Annealing effect of sulfonated polysulfone ionomer membranes on proton conductivity and methanol transport

Journal of Membrane Science 247 (2005) 103–110

Annealing effect of sulfonated polysulfone ionomer membranes on proton conductivity and methanol transport Ho Bum Parka , Hyun-Soo Shinb , Young Moo Leea , Ji-Won Rhimb,∗ a

b

National Research Laboratory for Membrane, School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, South Korea School of Chemical and Polymer Engineering, Hannam University, 133 Ojung-Dong, Daeduk-Gu, Daejon 306-791, South Korea

Received 27 August 2004; received in revised form 13 September 2004; accepted 13 September 2004 Available online 10 November 2004

Abstract Sulfonated polysulfone (SPSU) ionomer membranes were prepared via a solution sulfonation method using mixtures of chlorosulfonic acid (HSO3 Cl) and chlorotrimethylsilane ((CH3 )3 SiCl) as sulfonating agent. Ion exchange capacities (IECs) of SPSU ionomer membranes were controlled by varying the amount of HSO3 Cl. In the present study, we investigated the thermal transition behavior of SPSU membranes annealed at different temperatures (60 and 150 ◦ C) below glass transition temperature (Tg ) of SPSU ionomer membranes, and also studied the annealing effect on the proton conductivity and methanol transport through these ionomer membranes. The changes in Tg (◦ C), ion exchange capacity (IEC, mmol/g), water uptake content (%), proton conductivity (S/cm), and methanol permeability (cm2 /s) were reported and discussed. The heat-treatment of SPSU membranes at higher temperature led to the increase in Tg and simultaneously the decrease in water uptake, proton conductivity and methanol permeability. Here, it was found that the annealing below Tg accelerated the equilibrium process and physical aging of SPSU ionomer membranes and subsequently induced more compact chain packing structure, which eventually affected the proton and methanol transport through these ionomer membranes. © 2004 Elsevier B.V. All rights reserved. Keywords: Sulfonated polysulfone; Annealing; Ionomer membrane; Proton conductivity; Methanol permeability

1. Introduction Ion-conducting polymer membranes are very attractive and important materials owing to their versatile applications such as rechargeable batteries [1], electrochromic devices [2], ion exchangers [3], in water treatment [4], and in electrolysis [5]. One interesting application for these types of membranes is the polymer electrolyte membrane fuel cell (PEMFC), which will provide a clean, efficient, quiet, lightweight and high density power source useful a wide range of applications, from portable computers to electric vehicles. Principally, PEMFC is an assembly consisting of catalyzed elec∗

Corresponding author. E-mail address: [email protected] (J.-W. Rhim).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.09.023

trodes and polymer electrolyte membrane (PEM) and then operates at temperatures of 60–120 ◦ C [6]. The key component of PEMFC is undoubtedly PEM. The basic role of PEM is both an electrolyte medium for proton conduction [7] and a barrier to avoid the direct contact between fuels (hydrogen or methanol) and oxygen. Nafion® , only commercially available PEM, has been used in fuel cell applications since the perfluorinated ionomer membranes have a high chemical and electrochemical resistance at the harsh operating conditions, as well as high proton conductivity (∼0.1 S/cm). However, they have serious shortcomings. Most of all, their high costs limit a mass production for PEMFC. Another severe problem occurred in Nafion® membranes is the large amount of methanol-crossover in direct methanol fuel cell (DMFC) applications [8,9]. The high

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methanol permeability through the membranes leads to a decreased fuel cell performance owing to depolarization of the oxygen reducing cathode. To overcome the disadvantages of perfluorinated ionomer membranes, partially fluorinated and non-fluorinated hydrocarbon ionomer membranes such as sulfonated polyimides [10–13], sulfonated polyphosphazenes [14,15], sulfonated polyetheretherketones [16–18], and sulfonated polysulfones [19–27] have been evaluated as alternatives to Nafion® . Among them, the interest in polysulfone, as a membrane material, is mainly due to its excellent thermal and mechanical stability as well as its resistance to hydrolysis and oxidizing agents because sulfones are insensitive to hydrogenation and diphenylene sulfone moiety must confer thermal stability and oxidative resistance to the polymer backbone. Therefore, polysulfone has been used as the base of ion-exchange membranes via chemical modification by various sulfonation techniques. Sulfonated polysulfone (SPSf) ionomer membranes have been extensively studied and also tested for fuel cell applications [19–22,25,26]. The proton conductivity of the SPSf membranes was reported as a reasonable value up to 10−2 S/cm at 80 ◦ C [22,25]. In addition, the sulfonated polysulfone (SPSU) membranes have lower gas permeability and liquid (water and methanol) permeability than the sulfonated perfluorinated ionomers, which is one of advantages over the sulfonated perfluorinated ionomers. That is, lower permeability of gas and liquid prevents fuels from permeating from anode to cathode, and as a result can maximize the voltage in the region of low current densities in fuel cell tests. In the present study, the SPSU membranes were prepared using a direct sulfonation technique and then the annealing effect of the SPSU membranes was investigated on the proton conductivities and the methanol permeabilities. In general, it is well-known that a simple annealing process stabilizes the glassy polymer membrane, which causes a densification of the polymer matrix and leads to a restriction of chain mobility and consequently hinders the membrane plasticization caused by sorbing of gas and liquid molecules [28]. To the best of our knowledge, the annealing effect of PEM on their ion and liquid molecule transport was not yet reported and hence we will deal with it here.

2. Experimental 2.1. Materials Polysulfone (PSU, UDEL P-3500, Solvay Advanced Polymer LLC, Alpharetta, GA, USA) was dried for 24 h in a vacuum oven at 90 ◦ C prior to use. PSU is an amorphous, transparent, glassy polymer with a glass transition temperature of 185 ◦ C. In addition, Mo (g/mol) and Mw (g/mol) of PSU are 442 and 84,000, respectively. Chlorosulfonic acid (HSO3 Cl, CSA, 99%), chlorotrimethylsilane ((CH3 )3 SiCl, CTMS, 98%), triethylamine (TEA, 99.5%), and the other solvents (1,2-dichloroethane, DCE, HPLC grade; N-methyl-2-

pyrrolidone, NMP, 99%) were used as received from Aldrich (Milwaukee, WI, USA). 2.2. Synthesis procedure The sulfonated polysulfone were prepared from direct sulfonation method using CSA and CTMS [29]. The dried PSU was dissolved (10 g, 0.0226 mol) in 100 ml hydrous DCE in a 500 ml three-neck flask equipped with mechanical stirrer, condenser and nitrogen purge inlet. The resulting solution was sufficiently purged with nitrogen for 1 h and a mixture of CTMS in 20 ml DCE was added from an addition funnel over 10 min. A mixture of CSA in 20 ml DCE was then added dropwise for 30 min. The resulting solution was vigorously stirred for 12 h at 35 ◦ C. Gaseous HCl generated by substitution was carried with nitrogen stream and finally captured with TEA solution. After reaction, the reaction mixture was poured in a large methanol excess to precipitate the polymer into the acidic form SPSU. After filtration, the SPSU was rinsed several times using deionized water and methanol and dried at 60 ◦ C in a forced convection oven for 24 h and then was completely dehydrated in a vacuum oven for 3 days. In the sample designation (SPSU-X and SPSU-X-t), X (3–7) is denoted as the mole ratio of CSA and CTMS mixture (0.068, 0.090, 0.113, 0.136, and 0.158 mol) and PSU (0.0226 mol). That is, the degree of sulfonation was controlled by varying the amount of CSA and CTMS mixture (mole ratio = 1:1) added in the PSU solution. The overall scheme of sample preparation is illustrated in Fig. 1. 2.3. Membrane preparation SPSU membranes were prepared from 15% SPSU solutions in NMP. The SPSU solution was cast onto glass substrates using a Gardner knife and the cast films were subsequently dried in vacuum at 1 mbar and 60 ◦ C for 2 days. Then the SPSU membranes were carefully peeled off in an ice water bath. Note that the SPSU membranes were soaked in a shaking water bath for a week in order to minimize the effect of residual solvent (NMP). The wet SPSU membranes were completely dried in a vacuum oven at 60 ◦ C for 2 days (SPSU-X). Eventually, SPSU-X-t membranes were prepared by annealing at 150 ◦ C for 2 h. All membranes were sufficiently soaked in distilled and deionized water at room temperature for 24 h in order to make fully acidic SPSU form before measuring the proton conductivity and the methanol permeability. 2.4. Membrane characterization Glass transition temperatures (Tg s) of all SPSU membranes were measured using a differential scanning calorimeter (DSC) 2010 thermal analyzer (TA Instruments, New Castle, DE, USA) with a DSC module, purged with a nitrogen gas at a heating rate of 5 ◦ C/min. Thermogravimetric analysis (TGA) was performed in a nitrogen purge using a TGA

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Fig. 1. Preparation procedure of sulfonated polysulfone (SPSU) membranes used in this work.

2050 (TA Instruments). The amount of gases evolved during thermal degradation of the SPSU membranes was simultaneously detected using a Balzers Quadstar 422 high performance quardrupole mass spectrometer (Balzers Instruments, Vaduz, Liechtenstein). Water sorption experiments were carried out by immersing five sheets of all SPSU membranes in distilled and deionized water at 30 ◦ C for 24 h in order to understand the amount of water uptake (%) before and after heat treatment. For measurement, the membranes were taken out, wiped with tissue paper for experiment, and immediately weighed on a calibrated single pan Mettler microbalance. The amount of water uptake, WH2 O , was calculated using following equation: WH2 O (%) =

Ww − Wd × 100 Wd

(1)

where Ww and Wd are the weight of wet and dry samples, respectively. The amount of uptake of the SPSU membranes was calculated from the average value of five samples and water uptake measurements were found to be reproducible to 5%. The ion exchange capacity (IEC) was obtained from the titration of the released amount of proton (H+ ) of the preweighed polymer in an acid form in 1 M sodium chloride

(NaCl) with 0.01 M sodium hydroxide (NaOH) by using a phenolphthalein indicator. The IEC value was recorded as an average value for each sample in units of milliequivalents NaOH per gram of the polymer (mmol/g). 2.5. Proton conductivity measurement The impedances of all SPSU membranes were determined by using the electrode system, which contained a four-probe cell depicted in Fig. 2, connected with an impedance/gainphase analyzer (Solatron 1260, Solatron Analytical, UK) in combination with an electrochemical interface (Solatron 1287). A four-point-probe cell with two platinum wire outer current-carry electrodes and two platinum wire inner potential-sensing electrodes was mounted on a Teflon plate. Membrane samples were cut into strips that were approximately 1.0 cm wide and 4.0 cm long prior to mounting in the cell. The cell was placed in a thermo-humidity controlled chamber. A fixed AC current is passed between two outer electrodes, and the conductance of the material is calculated by the AC potential difference between the two inner electrodes. This method is relatively insensitive to the contact impedance and interfacial resistance at the current-carry electrode and is therefore well suited for measuring proton con-

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7.02 cm2 ) was clamped between the two compartments and these were kept under stirring during the experiment. A flux of methanol penetrates across the membrane as a result of the concentration difference between the two compartments. Under pseudo steady-state conditions, which prevailed during the experiment and for cB  cA , the methanol concentration in the receiving compartment as a function of time is given by following equation: DcB =

A DK × cA × D VB L

(3)

where c is concentration, A (cm2 ) and L (cm) the membrane area and thickness; D and K are the methanol diffusivity and partition coefficient between the membrane and the adjacent solution, respectively. The assumptions are made in this study that D inside the membrane is constant and K does not depend on concentration. The product DK is the membrane permeability (P). P=

1 DcB × × VB A cA Dt

(4)

where cB is measured several times during the experiment and the permeability is calculated from the slope of the straight line. The methanol concentrations were measured by gas chromatography with a thermal conductivity detector (Shimadtzu, model 14B, Tokyo, Japan). The temperature was kept constant in a thermostatic water bath.

Fig. 2. (a) A view of water vapor chamber for proton conductivity measurement and (b) a four-probe cell used for determining membrane conductivity.

ductance. The measurement was carried out in a sealed cell to maintain the finest hydrated condition at 60 ◦ C and relative humidity of 95% and the proton conductivity was calculated from following equation: σ=

l R×S

(2)

where σ is proton conductivity (S/cm); R, the bulk resistance or the ohmic resistance of membrane sample; l, the distance between counter electrode and working electrode (cm); S, the cross-sectional area of membrane sample (cm2 ); i, the current density, and X is the electric field strength, defined as the ratio of the measured potential difference to the infinitesimal distance between the points of attachment to the voltmeter. 2.6. Methanol permeability measurement A glass diffusion cell was used to determine the methanol permeability of SPSU membranes. The glass cell consisted of two compartments each approximately 35 ml, separated by a membrane. One compartment of the cell (VA ) was filled with a 2 M methanol solution in deionized water. The other (VB ) was filled with distilled water. The membrane (effective area

3. Results and discussion 3.1. Thermal transition The glass transition temperatures (Tg s) of PSU, SPSU-X, and SPSU-X-t are listed in Table 1. The Tg of PSU membrane was detected at 185 ◦ C. The sulfonation of PSU membrane leads to the increase of Tg up to 225 ◦ C. That is, the Tg of SPSU membranes increases with degree of sulfonation. The change of Tg after sulfonation is very similar to that reTable 1 Glass transition temperature (Tg ) of PSU, SPSU-X and SPSU-X-t membranes measured by differential scanning calorimeter (DSC) Sample code

Tg (◦ C)

PSU Untreated SPSU SPSU-3 SPSU-4 SPSU-5 SPSU-6 SPSU-7

185

Annealed SPSU SPSU-3-t SPSU-4-t SPSU-5-t SPSU-6-t SPSU-7-t

193 201 212 219 225 197 209 218 227 234

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Fig. 3. TGA thermograms of SPSU-X membranes in the range of temperature from 50 to 700 ◦ C in a nitrogen purge.

ported by Noshay and Robeson [30]. Usually, the Tg of sulfonated polymer depends on the degree of sulfonation, the existence of counterion and thermal history. The heat treatment of SPSU membranes at 150 ◦ C (SPSU-X-t) increases the Tg as compared with that of SPSU-X membranes. It indicates that the microstructure of the SPSU membranes might be denser or more compact after heat treatment at higher temperature. However, in the present study, there are a few probable factors to affect the Tg of SPSU membranes. They are the bound water molecules strongly bonded to sulfonic acid groups and the residual polar solvent (NMP) within SPSU membranes prepared at lower temperature (60 ◦ C). Although all samples were enough soaked in distilled and deionized water and then dried in vacuum oven in order to minimize the factors, even the minimum amount of those molecules within SPSU membranes may affect the Tg as plasticizers.

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Fig. 4. Thermogravimetry–mass spectroscopy (TG–MS) curve of SPSU-7-t membrane.

the mass spectra. Above 550 ◦ C, it is found that the yield of characteristic pyrolyzate with a hydroxyl end group such as phenol as well as benzene appears. It means the degradation of SPSU membranes by random bond cleavage of the aromatic ether or aromatic ether radicals. The TGA–MS curve demonstrates well that SPSU membranes have fairly good thermal stability below 200 ◦ C. 3.3. Proton conductivity The main factors to determine the ion conductivity in the ionomer membranes are generally the ion exchange functional sites (for instance, SO3 H) per unit volume and the water content within membranes. Therefore, it is very meaningful to consider the relationship between these factors and the proton conductivities of the sulfonated ionomer membranes. Figs. 5 and 6 show the proton conductivity at 60 ◦ C

3.2. Thermal degradation The sulfonation of PSU decreases significantly its thermal stability due to acid-catalyzed degradation. It was found that the onset weight-loss temperature of SPSU membranes decreased as the degree of sulfonation increased, as shown in Fig. 3. In the thermal decomposition curves, any difference was not found between SPSU-X and SPSU-X-t membranes and so TGA curves of the SPSU-X membranes are shown in Fig. 3. Generally the sulfonic acid group acts as an initiator for the degradation of SPSU membranes. Here the thermal stability of SPSU membrane was investigated again using a Thermogravimetric analysis/mass spectroscometer (MS). Fig. 4 demonstrates a TGA–MS curve of SPSU-7-t membrane. The initial weight loss at around 100 ◦ C is caused by evaporation of water molecules bound in a hydrophilic SPSU membrane. The secondary weight loss between 200 and 400 ◦ C is mainly due to the thermal activated decomposition of sulfonic acid groups in the polymer chains, which is confirmed by the evolution of SO and SO2 gas detected in

Fig. 5. Proton conductivity of the SPSU-X and the SPSU-X-t membranes as a function of IEC value.

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Fig. 6. Water uptake (%) in the SPSU-X and the SPSU-X-t membranes as a function of IEC value.

and the water content of SPSU membranes and constant relative humidity (RH) of 95% as a function of IEC value. As shown in Table 2, IEC values in the SPSU-X and the SPSUX-t membranes are almost the same, meaning that the desulfonation process did not occur at the annealing temperature of 150 ◦ C used in this study. Therefore, the amount of sulfonic acid groups is identical in the SPSU-X and the SPSU-X-t membranes. Here, the gradual increase of the amount of sulfonating agent (a mixture of CSA and CTMS) yields higher IEC values in the SPSU membranes, which leads to an increase in both the proton conductivity and the water uptake of the SPSU membranes. In all cases, the proton conductivities of the SPSU membranes are higher than 10−3 S/cm and, as expected, they increase with increasing of IEC value. For comparison, the proton conductivity of Nafion 117 membrane measured at 60 ◦ C is about 6.7 × 10−2 S/cm at a relative humidity of 95%. As shown in Table 2 and Fig. 5, in spite of same IEC values, the proton conductivities of the SPSU membranes are considerably affected by heat treatment. Namely, the proton conductivities of the SPSU membranes (SPSUX-t) annealed at 150 ◦ C are lower than those of SPSU memTable 2 Water uptake, IEC, and water molecules per SO3 − site (λ) of SPSU-X and SPSU-X-t membrane Sample code

Water uptake (%)

IEC (mmol/g)

Water molecules per SO3 − site (λ)

SPSU-3 SPSU-4 SPSU-5 SPSU-6 SPSU-7 SPSU-3-t SPSU-4-t SPSU-5-t SPSU-6-t SPSU-7-t

9.1 13.2 17.1 24.4 58.1 8.3 11.2 15.1 21.2 52.4

0.58 0.71 0.85 1.05 1.45 0.56 0.70 0.85 1.03 1.43

8.7 10.3 11.2 12.9 22.3 8.2 8.9 9.9 11.4 20.4

branes (SPSU-X) at 60 ◦ C. This means that the microstructure of SPSU membranes, particularly hydrophilic channel, is changed into more compact structure after annealing of the SPSU membranes at sub-Tg . Typically an amorphous glassy polymer such PSU is in a non-equilibrium state. Such a glassy polymer will always encompass, regardless of whether it is melt-extruded or solvent-cast, more than the theoretical minimal free volume obtained by the linear extrapolation from the liquid to the hypothetical equilibrium glassy state of the specific volume versus temperature curve. As a result, subjection of any glassy polymer like PSU to sub-Tg annealing will act to increase the packing density within membrane. Moreover, the proton conductivities of cation-exchange membranes such as sulfonated polymers depend largely on charge carriers such as protons and on their mobility which is mainly ensured by water molecules and their clusters. Therefore, the hydrophilic pathway through which protons can pass easily may be narrower in the annealed SPSU (SPSU-X-t) membranes by reducing water molecules-accessible space around sulfonic acid groups after annealing at sub-Tg . To confirm this assumption, the water contents in the SPSU-X and the SPSU-X-t membranes are shown in Fig. 6 as a function of IEC value. Here, the water content of the SPSU membranes increases with gradual addition of sulfonating agent (a mixture of CSA and CTMS), and the water content of the SPSU membranes are reduced after heat treatment, irrespective of IEC values. The reduction of water content in the SPSU-X-t membranes can be closely associated with the decrease of the proton conductivity. Table 2 shows the water molecules per SO3 − site (λ) of the SPSU-X and the SPSU-X-t membranes. The λ is defined as the number of water molecules per sulfonic acid site. The λ’s of the SPSU-X membranes are higher than those of the SPSU-X-t membranes in spite of same IEC values. This indicates that the molecular packing of the SPSU-X-t membranes is probably much denser than that of the SPSU-X membrane. As a result, the increase of packing density might lead to the reduction of free volume in the SPSU-X-t membranes and thus the water content and the proton conductivity decrease. Of course, as can be seen in the thermal transition results (Tg ), the differences in Tg between SPSU-X and SPSU-X-t membranes could be easily attributed plasticization of the membranes, i.e. SPSU-X membranes, which were dried at lower temperatures, could contain residual NMP and may water because polar solvent could be strongly bound to the sulfonic acid group by an acid-base mechanism, although all membranes were soaked and washed for very long time in order to minimize this effect. That is, membrane plasticization may affect the water content and the proton conductivity. However, as recently reported by Robertson [31], casting polar solvents such as DMF and DMAc strongly interact with the sulfonic acid groups of ionomers and then form a hydrogen-bonding complex, leading to reducing the number and/or mobility of protons available for proton transport. That is, these interactions dramatically reduce the proton conductivities of sulfonated polymer membranes. Consequently, in this work, the reduction of

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proton conductivity after annealing may be dominantly dependent upon the change of microstructure, resulting in close chain packing. The results are not very surprising, but it gives useful information on the preparation of optimized PEM material for fuel cell. Therefore, it is very important to consider the preparation conditions such as temperature, the selection of solvents and post treatment, as well as chemical structure and compositions in the design of PEMs. 3.4. Methanol permeability In the present study, we also considered the annealing effect on the transport of methanol molecules through the SPSU membranes. Recently, DMFC has been investigated as an interesting subject for a fuel-cell based mobile power supply system in the power range from a few watts to several hundred kilowatts. A major problem in DMFC is that methanol can transport easily through most of PEMs by means of (a) active transport together with the protons and their solvate waters called as electroosmotic drag, (b) diffusion through the water-filled domains within the PEM structure, and finally (c) diffusion through the PEM itself. Hence, the effective cut-off of methanol crossover through the PEMs is a main issue in a PEM design for DMFC. The methanol permeabilities (measured at 60 ◦ C) of the SPSU-X and the SPSU-X-t membranes are shown in Fig. 7 as a function of IEC value. The methanol permeability of all SPSU membranes is very dependent upon the degree of sulfonation and also lower than that of Nafion 117 (2.38 × 10−6 cm2 /s in 2 M methanol solution). The methanol transport through the SPSU membranes may be accelerated by the increase of water content in the SPSU membranes. Generally, the introduction of a hydrophilic moiety such as sulfonic acid group of polymer chains enhances the water solubility, and it is usually accompanied by the increase of degree of swelling by sorbed water. Thus, the more swelling

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of the membrane due to more water uptake can enlarge hydrophilic channels in the polymer matrix and also methanol molecules can pass through the hydrophilic domains, assuming that methanol can pass mainly through hydrophilic channels formed by water molecules and sulfonic acid groups. As to that, methanol transport behaviors in the SPSU-X-t membranes are good example to ensure above assumption, as compared with SPSU-X membranes. As shown in Fig. 7, the methanol permeabilities of the SPSU-X-t membranes are lower than those of the SPSU-X membranes. In the case of the SPSU membranes (SPSU-7 and SPSU-7-t) showing maximum proton conductivities, the methanol permeability of the SPSU-7 membrane is 7 × 10−7 cm2 /s and that of the SPSU7-t membrane is 4 × 10−7 cm2 /s. The reduction of methanol permeability in the SPSU-X-t membranes may be much related not only with the decrease of water content but also with the increase of packing density in the membrane after annealing process. 4. Conclusions In this work, the proton and methanol transport properties were investigated before and after annealing of SPSU membranes at sub-Tg . The annealing of SPSU membranes led to the decrease of water content, proton conductivity, and methanol permeability, as compared with SPSU membranes prepared at lower temperature. This may be due to denser structure in the annealed SPSU membranes than in the untreated SPSU membranes. That is, the annealing below Tg accelerates the stability of SPSU membranes and subsequently induced more compact structure, which also affects the methanol permeability as well as the proton conductivity. This result may exhibit the importance of a physical aging in the amorphous sulfonated glassy polymers on the proton conduction and the methanol transport. Acknowledgements This work was primarily supported by University Research Program of Ministry of Information & Communication in South Korea. H.B. Park and Y.M. Lee are also grateful to the financial support of National Research Laboratory Program of Korea Institute of Science and Technology for Evaluation and Planning.

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Fig. 7. Methanol permeability (cm2 /s) of the SPSU-X and the SPSU-X-t membranes as a function of IEC value.

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