The effects of aspect ratio and orientation on the mechanical properties of nanocomposites reinforced with carbon nanotubes

Many studies have been carried out on the effect of the addition of carbon nanotubes on the mechanical properties of polymer-based nanocomposites. A fundamental understanding of the mechanisms of reinforcement and the factors that control mechanical properties has been hampered by the lack of measurements on specimens with well-characterised nanotube microstructures. In the present study we have used a series of multi-walled carbon nanotubes (MWCNTs) with different aspect ratios (length/diameter) to prepare nanocomposites and determine their mechanical properties as a function of nanotube volume fraction. In addition a similar investigation has been carried out on the effect of MWCNT alignment for one type of nanotube, with nanotube orientation being determined from the quantitative analysis of transmission electron micrographs of microtomed sections of the nanocomposites. The findings have been analysed using a new theory that combines the rule of mixtures with shear lag theory. Overall it is predicted that the stiffness of the nanocomposites depends only upon the aspect ratio, orientation and volume fraction of the MWCNTs and is independent of their Young's modulus. Good agreement is found between the experimental data and theoretical analysis. This is of profound importance for our understanding of the mechanisms of reinforcement of CNT/polymer nanocomposites and points to how the properties of polymer-based nanocomposite systems may be optimised in the future.