Apparent conversion of adhesive resin in the hybrid layer, Part 1: Identification of an internal reference for Raman spectroscopy and the effects of water storage

Monomer conversion of adhesives in the hybrid layer is important to the quality and longevity of the dentin bond. In this study, degree of conversion and relative co‐monomer concentrations of both experimental and commercial adhesive resins were determined using Raman spectroscopy. The objectives were to identify stable Raman scattering peaks to use as internal references and to determine the effect of water storage on measured conversion and co‐monomer concentrations. The peak at 605 cm−1 did not change throughout polymerization and is associated with monomers in both adhesives. This peak was used as an internal reference for conversion and composition calculations before and after water storage. Conversion of the adhesive resins immediately after photopolymerization was ∼20% lower than that measured after 24‐h water storage. HEMA concentration (relative to bis‐GMA) in the adhesive resins immediately after photopolymerization was at least 5 wt % higher than that measured after 24‐h water storage. Elution of unreacted HEMA provides a reasonable explanation for the “supposed” change in conversion measurements, which is more appropriately described as apparent conversion. This apparent conversion will impact interpretation of physical properties and structure of the polymer, as well as increase the probability of water penetration and its reaction within the hybrid layer. © 2007 Wiley Periodicals, Inc. J Biomed Mater Res 2008