Simulations predict water uptake and transport in nanostructured ion exchange membranes

Ion exchange membranes are made up of polymers with charged groups randomly tethered to the backbone. In humid conditions, these membranes absorb water to form nanostructures: the polymers reside in a hydrophobic domain, while the charged groups and counterions reside in interconnected water-filled paths that facilitate ion and water transport. Consequently, these membranes find their application in energy storage and water filtration. Molecular dynamics simulations can be used to investigate the effect of polymer structure and hydration level on the nanostructured pore space and transport within it. Here, we investigate polystyrene-polymethylbutylene (PS-PMB) block copolymer membranes, with the PS block randomly sulfonated to an experimentally relevant level of 25 mole percent. The counterion distribution in the pore space is not uniform; counterions are mainly found near the polymer-water interface, which is decorated with negative sulfonate groups. We vary the water content in the membrane, and predict an equilibrium water uptake consistent with experiments. Surprisingly, even at a water content 8 times smaller than equilibrium, the aqueous paths remain connected; however, the pores shrink down to narrow ribbons. We measure both short-time and long-time diffusivity of counterions and water, and find that diffusivity increases with water content and is impacted by pore tortuosity.