DIELECTRIC-PROPERTIES ABOVE THE GLASS-TRANSITION FOR A SERIES OF EPOXIDE PREPOLYMERS

Dielectric properties above the glass transition have been investigated for a series of bisphenol-A type epoxide prepolymers (388 less-than-or-equal-to MBAR(w) less-than-or-equal-to 2640). Dielectric measurements were performed over a frequency range of 50 Hz-1 MHz using a vertical parallel plate cell which was constructed in the laboratory. The dielectric alpha-relaxation for each prepolymer fits the empirical model of the Havriliak-Negami equation. The temperature dependence of the dielectric relaxation time tau is described by the Williams-Landel-Ferry (WLF) equation as well as that of the direct current conductivity-sigma, which can be measured using the same cell. The relationship between tau and sigma, sigma . tau(m) = const, is derived from experimental results. The exponent m, which depends on the molecular weight of the prepolymer, is considered to correspond to the ratio of the segmental mobility to ionic mobility. The dielectric loss epsilon" can be used as an indicator of the direct current conduction in the temperatures where the ionic component in epsilon" becomes much larger than the dipole one.