Golub MA; International Symposium on Chemically Modified Surfaces (6th : 1995 Jun 19-21 : San Jose, CA).
Chem Modif Surf Symp Chem Modif Surf.
Ames Research Center, NASA, Moffett Field, CA 94035-1000, USA.
It was found that unless the surface of poly(tetrafluoroethylene) (PTFE) is virtually free of ubiquitous hydrocarbon contamination, anomalous changes in the XPS-derived F/C and O/C ratios, or the F/C, O/C and N/C ratios, are observed when PTFE films are exposed to an O2 or N2 radio-frequency plasma, respectively, for very short times (typically 1-5 min). Such anomalies, which have led to misinterpretations of changes in surface properties (e.g., contact angles, wettability) with time of plasma treatment, have taken the form of "spikes" (or sharp reversals in direction) in the F/C-, O/C- or N/C-exposure time plots for PTFE. In subsequent work, the use of near-pristine PTFE and Teflon PFA flims as well as the use of a monochromatized X-ray source for XPS surface characterization removed two factors which contributed to conflicting reports on the effect of exposure time on the F/C and O/C ratios for several Ar-plasma-treated fluoropolymers. Contrary to literature indications, a common pattern was found for PTFE and PFA: a nearly instantaneous, moderate drop in F/C ratio and a small rise in O/C ratio, followed by an almost steady-state surface composition (F/C approximately 1.4-1.6; O/C approximately 0.10-0.15) on prolonged exposure to an Ar plasma. The XPS C1s spectra for PTFE and PFA thus exposed for 20 min were similar and displayed a composite band of four curve-resolved peaks (approximately 285-290 eV) representing various CH, CC, CO, CN and CF functionalities in addition to a prominent peak at approximately 292.0 eV (CF2) and a minor peak at approximately 294.0 eV (CF3). The existence of "spikes" in the case of O2- or N2-plasma treatment of PTFE has been explained in terms of oxidation (or nitrogenation) and concomitant etching away of the hydrocarbon contamination so as to yield a fluoropolymer surface that is free of hydrocarbon while possessing a slightly oxidized (or nitridized), and perhaps cross-linked, fluorocarbon structure. In the case of the Ar plasma-treated fluoropolymers, a plausible mechanism for the F/C and O/C ratios to level off with exposure time to reach steady-state values involves a dynamic competition between plasma-induced degradation (C-C and C-F bond ruptures, crosslinking, CF3 formation) and etching (or regeneration) of the fluoropolymer surface.
- Biocompatible Materials
- Polyethylene Glycols
- Spectrum Analysis
- Surface Properties
- NASA Center ARC
- NASA Discipline Environmental Health
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