IN-SITU SPECTROELLIPSOMETRY STUDY OF THE CROSS-LINKING 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 nuc lear magnetic resonance (NMR) analysis. UV-visible real-time ellipsome try reveals the formation of an overlayer with higher refractive index , which is attributed to a densification of the polymer, and thus to c rosslinking at the surface. This assumption is confirmed by the decrea se in the extracted mass after treatment. The crosslinking mechanism i nvolves the elimination of methyl groups, and also the formation of ex omethylenic bends, as shown by NMR and IR ellipsometry. Spectroscopic ellipsometry appears as a valuable tool to study the interaction betwe en a plasma and a polymer, as UV-visible ellipsometry is sensitive to structural changes in the polymer, while IR ellipsometry can detect th e appearance and disappearance of chemical groups in a surface layer.