Polymer film characterization using quartz resonators

The dynamic behavior and the electrical response of a thickness-shear mode resonator with a polymer film of a few microns thickness coating one face are considered. With glassy polymers, this film moves synchronously with the oscillating resonator surface; with rubbery polymers, the upper portions of the film lag behind the driven resonator-film interface, causing shear deformation of the film. Interesting dynamic film behavior results, notably a film resonance when the acoustic phase shift across the film is an odd multiple of π/2. This dynamic behavior influences the electrical response of the resonator due to the coupling between shear displacement and the electric field in the piezoelectric quartz. An equivalent circuit model is derived that relates the near-resonant electrical characteristics of a polymer-coated resonator to the film properties. Measurements made on a polyisobutylene-coated resonator were interpreted with the model to obtain the film´s sheer storage and loss moduli, giving good agreement with previously reported values. In addition, changes in resonant frequency and damping observed at film resonance agree with model predictions