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.

Revealing Density Thresholds of Carbon Nanotube Cross-Links for Load Transfer: A Graph Theory Strategy

“…Owing to the existing CNT clusters, their spatial structures change with difficulty under tension, and both intra- and intertube chemical bonds in CNT clusters bear the external loading, which results in the observation of broken bonds and some local voids, even at small strains. A large-scale stress concentration occurs and extends with increasing strain, and then the local voids have nearly penetrated the CNT network . Extensive and violent fracture occurs suddenly when the network meets the load-carrying limit.…”

“…A large-scale stress concentration occurs and extends with increasing strain, and then the local voids have nearly penetrated the CNT network. 24 Extensive and violent fracture occurs suddenly when the network meets the load-carrying limit. This brittle failure is featured by a uniform brittle fracture cross-section, as shown in Figure 5d,f.…”

Chirality-Dependent Mechanical Properties of Bundles and Thin Films Composed of Covalently Cross-Linked Carbon Nanotubes

“…The tubular potential method ensures the absence of corrugation of the intertube potential for curved nanotubes, eliminates artificial intertube friction, and is capable of describing the self-assembly of nanotubes into continuous networks of entangled bundles. 66,72,74 The CL energy is defined assuming that each CL in an in-silico-generated material sample represents an individual CL in the real material. Then,…”

“…In our mesoscopic simulations, we focus attention only on the effects of CNT diameter and chirality as the effects of nanotube length and CL density were reported elsewhere. 72 Our major finding is that the mechanical performance of cross-linked CNT materials, like their electronic and optoelectronic properties, can be improved or optimized ondemand by tuning diameters of SWCNTs. We show, in particular, that the elastic modulus of CNT films increases, while the tensile strength decreases with increasing diameter of nanotubes at fixed material density and density of CLs.…”

“…These models enable simulations with large material samples and allow one to establish a direct relationship between the structure of the CNT network and material properties. The coarse-grained models of cross-linked CNT materials were used to determine the Young’s modulus of CNT mats and bundles, predict mechanical properties of CNT bundles, reveal the universal scaling parameters that govern the strength of CNT bundles, and study the effect of nanotube length on the elastic and inelastic properties of CNT films . None of the known mesoscopic simulations, however, addressed the effect of CNT diameter on the mechanical properties of pristine and cross-linked CNT materials.…”