Friction, surface oxidation, and polar free energy for polymer surfaces by chemical force microscopy

The surfaces of polystyrene (PS) films and polystyrene/poly(methyl methacrylate) (PS/PMMA) blend films have been studied using atomic force microscopy with probe tips coated with specific chemical functional groups (chemical force microscopy). PS films have been surface oxidized using an ultraviolet/ozone technique to give substrates with total surface oxygen levels between 0 and 20 atom % measured using XPS. XPS has also been used to study the chemistry of these substrates. AFM tips carrying hydroxyl-terminated (polar) and methyl-terminated (apolar) groups have been produced and used to characterize the statistical effects of oxidation surface chemistry. Scans have been acquired in frictional force mode with the tip moving orthogonal to the long axis of the cantilever. The torsional constant of the AFM cantilever has been used to derive absolute friction values for the systems studied. Increased frictional force is observed when surfaces of increasing polarity (defined by the oxygen level measured by XPS or by the water contact angle) are probed using the hydroxyl-terminated polar tips. Absolute coefficients of friction are shown to be directly proportional to the surface oxygen level and to the water contact angle, and these can therefore be used to approximate changes in polar free energy caused by oxidation. The phase-separated domains of PS and PMMA in blend thin films can be resolved at a submicrometer level using the CFM approach with a polar tip.