Water Absorption and Thermal-Mechanical Properties of Model Dentin Adhesives

Water is ubiquitous in the mouths of healthy patients and thus, it is imperative that we develop restorative materials that can function adequately in the presence of water. Objective: To determine the water effect on the photo-polymerization and dynamic-mechanical properties of model dentin adhesive. Methods: Composition of model resin: HEMA/bisGMA (45/55 w/w) with CQ and DMAEMA as photoinitiators. Distilled water at 0, 5, 8, 10, 12 and 16 wt% was added into the neat resins. Photo-polymerization of the model adhesives was analyzed using confocal Raman spectroscopy. Rectangular beam specimens (1x1x15mm^3) were prepared with a visible curing light chamber (LED Curebox) and used for the determination of glass transition temperature (Tg) and thermal-mechanical properties. Polymerized samples were stored at room temperature for 5 days (dark-cure). DMA (TA Instruments, Q800) analysis with a mini three-point bending measuring system was conducted over a temperature range of 0 – 200 degC with a ramping rate of 3 degC/min. The submersion clamp is used for the analysis of water-stored samples. The mass of each specimen was tracked during the drying/wetting process. Results: There was little difference in degree of conversion (DC) of model dentin adhesives cured in the presence of water (0-16 wt%), but there were substantial differences in the water absorbance and thermal-mechanical properties. For model adhesive specimens formulated with the same concentration of water, storage moduli were significantly lower (p < 0.05) for aqueous-aged as compared to air-stored samples. Conclusion: The significantly lower mechanical properties of the aqueous aged samples reflect the plasticizing effect of water. A similar degradation mechanism may occur clinically, i.e. on prolonged exposure of the restoration to oral fluids, water initially enters the matrix by diffusion into loosely cross-linked or hydrophilic domains or water may be trapped within the matrix during photopolymerization. NIH/NIDCR R01 DE014392.