Influence of hydrogen bonding on properties of BIS-GMA analogues

The influence of chemical structure on the important properties of composite matrix resins is being systematically investigated. This study addresses the relationships between pendent side chain structures, viscosity and curing shrinkage. In particular, viscosity is known to be greatly influenced by intermolecular interactions, such as hydrogen bonding, and free volume effects. In order to establish the relative importance of these factors, analogues of BIS-GMA were synthesized in which the pendent hydroxyl groups were replaced by trimethyl siloxyl, and by dimethyl, isopropyl siloxyl groups. The viscosities were determined with a cone and plate viscometer and curing shrinkages were determined gravimetrically. They were compared to previously determined values for BIS-GMA and its methyl and hydrogen substituted analogues. The high viscosity of BIS-GMA is drastically reduced by replacement of the hydroxyl group, or its substitution by silylation. The relatively smaller effects produced by varying the bulk of the substituted side chains indicates that the main effect on viscosity is due to the presence or absence of hydrogen bonding. Conversely, increasing the bulk of the side chain substituent has less effect on viscosity, but significantly reduces the curing shrinkage. Changes in curing shrinkages were explained in terms of effects of free volumes associated with the monomers.