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Transport properties of sulfonated poly (styrene-isobutylene-styrene) membranes with counter-ion substitution



In this study, the transport properties of poly(styrene-isobutylene-styrene) (SIBS) were determined as a function of sulfonation level (0–94.9%) and counter-ion substitution (Ba+2, Ca+2, Mg+2, Mn+2, Cu+2, K+1) for fuel cell applications. Increasing the sulfonation level improved the ion exchange capacity (IEC) of the membranes up a maximum (1.71 mequiv/g), suggesting a complex three-dimensional network at high sulfonation levels. Results show that proton conductivity increases with IEC and is very sensitive to hydration levels. Methanol permeability, although also sensitive to IEC, shows a different behavior than proton conductivity, suggesting fundamental differences in their transport mechanism. The incorporation of counter-ion substitution decreases both methanol and proton transport. Methanol permeability seems to be related to the size of the counter-ion studied, while proton conductivity is more sensitive to water content, which is also reduced upon the incorporation of counter-ions. To complement the studies, selectivity (i.e., proton conductivity/methanol permeability) of the studied membranes was determined and compared to Nafion® 117. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013