Orientational dynamics of water in the nafion polymer electrolyte membrane and its relationship to proton transport.

Orientational anisotropies are calculated from molecular dynamics simulations of bulk water and the Na(+) and H(+) forms of hydrated Nafion and then compared with corresponding experimental values. The extended jump model of Laage and Hynes is applied to water reorientations for each system, and the anisotropies are explored as a product of hydrogen bond restricted "wobble-in-a-cone" reorientations and that due to the discrete jumps of hydrogen bond reorganization. Additionally, the timescales of hydrogen bond switching and proton transport are presented for bulk water and the H(+) form of hydrated Nafion. The short time scale of proton hopping is found to be independent of Nafion water loading, suggesting the short time dynamics of proton hopping are relatively insensitive to the level of hydration. Furthermore, the long time decay for the forward rate of hydrogen bond switching is shown to be identical to the long time decay in the forward rate of proton hopping, for bulk water and all water loadings of Nafion investigated, suggesting a unified process.