Descripción de la Seguridad de la Información del CGT

The functionalization of carbon nanotubes aims to modify, chemically or otherwise, the surface in a controlled manner, which would lead to enhancements in structural or electronic of the materials. When carbon nanotubes applied as reinforcement into polymer, its interfacial bonding with the polymer matrix becomes a crucial issue for the load transfer within the composite. Chandra et al. [94] has observed that the thermo-mechanical transfer between fiber and matrix in conventional composites is influenced by both chemical and mechanical bonding. While chemical bonding arise from the formation of new phases, mechanical bonding occurs by interlocking of asperities. Apart from these two types of bonding, there is a third type of bonding that contributes to the load transfer mechanism,

which is Van der Waals bonding. Inside the nanotube based composites, these three types of bonding are considered to be the main transfer mechanism of load, and which governed the final properties of the composites. To ensure the nanotube based composites maintain sufficient strength, the nanotube must be well bonded to polymer interface. However, employing nanotubes as filler is challenging as they are difficult to disperse and dissolve in any organic and aqueous medium. Due to the strong attractive long-ranged Van der Waals interaction, nanotubes tend to aggregate and form ropes. Because of these highly entangled networks structures, the polymer matrix exhibits difficulties to penetrate into the nanotube structures.

In order to obtain a homogenous dispersion and achieve the full potential of nanotubes, two different possessing methods are introduced – physical/mechanical and chemical methods. The physical method implies the physically separation of the nanotube from each others. Meanwhile, the chemical technique implies the modification of the tube surface by surfactant or chemical treatment. Chemical treatments are always preferred in producing nanotube based composites. During which, the nanotube surface would be modified chemically and letting different chemical functional groups attached to its surface. These functionalized nanotubes not only would improve its dispersion throughout the solvent/the host composite material, but also improve the interfacial strength chemically due to the cross-linking created between the nanotubes and polymer matrix. Therefore, in turn, causing an enhancement in load transfer ability in the composites. Currently, there are several techniques widely adopted to modify the surface of carbon nanotubes. These techniques include solution oxidation method, sidewall functionalization, fluorination and high energy radical bombardment.

1.2.5.1 Solution oxidation method

Solution oxidation method [95-108] is the first method developed for chemical modification. Since this method does not require any particular equipment, it has been widely used for nanotube surface modification. Strong chemicals, such as concentrated HNO3,

KMnO4, HF, are used in this method to oxidize the carbon atoms on the surface of nanotube.

During the oxidation process, the carbon atoms at the end of the nanotube will be excited by the chemicals and causing its hybridization to alter, which allow other atoms or molecules to react with the excited atoms to form functional groups such as carboxyl or hydroxyl. The concentration of functional groups on the surface is found to be controlled successfully by using phase transfer catalyst to assist the process [109]. Though solution oxidation method is the simplest technique used for nanotube surface modification, the activation energy of carbon atoms at the end of functionalized nanotube is found to be lower than that of carbon atoms on the side wall because C-C bond is more serious deformed at the end. Besides, time consuming and chemical waste created during the process make this method unfavor to commercialize.

1.2.5.2 Sidewall functionalization

Since the solution oxidation method will only able to oxidize the end atoms on the surface of nanotube and leave the sidewall untouched, therefore, further modification is required to functionalize the sidewall of carbon nanotubes. In this method [110-117], carbon nanotubes and proper amount of aniline are mixed together with a magnetic stirrer. And Isoamyl nitrite is then added to the mixture to give the reactive arenediazonium species. The whole is heated to 60 ºC and vigorously stirred. In this way, the organic species would attach to the sidewall of carbon nanotubes by covalent reaction and serve as functional groups.

Although this method provides a solvent free condition, it involves the reactive and unstable chemicals which are not favor for storage and environment.

1.2.5.3 Fluorination [56-63]

Fluorination of carbon nanotubes occurs when fluorine atoms are made to covalently bond to the sidewalls of a nanotube. In order to achieve this propose [118-125], nanotubes are reacted with He diluted HF gas in the hot chamber, which is about 150-250C, for about 10 hours. Fluorine atom will attach to the surface of nanotubes and can be easily substituted by other function groups through further chemical reaction. This method is totally solvent free as it takes place in gas phase and easily acquired as the reaction can be run on large quantities of nanotubes. However, the evolution of hydrogen fluoride gas (HF) may happen due to the high processing temperature [126], and additional reaction has to be involved to substitute the fluorine atom, thus increases the complexity of the process.

1.2.5.4 High energy radical bombardment

Apart from the chemical method, physical method has also been developed for the purpose of modification. Researches [127-129] employed oxygen ion and argon ion as working radials and applied electrical field to accelerate the working radials to certain energy level. The accelerated ions were then utilized to bombard the surface of carbon nanotubes in the chamber. The carbon atoms on the surface would absorb the energy from radials and react with oxygen ions to form functional groups. This method is highly efficient and provides a solvent free condition. However, the high energy ions in bombardment process may cause damage to the structure of carbon nanotubes which leads to the degradation in the properties of nanotubes.