Vitrification and further structural relaxation in the isothermal curing of an epoxy resin

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Abstract
Isothermal curing of an epoxy resin based on diglycidyl ether of bisphenol A, using a hardener derived from phthalic anhydride, has been performed at curing temperatures between 30 and 130°C. Samples were cured isothermally at various intervals of time and analyzed by differential scanning calorimetry (DSC), the glass transition temperature Tg, and the conversion degree being determined by the residual enthalpy technique. The vitrification phenomenon and a further structural relaxation process, occurring at curing temperatures (Tc) lower than the maximum Tg (109°C), at which Tg equalizes Tc, have been studied at curing temperatures between 30 and 100°C. The structural relaxation process is analyzed by the endothermic peak that appears superposed on Tg in dynamic DSC scans. The area of this peak (Q) is a measure of the recovery enthalpy, and thus of the extent of the relaxation process. This process begins at higher curing times (tc) when Tc decreases because the vitrification of the system starts later. Both the enthalpy recovery (Q) and the temperature of the endothermic peak (Tm) increase with the annealing time (ta), calculated as the difference between tc and the time in which vitrification occurs, and tend to have a limiting value due to the fact that the system loses mobility when the free volume decreases during its asymptotic approach toward the metastable equilibrium state. Furthermore, the dependence of Q and Tm on ta at different Tc shows that the relaxation process in partially cured resins depends on the conversion degree of the system and consequently on the crosslinking density of the network.