Impact of Morphology on Water Dynamics in Hydrated Copolymers for Proton Exchange Membranes*

We study novel fluorine-free copolymers with transport properties rivaling those of perfluorosulfonic acid polymers that are promising as proton exchange membranes. These random copolymers have one monomer with a 5-carbon backbone and a pendant phenylsulfonate group and a second monomer with the same backbone and a pendant phenyl group. Using atomistic molecular dynamics simulations, we explore the effects of varying the relative humidity and the percent of sulfonated phenyl groups. We identify systems that form percolated nanostructures using a combination of visual inspection and cluster-based analyses. Total scattering functions reveal strong nanophase separation between the hydrophilic and hydrophobic domains as evidenced by the presence of an ionomer peak. We obtain the water diffusion coefficient, water rotational time, and nuclear magnetic resonance (NMR) T1 and T2 times for the copolymers that exhibit percolated nanostructures. We relate these dynamic properties to structural descriptors such as channel width distribution and fractal dimension to better understand the impact of nanostructure morphology on water dynamics.