Extent of polymerization of dental resins by differential scanning calorimetry

The traditional infrared spectroscopic methods for assessing the degree of polymerization of dental monomers are often hampered by the difficulties of sample preparation and, in the case of composites, by interference from the filler component. These difficulties may be circumvented by the use of another technique, differential scanning calorimetry (DSC). In this preliminary investigation, DSC was used to ascertain the degree of vinyl polymerization of an experimental monomer system consisting of seven parts BIS-GMA and three parts TEGDMA (triethylene glycol dimethacrylate). Thermally-activated polymerizations of this monomer system were studied using benzoyl peroxide (BP) as the initiator. Both the heating rate and the concentration of BP affected the percent of reacted vinyl groups. For a BP concentration of 0.39% and heating rates of 10 degrees/min and 2.5 degrees/min, conversions were 73 and 38%, respectively. Chemically-activated polymerizations using BP and fast-acting amine accelerators (e.g., p-t-butyl-N,N-dimethylaniline) gave approximately the same results (e.g., 50% conversion) as those obtained with slower-acting promoters (e.g., ascorbyl palmitate). Experimental difficulties are encountered in observing an exotherm with the very reactive accelerators unless the other parameters (e.g., BP or inhibitor content) involved in the reaction are adjusted accordingly. As a method for evaluating the performance of various dental monomers, initiator systems, and inhibitors, DSC has great potential utility.