RELAXATIONS IN THERMOSETS .10. ANALYSIS OF DIPOLAR RELAXATIONS IN DGEBA-BASED THERMOSETS DURING ISOTHERMAL CURE

Dielectric properties measured during isothermal curing of DGEBA-based thermosets using a mixture of aromatic amines as curing agent are analyzed. The evolution of the dielectric features of thermosets during curing and after a time when their dc conductivity has reached a negligibly small value are phenomenologically similar to the dielectric features of physically and chemically stable dipolar liquids and solids observed with increasing frequency or decreasing temperature. This equivalence is a consequence of the invariance of the dynamic behavior of dielectric susceptibility with respect to either the frequency of measurement or the relaxation time of the substance and demonstrates that crosslinking of a thermoset causes its relaxation time to increase monotonically. It is shown that the stretched exponential relaxation function formalism satisfactorily describes the dielectric results and that the value of its distribution parameter initially decreases and, after gelation, reaches a constant value, which we denote gamma, in the latter part of the cure. The value of the curing parameter, gamma, which lies between 0.2 and 0.4, monotonically decreases with increasing curing temperature and tends to a limiting value characteristic of a thermoset at higher temperatures. This is in contrast with the increase found in the corresponding representation in the Kohlrausch-Williams-Watts parameter beta with increasing temperature. The curing time dependence of the dipolar relaxation time tau has been determined and found to have the shape of an elongated S, with a well-defined point of inflexion, as tau increases during the cure, from a value characteristic of a liquid to an ultimate value characteristic of a glass.