A new method for characterizing the interphase regions of carbon nanotube composites

AbstractThe elastic properties of a carbon nanotube (CNT) reinforced composite are affected by many factors such as the CNT–matrix interphase. As such, mechanical analysis without sufficient consideration of these factors can give rise to incorrect predictions. Using single-walled carbon nanotube (SWCNT) reinforced Polyvinylchloride (PVC) as an example, this paper presents a new technique to characterize interphase regions. The representative volume element (RVE) of the SWCNT–PVC system is modeled as an assemblage of three phases, the equivalent solid fiber (ESF) mimicking the SWCNT under the van der Waals (vdW) forces, the dense interphase PVC of appropriate thickness and density, and the bulk PVC matrix. Two methods are proposed to extract the elastic properties of the ESF from the atomistic RVE and the CNT-cluster. Using atomistic simulations, the thickness and the average density of interphase matrix are determined and the elastic properties of amorphous interphase matrix are characterized as a function of density. The method is examined in a continuum-based three-phase model developed with the aid of molecular mechanics (MM) and the finite element (FE) method. The predictions of the continuum-based model show a good agreement with the atomistic results verifies that the interphase properties of amorphous matrix in CNT-composites could be approximated as a function of density. The results show that ignoring either the vdW interaction region or the interphase matrix layer can bring about misleading results, and that the effect of internal walls of multi-walled carbon nanotubes (MWCNTs) on the density and thickness of the dense interphase is negligible

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

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