Comparative wettability study of bulk and thin film of polytetrafluoroethylene after low energy ion irradiation

Abstract

A comparative wettability study was carried out on bulk polytetrafluoroethylene (PTFE) and physical vapor deposited PTFE-like thin films using low-energy Argon ion beam irradiation. Both bulk PTFE and PTFE-like thin film surfaces were irradiated with the beam energy of 300 and 800 eV for 5 min. The angle of incidence of the ion beam was varied from 0° to 70° with respect to the surface normal. Ion beam irradiation produced the tilted sharp-edged microstructures on PTFE sheets elongated in the direction of the beam. After irradiation with 800 eV at 20°, the bulk PTFE sheet became superhydrophobic, the water contact angle increased from 105° to 157°, and again decreased to 132° after irradiation at 70° Whereas nanoripple-like patterns formed on PTFE-like thin films after ion beam irradiation. The maximum wavelength of such ripples was found to be 212 nm for an 800 eV irradiated surface at an angle of 50° Beyond this angle, a clear transition from ripple to facet-like patterns was observed. The contact angles of the irradiated thin film were found to be varying from 105° to 112° up to 30° irradiation and again decreased to 83° after irradiation at 70° PTFE thin film showed strong anisotropic wettability behavior with a variation of 12° of contact angle values perpendicular to the ion beam direction, and water droplets did not roll off from the surface like in the case of bulk PTFE. X-ray photoelectron spectroscopy revealed that the bulk PTFE had C−C, and CF2 bonding on the surface, whereas PTFE thin films formed additional CF, CF‒CFn, and CF3 bonding, which was not observed in the bulk PTFE. After irradiation, additional CF-CFn and CF3 bonding in bulk PTFE were observed. However, in the case of thin films, ion irradiation causes severe chain scission, crosslinking, and defluorination of the surface.