Objectives: Through the use of combined techniques of near-IR spectroscopy and rheometry, this study investigates the role of chain-transfer reactions given by thiol additives on gel point conversion and correlates those findings to stress reduction efforts in methacrylate polymer networks.
Methods: Ethoxylated bis-phenol A dimethacrylate (BisEMA) was used alone (control) or combined with 3-methyl mercaptopropionate (MMP), benzene thiol (BT) or pentaerythritol-tetramercaptopropionate (PETMP) at 5, 10 or 20 mol% (thiol:methacrylate). Methacrylate conversion was followed in real-time (6165 cm-1) with stress and shear modulus development, using a tensometer and a photo-rheometer, respectively. Photocuring and near-IR access were provided by a novel optical apparatus. The rheometry experiments were conducted either in ambient conditions or under nitrogen purge to assess the effects of oxygen inhibition. Data were analyzed with 2-way ANOVA/Tukey's test (alpha=5%).
Results: Conversion values ranged from 68±2% (control) to 92±3% (MMP 20%), increasing with thiol concentration. Despite that, stress values decreased at higher thiol concentrations, in most cases achieving values lower than the control (1.67±0.3 MPa). For 5, 10 and 20 mol% thiol, respectively, stress (in MPa) was: MMP: 1.82±0.2, 1.75±0.1 and 1.38±0.1; BT: 0.49±0.1, 0.72±0.2 and 0.49±0.1; PETMP: 1.58±0.2, 1.31±0.3 and 1.11±0.2. This was credited to delays in gel point conversion (13±5%) and the onset of vitrification compared to the control. For 5, 10 and 20 mol% thiol, respectively, conversion at gel point reached: MMP - 13±5, 19±3 and 27±2%; BT – 38±2, 43±2 and 58±4%; PETMP – 16±3, 23±2 and 29±7%, at practical ambient conditions. When the dissolved oxygen was eliminated from the materials, gel point was determined to be 5.2±0.6% (control) and varied from 9-13% in MMP or PETMP and by 22-26% in BT.
Conclusions: Significant stress reductions were obtained through chain-transfer reactions that allowed for delay in gelation in thiol-modified methacrylate networks. Support: NIH/NIDCR-1R01DE018233.