Improving interfacial and mechanical properties of carbon nanotube-sized carbon fiber/epoxy composites

Abstract

Carboxyl-functionalized carbon nanotubes (CNTs-COOH) and the amine-functionalized carbon nanotubes (CNTs-NH2) were incorporated into the surface sizing agent of carbon fibers respectively, for modifying the interface between the fibers and the matrix. Experimental analyses of the interfacial shear strength, flexural properties, and inter-laminar shear strength revealed the beneficial effects of the CNTs-COOH and negative impacts of the CNTs-NH2 on the interfacial and mechanical properties of the resulting composites. These results were contrary to the effects of CNTs doped into the epoxy matrix of composites. Furthermore, a molecular dynamics simulations were performed to explore the mechanism of these performances from the viewpoint of the molecular structure, revealing that the interface between the CNT-sized carbon fibers and the epoxy was driven by multiple factors, including the CNT-epoxy interfacial binding energy (IBE), the packing density of the molecular chains, and the steric hindrance of the functionalized CNTs. According to these analyses, two kinds of functionalized CNTs showed different effects at the interface between carbon fiber and resin: the CNTs-COOH on the surface of the carbon fiber tightly bonded to the epoxy and provided effective load transfer between the matrix and fibers, whereas the agglomeration of the CNTs-NH2, the lower IBE between the CNTs-NH2 and the epoxy resin, and the steric hindrance produced by amine functional groups of the CNTs-NH2, resulting in hardly inhibiting the crack growth of the matrix under stress.