Salt and Water Diffusivities in Polymer Electrolyte Membranes

Motivated by experimental observation where transport properties of salt ions and water in charged polymer membranes exhibit an intriguing dependence on salt concentration, we have probed a series of atomistic molecular dynamics simulations of non-cross linked membranes in aqueous mono and divalent salt solutions along with mesoscale dissipative particle dynamics (DPD) simulations for crosslinked membranes at concentrations ranging from 0.06 to 1 M. We investigate the molecular level understanding the effects of both multi-component salt solutions and crosslinking of polymers on fundamental salt and water transport properties. Our finding shows that the diffusion of salt ions and water are influenced by cation sizes and salt concentrations. Divalent ions are more strongly coupled with ionic groups which reduce their motions as increasing concentration in fixed charged membranes. In crosslinked membranes, diffusion of salt ions and water are reduced significantly relative to non-crosslinked systems. However, the trends exhibited by the salt concentration dependence of diffusivities, and the coordination of the cations with anions, and with the polymer backbone remain qualitatively similar to those observed in non-crosslinked membranes.