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

H 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, λ. From Bloembergen, Purcell, and Pound analysis of the temperature-dependent H spin-lattice relaxation rates, the coexistence of two motional modes for λ < 9 was observed, which can be attributed to the nonfreezing 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 H NMR studies showed a steep increase in diffusion coefficients at 6 ≤ λ ≤ 9 followed by a linear increase with the increasing hydration level (λ ≥ 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 λ that can be divided into three characteristic regions. These include the formation of the first hydration shell at λ ≈ 3 (region I), the second hydration shell at λ ≈ 8 (region II), and finally (iii) the presence of bulk water above λ ≥ 10 (region III).