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Polymer Nanocomposites Containing Functionalised Multiwalled Carbon NanoTubes: a Particular Attention to Polyolefin Based Materials

Written By

Emmanuel Beyou, Sohaib Akbar, Philippe Chaumont and Philippe Cassagnau

Submitted: March 9th, 2012 Published: May 9th, 2013

DOI: 10.5772/50710

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Introduction

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 advantages of plastics. Carbon nanotubes are the third allotropic form of carbon and were synthesized 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 distinguished : 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 limitation 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 electron 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 because of the strong interaction between the polar moiety of the polymer chains and the surface 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 nanocomposite having properties much inferior to theoretical expectations are obtained. For example, the mechanical properties of polyethylene (PE) reinforced by carbon nanotubes do not improve significantly because the weak polymer-CNT interfacial adhesion prevents efficient 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.

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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 overcoming the inherent thermodynamic drive of nanotubes to bundle [40]. Several processing methods available for fabricating CNT/polymer composites based on either thermoplastic or thermosetting matrices mainly include solution mixing, melt blending, and in situ polymerisation (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 composite 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 solution processing methods are based on a general theme which can be summarised as:

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

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

  3. Controlled evaporation of solvent leaving a composite film.

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 solutions [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 polyvinylalcohol/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 dispersion. 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 selective 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

While solution processing is a valuable technique for both nanotube dispersion and composite 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 mixing. However, Bulk samples can then be fabricated by techniques such as compression moulding, injection moulding or extrusion. However it is important that processing conditions are optimised for the whole range of polymer–nanotube combinations. High temperature 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 blended 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 extruded through a slit die. The resulting film was then compression moulded to form a thin film.

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 radical 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 nanostructures could participate in PMMA polymerisation by acting as efficient radical scavengers. Dubois et al [53] applied the in situ polymerization to olefin monomers by anchoring methylaluminoxane, a commonly used co-catalyst in metallocene-based olefin polymerization onto carbon nanotubes surface. Then, the metallocene catalyst was added to the surface-activated 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 majority of reports using in situ polymerisation methods [54, 55], where the nanotubes are first dispersed 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 esterification 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. Solid-state mechanochemical pulverisation processes (using pan milling [57] or twin-screw pulverisation [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 advantage of possibly grafting the polymer on the nanotubes, which account in part for the observed 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 amphiphilic 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 latexes, 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 comprising 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, Mamedov et al [63] developed a fabrication method based on sequential layering of chemically modified nanotubes and polyelectrolytes to reduce phase separation and prepared composites with SWCNT loading as high as 50 wt %.

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2. Surface modifications of carbon nanotubes with polymers

CNTs are considered ideal materials for reinforcing fibres due to their exceptional mechanical properties. Therefore, nanotube−polymer composites have potential applications in aerospace science, where lightweight robust materials are needed [64]. It is widely recognised that the fabrication of high performance nanotube−polymer composites depends on the efficient load transfer from the host matrix to the tubes. The load transfer requires homogeneous 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 individual 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 strategies involve physical mixing in solution, in situ polymerisation of monomers in the presence 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 involved in the modification surface process. Figure 2 depicts a typical representation of such surface modifications.

2.1. Noncovalent attachment of polymers

The noncovalent attachment, controlled by thermodynamic criteria [68], which for some polymer chains is called wrapping, can alter the nature of the nanotube’s surface and make it more compatible with the polymer matrix. Non-covalent surface modifications are based mainly on weak interactions, such as van der Waals, π−π and hydrophobic interactions, between CNTs and modifier molecules. Non-covalent surface modifications are advantageous in that they conserve sp2-conjugated structures and preserve the electronic performance of CNTs. The main potential disadvantage of noncovalent attachment is that the forces between the wrapping molecule and the nanotube might be weak, thus as a filler in a composite the efficiency of the load transfer might be low.

Non-covalent modification approaches typically use organic mediating molecules that range from low molecular weight molecules to supra- molecules to polymers.

Figure 2.

Different routes for nanotubes’ functionalisation: sidewall covalent functionalisation (a); defect-group covalent functionalisation (b); noncovalent polymer wrapping (c); noncovalent pi-stacking (d).

2.1.1. Polymer wrapping

O’Connell et al. [68] reported that nanotubes could be reversibly solubilised in water by noncovalently associating them with a variety of linear polymers such as polyvinyl pyrrolidone (PVP) and polystyrene sulfonate (PSS). They demonstrated that the association between the polymer and the nanotubes is robust, not dependent upon the presence of excess polymer in solution, and is uniform along the sides of the tubes (Figure 1c). A general thermodynamic driving force for such wrapping in an aqueous environment has been identified [68].

Conjugated luminescent polymer poly-{(m-phenylenevinylene)-co-[(1,5-dioctyloxy-p-phenylene)-vinylene]} (PmPV) and its derivatives [69-71] have been successfully used for the wrapping around nanotubes on account of stabilising noncovalent bonding interactions, presumably as a result of π–π stacking (Figure 1d) and van der Waals interactions between PmPV and the surfaces of the nanotubes. Star et al [72] also synthesised the Stilbenoid dendrimers, a hyperbranched variant of the PmPV polymer, which exhibits an appropriate degree of branching, and it was found to be more efficient at breaking up nanotube bundles, provided it is employed at higher polymer-to-nanotube ratios than was the “parent” PmPV polymer.

In addition, the behavior of single walled and multi walled carbon nanotubes in aqueous solutions of Gum Arabic, a natural polysaccharide, has been described by Nativ-Roth et al [73]. They observed that while the as-prepared nanotube powders contain highly entangled ropes and bundles, the dispersions are mainly composed of individual tubes suggesting that the ability of Gum Arabic to exfoliate the bundles, and stabilize individual tubes in aqueous dispersions, can be utilized in the preparation of carbon nanotube-polymer composites. In the latter case, the dispersing polymer acts as a compatabilizer and as an adhesion promoter leading to strengthening of the matrix-nanotube interface.

It is clear from these accounts that noncovalent functionalisation of carbon nanotubes can be achieved without disrupting the primary structure of the nanotubes themselves.

2.1.2. Polymer absorption

Xia et al [74] has described a method to prepare polymer-encapsulated MWCNTs : it has been successfully prepared through ultrasonically initiated in situ emulsion polymerisations of n-butyl acrylate (BA) and methyl methacrylate (MMA) in presence of MWCNT. By employing the multiple effects of ultrasound, i.e., dispersion, pulverizing, activation, and initiation, the aggregation and entanglement of carbon nanotubes in aqueous solution can be broken down, while in situ polymerization of monomer BA or MMA on the surface of MWCNTs proceeds and the MWCNTs are coated by the formed polymer.

The hydrophilic regions of surfactants interact with polar solvent molecules, and the hydrophobic regions can adsorb onto nanotube surfaces [75]. Thus, the process of dispersing CNTs from aggregates, bundles, or ropes into separated individual CNTs depends strongly on the length of the hydrophobic regions and the types of hydrophilic groups of the surfactant. A topological, noncovalent solution to improving the dispersion of SWNTs by encasing them in cross-linkable surfactant micelles was demonstrated by Kang and Taton [16]. SWCNTs were dispersed in the dimethylformamide (DMF) solutions of amphiphilic poly(styrene)-block-poly(acrylic acid) copolymer. Water was added to the solutions and the poly(styrene)-block-poly(acrylic acid) copolymer wrapped the SWCNTs and formed micelle. Then the PAA blocks of the micellar shells were permanently crosslinked by addition of a water-soluble diamine linker and a carbodiimide activator. This encapsulation significantly enhances the dispersion of SWCNTs in a wide variety of polar and nonpolar solvents and polymer matrices [76]. Encapsulated SWNTs can be used as an alternative starting material to pure SWNTs for the production and investigation of nanotube composite materials.

2.2. Covalent attachment of polymers

Functionalisation of carbon nanotubes with polymers is a key issue to improve the interfacial interaction between CNTs and the polymer matrix when processing polymer/CNT nanocomposites. The covalent reaction of CNT with polymers is important because the long polymer chains help to dissolve the tubes into a wide range of solvents even at a low degree of functionalisation. There are two main methodologies for the covalent attachment of polymeric substances to the surface of nanotubes, which are defined as “grafting to” and ‘grafting from’ methods [76, 77]. The former relies on the synthesis of a polymer with a specific molecular weight followed by end group transformation. Subsequently, this polymer chain is attached to the graphitic surface of CNT. A disadvantage of this method is that the grafted polymer contents are limited because of high steric hindrance of macromolecules. The ‘grafting from’ method involved the immobilisation of initiators onto the substrate followed in situ surface polymerization to generate grafted polymer chains. Because the covalent attachment of the surface modifiers involves the partial disruption of the sidewall sp2 hybridization system, covalently modified CNTs inevitably lose some degree of their electrical and/or electronic performance properties [78].

2.2.1. ‘Grafting to’ method

Since the curvature of the carbon nanostructures imparts a significant strain upon the sp2 hybridised carbon atoms that make up their framework, the energy barrier required to convert these atoms to sp3 hybridisation is lower than that of the flat graphene sheets, making them susceptible to various addition reactions. Therefore, to exploit this chemistry, it is only necessary to produce a polymer-centred transient in the presence of CNT material. Alternatively, defect sites on the surface of oxidized CNTs, as open-ended nanostructures with terminal carboxylic acid groups, allow covalent linkages of oligomer or polymer chains. So, the ‘grafting to’ method involves the chemical reaction between as-prepared or commercially available polymers with reactive end groups and nanotubes’ surface functional groups or the termination of growing polymer radical, cation and anion formed during the polymerization of various monomers in the presence of CNTs or the deactivation of living polymer chain ends with the CNT surface.

For example, oxidized SWCNTs were grafted with amino-terminated poly (N-isopropylacrylamide) (PNIPAAm) by carbodiimide-activated reaction, which yielded a 8wt% polymer content[77]. In a different approach, oxidized MWCNTs were attached ontopolyacrylonitrile(PAN) nanoparticles through the reaction of the reduced cyano-groups of the polymer and the carboxylic moieties of CNT surface [79]. In addition, the amidation reaction was used for grafting of oligo-hydroxyamides to MWCNTs as described in figure 3 [80].

Ester-based linkages have been used by Baskaran et al. [81] by performing the reaction of hydroxy-terminated PS with thionyl chloridetreated MWCNTs, resulting in a hybrid containing 86wt% of CNTs. The esterification reaction was also used for grafting polyethylene glycol(PEG) chains to acylchloride-activated SWCNTs [82]. Silicone-functionalised CNT derivatives were prepared by opening terminal epoxy groups of functionalised polydimethylsiloxanes (PDMS) by the carboxylic groups of acid-treated MWCNTs [83]. Another example of the “grafting to” approach has been reported by Qin et al. [84] through the grafting of polystyrene with azide end group onto SWCNTs (Figure 4).

Figure 3.

Synthesis of oligo-hydroxyamide-grafted MWNT. Reproduced from [80] with permission of Elsevier.

Figure 4.

reaction of azide-terminatedpolystyreneonto CNTs surface. Reproduced from [80] with permission of ACS publications.

In an analogous approach, alkyne-decorated SWCNTs and PS-N3were coupled via [3+1] Huisgen cycloaddition between the alkyne and azide end groups [85]. A new method was developed by Hung et al. [86] for preparing polystyrene-functionalized multiple-walled carbon nanotubes (MWNTs) through the termination of anionically synthesized living polystyryllithium with the acyl chloride functionalities on the MWNTs. The acyl chloride functionalities on the MWNTs were in turn obtained by the formation of carboxyls via chemical oxidation and their conversion into acyl chlorides (Figure 5).

Lou et al. [87] reported the radical grafting of polyvinylpyridine chains onto the surface of nanotubes through the thermolysis of poly (2-vinylpyridine) terminated with a radical-stabilizing nitroxide (Figure 6), resulting in grafting densities up to 12 wt.-%.

2.2.2. ‘Grafting from’ technique

Mostly, it involves the polymerisation of monomers from surface-derived initiators on either MWCNTs or SWCNTs. These initiators are covalently attached using the various functionalisation reactions developed for small molecules [77]. Then, the polymer is bound via in situ radical, cationic, anionic, ring opening and condensation polymerizations. The advantage of ‘grafting from’ approach is that the polymer growth is not limited by steric hindrance, allowing high molecular weight polymers to be efficiently grafted as well as quite high grafting density [9].

Figure 5.

substitution reaction of living polystyryllithium anions with acyl choride-modified CNTs. Reproduced from [86] with permission of John Wiley and Sons.

Figure 6.

Radical grafting of TEMPO-end capped PVP to MWCNTs. Reproduced from [87] with permission of Elsevier.

Figure 7.

Anionic polymerisation of styrene onto carbon nanotubes

For example, Viswanathan et al [88]. have developed a procedure based on the SWCNT surface treatment with butyllithium providing initiating sites for the anionic polymerization of styrene (Figure 6).

The latter procedure eliminates the need for nanotube pretreatment prior to functionalization and allows attachment of polymer molecules to pristine tubes without altering their original structure.

In addition, polyethylenimine has been grafted onto the surface of MWNTs by performing a cationic polymerization of aziridine in the presence of amine-functionalized MWNTs (NH2–MWNTs [89]. The grafting of PEI was realized through two mechanisms, the activated monomer mechanism (AMM) or the activated chain mechanism (ACM), by which protonated aziridine monomers or the terminal iminium ion groups of propagation chains, respectively, are transferred to amines on the surface of MWNTs [89].

Bonduel et al. [90] reported the homogeneous surface coating of long carbon nanotubes by in situ polymerization of ethylene as catalyzed directly from the nanotube surface-treated by a highly active metallocene-based complex. It allowed for the break-up of the native nanotube bundles leading, upon further melt blending with HDPE, to high-performance polyolefinic nanocomposites [90]. In another attempt, an easy method for preparing polystyrene-grafted multi-walled carbon nanotubes (MWCNTs) with high graft yields was developed by using free radical graft polymerisation from photoinduced surface initiating groups on MWCNTs [91]. Conventional microscopy, including optical, atomic force, sanning electronic microscopy (SEM), and transmission electronic microscopy (TEM), reveal the dispersion state or quality of CNTs within a very limited area of a given nanofiller composite [67]. High resolution-TEM image of the MWCNTs-PS (Figure 8) shows that the surface of the MWCNTs-PS is covered with 4–5nm thick amorphous PS layers while the wall surface of purified MWCNTs was smooth, without any detectable polymer Layer [91].

Figure 8.

HR-TEM image of PS-g-MWCNT. Reprinted from [91] with permission of Elsevier.

Controlled radical polymerisation techniques such as nitroxide mediated polymerisation (NMP), atom transfer radical polymerisation (ATRP) and radical adition fragmentation transfer (RAFT) have been also used to graft polymer chains from the CNT surface [92-103] (see figure 9 as example).

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3. Carbon nanotubes nanocomposites based on Polyolefins

Polyethylene (PE) is one of the most widely used commercial polymer due to the excellent combination of low coefficient of friction, chemical stability and excellent moisture barrier properties [104]. The combination of a soft polymer matrix such as PE with nanosized rigid filler particles may provide new nanocomposite materials with largely improved modulus and strength. To improve the stiffness and rigidity of PE, CNTs can be used to make CNT/PE composites [104-107]. The mechanical properties of polyethylene (PE) reinforced by carbon nanotubes do not improve significantly because the weak polymer-CNT interfacial adhesion prevents efficient stress transfer from the polymer matrix to CNT [108-110]. The lack of functional groups and polarity of PE backbone results in incompatibility between PE and other materials such as glass fibres, clays, metals, pigments, fillers, and most polymers. A strategy for enhancing the compatibility between nanotubes and polyolefins consists in functionalising the sidewalls of CNT with polymers either by a ‘grafting to’ or a ‘grafting from’ approach. As discussed before, the “grafting from” approach involves the growth of polymers from CNT surfaces via in situ polymerisation of olefins initiated from chemical species immobilised on the CNT. As an example, Ziegler-Natta or metallocene catalysts for ethylene polymerisation can be immobilised on nanotubes to grow PE chains from their surface. However, covalent linkages or strong interactions between PE chains and nanotubes cannot be created during polymerisation [90, 111-113]. The “grafting to” technique involves the use of addition reactions between the polymer with reactive groups and the CNT surface. However, the synthesis of end-functionalized polyethylene (PE), which is necessary in the “grafting to” approach, is difficult [114]. Another promising route for a chemical modification of MWCNTs by PE is to use free radical initiators such as peroxides. The general mechanism of free radical grafting of vinyl compound from hydrocarbon chains detailed by Russell [115], Chung [116] and Moad [117] seems to express a widespread view. The grafting reaction starts with hydrogen abstraction by alkoxyl radicals generated from thermal decomposition of the peroxide. Then, the active species generated onto the hydrocarbon backbone react with unsaturated bonds located on the MWCNTs surface. This chemical modification is thus conceivable during reactive extrusion because the radicals’ lifetimes (in the range of few milliseconds) are compatible with typical residence time in an extruder (around one minute).

Figure 9.

ATRP ‘grafting from’ modification approach. Reproduced from [92] with permission of ACS publications.

3.1. Radical grafting of polyethylene onto MWCNTs

The main drawback of the free radical grafting is the low selectivity of the radical center, specially at high temperatures (in the range of 150-200°C, required for extrusion of polyethylene), leading to side reactions such as coupling and chain scission [115, 118]. Moreover, performing this chemical modification by reactive processing brings in many constraints inherent to the processing (e.g. short reaction time, viscous dissipation and high temperature). For instance, the difference of viscosity between the monomer and the molten polymer could enhance these side reactions. So, to separate these physical influences from the chemical modification, the grafting reaction can be predicted with a model compound approach based on a radical grafting reaction between peroxide-derived alkoxyl radicals, and a low molar mass alkane representing characteristics moieties of PE.

Figure 10.

General reactive pathways of free radical grafting of pentadecane onto MWCNTs ; Reproduced from [119] with permission of Elsevier.

3.1.1. A model compound approach through the use of pentadecane

Covalent functionalization of pentadecane-decorated multiwalled carbon nanotubes (MWCNTs) has been studied as a model compound approach for the grafting of poly (ethylene-co-1-octene) onto MWCNTs by reactive extrusion [119]. It was accomplished through radical addition onto unsaturated bonds located on the MWCNTs' surface using dicumyl peroxide as hydrogen abstractor. Pentadecane (C15H32) has been resorted as model for polyethylene because high boiling points of long chain alkanes permit study under high temperature conditions, typically over 150°C. It also gives clues about low viscosity at 150°C, on top of that the formed products in the grafting experiment can hence be analysed more easily than in the polymer melt. Figure 10 sums up main reactive pathways of free radical grafting of pentadecane onto MWCNTs with dicumyl peroxide as initiator. The hydrogen abstraction reactions from alkyl hydrocarbon bonds was studied starting from the reaction of DCP-derived radicals with pentadecane.

Figure 11.

Raman spectra of: p-MWCNTs (a) and penta-g-MWCNTs (b).

However, the alkoxy radicals can undergo additional reactions including β-scission leading to the formation of methyl radicals [117]. These latter preferentially induce coupling reaction (Fig. 10, route b and h) or attack onto the sp2 carbon of the MWCNTs (Fig. 10, route g) whereas cumyloxyl radicals are more prone to hydrogen abstraction from pentadecane [120]. The formed pentadecyl radicals through hydrogen abstraction are able to react with MWCNTs by radical addition onto sp2 carbon of theMWCNTs (Fig.10, main route a) and with other radical species via the common radical coupling reactions (Fig. 10, routes d1 and b). According to the results of Johnston [121,122], based on a study of the crosslinking reaction of poly (ethylene-co-1-octene) in the presence of DCP at 160 °C, coupling reactions are four times more prone to happen than scission reactions so the authors assumed that pentadecyl radicals do not undergo scission reactions [119]. Direct evidence for covalent sidewall functionalization has been found by Raman spectroscopy [123-124]. G band is a characteristic feature of the graphitic layers and corresponds to the tangential vibration of the carbon atoms. The second characteristic mode is a typical sign for defective graphitic structures (D band). The AD/AG ratio, which was defined as the intensity ratio of the D-band to G-band of CNTs, directly indicates the structural changes in nanotubes. Some authors used D to G area ratios (AD/AG) rather than intensity [125] which is a better indicator. The relatively high area ratio of the G band relative to D band for penta-g-MWCNT (AD/AG = 1.51) in comparison with that of p-MWCNT (i.e. (AD/AG =1.20) could be designated as an indicator of grafting species. The ratio between the G band and D band is a good indicator of the changes in chemistry of CNTs. Interestingly, Raman spectra of p-MWCNTs and penta-g-MWCNTs (Figure11a and 11b, respectively) showed two main peaks around 1350cm−1 (D band) and 1586cm−1 (G band). The relatively high intensity of the G band relative to D band (I G/I D=1.25) for penta-g-MWCNT sample in comparison with that of p-MWCNT (i.e. I G/I D=0.95) was designated as an indicator of grafting species [119].

It is important to determine whether the results of a CNT surface modification process agree qualitatively with expectations, and equally important is the need for a quantitative assessment of the extent and nature of surface modifications. The course of the generated radical species and the extent of the grafting reaction in regards to the DCP concentration and temperature can be studied through gas chromatography and thermogravimetric analysis (TGA) [119, 126]. TGA permits measurement of the total weight fraction of surface modifiers introduced onto the surfaces of CNTs if the surface-modified CNTs are free of impurities. Indeed, it is well known that heating functionalized nanotubes in an inert atmosphere removes the organic moieties and restores the pristine nanotubes structure. TGA can indicate the degree of surface modification because the type and quantity of surface modifier is identified. It was found that the higher grafting density, as high as 1.46 mmol/g, was obtained at 150°C. At higher temperatures, the grafting density decreases because the β-scission reaction of cumyloxyl radical accelerates as the temperature increases, leading to the formation of methyl radicals. These latter preferentially react by combination whereas cumyloxyl radicals are more prone to hydrogen abstraction from pentadecane. At 150°C, for initiator concentration higher than 3wt%, the grafting density decreases from 1.464mmol/g to 0.371mmol/g upon increasing DCP concentration up to 5%. Thus, to get high grafting efficiency, one should opt for optimal initiator concentration, i.e. 3wt%, and choose the most favourable reaction temperature, i.e. 150°C. Incorporation of TEMPO as radical scavenger in the grafting reaction of pentadecane onto MWCNTs serves two purposes: firstly, it actively suppresses radical combination reactions and hence promotes pentadecyl radicals’ addition to nanotubes (~16% increase in grafting density); and secondly, it effectively changes the polarity balance of the grafted species, making pentadecane and TEMPO functionalised nanotubes soluble in solvents such as THF and chloroform [126].

3.1.2. Synthesis of PE grafted carbon nanotubes via peroxide

To make full use of the strength of carbon nanotubes in a composite, it is important to have a high-stress transfer at the matrix-nanotube interface via strong chemical bonding, as discussed by Mylvaganam et al [127]. They have investigated the possible polyethylene-nanotube bonding with the aid of a quantum mechanics analysis with the polyethylene chains represented by alkyl segments, and the nanotubes modeled by nanotube segments with H atoms added to the dangling bonds of the perimeter carbons. Their study has predicted (i) covalent bond formation between alkyl radicals and carbon nanotubes is energetically favourable; and, (ii) this reaction may take place at multiple sites of nanotubes [126]. Hence one way to improve the load transfer of carbon nanotubes/PE composite via chemical bonds at the interface is to use free-radical generators such as peroxide or incorporate nanotubes by means of in situ polymerisation.

Figure 12 sums up main reactive pathways of free radical grafting of PE onto MWCNTs with dicumyl peroxide as initiator and TEMPO as radical scavenger.

Figure 12.

Reaction scheme for PE grafting onto MWCNTs with TEMPO as a radical scavenger. Reproduced from [128] with permission of John Wiley and Sons.

Figure 13.

TEM images of p-MWCNTs (1) and PE-g-MWCNTs (2). Reproduced from [128] with permission of John Wiley and Sons.

The formed PE-based radicals are able to react with MWCNTs by radical addition onto sp2 carbon of the MWCNTs (Figure 12) and with other radical species via the common radical coupling reactions. As discussed using a model compound approach (section 2.1.1), the presence of TEMPO radicals creates competitive combinations reactions (that are actually reversible reactions) which may favour the addition of PE-based radicals to MWCNTs (Figure 12). Before carrying out thermogravimetric analysis to gain a quantitative picture of the extent of nanotubes’ functionalisation, the adsorbed (non-covalently attached) PE chains were removed from the grafted ones (covalently attached) by extensive washings with dichlorobenzene (DCB). PE-grafted onto MWCNTs are well known to degrade at 300-540°C, which are nearly the same temperatures as pure PE reactants and the weight of grafted PE is estimated to be in the range 20-24% depending on the experimental procedure [128]. The corresponding grafting densities, calculated using a specific surface area (SSA) of 225m2/g for MWCNTs [115, 129] are varying from 1.1mg.m-2 to 1.4mg.m-2. LDPE grafting density on nanotubes is 1.1mg.m-2 while incorporation of TEMPO raises the grafting density to 1.4mg.m-2 [128]. Then, it is usual to examine the morphological structures of p-MWCNTs and PE-grafted MWCNTs by transmission electron microscopy (TEM). In these experiments, a few drops of dilute solutions of PE-grafted nanotubes in hot DCB are initially deposited onto a carbon-coated copper grid and further observed in a dried state after evaporation of the solvent (Figure 13).

A “grafting to” approach based on a radical process can also involve a polymer with reactive end groups.

3.1.3. Synthesis of PE grafted carbon nanotubes via end functionalised PE

Recently, D’Agosto and Boisson [130-134] developed new strategies that rely on a one step in situ functionalisation reaction within an ethylene polymerisation reactor to introduce a variety of functional groups including alkoxyamine [130-132] and thiol [132,134] functions at the end of polyethylene chains. Di-polyethylenyl magnesium compound (MgPE2) were prepared using a neodymium metallocene complex which catalysed polyethylene chain growth on magnesium compounds. MgPE2 was in situ reacted with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical or elemental sulphur to provide a macroalcoxyamine (PE-TEMPO) and polysulphur based product (PE-Sn-PE) respectively. PE-SH was obtained by simple reduction of PE-Sn-PE. According to these results, a strategy based on the use of those polyethylenes was investigated to generate radical-terminated chains formed either by thermal loss of a nitroxide (PE-TEMPO) or H-abstraction onto a thiol (PE-SH) and graft them onto CNTs (Figure 14) [128].

Indeed, Lou [87] showed an efficient attachment of poly(2-vinylpyridine) (P2VP) end-capped by TEMPO to CNT sidewalls by heating of TEMPO-terminated P2VP. Using the same strategy Liu [137] functionalized shortened CNTs with PS and poly[(tert-butyl acrylate)-b-styrene] and Wang [138] grafted poly(4-vinylpyridine-b-styrene) onto CNTs. In figure 14a, the homolytic cleavage of TEMPO-terminated PE leads to the formation of stable nitroxyl radicals and PE radicals. The reversible termination of the polyethylene chain is the key step for reducing the overall concentration of the radical chain end. The extremely low concentration of reactive chain ends is expected to minimize irreversible termination reactions, such as combination or disproportionation [135] (Figure 14a). Thiol-terminated polyethylene has been also grafted onto CNTs using a similar procedure. The thiol based compounds are widely used for controlling molar mass in free radical polymerizations via a chain transfer process. The chain transfer process displays two contiguous steps: transfer of the thiyl hydrogen to the growing polymer chain followed by re-initiation, whereby a thiyl radical adds to a monomeric double bond. In the presence of DCP-derived radicals and MWCNTs, thiyl radicals are formed and are expected to react by radical addition onto sp2 carbon of the MWCNTs. For both samples PE-TEMPO and PE-SH, the weight loss observed by thermogravimetric analysis (TGA) has been increased to 36% and 34%, respectively despite their low molar masses (e.g. 1400g/mol and 980g/mol) in comparison with that of LDPE (weight loss = 20-24% ; Mw = 90000g/mol ; see section 2.1.2.). These results indicated that the use of short end-functionalized PE chains has permitted a significant increase of the grafting density (e.g. 1.78 and 2.80μmol.m-2, respectively). These values are approximately increased by two orders of magnitude in comparison with the LDPE grafting density (e.g. 0.012μmol.m-2) suggesting that longer polymer chains cover a larger surface decreasing the grafting density, as previously described by Jerome et al [87,136] for the attachment of poly(2-vinylpyridine) (P2VP) and polystyrene (PS) onto MWCNTs. Indeed, they observed that PS grafting density decreases from 0.045μmol.m-2 to 0.01μmol.m-2 by increasing the molecular weight of PS-TEMPO from 3000g/mol to 30000g/mol [138]. The TEM observations (Figure 15) were consistent with the TGA results : the grafted polymer contents can be highered by using end-functionalized PE [129].

Figure 14.

Reaction scheme for end functionalised PE grafting onto MWCNTs: (a) via PE-TEMPO; (b) via PE-SH. Reproduced from [128] with permission of John Wiley and Sons.

Figure 15.

TEM images of PEf-TEMPO-g-MWCNTs (1) and PEf-SH-g-MWCNTs (2). Reproduced from [128] with permission of John Wiley and Sons.

3.2. Radical grafting of polypropylene on carbon nanotubes

Polypropylene (PP) is a widely used commercial polymer due to the excellent combination of mechanical resistance, chemical stability and excellent moisture barrier properties [104]. Although physical blending with CNTs is an economic way to modify polypropylene performance, compatibilizing agents are necessary for creating strong interface between filler particles and the polymer phase. Maleic anhydride grafted polypropylene (MA-g-PP) is often used as a compatibilizer which can improve the PP/CNTs composite properties by strong hydrogen bonding between hydroxyl groups located on the acidic-treated CNTs surface and anhydride groups of MA-g-PP [139, 140]. Recently, an original and simple method for promoting mobility sensitivity of carbon nanotubes (CNTs) to an external stress field in polypropylene (PP) matrix was developed [141]. In particular, an interfacial melt reaction initiated by free radicals were used as a tool to prepare PP/CNTs nanocomposites. The presence of tetrakis(phenylmethyl)thioperoxydi(carbothioamide) (TBzTD) increased the interfacial reaction between the PP chains and the CNTs. In addition, the grafted TBzTD to PP backbone could form a physical interaction with CNTs via a π–π interaction [141]. According to their previous results [119, 126] based on a study of the radical grafting of polyethylene derivatives onto MWCNTs, Farzi et al. investigated MWCNTs' sidewall functionalization by tetramethylpentadecane and PP in the presence of 1wt% DCP at 160°C.

3.2.2. A model compound approach through the use of 2,6,10,14-tetramethylpentadecane

Simlilarly to the pentadecane grafting procedure (see section 2.1.1.), 2,6,10,14-tetramethylpentadecane (TMP, C19H40) has been used as model for the grafting reaction of PP onto MWCNTs [129]. Thermolysis of dicumyl peroxide initiator performed in TMP and in presence of MWCNTs is depicted in Figure 16.

Figure 16.

Reaction scheme for the addition of TMP onto CNT in the presence of DCP. Reproduced from [129] with permission of Elsevier.

As shown in Figure 16, the formed peroxide radicals are prone to hydrogen abstraction from hydrocarbon substrates and it is expected that the active species generated onto the hydrocarbon backbone react with unsaturated bonds located on the MWCNTs surface keeping in mind that side reactions such as chain scission for PP derivatives may occur at high temperatures (Figure16) [115, 118]. For experiments conducted in dichlorobenzene (DCB) as solvent at 160°C with 1.5wt% DCP, TMP grafting density was as high as 0.92 mg/m2.

3.2.2. Synthesis of PP grafted carbon nanotubes via peroxide

Farzi et al. [129] have successfully grafted PP onto MWCNTs through a radical grafting reaction, carried out under similar experimental conditions to PE [128] and TMP [129] (1.5wt% DCP, 160°C) and using 1,2-dichlorobenzene (DCB) as solvent able to solubilize PP partially at elevated temperature. The corresponding PP-grafted nanotubes were analysed by TGA after a purification by soxhlet extraction in DCB. However, the authors were not able to obtain reproductible results with weight losses varying from 50% to 80% for the above-mentioned experiment. This behaviour was attributed to the purification procedure which did not permit to remove all the free PP chains. The authors have also speculated on the degradation behaviour of PP through the well-known β-scission reaction occurring in the presence of radical species therefore the authors were not able to give a PP grafting density. Then, the aforementioned PP coated MWCNTs have been dispersed within a commercially available PP matrix using a contra-rotating Haake Rheomixer and the amount of nanofiller in the final composites has been fixed to 3wt%. The evaluation of MWCNTs dispersion has been examined by using scanning electron microscopy (SEM) (Figure 17).

SEM images of the PP/PP-g-MWCNTs composites MWCNTs containing of 3wt% (Figure 17) demonstrated that there were still some areas where PP-g-MWCNTs were not found which was obviously connected with improper filler distribution. For a simple melt blend of PP with untreated MWCNTs, SEM images of the resulting material only showed clusters of a few tens micrometers of diameter evidencing a poor interfacial adhesion in the material (Figure 18), as reported by Lee [140] for untreated MWCNT/PP composites MWCNTs containing of 2wt%.

Figure 17.

SEM micrographs of PP/PP-g-MWCNT composites with MWCNTs loading of 3wt%. Reproduced from [129] with permission of Elsevier.

Figure 18.

SEM micrographs of PP/MWCNTs composites with MWCNTs loading of 3wt%. Reproduced from [129] with permission of Elsevier.

It was concluded from these results that the grafting of PP onto MWCNTs provided a low steric barrier against the strong intermolecular Van der Waals interactions among nanotubes within the PP matrix.

In a similar approach, isotactic polyrpopylene (iPP) was successfully grafted onto multiwalled carbon nanotubes (MWCNTs) by direct macroradical addition by sonication in hot xylene with BPO as an initiator [142]. It was found that both iPP macromolecular radicals and small-molecular benzoic acid free radicals were grafted onto MWNTs. iPP-g-MWNTs dispersed more uniformly in iPP than pristine MWNTs.

3.3. Rheological behaviour of polyolefin based carbon nanotubes nanocomposites

It has been well known for a century that the addition of fillers, mostly carbon black, to rubber compounds has a strong impact on the viscoelastic properties of materials. In recent years, polymer nanocomposites have been developed as a new class of composites [143]. Actually from a rheological point of view, a direct consequence of incorporation of fillers in molten polymers is the significant change in the steady shear viscosity behavior and the viscoelastic properties. The level of filler dispersion is expected to play a major role in determining the filler effects on non linear responses of nanocomposites. Generally speaking, thermoplastic polymers filled with nano-particles show a solid-like behavior response which includes a non–terminal zone of relaxation, apparent yield stress and a shear-thinning dependence of viscosity on particle concentration, aspect ratio and dispersion.

Since the melt rheological properties of filled polymers are sensitive to the structure, concentration, particle size, shape and surface characteristics of the fillers, rheology offers original means to assess the state of the dispersion in nanocomposites and to investigate the influence of flow conditions upon nano-filler dispersion itself [144]. As discussed previously, one of the most important challenges in filled polymer developments and applications is to obtain a homogeneous dispersion of CNT in polymer matrix by overcoming the van der Waals interaction between elementary tubes. As a result, it can be expected that the rheological percolation, and subsequently the non-linearity effect, depend on nanotube dispersion and aspect ratio. As matter of fact, a great level of activity in the domain of the rheology of polymers filled with CNT is reported in the more recent scientific literature. The rheological behavior of melt thermoplastic polymer filled with NTC was reported to depend on nanotube dispersion, aspect ratio and alignment under flow. However, among the different studies on liquid systems filled with carbon nanotubes two types of relaxation mechanisms of CNT must be differentiated according to the matrix viscosity: Do the carbon nanotubes behave as Brownian particles? The Doi-Edwards theory for dilute regime (the nantotubes are free to rotate without any contacts) allows the rotary diffusivity D r0 of a rod (Length: L and diameter: d) in an isotropic suspension to be calculated by equation (1), in which kB is the Boltzmann constant and ηm is the viscosity of the suspending medium

D r 0 = 3 k B T ( ln ( L / d ) 0.8 ) π η m L 3 E1

In semi-dilute regime, the rotary diffusivity D r can be written as equation (2), where A is a dimensionless constant whose value is generally large (A~1000).

D r = A D r 0 ( ν L 3 ) 2 E2

Consequently, the rotary diffusion of CNT varies according to matrix viscosity:

D r α 1 / η m E3

Actually, the rheological behavior of CNT suspension is observed close to the Doi-Edwards theory on the Brownian dynamics of rigid rods. However, it was observed that low shear deformations induced an aggregation mechanism, but these aggregates broke down at high shear, forming small aggregates with less entanglements [145]. The shear rheology of such carbon nanotube suspensions was reviewed by Hobbie [146] from the perspective of colloid and polymer science.

According to the Doi-Edwards theory, Marceau et al [147] have shown that the suspensions of CNT, at low concentration (ϕ=0.2%) and in low fluid matrix (ηm=5 Pa.s), behave as Brownian entities (Dr=5.0x10-5 s-1). The diffusion time of these CNTs is then τ r = 1 / 2 D r 10 4 s . If we imagine that these same CNT are dispersed in high viscous polymer matrix such as molten PP (ηm~1x103 Pa.s), their relaxation time will be then: τ r 2 x 10 6 s . The order magnitude of the relaxation time is then one month! Consequently, the carbon nanotubes cannot be considered anymore as Brownian entities in most of the papers that have been addressed to the viscoelastic behavior of carbon nanotubes dispersed in high viscous molten polymers. The main challenge in such nanocomposite systems is to control the dispersion of the nanotubes in high viscous fluid in order to have the lowest percolation threshold regarding the electrical properties. For example, by improving the CNT dispersion using functionalized single wall nanotubes, Mitchell et al [148] observed that the percolation threshold dropped from 3wt% to 1.5% in PS nanocomposites.

Actually, the dispersion of CNT in polymer matrix is strongly difficult mainly due to the nanotube-nanotube interactions higher than the nanotube-polymer interaction. However, optimal dispersion of CNTs can be achieved in polar polymers such as polyamides, polyesters or polycarbonate. This optimal dispersion is generally measured, at least from a qualitative point of view, from the electrical and/or rheological percolation threshold. Nevertheless, the dispersion of CNT in polyolefin (PP, PE or copolymer of ethylene) is most of the time a real challenge due to unfavorable and low nanotube-polymer interactions. On the other hand, the fact that CNT have a high aspect ratio and are not Brownian in polymer matrix leads to the conclusion that the different works of the literature are difficult to compare. The samples, studied in rheology or electric conductivity, have generally undergone different processing conditions. As a result, CNT nanocomposites are totally out of isotropic dispersion and the isotropic equilibrium of CNTs can never be achieved. However, general trends in CNT nancomposite can be described from the open literature.

From a sample preparation point of view, dispersion of CNTs in polyolefins were generally prepared via conventional melt processing, i.e melt blending in batch mixer or in twin screw extruder). Marginal methods may also be used as for example solid-state shear pulverization [149] or in situ lubrication methods [150]. Numerous studies [151-156] have been devoted to the linear viscoelasticity of PP nanocomposites based on CNT dispersion.

All of these papers reported an increase in shear viscosity and storage and loss moduli of the nanocomposites with increasing the CNT concentration as shown in Figure 19.

Furthermore, a general rheological trend for nanocomposites studied in most of these papers is the appearance of a transition from a liquid-like behaviour to a solid-like behaviour, i.e. the apparition of a plateau (second plateau modulus, G o = lim ω 0 G ' ( ω ) ) of the storage modulus at low frequency which is obviously higher than the loss modulus. Obviously, it is admitted that the increase of the CNT concentration is driving this transition. Above this critical transition, generally associated with the percolation threshold, these nanocomposites show a solid-like behavior response, which includes a non-terminal zone of relaxation leading to apparent yield stress and a shear-thinning dependence on viscosity | η * ( ω ) | α ω 1 .

Figure 19.

Variation versus frequency of the storage shear modulus G’(ω) and absolute complex viscosity η*(ω) at different concentrations of CNT (1 to 7%). Reproduced from [151] with permission of John Wiley and Sons.

This non-terminal frequency behavior is generally attributed to the formation of an interconnected nanotube network in the polymer matrix. Therefore, the solid-like behavior is associated nanotube–nanotube interactions which increase with the CNT content. Eventually, these interactions lead to percolation and the formation of an interconnected structure of nanotubes in the matrix. Due to the high aspect ratio of CNT (generally, L/d>150), the existence of this percolation threshold is expected at low concentration. For example, from Fig 19, the percolation threshold can be estimated to be less than 2% of CNT. This percolation threshold is generally observed in the range 0.5%-5% depending on CNT nature (aspect ratio, surface chemical modification) and on the processing methods for nanocomposite preparation.

If a lot of works have been devoted to the linear viscoelasticity of CNT nanocomposites whereas a few works have been reported on non-linear properties such as for example the melt flow instabilities. Interestingly, Palza et al [157] showed that carbon nanotubes modify the main characteristics of the spurt instability developed by the linear polyethylene. Furthermore, the sharkskin instability, developed in short chain branched polyethylene, is reduced at low amounts of MWCNT. Furthermore, the critical shear rate for the on-set of the spurt and the sharkskin instabilities decreases in the nanocomposites probably due to the physical interactions between the polymer and the nanofiller. Finally, at high shear rates, the gross melt fracture instability is completely erased in the nanocomposites based on the linear polymer whereas in short chain branched polyethylene the amplitude of this bulk distortion is rather moderated. Clearly, the carbon nanotubes have a drastic effect on the main flow instabilities observed in polyethylene. Consequently, the processing of CNT nanocomposites, i.e under non-linear deformation, is an open investigation domain.

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4. Conclusion

The most frequent method for preparing polymer nanotubes composites has been mixing nanotubes and polymer in a suitable solvent and to evaporate the solvent to form composite film. But to increase the advantages at its best, one needs: (i) high interfacial area between nanotubes and polymer; and, (ii) strong interfacial interaction. Unfortunately this solvent technique does not help much in achieving these targets; and as a result a nanocomposite having properties much inferior to theoretical expectations are obtained. In order to obtain higher contact area between nanotubes and polymer, the issue of dispersion needs to be addressed. Uniform dispersion of these nanotubes produces immense internal interfacial area, which is the key to enhancement of properties of interest. On the other hand, modification of nanotubes surface through functionalisation is required for creating an effective interaction with the host matrix and to make nanotubes soluble and dispersible.

The idea of grafting PE or PP with the help of peroxide during extrusion is exciting. It was shown that cumlyoxly radical generated by thermolysis of DCP can abstract hydrogen from polyolefin chains, thus creating polyolefin macroradicals. Then, these macroradicals add to the unsaturated carbon bonds on the surface of the nanotubes. The upside of this strategy is that radicals have short lifetimes which make the procedure possible in an extruder where the residence time is generally low. On the contrary, the downside is the low selectivity of radicals leading to a competition between radical combination reactions and radical addition reactions. Alkanes can be used as model for PE to perform the grafting of PE onto nanotubes. The results were interesting however the degree of PE grafting remained lower than the model (weight loss by TGA = 22% as compared to a weight loss of 30% in case of pentadecane). LDPE grafting density on nanotubes was 1.1mg.m-2 while incorporation of TEMPO raised the grafting density to 1.4mg.m-2. End functionalised PE can also be used for PE grafting onto nanotubes with dichlorobenzene as solvent. As emphasized by TEM images, a layer of considerable thickness has been grafted around the periphery of the nanotubes.

In order to follow the same strategy for nanotubes functionalisation with PP, tetramethylpentadecane has been selected as a model compound for PP. It was successfully grafted onto carbon nanotubes with a grafting density of 0.92 mg/m2. However, the grafting of PP onto nanotubes did not permit to obtain reproductible results. SEM images of the PP-g-MWCNTs nanocomposites with filler loadings of 3wt% in PP matrix did not show a significant improvement in MWCNTs dispersion within the PP matrix although sizes of the aggregates were slightly reduced.

In addition, it can be expected that the rheological percolation, and subsequently the non-linearity effect, depend on nanotube dispersion and aspect ratio. Low shear deformations induced an aggregation mechanism, but these aggregates broke down at high shear, forming small aggregates with less entanglements. In a high viscous polymer media, it was shown that carbon nanotubes could not be considered anymore as Brownian entities. A general rheological trend for CNTs-based nanocomposites is the appearance of a transition from a liquid-like behaviour to a solid-like behaviour increasing with the CNT content because it is associated to nanotube–nanotube interactions. Due to the high aspect ratio of CNT the percolation threshold can be expected to be less than 2% of CNT.

References

  1. 1. Iijima S. Helical microtubules of graphitic carbon. Nature 1991 354 56 58
  2. 2. Baughman R. H. Zakhidov A.A. de Heer W. A. Carbon Nanotubes--the Route Toward Applications. Science 2002 297 5582 787 792
  3. 3. Moniruzzaman M. Winey K. I. Polymer Nanocomposites Containing Carbon Nanotubes. Macromolecules 2006 39 16 5194 5205
  4. 4. Grossiord N. Loos J. Regev O. Koning C. E. Toolbox for dispersing carbon nanotubes into polymers to get conductive nanocomposites. Chemistry of Materials 2006
  5. 5. Ciardelli F. Coiai S. Passaglia E. Pucci A. Ruggeri G. Nanocomposites based on polyolefins and functional thermoplastic materials Polymer International 2008 57 6 805 836
  6. 6. Judeinstein P. Sanchez C. 1996 Hybrid organic-inorganic materials: A land of multi-disciplinarity Journal of Materials Chemistry 6 4 511 525
  7. 7. Connell M.O. Carbon Nanotubes Properties and Applications; CRC Press, 2006. 360 9780849327483.
  8. 8. Gogotsi Y. Carbon NanomaterialsCRC Press, 2006. 344 9780849393860
  9. 9. Spitalsky Z. Tasis D. Papagelis K. Galiotis C. 2010 Carbon nanotube-polymer composites: Chemistry, processing, mechanical and electrical properties. Progress in Polymer Science 35 3 357 401
  10. 10. Osswald S. Flahaut E. Gogotsi Y. In situ Raman spectroscopy study of oxidation of double- and single-wall carbon nanotubes Chemistry of Materials 2006 18 6 1525 1533
  11. 11. Yang Q. Wang L. Xiang W. Zhu J. Li J. Grafting polymers onto carbon black surface by trapping polymer radicals. Polymer 2007 48 10 2866 2873
  12. 12. Deng X. Jia G. Wang H. Sun H. Wang X. Yang S. Wang T. Liu Y. Translocation and fate of multi-walled carbon nanotubes in vivo. Carbon 2007 45 7 1419 1424
  13. 13. Shaffer M. S. P. Windle A. H. Fabrication and Characterization of Carbon Nanotube/Poly(vinyl alcohol) Composites. Advanced Materials 1999 11 11 937 941
  14. 14. Baskaran D. Mays J. W. Bratcher M.S. Noncovalent and nonspecific molecular interactions of polymers with multiwalled carbon nanotubes. Chemistry of Materials 2005 17 13 3389 3397
  15. 15. Haggenmuller R. Gommans H. H. Rinzler A. G. Fischer J. E. Winey K. I. Aligned single-wall carbon nanotubes in composites by melt processing methods. Chemical Physics. Letters 2000
  16. 16. Kang Y. J. Taton T. A. 2003 Micelle-encapsulated carbon nanotubes: A route to nanotube composites. Journal of the American Chemical Society 125 19 5650 5651
  17. 17. Star A. Stoddart J. F. Dispersion and solubilization of single-walled carbon nanotubes with a hyperbranched polymer. Macromolecules 2002 35 19 7516 7520
  18. 18. Barraza H. J. Pompeo F. O’Rear E. A. Resasco D. E. 2002 SWNT-filled thermoplastic and elastomeric composites prepared by miniemulsion polymerization. Nano Letters 2 8 797 802
  19. 19. Bonduel D. Mainil M. Alexandre M. Monteverde F. Dubois P. 2005 Supported coordination polymerization: a unique way to potent polyolefin carbon nanotube nanocomposites Chemical Communications 6 781 783
  20. 20. Park S. Yoon W. Lee B. Kim J. Jung Y. H. Do Y. Paik H. J. Choi I. S. Carbon Nanotubes as a Ligand in Cp2ZrCl2-Based Ethylene Polymerization. Macromolecular Rapid Communication 2006 27 1 47 50
  21. 21. Liu T. X. Phang I. Y. Shen L. Chow S. Y. Zhang W. D. Morphology and mechanical properties of multiwalled carbon nanotubes reinforced nylon-6 composites. Macromolecules 2004 37 19 7214 7222
  22. 22. Siochi E. J. Working D. C. Park C. Lillehei P. T. Rouse J. H. Topping C. C. Melt processing of SWCNT-polyimide nanocomposite fibers. Composites Part B. 2004 35 5 439 446
  23. 23. Potschke P. Fornes T. D. Paul D. R. Rheological behavior of multiwalled carbon nanotube/polycarbonate composites. Polymer 2002 43 11 3247 3255
  24. 24. Li C. Zhao Q. Deng H. Chen C. Wang K. Zhang Q. Chena F. Fua Q. 2011 Preparation, structure and properties of thermoplastic olefin nanocomposites containing functionalized carbon nanotubes Polymer international 60 11 1629 1637
  25. 25. Zhang Z. N. Zhang J. Chen P. Zhang B. Q. He J. S. Hu G. H. Enhanced interactions between multi-walled carbon nanotubes and polystyrene induced by melt mixing Carbon 2006 44 4 692 698
  26. 26. Shofner M. L. Khabashesku V. N. Barrera E. V. 2006 Chemistry of Materials. Processing and mechanical properties of fluorinated single-wall carbon nanotube-polyethylene composites. 18 4 906 913
  27. 27. Mahfuz H. Adnan A. Rangari V. K. Jeelani S. Manufacturing and characterization of carbon nanotube/polyethylene composites. International Journal of Nanoscience 2005 4 1 55 72
  28. 28. Ruan S. L. Gao P. Yu T. X. 2006 Ultra-strong gel-spun UHMWPE fibers reinforced using multiwalled carbon nanotubes. Polymer 47 5 1604 1611
  29. 29. Shofner M. L. Khabashesku V. N. Barrera E. V. 2006 Chemistry of Materials 18 906 913
  30. 30. Bahr J. L. Tour J. M. Processing and mechanical properties of fluorinated single-wall carbon nanotube-polyethylene composites. Chemistry of Materials 2001 13 4 3823 3824
  31. 31. Georgakilas V. Kordatos K. Prato M. Guldi D. M. Holzinger M. Hirsch A. J. Organic functionalization of carbon nanotubes. Journal of the American Chemical Society 2002 124 5 760 761
  32. 32. Peng H. Reverdy P. Khabashesku V. N. Margrave J. L. Sidewall functionalization of single-walled carbon nanotubes with organic peroxides. Chemical Communications 2003 362 363
  33. 33. Qin S. Qin D. Ford W. T. Resasco D. E. Herrera J. E. Functionalization of single-walled carbon nanotubes with polystyrene via grafting to and grafting from methods. Macromolecules 2004
  34. 34. Riggs J. E. Guo Z. Carroll D. L. Sun-P Y. Strong luminescence of solubilized carbon nanotubes. Journal of the American Chemical Society 2000 122 24 5879 5884
  35. 35. Umek P. Seo J. W. Hernadi K. Mrzel A. Pechy P. Mihailovic D. D. Forro L. Addition of carbon radicals generated from organic peroxides to single wall carbon nanotubes. Chemistry of Materials 2003 15 25 4751 4755
  36. 36. Liu Y. K. Yao Z. L. Adronov A. Functionalization of single-walled carbon nanotubes with well-defined polymers by radical coupling. Macromolecules 2005 38 4 1172 1179
  37. 37. Shaffer M. S. P. Koziol K. Polystyrene grafted multi-walled carbon nanotubes. Chemical Communications 2002
  38. 38. Kong H. Gao C. Yan D. Y. Controlled functionalization of multiwalled carbon nanotubes by in situ atom transfer radical polymerization. Journal of the American Chemical Society 2004 126 2 412 413
  39. 39. Baskaran D. Mays J. W. Bratcher M.S. Polymer-grafted multiwalled carbon nanotubes through surface-initiated polymerization. Angewandte Chemie International Edition 2004
  40. 40. Breuer O. Sundararaj U. Big returns from small fibers A review of polymer/carbon nanotube composites. Polymer Composites 2004 25 6 630 645
  41. 41. Tiwari I. Singh K. P. Singh M. An insight review on the application of polymer-carbon nanotubes based composite material in sensor technology. Russian Journal of General Chemistry 2009
  42. 42. Martínez- Hernández A.L. Velasco-Santos C. Castaño V.M. 2010 Carbon Nanotubes Composites: Processing, Grafting and Mechanical and Thermal Properties. Current Nanoscience 6 1 12 39
  43. 43. Jin L. Bower C. L. Zhou O. Applied Physics Letters 1998 73 9 1197 1199
  44. 44. Coleman J. N. Cadek M. Blake R. Nicolosi V. Ryan K. P. Belton C. 2004 High-performance nanotube-reinforced plastics: Understanding the mechanism of strength increase. Advanced Functional Materials 14 8 791 798
  45. 45. Cadek M. Coleman J. N. Ryan K. P. Nicolosi V. Bister G. Fonseca A. Reinforcement of polymers with carbon nanotubes: The role of nanotube surface area. Nano Letters 2004 4 2 353 356
  46. 46. Cadek M. Coleman J. N. Barron V. Hedicke K. Blau W. J. Morphological and mechanical properties of carbon-nanotube-reinforced semicrystalline and amorphous polymer composites. Applied Physics Letters 2002 81 27 5123 5125
  47. 47. Andrews R. Jacques D. Minot M. Rantell T. Fabrication of carbon multiwall nanotube/polymer composites by shear mixing. Macromolecular Materials and Engineering 2002 287 6 395 403
  48. 48. Breuer O. Sundararaj U. Big returns from small fibers A review of polymer/carbon nanotube composites. Polymer Composites 2004 25 6 630 645
  49. 49. Potschke P. Bhattacharyya A. R. Janke A. Goering H. Melt mixing of polycarbonate/multi-wall carbon nanotube composites. Composite Interfaces 2003 10 4 389 404
  50. 50. Andrews R. Jacques D. Qian D. L. Rantell T. 2002 Multiwall carbon nanotubes: Synthesis and application. Accounts of Chemical Research 35 12 1008 1017
  51. 51. Tang W. Santare M. H. Advani S. G. Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films. Carbon 2003 41 14 2779 2785
  52. 52. Jia Z. Wang Z. Xu C. Liang J Wei B. Wu D. Study on poly(methyl methacrylate)/carbon nanotube composites Materials Science and Engineering A 1999 271 1 395 400
  53. 53. Bonduel D. Mainil M. Alexandre M. Monteverde F. Dubois P. 2005 Supported coordination polymerization: a unique way to potent polyolefin carbon nanotube nanocomposites. Chemical Communications 41 6 781 783
  54. 54. Bryning M. B. Milkie D. E. Islam M. F. Kikkawa J. M. Yodh A. G. 2005 Thermal conductivity and interfacial resistance in single-wall carbon nanotube epoxy composites. Applied Physics Letters 87 87 161909 161911
  55. 55. Moniruzzaman M. Du F. Romero N. Winey K. I. Increased flexural modulus and strength in SWNT/epoxy composites by a new fabrication method. Polymer 2006 47 1 293 298
  56. 56. Zhu J. Kim J. Peng H. Margrave J. L. Khabashesku V. N. Barrera E. V. Improving the dispersion and integration of single-walled carbon nanotubes in epoxy composites through functionalization. Nano Letters 2003 3 8 1107 1113
  57. 57. Xia H. Wang Q. Li K. Hu G. H. Preparation of polypropylene/carbon nanotube composite powder with a solid-state mechanochemical pulverization process. Journal of Applied Polymer Science 2004 93 1 378 386
  58. 58. Kasimatis K. G. Nowell J. A. Dykes L. M. Burghardt W. R. Thillalyan R. Brinson L. C. Andrews R. Torkelson J. M. 2005 Morphology, thermal, and rheological behavior of nylon 11/multi-walled carbon nanotube nanocomposites prepared by melt compounding. Programme Making and Special Events Preprint 92 255 256
  59. 59. Regev O. El Kati P. N. B. Loos J. Koning C.E. Preparation of conductive nanotube-polymer composites using latex technology. Advanced Materials 2004 16 3 248 251
  60. 60. Lee J. U. Huh J. Kim K. H. Park C. Jo W. H. Aqueous suspension of carbon nanotubes via non-covalent functionalization with oligothiophene-terminated poly(ethylene glycol). Carbon 2007 45 5 1051 1057
  61. 61. Mcandrew T. Roger C. Bressand E. Laurent P. W. WO/2008/106572 2008
  62. 62. Vigolo B. Penicaud A. Coulon C. Sauder C. Pailler R. Journet C. Bernier P. Poulin P. Macroscopic fibers and ribbons of oriented carbon nanotubes. Science 2000 290 5495 1331 1334
  63. 63. Mamedov A.A. Kotov N.A. Prato M. Guldi D. M. Wicksted J. P. Hirsch A. Molecular design of strong single-wall carbon nanotube/polyelectrolyte multilayer composites. Nature Materials 2002 1 3 190 194
  64. 64. Calvert P. Nanotube composites- A recipe for strength. Nature 1999 399 6733 210 211
  65. 65. Hersam M.C. Progress towards monodisperse single-walled carbon nanotubes. Nature Nanotechnology 2008
  66. 66. Green M.J. Analysis and measurement of carbon nanotube dispersions: nanodispersion versus macrodispersion. Polymer Internantional 2010 59 10 1319 22
  67. 67. Kim S. W. Kim T. Kim Y. S. Choi H. S. Lim H. J. Yang S. J. Park C. R. Surface modifications for the effective dispersion of carbon nanotubes in solvents and polymers. Carbon 2012 50 8 3 33
  68. 68. O’Connell M.J. Boul P. Ericson L. M. Huffman C. Wang Y. Haroz E. Reversible water-solubilization of single-walled carbon nanotubes by polymer wrapping. Chemical Physics Letters 2001 342 3 265 271
  69. 69. Star A. Stoddart J. F. Steuerman D. Diehl M. Boukai A. Wong E. W. Preparation and properties of polymer-wrapped single-walled carbon nanotubes. Angewandte Chemie International Edition 2001 40 9 1721 1725
  70. 70. Steuerman D. W. Star A. Narizzano R. Choi H. Ries R. S. Nicolini C. Interactions between conjugated polymers and single-walled carbon nanotubes. Journal of Physical Chemistry B. 2002 106 12 3124 3130
  71. 71. Star A. Liu Y. Grant K. Ridvan L. Stoddart J. F. Steuerman D. W. Noncovalent side-wall functionalization of single-walled carbon nanotubes. Macromolecules 2003 36 3 553 560
  72. 72. Star A. Stoddart J. F. Macromolecules 2002 35 19 7516 7520
  73. 73. Nativ-Roth E. Levi-Kalisman Y. Regev O. Yerushalmi-Rozen R. On the route to compatibilization of carbon nanotubes. Journal of Polymer Engineering 2002 22 5 353 368
  74. 74. Xia H. S. Wang Q. Qiu G. H. Polymer-encapsulated carbon nanotubes prepared through ultrasonically initiated in situ emulsion polymerization. Chemistry of Materials 2003 15 20 3879 3886
  75. 75. Li X. Qin Y. Picraux S. T. Guo-X Z. Noncovalent assembly of carbon nanotube-inorganic hybrids. Journal of Material Chemistry 2011 21 21 25 47
  76. 76. Sahoo N. G. Rana S. Cho J. W. L. S. H. Chan Li. Polymer nanocomposites based on functionalized carbon nanotubes Progress in Polymer Science 2010 35 7 837 867
  77. 77. Kitano H. Tachimoto K. Anraku Y. J. Functionalization of single-walled carbon nanotube by the covalent modification with polymer chains. Journal of Colloid and Interface Science 2007 306 1 28 33
  78. 78. Park H. Zhao J. Lu J. P. Effects of sidewall functionalization on conducting properties of single wall carbon nanotubes. Nano Letters 2006 6 5 916 9
  79. 79. Han S. J. Kim B. Suh K. D. Electrical properties of a composite film of poly(acrylonitrile) nanoparticles coated with carbon nanotubes. Macromolecular Chemistry and Physics 2007 208 4 377 383
  80. 80. Zhou C. Wang S. Zhang Y. Zhuang Q. Han Z. situ preparation and continuous fiber spinning of poly(p-phenylene benzobisoxazole) composites with oligo-hydroxyamide-functionalized multi-walled carbon nanotubes Polymer 2008 49 10 2520 2530
  81. 81. Baskaran D. Mays J. W. Bratcher M.S. Polymer adsorption in the grafting reactions of hydroxyl terminal polymers with multi-walled carbon nanotubes. Polymer 2005 46 14 5050 5057
  82. 82. Zhao B. Hu H. Yu A. Perea D. Haddon R. C. Synthesis and characterization of water soluble single-walled carbon nanotube graft copolymers. Journal of American Chemical Society 2005 127 22 8197 8203
  83. 83. Zhang N. Xie J. Guers M. Varadan V. K. Chemical bonding of multiwalled carbon nanotubes to polydimethylsiloxanes and modification of the photoinitiator system for microstereolithography processing
  84. 84. Qin S. Qin D. Ford W. T. Resasco D. E. Herrera J. E. 2004 Functionalization of Single-Walled Carbon Nanotubes with Polystyrene via Grafting to and Grafting from Methods. Macromolecules 37 3 752 757
  85. 85. Li H. Cheng F. Duft A. M. Adronov A. 2005 Functionalization of single-walled carbon nanotubes with well-defined polystyrene by "click" coupling. Journal of American Chemical Society 127 41 14518 24
  86. 86. Huang H. M. Liu I. C. Chang C. Y. Tsai H. C. Hsu C. H. Tsiang R. C. C. 2004 Preparing a polystyrene-functionalized multiple-walled carbon nanotubes via covalently linking acyl chloride functionalities with living polystyryllithium Journal of Polymer Science Part A: Polymer Chemistry 42 22 5802 5810
  87. 87. Lou X. Detrembleur C. Pagnoulle C. Sciannamea V. Jerome R. Grafting of alkoxyamine end-capped (co)polymers onto multi-walled carbon nanotubes. Polymer 2004 45 18 6097 6102
  88. 88. Viswanathan G. Chakrapani N. Yang H. Wie B. Chung H. Cho K. Single-step in situ synthesis of polymer-grafted single-wall nanotube composites Journal of American Chemical Society 2003 9258 9259
  89. 89. Liu Y. Wu D. C. Zhang W. D. Jiang X. He C. B. Chung T. S. 2005 Polyethylenimine-grafted multiwalled carbon nanotubes for secure noncovalent immobilization and efficient delivery of DNA Angewandte Chemie-International Edition 44 30 4782 4785
  90. 90. Bonduel D. Mainil M. Alexandre M. Monteverde F. Dubois P. 2005 Supported coordination polymerization: a unique way to potent polyolefin carbon nanotube nanocomposites Chemical Communication 6 781 783
  91. 91. Park J. J. Park D. M. Youk J. H. Yu W. R. Lee J. Functionalization of multi-walled carbon nanotubes by free radical graft polymerization initiated from photoinduced surface groups. Carbon 2010 48 10 2899 2905
  92. 92. Yao Z. Braidy N. Botton G. A. Adronov A. 2003 Polymerization from the surface of single-walled carbon nanotubes- Preparation and characterization of nanocomposites. Journal of American Chemical Society 125 51 16015 16024
  93. 93. Kong H. Gao C. Yan D. 2004 Controlled functionalization of multiwalled carbon nanotubes by in situ atom transfer radical polymerization. Journal of American Chemical Society 126 2 412 413
  94. 94. Wang M. Pramoda K. P. Goh S. H. 2005 Enhancement of the mechanical properties of poly (styrene-co-acrylonitrile) with poly(methyl methacrylate)-grafted multiwalled carbon nanotubes Polymer 46 25 11510 11516
  95. 95. Baskaran D. Mays J. W. Bratcher MS 2004 Polymer-grafted multiwalled carbon nanotubes through surface-initiated polymerization. Angewandte Chemie-International Edition 43 16 2138 2142
  96. 96. Shanmugharah A. M. Bae J. H. Nayak R. R. Ryu S. H. 2007 Preparation of poly(styrene-co-acrylonitrile)-grafted multiwalled carbon nanotubes via surface-initiated atom transfer radical polymerization. Journal of Polymer Science Part A-Polymer Chemistry 45 3 460 470
  97. 97. Cui J. Wang W. P. You Y. Z. Liu C. Wang P. Functionalization of multiwalled carbon nanotubes by reversible addition fragmentation chain-transfer polymerization. Polymer 2004 45 26 8717 8721
  98. 98. Hong C. Y. You Y. Z. Pan C. Y. 2005Synthesis of water-soluble multiwalled carbon nanotubes with grafted temperature-responsive shells by surface RAFT polymerization. Chemistry of Materials 17 9 2247 2254
  99. 99. Wang G. J. Huang S. Z. Wang Y. Liu L. Qiu J. Li Y. 2007 Synthesis of water-soluble single-walled carbon nanotubes by RAFT polymerization. Polymer 48 3 728 733
  100. 100. Datsyuk V. Guerret-Piecourt C. Dagreou S. Billon L. Dupin J. C. Flahaut E. Double walled carbon nanotube/polymer composites via in-situ nitroxide mediated polymerisation of amphiphilic block copolymers. Carbon 2005 43 4 873 876
  101. 101. Zhao X. D. Fan X. H. Chen X. F. Chai C. P. Zhou Q. F. 2006 Surface modification of multiwalled carbon nanotubes via nitroxide-mediated radical polymerization. Journal of Polymer Science Part A-Polymer Chemistry 44 15 4656 4667
  102. 102. Dehonor M. Masenelli-Varlot K. Gonzalez-Montiel A. Gauthier C. Cavaille J. Y. Terrones H. 2005 Nanotube brushes: Polystyrene grafted covalently on CNx nanotubes by nitroxide-mediated radical polymerization. Chemical Communications 42 5349 5351
  103. 103. Fan D. Q. He J. P. Tang W. Xu J. T. Yang Y. L. 2007 Synthesis of polymer grafted carbon nanotubes by nitroxide mediated radical polymerization in the presence of spin-labeled carbon nanotubes. European Polymer Journal 43 1 26 34
  104. 104. Kaminsky W. Trends in polyolefin chemistry. Macromolecular Chemistry and Physics 2008 209 5 459 466
  105. 105. Mokashi V. V. Mokashi V. V. Qian D. Liu Y. J. A. 2007 study on the tensile response and fracture in carbon nanotube-based composites using molecular mechanics. Composites science and technology 67 3-4 530 540
  106. 106. Tang W. Z. Santare M. H. Advani S. G. 2003 Melt processing and mechanical property characterization of multi-walled carbon nanotube/high density polyethylene (MWNT/HDPE) composite films. Carbon 41 14 2779 2785
  107. 107. Yang B. X. Pramoda K. P. Xu G. Q. Goh S. H. 2007 Mechanical reinforcement of polyethylene using polyethylene-grafted multiwalled carbon nanotubes. Advanced Functional Materials 17 13 2062 2069
  108. 108. Shofner M. L. Khabashesku V. N. Barrera E. V. 2006 Processing and mechanical properties of fluorinated single-wall carbon nanotube-polyethylene composites. Chemistry of Materials 18 4 906 913
  109. 109. Mahfuz H. Adnan A. Rangari V. K. Jeelani S. Manufacturing and Characterization of Carbon Nanotube/Polyethylene Composites. International Journal of Nanoscience 2005 4 1 55 72
  110. 110. Ruan S. Gao P. Yu T. X. 2006 Ultra-strong gel-spun UHMWPE fibers reinforced using multiwalled carbon nanotubes. Polymer 47 5 1604 1611
  111. 111. Tong X. Liu C. Cheng H. M. Zhao H. Yang F. Zhang X. Surface Modification of Single-walled Carbon Nanotubes with Polyethylene via in situ Ziegler-Natta Polymerization. Journal of Applied Polymer Science 2004 92 6 3697 3700
  112. 112. Funk A. Kaminsky W. Polypropylene carbon nanotube composites by in situ polymerization. Composites Science and Technology 2007 67 5 906 915
  113. 113. Bonduel D. Bredeau S. Alexandre M. Monteverde F. Dubois P. Supported metallocene catalysis as an efficient tool for the preparation of polyethylene/carbon nanotube nanocomposites: effect of the catalytic system on the coating morphology. Journal of Materials Chemistry 2007 17 22 2359 2366
  114. 114. Dong J. Y. Hu Y. Design and synthesis of structurally well-defined functional polyolefins via transition metal-mediated olefin polymerization chemistry. Coordination Chemistry Reviews 2006
  115. 115. Russell K.E. 2002 Free radical graft polymerization and copolymerization at higher temperatures. Progress in Polymer Science 27 6 1007 1038
  116. 116. Chung T.C. Synthesis of functional polyolefin copolymers with graft and block structures. Progress in Polymer Science 2002 27 1 39 85
  117. 117. Moad G. The synthesis of polyolefin graft copolymers by reactive extrusion. Progress in Polymer Science 1999 24 1 81 142
  118. 118. Hettema R. Van Tol J. Janssen L. P. B. M. In-situ reactive blending of polyethylene and polypropylene in co-rotating and counter-rotating extruders. Polymer Engineering and Science 1999 39 9 1628 1641
  119. 119. Akbar S. Beyou E. Cassagnau P. Chaumont P. Farzi G. Radical grafting of polyethylene onto MWCNTs: A model compound approach. Polymer 2009 50 12 2535 2543
  120. 120. Badel T. Beyou E. Bounor-Legare V. Chaumont P. Flat J. J. Michel A. Melt grafting of polymethyl methacrylate onto poly(ethylene-co-1-octene) by reactive extrusion: Model compound approach. Journal of Polymer Science Part A-Polymer Chemistry 2007 45 22 5215 5226
  121. 121. Johnston R.T. Monte Carlo simulation of the peroxide curing of ethylene elastomers. Rubber Chemistry and Technology 2003 76 1 174 201
  122. 122. Johnston R.T. Modelling peroxide crosslinking in polyolefins Sealing Technology 2003 76 2 6 9
  123. 123. Osswald S. Havel M. Gogotsi Y. Monitoring oxidation of multiwalled carbon nanotubes by Raman spectroscopy. Journal of Raman Spectroscopy 2007 38 6 728 736
  124. 124. Pastine S. J. Okawa D. Kessler B. Rolandi M. Llorente M. Zettl A. Frechet J. M. J. 2008 A facile and patternable method for the surface modification of carbon nanotube forests using perfluoroarylazides. Journal of the American Chemical Society 130 13 4238 4239
  125. 125. Park M.J. Lee J. K. Lee B.S. Lee-W Y. Choi I. S. Lee-G S. Covalent Modification of Multiwalled Carbon Nanotubes with Imidazolium-Based Ionic Liquids Effect of Anions on Solubility. Chemistry of Materials 2006 18 1546 51
  126. 126. Akbar S. Beyou E. Chaumont P. Mélis F. 2010 Effect of a Nitroxyle-Based Radical Scavenger on Nanotube Functionalisation with Pentadecane A Model Compound Study for Polyethylene Grafting onto MWCNTs, Macromolecular Chemistry and Physics 211 22 2396 2406
  127. 127. Mylvaganam K. Zhang L. C. 2004 Nanotube functionalization and polymer grafting: An ab initio study Journal of Physical Chemistry B 108 39 5217 5220
  128. 128. Akbar S. Beyou E. Chaumont P. Mazzolini J. Espinosa E. D’Agosto F. Boisson C. 2011 Synthesis of Polyethylene-Grafted Multiwalled Carbon Nanotubes via a Peroxide-Initiating Radical Coupling Reaction and by Using Well-Defined TEMPO and Thiol End-Functionalized Polyethylenes Journal of Polymer Science Part A:Polymer Chemistry 49 4 957 965
  129. 129. Farzi G. Akbar S. Beyou E. Cassagnau P. Mélis F. 2009 Effect of radical grafting of tetramethylpentadecane and polypropylene on carbon nanotubes’ dispersibility in various solvents and polypropylene matrix Polymer 50 25 5901 5908
  130. 130. Lopez R. G. Boisson C. D’agosto F. Spitz R. Boisson F. Bertin D. Tordo P. 2004 Synthesis and characterization of macroalkoxyamines based on polyethylene Macromolecules 37 10 3540 3542
  131. 131. Lopez R. G. Boisson C. D’agosto F. Spitz R. Boisson F. Gigmes D. Bertin D. 2007 Catalyzed chain growth of polyethylene on magnesium for the synthesis of macroalkoxyamines Application to the production of block copolymers using controlled radical polymerization Journal of Polymer Science, Part A: Polymer Chemistry 45 13 2705 2718
  132. 132. Mazzolini J. Espinosa E. D’Agosto F. Boisson C. 2010Catalyzed chain growth (CCG) on a main group metal: an efficient tool to functionalize polyethylene Polymer Chemistry 1 6 793 800
  133. 133. D’Agosto F. Boisson C. 2010 A RAFT Analogue Olefin Polymerization Technique Using Coordination Chemistry Australian Journal of Chemistry 63 8 1155 1158
  134. 134. Mazzolini J. Mokthari I. Briquet R. Boyron O. Delolme F. Monteil V. Gigmes D. Bertin D. D’Agosto F. Boisson C. 2010 Thiol-End-Functionalized Polyethylenes Macromolecules 43 18 7495 7503
  135. 135. Hawker C. J. Bosman A. W. Harth E. 2001 New polymer synthesis by nitroxide mediated living radical polymerizations Chemical Review 101 12 3661 3688
  136. 136. Lou X. Detrembleur C. Sciannamea V. Pagnoulle C. Jerome R. 2004 Grafting of alkoxyamine end-capped (co)polymers onto multi-walled carbon nanotubes Polymer 45 18 6097 6102
  137. 137. Liu Y. Yao Z. Adronov A. 2005 Functionalization of single-walled carbon nanotubes with well-defined polymers by radical coupling Macromolecules 38 4 1172 1179
  138. 138. Wang H. C. Li Y. Yang M. 2007 Sensors for organic vapor detection based on composites of carbon nonotubes functionalized with polymers Sensors and Actuators B-Chemical 124 2 360 367
  139. 139. Wang Y. Wu J. Wei F. 2003 A treatment method to give separated multi-walled carbon nanotubes with high purity, high crystallization and a large aspect ratio Carbon 41 15 2939 2948
  140. 140. Lee S. H. Cho E. Jeon S. H. Youn J. R. 2007 Rheological and electrical properties of polypropylene composites containing functionalized multi-walled carbon nanotubes and compatibilizers Carbon 45 14 2810 2822
  141. 141. Wang Y. Wen X. Wan D. Zhang Z. Tang T. 2012 Promoting the responsive ability of carbon nanotubes to an external stress field in a polypropylene matrix: A synergistic effect of the physical interaction and chemical linking Journal Material Chemistry 22 3930 3938
  142. 142. Zheng J. Zhu Z. Qi j. Zhou Z. Li P. Peng M. 2011 Preparation of isotactic polypropylene-grafted multiwalled carbon nanotubes (iPP-g-MWNTs) by macroradical addition in solution and the properties of iPP-g-MWNTs/iPP composites Journal Material Science 46 648 658
  143. 143. Jancar J. Douglas J. F. Starr F. W. Kumar S. K. Cassagnau P. Lesser A. J. Sternstein S. S. Buehler M. J. 2010 Current issues in research on structure-property relationships in polymer nanocomposites Polymer 51 15 3321 3343
  144. 144. Cassagnau P. Melt rheology of organoclay and fumed silica nanocomposites Polymer 2008
  145. 145. Moreira L. Fulchiron R. Seytre G. Dubois P. Cassagnau P. 2010 Aggregation of Carbon Nanotubes in Semidilute Suspension Macromolecules 43 3 1467 1472
  146. 146. Hobbie E.K. Shear rheology of carbon nanotube suspensions Rheologica Acta 2010
  147. 147. Marceau S. Dubois P. Fulchiron R. Cassagnau P. 2009 Viscoelasticity of Brownian Carbon Nanotubes in PDMS Semidilute Regime Macromolecules 42 5 1433 1438
  148. 148. Mitchell C. A. Bahr J. L. Arepalli S. Tour J. M. Krishnamoorti R. 2002 Dispersion of functionalized carbon nanotubes in polystyrene Macromolecules 35 23 8825 8830
  149. 149. Pujari S. Ramanathan T. Kasimatis K. Masuda J. Andrews R. Torkelson J. M. Brinson L. C. Burghardt W. R. 2009 Preparation and Characterization of Multiwalled Carbon Nanotube Dispersions in Polypropylene: Melts Mixing versus Soli-State Shear Pulverization Journal of Polymer Science, Part B: Polymer Physics 47 14 1426 1436
  150. 150. Hong J. S. Hong I. G. Lim H. T. Ahn K. H. Lee S. J. 2012 In situ lubrication dispersion of Multi-walled carbon nanotubes in Polypropylene melts Macromolecular Material Engineering 297 3 279 287
  151. 151. Wu D. Sun Y. Wu L. Zhang M. 2008 Linear Viscoelasticity Properties and Crystallisation Behavior of Multi-Walled Carbon Nanotube/Polypropylene Composites Journal of Applied Polymer Science 108 3 1506 1513
  152. 152. Wu D. Sun Y. Zhang M. 2009 Kinetics Study on Melt Compounding of Carbon Nanotube/Polypropylene Nanocomposites Journal of Polymer Science, Part B: Polymer Physics 47 6 608 618
  153. 153. Jin S. H. Kang C. H. Yoon K. H. Bang D. S. Park Y. B. 2009 Effect of Compatibilizer on Morphology, Thermal and Rheological Properties of Polypropylene/Functionalized Multi-walled Carbon Nanotubes composite Journal of Applied Polymer Science 111 2 1028 1033
  154. 154. Menzer K. Hrausee B. Boldt R. Kretzschmar B. Weidisch R. Potschke P. 2011 Percolation Behaviour of multiwalled carbon nanotubes of altered length and primary agglomerate morphology in melt mixed isotactic polypropylene-based composites Composite Science and Technology 71 16 1936 1943
  155. 155. Hemmati M. Rahimi G. H. Kaganj A. B. Sepehri S. Rashidi A. M. 2008 Rheological and Mechanical Characterization of Multi-walled Carbon Nanotubes/Polypropylene Nanocomposites Journal of Macromolecular Science, Part B: Physics 47 6 1176 1187
  156. 156. Prashantha K. Soulestin J. Lacrampe M. F. Krawczak P. Dupin G. Claes M. 2009 Masterbatch-based multi-walled carbon nanotube filled polypropylene nanocomposites: Assessment of rheological and mechanical properties Composite Science and Technology 69 11-12 1756 1763
  157. 157. Palza H. Reznik B. Kappes M. Hennrich F. Naue I. F. C. Wilhelm M. 2010 Characterization of melt flow instabilities in polyethylene/carbon nanotube Polymer 51 16 3753 3761

Written By

Emmanuel Beyou, Sohaib Akbar, Philippe Chaumont and Philippe Cassagnau

Submitted: March 9th, 2012 Published: May 9th, 2013