Surface Tension and Surface Roughness of Supported Polystyrene Films

Surface-diffuse X-ray scattering has been measured from a series of polystyrene (PS) films supported on oxide-terminated Si substrates. Films of thicknesses ranging from 84 to 359 nm were investigated over a temperature range from 120 to 180 degreesC, all above the bulk PS glass transition temperature. The surface tension was extracted from the absolute intensity of diffuse scattering. The values show a slight excess of the surface tension (similar to20%) over the bulk material. Capillary wave theory is used to describe the observed diffuse scattering down to in-plane length scales of 2 nm, the limit of our resolution. The total surface roughness exceeds the value that would be expected from capillary wave theory, indicating the presence of excess roughness at length scales less than 2 nm.