Dynamic AFM on Viscoelastic Polymer Samples with Surface Forces

Abstract

Dynamic atomic force microscopy (dAFM) is widely used to characterize polymer viscoelastic surfaces in the air/vacuum environments; however, the link between the instrument observables (such as energy dissipation or phase contrast) and the nanoscale physical properties of the polymer surfaces (such as local viscoelasticity, relaxation, and adhesion) remains poorly understood. To shed light on this topic, we present a computational method that enables the prediction and interpretation of dAFM observables on samples with arbitrary surface forces and linear viscoelastic constitutive properties with a first-principles approach. The approach both accelerates the computational method introduced by Attard and embeds it within the tapping mode amplitude reduction formula (or, equivalently, frequency modulation frequency shift/damping formula) to recover the force history and instrument observables as a function of the set point amplitude or Z distance. The method is validated against other reliable computational codes. The role of surface forces and polymer relaxation times on the phase lag, energy dissipation, and surface deformation history is clarified. Experimental data on energy dissipation in tapping mode/amplitude modulation AFM (TM-AFM/AM-AFM) for different free amplitudes and set point ratios are presented on a three-polymer blend consisting of well-dispersed phases of polypropylene, polycarbonate, and elastomer. An approach to experimental validation of the computational results is presented and analyzed.

Nanomechanical mapping of soft materials with the atomic force microscope: methods, theory and applications

“…This limitation has motivated the introduction of the computational methods to describe in a self-consistent manner the tip-sample viscoelastic interactions and the associated deformations. 55,56 Computational solutions enable to explore the dynamic behaviour of complex models. This task is helped by the existence of two numerical codes dForce 57 and VEDA 58 that enable to simulate the response of a dynamic AFM for a variety of sample models.…”

“…Numerical simulations have been applied to determine the force based on a SLS response and to compare it with experiments performed with an amplitude modulation AFM on elastomers and polycarbonate samples. 56 4.4.1 Finite element method simulations. Finite element method (FEM) 59 is very useful to visualize the deformations generated by the tip on a variety of elastic and viscoelastic models.…”

Fast, quantitative and high resolution mapping of viscoelastic properties with bimodal AFM

“…These effects could be negligible in bulk measurements. Some considerations by Raman and co-workers 9 and Proksch et al 51 offer additional insights into this issue.…”

“…The understanding, characterization and mapping of the viscoelastic properties of soft matter at the nanoscale level is an active area of research in polymer science, nanolithography, mechanobiology and force microscopy. [1][2][3][4][5][6][7][8][9][10][11][12] The development of novel nanolithographies demands a fast and high resolution characterization of the viscoelastic properties of polymer resists. 12,13 In mechanobiology, there is some evidence that supports the influence of the cell's viscoelastic processes in its physiology.…”

On the frequency dependence of viscoelastic material characterization with intermittent-contact dynamic atomic force microscopy: avoiding mischaracterization across large frequency ranges

“…The peculiar types of behavior of viscoelastic materials have been recognized by AFM researchers since the early days of dynamic AFM, and various studies have explored different phenomena, such as energy dissipation and the proportion between elastic and viscous probe-sample interactions during the measurement [22][23][24][25][26][27]. A large number of publications have focused on the study of the phase signal (i.e., the lag of the oscillatory response of the cantilever with respect to the excitation, within amplitude-modulation AFM (AM-AFM)), which generally yields high-contrast images for dissipative materials [22].…”