Evidence of Temperature-Induced Subdiffusion of Water on the Micrometer Scale in a Nafion Membrane

To investigate the topological features of proton transport in perfluorosulfonic membranes, we performed time-dependent (t)H NMR pulsed gradient spin-echo experiments on four samples of Nafion-115 with lambda = 0.5, 3.2, 5.8, and 12.4 water molecules per sulfonic group at different temperatures ranging from 278 to 348 K. A subdiffusive behavior of water, < z(2) > = 2D(alpha)t(alpha) with 0.75 < alpha < 1, was observed above 320 K for each of the four samples. The onset of water subdiffusion with increasing temperature, basically independent of membrane hydration, supports the hypothesis of a drop in dimensionality for the diffusion space. Diffusion-diffraction effects on the NMR echo-signal attenuation confirm that proton displacement is restricted because of an interconnected pore structure With it coherence length of similar to 1 mu m, which depends on temperature and water content.