Fluorine Plasma Treatments of Poly(propylene) Films, 2-Modeling Reaction Mechanisms and Scaling

The surface properties of commodity hydrocarbon polymers such as poly(propylene) (PP) can be modified by functionalization with plasma-generated radicals and ions. For example, affixing fluorine to a hydrocarbon surface lowers surface energy and increases hydrophobicity. One such process is treatment of PP films in low-pressure, capacitively coupled plasmas (CCPs) sustained in F-2-containing gas mixtures. F atoms produced in the plasma abstract H atoms from the hydrocarbon and passivate the resulting alkyl sites producing C-F-n sites. Energetic ion and photon fluxes sputter and initiate crosslinking. In this paper, the plasma fluorination of PP in a CCP sustained in Ar/F-2 is discussed with results from a two-dimensional plasma hydrodynamics model. The surface reaction mechanism includes a hierarchy of H abstraction and F/F-2 passivation reactions, as well as crosslinking, and ion and photon-activated processes. Predictions for surface composition were compared to experiments. We found that the lack total fluorination with long plasma exposure is likely caused by crosslinking, which creates C-C bonds that might otherwise be passivated by F atoms. Increasing steric hindrances as fluorination proceeds also contribute to lower F/C ratios.