Determination of viscoelastic constants of thin films by acoustic plate modes

Viscoelastic properties of polymeric films supported by a piezoelectric plate have been studied by means of acoustic plate modes (APMs). A theoretical model is outlined, suitable for numerical calculation of velocity dispersion and attenuation of both Lamb and shear horizontal (SH) modes as a function of the complex elastic moduli of the film. Measurements have been taken on polymeric films deposited on a ST-cut quartz plate. Acoustic modes have been excited and detected by interdigital transducers (IDTs) positioned on the opposite surface of the plate. Two pairs of IDTs have been used, one parallel and one orthogonal to the X crystallographic direction of the plate, in order to generate and detect Lamb and SH modes, respectively. Measurements of phase velocity and attenuation of acoustic modes enable one to determine elastic moduli and dynamic viscosity of the films. The results obtained are shown to be consistent with those obtained by Brillouin scattering measurements taken from the same samples