Film mechanical resonance phenomenon during electrochemical deposition of polyaniline

Crystal admittance measurements during potentiodynamic electrodeposition from aqueous HCl of polyaniline films on a thickness shear mode resonator show the phenomenon of film resonance. This special situation arises when the combination of materials properties (film shear modulus and density) and sample nature (film thickness) is such that the acoustic phase shift across the film, φ = π/2 radians. Admittance spectra in the acoustically thick (but pre-film resonance) region were fitted to a standard Butterworth van Dyke (BvD) electrical equivalent circuit (with series L, C, and R elements) to yield film storage and loss shear moduli. From these values, it was possible to calculate φ of the growing film and thereby track the onset of film resonance. As φ approached π/2 radians, the admittance peak rapidly shifted to higher frequencies and changed dramatically in amplitude. An electrical equivalent circuit with parallel L, C, and R elements was used to model film resonance data; this simplified model ignores the contribution of the bathing electrolyte. Agreement of film shear moduli from both models, and of resonant film thickness from the parallel element equivalent circuit model and coulometric extrapolation of pre-resonant responses, supports the basic tenets of the film resonance model. At film resonance, the cyclic voltammetric response changed dramatically; it is speculated that this is a consequence of the unusual polymer dynamics at film resonance.