December 1-6, 2013 | Boston
Meeting Chairs: Charles Black, Elisabetta Comini, Gitti Frey, Kristi Kiick, Loucas Tsakalakos
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, TappingModetrade; AFM, quasistatic AFM indentation, HarmoniXtrade; resonance AFM, and PeakForcetrade; 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 modules(1) (http://www.nanohub.com) to now include viscoelasticity and hysteretic surface adhesion interactions (elastic case). Further we are using molecular dynamics to understand time dependence of tip-polymer contacts from first principles calculations.(1) Gaining insight into the physics of dynamic atomic force microscopy in complex environments using the VEDA simulator. Kiracofe, Daniel; Melcher, John; Raman, Arvind. Review of Scientific Instruments (2012), 83(1), 013702/1-013702/17. trade; (Trademark of Bruker-Nano)
Atomic force microscopes (AFM) can map the topography of surfaces with sufficient resolution to observe individual atoms. Mechanical property measurements with AFM have evolved from slow force volume to multiple-frequency based dynamic measurements using TappingMode and contact resonance. Recently, real-time control of the peak force of the tip-sample interaction has led to a fundamental change in AFM imaging, providing quantitative mapping of mechanical properties at unprecedented resolution. During material property mapping, the time scale of tip-sample interaction now spans from microseconds to seconds, tip sample forces can be controlled from piconewtons to micronewtons, and spatial resolution can reach sub-nanometer (where continuum mechanics fails). AFM has become a unique mechanical measurement tool having large dynamic range (1kPa to 100GPa in modulus) with the flexibility to integrate with other physical property characterization techniques in versatile environments.In particular, researchers have begun to take advantage of the wide range of deformation rates accessible to AFM in order to study time dependent properties of materials such as viscoelasticity . More traditional measurements with indentation DMA are usually limited in frequency to a few hundred Hz and have limited spatial resolution. In contrast, AFM measurements can extend from less than one Hz to KHz and beyond while retaining the high resolution needed to see the details in distribution of properties near domain boundaries in nanocomposites and other materials. This presentation will review this recent progress, providing examples that demonstrate the dynamic range of the measurements, and the speed and resolution with which they were obtained. Additionally, the effect of time dependent material properties on the measurements will be explored.------ M. E. Dokukin and I. Sokolov, Langmuir 28, 16060-71 (2012). K. K. M. Sweers, K. O. van der Werf, M. L. Bennink, and V. Subramaniam, Nanoscale 4, 2072 (2012).
In many fields including biology, polymer composites and nanomaterials there is high demand for studying of mechanical properties of soft matter on the micro and nanoscale. Force spectroscopy performed by the means of AFM presents fast reliable way of mapping of mechanical properties of the materials with resolution on the order of tens of nanometers. When mapping mechanical properties of soft materials one should consider the changes in the operating conditions (frequency and temperature) under which material is used, since these conditions change the behavior of the material under study. Therefore, more complex analysis of the mechanical properties should be involved to provide complete information about the mechanical properties of the material. Here we show how such analysis could be performed for several well studied polymeric materials. We have demonstrated the way of collection of the viscoelastic properties of the material with force spectroscopy using a constant loading rate. We have employed viscoelastic three-element model and applied it to analyze force-distance curves to get instantaneous and infinite moduli of the material as well as relaxation times in the single measurement. The measurements have been performed on the polymer materials known to present significant viscoelastic properties such as poly(n-butyl methacrylate).
Mechanics of soft materials at the nanoscale is important when