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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.

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SynopsisTriethylene glycol dimethacrylate, TEGDM, was cured to various dose levels (0.15-2.0 Mrad) by y-irradiation at 4OoC and its dynamic mechanical properties were evaluated. The resultant products had up to four transition phenomena dependent upon dose. The highest temperature transition (IV) was due to the ultimate glass transition. Transition I11 (ca. 40'C) was due to additional reaction occurring just above the cure temperature. Transition I (ca. -60°C) occurred at low doses and was associated with residual monomer. Transition I1 (ca. -1OOC) is due to association of -( CHzCHzO) -units in TEGDM monomer, and the associated regions persist in the crosslinked polymer.

TGDDM/DDS epoxy and T300/NARMCO 5208 composite specimens were exposed to 0.5 MeV electrons to dose levels up to 10,000 Mrad, and the effects of radiation on the mechanical properties were characterized using dynamic mechanical analysis (DMA). In nominally cured specimens there remain unreacted epoxide groups because the epoxy system vitrifies during the cure, preventing additional reaction. DMA shows that ionizing radiation continues the reactions of epoxide groups. Also, the ultimate glass transition is shown to decrease monotonically with radiation dose. The room temperature elastic modulus of the epoxy increases slightly with radiation, but at temperatures exceeding 100°C there is a slight decrease with radiation. There is a dynamic loss phenomena associated with the composites, not seen in the epoxy, that is thought to be due to the interphase region between the fiber and the matrix.