Synthesis and characterization of sulfonated poly(arylene ether)s with sulfoalkyl pendant groups for proton exchange membranes

Journal of Membrane Science 318 (2008) 271–279

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Synthesis and characterization of sulfonated poly(arylene ether)s with sulfoalkyl pendant groups for proton exchange membranes Jinhui Pang, Haibo Zhang, Xuefeng Li, Lifeng Wang, Baijun Liu, Zhenhua Jiang ∗ Alan G. MacDiarmid Institute, Institute of Chemistry, Jilin University, Changchun 130012, PR China

a r t i c l e

i n f o

Article history: Received 13 December 2007 Received in revised form 1 February 2008 Accepted 25 February 2008 Available online 29 February 2008 Keywords: Sulfonated polymers Poly(arylene ether ketone)s Proton exchange membranes Fuel cells

a b s t r a c t Side-chain-type sulfonated poly(arylene ether)s bearing sulfoalkyl pendant groups (PSA-SPAEs) as proton exchange membranes for fuel cells were prepared via nucleophilic substitution polycondensation reactions of sodium 3-(4-(2,6-difluorobenzoyl)phenyl)propane-1-sulfonate (SDFPPS), 4,4 -dihydroxyldiphenylether (DHDPE) and 4,4 -dichlorodiphenyl sulfone (DCDPS). The sulfonic acid group content (SC), as the quantity per repeat unit of polymer, ranging from 0.5 to 0.8, was readily controlled by changing the feed ratio of SDFPPS to DCDPS. Good thermal properties of PSA-SPAE copolymers were indicated by observing glass transition temperatures (Tg s) ranging from 210 to 230 ◦ C in sodium salt form and decomposition temperatures (Td s) over 220 ◦ C in acid-form and over 400 ◦ C in sodiumform in air. All PSA-SPAE membranes exhibited reasonable flexibility and tensile strength in the range of 41–78 MPa. PSA-SPAE copolymers bearing sulfonic acid groups on flexible side chains showed considerably reduced swelling ratio and improved proton conductivities. Proton conductivity curves parallel to those of Nafion 117 were obtained with proton conductivity of 10−1 S/cm at equivalent ion exchange capacities (IEC) of 1.47 and 1.64, comparable to Nafion 117. PSA-SPAE copolymer with SC 0.8 had the best data including PEM mechanical strength, water swelling and proton conductivity. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The electrolyte material used in current PEM fuel cells is a kind of hydrated sulfonic acid functionalized polymer. Because of the harsh conditions in operating fuel cells (high temperatures, high water activities, and the appearance of highly reactive oxidizing radicals), hydrolytic, thermoxidative, and (electro-) chemical stabilities are key issues in choosing the ionomers. Nowadays, the membranes materials are perfluorosulfonic acid (PFSA) polymers such as Nafion (DuPont), featuring superior stability compared to most hydrocarbon-based membranes. But their high water and methanol “crossover”, low proton conductivity, poor mechanical stability at elevated temperatures (T > 80 ◦ C) and low degrees of humidification are still disadvantages of these state-of-the-art membrane materials [1,2]. Hence, great efforts have been devoted to searching high-performance proton conducting polymers of low cost as alternative materials [3–5]. Sulfonation of aromatic polymers including sulfonated polysulfones, sulfonated poly(ether ether ketone)s [6,7], sulfonated polyimides [8,9], and sulfonated poly(benzimidazole) [10] is one of the available options.

∗ Corresponding author. Tel.: +86 431 85168886; fax: +86 431 85168868. E-mail addresses: [email protected], [email protected] (Z. Jiang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.02.051

Recently, those sulfonate aromatic polymers with sulfonic acid groups located in side chains (i.e., side-chain-type sulfonate polymer) have attracted much attention [11–14]. With this method, hydrophobic backbones and hydrophilic sulfonic acid groups are well separated, which makes it feasible to obtain more distinctly separated nano-phases and keeps these polymers with good water swelling stability and high conductivity [15–17]. Rikukawa and coworkers prepared SPEEK and sulfonated poly(4-phenoxybenzoyl-1,4-phenylene) (SPPBP) by post-sulfonation reactions of corresponding parent polymers [18]. They found that SPPBP, which had short pendant between polymer main chain and sulfonic acid groups, showed higher and more stable proton conductivity. Miyatake and coworkers synthesized novel sulfonated polyimide ionomers containing aliphatic groups both in the main chain and in the side chains [19–21]. They found that SPI with sulfonic groups introduced to flexible aliphatic side chains, the proton conduction activation energy (Ea ) was smaller than that of SPI with sulfonic groups directly attached to main chain. Some studies about sulfonated poly(arylene ether)s with sulfonic groups introduced to side chains have been also reported in recent years [22–28]. Most of them were obtained by graft or post-sulfonation process to introduce sulfonated group to polymer side chain, while direct copolymerization method was seldom adopted due to the limitation of sulfonated monomers.

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In previous work, we have reported the synthesis of sidechain-type sulfonated poly(arylene ether)s (SC-SPAE [29] and PSA-SPAE-6F [30]) via novel sulfonated monomers with direct copolymerization method. Two series of polymers both exhibit high proton conductivity and thermal and oxidative stability, which remain excellent dimensional stability in hot water. However, SCSPAE polymers do not exhibit perfect mechanical property, due to their wholly aromatic and high rigidity structure. When wholly aromatic rigidity side chain changes to flexible aliphatic side chain in monomer, the mechanical property of PSA-SPAE-6F was improved greatly. In our opinion, if diphenol structure of polymer main chain is adjusted (i.e., highly flexible diphenol), the polymer mechanical property will be further improved. In this paper, with novel sulfonated monomer, soduim-3-(4(2,6-difluorobenzoyl)phenyl)propane-1-sulfonate (DFPPS), 4,4 dichlorodiphenyl sulfone (DCDPS) and high flexible diphenol 4,4 -dihydroxyldiphenylether (DHDPE), we successfully prepared novel sulfonated poly(arylene ether)s with pendant sulfoalkyl groups (PSA-SPAE) by direct copolymerization method. These polymer membranes exhibited good dimensional stability, high proton conductivity, and reasonable tensile strength. With integrative performance above, they are promising for application as PEMFC. 2. Experimental 2.1. Chemicals and materials 2,6-Difluorobenzoyl chloride, 1-bromo-3-phenylpropane and 4,4 -dihydroxyldiphenylether (DHDPE) were purchased from Aldrich. 4,4 -dichlorodiphenyl sulfone (DCDPS) was purchased from Shanghai Chemical Factory. Other chemical reagents were purchased from Beijing Chemical Reagent and were purified by conventional method. K2 CO3 was dried at 120 ◦ C for 24 h before polymerization. 2.2. Synthesis of sulfonated monomer sodium 3-(4-(2,6-difluorobenzoyl)phenyl)propane-1-sulfonate (DFPPS) (Scheme 1) 3-(4-(2,6-Difluorobenzoyl)phenyl)propane-1-sulfonate (DFPPS) was synthesized according to a procedure described at previous reported [30]. Monomer’s structures were confirmed by 1 H NMR. Monomer A: (4-(3-bromopropyl)phenyl(2,6-difuorephenyl)methanone. 1 H NMR(CDCl ) ı: 2.19 (q, J = 7.0 Hz, 2H, H ); 2.87 (t, J = 7.5 Hz, 3 f 2H, He ); 3.4 (t, J = 6.5 Hz, 1H, Hg ); 7.00 (t, J = 8.3 Hz, 2H, Hb ); 7.32 (d, J = 8.2 Hz, 2H, Hd ); 7.45 (m, 1H, Ha ); 7.81(d, J = 8.1 Hz, 2H, Hc ).

Monomer B: soduim-3-(4-(2,6-difluorobenzoyl)phenyl)propane1-sulfonate (DFPPS). 1 H NMR (DMSO-d ) ı: 1.86 (q, J = 7.5 Hz, 2H, H ); 2.37 (t, J = 7.9 Hz, 6 f 2H, He ); 2.74 (t, J = 7.7 Hz, 1H, Hg ); 7.29 (t, J = 8.2 Hz, 2H, Hb ); 7.39 (t, J = 8.1 Hz, 2H, Hd ); 7.66 (m, 1H, Ha ); 7.71 (d, J = 8.1 Hz, 2H, Hc ). 2.3. Synthesis of sulfonated copolymers The following procedure represents a typical polymerization that’s about the polymer PSA-SPAE-50 preparation, as shown in Scheme 2. DFPPS (0.005 mol, 1.810 g), DCDPS (0.005 mol, 1.435 g), DHDPE (0.01 mol, 2.020 g), potassium carbonate (0.012 mol, 1.650 g), tetramethylene sulfone (TMS, 18 mL) and toluene (10 mL) were added into a 50 mL three-necked round-bottom flask equipped with a Dean-Stark trap and a nitrogen inlet. The reactor above was purged with nitrogen, and a slow flow of nitrogen was maintained during the entire reaction. The mixture was heated with continuous stirring. After reaction at 140 ◦ C for 2 h, water and toluene were removed by azeotropic distillation at 150 ◦ C. The system was then heated to 185 ◦ C, and the mixture was stirred at this temperature for 11 h. After the reaction, 5 mL of TMS was added to the mixture to lower solution viscosity. The solution was poured into 100 mL deionized water. The product was washed with hot deionized water and methanol, respectively, several times and treated in a Soxhlet extractor with alcohol at reflux. The resulting product was dried in vacuo at 80 ◦ C for 15 h. 4.3 g pure PSA-SPAE-50 was obtained (yield: 91%). 2.4. Membrane preparation Sodium-form PSA-SPAE (sodium-form) copolymer (1.0 g) was dissolved in N-methylpyrrolidinone (10 mL) overnight. The solution was then filtered with a fine glass frit filter funnel and cast directly onto clean glass plates. After carefully dried at 60 ◦ C for 10 h and vacuum dried at 120 ◦ C for 24 h, tough and flexible sodium-form films were obtained. The sodium-form PSA-SPAE membranes were transformed to their acid forms by proton exchange in 1 M H2 SO4 for 24 h at room temperature. The acidified films were then soaked in, and washed thoroughly with, deionized water. The thickness of membranes was in the range of 90–110 ␮m. 2.5. Characterizations 2.5.1. Copolymer analysis and measurements The viscosities of the obtained copolymers were determined using Ubbelohde viscometer in thermostatic container with the polymer concentration of 0.5 g/dL in NMP at 25 ◦ C. FT-IR spectra (film) were measured on a Nicolet Impact 410 Fourier-transform

Scheme 1. Synthesis of sulfonated monomer.

J. Pang et al. / Journal of Membrane Science 318 (2008) 271–279

273

Scheme 2. Synthesis of sulfonated poly(arylene ether)s with pendant sulfoalkyl groups (PSA-SPAE-X) by direct copolymerization.

infrared spectrometer. 1 H NMR and 13 C NMR experiments were carried out on a Bruker 510 spectrometer (1 H, 500 MHz, 13 C, 125 MHz) by using DMSO-d6 as solvent.

where Tdry and Ldry are the thickness and lengths of the dry membrane, respectively. Twet and Lwet refer to the membrane immersed in deionized water.

2.5.2. Thermal properties of membranes Differential scanning calorimeter (DSC) measurements were performed on a Mettler Toledo DSC821e instrument at a heating rate of 10 ◦ C/min from 50 to 300 ◦ C under nitrogen. The glasstransition temperatures (Tg ) of the copolymers were reported as the midpoint of the step transition in the second heating run. Thermogravimetric analysis (TGA) was employed to assess thermal stability of membranes with a PerkinElemer Pyris 1 thermal analyzer system. Before the analysis, the films were dried and kept in the TGA furnace at 120 ◦ C under an air atmosphere for 30 min to remove water. The samples were evaluated in the range of 100–800 ◦ C at a heating rate of 10 ◦ C/min in air.

2.5.4. Oxidative stability Oxidative stability of the membranes was tested by immersing the films into Fenton’s reagent (3% H2 O2 containing 2 ppm FeSO4 ) at 80 ◦ C. The oxidative stability was evaluated by their retained weight (RW) of membranes after treating in Fenton’s reagent for 1 h and the dissolved time (t) of polymer membranes into the reagent were used to evaluate oxidative resistance.

2.5.3. Water uptake and swelling ratio measurements The sample films (1 cm × 5 cm) were dried at 120 ◦ C for 24 h prior to the measurements. After measuring the lengths and weights of dry membranes, the sample films were soaked in deionized water to reach equilibrium at desired temperature. 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 was calculated by the following equation Water uptake (%) =

Wwet − Wdry Wdry

× 100%

where Wdry and Wwet are the weights of dried and wet samples, respectively. Dimensional changes of the copolymer membranes was investigated by immersing the sample films in deionized water to reach equilibrium at desired temperature. The change of film length and thickness were calculated from T =

Twet − Tdry Tdry

,

L =

Lwet − Ldry Ldry

2.5.5. Proton conductivity Proton conductivity measurements were conducted using ac impedance spectroscopy (Solatron-1260/1287 impedance analyzer) over a frequency range of 10–107 Hz with 50–500 mV oscillating voltage. A sheet of sulfonated membrane (15 mm × 10 mm) was placed in a test cell similar to that in previous reports [31]. Before measurement, the films were fully hydrated in water for 48 h. Temperature was controlled through a wrap-around resistance heater with a feed-back temperature controller. The impedance measurements were performed in water vapor with 100% relative humidity (RH) at desired temperature. The conductivity () of the films in the transverse direction was calculated by the following equation:

=

D LBR

where D is the distance between the two electrodes, L, B and R are the thickness, width and resistance of the film samples. 2.5.6. Methanol permeability Methanol permeability coefficient D (cm2 /s) was determined by using a cell that consists of two half cells separated by the membrane. Methanol (15 mol/L) was placed on one side of the diffusion cell (A cell) and water was placed on the other side (B cell). Magnetic stirrer was used on each compartment to ensure uniformity. Before test the membranes were immersed in deionized water for 24 h. The

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concentrations of the methanol in B cell were periodically determined by using SHIMADU GC-8A chromatograph. The methanol permeability coefficient was calculated as following formula [32]. CB (t) =

A DK CA (t − t0 ) VB L

where DK is the methanol diffusion coefficient (cm2 /s), CB (t) concentration of methanol in B cell (mol/L), CA concentration of methanol in A cell (mol/L), A membrane area (cm2 ) and L is thickness of membrane (cm). 2.5.7. Mechanical property The film’s (thickness about 90–110 ␮m) mechanical property was evaluated at room temperature and 40% relative humidity on SHIMADIU AG-I 1KN at the test speed of 2 mm/min, and the size of specimen was 20 mm × 4 mm. For each testing reported, three measurements were taken and average value was calculated at least. 2.5.8. Morphology Tapping mode AFM observations were performed with a Digital Dimension 3000 Instrument, using micro-fabricated cantilevers with a force constant of approximately 40 N/m. The ratio of amplitudes used in feedback control was adjusted to 0.6 of the free air amplitude for all the reported images. All samples were measured under relative humidity of 30%. 3. Results and discussion 3.1. Syntheses and characterization of monomer Despite the limited number of the available sulfonated monomers and the preparation difficulties of some sulfonated monomers, the direct copolymerization of sulfonated monomer with other non-sulfonated monomers has the potential for synthesizing random copolymers with a better control of sulfonation content (SC) and more defined chain structures in comparison with the copolymers by the post-sulfonation method [31–34]. Thus, it is interesting to synthesize sulfonated monomer with pendant sulfoalkyl groups. As shown in Scheme 1, a sulfonated monomer, sodium 3-(4-(2,6-difluorobenzoyl)phenyl)propane-1-sulfonate (DFPPS) was prepared via a two-step synthetic procedure: anhydrous aluminum chloride catalyzed Friedel-Crafts acylation of 1-bromo-3-phenylpropane with 2,6-difluorobenzoyl chloride and subsequently sulfonated with Na2 SO3 (total yield: 65%, m.p. = 184 ◦ C). Two monomer structures were confirmed by FT-IR, MS and 1 H NMR.

Fig. 1. FT-IR spectra of PSA-SPAE-X.

The FT-IR spectra of copolymers are shown in Fig. 1. The strong absorption band around 1670 cm−1 was assigned to the stretch vibration of carbonyl (sym C O) groups, at band of around 2900 cm−1 was observed weak absorption and assigned to the stretch vibration of methylene in flexible side chain. The band around 1043 cm−1 was assigned to the stretch vibration of sulfonic acid groups. 1 H NMR spectrum of PSA-SPAE-50 is shown in Fig. 2; and all the proton signals were unambiguously assigned from 1D and 2D H–H correlation NMR spectra. As expected, the resonance signals of the ortho sulfonyl and carbonyl protons appear at higher frequencies than those of the electron-rich protons such as the ortho ether linkage, because of deshielding from the sulfonyl or carbonyl groups. Alkyl protons appear at lowest frequencies. The 1 H–1 H COSY spectrum of PSA-SPAE-50 is shown in Fig. 3; Fig. 3(a) and (b) are aromatic and alkali region of the PSA-SPAE-50’s 1 H–1 H COSY spectrum, respectively. H-c,f,e,g,b,d,h,i,j could be easily assigned on the basis of their correlated signals via 1 H–1 H COSY spectrum. On the other hand, using 13 C NMR spectra also confirmed the structure of copolymer. The PSA-SPAE-50’s 13 C NMR spectrum is shown in Fig. 4. The spectrum data accords with the copolymer’s structure designed by us. The content of sulfonate or sulfonic acid (SC),

3.2. Synthesis and characterization of PSA-SPAEs The copolymerization of DFPPS, 4,4-dichlorodiphenylsulfone (DCDPS) and 4,4 -dihydroxyldiphenylether (DHDPE) was carried out in TMS at 185 ◦ C for 11 h under a pure nitrogen atmosphere. The resulting brown viscous solution was poured into water and sulfonated copolymers precipitated as swollen strings. After washed several times with deionized water and ethanol separately, pure PSA-SPAEs copolymers were obtained. The obtained copolymers showed high viscosities in the range of 0.9–1.1 dL/g, indicating their high molecular weights. The corresponding membranes (in sodium-form) were obtained by solvent-casting method in NMP solution (10 wt.%) and transformed to their acid-form by proton exchange in 1.0 N H2 SO4 solution for 24 h at room temperature. The acid-form membranes obtained were washed thoroughly with deionized water and kept in deionized water before measurement. The thickness of all membrane samples was in the range from 90 to 110 ␮m.

Fig. 2. 1 H NMR spectrum of PSA-SPAE-50 in DMSO-d6 (see Scheme 2 for proton labels).

J. Pang et al. / Journal of Membrane Science 318 (2008) 271–279

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Table 1 Some properties of PSA-SPAE-X copolymer Polymer

SCa

SCb

IECc (meq g−1 )

Tg (◦ C)

PSA-SPEA-80 PSA-SPEA-70 PSA-SPEA-60 PSA-SPEA-50

0.80 0.70 0.60 0.50

0.81 0.70 0.60 0.51

1.64 1.47 1.26 1.10

217 209 213 223

a b c

Td,onset (◦ C)

inh (dL/g)

Acid-form

Sodium-form

225 230 234 241

408 406 409 410

1.10 0.99 1.05 0.91

From percentage of sulfonated monomer (mol%). From calculation of 1 H NMR. Ion exchange content (IEC) was obtained form 1 H NMR.

expressed as the number of –SO3 Na(–SO3 H) groups per average repeat unit (R.U.) of the synthesized copolymers was evaluated by 1 H NMR. The experimentally determined SCs were found to be in close agreement with the pre-calculated SCs, expected from the monomer feed ratio (Table 1). As an example, the SC measurement technique was illustrated. Each signal in the spectrum was assigned to a single or to a group of protons of sulfonated (SR.U. ) or non-sulfonate (NSR.U. ) repeat unit by 1 H NMR, 13 C NMR and 1 H–1 H COSY experiments. The SCs were obtained by comparing the 1 H NMR signal intensity value of proton H-c + H-f (8.1 −7.6 ppm) with intensity value of proton signal H-i (1.5–2.0 ppm). The SC is derived from these signal intensities according to the following equation SC =

SR.U. (NSR.U. + SR.U. )

Therefore, SC =

2Ii (Ic + If − Ii ) + 2Ii

where 2 is a conversion factor in above equation multiplied the intensity value of H-i in order to compare intensities for equal number of protons for both NSR.U. and SR.U. Ic = intensity of H-c, If = intensity of H-f, Ii = intensity of H-i

Fig. 3. 1 H–1 H COSY spectra of polymer PSA-SPAE-50 (a) aromatic region, (b) alkyl region.

For example, the calculation of the SC = 0.505 for PSA-SPAE 50 was derived from the ratio of the integral signal intensity of SR.U. proton (H-i, Ii = 1H) to that of the integral intensity of NSR.U. proton (H-c, Ic = Ic + If − Ii = 1.96H). 3.3. Thermal properties In order to maintain their good mechanical strength, polymer materials are usually operated below the glass transition temperature (Tg ). Higher temperature fuel cell operation requires PEM materials with high Tg [35]. Only one transition temperature was found in the DSC curve before decomposition temperature for all the sodium-style samples, and they were in the range 209–223 ◦ C. Tg s of acid-style samples were not found before decomposition temperature (Td,onset > 200 ◦ C). In general Tg increases with sulfonated degree of polymer. In this study, the PSA-SPAE-50’s Tg was higher than PSA-SPAE-60’s and PSA-SPAE-70’s (Fig. 5), because PSASPAE-50 main chains contain more sulfone groups. The thermal stabilities of this series of polymers were studied by TGA in air (Table 1). PSA-SPAEs in the acid-form showed onset temperature of weight loss were all above 210 ◦ C. All sodium salt form of PSA-SPAEs showed higher thermal stability (Td,onset > 400 ◦ C). 3.4. Water uptake and swelling ratio

Fig. 4.

13

C NMR spectrum of copolymer PSA-SPAE-50.

Water uptake and swelling ratio of PEMs are closely related to IEC, proton conductivity, dimensional stability, and mechanical strength. The water within the membrane provides a carrier

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Fig. 5. DSC curves of PSA-SPAE-X (sodium-form).

for the proton and maintains high proton conductivity. However, excessive water uptake in a PEM leads to unacceptable dimensional change or loss of dimensional shape; which could lead to weakness or dimensional mismatch when incorporated into a membrane electrode assembly (MEA) [3,36]. Therefore, the preparation of sulfonated polymers with better water uptake and dimensional stability is one of the critical requirements for their application as PEMs. If the absorption of water can be restricted to a specific hydrophilic domain in the membrane, molecularly separated from the hydrophobic one, the latter domain may retain its cohesion and restrict the swelling. Thus, separating the hydrophilic sulfonic acid unit and the hydrophobic polymer main chain is to locate the sulfonic acid units at the chain ends of short side chains attached to the polymer main chain, which may be a good approach to limit the swelling and to control the morphology. PEMFCs are normally operated at temperatures from RT to 80 ◦ C based on the properties of state of art polymer electrolyte Nafion. However, since elevated operation temperatures will raise the tolerance ability of catalysts to CO, PEMs that can endure temperatures higher than 100 ◦ C are preferred. It can be seen from Figs. 6 and 7, absorption of PSA-SPAE-X membranes was from 12.2 to 76.7% in water with linear expansion from 4.5 to 26.3% depending on the SC and temperature. Due to moderate water absorption, PSA-SPAE-X polymer films do not exhibit unacceptable dimensional change or loss dimensional shape. Even though at 100 ◦ C, the polymer containing the most sulfonic acid groups (i.e., PSA-SPAE-80) film’s water uptake was 76.7% and swelling ratio was only 26.3%. The PAS-SPAE membranes showed isotropic membrane swelling in water, and the swelling ratios and  (the number of water molecules per sulfonic acid group (H2 O/SO3 H) of PSA-SPAE polymer and Nafion 117 are compared in Table 2. The PSA-SPAE copolymers with SC = 0.5 exhibited dimensional swelling of less than 10% and were adequately stable even in boiling water. The dimensional swelling of the copolymer (SC = 0.6, 0.7) were lower than Nafion 117 at the same condition. The dimensional swelling of PSA-SPAE-80 was

Fig. 6. Water uptake of PSA-SPAE-X as a function of temperature.

Fig. 7. Water swelling ratio of PSA-SPAE-X as a function of temperature.

smaller than Nafion 117. The  was in line with the trend of water uptake and PSA-SPAE-80’s  is similar to Nafion 117 at high temperature. In addition, PSA-SPAE copolymers showed considerably reduced swelling ratio compared to other main chain sulfonated copolymer [6,7]. PSA-SPAE-X copolymers exhibit moderate water absorption and excellent dimensional stability, and hence they are very promising for application as PEMFC. 3.5. Oxidative stability The oxidative stability of the copolymers was evaluated in Fenton’s reagent at 80 ◦ C. This method is regarded as one of the

Table 2 Water uptake and swelling ratio of the acid-form PSA-SPAE-X films Polymer

PSA-SPAE-80 PSA-SPAE-70 PSA-SPAE-60 PSA-SPAE-50 Nafion 117

80 ◦ C

Room temperature

100 ◦ C

W (%)

T (%)

L (%)



W (%)

T (%)

L (%)



W (%)

T (%)

L (%)



25.6 20.9 16.0 12.2 19.2

5.2 4.3 4.6 3.5 –

5.3 5.1 4.9 4.5 13.1

8.7 7.9 7.1 6.2 12.0

44.3 37.7 24.8 18.6 29.4

11.4 9.6 8.5 4.6 –

12.0 10.2 8.6 4.5 20.2

15.1 14.3 11.0 9.4 18.0

76.7 59.1 33.2 19.3 33.0

27.2 20.5 13.5 4.7 –

26.3 21.0 12.7 4.5 23.1

26.1 22.4 14.0 9.7 21.0

(W: water uptake, L: swelling ratio, : the number of mol of water molecules per mol of sulfonic acid).

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Table 3 Proton conductivity, oxidative stability, activation energy and methanol permeability Polymer

IEC (meq g−1 )

 (S/cm) ◦

PSA-SPAE-80 PSA-SPAE-70 PSA-SPAE-60 PSA-SPAE-50 Me-SPEEKDK Me-SPEEKK Nafion 117

1.64 1.47 1.26 1.10 1.61 1.80 0.91

Ea (kJ/mol) ◦

30 C

100 C

0.035 0.018 0.013 0.001 0.021 0.033 0.078

0.144 0.101 0.063 0.015 0.080 0.154 0.140

18.2 19.9 20.7 32.4 – – 9.7

Oxidative stability RW (%)

t (h)

96 97 99 – 98 97 98

4.5 >6.0 >6.0 >6.0 >6.0 2.5 >6.0

Methanol permeability (cm2 /s)

2.60 × 10−7 1.30 × 10−7 5.60 × 10−8 2.40 × 10−8 – – 1.61 × 10−6

t: copolymer membranes dissolve in Fenton’s reagent.

standard tests to gauge relative oxidative stability and to simulate accelerated fuel cell operating condition. It has been known that the oxidative attack by HO• and HOO• radicals mainly occurs in the hydrophilic domains to cause the degradation of polymer chains. All the polymers exhibited excellent oxidative stability, as shown in Table 3. Their weight was retained above 96% after treatment in Fenton’s reagent at 80 ◦ C for 1 h. And they were not dissolved in Fenton’s reagent within 4 h treatment at 80 ◦ C, which suggested their good oxidative stability. Although aliphatic compounds always have lower oxidative stability than aromatic compounds, the results of test indicated that these sulfonated polymers maintained good oxidative stability after introducing aliphatic sulfonic acid into aromatic polymer main chain. 3.6. Proton conductivity and methanol permeability The proton conductivity () of the PSA-SPAE-X series in acidform in the longitudinal direction was measured by ac impedance spectroscopy. All membranes were initially hydrated by immersion in deionized water for at least 24 h at room temperature. Some general trends were observed for all samples, as shown in Fig. 8. Like water uptake, the proton conductivities increased with temperature and SC. PSA-SPAE (-60, -70, -80) membranes showed proton conductivities higher than 10−2 S/cm at room temperature. PSA-SPAE (-70, -80) membranes showed proton conductivity higher than 0.1 S/cm at 100 ◦ C. PSA-SPAE-80 membrane (IEC = 1.64 meq g−1 ) exhibited high conductivity of 0.035 S/cm at room temperature and 0.14 S/cm at 100 ◦ C, which was comparable to Nafion 117. PSA-SPAE-50 was found to have obviously

Fig. 8. Proton conductivities of acid-form PSA-SPAE-X and Nafion 117 films as a function of temperature.

lower proton conductivity than others. Due to its high content of non-sulfonated moiety, PSA-SPAE-50 polymer might not have continuous hydrophilic domains. Compared with reported short aromatic side chain of poly(ether ketone)s (Me-SPEEKDK and MeSPEEKK) [36], PSA-SPAE-X indicated higher proton conductivity at same condition. As shown in Table 3, the proton conductivity of PSA-SPAE-80 (IEC = 1.64 meq g−1 ) membranes was higher than that of Me-SPEEKDK (IEC = 1.60 meq g−1 ) at the same test condition and was even higher than that of Me-SPEEKK (IEC = 1.80 meq g−1 ) at low temperature. This result may be associated with the pendant flexible aliphatic side chain of the new membranes, since the aliphatic chain moved easily than aromatic one, which is helpful to transport proton. At low temperature, it was very distinct. Fig. 8 also displays the Arrhenius plots of the conductivity of the analyzed membranes in the temperature range of 30–100 ◦ C. The activation energy (Ea ) of conductivity is calculated by fitting the Arrhenius equation  =  o exp(−Ea /RT). Curves are fitted linearly for Ea determination. The activation energy for proton conduction was found to be in the range of 32.4–18.2 kJ mol−1 (Table 3). PSA-SPAE-80’s Ea was 18.2 kJ mol−1 , which was higher than that of Nafion (9.7 kJ mol−1 ) (our measurement value is a little higher than reference reported (9.10 kJ mol−1 )), but smaller than that of the whole aromatic sulfonated polyimide ionomers (21 kJ mol−1 ) and sulfonated poly(ether ether ketone) ionomers (>30 kJ mol−1 ) [8,37]. Detailed mechanistic analyses are currently under investigation and will be reported elsewhere. Methanol permeability and proton conductivity are the two transport properties, which determine the fuel cell performance. Low methanol permeability and high proton conductivity are required for direct methanol fuel cells (DMFCs). The methanol

Fig. 9. Proton conductivity versus methanol permeability of PSA-SPAE films at room temperature.

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Table 4 Mechanical properties of the sulfonated polymers Polymer

Tensile strength (MPa)

PSA-SPAE-80 PSA-SPAE-70 PSA-SPAE-60 PSA-SPAE-50 PSA-SPAE-6F90 PSA-SPAE-6F80

44.4 78.4 58.2 48.5 45.6 52.4

± ± ± ± ± ±

1.22 1.50 1.52 1.37 1.46 1.66

Young’s modulus (GPa) 1.10 2.86 1.44 1.56 0.87 1.01

± ± ± ± ± ±

0.12 0.18 0.16 0.14 0.10 0.12

Elongation at break (%) 35.2 16.2 17.5 45.1 26.5 12.0

± ± ± ± ± ±

1.22 0.18 0.12 2.20 1.55 1.02

permeability values for 15 mol/L methanol of PSA-SPAE (80–50) at room temperature were in the range of 2.6 × 10−7 to 2.4 × 10−8 cm2 /s (Table 3), which were much lower than the value of Nafion 117 of 1.61 × 10−6 cm2 /s (data measured in our laboratories). As previously reported [38], we evaluated the obtained membranes according to a figure that has the logarithm of the proton conductivity as the ordinate and the logarithm of the reciprocal of methanol permeability as the abscissa. Fig. 9 shows the relationship of proton and the inverse of methanol permeability of polymeric films. All the membrane, with low methanol permeability, is located on the right-hand of the line. 3.7. Mechanical strength Good mechanical properties of the PEMs in either the anhydrous or hydrous states are one of necessary demands for their applications. The mechanical properties of PSA-PAE-X membrane were measured at room temperature and 30% RH. As shown in Table 4 the initial Young’s modulus for the membranes of PSA-SPAE50, -60, -70 and -80 are 1.59, 1.44, 2.86 and 1.10 GPa, respectively, which were much higher than Nafion 117. All the four membranes showed the elongation at break of 45.0, 17.5, 16.2 and 35.2%, and tensile strength ranging from 44.4 to 78.4 MPa. In general, with sulfonic acid group increasing, the polymer’s stress was increased and strain elongation was decreased. PSA-SPAE (-50, -60, -70) membranes exhibited this orderliness. PSA-SPAE-70 membrane exhibited much higher maximum stress (78 MPa) and elongation at break was as low as 16%. Because PSA-SPAE-80 copolymer contains more flexible aliphatic groups than others, the PSA-SPAE-80 membrane exhibited lower maximum stress (44.4 MPa) and larger elongation (35.2%). Compared with PSA-SPAE-6F film, the PSASPAE film exhibited improved mechanical properties (Table 4). The tension results showed they were strong and flexible membrane materials. 3.8. Morphology The phase structures of PSA-SPAE-X were characterized by AFM. The phase images of the acid-form PSA-SPAE-50 and PSA-SPAE-70 are shown in Fig. 10. The scale of the images was 500 nm × 500 nm. The phase images of PSA-SPAE-X membranes dark regions were assigned to softer regions, which represented the hydrophilic sulfonate groups. And the light regions were assigned to hydrophobic polymer backbone [31]. The size and continuity of these two regions had great influence on the transport properties of membranes. Hydrophilic size and continuity were found with SC of PSA-SPAE-X increasing from 0.5 to 0.8. For PSA-SPAE-50, the domains assigned to the hydrophilic ionic cluster were small in size and well separated from each other due to its low sulfonation content. Therefore, the membrane showed lower water uptake and proton conductivity. However, for PSA-SPAE-70 with higher sulfonation content, the hydrophilic/hydrophobic nano-phase separation became more obvious. The hydrophilic ionic domains were large in size (around 40 nm), and became continuous to some extent.

Fig. 10. AFM tapping phase images for PSA-SPAE copolymers (acid-form): (a) PSASPAE-50 (b) PSA-SPAE-70; scan boxes are 500 nm × 500 nm and phase scales are 0–10◦ .

4. Conclusions A series of sulfonated poly(arylene ether) copolymers containing sulfonic acid groups on aliphatic side chains were prepared from

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new sulfonated monomer and highly active diphenol via polycondensation reactions. All PSA-SPAE copolymers had good thermal stability with Td > 225 ◦ C (acid-form) and Tg > 209 ◦ C (sodiumform). A remarkable event was that the PSA-SPAE membranes showed low swelling ratio at high temperature. In addition, all polymers exhibited good tensile strength from 41 to 78 MPa, and the values were higher than perfluorosulfonic acid (PFSA) polymers. Due to the structure with a flexible aliphatic pendant between the hydrophilic sulfonic acid groups and the hydrophobic polymer main chain, the membranes exhibited high proton conductivities, less temperature dependence, and reasonable phase separate state. The PSA-SPAE-80 film with the highest sulfonation content among obtained copolymers showed proton conductivity of 0.1 S/cm and only 12% water swelling ratio at 80 ◦ C. Combined with its good mechanical property, these membranes may be a potential PEM materials for PEMFC applications. References [1] B.C.H. Steele, A. Heinzel, Materials for fuel-cell technologies, Nat. (Lond.) 414 (2001) 345–352. [2] M. Rikukawa, K. Sanui, Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers, Prog. Polym. Sci. 25 (2000) 1463–1502. [3] M.A. Hickner, H. Ghassemi, Y.S. Kim, B.R. Einsla, J.E. McGrath, Alternative polymer systems for proton exchange membranes (PEMs), Chem. Rev. 104 (2004) 4587–4612. [4] J. Roziere, D.J. Jones, Non-fluorinated polymer materials for proton exchange membrane fuel cells, Annu. Rev. Mater. Res. 33 (2003) 503–555. [5] Y. Yang, S. Holdcroft, Synthetic strategies for controlling the morphology of proton conducting polymer membranes, Fuel Cells 5 (2005) 171–186. [6] Y. Gao, G.P. Robertson, M.D. Guiver, S.D. Mikhailenko, X. Li, S. Kaliaguine, Synthesis of poly(arylene ether ether ketone ketone) copolymers containing pendant sulfonic acid groups bonded to naphthalene as proton exchange membrane materials, Macromolecules 37 (2004) 6748–6754. [7] P. Xing, G.P. Robertson, M.D. Guiver, S.D. Mikhailenko, S. Kaliaguine, Sulfonated poly(aryl ether ketone)s containing the hexafluoroisopropylidene diphenyl moiety prepared by direct copolymerization, as proton exchange membranes for fuel cell application, Macromolecules 37 (2004) 7960–7967. [8] K. Miyatake, H. Zhou, H. Uchida, M. Watanabe, Highly proton conductive polyimide electrolytes containing fluorenyl groups, Chem. Commun. (2003) 368. [9] Y. Chikashige, Y. Chikyu, K. Miyatake, M. Watanabe, Poly(arylene ether) ionomers containing sulfofluorenyl groups for fuel cell applications, Macromolecules 38 (2005) 7121–7126. [10] D.J. Jones, J. Roziere, Recent advances in the functionalisation of polybenzimidazole and polyetherketone for fuel cell applications, J. Membr. Sci. 185 (2001) 41–58. [11] H. Ghassemi, J.E. McGrath, Synthesis and properties of new sulfonated poly(pphenylene) derivatives for proton exchange membranes. I, Polymer 45 (2004) 5847–5854. [12] Y. Yin, J. Fang, H. Kita, K. Okamoto, Novel sulfoalkoxylated polyimide membrane for polymer electrolyte fuel cells, Chem. Lett. 32 (2003) 328–329. [13] Y. Yin, O. Yamada, Y. Suto, T. Mishima, K. Tanaka, H. Kita, K. Okamoto, Synthesis and characterization of proton-conducting copolyimides bearing pendant sulfonic acid groups, J. Polym. Sci., Part A: Polym. Chem. 43 (2005) 1545–1553. [14] Z. Hu, Y. Yin, S. Chen, O. Yamada, K. Tanaka, H. Kita, K. Okamoto, Synthesis and properties of novel sulfonated (co)polyimides bearing sulfonated aromatic pendant groups for PEFC applications, J. Polym. Sci., Part A: Polym. Chem. 44 (2006) 2862–2872. [15] R. Nolte, K. Ledjeff, M. Bauer, R. Muelhaupt, Partially sulfonated poly(arylene ether sulfone)—a versatile proton conducting membrane material for modern energy conversion technologies, J. Membr. Sci. 83 (1993) 211–220. [16] K.D. Kreuer, On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells, J. Membr. Sci. 185 (2001) 29–39.

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