RELAXATIONS IN THERMOSETS .13. EFFECTS OF POSTCURE AND AGING ON THE SUB-TG RELAXATIONS OF NONSTOICHIOMETRIC EPOXIDE-BASED THERMOSETS

The dielectric permittivity and loss of diglycidyl ether of bisphenol-A (DGEBA) cured with greater than and less than the stoichiometric amounts of diaminodiphenyl methane (DDM) have been measured over a temperature range 77-350 K prior to curing and gelation, after curing at about 340 K, and further aging for a predetermined period. The height of the gamma-relaxation peak monotonically decreases during the post-cure period and becomes masked by the contributions from the beta-relaxation peak, whose height, in turn, first increases on postcuring to a same maximum value for both nonstoichiometric thermosets and then decreases. This decrease is attributed to physical aging effects. The beta-relaxation peak shifts towards higher temperature on postcuring. Comparison between the changes in the dielectric properties of the saturated and starved thermosets show that while the gamma-relaxation process may be attributed to the motion of the epoxide dipolar groups of the unreacted DGEBA, the beta-relaxation process is not attributable entirely to the motion of - OH groups and of the unreacted amines in the thermoset. Explanations involving the chain and network packing in the structure of a thermoset are necessary for the observed behavior of the beta-relaxation process in amine saturated and starved thermosets.