(605b) The Effect of Polymer Equivalent Weight (EW) On Morphology in Hydrated Perfluorosulfonic Acid Membranes Via Molecular Dynamics Simulations

Molecular dynamics (MD) simulations have been performed to investigate the effects of polymer equivalent weight (EW) on the hydration and diffusion of protons in perfluorosulfonic acid (PFSA) membranes. Two groups of the PFSA polymer electrolytes were investigated at five different hydration levels (ë=H2O/SO3H) of 3, 6, 9, 15, and 22. We examine Nafion with EW of 744, 844, 944, and 1144 and the Short-side-chain (SSC) PFSA material with EW of 678, 778, 878, and 978. We probe the effect of EW changes on the local and global morphology of the hydrated membrane through pair correlation functions (PCFs) and water cluster distributions. We find that for both materials, an increase in the EW corresponds to a decrease in the connectivity of the aqueous domain and a decrease in the degree of confinement at low and intermediate water contents. At high water contents, the differences between the membranes are less pronounced. The relative connectivity is measure through an analysis of water cluster distributions. The confinement is measured through a set of PCFs including the S-S, water-water, and water-hydronium ion PCFs. When we compare Nafion and SSC with similar separation between side chains, we find that an increase in the side chain length has a qualitatively similar effect to that of a change in EW; namely that a decrease in the connectivity of the aqueous domain and a decrease in the degree of confinement. However, the quantitative effect of a change in side chain length on the membrane morphology is different than that of a change in EW. We also investigate the diffusivities of the hydronium ion and water as a function of this polymer architecture modification. Although we observe relatively clear trends between the change in EW and the change in membrane morphology, the effect of EW on transport properties is more complicated. As EW increases both connectivity and confinement decrease. A decrease in connectivity will hinder transport in the aqueous phase, however a decrease in confinement will enhance transport. The dominating term in this competition appears to be a function of water content as well as the time scale of interest.

Acknowledgments

The work is supported by a grant from the U. S. Department of Energy BES under the contract number DE-FG02-05ER15723. This research used resources of the Center for Computational Sciences at Oak Ridge National Laboratory, which is supported by the Office of Science of the DOE under Contract DE-AC05-00OR22725. This work also used resources of the National Institute for Computational Sciences (NICS), supported by NSF with agreement number: OCI 07-11134.