Molecular Dynamics Modeling of Carbon Nanotube Composite Fracture using ReaxFF

Abstract

Carbon nanotubes (CNTs) are well known to have exceptionally high mechanical properties when measured individually. Recently, CNT fiber composites have been enabled by the production of high-tex yarns in quantities on the order of kilometers. These high-tex CNT yarns have recently become comparable in specific stiffness and specific strength to carbon fiber. Despite these advancements, CNT yarns still have mechanical properties substantially lower than their CNT constituents. Closing this gap requires understanding load transfer between CNTs and the role of matrix binders such as amorphous carbon at the nanoscale. This work uses reactive molecular dynamics simulations to gain a nanoscale understanding of the key factors of CNT nanocomposite mechanical performance and to place more realistic upper bounds on the target properties. While molecular dynamics simulations using conventional force fields can predict elastic properties, the ReaxFF reactive force field can also model fracture behavior because of its ability to accurately describe bond breaking and formation during a simulation. The upper and lower bounds of CNT composite properties are investigated by comparing systems composed of CNTs continuously connected across the periodic boundary with systems composed of finite length CNTs. These lengths, effectively infinite for the continuous tubes and an aspect ratio of 13 for the finite length case, result from simulation limitations. Experimentally measured aspect ratios are typically on the order of 100,000, so the calculated results should represent upper and lower limits on experimental mechanical properties. Finally, the effect of various degrees of crosslinking to the amorphous carbon matrix is considered in an attempt to identify the amount of CNT-matrix covalent bonding that maximizes overall composite properties.