On the cohesion of carbon nanotubes in nanostructures

The research described herein has focused upon understanding and modeling of phenomena that control the cohesion in carbon nanotube-based systems. Of particular interest have been the effects of van der Waals interactions between graphene structures including graphite, single walled and multi-walled carbon nanotubes. The manifestations of these interactions are pervasive and very relevant for potential applications. Quantification of such interactions has been first attempted in flexural deformation of carbon nanotube arrays. The information obtained has been used to analyze applications that have received considerable attention in the literature like harmonic oscillators and carbon nanotube ropes. These applications have in common that they depend largely on efficient interfacial stress transfer between individual tubes and the theoretical results obtained for corroboration. A series of models to quantify the load transfer efficiency in multiwalled carbon nanotubes in various deformation modes have been developed and tested using experimental data reported in the literature. Conclusions from that analysis indicate that, in general, very weak interactions are present. The validity of continuum mechanics at the scales of interest was explored by means of a series of Molecular Dynamics simulations, aimed at testing the scaling of shear force with system size in concentric carbon nanotubes. Continuum mechanics requires such scaling to be linear, but factors such as commensurability and chirality of the interacting shells are shown to cause pronounced deviations from this expectation. A series of models based on nonlinear beam theory coupled to a Lennard-Jones potential were developed to understand recent molecular force spectroscopy analyses on the process of peeling a carbon nanotube from a graphitic substrate using Atomic Force Microscopy. It is hypothesized that the experimental results correspond to one of at least three possible peeling regimes identified that depend on the balance between adhesion forces and bending stiffness of the nanotubes. The combination of these research thrusts should provide the foundation for development of a fundamental understanding of stress transfer in multiwalled carbon nanotubes and bundles of single walled carbon nanotubes with far reaching consequences in mixing, separation and structural applications based on these systems