Chemically-modified carbon nanotubes (CNTs) exhibit a wide range of physical and chemical properties which makes them an attractive starting material for the preparation of super-strong and highly-conductive fibres and films. Much information is available across the primary literature, making it difficult to obtain an overall picture of the state-of-the-art. This volume brings together some of the leading researchers in the field from across the globe to present the potential these materials have, not only in developing and characterising novel materials but also the devices which can be fabricated from them. Topics featured in the book include Raman characterisation, industrial polymer materials, actuators and sensors and polymer reinforcement, with chapters prepared by highly-cited authors from across the globe. A valuable handbook for any academic or industrial laboratory, this book will appeal to newcomers to the field and established researchers alike.
Chemically-modified carbon nanotubes (CNTs) exhibit a wide range of physical and chemical properties which makes them an attractive starting material for the preparation of super-strong and highly-conductive fibres and films. Much information is available across the primary literature, making it difficult to obtain an overall picture of the state-of-the-art. This volume brings together some of the leading researchers in the field from across the globe to present the potential these materials have, not only in developing and characterising novel materials but also the devices which can be fabricated from them. Topics featured in the book include Raman characterisation, industrial polymer materials, actuators and sensors and polymer reinforcement, with chapters prepared by highly-cited authors from across the globe. A valuable handbook for any academic or industrial laboratory, this book will appeal to newcomers to the field and established researchers alike.
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Overview
Chemically-modified carbon nanotubes (CNTs) exhibit a wide range of physical and chemical properties which makes them an attractive starting material for the preparation of super-strong and highly-conductive fibres and films. Much information is available across the primary literature, making it difficult to obtain an overall picture of the state-of-the-art. This volume brings together some of the leading researchers in the field from across the globe to present the potential these materials have, not only in developing and characterising novel materials but also the devices which can be fabricated from them. Topics featured in the book include Raman characterisation, industrial polymer materials, actuators and sensors and polymer reinforcement, with chapters prepared by highly-cited authors from across the globe. A valuable handbook for any academic or industrial laboratory, this book will appeal to newcomers to the field and established researchers alike.
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Carbon Nanotubeâ"Polymer Composites
By Dimitrios Tasis
The Royal Society of Chemistry
Copyright © 2013 The Royal Society of ChemistryAll rights reserved.
ISBN: 978-1-84973-681-7
CHAPTER 1
Conducting Polymer-based Carbon Nanotube Composites: Preparation and Applications
SANG-HA HWANG, JEONG-MIN SEO, IN-YUP JEON, YOUNG-BIN PARK AND JONG-BEOM BAEK
1.1 Discovery of Conducting Polymers
Polymers have traditionally been considered to be good electrical insulators, and a variety of applications have relied on this insulating property. However, in 1958, Natta et al. succeeded in synthesizing polyacetylene (PA), a semiconducting conjugated polymer, which paved the way for the upsurge in conjugate polymer research that followed in the decades to come. Alan Heeger, Alan G. MacDiarmid and Hideki Shirakawa made the revolutionary discovery that plastic can be electrically conducting in the 1970s. As a result, they were jointly awarded the Nobel Prize in Chemistry in 2000. For a polymer to be electrically conducting, it must "imitate" a metal — that is, the electrons must be freely mobile and not bound to the atoms. One way to achieve this, is to have the polymer backbone consisting of alternating single and double bonds, called "conjugated double bonds", between carbon atoms. It must also be "doped", which means that electrons are removed (through oxidation) or introduced (through reduction). The resulting holes or extra electrons can move along the macromolecule, which would, in turn, make the polymer electrically conducting.
The chemical origins of such a remarkable difference in the material properties between various types of polymers can be readily rationalized. Traditional polymers, such as polyethylene or polypropylene, are made up of essentially σ-bonds; hence, a charge once created on any given atom on the polymer chain is not mobile (static charge). The presence of an extended π-conjugation in polymers, however, confers the required mobility to charges that are created on the polymer backbone (by the process of doping) and makes them electrically conducting. One problem is that, due to the presence of this extended conjugation along the polymer backbone, the chains are rigid and possess strong inter-chain interactions, resulting in insoluble and infusible materials. These conjugated polymers, hence, lack one of the most important and useful properties of polymers, namely the ease of processability. More recently, however, it was demonstrated that when lateral substituents were introduced even conjugated polymers can be made soluble (hence, processable) without significant decrease in their electrical conductivity. Another problem in technological application is the inherent chemical instability, especially, in the doped form, to ambient conditions. Today, conducting polymers that are stable even in the doped form have been developed. We shall highlight some specific examples of such systems and discuss some of their research progress and potential applications.
PA, in view of possessing the simplest molecular framework, has attracted the most attention, especially of physicists, with an emphasis on understanding the mechanism of conduction. However, its insolubility, infusibility and poor environmental stability had rendered it rather unattractive for technological applications. The technologically relevant front runners belong to essentially four families: polyaniline (PANI), polypyrrole (PPy), polythiophene (PT) and poly(phenylene vinylene) (PPV). PANI is rather unique as it is the only polymer that can be doped by a protonic acid and can exist in different forms depending upon the pH of the medium. A few of the more important conducting polymers and their molecular structures are shown in Figure 1.1.
1.2 Synthesis of Conducting Polymers
Conducting polymers can be synthesized either chemically or electrochemically, and each of their advantages and disadvantages are summarized in Table 1.1. Through the chemical polymerization approach, the conjugated monomers react with an excess amount of an oxidant in a suitable solvent, such as acid. The polymerization takes place spontaneously and requires constant stirring as the reaction progresses. The second method is via electrochemical polymerization, which involves placing both the counter and reference electrodes (such as platinum) into the solution containing the diluted monomer and electrolyte (the dopant) in a solvent. After applying a proper voltage, the polymer film immediately begins to form on the working electrode. A major advantage of chemical polymerization is associated with the mass reproducibility or scalability at a reasonable cost, which is a problem associated with electrochemical methods. On the other hand, an important feature of the electropolymerization technique is the direct formation of conducting polymer films that are highly conductive for use especially in electronic devices.
1.3 Conductivity and Doping of Conducting Polymers
In general, materials with conductivities less than 10-8 S cm-1 are considered as insulators, materials with conductivities between 10-8 and 103 S cm-1 are considered as semiconductors, and materials with conductivities greater than 103 S cm-1 are considered as conductors. Conducting polymers in their pristine (undoped) states are usually considered as semiconductors or insulators, having a band gap energy excessively high for the thermal excitation of a significant number of charge carriers. Therefore, undoped conducting polymers, such as PA and PT, show electrical conductivities of only 10-10–10-8 S cm-1 (Figure 1.2). Upon the doping of conducting polymers, there is a dramatic increase in the electrical conductivity by several orders of magnitude up to values of approximately 10-1 S cm-1, even at very low level of doping, for example, less than 1%. Subsequent doping to higher levels results in the saturation of the conductivity at values in the range 102–105 S cm-1, as shown in Figure 1.3. Heavily doped, stretch-oriented PA has the highest reported electrical conductivity among the conducting polymers, with the confirmed value of 80 000 S cm-1.
In the case of conventional inorganic crystalline semiconductors, such as silicon and germanium, doping occurs through the substitutional insertion of atoms (dopant) with a higher or lower valence band than the host semiconductor for ntype or p-type doping, respectively. For example, phosphorous (5 valence electrons) providing an extra electron to the silicon (4 valence electrons) lattice semiconductor band structure leads to n-type doping. However, doping in a conducting polymer is the process of oxidizing (p-doping) or reducing (n-doping) a neutral polymer and providing a counter anion or cation (i.e., dopant), respectively. Upon doping, a conducting polymer with a net charge of zero is produced due to the close association of the counter ions with the charged polymer backbone. This process introduces charge carriers, in the form of charged polarons (i.e., radical ions) or bipolarons (i.e., dications or dianions), into the polymer (Figure 1.4).
As summarized in Figure 1.5, charge injection by doping can be achieved in several ways. Each of the methods of doping illustrated in Figure 1.5 leads to unique and important phenomena. For chemical and electrochemical doping, the induced electrical conductivity is permanent until the carriers are either chemically compensated or until the carriers are purposely removed by "dedoping". In the case of photoexcitation, the photoconductivity is transient and lasts only until the excitations are either trapped or decay back to ground state. For charge injection at an interface, electrons reside in the π*-band, and holes reside in the π-band only as long as the bias voltage is applied. Doping also leads to a reversible shift in the electrochemical potential, thereby making possible polymer batteries and electrochemically active polymer electrodes, electrochromic phenomenon and polymer p–n junctions (light-emitting electrochemical cells). In the case of photodoping, the redistribution of oscillator interband (π*–π) transition provides a route to a non-linear optical (NLO) response. Real occupation of low energy excited states and virtual occupation of higher energy excited states (perturbation theory) lead to resonant and non-resonant NLO responses.
1.4 Conducting Polymers as Carbon Nanotube (CNT) Composite Matrices
The increasing demand for efficient products has driven a trend towards downsizing and lesser power consumption but greater performance. The progression relies upon searching for new desirable materials and the ability to make micro/nanoscale structures with high accuracy and precision. However, it is not so easy to satisfy these seemingly contradicting demands. For instance, current silicon-based semiconductor devices are reliable, and their performances have been constantly enhanced. However, they are reaching a barrier of modern quantum physics, as they approach the nanoscale. The search for new alternative materials and fabrication methods, therefore, is an urgent task for future developments.
CNTs and conducting polymers have both shown exceptional properties and characteristics. Coupling the two materials has furthermore revealed synergistic effects, which offers an attractive route to create a new breed of multifunctional materials with greater application potentials. Envisioned applications from CNT/conducting polymer composite systems involve mechanical, thermal, electrical, electrochemical features and their applications in various areas such as supercapacitors, sensors, organic light emitting diodes, solar cells, electromagnetic absorbers and advanced electronic devices.
Conducting polymers are conjugated polymers in doped states, which consist of alternating single and double bonds along its linear chains (sp2 hybridized structure). The conductivity of conducting polymers relies on these double bonds, which are sensitive to physical or chemical interactions. Similarly, CNTs also have sp2 hybridized bonds over the structure. CNTs possess unique structures and exhibit extraordinary electrical, optical, chemical and mechanical properties, which are somewhat complementary to those of conducting polymers. For instance, CNTs have a very long mean free path, ultra-high carrier mobility and can be either very good conductors or narrow-bandgap semiconductors depending on the chirality and diameter.
When mixed together, the two materials show a strong interfacial coupling via donor–acceptor binding and π–π interaction. Combining CNTs and conducting polymers in a composite has been found to affect their chemical and electronic structures. Beyond a simple physical combination of their properties, some synergistic effects and new features appear and can be developed into applications. From a chemistry standpoint, two possible impacts may take place in a CNT/conducting polymer system: either the CNTs are functionalized by the conducting polymers or the conducting polymers are modified (doped) with CNTs. Therefore, morphological modifications, electronic interactions, charge transfers or a combination of these effects may occur between the two constituents in the system.
Due to the nanoscale confinement in the system, the interaction via interfacial bonding is considered to play an essential role in the impacts. Morphologically, the interfacial interaction sites on the CNT surface are: (a) defect sites at the tube ends and side-walls; (b) covalent sidewall bindings; (c) non-covalent exohedral side-wall bindings; and (d) endohedral filling (Figure 1.6). Three routes have been commonly used for preparation of CNT/conducting polymer composites: (i) direct mixing; (ii) chemical polymerization; and (iii) electrochemical synthesis.
With regards to the development, studies on CNT/conducting polymer composites started in the early 1990s of the last century. Since 1992, Heeger and co-workersobserved the photovoltaic effects arising from the photo-induced charge transfer at the interface between conducting polymers as donors and a C60 film as acceptor, although the conversion efficiency was extremely low (less than 1%). However, the conversion efficiency of conducting polymer-based solar cell have been significantly improved (~6%) by further use of CNT derivatives acting as the electron acceptors in the CNT/conducting polymer composite matrices. The conversion efficiency is still moderate in comparison with that of inorganic systems, but the simple processing and low cost enable CNT/conducting polymer systems to be a promising choice for photovoltaic applications.
On the other hand, the introduction of CNTs into a polymer matrix improves the electrical conductivity, while possibly providing an additional active means for capacitive energy storage and secondary batteries. Gas sensors fabricated with single-walled carbon nanotube (SWCNT)/PPy nanocomposites have shown a higher sensitivity than that of PPy. The improvement stems from the effects of the increase of specific surface area and anion doping in the PPy matrix. The gas sensing capability in SWCNT/PANI composites has also been similarly improved. For biosensing applications, it has been demonstrated that the CNT/conducting polymer nanocomposites are very attractive as transducers, because they provide the best electron transfer and assure a faster ion mass transfer. In some host polymers, the CNT additives have served as a hole-blocking material, causing a shift of the recombination emission. The interaction between host polymer and CNT additive is considered to be the main reason accounting for all of these modifications.
Studies on CNT/conducting polymer systems will further contribute to the fundamental understanding of the nucleating capability of CNTs, epitaxial interaction and templated crystallization of the polymer at the CNT–polymer interface, and may ultimately lead to more efficient production of bulk nanocomposites. The combination of a strong CNT–polymer interaction, the nucleation ability of CNTs, the templating of polymer orientation of CNTs and crystallinity are all features that can be built on to develop high-performance composites. With that goal, the aim of this Chapter is to provide an outlook on the development and trend in future research on CNT/conducting polymer systems.
1.5 Applications of CNT/Conducting Polymer Composites
1.5.1 Supercapacitors
As shown in Figure 1.7, conducting polymers are one of three groups of candidate materials for supercapacitors (Figure 1.8) — along with carbon materials and metal oxides — due to their good electrical conductivity, large pseudo-capacitance and relatively low cost. The most commonly used conducting polymers include PANI, PPy and poly(3, 4ethylenedioxy thiophene) (PEDOT). The electrochemical capacitance and charge storage properties of conducting polymers have been studied by cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry. Conducting polymers have a very large specific capacitance that is close to ruthenium oxides, e.g. 775 F g-1 for PANI, 480 F g-1 for PPy and 210 F g-1 for PEDOT. However, conducting polymers commonly have poor mechanical stability due to repeated intercalation and depletion of ions during charging and discharging.
As one of the conducting and porous carbons, CNTs possess high mechanical strength, good electrical properties, high specific area and high dimensional ratios. Their application in electrochemical double-layer capacitors has been studied in detail. Aside from the three categories of pure materials, there is a new tendency to synthesize composite materials combining two or more pure materials for supercapacitors. The great promise is to combine CNTs with either metal oxides or conducting polymers. Composites of CNTs combined with RuO2, NiO and MnO have been prepared and have exhibited good potential for supercapacitor applications. However, composites consisting of CNTs and conducting polymers are even more interesting and promising, as they can combine two relatively affordable materials to gain the large pseudocapacitance of the conducting polymers, coupled with the conductivity and mechanical strength of CNTs.
The composites can be obtained both chemically and electrochemically. Using electropolymerization, CNT/conducting polymer nanocomposites can be readily deposited from a monomer-containing solution on to a CNT preform or from a CNT/monomer-containing solution on to a traditional conductive substrate. The first attempt to electrochemically deposited conducting polymers with CNTs was made in 1999, in which multi-walled carbon nanotube (MWCNT) electrodes were used for deposition of PANI films, and higher current density and more effective polymerization were found compared with those deposited on Pt electrode. In 2000, CNT/PANI co-axial nanowires were prepared electrochemically on aligned CNTs. The thickness of the PANI layer was estimated to be 40–50 nm by transmission electron microscopy (TEM) images, with the CNT framework expected to offer a higher mechanical strength as compared with the bare co-axial nanowires. Thin and uniform PPy films, which were also coated on individual CNTs of well-aligned CNT arrays, were produced via potentiodynamic polymerization in aqueous solution. The faradaic current for PPy deposition on CNTs is much higher than that on Ti or Pt substrate. The ion diffusion and migration pathways are shortened due to the unique structure of the electrode material. PPy-coated CNT array electrodes show significantly improved capacitance as compared with PPy coated on Ti or Pt substrates. In 2002, PPy was deposited on CNTs through galvanostatic oxidation of monomers in sulfuric acid in order to find an alternative and relatively cheap method to enhance the capacitance of CNTs. The PPy-modified electrodes had an elevated specific capacitance of 180 F g-1 as compared with 50 F g-1 for the unmodified CNT electrode. The charge loss of the PPy-modified CNT electrode was less than 20% after 2000 galvanostatic charging-discharging cycles.
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Excerpted from Carbon Nanotubeâ"Polymer Composites by Dimitrios Tasis. Copyright © 2013 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
Chapter 1, Conducting Polymer-Based Carbon Nanotube Composites: Preparation and Applications;
Chapter 2, Actuators and Infrared Sensors Based on Carbon Nanotube-Polymer Composites;
Chapter 3, Photoelectrical Responses of Carbon Nanotube-Polymer Composites;
Chapter 4, Chemical Functionalisation of Carbon Nanotubes for Polymer Reinforcement;
Chapter 5, Polymer-Grafted Carbon Nanotubes via Grafting from Approach;
Chapter 6, Metallic Single-Walled Carbon Nanotubes for Electrically Conductive Materials and Devices;
Chapter 7, Characterization of Dispersability of Industrial Nanotube Materials and their Length Distribution Before and After Melt Processing;
Chapter 8, Methods for Improving the Integration of Functionalized Carbon Nanotubes in Polymers;
Chapter 9, Raman Spectroscopy of Carbon Nanotube-Polymer Hybrid Materials