Methods: Blends of MMA/BisGMA with a photoinitiator were prepared. A near-IR transparent sleeve was bonded to the lower quartz rod of the tensometer and the volume in the sleeve above the rod was filled with the MMA/BisGMA resin. A second rod was slipped down into the sleeve to make a final resin thickness of 3mm. A fiber optic near-IR spectrometer and thermocouple were attached and used to measure the change in the intensity of the band at 6160cm-1and the reaction exotherm, respectively. The samples were irradiated through the lower rod using blue light for 2min and the development of the polymerization stress, temperature, and the degree of conversion were measured with time.
Results: Using the improved tensometer, a flowable 90/10mol% MMA/BisGMA resin reached an exotherm of almost 25oC, and degree of conversion of >95% based on the area under the NIR band at 6160cm-1. The measured stress initially dropped as the sample expanded from the exotherm. After ca. 60sec and 30% conversion, the sample stress rose dramatically as the sample gelled and contracted.
Conclusions: The tensometer provided simultaneous measurement of the stress development, exotherm, and degree of conversion in low viscosity BisGMA/MMA resins. In addition, the sensitivity of the tensometer allowed for the measurement of the thermal expansion of the curing sample and the auto acceleration effect of bulk photopolymerization. The application of the tensometer to flowable resins could help to characterize new classes of dental materials.