Study of UV Cure Kinetics Resulting from a Changing Concentration of Mobile and Trapped Radicals

Abstract

UV cure kinetics of the system bisphenol A dimethacrylate and a phenylphosphine oxide initiator are studied as a function of incident intensity (I) and initiator concentration (PI). The rate of polymerization, d(ln[M(t)])/dt, is found to be proportional to the intensity and initiator concentration raised to an exponential power of approximately 0.7 rather than the classical value of 0.5. The system does not obey classical steady-state kinetics but rather the rate of the reaction reaches a maximum shortly after gel and then decreases rapidly well before the reaction is quenched during the glass transition. These nonclassical results are proposed to be due to the coexistence of a varying ratio of the traditional bimolecular kinetics and a unimolecular trapped radical termination process due to the changing spatial/ dynamic heterogeneity arising from microgel formation in these systems. A model which focuses on the changing concentration of mobile active radicals is proposed. It uses four fitting parameters to describe changes in the total number of radicals minus the number of trapped immobile radicals. The model assumes a constant propagation rate constant, kp, as is generally assumed in traditional crosslinking reactions up to the reaction quench in the final stage of the cure. The model is similar in mathematical form to a more complex multiparameter free volume molecular approach in which both kp, kt, and a trapping rate constant change with reaction advancement. Our model is based on the changing mobile radical concentration, which reflects the changes in the spatial heterogeneity of these systems due to the formation of microgels at the beginning of the reaction. It predicts the initial rapid buildup of the total radical concentration to a constant value and then the existence of an increasing proportion of trapped radicals up to reaction quenching in the glass transition. It accurately describes the UV cure kinetics at varying incident intensities and varying initiator concentrations and its predictions of the changes in the total and trapped radical concentrations are similar to ESR measurements on other systems.