van der Waals interaction in systems involving oxidised polystyrene surfaces

The forces between polystyrene surfaces and silicon nitride or spherical silica probes in nitrogen and water have been measured using an atomic force microscope. The polystyrene surfaces were modified using a UV–ozone treatment which increased surface oxygen levels incrementally from 0 to 26 at.%. The Hamaker constant was determined for each system of probe/media and surface as a function of surface oxygen concentration. For each system involving untreated polystyrene, the measured Hamaker constant agreed well with that calculated using the Lifshitz theory. Fundamental material properties of the UV–ozone-treated surfaces were then obtained from the measured values of the Hamaker constant. Results obtained from three different probe/media/surface systems show a 15–45% reduction of refractive index and a 2–3% reduction of the dielectric permittivity with increasing surface oxygen concentration. This method of surface characterisation allowed the determination of refractive index and static dielectric permittivity with submicron resolution.