RELAXATIONS IN THERMOSETS .6. EFFECTS OF CROSS-LINKING ON SUB-TG RELAXATIONS DURING THE CURING AND AGING OF EPOXIDE-BASED THERMOSETS

The dielectric permittivity and loss of diglycidyl ether of bisphenol-A-based thermosets cured with diaminodiphenyl methane and diaminodiphenyl sulfone have been measured over a temperature range 77-400 K after curing or aging for a predetermined duration. Of the two sub-T(g) relaxations, the height of the gamma-relaxation peak monotonically decreases during both the cure and postcure periods, and the height of the beta-relaxation peak first increases to a maximum value and then decreases. This decrease is attributed to physical aging effects. The height of the alpha-relaxation peak decreases. The gamma- and beta-relaxation peaks become increasingly separated in temperature. A concept of accumulated equivalent curing time which is based upon known chemical kinetics has been introduced for use in both theoretical and practical aspects of the study of thermosets. It is shown that substantial curing of the sample occurs during its slow heating to the curing temperature. The use of this concept in the curing of thermosets is illustrated. A procedure for the analysis of the distribution of relaxation times from a set of results limited in both frequency and temperature range is described. The distribution parameter is 0.20 and 0.16 for the gamma and beta processes, respectively, and remains constant with postcuring and physical aging. The distribution parameter for the alpha process decreases from 0.60 to 0.36 on curing.