Viscoelastic analysis of organic thin films on quartz resonators

Abstract

By analyzing the frequency shifts and bandwidths of coated quartz resonators the viscoelastic coefficients of the coatings may be derived. The paper gives an overview of the experimental procedure and the data analysis. Some examples of the application of this technique in the range of soft condensed matter are described. The swelling and plastification of polymer films in solvent vapor was monitored in-situ. We reported on measurements of the acoustic thickness of polymer brushes, the thickness of which varies with the solvent quality. The acoustic thickness is significantly larger than the optical thickness because the dilute outer portions of the swollen layer affects the acoustic behavior of the brush much more than the optical behavior. Sandwiching polymer layers between quartz plates and rigid overlayers expands the range of the applicability of the technique at the end of low film thicknesses, because the rigid overlayer adds inertia to the composite film and therefore enhances the stress acting on the layer of interest. Finally, the combination with optical birefringence measurements in Fabry-Perot configuration gives access to the stress-birefringence inside the polymer film. Normal stress components proportional to the square of the oscillation amplitude have been measured for the first time.