Fast Water Diffusion and Long-Term Polymer Reorganization during Nafion Membrane Hydration Evidenced by Time-Resolved Small-Angle Neutron Scattering.

We report a small-angle neutron scattering study of liquid water sorption in Nafion membranes. The swelling of hydrophilic domains was measured on the nanoscale by combining in situ time-resolved and long-term static experiments, yielding kinetic curves recorded over an unprecedented time scale, from hundreds of milliseconds to several years. At low water content, typically below 5 water molecules per ionic group, a limited subdiffusive regime was observed and ascribed to nanoconfinement and local interactions between charged species and water molecules. Further ultrafast and thermally activated swelling due to massive liquid water sorption was observed and analyzed by using Fick's equation. The extracted mutual water diffusion coefficients are in good agreement with pulsed field gradient NMR self-diffusion coefficient values, evidencing a water diffusion-driven process due to concentration gradients within the Nafion membrane. Finally, after completion of the ultrafast regime, the kinetic swelling curves exhibit a remarkable long-term behavior scaling as the logarithm of time, showing that the polymer membrane can continuously accommodate additional water molecules upon hydration stress. The present nanoscale kinetics results provide insights into the vapor-versus-liquid sorption mechanisms, the nanostructure of Nafion, and the role of polymer reorganization modes, highlighting that the membrane can never reach a steady state.