Nanoscale structure and transport in simulated polyelectrolyte membranes

Polyelectrolyte membranes swell to absorb water, which forms interconnected paths that facilitate ion and water transport. Such membranes can be used for a variety of applications including electrolysis, energy storage, and water filtration. A key question about these membranes is how the polymer architecture affects the properties of the aqueous pore space and transport within it, which can be explored through well-designed simulations. Here, we investigate polystyrene-polymethylbutylene block copolymer membranes, with the polystyrene block randomly sulfonated to an experimentally relevant level (25 mol percent). We vary the amount of water in the membrane, and find strong circumstantial evidence that the equilibrium water uptake is about 16 water per sulfonate, consistent with experiments. Surprisingly, even at a water content 8x smaller, the pore structure remains connected, with geometry that shrinks down to narrow ribbons. We measure both short and long-time diffusivity of counterions and water, and find that diffusivity increases with water content. The counterion distribution in the pore space is not uniform; counterions are mainly found near the polymer-water interface, which is decorated with sulfonate anions. This non-uniform distribution may have important consequences for ion rejection described by Donnan exclusion.