Modifications of carbon nanotubes with polymers

Abstract

The chemical functionalizations of carbon nanotubes (CNTs) could enhance their chemical compatibility and dissolution properties, which enable both a more extensive characterization and subsequent chemical reactivity. The modifications with polymers could not only improve CNTs’ solubility and dispersibility but also the interfacial interaction to polymeric matrices in its composites. The main methods for the modification of CNTs with polymers are noncovalent attachment (polymer wrapping and absorption) and covalent attachment (“grafting to”and “grafting from”). The current states of the literatures in the field are presented in this review.

Introduction

The discovery of carbon nanotubes (CN) [1] and the prospect of developing novel carbon-based nanomaterials has excited worldwide interest among researchers [2]. The unique properties of carbon nanotubes (CNTs) make them attractive for applications in many scientific and technological fields such as electronic structures [3], [4], polymer composites [5], [6], [7], and biological systems [8].

However, the researches of carbon nanotube are challenged by several problems, such as: (i) mass production of material, (ii) control of length, diameter, and chirality, and (iii) manipulation for use in diverse technological fields. Issues regarding the synthesis and purification as well as the functionalizations and solubilizations of carbon nanotubes are relevant topics in the rapidly growing fields. While a large knowledge base exists for incorporating micron-sized carbon-based fibers, by comparison very little has been done for CNT incorporation. Certainly in the past few years as CNT supplies have become more plentiful, more efforts to successfully process CNTs using extrusion, ultrasonic radiation, high shear mixing and melt spinning have been developed to overcome this problem. Dispersion of the tubes is particularly important. Failure to disperse leads to a phase separation due to the strong surface interactions between the tubes. It is likely that chemical functionalizations would not only serve to facilitate dispersion, but could stabilize the CNTs and prevent agglomerations, which could lead to defects, from forming.

In many applications it is necessary to tailor the chemical nature of the nanotube’s walls in order to take advantage of their unique properties. There are two main approaches for the surface modification of CNTs. One is noncovalent attachment of molecules; the second is covalent attachment of functional groups to the walls of the nanotubes.

The noncovalent attachment, controlled by thermodynamic criteria [9], which for some polymer chains is called wrapping, can alter the nature of the nanotube’s surface and make it more compatible with the polymer matrix. The advantage of noncovalent attachment is that the perfect structure of the nanotube is not altered, thus its mechanical properties should not change. The main potential disadvantage of noncovalent attachment is that the forces between the wrapping molecule and the nanotube might be weak, thus as a filler in a composite the efficiency of the load transfer might be low.

The covalent attachment of functional groups to the surface of nanotubes can improve the efficiency of load transfer. However, it must be noted that these functional groups might introduce defects on the walls of the perfect structure of the nanotubes. These defects will lower the strength of the reinforcing component. Therefore, there will be a trade-off between the strength of the interface and the strength of the nanotube filler. The functional groups attached could be small molecules [10], [11], [12] or polymer chains. The chemical functionalizations is an especially attractive target, as it can improve solubility [13] and processibility and allows the unique properties of single-walled nanotubes (SWNTs) to be coupled to those of other types of materials. The ability to solubilize and separate discrete CNT molecules from the tight bundles they form would not only open new avenues in the field of nanotechnology, but would also aid in their purification, eventually allowing easier manipulation. Herein, we focus on recent developments in the modifications of CNTs with polymer.