Wettability control of PET surface by plasma-induced polymer film deposition and plasma/UV oxidation in ambient air

The surface modification of poly (ethylene terephthalate) (PET) film was achieved by two sequential atmospheric pressure plasma jet (APPJ) treatments: plasma-induced polymer coating with hexamethyldisiloxane (HMDSO) and plasma oxidation with nitrogen gas. For comparison with APPJ-oxidation, oxidation by excimer ultra violet (UV) light irradiation was performed. The sessile drop and Wilhelmy contact angle measurements revealed that the PET film subjected to APPJ-coating showed superior water repellency and subsequent APPJ- or UV-oxidation made the PET film super-hydrophilic. UV-oxidation was also performed on the PET film treated by APPJ-coating, covered by a metal mesh. The difference in the contact angles between the surface areas covered with and without the metal strip mask was evident based on the Wilhelmy contact angle measurement. Both hydrophobic and hydrophilic surfaces did not exhibit contact angle hysteresis and had excellent stability of wettability. Furthermore, the hydrophilic surface treated only by APPJ- or UV-oxidation exhibited contact angle hysteresis and hydrophobic recovery with prolonged storage. Field emission scanning electron microscopy (FE-SEM) and SEM observation revealed that the morphology of the PET surface became granular after APPJ-coating. From the analyses of X-ray photoelectron spectroscopy (XPS) and grazing-angle attenuated total reflectance Fourier transform infrared (GATR-FTIR) spectroscopy, the APPJ-coated film composed mainly of inorganic SiO2 with traces of carbon derived from CH3 group, which makes the coating surface more hydrophobic. The subsequent oxidation process of APPJ or UV light leads to the decomposition of the methyl group at the surface. The antifouling properties of the PET surface against soil particulates deposited from air and their removal in aqueous detergent solution, were improved by APPJ-coating and subsequent APPJ-oxidation, respectively.