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Incorporation of carbon nanotubes (CNTs) into a polymer matrix is a very attractive way to combine the mechanical and electrical properties of individual nanotubes with the advan‐ tages of plastics. Carbon nanotubes are the third allotropic form of carbon and were synthe‐ sized for the first time by Iijima in 1991 [1]. Their exceptional properties depend on the structural perfection and high aspect ratio (typically ca 100-300). Two types of CNTs are dis‐ tinguished : single-walled CNTs (SWCNTs) consist of a single graphene sheet wrapped into cylindrical tubes with diameters ranging from 0.7 to 2nm and have lengths of micrometers while multi-walled CNTs (MWCNTs) consist of sets of concentric SWCNTs having larger diameters [2-5]. The unique properties of individual CNTs make them the ideal reinforcing agents in a number of applications [6-9] but the low compatibility of CNTs set a strong limi‐ tation to disperse them in a polymer matrix. Indeed, carbon nanotubes form clusters as very long bundles due to the high surface energy and the stabilization by numerous of π−π elec‐ tron interactions among the tubes. Non covalent methods for preparing polymer/CNTs nanocomposites have been explored to achieve good dispersion and load transfer [10-12]. The non-covalent approaches to prepare polymer/CNTs composites via processes such as solution mixing [13,14], melt mixing [14,15], surfactant modification [16], polymer wrapping [17], polymer absorption [18] and in situ polymerization [19, 20] are simple and convenient but interaction between the two components remains weak. Relatively uniform dispersion of CNTs can be achieved in polar polymers such as nylon, polycarbonate and polyimide be‐ cause of the strong interaction between the polar moiety of the polymer chains and the sur‐

© 2013 Beyou et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Beyou et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

face of the CNTs [21-24]. Moreover, it was found that MWNTs disperse well in PS and form a network-like structure due to π-stacking interactions with aromatic groups of the PS chains [25]. However, it is difficult to disperse CNTs within a non polar polymer matrix such as polyolefins. To gain the advantages of CNTs at its best, one needs: (i) high interfacial area between nanotubes and polymer; and, (ii) strong interfacial interaction. Unfortunately the solvent technique does not help much in achieving these targets and, as a result, a nano‐ composite having properties much inferior to theoretical expectations are obtained. For ex‐ ample, the mechanical properties of polyethylene (PE) reinforced by carbon nanotubes do not improve significantly because the weak polymer-CNT interfacial adhesion prevents effi‐ cient stress transfer from the polymer matrix to CNT [26-28]. A strategy for enhancing the compatibility between nanotubes and polyolefins consists in functionalising the sidewalls of CNT to introduce reactive moieties and to disrupt the rope structure. Functional moieties are attached to open ends and sidewalls to improve the solubility of nanotubes [29-32] while the covalent polymer grafting approaches, including 'grafting to' [33-36] and 'grafting from' [37-39] that create chemical linkages between polymer and CNTs, can significantly improve dispersion and change their rheological behaviour. First, methods used for processing CNTs-based nanocomposites and for the functionalisation of carbon nanotubes (CNTs) with polymers will be described. This is followed by a review of the surface chemistry of carbon nanotubes in order to perform their dispersion in polyolefin matrix. Finally, general trends of the viscoelastic properties of CNTs/ polyolefin composites are discussed.

In general, agitation is provided by magnetic stirring, shear mixing, reflux or ultrasonication. Sonication can be provided in two forms, mild sonication in a bath or high-power sonication using a tip or horn. An early example of solution based composite formation is described by Jin *et al* [43]. By this method, high loading levels of up to 50wt% and reasonably good dispersions were achieved. A number of papers have discussed dispersion of nanotubes in polymer solu‐ tions [44-46]. This can result in good dispersion even when the nanotubes cannot be dispersed in the neat solvent. Coleman *et al* [44] used sonication to disperse catalytic MWCNT in polyvi‐ nylalcohol/H2O solutions, resulting in a MWCNT dispersion that was stable indefinitely. Films could be easily formed by drop-casting with microscopy studies showing very good disper‐ sion. Cadek *et al* [46] showed that this procedure could also be applied to arc discharge MWCNTs, double walled nanotubes (DWNTs) and High-Pressure CO Conversion (HiPCO) SWCNTs. They also showed that this procedure could be used to purify arc-MWCNTs by se‐

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**Figure 1.** Schematic representation of different steps of polymer/CNTs composite processing: solution mixing (a);

While solution processing is a valuable technique for both nanotube dispersion and compo‐ site formation, it is completely unsuitable for the many polymer types that are insoluble. Melt processing is a common alternative method, which is particularly useful for dealing with thermoplastic polymers (Figure 1b). This range of techniques makes use of the fact that thermoplastic polymers soften when heated. Amorphous polymers can be processed above their glass transition temperature while semi-crystalline polymers need to be heated above their melt temperature to induce sufficient softening. Advantages of this technique are its speed and simplicity, not to mention its compatibility with standard industrial techniques [47, 48]. Any additives, such as carbon nanotubes can be mixed into the melt by shear mix‐ ing. However, Bulk samples can then be fabricated by techniques such as compression

lective sedimentation during composite production.

melt mixing (b); *in situ* polymerisation (c).

**1.2. Melt mixing**

### **1. Methods to process polymer/carbon nanotubes composites**

Similar to the case of carbon nanotube/solvent suspensions, pristine carbon nanotubes have not yet been shown to be soluble in polymers illustrating the extreme difficulty of overcom‐ ing the inherent thermodynamic drive of nanotubes to bundle [40]. Several processing meth‐ ods available for fabricating CNT/polymer composites based on either thermoplastic or thermosetting matrices mainly include solution mixing, melt blending, and in situ polymeri‐ sation (figure 1) [41, 42].

#### **1.1. Solution blending**

The most common method for preparing polymer nanotube composites has been to mix the nanotubes and polymer in a suitable solvent before evaporating the solvent to form a com‐ posite film (Figure 1a). One of the benefits of this method is that agitation of the nanotubes powder in a solvent facilitates nanotubes' de-aggregation and dispersion. Almost all solu‐ tion processing methods are based on a general theme which can be summarised as:


In general, agitation is provided by magnetic stirring, shear mixing, reflux or ultrasonication. Sonication can be provided in two forms, mild sonication in a bath or high-power sonication using a tip or horn. An early example of solution based composite formation is described by Jin *et al* [43]. By this method, high loading levels of up to 50wt% and reasonably good dispersions were achieved. A number of papers have discussed dispersion of nanotubes in polymer solu‐ tions [44-46]. This can result in good dispersion even when the nanotubes cannot be dispersed in the neat solvent. Coleman *et al* [44] used sonication to disperse catalytic MWCNT in polyvi‐ nylalcohol/H2O solutions, resulting in a MWCNT dispersion that was stable indefinitely. Films could be easily formed by drop-casting with microscopy studies showing very good disper‐ sion. Cadek *et al* [46] showed that this procedure could also be applied to arc discharge MWCNTs, double walled nanotubes (DWNTs) and High-Pressure CO Conversion (HiPCO) SWCNTs. They also showed that this procedure could be used to purify arc-MWCNTs by se‐ lective sedimentation during composite production.

**Figure 1.** Schematic representation of different steps of polymer/CNTs composite processing: solution mixing (a); melt mixing (b); *in situ* polymerisation (c).

#### **1.2. Melt mixing**

face of the CNTs [21-24]. Moreover, it was found that MWNTs disperse well in PS and form a network-like structure due to π-stacking interactions with aromatic groups of the PS chains [25]. However, it is difficult to disperse CNTs within a non polar polymer matrix such as polyolefins. To gain the advantages of CNTs at its best, one needs: (i) high interfacial area between nanotubes and polymer; and, (ii) strong interfacial interaction. Unfortunately the solvent technique does not help much in achieving these targets and, as a result, a nano‐ composite having properties much inferior to theoretical expectations are obtained. For ex‐ ample, the mechanical properties of polyethylene (PE) reinforced by carbon nanotubes do not improve significantly because the weak polymer-CNT interfacial adhesion prevents effi‐ cient stress transfer from the polymer matrix to CNT [26-28]. A strategy for enhancing the compatibility between nanotubes and polyolefins consists in functionalising the sidewalls of CNT to introduce reactive moieties and to disrupt the rope structure. Functional moieties are attached to open ends and sidewalls to improve the solubility of nanotubes [29-32] while the covalent polymer grafting approaches, including 'grafting to' [33-36] and 'grafting from' [37-39] that create chemical linkages between polymer and CNTs, can significantly improve dispersion and change their rheological behaviour. First, methods used for processing CNTs-based nanocomposites and for the functionalisation of carbon nanotubes (CNTs) with polymers will be described. This is followed by a review of the surface chemistry of carbon nanotubes in order to perform their dispersion in polyolefin matrix. Finally, general trends

78 Syntheses and Applications of Carbon Nanotubes and Their Composites

of the viscoelastic properties of CNTs/ polyolefin composites are discussed.

**1. Methods to process polymer/carbon nanotubes composites**

sation (figure 1) [41, 42].

**1.1. Solution blending**

Similar to the case of carbon nanotube/solvent suspensions, pristine carbon nanotubes have not yet been shown to be soluble in polymers illustrating the extreme difficulty of overcom‐ ing the inherent thermodynamic drive of nanotubes to bundle [40]. Several processing meth‐ ods available for fabricating CNT/polymer composites based on either thermoplastic or thermosetting matrices mainly include solution mixing, melt blending, and in situ polymeri‐

The most common method for preparing polymer nanotube composites has been to mix the nanotubes and polymer in a suitable solvent before evaporating the solvent to form a com‐ posite film (Figure 1a). One of the benefits of this method is that agitation of the nanotubes powder in a solvent facilitates nanotubes' de-aggregation and dispersion. Almost all solu‐

tion processing methods are based on a general theme which can be summarised as:

**2.** Mixing of nanotubes and polymer in solution by energetic agitation.

**3.** Controlled evaporation of solvent leaving a composite film.

**1.** Dispersion of nanotubes in either a solvent or polymer solution by energetic agitation.

While solution processing is a valuable technique for both nanotube dispersion and compo‐ site formation, it is completely unsuitable for the many polymer types that are insoluble. Melt processing is a common alternative method, which is particularly useful for dealing with thermoplastic polymers (Figure 1b). This range of techniques makes use of the fact that thermoplastic polymers soften when heated. Amorphous polymers can be processed above their glass transition temperature while semi-crystalline polymers need to be heated above their melt temperature to induce sufficient softening. Advantages of this technique are its speed and simplicity, not to mention its compatibility with standard industrial techniques [47, 48]. Any additives, such as carbon nanotubes can be mixed into the melt by shear mix‐ ing. However, Bulk samples can then be fabricated by techniques such as compression moulding, injection moulding or extrusion. However it is important that processing condi‐ tions are optimised for the whole range of polymer–nanotube combinations. High tempera‐ ture and shear forces in the polymer fluid are able to break the carbon nanotubes bundles and CNTs can additionally affect melt properties such as viscosity, resulting in unexpected polymer degradation [49]. Andrews and co-workers [50] showed that commercial polymers such as high impact polystyrene, polypropylene and acrylonitrile–butadiene–styrene (ABS) could be melt processed with CVD-MWCNT to form composites. The polymers were blend‐ ed with nanotubes at high loading level in a high shear mixer to form master batches. An example of using combined techniques was demonstrated by Tang *et al* [51]. High density polyethylene pellets and nanotubes were melted in a beaker, then mixed and compressed. The resulting solid was broken up and added to a twin screw extruder at 170°C and extrud‐ ed through a slit die. The resulting film was then compression moulded to form a thin film.

An innovative latex fabrication method for making nanotube/polymer composites has been used by first dispersing nanotubes in water (SWCNT require a surfactant, MWCNT do not) and then adding a suspension of latex nanoparticles [59,60]. For example, PEG-based amphi‐ philic molecule containing aromatic thiophene rings, namely, oligothiophene-terminated poly(ethylene glycol) (TN-PEG) was synthesized, and its ability to disperse and stabilize pristine carbon nanotubes in water was shown.This promising method can be applied to polymers that can be synthesised by emulsion polymerisation or formed into artificial latex‐

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Finally, to obtain nanotube/polymer composites with very high nanotube loadings, Vigolo et al [62] developed a "coagulation spinning" method to produce composite fibers compris‐ ing predominately nanotubes. This method disperses SWCNT using a surfactant solution, coagulates the nanotubes into a mesh by wet spinning it into an aqueous poly(vinyl alcohol) solution, and converts the mesh into a solid fiber by a slow draw process. In addition, Ma‐ medov et al [63] developed a fabrication method based on sequential layering of chemically modified nanotubes and polyelectrolytes to reduce phase separation and prepared compo‐

CNTs are considered ideal materials for reinforcing fibres due to their exceptional mechani‐ cal properties. Therefore, nanotube−polymer composites have potential applications in aero‐ space science, where lightweight robust materials are needed [64]. It is widely recognised that the fabrication of high performance nanotube−polymer composites depends on the effi‐ cient load transfer from the host matrix to the tubes. The load transfer requires homogene‐ ous dispersion of the filler and strong interfacial bonding between the two components [65]. A dispersion of CNT bundles is called "macrodispersion" whereas a dispersion of individu‐ al nonbundled CNT is called a nanodispersion [66, 67]. To address these issues, several strategies for the synthesis of such composites have been developed. Currently, these strat‐ egies involve physical mixing in solution, *in situ* polymerisation of monomers in the pres‐ ence of nanotubes, surfactant-assisted processing of composites, and chemical functionalisation of the incorporated tubes. As mentioned earlier, 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. For this purpose, two main approaches for the surface modification of CNTs are adopted i.e. covalent and noncovalent, depending on whether or not covalent bonding between the CNTs and the functional groups and/or modifier molecules is in‐ volved in the modification surface process. Figure 2 depicts a typical representation of such

The noncovalent attachment, controlled by thermodynamic criteria [68], which for some pol‐ ymer chains is called wrapping, can alter the nature of the nanotube's surface and make it

**2. Surface modifications of carbon nanotubes with polymers**

es, e.g., by applying high-shear conditions [61].

sites with SWCNT loading as high as 50 wt %.

surface modifications.

**2.1. Noncovalent attachment of polymers**

#### **1.3. In Situ Polymerisation**

This fabrication strategy starts by dispersing carbon nanotubes in vinyl monomers followed by polymerising the monomers (Figure 1c). This method produces polymer-grafted CNTs mixed with free polymer chains resulting in a homogeneous dispersion of CNTs. In situ rad‐ ical polymerisation was applied for the synthesis of PMMA-based composites by Jia *et al* [52] using a radical initiator and the authors suggested that π-bonds of the CNT graphitic network were opened by the radical fragments of initiator and therefore the carbon nano‐ structures could participate in PMMA polymerisation by acting as efficient radical scaveng‐ ers. Dubois et al [53] applied the in situ polymerization to olefin monomers by anchoring methylaluminoxane, a commonly used co-catalyst in metallocene-based olefin polymeriza‐ tion onto carbon nanotubes surface. Then, the metallocene catalyst was added to the surfaceactivated CNTs and the course of ethylene polymerization was found to be similar to the one without the presence of pristine MWCNTs. Epoxy nanocomposites comprise the majori‐ ty of reports using in situ polymerisation methods [54, 55], where the nanotubes are first dis‐ persed in the resin followed by curing the resin with the hardener. Zhu *et al* [56] prepared epoxy nanocomposites by this technique using end-cap carboxylated SWCNTs and an ester‐ ification reaction to produce a composite with improved tensile modulus (E is 30% higher with 1 wt % SWCNT).

#### **1.4. Novel methods**

Rather than avoid the high viscosities of nanotube/polymer composites, some researchers have decreased the temperature to increase viscosity to the point of processing in the solid state. Sol‐ id-state mechanochemical pulverisation processes (using pan milling [57] or twin-screw pul‐ verisation [58]) have mixed MWCNTs with polymer matrices. Pulverisation methods can be used alone or followed by melt mixing. Nanocomposites prepared in this manner have the ad‐ vantage of possibly grafting the polymer on the nanotubes, which account in part for the ob‐ served good dispersion, improved interfacial adhesion, and improved tensile modulus.

An innovative latex fabrication method for making nanotube/polymer composites has been used by first dispersing nanotubes in water (SWCNT require a surfactant, MWCNT do not) and then adding a suspension of latex nanoparticles [59,60]. For example, PEG-based amphi‐ philic molecule containing aromatic thiophene rings, namely, oligothiophene-terminated poly(ethylene glycol) (TN-PEG) was synthesized, and its ability to disperse and stabilize pristine carbon nanotubes in water was shown.This promising method can be applied to polymers that can be synthesised by emulsion polymerisation or formed into artificial latex‐ es, e.g., by applying high-shear conditions [61].

Finally, to obtain nanotube/polymer composites with very high nanotube loadings, Vigolo et al [62] developed a "coagulation spinning" method to produce composite fibers compris‐ ing predominately nanotubes. This method disperses SWCNT using a surfactant solution, coagulates the nanotubes into a mesh by wet spinning it into an aqueous poly(vinyl alcohol) solution, and converts the mesh into a solid fiber by a slow draw process. In addition, Ma‐ medov et al [63] developed a fabrication method based on sequential layering of chemically modified nanotubes and polyelectrolytes to reduce phase separation and prepared compo‐ sites with SWCNT loading as high as 50 wt %.
