AFM-based adhesive indentation of polymer brushes: A phenomenological approach

Indentation-based mechanical characterization of ultra-thin polymer layers is widely performed by means of atomic force microscopy (AFM) using both conventional indenters and spherical colloidal probes. While there exists an extensive theoretical framework for extracting the material properties of bulk samples from the depth-sensing indentation data, in regard to a polymer brush, it is still challenging to interpret the contact force/indenter displacement curve in terms of its time-independent interaction potential density. The indentation process is assumed to be quasistatic, and the effects of viscoelastic deformation and time-dependent adhesion have been neglected. The novelty of this study lies in the Griffith's energy balance-based model of adhesive contact between an AFM probe and a polymer brush grafted onto a rigid substrate, which is applicable for finite deformations of polymer chains. A fitting-based analysis of experimental force/displacement curves available in the literature is presented.