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.
The simultaneous excitation and measurement of two eigenmodes in bimodal atomic force microscopy (AFM) during sub-micron scale surface imaging augments the number of observables at each pixel of the image...
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