Kinetics studies of methyl methacrylate photopolymerization initiated by titanium dioxide semiconductor nanoparticles

Kinetics of the photopolymerization of methyl methacrylate (MMA) initiated by TiO2 nanoparticles in aqueous suspensions was studied. The kinetics equation was established on basis of free radical chain mechanism. The observed dependences of polymerization rate on the variables of the concentration of TiO2 and the incident light intensity complied with the predictions from the kinetics equation. It was found that the polymerization rate as well as the number-average molecular weight of the synthetic polymers increased rapidly with increasing concentration of monomer. This phenomenon was explained by introducing a model of surface chain propagation. The effect of mass transfer was incorporated through addition of a diffusion flux term to the mass equilibrium of monomer. The initiation quantum efficiency was found to decrease with increasing incident light intensity. pH effects on the photopolymerization were investigated exclusively in this study. It was found that the rate of polymerization was pH dependent, while the configuration of polymer remains unchanged. The results from ESR spin-trapping analysis at various pH conditions suggested that trapping holes by OH− act as a competitive way towards OH radicals. With aids of these results, pH dependence of the initiation quantum efficiency was correlated to the influences of pH on the initiating species. Assuming that the photogenerated holes have higher initiation efficiency than OH radicals, the observed pH dependence of polymerization rate can be reasonably interpreted.