Interaction of long-lived reactive species from cold atmospheric pressure plasma with polymers: Chemical modification by ozone and reactive oxygen-nitrogen species

Abstract

Atmospheric pressure plasma (APP) sources are able to generate a variety of reactive species that have different effects on materials, such as functionalization, etching, and deposition. In this article, the authors study the effect of long-lived reactive neutral species on polymers using a model plasma-surface interaction system that consists of ultrathin (∼10 nm) polystyrene (PS) films and a surface microdischarge (SMD) reactor operated with various N2/O2 working gas mixtures. The authors characterized and quantified the reactive species generated by SMD using IR and UV absorption, and they found that O3, N2O5, N2O, and HNO3 are the dominant long-lived reactants near the target surface. When exposing PS films to these reactive species, the authors observed material responses including film thickness expansion, surface and bulk oxidation, and surface organic nitrate formation. The quantity of these changes varied with the N2/O2 working gas composition. By correlating material response with gas phase species, the authors find that the chemical modification of PS strongly depends on the density of O3 in the gas phase, which is indicative of an essential role of O3 in the remote APP treatment of polymers. Authors’ results show that O3 causes polymer surface oxidation, participates in the diffusion-reaction process in the polymer bulk, and results in aromatic ring cleavage and the formation of carbonyl groups. In contrast, they did not find a correlation between surface organic nitrate and individual long-lived reactive species mentioned above. This indicates that the organic nitrate formation on polymer surfaces might result from the interaction of multiple species, including O3 and nitrogen containing reactive species. A model for the interphase mass transfer of reactive species from gas to solid was also described.