Poly(arylene ether ether nitrile)s containing flexible alkylsulfonated side chains for polymer electrolyte membranes

Article abstract of DOI:10.1002/pola.26971

A series of poly(arylene ether ether nitrile)s with different chain lengths of the alkylsulfonates (SPAEEN-x: x refers number of the methylene units) are successfully synthesized for fuel cell applications. The polymers produced flexible and transparent membranes by solvent casting. The resulting membranes display a high thermal stability, oxidative stability, and higher proton conductivity than that of Nafion 117 at 80 °C and 95% relative humidity (RH). Furthermore, the SPAEEN-12 with the longest alkylsulfonated side chain exhibits a higher proton conductivity at 30% RH than that of SPAEEN-6 despite the lower IEC value, which indicates that the introduction of longer alkylsufonated side chains to the polymer main chain induces an efficient proton conduction by the formation of a well-developed phase-separated morphology.

Full text of DOI:10.1002/pola.26971

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Poly(arylene ether ether nitrile)s Containing Flexible Alkylsulfonated Side
Chains for Polymer Electrolyte Membranes
1
2
3
1
Li Sheng, Tomoya Higashihara, Mitsuru Ueda, Teruaki Hayakawa
1
Department 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
2
Department of Polymer Science and Engineering, Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa,
Yamagata 992–8510, Japan
3
Department of Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221–8686, Japan
Correspondence to: T. Higashihara (E-mail: thigashihara@yz.yamagata-u.ac.jp) or T. Hayakawa (E-mail: hayakawa.t.ac@
m.titech.ac.jp)
Received 2 July 2013; accepted 1 October 2013; published online 18 October 2013
DOI: 10.1002/pola.26971
ABSTRACT: A series of poly(arylene ether ether nitrile)s with dif-
ferent chain lengths of the alkylsulfonates (SPAEEN-x: x refers
number of the methylene units) are successfully synthesized
for fuel cell applications. The polymers produced flexible and
transparent membranes by solvent casting. The resulting mem-
branes display a high thermal stability, oxidative stability, and
lower IEC value, which indicates that the introduction of longer
alkylsufonated side chains to the polymer main chain induces
an efficient proton conduction by the formation of a well-
developed phase-separated morphology. V
2013 Wiley Periodi-
C
cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 21–29
ꢀ
higher proton conductivity than that of Nafion 117 at 80 C and
9
5% relative humidity (RH). Furthermore, the SPAEEN-12 with
KEYWORDS: alkylsulfonated side chain; fuel cell; poly(arylene
ether ether nitrile); polymer electrolyte membrane; proton
conductivity
the longest alkylsulfonated side chain exhibits a higher proton
conductivity at 30% RH than that of SPAEEN-6 despite the
INTRODUCTION The polymer electrolyte membrane (PEM) is
the key component of the PEMFC, because it serves as the
separator of the electrodes and transports protons from the
anode to the cathode. PEMs that have received the most
attention in recent years are fully fluorinated polymers with
acid-functionalized side chains, such as Dupont’s NafionVR
and Dow Chemical’s AquivionVR , due to their good proton
polymer main chain, that is, the higher ion exchange capacity
(IEC) value, is an effective way. Litt et al., for example,
1
6
reported the synthesis of poly(p-phenylene disulfonic acid)
2
1
with the IEC higher than 8.0 mequiv g . The proton con-
ductivity value was about 11 times higher than that of
ꢀ
Nafion
VR NR-212 even at 30% RH and 80 C, which has
never yet been reported. However, this membrane absorbed
a large amount of water, significantly swelled, and the proton
conductivity could not be measured above 65% RH. There-
fore, to prepare high performance PEMs for using under fuel
cell conditions cannot only focus on increasing the IEC.
Recently, the Okamoto and Watanabe groups reported a
series of side chain type sulfonated polyimides (SPIs) includ-
conductivity, excellent chemical stability and long-term dura-
bility. However, they suffer from some drawbacks, such as a
high production cost, poor thermomechanical properties
ꢀ
above 80 C, environmental incompatibility and high perme-
ability of the fuels and oxygen. To date, numerous sulfonated
aromatic hydrocarbon-based polymers, which contain sul-
fonic acid groups on the polymer main chain, have been
ing both the short side chains (sulfopropoxy and sulfobutoxy
1
–15
17–22
already reported as alternative membranes.
groups) and long side chains.
The resulting membranes
showed a higher hydrolytic stability compared to the main
chain type SPIs, because the sulfonic acid groups were
located on flexible side chains separated from the main
chains, which reduced the chances of nucleophilic attack by
water on the imide linkage, and also displayed some
improvement of proton conductivity by forming a well-
dispersed hydrophilic domains compared with the main
The low-cost productivity is an attracting advantage for
hydrocarbon-based polymers in comparison to the perfluoro-
polymers. However, most of them have failed to meet the
requirements of a high proton conductivity under a low rela-
tive humidity (RH) and durability under fuel cell operating
conditions. To improve the proton conductivity at a low RH,
increasing the content of the sulfonic acid groups in the
2
chain type ones. However, the proton conductivity of SPIs
V
C
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could not be improved even the number of aliphatic carbons
was washed with acetone. Then the solid product was
extracted from the sodium bromide and unconverted sodium
sulfite with boiling 95% ethanol, thoroughly washed with
2
0,21
were extended to 12,
probably due to the bulky imide
ring in the main chain, which increases the distance between
the sulfonic acid groups of the SPI. On the other hand,
appropriate high IEC, which is required for high proton
transport, is difficult to obtain for increasing the length of
ꢀ
acetone and then dried at 110 C for 24 h in vacuum.
1
Yield: 77%. H NMR (300 MHz, DMSO-d , d): 1.21–1.46 (4H,
6
2
0
ACH ACH A), 1.50–1.65 (2H, ACH ACH SO Na), 1.72–1.86
side chain on SPIs. Zhang group also reported side chain
2 2 2 2 3
2
3
(2H, BrCH ACH A), 2.37–2.46 (2H, ACH ASO Na), 3.47–
2 2 2 3
type polymers for PEMs, they chose poly(arylene ether sul-
fone)s with the cardo structure as the main chain and intro-
duced long side chains with the number of alkyl carbon of
2
3.56 (2H, BrACH A).
Synthesis of Poly(arylene ether ether nitrile) Containing
Methoxy Groups (MPAEEN)
The MPAEEN was synthesized by nucleophilic substitution
reaction. DFBN (1.67 g, 12.0 mmol), MHQ (1.26 g, 9.00
10. However, the proton conductivity was lower than that of
Nafion even in water due to low density of the alkylsulfonic
acid groups in the polymers.
In this study, the aromatic poly(arylene ether ether nitrile)s
mmol), MRS (0.420 g, 3.00 mmol), K CO (2.00 g, 14.4
2 3
(
SPAEEN-x) with high density of the alkylsulfonic acid groups
mmol), DMAc (15.0 mL), and toluene (15.0 mL) were added
into a dry 100 mL three-neck flask equipped with a nitrogen
inlet and a Dean-Stark trap. The reaction mixture was heated
were successfully prepared. The nitrile groups were intro-
duced into the polymer main chain to suppress the water
2
4
ꢀ
uptake according to the previous report. Furthermore, the
effect of chain lengths of alkylsulfonic acid groups on the
membrane properties, such as thermal stability, oxidative
stability, mechanical strength, water uptake, and proton con-
ductivity was investigated. We found that the membrane
SAPEEN-12 with the lower IEC value of 1.89 mequiv g
showed higher proton conductivity than the membrane
SPAEEN-6 with the higher IEC value of 2.32 mequiv g at
low RH by the formation of well-developed nanophase-sepa-
rated morphology, which was supported by atomic force
microscope (AFM) images.
to 140 C for 3 h. After dehydration and removal of toluene,
ꢀ
the reaction temperature was increased to 160 C. When the
solution viscosity had apparently increased, the mixture was
ꢀ
cooled to 120 C, then was slowly poured into a large excess
of deionized water. The fiber-like precipitate was filtered off,
washed with deionized water. The polymer was re-
precipitated from 2 wt % DMAc solution into ethanol, and
21
2
1
ꢀ
dried in vacuum at 100 C for 24 h.
Yield: 98%. FTIR (membrane): m2943–2835 (CAH), 2230
21
(
(
CBN), 1602 (C@C), 1192, 1240 (ArAOAAr), 1130 cm
OAC).
EXPERIMENTAL
Synthesis of Poly(arylene ether ether nitrile) Containing
Hydroxyl Groups (HPAEEN)
Materials
5
-Methoxyresorcinol (MRS) was purchased from Aldrich. 2,6-
Demethylation reaction was described as follow: MPAEEN
(3.20 g) was dissolved into 200 mL anhydrous 1,1,2,2-tetra-
chloroethane in a 500 mL three-neck flask with a nitrogen
inlet. The solution was cooled down to 0 C (ice bath) and 1
M solution of BBr in CH Cl was added dropwise. After 12
3 2 2
Difluorobenzonitrile (DFBN), methoxyhydroquinone (MHQ),
2
1
boron tribromide (17% in dichloromethane, ca. 1 mol L ),
,1,2,2-tetrachloroethane, 1,3-propanesultone, 1,4-butanesul-
tone, 1,6-dibromohexane, 1,10-dibromodecane, and
,12-dibromododecane were purchased from TCI. 1,1,2,2-Tet-
rachloroethane was distilled under reduced pressure from
ꢀ
1
1
h, the resulting polymer was filtered, washed with ethanol
and deionized water several times, and then dried under
ꢀ
finely ground CaCl
received.
2
. The other reagents were used as
vacuum at 100 C for 24 h.
Yield: 97%. FTIR (membrane): m3525–3260 (AOH), 2230
21
(
CBN), 1602 (C@C), 1192, 1240 cm (ArAOAAr).
Synthesis of Sodium 6-Bromohexylsulfonate, Sodium 10-
Bromodecylsulfonate, and Sodium 12-
Bromododecylsulfonate
Sodium 6-bromohexylsulfonate, sodium 10-bromodecyl-
sulfonate, and sodium 12-bromododecylsulfonate were syn-
Synthesis of Poly(arylene ether ether nitrile) with
Pendant Alkylsufonic Acid Groups (SPAEEN-x)
A typical synthetic procedure for SPAEEN-6 is described as
follows: HPAEEN (0.450 g, 2.00 mmol) was added into a 50
mL dry two-necked flask equipped with a Dean-Stark trap, a
condenser and a nitrogen inlet. Anhydrous DMSO (8.00 mL)
was then added via a syringe through a septum cap. After
2
5
thesized following the procedure described in a report.
typical example for the synthesis of sodium
-bromohexylsulfonate is as follows:
A
6
ꢀ
To a 50 mL flask with a reflux condenser and a nitrogen
inlet were added 1,6-dibromohexane (4.88 g, 20.0 mmol),
ethanol (10.0 mL), and deionized water (4.00 mL). The mix-
ture was heated to boiling temperature, and a solution of
sodium sulfite (0.756 g, 6.00 mmol) in deionized water (2.70
mL) was added dropwise, after refluxing for 2 h, most of sol-
vent was removed by evaporation. The remaining mixture
HPAEEN was completely dissolved in DMSO at 100 C, NaOH
(0.160 g, 4.00 mmol), and cyclohexane (8.00 mL) were
ꢀ
added. The mixture was heated at 100 C for 2 h to remove
water. After removal of cyclohexane and water, sodium 6-
bromo-hexylsulfonate (1.07 g, 4.00 mmol) and a suitable
quantity of potassium iodide (KI) were added into the mix-
ꢀ
ture. Then the reaction was kept at 100 C for 24 h. The
22
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SCHEME 1 Synthesis of sodium bromoalkylsulfonates.
2
resulting polymer was obtained by carefully pouring the
solution into water, filtered, washed thoroughly with water,
ethanol and dried under vacuum at 80 C for 10 h.
site ([H O]/[SO3 ]), often referred to as k value, and calcu-
2
lated according to the following equation:
ꢀ
Â
Ã
Â
À
ÁÃ
2
5WU ð%Þ310= 18 3IEC mmol g 21
k 5½H2Oꢁ= SO 3
Yield: 95%. FTIR (membrane): m2230 (CBN), 1602 (C@C),
(
2)
1192, 1240 (ArAOAAr), 1128, 1047 (O@S@O).
Dimensional change of a hydrated membrane was also inves-
tigated by placing the membrane in a thermocontrolled
humid chamber (80 C) at desired RH for 2 h, the changes
Membrane Formation and Proton Exchange
ꢀ
Membranes were prepared by casting the SPAEEN-x (in their
sodium form) solution in DMSO onto glass plates and dried
of length and thickness were calculated from the equation:
ꢀ
at 80 C for 24 h. Proton exchange was performed by
Dl 5 ðls2ldÞ=ld 3 100%
Dt 5 ðts2tdÞ=td 3 100%
(3)
(4)
ꢀ
immersing the membranes in 2.0 M H
2
SO
4
at 40 C for 3
days. The proton exchanged membranes were washed with
deionized water for 1 day.
where l and t are the length and thickness of the wet mem-
s
s
brane, respectively, l and t refer to those of the dried mem-
d
d
Measurements
H NMR spectrum was recorded on a JEOL JNM-AL 300
brane. Proton conductivity was measured using a two-probe
electrochemical impedance spectroscopy technique over the
frequency from 5 Hz to 1 MHz (Hioki 3532-80). The samples
were 8-mm long, 8-mm wide, and 68- to 109-lm thick. Pro-
ton conductivity (r) was calculated from the following
equation:
1
NMR spectrometer using DMSO-d6 as solvent and tetrame-
thylsilane (TMS) as an internal standard. Number- and
weight-average molecular weights (M and M ) were meas-
ured by gel permeation chromatography (GPC) on a Viscotek
TDA 302 system with a TSK-GEL a-M column and N,N-dime-
thylformamide (DMF) as eluent. Thermogravimetric analysis
n
w
r 5 d=ðtswsRÞ
(5)
(
TGA) was performed on a TA Instruments TG/DTA 7300 in
where d is the distance between the two electrodes, t and
s
nitrogen. Prior to measurement, all the samples were pre-
heated at 150 C for 30 min to remove any absorbed mois-
ture. Subsequently the samples were cooled to 50 C and
then heated to 550 C at a heating rate of 10 C min . Ion
exchange capacity (IEC) was measured by a titration method.
The samples were immersed in saturated NaCl solution at
room temperature for 3 days. After that, the NaCl solution
was directly titrated with 0.02 M NaOH using phenolphthal-
ein as pH indicator. Tensile measurements were performed
with a universal testing instrument (AGS-X 349–05489A, Shi-
w are the thickness and width of the membrane, and R is
s
ꢀ
the measured resistance. Tapping mode atomic force micro-
scope (AFM) images were taken by using Seiko Instruments
SPA-400 with a stiff cantilever of Seiko Instruments DF-20.
ꢀ
ꢀ
ꢀ
21
ꢀ
madzu, Japan) at 20 C and around 50% RH at a crosshead
speed of 2 mm/min. The oxidation stability of the mem-
branes was determined by Fenton’s test. The samples were
soaked in the Fenton’s reagent (3% H O containing 20 ppm
2
2
FeSO ) at room temperature. The disappearance time of the
4
samples was used to evaluate the radical oxidation stability
of the membrane. Water uptake of the membrane was meas-
ured by placing the membrane in a thermocontrolled humid-
ity chamber. The membranes were set at desired values and
kept constant for 2 h at each point. Then the membrane was
taken out, and quickly weighed on a microbalance. Water
uptake (WU) was calculated according to the following
equation:
WU 5 ðWs2WdÞ=Wd 3 100 wt %
(1)
where W and W are the weights of wet and dried mem-
s
d
branes, respectively. Water sorption was also expressed as
the average number of water molecules per ion exchange
1
FIGURE 1 H NMR spectra of sodium bromoalkylsulfonates.
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SCHEME 2 Synthesis of poly(arylene ether ether nitrile)s with pendant alkylsufonic acid groups (SPAEEN-x).
RESULTS AND DISCUSSION
other peaks are also well assigned to the corresponding
structures. Poly(arylene ether ether nitrile)s with pendant
alkylsufonic acid groups (SPAEEN-x, x refers the number of
methylene units) were prepared via a three-step reaction as
shown in Scheme 2. First, the polycondensation of DFBN
Synthesis of Sodium Bromoalkylsulfonates and Polymers
As shown in Scheme 1, various sodium bromoalkylsulfonates
2
5
were prepared according to the previous paper. The chemi-
cal structures of sodium 6-bromohexylsulfonate, sodium 10-
bromodecylsulfonate, and sodium 12-bromododecylsulfonate
ꢀ
with MRS was conducted at 160 C, leading to a low molecu-
lar weight polymer because of premature precipitation of the
polymer. Then, MHQ in the place of MRS was employed to
improve the solubility of the resulting polymer. Although the
high molecular weight polymer could be easily prepared
1
were confirmed by H NMR spectroscopy (Fig. 1). New peaks
at 2.36–2.46 ppm corresponding to the methylene protons
next to the ASO Na groups are clearly observed. All the
3
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1
FIGURE 2 H NMR spectra of (a) MPAEEN and (b) HPAEEN.
1
FIGURE 4 H NMR spectra of SPAEEN-x.
from DFBN and MHQ, the resulting polymer showed a lim-
ited solubility in halogenated hydrocarbons which are suita-
ble solvents for the demethylation of methoxy groups using
BBr . Finally, MPAEEN (M 5 41,000, M 5 116,000) was
fully proceeded. Finally, SPAEEN-x was obtained by the reac-
tion of the alkanesultones or sodium bromoalkylsulfonates
with HPAEEN in the presence of NaOH. Before adding the
alkanesultones or sodium bromoalkylsulfonates, the water in
the reaction mixture was removed by azeotropic distillation
with cyclohexane. The structures of the SPAEEN-x were also
3
n
w
prepared by the nucleophilic substitution polycondensation
ꢀ
of DFBN with MHQ and MRS in DMAc at 160 C.
The demethylation of the methoxy groups was conducted
using BBr in 1,1,2,2-tetrachloroethane. HPAEEN was precipi-
3
1
confirmed by the FTIR and H NMR spectroscopy. The FTIR
spectra of the SPAEEN-x in their sodium form are shown in
Figure 3. The SPAEEN-x show characteristic bands at 1047
tated during the reaction due to the polar nature of the phe-
nol groups. Figure 2 shows the H NMR spectra of MPAEEN
and HPAEEN. The characteristic methoxy protons at 3.79
ppm in the initial MPAEEN disappear in the HPAEEN, and
the peak around 10.35 ppm corresponding to the phenol
proton appears, indicating that the deprotection had success-
1
2
1
and 1128 cm assigned to the O@S@O stretching vibration
21
of the sulfonate groups. The band at 2230 cm correspond-
ing to the stretching vibrations of the nitrile group was also
1
observed. The H NMR spectra of SPAEEN-x are shown in
Figure 4, exhibiting characteristic methylene protons next to
FIGURE 3 FTIR spectra of SPAEEN-x in sodium form [(a)
SPAEEN-3; (b) SPAEEN-4; (c) SPAEEN-6; (d) SPAEEN-10; (e)
SPAEEN-12].
FIGURE 5 TGA curves of the SPAEEN-x membranes, crosslinked
2
6
sulfonated random (PBOS
4
-r-PSBOS
6
)
and block copolystyrene
2
7
derivatives (PSBOS -b-PnBOS3.2
1
)
in acid form under N
2
.
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TABLE 1 Thermal, Mechanical Properties, and Oxidative Stability of SPAEEN-x Membranes
Oxidative Stability
Maximum
Young’s
Elongation at
ꢀ
a
a
a
b
c
Samples
T
d 5% ( C)
Stress (MPa)
Modulus (MPa)
Break (%)
s (h)
RW (wt %)
d
SPAEEN-3
SPAEEN-4
SPAEEN-6
SPAEEN-10
SPAEEN-12
285
284
278
270
289
–
–
–
43
0
19
22
18
–
714
522
435
–
25
15
25
–
44
0
62
68
>95
>95
>80
>80
a
ꢀ
c
Measured at 20 C under 50% RH.
RW (wt %): Remaining weight: measured by soaking the membranes
b
ꢀ
s(h): Disappearance time: measured by soaking the membranes in
in Fenton’s reagent (3% H
d
2
O
2
containing 2 ppm FeSO
4
) at 80 C for 1 h.
Fenton’s reagent (3%
temperature.
H
2
O
2
containing 20 ppm FeSO
4
)
at room
Has not measured.
the sodium sulfonate group at 2.37 ppm, while no hydroxyl
protons were observed.
IEC, Water Uptake, and Dimensional Change
The IEC values were determined by a titration method, and
the measured IEC values of the SPAEEN-x membranes are in
2
1
Thermal Properties
the range of 1.89–2.69 mequiv g
(Table 2). The water
The thermal stability of the polymer SPAEEN-x in the acid
uptake (WU) and swelling ratio of the PEMs are very impor-
tant properties that are significantly related to the proton
conductivity and mechanical strength. Moreover, the IEC val-
ues of the sulfonated aromatic polymers generally dominate
the WU. The WU and hydration number (k) of SPAEEN-3 are
the highest among the studied membranes due to its high
forms was investigated by TGA analysis under nitrogen (Fig.
ꢀ
5
). All the samples were preheated at 150 C for 30 min in
the TGA furnace to remove any moisture. For comparison,
2
6
the crosslinked sulfonated random (PBOS -r-PSBOS ) and
4
6
2
7
block copolystyrene derivatives (PSBOS
1
-b-PnBOS3.2
)
hav-
ꢀ
ing flexible alkylsulfonated side chains and hydrophobic
alkoxy chains are also shown in Figure 5. All the SPAEEN-x
membranes showed a two-stage weight loss behavior. The
IEC (Fig. 6). However, the WU of SPAEEN-3 at 80 C under
95% RH is lower than 60%, which corresponds to the
absorption of <12 water molecules per a sulfonic acid
group. One plausible explanation is the presence of nitrile-
nitrile dipole interactions that combine to limit the WU,
ꢀ
first stage weight loss temperature that started from 220 C
is possibly associated with the loss of bound water and deg-
radation of the sulfonic acid groups, which is much higher
2
4
which has been reported by Pivovar group. In addition, the
interaction between the nitrile and sulfonic acid groups may
be important as nitrile groups have been found to associate
with sulfonic acid groups through bridging water molecules
ꢀ
26
than that of the PBOS -r-PSBOS (150 C) and PSBOS -b-
4
6
1
ꢀ
27
PnBOS3.2 (150 C). The second stage weight loss tempera-
ture at around 350 C is likely related to the degradation of
ꢀ
2
4,28
the alkyl side chain and polymer main chain. The 5 wt %
in specific spectroscopic studies.
On the other hand, the
weight loss temperatures (Td5%) of all the SPAEEN mem-
branes summarized in Table 1 are higher than 270 C. The
TGA results indicate that the SPAEEN-x have high thermal
WU and k dramatically decrease at 95% RH when the length
of the alkyl side chains is increased. The SPAEEN-3 mem-
brane, for example, has a WU around 57%, which is two
times higher than that of the SPAEEN-12 membrane (25%).
The dimensional change in all the SPAEEN-x membranes was
ꢀ
stability, which is high enough for the applications in
ꢀ
medium temperature (100–180 C) fuel cells.
TABLE 2 Ion Exchange Capacities (IEC), Water Uptake (WU), and Proton Conductivity (r) of the SPAEEN-x Membranes
21
a
a
IEC (mequiv g
)
WU (wt %)
r (mS/cm)
b
Samples
Theoretical
Measured
30% RH
95% RH
30% RH
95% RH
SPAEEN-3
SPAEEN-4
SPAEEN-6
SPAEEN-10
SPAEEN-12
2.87
2.76
2.57
2.24
2.11
2.69
2.61
2.32
1.99
1.89
5.65
5.37
4.67
2.31
2.89
57.3
53.5
35.6
26.7
25.0
1.71
212
226
175
150
137
1.06
0.416
0.750
0.877
a
ꢀ
b
Measured at 80 C.
By titration method.
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Table 1. The SPAEEN-3 membrane with the highest IEC value
resists dissolution in Fenton’s reagent for more than 40 h.
On the other hand, the membranes with longer alkylsulfo-
nated side chains show better oxidative stability than those
with the shorter side chains. The SPAEEN-12 membrane, for
example, does not disappear for up to 80 h. Furthermore,
the remaining weight of SPAEEN-10 and SPAEEN-12 after
soaking in Fenton’s reagent (3% H O aq., 2 ppm FeSO ) at
2
2
4
ꢀ
8
0 C for 1 h, is still greater than 95 wt %, which indicates
that the introduction of flexible alkylsufonic acids side chains
can improve the oxidative stability. Because the sulfonic acid
group is an electro-withdrawing group, when it is directly
attached to the aromatic ring of the polymer main chain, it
can reduce the electron density of the neighboring aromatic
groups by a resonance effect and facilitate the nucleophilic
displacement reaction. It would be reasonable to assume
that the oxidative degradation of the sulfonated poly(arylene
ether)s commences at the o-carbons to the ether linkages by
attack of the hydroxyl radicals. For the side chain type poly-
mers, the longer alkyl side chains could more effectively sep-
arate the polymer main chain from the sulfonic acid groups
to keep the main chain in a hydrophobic atmosphere that
prevents or reduces the chance of radical attack, and the
introduction of the ACN group, which decreases the water
FIGURE 6 Water uptake (A) and hydration number (k) (B) of
ꢀ
SPAEEN-x and Nafion 117 under different RH at 80 C.
ꢀ
also measured at different RHs (50–95%) and 80 C (Fig. 7).
The SPAEEN-x membranes show an anisotropic swelling
behavior with a slightly higher change in the thickness direc-
tion. The SPAEEN-3 membrane with the highest IEC among
these membranes displays a dimensional change lower than
13% even at 95% RH. This result indicates that the introduc-
tion of the ACN group and long alkylsulfonated side chains
can effectively suppress the swelling ratio of the membranes.
Oxidative Stability and Mechanical Properties
It is well known that the poor oxidative stability of the mem-
branes may cause failure during long-term fuel cell opera-
tions. Fenton’s test is a common method, which has been
widely used for evaluation the oxidative stability. The mem-
branes are generally immersed in 3% H2O2 containing 20
4 2 2
ppm FeSO at room temperature or 3% H O containing 2
ꢀ
ppm FeSO at elevated temperature (e.g., 80 C), and the oxi-
4
dative stability is characterized either by the disappearance
time of the membranes or by the weight loss of the mem-
branes as a function of time. In this study, the oxidative sta-
bility of all the prepared membranes was evaluated by
2 2
immersing the samples in Fenton’s reagent (3% H O aq., 20
ppm FeSO ) at room temperature. The resistance to oxida-
4
tion of the membranes was measured by observing their
dissolution behavior, and these results are summarized in
FIGURE 7 Dimensional change of SPAEEN-x: length change
ꢀ
(A) and thickness change (B) under different RH at 80 C.
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S/cm) at 95% RH. It is well known that the proton conduc-
tivity of the sulfonated aromatic hydrocarbon-based poly-
mers is mainly dependent on the IEC values, and the higher
IEC, the higher proton conductivity. The morphology of the
membrane is another factor which also affects the proton
conductivity. The longer flexible side chains are supposed to
favor a well-developed phase separation, which is expected
to contribute to the high proton conductivity. In this study,
the side chain type SPAEEN-x exhibit a higher proton con-
ductivity than that of the other sulfonated poly(ether sul-
1
0
fone)s (BNSH-67), which contain sulfonic acid groups on
the main chain. For example, the proton conductivity of the
2
1
23
SPAEEN-10 (IEC 5 1.99 mequiv g ) was 8.0 3 10
S
2
1
cm
, which was higher than that of the BNSH-67
IEC 5 2.18 mequiv g ) (10 S cm ) at 80 C and 50%
2
1
25
21
ꢀ
(
RH. The hydrophilic sulfonic acid groups are directly
attached to the main chains of BNSH-67, which could reduce
the hydrophobicity of the main chain, resulting in poor phase
separation. Compared with other side chain type polymers,
the conductivity of SPAEEN-12 is much higher than that of
sulfonated polyimide with long side chain 12(12) due to
the high density of the alkylsulfonic acids. All these results
indicate that introduction of flexible side chains and increas-
ing the density of the alkylsulfonic acids in the polymer
could effectively improve the proton conductivity.
FIGURE 8 Variation of proton conductivity of the SPAEEN-x,
1
0
ꢀ
BNSH-67, and Nafion 117 as a function of RH and 80 C.
uptake and swelling ratio, could also retard the entrance of
2
0
2
3
the oxidative agent into the membrane.
The tensile properties of the SPAEEN-x membranes were
measured and the results are listed in Table 1. It can be
seen that these membranes (except SPAEEN-3 and SPAEEN-
1
2) show maximum stress in the range of 18–22 MPa, which
As described above, the proton conductivity of the sulfonated
aromatic hydrocarbon-based polymers is mainly dependent
on the IEC values. However, in this study, the SPAEEN-6,
SPAEEN-10, and SPAEEN-12 containing an alkyl carbon more
than 4 display an interesting phenomenon, their proton con-
ductivities are independent on the IEC values at 30% RH
is similar to that of the crosslinked sulfonated polystyrene
with flexible alkylsulfonated side chains,
Young’s modulus of the SPAEEN-x membranes are much
higher than those of the latter due to the high chain rigidity
of aromatic polymer backbone.
2
6,27
while the
(
Table 2). Generally, the decrease of RH would cause the dis-
2
Proton Conductivity
connection of SO
3
clusters and the proton transportation
The proton conductivities of the SPAEEN-x membranes were
measured at 80 C under different RH (30–95%) conditions
by water molecular motion between clusters becomes domi-
nant for the membranes motion between clusters becomes
ꢀ
2
9
and the results are shown in Figure 8 and compared to that
of the Nafion 117 membrane. It can be seen that for all the
SPAEEN-x membranes, the proton conductivities increase
with the increasing RH. They show higher proton conductiv-
ities (0.137–0.212 S/cm) than that of Nafion 117 (0.118
dominant for the membranes, therefore, the conductivity of
these membranes mainly depends on water uptake, which is
affected by IEC. For the membranes which maintain the con-
nectivity of ion channel even at low RH, the protons trans-
ported easily through the channel. The SPAEEN-12
FIGURE 9 AFM image of SPAEEN-6 membrane (L) and SPAEEN-12 membrane (R).
28
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membrane contains the longest flexible aliphatic side chain,
which is expected to form ion channel for transporting
protons.
2 N. Asano, M. Aoki, S. Suzuki, K. Miyatake, H. Uchida, M.
Watanabe, J. Am. Chem. Soc. 2006, 128, 1762–1769.
3
S. Besse, P. Capron, G. Gebel, F. Jousse, D. Marsacq, M.
Pineri, C. Marestin, R. Mercier, J. New Mater Electrochem Sys-
tems 2002, 5, 109–112.
Morphology
4
C. Genies, R. Mercier, B. Sillion, N. Cornet, G. Gebel, M.
The microstructure of the SPAEEN-x membranes was investi-
gated by AFM. As shown in Figure 9, both SPAEEN-6 and
SPAEEN-12 membranes display the formation of phase-
separated structures, in which the alkylated sulfonic acids
would aggregate into hydrophilic clusters. In the AFM
images, more organized phase-separated morphology can be
observed for the SPAEEN-12 membrane than that of
SPAEEN-6. This indicates that the longer alkyl side chain
could induce the formation of well-organized nanodomains.
The formation of the domains in a series of SPAEEN mem-
branes would be to improve proton conductivity and oxida-
tive stability. The aggregation of the sulfonic acids supported
by longer flexible alkyl chains generates the well-developed
nanochannel liked morphology, which could improve the
proton conduction at low RH and also prevent from oxidiz-
ing the polymer main chain by hydroxyl radical attack.
Pineri, Polymer 2001, 42, 359–373.
5
T. Higashihara, K. Matsumoto, M. Ueda, Polymer 2009, 50,
5
341–5357.
6
J. M. Bae, I. Honma, M. Merata, T. Yamamoto, M. Rikukawa,
N. Ogata, Solid State Ionics, 2002, 147, 189–194.
7
8
9
J. A. Kerres, J. Membr. Sci. 2001, 185, 3–27.
V. Mehta, J. S. Cooper, J. Power Sources 2003, 114, 32–53.
K. Nakabayashi, K. Matsumoto, M. Ueda, J. Polym. Sci., Part
A: Polym. Chem. 2008, 46, 3947–3957.
0 K. Matsumoto, T. Nakagawa, T. Higashihara, M. Ueda, J.
Polym. Sci., Part A: Polym. Chem. ,47, 5827–5834.
1 K. Nakabayashi, T. Higashihara, M. Ueda, J. Polym. Sci.,
Part A: Polym. Chem. 2010, 48, 2757–2764.
1
1
1
2
2 K. Nakabayashi, T. Higashihara, M. Ueda, Macromolecules
010, 43, 5756–5761.
1
3 X. Guo, J. Fang, T. Watari, K. Tanaka, H. Kita, K. Okamoto,
Macromolecules 2002, 35, 6707–6713.
CONCLUSIONS
14 J. Fang, X. Guo, S. Harada, T. Watari, K. Tanaka, H. Kita, K.
Okamoto, Macromolecules 2002, 35, 9022–9028.
A series of SPAEEN-x copolymers containing different chain
lengths of alkylsulfonates have been prepared by a three-
step reaction, that is, polycondensation of DFBN containing a
strongly polar ACN group with MRS and MHQ, followed by
demethylation and the reaction with alkanesultones or
sodium bromoalkylsulfonates. The resulting membranes
exhibited a high thermal stability despite the full aliphatic
side chains, high oxidative stability against Fenton’s reagent
at room temperature, low water uptake and dimensional
change even at 95% RH. The SPAEEN-3 membrane with the
IEC of 2.69 mequiv g displayed the highest proton conduc-
tivity among these prepared membranes over the entire
range of RH from 30 to 95%. On the other hand, the
SPAEEN-12 membrane with the longer side chain displayed
a lower water uptake and higher proton conductivity (at
1
5 Y. Yin, J. Fang, H. Kita, K. Okamoto, Chem. Lett. 2003, 32,
3
28–329.
1
6 K. Si, D. Dong, R. Wycisk, M. Litt, J. Mater. Chem. 2012, 22,
2
0907–20917.
1
7 Y. Yin, Y. Suto, T. Sakabe, S. Chen, S. Hayashi, T. Mishima,
O. Yamada, K. Tanaka, H. Kita, K. Okamoto, Macromolecules
2006, 39, 1189–1198.
18 Z. Hu, Y. Yin, K. Okamoto, Y. Moriyama, A. Morikawa, J.
Membr. Sci. 2009, 329, 146–152.
19 Y. Yin, Q. Du, Y. Qin, Y. Zhou, K. Okamoto, J. Membr. Sci.
2
1
2
011, 367, 211–219.
2
0 T. Yasuda, Y. Li, K. Miyatake, M. Hirai, M. Nanasawa, M.
Watanabe, J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 3995–
4
005.
2
1 K. Miyatake, T. Yasuda, M. Hirai, M. Nanasawa, M.
Watanabe, J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 157–
30% RH) than those with the shorter side chain membrane
1
63.
SPAEEN-6 in spite of its lower IEC value. The introduction of
more than six aliphatic carbons as the side chain may
improve the proton conductivity especially at a low RH by
forming a well-developed phase separation.
2
2 A. Kabasawa, J. Saito, H. Yano, K. Miyatake, H. Uchida, M.
Watanabe, Electrochem. Acta 2009, 54, 1076–1082.
2
3 N. Gao, F. Zhang, S. B. Zhang, J. Liu, J. Membr. Sci. 2011,
3
72, 49–56.
2
4 D. S. Kim, Y. S. Kim, M. D. Guiver, B. S. Pivovar, J. Membr.
Sci. 2008, 321,199–208.
ACKNOWLEDGMENTS
2
5 C. S. Marvel, C. F. Bailey, M. S. Sparburg, J. Am. Chem.
This work was supported by Industrial Technology Research
Grant Program in 2011 (#11B02008c) from New Energy and
Industrial Technology Development Organization (NEDO) of
Japan.
Soc. 1927, 49, 1833–1837.
26 L. Sheng, T. Higashihara, S. Nakazawa, M. Ueda, Polym.
Chem. 2012, 3, 3289–3295.
27 L. Sheng, T. Higashihara, R. Maeda, T. Hayakawa, M. Ueda,
J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2216–2224.
2
8 Y. S. Kim, D. S. Kim, B. Liu, M. D. Guiver, B. S. Pivovar, J.
REFERENCES AND NOTES
Electrochem. Soc. 2008, 155, B21–B26.
1
J. Roziere, D. J. Jones, Annu. Rev. Mater. Res. 2003, 33, 503–
29 K. Hase, T. Nakano, A. Koiwai, ECS Trans. 2007, 11, 159–
555.
164.
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