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

This study uses variable temperature 19 F solid‐state nuclear magnetic resonance (SSNMR) spectroscopy to determine the influence of electrostatic interactions on the T 1 , T 1ρ , and T 2 values of Nafion®. Because of a “homogenizing” of the T 1 's as a result of spin diffusion, it was not possible to resolve from the T 1 experiments the relative motions of the side‐ and main‐chain. The initial increase in T 1ρ as a function of increasing temperature has been attributed to backbone rotations that increase with increasing temperature. The maxima observed in the T 1ρ 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 1ρ values were observed for the main‐chain at high temperatures, which has been attributed to a thermally activated ion‐hopping process. The results of T 2 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.