Structural analysis on photopolymerized dental resins containing nanocomponents

The incorporation of nanoscale layered silicates, as montmorillonite (MMT), into polymers has attracted great attention due to their ability to improve mechanical, thermal, and barrier properties of many polymers. Inserting these nanocomponents into dental resins could lead to materials with improved wearing resistance under masticatory attrition, among other advantages. Thus, the aim of this work was to investigate the process of incorporating layered silicates into dental resins by studying the influence of the concentration of these nanocomponents and the type of chemical species, used to modify the clays, in the morphology of the systems. Three types of clay were inserted in a BisGMA/TEGDMA photopolymerized resin system. The samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared spectroscopy (FTIR) and small angle X-ray scattering (SAXS). FTIR and TGA results confirmed the presence of the modifier agents in MMT. SEM micrographs indicated a homogeneous distribution of the layered silicates throughout the dental resin after photopolymerization and a high degree of interfacial adhesion. SEM micrographs also showed that the mixing process used to add clay into dental monomers was not successful in breaking agglomerates and was also responsible for forming new agglomerates particularly in highly concentrated materials. XRD and SAXS patterns showed the development of a partially exfoliated/intercalated clay structure after photopolymerization of the dental resin. Exfoliation, together with intercalation, was even observed in materials having concentrations of MMT as high as 16 wt.%. Exfoliation in highly filled materials was attributed to the formation of a microstructure composed of large agglomerates, in which most of the MMT layers were concentrated and restricted to exfoliate, and few small particles disseminated throughout the polymer matrix that were able to exfoliate. SAXS results also suggested that the modification of MMT with species containing polar groups was more successful in producing exfoliated structures than MMT having highly hydrophobic incorporated species. The insertion of vinyl groups into MMT galleries led to the production of dental resins containing large and compact agglomerates with intercalated layers, as suggested by determining the fractal number of the materials. The possibility of producing photopolymerized dental resins containing exfoliated nanolayers can potentially be useful in controlling important properties of dental materials such as resistance to attrition, moisture absorption, polymerization shrinkage, coefficient of thermal expansion, among others.