Examination of mass and modulus contributions to thickness shear mode and surface acoustic wave vapour sensor responses using partition coefficients

Abstract

New data are reported on vapour sorption by four different polymer thin films on thickness shear mode (TSM) and surface acoustic wave (SAW) devices. The polymers examined were poly(isobutylene), PIB, poly(epichlorohydrin), PECH, poly(butadiene), PBD, and poly(dimethylsiloxane), PDMS. The test vapours for the TSM sensor experiments were benzene, toluene, hexane, cyclohexane, chloroform, dichloromethane, carbon tetrachloride and butan-2-one, tested at several concentrations from P/P0=ca. 0.03 to 0.64. Measurements on the TSM devices using oscillators were complemented with measurements by impedance analysis. Acoustic wave sensor data were converted to apparent partition coefficients calculated assuming the response was due to mass-loading only. Values derived from SAW and TSM measurements were compared with one another, and with other partition coefficient data determined by non-acoustic wave techniques. It was found that TSM devices coated with PIB, PECH, and PBD (film thickness <1 µm) behaved as gravimetric sensors at low vapour concentrations. The PDMS-coated TSM sensor displayed significant losses and unusual behaviour suggestive of other effects. SAW sensors with PIB and PECH films gave responses indicating that modulus effects were contributing to the response, in addition to mass-loading effects. This enhancement appeared to be at least a factor of two, suggesting the mass-loading accounted for ca. 50% of the SAW response. The responses of PBD and PDMS-coated SAW devices were not enhanced relative to the expected mass-loading responses. Differences in behaviour among the test polymers on SAW devices are discussed in terms of the differing viscoelastic properties of these materials.