Processing and interpretation of core-electron XPS spectra o complex plasma-treated polyethylene-based surfaces using a theoretical peak model

Interpretation of X-ray photoelectron spectroscopy (XPS) spectra of complex material surfaces, such as those obtained after surface plasma treatment of polymers, is confined by the available references. The limited understanding of the chemical surface composition may impact the ability to determine suitable coupling chemistries used for surface decoration or assess surface-related properties like biocompatibility. In this work, XPS is used to investigate the chemical composition of various ultra-high-molecular-weight polyethylene (UHMWPE) surfaces. UHMWPE doped with alpha-tocopherol or functionalised by active screen plasma nitriding (ASPN) was investigated as a model system. Subsequently, a more complex combined system obtained by ASPN treatment of alpha-tocopherol doped UHMWPE was investigated. Through ab initio orbital calculations and by employing Koopmans' theorem, the core-electron binding energies (CEBEs) were evaluated for a substantial number of possible chemical functionalities positioned on PE-based model structures. The calculated Delta CEBEs showed to be in reasonable agreement with experimental reference data. The calculated Delta CEBEs were used to develop a material-specific peak model suitable for the interpretation of merged high-resolution C 1 s, N 1 s and O 1 s XPS spectra of PE-based materials. In contrast to conventional peak fitting, the presented approach allowed the distinction of functionality positioning (i.e. centred or end-chain) and evaluation of the long-range effects of the chemical functionalities on the PE carbon backbone. Altogether, a more detailed interpretation of the modified UHMWPE surfaces was achieved whilst reducing the need for manual input and personal bias introduced by the spectral analyst.

Automated summary

This study focuses on the interpretation of X-ray photoelectron spectroscopy (XPS) spectra of different ultra-high-molecular-weight polyethylene (UHMWPE) surfaces. By employing ab initio orbital calculations and Koopmans' theorem, the core-electron binding energies (CEBEs) of various possible chemical functionalities on PE-based model structures were evaluated. The calculated Delta CEBEs were used to develop a material-specific peak model for the interpretation of XPS spectra of PE-based materials.