RELAXATIONS IN THERMOSETS .23. DIELECTRIC STUDIES OF CURING KINETICS OF AN EPOXIDE WITH DIAMINES OF VARYING CHAIN LENGTHS

The curing kinetics of two thermosets of diglycidyl ether of Bisphenol A with propylenediamine and hexamethylenediamine have been studied at several temperatures and measurement frequencies. The results have been analyzed both in the permittivity and complex modulus formalisms, and a general pattern of curing kinetics has been observed. At the earlier stages of curing, the dc conductivity decreases according to a scaling law with a critical exponent which increases with the curing temperature. This decrease can also be described by a singularity equation. The analysis has been used to obtain the gelation time of the thermoset and its variation with the temperature and molecular size of the curing agent. The complex permittivity irreversibly changes as the thermoset cures, but these changes are observable only in the postgel period. The permittivity follows a stretched exponential relaxation function with a curing parameter whose value is both temperature and chain length dependent. The relaxation time increases with curing time according to a curve with a sigmoidal shape. This curve shifts to shorter times with an increase in the curing temperature. Measurements for different frequencies show that the various parameters for the curing kinetics are independent of the frequency of measurements. These results are discussed, and information on the effects of molecular chain length on the curing kinetics has been obtained.