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

Abstract

To investigate the topological features of proton transport in perfluorosulfonic membranes, we performed time-dependent 1H NMR pulsed gradient spin−echo experiments on four samples of Nafion-115 with λ = 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, <z2> = 2Dαtα with 0.75 < α < 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 a coherence length of ∼1 μm, which depends on temperature and water content.