Evaluation of cross-linking and degradation processes occurring at polymer surfaces upon plasma activation via size-exclusion chromatography

Abstract

Plasma activation is an attractive and widely used technique for the surface modification of polymeric substrates. Despite being extensively explored, the fundamentals of plasma activation processes remain not completely unraveled yet, mainly because of the complex physicochemical changes occurring at the polymer surface. In general, plasma treatment leads to a surface functionalization and/or polymer structure modification. The occurrence of plasma surface functionalization was elaborately studied via X-ray photoelectron spectroscopy (XPS), while the second process was in contrast investigated to a much lower extent. Therefore, this study aims at investigating the relative importance of polymer degradation and cross-linking upon standard plasma treatment conditions via size-exclusion chromatography (SEC). To do so, poly(methyl methacrylate) (PMMA) was subjected to different low and medium pressure dielectric barrier discharge (DBD) plasma treatments sustained in three different gases (Ar, He, N2), and poly(2-ethyl-2-oxazoline) (PEtOx) was treated with Ar plasma at medium pressure. Water contact angle (WCA) measurements were carried out to determine the surface chemistry equilibrium region for Ar plasma treatment at both pressures, after which WCA and XPS measurements were used to evaluate the surface chemistry modification upon the different treatments. The wettability of both PMMA and PEtOx increased for all treatments, which could be linked to the introduction of polar functionalities on the surface, as observed by XPS. By taking into consideration these chemistry changes, all SEC measurements indicated that polymer degradation was the dominant process, while some polymer coupling was observed for Ar plasma treatment at medium pressure. For PMMA treatment at low pressure, a similar degradation was observed for all gases. This was in contrast to the medium pressure treatments for which the Ar plasma induced a more significant chain scission in comparison to N2 and He. This could probably be attributed to the plasma properties, as Ar DBDs at medium pressure are known to consist of highly energetic microdischarges which are typically absent for the other treatments. Additionally, the different degradation behavior of PMMA and PEtOx highlighted the prominent influence of the initial chemical polymer structure on the plasma activation response. As such, this study further establishes the effective use of SEC in the evaluation of polymer degradation and cross-linking upon plasma activation.