Dielectric relaxation during isothermal curing of epoxy resin with an aromatic amine

The isothermal curing behavior of an epoxy resin system has been monitored by a dielectric measurement whose sensor consisted of a vertical parallel‐plate cell based on a three‐terminal method. The materials used in the isothermal cure were the diglycidyl ether of bisphenol‐A (DGEBA), which was purified from Epon 825 by recrystallization, and 4,4′‐diamino diphenyl methane (DDM). A dielectric relaxation was observed during each isothermal cure at temperatures of 70,80,90, and 100°C, which were below the glass transition temperature (Tg) of the reactive DGEBA–DDM system at the gel point. The relaxation is considered to be caused by the transformation from a liquid state to an ungelled glassy state as a result of an increase in molecular weight because gelation followed by a rubbery state did not exist in the temperature range studied. The dielectric relaxation for the DGEBA–DDM system fits the empirical model of the Havriliak–Negami equation. The Tg of the DGEBA–DDM system, which was estimated from the dielectric relaxation time, agreed with the one experimentally determined by differential scanning calorimetry (DSC).