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 × 109 Hz), temperatures (173–363 K) and water contents (0.065–0.46, g of water per gram of dry material). The secondary dipolar mechanisms (γ and βsw) 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 dc 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. ©1995 John Wiley & Sons, Inc.

A model based on the aspect of the distribution of the length of conduction paths accessible for electric charge flow reproduces the universal power-law dispersive ac conductivity observed in polymer networks and, generally, in disordered matter.Power exponents larger than unity observed in some cases are physically acceptable within this model. A saturation high frequency region is also predicted, in agreement with experimental results. There does not exist a 'universal fractional power law' (and is useless searching for a unique common critical exponent), but a qualitative universal behavior of the ac condductivity in disordered media.

We investigate the dielectric behavior of sandstone, which consists of a porous matrix with a small amount of inherent humidity, by the thermal-stimulated depolarization current technique. Nine different relaxation mechanisms are detected by the thermal sampling scheme, and are characterized. The activation energy distribution and the pre-exponential factor are obtained by analyzing the signals under the constraint of a normal distribution in the activation energy. The drying of the specimen at elevated temperature under dynamic vacuum affects some of the relaxation mechanisms. The model of freely rotating dipoles may not account for all the drying-sensitive mechanisms. It is probable that water molecules are organized in a way that provide either conductive layers over the surface of the grains or for conductive inclusions inside the bulk. Longdistance charge-transport mechanisms are also affected by the removal of the humidity.

Isotherms of the imaginary part of the permittivity from 10−2to2×106Hz from liquid nitrogen to room temperature for fresh and thermally aged specimens of conducting polypyrrole reveal a dielectric loss peak, which is affected by the reduction of conducting grains with aging. Charge trapping at the interfaces separating the conductive islands seems invalid. Thermal aging indicates that macroscopic conductivity and short range one have different aging evolution. The first (dc conductivity) is dominated by the tunneling of the carriers between neighboring grains through the intermediate insulating barriers, though the second (ac conductivity) is due to a backward-forward movement of the carriers and is controlled by the intrachain transport of them and their hopping between the chains.

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