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

H-1 NMR spectroscopy is employed to study the longrange (diffusion) and short-range (relaxation) motions of water in the hydrophilic channels of Nafion at different stages of hydration, 2water. From Bloembergen, Purcell, and Pound analysis of the temperature-dependent H-1 spin-lattice relaxation rates, the coexistence of two motional modes for lambda(water) < 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-1 NMR studies showed a steep increase in diffusion coefficients at 6 <= lambda(water) <= 9 followed by a linear increase with the increasing hydration level (lambda(water) >= 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 lambda(water) that can be divided into three characteristic regions. These include the formation of the first hydration shell at lambda(water) approximate to 3 (region I), the second hydration shell at 2water lambda(water) approximate to 8 (region II), and finally (iii) the presence of bulk water above lambda(water) >= 10 (region III).