The Glass Transition Temperature in Dental Composites

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In dentistry, two types of composites are used: lightand chemical-cured. Light-cured composites are preferred by dentists, mainly due to their longer working time before the curing process is initiated by exposure to visible light. Such materials have been indicated for various clinical procedures such as aesthetic and functional restorations of anterior and posterior teeth, preventive restorations, cementation of endodontic posts, indirect restorations with prostheses, and bonding of orthodontic appliances. The importance of the curing efficiency to the performance of light-cured composites is well established. The physical and mechanical properties of these materials are influenced by the level of monomer to polymer conversion achieved during the polymerization process (Manffezzoli et al., 1994). Light intensity and irradiation time are important factors to achieve an appropriate degree of conversion (DC) of photoactivated composites (Rueggeberg et al., 1992). DC also depends on the medium in which the material is being cured. An important physical property of the cured matrix is indicated by the glass transition temperature (Tg). The Tg of a dental composite is only of relevance if it lies within the range of intraoral temperatures (Knox et al., 2000; Moore et al., 1999). Inadequate polymerization determines a low final Tg in material. Intraoral temperatures that exceed the Tg may result in material softening and, consequently, in failure of the clinical procedure (Rueggeberg et al., 1992). Glass transitions are one of the most important phenomena in polymeric materials because they determine the physical state and final mechanical properties of the material. The Tg represents a point at which dramatic changes take place in polymer properties. Tg is the temperature at which amorphous polymers undergo a transition from a glassy to a rubbery state. At temperatures lower than Tg, only localized molecular movements are observed (secondary relaxations); for T > Tg, the flow of chains occurs. Between both (i.e., T = Tg), a rather complicated behavior known as the primary (or ) relaxation is observed. Moreover, crystallization is possible for a great number of polymers so that such materials are formed by two phases differing highly in their rheological properties. In the case of totally amorphous polymers, the occurrence of localized molecular motion below Tg in a glassy polymer ( relaxation) is considered a precursor of the large-scale molecular motion ( relaxation). In the case of amorphous polymers, the former is attributed to the availability of a set of configurational states through the faster, uncorrelated, rotational motions of the smallest unit of the chains within the randomly distributed sites of frozen-in density fluctuations (McCrum et al., 1991).