Kinetics of atomic force microscope probe interaction with inhomogeneous polymer surface in fast nano-indentation regime

Abstract

Atomic force microscopy (AFM) is a powerful tool for studying the structural and physical-mechanical properties of surfaces. The AFM experiment consists in detecting the interaction between the probe and the material. The probe is an elastic cantilever fixed horizontally at one end; a tip is located at the free end of the cantilever. The interaction of the tip with the surface causes deflection of the cantilever, which is interpreted depending on the experimental conditions. High-speed indentation atomic force microscopy methods allow obtaining maps of three-dimensional relief and physical-mechanical properties of the material in high resolution at micro- and nanoscale levels. In this case, the probe is rapidly pressed on the material and the interaction is recorded at each point of the surface of the studied area. The user sets the speed at which the probe approaches the material surface. This value is assumed to be constant and is called the indentation rate. However, this is not true: the tip speed at the loose end of the cantilever depends in general on many factors. As an example of investigation of polyurethane surface (heterogeneous two-phase polymer) it is shown that the probe-material interaction speed nonlinearly depends on the surface area, indentation depth and direction, nominal speed of experiment, cantilever properties. This affects the measurement of relief topography, stiffness, Van der Waals forces and adhesion between the probe and the material. Thus, for a correct quantitative interpretation of the results, the interaction speed of the tip with the material must be taken into account; this statement is especially important when working with soft polymers. The presented methods can be useful in the study of a wide class of soft heterogeneous materials.