Method: Equimolar mixtures of BisGMA with a series of mono-methacrylates, including isostearyl (ISMA), with 0.1 wt% of various photo-initiators (2,2-dimethoxy-2-phenylacetophenone (DMPA), hydroxy-2-methylpropiophenone (HMP) or HMP modified by reaction with lauryl isocyanate (HMP-C12)) were heated until clear and then allowed to phase separate provided by the equilibrium thermodynamic partitioning of monomers and initiator at 23°C. 1,14-Tetradecanediol dimethacrylate (TDDMA), which is compatible with BisGMA and ISMA, was added to create new equilibrium phase-separated resin compositions. Comonomer compositions of the individual phases were determined by 1H NMR and photo-induced reaction kinetics measured by real-time near-infrared spectroscopy. Mechanical properties of the isolated photo-cured polymers were obtained with a universal testing machine.
Result: Equilibrium BisGMA/ISMA molar ratios within the separate phases are 87:13 and 3:97. Initiated with DMPA, the BisGMA-rich phase reached 55±3% conversion with a modulus of 1306±114 MPa. The ISMA-rich phase achieved 84±2% conversion but produced weak polymers with a modulus too low to measure. In a ternary comonomer mixture with TDDMA added to BisGMA/ISMA, TDDMA primarily partitions into the ISMA-rich phase to give a BisGMA/ISMA/TDDMA composition of 6:58:36, which provided a polymer with a modulus of 437±18 MPa at 89±2% conversion. A phase-separated comonomer formulation prepared with either DMPA, HMP or HMP-C12 showed high reaction rates in both phases, fastest polymerization in the ISMA-poor phase or enhanced rate in the ISMA-rich phase, respectively.
Conclusion: The comonomer selection as well as the initiator choice can be used to control individual phase compositions and reactivity in heterogeneous systems.
Keywords: Dental materials, Polymerization and Polymers
See more of: Dental Materials 6: Polymer-based Materials-Chemistry and Composition