Characterization of polydimethacrylates and their composites by dynamic mechanical analysis.

The objective was to characterize, by dynamic mechanical analysis (DMA), the polymeric materials used to restore teeth and, in particular, to assign a glass transition temperature, Tg. Generally, in DMA work, Tg is taken as the temperature at which the internal damping, i.e., tan delta, has a maximum value. However, in the present work it was shown that the classical method of assigning Tg corresponds more closely to an assignment based on changes in elastic modulus. On this basis, a bis-GMA copolymer and a proprietary dental composite, both polymerized by exposure to light, were assigned Tg values of -25 degrees C and +10 degrees C, respectively. The composite had an even lower value, Tg = -10 degrees C, after saturation with water. The value of Tg can be increased considerably by treatments which are known to increase the degree of polymerization, i.e., by heating or by gamma-irradiation. The present findings imply that composites made by photopolymerization may undergo marked changes in mechanical state (becoming glassy, leathery, and rubbery) in the oral cavity, due to changes in temperature from about 0 degree C to 50 degrees C. It is not known which of these states would give best service properties, such as wear resistance, but in any case variation of properties in service would appear to be undesirable.