SPMW Interplay between theory and experiment in AFM nanomechanical studies of polymers
30 Nov 2006 | Online Presentations | Contributor(s): Sergei Magonov, Sergey Belikov
High-resolution imaging of surfaces and compositional mapping of heterogeneous materials are the main functions of atomic force microscopy (AFM) in studies of polymer materials. Compositional mapping is mostly based on differences of mechanical properties of the sample components yet quantitative analysis of these properties remains a challenging issue. The same is true for nanoindentation studies, which complement the main AFM functions. We believe that advances in high-resolution imaging and nanomechanical analysis should be based on a rigorous interplay between theory and experiment. For this purpose, KBM (Krylov-Bogoliubov-Mitropolsky) modeling of tapping mode AFM was performed and applied for analysis of the experimental data. In the first step, a simulation of AFM imaging of molecular lattices and defects was performed for polydiacetylene crystal and compared with experimental images obtained on this sample[1,2]. This comparison suggests that an extreme care should be taken in the analysis of the molecular- and atomic-scale images to avoid misleading conclusions. In the second step, we performed the analysis of experimental amplitude and phase curves and phase images of polymer blends. The KBM model describes tapping mode amplitude and phase curves in terms of three stationary solutions (two stable nodes, one saddle) verified by the experiments. A simple model of energy dissipation (adhesion avalanche) was also considered. Different phase and dissipation contrast of images of multilayer polymer blends suggests that phase imaging is better for compositional mapping whereas understanding of the dissipative processes is indispensable for a quantitative description of tip-sample forces. Further steps in the development of this interplay include an extension of KBM to frequency modulation mode and a consideration of force models describing a transition from Lennard-Jones solid to elastic solid .