Collapse of a polymer brush in a poor solvent probed by noise analysis of a scanning force microscope cantilever

We have investigated the swelling of a polystyrene polymer brush in mixtures of toluene and methanol. We varied the solvent quality for polystyrene via the methanol weight fraction. As the solvent quality decreases, the blush collapses. The brush was synthesized by the "grafting from" method and has a thickness of 66 nm in the dry state. The blush's viscoelastic profile was probed by analyzing the noise spectrum of a scanning force microscope (SFM) cantilever. The noise power spectrum displays the cantilever resonance. By fitting Lorentz curves to the spectra, one obtains an effective spring constant and a friction coefficient, which are measures of the elastic and the viscous interaction between the cantilever and the sample. The effective spring constant of the cantilever increases, when it comes into contact with the brush. This allows for an assessment of the brush thickness. The thickness in the swollen state (pure toluene) is about five times larger than the thickness in the dry state.