Synthesis and characterization of "non-shrinking" nanocomposites for dental applications

Abstract

The aim of this study was to design and develop novel low shrinkage nanocomposites based on SSQ (Polyhedral Silsesquioxane) for dental applications. SSQ based nanocomposites with various types of methacrylate and/or epoxide functionalities were synthesized based on inexpensive starting materials. The 8 synthesized SSQ compounds obtained in high yield were viscous liquids at room temperature and formed soluble hybrids when formulated with existing dental-based monomers in different proportions. The synthesized materials were characterized chemically using FTIR, NMR, DSC, TGA and SEC to confirm polymer structure and purity. Physico-mechanical properties such as post-gel polymerization shrinkage, indentation hardness and modulus of the synthesized materials and their formulated neat resins were then investigated and compared with unfilled 1:1 (control) Bis-GMA / TEGDMA materials (typical monomers used in dental composites). All samples investigated were cured using a dental light-curing unit at 500mW/cm2 for 40 seconds. At all time intervals, shrinkage associated with the control was found to be significantly higher than all SSQ based materials and their formulated neat resins. However, both hardness and modulus of the control were found to be significantly higher than all SSQ based materials and most of the formulated neat resins. It was observed that the addition of as little as 5 wt% SSQ nanocomposites into the control monomers significantly reduced polymerization shrinkage while maintaining useful mechanical properties. Based on the study results, four promising materials were selected and developed into experimental nanocomposites (S1 a?? S4) by reinforcing with 63 wt% of commercial fillers. The experimental nanocomposites (S1 a?? S4) were then characterized for their physico-mechanical properties such as polymerization shrinkage, hardness, modulus, depth of cure, degree of conversion and water sorption. Results obtained were compared with various commercial dental composites (Filtek Supreme [FS], Filtek Flow [FF] and Filtek A110 [A110]). At 60 minutes post-gel polymerization, shrinkage associated with the experimental nanocomposites and commercial products ranged from (0.31 A? 0.03) to (0.42 A? 0.03)% and (0.54 A? 0.03) to (0.84 A? 0.07)% respectively. At all time intervals, shrinkage associated with the experimental materials was found to be significantly lower than the commercial products with depth of cure greater than 2mm obtained for all materials. No significant difference in hardness was observed between S1, A110 and FF. Modulus associated with S1 and S4 was found to be higher if not equal to A110 and FF. The degree of conversion of S4 was also found to be higher than A110. Water sorption obtained for all experimental nanocomposites was found to be significantly lower than the commercial products and met the ISO requirement of less than 40 I?g/mm3. With the results obtained, we conclude that SSQ based nanocomposites show potential for use as dental restoratives and present a promising approach to achieve novel low/nona??shrinking nanocomposite based dental materials.