Relaxation and crystallization of water in a hydrogel

Abstract

The dielectric spectroscopy of 48.3 wt.% water in a terpolymer hydrogel shows that its α-relaxation has a distribution of relaxation times, with a temperature-independent half width, and an Arrhenius rate dependence. The crystallization of this water was followed by the time and temperature dependence of its dielectric features. It begins slowly at 207 K and does not reach completion on heating up to 270 K. In this temperature range supercooled water and ice coexist, the amount of ice increases with increasing temperature, and the dc conductivity decreases with increase in the crystallized fraction, but the dielectric relaxation rate remains unaltered. It is shown that these features can be more easily discerned in plots of the electrical modulus than of the relative permittivity. The results indicate a continuous variation of the energetic states of water molecules from the surface-bound to the bulk state inside the cavities of the hydrogel. It is pointed out that water also exists at grain junctions in the polycrystalline ice formed and that their reduction on grain-growth may be responsible for the time-dependent crystallization of water sequestered in the pores of the polymer.