Proton conductivities and methanol permeabilities of membranes made from partially sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymers

A series of partially sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymers (SSEBS) was synthesized and the sulfonation was characterized qualitatively by a Fourier transform-infra red (FT-IR) and quantitatively by an elementary analysis. The membranes from synthesized SSEBS were prepared by solution casting method and their proton conductivities and methanol permeabilities were measured. It was found that the proton conductivity and methanol permeability increase abruptly when the sulfonation degree on polystyrene end blocks exceeds 15 mol% and the proton conductivity equivalent to that of Nafion 117 is obtained for SSEBS with 34 mol% sulfonation while the methanol permeability is decreased more than a half. The water and methanol uptakes of SSEBS and Nafion membranes were directly evaluated and the results showed that, in both cases, the membranes favors methanol much more than water, indicating that it would not be an easy task to develop a promising proton exchange membrane from SSEBS which could be utilized for the direct methanol fuel cell (DMFC). The microstructures of SSEBS and Nafion membranes were examined by small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) and the difference between them is discussed too.