DIELECTRIC-RELAXATION SPECTROSCOPY IN POLY(HYDROXYETHYL ACRYLATES)/WATER HYDROGELS

The electrical and dielectric properties of poly(hydroxyethyl acrylate), PHEA, hydrogels were studied by means of dielectric relaxation spectroscopy in wide ranges of frequencies (5-2 X 10(9) Hz), temperatures (173-363 K) and water contents (0.065-0.46, g of water per gram of dry material). The secondary dipolar mechanisms (gamma and beta(sw)) and the dc conductivity mechanism were studied in detail by analyzing the dielectric susceptibility data within the complex permittivity formalism, the modulus formalism, and the complex impedance formalism. For both mechanisms molecular mobility was found to increase with increasing temperature or increasing water content (T-f-h superposition principle). The energy parameters and the shape parameters of the response were determined for both mechanisms at several water contents and temperatures. The temperature dependence of de conductivity was found to change from Vogel-Tamman-Fulcher (VTF) type to Arrhenius type at water contents of ca. 0.30. At water contents lower than about 0.30 the hydrogels are homogeneous whereas at higher water contents a separate water phase appears. In terms of the strong/fragile classification scheme our results suggest that the PHEA hydrogels are fragile systems. (C) 1995 John Wiley and Sons, Inc.