The Influence of Cage Silsesquioxanes with Different Amounts of Glycidyl Groups on the Properties of Epoxy Nanocomposites

In order to determine which of the factors (the presence of silsesquioxane (SQ) cores or the increase in the degree of matrix cross-linking) has a greater influence on the change in the properties of the epoxy composite, four SQ derivatives were synthesized. The subsequent silsesquioxanes possessed increasing amounts of glycidyloxypropyl groups responsible for cocross-linking with the epoxy resin. SQ epoxy derivatives were used to produce epoxy composites with different modifier contents (1, 5, and 10% by weight relative to the resin). All produced resin mixtures were cross-linked using a commercial amine hardener. All cured composites showed high homogeneity and transparency. FT-IR analysis of the materials showed that in all samples, there was a complete conversion of epoxy groups, which indicates a complete reaction of the hardener with both the resin and the modifiers. For almost all composites, an increase in the water contact angle (WCA) and a decrease in the surface free energy (SFE) of the materials were observed compared with the unmodified reference. The presence of SQ derivatives in the epoxy matrix (especially in the highest concentration) significantly slowed thermal degradation (up to 55%). Modification with all SQs did not significantly affect the change in the glass transition temperatures of the composites (maximum of 16.8 °C). In addition, the presence of silsesquioxanes had a marginal effect on the changes in HRR and THR parameters; however, in higher concentrations, SQ had a clear effect on reducing HRC values (up to 33%). Composites based on SQ, in comparison to raw resin, showed practically no change in color, gloss, or surface roughness. However, they were characterized by slightly increased hardness (from 2.1 to 6.0%).