## 3 Results and Discussion  
### Changes in Film Wettability and Surface Free Energy--Effectiveness of Activation  
The water contact angle (WCA) is the basic parameter used to characterize plasma-activated materials, determining their wettability. Although PLA is a relatively hydrophobic polymer with a static water contact angle of about 80\({}^{\circ}\)[58], all the films tested were hydrophilic substrates even before the plasma activation process (Figure 1), as the WCA value was less than 90\({}^{\circ}\). Obviously, the smaller the WCA, the better the wettability [56], so it can be concluded based on the results obtained that NTSS film would potentially produce good quality prints even without modification, while BCP film would be more difficult and TCL film the most difficult for printing and finishing processes. Nevertheless, printing inks would also have a significant impact [37]. The type of material had no significant effect on the WCA value obtained by plasma modification of the films (Figure 1). As a result of both oxygen and argon plasma activation  
Figure 1: Contact angle of PLA film with water, diiodomethane, and ethylene glycol before and after plasma activation. for all the polylactide films tested, the WCA decreased significantly (from a value of 62.5-76.2 before activation to a value of 37.3-44.2), with the largest changes occurring for the TCL film (WCA reduction of up to about 50%) characterized by the initially largest contact angle, and the smallest for the NTSS film with the initially smallest contact angle (WCA reduction of 40%). A slightly higher efficiency of oxygen plasma compared to argon plasma was observed for all films. This may be due to the fact that the water contact angle depends primarily on the chemical structure and morphology of the polymer surface, and it was with oxygen plasma activation that the surface became more polar (Figure 2). Nevertheless, larger changes in roughness occurred with argon activation (Figure 3). Considering that the tested films are well-wettable materials, the roughness may have a noticeable effect on their water contact angle. With argon activation, physical etching occurs, rather than reactive etching as with oxygen activation, which may lead to increased surface roughness. Contact angle with diiodomethane is commonly determined alongside WCA, since knowledge of these two values makes it possible to calculate the value of surface free energy (SFE) and its polar and dispersion components [57]. There is a correlation between the components of SFE and the printability of materials, namely, the higher the polar component and the lower the dispersion component, the better the print quality [59]. In  
Figure 3: Film roughness before and after plasma activation. Figure 2: Changes in surface free energy of the film and its components due to plasma activation of the material. contrast to WCA (40-51% reduction), diiodomethane CA did not decrease as significantly (10-20% reduction). In contrast, ethylene glycol CA had values reduced by up to 70-84% as a result of plasma activation. Such a large reduction in contact angle with ethylene glycol is due to the fact that it is a polar liquid like water, for which a significant improvement in substrate wetting was also observed as a result of its plasma activation, but with a lower surface tension (48.8 mN/m). Moreover, due to its much lower vapor pressure than water, the contact angle can be monitored for 30 min without the effect of evaporation, which can be a problem in WCA measurements [60]. Changes in the contact angle with diiodomethane, despite the fact that its surface tension (50.8 mN/m) is only slightly different from that of glycolic ethylene, are much smaller, due to the fact that it is a non-polar liquid with completely different wetting properties. The type of material and SFE values of non-activated films had no significant effect on the SFE values of oxygen plasma-modified films (Figure 2). In the case of argon plasma activation for NTSS films, slightly better results were obtained.