Shull Research Group

The QCM is a versatile tool that can quantify the mechanical properties of thin-film polymer glasses, gels, and complexes in both air and liquid environments as a function of external conditions, such as changing temperature or pH/ionic strength. You can also measure the properties of bulk viscous liquids. You can also study electrochemical stimuli to materials using the E-QCM, while still being able to measure their viscoelastic properties (neat!). The films have to be smooth and homogeneous and are typically in the range of 800 nm to 5 microns depending on how stiff the material is. Stiff materials, such as polymer glasses like PS at room temperature, require about a 5 micron film. Soft hydrogels require about 1 micron films. 

Absolutely! Downloading and running QCMDAnalyze.m will launch the MATLAB GUI for analyzing data from QCM-D instruments to extract the fixed frequency rheological properties (shear modulus and phase angle) of a thin film or a bulk medium in contact with the crystal. This code uses the power-law analysis developed by Shull et al., which is a more realistic description of materials than the commonly used Voigt model. The detailed approach was outlined by DeNolf et al. and Martin et al. This approach to solving QCM-D data is more accurate and robust than those analysis provided with commercial instruments because it uses minimal fit parameters, makes the fewest assumptions, uses MATLAB’s powerful numerical solvers, and provides physical insight into the experiment and material parameters. Additionally, this analysis also provides checks and balances for accurate viscoelastic analysis of polymer films whose properties can span from glasses to soft gels in air or immersed in liquids, and also viscous polymer solutions.

Film thickness, sample quality, and film thickness again! First and foremost your sample needs to be as smooth and homogeneous as possible. Typically samples with uneven thicknesses, particles, rips or tears, especially near the center of the electrode are undesirable. Additionally, your film must be in the correct thickness range. Look to this publication for details on the thickness limits. Generally, you need a thicker sample for stiffer materials (~5 microns for polymer glasses) and a thinner film for softer materials (~1 micron for hydrogels). One must have an approximate idea of what properties to expect from the material of interest to aim for the correct thickness. 

If your film is too thin, then you cannot calculate mechanical properties. However, the mass information contained in the frequency shifts is still accurate. If you are only interested in mass transfer from your material, then you should aim to make your film thin. But what is thin? Good question. It depends on your material of interest. If your material is glassy, then anything less than about 3 microns is “thin.”  If your material is soft then anything less than about 500 nm is “thin.” When your sample is in this “thin-film” limit, it is in the Sauerbrey regime, where the Sauerbrey equation gives you an accurate measure of the mass transfer to/from your material.  

The QCM is a fixed high-frequency technique. DMA and shear rheometry are relatively low-frequency techniques, but you could use time-temperature superposition to access a larger frequency domain. The QCM provides a direct measurement of the high-frequency material response, which is often difficult to access. The high-frequency also provides very high sensitivity and resolution to changes in material properties, often surpassing the resolution that can be attained using conventional techniques. If the frequency effects are considered appropriately, then the QCM is complementary to any other lower frequency mechanical measurement approach. Also, the QCM provides information about the mass transfer simultaneously. This is a significant advantage over other mechanical tests.