Mesoscopic computational model of covalent cross-links and mechanisms of load transfer in cross-linked carbon nanotube films with continuous networks of bundles

Abstract

An “effective bond model” of covalent cross-links between carbon nanotubes (CNTs) is developed for mesoscopic simulations of CNT materials. The cross-links are represented by discrete stretchable bonds between mesoscopic nanotube elements. These bonds induce forces in both the normal and tangential directions to the CNT surfaces. A general approach for fitting the model parameters based on results of atomistic simulations describing the pullout of a nanotube from a bundle is suggested. This approach is used to obtain sets of the best-fit parameters, recommended for simulations with (26,0) CNTs when cross-links are predominantly formed by single interstitial atoms. It is shown that quantitative agreement between the atomistic and mesoscopic simulations can be achieved if the equilibrium length of the mesoscopic cross-link bond is smaller than the equilibrium gap between pristine CNTs and an additional fitting parameter, which controls the direction of the cross-link force with respect to the geometrical bond, is introduced into the model. The developed model is used to simulate stretching and compression of a thin CNT film with a continuous network of bundles and to reveal the microscopic mechanisms of the mechanical load transfer. It is found that during stretching the cross-links create a transient percolating “load transfer network” that includes relatively small number of strained CNT segments, participating in the load transfer, while bending modes of CNTs are not activated. The structure of this network evolves with increasing strain, when breaking of certain cross-links irreversibly activates new paths of load transfer. During compression, mechanical response of the nanotube network is determined by the trade-off between stretching and bending of individual nanotubes, while growing correlations between motions of large bundles eventually result in the collective bending of the whole film.