Proton Exchange Membranes for a Direct Methanol Fuel Cell Based on Sulfonated Styrene-(ethylene-butylene)- Styrene/Polyvinylidene Fluoride Blends N. Seeponkai, J. Wootthikanokkhan Division of Materials Technology, School of Energy Environment and Materials, King Mongkut’s University of Technology, Thonburi, Bangkok 10140, Thailand Received 16 October 2008; accepted 4 October 2009 DOI 10.1002/app.31561 Published online 17 March 2010 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Proton-conducting membranes for a direct methanol fuel cell, based on styrene-(ethylene-butylene)- styrene (SEBS) triblock copolymer blended with polyviny- lidene fluoride (PVDF), have been developed. First, the partially sulfonated SEBS with a variety of degrees of sub- stitution was prepared by reacting the SEBS with propio- nyl sulfate. Then, the sulfonated SEBS was blended with PVDF at various blending ratios and fabricated by using a solution casting technique. The water uptake, proton con- ductivities, methanol permeabilities, and mechanical prop- erties of the blend membranes were measured by using gravimetry, impedance analysis, gas chromatography, and tensile test, respectively. It was found that water uptake of the blend membranes increased with the sulfonated SEBS content, at the expense of their methanol resistance. The proton conductivity of the blend membranes, however, did not change linearly with the sulfonated SEBS content. This was related to poor compatibility between the two polymers in the blend membranes. However, by adding 5 wt % of poly(styrene)-b-poly(methyl methacrylate) block copolymer, compatibility, proton conductivity, and metha- nol resistance of the blend membrane increased. V C 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 393–399, 2010 Key words: blends; block copolymers; compatibility; membranes INTRODUCTION Direct methanol fuel cell (DMFC) is a kind of proton exchange membrane fuel cell (PEMFC), which uses methanol as a fuel to generate electricity without the use of any reforming unit. Currently, Nafion V R , which is a kind of perfluorinated polymer with some sul- fonic groups, is commercially available and has been widely used as an electrolyte polymeric membrane in the PEMFC. This is attributed to the fact that Nafion V R is highly proton-conductive and thermally stable. However, the Nafion membrane also has some disadvantages in relation to DMFC applica- tions. For example, the methanol resistance of the Nafion membrane is not sufficiently high. It was reported that about 40% of the methanol was lost through the process called ‘‘methanol crossover. 1,2 ’’ Beside this, the cost of the commercial Nafion mem- brane is considerable, and the proton conductivity of the membrane tends to decrease at operating tem- peratures above 100  C. 3 Consequently, several efforts have been made to develop some new proton-conducting membranes to be used as a replacement for the Nafion membrane in DMFC applications. 4–12 Our interest in this research area has been involved with the develop- ment of DMFC membranes from various types of polymeric systems. This include, sulfonated poly (vinyl alcohol) (PVA) membranes, 13,14 sulfonated poly(ether etherketone) (PEEK)/polyvinylidene fluo- ride (PVDF) blend membrane, 15 and sulfonated polystyrene/PVDF blend membranes. 16 The former membrane system based on PVA is interesting with respect to DMFC applications. The proton conduc- tivity of the material can be induced by reacting the polymer with some sulfonating agents, such as sul- fosuccinic acid, sulfophthalic acid, and sulfoacetic acid. 13 Furthermore, the methanol resistance of the sulfonated PVA membrane can be further enhanced by mixing the polymer with some nanoclays, to obtain a form of nanocomposite membrane. 14 How- ever, the stability of the material under actual DMFC operating conditions has yet to be explored and verified. This is partly attributed to the fact that the sulfonated PVA contains some ester bonds within the molecule that are rather sensitive to hy- drolysis under warm and acidic conditions. Correspondence to: N. Seeponkai (puy_namon@hotmail. com). Contract grant sponsor: The Commission of Higher Education, the Ministry of Education, Royal Thai Government; contract grant number: RMU4880043. Contract grant sponsor: Thailand Research Fund (TRF). Journal of Applied Polymer Science, Vol. 117, 393–399 (2010) V C 2010 Wiley Periodicals, Inc.