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Nafion β-relaxation was studied by detailed dielectric spectroscopy measurements carried out in a wide range of both temperature and relative humidity. Cast and extruded Nafion were compared in the H+, Na+, and Cs+ forms to evaluate contributions arising from membrane morphology and ionic interactions. The experimental data indicated that β-relaxation is associated with the motion of side chains of the ionomer. The unusual shift to lower frequencies of the β-relaxation was shown to be more pronounced for samples containing high water content and at temperatures above the α-relaxation, evidencing that the electrostatic repulsions between bare sulfonic groups reduce the side chain mobility. A pronounced increase of relaxation times was observed at relative humidity >60% suggesting a change of the morphology of the ionomer, in agreement with recent descriptions of the Nafion structure by small angle X-ray scattering analyzes.

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