THE PREDOMINANT ROLE OF SWELLING-INDUCED MODULUS CHANGES OF THE SORBENT PHASE IN DETERMINING THE RESPONSES OF POLYMER-COATED SURFACE ACOUSTIC-WAVE VAPOR SENSORS

The sorption of vapors by fluoropolyol, poly(epichlorohydrin), and poly(isobutylene) is examined by gas-liquid chromatography (GLC), and these results are compared with the responses of surface acoustic wave (SAW) vapor sensors coated with the same polymers. The sensor responses exceed those which can be attributed to gravimetric effects, indicating that the SAW devices are responding to some other change in the coating properties. A model is developed to estimate the effect of polymer swelling on SAW sensor responses. The model is based on the use of partition coefficients determined by GLC as an independent measure of polymer mass loading, and polymer thermal expansion on SAW surfaces as a measure of volume change effects which is independent of mass loading effects. Both experimental comparisons and the model indicate that swelling effects can be ca. 4 times greater than mass-loading effects. The likely mechanism by which swelling influences the SAW sensor response is via reductions in the modulus of the polymer overlayer.