On the heterogeneous structure of thermally cured bis-GMA/TEGDMA resins

In this paper we report on the morphogenesis of dental resins formed by a thermally initiated radical cross-linking copolymerization of model dental resins. These systems were considered reasonable model simplifications of the actual resins used in commercial filling composites, veneers, etc. Effect of bis-GMA content varying from 0 to 100% on the degree of conversion and morphology of the cured resin was investigated. Dynamic scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), dynamic mechanical thermal analysis (DMTA) and high temperature solvent extraction experiments were performed in order to determine the nature and location of unreacted unsaturations in relation to bis-GMA concentration. The interval of cure temperatures, ranging from 23 to 250 degrees C, may seem irrelevant from the clinical point of view but is instrumental for an understanding of the processes involved. Single glass transition temperature (Tg) found in DMTA experiments showed that the copolymerization process between bis-GMA and TEGDMA is random in nature, however, the overall cure reaction is inhomogeneous and this inhomogeneity is increasing with increasing bis-GMA molar fraction. Heterogeneous character of the cure was reflected into the network morphogenesis characterized by microgellation preceded by cyclization and, following connection of microgels. These cure kinetics resulted in a heterogeneous morphology of the cured resin. Moreover, it was shown that the cure kinetics controls to a great extent the degree of conversion of the reactive vinylidene C=C bonds in a fully cured resin. A decrease in bis-GMA concentration in the resin mixture led to a higher degree of conversion, however, even a neat TEGDMA homopolymer did not exhibit C=C conversion greater than 65-70% under the cure conditions used. Leaching out tests, consisting of an extraction of unreacted monomer from a finely ground cured resin in boiling cyclohexanone at 160 degrees C over a period of 5 d, revealed total weight loss of the order of 3.2-5.6 wt % regardless of the total conversion. No uncured monomer was detected in the extract solution using FTIR spectroscopy, however, the method used is not sensitive to the presence of oligomers. It was concluded that the remaining unreacted double bonds are the inevitable result of the diffusion controlled heterogeneous cure reaction. The low rate of diffusion in the resin beyond its gel point is controlled by high monomer rigidity, low mobility and radius of gyration of the monomers. The unreacted C=C bonds exist in the form of pendant, side groups chemically attached to the network. In addition, a hypothesis has been proposed that for concentrations of rigid bis-GMA monomers above 50 wt %, preferably only one methacrylate group of its molecule has reacted on the time span of the experiment performed and has been incorporated in the cured network. Hence, a substantial part of unsaturation exists in the solid polymer in the form of pendant groups formed by mechanically ineffective bis-GMA monomers. The results are in good qualitative agreement with computer simulations based on the kinetic gelation model.