## 3 Results and Discussion
### Study of the Adhesion of the Reinforcing Layer to the Hydrogel Substrate
As can be seen (Figure 4a), with an increase in the thickness of the hydrogel layer, the value of the specific amount of modifying film slightly increases. The value of the specific amount of modifying film increases with an increase in the content of the PA-6/PVP mixture in the modifying solution (Figure 4b) and the concentration of HCOOH (Figure 4c). During the formation of the reinforcing layer on the hydrogel film, two processes take place--the deposition of the polyamide/PVP polymer mixture with water (the hydrogel substrate is in a swollen state) and the formation of physical intermolecular bonds between the polyamide and PVP of the proposed mixture and the functional groups of the pHEMA-gr-PVP copolymer of membrane substrate. It is obvious that increasing the content of the PA-6/PVP mixture in the modifying solution will contribute to the intensification of these processes. An increase in the concentration of HCOOH in the modifying solution may indicate the fact of a strong interaction of the acid, as a solvent, with polyamide, as a result of which the part of intermolecular interaction of PA-6 with PVP decreases, which affects the conformation of polyamide macromolecules. A decrease in the interaction of PA-6 with PVP causes the straightening of PA macromolecules and the strengthening of their interaction with the functional groups of the pHEMA-gr-PVP copolymer. In this case, water acts as a diluent of the solution and does not act as a co-solvent, weakening the acid solvation of polyamide macromolecules. Figure 3: The effect of the HEMA:PVP:H\({}_{2}\)O ratio in the hydrogel substrate on the value of the specific amount of the modifying film (\(A\), kg/m\({}^{2}\)). A significant influence of the molecular weight of PVP on the value of the specific amount of modifying film was established. In particular, PVP of different molecular weights was used, both for obtaining a hydrogel substrate and in a modifying solution. The use in the original polymer-monomer composition of PVP with MM = 360 \(\times\) 10\({}^{3}\) contributes to the growth of \(A\) (Figure 4d, curve 3). In this case, the macrochains of the high-molecular-weight PVP loosen the polymer network (due to the grafting of polyHEMA to PVP and the formation of a polymer network with a lower cross-linking density), into the free space of which the formate solution of the PA-6/PVP polymer mixture diffuses. When using PVP with a lower molecular weight (12 \(\times\) 10\({}^{3}\) g/mol) in a polymer-monomer composition, the value of the specific amount of modifying film is significantly lower (Figure 4d, curve 1). It is possible that the modifying solution penetrates into the surface layer of the hydrogel substrate in a smaller amount, forming a film on its surface. As the study results show, the adhesion of the layer based on PA-6/PVP is higher in composite membranes, for the reinforcement of which modifying solutions of PVP of lower molecular weight were used. At the same time, it can be noted that the modifying solution containing PVP with MM = 12 \(\times\) 10\({}^{3}\) g/mol diffuses into the hydrogel at a higher rate; however, the formation of a dense film based on PA-6/PVP reduces \(A\) (Figure 4d, curve 2). Figure 4: Change in the value of the specific amount of modifying film over time (\(A\), kg/m\({}^{2}\)) depending on the thickness of the hydrogel substrate (**a**), the concentration of the modifying solution (**b**), the concentration of formic acid (**c**) and the molecular weight of PVP (**d**); (**a**)—\(\delta\), mm: 1—0.35, 2—0.5; (**b**)—[PA-6/PVP]:FA, wt.%: 1—7.93, 2—10:90; (**c**)—[FA], wt.%: 1—75, 2—80; (**d**)—MMVP, g/mol: Hydrogel substrate: 1, 2—12 \(\times\) 10\({}^{3}\), 3—360 \(\times\) 10\({}^{3}\); modifying solution: 1—360 \(\times\) 10\({}^{3}\), 2, 3—12 \(\times\) 10\({}^{3}\).