Journal of Dental Research, Vol 66, 1032-1035, Copyright © 1987 by International & American Associations for Dental Research Online Journals
Characterization of polydimethacrylates and their composites by dynamic mechanical analysis
T. W. Wilson and D. T. Turner
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.
