| Photopolymer-based adhesive systems have been widely used in dental practice during past several decades because of their better esthetical properties, lower environmental impact, and more conservative restoration. The immediate bonding effectiveness of today's adhesive is good; however, its earlier failure or long-time durability remains a big issue. Previous studies showed that the hybrid layer (HL) could be the weak link in the bonding. Many studies have focused on the acquisition of physical and morphologic properties of HL, but chemistry-related information within HL is still lacking, especially from aged specimens. Therefore, a better chemical understanding will greatly benefit the future development of dentin adhesive systems. In this project, dentin-adhesive bond in HL was studied using Raman spectroscopy. The main objectives included: evaluation of adhesive conversion in HL, detection of unreacted monomer elution, determination of adhesive distribution within HL, and demonstration of the effect of storage media and time on the durability of adhesive in HL.; Monomer conversion in adhesive specimens was determined quantitatively and reliably using an internal reference peak in the Raman spectrum. Measurements of adhesive resin conversion within HL were found to be artificially high due to elution of unreacted HEMA from the polymer matrix during water storage, which was confirmed using gas chromatography/mass spectrometry.; A novel method based on Raman principles was developed to determine the absolute molar concentration (MC) of monomer distribution within HL. The method was validated using an analogous liquid three-component system. Results showed that less than 10% of monomers penetrated to the center of HL.; Absolute MCs for specimens stored in three different incubation media for 24 h and 24 wk were obtained and compared. Degradation of adhesive polymer network was observed in the enzymatic incubation media after 24 wk. The adhesive degradation patterns within HL obtained from the Raman study were correlated with the physical and morphological results obtained in another part of the study. The Raman results partially agreed with those from the micro-tensile tests. |
