Surface Modification of Nanotube Fillers E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Serie: Polymer Nano-, Micro- and Macrocomposites
• Sprache: Englisch

Seitenzahl: 471

Veröffentlichungsjahr: 2011

Beschreibung

The book series "Polymer Nano-, Micro- and Macrocomposites" provides complete and comprehensive information on all important aspects of polymer composite research and development, including, but not limited to, synthesis, filler modification, modeling, characterization as well as application and commercialization issues. Each book focuses on a particular topic and gives a balanced in-depth overview of the respective subfield of polymer composite science and its relation to industrial applications. With the books the readers obtain dedicated resources with infomation relevant to their research, thereby helping to save time and money.
In this first volume in the series, authors from leading academic institutions and companies share their first-hand knowledge of nanotube-surface enhancements for use in polymer composites. All the important methods for the functionalization of nanotube fillers, including polymer wrapping, non-covalent modification with nanoparticles, silica layers or entrapped micelles, chemically induced growth of multilayers, techniques based on covalent bonding, such as polmer or quantum dot attachment, and direct polymerization approaches are covered.

Leseprobe

Table of Contents

Cover

Table of Contents

Related Titles

Title page

Copyright page

Preface

List of Contributors

1 Carbon Nanotubes Surface Modifications: An Overview

1.1 Introduction

1.2 Noncovalent Functionalization of Nanotubes

1.3 Covalent Modifications of Carbon Nanotubes

2 Modification of Carbon Nanotubes by Layer-by-Layer Assembly Approach

2.1 Introduction

2.2 Layer-by-Layer Modification of the Carbon Nanotubes

2.3 LbL Assembly on CNTs Using Click Chemistry

2.4 LbL Assembly on Vertically Aligned (VA) MWNTs

2.5 LbL on CNTs of Biological Molecules

2.6 LbL on CNTs for Template Development

2.7 Applications of LbL-Modified CNTs

2.8 Conclusions

3 Noncovalent Functionalization of Electrically Conductive Nanotubes by Multiple Ionic or π–π Stacking Interactions with Block Copolymers

3.1 Noncovalent Functionalization of CNTs by π–π Stacking with Block Copolymers Bearing Pyrene Groups

3.2 Noncovalent Functionalization of Electrically Conducting Nanotubes by Multiple Ionic Interactions with Double-Hydrophilic Block Copolymers

Acknowledgment

4 Modification of Nanotubes with Conjugated Block Copolymers

4.1 Introduction

4.2 Synthesis of P3HT Block Copolymer

4.3 Dispersion of CNTs

4.4 Conclusions and Perspective

Acknowledgments

5 Theoretical Analysis of Nanotube Functionalization and Polymer Grafting

5.1 Introduction

5.2 Theoretical Techniques in Modeling Nanotube Functionalization

5.3 Functionalizing Carbon Nanotubes through Mechanical Deformation

5.4 Functionalizing Carbon Nanotubes via Chemical Modification

5.5 Polymer Grafting

5.6 Summary

6 Covalent Binding of Nanoparticles on Carbon Nanotubes

6.1 Introduction

6.2 Covalent Binding of Quantum Dots on CNTs

6.3 Covalent Binding of Magnetic Nanoparticles on CNTs

6.4 Covalent Binding of Gold Nanoparticles on CNTs

6.5 Growth of Poly(amidoamine) Dendrimers on Carbon Nanotubes

6.6 Application of Nanoparticles-Conjugated CNTs Composites

6.7 Concluding Remarks

Acknowledgments

7 Amine-Functionalized Carbon Nanotubes

7.1 Introduction

7.2 Functionalization Strategies for CNTs

7.3 Importance of Amine Functionalization

7.4 Methods of Amine Functionalization

7.5 Characterization Techniques

7.6 Degree of Amine Functionalization

7.7 Changes in the Band Structure

7.8 Applications of Amine-Functionalized CNTs

7.9 Limitations of Amine-Functionalized CNTs

7.10 Conclusions

8 Functionalization of Nanotubes by Ring-Opening and Anionic Surface Initiated Polymerization

8.1 Introduction

8.2 Surface-Initiated Polymerization

8.3 Conclusions

9 Grafting of Polymers on Nanotubes by Atom Transfer Radical Polymerization

9.1 Introduction

9.2 Grafting of Polymers on CNTs by ATRP

9.3 Functionalization of CNTs by Other CRPs

9.4 Grafting of Polymers on Other Nanotubes by ATRP

9.5 Conclusions

Acknowledgments

10 Polymer Grafting onto Carbon Nanotubes via Cationic Ring-Opening Polymerization

10.1 Introduction

10.2 Cationic Ring-Opening Polymerization of Cyclic Monomers in the Presence of Chain Transfer Reagents in One Pot

10.3 Preparation of Polymers-Grafted Carbon Nanotubes

10.4 Applications of Polymer-Grafted CNTs

11 Plasma Deposition of Polymer Film on Nanotubes

11.1 Introduction

11.2 Principle and Experiment

11.3 Results

11.4 Summary

12 Functionalization of Carbon Nanotubes by Polymers Using Grafting to Methods

12.1 Introduction

12.2 Overview of the “Grafting to” Methods

12.3 Conclusions

Acknowledgments

13 Organic Functionalization of Nanotubes by Dipolar Cycloaddition

13.1 Introduction

13.2 The Case of Azomethine Ylide

13.3 The Case of Pyridinium Ylides

13.4 The Case of Nitrile Oxide

13.5 The Case of Nitrone

13.6 The Case of Nitrile Imines

13.7 Conclusions

Index

Related Titles

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2010

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The Editor

Dr. Vikas Mittal

Berner Weg 26

67069 Ludwigshafen

Germany

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

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ISBN: 978-3-527-32878-9

ISBN oBook: 978-3-527-63508-5

ISBN ePDF: 978-3-527-63510-8

ISBN ePub: 978-3-527-63509-2

ISBN Mobi: 978-3-527-63511-5

ISSN: 2191-0421

Preface

Nanotubes have emerged as a very high potential inorganic reinforcement for the polymer materials to enhance mechanical, electrical, and transport properties of the pristine polymers. Though they are relatively new class of polymer fillers, but the factors like matrix compatibility, uniform distribution in the polymer matrix, etc., are same as conventional fillers. In order to achieve uniform dispersion of nanotubes in the polymer matrices thus to achieve their full potential, it is very important to ensure their surface compatibility with the polymer matrix. As polymer matrices may have different polarity and functional groups present in their structure, therefore, the surface of nanotubes is required to be altered accordingly. There are numerous ways of surface functionalization reported in the literature, the compilation of which is presented in this book. Physical and chemical modes of functionalization generate different surface properties of nanotubes and hence different ways of interaction with the polymer matrices. Different modes of surface functionalization of nanotubes have their own advantages and limitations; the choice of the functionalization method is based on the application at hand or based on the required properties.

Chapter 1 presents an overview of the different nanotube surface modification techniques. Advantages and limitations of these modification techniques have been pointed. The use of modified fillers with a variety of polymer matrices has been demonstrated. Chapter 2 details the layer-by-layer assembly approach for the noncovalent modification of nanotubes. Thus, the surface properties of nanotubes can be controlled by the nature of the layer associated with the surface. Chapter 3 describes the noncovalent functionalization of electrically conductive nanotubes by multiple ionic or π–π stacking interactions using block copolymers. Use of conjugated block copolymers for the surface modification of nanotubes has been reported in Chapter 4. Chapter 5 presents in-depth analysis of the theoretical analysis of nanotube modifications and polymer grafting on the surface of nanotubes. Chapter 6 describes the interesting concept of covalent binding of nanoparticles on the surface of nanotubes. Thus, by controlling the surface properties of nanoparticles, the similar behavior can also be induced on the nanotube surface. Chapter 7 presents the amine-functionalized carbon nanotubes. Such nanotubes can be used for subsequent chemical reactions to generate a variety of functional groups on the surface of nanotubes. Functionalization of nanotubes by ring-opening and anionic-surface-initiated polymerization has been described in Chapter 8. The use of living atom transfer radical polymerization for the grafting of polymers on nanotube surfaces has been reported in Chapter 9. Cationic ring-opening polymerization has been demonstrated in Chapter 10 for the polymer grafting on the nanotube surface. Use of plasma for the deposition of polymer films on the surface of nanotubes has been discussed in Chapter 11. Grafting to method has been described for the functionalization of carbon nanotubes in Chapter 12. Dipolar cycloaddition processes for the organic functionalization of nanotube surfaces have been the focus of Chapter 13.

It gives me immense pleasure to thank Wiley-VCH for kind acceptance to publish the book. I dedicate this book to my mother for being constant source of inspiration. I express heartfelt thanks to my wife Preeti for her continuous help in co-editing the book as well as for her ideas to improve the manuscript.

Vikas Mittal

List of Contributors

Chuh-Yung Chen

National Cheng Kung University

Department of Chemical Engineering

Room 93B15, 70101, Tainan

Taiwan

Daxiang Cui

Department of Bio-Nano Science and Engineering

National Key Laboratory of Micro/Nano Science and Technology

Institute of Micro/Nano Science and Technology

800 Dongchuan Road

Shanghai, 200240

China

Christophe Detrembleur

University of Liège

Center for Education and Research on Macromolecules (CERM)

Sart-Tilman B6a

4000 Liège

Belgium

Chao Gao

Zhejiang University

Department of Polymer Science and Engineering

MOE Key Laboratory of Macromolecular Synthesis and Functionalization

38 Zheda Road

Hangzhou, 310027

China

Vassilios Georgakilas

NCSR “Demokritos”

Institute of Materials Science

15310 Ag. Paraskevi, Athens

Greece

Dimitrios Gournis

University of Ioannina

Department of Materials Science and Engineering

45110 Ioannina

Greece

Nikos Hadjichristidis

University of Athens

Department of Chemistry

Panepistimiopolis Zografou

Athens 15771

Greece

Qun Huo

University of Central Florida

Nanoscience Technology Center and the Department of Chemistry

12424 Research Parkway Suite 400

Orlando, FL 32826

USA

Vaibhav Jain

Naval Research Laboratory

Optical Sciences Division

Washington, DC

USA

Christine Jérôme

University of Liège

Center for Education and Research on Macromolecules (CERM)

Sart-Tilman B6a

4000 Liège

Belgium

Robert Jérôme

University of Liège

Center for Education and Research on Macromolecules (CERM)

Sart-Tilman B6a

4000 Liège

Belgium

Ramaiyan Kannan

National Chemical Laboratory

Physical and Materials Chemistry Division

Pune 411008

India

Akshay Kokil

University of Massachusetts Lowell

Department of Chemistry

Center for Advanced Materials

Lowell, MA

USA

Jianhua Liu

University of Central Florida

Nanoscience Technology Center and the Department of Chemistry

12424 Research Parkway Suite 400

Orlando, FL 32826

USA

Ye Liu

Institute of Materials Research and Engineering

3 Research Link

Singapore 117602

Vikas Mittal

BASF SE, Polymer Research

67056 Ludwigshafen

Germany

and

Chemical Engineering Program

The Petroleum Institute

2533, Abu Dhabi

UAE

Kausala Mylvaganam

The University of New South Wales

School of Mechanical and Manufacturing Engineering

J17, NSW 2052

Australia

Petar Petrov

Bulgarian Academy of Sciences

Institute of Polymers

Akad G Bonchev Str

Block 103A, BG-1113 Sofia

Bulgaria

Vijayamohanan K Pillai

National Chemical Laboratory

Physical and Materials Chemistry Division

Pune 411008

India

Dimitrios Priftis

University of Athens

Department of Chemistry

Panepistimiopolis Zografou

Athens 15771

Greece

Georgios Sakellariou

University of Athens

Department of Chemistry

Panepistimiopolis Zografou

Athens 15771

Greece

Levon Terlemezyan

Bulgarian Academy of Sciences

Institute of Polymers

Akad G Bonchev Str

Block 103A, BG-1113 Sofia

Bulgaria

Jean-Michel Thomassin

University of Liège

Center for Education and Research on Macromolecules (CERM)

Sart-Tilman B6a

4000 Liège

Belgium

Binh Tran

University of Central Florida

Nanoscience Technology Center and the Department of Chemistry

12424 Research Parkway Suite 400

Orlando, FL 32826

USA

Decheng Wu

Institute of Materials Research and Engineering

3 Research Link

Singapore 117602

Lei Zhai

University of Central Florida

Nanoscience Technology Center and the Department of Chemistry

12424 Research Parkway Suite 400

Orlando, FL 32826

USA

Liang Chi Zhang

The University of New South Wales

School of Mechanical and Manufacturing Engineering

J17, NSW 2052

Australia

Jianhua Zou

University of Central Florida

Nanoscience Technology Center and the Department of Chemistry

12424 Research Parkway Suite 400

Orlando, FL 32826

USA

1

Carbon Nanotubes Surface Modifications: An Overview

Vikas Mittal

1.1 Introduction

Carbon nanotubes are allotropes of carbon and are regarded as the ultimate carbon fibers [1–3]. The credit for realizing the nanotubes in an arc discharge apparatus is given generally to Iijima who successfully proved the existence of first multiwalled carbon nanotubes (MWCNTs) mixed with other forms of carbon [4], though the existence of these materials was realized earlier also, for example, by Endo in 1976 [5]. Subsequently, single-walled carbon nanocomposites (SWCNTs) were discovered and a significant research effort followed thereafter [6–9]. Carbon nanotubes have unique mechanical, electrical, magnetic, optical, and thermal properties [10]. Although the organic–inorganic nanocomposites can conventionally contain inorganic fillers which differ by the virtue of their primary particle dimensions [11–16], nanotubes containing composites generate much high-end application potential thus signifying the importance of nanotubes. The synthesis methods for the generation of nanotubes include high-temperature evaporation using arc-discharge [17–19], laser ablation [20], chemical vapor deposition, etc. [21–26].

Owing to their inert nature, the nanotubes tend to form bundles with each other and thus do not disperse well in the organic matrices in their pristine state. Suitable enhancement of the surface of the nanotubes is thus required in order to optimize their dispersion in the organic matrices. Out of various possible ways to achieve the surface functionalization, noncovalent means of surface modification are quite common. In this methodology, polymer chains are wrapped around the nanotubes or various surfactant molecules are adsorbed on the surface of nanotubes. In this case, the modification molecules are only physically bound to the surface. In the covalent means of surface modification, polymer chains can be grafted to or from the surface of the nanotubes. Controlled living polymerization methods have also been used to control the architecture of these grafts. These methods generate surface modifications which are chemically bound to the surface of the tubes, but such chemical means of modifications also disturb the structural homogeneity of the tubes, thereby decreasing the mechanical properties. Thus different surface functionalization methods have their own benefits and limitations, and the choice of these methods is dictated by the applications required from the generated nanocomposite materials. Pi–pi stacking is also one of the means to organophilize the nanotubes. The surface modification of the nanotubes forms a critical phase in the nanocomposite synthesis as the interactions of the surface modification molecules with the polymer chains dictate if the nanotubes can be homogenously dispersed in the organic phase or not. Therefore, various methods of surface modification of nanotubes are required to be reviewed in detail in order to tune the nanocomposite morphology and subsequent properties.

1.2 Noncovalent Functionalization of Nanotubes

Noncovalent mode of nanotube functionalization has received a lot of academic interest owing to the noninvasive mode of surface functionalization, which keeps the original nanotube properties intact. However, for some load-bearing applications, the presence of physically bound modifier molecules on the surface can also be a concern. The following studies detail the noncovalent functionalization of nanotubes for eventual dispersion in polymer matrices.

Zhu et al. reported the noncovalent functionalization of nanotubes based on the colloid stabilization principles [27]. In this approach, charged inorganic ZrO2 nanoparticles were used as stabilizing media as shown in Figure 1.1. HiPco SWNTs were first purified following a two-step procedure in which the nanotubes were heated in an O2 atmosphere at 300 °C and subsequent removal of metal catalysts in HCl at 60 °C for 2 h. The purified SWCNTs were then mixed with ZrO2 nanoparticle aqueous solutions and were sonicated. The suspensions were allowed to stand for few hours to few days to remove the unstable large bundles of nanotubes. After this gravity-driven sedimentation of the uncoated nanotubes, the suspensions of nanotubes with nanoparticles were observed to be very stable for long periods of time and the suspensions were transparent. The microscopic investigations by AFM also confirmed the existence of single nanotubes in the dispersed mode. It was suggested that charge repulsion originating from ZrO2 particles can be the most plausible phenomena and there was no direct evidence of nanoparticle haloing of ZrO2 around SWNTs. The external stimuli were observed to self-assemble the dispersed nanotubes into macroscopic materials in solution indicating that the functionalization of the nanotubes with the nanoparticles was very efficient in tuning the surface properties.

Figure 1.1 Schematic representation of dispersion of nanotubes in water by using inorganic nanoparticles.

Reproduced from Ref. [27] with permission from American Chemical Society.

Proteins (like streptavidin) adsorb spontaneously on the surface of the nanotubes [28]. Nonspecific binding of the proteins has also been proven microscopically on nanotubes after exposure of the nanotubes with solution of streptavidin. A number of polymer systems have been used for the prevention of nonspecific binding of proteins on the surfaces by forming coatings and self-assembled monolayers. In an interesting study on the interactions of proteins with nanotubes (Figure 1.2), it was demonstrated that by noncovalent functionalization of nanotubes, nonspecific binding of the proteins on the nanotube surface could be eliminated and specific binding of proteins after surface functionalization could be achieved [29]. Poly(ethylene glycol) is the most commonly used polymer to eliminate the nonspecific binding of the proteins on the surface [30–32]. However, it was observed by the authors that the poly(ethylene glycol) though irreversibly adsorbed on the nanotube surface, but appreciable adsorption of streptavidin was still observed indicating that the coverage of nanotube with the polymer was not optimum. The authors suggested that surfactant can be used for improving the coverage of the surface with poly(ethylene glycol). Triton-X surfactant containing an aliphatic chain and a short hydrophilic poly(ethylene glycol) unit was used to bind to the nanotube surface by hydrophobic interactions. Its adsorption alone on the surface was observed to resist the nonspecific binding of the proteins on the surface and after the adsorption of poly(ethylene glycol) on the surface, no streptavidin adsorption was observed. The authors also suggested means to specifically bind proteins on the surface by following the similar noncovalent functionalization of nanotubes. Biotin moiety was attached to the poly(ethylene glycol) chains by using amine-functionalized poly(ethylene glycol) and covalently linking it with an amine reactive biotin reagent. As streptavidin demonstrates high affinity to biotin, the triton-PEG-biotin functionalized nanotubes were ideal substrates for the very specific adsorption of streptavidin on them.

Figure 1.2 (a) Functionalization of SWNTs for preventing nonspecific binding of protein and (b) strategy for introducing selective binding of streptavidin with prevention of nonspecific binding.

Reproduced from Ref. [29] with permission from American Chemical Society.

Polymer wrapping method was reported by O’Connell et al. for the noncovalent functionalization of polymer nanotubes [33]. The SWCNTs were dispersed in 1% sodium dodecyl sulfate (SDS) in water by using shear mixer and ultrasonication followed by the addition of aqueous solution of poly(vinyl pyrrolidone) (PVP). Excess amount of the polymer was used for wrapping process, and the excess of SDS and PVP were subsequently removed by ultracentrifugation. The AFM images shown in Figure 1.3 confirm that most of the nanotubes existed as single tubes associated with at most a single layer of polymer and only a smaller number of aggregates consisting of more than one nanotube. The suspensions were observed to be stable for months and once dried, the nanotubes were reported to disperse in water with minimal ultrasonication. Nonwrapped nanotubes were observed to assemble into mats of tangled and seemingly endless ropes, whereas it was not the case in the polymer-wrapped nanotubes. The modified nanotubes were also tested for the strength of bonding between the polymer and the nanotubes. The modified tubes were subjected to a cross flow in the channel of field-flow fractionation device and there was no effect on the nature of the nanotubes. The authors suggested that the nanotube wrapping is driven largely by the thermodynamic drive to eliminate the hydrophobic interface between the tubes and the dispersion medium. The molecular level image of the nanotubes wrapped with polymer chains is also demonstrated in Figure 1.4. The authors suggested that the helical wrapping of the chains around the nanotubes takes place due to the free rotation about the backbone bonds. However, single tight coil was suggested to generate significant bond angle strain in the polymer backbone. Therefore polymer coverage by multihelical wrapping was suggested. This mode of wrapping provides high extents of surface coverage with low backbone strain and locally multiple strands of polymer coil around the nanotube close to their nascent backbone curvature.

Figure 1.3 AFM images of PVP–SWNT system adsorbed on a functionalized substrate. Top two images correspond to 5 µm height and amplitude images, respectively, whereas bottom two images correspond to 1 µm counterparts.

Reproduced from Ref. [33] with permission from Elsevier.

Figure 1.4 Possible arrangements of the polymer wrapping around the nanotubes. Top image corresponds to a double helix, the middle scheme is a triple helix, whereas the bottom arrangement shows switch backs, which allow multiple parallel wrapping strands to come from the same polymer chain owing to the backbone bond rotations.

Reproduced from Ref. [33] with permission from Elsevier.

Apart from polymer layer medications, the coverage of the nanotube surfaces with an inorganic silica layer has also been reported and the generated silica layer was bound noncovalently to the nanotubes [34]. Functionalizing the sidewalls of carbon nanotubes with a layer of SiO2 also opens the possibilities of further modification of the nanotube surfaces by utilizing the surface reactions available for silica surfaces. The process of nanotube functionalization is demonstrated in Figure 1.5. A promoter, 3-aminopropyltriethanoxysilane (APTES), was absorbed on the sidewalls of the nanotubes by the interaction of sidewalls of nanotubes with the amine groups followed by the polymerization of the molecule by heat treatment. This, thus, generates an irreversible coating on the nanotube surfaces. As a next step, thin shell of SiO2 of thickness roughly 1 nm was generated around the nanotubes by using tetraethyl orthosilicate as the precursor. The thickness of the coating can be adjusted by controlling the reaction time and the concentration of tetraethyl orthosilicate. This, thus, generates two advantages: first, the thin silica layer is bound noncovalently to the surface; and second, this can then further allow using the SiO2 functionalization chemistries to generate different functionalities on the SiO2 surface. The nanotubes were grown by chemical vapor deposition method in this study, and the authors also opined that the similar inorganic surface modifications can also be extended to other oxide materials. Similarly, norbornene polymerization was achieved on the nanotube surface via noncovalently bound ring-opening metathesis initiator [35].

Figure 1.5 Growth of thin SiO2 films on the walls of single-walled carbon nanotubes by using 3-APTES as promoter.

Reproduced from Ref. [34] with permission from American Chemical Society.

A graft copolymer polystyrene-g-(glycidyl methacrylate-co-styrene) (PS-g-(GMA-co-St)) was reported to noncovalently modify the surface of nanotubes [36]. The graft copolymer was synthesized by free radical melt grafting of GMA on PS chains. The PS chains owing to their affinity of the surface of the MWNTs led to the noncovalent modification of the nanotubes. The MWNTs, which were produced by catalytic pyrolysis of propylene in this study, were dispersed in THF by mixing with excess of copolymer. The surface modification allowed the nanotubes to be dispersed in a variety of polar and nonpolar solvents. Also, when the acid-treated MWNTs were used, the presence of GMA in the copolymer provided the epoxy functional groups to which the acidic groups present on the surface of nanotubes could covalently attach to form esters. Figure 1.6 shows the nanotubes functionalized by the copolymer by following both the above-mentioned functionalization approaches. The nanotubes, which had only noncovalent functionalization, were observed to be uniformly modified on the surface by the polymer layer. However, in the nanotubes, which were acid-treated and hence had a covalent binding of the polymer chains, the polymer was bound to the surface only at sites where the carboxyl groups were present. Different modification mechanisms were also observed to significantly affect the solubility characteristics of the nanotubes. The noncovalently modified nanotubes were observed to swell well in different polar and nonpolar solvents, whereas the covalently functionalized nanotubes were swollen in solvents in which polystyrene is soluble. In the case of noncovalently functionalized nanotubes, the polystyrene component of the copolymer was attached to the nanotubes surface, thus leaving the chains containing gylcidyl methacrylate dangling away from the nanotube surface. As glycidyl methacrylate is readily soluble in many solvents, the swelling of noncovalently functionalized nanotubes was improved significantly. However, in the case of covalently functionalized nanotubes, the polystyrene chains formed the external “capsules” owing to the chemical reaction of the GMA block of the copolymer with the surface of nanotubes. Thus, the swellability of nanotubes was dictated by the swelling or solubility characteristics of polystyrene chains.

Figure 1.6 High-resolution TEM images of nanotubes modified by the polymer (a) noncovalently and (b) covalently.

Reproduced from Ref. [36] with permission from Elsevier.

In another study, the authors stressed the need of noncovalent functionalization of nanotubes to retain their attractive electronic and mechanical properties. To achieve this, the authors used an approach for the functionalization of nanotubes by using polymer multilayers [37]. The formation of polymer electrolyte layers has been reported in the literature as a promising strategy to functionalizing the surfaces [38–45]. The thickness of the layers depends on the nature of the polymer used for adsorption as well as pH tuning and the ionic strength of the solution used for adsorption. In this study, hydrolyzed-poly(styrene-alt-maleic anhydride) (h-PSMA) was adsorbed noncovalently onto nanotubes surface via hydrophobic interactions. As the copolymer contained carboxylic acid groups, these groups could be used to further attach a second layer of polyethyleneimine covalently forming a cross-linked polymer bilayer. The same process was followed to attach second layer of h-PSMA and subsequently a second layer of polyethyleneimine. In the end, a layer of polyacrylic acid was generated on the surface of the nanotubes. The functionalization of the surfaces by the polymer multilayers was confirmed by using polarized infrared (IR) internal reflectance spectroscopy. Gold nanoparticles could also be immobilized on the surface by generating a third layer of polyethyleneimine. As gold particles were negatively charged, they could bind to the positive charges in the polyethyleneimine layer by electrostatic interactions. A control sample which was untreated substrates was also immersed in aqueous suspension of gold nanoparticles. A negligible amount of gold nanoparticles was observed to attach to this sample, thus confirming the tuning of the surface properties by using polymer multilayers approach. Figure 1.7 shows the functionalization of nanotubes by the polymer multilayers approach. Thus, this noncovalent mode of surface functionalization retains the properties of nanotubes as well as generates reactive functional groups on the surface to tune the properties of nanotubes accordingly.

Figure 1.7 Functionalization of graphite and nanotube surfaces: (a) fluorescence micrograph of polymer-coated graphite, (b) SEM image of gold nanoparticles immobilized on polymer-coated graphite, (c) SEM image of substrate-grown SWNTs, (d)–(f) SEM images of gold nanoparticles immobilized on polymer-coated SWNTs, and (g) TEM image of gold nanoparticles immobilized on polymer-coated MWNTs.

Reproduced from Ref. [37] with permission from American Chemical Society.

Chen et al. reported the pi-stacking approach to immobilize biomolecules on carbon nanotubes [46]. Noncovalent functionalization of the sidewalls of SWCNTs was achieved, subsequent immobilization of various biological molecules was observed, and the process was reported to be controllable and site-specific.

For the noncovalent functionalization of nanotubes, a bifunctional molecule, 1-pyrenebutanoic acid, succinimidyl ester was irreversibly adsorbed onto the inherently hydrophobic surfaces of SWNTs in dimethylformamide or methanol. The pyrenyl group is highly aromatic in nature and, therefore, is known to interact strongly with the basal plane of graphite via pi-stacking [46, 47]. The authors also observed the similar interaction of the pyrenyl group with the nanotubes surface. It was also observed that the ester molecule was irreversibly adsorbed on the surface and resisted desorption in aqueous solutions.

Succinimidyl ester groups are highly reactive to nucleophilic substitution by primary and secondary amines present in excess on the surface of most proteins; therefore the functionalization of nanotubes with these groups opens the possibilities for nanotubes to generate specific interactions with the biomolecules. Figure 1.8 demonstrates these functionalization processes. The authors suggested that the proteins immobilization on nanotubes involves the nucleophilic substitution of N-hydroxysuccinimide by an amine group on the protein, resulting in the formation of an amide bond. Successful immobilization of both ferritin and streptavidin proteins could be achieved by this method, thus indicating the versatility of this functionalization approach to make the surfaces of nanotubes more usable. Apart from proteins, any other chemical molecules can also be reacted on the surfaces treated with ester and thus the surface properties can be tuned or controlled according to the requirement.

Figure 1.8 Irreversible adsorption of 1-pyrenebutanoic acid, succinimidyl ester on the side walls of nanotubes.

Reproduced from Ref. [46] with permission from American Chemical Society.

Kang et al. [48] reported an interesting approach of encapsulating polymer nanotubes within cross-linked, amphiphilic copolymer micelles, similarly reported by other researchers [49, 50]. The modified nanotubes could be dispersed in a variety of polar and nonpolar solvents and polymer matrices. The authors used an amphiphilic poly(styrene)-block-poly(acrylic acid) copolymer, which was dissolved in dimethylformamide (DMF), a solvent in which both the blocks of the copolymer are soluble and do not form micelles. The nanotubes were subsequently suspended in the copolymer solution by ultrasonication. Micellization of the copolymer was achieved by the gradual addition of water to the suspension, leading to the encapsulated nanotubes inside these micelles. The poly(acrylic acid) blocks were subsequently cross-linked by the addition of diamine linker and carbodiimide activator. The nanotubes were observed to remain dispersed throughout the cross-linking process. It was also observed that full cross-linking of poly(acrylic acid) blocks was not necessary to achieve stabilization; conversion of 25% acid groups to amides was enough to bring about the stability of nanotube suspension. The treated nanotubes were washed off the excess reagents and empty micelles, and the encapsulated nanotubes could be successfully redispersed in different solvents. Apart from that, these suspensions were stable over long period of time as neither the visible absorbance nor the scattered light intensity of the suspensions changed as a function of time. It was hypothesized that high solubility of these encapsulated nanotubes resulted because of solvation of at least one block of the encapsulating copolymer in the solvents used. The authors reported in some cases the solvation of even both the blocks of the copolymer in the solvent. The solvents were also analyzed and it was confirmed that no polymer was desorbed from the surface of nanotubes. Figure 1.9 shows the schematic of the encapsulation process.

Figure 1.9 Encapsulation of nanotubes by using poly(styrene)-block-poly(acrylic acid) copolymer.

Reproduced from Ref. [48] with permission from American Chemical Society.

Many other studies have been reported for the noncovalent functionalization of nanotubes. Conjugated luminescent polymer poly(metaphenylenevinylene) (PmPv) could be coated on to the nanotubes surface [51]. Water-soluble polyvinyl alcohol was coated around the nanotubes, thus allowing the dispersion of nanotubes in water [52]. Carbon nanotubes were also wrapped with starch to generate starched nanotubes [53]. Long-chain polymers like Gum Arabic were also physically adsorbed to disperse the nanotubes individually in solvents [54]. Aromatic polyimides were also used as functionalizing polymer to achieve noncovalent functionalization of carbon nanotubes [55].

1.3 Covalent Modifications of Carbon Nanotubes

Covalent functionalization of nanotubes though may produce defects in the wall structure of the nanotubes, this mode of surface functionalization is required when the load-transfer properties are concerned. Thus, the choice of the modifications on the nanotubes surface may be a direct result of the properties required from them. Following paragraphs explain the various methodologies reported in the literature in order to achieve covalent functionalization of nanotubes.

Velasco-Santos et al. reported the advantages of chemical functionalization of nanotubes in improving the composite properties [56]. In the study, in situ polymerization of methyl methacrylate (MMA) was achieved both in the presence of unfunctionalized and acid-functionalized nanotubes. Interesting phenomena regarding the mode of polymerization and resulting composite properties were reported. Some studies have earlier reported that the unfunctionalized nanotubes can have chemical interactions with the polymer through opening of the pi bonds in the nanotube structure and subsequently nanotube can take part in polymerization reaction, thus grafting the polymer chains covalently bound to the clay surface [57, 58]. The authors also observed in this study that the position of vibration bands in the IR spectra was observed to increase, indicating that the pi bonds in the nanotubes could have been opened. However, it was suggested that the covalent functionalization provides much more attractive route as the generated reactive groups on the tips and surface of nanotubes can be much more useful in binding the nanotubes to the polymer chains during the course of polymerization. Composites generated using functionalized nanotubes were confirmed through IR spectroscopic characterization to have a chemical connection between the formed polymer and nanotubes. It was concluded that covalent surface functionalization in the presence of in situ polymerization to attach the polymer chains covalently to the surface of nanotubes is the best method for generating nanotube nanocomposites.

As one-dimensional materials have strong potential as building blocks in electronics, nanohybrid materials from CNTs and nanoparticles were reported in which the nanoparticles were covalently immobilized on the nanotubes surface [59–61]. To achieve theses functionalizations, the nanotubes were treated with nitric acid followed by thionyl chloride to generate COCl groups on the surface. Magnetite colloid solution was prepared in ethanol solution, to which 3-aminopropyltrimethoxysilane (APTS) was added with rapid stirring. APTS-coated magnetite nanoparticles were dried under vacuum at room temperature, which resulted in amine-terminated magnetite nanoparticles. These particles were reacted with COCl-treated nanotubes at room temperature using ultrasonification as explained in Figure 1.10. Transmission electron microscopy observations also revealed that the magnetite nanoparticles were deposited and homogenously distributed on the surface of the carbon nanotubes. It also demonstrated the effectiveness of the pretreatment of nanotubes in generating active sites on the carbon nanotubes. The resulting nanohybrids were also easily dispersible in aqueous solvents and were observed to be stable for more than 2 months; no precipitation of the inorganic material was observed. It should be noted that noncovalent immobilization of nanoparticles on the nanotubes surface has also been reported, but the covalent functionalization was reported to be more efficient and stronger in interactions between the components.

Figure 1.10 Covalent attachment of magnetite nanoparticles on the carbon nanotube surface.

Reproduced from Ref. [61] with permission from Elsevier.

Covalent functionalization method was reported for the modification of SWCNTs with enzymes and amines [62]. In a typical experiment, SWNTs were refluxed in 4 M HNO3 for 24 h and were exposed to 1 M HCl. The carboxylated SWNTs were then filtered and washed with water followed by drying in air. Acylation reactions with the acid-treated nanotubes were carried out by stirring the nanotubes in a 20 : 1 mixture of thionyl chloride and DMF at 70 °C for 24 h. The nanotubes were then filtered, washed with anhydrous THF, and dried under vacuum at room temperature. The so-obtained nanotubes were then reacted with desired amines using DMF as solvent, at 110 °C. The amines were used in excess, which after the reaction was washed first with DMF, followed by anhydrous THF. In the case of the enzymes, the reactions were carried out at low temperatures. Successful reactions of the following molecules with the surface groups on nanotubes could be achieved: porcine pancrease lipase (PPL), amino lipase (AK), cis-myrtanylamine, 2,4-dinitroaniline, 2,6-dinitroaniline, N-decyl-2,4,6-trinitroaniline, and N-(3-morpholinopropyl)-2,4,6-trinitroaniline). It was observed that the generated defects and covalent binding of amines on the surface after functionalization caused the reduction in electrical conductivity in the functionalized nanotubes. It was also observed that both the single nanotubes as well as bundles could be grafted, indicating that initial homogenous dispersion of nanotubes during functionalization is immensely important. Successful immobilization of a variety of amine as well as biological entities on the surface confirmed the potential as well as robustness of the functionalization method. Figure 1.11 shows the SEM images of the untreated and treated nanotubes [62].

Figure 1.11 SEM images of (a) pristine SWNTs, (b) cis-myrtanylamine functionalized SWNTs, (c) AK functionalized SWNTs, and (d) PPL functionalized SWNTs.

Reproduced from Ref. [62] with permission from Elsevier.

Grafting from the surface methods has been studied widely in detail and can also be directly applied on the nanotubes [63–68]. Qin et al. reported the polymerization of n-butyl methacrylate from the surface of nanotubes by using controlled living polymerization method [69]. To achieve such surface grafting, nanotubes were first refluxed in nitric acid, which generates acid functional groups on the sidewalls of the tubes. These acid groups were subsequently reacted with thionyl chloride to convert them into acyl chloride groups. An atom transfer radical polymerization agent 2-hydroxyethyl-2-bromopropionate (HEBP) was synthesized by the reaction of ethylene glycol and 2-bromopropionlybromide. The acyl chloride groups on the surface of nanotubes were then reacted with the ATRP initiator. After the ATRP ignition moiety was bound to the surface of nanotubes, these nanotubes could then be used to graft brushes of poly(n-butyl methacrylate) in the presence of CuCl and BiPy. Figure 1.12 shows the schematic of the functionalization process. The grafted brushes had significant effect on the solubility properties of nanotubes. This process also offers many other advantages: the amount of ATRP initiator on the surface can be controlled, the chain length of the grafts can also be tuned according to requirement, and a large number of monomers can be polymerized to graft brushes of various functionalities from the surface of nanotubes. Gao et al. also followed the similar grafting from the surface approach and polymerized MMA on the surface by using atom transfer radical polymerization [70]. Figure 1.13 shows the images of the pristine nanotubes as well as treated nanotubes where reaction conditions have been altered to change the grafting characteristics. Polymer thicknesses from 3.8 to 14 nm were grafted by changing the feed ratios, and the nanotubes after functionalization with poly(methyl methacrylate) could also be further reacted to form another layer of hydroxyethyl methacrylate (HEMA).

Figure 1.12 Atom transfer radical polymerization method from the grafting of poly(n-butyl acrylate) chains from the surface of nanotubes.

Reproduced from Ref. [69] with permission from American Chemical Society.

Figure 1.13 (a) Pristine nanotubes and (b–f) nanotubes modified with varying thicknesses of polymer film grafted from the surface.

Reproduced from Ref. [70] with permission from American Chemical Society.

Ge et al. reported the chemical grafting of polyetherimides on the surface of nanotubes [71]. A polyetherimide (BisADA-DCB) was synthesized via the condensation polymerization of 4,4′-bis(4,4′-isopropylidene diphenoxy)-bis(phthalic anhydride) (BisADA) and 2,2′-dichloro-4,4′-biphenyldiamine (DCB) in m-cresol with isoquinonline at elevated temperatures via one-step imidization. The resulting BisADA-DCB containing diamine endgroups was observed to be soluble in many solvents such as chloroform, methylene chloride, THF, and cyclopentanone. The carboxylic acid functionalized MWNTs were reacted with polyetherimide and new characteristic bands were detected in the attenuated total reflection Fourier transform IR spectroscopy. Transmission electron microscopy was also used to investigate the MWNTs before and after chemical grafting. A large population of MWNTs was coated by BisADA-DCB. A thin layer of BisADA-DCB with a thickness of 5–20 nm was observed. Figure 1.14 shows the schematic of functionalization process.

Figure 1.14 Functionalization of MWNT with polyetherimide.

Reproduced from Ref. [71] with permission from American Chemical Society.

Attachment of diglycidyl ether of bisphenol A on the nanotubes surfaces was studied [72] to covalently functionalize the nanotubes. MWCNTs with acidic groups on the surface were reacted with epoxide-terminated molecule as shown in Figure 1.15. As the molecule had epoxy groups in both the ends, both the groups could take part in the functionalization reactions, which was also confirmed with IR spectrometry. It was observed that the epoxide groups reacted either by the epoxide–epoxide reactions followed by the reaction of the epoxide ring with the carboxylic acid group on the nanotube, or both the epoxide groups can react with the acid groups present on the nanotube surface.

Figure 1.15 Epoxy functionalization of the nanotubes.

Reproduced from Ref. [72] with permission from American Chemical Society.

A novel approach to in situ surface functionalization of the nanotubes by the attachment of polystyrene chains to SWCNTs based on anionic polymerization scheme was reported by Viswanathan et al. [73] as shown in Figure 1.16. As produced nanotubes without any purification or surface treatment were used for the functionalization process, these procedures were suspected to introduce functionalities that hinder carbanion formation. To achieve the grafting, SWCNTs were dispersed by sonication in purified cyclohexane to which a slight excess of butyl lithium was added. This introduced carbanions on the SWCNT surface, which help to exfoliate the nanotube bundles because of the repulsion between the negatively charged nanotubes and also provided initiating sites for the polymerization of styrene. The small diameters of nanotubes offer high reactivities for the carbanion addition reactions owing to their high curvatures. Raman spectroscopy was used to confirm the evidence of formation of carbanions and subsequent attachment of polymer chains. On addition of styrene, both free butyl lithium and the nanotube carbanions were observed to initiate polymerization, resulting in an intimately mixed composite system. The polymerization could efficiently be terminated by the addition of degassed n-butanol.

Figure 1.16 Covalent functionalization of nanotubes by anionic polymerization approach.

Reproduced from Ref. [73] with permission from American Chemical Society.

Low-molecular-weight chitosan, a natural and green polymer, was covalently bound to the sidewalls of MWCNTs by a nucleophilic substitution reaction on the surface of nanotubes [74]. Amino and primary hydroxyl groups of chitosan were reported to contribute mainly to the formation of MWNT–chitosan structures. These groups reacted with COCl groups generated on the nanotube surfaces by acid treatment. By thermogravimerty, the extent of chitosan in the organically modified nanotubes was approximately 58 wt%, indicating a significant amount of chitosan grafting to the surface. It was observed from XPS data that a nitrogen-to-carbon ratio of 0.091 was present in the modified nanotubes, which was used to estimate the average degree of functionalization in the nanotubes. On the basis of this information, it is calculated that approximately four molecular chains of the LMCS were attached to 1000 carbon atoms of the nanotube sidewalls. The functionalized nanotubes were soluble in a variety of solvents like dimethylformamide, dimethyl acetamide, dimethylsulfoxide, and acetic acid aqueous solution. Figure 1.17 also shows the TEM images of the nanotubes before and after the modification of the tubes with chitosan.

Figure 1.17 TEM images of (a) raw MWNTs, (b) cut and purified MWNTs, and (c) the MWNT-modified with chitosan. Arrows indicate the typical features of the attached polymer.

Reproduced from Ref. [74] with permission from American Chemical Society.

Microwave irradiation for the acid functionalization of both single and MWCNTs was also reported, which caused the generation of significant amount of hydrophilic functional groups on the surface [75]. Immobilization of DNA on the nanotubes was reported by covalent functionalization of nanotubes with polyethylenimine (PEI) [76]. Plasma deposition of polymers as thin films on the surface of nanotubes has also been reported [77, 78]. Fluorinated SWCNTs have also been reported to demonstrate higher potential for further derivatization due to higher reactivity than the unfunctionalized nanotubes [79–81].

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2

Modification of Carbon Nanotubes by Layer-by-Layer Assembly Approach

Vaibhav Jain and Akshay Kokil

2.1 Introduction

The discovery of carbon nanotubes (CNTs) in 1991 by Iijima [1] offered an excellent material that possesses exceptional mechanical and electrical properties [2, 3]. As opposed to the spherical nature of fullerenes [4], CNTs can be visualized as seamless rolled-up hollow cylinders of graphite sheets. They are obtained as single-walled tubes (SWCNTs), represented by a hollow single cylinder or as multiwalled tubes (MWCNTs), which are a collection of multiple graphene concentric hollow cylinders. CNTs are mainly synthesized by three methods: electric arc discharge, laser ablation, or chemical vapor deposition (CVD). From the time of their discovery, CNTs have had a significant impact on research both in academia and in industry. Due to their outstanding mechanical, thermal, and electronic properties, CNTs offer wide-ranging applications in materials science, molecular electronics, and in the biomedical sciences [2, 5]. Around the world, significant strides have been made in CNT research and this area is ever-evolving. Newer applications have been envisioned for CNTs owing to their superior properties. Although great strides have been achieved in the development of CNTs, their covalent functionalization still requires at times intricate protocols, which can limit their applicability. Covalent functionalization of CNTs can also have a degradative effect on their electronic properties. Noncovalent functionalization of CNTs aids in preserving the sp2 hybridized structure of the nanotube and thus maintains their novel electronic properties. Deposition of polymers and small molecules on CNTs in layers introduces a relatively facile technique for CNT functionalization.

Layer-by-layer (LbL) deposition of polymeric thin films (also known as ionic self-assembled multilayers (ISAMs) of polyelectrolytes) is a technique developed by Decher et al., [6, 7] which uses the attractive forces between the molecules of opposite charges to form the films [8, 9]. Figure 2.1 shows the steps of the fabrication process along with its brief description. In this technique, a high concentration of polyelectrolytes (PE) is deposited on the substrate that was carrying a negative charge. These highly concentrated solutions lead to excess adsorption, which means that there is complete charge neutralization and resaturation bringing charge reversal [10]. Brent et al. proved this phenomenon by adopting a surface-force measurement technique [11]. Alternate deposition of these highly concentrated PE of positive and negative charge increases the thickness of the film and also provides flexibility in choosing the number of bilayers while still maintaining nanoscale control of the thickness. One of the important advantages of the LbL film assembly is that it is very easy to use without the need of any kind of expensive or special apparatus. Various combinations of PE with other materials such as gold colloids [12], silica [13, 14], clay minerals [15], proteins [16, 17], viruses [18], dendrimers [19], etc., have also proven that the LbL technique is very flexible and easy to use.

Figure 2.1 (a) Schematic of the film deposition process using slides and beakers. Steps 1 and 3 represent the adsorption of a polyanion and polycation, respectively, and steps 2 and 4 are washing steps. The four steps are the basic buildup sequence for the simplest film architecture, (A/B)n. The construction of more complex film architectures requires only additional beakers and a different deposition sequence. (b) Simplified molecular picture of the first two adsorption steps, depicting film deposition starting with a positively charged substrate. Counterions are omitted for clarity. The polyion conformation and layer interpenetration are an idealization of the surface charge reversal with each adsorption step.

Soon after Decher’s work, Rubner et al. demonstrated the ability to deposit thin uniform LbL films of conductive materials (polymers like polyaniline, polypyrrole, etc.) on substrates of different shapes and sizes [20–22]. Thin-film formation by molecular self-assembly of p-type-doped electrically conductive polymers with either a conjugated or nonconjugated polyanion was demonstrated. Partially doped conjugated polymers have delocalized defect sites along the polymer backbone that carry positive charges (polarons and bipolarons) and that charge has been used with the negative charge of the polyanion to deposit the multilayers [23]. The range of the applications of LbLs is very broad, but a few of the most common ones that have been demonstrated in the last 15 years are thin films for nonlinear optics [23, 24], selective area patterning [25], sensors [26], electrochromic films [27, 28], electrocatalysis [29], thin films for light-emitting diodes [30], antireflection coatings [31], and active enzymes [32].

A change in pH, ionic strength, and salt concentration of the PE solution results in changing the thickness of the LbL film bilayers. Rubner et al. have explained the influence of changing pH of the solution on the film morphology, physical properties, and composition [33, 34]. Variation in the charge density of a weak PE (e.g., poly(allylamine hydrochloride) and poly(acrylic acid)) can result in a change in the adsorption behavior, that finally results in a change in the behavior of the films resulting in very thick to very thin films over a narrow pH range [35]. At low pHs, positively charged PE (polycations) are highly ionized, but lose protons as the pH goes higher than a characteristic pH known as the pKa