A critical review on nanotube and nanotube/nanoclay related polymer

composite materials

Kin-tak Lau a,*, Chong Gu b, David Hui c

a Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, Chinab Department of Chemical Engineering, Massachusetts Institute of Technology (MIT), Cambridge, MA, USA

c Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA

Received 11 July 2005; received in revised form 18 August 2005; accepted 19 August 2005

Available online 3 April 2006

Abstract

Since the last decade, research activities in the area of nano-materials have been increased dramatically. More than a 1000 of journal articles in

this area have been published within the last 3 years. Materials scientists and researchers have realized that the mechanical properties of materials

can be altered at the fundamental level, i.e. the atomic-scale. Carbon nanotubes (hereafter called ‘nanotubes’) have been well recognized as nano-

structural materials that can be used to alter mechanical, thermal and electrical properties of polymer-based composite materials, because of their

superior properties and perfect atom arrangement. In general, scientific research related to the nanotubes and their co-related polymer based

composites can be distinguished into four particular scopes: (i) production of high purity and controllable nanotubes, in terms of their size, length

and chiral arrangement; (ii) enhancement of interfacial bonding strength between the nanotubes and their surrounding matrix; (iii) control of the

dispersion properties and alignment of the nanotubes in nanotube/polymer composites and (iv) applications of the nanotubes in real life. Although,

so many remarkable results in the above items have been obtained recently, no concluding results have so far been finalized. In this paper, a critical

review on recent research related to nanotube/polymer composites is given. Newly-adopted coiled nanotubes used to enhance the interfacial

bonding strength of nanocomposites are also discussed. Moreover, the growth of nanotubes from nanoclay substrates to form exfoliated

nanotube/nanoclay polymer composites is also introduced in detail.

q 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Nano-structures; B. Mechanical properties; Nanotubes; Nanoclays; Nanocomposites

1. Introduction

Since, the discovery of carbon nanotubes (hereafter called

‘nanotubes’) by Iijma [1], researches related to the nanotubes

and their co-related composite materials have been dramati-

cally increased. The arguments for the true mechanical

properties of both single-walled and multi-walled nanotubes

never cease. Whether chemical bonding between the nanotubes

and their surrounding polymer-based matrix in the composites

exits or not, is another disputable topic that researchers have to

solve before applying the nano-structural materials to real life.

Because of the high tensile modulus, the single-walled

nanotube has been regarded as one of the ultra-strong materials

in the World. The single-walled nanotube is supposed to be

1359-8368/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesb.2006.02.020

* Corresponding author. Tel.: C86 852 2766 7730; fax: C86 852 2365 4703.

E-mail address: mmktlau@polyu.edu.hk (K.-t. Lau).

formed by rolling a graphene sheet and has a Yong’s modulus

of about 1 TPa [2]. Another work also reported the Young’s

modulus of 4.7 TPa [3]. However, some computational studies

found that the true moduli of the nanotubes were far below the

estimated values obtained from the graphene sheet. Molecular

dynamics (MD) simulation is one of the useful tools to estimate

the physical, mechanical and thermal properties of the

nanotubes, because it is able to reproduce the realistic nanotube

structures. Several kinds of local defects, such as Stone Waals

defect and dislocation of carbon atoms may influence the

properties of the nanotubes, which have been discussed in some

computational work [4,5]. Unfortunately, the accuracy of the

calculation is highly dependant on the initial boundary

condition applied to the simulated models and the sizes of

the systems. Also, the weak van der Waals interaction between

layers of multi-walled nanotubes causes the reduction of the

mechanical strength subject to a uniaxial tensile load in

nanocomposites. Besides, many theoretical works using the

continuum mechanics approach have been done to comprehen-

sively investigate all the parameters that influence the

Composites: Part B 37 (2006) 425–436

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K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436426

properties of nano-materials and to anticipate their macro-scale

properties. However, this method is somehow inaccurate and

has to be combined with the MD simulation. The time required

for MD simulation is typically long and the investment on

facilities is also huge.

Although so many efforts, focusing on different aspects of

nanotubes and their co-related polymer-based composites,

have been paid to date, still no convergent results were

obtained. This may be caused by the use of different

approaches in theoretical and computational analyses. Besides,

no general testing standards for such tiny structural materi-

als/reinforcements have been set up as references for all

scientists and researchers, and this indeed is the major problem

that they are presently facing. In this paper, it is intended to

summarize recent research achievements related to the

nanotubes and their co-related structures in nanocomposites,

for easing readers to reference. Several important aspects that

influence the properties of nanotube/polymer composites will

also be discussed in detail.

2. Mechanical properties of nanotubes

Carbon nanotube has been well recognized as one of the

ultra-strong materials in the World, which has been proven by

both simulations and experimental measurements [6]. The

extreme small size makes it suitable to be embedded into any

type of light weight and soft materials as reinforcements to

form strong and light nanocomposites. Since the authors

published the first review article [7], numerous researches have

been started focusing on the feasibility of using these nano-

structural materials to strengthen polymer-based composites.

However, the true mechanical properties of nanotubes such as

their Young’s modulus, yield strength, ultimate strength,

elastic properties and even fracture behaviour are still uncertain

to date. This actually induces many arguments in whether the

nanotubes are suitable to be used as nano-reinforcements for

the nanocomposites or not.

Experiments conducted previously showed that the Young’s

moduli of nanotubes range from 270 to 950 GPa. Such a large

Fig. 1. Tensile strength test o

discrepancy was due to the different sizes, lengths and numbers

of wall layers used in different tests. However, it is hard to

produce identical nanotubes even in the same experiment. In

Fig. 1, a typical tensile test for multi-walled nanotubes

conducted by Ruoff and Lorents is shown [8]. Since, it have

been reported that inner layers of multi-walled nanotubes

cannot effectively take any tensile loads applied at the both

ends, because the stress transferability between the layers of

the nanotubes is very weak [9] and only the outmost layer of

the nanotubes takes the entire load. Therefore the failure of the

multi-walled nanotubes could start at the outmost layer by

breaking the bonds among carbon atoms, as described in Fig. 2.

The relations between the geometrical dimensions of the

nanotube, e.g. the size, number of wall layers and length of the

nanotubes and their mechanical properties have not been

worked out yet. Moreover, in some scenarios, substrates

remain inside the nanotubes may cause contamination, which

would be one of the potential hazards to nanotube/polymer

composites.

In the early stage, empirical force potentials were used in

MD simulations to calculate the Young’s modulus of single-

walled nanotubes and the estimated value was almost four

times greater than that of diamond. As described in the

introduction section, this kind of simulation was based mainly

on the structure of a perfect graphene sheet with complete

hexagonal carbon atom arrangement, while interactions

between atoms in the circular configuration were not

comprehensively studied. In those calculations, two common

approaches based on quantum mechanics and molecular

mechanics were used. Both of them attempt to capture the

variation of system energy associated with the change in

atomic positions by following Newton’s second law (ForceZmass!acceleration). For a single-walled nanotube, the mutual

interactions between atoms are basically described by the force

potentials from both bonding and non-bonding interactions as

defined in Ref. [9]. Essentially, the bonding energy described at

the atomic scale is the sum of four different interactions,

namely bond stretching (Ur), angle variation (Uq), inversion

(Uu) and torsion (Ut). A schematic illustration of each energy

f multi-walled nanotube.

Fig. 2. Stretching process of a triple-walled nanotubes in MD simulation. The nanotube was at (a) an unloaded and (b) stretched till failure conditions.

Fig. 3. Bond structures and corresponding energy terms of a graphene cell.

K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436 427

term and the corresponding bond structure for a grapheme cell

is shown in Fig. 3. The most commonly used functional forms

are:

Ur Z1

2

X

i

KiðdRiÞ2; (1)

Uq Z1

2

X

i

CjðdqjÞ2; (2)

Uu Z1

2

X

k

BkðdukÞ2; (3)

and

Ut Z1

2

X

i

Ai½1CcosðnitiKfiÞ�; (4)

K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436428

where dRi is the elongation of the bond identified by the label i,

Ki is the force constant associated with the stretching of the ‘i’

bond, and dqj and duk are the variance of bond angle j and

inversion angle k, respectively. Cj and Bk are force constants,

associated with angle variance and inversion, respectively. Ai is

the ‘barrier’ height to rotation of the bond i and ni is the

multiplicity, which gives the number of minimums as the bond

is rotated within the range of 2p [9].

To determine the tensile modulus of a single-walled

nanotube subject to uniaxial loadings, it is useful to make

observation at small strains. In this case, since the torsion, the

inversion, the van der Waals, and the electrostatic interactions

energy terms are small and neglectable compared with the

bond stretching and the angle variation terms, the total energy

of the single-walled nanotube can be reduced to:

ETotal Z1

2

X

i

KiðdRiÞ2 C

1

2

X

j

CjðdqjÞ2 (5)

The force constants Ki and Ci can be obtained from quantum

mechanics (ab initio). The average macroscopic elastic

modulus and Poisson’s ratio of a single-walled nanotube

were estimated to be about 1.347 TPa and 0.261, respectively

[10]. It is also found that the Poisson’s ratio of the single-

walled nanotubes decreases with increasing diameter (see

Fig. 4). Such calculations may be performed using either the

force or the energy approach, by measuring the mechanical

forces between carbon atoms in nanotubes with different chiral

arrangements.

Molecular mechanics simulations predicted that the fracture

strain and stress of a zigzag nanotube were between 10–15%,

and 65–93 GPa, respectively [11]. Brittle failures of the

nanotubes were also found in the simulation and the results

agreed with the experimental measurements. However, another

research using a continuum theory of fracture nucleation

demonstrated that the breaking strain of a single-walled

nanotube was about 55%, in which the fracture nucleation

was assumed to be the bifurcation instability of a homo-

geneously deformed nanotube at this strain level [12].

Fig. 4. MD predictions on a single-walled nanotubes with different tube

diameters [10].

Belytschko et al. [11] found a shear cracking of the nanotubes

along the G458 directions with the existence of a 5/7/7/5

dislocation (see Fig. 5). It is also concluded that the chiral

arrangement of the nanotubes could not significantly influence

their mechanical strength. Pantano et al. [13] have provided a

comprehensive review on the mechanics of the deformation of

nanotube structures investigated through MD simulations and

finite element (FE) analysis, in which local buckling of the

multi-walled nanotubes at their inner bending face and radial

deformations of adsorbed nanotubes in relation to their size and

adjacent components have been discussed. In their study, it has

been proved that FE models could be used to simulate the

structural behaviour of nanotubes and the results were

comparable with the atomic models for various configurations.

It is also concluded that the use of shell theory associated with

appropriate boundary constraints applied to the FE models can

simulate the true status of the nanotubes. Besides, in their

simulations, the wall-to-wall shear resistance was ignored,

because many experimental observations in the past have

proved that only a very weak van der Waals interaction existed

between layers of the nanotubes. The shape of the deformed

nanotubes was well agreed with experimental observation

through TEM. In Fig. 6, a TEM observed bent multi-walled

nanotubes and a corresponding image captured from the

simulation are compared.

Although, so many researchers have been striving hard to

look for ways to investigate the mechanical properties of the

nanotubes for nanocomposite applications, no concluding

results have been made so far to provide an exact solution on

this aspect, since the quantitative measurements are unavail-

able due to the small physical size of the nanotube and the

combination of different parameters involved, such as the

chiral arrangement, the number of wall layers, the layer

thickness and the assumed space between layers. Another

possible reason for the difference in simulation results may be

caused by the definition of the mechanical properties, e.g. the

Young’s modulus, in the microscopic scale, which may be

different from the one in macroscopic scale. In Table 1, a

summary of the mechanical properties obtained from exper-

imental measurements, molecular dynamic (MD) simulations

and FE modelings are given. The difficulty in estimating the

Young’s modulus of the multi-walled nanotubes is that it is

highly dependent on the condition of the outermost layer of the

nanotubes, since all the inner layers may not be able to

effectively take loads [24]. This is further proved by Fig. 7,

which shows that a slipping occurs between the outermost layer

and inner layers when the load is applied. Accordingly, Lau

et al. [25] have proposed that the Young’s modulus of the

multi-walled nanotubes (MWNTs) used in polymer composites

can be estimated by regarding the outmost layer as a single-

walled nanotube since the strain in all the inner layers will not

be affected when the load is applied, i.e.

EMWNTjd0ZESWNTjd ; (6)

where d0 and d represent the diameters of the outmost layer of

the multi-walled and single-walled nanotubes, respectively.

Fig. 5. Crack formation in the [40,40] armchair nanotube with 5/7/7/5 defect [11].

K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436 429

Eq. (6) shows that the Young’s modulus of MWNTs used in the

nanocomposites is equivalent to the Young’s modulus

established by a single walled nanotube with the same

outermost diameter (d0Zd).

3. Stress transfer properties of nanotube/polymercomposites

Although, the nanotubes have been regarded as ultra-strong

nano-reinforcements to enhance the mechanical performance,

electrical and thermal properties of nanocomposites, their

applications are still very limited due to many uncertainties

such as the dispersion properties, alignments and stress transfer

properties of the nanotubes in the nanocomposites, which are

very difficult topics that most researchers have been facing for

many years. In the traditional way, fibre pullout test has been

well recognized as a standard method to investigate the

Fig. 6. (Top) TEM image of a buckled MWNT nanotubes and (bottom) image

captured from FE bending simulation of a 14-wall MWNT.

interfacial bonding properties of advanced composite

materials. However, due to the size limitation, good

performance of such test for nanotube/polymer nanocompo-

sites seems impossible. Even though, several tensile tests on

nanotube/polymer nanocomposites have been reported in the

previous literatures to study the bonding behaviour between the

nanotubes and the matrix [26–28], in which the interfacial

shear strength ranging from 35 to 376 MPa was reported,

depending on the diameter of the nanotubes and the number of

wall layers. Lau and Hui [29] have found that most of

nanotubes were pulled out during the tensile testing. Since the

perfect atomic architecture is formed on the surface of the

nanotubes, it is difficult to break the carbon–carbon bonds

without the use of chemical agencies. However, attaching other

elements on the surface of the nanotubes may distort their

extraordinary performances. Research in this area has been

conducted for several years, and many works are still ongoing.

Recently, Wagner [30], Lau [31], Haque and Ramasetty

[32], and Gao and Li [33] have calculated the interfacial

bonding strength of nanotube/polymer composites using

fundamental shear lag models. It is concluded that the single-

walled zigzag nanotubes would induce higher interfacial

bonding stress at both bonded end regions. The stress transfer

length would be affected by the diameter and type of

nanotubes. An optimal aspect ratio of 1000 would provide

efficient load transfer in the related nanocomposite structures.

In MD simulation without considering the atomic bonding

between the nanotubes and the matrix, it was found that non-

bond interactions consists of electrostatic and van der Waals

interactions, deformation induced by these forces, as well as

stress/deformation arising from mismatch in the coefficients of

thermal expansion [34]. All of these parameters affect the

interfacial bonding properties between the nanotubes and the

matrix. In most MD simulations, Lennard-Jones 6–12

potentials have been popularly used in modeling the non-

bond interactions within the nanotubes and between the

Table 1

Mechanical properties of nanotubes addressed in different literatures

Author E (TPa) n Year Method Ref.

Yakoson 5.5 0.19 1996 MD [14]

Zhou et al. 0.77 0.32 2001 Theoretical [15]

Lu 1.0 0.28 1997 MD [2]

Tu 4.7 0.34 2002 Theoretical [3]

Chnag and Gao 1.33 0.26 2003 MD [10]

Kristnan et al. 1.25 – 1998 Theoretical [16]

Li and Chou 1.05 – 2003 FEM [17]

Yu et al. 0.27–0.95 – 2000 Experimental [18]

Li et al. 0.79 – 2000 Experimental [19]

Demczyk et al. 0.9 – 2002 Experimental [20]

Natsuki et al. 0.73–1.1 – 2003 Molecular and solid

mechanics

[21]

Li et al. 0.8 – 2005 Experimental [22]

Tserpes et al. 2.3–2.4 – 2005 Structural mechanics

and FEM

[23]

Lau et al. 0.44 – 2004 MD [24]

K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436430

polymer matrix and the nanotubes [25,35]. Frankland et al. [35]

have found that even a relatively low density of cross-links (as

shown in Fig. 8) can have a large influence on the properties of

nanotube/polymer interface. They also found that the tensile

strength of the nanotubes at the functionalization level could

not have a significant difference. Besides, the nano-mechanical

interlocking was also observed at the nanotube/polymer

interface. With the help of thermal mismatch in coefficients

of thermal expansion between the two materials, such

interlocking after being cured could substantially increase the

friction at the interface and thus increase the pullout strength of

the nanotubes. A physical pullout test was conducted by Barber

et al. [36] using AFM to pull a nanotube, which had been cured

on a polyethylene-butene sheet. It was found that the average

Fig. 7. Schematic illustration of the deformation shapes of

interfacial stress, which was required to entirely remove the

nanotube, was about 47 MPa. Fig. 9 shows a load-time curve of

a nanotube pulled out from the sheet.

MD simulations normally take very long computational

time and require powerful computer facilities, which inevitably

creates a barrier for adopting this technique for practical

applications. Other equivalent-continuum models, constitutive

models, equivalent-truss models (see Fig. 10) and MD

associated with FE models [37] have appeared gradually in

the past few years [38]. MD simulations are normally used for

studying the physics of condensed matter systems in which the

forces acting on particles in a defined cell are calculated and the

classical Newtonian equations of motion are integrated

numerically. Equivalent-continuum models are based on the

nanotubes subject to different load applications [25].

Fig. 8. Illustration of the cross-linked system between the nanotube and matrix [35].

K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436 431

equilibrium molecular structure obtained from the MD

simulations and are used to predict the bulk mechanical

behaviour of nano-structured materials. Liu and Chen [37]

have applied the concept of representative volume elements

(RVEs) to extract the mechanical properties of the nanotube/

polymer composites based on 3D elasticity theory and FE. In

their RVE approach, a single walled nanotube with surround-

ing polymer matrix was modeled, with properly applied

boundary and interface conditions to account for the effects

of the surrounding matrix. This RVE model can be employed to

study the interactions of the nanotubes with the matrix, to

investigate the load transfer mechanism or to evaluate the

effective materials properties of the nanocomposites.

Equivalent truss models have been more popular because

the energies (bond and non-bond interactions) used to hold

different atoms in the nanotubes could be simulated as FE truss

members. This technique provides a short processing time and

high accuracy in calculation. Most of these analyses mainly

focused on the determination of an effective embedding length

of nanotubes in nanocomposites, in order to allow the total load

Fig. 9. Typical plot of pullout force against pullout time of the nanotube

embedded in the polymer [36].

to transfer from the matrix to the nanotubes [39]. However, the

influence due to the sliding of layers inside the multi-walled

nanotubes was not well discussed elsewhere for the develop-

ment of nanotube related nanocomposites. Besides, the

instability of nanotubes at different temperatures may cause

the distortion of the nanotubes during applications, particularly

in some high precision instruments. In Fig. 11, the structures of

the simulated single-walled nanotubes at different temperatures

are shown. Gou and Lau [40] have provided a comprehensive

review on recent researches on the modeling and simulation of

nanotube/polymer interface for nanocomposite materials.

4. Novel coiled nanotubes and nanotube/nanoclay polymer

composites

Since, many studies have addressed that there is no chemical

bonding between the nanotubes and the matrix, and it is also

hard to take the benefit from the inner layers of multi-walled

nanotubes because of the very weak bonding between the

layers, few researches have been reported to investigate the

bonding properties of nanocomposites. The growth of

nanotubes from carbon fibres would be of interest to many

researchers in the advanced composite field [41]. Although,

this method can enhance the bonding between the carbon fibre

and the matrix, other properties of the nanotubes, such as the

strength, were not fully used in the composite materials.

One of the other possible ways to enhance the bonding

strength between the nanotubes and the matrix is to make use

of the nano-mechanical interlocking of the nanotubes by

changing their configuration and/or surface morphology.

Recently, Lu et al. [42–43] produced coiled carbon nanotubes

(herewith called ‘coiled nanotubes’) by using a reduced

pressure catalytic chemical vapour deposition (CCVD)

method. Dissimilar coiled nanotubes were prepared and

fabricated by CCVD on finely divided Co nano-particles

supported by silica gel under reduced pressure and at low gas

Fig. 10. Equivalent-continuum modeling of effective fibre [37].

K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436432

flow rates. In their work, high aspect ratio coiled multi-walled

nanotubes were produced. In Fig. 12, coiled nanotubes with

various pitch lengths and diameters are shown. Since the spring

stiffness of the coiled nanotubes is highly dependent on the

shear strength, sliding of the inner layers becomes less

significant to the performance of the whole structures, and

the overall shear stiffness of the nanotubes can then be fully

used to strengthen the nanotube related composite structures.

A high Young’s modulus of coiled nanotube, 0.7 TPa, was

obtained and it has been anticipated that adding a small amount

of these coiled nanotubes, instead of the straight ones, to the

polymer-based materials could improve their thermal and

mechanical properties, as well as the fracture toughness [44].

It was found that the glass transition temperature (Tg) and

transition enthalpy (DH) of epoxy after being added a small

Fig. 11. Distortion of a nanotube at different temperature conditions (by MD

simulation).

amount of coiled nanotubes decreased comparing with a

pristine sample. As indicated in Table 2, it is obvious that the

Tg and DH of straight SWNT/epoxy and MWNT/epoxy

composites are higher than the coiled one. It is inferred that

during the glass transition process, SWNTs can act as a heat

sink to accelerate the heat absorption of the epoxy, while coiled

nanotubes act as heat-shielding fillers and prevent the epoxy

from exchanging energy with outside system. The results

indicate that the coiled nanotubes can be used to develop heat

shielding polymer-based composite structures. Besides, it was

also found that the hardness of coiled nanotube/epoxy

composites increased compared with a pristine epoxy sample

by 60%. However, the flexural strength of the coiled nanotube

composites decreased by 18.2%. Even though, as compared

with the results from a straight SWNT/epoxy sample (dropped

by 32.3%), the use of coiled nanotubes as nano-reinforcement

for nanocomposites is still a better choice. In Fig. 13, the

fracture surfaces of two different samples are shown. It is

obvious that the coiled nanotube/epoxy composite was

fractured in a more brittle nature while pullout of the nanotubes

in SWNT/epoxy composite was still found.

As mentioned in the previous sections, the nanotubes have

been well recognized as ultra-strong nano-reinforcements for

advanced composite materials. However, the production of

well aligned and well dispersed nanotube/polymer nanocom-

posites is hardly achieved since agglomeration happens all the

time during the manufacturing process although ultrasonic

sonication and pressurization are adopted [45]. Nanoclay

(nano-montmorillonite) is another alternative used to produce

high strength and thermal stable nanocomposites because of its

exfoliated structural forms in soft polymer-based matrix.

However, a difficulty still exists in producing such exfoliated

planar structures throughout the whole composite materials,

Fig. 12. Coil nanotubes with different diameters and pitch lengths.

Fig. 13. SEM images on the fracture surface of (a) straight nanotube/epoxy and

(b) coiled nanotube/epoxy samples.

K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436 433

particularly for mass production and in the twin-screwing

injection of thermo-plastic products [46]. Recently, a novel

nanocomposite has been developed by Lau et al. [47] through

growing nanotubes from nanoclay platelets. During the

growing process, exfoliated nanoclay structures were formed

due to the growth of the nanotube between the platelets. These

nanotube/nanoclay nano-particles would be used as strong

nano-reinforcements for polymer-based composite materials.

In their work, Co(OH)2 particles were formed on the surface of

nanoclay layers with pH-controlled ion precipitation. The

bructile-like phase of Co(OH)2 colloidal particles obtained

exhibited a tendency of irregular growth with increasing pH.

The participation of Co(OH)2 colloidal particles on the

nanoclay surface led to the formation of a weakly-ordered

layered structure in the nanoclay as evidenced by the change of

(001) reflections in the nanoclays structure. The catalysis of the

Table 2

Thermal and mechanical properties of nanotube/epoxy samples

Sample type Tg (8C) DH (J gK1) Flexural

strength

(MPa)

Micro-hard-

ness (HV)

Pure epoxy 54.47 7.282 74.3 10.8

SWNT/

epoxy

57.34 7.852 50.3 12.9

MWNT/

epoxy

55.28 6.752 – –

Coiled NT/

epoxy

50.94 0.745 60.8 16.6

produced Co(OH)2-nanoclay hybrid was proved by the growth

of nanotubes with CVD method. Under the control of the pH

value, the resultant nanotubes created a network-like structure

linking the nanoclay flakes and enhanced the separation of the

nanoclay platelets, and thus formed exfoliated structures. In

Figs. 14, the growth mechanism of nanotubes from nanoclay

layers and the resultant nanotube/nanoclay particle through

SEM observation are shown, respectively. In the figure, the

produced nanotubes are entangled with nanoclay within a large

area and also dispersed in the nanoclay without aggregation.

Additionally, coil-shaped nanotubes with varying coil pitch

can also be seen, which is a phenomenon possibly caused by

the instable nucleation of hexagonal carbon ring of graphite

during the CVD growth. These novel nano-particles could be

used in polymer-based composite materials as nano-reinforce-

ments to strengthen their mechanical properties, and/or at the

same time, alter their thermal and electrical properties.

Apart from the stress transfer, mechanical, electrical and

thermal properties of nanotube related polymer composites, the

design of a proper manufacturing process of the composites is

also a crucial factor to create good physical interactions

between the nanotubes and the matrix. Lau et al. [48] have

found that acetone would be a better solvent used to disperse

nanotubes into epoxy-based composites because the use of

Fig. 14. FE-SEM image of the nanotube/nanoclay composite at pH 9.5.

K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436434

DMF and ethanol would influence mechanical performance of

the composites during the pre-curing process of the compo-

sites. The solvent effects are in the order of DMFOethanolOacetone, which is consistent with the order of their boiling

points. Un-evaporated solvents remained in the resin/hardener

mixtures could degrade their pre-designed mechanical

properties.

Fig. 15. Characteristics of PS-3 at different temperatures [50].

5. Potential applications of nanotube related composites

The application of nanotubes for the composite industry is

huge, ranging from the improvement of mechanical properties

to the alternations of thermal and electrical properties of

traditional polymeric-based composite materials. Each appli-

cation only needs a small amount of nanotubes to be added into

the polymer based materials. Numerous researches have been

conducted in these areas, and several excellent results have

been reported in the past few years. Apart from the

improvement of the mechanical properties of the composites,

it has also been proved that the electrical conductivity

increased with the amount of nanotubes used in epoxy-based

materials [49,50]. By combining with conductive polymer,

such as Polyaniline (PANI), nanotubes can be used to design

sensitive electrochemical sensors [51]. It was observed that

with an increase of the nanotube concentration, the conduc-

tivity of PANI/nanotube films and the current level in the

metal-semiconductor devices increase, even at an elevating

temperature condition, as indicated in Fig. 15. Besides, the fire

behaviour of polyamide 6 was also improved by adding a small

amount of nanotubes into it, due to the increase of the melt

viscosity that prevent dripping and flowing but hinder the

decomposition of volatiles feeding the flame [52]. Raffaelle et

al. [53] have reported that nanotubes can also be used for power

applications, such as proton exchange membrane (PEM) fuel

cells, polymeric solar cells (Fig. 16), LiC batteries, and

thermionic emitters. However, besides those positive responses

from many previous literatures, it was also found that the

nanotubes would be more toxic than other carbon particles or

quartz dust when being absorbed into the lung tissue [54]. For

example, in Fig. 17, it shows that the nanotubes are capable of

intracellular localization and consequently cause irritation in

human epidermal keratinocytes (HEK) [55].

However, those results are obtained at different controlled

environments. For an instance, the manufacturing process of

Fig. 16. Application of SWNTs to polymeric solar cells [54].

Fig. 17. TEM image of human epidermal keratinocytes. Intracellular localization of the MWNT[55].

K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436 435

nanotube/polymer composites was controlled inside the

laboratory and all the samples made were very small

(w20 mm in diameter in average). Without such constraint,

the application of these nanocomposites in real life for mass

production in harsh manufacturing environments would be

another big challenge in the future. Besides, the control of the

dispersion properties and the alignment of nanotubes are still

major problems that have not been solved in producing macro-

scale polymer-based composites.

6. Conclusion

In the past decade, numerous researches have been

conducted on the mechanical, thermal and electrical properties

of carbon nanotubes and the corresponding applications. Also,

different types of nanotubes such as straight, coiled and

bamboo types are mixed with nano-clays to form nanotube/

nanoclay composites and further used for different appli-

cations. However, no matter which type of the nanotubes is

used in composite materials, the alignment, dispersion and

interfacial bonding properties of the nanotubes in matrix is the

most important issue. Most of the works done at the early stage

focused mainly on the feasibility of using straight type

nanotubes as nano-reinforcements for composite materials.

However, due to the weak bonding between the straight type

nanotubes and the matrix, coiled nanotubes appeared to be a

better choice because of their mechanical interlocking proper-

ties, which can be used to overcome the less-bonding problem

of their straight type counterpart. The latest development on

growing nanotubes from nanoclays also opened a new direction

for nanocomposites.

The nanotube/polymer composites have been investigated

for more than 10 years. Different types of works related to these

materials can be found in more than a 1000 literatures and in

many different disciplines. In this paper, a critical review on the

interfacial bonding properties between the nanotubes and the

matrix, coiled nanotubes and nanotube/nanoclay composites

are given. It is not hard to anticipate than more works will be

emerged in the near future. However, the practical use of these

materials will have to wait for a long period, until the

aforementioned concerns addressed in this paper are com-

pletely solved.

Acknowledgements

This project is fully supported by Research Grant Council of

Hong Kong (B-Q856).

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