Progress in Polymer Science

Progress in Polymer Science 35 (2010) 837–867
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Progress in Polymer Science
journal homepage: www.elsevier.com/locate/ppolysci
Polymer nanocomposites based on functionalized carbon nanotubes
Nanda Gopal Sahoo a , Sravendra Rana b , Jae Whan Cho b,∗ , Lin Li a,∗∗ , Siew Hwa Chan a
a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore
b Department of Textile Engineering, Konkuk University, Seoul 143-701, Republic of Korea
article info
Article history:
Received 5 August 2008
Received in revised form 10 March 2010
Accepted 12 March 2010
Available online 16 March 2010
Keywords:
Nanocomposites
Carbon nanotubes
Mechanical properties
Functionalization
Contents
abstract
Carbon nanotubes (CNTs) exhibit excellent mechanical, electrical, and magnetic properties
as well as nanometer scale diameter and high aspect ratio, which make them an ideal
reinforcing agent for high strength polymer composites. However, since CNTs usually form
stabilized bundles due to Van der Waals interactions, are extremely difficult to disperse
and align in a polymer matrix. The biggest issues in the preparation of CNT-reinforced
composites reside in efficient dispersion of CNTs into a polymer matrix, the assessment of
the dispersion, and the alignment and control of the CNTs in the matrix. There are several
methods for the dispersion of nanotubes in the polymer matrix such as solution mixing,
melt mixing, electrospinning, in-situ polymerization and chemical functionalization of the
carbon nanotubes, etc. These methods and preparation of high performance CNT-polymer
composites are described in this review. A critical comparison of various CNT functionalization
methods is given. In particular, CNT functionalization using click chemistry and the
preparation of CNT composites employing hyperbranched polymers are stressed as potential
techniques to achieve good CNT dispersion. In addition, discussions on mechanical,
thermal, electrical, electrochemical and optical properties and applications of polymer/CNT
composites are included.
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction ........................................................................................................................ 838
2. Functionalization of CNT........................................................................................................... 838
2.1. Defect functionalization .................................................................................................... 838
2.2. Non-covalent functionalization ............................................................................................ 839
2.3. Covalent functionalization ................................................................................................. 840
2.4. Functionalization using click chemistry ................................................................................... 842
3. Preparation methods of polymer/CNT nanocomposites .......................................................................... 847
3.1. Solution mixing ............................................................................................................ 847
3.2. Melt mixing................................................................................................................. 848
3.3. In-situ polymerization ..................................................................................................... 848
4. Preparation of CNT nanocomposites using dendritic polymers ................................................................... 850
4.1. CNT nanocomposites via covalently functionalized CNT-dendritic polymers ............................................ 850
4.2. CNT nanocomposites via non-covalently functionalized CNT-dendritic polymers ....................................... 852
∗ Corresponding author. Tel.: +82 2 450 3513; fax: +82 2 457 8895.
∗∗ Corresponding author. Tel: +65 67906285; fax: +6794 2035.
E-mail addresses: jwcho@konkuk.ac.kr (J.W. Cho), mlli@ntu.edu.sg (L. Li).
0079-6700/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.progpolymsci.2010.03.002

838 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
5. Mechanical properties of polymer/CNT nanocomposites ......................................................................... 853
5.1. Polyurethane/CNT composites ............................................................................................. 853
5.2. Polyimide/CNT nanocomposites ........................................................................................... 855
6. Electrical conductivity of polymer/CNT nanocomposites ......................................................................... 856
7. Optical properties of polymer/CNT nanocomposites.............................................................................. 857
8. Applications ........................................................................................................................ 858
9. Concluding remarks................................................................................................................ 859
Acknowledgments ................................................................................................................. 859
References ......................................................................................................................... 859
1. Introduction
Since the discovery of carbon nanotubes (CNTs) in
1991 by Iijima [1], they have received much attention for
their many potential applications, such as nanoelectronic
and photovoltaic devices [2,3], superconductors [4], electromechanical
actuators [5], electrochemical capacitors
[6], nanowires [7], and nanocomposite materials [8,9]. Carbon
nanotubes may be classified as single-walled carbon
nanotubes (SWNTs) [10,11], double-walled carbon nanotubes
(DWNTs) [12,13] or multi-walled carbon nanotubes
(MWNTs) [1]. SWNT and DWNT comprise cylinders of one
or two (concentric), respectively, graphene sheets, whereas
MWNT consists several concentric cylindrical shells of
graphene sheets. CNTs are synthesized in a variety of ways,
such as arc discharge [10], laser ablation [14], high pressure
carbon monoxide (HiPCO) [15], and chemical vapor
deposition (CVD) [16,17]. CNTs exhibit excellent mechanical,
electrical, thermal and magnetic properties [18,19]. The
exact magnitude of these properties depends on the diameter
and chirality of the nanotubes and whether they are
single-walled, double-walled or multi-walled form. Typical
properties of CNTs are collected in Table 1 [20–25].
Because of these excellent properties, CNTs can be used
as ideal reinforcing agents for high performance polymer
composites. Ajayan et al. [26] reported the first polymer
nanocomposites using CNTs as a filler. The number of articles
and patents in polymer composites containing CNTs
is increasing every year [27]. Various polymer matrices
are used for composites, including thermoplastics [28–30],
thermosetting resins [31,32], liquid crystalline polymers
[33,34], water-soluble polymers [35], conjugated polymers
[3], among others. The properties of polymer composites
that can be improved due to presence of CNTs include tensile
strength [36,37], tensile modulus [38,39], toughness
[40], glass transition temperature [41,42], thermal conductivity
[43,44], electrical conductivity [45,46], solvent
resistance [47], optical properties [48,49], etc.
Table 1
Typical properties of CNTs [20–25].
Property SWNT DWNT MWNT
Tensile strength (GPa) 50–500 23–63 10–60
Elastic modulus (TPa) ∼1 – 0.3–1
Elongation at break (%) 5.8 28 –
Density (g/cm 3 ) 1.3–1.5 1.5 1.8–2.0
Electrical conductivity (S/m) ∼10 6
Thermal stability >700 ◦ C (in air)
Typical diameter 1 nm ∼5nm ∼20 nm
Specific surface area 10–20 m 2 /g
Progress in polymer/carbon nanotube composite
research considered here will included studies on
functionalization of CNTs their mechanical, electrical
conductivity and optical properties, and applications of
polymer/CNT composites.
2. Functionalization of CNT
Since CNTs usually agglomerate due to Van der Waals
force, they are extremely difficult to disperse and align in
a polymer matrix. Thus, a significant challenge in developing
high performance polymer/CNTs composites is to
introduce the individual CNTs in a polymer matrix in order
to achieve better dispersion and alignment and strong
interfacial interactions, to improve the load transfer across
the CNT-polymer matrix interface. The functionalization of
CNT is an effective way to prevent nanotube aggregation,
which helps to better disperse and stabilize the CNTs within
a polymer matrix. There are several approaches for functionalization
of CNTs including defect functionalization,
covalent functionalization and non-covalent functionalization
[50]. These functionalization methods will be
summarized here.
2.1. Defect functionalization
CNTs are purified by oxidative methods to remove
metal particles or amorphous carbon from the raw materials
[51,52]. In these methods, defects are preferentially
observed at the open ends of CNTs. The purified SWNTs
contain oxidized carbon atoms in the form of –COOH group
[53,54]. In this oxidizing method, SWNTs are broken to very
short tubes (pipes) of lengths 100–300 nm [55]. Mawhinney
et al. [56] studied surface defect site density on SWNTs
by measuring the evolution of CO 2(g) and CO(g) on heating
to 1273 K. The results indicated that about 5% of the carbon
atoms in the SWNTs are localized at defective sites.
Acid base titration method [57] was used to determine
that the percentage of acidic sites of purified SWNTs was
about 1–3%. However, the defective sites created at the
CNT surfaces by this method are extremely sparse, and can
not promote good dispersion in the polymer/CNT composites.
However, they can be used for covalent attachment of
organic groups by converting them into acid chlorides and
subsequently reacting with amines to give amides [58,59].
The functionalized CNTs are more soluble in organic solvents
than the raw CNTs. Most of the SWNT bundles
exfoliate to give individual SWNT macromolecules [8] if the
reaction time of acid chloride group with amines is at ele-

Table 2
Non-covalent functionalization of CNTs.
N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 839
Polymer or surfactant Nanotube type Preparation method References
Surfactants
Cationic (CTAB) SWNT Microemulsion [332]
Cationic (CTAB) SWNT Ultrasonication [333]
Cationic (CTVB) SWNT Sonication [334]
Cationic (MATMAC) MWNT Emulsion polymerization [335,336]
Anionic (SDS) MWNT Ultrasonication [61]
Anionic (SDBS) SWNT Ultrasonication [68,70]
Anionic (SDBS) SWNT Bath sonicaton [71]
Non-ionic (Triton X-100) MWNTSWNT Ultrasonication [337,338]
Non-ionic (Triton X-305)
Biomacro-molecules
MWNT Ultrasonication [339]
Proteins/DNA MWNT Immobilization [64]
Glucose (Dextran) SWNT Dialysis [340]
ˇ-1,3-glucans SWNT Electroactive interaction [341]
Chitosan
Polymers
SWNT Ultrasonication [342]
Poly(4-vinyl pyridine) SWNT Sol–gel chemistry [343]
Poly(phenyl acetylene) MWNT Solution mixing [344]
Poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) MWNT Solution mixing [83]
SWNT [85]
Poly(styrene)-poly(methacrylic acid) SWNT
MWNT
Solution mixing [90]
vated temperature for 4 days and these SWNTs are soluble
in organic solvents.
2.2. Non-covalent functionalization
Non-covalent functionalization of nanotubes is of particular
interest because it does not compromise the
physical properties of CNTs, but improves solubility
and processability. This type of functionalization mainly
involves surfactants, biomacromolecules or wrapping with
polymers (Table 2). In the search for non-destructive
purification methods, nanotubes can be transferred to the
aqueous phase in the presence of surfactants [60,61]. In this
case, the nanotubes are surrounded by the hydrophobic
components of the corresponding micelles. The interaction
becomes stronger when the hydrophobic part of the
amphiphilic contains an aromatic group.
CNTs can be well dispersed in water using anionic,
cationic, and non-ionic surfactants [68–71]. Anionic surfactants
such as sodium dodecylsulfate (SDS) [72–74] and
sodium dodecylbenzene sulfonate (NaDDBS) [75,76] are
commonly used to decrease CNT aggregation in water. The
interaction between the surfactants and the CNTs depends
on the nature of the surfactants, such as its alkyl chain
length, headgroup size, and charge. SDS has a weaker interaction
with the nanotube surface compared to that of
NaDDBS and Triton-X100 because it does not have a benzene
ring. Indeed -stacking interaction of the benzene
rings on the surface of graphite increases the binding and
surface coverage of surfactant molecules to graphite significantly
[71]. NaDDBS disperses better than Triton-X100
because of its head group and slightly longer alkyl chain.
Fig. 1 represents the adsorption of different surfactants
onto the nanotube surfaces.
Direct non-surfactant mediated immobilization of protein
and DNA on CNTs has been reported [64]. The
hydrophobic regions of the proteins are probably important
for adsorption. A controlled and specific method for
immobilizing proteins onto non-covalently functionalized
SWNTs has been developed [62]. The mechanism of protein
immobilization on nanotubes involves the nueleophilic
substitution of N-hydroxysuccinimide by an amine group
on the protein. In this case, non-covalent functionalization
is accomplished by the interaction of delocalized
-bonds on the CNTs wall due to sp 2 hybridization with
-bonds of polymer molecules of the matrix [62,63]. CNTs
Fig. 1. Schematic representation of how surfactants may adsorb onto the nanotube surface. Reprinted with permission from Ref. [71]. Copyright 2003,
American Chemical Society, USA.

840 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Fig. 2. A bundled pair of copolymer encapsulated SWNTs from unpurified material (a) and purified encapsulated SWNTs (b). Reprinted with permission
from Ref. [87]. Copyright 2003, American Chemical Society, USA.
can be successfully solubilized in organic [65] or aqueous
[66] solvents after non-covalent functionalization, partly
attributed to the much better coverage of functional groups
[67].
The dispersion of CNT in both water [77,78] and organic
solvents [79] may be enhanced by the physical association
of polymers with CNT. This can be explained by
the ‘wrapping’ mechanism [78] that attributed to specific
interactions between the polymer and the CNTs.
Polymers can wrap around CNTs, forming supramolecular
complexes [80–82]. In these cases, -stacking
interactions between the polymer and the nanotube surface
are responsible for the close association of the
structures. Blau and co-workers [83–86] prepared a
nanotube-polymer hybrid by suspended SWNTs in organic
solvents poly(p-phenylenevinylene-co-2,5-dioctyloxy-mphenylenevinylene)
to wrap the copolymer around the
nanotubes. The electrical properties of these hybrids were
improved relative to those of the individual components. A
non-covalent method has been used to modify SWNTs by
encapsulating SWNTs within crosslinked and amphiphilic
poly(styrene)-block-poly(acrylic acid) copolymer micelles
(Fig. 2) [87]. This encapsulation significantly enhanced
the dispersion of SWNTs in a wide variety of polar and
non-polar solvents and polymer matrices because the
copolymer shell was permanently fixed. Thus, encapsulated
SWNTs may be stabilized with respect to typical
polymer processing and recovery from the polymer matrix.
Non-wrapping approaches have also been used for dispersion
and solubility of CNT in different media [88,89]. In
these cases, copolymers of various different structures and
compositions act efficiently as stabilizers, and may be tailored
so as to disperse the tubes in a variety of solvents.
Nativ-Roth et al. [90] suggested that the block copolymers
adsorbed to the nanotubes by a non-wrapping mechanism,
and the solvophilic blocks act as a steric barrier that
leads to the formation of stable dispersions of individual
SWNTs and MWNTs above a threshold concentration of
the polymer. The strong – interaction between polymer
backbone and nanotube surface led to soluble SWNTs. The
main potential disadvantage of non-covalent attachment
is that the forces between the wrapping molecule and the
nanotube might be weak, thus as a filler in a composite the
efficiency of the load transfer might be low.
2.3. Covalent functionalization
Because of the -orbitals of the sp 2 -hybridized C atoms,
CNTs are more reactive than those with a flat graphene
sheet, they have an enhanced tendency to covalently attach
with chemical species [91]. In the case of covalent functionalization,
the translational symmetry of CNTs is disrupted
by changing sp 2 carbon atoms to sp 3 carbon atoms, and
the properties of CNT, such as electronic and transport
are influenced [92]. But this functionalization of CNT can
improve solubility as well as dispersion in solvents and
polymer. Covalent functionalization can be accomplished
by either modification of surface-bound carboxylic acid
groups on the nanotubes or by direct reagents to the side
walls of nanotubes (Table 3). Generally, functional groups
such as –COOH or –OH are created on the CNTs during
the oxidation by oxygen, air, concentrated sulfuric acid,
nitric acid, aqueous hydrogen peroxide, and acid mixture
[55,93]. The surface of the acid treated MWNTs indicates
the presence of some defects in the carbon–carbon bonding
associated with the formation of carboxylic acid groups on
the surface, while the raw MWNTs show uniform surfaces
and a clear diffraction pattern because of their perfect lattice
structure of carbon–carbon bonds (Fig. 3) [94–96]. The
number of –COOH groups on the surface of CNT depends
on acid treatment temperature and time, increasing with
increasing temperature [95]. The extent of the induced
–COOH and –OH functionality also depends on the oxidation
procedures and oxidizing agents [97]. Nanotube ends
can be opened during the oxidation process.
The presence of carboxylic acid groups on the nanotube
surface is more convenient than others because a
variety of chemical reactions can be conducted with this
group. The presence of –COOH or –OH groups on the nanotube
surface helps the attachment of organic [58,59,98]
or inorganic materials, which is important for solubilizing
nanotubes. CNTs have been covalently functionalized

Table 3
Covalent functionalization of CNTs.
N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 841
Nanotube type Preparation approach Polymer/organic molecules Catalyst/reagent References
MWNT Grafting from (ROP) Poly(-caprolactone) Sn(Oct)2 [138]
MWNT Grafting from (ROP) Poly(L-lactide) Sn(Oct)2 [139]
SWNT
MWNT
Grafting from (ROP) Nylon-6 Sodium [125,126]
MWNT Grafting from (ATRP) Poly(methyl methacrylate) AIBN [127]
MWNT Grafting from (ATRP) Polystyrene CuBr [129]
MWNT Grafting from (ATRP) Polystyrene Cu(I)Br/PMDETA [130]
MWNT Grafting from (ATRP) Poly(acrylic acid) Cu(I)Br/PMDETA [131]
MWNT Grafting from (ATRP) Poly(tert-butyl acrylate) Cu(I)Br/PMDETA [132]
MWNT Grafting from (ATRP) Poly(N-isopropylacrylamide) Cu(I)Br/PMDETA [133]
MWNT Grafting from (ATRP) Glycerol Monomethacrylate Cu(I)Br/PMDETA [136]
SWNT
MWNT
Grafting to Polyethylene glycol – [111,121]
MWNT Grafting to Polyimide – [117]
SWNT Grafting to Poly(amido amine) – [122]
MWNT Grafting to Poly(-caprolactone)-diol – [345]
SWNT
MWNT
Grafting to Poly(vinyl acetate-co-vinyl alcohol) – [114]
MWNT Grafting to Poly(2-vinyl pyridine) – [119]
SWNT Cycloaddition of azomethine ylides 3,4-dihydroxybenzaldehyde N-methylglycine [346]
MWNT Cycloaddition of azomethine ylides 7-bromo-9,9-dioctyl fluorine-2-carbaldehyde L-lysine [347]
SWNT
MWNT
Cycloaddition of azomethine ylides Amino-acid Paraformaldehyde [348]
SWNT
MWNT
Cycloaddition of azomethine ylides Peptides, Nucleic acids R-NHCH2COOH [349]
MWNT [4 + 2] Cycloadditions 3,6-diaminotetrazine Temp. [350]
SWNT [4 + 2] Cycloadditions Triazolinedione Temp. [351]
SWNT [4 + 2] Cycloadditions 2,3-dimethoxy-1,3-butadiene Cr(CO)6 [352]
SWNT [2 + 1] Cycloadditions Alkyl azidoformate Nitrene [106]
Dipyridyl imidazolidene Carbene
SWNT
MWNT
[2 + 1] Cycloadditions Dichlorocarbene Carbene [353]
SWNT [2 + 1] Cycloadditions PEG di-azidocarbonate ester Nitrene [354]
SWNT Radical additions Polystyrene Nitroxide [355]
SWNT Radical additions Methoxyphenylhydrazine Microwave [356]
with thiocarboxilic and dithiocarboxylic esters that help
crosslinking between CNTs [99]. CNTs can be functionalized
at end caps or at the sidewall to enhance their dispersion
as well as solubilization in solvents and in polymer
matrices [91,100]. SWNTs were fluorinated at their side
walls by passing elemental fluorine at different temperatures
[101]. The fluorinated SWNTs exhibited improved
solubility in isopropanol or dimethyl formamide by ultrasonication
[102,103]. Fluorinated SWNTs may be converted
to side wall alkylated SWNTs by reaction with Grignard
reagent or alkyllithium compounds that are soluble in chloroform
[104]. SWNTs have also been solubilized by direct
functionalization of their side walls by nitrenes [105,106],
carbenes [106], and arylation [107,108].
Functionalization of CNTs with polymer molecules
(polymer grafting) is particularly important for processing
of polymer/CNT nanocomposites [109,110]. Two main
categories “grafting to” and “grafting from” approaches
have been reported for the covalent grafting of polymers to
nanotubes. The “grafting to” approach is based on attachment
of as-prepared or commercially available polymer
molecules on the CNT surface by chemical reactions, such
as amidation, esterification, radical coupling, etc. The polymer
must have suitable reactive functional groups for
preparation of composites in this approach. Fu et al. [111]
reported functionalization of CNTs by using “grafting to”
method. They refluxed CNTs containing carboxylic acid
groups with thionyl chloride to convert acid groups to
acylchlorides. Then, the CNTs with surface-bound acylchloride
moieties were used in the esterification reactions
with the hydroxyl groups of dendritic poly(polyethylene
glycol) polymer. Another example of the “grafting to”
approach has been reported by Qin et al. [112]. They
grafted SWNTs with polystyrene (PS) with functionalized
end groups PS (–N 3), via a cycloaddition reaction.
Polymer grafted CNTs have been formed by covalently
attaching nanotubes to highly soluble linear polymers, such
as poly(propionylethylenimine-co-ethylenimine) (PPEI-EI)
via amide linkages or poly(vinyl acetate-co-vinyl alcohol)
(PVA-VA) via ester linkages [113,114]. The resulting PVAgrafted
CNTs were soluble in PVA solution and PVA-CNT
nanocomposites films showed very high optical quality

842 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Fig. 3. TEM pictures of raw MWNTs (upper) and surface-modified MWNTs (lower). Reprinted with permission from Ref. [95]. Copyright 2005, Wiley-VCH,
Germany.
without any observable phase separation. Many other linear
polymers such as poly(sodium 4-stryrenesulfonate)
[115], poly(methyl methacrylate) (PMMA) [116], polyimide
[117,118], poly-(2-vinylpyridine) [119], PPEI-EI
[120], and poly(m-aminobenzenesulfonic acid) [35] as
well as dendrons [121], dendrimers [122], and hyperbranched
polymers [123] have been successfully bonded to
CNTs.
A novel route to polymer reinforcement via preparation
of polymer-functionalized nanotubes using organometallic
approach has been reported [124]. In the work, CNTs were
first functionalized by organometallic n-butyl lithium, and
then covalently attached to a chlorinated polypropylene
via a coupling reaction. The main limitation of the “grafting
to” method is that the grafted polymer content is quite low
due to the relatively small fraction of active sites on the
CNT, and the depressing effects of steric hinderence in the
reactivity of polymer [123].
In the “grafting from” approach, the polymer is
bound to the CNT surface by in-situ polymerization of
monomers in presence of reactive CNTs or CNT supported
initiators. The main advantage of this approach
is that the polymer-CNTs composites can be prepared
with high grafting density. This approach has been
used successfully to graft many polymers such as
polyamide 6 [125,126], PMMA [127,128], PS [129,130],
poly(acrylic acid) (PAA) [131], poly-(tert-butyl acrylate)
[132], poly(N-isopropylacrylamide) (NIPAM) [133], poly(4vinylpyridine)
[134], and poly(N-vinylcarbazole) [135] on
CNT via radical, cationic, anionic, ring-opening, and condensation
polymerizations. Gao et al. [136] described
the functionalization of MWNTs with a hydrophilic polymer,
glycerol monomethacrylate (GMA) by the “grafting
from” approach. In this work, the oxidized MWNTs
were treated with thionyl chloride, glycol, and 2-
bromo-2-methylpropionyl bromide to produce MWNT-Br
macroinitiators for the atom transfer radical polymerization
of GMA as shown in Scheme 1. The grafted polymer
content can be controlled by the feed ratio of monomer
to macroinitiators. The hydroxyl groups of the polyGMA
chains grafted on the MWNTs are highly active and can be
further converted to carboxylic acid groups. MWNTs were
covalently functionalized with poly(L-lysine) by a surface
initiated ring-opening polymerization method [137]. Zeng
et al. [138] reported “grafting from” approach based on
in-situ ring-opening polymerization of -caprolactone to
covalently graft biodegradable poly(-caprolactone) onto
CNT surfaces. CNT-graft-poly(L-lactide) by using surfaceinitiated
ring-opening polymerization has been studied by
Chen et al. [139].
A few additional techniques for functionalization of
CNTs into polymer matrixes have been utilized. Yan et al.
[140] used Ar plasma for the generation of defect sites
in the SWNT caps and sidewalls, with subsequent UVgrafting
of 1-vinylimidazole from the defect sites. Yang et
al. [141] obtained soluble MWNTs via amidation reaction
of octadecylamine with purified MWNTs when they were
mixed with copolymers of methyl and ethyl methacrylate
(poly(MMA-co-EMA)).
2.4. Functionalization using click chemistry
“Click” chemistry, coined for the Huisgen [3 + 2] dipolar
cycloaddition reaction [142], is an ideal reaction for
material synthesis and modification and for self assembly
of nanomaterials. A Cu (I)-catalyzed Huisgen [3 + 2] dipolar
cycloaddition reaction between terminal alkynes and
azides resulting in the formation of 1,2,3-triazoles has been
utilized elegantly in recent years [143,144]. This reaction is
very useful for synthesizing small molecules [145], den-

N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 843
Scheme 1. Functionalization of MWNTs with PolyGMA by atom transfer radical polymerization. Reprinted with permission from Ref. [136]. Copyright
2005, American Chemical Society, USA.
drimers [146,147], dendronized polymers [148–150], and
biologically derived macromolecular structures [151]. Click
chemistry benefits by the facile introduction of azide and
alkyne groups into organic and polymer molecules, the stability
of these groups to many reaction conditions, and the
tolerance of the reaction in presence of other functional
groups. The main advantages of click chemistry are its toleration
of other functional groups, a short reaction time,
and high yield, high purity, and regiospecificity, as well
as its suitability for use under aqueous conditions [152].
Azides and acetylenes are stable across a broad range of
organic reaction conditions and in biological environments,
yet they are highly energetic functional groups and the
products are screened directly from the reaction mixture
because no protecting groups are used. The triazole is used
as a rigid linker that can mimic the atom placement and
electronic properties of a peptide bond without the same
susceptibility to hydrolytic cleavage [153,154], as shown in
Scheme 2.
Apart from the synthetic promise, triazole moieties
are relatively stable to metabolic degradation and can
participate in hydrogen bonding, which may be useful
in the context of bimolecular targets and solubility, and
is extremely stable to hydrolysis, oxidation or reduction
[144]. Click chemistry may be an ideal modular methodology
for the introduction of a wide variety of molecules
onto the surface of CNTs [155–157] as shown in Fig. 4. By
applying this approach, one can easily functionalize CNTs
with desired molecules, which enhance their importance
from nanoelectronics to nanobiotechnology.
An application of the Huisgen cycloaddition to the functionalization
of SWNTs with PS was reported by Adronov
and co-workers [155]. To achieve a high degree of functionalization
by using alkyne groups on the nanotube surface
with the Pschorr-type arylation, subsequent introduction
of PS was achieved by first installing azide functionality
at the polymer chain end. The Cu(I)-catalyzed formation
of 1,2,3-triazoles by coupling azide-terminated polymer
and alkyne-functionalized SWNTs was found to occur in
an efficient manner under a variety of favorable conditions.
This resulted in relatively high nanotube graft densities, full
control over polymer molecular weight, and good solubility
in organic solvents. The grafted PS was further modified
via sulfonation and the sulfonated PS grafted CNTs were
highly soluble in aqueous medium and insoluble in organic
medium [158].
Cho and co-workers [156] focused on the covalent
attachment of bioactive molecules with SWNTs (Scheme 3).
The functionalization of the SWNTs was achieved by covalently
bonded organic molecules derived from amino acids
through click chemistry. The alkyne-functionalized SWNTs
were prepared by the treatment of p-amino propargyl
ether with SWNTs using a solvent free diazotization procedure
to produce alkyne functionalized CNTs. In order
Scheme 2. The coupling of azides and alkynes created triazole linkages in absence and presence of Cu(I) catalyst, the useful topological and electronic
feathers with nature’s ubiquitous amide connectors. Reprinted with permission from Ref. [153,154]. Copyright 2002, Wiley-VCH, Germany and Copyright
2004, American Chemical Society, USA.

844 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Fig. 4. Typical scheme of click chemistry for functionalization of carbon nanotube [156].
to attach different azides derived from amino acids, a
series of well-defined chiral azides from corresponding
-amino acids were prepared. The alkyne functionalized
SWNTs in dimethylformamide (DMF)-pyridine-butanol
solution were treated with azide-functionalized amino
acid compound, followed by CuI, ascorbic acid and N,N ′ -
diisopropylethylamine. The 1,2,3-triazole ring has been
utilized as a linker between the chiral molecules and
SWNTs, which can provide a new strategy for the attachment
of bioactive molecules like peptides, polysaccharides
and others [156]. They also functionalized SWNTs by
the covalent attachment of azide moiety containing
polyurethane (PU) with alkyne functionalized SWNTs using
click chemistry approach. The CNTs were functionalized
with PCL-based PU using click chemistry, followed by the
reaction of alkyne-decorated CNTs and azide-moiety containing
PCL-diol [159].
Hybrid materials based on CNTs and metal nanoparticles
are under consideration for major roles for several
applications, including catalytic, optical, electronic and
magnetic applications [160–162]. Gao and co-workers
[162] reported magnetic nanohybrids from magnetic
nanoparticles and polymer coated nanomaterials via a
Cu(I)-catalyzed azide alkyne cycloaddition reaction. The
nanohybrids were prepared from Fe 3O 4 nanoparticles
and polymer coated MWNTs. The authors prepared controlled
size Fe 3O 4 nanoparticles then functionalized with
azide moieties (Fe 3O 4-N 3) and finally with alkyne moieties
(Fe 3O 4-Alk). MWNTs were separately modified with polymer
containing abundant azide groups (MWNT-pAz) and
polymer containing abundant of alkyne groups (MWNT-
pAlk). (Fe 3O 4-N 3) and (Fe 3O 4-Alk) were coupled with
polymer coating nanotubes to give magnetic nanohybrids
of MWNT-pAz@Fe 3O 4 and MWNT-pAlk@Fe 3O 4,
respectively. Fig. 5 presents the TEM images of both
MWNT-pAz and MWNT-pAlk, evenly decorated with
magnetic nanoparticles. Cho and co-workers [163] prepared
gold nanoparticles functionalized SWNTs using a
click chemistry approach. Gold nanoparticles containing
octanethiol were prepared by the reduction of tetrachloroauric
acid using sodium borohydride in presence of
alkanethiol. The alkyl thiol protected gold nanoparticles
were further treated with azidoundecane thiol to yield
azide moiety containing gold nanoparticles, which were
reacted with alkyne functionalized SWNTs. Campidelli et
al. [157] decorated SWNTs with phthalocyanine using a
click coupling approach. The authors functionalized SWNTs
with 4-(2-trimethylsilyl)ethynylaniline in the presence of
isoamyl nitrite, which were then treated with azide moiety
containing Zinc-phthalocyanine (ZnPc) in the presence
of CuSO 4 and sodium ascorbate to give the nanotubephthalocyanine
hybrid. They also studied the photovoltaic
properties of synthesized materials and observed that the
photocurrent of SWNT-ZnPc was higher and more stable
and reproducible than that of pristine SWNTs.
Layer-by-layer (LbL) covalent functionalization of
MWNTs was achieved by Gao and co-workers [164].
The clickable polymer poly(2-azidoethyl methacrylate)
was synthesized by atom transfer radical
polymerization (ATRP) and another clickable polymer
poly(propargyl methacrylate) was synthesized by
reverse addition–fragmentation chain transfer (RAFT)
Scheme 3. Scheme of application of click chemistry for synthesis of bioactive molecule bonded SWNT. (a) (i) CbzCl, 2 M NaOH solution, 0 ◦ C, 2 h, then 2 M
HCl, pH 3, (ii) SOCl2, MeOH, RT, 8 h (iii) NaBH4 (excess), MeOH, RT over night. (b) (i) TsCl (1.2 equiv.), Et3N (1.3 equiv.), 0 ◦ C to RT 4 h (ii) NaN3, DMSO:1,4dioxane,
80 ◦ C, 16 h, 78–81% yields in two steps. (c) CuI (0.15 M), ascorbic acid (0.08 M) and DIPEA (0.17 M) in DMF-pyridene-butanol, RT, 14 h, followed by
80 ◦ C for 4 h [156].

N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 845
Fig. 5. TEM images of (a) Fe3O4-N3, (b and c) MWNT-pAlk@Fe3O4, (d) MWNT-pAz@Fe3O4. Reprinted with permission from Ref. [162]. Copyright 2009, The
Royal Society of Chemistry, UK.
polymerization. Both polymers were alternately coated
on alkyne-modified MWNTs using the reliable Cu(I)catalyzed
Huisgen 1,3-dipolar cycloaddition reaction.
Poly(2-azidoethyl methacrylate) was clicked on the
preprepared alkyne-modified MWNTs as the first polymer
layer. Alkyne side groups containing poly(propargyl
methacrylate) were coated as the second polymer layer
via click coupling. Poly(2-azidoethyl methacrylate) was
clicked on the second polymer layer coated MWNTs,
and the residual clickable azido groups of third layer
coated MWNTs were further clicked with alkyne-modified
rhodamine B and monoalkyne-terminated PS. The post
modification supports the usefulness of functionalized
MWNTs as a nanoplatform for further molecular design
and material synthesis. Due to formation of a crosslinked
polymer network, the covalent linkage offers several
advantages, such as high stability and good control
over the quantity and thickness of the polymeric layers.
The authors also reported a clickable macroinitiator
for building the amphiphillic polymer brushes on CNTs
[165]. The azido and bromo groups functionalized CNTs
were prepared by the reaction of poly(3-azido-2-(2-
bromo-2-methylpropanoyloxy)propyl methacrylate with
alkynated CNTs. Both the ATRP and click coupling could
be achieved by a one pot procedure using bromo and
azido moieties as initiators for PS and PEG grafting (Fig. 6).
The reaction could be easily accomplished with SWNTs
and MWNTs using “grafting to” and “grafting from”
approaches for functionalization with multiple kinds of
polymers.
Functionalization of CNTs with stimuli-responsive
materials is expected to be useful for manufacturing
advanced biosensors and bioprobes [166]. Li and
co-workers [167] introduced an alkyne functionalized nanotube
surface using a carbamate linkage. The azide moiety
containing a thermoresponsive diblock copolymer composed
of N,N-dimethylacrylamide (DMA) and NIPAM was
covalently attached with alkynated MWNTs via the Cu(I)catalyzed
[3 + 2] Huisgen cycloaddition. The copolymer
containing hydrophilic DMA, as well as a smart NIPAM
block, is capable of forming micelles with response to
changes in the aqueous solution temperature. The micelles
size and transition temperature can be controlled through
NIPAM block length. Due to the higher azide concentra-

846 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Fig. 6. Synthesis of amphiphilic/Janus polymer brushes-grafted multiwalled and single-walled carbon nanotubes (MWNTs and SWNTs) by a combination of
click chemistry and atom transfer radical polymerization (ATRP) approach. Reprinted with permission from Ref. [165]. Copyright 2008, American Chemical
Society, USA.
tion on their periphery, micelles afford improved grafting
efficiency and solubility of nanotubes, compared to coils in
solution.
The oligosaccharide ˇ-cyclodextrin is well known
to encapsulate biological molecules in its hydrophobic
cavities in an aqueous solution, enhancing its utility as
a drug carrier and enzyme mimic. Zheng et al. reported
a ˇ-cyclodextrin-modified SWNT nanohybrid through
Huisgen cycloaddition [168]. Mono-6-(p-toluenesulfonyl)-
ˇ-cyclodextrin, prepared by reaction of ˇ-cyclodextrin
with p-toluenesulfonyl chloride, was treated with
sodium azide to convert it to an azide-functionalized
cyclodextrin. Purified SWNTs were reacted with p-(2-
propynyloxy)-benzenamine in o-dichlorobenzene (ODCB)
using a diazotization coupling procedure to produce
alkyne-functionalized SWNTs. The azide-functionalized
cyclodextrin was further reacted with alkynated SWNTs
via click coupling. The ˇ-cyclodextrin functionalized
SWNTs show good solubility in water, enhancing
their biological importance for drug delivery applications.
Though there are not so many examples of employing
the click chemistry for CNT functionalization, related
papers are increasingly reported recently. Scheme 4 shows
the attachment of different functionalities on CNTs via click
coupling. The wide range of attached molecules enhances

N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 847
Scheme 4. The attachment of different functionalities on CNTs via click coupling.
the importance of click chemistry and opens the new vista
of CNTs based nanomaterials.
3. Preparation methods of polymer/CNT
nanocomposites
As emphasized in the preceding, the dispersion of CNTs
in polymer matrices is a critical issue in the preparation
of CNT/polymer composites. Better reinforcing effects of
CNTs in polymer composites will be achieved if they do
not form aggregates and as such, they must be well dispersed
in polymer matrixes. Currently there are several
methods used to improve the dispersion of CNTs in polymer
matrices such as solution mixing, melt blending, and
in-situ polymerization method.
3.1. Solution mixing
In this approach, a dispersion of CNTs in a suitable solvent
and polymers are mixed in solution. The CNT/polymer
composite is formed by precipitation or by evaporation
of the solvent. It is well known that it is very difficult
to properly disperse pristine CNTs in a solvent by simple
stirring. A high power ultrasonication process is more
effective in forming a dispersion of CNTs. Ultrasonic irradiation
has been extensively used in dispersion, emulsifying,
crushing, and activating the particles. By taking advantage
of the multi-effects of ultrasound, the aggregates and
entanglements of CNTs can be effectively broken down. For
example, Li et al. [169] used a simple solution–precipitation
technique to improve the dispersion of CNTs in a poly-
carbonate solution by sonication at a frequency of 20 kHz
for 10 min. They showed that the CNTs were uniformly
dispersed in polycarbonate matrix on its consolidation. In
this case, ultrasonic wave as well as mechanically stirring
played important roles in the formation of the composites
with a uniform particle size. The chemical effects of ultrasound
are associated with the rapid (microsecond time
scale), violent collapse of cavitation bubbles created as
the ultrasonic waves pass through a liquid medium [170].
Sonochemical theory and the corresponding studies suggested
that ultrasonic cavitation can generate a high local
temperature of 5000 K and a local pressure of 500 atm
[171], which is a very rigorous environment. Safadi et al.
[172] dispersed MWNTs in PS using ultrasonication and
dismembrator at 300 W for 30 min. Uniform dispersions of
CNTs in PS were achieved by using sonication. Recently Cho
and co-workers successfully prepared PU/MWNTs composites
with better dispersion of CNTs up to 20 wt% in PU
[173]. In the research, the necessary weight fractions of
carboxylate MWNTs were first dispersed in DMF solution
under sonication at room temperature for 1 h using a high
power ultrasonic processor. Thereafter, PU was added into
this solution and stirred for 1 h. The mixtures were then
sonicated again for 1 h. The SEM photographs of the crosssectional
fracture of composites of the achieved dispersion
for the investigated MWNTs are shown in Fig. 7. The homogeneous
dispersion in the composites was achieved by the
addition of a higher amount of MWNT-COOH (20 wt%).
Using a proper surfactant is another efficient method to
disperse higher loading of nanotubes [70,174,175]. The
use of non-ionic surfactants such as polyoxyethylene-8-

848 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Fig. 7. SEM micrographs of PU/CNT composites: (a) 10 wt% pristine MWNT and (b) 20 wt% MWNT-COOH. Reprinted with permission from Ref. [173].
Copyright 2006, Wiley-VCH, Germany.
lauryl has been demonstrated to improve the dispersion
and strong interaction between MWNTs and epoxy resins
[42].
Barrau et al. [176] found that amphiphilic palmitic acid
facilitate an efficient dispersion of CNTs in an epoxy matrix.
The hydrophobic part of palmitic acid is absorbed onto
the nanotube surface, whereas the hydrophilic head group
induces electrostatic repulsions between nanotubes, preventing
their aggregation. The co-solvent also affects the
dispersion of nanotubes in polymer matrix. Recently, Camponeschi
et al. [177] reported on the use of trifluoroacetic
acid as a co-solvent for the dispersion of MWNTs in a
conjugated polymer poly(3-hexylthiophene), and PMMA
through a solution process. Scanning electron microscopy
(SEM), optical microscopy, and light transmittance studies
revealed that the better dispersion of CNTs in polymer
matrixes was obtained by using trifluoroacetic acid.
Many other polymer composites such as PU/CNT [178],
PS/CNT [179–181], epoxy/CNT [180,182,183], Poly(vinyl
alcohol)/CNT [184], P(MMA-co-EMA)/CNT [141], polyacrylonitrile/CNT
[185], and polyethylene(PE)/CNT [186] have
been fabricated by this method.
3.2. Melt mixing
For solution mixing, the matrix polymer must be soluble
in at least one solvent. This is problematic for many
polymers. Melt mixing is a common and simple method,
particularly useful for thermoplastic polymers. In melt processing,
CNTs are mechanically dispersed into a polymer
matrix using a high temperature and high shear force
mixer or compounder [187]. This approach is simple and
compatible with current industrial practices. The shear
forces help to break nanotube aggregates or prevent their
formation. Zhang et al. [188] prepared nylon-6/MWNTs
composites containing 1 wt% MWNTs via a melt compounding
method using a Brabender twin-screw mixer.
SEM image showed a homogeneous dispersion of MWNTs
achieved through the matrix polymer, associated with
significant enhancements in mechanical properties. Bocchini
et al. [189] fabricated MWNTs/linear low density
polyethylene (LLDPE) nanocomposites via melt-blending
using a Brabender Plasticorder internal mixer. MWNTs
dispersed in LLDPE delay thermal and oxidative degradation
with respect to that for virgin LLDPE. PE/MWNT
nanocomposites were prepared using twin screw melt
compounding [190]. Microscopic observations across the
length scales and X-ray diffraction measurements indicate
that the MWNTs are very well distributed and dispersed in
the PE matrix. Melt mixing has been successfully applied
for the preparation of different polymers-CNT composites
such as polypropylene/CNT [191–193], high density
PE/CNT [194], polycarbonate/CNT [195–197], PMMA/CNT
[198–201], polyoxymethylene/CNT [202], polyimide/CNT
[203], PA6/CNT [204], etc. The disadvantage of this method
is that the dispersion of CNTs in a polymer matrix is quite
poor compared to the dispersion that may be achieved
through solution mixing. In addition, the CNTs must be
lower due to the high viscosities of the composites at higher
loading of CNTs.
3.3. In-situ polymerization
In this polymerization method, the CNTs are dispersed
in monomer followed by polymerization. A higher percentage
of CNTs may be easily dispersed in this method, and
form a strong interaction with the polymer matrixes. This
method is useful for the preparation of composites with
polymers that can not be processed by solution or melt
mixing, e.g., insoluble and thermally unstable polymers.
Hu et al. [205] synthesized MWNT-reinforced polyimide
nanocomposites by in-situ polymerization of monomers in
the presence of acylated MWNTs, as shown in Scheme 5.In
this work, MWNTs were functionalized with acyl groups,
and then reacted with 3,3 ′ ,4,4 ′ -biphenyltetracarboxylic
dianhydride to form MWNT-poly(amic acid). The final
MWNT-polyimide nanocomposite films were obtained
by imidization of MWNT-poly-(amic acid) at 350 ◦ C for
1 h under vacuum. In this method, the CNTs were uniformly
dispersed in polymer matrix. Recently, Cho et
al. fabricated MWNTs-reinforced polyimide nanocomposites
by in-situ polymerization using 4,4 ′ -oxydianilline,
MWNT-COOH, and pyromellitic dianhydride followed by
casting, evaporation, and thermal imidization [206]. A

Scheme 5. Outline of the preparation of MWNT-polyimide nanocomposite
films. Reprinted with permission from Ref. [205]. Copyright 2006,
Wiley-VCH, Germany.
homogeneous dispersion of MWNT-COOH was achieved
in polyimide matrix as evidenced by scanning electron
microscopy (Fig. 8). This method has been widely used
for the preparation of PMMA-CNT composites [207–210].
Jia et al. [207] first synthesized PMMA by in-situ radical
polymerization method. They used free radical initiator of
2,2 1 -azobisisobutyronitrile, AIBN, to initiate open CNTs bonds
to participate in PMMA polymerization, forming a
strong interface between the CNTs and the PMMA matrix.
Other researchers used similar free radical initiator AIBN
to prepare SWNT-PMMA composites by in-situ polymerization
[127,211].
Conducting polymers are attached to CNTs surfaces
by in-situ polymerization to improve the processability,
and electrical, magnetic and optical properties of CNTs
[212–215]. Cho and co-workers [216] described a simple
approach to the synthesis of MWNT/polypyrrole (PPy) nanotubes
by the in-situ chemical polymerization of pyrrole on
the CNTs using ferric chloride as an oxidant. They investigated
the effect of the monomer concentration on the
coating and properties of the resulting complex nanotubes.
By changing the pyrrole/MWNT ratio, the layer thickness
of PPy could be easily controlled in MWNT-PPy complex
nanotubes, as shown in Fig. 9. Long et al. [213] synthe-
N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 849
Fig. 8. SEM images of PI/MWNTs nanocomposites: PI/MWNT-COOH
5 wt%. Reprinted with permission from Ref. [206]. Copyright 2007, Elsevier
Science Ltd., UK.
sized the CNT/PPy nanocables through an in-situ chemical
oxidative polymerization method. They showed that the
conductivity of nanocables increased with increasing nanotube
weight percentage.
In-situ polymerization method has also been used
for the preparation of polyurethane/CNT nanocomposites.
PU/MWNT composites were synthesized by two in-situ
polymerization methods [217]. In one method, a calculated
amount of carboxylate MWNT and 1,4-butanediol (BD) was
added to a prepolyurethane solution in the subsequent
chain-extension step. In the second method, the necessary
weight fractions of MWNTs were first dispersed in
poly(-caprolactone)diol (PCL). Thereafter, 4,4 ′ -methylene
bis(phenylisocyanate) (MDI) was added into this mixture.
The chain extender BD was added to this prepolymer
and the final PU-MWNTs composite was synthesized. The
MWNTs were relatively well dispersed in the PU matrix
of the PU-MWNT sample in second method. Xia and Song
[218] found that MWNTs could be individually dispersed in
Fig. 9. TEM photographs of PPy-coated MWNTs: (a) MWNT:PPy = 1:2; (b) MWNT:PPy = 1:5. Reprinted with permission from Ref. [216]. Copyright 2007,
Elsevier Science Ltd., UK.

850 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Fig. 10. FT-IR spectra of samples obtained (a) during prepolymerization (first step) with reaction time and (b) during reaction of prepolymer with
functionalized MWNTs (second step). Reprinted with permission from Ref. [220]. Copyright 2006, Wiley-VCH, Germany.
the PU matrix by in-situ polymerization method with the
aid of a dispersing agent. But SWNTs were not dispersed
well in this method. Later they synthesized PU/SWNTs
nanocomposites using PU-grafted SWNT which improved
the dispersion of SWNT in the PU matrix and strengthened
the interfacial interaction between the PU and SWNT [219].
A novel concept has been proposed to prepare
PU-MWNTs composite via in-situ polymerization of a prepolymer
in the presence of carboxylated MWNTs [220].
Synthesis of PU/MWNT nanocomposites was carried out
in a two-step process as follows. First, prepolymer was
prepared from a reaction of MDI and PCL at 80 ◦ C for
90 min in a four-neck cylindrical vessel equipped with a
mechanical stirrer. In the second stage, a calculated amount
of carboxilated MWNTs was added to the prepolymer at
110 ◦ C, and they were reacted for 150 min to obtain the
final crosslinked MWNT-PU nanocomposite. In this case,
any chain extender was not used at the second stage reaction.
PU chains were crosslinked to MWNTs by a reaction
between the carboxylic acid groups of the MWNTs and
the NCO groups of prepolyurethane (Fig. 10). These PUcrosslinked
MWNTs composites were never dissolved in PU
solvents such as N,N-dimethylformamide, dimethyl sulfoxide,
dimethylacetamide, or tetrahydrofuran.
The in-situ polymerization of caprolactam in the presence
of SWNTs allowed the continuous spinning of
SWNTs-PA6 fibers [221]. In addition, caprolactam is an
excellent solvent for carboxylic acid-functionalized SWNTs
(SWNTs-COOH). This allows the efficient dispersion of the
SWNTs and subsequent grafting of PA6.
4. Preparation of CNT nanocomposites using
dendritic polymers
Due to their three-dimensional globular and spherelike
structural architectures, dendritic polymers (DP) such
as dendrimeric and hyperbranched polymers, have generated
great excitement in polymer research, owing to
their wide range of applications from drug delivery to
chemical sensors [222–224]. Dendrimers have unique size,
controlled and symmetric structure with ideally branch-
ing units without any structural defects [225,226], but
require a multi-step synthesis reaction, whereas the Hyperbranched
polymers exhibit a randomly branched structure,
with a single step synthesis process [227,228]. Recently,
dendritic polymers have been used to enhance the dispersion
of CNTs in polymer matrices, taking advantage
of their highly functionalized three-dimensional globular,
non-entangled structures. They exhibit higher solubility
and lower viscosity in the melt and solution states compared
to linear polymers of the same molar mass [229,230].
Dendritic polymer/CNT nanocomposites may be formed via
covalent and non-covalent functionalization of CNTs. Dendritic
polymers are effective for enhancing mechanical and
electrical properties of polymer nanocomposites because
the pristine CNTs can be used to obtain well dispersed CNTs
in polymer matrix, without any CNT modification.
4.1. CNT nanocomposites via covalently functionalized
CNT-dendritic polymers
Grafting of dendritic polymers on CNTs is a novel
approach for fabricating the nanomaterials and nanodevices
[231–234]. Newkome and co-workers [235]
fabricated unique CdS quantum dot composite assemblies
using dendronized SWNTs. Acyl chloride functionalized
MWNTs were treated with amino-polyester dendron to
prepare [(Den)n-SWNT]. The ester groups were further
changed into carboxylic groups using formic acid, and
reacted with Cd(NO 3) 2 to generate encapsulated CdS quantum
dots tethered to the SWNT surface. An electrode
material based on hyperbranched polymer-functionalized
MWNTs for lithium batteries showed good reversible
capacities and excellent capacity retention [236]. Sun and
co-workers [237], prepared the dendron functionalized
CNTs via amidation and esterification and their photophysical
properties were studied. Prato and co-workers
[238] reported the synthesis of SWNTs functionalized with
polyamidoamine dendrimers. The dendrimers present on
the nanotube sidewalls have been further functionalized
with porphyrin moieties, and the photophysical properties
of nanoconjugates have studied. Under visible light irra-

N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 851
Scheme 6. Synthetic process for the MWNT-hyperbranched polyether nanohybrids. Reprinted with permission from Ref. [233]. Copyright 2004, American
Chemical Society, USA.
diation, porphyrin-SWNT nanoconjugate give rise to fast
charge separation (1.5 ± 0.5) × 10 10 s −1 evolving from the
photoexcited porphin chromophores. Haddleton and coworkers
[239] described a poly(amidoamine) dendrimer
(PAMAM) functionalized MWNTs via ester linkage. The
dendron functionalized MWNTs were used as a template
for the deposition of silver nanoparticles on the
MWNT surface. Park and co-workers [240] studied the
antimicrobial effects of silver nanoparticles functionalized
PAMAM-MWNTs nanohybrids. The dendritic-MWNTs/Ag
was found to have a stronger antimicrobial effect than
dendritic-MWNTs. The role of the architecture of dendritic
molecules on CNT photoelectrical behavior has also been
discussed [241]. These super-structured CNTs exhibit a
peculiar response to electron beams. Gao and co-workers
[233] prepared multihydroxyl hyperbranched polymer on
the surface of MWNTs. In situ ring-opening polymerization
was employed for growing multihydroxyl dendritic macromolecules
on the surface of MWNTs, as shown in Scheme 6.
Tree like multihydroxy hyperbranched polyether were
covalently grafted on the MWNTs using MWNTs-OH as an
initiator. The amount of polymer grafted is controllable
using this approach. The molecular weight of the grafted
hyperbranched polymer increases with increasing a feed
ratio of the monomer.
Biocompatible polymers on CNTs are arousing
interest due to the great significance of the nanohybrids
in bionanotechnology [242,243]. Mueller and
co-workers [244] prepared hyperbranched glycopolymers
functionalized MWNTs by atom transfer radical
polymerization of 3-O-methacryloyl-1,2:5,6-di-Oisopropylidene-D-glucofuranose
(MAIG) and self
condensing vinyl copolymerization of MAIG and AB*
inimer, 2-(2-bromoisobutyryloxy)ethyl methacrylate. The
hyperbranched glycopolymers are biocompatible and
water soluble, and the resulting polymer-functionalized
MWNTs would be very useful for bionanotechnology
applications. Hyperbranched poly(citric acid) (PCA)
grafted MWNTs based nanocomposites were synthesized
by Hekmatara and co-workers [245]. The CNT-g-PCA
nanocomposites were soluble in water freely. Hong et al.
[246] reported the coating of MWNTs with hyperbranched
polymer shell by self-condensing vinyl polymerization
(SCVP) of 2-((bromobutyryl)-oxy) ethyl acrylate via
ATRP. The synthesized hyperbranched polymers have
a large number of functional groups facilitating further
functionalization of MWNTs and providing a method to
homogeneously disperse MWNT conducting layers within
electroluminescent devices.
Core–shell nanostructures were prepared by Xie and
co-workers [234,247] with MWNTs and hyperbranched
poly(urea-urethane)s (HPU) as the hard core and the soft
shell, respectively The authors have synthesized HPUfunctionalized
MWNTs by a polycondensation method;
the solution rheology of HPU functionalized MWNTs were
studied. A large number of proton-donor and protonacceptor
groups were located in the HPU functionalized
MWNT; intra- and intermolecular H-bonds were easily
formed by their interactions. At low temperature, shearing
forces induce conversion from intra- to intermolecular Hbonds.
The rheological behavior of the HPU-functionalized
MWNT solutions showed a strong dependence on concentration,
temperature, and thermal and shearing prehistory.
Baek and co-workers [248,249] prepared hyperbranched
poly(ether ketone)s (PEK) grafted MWNTs using an in
situ polymerization method. Hyperbranched ether–ketone
polymer containing different monomer ratios were synthesized
in the presence of MWNTs to afford PEK-g-MWNTs
nanocomposites. Due to the molecular architecture of
hyperbranched polymers, the morphology of the nanocomposites
resembles mushroom-like clusters on MWNTs
stalks [250]. The resultant nanocomposites were soluble in
most strong acids, such as trifluoroacetic acid, methanesulfonic
acid and sulfuric acid. UV curable functional groups
containing hyperbranched polymers were synthesized for
modification of CNTs [251]. The hyperbranched polyester

852 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Fig. 11. Current AFM images of linear (a) and hyperbranched (b) polyurethane nanocomposites containing pristine multi-walled carbon nanotubes of
20 wt% [254].
functionalized MWNTs were reacted with difunctional
molecules synthesized from toluene 2,4-diisocyanate and
hydroxylethyl acrylate to get UV curable hyperbranched
polymer. The modified MWNTs containing large amount
of UV-curable acrylate group were dispersed with UV curable
aliphatic urethane acrylate resins. In the presence of
UV irradiation, crosslinking reaction developed between
MWNTs and acrylate resins, leading to the covalent bonding
of MWNTs to the matrix, a more stable coupling than
simply physical bonding. Due to the excellent mechanical
properties of MWNTs, both the tensile strength and toughness
of the nanocomposites were enhanced by nearly 41%
and 105%, respectively, with only 0.1 wt% MWNTs.
4.2. CNT nanocomposites via non-covalently
functionalized CNT-dendritic polymers
The disruption of electronic conjugation by covalent
modification motivates research on non-covalent functionalization
of CNTs. In fact, excellent modification of
CNTs through non-covalent interaction between the system
of CNT and the functional group of polymer can
be achieved. Star and Stoddart [252] studied the dispersion
and solubilization of SWNTs with a hyperbranched
polymer (poly[(m-phenylenevinylene)-co-(2,5-dioctoxyp-phenylene)
vinylene] (PmPV). The hyperbranched polymer
exhibits more efficiency at dispersing nanotube
bundles than its parent PmPV polymer. The branching
of PmPV makes it less efficient for wrapping bundles of
SWNTs. They studied the molecular modeling of hyperbranched
polymer and found that the pockets provided
by the hyperbranched polymer offer a better fit for the
SWNTs. Cho and co-workers [253] also prepared the hyperbranched
polyurethane (HBPU)-MWNTs nanocomposites
using an in situ polymerization method. They prepared
different hard segment containing PU nanocomposites via
a two step process. Novel dispersion of MWNTs in the
HBPU matrix was observed, as well as good solubility
of nanocomposites in organic solvents. Because of the
good dispersion of MWNTs in the polymer matrix, the PU
nanocomposites showed dominant shape recovery properties
of 84–96%, and enhanced mechanical properties.
They also studied the dispersion of high concentration
MWNTs (5–40 wt%) using an HBPU matrix [254]. The
MWNTs dispersion was analyzed on the basis of surface
morphology using current AFM images (Fig. 11), including
dispersions of 20 wt% MWNT in linear, as well as hyperbranched
polymer matrix. The aggregated MWNTs were
observed in linear PU/MWNTs nanocomposites, whereas
well dispersed MWNTs were observed for HBPU/MWNTs
nanocomposites. Compared to linear PU nanocomposites,
the hyperbranched nanocomposites give remarkable high
electrical conductivity.
Nakamoto and co-workers [255] presented the construction
of insulating conducting wire composed of
SWNTs and phenolic polymers as a conducting wire
and as an insulating coating, respectively. A hyperbranched
phenolic polymer (HBP) was synthesized from
3,4,5-trimethoxytoluene (A 2) and 1,3,5-tribromomethyl-
2,4,6-trimethoxybenzene (B 3) using a Lewis acid-catalyzed
polycondensation method. Fig. 12 shows the TEM and AFM
images for the samples. The images show the covering
of the SWNT surface by HBPs, which indicates solubilization
of SWNTs with physical adsorption of HBPs. In the
presence of HBP polymer, SWNTs were homogeneously
dispersed in DMF solution, in contrast to the use of linear
polymer, where SWNTs were insoluble in DMF, even
after sonication. The authors also studied the impact of solvent
on the solubility of SWNTs with HBP. The SWNTs were
sparsely soluble in THF solution with HBP, whereas typically
SWNTs van Hove singularities were found with HBP in
DMF. A non-covalent method for preparing the crosslinked
amphiphillic hyperbranch polymer micelle-encapsulated
CNTs was proposed to improve the dispersion of CNTs in
water [256]. Fig. 13 shows the TEM images of uncrosslinked
and crosslinked encapsulated CNTs. Zheng and co-workers
[257] reported an MWNTs based solvent-free nanofluids at
room temperature using hyperbranched poly(amine-ester)
(HPAE). Acid functionality containing MWNTs were homogeneously
dispersed in HPAE matrix. The ternary amine
of HPAE were protonated with the carboxylic groups of
MWNTs via an acid–base reaction, which leads to ionic
attachment on the surface of MWNTs as well as hydrogen
bonding interaction between the –OH, C O groups

N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 853
Fig. 12. (a) TM-AFM height image of SWNTs solubilized by extended HBP. (b) Height profile along dash line in (a). (c) Proposed structure of hybrids
composed of HBP and SWNTs on mica surface. The extended TM-AFM (d) height and (e) phase images in box area in (a). Reprinted with permission from
Ref. [255]. Copyright 2008, Elsevier Science Ltd., UK.
and the ternary amine groups to form a homogeneous
hybrid system. The preparation of fluorinated oligomeric
aggregates/CNTs nanocomposites possessing a higher dispersibility
and stability in water is of particular interest
for developing new fluorinated functional materials [258].
Sawada et al. [259] synthesized a dendrimer copolymer
containing fluoroalkyl segments [260] for the dispersion of
SWNTs in water. They demonstrated that the fluorinated
dendrimer block copolymers could form new fluorinated
molecular aggregates to have higher dispersion ability,
not only for SWNTs and fullerenes, but also for magnetic
nanoparticles in water. Valentini et al. [261] reported
naphthalenediimide modified electrically conducting dendrimer
poly(amidoamine)/SWNTs nanocomposites. The
electric conductance of SWNTs drastically increased upon
adsorption of the conducting dendrimer, owing to the
interaction between SWNTs and the conducting dendrimer.
The effective immobilization of glucose oxidase (GOx)
on the CNT surface is a key issue for developing a glucosebased
biosensor [262–264]. Zhu and co-workers [265]
prepared the glucose biosensor based on layer-by-layer
electrostatic adsorption of glucose oxidase and dendrimerencapsulated
Pt nanoparticles on MWNTs. LBL technique
provide a favorable microenvironment to keep the bioactivity
of glucose oxidase, and prevent enzyme molecule
leakage. The enzyme electrode showed good characteristics,
such as short response time, high sensitivity, and
stability. The (GOx/Pt-DENs) 4/CNTs electrode showed bet-
ter chronoampermetric response than the unmodified
CNTs electrode.
5. Mechanical properties of polymer/CNT
nanocomposites
As remarked above, their extraordinary mechanical
properties and large aspect ratio make CNTs excellent candidates
for the development of CNT-reinforced polymer
nanocomposites. Indeed, a wide range of polymer matrixes
have been used for the development of such nanocomposites.
This section focuses on the mechanical properties of
composites of CNT composites in two polymer matrixes
well represented in the literature.
5.1. Polyurethane/CNT composites
Polyurethane is one of the most versatile materials
today. It is widely used in coatings, adhesives, shape
memory polymers, medical fields and composites. PU is
consisting of alternating hard and soft segments. The
hard segment is composed of alternating diisocyanate and
chain-extender molecules (i.e., diol or diamine), whereas
the soft segment is formed from a linear, long-chain diol.
Phase separation occurs in PUs because of the thermodynamic
incompatibility of the hard and soft segments.
PU/CNT composites [266–275] are of significant current
interest. The mechanical properties for different PU composites
are summarized in Table 4.

854 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Fig. 13. TEM images of (A) empty micelles, (B) uncrosslinked e-MWNT, and (C and D) crosslinked e-MWNT. Reprinted with permission from Ref. [256].
Copyright 2008, Elsevier Science Ltd., UK.
Incorporation of CNTs into PU can dramatically increase
the tensile strength and modulus. For example, the addition
of carboxylated MWNTs into the PU matrix by solvent mixing
improved the tensile strength and modulus of the PU
matrix [173,276] as shown in Fig. 14. The tensile strength
of a composite containing 10 wt% of MWNT-COOH was
enhanced by 108% as compared to pure PU, while an
Table 4
Mechanical properties of PU-CNT composites.
Nanotube type Preparation method % of modulus improvement at
1% CNT
increase of 68% was achieved by incorporating the same
amount of raw MWNTs in the PU matrix. Finally, the tensile
strength and modulus of nanocomposites increased
from 7.6 MPa in pure PU to 21.3 MPa (an increase of 180%)
and 50 to 420 MPa (an increase of 740%), respectively,
when the functionalized MWNTs content reached 20 wt%
in composites. The hydrophilic functional groups on the
% of tensile strength
improvement at 1% CNT
SWNT Solution 25 50 [357]
MWNT-functional Solution 140 20 [162]
MWNT-functional Solution – 63 [279]
SWNT-functional Electrospining 250 104 [281]
MWNT Addition polymerization 561 397 [280]
MWNT In-situ polymerization 35 114 [277]
MWNT-functional In-situ polymerization 54 25 [358]
MWN-functional Solution 12 6 [274]
MWNT In-situ polymerization 90 90 [359]
MWNT-functional In-situ polymerization 40 7 [266]
Reference

Fig. 14. Stress–strain profiles of PU composites at different MWNT
loading. Reprinted with permission from Ref. [173]. Copyright 2006,
Wiley-VCH, Germany.
MWNTs were helpful in improving the interaction with
–CONH– groups in PU. Therefore, the strong interaction
between the functionalized MWNTs and the PU matrix
greatly enhanced the dispersion as well as the interfacial
adhesion, thus strengthening the overall mechanical
performance of the composite. The mechanical properties
of composites depends on the acid treatment temperature
of the CNTs. Composites containing MWNTs reacted
at 90 ◦ C resulted in a greater increased modulus compared
with those at 140 ◦ C, indicating that severe surface
modification lowers mechanical properties [95]. Kuan et
al. [277] incorporated amino functionalized MWNTs into
waterborne PU. They found an increase in modulus from
77 MPa for the polymer to 131 MPa for a 4 phr composite
(an increase of 70%) and a tensile strength increase
from 5.1 MPa to 18.9 MPa (an increase of 270%) at the same
loading level. Covalent bond formation between amino
functionalized MWNTs and PU promoted increased interfacial
strength and tensile strength. MWNT is more effective
to the improvement of modulus, whilst SWNT is better for
elongation and tensile strength. The different reinforcing
effects of MWNT and SWNT on PU were correlated to the
shear thinning exponent and the shape factor of CNTs in
polyol dispersion.
Polymer grafting is very effective in increasing dispersion
and the mechanical properties of composites due to
its strong chemical bonding between polymer and CNTs.
Xia et al. [278] studied polycaprolactone-based PU-grafted
Table 5
Mechanical properties of PI-CNT composites.
N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 855
SWNTs (SWNT-g-PU) and poly(propylene glycol)-grafted
MWNTs into PU by in-situ polymerization. Mechanical
property improvements were observed in both cases. The
incorporation of 0.7 wt% SWNT-g-PU into PU improved
the Young’s modulus by ∼278% and ∼188% compared to
the pure PU and ungrafted pristine SWNT/PU composites,
respectively. This is due to the better dispersion of SWNTg-PU
and MWNT-g-PU and stronger interfacial interactions
between the CNTs and PU. Wang and Tseng [279] also
found that adding 1–10 wt% PU functionalized MWNT to
PU increased the tensile strength by 63–210%. The storage
modulus and soft segment Tg (from tan ) increased with
increasing PU-functionalized MWNT in the PU. The Tg of
the soft segments of the nanocomposite films shifted from
−20 to −5 ◦ C, suggesting that PU functionalized MWNTs
are compatible with the amorphous regions of the soft
segments in the PU matrix. Recently McClory et al. [280]
reported thermosetting PU-MWNTs nanocomposites by an
addition polymerization reaction. The Young’s modulus
increased by 97 and 561% on the addition of 0.1 wt% and
1 wt% MWNTs in the PU, respectively, whereas ultimate
tensile strength increased by 397% when either 0.1 or 1 wt%
MWNTs added to PU. In this composite, the percentage
of elongation-at-break increased from 83 to 302% on the
addition of 0.1 wt% CNT compared to pure PU resin.
Improvement of mechanical properties has been
reported for melt-processed composite fibers. Young’s
modulus of composite fibers increased by 27 folds compared
to unfilled PU fiber. Sen et al. [281] studied the
fabrication of membranes of SWNT-filled PU by the
electrospinning technique. The tensile strength of esterfunctionalized
SWNT/PU membranes was enhanced by
∼104%, and the tangent modulus improved by ∼250%
compared to PU membrane. So, these enhancements in
mechanical properties can be attributed to the high dispersion
of CNTs through the polymer matrix and good
interfacial interaction between CNT and PU.
5.2. Polyimide/CNT nanocomposites
Polyimide (PI) is a candidate polymer for a variety of
applications such as packaging materials, circuit boards,
and interlayer dielectrics due to their good dielectric
properties, flexibility, high glass transition temperature,
excellent thermal stability, and radiation resistance. Polyimides
serve as excellent polymer matrices for polymeric
CNT nanocomposites [282,283]. The mechanical properties
for different PI composites are summarized in Table 5.
Nanotube type Sample type CNT content (wt%) % of modulus improvement % of tensile strength improvement Reference
SWNT-functional Film 1 89 9 [287]
MWNT-functional Film 5 33 7 [206]
MWNT-functional Film 5 – 40 [285]
MWNT-functional Film 0.5 110 100 [289]
MWNT-functional Film 0.5 60 61 [290]
MWNT-functional Film 6.98 61 31 [284]
SWNT Film 1 10 10 [203]
SWNT Rod 1 0 11 [203]
SWNT Fiber 1 45 0 [203]
MWNT-functional Film 7.5 52 21 [282]

856 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Most studies report nanocomposites with CNT result
inimprovements in the mechanical properties of PI. For
example, in situ polymerized PI containing 5 wt% MWNT-
COOH showed increase in modulus (33%) and tensile
strength (7%) as compared to neat PI [206]. However,
compared to that of the neat PI, the modulus and tensile
strength of raw CNT/PI nanocomposites showed only
a slight increase.The better improvement in the tensile
strength and modulus of PI/MWNT-COOH may be caused
by the strong interactions between PI matrix and MWNT-
COOH. As discussed above, the functionalized MWNTs
prepared by acid treatment contain –COOH groups, which
facilitate improved interaction with –O– groups in the PI
chain.
Another study revealed moderate increases in the modulus
and tensile strength of PI when mixed with plasma
modified MWNT [289]. The addition of 0.5 wt% of the
plasma modified MWNT to the PI increased the modulus
from 2.17 to 4.56 GPa and the tensile strength from 124.5
to 249 MPa, or increases of 110% and 100%, respectively.
These impressive results were attributed to chemical bond
formation between the plasma modified MWNT and the
PI. When the modified MWNTs content was higher (above
0.5 wt), the modulus and tensile strength were decreased
at high modified MWNT content. This result is consistent
with other reports [285,286]. Zhu et al. [285] found that the
tensile strength of the PI/MWNT composites increased with
increased MWNT content up to 5 wt%, then decreased with
further increase in MWNTs. The incorporation of 5 wt%
MWNTs into PI resulted in an enhanced tensile strength by
40% compared to pure PI, attributed to good dispersion of
MWNTs in the nanocomposite. At higher level of MWNTs,
the MWNTs could not be well-dispersed, agglomerating
to large clusters and resulting in a decrease of the tensile
strength. Jiang et al. [286] also observed that the Young
modulus of PI/MWNT composites improved by adding up
to 1.89 vol% MWNT and decreased with MWNT content
further increase.
A study utilizing SWNTs reported that the mechanical
properties of SWNT/PI composites with a low level
of SWNTs (0.30 wt%) showed a slight increase (5% tensile
strength and 18% Young modulus) compared to PI, whereas
composites with higher SWNT level (1 wt%) showed significant
improvement of the mechanical properties (9% tensile
strength and 90% Young modulus) [287].
The mechanical properties of PI/CNT composites
depend on the nature of any functionalization of the CNT.
At lower level of MWNT (up to 0.99 wt%), the tensile
properties of amine-modified MWNT/PI composites was
higher than that of acid-modified MWNT/PI composites
[284]. However, acid-modified MWNTs improved the tensile
properties of the PI more than amine-modified MWNT
for MWNTs content above 2.44 wt%. The acid-modified
MWNTs may form hydrogen bonds with the C O bonds of
the PI molecules. However, the bonding of amine-modified
MWCNT to polyamic acid may reduce its imidization. The
mechanical strength of polyamic acid is less than that of
PI and polyamic acid is more brittle than PI. So, aminemodified
MWNT is added to the PI matrix may affect the
mechanical properties of the polymer. The same group
also reinforced PI by the addition of vinyltriethoxysi-
lane functionalized MWNT [290]. They observed that the
modulus and tensile strength of 0.5 wt% functionalized
MWNT-PI composites increased 60 and 61%, respectively,
as compared to neat PI. This improvement of mechanical
properties depends on the ratio of vinyltriethoxysilane to
MWNTs. When the ratio of vinyltriethoxysilane to MWCNT
is 2:1, the composite showed better tensile properties than
other composites with different ratios and neat PI, because
only this ratio of vinyltriethoxysilane to MWCNT can form
interpenetrating network in PI matrix.
A number of other studies have observed increases
in modulus, but either no increase or decrease in tensile
strength [203,288]. Compared with the neat polymer,
the addition of CNTs resulted in increase in the modulus
and decrease in the tensile strength (18%). However,
the increase in the elastic modulus was small, e.g. 37%
for 14.3 wt% of CNT addition [288]. The improvement of
mechanical property SWNT/PI nanocomposite depends on
sample type, such as film, rod and fiber [203]. The tensile
modulus, ultimate strength, and elongation-at-break
were increased for the composite films with 1 wt% SWNT.
In case of extruded composite rod (∼1 mm in diameter),
there was no significant change in mechanical properties.
But the mechanical properties were significantly changed
when the extruded rods were drawn down to much smaller
diameters. Tensile strength and modulus were increased
with decreasing fiber diameter because of increased alignment
induced by the post extrusion fiber drawing process.
6. Electrical conductivity of polymer/CNT
nanocomposites
CNTs exhibit the high aspect ratio and high conductivity,
which makes CNTs excellent candidates for
conducting composites. Percolation theory predicts that
there is a critical concentration at which composites containing
conducting fillers in insulating polymers become
electrically conductive. According to percolation theory,
c = A(V − Vc) ˇ , where c is the conductivity of the composites,
V is the CNT volume fraction, Vc is the CNT
volume fraction at the percolation threshold, and A and
ˇ are fitted constant. The percolation threshold has been
reported to ranging from 0.0025 wt% [291] to several wt%
[302]. The percolation threshold for the electrical conductivity
in polymer-CNT nanocomposites depends on
dispersion [291,292,183], alignment [293,294], aspect ratio
[292,295,296], degree of surface modification [95] of CNTs,
polymer types [301] and composite processing methods
[292]. The electrical conductivity and percolation threshold
of CNT-polymer composites are shown in Table 6.
The aligned CNTs in epoxy decrease the percolation
threshold by an order of magnitude compared to entangled
nanotubes [291]. The entangled CNTs were dispersed
completely during the shear intensive stirring process,
whereas aligned CNTs led to a superior dispersion after
shear-intensive processing. The well dispersed CNTs easily
form conductive paths due to their relatively homogeneous
dispersion of the nanoparticles, compared with that for the
aggregated CNTs. The percolation threshold in CNT/epoxy
composites with palmitic acid reduced by 2 compared to
those composites without palmitic acid, attributed to the

Table 6
Electrical conductivity of CNT-polymer composites.
N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 857
Polymer type Nanotube type Percolation threshold (wt%) CNT content (wt%) Maximum conductivity (S/m) Reference
Polystyrene MWNT 0.15–0.2 2 10 3 [300]
Polycarbonate SWNT-functional 0.11 7 4.81 × 10 2 [301]
Polystyrene SWNT-functional 0.045 7 6.89 [301]
Poly(vinyl acetate) XD grade CNT – 20 4.8 × 10 3 [361]
Poly(methyl methacrylate) MWNT 0.3 40 3 × 10 3 [362]
Poly(methyl methacrylate) SWNT 0.17 10 1.7 × 10 3 [363]
Poly(methyl methacrylate) SOCl2-doped SWNT 0.17 13.5 10 4 [363]
Poly(vinyl alcohol) MWNT 5–10 60 100 [302]
Poly(vinyl acetate) SWNT 0.04 4 ≈15 [360]
Epoxy MWNT 0.0025 1 2 [291]
efficient dispersion of CNTs in the epoxy matrix promoted
by the palmitic acid [176]. The electrical conductivity of
SWNT/epoxy nanocomposites with SWNTs aligned under a
25 T magnetic field was increased by 35% compared to similar
composites without magnetic aligned SWNTs [293].In
contrast, Fangming et al. [294] found that the electric conductivity
of the aligned CNT in PMMA was 10 −10 S/cm and
unaligned CNT was 10 −4 S/cm for 2% SWNT/PMMA composites.
This indicates that the alignment of the nanotubes
in the composite decreased the electrical conductivity and
also the percolation threshold. The reason is that there
are fewer contacts between the nanotubes when they are
highly aligned in the composites, and so aligned composites
require more nanotubes to reach the percolation threshold.
The aspect ratio of the CNT has a tremendous influence
on the percolation threshold of the polymer nanocomposites
without varying other important parameters, such as
the polymer matrix or the dispersion and aggregation state
of the CNTs. Recently Grossiord et al. [300] reported that the
percolation threshold of polystyrene/MWNT nanocomposites
made from 2 wt% high-aspect ratio nanotubes grown
in vertically aligned films was 0.15–0.5 wt%, which is 5
times smaller than that for low aspect ratio industricallyproduced
nanotubes polymer composites. They found
an electrical conductivity of 10 3 S/m with 2 wt% CNTs.
The percolation threshold was observed at 0.045 wt%
non-covalently functionalized, soluble SWNT loading for
a SWNT-PS composite, with a maximum conductivity
of 6.89 S/m at 7 wt% of nanotube loading. The same
preparation method for a SWNT-PC composite resulted
in a material for which the conductivity increased to
4.81 × 10 2 S/m at 7 wt% of nanotubes, with a very low percolation
threshold reached at 0.11 wt% of nanotubes [301].
It is well known that chemical functionalization disrupts
the extended -conjugation of nanotubes and hence
reduces the electrical conductivity of functionalized CNTs.
Silane-functionalized CNT/epoxy nanocomposites showed
lower electrical conductivity than that of the untreated
CNTs composites at identical nanotube content [297]. Cho
et al. [94] reported that the electrical conductivity of the
surface-modified MWNT composites was lower than that
of the unreacted MWNT composites that had the same nanotube
content. This is attributed to increased defects in
the lattice structure of carbon-carbon bonds on the nanotube
surface as a result of the acid treatment. In particular,
the severe modification of the nanotubes significantly lowered
the conductivity. Several researchers reported that the
functionalization of CNTs can improve the electrical con-
ductivity of the composites [298,299]. Tamburri et al. [298]
found that the functionalization SWNTs with –COOH and
–OH groups enhanced the composites conductivity compared
to untreated SWNTs.
7. Optical properties of polymer/CNT
nanocomposites
CNTs exhibit unique one-dimensional p-electron conjugation,
mechanical strength, and high thermal and
chemical stability, which make them very attractive for
use in many applications. Optical limiting, an important
non-linear optical behavior, can develop with increasing
input fluence of a light pulse, such that the transmitted
fluence tends to a constant, independent of the input fluence.
For dispersions of CNTs in a number of solvents, it
appears that optical limiting is principally due to the nonlinear
scattering due to bubbles formed by light absorption
induced heating, although sublimation, a gradual reduction
in size for a CNT at high temperature, may contribute, in
contrast to a minimal role of reverse saturable absorption
(RSA), an effect dependent on absorption by excited electronic
states [303–305]. O’Flaherty et al. [306] reported that
the optical limiting of the poly(9,9-di-n-octylfluorenyl-2,7diyl)/MWNTs
samples is dependent on the mass fraction
of CNTs. The magnitude of the non-linear effect increased
systematically when the mass fraction of the nanotubes
increased from 0.011 to 0.038. The non-linear optical
extinction of nanosecond laser pulses by a set of conjugated
copolymer poly(para-phenylenevinylene-co-2,5dioctyloxy-meta-phenylenevinylene)/MWNTs
composites
dispersed in solution has been reported [307]. In this case,
either the MWNTs or the polymer dominates the nonlinear
response of the composite, depending on the relative
mass of polymer to nanotube. The non-linear extinction
was 0% for 0, 1.3, and 2.5 wt% MWNT at 10 J/cm 2 , whereas
the samples with 3.6 wt% and 5.9 wt% MWNTs were a normalized
non-linear extinction of 12% and 41%, respectively.
Thus the materials display a dramatic improvement in the
optical-limiting performance when the MWNT mass content
is increased from 1.3% to 3.6%.
Several polymer-coated and polymer-grafted MWNTs
were synthesized and their non-linear optical properties
of composites were investigated using 532 nm nanosecond
laser pulses [110]. The authors reported that the
optical limiting thresholds of the all composite samples
to be approximately 1 J cm −2 , which is similar to that
of the MWNT-DMF suspension. The results showed that

858 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867
Fig. 15. Optical and electrical properties of CNT/S-PEEK composite films: (a) transparent film on a labeled paper and (b) optical transmittance measurement.
Reprinted with permission from Ref. [311]. Copyright 2008, American Chemical Society, USA.
the polymers and processing methods do not change the
non-linear optical properties of MWNTs. The optical limiting
performance of double-C60-end-capped poly(ethylene
oxide)/MWNTs were also studied and found better
than the aqueous MWNTs suspension [308]. In case of
SWNT/poly(3-octylthiophenes) composite films, the optical
properties did not significantly change with low fraction
of CNT changes [309]. In a study of the optical properties
of poly(para phenylene vinylene) (PPV)/CNT composites
[310], different concentration SWCNT-PPV films were prepared
using a solution mixing process, heated at 120 ◦ C.
The authors observed that the optical spectra at 120 ◦ C PPV
conversion temperature was dramatically changed with
respect to those obtained at 300 ◦ C. It was also found that
the effect of the low conversion temperature on all the optical
spectra was similar to that of increasing the nanotube
concentration in standard PPV.
A novel preparation of optically transparent CNT/
polymer composites with highly aligned nanotubes has
reported recently [311]. First, CNT arrays were grown on
silicon by a CVD process. Uniform CNT sheets were then
pulled out of the arrays and stabilized on glass. Composite
films were finally produced by spin-coating or casting
polymer solutions onto the CNT sheets, followed by evaporation
of the solvents. Film thickness was controlled by
varying the concentration of polymer solutions and coating
times. PS, PMMA, and sulfonated poly(ether ether ketones)
derived composite films with more than 80% optical transparency
were prepared using this technique (Fig. 15). These
results suggest that the optical properties of the composites
can be tailored in a predetermined manner by controlling
Table 7
Application of CNT-polymer composites.
the nanotube content, orientation and precursor conversion
temperature, thus opening a pathway for developing
optically functional materials.
8. Applications
As developed in the preceding sections, because of
their excellent mechanical, electrical, and magnetic properties,
as well as nanometer scale diameter and high
aspect ratio, CNTs can be very useful materials in composites
to improve a particular property for specific
applications (Table 7). The addition of CNTs to conjugated
polymers was found to improve the quantum
efficiency of -conjugated polymers because the interaction
between the highly delocalized -electrons of
CNTs and the -electrons correlated with the lattice of
the polymer skeleton [3]. Such composites are widely
used in photovoltaic devices [312] and light-emitting
diodes [313]. CNT-conducting polymer composites have
a potential application in supercapacitors [314,315]. The
PANI/MWNTs composites electrodes showed much higher
specific capacitance (328 F g −1 ) than pure PANI electrodes
[316]. The capacitances of a CNT-polypyrrole composite-
CNT-poly(3-methyl-thiophene) composite based supercapacitor
prototype and a CNTs-CNTs-polypyrrole based
hybrid supercapacitor prototype were 87 and 72 F g −1 ,
respectively, much larger the 21 F g −1 for the CNTs/CNTs
corresponding supercapacitor prototype due to the Faraday
effect of the conducting polymers [317].
CNTs are also widely used in actuators [318,319].
The addition of CNTs to PANI fibers increased the elec-
Nanotube type Polymer type Applications References
SWNT Poly(3-octylthiophene) Photovoltaic devices [312]
MWNT; SWNT Polyaniline, polypyrrole, poly-(3,4-ethylenedioxythiophene),
poly(3-methyl-thiophene
Supercapacitors [314,316,317,364,365]
SWNT Nafion Actuators [318]
MWNT Poly(vinyl alcohol), poly(2-acrylamido-2-methyl-1-propanesulfonic acid) [366]
MWNT Nafion Fuel cell [367]
MWNT-functional Sulfonated poly(arylene sulfone), [368]
SWNT; MWNT Polypyrrole Biosensors [369,324]
SWNT Poly(methyl methacrylate) Biocatalytic films [370]
SWNT-functional;
MWNT-functional
DNA (polynucleotide) Gene delivery [371]

tromechanical actuation because the CNTs improved the
mechanical, electronic, and electrochemical properties of
the PANI fibers [320].
Composites based on CNTs are studied for a variety
of sensor applications [321,322]. For example, polypyrrole
or PANI deposited on single-walled CNT networks
that can be used as solid state pH sensors [323]. A DNA
sensor was created from a composite of polypyrrole and
CNTs functionalized with carboxylic groups to covalently
immobilize DNA onto CNTs [324]. In another example,
polypyrrole films doped with CNTs functionalized with
oligonucleotides were successfully implemented for DNA
biosensors using direct impedance measurements [325].In
general, the presence of CNTs tends to increase the overall
sensitivity and selectivity of biosensors. The thermal transport
properties of polymer composites can be improved
with the addition of CNTs due to the excellent thermal
conductivity of CNTs. Such composite are quite attractive
for usages as printed circuit boards, connectors, thermal
interface materials, heat sinks, lids, housings, etc. [8].
The superior properties of CNTs are not limited to
electrical and thermal conductivities, but also include
mechanical properties, such as stiffness, toughness, and
strength. CNTs with their high aspect ratio and excellent
mechanical properties have the potential to strengthen and
toughen hydroxyapatite without offsetting its bioactivity,
thus opening up a wider range of clinical applications [326].
Bianco et al. studied the application of CNTs as new vectors
for the delivery of therapeutic molecules [327,328].
CNTs have been shown to cross cell membranes easily and
to deliver peptides, proteins, and nucleic acids into cells
[329,330].
CNTs were employed to reinforce the interfaces
between ultra high molecular weight PE polymer particles,
enhancing composite strength, stiffness, impact toughness
as well as structural damping [331]. These composites
are attractive for applications in aerospace and naval
engineering. The high strength and toughness-to-weight
characteristics of CNTs may also prove valuable as part
of composite components in fuel cells that are deployed
in transport applications, where durability is extremely
important.
9. Concluding remarks
There are several approaches for developing high
performance CNT-polymer nanocomposites utilizing the
unique properties of CNTs. The critical challenge is the
development of methods to improve the dispersion of CNTs
in a polymer matrix because their enhanced dispersion in
polymer matrices greatly improves the mechanical, electrical
and optical properties of composites. Despite various
methods, such as melt processing, solution processing, insitu
polymerization, and chemical functionalization, there
are still opportunities and challenges to be found in order to
improve dispersion and modify interfacial properties. One
of challenges is to achieve the optimal functionalization of
CNTs, which can maximize interfacial adhesion between
CNTs and the polymer matrix. A specific functionalization
of CNTs is required for strong interfacial adhesion between
CNTs and a given polymer matrix, which may also simul-
N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 859
taneously improve the dispersion of CNTs in the polymer
matrix.
The mechanical properties of CNT-polymer nanocomposites
may be compromised between carbon–carbon
bond damage and increased CNT-polymer interaction due
to CNT functionalization. Similarly, electrical conductivity
of a CNT-polymer nanocomposite is determined by the negative
effect of carbon–carbon bond damage and the positive
effect of improved CNT dispersion due to CNT functionalization.
In either case, the choice and control of tailored
functionalization sites for chemical modification of CNTs
are necessary. As an example, selective CNT functionalization
can be achieved via click chemistry by preparing
azide-functionalized polymers. However, this may be limited
in the practical use owing to the need for multi-step
reactions for azide and alkyl groups to apply click chemistry.
The employment of hyperbranched polymers for
improving CNT dispersion may be also useful because it
can result in enhanced electrical conductivity, as well as
mechanical properties of nanocomposites, without modification
of CNT.
In practice, the problems regarding melt processing
need to be solved, as melt mixing is the most common commercial
method used to prepare CNT-polymer composites.
To achieve the best performance of CNT-polymer composites,
it is important to choose the CNT functionalization
method, a suitable polymer matrix for CNT dispersion and
molecular interaction control with CNTs as well as polymer
composite processing conditions such as temperature,
shear rate, shear force, and mixing time. In conclusion, the
CNT functionalization and matrix polymer design for dispersion
of CNTs and interfacial adhesion between CNTs and
a polymer matrix are the key challenges for development
of high performance CNT composites.
Acknowledgments
This work was supported by the SRC/ERC Program
of MOST/KOSEF (R11-2005-065) and A*STAR SERC Grant
(0721010018).
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