**3. Synthesis of CNTs**

strongest metallic alloy known. Experimental values of Young's modulus for SWCNTs are re‐ ported as high as to 1470 GPa and 950 GPa [3, 4] for MWCNTs, nearly 5 times of steel. There are no direct mechanical testing experiments that can be done on individual nanotubes (nanoscopic specimens) to determine directly their axial strength. However, the indirect experiments like AFM provide a brief view of the mechanical properties as well as scanning probe techniques that can manipulate individual nanotubes, have provide some basic answers to the mechanical be‐ havior of the nanotubes [5]. The analysis performed on several MWCNTs gave average Young's modulus values of 1.8 TPa, which is higher than the in – plane modulus for single crystal graph‐ ite. So the high stiffness and strength combined with low density implies that nanotubes could serve as ideal reinforcement in composite materials and provide them great potential in applica‐

The nanometer dimensions of CNTs, together with the unique electronic structure of a graphene sheet, make the electronic properties of these one-dimensional (1D) structures extraordinary. The one dimensional structure of CNTs helps them in making a good electric conductor. In a 3D conductor the possibility of scattering of electrons is large as these can scatter at any angle. Espe‐ cially notable is the fact that SWCNTs can be metallic or semiconducting depending on their structure and their band gap can vary from zero to about 2 eV, whereas MWCNTs are zero-gap metals. Thus, some nanotubes have conductivities higher than that of copper, while others be‐ have more like silicon. Theoretically, metallic nanotubes having electrical conductivity of 105

er than copper metal and hence can be used as fine electron gun for low weight displays. Due to the large diameter of MWCNTs, their transport properties approaches those of turbostatic graphite. Theoretical study also shows that in case of MWCNTs the overall behavior is deter‐ mined by the electronic properties of the external shell. Conductivities of individual MWCNTs

ermost shells or the presence of defects [7].The electronic properties of larger diameter MWCNTs approach those of graphite. Nanotubes have been shown to be superconducting at low temperatures. As probably CNTs are not perfect at ends and end defects like pentagons or heptagons are found to modify the electronic properties of these nanosystems drastically. There is great interest in the possibility of constructing nanoscale electronic devices from nanotubes and some progress is being made in this area. SWCNTs have been recently used to form conduct‐ ing and semiconducting layers (source, drain and gate electrodes) in thin films transistors. So the high electrical conductivity of CNTs makes them an excellent additive to impart electrical con‐ ductivity in otherwise insulating polymers. Their high aspect ratio means that a very low load‐ ing is needed to form a connecting network in a polymer compared to make them conducting.

CNTs are expected to be very good thermal conductors along the tube, but good insulators laterally to the tube axis. Experiments on individual tubes are extremely difficult but meas‐ urements show that a SWCNT has a room-temperature thermal conductivity along its axis

tions such as aerospace and other military applications.

194 Syntheses and Applications of Carbon Nanotubes and Their Composites

S/m can carry an electric current density of 4 × 109

have been reported to range between 20 and 2 × 107

**2.3. Thermal properties of CNTs**

**2.2. Electrical properties of CNTs**

106

A variety of synthesis methods now exist to produce carbon nanotubes. The three main pro‐ duction methods used for synthesis of CNTs are d.c. arc discharge, laser ablation and chemi‐ cal vapor deposition (CVD).

#### **3.1. d.c. arc discharge technique**

to

A/cm2 which is more than 1000 times great‐

S/m [6], depending on the helicities of the out‐

The carbon arc discharge method, initially used for producing C60 fullerenes, is the most common and perhaps easiest way to produce carbon nanotubes as it is rather simple to un‐ dertake. In this method two carbon rods placed end to end, separated by approximately 1mm, in an enclosure that is usually filled with inert gas (helium, argon) at low pressure (be‐ tween 50 and 700 mbar) as shown in Figure 1. Recent investigations have shown that it is also possible to create nanotubes with the arc method in liquid nitrogen [9]. A direct current of 50 to 100 A driven by approximately 20 V creates a high temperature (~4000K) discharge between the two electrodes. The discharge vaporizes one of the carbon rods (anode) and forms a small rod shaped deposit on the other rod (cathode). Large-scale synthesis of MWCNTs by a variant of the standard arc-discharge technique was reported by Ebbesen and Ajayan [10]. A potential of 18 V dc was applied between two thin graphite rods in heli‐ um atmosphere. At helium pressure of ~500 Torr, the yield of nanotubes was maximal of 75% relative to the starting graphitic material. The TEM analysis revealed that the samples consisted of nanotubes of two or more concentric carbon shells. The nanotubes had diame‐ ters between 2 and 20 nm, and lengths of several micrometers. The tube tips were usually capped with pentagons.

If SWCNT are preferable, the anode has to be filled with metal catalyst, such as Fe, Co, Ni, Y or Mo. Experimental results show that the width and diameter distribution depends on the composition of the catalyst, the growth temperature and the various other growth condi‐ tions. If both electrodes are graphite, the main product will be MWCNTs. Typical sizes for MWCNTs are an inner diameter of 1-3nm and an outer diameter of approximately 10nm. Because no catalyst is involved in this process, there is no need for a heavy acidic purifica‐ tion step. This means MWCNT can be synthesized with a low amount of defects.

from the arc discharge technique as shown in Figure 2. In this technique, the carbon material deposits on the chamber and cathode. The arcing process resulted in the formation of weblike deposits on the inner walls of the arc chamber. A typical SEM micrograph of such de‐ posit (Figure 2a) revealed the presence of SWNT bundles along with the amorphous carbon and catalyst particles. Rod-like microstructures aligned preferentially along the length of the cathode were also found at the tip of the cathode as shown in Figure 2b. The inset in Figure 2b shows the presence of graphitized carbon and sharp needle-like nanostructure when these rods are powdered. Upon detailed electron microscopic examination, these needles ex‐

Carbon Nanotubes and Their Composites http://dx.doi.org/10.5772/52897 197

**Figure 2.** (a) SEM micrograph of the chamber deposit showing the presence of long and flexible carbon nanotubes. (b) SEM micrograph of the cathode deposit showing the presence of rod-like microstructures. The inset figure shows the presence of needle-like nanostructures present within each microstructure. (c) TEM micrograph of a single needle-

like nanostructure (Reprinted with permission from Elsevier (11))

hibited the MWCNT structure with an outer diameter of 20–25 nm (Figure 2c).

**Figure 1.** Schematic diagram of dc-arc discharge set-up

In most of the studies, SWCNTs are synthesized using the dc-arc discharge process by filling the catalyst powder into a hole drilled in a graphite elctrode act as an anode and arcing takes place between this anode and a pure graphite based cathode in optimized chamber conditions. In one of the study by Mathur et al. [11] SWCNTs and MWCNTs were synthe‐ sized simultaneously in a single experiment selectively. In their experiment, However, in‐ stead of filling the catalyst powder into a hole drilled in a graphite electrode; they prepared a catalyst/graphite composite electrode. Coke powder, catalyst powder, natural graphite powder and binder pitch were thoroughly mixed together in a ball mill in appropriate pro‐ portions and molded into green blocks using conventional compression molding technique. A mixture of Ni and Co powders was used as catalyst. The green blocks were heated to 1200o C in an inert atmosphere to yield carbonized blocks with varying compositions of coke, natural graphite powder, Ni and Co. These electrodes were used as the anodes in the arcing process and a high density graphite block was used as the cathode. A uniform gap of 1– 2 mm was maintained between the electrodes during the arcing process with the help of a stepper motor for a stable arc-discharge (dc voltage 20–25 V, current 100–120 A, 600 torr he‐ lium).The SWCNTs yield was found to be doubled in this case.

#### *3.1.1. Characteristics of CNTs produced by d.c. arc discharge technique*

Arc discharge is a technique that produces a mixture of components and requires separating nanotubes from the soot and the catalytic metals present in the crude product. In this techni‐ que both SWCNT and MWCNT can be produced and it has been described by several re‐ searchers.

The scanning electron microscope (SEM) and transmission electron microscope (TEM) are generally used to observe the physical appearance of any carbon based soot. Similarly, Ma‐ thur et al. [11] used SEM and TEM for the observation of SWCNT and MWCNT produced from the arc discharge technique as shown in Figure 2. In this technique, the carbon material deposits on the chamber and cathode. The arcing process resulted in the formation of weblike deposits on the inner walls of the arc chamber. A typical SEM micrograph of such de‐ posit (Figure 2a) revealed the presence of SWNT bundles along with the amorphous carbon and catalyst particles. Rod-like microstructures aligned preferentially along the length of the cathode were also found at the tip of the cathode as shown in Figure 2b. The inset in Figure 2b shows the presence of graphitized carbon and sharp needle-like nanostructure when these rods are powdered. Upon detailed electron microscopic examination, these needles ex‐ hibited the MWCNT structure with an outer diameter of 20–25 nm (Figure 2c).

**Figure 1.** Schematic diagram of dc-arc discharge set-up

196 Syntheses and Applications of Carbon Nanotubes and Their Composites

lium).The SWCNTs yield was found to be doubled in this case.

*3.1.1. Characteristics of CNTs produced by d.c. arc discharge technique*

1200o

searchers.

In most of the studies, SWCNTs are synthesized using the dc-arc discharge process by filling the catalyst powder into a hole drilled in a graphite elctrode act as an anode and arcing takes place between this anode and a pure graphite based cathode in optimized chamber conditions. In one of the study by Mathur et al. [11] SWCNTs and MWCNTs were synthe‐ sized simultaneously in a single experiment selectively. In their experiment, However, in‐ stead of filling the catalyst powder into a hole drilled in a graphite electrode; they prepared a catalyst/graphite composite electrode. Coke powder, catalyst powder, natural graphite powder and binder pitch were thoroughly mixed together in a ball mill in appropriate pro‐ portions and molded into green blocks using conventional compression molding technique. A mixture of Ni and Co powders was used as catalyst. The green blocks were heated to

 C in an inert atmosphere to yield carbonized blocks with varying compositions of coke, natural graphite powder, Ni and Co. These electrodes were used as the anodes in the arcing process and a high density graphite block was used as the cathode. A uniform gap of 1– 2 mm was maintained between the electrodes during the arcing process with the help of a stepper motor for a stable arc-discharge (dc voltage 20–25 V, current 100–120 A, 600 torr he‐

Arc discharge is a technique that produces a mixture of components and requires separating nanotubes from the soot and the catalytic metals present in the crude product. In this techni‐ que both SWCNT and MWCNT can be produced and it has been described by several re‐

The scanning electron microscope (SEM) and transmission electron microscope (TEM) are generally used to observe the physical appearance of any carbon based soot. Similarly, Ma‐ thur et al. [11] used SEM and TEM for the observation of SWCNT and MWCNT produced **Figure 2.** (a) SEM micrograph of the chamber deposit showing the presence of long and flexible carbon nanotubes. (b) SEM micrograph of the cathode deposit showing the presence of rod-like microstructures. The inset figure shows the presence of needle-like nanostructures present within each microstructure. (c) TEM micrograph of a single needlelike nanostructure (Reprinted with permission from Elsevier (11))

The nature of the soot can be identified using Raman spectrometer and generally used for confirmation of the quality of CNTs. The nature of these two deposits obtained using this arc discharge process was confirmed from their respective Raman spectrum (Figure 3). The Raman spectrum of the chamber deposit showed the characteristic radial breathing and tan‐ gential bands at 165–183 and 1591 cm-1, respectively. The strong G-band at 1580 cm-1 in the Raman spectrum of the cathode deposit and its TEM image depicted in Figure 2c, confirmed that the cathode deposit predominantly contained MWCNTs. The prominent D-band around 1350 cm-1seen in both spectrum is attributed to the presence of disordered carbona‐ ceous material present in the as-prepared deposits. In their study, Mathur et al. [11] show that SWCNTs deposit on the arc chamber and MWCNTs on cathode deposit.

cene. The furnace provided a constant temperature zone of 18 cm in the centre as shown in

solution containing a mixture of ferrocene and toluene in particular proportion (0.077 g fer‐ rocene in 1 ml toluene) was injected in the reactor at a point where the temperature was

**Figure 4.** Schematic diagram of the CVD reactor along with the temperature profile (Reprinted with permission from

CNTs are produced in the form of big bundles using CVD technique. The physical appear‐ ance of the as produced CNTs is shown in Figure 5a and Figure 5b for SEM and TEM re‐ spectively. Figure 5a shows a big CNT bundle of length >300µm and the inset image of Figure 5a shows very good quality of uniform CNTs. Figure 5b shows the TEM image of as produced CNTs confirming the presence of MWCNT with metallic catalytic impurites either

Further confirmation of the quality and type of CNTs can be obtained using Raman spec‐ trometer as shown in Figure 6. This shows the tangential band at 1580 cm-1 (G band) of high intensity and the disorder-induced band at 1352 cm-1 (D band) as a perfect MWCNT nature [13]. The ratio of intensity of G to D band gives the information regarding the quality of the CNTs. The high value of intensity ratio of G/D band confirms the better quality of CNTs.

justed so that the maximum amount of precursor is consumed inside the desired zone.

C. Argon was also fed along with the charge as a carrier gas and its flow rate was ad‐

C. Once the temperature was reached, the

Carbon Nanotubes and Their Composites http://dx.doi.org/10.5772/52897 199

Figure 4. The reaction zone was maintained at 750o

*3.2.1. Characteristics of CNTs Produced by CVD Technique*

on the tip of the tube or in the cavity of of CNTs (inset of Figure 5b).

200o

Elsevier (12))

**Figure 3.** Room temperature Raman spectrum of the chamber and cathode deposit (Reprinted with permission from Elsevier (11))

#### **3.2. Chemical vapor deposition**

Pyrolysis of organometallic precursors such as metallocenes (e.g. ferrocene) in a furnace pro‐ vides a straight forward procedure to prepare CNT by CVD technique. Different hydrocar‐ bons, catalyst and inert gas combinations have been used by several researchers in the past for the growth of CNT by CVD technique. In one of the study by Mathur et al. [12], CNTs were grown inside the quartz reactor by thermal decomposition of hydrocarbons, e.g. tol‐ uene in presence of iron catalyst obtained by the decomposition of organometallic like ferro‐ cene. The furnace provided a constant temperature zone of 18 cm in the centre as shown in Figure 4. The reaction zone was maintained at 750o C. Once the temperature was reached, the solution containing a mixture of ferrocene and toluene in particular proportion (0.077 g fer‐ rocene in 1 ml toluene) was injected in the reactor at a point where the temperature was 200o C. Argon was also fed along with the charge as a carrier gas and its flow rate was ad‐ justed so that the maximum amount of precursor is consumed inside the desired zone.

**Figure 4.** Schematic diagram of the CVD reactor along with the temperature profile (Reprinted with permission from Elsevier (12))

#### *3.2.1. Characteristics of CNTs Produced by CVD Technique*

The nature of the soot can be identified using Raman spectrometer and generally used for confirmation of the quality of CNTs. The nature of these two deposits obtained using this arc discharge process was confirmed from their respective Raman spectrum (Figure 3). The Raman spectrum of the chamber deposit showed the characteristic radial breathing and tan‐ gential bands at 165–183 and 1591 cm-1, respectively. The strong G-band at 1580 cm-1 in the Raman spectrum of the cathode deposit and its TEM image depicted in Figure 2c, confirmed that the cathode deposit predominantly contained MWCNTs. The prominent D-band around 1350 cm-1seen in both spectrum is attributed to the presence of disordered carbona‐ ceous material present in the as-prepared deposits. In their study, Mathur et al. [11] show

**Figure 3.** Room temperature Raman spectrum of the chamber and cathode deposit (Reprinted with permission from

Pyrolysis of organometallic precursors such as metallocenes (e.g. ferrocene) in a furnace pro‐ vides a straight forward procedure to prepare CNT by CVD technique. Different hydrocar‐ bons, catalyst and inert gas combinations have been used by several researchers in the past for the growth of CNT by CVD technique. In one of the study by Mathur et al. [12], CNTs were grown inside the quartz reactor by thermal decomposition of hydrocarbons, e.g. tol‐ uene in presence of iron catalyst obtained by the decomposition of organometallic like ferro‐

Elsevier (11))

**3.2. Chemical vapor deposition**

that SWCNTs deposit on the arc chamber and MWCNTs on cathode deposit.

198 Syntheses and Applications of Carbon Nanotubes and Their Composites

CNTs are produced in the form of big bundles using CVD technique. The physical appear‐ ance of the as produced CNTs is shown in Figure 5a and Figure 5b for SEM and TEM re‐ spectively. Figure 5a shows a big CNT bundle of length >300µm and the inset image of Figure 5a shows very good quality of uniform CNTs. Figure 5b shows the TEM image of as produced CNTs confirming the presence of MWCNT with metallic catalytic impurites either on the tip of the tube or in the cavity of of CNTs (inset of Figure 5b).

Further confirmation of the quality and type of CNTs can be obtained using Raman spec‐ trometer as shown in Figure 6. This shows the tangential band at 1580 cm-1 (G band) of high intensity and the disorder-induced band at 1352 cm-1 (D band) as a perfect MWCNT nature [13]. The ratio of intensity of G to D band gives the information regarding the quality of the CNTs. The high value of intensity ratio of G/D band confirms the better quality of CNTs.

reactor, as the vaporized carbon condenses. The yield of nanotube synthesis by this process

Carbon Nanotubes and Their Composites http://dx.doi.org/10.5772/52897 201

The laser-ablation prepared samples usually contain >70% nearly endless, highly tangled ropes of SWCNTs along with nanoscale impurities. The SWCNTs formed in this case are bundled together by van der Waals forces. Laser vaporisation results in a higher yield for SWCNT synthesis and a narrower size distribution than SWCNTs produced by arc-dis‐ charge [15]. The nanotubes generated by the laser ablation and arc discharge technique are relatively impure, with presence of unwanted carbonaceous impurities and not operated at

Compared to other methods for synthesis of CNTs, more parameters, including tempera‐ ture, feeding gases, flow rate, catalyst components and heating rate are accessible to control the growth process in CVD. By changing the growth conditions, we can control the proper‐ ties of the produced CNTs such as length, orientation and diameter to some extent. It has been observed that the gas phase processes produces CNTs with fewer impurities and are most amenable to large scale processing. So the gas phase techniques such as CVD, for nanotube growth offer the greatest potential for scaling up nanotube production for process‐

During CNTs synthesis, impurities in the form of catalyst particles, amorphous carbon and non tubular fullerenes are also produced. The most of the production methods involve the use of catalysts which are normally transition metals (Fe, Co, Ni or Y), these remains in the

**Figure 7.** Schematic diagram of Laser ablation set-up for CNT synthesis

*3.3.1. Characteristics of CNTs produced by laser ablation technique*

higher scale; therefore, the overall production costs are high.

is roughly 70%.

ing of composites.

**4. Purification**

**Figure 5.** (a) SEM image of aligned CNT bundle synthesized by CVD technique.The inset figure shows the very good quality of uniform CNTs (b) TEM image of as grown MWCNT and inset image shows the MWCNTs with encapsulated metallic impurities

**Figure 6.** Raman spectrum of CVD-grown MWCNTs.

#### **3.3. Laser ablation**

In the laser ablation process, a pulsed laser vaporizes a graphite target containing small amounts of a metal catalyst [14] as shown in Figure 7. The target is placed in a furnace at roughly 1200°C in an inert atmosphere. The nanotubes develop on the cooler surface of the reactor, as the vaporized carbon condenses. The yield of nanotube synthesis by this process is roughly 70%.

**Figure 7.** Schematic diagram of Laser ablation set-up for CNT synthesis

#### *3.3.1. Characteristics of CNTs produced by laser ablation technique*

The laser-ablation prepared samples usually contain >70% nearly endless, highly tangled ropes of SWCNTs along with nanoscale impurities. The SWCNTs formed in this case are bundled together by van der Waals forces. Laser vaporisation results in a higher yield for SWCNT synthesis and a narrower size distribution than SWCNTs produced by arc-dis‐ charge [15]. The nanotubes generated by the laser ablation and arc discharge technique are relatively impure, with presence of unwanted carbonaceous impurities and not operated at higher scale; therefore, the overall production costs are high.

Compared to other methods for synthesis of CNTs, more parameters, including tempera‐ ture, feeding gases, flow rate, catalyst components and heating rate are accessible to control the growth process in CVD. By changing the growth conditions, we can control the proper‐ ties of the produced CNTs such as length, orientation and diameter to some extent. It has been observed that the gas phase processes produces CNTs with fewer impurities and are most amenable to large scale processing. So the gas phase techniques such as CVD, for nanotube growth offer the greatest potential for scaling up nanotube production for process‐ ing of composites.

### **4. Purification**

**a b**

**Figure 5.** (a) SEM image of aligned CNT bundle synthesized by CVD technique.The inset figure shows the very good quality of uniform CNTs (b) TEM image of as grown MWCNT and inset image shows the MWCNTs with encapsulated

In the laser ablation process, a pulsed laser vaporizes a graphite target containing small amounts of a metal catalyst [14] as shown in Figure 7. The target is placed in a furnace at roughly 1200°C in an inert atmosphere. The nanotubes develop on the cooler surface of the

metallic impurities

**Figure 6.** Raman spectrum of CVD-grown MWCNTs.

200 Syntheses and Applications of Carbon Nanotubes and Their Composites

**3.3. Laser ablation**

During CNTs synthesis, impurities in the form of catalyst particles, amorphous carbon and non tubular fullerenes are also produced. The most of the production methods involve the use of catalysts which are normally transition metals (Fe, Co, Ni or Y), these remains in the resulting nanotubes as spherical or cylindrical particles after experiments. Through careful control of process parameters one could minimize the formation of amorphous carbon parti‐ cles, so that the main impurities in CNTs are the remaining catalyst particles. However as most of these catalytic particles may either hide in internal cavity or stick firmly to the walls of CNTs, it is almost impossible to get rid of these effectively without damaging the nano‐ tubes. Several purification methods have been tried to overcome these impurities. In one of the study by Mathur et al. [11], SWCNT soot prepared by dc arc discharge process was puri‐ fied by removing various forms of impurities, such as amorphous carbon, graphitic nano‐ shells and catalyst particles present in the chamber deposit by applying a judicious combination of wet and dry chemical methods (acid treatment and oxidation). In this proc‐ ess, initially SWCNT soot were oxidized at 350 o C for 6h in air which remove the amorphous carbon followed by refluxing in HCl for the removal of metallic impurities like Ni and Co and again oxidation at 550o C for 30 min for the removal of graphitic nanoshell.The final product gives 97% purified SWCNT. The MWCNTs produced by CVD technique contains mainly ~10% metallic impruites which can be removed by heating it in the inert atmosphere at 2500o C in graphitization furnace. This process gives >99% pure MWCNTs and also helps in annealing out the defects in the tubes. This graphitization process at high temperature can also be useful for removal of impurities in the arc discharge produced SWCNTs soots with the combination of other purification steps.

obtained electrically conductive nanocomposites by dispersing CNT and PMMA in toluene, followed by the drop casting on substrate. The choice of solvent is generally made based on the solubility of the polymer. The solvent selection for nanotube dispersion also had a signif‐ icant influence on the properties of the nanocomposites and studied by Lau and co-workers [20].Their results demonstrates that, contrary to the general belief that small traces of CNTs alone will serve to strengthen the epoxy composites, the choice of the solvent used in the dispersion of CNTs also can have a significant impact. The change trend of the mechanical properties was found to be related to the boiling point of respective solvent used. In the samples observed in their study, only acetone-dispersed nanocomposites displayed im‐ provements in flexural strength over the pure epoxy, while ethanol and DMF used in CNTs dispersion actually countered the benefits of CNTs in the resulting nanocomposites.It is rea‐ sonable that, easier the solvent can evaporate, less solvent will remain to affect the curing reaction. Their results of thermogravimetric analysis (TGA) proved the existence of residual solvent in the resulting nanocomposites. Further evidence of the solvent influence was ob‐ tained by Fourier transform infrared (FTIR) spectra, which displayed the difference in the molecular structure of the final nanocomposites depending on the solvent used. The solvent influence is attributed to the different amount of unreacted epoxide groups and the extent of cure reaction in the manufacturing process. The presence of residual solvent may alter the reaction mechanism by restricting the nucleophile–electrophile interaction between the hardener and epoxy, henceforth, affect the cross-linking density and thus degrade the trans‐ port properties [21]and mechanical properties of the cured structures. The residual solvent may absorb some heat energy from the composite systems in the pre-cured process, causing a change in local temperature. Nanocomposites with other thermoplastic materials with en‐ hanced properties have been fabricated by solvent casting [16-18, 22]. The limitation of this method is that during slow process of solvent evaporation, nanotubes may tend to agglom‐ erate, that leads to inhomogeneous nanotube distribution in polymer matrix. The evapora‐ tion time can be decreased by dropping the nanotube/polymer suspension on a hot substrate (drop casting) [19]or by putting suspension on a rotating substrate (spin-casting) [23]. Du et al. [24] developed a versatile coagulation method to avoid agglomeration of CNTs in PMMA-CNT nanocompositses that involves pouring a nanotube/polymer suspension into an excess of solvent. The precipitating polymer chains entrap the CNT, thereby preventing

Carbon Nanotubes and Their Composites http://dx.doi.org/10.5772/52897 203

The alternative and second most commonly used method is melt mixing, which is mostly used for thermoplastics and most compatible with current industrial practices. This techni‐ que makes use of the fact that thermoplastic polymers softens when heated. Melt mixing uses elevated temperatures to make substrate less viscous and high shear forces to disrupt the nanotubes bundle. Samples of different shapes can then be fabricated by techniques such as compression molding, injection molding or extrusion. Andrews and co-workers [25] formed composites of commercial polymers such as high impact polystyrene, polypropy‐ lene and acrylonitrile–butadiene–styrene (ABS) with MWCNT by melt processing. Initially these polymers were blended in a high shear mixer with nanotubes at high loading level to

the CNT from bundling.

*5.1.2. Melt mixing method*
