at High Temperatures and Low Humidities
Ching-Nan Chuang,
1,2Liang Chao,
3Ying-Jie Huang,
4Tar-Hwa Hsieh,
1,2Hung-Yi Chuang,
1,2Shu-Chi Lin,
1,5Ko-Shan Ho
11Department of Chemical and Material Engineering, National Kaohsiung University of Applied Sciences,
415 Chien-Kuo Road, Kaohsiung 807, Taiwan
2Institute of Polymer Science and Engineering, National Taiwan University, Section 1, 4 Roosevelt Road,
Taipei, Taiwan
3Center for General Education, Technology and Science Institute of Northern Taiwan, Peito, Taipei 11202, Taiwan 4Institute of Nanotechnology, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, Taiwan
5Group of Bio-Instructor, Madou Junior High School, 36 Nan-Shi District, Madou, Tainan County 721, Taiwan
Received 11 April 2007; accepted 18 September 2007 DOI 10.1002/app.27437
Published online 10 December 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The synthesis of a
p-toluidine/formalde-hyde (PTF) resin was performed, and the effects of the molar ratio of the individual monomers and the polymer-ization conditions on the structure of the PTF resin were studied. Fourier transform infrared and 13C-NMR spectra were used to characterize the PTF. Wide-angle X-ray dif-fraction patterns revealed the crystalline structures of vari-ous PTFs. Polarized optical microscopy revealed that the molar ratio of the monomers had a strong effect on the crystalline morphologies. A longer polymerization time turned out a polymer with a higher intrinsic viscosity and molecular weight, which led to differences in the proton
conductivity. All of the PTFs showed a higher proton con-ductivity than a commercial Nafion membrane at 90–1008C and 0% relative humidity. The proton conductivity of the PTF series could be improved by sulfonation with sulfuric acid and could be maintained after blending with polyur-ethane. Pure methanol could be used as a fuel source because of the insolubility and nonwetting properties of PTF in methanol to increase the output current density for a PTF membrane electrode assembly. Ó 2007 Wiley Periodi-cals, Inc. J Appl Polym Sci 107: 3917–3924, 2008
Key words: membranes; resins; solid state structure
INTRODUCTION
Fuel-cell technology has been considered a promis-ing alternative for supplypromis-ing clean and highly effi-cient power instead of the burning of fossil oil.1–3 There are several kinds of fuel cells available: alka-line fuel cells, phosphoric acid fuel cells, solid oxide fuel cells, molten carbonate fuel cells, proton-exchange membrane fuel cells, and direct methanol fuel cells. Among the these types of fuel cells, pro-ton-exchange membrane fuel cells and direct metha-nol fuel cells are known to perform by the use of proton-exchange membranes (PEMs) as solid-state electrolytes for the conveyance of protons between electrodes. The most commonly used and commer-cially available PEMs are based on a perfluorinated ionomer, such as Nafion from Dupont; a partially
fluorinated polymer, such as polyvinylene fluoride-grafted-sulfonated polystyrene (PVDF-g-SPS);4 a hydrocarbon polymer, such as sulfonated poly(ether ether ketone),5–7 poly(phenylene oxide),8,9 or poly-benzimidazole;10–13 and a block copolymer, such as sulfonated poly(styrene isobutylene styrene).14 Poly-mers based on solid-state electrolytes can be used in lots of high-technical areas and have attracted much attention in past decades. Most studies have focused on the design of novel polymer materials possessing high ionic conductivities and better mechanical properties, durability, and chemical and thermal re-sistance for industrial purposes.15–17 For a regular PEM, proton movement comes from the help of the abundant water molecules absorbed around the PEM, and this means that the membrane should absorb as much water as it can to maintain the high ionic conductivity. However, water evaporates easily because of the heat that evolves from the perform-ance of the fuel cells, which can largely slow down the movement of the protons and the ionic conduc-tivity. In other words, the lifetime of this kind of fuel cell will be shortened unless it can preserve its high humidity during or after performance from
Correspondence to: K.-S. Ho (hks@cc.kuas.edu.tw).
Contract grant sponsor: National Science Council (Taiwan, Republic of China); contract grant numbers: 95-2221-E-151-055 and 95-2622-E-151-025-CC3.
Journal of Applied Polymer Science, Vol. 107, 3917–3924 (2008) V
the aqueous fuel, which needs a good fuel-barrier membrane and a high-water, low-fuel-concentration fuel source, which results in a limited output current or voltage of the fuel cell.
Therefore, we refer to the performance of a dry, solid-state membrane used in lithium batteries. Most of these membranes are made of poly(ethylene oxide) (PEO) derivatives that can convey lithium ions through coordinate bonding under totally dry conditions. The studies on PEO started with the work of Wright and coworkers18,19 in 1975. They found that PEO complexed with alkali metal salts (e.g., KSCN) could exhibit high ionic conductivity even at elevated temperatures. Armand et al.20(a) rec-ognized the potential applications of these polymer electrolytes as separators in solid-state batteries and then found that ions are actually transported within the helical channels of PEO molecules. Hardy and Shriver20(b) reported that ions could move faster in an amorphous polymer electrolyte with a low glass-transition temperature (Tg). A complexation similar
to the coordination bonding between lithium ions and the etheryl groups of PEO can occur between the protons and amino groups of a p-toluidine/form-aldehyde (PTF) resin, whose crystallinity and molec-ular weight (related to Tg) can be altered by changes
in the reaction (polymerization) conditions, such as the reactant ratio and time. In this study, a PEM with good proton conductivity at high temperatures [from room temperature (RT) to 1008C] under low-humidity conditions was prepared.
EXPERIMENTAL Materials and reagents
The p-toluidine crystals were purchased from Acros (Taiwan) without any further purification. The other chemicals were all obtained from Aldrich and used as received, including formaldehyde (37.2 wt % aqueous solution), concentrated sulfuric acid (98%), methanol, and toluene.
Synthesis and sample preparation
Preparation of the PTF resin with different reaction times
Similarly to the preparation of novolac-type phenol formaldehyde through condensation polymeriza-tion,21 the PTF resin was obtained by the polymer-ization of p-toluidine with an equivalent amount of formaldehyde in the presence of acids, as illustrated in Scheme 1. p-Toluidine (21.4 g, 0.2 mol) was added to a 500-mL, four-necked flask fitted with a reflux condenser and a mechanical stirrer. After the reac-tant was heated to 1208C in the reactor, 2 g of
con-centrated sulfuric acid was slowly added, and a 37.2% solution of formaldehyde (12.6 mL, 0.16 mol) was added dropwise within 60 min. The reaction lasted 6 and 24 h, respectively, before the flask was opened and an additional 2 g of concentrated sulfu-ric acid was introduced to absorb (remove) the excess or condensed water. The obtained PTF resin was immersed in toluene and filtered to remove re-sidual p-toluidine. The cake was dried in a vacuum oven at 608C for 24 h. The dried sample was charac-terized with Fourier transform infrared (FTIR) and NMR spectrometry, respectively.
Preparation of the PTF resin with different molar ratios
Formaldehyde (37.2% aqueous solution) and p-tolui-dine (analytical grade) in various molar ratios (0.8/1, 1/1, and 1.2/1) were mixed together in the presence of acids in a 500-mL, four-necked flask fitted with a reflex condenser and a mechanical stirrer. The mix-ture was heated to 1208C and stayed there for 24 h. The mixture was then cooled to RT and put into a toluene pool to remove residual p-toluidine; this was followed by filtration and drying in vacuo at 608C for 24 h. The dried sample was characterized with FTIR and 13C-NMR spectra.
Preparation of polyurethane (PU) and its blend with the PTF resin
PU in an N-methylpyrrolidone (NMP) solution was prepared through the mixing of equal moles of poly-tetramethylene oxide (PTMO) 650 with MDI in NMP. The obtained NMP solution of PU was put into different proportions of PTF and stirred over-night. The blend solution was cast onto a Petri dish before being dried in a vacuum oven at 508C for at least 48 h to obtain a PTF/PU polyblending film.
Measurements FTIR spectrometry
The FTIR spectra of various samples were obtained with a PerkinElmer 69002 FTIR instrument with a re-solution of 4 cm21 and 16 scans in dry KBr. The scan was performed from 4000 to 400 cm21.
13C-NMR spectroscopy
For characterization by 13C-NMR spectra, different PTFs prepared under different polymerization condi-tions were first dissolved in the solvent deuterated dimethyl sulfoxide and subjected to the regular measuring procedures of a Varian Unity Plus NMR spectrometer, which was operated at 400 MHz. The
13C chemical shifts were externally referenced to
tet-ramethylsilane.
Wide-angle X-ray diffraction (WAXD)
A Rigaku D-2000 X-ray generator (Japan) with a Cu Ka target (l 5 1.5406 A˚ ) was used to obtain X-ray diffraction patterns at 28/min. The scattering angle (2y) ranged from 2 to 408.
Polarized optical microscopy (POM)
The samples were first dissolved in methanol and onto a microscope glass plate (7.6 3 2.54 3 0.1 cm) to evaporate methanol and then were dried in a vac-uum oven at 608C for 24 h. An Olympus model BH-2 polarized optical microscope (Japan) was used to take pictures of PTF resins with different reaction ratios at a magnification of 2003.
Intrinsic viscosity
The PTF resin with different polymerization times was dissolved in concentrated sulfuric acid (0.6 g/ dL) at 308C, and the intrinsic viscosity was measured with a Cannon-Fenske viscometer according to eq. (1): ½h ¼1 C ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ðhsp ln hrÞ q (1)
where [Z] is the intrinsic viscosity, Zspis the specific
viscosity, and Zris the relative viscosity.
Electrochemical impedance spectroscopy
The proton conductivity was measured with an elec-trochemical alternating-current impedance spectros-copy technique over the frequency range of 100 Hz to 100 MHz. The resistance of PTF was measured with an Autolab PGSTAT 30 impedance analyzer (The Netherlands). The proton conductivity was determined with eq. (2):
rðS=cmÞ ¼ d
Z0o3A (2) where s is the conductivity, Z0o is the extrapolated real-part of the impedance when the imaginary
im-pedance is zero (the intercept crosses the Z0 axis), A is the cross-sectional area, and d is the thickness of the PTF sample.
RESULTS AND DISCUSSION Characterization
The characteristic absorption peaks of the PTF resin with different molar ratios are shown in Figure 1. Figure 1 reveals the presence of stretching at 3326 cm21, which belongs to the amine group of polymerized p-toluidine of PTF. Also, there are char-acteristic absorption peaks at 1120 cm21 for ether groups and at 601 and 1191 cm21 corresponding to the asymmetric and symmetric stretching of com-plexed sulfonic groups. Except for the ortho–ortho linking (830 cm21), the para-methyl groups of p-tolui-dine can be found at 820 cm21 from the spectral analysis. Only Figure 1(c) demonstrates 3422 cm21 due to the presence of the stretching OH group obtained from excess formaldehyde, which can form the etheryl backbone and hydroxyl chain ends and accounts for the better water solubility.22 These hydroxyl chain ends allow us to perform any modifi-cation of the PTF membrane, such as changes in the methanol-barrier property and the attachment of hydrophobic groups. The assignments of the infrared spectrum are listed in Table I.
The 13C-NMR spectra were used to characterize PTF prepared with different polymerization times and various monomer ratios. Figure 2(a) reveals the resonance peaks of PTF with less and equal molar amounts of formaldehyde. The corresponding car-bons are numbered with the attached chemical struc-ture. The aromatic carbons of PTF can be seen around 110–150 ppm (nos. 1–8). The methylene
car-Figure 1 FTIR spectra of the PTF resin with different molar ratios of formaldehyde to p-toluidine: (a) 1/0.8, (b) 1/1, and (c) 1.2/1.
bons of PTF, which link benzene rings together, are significantly demonstrated around 38–42 ppm (nos. 10 and 11). The para-substituting methyl to the aro-matic ring is approximately located at 20 ppm (no. 9). The spectrum of PTF with excess formaldehyde, which is illustrated in Figure 2(b) (which shows the molar ratio of formaldehyde to p-toluidine), reveals
extra resonance peaks around 50 ppm for hydroxyl and etheryl methylenes and wide-ranging aromatic carbon resonance peaks around 110–170 ppm. The additional resonance peak comes from the etheryl linkages derived from the excess formaldehyde, where hydroxyl chain ends can condense into ether. In other words, they can terminate the PTF mole-cules with methanol groups, and this makes its NMR spectrum more complicated. The presence of several peaks in this region indicates that there are hydrogen-bonded and non-hydrogen-bonded metha-nol chain ends present.
WAXD patterns
If aniline is used for polymerization with formalde-hyde, it will produce a resin without any crystal-linity because of the presence of the ortho–para polymerization, except for the ortho–ortho poly-merization of aniline monomers.22–24 However, crystallinity can be found if aniline is replaced by p-toluidine, for which only ortho–ortho polymerization is possible. The WAXD patterns of PTF resins of dif-ferent monomer ratios are shown in Figure 3. In Fig-ure 3, different molar ratios show different crystal-line pattern peaks. The polymerization with a form-aldehyde/p-toluidine molar ratio of 0.8/1 exhibits a characteristic peak at 2y 5 5.658, which is the PTF resin layer-to-layer distance. When the molar ratio is 1, a new characteristic peak is revealed with 2y 5 4.758. Excess formaldehyde [Fig. 3(c); formaldehyde/ p-toluidine 5 1.2/1] shows an insignificant peak in a low-angle region due to the presence of the softer etheryl groups of the backbone, which make it more difficult to maintain a well-defined crystalline structure. However, the methanol terminal chain ends can create strong hydrogen bonds between the molecules, destroy the original crystalline structure,
TABLE I
Assignments of the Infrared Spectrum of PTF
Frequency (cm21) Assignment 3422 n(OH) 3500–3200 n(NH of or ) 2960–2850 nasand nsy(CH3) 2920 n(CH2) 1600 n(C¼¼C),benzene ring 1600–1575 d(NH of ) 1450 d(CH2) 1370–1250 n(CN) 1120 n(O)
1191 d(O¼¼S¼¼O) of SO3
620 d(S¼¼O) of SO3
830 (o,o0,p)-Substitution
820 p- and (o,p)-substitution
773 o-Substitution
Figure 2 13C-NMR spectra of PTF with different molar ratios of formaldehyde to p-toluidine: (a) 1/0.8 or 1/1 and (b) 1.2/1.
Figure 3 X-ray diffraction patterns of the PTF resin with different molar ratios of formaldehyde to p-toluidine: (a) 1/0.8 (b) 1/1, and (c) 1.2/1.
and form a smaller structure that contributes to the high-angle diffraction peaks for PTF prepared with excess formaldehyde (formaldehyde/p-toluidine 5 1.2/1 mol/mol), and the small-size crystal has a spherulitic structure, which is discussed in the fol-lowing discussion about POM.
POM
Figure 4 presents POM images of PTF resins with different reaction times and monomer ratios. The brown color of the crystalline structure of the pure p-toluidine monomer [Fig. 4(a)] can be clearly identi-fied with POM, and it provides a reference for checking the crystalline structures of PTF prepared under various polymerization conditions. When the monomer ratio for the two monomers is 1, it exhibits a needlelike structure [Fig. 4(b)], which contributes to the X-ray diffraction pattern of Figure 3(a). How-ever, when excess formaldehyde is present during the polymerization, the needlelike structure is destroyed because of the formation of strong hydro-gen bonds between the chain ends, and a spherulite-like Maltese cross structure is displayed [Fig. 4(c) and Scheme 2].
Intrinsic viscosity
Table II presents the intrinsic viscosity, which is related to the molecular weight, of the PTF resin pre-pared with polymerization times of 24 and 6 h. The measurement was carried out with PTF dissolved in concentrated sulfuric acid solutions. It indicates that a longer polymerization time results in a higher intrinsic viscosity and a higher molecular weight.
Proton conductivity
Figure 5 presents typical Cole–Cole plots, and the extrapolated bulk resistance of the PTF prepared under various polymerization conditions is listed in Table II. According to the Cole–Cole plots obtained at 1008C for all samples, PTF polymerized for 6 and 24 h showed extrapolated bulk resistance of 214 and 365 O, respectively, at about 30% humidity. The
pro-Figure 4 POM images of p-toluidine and PTF with differ-ent molar ratios of formaldehyde to p-toluidine (magnifica-tion 5 2003): (a) 0/1, (b) 1/1, and (c) 1.2/1. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
Scheme 2 Possible spherulitic structure nucleated and grown by chain-end hydrogen bonding between diol-like PTF molecules.
ton conductivity of PTF was determined with eq. (2) and turned out to be 1.423 1024and 1.393 1024S/ cm, respectively. This illustrates that the proton con-ductivity of a PTF sample is dependent on the poly-merization time, which can control the molecular weight of the resultant polymers. Obviously, the polymer with the larger molecular weight demon-strates poorer proton conductivity because of its higher Tg, which can deter the movement of the
transporting protons. The proton conductivity of PTF prepared with different monomer ratios (formal-dehyde/p-toluidine) was found to be 1.39 3 1024, 1.47 3 1024, and 1.76 3 1024 S/cm, respectively, at RT, as shown in Table II. This demonstrates that the presence of excess formaldehyde can create more etheryl linkages along the backbones to effectively soften the PTF backbone for higher proton conduc-tivity. However, the proton conductivity in the 1024 S/cm range is not high enough to obtain a fuel cell because even when the thickness is thinned to 25 mm to reduce resistance loss, a conductivity of 1024 S/ cm can be converted into 25 O cm2, and for either hydrogen or methanol single fuel cells, the open circuit voltage (maximum voltage) is just around 1.2 V, and
the maximum current is still lower than 0.05 A/cm2 without consideration of other sources of polar-ization losses. The low proton conductivity may come from two main factors. One is the lack of effec-tive transporting channels for protons due to the dense/crystalline structure of PTF, and the other originates from the low numbers of proton-trans-porting media such as sulfonic groups. To achieve higher numbers of proton-transporting media, more sulfuric acid can be added at the later stage of high-temperature refluxing during polymerization, which can lead to a sulfonation reaction, create sulfonic groups for the benzene rings, and increase the pro-ton conductivity to more than 1 order of 2.11 3 1023 S/cm, as listed in Table II. To create conducting channels for protons, PU has been mixed with PTF. Interestingly, even though PU is not capable of con-ducting any protons, the proton conductivity only slightly decrease and remains almost the same as that of the pure PTF according to Table II when an equal weight percentage of PU or less is present with PTF. It is believed that introducing PU can sep-arate the ordered PTF structure and create effective channels, allowing the protons to go through the PU
TABLE II
Proton Conductivity (s) and Intrinsic Viscosity ([Z]) of PTF and Nafion Measured at RT (258C, 30% RH) and 90–1008C (0% RH) PEM [Z] (dL/g) s 3 103(S/cm) RT 90–1008C Nafion 3 0.0125 PTF (1 : 0.8) for 6 h 0.0471 0.142 0.139 PTF (1 : 0.8) for 24 h 0.0667 0.139 0.135 PTF (1 : 1) for 24 h — 0.147 0.138
PTF with excess CH2O for 24 h — 0.176 0.136
PTF with excess CH2O and sulfuric acid for 24 h — 2.11 3.62
PU prepolymer blended with 70% PTF — 6.5 —
PU prepolymer blended with 60% PTF — 3.0 —
PU prepolymer blended with 50% PTF — 1.54 —
matrix because of the incompatibility between PTF and PU, as shown in Figure 6. Besides, the presence of PU can improve the mechanical and methanol-barrier properties of the final blended membrane and keep the proton conductivity.
For comparison with a commercial PEM, Table II also shows the proton conductivity for Nafion, which decreases rapidly from RT (3 3 1023) to 1008C (1.25 3 1025 S/cm) because of the depletion and evaporation of absorbed water molecules, which are the main conveying media for the protons for a Nafion membrane. However, the proton conductivity of the PTF series shows no significant dependence on the temperature from RT to 1008C, and this allows the adoption of non-water-containing pure methanol as the fuel source to maximize the output power for a PTF-made membrane electrode assem-bly (MEA); moreover, the MEA can be operated under a dry condition that always occurs at a high temperature when most of the water product has been evaporated. Therefore, although the proton conductivity of some PTF series listed in Table II is not as high as that of Nafion at RT, the output power of the resultant MEA could be higher than that of the Nafion MEA because the input methanol concentration of the PTF MEA can be elevated to even 100% because of its insolubility and nonwetting property in methanol, which can significantly increase the output current or voltage, which is much higher than that of a regular Nafion MEA. In other words, without consideration of the polariza-tion losses, the proton conductivity of 1024 S/cm from RT to 1008C is equivalent to 1022 S/cm if pure methanol is used instead of the 1–3% methanol aqueous solution used for a regular Nafion MEA because of its poor methanol-barrier property and strong humidity-dependent proton conductivity, which needs a continuous supply of water from the input aqueous fuel on account of the significant
hu-midity loss at the high-performance temperature (which could reach 858C) of regular fuel cells. Besides, some of the blended films have proton con-ductivity of the order of 1023 S/cm, which would definitely produce a higher current density for a direct methanol fuel cell whether it is operated at high or low humidities and temperatures.
CONCLUSIONS
The synthesis of a PTF resin by condensation poly-merization in the presence of sulfuric acid has been described, and the effects of the molar ratio of the individual monomers (p-toluidine and formalde-hyde) and condensation polymerization conditions on the structure of the PTF resin have been studied.
Spectral studies on characterization by FTIR and
13C-NMR have been performed and prove the
pres-ence of the PTF resin. WAXD and POM have been used to examine the effects of the molar ratio and reaction time on the crystallinity and crystalline structure of PTF. It has also been found that PTF can proceed to crystallization because of the absence of isomers in the ortho–para positional polymerization of p-toluidine instead of aniline monomer, which always gives the aniline formaldehyde resin, which is not crystallizable at all. A PTF resin with a high molecular weight has poorer proton mobility because of its stiffer structure.
Nafion shows lower proton conductivity than all PTF membranes at temperatures higher than 908C, but the PTF series maintains its proton conductivity from RT to at least 1008C with an insignificant de-pendence on humidity; this allows the use of non-water-containing pure methanol as a fuel source. PTF molecules can become softer and proton con-ductivity can be improved by introduction into the backbones with a soft etheryl group from the con-densation of the chain-end methanol groups of PTF prepared with excessive formaldehyde.
The low proton conductivity of 1024 S/cm of the PTF series can be improved by sulfonation with sul-furic acid to 1023S/cm, which can be maintained af-ter blending with PU, which can create open proton conducting channels for pure PTF
Because pure methanol can be used as a fuel source on account of the insolubility and nonwetted property of PTF in it, the output current density is able to be significantly increased if a PTF MEA is assembled.
Future work will cover the copolymerization of PTF diol with PU prepolymer to effectively destroy the ordered structure of PTF for a higher proton con-ductivity and good mechanical and methanol-barrier properties.
Figure 6 Scanning electronic microscopy picture of the cross-sectional area of 50% PTF mixed with the PU pre-polymer.
References
1. Kordesch, K.; Simader, G. Fuel Cells and Their Applications; VCH: Weinheim, 1996.
2. Cleghorn, C.; Ren, X.; Springer, T. E.; Wilson, M. S.; Zawodzin-ski, C.; ZawodzinZawodzin-ski, T. A.; Gottesfeld, S. Int J Hydrogen Energy 1997, 22, 1137.
3. Ren, X.; Zelency, P.; Thomas, S.; Davey, J.; Gottesfeld, S. J Power Sources 2000, 86, 111.
4. Hietala, S.; Maunu, S. L.; Sundholm, F. J Polym Sci 1999, 38, 1741. 5. Alberti, G.; Casciola, M.; Massinelli, L.; Bauer, B. J Membr Sci
2001, 185, 73.
6. Zaidi, S. M.; Mikhailenko, S. D.; Robertson, G. P.; Guiver, M. D.; Kaliaguine, S. J Membr Sci 2000, 173, 17.
7. Mikhailenko, S. D.; Zaidi, S. M.; Kaliaguine, S. Catal Today 2001, 67, 225.
8. Smitha, B.; Sridhar, S.; Khan, A. A. J Membr Sci 2003, 225, 63. 9. Mohr, R.; Kudela, V.; Schauer, J.; Richau, K. Desalination 2002,
147, 191.
10. Staiti, P. Mater Lett 2001, 47, 241.
11. Jones, D. J.; Roziere, J. J Membr Sci 2001, 85, 41.
12. Weng, D.; Wainright, J. S.; Landau, U.; Savinell, R. F. J Electro-chem Soc 1996, 143, 1260.
13. Li, Q.; He, R.; Jensen, J. O.; Bjerrum, N. J. Fuel Cells 2004, 4, 147.
14. Elabd, Y. A.; Napadensky, E.; Sloan, J. M.; Crawford, D. M.; Walker, C. W. J Membr Sci 2003, 217, 227.
15. Murata, K.; Izuchi, S.; Yoshihisa, Y. Electrochim Acta 2000, 45, 1501.
16. Wright, P. V. Electrochim Acta 1998, 43, 1137.
17. Wen, T. C.; Wang, Y. J.; Cheng, T. T.; Yang, C. H. Polymer 1999, 40, 3979.
18. Fenton, D. E.; Parke, J. M.; Wright, P. V. Polymer 1973, 14, 589. 19. Wright, P. V. J Br Polym 1975, 7, 319.
20. (a) Armand, M. B.; Chabagno, J. M.; Duclot, M. J. In Fast Ion Transport in Solids; Vashista, P.; Mundy, J. N.; Shenoy, G. K., Eds.; Elsevier: New York, 1979; p 131; (b) Hardy, L. C.; Shriver, D. F. J Am Chem Soc 1985, 107, 3823.
21. Moore, J. A. Macromolecular Syntheses; Wiley: Chichester, England, 2000; p 215.
22. Ho, K. S.; Hsieh, T. H.; Kuo, C. W.; Lee, S. W.; Lin, J. J.; Huang, Y. J. J Polym Sci Part A: Polym Chem 2005, 43, 3116. 23. Ho, K. S.; Hsieh, T. H.; Kuo, C. W.; Lee, S. W.; Huang, Y. J.;
Chuang, C. N. J Appl Polym Sci 2007, 103, 2120.
24. Huang, Y. J.; Hsieh, T. H.; Wang, Y. Z.; Chuang, C. N.; Chen, Y. P.; Huang, P. T.; Ho, K. S. J Appl Polym Sci 2007, 104, 773.
