Modification of polystyrene with a highly reactive cold oxygen plasma

Abstract

Samples of biaxially oriented polystyrene (PS) were exposed to an extremely non-equilibrium oxygen plasma. The neutral gas kinetic temperature was 70 °C, the electron temperature was about 3 eV, the plasma density was about 2 × 10 15 m − 3 , and the neutral oxygen atom density was 4 × 10 21 m − 3 . The plasma was created with an electrodeless radiofrequency discharge and the samples were kept at a floating potential. The plasma treatment time was between 1 and 50 s. The samples were characterized by high-resolution X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), microbalance, and water contact-angle (WCA) measurements. The etching rate was found to be relatively independent of the treatment time, at about 9 nm/s. The corresponding probability for oxidation with O atoms was calculated from the known flux of O atoms onto the surface, and was close to 6 × 10 − 4 . The AFM did not show any appreciable increase in the surface roughness due to the plasma treatment. Both the XPS and the WCA measurements showed a well-activated surface after just a second of plasma treatment. The rapid functionalization was explained by the huge flux of O atoms onto the sample surface. The tilting of the samples during the XPS characterization made it possible to conclude that the oxygen-containing functional groups were concentrated at the surface of the film, at a depth much smaller than that examined by XPS. The functional groups found using XPS were the standard C–O, O–C–O, C = O, O–C═O as well as –O–CO–O–. The appearance of functional groups with a high degree of oxidation, like carboxyl and carbonate, was attributed to the oxygen's interaction with the aromatic phenyl rings. The ageing results showed that the concentration of the oxygen functional groups did not change much, but the contact angle of the water drop increased moderately during the first day and remained almost unchanged after subsequent ageing.