**7. Properties of the nanocomposites**

#### **7.1. Mechanical properties of MWCNTs polymer nanocomposites**

Different thermoplastic and thermoset polymer matrices have been tried to realize the supe‐ rior mechanical properties of CNTs for development of light weight strong material. NASA scientists are considering CNT-polymer composite for space elevator. To date, a volume of literature is available on the improvement of mechanical performance of polymers with ad‐ dition of CNTs. The first study for formation of CNT-polymer composites was carried out by Ajayan et al. [69]. CNTs were aligned within the epoxy matrix by the shear forces in‐

duced by cutting with a diamond knife, however no quantitative mechanical measurements were made. The first true study for tensile and compression properties of CNT polymer composites was carried by Schadler et al. [75] with epoxy. On addition of 5-wt% MWCNTs the tensile modulus increased from 3.1 GPa to 3.71 GPa. and compression modulus in‐ creased from 3.63 to 4.5 GPa. However, no significant increases in toughness values were observed. Bai et al. [76] observed doubling of Young's modulus from 1.2 to 2.4 GPa and sig‐ nificant increase in strength from 30 to 41 MPa on addition of 1 wt.% MWCNTs. Also excel‐ lent matrix–nanotube adhesion was confirmed by the observation of nanotube breakage during fracture surface studies. Zhou et al. [77] reported steady increase of flexural modulus in CNT-epoxy composite with higher CNT weight percentage and found an improvement of 11.7% in modulus with 0.4 wt% loading of CNTs and 28% enhancement in flexural strength with 0.3 wt% loading. Garg et al. [31] reported an increment of 155 % in flexural strength of epoxy with addition of merely 0.3% amine functionalized MWCNTs and an increment of 38% in flexural modulus. Mathur et al. [13] reported an increment of 158% in flexural strength of phenolic with addition of 5 vol% of MWCNT. Colemann et al. [16] reviewed the mechanical properties of a large number of CNT reinforced polymer (thermoplastic and thermosetting) composites fabricated by various methods and reported enhancement in me‐ chanical properties. Du et al. [78] studied the experimental results for mechanical perform‐ ance of CNTs nanocomposites carried out by different research groups and observed that the gains are modest and far below the simplest theoretical estimates. Haggenmueller [79] applied the Halpen Tsai composite theory to CNT nanocomposites and observed that the ex‐ perimental elastic modulus is smaller than predicted by more than one order. It is attributed to the lack of perfect load transfer from nanotubes to matrix due to non uniform dispersion and small interfacial interaction. Although chemical functionalization of CNTs has sorted out those problems to an extent yet the best results have to be achieved. Also aspect ratio is other source of uncertainty in mechanical properties. Defects on the CNT surface also ex‐ pected to influence the mechanical properties significantly. The methods of handling nano‐ tubes, including acid treatments and sonication for long time are known to shorten nanotubes results in decreasing aspect ratio and are detrimental to mechanical properties. The mechanical properties of CNT based composites increased upto a certain loading of CNTs and beyond which it starts decreasing. This may be because of increase in viscosity of polymers at higher CNTs loading and also cause some surface of CNTs not to be completely covered by polymers matrix due to the large specific surface area of CNTs.Therefore, few studies have been carried out to disperse high loading of CNTs in polymer matrices with improved mechanical properties. Bradford et al. [66] reported 400 MPa (tensile strength) and 22.3 GPa (tensile modulus) for 27vol% of MWCNT–epoxy composites. Feng et al. [80] also reported 183% and 408% improvement in tensile strength and tensile modulus respec‐ tively at 39.1 wt% SWCNTs loading compared with those of the neat epoxy. Ogasawara et al. [67] found 50.6 GPa and 183 MPa modulus and ultimate tensile strength respectively of CNT (21.4 vol.%)/epoxy composite. These values were 19 and 2.9 times those of the epoxy resin respectively.

Recently, Feng et al., [64] reported a mixed curing assisted layer by layer method to synthe‐ size MWCNT/epoxy composite film with a high CNT loading from ~15 to ~36 wt.-%. The mixed-curing-agent consists of two types of agents, one of which is responsible for the parti‐ al initial curing at room temperature to avoid agglomeration of the CNTs, and the other for complete curing of epoxy resin at high temperature to synthesize epoxy composite films with good CNT dispersion. In another study by Feng et al. [65] upto ~39.1 wt. % SWCNTepoxy composites were fabricated using same mixed curing layer by layer method and their mechanical properties were enhanced significantly. Bradford et al. [66] reported a method to quickly produce macroscopic CNT composites with a high volume fraction upto 27% of mil‐ limeter long, well aligned CNTs. Specifically, they used the novel method, shear pressing, to process tall, vertically aligned CNT arrays into dense aligned CNT preforms, which are sub‐ sequently processed into composites. In another study by Ogasawara et al. [67] aligned MWCNT/epoxy composites were processed using a hot-melt prepreg method. Vertically aligned ultra-long CNT arrays (forest) were converted to horizontally aligned CNT sheets by pulling them out. An aligned CNT/epoxy prepreg was fabricated using hot-melting with

B-stage cured epoxy resin film. The final composites contains 21.4 vol% of MWCNTs.

The other key challenge is to understand the effect of nanotube alignment on nanocompo‐ sites properties because the nanotubes have asymmetric structure and properties. Like other one-dimensional fiber fillers CNTs displays highest properties in the oriented reinforced di‐ rection and the mechanical, electrical, magnetic and optical performance of its composites are linked directly to their alignment in the matrix. So to take the full advantage of excellent properties of CNTs these should be aligned in a particular direction. For example, the align‐ ment of CNT increases the elastic modulus and electrical conductivity of nanocomposites

Several methods like application of electric field during composite formation and carbon arc discharge [68], composite slicing [69], film rubbing [70], chemical vapor deposition [71, 72], mechanical stretching of CNT-polymer composites [73] and magnetic orientation [74] have been reported for aligning nanotubes in composites. Electrospinning is also an effective

Different thermoplastic and thermoset polymer matrices have been tried to realize the supe‐ rior mechanical properties of CNTs for development of light weight strong material. NASA scientists are considering CNT-polymer composite for space elevator. To date, a volume of literature is available on the improvement of mechanical performance of polymers with ad‐ dition of CNTs. The first study for formation of CNT-polymer composites was carried out by Ajayan et al. [69]. CNTs were aligned within the epoxy matrix by the shear forces in‐

**6.5. Alignment of CNTs in polymer matrix**

210 Syntheses and Applications of Carbon Nanotubes and Their Composites

along the nanotube alignment direction.

method for the alignment of CNTs in polymer matrix.

**7.1. Mechanical properties of MWCNTs polymer nanocomposites**

**7. Properties of the nanocomposites**

Andrews et al. [80] prepared aligned CNT/pitch composites and found the significant im‐ provement in the electrical and mechanical properties especially due to orientation of CNTs. Du et al. [78] compared the mechanical performance of randomly oriented and aligned CNTs polymer composites. Their study revealed that in aligned CNT polymer nanocompo‐ sites tensile strength and modulus even reached to 3600 MPa and 80 GPA respectively which is much higher than the general value of 100 MPa and 6 GPa in case of randomly ori‐ ented CNT polymer nanocomposites. They also observed that the mechanical properties are always higher for aligned CNTs composites with higher loading while the case is different for isotropic CNT polymer composites.

tant to improve the nanotube dispersion and reduced the threshold concentration from 0.18 to 0.08 wt%. To study the effect of aspect ratio on electrical conductivity of CNT nanocom‐ posites Bai et al. [84] pretreated MWCNTs to alter their aspect ratios before preparing ep‐ oxy/MWCNTs composites and found that the threshold concentration varied from 0.5 to > 4 wt % with decreasing aspect ratio. The effect of alignment of CNTs in polymer composites was also studied. Du et al. [24] found some contradictory results with respect to alignment of rod like fillers and observed the lowest percolation threshold and maximum conductivity with their random orientation. They found that the electrical conductivity of 2 wt% CNT/ PMMA nanocomposites decrease significantly (from ~10-4 to ~10-10 S/cm) when CNTs were highly aligned. In contrast Choi et al. [85] observed that the nanotube alignment increased the conductivity of a 3 wt% CNT/epoxy composites from ~10-7 to ~10-6S/cm. In most of the cases the CNT nanocomposites with isotropic nanotubes orientation have greater electrical conductivity than the nanocomposites with highly aligned CNTs especially at lower CNT loadings. By alignment of CNTs in polymers, the percolation pathway is destroyed as aligned CNTs seldomly intersects each other. At higher CNTs loading the conductivity is

Carbon Nanotubes and Their Composites http://dx.doi.org/10.5772/52897 213

more in case of aligned CNTs as compared to randomly oriented CNTs.

**Figure 9.** General trend of electrical conductivity of CNT polymer composites

The study carried out by different researchers also revealed that the composites with ther‐ moplastic polymers have higher conductivity as compared to that of thermosetting poly‐ mers above percolation threshold. Transport properties in CNT-PMMA composites have been reported by Stephan et al. [86] and Benoit et al. [19] where low percolation threshold of 0.5 wt% and 0.33 wt% respectively were obtained. Singhai et al. [87] found that increase in number of defects lead to a decrease in conductivity. However Lau et al. [88] concluded that fuctionalization of CNTs can enhance the electrical conductivity of MWCNTs. The reason at‐ tributed to this phenomenon is electron transfer from the carbon atoms on MWCNTs to functionalized groups attached to the surface favorably promoting conductivity. The study

#### **7.2. Electrical properties of MWCNTs polymer nanocomposites**

CNTs because of their extraordinary electrical conductivity are also excellent additive to im‐ part electrical conductivity to polymer. Many experimental results shows that the conduc‐ tive CNT composites can be constructed at low loading of CNTs due to low percolation threshold originated from the high aspect ratio and conductivity of CNTs [70, 78]. Figure 9 shows the general trend of electrical conductivity of CNT- polymer nanocomposites. It can be found from almost all the experimental results and also obvious from figure that CNT nanocomposites exhibit a typical percolation behavior and CNT reinforcement to polymers can increase the conductivity of resulting composites to several order of magnitude or even some times higher than ten orders of magnitude.

According to percolation theory the conductivity follow the following power law close to threshold percolation.

$$
\sigma = \sigma o(p - p\_o)^t \qquad \text{ for} \quad p > p\_o
$$

where *σ* is the composite conductivity, *σ<sup>o</sup>* is a constant*, p* the weight fraction of nanotubes, *po* is the percolation threshold and *t* the critical exponent [81]. Theoretical and experimental re‐ sults have shown that percolation laws are applicable to CNT-based composites and that the enhanced maximum conductivity and percolation can be achieved with significantly lower filler concentrations than with other carbon and other conductive fillers. Depending on the polymer matrix, the processing technology and the type of nanotubes used, recent experi‐ mental studies have achieved percolation thresholds between 0.0021 to 9.5% by weight and critical exponents varying from 0.9 to 7.6 [78].

Sandler et al. [82] observed the percolation threshold of CNTs/epoxy nanocomposites be‐ tween 0.0225 and 0.04 wt %. They further observed very low percolation threshold at 0.0025 wt% for aligned CNT- epoxy composites [83]. The current voltage behavior measurements exhibited non-ohmic behavior, which is most likely due to tunneling conduction mecha‐ nism. The main mechanism of conduction between adjacent nanotubes is probably electron hopping when their separation distance is small. At concentration greater than percolation threshold, conducting paths are formed through the whole nanocomposites, because the dis‐ tance between the conductive CNT filler (individual or bundles) is small enough to allow efficient electron hopping.

The electrical conductivity of CNT/polymer composites also effected by dispersion and as‐ pect ratio of CNTs and was studied by Barrau et al. [56]. They used palmitic acid as surfac‐ tant to improve the nanotube dispersion and reduced the threshold concentration from 0.18 to 0.08 wt%. To study the effect of aspect ratio on electrical conductivity of CNT nanocom‐ posites Bai et al. [84] pretreated MWCNTs to alter their aspect ratios before preparing ep‐ oxy/MWCNTs composites and found that the threshold concentration varied from 0.5 to > 4 wt % with decreasing aspect ratio. The effect of alignment of CNTs in polymer composites was also studied. Du et al. [24] found some contradictory results with respect to alignment of rod like fillers and observed the lowest percolation threshold and maximum conductivity with their random orientation. They found that the electrical conductivity of 2 wt% CNT/ PMMA nanocomposites decrease significantly (from ~10-4 to ~10-10 S/cm) when CNTs were highly aligned. In contrast Choi et al. [85] observed that the nanotube alignment increased the conductivity of a 3 wt% CNT/epoxy composites from ~10-7 to ~10-6S/cm. In most of the cases the CNT nanocomposites with isotropic nanotubes orientation have greater electrical conductivity than the nanocomposites with highly aligned CNTs especially at lower CNT loadings. By alignment of CNTs in polymers, the percolation pathway is destroyed as aligned CNTs seldomly intersects each other. At higher CNTs loading the conductivity is more in case of aligned CNTs as compared to randomly oriented CNTs.

**Figure 9.** General trend of electrical conductivity of CNT polymer composites

Du et al. [78] compared the mechanical performance of randomly oriented and aligned CNTs polymer composites. Their study revealed that in aligned CNT polymer nanocompo‐ sites tensile strength and modulus even reached to 3600 MPa and 80 GPA respectively which is much higher than the general value of 100 MPa and 6 GPa in case of randomly ori‐ ented CNT polymer nanocomposites. They also observed that the mechanical properties are always higher for aligned CNTs composites with higher loading while the case is different

CNTs because of their extraordinary electrical conductivity are also excellent additive to im‐ part electrical conductivity to polymer. Many experimental results shows that the conduc‐ tive CNT composites can be constructed at low loading of CNTs due to low percolation threshold originated from the high aspect ratio and conductivity of CNTs [70, 78]. Figure 9 shows the general trend of electrical conductivity of CNT- polymer nanocomposites. It can be found from almost all the experimental results and also obvious from figure that CNT nanocomposites exhibit a typical percolation behavior and CNT reinforcement to polymers can increase the conductivity of resulting composites to several order of magnitude or even

According to percolation theory the conductivity follow the following power law close to

where *σ* is the composite conductivity, *σ<sup>o</sup>* is a constant*, p* the weight fraction of nanotubes, *po* is the percolation threshold and *t* the critical exponent [81]. Theoretical and experimental re‐ sults have shown that percolation laws are applicable to CNT-based composites and that the enhanced maximum conductivity and percolation can be achieved with significantly lower filler concentrations than with other carbon and other conductive fillers. Depending on the polymer matrix, the processing technology and the type of nanotubes used, recent experi‐ mental studies have achieved percolation thresholds between 0.0021 to 9.5% by weight and

Sandler et al. [82] observed the percolation threshold of CNTs/epoxy nanocomposites be‐ tween 0.0225 and 0.04 wt %. They further observed very low percolation threshold at 0.0025 wt% for aligned CNT- epoxy composites [83]. The current voltage behavior measurements exhibited non-ohmic behavior, which is most likely due to tunneling conduction mecha‐ nism. The main mechanism of conduction between adjacent nanotubes is probably electron hopping when their separation distance is small. At concentration greater than percolation threshold, conducting paths are formed through the whole nanocomposites, because the dis‐ tance between the conductive CNT filler (individual or bundles) is small enough to allow

The electrical conductivity of CNT/polymer composites also effected by dispersion and as‐ pect ratio of CNTs and was studied by Barrau et al. [56]. They used palmitic acid as surfac‐

for isotropic CNT polymer composites.

212 Syntheses and Applications of Carbon Nanotubes and Their Composites

some times higher than ten orders of magnitude.

critical exponents varying from 0.9 to 7.6 [78].

threshold percolation.

*σ* = *σo*(*p* − *po*)*<sup>t</sup>* for *p* > *po*

efficient electron hopping.

**7.2. Electrical properties of MWCNTs polymer nanocomposites**

The study carried out by different researchers also revealed that the composites with ther‐ moplastic polymers have higher conductivity as compared to that of thermosetting poly‐ mers above percolation threshold. Transport properties in CNT-PMMA composites have been reported by Stephan et al. [86] and Benoit et al. [19] where low percolation threshold of 0.5 wt% and 0.33 wt% respectively were obtained. Singhai et al. [87] found that increase in number of defects lead to a decrease in conductivity. However Lau et al. [88] concluded that fuctionalization of CNTs can enhance the electrical conductivity of MWCNTs. The reason at‐ tributed to this phenomenon is electron transfer from the carbon atoms on MWCNTs to functionalized groups attached to the surface favorably promoting conductivity. The study carried out by Grimes et al. [89] revealed that the electrical response of as fabricated MWCNTs is significantly influenced by the presence of residual catalyst metal particles.

by using MWCNT grown carbon fibre fabric based epoxy composites with improved me‐ chanical properties [91].The effect of the length (aspect ratio) of CNTs on EMI SE of compo‐ sites was also studied by few researchers. Huang et al. [92] reported EMI SE of 18 dB with 15 wt.-% small CNTs and 23-28 dB with 15 wt% long CNTs in X band (8-12.4 GHz). Li et al. [93] also observed that SE with long length CNTs is more as compared to small length CNTs at the same 15wt % loading composites. The residual catalyst metal particle in the cavity of

Carbon Nanotubes and Their Composites http://dx.doi.org/10.5772/52897 215

There are few additional advantages of using MWCNTs as EMI shielding material. The EMI SE also depends on the source of origin of electromagnetic waves. Electrically conducting material can effectively shield the electromagnetic waves generated from an electric source, whereas magnetic materials effectively shield the electromagnetic waves generated from a magnetic source. The MWCNTs exhibits electrical properties because of presence of pi elec‐ trons and magnetic properties because of the presence of catalytic iron particles in tubes. Al‐ so one common problem experienced with commonly used composite materials for EMI shielding is build up of heat in the substance being shielded. The possible solution for this is to add thermal conducting material. Composites with MWCNTs can easily overcome this

As discussed above that the CNTs have thermal conductivity as high as 6600W/mK predict‐ ed for SWCNTs [94] at room temperature and have experimental value 3000W/mK for iso‐ lated MWCNT. So it is quite expected that the reinforcement of CNTs can significantly enhance the thermal properties of CNT-polymer nanocomposites. The improvement in ther‐ mal transport properties of CNT polymer composites leads their applications for usage as

Synthesis of high quality and reproducible CNTs is still remain a very importnat issue. Chemical vapor deposition has been found an efficient process for the synthseis of bulk quantity of CNTs. The CNT-polymer composites have been developed with improved me‐ chanical properties but for actual structural applications, these have to compete with the ex‐ isting carbon fibre based composites. Dispersion of high loading of CNTs and their alignment in any polymer matrix without sacrificing their mechanical properties is still a challenge for using CNTs in high performance composites for specific applications such as as automobile, defence, aerospace, sports etc. CNT- carbon fibres-polymer multiscale com‐ posites could be an alternative route for further improvement in the mechanical properties of the composites over commercially available CF-polymer composites. Till then electrical properties of CNT polymer composites provides exciting possibility as antistatic and electro‐

CNTs also effects the SE of the composites.

problem as it has high thermal conductivity.

magnetic interference shielding material.

**8. Conclusion**

**7.4. Thermal properties of MWCNTs polymer nanocomposites**

printed circuit boards, connectors, thermal interface materials, heat sinks.

#### **7.3. EMI shielding proerties of MWCNTs polymer nanocomposites**

The electrical conductivity of CNT reinforced polymer composites makes them a very suita‐ ble candidate to be employed for electromagnetic interference (EMI) shielding. EMI is the process by which disruptive electromagnetic energy is transmitted from one electronic de‐ vice to another via radiation or conduction. As we all know that the electromagnetic waves produced from some electronic instrument have an adverse effect on the performance of the other equipments present nearby causing data loss, introduction of noise, degradation of picture quality etc. The common example is the appearance of noise in television signal when a telephone or mobile rings. Also recent reports of deterious effects of electromagnetic radiations on electro medical devices have caused concern among health care providers. The overlapping of signals transmitted in air traffic system with signals from other electronic equipments became cause of several accidents in past. Also mobile phones and passing taxi radios have been known to interfere with anti-skid braking system (ABS), airbags and other electronic equipments causing drivers to lose control. In today's scenario where rapid com‐ munication is required, there is an increase in electromagnetic radiations within the spec‐ trum in which the wireless, cordless and satellite system operates. So it a strong desire to shield electronics equipments from the undesired signals. Problems with EMI can be mini‐ mized or sometime eliminated by ensuring that all electronic equipments are operated with a good housing to keep away unwanted radio frequency from entering or leaving. The shielding effectiveness (SE) of the shielding material is its ability to attenuate the propaga‐ tion of electromagnetic waves through it and measured in decibels (dB) given by

*SE*(dB) = −10 log(*Pt* / *P*0),

where *P*<sup>t</sup> and *P*0 are, respectively, the transmitted and incident electromagnetic power. A SE of 10 dB means 90% of signal is blocked and 20 dB means 99% of signal is blocked.

One of the important criterion for a material to be used for EMI shielding material is that it should be electrically conducting. Because of their high electrical conductivity metals have been used for past several years as EMI shielding materials. But the shortcomings of metals like heavy weight, physical rigidity and corrosion restricts their use. The most notable sub‐ stance that could overcome these shortcomings is the CNT-polymer composites. As dis‐ cussed in previous sections these are electrically conductive, having low density, corrosion resistant and can be molded in any form. Due to easy processing and good flexibility, CNT– polymer composites have been employed for application as promising EMI shielding mate‐ rials. The SE of the CNT-polymer composites depends on various factors like,type of CNTs (either SWCNT or MWCNT), aspect ratio of CNTs, quality of CNTs, thickness and electrical conductivity of the shielding material. Several studies have been reported on EMI shielding properties of randomly oriented CNT based polymer composites. Mathur and co-workers [18] have prepared MWNT-PMMA and MWNT-PS composites and observed 18dB and 17dB SE respectively with 10-wt % MWCNT loading. Singh et al. [90] reported a SE of 51 dB by using MWCNT grown carbon fibre fabric based epoxy composites with improved me‐ chanical properties [91].The effect of the length (aspect ratio) of CNTs on EMI SE of compo‐ sites was also studied by few researchers. Huang et al. [92] reported EMI SE of 18 dB with 15 wt.-% small CNTs and 23-28 dB with 15 wt% long CNTs in X band (8-12.4 GHz). Li et al. [93] also observed that SE with long length CNTs is more as compared to small length CNTs at the same 15wt % loading composites. The residual catalyst metal particle in the cavity of CNTs also effects the SE of the composites.

There are few additional advantages of using MWCNTs as EMI shielding material. The EMI SE also depends on the source of origin of electromagnetic waves. Electrically conducting material can effectively shield the electromagnetic waves generated from an electric source, whereas magnetic materials effectively shield the electromagnetic waves generated from a magnetic source. The MWCNTs exhibits electrical properties because of presence of pi elec‐ trons and magnetic properties because of the presence of catalytic iron particles in tubes. Al‐ so one common problem experienced with commonly used composite materials for EMI shielding is build up of heat in the substance being shielded. The possible solution for this is to add thermal conducting material. Composites with MWCNTs can easily overcome this problem as it has high thermal conductivity.

#### **7.4. Thermal properties of MWCNTs polymer nanocomposites**

As discussed above that the CNTs have thermal conductivity as high as 6600W/mK predict‐ ed for SWCNTs [94] at room temperature and have experimental value 3000W/mK for iso‐ lated MWCNT. So it is quite expected that the reinforcement of CNTs can significantly enhance the thermal properties of CNT-polymer nanocomposites. The improvement in ther‐ mal transport properties of CNT polymer composites leads their applications for usage as printed circuit boards, connectors, thermal interface materials, heat sinks.
