Objectives: Evaluate the application of thiol-ene chemistry to the setting mechanism in elastomeric dental impression materials by functionalizing siloxane oligomers.
Methods: Thiol and ene-functionalized siloxanes were synthesized with varying fractions of urethane, acrylate, and allyl components. The effect of each of these moieties on the kinetic and mechanical properties (rubbery modulus, toughness) of the resulting thiol-ene network was investigated by Fourier Transform Infrared Spectroscopy (FTIR) and Dynamic Mechanical Analysis (DMA). Results: Using a 66.5 mol% allylurethane/28.5 mol% diallylamino/5 mol% acrylate siloxane oligomer as a baseline for comparisons (11.9 MPa ± 1.6, 88 kJ/m2 ± 5), omitting urethane proved to increase thiol and ene conversions by removing diffusion limitations but also decreased network rubbery moduli (E') and toughness (TEB) (6.21 MPa ± 0.6, 15 kJ/m2 ± 6). Increasing the acrylate content had negligible effects on the reaction rate and the thiol/ene conversions; however, the acrylate functionality increased the rubbery modulus (14.5 MPa ± 0.2) by decreasing the molecular weight between crosslinks and reduced the toughness (36 kJ/m2 ± 12), as the urethane concentration was dropped. Halving the allyl functionality decreased the rubbery modulus (11.4 MPa ± 0.4) by reducing the crosslink density (XLD) and greatly diminishing the toughness (17 kJ/m2 ± 6).
Statistical Analysis: One-way Analysis of Variance, or ANOVA, was carried out at the 99% confidence level (α=0.01) to determine whether statistical differences in means existed in regards to %allyl conversion, Tg, E' (at 33 °C), XLD, and TEB. Discrepancies were then identified using the Fisher Least Significant Difference (LSD) method.
Conclusions: Thiol-ene functionalized siloxanes have great potential as impression materials. The flexible Si-O bond allows these materials to exist as elastomers at operational temperatures (24 °C to 37 °C), and the incorporation of urethanes confers statistically relevant increases in toughness. The authors gratefully thank Septodont for supporting this research.