Thermomechanical Studies of Photoinitiated Cationic Polymerization of a Cycloaliphatic Epoxy Resin

摘要

Thin films of 3,4-epoxycyclohexylmethyl 3',4'-epoxycyclohexane carboxylate were maintained at isothermal temperatures ranging from 0 to 50℃ during UV irradiation (1.1 J/cm~2). Under those conditions the maximum cure rate was achieved rapidly; however, only a conversion level of less than 10% was obtained before the reaction rate slowed dramatically. This low level of cure is not due to the loss of molecular mobility that is normally associated with a diffusion controlled reaction since the glass transition temperature, T_g, of the epoxide was at least 40℃ below its reaction temperature. Raising the sample temperature above 60℃ resulted in a sharp increase in the conversion level. Irradiation of films maintained at 100℃ yielded a conversion of more than 80%. Using an UV conveyor belt system to expose 1 mm thick films to the same dose of UV irradiation as used with the thin films, the dark reaction effect was observed. After five hours of storage in the dark at ambient temperature, the T_g increased from -20 to 23℃ and the modulus rose from 0.2 to 1 GPa. Calorimetric traces taken after various ambient storage times indicated that two major processes were occurring as indicated by the presence of two exothermic maxima in the DSC dynamic curing curves. The maxima were located near 60 and 110℃, when a heating rate of 5℃/min was used. The ultimate T_g for this epoxide is ～190℃.