In situ spectroellipsometry study of the crosslinking of polypropylene by an argon plasma

The exposure of polypropylene to an argon plasma is studied by in situ spectroscopic ellipsometry in the UV-visible and in the IR range. The results are correlated with amorphous phase extraction and proton nuclear magnetic resonance (NMR) analysis. UV-visible real-time ellipsometry reveals the formation of an overlayer with higher refractive index, which is attributed to a densification of the polymer, and thus to crosslinking at the surface. This assumption is confirmed by the decrease in the extracted mass after treatment. The crosslinking mechanism involves the elimination of methyl groups, and also the formation of exomethylenic bonds, as shown by NMR and IR ellipsometry. Spectroscopic ellipsometry appears as a valuable tool to study the interaction between a plasma and a polymer, as UV-visible ellipsometry is sensitive to structural changes in the polymer, while IR ellipsometry can detect the appearance and disappearance of chemical groups in a surface layer.