Dynamic AFM on viscoelastic polymer samples with surface forces
Abstract
The dynamics of an oscillating Atomic Force Microscopy (AFM) tip tapping on a polymer surface are key to understanding the nanoscale physical properties of polymer samples. This is because dynamic AFM observables such as energy dissipation or phase contrast are linked to nanoscale physical properties such as local viscoelasticity and adhesion. Attard and co-workers [1-5] developed a rigorous, mathematical model to compute surface deformation and forces due to a prescribed motion of an axisymmetric AFM tip. The model included tip-sample surface forces through a Lennard-Jones pressure term as well as sample deformation through a linear viscoelastic constitutive model. We adapt this formalism and develop an approach to model the physics of oscillating tips interacting with polymer samples in amplitude or frequency modulation AFM (AM-AFM, FM-AFM). The approach is validated against other computational codes. The predictions are compared with data acquired in AM-AFM on an elastomer (E) polycarbonate (PC) polypropylene (PP) blend where the constitutive properties of each component are determined over a wide frequency range at room temperature using DMA measurements and time-temperature superposition. The theoretical and computational approach presented in this work not only does away with artefacts arising from the use of ad hoc viscoelastic contact mechanic models [6] but it also provides deep insight into the role of surface forces and polymer relaxation times on the dynamic AFM observables.
Degree
M.S.M.E.
Advisors
Raman, Purdue University.
Subject Area
Mechanical engineering
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