Comparative study between in-plasma and post-plasma chemical processes occurring at the surface of UHMWPE subjected to medium pressure Ar and N2 plasma activation

Surface chemical properties triggered by plasma activation of polymers were extensively investigated. However, clear distinctions between particular mechanisms occurring in-plasma and post-plasma treatment performed at medium pressure are up-to-the-minute missing and are therefore constituting the study aim. Polyethylene films were subjected to N-2 and Ar plasma using a dielectric barrier discharge operating at 5.0 kPa with a minimally oxygen-contaminated chamber. A distinctive combination of optical emission spectroscopy and in-situ X-ray photoelectron spectroscopy (XPS) was used to characterize in-plasma species/surface interactions. Post-plasma oxidation was unraveled by exposing plasma-treated samples to ambient air and performing ex-situ XPS. In-situ XPS showed remarkably high nitrogen (20.6%) with negligible oxygen incorporation (2.8%) thus exceptionally enhancing N-selectivity when comparing to the state-of-the-art. More oxygen (5.9%) was however detected after Ar activation due to high energy metastables dissociating O-2 impurities. Upon exposure to ambient air, post-plasma oxidation occurred producing more oxygen mainly in the form of O-C-O and N-C-O groups on N-2-activated surfaces and C-O groups on Ar-activated surfaces. Above a certain storage time, physical processes of polymeric chain reorientation prevailed over chemical processes. Overall, in- and post-plasma surface interactions were discriminated in this work that constitutes a perfect-picture reference for polymer plasma activation.