## 3 Results and Discussion
### Properties of Composite Two-Layer Polyamide/Hydrogel Membranes  
#### 3.2.1 Strength and Elasticity of Composite Membranes  
One of the most important operational characteristics of polymer membranes is the physico-mechanical properties, which largely determine the areas of their use, as well as the operating conditions. The strength characteristics of the obtained two-layered films were characterized by the breaking stress (\(\sigma\), MPa), elasticity--by the relative elongation at break (\(\varepsilon\), %). Test samples were used in a swollen state. It was found (Figure 4d) that increasing PVP content in the original composition and in the structure of the hydrogel substrate provides an increase in the value of the specific amount of modifying film. At the same time, an increase in the PVP content in the formulation of the copolymer of the hydrogel substrate causes a decrease in its mechanical strength and an increase in elasticity (Figure 2). To establish the optimal content of the reinforcing layer, films were obtained from a modifying solution with the composition: PA-6:PVP = 95:5 wt.% with different concentrations of PA-6:PVP in formic acid (Figure 7). It was found that the properties of films based on the mixture of polyamide with PVP change in an extreme dependence depending on the content of the PA-6 mixture and PVP in the modifying solution. The maximum values of ultimate stress and relative elongation during break are characteristic of films obtained with a concentration of the polymer mixture [PA-6/PVP] = 7 wt.%. From the given composition of the modifying solution, the influence of the PMC formulation and the duration of modification on the strength characteristics of two-layer membranes was investigated (Figure 8). The results presented in Figure 2 indicate a decrease in the strength of hydrogel films with an increase in the concentration of polyvinylpyrrolidone in the original composition. In the case of composite two-layer films, the influence of PVP content in the hydrogel substrate has an extreme character--the two-layer membranes with the formulation of PMC in the hydrogel substrate HEMA:PVP:H\({}_{2}\)O = 48:12:40 wt.% are characterized by the highest break strength (Figure 8, curve 2). The strength of composite films increases with an increase in the contact time of the membrane substrate with the PA-6/PVP solution. These changes are associated with an increase in the value of the specific amount of modifying film on the surface layer of the hydrogel substrate (Figure 4) and an increase in the thickness of the reinforcing layer of higher strength due to the formation of intermolecular physical bonds. Figure 7: The effect of the concentration of PA-6/PVP mixture in formic acid on the physico-mechanical characteristics of PA-6/PVP films (PA-6:PVP = 95:5 wt.%; MM\({}_{\text{PVP}}\) = 12 \(\times\) 10\({}^{3}\) g/mol; [FA] = 80 wt.%). The dependence of the strength characteristics of two-layer films on the MM\({}_{\rm PVP}\) and the thickness of the membrane substrate was established. As the results show (Figure 9), in the case of using PVP with MM = 360 \(\times\) 10\({}^{3}\) g/mol (sample 2) for the synthesis of a hydrogel substrate, the strength of two-layer films is higher, compared to films whose hydrogel substrate was obtained in the presence of PVP with MM = 12 \(\times\) 10\({}^{3}\) g/mol (sample 1). At the same time, the strength characteristics of composite membranes are affected by the different molecular weight of PVP, which is present in the modifying solution (samples 1 and 3). This can be explained by the fact that PVP with MM = 360 \(\times\) 10\({}^{3}\) g/mol in the hydrogel film contributes, as noted above, to the loosening of the polymer network, the free volume of which is filled with an interpolymer based on PVP with MM = 12 \(\times\) 10\({}^{3}\) g/mol with the formation of a rigid layer.