Swelling of a polymer brush probed with a quartz crystal resonator

We have studied the temperature dependence of swelling of a polystyrene brush in cyclohexane with quartz resonators and ellipsometry. The swelling of the layer, which has been covalently bound to a silicon oxide surface, continuously increases as the temperature is raised. The data from the quartz resonator display a ``film resonance,'' which indicates that the thickness of the brush is on the order of the wavelength of transverse sound. The quantitative analysis shows that the acoustic thickness as determined with the quartz resonator is higher than the optical thickness from ellipsometry. We argue that this difference is partly due to the high contrast achieved in acoustical measurements, which makes the quartz resonator most sensitive to dilute regions in the tail of the segment density profile. The relation between the acoustic thickness and the hydrodynamic thickness is discussed.