Investigation into polymerization shrinkage stress and development of novel dental resins

Although dimethacrylate-based dental composites have become widely utilized in restorative dentistry, polymerization shrinkage and shrinkage stress still remain as the foremost shortcomings of current dental composites despite that various approaches have been proposed to reduce the shrinkage stress. However, numinous conflicting observations and interpretations are presented in this field, with controversies frequently arising because polymerization shrinkage stress and functional group conversion have never been measured simultaneously. Recognizing the critical role of functional group conversion on network evolution and material properties, this research has focused on probing the origins and control of shrinkage stress development, especially its relationship with the extent of polymerization, and exploring novel dental resin systems designed to improve curing performance and reduce shrinkage stress.; To probe the interrelationships between shrinkage stress development and the polymerization progress, we developed a novel experimental technique. This technique is capable of measuring the shrinkage stress, using a novel cantilever beam based tensometer, and conversion, using a remote near infrared system, simultaneously in real-time on the same sample. The results from both the filled and unfilled resin systems showed that a majority of the shrinkage stress develops during and after the vitrification stage, with a dramatic increase linked to the asymptotic approach of conversion to its limiting value.; With this novel technique, the effects of the photocuring protocol on the stress and polymerization development have been evaluated. The results show that the magnitude of the shrinkage stress strongly depends on the final conversion and depends little on the initiation rate or curing conditions. Moreover, since a majority of the shrinkage stress develops during and after the vitrification stage, when little stress relaxation is possible, the ultimate benefit of using lower initial light intensity to relieve the shrinkage stress is very limited. In addition, the effects of filler content on the developed shrinkage stress and in situ polymerization kinetics have been investigated, which revealed that the filler has both kinetic and physical/mechanical impacts on the stress development.; Based on the enhanced understanding of stress development and control, novel dental resin formulations have been explored. The novel crosslinked thiol-ene systems have shown dramatically reduced shrinkage stress, significantly increased functional group conversion, and decreased leachable species, which are greatly beneficial for use as future dental restorative resins. In addition, several mono(meth)acrylates that contain secondary functionalities have shown very promising potential as alternative reactive diluents.