FRACTURE AND STRUCTURE OF HIGHLY CROSS-LINKED POLYMER COMPOSITES

The fracture toughness of silica filled dimethacrylate resins has been found to increase with the degree of cure of the network, and was previously attributed to increased crosslinking or the concomitant reduction in unreacted monomer (sol species). Studies of the dependence of the polymerization on the photoinitiator concentration and cure time of dimethacrylate resins illustrate the interrelation of the wt % sol and the degree of cure and reveal that a considerable amount of residual monomer remains in undercured networks, as predicted by simple gelation theory. In an attempt at separating the effects of residual monomer and crosslinking on the fracture behavior, several series of filled dimethacrylate and epoxy resins were studied in which the crosslink density and sol levels were independently varied without introducing significant changes in the chemical composition of the network. Although the fracture energy and toughness were raised by increasing the crosslink density in the epoxy resins, no significant variation was found for the dimethacrylates. Saturated analogues of dimethacrylates (used to represent residual monomer) significantly impaired the fracture resistance, suggesting that the reduction in residual monomer is responsible for the improved fracture toughness observed with postcured dimethacrylate networks.