Concurrent restructuring and oxidation of the surface of n-hexane plasma polymers during aging in air

Abstract

Angle-dependent XPS and air/water contact angle (CA) measurements were performed on specimens of n-hexane plasma polymers at various times after fabrication in order to monitor the aging of the surfaces in contact with air. XPS revealed incorporation of oxygen over extended periods of time. The depth distribution of O changed in the course of aging. CAs decreased over the first 3 weeks, then increased again, and finally stabilized at 5 weeks. These results were interpreted in terms of two concurrent processes: spontaneous oxidation (initiated by trapped radicals), which increased the surface polarity, and surface restructuring, which caused the partial removal of polar groups from the interface with air. The former process made a larger contribution to the overall aging of the surfaces but the latter process also contributed measurably. Over the first 3 weeks, oxidation was rapid and surface restructuring was not competitive. Subsequently, the oxygen uptake slowed down markedly, and the outermost surface layers became oxygen depleted relative to the deeper layers, due to partial surface reorientation. The surface topography, assessed by STM, did not change on aging. The aging of n-hexane plasma polymer surfaces thus is due to superimposed effects arising from post-deposition oxidative processes and partial surface reorientation.