The absorption and desorption of chlorobenzene, odichlorobenzene, and chloroform in poly[n-butyl methacrylate] (PBMA) was studied in polymer-coated 104 MHz surface acoustic wave (SAW) sensors, and in freestanding polymer films by thermogravimetric analysis (TGA). The sorption processes were analyzed by a Fickian simulation and best-fit partition, and diffusion coefficients were derived. Good correlations were found between simulated and observed data. Partition coefficients derived from SAW response were independent of coating thickness and were found to be about two to three times bigger than those derived from the gravimetric response. In contrast, the diffusion coefficients increased linearly with coating thickness in the range 70-560 kHz. For the thickest polymer coating, SAW-derived diffusion coefficients were comparable with TGA-related diffusion coefficients. This study reconfirms the finding for other polymers that the response of SAW chemosensors is higher than that anticipated from a mass change only. The viscoelastic effect is again more pronounced than the gravimetric effect. Moreover, the similarity of diffusion coefficients obtained at higher polymer thicknesses suggests that the rate of change of the SAW viscoelastic component is similar to that of the gravimetric element. It is fair to assume that both processes originate from the same event: the absorption of the analyte in the polymer. In this view the polymer-coated SAW sensor may be regarded as an enhanced gravimetric sensor.

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Current micromachining and IC manufacturing technology has enabled the development of a wide range of acoustic sensors and devices operating at frequencies up to several GHz. Accurate measurements of the vibrational behavior of these devices is often complicated by their small size, intricate geometry and high operating frequencies. To address these issues a measurement system based on a fiber delivered heterodyne laser interferometer has been developed to perform spatially dense measurements of the surface displacement at high frequencies. With this system, noncontact displacement measurements can be made with sub-angstrom sensitivity for a spot several micrometers in diameter. Evaluation of the system is made with 2-D scans of the normal surface displacement of a 110-MHz delay line surface acoustic wave (SAW) chemical vapor sensor. Measured displacement levels of the uncoated quartz substrate for this device are typically less than 1 nm for acoustic wavelengths of 29 mum. Limitations of this method are discussed, including the effect of spatial filtering brought about by the finite optical spot size. [Work sponsored by the Office of Naval Research.] a)National Research Council-NRL Research Associate. b)Also with SFA, Inc., Landover, MD 20785.