Highly phosphonated poly(N-phenylacrylamide) for proton exchange membranes

Article abstract of DOI:10.1002/pola.24422

A novel highly phosphonated poly(N-phenylacrylamide) (PDPAA) with an ion-exchange capacity (IEC) of 6.72 mequiv/g was synthesized by the radical polymerization of N-[2,4-bis(diethoxyphosphinoyl)phenyl]acrylamide (DEPAA), followed by the hydrolysis with tr

Full text of DOI:10.1002/pola.24422

Highly Phosphonated Poly(N-phenylacrylamide) for
Proton Exchange Membranes
NAMIKO FUKUZAKI,1 KAZUHIRO NAKABAYASHI,1 SATOSHI NAKAZAWA,2 SHIGEAKI MURATA,2
TOMOYA HIGASHIHARA,1 MITSURU UEDA1
1Department of Organic and Polymeric Materials, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
2Higashifuji Technical Center, Toyota Motor Corporation, 1200 Mishuku, Susono, Shizuoka 410-1193, Japan
Received 21 July 2010; accepted 26 September 2010
DOI: 10.1002/pola.24422
Published online 11 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A novel highly phosphonated poly(N-phenylacryla-
mide) (PDPAA) with an ion-exchange capacity (IEC) of 6.72
mequiv/g was synthesized by the radical polymerization of N-
[2,4-bis(diethoxyphosphinoyl)phenyl]acrylamide (DEPAA), fol-
lowed by the hydrolysis with trimethylsilyl bromide. Then, the
crosslinked PDPAA membrane was successfully prepared by
the electrophilic substitution reaction between the aromatic
rings of PDPAA and the carbocation formed from hexamethox-
ymethylmelamine (CYMEL) as a crosslinker in the presence of
methanesulfonic acid. The crosslinked PDPAA membrane had
high oxidative stability against Fenton’s reagent at room tem-
perature. The proton conductivity of the crosslinked PDPAA
membrane was 8.8 ꢀ 10ꢁ2 S/cm at 95% relative humidity (RH)
and 80 ꢂC, which was comparable to Nafion 112. Under low
RH, the crosslinked PDPAA membrane showed the proton con-
ductivity of 1.9 ꢀ 10ꢁ3 and 4.7 ꢀ 10ꢁ5 S/cm at 50 and 30% RH,
respectively. The proton conductivity of the crosslinked PDPAA
membrane lied in the highest class among the reported phos-
phonated polymers, and, consequently, the very high local
concentration of the acids of PDPAA (IEC ¼ 6.72 mequiv/g)
achieved high and effective proton conduction under high RH.
C
V
2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
49: 93–100, 2011
KEYWORDS: crosslinking; crosslinking reaction; high-perform-
ance polymers; phosphonated polymers; polyacrylamides;
polymer electrolyte fuel cells; polymer electrolyte membranes;
proton conductivity; radical polymerization
INTRODUCTION Polymer electrolyte fuel cells (PEFCs) have
attracted considerable attention in recent years because of
their potential as future power sources for automotive, sta-
tionary, and portable applications. The function of PEFCs is
basically to separate the anode and the cathode sides and to
conduct protons under a wide range of operation condi-
tions.1–3 In the case of automotive application, the guideline
effort gave some high-performance PEFC materials based on
sulfonated polymers.9,13,15,17 High proton conduction under
high temperature (>90 C) and low RH conditions, however,
is a tough issue for sulfonated polymers to overcome
because their proton transportation is dominated by water
molecules associated with sulfonic acid groups. Conse-
quently, a drastic decrease in proton conductivity under low
water uptake conditions is inevitable.
ꢂ
of proton conductivity is around 1.0 ꢀ 10ꢁ1 S/cm at 120 C
ꢂ
under 20–30% relative humidity (RH). State-of-the-art PEFC
Recently, phosphonated polymers have been attractive for
several applications.21,22 Especially, in the field of PEFCs,
phosphonated polymers are one of the candidates to solve
the aforementioned problem because they show the high
degree of self-dissociation and hydrogen bonding because of
their amphoteric properties, which enables proton conduc-
tion even under anhydrous conditions.23–26 Additionally, as
well as for sulfonated polymers, a high concentration of
acids is important to form large hydrogen-bonded aggre-
gates, which provide proton transportation channels, accord-
ing to the previous report.25
R
membranes are perfluorinated polymers such as NafionV and
R
FlemionV, which show high proton conductivity under a
wide range of RH at moderate operation temperature
because of the well-defined phase separation between the
hydrophilic main chains and the hydrophobic side chains.4
However, they have some drawbacks, such as limited opera-
tion temperature (<80 ꢂC) and high cost. Hence, acid-func-
tionalized aromatic hydrocarbon polymers have been widely
investigated as alternative nonfluorinated polymeric materi-
als,5–7 and a number of sulfonated aromatic polymers, such
as poly(phenylene)s,8,9 poly(ether ether ketone)s,10–12 poly
(ether sulfone)s,13–17 and polyimides,18–20 have been devel-
oped as candidates for PEFC materials. Up to now, intensive
Recently, Parvole and Jannasch reviewed the synthetic
approaches and properties of polymers containing
Correspondence to: M. Ueda (E-mail: ueda.m.ad@m.titech.ac.jp)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 93–100 (2011) 2010 Wiley Periodicals, Inc.
PDPAA FOR PROTON EXCHANGE MEMBRANES, FUKUZAKI ET AL.
93

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
phosphonic acid groups and concluded that the concentra-
tion of phosphonic acid groups in membranes should be suf-
ficiently high to enable the formation of percolating hydro-
gen-bonded networks for proton transportation.25 Based on
these findings, membranes with very high local concen-
trations (ion exchange capacity, IEC, up to 5.3 mequiv/g) of
immobilized proton-conducting phosphonic acid by grafting
poly(vinylphosphonic acid) side chains on polysulfones were
prepared and showed high proton conductivity of 5 mS/cm
under nominally dry conditions and up to 93 mS/cm under
100% RH at 120 C. However, the synthesis of these poly-
mers is not straightforward because phosphonated polymers
are prepared by modification of premade polymers. Addi-
tionally, the structure of polymers, especially the grafting
positions, is not clear, and less repeatability of polymer reac-
tions may also be a matter of concern.
matography (ethyl acetate:hexane ¼ 50:1) to give 2,4-bis
(diethoxyphosphinoyl)aniline (DEPA; 5.02 g, 68.8%).
1H NMR (CDCl3, d, ppm): 7.74–7.84 (1H, t), 7.48–7.54 (1H,
t), 6.68–6.74 (1H, m), 6.05 (2H, s), 3.89–4.11 (8H, m), 1.20–
1.26 (12H, m).
Synthesis of N-[2,4-Bis(diethoxyphosphinoyl)phenyl]
acrylamide (DEPAA)
Acryloyl chloride (1.22 mL, 15 mmol) was added in small
portions to a THF solution (9.5 mL) of DEPA (3.45 g,
9.0 mmol) and pyridine (1.22 mL, 1.5 mmol) at 0 ꢂC. The
mixture was stirred at room temperature for 1 h. Water was
added into the mixture and extracted with dichloromethane.
The organic layer was washed with saturated aqueous
NaHCO3 solution and water and then dried over MgSO4. Af-
ter evaporation, the residue was purified by silica gel column
chromatography (ethyl acetate) to give DEPAA (1.32 g,
33.3%).
26
ꢂ
Herein, we report a facile synthesis of highly phosphonated
poly(N-phenylacrylamide) (PDPAA) with
a high concen-
m.p. 142–143 ꢂC. FTIR (KBr, cmꢁ1): 3428.8, 3382.5, 1697.0,
1635.0, 1619.9, 1592.9, 1496.5, 1473.3, 1292.0, 1176.4,
1099.2, 1010.5, 825.4. 1H NMR (CDCl3, d, ppm): 11.20 (1H,
s), 8.84–8.90 (1H, m), 8.02–8.11 (1H, t), 7.89–7.96 (1H, t),
6.44–6.50 (1H, d), 6.26–6.35 (1H, q), 5.81–5.85 (1H, d),
4.03–4.21 (8H, m), 1.31–1.37 (12H, m). 13C NMR (300 MHz,
CDCl3, d, ppm): 164.7, 146.4, 137.5, 137.0, 132.4, 128.4,
120.9, 120.7, 120.6, 63.1, 16.7. Anal. Calcd. for C17H27NO7P2:
C, 48.69%; H, 6.49%; N, 3.34%; Found: C, 48.61%; H, 6.28%;
N, 3.24%.
tration of phosphoric acids by the radical polymerization of
N-[2,4-bis(diethoxyphosphinoyl)phenyl]acrylamide (DEPAA),
followed by the hydrolysis with trimethylsilyl bromide. The
crosslinked PDPAA membrane was successfully prepared by
the electrophilic substitution reaction of the aromatic rings
of PDPAA and the carbocation formed from hexamethoxy-
methylmelamine (CYMEL) as the crosslinker in the presence
of methanesulfonic acid because PDPAA with an IEC of 6.72
mequiv/g was soluble in water. The obtained crosslinked
membrane showed high proton conductivity of 8.8 ꢀ 10ꢁ2
ꢂ
S/cm, comparable to Nafion 112 at 95% RH and 80 C. Addi-
Polymerization of DEPAA (PDEPAA)
tionally, the properties of the crosslinked PDPAA membrane,
such as thermal stability, oxidative stability, and water
uptake, are also discussed in detail.
DEPAA (0.63 g, 1.5 mmol), AIBN (0.01 g, 0.06 mmol), and
distilled DMF (0.6 mL) were placed in a glass tube using
standard freeze-evacuate-thaw procedures. The reaction mix-
ture was heated at 60 ꢂC for 48 h and then cooled to room
temperature. The viscous polymer solution was dissolved in
DMF and poured in water. The precipitate was collected and
dried in vacuo at 50 ꢂC for 24 h to give PDEPAA (0.49 g,
78.0%).
EXPERIMENTAL
Materials
Dicyclohexylmethylamine was purchased from Sigma-Aldrich.
2,4-Dibromoaniline, triphenylphosphine, diethyl phosphate,
acryloyl chloride, and trimethylsilyl bromide were purchased
from TCI. Distilled ethanol, palladium (II) acetate, pyridine,
and 2,20-azobis(isobutyronitrile) (AIBN) were purchased
from Wako. N,N-Dimethylformamide (DMF) was distilled
from calcium hydride before use. Tetrahydrofuran (THF) was
refluxed over sodium benzophenone for 12 h and then dis-
tilled. Other solvents and reagents were used without further
purification.
FTIR (KBr, cmꢁ1): 3401.82, 2985.3, 1708.6, 1299.8, 1253.5,
1049.1, 964.2, 794.2. 1H NMR (CDCl3, d, ppm): 10.86 (1H,
bs), 8.23 (1H, m), 7.87 (1H, t), 7.54 (1H, bs), 4.12–4.03 (8H,
m), 2.33–1.05 (15H, m).
Synthesis of PDPAA by Hydrolysis of PDEPAA
PDEPAA (0.42 g, 1.0 mmol) was first dissolved in distilled
chloroform (5.0 mL). Then, trimethylsilyl bromide (2.6 mL,
19.7 mmol) was added dropwise at 5 ꢂC, and the mixture
was stirred at 40 ꢂC for 24 h. After the evaporation, the
alcoholysis of the silylated intermediate was performed by
adding an excess of methanol (5.0 mL). The mixture was
stirred at room temperature for 6 h, and then the solvent
was evaporated. The residue was poured into acetone. The
Monomer Synthesis
Synthesis of 2,4-Bis(diethoxyphosphinoyl)aniline (DEPA)
To a freshly distilled ethanol (120 mL) solution of 2,4-dibro-
moaniline (5.02 g, 20 mmol), palladium (II) acetate (0.45 g,
2.0 mmol), and triphenylphosphine (1.57 g, 6.0 mmol) were
added diethyl phosphite (12.4 mL, 96 mmol) and dicyclohex-
ylmethylamine (12.7 mL, 60 mmol). The mixture was
refluxed under nitrogen atmosphere for 48 h. After the evap-
oration, the residue was dissolved with dichloromethane,
washed with 2 M HCl and water, and then dried over MgSO4.
After evaporation, the residue was purified by silica gel chro-
ꢂ
precipitate was collected and dried in vacuo at 40 C for 24
h (0.3 g, 97.8%).
FTIR (KBr, cmꢁ1): 3448.1, 2275.6, 1673.9, 1585.2, 1519.6,
1380.8, 1006.7. 1H NMR (CD3OD, d, ppm): 8.02 (2H, bs),
7.50 (1H, bs), 2.54–2.01 (3H, m).
94
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 1 Synthesis of DEPAA.
Membrane Preparation and IEC
dissolved time of the membranes into the reagent was used
to evaluate their oxidative stability.
A methanol solution of PDPAA was cast onto a flat glass
plate. Drying the solution at 80 ꢂC for 12 h under reduced
pressure gave a transparent, flexible, and tough membrane.
IEC value of the polymer was determined by titration with
0.02 M aq. NaOH. The point of neutralization between the
phosphonic acids and 0.02 M aq. NaOH was determined at
pH ¼ 8.0.
Measurement
1H (300 MHz) and 13C spectra (75 MHz) were recorded with
a Bruker DPX300S spectrometer. 31P spectrum (162 MHz)
was recorded with a JEOL. Fourier transform infrared (FTIR)
spectra were obtained with a Horiba FT-120 spectrophotom-
eter. Thermal analysis was performed on a Seiko EXSTAR
6000 TG/DTA 6300 thermal analyzer at a heating rate of
10 ꢂC/min for thermogravimetry (TG) and differential ther-
mal analysis (DTA). Molecular weight measurement was per-
formed via gel permeation chromatography (Waters) with
two polystyrene gel columns (shodex KD-806M). N-Methyl-2-
pyrrolidinone containing 0.01 M LiBr was used as a solvent
at a flow rate of 0.5 mL/min. Mn and Mw were calibrated by
standard polystyrene samples.
Preparation of Crosslinked Membrane
The optimized condition to prepare the crosslinked mem-
branes is as follows: methanol solution of PDPAA (0.15 g,
0.5 mmol), 7 wt % of CYMEL for PDPAA, and 0.07 wt % of
methanesulfonic acid for PDPAA was casted onto a Teflon
sheet. Drying the film at 40, 60, 80, 100, 120, and 150 C for
6 h (for 1 h at each temperature) under ambient atmosphere
gave a transparent membrane.
ꢂ
Water Uptake
RESULTS AND DISCUSSION
The humidity dependence of water uptake was measured by
plating the membrane in a thermocontrolled humid chamber
for 4 h. Then, the membrane was taken out and quickly
weighed on a microbalance. Water uptake was calculated
from:
Monomer Synthesis
The monomer DEPAA was prepared by a two-step procedure
using 2,4-dibromoaniline as the starting material (Scheme
1). 2,4-Dibromoaniline was reacted with diethyl phosphite
through Pd-catalyzed phosphorylation27 to afford DEPA,
which was further reacted with acryloyl chloride to yield
DEPAA. The structure of DEPAA was characterized by the
elemental analysis, FTIR, and 1H NMR spectroscopies. The
FTIR spectrum of DEPAA showed characteristic absorptions
at 1697 and 1635 cmꢁ1, which are assignable to an amide
carbonyl group and a carbon–carbon double bond, respec-
tively. Figure 1 shows the 1H NMR spectrum of DEPAA. All
peaks agree with the assignment to the expected structure
of DEPAA.
WU ¼ ðWs ꢁ WdÞ=Wd ꢀ 100 wt %;
where Ws and Wd are the weights of wet and dried mem-
brane, respectively.
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 under a thermocontrolled humid chamber. Proton
conductivity (r) was calculated from:
Polymer Synthesis
The synthesis of PDPAA was carried out by the radical poly-
merization of DEPAA using AIBN as a radical initiator in
DMF, followed by the hydrolysis of the phosphonic acid ester
using trimethysilyl bromide (Scheme 2). The polymerization
proceeded smoothly and yielded high molecular weight of
PDEPAA. The number-average and weight-average molecular
weights of PDPAA were 101,000 and 207,000, respectively,
estimated by GPC based on polystyrene standards. PDPAA
was obtained as white solid and soluble in methanol and
water.
r ¼ d=ðLswsRÞ;
where d is the distance between the two electrodes, Ls and
ws are the thickness and width of the membrane, and R is
the resistance value measured.
Oxidative Stability
Oxidative stability of the membranes was tested by immers-
ing the films into Fenton’s reagent (3% H2O2 aqueous solu-
tion containing 20 ppm FeSO4) at room temperature. The
The chemical structure of PDPAA was confirmed by FTIR,
1H, and 31P NMR spectroscopies. Figure 2 shows the 1H
PDPAA FOR PROTON EXCHANGE MEMBRANES, FUKUZAKI ET AL.
95

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 1H NMR spectrum of DEPAA in CDCl3.
NMR spectra of PDEPAA and PDPAA. The signals of the
acrylate group (d ¼ 5.81–6.50) disappear in the 1H NMR
spectrum of PDEPAA, and the signals corresponding to the
phosphonic ethyl ester group can be observed at d ¼ 4.12–
4.03 and 2.33–1.05. This spectrum suggests that the vinyl
polymerization proceeded completely without the decom-
position of the monomer, and the signals of the phosphonic
ethyl ester disappear completely in the 1H NMR spectrum
of PDPAA after the hydrolysis. Additionally, the 31P NMR
spectrum has two peaks corresponding to the characteris-
tic phosphonic acids at d ¼ 10.0ꢁ20.0, which also supports
that the whole reaction proceeded without any decomposi-
tion and side reaction to give the desired PDPAA (Fig. 3).
In the FTIR spectra (Fig. 4), the disappearance of the
absorption at around 1635 cmꢁ1 assignable to the vinyl
group was confirmed, and the characteristic absorption at
1253 cmꢁ1 corresponding to the phosphonic ester disap-
pears after the hydrolysis, resulting in the appearance of
the absorption at 2275 cmꢁ1 corresponding to the phos-
phonic acids. The IEC value of PDPAA was determined to
be 6.72 mequiv/g (theoretical IEC value ¼ 6.49 mequiv/g)
FIGURE 2 1H NMR spectra of PDEPAA (top) in CDCl3 and
PDPAA (bottom) in CD3OD.
by titration with 0.02 M aq. NaOH, which also indicates the
complete hydrolysis.
As PDPAA itself was soluble in water, the crosslinked PDPAA
membrane was prepared by the electrophilic substitution
SCHEME 2 Synthesis of PDPAA.
96
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 3 31P NMR spectrum of PDPAA.
FIGURE 4 FTIR spectra of PDEPA and PDPAA.
reaction between the aromatic rings of PDPAA and the
carbocation formed from CYMEL as the crosslinker in the
presence of methanesulfonic acid (Scheme 3).28 Additionally,
this crosslinking system, which little affects IEC values,
enables to obtain a crosslinked membrane with high IEC
value. As a result of optimization of the crosslinking reaction
conditions, such as the amount of CYMEL, reaction time, and
reaction temperature, a fine crosslinked PDPAA membrane
could be prepared as follows: a methanol solution of PDPAA,
7 wt % of CYMEL for PDPAA, and 0.07 wt % of methane-
sulfonic acid for PDPAA were cast onto a Tefloꢂn sheet. The
film was dried at 40, 60, 80, 100, 120, and 150 C totally for
6 h (for 1 h at each temperature) under ambient atmosphere
to give a fine membrane. After the crosslinking reaction,
the obtained crosslinked PDPAA membrane became insolu-
ble in water, which indicates that the crosslinking reaction
proceeded successfully.
of PDPAA and crosslinked PDPAA under nitrogen atmo-
sphere. In PDPAA, the two-stꢂep weight loss of the polymer
ꢂ
is observed from 210 to 350 C and above 350 C. The first
weight loss is due to the elimination of the phosphonic acid
groups, and the second is attributed to the degradation of
polymer backbones. On the other hand, the similar TG curve
was observed in the crosslinked PDPAA. The weight loss
due to the polymer decomposition starts from around
210 ꢂC, which indicates that the crosslinked PDPAA has
enough thermal stability for fuel cell operation.
Oxidative Stability
The oxidative stability of the crosslinked PDPAA membrane
was evaluated in Fenton’s reagent (a 3% H2O2 aqueous solu-
tion containing 20 ppm FeSO4) at room temperature. Gener-
ally, sulfonated polymers with high IEC value have been
known to demonstrate low oxidative stability against Fen-
ton’s reagent.12,14,17 On the other hand, the crosslinked
PDPAA membrane kept the shape of the membrane for 24 h,
and it took 48 h for complete dissolution, indicating that the
Thermal Properties of PDPAA
The thermal properties of PDPAA and crosslinked PDPAA
were evaluated by TG analysis. Figure 5 shows the TG curves
SCHEME 3 Preparation of the crosslinked PDPAA membrane.
PDPAA FOR PROTON EXCHANGE MEMBRANES, FUKUZAKI ET AL.
97

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 Humidity dependence of hydration number (k) of
crosslinked PDPAA membrane at 80 ꢂC.
FIGURE 5 TG curves of PDEPAA and PDPAA under nitrogen
atmosphere.
high and effective proton conduction. However, excess water
uptake induces unacceptable dimensional changes and a
decrease in the mechanical strength. The humidity depend-
ence of water uptake wꢂas investigated for the crosslinked
PDPAA membrane at 80 C. As shown in Figure 6, the cross-
linked PDPAA membrane shows moderate water uptake
(50.0 wt %) at 95% RH, which corresponds to k (the num-
ber of water molecules per an acid group) ¼ 4.3 (Fig. 7).
Decreasing RH, the water uptake decreases to 12.0, 8.3, and
3.8 wt %, which corresponds to k ¼ 1.0, 0.7, and 0.3 at 70,
50, and 30% RH, respectively. Considering that the cross-
linked PDPAA membrane has quite high IEC value (6.72
mequiv/g), the water uptake is expected to be higher in a
wide range of RH (especially under low RH) as well as
crosslinked PDPAA membrane has excellent oxidative stabil-
ity against Fenton’s reagent regardless of quite high IEC
value.
Water Uptake
The water uptake behavior of acid-functionalized polymers
is a significant factor for proton conductivity. Although the
proton conduction of phosphonic acids is considered to be
mainly derived from their own self-dissociation and hydro-
gen bonding, high water uptake can contribute to the
enhancement of proton conductivity. Accordingly, even
phosphonated polymers need high water uptake to achieve
FIGURE 6 Humidity dependence of water uptake of crosslinked
PDPAA membrane at 80 ꢂC.
FIGURE 8 Humidity dependence of proton conductivity of
crosslinked PDPAA and Nafion 112 membranes at 80 ꢂC.
98
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
sulfonated polymers with high IEC values.15,27 This is prob-
ably because of its weaker acidity that that of sulfonated
polymers, and PDPAA is likely not to form the water-swollen
phase, which efficiently retains water in the membrane, and
the compact crosslinked structure may attribute to this
behavior.
3 Hickner, A. H.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.;
MaGrath, J. E. Chem Rev 2004, 104, 4587–4611.
4 Savadoga, O.
J New Mater Electrochem Syst 1998, 1,
47–66.
5 Rikukawa, M.; Sanui, K. Prog Polym Sci 2000, 25,
1463–1502.
Proton Conductivity
6 Yin, Y.; Yamada, O.; Tanaka, K.; Okamoto, K. Polym J 2006,
The humidity dependence of the proton conductivity of the
crosslinked PDPAA membrane was measured at 80 ꢂC.
Generally, the proton conductivity of the membrane is con-
sidered to play a significant role in the performance of fuel
cells. As shown in Figure 8, the crosslinked PDPAA mem-
brane shows high proton conductivity, comparable to Nafion
112 at 95% RH, and still maintains relatively high proton
conductivity of 2.8 ꢀ 10ꢁ2 S/cm at 70% RH, indicating that
the water-assisted proton transportation similar to that in
conventional sulfonated polymers can contribute to high pro-
ton conductivity under high RH. Under low RH, the proton
conductivity of 1.9 ꢀ 10ꢁ3 and 4.7 ꢀ 10ꢁ5 S/cm is observed
at 50 and 30% RH, respectively. The proton conductivity of
the crosslinked PDPAA membrane is strongly dependent on
RH, which implies that the formation of percolating hydro-
gen-bonded networks for proton transportation is deficient
under low RH. Yet, to the best of our knowledge, the proton
conductivity of the crosslinked PDPAA membrane still lies in
one of the highest class among the reported phosphonated
polymers,26,29–32 which clearly demonstrates that the very
high local concentration of the acids of PDPAA (IEC ¼ 6.72
mequiv/g) was effective for high proton conduction.
38, 197–219.
7 Higashihara, T.; Matsumoto, K.; Ueda, M. Polymer 2009, 50,
5341–5357.
8 Ghassemi, H.; McGrath, J. E. Polymer 2004, 45, 5847–
5854.
9 Seesukphronrarak, S.; Ohira, A. Chem Commun 2009,
4744–4746.
10 Zaidi, S. M. J.; Mikhailenko, S. D.; Robertson, G. P.; Guiver,
M. D.; Kaliaguine, S. J Membr Sci 2000, 173, 17–34.
11 Liu, B.; Robertson, G. P.; Guiver, M. D.; Hu, W.; Jiang, Z.
Macromolecules 2007, 40, 1934–1944.
12 Matsumoto, K.; Higashihara, T.; Ueda, M. Macromolecules
2008, 41, 7560–7565.
13 Nakabayashi, K.; Higashihara, T.; Ueda, M. J Polym Sci Part
A: Polym Chem 2010, 48, 2757–2764.
14 Matsumoto, K.; Higashihara, T.; Ueda, M. J Polym Sci Part
A: Polym Chem 2009, 47, 3444–3453.
15 Matsumoto, K.; Nakagawa, T.; Higashihara, T.; Ueda, M.
J Polym Sci Part A: Polym Chem 2009, 47, 5827–5834.
16 Matsumoto, K.; Higashihara, T.; Ueda, M. Macromolecules
2009, 42, 1161–1166.
CONCLUSIONS
17 Bae, B.; Miyatake, K.; Watanabe, M. Macromolecules 2010,
A novel highly phosphonated PDPAA with an IEC of 6.72
mequiv/g was successfully prepared by the radical polymer-
ization of DEPAA, followed by the hydrolysis with trimethyl-
silyl bromide. Then, the crosslinked PDPAA membrane was
prepared by the reaction of PDPAA with CYMEL as the
crosslinker in the presence of methanesulfonic acid. The
crosslinked PDPAA membrane showed high oxidative stabil-
ity against Fenton’s reagent at room temperature and moder-
ate water uptake at 95% RH. However, the water uptake
significantly decreased from 12.0 to 3.8 wt % at 70 to 30%
RH in spite of quite high IEC value. These results implied
that the crosslinked compact structure suppressed water
swelling. The crosslinked PDPAA membrane showed high
proton conductivity of 8.8 ꢀ 10ꢁ2 S/cm at 95% RH, which
was comparable to Nafion 112 under the same conditions.
Although the polymer has interesting properties, the proton
conductivity of crosslinked PDPAA under low RH is required
to improve for fuel cell applications.
43, 2684–2691.
18 Chan, S.; Yin, Y.; Kita, H.; Okamato, K. J Polym Sci Part A:
Polym Chem 2007, 45, 2797–2811.
19 Miyatake, K.; Yasuda, T.; Hirai, M.; Nanasawa, M.; Wata-
nabe, M.
157–163.
J Polym Sci Part A: Polym Chem 2007, 45,
20 Li, N.; Yasuda, T.; Cui, Z.; Zhang, S.; Li, S. J Polym Sci Part
A: Polym Chem 2008, 46, 2820–2832.
21 Cetal, Y.; Degrange, M.; Pluart, L. L.; Madec, P.-J.; Pham, T.-
N.; Chen, F.; Cook, W. D. J Polym Sci Part A: Polym Chem
2009, 47, 5258–5271.
22 Lam, O. A.; David, G.; Hervaud, Y.; Boutevin, B. J Polym Sci
Part A: Polym Chem 2009, 47, 5090–5100.
23 Schuster, M.; Rager, T.; Noda, A.; Kreuer, K. D.; Maier, J.
Fuel Cells 2005, 5, 355–365.
24 Steininger, H.; Schuster, M.; Kreuer, K. D.; Kaltbeitzel, A.;
Bingol, B.; Meyer, W. H.; Schauff, S.; Brunklaus, G.; Maier, J.;
¨
Spiess, H. W. Phys Chem Chem Phys 2007, 9, 1764– 1773.
REFERENCES AND NOTES
25 Parvole, J.; Jannasch, P. In Advances in Fuel Cells; Zhao, T.
S.; Kreuer, K.-D.; Nguyen, T. V., Eds.; Elsevier: New York, 2007;
pp 120–185.
1 Advances in Fuel Cells; Zhao, T. S.; Kreuer, K.-D.; Nguyen, T.
V., Eds.; Elsevier: New York, 2007.
26 Parvole, J.; Jannasch, P. Macromolecules 2008, 41, 3893–
2 Maunitz, K. A.; Moore, R. B. Chem Rev 2004, 104,
3903.
4535–4586.
PDPAA FOR PROTON EXCHANGE MEMBRANES, FUKUZAKI ET AL.
99

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
27 Bessmertnykh, A.; Doualihy, C. D.; Guilard, R. Chem Lett
30 Parvole, J.; Jannasch, P. J Mater Chem 2008, 18, 5547–5556.
2009, 38, 738–739.
31 Tayouo, R.; David, G.; Ame´duri, B.; Rozie`re, J.; Roualde`s, S.
28 Endo, K.; Ogura, T.; Higashihara, T.; Ueda, M. Polym J 2009,
Macromolecules 2010, 43, 5269–5276.
41, 808–809.
32 Wagner, T.; Manhart, A.; Deniz, N.; Kaltbeitzel, A.; Wanger,
M.; Brunklaus, G.; Meyer, W. H. Macromol Chem Phys 2009,
210, 1903–1914.
29 Perrin, R.; Elomaa, M.; Jannasch, P. Macromolecules 2009,
42, 5146–5154.
100
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA