Broadband dielectric investigation on poly(vinyl pyrrolidone) and its water mixtures.

Broadband dielectric spectroscopy and differential scanning calorimetry measurements have been performed to study the molecular dynamics poly (vinyl pyrrolidone) and its water solutions in a wide range of concentrations (0 wt %<w(c)<55 wt %) and in the temperature range from 140 to 500 K. The dry material was completely characterized showing the presence of two relaxations in the supercooled state. The slow one has the characteristics of a Johari-Goldstein-type relaxation. On the other hand, the low temperature water dynamics in the mixtures evidences a prominent loss peak due to the reorientation of water molecules inside the polymer matrix for all the hydration levels. We show that the relaxation times are almost water concentration (w(c)) independent from w(c)>20 wt % suggesting that this dynamical process is dominated by water-water interactions. In addition, the temperature dependence of the water relaxation times exhibits a crossover from non-Arrhenius to Arrhenius behavior during cooling throughout the glass transition range, which has been interpreted as due to the constrains imposed by the rigid polymer matrix on the water molecules dynamics.