Chemical characterization of plasma-activated polymeric surfaces via XPS analyses: A review

During the last few decades, non-thermal plasma surface functionalization of polymeric materials has increasingly earned a high-flying position in a wide range of application fields. Nonetheless, given the diversity of chemical reactions occurring between the surface and the multitude of active species present in plasma, a chaotic insertion of non-specific surface functional groups might befall. Therefore, achieving controlled surface chemistries can be a challenging approach demanding excessive optimization of the working parameters. In fact, correlations between the used working parameters, the generated plasma active species and the induced surface chemistry should be carefully analyzed for a deeper fundamental understanding of the plasma-surface interactions. To do so, researchers have been employing a broad range of surface analytical techniques with X-ray photoelectron spectroscopy (XPS) being the most widely used since it accurately determines the surface chemical composition at a depth approximately equaling the region depth affected by the plasma activation (a few nanometers). This review paper is therefore dedicated to provide an extensive overview on the different XPS measurement capabilities applied to chemically characterize plasma-activated polymeric surfaces. Beside the typical measurements determining the surface elemental composition, more advanced XPS analyses will be discussed such as peak fitting, XPS mapping, angle resolved XPS, derivatization reactions combined with XPS analyses and SEM-like imaging capabilities of XPS used for 3D scaffolds. Moreover, clear distinctions between post-plasma and exclusive in-plasma surface interactions are also made via a literature overview involving XPS analyses undertaken in situ. Finally, the well-known ageing effect of plasma-activated surfaces is deeply tackled through XPS measurements performed after relatively prolonged storage times. The limitations associated with some of the reported XPS analyses are also comprehensively discussed. An extended knowledge on plasma surface interactions could be as such gained. Overall, this review constitutes a perfect-picture reference for all future studies involving a plasma activation of polymers in particular and XPS analyses in general.