Viscoelastic properties of thin films probed with a quartz-crystal resonator

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Abstract
We report on the application of piezoelectric quartz-crystal resonators to the viscoelastic characterization of thin organic layers. A passive measurement of the crystal’s electrical impedance allows one to determine the frequencies of its resonances as well as the corresponding damping constants. The accessible resonances span a frequency range from about 4 MHz to about 100 MHz. A parallel ellipsometer setup is used for the simultaneous determination of optical thicknesses. When one side of the quartz plate is coated with a viscoelastic layer, the frequencies and the damping constants of the acoustic modes change. While in the limit of ultrathin films these changes depend only on the mass, thicker films show pronounced viscoelastic effects. We inverted these dependences to obtain the complex and anisotropic shear compliance of a film with a thickness of about 1.75 μm. Results are presented for a Langmuir-Bodgett-Kuhn film consisting of the molecular composite poly(γ-methyl-L-glutamate-co-γ-n-octadecyl-L-glutamate). These rodlike molecules have a rigid backbone and flexible side chains. With respect to its mechanical and rheological behavior the substance is frequently called a ‘‘molecularly reinforced liquid.’’ The results show that the large-scale viscoelastic behavior is not a liquidlike one, as would have been suggested by simple composite models. A possible interpretation is the existence of vitrified states in the side-chain regions.