Methods to Control Composition, Kinetics and Properties in Phase-separated Resins

Objective: Methacrylate comonomers that undergo thermodynamically-induced phase separation during polymerization have been used to achieve dental polymers that display high conversion and reduced shrinkage. This study was designed to demonstrate the ability to control both the final properties of the respective phases (based on comonomer composition and conversion) and the relative rates those properties evolve. 

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.