Photopolymerization Behavior of Thiol-Acrylate Monomers in Clay Nanocomposites

This study investigates the influence of organoclays modified with polymerizable dispersants on the photopolymerization and physical properties of a binary thiol-acrylate copolymer system. Real-time infrared spectroscopy (RTIR) was used to characterize polymerization behavior, while dynamic mechanical analysis (DMA) and photorheometry were utilized to investigate mechanical and shrinkage properties. Adding nonpolymerizable organically modified clays into thiol-acrylate system leads to no significant changes in photopolymerization rate as compared to the unfilled polymer. Lower thiol conversion but equivalent acrylate conversions as the neat formulation are observed when nonreactive organoclay is added. On the other hand, for organoclays with incorporated acrylate functionalities, higher photopolymerization rates are achieved, As the concentration of acrylated organoclays increase, higher thiol conversion also occurs. The higher polymerization rates are due to reduced termination rates resulting from lower mobility radicals formed on the clay surface. Increases in Young's modulus are observed with addition of nonpolymerizable organoclays, and much higher Young's modulus is obtained with polymerizable organoclays incorporated into the polymer matrices. Polymerization-induced shrinkage is significantly reduced upon incorporating polymerizable organoclay into the photopolymer nanocomposite. An increased tendency toward step-growth polymerizations upon adding thiolated organoclays lead to reduction in shrinkage even with the higher Young's modulus achieved in these nanocomposites.