A Literature Review on
Dispersion of Carbon Nanotubes and Their
Polymer Composites
Jian Zhao
Department of Materials Science and Engineering
University of Cincinnati
October 2001
1
Table of Content
1. Introduction
2. Thermodynamics of Nanophase Carbon
3. Methods for dispersion of nanotubes
4. Nanotube-polymer composites
5. Summery
1. Introduction
The carbon nanotubes discovered by lijima
[1]
have many interesting properties such
as high mechanical strength, and a remarkable electronic structure. These make it
2
have a wide range of potential uses. Carbon nanotubes occur in two distinct forms,
single-walled nanotubes(SWNT), which are composed of a graphene sheet rolled into
a cylinder (Figure 1) and multi-walled nanotubes (MWNT), which consist of multiple
concentric graphene cylinders. Compared with multiwalled nanotubes, single-walled
nanotubes are expensive and difficult to obtain and clean, but they have been of great
interest owing to their expected novel electronic, mechanical, and gas adsorption
properties [2].
Figure 1. The cylinder-like structure model of the rolled graphene sheet (Lijima, S.,
Brabec, C., Maiti, A. and Bernhole, J., Journal of Chemical Physics, 1996, 104, 2089)
There are several methods for preparation of carbon nanotubes. Arc-discharge and
laser ablation methods have been widely used in the earlier period of time. However,
the large-scale production of carbon nanotubes with controlled conformation still
remains challenging. In the past few years, chemical vapour deposition(CVD) has
been used as a promising solution. The process is simpler and has a higher
3
productivity than the arc-charge process. However, the carbon nanotubes produced by
the catalytic process are usually thicker than those by the arc-discharge process and
often consist of large aggregates [2-4].
Carbon nanotube based materials have inspired scientists for a range of potential
applications
[3-5]
. The use of carbon nanotubes in polymer/carbon nanotube
composites has attracted wide attention
[6,7]
. The carbon nanotubes have unique
atomic structure, very high aspect ratio, and extraordinary mechanical properties
(strength and flexibility). These properties make them ideal reinforcing fibers in
nanocomposites[8]. “Carbon nanotube reinforced composites have been investigated
for flame-retardant performance [9], improved electrical conductivity and electrostatic
charging behavior, optical emitting devices
[10,11]
, and in lightweight, high strength
composites [9]”.
However, Colloidal materials such as carbon nanotubes do not spontaneously suspend
in polymers, thus the chemistry and physics of filler dispersion become a major issue.
In the case of polymers filled with carbon nanotubes, the research challenge is
particularly tremendous due to the unique character of these unusual materials. Due to
strong attractive interaction, nanotubes aggregate to form bundles or “ropes” (Figure
2) that are very difficult to disrupt
[12]
. In case of single nanotubes, they are only 1-3
nm in diameter, however, since they like to assemble into ropes, which consist of
many nanotubes, are most likely 10-200 nm in diameter. Furthermore, ropes are
4
tangled with one another like spaghetti or polymers. With high shear, these ropes can
be untangled, but it is extremely difficult to further disperse at the single tube level.
Figure2. TEM image of partially exfoliated single-wall carbon nanotubes
showing the rope-like structure. From reference 12.
Due to the low entropy of mixing, rigid molecules of high molecule weight require
strong attractive interactions to disperse. Since the connectivity and rigidity of
macromolecules drastically reduces the number of configurations available in the
dispersed state, mixing becomes a problem. In the case of rigid fillers dispersed into
stiff polymers, the problem is compounded in that neither species gains entropy on
dispersion.
In this review, the origin of dispersion problem, mainly in aspect of thermodynamics
is explored. Several methods have been reported to solve the problem, through
functionalization of carbon nanotubes or strong acids, certain solvents and
surfactants. Finally, carbon nanotube-polymer composites with a variety of polymers
and fabrication methods are reviewed.
5
2. Thermodynamics of Nanophase Carbon
Several factors make the dispersion of nanophase carbon particularly troublesome.
These factors are dominated by strong attraction between carbon species of both
enthalpic and entropic origin. In addition, the low dismensionality of carbon
nanotubes leads to an enhancement of these attractive forces.
The origin of the attractive forces between graphilic structures is well known. Due to
the extended pi electron system, these systems are highly polarizable, and thus subject
to large attractive van der Waals forces. These forces are responsible for the
secondary bonding that holds graphitic layers together. In the case of carbon
nanotubes, these forces lead to so called “ropes”. Extended structures are formed by
side-by-side aggregation of the nanotubes in ropes.
When suspended in a polymer, an attractive force between filler particles also arises
due to pure entropic factors [13,14]. Polymer chains in the corona region of the colloidal
filler suffer an entropic penalty since roughly half of their configurations are
precluded. Therefore there is a depletion of polymer in the corona, thus results in an
osmotic pressure forcing the filler particles together. This effective attraction is
intrinsic to colloids dispersed in polymers.
Finally, the linear structure of carbon nanotubes leads to cooperatively enhanced
forces because rods interact along a line.
6
3. Dispersion methods
Dispersion of carbon nanotubes is particularly intractable because the forces
described above are especially large. The advantages of carbon nanotube fillers have
not been fully realized because of the difficulty of obtaining fully dispersed
nanotubes. Also the lack of affinity of the nanotubes for polymer matrix inhibits load
transfer from the matrix to nanotubes [15].
The situation is improved, since strategies have been devised to overcome barriers to
dispersion. A lot of methods have been reported to solve the problem of dispersion,
through functionalization of carbon nanotubes or strong acids, certain solvents and
surfactants.
3.1 Dispersion in solvents and surfactants
Solution-phase handling would be exceptionally useful for many of carbon nanotubes
applications. To date, the best solvents reported for generating SWNT dispersions are
amides, particularly N,N-dimethylformamide(DMF) and N-methylpyrrolidone(NMP)
[16,17]
. However, the dispersions aggregate on a time-scale of days. Ausman et al
investigated the room-temperature solubility of SWNTs in a variety of solvents. It
was found that a class of non-hydrogen-bonding Lewis bases could provide better
solubility [18], but they just pointed a possible way towards better solvents capable for
7
solvating pristine tubes. The problem that pristine SWNTs are insoluble in all
solvents is still not overcome.
With aid of surfactants, carbon nanotubes have been solubilized in water. Surfactants
can deposit on the surface of nanotubes
[20,21]
[19]
and help form stable colloidal dispersion
. The surfactant acts as a coupling agent and may introduce a steric repulsive
force between the carbon nanotubes. The repulsive force overcomes the van der
Waals attractive force between the carbon surfaces. A polyelectrolyte-surfactantMWNTs complex can help micrometer-length multi-walled carbon nanotubes
dissolve in organic solvents by forming a lamellar structure
[22]
. However, removing
the surfactant afterward is problematic although the instinct structure of nanotube is
not destructed.
SWNTs have been solubilized by functionalization of nanotubes. For example,
nanotubes could be solubilized well by functionalizing the end-caps with long
aliphatic amines
[23]
. Further, it has been reported that SWNTs have been solubilized
by functionalizing their sidewalls with fluorine
[24]
and with alkanes
[16]
. However,
the above reactions caused the opening of the nanotubes tips or detrimental damage to
their sidewalls owing to the harsh conditions
[25,26]
. With progress of research on
functionalization of nanotubes, these problems are being solved (For detail, see
section 3.3: Functionalization of nanotubes).
3.2 Dispersion in strong acids
8
In the untreated state nanotubes may be dispersed using ultrasound, but do not remain
in quiescent suspension at high concentration. And use of ultrasound has been shown
to cause defects at nanotubes[27]. Appropriate chemical oxidation of their surfaces,
with the mixture of concentric acid and sulfuric acid, can introduces oxygencontaining functional groups onto surface. During this process, open ends are formed
in the oxidizing environment. After such treatment, the nanotubes form a welldispersed electrostatically stabilized colloid in water and ethanol [28].
It has been indicated that through chemical reaction, SWNT bundles form
intercalation compounds with HNO3 after they are immersed in nitric acid solution
for a short period of time. The inter-nanotube spacing within the bundles is expanded
due to intercalation. Individual SWNTs can be exfoliated from the bundles after
longer exposure to HNO3. It is demonstrated that nanotubes consisting of aggregates
can be easily dispersed to individual fibers by treatment with strong acids [12]
More important is that the presence of oxidizing groups helps the attachment of
organic or inorganic materials to the surface that is important to soluble nanotubes,
self-assembly on surfaces and chemical sensor, although destruction of structure of
nanotubes makes them less intriguing for some applications.
3.3 Surface Functionalization
9
The intrinsic polarity of graphitic structures can be overcome by chemical means.
That is, if the pi system were disrupted through defects, the polarizability and
resulting van der Waals forces would be reduced. Surface functionalization might
achieve this goal. However, the chemical modification of SWNTs at the molecular
level has been little studied to date. This is largely limited because it is difficult to
obtain pure SWNTs, and SWNTs do not dissolve in solvents.
Several studies of chemical modification of carbon nanotubes have been reported.
Chen et al. reported on the derivatization of SWNTs dissolved in organic solutions
with thioylchroride and octadecylamine[23]. Reaction of soluble SWNTs with
dichlorocarbene led to functionalization of the nanotubes. They found that by
solution-phase near-infrared spectroscopy, the band gaps of some types of SWNTs
could be investigated directly. This type of near–infrared spectroscopy allows the
study of the effects of chemical modifications on the band gaps of SWNTs. This is
very important to the molecular design of new SWNT-based materials.
Wong et al
[29]
reported modification of multiwall carbon nanotubes via amide bond.
The amide bond forms between amine and carboxy functional groups bonded to the
open ends of MWNTs. With the use of these modified nanotubes as AFM tips, the
binding force between single protein-ligands pairs can be measured, based on
molecular interaction.
10
The polymer-bound carbon nanotubes can be formed by covalently attaching
nanatubes to highly soluble linear polymers, such as poly(propionylethylenimine-coethylenimine) (PPEI-EI) via amide linkages or poly(vinyl acetate-co-vinyl alcohol)
(PVA-VA) via ester linkages. The samples of polymer bound nanotubes are soluble
in both organic solvents and water, and highly colored homogenous solutions are
formed [30,31].
However, the above several approaches have the drawback that acid-treated end-caps,
the most convenient chemical “handles” for further modification, are tied-up.
It has been demonstrated that carbon nanotubes can be covalently fluorinated within
the temperature range from 250C to 400C
[24]
. And fluorine can be effectively
removed from the SWNTs using anhydrous hydrazine. Boul et al reported that
sidewall-alkylated nanotubes can be obtained by reacting sidewall-fluorinated
nanotubes with alkyl magnesium synthesis or by reaction with alkyllithium
precursors[16].
When Koshio et al investigated the purification of SWNTs by utrasonication of
SWNTs in a monochlorobenzene (MCB) solution of poly(methyl methacrylate)
(PMMA)
[32]
, they found that SWNTs reacted with the organic liquid during
ultrasonic process. They explained that ultrasound forms hot spots in the mixture of
SWNTs and organic liquids. At the hot spots, Organic molecules, such as MCB and
PMMA, are decomposed. Meantime, Reactive species form and then the sidewalls of
11
the SWNTs are damaged. Carbon-dangling bonds form under high temperature and
pressure. The reactive organic species react with the dangling bonds of SWNTs. They
believed that this simple ultrasonication technique should achieve surface
functionalization of SWNTs in various ways
[33]
. In these latter cases, the sidewall
functionalization coverage is high, resulting naturally in a modification of the
intrinsic SWNT properties.
For application requiring the high conductivity of carbon nanotubes, all the above
methods are not attractive. Another strategy that scientists have begun to explore is
to attach organic molecules to these tubular nanostructures in a noncovalent way in
order to preserve the nanotuibes  networks- and thus their electronic characteristics.
Dai and coworkers have found a “simple and general approach for noncovalently
anchoring aromatic molecules to sidewalls of SWNTs” (Figure 3). They used a
molecule containing a planar pyreny group. The pyreny group irreversibly absorbs to
the surface of a SWNT with -stacking forces. The molecule’s tail is tipped with a
succinimidyl ester group. While an amine group attacks the ester function, the ester
group is readily displaced and an amide bond forms
[34]
. This may be very useful not
only for immobilizing proteins or DNA, but also for solubilizing carbon nanotubes.
12
Figure 3. 1-Pyrenebutanoic Acid, Succinimidyl Ester 1 Irreversibly Adsorbing
onto the Sidewall of a SWNT via -Stacking. From reference 34.
Staddart et al produced bundles of SWNTs that have a conjugated polymer helically
wrapped around. The polymer is a poly(m-phenylenevinylene) with octyloxy chains.
After SWNTs were added to a solution of this polymer and then the mixture was
sonicated, a stable suspension of nanotubes was produced. They believe that “the
polymer wraps itself around the SWNT bundles, with the phenylene rings and vinyl
units of the polymer backbone”. The polymer hugs the nanotube surfaces, as a result
of - interactions [35]. Since the suspension can be considered as a polymer-nanotube
composite, it is mentioned in more detail on next section together with other polymer
solutions that have similar behaviors (Page 17). Actually, due to some analogies
between polymer solution and carbon nanotube dispersion in term of an
entanglement-like transition and flow-induced alignment
13
[35]
, we can imagine that
molecules of high molecular weight can wrap themselves around the surfaces of
nanotubes.
Scientists are enthusiastic to explore potential applications of both noncovalent
modification approaches, which can manipulate nanotubes into ordered array without
destroying their instinct structure.
4. Nanotube-polymer composites
Due to agglomerate structures, carbon nanotubes are very difficult to break down
physically. Therefore, fabrication of homogeneous nanocomposites with carbon tubes
remains a technical challenge. Generally, nanotubes-polymer composites were
fabricated by direct mixing or in situ polymerization. Obviously, in situ
polymerization is a better way for homogenous dispersions because nanotubes are
more likely to disperse in a precursor monomer than in the polymer.
The carbon nanotubes-polymer composites were initially reported by Ajayan et al [36].
They just mechanically mixed the purified MWNTs with epoxy resin. Since then,
attention has been paid to composite materials with uniform and high nanotubes
loading. Carbon nanotube epoxy composites are most widely studied as
nonconjugated polymer-based composites.
14
Sandler et al reported that untreated carbon nanotubes were dispersed in an epoxy
matrix. The use of carbon nanotubes not only reduces the percolation threshold to
below 0.04 wt%, but also increases the overall conductivity. Although ultrasound in
ethanol and the intense stirring process improves the dispersion of the nanotubes, it is
not impossible to break up all the entanglements of the carbon nanotubes. Even on the
millimeter scale the distribution of nanotubes is not uniform
[37]
. Gong et al reported
that using surfactants as wetting agents might improve dispersion
and
thermomechanical properties of carbon nanotubes/polymer(epoxy) composites, but
even with the addition of the surfactant, complete homogeneous dispersion of the
nanotubes was not obtained
[38]
. Better surfactants and different concentration of
polymer were not investigated in this case.
Composite films of poly(vinyl alcohol) and nanotubes were prepared. Followed by
casting to make films, chemically treated nanotubes were carefully mixed with
aqueous poly(vinyl alcohol) solution, [CH2CH(OH)]N(PVOH) in order to prevent
reaggregation. Each nanotube must be covered with a layer of polymer to form a
stable mixture before it is able to interact with a number of other nanotubes. It was
believed that “in colloidal terms, the absorbed polymer sterically stabilizes the
nanotube dispersion and protects it against bridging flocculation and depletion
aggregation. The electrical conductivies of composites show typical percolation
behavior and the presence of the nanotubes stiffens the material, particularly at high
temperature”
[39]
. However, the plausible mechanism seems too simple and was not
confirmed directly.
15
PMMA/nanotubes composites were fabricated by in situ process. In the process
treated nanotubes can be initiated by AIBN and open their -bonds. With the opened
-bonds nanotubes can link with the PMMA, thus obstruct the growth of PMMA and
produce a C-C bond between the nanotubes and the PMMA. The dispersion ration of
nanotubes in the PMMA matrix is improved and the properties of the composites rise
due to high interfacial strength [40].
Thin film of PMMA/SWNT composite was also fabricated by spin coating. It was
found that the polymers intercalated between nanotubes into bundles. At low
nanotubes concentration, amorphous carbon was dispersed well, thus more uniform
films were prepared
[41]
. A combination of solvent casting and melt mixing was used
to fabricate SWNT/PMMA composites. “Melt mixing produced compositionally
uniform films on the micrometer scale, while the films prepared by solvent casting
were heterogeneous”. Mechanical properties and electrical conductivity of the aligned
nanocomposite fibers were improved by melt spinning[42]. Drawback of this method is
that melt spinning described by the authors is too complex and time-consuming.
MWNTs can be dispersed well through polystyrene matrices by an ultrasonic assisted
solution-evaporation method. TEM images of the composite films indicate that the
nanotubes are homogeneously distributed at ~1m length scale. Only 1% nanotube
addition increases the polymer mechanical properties significantly. In situ TEM show
16
that the external load can be effectively transmitted from the matrix to the nanatubes.
[43]
.
The combination of carbon nanotubes with -conjugated polymers is of interest
because their -conjugation enables the polymers to be used as an active material for
light-emitting diode(LEDs), field-effect transistor(FETs), and photovoltaic device et
al. Chen et al reported a novel electrochemical method for the synthesis of a
nanotubes-polyppyrrole composite that has a high concentration of well-dispersed
nanotubes. High-resolution TEM revealed a remarkably uniform PPy coating on
individual nanotubes, indicating that nanotubes are wetted by the continuous polymer
phase. Although further mechanism was not investigated, the uniform coating on
individual nanotubes promises controlled modification of the surface of nanotubes.
This is “the first example of anionic nanotubes acting as a strong and conductive
dopant in the polymerization of a conducting polymer” [44].
Ago et al reported that a uniform film of MWNTs and poly(p-phenylene
vinylene)(PPV) widely studied was prepared by spin-coating highly concentrated
MWNT dispersion[45]. An atomic force image shows that PPV covers the surface of
MWNTs and forms a well-mixed composite. Results suggest that MWNT can act as a
good electrode due to the formation of a complex interpenetrating network with
polymer chains. Some investigation of electronic properties of the composites is
beyond my knowledge.
17
Figure 4. Chemical structures of some polymers studied
Carbon nanotubes can be solubilized in the solution of Poly(p-phenlyenevinylene-co2,5-dioctoxy-m-phenylenevinylene)
(PMPV),
a
variation
on
the
poly(para-
phenylenevinylene)(PPV). The copolymer has an added dihedral angle that makes the
chain tend to spiral into helix. Polymer/toluene solution can act as a solvent for
nanotubes. The suspension can be considered as a polymer-nanotube composite. It
was found that the formed organic composites show an increase of the conductivity
by ten orders of magnitude [46-49].
In the case of MWNT, it has been demonstrated that wrapping of polymer ropes
around the tube lattice occurs in a well-ordered periodic fashion and keep them
suspended in solution indefinitely. Crystalline polymer nucleates from nanotube
defects and each nanotube has a uniform coating of polymer. In the case of SWNT
hybrids, the pitch of the polymer coil can accommodate individual nanotubes. SWNT
ropes are destroyed due to an intercalation process of polymer. At low loading
fractions, the tubes are sufficiently isolated and are unable to reaggregate. “The
18
polymer interact strongly with nanotubes of within a diameter range of 1.35-1.55 nm
and weakly with tubes of smaller and higher diameters”. However, the exact nature of
polymer interaction with SWNT is not reported in these papers
[46-49]
. And the use of
SWNT for mechanical reinforcement and thermal stabilization of the conjugated
polymer were not further studied.
Figure 5. Molecular model shows
how conjugated polymer would
helically wrap itself around a carbon
nanotube.
From C&EN/MAY 7,2001, page 15
The optical polymer characteristics have been established for nanotube solubility,
based on research on PmPV and its derivatives
[50]
. The combination of the -
conjugation, twist allowing backbone and solubilizing side groups, results in a
successful polymer conformation (a flat helix, with exposed backbone and outward
pointing side groups). It was also suggested that the binding of conjugated polymers
is the combination of electrostatic forces and van der Waals forces. It is not surprising
that the forces are stronger than van der Walls binding for nonconjugated polymers,
19
which, to some extent, may explain that nanotubes are generally dispersed better in
conjugated polymers.
Except PmPV, Poly(hydroxy amino ethers) (PHAE)[51] and poly(fluorinediooctane)[50] have been reported to bind well with MWNTs in composite material.
PMMA, an amorphous random-coil polymer, was not able to hold nanotubes in
suspension
[52]
. Additionally, atactic polystyrene was found not to suspend nanotubes
well. Poly(phenylacetylene) (PPA) polymerized in situ with MWNT has been
reported to dissolve short nanotubes by forming helical wrapping around them[53,54].
Based on the mechanisms of adhesion of these composites at the molecular level, it
was found that helical polymer conformations are essential in determining the
strength of the interface. Hydrogen bonds interactions with the -bond network of
pristine carbon nanotubes improve binding to the surface. Phenyl side-groups interact
strongly with the surface as well, but a regular arrangement around nanotube and
good contact is necessary. These characteristics are a bit consistent with results
reported by Panhuis [50]. In addition, it was shown that isolated nanotubes are desired
for dispersion in a matrix, because of weak frictional interactions between layers of
MWNTs and between SWNTs in bundles.
Based on overview of literatures, it is found that interaction between pristine
nanotubes and polymers is dependent on the choice of matrix polymer and also
polymer conformation, thus the molecular structure may play a critical role in the
interaction. Even with “best polymer”, pristine graphite may not form strong
20
interfaces. It have been proposed that funtionalizing nanotubes or chemical bonding
might increase the interaction with polymer matrix, but it was found that some
mechanical properties decrease after covalent chemical modification
[55]
, and the
structure of nanotubes would be destroyed partially. It seems that MWNTs are more
suitable for chemical treatment because the inner graphene layers can be unreacted,
thus the essential electronic structure can be retained, but less excellent properties
than SWNT and weak interaction between layers make them not attractive for a lot of
applications. Few mechanisms about adhesion, load transfer and deformation were
investigated, which make it difficult to accurately predict behaviors of nanotubepolymer composites and fabricate “ideal” nanocomposites. Therefore, synthesis of
more dispersible nanotubes, a more complete understanding of interfacial chemistry
and dispersion mechanism and better understanding of interaction between polymer
matrix and carbon nanotubes should be included in future work.
5. Summary
More and more interest exists in producing composites using carbon nanotubes as
filler. Such nanocomposites would exhibit superior performance, but dispersion of
nanotubes and fabrication of homogenous polymer composites remain a technical
challenge. It is worth intensive investigation in order to fully exhibit the excellent
properties of their polymer composites.
It is proved to be feasible to disperse
nanotubes well into polymer through better choices of surface modification, polymers
and fabrication methods. Almost all types of nanocomposites, which are
21
homogenously dispersed, can be obtained with improved mechanical properties and
electrical properties etc. It is fortunate that theoretical and chemical tools have just
been developed that can solve the problem of dispersion.
The potential applications for these nanotubes-polymer composites are widespread
and can be found both government and commercial systems. Some of these
applications include thermally conductive adhesives, solar collectors, aerospace
structural panels, automotive components, high stiffness-to-weight space mirror
substrates or even totally new ones, which we even can not imagine.
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