DETERMINATION OF VISCOELASTIC CONSTANTS OF THIN FILMS BY ACOUSTIC PLATE MODES

Viscoelastic properties of polymeric films supported by a piezcelectric 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, enabled us to determine elastic moduli and dynamic viscosity of the films. The resulta obtained have shown to be consistent with those relative to Brillouin Scattering measurements, taken from the same samples.