In general, long-length CNTs show a flexible fibre-like character with tube buckling and entangled configuration. Therefore, its processing inside polymer matrix is a bit difficult in terms of de-entanglements. The long length ensures lower electrical percolation threshold, good mechanical properties due to the large-distance stress transfer and crack propagation prevention ability. However, it can be easily aligned and tends to show better interfacial polarization. Like the length of CNTs, diameters also affect the properties, for example SWCNTs, large diameter offers large interfacial surface area per unit length but less stiffness.
Another important parameter is aspect ratio i. However, it could be bit elusive and care has to be taken while talking about aspect ratio because a short-length and small-diameter CNT can have same aspect ratio as a long-length and large-diameter CNT, but their properties and translated effect on composites may be altogether different.
Carbon Nanotube-Polymer Composites (RSC Publishing)
Nevertheless, CNTs are widely used as very small, high aspect ratio conductive additives for plastics of all types. The high aspect ratio up or more gives CNTs an edge over other conductive fillers e. A perfect single-wall CNT is only a theoretical construction and even perfect hexagonal sp 2 structure possesses different types of defects. Generally, a defective site has a high reactivity meaning that at those points, the chemical reactions are favoured.
They are localized mainly at tube ends and near tube bending locations. Defects are very important because they can modify the electronic properties of the nanotubes thereby influencing their applications. The defects can also be present along the sidewall, and after acid treatment, it is possible to open nanotube ends and attach chemical functional groups facilitating the interaction of CNTs with other moieties. In addition to being important for the potential applications of nanotubes, defects also play a key role in the functionalization process. As mentioned in the introduction of this chapter, Iijima discovered the catalyst-free formation of multiwall carbon nanotubes MWCNTs , while investigating the other carbon nanostructures formed along with the fullerenes and more particularly, the solid carbon deposit forming onto the cathode.
This technique produces a complex mixture of components including non-tubular forms of the carbon such as nanoparticles, fullerene-like structures including C 60 , multiwall shells, amorphous carbon, etc. Therefore, this requires further purification to separate the CNTs from the soot and the residual catalytic metals present in the crude product.
In this technique, the high yield production of CNTs depends on the uniformity of the plasma arc, and the temperature of the deposit forming on the carbon electrode. The laser ablation technique was used successfully to synthesize fullerene for the first time in by Kroto et al.
The target is vapourized more uniformly by a second laser pulse. The use of two successive laser pulses minimizes the amount of carbon deposited as soot. The second laser pulse breaks up the larger particles ablated by the first one and feeds them into the growing nanotube structure. It is possible to vary the average nanotube diameter and size distribution by varying the growth temperature, the catalyst composition and other process parameters.
Both these solid carbon source-based synthesis methods have some drawbacks.
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One issue is the scaling up of the process to the industrial level. Secondly, the synthesized CNTs have impurities of metal catalyst particles and unwanted carbon forms such as fullerenes, amorphous carbon, multiwall shells, single-wall nanocapsule and need further purification. Chemical vapour deposition CVD of gaseous carbon source hydrocarbons, CO over a metal catalyst is a classical method that has been used to produce various carbon materials such as carbon fibres and filaments for a long time [ 48 ].
In this technique, a carbon source is taken in the gas phase and an energy source such as plasma or a resistively heated coil is employed to transfer energy to a gaseous carbon molecule. The CVD technique uses hydrocarbons such as methane, carbon monoxide or acetylene as carbon source. The growth starts after two gases are passed through the chamber, that is hydrocarbon gas and the other a carrier gas such as nitrogen, hydrogen or argon.
The advantages of the CVD process are low power input, low-temperature range, relatively high purity and most importantly, the possibility to scale up the process. Depending on their chirality and diameter, CNTs can be either metallic or semiconducting in their electrical behaviour [ 20 , 27 , 51 , 52 ]. Because of its nanoscale cross-section, electrons propagate only along the tube axis.
Polymer Composite Containing Carbon Nanotubes and their Applications.
As a result, carbon nanotubes are frequently referred to as one dimensional conductor. All nanotubes are expected to be very good thermal conductors along the tube axis, exhibiting a property known as ballistic conduction, but good insulators laterally to the tube axis [ 55 — 58 ].
CNTs are the strongest and the stiffest materials yet discovered in terms of tensile strength and elastic modulus, respectively [ 20 , 27 , 60 — 62 ]. This strength is a direct consequence of covalent sp 2 bonds formed between the individual carbon atoms. It has been shown that CNTs are very strong in the axial direction. On the other hand, there was evidence that in the radial direction, they are rather soft. The first transmission electron microscope observation of radial elasticity suggested that even the van der Waals forces can deform two adjacent nanotubes [ 64 ].
The results show that MWCNTs are radially deformable to a large extent notwithstanding their axial rigidness and strength under tensile load. Such catalytic particles are formed either in situ during the growth of CNTs e. As the magnetism is due to the metallic impurities, CNTs tend to lose magnetic character upon treatment under harsh conditions, for example acidic treatment such as functionalization [ 27 , 67 ] or high-temperature annealing [ 68 ].
Reproduced from  with permission from Wiley.
The obtained g values are very near to the characteristic g values of ferromagnetic materials. It is suggested that presence of iron catalyst inside CNTs may be helpful in their magnetic manipulation, for example for magnetic alignment of CNTs in composites [ 20 , 70 ] or for targeted drug delivery [ 69 ]. Further, the magnetic properties also contribute towards electromagnetic energy absorption and play a role in improving its contribution towards total microwave shielding effectiveness [ 21 , 71 ].
The pure CNTs are made up of graphitic cylindrical walls made up of carbon atoms. Therefore, being of non-polar in nature, the surfaces of CNTs especially MWCNTs are highly hydrophobic in nature with good affinity towards non-polar materials such as hydrocarbons, parrafins, oils or organic solvents. As CNTs also display relatively high-specific surface area, when 3D shape architectured, due to the inherent hydrophobicity they can be useful for water purification applications, for example for separation of non-polar pollutants such as oils, solvents or even organic dyes.
Owing to their chemical inertness, CNTs are difficult to disperse in water and in organic media, and they pose high resistance to wetting. Historically, unlike fullerenes, their chemistry was considered very poor for a long time. Difficulties also arise in making composites of such inert nanotubes with other materials which is important for many device applications.
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A suitable functionalization of the nanotubes i. Functionalization enhances the solubility and processibility, and allows combining the unique properties of nanotubes with those of other materials [ 20 , 26 , 27 , 72 ]. It also improves the interaction of the nanotube with other entities, such as a solvent, polymer and other organic molecules and also with other nanotubes.
A functionalized nanotube displays different mechanical and electrical properties as compared to pristine nanotube and thus may be utilized for several applications. Reproduced from  with permission from Springer-Verlag Berlin. Depending upon the site of interaction, it can be of two types—covalent sidewall functionalization and defect functionalization. Direct covalent sidewall functionalization involves a change of hybridization from sp 2 to sp 3 and the loss of conjugation.
Defect functionalization is based on the transformations of defect sites already present. In addition to these, oxidative purification generated oxygenated sites are also considered as defects. SWCNTs demonstrate low dispersability, and they occur in the form of bundles. This situation warrants the use of a highly reactive reagent for the covalent bond formation at the sidewalls. It is not possible to tell beforehand that whether these addition reactions are more likely to take place at defect sites or intact hexagonal regions of sidewall. Several covalent routes have been taken for covalent functionalization such as oxidative purification [ 74 , 75 ], amidation [ 76 ], esterification [ 77 ], thiolation [ 78 ], halogenation [ 79 — 81 ], hydrogenation [ 82 ], cycloadditions [ 81 , 83 — 87 ], electrochemical functionalization [ 88 , 89 ].
Covalent functionalization suffers from the drawback of damaging the structure of CNTs. Broadly, surfactants and polymers are utilized for this non-destructive functionalization of CNTs. Surface active molecules such as sodium dodecylsulfate SDS or benzylalkonium chloride have been successfully used for the formation of non-covalent aggregates [ 90 — 92 ]. On the other side, polymer wrapping around CNTs can be accomplished through the use of polymers such as poly m -phenylene- co -2, 5-dioctoxy- p -phenylenevinylene PmPV, 2 in organic solvents such as chloroform.
The covalent and non-covalent functionalizations are essentially exohedral derivatizations. The hollow inner cavity of SWCNTs serves as a capillary for the storage of nanoparticles and fullerenes, etc. The rich endohedral chemistry of SWCNTs is amply illustrated by the incorporation of fullerenes and metallofullerenes in their cavity [ 94 , 95 ].
The spectacular properties of CNTs such as their high strength and stiffness make them ideal candidates for structural applications. At present, polymer nanocomposite is one of the biggest application areas for CNTs. The extraordinary properties of CNTs coupled with easily tailorable characteristics of polymers give rise to truly versatile CNT-polymer nanocomposites [ 20 , 21 , 26 , 27 , 96 — ].
The emergence of CNTs as filler materials has contributed in the realization of CNT-polymer nanocomposites as next generation advanced structural material. By definition, a composite is a multiphase material formed from a combination of materials which differ in composition or form, retain their own chemical and physical properties, and maintain an interface between components which act in concert to provide improved specific or synergistic characteristics not obtainable by any of the original components acting alone [ 20 , , ].
Any composite is composed of two categories of materials: the reinforcement or filler and the matrix. The reinforcements contribute useful properties mechanical, electrical, thermal, optical, etc.
The limitations of traditional micrometer-scale polymer composites have prompted a considerable research effort to focus on polymer nanocomposites. Historically, nanocomposites are not entirely new as some nanocomposites such as carbon black and fumed silica-filled polymers [ , ] have been used a long time ago. However, the discovery of CNTs added an additional impetus in the polymer nanocomposite research promising hitherto unknown potential for various applications. Comparison and contrast of advanced fibre reinforcements vs.
A variety of synthesis techniques are in practice for incorporation of CNTs into various polymeric host matrices [ 14 , 20 , 21 , 26 , 27 , 97 , , — ]. The main motive is to deagglomerate the CNTs and realize their uniform dispersion inside polymer matrix. Currently, there is no single technique which is universally applicable to all the situations.
Depending upon the thermal or chemical properties of matrix polymer, ease of its synthesis from suitable monomer, desired performance indices of composites and cost constraints, with some trade-off in properties, one can choose the suitable processing method for a particular case. This section briefly describes the important processing techniques for synthesis of CNT-based polymer nanocomposites.
The solution processing is the most common technique to form CNT-based polymer nanocomposites which exploits intensive agitation e. It is important to note that this technique is limited to the polymers which are soluble in solvent s. The choice of solvent is mainly governed by solubility of matrix polymer. Further, the solvent for CNTs and polymer dispersion may be same or different, but should be of good missibility to realize intimate mixing between phases. In many cases, CNTs are not separately dispersed rather they are directly added to polymer solution followed by intensive mixing before film casting.
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