 7.91 7.89 7.88 7.84 7.83 7.80 7.65
Lot 3, high mol wt. 164.16 8.26 51.87 13.72 1.00
8.10 8.00 7.90 7.80 7.70 7.60 ppm (t1)
Figure 2.17. 1H-NMR spectra (from 600MHz except lot 1(300MHz)) of PPDSA, disodium salt of lots 1, 2, and 3 (high and low molecular weight polymer) in D2O at 25°C (concentration of PPDSA: 4.4 g/dL). Integrations of peaks of each spectrum were normalized using the isolated peak at 7.65ppm, in each spectrum.
88 Chapter 2
SO3H b a 141.9 136.6 130.6 c n HO3S
ab c
160 150 140 130 120 110 100 ppm (t1)
Figure 2.18. 13C-NMR spectrum (600MHz) of PPDSA, diprotonated form (lot 2) in D2O at 25°C.
89 Chapter 2
2.4.2.2.2. Rheological properties
Rheograms for aqueous PPDSA solutions at different concentrations
were taken and are shown in Figure 2.19. Because PPDSA is rigid rod liquid
crystalline polymer, a polymer solution at 38.51g/dL shows characteristic shear
rate dependent viscosity and has two regions (the shear thinning and Newtonian
plateau) in its viscosity-shear rate plot. However, the effect of polymer solution
concentration on the shear rate dependent viscosity could not be studied. When
the polymer solutions were diluted to the range of 0.48~19.26g/dL, viscosities
measured below 0.1s-1 were scattered; the rheometer had reached its sensitivity
limit.
1.E+02
38.51 g/dL 1.E+01 19.26 g/dL
3.85 g/dL 1.E+00 0.48 g/dL
1.E-01
1.E-02 Viscosity (Pa*s) Viscosity
1.E-03
1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 Shear Rate (1/s)
Figure 2.19. Rheograms for aqueous PPDSA (lot 3, high molecular weight polymer) solutions at different concentrations and shear rates.
90 Chapter 2
2.4.2.2.3. PPDSA Viscosity results
The PPDSA solutions have an abnormal upturn of the reduced viscosity with decreasing concentration independent of the cation species, solvent or salt concentration (Figure 2.20). The reduced viscosities are almost constant at high concentration and rise at low concentration. The effects of shear rate, salt concentration, solvent, and cation species on the reduced viscosity will be shown in this section.
2.500 LOT 1_H_water LOT 2_Li_DMF LOT 2_Li_0.1M-LiBr-DMF 2.000 LOT 2_Li_0.5M LiBr-DMF LOT 2_TBP_DMF /c )
sp LOT 2_Li_0.1M LiBr-DMF/NMP(33/67) η LOT 2_Li_0.3M LiCl-water 1.500 LOT 2_H_water LOT 3_high mol. wt. polymer_H_water LOT 3_high mol. wt. polymer_Na_0.3M LiCl-water 1.000 Reduced viscosity( Reduced viscosity( 0.500
0.000 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Concentration (g/dL)
Figure 2.20. Reduced viscosities as a function of concentration for different salt forms of PPDSA (lots 1, 2 and 3) in different solvents at 35°C.
91 Chapter 2
2.4.2.2.3.1. Effect of shear rate
The effect of shear rate on the reduced viscosity was studied using viscometers having different shear rates (3537, 2697 and 1707 s-1) in D.I. water.
A plot of viscosity vs. shear rate for an aqueous solution of high molecular weight
lot3 PPDSA in water is shown in Figure 2.21. This plot show that, as would be
expected for high molecular weight rigid rod materials, the measured viscosity
decreases as the shear rate increases
2.5 Lot 3_high mol. wt._1C C282_shear rate=3537(1/s) Lot 3_high mol. wt._1 J659_shear rate=2697(1/s) 2 Lot 3_high mol. wt._0C C453_shear rate=1707(1/s)
1.5 /c (dL/g) /c
sp 1 η
0.5
0 0 0.2 0.4 0.6 0.8 Concentration (g/dL)
Figure 2.21. Reduced viscosities of PPDSA, diprotonated (lot 3, high molecular weight polymer) in D.I. water at 35°C using different viscometers (shear rates of viscometers = 3537, 2697 and 1707s-1 in D.I. water, see Table 2.1).
92 Chapter 2
2.4.2.2.3.2. Effect of salt concentration on viscosity
The salt concentration effect on the reduced viscosities of PPDSA was
studied in DMF. When a salt solution (LiBr in DMF) was used, the reduced
viscosity of lot 2 PPDSA, decreased due to the shielding of the sulfonic acids on
the polymer backbone by the added salt. (Figure 2.22)
Polymer-salt solutions with lithium salt concentrations between 0.1 and
1.0 M have almost constant reduced viscosities in the concentration range 0.2 to
0.6 g/dL, solutions.
1.2 Lot 2_diLi in DMF Lot 2_diLi in 0.1M LiBr-DMF solution 1.0 Lot 2_diLi in 0.5M LiBr-DMF solution Lot 2_diLi in 1.0M LiBr-DMF solution
0.8
/c 0.6 sp η
0.4
0.2
0.0 0.00.20.40.60.8 c (g/dL)
Figure 2.22. Effect of salt concentration on reduced viscosities of PPDSA (lot 2) in DMF. The viscosities were measured at 35°C using the 0C C453 viscometer.
93 Chapter 2
2.4.2.2.3.3. Effect of solvent
The reduced viscosities of lot 2 PPDSA in 0.1M LiBr-DMF and -DMF/NMP
(33/67, v/v) solutions were measured using the same experimental conditions
(polymer sample, cation species, salt concentration, viscometer and
temperature) (Figure 2.23). The effect of solvent is small, but, the reduced
viscosity of polymer in DMF is slightly higher at high concentration.
0.20
0.19
0.18
0.17
0.16 /c (dL/g) /c sp
η 0.15
0.14 Lot 2_diLi in 0.1M LiBr-DMF solution 0.13 Lot 2_diLi in 0.1M LiBr-DMF/NMP solution 0.12 0.0 0.2 0.4 0.6 0.8 Concentration (g/dL)
Figure 2.23. Effect of solvent on the reduced viscosity of PPDSA (lot 2). The viscosities were measured at 35°C using 0C C434 viscometer.
94 Chapter 2
2.4.2.2.3.4. Effect of cationic species
The reduced viscosities of PPDSA (lot 1), diprotonated form and disodium salt were measured under the same conditions. The effect of cation (protonated
form vs. sodium salt) on the reduced viscosity of PPDSA is small (Figure 2.24)
0.20 0.18 Lot 1_diprotonated form in water 0.16 Lot 1_diNa in water 0.14 0.12 0.10 /c (dL/g)
sp 0.08 η 0.06 0.04 0.02 0.00 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Concentration (g/dL)
Figure 2.24. Effect of PPDSA counterion on reduced viscosities of PPDSA (lot 1). Viscosities were measured at 35°C using 0C C453 viscometer.
95 Chapter 2
2.4.2.2.3.5. Effect of polymer molecular weight
The reduced viscosities of diprotonated high and low molecular weight polymers of lot 3 are compared with diprotonated lot 2 and lot 1 in Figure 2.25.
The reduced viscosities at about 0.40~0.44 g/dL were 0.68 dl/g (high molecular weight polymer in lot 3), 0.26 dl/g (low molecular weight polymer in lot 3), 0.21 dl/g (lot 2), and 0.07 dl/g (lot 1). The lot 3 low molecular weight polymer (from soluble fraction) has a viscosity similar to that of the polymer made in NMP, lot 2 that precipitated during polymerization.
2.5
Lot 1
2.0 Lot 2
Lot 3-high mol.wt. 1.5 Lot 3-low mol.wt. /c (dL/g) /c
sp 1.0 η
0.5
0.0 0.0 0.2 0.4 0.6 0.8 Concentration (g/dL)
Figure 2.25. Comparison of the reduced viscosities of lots 1, 2 and 3, diprotonated PPDSA in D.I. water at 35°C using 0C C453 viscometer. The each polymer was dissolved in D.I. water and diluted with D.I. water.
96 Chapter 2
2.4.2.2.4. GPC (Gel Permeation Chromatography)
Gel permeation chromatography (GPC) was used to obtain the relative
molecular weight of the resulting polymers compared to that of polystyrene (PS).
A GPC chromatogram of DBBDSA-Li (Figure 2.26) was taken to compare its
elution time with that of the polymer; its elution time is 27.3 mins. It absorbed UV
light in the range of 270 to 300nm (maximum absorbance at 280nm) and does
not absorb above 300nm.
0.10
0.08 270 27.3 min 280 0.06 290 0.04 300 310 0.02 320 Intensity (AU) Intensity
0.00 W
a 270 v e 280 l e 290 n g 300 t h 310 m 320 ( n m 15 20 25 30 35 ) Elution time (min)
Figure 2.26. GPC chromatogram of DBBDSA-Li in DMF: the pumping speed was 0.7 mL/min and the column chamber temperature was 35°C. 0.01M LiBr-DMF solution was used as the eluent.
GPC test conditions for PPDSA (lot 1), disodium salt are summarized in
Table 2.7. Because salt solutions have been frequently used in GPC studies to break association in polyelectrolyte polymers, 0.01M LiBr-DMF solution was used as an eluent and compared with DMF. GPC chromatograms of the polymer under
97 Chapter 2 different test conditions are shown in Figure 2.27 and the elution times of the polymer are summarized in Table 2.8.
When chromatograms of the polymer (Figure 2.27, a1 or a2) are compared with that of the monomer (Figure 2.20), their elution times are almost that of monomer. At low column temperature (35°C), one peak can be seen in
Figure 2.27, b1 and b2. At high column temperature (150°C) with the other conditions, the same two peaks are visible and their relative intensity changed with polymer concentration.
Conc. of Conc. of Column Solvent for sample sample temperature Eluent sample = 1.0 g/L = 0.125 g/L (°C) Chart ID in Figure 2.21 0.01M LiBr- 35 a1 a2 DMF DMF/water 35 (67/33) DMF b1 b2
150 DMF c1 c2
Table 2.7. GPC test conditions for PPDSA (lot 1), disodium salt. The elution speed was 0.7 mL/min. Chart IDs are notations in Figure 2.23.
98 Chapter 2
a1) a2)
0.030 0.004 0.025 0.003 0.020 270 0.015 280 0.002 270 290 280 0.010 300 0.001 290 0.005 310 300 Intensity (AU) 0.000 320 Intensity (AU) 0.000 310 330 320 -0.005 340 -0.001 330 -0.010 350 339 280 W 280 350 a 300 ve 300 W 320 le 320 a ng ve 340 th 339 len ( gth nm (n 10 15 20 25 30 35 ) 10 15 20 25 30 35 m) Elution time (min) Elution time (min) b1) b2)
0.05 0.006 0.04 0.005 0.03 270 0.004 270 280 280 0.02 290 0.003 300 0.002 290 0.01 310 300 320 0.001 Intensity (AU) Intensity (AU) 0.00 310 330 0.000 340 320 -0.01 W 350 -0.001 a 330 ve 280 W 280 le n 300 av 300 340 g ele th 320 n 320 gt 350 (n 340 h ( 340 m nm ) 10 15 20 25 30 35 ) 10 15 20 25 30 35 Elution time (min) Elution time (min)
c1) c2) 0.030 0.0035 0.0030 0.025 0.0025 0.020 0.0020 0.015 0.0015 270 0.0010 270 0.010 280 0.0005 280 290 290 0.0000 300 0.005 300 -0.0005 310
310 Intensity(AU) Intensity (AU) 0.000 -0.0010 320 -0.005 320 -0.0015 330 330 -0.0020 340 -0.010 340 350 280 350 W 300 280 av W el 320 av 300 en ele g 340 ng 320 th th 340 ( (nm nm 10 15 20 25 30 35 ) 10 15 20 25 30 35 ) Elution time (min) Elution time (min)
Figure 2.27. GPC chromatograms of PPDSA (lot 1), disodium salt under different elution conditions: the elution speed was 0.7mL/min. Test conditions are summarized in Table 2.7.
99 Chapter 2
elution time in min Column (relative molecular weight) Sample temperature Eluent solvent (°C) of 1.0g/dL of 0.125g/dL sample sample
0.01M 35 27.74 27.05 LiBr-DMF
DMF/water 20.25 20.18 35 DMF (67/33) (171,900) (179,300)
18.93 18.99 150 DMF 27.53 27.40
Table 2.8. Elution time and calculated relative molecular weights of PPDSA (lot 1), disodium salt under different test conditions: polymer molecular weights relative to PS standards were calculated using a calibration curve. Monomer elution time in 0.01MLiBr-DMF was 27.3 min.
100 Chapter 2
2.4.2.2.5. Water uptake
2.4.2.2.5.1. Experimental set-up
To set up the drying conditions for the water uptake test, two types of drying conditions were checked: A) After drying in the oven (over 100°C for 1 day) and measuring the weights of the weighing bottles and caps, polymer films
were put in the bottles and dried at 90°C/ 24hours under vacuum. The bottle
containing the film was cooled in a desiccator having molecular sieves (4Å). The bottle was capped with its pre-weighed cap and the weight of the capped weighing bottle containing the film was taken, B) the same procedure was used but the film was dried further at 150°C for 1hour. The final data are shown in
Table 2.9. The weights of the dried samples did not change after using condition
B (the difference is only below 3% of the weight of dried film). Therefore, the drying conditions for further tests used “90°C/ 24hours/ under vacuum (condition
A)”.
Weight of sample #1 Weight of sample #2 Drying condition (g) (g)
Condition A 0.0483 0.0990 (90°C/24hrs/under vacuum)
Condition B 0.0498 0.0980 (150°C/1hr/under vacuum)
Table 2.9. Results of drying conditions test using PPDSA film (lot 2).
101 Chapter 2
Before running the water uptake test, the equivalent weight (Eq. Wt., molecular weight per mole of sulfonic acid) of dried sample was evaluated as below. After drying the polymer films and recording their weight, 5ml of 2M aqueous NaCl solution was added to each bottle. The resulting solution was titrated with a standardized sodium hydroxide solution using phenolphthalein as the end-point indicator. The measured equivalent weights are shown in Table
2.10. The difference between the average measured equivalent weight at 0%RH and the theoretical value (118.13 g/ [SO3H]) is 21 g/ [SO3H]. Some water could not be removed by the drying process; lambda (λ, the number of water molecules on one sulfonic acid) is 1.2. That means that PPDSA film holds 1.2 water molecules per sulfonic acid group very strongly.
Equivalent Weight of [SO H] Sample # 3 weight dried film (g) (mole) (g/[SO3H])
#1 0.0482 3.46E-04 139
#2 0.0471 3.38E-04 140 #3 0.0480 3.42E-04 140 #4 0.0517 3.74E-04 138 #5 0.0547 3.95E-04 138 #6 0.0522 3.72E-04 140 Average 139.0 ± 0.4
Table 2.10. Equivalent weight of dried PPDSA films (lot 2). The drying conditions were condition A as described previously. The theoretical equivalent weight of the anhydrous polymer is 118.13 g/[SO3H]. λ for the dried sample is 1.2.
102 Chapter 2
2.4.2.2.5.2. Evaluation of water uptake and lambda (λ)
To decide the minimum storage time needed for equilibrating at controlled
relative humidities, the weight changes of the polymer films were monitored for 6
days. In all the tested relative humidities (15 to 75%RH), the weights were
constant after one day’s equilibration (Figure 2.28). So the equilibration time in
the humidity chambers was fixed at 1 day.
140 15%RH 35%RH 120 50%RH 75%RH 100
80
60
40
20 Weight change to weight dried film (%) 0 0 24487296120144 Storage time (hours)
Figure 2.28. Tests to determine the minimum storage time in controlled humidity chamber for water uptake test. Diprotonated PPDSA, (lot 2) was used.
The water uptake test at different humidities was carried out using the pre-established drying conditions and equilibrium time; the results are shown in
Table 2.11. Two lots of PPDSA films (lots 1 and 2) were studied for the effect of polymer molecular weight on lambda (Figure 2.29). Even though lot 2 had a higher molecular weight and higher reduced viscosity (about 3 times that of lot 1,
103 Chapter 2
in viscosity results section), λ for both lots was identical within experimental error from 15 to 50%RH. But, at 75%RH, lot 1 polymer absorbed more water than lot 2 polymer. This could be because the lower molecular weight polymer was more soluble (greater ∆S of mixing).
W - W %RH W (g)*1 W (g)*2 2 1 [SO H] (mole) *4 Lambda*5 1 2 (g)*3 3
11 0.0164 0.0203 0.0039 1.20E-04 3.0
15 0.0471 0.0662 0.0191 3.38E-04 4.3
35 0.0480 0.0734 0.0254 3.42E-04 5.3
50 0.0517 0.0876 0.0359 3.74E-04 6.5
75 0.0547 0.1064 0.0517 3.95E-04 8.5
100 0.0522 0.2226 0.1704 3.72E-04 26.7
Table 2.11. Water uptake and lambda evaluation for PPDSA (lot 2). *1. W1 : the weight of the dried sample; 2. W2 : the weight of the equilibrated sample; 3. weight of absorbed water = W2 - W1; 4. [SO3H] was measured by titration with the standardized aqueous NaOH solution; 5. Lambda (λ) = 1.2 (λ of the dried film) + [(W3 - W2)/18]/[SO3H]].
104 Chapter 2
12.0
10.0
8.0
6.0 Lambda 4.0
PPDSA_LOT1 2.0 PPDSA_LOT2 0.0 020406080100 Relative humidity (%)
Figure 2.29. Lambda (λ) of PPDSA films as a function of relative humidity. Lot 2 PPDSA had higher molecular weight than lot 1.
105 Chapter 2
2.4.2.2.6. Dimensional changes with water uptake
The dimensional changes of PPDSA in three directions at different relative humidities are summarized in Table 2.12, a). To compare the degree of dimensional change at different humidities with each other, data are normalized to the dimensions of the dried film for the 15%RH test condition, shown in Table
2.12, b). PPDSA does not expand isotropically (Figure 2.30). From 15 to 50%RH, the X and Y directional changes are almost double the changes in the Z direction.
At 75%RH, the expansion was 23% in the X direction, 28% in the Y direction and
30% in the Z direction compared to that at 0%RH. The volume of the equilibrated film sharply increased from 0 to 15%RH and doubled its original volume at
75%RH.
106 Chapter 2
a)
Dimensions dry (before equilibration) Dimensions after equilibration Direction 15%RH 35%RH 50%RH 75%RH 15%RH 35%RH 50%RH 75%RH
X direction 1.80 2.22 2.17 1.78 2.08 2.58 2.55 2.19 (mm)
Y direction 2.47 2.65 2.34 3.17 2.80 3.08 2.90 4.07 (mm)
Z direction 397 404 378 427 423 443 430 554 (um)
b) Relative humidity 0%RH 15%RH 35%RH 50%RH 75%RH
Lambda 1.2 4.3 5.3 6.5 8.5
X direction 100% 116% 116% 118% 123%
Y direction 100% 113% 116% 124% 128%
Z direction 100% 106% 110% 114% 130%
Volume 100% 139% 148% 166% 205%
Table 2.12. Dimensional changes of PPDSA (lot 2) at different humidities; a) in length (mm or um) and b) as % of dried film dimensions. The X and Y directions are perpendicular and parallel to the casting direction, respectively. The Z direction is the thickness direction.
107 Chapter 2
150
X-direction 140 Y-di recti on Z-direction
130
120
110 Increase ratio to 0%RH dimension (%) dimension 0%RH to ratio Increase 100 0 1020304050607080 Relative Humidity (%)
Figure 2.30. Plots of dimensional changes of PPDSA film (lot 2) vs. relative humidity .
108 Chapter 2
2.4.2.2.7. Differential Scanning Calorimetry (DSC)
Before running DSC measurements on our conditioned films, the heat of
melting and vaporization of bulk water was studied. The theoretical heat of
melting and vaporization are 333.5 J/g and 2838 J/g. 60 Our measured heats of
melting and vaporization of bulk water were 309.7 J/g and 2125 J/g. Since these
are close to the reference values, it showed that the experimental setup was
adequate.
Chilled PPDSA films humidified between 15 and 75%RH had no endothermic peak between -50 to 10°C (Figure 2.31). The absorbed water molecules do not freeze even at -50°C.
High temperature scans were also run on the PPDSA films (Figure 2.32).
The curves have endotherms at 111~120°C and 152~160°C. Endothermic shoulders above 240~250°C correspond to the decomposition of sulfonic acid groups.
109 Chapter 2
2 a) 0
-2
-4
-6
-8 W/g -10
-12
-14
-16 bulk water_2.38mg -18 -60 -40 -20 0 20 40 60 80
Temperature (C)
0.1 b) 0
-0.1
-0.2
-0.3
W/g -0.4 0%RH 15%RH -0.5 35%RH -0.6 50%RH 75%RH -0.7 bulk water_2.38mg -0.8 -80 -60 -40 -20 0 20 40 60 Temperature (C)
Figure 2.31. DSC thermograms for melting of a) bulk water and b) absorbed water in equilibrated PPDSA films (lot 3, high molecular weight polymer) at different humidities. Heating rate is 10°C/min after cooling to -50°C and holding there for 30 mins. The N2 flow rate is 60 mL/min.
110 Chapter 2
5 a) 0
-5
-10
-15
-20
Heat Flow(W/g) -25
-30
-35 Bulk water_2.38mg
-40 0 25 50 75 100 125 150 175 200 225 250 275 300 Temperature (°C)
0.5
b) 0
-0.5
-1
-1.5
Heat Flow (W/g) Flow Heat 0%RH -2 15%RH 35%RH 50%RH -2.5 75%RH Bulk water_2.38mg -3 0 25 50 75 100 125 150 175 200 225 250 275 300 Temperature (°C)
Figure 2.32. DSC thermograms for vaporization of a) bulk water and b) absorbed water in equilibrated PPDSA films (lot 3, high molecular weight polymer) at different humidities. Heating rate is 10°C/min. The N2 flow rate is 60 mL/min.
111 Chapter 2
2.4.2.2.8. Thermogravimetric analysis (TGA)
PPDSA has high a decomposition temperature (about 304°C) and loss about 13% of its weight before decomposition (Figure 2.33). Above 304°C, decomposition proceeds rapidly and about 48% are lost compared to the initial weight. The weight loss up to 304°C corresponds to about one water molecule per sulfonic acid (λ of dried film = 1.2); the second weight loss corresponds to the decomposition of sulfonic acids on the polymer backbone.
Sample: PU7 File: D:...\PU7_A2-17H_full data_TGA.txt Size: 3.3290 mg TGA Operator: Lee Method: Ramp Run Date: 2006-07-25 15:16 Comment: PU7 A2-17(H) Instrument: TGA Q500 V6.3 Build 189 120 1.0
100 0.8
303.98°C 80 86.92% 0.6 Weight (%) 60 354.18°C 0.4 51.50% Deriv. Weight (%/°C) Weight Deriv.
499.79°C 42.20% 40 0.2
20 0.0 0 200 400 600 800 Temperature (°C) Universal V4.1D TA Instruments
Figure 2.33. TGA thermogram of PPDSA film (lot 2). The sample was dried at 90°C for 1 day. Heating rate is 10°C/min under N2 (60mL/min).
112 Chapter 2
2.4.2.2.9. Wide angle X-ray diffraction (WAXD)
2.4.2.2.9.1. WAXD sample preparation method
In order to obtain reliable WAXD data, the humidified polymer needed to
maintain its water content during the measurement, and the equilibration should
allow the sample to expand freely to its equilibrium dimension.
Several materials were examined for sealing the humidified polymer sample to get reproducible WAXD data. A pre-test control run was made by stacking two sheets of sealing material without polymer film and recording their diffraction pattern using the parameters given in Table 2.13. The diffractograms of each material are shown in Figure 2.34. Mylar films (Mylar® C) were provided by
Dupont Tenijin Films Company. Because Mylar (PET) and Kapton (polyimide)
films have some degree of crystallinity, even if the thickness is very thin (Mylar®
C: ~4.5 um), any PPDSA diffraction peaks could be concealed under the intense peaks from the sealing polymer. Cover glass (~100um) was a possible sealing material. But, its thickness (~100um) and composition (SiO2) generated so much
scattering that the PPDSA peaks could not be seen cleanly. Finally, PVC was
found to be the most suitable sealing material for maintaining the humidity of
sample while obtaining high quality data. This sealing material was also used for
sample preparations in the 2 dimensional X-ray and the optical polarizing
microscopy experiments.
113 Chapter 2
Parameters Setting values Start angle (°) 0.2 Stop angle (°) 35 Power 30kV / 30mA Sampling width (°) 0.1 Scanning speed (°/min) 0.5 Div. slit (mm) 2 Div. H. L. slit (mm) 5 Rec. slit (mm) Open Sct. Slit (mm or °) Open
Table 2.13. WAXD test conditions for studies to select sealing materials. The reflection mode was used.
250000
PVC_16um
200000 Mylar(PET)_10um Mylar(PET)_8um
Mylar(PET)_4.5um
150000 Kapton_54um
Cover glass_100um
100000 Intensity (a. u.) (a. Intensity
50000
0 0 5 10 15 20 25 30 35 2 theta (degree)
Figure 2.34. WAXD diffractograms of materials studied for sealing.
114 Chapter 2
2.4.2.2.9.2. WAXD diffractogram of PPDSA
The equilibration of dried film at different humidities and the sealing method used with PVC films for the WAXD experiments followed the procedures
as shown in Scheme 2.5. From Table 2.14 and Figures 2.35 and 2.36, we can
assign six peaks (A, B, C1, D, E, and F) in the transmission mode and six peaks
(A, C1, C2, D, E, and F) in the reflection mode. Of all the peaks, only peak A
changes with the film water content; the peak positions (2θ in WAXD) decrease with increasing relative humidity. The other peaks, (B, C1, C2, D, E, and F), are independent of the water content.
Peak A is narrow and large, and thus suitable for analyzing the d spacing change as a function of relative humidity. It is very intense and sharp in the transmission mode and almost nonexistent in the reflection mode. The average value and standard deviation of the d spacings in the transmission mode at different relative humidities are listed in Table 2.15, a).
Peak B is broad and shown in the transmission mode. In the reflection
mode, it might be concealed by the broad C1 and C2 peaks. The d spacings for
the B peaks changed within ~1 Å. But, when deviations for peaks of B, Table 2.14,
and their breadth are considered, the d spacing changes are within experimental
error ranges. The average and standard deviation are 6.15 ± 0.29 (for parallel
samples) and 6.25 ± 0.34 (for perpendicular samples) Å (Table 2.15, b)).
115 Chapter 2
The peaks C1, C2, D, E and F are easily seen in the reflection mode. In the transmission mode these peaks are very broad and less intense compared to peak A. The average and standard deviation of the d spacings in the reflection mode of these peaks are listed in Table 2.15, b).
116 Chapter 2 a) Transmission mode
Peak %RH Parallel1 Perpendicular2 2theta (°) d-spacing (Å) 2theta (°) d-spacing (Å)
A011.12 ±1.7 7.96 ±1.04 10.91 ±0.7 8.11 ±0.52
11 10.14 ±0.3 8.72 ±0.26 10.04 ±0.3 8.81 ±0.26 15 9.84 ±0.3 8.99 ±0.24 9.95 ±0.3 8.89 ±0.23 35 9.28 ±0.2 9.53 ±0.24 9.39 ±0.3 9.41 ±0.27 50 8.66 ±0.3 10.21 ±0.31 8.86 ±0.3 9.99 ±0.29 75 8.02 ±0.2 11.02 ±0.29 7.98 ±0.3 11.08 ±0.34 B015.70 ±1.7 5.64 ±0.56 15.41 ±1.7 5.75 ±0.57 11 14.88 ±2.7 5.95 ±0.90 15.00 ±1.9 5.91 ±0.66 15 14.10 ±1.4 6.28 ±0.55 14.03 ±1.5 6.32 ±0.59 35 14.07 ±1.8 6.29 ±0.72 13.72 ±1.6 6.45 ±0.67 50 14.09 ±1.8 6.29 ±0.70 13.63 ±1.8 6.50 ±0.74 75 13.81 ±2.4 6.41 ±0.94 13.43 ±2.3 6.59 ±0.95 C1 0 17.84 ±1.5 4.97 ±0.37 17.22 ±1.4 5.15 ±0.39 11 17.07 ±1.0 5.19 ±0.28 16.95 ±0.8 5.23 ±0.22 15 16.49 ±1.3 5.38 ±0.38 16.63 ±1.1 5.33 ±0.33 35 17.04 ±2.4 5.20 ±0.64 16.28 ±2.2 5.44 ±0.65 50 NA NA 16.43 ±2.7 5.40 ±0.75 75 NA NA 17.14 ±2.6 5.18 ±0.68 D022.66 ±1.4 3.93 ±0.22 22.87 ±1.9 3.89 ±0.30 11 22.81 ±2.3 3.90 ±0.35 23.70 ±1.5 3.75 ±0.22 15 23.40 ±2.5 3.80 ±0.36 23.78 ±4.5 3.74 ±0.59 35 22.87 ±2.9 3.89 ±0.43 23.15 ±3.3 3.84 ±0.48 50 23.35 ±3.0 3.81 ±0.43 22.91 ±3.6 3.88 ±0.52 75 22.89 ±2.8 3.88 ±0.41 22.70 ±2.8 3.92 ±0.42 E025.24 ±1.9 3.53 ±0.24 25.40 ±1.3 3.51 ±0.17 11 24.28 ±2.3 3.67 ±0.31 25.22 ±0.7 3.53 ±0.09 15 NA NA NA NA 35 NA NA NA NA 50 NA NA NA NA 75 NA NA NA NA F028.28 ±4.6 3.16 ±0.43 27.70 ±1.4 3.22 ±0.15 11 NA NA 26.92 ±1.1 3.31 ±0.13 15 27.46 ±3.7 3.25 ±0.38 28.80 ±2.6 3.10 ±0.25 35 27.58 ±4.2 3.23 ±0.41 27.81 ±4.0 3.21 ±0.39 50 28.11 ±4.1 3.18 ±0.40 28.02 ±4.8 3.19 ±0.46
75 28.02 ±4.7 3.19 ±0.45 28.01 ±4.2 3.19 ±0.40
117 Chapter 2 b) Reflection mode
Peak %RH Parallel1 Perpendicular2 2theta (°) d-spacing (Å) 2theta (°) d-spacing (Å)
A010.48 ±0.3 8.44 ±0.26 9.70 ±0.4 9.12 ±0.33
15 9.76 ±0.2 9.07 ±0.21 9.91 ±0.6 8.93 ±0.49 35 9.18 ±0.3 9.64 ±0.30 9.21 ±0.7 9.60 ±0.66 50 9.01 ±0.2 9.81 ±0.25 8.72 ±0.6 10.15 ±0.60 75 7.93 ±0.5 11.16 ±0.60 8.04 ±0.3 11.00 ±0.33 C1 0 16.90 ±1.6 5.25 ±0.45 16.75 ±1.4 5.29 ±0.41 15 16.90 ±1.5 5.25 ±0.43 16.97 ±1.4 5.23 ±0.41 35 16.78 ±1.4 5.28 ±0.40 16.75 ±1.9 5.29 ±0.53 50 16.80 ±1.2 5.28 ±0.35 16.82 ±1.4 5.27 ±0.40 75 16.84 ±1.3 5.27 ±0.36 16.90 ±0.9 5.25 ±0.26 C2 0 18.71 ±0.8 4.74 ±0.18 18.57 ±1.3 4.78 ±0.31 15 18.66 ±0.7 4.75 ±0.16 18.64 ±0.9 4.76 ±0.22 35 18.61 ±0.9 4.77 ±0.22 18.72 ±0.5 4.74 ±0.13 50 18.68 ±1.0 4.75 ±0.23 18.69 ±1.2 4.75 ±0.28 75 18.72 ±1.0 4.74 ±0.24 18.66 ±0.9 4.76 ±0.22 D023.67 ±1.3 3.76 ±0.19 23.77 ±1.0 3.74 ±0.15 15 23.82 ±1.2 3.74 ±0.17 23.83 ±1.2 3.73 ±0.18 35 23.39 ±1.6 3.80 ±0.23 24.08 ±1.3 3.70 ±0.19 50 23.55 ±1.3 3.78 ±0.20 23.63 ±1.2 3.76 ±0.18 75 23.55 ±1.3 3.78 ±0.19 23.59 ±1.4 3.77 ±0.20 E025.82 ±1.2 3.45 ±0.16 25.78 ±1.6 3.46 ±0.19 15 25.77 ±1.0 3.46 ±0.13 25.97 ±1.7 3.43 ±0.20 35 25.82 ±1.0 3.45 ±0.13 26.18 ±1.1 3.40 ±0.13 50 25.80 ±1.2 3.45 ±0.16 25.89 ±1.7 3.44 ±0.21 75 25.78 ±1.2 3.46 ±0.15 26.14 ±1.5 3.41 ±0.19 F028.87 ±6.9 3.09 ±0.58 29.57 ±5.4 3.02 ±0.45 15 28.42 ±6.1 3.14 ±0.54 29.57 ±5.7 3.02 ±0.48 35 29.53 ±4.3 3.03 ±0.37 30.66 ±0.9 2.92 ±0.08 50 33.40 ±4.8 2.68 ±0.32 29.90 ±4.9 2.99 ±0.41 75 30.90 ±4.0 2.89 ±0.32 29.52 ±2.9 3.03 ±0.26
Table 2.14. WAXD peak data of PPDSA (lot 2) in a) transmission mode and b) reflection mode. Note) 1 and 2: Parallel and perpendicular correspond to the X- ray beam to the casting direction. ±deviation in degree was calculated using 0.5*FWHH of each deconvoluted peak, and that in Å was calculated with maximum and minimum of 2θ using Bragg’s law.
118 Chapter 2
50000 a) 45000 0%RH 11%RH 40000 A 15%RH 35000 35%RH 50%RH 30000 75%RH 25000
20000 Intensity (a.u.) 15000
10000 DE BC1 F 5000
0 0 5 10 15 20 25 30 35 2 theta (degree)
60000 b) 0%RH 50000 11%RH A 15%RH 35%RH 40000 50%RH 75%RH 30000
Intensity (a.u.) 20000
D E BC1 F 10000
0 0 5 10 15 20 25 30 35 2 theta (degree)
Figure 2.35. WAXD diffractograms in transmission mode of PPDSA (lot 2); a) from parallel sample and b) from perpendicular sample.
119 Chapter 2
25000 a) 0%RH 15%RH E 20000 35%RH 50%RH 75%RH C2 C1 15000 D
10000 F
Intensity (a.u.) A
5000
0 0 5 10 15 20 25 30 35 2 theta (degree)
25000 b) 0%RH 15%RH C1 C2 20000 35%RH 50%RH 75%RH
15000 D E
10000 Intensity (a.u.) A F
5000
0 0 5 10 15 20 25 30 35 2 theta (degree)
Figure 2.36. WAXD diffractograms in reflection mode of PPDSA (lot 2); a) from parallel sample and b) from perpendicular sample.
120 Chapter 2
a)
d spacing of peak A (value ± deviation)(Å) %RH Parallel Perpendicular
0 7.96 ± 1.04 8.11 ± 0.52 11 8.72 ± 0.26 8.81 ± 0.26 15 8.99 ± 0.24 8.89 ± 0.23 35 9.53 ± 0.24 9.41 ± 0.27 50 10.21 ± 0.31 9.99 ± 0.29 75 11.02 ± 0.29 11.08 ± 0.34
b)
d spacing (average ± standard deviation)(Å) Peak Parallel Perpendicular
B 6.15 ± 0.29 6.25 ± 0.34 C1 5.26 ± 0.02 5.27 ± 0.03 C2 4.75 ± 0.01 4.76 ± 0.01 D 3.77 ± 0.03 3.74 ± 0.03 E 3.45 ± 0.00 3.43 ± 0.02 F 2.97 ± 0.18 2.99 ± 0.05
Table 2.15 Deconvolution results of d spacing of peaks: a) d spacings of peak A from the transmission mode spectra (Table 2.14) at different relative humidities, b) Average of d spacings of B, C1, C2, D, E and F peaks. Average and standard deviation (Å) of d spacings in b) were calculated using data in Table 2.14.
121 Chapter 2
2.4.2.2.10. 2D X-ray diffraction
Two dimensional X-ray diffraction is a very useful technique to characterize the orientation of polymer chains in a film. If polymer chains align,
several spots along the equator could possibly be seen in the X-ray rather than a
ring. Similarly, repeats along the polymer chain could produce several spots
along the meridion.61
The 2D X-ray spectra of PPDSA at different relative humidities are shown
in Figure 2.37 and the d spacings of the rings are listed in Table 2.16 . However,
all the PPDSA films (at relative humidities from 0 to 75%) show rings for the long
spacings instead of spots. The circle dimension for spectra taken at 75%RH was
broader than that of the others, probably because the 2D X-ray exposure time
(24 hours) for all samples was much longer than for the WAXD test (8 hours),
and the high humidity sample could have been slightly dehydrated, changing the
d spacing.
Meridional Equatorial
%RH radius of ring radius of ring d-spacing (Å) d-spacing (Å) (mm) (mm)
0 13.98 ± 1.3 8.11 ± 0.68 13.87 ± 1.3 8.17 ± 0.67 15 13.22 ± 0.4 8.61 ± 0.25 12.89 ± 0.5 8.84 ± 0.34 35 12.55 ± 0.4 9.04 ± 0.25 12.45 ± 0.4 9.12 ± 0.30 50 11.49 ± 0.4 9.92 ± 0.29 11.74 ± 0.4 9.71 ± 0.30
Table 2.16. d spacing (value ± deviation) from 2D X-ray spectra of PPDSA (lot 2) at different humidities.
122 Chapter 2
at 0%RH at 15%RH
at 35%RH at 50%RH
Figure 2.37. 2D X-ray diffraction images of PPDSA (lot 2) at different relative humidities. Outside rings are assigned to CaF2 (for reference) and the yellow solid lines show the diameter of the CaF2 diffraction. Red dotted lines show the diameter of diffraction from the sample at 0 %RH.
123 Chapter 2
12.0
2D_Meridional 11.0 2D_Equatorial
10.0
9.0 d (Å) spacing 8.0
7.0
6.0 0 102030405060 Relative Humidity (%RH)
Figure 2.38. Plot of meridional and equatorial d spacing calculated from 2D X-ray photographs vs. relative humidity,
124 Chapter 2
2.4.2.2.11. Optical polarizing microscope
PPDSA is birefringent due to the liquid crystalline organization of its rigid rod polymer backbone. The birefringence is seen with humidity controlled films and is independent of the humidity (Figures 2.39 and 2.40). One interesting phenomenon is observed; the wet film surface consists of small domains (Figure
2.39. b)). But, since lot 2 polymer was dark brown and the film was translucent,
we could not obtain detailed information from that sample.
a) b)
Figure 2.39. OPM images of PPDSA (lot 2) under cross-polarized light: a) X100, b) X500. The film was cast from aqueous solution and sealed with PVC film after reaching equilibrium at 50%RH. The thickness of sample was ~200um.
PPDSA film from lot 3 (high molecular weight polymer) was clearer (but still translucent) and showed birefringence independent the degree of film humidity. On the 0%RH sample (Figure 2.40, a)), even though there are several cracks on the surface, some regions are brighter than other regions. At 15%RH sample (Figure 2.40, b)), the image was similar to that of the dried film except that some local orientation (red circles) was seen in few regions. At 35%RH, the
125 Chapter 2 sample showed relatively larger area with local orientation (red circles in Figure
2.40, c)). However, most of regions show general birefringence. At 50%RH, all the areas are organized locally, but the local orientation is random (Figure 2.40, d)). As the water content increases, polymer chains organize over relatively larger area and the film has visible domain structure. The polymer film at 75%RH, shows the orientation in domains in all areas (Figure 2.40. e)) which is easily detectable. The size of the domain depends on the humidity.
126 Chapter 2
a)
127 Chapter 2
b)
128 Chapter 2
c)
129 Chapter 2
d)
130 Chapter 2
e)
Figure 2.40. OPM images of PPDSA films (lot 3, high molecular weight polymer) under cross-polarized light (X100): a) at 0%RH, b) at 15%RH, c) at 35%RH, and d) at 50%RH and e) at 75%RH. The film thickness was ~200um before humidifying.
131 Chapter 2
Aqueous solutions of PPDSA are expected to form a lyotropic liquid
crystalline phase due to its rigid rod structure. Figure 2.41 shows that an aqueous
PPDSA solution is lyotropic; it has the typical birefringent Schlieren texture of a
nematic liquid crystalline phase.62
Figure 2.41. OPM image of PPDSA (lot 3, high molecular weight polymer aqueous solution (38.51 g/dL) (X100) under cross-polarized light.
132 Chapter 2
2.4.2.2.12. Proton conductivity measurements
2.4.2.2.12.1. Theoretical basis
The proton conductivity of a thin polymer membrane cannot be evaluated
using DC measurements as is done for a simple resistor.63 When the membrane
is in contact with the metal electrodes, the interfacial impedance of the
metal/ionomer interface can be significant relative to the resistance within the
polymer membrane. This phenomenon is usually modeled using the equivalent
circuit as shown in Scheme 2.7 where the resistors and capacitors in parallel
represent the interfacial impedances between the electrodes and membrane.
Scheme 2.7. Equivalent circuit for an AC impedance conductivity measurement. The electrode interface can be modeled as a resistor with high impedance (R1 and R3, where R1≈R3) in parallel with a capacitor (C1 and C3, where C1≈ C3). The membrane is modeled as a simple resistor (R2). The electrodes and membrane are modeled as impedances in series.
133 Chapter 2
The resistor in series with the two interfacial resistor/capacitor impedances represents the resistance to the proton motion through the membrane. The impedance of a resistor is simply its resistance while the impedance of a capacitor is given by Equation 2.6, where j is (-1)1/2, ω is the frequency at which
the voltage is cycled in Hz and C is the capacitance in Farads.64 The resulting circuit can then be simplified to three impedances in series (Equation 2.7).
1 Z = (Equation 2.6) c jωC
Ztotal = Z1 + Z 2 + Z 3 (Equation 2.7)
The resulting impedance is explicitly given by:
⎡ R1 − jωC1 R1 ⎤ Z total = 2⋅ ⎢ 2 ⎥ + R2 ; ⎣⎢1+ ()ωC1 R1 ⎦⎥
⎡ 2R1 ⎤ ⎡ 2ωC1R1 ⎤ Ztotal = ⎢R2 + 2 ⎥ − j⎢ 2 ⎥ (Equation 2.8) ⎣ 1+ ()ωC1R1 ⎦ ⎣1+ ()ωC1R1 ⎦
At the high frequency limit, the imaginary part of Equation 3.30 becomes 1/ωCR,
which tends toward zero. The real part of that equation is left with only R2, since
2 R/[1+(ωC1R1) ] also becomes insignificant at high frequencies. Therefore, using
134 Chapter 2 this technique, it is possible to separate the electrode/membrane contact impedance leaving only the membrane resistance, R2. R2 (membrane resistance) in Equation 2.8 is used to evaluate the proton conductivity.65
The polyelectrolyte resistance is usually determined from the high frequency intersection with the real axis in an impedance plot. In some cases, when the impedance does not intercept the real axis at high frequencies, the membrane resistances were obtained from curve-fitting in Z-viewer program
(Figure 2.42). Examples of the detailed complex plots obtained from 4-probe impedance measurement using lot 2 PPDSA are shown in Figure 2.43.
a) b)
Figure 2.42. Example of AC impedance measurement results for parallel-cut film at 50°C and 50%RH with 50mV AC amplitude: a) Complex plot for Z” (imaginary impedance) as a function of Z’ (real impedance), b) Bode plot for IZI (or theta) as a function of frequency.
135 Chapter 2
a) At room temperature
-4000 -400
15%RH 35%RH -3000 -300
-2000 Z''
-200 Z''
-1000
-100 0
1000 0 7000 8000 9000 10000 11000 12000 13000 1400 1500 1600 1700 1800 1900 Z' Z'
-300 -300
50%RH 75%RH
-200 -200 Z'' Z''
-100 -100
0 0 500 600 700 800 900 300 400 500 600 700 Z' Z'
136 Chapter 2
b) At 50°C
-1250 -200
15%RH 35%RH -1000 -150
-750
-100 Z'' Z''
-500
-50 -250
0 0 2000 2250 2500 2750 3000 3250 3500 450 500 550 600 650 700 Z' Z'
-150 -250
50%RH 75%RH -200
-100 -150 Z'' Z''
-100 -50
-50
0 0 250 300 350 400 0 50 100 150 200 250 Z' Z'
137 Chapter 2
c) At 75°C
-250 -100 15%RH 35%RH -200 -75
-150
Z'' -50 Z''
-100
-25 -50
0 0 0 50 100 150 200 250 200 250 300 Z' Z'
-100 -100
50%RH 75%RH -75 -75
-50 Z'' -50 Z''
-25 -25
0 0 200 250 300 200 250 300 Z' Z'
Figure 2.43. Complex plots for PPDSA films (lot 2) obtained from 4-probe impedance measurement at different humidities (15, 35, 50 and 75%RH) and at different temperatures (room temperature, 50 and 75°C)
138 Chapter 2
2.4.2.2.12.2 Effect of the plastic plate used in cell
The effect of the plastic plate was studied using different cell assembly
types, shown in Scheme 2.4. The conductivity difference between two types of
cell assemblies, is shown in Figure 2.44. is negligible. Thus, the plastic plate does not have any interference in measuring the conductivity.
1.0E+01
1.0E+00
1.0E-01
1.0E-02
1.0E-03
Lot 2_┴_25°C_through the plastic plate Ionic Conductivity (S/cm) Conductivity Ionic 1.0E-04 Lot 2_┴_25°C_on the plastic plate
Lot 2_//_25°C_on the plastic plate 1.0E-05 0 102030405060708090100 Relative Humidity (% )
Figure 2.44. Conductivity at different relative humidities: Effect of the plastic plate used in the cell assembly. Lot 2 polymer was used in these measurements.
139 Chapter 2
2.4.2.2.12.3 Effect of molecular weight on the proton conductivity
In this section, the conductivity dependence on the polymer molecular
weigh is studied using PPDSA films from different polymerization lots, shown in
Table 2.17. The proton conductivities of PPDSA film rise with the molecular
weight of polymer under the same test conditions (Table 2.17 and Figure 2.45).
But, once the polymer molecular weight reaches a certain level, the conductivity does not change much. The order of conductivities is lot 3 (high molecular weight
polymer) ≈lot 2 > lot 1.
Conductivity (S/cm) Relative Temperature humidity Lot 3 (°C) (%RH) Lot 1 Lot 2 (high mol. wt. polymer) 15 1.1E-03 8.9E-03 1.2E-02 35 5.5E-02 5.3E-02 5.8E-02 25 50 6.5E-02 1.6E-01 1.1E-01 75 1.8E-01 1.9E-01 2.6E-01 15 1.2E-02 3.3E-02 4.2E-02 35 1.8E-01 1.5E-01 1.8E-01 50 50 NA 3.2E-01 2.5E-01 75 NA NA NA 15 3.1E-02 9.2E-02 8.7E-02 35 4.1E-01 3.5E-01 2.9E-01 75 50 NA 1.0E+00 4.5E-01 75 NA NA NA
Table 2.17. Proton conductivities of PPDSA films from lots 1, 2, and 3 at different conditions. The PPDSA films were cut parallel to the casting direction.
140 Chapter 2
1.E+01
1.E+00
1.E-01
1.E-02 PPDSA(lot 1)_25°C PPDSA(lot 1)_50°C PPDSA(lot 1)_75°C 1.E-03 PPDSA(lot 2)_25°C
Ionic Conductivity (S/cm) PPDSA(lot 2)_50°C PPDSA(lot 2)_75°C 1.E-04 PPDSA(lot 3)_25°C PPDSA(lot 3)_50°C PPDSA(lot 3)_75°C 1.E-05 0 102030405060708090100
Relative Humidity (%)
Figure 2.45. Proton conductivities of PPDSA film from different lots at different relative humidities and temperatures. Current was flowing parallel to film surface (the in-plane measurement). The lot 3 sample is high molecular weight polymer.
141 Chapter 2
2.4.2.2.12.4 Effect of the casting direction on conductivity
The effect of film casting direction on the conductivity was studied; The results are listed in Table 2.18 and shown in Figure 2.46. Since PPDSA is a rigid rod liquid crystalline polymer, chains can be organized with respect to the casting direction and the film properties (conductivity, mechanical properties, etc) could be affected by the degree of orientation. The polymer conductivities (lots 2 and 3) were independent of the X and Y directions. If the rigid rod polymer chain were aligned parallel to the casting direction, the measured conductivities in that direction might be higher than the conductivity at right angles because the proton mobility should be higher parallel to the chain direction. Since the conductivity is independent of the film orientation, PPDSA film is isotropic in the X and Y directions.
142 Chapter 2
Conductivity (S/cm) Relative Temperature humidity Lot 2 Lot 3_high mol. Wt. polymer (°C) (%RH) Parallel Perpendicular Parallel Perpendicular
15 8.9E-03 8.4E-03 1.2E-02 1.1E-02
35 5.3E-02 3.5E-02 5.8E-02 7.6E-02 25 50 1.6E-01 9.4E-02 1.1E-01 1.1E-01
75 1.9E-01 3.0E-01 2.6E-01 2.4E-01
15 3.3E-02 2.8E-02 4.2E-02 4.6E-02
35 1.5E-01 9.5E-02 1.8E-01 2.4E-01 50 50 3.2E-01 2.8E-01 2.5E-01 2.5E-01
75 NA NA NA NA
15 9.2E-02 1.0E-01 8.7E-02 1.2E-01
35 3.5E-01 2.1E-01 2.9E-01 3.0E-01 75 50 1.0E+00 5.1E-01 4.5E-01 3.4E-01
75 NA NA NA NA
Table 2.18. The membrane conductivities of PPDSA films of lots 2 and 3 at different temperatures and humidities. Parallel and perpendicular mean the measuring direction is parallel (or perpendicular) to the casting direction, respectively.
143 Chapter 2
a) 1.E+01
1.E+00
1.E-01
1.E-02 PPDSA(lot 2)_//_25°C PPDSA(lot 2)_//_50°C
1.E-03 PPDSA(lot 2)_//_75°C PPDSA(lot 2)_┴_25°C
Ionic Conductivity (S/cm) Conductivity Ionic 1.E-04 PDBSA(lot 2)_┴_50°C
PDBSA(lot 2)_┴_75°C 1.E-05 0 102030405060708090100 Relative Humidity (%)
1.E+01 b) 1.E+00
1.E-01
1.E-02 PPDSA(lot 3)_//_25°C PPDSA(lot 3)_//_50°C
1.E-03 PPDSA(lot 3)_//_75°C PPDSA(lot 3)_┴_25°C
Ionic Conductivity (S/cm) Conductivity Ionic 1.E-04 PPDSA(lot 3)_┴_50°C PPDSA(lot 3)_┴_75°C 1.E-05 0 102030405060708090100 Relative Humidity (%)
Figure 2.46. Effect of the casting direction vs. measuring direction on conductivity. a) lot 2, and b) lot 3 (high molecular weight polymer).
144 Chapter 2
2.4.2.2.13. Mechanical properties
The mechanical properties of PPDSA were studied using humidified
PPDSA films. Stress-strain curves (Figure 2.47) show that the material behaves as would be expected from a rigid rod liquid crystalline polymer. The dried film was relatively brittle with a high Young’s modulus and low elongation at break.
Young’s modulus as well as the stress and strain at break depend on the relative humidity, reflecting the plasticizing effect of the water molecules in the polymer film. PPDSA equilibrated at 15%RH was brittle with a high break force (6.88
MPa) and Young’s modulus (1650 MPa). However, 1.6 GPa is a low modulus for any rigid polymer, much less a liquid crystal polymer. It is should be 5~20GPa unless plasticized. As the water contents increased (at 35%RH), modulus is decreased to 30MPa. The film at 50%RH could not be measured because it dried rapidly under lab condition after 20~30mins, and it was too soft to run an accurate test.
145 Chapter 2
8 ––––– · Lot3_15%RH(6.2mm*1.38mm*141um) ––––––– Lot3_35%RH(9.89mm*2.39mm*234um)
6
4
Stress (MPa) 1650MPa 1653MPa
2
30MPa 30.04MPa
0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Strain (%) Universal V4.1D TA Instruments
Figure 2.47. Stress-strain curves of humidified PPDSA film (lot 3, high molecular weight polymer) at 15 and 35%RH at room temperature.
146 Chapter 2
2.5. Discussion
2.5.1. Reaction conditions for DBBDSA synthesis
The new monomer, DBBDSA (1,4-dibromo-2,5-benzenedisulfonic acid) was sulfonated using fuming sulfuric acid (Scheme 2.8). Other reagents
(concentrated sulfuric acid and chlorosulfonic acid) were also tried, but only
meta-substituted material was obtained, or the reaction did not work well.
SO3 Na Fumming Sulfuric acid Salt out Br Br Br Br 220~230C, 24hrs with Na2CO3
Na O3S
Scheme 2.8. Synthesis reaction scheme for DBBDSA, disodium salt (DBBDSA- Na).
The method used to make disulfonated DBB needed careful control of the reaction conditions due to the required high temperature. The reaction system was purged with inert gas (Ar gas, purged after drying with molecular sieves
(4Å)). Otherwise, the reactant and product could be oxidized in the highly acidic reaction medium at high temperature by oxygen, and only by-products could be obtained.
The purification of disulfonated DBB also had to take into account the high solubility of the product. The normal salting out process uses NaCl to precipitate the sulfonated product. But, disulfonated monomer was very soluble in
147 Chapter 2
aqueous solution, and the salting out using NaCl did not work well. So, this
disulfonated monomer was salted out by Na2SO4 formed by adding Na2CO3.
The next consideration was the separation of para-substituted DBB from
the mixture of meta- and para-substituted DBB. This purification is very important
if one wishes to get high molecular weight polymer through Ullmann coupling.
Previous studies51 reported that a halide meta to the sulfonic acid group was not involved efficiently in the coupling reaction. Unfortunately, these disulfonated compounds are ionic materials and could not be separated using silica gel column chromatography. But, their different solubilities, due to different symmetry of the chemical structures made the para-substituted sodium salt almost
insoluble in ethanol and the isomer could be separated using Soxhlet extraction.
The extraction process was monitored by a characterization of the solids
remaining in the thimble using 1H- and 13C-NMR after extraction for 2~3 days.
This extraction process was repeated until the only desired product was
remained in the thimble. Yields (Table 2.5) of the para-and meta-substituted
compounds were calculated after a complete extraction.
Low total yields are possibly due to meta-substituted compound and/or monosulfonated compound remaining soluble in the salting out process. The amount of water-insoluble solid that was filtered before salting out was less than
0.10g in lot 11. So, most of DBB was consumed in sulfonation. In the salting out process, the para-substituted DBB was expected to crystallize easily from solution since it seems to be much less soluble than the meta-substituted compound. Some fraction of the meta-substituted compound and/or
148 Chapter 2
monosulfonated compound (such as 1,4-dibromo-2-benzenesulfonic acid) could
stay in solution, The salted out solid had no monosulfonated compound, based
on its 1H-NMR spectrum. However, because the remaining solution after salting out was not analyzed further, we do not know if there was any monosulfonated
compound.
In Table 2.5, different reaction conditions were studied to increase the
yield of DBBDSA (1,4-dibromo-2,5-benzenedisulfonic acid). At conventional
temperatures (120°C for lot 1 and 180°C for lot 2, Table 2.5) with a mole ratio of
[SO3]/[DBB] (1,4-dibromobenzene) of 5.3, the major product was the meta-
substituted material and the minor one was the para-substituted material (yields
were about 10 and 19%). When the reaction temperature was increased to
225°C (lot 3, Table 2.5), the yield of para-substituted DBB increased to about
28%. From these results, we can assume that high reaction temperature favors
the 2nd sulfonation para to the first sulfonic acid.
The effect of mole ratio of [SO3]/[DBB] on the yield of para-substituted
DBB was tested to determine the best reaction conditions. Reactions for 24
hours using low mole ratios of [SO3]/[DBB] (3.5 for lot 5 and 2.5 for lot 7)
increased the yield of para-substituted DBB to 37~38%, the highest yield, from
that using high mole ratios (5.6 for lot 3). However, when the mole ratio of
[SO3]/[DBB] was reduced to 2.2 (lot 9), the yield did not improve. Therefore, a
mole ratio of [SO3]/[DBB] of 2.5 was a good condition to give the best yield of
para-substituted DBB.
149 Chapter 2
The effect of reaction time was tested with a low mole ratio of [SO3]/[DBB]
of 3.5 at 225°C. Longer reaction time (24 hours for lot 5) had a slightly higher
yield than that for a short reaction time (10 hours for lot 4). But, when the
reaction was scaled up (lot 6; 4 times to that of lot 5) the yield dropped to 29%.
However, at the lower mole ratio of 2.5, using the same amount of
dibromobenzene, the yield of para-substituted DBB increased to 38% (lot 7). A
further scale-up (lot 8; 8 times to that of lot 5) gave reasonable yield of para-
substituted DBB (32%). Therefore, the best conditions found (225°C,
[SO3]/[DBB] =2.5, 24hours) for the synthesis of para-substituted DBB were used
in subsequent reactions.
DBBDSA salts were polymerized in different organic solvents using activated copper powder. The goal was to make high molecular polymer. Ion exchange provided an easy way to convert one cationic species to another. The ammonium (benzyltrimethylammonium, BTMA) and phosphonium
(tetrabutylphosphonium, TBP) salts were used to, hopefully, increase the solubility of the polymer during the coupling reaction. PPSA polymer from earlier reactions (Ullmann coupling of bis (benzyltrimethylammonium) salt of 4,4-
’dibromo-3,3’- biphenyl disulfonic acid in NMP) always precipitated during
polymerization and it was difficult to obtain high molecular weight.51
150 Chapter 2
2.5.2. Polymerization conditions for PPDSA synthesis
The nickel catalyzed coupling method reported by Percec40, 66, 67 was
tested for 1,4-dichloro-2-benzenesulfonic acid as a possible synthetic route for
PPPS. The nickel catalyst (NiCl2(dppf), dichloro[1,1’-bis(diphenylphosphino)- ferrocene]nickel) was used for these reactions because it is an effective catalyst for Suzuki coupling and for other cross-coupling reactions. Besides, the dppf ligand circumvented the possibility of exchange reactions on the ArPdL2I
(L=triphenylphosphine) that could produce a branched rather than a linear material.68 But, this nickel catalytic coupling system did not work with monomers
having ionic groups (organic or inorganic salts of 1,4-dichloro-2-benzenesulfonic
acid); The problem was the selection of base used in reaction. Because the
base cation exchanges with the cation in the monomer, a base having the same
cation as the monomer should be used in the reaction to maintain the reactivity of
monomer and/or polymer. This limitation generated solubility problems in the
monomer and resulting polymer, and decreased the compatibility with organic
solvents.
The failure of the above synthetic route for synthesis of sulfonated
aromatic polymers due to its complexity and incompatibility led me to test the
Ullmann coupling reaction. The Ullmann coupling reaction between sulfonated
biphenyl monomer had already been proved out in Litt group.51
At first, the coupling reaction was tested with DBBDSA-BTMA in NMP
(pretest lot, Table 2.6). But after 1~2 hours of reaction, all polymer had
precipitated even if mechanical stirring was used, and there was no further
151 Chapter 2
polymerization (lot 1). The molecular weight of precipitated polymer was very low
(reduced viscosity at 0.2 g/dL of polymer, disodium salt from lot 1 in D.I water at
35°C: 0.10 dL/g), and a cast film was very brittle.
From these results and the previous results51, the solution behavior of
polymers made using different cationic species and reaction solvents needed to
be studied by viscosity measurements at different concentrations. More results from viscosity measurements will be discussed in the next section. The solubility of resulting polymer from lot 2 with different cations was tested in order to optimize reaction conditions to get high molecular weight polymer. In the Ullmann coupling reaction, the available organic solvents were limited to DMF, DMAC and
NMP54, and the matching of the salt form of monomer and resulting polymer with
the organic solvent during coupling was expected to be the key parameter for
production of high molecular weight polymer.
The main reason for using the ammonium (BTMA) or phosphonium (TBP) salt form of DBBDSA was to increase the solubility of the polymer in reaction medium to get high molecular weight polymer. But, solubilities of diBTMA or diTBP salts of the resulting polymers in reaction medium were different from our expectations. The polymer from lot 1, made using DBBDSA-BTMA, precipitated during polymerization after about 2 hours; it has the lowest reduced viscosity,
Figure 2.27. Lot 2 polymer, made using DBBDSA-TBP, also precipitated during
reaction and had a reduced viscosity similar to the low molecular weight polymer of lot 3. So, the DBBDSA counterion affects the polymer solubility during the reaction.
152 Chapter 2
The other factor considered for making high molecular weight polymer
was the monomer concentration in the reaction system. PPDSA has a sharp
increase of viscosity at low concentrations in aqueous or organic solvents,
independent of the presence of salt: the viscosity of the polymer at or below 0.1
g/dL is higher than that at about 0.4g/dL, shown in section 2.4. So, a low
concentration of monomer in the reaction medium is important for increasing the
polymer solubility during polymerization.
Therefore, the best conditions were determined to be: DBBDSA-Li in dried DMF at a concentration of 0.05mole/L (2.0 g/dL). These conditions produced the highest molecular weight PPDSA (lot 3, Table 2.6) with the highest reduced viscosity found, 0.67dL/g (Table 2.19).
153 Chapter 2
Sample η Lot concentration red Remarks (dL/g) (g/dL)
Lot 1 0.172 0.092
Lot 2 0.185 0.267
High molecular weight polymer 0.202 0.671 (fraction insoluble in DMF) Lot 3 Low molecular weight polymer 0.179 0.330 (fraction soluble in DMF)
Table 2.19. Reduced viscosities of PPDSA, diprotonated form. Viscosities were measured in D.I water at 35°C using viscometer 0C C453.
2.5.2.1. Proposed Ullmann coupling mechanism
Previous studies on the Ullmann coupling efficiency as a function of a second substituent reported that an electron withdrawing group promoted aryl- aryl coupling by decreasing the electron density of the carbon directly linked the halogen atom and ortho-substituted aryl halides reacted more efficiently than meta-substituted counterparts.53, 54 Fanta and coworkers54 proposed that
coordination of the electron-withdrawing group ortho to the carbon-halogen bond
with the copper surface facilitated the transfer of electron density from the copper
to the carbon attached to the halogen. This complex then acted as the
nucleophile to attack another molecule of aryl halide, which resulted in the
desired carbon-carbon bond formation. The proposed Ullmann coupling reaction
mechanism for PPDSA can be summarized in three steps (see Figure 2.48):
154 Chapter 2
1) Formation of the copper-aryl halide complex to create a nucleophilic carbon
2) Nucleophilic attack of the copper-aryl complex on another aryl halide molecule.
3) Formation of copper halide and release of the newly formed diaryl group.
Br EWG Br EWG Br EWG
EWG Br EWG EWG Br
Br Cu Cu Cu Cu Cu Cu
1 2
EWG EWG
Br Br
EWG EWG Br Br Cu Cu Cu
3
Figure 2.48. Proposed Ullmann coupling reaction mechanism for PPDSA.
155 Chapter 2
2.5.3. PPDSA solution properties
2.5.3.1. Rheological properties and solution OPM image
The flow behavior of liquid crystalline polymers (LCPs) is certainly very complex and is still being studied. Even in steady state simple shear flow, such materials display a variety of peculiar features, many of which are still not completely understood. As the most classical example, shear rate (ŕ) dependent viscosity (η) can be considered for LCPs (Figure 2.49). Anisotropic solutions of
LCPs have shear rate dependent viscosity at low shear rates, while isotropic solutions have none.
Figure 2.49. Shear rate dependent viscosity of isotropic (---) and anisotropic (—) solutions of poly(p-phenylene terephthalamide in sulfuric acid at 60°C.69
The viscosity of dilute solutions of rodlike particles has been theoretically
treated for the case of very low shear stress by Simha.70 Hennans71 reported that
the viscosity of PBLG (poly-r-benzyl-L-glutamate) in m-cresol reached a maximum at the concentration where the phase transition from isotropic to
156 Chapter 2
anisotropic occurred. This was confirmed by Kiss and Porter for the same
polymer.74 Similar observations were made by other workers, such as Papkov, in
their study of poly-p-benzamide solutions.75, 76 Aharoni77 described the entire viscosity-concentration curve covering the isotropic, biphasic and anisotropic phases in terms of axial ratio. Nevertheless, there was no successful theoretical interpretation of this viscosity-concentration curve until Doi78 extended the Doi-
Edwards rigid rod solution theory to the entire concentration range and formulated a molecular basis for the relationship between viscosity and order parameter (S). The order parameter (S) is defined the degree of order of LCP chains
S = 1/ 2()3 < cos2θ > −1 (Equation 2.9)
S is zero for a random distribution of directors; it is unity if all the directors are aligned perfectly. Because PPDSA is a rigid rod liquid crystalline polymer, its polymer solution behavior may follow the Doi-Edwards model.78 According to this
model, the viscosity increase is probably due to increased rate of shear
relaxation as concentration decreases.
Typically, flow curves of LCP solutions display three regions: shear
thinning, a Newtonian plateau and shear thinning, proposed by Onogi and
Asada79 (Figure 2.49). This result has become a scaffold for interpreting the flow
behavior of LCPs.80, 81 At low shear rates, η is a decreasing function of ŕ (shear rate), the so-called “region I” in the Onogi and Asada nomenclature. A plateau
157 Chapter 2
then follows at intermediate shear rates (region II). The LCPs again becomes
shear thinning (region III) at still larger values of the gradient. The three region
behavior is not always observed. As pointed out by Wissbrun82, some LCPs
showed regions II and III only, whereas in others83 a single shear thinning curve
was observed over many decades of shear rate.
Figure 2.49. Typical three regions of flow behavior of LCP solution.79
At high concentration (38.51g/dL, Figure 2.19), two apparent regions are
observed; shear thinning (1.0 X 10-3 to 1.0 s-1) and a Newtonian plateau (10 to
100s-1). At that concentration, most of polymer chains should be in ordered domains in solution. Based on the Onogi and Asada model79, the viscosity
dependence can be interpreted in terms of the response of the domains to the shear forces. As the shear rate increases, the domains aligned and viscosity decreases. The isotropic samples had only monotonic curves while biphasic and anisotropic sample had two or three regions. This anisotropy can be seen in the
OPM image of polymer solution at same concentration. The solution OPM
158 Chapter 2
images had the typical birefringent texture of a nematic liquid crystalline polymer
solution. Different colors are due to rotation of the direction of the order
parameter with respect to the polarizing plane.
At lower concentrations (0.48~19.26 g/dL), reliable viscosities at low
shear rates could not be recorded. The shear thinning of diluted polymer solution
was studied using viscometers having different shear rates and will be discussed
in the next section.
These results demonstrated several important solution properties of
PPDSA. First, PPDSA aqueous solutions at high concentrations have
characteristic shear dependent viscosity. Viscosity is a negative function of shear
rate in the range of 1.0 X 10-3 to 1.0 s-1, as shown in Figure 2.19. Second, high
concentration PPDSA solution is anisotropic (lyotropic), shown by the presence
of two distinct regions (shear thinning and Newtonian plateau) in the plot of
viscosity vs. shear rate. This anisotropy is supported by other results. In OPM
(optical polarizing microscopy) images, PPDSA solution (at 38.51g/dL) has a typical nematic structure (section 2.4). Third, the driving force for this organization may come from the its rigid rod structure.
159 Chapter 2
2.5.3.2. Viscosities of PPDSA
2.5.3.2.1. Abnormal behavior in the reduced viscosity of PPDSA
Various polyelectrolyte solutions84-89 have similar behaviors, the
“polyelectrolyte effect” in viscosity measurement. But, the viscosity for random
coil polyelectrolytes change, depending on the salt concentration, and the
“polyelectrolyte effect” disappears as the salt concentration is increased.
However, the sharp increase of specific viscosity with decreasing concentration of PPDSA was not “the polyelectrolyte effect”, because this type of behavior should disappear at high salt concentrations.
The same phenomenon can be seen in nitrated poly(p-phenylene)
(PPP)86 and sulfonated PPP90. These two types of linear, rod-like nitrated PPP’s in Yamamoto’s report were synthesized using Kovacic’s recipe for cationic
91- polymerization of benzene in the presence of AlCl3 and CuCl2 method (PPP-a),
93 and Ni-catalyzed organometallic dehalogenative polycondensation of p-
dibromobenzene with magnesium (PPP-b)94, 95. Their viscosity behavior in salt
solutions showed the same trends as were found in zero salt concentration. They
claimed that the strongly electron-withdrawing NO2 group seems to induce a highly polarized structure in the π - conjugated polymer which might be the reason for the polyelectrolyte-like behavior of nitrated PPP. They stated that, in contrast to nitrated PPP, nitrated polystyrene does not show a polyelectrolyte-like behavior.95
Another example is sulfonated PPP [poly(p-biphenylene-3,3’ disulfonic acid), PBPDSA]. This polymer was prepared using 4,4’-dibromobiphenyl-3,3’-
160 Chapter 2
disulfonic acid using the Ullmann coupling reaction.90 The same pattern can be seen in Figure 2.50.
From the two examples, it is suggested that PPDSA’s abnormal viscosity comes from effects of a rigid rod polymer backbone.
Another useful result from viscosity measurements concerns the monomer concentration in the reaction mixture. If polymer chains can have more interaction each other in the presence of copper powder, the rate of polymerization possibly increases and high molecular weight polymer can be obtained. High molecular weight usually improves the polymer mechanical
properties. In fuel cell application, a higher pressure of fuel gas (hydrogen gas or
oxygen gas) generates higher reaction rates, and thus needs a stronger
membrane. Also, good mechanical strength is an important factor in the MEA
(membrane-electrode-assembly) process because hot pressing is usually applied.
161 Chapter 2
a)
b)
Figure 2.50. Viscosity plots for PBPDSA at different salt concentrations. a) in water, b) in 1.0M LiCl solution.90 PBPDSA chemical structure was below.
SO3H
H3OS n
162 Chapter 2
2.5.3.2.2. Shear thinning of PPDSA
The PPDSA aqueous solutions show some degree of shear thinning in
the plot of reduced viscosity measured vs. concentration (Figure 2.21) using different viscometers. This plot show that, as would be expected for high molecular weight rigid rod materials, the measured viscosity decreases as the shear rate increases due to shear induced orientation of the rods in solution.96, 97
The shear rate dependence of the apparent viscosity of a nematic liquid crystal in Poiseuille flow also depends on its orientation at the wall. This is shown in Figure 2.51 (Fisher and Fredrickson98), for the case of small molecule perpendicular wall orientation. Evidently, there is a large effect of capillary diameter. Fisher and Fredrickson were able to correlate their data by assuming that there is a layer of fluid at the wall where the molecular orientation is perpendicular. This high viscosity layer surrounds a core of low viscosity fluid, corresponding to a parallel orientation. With this model they could calculate wall layer thicknesses, independent of capillary diameter but decreasing with increasing shear rate. At low shear rates, the layer thickness was about 7 um, perhaps approaching the value of 20 um estimated from static optical measurements.
163 Chapter 2
Figure 2.52. Viscosity behavior of nematic p-azoxyanisole in tubes surface treated for perpendicular orientation: shear rate of tubes were calculated using equation 4Q/πR3 (Q is the flow rate in mL/sec and R is the capillary radius in cm).96
2.5.3.2.3. Effect of salt concentration on viscosity
The salt concentration effect on the reduced viscosities of PPDSA in DMF was studied (Figure 2.22). The shielding by the added salt decreases the repulsive columbic forces between the polymer chains, reducing their effective diameter, and thus decreasing interactions.
Basically, random coil polyelectrolyte polymer solutions have an increase of reduced viscosity in the absence of added salt, but show the normal behavior
(increasing viscosity with concentration) when salt is added. But, PPDSA has the same concentration-viscosity profile whether or not salt is added. Both with and without salt, the degree of local orientation decreases as the polymer concentration decreases to below 0.1 g/dL, and the viscosity increases. This is expected from rigid rod polymer solutions.
164 Chapter 2
If the orientation is still high, the reduced viscosity should not change much at low concentrations. Without salt, as the solution is diluted, the ionic radius of the polymer increases (Debye-Hückel layer). The polymer occupied more volume and thus increases viscosity even if the chains are still mostly organized.
2.5.3.2.4. Effect of solvent
: 0.1M LiBr-DMF solution vs. 0.1M LiBr- DMF/NMP (33/67,v/v)
The effect of solvent on reduced viscosities was studied to help select the best polymerization solvent (Figure 2.23). The effect of solvent is small, but, in
DMF (higher dielectric constant), polymer chain could have larger ionic atmosphere around it and more interactions with other chains. Thus the reduced viscosity of the polymer in DMF is slightly higher
2.5.3.2.5. Effect of cationic species
The effect of cation (protonated form vs. sodium salt) on the reduced viscosity of PPDSA is small (Figure 2.24). But the lower reduced viscosity of the protonated form means that the local high concentration of acid groups on the chain forces many to become neutral (-SO3H) rather than ionic. This could decrease the ionic atmosphere around the chain and lower interactions with other chains, without changing other aspects of the orientation.
165 Chapter 2
2.5.3.2.6. Modified Huggins equation for rigid rod polymer model
Polyelectrolytes exhibit a number of anomalous characteristic properties
when compared to neutral polymer solutions; these have been extensively
studied over the past fifty years.99-102 Polyelectrolytes show unique rheological
properties, as evidenced by their reduced viscosity which shows both a higher
value at low concentrations103-113 and a lower value at high concentrations114-115.
For polyelectrolyte polymers, Fuoss proposed a phenomenological equation, the Fuoss law116 in 1951 (Equation 2.10). In the semi-dilute and
moderately dilute “salt-free” polyelectrolyte solution, Equation 2.10 should govern
the relation between the reduced viscosity (ηsp/c) and the concentration (c) (ηsp =
(η – η0)/ η0 is the specific viscosity, with η0 the solvent viscosity). According to this
1/2 expression, a straight line is obtained when c / ηsp is plotted vs. c . Furthermore,
it is usually assumed that this line can be extrapolated to zero concentration and
that the intercept at c = 0 gives the inverse of the intrinsic viscosity (Equation
2.11). 117
η sp A = (Equation 2.10) c ()1+ B C
c B 1 B 1 = c + ≈ c + (Equation 2.11) η sp A A A []η
But, PPDSA has an abnormal reduced viscosity curve that increases with
decreasing concentration even in salt solution. Thus, the normal extrapolation to
166 Chapter 2 zero concentration to obtain the intrinsic viscosity is not useful. It might be possible to modify the general Huggins equation to include local organization of the rigid rod polyelectrolyte.
To modify the general Huggins equation, the concentration of organized
(or associated) particles (P) should be considered instead of the concentration of individual polymer chains (C) because PPDSA can organize and make aggregates. The concentration parameters are set as below;
Pn : number of organized particle (= concentration of particle) [mole/L],
Xn : degree of organization,
C : concentration of individual polymer chain [mole/L].
Polymer chains in solution are associated and form organized aggregates in the high concentration region with an association constant (K) (Equations 2.12 and 2.13). The degree of association can be derived from Equations 2.12 and
2.13 as Equation 2.14. The concentration of polymer (C) is converted to a function of P (Equation 2.17) using Equations 2.15 and 2.16.
Pn Pn−1 + P1 → Pn ; K = (Equation 2.12) Pn−1 × P1
n−1 P1 Pn = ()K × P1 × P1 ; ∑ Pn = P = (Equation 2.13) 1 − K × P1
167 Chapter 2
1 1 X = = = 1 + KP (Equation 2.14) n 1 − KP KP 1 1 − 1 + KP
⎛ P ⎞ ⎜ 1 ⎟ C = P× X n = ()1+ KP ×⎜ ⎟ = P()1+ KP (Equation 2.15) ⎝1− KP1 ⎠
− 1 + 1 + 4KC 2 (KC ) C P = ≈ ≈ (Equation 2.16) 2K 2K K
η sp 2 = []η + k' []η P (Equation 2.17) P
Using Equation 2.16, the general Huggins equation for concentration of molecules (Equation 2.17) can be converted to a modified Huggins equation
(Equation 2.18). A slope, [(k’* [η]2)/K ], and an intercept, [[η]/K0.5], is obtained
0.5 0.5 when the ηsp/C is plotted to C .
2 η sp []η k'[]η 0.5 = + C (Equation 2.18) C 0.5 K 0.5 K
The meaning of the slope and intersection in Equation 2.18 is not clearly understood yet and more studies are needed.
However, using the Equation 2.18, viscosities under different conditions can be compared more easily than plats using the usual coordinates. The effect of factors (molecular weight of polymer, cationic species, solvent or salt concentration) can be seen more clearly (Figure 2.53 ~ 2.56).
168 Chapter 2
0.700
Lot 1 0.600 Lot 2
Lot 3_low mol. wt. y = 0.34x + 0.22 0.500 Lot 3_high mol. wt.
) 0.400 1/2 /(c sp
η 0.300
0.200 y = 0.11x + 0.09
y = 0.09x + 0.08 0.100
y = 0.03x + 0.03 0.000 0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 c1/2
0.5 0.5 Figure 2.53. Plot of ηsp/C to C of aqueous PPDSA, solution (lots 1, 2 and 3) at 35°C. Each polymer was dissolved in D.I. water and diluted with D.I. water. Viscometer 0C C453 was used. All polymers were the diprotonated form.
169 Chapter 2
0.06
y = 0.028x + 0.031 0.05
0.04 )
1/2 y = 0.028x + 0.026 0.03 /(c sp η 0.02
Lot 1_diNa in water 0.01 Lot 1_diprotonated in water
0 00.20.40.60.81
c1/2
Figure 2.54. Effect of PPDSA counterion on reduced viscosities of PPDSA (lot 1) 0.5 0.5 in plot of ηsp/C vs. C . Viscosities were measured at 35°C using 0C C453 viscometer.
0.140
0.120 y = 0.14x + 0.01
0.100
) 0.080 1/2
/(c y = 0.11x + 0.02
sp 0.060 η
0.040
0.020 Lot2_diLi in 0.1M LiBr-DMF solution Lot 2_diLi in 0.1M LiBr-DMF/NMP solution 0.000 0.00 0.20 0.40 0.60 0.80 1.00 c1/2
Figure 2.55. Effect of solvent on the reduced viscosity of PPDSA in plot of 0.5 0.5 ηsp/C vs. C . Viscosities were measured at 35°C using 0C C434 viscometer.
170 Chapter 2
0.8 a) Lot 3_high mol. wt._diprotonated form in water 0.7 Lot 3_high mol. wt._disodium salt form in 0.3M LiCl-water 0.6
0.5 y = 0.34x + 0.22 ) 1/2 0.4 /(c sp η 0.3
0.2 y = 0.34x + 0.00
0.1
0 0 0.2 0.4 0.6 0.8 1 1.2
c1/2
b) 0.24 Lot 2_diLi in DMF
0.20 Lot 2_diLi in 0.1M LiBr-DMF solution Lot 2_diLi in 0.5M LiBr-DMF solution
0.16
) y = 0.14x + 0.04 1/2
/(c 0.12 sp η
0.08 y = 0.14x + 0.01 y = 0.14x + 0.01
0.04
0.00 0.00 0.20 0.40 0.60 0.80 1.00 c1/2
Figure 2.56. Effect of salt concentration on the reduced viscosity of PPDSA 0.5 0.5 solution in plot of ηsp/C vs. C ; a) in D. I. water, B) in DMF. The viscosities were measured at 35°C using 0C C434 viscometer
171 Chapter 2
In Figures 2.53~2.56, the slopes for PPDSA from lots 1, 2, 3 (high and low molecular weight) is independent of the cation and added salt. It is constant for a given polymer and the intercept depends on the conditions. Different solvents give different slopes. However, the more study about PPDSA reduced viscosity is needed to be fully understood.
172 Chapter 2
2.5.4. NMR spectra of PPDSA
2.5.4.1. 1H-NMR spectra of PPDSA
The polymer, disodium salt form chemical structure from different polymerizations was studied using 1H-NMR. There should be one proton peak in
the polymer NMR spectrum if the monomer had coupled at the 1and 4 positions.
However, many peaks are observed in the PPDSA 1H- NMR spectra. To analyze those efficiently, the spectra were deconvoluted using ACD labs Curve processing module (version 9.05). The resulting deconvoluted spectra are shown in Figure 2.57. The deconvolution results (peak position (ppm and Hz), height,
FWHH (full width at half height), and area) are listed in Table 2.20. The viscosities of polymers obtained from the different lots were listed in Table 2.19: the viscosity order was lot 3 (high molecular weight polymer) > lot 3 (low molecular weight polymer) > lot 2 > lot 1.
173 Chapter 2
Lot 1
Lot 2
Lot 3 low mol wt.
Lot 3 high mol wt.
G1 G2 G3 G4 G5
Figure 2.57. Deconvoluted 1H-NMR spectra of PPDSA, disodium salt, of lots 1, 2, and 3 (high and low molecular weight) in D2O at 25°C (concentration of PPDSA: 4.4 g/dL).
174 Chapter 2 a2) lot 1 Peak ppm Hz Height FWHH Area group G1 8.03 2411.00 0.0040 3.32 6,817,059 8.02 2407.67 0.0124 4.50 28,848,732 8.01 2403.80 0.0113 4.10 24,147,804 8.00 2401.27 0.0035 1.82 3,311,630 G2 8.00 2399.80 0.0141 5.73 41,927,820 7.98 2394.78 0.0073 2.35 8,951,064 7.97 2390.14 0.0047 3.67 8,955,564 7.97 2392.36 0.0123 3.98 25,428,308 7.95 2386.43 0.0040 1.99 4,104,525 G3 7.90 2369.94 0.0017 8.37 7,510,379 7.88 2363.91 0.0052 6.48 17,634,718 7.87 2360.26 0.0025 3.34 4,278,274 7.86 2357.65 0.0024 2.57 3,142,785 G4 7.84 2352.37 0.0082 4.63 19,669,288 7.83 2348.79 0.0096 3.90 19,496,284 7.82 2346.33 0.0069 3.65 12,998,876 7.81 2342.08 0.0043 2.33 5,148,569 7.81 2343.92 0.0073 2.65 9,983,361 7.80 2339.71 0.0085 3.15 13,971,322 7.79 2337.66 0.0079 3.32 13,568,943 7.78 2335.30 0.0089 3.37 15,641,214 7.77 2332.35 0.0060 2.83 8,743,340 G5 7.70 2309.61 0.0018 2.75 2,597,099 7.65 2294.91 0.0038 5.10 10,092,059 7.61 2282.86 0.0008 4.54 1,808,609 7.55 2264.74 0.0009 10.27 4,801,338
a2) lot 2 Peak ppm Hz Height FWHH Area group G1 8.05 4828.06 0.0029 4.53 19,818,784 8.04 4820.78 0.0127 8.56 161,791,920 8.03 4812.73 0.0189 11.61 326,738,528 8.01 4803.71 0.0033 5.93 29,145,302 G2 8.00 4797.25 0.0052 3.65 28,090,832 7.99 4789.05 0.0061 5.86 53,018,016 7.99 4792.47 0.0039 3.68 21,564,170 7.98 4783.43 0.0062 5.57 51,373,988 G3 7.91 4741.47 0.0079 7.08 83,561,912 7.89 4734.08 0.0091 8.41 113,835,912 7.88 4722.89 0.0067 7.26 72,300,328 7.88 4727.70 0.0097 6.31 91,226,808 7.86 4715.83 0.0033 4.94 23,965,618 7.85 4709.09 0.0061 5.06 45,966,776 7.84 4701.49 0.0063 4.25 39,817,832 G4 7.83 4698.40 0.0039 4.49 25,851,934 7.83 4693.55 0.0045 4.48 29,957,746 7.81 4682.11 0.0036 3.81 20,725,172 7.80 4679.44 0.0062 6.08 56,540,508 7.79 4673.66 0.0088 5.72 75,324,496 G5 7.65 4587.31 0.0052 4.35 33,957,900
175 Chapter 2
a3) lot 3, low molecular weight polymer Peak ppm Hz Height FWHH Area group G1 8.05 4827.18 0.0023 4.05 15,555,501 8.04 4818.85 0.0135 6.79 150,192,544 8.02 4810.08 0.0181 8.18 243,911,072 8.01 4801.12 0.0038 7.45 46,321,892 G2 8.00 4794.69 0.0030 2.28 11,281,102 7.99 4791.06 0.0042 3.61 24,660,368 7.98 4782.85 0.0056 2.57 23,767,430 7.98 4787.73 0.0021 2.35 8,038,108 7.97 4780.89 0.0021 6.00 20,308,752 G3 7.90 4740.33 0.0035 5.14 29,320,960 7.89 4733.42 0.0037 5.32 32,321,700 7.88 4723.50 0.0042 8.28 56,851,896 7.88 4727.01 0.0074 3.67 44,437,080 7.85 4707.98 0.0016 5.05 12,952,019 G4 7.83 4698.10 0.0036 3.95 23,409,660 7.82 4691.92 0.0054 2.42 21,293,140 7.80 4675.41 0.0039 5.14 32,820,444 7.80 4680.46 0.0034 5.27 29,877,302 7.79 4672.53 0.0045 3.17 23,396,712 G5 7.65 4587.38 0.0018 3.93 11,318,210 a4) lot 3, high molecular weight polymer
Peak ppm Hz Height FWHH Area group G1 8.07 4836.58 0.0015 21.69 12,057,552 8.04 4824.15 0.0032 19.79 24,162,652 8.03 4813.75 0.0037 14.12 25,596,840 8.01 4800.86 0.0004 11.46 1,971,984 G2 7.99 4792.21 0.0012 33.30 14,991,975 7.95 4768.10 0.0006 23.58 5,878,794 G3 7.92 4749.82 0.0017 18.27 12,459,352 7.89 4732.81 0.0022 17.64 18,286,900 7.86 4710.67 0.0008 23.10 6,818,583 G4 7.82 4689.25 0.0006 18.50 4,868,618 7.79 4671.66 0.0007 11.53 3,263,568 7.77 4660.39 0.0005 19.53 4,861,694 7.73 4635.79 0.0002 46.74 4,166,306 G5 7.65 4586.57 0.0002 11.03 979,906
176 Chapter 2
Peak Lot 1 Lot 2 Lot 3_low mol. wt. Lot 3_high mol. wt. b) Group ppm FWHH ppm FWHH ppm FWHH ppm FWHH G1 8.07 21.69 8.05 4.53 8.05 4.05 8.04 8.56 8.04 6.79 8.04 19.79 8.03 3.32 8.03 11.61 8.03 14.12 8.02 4.50 8.02 8.18 8.01 4.10 8.01 5.93 8.01 7.45 8.01 11.46 8.00 1.82 G2 8.00 5.73 8.00 3.65 8.00 2.28 7.99 5.86 7.99 3.61 7.99 33.30 7.99 3.68 7.98 2.35 7.98 5.57 7.98 2.57 7.98 2.35 7.97 3.67 7.97 6.00 7.97 3.98 7.95 1.99 7.95 23.58 G3 7.92 18.27 7.91 7.08 7.90 8.37 7.90 5.14 7.89 8.41 7.89 5.32 7.89 17.64 7.88 6.48 7.88 7.26 7.88 8.28 7.86 23.10 7.88 6.31 7.88 3.67 7.87 3.34 7.86 2.57 7.86 4.94 7.85 5.06 7.85 5.05 7.84 4.25 G4 7.84 4.63 7.83 3.90 7.83 4.49 7.83 3.95 7.83 4.48 7.82 3.65 7.82 2.42 7.82 18.50 7.81 2.33 7.81 3.81 7.81 2.65 7.80 3.15 7.80 6.08 7.80 5.14 7.80 5.27 7.79 3.32 7.79 5.72 7.79 3.17 7.79 11.53 7.78 3.37 7.77 2.83 7.77 19.53 7.73 46.74 7.70 2.75 7.69 3.36 G5 7.70 2.75 7.65 5.10 7.65 4.35 7.65 3.93 7.65 11.03 7.61 4.54 7.55 10.27
Table 2.20. a1~a4) Deconvolution results of 1H-NMR spectra from different lots, b) comparison of FWHH of the peak groups in the deconvoluted spectra, Figure 2.57.
177 Chapter 2
As the polymer molecular weight increases, two changes can be seen in
NMR spectra (Figure 2.57 and Table 2.20, a1)). The first is the change in the 7.65
ppm peak area (G5), and the number of peaks near 7.65 ppm. In the spectrum of
lot 1, the peak area at 7.65 ppm is relatively large, and three small peaks around
7.65 ppm have visible intensities. As the molecular weight increases, the area of the 7.65 ppm peak diminishs, and the adjacent peaks almost disappear. The change of the area ratios for the different lots is summarized in Table 2.21. The area ratios of peaks in G5 (7.5~7.73 ppm) to peaks from G1 to G4 (8.0~7.73 ppm change from 0.063 to 0.007. The peaks between 7.5~7.7 ppm probably belong to protons at the ends of the polymer chain. Xns (the number average degree of
polymerization) for different lots are calculated using these area ratios: the
highest value is 142 (lot 3, high molecular weight polymer).
Second is that all the peaks become broader (larger FWHH), as the
molecular weight increases (Table 2.20, b)). As the polymer molecular weight
increases, the molecular relaxation time becomes longer and the peaks become
broader. These FWHH changes are expected from the viscosity results.
The 1H-NMR spectra of PPDSA are not fully understood and a full
analysis that could characterize its chain stereochemistry was not undertaken.
However, the 13C-NMR spectrum, next section, shows that the polymer contains
only 1,4-phenylene units.
178 Chapter 2
2) Sum of areas of ‾ 1) Sum of areas X n Lots peaks of peaks in G5 = Ratio of in G1,G2, G3 and G4 (2)/(1)
Lot 1 19,299,104 304,279,857 16
Lot 2 33,957,900 1,370,616,582 40
Lot 3, low mol. wt. 11,318,210 850,717,682 75 polymer Lot 3, high mol. 979,906 139,384,817 142 wt. polymer
Table 2.21. Area ratios of peaks in G5 (7.5~7.73 ppm) to peaks in G1, G2, G3 and G4 (8.0~7.73 ppm) and the number average degree of polymerization of different lots.
179 Chapter 2
2.5.4.2. 13C-NMR spectrum of PPDSA
The 13C-NMR spectrum was very useful for characterizing the polymer
chemical structure. Assigned peaks in the 13C-NMR spectra of monomer
(DBBDSA-Li) and the resulting polymer (PPDSA) are summarized in Table 2.22
and shown in Figure 2.58.
Changes in the chemical shifts of peaks a and c confirmed that aromatic coupling reaction between monomers had happened with a loss of Br; 1) The peak c for carbon bonded to Br (118.3 ppm) in monomer disappeared and a new peak c (130.62 ppm) appeared. 2) After coupling, the electron density of carbons
connected to the sulfonic acid group slightly increased due to loss of halide, and
peak a is shifted about 3 ppm to higher field.
notation of Monomer Polymer Remarks peaks (DBBDSA-Li) (PPDSA)
C-SO H Peak a 144.8 ppm 141.9 ppm 3 in monomer and polymer
C-H Peak b 135.2 ppm 136.6 ppm In monomer and polymer
C-Br in monomer Peak c 118.3 ppm 130.62 ppm C-C in polymer
Table 2.22. Chemical shifts of peaks in the 13C-NMR spectra of monomer (DBBDSA-Li, Figure 2.8) and its polymer (PPDSA, Figure 2.18).
180 Chapter 2
SO3H b
141.9 136.6 130.6 a
c n abc HO3S
160 150 140 130 120 110 100 ppm (t1)
SO3 Li 144.8 135.2 118.3 b a Br Br c b Li O3S
a c
160 150 140 130 120 110 100 ppm (t1)
Figure 2.58. Comparison of chemical shifts of peaks in the 13C-NMR spectra of monomer (DBBDSA-Li) and its polymer (PPDSA), lot 2.
181 Chapter 2
2.5.5. Determination of relative molecular weight of PPDSA using GPC
When the elution times of polymer and monomer are compared, their elution times are almost the same (27.3 min for monomer, Figure 2.26 vs. 27.74
(1.0 g/dL) and 27.05 (0.125 g/dL) mins for PPDSA (lot 1), Figure 2.27). This does not make sense because the polymer structure was confirmed by the 13C-NMR
spectrum. Further evidence that the polymer has a reasonable molecular weight
is that coherent films can be cast. In addition, the polymer absorbs strongly at
longer UV wavelength (> 300nm), where the monomer does not absorb.
Inclusion of salt in the eluent affects the elution time of the polymer. At both 1.0 and 0.125 g/dL, the polymers come out at shorter elution times when pure DMF was used as an eluent instead of 0.01M LiBr-DMF solution. The change of column chamber temperature also has an effect on elution time; polymer has one peak (about 20 mins) in the chromatogram at 35°C (Figure
2.27,b1), while at 150°C, the polymer has two major peaks (about 19 and 27
mins) (Figure 2.27,c1). This temperature effect can be also seen at low polymer concentrations (0.125 g/dL, Figures 2.27,b2 and c2). When the polymer
concentration was 0.125 g/dL, the relative peak intensities at 150°C and long
elution time are higher than in case of 1.0 g/dL (Figures 2.27, c1 and c2).
Based on the GPC results, the relative molecular weight of PPDSA measured using GPC is not reliable because it depends on the eluent salt concentration. The effect of test conditions (salt concentration, temperature and polymer concentration) on the elution time of polymers is not clarified yet.
However, the results obtained using pure DMF show that we have good polymer.
182 Chapter 2
2.5.6. Water retention of PPDSA film at different relative humidities
2.5.6.1. Lambda of PPDSA
In the operation of a PEMFC, membrane hydration is critical to the fuel
cell performance since it determines proton conductivity,118-120 methanol
permeability121-123 and electro-osmotic drag124, 125, 126 However, the degree of
water absorption on a mass basis does not correlate well with those properties,
especially when comparisons are made between different macromolecular systems. When membrane properties are studied using lambda (λ, the number of water molecules on one sulfonic acid) as a measure of the water retention, the comparison of the proton conductivity and morphological changes with different polymer systems can be more useful.
Lambda, λ for different molecular weight PPDSAs (lots 1 and 2) was measured at different relative humidities (Figure 2.59). λ for both lots was
identical within experimental error from 15 to 50%RH. It is reasonable to suppose
that both lots had the same supramolecular organization and thus the same
water-absorbing ability. The dried film (0%RH) had 1.2 water molecules even
after the film was dried under vacuum at 90°C for 1 day, and at 150°C for 1 hour.
These waters are very tightly bound.
The most important result is that PPDSA in this relative humidity range holds almost two more water molecules per sulfonic acid than Nafion 117, due to its high sulfonic acid concentration (= low equivalent weight (Eq. wt.) of 118.13 g/[SO3H] and frozen-in free volume. Because the proton conductivity in such
membranes is strongly dependent on lambda (i.e. water content), which is the
183 Chapter 2
medium for proton transport, high lambda at low humidity is a key property
needed to improve Fuel Cell performance.
12.0
10.0
8.0
6.0 Lambda 4.0
PPDSA_LOT1 2.0 PPDSA_LOT2 Nafion 117 0.0 020406080100 Relative humidity (%)
Figure 2.59. Lambda (λ) of PPDSA films and Nafion 117 as a function of relative humidity. Lot 2 PPDSA had about 3 times higher molecular weight than lot 1. The data for Nafion 117 is from reference 124.
184 Chapter 2
2.5.6.2. State of water in PPDSA
Lambda of membranes is an important factor affecting their mechanical and conductivity properties. But, more useful results for fuel cell applications is how strongly the absorbed water molecules are bound to polymer. It was proposed in several papers that the state of water127-130 confined in the
membrane plays a more significant role in determining the membrane transport
properties than had been previously suggested. Protons diffuse through a water
matrix absorbed in the polymer sulfonic acid membrane, likely by a combination
of both hopping and vehicle mechanisms (Figure 2.60). Proton transport for any
well hydrated sulfonic acid-based membrane has been suggested to involve the
two mechanisms.
a) b)
Figure 2.60. Schematic of proton transport mechanisms; a) the vehicle mechanism, b) the Grotthuss or hopping mechanism.131
The state of water in polymers has been extensively studied, but mainly in the context of hydrogels. It is generally defined as follows: (1) nonfreezing water,
water that is strongly bound to the polymer chain and has a role in effective glass
transition reduction (plasticization); (2) freezable loosely bound water, water that
is weakly bound to the polymer chain or interacts weakly with nonfreezing water,;
185 Chapter 2
it displays relatively broad melting endotherms and (3) free water, water that is
not intimately bound to the polymer chain and behaves like bulk water, i.e., a
sharp melting point at 0°C.
The effect of electro-osmotic drag coefficient on fuel cell performance
must also be considered for water management. The electro-osmotic drag
coefficient is defined as the number of water molecules carried per proton across the membrane when there is no water concentration gradient132 (Figure 2.61) and is depends on the state of the water molecules, reaching a maximum of four waters per proton when there is free water.
Many researchers have expended much effort in understanding the
“water balance” problem in fuel cell systems. The water distribution through the fuel cell is determined by the interplay between the water uptake of the membrane, the tendency of the protonic current to transport water from the anode to the cathode (electro-osmotic drag), and the diffusion of water down water activity gradients.133 If the free water content in the membrane becomes
too high at high relative humidity, the electro-osmotic drag coefficient will
proportionally increase and then the water gradient in the membrane could
become seriously unbalanced. The performance (the protonic current (or
transport)) could be suppressed if the membrane resistance rises in the water
poor section. Therefore, comparison of the state of absorbed water between
similar classes of polyelectrolyte membranes could be assisted by DSC
measurements, determining weakly bound and free water..
186 Chapter 2
H2O production
Electro-osmotic drag
+ H (H2O)n
H+ transfer Water diffusion
Anode Membrane Cathode .
Figure 2.61. Schematic of water gradient in reduction and oxidation in Fuel Cell.133
Figure 2.62 shows typical DSC curves reported for two polyelectrolytes
(sulfonated poly(arylene ether sulfone) and Nafion). The samples with high water
content had two overlapping endotherm peaks, a broad melting peak range from
-30~10°C, assigned to weakly bound water and a sharp melting peak at 0°C due
to the free water. 132 Nafion 1135 spectra show free water molecules at 24.8%
water uptake, and some loosely bound water at 11.0% (λ ~7) and 18.0% (λ ~11)
water uptake. BPSH-40 spectra show free water molecules at 36.4% water
uptake, and loosely bound water at 24.0% water uptake (λ ~8), but only strongly
bound water (λ ~6) at 20% water and below.
.
187 Chapter 2
a) b)
Figure 2.62. Pressure DSC thermograms for (a) BPSH-40 and (b) Nafion-1135 as a function of water content. BPSH is bisphenol-based wholly aromatic sulfonated poly(arylene ether sulfone) statistical copolymer; the 40 in BPSH-40 is the mole percent of disulfonated monomer.134
The state of water in PPDSA films was studied using low and high
temperature scanning DSC measurements, Figures 2.31 (low temperature scan)
and 2.32 (high temperature scan). Equilibrated PPDSA films from 15 to 75%RH
had no endothermic peak in the low temperature scans from -50 to 60°C.This
result is very interesting because PPDSA that was equilibrated at 75%RH had
about 9 water molecules per acid group. Both the Nafion and BPSH-40 started
showing endothermic peaks at λ ~7. This is a very important result for low
temperature applications of PEMFC because the operating conditions for vehicle
are sometimes below -20°C. In fact, to get high conductivity using synthetic
188 Chapter 2
polymeric membrane and Nafion, high humidity was essential factor. The large
amount of free water in the membrane can increase not only the proton
conductivity but also the electro-osmotic drag coefficient. This is not good for
long-term Fuel Cell performance due to poor water management. PPDSA is a
potential candidate for PEMFC and meets the protonic conductivity targets
proposed by U.S. Department of Energy.135
The high temperature scanning curves had endotherms with maxima at
111~120°C and 152~160°C. The scanning results from 100 to 150°C are much more important than the low temperature scanning results. High temperature operation is essential to decrease the poisoning of Pt catalyst. But, Nafion and most of the other reported membranes had a sharp drop of proton conductivity at high temperature due to the loss of water. However, PPDSA has high water affinity above 120°C, based on these DSC results and should meet or exceed the
DOE protonic conductivity targets.133
Tightly bound water molecules (λ=1.2) could also be seen in these results.
These did not freeze down to -50°C, but vaporized above 150°C. A TGA thermogram (Figure 2.33) of the dried film showed the strongly bound water. The first weight loss before decomposition (about 304°C) was about 13%, corresponding to the loss of one water molecule.
These DSC thermograms confirmed that there is no free water in the humidified PPDSA films, up to 75%RH. The binding strengths of the water molecules in humidified PPDSA can be divided into two regions. Based on these
189 Chapter 2 results, it is possible that PPDSA may have an electro-osmotic drag coefficient less than 1 between 15 to 75%RH, combined with high proton conductivity.
190 Chapter 2
2.5.7. The presence of frozen-in free volume in PPDSA
Lambda is in PPDSA about two higher than in Nafion 117 between 15 and
75%RH. These absorbed water molecules (λ ~8.5) were tightly bound in the
polymer (no endotherm showing weakly bound or free water) based on the DSC
results. In this section, possible reasons for the high water retention of PPDSA will be discussed.
In the first part, macroscopic studies using the dimensional and weight
changes of films at different humidities will be discussed. The second part will
cover the microscopic studies using the X-ray data and a packing model study.
2.5.7.1. Macroscopic studies
2.5.7.1.1. Dimensional changes of PPDSA film at different humidities
Membrane dimension stability is a very important part of a robust design
for a fuel cell. Because the membrane-electrode assembly (MEA) is put in a
sandwich structure of two gas diffusion layers and bipolar plates, fuel cell
membranes must have dimensional stability under a variety of conditions (e.g.
high temperature and high humidity).
PPSA made earlier in our lab90 had a lamellar structure in the solid state,
and the homopolymer had a large expansion in thickness (at 75%RH and room
temperature, 80 % increase in thickness direction, and 5 and 6 % increase in
other directions compared to the dimensions at 15%RH). The unique solid state
structure was deduced from its anisotrpic expansion.
191 Chapter 2
However, PPDSA expands almost isotropically (Figure 2.30). At 75%RH, the expansion was 23% in the X direction, 28% in the Y direction and 30% in the
Z direction compared to the dimensions at 0%RH. To study the dimensional changes, these individual experimental data points at a specific humidity were normalized to the dried film dimensions at 15%RH; they are listed in Table 2.23.
192 Chapter 2
a)
Measure before humidification Measure after humidification Dimension 15%RH 35%RH 50%RH 75%RH 15%RH 35%RH 50%RH 75%RH
X direction 1.80 2.22 2.17 1.78 2.08 2.58 2.55 2.19 (mm)
Y direction 2.47 2.65 2.34 3.17 2.80 3.08 2.90 4.07 (mm)
Z direction 397 404 378 427 423 443 430 554 (um)
Volume 1.77 2.37 1.91 2.41 2.46 3.52 3.17 4.94 (cc) E-03 E-03 E-03 E-03 E-03 E-03 E-03 E-03
b)
Normalized volume of film Volume of film (cc) Normali (cc) Relative λ -zation humidity before after factor before after equilibration equilibration equilibration equilibration
0%RH 1.2 1.77E-03 1.77E-03 1.00 1.77E-03 1.77E-03
15%RH 4.3 1.77E-03 2.46E-03 1.00 1.77E-03 2.46E-03
35%RH 5.3 2.37E-03 3.52E-03 0.74 1.77E-03 2.62E-03
50%RH 6.5 1.91E-03 3.17E-03 0.92 1.77E-03 2.93E-03
75%RH 8.5 2.41E-03 4.94E-03 0.73 1.77E-03 3.62E-03
Table 2.23. Dimensional changes of PPDSA (lot 2) at different humidities. a) Original data, b) data normalized to the dimensions of the dried film for 15%RH, The X and Y directions are perpendicular and parallel to the casting direction, respectively. The Z direction is the thickness direction.
193 Chapter 2
2.5.7.1.2. Density of the absorbed water in PPDSA at different humidities
To analyze the volume and weight changes of films as a function of relative humidity and calculate the density of the absorbed water in the polymer,
the weight and volume of equilibrated films should be measured on the same sample. However, these measurements were very difficult because the equilibrated films were so soft that the error in the measured volume of the films
could be large.
To use the volume and weight changes of films measured using different
samples, the measured volume and weight changes needed to be converted to
molar quantities. This conversion is possible if we know the density of polymer at
0%RH, the equivalent weight at 0%RH, and the volume and weight changes of
the films at different humidities.
The volumes and weights of dried samples (0%RH) were measured and
are listed in Table 2.24. The polymer density at 0%RH was calculated using
these data and is shown in same table. The sample sizes were different so a
weighted average polymer density at 0%RH was determined (1.32 g/cc).
Weight of Length Width Thickness Density sample film (g) (mm) (mm) (um) (g/cc)
#1 0.0084 3.53 4.05 452.7 1.30
#2 0.0093 3.54 4.31 476.0 1.28
#3 0.0029 2.25 2.23 403.0 1.44
#4 0.0023 1.73 2.39 388.0 1.44
#5 0.0028 1.71 2.82 444.7 1.31
Table 2.24. Weight and dimensions of PPDSA (lot 2) at 0 %RH. The calculated PPDSA density at 0 %RH is 1.32 g/cc (weighted average).
194 Chapter 2
The weight changes of films in the water uptake test were normalized to the weight for the dried film used for the 15%RH measurement and are shown in
Table 2.25. Because the same samples were used for the sulfonic acid titration to determine the [SO3H] concentration after the water uptake, the normalization was needed to compare the sample weight changes at different humidities.
Weight of film(g) Normali Normalized weight of film (g) Relative λ -zation humidity before after before after factor equilibration equilibration equilibration equilibration
0%RH 1.2 4.71E-02 4.71E-02 1.00 4.71E-02 4.71E-02
15%RH 4.3 4.71E-02 6.43E-02 1.00 4.71E-02 6.43E-02
35%RH 5.3 4.80E-02 7.05E-02 0.98 4.71E-02 6.92E-02
50%RH 6.5 5.17E-02 8.52E-02 0.91 4.71E-02 7.76E-02
75%RH 8.5 5.47E-02 1.05E-01 0.86 4.71E-02 9.07E-02
Table 2.25. Weight changes of PPDSA (lot 2) at different humidities. a) Original data from water uptake results (Table 2.11), b) data normalized to weight of the dried film used for the 15%RH equilibration.
Using the polymer density at 0%RH (1.32 g/cc from Table 2.24), the normalized volume changes (Table 2.23), the normalized weight changes (Table
2.25) and the equivalent weight of the dried film (139.0 g/[SO3H] from Table
2.10 ), the molar volume and weight changes for a given polymer at different humidities could be obtained and are shown in Table 2.26. Plots of molar weight and molar volume versus lambda are shown in Figure 2.63 and their average and standard deviation are listed in Table 2.27..
195 Chapter 2
a)
Normalized Molar volume of volume of film film (cc) Relative (cc) Normalization λ humidity factor after equilibration after equilibration
0%RH 1.2 1.77E-03 5.97E+04 105 15%RH 4.3 2.46E-03 5.97E+04 147 35%RH 5.3 2.62E-03 5.97E+04 156 50%RH 6.5 2.93E-03 5.97E+04 175 75%RH 8.5 3.62E-03 5.97E+04 216
b)
Normalized Molar weight of weight of film film (g) Relative (g) Normalization λ humidity factor after equilibration after equilibration
0%RH 1.2 4.71E-02 2.95E+03 139 15%RH 4.3 6.43E-02 2.95E+03 190 35%RH 5.3 6.92E-02 2.95E+03 204 50%RH 6.5 7.76E-02 2.95E+03 229 75%RH 8.5 9.07E-02 2.95E+03 268
Table 2.26. Molar volume (a) and weight (b) changes of PPDSA (lot 2) at different humidities.
196 Chapter 2
a) 300
250 y = 17.6x + 115.3
200
150
100
50 Molar weight of PPDSA of (g) Molar weight 0 0.0 2.0 4.0 6.0 8.0 10.0 Lambda b)
250
200 y = 94.3e 0.097x
150
100
50
Molar volume of PPDSA (cc) 0 0.0 2.0 4.0 6.0 8.0 10.0 Lambda
Figure 2.63. Plots of molar weight (a) and molar volume (b) vs. lambda for PPDSA (lot 2) using data in Table 2.26.
197 Chapter 2
a)
Estimated 95% range(univar) Characteristics Std dev Value Max Min
Slope 17.6 0.7 19.7 15.5
Intercept 115.3 3.8 127.3 103.8 b) Estimated 95% range(univar) Characteristics Std dev value Max Min Exponent 0.097 0.004 0.109 0.086 constant 94.3 2.2 101.4 87.2
Table 2.27. Statistical results of fitting line and exponential curve for plots in Figure 2.63. These results were calculated using MacroMath® Scientist (version 2.01) software.
The molar weight increase fits on a straight line and the slope is close to the theoretical value (18, the molecular weight of water). However, the molar volume does not follow the straight line, but an exponential curve. This means that the density of the absorbed water is above 1.0, the volume increase is smaller than expected due to frozen-in free volume in the polymer solid structure. Figure 2.64
shows that the measured molar volume from 15 to 75%RH is smaller than the
calculated values.
The densities of the absorbed water at different humidities were
calculated and are shown in Table 2.29 and Figure 2.65. The line shows in Figure
2.64 is the change on volume with weight if there is no loss of volume, water density is 1.0 g/cc. Because the changes of molar weight and volume come from
198 Chapter 2
absorbing water molecules, the measured volumes are all below the line,
showing that absorbed water has a smaller volume and higher density than free water.
199 Chapter 2
∆ ∆ Measured Measured Relative [Measured ∆ [Molar [Calculated molar molar humidity λ molar weight] molar volume weight (%RH) volume] (g) volume] (cc) (g) (cc) (cc)
0 1.2 105 0 139 0 0 15 4.3 147 42 190 51 51 35 5.3 156 51 204 65 65 50 6.5 175 70 229 90 90 75 8.5 216 111 268 129 129
Table 2.28. Measured and calculated molar volume changes of PPDSA (lot 2) at different humidities. The calculated molar volumes are obtained from the change of molar weight and the density of free water (1.0 g/cc).
140
120
100
80
60
40
20 [Measured molar volume] (cc) volume] molar [Measured ∆ 0 0 20 40 60 80 100 120 140 ∆ [calculated molar volume] (cc)
Figure 2.64. Plot of the measured molar volume changes vs. the calculated molar volume changes of PPDSA (lot 2) at different humidities. Blue dotted line shows the expected molar volume when there is no free volume in PPDSA.
200 Chapter 2
Relative density of ∆ [Molar volume] ∆ [Molar weight] humidity λ absorbed (cc) (g) (%RH) water (g/cc)
0 1.2 0 0 15 4.3 42 51 1.22 35 5.3 51 65 1.28 50 6.5 70 90 1.30 75 8.5 111 129 1.16
Table 2.29. Densities of the absorbed water in PPDSA (lot 2) at different humidities.
140
120
100
80
60 [Molar weight] (g) weight] [Molar
∆ 40
20
0 0 20 40 60 80 100 120 140 ∆ [Molar volume] (cc)
Figure 2.65. Plot of the molar weight change vs. the molar volume change for PPDSA (lot 2). The blue dotted line shows the weight increase for a given volume when the density of the absorbed water is 1.0 g/cc.
201 Chapter 2
2.5.7.2. Microscopic studies.
2.5.7.2.1. Analysis of X-ray diffractogram of PPDSA
The determination of the solid state structure of PPDSA at different humidities is needed to better understand the water retention properties of
PPDSA. Other sulfonated rigid rod poly(p-phenylene)s have such properties.51
But, this reference did not show WAXD data at different relative humidities; the environmental humidity was not controlled during the WAXD experiments. In this thesis, the PVC sealing method ensured that the samples were kept at controlled humidities, and reasonable spectra were obtained.
In the WAXD spectra (Figure 2.14, and Tables 2.35 and 2.36), the intensities and breadths of peak A at different humidities depend on the X-ray acquisition mode: intense and sharp peaks in the transmission mode vs. weak and broad peaks in the reflection mode. A possible reason for weak intensities and broader peaks A in the reflection mode might be that most of chains are oriented relatively perpendicular to the film surface.
The d spacing for peak A is a function of relative humidity (relative to lambda; the number of absorbed water molecules per sulfonic acid group) and information about the solid state morphology can be extracted. A plot of the d spacing of peak A versus lambda is shown in Figure 2.66. The d spacing changes from 15 to 75%RH are from ~8 to ~11 Å, and are proportional to lambda
(λ: 4.3 to 8.5) while the d spacing change in the 0 ~ 11%RH does not follow the curve above 15%RH (Table 2.30). The d spacings measured using WAXD data and 2D X–ray spectra are within experimental error (Table 2.31 and Figure 2.67).
202 Chapter 2
Further analysis of the d spacing changes for peak A at different humidities using model studies will be discussed in following section.
d spacing of peak A from transmission mode spectra (value ± deviation)(Å) Lambda Parallel Perpendicular
1.2 7.96 ± 1.04 8.11 ± 0.52 3.0 8.72 ± 0.26 8.81 ± 0.26 4.3 8.99 ± 0.24 8.89 ± 0.23 5.3 9.53 ± 0.24 9.41 ± 0.27 6.5 10.21 ± 0.31 9.99 ± 0.29 8.5 11.02 ± 0.29 11.08 ± 0.34
Table 2.30. The d spacing change of PPDSA (lot 2) as a function of lambda. Deviation was calculated with 2θ±0.5*FWHH using Bragg’s law.
d spacing calculated d spacing calculated using WAXD data (Å) using 2D X-ray data (Å) %RH λ Parallel Perpendicular Meridional Equatorial
0 1.2 7.96 ± 1.04 8.11 ± 0.52 8.11 ± 0.68 8.17 ± 0.67
15 4.3 8.99 ± 0.24 8.89 ± 0.23 8.61 ± 0.25 8.84 ± 0.34
35 5.3 9.53 ± 0.24 9.41 ± 0.27 9.04 ± 0.25 9.12 ± 0.30
50 6.5 10.21 ± 0.31 9.99 ± 0.29 9.92 ± 0.29 9.71 ± 0.30
Table 2.31. Comparison of d spacings calculated from the WAXD data with those calculated from the 2D X-ray spectra. Value and deviation (Å) of d spacings were given in Tables 2.14 (WAXS data) and 2.16 (2D X-ray data).
203 Chapter 2
12.0
11.0
10.0
9.0 d spacing (Å) d spacing 8.0 Transmission_perpendicular Transmission_parallel 7.0 Reflection_perpendicular Reflection_parallel 6.0 0.0 2.0 4.0 6.0 8.0 10.0 Lambda
Figure 2.66. Plot of d spacings vs. lambda for peak A at different humidities (0 to 75%RH). Error bars show the standard deviation of the d spacings, in Table 2.14.
204 Chapter 2
12.0 Transmission_perpendicular 11.0 Transmission_parallel 2D_Meridional 2D_Equatorial 10.0
9.0 d spacing (Å) d spacing 8.0
7.0
6.0 0.01.02.03.04.05.06.07.0 Lambda
Figure 2.67. Comparison of d spacings calculated from the 2D X-ray spectra with the d spacing from WAXD data (in transmission mode) at different humidities (0 to 50%RH).
205 Chapter 2
On the other hand, peaks B, C, C1, C2, D, E and F are independent with the relative humidity (Table 2.15) and provide information about the structure of the PPDSA backbone. These peaks are broad in the transmission and sharp in
the reflection mode. A wide breadth in the transmission mode is probably due to
the perpendicular orientation of polymer chains to the film surface.
The peak C1 is assigned to the distance between sulfonic acid groups on one and next benzene ring along with chain(simulation value; 5.21 to 5.56Å: d spacing from WAXD; 5.26±0.02Å from the parallel and 5.27±0.02 Å from the perpendicular beam direction to the casting direction) (Table 2.15 and Figure
2.68, a)). The peak C2 can possibly be assigned to the sulfur to sulfur length through one benzene ring. The d spacing for peak C2 probably is the diameter of a cylinder made by sulfonic acids on the rigid rod backbone (calculated value;
4.78 Å: d spacing from WAXD; 4.75±0.01Å for the beam parallel and
4.76±0.01Å for the beam perpendicular to the casting direction) (Table 2.15 and
Figure 2.68,b)). However, peak assignments for peaks B, D, E and F require more study.
The directional dependence of the X and Y chain orientation in the
PPDSA films was tested using samples orientations, with the X-ray beam parallel
(or perpendicular) to the casting direction. In the transmission mode, peak positions and relative intensities are about the same within the experimental error, so the PPDSA film is reasonably isotropic in the X and Y directions (Figure 2.35 and Table 2.15). Since the polydomains are perpendicularly oriented to the surface, the PPDSA film is reasonably isotropic in the X and Y directions. 2-D X-
206 Chapter 2
ray spectra and dimensional changes at different humidities support the isotropy
finding. Equilibrated films at different relative humidities have only ring in the 2D
X-ray spectra and the d spacings agree with those from WAXD. In addition,
equilibrated films had an almost isotropic dimensional expansion in the X, Y and
Z directions.
In the reflection mode, the positions of peaks C1, C2, D, E and F were almost same, but their intensities varied with the beam direction; the C1 and C2
peaks are more intense than other peaks in the parallel beam direction. However
these directional dependencies for peaks C1, C2, D, E and F require more study.
207 Chapter 2
a)
Peak C1
b)
Peak C2
Imaginary cylinder made by sulfonic acids Figure 2.68. Schematic for a) peak C1, and b) peak C2; Gray, white, orange, red, and brown ball in ball-and-stick rendering model are C, H, S, O, and Br, respectively.
208 Chapter 2
2.5.7.2.2. Model study: Hexagonal packing model
We propose that PPDSA packs hexagonally in the solid state because the solution OPM image showed a typical nematic liquid crystalline structure. In addition, the organized regions in the films OPM images increased with relative humidity; at 75%RH, all the area was organized (Figure 2.40,e)). A similar structure for a different class of sulfonated polymer was described by Rulken et al.136 They synthesized the sulfonated poly(p-phenylene) shown in Figure 2.69
and characterized it by X-ray techniques. They reported that three polymer
backbones were associated into one column and such columns were packed
hexagonally.
a) CH3 SO3 Na
H3C Na O3S n
b)
Figure 2.69. a) The chemical structure of a sulfonated poly(p-phenylene), b) Model of the hexagonal column made by three strands of sulfonated poly(p- phenylene).136 R is about 10Å.
209 Chapter 2
A schematic of the hexagonal packing for PPDSA is shown in Figure 2.70.
The volume change per PPDSA sulfonic acid group (V/SO3H) at different relative
humidities, from the experimental data, should follow Equation 2.19, if the density
of absorbed water is 1.0 g/cc.
24 V (V polymer +VH 2O ) V polymer ⎛ 18×10 ⎞ = = + ⎜ ⎟×λ ⎜ 23 ⎟ (Equation 2.19) SO3 H N A 0.6 ⎝ 6.022×10 ⎠
23 where, NA is Avogadro’s constant (6.022X10 ), Vpolymer is the volume of PPDSA
per acid group, and VH2O is the volume change due to the absorbed water.
(2/√3)D
D (d spacing from WAXD)
Bundle of polymer chains or one polymer
Figure 2.70. Schematic of the proposed hexagonal packing structure in solid PPDSA. Brown filled circles represent a bundle of polymer chains or one polymer chain. D is the d spacing found using WAXD. The distance between two brown filled circles was calculated as (2/√3)D.
210 Chapter 2
Using this hexagonal packing model, the volume of a bundle of chains (or one chain) per sulfonic acid group can be calculated. Before obtaining the volume change from the change of d spacing, the cross-sectional area is calculated in terms of d spacing; the cross-sectional area of the hexagon (S) is 6 times the area of the blue lined triangle in Figure 2.70 (Equations 2.20 and 2.21).
1 ⎛ 2D ⎞ D 2 s = × ⎜ ⎟ × D = (Equation 2.20) 2 ⎝ 3 ⎠ 3
2 ⎛ D ⎞ 2 S = 6× s = 6×⎜ ⎟ = 2 3 × D (Equation 2.21) ⎝ 3 ⎠
However, the hexagonal area (S) consists of three polymer chains (or bundles).
Therefore, the cross-sectional area per unit chain (or bundle) (A) is calculated using Equation 2.22. The volume of a chain (or bundle) per SO3H (V) can then be calculated using Equation 2.23.
S ⎛ 2 ⎞ 2 A = = ⎜ ⎟ × D (Equation 2.22) 3 ⎝ 3 ⎠
⎛ 2 ⎞ 2 V = 2.1× ⎜ ⎟ × D (Equation 2.23) ⎝ 3 ⎠ where 2.1 (Å) is the length along the backbone per sulfonic acid group, see
Figure 2.71.
211 Chapter 2
SO3HSO3HSO3H
HO3S HO3S HO3S
4.2Å
Figure 2.71. Molecular structure of PPDSA. The length of the repeating unit is 4.2Å. Then, the length per one sulfonic acid is 2.1 Å.
Using Equations 2.19, 2.22 and 2.23, D2 [(d spacing)2] is expressed as a function
of lambda (Equation 2.24), and the volume of polymer(only) was calculated from
the intercept of Equations 2.25 or 2.26). The water density determines the slope,
but the intercept (λ=0) is not influenced by the water density since Vpolymer is
2 D0 /0.6879 .
V V ⎛ 18×1024 ⎞ ⎛ 2 ⎞ A = = polymer + ⎜ ⎟× λ = ⎜ ⎟× D2 ⎜ 23 ⎟ (Equation 2.24) 2.1 0.6× 2.1 ⎝ 2.1× 6.022×10 ⎠ ⎝ 3 ⎠
2 λ = 0.08112 × D − 0.0558 ×V polymer (Equation 2.25)
2 D = 12.3266 × λ + 0.6879 ×V polymer (Equation 2.26)
212 Chapter 2
Before using the hexagonal packing model, we need to verify that the
model is appropriate. The first proof is seen in a plot of the molar volume
changes vs. (d spacing)2. If the hexagonal packing model is correct for solid
PPDSA, the slope of the plot of the ratio of molar volume change vs. (ratio of d spacing change)2 should be 1.0. Data are listed in Table 2.32 and their plot is
shown in Figure 2.72; the slope is 1.03±0.01. This means that the macroscopic results (the molar volume) matched well with the microscopic results (d spacing)
and the hexagonal model is suitable for PPDSA.
d spacing [(d ) Molar film (V ) %RH λ from %RH %RH /(d )]2 volume (cc) /(V ) WAXD 0%RH 0%RH
0 1.2 7.96 1.00 105 1.00 15 4.3 8.99 1.23 147 1.39 35 5.3 9.53 1.43 156 1.48 50 6.5 10.21 1.64 175 1.66 75 8.5 11.02 1.90 216 2.05
Table.2.32. Ratio of molar volume changes and (d spacing)2 of PPDSA (lot 2) at different humidities. d spacings were obtained using the parallel beam direction sample in the transmission mode. dx%RH is the d spacing at x%RH and Vy%RH is the molar film volume at y%RH.
213 Chapter 2
2.5
2
1.5
1
(molar volume_%RH) Y=1.03X /(molar volume_0%RH) /(molar 0.5 standard deviation : 0.01 95% range (univar) : 1.00 ~ 1.08 0 0.0 0.5 1.0 1.5 2.0 2.5 [(d spacing_%RH)/(d spacing_0%RH)]2
Figure 2.72. Plot of the ratio of molar volume change of PPDSA (lot 2) vs. (ratio of d spacing change)2 at different humidities.
The second proof relates to the dependence of expansion order of the d
spacings versus lambda. If the film has two dimensional expansion, the
hexagonal packing model is acceptable. Because the polymer film absorbs water
and the inter-chain distance (d spacing for peak A) increases with increasing
relative humidity, the d spacings at a specific relative humidity (ds%RH,A) can be expressed in terms of a function of lambda (Equation 2.27).
ds%RH ,A = ds0%RH + f (λ) (Equation 2.27)
where ds0%RH,A is the d spacing for peak A at 0 %RH and f(λ) is a function of
lambda.
214 Chapter 2
After calculating the polymer density at 0%RH (dpolymer at 0%RH, 1.32 g/cc, see in Table 2.24) and measuring the equivalent weight at 0 %RH, Equation 2.27 can be expanded to Equations 2.28 and 2.29.
n ⎛ d ⎞ ⎛ Δλ ×18× d ⎞ ⎜ %RH ,A ⎟ ⎜1 polymer,0%RH ⎟ ⎜ ⎟ = ⎜ + ⎟ (Equation 2.28) ⎝ d 0%RH ,A ⎠ ⎝ 139.35 ⎠
⎛ d ⎞ ⎛ Δλ ×18× d ⎞ ln⎜ %RH ,A ⎟ ln⎜1 polymer,0%RH ⎟ ⎜ ⎟ = n × ⎜ + ⎟ (Equation 2.29) ⎝ d 0%RH ,A ⎠ ⎝ 139.35 ⎠
where n (in Equations 2.28 and 2.29) is the local order of dimensional expansion
(proportional to the swelling), ∆λ is the lambda difference between 0%RH and a specific %RH, and 139.35 g/[SO3H] is the polymer equivalent weight at 0%RH.
The average value of n in Figure 2.73 (plotted using Equation 2.29) and Table
2.33 was 0.55 which means that PPDSA has two dimensional swelling.
Poly(biphenylene disulfonic acid), studied previously in our lab, swelled linearly with λ.90
215 Chapter 2
0.35 a) 0.3 y = 0.54x - 0.11 0.25
0.2
ln(Y) 0.15 y = 0.58x - 0.16 0.1
0.05 Transmission_parallel Transmission_perpendicular 0 0 0.2 0.4 0.6 0.8 1 ln(X)
0.4 Reflection_parallel b) 0.35 Reflection_perpendicular 0.3
0.25
0.2 y = 0.52x - 0.16
ln(Y) 0.15
0.1
0.05 y = 0.54x - 0.25 0
-0.05 0 0.2 0.4 0.6 0.8 1 ln(X)
Figure 2.73. Plots of the ratio of d-spacing at different humidities (from 15%RH to 75%RH) as a function of lambda. a) using data from the transmission mode, b) using data from the reflection mode; X is [1+ (∆λ X 18 X dpolymer at 0%RH)/139.35]. Y is the ratio of d-spacings (for peak A) at different humidities to that at 0%RH. The average, n in Equation 2.29 is 0.55.
216 Chapter 2
Value ± standard deviation Diffraction mode Direction Slope Intercept
Parallel 0.54 ± 0.02 -0.11 ± 0.01 Transmission mode Perpendicular 0.58 ± 0.02 -0.16 ± 0.01
Parallel 0.52 ± 0.09 -0.16 ± 0.06 Reflection mode Perpendicular 0.54 ± 0.03 -0.25 ± 0.02
Table 2.33. Statistical results of slopes and intercepts in Figure 2.73. Values and standard deviations were calculated using MacroMath® Scientist (version 2.01) software.
The third proof can be shown using plots of (d spacing)2 versus lambda. If
the plot shows a 1st order function of (d spacing)2 to lambda, it also means that
the hexagonal model is applicable. Figure 2.74 shows that (d spacing)2 is linearly
proportional to lambda above 15%RH.
Based on these three proofs, the hexagonal model can be used to study
the solid state structure of PPDSA. The first important result is that the hexagonal
packing model fits well with the experimental WAXD data because the slope is
almost constant in the D2 vs. lambda plots (Figure 2.74). By using intercepts in
Table 2.34, the volume of one chain (or bundle of chains) (equivalent volume in
cc/SO3H) was (53.96 ± 4.62) cc/ SO3H, This result implies that the diffracting
species consists of a single polymer chain. It is suggested that the sulfonic acid
groups surround the polymer backbone and their large electrostatic repulsive
forces keep the polymer chains separate.
217 Chapter 2
The second important result concerns the density of absorbed water in
the PPDSA films. The slope of Equation 2.26 using this model, if water density is
1.0g/cc, is 12.3266. The ratio of the model slope to that obtained from the WAXD data is the average density of “absorbed” water in the system, 1.23 g/cc. This is a very meaningful result. The observation was that the absorbed water has a higher density (smaller volume) than free water. This means that the PPDSA film has molecular size voids that can be filled with water, and the process is not finished even when λ=9. This frozen-in free volume was shown in the weight and dimensional changes in macroscopic studies. The molar volume did not increase in proportion to weight increase with absorbed water.
The most important conclusion is that PPDSA is reasonably isotropic in the X and Y directions and perpendicularly oriented in the Z direction. In surface conductivity measurement, the effect of direction (parallel or perpendicular to the casting direction) should negligible. But, the through-plane conductivity might be higher than the surface conductivity due to the orientation of the polymer chains perpendicular to the surface. In the through-plane conductivity measurements, protons might be able to move within the hexagonal structure without hindrance.
218 Chapter 2
a) 140
120 y = 9.8x + 39.2 )
2 100 (Å 2 80 y = 10.6x + 32.4
60
(d spacing) (d 40 PPDSA (lot 2)_Transmission_parallel 20 PPDSA (lot 2)_Transmission_perpendicular 0 0.0 2.0 4.0 6.0 8.0 10.0 Lambda
140 b) 120
y = 9.8x + 38.7
) 100 2 (Å 2 80 y = 9.9x + 38.3
60
(d spacing) (d 40 PPDSA(lot 2)_Reflection_parallel 20 PPDSA(LOT2)_Reflection_perpendicular 0 0.0 2.0 4.0 6.0 8.0 10.0 Lambda
Figure 2.74. Plots of (d spacing)2 vs. lambda for PPDSA (lot 2) at different humidities using a) data from WAXD transmission mode and b) data from WAXD reflection mode. Lambda is obtained from the water uptake measurements and acid-base titration. d spacings was obtained from the WAXD spectra of the humidity controlled PPDSA films.
219 Chapter 2
Value ± standard deviation Diffraction mode Direction Slope Intercept
Parallel 9.8 ± 0.31 39.2 ± 2.00 Transmission mode Perpendicular 10.6 ± 0.47 32.4 ± 3.01
Parallel 9.9 ± 1.67 38.3 ± 10.62 Reflection mode Perpendicular 9.8 ± 0.48 38.7 ± 3.04
Table 2.34. Values of slopes and intercepts in Figure 2.74. Values and standard deviations were calculated using MacroMath® Scientist (version 2.01) software.
220 Chapter 2
2.5.7.2.3. Frozen-in free volume in PPDSA
In analysis using the hexagonal packing model, (d spacing)2 [D2] linearly increases with lambda as a 1st order function and follows Equation 2.26 from λ of
4.3 (15%RH) to 8.5 (75%RH). But, D2 has large positive deviation from this
straight line in the low lambda and D2 increases are small, compared to those
when λ > 4 (Figure 2.74). In other words, the polymer volume increases very little
at low lambda.
To analyze and study this gradual (d spacing)2 change, a new relating
lambda and D2 is presented in Equations 2.30 and 2.31 based on Equation 2.26.
If lambda is large, D2 changes using Equation 2.30 approximate Equation 2.26.
To fit the experimental data at low lambda, Equation 2.30 is modified by adding one more λ function (Equation 2.31).
2 D = 12.3266 * λ + I1 (Equation 2.30)
2 D = 12.3266 *λ + I1 + I 2 (1− tanh(K *λ)) (Equation 2.31)
2 2 2 2 where D , λ and K are (d spacing) , lambda and a constant. I1 is D at λ = 0 if D
2 follows a straight line (Equation 2.30) and I2 is the deviation of D from Equation
2.30 at λ = 0
® The parameters (I1, I2 and K) in Equation 2.31 were calculated using MacroMath
Scientist (version 2.01) software and are shown in Table 2.35. The standard deviation of I2 is about 6 % and the calculated curve fits well with the
221 Chapter 2 experimental data. D2 from the deconvoluted transmission mode spectra and from the curve fitting, and lambda are listed in Table 2.365 and plotted as function of lambda in Figure 2.75.
Standard 95% range (univar) Parameters Estimated value deviation Max. Min.
I1 13.1 3.4 20.9 5.4
I2 47.8 3.1 54.9 40.7 K 0.18 0.03 0.25 0.11
Table 2.35. Curve fitting results for parameters in Equation 2.31.
(d spacing)2 (d spacing)2 Lambda _measured using _calculated by scientist software deconvoluted peak positions
1.2 63.34 65.36 1.2 65.80 65.36 3 76.08 73.92 3 77.70 73.92 4.3 80.87 82.43 4.3 79.01 82.43 5.3 90.89 90.36 5.3 88.64 90.36 6.5 104.17 101.26 6.5 99.72 101.26 8.5 121.44 121.92 8.5 122.84 121.92
Table 2.36. Changes of (d spacing)2 with lambda.
222 Chapter 2
140.0
120.0
100.0 2 80.0
60.0 (d spacing) (d
40.0
20.0 Measured data Calculated data including the frozen-in free volume Calculated data ignoring the frozen-in free volume 0.0 0.0 2.0 4.0 6.0 8.0 10.0 Lambda
Figure 2.75. Plots of (d spacing)2 of PPDSA (lot 2) vs. lambda. Red circles are experimental data. Blue dotted line shows the calculated data using Equation 2.30. Red solid line shows the calculated data using Equation 2.31.
The intercepts in Figure 2.75 are for one polymer chain volume per sulfonic acid (equivalent volume). The difference in the straight and curved lines means that the polymer at λ = 0 has unoccupied volume and can accommodate water molecules without much inter-chain distance change. Since PPDSA has two sulfonic acids per repeating unit, polymer chains might be kept apart due to the steric repulsion of rigid rod structure, and the polymer has unoccupied
223 Chapter 2
volume (i. e. the frozen-in free volume). The size of free volume was calculated
and is shown in Table 2.37. At λ=0, PPDSA can absorb about four water
molecular per sulfonic acid. This frozen-in free volume can be seen in Figure
2.59. Lambda from 0 to 15%RH (λ=4.5) sharply increased, but gradually
increased above 15%RH. Other evidence is the PPDSA has no free water up to
75%RH. Due to the frozen-in free volume, absorbed water molecules were tightly
bound and free water was not detected in DSC experiments
The intercept of the straight line should represent the polymer chain
cross-section per sulfonic acid. Equivalent volume at λ=0 (32±8 Å3) is
comparable the result calculated using WAXD data (peak C2). The calculated
equivalent volume using WAXD data (37 Å3) in the error range of that from
intercept data.
224 Chapter 2
Estimated value± Estimated Characteristics Unit standard deviation value Max. Max. Ignoring free 13.1 16.6 9.7 volume (d spacing)2 Å2 Including free 61.0 67.5 54.4 volume
Calc. using I1 (in case of having 31.9 40.2 23.6 Polymer no free volume) volume per Å3
sulfonic acid Calc. using I1 + I2 (in case of having 147.8 163.7 132.0 free volume)
Frozen-in Free volume Å3 115.9 140.1 91.8 per sulfonic acid
# of water molecules accommodated 3.9 4.7 3.1 in frozen-in free volume per sulfonic acid
Table 2.37. The frozen-in free volume in PPDSA at λ=0. Polymer volumes per sulfonic acid were calculated using Equation 2.23.
225 Chapter 2
2.5.8. Effect of water contents on proton conductivity of PPDSA
The most important result is that the polymers have higher conductivities
at low humidity (15%RH) compared to Nafion 117 (Table 2.38 and Figure 2.76).
The conductivities are plotted in terms of the relative humidity and lambda.
Lambda might not change much in test temperature range (from 25 to 75°C)
because most of absorbed water can vaporize from 120°C under lab atmosphere.
The proton conductivities of PPDSA (lots 2 and 3) were about 102 times higher than that of Nafion 117 at 15%RH and room temperature, reaching about 0.1
S/cm at 50%RH and room temperature. These results meet the DOE guidelines for high temperature fuel cells. Three main reasons for the high conductivities at low humidity are suggested: 1) the higher lambda (water content) of PPDSA film compared to that of Nafion at the same conditions, 2) high IEC (ion exchange capacity) of PPDSA and 3) the nano-size proton transfer channels within the film.
The first reason is high lambda for PPDSA. Lambda for PPDSA is about two waters higher than those of Nafion 117 between 15 and 75%RH at room
temperature. Theoretically, the water molecules in the fuel cell membrane can be
considered as the carrier phase in the Grotthuss mechanism (the hopping of
+ hydrogen–bonded proton, H3O ) or the vehicle mechanism (the pure diffusion of
+ hydrated protons, [H (H2O)n]). The proton conductivity is proportional to the
lambda.
The second reason is high IEC of PPDSA (8.46 meq/g). Xinhuai Ye et. al.137 studied the effect of IEC on the proton conductivity of sulfonated polyimide.
They synthesized sulfonated polyimides with different sulfonation degrees. Their
226 Chapter 2
IEC values were 2.54, 2.81 and 3.08, and the proton conductivity of these
polymers increased with IEC, especially at high temperature (140°C). In fact, to
have high conductivity, IEC is a very important consideration in polymer design.
The nano-size channels (much less than 8 - 11Å) due to the hexagonal packing of polymer molecules in the film could also help to increase the conductivity. The absorbed water molecules are partly held by hydrogen bonding
with the sulfonic acids. The frozen-in free volume, suggested in previous section,
due to the hexagonal packing of the PPDSA backbones helps hold the water
molecules efficiently at low humidity. Also, the linear rigid rod structure of PPDSA
can decrease the mean-free-path of the mobile ion. No matter whether the H+
mobility is due to the Grotthuss mechanism or the vehicle mechanism, the short
distance between the adjacent sulfonic acids and the straight path can increase
the proton transport velocity.
PPDSA conductivities are isotropic in the X and Y direction, as shown in
Figure 2.46. This directional independence can be expected from the ring
patterns in 2D-Xray spectra and the isotropic dimensional expansion. However, polymer chains are perpendicularly oriented to the surface, as shown in the
WAXD spectra using transmission versus reflection data, and through-plane conductivity (in the Z direction) should be the same or higher than in-plane conductivity.
227 Chapter 2
Conductivity (S/cm) Relative Temperature humidity PPDSA (°C) Nafion (%RH) 117 Lot 1 Lot 2 Lot 3
15 1.1E-03 8.9E-03 1.2E-02 8.5E-05 35 5.5E-02 5.3E-02 5.8E-02 4.0E-03 25 50 6.5E-02 1.6E-01 1.1E-01 1.0E-02 75 1.8E-01 1.9E-01 2.6E-01 3.0E-02 15 1.2E-02 3.3E-02 4.2E-02 35 1.8E-01 1.5E-01 1.8E-01 50 50 NA 3.2E-01 2.5E-01 75 NA NA NA 15 3.1E-02 9.2E-02 8.7E-02 35 4.1E-01 3.5E-01 2.9E-01 75 50 NA 1.0E+00 4.5E-01 75 NA NA NA
Table 2.38. Proton conductivities of different lots of PPDSA films and Nafion 117 at various temperatures and relative humidities. The PPDSA films were cut parallel to the casting direction.
228 Chapter 2
a) 1.E+01
1.E+00
1.E-01
1.E-02 PPDSA(lot 1)_25°C PPDSA(lot 1)_50°C PPDSA(lot 1)_75°C 1.E-03 PPDSA(lot 2)_25°C PPDSA(lot 2)_50°C Ionic Conductivity (S/cm) Conductivity Ionic PPDSA(lot 2)_75°C PPDSA(lot 3)_25°C 1.E-04 PPDSA(lot 3)_50°C PPDSA(lot 3)_75°C Nafion 117 1.E-05 0 102030405060708090100
Relative Humidity (%)
b) 1.E+01
1.E+00
1.E-01
1.E-02 PPDSA(lot 1)_25°C PPDSA(lot 1)_50°C PPDSA(lot 1)_75°C PPDSA(lot 2)_25°C 1.E-03 PPDSA(lot 2)_50°C Ionic Conductivity (S/cm) PPDSA(lot 2)_75°C PPDSA(lot 3)_25°C 1.E-04 PPDSA(lot 3)_50°C PPDSA(lot 3)_75°C Nafion 117
1.E-05 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 Lambda
Figure 2.76. Proton conductivities of PPDSA films from different lots under conditions: a) as a function of relative humidity, and b) as a function of lambda. Current was flowing parallel to film surface (the in-plane measurement). Lot 3 sample is the high molecular weight polymer.
229 Chapter 2
2.5.8.1. Activation energy for proton transport in PPDSA
From the conductivities of PPDSA between 15 and 50%RH at room
temperature, 50 and 75°C, the activation energy for proton transport can be
obtained using the Arrhenius equation. The relationship between ionic
conductivity and temperature can be expressed in Equation 2.32.138-140
⎛ E ⎞ ⎛ E ⎞ 1 σ = A× exp⎜− a ⎟ ; − lnσ = ⎜ a ⎟ − ln A (Equation 2.32) ⎝ RT ⎠ ⎝ R ⎠ T
where σ, A, Ea, R and T denote the ionic conductivity, pre-exponential factor, the
activation energy for ionic conduction, gas constant, and temperature,
respectively. This equation indicates that the conductivity of the membrane
should increase as the temperature of the sample increases. The temperature
dependence becomes more significant when the activation energy is high. Ea is a measure of how easily proton conduction occurs. Lower value of Ea indicates that proton conductivity occurs readily and the barriers to proton conduction are low, whereas high Ea indicates that protons can not move easily through the polymer.
The proton conductivity vs. temperature plots, using PPDSA from different
lots with measurements in the X (the measuring direction is parallel to the casting
direction) and Y (the measuring direction is parallel to the casting direction)
direction at 15 ~ 50%RH, are shown in Figures 2.77 for lot 2 polymer and 2.78 for
lot 3 polymer. The raw data used in these figures are listed in Tables 2.39. The
activation energies (Ea) for proton transport were calculated and are shown in
Table 2.40 with their average value and standard deviation. Because the films
230 Chapter 2
lost shape at 75%RH and 75°C, the conductivities for that condition are not included. The temperature dependence of proton conductivity was larger at low humidity for all measurements, as expected.
When the activation energies (Ea) for lot 2 and lot 3 (high molecular
weight polymer) are compared, the high molecular weight polymer’s Ea is lower than that for lot 2. The effect of molecular weight is bigger and the activation energy difference is about 8kJ/mole as the relative humidity increases to 50%RH
231 Chapter 2
6.0
5.0 y = 4809x - 11.6 4.0
3.0 y = 3629x - 9.2 (S/cm)] σ 2.0 y = 3500x - 9.8 -ln[
1.0 Lot 2_15%RH
Lot 2_35%RH 0.0 Lot 2_50%RH -1.0 0.0025 0.0030 0.0035 0.0040 0.0045 1/T (K-1)
Figure 2.77. The ln[σ (conductivity)] plot for PPDSA strips (lot 2) as a function of temperature (1/T).
232 Chapter 2
6.0
5.0 y = 4437x - 10.6 4.0
y = 3043x - 7.7
(S/cm)] 3.0 σ y = 2574x - 6.6 -ln[ 2.0 Lot 3_15%RH 1.0 Lot 3_35%RH Lot 3_50%RH 0.0 0.0025 0.0030 0.0035 0.0040 0.0045 1/T (K-1)
Figure 2.78. The ln[σ (conductivity)] plot for PPDSA strips (lot 3, high molecular weight polymer) as a function of temperature (1/T).
233 Chapter 2
a)
-ln[σ(S/cm)]
T T 1/T 15%RH 35%RH 50%RH (°C) (K) (K-1) Perpendi Perpendi Perpendi Parallel Parallel Parallel -cular -cular -cular
25 298 0.0034 4.72 4.78 2.93 3.36 1.82 2.36
50 323 0.0031 3.41 3.56 1.90 2.36 1.15 1.28
75 348 0.0029 2.38 2.26 1.06 1.58 -0.04 0.67
b)
-ln[σ(S/cm)]
T T 1/T 15%RH 35%RH 50%RH (°C) (K) (K-1) Perpendi Perpendi Perpendi Parallel Parallel Parallel -cular -cular -cular
25 298 0.0034 4.43 4.54 2.84 2.57 2.24 2.19
50 323 0.0031 3.17 3.09 1.72 1.42 1.40 1.39
75 348 0.0029 2.44 2.10 1.22 1.21 0.79 1.08
Table 2.39. ln[σ (conductivity)] for PPDSA films (lots 2 (a) and 3, high molecular weight polymer (b)) at different temperatures: Parallel and perpendicular mean the conductivity measuring direction is parallel (or perpendicular) to the casting direction, respectively.
234 Chapter 2
Relative Slope in Eq.2.32 Intercept in Eq.2.32 Lot humidity (value ± standard (value ± standard Ea (kJ/mole) (%RH) deviation) deviation)
15 4809 ± 261 -11.6 ± 0.8 40.0 ± 2.2
Lot 2 35 3629 ± 581 -9.2 ± 1.8 30.2 ± 4.8
50 3500 ± 669 -9.8 ± 2.1 29.1 ± 5.6
15 4437 ± 259 -10.6 ± 0.8 36.9 ± 2.2
Lot 3 35 3043 ± 395 -7.7 ± 1.2 25.3 ± 3.3
50 2574 ± 215 -6.6 ± 0.7 21.4 ± 1.8
Table 2.40. Calculated Ea (activation energy) for PPDSA films (lots 2 and 3) at different humidities. Values and standard deviations of slope and intercept were calculated using MacroMath® Scientist (version 2.01) software.
235 Chapter 2
45.0
40.0
35.0
30.0
25.0
20.0 (kJ/mole) a
E 15.0
10.0 Lot 2
5.0 Lot 3
0.0 0 102030405060 Relative humidity (%RH)
Figure 2.79. Activation energy in PPDSA films (lots 2 and 3) as a function of relative humidity. The lot 3 polymer is the high molecular weight fraction.
The activation energy of Nafion 117 in liquid water with a lambda of 22.0
141 is 2.3 kcal/mole (= 9.6kJ/mole) . Other values for the Ea of Nafion in acidic
liquid electrolyte or water were in the range of 10.3 ~ 13.5 kJ/mole.141-144 It is well known that proton conduction in Nafion membrane is governed by two mechanisms. One is a proton hopping (Grotthuss) mechanism, and the other is a
+ pure diffusion of hydrated protons, [H (H2O)n]. It has been suggested that
transport of H+ by a hopping mechanism contributes more to conduction at high
236 Chapter 2
water content, but little protonic hopping is expected at low water content.124
Thus, the proton conduction in Nafion 117 at high humidity is explained by hopping mechanism (through hydrogen-bonded water molecules that are strongly localized near the sulfonic acid). Lot 3 PPDSA at 50%RH (λ = 6.5) and room temperature has an activation energy of 21.4 ± 1.8 kJ/mole, close to that of
Nafion (19~22 kJ/mole) at 50%RH.145, 146
2.5.8.2. Corrected proton conductivity
Basically, the electric current equals the rate at which charge crosses a plane perpendicular to its flow. This can then be set equal to the product of four terms as below. It is clear that the current is proportional to each factor. 147
Electric current = rate at which charge crosses any plane
= (number of carriers per unit volume) X (cross-sectional area)
X (charge on each carrier) X (average carrier speed)
The number of carriers per unit volume is inversely related to lambda (λ,
the number of water molecules per sulfonic acid group), which is the carrier
matrix in a proton exchange membrane, and the volume, which related with the
inter-chain distance. These data can be generated by measuring water uptake,
dimensional changes and WAXD at different humidities. After the film is
equilibrated at a specific humidity, it has an isotropic expansion as shown in
dimensional change measurements (Figure 2.30). The film resistance measured
237 Chapter 2
in the impedance measurements is affected by dimensional changes as shown in
Equation 2.33. The conductivity (σcorrected) corrected to constant volume can be
obtained using Equation 2.34 by considering the dimensional changes (Table
2.12)
1 x + Δx ⎛ V ⎞ 3 () ⎜ ⎟ R = R0 * ; R0 = R *⎜ ⎟ (Equation 2.33) ()()y + Δy z + Δz ⎝V0 ⎠
where R is the measured membrane resistance using impedance measurements
and R0 is the corrected membrane resistance.
1 3 ⎛V0 ⎞ σ corrected = σ *⎜ ⎟ (Equation 2.34) ⎝ V ⎠
where σ and σcorrected are the calculated conductivity using R and the corrected
conductivity if there were no dimensional changes; V is the volume of a film after
equilibration at a specific humidity and V0 is the volume of the film before
equilibration.
The measured and corrected conductivities are compared in Table 2.41 and are shown in Figure 2.80. From the conductivities corrected for volume change, it can be seen that the volume corrected conductivities are almost the same as the uncorrected conductivities.
238 Chapter 2
a) Relative Molar volume of film humidity (V /V)1/3 (cc) 0 (%RH) 0 105
15 147 0.90
35 156 0.88
50 175 0.84
75 216 0.79
b)
Relative σ σ humidity measured σ _// measured σ _┴ _// corrected _┴ corrected (%RH) 15 8.9E-03 8.0E-03 8.4E-03 7.5E-03
35 5.3E-02 4.7E-02 3.5E-02 3.0E-02
50 1.6E-01 1.4E-01 9.4E-02 8.0E-02
75 1.9E-01 1.5E-01 3.0E-01 2.4E-01
Table 2.41. Corrected conductivities for PPDSA film (lot 2) at 25°C and different humidities; a) the volume ratio at different humidities, b) the corrected conductivity at different humidities.
239 Chapter 2
1.E+01 a)
1.E+00
1.E-01
1.E-02
1.E-03
σ_//_25°C Ionic Conductivity (S/cm) 1.E-04 σ_corrected_//_25°C
1.E-05 0 102030405060708090100 Relative Humidity (%)
1.E+01 b)
1.E+00
1.E-01
1.E-02
1.E-03
σ_┴_25°C Ionic Conductivity (S/cm) 1.E-04 σ_corrected_┴_25°C
1.E-05 0 102030405060708090100 Relative Humidity (%)
Figure 2.80. Corrected and uncorrected conductivities of PPDSA film (lot 2) using films cut a) parallel and (b) perpendicular to the casting direction.
240 Chapter 2
2.5.8.3. Intrinsic conductivity: Proton conductivity in the aqueous phase
To directly compare the proton conductivity of a new membrane with others, we should consider three aspects. The first aspect is that PEM membranes vary greatly in composition and equivalent weight. A second is that morphology affects conductivity greatly at equivalent water content. In addition, fluorocarbon sulfonic acids are much stronger acids than the aromatic sulfonic acids.
Intrinsic conductivities of the polyelectrolyte membrane can be compared if one considers only the aqueous phase in each polymer. This removes the complication of widely differing equivalent weights for different PEMs. It dose not compensate for the differing morphologies and acidities, but by removing one complicating factor, it may be easier to understand the influence of the other factors on conductivity.
The conductivity in the aqueous phase (intrinsic conductivity) is expressed as Equation 2.35 by consideration of the volume change due to absorbing water. The data are listed in Table 2.42 and shown in Figure 2.81
⎛ V ⎞ ⎜ ⎟ σ aq. phasec = σ measured ×⎜ ⎟ (Equation 2.35) ⎝Vwater ⎠
where σaq. phase and σmeasured are the conductivity in the aqueous phase and the
measured conductivity using impedance measurement; V and Vwater are the volume of a film after equilibration at a specific humidity and the volume increase after absorbing water.
241 Chapter 2
The intrinsic conductivities for PPDSA (lot 2) at 15%RH are about 5 times higher and above 35%RH, they are about 2 times higher than the measured conductivities. When compared with Nafion and PBPDSA (previously made in our lab)90, PPDSA has lower intrinsic conductivities from 15 to 35%RH (λ: 4.3 to 5.3) and same order of intrinsic conductivities at and above 50%RH. The possible reason is that the fraction of ionized acid in PPDSA is lower than in Nafion because Nafion is superacid and is almost completely ionized even at λ’s of 2 to
3.148 Above 50%RH (λ=6.5), the fraction of acid ionized increases and then the intrinsic conductivities PPDSA reach that of Nafion.
242 Chapter 2
a) Relative Molar volume of film humidity V V/V (cc) water water (%RH) 0 105 15 147 42 3.53 35 156 51 3.08 50 175 70 2.52 75 216 111 1.95
b)
Relative σ σ σ σ humidity λ measured aq. phase measured aq.ohase _// _// _┴ _┴ (%RH) 15 4.3 8.9E-03 3.1E-02 8.4E-03 3.0E-02
35 5.3 5.3E-02 1.6E-01 3.5E-02 1.1E-01
50 6.5 1.6E-01 4.1E-01 9.4E-02 2.4E-01
75 8.5 1.9E-01 3.8E-01 3.0E-01 5.9E-01
Table 2.42. Conductivities in aqueous phase for PPDSA film (lot 2) at 25°C and different humidities.
243 Chapter 2
1.E+01 a) σ-measured_//_25°C
σ_aqueous phase_//_25°C ) 1.E+00
1.E-01
1.E-02 Ionic Conductivity (S/cm Conductivity Ionic
1.E-03 0.0 2.0 4.0 6.0 8.0 10.0 Lambda
b) 1.E+01 σ-measured_┴_25°C
) σ_aqueous phase_┴_25°C 1.E+00
1.E-01
1.E-02 Ionic Conductivity (S/cm
1.E-03 0.0 2.0 4.0 6.0 8.0 10.0 Lambda
Figure 2.81. Comparison of the aqueous phase conductivities and the measured conductivities for PPDSA film (lot 2) with a) parallel and (b) perpendicular cut films to the casting direction.
244 Chapter 2
1.E+00
1.E-01
1.E-02
PPDSA (lot 2)_measured_//
1.E-03 PPDSA (lot 2)_aqueous phase_// PPDSA (lot 2)_measured_┴ PPDSA (lot 2)_aqueous phase_┴ Ionic Conductivity (S/cm) Conductivity Ionic PBPDSA_measured 1.E-04 PBPDSA_aqueous phase Nafion_measured Nafion_aqueous phase
1.E-05 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 Lambda
Figure 2.82. The aqueous phase conductivities of PPDSA (lot 2), Nafion 117 and PBPDSA [poly(biphenyl-2,2’-disulfonic acid)] made in our lab.90
245 Chapter 2
2.5.9. Effect of water content on mechanical properties
Water content has a large effect on PPDSA mechanical properties. A dried film was so brittle that it could not be measured. PPDSA equilibrated at
15%RH was brittle with a high break force (6.88 MPa at 1.1% elongation) and
Young’s modulus (1665 MPa). The low elongation could be due to low molecular weight as well as the rigid rod structure. However, as the water content increased
(35%RH), Young’s modulus was significantly reduced to 31.4 MPa. This means
that the absorbed water acts as a plasticizer
The low modulus (1.6GPa at 15%RH and 31.4MPa at 35%RH) compared
to any rigid polymer, less than liquid crystal polymer (5~20GPa) can be explained
by the polymer morphology and the plasticizing effect of the absorbed water.
Since polymer chains have relatively perpendicular orientations to the film
surface based on the 2D X-ray and WAXD results, PPDSA can be elongated by
low modulus. PBPDSA (previously made in our lab)90 had a lamellar structure
parallel to the surface and had a modulus of about 430 MPa at 40%RH. The
large drop of modulus with water content is due to the plasticizing effect. As more
water molecules are absorbed in the polymer, the inter-chain distance increases
and the polymer chains can slide, with low modulus.
246 Chapter 2
2.5.10. Thermal stability of PPDSA
PPDSA is expected to have good thermal stability because it is an aromatic polymer. As shown for polyphenylene sulfide (PPS) with a degree of
sulfonation, m =2,149 highly sulfonated polymers have higher thermal stability
than polymers with low degrees of sulfonation. The decomposition temperature of
highly sulfonated PPS (m=2.0) was 265°C, 125°C higher than that of PPS
(m=0.6) and 75°C higher than that of perfluorosulfonic acid ionomer (Nafion).
This is attributed to the stronger C-S bond strength in PPS (m=2.0) due to the two electron-withdrawing sulfonic acid substituents on one phenyl ring.
PPDSA decomposed at about 304°C, which is 114°C higher than that of the perfluorosulfonate ionomer (Nafion) decomposition temperature.149 The first
weight loss is due to vaporization of tightly bound water (λ=1.2). This vaporization
can be seen in its DSC high temperature scan curve (Figure 2.32, b)). The
second weight loss is shown in DSC scan curve and is possibly due to
decomposition of sulfonic acids. The thermal behavior of PBPDSA90 was studied
using TGA –MS by Litt’s group; SO2 was detected at 245°C and above, indicating loss of sulfonic acid.
247 Chapter 2
2.6. Conclusions
1,4-Dibromo-2,5-benzenedisulfonic acid, DBBDSA was made by sulfonation of 1,4-dibromobenzene with fuming sulfuric acid at 225°C. The chemical structures of DBBDSA-Li (dilithium salt), -Na (disodium salt), -BTMA
(benzyltrimethylammonium salt) and -TBP (tetrabutylphosphonium salt) were characterized by 1H-, 13C-NMR and FT-IR. The maximum yield of desired product
was about 35 to 38% in both small and large scale reactions.
Using the new monomer, PPDSA, [poly(p-phenylene-2,5-disulfonic acid)]
were made using the copper mediated Ullmann coupling reaction. Based on the viscosities of several salt forms of PPP in different solvents, reaction conditions needed to produce high molecular weight polymer were found. Higher molecular weight PPDSA was obtained (reduced viscosity: 0.671 dL/g at 0.202 g/dL in D2O
at 35°C) from the reaction of DBBDSA-Li in dried DMF (0.05 mole/L) at 135°C.
The chemical structure was studied by 1H- and 13C-NMR. 1H-NMR spectra
showed that as the molecular weight of polymer increased, all peaks became
broader (larger FWHH) and the relative area of peaks near 7.65 ppm noticeably
decreased. The number average of degree of polymerization of the polymer (lot 3,
high molecular weight polymer) was 142 by calculation using the area ratio of
deconvoluted peak areas. The structure of PPP was characterized by 13C-NMR and confirmed the coupling between monomers at 1 and 4 positions.
From rheometric measurements, PPDSA solutions (38.51 g/dL) had shear dependent viscosity and two regions (shear thinning and Newtonian
248 Chapter 2
plateau) in its viscosity-shear rate plot as expected from rigid rod polymer
structures.
GPC was used to calculate the molecular weights of the polymers relative to PS standards. However, the elution time of the polymer and the monomer
were almost same. In addition, GPC chromatograms and elution times were
greatly affected by temperature, polymer concentration and salt concentration.
These made an interpretation of GPC chromatograms difficult.
The solution properties were those expected from a linear rigid rod polymer. Reduced viscosity showed an upturn as concentration decreased and this behavior was not affected by the presence of salt in the solution. PPDSA aqueous solutions showed shear thinning, which is characteristic of linear rigid
rod polymers. A modified Huggins equation was applied to study the viscosity
behavior of PPDSA, but more study is needed to understand the system fully.
Membrane properties of PPDSA were characterized in terms of water
content. PPDSA absorbed about two waters per sulfonic acid more than Nafion
from 15 to 75%RH at room temperature. It absorbed more water at a given
relative humidity than other aromatic sulfonic acid polyelectrolytes. DSC high and
low temperatures scanning curves showed that absorbed water molecules (λ ~ 9
at 75%RH) did not freeze after cooling to -50°C. There were two vaporization endotherms on heating; the lower one was at about 120~130°C and the higher
one was about 150~160°C. These could be assigned to strongly bound water
with different binding strengths. TGA result showed that the dried polymer had about one water molecule per sulfonic acid. The polymer started decomposing at
249 Chapter 2
about 304°C. This is excellent stability compared to other sulfonic acid
polyelectrolytes.
High water content and strong binding power were related to the frozen-in
free volume in PPDSA. The frozen-in free volume at different relative humidities was studied using macroscopic and microscopic methods, dimensional/weight changes measurements and X-ray (WAXD and 2D X-ray).
Dimensional/weight change measurements showed that the PPDSA molar volume did not increase proportionally with the change of its molar weight.
From X-ray results in the transmission mode, the long spacing increase from 8 to 11 Å as the relative humidity increased from 0 to 75%. WAXD reflection spectra showed peaks only at higher angles (smaller d spacings) that did not vary with relative humidity. These are probably related to the sulfonic acid organization around the backbone. The long spacing was barely visible in these spectra, showing that there were very few domains with chain axes parallel to the film surface. Flat plate X-ray scans showed only the long spacing; this was a complete ring. This implies that the nematic domains were organized with axes from ~45 to 90° to the film surface. When a polarizing optical microscope was used to study PPDSA, it was birefringent over the whole relative humidity range.
Local domain orientation would be observed even at 15%RH and domain size increased as the relative humidity increased.
To study a relationship between the inter-chain distance and lambda at different humidities, a hexagonal packing model was proposed and verified using micro- and macroscopic data. Based on the results from the model study, PPDSA
250 Chapter 2
had a frozen-in free volume about four water molecules at λ = 0 can be filled with water. The sharp increase of lambda from 0 to 15%RH was possibly explained by this free volume. The calculated density of water using this model was 1.23 g/cc.
In addition, permanent nano-size proton conducting channels can be formed in the hexagonal structure and these nano-size channels can provide high water binding ability and high proton conductivity at low humidity.
The water content affects the proton conductivity and mechanical properties. PPDSA had outstanding proton conductivity when compared to other membrane materials; 0.01 S/cm at 15%RH and room temperature (which is 102 times higher than that of Nafion) and 0.1 S/cm at 15%RH and 75°C. Conductivity was above 0.1 S/cm at 75%RH and room temperature. The conductivity activation energy of PPDSA at λ=6.5 was 21.4 ± 1.8 kJ/mole. Even if the volume changes were considered, the PPDSA conductivities did not change much. The
PPDSA intrinsic conductivities were affected the fraction of acid ionized. The conductivities were lower than Nafion at low lambda and almost same as those of Nafion above λ = 6.5.
The low modulus of humidified PPDSA was explained as due to the by plasticizing effect of the absorbed water and the perpendicular orientation of polymer. Film humidified at 35%RH had a very lower modulus (31.4 MPa).
However, PPDSA is water-soluble at high humidity (above 75%RH) and can not be used as such in fuel cells. The next chapter will discuss strategies to control PPDSA solubility.
251 Chapter 2
2.7. References
1. Eisenberg, A., Yeager, H. L., Eds. Perfluorinated Ionomer Membranes; ACS
Symposium Series 180; American Chemical Society: Washington, DC, 1982.
2. Mauritz, K. A., Macromolecules, 1989, 22, 4483.
3. Eisenberg, A., Hird, B., Moore, R. B., Macromolecules, 1990, 23, 4098.
4. Perusich, S. A., Avakian, P., Keating, M. Y., Macromolecules, 1993, 26, 4756.
5. Kreuer, K. D., Chem. Mater., 1996, 8, 610.
6. Yea, S. C., Eisenberg, A., J. Appl. Polym. Sci., 1977, 21, 875.
7. Marvel, C. S., Artzell, G. E., J. Am. Chem. Soc. 1959, 81, 448.
8. Kovacic, P.; Kyriakis, A., J. Am. Chem. Soc. 1963, 85, 454.
9. Yamamoto, T., Hayashi, Y., Yamamoto, Y., Bull. Chem. Soc. Jpn., 1978, 51,
2091.
10. Yamamoto, T., Morita, A., Miyazaki, Y., Maruyama, T., Wakayama, H., Zhou,
Z., Nakamura, Y., Kanbara, T., Sasaki, S., Kubota, K., Macromolecules, 1992,
25, 1214.
11. J. A. Kerres, J. Membr. Sci., 2001, 185, 3.
12. M. Rikukawa, K. Sanui, Prog. Polym. Sci., 2000, 25, 1463.
13. O. Savadogo J. New Matter. Electrochem., 1998, 1, 66.
14. Jin X., Bishop M., Ellis T., Karasz F. B., Polym. J., 1985, 17, 4.
15. Lee J, Marvel C. S., J. Polym. Sci. Polym. Chem. Ed., 1984, 22, 295.
16. Noshay, A., Robeson, L.M., J. Appl. Polym. Sci., 1976, 20, 1885.
17. Johnson BC., Ylgor I., Iqbal M., Wrightman JP., Lliyd DR., McGrath JE., J.
Polym. Sci. Polym. Chem. Ed., 1984, 22, 72.
252 Chapter 2
18. Litter M., Marvel C.S., J. Polym. Sci. Polym. Chem. Ed., 1985, 23, 2205.
19. Makowsky H, Lundberg R. D., Singahl G. H., US Patent, 1975, US3870841.
20. Bishop M. T., Karasz F. E., Russo P. S., Langley K. H., Macromolecules, 1985,
18, 86.
21. Devaux J., Delimoy D., Daoust D., Legras R., Mercier J.P., Strazielle C., Neild
E., Polymer, 1985, 26, 1994.
22. Hickner, A.M., Ghassemi H., Kim, Y. S., Einsla B. R., McGrath J.E., Chem.
Rev., 2004, 104, 4587-4612.
23. Giuffre, L., Montoneri, E., Modica, G., Tempesti, E., Hydrogen Energy, 1983,
8, 419.
24. Montoneri, E., Modica, G., Giuffre, L., Peraldo-Bicelli, C., Maffi, S., J. Polym.
Mater. 1987, 11, 263.
25. Montoneri, E., J. Polym. Sci., 1989, A27, 3043.
26. Hruszka, P., Jurga, J., Brycki, B., Polymer, 1992, 33, 248.
27. Kenji Miyatake, Hiroyuki Iyotani, Kimihisa Yamamoto, and Eishun Tsuchida,
Macromolecules, 1996, 29, 6969-6971.
28. Kenji Miyatake, Eiichi Shouji, Kimihisa Yamamoto, and Eishun Tsuchida,
Macromolecules, 1997, 30, 2941-2946.
29. Gilbert, E. E., Sulfonation and Related Reactions, interscience, New York,
1965, 425-438.
30. Sone Y., et. al., J. Electrochem. Soc., 1996, 143(4), 1254.
31. Kovacic, P., Jones, M. B., Chem. Rev., 1987, 87, 357.
32. Noren, G. K., Stille, J. K., Mucromol. Reu., 1971, 5, 385.
253 Chapter 2
33. Tourillion, Ci., In Handbook of Conducting Polymers; Skotheim, T. A, Ed.;
Dekkcr: New York, 1986.
34. Baughman, R. H., Bredas, J. L., Chance, R. R., Elsenbaumer, R. L.,
Shacklette, L. W., Chem. Reu., 1982, 82, 209.
35. Krivoshei, I. V., Skorobogatov, V. M., Polyacetylene and Polyarylenes;
Polymer Monographe; Gordon and Breach Science Publishers: Philadelphia,
PA, 1991; Vol. 10.
36. Berresheim A. J., Muller M., Mullen K., Chem Rev., 1999, 99, 1747-1785.
37. Watson M. D., Fechtenkotter A., and Mullen K., Chem. Rev., 2001, 101, 1267.
38. Wu, J., Grimsdale, A. C., Mullen K., J. Mater. Chem., 2005, 15, 41–52.
39. Percec, V., Pugh, C., Cramer, E., Weiss, R., Polym. Prepr. (Am.Chem. Soc.,
Div. Polym. Chem.), 1991, 32 (l), 329.
40. Percec, V., Okita, S., Weiss, R., Macromolecules, 1992, 25, 1816-1823.
41. Percec, V., Bae, J.-Y., Zhao, M., Hill, D.H., Macromolecules, 1995, 28, 6726.
42. Bloom, P., Jones, C.J., Sheares, V.V., Macromolecules, 2005, 38(6), 2159.
43. Rehan, M., Schlüter, A. D., Wegner, G., Feast W. J., Polymer, 1989, 30, 1054.
44. Schlüter A. D., J. Polym. Sci., part A, 2001, 39, 1533-1566.
45. Goodson, F. E., Wallow, T. I., Novak, B.M., Macromolecules, 1998, 31, 2047.
46. Rulkens, R., Schulze, M., Wegner, G., Macromol. Rapid Commun., 1994, 15,
669-676.
47. Felix E. Goodson, Thomas I. Wallow, Bruce M. Novak, Macromolecules,
1998, 31, 2047-2056.
254 Chapter 2
48. Cimrova, V., Schmidt, W., Rulkens, R., Schulze, M., Meyer, W., Neher, D.,
Adv. Mater., 1996, 8(7), 585-588.
49. Rulkens, R., Wegner, G., Enkelmann, V., Schulze, M., Ber. Bunsenges. Phys.
Chem., 1996, 100(6), 707-714.
50. Vanhee, S., Rulkens, R., Lehmann, U., Rosenauer, C., Schulze, M., Kohler,
W.,Wegner, G., Macromolecules, 1996, 29, 5136-5142.
51. Sergio Granados-focil, Ph. D thesis, Case Western Reserve University,
January, 2006.
52. Ullmann, F., Bielecki, J., Chem. Ber., 1901, 34, 2174.
53. Fanta, P.E., Synthesis, 1974, 1, 4.
54. Fanta, P.E., Chem. Rev., 1964, 64, 613.
55. Hassan, J., Sevignon, M., Gozzi, C., Schultz, E., Lemaire, M., Chem. Rev.,
2002, 102, 1359-1469.
56. Granados-Focil, S., Litt, M. H., Polym. Mat. Sci. and Eng., 2003, 89, 438-439.
57. Granados-Focil, S., Litt, M. H., Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem.,
2004, 49(2).
58. Vogel A.I., Vogel’s Textbook of practical chemistry, Longman, London, 1989,
426.
59. Chen, L. M. S. Thesis, Case Western Reserve University, 1999.
60. Handbook of Chemistry and Physics, David R. Lide, CRC press, 77th Ed. 6-8
255 Chapter 2
61. N. Kasai, M. Kakudo, X-ray Diffraction by Macromolecules, Kodansha and
Springer, 2005.
62. D. Demus, L. Richter, Textures of Liquid Crystals, 1978, VEB Deutscher
Verlag fur Grundstoffindustrie, Leipzig, German Democratic Republic.
63. B. D. Cahan , J. S. Wainright, J. Electrochem. Soc., 1993, 140, 185.
64. R. E. Thomas, A. J. Rosa, The Analysis and design of linear circuits, 2nd Ed.,
Prentice Hall, 433-459, 1998.
65. A. J. Bard, L. R. Faulkner, Electrochemical methods, Fundamentals and
Applications, 2nd edition, John wiley & sons, INC.2001.
66. Virgil Percec, Jin-Young Bae, Mingyang Zhao, and Dale H. Hill, J. Org.
Chem., 1995, 60, 176-185.
67. Virgil Percec, Jin-Young Bae, Mingyang Zhao, and Dale H. Hill,
Macromolecules, 1996, 29, 3727-3735.
68. Felix E. Goodson, Thomas I. Wallow, and Bruce M. Novak, Macromolecules,
1998, 31, 2047-2056.
69. D. G. Baird, J. Rheology., 1980, 24,465.
70. Simha, R., Phys. Chem., 1940, 4425.
71. Henans, J. Jr., J. Colloid Sci., 1962, 17, 638.
74. Kiss, G., Porter, R., J. Polym. Sci., Polym. Phys. Ed., 1980, 18, 36.
75. Papkov, S. P., Kulichikhim, V. G., Kalmykova, V. D. and Malkin, A. Ya., J.
Polym. Sci. , Polym. Phys. Ed., 1974 U, 1753.
76. Papkov, S., V. Kulichikhin, A. Malkin, V. Kalmykova, A. Volokhina, and L.
Gudin, Vysokomolekuljarnye Soedinenija (URSS), 1972, 14-N4, 244.
256 Chapter 2
77. Aharoni, S. M., Polymer, 1980, 21 1413.
78. Doi, M., Edwards, S. F., The Theory of Polymer Dynamics, Oxford University
Press, New York, 1986.
79. Onogi, S. and Asada, T., pp. 127-147 in Rheology, Vol. I, Astarita, G.,
Marmcci, G. and Nicolais, L., Eds., Plenum, New York (1980).
80. L. Walker, N. Wagner, J. Rheology, 1994, 38(5), 1525.
81. M. Dadmun, C. C. Han, Macromolecules, 1994, 27, 7522.
82. K. F. Wissbrun, J. Rheology, 1981, 25(6), 619.
83. D. S. Kalika, D. W. Giles, M. M. Denn, J. Rheol. 1990, 34, 139- 154.
84. D. G. Peiffer, R. D. Lundberg, J. of Poly. Sci.: Polym. Chem. Ed., 1984, 22,
1757.
85. N. Sarkar, L. D. Kershner, Journal of Applied Polymer Science, 1996, 62, 393.
86. T. Yamamoto, B-K. Choi, K. Kubota, Polymer Journal, 2000, 32(7), 619.
87. P. J. Flory, Principle of polymer chemistry, Cornell University Press, Ithaca, N.
Y. 1953.
88. S. A. Rice, M. Nagasawa, Polyelectrolyte solutions, Academic press, New
York, N. Y. 1969.
89. Förster, M. Schmidt, Adv. Polym. Sci., 1995, 120, 51.
90. Segio Granados-focil, Ph. D thesis, Case Western Reserve University,
January, 2006.
91. P. Kovacic, A. Kyriakis, J. Am. Chem. Soc., 1963, 85, 454.
92. P. Koacic, J Oziomeck, J. Org. Chem., 1964, 29, 100.
257 Chapter 2
93. C. F. Hsing, I. Khoury, M. Bezoari, P. Kovacic, J. Poly. Sci.; Polym. Chem. Ed.,
1985, 26, 1628.
94. T. Yamamoto, A. Yamamoto, Chem. Lett., 1977,353.
95. T. Yamamoto, Y. Hayashi, A. Yamamoto, Bull. Cehm. Soc. Jpn., 1978, 51,
2091.
96. Moore W.R., Prog. Polym. Sci., 1967, 1, 1-457.
97. G. Liu, X. Yan, S. Duncan, Macromolecules, 2003, 36, 2049.
98. J. Fisher, A. G. Fredrickson, Mol. Cryst. Liq. Cry.,1969, 8, 267.
99. David C. Boris, Ralph H Colby, Macromolecules, 1988, 31, 5746-5755.
100. M. Nagasawa, In Polyelectrolytes, Selegny, E., Ed., Dordrecht, Netherlands,
1974, Vol. I.
101. A. Katchalsky, Z. Alexandrowicz, O. Kedem, In Chemical Physics of Ionic
Solutions, Conway, B. E., Barradas, R. G.,Eds., Wiley, New York, 1966.
102. F. Oosawa, Polyelectrolytes, Marcel Dekker, New York, 1971.
103. H. Eisenberg, J. Pouyet, J. Polym. Sci., 1954, 13, 85.
104. J. A. Butler, F. R. Robins, K. V. Shooter, Proc. R. Soc., London 1957, A241,
299.
105. H. Eisenberg, G. R. Mohan, J. Phys. Chem., 1959, 63, 671.
106. C. Rochas, A. Domard, M. Rinaudo, Polymer, 1979, 20, 76.
107. M. Tricot, B. Maquet, M. Devaleriola, C. Houssier, Polymer, 1984, 25, 1397.
108. J. Cohen, Z. Priel, Y. Rabin, J. Chem. Phys., 1988, 88, 7111.
109. J. Yamanaka, H. Matsuoka, H. Kitano, M. Hasegawa, N. Ise J. Am. Chem.
Soc., 1990, 112, 587.
258 Chapter 2
110. J. Yamanaka, H. Araie, H. Matsuoka, H. Kitano, N. Ise, T. Yamaguchi, S.
Saeki, M. Tsubokawa, Macromolecules, 1991, 24, 6156.
111. H. Vink, Polymer, 1992, 33, 3711.
112. J. L. Ganter, M. Milas, M. Rinaudo, Polymer, 1992, 33, 113.
113. K. C. Tam, C. J. Tiu, Non-Newtonian Fluid Mech. 1993, 46,275.
114. Fernandez Prini, R., Lagos, A. E., J. Polym. Sci.: Part A, 1964, 2, 2917.
115. M. G. Oostwal, Ph. D. Thesis, University of Leiden, 1994.
116. R. M. Fuoss, Discuss. Faraday Soc., 1951, 11, 125.
117. Y. Rabin, Journal of polymer Science: Part C: Polymer Letters, 1988, 26,
397-399.
118. M. Elomaa, S. Hietala, M. Paronen, N. Walsby, K. Jokela, R. Serimaa, M.
Torkkeli, T. Lehtinen, G. Sundholm, R. J. Sundholm, Mater. Chem., 2000, 10,
2678-2684.
119. R. W. Kopitzke, C. A. Linkous, H. R. Anderson, G. L. Nelson, J. Electrochem.
Soc., 2000, 147, 1677-1681.
120. P. D. Beattie, F. P. Orfino, V. I. Basura, K. Zychowska, J. F. Ding, C. Chuy, J.
Schmeisser, S. J. Holdcroft, Electroanal. Chem., 2001, 503, 45-56.
121. B. Pivovar, Y. Wang, E. L. Cussler, J. Membr. Sci., 1999, 154, 155-162.
122. D. Rivin, C. E. Kendrick, P. W. Gibson, N. S. Schneider, Polymer, 2001, 42,
623-635.
123. J. Kim, B. Kim, B. Jung, J. Membr. Sci., 2002, 207, 129-137
124. T. A. Zawodzinski, C. Derouin, S. Radzinski, R. J. Sherman, V. T. Smith, T. E.
Springer, S. Gottesfeld, J. Electrochem. Soc., 1993, 140, 1041.
259 Chapter 2
125. T. A. Zawodzinski, T. E. Springer, J. Davey, R. Jestel, C. Lopez, J. Valerio, S.
Gottesfeld, J. Electrochem. Soc., 1993, 140, 1981-1985.
126. X. Ren, S. Gottesfeld, J. Electrochem. Soc., 2001, 148, A87-A93.
127. F. X. Quinn, E. Kampff, G. Smyth, V. J. McBrierty, Macromolecules, 1988, 21,
3192-3198.
128. G. Smyth, F. X. Quinn, V. J. McBrierty, Macromolecules, 1988, 21, 3198-
3216.
129. R. M. Hodge, T. J. Bastow, G. H. Edward, G. P. Simon, A. J. Hill,
Macromolecules, 1996, 29, 8137-8143.
130. B. Lafitte, L. E. Karlsson, P. Jannasch, Macromol. Rapid Commun. 2002, 23,
896-900.
131. Bryan S. Pivovar, Polymer, 2006, 47, 4194–4202.
132. T. A. Zawodzinski Jr, T. E. Springer, F. Uribe, S. Gottesfield, Solid state
ionics, 1993, 60, 199-211.
133. Thomas A. Zawodzinski, John Davey, Judith Valerio, Shimshom Gottesfeld,
Electrochima Acta., 1995, 40, 297-302.
134. Yu Seung Kim, Limin Dong, Michael A. Hickner, Thomas E. Glass, Vernon
Webb, James E. McGrath, Macromolecules, 2003, 36(17), 6281.
135. 2007 Multi-Year Research, Development and Demonstration Plan, U.S.
Department of Energy, 2007.
136. R. Rulkens, G. Wegner, T. Thurn-Albrecht, Langmuir, 1999, 15, 4022-4025
137. Xinhuai Ye, He Bai, W.S. Winston Ho, Journal of Membrane Science, 2006,
279, 570–577.
260 Chapter 2
138. N. Miyake, J. S. Wainright, R. F. Savinell, J. Electrochem. Soc., 2001, 148
(8), A898–A904.
139. Y. Sone, P. Ekdunge, D. Simonsson, J. Electrochem. Soc., 1996, 143 (4),
1254–1259.
140. V. V. Binsu, R. K. Nagarale, Vinod K. Shahi, P. K. Ghosh, Reactive &
Functional Polymers, 2006, 66, 1619–1629.
141. Michael A. Hickner, Cy H. Fujimoto, Chris J. Cornelius, Polymer, 2006, 47,
4238–4244.
142. Rune Halseid, Preben J. S. Vie, Reidar Tunold, Journal of The
Electrochemical Society, 2004, 151(3), A381-A388.
143. J. Halim, F. N. Büchi, O. Haas, M. Stamm, G. G. Scherer, Electrochim. Acta.,
1994, 39, 1303.
144. V. Tricoli, N. Carretta, M. Bartolozzi,, J. Electrochem. Soc., 2000, 147, 1286.
145. M. Cappadonia, J. W. Erning, S. M. Saberi Niaki, and U. Stimming, Solid
State Ionics, 1995, 77, 65.
146. W. B. Johnson and W. Liu, in High Temperature Materials, S. Singhal, Editor,
PV 2002-5, p. 132, The Electrochemical Society Proceedings Series,
Pennington, NJ (2002).
147. Heith B. Oldham, Jan C. Myland, Fundamentals of Electrochemical Science,
Chapter 1. The conduction of electricity, Academic press, INC, 1994.
148. Paddison, S., Annu. Rev. Mater. Res., 2003,33,289
149. Kenji Miyatake, Hiroyuki Iyotani, Kimihisa Yamamoto, Eishun Tsuchida,
Macromolecules, 1996, 29, 6969-6971.
261 Chapter 3
Chapter 3. General conclusions and future work
3.1. General conclusions
A new disulfonated monomer, DBBDSA (1,4-dibromo-2,5-benzene-
disulfonic acid) was synthesized by sulfonation of 1,4-dibromobenzene with
fuming sulfuric acid at a high temperature. The crude product was a mixture of
1,4-dibromo-2,5-benzenedisulfonic acid and 1,4-dibromo-2,6-benzenedisulfonic
acid. Pure para DBBDSA was separated using Soxhlet extraction by ethanol. The
best yield of DBBDSA disodium salt was about 38 %. DiBTMA
(benzyltrimethylammonium), dilithium and diTBP (tetrabutylphosphonium) salt forms of DBBDSA were synthesized and characterized
Ullmann coupling of DBDBSA salts was used to synthesize the rigid rod
liquid crystalline polymer, PPDSA [poly(p-phenylene-2,5-disulfonic acid)]. Low molecular weight polymers were obtained when diBTMA (or diTBP) salts were reacted in NMP, because the resulting polymers precipitated during coupling and probably could not react further. The best results were obtained when the dilithium salt of DBBDSA was reacted in DMF. The criteria used to assess molecular weight were the polymer viscosity and solubility
Solution behavior was observed using rhelology and viscosity
measurements at different polymer concentrations. In steady state rheological
studies, PPDSA solutions had shear dependent viscosities. There were two distinct regions in the viscosity-shear rate plots. Concentrated PPDSA solution
262 Chapter 3
and the humidified solid polymer had typical nematic liquid crystalline
organization in OPM images. However, even at concentrations below 1 g/dL, the
rigid rod structures increased polymer relaxation times and the solutions showed
strong shear dependence. An abnormal increase of reduced viscosity in the low
concentration region was observed under all conditions (shear rate, salt concentration, solvent, cationic species).
The PPDSA, disodium salt 1H-NMR spectra showed many overlapped peaks instead of single expected peak. Based on the peak deconvolution results,
as the polymer molecular weight increased, all peaks became broader and most
peaks at high field disappeared. The Xns (number average degree of
polymerization) for different lots were calculated using peak area ratios and these
1 were consistent with viscosity results. The highest Xn was 142. But, PPDSA H-
NMR spectra could not be fully understood due to the local organization in solution. However, 13C-NMR spectrum showed that the resulting polymer had
three kinds of carbon which matched well with the chemical structure expected
for PPDSA.
PPDSA has a hexagonally packed nematic liquid crystal polydomain
structure with frozen-in free volume. This was shown using WAXD, 2D X-ray,
dimensional changes and lambda as a function of the relative humidity and
proved. The hexagonal packing structure in the domains was supported by
WAXD results and model study. The inter-chain distance increased with relative
humidity between 15 and 75%RH; the square of the inter-chain distance was a
first order function of lambda and fit well with the hexagonal packing model. The
263 Chapter 3
analysis from the WAXD results, dimensional changes suggested that in addition
to its hexagonal packing, PPDSA had the frozen-in free volume. This was proved
by the determination that absorbed water in humidified samples had higher
density than pure water. There was little change of d-spacing with lambda at low
humidities. About four water molecules could be absorbed with almost no change
in d spacing.
WAXD spectra depended on the test mode. The single reflection of peak
A, ring (corresponding to the inter-chain distance) shown in 2D-Xray images
proved that PPDSA had a polydomain structure and most of chains were oriented
relatively perpendicular to the surface of film.
PPDSA, a highly sulfonated polymer, has a very high IEC (ion exchange capacity). Its theoretical IEC is 8.46 meq/g. The high IEC and the frozen-in free
volume increase the water uptake of polymer; PPDSA lambda between 15 and
75%RH was about 2 higher than Nafion 117 and most aromatic PEMs. Such a high lambda increases proton conductivity. PPDSA had higher proton
conductivity than Nafion 117 between 15 and 75%RH. Based on the activation
energy for proton transport and the conductivities in aqueous phase, PPDSA had
lower intrinsic conductivities than Nafion 117 at lambda <6.5 and similar intrinsic
conductivities at lambda ≥6.5.
264 Chapter 3
PPDSA held absorbed water molecules (λ~9) so tightly that there was no
free water. This was shown using DSC: the absorbed water showed no
endotherm peak in DSC scanning from -50 to 60°C and two endotherm peaks at
110 and 150°C. PPDSA had high thermal stability; the polymer started decomposing at about 304°C at 10°C/min. It was stable for at least one hour at
240°C.
Rigid rod liquid crystalline PPDSA had better conductivity at low
humidities than Nafion 117 due to its high level of sulfonation and the frozen-in
free volume. However, PPDSA is water soluble above 75%RH and has very low
elongation in the dried state. But, PPDSA is a promising candidate since it meets the low and high temperature fuel cell conductivity requirements. It is an attractive membrane material with high potential.
265 Chapter 3
3.2. Future work
The synthesis of water insoluble polymers based on PPDSA should be
the top priority for the next stage of research. Control of polymer solubility can be
achieved by the grafting hydrophobic groups on the homopolymer or co-
polymerization with new non-ionic co-monomers to get random or block
copolymers.
Graybill2 reported that a sulfonic acid was able to react with aromatic
hydrocarbons using polyphosphoric acid. This method was used our lab (Sergio
Granados-Focil)3 to graft aromatic groups on PPSA and water insoluble polymer could be obtained. But, it was difficult to control the reaction conditions because
most hydrocarbons have poor solubility in polyphosphoric acid and the reaction
system is heterogeneous. The degree of grafting depended strongly on mixing
rates.
Copolymerization should give better synthetic control over the reaction to
make water insoluble polymer. Because a co-monomer could be polymerized in
the Ullmann coupling reaction, the ratio of co-monomer and the copolymer type
(block or random) could be controllable. Besides, if the copolymerization is
successful, precipitation of the non-ionic polymer block during coupling might be
prevented, perhaps by changing the reaction solvent.
Sulfone co-monomers are the first choice. Sulfone compounds are known
to be insoluble in water and are thermally stable material. Also, the only chance
to get good copolymers is to keep the co-monomer structure as close as possible
to that of DBBDSA, especially electronically. Recently, 1,4-dibromo-2,5-
266 Chapter 3
dibenzenesulfonyl chloride was synthesized using chlorosulfonic acid
(yield :~82%; m.p.: 218°C). Among several Fridel-Craft reactions that could use
this disulfonyl chloride, reaction conditions using ZnCl2 were established in our
lab (Kun Si) using a modified method from reference 4. Thus, co-monomers with
different aryl groups (R1 in Figure 3.1).could be synthesized using this method.
If the sulfone co-monomer could not be copolymerized with DBBDSA
using Ullmann coupling, Suzuki coupling5,6 to co-monomer could be used to make a comonomer with the same end groups as DBBDSA (Figure 3.2).
We can expect several advantages from copolymers containing aromatic
sulfones. First, water-insoluble polymer can be made. Second, the final
properties (conductivity, solubility, dimensional stability) can be controlled by
adjusting the ratio of sulfone comonomer. Third, if a polymer with a low fraction of sulfone is water-soluble, after casting a film, it can be heated to 200°C to
crosslink the system. Last, such co-monomers can maintain the nano-size
conducting channels in film that can not collapse at low humidity.
The next step after the synthesis of water insoluble polymer would be its
characterization, as was done for PPDSA. After characterization, the production
of an MEA must be optimized. The process will be different from that for the normal PEMs because of the polymer’s poor elongation. Finally, this new polymer will be incorporated into a fuel cell and its performance will be tested.
267 Chapter 3
R1 SO2
Br Br R1:
O2S R1
Figure 3.1. Possible structures of sulfone co-monomers.
R1 R1 R1 SO2 SO2 SO2
Br Br
O2S O2S O2S R1 R1 R1
Figure 3.2. Possible structure of sulfone co-monomer using Suzuki coupling.
268 Chapter 3
3.3. References
1. 2007 Multi-Year Research, Development and Demonstration Plan, U.S.
Department of Energy, 2007.
2. Graybill B.M., Journal of Organic Chemistry, 1967, 32(9), 2931-2933.
3. Segio Granados-focil, Ph. D thesis, Case Western Reserve University, January,
2006.
4. B. P. Bandgar, S. P. Kasture, Synthetic communications, 2001, 31(7), 1065-
1068.
5. N. Miyamura, A. Suzuki, Chem. Rev., 1995, 95, 2457.
6. A. B. Morgan, J. L. Jurs, J. M. Tour, Journal of Applied Polymer Science, 2000,
76, 1257.
269 Bibliography
1. 2007 Multi-Year Research, Development and Demonstration Plan, U.S.
Department of Energy, 2007
2. A. B. Morgan, J. L. Jurs, J. M. Tour, Journal of Applied Polymer Science, 2000,
76, 1257.
3. A. D, child, J. R. Reynolds, Macromolecules, 1994, 27, 1975.
4. A. J. Bard, L. R. Faulkner, Electrochemical methods, Fundamentals and
Applications, 2nd edition, John wiley & sons, INC.2001.
5. A. J. Chalk, A. S. Hay, J. Polm. Sci., A, 1982, 7, 5843.
6. A. Katchalsky, Z. Alexandrowicz, O. Kedem, In Chemical Physics of Ionic
Solutions, Conway, B.E., Barradas, R. G.,Eds., Wiley, New York, 1966.
7. A. Noshay, L. M. Robeson, J. Appl. Polym. Sci., 1976, 20, 1885.
8. Aharoni, S. M., Polymer, 1980, 21 1413.
9. B. C. H. Steele, J. Mater. Sci., 2001, 36, 1053?1068.
10. B. C. Johnson, I. Ylgor, M. Iqbal, J. P. Wrightman, D. R. Lloyd, J. E. McGrath,
J. Polym. Sci., Polym. Chem. Ed., 1984, 22, 72.
11. B. D. Cahan , J. S. Wainright, J. Electrochem. Soc., 1993, 140, 185.
12. B. Lafitte, L. E. Karlsson, P. Jannasch, Macromol. Rapid Commun. 2002, 23,
896-900.
13. B. P. Bandgar, S. P. Kasture, Synthetic communications, 2001, 31(7), 1065-
1068.
14. B. Pivovar, Y. Wang, E. L. Cussler, J. Membr. Sci., 1999, 154, 155-162.
270 15. Baughman, R. H., Bredas, J. L., Chance, R. R., Elsenbaumer, R. L.,
Shacklette, L. W., Chem. Reu., 1982, 82, 209.
16. Berresheim A. J., Muller M., Mullen K., Chem Rev., 1999, 99, 1747-1785.
17. Bishop M. T., Karasz F. E., Russo P. S., Langley K. H., Macromolecules, 1985,
18, 86.
18. Bloom, P., Jones, C.J., Sheares, V.V., Macromolecules, 2005, 38(6), 2159.
19. Bryan S. Pivovar, Polymer, 2006, 47, 4194-4202.
20. C. Bially, D. Williams, F. E. Karasz, W. J. MacKnight, Polymer, 1987, 28, 1009.
21. C. E. Bunker, H. W. Rollins, B. J. Ma, J. Photochem. Photobiol. A, 1999, 126,
71.
22. C. F. Hsing, I. Khoury, M. Bezoari, P. Kovacic, J. Poly. Sci.; Polym. Chem. Ed.,
1985,26, 1628
23. C. Mottet, A. Revillon, P. Le Perchec, M. E. Lauro, A. Guyot, Polym. Bull.,
1982, 8, 511
24. C. Rochas, A. Domard, M. Rinaudo, Polymer, 1979, 20, 76.
25. Carrette L., Friedrich K. A., Stimming U., Fuel Cells, 2001, 1(1), 5-39.
26. Chen, L. M. S. Thesis, Case Western Reserve University, 1999.
27. Cimrova, V., Schmidt, W., Rulkens, R., Schulze, M., Meyer, W., Neher, D.,
Adv. Mater.,1996, 8(7), 585-588.
28. Cornet N., Diat O., Gebel G., Jousse F., Marsacq D., Mercier R., Pineri M., J.
New Mater. Electrochem. Syst., 2000, 3, 33.
29. D. Demus, L. Richter, Textures of Liquid Crystals, 1978, VEB Deutscher
Verlag fur Grundstoffindustrie, Leipzig, German Democratic Republic.
271 30. D. G. Baird, J. Rheology., 1980, 24,465.
31. D. G. Peiffer, R. D. Lundberg, J. of Poly. Sci.: Polym. Chem. Ed., 1984, 22,
1757.
32. D. Rivin, C. E. Kendrick, P. W. Gibson, N. S. Schneider, Polymer, 2001, 42,
623-635.
33. D. S. Kalika, D. W. Giles, M. M. Denn, J. Rheol. 1990, 34, 139- 154.
34. David C. Boris, Ralph H Colby, Macromolecules, 1988, 31, 5746-5755.
35. Devaux J., Delimoy D., Daoust D., Legras R., Mercier J.P., Strazielle C., Neild
E., Polymer, 1985, 26, 1994.
36. Doi, M., Edwards, S. F., The Theory of Polymer Dynamics, Oxford University
Press, New York, 1986.
37. Dunn, S. Peterson, J. A., Micropower, the next Electrical Era, 2000, World
Watch Institute, Washington D.C.
38. E. E. Gilbert, in Sulfonation and Related Reactions (New York: Wiley-
Interscience, 1965 p. 78
39. Eisenberg, A., Hird, B., Moore, R. B., Macromolecules, 1990, 23, 4098.
40. Eisenberg, A., Yeager, H. L., Eds. Perfluorinated Ionomer Membranes; ACS
Symposium Series 180;American Chemical Society: Washington, DC, 1982.
41. F. Oosawa, Polyelectrolytes, Marcel Dekker, New York, 1971.
42. F. X. Quinn, E. Kampff, G. Smyth, V. J. McBrierty, Macromolecules, 1988, 21,
3192-3198.
43. Fang J., Guo X., Harada S., Watari T., Tanaka K., Kita H., Okamoto, K.-I.
Macromolecules, 2002, 35, 9022.
272 44. Fanta, P.E., Chem. Rev., 1964, 64, 613.
45. Fanta, P.E., Synthesis, 1974, 1, 4.
46. Felix E. Goodson, Thomas I. Wallow, and Bruce M. Novak, Macromolecules,
1998, 31, 2047-2056
47. Fernandez Prini, R., Lagos, A. E., J. Polym. Sci.: Part A, 1964, 2, 2917.
48. Forster, M. Schmidt, Adv. Polym. Sci., 1995, 120, 51.
49. G. Gebel, J. Lambard, Macromolecules, 1997, 30, 7914-7920.
50. G. Gebel, Polymer, 2000, 41, 5829.
51. G. Liu, X. Yan, S. Duncan, Macromolecules, 2003, 36, 2049.
52. G. Smyth, F. X. Quinn, V. J. McBrierty, Macromolecules, 1988, 21, 3198-3216.
53. Genies C., Mercier R., Sillion B., Cornet N., Gebel G., Pineri M., Polymer,
2001, 42, 359.
54. Genies C., Mercier R., Sillion B., Petiaud R., Cornet N., Gebel G., Pineri M.,
Polymer, 2001,42, 5097-5101.
55. Gilbert, E. E., Sulfonation and Related Reactions, interscience, New York,
1965, 425-438
56. Giuffre, L., Montoneri, E., Modica, G., Tempesti, E., Hydrogen Energy, 1983,
8, 419.
57. Goodson, F. E., Wallow, T. I., Novak, B.M., Macromolecules, 1998, 31, 2047.
58. Granados-Focil, S., Litt, M. H., Polym. Mat. Sci. and Eng., 2003, 89, 438-439.
59. Granados-Focil, S., Litt, M. H., Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem.,
2004, 49(2).
60. Graybill B.M., Journal of Organic Chemistry, 1967, 32(9), 2931-2933.
273 61. Grot W. G., U.S. Patent, 1984, 4, 433, 082.
62. Grove, W. R., Phil. Mag. Ser. 1839, 3(14), 127-130.
63. Guo X., Fang J., Watari T., Tanaka K., Kita H., Okamoto, K.-I.,
Macromolecules, 2002, 35, 6707
64. H. Eisenberg, G. R. Mohan, J. Phys. Chem., 1959, 63, 671.
65. H. Eisenberg, J. Pouyet, J. Polym. Sci., 1954, 13, 85.
66. H. Kobayashi, H. Tomita, H. Moriyama, A. Kobayashi, T. Watanabe, J. Am.
Cehm. Soc., 1994, 116, 3153
67. H. Vink, Polymer, 1992, 33, 3711.
68. Handbook of Chemistry and Physics, David R. Lide, CRC press, 77th Ed. 6-8
69. Hassan, J., Sevignon, M., Gozzi, C., Schultz, E., Lemaire, M., Chem. Rev.,
2002, 102, 1359-1469
70. Heith B. Oldham, Jan C. Myland, Fundamentals of Electrochemical Science,
Chapter 1. The conduction of electricity, Academic press, INC, 1994.
71. Heitner-Wirguin, C. J. Membr. Sci., 1996, 120, 1-33.
72. Henans, J. Jr., J. Colloid Sci., 1962, 17, 638.
73. Hickner, A.M., Ghassemi H., Kim, Y. S., Einsla B. R., McGrath J. E., Chem.
Rev., 2004, 104, 4587-4612.
74. Hruszka, P., Jurga, J., Brycki, B., Polymer, 1992, 33, 248.
75. J. A. Butler, F. R. Robins, K. V. Shooter, Proc. R. Soc., London 1957, A241,
299.
76. J. A. Elliott, S. Hanna, A. M. S. Elliott, G. E. Cooley, Polymer, 2000, 42, 2251-
2253.
274 77. J. A. Kerres, J. Membr. Sci., 2001, 185, 3.
78. J. Cohen, Z. Priel, Y. Rabin, J. Chem. Phys., 1988, 88, 7111.
79. J. Fisher, A. G. Fredrickson, Mol. Cryst. Liq. Cry.,1969, 8, 267.
80. J. Halim, F. N. Buchi, O. Haas, M. Stamm, G. G. Scherer, Electrochim. Acta.,
1994, 39,1303
81. J. Kim, B. Kim, B. Jung, J. Membr. Sci., 2002, 207, 129-137
82. J. L. Ganter, M. Milas, M. Rinaudo, Polymer, 1992, 33, 113.
83. J. Lee, C. S. Marvel, J. Polym. Sci., Polym. Chem. Ed., 1984, 22, 295.
84. J. Yamanaka, H. Araie, H. Matsuoka, H. Kitano, N. Ise, T. Yamaguchi, S.
Saeki, M. Tsubokawa, Macromolecules, 1991, 24, 6156.
85. J. Yamanaka, H. Matsuoka, H. Kitano, M. Hasegawa, N. Ise J. Am. Chem.
Soc., 1990, 112,587
86. Jin X., Bishop M., Ellis T., Karasz F. B., Polym. J., 1985, 17, 4.
87, Johnson BC., Ylgor I., Iqbal M., Wrightman JP., Lliyd DR., McGrath JE., J.
Polym. Sci. Polym.Chem. Ed., 1984, 22, 72.
88. K. C. Tam, C. J. Tiu, Non-Newtonian Fluid Mech. 1993, 46,275.
89. K. F. Wissbrun, J. Rheology, 1981, 25(6), 619.
90. Kenji Miyatake, Eiichi Shouji, Kimihisa Yamamoto, and Eishun Tsuchida,
Macromolecules,1997, 30, 2941-2946.
91. Kenji Miyatake, Hiroyuki Iyotani, Kimihisa Yamamoto, and Eishun Tsuchida,
Macromolecules, 1996, 29, 6969-6971.
92. Kiss, G., Porter, R., J. Polym. Sci., Polym. Phys. Ed., 1980, 18, 36.
93. Kovacic, P., Jones, M. B., Chem. Rev., 1987, 87, 357.
275 94. Kovacic, P.; Kyriakis, A., J. Am. Chem. Soc. 1963, 85, 454.
95. Kreuer, K. D., Chem. Mater., 1996, 8, 610.
96. Krivoshei, I. V., Skorobogatov, V. M., Polyacetylene and Polyarylenes;
Polymer Monographe; Gordon and Breach Science Publishers: Philadelphia,
PA, 1991; Vol. 10.
97. L. Walker, N. Wagner, J. Rheology, 1994, 38(5), 1525.
98. Lee J, Marvel C. S., J. Polym. Sci. Polym. Chem. Ed., 1984, 22, 295.
99. Litter M., Marvel C.S., J. Polym. Sci. Polym. Chem. Ed., 1985, 23, 2205.
100. M. Cappadonia, J. W. Erning, S. M. Saberi Niaki, and U. Stimming, Solid
State Ionics,1995, 77, 65.
101. M. Dadmun, C. C. Han, Macromolecules, 1994, 27, 7522.
102. M. Elomaa, S. Hietala, M. Paronen, N. Walsby, K. Jokela, R. Serimaa, M.
Torkkeli, T. Lehtinen, G. Sundholm, R. J. Sundholm, Mater. Chem., 2000, 10,
2678-2684.
103. M. G. Oostwal, Ph. D. Thesis, University of Leiden, 1994.
104. M. I. Litter, C. S. Marvel, J. Polym. Sci., Polym. Chem. Ed., 1985, 23, 2205.
105. M. Nagasawa, In Polyelectrolytes, Selegny, E., Ed., Dordrecht, Netherlands,
1974, Vol.
106. M. R. Tant, K. P. Darst, K. D. Lee, C. W. Martin, ACS Sym. Ser. 1989, 395,
370.
107. M. Rikukawa, K. Sanui, Prog. Polym. Sci., 2000, 25, 1463.
108. M. Tricot, B. Maquet, M. Devaleriola, C. Houssier, Polymer, 1984, 25, 1397.
109. Makowsky H, Lundberg R. D., Singahl G. H., US Patent, 1975, US3870841.
276 110. Marvel, C. S., Artzell, G. E., J. Am. Chem. Soc. 1959, 81, 448.
111. Mauritz, K. A., Macromolecules, 1989, 22, 4483.
112. Michael A. Hickner, Cy H. Fujimoto, Chris J. Cornelius, Polymer, 2006, 47,
4238?4244.
113. Montoneri, E., J. Polym. Sci., 1989, A27, 3043.
114. Montoneri, E., Modica, G., Giuffre, L., Peraldo-Bicelli, C., Maffi, S., J. Polym.
Mater. 1987,11, 263.
115. Moore W.R., Prog. Polym. Sci., 1967, 1, 1-457.
116. N. Kasai, M. Kakudo, X-ray Diffraction by Macromolecules, Kodansha and
Springer, 2005.
117. N. M.Belomoina, A. L. Rusanov, Vysokomol. Sodein., Ser. B, 1996, 38, 355.
118. N. Miyake, J. S. Wainright, R. F. Savinell, J. Electrochem. Soc., 2001, 148
(8), A898-A904.
119. N. Miyamura, A. Suzuki, Chem. Rev., 1995, 95, 2457.
120. N. Sarkar, L. D. Kershner, Journal of Applied Polymer Science, 1996, 62,
393.
121. Noren, G. K., Stille, J. K., Mucromol. Reu., 1971, 5, 385.
122. Noshay, A., Robeson, L.M., J. Appl. Polym. Sci., 1976, 20, 1885.
123. O. Savadogo J. New Matter. Electrochem., 1998, 1, 66.
124. Onogi, S. and Asada, T., pp. 127-147 in Rheology, Vol. I, Astarita, G.,
Marmcci, G. and Nicolais, L., Eds., Plenum, New York (1980).
125. P. D. Beattie, F. P. Orfino, V. I. Basura, K. Zychowska, J. F. Ding, C. Chuy, J.
Schmeisser,S. J. Holdcroft, Electroanal. Chem., 2001, 503, 45-56.
277 126. P. J. Flory, Principle of polymer chemistry, Cornell University Press, Ithaca,
N. Y. 1953.
127. P. Koacic, J Oziomeck, J. Org. Chem., 1964, 29, 100.
129. P. Kovacic, A. Kyriakis, J. Am. Chem. Soc., 1963, 85, 454.
130. Paddison, S., Annu. Rev. Mater. Res., 2003, 33, 289
131. Papkov, S. P., Kulichikhim, V. G., Kalmykova, V. D. and Malkin, A. Ya., J.
Polym. Sci., Polym. Phys. Ed., 1974, U, 1753.
132. Papkov, S., V. Kulichikhin, A. Malkin, V. Kalmykova, A. Volokhina, and L.
Gudin, Vysokomolekuljarnye Soedinenija (URSS), 1972, 14-N4, 244.
133. Percec, V., Bae, J.-Y., Zhao, M., Hill, D.H., Macromolecules, 1995, 28, 6726.
134. Percec, V., Okita, S., Weiss, R., Macromolecules, 1992, 25, 1816-1823.
135. Percec, V., Pugh, C., Cramer, E., Weiss, R., Polym. Prepr. (Am.Chem. Soc.,
Div. Polym.Chem.),1991, 32 (l), 329.
136. Perusich, S. A., Avakian, P., Keating, M. Y., Macromolecules, 1993, 26, 4756.
137. R. E. Thomas, A. J. Rosa, The Analysis and design of linear circuits, 2nd
Ed., PrenticeHall,433-459, 1998.
138. R. M. Fuoss, Discuss. Faraday Soc., 1951, 11, 125.
139. R. M. Hodge, T. J. Bastow, G. H. Edward, G. P. Simon, A. J. Hill,
Macromolecules, 1996, 29, 8137-8143.
140. R. Rulkens, G. Wegner, T. Thurn-Albrecht, Langmuir, 1999, 15, 4022-4025
141. R. W. Kopitzke, C. A. Linkous, H. R. Anderson, G. L. Nelson, J. Electrochem.
Soc., 2000, 147,1677-1681.
278 142. Rehan, M., Schluter, A. D., Wegner, G., Feast W. J., Polymer, 1989, 30,
1054.
143. Rikukawa R. M., Sanui K., Prog. Polym. Sci., 2000, 1463-1502.
144. Robeson, L. M., Matzner, M., US Patent, 1983, US4,380,598.
145. Rulkens, R., Schulze, M., Wegner, G., Macromol. Rapid Commun., 1994, 15,
669-676.
146. Rulkens, R., Wegner, G., Enkelmann, V., Schulze, M., Ber. Bunsenges.
Phys. Chem., 1996, 100(6), 707-714.
147. Rune Halseid, Preben J. S. Vie, Reidar Tunold, Journal of The
Electrochemical Society, 2004, 151(3), A381-A388.
148. Rusanov A. L., Likhatchev D. Y., Mullen K., Russian Chemical Reviews,
2002, 71(9), 761-774
149. S. A. Rice, M. Nagasawa, Polyelectrolyte solutions, Academic press, New
York, N. Y. 1969.
150. S. C. Yeo, A. Eisenberg, J. Appl. Polym. Sci., 1977, 21, 875-98.
151. Savadogo O., J. New Mater. Electrochem. Syst., 1998, 1, 47.
152. Schluter A. D., J. Polym. Sci., part A, 2001, 39, 1533-1566.
153. Segio Granados-focil, Ph. D thesis, Case Western Reserve University,
January, 2006.
154. Simha, R., Phys. Chem., 1940, 4425.
155. Sone Y., et. al., J. Electrochem. Soc., 1996, 143(4), 1254.
156. T. A. Zawodzinski Jr, T. E. Springer, F. Uribe, S. Gottesfield, Solid state
ionics, 1993, 60,199-211.
279 157. T. A. Zawodzinski, C. Derouin, S. Radzinski, R. J. Sherman, V. T. Smith, T. E.
Springer, S. Gottesfeld, J. Electrochem. Soc., 1993, 140, 1041.
158. T. A. Zawodzinski, T. E. Springer, J. Davey, R. Jestel, C. Lopez, J. Valerio, S.
Gottesfeld,J. Electrochem. Soc., 1993, 140, 1981-1985.
159. T. D. Gierke, W. Y. Hsu, ACS Symp. Ser., 180, American Chemical Society,
Washington, DC, 1982.
160. T. D. Gierke, W. Y. Hsu, Macromolecules, 1982, 15, 101-105.
161. T. Ogawa, C. S. Marvel, J. Polym. Sci. Polym. Chem. Ed., 1985, 23, 1231.
162. T. Yamamoto, A. Yamamoto, Chem. Lett., 1977,353.
163. T. Yamamoto, B-K. Choi, K. Kubota, Polymer Journal, 2000, 32(7), 619.
164. T. Yamamoto, Y. Hayashi, A. Yamamoto, Bull. Cehm. Soc. Jpn., 1978, 51,
2091.
165. Thomas A. Zawodzinski, John Davey, Judith Valerio, Shimshom Gottesfeld,
Electrochima Acta., 1995, 40, 297-302.
166. Tourillion, Ci., In Handbook of Conducting Polymers; Skotheim, T. A, Ed.;
Dekkcr: New York, 1986.
167. U.S. patent, 4 634 530.
168. Ueda M., Toyota H., Ochi T., Sugiyama J., Yonetake K., Masuko T.,
Teramoto T., J. Polym. Sci.,Polym. Chem. Ed., 1993, 31, 853.
169. Ullmann, F., Bielecki, J., Chem. Ber., 1901, 34, 2174.
170. V. A. Sochilin, A. V. Pdbalk, V. I. Semenov, M. A. Sevast¡¯yanov, I. E.
Kardash, Vysokomol.Sodein., Ser. A, 1993, 35, 1480.
171. V. Tricoli, N. Carretta, M. Bartolozzi, J. Electrochem. Soc., 2000, 147, 1286.
280 172. V. V. Binsu, R. K. Nagarale, Vinod K. Shahi, P. K. Ghosh, Reactive &
Functional Polymers, 2006, 66, 1619-1629.
173. Vanhee, S., Rulkens, R., Lehmann, U., Rosenauer, C., Schulze, M., Kohler,
W.,Wegner, G., Macromolecules, 1996, 29, 5136-5142.
174. Virgil Percec, Jin-Young Bae, Mingyang Zhao, and Dale H. Hill, J. Org.
Chem., 1995, 60,176-185
175. Virgil Percec, Jin-Young Bae, Mingyang Zhao, and Dale H. Hill,
Macromolecules, 1996, 29, 3727-3735
176. Vogel A.I., Vogel’s Textbook of practical chemistry, Longman, London, 1989,
426.
177. W. A. Thaler, J. Polym. Sci., 1982, 20, 875.
178. W. A. Thaler, Macromolecules, 1983, 16, 623.
179. W. B. Johnson and W. Liu, in High Temperature Materials, S. Singhal, Editor,
PV 2002-5, p. 132, The Electrochemical Society Proceedings Series,
Pennington, NJ (2002).
180. Wang F., Hickner M., Ji Q., Harrison W., Mecham J.,Zawodzinski T. A.,
McGrath, J. E., Macromol. Symp., 2001, 175, 387.
181. Wang F., Hickner M., Kim Y. S., Zawodzinski T. A., McGrath J. E., J. Membr.
Sci., 2002,197, 231
182. Watson M. D., Fechtenkotter A., and Mullen K., Chem. Rev., 2001, 101,
1267.
183. Wu, J., Grimsdale, A. C., Mullen K., J. Mater. Chem., 2005, 15, 41?52.
184. X. Jin, M. T. Bishop, T. S. Ellis, F. E. Karasz, Br. Polym. J., 1985, 17, 4.
281 185. X. L. Wei, Y. Z. Wang, S. M. Long, C Bobeczko, A. J. Epstein, J. Am. Chem.
Soc., 1996,118, 2545
186. X. Ren, S. Gottesfeld, J. Electrochem. Soc., 2001, 148, A87-A93.
187. Xinhuai Ye, He Bai, W.S. Winston Ho, Journal of Membrane Science, 2006,
279, 570-577.
188. Y. Rabin, Journal of polymer Science: Part C: Polymer Letters, 1988, 26,
397-399.
189. Y. Sone, P. Ekdunge, D. Simonsson, J. Electrochem. Soc., 1996, 143 (4),
1254?1259.
190. Yamamoto, T., Hayashi, Y., Yamamoto, Y., Bull. Chem. Soc. Jpn., 1978, 51,
2091.
191. Yamamoto, T., Morita, A., Miyazaki, Y., Maruyama, T., Wakayama, H., Zhou,
Z., Nakamura,Y., Kanbara, T., Sasaki, S., Kubota, K., Macromolecules, 1992,
25, 1214.
192. Yea, S. C., Eisenberg, A., J. Appl. Polym. Sci., 1977, 21, 875.
193. Yin Y., Fang J., Cui Y., Tanaka K., Kita H., Okamoto, K.-I., Polymer, 2003, 44,
4509.
194. Yu Seung Kim, Limin Dong, Michael A. Hickner, Thomas E. Glass, Vernon
Webb, James E. McGrath,Macromolecules, 2003, 36(17), 6281.
195. Z. Qi, M. C. Lefebre, P. G. pickup, J. Electrochem., 1998, 459, 9.
196. Z. Qi, P. G. Pickup, Chem. Commun., 1998, 1, 15.
197. Zalbowitz, M., S. Thomas, Fuel Cells: Green Power, 1999, U.S. Department
of Energy.
282 198. Zhang Y., Litt M., Savinell R. F., Wainright J. S., Vendramint J., Polym.
Prepr., 2000,41, 1561
199. Zhang Y., Ph.D. Thesis, Case Western Reserve University, 2001, p151.
200. Zhang, Y., Litt M., Savinell R. F., Wainright J. S., Polym. Prepr., 1999, 40,
480.
201. Zhou W., Watari T., Kita H., Okamoto K.-I., Chem. Lett., 2002, 5, 534.
283