Correlation between morphology, water uptake, and proton conductivity in radiation grafted proton exchange membranes

A small-angle neutron scattering (SANS) investigation of saturated aqueous proton exchange membranes is presented. Our membranes were synthesized by radiation-induced grafting of poly(ethylene-alt-tetrafluoroethylene) (ETFE) with styrene in the presence of crosslinker (divinylbenzene, DVB) and the polystyrene was sulfonated subsequently. The contrast variation method was used to understand the relationship between morphology, water uptake, and proton conductivity. We find that the membranes are separated into two phases, mostly following the morphology already defined in the semi-crystalline ETFE base film. The amorphous phase hosts the water and swells upon hydration, swelling being inversely proportional to the degree of crosslinking. Proton conductivity and volumetric fraction of water are related by a power law, indicating a percolated and most likely random network of finely dispersed aqueous pores in the hydrophilic domains.