Dielectric properties and viscoelastic response in two-phase polymers

An electrorheological model is applied to results from dynamic mechanical analysis in order to relate low-frequency dielectric properties to viscoelastic response. The model describes polymers with two phases at the mesoscopic scale, with polar groups embedded in one of the phases. Calculations use an electrorheological representation, based on the Voigt model, to describe the two phases. Low-frequency electrical and mechanical stresses are coupled through the permanent dipole moment of polar groups embedded in one of the phases. In the present model, low-frequency dielectric relaxation is calculated as the superposition of two Debye processes, corresponding to the interaction of the polar groups with the two phases. Induced polarization in the non-polar zones contributes only to high-frequency (relaxed) permittivity. The parameters of the relaxation processes are obtained from the electrorheological model using viscoelastic data from dynamic mechanical analysis. The model was applied to commercial EPDM (ethylene-propylene-diene monomer) employed as housing in electrical insulators rated at 33 kV. This rubber has two phases (amorphous and semi-crystalline), with polar groups embedded in the amorphous phase. Samples aged for 10 years in active transmission lines were studied, and compared with new samples. A good agreement was found between the calculated and measured dielectric spectra, within the low-frequency range where the model is applicable.