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.