A study of the evolution of mechanical properties and structural heterogeneity of polymer networks formed by photopolymerizations of multifunctional (meth)acrylates

A study of the mechanical properties and the structural heterogeneity of crosslinked polymers formed by photopolymerization of multifunctional monomers is described. By using living radical polymerizations, networks with no trapped carbon radicals have been synthesized. These crosslinked networks, which have no trapped free radicals can be heated without inducing further reaction and crosslinking. This feature makes the living radical polymerizations very useful in the characterization of structure and properties during and after the polymerization. In this work, living radical polymerizations have been used to study the mechanical properties of networks formed by home-and copolymerization of diethyleneglycol dimethacrylate (DEGDMA) and poly(ethyleneglycol 600) dimethacrylate (PEG600DMA) with n-octyl methacrylate (OcMA) using dynamic mechanical analysis. Further, an acrylate copolymer system consisting of n-heptyl acrylate (HepA) and diethylene glycol diacrylate (DEGDA) has also been examined. The glass transition temperature of the copolymers was characterized as a function of composition as well as size of crosslinking agent in these copolymers. By performing frequency scan experiments, the distribution of relaxation times of the crosslinked polymers were characterized. From such analyses, the structural heterogeneity as measured by the width of the distribution of relaxation times of the networks was characterized as a function of the comonomer composition in the copolymers. Evidence that the dependence of the glass transition temperature on the crosslinking density is not straightforward is presented. Also, the results indicate that the structural heterogeneity of the materials increases as the crosslinking density of the copolymer is increased. (C) 1998 Elsevier Science Ltd. All rights reserved.