## 3 Results and Discussion
### Study of the Adhesion of the Reinforcing Layer to the Hydrogel Substrate  
When obtaining multilayer composite films, the main factor that affects their operational characteristics is adhesion between layers, which, of course, will depend on the technological parameters of the process of obtaining composites. In this case, the main factors affecting surface adsorption are the composition and thickness (\(\delta\), mm) of the hydrogel membrane substrate, the composition of the modifying solution, as well as the duration of the modification stage (\(\tau_{\text{mod}}\), min), i.e., the time the hydrogel substrate is kept in the modifying solution. In the work, the adhesion strength of the reinforcing layer to the hydrogel substrate was estimated based on the study of the value of the specific amount of modifying film (\(A\), kg/m\({}^{2}\)) on the surface of the hydrogel substrate. First of all, the influence of the composition of the hydrogel substrate, in particular the content of PMC and water (solvent) in the original composition, on the specific amount of the modifying film was investigated (Figure 3). In Figure 3, the results of the study of a composite membrane with a hydrogel substrate (thickness \(\delta\) = 0.5 mm) are presented, for the synthesis of which PVP of MMVP = 360 \(\times\) 10\({}^{3}\) was used. A solution with the following characteristics was used as a modifying solution: (PA-6/PVP):FA = 7:93 wt.%; PA-6:PVP = 95:5 wt.%; MMVP = 12 \(\times\) 10\({}^{3}\); [FA] = 80 wt.%; \(\tau_{\text{mod}}\) = 5 min. It was found that with an increase in the amount of PMC in the hydrogel, the \(A\) value (with the same formulation of the modifying layer) increases, which is caused by intermolecular interactions and the resulting conformational changes that occur during saturation of the hydrogel surface with polyamide macromolecules. At the same time, this is due to the fact that the part of the PVP that did not take part in the grafting reaction is washed out during hydration [31, 48], which leads to the formation of additional pores that are filled with a modifying solution. However, an increase in the solvent content causes a decrease in the adhesion of the reinforcing layer to the hydrogel substrate and correspondingly, the reduction of \(A\). The best value of the specific amount of modifying film has hydrogel films obtained on the basis of the original composition HEMA:PVP:H\({}_{2}\)O = 48:12:40 wt.%. An increase in the amount of polyvinylpyrrolidone in the original composition negatively affects its technological properties (flowability and dosage deteriorate) and polymerization conditions (the time of PVP dissolution and air removal from the composition increases). Figure 2: The effect of PMC formulation on the physico-mechanical properties of hydrogel membranes based on pHEMA-gr-PVP copolymers. The connection of the reinforcing layer with the hydrogel substrate is ensured, mainly, due to the sorption of the modifying solution by the hydrogel pores. Therefore, it is natural that the adhesion of the reinforcing layer to the hydrogel substrate will depend on such a technological parameter of the process as the duration of the modification--the exposure time of the hydrogel substrate in the modifying solution (Figure 4). Composite membranes were studied, which consist of a hydrogel substrate based on the original composition HEMA:PVP:H\({}_{2}\)O = 48:12:40 wt.% and a reinforcing layer, for the formation of which a modifying solution with a ratio of PA-6:PVP = 95:5 wt.% was used. According to the obtained results, an increase in the contact time of the hydrogel membrane substrate with the modifying solution causes an increase in the value of the specific amount of the modifying film. In particular, the influences of the thickness of the hydrogel substrate (Figure 4a), the concentration of the modifying solution (Figure 4b), the concentration of formic acid (Figure 4c) and the molecular weight of PVP (in PMC and the modifying solution) (Figure 4d) on \(A\) and its change over time were investigated.