Atomic force microscopy nanoindentation kinetics and subsurface visualization of soft inhomogeneous polymer.

Modern techniques of nanoindentation by atomic force microscopy (AFM) produce maps of topography and physical-mechanical properties of the material. Analysis of the interaction rate of the AFM tip with the soft surface reveals the surface and subsurface structure and expands standard analysis of the material behavior. Phase-separated polymer (polyurethane, elastic modulus-6 MPa) is studied. Reversible inelastic changes of the surface at different stages of indentation were established in dependence on peculiarities of velocity and position of the AFM-tip in the material: uniform soft nanofilm covering the outer surface gradually passes into fibrillar heterogeneous structure of the polymer. The point of stable mechanical contact is defined, and the elastic moduli of soft and hard blocks of the polymer are estimated using certain intervals of the indentation. The presented methods of surface analysis are useful in the study of a wide class of soft heterogeneous materials.