dispersion and alignment of carbon nanotubes in polymer matrix a review

Dispersion and alignment of carbon nanotubes in polymer

matrix: A review

Xiao-Lin Xiea,b, Yiu-Wing Maia,*, Xing-Ping Zhoub

aCenter for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and

Mechatronic Engineering J07, University of Sydney, Sydney, NSW 2006, AustraliabDepartment of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, China

Received 28 November 2004; accepted 20 April 2005

Abstract

Polymer/carbon nanotube (CNT) composites are expected to have good processability characteristics of the

polymer and excellent functional properties of the CNTs. The critical challenge, however, is how to enhance

dispersion and alignment of CNTs in the matrix. Here, we review recent progress and advances that have been made

on: (a) dispersion of CNTs in a polymer matrix, including optimum blending, in situ polymerization and chemical

functionalization; and (b) alignment of CNTs in the matrix enhanced by ex situ techniques, force and magnetic fields,

electrospinning and liquid crystalline phase-induced methods. In addition, discussions on mechanical, thermal,

electrical, electrochemical, optical and super-hydrophobic properties; and applications of polymer/CNT composites

are included. Enhanced dispersion and alignment of CNTs in the polymer matrix will promote and extend the

applications and developments of polymer/CNT nanocomposites.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Polymer nanocomposites; Carbon nanotubes; Dispersion; Alignment; Applications

1. Introduction

The discovery of carbon nanotubes can be traced back to the origin of fullerene chemistry

(buckyball, C60) in 1985 [1]. Fullerenes have provided an exciting new insight into carbon

nanostructures built from sp2 carbon units based on geometric architectures. In 1991, Iijima [2]

discovered carbon nanotubes (CNTs) that are elongated fullerenes where the walls of the tubes are

hexagonal carbon and often capped at each end. There are two types of CNTs: multi-walled carbon

nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). The former consists of two or

more concentric cylindrical shells of graphene sheets coaxially arranged around a central hollow core

with interlayer separations as in graphite. In contrast, SWCNT comprises a single graphene cylinder.

Both SWCNTs and MWCNTs have physical characteristics of solids and are micro-crystals with high

aspect ratios of 1000 or more, although their diameter is close to molecular dimensions. Table 1 shows

theoretical and experimental properties of carbon nanotubes [3,4].

From Table 1, it is clear that CNTs have unique mechanical, electrical, magnetic, optical and

thermal properties. In some special applications, such as space explorations, high-performance

lightweight structural materials are required, and they can be developed by adding CNTs to polymers

or other matrix materials. Moreover, although graphite is a semi-metal, CNTs can be either metallic or

Materials Science and Engineering R 49 (2005) 89–112

* Corresponding author. Tel.: +61 2 9351 2290; fax: +61 2 9351 3760.

E-mail address: [email protected] (Y.-W. Mai).

0927-796X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.mser.2005.04.002

semi-conducting due to the topological defects from the fullerene-like end caps in CNTs (pentagons in

a hexagonal lattice). Thus, the physico-mechanical properties of CNTs are dependent upon their

dimensions, helicity or chirality. The syntheses, structures, properties and applications of CNTs have

been discussed in several books [5,6].

The superior properties of CNTs offer exciting opportunities for new composites. NASA has

invested large sums of money to develop carbon nanotube-based composites for applications such as the

Mars mission [6]. Recently, polymer/CNT composites have attracted considerable attention owing to

their unique mechanical, surface and multi-functional properties, and strong interactions with the matrix

resulting from the nano-scale microstructure and extremely large interfacial area. Wagner et al. [7]

experimentally studied the fragmentation of MWCNTs embedded within thin polymeric films under both

compressive and tensile strains. They found that the polymer/CNTinterfacial shear stress is of the order of

500 MPa, which is much larger than traditional fiber composites. It suggests good bonding between CNTs

and polymer matrix [8]. However, CNTs are easy to agglomerate, bundle together and entangle, leading to

many defect sites in the composites, and limiting the efficiency of CNTs on polymer matrices [9]. Salvetat

et al. [10] studied the effect of dispersion of CNTs on the mechanical properties of polymer/CNT

composites, and found that poor dispersion and rope-like entanglement of CNTs led to drastic weakening

of the composites. Thus, alignment of CNTs in the matrix has a predominant role on the mechanical and

functional properties of polymer/CNT composites [11,12]. To-date, the main challenges are to improve

the dispersion and alignment of CNTs in a polymer matrix when processing these nanocomposites.

Recently, Thostenson et al. [13] reviewed CNTs and their composites. Our review given below is

complementary to theirs and we focus on recent progress made towards the improvement of dispersion

and alignment of CNTs in polymer matrices. Also, thermal–physical–mechanical properties and

applications of polymer/CNT composites are discussed.

2. Enhancement of dispersion of CNTs in polymer matrices

To maximize the advantage of CNTs as effective reinforcements in high strength composites,

they should not form aggregates and must be well dispersed to prevent slippage [14]. There are several

techniques to improve the dispersion of CNTs in polymer matrices, such as by optimum physical

blending, in situ polymerization and chemical functionalization.

90 X.L. Xie et al. / Materials Science and Engineering R 49 (2005) 89–112

Table 1

Theoretical and experimental properties of carbon nanotubes

Property CNTs Graphite

Specific gravity 0.8 g/cm3 for SWCNT;

1.8 g/cm3 for MWCNT (theoretical)

2.26 g/cm3

Elastic modulus �1 TPa for SWCNT; �0.3–1 TPa for MWCNT 1 TPa (in-plane)

Strength 50–500 GPa for SWCNT; 10–60 GPa for MWCNT

Resistivity 5–50 mV cm 50 mV cm (in-plane)

Thermal conductivity 3000 W m�1 K�1 (theoretical) 3000 W m�1 K�1 (in-plane),

6 W m�1 K�1 (c-axis)

Magnetic susceptibility 22 � 106 EMU/g (perpendicular with plane),

0.5 � 106 EMU/g (parallel with plane)

Thermal expansion Negligible (theoretical) �1 � 10�6 K�1 (in-plane),

29 � 10�6 K�1 (c-axis)

Thermal stability >700 8C (in air); 2800 8C (in vacuum) 450–650 8C (in air)

Specific surface area 10–20 m2/g

2.1. Optimum physical blending

The widely used compounding technique to prepare conventionally filled polymers is still the

most convenient and practical way when nano-sized fillers are considered to replace micron-sized

fillers for high-performance polymers. But the dispersion of nano-fillers in polymer matrix is more

difficult than that of micro-fillers due to the strong tendency to agglomerate for the nano-fillers.

For polymer/CNT composites, the high power dispersion methods, such as ultrasound and high

speed shearing, are the simplest and most convenient to improve the dispersion of CNTs in a polymer

matrix. For example, Qian et al. [9] made use of a simple solution-evaporation method assisted by high

energy sonication to prepare polystyrene (PS)/MWCNT composite films, in which MWCNTs were

dispersed homogeneously in the PS matrix. Similarly, Sandler et al. [15] dispersed CNTs in epoxy

under high speed stirring (2000 rpm) for 1 h, and proved that intense stirring was an effective process

to achieve dispersed CNTs uniformly in epoxy. Besides, adding a proper compatibilizer to polymer/

CNT composites is also another efficient method. Xie et al. [16] prepared polypropylene (PP)/CNT

composites compatibilized with maleic anhydride grafted styrene–(ethylene-co-butylene)–styrene

copolymer (MA-SEBS) by using a combination of ball milling and melt mixing. In Fig. 1, the viscosity

of PP melts increases with CNTs loading but appears to flatten beyond 5 wt.%. When MA-SEBS is

added to PP/CNT composites, the melt viscosity of PP/CNT/MA-SEBS composites is further

increased. Also, the surface electrical resistance of PP/CNT/MA-SEBS composites in Fig. 2 shows

that CNTs increase the electrical conductivity of these nanocomposites, and the addition of MA-SEBS

improves further their electrical conductivity. Thereby, the static electrical charges on the surface of

the nanocomposites are easy to remove. Thus, the anti-static property of the nanocomposite is

improved. These results confirm that MA-SEBS acts as a compatibilizer to enhance formation of a

perfect percolation network and dispersion of CNTs in the matrix. Figs. 3 and 4 show that CNTs

reinforce the PP matrix due to their large aspect ratio and high stiffness, but the ductility for PP/CNT

composites is very low, Table 2. MA-SEBS improves the interaction and interfacial adhesion between

CNTs and PP, and thus improves the tensile strength and ductility but reduces the stiffness of the PP/

CNT/MA-SEBS composites.

In a further study on ultra-high molecular weight polyethylene (UHMWPE)/CNT composites,

Xie et al. [17] have also confirmed that MA-SEBS is an effective compatibilizer, enhances the

dispersion of CNTs in the UHMWPE matrix, and speeds up crystallization of the UHMWPE phase.

Similarly, Jin et al. [18] coated MWCNTs by poly(vinylidene fluoride) (PVDF) and then melt-blended

X.L. Xie et al. / Materials Science and Engineering R 49 (2005) 89–112 91

Fig. 1. Relationship between torque and CNT content for PP/CNT composite melt with and without MA-SEBS. After [16].

with poly(methyl methacrylate) (PMMA). PVDF was found to be a compatibilizer and assist

dispersion of MWCNTs in PMMA and served as an adhesive to increase their interfacial bond

strength, hence greatly improving the mechanical properties of PMMA/MWCNT composites.

Additionally, surfactants can be used as a dispersing agent to improve the dispersion of CNTs in

processing polymer/CNT composites. Gong et al. [19] used a non-ionic surfactant, polyoxyethylene-

8-lauryl, as a processing aid for epoxy/CNT composites due to the strong interactions between carbon

of CNTs and the hydrophobic segment of the surfactant via van der Waals forces, as well as epoxy and

the hydrophilic segment of the surfactant via hydrogen bonding. The homogeneous dispersion of

CNTs in epoxy improved the thermomechanical properties of the composites. With only 1 wt.% CNTs

in the epoxy/CNT composites, the glass transition temperature was increased by 25 8C (from 63 to

88 8C) and the elastic modulus by over 30%.

In sum, good dispersion of CNTs in polymer matrices can be achieved by means of high power

dispersion, compatibilizer, polymer-assisted melt blending, and surfactants. We believe the addition of


Fig. 2. Relationship between surface resistance and CNT content for PP/CNT composite melt with and without MA-SEBS.After [16].

Fig. 3. Relationship between modulus and CNT content for PP/CNT composite melt with and without MA-SEBS. After[16].

compatibilizers, surfactants, etc., in conjunction with melt blending is most effective and will be

widely used to enhance the dispersion and interfacial bonding of many polymer/CNT composites.

2.2. In situ polymerization

To improve the processability, electrical, magnetic and optical properties of CNTs, some

conjugated or conducting polymers are attached to their surfaces by in situ polymerization. Tang

and Xu [20] synthesized poly(phenylacetylene)-wrapped carbon nanotubes (PPA-CNTs), which were

soluble in organic solvents, such as tetrahydrofuran, toluene, chloroform and 1,4-dioxane. Meanwhile,

the fullerene tips and graphene sheets of CNTs may undergo nonlinear optical (NLO) absorption

processes, and the cylindrical bodies of CNTs with a high aspect ratio may also function as light

scattering centers. Both NLO absorption and light scattering of CNTs protect dramatically the PPA

chains from being photo-degraded under severe laser irradiation. Therefore, PPA-CNTs show strong

photo-stabilization effect. It is interesting that PPA-CNTs can be macroscopically processed, and

shearing of their solutions readily aligns CNTs along the applied mechanical force direction. Fan et al.

[21] synthesized conducting polypyrrole-coated carbon nanotubes (PPY-CNTs), and found that the

magnetization of PPY-CNTs is the sum of the two components, PPY and CNTs. Star et al. [22]

synthesized poly(metaphenylenevinylene)-wrapped single-walled carbon nanotubes (PmPV-

SWCNTs), and UV–vis absorption spectra confirmed p–p interactions between SWCNT and fully

conjugated PmPV backbone. The results indicate that the photo-excited PmPV has a dipole moment

that alters the local electric field at the surface of SWCNTs.

Cochet et al. [23] synthesized polyaniline (PANI)/MWCNT composites by in situ polymerization

in the presence of MWCNTs. Their results reveal the site-selective interaction between the quinoid

ring of PANI and MWCNTs, thus opening the way for charge transfer processes, and improving the


Fig. 4. Relationship between tensile strength and CNT content for PP/CNT composite melt with and without MA-SEBS.After [16].

Table 2

Elongation-at-break for PP/CNT and PP/CNT/MA-SEBS composites

CNT content (%) 0 0.5 1.0 2.0 5.0 10

PP matrix % >146 5.69 7.59 9.99 11.73 10.04

PP90/MA-SEBS10 matrix % >146 25.2 30.18 59.06 54.30 39.80

electric properties of PANI/MWCNT composites. Xiao and Zhou [24] deposited polypyrrole (PPY) or

poly(3-methylthiophene) (PMeT) on the surfaces of the MWCNTs by in situ polymerization. The

Faraday effect of the conducting polymer enhances the performance of super-capacitors with

MWCNTs deposited with the conducting polymer.

High-performance structural composites based on CNTs and polymer have also been prepared by

in situ polymerization. Jia et al. [25] first synthesized PMMA/CNT composites by in situ polymer-

ization of MMA with CNTs present. Later, Park et al. [26], Velasco-Santos et al. [27], and Jang et al.

[28] polymerized in situ polyimide (PI)/SWCNT, PMMT/MWCNT, and liquid crystalline epoxide

(LCE)/MWCNT composites. PI/SWCNT composite films were anti-static and optically transparent

with significant conductivity enhancement (10 orders) at a very low loading (0.1 vol.%). For PMMA/

MWCNT composites containing 1 wt.% MWCNT, the storage modulus at 90 8C is increased by an

outstanding 1135%, and the glass transition temperature is raised exceptionally by �40 8C. For LCE/

MWCNT composites containing 1 wt.% MWCNT, the nematic phase temperature of liquid crystalline

epoxide (LCE) is decreased. When the MWCNTs are modified by chemical surface oxidation

(oxMWCNT), this LCE nematic phase temperature of LCE/oxMWCNT composites is shifted to a

lower temperature. This is because the molecular alignment of the LCE for evolution of the LC phase

is partially restricted by the interaction between the MWCNTs and LCE. In LCE/oxMWCNT

composites, the LC domains form at a lower temperature.

2.3. Chemical functionalization

In addition, the surfaces of CNTs have to be chemically functionalized (including grafting

copolymerization) to achieve good dispersion in polymer/CNT composites and strong interface

adhesion between surrounding polymer chains, even though the interface area is very large. CNTs are

assembled as ropes or bundles, and there are some catalyst residuals, bucky onions, spheroidal

fullerenes, amorphous carbon, polyhedron graphite nano-particles, and other forms of impurities in as-

grown CNTs. Thus, purification, ‘‘cutting’’ or disentangling, and activation treatments are needed

before chemical functionalization.

2.3.1. Purification

The first method to purify MWCNTs is based on oxidation [4,29], which is observed to occur

preferentially at the nanotube ends and on nano-particles that have a high concentration of topological

defects. MWCNTs are purified by burning away of these tube ends, nano-particles and amorphous

carbon at >700 8C in air or oxygen. However, the yield is extremely low (<5%). Hiura et al. [30]

purified CNTs by a mixture of concentrated sulfuric acid and potassium permanganate, but this is still

not proven to be a good method for large-scale separation. Tohji et al. [31] suggested a purification

method that included hydrothermal treatment along with extraction of fullerenes, thermal oxidation and

dissolution in 6 M hydrochloric acid. To prevent CNTs from being destroyed during purification,

Bonard et al. [32], Bandow et al. [33] and Duesberg et al. [34] dispersed the CNTs in polar solvents

assisted by surfactants, such as sodium dodecyl sulfate, followed by micro-filtration and size exclusion

chromatography. These methods may be possible to separate CNTs in selected sizes without destruc-

tion. Yamamoto et al. [35] used AC electrophoresis to treat the CNTs dispersed in iso-propyl alcohol.

They found that the separation from impurity particles depended on the frequency of the applied field.

At the same time, it is worth noting that CNTs align along the electric field direction. Coleman et al. [36]

and McCarty et al. [37] used a functional organic polymer, poly(m-phenylene-co-2,5-dioctoxy-p-

phenylenevinylene) (PmPV), as a filtration system to purify CNTs. They found that the solution of

PmPV is capable of suspending nanotubes indefinitely whilst the accompanying amorphous graphite


separates out. Although these novel techniques are only suited to treat small amounts of the

nanocomposite, they are definite advances to manipulate CNTs.

Liu et al. [38] developed a purification method that consisted of refluxing in 2.6 M nitric acid and

re-suspending CNTs in pH 10 water with surfactant followed by filtration with a cross-flow filtration

system. This may be an efficient method to purify CNTs owing to the combined advantages of the

chemical and physical methods.

2.3.2. ‘‘Cutting’’ or disentangling

CNTs are long and entangled as ropes or in bundles. The ‘‘cutting’’ process is necessary for CNTs

to disentangle, open up the tubes and provide active sites for chemical functionalization. Liu et al. [38]

chose a 3:1 mixture of concentrated sulfuric acid and nitric acid to ‘‘cut’’ CNTs into �150 nm in

length, Fig. 5. The short and open-ended CNTs are then treated by a 4:1 mixture of concentrated

sulfuric acid and 30% aqueous hydrogen peroxide. Thus, more functional groups such as carboxylic

acid and hydroxyl groups are formed on the CNT surface. Later, Shaffer et al. [39,40] used a 3:1

mixture of concentrated sulfuric acid and nitric acid to ‘‘cut’’ CNTs and produced an electrostatic

stabilized dispersion of CNTs in water with an average length of 1.1 mm, Fig. 6. Chen et al. [41] also

obtained disentangled SWCNTS with an average length >1 mm. Obviously, it is necessary to control

and adjust the processing conditions to obtain short CNTs with different length.

2.3.3. Activation treatments

In general, carboxylic acid and hydroxyl groups could be formed on the surface or open ends of

CNTs during the oxidation process by oxygen, air, concentrated sulfuric acid, nitric acid and 30%

aqueous hydrogen peroxide, and concentrated sulfuric acid and its mixture [38].

To further activate the carboxylic acid groups on the surface of CNTs, they are converted into acyl

chloride groups by reaction with thionyl chloride at room temperature [38]. Wu et al. [42] converted

the hydroxyl groups on the surface of CNTs into hydroxymethyl groups (–CH2OH) by the

formalization reaction with formaldehyde as below:


Fig. 5. Tapping mode AFM images of ‘‘cut’’ CNTs by Liu et al. After [38].

By using a redox initiating system, which consists of cerium ions and hydroxymethyl groups (–

CH2OH) on the surface of CNTs, free radical graft polymerization of vinyl monomers could occur.

Zhou [43] also converted carboxylic acid and hydroxyl groups on the surface of CNTs into vinyl

groups (–CH CH2) by reaction with 3-isopropenyl-a,a-dimethylbenzyl isocyanate:

Because of the presence of these vinyl groups (–CH CH2) on CNTs, some vinyl monomers could

be grafted on the surface of CNTs.

Besides the above activation methods, Chen et al. [44] activated CNTs surface by plasma

modification. Acetaldehyde and ethylenediamine vapors were plasma polymerized on the surface of

CNTs, thereby introducing active aldehyde (–CHO) and amino (–NH2) groups.

Moreover, Mickelson et al. [45] added functional groups to the side walls of SWCNTs by fluorine

at elevated temperatures. These fluorinated SWCNTs could be solvated in alcohols and reacted with

other species, particularly strong nucleophiles, such as alkylithium reagents. Mawhinney et al. [46]

reported that carboxylic acids, anhydrides, quinines, and esters can also be introduced to CNTs by

ozone oxidation. Bahr et al. [47] successfully functionalized SWCNTs with electrochemical reduction

of aryl diazonium compounds, resulting in a free radical that could attach to the SWCNT surface.

Georgakilas et al. [48] discovered a method to functionalize SWCNTs with 1,3-dipolar cycloaddition

of azomethine ylides. Chen et al. [49] and Holzinger et al. [50] functionalized SWCNTs by direct

additions based on nucleophilic carbenes, cycloaddition of nitrenes and radicals. These functionaliz-

ing methods have been reviewed by Bahr and Tour [51].


Fig. 6. Tapping mode AFM images of ‘‘cut’’ CNTs by Chen et al. [41].

2.3.4. Chemical functionalization

Riggs et al. [52] and Lin et al. [53] synthesized polymer grafted CNTs based on acylated CNTs

with poly(propionylethylenimine-co-ethylenimine) (PPEI-EI), poly(vinyl acetate-co-vinyl alcohol)

(PVA-VA) and poly(vinyl alcohol) (PVA), respectively. They found that PVA grafted CNTs were

soluble in PVA solution, PVA-CNT nanocomposite films so-formed are of high optical quality without

any observable phase separation. The result indicates that chemical graft functionalization of CNTs by

matrix polymer was an effective way to achieve homogeneous dispersion for high-performance

polymer/CNT nanocomposites. Cao et al. [54] synthesized dodecylamine grafted CNTs with

dodecylamine and acylated CNTs, and found that these grafted CNTs were miscible with poly-

vinylbutyral (PVB) caused by the interaction between functional groups in polymer and long

dodecylamine chains attached to the ends of CNTs. Also, Hill et al. [55] functionalized CNTs by

the grafting reaction between carboxylic acid-bound CNTs and poly(styrene-co-p-(4-(40-vinylphe-

nyl)-3-oxabutanol)) (PSV). These PSV-grafted CNTs are soluble in common organic solvents and are

homogeneously dispersed in the PS matrix. This offers an example for the widely held expectation that

the solubility of CNTs will enable the preparation of desirable polymer/CNT nanocomposites.

Meanwhile, Mitchell et al. [56] synthesized 4-(10-hydroxydecyl) benzoate-SWCNTs by in situ

reaction of organic diazonium compounds, and prepared the composites based on the functionalized

SWCNTs and PS. These nanocomposites show a percolated SWCNT network structure at 1 vol.%

nanotubes, whilst for the unfunctionalized SWCNT/PS nanocomposites this occurs at twice SWCNTs

loading. The reason is because the chemical functionalization of CNTs enhances their interaction with

the polymer matrix and improves their dispersion in the composites.

Recently, Viswanathan et al. [57] reported that introduction of carbanions on the SWCNTs

surface by treatment with an anionic initiator served to exfoliate bundles of SWCNTs, and provided

initiating sites for polymerization of styrene. Qin et al. [58] grafted poly(n-butyl methacrylate)

(PBMA) brushes on the ends and sidewalls of SWCNTs by using atom transfer radical polymerization

(ATRP). Kong et al. [59] successfully functionalized MWCNTs with various contents of poly(methyl

methacrylate) layers by in situ ATRP grafting polymerization (Fig. 7).

In summary, chemical functionalization can promote good dispersion in the polymer/CNT

composites and will play a dominant role in future development and applications of these nano-

composites.

3. Alignment of CNTs in polymer matrix

In common with conventional fiber composites, both mechanical properties, such as stiffness and

strength, and functional properties, such as electrical, magnetic and optical properties, of polymer/

CNT nanocomposites are linked directly to the alignment of carbon nanotubes in the matrix. This is a

topic that has received much recent attention. Andrews et al. [60] prepared drawn isotropic petroleum


Fig. 7. Schematic of functionalized MWCNTs with PMMA layers by in situ ATRP. After [59].

pitch/SWCNT composite fibers with 5 wt.% SWCNTs. The tensile strength, modulus and electrical

conductivity of these pitch composite fibers are enhanced by �90, �150 and 340%, respectively, due

to the orientation of the SWCNTs. These results confirm a new efficient method to develop high-

performance carbon fiber composites and the interest now is to develop effective techniques to align

the CNTs in the polymer matrix.

3.1. Ex situ alignment of CNTs

‘‘Ex situ’’ is used to mean that the CNTs are aligned in advance, then compounded with the

polymer matrix by in situ polymerization of some monomers. Feng et al. [61] prepared well-aligned

polyaniline (PANI)/MWCNT composite films by in situ polymerization of aniline in the presence of

aligned MWCNTs, as shown in Fig. 8. It offers a general route for controlled assembly of organized

nanocomposites and devices. Obviously, the challenge for the method is how to align CNTs in the first

place. These methods are described below.

3.1.1. Filtration

de Heer et al. [62] dispersed CNTs in ethanol, and prepared aligned CNT films by drawing the

suspension through a 0.2 mm-pore ceramic filter and transferring the uniformly aligned CNTs on the

filter to a polymer surface. Later, Watters et al. [63], Casavant et al. [64] and Fischer et al. [65]

prepared thick macroscopic membranes (10 mm thick, 125 cm2 surface area) of aligned SWCNTs via

high-pressure filtration of aqueous surfactant-suspended SWCNTs in a magnetic field of 7 T.

3.1.2. Plasma-enhanced chemical vapor deposition (PECVD)

Ren et al. [66–68] synthesized large arrays of well-aligned CNTs on glass by direct-current power

generated plasma-enhanced chemical vapor deposition (PECVD), where the glass substrate is first

coated with a layer of nickel catalyst. Later, microwave plasma is also used to align CNTs on Si, Ni,

and Fe–Ni–Cr alloy substrates during growth by microwave plasma-enhanced chemical vapor

deposition (MPECVD) [69–72].

3.1.3. Template

Lie et al. [73] used meso-porous silica containing iron nano-particles embedded in the pores as a

template, and synthesized large-scale aligned CNTs by chemical vapor deposition (CVD). The aligned

CNTs are easily cleaved along the growth direction of the nanotubes. Later, Li et al. [74] reported that

highly ordered arrays of parallel CNTs were grown by pyrolysis of acetylene on cobalt within a

hexagonal close-pack nano-channel alumina template at 650 8C. Similarly, Sohn et al. [75] synthe-

sized well-aligned CNTs and patterned selective grown CNTs with a porous Si substrate and a

patterned Fe film on Si substrate by thermal chemical vapor deposition. Also, Yuan et al. [76] prepared


Fig. 8. Schematic of preparing well-aligned PANI/MWCNT composites. After [58].

highly ordered mono-dispersed CNTs based on self-ordered hexagonal nano-pore alumina templates

by thermal decomposition of ethylene, Wang et al. [77] and Wei et al. [78] fabricated 3D controllable

aligned CNTs as shown in Fig. 9. Additionally, Schlittler et al. [79] reported the self-assembly of

single crystals of SWCNTs using thermolysis of nano-patterned precursors.

It is noted that CNTs are useful for downsizing circuit dimensions, especially when the CNTs are

aligned, thus paving a way for the development of novel nano-electronic applications. For example,

CNT field effect transistors are fabricated by lithographically applying electrodes to CNTs that are

either randomly distributed on a silicon substrate or positioned on the substrate with an atomic force

microscope [80–82]. Integrated nano-electronic devices involve two or more CNT field effect

transistors and provide the basis for large-scale integration [81,82].

As discussed above, the methods to align CNTs all require porous substrates. However, Ago et al.

[83] reported that Co nano-particles chemically synthesized by a reverse micelle method catalyzed the

growth of MWCNTs aligned perpendicular to a substrate. Yanagi et al. [84] used highly oriented

pyrolytic graphite (HOPG) as a template to prepare self-oriented short SWCNTs deposited on HOPG.

Obviously, these methods offer processable approaches to align CNTs arrays over a large area.

Summing up, highly ordered CNTs are useful for many applications, such as field emission

displays and sensors, data storage, and light-emitters. This also provides basic techniques to develop

well-aligned polymer/CNT composites.

3.2. Force field-induced alignment of CNTs

Ajayan et al. [85] developed a simple method to produce aligned arrays of CNTs by ‘‘cutting’’ an

epoxy/CNT nanocomposite. Their research results demonstrated the nature of rheology in composite

media on nanometer scales and flow-induced anisotropy produced by the ‘‘cutting’’ process. The fact

that CNTs do not break and are straightened after the cutting process also suggests that they have

excellent mechanical properties. However, the orientation of CNTs in epoxy/CNT composite is

affected by the thickness of the slices, and the alignment effect becomes less pronounced with

increasing slice thickness.

Vigolo et al. [86] dispersed SWCNTs in a surfactant solution (sodium dodecyl sulfate, SDS),

which was slowly injected through a syringe needle into a PVA solution. Because the latter is more


Fig. 9. Schematic of fabricated 3D controllable aligned CNTs. After [77].

viscous than the SWCNT dispersion, and there is a shear contribution in the flow at the tip of the

syringe needle, the flow-induced alignment is maintained by the PVA solution, and SWCNTs are

rapidly stuck together as they are injected out from the syringe. By pumping the polymer solution from

the bottom, meter-long ribbons are easily drawn, and well-oriented PVA/CNT composite fibers and

ribbons are formed by a simple process. It offers a method to align CNTs by a flow field.

Lin et al. [11] aligned CNTs in composites by uniaxially stretching polymer/CNT composite

films at 100 8C. Haggenmuller et al. [87] prepared PMMA/SWCNT composite films and fibers by melt

processing. They found the composite films showed higher conductivity along the flow direction than

perpendicular to it; the elastic modulus and yield strength of PMMA/SWCNT composite fibers

increased with draw ratio, and SWCNTs in the composite fibers are well aligned. Thostenson and

Chou [88] prepared PS/MWCNT composite films by extruding the composite melt through a

rectangular die and drawing the film prior to cooling. Compared with the drawn PS film, the tensile

strength and modulus of the PS/MWCNT composite films were increased by 137 and 49%,

respectively. However, Safadi et al. [89] prepared PS/MWCNT composite films by spin casting at

high speed (2200 rpm) and found that MWCNTs were aligned in specific angles relative to the radial

direction: 458 and 1358 on average. The presence of �2.5 vol.% MWCNTs doubles the tensile

modulus, and transforms the film from insulating to conducting. It is noted that the CNTs have higher

orientation than the polymer matrix during melt-drawing of the polymer/CNT composites [90].

Finally, Cooper et al. [91] claimed that they aligned CNTs in PMMA/CNT composites by melt

extrusion. It provides an easy and effective method to develop high-performance composites for the

materials industry.

3.3. Magnetic field-induced alignment of CNTs

Kimura et al. [92] were first to use a high magnetic field to align MWCNTs in a polyester matrix and

obtained electrically conductive and mechanically anisotropic composites. They dispersed MWCNTs in

the monomer solution of unsaturated polyester, then applied a constant magnetic field of 10 T to align the

nanotubes. Polymerizing this MWCNT-monomer dispersion under the magnetic field freezes the

alignment of MWCNTs in the polyester matrix. It offers a new method to prepare special composites

with anisotropic electric and mechanical properties based on the anisotropic nature of MWCNTs. Choi

et al. [12] prepared aligned MWepoxy/CNT nanocomposites under a 25 T magnetic field. Their thermal

and electrical properties along the magnetic field alignment direction are increased by 10 and 35%,

compared with those epoxy/MWCNT nanocomposites without the application of a magnetic field.

As shown in Fig. 10, Takahashi et al. [93] applied a magnetic field to the processing apparatus,

such as a hot-press machine, and prepared polycarbonate (PC)/carbon fiber nanocomposites. Based on

wide angle X-ray diffraction (WAXD) by a step-scanning method, the orientation coefficients (S) of

carbon fiber (150 nm) and PC crystallite in the magneto-oriented sample were estimated from the

following equations:

S ¼ 12ð3 cos2 u � 1Þ (1a)

cos2 u ¼R p=2

0IðfÞ cos2 f sinf df

R p=2

0IðfÞ sinf df

(1b)

where u is the angle between carbon fiber or PC crystallite and magnetic direction, and I(F) is the

azimuthal intensity distribution. They found the orientation coefficients (S) of the carbon fibers and the


crystallites were 0.64 and 0.65, respectively. Since PC is usually categorized as an amorphous polymer

due to its extremely slow crystallization without any additives, these results indicate that CNTs are

aligned by the applied magnetic field which also accelerates the crystallization of PC and the

orientation of its crystallites in the magnetic field direction.

3.4. Electrospinning-induced alignment of CNTs

Electrospinning is an electrostatic method for the fabrication of long organic fibers. Recently, the

electrospinning process [94–99] was used to align CNTs in polymer nanofibers, in which a high direct-

current (dc) voltage (e.g. 25 kV) is generated between a negatively charged polymer fluid and a

metallic fiber collector for random orientation or nano-scale fibril alignment so that a continuous yarn

is manufactured along with the fiber mats [95]. The process of electrospinning-induced alignment of

CNTs is shown in Fig. 11. It is seen that the polyacrylonitrile (PAN)/SWCNT nanofibers are smooth

and uniform, and SWCNTs are aligned along the direction of the nanofibers as shown in Fig. 12.

Sen et al. [96] fabricated SWCNT-reinforced polystyrene (PS) nanofibers and poly-urethane (PU)

membranes using the electrospinning process. They found that the SWCNT bundles are oriented


Fig. 10. Optical micrograph (a) and wide angle X-ray diffraction (WAXD) pattern (b) of PC/carbon fiber nanocompositestreated in molten state under a magnetic field. After [93].

Fig. 11. Schematic of the electrospinning process. After [95].

parallel to the PS/SWCNT nanofiber axis. The tensile strength and the modulus of PU/ester

functionalized SWCNT composite membranes are enhanced by 104 and 250% as compared to

electrospun pure PU membranes. They have also developed a flexible approach to the alignment of

SWNT that allows their hierarchical assembly using electrospinning, and orientation of SWNT is

directionally controllable. Thus, cross-aligned nanofibers are shown in Fig. 13 [97].

A theoretical model is presented for the behavior of rod-like particles representing CNTs in

electrospinning. Initially, the rods are randomly oriented, but due to the ‘‘sink-like’’ flow in a wedge-

like jet they are gradually oriented mainly along the streamlines, so that straight CNTs are sucked into

the electrospun jet and become oriented [94].

3.5. Liquid crystalline phase-induced alignment of CNTs

Due to the unique molecular structure of liquid crystals (LC), the liquid crystalline phase is easy

to orient along the applied field, such as force, electric and magnetic fields. Following this principle,

Kawasumi et al. [100,101] prepared a nematic low molar mass liquid crystal/clay composite, and used

the low molar mass LC to induce the orientation of clay layers along the direction of low-frequent

electric field. As shown in Fig. 14, when a low frequency electric field was applied to the LC/clay

system, the clay layers aligned parallel to the electric field. Even after the electric field was switched

off, the oriented clay layers maintained their orientation due to their bulkiness.

Recently, we prepared liquid crystalline copolyester/clay nanocomposites [102]. Under shear, the

LCP domains are stretched into ultra-fine oriented LCP/clay bands, whose width is about 140 nm.


Fig. 12. TEM images of PAN/SWCNT nanocomposite fibrils. After [92]. (The average diameter of SWCNTs is 1.3 nm).

Fig. 13. AFM images of: (a) cross-aligned SWNTs/PVP composite fibers; (b) cross-aligned SWNT array. After [97].

Simultaneously, the exfoliated and randomly distributed clay layers are well aligned along the

shearing direction induced by the LCP. Fig. 15 is AFM image of the sheared LCP/clay nanocomposite.

It can be seen that exfoliated clay layers are oriented along the force field induced by LCP, and there

are no exfoliated clay layers in the transverse direction of the ultra-fine oriented LCP/clay band. This

technique thus provides an efficient route to prepare high-performance structural polymer nanocom-

posites.

Similarly, Lynch and Patrick [103] oriented the nematic low molar mass LCs in an electric field

and used these matrices to align the suspended MWCNTs. As shown in Fig. 16, the MWCNTs were

oriented along the direction of applied electric field, and a 1.8 V/mm electric field was strong enough

to overcome the orientational effect of the grooves, which were perpendicular to the electric field.

Song et al. [104] first demonstrated the nematic liquid crystallinity of MWCNTs in water.

Examination of a series of aqueous dispersions with different MWCNTs concentrations showed a

phase transition from isotropic to a Schlieren texture typical of lytropic nematic liquid crystals above a

critical concentration of �4.3% by volume. Later, Mrozek et al. [105] reported that the ordered

SWCNTs could be used to organize the liquid crystalline polymers (LCPs) via kinetic seeding of

homogeneous liquid crystal domains from an LCP melt. Based on these reported results, we believe

that CNTs in the liquid crystalline state can improve both liquid crystallinity and orientation of LCPs.

It is obvious that the alignment of CNTs will induce anisotropy in structural, mechanical,

electrical and thermal properties of polymer/CNT nanocomposites. For example, the ratios of

electrical conductivity and thermal conductivity between the aligned and transverse directions of

aligned MWCNT/epoxy nanocomposites are 1.4 and 1.9 corresponding, respectively, to the applica-


Fig. 14. Schematic of LC/clay composite system at different states. After [101].

Fig. 15. AFM image of LCP/clay nanocomposite containing 5 wt.% clay.

tion of a 15 and 25 T magnetic field [12]. The temperature-dependent anisotropy is, however, very

weak. Research on anisotropy of aligned polymer/CNT nanocomposites has just begun and there is

little published work to-date. We believe the anisotropic mechanical, thermal and electric properties of

polymer/CNT nanocomposites will be extremely useful in many structural and functional materials

and devices.

4. Properties and applications of polymer/CNT nanocomposites

CNTs have been proposed for many potential applications including conductive and high-

strength composites; energy storage and energy conversion devices; sensors; field emission displays

and radiation sources; hydrogen media; and nanometer-sized semi-conductor devices, probes, and

interconnects, etc. [81]. Polymer/CNT nanocomposites are expected to have good processability of the

polymers and high mechanical and functional properties of the CNTs. Continuing advances on

dispersion and alignment of CNTs in polymer matrices will further promote developments in and

expand the range of applications of these nanocomposites.

4.1. Mechanical properties

Incorporation of CNTs into a polymer matrix can potentially provide structural materials with

dramatically increased modulus and strength. For example, adding 1 wt.% MWCNTs in the PS/

MWCNT composite films by the solution-evaporation method, results in 36–42 and �25% improve-

ments in tensile modulus and break stress, respectively [9]. Biercuk et al. [106] have observed a

monotonic increase of resistance to indentation (Vickers hardness) by up to 3.5 times on adding

2 wt.% SWCNTs in epoxy resin. Cadek et al. [107] also found that adding 1 wt.% MWSNTs to

polyvinyl alcohol (PVA) increased the modulus and hardness by 1.8 times and 1.6 times, respectively.

As discussed in Sections 2 and 3, the homogeneous dispersion and alignment of CNTs in polymer

matrices are significant to enhance the effectiveness of reinforcement. For example, for PMMA/

MWCNT composites containing 1 wt.% MWCNT, the storage modulus at 90 8C is increased by an


Fig. 16. AFM image of LC/MWCNTs under 1.8 V/mm electric field (5 mm � 5 mm). After [103].

outstanding 1135% due to the homogeneous dispersion enhanced by in situ polymerization [27]. The

tensile strength and modulus of melt drawn PS/MWCNT composite films are, respectively, increased

by 137 and 49% compared to the drawn PS film [88].

In general, inclusion of CNTs in polymer matrices leads to reductions in impact toughness of

composites [108,109] and there is an additional effect due to the orientation of the CNTs [110].

However, opposite effects are also reported, which claim an improvement in toughness [111]. For

example, Ruan et al. [112] have shown that, with 1 wt.% MWCNTs in UHMWPE, the toughness (that

is, area under tensile stress–strain curve) was increased 150% and ductility by 104% due to the

enhanced chain mobility in UHMWPE induced by MWCNTs. Similarly, Weisenberger et al. [113]

found that addition of 1.8 vol.% MWCNTs enhanced the energies to yield and to break by about 80%

for an aligned polyacylonitrile/MWCNT fiber. Dalton et al. [114] prepared super-tough polyvinyl

alcohol/SWCNT composite fiber containing 60 wt.% SWCNTs caused by slippages between SWCNT

bundles. Gorga and Cohen [110] proposed a failure mechanism, in which the orientation of MWCNTs

(normal to the direction of craze/crack growth) enabled them to toughen PMMA by crake-wake

bridging. Assouline et al. [115] found that addition of 1 wt.% MWCNTs in polypropylene (PP) matrix

increased the composite toughness due to the fibrillar crystal structure of PP induced by MWCNTs.

Recently, Blake et al. [116] modified butyl-lithium-functionalized MWCNTs with chlorinated

polypropylene (CPP) to produce nanotubes covalently bonded to chlorinated polypropylene

(MWCNT/CPP), and compounded the CPP/MWCNT with the CPP/tetrahydrofuran (THF) solution

to obtain CPP/MWCNT composites. They claimed that as the MWCNT content was increased to

0.6 vol.%, the modulus increased by three times compared to pure CPP from 0.22 to 0.68 GPa, and

both tensile strength and toughness (measured by the area under the stress–strain curve) increased by

3.8 times (from 13 to 49 MPa) and 4 times (from 27 to 108 J/g), respectively. These results show that

the covalent functionalization of CNTs enables both efficient dispersion and excellent interfacial stress

transfer [116].

A few words of caution should be made here on the definition and measurement of toughness of

CNTs-polymer nanocomposites. From the above, it is seen, for convenience, that toughness is variably

defined by the Izod impact strength (J/m) or the area under the tensile stress–strain curve (J/m3)

depending on the background and experience of the researchers. However, these two parameters do not

give the correct toughness values. For proper toughness measurement techniques, the readers should

refer to [117].

4.2. Thermal properties

The addition of CNTs could increase the glass transition, melting and thermal decomposition

temperatures of the polymer matrix due to their constraint effect on the polymer segments and chains.

It is important to improve the thermal endurance of polymer composites. Thus, with a surfactant,

adding 1 wt.% CNTs to epoxy increases the glass transition temperature from 63 to 88 8C [19].

Similarly, with 1 wt.% well-dispersed SWCNTs, the glass transition temperature of PMMA is

increased by �40 8C [27]. Because CNTs act as nucleation sites in the matrix, their inclusion

enhances polymer crystallization and increases the melting temperature [107]. Kashiwagi et al. [118]

found that the thermal decomposition temperature of polypropylene (PP) at peak weight loss in

nitrogen was increased by �12 8C with 2 vol.% MWCNTs, and that MWCNTs significantly reduced

the PP heat release rate making it as effective a fire-retardant as PP/PP-g-MA/clay.

Also, the incorporation of CNTs could improve the thermal transport properties of polymer

composites due to the excellent thermal conductivity of CNTs. This offers an opportunity for polymer/

CNT composites for usages as printed circuit boards, connectors, thermal interface materials, heat


sinks, lids and housings, and high-performance thermal management from satellite structures down to

electronic device packaging. Biercuk et al. [106] found that 1 wt.% un-purified SWCNTs in epoxy

showed a 70% increase in thermal conductivity at 40 K, rising to 125% at room temperature. Choi

et al. [12] observed that the thermal conductivity of epoxy increased by up to 300% with 3 wt.%

SWCNTs. It is noted that aligned MWCNTs will further improve the thermal conductivity. Compared

to the epoxy/MWCNT nanocomposites without an applied magnetic field, the alignment of MWCNTs

under a 25 T magnetic field has lead to a 10% increase in thermal conductivity [12].

4.3. Electrical and electrochemical properties

The first realized major commercial application of carbon nanotubes is their use as electrically

conducting components in polymer composites [81]. It is reported that GE Plastics has been using

CNTs in a poly(phenylene oxide) (PPO)/polyamide (PA) blend for automotive mirror housings for

Ford [119] to replace conventional micron-size conducting fillers, which would require loadings as

high as 15 wt.% to have a satisfactory anti-static property but which would impart poor mechanical

properties and a high density to the composite.

Coleman et al. [120] showed that the electric conductivity of poly(p-phenlyenevinylene-co-2,5-

dioctoxy-m-phenylenevinylene) (PMPV) polymer could be dramatically increased by up to ten orders

of magnitude when 8 wt.% CNTs were added, indicative of percolating behavior [121]. The in situ

polymerized polyimide (PI)/SWCNT composite films exhibited significant conductivity enhancement

(10 orders) at a very low loading (0.1 vol.%) without significantly sacrificing optical transmission [26].

Since their mechanical properties and thermal stability were also improved by addition of SWCNTs,

they are potentially useful in a variety of aerospace and terrestrial applications. Again, alignment of

CNTs affects the electrical properties of polymer/CNT nanocomposites. Thus, in epoxy/MWCNT

nanocomposites with MWCNTs aligned under a 25 T magnetic field leads to a 35% increase in electric

conductivity compared to those similar composites without magnetic aligned CNTs [12]. Improve-

ments on the dispersion and alignment of CNTs in a polymer matrix could decrease the percolation

threshold value [12,15,122].

Recently, super-capacitors are attracting great attention because of their high capacitance and

potential applications in electronic devices. It has been reported that the performance of super-

capacitors with MWCNTs deposited with conducting polymers as active materials is greatly enhanced

compared to electric double-layer super-capacitors with CNTs due to the Faraday effect of the

conducting polymer [24]. Besides these, polymer/CNT nanocomposites could have many potential

applications in electrochemical actuation, electromagnetic interference shielding (EMI), wave

absorption, electronic packaging, self-regulating heater, and PTC resistors, etc. [123,124].

4.4. Optical and photovoltaic properties

Nonlinear optical organic materials, such as porphyrins, dyes and phthalocyanines, provide

optical limiting properties for photonic devices to control light frequency and intensity in a predictable

manner. However, these are narrow band optical materials. Carbon nanotubes, both SWCNTs and

MWCNTs, have been studied [2]. O’Flaherty et al. [125] noted that the optical limit saturated at

carbon nanotubes exceeding 3.8 wt.%, relative to the polymer mass. Goh et al. [126] found that the

aqueous MWCNT suspension showed only week optical limit towards laser at 532 nm operating at

20 Hz, but its mixture with double-C60-end-capped poly(ethylene oxide) (FPEOF) solution displayed

enhanced optical limiting responses at 532 and 1064 nm. Polymer/CNT composites could also be used

to protect human eyes, optical elements, optical sensors and optical switching. Chen et al. [127]


demonstrated the ultra-fast optical switching property of polyimide (PI)/SWCNT composites at

1.55 mm.

Another potentially important application of CNTs is in polymer-based light-emitting devices.

The advantages for organic light-emitting diodes (OLEDs) based on conjugated polymers are low cost,

low operating voltage, excellent processability and flexibility. However, their low quantum efficiency

and stability have limited their applications and developments. Riggs et al. [52] observed strong

luminescence of soluble polymer grafted CNTs. Ago et al. [128] revealed the electronic interaction

between CNTs and photo-exited polymers, such as poly(p-phenylene vinylene) (PPV), and proposed

‘‘hole collecting’’ properties of MWCNTs from PPV at the composite interface via the non-radiative

energy transfer of singlet excitons from PPV to MWCNTs. Woo et al. [129] found that SWCNTs in

poly(m-phenylene vinylene-co-2,5-dioctoxy-p-phenylene vinylene) (PmPV) were responsible for

blocking hole transport in the composite by forming hole traps in the polymer matrix induced by

image charge effects between SWCNTs and the charge carriers. When the PmPV/MWCNT composite

is used as an electron-transport layer in OLEDs based on poly(2,5-dimethoxy-1,4-phenylene-

vinylene-2-methoxy-5(20-ethylhexyloxy)-1,4-phenylene-vinylene) (M3EH-PPV), the brightness of

OLEDs is greatly enhanced due to the low potential barrier reduced by the incorporation of CNTs in

the PmPV/MWCNT composite [130]. Kim et al. [131] observed that the device qualities, such as

external quantum efficiency, were improved by 2–3 times for up to 0.2 wt.% SWCNTs in OLEDs.

They ascribed this to the facile hole injection and the polymer–SWCNT interactions which stiffened

the polymer chains. Subsequently, the better hole transport in the metallic SWCNT–polymer medium

induced more efficient single exciton formation at or near the interface region.

Besides, CNTs are also widely used in organic photovoltaic devices. Doping with 6 wt.%

chemical functionalized MWCNTs by grafting dodecylamine chains, the photosensitivity of oxoti-

tanium phthalocyanine (TiOPc) is five-fold higher than that of un-doped TiOPc when exposed to

570 nm wavelength [54]. It is beneficial to design photo-conductive devices with high efficiency of

charge carrier generation. Clearly, polymer/CNT nanocomposites represent an alternative class of

organic semi-conducting materials that are promising for organic photo-voltaic cells and devices with

improved performance [132,133].

4.5. Super-hydrophobic properties

Li et al. [134] first observed the contact angle for water on an aligned CNT film by the pyrolysis of

metal phthalocyanines is about 1288. After the CNT film is modified through immersion in a methanol

solution of hydrolyzed fluoroalkylsilane, the modified aligned CNT films show super-‘‘amphiphobic’’

properties, the contact angles for water and rapeseed oil on the film are 1718 and 1618, respectively. In


Fig. 17. SEM images of aligned CNT films by pyrolysis of metal phthalocyanines: (a) top view; and (b) cross-sectional view.After [134].

general, the contact angle (u) of a flat surface and that of a suitably rough surface (ur) is given by

cos ur ¼ r f1 cos u � f2 (2)

where r is the surface roughness factor, f1 and f2 are fractions of CNTs and air on the aligned CNT

films. From Fig. 17, the structure of aligned CNT films leads to a sufficiently rough surface that the

liquid may trap air to give a composite surface effect. The very large fraction (f1) of air and the very

small fraction (f2) of CNTs on the films give rise to large contact angles for water and oil.

Following the above study, Sun et al. [135] and Feng et al. [136] prepared lotus-like and

honeycomb-like aligned CNT films with a combination of micro- and nano-structures (see Fig. 18) and

they all displayed super-hydrophobicity. Thus, it is expected that well-aligned CNT compounded

polymer films or coatings should have wide applications including super-hydrophobic surfaces to

textiles, coatings, gene delivery, micro-fluid channels, non-wetting liquid transfer and so forth.

5. Concluding remarks

Enhanced dispersion and alignment of CNTs in polymer matrices greatly improve mechanical,

electric, thermal, electrochemical, optical and super-hydrophobic properties of polymer/CNT com-

posites. Thus, the critical challenge is the development of means and ways to promote and increase the

dispersion and alignment of CNTs in the matrix. Optimum physical blending, in situ polymerization

and chemical functionalization could improve the dispersion of CNTs, but the last method is the most

efficient. The alignment of CNTs in a polymer matrix could be increased by ex situ alignment due to

force, electrical and magnetic field-induced methods, and liquid crystalline phase could also induce

the orientation of CNTs along the direction of the applied field. Currently, we are investigating the

liquid crystalline polymer grafted CNTs, and we anticipate that the modified CNTs will be compatible

with the polymer matrix, and easy to be aligned along the field direction in a polymer matrix due to the

high compliance of the CNTs. This seems to be a promising alignment method of CNTs not yet

explored.

Acknowledgements

We wish to thank the Australian Research Council (ARC) and the National Natural Science

Foundation of China (20474021 and 20490220) for the continuing financial supports of our individual

projects on polymer nanocomposites. Sincere thanks are due to our colleagues at the CAMT and


Fig. 18. SEM images of lotus-like and honeycomb-like aligned CNT films: (a) top view of lotus-like aligned CNT films; (b)enlarged view of single micro-papilla of the lotus-like aligned CNT films; and (c) top view of honeycomb-like aligned CNTfilms. After [136].

HUST for many useful suggestions and comments on the manuscript. YWM is Australian Federation

Fellow funded by the ARC tenable at the University of Sydney. XLX is Visiting Scholar to the CAMT

supported by the ARC when this review was completed.

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Xiao-Lin Xie is a professor in the Department of Chemistry at Huazhong University of Science and Technology (HUST),China. He is Vice-Director of the Rheology Sectional Committee and Member of the Applied Chemistry SectionalCommittee of the Chinese Society of Chemistry. Prof. Xie’s research interests are on polymer composites and liquidcrystalline polymers. In 1995, he received a PhD degree from Sichuan University, which was followed by 2 yearspostdoctoral research at Zhejiang University. From April 2003 to March 2004, he was Visiting Scholar in the University ofSydney Centre for Advanced Materials Technology and undertook studies on polymer–matrix nanocomposites.

Yiu-Wing Mai was a graduate of Hong Kong and Sydney universities. He now holds a University Chair in Sydney Universityand is an Australian Federation Fellow. He is also Director of the Centre for Advanced Materials Technology at SydneyUniversity since 1988. Professor Mai has a general research interest on the processing-structure–property relationships of arange of engineering materials including nanocomposites. He is listed in the ISI HighlyCited.com database as a highly citedresearcher in Materials Science.

Xing-Ping Zhou is a lecturer in the Department of Chemistry, Huazhong University of Science and Technology (HUST),China. His research interest is on polymeric materials. He received a masters degree from HUST in 2001 and was a researchassistant in the Department of Physics and Materials Science, City University of Hong Kong during the period November2000 to February 2001. He is now a PhD student in HUST.


dispersion and alignment of carbon nanotubes in polymer matrix a review

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