Effects of the nanotube length and network morphology on the deformation mechanisms and mechanical properties of cross-linked carbon nanotube films

Abstract

The effects of the carbon nanotube (CNT) length and material structure on the mechanical properties of free-standing thin CNT films with continuous networks of bundles of nanotubes and covalent cross-links are studied in large-scale simulations. The simulations are performed based on a dynamic mesoscopic model that accounts for stretching and bending of CNTs, van der Waals interaction between nanotubes, and inter-tube cross-links. It is found that the tensile modulus and strength of the CNT films strongly increase with increasing CNT length, but the effect of the nanotube length is altered by the cross-link density. The mutual effect of the nanotube length and cross-link density on the modulus and strength is primarily determined by a single parameter that is equal to the average number of cross-links per nanotube. The modulus and strength, as functions of this parameter, follow the power-type scaling laws with strongly different exponents. The film elongation at the maximum stress is dominated by the value of the cross-link density. The dispersion of nanotubes without formation of thick bundles results in a few-fold increase in the modulus and strength. The variation of the film properties is explained by the effects of the CNT length, cross-link density, and network morphology on the network connectivity. The in-plane compression results in the collective bending of nanotubes and folding of the whole film with only minor irreversible changes in the film structure. Depending on the CNT length, the reliefs of the folded films vary from a complex two-dimensional landscape to a quasi-one-dimensional wavy surface.