Variable temperature 19F solid-state NMR study of the effect of electrostatic interactions on thermally-stimulated molecular motions in perfluorosulfonate lonomers

This study uses variable temperature F-19 solid-state nuclear magnetic resonance (SSNMR) spectroscopy to determine the influence of electrostatic interactions on the T-1, T-1 rho, and T-2 values of Nafion (R). Because of a "homogenizing" of the T-1's as a result of spin diffusion, it was not possible to resolve from the T, experiments the relative motions of the side- and main-chain. The initial increase in T-1 rho, as a function of increasing temperature has been attributed to backbone rotations that increase with increasing temperature. The maxima observed in the T-1 rho, plots suggest a change in the dominant relaxation mechanism at that temperature. The similarity in relaxation behavior of the side- and main-chains suggests that the motions are dynamically coupled, because of the fact that the side-chain is directly attached to the main-chain. Two T-1 rho, values were observed for the main-chain at high temperatures, which has been attributed to a thermally activated ion-hopping process. The results of T2 studies show that correlated motions of the side- and main-chain exist at low temperatures. However, at elevated temperatures the T-2 values for the side-chain increase rapidly while remaining relatively constant for the main-chain, indicating an onset of mobility of the side-chains. (c) 2007 Wiley Periodicals, Inc.