Polymerization Induced Phase Separation in Low Shrinkage Dental Resin

Polymerization induced phase separation (PIPS) is the process whereby an initially homogeneous solution undergoes phase separation during polymerization as a result of the progressive reduction in polarity and entropy as conversion and molecular weight of the polymerizing species increase. Objective: Recently, in order to reduce polymerization shrinkage, new resins monomers with different chemistries have been introduced. An example is N'Durance from Septodont which uses a dimer-acid chemistry to produce a dental composite with 1.2%-1.4% volume shrinkage and high levels of conversion (70-80%). Formulations containing Dimer Acid derived monomers claim to undergo PIPS to generate heterogeneous polymer networks. In this study, real time conversion and phase separation during polymerization were measured simultaneously in formulations using Dimer Acid Chemistry, Methods: Experimental resins containing ethoxylated bisphenol A dimethacrylate(Bis-EMA) , dimer dicarbamate dimethacrylate(DDCDMA) and urethane dimethacrylate(UDMA) in 60:25:15(C1) and 55:25:20(C2)wt% ratio were light activated using conventional camphorquinone/ethyl-4-dimethylamino benzoate system in a 0.4/0.8wt% ratio.  One millimeter thick specimens were cured between glass slides with an Acticure 4000(Mississauga, Ontario) curing light with no filter, at irradiances of 50, 150 and 200 mW/cm². Simultaneous real time conversion(near-IR spectroscopy, Nexus 670: Thermo Nicolet, Waltham, MA) and visible light transmission(UV-Vis spectroscopy, Ocean Optics USB 2000, Dunedin, FL) measurements were conducted throughout the polymerization process via fiber optic cables(1mx1mm diameter). An optical bench was used to warrant a stable, repeatable orientation and alignment of the specimens, curing light, lenses and fiber optic cables, Results: Table below summarizes results of representative examples

Conclusion: The photo initiated resin systems tested produced final conversions, polymerization rates and visible light transmission profiles that varied considerably according to the resin compositions and the irradiance levels used.