**1. Introduction**

Carbon nanotubes describes a specific topic within solid-state physics, but is also of interest in other sciences like chemistry or biology. Actually the topic has floating boundaries, because we are at the molecule level. In the recent years carbon nanotubes have become more and more popular to the scientists. Initially, it was the spectacularly electronic properties, that were the basis for the great interest, but eventually other remarkable properties were also discovered.

The first CNTs were prepared by M. Endo in 1978, as part of his PhD studies at the Universi‐ ty of Orleans in France. Although he produced very small diameter filaments (about 7 nm) using a vapour-growth technique, these fibers were not recognized as nanotubes and were not studied systematically. It was only after the discovery of fullerenes, C60, in 1985 that re‐ searchers started to explore carbon structures further. In 1991, when the Japanese electron microscopist Sumio Iijima [1] observed CNTs, the field really started to advance. He was studying the material deposited on the cathode during the arc-evaporation synthesis of full‐ erenes and came across CNTs. A short time later, Thomas Ebbesen and Pulickel Ajayan, from Iijima's lab, showed how nanotubes could be produced in bulk quantities by varying the arc-evaporation conditions. However, the standard arc-evaporation method only pro‐ duced only multiwall nanotubes. After some research, it was found that the addition of met‐ als such as cobalt to the graphite electrodes resulted in extremely fine single wall nanotubes.

The synthesis in 1993 of single-walled carbon nanotubes (SWNTs) was a major event in the development of CNTs. Although the discovery of CNTs was an accidental event, it opened the way for a flourishing research into the properties of CNTs in labs all over the world, with many scientists demonstrating promising physical, chemical, structural, and optical properties of CNTs.

© 2013 Tarawneh and Ahmad; 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 Tarawneh and Ahmad; 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.

CNTs exhibit a great range of remarkable properties, including unique mechanical and elec‐ trical characteristics. These remarkable high modulus and stiffness properties have led to the use of CNTs to reinforce polymers in the past few years. Both theoretical (e.g. molecular structural mechanics and tight-binding molecular dynamics) and experimental studies have shown SWCNTs to have extremely high elastic modulus (≈1 TPa) [2-3]. The tensile strength of SWCNTs estimated from molecular dynamics simulation is ≈150 MPa [4]. The experimen‐ tal measurement of 150 MPa was found for the break strength of multi-walled carbon nano‐ tubes (MWCNTs) [5].

The carbon nanotube additions to polyurethane (PU) improve the mechanical properties such as increased modulus and yield stress, without loss of the ability to stretch the elasto‐ mer above 1000% before final failure; the addition of CNTs increases the modulus and strength of PU without degrading deformabilty. The elongation at break decreases very slightly with CNT loading up to 17 wt.%. At this filler loading, the nanocomposite still

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Theoretical prediction showed an extremely high thermal conductivity (6000 W/mK) of an isolated SWCNTs [11]. High thermal conductivity of the CNTs may provide the solution of thermal management for the advanced electronic devices with narrow line width. Revealed the thermal conductivity of epoxy-based composites reinforced with 1.0 wt.% SWCNTs in‐ creased over 125% reaching a value of ~0.5 W/mK [12]. The variation of thermal conductivi‐ ty with the values of 35 and 2.3 W/mK for a densepacked mat and a sintered sample, respectively [13]. High thermal conductivity of 42 and ~18W/mK of the aligned and the ran‐ dom bucky paper mats, respectively. However, the thermal conductivity drops significantly by almost an order of magnitude when the aligned bucky paper mats were loaded with ep‐

oxy, the volume fraction of the aligned bucky paper composites is about 50%[14].

thermal transfer more significantly than that of the nanotubes themselves [16].

non mean free path at higher temperature [17].

Developed an infiltration method to produce CNTs/epoxy composites and showed a 220% increase in thermal conductivity (~0.61 W/mK) at 2.3 wt.% SWCNT loading, and they found that the electrical resistance between SWCNT-polymer is more severe than that of SWCNT– SWCNT [15]. Prepared SWCNT and MWCNT films and reported the thermal conductivity of 1.64 and 1.51 W/mK, respectively; they concluded that the intra-tube spacing affects the

The thermal conductivities of composites reinforced with 1.0 wt.% SWCNTs and 4.0 wt.% MWCNTs are 2.43 W/mK and 3.44 W/mK, respectively. Composites reinforced with the un‐ purified CNTs have higher thermal conductivity than that of the purified CNTs reinforced composite. This is attributed to the generation of defects on the CNT surface during acid treatment. Moreover, due to longer phonon propagation length, it is found that thermal con‐ ductivity increases with temperatures over the range from 25 to 55°C for both SWCNTs/ Poly (methyl methacrylate) PMMA and MWCNTs/PMMA composites. However, the ther‐ mal conductivities of CNT films decrease with increasing temperature, which results from phonon scattering during transfer due to the presence of defects coupled with smaller pho‐

The differences in the composite manufacturing methods, powder-(MWCNTs and ball mil‐ led SWCNTs) or liquid- (chemically treated SWCNTs) based approach, can not account for the differences in the properties, since both methods were used for the SWNT-composites and resulted in similar thermal behaviour [18]. Thus, they concluded that in this case, there must be a very large interface resistance to the heat flow associated with poor phonon cou‐ pling between the stiff nanotubes and the (relatively) soft polymer matrix. In addition it is possible that the phonon vibrations in the SWCNTs are dampened by the matrix interaction, while in the MWCNTs the phonons can be carried in the inner walls without hindrance.

maintains a very high value of elongation at rupture, i.e. 1200% [10].

The remarkable properties of CNTs offer the potential for improvement of the mechanical properties of polymers at very low concentrations. In practice, MWCNTs are preferred over SWCNTs as the reinforcing fillers for polymers due to their lower production cost. Howev‐ er, slippage between the shells of MWCNTs would undermine the capability of the fillers to bear the external applied load.

Mixed 1 wt.% MWCNTs with polystyrene (PS) in toluene via ultrasonication, achieved about 36–42% increase in the elastic modulus and a 25% increase in the tensile strength of the PS–MWCNT film compared to pure PS [6]. They found that nanotube fracture and pull‐ out are responsible for the failure of the composite. The fracture of MWCNTs in a PS matrix implies that certain load transfer from the PS to the nanotubes has taken place. However, the pullout of MWCNTs from the PS matrix indicating that the PS–nanotube interfacial strength is not strong enough to resist debonding of the fillers from the matrix. It is consid‐ ered that some physical interactions exist at the PS–MWCNT interface, thereby enabling load transfer from the matrix to the fillers.

The additions of 0.25– 0.75 wt.% SWCNTs to polypropylene (PP) considerably its tensile strength and stiffness as well as storage modulus. The elongation at break reduces from 493 (PP) to 410% with the addition of 0.75 wt.% filler, corresponding to -17% reduction in ductil‐ ity. At 1 wt.% SWNT, both stiffness and strength are significantly reduced due to the forma‐ tion of aggregates [7].

The morphology and mechanical properties of the melt-compounded polyamide 6 (PA6)– MWNT nanocomposites were studied by [8]. The MWCNTs were purified by dissolving the catalyst in hydrochloric acid followed by refluxing in 2.6 Mnitric acids to increasing the car‐ boxylic and hydroxyl groups. It was also found that with the addition of only 1 wt.% MWCNTs, the tensile modulus and the tensile strength are greatly improved by ≈115 and 120%, respectively compared to neat PA6. The tensile ductility drops slightly from 150 to 125%. They attributed the improvements of these mechanical properties to a better disper‐ sion of MWCNTs in PA6 matrix, and to a strong interfacial adhesion between the nanofillers and PA6 matrix which leads to favorable stress transfer across the polymer to the MWCNTs.

The influence of SWNT and carbon nanofiber additions on the mechanical performances of silicone rubber was reported by [9]. They reported that SWCNTs are effective reinforce‐ ments for silicone rubber due to their large aspect ratio and low density. The initial modulus (measured by fitting a straight line to the data below 10% strain) tends to increase almost linearly with increasing filler content. The effect of SWCNT and carbon fiber additions on the tensile ductility of silicone rubber is shown that the strain to failure drops from 325 to 275% upon loading with 1 wt.% SWCNTs, corresponding to ≈15% reduction.

The carbon nanotube additions to polyurethane (PU) improve the mechanical properties such as increased modulus and yield stress, without loss of the ability to stretch the elasto‐ mer above 1000% before final failure; the addition of CNTs increases the modulus and strength of PU without degrading deformabilty. The elongation at break decreases very slightly with CNT loading up to 17 wt.%. At this filler loading, the nanocomposite still maintains a very high value of elongation at rupture, i.e. 1200% [10].

CNTs exhibit a great range of remarkable properties, including unique mechanical and elec‐ trical characteristics. These remarkable high modulus and stiffness properties have led to the use of CNTs to reinforce polymers in the past few years. Both theoretical (e.g. molecular structural mechanics and tight-binding molecular dynamics) and experimental studies have shown SWCNTs to have extremely high elastic modulus (≈1 TPa) [2-3]. The tensile strength of SWCNTs estimated from molecular dynamics simulation is ≈150 MPa [4]. The experimen‐ tal measurement of 150 MPa was found for the break strength of multi-walled carbon nano‐

The remarkable properties of CNTs offer the potential for improvement of the mechanical properties of polymers at very low concentrations. In practice, MWCNTs are preferred over SWCNTs as the reinforcing fillers for polymers due to their lower production cost. Howev‐ er, slippage between the shells of MWCNTs would undermine the capability of the fillers to

Mixed 1 wt.% MWCNTs with polystyrene (PS) in toluene via ultrasonication, achieved about 36–42% increase in the elastic modulus and a 25% increase in the tensile strength of the PS–MWCNT film compared to pure PS [6]. They found that nanotube fracture and pull‐ out are responsible for the failure of the composite. The fracture of MWCNTs in a PS matrix implies that certain load transfer from the PS to the nanotubes has taken place. However, the pullout of MWCNTs from the PS matrix indicating that the PS–nanotube interfacial strength is not strong enough to resist debonding of the fillers from the matrix. It is consid‐ ered that some physical interactions exist at the PS–MWCNT interface, thereby enabling

The additions of 0.25– 0.75 wt.% SWCNTs to polypropylene (PP) considerably its tensile strength and stiffness as well as storage modulus. The elongation at break reduces from 493 (PP) to 410% with the addition of 0.75 wt.% filler, corresponding to -17% reduction in ductil‐ ity. At 1 wt.% SWNT, both stiffness and strength are significantly reduced due to the forma‐

The morphology and mechanical properties of the melt-compounded polyamide 6 (PA6)– MWNT nanocomposites were studied by [8]. The MWCNTs were purified by dissolving the catalyst in hydrochloric acid followed by refluxing in 2.6 Mnitric acids to increasing the car‐ boxylic and hydroxyl groups. It was also found that with the addition of only 1 wt.% MWCNTs, the tensile modulus and the tensile strength are greatly improved by ≈115 and 120%, respectively compared to neat PA6. The tensile ductility drops slightly from 150 to 125%. They attributed the improvements of these mechanical properties to a better disper‐ sion of MWCNTs in PA6 matrix, and to a strong interfacial adhesion between the nanofillers and PA6 matrix which leads to favorable stress transfer across the polymer to the MWCNTs. The influence of SWNT and carbon nanofiber additions on the mechanical performances of silicone rubber was reported by [9]. They reported that SWCNTs are effective reinforce‐ ments for silicone rubber due to their large aspect ratio and low density. The initial modulus (measured by fitting a straight line to the data below 10% strain) tends to increase almost linearly with increasing filler content. The effect of SWCNT and carbon fiber additions on the tensile ductility of silicone rubber is shown that the strain to failure drops from 325 to

275% upon loading with 1 wt.% SWCNTs, corresponding to ≈15% reduction.

tubes (MWCNTs) [5].

tion of aggregates [7].

bear the external applied load.

load transfer from the matrix to the fillers.

118 Syntheses and Applications of Carbon Nanotubes and Their Composites

Theoretical prediction showed an extremely high thermal conductivity (6000 W/mK) of an isolated SWCNTs [11]. High thermal conductivity of the CNTs may provide the solution of thermal management for the advanced electronic devices with narrow line width. Revealed the thermal conductivity of epoxy-based composites reinforced with 1.0 wt.% SWCNTs in‐ creased over 125% reaching a value of ~0.5 W/mK [12]. The variation of thermal conductivi‐ ty with the values of 35 and 2.3 W/mK for a densepacked mat and a sintered sample, respectively [13]. High thermal conductivity of 42 and ~18W/mK of the aligned and the ran‐ dom bucky paper mats, respectively. However, the thermal conductivity drops significantly by almost an order of magnitude when the aligned bucky paper mats were loaded with ep‐ oxy, the volume fraction of the aligned bucky paper composites is about 50%[14].

Developed an infiltration method to produce CNTs/epoxy composites and showed a 220% increase in thermal conductivity (~0.61 W/mK) at 2.3 wt.% SWCNT loading, and they found that the electrical resistance between SWCNT-polymer is more severe than that of SWCNT– SWCNT [15]. Prepared SWCNT and MWCNT films and reported the thermal conductivity of 1.64 and 1.51 W/mK, respectively; they concluded that the intra-tube spacing affects the thermal transfer more significantly than that of the nanotubes themselves [16].

The thermal conductivities of composites reinforced with 1.0 wt.% SWCNTs and 4.0 wt.% MWCNTs are 2.43 W/mK and 3.44 W/mK, respectively. Composites reinforced with the un‐ purified CNTs have higher thermal conductivity than that of the purified CNTs reinforced composite. This is attributed to the generation of defects on the CNT surface during acid treatment. Moreover, due to longer phonon propagation length, it is found that thermal con‐ ductivity increases with temperatures over the range from 25 to 55°C for both SWCNTs/ Poly (methyl methacrylate) PMMA and MWCNTs/PMMA composites. However, the ther‐ mal conductivities of CNT films decrease with increasing temperature, which results from phonon scattering during transfer due to the presence of defects coupled with smaller pho‐ non mean free path at higher temperature [17].

The differences in the composite manufacturing methods, powder-(MWCNTs and ball mil‐ led SWCNTs) or liquid- (chemically treated SWCNTs) based approach, can not account for the differences in the properties, since both methods were used for the SWNT-composites and resulted in similar thermal behaviour [18]. Thus, they concluded that in this case, there must be a very large interface resistance to the heat flow associated with poor phonon cou‐ pling between the stiff nanotubes and the (relatively) soft polymer matrix. In addition it is possible that the phonon vibrations in the SWCNTs are dampened by the matrix interaction, while in the MWCNTs the phonons can be carried in the inner walls without hindrance.

The precise sectioning of CNTs provides an effective way to shorten carbon nanotubes with controlled length and minimum sidewall damage [19]. For shortened nanotubes they found that they are easily dispersed into polymer matrices, which effectively improved the perco‐ lation. The minimum CNT sidewall damage and improved percolation in short SWCNT composites led to an obvious improvement of thermal conductivity. Hence, their research suggests an effective way to improve dispersion of CNTs into polymer matrices and also re‐ tain the perfect electronic structure of the CNTs, resulting in desired functional materials.

The synthesized carbon nanotubes usually exist as agglomerates of the size of several hun‐ dred micrometers [25]. Such entanglements make it difficult to disperse nanotubes uniform‐ ly in a polymer matrix. To overcome the dispersion problem, it is necessary to tailor the chemical nature of the nanotube surface. One of the most straightforward methods for nano‐ tube dispersion is direct mixing; however, it does not always yield a homogeneous distribu‐ tion of nanotubes because of the lack of compatibility between the MWCNTs and polymer matrix. Solution processing has been a commonly used method in fabrication of the welldispersed carbon nanotube composites. However, it is hard to achieve homogeneous disper‐ sion of nanotubes in a polymer matrix because carbon nanotubes are insoluble and bundled.

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Chemical functionalization of the MWCNTs surface increases the interfacial interaction be‐ tween MWCNTs and the polymer matrix. This enhances the adhesion of the MWCNTs in various organic solvents and polymers, reduces the tendency to agglomerate, and improves dispersion. The improved interactions between MWCNTs and the polymer matrix govern the load-transfer from the polymer to the nanotubes and, hence, increase the reinforcement efficiency. Attachment of oxygen containing functional groups (i.e., carboxyl groups, car‐ bonyl groups, hydroxyl groups, etc.) on the surface of the MWCNTs could be achieved by

The chemically functionalized MWCNTs can be easily mixed with the polymer matrix. Acid treatment of the nanotube is an especially well-known technique to remove catalytic impuri‐ ties, generate functional groups on open ends or sidewalls of nanotubes, and facilitate good

The emergence of thermoplastic elastomers (TPEs) is one of the most important develop‐ ments in the area of polymer science and technology. TPEs are a new class of material that combines the properties of vulcanized rubber with the ease of processability of thermoplas‐ tics [26]. Thermoplastic elastomers can be prepared by blending thermoplastic and elasto‐ mers at a high shear rate. Thermoplastics, for example, polypropylene (PP), polyethylene (PE) and polystyrene (PS), and elastomers, such as ethylene propylene diene monomer (EPDM), natural rubber (NR) and butyl rubber (BR), are among the materials used in ther‐

Blends of natural rubber (NR) and polypropylene (PP) have been widely reported by previ‐ ous researchers [26]. According to them, polypropylene is the best choice for blending with natural rubber due to its high softening temperature (150°C) and low glass transition tem‐ perature (-60°C, is Tg for NR), which makes it versatile in a wide range of temperatures. Even though NR and PP are immiscible, their chemical structure is nearly the same. Thus, stable dispersion of NR and PP is possible. Incompatibility between NR and PP can be over‐ come by the introduction of a compatibiliser that can induce interactions during blending. Compatibility is important as it may affect the morphology, mechanical and thermal proper‐ ties of the blends. Among the commonly used compatibilisers are dicumyl peroxide (DCP), m-phenylene bismaleimide (HVA-2) and liquid natural rubber (LNR). Apart from compati‐ bility, mixing torque and curing are interrelated in determining the homogeneity of the

applying several chemical treatments.

moplastic elastomer blends.

TPNR blend.

dispersion of MWCNTs in polymeric solutions or melts.

Accurate measurement of the thermal conductivity of composites and nanocomposites can be done using the transient hot-wire technique which is capable of measuring the thermal conductivity of solid materials in an absolute way. The enhancement in the thermal conduc‐ tivity was measured as 27% in relation to the thermal conductivity of the epoxy-resin poly‐ mer, which is satisfactory taking into account the low volume fraction (28%) of the glass fibres used in the composite [20]. They reported that when 2% by weight C-MWNT were mixed with the epoxy-resin, the enhancement of thermal conductivity was 9% while using both glass fibres and C-MWNT the enhancement was 48%.

For sufficient enhancement of most of the nanocomposites' properties, the dispersion of the CNTs should be very fine in the polymer matrix, which means that the surface of interaction between the filler and the matrix should be optimised. However, this is difficult to achieve since their long length results in them becoming entangled. Moreover, their very large sur‐ face-to-volume ratio and strong van der Waals interactions keep them tied together, which in most cases leads to the formation of large agglomerates in polymer matrices. The interfa‐ cial adhesion between CNTs and the polymeric matrix is also crucial. In order to increase the interfacial adhesion between the polymer and the CNTs various routes of surface modi‐ fication of the nanotubes have been considered. One is non-covalent functionalisation of molecules and the other is covalent functionalisation from the walls of the nanotubes. Noncovalent functionalisation is based on weak Van der Waals forces [21]. The advantage of non-covalent functionalisation is that the perfect structure of the nanotubes is not altered while the covalent attachment can greatly improve the load transfer to the matrix; however, it usually introduces structural defects on the nanotubes' surface.

Although both probe style and bath style ultrasonic systems can be used for dispersing CNTs, it is widely believed that the probe style ultrasonic systems work better for dispers‐ ing CNTs [22]. It is also widely known that adding a dispersing reagent (surfactant) into the solution will accelerate the dispersion effect.

The most common procedure used for covalent attachment of reactive groups is the treat‐ ment with inorganic acids. Usually the nanotubes are refluxed with a nitric acid solution or a mixture of nitric and sulfuric acid, sometimes concurrently with the application of high power sonication [23]. These oxidative treatments usually result in shortening of the CNTs' length and formation of surface reactive groups, such as hydroxyl, carbonyl and mainly car‐ boxylic acid. Oxidation of the nanotubes starts at the tips and gradually moves towards the central part of the tube and the layers are removed successively [24].

The synthesized carbon nanotubes usually exist as agglomerates of the size of several hun‐ dred micrometers [25]. Such entanglements make it difficult to disperse nanotubes uniform‐ ly in a polymer matrix. To overcome the dispersion problem, it is necessary to tailor the chemical nature of the nanotube surface. One of the most straightforward methods for nano‐ tube dispersion is direct mixing; however, it does not always yield a homogeneous distribu‐ tion of nanotubes because of the lack of compatibility between the MWCNTs and polymer matrix. Solution processing has been a commonly used method in fabrication of the welldispersed carbon nanotube composites. However, it is hard to achieve homogeneous disper‐ sion of nanotubes in a polymer matrix because carbon nanotubes are insoluble and bundled.

The precise sectioning of CNTs provides an effective way to shorten carbon nanotubes with controlled length and minimum sidewall damage [19]. For shortened nanotubes they found that they are easily dispersed into polymer matrices, which effectively improved the perco‐ lation. The minimum CNT sidewall damage and improved percolation in short SWCNT composites led to an obvious improvement of thermal conductivity. Hence, their research suggests an effective way to improve dispersion of CNTs into polymer matrices and also re‐ tain the perfect electronic structure of the CNTs, resulting in desired functional materials.

Accurate measurement of the thermal conductivity of composites and nanocomposites can be done using the transient hot-wire technique which is capable of measuring the thermal conductivity of solid materials in an absolute way. The enhancement in the thermal conduc‐ tivity was measured as 27% in relation to the thermal conductivity of the epoxy-resin poly‐ mer, which is satisfactory taking into account the low volume fraction (28%) of the glass fibres used in the composite [20]. They reported that when 2% by weight C-MWNT were mixed with the epoxy-resin, the enhancement of thermal conductivity was 9% while using

For sufficient enhancement of most of the nanocomposites' properties, the dispersion of the CNTs should be very fine in the polymer matrix, which means that the surface of interaction between the filler and the matrix should be optimised. However, this is difficult to achieve since their long length results in them becoming entangled. Moreover, their very large sur‐ face-to-volume ratio and strong van der Waals interactions keep them tied together, which in most cases leads to the formation of large agglomerates in polymer matrices. The interfa‐ cial adhesion between CNTs and the polymeric matrix is also crucial. In order to increase the interfacial adhesion between the polymer and the CNTs various routes of surface modi‐ fication of the nanotubes have been considered. One is non-covalent functionalisation of molecules and the other is covalent functionalisation from the walls of the nanotubes. Noncovalent functionalisation is based on weak Van der Waals forces [21]. The advantage of non-covalent functionalisation is that the perfect structure of the nanotubes is not altered while the covalent attachment can greatly improve the load transfer to the matrix; however,

Although both probe style and bath style ultrasonic systems can be used for dispersing CNTs, it is widely believed that the probe style ultrasonic systems work better for dispers‐ ing CNTs [22]. It is also widely known that adding a dispersing reagent (surfactant) into the

The most common procedure used for covalent attachment of reactive groups is the treat‐ ment with inorganic acids. Usually the nanotubes are refluxed with a nitric acid solution or a mixture of nitric and sulfuric acid, sometimes concurrently with the application of high power sonication [23]. These oxidative treatments usually result in shortening of the CNTs' length and formation of surface reactive groups, such as hydroxyl, carbonyl and mainly car‐ boxylic acid. Oxidation of the nanotubes starts at the tips and gradually moves towards the

both glass fibres and C-MWNT the enhancement was 48%.

120 Syntheses and Applications of Carbon Nanotubes and Their Composites

it usually introduces structural defects on the nanotubes' surface.

central part of the tube and the layers are removed successively [24].

solution will accelerate the dispersion effect.

Chemical functionalization of the MWCNTs surface increases the interfacial interaction be‐ tween MWCNTs and the polymer matrix. This enhances the adhesion of the MWCNTs in various organic solvents and polymers, reduces the tendency to agglomerate, and improves dispersion. The improved interactions between MWCNTs and the polymer matrix govern the load-transfer from the polymer to the nanotubes and, hence, increase the reinforcement efficiency. Attachment of oxygen containing functional groups (i.e., carboxyl groups, car‐ bonyl groups, hydroxyl groups, etc.) on the surface of the MWCNTs could be achieved by applying several chemical treatments.

The chemically functionalized MWCNTs can be easily mixed with the polymer matrix. Acid treatment of the nanotube is an especially well-known technique to remove catalytic impuri‐ ties, generate functional groups on open ends or sidewalls of nanotubes, and facilitate good dispersion of MWCNTs in polymeric solutions or melts.

The emergence of thermoplastic elastomers (TPEs) is one of the most important develop‐ ments in the area of polymer science and technology. TPEs are a new class of material that combines the properties of vulcanized rubber with the ease of processability of thermoplas‐ tics [26]. Thermoplastic elastomers can be prepared by blending thermoplastic and elasto‐ mers at a high shear rate. Thermoplastics, for example, polypropylene (PP), polyethylene (PE) and polystyrene (PS), and elastomers, such as ethylene propylene diene monomer (EPDM), natural rubber (NR) and butyl rubber (BR), are among the materials used in ther‐ moplastic elastomer blends.

Blends of natural rubber (NR) and polypropylene (PP) have been widely reported by previ‐ ous researchers [26]. According to them, polypropylene is the best choice for blending with natural rubber due to its high softening temperature (150°C) and low glass transition tem‐ perature (-60°C, is Tg for NR), which makes it versatile in a wide range of temperatures. Even though NR and PP are immiscible, their chemical structure is nearly the same. Thus, stable dispersion of NR and PP is possible. Incompatibility between NR and PP can be over‐ come by the introduction of a compatibiliser that can induce interactions during blending. Compatibility is important as it may affect the morphology, mechanical and thermal proper‐ ties of the blends. Among the commonly used compatibilisers are dicumyl peroxide (DCP), m-phenylene bismaleimide (HVA-2) and liquid natural rubber (LNR). Apart from compati‐ bility, mixing torque and curing are interrelated in determining the homogeneity of the TPNR blend.

Mechanical blending of PP and NR with the addition of LNR as a compatibiliser has been reported to be optimal at a temperature of 175-185°C and a rotor speed of 30-60rpm. The percentage of LNR used depends on the ratio of NR to PP. For a NR:PP ratio of 30:70 the best physical properties are obtained at 10% LNR [27]. The compatibiliser helps to induce the interaction between the rubber and plastic interphase and thereby increases the homoge‐ neity of the blend.

**2. Experiment Details**

(MWCNTs).

MWCNTs.

24hours [31].

**2.2. Acid Treatment of MWCNTs**

photochemical degradation technique.

Polypropylene, with a density of 0.905 g cm-3, was supplied by Propilinas (M) Sdn. Bhd, natural rubber was supplied by Guthrie (M) Sdn. Bhd, and polypropylene (PP) with a densi‐ ty of 0.905 g/cm3 was supplied by Polipropilinas (M) Sdn. Bhd were used in this research. Maleic anhydride–grafted–polypropylene (MAPP) with a density of 0.95 g/cm3 was sup‐ plied from Aldrich Chemical Co., USA. Liquid natural rubber (LNR) was prepared by the

Characterization and Morphology of Modified Multi-Walled Carbon Nanotubes Filled Thermoplastic Natural Rubber

A Multi-walled carbon nanotubes (MWCNTs) were provided by Arkema (Graphis‐ trengthTM C100). Table 1 shows the properties of multi-walled carbon nanotubes

**MWCNTs Purity Length Diameter Manufactured**

**2.1. Preparation of TPNR-Multi-Walled Carbon Nanotubes (MWCNTs) Composite**

Mixing was performed by an internal mixer (Haake Rheomix 600P). The mixing tempera‐ ture was 180°C, with a rotor speed of 100 rpm and 13 min mixing time. The indirect techni‐ que (IDT) was used to prepare nanocomposites, this involved mixing the MWCNTs with LNR separately, before it was melt blended with PP and NR in the internal mixer. TPNR nanocomposits were prepared by melt blending of PP, NR and LNR with MWCNTs in a ra‐ tio of 70 wt% PP, 20 wt% NR and 10wt% LNR as a compatibiliser and 1,3,5 and 7%

Two types of MWCNTs were introduced to the TPNR which is untreated MWCNTs (MWCNTs 1) and treated MWCNTs (MWCNTs 2), MWCNTs 2 were treated by immersing neat MWCNTs in a mixture of nitric and sulfuric acid with a molar ratio of 1:3, respectively. In a typical experiment, 1g of raw MWCNTs was added to 40ml of the acid mixture. Then, the oxidation reaction was carried out in a two-necked, round-bottomed glass flask equip‐ ped with reflux condenser, magnetic stirrer and thermometer. The reaction was carried out for 3 hours at 140°C. After that, this mixture was washed with distilled water on a sintered glass filter until the pH value was around 7, and was dried in a vacuum oven at 70°C for

Catalytic Chemical Vapour Deposition (CVD)

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MWCNTs "/90% 0.1-10 μm 10-15 nm.

**Table 1.** Properties of multi-walled carbon nanotubes (MWCNTs).

MWCNTs/TPNR composites with different amounts of MWCNT were prepared and their thermal properties have been investigated by [28]. The higher thermal conductivity was ach‐ ieved in the samples with 1 and 3wt% of MWCNTs compared to the pristine TPNR. Any sample with MWCNTs content higher than 3wt% caused the conductivity to decrease. In addition, the improvement of thermal diffusivity and specific heat was also achieved at the same percentage. DMA confirmed that the glass transition temperature (Tg) increased with the increase in the amount of MWCNTs.

The tensile strength, tensile modulus, and also the impact strength of TPNR/MWCNTs are improved significantly while sacrificing high elongation at break by incorporating MWCNTs. The reinforcing effect of MWCNTs was also confirmed by DMA where the addi‐ tion of nanotubes has increased the storage modulus, the loss modulus, and also the glass transition temperature (Tg). Homogeneous dispersion of MWCNTs throughout the TPNR matrix and strong interfacial adhesion between MWCNTs and matrix as confirmed by SEM images are proposed to be responsible for the significant mechanical enhancement [29].

The reinforcing effect of two types of MWCNTs has also confirmed by dynamic mechanical analysis where the addition of nanotubes have increased in the storage modulus E', and the loss modulus E'', in the addition the glass transition temperature (Tg) increased with an in‐ crease in the amount of MWCNTs. The addition of MWCNTs in the TPNR matrix improved the mechanical properties. The tensile strength and elongation at break of MWCNTs 1 in‐ creased by 23%, and 29%, respectively. The Young's modulus had increased by increasing the content of MWCNTs. For MWCNTs 2 the optimum result of tensile strength and Young's modulus was recorded at 3% which increased 39%, and 30%, respectively. The laser flash technique was used to measure the thermal conductivity, thermal diffusivity and spe‐ cific heat, from the results obtained. The high thermal conductivity was achieved at 1 wt% and 3 wt% of MWCNTs compared with TPNR after 3 wt% it decreased, also the improve‐ ment of thermal diffusivity and specific heat was achieved at the same percentage. The MWCNTs 1 and 2/TPNR nanocomposites were fabricated and the tensile and properties were measured [30].

In this chapter, the effect of multi-walled carbon nanotubes with and without acid treatment on the properties of thermoplastic natural rubber (TPNR) was investigated. Two types of MWCNTs were introduced into TPNR, which are untreated multi-walled carbon nanotubes (UTMWCNTs) (without acid treatment) and treated multi-walled carbon nanotubes (TMWCNTs) (with acid treatment). Using this method, MWCNTs are dispersed homogene‐ ously in the TPNR matrix in an attempt to increase the properties of these nanocomposites. The effect of MWCNTs on the mechanical and thermal properties of TPNR nanocomposites is reported in this chapter.
