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, China b 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 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 www.elsevier.com/locate/compositesb 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).
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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.
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-