Synthesis and characterization of benzoxazine-containing, crosslinkable, and sulfonated polymer through Diels-Alder reaction for direct methanol fuel cells

Article abstract of DOI:10.1016/j.polymer.2013.02.037

Introduction of cross-linked structure to sulfonated polymers is an effective approach to reduce the methanol permeability of the polymers for application in direct methanol fuel cells. In this work, a crosslinkable sulfonated polymer (sPBz) has been prep

Full text of DOI:10.1016/j.polymer.2013.02.037

Polymer 54 (2013) 2096e2104
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Polymer
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Synthesis and characterization of benzoxazine-containing, crosslinkable, and
sulfonated polymer through DielseAlder reaction for direct methanol fuel cells
,
Nan-Hsun Chen a, Hsieh-Yu Li b, Juin-Yih Lai a, Ying-Ling Liu b
*
a Department of Chemical Engineering and R&D Center for Membrane Technology, Chung Yuan Christian University, Chungli, Taoyuan 32023, Taiwan
b Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Introduction of cross-linked structure to sulfonated polymers is an effective approach to reduce the
methanol permeability of the polymers for application in direct methanol fuel cells. In this work, a
crosslinkable sulfonated polymer (sPBz) has been prepared through DielseAlder reaction with a sulfo-
nated bismaleimide and a benzoxazine-containing bisfuran as monomers. The polymer possesses both
sulfonic acid groups and crosslinkable benzoxazine groups. The sulfonic acid group reduces the tem-
perature of the thermal polymerization of benzoxazine groups, and the polymerization of benzoxazine
group results in cross-linked sPBz polymer (CR-sPBz). CR-sPBz has showed good film formability, high
mechanical strength, flexibility, and solvent resistance. The methanol permeability and proton conduc-
Received 30 October 2012
Received in revised form
5 February 2013
Accepted 20 February 2013
Available online 27 February 2013
Keywords:
Benzoxazine
Proton exchange membrane
Direct methanol fuel cell
tivity of CR-sPBz is 2.52*10ꢀ7 cm2 ꢀ1 and 48 mS cmꢀ1 (60 ꢁC), respectively, corresponding to a selectivity
s
(proton conductivity/methanol permeability) of 1.86*105 S s cmꢀ3. The selectivity of CR-sPBz is about 3.4
times of the value of the commercially-available Nafion 117 membrane. The high selectivity of CR-sPBz
warrants its performance and suitability for use as a proton exchange membrane for direct methanol
fuel cells.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
drastically reduces the DMFC performance employing Nafion
membranes as PEMs. To reduce the methanol permeability of PEMs
Direct methanol fuel cell (DMFC) is
a
promising and
for DMFCs, modification of Nafion and other polyelectrolyte
membranes has been performed with polymer blends [3e5],
addition of inorganic fillers [6e12], and formation of electrostatic-
assembly multilayer membranes [13e15]. These efforts effectively
reduce the methanol permeability of PEMs. However, the proton
conductivities of the PEMs usually decrease simultaneously, as
second components which are not proton-conductive have been
added to the PEMs, so as to reduce their densities of sulfonic acid
groups and ion-exchange capacities.
Design and synthesis of new polyelectrolytes for use as PEMs is
another approach to effectively improve the DMFC performance.
Highly sulfonated polymers exhibiting high ion-exchange capacities
and high proton conductivities have been prepared [16,17]. In-
creases in the degree of sulfonation usually result in decreases in
membrane stability in methanol aqueous solutions and increases in
their methanol permeability. Proton conductivity and methanol
permeability are trade-offs for the prepared PEMs. This problem
might be overcome with microphase-separated polyelectrolytes
possessing sulfonated hydrophilic and non-sulfonated hydrophobic
domains [18,19]. The hydrophilic domains serve as proton-
conductive channels warranting the high proton conductivity, and
the hydrophobic domains stabilize the membranes in methanol
environmental-friendly power source which could be operated
with a relatively light and simple system providing efficient energy
density. Methanol fed to DMFCs catalytically reacts with water to
generate CO2, electrons, and protons. Protons transport from
cathode to anode through a proton exchange membrane (PEM) and
electrons move through an outer circuit from anode to cathode. As
a result, PEM could be the key component of a DMFC [1,2]. Like
other fuel cells employing PEMs, the proton conductivities of PEMs
are critical for the cell performance of DMFC. Nafion poly-
electrolytes have been the commercially-available, widely studied,
and reference materials as PEMs, as they have high proton con-
ductivities, good chemical resistance, and high mechanical
strength. On the other hand, the fed methanol fuel could cross the
PEM in DMFC, resulting in a significant loss of fuel cell performance.
As a result, the ability of PEMs to resist to methanol crossover be-
comes another critical and important issue of PEMs for DMFCs.
The commercial Nafion membranes have methanol perme-
ability of about 10ꢀ6 cm2 sꢀ1 [1]. The high methanol permeability
* Corresponding author. Tel.: þ886 3 5711450; fax: þ886 3 5715408.
E-mail address: liuyl@mx.nthu.edu.tw (Y.-L. Liu).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.02.037

N.-H. Chen et al. / Polymer 54 (2013) 2096e2104
2097
fuels. On the other hand, formation of cross-linked networks in
PEMs could be more effective to reduce the methanol permeability
of PEMs [20e25]. Luo et al. [20] reported cross-linked sulfonated
polyetheretherketone which showed a methanol permeability of
1.3*10ꢀ7 cm2 sꢀ1. Na’s group reported that cross-linked sulfonated
poly(arylene ether ketone)s showed a reduction of the methanol
permeability of the polyelectrolytes from about 10e15*10ꢀ7 cm2 sꢀ1
to about 1e2*10ꢀ7 cm2 sꢀ1 [22e25]. This modification was still
accompanied with slight decreases in the proton conductivity of the
PEMs, due to the addition of the non-sulfonated crosslinking agents.
This could be further improved with incorporation of self-
crosslinkable groups to the polyelectrolyte chains [26e29]. Sulfo-
nated polyphosphazenes possessing hydroxymethylphenoxy
groups could undergo self-crosslinking reaction [26]. The cross-
linked PEMs showed a methanol permeability of 1.0*10ꢀ7 cm2 sꢀ1
and a proton conductivity of 1.8 mS cmꢀ1. Lee et al. [28] prepared
ethynyl-terminated polyelectrolytes which showed a methanol
permeability of 1.1*10ꢀ7 cm2 sꢀ1 and a proton conductivity of
54 mS cmꢀ1 after self-crosslinking reaction.
Benzoxazine groups can undergo ring-opening crosslinking re-
action under heat without release of volatile small molecules [30].
Incorporation of benzoxazine groups to polymer chains has been
reported for preparation of crosslinkable polymers [31e33]. Poly-
benzoxazines possess high density of inter- and intra-molecular
hydrogen bonds which help to maintain water molecules in poly-
electrolytes. As a result, benzoxazine-based cross-linked poly(2,5-
benzimidazole) has been reported for the application of proton
exchange membranes [34]. Polymers possessing benzoxazine link-
ages in the main chains as curable groups have been widely studied
[32,33,35e37]. Polymer films exhibiting high glass transition tem-
peratures and flexibility have been reported [36,37]. As cross-linked
polymers are usually brittle and not easy to be fabricated into
membranes, the flexible cross-linked polymers from main-chain
polybenzoxazine precursors could be suitable candidates as PEMs
in fuel cells. On the other hand, the high temperatures required for
benzoxazine polymerization (above 200 ꢁC) is also a problem. This
problem could be overcome with incorporation of sulfonic acid
groups, which could effectively catalyze the reaction and reduce the
polymerization temperatures of benzoxazines.
acetic anhydride (Tedia), and sodium acetate (Showa) were used as
received. Reagent-grade solvents were dried and purified with the
standard methods prior to use. Bis(3-furfuryl-3,4-dihydro-2H-1,3-
benzoxazinyl) isopropane (BPA-FBz) was prepared in the Labora-
tory according to the reported method using furfurylamine and
bisphenol-A as raw materials [41].
2.2. Characterization
Fourier transform infrared (FT-IR) spectra were obtained with
Perkin Elmer Spectrum One FT-IR. 1H nuclear magnetic resonance
(NMR) spectra were recorded with a Brüker MSL 300 NMR spec-
trometer (300 MHz). Transmission electron microscopy (TEM) ob-
servations were performed with a Hitachi H-7500 TEM. Samples
were microtoned into slices in about 100 nm thickness. Differential
scanning calorimetric (DSC) thermograms were recorded with a
TA-Q100 DSC instrument under a nitrogen flow of 40 mL minꢀ1. The
temperature-modulated mode was applied to the measurement
with a heating rate of 2 ꢁC minꢀ1 and a modulated temperature of
1
ꢁC. Storage modulus and glass transition temperature were
measured with a dynamic mechanical analyzer (DMA, TA-DMA
2980). The stressestrain curve was measured with an Instron
5543 analyzer at an elongation rate of 0.5 mm minꢀ1
.
2.3. Measurement of methanol and water uptakes
The membrane was dried in a vacuum oven for 6 h to remove
the water molecules held in the membrane. The initial weight (W0)
and length (L0) of the dry membranes were measured before it was
immersed in methanol or water at 30 ꢁC. At various elapsed times,
the membrane was wiped dry with lance tissues and its weight
(Wt) and thickness (Lt) were recorded. The equilibrium Wt and Lt
values have been taken for calculation. The uptake and relative
dimensional change were calculated as the following equations:
Uptakesð%Þ ¼ ðWt ꢀ WoÞ=Woꢂ100%
Dimensional changeð%Þ ¼ ðLt ꢀ LoÞ=Loꢂ100%
Basing on the above discussion, in this work we report the
synthesis and characterization of sulfonated DielseAlder polymers
possessing self-crosslinkable benzoxazine groups for use as PEMs
for DMFCs. The sulfonated polybenzoxazine precursors have been
obtained through DielseAlder reaction using a sulfonated bisma-
leimide and a benzoxazine-containing bisfuran compound as
monomers (Scheme 1) [36,38]. The obtained polymer possesses
thermally-crosslinkable benzoxazine groups in the main chains and
sulfonic acid pendent groups. The sulfonic acid groups catalyze the
ring-opening addition reaction of benzoxazine groups [39,40] to
reduce the polymerization temperature of the benzoxazine groups
to about 120 ꢁC, compared to the high reaction temperatures
(above 200 ꢁC) of other benzoxazine compounds [30]. The cross-
linked PEM from the sulfonated DA polymer shows a methanol
permeability of 2.52*10ꢀ7 cm2 sꢀ1 and a proton conductivity of
2.4. Measurement of proton conductivity
A system based on a Solartron SI 1287 electrochemical interface
equipped with a Solartron 1255B frequency response analyzer was
applied to measure the proton conductivities of the obtained
membranes. The frequency range was 100e106 Hz and the
oscillation amplitude was 10 mV. A sample in an area of 1 cm2
was placed in an open and temperature-controlled cell. The cell
was clamped between two blocking stainless steel electrodes with
a permanent pressure of about 3 kg/cm2. Specimens were soaked in
de-ionized water before the tests. The conductivity (
samples in the transverse direction was calculated from the
impedance data, with the relationship
s) of the
s
¼ d/RS, where d and S are
48 mS cmꢀ1 at 60 ꢁC, corresponding to
a selectivity of
the thickness and face area of the sample, respectively, and R was
derived from the low intersection of the high frequency semicircle
on a complex impedance plane with the Re (Z) axis. The measure-
ment was carried out at a 95% relative humidity.
1.86*105 S s cmꢀ3. The selectivity is w3.4 times of the value
measured with the commercially-available Nafion 117 membrane.
2. Experimental
2.5. Measurement of methanol permeability
2.1. Materials
Methanol permeability was measured with a side-by-side
equipment. The amounts of methanol permeation from a 3 M
methanol aqueous solution to pure water through the membranes
were determined with a gas chromatography (GL Sciences, GC
4,40-Bis(4-aminophenoxy)biphenyl (BAPB) and maleic anhy-
dride were purchased from Aldrich and Showa Co., respectively.
Reagent-grade sulfuric acid (Fisher), fumed sulfuric acid (Merck),

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N.-H. Chen et al. / Polymer 54 (2013) 2096e2104
Scheme 1. Preparation of sulfonated bismaleimide monomer sBMIPB and the corresponding benzoxazine-containing sulfonated polymer sPBz through DielseAlder reaction.
390B). The first Fick’s law was applied for the calculation of
methanol permeability.
with water, and dried at 90 ꢁC under vacuum for 3 h. Sulfonated
BAPB (sBAPB) was obtained with a yield of 100%. FTIR (KBr): 631,
1015, 1092, and 1177 cmꢀ1 (eSO3H), 1470 cmꢀ1 (trisubstituted
phenyl group); 1H NMR (DMSO-d6, ppm):
d
¼ 7.96 (2H),
d
¼ 7.41
2.6. Preparation of sulfonated BAPB [42]
(2H),
d
¼ 6.75 (4H),
d
¼ 6.67 (2H),
d
¼ 6.58 (4H), ¼ 4.87 (4H).
d
BAPB (11 g, 30 mmol) dissolved in 18 mL of sulfuric acid was
charged into a round-bottom flask and kept at about 0 ꢁC with ice.
4.2 mL of fumed sulfuric acid was added to the flask dropwise. The
reactant mixture was kept at 0 ꢁC for 0.5 h, heated to 50 ꢁC slowly,
and then reacted at 50 ꢁC for 2 h. After being cooled to room
temperature, the solution was poured into an excess iceewater
mixture. The precipitate was collected with filtration, washed
2.7. Preparation of sulfonated 4,40-bis(4-maleimidophenoxy)
biphenyl(sBMIPB)
sBAPB (10.5 g, 20 mmol) was mixed with 40 mL DMF and 5 mL
TEA. After reaching a complete dissolution, the solution was
charged in to a 100 mL round-bottom flask and then icy cooled.

N.-H. Chen et al. / Polymer 54 (2013) 2096e2104
2099
Maleic anhydride (3.92 g, 40 mmol) was added to the solution
dropwise. The mixture was reacted at room temperature for 3 h.
The product was collected with precipitation from acetone and
filtration, washed with hot water and acetone, and then dried at
50 ꢁC under vacuum. The amic acid intermediate (10.0 g,
11.5 mmol), sodium acetate (2.0 g), and acetic anhydride (40 mL)
were dissolved in 50 mL DMF. The solution was charged into a
250 mL round-bottom flask and then reacted at 70 ꢁC for 5 h
under a nitrogen atmosphere. The product was collected with
precipitation from acetone and filtration. The crude product was
purified with repeating the dissolutioneprecipitation process
twice. Final sBMIPB product was obtained with a yield of 76%. FTIR
(KBr): 689 (CeH), 1712 (eC]O symmetric stretching) and
1772 cmꢀ1 (eC]O asymmetric stretching); 1H NMR (DMSO-d6,
ppm):
d
¼ 8.07 (2H),
d
¼ 7.60 (2H),
d
¼ 7.21 (4H),
d
¼ 7.16 (H),
d
¼ 7.04 (4H),
d
¼ 6.98 (4H).
2.8. Preparation of benzoxazine-containing sulfonated polymer
Sulfonated polymer was prepared through DielseAlder reaction
[36,38] using BPA-FBz and sBMIPB as monomers. BPA-FBz (3.76 g,
8 mmol) and sBMIPB (5.84 g, 8 mmol) were dissolved in 40 mL
NMP, respectively. The solutions were mixed together and then
charged into a 250 mL round-bottom flask. After reaction at 75 ꢁC
for 5 days, the polymer was precipitated from methanol, washed
with methanol and acetone, and dried at 50 ꢁC under vacuum. The
product was obtained with a yield of about 58% and was coded as
sPBz. FTIR (KBr): 1712 and 1772 cmꢀ1 (C]O of maleimide group),
1190 cmꢀ1 (CeN of benzoxazine ring), 1399 cmꢀ1 (CeN of mal-
eimide group), 1506 cmꢀ1 (di-substituted phenyl groups),
1467 cmꢀ1 (tri-substituted phenyl groups), 631, 1015, 1092, and
1177 cmꢀ1) (eSO3H); 1H NMR (DMSO-d6, ppm):
¼ 5.08 (benzoxazine rings), ¼ 1.52 (eCH3), ¼ 2.98 to
CHe), ¼ 6.62 (eCH ¼ CHe).
¼ 5.26 (eCHeOe) and
d
¼ 4.23 and
d
d
d
d
¼ 3.40 (e
d
d
2.9. Preparation of cured sPBz membrane (CR-sPBz)
sPBz was dissolved in DMSO to result in a 20 wt% solution. The
polymer solution was cast into a film on a glass plate with a casting
knife. The sample was then thermally treated in a vacuum oven at
80 ꢁC for 2 h and 120 ꢁC for 5 h to result in the cured polyelectrolyte
membrane (CR-sPBz).
3. Results and discussion
3.1. Synthesis and characterization of sulfonated monomer and
polymer
Fig. 1. FTIR and 1H NMR characterization of sulfonated compounds of sBAPA and
sBMIPB.
Preparation of the sulfonated bismaleimide compound sBMIPB
has been carried out with a two-step method. The first reaction is
sulfonation of BAPB with sulfuric acid [42], and the second step is
maleimidization of the sulfonated BAPB (sBAPB) with maleic an-
hydride. The obtained product sBMIPB could react with the bis-
furan compound BPA-FBz through DielseAlder reaction to result in
sulfonated and benzoxazine-containing polymer sPBz. The syn-
thetic route is shown in Scheme 1. Characterization of sBAPB and
sBMIPB has been performed with FTIR and 1H NMR. The results are
shown in Fig. 1. Sulfonation of BAPA results in the absorption peaks
of sulfonic acid groups and tri-substituted phenyl groups in FTIR
spectrum. Transformation of the amine groups of sBAPA to the
maleimide groups could be characterized with the appearance of
the absorption peaks of maleimide groups and the disappearance of
the absorption peaks of amino groups in the FTIR spectrum of
sBAPB. The results of FTIR analysis support to the functional group
transformation and the chemical structure of the prepared
compounds. The 1H NMR spectra of the compounds further char-
acterize their chemical structures. In the spectrum of sBAPB, the
protons at the ortho-position to the eSO3H group show a resonance
peak at
d
¼ 7.96 ppm. The protons of the un-sulfonated phenyl
groups show resonance peaks at
d
¼ 6.58 ppm and
d
¼ 6.75 ppm.
These two peaks have the same peak area, which is twice of the
peak area of the other 3 peaks. The results suggest sBAPB has two
sulfonic acid groups and support to the chemical structure of sBAPB
shown in Scheme 1. Moreover, the resonance peaks at
d
¼ 4.87 ppm
associate to the protons of the amine groups. The peaks disappear
in the 1H NMR spectrum of sBMIPB, demonstrating the perfor-
mance of maleimidization reaction of sBAPA with maleic anhy-
dride. The resonance peaks corresponding to the maleimide groups
appearing in the spectrum also support to the formation of mal-
eimide groups.

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N.-H. Chen et al. / Polymer 54 (2013) 2096e2104
Fig. 2 shows the FTIR and 1H NMR spectra of sPBz. The absorp-
3.2. Properties of sPBz and cured sPBz
tion peak of eCeH of maleimide group at 668 cmꢀ1 and the peaks
of furan group at 773 and 1583 cmꢀ1 do not appear in the FTIR
spectrum of sPBz. The result gives the demonstration of the
occurrence of DielseAlder reaction. In the 1H NMR analysis, sPBz
The solubility test of sPBz has been performed by placing 10 mg
of sPBz in 1 mL of various solvents. sPBz dissolves in 1-methyl-2-
pyrolidone (NMP) and dimethylsulfoxide well at about 25 ꢁC sPBz
is also readily soluble in water at 60 ꢁC. This result means sPBz
could be further fabricated with a solutionꢀp1rocess. The inherent
shows resonance peaks at
d
¼ 4.23 ppm and
d
¼ 5.08 ppm associ-
ating to the eCH2e protons of benzoxazine rings, demonstrating
the presence of benzoxazine groups in the polymer chains.
Nevertheless, the relatively low fraction of the eCH2e resonance
peaks of benzoxazine rings indicates some of the benzoxazine rings
might be opened during the DA reaction due to the catalytic effect
of the sulfonic acid groups. This result could have somewhat effect
on reduction of the crosslinking densities of the final cross-linked
polymer, but should not affect the properties for PEM applications.
Moreover, the characteristic peaks of DA adduct units of maleimide
viscosity of sPBz at 25 ꢁC is 0.97 g (dL)
(measured with a
0.5 g dLꢀ1 solution of NMP). The high inherent viscosity also in-
dicates that sPBz could show good film formability. Nevertheless,
here we do not have a molecular weight data of sPBz from gel
permeation chromatography (GPC) analysis as the poor solubility of
sPBz in the common solvents (tetrahydrofuran (THF) and N,N-
dimethylformamide (DMF)) for GPC. The poor solubility of sPBz in
THF and DMF might be due to the strong interaction of the ben-
zoxazine and sulfonic acid groups.
and furan groups are
¼ 5.26 ppm (eCHeOe) and
d
¼ 2.98 to
d
¼ 3.40 ppm (eCHe),
d
d
¼ 6.62 ppm (eCH ¼ CHe). The
Fig. 3 shows the temperature-modulated DSC thermograms of
sPBz. No obvious glass transition behavior could be seen with the
reversible heat flow curve. The non-reversible heat flow curve
shows an endothermic peak at about 65 ꢁC, which corresponds to
the retro-DA reaction of the DA adduct linkages of sPBz. The tem-
perature observed here is somewhat lower than the temperatures
results support to the performance of DA polymerization and the
chemical structure of sPBz. Moreover, the strong interaction be-
tween the used solvents and the polymer chain results in the dif-
ficulty of completed solvent removal and the resonance peaks of
the residual solvents in the NMR spectrum.
Fig. 2. FTIR and 1H NMR characterization of benzoxazine-containing and sulfonated polymer sPBz.

N.-H. Chen et al. / Polymer 54 (2013) 2096e2104
2101
Fig. 3. Temperature-modulated DSC thermograms of sPBz.
observed for the retro-DA reaction (115 ꢁC). The reduced reaction
temperature could be attributed to the electron-withdrawing effect
of the sulfonate groups. On the other hand, the exothermic peak
appearing in the non-reversible heat flow curve at about 120 ꢁC
indicates the occurrence of the ring-opening addition reaction of
benzoxazine groups. The reaction temperature is much lower than
the temperatures reported to benzoxazine polymerization (230 ꢁC)
of the un-sulfonated polybenzoxazine analog [32]. As the ring-
opening polymerization of benzoxazine groups is highly catalyzed
with acidic groups [39,40], the sulfonic acid groups of sPBz
contribute to the catalytic effect so as to reduce the benzoxazine
polymerization temperature of SPBz. Moreover, the acidic phenolic
groups generated from ring-opening reaction of benzoxazine might
also contribute to lower the polymerization temperature of ben-
zoxazine rings. This catalytic effect and the relatively low temper-
ature of benzoxazine polymerization of sPBz are significant for
further fabrication of sPBz into cross-linked proton exchange
membrane. In this work, sPBz has been cured at 80 ꢁC for 2 h and
120 ꢁC for 5 h to result in the cured polyelectrolyte membrane (CR-
sPBz). Characterization and discussion on the reactions and struc-
tures of the DA reaction based polybenzoxazine have been reported
previously [36]. In the curing process, the occurrence of the retro-
DA reaction might break down the sPBz chain in a certain extent.
Nevertheless, this feature might not affect the cured product, as the
crosslinking reaction (polymerization of benzoxazine groups) soon
follows to result in cross-linked structures. The maleimide groups
generated from the retro-DA reaction might also perform a cross-
linking reaction. As a result, after the crosslinking reaction, the CR-
sPBz product is constructed with a three-dimensional network
composed of cross-linked polybenzoxazine and polymaleimide
structures. The contents of thermally-reversible DA adducts should
be very few, so as not to affect the properties of the cross-linked CR-
sPBz membrane for using as PEMs. It is noteworthy that the low
curing temperature of sPBz is critical and necessary in this work
since the sulfonic acid groups have relatively poor thermal stability.
Fig. 4 shows the DMA thermograms of CR-sPBz samples under
different curing conditions. For the sample cured at 120 ꢁC for 2 h, a
sharp increase in the storage modulus appears in the DMA ther-
mogram, indicating further curing reaction of benzoxazine groups
occurs in the heating scan. The curing reaction of sPBz is not
complete under the curing condition of 120 ꢁC for 2 h. While
increasing the reaction time to 5 h, the curing reaction conversion
increases as modulus increase does not appear at about 120 ꢁC for
the sample cured at 120 ꢁC for 5 h. The modulus increase at about
250 ꢁC in the DMA thermogram of the sample cured at 120 ꢁC for
Fig. 4. DMA thermograms of sPBz after thermal treatment at (a) 120 ꢁC for 2 h and (b)
120 ꢁC for 5 h.
5 h could be attributed to the crosslinking reaction of maleimide
groups which generate from the retro-DA reaction of maleimide-
furan adducts. Nevertheless, this reaction should not affect the
properties of the CR-sPBz membrane for DMFC, as the reaction
temperature is much higher than the temperatures for PEM
application. Even though the maleimide polymerization performs
for the CR-sPBz membranes, the major effect on the membrane
Fig. 5. Stressestrain curve of dry CR-sPBz membrane.

2102
N.-H. Chen et al. / Polymer 54 (2013) 2096e2104
Fig. 6. Photographs of cross-linked CR-sPBz film. Picture (a) showing the film formability, picture (b) showing the flexibility, and picture (c) showing the insolubility in NMP.
could be an increase in the crosslinking density of the membrane.
This reaction might not have obvious fatal effect in this work.
It is noteworthy that CR-sPBz exhibits a high storage modulus of
about 3.0 GPa at about 50e150 ꢁC, the temperature range for most
PEM application. On the other hand, the stressestrain curve of the
cross-linked polymer (dry sample) has been recorded, with an
Instron instrument, as shown in Fig. 5. CR-sPBz shows a typical
stressestrain curve of cross-linked polymers in the test, with a
maximum stress of 49 MPa and an elongation at break of about
3.7%. The Young’s modulus of the CR-sPBz membrane is about
1.7 GPa. After the curing reaction, CR-sPBz is stable and becomes
insoluble in NMP (Fig. 6). Almost no soluble fraction could be found
in the solution test. The gel fraction of the CR-sPBz sample is almost
100%. Besides the high mechanical strength and Tg, CR-sPBz
membrane also shows high flexibility (Fig. 6b), like other cross-
linked polybenzoxazines possessing benzoxazine groups in the
main chains [36,37,43]. The flexibility of CR-sPBz is very impressive
and attractive for using as a proton exchange membrane for DMFC,
as in general highly cross-linked and strong polymer membranes
could be stiff and brittle.
3.3. Characterization of CR-sPBz for DMFC application
The value of ion-exchange capacity (IEC) of sPBz measured with
titration is about 1.48 meq gꢀ1, which is close to the theoretical
value of 1.62 meq gꢀ1. However, the IEC value of CR-sPBz from
titration measurement is 0.94 meq gꢀ1. The derivation of the
experimental IEC value of CR-sPBz could be resulted from the cross-
linked structures of CR-sPBz, which interfere the ion exchanging
operation in the titration test. The water uptake of CR-sPBz is about
21 wt%, compared to the water uptake of 36 wt% measured with
Nafion 117 at 80 ꢁC. The hydrated CR-sPBz does not exhibit any
endothermic peak at about 0 ꢁC in its DSC scan, indicating that the
Table 1
Properties of CR-sPBz and Nafion 117 membranes.
Sample
Methanol Dimensional Proton
Methanol
Relative
uptake
(wt%)
change upon conductivity permeability
selectivity
methanol
uptake (%)
at 60 ꢁC,
(10ꢀ7 cm2 sꢀ1
)
RH ¼ 95%
(mS cmꢀ2
)
CR-sPBz
Nafion 117 15
20
1.6
5.9
55
85
2.52
13.1
3.4
1.0
Fig. 7. Morphological micrographs of CR-sPBz with TEM (a and b) and AFM (c).

N.-H. Chen et al. / Polymer 54 (2013) 2096e2104
2103
hydrated CR-sPBz membrane almost does not possess free water.
The near 100% bound water in CR-sPBz is effective for proton
transportation through the membrane. CR-sPBz shows a dimen-
sional change of about 6.0% upon hydration, which is much lower
than the value (9.4%) measured with Nafion 117. Meanwhile, the
methanol uptake and dimensional change upon methanol ab-
sorption of CR-sPBz are 20 wt% and 1.6%, respectively, where the
values for Nafion 117 are 15 wt% and 5.9% (Table 1). The cross-linked
structure of CR-sPBz stabilizes the membrane in water and meth-
anol and depresses its dimensional change in water and methanol.
CR-sPBz demonstrates better dimensional stability upon absorp-
tion of water and methanol than does Nafion.
Fig. 7 shows the micrographs of CR-sPBz for morphology
studies. Individual sulfonic domain in the size of about 10 nm ap-
pears in the TEM micrographs of the PbSO4-stained CR-sPBz sam-
ple. The domain size is similar to other polyelectrolyte membranes
reported in the literature [44,45]. Proton-conducting paths
composed of the sulfonic acid domains appear in the membrane, as
shown with the TEM and AFM micrographs. As a result, the
microphase separation of the proton-conductive domains and the
resulting proton pathways contribute to the hopping trans-
portation of protons through the membranes. Fig. 8 shows the
proton conductivity of CR-sPBz membrane. At 60 ꢁC and relative
humidity (RH) of 90%, CR-sPBz shows a proton conductivity of
45 mS cmꢀ1, which is lower than that of Nafion 117 but is high
enough for DMFC application [28]. Nevertheless, the proton con-
ductivity of CR-sPBz approaches to the value of Nafion 117 while
decreasing the environmental RH to 30e50%. Under the condition
of RH ¼ 95%, the proton conductivity of CR-sPBz increases from 32
to 47 mS cmꢀ1 with increasing the temperature from 20 to 60 ꢁC.
On the other hand, the methanol permeability of CR-sPBz is
2.52*10ꢀ7 cm2 ꢀ1, which is about one fifth of the value of Nafion
s
117. The relatively low methanol permeability of CR-sPBz is
attractive and critical for its application as a proton exchange
membrane for DMFC.
As the proton conductivity and methanol permeability are
trade-offs for a proton exchange membrane for DMFC, the selec-
tivity defined as the ratio of these two factors has been taken as a
performance index of the proton exchange membranes for DMFC.
As a result, the selectivity of CR-sPBz is 1.86*105 S s cmꢀ3, which is
about 3.4-fold of the selectivity of Nafion 117. The high selectivity of
CR-sPBz suggests its suitability for using as a high performance
proton exchange membrane for DMFC.
4. Conclusion
A sulfonated polymer containing benzoxazine groups has been
prepared through DielseAlder reaction. Introduction of benzox-
azine groups to sulfonated polymer is effective for preparation of
crosslinkable polyelectrolyte for DMFC application. The sulfonic
acid groups catalyze the thermal polymerization of benzoxazine
groups, and reduce the temperature of fabrication of sPBz into
highly cross-linked and flexible proton exchange membrane. CR-
sPBz exhibits individual sulfonic domains in the size of about
10 nm contributing to proton conduction through the membrane.
Meanwhile, the cross-linked structure depress the chain mobility
and methanol permeability of CR-sPBz, As a result, CR-sPBz shows a
selectivity which is about 3.4-fold of that of Nafion 117, warranting
its suitability for DMFC application.
Acknowledgments
The authors thank the National Science Council (Grant NSC 100-
2221-E-007-127-MY3; NSC 101-2623-E-007-013-IT) and the Min-
istry of Economic Affairs (Grant 101-EC-17-A-09-S1-198) for
financial support on this work, and thank Professor Yung Chang
(Chung Yuan Christian University, Taiwan) for his kind assistance
on AFM measurements.
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