## 3 Results and Discussion
### Mechanical and Thermophysical Properties  
The dependence of the tensile mechanical properties of the TiO\({}_{2}\)/EP nanocomposite on the concentration and size of the TiO\({}_{2}\) particles is shown in Figure 8a-c. It is evident that the inclusion of a small amount of nanoparticles (1 wt. %) in the studied Systems leads to an improvement in all mechanical properties. The transfer of the mechanical loading from the matrix to the harder nanoscale filler is provided by interfacial interactions leading to the improvement of the mechanical properties [67]. The results obtained confirm the formation of sufficiently strong interfacial interactions, because their absence would lead to a sharp drop in tensile strength even at low concentrations of TiO\({}_{2}\), as shown in the work by [PERSON] et al. [47]. Figure 7: IR spectrum of cured ED-20 (1); difference spectrum obtained as a result of subtraction of the spectrum of the TiO\({}_{2}\)/EP nanocomposite (3 wt. % TiO\({}_{2}\)\(d_{\text{av}}\) = 46 nm) from the spectrum of the cured ED-20 (2). Nevertheless, Figure 8a shows that the limiting value of the tensile strength for System I is reached in the range of 1-3 wt. % TiO\({}_{2}\), while, for System II, the tensile strength continues to increase even at a concentration of 4 wt. % TiO\({}_{2}\). This material behavior may be caused by the aggregation of nanoparticles, as noted earlier. It is known that larger Van der Waals forces act between smaller particles. The result is the reduced ability of the nanoparticles to disperse and the formation of aggregates. Smaller particles aggregate actively at lower concentrations than larger ones [39]. This fact explains the decrease in tensile strength at TiO\({}_{2}\) concentrations exceeding 1-3 wt. % in System I and 5 wt. % in System II. It is logical that, at the maximum nanoparticle concentration, their interaction with each other prevails over their interaction with epoxy matrix, thus forming clusters of several hundred nanometers in size. It is important to note that the original nanoparticles have a close true density (4.2495 g/cm\({}^{3}\) for \(d_{\text{av}}\) = 46 nm and 4.2700 g/cm\({}^{3}\) for \(d_{\text{av}}\) = 100 nm) contrary to their bulk density (0.187 g/cm\({}^{3}\) for \(d_{\text{av}}\) = 46 nm and 0.457 g/cm\({}^{3}\) for \(d_{\text{av}}\) = 100 nm). Therefore, the less bulk density, the more volumetric filling with the same weight filling. A larger volume filling with smaller particles leads to a decrease in the distance between the particles, which also contributes to their aggregation. These aggregates act as defects of the nanocomposite materials leading to the loss of high mechanical characteristics. The  
Figure 8: Mechanical properties: tensile strength (**a**), elasticity modulus (**b**), and elongation under maximum tension (**c**); and Charpy impact strength (**d**). Symbols: --TiO\({}_{2}\)/EP of System I (\(d_{\text{av}}\) = 46 nm), II--TiO\({}_{2}\)/EP of System II (\(d_{\text{av}}\) = 100 nm), and III--TiO\({}_{2}\)/EP of System III (\(d_{\text{av}}\) = 46 nm + 1&61). addition of surfactants in System III improves the dispersion of nanoparticles, having a direct impact on the physical properties, including the tensile strength. In System III, the ultimate tensile strength is reached at 1 wt. % TiO\({}_{2}\) content and is the maximum among the considered Systems. With a further increase in the TiO\({}_{2}\) content, the tensile strength decreases to 68.5 MPa (5 wt. %). The necessity of a high degree of dispersion of nanoparticles and strong interfacial interactions in the synthesis of nanocomposites is demonstrated in the review by [PERSON] and [PERSON] [68]. As expected, the elasticity modulus of Systems I and II increases with the TiO\({}_{2}\) concentration (Figure 8b). For System II, the increase in the elasticity modulus was 10-15% higher than for System I. Thus, the increase in the elasticity modulus additionally confirms the formation of sufficiently strong interactions between the nanoparticles and the matrix [69,70].