The application of acoustic waves to investigate the viscoelastic properties of thin films

Abstract

Thickness shear mode (TSM) quartz resonators are not only used extensively as accurate timing devices, but also in a variety of sensing applications. This work describes a series of experiments on the measurement of the viscoelastic properties of thin films in contact with such resonators. A technique was developed that allowed the calculation of the complex shear modulus of a polymer layer from a measurement of the impedance spectrum of the resonator around the series resonance point. This method was applied particularly for the characterisation and comparison of the properties of polyethylene oxide (PEO) and polydimethylsiloxane (PDMS) polymer layers. The values for the shear moduli of these two layers clearly indicated that, at room temperature, the PEO layer was in the glassy state, while the PDMS was in the rubbery state. This was confirmed by the very high values observed with PDMS for the impedance minimum at the series resonant frequency, indicative of the high damping of the acoustic wave by this lossy material. Cooling of the PDMS layer to temperatures below -60°C induced a transition to the glassy state and this transition was clearly observed from the impedance measurements. The changes induced in the viscoelastic properties of polymer films during the in-diffusion of organic vapours was also studied with this technique. This produced the dramatic result that in the case of PDMS the induced frequency shift was not dominated by the mass loading, but rather by changes in the shear modulus of the polymer. In this case the frequency shift was not a negative shift, as conventionally predicted by the Sauerbrey equation, but a positive shift due to the softening of the polymer by the organic vapours.