Relaxations in thermosets. XIII. Effects of post‐cure 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 γ‐relaxation peak monotonically decreases during the post‐cure period and becomes masked by the contributions from the β‐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 β‐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 γ‐relaxation process may be attributed to the motion of the epoxide dipolar groups of the unreacted DGEBA, the β‐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 β‐relaxation process in amine saturated and starved thermosets.