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: [email protected] (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).
References
[1] Iijima S. Helical mircotubles of graphitic carbon. Nature (London)
1991;354:56–8.
[2] Lu JP. Elastic properties of single and multilayered nanotubes. J Phys
Chem Solids 1997;58:1649–52.
[3] Tu ZC, Ou-yang ZC. Single-walled and multiwalled carbon nanotubes
viewed as elastic tubes with effective Young’s moduli dependent on layer
number. Phys Rev B 2002;65:233–407.
[4] Belytschko T, Xiao SP, Schatz GC, Ruoff RS. Atomistic simulations of
nanotube fracture. Phys Rev B 2002;65:235430.
[5] Maiti A, Svizhenko A, Anantram MP. Electronic transport through carbon
nanotubes: effects of structural deformation and tube chirality. Phys Rev
Lett 2002;88(12):126905.
[6] Li Y, Wang K, Wei JQ, Gu Z, Wang ZC, Luo JB, et al. Tensile properties
of long aligned double-walled carbon nanotube strands. Carbon 2005;43:
31–5.
[7] Lau KT, Hui D. The revolutionary creation of new advanced materials—
carbon nanotube composites. Compos, Part B 2002;33:263–77.
[8] Ruoff RS, Lorents DC. Mechanical and thermal properties of carbon
nanotubes. Carbon 1995;33:925–30.
[9] Lau KT, Chipara M, Ling HY, Hui D. On the effective elastic moduli of
carbon nanotubes for nanocomposite structures. Compos Part B Eng
2004;35:95–101.
[10] Chang TC, Gao HJ. Size-dependent elastic properties of a single-walled
carbon nanotube via molecular mechanics model. J Mech Phys Solids
1997;51:1059–74.
[11] Belytschko T, Xiao SP, Schatz GC, Ruoff. Atomistic simulations of
nanotube fracture. Phys Rev B 2002;65:235430.
[12] Zhang P, Huang Y, Gao H, Hwang KC. Fracture nucleation in single-wall
carbon nanotubes under tension: a continuum analysis incorporating
interatomic potentials. Trans ASME 2002;69:454–8.
K.-t. Lau et al. / Composites: Part B 37 (2006) 425–436436
[13] Pantano A, Parks DM, Boyce MC. Mechanics of deformation of single-
and multi-wall carbon nanotubes. J Mech Phys Solids 2004;52(4):
789–821.
[14] Yakobson BI, Brabec CJ, Bernhoc J. Nanomechanics of carbon tubes:
instabilities beyong linear response. Phys Rev Lett 1996;76(14):2511–4.
[15] Zhou G, Duan WH, Gu BL. First-principles study on morphology and
mechanical properties of single-walled carbon nanotube. Chem Phys Lett
2001;333:344–9.
[16] Krishnan A, Dujardin E, Ebbesen TW, Yianilos PN, Treacy MMJ.
Young’s modulus of single-walled nanotubes. Phys Rev B 1998;58(20):
14013–9.
[17] Li CY, Chou TW. Elastic moduli of multiwalled carbon nanotubes and the
effect of van der Waals forces. Compos Sci Technol 2003;63(11):
1517–24.
[18] Yu MF, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS. Strength and
breaking mechanics of multiwalled carbon nanotubes under tensile load.
Science 2000;287:637–40.
[19] Li F, Cheng HM, Bai S, Su G, Dresselhaus MS. Tensile strength of single-
walled carbon nanotubes directly measured from their macroscopic ropes.
Appl Phys Lett 2000;77(20):3161–3.
[20] Demczyk BG, Wang YM, Cumings J, Hetman M, Han W, Zettle A, et al.
Mater Sci Eng A 2002;334:173–8.
[21] Natsuki T, Tantrakarn K, Endo M. Prediction of elastic properties for
single-walled carbon nanotubes. Carbon 2003.
[22] Li YJ, Wang KKL, Wei JQ, Gu ZY, Wang ZC, Luo JB, et al. Tensile
properties of long aligned double-walled carbon nanotube strands. Carbon
2005;43:31–5.
[23] Tserpes KI, Papanikos P. Finite element modelling of single-walled
carbon nanotubes. Compos Part B 2005;36:468–77.
[24] Lau KT, Gu C, Gao GH, Ling HY, Reid SR. Stretching process of single-
and multi-walled carbon nanotubes for nanocomposite applications.
Carbon 2004;42:423–60.
[25] Gou J, Lau KT. Modeling and simulation of carbon nanotube/polymer
composites. In: Rieth Michael, Schommers Wolfram, editors. A chapter
in the handbook of theoretical and computational nanotechnology.
American Scientific Publishers in 2005; 20051-58883-042-X.
[26] Qian D, Dickey EC, Andrew R, Rantell T. Load transfer and deformation
mechanisms in carbon nanotube–polystyrene composites. Appl Phys Lett
2000;76(2):2868–70.
[27] Cooper CA, Cohen SR, Barber AH, Wagner HD. Detachment of
nanotubes from a polymer matrix. Appl Phys Lett 2002;81(20):3873–5.
[28] Xu XJ, Thwe MM, Shearwood C, Liao K. Mechanical properties and
interfacial characteristics of carbon nanotube reinforced epoxy thin films.
Appl Phys Lett 2002;81(15):2833–5.
[29] Lau KT, Hui D. Effectiveness of using carbon nanotubes as nano-
reinforcements for advanced composite structures. Carbon 2002;40:
1597–617.
[30] Wagner HD. Nanotube-polymer adhesion: a mechanics approach. Chem
Phys Lett 2002;361:57–61.
[31] Lau KT. Interfacial bonding characteristics of nanotube/polymer
composites. Chem Phys Lett 2003;370:399–405.
[32] Haque A, Ramasetty A. Theoretical study of stress transfer in carbon
nanotube reinforced polymer matrix composites. Compos Struct
2005;71(1):68–77.
[33] Gao XL, Li K. A shaer-lag model for carbon nanotube-reinforced polymer
composites. Int J Solids Struct 2005;42:1649–67.
[34] Liao K, Li S. Interfacial characteristics of a carbon nanotube–polystyrene
composite system. Appl Phys Lett 2001;79(25):4225–8.
[35] Wong M, Paramsothy M, Xu XJ, Ren Y, Li S, Liao K. Physical
interactions at carbon nanotube–polymer interface. Polymer 2003;44:
7757–64.
[36] Barber AH, Wagner HD. Measurement of carbon nanotube–polymer
interfacial strength. Appl Phys Lett 2003;82(23):4140–1.
[37] Liu YJ, Chen XL. Continuum models of carbon nanotube-based
composites using the boundary element method. Electron J Bound
Elem 2003;1(2):316–35.
[38] Odegard GM, Gates TS, Wise KE, Pack C, Siochi EJ. Constitutive
modelling of nanotube-reinforced polymer composites. Compos Sci
Technol 2003;63(11):1671–87.
[39] Wan H, Delale F, Shen L. Effect of CNT length and CNT-matrix
interphase in carbon nanotube (CNT) reinforced composites. Mech Res
Commun 2005;32(5):481–489.
[40] Gou J, Lau KT. Modelling and simulation of carbon nanotube/polymer
composites. In: Rieth Michael, Schommers Wolfram, editors. Handbook
of theoretical and computational nanotechnology. American Scientific
Publishers; 2005 (chapter 14).
[41] Zhao ZG, Ci LJ, Cheng HM, Bai JB. The growth of multiwalled carbon
nanotubes with different morphologies on carbon fibres. Carbon 2005;43:
651–73.
[42] Lu M, Li HL, Lau KT. Formation and growth mechanism of dissimilar
coiled carbon nanotubes by reduced pressure catalytic chemical vapour
deposition. J Phys Chem B 2004;108:6186–92.
[43] Lu M, Lau KT, Xu JC, Li HL. Coiled carbon nanotubes growth and DSC
study in rpoxy-based composites. Colloids Surf A 2005;257–258:339–43.
[44] Volodin A, Ahlskog M, Seynaeve E, Van Haesendonck C, Fonseca A,
Nagy JB. Imaging the elastic properties of coiled carbon nanotubes with
atomic force microscopy. Phys Rev Lett 2000;84(15):3342–5.
[45] Mukhopadhyay K, Dwivedi CD, Mathur GN. Conversion of carbon
nanotubes to carbon fibers by sonication. Carbon 2002;40:1373–6.
[46] Lam CK, Cheung HY, Ling HY, Lau KT. Effects of ultrasonic sonication
in nanoclay clusters of nanoclays/epoxy composites. Mater Lett 2005;
59(11):1369–72.
[47] Lau KT, Lu M, Qi JQ, Zhao DD, Cheung HY, Lam CK. et al., Cobalt
hydroxide colloidal particles precipitation on nanoclay layers for the
formation of novel nanocomposites of carbon nanotubes/nanoclay.
Compos Sci Technol 2006;66(3–4):450–458.
[48] Lau KT, Lu M, Lam CK, Cheng HY, Li HL. Thermal and mechanical
properties of single-walled carbon nanotube bundle-reinforced epoxy
nanocomposites: the role of solvent for nanotube dispersion. Compos Sci
Technol 2005;65:719–25.
[49] Kim YJ, Shin TS, Choi HD, Kwon JH, Chung YC, Yoon HG. Electrical
conductivity of chemically modified multiwalled carbon nanotube/epoxy
composites. Carbon 2005;43:23–30.
[50] Ramamurthy PC, Malshe AM, Harrell WR, Gregory RV, McGuire K,
Rao AM. Polyaniline/single-wall carbon nanotube composite electronic
devices. Solid State Electron 2004;48:2019–24.
[51] Guo M, Chen H, Li J, Tao B, Yao S. Fabrication of polyaniline/carbon
nanotube composite modified electrode and its electrocatalytic property to
the reduction of nitrite. Anal Chim Acta 2005;532(1):71–77.
[52] Schartel B, Potschke P, Knoll U, Abdel-Goad M. Fire behaviour of
polyamide 6/multiwall carbon nanotube nanocomposites. Eur Polym J
2005;41(5):1061–1070.
[53] Monteiro-Riviere NA, Nemanich RJ, Inman AO, Wany YY, Riviere JE.
Multi-walled carbon nanotube interactions with human epidermal
keratinocytes. Toxicology 2005;155:377–84.
[54] Raffaelle RP, Landi BJ, Harris JD, Bailey SG, Hepp AF. Carbon
nanotubes for power applications. Mater Sci Eng B 2005;116:
233–43.
[55] Monteiro-Riviere NA, Nermanich RJ, Inman AO, Wang YY, Riviere JE.
Multi-walled carbon nanotube interactions with human epidermal
keratinocytes. Toxicol Lett 2005;155:377–84.