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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.

The effect of nanotube chirality on the mechanical properties of materials composed of single-walled carbon nanotubes (CNTs) is poorly understood since the interfacial load transfer in such materials is strongly dependent on the intertube interaction and structure of the nanotube network. Here, a combined atomistic-mesoscopic study is performed to reveal the effect of CNT diameter on the deformation mechanisms and mechanical properties of CNT bundles and low-density CNT films with covalent cross-links (CLs). First, the pullout of the central nanotube from bundles composed of seven (5,5), (10,10), (20,20), (17,0), and (26,0) CNTs is studied in molecular dynamics simulations based on the ReaxFF force field. The simulations show that the shear modulus and strength increase with decreasing CNT diameter. The results of atomistic simulations are used to parametrize a mesoscopic model of CLs and to perform mesoscopic simulations of in-plane tension and compression of thin films composed of thousands of cross-linked CNTs. The mechanical properties of CNT films are found to be strongly dependent on CNT diameter. The film modulus increases as the CNT diameter increases, while the tensile strength decreases. The in-plane compression is characterized by collective bending of whole films and order-of-magnitude smaller compressive strengths. The films composed of (5,5) CNTs exhibit the ability for large-strain compression without irreversible changes in the material structure. The stretching rigidity of individual nanotubes and volumetric CL density are identified as the key factors that dominate the effect of CNT chirality on the mechanical properties of CNT films. The film modulus is affected by both CL density and stretching rigidity of CNTs, while the tensile strength is dominated by CL density. The obtained results suggest that the on-demand optimization of the mechanical properties of CNT films can be performed by tuning the nanotube chirality distribution.

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