## 4 Conclusions  
Epoxy composites based on epoxy dian resin and nanocrystalline titanium dioxide were obtained and characterized by varying its content in the polymer matrix (1-5 wt. %) and particle size (\(d_{\text{av}}\) = 46 and 100 nm). TiO\({}_{2}\) nanoparticles were prepared by the plasma-chemical method and dispersed in the native epoxy resin using an ultrasonic treatment. The TiO\({}_{2}\)/EP nanocomposites were obtained by a curing reaction according to a two-step temperature regime in the presence of an aromatic diamine DDM. According to the IR spectroscopy data, it was shown for the first time that mainly the polar (nonconjugated C-N and conjugated = C-N and = C-O-C-) groups as well as the unsaturated hydrocarbon groups of the polymer matrix are involved in the interaction with the TiO\({}_{2}\) nanoparticles, indicating the interfacial layer formation and, consequently, improvement of the mechanical properties. The difference in IR spectra show that, in the interface interactions, mainly the conjugated and ordinary C-N bonds of the matrix as well as the aromatic groups are involved. The study of the mechanical properties of the obtained epoxy nanocomposites showed that the introduction of TiO\({}_{2}\) nanoparticles into the epoxy resin simultaneously improved the elasticity modulus, tensile strength, and fracture toughness of the TiO\({}_{2}\)/EP composites, with the nature of changes and quantitative parameters depending on both the content of the dispersed phase and the size of the TiO\({}_{2}\) nanoparticles. It was noted that, in the case of System I (\(d_{\text{av}}\) = 46 nm), the tendency for the nanoparticles to aggregate during curing is higher than for System II with larger particle sizes in the initial state, which is correspondingly reflected in the mechanical properties of the obtained epoxy composites. Using a Pluronic L61 surfactant in System III improved the nanoparticle distribution and \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Particle Size, nm**} & \multicolumn{8}{c}{**TiO\({}_{2}\) wt. \%**} \\ \cline{2-9}  & **0** & **1** & **2** & **3** & **4** & **5** & **10** & **15** \\ \hline
46 (System I) & 172 & 168 & 169 & 171 & 171 & 173 & - & - \\
100 (System II) & - & - & 168 & 168 & 166 & - & - \\
21 ([PERSON], 2013) [73] & 51.3 & 61 & 60.5 & - & - & 63.5 & 60.5 & \\
48 ([PERSON], 2018) [44] & 62 & 65 & 68 & 71 & - & 75 & 80 & 63 \\
48 ([PERSON], 2018) [45] & 70 & 77 & 80 & 85 & - & 89 & 100 & 82 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Glass transition temperatures (\({}^{\circ}\)C) for TiO\({}_{2}\)/EP nanocomposites of Systems I and II. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Nanoparticle Size, nm**} & \multicolumn{8}{c}{**TiO\({}_{2}\) wt. \%**} \\ \hline
46 (System I) & 0 & 1 & 2 & 3 & 4 & 5 \\
100 (System II) & 347 & 369 & 368 & 367 & 368 & 368 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Decomposition onset temperatures for TiO\({}_{2}\)/EP nanocomposites. prevented the formation of large aggregates, resulting in an improved tensile strength but lower elasticity modulus, contrary to Systems without the surfactant. The addition of TiO\({}_{2}\) nanoparticles significantly increases the impact toughness of the epoxy polymer (3.1 times for System I and 4.9 times for System II). This is probably due to a lower degree of nanoparticles agglomeration and their more homogeneous distribution in the polymer matrix in the case of System II. Thus, for the first time, the simultaneous improvement of the strength properties and impact toughness of the composites has been shown for the TiO\({}_{2}\)/EP system. Probably a necessary condition for such behavior is the absence of a tendency for the agglomeration of nanoparticles, without using, for example, surfactants or modifying agents preventing particle agglomeration, which was demonstrated in the example of System II.