Modification of the PTFE wettability by oxygen plasma treatments: influence of the operating parameters and investigation of the ageing behaviour

The effects of O2 plasma treatments on poly(tetrafluoroethylene) (PTFE) sheets are deeply investigated. The chemical modifications owing to the plasma treatment are studied by means of attenuated total reflectance Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy (XPS), while the surface topography of the plasma-treated samples is assessed by atomic force microscopy (AFM) analyses. Results are correlated with the wettability characteristics of the plasma-modified PTFE sheets, which is studied by means of contact angle (CA) and roll-off angle measurements. Finally, the ageing of the plasma-treated surfaces is investigated. The RF power input and the treatment time influence the chemical/morphological characteristics and the wettability of the plasma-treated samples. PTFE samples treated at low-power input (up to 50 W) are more hydrophilic than the untreated one. The XPS analysis reveals that OH functionalization of the outermost layers of polymeric chains decreases by increasing the RF power input, and dynamic CAs gradually increase. The AFM analysis shows a strong increase in the surface roughness as a consequence of the differential etching of the PTFE surface for samples treated at power inputs > 100W, with formation of globular structures. XPS analyses of the aged samples reveal a post-plasma oxidation. However, the dynamic CAs measured after 30 days are greater than those measured immediately after treatment, and for power input >100W the aged plasma-treated PTFE surfaces are super-hydrophobic. This result suggests a decrease in the surface density of the exposed hydroxyl groups during ageing time, as a consequence of surface adaptation.