Multiscale modelling of elastic properties of non-bonded single-walled carbon nanotube polymer matrix composites

Abstract

Single walled carbon nanotubes (SWCNTs) have attracted great attention for new generation of advanced polymer matrix composites (PMC). Nevertheless, diverse experimental reports on the elastic properties of SWCNT-PMC, have also initiated extensive theoretical investigations aiming at revealing their reinforcement mechanisms and optimizing their mechanical properties. It has been reported that the overall stiffness of SWCNT-PMC is significantly affected by the interphase and waviness of SWCNTs. Although the impact of a pre-determined interphase on the stiffness of SWCNT-PMC has been studied thoroughly, little is known about the elastic properties of the interphase layer. Moreover, the waviness of SWCNTs has been unrealistically assumed to be regular wave-shaped fibres. To accurately predict the elastic properties of non-bonded SWCNT-PMC, this thesis has developed a comprehensive multiscale numerical strategy to address both interphase and waviness with minimal simplifications. First, the stiffnesses of SWCNTs and polymer matrices are investigated through atomistic simulations. This leads to the conclusion that, except the transverse Young's modulus, all other elastic quantities of SWCNTs under the vdW forces increase with the pressure rise. The study also confirms that molecular mechanics (MM) can only provide acceptable results for polymers under specific conditions. The multiscale investigations are then carried out in two stages. In Stage 1, a cubic nanoscale representative volume element (NRVE) of a polymer matrix with SWCNT is characterized through atomistic simulations. Using the results of individual constituents, a three-phase continuum finite element (FE) model, consisting of the bulk matrix, the dense interphase matrix and the SWCNT under van der walls (vdW) force, is developed successfully. The study shows that the average density of the interphase can be used as a parameter to determine the mechanical properties of the dense interphase matrix. In Stage 2, the NRVE model is used as a basic solid element for the wavy SWCNTs in a cubic micro-scale representative volume element (MRVE) composite. A new indicator for the waviness is defined and quantified from micrograph images. The study confirms that the models established produce results consistent with experiments, that aligned SWCNTs are remarkable stiffeners, and that the interphase region of non-bonded SWCNT-PMC can be ignored only if the SWCNT diameter is (10, 10) or smaller.