Photoinduced Stress Relaxation in Crosslinked Polymers

Objectives: Crosslinked polymers are limited by their inability to eliminate residual stresses due to polymerization shrinkage, causing deleterious effects when used as dental restorative materials. Photochemically altering the topology of polymer networks offers a path to stress elimination which has otherwise proven difficult to obtain with this class of polymers. We have developed a strategy which is uniquely able to alter the topology of a network while the polymer chemistry and network connectivity remains unchanged(1). The potential of this strategy to produce glassy polymeric materials which are able to relieve shrinkage stress during and after their polymerization was investigated. Methods: Crosslinked polymers containing addition-fragmentation chain transfer (AFCT) functionalities were synthesized monomers containing addition-fragmentation functionalities incorporated in the monomer structure. The model formulation used was 75wt% of a stoichiometric ratio of pentaerythritol tetra(3-mercaptopropionate) and 2-methylene-propane-1,3-di(thioethylvinylether) and 25wt% 2-methyl-7-methylene-1,5-dithiacyclooctane. The introduction of radicals via photocleavage of residual photoinitiator in the polymer matrix reacted via AFCT through mid-chain functional groups. Stress relaxation behavior was then determined by straining cured samples to varying degrees and observing the stress evolution during irradiation. Results: The final percentage of stress relaxation during irradiation of strained samples of the model formulation was found to increase as the initial stress was raised; however, the residual stress after irradiation was found to be lower at reduced initial stress. At the highest strain examined (0.075), almost 75% of the initial stress was relieved after the first minute of irradiation; almost 85% of the initial stress was relieved after the second minute. Conclusions: The incorporation of addition-fragmentation functionalities in the backbone of crosslinked polymers were found to be capable of dramatically reducing stress introduced during irradiation. (Supported by NIH grants DE10959 and DE12998) 1. T. F. Scott, A. D. Schneider, W. D. Cook, C. N. Bowman, Science 308, 1615 (2005).