413e The Effect of Polymer Structure and Morphology on Proton Hydration and Diffusion in Perfluorosulfonic Acid Membranes Via Molecular Dynamics Simulations

Junwu Liu¹, Shengting Cui², Stephen J. Paddison², David J. Keffer², and Brian J. Edwards². (1) Department of Chemical Engineering, University of Tennessee, 1512 Middle Drive, Knoxville, TN 37996, (2) Chemical Engieering, University of Tennessee, 1512 Middle Drive, Knoxville, TN 37996

Abstract

Molecular dynamics (MD) simulations have been performed to investigate the effects of polymer architecture on the hydration and diffusion of protons in perfluorosulfonic acid (PFSA) membranes. The structure of the PFSA ionomers were varied in three distinct ways. First, the length of the side chain was varied to correspond to Nafion with its relatively long side-chain, -OCF2CFCF3OCF2CF2SO3H, and to the Short-Side-Chain (SSC) PFSA, -OCF2CF2SO3H. Second, the equivalent weight of the polymer electrolyte was varied in order to probe the effect of polymer equivalent weight. Finally, the molecular weight of the polymer was varied in order to quantify the effects in the molecular simulations on simulated molecules much shorter than those in real systems. We investigate these structure/property relationships as functions of the degree of hydration, including lambda values (ratios of H2O/SO3H) of 3, 6, 9, 15, and 22.

We characterize the effect of these structural changes on the local and global morphology of the aqueous phase, through pair correlation functions and water cluster size distributions, respectively. We also analyze the diffusivities of the hydronium ion and water as a function of polymer architecture. The results from the molecular-level simulations are compared with available experimental data.