Low-frequency dynamics of water absorbed in Nafion membranes as a function of temperature

We performed a neutron scattering study to investigate the dynamical behaviour of water absorbed in Nafion at low hydration levels (λ = 6, λ = moles of water/moles of sulphonic acid sites) as a function of temperature in the range 200–300 K. To single out the signal of the confined water, the measurements were performed on samples hydrated with both H2O and D2O in the same temperature range. Due to the strong incoherent scattering cross section of hydrogen atoms with respect to deuterium, in the difference spectra the contribution from the Nafion membrane is subtracted out and most of the spectra originates from absorbed water. The estimated dynamical susceptibility exhibits features that resemble those of bulk water. In particular, the spectra display a bump at around 1 meV, possibly related to the α relaxation, the intensity of which is markedly affected by the temperature change. Two features due to the phonon-like collective hydrogen bond network dynamics are visible at approximately 7 meV and 25 meV.