Proton conducting membranes prepared by radiation grafting of styrene and various comonomers

Proton conducting membranes are synthesized by radiation grafting a fluoropolymer matrix, poly(ethylene-alt-tetrafluoroethylene) (ETFE), with styrene and various non-crosslinking comonomers: methacrylonitrile (MAN), acrylonitrile (AN), methyl methacrylate (MMA), and methacrylic acid (MAA). During sulfonation, hydrolysis of nitrile and ester groups takes place to different extents. Solid-state NMR spectroscopy confirms the cyclic ketone structure formed in styrene/MAA co-grafted membranes. The comonomer influence on the membrane properties is studied by characterizing the ion exchange capacity (IEC), water uptake, proton conductivity, and the nano-scale morphology via small-angle X-ray scattering (SAXS). In water swollen state, the proton conductivities of grafted membranes with similar IEC are comparable, while at reduced relative humidity (<80%) the co-grafted membranes exhibit lower proton conductivity compared to the styrene grafted membrane, regardless of the type of comonomer. It is proposed, based on the SAXS analysis, that this is a consequence of an increased average distance between the acid groups in co-grafted membranes.