Novel hydroxide-conducting polyelectrolyte composed of an poly(arylene ether sulfone) containing pendant quaternary guanidinium groups for alkaline fuel cell applications

Article abstract of DOI:10.1021/ma100260a

Poly(arylene ether sulfone)s were functionalized with quaternary guanidinium groups in order to investigate their properties as novel polymeric hydroxide exchange membrane materials. The quaternized polymers were synthesized via chloromethylation of poly(

Full text of DOI:10.1021/ma100260a

3890 Macromolecules 2010, 43, 3890–3896
DOI: 10.1021/ma100260a
Novel Hydroxide-Conducting Polyelectrolyte Composed of an
Poly(arylene ether sulfone) Containing Pendant Quaternary Guanidinium
Groups for Alkaline Fuel Cell Applications
Junhua Wang,†,‡ Shenghai Li,† and Suobo Zhang*,†
†Key Laboratory of Advanced Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun 130022, China, and ‡Graduate School of Chinese Academy of Sciences,
Beijing 100039, China
Received February 2, 2010; Revised Manuscript Received March 15, 2010
ABSTRACT: Poly(arylene ether sulfone)s were functionalized with quaternary guanidinium groups in
order to investigate their properties as novel polymeric hydroxide exchange membrane materials. The
quaternized polymers were synthesized via chloromethylation of poly(arylene ether sulfone)s, followed by
reactions with pentamethylguanidine. The resulting quaternized polymers PSGCl-x (where x represents the
number of the quaternary guanidinium groups/repeat units) presented an elevated molecular weight and
exhibited an outstanding solubility in polar aprotic solvents. Consequently flexible and tough membranes of
PSGCl-x with varying ionic content could be prepared by casting from the DMSO solution. Novel anion
exchange membranes, PSGOH-x, were obtained by an anion exchange of PSGCl-x with 1 M NaOH at room
temperature. The membranes displayed a high ionic conductivity and an excellent chemical stability. The
obtained alkaline anion exchange membranes (AEMs) showed conductivities almost above 10-2 S cm-1 at
room temperature; the hydroxide conductivity of PSGOH-1.4 was for instance found to be 6.7 ꢀ 10-2 S cm-1
.
Introduction
Recently, many kinds of anion exchange membranes based on
polymers containing quaternary ammonium groups applied for
the alkaline fuel cells have been developed, such as quaternized
poly(ether sulfone),11-19 quaternized poly(2,6-dimethyl-1,4-phe-
nylene oxide),20 quaternized poly(phthalazinone ether sulfone
ketone),21 quaternized poly(phenylene),22 and radiation-grafted
PVDF, ETFT, and FEP,23-25 etc. The quaternized polysulfone is
typical of these AEMs and has been researched extensively. The
first anion exchange membranes based on the polysulfone, Udel
1700 (Amoco), were reported by Zschocke and Quellmalz, who
described the material’s alkaline stability and demonstrated its
use in electrodialysis.11 They werepreparedbychloromethylation
of the parent polysulfone and followed by reaction with trimethyl-
amine to form benzyltrimethylammonium groups. Zhuang et al.
further developed this kind of quaternized polysulfone and used
this material to produce H2/O2 alkaline fuel cells successfully.18
However, compared with PEMs, the low ionic conductivity of
quaternary ammonia polysulfone has become an obstacle of the
further development for alkaline fuel cell applications.
One of the approaches to promote the hydroxide conductivity
of quaternized polymers is to increase the basicity of the func-
tional groups. For the quaternary ammonium hydroxide unit, an
increase in its basicity leads to an augmentation of both the
number of dissociated hydroxides and the water uptake, thus
facilitating the ion conductivity under similar humidified condi-
tions. Recently, Yan et al. synthesized a new quaternary phos-
phonium based ionomer that is highly hydroxide conductive and
is soluble in low-boiling-point water-soluble solvents: tris(2,4,6-
trimethoxyphenyl)polysulfone-methylene quaternary phos-
phonium hydroxide.26 Pentaalkylguanidine is another kind of
organic superbase. For example, the simplest member, penta-
methylguanidine, possesses a pKa value equal to 13.8 while the
value of trimethylamine is only 10.8.27 The strong basicity of
pentamethylguanidine suggests that quaternary guanidinium
hydroxide is a very strong base. Additionally, the positive charge
Polymer electrolyte fuel cells (PEFCs), which convert chemical
energy to electrical energy, are regarded as promising future
power sources owing to their advantages, such as their high
efficiency, high energy density, quiet operation, and environ-
mental friendliness.1-3 Up until now, the most advanced PEFCs
are based on proton exchange membranes (PEMs), in particular
Nafion membranes. Although the PEMs exhibit excellent che-
mical, mechanical, and thermal stabilities as well as high ionic
conductivity, some specific limitations exist for PEMs which
impede the commercialization of PEMFCs. Limitations of the
use of acidic polymeric electrolytes include (1) slow electrode
kinetics, (2) CO poisoning of Pt and Pt-based electrocatalysts at
low temperatures, (3) high costs of the membranes and catalysts,
and (4) high fuel permeabilities.4-6
To overcome these hurdles, there has been a growing interest
in developing alkaline anion exchange membranes (AEMs) for
fuel cell applications.7-10 AEMs are designed to provide suffi-
cient hydroxyl ions for ion exchange during electrochemical
reactions in alkaline fuel cells. AEMFCs have numerous advan-
tages over PEMFCs, including performance and cost. In a basic
environment, the cathode oxygen reduction overpotential can be
significantly reduced, leading to high fuel cell efficiencies. More-
over, catalysts in basic medium are more durable,7 and the
inherently faster kinetics of oxygen reduction reactions in an
alkaline fuel cell renders it possible to employ non-precious
metals as catalysts, thereby leading to the use of noble metals
such as Pt and Ru, being avoided.8 Further, AEMFCs offer fuel
flexibility (e.g., methanol, ethanol, ethylene glycol, etc.) due to
their low overpotentials for hydrocarbon fuel oxidation and
reduced fuel crossover.9,10
*To whom correspondence should be addressed. E-mail: sbzhang@
ciac.jl.cn.
pubs.acs.org/Macromolecules
Published on Web 03/22/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 8, 2010 3891
in the guanidinium hydroxides is delocalized over one carbon and
three nitrogen atoms, which gives them a high degree of thermal
and basic stability.28 However, to the best of our knowledge, the
potential of this or related polymers as a hydroxide-conducting
material has not been investigated so far. As a consequence, the
objective of this study was to synthesize and characterize novel
poly(arylene ether sulfone)s containing quaternary guanidinium
hydroxide groups and evaluate their properties as alkaline anion
exchange membrane materials.
Synthesis of Polysulfone Containing Quaternary Guanidinium
Chloride (PSGCl-x). The following represents a typical proce-
dure for the synthesis of guanidinium of PSGCl-x. PSGCl-1.2
(1.0 g, 2.4 mmol) was dissolved in 20 mL of DMSO, and then
1,1,2,3,3-pentamethylguanidine (0.7 g, 4.8 mmol) was added.
The reaction mixture was stirred at room temperature for 12 h
and then poured into distilled water. The precipitate was filtered
off, washed with deionized water thoroughly, and dried in
vacuum oven for 24 h at 60 °C to give product with 95% of
yield. NMR was used to confirm the synthesis of PSGCl-1.2, and
the degree of conversion of -CH2Cl group was close to 100%.
Membrane Casting and Preparation of Polysulfone Containing
Guanidinium Hydroxide (PSGOH-x). PSGCl-x membranes
were cast from DMSO solutions (4 wt %) in a custom-built flat
glass dish. The membranes were first dried at 40 °C for 4 h and at
80 °C for 12 h and then vacuum-dried at 100 °C for 24 h.
PSGOH-x membranes were obtained by treating PSGCl-x in
1 M NaOH at room temperature for 48 h to exchange the
chloride ions for hydroxide ions. Then, the membranes were
washed thoroughly to remove residual NaOH and immersed in
deionized water for 48 h prior to analysis.
Experimental Section
Materials. 4,40-Biphenol, 4,40-dichlorodiphenyl sulfone, chloro-
methyl methyl ether, and 1,1,3,3-tetramethylurea were pur-
chased from Aldrich. Dimethyl sulfoxide (DMSO) and N,N-
dimethylacetamide (DMAc) were stirred over CaH2 for 24 h,
˚
then distilled under reduced pressure, and stored over 4 A
molecular sieves under a nitrogen atmosphere. Toluene and
acetonitrile were dried by refluxing over sodium or CaH2 and
distilled prior to use. All other reagents were obtained from
commercial sources and used as received. The unfunctiona-
lized poly(arylene ether sulfone) was prepared as described by
Harrison et al.29
1
Measurements. H and 13C NMR spectra were measured at
300 MHz on an AV300 spectrometer. FT-IR spectra were re-
coded on membrane samples of thickness 10 μm using a Bio-Rad
digilab Division FTS-80 FT-IR spectrometer. Elemental ana-
lyses were performed on an Elemental Analyses MOD-1106.
The reduced viscosities were determined with an Ubbelohde
capillary viscometer at 30 ( 0.1 °C on 0.5 g dL-1 concentrations
of polymer in DMSO. Thermogravimetric analysis (TGA) was
performed in nitrogen with a Perkin-Elmer TGA-2 thermogravi-
metric analyzer at a heating rate of 10 °C min-1. Molecular
weights were also determined by gel permeation chromato-
graphy (GPC) using a Waters 515 HPLC pump, coupled with
a Waters 410 differential refractometer detector and a Waters
996 photodiode array detector operating at a wavelength of
260 nm. CHCl3 was used as eluent. Tensile measurements were
carried out with an Instron-1211 mechanical testing instrument
Synthesis of 1,1,2,3,3-Pentamethylguanidine (PMG). The Vils-
meyer salt (C-chloro-N,N,N0N0-tetramethylformamidine) was
obtained by reaction of 1,1,3,3-tetramethylurea with oxalyl
chloride in toluene.30 In a nitrogen atmosphere, to a solution
of Vilsmeyer salt (34.2 g, 0.20 mol) in dry acetonitrile, excessive
methylamine gas was slowly passed through the reaction system
under room temperature. After the mixture was refluxed for 2 h,
5 equiv of 8 M NaOH solution was added under vigorous
stirring. The mixture was filtered; after removal of the solvent
as well as excess methylamine, the residue was distilled under
reduced pressure to afford 21.9 g (0.17 mol, yield 85%) of
1
1,1,2,3,3-pentamethylguanidine as a colorless liquid. H NMR
(CD3Cl): δ 2.90 (3H, s), 2.72 (6H, s) and 2.59 (6H, s). 13C NMR
(CD3Cl): δ 161.24, 39.25, 38.64, and 36.48. Anal. Calcd for
C6H15N3 (129.2): C, 55.78%; H, 11.70%; N, 32.52%. Found: C,
55.93%; H, 11.78%; N, 31.96%.
at a speed of 1 mm min-1
.
Ion Exchange Capacity (IEC). The membranes in the hydro-
xide form were immersed in 100 mL of 0.1 M HCl standard for
48 h. The solutions were then titrated with a standardized
NaOH solution using phenolphthalein as an indicator.
Water Uptake and Swelling Ratio Measurements. The mem-
branes were vacuum-dried at 100 °C for 10 h until constant
weight to obtain the dry material. They were then immersed in
deionized water at given temperature for 4 h. After this time, the
membranes were taken out, wiped with tissue paper, and quickly
weighed on a microbalance. The water uptake of membranes
was calculated according to
Synthesis of Model Compound. To a 100 mL round-bottomed
flak were charged 6.32 g (0.05 mol) of 1-(chloromethyl)benzene,
6.46 g (0.05 mol) of 1,1,2,3,3-pentamethylguanidine, and 20 mL
of toluene. The solution was stirred at room temperature for 4 h.
The precipitate was filtered, washed with toluene, and dried in a
vacuum. The product was obtained in a yield of 96% (12.3 g). 1H
NMR (DMSO-d6): δ 7.45-7.32 (5H, m), 4.52-4.16 (2H, m) and
3.02-2.75 (15H, m). 13C NMR (DMSO-d6): δ 163.49, 136.60,
130.00, 129.89, 129.30, 56.31, 40.84, and 38.50. Anal. Calcd for
C16H36N6 (255.8): C, 61.04%; H, 8.67%; N, 16.43%; Cl,
13.86%. Found: C, 61.01%; H, 8.62%; N, 16.31%; Cl, 13.92%.
Synthesis of Polysulfone Containing Chloromethyl Groups
(PSCM-x). The DS values of the polymers, where DS repre-
sented the degree of substitution (the number of chloromethyl
groups/repeat units), were controlled by adjusting the amount
of ZnCl2. A typical synthesis procedure of PSCM-1.0, where 1.0
refers to the DS value, was as follows. PS (1.0 g, 2.5 mmol) was
dissolved in 30 mL of 1,1,2,2-tetrachloroethane in a three-
necked flask equipped with a stirrer, a condenser, and a drop-
ping funnel under nitrogen. Anhydrous zinc chloride (0.33 g,
5 mmol) was dissolved into 8 g of chloromethyl methyl ether
(CMME, 40 equiv). The zinc chloride solution in CMME was
dripped into the flask by the addition funnel. Because chloro-
methyl methyl ether is a known toxic, carcinogenic substance,
caution must be used to avoid any contact with or inhaling it.
The resulting solution was heated to reflux for 4 h in the dark.
After cooling to room temperature the reaction was poured into
excessive methanol (0.5 L). The fiber-like precipitate was col-
lected by filtration and washed with deionized water and
methanol three times prior to being dried under vacuum to
produce the final product. Yield: 98%.
water uptake ð%Þ ¼ ½ðWwet - WdryÞ=Wdryꢁ ꢀ 100%
ð1Þ
where Wdry and Wwet are the weight of the dry and the
corresponding water-swollen membranes, respectively.
The water swelling ratio of the membranes was investigated
by immersing the round-shaped samples into water at given
temperature for 4 h, and the swelling ratio was calculated from
swelling ratio ð%Þ ¼ ½ðlwet - ldryÞ=ldryꢁ ꢀ 100%
ð2Þ
Here, ldry and lwet are the diameter of the dry and wet samples,
respectively.
Hydroxide Conductivity. The hydroxide conductivity (σ, S
cm-1) ofeachmembranecoupon(size:1cmꢀ 4 cm) wasobtained
using σ = d/LsWsR, where d is the distance between reference
electrodes and Ls and Ws are the thickness and width of the
membrane, respectively. The resistance value (R) was measured
over the frequency range from 1 mHz to 100 kHz by four-point
probe alternating current (ac) impedance spectroscopy using
an electrode system connected with an impedance/gain-phase

3892 Macromolecules, Vol. 43, No. 8, 2010
Wang et al.
Scheme 1. Synthetic Route of 1,1,2,3,3-Pentamethylguanidine (PMG)
Table 1. Reaction Conditions and Resultant Molecular Weights from
the Chloromethylationa
tempera- equivalents
sample time (h) ture (°C) of CMME DSb (g mol-1
Mw ꢀ 104
gelation
(%)d
)
1
2
3
4
5
6
4
4
4
4
4
8
60
60
60
40
70
60
20
30
40
40
40
40
1.24
1.65
1.96
0.86
2.00
1.96
133
134
126
121
134
144
11
6
-
-
35
8
c
analyzer (Solatron 1260) and an electrochemical interface
(Solatron 1287, Farnborough Hampshire, ONR, UK). The
membranes and the electrodes were set in a Teflon cell, and the
distance between the reference electrodes was 1 cm. The cell was
placed in a thermo-controlled chamber in liquid water for
measurement. Conductivity measurements under fully hydrated
conditions were carried out with the cell immersed in liquid water.
All samples were equilibrated in water for at least 24 h prior to the
conductivity measurement. At a given temperature, the samples
were equilibrated for at least 30 min before any measurements.
Repeated measurements were then taken at that given tempera-
ture with 10 min interval until no more change in conductivity
was observed.
a Reaction conditions: PS (1 g, 2.5 mmol), ZnCl2 (0.86, 5 mmol),
1,1,2,2-tetrachloroethane (30 mL). b Degree of substitution = (number
of the chloromethyl groups/repeat units). c No gelation. d After the
reaction finishing, the gel-like precipitates were collected by filtration
and dried under vacuum. Gelation was determined by simple weighing.
Alkaline Stability. PSGOH-x samples were kept in a NaOH
solution (1 M) at 25 or 60 °C for 48 h. Then, they were washed
with deionized water to remove the sorbed sodium hydroxide
and immersed in deionized water for at least 48 h prior to
measure for ionic conductivity analysis.
Results and Discussion
The synthetic route of 1,1,2,3,3-pentamethylguanidine (PMG)
is outlined in Scheme 1. C-Chloro-N,N,N0N0-tetramethylforma-
midine (the Vilsmeier salt) was prepared by reaction of 1,1,3,3-
tetramethylurea with oxalyl chloride in toluene at a yield of 80%.
The Vilsmeier salt reacted quickly with dry methylamine gas at
30 °C, giving pentamethylguanidine (PMG) in 85% yield. The
chemical structure and elemental composition of PMG were
1
confirmed by elemental analysis as well as H and 13C NMR
spectroscopies.
As shown in Scheme 2, the preparation of AEMs involves
several chemical reactions: chloromethylation, quaternization,
and alkalization. Among them, chloromethylation and quaterni-
zation are two key reactions that determine the ionic conductiv-
ity.26 The chloromethylation of polymer is a valuable procedure
that was established nearly four decades ago.31 Chloromethy-
lated polymer is the starting point for the synthesis of polymer-
bound reagents for ion-exchange resins due to the high reactivity
of the tethered chloromethyl group. As has been reported, the
cross-linking reaction often takes place rapidly during chloro-
methylation of polysulfone resin as the active aromatic ring
attacks the chloromethyl group in a Friedel-Crafts alkylation
fashion, which in turn causes gelation.17,19 The formed gel is not
useful for further functionalization and membrane preparation.
Therefore, it is of high importance to study the effecting factors
on chloromethylation and obtain different DS values of chloro-
methylated polysulfone without gelation.
The chloromethylation of polysulfone was carried out in
1,1,2,2-tetrachloroethane using chloromethyl methyl ether as a
chloromethylation reagent and ZnCl2 as a catalyst. The effects of
reaction temperature, reaction time, and the amount of CMME
on the chloromethylation were investigated, and the results
for six different batches of chloromethylated poly(arylene ether
sulfone)s, PSCM-x, are summarized in Table 1, where x refers to
the DS values. 1H NMR analysis (Figure 1) indicated that
chloromethylation of PS occurred at the activated positions of
the biphenyl moieties. The signals with chemical shifts δ 7.0-7.9
were assigned to the aromatic hydrogen of PSCM-x. Resonances
due to chloromethyl groups were detected at δ 4.6. The DS values
of the polymers were calculated from the ratio of the integration
of Hf to Ha. Reaction temperatures higher than 60 °C, reaction
times longer than 8 h, or the amount of CMME used less than
Figure 1. 1H NMR spectra of PSCM-0 (a), PSCM-1.0 (b), and PSCM-
1.96 (c) in CDCl3.
40 equiv resulted in gels, presumably because of cross-linking via
Friedel-Crafts alkylation by the chloromethyl groups in the
presence of a Lewis acid. The optimized reaction conditions was
as follows: PS (1 g, 2.5 mmol), CMME (8 g, 100 mmol, 40 equiv),
ZnCl2 (0.68 g, 5.0 mmol, 2 equiv), 1,1,2,2-tetrachloroethane
(30 mL), 60 °C. Under this optimized condition, the reaction
was rather fast; it took ∼4 h for the DS to reach 1.96 (close to the
theoretical maximum value) without gelating. On the other hand,
we noticed that ZnCl2 affected the chloromethylation of poly-
sulfone directly and could determine the DS values by controlling
the amount of ZnCl2 used under this reaction condition. As
shown in Figure 2, increasing the amount of ZnCl2 increased
the number of the tethered chloromethyl group. For example, the
gained DS values of PSCM were approximately 0.5, 1.0, and
2.0, with the used equiv of ZnCl2 0.5, 1.0, and 2.0 in 4 h,
respectively.
The PSCM-x was converted to a quaternized poly(arylene
ether sulfone)s (PSGCl-x) by reaction with 1,1,2,3,3-penta-
methylguanidine (PMG). The mole ratio of PMG to the chloro-
methyl groups was set to be 2:1 to ensure a complete conversion
of the -CH2Cl units into the quaternary guanidinium moieties.
In addition, the viscosity of PSGCl-x was even greater than the
parent polymer in accordance with the reaction. This was due to
the positive charges (the quaternary guanidinium groups) located
along the polymeric chains that repulsed each other in solution,
thereby inducing chain extension. The structure of the ionomers
was confirmed by NMR and IR spectroscopies. To obtain precise
data regarding the chemical shifts of 1H and 13C NMR, the model

Article
Scheme 2. Synthetic Route of PS, PSCM-x, and PSGOH-x
Macromolecules, Vol. 43, No. 8, 2010 3893
Figure 2. Effect of the amount of ZnCl2 on the DS of PSCM. Reaction
conditions: (a) PS (1 g, 2.5 mmol), CMME (8 g, 0.1 mol, 40 equiv),
ZnCl2 (0.17 g, 1.25 mmol, 0.5 equiv), 1,1,2,2-tetrachloroethane (30 mL),
60 °C; (b) PS (1 g, 2.5 mmol), CMME (8 g, 0.1 mol, 40 equiv), ZnCl2
(0.34 g, 2.5 mmol, 1.0 equiv), 1,1,2,2-tetrachloroethane (30 mL), 60 °C;
(c) PS (1 g, 2.5 mmol), CMME (8 g, 0.1 mol, 40 equiv), ZnCl2 (0.68 g,
5.0 mmol, 2.0 equiv), 1,1,2,2-tetrachloroethane (30 mL), 60 °C.
Figure 3. 1H NMR and 13C NMR spectra of model compound (a, b)
and PSGCl-1.2 (c) in DMSO-d6.
Scheme 3. Synthetic Route of Obtaining a Model Compound
temperature, giving rise to guanidinium chloride in quantitative
yield. As shown in Figure 3a,b, the peaks around δ 4.52-4.16
(2H, m) and 3.02-2.75 (15H, m) could be assigned to the chemical
shifts of the protons on the methylene and methyl groups,
respectively. The integration ratio of H (-CH2-) to H (-CH3)
was close to 2:15, as expected for the composition of the model
compounds. In the 13C NMR spectrum, the peak at δ 163.49 ppm
was attributed to the chemical shift of the central carbon atom in
the guanidinium group, and the peak at δ 56.31 ppm was assigned
to the carbon atom in the methylene groups. The two peaks
around δ 40.84 and 38.50 ppm were attributed to the carbon atom
compound was synthesized by the reaction shown in Scheme 3.
The PMG reacted quickly with 1-(chloromethyl)benzene at room

3894 Macromolecules, Vol. 43, No. 8, 2010
Wang et al.
Table 2. Mechanical Properties of PSGOH-xa
tensile strength
(MPa)
tensile modulus elongation at break
(GPa)
sample
(%)
PSGOH-0.4
PSGOH-0.6
PSGOH-0.7
PSGOH-1.0
PSGOH-1.2
PSGOH-1.4
PSGOH-1.5
67.6
60.1
55.0
46.3
39.7
15.8
-
1.70
1.61
1.53
1.20
0.69
0.33
-
10.4
13.5
18.3
26.7
35.5
71.4
-
a Measured at 30 °C, 50% RH.
low boiling point solvent such as ethanol and (n- or 2-)propanol
since a soluble ionomer could be used in the catalyst layer to build
an efficient three-phase boundary and thus drastically improve
the utilization of the catalyst particles and reduce the internal
resistance.26
The thermal stability of the synthesized PSGCl and PSGOH
ionomers was studied under identical drying and heating condi-
tions. The typical TGA curves are shown in Figure 5. As a result
of the strong hydrophilicity of the quaternary guanidinium
groups attracting water from the atmosphere, a slight weight loss
was observed between 80 and 120 °C, corresponding to the
evaporation of absorbed water. This behavior has been com-
monly found in other polymeric ion exchange membranes. The
first weight loss (5%) observed for PSGCl-1.2 at 286 °C was
related to the degradation of quaternary guanidinium groups,
whereas the corresponding value for PSGOH-1.2 was around
185 °C. In comparison, the mass loss for the PSQNOH prepared
from trimethylamine and the same PSCM started its mass loss at
around 165 °C.17 This was due to a lower thermal stability of the
quaternary ammonium hydroxide as opposed to the quaternary
guanidinium hydroxide. The results demonstrate that the
PSGOH had a good thermal stability and a higher degradation
temperature.
Figure 4. FT-IR spectra of PS, PSCM-1.2, and PSGOH-1.2.
The mechanical properties for the PSGOH membranes over a
range of IEC values are shown in Table 2. The samples displayed
values of tensile stress at maximum load of 15.8-67.6 MPa,
Young’s moduli of 0.33-1.70 GPa, and values of elongation at
break of 10.4-71.4%. At high IEC values (>2.25 mequiv g-1),
gel-like mechanical properties were observed. This was the
result of the materials absorbing large amounts of the equilib-
rium water. The water sorption of these membranes increased
with an increasing ion exchange capacity due to the strong
hydrophilicity of the guanidinium groups. For example, the
tensile stress and Young’s moduli of PSGOH-1.4 decreased to
respectively 15.8 MPa and 0.33 GPa. This mechanical instabil-
ity was the result of a morphological change at the percolation
threshold in which the hydrophilic domains became intercon-
nected.
The water uptake of sulfonated polymers is known to have a
profound effect on the conductivity and mechanical properties of
PEMs. The same is true for AEMs. Water molecules dissociate
the alkaline functionality and facilitate hydroxide transport.
However, excessively high levels of water uptake can result in
membrane fragility and dimensional change, which lead to the
loss of mechanical properties. Basically, the amount of water
uptake of PSGOH-x is strongly dependent upon the amount of
quaternary guanidinium hydroxide groups and is also related to
the IEC values. The water uptake was measured from the ratio of
the weight of water absorbed by the membrane when immersed in
water, with respect to the dry membrane weight. This data are
reported in Table 3 along with the swelling ratios. As expected,
the water uptake increased with increasing IEC. The water
uptake of the PSGOH-x membranes with IEC values between
0.79 and 2.15 was in the range of 12-88%. The results also
demonstrate the temperature-dependent water uptake and
Figure 5. TGA curves for PSGOH-1.2 (a), PSGCl-1.2 (b), and
PSQNOH (c) in N2.
in the methyl groups. These characteristic chemical shifts could
all be found in the 1H spectra of PSGCl-1.2 (Figure 3c).
The FT-IR spectra of PS, PSCM-1.2, and PSGCl-1.2 are shown
in Figure 4. The absorption bands at 2977-2850 cm-1 were
characteristic of the -CH2- and -CH3 groups. The absorption
bands at 723-681 cm-1 arise from stretching vibrations of C-Cl
bonds. The broad band at 3415 cm-1 was assigned to the
stretching vibration of O-H bonds, and the band at 1564 cm-1
was attributed to vibrations of CdN in the guanidinium groups.
After amination, the intensities of the bands at 754-681 cm-1
were found to substantially disappear, and new absorption bands
appeared at 3415 and 1564 cm-1. This result confirmed the
complete conversion of the -CH2Cl groups; guanidinium units
had thus been successfully introduced into the polymers.
The PSGCl ionomers exhibited outstanding solubility in polar
aprotic solvents such as NMP, DMF, DMAc, and DMSO. Then,
PSGCl membranes could be cast from DMSO solutions in a flat
glass dish. The drying procedure was optimized to produce flat,
transparent membranes while quantitatively removing the cast-
ing solvent. PSGOH membranes (in the hydroxide form) were
obtained by treating PSGCls in 1 M NaOH at room temperature
for 48 h. The PSGOH ionomers were soluble in NMP, DMAc,
and DMSO; meanwhile, PSGOH-100 was soluble in 2-propanol.
In the preparation of alkaline exchange membrane fuel cells
(AEMFCs), a required property for AEMs was their solubility in

Article
Macromolecules, Vol. 43, No. 8, 2010 3895
Table 3. Ion Exchange Capacity, Water Uptake, Swelling Rate, and Hydroxide Conductivity of PSGOH Membranes
water uptake (%) swelling rate (%)
IEC (mequiv g-1 conductivity (S cm-1
20 °C 60 °C 20 °C 60 °C 60 °C
)
)
sample
theoreticala
experimental
20 °C
PSGOH-0.4
PSGOH-0.6
PSGOH-0.7
PSGOH-1.0
PSGOH-1.2
PSGOH-1.4
PSQNOH-1.0
0.86
1.21
1.37
1.79
2.00
2.25
2.45
0.79
1.17
1.34
1.68
1.89
2.15
1.85
12
17
24
39
55
88
44
15
19
39
65
7
8
0.005
0.012
0.018
0.033
0.045
0.067
0.029
0.008
0.023
0.035
0.051
0.074
11
13
16
20
31
17
12
16
23
46
-
107
b
-
68
-
0.044
22
a Calculated from 1H NMR. b Swelled to gel-like, could not be measured.
presented the hydroxide conductivities ranging from 0.5 to 6.7 ꢀ
10-2 S cm-1 at 20 °C. Their hydroxide conductivities increased
with the IEC values and water uptake. At low IEC values, the
conductivity of the membranes was rather low and increased in a
gradual fashion. In this regime, the ionic conductivity was
thought to be limited by the connection between ionic domains,
and as the IEC value increased, the conductivity increased rapidly
as the volume fraction of water and concentration of ionic groups
in the membranes was raised. In comparison, PSGOH-1.2 (IEC=
1.89 mequiv g-1) showed a hydroxide conductivity of 4.5 ꢀ 10-2
S cm-1 at 20 °C, which was higher than that of the aforemen-
tioned PSQNOH (2.9 ꢀ 10-2 S cm-1, IEC = 1.85 mequiv g-1).
The reason could be the higher basicity of quaternary guanidi-
nium hydroxides leading to an augmentation of both the number
of dissociated hydroxides and water molecules. In addition, the
temperature dependence of the hydroxide conductivities of
PSGOH-x measured in liquid water also is summarized in
Table 3. As expected, an increase in temperature resulted in an
increase in hydroxide conductivity based on the simplified diffu-
sion mechanism and thermal motion of hydroxides in channels
within membranes. For example, the ionic conductivity of
PSGOH-1.2 increased from 4.5 to 7.4 ꢀ 10-2 S cm-1, correspond-
ing to a temperature rise from 20 to 60 °C.
Figure 6. Water uptake and hydroxide anion conductivity versus ion
exchange capacity: (9) water uptake; (b) conductivity.
Besides the excellent thermal stability and high ionic conduc-
tivity, PSGOHs also had good alkaline stability. The PSGOH
membranes maintain their ionic conductivity after immersion in
1 M NaOH solution at 60 °C for 48 h (Figure 7). The thermal
stability of PSGOH is high enough for use in AEMFCs, since the
operating temperature of current AEMs has typically been
limited to 50 or 60 °C. No obvious decrease of ionic conductivity
was observed for PSGOHs after immersion in either DI water or
1 M NaOH for 30 days at room temperature, indicating good
long-term stability.
Conclusions
In summary, poly(arylene ether sulfone)s were successfully
modified with quaternary guanidinium groups through direct
chloromethylation followed by reaction with pentamethylgua-
nidine. We synthesized the high DS of chloromethylated
polysulfone without gelation and first found that ZnCl2
affected the chloromethylation of polymer directly and could
determine the DS values by controlling the amount of ZnCl2
used. The obtained quaternized ionomers exhibited an excel-
lent solubility and formed flexible and tough membranes of
varying ionic content by casting from a DMSO solution. These
novel AEMs based on quaternary guanidinium hydroxide
groups, denoted PSGOHs, presented excellent thermal stabi-
lities, high ionic conductivities, and good alkaline stabilities.
The preliminary properties have demonstrated their potential
as electrolytes for hydroxide anion exchange membrane fuel
cells. The obtained results will aid in designing superior
membranes, e.g., the cross-linked or side chain polymers based
on varied quaternary guanidinium groups for improved mem-
brane performances.
Figure 7. Stability of ionic conductivity in 1 M NaOH at 60 °C.
swelling ratio for the in-plane direction of the PSGOH mem-
branes. An increase in water uptake with increase temperature
was also observed. For example, at 60 °C, PSGOH-1.2 showed
107% water uptake and 46% swelling ratio for the in-plane
direction, which was much higher than that at 20 °C (55% water
uptake and 20% swelling ratio).
The hydroxide conductivities of the PSGOHs were measured
after the membranes were initially hydrated by immersion in
deionized water for 24 h at room temperature. The hydroxide
conductivity values reported in Table 3 are expressed in Figure 6
as functions of the IEC. In general, a conductivity above
10-2 S cm-1 is required for hydroxide exchange membrane
materials used in fuel cells. PSGOH-0.4, 0.6, 1.0, 1.2, and 1.4

3896 Macromolecules, Vol. 43, No. 8, 2010
Wang et al.
Acknowledgment. We gratefully acknowledge the National
Basic Research Program of China (No. 2009CB623401), the
National Science Foundation of China (No. 20802071,
50825302, 50973106), China Postdoctoral Science Foundation
(200801354), Scientific and Technological Planning Project of
Jilin Province (No. 20080117), and the Development of Scientific
and Technoloical Project of Jilin Province (No. 20080620) for the
financial support.
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