2.4.2. XPS Measurements

To assess the exclusive in-plasma-induced surface chemical composition of the UHMWPE films, XPS measurements were performed without any prior exposure of the treated samples to ambient air using a PHI Versaprobe II spectrometer connected to the DBD reactor. After a direct transfer of the treated samples to the XPS machine, their surface was excited over an area of 500 × 500 μm² with an X-ray beam (size: 100 μm) generated from an Al K_α X-ray source (hν = 1486.6 eV) operating at a power of 20 W. During the measurement process, the XPS main chamber was maintained at a pressure of at least 10⁻⁹ kPa. The detection of the ejected photoelectrons was executed by a hemispherical analyzer placed at an angle of 45° with respect to the normal of the UHMWPE sample surface. Survey scans and individual high-resolution spectra (C1s and N1s) were acquired with pass energies of 187.85 eV (step size = 0.8 eV) and 23.50 eV (step size = 0.1 eV), respectively. All XPS data reported in this paper represent the average calculated from six measurement points that were randomly selected on each sample for every condition. The detection and quantification of the elements present on the surfaces were conducted by analyzing the survey scans using the Multipak (Version 9.6.1) software while putting into play an interacted Shirley background and the relative sensitivity factors supplied by the manufacturer of the XPS instrument. The same software was also employed to curve fit the high-resolution C1s peaks after calibrating the energy scale with respect to the hydrocarbon component (285.0 eV). To do so, Gaussian–Lorentzian peak shapes (%Gaussian > 80%) were adopted with a full width at half maximum (FWHM) of each line shape restricted below 1.5 eV.