## 3 Results and Discussion
### EDS Analysis of the Chemical Composition of the Plasma Treated PVC Catheters  
The EDS study of the chemical composition of the pristine and plasma-treated PVC catheters indicated the growth of the concentration of oxygen in the near surface layers of plasma-treated PVC catheters, as demonstrated in Figure 8. The presence of oxygen in the pristine PVC is reasonably explained by its oxidation under high-temperature extrusion processing [47].  
The growth of the concentration of oxygen from 7 to 9 atomic percent was registered, as shown in Figure 8. It should be emphasized that this tendency was kept for all kinds of investigated plasma discharges. The concentration of oxygen in the surface layer of catheters grew with the time of treatment, as shown in Figure 8. These results coincide with the finding reported by the other research groups [48]. It was demonstrated that when PVC is treated with argon plasma, unsaturated bonds are created at the surface [49]. It was also shown that the pendant groups of PVC are removed by the argon plasma treatment [49]. This resulted in the formation of unsaturated bonds and cross-links in the modified layer and the outermost top layer of the polymer became oxidized after exposure to air due to a reaction between long-living radicals and oxygen [49]. It is reasonable to suggest that the similar mechanism also takes place under the cold air plasma treatment, thus, giving rise to the change in the wettability of PVC, discussed in detail in the previous sections.  
It was instructive to study the surface composition of the plasma-treated PVC catheters after one week of ageing under ambient conditions. It is recognized from the data presented in Figure 9 and its comparison to the data shown in Figure 8, that the concentration of oxygen in the near-surface layers of the plasma-treated catheters decreased very slightly. This observation is quite expectable as oxidation of PVC emerging from the plasma treatment is an irreversible process [48; 49]. Consider that the time of ageing was markedly larger that the characteristic time of the hydrophobic recovery, supplied in Table 1. Thus, we conclude, that the process of the hydrophobic recovery hardly correlated with the oxidation of the polymer arising from the cold plasma treatment.  
Hydrophobic recovery is a complex relaxation process including restoring of mechanical, electrical, and surface properties of plasma-treated polymers [50]. The relatively  
Figure 8: The surface concentration of oxygen (atomic percent) in the surface layer of catheters vs. the duration of plasma treatment as established immediately after the treatment with SEM/EDS spectroscopy is depicted.  
Figure 9: The surface concentration of oxygen (atomic percent) in the surface layer of catheters vs. the time of plasma treatment as established with SEM/EDS spectroscopy one week after the treatment is presented.  
\"long\" times inherent to the kinetics of the hydrophobic recovery of the plasma-treated PVC (\(\tau\cong 1.0\) day) hint to the assumption that the effect is constituted to a large extent by the bulk mobility of polymer chains [50]. The surface mobility of polymer chains is described by the markedly shorter characteristic times [50].