Structure and Dynamics of Hydrated Precise Sulfophenylated Polyethylene Polymers from Simulation

Design of novel materials for proton exchange membranes can be aided by a greater understanding of the molecular mechanisms of ion transport. We study a precisely-spaced sulfophenylated polyethylene polymer (p5PhSA) in which sulfonate anions are covalently bound to the pendant phenyl groups and are neutralized by hydronium ions. We perform molecular dynamics simulations at different water contents. Sulfonate groups and water molecules form percolating nanochannels that phase separate from the polymer backbone, allowing for transport of hydronium ions through the system. The inomer peak in computed structure factors exhibits a shift in position to lower wave vector and quenching in magnitude relative to the amorphous halo with increasing water content, similar to x-ray scattering data. We use computed partial structure factors to better understand these changes. Higher water content also results in hydronium ions being increasingly coordinated by water and decreasingly coordinated by sulfonate groups. Finally, rotation rates and diffusion constants of water molecules are found to exhibit exponential decay as a function of sulfonate to water molar ratio.