Effect of Sulfonation On the Transport Properties of Poly(styrene-isobutylene-styrene) and Poly(styrene-isoprene-styrene) Membranes

In this study, physical blends of sulfonated and unsulfonated linear poly(styrene-isobutylene-styrene) (SIBS) and radial poly(styrene-isoprene-styrene) (SIS) were casted and analyzed as function of sulfonation level (0-60.8%). Increasing the amount of sulfonic groups improved the ion exchange capacity (IEC) of the membranes, which can be related to their transport properties. To confirm and determine accurate sulfonation levels, SIBS-SIS membranes were characterized using Fourier transform infrared spectroscopy (FTIR) and elemental analysis (EA). Thermogravimetrical analysis (TGA) was performed in order to evaluate the thermal stability, which increases with sulfonation. Absorption limitations and their effect on the membrane’s transport properties were investigated through water swelling experiments. Transport properties for direct methanol fuel cell (DMFC) applications (methanol permeability and proton conductivity), were measured and compared to Nafion®.  The results suggest that the combination of ionic sulfonic groups with unique morphological arrangements can lead to high selectivities for DMFC applications.