## 3 Results and Discussion
### Properties of Composite Two-Layer Polyamide/Hydrogel Membranes
#### 3.2.2 Swelling Capacity and Osmotic Permeability of Composite Membranes  
Important operational characteristics of hydrogel materials are their swelling in solvents. Copolymers pHEMA-gr-PVP are characterized by a relatively high ability to adsorb water and other solvents and high osmotic permeability for dissolved low molecular weight substances [31]. Therefore, an important task was to investigate how the presence of a reinforced layer on the surface of hydrogel films will affect these characteristics. The swelling capacity of the obtained materials was characterized by their water content (W, %) (Figure 10). For research, composite membranes were used, 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 was used with the ratio (PA-6/PVP):FA = 7:93 wt.% and PA-6:PVP = 95:5 wt.%. The research shows that an increase in the exposure time of hydrogel films in a modifying solution (Figure 10a) leads to a decrease in the water content of composite membranes. This occurs naturally since the sorption capacity of membranes depends on the volume of their pores. In this case, with an increase in the modification time of the hydrogel substrate, the amount of polyamide in the pores of the substrate layer, which is in contact with the modifying solution, proportionally increases. Previous studies have established that together with an increase in the molecular weight of PVP, the swelling capacity of hydrogels based on pHEMA-gr-PVP copolymers increases as well [31]. This regularity is also characteristic of the obtained composite materials. As can be seen (Figure 10b), the composite films, the hydrogel substrate of which was obtained using PVP with MM\({}_{\text{PVP}}\) = 360 \(\times\) 10\({}^{3}\) g/mol, are characterized by higher water content compared to the substrates obtained in the presence of PVP with MM\({}_{\text{PVP}}\) = 12 \(\times\) 10\({}^{3}\) g/mol. The permeability of the obtained two-layer films was characterized by the results of the study of the process of osmosis of a model substance (sodium chloride) from an aqueous solution. It was found that the osmotic permeability of the composite membranes deteriorates compared to the unmodified hydrogel substrate (Figure 11a). It happens  
Figure 10: Dependence of water content (W, %) of composite membranes on modification time (**a**) and molecular weight of PVP (**b**). (**a**)—\(\tau_{\text{mod}}\), min: 1—1; 2—3; 3—5; 4—7; (**b**)—MM\({}_{\text{PVP}}\), g/mol: hydrogel substrate: 1—12 \(\times\) 10\({}^{3}\), 2, 3—360 \(\times\) 10\({}^{3}\); \(\underline{\text{mod}}\)ifying solution: 1, 2—360 \(\times\) 10\({}^{3}\), 3—12 \(\times\) 10\({}^{3}\). naturally that the decrease in osmotic permeability occurs with an increase in the content of the polyamide/PVP mixture in the modifying solution. This is a result of the fact that as the content of PA-6/PVP in the modifying solution increases, as mentioned in Section 3.1, a denser PA-6/PVP layer is formed on the hydrogel substrate. Naturally, with an increase in the content of polyvinylpyrrolidone in the composition of the hydrogel substrate, the osmotic salt permeability increases (Figure 11b). As in the case of the effect on adhesion strength and physico-mechanical characteristics, the osmotic permeability of composite membranes is also affected by the molecular weight of PVP (Figure 12). Figure 11: Dependence of osmotic salt permeability, G/S (**a**), salt permeability \(\alpha_{NaCl}\), and water permeability \(K_{H2O}\) coefficients (**b**) of composite membranes on the concentrations of PA-6/PVP in the composition of the modifying solution. Hydrogel substrate: HEMA:PVP:H\({}_{2}\)O = 48:12:40 wt.%; MM\({}_{\text{PVP}}\) = 360 \(\times\) 10\({}^{3}\) g/mol; modifying solution: PA-6:PVP = 95:5 wt.%; [FA] = 80 wt.%; MM\({}_{\text{PVP}}\) = 12 \(\times\) 10\({}^{3}\) g/mol; \(\tau_{\text{mod}}\) = 5 min.