RELAXATIONS IN THERMOSETS .20. DIELECTRIC AND CURING KINETICS OF ELASTOMER-CONTAINING EPOXIDE THERMOSET COMPOSITES

The dielectric properties of composites of amine-terminated butadiene acrylonitrile elastomer and diglycidyl ether of bisphenol-A during their curing with diamino diphenyl methane have been measured. As the composite cures, both its permittivity, epsilon', and loss, epsilon", initially decrease and then increase with time. These changes are attributed to the onset of phase separation of the elastomer in the thermosetting mixture, which occurs before the gelation. The time for this onset slightly decreases with increase in the amount of elastomer and the rate of chemical reactions after the phase separation becomes slower as the elastomer content is increased. The permittivity and loss of the cured thermoset increase with the increase in the elastomer content. A prominent relaxation process in the cured thermosets is the alpha-relaxation peak of the elastomer, whose T(g) is 223 K. This peak is followed by a shoulder at lower temperatures, whose height decreases with increase in the elastomer content, and which is mainly a reflection of the gamma- and beta-relaxation processes in the pure thermoset and a sub-T(g) relaxation in the elastomer. The dielectric properties of the cured thermosets monotonically change with the amount of elastomer in it. This indicates that the inversion of a thermoset matrix to an elastomer matrix, as the elastomer concentration is increased, is virtually undetectable by dielectric measurements.