**5. Nanocomposites**

Because of the high strength and stiffness of CNTs, they are ideal candidates for structural applications. For example, they may be used as reinforcements in high strength, low weight and high performance composites. Presently there is a great interest in exploiting the excit‐ ing properties of these CNTs by incorporating them into some form of polymer matrix.

#### **5.1. Composite fabrication techniques**

A large number of techniques have been used for the fabrication of CNT-polymer nanocom‐ posites based on the type of polymer used.

#### *5.1.1. Solvent casting*

The solution casting is most valuable technique to form CNTs/polymer nanocomposites. However, its use is restricted to polymers that are soluble. Solvent casting facilitates nano‐ tube dispersion and involves preparing a suspension of CNTs in the desirable polymer solu‐ tion via energetic agitation (magnetic stirring or sonication) and then allowing the solvent to evaporate to produce CNT-polymer nanocomposites. A lot of study is available in open lit‐ erature for the formation of CNT nanocomposites by this method [16-18]. Mathur et al. [18] cast the solutions of the MWCNT/polystyrene (PS)/toluene and MWCNT/ polymethyl meth‐ acrylate (PMMA)/toluene suspensions after sonication into a petry dish to produce nano‐ tubes composites with enhanced electrical and mechanical properties. Benoit et al. [19] 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 the CNT from bundling.

#### *5.1.2. Melt mixing method*

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‐

carbon followed by refluxing in HCl for the removal of metallic impurities like Ni and Co

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

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

Because of the high strength and stiffness of CNTs, they are ideal candidates for structural applications. For example, they may be used as reinforcements in high strength, low weight and high performance composites. Presently there is a great interest in exploiting the excit‐ ing properties of these CNTs by incorporating them into some form of polymer matrix.

A large number of techniques have been used for the fabrication of CNT-polymer nanocom‐

The solution casting is most valuable technique to form CNTs/polymer nanocomposites. However, its use is restricted to polymers that are soluble. Solvent casting facilitates nano‐ tube dispersion and involves preparing a suspension of CNTs in the desirable polymer solu‐ tion via energetic agitation (magnetic stirring or sonication) and then allowing the solvent to evaporate to produce CNT-polymer nanocomposites. A lot of study is available in open lit‐ erature for the formation of CNT nanocomposites by this method [16-18]. Mathur et al. [18] cast the solutions of the MWCNT/polystyrene (PS)/toluene and MWCNT/ polymethyl meth‐ acrylate (PMMA)/toluene suspensions after sonication into a petry dish to produce nano‐ tubes composites with enhanced electrical and mechanical properties. Benoit et al. [19]

C in graphitization furnace. This process gives >99% pure MWCNTs and also helps

C for 6h in air which remove the amorphous

C for 30 min for the removal of graphitic nanoshell.The final

ess, initially SWCNT soot were oxidized at 350 o

202 Syntheses and Applications of Carbon Nanotubes and Their Composites

the combination of other purification steps.

**5.1. Composite fabrication techniques**

posites based on the type of polymer used.

and again oxidation at 550o

**5. Nanocomposites**

*5.1.1. Solvent casting*

at 2500o

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 form master batches that were thereafter diluted with pure polymer to form lower mass fraction samples. Compression molding was used to form composite films. A similar combi‐ nation of shear mixing and compression molding is studied by many other groups dis‐ cussed elsewhere [16]. Also Meincke et al. [5] mixed polyamide-6, ABS and CVD-MWCNT in a twin screw extruder at 260o C and used injection molding to make nanocomposites. Tang et al. [26] used both compression and twin-screw extrusion to form CNT/polyethylene composites. Although melt-processing technique has advantages of speed and simplicity, it is not much effective in breaking of agglomeration of CNTs and their dispersion. Bhatta‐ charyya et al. [27] made 1 wt% CNT/polypropylene (PP) nanocomposites by melt mixing, but found that melt mixing alone did not provide uniform nanotube dispersion. Niu et el. [28] studied both methods to prepare polyvinylidene fluoride (PVDF)-CNT nanocomposites to study electrical properties and found it better in composites formed by solution casting.

MWCNT-polycarbonate composite using solvent casting followed by compression mould‐

Carbon Nanotubes and Their Composites http://dx.doi.org/10.5772/52897 205

The other less commonly known methods for CNT- polymer nanocomposites formation are twin screw pulverization [41], latex fabrication [42], coagulation spinning [43] and electro‐

**6. Challenges in MWCNT polymer composites fabrication and possible**

Although these fabrication methods helped to enhance the properties of CNT reinforced composites over neat polymer but there are several key challenges that hinders the excellent

Disperion of nanoscale filler in a matrix is the key challenge for the formation of nanocomposite. Dispersion involves separation and then stabilization of CNTs in a medium. The methods de‐ scribed above for the nanocomposites fabrication require CNTs to be well dispersed either in solvent or in polymer for maximizing their contact surface area with polymer matrix. As CNTs have diameters on nanoscale the entanglement during growth and the substantial van der Waals interaction between them forces to agglomerate into bundles. The ability of bundle for‐ mation of CNTs with its inert chemical structure makes these high aspect ratio fibers dissolving in common solvents to form solution quite impossible. The SEM of MWCNTs synthesized by CVD technique seems to be highly entangled and the dimensions of nanotube bundles is hun‐ dreds of micrometer. This shows several thousands of MWCNTs in one bundle as shown in Fig‐ ure 5a. These bundles exhibits inferior mechanical and electrical properties as compared to individual nanotube because of slippage of nanotubes inside bundles and lower aspect ratio as compared to individual nanotube. The aggregated bundles tend to act as defect sites which ad‐ versely affect mechanical and electrical properties of nanocomposites. Effective separation re‐ quires the overcoming of the inter-tube van der Waal attraction, which is anomalously strong in CNT case. To achieve large fractions of individual CNT several methods have been employed. The most effective methods are by attaching several functional sites on the surface of CNTs through some chemical treatment or by surrounding the nanotubes with dispersing agents such as surfactant. Thereafter the difficulty of dispersion can be overcome by mechanical/physical means such as ultrasonication, high shear mixing or melt blending. Another obstacle in dispers‐ ing the CNTs is the presence of various impurities including amorphous carbon, spherical full‐ erenes and other metal catalyst particles. These impurities are responsible for the poor

The second key challenge is in creating a good interface between nanotubes and the poly‐ mer matrix. From the research on microfiber based polymer composites over the past few

ing for the enhancement in the impact properties.

CNT properties to be fruitful in polymer composite formation.

properties of CNTs reinforced composites [45].

**6.2. Adhesion between CNTs and polymer**

phoretic deposition [44].

**solutions**

**6.1. Dispersion**

#### *5.1.3. In-situ polymerization*

In addition to solvent casting and melt mixing the other method which combines nanotubes with high molecular weight polymers is in-situ polymerization starting with CNTs and mono‐ mers. In-situ polymerization has advantages over other composite fabrication methods. A stronger interface can be obtained because it is easier to get intimate interactions between the polymer and nanotube during the growth stage than afterwards [29, 30].The most common in situ polymerization methods involve epoxy in which the monomer resins and hardeners are combined with CNTs prior to polymerizing [31]. Pande and coworkers [32] performed the insitu polymerization of MWCNT/ PMMA composites for the enhancement in flexural strength and modulus of composites. Li et al. [33] reported the fabrication and characterization of CNT/ polyaniline (PANI) composites. Xiao and Zhou [34]deposited polypyrrolre (PP) and poly(3 methylthiophene) (PMet) on the surface of MWCNTs by in situ polymerization. Saini et al. [35] reported fabrication process of highly conducting polyaniline (PANI)–(MWCNT) nanocompo‐ sites by in-situ polymerization. This material was used in polystyrene for the fabrication of MWCNT-PANI-PS blend for microwave absorption [36]. Moniruzzaman [17] reported many other studies of in-situ polymerization of CNTs with different polymers. Generally, in situ poly‐ merization can be used for the fabrication of almost any polymer composites containing CNT that can be non-covalently or covalently bound to polymer matrix. This technique enables the grafting of large polymer molecules onto the walls of CNT. This technique is particularly impor‐ tant for the preparation of insoluble and thermally unstable polymers, which cannot be process‐ ed by solution or melt processing.

Some studies have been also carried out using combined methods, such as solvent casting in conjunction with sonication, followed by melt mixing. Haggenmueller et al. [37] observed considerable nanotube dispersion in CNT-polymer nanocomposites using combination of solvent casting and melt mixing. Pande et al. [32, 38] also prepared MWCNT bulk compo‐ sites with PMMA and PS using a two-step method of solvent casting followed by compres‐ sion molding and obtained better electrical and mechanical properties. Singh et al. [39] also prepared MWCNT-LDPE composites using solvent casting followed by compression moulding and obtained better electrical conductivity. Jindal et al. [29, 40] prepared MWCNT-polycarbonate composite using solvent casting followed by compression mould‐ ing for the enhancement in the impact properties.

The other less commonly known methods for CNT- polymer nanocomposites formation are twin screw pulverization [41], latex fabrication [42], coagulation spinning [43] and electro‐ phoretic deposition [44].
