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Acoustic wave microsensors are capable of sensing transition behaviors of homogeneous amorphous polymers applied as thin films to the surface of the sensor. The device and wave characteristics of four such microsensors, and examples of their uses in monitoring polymer properties, are described. The devices are the thickness-shear mode (TSM), surface acoustic wave (SAW), flexural plate wave (FPW), and shear horizontal acoustic plate mode (SH-APM) devices. Using the FPW device, the change in the polymer coefficient of thermal expansion at the static glass transition temperature is sensed as a change in slope of the frequency-temperature plot. This behavior reflects the fact that the frequency measures acoustic velocity, which decreases as the polymer modulus decreases. Modulus decreases as the polymer volume increases. Frequency-dependent relaxation properties are detected with a sigmoidal change in slope of the frequency-temperature plot and a minimum in signal amplitude. The minimum in signal amplitude is analogous to the maximum in the loss tangent in dynamic mechanical analysis, or the maximum in sound damping in bulk-wave ultrasonic studies.
acoustic wave, microsensor, thin film, glass transition, thickness-shear mode (TSM), surface acoustic wave (SAW), flexural plate wave (FPW), shear horizontal acoustic plate mode (SH-APM), ultrasonic
Pacific Northwest Laboratory, Richland, WA