Modification of poly-HEMA with nonreactive POSS derivatives by in situ photopolymerization

Abstract

Photo-induced polymerization of 2-hydroxyethyl methacrylate (HEMA) in the presence of various amounts of nonreactive polyhedral oligomeric silsesquioxanes (POSS) functionalized with glycidyl, fluoroalkyl or hydroxyl groups was investigated. HEMA/POSS systems were characterized before, during and after the photocuring, with the special emphasis on the photopolymerization kinetics (measured by isothermal differential scanning calorimetry). It was found that the introduction of tested POSS derivatives into HEMA strongly affects the photopolymerization kinetics (enhancement of the gel effect, increase in the polymerization rate and conversion), mainly due to the increase in the viscosity of the initial formulation which leads to a reduction in the termination rate coefficient. However, interactions HEMA–POSS cause also a slight increase in the propagation rate coefficient. The behavior of the polymerization rate coefficients during the reaction suggests that POSS cages may mitigate the inhibitory effect of viscosity on the diffusion of macroradicals by exerting a slip effect. The materials produced are microcomposites due to the partial phase separation occurring during the curing process. Small amounts of added POSS modifiers cause plasticization of the material; at higher loads, POSS domains behave like nanofiller aggregates that increase the glass temperature. The nonreactive POSS have very little effect on thermal decomposition of the poly-HEMA matrix, which can result in a degree from the phase separation; the latter is also the main cause of the deterioration of the mechanical properties of composites compared to a pure polymer matrix.