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Pendant crosslinked poly(aryl ether sulfone) copolymers sulfonated on backbone for proton exchange membranes.


Fuel cells are considered to be clean and efficient power sources in portable and stationary electronic applications due to the direct conversion of the chemical energy into electric energy [1-3]. Interest has recently increased in the development of new polymer electrolyte membranes (PEMs) for fuel cells, as PEM is one of the most important components in the fuel cell system [4-7]. To be applied in polymer electrolyte membranes fuel cells (PEMFCs), the advanced membrane materials should meet the following requirements: (i) high ionic conductivity; (ii) low fuel permeability; (iii) good thermal and hydrolytic stability; (iv) excellent electrochemical stability in an aggressive environment; (v) substantial morphological and dimensional stability; (vi) outstanding mechanical properties in both the dry and hydrated states; (vii) sufficient water uptake and moderate swelling; (viii) suppressed water transport through diffusion and electroosmosis; (ix) easy fabrication to form the membrane electrode assembly (MEA), and finally, more importantly from a practical point of view; (x) a competitive low-cost and sufficient long-term durability [7, 8]. Currently, typically used membrane materials for PEMFC are perfluorinated copolymer s (such as Nafion) because of their excellent mechanical properties and chemical stability. However, high cost, poor water management capability, and loss of proton conductivity at temperatures above 80[degrees]C as well as high methanol permeability limit their commercial development [9-12].

Many research groups are currently focusing their work on the development of proton conducting nonfluorinated hydrocarbon membranes, especially based on sulfonated aromatic main chain polymers [13-15]. These polymers include different conventionally sulfonated engineering plastics such as sulfonated poly(arylene ether ketone) (sPEK) [16, 17], sulfonated poly(arylene ether sulfone) (sPES) [18, 19], sulfonated poly(phenyl sulfones)

[20, 21], and so on, which have shown promising results. Compared to the Nafion membrane, sulfonated aromatic polymers are required to have a high degree of sulfonation (DS) to exhibit a high enough proton conductivity for PEM application, whereas the membranes so high DS that they are soluble or highly swellable in methanol, which is detrimental to the direct methanol fuel cells (DMFC) MEA [22]. Crosslinking of the membrane represents a simple and effective method of preventing undesirable swelling of highly sulfonated aromatic polymers [23]. Wang et al. [24] found that the polymers with pendant sulfonic acid groups are more stable to hydrolysis than those with sulfonic acid groups directly attached on the polymer (aryl) backbone. Therefore, the physical and chemical stability of the crosslinked sulfonated poly(arylene ether ketone) (CSPAEK) membranes can be controlled by appropriately balancing the number of sulfonic acid groups attached on the polymer backbone and that of the pendant positions, as well as by changing DS. To increase the proton conductivity to maintain high thermal, mechanical, and hydrolytic resistance, not only the backbone (aryl) but also the pendant sulfonation may be a good trial. Fang et al. [25] reported the preparation of crosslinked membranes based on the condensation reaction of sulfonic acid groups (-S[O.sub.3]H) with electron-rich phenyl rings in the same polymer matrix. Their results showed that these modifications provided improvements in mechanical properties and dimensional stability. However, the disadvantage was the loss of sulfonic acid groups, which are required to achieve high proton conductivity.

In this study, we synthesized a series of sulfonated poly(aryl ether sulfone) copolymers (SPSFs) containing phenyl pendant groups with sulfonic acid groups on the backbone. The DS is controlled by changing the feed ratios of sulfonated to unsulfonated monomers. Postpolymerization reactions were used to produce a series of crosslinked SPSFs (CSPSFs). We use 4,4'-thiodibenzoic acid (TDA) as a crosslinker and the carboxylic acid groups in TDA can undergo Friedel-Craft acylation with the phenyls pendent rings in SPSFs to prepare PEMs for fuel cell applications.



Bisphenol A (BPA), phenol, acetophenone, 98% sulfuric acid, anhydrous potassium carbonate, dichloromethane, methylbenzene (MB), N,N-dimethylformamide (DMF), fuming sulfuric acid (30% S[O.sub.3]) (all from Chengdu Ke Long Co.), which were used as received. Mercaptoacetic acid (analytically pure, from aladdin), 4,4'-dichlorodiphenyl sulfone (DCDPS), (AR, JiangSu Yansheng Chemical Industry Company). A-methyl-2-pyrrolidone (NMP), JiangSu NanJing JinLong Chemical Industry Company. The disodium 3,3'-disulfonate-4, 4'-dichlorodiphenylsulfone (SDCDPS) [26], 1,1-bis(4-hydroxyphenyl)-1-phenylethane (PhBPA) monomer [27, 28], and 4,4'-thiobenzoic acid (TDA) [29] monomer used in the investigation was synthesized according to the procedure described by previous literature.


Synthesis of Sulfonated Poly(aryl ether sulfone) Copolymers (SPSFs). The sulfonated poly(aryl ether sulfone) copolymers (SPSFs) with different DS were synthesized via an aromatic nucleophilic substitution copolymerization of two different bisphenol (BPA and PhBPA) monomers, various ratios of disulfonated monomers (SDCDPS, monomer x) to nonsulfonated monomers (DCDPS, monomer 100-x) and 1.2 mole ratios of [K.sub.2]C[O.sub.3] in a N-methyl-2-pyrrolidone (NMP)/toluene solvent system (as shown in Scheme 1). In this work, NMP was selected as the reaction solvent and the monomers concentration in NMP was maintained at 25-30 wt% to obtain a rapid reaction. The polymerization was carried out in an inert atmosphere of nitrogen in a 250 ml three-necked round bottom flask fitted with a mechanical stirrer and a Dean-Stark trap with a reflux condenser. The reaction mixture was first heated to 160[degrees]C for 5 h to remove the resulting water, then the temperature was raised to 195[degrees]C and kept for another 14 h to complete the reaction. The reaction solution was poured into 1500 ml water to precipitate fibrous solid when the reactor solution was cooled down to 100[degrees]C. The crude product was pulverized to powder and extracted with water and ethanol for 48 h, respectively. After drying under vacuum at 80[degrees]C for 48 h, the yield was about 95.6%.

Preparation of SPSFs and Crosslinked SPSFs (CSPSFs) Filins. The sodium form of SPSF was dissolved in N,N-DMF with 15 wt% concentration and cast onto clean glass plates followed by heating at 60[degrees]C for 6 h and 120[degrees]C for 6 h to remove the solvent. For the crosslinked membranes, acid-form SPSF and TDA (various ratios) were dissolved in DMF with 15 wt%. DMF was slowly removed in the same manner as described above. The crosslinking bonds were formed by a Friedel-Craft reaction by heating to 160[degrees]C and holding for 10 h under vacuum [30]. All membranes were acidified with 1 M [H.sub.2]S[O.sub.4] for 24 h, thoroughly washed with deionized water and then dried under vacuum at 80[degrees]C for 48 h before use.

Polymer Characterization

Fourier-transform infrared (FTIR) spectra of the samples were recorded on a Nicolet NEXUS 670 spectrophotometer within the wavenumber range of 4000-400 [cm.sup.-1]. The samples of polymer were prepared in the pellet form by mixing the dry powder material with KBr. [sup.1]H NMR spectra were recorded on a 400 MHz Bruker A VANCE 400S spectrometer using deuterated dimethyl sulfoxide (DMSO-[d.sub.6]) as solvent and tetramethylsilane (TMS) as internal standard. Ion exchange capacity (IEC) of SPSF membranes were calculated from the peak area in the [sup.1]H NMR spectra. Titration was also used for the determination of IEC. A piece of SPSF membrane was equilibrated in large excess of 0.05 M NaCl aqueous solution for overnight. The released HCl by the ion exchange was titrated with standard 0.01 M NaOH aqueous solution. The [IEC.sub.T] (calculate the IEC by titration) value was obtained from following equation:

[IEC.sub.T] = Consumed NaOH X molarity NaOH / Weight dried membrane (meq/g)

The IEC values (calculated by theoretical and H NMR) from the degrees of sulfonation were obtained from following equation:

IEC = DS x 1000 / 473 + 102DS


Gel permeation chromatography (GPC) was performed on coupled SDV columns from Polymer Standard Service (GRAM 3000, 1000, and 100) with UV detector (Soma S-3702) and RI detector (ERC 7512; ERMA), calibrated with polystyrene (Polymer Standard Service). The intrinsic viscosities were determined from 0.5 g [dl.sup.-1] NMP solution of sulfonated polymer using a Cannon-Ubbelodhe viscometer at 30[degrees]C. The glass transition temperature of sulfonated polymer was measured on a NETZSCH DSC 200 PC thermal analysis equipment at a heating rate of 10[degrees]C [min.sup.-1] from 50 to 400[degrees]C under nitrogen. The thermal stability was conducted on a TGA Q500 V6.4 Build 193 thermal analysis equipment under nitrogen from 50 to 800[degrees]C at a heating rate of 10[degrees]C [min.sup.-1].

The Proton Conductivities. The membranes were estimated from ac impedance spectroscopy data, obtained over a frequency range of 1-[10.sup.7] Hz with oscillating voltage of 100 mV, using a Solartron 1260 gain phase analyzer. Specimens in the form of 20 x 10 [mm.sup.2] strips were soaked in deionized water for at least 24 h prior to the test. Each specimen was placed in a temperature-controlled cell open to the air by a pinhole, where it was equilibrated at 100% RH at ambient pressure. Each end of the membrane strip was clamped in a frame between two stainless steel electrodes. Measurements, in four-point mode, were carried out after sample conditioning in the closed cell overnight or longer. The conductivity (a) of the samples in the longitudinal direction was calculated, using the relationship:

[sigma] = L/RDW

where L is the distance between the electrodes and D and W are the thickness and width of the sample stripe, respectively. R was derived from the low intersect of the high-frequency semicircle on a complex impedance plane with the Re (z) axis.

Water Uptake and Swelling Ratio Measurements. The membranes were dried at 120[degrees]C overnight prior to the measurements. After measuring the lengths and weights of dry membranes, the sample films were soaked in deionized water for 24 h at different temperatures (20, 30, 40, 50, 60, 70, 80, and 90[degrees]C). Before measuring the lengths and weights of hydrated membranes, the water was removed from the membrane surface by blotting with a paper towel.

The water uptake content was calculated by

water uptake (%) = [[omega].sub.wet] - [[omega].sub.dry] / [[omega].sub.dry] x 100%

where [[omega].sub.dry] and [[omega].sub.wet] are the weights of dried and wet samples, respectively.

The swelling ratio was calculated by

swelling ratio (%) = [l.sub.wet] - [l.sub.dry]/[l.sub.dry] x 100%

where [l.sub.dry] and [l.sub.wet] are the lengths of dry and wet samples, respectively.

Oxidative Stability and Methanol Permeability. The oxidative stability of the membranes was tested by immersing the films into Fenton's reagent (3% [H.sub.2][O.sub.2] containing 2 ppm FeS[O.sub.4]) at 80[degrees]C. Their retained weights (RW) of membranes after treating in Fenton's reagent for 1 h and the dissolved time (r) of polymer membranes into the reagent were used to evaluate oxidative resistance.

The methanol permeability was measured using a glass diffusion cell consisting of two reservoirs. One side of the reservoir (reservoir A) was filled with 2 M methanol solution and the other (reservoir B) with DI water. The membrane was clamped in the middle of the two reservoirs. The solution in each reservoir was continuously stirred to maintain unifonn concentration. The concentration of the permeate (methanol) in reservoir B, [C.sub.B], was continuously measured by a GC-14C gas chromatograph (SHIMADZU, Tokyo, Japan). The diffusion coefficient of methanol, D, was determined from the slope of the linear plot of [C.sub.B] vs. time, t, based on the following equation:

[C.sub.B](t) -A/[V.sub.B] DK/L [C.sub.A] (t - [t.sub.0])

Here, A ([cm.sup.2]) and L (cm) are the area and thickness of the membrane, respectively, [V.sub.B] ([cm.sup.3]) is the volume of the reservoir B, [C.sub.A] is the methanol concentration in the reservoir A, and K is the distribution coefficient. The product DK is equivalent to the methanol permeability.

Mechanical Properties. The mechanical properties of dry and wet membranes were measured at room temperature on an Instron 5565 instrument at a strain rate of 5 mm/min, and a 1 KN load cell was used. The size of samples was 20 mm x 4 mm. The samples in wet state were obtained by immersing them in water for 48 h and the samples in the dry state were obtained by putting samples in vacuum oven at 100[degrees]C for 24 h.

Morphology. The membrane samples were stained with lead ions by ion exchange of the sulfonic acid groups in 0.5 M lead acetate aqueous solution, rinsed with deionized water, and dried in vacuum oven for 12 h. Then the dried sample was embedded in epoxy resin and sectioned using a microtome to yield a 90-nm thick sample which was placed on copper grids. Images were taken on an ultrahigh-resolution transmission electron microscope (Tecnai G2 F20) using an accelerating voltage of 200 kV.


Synthesis and Characterization of SPSFs

A series of copolymers with different DS were prepared by aromatic nucleophilic substitution polycondensation with different ratios of monomers x (SDCDPS), 100x (DCDPS), and two different bisphenol (BPA and PhBPA) monomers in a NMP/toluene solvent system. The polymerization results and analytical data were displayed in Table 1. From Table 1, we found that all copolymers exhibited high intrinsic viscosity, which indicated the polymers had high molecular weight. Moreover, the intrinsic viscosities of copolymers increased with the increment of sodium sulfonate group content because the sulfonated groups enhanced intermolecular associations.

FTIR and [sup.1]H NMR were used to confirm structure of SPSF-x. The successful introduction of the sodium sulfonate groups of copolymers was confirmed by FTIR spectroscopy (Fig. 1). From the difference between the curves, it was obvious to see that the symmetric and asymmetric vibrations of the sulfonic acid group appeared at 1167 and 1023 [cm.sup.-1] enhancement with the increase of the DS, respectively. The [sup.1]H NMR spectra of SPSF-x in DMSO-[d.sub.6] are displayed in Fig. 2. The pendant and isopropylidene -C[H.sub.3] protons ([H.sub.11], [H.sub.10]) appear at 2.21 and 1.65 ppm, respectively. Aromatic protons ([H.sub.3], [H.sub.4], [H.sub.12], [H.sub.13], [H.sub.14] and [H.sub.6], [H.sub.7], [H.sub.8], [H.sub.9]) at the ortho position of the electron donating groups appeared at high field area (7.4-7.2 and 7.2-6.8 ppm) and while [H.sub.5] and [H.sub.2] at the ortho position of the electron withdrawing sulfone groups were at low field area (8.0-7.9 ppm). The proton located at sulfonated groups (Hi) was deshielded due to the strongly electron-withdrawing effects of -S[O.sub.3]H and -S[O.sub.2]-, their signals transferred into low field area (8.27 ppm).

Preparation of Crosslinked Membranes

The preparation of crosslinked SFSF (CSPSF) membranes was performed by two steps. SPSF solutions in DMF blending with a certain weight ratios of TDA were casted into films and followed by heating at 60[degrees]C for 6 h and 120[degrees]C for 6 h to remove the solvent, then subsequently thermally treated at high temperature (160[degrees]C) in vacuum for 10 h. The crosslinking reaction of SPSF was obtained by a Friedel-Craft reaction at 160[degrees]C between carboxylic acid groups in the TDA and the nucleophilic phenyl rings in the SPSF. The crosslinking mechanism is shown in Scheme 1. Under acidic conditions, a carboxylic acid group on a phenyl ring is very reactive towards an aromatic ring. An electrophilic aromatic substitution reaction could occur between them in SPSF, resulting in a crosslinking structure. The sulfonic acid groups in SPSF not only serve as proton transport facilitators but also as a benign solid catalyst for the Friedel-Craft reaction at higher temperature.

Figure 3 shows the FTIR spectra of TDA, SPSF60 and CSPSF60 membranes. All of the spectra showed the absorption at 1591 and 1488 [cm.sup.-1] , apparently corresponding to stretching vibrations of C=C in aromatic ring. SPSF60 exhibited two characteristic bands at 1167 and 1028 [cm.sup.-1] for asymmetric and symmetric O=S=O stretching vibrations of sulfonic acid groups. These characteristic bands for sulfonic acid groups were also observed in the spectrum of a crosslinked membrane. The absorption band at 1082 and 1090 [cm.sup.-1] was attributed to the C--S--C stretching of TDA and CSPSF60, respectively. Finally, in comparison with SPSF60, the strong absorption at 1680 and 1665 [cm.sup.-1] due to the C=0 asymmetric stretching of the carboxylic acid groups (COOH) in TDA and of the ketone in CSPSF60 membrane, respectively. All of these results suggested that the crosslinking of SPSF was achieved successfully with the assistance of TDA.

The qualitative solubilities of the SPSFs and CSPSFs are summarized in Table 2. We found that the SPSFs membranes can be easily dissolved in common organic polar solvents, such as DMAc, NMP, and DMF, while CSPSFs membranes become insoluble in these solvents even at 80[degrees]C after fully thermal-treated at 160[degrees]C. This result also suggested the formation of a crosslinked network among CSPSFs polymer chains in the membranes.

Thermal and Mechanical Properties

The thermal stability of crosslinked membranes, which is critical for the durability during fuel cell operation at high temperature, was evaluated using TGA experiments and the results are presented in Fig. 4. All the membranes exhibited three-step weight loss. The first weight loss observed from room temperature to 200[degrees]C was associated with the weight loss of absorbed water. This weight loss increased with increasing DS, implying that the dried samples with a higher DS absorbed more water because of their higher hydrophilicity. Comparing with the crosslinked membranes, the pristine membrane exhibited much weight loss in this region due to its high water uptake, and these water molecules seemed to be bound directly to the sulfonic acid group via hydrogen bonds. The second weight loss region from 270 to 400[degrees]C corresponded to the loss of sulfonic acid groups. In the third weight loss region (>450[degrees]C), the polymer went through further degradation, and this region was attributed to the decomposition of the main chain of the copolymer. All the membranes show excellent thermal stability before 250[degrees]C, which is higher than that of Nafton 117. The crosslinked backbone of SPSFs makes it much stable than the flexible main chain structure.

The glass transition temperature ([T.sub.g]) of the SPSFs and CSPSFs were investigated using Differential scanning calorimetry (DSC) as shown in Fig. 5. The glass transition temperature ([T.sub.g]) of the SPSFs increased with an elevated IEC values and the [T.sub.g] of the crosslinked membranes was greatly increased comparing with that of the uncrosslinked membranes. As shown in Fig. 5, all the [T.sub.g]s of the SPSFs and CSPSFs were about 200-250[degrees]C, which are much lower than the decomposition temperature. The thermal properties of the CSPSF membranes studied in this work were high enough for serving as a PEM in fuel cell applications.

The mechanical properties of crosslinked membranes were compared to the SFSFs in both the dry and wet state. As shown in Table 3, the mechanical properties of all membranes in the wet state are not as good as those of membranes in dry state. All the crosslinking membranes showed much higher Young's modulus and tensile strength than that of SPSFs, which satisfied the demands of DMFC. However, the elongation at break decreased. In general, three-dimensional network made by crosslinking restricts the molecular chain motion of the polymer, and improves the mechanical performances of the membranes.

IEC, Water Uptake, and Swelling Ratio

IEC value is very closely related to dimensional stability and proton conductivity of PEM under fuel cell operation condition. Although the level of hydration of proton exchange membranes is highly dependent on the IEC, at high levels of hydration the mechanical properties are typically compromised and acid concentration is reduced because of the high water uptake and swelling ratio. Thus SPSFs was designed to prepare with appropriate IEC values between 0.41 and 2.11 mequiv [g.sup.-1]. And the experimental IEC ([IEC.sub.T] and [IEC.sub.N]) values of SPSFs determined by titration and [sup.1]H NMR were agreed well with calculated IEC ([IEC.sub.CAL]) values (Table 1).

For most of the ionic conductive polymers, water is very important for the transport of ions through the membranes. Appropriate water uptake of the membrane is necessary to provide a carrier for ions and maintain high ionic conduction. However, excessive water uptake in the membranes usually leads to poor mechanical properties and high fuel permeability. The water uptake of the prepared membranes with different DS and crosslinking densities is shown in Fig. 6. The water uptake of the CSPSF membrane increased with increasing IEC and temperature values at 10% crosslinking density was shown in Fig. 6a. As expected, the water uptake decreased with increasing crosslinking density. For example, the water uptake and swelling ratio of SPSF30 were 93.2% and 32.9% at 90[degrees]C, respectively, but they were merely 32.9% and 8.7% after forming crosslinked network structure values at 40% crosslinking degree. The water uptake of the crosslinked membranes was greatly reduced compared with that of the uncrosslinked membranes. However, an excessively high crosslinking density makes the molecular structure of the membrane so tight that the water uptake may be greatly reduced in a brittle state, as shown in Fig. 6b.

Proton Conductivity and Oxidative Stability

The proton conductivity is a critical parameter for the proton exchange membrane used in fuel cells. The proton transport in membranes requires both well connected ion channels and proper contents of boned water. Figure 7a shows the temperature dependence of proton conductivity of membranes (CSPSF30) with different crosslinking degrees (CD). Increase of the crosslinking degree leads to a slight decrease in proton conductivity, and the effect of DS on the proton conductivity of CSPSF with a fixed CD of 10% is also illustrated in Fig. 7b. The proton conductivity increases considerably with increasing temperature and DS because the water uptake increases and thus protons are more feasibly transferred through enlarged ion channels. In Fig. 7a, the conductivity is decreased with increasing crosslinking density of the membrane. However, this crosslinking density effect on the proton conductivity was not as strong as the DS effect, as the sulfonic acid groups were attached on the crosslinker in this study to minimize such a negative effect. The uncrosslinking membrane showed a very high conductivity of 113.2 mS [cm.sup.-1] at 90[degrees]C, which was higher than that of Nafion (~ 100 mS [cm.sup.-1]) [31] under the same condition. However, the proton conductivity of the crosslinking membranes was lower than that of Nafion 117.

The oxidative stability of the membranes SPSFs and CSPSFs were evaluated in a hot Fenton's reagent (80[degrees]C) for 1 h as an accelerated test. The results are summarized in Table 3. Crosslinking generally caused significant improvement of oxidative stability. This is consistent with the change of water uptake we have discussed above.

Therefore, we can presume that the oxidative stability greatly depended on the water content absorbed by the membranes. The crosslinking network structure decreases the water uptake of the films, which probably makes the polymer chain to be less attacked by water molecules containing oxidizing species and hence the oxidative stability increases.

Methanol Permeability and Selectivity

Membranes for the practical application in DMFC must possess both high proton conductivity and high methanol barrier. In Fig. 8, the crosslinking density effect on the methanol permeability of membranes prepared is shown. The pristine membrane (SPSF30) showed the highest methanol permeability of 6.2 x [10.sup.-7] [cm.sup.2] [s.sup.-1] and after crosslinking, the methanol permeability substantially decreased from 4.3 to 1.8 x [10.sup.-7] [cm.sup.2] [s.sup.-1]. The results showed a same tendency as the water uptake. This phenomenon indicated that the methanol transport in the same ion channels as water which are compact after crosslinking.

In fuel cell application, higher proton conductivity but lower methanol permeability is preferable for PEM. However, as proton conductivity and methanol permeability generally behave in a similar trend, selectivity, the ratio of proton conductivity to methanol permeability, is a good measure of membranes' effectiveness in fuel cell. As shown in Figs. 7a and 8, the crosslinking degrees contributed a lot to the proton conductivity and the methanol permeability of the membrane, which determined the selectivity. In Fig. 9, the selectivity of the CSPSF membrane is illustrated according to the crosslinking density. Selectivity increases with increasing crosslinking density and shows the maximum at 20%, followed by a decrease afterwards. However, an excessively high crosslinking density makes the molecular structure of the membrane so tight that the proton conductivity and methanol permeability may be greatly reduced. In 0-20%, the proton conductivity was less strongly influenced by crosslinking than methanol permeability, but at 30%, the conductivity reduction was much more significant than methanol permeability. Therefore, the CSPSF system with 20% crosslinking was much more suitable for use in fuel cells.

Morphologies of Membranes

The hydrophilic/hydrophobic microphase separation morphology is particularly important for PEM materials because it affects the water uptake and the proton transport pathway in the PEM. The morphology of the CSPSF membranes with different DS values was studied extensively by TEM. The CSPSF membranes were first treated by soaking 0.5 M lead acetate aqueous solution for 24 h, and then the dried sample was embedded in epoxy resin and sectioned using a microtome to yield a 90-nm thick sample which was placed on copper grids prior to measurement. As is clearly seen in Fig. 10, CSPSF20 and CSPSF 40 exhibited spherical ionic clusters (dark areas) and the diameter of spherical ionic clusters is increasing with the sulfonation degree increased. This kind of hydrophilic/hydrophobic microphase separation morphology may explain that the dimension of the ionic clusters increase with increasing DS. The results strongly confirmed the experimental data revealing that the proton conductivity was enhanced with increasing DS.


A series of sulfonated poly(aryl ether sulfone) copolymers containing phenyl pendant groups with sulfonic acid groups on the backbone were synthesized by nucleophilic polycondensation. Varying the feed ratios of sulfonated monomers to nonsulfonated monomers could control DS of the copolymers. All copolymers possessed relatively high molecular weight and could be cast into tough membranes. Post-crosslink reactions are carried out with 4,4'TDA as a crosslinker and the carboxylic acid groups in TDA can undergo Friedel-Craft acylation with the phenyls pendent rings in sulfonated poly(arylene ether sulfone)s copolymers to prepare PEMs for fuel cell applications. Crosslinked copolymers exhibited higher [T.sub.g]s than those of uncrosslinked copolymers with same DS. All copolymers exhibited good thermal stabilities. The proton conductivity of CSPSFs membranes increases considerably with increasing temperature and sulfonation degrees decreases considerably with the crosslinking degree increasing. Transmission electron microscopic observations revealed that CSPSFs membranes form welldefined microphase separated structures. In conclusion, the CSPSFs membranes show very good prospects in proton exchange membranes fuel cell (PEMFC) application.


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Dongsheng Li, (1) Zhimin Li, (2) Fei Hu, (2) Shengru Long, (1) Gang Zhang, (1) Jie Yang (1,3)

(1) Institute of Materials Science and Technology, Analysis and Testing Center, Sichuan University, Chengdu 610064, China

(2) College of Polymer Materials Science and Engineering, Sichuan University, Chengdu 610065, China

(3) State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China

Correspondence to: Gang Zhang; e-mail: (or) Jie Yang; e-mail:

DOI 10.1002/pen.23754

Published online in Wiley Online Library (

TABLE 1. The DS, inherent viscosities, average molecular weights,
polydispersity index (PDI), and 1EC of SPSF copolymers.

          DS (a)   [[eta].sub.inh] (b)      [M.sub.n]
Polymer    (%)       (dL [g.sup.-1])     (g [mol.sup.-1])   PDI

SPSF0       0             0.74                48,456        1.87
SPSF10      20            0.86                42,376        2.03
SPSF20      40            0.87                45,904        1.94
SPSF30      60            0.90                39,438        1.88
SPSF40      80            0.92                41,045        2.10
SPSF50     100            1.01                42,376        1.78
SPSF60     120            1.16                38,903        1.91

                IEC (mmol [g.sup.-1])

Polymer   [IEC.sub   [IEC.sub.N]   [IEC.sub.T]
           .CAL]        (c)            (d)

SPSF0        0            0             0
SPSF10      0.41        0.38          0.32
SPSF20      0.79        0.75          0.69
SPSF30      1.15        1.20          1.06
SPSF40      1.49        1.44          1.31
SPSF50      1.81        1.77          1.59
SPSF60      2.11        2.05          1.98

(a) DS refers to the number of sulfonic acid group on every
polymer repeat unit.

(b) Viscosity before crosslinked: 0.5 g d[L.sup.-1]
in NMP solution at 30 [+ or -] 0.1[degrees]C.

(c) IEC from [sup.1]H NMR spectra.

(d) IEC from titration.

TABLE 2. Solubility of the SPSFs and CSPSFs.

Membranes            Methanol

            25[degrees]C   80[degrees]C

SPSF20           -              -
SPSF40           -              -
SPSF60           -              -
CSPSF20          -              -
CSPSF40          -              -
CSPSF60          -              -

Membranes               NMP

            25[degrees]C   80[degrees]C

SPSF20           +              +
SPSF40           +              +
SPSF60           +              +
CSPSF20          -             + -
CSPSF40          -             + -
CSPSF60          -             + -

Membranes               DMF

            25[degrees]C   80[degrees]C

SPSF20           +              +
SPSF40           +              +
SPSF60           +              +
CSPSF20          -             + -
CSPSF40          -             + -
CSPSF60          -             + -

Membranes              DMAc

            25[degrees]C   80[degrees]C

SPSF20           +              +
SPSF40           +              +
SPSF60           +              +
CSPSF20         + -            + -
CSPSF40         + ~            + -
CSPSF60         + -            + -

+ : soluble; insoluble; + -; swollen.

TABLE 3. Mechanical properties and oxidative stability
of SPSF and CSPSF copolymers.

                     Tensile strength   Young's modulus
                           (MPa)            (GPa)

Polymer     CD (%)    Dry        Wet     Dry      Wet

SPSF 10       0       33.9       26.6    1.23     1.03
SPSF30        0       22.3       17.5    0.85     0.64
SPSF50        0       14.4        8.6    0.64     0.37
CSPSFI0       20      50.8       46.9    1.89     1.47
CSPSF30       20      34.2       26.6    1.05     0.83
CSPSF50       20      20.4       12.7    0.87     0.73
Nafian 117    --       --         --      --       --

             Elongation at
               break (%)      Oxidative stability

Polymer      Dry      Wet     RW (%)     T (h) (b)

SPSF 10       7.3    10.4       98           >6
SPSF30        9.1     8.5       95     [approximately
                                        equal to] 6
SPSF50       12.4    11.2       87            4
CSPSFI0       4.7     6.3       99           >10
CSPSF30       6.8     5.3       99           >8
CSPSF50       9.6     8.5       97           >7
Nafian 117     --      --       97           >6

CD: crosslinking density (on the basis of TDA
to acid-form SPSF mole ratios).

(a) Retained weights of membranes after treating
in Fenton's reagent for 1 h.

(b) The dissolved time of polymer membranes.
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Author:Li, Dongsheng; Li, Zhimin; Hu, Fei; Long, Shengru; Zhang, Gang; Yang, Jie
Publication:Polymer Engineering and Science
Article Type:Report
Date:Sep 1, 2014
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