Block copolystyrene derivatives having flexible alkylsulfonated side chains and hydrophobic alkoxy chains as a proton exchange membrane for fuel cell application

Article abstract of DOI:10.1002/pola.26603

A series of block copolystyrene derivatives, poly{[4-(4-sulfobutyloxy) styrene]_x-block-[4-(n-butoxystyrene)]_y} (PSBOS _x-b-PnBOS_y), containing a flexible alkylsufonated side chain and hydrophobic alkoxy chain with various ion exchange capacities (IECs) have been synthesized based on living anionic polymerization. The resulting crosslinked membranes were prepared using 4,4′-methylene-bis[2,6- bis(hydroxyethyl)phenol] as the crosslinker in the presence of methanesulfonic acid. The crosslinked PSBOS_2.2-b-PnBOS₁ membrane with IEC of 2.89 mequiv g^-1 displays a high proton conductivity (0.01 S cm^-1) at 30% relative humidity and 80 °C, which is comparable to that of Nafion. The well-developed phase separation and the continuous hydrophilic domains in the crosslinked PSBOS_2.2-b-PnBOS₁ membranes have been observed in a transmission electron microscope image. Moreover, the dynamic mechanical analysis measurement and Fenton's reagent testing show that the crosslinked PSBOS_x-b-PnBOS_y membranes have good mechanical properties and oxidative stability. These results indicate that the introduction of flexible alkylsulfonated side chains to the polystyrene main chains positively affect both the proton conductivity and oxidative stability.

Full text of DOI:10.1002/pola.26603

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Block Copolystyrene Derivatives Having Flexible Alkylsulfonated Side
Chains and Hydrophobic Alkoxy Chains as a Proton Exchange Membrane
for Fuel Cell Application
Li Sheng, Tomoya Higashihara, Rina Maeda, Teruaki Hayakawa, Mitsuru Ueda
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
Correspondence to: M. Ueda (E-mail: ueda.m.ad@m.titech.ac.jp)
Received 14 December 2012; accepted 19 January 2013; published online 19 February 2013
DOI: 10.1002/pola.26603
ABSTRACT: A series of block copolystyrene derivatives,
poly{[4-(4-sulfobutyloxy)styrene]x-block-[4-(n-butoxystyrene)]y}
(PSBOSx-b-PnBOSy), containing a flexible alkylsufonated side
chain and hydrophobic alkoxy chain with various ion exchange
capacities (IECs) have been synthesized based on living anionic
polymerization. The resulting crosslinked membranes were
prepared using 4,40-methylene-bis[2,6-bis(hydroxyethyl)phenol]
as the crosslinker in the presence of methanesulfonic acid. The
crosslinked PSBOS2.2-b-PnBOS1 membrane with IEC of 2.89
PnBOS1 membranes have been observed in a transmission
electron microscope image. Moreover, the dynamic mechanical
analysis measurement and Fenton’s reagent testing show that
the crosslinked PSBOSx-b-PnBOSy membranes have good me-
chanical properties and oxidative stability. These results indi-
cate that the introduction of flexible alkylsulfonated side chains
to the polystyrene main chains positively affect both the proton
C
V
conductivity and oxidative stability.
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mequiv gꢀ1 displays a high proton conductivity (0.01 S cmꢀ1
)
at 30% relative humidity and 80 ꢁC, which is comparable to
that of Nafion. The well-developed phase separation and the
continuous hydrophilic domains in the crosslinked PSBOS2.2-b-
KEYWORDS: anionic polymerization; block copolymers; poly-
electrolytes; proton conductivity; proton exchange membrane
INTRODUCTION Polymer electrolyte membrane fuel cells
(PEMFCs) have received tremendous attention because of
their potential to achieve higher efficiencies than current
power sources using a renewable fuel at a low environmen-
tal cost.1–3 In PEMFCs, the perfluorosulfonic acid polymers,
mechanical strength.9 Recently, some investigations of PEMs
revealed that the proton transport properties are dependent
on the polymer morphology, in particular, the nanochannel
that contain the sulfonic acid groups.10–14 The ‘‘hydrated’’
protons can efficiently pass through the proton nanochan-
nels,15–17 which should increase the proton conductivity
even at a low relative humidity (RH).
R
such as NafionV (DuPont), have been widely utilized as pro-
ton exchange membranes (PEMs), serving as both a cell sep-
arator and an electrolyte. However, the high cost, high meth-
anol/gas diffusion, and the limited operating temperature of
the perfluorinated polymers have significantly prevented
their widespread use.4–6 Thus, intensive efforts have been
devoted to the development of low-cost proton-conducting
sulfonated hydrocarbon polymers used at elevated tempera-
tures to reduce the impurity poisoning of the PEMFCs.7,8
The block copolymers containing perfectly alternating hydro-
philic and hydrophobic segments have received increasing
attention because of the enhanced conductivity when com-
pared to the similar structured random copolymers with
similar IEC values. It is well established that block copoly-
mers, having immiscible segments, self-assemble into a range
of ordered, periodic structures through the process of micro-
phase separation.18,19 The morphology and properties of
PEMs based on sulfonated polystyrene-containing block
copolymers have been extensively studied.20–29 However, the
oxidative stability and mechanical strength have been far
less investigated in these reports because polystyrene is not
considered to be an ideal material for PEM application due
to its poor oxidative stability of the benzylic CAH bond. On
the other hand, our previous study showed an interesting
The proton conductivity could be one of the most significant
characteristics of the PEMs for use in PEMFCs. The proton
conductivity of sulfonated hydrocarbon polymers is strongly
dependent on the sulfonic acid group contents, that is, the
ion exchange capacity (IEC) values. These polymers show a
suitable conductivity only at high IEC values. However, high
IEC values would result in some negative effects on the
properties of the PEMs, such as dimensional stability and
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finding, the introduction of flexible alkylsulfonated side
chains to the random copolystyrene main chains improved
the oxidative stability probably due to prevention or reduc-
tion of the hydroxyl radical attack on the polymer main
chain, and also increased the proton conductivity at a low
RH.30 An attractive way to obtain a well-organized phase
separation is to fabricate block copolymers consisting of
hydrophilic and hydrophobic blocks. Some sulfonated multi-
block copolymers have been reported, which form nano-size
water channels for proton transportation, resulting in less
dependence of the humidity on the proton conductivity, as
well as improving the mechanical properties compared to
the random copolymers with the same IEC.37 In this study, a
series of block copolystyrene derivatives having flexible
alkylsulfonated side chains and hydrophobic alkoxy chains
have been synthesized and the relationship between the
block polymer structure and the membrane properties, such
as the water uptake (WU), oxidative stability, mechanical
properties, proton conductivity, morphology, was also investi-
gated in detail.
filled with nitrogen three times. Then anhydrous DMF (150
mL) and 1-bromobutane (32.5 mL, 300 mmol) were added
via syringe through the septum cap. The reaction mixture
was magnetically stirred at 60 ꢁC under nitrogen overnight.
The mixture was cooled to room temperature, and then was
poured into water. After the mixture was extracted by ethyl
ether, the organic layer was washed with water three times
and dried with magnesium sulfate. The mixture was filtered,
and the solvent was evaporated under educed pressure. The
product (BOS) was isolated by column chromatography on
silica gel (hexane).
Yield 31.7 g, (90%). 1H NMR (300 MHz, DMSO-d6, d): 0.96–
1.95 (t, 3H, ACH2CH3A), 1.44–1.58 (m, 2H, ACH2CH2CH3A),
1.70–1.83 (m, 2H, ACH2CH2CH3A), 4.00–4.09 (t, 2H,
AOCH2CH2A), 5.13–5.21 (d, 1H, ACH¼¼CH2), 5.65–5.76 (d,
1H, ACH¼¼CH2), 6.65–6.79 (q, 1H, ACH¼¼CH2), 6.91–7.00 (d,
2H, ACHA in Ph), 7.40–7.48 (d, 2H, ACHA in Ph).
BOS was further purified by washing sequentially with
diluted aqueous base, water followed by drying with magne-
sium sulfate, and distilled under reduced pressure over
finely ground CaH2 and stored in the glovebox.
EXPERIMENTAL
Materials
4-Vinylphenyl acetate, 1,4-butanesultone, 1-bromobutane, 4-
tert-butoxystyrene (tBOS), 1,4-dioxane, hydrochloric acid
(HCl), and dimethylsulfoxide (DMSO, anhydrous) were pur-
chased from TCI. sec-Butyllithium in cyclohexane, n-hexane
Synthesis of PtBOSx-b-PnBOSy (x and y Refer to the
Molar Ratio of Each Unit Calculated from Gel
(s-BuLi), and di-n-butylmagnesium (1.0
M solution in
Permeation Chromatography)
heptanes) were purchased from Sigma-Aldrich. tBOS was
purified by washing sequentially with diluted aqueous base,
water followed by drying with magnesium sulfate (dried),
and distilled under reduced pressure from finely ground
CaH2 and stored in the glovebox under the purified nitrogen
atmosphere. The other reagents were used as received.
Hydrazine hydrate, 2,20-azobis(isobutyronitrile) (AIBN), and
tetrahydrofuran (THF, super dehydrated) were purchased
from Wako. 4,40-Methylene-bis[2,6-bis(hydroxyethyl)phenol]
(MBHP) was prepared according to the previous report.33
PtBOSx-b-PnBOSy block copolymers composed of different
length blocks were synthesized through anionic polymeriza-
tion in THF at ꢀ78 ꢁC using sec-BuLi as an initiator. All the
polymerizations were performed in the glovebox filled with
purified nitrogen (O2 < 1 ppm, H2O < 1 ppm). The polymer-
ization temperature was easily controlled by a cooling reac-
tor (UC reactor, Techno-Sigma) inside the glovebox. A general
procedure is described below using PtBOS1-b-PnBOS1.3, con-
sisted of a poly(4-tert-butoxystyrene) (PtBOS) segment with
the target Mn of 20 ꢂ 103 and a poly(n-butoxystyrene)
(PnBOS) segment with an Mn of 30 ꢂ 103, as an example.
Synthesis of p-(n-Butoxy)styrene
To a 100-mL dry one-necked flask were added anhydrous
THF (10.0 mL) and tBOS (1.07 mL). A suitable quantity of
di-n-butylmagnesium was also added at ꢀ78 ꢁC to remove
residual protic sources, followed by adding the sec-BuLi
(0.050 mL, 0.050 mmol) solution in cyclohexane to initiate
the polymerization. A bright yellow-orange color appeared
corresponding to the carbanion. The polymerization was
allowed to perform for 1 h and some drops of polymer solu-
tion were transferred to ethanol for measuring molecular
weight by gel permeation chromatography (GPC). The THF
(10.0 mL) solution of (n-butoxy)styrene (nBOS; 2.14 mL)
and a suitable quantity of di-n-butylmagnesium was trans-
4-Vinylphenol (VPO) was prepared from 4-vinylphenyl ace-
tate according to the procedures in the literature with minor
modification.34 After stirring the ethanol/NaOH solution of
4-vinylphenyl acetate at room temperature for 5 h, the sol-
vent was evaporated, then the residue was dissolved in ethyl
acetate. The organic layer was washed with a citric acid so-
lution and water several times, followed by drying with an-
hydrous magnesium sulfate. The mixture was filtered, and
the solvent was evaporated under reduced pressure. The
product was isolated by drying under reduced pressure at
room temperature for 3 h.
Yield: 85%. 1H NMR (300 MHz, DMSO-d6, d): 5.01–5.07 (d,
1H, ACH¼¼CH2), 5.53–5.61 (d, 1H, ACH¼¼CH2), 6.56–6.68 (q,
1H, ACH¼¼CH2), 6.72–6.79 (d, 2H, ACHA in Ph), 7.24–7.31
(d, 2H, ACHA in Ph), 9.37–9.45 (s, 1H, AOHA).
ꢁ
ferred into the above PtBOS anion solution at ꢀ78 C. Upon
the addition of nBOS the color changed immediately to the
bright yellow. After another 2 h, ethanol was added to termi-
nate the polymerization. The block polymer was isolated by
ꢁ
To a 250-mL dry flask were added VPO (24.0 g, 200 mmol)
and K2CO3 (41.4 g, 300 mmol). This flask was evacuated and
precipitation in methanol, filtered, and dried at 40 C for 20
h in vacuum.
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Synthesis of PHSx-b-PnBOSy
IEC was measured by a titration method. The samples were
immersed in saturated NaCl solution at room temperature
for 3 days. After that, the samples were not taken out and
the NaCl solution was directly titrated with 0.02 M NaOH
using phenolphthalein as pH indicator.
The tert-butyl group of PtBOS1-b-PnBOS1.3 (2.00 g, 4.94
mmol of PtBOS) was deprotected by treating with hydrochlo-
ric acid (37 wt %, 3.00 mL) in 1,4-dioxane (26.0 mL) at 80
ꢁC overnight, followed by cooling to room temperature. The
mixture was carefully poured into water with stirring, then
the polymer was washed with water several times, and dried
The oxidation stability of the membranes was determined by
Fenton’s test. The samples were soaked in the Fenton’s rea-
gent (3% H2O2 containing 20 ppm FeSO4) at room tempera-
ture. The disappearance time of the samples was used to
evaluate the radical oxidation stability of the membranes.
ꢁ
at 80 C for 24 h in vacuum. Yield 1.67 g, (97%).
Synthesis of PSBOSx-b-PnBOSy
The mixture of PHS1-b-PnBOS1.3 (0.872 g, 2.50 mmol), so-
dium hydride (0.120 g, 5.00 mmol), and 1,4-butanesultone
(0.681 g, 5.00 mmol) were stirred in THF (8.00 mL) at 60 C
overnight, and then another 8 mL DMSO was added into the
WU of PSBOSx-b-PnBOSy membranes was measured by plac-
ing the membrane in a thermocontrolled humid chamber.
The membranes were set at desired values and kept con-
stant for 2 h at each point. Then the membrane was taken
out, and quickly weighed on a microbalance. WU was calcu-
lated according to the following equation:
ꢁ
ꢁ
mixture. The mixture was stirred at 80 C for 24 h, and was
then poured into methanol. The isolated polymer was dried
ꢁ
at 60 C for 5 h in vacuum. Yield 1.24 g, (98%).
Crosslinked Membrane Formation and Proton Exchange
The PSBOSx-b-PnBOSy in their sodium salt form and the
crosslinker, MBHP (3 mol % to the polymer) were dissolved
in DMSO/THF (5/1 w/w) to give a 5–8 w/v % solution,
then one drop of methanesulfonic acid in DMSO solution was
added. The solution was cast onto a glass plate and heated
at 80 ꢁC for 36 h to give a transparent membrane. The as-
cast membrane was immersed in water to remove any resid-
ual solvent.
WU ¼ ðWs ꢀ WdÞ=Wd ꢂ 100 wt %
(1)
where Ws and Wd are the weights of wet and dried mem-
branes, respectively. Water sorption was also expressed as
the average number of water molecules per ion exchange
site ([H2O]/[SOꢀ3 ]), often referred to as k value, and calcu-
lated according to the following equation:
k ¼ ½H2Oꢃ=½SOꢀ3 ꢃ ¼ WU ð%Þ ꢂ 10=½18 ꢂ IEC ðmmol gꢀ1Þꢃ (2)
Proton exchange treatment was performed by immersing the
crosslinked PSBOSx-b-PnBOSy membrane in 1 M sulfuric acid
solution at room temperature for 3 days. The membrane was
taken out, and rinsed with deionized water till the rinsed
water became neutral.
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, and the
changes of length and thickness were calculated from:
Dl ¼ ðls ꢀ ldÞ=ld ꢂ 100%
Dt ¼ ðts ꢀ tdÞ=td ꢂ 100%
(3)
(4)
Measurements
1H NMR spectrum was recorded on a Bruker DPX300S spec-
trometer using CDCl3 or DMSO-d6 as solvent. Number- and
weight-average molecular weights (Mn and Mw) were meas-
ured by GPC on a Hitachi LC-7000 system equipped with
polystyrene gel columns (TSKgel GMHHR-M) eluted with
chloroform (CHCl3) at a flow rate of 1.0 mL minꢀ1 calibrated
by standard polystyrene samples.
where ls and ts are the length and thickness of the wet mem-
brane, respectively, ld and td refer to those of the dried
membrane.
Proton conductivity was measured using a two-probe elec-
trochemical impedance spectroscopy technique over the fre-
quency from 5 Hz to 1 MHz (Hioki 3532-80). The samples
were 8 mm long, 8 mm wide, and 30–55 lm thick. Proton
conductivity (r) was calculated from the following equation:
Thermogravimetric analysis (TGA) was performed in nitro-
gen with a Seiko EXSTAR 6000 TG/DTA 6300 thermal instru-
ment. Before measurement, all the samples were preheated
ꢁ
at 100 C for 30 min to remove any absorbed moisture. Sub-
sequently, the samples were cooled to 50 ꢁC and then
r ¼ d=ðtswsRÞ
(5)
ꢀ1
ꢁ
ꢁ
reheated to 550 C at a heating rate of 10 C min
.
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.
Dynamic mechanical analysis (DMA) was measured on a
Seiko DMA 6300 apparatus in tension mode at a frequency
ofꢁ1.0 Hz under nitrogen atmosphere at a heating rate of
2 C minꢀ1. The samples were 10 mm long, 4 mm wide, and
35–52 lm thick.
For transmission electron microscopy (TEM) observations,
the membranes were stained with lead ions by ion exchange
of the sulfonic acid groups in a 2-wt % Pb(NO3)2 aqueous
solution for 3 days, immersed into deionized water for 1
days, and dried at 60 ꢁC overnight. The stained membranes
were embedded in an epoxy resin, sectioned to 90 nm thick-
ness at ꢀ50 ꢁC using a microtome Ultracut UCT equipped
Thermomechanical analysis (TMA) was measured on a Seiko
TMA/SS 6100 apparatus at room temperature and 60% RH.
For each kind of membranes, two samples were used for
measurements and the samples were 12 mm long, 3 mm
wide, and 35–60 lm thick.
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PnBOS3.2, for example, had the number-averaged molecular
weight of 1.11ꢂ105 and PDI of 1.06. The PtBOSx-b-PnBOSy
was converted to PHSx-b-PnBOSy by treatment with an HCl
solution, and the PSBOSx-b-PnBOSy was produced by the
reaction of PHSx-b-PnBOSy with the 1,4-butanesultone. The
chemical structure of the synthesized nBOS was confirmed
by 1H NMR (Fig. 1). The characteristic peaks of the methyl
protons at 1.01 ppm and methylene protons next to the
ether group at 4.04 ppm were observed. The other peaks
were in good agreement with the proposed structure. Figure
2 shows the 1H NMR spectra of the block copolymer (a)
before and (b) after hydrolysis. The characteristic signal
around 1.28 ppm assigned to the tert-butyl protons in the
initial PtBOSx-b-PnBOSy block copolymer disappeared in the
deprotected PHSx-b-PnBOSy copolymer, and a broad peak
around 8.77 ppm corresponding to the phenol protons
appears after the acidic cleavage, indicating that the depro-
tection was completed. In addition, the compositions of the
PtBOSx-b-PnBOSy block polymers were determined using the
relative intensities between the aromatic protons of the phe-
nyl rings and the methyl group of the PnBOSy, which were
SCHEME 1 Synthesis of p-(n-butoxy)styrene (nBOS).
with a cryo unit of Leica EMFCS, and placed on a copper
grid. The TEM images were taken on a Hitachi H7650 with
an accelerating voltage of 100 kV.
Small-angle X-ray scattering (SAXS) was performed using
Bruker NanoSTAR. The scattering was recorded by a charge-
coupled device. The intensity of scattering peaks is a func-
tion of the magnitude of scattering vector q ¼ 4psinh/k,
where k is the X-ray wavelength of 1.54 Å (Cu Ka radiation)
and 2e` is scattering angle. The d-spacing was calculated
from Bragg’s equation, d ¼ 2ð/q* with q* being the position
of primary scattering peak.
1
similar to that determined from the GPC. The H NMR spec-
trum of PSBOSx-b-PnBOSy showed characteristic methylene
protons next to the sodium sulfonate group at 2.73 ppm and
the hydroxyl protons completely disappeared (Fig. 3).
RESULTS AND DISCUSSION
Synthesis of PHSx-b-PnBOSy and PSBOSx-b-PnBOSy
nBOS was prepared via a two-step reaction (Scheme 1), that
is, the hydrolysis of the acetate group of 4-vinylphenyl ace-
tate, followed by the reaction with 1-bromobutane. As shown
in Scheme 2, a series of block copolystyrene derivatives,
PtBOSx-b-PnBOSy, were prepared by the anionic polymeriza-
tion of nBOS and tBOS using sec-BuLi as the initiator at
Preparation of Crosslinked PSBOSx-b-PnBOSy Membranes
The crosslinked PSBOSx-b-PnBOSy membranes were prepared
according to our previous report with minor modification
(Scheme 3).30 A solution of PSBOSx-b-PnBOSy, 3 mol% of
MBHP and a catalytic amount of methanesulfonic acid in
ꢁ
ꢁ
ꢀ78 C. The length of each unit was adjusted by controlling
DMSO was cast on a glass plate. The film was dried at 80 C
the molecular weight of PtBOSx and PnBOSy (Table 1). All
the prepared block polymers displayed fairly high molecular
weights and a low polydispersity index (PDI). PtBOS1-b-
for 36 h under ambient conditions, and the obtained mem-
brane was then immersed in water. After the crosslinking
reaction, the film became insoluble in water, NMP, and
SCHEME 2 Synthesis of PHSx-b-PnBOSy and PSBOSx-b-PnBOSy.
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TABLE 1 The Molecular Weights and the Compositions of PtBOSx-b-PnBOSy
Samples
Target Mn
Mn (PDI)a of PtBOS
Total Mn (PDI)a
PtBOS2.2-b-PnBOS1
PtBOS1-b-PnBOS1.3
PtBOS1-b-PnBOS3.2
30,000–10,000
20,000–30,000
30,000–90,000
27,600 (1.19)
21,600 (1.05)
26,400 (1.06)
40,300 (1.16)
49,900 (1.04)
111,000 (1.06)
a
Determined from GPC.
DMSO, which indicated that the crosslinking reaction suc-
cessfully took place.
the weight loss of the membranes as a function of time. In
this study, 3% hydrogen peroxide containing 20 ppm FeSO4
was used as Fenton’s reagent at room temperature. Table 2
shows the disappearance time of the crosslinked PSBOSx-b-
PnBOSy membranes (the size of each sheet: 10 mm long and
7 mm wide). The crosslinked PSBOS1-b-PnBOS3.2 membrane
with the lowest IEC value resisted soaking in Fenton’s rea-
gent for more than 70 h. It can be seen from Table 2, the
oxidative stability is closely related to the IEC levels, and the
higher IEC, the lower oxidative stability. However, all the pre-
pared crosslinked membranes displayed a better oxidative
stability against Fenton’s reagent compared to the other sul-
fonated polymers with similar IEC values such as the poly
(ether sulfone)31,32 which indicated that the introduction of
flexible side chains can effectively improve the oxidative sta-
bility. One of the reasons for the high oxidative stability is
probably that the main chains with the benzylic CAH bonds
are well separated from the sulfonic acids, by which the
hydroxy radical attack on the membrane could be signifi-
cantly suppressed. It is interesting to compare the stability
of the block copolystyrene membranes to that of the random
copolystyrene which we have already reported.30 Although
the crosslinked PSBOS2.2-b-PnBOS1 membranes had a slightly
higher IEC than the crosslinked PBOS3-r-PSBOS7, the stability
was much better for the former than for the latter, which
indicated that the more organized morphologies may also
enhance the oxidative stability.
Thermal Stability and Mechanical Properties
The thermal stability of the crosslinked PSBOSx-b-PnBOSy
membranes in their proton form was evaluated by TGA
under nitrogen (Fig. 4). Before the measurement, the sam-
ples were preheated at 100 ꢁC for 30 min to remove any
absorbed moisturꢁe. A three-step weight loss was observed
ꢁ
ꢁ
from 150 to 200 C, from 250 to 350 C, and above 350 C.
The first stage weight loss is due to the dehydration between
the sulfonic acids and desulfonation reaction.35,36 The sec-
ond- and third-stage weight losses are attributed to the
decomposition of the polymer side chains and main chains,
respectively.
The mechanical property was characterized by the maximum
storage modulus (E0), maximum stress (MS), and elongation
at break (EB). These results are listed in Table 2. The E0 and
MS values increased when the decreasing IEC values of the
crosslinked PSBOSx-b-PnBOSy membranes. The E0 and MS of
the crosslinked PSBOS1-b-PnBOS1.3 are 371 and 30 MPa,
respectively. The MS value is two times higher than the
crosslinked PSBOS2.2-b-PnBOS1.
Oxidative Stability
Fenton’s test is a common method which has been widely
used for evaluating the oxidative stability of PEMs. The mem-
branes are generally immersed in 3% hydrogen peroxide
containing 20 ppm FeSO4 at room temperature or 3% hydro-
gen peroxide containing 3 ppm FeSO4 at elevated tempera-
ture (e.g., 80 ꢁC), and the oxidative stability is characterized
either by the disappearance time of the membranes or by
FIGURE 2 1H NMR spectra of PtBOSx-b-PnBOSy (a) and PHSx-
b-PnBOSy (b).
FIGURE 1 1H NMR spectrum of nBOS.
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FIGURE 4 TGA curves of the crosslinked PSBOSx-b-PnBOSy (in
their proton form) in nitrogen.
FIGURE 3 1H NMR spectrum of PSBOSx-b-PnBOSy.
mequiv gꢀ1) displays a low water uptake (<2 wt %), corre-
sponding to ꢄ1 water molecule per sulfonic acid group at
30% RH.
Ion Exchange Capacity, Water Uptake, and Dimensional
Change
The IEC values of the crosslinked PSBOSx-b-PnBOSy mem-
branes were determined by a titration method, and the
measured IECs were in the range of 1.39–2.89 mequiv gꢀ1
(Table 1). It is well known that the water uptake of most ar-
omatic PEMs strongly affects the proton conductivity because
water within the membrane acts as a carrier of the protons.
The water uptakes and hydration number (k) of the cross-
linked PSBOSx-b-PnBOSy membranes and Nafion 117 are
shown in Figure 5(A,B), respectively. As shown in Figure
5(A), the water uptakes of the membranes are in the follow-
The dimensional change in the crosslinked membranes was
measured in both the length and thickness directions [Fig.
6(A,B)]. All the crosslinked membranes exhibit an isotropic
swelling, and the dimensional change increases with the
increasing IEC and RH. The crosslinked PSBOS2.2-b-PnBOS1
membrane with the highest IEC among these membranes
displays a dimensional change lower than 20% even at 95%
RH.
Proton Conductivity
ing order: PSBOS2.2-b-PnBOS1
> PSBOS1-b-PnBOS1.3 >
Figure 7 shows the variation in the proton conductivity of
the crosslinked PSBOSx-b-PnBOSy membranes at 80 ꢁC as a
function of RH, in which the data of Nafion 117 obtained in
PSBOS1-b-PnBOS3.2, which is just consistent with the IEC
order, and increases with the increasing RH. The crosslinked
PSBOS2.2-b-PnBOS1 membrane with the highest IEC value
(2.89 mequiv gꢀ1) shows the highest water uptake among
these membranes. The water uptake is around 70 wt % at
95% RH, which corresponds to an absorption of 12 water
molecules per sulfonic acid group. The water uptake and k
dramatically decreases when decreasing the RH, The
PSBOS1-b-PnBOS3.2 membrane with the low IEC value (1.39
TABLE 2 Ion exchange capacities (IEC), disappeared time (s)
after Fenton’s test, maximum stress (MS), elongation at break
(EB), and maximum storage moduli (E0s) of the crosslinked
PSBOSx-b-PnBOSy membranes
IEC (mequiv
gꢀ1
)
s
E0
MSc
EBc
(%)
Polymer
Cal.
2.97
Titr.a
2.89
(h)b
(108 Pa)
(MPa)
PSBOS2.2
-
54
70
71
–
2.25
15
30
45
26
15
7
b-PnBOS1
PSBOS1-b-
PnBOS1.3
2.06
1.22
2.03
1.39
0.91
3.71
PSBOS1-b-
PnBOS3.2
Nafion 117d
7.95
8
e
–
0.4–0.5
200
a
By titration method.
b
Measured by soaking the membranes in Fenton’s reagent (3% H2O2
containing 20 ppm FeSO4) at room temperature.
c
Measured at room temperature and 60% relative humidity.
Cited from ref. 30.
d
SCHEME 3 Chemical structures of the crosslinker and cross-
e
linked membrane.
No data in this reference.
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FIGURE 5 (A) Water uptake and (B) hydration number (k) of
crosslinked PSBOSx-b-PnBOSy membranes and Nafion 117 at
different RH and 80 ꢁC.
FIGURE 6 Dimensional change of crosslinked PSBOSx-b-
PnBOSy membranes at different RH and 80 ꢁC: (A) change in
length and (B) change in thickness.
Notably, the difference between the proton conductivity and
IEC value is smaller for the PSBOSx-b-PnBOSy block copoly-
mers than for the PBOSx-r-PSBOSy random copolymers at a
low RH.30 This is likely related to the differences in
this study and the crosslinked PBOS2-r-PSBOS8 obtained in
our previous work30 are presented for comparison. Although
the proton conductivities of the crosslinked PSBOS1-b-
PnBOS1.3 and PSBOS1-b-PnBOS3.2 membranes are lower than
that of Nafion 117 in the RH range below 70%, these mem-
branes exhibit higher or comparable proton conductivities
compared to Nafion 117 at 95% RH. On the other hand, the
crosslinked PSBOS2.2-b-PnBOS1 membrane shows a fairly
high proton conductivity, which is comparable to that of
Nafion 117 at 30% RH and higher than that of Nafion 117 in
the RH range above 50%, especially, almost 2.5 times higher
at 95% RH. Furthermore, the block copolymers display
higher proton conductivities than the corresponding random
copolymers despite the lower IEC values. The crosslinked
PSBOS2.2-b-PnBOS1 membrane has the lower IEC value of
2.89 mequiv gꢀ1 than, for example, that of the random copol-
ymer PBOS2-r-PSBOS8 membrane (3.03 mequiv gꢀ1), how-
ever, the proton conductivities of the former membrane are
much higher than the latter membrane over the entire RH
range. The proton conductivity of the block copolymers fol-
lows the typical proton conductivity behavior of the sulfo-
nated aromatic polymers, depending on the IEC values of the
membranes, the higher IEC, the higher proton conductivity.
FIGURE 7 Proton conductivity of crosslinked PSBOSx-b-
PnBOSy membranes and Nafion 117 at different RH and 80 ꢁC.
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FIGURE 8 TEM image of the crosslinked PSBOS2.2-b-PnBOS1 membrane (L) and PSBOS1-b-PnBOS1.3 membrane (R), scale bar ¼
100 nm.
morphology of the random and block copolymers. The
PSBOSx-b-PnBOSy membranes display a more well-organized
microphase separation than that of the random PBOS2-r-
PSBOS8. Although the crosslinked PSBOS1-b-PnBOS1.3 has a
low IEC (2.03 mequiv gꢀ1), the phase separation between
the hydrophilic and hydrophobic domains is clearly observed
and the matrix of the hydrophilic domains is well connected
to each other.
agreement with the averaged distance between the domains
observed in the TEM images. Moreover, the SAXS line profile
of the crosslinked PSBOS1-b-PnBOS1.3 membranes obviously
displayed the first- and second-order peaks with q values of
0.019 and 0.034 Åꢀ1, thus having the q value ratio of these
peaks of 1:H3. This is associated with the formation of a
hexagonally packed microphase-separated structure in the
membrane.
Transmission Electron Microscopy and Small-Angle
X-Ray Scattering
CONCLUSIONS
A series of PSBOSx-b-PnBOSy block copolymers having flexi-
ble alkylsulfonated side chains and hydrophobic alkoxy
chains were prepared by anionic polymerization of nBOS and
tBOS, the deprotection of the tert-butyl ether group, followed
by the reaction of the resulting PHSx-b-PnBOSy with 1,4-buta-
nesultone. The crosslinked PSBOSx-b-PnBOSy membranes
The nanostructure and morphology of the crosslinked
PSBOS2.2-b-PnBOS1 and PSBOS1-b-PnBOS1.3 membranes were
investigated by TEM using cross-sectional slices of dry mem-
branes and SAXS. The membranes were stained in an aque-
ous lead acetate solution before imaging; therefore, the dark
areas in the TEM correspond to the region of hydrophilic
domains containing sulfonic acid groups. As can be seen in
Figure 8, a phase separation between the hydrophilic and
hydrophobic domains of both membranes is clearly
observed. The interdistance of each domain averages 35 nm
for the crosslinked PSBOS2.2-b-PnBOS1 membrane and 30 nm
for the crosslinked PSBOS1-b-PnBOS1.3 membrane. The
amphiphilic nature of PSBOSx-b-PnBOSy leads to an enhanced
phase separation and aggregation of the ionic cluster. More-
over, the matrices of the hydrophilic domains are more well
connected to each other than that of random copolystyrene
derivative.30 Figure 9 shows the SAXS intensity profiles for
the crosslinked PSBOSx-b-PnBOSy (in their proton form) in
which the membrane was solution-cast from a mixture of
DMSO/THF (5/1 w/w). The SAXS profile also strongly sup-
ported the formation of a microphase-separated nanostruc-
ture in the resulting membranes. For example, the first scat-
tering peaks of the crosslinked PSBOS2.2-b-PnBOS1 and
PSBOS1-b-PnBOS1.3 membranes at 0.016 and 0.019 Åꢀ1
show the formation of the nanostructure with the domain
spacing of ꢄ39 and 33 nm, respectively, which are in good
FIGURE 9 Small-angle X-ray scattering profiles for crosslinked
PSBOS2.2-b-PnBOS1 and PSBOS1-b-PnBOS1.3 membranes.
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13 Y. Chang, G. F. Brunello, J. Fuller, M. Hawley, Y. S. Kim, M.
D. Miller, M. A. Hickner, S. S. Jang, C. Bae, Macromolecules
2011, 44, 8458–8469.
were prepared by the reaction of PSBOSx-b-PnBOSy with
MBHP as the crosslinker in the presence of methanesulfonic
acid. The crosslinked PSBOSx-b-PnBOSy membranes showed a
higher oxidative stability against Fenton’s reagent at room
temperature, lower water uptake at 95% RH, and good me-
chanical property than those of the corresponding cross-
linked PBOS2-r-PSBOS8 random copolymer membranes. Fur-
thermore, the crosslinked PSBOS2.2-b-PnBOS1 membrane
exhibited a higher proton conductivity than Nafion 117 over
the entire RH range. TEM images showed a well-developed
phase separation of the crosslinked PSBOS2.2-b-PnBOS1 and
PSBOS1-b-PnBOS1.3 membranes.
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Grant Program in 2011 (#11B02008c) from New Energy and
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