Interaction of chlorine radicals with polyethylene and hydrocarbon thin films under vacuum conditions––a comparison with atomic oxygen reactivity

The surface reactions of atomic chlorine and oxygen with hydrocarbon-based polymers and organic thin films under vacuum conditions have been investigated with in situ X-ray photoelectron spectroscopy (XPS). The interaction of chlorine radicals (Cl(2P)) with polyethylene (PE) under vacuum conditions produces a partially chlorinated layer containing both CCl and CCl2 groups whose concentration was maximized at the surface. Compared to higher-pressure photochlorination experiments where the flux of chlorine atoms is higher, the maximum extent of PE chlorination as measured by the C:Cl XPS ratio and the evolution of the C(1s) region was reduced in the present study while the surface selectivity of the reaction was enhanced. This influence of chlorine atom flux on the extent of chlorination and surface selectivity can be rationalized by a simple stochastic model of the PE chlorination process that incorporates steric effects associated with the production of mono and dichlorinated carbon atoms as well as cross-linking reactions between carbon-containing radicals. During the reaction of PE with atomic oxygen (O(3P)), a concentration gradient of oxygen-containing carbon functionality is also observed in the near surface region. Experiments carried out on hydrocarbon thin films based on self-assembled monolayers (SAMs) reveal that chlorination proceeds without erosion. In contrast, the incorporation of new carbon containing-oxygen functionalities during reactions of hydrocarbon films with atomic oxygen occurs in competition with carbon erosion.