## 3 Results and Discussion
### Changes in Selected Film Properties as a Result of Plasma Modification
Figure 4: Surface topology of PLA films: (**a**) NTSS before plasma activation, (**b**) NTSS after oxygen plasma activation, (**c**) NTSS after argon plasma activation, (**d**) BCP before plasma activation, (**e**) BCP after oxygen plasma activation, (**f**) BCP after argon plasma activation, (**g**) TCL before plasma activation, (**h**) TCL after oxygen plasma activation, (**i**) TCL after argon plasma activation. Figure 5: Weight change in samples due to plasma activation. The higher tensile strength values of films activated by argon plasma compared to those modified by oxygen plasma can be explained by the changes that these gases cause in the material. Namely, if the plasma comes from an inert gas such as argon, it generally leads to crosslinking. With reactive gases such as oxygen, on the other hand, chemical changes on the surface predominate, such as the formation of peroxide, hydroperoxide, carboxyl, or hydroxyl groups [41]. A higher number of hydrophilic end groups is one of the factors accelerating degradation [58]. A previous study that also included XPS analysis [20] found that oxygen plasma activation led to the formation of a significant number of carboxyl groups, with an increase in activation time resulting in a higher proportion of hydroxyl groups. Determination of the detailed chemical changes taking place in the material is, however, beyond the scope of this paper. Color change is another parameter showing the aging of plastics. Yellowing or browning of the sample is a natural reaction of polymers to both thermal and UV aging [65]. Nevertheless, none of the films tested showed significant color changes (Figure 7). The observed differences in film lightness (L*) were within the range of measurement error. Moreover, as a result of plasma activation with both oxygen and argon plasma, the lightness did not change significantly for any of the materials tested. The color changes that were observed for all of the samples were not significant or noticeable to the naked eye, as evidenced by the very low values of the color difference \(\Delta\)E of the non-activated and plasma-activated films. The highest \(\Delta\)E values of 0.7 and 0.6 were recorded for BCP film activated with oxygen and argon plasma, respectively. For the other substrates, the \(\Delta\)E  
Figure 6: Changes in tensile strength in MD of the film due to plasma activation. Figure 7: Color changes of the film due to plasma activation (the axes a* and b* describe the green-red and blue-yellow opponent colors, respectively). value was below 0.5, with similar slightly greater color changes occurring due to oxygen activation than due to argon activation. This can be justified by the fact that the color change is the result of a radical oxidation reaction [65,66]. In addition to color, gloss is an important parameter characterizing quality. All of the tested films are materials with very high gloss, although for NTSS film, much lower values were recorded for measurements obtained with a measurement geometry of 20\({}^{\circ}\) (Figure 8). Moreover, for this material, quite large differences were observed for the measured values in the transverse and longitudinal directions, both for non-activated samples and those subjected to different types of activation. NTSS films, unlike BCP and TCL films, gained luster as a result of activation. Nevertheless, for the latter, no negative effect of activation can be concluded based on the obtained results, as the recorded changes are considered insignificant and are invisible to the observer. The mechanism of film degradation depends on a number of interrelated factors [58], whereby additives used in film production and plasticizers can affect the degradation of the material, as confirmed by the results obtained in this study. Moreover, the degradation rate of PLA depends on its crystallinity, molecular weight and its distribution, morphology, rate of water diffusion into the polymer, and stereoisomer content [67].