Water Activity Coefficient and Proton Mobility in Hydrated Acidic Polymers

Abstract

Water vapor sorption and proton conductivity data of composite and cross-linked sulfonated aromatic polymers, sulfonated (poly ether ether ketone) (SPEEK) and sulfonated (poly phenyl sulfone) (SPPSU), determined at various temperatures and relative humidities, are used to determine effective water activity coefficients and proton mobilities in these ionomers. The data are challenged with classical physico-chemical laws for electrolyte solutions. The concentration dependence of water activity coefficients in hydrated aromatic polymers is related to an internal pressure exerted by the polymer, which is of the order of the tensile strength of the membranes. The strong concentration dependence of proton mobility is attributed to the particular environment that protons experience inside the nanometric hydrated channels with immobilization of protons near sulfonate groups situated on the channel boundaries. At high proton concentration, the percolation threshold of hydrated channels is observed in SPEEK, which presents hydrated nanochannels with higher tortuosity than Nafion or SPPSU. In SPEEK–silylated poly-phenyl-sulfone (SiPPSU) composites, this percolation threshold is not observed, pointing to special interface paths for conduction.