Effect of local material properties on tapping mode atomic force microscopy

The phase image produced by Atomic Force Microscopy is very important in the study of polymeric materials. This image is a map of the spatial variation in the phase lag between the tip oscillation and the driving oscillation, resulting from variations in local properties. We have studied the effect of local material properties and operating conditions by simulating the AFM imaging procedure and by experiment. For hard elastic materials the dominant interaction force between the tip and the sample is the elastic repulsive force. In the case of polymers, which are softer and viscoelastic in nature, both the modulus and the viscosity influence the phase lag with the viscous force often dominating. Additionally, as expected, the phase lag for soft, viscoelastic domains is higher than for high elastic domains. Experiments were performed with silicon (hard and elastic) and polybutadiene (soft and viscoelastic) that validate our simulation results. Experiments also illustrate that the thickness of a polymer film on a substrate influences the phase lag when it is on the order of the radius of gyration of the molecules. This property makes it possible for the AFM to detect sub-surface features through polymer films under certain conditions. The eventual goal of this work is to enable the quantitative as well as qualitative interpretation of phase images of polymeric materials.