## 3 Results and Discussion
### Pp-PEG Thin Film Characterization  
The properties of fabricated pp-PEG films were explored by using specific investigation techniques, such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and high-performance liquid chromatography (HPLC). To establish correlations with the results related to optical and electrical plasma characterization, the experimental conditions for producing pp-PEG films were similar to those used previously, i.e., atmospheric-pressure DBD working in a He/EG vapor mixture with the applied voltage covering the frequency range from 2 kHz to 20 kHz. It was found that a key factor affecting the morphology of the obtained pp-PEG films is the signal frequency that modulates the plasma discharge. In a low-frequency domain (2-6 kHz), the morphology of polymerized films presents a smooth appearance, as can be observed from the SEM image presented in Figure 7a. For this frequency domain, the thickness determined by cross-section SEM analysis shows a slow increasing trend, with values ranging from 500 nm to 700 nm (Figure 7b). As the discharge frequency increases (6-10 kHz), the morphology of the pp-PEG film changes to porous, while the thickness exhibits a sharp increase up to 27 \(\upmu\)m (Figure 7b). The inset in Figure 7b shows that a porous pp-PEG film with a thickness of 1.07 \(\upmu\)m is grown for an applied discharge frequency of 8 kHz. Additionally, it can be observed that the film shows a porous appearance only toward the surface in a range of several hundred nanometers (i.e., 368 nm), while going deeper, the polymerized film presents a compact aspect. The porous morphology of the obtained pp-PEG films could be related to a plasma-induced monomer fragmentation that becomes stronger as the discharge frequency increases (Figure 7c). The increase in the porosity of the films with increasing frequency could be due to the accumulation of defects in the assembly of monomer fragments through a mechanism of random (poly)recombination. This leads to the formation of irregular polymer chains and structures at the macroscopic scale on the surface of the polymer [43]. Figure 7: SEM-derived results of pp-PEG obtained films: (**a**) SEM image of a pp-PEG film obtained in atmospheric-pressure DBD at 2 kHz; (**b**) thickness variation of the obtained films with frequency with an inset showing the cross-section of the sample obtained at 8 kHz; and (**c**) SEM topography of the sample obtained at 8 kHz. To reveal the chemical groups present in the structure of the pp-PEG films, FTIR transmittance spectra were recorded for samples polymerized at different DBD discharge frequencies. IR spectra show the presence of various chemical groups with different vibration modes (Figure 8), such as -C-H in-plane deformation bending (873 cm\({}^{-1}\)), -C-O stretching (1046 cm\({}^{-1}\) and 1299 cm\({}^{-1}\)), -CH\({}_{2}\)- symmetrical deformation (1456 cm\({}^{-1}\) and 1419 cm\({}^{-1}\)), -C=O stretching (1714 cm\({}^{-1}\)) and -C-H bonds (2961 cm\({}^{-1}\)). The broad band ranging from 3600 cm\({}^{-1}\) to 3200 cm\({}^{-1}\) (Figure 8b) was assigned to various -OH vibration modes in the film volume [4]. All identified components are in good agreement with the starting ethylene glycol monomer whose IR spectrum was added for comparison. It can also be observed that the C=O bond appears only in plasma-polymerized films, reminiscent of the formation of -COOH groups on the surface. This is presumably due to the formation of chemical bonds between the plasma-generated fragments of the monomer and oxygen impurities during the atmospheric-pressure polymerization process. Moreover, for higher frequencies, the emerging of the band corresponding to -CH\({}_{2}\)- symmetrical deformation (1456 cm\({}^{-1}\) and 1419 cm\({}^{-1}\)) clearly shows a stronger tendency of monomer fragmentation induced by the DBD plasma (Figure 8a).