Effect of viscoelastic properties of polymers on the phase shift in tapping mode atomic force microscopy

On the basis of the frequency-sweep tapping mode atomic force microscopy (T MAFM) experiments for polystyrene (PS) and poly(dimethylsiloxane) (PDMS), t he validity of the harmonic approximation for describing the phase shift of TMAFM was tested by evaluating the analytical expressions derived from the approximation. Our study shows that the phase shift is almost quantitative ly described by the harmonic approximation. By analyzing the results of the se experiments, it was also examined how the viscoelastic properties of a p olymer affect the nature of the tip-sample interaction. The nature of the t ip-sample interaction is strongly affected by the sharpness of the tip and by the elasticity, viscosity, and plasticity of a polymer sample. On a stro ngly glassy, brittle sample with a low impact strength, such as PS, the pha se shift can be dominated by the attractive force with the use of a dull ti p, while on a strongly elastic sample, such as PDMS, the phase shift, can b e dominated by the indentation force with the use of a sharp tip.