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Viscoelastic Contact Resonance Force Microscopy of Polymers in Liquid
A.B. Churnside, R.C. Tung, A. Aziz, S. Bryant, J.P. Killgore
NIST, US
Keywords: atomic force microscopy, polymers, liquid, water
Summary:
Contact resonance force microscopy (CR-FM) is an AFM-based technique for quantitatively determining the nanoscale mechanical properties of materials. In CR-FM, the resonant properties of an AFM cantilever in contact with the surface are interpreted as viscoelastic properties of the surface. Measurements in liquid environments are key to characterizing many materials. However, liquid CR-FM is complicated by spurious cantilever resonances and difficulty in separating liquid from material effects. Direct cantilever excitation yields well-separated contact resonance peaks in liquid. Recently we developed a model to compensate for the hydrodynamics in contact resonance. Here, we apply corrected CR-FM to loss tangents (tan(δ)) in water. Specifically, we compared the tan(δ) of polystyrene and polypropylene measured in air and water. The uncorrected measured tan(δ) values in water were higher than in air. After applying the hydrodynamic correction, the values in air agreed with those in water. This establishes the technique on stiffer polymers (elastic modulus in the GPa range) whose material properties do not change in water. We then calculated loss tangents in the range of 0.1 to 1 for different hydrogels. This demonstrates the usefulness of CR-FM for soft (elastic modulus in the kPa range) materials whose properties are best measured in liquid.
