The study of functionalizations of graphene and carbon nanotubes and their applications in nanocomposites

Abstract

Up to date, the nano-particles and nano-materials have been established as an independent research discipline. Among them, carbon nano-materials, especially the carbon nanotubes (CNTs) and graphene have drawn tremendous research interest in the past few decades due to their exceptional mechanical, thermal, electrical and optical properties. The CNTs and graphene thus have shown a great promise in preparing the nano-scaled sensors, conductors, capacitors, batteries and other apparatuses. In addition, one of the most significant applications of the CNTs and graphene is to incorporate them into some polymers as the reinforcing fillers. Compared with other nano-fillers like carbon black and silica, the CNTs and graphene have much higher Young's Modulus, aspect ratio, thermal and electrical conductivity, etc. With fairly a little amount of these nano-fillers being incorporated into the polymer matrices, generally no more than 5% in weight content, the corresponding properties of the prepared nano-composites can be enhanced largely. Based on these facts, the CNTs and graphene are viewed as the most desirable nano-fillers since they emerged. However, one major bottleneck for the practical application of CNTs is their poor dispersibility in solvents and/or polymer matrices. This drawback is mainly resulted from the strong van der Waals interactions among the CNTs and graphene. Due to these reasons, aggregations of CNTs bundles and graphene sheets are more probably observed in the polymer matrices or solvents rather than well-dispersed individuals. The precipitation and aggregation of the CNTs and graphene significantly affect the mechanical and electrical properties of the polymer nano-composites due to microphase segregation, inefficient load transfer, and limited percolation network formation. Hence, the functionalizations of the CNTs and graphene have to be done before applying them in preparing nano-composites. Generally, using some pre-treatment methods, physical and/or chemical, can help improve the dispersion of the nano-scaled materials. Specifically, physical approaches include adsorbing or mixing certain types of polymers, electrolyte, surfactants, and biomolecules onto the surfaces of the CNTs and graphene. These foreign molecules act as the compatibilizer to weaken the van der Waals forces and to increase the dispersability of the CNTs and graphene in the solvents and polymer matrices. On the other hand, the chemical functionalizations of the CNTs and graphene also play a very crucial role in improving not only the dispersion but also the inherent properties of them. Being covalently grafted by some small molecules or polymers onto the surface, the CNTs and graphene will have obvious improvement in their dispersions in the solvents and the adhesions to the polymer matrices. More significantly, the introduced molecules can even improve the inherent properties of the CNTs and graphene. Since both CNTs and graphene can be easily acidified by strong oxidants, a number of chemical reactions can be introduced to the CNTs and graphene. The present thesis focuses on the functionalizations of the CNTs and graphene, which include physical (non-covalent) functionalizations and chemical (covalent) functionalizations. The prepared functionalized CNTs and graphene are then used as the reinforcing fillers to fabricate polymer nano-composites. The corresponding properties of these nano-composites are studied and investigated subsequently.