The influence of nanosilica surface modifications with CH3- and NH2-groups on the properties of epoxy-silica composites

The influence of the chemical nature of functional groups immobilized on the surface of silica nanoparticles on the physicochemical and mechanical properties of epoxy composites has been examined. Our work was formed on the basis of the assumption about the high role of the nature of the filler surface in the formation of the mechanical properties of polyepoxide. Modification of nanosilica with amino-groups (–NH2) should contribute to improving the characteristics of the composite due to the reactive affinity for epoxy and amine fragments of the matrix. In contrast, methyl groups (–CH?) should increase compatibility with the organic phase (oleophilicity), but due to chemical inertness, reduce the ability to interact in the polymer network. It was experimentally found that the introduction of 1–4 wt. % of unmodified nanosilica leads to a decrease in compressive strength, a decrease in chemical stability in acetone and an increase in the brittleness of the composite. At the same time, the indicators of shrinkage, thermal stability and resistance to an oxidizing environment remained unchanged or improved slightly. Modification of silica nanoparticles with methyl groups (at filling of 1–2 wt. %) provided partial recovery of compressive strength, increased thermal resistance and plasticity of the composite, but did not provide a significant improvement in solution stability or reduction in shrinkage. The most pronounced positive effects were achieved when using amino-nanosilica (modified with amino groups): a significant increase in compressive strength was observed to the level or higher than that of the unfilled resin, an increase in the elastic modulus, as well as a significant improvement in resistance to aggressive environments, fire resistance and reduction in shrinkage. Microscopic studies (optical microscopy, scanning electron microscopy, atomic force microscopy) confirmed that the introduction of unmodified nanofiller causes a decrease in transparency, the appearance of agglomerates and zones of heterogeneity. On the contrary, functionalization, especially with amino groups, provides a more homogeneous distribution of particles in the polymer matrix. The results obtained confirm the key role of the physicochemical properties of the nanosilica surface in the processes of structure formation of epoxy composites. Thus, optimal modification of the filler surface with chemically active groups opens up opportunities for targeted control of the properties of composites.