Fracture properties of thin papers made from graphene oxide and carbon nanotubes

Abstract

Graphene - one atom thick sp² bonded carbon atoms arranged in a honeycomb lattice structure having exceptionally high in-plane electronic mobility, mechanical strength, and thermal conductivity - has attracted significant attention since its first discovery in 2004. Graphene oxides (GO), consisting of oxygen containing functional groups on its basal planes and edges and comprising of sp² and sp³ hybridized carbon atoms, provides well dispersed individual sheet in water and organic solvents. It is widely used as a building block in composites, mechanical actuators, nano-robots and paper-like materials. In recent years, GO-based paper materials have attracted much interest because of their outstanding strength, stiffness and high degree of flexibility. This paper-like material may be used as sealants, actuators, and bio-compatible substrates, flexible substrates with high chemical and thermal stability. The lateral dimensions of GO sheets have significant impact in controlling their properties and applications. The mechanical properties of GO papers have been extensively studied mostly in uni-axial tensile mode. However, no study has been carried out in other modes of deformation, such as shear. It is also important to differentiate the fracture mode or the conditions under which fracture occurs. When paper-like carbon materials are used in engineering applications, proper understanding their fracture toughness becomes essential. Like GO papers, papers made from carbon nanotubes (CNTs), or bucky papers, is a viable engineering material due to their useful mechanical, electrical and thermal and many other properties. The GO sheets in an aqueous media act as a surfactant because of their high aspect ratio and amphiphilic nature. This surfactant-like effect helps CNTs to be easily dispersed and absorbed onto the GO sheets in water and stable GO/CNT hybrid dispersions can be easily produced. This research focuses on the fracture behavior of GO papers, bucky papers and their hybrid papers in mode-I and mode-III loading. The effect of GO sheet size and CNT content on fracture and tearing toughness of the papers are evaluated using double-edge-notch tension (DENT) and trouser tear specimens, respectively. The tear studies of all paper specimens under the mode-III loading exhibited stick-slip tearing. The concept of linear elastic fracture mechanics (LEFM) is applied to measure the fracture toughness of GO papers. GO papers made from large sheets give higher fracture and tearing toughness than those made from small GO sheets. About 66% enhancement of fracture toughness and 70 % enhancement of tearing toughness are observed. Hybridization also enhances these two properties. Fracture and tearing toughness increased by 22% and 15% after hybridization with 5 wt% CNTs, compared to the unsorted GO papers. The failure mechanisms taking place during the fracture toughness and trouser tearing tests are identified from the microscopic examination of the fracture surfaces. Cleavage failure and CNT pullout are the dominant failure mechanisms under mode-I loading of GO papers and bucky papers, respectively. In case of tear, combination of cohesive/adhesive failure of GO paper and CNT pullout are the dominant failure mechanism for bucky paper specimens.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology