Frequency-independent and frequency-dependent polymer transitions observed on flexural plate wave ultrasonic sensors

Abstract

The flexural plate wave device Is capable of sensing transit Ion behaviors of homogeneous amorphous polymers applied as thin films to its surface. The changes In polymer properties at the static glass transition temperature are sensed as a change In slope of the frequency-temperature plot. Frequency-dependent relaxation properties are detected with a sigmoidal change In slope of the frequency-temperature plot and a minimum In signal amplitude. The transitions observed using poly(vinyl acetate), poly(/erf-butyl acrylate), poly(lsobutylene), and poly (vinyl propionate) are correlated with the properties of these polymers known by Independent methods. The responses of the flexural plate wave device at 5 MHz are In accord with the results of conventional bulk-wave ultrasonic experiments at similar frequencies. The relevance of polymer transition processes to the performance of polymer-coated acoustic vapor sensors Is discussed.

Acoustic Wave Sensors

“…FPW devices (see Figure 2-1) are plate wave devices with plates that are only a few percent of an acoustic wavelength thick (typically 2-3 fim thick) [63][64][65][66][67][68][69][70]. The plate is a composite structure consisting of a silicon nitride layer, an aluminum ground plane, and a sputtered zinc oxide piezoelectric layer, all supported by a silicon substrate.…”

“…(The surface-parallel motion is parallel to the wave propagation direction) Typical device resonant frequencies are 2-7 MHz. FPW devices have been used in a great variety of applications, including vapor sensing, observation of polymer glass transitions, liquid properties such as density and viscosity, and biosensing [63,64,67,[70][71][72][73][74][75][76]. In addition, these devices have interesting capabilities for pumping fluids and moving solids [77,78].…”

“…This signal change occurs with no mass per unit area change. Sensitivity to polymer thermal expansion can also lead to observation of the polymer static glass-to-rubber transition (discussed further in Section 4.5.2) [25,64].…”

Detection of phase transition in polyethylene glycol using a multimodal micromechanical acoustic resonator

“…A similar negative trend in frequency has been observed in flexural plate wave resonators when the coated polymer transitions from a harder (higher stiffness) physical state to a softer rubbery (less stiff) physical state. 22 As the PEG mass completely melts, the effect of stiffness becomes negligible (liquid only applies viscous drag which effects the associated liquid mass loading and dissipation). In the case of M2, as the drop completely melts, there is a sudden decrease in the mass loading as only a thin layer of liquid 26 remains in shear contact with the vibrating surface of the resonator.…”

“…Indeed, microcantilevers have been previously used to detect transitions among different rotator phases in nanogram samples of paraffin wax, 21 and in a separate study, flexural plate wave resonators were used to characterize glass transition in different polymeric glues. 22 In both the cases, resonators were uniformly coated with the samples and exposed to temperature variations. In the present study, instead of a uniform layer, phase transition (solid to liquid and vice versa) in a microdroplet of a waxy crystalline polymer, polyethyelene glycol (PEG) 1000, is studied by observing the changes in the resonant parameters of two distinct piezoelectrically transduced resonant modes (flexural and bulk) of a beam resonator.…”