Comprehensive Picture of Water Dynamics in Nafion Membranes at Different Levels of Hydration

1H NMR spectroscopy is employed to study the long-range (diffusion) and short-range (relaxation) motions of water in the hydrophilic channels of Nafion at different stages of hydration, λwater. From Bloembergen, Purcell and Pound (BPP) analysis of the temperature-dependent 1H spin-lattice relaxation rates, the coexistence of two motional modes for λwater < 9 was observed, which can be attributed to the non-freezing behavior of water molecules. At higher hydration levels, a clear transition between two different motional modes was detected associated with the freezing of bulk water. The hydration-level-dependent diffusion 1H NMR studies showed a steep increase in diffusion coefficients at 6 ≤ λwater ≤ 9 followed by a linear increase with increasing hydration level (λwater ≥ 12). Taken together the correlation of long- and short-range motion studies allowed us to establish a comprehensive picture of the hydration shell formation for the sulfonic acid groups in Nafion dependent on λwater that can be divided into three characteristic regions. These include the formation of the first hydration shell at λwater ≈ 3 (region I) and the second hydration shell at λwater ≈ 8 (region II), and finally, the presence of bulk water above λwater ≥ 10 (region III).