Dynamic behavior of ultra-thin polymer films deposited on surface acoustical wave devices

This paper reports the experimental results for polymer-coated surface acou stic wave (SAW) that were exposed to various gases (carbon dioxide, methane , ethane). The polymers used to coat the SAW devices were polycarbonate (PC ; glassy), polyisobutylene (PIB; rubbery), and polydimethylsiloxane (PDMS; most rubbery). It was observed that the direction of the frequency shift of the SAW delay line oscillator for the SAW filters coated with PC and PIE c ould be described from existing work by Wohltjen [H. Wohltjen, Mechanism of operation and design considerations for surface acoustic wave device vapor sensors, Sensors and Actuators A 5 (1984) 307-325] in which the modulus te rms and the electrical terms are insignificant compared to the mass loading terms. However, for the PDMS-coated SAW this was not the case. In every ex periment performed. the frequency shift was positive, exactly opposite of w hat was predicted by the Wohltjen's equation for acoustically thin, perfect ly elastic films. It is felt that operation at high frequencies causes chan ges in the oscillation frequency due to changes in the modulus term in Wohl tjen's equation to be comparable to the change in frequency due to the mass loading (Delta hp') term. This is especially relevant if the solubility of the penetrant gases is low. (C) 1999 Elsevier Science S.A. All rights rese rved.