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Functionalization of carbon nanotubes

via plasma post-discharge surface treatment:

Implication as nanofiller in polymeric matrices

Anne acadmique 2009-2010

Benoit RUELLE

Dissertation prsente en vue de

lobtention du grade acadmique

de Docteur en Sciences

Directeurs de thse : Prof. M. HECQ

Prof. Ph. DUBOIS

LLaabboorraattooiirreeddeeCChhiimmiieeIInnoorrggaanniiqquueeeett

AAnnaallyttiiquuee

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A ma famille et Emilie

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Remerciements

A l'issue de la rdaction de cette recherche, je suis convaincu que la thse est loin d'tre un

travail solitaire. En effet, je n'aurais jamais pu raliser ce travail sans le soutien d'un grand

nombre de personnes dont la gnrosit, la bonne humeur et l'intrt manifests l'gard de

ma recherche m'ont beaucoup aids et permis de progresser.

En premier lieu, je tiens remercier mes directeurs de thse, les Professeurs Michel Hecq et

Philippe Dubois qui mont offert leur confiance et lopportunit de pouvoir raliser cette thse

dans dexcellentes conditions au sein de leurs laboratoires respectifs. Le fait de savoir que

leurs portes taient toujours grandes ouvertes ma t dune grande aide.

Jaimerais remercier plus particulirement trois personnes dont lapport et lcoute ont t

plus que primordiaux ce travail. Merci Carla Bittencourt pour toutes ses discussions

concernant les nanotubes de carbone, ses relectures et les magnifiques analyses XPS et TEM

haute rsolution. Un grand merci galement Thomas Godfroid pour les transformations de la

chambre plasma micro-onde, lide de la Nanotube Dance Party, sa science du plasma micro-

onde, ses conseils et encouragements dans les moments difficiles. Enfin, dzikuj Sophie

Peeterbroeck pour son clairage concernant les composites polymres base de nanotubes, sa

patience, son soutien sans failles, ses relectures et, surtout, son amiti.

I would like to thank Professor Vittoria Vittoria for having welcomed me in the Department

of Chemical and Food Engineering in Salerno University. I am very grateful to Doctor

Giuliana Gorrasi and Professor Salvatore De Pasquale for their welcome, their kindness and

for having shared their enthusiasm. Special thanks to Giovanni, Salvatore, Dante and Luigi

for their help, their jokes and, of course, the Italian coffee breaks which are become a tradition

for me. Finally, a big thank to my friend Stefano for his welcome and the enjoyable time that I

spent with him and his friends. Grazie mille Salerno.

I would like to thank Doctor Hans Miltner, from the VUB, for his collaboration in the

crystallization studies and Professor Gustaaf van Tendeloo for the high resolution TEM

images performed in the EMAT in University of Antwerp.

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Merci au Professeur Giovanni Camino, au Docteur Frdric Luizi, au Docteur Rony Snyders

et au Docteur Didier Villers davoir accept de faire partie de mon jury de thse.

Merci au F.R.I.A. (Fonds pour la Formation la Recherche dans lIndustrie et dans

lAgriculture) pour le financement de cette thse.

Je tiens aussi remercier les membres du SMPC pour leur sympathie et les moments partags,

et plus spcialement Ben, Nathalie, Deborah, Fabian, Fouad, Samira, Leila, Manu, Mounch,

Magali, Anne-Lise et Stphane.

Jaimerais galement remercier toute lquipe du LCIA, Rony, Stephanos, Monsieur Dauchot,

Corinne, Fabian, Yoann, Damien, Denis, Matthieu, David, Greg, Fabien, Dany, Adil,

Philippe, Daphn, Axel, Alisson, Alice, Damien, Xavier, Cristina ainsi que Mickal, Rachel,

Anne-so et Christine pour lambiance chaleureuse et joyeuse quils ont fait rgner au sein du

service ainsi que les coups de main ou conseils quils ont pu me prodiguer.

Jaimerais aussi remercier plus particulirement Laurent, mon compre depuis sept ans dj,

pour son soutien (surtout lors du retour du FRIA), son coute et nos dlires. Une pense

galement mes amis, Julien, Delphine, Melissa, Nathalie, Philippe, Renaud, Ahmed et Maria

pour tous ces moments partags et assez souvent folkloriques

Enfin, je tiens remercier du plus profond de mon cur ma famille qui ma soutenu et

encourag tout au long de mes tudes en tant tout moment mes cts.

Pour finir, je te remercie Milie, pour ton soutien quotidien, tout ce que tu mapportes, tes

relectures et de me supporter, surtout ces derniers mois

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TABLE OF CONTENT

TABLEOFCONTENT

I.

INTRODUCTION....................................................................................................................1

II.STATE OF THE ART............................................................................................................5

II.A. Carbon Nanotubes ............................................................................................................5

II.A.1. Structure of carbon nanotubes....................................................................................6

II.A.2. Carbon nanotubes : Methods of production.............................................................9

II.A.3. Properties of carbon nanotubes................................................................................11

II.A.4. A large range of applications ....................................................................................14

II.B. Functionalization of carbon nanotubes......................................................................18

II.B.1. Interests and types of functionalization ...................................................................18

II.B.2. Non-covalent functionalization with surfactant or polymer.................................19

II.B.3. Covalent functionalization ........................................................................................21

II.B.3.a. End and defect-side chemistry .................................................................................21

II.B.3.b.

Sidewall functionalization .......................................................................................24

II.C. Plasma treatment of carbon nanotubes......................................................................28

II.C.1. Introduction to plasma : the fourth state of matter................................................28

II.C.2. Characteristics and principal applications of plasma............................................29

II.C.3. Applications of plasma into materials surface functionalization.........................32

II.D. Polymer nanocomposites-based on carbon nanotubes............................................34

II.D.1. Generalities on polymer nanocomposites...............................................................34

II.D.2. Melt blending of polymer matrices with carbon nanotubes.................................36

II.D.3. Interfacial compatibilization: improvement of CNTs dispersion........................37

II.D.4. Carbon nanotube-based nanocomposites: applications.........................................45

III.OBJECTIVES & PROPOSED RESEARCH STARTEGIES............................54

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TABLE OF CONTENT

IV.RESULTS & DISCUSSION...........................................................................................58

IV.A. Microwave plasma post-discharge treatment...........................................................58

IV.A.1.

Microwave plasma experimental set-up and treatment parameters....................59

IV.A.2. Atomic nitrogen treatment of carbon nanotubes....................................................60

IV.A.2.a. Treatment of CNTs pressed pellet ........................................................................61

IV.A.2.b. Morphology of nitrogen-treated carbon nanotubes ...............................................65

IV.A.2.c. Treatment of CNT powder ....................................................................................67

IV.A.2.d. Origin of oxygen surface contamination ...............................................................69

IV.A.3. Quantification of primary amine functions grafted on CNTs..............................72

IV.A.3.a.

Limits of XPS N1s peak deconvolution ................................................................73

IV.A.3.b. Chemical derivatization of primary amines ..........................................................74

IV.A.3.c. Post-plasma reaction with hydrogen gas ...............................................................78

IV.A.3.d. Introduction of H2in the -wave plasma discharge ..............................................78

IV.A.3.e. Introduction of H2in the -wave plasma post-discharge ......................................83

IV.A.3.f. Stability of primary amines grafted on CNTs .......................................................86

IV.A.4. Influence of treatment time on nitrogen and NH2functions grafting...............88

IV.A.5.

Conclusion..................................................................................................................91

IV.B. Preparation and characterization of CNTs-g-PCL nanohybrids.........................92

IV.B.1. Preparation of CNTs-g-PCL nanohybrids..............................................................92

IV.B.2. CNTs-g-PCL nanohybrids characterization...........................................................96

IV.B.2.a. Content in PCL grafted on CNTs surface and free PCL chains ............................96

IV.B.2.b. Solvent dispersibility of CNTs-g-PCL nanohybrids .............................................98

IV.B.2.c.

Morphology of CNTs-g-PCL nanohybrids ...........................................................99

IV.B.3. Conclusion................................................................................................................104

IV.C. Carbon nanotubes-based nanocomposites.....................................................................105

IV.C.1. Thermal properties of CNTs-based nanocomposites..........................................107

IV.C.1.a. Influence of CNTs on the thermal degradation of nanocomposites ....................107

IV.C.1.b. Thermal degradation of PCL nanocomposites ....................................................108

IV.C.1.c.

Thermal degradation of HDPE nanocomposites .................................................111

IV.C.1.d. DSC analyses of PCL nanocomposites ...............................................................113

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TABLE OF CONTENT

IV.C.1.e. DSC analyses of HDPE nanocomposites ............................................................116

IV.C.1.f. Conclusion ..........................................................................................................118

IV.C.2. Electrical properties of CNTs-filled nanocomposites.........................................119

IV.C.2.a.

Electrical properties of nanocomposites and percolation theory ........................119

IV.C.2.b. Electrical resistivity measurements range ...........................................................123

IV.C.2.c. Electrical properties of PCL nanocomposites .....................................................124

IV.C.2.d. Electrical properties of HDPE nanocomposites ..................................................132

IV.C.2.e. Conclusions .........................................................................................................139

V. GENERAL CONCLUSIONS & PERSPECTIVES.............................................143

VI.

EXPERIMENTAL PART & INSTRUMENTATION........................................147

VI.A. Materials ..........................................................................................................................147

VI.B. Preparation of samples .................................................................................................147

VI.C. Instrumentation.............................................................................................................150

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I. INTRODUCTION 1

1. INTRODUCTION

Since their first observation in 19911, carbon nanotubes (CNTs) have attracted a lot of

attention of many researchers and engineers around the world owing to their exceptional

properties. Their outstanding physical properties are a direct result of the near-perfect

structure of the CNTs, which at atomic scale can be thought of as a hexagonal sheet of carbon

atoms rolled into a seamless one-dimensional cylindrical shape. Due to this structure, no

previous material has displayed the combination of superlative properties attributed to them2.

Besides their extremely small (nanometric) size, their excellent electrical and thermal

conducting performances combined with their high toughness and transverse flexibility allow

for envisaging their use in a wide variety of applications like, for example, electronic

components, chemical and biological sensors, chemical and genetic probes, field emission

tips, mechanical memories, hydrogen and ion storage components, etc3. Carbon nanotubes are

thus expected to transmit their unique properties in multidisciplinary fields and should play a

key role in the nanotechnologies of the 21stcentury.

Offering in the same time a high aspect ratio (length-to-diameter) and a low density, this

allotropic variety of carbon is an ideal candidate as advanced filler materials in composites.

Indeed, CNTs can provide a three-dimensional conductive network through the polymer

matrix with exceedingly low percolation thresholds4. Furthermore, it has been suggested that

their high thermal conductivity can be exploited to make thermally conductive composites5.

In addition, the mechanical enhancement of polymer materials using CNTs as reinforcing

nanofillers should be exploited in the very near future6.

A key parameter for producing nanocomposite materials with largely improved

physical properties relies on the extent of dispersion of the individual CNTs. However, the

CNTs tend to form long bundles thermodynamically stabilized by numerous van der Waals

forces between the tubes and by entanglements, which occur during their synthesis. These

aggregates constitute a handicap in the majority of CNTs applications due to their low

solubility in water and organic solvents. In the case of their use as nanofillers, the

homogeneous dispersion of CNTs in polymer matrices is relatively difficult to achieve within

a large majority of polymer7.

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I. INTRODUCTION 2

Figure 1- 1 : TEM micrograph of CNTs bundle/aggregate

The functionalization of the CNT sidewalls represents a solution in order to tune the

interactions between CNTs and the host polymer matrix and improve their dispersion ability.

This surface modification can be divided into two main approaches.

One is the non-covalent functionalization, used in different techniques such as the addition of

surfactants8, polymer wrapping

9or polymerization-filling technique (PFT)

10, which allows the

unaltered CNTs for preserving their physical properties. However, the interaction between thewrapping molecules and the nanotubes remain generally weak limiting the efficiency of the

properties transfer between the nanotubes and the host polymer, in particular a low load

transfer for mechanical properties.

The other approach relies on the covalent grafting of functional groups along the CNT

sidewalls. These chemical functions are interesting to improve the interactions with the

polymer matrix11

and can be used as anchoring sites for polymer chains8. The covalent

bonding between CNTs and the polymer allows obtaining an optimal interfacial strength and

thus a better load transfer. The covalent functionalization will influence the CNTs properties

and depends on the location of functionalization sites. Indeed, the functionalization can take

place on already existing CNT structural defects or defect sites can be created by the grafting

of chemical functions. In this last case, the treatment for grafting can degrade the CNTs

physical properties if the extent of functionalization is too important leading to damaged

CNTs structure12

.

In the majority of so far reported functionalization processes such as chemical

functionalization performed in solvent13

or by ball milling14

, only a very tiny fraction of

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I. INTRODUCTION 3

carbon atoms along the nanotubes are functionalized. And some targeted reactive functions,

like amine groups, most often require time-consuming and costly multistep reactions that

rarely result in a controlled covalent functionalization process.

The aim of this research is to develop a non destructive and selective covalent

functionalization of CNTs and to disperse these treated CNTs homogenously within a

polymer matrix with covalent bonds at the interface polymer/nanotube with the preservation

of electrical properties of CNTs. It is thus important to control the quantity and the nature of

the grafted functions since it is necessary to have enough superficial chemical groups while

preserving inherent CNTs properties.

In the first part of the study, the treatment of CNTs via an original microwave plasma process

is developed to readily functionalize the CNTs surface with primary amine groups in one

single step. The use of plasma, which is an ionized gas consisting in ions, electrons and

neutral species, allows to treat the CNTs with a relative low time and environmentally

friendly process without any use of solvent. The quantity of grafted nitrogenated groups, and

particularly primary amines, was determined by x-ray photoelectron spectroscopy (XPS). The

covalent character of grafted nitrogen groups was studied by high-resolution ultraviolet

photoelectron spectroscopy (HR-UPS).

The so-grafted primary amine groups are used as initiation sites for promoting the ring

opening polymerization (ROP) of lactone monomers (i.e. -caprolactone) yielding polyester-

grafted CNT nanohybrids. The morphology of the recovered nanohybrids and the quantity of

grafted polymer chains were studied by transmission electron microscopy (TEM) and

thermogravimetric analysis (TGA), respectively.

Finally, these nanohybrids are used as highly filled masterbatches to be dispersed in the

molten state within either poly(-caprolactone) (PCL) or high-density polyethylene (HDPE)

matrices to obtain high performance nanocomposites. The thermal behavior as well as their

electrical properties of the so-produced nanocomposites were characterized and correlated to

their morphology.

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I. INTRODUCTION 4

References

1Iijima S.,Nature 1991, 354, 56-58

2Thostenson E., Ren Z., Chou T.-W., Composites Sci. Tech. 2001, 61, 1899-1912

3Collins P. G., Avouris P., Sci. Am. 2000, 12, 62-694Kilbride B. E., Coleman J. N., Fraysse J., Fournet P., Cadek M., Drury A.,J. Appl. Phys. 2002, 92, 4024-4030

5Bagchi A., Nomura S., Compos. Sci. Technol. 2006, 66, 1703-17126Wang W., Ciselli P., Kuznetsov E., Peijs T., Barber A. H., Philos. Trans. Roy. Soc. A 2008, 368, 1613-16267Andrews R., Weisenberger M. C., Curr. Opin. Solid State Mater. Sci. 2004, 8, 31-378Hirsch A.,Angew. Chem. Int. Ed. 2002, 41, 1853-18599Liu P.,Eur. Polym. J.2005, 41, 2693-270310Bonduel D., Mainil M., Alexnadre M., Monteverde F., Dubois Ph., Chem. Commun. 2005, 781-78311Thostenson E. T., Li C., Chou T. W., Compos. Sci. Technol. 2005, 65, 491-51612Fu K., Huang W., Lin Y., Riddle L. A., Carroll D. L., Sun Y. P.,Nano Lett. 2001, 1, 439-44113Banerjee S., Hemraj-Benny T., Wong S. S.,Adv. Mater.2005, 15, 3334-333914

Konya Z., Vesselenyi I., Niesz K., Kukovecz A., Demortier A., Fonseca A., Delhalle J., Mekhalif Z., Nagy J.B., Koos A. A., Osvath Z., Kocsonya A., Biro L. P., Kiricsi I., Chem. Phys. Lett. 2002, 360, 429-435

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II. STATE OF THE ART 5

II.

STATE OF THE ART

II.A.

Carbon nanotubes

First TEM evidence for the tubular nature of nano-sized carbon filaments was

published in 19521 and many other reports have followed over the next forty years 2,3,4.

However, these fibers were not recognized as nanotubes and were not studied systematically.

It was only after the discovery of fullerenes in 19855 that carbon nanostructures were

explored further and attracted global scientific attention. Interest in carbon nanotube research

began in 1991 when the Japanese electron microscopist Sumio Iijima6

In 1993, two distinctive research groups demonstrated independently the ability to produce

single-sheet tubules, coined as single-walled carbon nanotubes (SWNTs)

observed tubular

features in carbon soot produced in an arc discharge. The structure of these tubules is

composed by concentric multiwalls forming one cylinder defining a multi-walled nanotube

(MWNT).

7,8

Promising physical, chemical and electronic properties of carbon nanotubes have been

highlighted in the next years, leading to a great expansion of the breadth and range of research

dedicated to these nanoparticles over the past several years (Figure 2-1). Once considered a

novelty, carbon nanotubes are today a key foundational material of the nanotechnology

revolution.

.

Figure 2- 1 : Number of publications on carbon nanotubes by year (www.isiknowledge.com)

0

1000

2000

3000

4000

5000

6000

7000

8000

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Numberofpublications

Year

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II.A.1.Structure of carbon nanotubes

Carbon nanotubes (CNTs) are an allotropic form of carbon like diamond, graphite or

fullerenes. CNTs are composed entirely of sp bonds, with each carbon atom joined to three

neighbours forming a macromolecule of carbon, atoms.

An ideal nanotube can be thought of as a sheet of graphite (a hexagonal lattice of carbon)

rolled into a seamless cylinder (Figure 2-2). With a diameter varying from 1 to 5 nm 8, the

cylinder can have typically length of about several microns and each end can be capped with a

hemisphere of fullerene-like structure. This nanostructure presents therefore a high aspect

ratio (length-to-diameter), which allows considering CNT as one-dimensional object.

Figure 2- 2 : Formation of a nanotube by a graphene rolling up 9

CNTs can be divided in two categories, single-walled nanotubes (SWNTs), which are

constituted by the fundamental cylinder structure, and multi-walled nanotubes (MWNTs)

consisting of multiple rolled graphite layers (Figure 2-3). The structure of MWNTs can be

described by the Russian Doll modelwhere carbon sheets are arranged in different coaxial

tubes (number of tubes can vary from 2 to 50), or by the Parchment model, a single sheet is

rolled in around itself like a scroll of parchment

10

. The interlayer distance in MWNTs is close

to 3.3, similar to graphene layers in graphite.

Figure 2- 3 : Schematic representation of a single-wall (a) and a multi-wall carbon nanotube (Russian Doll model) (b)(from www.lps.u-psud.fr/IMG/pdf_coursUVSQ2008.pdf, 2009-05)

a b

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The structure of an individual carbon nanotube can be specified in terms of a vector, the chiral

vector Chthat joins two crystallographically equivalent sites on the original graphene sheet11

(Figure 2-4). When this hexagonal lattice is rolled up, the two end-points of the vector are

superposed to form the cylinder. The vector Chcan be described by the following relation:

h =1 + 2 (II.1)

where a1and a2are unit vectors of the hexagonal lattice and n and m are translational indices.

Each nanotube topology is usually characterized by these two integer numbers (n,m) leading

to a primary symmetry classification of nanotubes. A chiral nanotube is defined by a nanotube

whose mirror image cannot be superposed to the original one. On the contrary, the structure of

an achiral nanotube is undistinguishable from the mirror image one. There are only two cases

of achiral nanotubes: armchair (n,n) and zig-zag (n,0) nanotubes (Figure 2-5). The nanotube

structure can be also defined by the chiral angle between the vectors Chand a1, with values

of in the range 0-30 due to the hexagonal symmetry of the lattice. The two limit cases

correspond to achiral nanotubes with values of 0 and 30 respectively for zig-zag and

armchair classes.

Figure 2- 4 : Schematic diagram showing how a hexagonal sheet of graphite is rolled to form a carbon nanotube12

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Figure 2- 5 : Illustrations of the atomic structure of armchair, zig-zag and chiral nanotubes

(from www.cobweb.ecn.purdue.edu, 2009-06)

In the multi-walled carbon nanotubes (Russian doll model), each individual concentric

tube can have different chirality.

This primary symmetric classification of CNTs is very important because variations in the

nanotube morphology can lead to changes in the properties of the nanotube. For example, the

structure of a CNT determines its electronic behavior. Indeed, the electronic properties of a

nanotube follow a general rule: if (n-m) is a multiple of 3, then the tube exhibits a metallic

behavior, otherwise it is an electrical semiconductor13

Investigations of the influence of chirality on the mechanical properties of CNTs have also

been reported

. So, armchair nanotubes present a

metallic behavior while zig-zag and chiral nanotubes can be either metallic or semi-

conducting.

14

. Although the chirality has a relatively small influence on the elastic stiffness,

the Stone-Wales (Figure 2-6) transformation plays a key-role in the nanotube plastic

deformation under tension. This transformation corresponds to a reversible diatomic

interchange where the resulting structure is built upon two pentagons and two heptagons in

pairs.

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Figure 2- 6 : The Stone-Wales transformation occurring in an armchair nanotube under axial tension11

The Stone-Wales transformation introduces a new defect in the nanotube structure, the

heptagon. Heptagons allow for concave areas within the nanotube. Thus, the heptagonal

defects in nanotubes can result in many possible equilibrium shapes. Indeed, most nanotubes

are not straight cylinders with hemispherical caps.

II.A.2.Carbon nanotubes: Methods of production

The morphology and properties of carbon nanotubes are closely related to their

method of production. The first developed production methods were the arc discharge and

laser vaporization processes. However, majority of works has been devoted to chemical vapor

deposition (CVD) techniques since they can be readily scaled up for industrial production.

The arc dischargemethod consists to generate an electric arc discharge between two

graphite electrodes under an inert gas atmosphere (Ar, He) 15. This method requires very high

temperature (>5000C) andproduces a mixture of different structures, including fullerenes,

onion-like carbon particles and some graphite sheets. It is necessary to separate the carbon

nanotubes from the soot and catalytic metals present in the crude product. Although it is

possible to selectively grow SWNTs or MWNTs, carbon nanotubes produced by this method

are normally entangled with poor control over the length and diameter16

In the laser ablationmethod, a graphite target is vaporized by laser irradiation under

flowing inert gas atmosphere at high temperature (near 1200C)11. Laser ablation has become

the common method for producing SWNTs

.

17, the nanotubes produced are very pure.

However, as for the arc discharge technique, the quantities of the produced sample are limited

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by the size of the used carbon source. In addition, subsequent purification steps are necessary

to separate the nanotubes from undesirable by-products.

These limitations have motivated the development of gas-phase techniques, such as

chemical vapor deposition (CVD)18,19

(Figure 2-7), where CNTs are formed by the

decomposition of a hydrocarbon-containing gas onto a supported catalyst (e.g., Fe/Co on

alumina). The synthesis of CNTs is often thermally or plasma enhanced. This gas-phase

technique is a continuous process which allows to produce high purified nanotubes,

minimizing subsequent purification steps, with limited control over the structure and

diameter. The CVD technique is suitable for a large-scale industrial production of nanotubes

and can be also used to form vertically aligned carbon nanotubes. The MWNTs used in this

work are industrially produced by CVD and supplied by Nanocyl S.A.

Figure 2- 7 : Scheme of a CVD reactor: hydrocarbon gas is decomposed over a metal catalyst

in a quartz tube heated by a furnace at 600-1000C

(fromhttp://www.ifw-dresden.de,2009-06)

High pressure conversion of carbon monoxide (HiPCO)technique is considered as

an improved CVD process. This production method consists of the gas-phase growth of

SWNTs with carbon monoxide as a carbon source at high temperature and pression20

.

These different methods are still under development. There are numerous variations of thesetechniques operating under different conditions, with different set-up, and process parameters.

For example, a group of researchers have recently developed an effective method for growing

SWNTs via a metal catalyst-free CVD process on a sputtering deposited SiO2 film21

. Each

technique provides diverse advantages and disadvantages over the quality and nature of the

so-recovered CNTs.

In the reported synthesis methods, by-products such as amorphous carbon, carbon

nanoparticles and catalytic metal particles are often present in the CNT powder. If the
http://www.ifw-dresden.de/http://www.ifw-dresden.de/http://www.ifw-dresden.de/http://www.ifw-dresden.de/

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quantity of these by-products is high, several purification steps can be necessary before an

acceptable purity level. Most common purification approaches are based on the difference in

resistance to thermal and/or chemical treatments between carbon nanotubes and by-products,

such as oxidation22,23,24, acid treatment25, annealing26,27. Unfortunately, these purification

methods very often impair carbon nanotubes by the introduction of structural defects, the

nanotube tips opening and/or oxidation of the CNTs surface. A method to overcome this

drawback consists to physically separate the different nanostructures by ultrasonic vibrations,

however prolonged sonication has been reported to can introduce defects and can lead to the

destruction of CNT structure, in addition to the formation of amorphous carbon28

Alternative non-destructive methods such as microfiltration

.29 or size exclusion

chromatography30,31

can be used on SWNTs but these techniques are prohibitively slow and

limit the quantity in produced CNTs.

II.A.3.Properties of carbon nanotubes

Carbon nanotubes have gained in interest as nanoscale materials due to their

exceptional physical properties such as their very high Youngs modulus, their ultimate

strength and their high electric and thermal conductivity. However, directly measuring the

properties of these nanoparticles is very difficult by conventional methods. Therefore, several

properties have been first evaluated using by mathematical modelling.

When CNTs were discovered, their structure immediately encouraged speculation

about their potential mechanical properties. Indeed, the sp2carbon-carbon bond is one of the

strongest and, so, a fiber formed by these axially oriented covalent bonds would be an

extremely strong material. Theoretical investigations have led to estimated Youngs modulus

of CNTs in the range of 1 TPa 32. Different experimental methods have been developed to

measure the elastic properties of individual nanotubes. One method allows the determination

of the CNT stiffness by the observation of the amplitude of their intrinsic thermal vibrations

in a transmission electronic microscope (TEM). Average Youngs modulus values of 1.8

TPa33 and 1.25 TPa34 were obtained for MWNTs and SWNTs respectively. A second

technique consists in directly measuring the stiffness of MWNTs clamped at one extremity

using atomic force microscopy (AFM)35leading to an average value of 1.28 0.59 TPa. A

similar method by AFM, consisting now in clamping CNTs at both ends across a pore of an

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alumina membrane, was used by Salvetat et al. 36

The tensile strength of CNTs has also been estimated by different theoretical calculations, an

average value of 100 GPa was achieved

. By this way, Youngs modulus values of

810 410 GPa for MWNTs have been measured.

37,38,39. Experimental studies using AFM have allowed

to confirm this value ranging from 10 to 200 GPa as measured for the CNT tensile

strength40,41, much higher them steel (~1 GPa). Taking into account a study carried out on 26

CVD-MWNTs, Barber et alhave concluded that the CNT strengthening mechanism can be

associate to the interaction between the walls of the nanotubes42

. For illustration, a tensile

strength of 200 GPa for MWNT corresponds to the ability to endure a weight of 20 tons on a

cable with a cross-section of 1 mm. Carbon nanotubes thus represent the strongest and

stiffest materials in terms of tensile strength and elastic modulus. To highlight the nanotube

mechanical properties, Youngs modulus and tensile strength values of SWNTs, MWNTs and

other materials (stainless steel and Kevlar) are compared in Table 2-1.

Material Youngs Modulus (TPa) Tensile Strength (GPa) Elongation at Break (%)

Stainless Steel43 ~ 0.2 0.3 - 1.5 10 - 30

Kevlar44 ~ 0.15 ~ 2.8 ~ 2

SWNT ~ 1 13 - 53 ~ 16MWNT 0.8 - 1.8 10 - 200 /

Table 2- 1 : Comparison of mechanical properties of different materials

The above mechanical properties of CNTs refer to axial properties of the nanotube.

Simple geometrical considerations suggest that CNTs should be much softer in the radial

direction than along the tube axis. Indeed, TEM observations of radial elasticity show that

even van der Waals forces can deform two adjacent nanotubes45. Nanoindentation

experiments with AFM indicated Youngs modulus of the order of several GPa46in the radial

axis. In spite of these excellent mechanical properties, carbon nanotubes possess also a

remarkable flexibility. Besides their experimental observations, Iijima et al47

simulated the

deformation properties of nanotubes bent to large angles. The bending is completely

reversible up to angles in excess of 110, despite the formation of complex kink shapes.

Figure 2-8 shows experimental and theoretical results, demonstrating the exceptional

resilience of CNTs at large strain.

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Figure 2- 8 : HR-TEM micrograph and computer simulation of nanotube bend 47

Carbon nanotubes are expected to be very good thermal conductors along the tube, but

good insulators laterally to the tube axis. It was demonstrated that thermal conductivity of

CNTs is determined primarily by phonons48. Experimental thermal conductivity measured on

individual MWNTs reaches 3300 W/(m.K)49

, which is about twice as high as diamond.

Moreover, CNTs are thermally stable up to 2800C in vacuumand up to 700C under air11

.

Other important physical properties of CNTs are their electronic properties, which are

dependent on the molecular structure (chirality) of CNTs as explained previously in paragraph

II.A.1. It was demonstrated that (n,m) carbon nanotubes are metallic when n-m is equal to a

multiple of 3, others are semi-conducting12.

The determination of electronic properties of MWNTs is a complicated task as the carbon

sheets in the MWNTs can have different electronic character and chirality. However,Bachtold et al.50 demonstrated that current flows only through the outer-shell of MWNTs

based on measurements of the Aharonove-Bohm oscillation. In studies of MWNTs with outer

shell side-bonded to metal electrodes, it was also concluded that the electrical transport is

dominated by outer-shell conduction51

The diameter being in the nanometer range gives rise to quantum effects in carbon nanotubes.

In fact, the unique electrical properties of nanotubes arise from the confinement of the

electrons in tubes, which allows motion only along the tube axis. The scattering processes are

therefore reduced in the tube and metallic CNTs can carry enormous current densities up to

.

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109A/cm without being destroyed52. This density is about 2-3 orders of magnitude higher

than the current density of metals such as copper. A high electrical conductivity value of 103

S/cm is often measured by both two- and four-probe processes on individual SWNTs53 and

MWNTs54,55,56,57

.

II.A.4.A large range of applications

The exceptional properties of carbon nanotubes can be potentially used in many

applications ranging from nanodevices to macroscopic materials. First commercial

applications have very recently appeared, including, e.g. motor vehicle fuel system

components and specialized sport equipments. The world demand is expected to reach $200

million end of 2009 and by 2020 will likely approach $9 billion 58

. Figure 2-9 depicts the

predicted world nanotube demand by market.

Figure 2- 9 : Scheme of world demand in nanotubes by market in 2009 (redrawn from ref. 57)

One of the first proposed applications for carbon nanotubes was the use of MWNTs as

reinforcement or as electrically conductive componentsin polymer composite materials

59

.The combination of their unique properties and their light mass allows to consider nanotubes

as ideal fillers to form composite materials with improved mechanical strength and electrical

conductivity. The use of CNTs in this field of application is discussed with more details in the

section II.D.

With the highest current density as compared to any metal, carbon nanotubes are ideal

candidates for the next generation of nanoelectronic applications. An electronic circuit

constituted of wires and intramolecular junctions fully based on CNTs can be suggested.

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These components can not only connect different CNTs for integration, but can also act as

functional building blocks in the circuit, such as rectifiers, switches, amplifiers, etc 60. It was

shown that carbon nanotube-based field effect transistors (FETs) have excellent operating

characteristics that are as good as or better than currently used silicon devices61. However, a

drawback for electronic applications is the SWNT band gap dependance on its chirality. For

an optimal device performance, it is necessary to be able to grow a specific type of tube at a

defined position and direction of alignment, with near 100% yield. Control of growth position

and alignment can be achieved now, however the control of chirality still an issue. It has been

reported that design of the catalyst in CVD process can be used in the control of the tube

chirality62,63

.

Carbon nanotubes have emerged as a promising class of electron field emitters,

because they possess a low threshold electric field for emission and they are stable at high

current density. A cathode made with aligned CNTs can be used to shoot electrons under

vacuum to the targeted anode. Depending on its nature, the electrode can produce visible light

or X-ray 64

Carbon nanotubes with superior field emission properties can be used for the development of

the next generation flat-panel display TV based on field emission

. The small size of these electron guns enables miniaturisation of elements in

display technologies (Field Emission Display or FED) and instrumentation based on X-ray.

65

(Figure 2-10).

Figure 2- 10 : CNTs-based nano-emissive display (NED) prototype

(Image: Motorola Labs)

Carbon nanotubes present also some advantages for sensing applications. Their small

size with very large surface, high sensitivity via defects, fast response and good reversibility

at room temperature enable them to be used as a resistive chemical sensor66. Actually, the

conductivity of CNTs changes when they are exposed to certain gaseous chemical species

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(NO2, NH3)67. It has been reported that the sensor selectivity can be tuned by grafting onto the

CNTs chemical species that will selectivety interact. One promising example is DNA-

decorated carbon nanotube transistors acting as chemical sensors68, wherein attached single-

strand DNA (ss-DNA) acts as a chemical recognition site and nanotubes as electronic read-out

components. Response of these systems differs in sign and magnitude for different gases and

odours and can be tuned by choosing the base sequence of the ss-DNA. For example, SWNTs

coated with ss-DNA have been used for sensing of a low concentration of dopamine in the

presence of excess ascorbic acid69. CO2 detection using SWNTs and a

microelectromechanical system has been demonstrated70and H2sensing has been carried out

at room temperature using SWNTs films71

Techniques for the attachment of MWNTs at the tips of atomic force microscopy (AFM)

cantilevers have been developed for their use as scanning probes

.

72. These modified tips

present a number of advantages, including crash-proof operation and the capability of imaging

deep structures inaccessible to conventional tips. Moreover, the extremity of nanotube

modified tips can be functionalized with distinct chemical groups and used to demonstrate

nanoscale imaging with the ability to discriminate local chemistry of the surface being

probed73

.

Carbon nanotubes are now considered for energy storage and productionbecause of

their small dimensions, smooth surface topology and perfect surface specificity since only the

graphite planes are exposed in their structure74. It has been claimed that CNTs could represent

ideal candidates for the storage of hydrogen due to their large surface coupled with tubular

structure suggesting important capillarity effects. A storage capacity near 6.5% by weight, if

readily cyclable, is required for automotive applications. Theoretical estimates suggest a

storage capacity of 8-10wt%, but it turns out that experimental values are less than 1wt%75.

Thus, it seems that a consensus has been reached now to say that CNTs are not a usefulhydrogen storage medium76. However, some studies demonstrate that Ti coating could

increase the capacity storage of nanotubes up to 10wt%77

.

Another major area of research on CNTs concerns biomedical materials and devices.

Many applications for CNTs have been proposed including biosensors (examples of DNA

decorated CNTs), drug and vaccine delivery vehicles and novel biomaterials78. For example,

SWNTs can be used as DNA transporters and near-infrared (NIR) photothermal agents79

.Actually, folic acid functionalized carbon nanotubes can selectively kill cancer cells because

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of the NIR light heating effect of SWNTs. The transporting capabilities of CNTs combined

with suitable functionalization chemistry and their intrinsic optical properties can lead to

novel nanomaterials for drug delivery and cancer therapy.

However, many uncertain factors may limit the biomedical applications of CNTs like their

aggregation and their questionable biocompatibility. It has been reported80that CNTs possess

some degree of toxicity (in vitro and in vivo), predominately due to the presence of residual

transition metal catalysts. Pristine CNTs has been shown to have minimal cytotoxicity at high

concentration, while functionalized CNTs are not toxic81

. These initial results urge caution

when handling CNTs and the introduction of safety measures in manufacturing facilities and

laboratories should be seriously considered.

As discussed before, carbon nanotubes could be used in a large range of diversified

applications, however it remains several drawbacks to be overcome such as their tendency to

aggregate. Indeed, CNTs form long bundles thermodynamically stabilized by numerous -

interactions between sidewalls. One solution consists to directly design carbon nanotubes

during their synthesis depending on the targeted application. For instance, the development of

techniques for the controlled growth of CNTs aligned on suitable substrates has received

special attention as well-defined CNTs architectures are important for applications such as

electronic displays or electron field emitters82

. Figure 2-11 shows an example of very well

defined vertically aligned CNTs using nanolithography. This example demonstrates the high

control reached on the growth of CNTs aligned on a flat surface.

Figure 2- 11 : Each Nanobama face is made of 150 million vertically aligned CNTs and measures 0.5mm83

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For other applications requiring non-aligned carbon nanotubes, the functionalization of

CNTs represents a solution to disaggregate them. The functionalization consists to graft

functional groups or polymer chains to CNTs surface. The different ways approached so far to

functionalize carbon nanotubes are presented in the next section.

II.B. Functionalization of carbon nanotubes

II.B.1. Interests and types of functionalization

As previously discussed, the CNTs unique properties make them desirable for many

different applications. However, to exploit as much as possible these properties, most of the

applications require the functionalization of carbon nanotubes, such as changing the surface

properties to make nanotubes soluble in different media, or attaching functional groups or

polymer chains for specific utilizations of modified nanotubes.

The CNT functionalization methods can be divided into two major groups. The first

one is the functionalization from inside (endohedral functionalization) meaning that CNTs are

treated by filling their inner empty cavity with different molecules 84 or nanoparticles85

(Figure 2-12).

Figure 2- 12 : Schematic representation (a) and HRTEM image (b) of a SWNT filled with C60fullerenes

(from www.fz-juelich.de/iff/datapool/iee/TEM.jpg)

The introduction of these particles, for instance Buckminsterfullerene C60, can be achieved by

spontaneous penetration of nanoparticles or by wet chemistry, where nanotubes are filled with

some compounds, which react under particular thermal or chemical conditions and produce

inside the nanoparticles.

a

b

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The second group is the exohedral functionalization, which consists to graft molecules

on the outer surface of nanotubes. Several approaches have been developed and include defect

functionalization (Figure 2-13A), covalent functionalization (Figure 2-13B) and noncovalent

functionalization with surfactants (Figure 2-13C) or polymers (Figure 2-13D)86

.

Figure 2- 13 : Functionalization possibilities for CNTs: defect functionalization (A),

covalent sidewall functionalization (B), noncovalent functionalization with surfactants (C)

and polymer wrapping (D)86

The different types of exohedral functionalization can be classified via the nature of the

interactions between the surface of carbon nanotubes and the functional groups or polymer

chains. These interactions can rely upon covalentor non-covalent bonds.

II.B.2. Non-covalent functionalization with surfactant or polymer

In comparison to a covalent bond, the energy released in the formation of a non-

covalent bond is lower. However, in supramolecular systems like carbon nanotubes, the

number of these bonds can be high and become dominant. The noncovalent interaction is

based on van der Waals forces or - stacking and it is controlled by thermodynamics.

The great advantage of this type of functionalization relies upon the possibility of attaching

various groups without disturbing the electronic system of the rolled graphene sheets of

CNTs87

.

CNT

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The formation of non-covalent aggregates with surfactants is a suitable method for dispersing

individual nanotubes in aqueous88 or organic solvents89. To solubilize CNTs in aqueous

media, the used surfactant is an amphiphilic molecule whose the hydrophobic part is oriented

toward the surface of CNTs, whereas the polar moiety interacts with solvent molecules90.

Moore et al.91

Another example of non-covalent functionalization is the anchoring through strong -

stacking interactions of polynuclear aromatic compounds to the side-walls of CNTs, such as

planar pyrene moiety

studied various aqueous dispersions of CNTs with anionic, cationic and non-

ionic surfactants. They show that the ability of surfactant to disperse individual CNTs is

linked to the size of the hydrophilic groups due to an enhanced steric stabilization.

92

. (Figure 2-14).

Figure 2- 14 : Interaction of nanotubes with pyrene derivatives93

Carbon nanotubes can be also wrapped with polymer chains to form supramolecular

complexes of CNTs. These structures can be formed by solution mixing of nanotubes with

polymer chains. For example, the wrapping of SWNTs with polymer bearing polar side-

chains, such as polyvinylpyrrolidone (PVP) or polystyrenesulfonate (PSS), leads to the

suspension of SWNT/polymer complexes in water

94. In 2003, this method was used to wrap

SWNTs with DNA oligonucleotide sequences to improve separation of metallic versus

semiconducting nanotubes95

Another way to get these nanotubes/polymer complexes is the in situ polymerization directly

in presence of CNTs. The polymerization-filling technique (PFT) allows to produce carbon

nanotubes coated with polyethylene

.

96. First, methylaluminoxane (MAO), a commonly used

co-catalyst in metallocene-based olefin polymerization, is anchored on the nanotubes surface.

Then, the metallocene catalyst (e.g., Cp2*ZrCl2) is added. Finally, the polymerization of

ethylene is started, polyethylene is formed near the CNT surface and, with increasingmolecular mass, precipitates onto nanotubes. The bundles of CNTs are destructed along with

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the formation of the polyethylene coating on nanotube surface. The different steps of this

technique are represented in figure 2-15.

Figure 2- 15 : Schematic representation of the polymerization-filling technique (PFT) applied to CNTs96

Although the non-covalent functionalization allows the preservation of the electronic structureof CNTs, the weakness of interface between nanotubes and physisorbed polymer chains is a

limitation for applications involving mechanical reinforcement97

.

II.B.3. Covalent functionalization

Two major groups of chemical functionalization of CNTs via covalent attachment can

be distinguished, the end and defect-group chemistry and the sidewall functionalization.

II.B.3.a. End and defect-side chemistry

The functionalization via end and defect-side chemistry consists to graft functional

group directly on the already existing defects in the structure of CNTs. Indeed, carbon

nanotubes are generally described as perfect graphite sheets rolled into nanocylinders. In

reality, all CNTs present defects and can be curved. Typically, around 1-3% of carbon atoms

of a nanotube are located at a defect site98.

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The end caps of nanotubes can be composed of highly curved fullerene-like hemispheres,

which are highly reactive as compared to the sidewalls 99. The sidewalls themselves contain

defect sites such pentagon-heptagon pairs called Stone-Wales defects (see previously

discussed Figure 2-6), sp3-hybridized defects and vacancies in the nanotube lattice100

. The

most frequently encountered type of defect is the so-called Stone-Wales (or 7-5-5-7) defect

which leads to a local deformation of the nanotube curvature. Addition reactions are most

favored at the carbon-carbon double bonds in these positions. The different typical defects are

showed in the Figure 2-1686.

Figure 2- 16 : Typical defects in a SWNT

Five- or seven-membered rings in the carbon framework, instead of the normal six-

membered rings, leading to a bend in the tube (Figure 2-16, arrow A). The sp3-hybridized

defects is indicated with the letter B (R=H and OH). Carbon framework can be damaged by

oxidative conditions, which leaves COOH groups on the edge of a hole (Figure 2-16,C).

Figure 2-16D shows open end of the SWNT terminated with COOH groups. Other

terminal groups, such as OH, -H and =O, are also possible.

Frequently, these intrinsic defects are supplemented by oxidative damages to the

nanotube framework by strong acids during the purification step. These acids leave vacancies

functionalized with oxygenated functional groups such as carboxylic acid, ketone, alcohol and

ester groups101. The oxidative introduced carboxyl groups represent useful sites for further

modifications in organic solvents such as the coupling of molecules through the creation of

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amide and ester bonds. Figure 2-17 represents common functionalization routes used to

derivatize SWNTs at ends and defect sites through solution-based chemistry.

By this way, nanotubes can be flanked with a wide range of functional moieties, for which

bifunctional molecules are often utilized as linkers. This method was used to graft amine

moieties onto carbon nanotubes via the reaction with diamines such as triethylenetetramine102,

ethylenediamine103or 1,6-hexamethylenediamine104. Another approach involves reduction of

the carboxyl groups to hydroxyls, followed by transformation into amino groups via

phthalimide coupling and hydrolysis105. Gromov et al. have developed two other approaches,

using amino-decarboxylation substitution, to get primary amino groups directly attached on

the carbon nanotube surface106

. The carboxylic groups, in majority at CNTs open ends, are

replaced with amino groups via Hofmann rearrangement of carboxylic acid amide or via

Curtius reaction of carboxylic acid chloride with sodium azide.

Figure 2- 17 : Schematic of common functionalization ways at ends and defect sites107

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One advantage of the defect-side chemistry is that functionalized carbon nanotubes retain

their pristine electronic and mechanical properties108

. However, the repartition of grafted

functions is not homogenous along CNTs. Indeed, the carboxylic groups, used as anchoring

sites, are placed in majority at the extremities of carbon nanotubes.

II.B.3.b. Sidewall functionalization

The second group of covalent grafting reactions is the sidewall functionalization, which

consists to graft chemical groups through reactions onto the -conjugated skeleton of CNTs.

First covalent sidewall functionalization was carried out on the basis of well-developed

addition chemistry on fullerenes. However, it is predicted that the sidewall addition chemistry

of CNTs differs from that of fullerenes, even though both are curved, conjugated carbon

systems109. Indeed, the chemical reactivity in strained carbon systems arises from two factors,

the pyramidalization at the carbon atoms and the -orbital misalignment between adjacent

carbon atoms110. In fullerenes, the former is the most influent factor due to their pronounced

curvature. The relief of pyramidalization strain energy results in energetically favorable

addition reactions onto fullerene structures. In carbon nanotubes, the pyramidalization strain

is not as acute and -orbital misalignment is expected to have a greater influence 111

. This

misalignment, associated with bonds at an angle to the tube circumference (bonds that are notperpendicular or parallel to the tube axis), is the origin of torsional strain in CNTs. The relief

of this strain controls the extent of reactivity of carbon nanotubes. These two factors

influencing the reactivity of CNTs are represented in Figure 2-18. We can observe that the C-

C bond in CNTs structure will be more or less reactive in function of its angle in to the tube

circumference.

The strain in CNTs arises thus from these factors which scale inversely with tubediameter86, thinner CNTs are more reactive than larger tubes. However, the reactivity of CNT

sidewalls remains low and sidewall-functionalization is only successful if a highly reactive

reagent is used, whereas the nanotube caps are quite reactive due to their fullerene-like

structure. Another constraint for sidewall functionalization is the tendency of CNTs to form

bundles and to limit the available nanotube surface for the grafting of chemical reagents.

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Figure 2- 18 : Pyramidalization angles (p) and the -orbital misalignment angles () along C1-C4 in the (5,5) SWNT

and its capping fullerene C60112

A large majority of covalent sidewall functionalizations is carried out in organic

solvent, which allows the utilization of sonication process to improve the dispersion of CNTsand, thus, the available surface of carbon nanotubes. However, precipitation immediately

occurs when this process is interrupted

113. The required reactive species such as carbenes,

nitrenes or radicals, are in general made available through thermally activated reactions114.

However, the addition mechanisms are not understood completely yet. Normally, the addition

reaction could be initiated exclusively on the intact sidewall or in parallel at defect sites. Most

common sidewall functionalizations carried out in organic solvents, such as carbene115 or

nitrene116

[2+1] cycloaddition reactions or radical additions via diazonium salts117

forinstance, are represented in Figure 2-19.

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Figure 2- 19 : Schematic describing various common covalent sidewall functionalization reactions of CNTs

using organic solvents107

The first studied sidewall functionalization is the fluorination of CNTs. Carbon

nanotubes have been fluorinated by fluorine in the range between room temperature and

600C118,119,120. This reaction is very useful because further nucleophilic substitutions can be

easily accomplished and open a flexible approach for providing the CNT sidewalls with

a

a'

PristineCNTs

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various types of functional groups121

Among these reactions, several diamines are reported to react with fluoronanotubes via

nucleophilic substitution reactions

. Some examples of these reactions are illustrated in the

lower part of Figure 2-19a.

122, leading to the formation of amino-functionalized CNTs

(Figure 2-19a). However, by using a bifunctional reagent such as diamines with sufficiently

long carbon chain, the nanotubes can be covalently cross-linked with each other. Modification

with amino-containing substituents was developed by the use of radical process of photolysis

of acetonitrile123. The 1,3-dipolar cycloaddition reaction can be also used to graft linkers, with

amino groups at their ends, uniformly distributed around sidewalls124

.

However, carbon nanotube functionalizations carried out in organic solvent present

some inconvenients. The first one is the necessity to agitate carbon nanotubes by sonication

process to get a good dispersion of them in organic solvent. Indeed, intensive or too long

sonication causes severe damages to the tube walls and the average length of CNTs can be

reduced125

. Secondly, this functionalization inorganic solvent requires generally multistep

time-consuming reactions. Finally, the use of organic solvent is polluting and difficult to

upgrade for industrial processes.

To overcome the drawbacks of the wet chemistry functionalization, dry processes

are being developed. The ball-milling of MWNTs in reactive atmospheres was shown to

functionalize nanotubes with different chemical groups, such as amides, carbonyls, thiols and

mercaptans for instance, depending on the used atmosphere126. Using this method, CNTs are

broken and radicals formed on their newly created extremities can then react with the

introduced gas. A relatively long time (min. 24h) is necessary to get an enough quantity of

functions onto CNTs. However, the average length of CNTs decreases for increasing

treatment and amorphous carbon is observed after the breaking process127

. Another drawbackis the presence of functional groups exclusively on the ends of cup-stacked carbon nanotubes.

An alternative dry approach to chemical modification of CNTs uses plasma

discharges. Generalities on plasma processes and their applications in CNT functionalization

are presented in the next chapter.

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II.C. Plasma treatment of carbon nanotubes

II.C.1. Introduction to plasma: the fourth state of matter

Plasma is physically defined as an ionized gas with an equal number of positivelyand negatively charged particles. It consists of free electrons, ions, radicals, UV-radiation and

various highly excited neutral and charged species128. The entire plasma is electrically neutral

but its behavior is complex because the particle motions are controlled by electric and

magnetic fields. In 1879, Sir William Crookes firstly defined this state of ionized gas as the

fourth state of matter after solid, liquid and gas (in order of increasing energy). The nature of

the cathode ray matter was in the Crookes tube subsequently identified by Sir J. J. Thomson

in 1897129

and called plasma by Irving Langmuir when he studied the electrified gases invacuum tubes in 1928130

According to gas temperature, plasma can be classified into thermal (or equilibrium)

plasma, which is characterized by being fully ionized (gas temperature Tg = electron

temperature Te) or cold or (non-equilibrium) plasma with the gas only partially ionized

(TgTe). Thermal plasma implies that the temperature of all active species (electrons, ions

and neutrals) is the same. This is, for example, true for stars as well as for fusion plasma.

High temperatures are required to form these equilibrium plasmas, typically ranging from

4000K to 20000K

. It is often considered that 99% of the matter in the universe is in the

plasma state. This estimate may not be very accurate, but it is certainly a reasonable one in

view of the fact that stellar interiors and atmospheres, gaseous nebulae and much of the

interstellar medium are plasmas.

131

In the non-thermal or cold plasmas, the temperature of neutral and positively charged

species is low, while the electrons are in much higher temperature because they are light and

easily accelerated by the applied electromagnetic fields. As a result the generated plasma is in

the state of non-equilibrium and the reaction may proceed at low temperature. Indeed, the

electrons can reach temperatures of 104 105 K (1-10 eV) while the temperature of the gas

can be as low as room temperature

.

132. Figure 2-20 represents the wide range of plasma types

in function of their electron densities and their temperatures.

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Figure 2- 20 : Diverse types of plasma represented in function of electron density and temperature133

II.C.2.Characteristics and principal applications of plasma

The electrically induced discharge in gas is the most common method for plasma

ignition. Direct current (DC) discharges, pulsed DC discharges, radio frequency (RF)

discharges (13.56 MHz) and microwave discharges (2.45 GHz) represent the plasma

categorization based on electric apparatus134. These common standard frequencies were

chosen in order to avoid interfering with telecommunication. The basic feature of a variety of

electrical discharges is that they produce plasmas in which the majority of the electrical

energy primarily goes into production of energetic electrons, instead of heating the entire gas

stream135. These energetic electrons induce ionization, excitation and molecular fragmentation

process of the background gas molecules to produce excited species which create a

chemically-richenvironment. Due to their essential role, the electrons are therefore

considered to be the primary agents in the plasma.

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Plasma processes provide a cost effective and environmentally friendly alternative to

many important industrial processes because the method produces no waste products and in

most cases exposes operators to no significant hazards. Plasmas find thus well-established use

in industrial applications, however they are also gaining more interest in the field of life

sciences, related to environmental issues136 and biomedical applications137,138. Display

technologies, which are based on microdischarges, have been developed and offer the

possibility of large lightweight, flat TV monitors139. Another application field of plasmas is in

analytical spectrochemistry for the trace analysis of solids, liquids and gases140. Atomic

lasers141, ozone generation142, light systems (by example fluorescence lamps)143and particle

sources144

Gas discharge plasmas can be used in a large variety of applications requiring surface

modification. Plasma processing is generally used for film deposition

are also developed.

145,146, etching147 and

may also be used for resist development and removal 148. Surface modification by plasmas

plays a crucial role in the microelectronics industry, for the microfabrication of integrated

circuits130, and in materials technology149

All plasmas that are created by the injection of microwave power, electromagnetic

radiation in the frequency range of 300 MHz to 10 GHz, can in principle be called

microwave induced plasmas (MIPs)

.

150

Among these plasmas, surface wave discharges (SWDs) can be used when high-

density plasmas have to be created. SWDs are sustained by waves that are conducted parallel

to the surface of the dielectric walls surrounding the plasma. The most common SWD

configuration is the surfatron, but the waveguide used in this work is a surfaguide

. This is, however, a general term that comprises

several different plasma types, such as cavity induced plasma, atmospheric plasma torches,

etc. A striking feature of MIPs is the wide range of operational conditions that can be applied:

dependent on the plasma source, power levels can range from a few Watts up to several

hundreds of kiloWatts, the discharge pressure might range from less than 10-2Pa up to several

times atmospheric pressure, whereas many different discharge gases might be used (both

noble and molecular gases). A result of this broad range of operational conditions is that

plasmas with widely varying plasma parameters (such as electron density, electron

temperature, gas temperature, ionization degree and chemical composition) can be created.Moreover, the geometry of the plasmas can be largely influenced. Because practical

applications as well as laboratory research usually need plasmas with specific characteristics

and a controllable geometry, MIPs are in many cases preferable to other plasmas.

151

. Themicrowave induced plasmas present a higher density of electron in comparison of other type

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of plasmas such as RF plasmas by instance, because electron are more easily created with

microwaves (at same power) and they present a more important density.

Figure 2- 21 : Ar + N2surface microwave discharge

The high electron density of the MIPs is a key parameter for the creation of atomic

nitrogen in Ar+N2microwave plasma Godfroid et al. have demonstrated that production of

atomic nitrogen in Ar+N2SWD is achieved by two electronic mechanisms152

. The first is thedissociation by direct electronic impact, which is achieved by collision between a nitrogen

molecule and an electron:

+ 2 ,+g n=0 + (4) + (4) (II.2)

this reaction leads to the production of atomic nitrogen in the fundamental state N(4S).

The second one is called dissociative recombination and is achieved by the recombination

between an electron and an ionized nitrogen molecule:

+ 2+ (4) + (4) (II.3)

Finally, the set-up and parameters of the microwave discharge have been studied to get the

higher nitrogen dissociation rate153

. The most influential parameters proved to be the power

supply pulse, the dilution of N2in Ar and the total gas flow.

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II.C.3.Applications of plasma into materials surface functionalization

Compared with other chemical modification methods, the plasma induced

functionalization present interesting properties and is known as is an environmentally

friendly, solvent-free and time efficient process. Moreover, this treatment allows the grafting

of a wide range of different functional groups depending on plasma parameters such as power,

gas used, duration of treatment and pressure. Moreover, this method provides the possibility

of scaling up to produce large quantities necessary for commercial use. Plasma treatment has

been widely used for surface activation of various materials, ranging from organic polymers

to ceramics and metals. In this domain, we can cite the polymer surface functionalization in

pulsed and continuous nitrogen microwave plasma, under admixture of hydrogen154

The surface modification of carbon nanotubes can be also carried out through a wide

range of plasma processes. In addition to the already cited advantages of plasma treatment,

the amount of functional groups can also be tailored. This is important since having saturation

of these groups on the surface can alter the electronic conductivity of nanotube material. For

instance, the resistivity of CNTs increases with the degree of fluorination

. This

process allows grafting nitrogen functional groups on polystyrene with a high selectivity in

primary amine groups.

155. The most

common plasma treatment of CNTs is the low pressure RF cold plasma which is successfullyused to bind oxygen155,156,157, hydrogen158and fluorine groups159,160

It has been observed that a complete purification of carbon nanotubes powder can be

reached out after CNTs treatment in glow discharges (RF or MW). Indeed, amorphous carbon

domains are eliminated and the impurities are removed by ion bombardment and irradiation in

an O2 RF plasma

.

161. However, it was also found that the average diameter of MWNTs

decreases with treatment duration in three steps. At first, ion bombardment causes the creation

of vacancies and interstitials in MWNTs, the superficial structure, including producedamorphous carbon, are thus lost. Secondly, the ion beams continue to react with amorphous

carbon until they are peeled totally from the CNTs. The last step is the oxidation of

amorphous carbon under O2plasma irradiation in CO2, inducing the decrease of the MWNTs

average diameter. The nature of the plasma gas is important because it is known that

oxygenated ions or radicals are very reactive species in etching processes. However,

destruction of carbon nanotube sidewalls is also observed for other less reactive plasma gas

such as CF4162 or Ar163. Moreover, it was shown that UV photons promote the

defunctionalization of moieties grafted on CNTs164. Figure 2-22 represents high resolution

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transmission electronic microscopy images of MWNTs before and after Ar plasma discharge.

We can observe that ion bombardment and irradiation in Ar plasma deforms the graphite

layers and destructs the MWNTs. A solution consists to reduce the power supplied to the

plasma or the duration of the treatment to limit destruction of sidewalls. However, these limits

are very restricting to get a suitable and controlled functionalization of carbon nanotubes.

Figure 2- 22 : HRTEM images of MWNTs (a) before and (b) after Ar microwave plasma treatment for 3 minutes162

Another solution is to avoid the functionalization of CNTs directly inside the plasma

where the density of high energetic ions is very high and, instead place the CNTs outside the

discharge production zone to reduce the detrimental effects associated with ion bombardment

and irradiation. In this approach, radicals become most important reactive species to graft

functional moieties onto CNTs surface. As aforementioned, the application of microwave

discharge sources for the production of intense beams of atomic, radical and metastable

species is well established. For example, carbon nanotubes have been hydrogenated by atomic

hydrogen generated in a H2microwave plasma163. In this type of functionalization, the exact

location of the sample and its distance from the plasma discharge zone become a key-factor in

determining the extent of functionalization. Khare et al. have studied the exposition of

SWNTs to microwave-generated N2 plasma, by placing CNTs at different distances from

discharge.165 At the shortest distance (1cm), they observed the highest concentration in

nitrogen groups but, also, a loss of integrity of SWNTs due to highly reactive species, like

N2+. At intermediate distances, the incorporation of nitrogen, but also of high quantity of

oxygen, onto the CNTs is obtained while functionalization was not observed for the maximal

distance of 7 cm due to total recombination of atomic nitrogen. It is thus important to place

a b

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CNTs outside the glow discharge zone at a minimal distance to avoid interaction with highly

reactive ions and UV photons. With this type of treatment, radicals become the most

important reactive species to graft functional groups onto carbon nanotubes. The efficiency of

this approach depends on the extent of radicals production, in our case of atomic nitrogen.

Within the scope of this work, the functionalization of carbon nanotubes will be studied by

this approach with microwave plasma allowing a high efficiency of atomic nitrogen

production139.

II.D. Polymer nanocomposites-based on carbon nanotubes

II.D.1.Generalities on polymer nanocomposites

A nanocomposite is defined as a composite material where at least one of the

dimensions of one of its constituents is on the nanometer size scale166

The particularity of nanofillers is that they have at least one of their dimensions in

the same range than the radius of gyration of typical polymer chains. Moreover, when particle

size decreases to nanometer size, the specific surface area of fillers drastically increases and,

so, significantly increases the resulting interfacial volumes. These regions can have distinctly

different properties from the bulk polymer and can represent substantial volume fraction ofthe matrix for nanoparticles with surface areas in the order of hundreds of m/g, like carbon

nanotubes (Figure 2-23). The essential of the entire polymer matrix in a nanocomposite can

behave as if it was part of interface with specific properties. Another key-advantage is that

the improvement of the matrix properties can be reached at low nanofiller contents in

comparison to more conventional microfiller-based composites. These considerations on the

nanoparticle specificities explain why these fillers can have remarkably high impact on the

final properties of nanocomposites

. This term implies also

the combination of two (or more) distinct materials, such as a ceramic and a polymer. The

challenge and interest in the development of nanocomposite is to find ways to create

macroscopic components that benefit from the unique properties of the very small objects

dispersed within them. Natural materials, such as bone or wood, are good examples of the

successful implementation of this concept, offering excellent mechanical properties compared

to those of their separated constituents.

167

.

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Particle size 10m 1m 10nm

Volume content 30% 30% 3%

Number of particles ~10 ~10 ~10

Interface ~0.1m/g ~1m/g ~100m/g

Aspect ratio ~20 ~100 ~1000

Figure 2- 23 : Influence of particle characteristics on various geometric factors168

Carbon nanotubes (CNTs) are considered as ideal candidates to prepare polymer

nanocomposites presenting high-performance properties due to the combination of their

remarkable physical properties with their low density. The resulting CNTs-based materials

can be substantially stronger and lighter than conventional carbon fiber-reinforced polymer

composites currently used in many applications

169. The incorporation of CNTs also provide

electrical conductivity to insulating polymer matrix170, delay thermal degradation of the

polymer matrix and act as efficient flame retardant additives171

.

Nanocomposite properties depend on several factors: the polymer nature, the

synthetic process of CNTs, the nanotube purification process (if any), the amount and type of

impurities on nanotubes or their aspect ratio. However, the key-factor remains the quality of

CNTs dispersion within the host polymer matrix172,173,174

. The variations in nanotube and

nanotube/polymer properties account for many of the apparent inconsistencies in the

literature. However, these variations are difficult to quantify. In this work, we have performed

our studies using CNTs from the same batch, thereby reducing the variability betweensamples and clarifying overall trends.

Several methods have been used to disperse carbon nanotubes in polymer

matrices175

. The most straightforward method is the direct melt blending. Two other methods

are the solution-mediated processes and in-situ polymerization however these techniques are

less versatile and more environmentally contentious.

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II.D.2.Melt blending of polymer matrices with carbon nanotubes

The melt blending approach is a very suitable technique for industrial scale processes

because of its high speed and simplicity176

The optimization of processing conditions is very important because CNTs can affect melt

properties such as viscosity, resulting in unexpected polymer degradation because of

excessive shear rates

. The strategy consists in blending a molten

thermoplastic with carbon nanotubes in order to optimize the polymer-nanotube interactions

and to form a nanocomposite. Amorphous polymers, such as polystyrene (PS), can be

processed above their glass transition temperature while semi-crystalline polymers, such as

polyethylene (PE), need to be heated above their melt temperature to induce sufficient

softening. In general, melt processing involves the melting of polymer to form a viscous

liquid. Any additives, such as carbon nanotubes, can be mixed into the melt by shear mixing.

Bulk samples can then be fabricated by techniques such as compression molding, injection

molding or extrusion.

177. Successful examples of melt blended compositions include

poly(methyl methacrylate)/MWNTs178, polycarbonate/MWNTs179, polypropylene/SWNTs180,

nylon-6/MWNTs181,182, polypropylene/SWNTs183 or polyimide/SWNTs composites184

. All

these examples show some interesting effects in terms of mechanical reinforcement,

crystallization behavior or electrical and thermal percolation thresholds but these effects arealways dependent on the studied matrix. Most of time the melt blending method does not give

very good dispersion of the nanofillers in the majority of polymer matrices because the

aggregates of CNTs are highly stabilized and the compatibility between the polymer chains

and carbon nanotubes is low.

As discussed before, carbon nanotubes have important surface properties, which

affect their dispersion within a polymer matrix

185

. Indeed, many van der Waals interactions(essentially - interactions) between CNTs hold them together in bundles of 50 to few

hundred individual nanotubes packed in a hexagonal array (Figure 2-24a). The average

diameter of a bundle is typically between 10 and 100 nm. Unfortunately, CNTs keep their

tendency to aggregate in large majority of polymer host matrices (Figure 2-24b). These

entanglements and bundles lead to many defect sites in the composites and limit the efficiency

of properties transfer from nanotubes to polymer composites186. Baughman et al. proved that

bundling results in diminished mechanical and electrical properties as compared to theoretical

predictions based on the dispersions of individual SWNTs58.

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Figure 2- 24 : Transmission electron microscopy (TEM) images of (a) a SWNTs rope (axial view)187

(b) bundle-like aggregate of MWNTs subsisting in PE matrix after melt blending

and

The challenge is thus to incorporate individual CNTs, or at least relatively thin

nanotubes bundles, inside the polymer matrix. Different techniques are proposed in the

literature to overcome this drawback. One of them is the modification of CNTs sidewalls via a

pre-treatment of CNTs.

II.D.3. Interfacial compatibilization: improvement of CNTs dispersion

This second approach implies a modification of the CNTs by sidewall

functionalization. In addition to pre-disaggregation of CNTs bundles, the interfacial

compatibilization also improves the interactions polymer-filler107. Indeed, due to the relatively

smooth grapheme-like surface of nanotubes, there is a lack of interfacial adhesion between the

polymer matrix and CNTs. A crucial parameter for the production of this type of carbon

nanotube-based nanocomposites is the control of the degree of grafting/functionalization on

the CNTs walls. For example, it was shown by molecular dynamics that a very high number

of functional groups covalently attached onto the CNT surface can decrease the maximum

buckling force of nanotubes by about 15% and therefore reduce the mechanical properties of

the final nanotube-based composites188. Moreover, the degree of covalent functionalization

should not be too high in order not to significantly disturb the electron system of nanotubes,

and thus affect the electrical properties, but it should be high enough to provide good

compatibility between the filler and polymer matrix. However, several researchers have

200nm

a b

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reported that functionalization can improve the electrical properties of nanocomposites189. It

appears that the disadvantages of functionalization with respect to CNTs conductivity can be

outweighed by the improved dispersion190 and some grafted functional groups can have a

doping effect if the linked heteroatom is an electron donor as nitrogen for instance191

Covalent functionalization can be performed by either defect-side chemistry, modification of

surface-bound reactive groups, e.g., carboxylic acid groups or direct sidewall

functionalization. Some pertinent examples are exposed hereafter.

. It is

thus important to evaluate the content and to determine the exact location of grafted

functional groups on the CNTs surface in order to control the carbon nanotube integrity and to

avoid intensive lost of properties.

Hydroxyl-functionalized carbon nanotubes have been used to reinforce poly(vinyl alcohol)

(PVA)192. The idea is that the OH groups from SWNTs form hydrogen bonds with the OH

of the PVA allowing a twofold increase of the modulus

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