The predominant role of swelling-induced modulus changes of the sorbent phase in determining the responses of polymer-coated surface acoustic wave vapor sensors

Abstract

The sorption of vapors by fluoropotyol, poly(epichlorohydrln), and poly(lsobutytene) 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.

Analysis of Liquid-Phase Chemical Detection Using Guided Shear Horizontal-Surface Acoustic Wave Sensors

“…17 Changes in the viscoelastic properties of the chemically sensitive or waveguiding layer may also contribute to the observed frequency shift when guided SH-SAW sensors are exposed to aqueous analytes. This effect has been noted in gas-phase SAW sensor studies where the viscoelastic contribution to the observed frequency shift was defined in terms of swelling-induced modulus changes 18,19 as, accessed by following the link in the citation at the bottom of the page.…”

“…The contribution of mass loading to the sensor response is well accepted in this field while the contribution of the viscoelastic property of polymer coating is still under investigation, even in gasphase sensors. 18,19,[25][26][27] Unfortunately, results in gas phase cannot be totally adapted or used to explain liquid-phase responses. For example, ongoing work in liquid-phase sensing has also shown that for a rubbery polymer such as PDMS viscoelasticity changes can clearly dominate sensor response.…”

“…18,19 Δfs represents the frequency shift due to the amount of sorbent phase; K is the partition coefficient, the ratio of the concentration of analyte in the sorbent phase to the concentration of the analyte in the vapor phase, CA; ρs is the density of the sorbent phase; ρA is the density of the vapor as a liquid; ASAW represents the kilohertz change in frequency due to a 1 °C change in temperature per kilohertz of coating on the device surface; α is the coefficient of thermal expansion of the polymer, i.e., the fractional volume increase per degree; and fA is the fractional free volume of the diluent (vapor) as a liquid. The parameters in eq 2 consist of those that are vapordependent, fA, CA, and ρA, and those that are strictly polymerdependent, ρs, ASAW, and α.…”

“…As a result, the propagating wave senses the film as a stiff glassy material rather than as a soft rubbery material. 19 However, upon absorption of water or analytes into the polymer matrix, G' is expected to decrease, while G' ' should increase as the polymer becomes lossier. Also different analytes result in different changes in the polymer viscoelasticity, due in part to differences in their partition coefficients.…”

Prediction of Vapor-Liquid Equilibrium of Polypropylene Oxide Solution Systems by Cubic Equations of State

“…The Phase behavior of polymer solutions are of extreme importance for the development of in several polymer processing and many polymers are produced in solution [1], [2]. The polymer devolatilization and other polymeric membrane separation processes [3], recovery of organic vapors from waste-air streams using a polymeric membrane [4], and pervaporation [5] may be have a few residual solvents.…”