The heterogeneity in networks produced by polymerization-induced phase separation (PIPS) can be controlled by the sequence of phase formation, determining domain size and distribution and dramatically affecting properties. Objectives: sol/gel analysis is used here to follow the compositional drift during heterogeneous network formation, to ultimately tailor methacrylate materials with reduced shrinkage and stress.
Methods: BisGMA was copolymerized with either isodecyl-methacrylate (BIDMA) or butyl-methacrylate (BBuMA) at 10, 20, 30 or 40 mol% (BisGMA0). Photopolymerization kinetics was followed in real-time by near-IR in discs (1 mm thick, 15 mm in diameter), polymerized to 30%, 60% or to their limiting conversion (controlled by exposure time). The mass of each polymer specimen was recorded before and after storage in dichloromethane (with 0.1 wt% butyl-hydroxyethyltoluene) for 4 weeks. The composition of the extract (sol), from which the gel composition was calculated (BisGMAgel), was determined by 1H-NMR.
Results: Mass of leachable components decreased as the conversion increased for all materials. At 30% conversion, the homogeneous materials (BBuMA) showed mass loss ranging between 50-64%, approximately 10-15% lower than the heterogeneous materials (BIDMA, as determined by haziness in the polymer), with greater mass loss for the lowest BisGMA0. At 60% conversion, mass loss ranged between 22-35% for BBuMA and 26-39% for BIDMA. At high conversion (80-90%), all groups showed similar mass loss (5-10wt%). For BIDMA, 1H-NMR shows increased contribution of BisGMA to network formation (BisGMAgel) with higher BisGMA0 and towards the limiting conversion.
Conclusions: The greater mass loss for BIDMA materials at low conversion indicates that a BisGMA-rich phase was formed, with greater contribution of crosslinks as opposed to polymer chain growth, which would have been observed had the original ratio of the monomethacrylate been uniformly maintained. This demonstrates the early-stage onset of heterogeneous structure that produce reaction kinetics that differ based on phase composition. Support: NIH/NIDCR- R01DE014227.