Predicting Contrast in Dynamic Atomic Force Microscopy of Polymeric Systems

G. Meyers
The Dow Chemical Company, US

Keywords: AFM, atomic force microscopy

Summary:

In spite of the growing importance of AFM in materials research, the link between a dynamic AFM “image” and the underlying material structure and properties remains tenuous. In part, this is because of an incomplete connection between the observables in dynamic AFM (probe tip amplitude, phase) on one hand, and material properties [local mechanical response (elasticity, anelasticity, viscosity and plasticity), charge density, magnetic dipole, and topography] and feedback control on the other hand. Without accurate descriptions of tip-sample interactions coupled with probe dynamics, attempts to quantify precisely nanoscale material properties via AFM mapping will remain prone to large uncertainty. The current state of AFM imaging for polymer morphology is at a cross-road. There now exists a multiplicity of imaging options that take advantage of resonant or non-resonant properties of vibrating cantilevers in contact with surfaces. With respect to mechanically resolved imaging we now have the ability to probe surfaces over a variety of time, force, and displacement regimes. While this is exciting we also are faced with the challenge of determining the best imaging mode for the problem at hand. For example, in order to resolve the phase morphology in a complex blend we may try Force Modulation AFM, TappingMode™ (Trademark of Bruker-Nano) AFM, quasistatic AFM indentation, HarmoniX™ resonance AFM, and PeakForce™ Tapping imaging. The last two are newer dynamic methods and may provide quantitative mechanical mapping. Much of the trial and error approaches to optimizing contrast in AFM imaging of polymer systems are now just that, left to empirical testing. We know that polymeric materials will have frequency and strain rate dependent properties and so we expect that imaging contrast in blends might be improved by understanding and controlling experimental conditions to take advantage of these dependencies. The need therefore is to be able to simulate the various imaging modes on model heterogeneous surfaces where the material property descriptors can be used in conjunction with appropriate contact mechanical models and cantilever dynamic models. Specifically we have enhanced the continuum based models available through the VEDA suite of simulation modules1 (http://www.nanohub.com) to now include viscoelastic model simulations. Further we are using molecular dynamics to understand time dependence of tip-polymer contacts from first principles calculations.