Polymer electrolyte membranes based on polystyrenes with phosphonic acid via long alkyl side chains

Article abstract of DOI:10.1002/pola.26246

A series of polystyrenes with phosphonic acid (5) via long alkyl side chains (4, 6, and 8 methylene units) were prepared by the radical polymerization of the corresponding diethyl ω-(4-vinylphenoxy)alkylphosphonates, followed by the hydrolysis with trimethylsilyl bromide. The resulting phosphonated polystyrene membranes had a high oxidative stability against Fenton's reagent at room temperature. The membranes prepared from 5 exhibited a very low water uptake, similar to that of Nafion 117 over the wide range of 30 to 80% relative humidity (RH). The proton conductivities of these membranes are lower than that of Nafion 117 in the range of 30 to 90% RH, but comparable or higher than those of the reported phosphonated polymers with higher IEC values, such as the phosphonated poly(N-phenylacrylamide) (PDPAA, IEC: 6.72 mequiv/g) and fluorinated polymers with pendant phosphonic acids (M47, IEC: 8.5 mequiv/g), at low RH conditions despite the much lower IEC values (3.0-3.8 mequiv/g) of these membranes. These results suggest that the flexible pendant side chains of 5 would contribute to the formation of hydrogen-bonding networks by considering the very low water uptake of these polymers.

Full text of DOI:10.1002/pola.26246

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Polymer Electrolyte Membranes Based on Polystyrenes with Phosphonic
Acid via Long Alkyl Side Chains
Youko Tamura,1 Li Sheng,1 Satoshi Nakazawa,2 Tomoya Higashihara,1 Mitsuru Ueda1
1Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology,
Meguro-Ku, Tokyo 152-8552, Japan
2Higashifuji Technical Center, Toyota Motor Corporation, Susono, Shizuoka 410-1193, Japan
Correspondence to: M. Ueda (E-mail: ueda.m.ad@m.titech.ac.jp)
Received 7 May 2012; accepted 26 June 2012; published online
DOI: 10.1002/pola.26246
ABSTRACT: A series of polystyrenes with phosphonic acid (5)
via long alkyl side chains (4, 6, and 8 methylene units) were
prepared by the radical polymerization of the corresponding
diethyl x-(4-vinylphenoxy)alkylphosphonates, followed by the
hydrolysis with trimethylsilyl bromide. The resulting phospho-
nated polystyrene membranes had a high oxidative stability
against Fenton’s reagent at room temperature. The membranes
prepared from 5 exhibited a very low water uptake, similar to
that of Nafion 117 over the wide range of 30 to 80% relative
humidity (RH). The proton conductivities of these membranes
are lower than that of Nafion 117 in the range of 30 to 90% RH,
but comparable or higher than those of the reported phospho-
nated polymers with higher IEC values, such as the phospho-
nated poly(N-phenylacrylamide) (PDPAA, IEC: 6.72 mequiv/g)
and fluorinated polymers with pendant phosphonic acids (M47,
IEC: 8.5 mequiv/g), at low RH conditions despite the much
lower IEC values (3.0–3.8 mequiv/g) of these membranes.
These results suggest that the flexible pendant side chains of 5
would contribute to the formation of hydrogen-bonding
networks by considering the very low water uptake of these
C
V
polymers.
2012 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 000: 000–000, 2012
KEYWORDS: alkylphosphonic acids; polyelectrolytes; polymer
electrolyte fuel cells; polystyrene; proton conductivity; radical
polymerization
INTRODUCTION Proton exchange membrane fuel cells
(PEMFCs) are considered one of the most promising power
sources for both mobile and stationary applications due to
their high energy conversion efficiency and low pollution.1–3
The proton exchange membrane (PEM) is an indispensable
component of a PEMFC to transport protons from the anode
to the cathode. Currently, the state-of-the-art PEMs are sulfo-
such as sulfonated poly(ether ether ketone)s (SPEEKs),4,5
sulfonated polyphenylenes,6,7 sulfonated polysulfones
(SPSFs),8,9 sulfonated polyimides (SPIs),10,11 and some of
them show comparable or even higher proton conductivities
than that of Nafion in water or under a high relative humid-
ity (RH). However, the proton conductivity significantly
decreased when the RH decreased, which is too low to meet
the requirement for practical use. On the other hand, the
PEMFC operation under reduced RH can increase the effi-
ciency of the system and provide a greatly simplified water
management. The development of PEMs having a high pro-
ton conductivity at a low RH is currently in strong demand
for PEMFCs.
R
R
nated perfluoropolymers (e.g., NafionV and FlemionV), which
show high proton conductivities over a wide range of rela-
tive humidities at a moderate operating temperature due to
the well-defined phase separation between the hydrophobic
main chains and the hydrophilic side chains. However, sev-
eral drawbacks, such as high cost, high methanol permeabil-
ity, and limited operating temperature (<80 ꢀC), restrict
their practical use in PEMFCs. Thus, the development of
well-balanced alternatives, which should meet several
requirements (e.g., high proton conductivity, high membrane
stability, low fabrication cost, etc.), is essential today for the
realization of PEMFCs. In recent years, the hydrocarbon-
based polymer exchange membranes have been proposed for
PEMs due to their flexibility in molecular design, synthesis,
and cost-performance. Many efforts have been made to study
a wide variety of sulfonated hydrocarbon-based polymers,
Studies of the phosphonated polymers have drawn consider-
able attention in past years. The phosphonated polymers are
recognized as one of the materials for PEMs which may
potentially show some crucial advantages over the sulfo-
nated membranes.12–19 They have the specific ability of facil-
itating proton conductivity at a low RH or even under anhy-
drous conditions20–23 because the phosphonic acid provides
amphoteric properties, and shows a high degree of self-
dissociation and hydrogen bonding, and the proton is
C
V
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transported through structure diffusion (the Grotthus mecha-
nism24,25). Recently, Yoon et al. reported that the poly(vinyl-
benzyloxy-alkylphosphonic acid)s showed the proton con-
ductivity of 3 ꢁ 10ꢂ4 S/cm at 140 ꢀC under nominally
anhydrous conditions. They concluded that the high proton
conductivity may be attributed to the highly aggregated pro-
ton conducting acid channels with mobile hydrogen bond
networks. The flexible side chains may have a positive effect
on the aggregation of the protogenic group and hydrogen
bond formation.26 Quite recently, we reported that PEMs
based on polyacrylates with phosphonic acid via long alkyl
side chains exhibited a very high proton conductivity compa-
rable to that of the Nafion 117 membrane in the range of 30
to 80% RH at 80 ꢀC, regardless of the significant low water
uptake behavior.27 However, an ester linkage may have a
problem regarding hydrolytic stability, thus other robust
linkages are preferable. To remedy this issue, alkylphos-
phonic acids were introduced to the polystyrenes through
the more stable ether linkages than the ester ones.
7.29–7.39 (2H, d), 6.79–6.90 (2H, d), 6.60–6.72 (1H, q),
5.55–5.66 (1H, d), 5.07–5.16 (1H, d), 4.01–4.17 (4H, m),
3.91–4.00 (2H, t), 1.40–1.85 (8H, m), 1.28–1.38 (6H, t).
3c: FT-IR (Si, cmꢂ1): 2927, 1608, 1511, 1469, 1388, 1254,
1176, 1041, 957. 1H NMR (CDCl3, d, ppm): 7.28–7.38 (2H,
d), 6.79–6.88 (2H, d), 6.58–6.72 (1H, q), 5.54–5.65 (1H, d),
5.06–5.15 (1H, d), 4.00–4.17 (4H, m), 3.89–3.99 (2H, t),
1.25–1.83 (20H, m).
Synthesis of Poly(diethyl 4-(4-
vinylphenoxy)alkylphosphonate) (4)
A typical procedure for the synthesis of poly(diethyl 4-(4-
vinylphenoxy)butylphosphonate) (4a) is as follows.
The monomer 3a (1.00 g, 3.22 mmol), 2,2’-azobis(isobutyro-
nitrile) (AIBN) (0.0211 g, 0.129 mmol), and dry toluene
(1.30 mL) were placed in a sealed glass tube after standard
freeze-evacuate-thaw procedures. The mixture was stirred at
60 ꢀC for 24 h. After cooling to room temperature, the mix-
ture was diluted with toluene and then poured in hexane.
ꢀ
In this study, we synthesized a series of polystyrenes with
phosphonic acids via 4, 6, and 8 methylene spacers. More-
over, the relationship between the length of the alkyl chain
and the properties of the membranes, such as water uptake,
proton conductivity, and morphology, was also investigated.
The precipitate was collected and dried in vacuo at 40 C for
10 h to give 4a (0.88 g, 88 %).
Synthesis of Poly(4-(4-vinylphenoxy)
alkylphosphonic acid) (5)
A typical procedure for the synthesis of poly(4-(4-vinylphe-
noxy)butylphosphonic acid) (5a) is as follows.
EXPERIMENTAL
Materials
To the chloroform solution (8.3 mL) of 4a (0.52 g, 1.66
Diethyl 4-bromobutylphosphonate (2a), diethyl 6-bromohex-
ylphosphonate (2b), and diethyl 8-bromooctylphosphonate
(2c) were prepared according to a previous report.28 Other
solvents and reagents were used as received.
mmol) was added dropwise trimethylsilyl bromide (1.2ꢀ9 mL,
ꢀ
9.95 mmol) at 0 C, and the mixture was stirred at 40 C for
24 h. After evaporation, the residue was dissolved in metha-
nol and the solution was stirred at room temperature for 6
h. Then methanol was evaporated and the obtained product
was then dried in vacuo at 40 ꢀC for 24 h to produce 5a
(0.42 g, 98 %). FT-IR (Si, cmꢂ1): 2946, 2333, 1612, 1508,
1238, 1106, 933, 825. 1H NMR (MeOD, d, ppm): 4.36 (2H,
broad), 2.54–1.50 (5H, m).
Synthesis of Diethyl 4-(4-Vinylphenoxy)
alkylphosphonate (3)
The monomer, diethyl 4-(4-vinylphenoxy)butylphosphonate
(3a), was prepared as follows: 2a (1.53 g, 5.1 mmol), 4-
vinylphenol (1) (0.674 g, 5.6 mmol), potassium carbonate
(1.41 g, 10.2 mmol), and 18-crown-6 (0.03 g, 0.11 mmol)
were dispersed in dry acetone (10.0 mL). Then the mixture
was refluxed for 17 h. After evaporation of the solvent, dis-
tilled water was added. The aqueous layer was extracted
two times with toluene. The organic layer was washed with
water, dried over anhydrous magnesium sulfate, and filtered.
The organic layer was concentrated under reduced pressure.
The product was isolated by column chromatography on
silica gel (ethyl acetate/n-hexane ¼ 2/1 v/v). Yield: 1.34 g
(77%). FT-IR (Si, cmꢂ1): 2981, 1735, 1608, 1511, 1473,
1392, 1245, 1176, 1025, 960. 1H NMR (CDCl3, d, ppm):
7.30–7.38 (2H, d), 6.80–6.87 (2H, d), 6.60–6.72 (1H, q),
5.55–5.65 (1H, d), 5.08–5.16 (1H, d), 4.02–4.17 (4H, m),
3.93–4.01 (2H, t), 1.71–1.95 (6H, m), 1.27–1.39 (6H, t).
Membrane Preparation and Ion Exchange Capacity (IEC)
Methanol solutions of 5 were cast onto flat Teflon sheets.
ꢀ
Drying the solution at 80 C for 12 h gave transparent mem-
branes. IEC values of 5 were determined by titration with
0.02 M NaOH (aq).
Measurements
1H (300 MHz) and 13C (75 MHz) spectra were recorded on
a Bruker DPX300S spectrometer using CDCl3 or MeOD-d4
as solvent and tetramethylsilane as reference. Fourier
transform-infrared (FT-IR) spectra were obtained with a
Horiba FT-120 Fourier transform spectrophotometer. Num-
ber- and weight-average molecular weights (Mn and Mw)
were measured by gel permeation chromatography (GPC) on
a Hitachi LC-7000 system equipped with polystyrene gel col-
umns (TSKgel GMHHR-M) eluted with N, N-dimethylforma-
mide (DMF) containing 0.01 M LiBr at a flow rate of
1.0 mL/min calibrated by standard polystyrene samples.
Thermogravimetric analysis (TGA) was measured in N2 on a
Seiko EXSTAR 6000 TG/DTA 6300 thermal analyzer at a
Other monomers, diethyl 6-(4-vinylphenoxy)hexylphospho-
nate (3b) and diethyl 8-(4-vinylphenoxy)octylphosphonate
(3c) were obtained by the similar procedure described
above.
3b: FT-IR (Si, cmꢂ1):2947, 1735, 1608, 1508, 1473, 1389,
1238, 1107, 933, 825, 771, 725. 1H NMR (CDCl3, d, ppm):
ꢀ
heating rate of 10 C/min.
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Oxidative Stability
The radical oxidative stability of the membranes was tested
by immersing the films into Fenton’s reagent (3% H2O2
aqueous solution containing 20 ppm FeSO4) at room temper-
ature. The remaining weight after they were soaked in Fen-
ton’s reagent at room temperature for 24 h was used to eval-
uate the oxidative stability of the membranes.
RESULTS AND DISCUSSION
SCHEME 1 Synthesis of phosphonated monomers 3a, 3b and 3c.
Synthesis of Monomer
As shown in Scheme 1, three phosphonated monomers, 3a,
3c, and 3c, were easily prepared by the solid state phase
transfer catalyst reaction of 1 and corresponding diethyl x-
bromoalkylphosphonates (2). Their chemical structures were
characterized by FT-IR, 1H NMR, and 13C NMR spectroscop-
ies. Figure 1 shows the 1H NMR spectra of the three mono-
mers 3, and the typical 13C NMR spectrum of 3a is shown in
Figure 2. All peaks could be assigned to the expected struc-
ture of the monomers 3, indicating that the monomers were
successfully synthesized.
Water Uptake
The humidity dependence of water uptake was measured by
placing the membrane in a thermo-controlled humid cham-
ber for 4 h. Then the membrane was taken out, and quickly
weighed on a microbalance. Water uptake (WU) was calcu-
lated according to the following equation:
WU ¼ ðWs ꢂ WdÞ=Wd ꢁ 100 wt %
where, Ws and Wd are the weights of wet and dried mem-
branes, respectively.
Synthesis of Polymers
The polymer synthesis was done by the radical polymeriza-
tion of the monomers 3 using AIBN as the initiator in DMF,
followed by hydrolysis of the phosphonic acid ester using
trimethysilyl bromide (Scheme 2). The polymerization
smoothly proceeded and produced the corresponding poly
(diethyl x-(4-vinylphenoxy)alkylphosphonate)s (4). The
number-average and weight-average molecular weights of 4
estimated by GPC based on the polystyrene standards are
listed in Table 1. The results indicated that high molecular
weight polymers were successfully prepared.
Proton Conductivity
Proton conductivity in plane direction of the membrane was
determined using an electrochemical impedance spectros-
copy technique over the frequency from 5 Hz to 1 MHz
(Hioki 3532-80). A two-point probe conductivity cell with
two platinum plate electrodes was fabricated. The cell was
placed in a thermo-controlled humid chamber at 80 ꢀC for
4 h before the measurement. Proton conductivity (r) was
calculated from the following equation:
The chemical structures of 4 were confirmed by FT-IR and
1H NMR spectroscopies. In the FT-IR spectra of 4a and 5a
(Fig. 3), the disappearance of the absorption at around 1608
cmꢂ1 due to the vinyl group was confirmed and the charac-
teristic absorption at 1033 cmꢂ1 corresponding to the phos-
phonic ester disappeared after the hydrolysis, while the
r ¼ d=ðtswsRÞ
where d is the distance between the two electrodes, ts and
ws are the thickness and width of the membrane, and R is
the resistance measured.
FIGURE 1 1H NMR spectra of phosphonated monomers: (A) 3a, (B) 3b, and (C) 3c.
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SCHEME 2 Synthesis of 5a, 5b, and 5c.
sentatively measured by thermomechanical analysis (TMA)
at room temperature showing a tensile strength of 10 MPa
at break, which is inferior to that of Nafion 117 (ꢃ30 MPa).
Ion Exchange Capacity (IEC) and Oxidative Stability
The IEC was measured based on the acid–base titration
method. The samples were immersed in a saturated NaCl so-
lution at room temperature for 2 days. The samples were
then removed and the NaCl solution was titrated with 0.02
M NaOH until pH ¼ 7. The IEC values (Table 1) decreased in
the order 5a, 5b, and 5c because of the different lengths of
the side chain.
FIGURE 2 13C NMR spectrum of 3a.
absorption at 2333 cmꢂ1 corresponding to the phosphonic
1
acids appeared. The H NMR spectra of 4a and 5a are shown
in Figure 4. The signals of the characteristic vinyl group (d
¼ 5.07–6.72) disappeared in the 1H-NMR spectrum of 4a,
and the signals corresponding to the phosphonic ethyl ester
group are observed at 4.17 to 4.02 ppm and 1.39 to 1.27
ppm. This result suggests that the vinyl polymerization com-
pletely proceeded without the decomposition of the mono-
mer. On the other hand, the signals of the phosphonic ethyl
ester at 4.17 to 4.02 ppm and 1.39 to 1.27 ppm completely
disappeared in the 1H-NMR spectrum of 5a after the hydro-
lysis, which also supports the fact that the hydrolysis pro-
ceeded without any decomposition or side reaction to give
the desired 5a.
The radical oxidative stability of the polymers 5 was eval-
uated by the remaining weight after they were soaked in
ꢀ
Fenton’s reagent at room temperature for 24 h and at 80 C
for 1 h, the results of which are listed in Table 1. All the
membranes showed more than 94% remaining weights after
Fenton’s test. Generally, polystyrene is not considered an
ideal material for sulfonated PEM application due to its poor
oxidative stability of the benzylic C-H bond,29 and many
common sulfonated hydrocarbon polymers have been
reported to completely dissolve in Fenton’s reagent within a
few hours.30,31 The results of Fenton’s test in this study indi-
cated that the polymers 5 have an excellent radical oxidative
stability, probably due to their quite low water uptake as
will be discussed later.
Thermal Properties
The thermal stability of the polymers 5 was investigated by
TGA under nitrogen. Before the measurement, the samples
were preheated at 100 ꢀC for 30 min to remove the
absorbed moisture. As shown in Figure 5, all the samples
displayed a two-stage weight loss behavior. The first stage
weight loss in the approximate range of 200 to 310 ꢀC is
ascribed to the elimination of the phosphonic acid groups.
The second stage weight loss starting from ꢃ400 ꢀC is
attributed to the decomposition of the polymer main chain.
The TGA results indicated that the polymers 5 have a suffi-
cient thermal stability which well meets the requirement for
use in fuel cells. The mechanical property of 5b was repre-
Water Uptake and Hydration Number
The water uptake properties of the PEMs have a strong
influence on their proton conductivity because water mole-
cules play an important role as a proton transportation
carrier in membranes. The water uptakes of 5a, 5b, and 5c
TABLE 1 Ion Exchange Capacities (IECs), Molecular Weight (Mn and Mw) and Remaining Weight (RW) after Fenton’s Test of
Membranes Prepared from Polymers 5
IEC (mequiv gꢂ1
)
Measureda
Mn (KDa)
Mw (KDa)
Mw/Mn
RWc (%)
RWd (%)
b
b
b
Polymer
Theoretical
5a
5b
5c
a
3.90
3.52
3.20
3.80
3.42
3.00
50.2
45.8
48.0
90.0
86.8
79.0
1.79
1.90
1.65
98.3
98.5
98.4
94.2
95.5
96.3
d
By titration method.
Measured by soaking the membranes in Fenton’s reagent (3% H2O2
containing 2 ppm FeSO4) at 80ꢀC for 1 h.
b
c
Determined by GPC based on polystyrene standards.
Measured by soaking the membranes in Fenton’s reagent (3% H2O2
containing 20 ppm FeSO4) at room temperature for 24 h.
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FIGURE 3 FTIR spectra of 4a and 5a.
are shown in Figure 6(A), and compared with that of Nafion
117. All the membranes show lower water uptakes (<25%)
even at 90% RH, leading to an excellent dimensional stabil-
ity. The water uptake of 5c is comparable to that of Nafion
117 even at 90% RH. Moreover, their water uptakes are sim-
ilar to that of Nafion 117 in the wide range of 30 to 80%
RH. In our previous report,32 the phosphonated poly(N-phe-
nylacrylamide) (PDPAA) showed a higher water uptake
which was almost two times higher than that of 5 over the
entire range of RH due to the higher IEC value (IEC ¼ 6.72
mequiv/g).
FIGURE 5 TGA curves of 5a, 5b, and 5c.
It can be seen that for all the membranes, the proton con-
ductivity increases with the increasing RH. The 5a mem-
brane with four methylene spacer units showed a slightly
higher proton conductivity due to its higher IEC value than
the other two membranes. The proton conductivity of 5 is
lower than that of Nafion 117, which is likely related to the
differences in the acidity between the sulfonic acid and phos-
phonic acid. For comparison, the proton conductivities of the
reported phosphonated polymers with much higher IEC val-
ues, such as PDPAA32 (IEC: 6.72 mequiv/g) and M4733 (IEC:
8.4 mequiv/g), and their chemical structures are shown in
Figure 7. It should be noted that the proton conductivities of
5 are higher than that of M47 in the range of 50% to 95%
RH, even though they do not possess fluorinated segments
as does M47 and have twice the lower IEC value. The high
proton conductivity of 5 may be attributed to a higher
degree of hydrophilic/hydrophobic microphase separation
assisted by long spacer units. On the other hand, although
PDPAA showed a high proton conductivity at 70 to 95% RH
comparable to Nafion 117, its conductivity sharply dropped
when the RH decreased regardless of the high IEC values
(6.72 mequiv/g), which means the water-assisted mechanism
works in proton conduction. However, the membranes 5
Furthermore, the k value, which is the number of water mol-
ecules per an acid group, is plotted versus RH [Fig. 6(B)].
The k values decrease with the increasing alkyl chain length
of alkylphosphonic acids. Compared with the Nafion 117
membrane, significantly lower k values are observed in all
the membranes over the entire range of RH, probably due to
the low acidity of phosphonic acid. The high degree of
hydrogen bonding and the lower acidity of the phosphonic
acid groups can limit the water uptake behavior of the mem-
brane in the phosphonate polymers.
Proton Conductivity
Proton conductivity is the primary property that determines
the PEM performance. The proton conductivity of three
membranes with different side chain lengths was measured
at 80 ꢀC under different RH (30–90%) conditions and the
results are shown in Figure 7.
FIGURE 4 1H NMR spectra of the 4a and 5a.
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FIGURE 8 Morphology of 5b studied by AFM.
tivity probably due to lower IECs (3.0–3.80) than that (4.1–
5.44) of PEMs based on polyacrylates.27 Thus, the mem-
branes 5 with the IEC values over 4.5 would be prepared to
form well-connected hydrogen-bonding networks.
Atomic Force Microscopic Observation
FIGURE 6 (A) Water uptake and (B) hydration numbers (k) of
three membranes 5 under different RH.
The tapping mode phase image of the surface of the 5b
membrane was recorded as a representative example under
ambient conditions on the 500 ꢁ 500 nm scale to investigate
its morphological characteristics. The dark and bright
regions were assigned to the flexible structure corresponding
to the hydrophilic domains with phosphonic acid groups
containing water and the rigid structure corresponding
to the hydrophobic domains, respectively. As can be seen in
Figure 8, continuous hydrophilic domains are clearly
observed, implying that the long and flexible hydrophilic side
chains induce the aggregation to form wide hydrophilic
channels.
exhibit much higher proton conductivity than that of PDPAA
at 30% RH, although they exhibit a lower proton conductiv-
ity than that of Nafion 117. These results suggest that the
flexible pendant side chains of 5 would contribute to the for-
mation of hydrogen-bonding networks by taking into consid-
eration of a very low water uptake of these polymers. On the
other hand, compared with the proton conductivity of PEMs
based on polyacrylates with phosphonic acid via long alkyl
side chains, the membranes 5 showed lower proton conduc-
CONCLUSIONS
Polystyrenes with phosphonic acid via long alkyl side chains
were developed for PEMs. These membranes showed a very
low water uptake, similar to that of Nafion 117 in the wide
range of 30 to 80% RH, and high oxidative stability against
Fenton’s reagent. Moreover, the proton conductivities of
these membranes were comparable or higher than those of
the reported phosphonated polymers, PPPA and M47,
under low RH conditions even though the IEC values of
these membranes are much lower than those of PPPA and
M47. These results proved that the flexible pendant side
chains induced the self-assembly of the hydrophilic domains
(i.e., the formation of hydrogen-bonding networks) to achieve
good proton transportation derived from the hydrogen-bond-
ing networks.
FIGURE 7 Proton conductivity as a function of relative humid-
ity of membranes 5, Nafion 117, PDPAA32 at 80ꢀC and M4733 at
90ꢀC.
ACKNOWLEDGMENTS
This work is supported by Toyota Motor Corporation.
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19 Itoh, T.; Hirai, K.; Tamura, M.; Uno, T.; Kubo, M.; Aihara, Y.
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