Carbon Nanotubes: Supramolecular Mechanics
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Supramolecular interactions are usually short-range (near contact) forces between the molecules, too weak to cause the intermolecular changes or bond formation between the particles. At the same time, their strength and range are sufficient to cause self-assembly of the molecular units or certain elastic deformations of these building blocks. Because of the weakness of supramolecular interactions, the resulting organization is sensitive to external conditions and ensures certain ease of transformations in response to external fields. Well known is a fascinating class of liquid crystals, whose “liquid” aspect is exactly a result of relatively small energy of intermolecular forces. Nanoparticles and especially nanotubes offer a kind of building blocks that can be efficiently glued by weak supramolecular forces in a self-assembly process. In case of nanotubes, because of their hollow structure and large aspect ratio, same forces or interaction between them can cause deformations of bending, torsion, flattening polygonization, etc. In turn, the degree of such deformation can enhance (like in case of partial polygonization of nanotubes in the array bundles) supramolecular attraction or decrease it under other circumstances. Supramolecular behavior of nanotubes is therefore intrinsically coupled with their own mechanical properties which we will discuss in some detail.
Unique mechanical properties of carbon nanotubes indirectly lead to unusual role of the forces between them. These supramolecular interactions are normally weak and, for most molecular species, are easily overcome by thermal agitation. However, they turn out to be significant in case of nanotubes, building up very strong attractive potentials over the extensive aligned contacts (∼ 0.5 eV/nm). The general aspects and issues of supramolecular mechanics are discussed, including an overview of the mechanical properties of nanotubes, especially linear elastic parameters and their relation with persistence length, nonlinear elastic instabilities, and buckling. Furthermore, inelastic relaxation, yield strength, and fracture mechanisms, including their kinetic theory, are also discussed. A discussion of theoretical and computational studies is supplemented by brief summaries of experimental results for the entire range of the deformation amplitudes. Atomistic scenarios of “strong” supramolecular interactions, when polymerization and coalescence welding occur, are also discussed because of their significance in potential applications.