SHALLOW REORIENTATION IN THE SURFACE DYNAMICS OF PLASMA-TREATED FLUORINATED ETHYLENE PROPYLENE POLYMER

Samples of fluorinated ethylene propylene (FEP) copolymer have been treated in plasma atmospheres that induce oxidative modification of the surface. Contact angles changed markedly within short plasma treatment times, and XPS demonstrated the incorporation of oxygen-containing chemical groups on the FEP surface. The contact angles, measured after various storage times in air to assess the long-term stability of the modified surfaces, increased markedly within a 3-week period whereas the changes in XPS were considerably less pronounced. Surface recontamination appeared unlikely on the basis of XPS analysis and the surface reversal was attributed to FEP chain dynamics, which causes rearrangement toward a lower energy surface. The surface reorientation of plasma-treated FEP samples did not proceed to completion; some of the polar groups are believed to be attached to immobile sites on the surface. The observation that the surface energy decreases while the oxygen content as measured by XPS increases on storage is proposed to be related to two factors. First, the driving force for reorientational movement of treated polymer chains in FEP acts over a depth smaller than the XPS signal depth; the mobile oxygen-containing groups thus move away from the immediate surface but stay within the XPS analysis depth. Second, oxidative reactions are believed to continue for extended periods of time following plasma treatment; these reactions, which originate from radicals, cause uptake of oxygen over entended periods of time.