## 3 Results and Discussion
### Mechanical and Thermophysical Properties
It should be noted that, in System I, the elasticity modulus continues to increase at concentrations of TiO\({}_{2}\) above 3 wt. %, while the strength, on the contrary, decreases. A similar result has been published in the articles [47,71], in which the nanoparticle addition leads to an increase in the elasticity modulus while the strength decreases. In System III, due to the addition of a surfactant, the elasticity modulus is expected to decrease. A distinction between Systems I and II was also observed when considering the relative elongation in the tensile test (Figure 8c). It was shown that, for System I, a maximum elongation was observed in the 1 wt. % TiO\({}_{2}\) concentration, while, for System II, it was observed in the 4 wt. % TiO\({}_{2}\) concentration. This behavior may be related to the specific surface area of the nanoparticles and, consequently, to the different content of the specific interface area in the composites. It seems that there is a limit of the interface specific area that ensures the best mechanical properties of nanocomposites to be achieved. This regularity may be related to the difference in the volumetric filling, which was mentioned above. A further decrease in the tensile strain can be explained by the fact that, with an increase in the concentration of nanoparticles and their aggregates, the mobility of the polymer chains decreases. The result is the more brittle behavior of the material. The elongation in System III does not have a pronounced maximum, contrary to Systems I and II. The addition of TiO\({}_{2}\) and L61 increases the elongation to values close to the obtained values for Systems I and II, but when the concentration of nanoparticles increases in the studied range, the numerical value of the characteristic changes weakly. The addition of TiO\({}_{2}\) significantly increases the impact toughness of the epoxy polymer from 7 kJ/m\({}^{2}\) to 22 kJ/m\({}^{2}\) for System I and up to 34 kJ/m\({}^{2}\) for System II, as shown in Figure 8d. The critical point for System I becomes the 2 wt. % TiO\({}_{2}\), while, in System II, a sharp drop in the characteristic is observed above 4 wt. %. System II with larger nanoparticles showed the best impact toughness of the composites. The impact toughness of a material is related to the energy required to fracture a material and its numeric value the ability of the material to resist the impact. Usually, the impact properties of polymers are improved by smaller particles, because large particles can act as centers of stress concentration and, consequently, crack formation centers [21]. However, the greater activity of small particles facilitates their agglomeration and, consequently, hampers their dispersion. The agglomerated particles act as the centers of formation of large defects, which leads to a decrease in the impact toughness. Nevertheless, well-dispersed TiO\({}_{2}\) nanoparticles can block crack propagation through the polymer and, as a consequence, increase the impact toughness of the nanocomposite. The dependences of the Tg and thermal stability on the TiO\({}_{2}\) concentration were investigated. The Tg value for the cured epoxy resin was 172 \({}^{\circ}\)C and practically did not change with the increase of TiO\({}_{2}\) concentration (Table 2). The Tg value of the cross-linked polymers is closely related to the cross-linking density and mobility of the polymer chain segments. The inclusion of nanofillers usually leads to a decrease in the cross-linking degree and consequently decreases the Tg. The aggregation of nanofillers in the polymer matrix reduces the interface and produces an additional free volume that promotes the chain segments' movement. This phenomenon was observed in earlier studies on epoxy composites [41,72]. In the studied TiO\({}_{2}\)/EP nanocomposites, nanoparticles also form large aggregates; however, T\({}_{\text{g}}\) does not demonstrate a decreasing tendency.