DETERMINATION OF GLASS-TRANSITION TEMPERATURE FROM DIELECTRIC ANALYSIS FOR A SERIES OF EPOXIDE OLIGOMERS

Dielectric properties have been investigated for a bisphenol-A type epoxide oligomer, whose weight average molecular weight (M(w)BAR) was 9454. The dielectric alpha-relaxation of the oligomer was found to be governed by the Havriliak-Negami equation as well as the same series of oligomers with smaller M(w)sBAR (388 less-than-or-equal-to M(w)BAR less-than-or-equal-to 3903). The dielectric relaxation times (tau(S)) for the oligomers with different M(w)sBAR (1396 less-than-or-equal-to M(w)BAR less-than-or-equal-to 9454) can be expressed by the Williams-Landel-Ferry (WLF) equation as a function of the glass transition temperature (T(g)) at fixed temperatures from 70 to 100-degrees-C. The finding indicates that the T(g) of the epoxide oligomer is calculated from the tau through the WLF equation, providing the relation between T(g) and tau. The same type of WLF equation was also successfully applied to describe the T(g) dependence of the practical dielectric relaxation time (tau(p)), which was obtained from the peak of the dielectric loss vs. frequency curve. The tau(p) can be calculated more easily than the tau, based on the Havriliak-Negami equation, not only in the measurement of epoxide oligomer, but also in that of the reactive epoxy resin systems during curing. The T(g) of an epoxy-aromatic amine system, which was determined from the tau(p) nondestructively detected in the dielectric cure monitoring, was consistent with the T(g) experimentally measured by differential scanning calorimetry (DSC).