Surface Chemical Characterization of Plasma-Chemically Deposited Polymer Films by Time of Flight Static Secondary Ion Mass Spectrometry (2005)

Plasma deposited polymer films from simple organic molecules with a polymerizeable C=C bond, styrene, ethylene, allylamine, and allyl alcohol, were studied by Time of Flight Static Secondary Ion Mass Spectrometry (ToF- SSIMS) before and after exposure to ambient air. ToF-SSIMS analysis of the plasma deposited polymer films was performed immediately after their preparation without any air contact, i.e., under so called in-situ conditions. The influence of the external plasma parameters, duty cycle, plasma power, monomer flow rate, and reaction pressure, on the secondary ion mass spectra of plasma deposited films was investigated. From these data, information on the chemical character of the surface of plasma deposited films was derived. In detail, the unsaturated, branched and cross-linked character of the plasma polymers was described. The problem of the retention of relevant structure units, e.g., functional groups, in plasma deposited polymers, was elucidated in detail using SSIMS data. Harder plasma conditions, which can be obtained when high duty cycles, high plasma power, and low monomer flow rates are employed, result in an increase in the unsaturated, branched and/or cross- linked character of the investigated plasma polymers. Additionally, the retention of the relevant structure groups, e.g., aromatic rings in the case of styrene and side chain groups in the case of allylamine and allyl alcohol plasma polymers, decreases when the plasma conditions become harder. Plasma polymers with high functional group retention and a minimum degree of unsaturation and branching and cross-linking were obtained at the mildest plasma conditions applied in this study. Plasma copolymerization was investigated by using allyl alcohol, or allylamine as functional group bearing monomers and styrene, or ethylene as chain extending monomers. It was found that through a careful selection of the feed gas composition the concentration of surface functional groups in plasma deposited films can be controlled. In addition, the ToF-SSIMS spectra of the plasma deposited copolymer films were found to be partially different from a superposition of the spectra of the respective plasma homopolymers. This suggests that interactions and recombination reactions are taking place between the monomer molecules in the plasma or during their respective plasma deposition process. Since the plasma polymers contain radical sites, extensive oxygen incorporation occurs when they are exposed to ambient air. It was found that the radical density of the plasma polymers and their ageing behavior is influenced by the nature of the monomer and the external plasma parameters. In the case of plasma deposited polymers prepared from olefinic and aromatic monomers, ethylene and styrene, respectively, a relation between the regularity of the plasma polymers and the amount of oxygen incorporation was found. The oxygen uptake of these plasma polymers increases when the plasma conditions become harder. On the contrary, the oxygen uptake of the plasma deposited allylamine films did not depend on the external plasma parameters. In the case of plasma deposited allyl alcohol films, it was found that their ageing on air is probably defined by two competing processes, i.e. the oxygen uptake and the diffusion of low molecular weight oxidized species into the bulk together with re-orientations of oxygen functional groups. The ageing behavior of plasma deposited copolymers films was found to be dependent on the feed gas composition.