## 3 Results and Discussion
### Characterization of BNC/\(\kappa\)-Car Composites
#### 3.2.4 Water Holding Capacity, Water Retention Rate, and Swelling Properties
After 3 h, when BNC was completely hydrated, the degrees of swelling of BNC/\(\kappa\)-Car\({}_{2.0}\) and BNC/\(\kappa\)-Car\({}_{4.0}\) were only 55 and 53%, respectively. This difference may be ascribed to the high concentration of \(\kappa\)-Car in surface layers since the latter polymer swells quickly but binds less water. The occurrence of negative charges on its coils causes that water molecules to bind strongly and their diffusion to the interior of the membrane takes more time. BNC/\(\kappa\)-Car\({}_{2.0}\) and BNC/\(\kappa\)-Car\({}_{4.0}\) absorbed 70.52 and 95.65 \(\mathrm{S_{Water}}/\mathrm{S_{dry\;mass}}\), respectively. The difference in the swelling ratio is a consequence of different porosity and contact surface area, which are greater in the case of BNC/\(\kappa\)-Car\({}_{4.0}\) and therefore, the latter composite bound more water. The water retention rate of BNC and its composites is a vital parameter in case of medical applications [50]. Like WHC, WRR is affected by the thickness and structure of fibers as well as the presence of additives inside the membrane [33]. Interestingly, the WRR profiles of the BNC/\(\kappa\)-Car composites obtained in this study were different from the WHC profiles. In case of BNC/\(\kappa\)-Car prepared by the first method, the rate of water evaporation decreased with the increase in \(\kappa\)-Car concentration, with an exception of BNC/\(\kappa\)-Car\({}_{0.8}\) (Figure 7). The similar observations were reported by [51] who studied the effect of an increasing concentration of a single sugar on the \(\alpha\)-linked glucuronic acid-based oligosaccharides. The positive impact of carrageenan on WRR of carrageenan/hydroxymethyl cellulose composites was also reported [52]. In this study, the differences between the composites were visible after 6 h of drying process. Presumably, the molecules of water, which were physically attached to the surface of membranes, were evaporated within this  
Figure 5: Water holding capacity of BNC and BNC/\(\kappa\)-Car composites (The data are presented as the means \(\pm\) SD from at least three independent experiments; the means that are marked with different letters are significantly different). time. Noteworthy, these molecules accounted for less than 20% of bound water. The diffusion of the remaining water molecules, which were entrapped between the fibers, took either 36 h (BNC) or 48 h (BNC/\(\kappa\)-Car\({}_{0.2}\) and BNC/\(\kappa\)-Car\({}_{0.5}\)). The composite BNC/\(\kappa\)-Car\({}_{0.8}\) was characterized by the significantly lower value of WRR that may be ascribed to the high thickness of fibers and enhanced porosity, which were visible in the SEM images. Presumably, these parameters had the greater impact on WRR than the presence of \(\kappa\)-Car. Because of the lower surface area and expanded pores, water molecules that are bound either to the surface or inside the membrane may be removed much easier. WRR values of BNC/\(\kappa\)-Car\({}_{2.0}\) and BNC/\(\kappa\)-Car\({}_{4.0}\) were similar and water evaporation took up to 48 h. The effect of \(\kappa\)-Car incorporation into the surface layers of membranes was noticeable. The diffusion of water was hindered like in case of the swelling behavior of modified BNC and within 24 h, BNC/\(\kappa\)-Car\({}_{2.0}\), BNC/\(\kappa\)-Car\({}_{4.0}\) and BNC lost 55, 60 and 68% of bound water, respectively. Figure 6: Swelling ratio of BNC and BNC/\(\kappa\)-Car composites. (The data are presented as the means \(\pm\) SD from at least three independent experiments).