polymer nanocomposites based on functionalized carbon nanotubes

Progress in Polymer Science 35 (2010) 837–867

Contents lists available at ScienceDirect

Progress in Polymer Science

journa l homepage: www.e lsev ier .com/ locate /ppolysc i

Polymer nanocomposites based on functionalized carbon nanotubes

Nanda Gopal Sahooa, Sravendra Ranab, Jae Whan Chob,∗, Lin Lia,∗∗, Siew Hwa Chana

a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singaporeb Department of Textile Engineering, Konkuk University, Seoul 143-701, Republic of Korea

a r t i c l e i n f o

Article history:Received 5 August 2008Received in revised form 10 March 2010Accepted 12 March 2010Available online 16 March 2010

Keywords:NanocompositesCarbon nanotubesMechanical propertiesFunctionalization

a b s t r a c t

Carbon nanotubes (CNTs) exhibit excellent mechanical, electrical, and magnetic proper-ties as well as nanometer scale diameter and high aspect ratio, which make them an idealreinforcing agent for high strength polymer composites. However, since CNTs usually formstabilized bundles due to Van der Waals interactions, are extremely difficult to disperseand align in a polymer matrix. The biggest issues in the preparation of CNT-reinforcedcomposites reside in efficient dispersion of CNTs into a polymer matrix, the assessment ofthe dispersion, and the alignment and control of the CNTs in the matrix. There are severalmethods for the dispersion of nanotubes in the polymer matrix such as solution mixing,melt mixing, electrospinning, in-situ polymerization and chemical functionalization of thecarbon nanotubes, etc. These methods and preparation of high performance CNT-polymercomposites are described in this review. A critical comparison of various CNT functional-ization methods is given. In particular, CNT functionalization using click chemistry and thepreparation of CNT composites employing hyperbranched polymers are stressed as poten-
tial 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.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8382. Functionalization of CNT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 838

2.1. Defect functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8382.2. Non-covalent functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8392.3. Covalent functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8402.4. Functionalization using click chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 842

3. Preparation methods of polymer/CNT nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8473.1. Solution mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 847
3.2. Melt mixing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8483.3. In-situ polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 848
4. Preparation of CNT nanocomposites using dendritic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8504.1. CNT nanocomposites via covalently functionalized CNT-dendritic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8504.2. 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: [email protected] (J.W. Cho), [email protected] (L. Li).

0079-6700/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.progpolymsci.2010.03.002

dx.doi.org/10.1016/j.progpolymsci.2010.03.002

http://www.sciencedirect.com/science/journal/00796700

http://www.elsevier.com/locate/ppolysci

mailto:[email protected]

mailto:[email protected]

dx.doi.org/10.1016/j.progpolymsci.2010.03.002

8. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8589. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 859

. . . . . . . .. . . . . . . .

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

Since the discovery of carbon nanotubes (CNTs) in1991 by Iijima [1], they have received much attention fortheir many potential applications, such as nanoelectronicand photovoltaic devices [2,3], superconductors [4], elec-tromechanical actuators [5], electrochemical capacitors[6], nanowires [7], and nanocomposite materials [8,9]. Car-bon nanotubes may be classified as single-walled carbonnanotubes (SWNTs) [10,11], double-walled carbon nan-otubes (DWNTs) [12,13] or multi-walled carbon nanotubes(MWNTs) [1]. SWNT and DWNT comprise cylinders of oneor two (concentric), respectively, graphene sheets, whereasMWNT consists several concentric cylindrical shells ofgraphene sheets. CNTs are synthesized in a variety of ways,such as arc discharge [10], laser ablation [14], high pres-sure carbon monoxide (HiPCO) [15], and chemical vapordeposition (CVD) [16,17]. CNTs exhibit excellent mechani-cal, electrical, thermal and magnetic properties [18,19]. Theexact magnitude of these properties depends on the diam-eter and chirality of the nanotubes and whether they aresingle-walled, double-walled or multi-walled form. Typicalproperties of CNTs are collected in Table 1 [20–25].

Because of these excellent properties, CNTs can be usedas ideal reinforcing agents for high performance polymercomposites. Ajayan et al. [26] reported the first polymernanocomposites using CNTs as a filler. The number of arti-cles and patents in polymer composites containing CNTsis increasing every year [27]. Various polymer matricesare 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 ten-sile strength [36,37], tensile modulus [38,39], toughness[40], glass transition temperature [41,42], thermal con-ductivity [43,44], electrical conductivity [45,46], solventresistance [47], optical properties [48,49], etc.
Table 1Typical properties of CNTs [20–25].

Property SWNT DWNT MWNT

Tensile strength (GPa) 50–500 23–63 10–60Elastic modulus (TPa) ∼1 – 0.3–1Elongation at break (%) 5.8 28 –Density (g/cm3) 1.3–1.5 1.5 1.8–2.0Electrical conductivity (S/m) ∼106

Thermal stability >700 ◦C (in air)Typical diameter 1 nm ∼5 nm ∼20 nmSpecific surface area 10–20 m2/g

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 859. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 859

Progress in polymer/carbon nanotube compositeresearch considered here will included studies onfunctionalization of CNTs their mechanical, electricalconductivity and optical properties, and applications ofpolymer/CNT composites.

2. Functionalization of CNT

Since CNTs usually agglomerate due to Van der Waalsforce, they are extremely difficult to disperse and align ina polymer matrix. Thus, a significant challenge in devel-oping high performance polymer/CNTs composites is tointroduce the individual CNTs in a polymer matrix in orderto achieve better dispersion and alignment and stronginterfacial interactions, to improve the load transfer acrossthe CNT-polymer matrix interface. The functionalization ofCNT is an effective way to prevent nanotube aggregation,which helps to better disperse and stabilize the CNTs withina polymer matrix. There are several approaches for func-tionalization of CNTs including defect functionalization,covalent functionalization and non-covalent functional-ization [50]. These functionalization methods will besummarized here.

2.1. Defect functionalization

CNTs are purified by oxidative methods to removemetal particles or amorphous carbon from the raw mate-rials [51,52]. In these methods, defects are preferentiallyobserved at the open ends of CNTs. The purified SWNTscontain oxidized carbon atoms in the form of –COOH group[53,54]. In this oxidizing method, SWNTs are broken to veryshort tubes (pipes) of lengths 100–300 nm [55]. Mawhin-ney et al. [56] studied surface defect site density on SWNTsby measuring the evolution of CO2(g) and CO(g) on heatingto 1273 K. The results indicated that about 5% of the car-bon atoms in the SWNTs are localized at defective sites.Acid base titration method [57] was used to determinethat the percentage of acidic sites of purified SWNTs wasabout 1–3%. However, the defective sites created at theCNT surfaces by this method are extremely sparse, and cannot promote good dispersion in the polymer/CNT compos-ites. However, they can be used for covalent attachment oforganic groups by converting them into acid chlorides and

838 N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867

5. Mechanical properties of polymer/CNT nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8535.1. Polyurethane/CNT composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8535.2. Polyimide/CNT nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 855

6. Electrical conductivity of polymer/CNT nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8567. Optical properties of polymer/CNT nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 857

subsequently reacting with amines to give amides [58,59].The functionalized CNTs are more soluble in organic sol-vents than the raw CNTs. Most of the SWNT bundlesexfoliate to give individual SWNT macromolecules [8] if thereaction time of acid chloride group with amines is at ele-

N.G. Sahoo et al. / Progress in Polymer Science 35 (2010) 837–867 839

Table 2Non-covalent functionalization of CNTs.

Polymer or surfactant Nanotube type Preparation method References

SurfactantsCationic (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) MWNT Ultrasonication [339]

Biomacro-moleculesProteins/DNA MWNT Immobilization [64]Glucose (Dextran) SWNT Dialysis [340]ˇ-1,3-glucans SWNT Electroactive interaction [341]Chitosan SWNT Ultrasonication [342]

PolymersPoly(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]SW

SWM

vi

2

tpaippaccta

cfsci

FA

Poly(styrene)-poly(methacrylic acid)

ated temperature for 4 days and these SWNTs are solublen organic solvents.

.2. Non-covalent functionalization

Non-covalent functionalization of nanotubes is of par-icular interest because it does not compromise thehysical properties of CNTs, but improves solubilitynd processability. This type of functionalization mainlynvolves surfactants, biomacromolecules or wrapping witholymers (Table 2). In the search for non-destructiveurification methods, nanotubes can be transferred to thequeous phase in the presence of surfactants [60,61]. In thisase, the nanotubes are surrounded by the hydrophobicomponents of the corresponding micelles. The interac-ion becomes stronger when the hydrophobic part of themphiphilic contains an aromatic group.

CNTs can be well dispersed in water using anionic,
ationic, and non-ionic surfactants [68–71]. Anionic sur-actants such as sodium dodecylsulfate (SDS) [72–74] andodium dodecylbenzene sulfonate (NaDDBS) [75,76] areommonly used to decrease CNT aggregation in water. Thenteraction between the surfactants and the CNTs depends
ig. 1. Schematic representation of how surfactants may adsorb onto the nanotmerican Chemical Society, USA.

NT [85]

NT Solution mixing [90]WNT

on the nature of the surfactants, such as its alkyl chainlength, headgroup size, and charge. SDS has a weaker inter-action with the nanotube surface compared to that ofNaDDBS and Triton-X100 because it does not have a ben-zene ring. Indeed �-stacking interaction of the benzenerings on the surface of graphite increases the binding andsurface coverage of surfactant molecules to graphite sig-nificantly [71]. NaDDBS disperses better than Triton-X100because of its head group and slightly longer alkyl chain.Fig. 1 represents the adsorption of different surfactantsonto the nanotube surfaces.

Direct non-surfactant mediated immobilization of pro-tein and DNA on CNTs has been reported [64]. Thehydrophobic regions of the proteins are probably impor-tant for adsorption. A controlled and specific method forimmobilizing proteins onto non-covalently functionalizedSWNTs has been developed [62]. The mechanism of proteinimmobilization on nanotubes involves the nueleophilic
substitution of N-hydroxysuccinimide by an amine groupon the protein. In this case, non-covalent functionaliza-tion is accomplished by the interaction of delocalized�-bonds on the CNTs wall due to sp2 hybridization with�-bonds of polymer molecules of the matrix [62,63]. CNTs
ube surface. Reprinted with permission from Ref. [71]. Copyright 2003,


d mater
Fig. 2. A bundled pair of copolymer encapsulated SWNTs from unpurifiefrom Ref. [87]. Copyright 2003, American Chemical Society, USA.
can be successfully solubilized in organic [65] or aqueous[66] solvents after non-covalent functionalization, partlyattributed to the much better coverage of functional groups[67].

The dispersion of CNT in both water [77,78] and organicsolvents [79] may be enhanced by the physical associ-ation of polymers with CNT. This can be explained bythe ‘wrapping’ mechanism [78] that attributed to spe-cific interactions between the polymer and the CNTs.Polymers can wrap around CNTs, forming supramolec-ular complexes [80–82]. In these cases, �-stackinginteractions between the polymer and the nanotube sur-face are responsible for the close association of thestructures. Blau and co-workers [83–86] prepared ananotube-polymer hybrid by suspended SWNTs in organicsolvents poly(p-phenylenevinylene-co-2,5-dioctyloxy-m-phenylenevinylene) to wrap the copolymer around thenanotubes. The electrical properties of these hybrids wereimproved relative to those of the individual components. Anon-covalent method has been used to modify SWNTs byencapsulating SWNTs within crosslinked and amphiphilicpoly(styrene)-block-poly(acrylic acid) copolymer micelles(Fig. 2) [87]. This encapsulation significantly enhancedthe dispersion of SWNTs in a wide variety of polar andnon-polar solvents and polymer matrices because thecopolymer shell was permanently fixed. Thus, encapsu-lated SWNTs may be stabilized with respect to typicalpolymer processing and recovery from the polymer matrix.

Non-wrapping approaches have also been used for dis-persion and solubility of CNT in different media [88,89]. Inthese cases, copolymers of various different structures andcompositions act efficiently as stabilizers, and may be tai-lored so as to disperse the tubes in a variety of solvents.Nativ-Roth et al. [90] suggested that the block copolymersadsorbed to the nanotubes by a non-wrapping mecha-nism, and the solvophilic blocks act as a steric barrier that
leads to the formation of stable dispersions of individualSWNTs and MWNTs above a threshold concentration ofthe polymer. The strong �–� interaction between polymerbackbone and nanotube surface led to soluble SWNTs. Themain potential disadvantage of non-covalent attachment
ial (a) and purified encapsulated SWNTs (b). Reprinted with permission

is that the forces between the wrapping molecule and thenanotube might be weak, thus as a filler in a composite theefficiency of the load transfer might be low.

2.3. Covalent functionalization

Because of the �-orbitals of the sp2-hybridized C atoms,CNTs are more reactive than those with a flat graphenesheet, they have an enhanced tendency to covalently attachwith chemical species [91]. In the case of covalent function-alization, the translational symmetry of CNTs is disruptedby changing sp2 carbon atoms to sp3 carbon atoms, andthe properties of CNT, such as electronic and transportare influenced [92]. But this functionalization of CNT canimprove solubility as well as dispersion in solvents andpolymer. Covalent functionalization can be accomplishedby either modification of surface-bound carboxylic acidgroups on the nanotubes or by direct reagents to the sidewalls of nanotubes (Table 3). Generally, functional groupssuch as –COOH or –OH are created on the CNTs duringthe oxidation by oxygen, air, concentrated sulfuric acid,nitric acid, aqueous hydrogen peroxide, and acid mixture[55,93]. The surface of the acid treated MWNTs indicatesthe presence of some defects in the carbon–carbon bondingassociated with the formation of carboxylic acid groups onthe surface, while the raw MWNTs show uniform surfacesand a clear diffraction pattern because of their perfect lat-tice structure of carbon–carbon bonds (Fig. 3) [94–96]. Thenumber of –COOH groups on the surface of CNT dependson acid treatment temperature and time, increasing withincreasing temperature [95]. The extent of the induced–COOH and –OH functionality also depends on the oxida-tion procedures and oxidizing agents [97]. Nanotube endscan be opened during the oxidation process.

The presence of carboxylic acid groups on the nan-otube surface is more convenient than others because a
variety of chemical reactions can be conducted with thisgroup. The presence of –COOH or –OH groups on the nan-otube surface helps the attachment of organic [58,59,98]or inorganic materials, which is important for solubiliz-ing nanotubes. CNTs have been covalently functionalized


Table 3Covalent functionalization of CNTs.

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 Grafting from (ROP) Nylon-6 Sodium [125,126]MWNT

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 Grafting to Polyethylene glycol – [111,121]MWNT

MWNT Grafting to Polyimide – [117]SWNT Grafting to Poly(amido amine) – [122]MWNT Grafting to Poly(�-caprolactone)-diol – [345]

SWNT Grafting to Poly(vinyl acetate-co-vinyl alcohol) – [114]MWNT

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 Cycloaddition of azomethine ylides Amino-acid Paraformaldehyde [348]MWNT

SWNT Cycloaddition of azomethine ylides Peptides, Nucleic acids R-NHCH2COOH [349]MWNT

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 [2 + 1] Cycloadditions Dichlorocarbene Carbene [353]MWNT

zidocarbnephenylh

wcasmwasstrrfc

(ichnm

SWNT [2 + 1] Cycloadditions PEG di-aSWNT Radical additions PolystyreSWNT Radical additions Methoxy

ith thiocarboxilic and dithiocarboxylic esters that helprosslinking between CNTs [99]. CNTs can be functionalizedt end caps or at the sidewall to enhance their disper-ion as well as solubilization in solvents and in polymeratrices [91,100]. SWNTs were fluorinated at their sidealls by passing elemental fluorine at different temper-

tures [101]. The fluorinated SWNTs exhibited improvedolubility in isopropanol or dimethyl formamide by ultra-onication [102,103]. Fluorinated SWNTs may be convertedo side wall alkylated SWNTs by reaction with Grignardeagent or alkyllithium compounds that are soluble in chlo-oform [104]. SWNTs have also been solubilized by directunctionalization of their side walls by nitrenes [105,106],arbenes [106], and arylation [107,108].

Functionalization of CNTs with polymer moleculespolymer grafting) is particularly important for process-
ng of polymer/CNT nanocomposites [109,110]. Two mainategories “grafting to” and “grafting from” approachesave been reported for the covalent grafting of polymers toanotubes. The “grafting to” approach is based on attach-ent of as-prepared or commercially available polymer
onate ester Nitrene [354]Nitroxide [355]

ydrazine Microwave [356]

molecules on the CNT surface by chemical reactions, suchas amidation, esterification, radical coupling, etc. The poly-mer must have suitable reactive functional groups forpreparation of composites in this approach. Fu et al. [111]reported functionalization of CNTs by using “grafting to”method. They refluxed CNTs containing carboxylic acidgroups with thionyl chloride to convert acid groups toacylchlorides. Then, the CNTs with surface-bound acylchlo-ride moieties were used in the esterification reactionswith the hydroxyl groups of dendritic poly(polyethyleneglycol) polymer. Another example of the “grafting to”approach has been reported by Qin et al. [112]. Theygrafted SWNTs with polystyrene (PS) with functional-ized end groups PS (–N3), via a cycloaddition reaction.Polymer grafted CNTs have been formed by covalentlyattaching 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 PVA-grafted CNTs were soluble in PVA solution and PVA-CNTnanocomposites films showed very high optical quality


s (lower
Fig. 3. TEM pictures of raw MWNTs (upper) and surface-modified MWNTGermany.
without any observable phase separation. Many other lin-ear polymers such as poly(sodium 4-stryrenesulfonate)[115], poly(methyl methacrylate) (PMMA) [116], poly-imide [117,118], poly-(2-vinylpyridine) [119], PPEI-EI[120], and poly(m-aminobenzenesulfonic acid) [35] aswell as dendrons [121], dendrimers [122], and hyper-branched polymers [123] have been successfully bonded toCNTs.

A novel route to polymer reinforcement via preparationof polymer-functionalized nanotubes using organometallicapproach has been reported [124]. In the work, CNTs werefirst functionalized by organometallic n-butyl lithium, andthen covalently attached to a chlorinated polypropylenevia a coupling reaction. The main limitation of the “graftingto” method is that the grafted polymer content is quite lowdue to the relatively small fraction of active sites on theCNT, and the depressing effects of steric hinderence in thereactivity of polymer [123].

In the “grafting from” approach, the polymer isbound to the CNT surface by in-situ polymerization ofmonomers in presence of reactive CNTs or CNT sup-ported initiators. The main advantage of this approachis that the polymer-CNTs composites can be preparedwith high grafting density. This approach has beenused successfully to graft many polymers such aspolyamide 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(4-vinylpyridine) [134], and poly(N-vinylcarbazole) [135] onCNT via radical, cationic, anionic, ring-opening, and con-
densation polymerizations. Gao et al. [136] describedthe functionalization of MWNTs with a hydrophilic poly-mer, glycerol monomethacrylate (GMA) by the “graftingfrom” approach. In this work, the oxidized MWNTswere treated with thionyl chloride, glycol, and 2-
). Reprinted with permission from Ref. [95]. Copyright 2005, Wiley-VCH,

bromo-2-methylpropionyl bromide to produce MWNT-Brmacroinitiators for the atom transfer radical polymeriza-tion of GMA as shown in Scheme 1. The grafted polymercontent can be controlled by the feed ratio of monomerto macroinitiators. The hydroxyl groups of the polyGMAchains grafted on the MWNTs are highly active and can befurther converted to carboxylic acid groups. MWNTs werecovalently functionalized with poly(L-lysine) by a surfaceinitiated ring-opening polymerization method [137]. Zenget al. [138] reported “grafting from” approach based onin-situ ring-opening polymerization of �-caprolactone tocovalently graft biodegradable poly(�-caprolactone) ontoCNT surfaces. CNT-graft-poly(L-lactide) by using surface-initiated ring-opening polymerization has been studied byChen et al. [139].

A few additional techniques for functionalization ofCNTs into polymer matrixes have been utilized. Yan et al.[140] used Ar plasma for the generation of defect sitesin the SWNT caps and sidewalls, with subsequent UV-grafting of 1-vinylimidazole from the defect sites. Yang etal. [141] obtained soluble MWNTs via amidation reactionof octadecylamine with purified MWNTs when they weremixed 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] dipo-lar cycloaddition reaction [142], is an ideal reaction formaterial synthesis and modification and for self assembly
of nanomaterials. A Cu (I)-catalyzed Huisgen [3 + 2] dipo-lar cycloaddition reaction between terminal alkynes andazides resulting in the formation of 1,2,3-triazoles has beenutilized elegantly in recent years [143,144]. This reaction isvery useful for synthesizing small molecules [145], den-


S r radica2

dbcabtgeaaAoypbaesS

apii[oo

Sf2

cheme 1. Functionalization of MWNTs with PolyGMA by atom transfe005, American Chemical Society, USA.

rimers [146,147], dendronized polymers [148–150], andiologically derived macromolecular structures [151]. Clickhemistry benefits by the facile introduction of azide andlkyne groups into organic and polymer molecules, the sta-ility of these groups to many reaction conditions, and theolerance of the reaction in presence of other functionalroups. The main advantages of click chemistry are its tol-ration of other functional groups, a short reaction time,nd high yield, high purity, and regiospecificity, as wells its suitability for use under aqueous conditions [152].zides and acetylenes are stable across a broad range ofrganic reaction conditions and in biological environments,et they are highly energetic functional groups and theroducts are screened directly from the reaction mixtureecause no protecting groups are used. The triazole is useds a rigid linker that can mimic the atom placement andlectronic properties of a peptide bond without the sameusceptibility to hydrolytic cleavage [153,154], as shown incheme 2.

Apart from the synthetic promise, triazole moietiesre relatively stable to metabolic degradation and canarticipate in hydrogen bonding, which may be useful

n the context of bimolecular targets and solubility, and
s extremely stable to hydrolysis, oxidation or reduction144]. Click chemistry may be an ideal modular method-logy for the introduction of a wide variety of moleculesnto the surface of CNTs [155–157] as shown in Fig. 4. By
cheme 2. The coupling of azides and alkynes created triazole linkages in abseeathers with nature’s ubiquitous amide connectors. Reprinted with permission f004, American Chemical Society, USA.

l polymerization. Reprinted with permission from Ref. [136]. Copyright

applying this approach, one can easily functionalize CNTswith desired molecules, which enhance their importancefrom nanoelectronics to nanobiotechnology.

An application of the Huisgen cycloaddition to the func-tionalization of SWNTs with PS was reported by Adronovand co-workers [155]. To achieve a high degree of function-alization by using alkyne groups on the nanotube surfacewith the Pschorr-type arylation, subsequent introductionof PS was achieved by first installing azide functionalityat the polymer chain end. The Cu(I)-catalyzed formationof 1,2,3-triazoles by coupling azide-terminated polymerand alkyne-functionalized SWNTs was found to occur inan efficient manner under a variety of favorable conditions.This resulted in relatively high nanotube graft densities, fullcontrol over polymer molecular weight, and good solubil-ity in organic solvents. The grafted PS was further modifiedvia sulfonation and the sulfonated PS grafted CNTs werehighly soluble in aqueous medium and insoluble in organicmedium [158].

Cho and co-workers [156] focused on the covalentattachment of bioactive molecules with SWNTs (Scheme 3).The functionalization of the SWNTs was achieved by cova-lently bonded organic molecules derived from amino acids
through click chemistry. The alkyne-functionalized SWNTswere prepared by the treatment of p-amino propargylether with SWNTs using a solvent free diazotization pro-cedure to produce alkyne functionalized CNTs. In order
nce and presence of Cu(I) catalyst, the useful topological and electronicrom Ref. [153,154]. Copyright 2002, Wiley-VCH, Germany and Copyright


for fun
Fig. 4. Typical scheme of click chemistry
to attach different azides derived from amino acids, aseries of well-defined chiral azides from corresponding�-amino acids were prepared. The alkyne functionalizedSWNTs in dimethylformamide (DMF)-pyridine-butanolsolution were treated with azide-functionalized aminoacid compound, followed by CuI, ascorbic acid and N,N′-diisopropylethylamine. The 1,2,3-triazole ring has beenutilized as a linker between the chiral molecules andSWNTs, which can provide a new strategy for the attach-ment of bioactive molecules like peptides, polysaccharidesand others [156]. They also functionalized SWNTs bythe covalent attachment of azide moiety containingpolyurethane (PU) with alkyne functionalized SWNTs usingclick chemistry approach. The CNTs were functionalizedwith PCL-based PU using click chemistry, followed by thereaction of alkyne-decorated CNTs and azide-moiety con-taining PCL-diol [159].

Hybrid materials based on CNTs and metal nanopar-ticles are under consideration for major roles for severalapplications, including catalytic, optical, electronic andmagnetic applications [160–162]. Gao and co-workers[162] reported magnetic nanohybrids from magneticnanoparticles and polymer coated nanomaterials via aCu(I)-catalyzed azide alkyne cycloaddition reaction. Thenanohybrids were prepared from Fe3O4 nanoparticlesand polymer coated MWNTs. The authors prepared con-
trolled size Fe3O4 nanoparticles then functionalized withazide moieties (Fe3O4-N3) and finally with alkyne moieties(Fe3O4-Alk). MWNTs were separately modified with poly-mer containing abundant azide groups (MWNT-pAz) andpolymer containing abundant of alkyne groups (MWNT-
Scheme 3. Scheme of application of click chemistry for synthesis of bioactive moHCl, pH 3, (ii) SOCl2, MeOH, RT, 8 h (iii) NaBH4 (excess), MeOH, RT over night. (b)dioxane, 80 ◦C, 16 h, 78–81% yields in two steps. (c) CuI (0.15 M), ascorbic acid (0.080 ◦C for 4 h [156].

ctionalization of carbon nanotube [156].

pAlk). (Fe3O4-N3) and (Fe3O4-Alk) were coupled withpolymer coating nanotubes to give magnetic nanohy-brids of MWNT-pAz@Fe3O4 and MWNT-pAlk@Fe3O4,respectively. Fig. 5 presents the TEM images of bothMWNT-pAz and MWNT-pAlk, evenly decorated withmagnetic nanoparticles. Cho and co-workers [163] pre-pared gold nanoparticles functionalized SWNTs using aclick chemistry approach. Gold nanoparticles containingoctanethiol were prepared by the reduction of tetra-chloroauric acid using sodium borohydride in presence ofalkanethiol. The alkyl thiol protected gold nanoparticleswere further treated with azidoundecane thiol to yieldazide moiety containing gold nanoparticles, which werereacted with alkyne functionalized SWNTs. Campidelli etal. [157] decorated SWNTs with phthalocyanine using aclick coupling approach. The authors functionalized SWNTswith 4-(2-trimethylsilyl)ethynylaniline in the presence ofisoamyl nitrite, which were then treated with azide moi-ety containing Zinc-phthalocyanine (ZnPc) in the presenceof CuSO4 and sodium ascorbate to give the nanotube-phthalocyanine hybrid. They also studied the photovoltaicproperties of synthesized materials and observed that thephotocurrent of SWNT-ZnPc was higher and more stableand reproducible than that of pristine SWNTs.

Layer-by-layer (LbL) covalent functionalization ofMWNTs was achieved by Gao and co-workers [164].
The clickable polymer poly(2-azidoethyl methacry-late) was synthesized by atom transfer radicalpolymerization (ATRP) and another clickable poly-mer poly(propargyl methacrylate) was synthesized byreverse addition–fragmentation chain transfer (RAFT)
lecule bonded SWNT. (a) (i) CbzCl, 2 M NaOH solution, 0 ◦C, 2 h, then 2 M(i) TsCl (1.2 equiv.), Et3N (1.3 equiv.), 0 ◦C to RT 4 h (ii) NaN3, DMSO:1,4-8 M) and DIPEA (0.17 M) in DMF-pyridene-butanol, RT, 14 h, followed by


F NT-pAzR

pocPpmmvcacrmMapaoTf[w

ig. 5. TEM images of (a) Fe3O4-N3, (b and c) MWNT-pAlk@Fe3O4, (d) MWoyal Society of Chemistry, UK.

olymerization. Both polymers were alternately coatedn alkyne-modified MWNTs using the reliable Cu(I)-atalyzed Huisgen 1,3-dipolar cycloaddition reaction.oly(2-azidoethyl methacrylate) was clicked on thereprepared alkyne-modified MWNTs as the first poly-er layer. Alkyne side groups containing poly(propargylethacrylate) were coated as the second polymer layer

ia click coupling. Poly(2-azidoethyl methacrylate) waslicked on the second polymer layer coated MWNTs,nd the residual clickable azido groups of third layeroated MWNTs were further clicked with alkyne-modifiedhodamine B and monoalkyne-terminated PS. The postodification supports the usefulness of functionalizedWNTs as a nanoplatform for further molecular design

nd material synthesis. Due to formation of a crosslinkedolymer network, the covalent linkage offers severaldvantages, such as high stability and good control
ver the quantity and thickness of the polymeric layers.he authors also reported a clickable macroinitiatoror building the amphiphillic polymer brushes on CNTs165]. The azido and bromo groups functionalized CNTsere prepared by the reaction of poly(3-azido-2-(2-
@Fe3O4. Reprinted with permission from Ref. [162]. Copyright 2009, The

bromo-2-methylpropanoyloxy)propyl methacrylate withalkynated CNTs. Both the ATRP and click coupling couldbe achieved by a one pot procedure using bromo andazido moieties as initiators for PS and PEG grafting (Fig. 6).The reaction could be easily accomplished with SWNTsand MWNTs using “grafting to” and “grafting from”approaches for functionalization with multiple kinds ofpolymers.

Functionalization of CNTs with stimuli-responsivematerials is expected to be useful for manufactur-ing advanced biosensors and bioprobes [166]. Li andco-workers [167] introduced an alkyne functionalized nan-otube surface using a carbamate linkage. The azide moietycontaining a thermoresponsive diblock copolymer com-posed of N,N-dimethylacrylamide (DMA) and NIPAM wascovalently attached with alkynated MWNTs via the Cu(I)-catalyzed [3 + 2] Huisgen cycloaddition. The copolymer
containing hydrophilic DMA, as well as a smart NIPAMblock, is capable of forming micelles with response tochanges in the aqueous solution temperature. The micellessize and transition temperature can be controlled throughNIPAM block length. Due to the higher azide concentra-


ed and s. Reprin

Fig. 6. Synthesis of amphiphilic/Janus polymer brushes-grafted multiwallclick chemistry and atom transfer radical polymerization (ATRP) approachSociety, USA.

tion on their periphery, micelles afford improved graftingefficiency and solubility of nanotubes, compared to coils insolution.

The oligosaccharide ˇ-cyclodextrin is well knownto encapsulate biological molecules in its hydrophobiccavities in an aqueous solution, enhancing its utility asa drug carrier and enzyme mimic. Zheng et al. reporteda ˇ-cyclodextrin-modified SWNT nanohybrid through
Huisgen cycloaddition [168]. Mono-6-(p-toluenesulfonyl)-ˇ-cyclodextrin, prepared by reaction of ˇ-cyclodextrinwith p-toluenesulfonyl chloride, was treated withsodium azide to convert it to an azide-functionalizedcyclodextrin. Purified SWNTs were reacted with p-(2-
ingle-walled carbon nanotubes (MWNTs and SWNTs) by a combination ofted with permission from Ref. [165]. Copyright 2008, American Chemical

propynyloxy)-benzenamine in o-dichlorobenzene (ODCB)using a diazotization coupling procedure to producealkyne-functionalized SWNTs. The azide-functionalizedcyclodextrin was further reacted with alkynated SWNTsvia click coupling. The ˇ-cyclodextrin functionalizedSWNTs show good solubility in water, enhancingtheir biological importance for drug delivery appli-cations.

Though there are not so many examples of employ-ing the click chemistry for CNT functionalization, relatedpapers are increasingly reported recently. Scheme 4 showsthe attachment of different functionalities on CNTs via clickcoupling. The wide range of attached molecules enhances


nt funct

to

3n

ioCnpmmi

3

vcotpeacteet

Scheme 4. The attachment of differe

he importance of click chemistry and opens the new vistaf CNTs based nanomaterials.

. Preparation methods of polymer/CNTanocomposites

As emphasized in the preceding, the dispersion of CNTsn polymer matrices is a critical issue in the preparationf CNT/polymer composites. Better reinforcing effects ofNTs in polymer composites will be achieved if they doot form aggregates and as such, they must be well dis-ersed in polymer matrixes. Currently there are severalethods used to improve the dispersion of CNTs in poly-er matrices such as solution mixing, melt blending, and

n-situ polymerization method.

.1. Solution mixing

In this approach, a dispersion of CNTs in a suitable sol-ent and polymers are mixed in solution. The CNT/polymeromposite is formed by precipitation or by evaporationf the solvent. It is well known that it is very difficulto properly disperse pristine CNTs in a solvent by sim-le stirring. A high power ultrasonication process is moreffective in forming a dispersion of CNTs. Ultrasonic irradi-tion has been extensively used in dispersion, emulsifying,
rushing, and activating the particles. By taking advan-age of the multi-effects of ultrasound, the aggregates andntanglements of CNTs can be effectively broken down. Forxample, Li et al. [169] used a simple solution–precipitationechnique to improve the dispersion of CNTs in a poly-
ionalities on CNTs via click coupling.

carbonate solution by sonication at a frequency of 20 kHzfor 10 min. They showed that the CNTs were uniformlydispersed in polycarbonate matrix on its consolidation. Inthis case, ultrasonic wave as well as mechanically stirringplayed important roles in the formation of the compositeswith a uniform particle size. The chemical effects of ultra-sound are associated with the rapid (microsecond timescale), violent collapse of cavitation bubbles created asthe ultrasonic waves pass through a liquid medium [170].Sonochemical theory and the corresponding studies sug-gested that ultrasonic cavitation can generate a high localtemperature 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 anddismembrator at 300 W for 30 min. Uniform dispersions ofCNTs in PS were achieved by using sonication. Recently Choand co-workers successfully prepared PU/MWNTs com-posites with better dispersion of CNTs up to 20 wt% in PU[173]. In the research, the necessary weight fractions ofcarboxylate MWNTs were first dispersed in DMF solutionunder sonication at room temperature for 1 h using a highpower ultrasonic processor. Thereafter, PU was added intothis solution and stirred for 1 h. The mixtures were thensonicated again for 1 h. The SEM photographs of the cross-sectional fracture of composites of the achieved dispersionfor the investigated MWNTs are shown in Fig. 7. The homo-
geneous dispersion in the composites was achieved by theaddition of a higher amount of MWNT-COOH (20 wt%).Using a proper surfactant is another efficient method todisperse higher loading of nanotubes [70,174,175]. Theuse of non-ionic surfactants such as polyoxyethylene-8-


NT and
Fig. 7. SEM micrographs of PU/CNT composites: (a) 10 wt% pristine MWCopyright 2006, Wiley-VCH, Germany.
lauryl has been demonstrated to improve the dispersionand strong interaction between MWNTs and epoxy resins[42].

Barrau et al. [176] found that amphiphilic palmitic acidfacilitate an efficient dispersion of CNTs in an epoxy matrix.The hydrophobic part of palmitic acid is absorbed ontothe nanotube surface, whereas the hydrophilic head groupinduces electrostatic repulsions between nanotubes, pre-venting their aggregation. The co-solvent also affects thedispersion of nanotubes in polymer matrix. Recently, Cam-poneschi et al. [177] reported on the use of trifluoroaceticacid as a co-solvent for the dispersion of MWNTs in aconjugated polymer poly(3-hexylthiophene), and PMMAthrough a solution process. Scanning electron microscopy(SEM), optical microscopy, and light transmittance studiesrevealed that the better dispersion of CNTs in poly-mer 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(vinylalcohol)/CNT [184], P(MMA-co-EMA)/CNT [141], polyacry-lonitrile/CNT [185], and polyethylene(PE)/CNT [186] havebeen fabricated by this method.

3.2. Melt mixing

For solution mixing, the matrix polymer must be sol-uble in at least one solvent. This is problematic for manypolymers. Melt mixing is a common and simple method,particularly useful for thermoplastic polymers. In melt pro-cessing, CNTs are mechanically dispersed into a polymermatrix using a high temperature and high shear forcemixer or compounder [187]. This approach is simple andcompatible with current industrial practices. The shearforces help to break nanotube aggregates or prevent theirformation. Zhang et al. [188] prepared nylon-6/MWNTscomposites containing 1 wt% MWNTs via a melt com-pounding method using a Brabender twin-screw mixer.
SEM image showed a homogeneous dispersion of MWNTsachieved through the matrix polymer, associated withsignificant enhancements in mechanical properties. Boc-chini et al. [189] fabricated MWNTs/linear low densitypolyethylene (LLDPE) nanocomposites via melt-blending
(b) 20 wt% MWNT-COOH. Reprinted with permission from Ref. [173].

using a Brabender Plasticorder internal mixer. MWNTsdispersed in LLDPE delay thermal and oxidative degra-dation with respect to that for virgin LLDPE. PE/MWNTnanocomposites were prepared using twin screw meltcompounding [190]. Microscopic observations across thelength scales and X-ray diffraction measurements indicatethat the MWNTs are very well distributed and dispersed inthe PE matrix. Melt mixing has been successfully appliedfor the preparation of different polymers-CNT compos-ites such as polypropylene/CNT [191–193], high densityPE/CNT [194], polycarbonate/CNT [195–197], PMMA/CNT[198–201], polyoxymethylene/CNT [202], polyimide/CNT[203], PA6/CNT [204], etc. The disadvantage of this methodis that the dispersion of CNTs in a polymer matrix is quitepoor compared to the dispersion that may be achievedthrough solution mixing. In addition, the CNTs must belower due to the high viscosities of the composites at higherloading of CNTs.

3.3. In-situ polymerization

In this polymerization method, the CNTs are dispersedin monomer followed by polymerization. A higher percent-age of CNTs may be easily dispersed in this method, andform a strong interaction with the polymer matrixes. Thismethod is useful for the preparation of composites withpolymers that can not be processed by solution or meltmixing, e.g., insoluble and thermally unstable polymers.Hu et al. [205] synthesized MWNT-reinforced polyimidenanocomposites by in-situ polymerization of monomers inthe presence of acylated MWNTs, as shown in Scheme 5. Inthis work, MWNTs were functionalized with acyl groups,and then reacted with 3,3′,4,4′-biphenyltetracarboxylicdianhydride to form MWNT-poly(amic acid). The finalMWNT-polyimide nanocomposite films were obtainedby imidization of MWNT-poly-(amic acid) at 350 ◦C for1 h under vacuum. In this method, the CNTs were uni-
formly dispersed in polymer matrix. Recently, Cho etal. fabricated MWNTs-reinforced polyimide nanocompos-ites by in-situ polymerization using 4,4′-oxydianilline,MWNT-COOH, and pyromellitic dianhydride followed bycasting, evaporation, and thermal imidization [206]. A


SpW

himfJp2bsOti

ba[aottcBon

FE

cheme 5. Outline of the preparation of MWNT-polyimide nanocom-osite films. Reprinted with permission from Ref. [205]. Copyright 2006,iley-VCH, Germany.

omogeneous dispersion of MWNT-COOH was achievedn polyimide matrix as evidenced by scanning electron

icroscopy (Fig. 8). This method has been widely usedor the preparation of PMMA-CNT composites [207–210].ia et al. [207] first synthesized PMMA by in-situ radicalolymerization method. They used free radical initiator of,21-azobisisobutyronitrile, AIBN, to initiate open CNTs �-onds to participate in PMMA polymerization, forming atrong interface between the CNTs and the PMMA matrix.ther researchers used similar free radical initiator AIBN

o prepare SWNT-PMMA composites by in-situ polymer-zation [127,211].

Conducting polymers are attached to CNTs surfacesy in-situ polymerization to improve the processability,nd electrical, magnetic and optical properties of CNTs212–215]. Cho and co-workers [216] described a simplepproach to the synthesis of MWNT/polypyrrole (PPy) nan-tubes by the in-situ chemical polymerization of pyrrole onhe CNTs using ferric chloride as an oxidant. They inves-igated the effect of the monomer concentration on the
oating and properties of the resulting complex nanotubes.y changing the pyrrole/MWNT ratio, the layer thicknessf PPy could be easily controlled in MWNT-PPy complexanotubes, as shown in Fig. 9. Long et al. [213] synthe-
ig. 9. TEM photographs of PPy-coated MWNTs: (a) MWNT:PPy = 1:2; (b) MWNlsevier Science Ltd., UK.

Fig. 8. SEM images of PI/MWNTs nanocomposites: PI/MWNT-COOH5 wt%. Reprinted with permission from Ref. [206]. Copyright 2007, ElsevierScience Ltd., UK.

sized the CNT/PPy nanocables through an in-situ chemicaloxidative polymerization method. They showed that theconductivity of nanocables increased with increasing nan-otube weight percentage.

In-situ polymerization method has also been usedfor the preparation of polyurethane/CNT nanocomposites.PU/MWNT composites were synthesized by two in-situpolymerization methods [217]. In one method, a calculatedamount of carboxylate MWNT and 1,4-butanediol (BD) wasadded to a prepolyurethane solution in the subsequentchain-extension step. In the second method, the neces-sary weight fractions of MWNTs were first dispersed inpoly(�-caprolactone)diol (PCL). Thereafter, 4,4′-methylenebis(phenylisocyanate) (MDI) was added into this mixture.The chain extender BD was added to this prepolymer
and the final PU-MWNTs composite was synthesized. TheMWNTs were relatively well dispersed in the PU matrixof the PU-MWNT sample in second method. Xia and Song[218] found that MWNTs could be individually dispersed in
T:PPy = 1:5. Reprinted with permission from Ref. [216]. Copyright 2007,


n (firstf. [220]

Fig. 10. FT-IR spectra of samples obtained (a) during prepolymerizatiofunctionalized MWNTs (second step). Reprinted with permission from Re

the PU matrix by in-situ polymerization method with theaid of a dispersing agent. But SWNTs were not dispersedwell in this method. Later they synthesized PU/SWNTsnanocomposites using PU-grafted SWNT which improvedthe dispersion of SWNT in the PU matrix and strengthenedthe interfacial interaction between the PU and SWNT [219].

A novel concept has been proposed to preparePU-MWNTs composite via in-situ polymerization of a pre-polymer in the presence of carboxylated MWNTs [220].Synthesis of PU/MWNT nanocomposites was carried outin a two-step process as follows. First, prepolymer wasprepared from a reaction of MDI and PCL at 80 ◦C for90 min in a four-neck cylindrical vessel equipped with amechanical stirrer. In the second stage, a calculated amountof carboxilated MWNTs was added to the prepolymer at110 ◦C, and they were reacted for 150 min to obtain thefinal crosslinked MWNT-PU nanocomposite. In this case,any chain extender was not used at the second stage reac-tion. PU chains were crosslinked to MWNTs by a reactionbetween the carboxylic acid groups of the MWNTs andthe NCO groups of prepolyurethane (Fig. 10). These PU-crosslinked MWNTs composites were never dissolved in PUsolvents such as N,N-dimethylformamide, dimethyl sulfox-ide, dimethylacetamide, or tetrahydrofuran.

The in-situ polymerization of caprolactam in the pres-ence of SWNTs allowed the continuous spinning ofSWNTs-PA6 fibers [221]. In addition, caprolactam is anexcellent solvent for carboxylic acid-functionalized SWNTs(SWNTs-COOH). This allows the efficient dispersion of theSWNTs and subsequent grafting of PA6.

4. Preparation of CNT nanocomposites usingdendritic polymers

Due to their three-dimensional globular and sphere-like structural architectures, dendritic polymers (DP) such
as dendrimeric and hyperbranched polymers, have gen-erated great excitement in polymer research, owing totheir wide range of applications from drug delivery tochemical sensors [222–224]. Dendrimers have unique size,controlled and symmetric structure with ideally branch-
step) with reaction time and (b) during reaction of prepolymer with. Copyright 2006, Wiley-VCH, Germany.

ing units without any structural defects [225,226], butrequire a multi-step synthesis reaction, whereas the Hyper-branched polymers exhibit a randomly branched structure,with a single step synthesis process [227,228]. Recently,dendritic polymers have been used to enhance the dis-persion of CNTs in polymer matrices, taking advantageof their highly functionalized three-dimensional globular,non-entangled structures. They exhibit higher solubilityand lower viscosity in the melt and solution states com-pared to linear polymers of the same molar mass [229,230].Dendritic polymer/CNT nanocomposites may be formed viacovalent and non-covalent functionalization of CNTs. Den-dritic polymers are effective for enhancing mechanical andelectrical properties of polymer nanocomposites becausethe pristine CNTs can be used to obtain well dispersed CNTsin polymer matrix, without any CNT modification.

4.1. CNT nanocomposites via covalently functionalizedCNT-dendritic polymers

Grafting of dendritic polymers on CNTs is a novelapproach for fabricating the nanomaterials and nan-odevices [231–234]. Newkome and co-workers [235]fabricated unique CdS quantum dot composite assembliesusing dendronized SWNTs. Acyl chloride functionalizedMWNTs were treated with amino-polyester dendron toprepare [(Den)n-SWNT]. The ester groups were furtherchanged into carboxylic groups using formic acid, andreacted with Cd(NO3)2 to generate encapsulated CdS quan-tum dots tethered to the SWNT surface. An electrodematerial based on hyperbranched polymer-functionalizedMWNTs for lithium batteries showed good reversiblecapacities and excellent capacity retention [236]. Sun andco-workers [237], prepared the dendron functionalizedCNTs via amidation and esterification and their photo-physical properties were studied. Prato and co-workers
[238] reported the synthesis of SWNTs functionalized withpolyamidoamine dendrimers. The dendrimers present onthe nanotube sidewalls have been further functionalizedwith porphyrin moieties, and the photophysical propertiesof nanoconjugates have studied. Under visible light irra-


S nohybriC

dcpw(dpMaPwdmdp[twmTciuhr

ibcmpicihwMa

cheme 6. Synthetic process for the MWNT-hyperbranched polyether nahemical Society, USA.

iation, porphyrin-SWNT nanoconjugate give rise to fastharge separation (1.5 ± 0.5) × 1010 s−1 evolving from thehotoexcited porphin chromophores. Haddleton and co-orkers [239] described a poly(amidoamine) dendrimer

PAMAM) functionalized MWNTs via ester linkage. Theendron functionalized MWNTs were used as a tem-late for the deposition of silver nanoparticles on theWNT surface. Park and co-workers [240] studied the

ntimicrobial effects of silver nanoparticles functionalizedAMAM-MWNTs nanohybrids. The dendritic-MWNTs/Agas found to have a stronger antimicrobial effect thanendritic-MWNTs. The role of the architecture of dendriticolecules on CNT photoelectrical behavior has also been

iscussed [241]. These super-structured CNTs exhibit aeculiar response to electron beams. Gao and co-workers233] prepared multihydroxyl hyperbranched polymer onhe surface of MWNTs. In situ ring-opening polymerizationas employed for growing multihydroxyl dendritic macro-olecules on the surface of MWNTs, as shown in Scheme 6.

ree like multihydroxy hyperbranched polyether wereovalently grafted on the MWNTs using MWNTs-OH as annitiator. The amount of polymer grafted is controllablesing this approach. The molecular weight of the graftedyperbranched polymer increases with increasing a feedatio of the monomer.

Biocompatible polymers on CNTs are arousingnterest due to the great significance of the nanohy-rids in bionanotechnology [242,243]. Mueller ando-workers [244] prepared hyperbranched glycopoly-ers functionalized MWNTs by atom transfer radical

olymerization of 3-O-methacryloyl-1,2:5,6-di-O-sopropylidene-D-glucofuranose (MAIG) and selfondensing vinyl copolymerization of MAIG and AB*
nimer, 2-(2-bromoisobutyryloxy)ethyl methacrylate. Theyperbranched glycopolymers are biocompatible andater soluble, and the resulting polymer-functionalizedWNTs would be very useful for bionanotechnology
pplications. Hyperbranched poly(citric acid) (PCA)

ds. Reprinted with permission from Ref. [233]. Copyright 2004, American

grafted MWNTs based nanocomposites were synthesizedby Hekmatara and co-workers [245]. The CNT-g-PCAnanocomposites were soluble in water freely. Hong et al.[246] reported the coating of MWNTs with hyperbranchedpolymer shell by self-condensing vinyl polymerization(SCVP) of 2-((bromobutyryl)-oxy) ethyl acrylate viaATRP. The synthesized hyperbranched polymers havea large number of functional groups facilitating furtherfunctionalization of MWNTs and providing a method tohomogeneously disperse MWNT conducting layers withinelectroluminescent devices.

Core–shell nanostructures were prepared by Xie andco-workers [234,247] with MWNTs and hyperbranchedpoly(urea-urethane)s (HPU) as the hard core and the softshell, respectively The authors have synthesized HPU-functionalized MWNTs by a polycondensation method;the solution rheology of HPU functionalized MWNTs werestudied. A large number of proton-donor and proton-acceptor groups were located in the HPU functionalizedMWNT; intra- and intermolecular H-bonds were easilyformed by their interactions. At low temperature, shearingforces induce conversion from intra- to intermolecular H-bonds. The rheological behavior of the HPU-functionalizedMWNT solutions showed a strong dependence on concen-tration, temperature, and thermal and shearing prehistory.Baek and co-workers [248,249] prepared hyperbranchedpoly(ether ketone)s (PEK) grafted MWNTs using an insitu polymerization method. Hyperbranched ether–ketonepolymer containing different monomer ratios were synthe-sized in the presence of MWNTs to afford PEK-g-MWNTsnanocomposites. Due to the molecular architecture ofhyperbranched polymers, the morphology of the nanocom-posites resembles mushroom-like clusters on MWNTs
stalks [250]. The resultant nanocomposites were soluble inmost strong acids, such as trifluoroacetic acid, methanesul-fonic acid and sulfuric acid. UV curable functional groupscontaining hyperbranched polymers were synthesized formodification of CNTs [251]. The hyperbranched polyester


rethane
Fig. 11. Current AFM images of linear (a) and hyperbranched (b) polyu20 wt% [254].
functionalized MWNTs were reacted with difunctionalmolecules synthesized from toluene 2,4-diisocyanate andhydroxylethyl acrylate to get UV curable hyperbranchedpolymer. The modified MWNTs containing large amountof UV-curable acrylate group were dispersed with UV cur-able aliphatic urethane acrylate resins. In the presence ofUV irradiation, crosslinking reaction developed betweenMWNTs and acrylate resins, leading to the covalent bond-ing of MWNTs to the matrix, a more stable coupling thansimply physical bonding. Due to the excellent mechanicalproperties of MWNTs, both the tensile strength and tough-ness of the nanocomposites were enhanced by nearly 41%and 105%, respectively, with only 0.1 wt% MWNTs.

4.2. CNT nanocomposites via non-covalentlyfunctionalized CNT-dendritic polymers

The disruption of electronic conjugation by covalentmodification motivates research on non-covalent func-tionalization of CNTs. In fact, excellent modification ofCNTs through non-covalent interaction between the �-system of CNT and the functional group of polymer canbe achieved. Star and Stoddart [252] studied the disper-sion and solubilization of SWNTs with a hyperbranchedpolymer (poly[(m-phenylenevinylene)-co-(2,5-dioctoxy-p-phenylene) vinylene] (PmPV). The hyperbranched poly-mer exhibits more efficiency at dispersing nanotubebundles than its parent PmPV polymer. The branchingof PmPV makes it less efficient for wrapping bundles ofSWNTs. They studied the molecular modeling of hyper-branched polymer and found that the pockets providedby the hyperbranched polymer offer a better fit for theSWNTs. Cho and co-workers [253] also prepared the hyper-branched polyurethane (HBPU)-MWNTs nanocompositesusing an in situ polymerization method. They prepareddifferent hard segment containing PU nanocomposites viaa two step process. Novel dispersion of MWNTs in the
HBPU matrix was observed, as well as good solubilityof nanocomposites in organic solvents. Because of thegood dispersion of MWNTs in the polymer matrix, the PUnanocomposites showed dominant shape recovery prop-erties of 84–96%, and enhanced mechanical properties.
nanocomposites containing pristine multi-walled carbon nanotubes of

They also studied the dispersion of high concentrationMWNTs (5–40 wt%) using an HBPU matrix [254]. TheMWNTs dispersion was analyzed on the basis of surfacemorphology using current AFM images (Fig. 11), includingdispersions of 20 wt% MWNT in linear, as well as hyper-branched polymer matrix. The aggregated MWNTs wereobserved in linear PU/MWNTs nanocomposites, whereaswell dispersed MWNTs were observed for HBPU/MWNTsnanocomposites. Compared to linear PU nanocomposites,the hyperbranched nanocomposites give remarkable highelectrical conductivity.

Nakamoto and co-workers [255] presented the con-struction of insulating conducting wire composed ofSWNTs and phenolic polymers as a conducting wireand as an insulating coating, respectively. A hyper-branched phenolic polymer (HBP) was synthesized from3,4,5-trimethoxytoluene (A2) and 1,3,5-tribromomethyl-2,4,6-trimethoxybenzene (B3) using a Lewis acid-catalyzedpolycondensation method. Fig. 12 shows the TEM and AFMimages for the samples. The images show the coveringof the SWNT surface by HBPs, which indicates solubiliza-tion of SWNTs with physical adsorption of HBPs. In thepresence of HBP polymer, SWNTs were homogeneouslydispersed in DMF solution, in contrast to the use of lin-ear polymer, where SWNTs were insoluble in DMF, evenafter sonication. The authors also studied the impact of sol-vent on the solubility of SWNTs with HBP. The SWNTs weresparsely soluble in THF solution with HBP, whereas typi-cally SWNTs van Hove singularities were found with HBP inDMF. A non-covalent method for preparing the crosslinkedamphiphillic hyperbranch polymer micelle-encapsulatedCNTs was proposed to improve the dispersion of CNTs inwater [256]. Fig. 13 shows the TEM images of uncrosslinkedand crosslinked encapsulated CNTs. Zheng and co-workers[257] reported an MWNTs based solvent-free nanofluids atroom temperature using hyperbranched poly(amine-ester)(HPAE). Acid functionality containing MWNTs were homo-
geneously dispersed in HPAE matrix. The ternary amineof HPAE were protonated with the carboxylic groups ofMWNTs via an acid–base reaction, which leads to ionicattachment on the surface of MWNTs as well as hydro-gen bonding interaction between the –OH, C O groups


F BP. (b)c ) heightR

ahapfScSdmnnndeaid

obpeeptlis

ig. 12. (a) TM-AFM height image of SWNTs solubilized by extended Homposed of HBP and SWNTs on mica surface. The extended TM-AFM (def. [255]. Copyright 2008, Elsevier Science Ltd., UK.

nd the ternary amine groups to form a homogeneousybrid system. The preparation of fluorinated oligomericggregates/CNTs nanocomposites possessing a higher dis-ersibility and stability in water is of particular interestor developing new fluorinated functional materials [258].awada et al. [259] synthesized a dendrimer copolymerontaining fluoroalkyl segments [260] for the dispersion ofWNTs in water. They demonstrated that the fluorinatedendrimer block copolymers could form new fluorinatedolecular aggregates to have higher dispersion ability,

ot only for SWNTs and fullerenes, but also for mag-etic nanoparticles in water. Valentini et al. [261] reportedaphthalenediimide modified electrically conducting den-rimer poly(amidoamine)/SWNTs nanocomposites. Thelectric conductance of SWNTs drastically increased upondsorption of the conducting dendrimer, owing to thenteraction between SWNTs and the conducting den-rimer.

The effective immobilization of glucose oxidase (GOx)n the CNT surface is a key issue for developing a glucose-ased biosensor [262–264]. Zhu and co-workers [265]repared the glucose biosensor based on layer-by-layerlectrostatic adsorption of glucose oxidase and dendrimer-ncapsulated Pt nanoparticles on MWNTs. LBL technique
rovide a favorable microenvironment to keep the bioac-ivity of glucose oxidase, and prevent enzyme moleculeeakage. The enzyme electrode showed good character-stics, such as short response time, high sensitivity, andtability. The (GOx/Pt-DENs)4/CNTs electrode showed bet-
Height profile along dash line in (a). (c) Proposed structure of hybridsand (e) phase images in box area in (a). Reprinted with permission from

ter chronoampermetric response than the unmodifiedCNTs electrode.

5. Mechanical properties of polymer/CNTnanocomposites

As remarked above, their extraordinary mechanicalproperties and large aspect ratio make CNTs excellent can-didates for the development of CNT-reinforced polymernanocomposites. Indeed, a wide range of polymer matrixeshave been used for the development of such nanocompos-ites. This section focuses on the mechanical properties ofcomposites of CNT composites in two polymer matrixeswell represented in the literature.

5.1. Polyurethane/CNT composites

Polyurethane is one of the most versatile materialstoday. It is widely used in coatings, adhesives, shapememory polymers, medical fields and composites. PU isconsisting of alternating hard and soft segments. Thehard segment is composed of alternating diisocyanate andchain-extender molecules (i.e., diol or diamine), whereasthe soft segment is formed from a linear, long-chain diol.
Phase separation occurs in PUs because of the thermo-dynamic incompatibility of the hard and soft segments.PU/CNT composites [266–275] are of significant currentinterest. The mechanical properties for different PU com-posites are summarized in Table 4.


and (C a
Fig. 13. TEM images of (A) empty micelles, (B) uncrosslinked e-MWNT,Copyright 2008, Elsevier Science Ltd., UK.
Incorporation of CNTs into PU can dramatically increasethe tensile strength and modulus. For example, the addition
of carboxylated MWNTs into the PU matrix by solvent mix-ing improved the tensile strength and modulus of the PUmatrix [173,276] as shown in Fig. 14. The tensile strengthof a composite containing 10 wt% of MWNT-COOH wasenhanced by 108% as compared to pure PU, while an
Table 4Mechanical properties of PU-CNT composites.

Nanotube type Preparation method % of modulus im1% CNT

SWNT Solution 25MWNT-functional Solution 140MWNT-functional Solution –SWNT-functional Electrospining 250MWNT Addition polymerization 561MWNT In-situ polymerization 35MWNT-functional In-situ polymerization 54MWN-functional Solution 12MWNT In-situ polymerization 90MWNT-functional In-situ polymerization 40

nd D) crosslinked e-MWNT. Reprinted with permission from Ref. [256].

increase of 68% was achieved by incorporating the sameamount of raw MWNTs in the PU matrix. Finally, the ten-
sile strength and modulus of nanocomposites increasedfrom 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
provement at % of tensile strengthimprovement at 1% CNT

Reference

50 [357]20 [162]63 [279]

104 [281]397 [280]114 [277]

25 [358]6 [274]

90 [359]7 [266]

N.G. Sahoo et al. / Progress in Polymer

FlW

M–bgcpotapmaw7iflfcteesp

siX

properties, flexibility, high glass transition temperature,

TM

ig. 14. Stress–strain profiles of PU composites at different MWNToading. Reprinted with permission from Ref. [173]. Copyright 2006,

iley-VCH, Germany.

WNTs were helpful in improving the interaction withCONH– groups in PU. Therefore, the strong interactionetween the functionalized MWNTs and the PU matrixreatly enhanced the dispersion as well as the interfa-ial adhesion, thus strengthening the overall mechanicalerformance of the composite. The mechanical propertiesf composites depends on the acid treatment tempera-ure of the CNTs. Composites containing MWNTs reactedt 90 ◦C resulted in a greater increased modulus com-ared with those at 140 ◦C, indicating that severe surfaceodification lowers mechanical properties [95]. Kuan et

l. [277] incorporated amino functionalized MWNTs intoaterborne PU. They found an increase in modulus from

7 MPa for the polymer to 131 MPa for a 4 phr compos-te (an increase of 70%) and a tensile strength increaserom 5.1 MPa to 18.9 MPa (an increase of 270%) at the sameoading level. Covalent bond formation between aminounctionalized MWNTs and PU promoted increased interfa-ial strength and tensile strength. MWNT is more effectiveo the improvement of modulus, whilst SWNT is better forlongation and tensile strength. The different reinforcingffects of MWNT and SWNT on PU were correlated to thehear thinning exponent and the shape factor of CNTs inolyol dispersion.

Polymer grafting is very effective in increasing disper-ion and the mechanical properties of composites due tots strong chemical bonding between polymer and CNTs.ia et al. [278] studied polycaprolactone-based PU-grafted

able 5echanical properties of PI-CNT composites.

Nanotube type Sample type CNT content (wt%) % of modulus

SWNT-functional Film 1 89MWNT-functional Film 5 33MWNT-functional Film 5 –MWNT-functional Film 0.5 110MWNT-functional Film 0.5 60MWNT-functional Film 6.98 61SWNT Film 1 10SWNT Rod 1 0SWNT Fiber 1 45MWNT-functional Film 7.5 52

Science 35 (2010) 837–867 855

SWNTs (SWNT-g-PU) and poly(propylene glycol)-graftedMWNTs into PU by in-situ polymerization. Mechanicalproperty improvements were observed in both cases. Theincorporation of 0.7 wt% SWNT-g-PU into PU improvedthe Young’s modulus by ∼278% and ∼188% compared tothe pure PU and ungrafted pristine SWNT/PU composites,respectively. This is due to the better dispersion of SWNT-g-PU and MWNT-g-PU and stronger interfacial interactionsbetween the CNTs and PU. Wang and Tseng [279] alsofound that adding 1–10 wt% PU functionalized MWNT toPU increased the tensile strength by 63–210%. The storagemodulus and soft segment Tg (from tan �) increased withincreasing PU-functionalized MWNT in the PU. The Tg ofthe soft segments of the nanocomposite films shifted from−20 to −5 ◦C, suggesting that PU functionalized MWNTsare compatible with the amorphous regions of the softsegments in the PU matrix. Recently McClory et al. [280]reported thermosetting PU-MWNTs nanocomposites by anaddition polymerization reaction. The Young’s modulusincreased by 97 and 561% on the addition of 0.1 wt% and1 wt% MWNTs in the PU, respectively, whereas ultimatetensile strength increased by 397% when either 0.1 or 1 wt%MWNTs added to PU. In this composite, the percentageof elongation-at-break increased from 83 to 302% on theaddition of 0.1 wt% CNT compared to pure PU resin.

Improvement of mechanical properties has beenreported for melt-processed composite fibers. Young’smodulus of composite fibers increased by 27 folds com-pared to unfilled PU fiber. Sen et al. [281] studied thefabrication of membranes of SWNT-filled PU by theelectrospinning technique. The tensile strength of ester-functionalized SWNT/PU membranes was enhanced by∼104%, and the tangent modulus improved by ∼250%compared to PU membrane. So, these enhancements inmechanical properties can be attributed to the high dis-persion of CNTs through the polymer matrix and goodinterfacial interaction between CNT and PU.

5.2. Polyimide/CNT nanocomposites

Polyimide (PI) is a candidate polymer for a variety ofapplications such as packaging materials, circuit boards,and interlayer dielectrics due to their good dielectric

excellent thermal stability, and radiation resistance. Poly-imides serve as excellent polymer matrices for polymericCNT nanocomposites [282,283]. The mechanical propertiesfor different PI composites are summarized in Table 5.

improvement % of tensile strength improvement Reference

9 [287]7 [206]

40 [285]100 [289]

61 [290]31 [284]10 [203]11 [203]

0 [203]21 [282]

Polyme
856 N.G. Sahoo et al. / Progress in
Most studies report nanocomposites with CNT resultinimprovements in the mechanical properties of PI. Forexample, in situ polymerized PI containing 5 wt% MWNT-COOH showed increase in modulus (33%) and tensilestrength (7%) as compared to neat PI [206]. However,compared to that of the neat PI, the modulus and ten-sile strength of raw CNT/PI nanocomposites showed onlya slight increase.The better improvement in the tensilestrength and modulus of PI/MWNT-COOH may be causedby the strong interactions between PI matrix and MWNT-COOH. As discussed above, the functionalized MWNTsprepared by acid treatment contain –COOH groups, whichfacilitate improved interaction with –O– groups in the PIchain.

Another study revealed moderate increases in the mod-ulus and tensile strength of PI when mixed with plasmamodified MWNT [289]. The addition of 0.5 wt% of theplasma modified MWNT to the PI increased the modulusfrom 2.17 to 4.56 GPa and the tensile strength from 124.5to 249 MPa, or increases of 110% and 100%, respectively.These impressive results were attributed to chemical bondformation between the plasma modified MWNT and thePI. When the modified MWNTs content was higher (above0.5 wt), the modulus and tensile strength were decreasedat high modified MWNT content. This result is consistentwith other reports [285,286]. Zhu et al. [285] found that thetensile strength of the PI/MWNT composites increased withincreased MWNT content up to 5 wt%, then decreased withfurther increase in MWNTs. The incorporation of 5 wt%MWNTs into PI resulted in an enhanced tensile strength by40% compared to pure PI, attributed to good dispersion ofMWNTs in the nanocomposite. At higher level of MWNTs,the MWNTs could not be well-dispersed, agglomeratingto large clusters and resulting in a decrease of the tensilestrength. Jiang et al. [286] also observed that the Youngmodulus of PI/MWNT composites improved by adding upto 1.89 vol% MWNT and decreased with MWNT contentfurther increase.

A study utilizing SWNTs reported that the mechani-cal properties of SWNT/PI composites with a low levelof SWNTs (0.30 wt%) showed a slight increase (5% tensilestrength and 18% Young modulus) compared to PI, whereascomposites with higher SWNT level (1 wt%) showed signifi-cant improvement of the mechanical properties (9% tensilestrength and 90% Young modulus) [287].

The mechanical properties of PI/CNT compositesdepend on the nature of any functionalization of the CNT.At lower level of MWNT (up to 0.99 wt%), the tensileproperties of amine-modified MWNT/PI composites washigher than that of acid-modified MWNT/PI composites[284]. However, acid-modified MWNTs improved the ten-sile properties of the PI more than amine-modified MWNTfor MWNTs content above 2.44 wt%. The acid-modifiedMWNTs may form hydrogen bonds with the C O bonds ofthe PI molecules. However, the bonding of amine-modifiedMWCNT to polyamic acid may reduce its imidization. The
mechanical strength of polyamic acid is less than that ofPI and polyamic acid is more brittle than PI. So, amine-modified MWNT is added to the PI matrix may affect themechanical properties of the polymer. The same groupalso reinforced PI by the addition of vinyltriethoxysi-
r Science 35 (2010) 837–867

lane functionalized MWNT [290]. They observed that themodulus and tensile strength of 0.5 wt% functionalizedMWNT-PI composites increased 60 and 61%, respectively,as compared to neat PI. This improvement of mechanicalproperties depends on the ratio of vinyltriethoxysilane toMWNTs. When the ratio of vinyltriethoxysilane to MWCNTis 2:1, the composite showed better tensile properties thanother composites with different ratios and neat PI, becauseonly this ratio of vinyltriethoxysilane to MWCNT can forminterpenetrating network in PI matrix.

A number of other studies have observed increasesin modulus, but either no increase or decrease in ten-sile strength [203,288]. Compared with the neat polymer,the addition of CNTs resulted in increase in the modu-lus 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 ofmechanical property SWNT/PI nanocomposite depends onsample type, such as film, rod and fiber [203]. The ten-sile modulus, ultimate strength, and elongation-at-breakwere 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 changedwhen the extruded rods were drawn down to much smallerdiameters. Tensile strength and modulus were increasedwith decreasing fiber diameter because of increased align-ment induced by the post extrusion fiber drawing process.

6. Electrical conductivity of polymer/CNTnanocomposites

CNTs exhibit the high aspect ratio and high con-ductivity, which makes CNTs excellent candidates forconducting composites. Percolation theory predicts thatthere is a critical concentration at which composites con-taining conducting fillers in insulating polymers becomeelectrically conductive. According to percolation theory,�c = A(V − Vc)ˇ, where �c is the conductivity of the com-posites, V is the CNT volume fraction, Vc is the CNTvolume fraction at the percolation threshold, and A andˇ are fitted constant. The percolation threshold has beenreported to ranging from 0.0025 wt% [291] to several wt%[302]. The percolation threshold for the electrical con-ductivity in polymer-CNT nanocomposites depends ondispersion [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 thresholdof CNT-polymer composites are shown in Table 6.

The aligned CNTs in epoxy decrease the percolationthreshold by an order of magnitude compared to entan-gled nanotubes [291]. The entangled CNTs were dispersedcompletely during the shear intensive stirring process,whereas aligned CNTs led to a superior dispersion aftershear-intensive processing. The well dispersed CNTs easily
form conductive paths due to their relatively homogeneousdispersion of the nanoparticles, compared with that for theaggregated CNTs. The percolation threshold in CNT/epoxycomposites with palmitic acid reduced by 2 compared tothose composites without palmitic acid, attributed to the


Table 6Electrical conductivity of CNT-polymer composites.

Polymer type Nanotube type Percolation threshold (wt%) CNT content (wt%) Maximum conductivity (S/m) Reference

Polystyrene MWNT 0.15–0.2 2 103 [300]Polycarbonate SWNT-functional 0.11 7 4.81 × 102 [301]Polystyrene SWNT-functional 0.045 7 6.89 [301]Poly(vinyl acetate) XD grade CNT – 20 4.8 × 103 [361]Poly(methyl methacrylate) MWNT 0.3 40 3 × 103 [362]Poly(methyl methacrylate) SWNT 0.17 10 1.7 × 103 [363]

ebS2icdupiaahr

oitopiitpaTnaopi4c

rrSlCesoototef

Poly(methyl methacrylate) SOCl2-doped SWNT 0.17Poly(vinyl alcohol) MWNT 5–10Poly(vinyl acetate) SWNT 0.04Epoxy MWNT 0.0025

fficient dispersion of CNTs in the epoxy matrix promotedy the palmitic acid [176]. The electrical conductivity ofWNT/epoxy nanocomposites with SWNTs aligned under a5 T magnetic field was increased by 35% compared to sim-

lar composites without magnetic aligned SWNTs [293]. Inontrast, Fangming et al. [294] found that the electric con-uctivity of the aligned CNT in PMMA was 10−10 S/cm andnaligned CNT was 10−4 S/cm for 2% SWNT/PMMA com-osites. This indicates that the alignment of the nanotubes

n the composite decreased the electrical conductivity andlso the percolation threshold. The reason is that therere fewer contacts between the nanotubes when they areighly aligned in the composites, and so aligned compositesequire more nanotubes to reach the percolation threshold.

The aspect ratio of the CNT has a tremendous influencen the percolation threshold of the polymer nanocompos-tes without varying other important parameters, such ashe polymer matrix or the dispersion and aggregation statef the CNTs. Recently Grossiord et al. [300] reported that theercolation threshold of polystyrene/MWNT nanocompos-

tes made from 2 wt% high-aspect ratio nanotubes grownn vertically aligned films was 0.15–0.5 wt%, which is 5imes smaller than that for low aspect ratio industrically-roduced nanotubes polymer composites. They foundn electrical conductivity of 103 S/m with 2 wt% CNTs.he percolation threshold was observed at 0.045 wt%on-covalently functionalized, soluble SWNT loading forSWNT-PS composite, with a maximum conductivity

f 6.89 S/m at 7 wt% of nanotube loading. The samereparation method for a SWNT-PC composite resulted

n a material for which the conductivity increased to.81 × 102 S/m at 7 wt% of nanotubes, with a very low per-olation threshold reached at 0.11 wt% of nanotubes [301].

It is well known that chemical functionalization dis-upts the extended �-conjugation of nanotubes and henceeduces the electrical conductivity of functionalized CNTs.ilane-functionalized CNT/epoxy nanocomposites showedower electrical conductivity than that of the untreatedNTs composites at identical nanotube content [297]. Chot al. [94] reported that the electrical conductivity of theurface-modified MWNT composites was lower than thatf the unreacted MWNT composites that had the same nan-tube content. This is attributed to increased defects in
he lattice structure of carbon-carbon bonds on the nan-tube surface as a result of the acid treatment. In particular,he severe modification of the nanotubes significantly low-red the conductivity. Several researchers reported that theunctionalization of CNTs can improve the electrical con-
13.5 104 [363]60 100 [302]

4 ≈15 [360]1 2 [291]

ductivity of the composites [298,299]. Tamburri et al. [298]found that the functionalization SWNTs with –COOH and–OH groups enhanced the composites conductivity com-pared to untreated SWNTs.

7. Optical properties of polymer/CNTnanocomposites

CNTs exhibit unique one-dimensional p-electron con-jugation, mechanical strength, and high thermal andchemical stability, which make them very attractive foruse in many applications. Optical limiting, an importantnon-linear optical behavior, can develop with increasinginput fluence of a light pulse, such that the transmittedfluence tends to a constant, independent of the input flu-ence. For dispersions of CNTs in a number of solvents, itappears that optical limiting is principally due to the non-linear scattering due to bubbles formed by light absorptioninduced heating, although sublimation, a gradual reductionin size for a CNT at high temperature, may contribute, incontrast to a minimal role of reverse saturable absorption(RSA), an effect dependent on absorption by excited elec-tronic states [303–305]. O’Flaherty et al. [306] reported thatthe optical limiting of the poly(9,9-di-n-octylfluorenyl-2,7-diyl)/MWNTs samples is dependent on the mass fractionof CNTs. The magnitude of the non-linear effect increasedsystematically when the mass fraction of the nanotubesincreased from 0.011 to 0.038. The non-linear opticalextinction of nanosecond laser pulses by a set of con-jugated copolymer poly(para-phenylenevinylene-co-2,5-dioctyloxy-meta-phenylenevinylene)/MWNTs compositesdispersed in solution has been reported [307]. In this case,either the MWNTs or the polymer dominates the non-linear response of the composite, depending on the relativemass of polymer to nanotube. The non-linear extinctionwas 0% for 0, 1.3, and 2.5 wt% MWNT at 10 J/cm2, whereasthe samples with 3.6 wt% and 5.9 wt% MWNTs were a nor-malized non-linear extinction of 12% and 41%, respectively.Thus the materials display a dramatic improvement in theoptical-limiting performance when the MWNT mass con-tent is increased from 1.3% to 3.6%.

Several polymer-coated and polymer-grafted MWNTswere synthesized and their non-linear optical properties
of composites were investigated using 532 nm nanosec-ond laser pulses [110]. The authors reported that theoptical limiting thresholds of the all composite samplesto be approximately 1 J cm−2, which is similar to thatof the MWNT-DMF suspension. The results showed that


a) transpemical S

Fig. 15. Optical and electrical properties of CNT/S-PEEK composite films: (Reprinted with permission from Ref. [311]. Copyright 2008, American Ch

the polymers and processing methods do not change thenon-linear optical properties of MWNTs. The optical limit-ing performance of double-C60-end-capped poly(ethyleneoxide)/MWNTs were also studied and found betterthan the aqueous MWNTs suspension [308]. In case ofSWNT/poly(3-octylthiophenes) composite films, the opti-cal properties did not significantly change with low fractionof CNT changes [309]. In a study of the optical propertiesof poly(para phenylene vinylene) (PPV)/CNT composites[310], different concentration SWCNT-PPV films were pre-pared using a solution mixing process, heated at 120 ◦C.The authors observed that the optical spectra at 120 ◦C PPVconversion temperature was dramatically changed withrespect to those obtained at 300 ◦C. It was also found thatthe effect of the low conversion temperature on all the opti-cal spectra was similar to that of increasing the nanotubeconcentration in standard PPV.

A novel preparation of optically transparent CNT/polymer composites with highly aligned nanotubes hasreported recently [311]. First, CNT arrays were grown onsilicon by a CVD process. Uniform CNT sheets were thenpulled out of the arrays and stabilized on glass. Compos-ite films were finally produced by spin-coating or castingpolymer solutions onto the CNT sheets, followed by evap-oration of the solvents. Film thickness was controlled byvarying the concentration of polymer solutions and coating
times. PS, PMMA, and sulfonated poly(ether ether ketones)derived composite films with more than 80% optical trans-parency were prepared using this technique (Fig. 15). Theseresults suggest that the optical properties of the compositescan be tailored in a predetermined manner by controlling
Table 7Application of CNT-polymer composites.

Nanotube type Polymer type

SWNT Poly(3-octylthiophene)MWNT; SWNT Polyaniline, polypyrrole, poly-(3,4-ethylenedioxythio

poly(3-methyl-thiopheneSWNT NafionMWNT Poly(vinyl alcohol), poly(2-acrylamido-2-methyl-1-prMWNT NafionMWNT-functional Sulfonated poly(arylene sulfone),SWNT; MWNT PolypyrroleSWNT Poly(methyl methacrylate)SWNT-functional;

MWNT-functionalDNA (polynucleotide)

arent film on a labeled paper and (b) optical transmittance measurement.ociety, USA.

the nanotube content, orientation and precursor conver-sion temperature, thus opening a pathway for developingoptically functional materials.

8. Applications

As developed in the preceding sections, because oftheir excellent mechanical, electrical, and magnetic prop-erties, as well as nanometer scale diameter and highaspect ratio, CNTs can be very useful materials in com-posites to improve a particular property for specificapplications (Table 7). The addition of CNTs to �-conjugated polymers was found to improve the quantumefficiency of �-conjugated polymers because the inter-action between the highly delocalized �-electrons ofCNTs and the �-electrons correlated with the lattice ofthe polymer skeleton [3]. Such composites are widelyused in photovoltaic devices [312] and light-emittingdiodes [313]. CNT-conducting polymer composites havea potential application in supercapacitors [314,315]. ThePANI/MWNTs composites electrodes showed much higherspecific capacitance (328 F g−1) than pure PANI electrodes[316]. The capacitances of a CNT-polypyrrole composite-CNT-poly(3-methyl-thiophene) composite based super-capacitor prototype and a CNTs-CNTs-polypyrrole basedhybrid supercapacitor prototype were 87 and 72 F g−1,
respectively, much larger the 21 F g−1 for the CNTs/CNTscorresponding supercapacitor prototype due to the Fara-day 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-

Applications References

Photovoltaic devices [312]phene), Supercapacitors [314,316,317,364,365]

Actuators [318]opanesulfonic acid) [366]

Fuel cell [367][368]

Biosensors [369,324]Biocatalytic films [370]Gene delivery [371]

Polymer

tmt

ortsCipobgspwcfi

emsmttBtCt[

beaaecoii

9

pudiptmsaioCCoC

N.G. Sahoo et al. / Progress in

romechanical actuation because the CNTs improved theechanical, electronic, and electrochemical properties of

he PANI fibers [320].Composites based on CNTs are studied for a variety

f sensor applications [321,322]. For example, polypyr-ole or PANI deposited on single-walled CNT networkshat can be used as solid state pH sensors [323]. A DNAensor was created from a composite of polypyrrole andNTs functionalized with carboxylic groups to covalently

mmobilize DNA onto CNTs [324]. In another example,olypyrrole films doped with CNTs functionalized withligonucleotides were successfully implemented for DNAiosensors using direct impedance measurements [325]. Ineneral, the presence of CNTs tends to increase the overallensitivity and selectivity of biosensors. The thermal trans-ort properties of polymer composites can be improvedith the addition of CNTs due to the excellent thermal

onductivity of CNTs. Such composite are quite attractiveor usages as printed circuit boards, connectors, thermalnterface materials, heat sinks, lids, housings, etc. [8].

The superior properties of CNTs are not limited tolectrical and thermal conductivities, but also includeechanical properties, such as stiffness, toughness, and

trength. CNTs with their high aspect ratio and excellentechanical properties have the potential to strengthen and

oughen hydroxyapatite without offsetting its bioactivity,hus opening up a wider range of clinical applications [326].ianco et al. studied the application of CNTs as new vec-ors for the delivery of therapeutic molecules [327,328].NTs have been shown to cross cell membranes easily ando deliver peptides, proteins, and nucleic acids into cells329,330].

CNTs were employed to reinforce the interfacesetween ultra high molecular weight PE polymer particles,nhancing composite strength, stiffness, impact toughnesss well as structural damping [331]. These compositesre attractive for applications in aerospace and navalngineering. The high strength and toughness-to-weightharacteristics of CNTs may also prove valuable as partf composite components in fuel cells that are deployedn transport applications, where durability is extremelymportant.

. Concluding remarks

There are several approaches for developing higherformance CNT-polymer nanocomposites utilizing thenique properties of CNTs. The critical challenge is theevelopment of methods to improve the dispersion of CNTs

n a polymer matrix because their enhanced dispersion inolymer matrices greatly improves the mechanical, elec-rical and optical properties of composites. Despite various

ethods, such as melt processing, solution processing, in-itu polymerization, and chemical functionalization, therere still opportunities and challenges to be found in order tomprove dispersion and modify interfacial properties. One
f challenges is to achieve the optimal functionalization ofNTs, which can maximize interfacial adhesion betweenNTs and the polymer matrix. A specific functionalizationf CNTs is required for strong interfacial adhesion betweenNTs and a given polymer matrix, which may also simul-
Science 35 (2010) 837–867 859

taneously improve the dispersion of CNTs in the polymermatrix.

The mechanical properties of CNT-polymer nanocom-posites may be compromised between carbon–carbonbond damage and increased CNT-polymer interaction dueto CNT functionalization. Similarly, electrical conductivityof a CNT-polymer nanocomposite is determined by the neg-ative effect of carbon–carbon bond damage and the positiveeffect of improved CNT dispersion due to CNT functional-ization. In either case, the choice and control of tailoredfunctionalization sites for chemical modification of CNTsare necessary. As an example, selective CNT functional-ization can be achieved via click chemistry by preparingazide-functionalized polymers. However, this may be lim-ited in the practical use owing to the need for multi-stepreactions for azide and alkyl groups to apply click chem-istry. The employment of hyperbranched polymers forimproving CNT dispersion may be also useful because itcan result in enhanced electrical conductivity, as well asmechanical properties of nanocomposites, without modi-fication of CNT.

In practice, the problems regarding melt processingneed to be solved, as melt mixing is the most common com-mercial method used to prepare CNT-polymer composites.To achieve the best performance of CNT-polymer compos-ites, it is important to choose the CNT functionalizationmethod, a suitable polymer matrix for CNT dispersion andmolecular interaction control with CNTs as well as poly-mer composite processing conditions such as temperature,shear rate, shear force, and mixing time. In conclusion, theCNT functionalization and matrix polymer design for dis-persion of CNTs and interfacial adhesion between CNTs anda polymer matrix are the key challenges for developmentof high performance CNT composites.

Acknowledgments

This work was supported by the SRC/ERC Programof MOST/KOSEF (R11-2005-065) and A*STAR SERC Grant(0721010018).

References

[1] Iijima S. Helical microtubules of graphitic carbon. Nature1991;354:56–8.

[2] Bachtold A, Hadley P, Nakanishi T, Dekker C. Logic circuits withcarbon nanotube transistors. Science 2001;294:1317–20.

[3] Ago H, Petritsch K, Shaffer MSP, Windle AH, Friend RH. Compos-ites of carbon nanotubes and conjugated polymers for photovoltaicdevices. Adv Mater 1999;11:1281–5.

[4] Kasumov AY, Deblock R, Kociak M, Reulet B, Bouchiat H, Khodos I, etal. Supercurrents through single-walled carbon nanotubes. Science1999;284:1508–11.

[5] Baughman RH, Cui C, Zakhidov AA, Iqbal Z, Barisci JN, Spinks GM,et al. Carbon nanotube actuators. Science 1999;284:1340–4.

[6] Niu C, Sichel EK, Hoch R, Moy D, Tennet H. High power electrochem-ical capacitors based on carbon nanotube electrodes. Appl Phys Lett1997;70:1480–2.

[7] Ajayan PM, Iijima S. Capillarity-induced filling of carbon nanotubes.
Nature 1993;361:333–4.
[8] Xie XL, Mai YW, Ping X. Dispersion and alignment of carbonnanotubes in polymer matrix: a review. Mater Sci Eng Rep2005;49:89–112.

[9] Andrews R, Weisenberger MC. Carbon nanotube polymer compos-ites. Curr Opin Solid State Mater Sci 2004;8:31–7.

Polyme
[10] Iijima S, Ichihashi T. Single-shell carbon nanotubes of 1-nm diam-eter. Nature 1993;363:603–5.

[11] Bethune DS, Kiang CH, Devries MS, Gorman G, Savoy R, VazquezJ, et al. Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls. Nature 1993;363:605–7.

[12] Sugai T, Yoshida H, Shimada T, Okazaki T, Shinohara H. Newsynthesis of high-quality double-walled carbon nanotubes by high-temperature pulsed arc discharge. Nano Lett 2003;3:769–73.

[13] Bandow S, Takizawa M, Hirahara K, Yudasaka M, Iijima S. Ramanscattering study of double-wall carbon nanotubes derived from thechains of fullerenes in single-wall carbon nanotubes. Chem PhysLett 2001;337:48–54.

[14] Guo T, Nikolaev P, Thess A, Colbert DT, Smalley RE. Catalytic growthof single-walled nanotubes by laser vaporization. Chem Phys Lett1995;243:49–54.

[15] Nikolaev P, Bronikowski MJ, Bradley RK, Rohmund F, Colbert DT,Smith KA, et al. Gas-phase catalytic growth of single-walled carbonnanotubes from carbon monoxide. Chem Phys Lett 1999;313:91–7.

[16] Joseyacaman M, Mikiyoshida M, Rendon L, Santiesteban JG. Cat-alytic growth of carbon microtubules with fullerene structure. ApplPhys Lett 1993;62:657–9.

[17] Huang SM, Woodson M, Smalley RE, Liu J. Growth mechanism oforiented long single walled carbon nanotubes using fast-heatingchemical vapor deposition process. Nano Lett 2004;4:1025–8.

[18] Treacy MM, Ebessen TW, Gibson JM. Exceptionally high Young’smodulus observed for individual carbon nanotubes. Nature1996;381:678–80.

[19] Saito R, Dresselhaus G, Dresselhaus MS. Physical properties of car-bon nanotubes. London: Imperial College Press; 1998.

[20] Chae HG, Liu J, Kumar S. Carbon nanotubes properties and applica-tions. In: O’Connell MJ, editor. Carbon nanotube-enabled materials.Boca Raton: Taylor & Francis Group, LLC; 2006.

[21] Ma C, Zhang W, Zhu Y, Ji L, Zhang R, Koratkar N, et al. Alignment anddispersion of functionalized carbon nanotubes in polymer compos-ites induced by an electric field. Carbon 2008;46:706–10.

[22] Ajayan PM, Schadler LS, Braun PV. Nanocomposite science and tech-nology. Weinheim, Germany: Wiley-VCH, GmbH & Co. KgaA; 2003.

[23] Lu JP. Elastic properties of carbon nanotubes and nanoropes. PhysRev Lett 1997;79:1297–300.

[24] Li Y, Wang K, Wei J, Gu Z, Wang Z, Luo J, et al. Tensile propertiesof long aligned double-walled carbon nanotube strands. Carbon2005;43:31–5.

[25] Yu MF, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS. Strengthand breaking mechanism of multiwalled carbon nanotubes undertensile load. Science 2000;287:637–40.

[26] Ajayan PM, Stephan O, Colliex C, Trauth D. Aligned carbon nanotubearrays formed by cutting a polymer resin-nanotube composite. Sci-ence 1994;265:1212–4.

[27] Moniruzzaman M, Winey KI. Polymer nanocomposites containingcarbon nanotubes. Macromolecules 2006;39:5194–205.

[28] Jamal A, Ali R, Somayeh M. Preparation and characterization of lin-ear low density polyethylene/carbon nanotube nanocomposites. JMacromol Sci Part B: Phys 2007;46:877–89.

[29] Li L, Li CY, Ni C, Rong LX, Hsiao B. Structure and crystallizationbehavior of Nylon 66/multi-walled carbon nanotube nanocompos-ites at low carbon nanotube contents. Polymer 2007;48:3452–60.

[30] Morales-Teyssier O, Sanchez-Valdes S, Ramos-de Valle LF. Effect ofcarbon nanofiber functionalization on the dispersion and physicaland mechanical properties of polystyrene nanocomposites. Macro-mol Mater Eng 2006;291:1547–55.

[31] Gryshchuk O, Karger-Kocsis J, Thomann R, Konya Z, Kiricsi I. Mul-tiwall carbon nanotube modified vinylester and vinylester-basedhybrid resins. Compos Part A 2006;37:1252–9.

[32] Ghose S, Watson KA, Delozier DM, Working DC, Siochi EJ, Con-nell JW. Incorporation of multi-walled carbon nanotubes into hightemperature resin using dry mixing techniques. Compos Part A2006;37:465–75.

[33] Mrozek RA, Kim BS, Holmberg VC, Taton TA. Homogeneous, coaxialliquid crystal domain growth from carbon nanotube seeds. NanoLett 2003;3:1665–9.

[34] Bliznyuk VN, Singamaneni S, Sanford RL, Chiappetta D, Crooker B,Shibaev PV. Matrix mediated alignment of single wall carbon nan-otubes in polymer composite films. Polymer 2006;47:3915–21.

[35] Zhao B, Hu H, Haddon RC. Synthesis and properties of a water-soluble single-walled carbon nanotube-poly(m-aminobenzenesulfonic acid) graft copolymer. Adv Funct Mater 2004;14:71–6.

[36] Kanagaraj S, Varanda FR, Zhil’tsova TV, Oliveira MSA,Simoes JAO. Mechanical properties of high density polyethy-

r Science 35 (2010) 837–867

lene/carbon nanotube composites. Compos Sci Technol 2007;67:3071–7.

[37] Kim JY, Han SI, Kim SH. Crystallization behaviors and mechani-cal properties of poly(ethylene 2,6-naphthalate)/multiwall carbonnanotube nanocomposites. Polym Eng Sci 2007;47:1715–23.

[38] Jin SH, Park YB, Yoon KH. Rheological and mechanical propertiesof surface modified multi-walled carbon nanotube-filled PET com-posite. Compos Sci Technol 2007;67:3434–41.

[39] Wang Z, Ciselli P, Peijs T. The extraordinary reinforcing efficiencyof single-walled carbon nanotubes in oriented poly(vinyl alcohol)tapes. Nanotechnology 2007;18, 455709/1–9.

[40] Yang BX, Shi JH, Pramoda KP, Goh SH. Enhancement of stiffness,strength, ductility and toughness of poly(ethylene oxide) usingphenoxy-grafted multiwalled carbon nanotubes. Nanotechnology2007;18, 125606/1–7.

[41] Pham JQ, Mitchell CA, Bahr JL, Tour JM, Krishanamoorti R, Green PF.Glass transition of polymer/single-walled carbon nanotube com-posite films. J Polym Sci Part B Polym Phys 2003;41:3339–45.

[42] Gong XY, Liu J, Baskaran S, Voise RD, Young JS. Surfactant-assistedprocessing of carbon nanotube/polymer composites. Chem Mater2000;12:1049–52.

[43] Singh IV, Tanaka M, Zhang J, Endo M. Evaluation of effectivethermal conductivity of CNT-based nano-composites by ele-ment free Galerkin method. Int J Numer Meth Heat Fluid Flow2007;17:757–69.

[44] Sergei S, Xue L, Rahmi O, Pawel K, David GC. Role of thermal bound-ary resistance on the heat flow in carbon-nanotube composites. JAppl Phys 2004;95:8136–44.

[45] Sankapal BR, Setyowati K, Chen J, Liu H. Electrical properties of air-stable, iodine-doped carbon-nanotube-polymer composites. ApplPhys Lett 2007;91, 173103/1–3.

[46] Grossiord N, Miltner HE, Loos J, Meuldijk J, Van Mele B, KoningCE. On the crucial role of wetting in the preparation of con-ductive polystyrene-carbon nanotube composites. Chem Mater2007;19:3787–92.

[47] Guo H, Sreekumar TV, Liu T, Minus M, Kumar S. Structure and prop-erties of polyacrylonitrile/single wall carbon nanotube compositefilms. Polymer 2005;46:3001–5.

[48] Ichida M, Mizuno S, Kataura H, Achiba Y, Nakamura A. Anisotropicoptical properties of mechanically aligned single-walled carbonnanotubes in polymer. Appl Phys A 2004;78:1117–20.

[49] Weglikowska UD, Kaempgen M, Hornbostel B, Skakalova1 V, WangJ, Liang J, et al. Conducting and transparent SWNT/polymer com-posites. Phys Stat Sol B 2006;243:3440–4.

[50] Hirsch A. Functionalization of single-walled carbon nanotubes.Angew Chem Int Ed 2002;41:1853–9.

[51] Nalwa HS, editor. Handbook of nanostructured materials and nan-otechnology, vol. 5. New York: Academic Press; 2000.

[52] Ebbesen TW, Ajayan PM, Hiura H, Tanigaki K. Purification of nan-otubes. Nature 1994;367:519–1519.

[53] Hiura H, Ebbesen TW, Tanigaki K. Opening and purification of car-bon nanotubes in high yields. Adv Mater 1995;7:275–6.

[54] Kuznetsova A, Mawhinney DB, Naumenko V, Yates JT, Liu J, SmalleyRE. Enhancement of adsorption inside of single-walled nanotubes:opening the entry ports. Chem Phys Lett 2000;321:292–6.

[55] Liu J, Rinzler AG, Dai H, Hafner JH, Bradley RK, Boul PJ, et al. Fullerenepipes. Science 1998;280:1253–6.

[56] Mawhinney DB, Naumenko V, Kuznetsova A, Yates Jr JT, Liu J,Smalley RE. Surface defect site density on singale walled carbonnanotubes by titration. Chem Phys Lett 2000;324:213–6.

[57] Hu H, Bhowmik P, Zhao B, Hamon MA, Itkis ME, Haddon RC.Determination of the acidic sites of purified single-walled carbonnanotubes by acid-base titration. Chem Phys Lett 2001;345:25–8.

[58] Hemon MA, Chen J, Hu H, Chen Y, Itkis ME, Rao AM, et al. Dissolutionof single-walled carbon nanotubes. Adv Mater 1999;11:834–40.

[59] Chen J, Hamon MA, Hu H, Chen Y, Rao AM, Eklund PC, et al.Solution properties of single-walled carbon nanotubes. Science1998;282:95–8.

[60] Bandow S, Rao AM, Williams KA, Thess A, Smalley RE, Eklund PC.Purification of single-wall carbon nanotubes by microfiltration. JPhys Chem B 1997;101:8839–42.

[61] Duesberg GS, Burghard M, Muster J, Philipp G. Separation of car-bon nanotubes by size exclusion chromatography. Chem Commun
1998:435–6.
[62] Chen RJ, Zhang Y, Wang D, Dai H. Noncovalent sidewall func-tionalization of single-walled carbon nanotubes for proteinimmobilization. J Am Chem Soc 2001;123:3838–9.

[63] Lordi V, Yao N. Molecular mechanics of binding in carbon-nanotube-polymer composites. J Mater Res 2000;15:2770–9.

Polymer
[64] Guo Z, Sadler PJ, Tsang SC. Immobilization and visualization of DNAand proteins on carbon nanotubes. Adv Mater 1998;10:701–3.

[65] O’Connell MJ, Boul P, Ericson LM, Huffman C, Wang Y, Haroz E, et al.Reversible water-solubilization of single-walled carbon nanotubesby polymer wrapping. Chem Phys Lett 2001;342:265–71.

[66] Steuerman DW, Star A, Narizzano R, Choi H, Ries RS, Nicolini C,et al. Interactions between conjugated polymers and single-walledcarbon nanotubes. J Phys Chem B 2002;106:3124–30.

[67] Star A, Stoddart JF, Steuerman D, Diehl M, Boukai A, Wong EW, etal. Preparation and properties of polymer-wrapped single-walledcarbon nanotubes. Angew Chem Int Ed 2001;40:1721–5.

[68] Paredes JI, Burghard M. Dispersions of individual single-walled car-bon nanotubes of high length. Langmuir 2004;20:5149–52.

[69] Duesberg GS, Muster J, Krstic V, Burghard M, Roth S. Chromato-graphic size separation of single-wall carbon nanotubes. Appl PhysA 1998;67:117–9.

[70] Moore VC, Strano MS, Haroz EH, Hauge RH, Smalley RE, SchmidtJ, et al. Individually suspended single-walled carbon nanotubes invarious surfactants. Nano Lett 2003;3:1379–82.

[71] Islam MF, Rojas E, Bergey DM, Johnson AT, Yodh AG. High weightfraction surfactant solubilization of single-wall carbon nanotubesin water. Nano Lett 2003;3:269–73.

[72] Yurekli K, Mitchell CA, Krishnamootri R. Small angle neutronscattering from surfactant assisted aqueous dispersion of carbonnanotubes. J Am Chem Soc 2004;126:9902–3.

[73] Jiang L, Gao L, Sun J. Production of aqueous colloidal dispersions ofcarbon nanotubes. J Coll Interf Sci 2003;260:89–94.

[74] Poulin P, Vigolo B, Launois P. Films and fibers of oriented single wallnanotubes. Carbon 2002;40:1741–9.

[75] Sáfar GAM, Ribeiro HB, Malard LM, Plentz FO, Fantini C, Santos AP,et al. Optical study of porphyrin-doped carbon nanotubes. ChemPhys Lett 2008;462:109–11.

[76] Tan Y, Resasco DE. Dispersion of single-walled carbon nanotubes ofnarrow diameter distribution. J Phys Chem B 2005;109:14454–60.

[77] Bandyopadhyaya R, Nativ-Roth E, Regev O, Yerushalmi-Rozen R.Stabilization of individual carbon nanotubes in aqueous solutions.Nano Lett 2002;2:25–8.

[78] Pei X, Hu L, Liu W, Hao J. Synthesis of water-soluble carbonnanotubes via surface initiated redox polymerization and their tri-bological properties as water-based lubricant additive. Eur PolymJ 2008;44:2458–64.

[79] Wu HX, Qiu XQ, Cao WM, Lin YH, Cai RF, Qian SX. Polymer-wrappedmultiwalled carbon nanotubes synthesized via microwave-assisted in situ emulsion polymerization and their optical limitingproperties. Carbon 2007;45:2866–72.

[80] Cheng F, Imin P, Maunders C, Botton G, Adronov A. Soluble,discrete supramolecular complexes of single-walled carbon nan-otubes with fluorene-based conjugated polymers. Macromolecules2008;41:2304–8.

[81] Yang L, Zhang B, Liang Y, Yang B, Kong T, Zhang LM. In situ syn-thesis of amylose/single-walled carbon nanotubes supramolecularassembly. Carbohydrate Res 2008;343:2463–7.

[82] Ogoshi T, Yamagishi TA, Nakamoto Y. Supramolecular single-walled carbon nanotubes (SWCNTs) network polymer made byhybrids of SWCNTs and water-soluble calix[8]arenas. Chem Com-mun 2007:4776–8.

[83] Curran SA, Ajayan PM, Blau WJ, Carroll DL, Coleman JN, Dalton AB, etal. A composite from poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) and carbon nanotubes: a novel material formolecular optoelectronics. Adv Mater 1998;10:1091–3.

[84] Coleman JN, Dalton AB, Curran S, Rubio A, Davey AP, Drury A, etal. Phase separation of carbon nanotubes and turbostratic graphiteusing a functional organic polymer. Adv Mater 2000;12:213–6.

[85] McCarthy B, Coleman JN, Czerw R, Dalton AB, Carroll DL, Blau WJ.Microscopy studies of nanotube-conjugated polymer interactions.Synth Met 2001;121:1225–6.

[86] Dalton AB, Stephan C, Coleman CSJN, McCarthy B, AjayanPM, Lefrant S, et al. Selective interaction of a semiconjugatedorganic polymer with single-wall nanotubes. J Phys Chem B2000;104:10012–6.

[87] Kang Y, Taton TA. Micelle-encapsulated carbon nanotubes: a routeto nanotube composites. J Am Chem Soc 2003;125:5650–1.

[88] Shvartzman-Cohen R, Nativ-Roth E, Baskaran E, Levi-Kalisman
Y, Szleifer I, Yerushalmi-Rozen R. Selective dispersion of single-walled carbon nanotubes in the presence of polymers: therole of molecular and colloidal length scales. J Am Chem Soc2004;126:14850–7.
[89] Shvartzman-Cohen R, Kalisman YL, Navi-Roth E, Yerushalmi-Rozen R. Generic approach for dispersing single-walled carbon

Science 35 (2010) 837–867 861

nanotubes: the strength of a weak interaction. Langmuir2004;20:6085–8.

[90] Nativ-Roth E, Shvartzman-Cohen R, Bounioux C, Florent M, ZhangD, Szleifer I, et al. Physical adsorption of block copolymers toSWNT and MWNT: a nonwraping mechanism. Macromolecules2007;40:3676–85.

[91] Niyogi S, Hamon MA, Hu H, Zhao B, Bhowmik P, Sen R, etal. Chemistry of single-walled carbon nanotubes. Acc Chem Res2002;35:1105–13.

[92] Park H, Zhao J, Lu JP. Effects of sidewall functionalization on con-ducting properties of single wall carbon nanotubes. Nano Lett2006;6:916–9.

[93] Zhang X, Sreekumar TV, Liu T, Kumar S. Properties and structure ofnitric acid oxidized single wall carbon nanotube films. J Phys ChemB 2004;108:16435–40.

[94] Cho JW, Kim JW, Jung YC, Goo NS. Electroactive shape-memorypolyurethane composites incorporating carbon nanotubes. Macro-mol Rapid Commun 2005;26:412–6.

[95] Georgakilas V, Kordatos K, Prato M, Guldi DM, Holzingger M, HirschA. Organic functionalization of carbon nanotubes. J Am Chem Soc2002;124:760–1.

[96] Zhang Y, Shi Z, Gu Z, Iijima S. Structure modification of single-wallcarbon nanotubes. Carbon 2000;38:2055–9.

[97] Datsyuk V, Kalyva M, Papagelis K, Parthenios J, Tasis D, Siokou A,et al. Chemical oxidation of multiwalled carbon nanotubes. Carbon2008;46:833–40.

[98] Chen J, Rao AM, Lyuksyutov S, Itkis ME, Hamon MA, Hu H, et al.Dissolution of full-length single-walled carbon nanotubes. J PhysChem B 2001;105:2525–8.

[99] Vasiliev I, Curran SA. Cross linking of thiolated carbon nanotubes:an ab initio study. J Appl Phys 2007;102, 024317/1–5.

[100] Zhou O, Fleming RM, Murphy DW, Chen CH, Haddon RC,Ramirez AP, et al. Defects in carbon nanostructures. Science1994;263:1744–7.

[101] Mickelson ET, Huffman CB, Rinzler AG, Smalley RE, Hauge RH, Mar-grave JL. Fluorination of single-wall carbon nanotubes. Chem PhysLett 1998;296:188–94.

[102] Mickelson ET, Chiang IW, Zimmerman JL, Boul PJ, Lozano J, Liu J, etal. Solvation of fluorinated single-wall carbon nanotubes in alcoholsolvents. J Phys Chem B 1999;103:4318–22.

[103] Kelly KF, Chiang IW, Mickelson ET, Hauge RH, Margrave JL, WangX, et al. Insight into the mechanism of sidewall functionaliza-tion of single-walled nanotubes: an STM study. Chem Phys Lett1999;313:445–50.

[104] Boul PJ, Liu J, Mickelson ET, Huffman CB, Ericson LM, Chiang IW, etal. Reversible sidewall functionalization of buckytubes. Chem PhysLett 1999;310:367–72.

[105] Bahr JL, Mickelson ET, Bronikowski MJ, Smalley RE, Tour JM. Disso-lution of small diameter single-wall carbon nanotubes in organicsolvents? Chem Commun 2001:193–4.

[106] Holzinger M, Vostrowsky O, Hirsch A, Hennrich F, Kappes M, WeissR, et al. Sidewall functionalization of carbon nanotubes. AngewChem Int Ed 2001;40:4002–5.

[107] Bahr JL, Yang J, Kosynkin DV, Bronikowski MJ, Smalley RE, Tour JM.Functionalization of carbon nanotubes by electrochemical reduc-tion of aryl diazonium salts: a bucky paper electrode. J Am ChemSoc 2001;123:6536–42.

[108] Bahr JL, Tour JM. Highly functionalized carbon nanotubesusing in-situ generated diazonium compounds. Chem Mater2001;13:3823–4.

[109] Baek JB, Lyons CB, Tan LS. Grafting of vapor-grown carbonnanofibers via in situ polycondensation of 3-phenoxybenzoic acidin poly(phosphoric acid). Macromolecules 2004;37:8278–85.

[110] Jin Z, Sun X, Xu G, Goh SH, Ji W. Nonlinear optical properties ofsome polymer/multi-walled carbon nanotube composites. ChemPhys Lett 2000;318:505–10.

[111] Fu K, Huang W, Lin Y, Riddle LA, Carroll DL, Sun YP. Defunctionaliza-tion of functionalized carbon nanotubes. Nano Lett 2001;1:439–41.

[112] Qin S, Qin D, Ford WT, Resasco DE, Herrera JE. Functional-ization of single-walled nanotubes with polystyrene via graft-ing to and grafting from methods. Macromolecules 2004;37:752–7.

[113] Riggs JE, Guo Z, Carroll DL, Sun YP. Strong luminescence of solubi-
lized carbon nanotubes. J Am Chem Soc 2000;122:5879–80.
[114] Lin Y, Zhou B, Fernando KAS, Liu P, Allard LF, Sun YP. Polymeric car-bon nanocomposites from carbon nanotubes functionalized withmatrix polymer. Macromolecules 2003;36:7199–204.

[115] Qin S, Qin D, Ford WT, Herrera JE, Resasco DE, Bachilo SM, et al. Solu-bilization and purification of single-wall carbon nanotubes in water

Polyme
by in situ radical polymerization of sodium 4-styrenesulfonate.Macromolecules 2004;37:3965–7.

[116] Koshio A, Yudasaka M, Zhang M, Iijima S. A simple way to chem-ically react single-wall carbon nanotubes with organic materialsusing ultrasonication. Nano Lett 2001;1:361–3.

[117] Qu LW, Lin Y, Hill DE, Zhou B, Wang W, Sun X, et al. Polyimide-functionalized carbon nanotubes: synthesis and dispersion innanocomposite films. Macromolecules 2004;37:6055–60.

[118] Park C, Ounaies Z, Watson KA, Crooks RE, Smith J, Lowther SE, et al.Dispersion of single wall carbon nanotubes by in situ polymeriza-tion under sonication. Chem Phys Lett 2002;364:303–8.

[119] Lou XD, Detrembleur C, Pagnoulle C, Jerome R, Bocharova V, KiriyA, et al. Surface modification of multiwalled carbon nanotubes bypoly(2-vinylpyridine): dispersion, selective deposition, and deco-ration of the nanotubes. Adv Mater 2004;16:2123–7.

[120] Riggs JE, Walker DB, Carroll DL, Sun Y-P. Optical limiting proper-ties of suspended and solubilized carbon nanotubes. J Phys Chem B2000;104:7071–6.

[121] Sun Y-P, Huang W, Lin Y, Fu K, Kitaygorodskiy A, Riddle LA, etal. Soluble dendron-functionalized carbon nanotubes: preparation,characterization, and properties. Chem Mater 2001;13:2864–9.

[122] Sano M, Kamino A, Shinkai S. Construction of carbon nanotube starswith dendrimers. Angew Chem Int Ed 2001;40:4661–3.

[123] Cao L, Yang W, Yang J, Wang C, Fu S. Hyperbranchedpoly(amidoamine)-modified multi-walled carbon nanotubes viagrafting-from method. Chem Lett 2004;33:490–1.

[124] Blake R, Gunko YK, Coleman J, Cadek M, Fonseca A, Nagy JB, etal. A generic organometallic approach toward ultra-strong carbonnanotube polymer composites. J Am Chem Soc 2004;126:10226–7.

[125] Qu L, Veca LM, Lin Y, Kitaygorodskiy A, Chen B, McCall AM, et al.Soluble nylon-functionalized carbon nanotubes from anionic ring-opening polymerization from nanotube surface. Macromolecules2005;38:10328–31.

[126] Yang M, Gao Y, Li H, Adronov A. Functionalization of multiwalledcarbon nanotubes with polyamide 6 by anionic ring-opening poly-merization. Carbon 2007;45:2327–33.

[127] Park SJ, Cho MS, Lim ST, Cho HJ, Jhon MS. Synthesis and dispersioncharacteristics of multi-walled carbon nanotube composites withpoly(methyl methacrylate) prepared by in-situ bulk polymeriza-tion. Macromol Rapid Commun 2003;24:1070–3.

[128] Yao Z, Braidy N, Botton GA, Adronov A. Polymerization from the sur-face of single-walled carbon nanotubes—preparation and charac-terization of nanocomposites. J Am Chem Soc 2003;125:16015–24.

[129] Baskaran D, Mays JW, Bratcher MS. Polymer-grafted multi-walled carbon nanotubes through surface-initiated polymeriza-tion. Angew Chem Int Ed 2004;43:2138–42.

[130] Kong H, Gao C, Yan D. Functionalization of multiwalled carbonnanotubes by atom transfer radical polymerization and defunction-alization of the products. Macromolecules 2004;37:4022–30.

[131] Kong H, Luo P, Gao C, Yan D. Polyelectrolyte-functionalizedmultiwalled carbon nanotubes: preparation, characterization andlayer-by-layer self-assembly. Polymer 2005;46:2472–85.

[132] Kong H, Gao C, Yan D. Constructing amphiphilic polymer brusheson the convex surfaces of multi-walled carbon nanotubes byin situ atom transfer radical polymerization. J Mater Chem2004;14:1401–5.

[133] Kong H, Li W, Gao C, Yan D, Jin Y, Walton DRM, et al. Poly(N-isopropylacrylamide)-coated carbon nanotubes: temperature-sensitive molecular nanohybrids in water. Macromolecules2004;37:6683–6.

[134] Qin S, Qin D, Ford WT, Herrera JE, Resasco DE. Grafting of poly(4-vinylpyridine) to single-walled carbon nanotubes and assembly ofmultilayer films. Macromolecules 2004;37:9963–7.

[135] Wu W, Zhang S, Li Y, Li J, Liu L, Qin Y, et al. PVK-modified single-walled carbon nanotubes with effective photoinduced electrontransfer. Macromolecules 2003;36:6286–8.

[136] Gao C, Vo CD, Jin YZ, Li W, Armes SP. Multihydroxy polymer-functionalized carbon nanotubes: synthesis, derivation, and metalloading. Macromolecules 2005;38:8634–48.

[137] Li J, He WD, Yang LP, Sun XL, Hua Q. Preparation of multi-walled carbon nanotubes grafted with synthetic poly(L-lysine)through surface-initiated ring opening polymerization. Polymer2007;48:4352–60.

[138] Zeng HL, Gao C, Yan DY. Poly(�-caprolactone)-functionalized car-bon nanotubes and their biodegradation properties. Adv FunctMater 2006;16:812–8.

[139] Chen GX, Kim HS, Park BH, Yoon JS. Synthesis of poly(L-lactide)-functionalized multiwalled carbon nanotubes by ring-openingpolymerization. Macromol Chem Phys 2007;208:389–98.

r Science 35 (2010) 837–867

[140] Yan YH, Cui J, Chan-Park MB, Wang X, Wu QY. Systematic studies ofcovalent functionalization of carbon nanotubes via argon plasma-assisted UV grafting. Nanotechnology 2007;18, 115712/1–7.

[141] Yang J, Hu J, Wang C, Qin Y, Guo Z. Fabrication and characterizationof soluble multi-walled carbon nanotubes reinforced P(MMA-co-EMA) composites. Macromol Mat Eng 2004;289:828–32.

[142] Huisgen R. 1,3-Dipolar cycloadditions-introduction, survey, mech-anism in 1,3-dipolar cycloaddition chemistry. In: Padwa A, editor.1,3-Dipolar cycloaddition chemistry, vol. 1. New York: Wiley; 1984.p. 1–176.

[143] Kolb HC, Finn MG, Sharpless KB. Click chemistry: diverse chem-ical function from a few good reactions. Angew Chem Int Ed2001;40:2004–21.

[144] Kolb HC, Sharpless KB. The growing impact of click chemistry ondrug discovery. Drug Disc Today 2003;8:1128–37.

[145] Tornøe CW, Christensen C, Meldal M. Peptidotriazoles on solidphase: [1,2,3]-triazoles by regiospecific copper(I)-catalyzed 1,3-dipolar cycloadditions of terminal alkynes to azides. J Org Chem2002;67:3057–64.

[146] Joralemon MJ, OReilly RK, Matson JB, Nugent AK, Hawker CJ, Woo-ley KL. Dendrimers clicked together divergently. Macromolecules2005;38:5436–43.

[147] Wu P, Feldman AK, Nugent AK, Hawker CJ, Scheel A, Voit B, et al.Efficiency and fidelity in a click-chemistry route to triazole den-drimers by the copper(I)-catalyzed ligation of azides and alkynes.Angew Chem Int Ed 2004;43:3928–32.

[148] Helms B, Mynar JL, Hawker CJ, Frechet JMJ. Dendronized lin-ear polymers via click chemistry. J Am Chem Soc 2004;126:15020–1.

[149] Malkoch M, Schleicher K, Drockenmuller E, Hawker CJ, Russell TP,Wu P, et al. Structurally diverse dendritic libraries: a highly efficientfunctionalization approach using click chemistry. Macromolecules2005;38:3663–78.

[150] Lee JW, Kim JH, Kim BK, Shin WS, Jin SH. Synthesis of Frechet typedendritic benzyl propargyl ether and Frechet type triazole den-drimer. Tetrahedron 2006;62:894–900.

[151] Link AJ, Vink MKS, Tirrell DA. Presentation and detection of azidefunctionality in bacterial cell surface proteins. J Am Chem Soc2004;126:10598–602.

[152] Kumar I, Rode CV. Efficient synthesis of fused 1,2,3-triazolo-delta-lactams using huisgen [3+2] dipolar cycloaddition click-chemistryin water. Chem Lett 2007;36:592–3.

[153] Rostovtsev VV, Green LG, Fokin VV, Sharpless KB. A stepwisehuisgen cycloaddition process: copper(I)-catalyzed regioselectiveligation of azides and terminal alkynes. Angew Chem Int Ed2002;41:2596–9.

[154] Horne WS, Yadav MK, Stout CD, Ghadiri MR. Heterocyclic peptidebackbone modifications in an alpha-helical coiled coil. J Am ChemSoc 2004;126:15366–7.

[155] Li H, Cheng F, Duft AM, Adronov A. Functionalization of single-walled carbon nanotubes with well-defined polystyrene by clickcoupling. J Am Chem Soc 2005;127:14518–24.

[156] Kumar I, Rana S, Rode CV, Cho JW. Functionalization of single-walled carbon nanotubes with azides derives from amino acidsusing click chemistry. J Nanosci Nanotech 2008;8:3351–6.

[157] Campidelli S, Ballesteros B, Filoramo A, Dıaz DD, Torre G, Torres T,et al. Facile decoration of functionalized single-wall carbon nan-otubes with phthalocyanines via click chemistry. J Am Chem Soc2008;130:11503–9.

[158] Li H, Adronov A. Water soluble SWCNTs from sulfonation ofnanotube-bound polystyrene. Carbon 2007;45:984–90.

[159] Rana S, Cho JW. Synthesis and characterization of polyurethane-functionalized carbon nanotubes with shape memory effect usingclick chemistry. Proceed intern confer intelligent textiles 2007, Seoul,Korea; 2007. p. 142–3.

[160] Wildgoose GG, Banks CE, Compton RG. Metal nanoparticles andrelated materials supported on carbon nanotubes: methods andapplications. Small 2006;2:182–93.

[161] Voggu R, Suguna P, Chandrasekaran S, Rao CNR. Assembling cova-lently linked nanocrystals and nanotubes through click chemistry.Chem Phys Lett 2007;443:118–21.

[162] He H, Zhang Y, Gao C, Wu J. Clicked magnetic nanohybrids with asoft polymer interlayer. Chem Commun 2009:1655–7.

[163] Rana S, Kumar I, Yoo HJ, Cho JW. Assembly of gold nanoparticles onsingle-walled carbon nanotubes by using click chemistry. J NanosciNanotech 2009;9:3261–3.

[164] Zhang Y, He H, Gao C, Wu J. Covalent layer-by-layer functionaliza-tion of multiwalled carbon nanotubes by click chemistry. Langmuir2009;25:5814–24.

Polymer
[165] Zhang Y, He H, Gao C. Clickable macroinitiator strategy tobuild amphiphilic polymer brushes on carbon nanotubes. Macro-molecules 2008;41:9581–94.

[166] Hong CY, Pan CY. Functionalized carbon nanotubes responsive toenvironmental stimuli. J Mater Chem 2008;18:1831–6.

[167] Liu J, Nie Z, Gao Y, Adronov A, Li H. Click coupling betweenalkyne-decorated multiwalled carbon nanotubes and reactivePDMA-PNIPAM micelles. J Polym Sci Part A Polym Chem2008;46:7187–99.

[168] Guo Z, Liang L, Liang JJ, Ma YF, Yang XY, Ren DM, et al. Covalently�-cyclodextrin modified single-walled carbon nanotubes: a novelartificial receptor synthesized by click chemistry. J Nanopart Res2008;10:1077–83.

[169] Li C, Pang XJ, Qu MZ, Zhang QT, Wang B, Zhang BL, et al. Fab-rication and characterization of polycarbonate/carbon nanotubescomposites. Compos Part A 2005;37:1485–9.

[170] Xia H, Wang Q, Qiu G. Polymer-encapsulated carbon nanotubesprepared through ultrasonically initiated in situ emulsion polymer-ization. Chem Mater 2003;15:3879–86.

[171] Suslick KS. Sonochemistry. Science 1990;247:1439–45.[172] Safadi B, Andrews R, Grulke EA. Multiwalled carbon nanotubes

polymer composites: synthesis and characterization of thin films.J Appl Polym Sci 2002;84:2660–9.

[173] Sahoo NG, Jung YC, Yoo HJ, Cho JW. Effect of functional-ized carbon nanotubes on molecular interaction and propertiesof polyurethane composites. Macromol Chem Phys 2006;207:1773–80.

[174] Vigolo B, Penicaud A, Coulon C, Sauder C, Pailler R, Journet C, etal. Macroscopic fibers and ribbons of oriented carbon nanotubes.Science 2000;290:1331–4.

[175] Saran N, Parikh K, Suh DS, Munoz E, Kolla H, Manhor SK. Fabri-cation and characterization of thin films of single-walled carbonnanotubes bundles on flexible plastic substrates. J Am Chem Soc2004;126:4462–3.

[176] Barrau S, Demont P, Perez E, Peigney A, Laurent C, Laca-banne C. Effect of palmitic acid on the electrical conductivityof carbon nanotubes-epoxy resin composites. Macromolecules2003;36:9678–80.

[177] Camponeschi E, Florkowski B, Vance R, Garrett G, GarmestaniH, Tannenbaum R. Uniform directional alignment of single-walled carbon nanotubes in viscous polymer flow. Langmuir2006;22:1858–62.

[178] Chen W, Tao X. Self-organizing alignment of carbon nan-otubes in thermoplastic polyurethane. Macromol Rapid Commun2005;26:1763–7.

[179] Qian D, Dickey EC, Andrews R, Rantell T. Load transfer and defor-mation mechanisms in carbon nanotube-polystyrene composites.Appl Phys Lett 2000;76:2868–70.

[180] Wong M, Paramsothy M, Xu XJ, Ren Y, Li S, Liao K. Physi-cal interaction at carbon nanotube-polymer interface. Polymer2003;44:7757–64.

[181] Kota AK, Cipriano BH, Duesterberg MK, Gershon AL, PowellD, Raghavan SR, et al. Electrical and reheological perco-lation in polystyre/MWCNT nanocomposites. Macromolecules2007;40:7400–6.

[182] Sandler J, Shaffer MSP, Prasse T, Bauhofer W, Schulte K, WindleAH. Development of a dispersion process for carbon nanotubes inan epoxy matrix and the resulting electrical properties. Polymer1999;40:5967–71.

[183] Song YS, Youn JR. Influence of dispersion states of carbon nan-otubes on physical properties of epoxy nanocomposites. Carbon2005;43:1378–85.

[184] Shaffer MSP, Windle AH. Fabrication and characterization ofcarbon nanotube/poly(vinyl alcohol) composites. Adv Mater1999;11:937–41.

[185] Chae HG, Sreekumar TV, Uchida T, Kumar S. A compar-ison of reinforcement efficiency of various types of car-bon nanotubes in polyacrylonitrile fiber. Polymer 2005;46:10925–35.

[186] Bin Y, Kitanaka M, Zhu D, Matsuo M. Development of highlyoriented polyethylene filled with aligned carbon nanotubesby gelation/crystallization from solutions. Macromolecules2003;36:6213–9.

[187] Andrews R, Jacques D, Minot M, Rantell T. Fabrication of carbonmultiwall nanotube/polymer composites by shear mixing. Micro-mol Mater Eng 2002;287:395–403.

[188] Zhang WD, Shen L, Phang IY, Liu T. Carbon nanotubes reinforcednylon-6 composite prepared by simple melt-compounding. Macro-molecules 2004;37:256–9.

Science 35 (2010) 837–867 863

[189] Bocchini S, Frache A, Camino G, Claes M. Polyethylene thermaloxidative stabilisation in carbon nanotubes based nanocomposites.Eur Polym J 2007;43:3222–35.

[190] McNally T, Potschkeb P, Halley P, Murphy M, Martin D, Bell SEJ, et al.Polyethylene multiwalled carbon nanotube composites. Polymer2005;46:8222–32.

[191] Lopez Manchado MA, Valentini L, Biagiotti J, Kenny JM. Thermaland mechanical properties of single-walled carbon nanotubes-polypropylene composites prepared by melt processing. Carbon2005;43:1499–505.

[192] Valentini L, Biagiotti J, Kenny JM, Santucci S. Morphologicalcharacterization of single-wall carbon nanotube/polypropylenecomposites. Compos Sci Technol 2003;63:1149–53.

[193] Zhang H, Zhang Z. Impact behaviour of polypropylene filled withmulti-walled carbon nanotubes. Eur Polym J 2007;43:3197–207.

[194] Tang W, Santare MH, Advani SG. Melt processing and mechanicalproperty characterization of multi-walled carbon nanotube/highdensity polyethylene (MWNT/HDPE) composite films. Carbon2003;41:2779–85.

[195] Potschke P, Fornes TD, Paul DR. Rheological behavior of mul-tiwalled carbon nanotube/polycarbonate composites. Polymer2002;43:3247–55.

[196] Potschke P, Bhattacharyya AR, Janke A. Morphology and electri-cal resistivity of melt mixed blends of polyethylene and carbonnanotube filled polycarbonate. Polymer 2003;44:8061–9.

[197] Pötschke P, Bhattacharyya AR, Janke A. Melt mixing of polycarbon-ate with multiwalled carbon nanotubes: microscopic studies on thestate of dispersion. Eur Polym J 2004;40:137–48.

[198] Jin Z, Pramoda KP, Xu G, Goh SH. Dynamic mechanical behav-ior of melt-processed multi-walled carbon nanotube/poly(methylmethacrylate) composites. Chem Phys Lett 2001;337:43–7.

[199] Zeng J, Saltysiak B, Johnson WS, Schiraldi DA, Kumar S. Process-ing and properties of poly(methyl methacrylate)/carbon nano fibercomposites. Compos Part B 2004;35:173–8.

[200] Haggenmueller R, Gommans HH, Rinzler AG, Fischer JE, Winey KI.Aligned single-wall carbon nanotubes in composites by melt pro-cessing methods. Chem Phys Lett 2000;330:219–25.

[201] Gorga RE, Cohen RE. Toughness enhancements in poly(methylmethacrylate) by addition of oriented multiwall carbon nanotubes.J Polym Sci Part B Polym Phys 2004;42:2690–702.

[202] Zeng Y, Ying Z, Du J, Cheng HM. Effects of carbon nanotubes onprocessing stability of polyoxymethylene in melt-mixing process.J Phys Chem C 2007;111:13945–50.

[203] Siochi EJ, Working DC, Park C, Lillehei PT, Rouse JH, Topping CC,et al. Melt processing of SWNT-polyimide nanocomposite fibres.Compos Part B 2004;35:439–46.

[204] Meincke O, Kaempfer D, Weickmann H, Friedrich C, VathauerM, Warth H. Mechanical properties and electrical conductivity ofcarbon-nanotube filled polyamide-6 and its blends with acryloni-trile/butadiene/styrene. Polymer 2004;45:739–48.

[205] Hu N, Zhou H, Dang G, Rao X, Chen C, Zhang W. Efficient dispersionof multi-walled carbon nanotubes by in situ polymerization. PolymInt 2007;56:655–9.

[206] So HH, Cho JW, Sahoo NG. Effect of carbon nanotubes on mechanicaland electrical properties of polyimide/carbon nanotubes nanocom-posites. Eur Polym J 2007;43:3750–6.

[207] Jia Z, Wang Z, Xu C, Liang J, Wei B, Wu D, et al. Study on poly(methylmethacrylate): carbon nanotube composites. Mater Sci Eng A1999;271:395–400.

[208] Putz KW, Mitchell CA, Krishnamoorti R, Green PF. Elastic modu-lus of single-walled carbon nanotube/poly(methyl methacrylate)nanocomposites. J Polym Sci Part B Polym Phys 2004;42:2286–93.

[209] Sung JH, Kim HS, Jin HJ, Choi HJ, Chin IJ. Nanofibrous membranesprepared by multiwalled carbon nanotube/poly(methyl methacry-late) composites. Macromolecules 2004;37:9899–902.

[210] Hwang GL, Shieh YT, Hwang KC. Efficient load transfer to polymer-grafted multiwalled carbon nanotubes in polymer composites. AdvFunct Mater 2004;14:487–91.

[211] Velasco-Santos C, Martinez-Hernandez AL, Fisher FT, Ruoff R, Cas-tano VM. Improvement of thermal and mechanical properties ofcarbon nanotube composites through chemical functionalization.Chem Mater 2003;15:4470–5.

[212] Karim MR, Lee CJ, Park YT, Lee MS. SWNTs coated by con-ducting polyaniline: synthesis and modified properties. Synt Met2005;151:131–5.

[213] Long Y, Chen Z, Zhang X, Zhang J, Liu Z. Electrical properties of multi-walled carbon nanotube/polypyrrole nanocables: percolation-dominated conductivity. J Phys D Appl Phys 2004;37:1965–9.

Polyme
[214] An KH, Jeong SY, Hwang HR, Lee YH. Enhanced sensivity of a garsensor incorporating single-walled carbon nanotube-polypyrrolenanocomposites. Adv Mater 2004;16:1005–9.

[215] Fan J, Wan M, Zhu D, Chang B, Pan Z, Xie S. Synthesis, character-izations, and physical properties of carbon nanotubes coated byconducting polypyrrole. J Appl Polym Sci 1999;74:2605–10.

[216] Sahoo NG, Jung YC, So HH, Cho JW. Polypyrrole coated carbonnanotubes: synthesis, characterization, and enhanced electricalproperties. Synth Met 2007;157. p. 179–374.

[217] Yoo HJ, Jung YC, Sahoo NG, Cho JW. Electroactive shape memorypolyurethane nanocomposites from in-situ polymerization withcarbon nanotubes. J Macromol Sci Part B Phys 2006;45:441–51.

[218] Xia H, Song M. Preparation and characterization of polyurethane-carbon nanotube composites. Soft Matter 2005;1:386–94.

[219] Xia H, Song M. Preparation and characterization of polyurethanegrafted single-walled carbon nanotubes and derived polyurethanenanocomposites. J Mater Chem 2006;16:1843–51.

[220] Jung YC, Sahoo NG, Cho JW. Polymeric nanocomposites ofpolyurethane block copolymers and functionalized multi-walledcarbon nanotubes as crosslinkers. Macromol Rapid Commun2006;27:126–31.

[221] Gao J, Itkis ME, Yu A, Bekyarova E, Zhao B, Haddon RC. Continu-ous spinning of a single-walled carbon nanotube-nylon compositefiber. J Am Chem Soc 2005;127:3847–54.

[222] Tomalia DA, Baker H, Dewald J, Hall M, Kallos G, Martin S, etal. Dendritic macromolecules: synthesis of starburst dendrimers.Macromolecules 1986;19:2466–8.

[223] Cloninger MJ. Biological applications of dendrimers. Curr OpinChem Biol 2002;6:742–8.

[224] Ishizu K, Tsubaki K, Mori A, Uchida S. Architecture of nanostruc-tured polymers. Prog Polym Sci 2003;28:27–54.

[225] Zeng F, Zimmerman SC. Dendrimers in superamolecular chemistry:from recognition to self-assembly. Chem Rev 1997;97:1681–712.

[226] Matthews OA, Shipway AN, Stoddart JF. Dendrimers-branchingout from curiosities into new technologies. Prog Polym Sci1998;23:1–56.

[227] Gao C, Yan D. Hyperbranched polymers: from synthesis to applica-tions. Prog Polym Sci 2004;29:183–275.

[228] Hawker CJ, Frechet JMJ. Preparation of polymers with controlledmolecular architecture. A new convergent approach to dendriticmacromolecules. J Am Chem Soc 1990;112:7638–47.

[229] Huang F, Gibson HW. Formation of supramolecular hyperbranchedpolymer from self-organization of an AB2 monomer contain-ing a crown ether and two paraquat moieties. J Am Chem Soc2004;126:14738–9.

[230] Yates CR, Hayes W. Synthesis and applications of hyperbranchedpolymers. Eur Polym J 2004;40:1257–81.

[231] Feng QP, Xie XM, Liu YT, Zhao W, Gao YF. Synthesis ofhyperbranched aromatic polyamide-imide and its grafting ontomultiwalled carbon nanotubes. J Appl Polym Sci 2007;106:2413–21.

[232] Davis JJ, Coleman KS, Azamian BR, Bagshaw CB, Green MLH. Chem-ical and biochemical sensing with modified single walled carbonnanotubes. Chem Eur J 2003;9:3732–9.

[233] Xu Y, Gao C, Kong H, Yan D, Jin YZ, Watts PCP. Growing mul-tihydroxyl hyperbranched polymers on the surfaces of carbonnanotubes by in situ ring-opening polymerization. Macromolecules2004;37:8846–53.

[234] Yang Y, Xie X, Yang Z, Wang X, Cui W, Yang J, et al.Controlled synthesis and novel solution rheology of hyper-branched poly(urea-urethane)-functionalized multiwalled carbonnanotubes. Macromolecules 2007;40:5858–67.

[235] Hwang SH, Moorefield CN, Wang P, Jeong KU, Cheng SZD, KottaKK, et al. Dendron-tethered and templated CdS quantum dots onsingle-walled carbon nanotubes. J Am Chem Soc 2006;128:7505–9.

[236] Wang X, Liu H, Jin Y, Chen C. Polymer-functionalized multi-walledcarbon nanotubes as lithium intercalation hosts. J Phys Chem B2006;110:10236–40.

[237] Martin RB, Qu L, Lin Y, Harruff BA, Bunker CE, Gord JR, et al. Func-tionalized carbon nanotubes with tethered pyrenes: synthesis andphotophysical properties. J Phys Chem B 2004;108:11447–53.

[238] Campidelli S, Sooambar C, Lozano DE, Ehli C, Guldi DM, PratoM. Dendrimer-functionalized single-wall carbon nanotubes: syn-
thesis, characterization, and photoinduced electron transfer. J AmChem Soc 2006;128:12544–52.
[239] Tao L, Chen G, Mantovani G, York S, Haddleton DM. Modificationof multi-wall carbon nanotube surfaces with poly(amidoamine)dendrons: synthesis and metal templating. Chem Commun2006:4949–51.

r Science 35 (2010) 837–867

[240] Yuan W, Jiang G, Che J, Qi X, Xu R, Chang MW, et al. Deposition ofsilver nanoparticles on multiwalled carbon nanotubes grafted withhyperbranched poly(amidoamine) and their antimicrobial effects.J Phys Chem C 2008;112:18754–9.

[241] Valentini L, Trentini M, Mengoni F, Alongi J, Armentano I,Ricco L, et al. Synthesis and photoelectrical properties of carbonnanotube–dendritic porphyrin light harvesting molecule systems.Diamond Rel Mater 2007;16:658–63.

[242] Gu L, Lin Y, Qu L, Sun YP. Carbon nanotubes as a scaffold to displaypaired sugars in solution. Biomacromolecules 2006;7:400–2.

[243] Yang H, Wang SC, Mercier P, Akins DL. Diameter selective dispersionof single-walled carbon nanotubes using a water soluble biocom-patible polymer. Chem Commun 2006:1425–7.

[244] Gao C, Muthukrishnan S, Li W, Yuan J, Xu Y, Mueller AHE. Linearand hyperbranched glycopolymer-functionalized carbon nan-otubes: synthesis, kinetics, and characterization. Macromolecules2007;40:1803–15.

[245] Adeli M, Bahari A, Hekmatara H. Carbon nanotube-graft-poly(citricacid) nanocomposites. Nano 2008;3:37–44.

[246] Hong CY, You YZ, Wu D, Liu Y, Pan CY. Multiwalled carbonnanotubes grafted with hyperbranched polymer shell via SCVP.Macromolecules 2005;38:2606–11.

[247] Yang Y, Xie X, Wu J, Yang Z, Wang X, Mai YW. Multiwalledcarbon nanotubes functionalized by hyperbranched poly(urea-urethane)s by a one-pot polycondensation. Macromol RapidCommun 2006;27:1695–701.

[248] Choi JY, Oh SJ, Lee HJ, Wang DH, Tan LS, Baek JB. In-situgrafting of hyperbranched poly(ether ketone)s onto multiwalledcarbon nanotubes via the A3 + B2 approach. Macromolecules2007;40:4474–80.

[249] Choi JY, Han SW, Huh WS, Tan LS, Baek JB. In situ graft-ing of carboxylic acid-terminated hyperbranched poly(ether-ketone) to the surface of carbon nanotubes. Polymer 2007;48:4034–40.

[250] Jeon IY, Tan LS, Baek JB. Nanocomposites derived from in situgrafting of linear and hyperbranched poly(ether-ketone)s contain-ing flexible oxyethylene spacers onto the surface of multiwalledcarbon nanotubes. J Polym Sci Part A Polym Chem 2008;46:3471–81.

[251] Zhou W, Xu J, Shi W. Surface modification of multi-wall carbonnanotube with ultraviolet-curable hyperbranched polymer. ThinSolid Films 2008;516:4076–82.

[252] Star A, Stoddart JF. Dispersion and solubilization of single-walledcarbon nanotubes with a hyperbranched polymer. Macromolecules2002;35:7516–20.

[253] Rana S, Karak N, Cho JW, Kim YH. Enhanced dispersion of car-bon nanotubes in hyperbranched polyurethane and properties ofnanocomposites. Nanotechnology 2008;19, 495707/1–8.

[254] Kim HH, Rana S, Cho JW, Lee JY. High concentration carbon nan-otubes in hyperbranched polyurethane and their characteristics,submitted.

[255] Ogoshi T, Saito T, Yamagishi T, Nakamoto Y. Solubilization of single-walled carbon nanotubes by entanglements between them andhyperbranched phenolic polymer. Carbon 2009;47:117–23.

[256] Zhou W, Lv S, Shi W. Preparation of micelle-encapsulatedsingle-wall and multi-wall carbon nanotubes with amphiphilichyperbranched polymer. Eur Polym J 2008;44:587–601.

[257] Zhang J, Zheng Y, Yu P, Mo S, Wang R. The synthesis offunctionalized carbon nanotubes by hyperbranched poly(amine-ester) with liquid-like behavior at room temperature. Polymer2009;50:2953–7.

[258] Sawada H, Shindo K, Iidzuka J, Ueno K, Hamazaki K. Solu-bilization and applications of single-walled carbon nanotubesinto aqueous and organic media by the use of nanometer size-controlled fluoroalkyl end-capped oligomeric aggregates. EurPolym J 2005;41:2232–9.

[259] Sawada H, Naitoh N, Kasai R, Suzuki M. Dispersion of single-walled carbon nanotubes in water by the use of novel fluorinateddendrimer-type copolymers. J Mater Sci 2008;43:1080–6.

[260] Yoshioka H, Suzuki M, Mugisawa M, Naitoh N, Sawada H. Synthesisand applications of novel fluorinated dendrimer-type copolymerby the use of fluoroalkyanoyl peroxide as a key intermediate. J CollInterf Sci 2007;308:4–10.

[261] Valentini L, Armentano I, Ricco L, Alongi J, Pennelli G, Mariani A,et al. Selective interaction of single-walled carbon nanotubes withconducting dendrimer. Diamond Rel Mater 2006;15:95–9.

[262] Zheng H, Xue H, Zhang Y, Shen Z. A glucose biosensor based onmicroporous polyacrylonitrile synthesized by single rare-earth cat-alyst. Biosens Bioelectron 2002;17:541–5.

Polymer
[263] Yi X, Xian JH, Yuan CH. Direct electrochemistry of horseradishperoxidase immobilized on a colloid/cysteamine-modified goldelectrode. Anal Biochem 2000;278:22–8.

[264] Xu JJ, Chen HY. Amperometric glucose sensor based onglucose oxidase immobilized in electrochemically generatedpoly(ethacridine). Anal Chim Acta 2000;423:101–6.

[265] Xu L, Zhu Y, Tang L, Yang X, Li C. Biosensor based onself-assembling glucose oxidase and dendrimer-encapsulated Ptnanoparticles on carbon nanotubes for glucose detection. Electro-analysis 2007;19:717–22.

[266] Sahoo NG, Jung YC, So HH, Cho JW. Synthesis of polyurethanenanocomposites of functionalized carbon nanotubes by in-situ polymerization methods. J Korean Phys Soc 2007;51:S1–6.

[267] Potschke P, Haussler L, Pegel S, Steinberger R, Scholz G. Thermo-plastic polyurethane filled with carbon nanotubes for electricaldissipative and conductive applications. Kauts Gummi Kunst2007;60:432–7.

[268] Lee CH, Liu JY, Chen SL, Wang YZ. Miscibility and proper-ties of acid-treated multi-walled carbon nanotubes/polyurethanenanocomposites. Polymer J 2007;39:138–46.

[269] Koerner H, Liu W, Alexander M, Mirau P, Dowty H, Vaia RA.Deformation-morphology correlations in electrically conductivecarbon nanotube-thermoplastic polyurethane nanocomposites.Polymer 2005;46:4405–20.

[270] Ryszkowska J, Jurczyk-Kowalska M, Szymborski T, Kurzydlowski KJ.Dispersion of carbon nanotubes in polyurethane matrix. Physica E2007;39:124–7.

[271] Meng Q, Hu J, Zhu Y. Shape-memory polyurethane/multiwalledcarbon nanotube fibers. J Appl Polym Sci 2007;106:837–48.

[272] Chen XH, Chen XJ, Lin M, Zhong WB, Chen XH, Chen ZH.Functionalized multi-walled carbon nanotubes prepared by insitu polycondensation of polyurethane. Macromol Chem Phys2007;208:964–72.

[273] Xiong JW, Zheng Z, Qin X, Li M, Li HQ, Wang X. The thermal andmechanical properties of a polyurethane/multi-walled carbon nan-otube composite. Carbon 2006;44:2701–7.

[274] Kwon JY, Kim HD. Preparation and properties of acid-treatedmultiwalled carbon nanotube/waterborne polyurethane nanocom-posites. J Appl Polym Sci 2005;96:595–604.

[275] Sahoo NG, Jung YC, Cho JW. Electroactive shape memoryeffect of polyurethane composites filled with carbon nan-otubes and conducting polymer. Mater Manufac Proc 2007;22:419–23.

[276] Sahoo NG, Jung YC, Yoo HJ, Cho JW. Influence of carbon nanotubesand polypyrrole on the thermal, mechanical and electroactiveshape-memory properties of polyurethane nanocomposites. Com-pos Sci Technol 2007;67:1920–9.

[277] Kuan HC, Ma CCM, Chang WP, Yuen SM, Wu HH, Lee TM. Synthesis,thermal, mechanical and rheological properties of multiwall car-bon nanotube/waterborne polyurethane nanocomposite. ComposSci Technol 2005;65:1703–10.

[278] Xia H, Song M, Jin J, Chen L. Poly(propylene glycol)-grafted multi-walled carbon nanotube polyurethane. Macromol Chem Phys2006;27:1945–52.

[279] Wang TL, Tseng CG. Polymeric carbon nanocomposites frommultiwalled carbon nanotubes functionalized with segmentedpolyurethane. J Appl Polym Sci 2007;105:1642–50.

[280] McClory C, McNally T, Brennan GP, Erskine J. Thermosettingpolyurethane multiwalled carbon nanotube composites. J ApplPolym Sci 2007;105:1003–11.

[281] Sen R, Zhao B, Perea D, Itkis ME, Hu H, Love J, et al. Prepara-tion of single walled carbon nanotube reinforced polystyrene andpolyurethane nanofibers and membranes by electrospinning. NanoLett 2004;4:459–64.

[282] Yuen SM, Ma CCM, Chiang CL, Lin YY, Teng CC. Preparation andmorphological, electrical, and mechanical properties of polyimide-grafted MWCNT/polyimide composite. J Polym Sci Part A PolymChem 2007;45:3349–58.

[283] Ounaies Z, Park C, Wise KE, Siochi EJ, Harrison JS. Electricalproperties of single wall carbon nanotube reinforced polyimidecomposites. Compos Sci Technol 2003;63:1637–46.

[284] Yuen SM, Ma CCM, Lin YY, Kuan HC. Preparation, morphol-
ogy and properties of acid and amine modified multiwalledcarbon nanotube/polyimide composite. Compos Sci Technol2007;67:2564–73.
[285] Zhu BK, Xie SH, Xu ZK, Xu YY. Preparation and properties of thepolyimide/multi-walled carbon nanotubes nanocomposites. Com-pos Sci Technol 2006;66:548–54.

Science 35 (2010) 837–867 865

[286] Jiang X, Bin Y, Matsuo M. Electrical and mechanical properties ofpolyimide-carbon nanotubes composites fabricated by in situ poly-merization. Polymer 2005;46:7418–24.

[287] Yu A, Hu H, Bekyarova E, Itkis ME, Gao J, Zhao B, et al. Incorporationof highly dispersed single-walled carbon nanotubes in a polyimidematrix. Compos Sci Technol 2006;66:1190–7.

[288] Ogasawara T, Ishida Y, Ishikawa T, Yokota R. Characterizationof multi-walled carbon nanotube/phenylethynyl terminated poly-imide composites. Compos Part A 2004;35:67–74.

[289] Chou WJ, Wang CC, Chen CY. Characteristics of polyimide-basednanocomposites containing plasma-modified multi-walled carbonnanotubes. Compos Sci Technol 2008;68:2208–13.

[290] Yuen SM, Ma CHM, Chiang CL, Teng CC, Yu YH. Poly(vinyl-triethoxysilane) modified MWCNT/polyimide nanocomposites—preparation, morphological, mechanical, and electrical properties.J Polym Sci Part A Polym Chem 2008;46:803–16.

[291] Sandler JKW, Kirk JE, Kinloch IA, Shaffer MSP, Windle AH. Ultra-lowelectrical percolation threshold in carbon-nanotube-epoxy com-posites. Polymer 2003;44:5893–9.

[292] Li J, Ma PC, Chow WS, To CK, Tang BZ, Kim JK. Correlations betweenpercolation threshold, dispersion state, and aspect ratio of carbonnanotubes. Adv Funct Mater 2007;17:3207–15.

[293] Choi ES, Brooks JS, Eaton DL, Al Haik MS, Hussaini MY, GarmestaniH, et al. Enhancement of thermal and electrical properties of carbonnanotube polymer composites by magnetic field processing. J ApplPhys 2003;94:6034–9.

[294] Fangming D, Fisher JE, Winey KI. Coagulation method for preparingsingle-walled carbon nanotube/poly(methyl methacrylate) com-posites and their modulus, electrical conductivity, and thermalstability. J Polym Sci Part B Polym Phys 2003;41:3333–8.

[295] Bai JB, Allaoui A. Effect of the length and the aggregate size ofMWNTs on the improvement efficiency of the mechanical and elec-trical properties of nanocomposites-experimental investigation.Compos Part A 2003;34:689–94.

[296] Bryning MB, Islam MF, Kikkawa JM, Yodh AG. Very low conductivitythreshold in bulk isotropic single-walled carbon nanotube-epoxycomposites. Adv Mater 2005;17:1186–91.

[297] Ma PC, Kim JK, Tang BZ. Effects of silane functionalization on theproperties of carbon nanotube/epoxy nanocomposites. Compos SciTechnol 2007;67:2965–72.

[298] Tamburri E, Orlanducci S, Terranova ML, Valentine F, PalleschiG, Curulli A, et al. Modulation of electrical properties in single-walled carbon nanotube/conducting polymer composites. Carbon2005;43:1213–21.

[299] Li S, Qin Y, Shi J, Guo ZX, Li Y, Zhu D. Electrical properties ofsoluble carbon nanotube/polymer composite films. Chem Mater2005;17:130–5.

[300] Grossiord N, Loos J, Laake LV, Maugey M, Zakri C, Koning CE, etal. High-conductivity polymer nanocomposites obtained by tailor-ing the characteristics of carbon nanotube fillers. Adv Funct Mater2008;18:3226–34.

[301] Ramasubramaniam R, Chen J, Liu H. Homogeneous carbon nan-otube/polymer composites for electrical applications. Appl PhysLett 2003;83:2928–30.

[302] Li HC, Lu SY, Syue SH, Hsu WK, Chang SC. Conductivity enhancementof carbon nanotube composites by electrolyte addition. Appl PhysLett 2008;93, 033104/1–3.

[303] Mishra SR, Rawat HS, Mehendale SC, Rustagi KC, Sood AK, Bandy-opadhyay R, et al. Optical limiting in single-walled carbon nanotubesuspensions. Chem Phys Lett 2000;317:510–4.

[304] Vivien L, Riehl D, Hache F, Anglaret E. Nonlinear scattering ori-gin in carbon nanotube suspensions. J Nonlinear Opt Phys Mater2000;9:297–307.

[305] Sun X, Xiong Y, Chen P, Lin J, Ji W, Lim JH, et al. Investigation of anoptical limiting mechanism in multiwalled carbon nanotubes. ApplOpt 2000;39:1998–2001.

[306] O’Flaherty SM, Murphy R, Hold SV, Cadek M, Coleman JN, BlauWJ. Material investigation and optical limiting properties ofcarbon nanotube and nanoparticle dispersions. J Phys Chem B2003;107:958–64.

[307] O’Flaherty SM, Hold SV, Brennan ME, Cadek M, Drury A, Coleman JN,et al. Nonlinear optical response of multiwalled carbon-nanotubedispersions. J Opt Soc Am B 2003;20:49–58.

[308] Goh HW, Goh SH, Xu GQ, Lee KY, Yang GY, Lee YW, et al. Optical lim-iting properties of double-C60-end-capped poly(ethylene oxide),double-C60-end-capped poly(ethylene oxide)/poly(ethyleneoxide) blend, and double-C60-end-capped poly(ethyleneoxide)/multiwalled carbon nanotube composite. J Phys Chem B2003;107:6056–62.

Polyme
[309] Kymakis E, Amaratunga GAJ. Optical properties of polymer-nanotube composites. Synt Metal 2004;142:161–7.

[310] Mulazzi E, Perego R, Aarab H, Mihut L, Faulques E, Lefrant S, et al.Optical properties of carbon nanotube-PPV composites: influenceof the PPV conversion temperature and nanotube concentration.Synth Met 2005;154:221–4.

[311] Peng H. Aligned carbon nanotube/polymer composite films withrobust flexibility, high transparency, and excellent conductivity. JAm Chem Soc 2008;130:42–3.

[312] Kymakis E, Amaratunga GAJ. Single-wall carbon nanotube/conjugated polymer photovoltaic devices. Appl Phys Lett2002;80:112–4.

[313] Fournet P, Coleman JN, Lahr B, Drury A, Blau WJ, O’Brien DF, etal. Enhanced brightness in organic light-emitting diodes using acarbon nanotube composite as an electron-transport layer. J ApplPhys 2001;90:969–75.

[314] Frackowiak E, Khomenko V, Jurewicz K, Lota K, Béguin F. Super-capacitors based on conducting polymers/nanotubes composites. JPower Source 2006;153:413–8.

[315] Zhou CF, Kumar S, Doyle CD, Tour JM. Functionalized sin-gle wall carbon nanotubes treated with pyrrole for electro-chemical supercapacitor membranes. Chem Mater 2005;17:1997–2002.

[316] Dong B, He BL, Xu CL, Li HL. Preparation and electrochemi-cal characterization of polyaniline/multi-walled carbon nanotubescomposites for supercapacitor. Mater Sci Eng B 2007;143:7–13.

[317] Xiao Q, Zhou X. The study of multiwalled carbon nanotubedeposited with conducting polymer for supercapacitor. Elec-trochim Acta 2003;48:575–80.

[318] Landi BJ, Raffaelle RP, Heben MJ, Alleman JL, VanDerveer W, GennettT. Single wall carbon nanotube-Nafion composite actuators. NanoLett 2002;2:1329–32.

[319] Koerner H, Price G, Pearce NA, Alexander M, Vaia RA. Remotelyactuated polymer nanocomposites—stress-recovery of carbon-nanotube-filled thermoplastic elastomers. Nature Mater2004;3:115–20.

[320] Mottaghitalab V, Xi B, Spinks GM, Wallace GG. Polyaniline fibrescontaining single walled carbon nanotubes: enhanced performanceartificial muscles. Synth Met 2006;156:796–803.

[321] Chen HW, Wu RJ, Chan KH, Sun YL, Su PG. The application ofCNT/Nafion composite material to low humidity sensing measure-ment. Sensor Actuat B-Chem 2005;104:80–4.

[322] Cai H, Cao X, Jiang Y, He P, Fang Y. Carbon nanotube-enhanced elec-trochemical DNA biosensor for DNA hybridization detection. AnalBioanal Chem 2003;375:287–93.

[323] Ferrer-Anglada N, Kaempgen M, Roth S. Transparent and flexiblecarbon nanotube/polypyrrole and carbon nanotube/polyaniline pHsensors. Phys Stat Sol B 2006;243:3519–23.

[324] Cheng G, Zhao J, Tu Y, He P, Fang Y. A sensitive DNA electrochemicalbiosensor based on magnetite with a glassy carbon electrode mod-ified by muti-walled carbon nanotubes in polypyrrole. Anal ChimActa 2005;533:11–6.

[325] Cai H, Xu Y, He PG, Fang YZ. Indicator free DNA hybridizationdetection by impedance measurement based on the DNA-dopedconducting polymer film formed on the carbon nanotube modifiedelectrode. Electroanalysis 2003;15:1864–70.

[326] White AA, Best SM, Kinloch LA. Hydroxyapatite–carbon nanotubecomposites for biomedical applications: a review. Int J Appl CeramTechnol 2007;4:1–13.

[327] Bianco A, Kostarelos K, Partidos CD, Prato M. Biomedical appli-cations of functionalised carbon nanotubes. Chem Commun2005:571–7.

[328] Pastorin G, Kostarelos K, Prato M, Bianco A. Functionalized carbonnanotubes: towards the delivery of therapeutic molecules. J BiomedNanotech 2005;1:133–42.

[329] Shi Kam NW, Jessop TC, Wender PA, Dai H. Nanotube molec-ular transporters: internalization of carbon nanotube-proteinconjugates into mammalian cells. J Am Chem Soc 2004;126:6850–1.

[330] Pantarotto D, Partidos CD, Graff R, Hoebeke J, Briand JP, Prato M, etal. Synthesis, structural characterization, and immunological prop-erties of carbon nanotubes functionalized with peptides. J Am ChemSoc 2003;125:6160–4.

[331] Kireitseu M, Hui D, Tomlinson G. Advanced shock-resistant andvibration damping of nanoparticle-reinforced composite material.Compos Part B 2008;39:128–38.

[332] Das D, Das PK. Superior activity of structurally deprived enzyme-carbon nanotube hybrids in cationic reverse micelles. Langmuir2009;25:4421–3.

r Science 35 (2010) 837–867

[333] Shin JY, Premkumar T, Geckeler KE. Dispersion of single-walled car-bon nanotubes by using surfactants: are the type and concentrationimportant? Chem Eur J 2008;14:6044–8.

[334] Doe C, Choi SM, Kline SR, Jang HS, Kim TH. Charged rod-likenanoparticles assisting single-walled carbon nanotube dispersionin water. Adv Funct Mater 2008;18:2685–91.

[335] Park I, Park M, Kim J, Lee H, Lee MS. Multiwalled carbon nanotubesfunctionalized with PS via emulsion polymerization. Macromol Res2007;15:498–505.

[336] Park I, Lee W, Kim J, Park M, Lee H. Selective sequestering ofmulti-walled carbon nanotubes in self-assembled block copolymer.Sensor Actuat B-Chem 2007;126:301–5.

[337] Kim HS, Park WI, Kang M, Jin HJ. Multiple light scattering mea-surement and stability analysis of aqueous carbon nanotubedispersions. J Phys Chem Solid 2008;69:1209–12.

[338] Wang H, Zhou W, Ho DL, Winey KI, Fischer JE, Glinka CJ, et al. Dis-persing single-walled carbon nanotubes with surfactants: a smallangle neutron scattering study. Nano Lett 2004;4:1789–93.

[339] Lisunova MO, Lebovka NI, Melezhyk OV, Boiko YP. Stability of theaqueous suspensions of nanotubes in the presence of nonionic sur-factant. J Coll Interf Sci 2006;299:740–6.

[340] Barone PW, Strano MS. Reversible control of carbon nanotubeaggregation for a glucose affinity sensor. Angew Chem Int Ed2006;45:8138–41.

[341] Numata M, Sugikawa K, Kaneko K, Shinkai S. Creation of hierar-chical carbon nanotube assemblies through alternative packingof complementary semi-artificial ˇ-1,3-glucan/carbon nanotubecomposites. Chem Eur J 2008;14:2398–404.

[342] Yan LY, Poon YF, Chan-Park MB, Chen Y, Zhang Q. Individ-ually dispersing single-walled carbon nanotubes with novelneutral pH water-soluble chitosan derivatives. J Phys Chem C2008;112:7579–87.

[343] Rouse JH. Polymer-assisted dispersion of single-walled carbon nan-otubes in alcohols and applicability toward carbon nanotube/sol-gel composite formation. Langmuir 2005;21:1055–61.

[344] Yuan WZ, Sun JZ, Dong Y, Haussler M, Yang F, Xu HP, et al. Wrappingcarbon nanotubes in pyrene-containing poly(phenylacetylene)chains: solubility, stability, light emission, and surface photovoltaicproperties. Macromolecules 2006;39:8011–20.

[345] Jana RN, Cho JW. Thermal stability, crystallization behav-ior and phase morphology of poly(�-caprolactone)-diol-graftedmulti walled carbon nanotubes. J Appl Polym Sci 2008;110:1550–8.

[346] Georgakilas V, Bourlinos A, Gournis D, Tsoufis T, Trapalis C,Mateo-Alonso A, et al. Multipurpose organically modified carbonnanotubes from functionalization to nanotube composites. J AmChem Soc 2008;130:8733–40.

[347] Xu G, Zhu B, Han Y, Bo Z. Covalent functionalization of multi-walledcarbon nanotube surfaces by conjugated polyfluorenes. Polymer2007;48:7510–5.

[348] Georgakilas V, Tagmatarchis N, Pantarotto D, Bianco A, Briand JP,Prato M. Amino acid functionalisation of water soluble carbon nan-otubes. Chem Commun 2002:3050–1.

[349] Bianco A, Kostarelos K, Prato M. Applications of carbon nanotubesin drug delivery. Curr Opin Chem Biol 2005;9:674–9.

[350] Hayden H, Gun’ko YK, Perova TS. Chemical modification of multi-walled carbon nanotubes using a tetrazine derivative. Chem PhysLett 2007;435:84–9.

[351] Menard-Moyon C, Gross M, Bernard M, Turek P, Doris E, MioskowskiC. Unexpected outcome in the reaction of triazolinedione with car-bon nanotubes. Eur J Org Chem 2007:4817–9.

[352] Menard-Moyon C, Dumas FO, Doris E, Mioskowski C. Func-tionalization of single-wall carbon nanotubes by tandem high-pressure/Cr(CO)6 activation of diels-alder cycloaddition. J Am ChemSoc 2006;128:14764–5.

[353] Cho E, Kim H, Kim C, Han S. Ab initio study on the carbon nan-otube with various degrees of functionalization. Chem Phys Lett2006;419:134–8.

[354] Holzinger M, Steinmetz J, Samaille D, Glerup M, Paillet M, BernierP, et al. [2+1] Cycloaddition for cross-linking SWCNTs. Carbon2004;42:941–7.

[355] Liu Y, Yao Z, Adronov A. Functionalization of single-walled carbonnanotubes with well-defined polymers by radical coupling. Macro-
molecules 2005;38:1172–9.
[356] Liu J, Zubiri MR, Vigolo B, Dossot M, Fort Y, Ehrhardt JJ, et al. Efficientmicrowave-assisted radical functionalization of single-wall carbonnanotubes. Carbon 2007;45:885–91.

[357] Buffa F, Abraham GA, Grady BP, Resasco D. Effect of nanotubefunctionalization on the properties of single-walled carbon nan-

Polymer
otube/polyurethane composites. J Polym Sci Part B Polym Phys2007;45:490–501.

[358] Kwon J, Kim H. Comparison of the properties of waterbonepolyurethane/multiwalled carbon nanotube and acid-treatedmultiwalled carbon nanotube composites prepared by in situ poly-merization. J Polym Sci Part A Polym Chem 2005;43:3973–85.

[359] Guo SZ, Zhang C, Wang WZ, Liu TX, Tjiu WC, He CB, et al. Prepa-ration and characterization of polyurethane/multiwalled carbonnanotube composites. Polym Polym Compos 2008;16:501–7.

[360] Grunlan JC, Mehrabi AR, Bannon MV, Bahr JL. Water-based single-walled-nanotube-filled polymer composite with an exceptionallylow percolation threshold. Adv Mater 2004;16:150–3.

[361] Yu C, Kim YS, Kim D, Grunlan JC. Thermoelectric behav-ior of segregated-network polymer nanocomposites. Nano Lett2008;8:4428–32.

[362] Kim HM, Kim K, Lee CY, Joo J, Cho SJ, Yoon HS, et al. Electricalconductivity and electromagnetic interference shielding of mul-tiwalled carbon nanotube composites containing Fe catalyst. ApplPhys Lett 2004;84:589.

[363] Skakalova V, Dettlaff-Weglikowska U, Roth S. Electrical and
mechanical properties of nanocomposites of single wall carbonnanotubes with PMMA. Synth Met 2005;152:349–52.
[364] Frackowiak E, Jurewicz K, Delpeux S, Beguin F. Nanotublar materialsfor supercapacitors. J Power Source 2001;97–98:822–5.

[365] An KH, Jeon KK, Heo JK, Lim SC, Bae DJ, Lee YH. High-capacitance supercapacitor using a nanocomposite electrode of

Science 35 (2010) 837–867 867

single-walled carbon nanotube and polypyrrole. J Electrochem Soc2002;149:A1058–62.

[366] Dai CA, Hsiao CC, Weng SC, Kao AC, Liu CP, Tsai WB, et al. A mem-brane actuator based on an ionic polymer network and carbonnanotubes: the synergy of ionic transport and mechanical prop-erties. Smart Mater Struct 2009;18, 85016/1–13.

[367] Thomassin JM, Kollar J, Caldarella G, Germain A, Jerome R, Detrem-bleur C. Beneficial effect of carbon nanotubes on the performancesof Nafion membranes in fuel cell applications. J Membr Sci2007;303:252–7.

[368] Joo SH, Park CP, Kim EA, Lee YH, Chang H, Seung D, et al.Functionalized carbon nanotube-poly(arylene sulfone) compositemembranes for direct methanol fuel cells with enhanced perfor-mance. J Power Sources 2008;180:63–70.

[369] Cosnier S, Ionescu RE, Holzinger M. Aqueous dispersions of SWCNTsusing pyrrolic surfactants for the electro-generation of homo-geneous nanotube composites. Application to the design of anamperometric biosensor. J Mater Chem 2008;18:5129–33.

[370] Rege K, Raravikar NR, Kim DY, Schadler LS, Ajayan PM,Dordick JS. Enzyme-polymer-single walled carbon nan-
otube composites as biocatalytic films. Nano Lett 2003;3:829–32.
[371] Sing R, Pantarotto D, McCarthy D, Chaloin O, Hoebeke J, PartidosCD, et al. Binding and condensation of plasmid DNA onto functional-ized carbon nanotubes: toward the construction of nanotube-basedgene delivery vectors. J Am Chem Soc 2005;127:4388–96.

polymer nanocomposites based on functionalized carbon nanotubes

Documents