Sub-Tg annealing studies of rubber-modified and unmodified epoxy systems
Epoxy network systems based on DGEBA (bisphenol_A_diglycidyl ether) and NMA (nadic methyl anhydride) systems modified with the low molecular weight CTBN (carboxyl_terminated butudiene_acrylonitrile copolymer)rubbers were prepared and studied. It was found that below the glass transition of the epoxy matrix these materials displayed time_dependent changes in their mechanical properties; specifically, the strain to break as well as the rate of stress relaxation were observed to decrease in a near_linear behavior with the logarithm of time at sub_T g annealing. Calorimetric methods clearly showed a simultaneous decrease in enthalpy with time that behaved in a similar fashion as the time_dependent mechanical properties. All the calorimetric and mechanical data are qualitatively related. The importance of this phenomena is considered in view of the widespread use of epoxys. Similar behavior is expected for other network glasses thermally quenched into a nonequilibrium state.