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 multifunction al monomers is described. By using living radical polymerizations, net works with no trapped carbon radicals have been synthesized. These cro sslinked networks, which have no trapped free radicals can be heated w ithout 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 t his work, living radical polymerizations have been used to study the m echanical properties of networks formed by home-and copolymerization o f diethyleneglycol dimethacrylate (DEGDMA) and poly(ethyleneglycol 600 ) dimethacrylate (PEG600DMA) with n-octyl methacrylate (OcMA) using dy namic mechanical analysis. Further, an acrylate copolymer system consi sting of n-heptyl acrylate (HepA) and diethylene glycol diacrylate (DE GDA) has also been examined. The glass transition temperature of the c opolymers was characterized as a function of composition as well as si ze of crosslinking agent in these copolymers. By performing frequency scan experiments, the distribution of relaxation times of the crosslin ked polymers were characterized. From such analyses, the structural he terogeneity as measured by the width of the distribution of relaxation times of the networks was characterized as a function of the comonome r composition in the copolymers. Evidence that the dependence of the g lass transition temperature on the crosslinking density is not straigh tforward is presented. Also, the results indicate that the structural heterogeneity of the materials increases as the crosslinking density o f the copolymer is increased. (C) 1998 Elsevier Science Ltd. All right s reserved.