Ageing of Oxygen-Plasma-Treated Polytetrafluoroethylene Surfaces: Revealing a Novel Link Between Morphological Evolution and Wettability

Despite the fact that oxygen plasma treatments are widely used to modify the surface properties of polytetrafluoroethylene (PTFE), the long-term stability of these surface modifications has not been fully investigated. Specifically, the roles of morphological restructuring and chemical modifications at the surface remain to be understood. In this work, we treat commercial PTFE samples using O2 plasmas at different discharge pressures to investigate their surface modifications and subsequent ageing at atmospheric pressure. We provide direct evidence that ageing behavior is governed by nanoscale and microscale restructuring of the plasma-modified interface, revealing a novel link between morphology dynamics and wettability properties. To capture this surface evolution, the modified interface was characterized using water contact angle (WCA) measurements, scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, and mass spectrometry (MS). Initial plasma treatment enhances PTFE hydrophobicity, shifting the WCA from θc≃105° to a highly hydrophobic state of θc≃135°. By monitoring the samples in contact with air over a 67-day period a gradual transition toward hydrophilicity was revealed, with WCAs stabilizing at θc≃70° after approximately 20 days. SEM observations identified time-dependent morphological degradation of plasma-induced nanostructures, while qualitative and quantitative FTIR analysis—utilizing the Specified Area Under Band (SAUB) method—confirmed corresponding shifts in carbonyl and hydrocarbon indices. These results demonstrate that ageing kinetics are a direct function of plasma pressure. The transition is further supported by a phenomenological fractal model, which confirms a morphological shift from an initial fractal surface (ds≃2.25) toward a standard flat geometry (ds=2). Furthermore, calculations indicate a sign reversal in solid-gas interface tension parameters, reflecting the changed chemical nature of the surface. We conclude that the loss of hydrophobicity is driven by a synergistic interplay between morphological relaxation and chemical restructuring.