Frictional force microscopy of oxidized polystyrene surfaces measured using chemically modified probe tips

Chemical force microscopy (CFM) using probe tips that have been chemically functionalized to give hydroxylated (polar) or methylated (apolar) surfaces has been used to investigate frictional properties of polystyrene (PS) fil ms that have been oxidatively modified to varying degrees using an ultravio let-ozone (UVO) treatment. Oxygen chemisorption levels and functional group chemistry of the PS films have been followed using X-ray photoelectron spe ctroscopy (XPS), and the resulting data have been correlated with lateral ( frictional) force measurements for each surface type. XPS analysis showed t hat UVO treatments introduced polar species at the film surfaces via the fo rmation of C-O, C=O, and O-C=O functional groups. CFM was performed on the native and treated films using tips that were either hydroxyl- or methyl-te rminated. The treated films exhibit substantially higher friction than untr eated films when imaging with the hydroxyl-terminated tips. Friction is red uced when nonpolar methyl-functionalized tips are employed. The higher fric tional forces and coefficients measured for the oxidized surfaces using the hydroxyl-terminated tip are due to hydrogen bonding between the polar hydr oxyl groups of the tip and the oxygen functional groups of the PS surfaces.