State of water in nation 117 membranes studied by microwave dielectric relaxation spectroscopy

Complex dielectric permittivity spectra, ε^*(ω), of hydrated Nafion 117 membranes have been measured using the transmission line method over the frequency range of 0.045-26 GHz and at temperatures ranging from 25 to 45 °C. The spectra were well-fitted by a superposition of one to three Debye relaxation processes, depending on the water content. The first high-frequency dispersion step, with a relaxation time τ≈8 ps at 25 °C, shows the characteristic dynamics of bulk water and is attributed to the cooperative relaxation of hydrogen bonded networks of "free" water in the membrane. The free water is assumed to exist at the center of the ion cluster regions or pores. The second step, having a relaxation time of τ≈30 ps at 25 °C, is attributed to the water molecules in the second hydration shell of the sulfonate ions. The remaining relaxation process occurs at low-frequency, having a relaxation time of τ ≈ 90 ps, and is postulated to result from the motion of the water molecules hydrophobically- associated with the perfluorinated side chains and polymer backbone.