Mesoscale Structural and Mechanical Simulations of Cross-Linked Carbon Nanotube Materials

Abstract

Relatively poor mechanical properties of carbon nanotube (CNT) bulk materials can be improved by formation of bonds or covalent cross-links (CLs) between nanotubes. In this work, an “effective bond model” of covalent CLs between carbon nanotubes is developed for mesoscopic simulations of cross-linked CNT materials. A general approach for fitting the CL model parameters based on results of atomistic simulations is developed. The best-fit parameters of the CL model are found. The developed effective bond model of CLs is included into a dynamic mesoscopic model of CNT materials, where each nanotube is represented in the form of a chain of stretchable cylindrical segments. The mesoscopic force field in this model accounts for stretching and bending of CNTs, van der Waals interaction between nanotubes, and inter-tube CLs. The model is applied to generate and equilibrate in silico pristine and cross-linked CNT fiber and film samples with structural characteristics close to observed in experiments. The structural parameters of CNT fibers and films, including the average bundle size, Herman orientation factor, and tortuosity, are calculated. The quasi-static simulations of large-scale cross-linked CNT films are performed to reveal the load transfer mechanism, as well as effects of CNT length, CL density, material density, and network morphology on mechanical properties under conditions of quasi-static deformation. It is found that stretching of CNT segments is the dominant mode of load transfer in cross-linked CNT film during their stretching, while bending and buckling is the dominant mode of load-transfer during compression. Both tensile modulus and strength of CNT films increase strongly with increasing CNT length. The effect of the nanotube length on mechanical properties, however, is altered by the density of CLs. The mutual effect of the nanotube length and CL density on modulus and strength is described by power scaling laws, where the modulus and strength are functions of the average number of CLs per nanotube, i.e., the product of the CNT length and CL linear density. The exponents in the scaling laws for the modulus and strength are strongly different from each other. The material density of the film samples weakly affects the specific mechanical properties. The dispersion of nanotubes in the films without formation of thick bundles results in the few-fold increase of the modulus and strength. In qualitative agreement with experimental observations, the in-plane compression of a large thin CNT film results in collective bending of nanotubes and folding of the whole film with minor irreversible structural changes. Depending on the CNT length, the reliefs of the compressed films vary from quasi-one-dimensional wavy surface to complex two-dimensional landscape.