Atomic force microscopy and wettability study of oxidized patterns at the surface of polystyrene

The surface properties of patterned surfaces made by a combination of photo lithography and oxygen plasma treatment of polystyrene (PS) were investigat ed. PS and plasma-treated PS (PSox) were first characterized using X-ray ph otoelectron spectroscopy and the study of wetting dynamics (Wilhelmy plate method) in water and in solutions of different pH, The results indicated th at the PSox surface may be viewed as covered with a polyelectrolyte-like ge l, which swells depending on pH. It was then shown, using atomic force micr oscopy (AFM), that the adhesion force: measured on PS with a silicon tip in water was higher compared with that measured on PSox. This feature allowed imaging of the oxidation patterns using the adhesion mapping mode. The ori gin of the pulloff force contrast, which could not be explained by combinin g Johnson-Kendall-Roberts theory and thermodynamic considerations, was attr ibuted to repulsion between the tip and hydrated polymer chains present on the oxidized surface. Imaging was also performed in the lateral force mode, a higher friction being recorded on PS than on PSox. (C) 1999 Academic Pre ss.