Use of SAW devices to monitor viscoelastic properties of materials

Preliminary data are presented which demonstrate the potential sensitivity of SAW (surface acoustic wave) to the elastic properties of polymer coatings. 158-MHz dual SAW devices were used to demonstrate the sensitivity of the SAW to changes in coating elastic properties, and to identify the T_g and/or T_m of several polymer coatings. Data for PCAP (poly(caprolactone)) and FPOL (fluoropolyol) indicate that the SAW frequency is sensitive to changes in the shear modulus of the coatings. The steady decrease in frequency with increased temperature observed for FPOL-coated devices may be the result of increased surface coverage due to expansion of the film. Increased surface coverage would result in an increase in surface wave interaction with the film and a corresponding steady decrease in frequency. This indicates that these polymer films may not be true lossless films. It is suggested that in light of this, previous assumptions made in the development of predictive equations using SAW data must be reevaluated. It is concluded that the sensitivity of SAWs to elastic properties is indicative of their potential as monitors for cure processes, or other applications where changes in the elasticity of materials are of interest