Polyethylene (PE) film is a colorless, odorless, tasteless, translucent and non-toxic insulating material, which is widely used to make packaging bags, food bags and various containers. However, since there are no polar groups in polyethylene molecular chains, it presents poor surface wettability and is classified as a typical difficult-to-bond material. It can only achieve favorable hydrophilicity after surface treatment, which greatly restricts its application in medical, hygienic and other industrial fields.

As a clean dry treatment process, low-temperature plasma technology boasts superior environmental friendliness and cost advantages, making it a common method for material surface modification with huge market potential and development prospects. When hydrophobic polymer materials are treated with low-temperature plasma, surface etching occurs and surface roughness is improved, which is conducive to enhancing hydrophilicity. Meanwhile, polar groups are grafted onto the material surface after treatment, effectively raising surface energy and eliminating the weak boundary layer on product surfaces.

Chemical Composition and Wettability Characterization of PE Films Before and After Low-temperature Plasma Treatment

Air plasma was adopted to modify PE films to endow them with desirable hydrophilicity. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were carried out to analyze the chemical composition of modified PE films. As shown in Figure 1(a), only carbon element was detected on the surface of pristine PE films, while oxygen and nitrogen elements were additionally found on plasma-treated PE films, indicating that oxygen-containing polar functional groups were successfully grafted onto PE films after air plasma modification.

Table 1 presents the contents of carbon, nitrogen and oxygen elements of air plasma-treated PE films labeled M-1 and M-2. The oxygen and nitrogen contents of M-1 are 2.38% and 0.51% respectively, while those of M-2 reach 19.39% and 2.83%. The peak fitting analysis of C1s carbon peaks (Figure 1(b)) shows extra characteristic peaks at 285.92 eV (C-O/C-N bonds) and 288.52 eV (C=O/O-C=O bonds), further verifying the existence of oxygen-containing functional groups on plasma-modified PE films.

FTIR spectra of PE films before and after air plasma modification are displayed in Figure 1(c). The characteristic peak of pristine PE films appears at 1490 cm⁻¹, corresponding to the bending vibration of methylene groups (-CH₂-). Two new absorption peaks emerge at 3608 cm⁻¹ and 1710 cm⁻¹ on plasma-treated PE films, which are assigned to hydroxyl vibration and carbonyl (C=O) stretching vibration of carboxylic groups respectively, confirming the grafting of oxygen-containing functional groups. With the increase of plasma treatment intensity, the intensity of C=O absorption peak at 1720 cm⁻¹ rises, proving that more oxygen-containing functional groups are grafted on PE films by optimizing plasma treatment parameters. This phenomenon is attributed to the reaction between free radicals generated on PE film surfaces during plasma treatment and oxygen/nitrogen in air, thus grafting oxygen-containing and nitrogen-containing groups.Figure 1(d) illustrates the wettability of pristine PE films and PE films modified under different plasma parameters.Plasma treatment can significantly improve the surface energy and hydrophilicity of PE films. Static water contact angle was used to evaluate the surface hydrophilic variation. The water contact angle of untreated PE films is 124°. The contact angle gradually decreases after plasma treatment, among which the water contact angle of the upper surface treated at 40 W for 7 s is 85°, and that of the lower surface treated at 90 W for 180 s is only 6°. Plasma treatment removes the weak boundary layer, forms uneven rough surfaces and breaks chemical bonds to generate free radicals. The increased surface roughness and specific surface area caused by etching effect greatly facilitate surface wetting.

Figure 1 Changes in chemical composition and wettability of PE films before and after plasma treatment
Table 1 Contents of carbon, nitrogen and oxygen elements of PE films modified by air plasma with different intensities (M-0, M-1, M-2)

Structural and Morphological Characteristics of PE Films Before and After Low-temperature Plasma Treatment

Scanning electron microscopy (SEM) was used to explore the effects of different plasma treatment intensities on the morphology of pristine PE films. Figure 2(a) and 2(d) are the SEM images of PE film upper surfaces before and after air plasma treatment respectively. No obvious morphological changes are observed, because the upper surface is treated with low power and short duration, which only improves surface wettability without altering the structural morphology.

Figure 2(b) and 2(e) show the SEM images of PE film lower surfaces before and after treatment. Obvious morphological differences can be seen: the original surface structure disappears, fine branch-like structures are etched and stacked, and surface pore size expands distinctly with greatly increased surface porosity. High-energy active particles such as electrons and ions in plasma bombard the film surface, breaking C-C and C-H bonds on molecular chains. Amorphous and crystalline regions are etched at different rates. Gaseous substances decomposed from etched sputtered substances are excited in plasma and diffuse back to the film surface, resulting in simultaneous etching and repolymerization, thus forming large pores and stacked protrusions on film surfaces. The cross-sectional SEM images in Figure 2(c) and 2(f) reveal that the lower surfaces modified by intensive plasma treatment possess looser pore structures.

Figure 2 SEM morphologies of surface and cross-section of PE films before and after plasma treatment: (a, d) Upper surfaces of pristine PE film and M-2 modified PE film; (b, e) Lower surfaces of pristine PE film and M-2 modified PE film; (c, f) Cross-sections of pristine PE film and M-2 modified PE film

In this study, low-temperature plasma equipment was used for surface modification of pristine polyethylene films. XPS, FTIR and contact angle measuring instrument were applied to characterize chemical composition and surface wettability. The test results confirm that oxygen and nitrogen elements are successfully introduced into PE films via plasma treatment, and hydroxyl groups, carbonyl groups and other oxygen-containing polar functional groups are verified by FTIR analysis. Water contact angle tests prove that PE films can be converted from superhydrophobic to hydrophilic by increasing treatment power or duration on upper surfaces, while excessive treatment on lower surfaces can turn superhydrophobic surfaces into superhydrophobic ones.

SEM and pore structure analysis results show that low-intensity plasma treatment exerts negligible influence on the upper surface morphology of PE films. With the extension of plasma treatment time, micropores on lower surfaces gradually evolve into macropores, and physical etching on weak boundary layers causes surface accumulation and roughening. Pore size analysis indicates that the proportion of large pores increases with etching time, while the pore size tends to be stable with further prolonged treatment time.