FTIR and ESR Spectroscopic Studies of the Photopolymerization of Vinyl Ester Resins

The reaction kinetics during photopolymerization of bisGMA/styrene blends were investigated using FTIR and ESR spectroscopy. Increased styrene concentration reduced the polymerization rate of both methacrylate and styrene due to an increase in the termination rate and due to the stability of the styryl radical. Raised styrene concentrations also increased the final methacrylate conversion, but the final styrene conversion decreased because styrene plasticized the network, allowing methacrylate conversion to rise at higher styrene concentrations; however, the copolymerization did not proceed far enough to achieve increased conversions of styrene. The final concentration of radicals was reduced at higher styrene concentrations because of an increase in the bimolecular termination rate for networks with low cross-link densities. The proportion of styryl radicals trapped in the vitrified matrix was found to be markedly higher than the proportion predicted from the proportion of styrene monomer in the feed resin or from the copolymerization rate constants, due to the higher mobility of styrene relative to the methacrylate groups. Increased isothermal cure temperature resulted in a raised polymerization rate, increased conversion, and a decreased final concentration of radicals. This was attributed to the effects of an increase in the propagation rate and the bimolecular termination rate. The proportion of styryl radicals trapped in the vitrified matrix was found to increase with raised isothermal cure temperature because the fraction of styrene reacting increased relative to the methacrylate groups. The concentration of radicals released by photocleavage of the photoinitiator was sufficient to account for the increase in the concentration of radicals even after the sample entered the vitrification region.