The increasing demand for efficient machines has driven a trend towards the miniaturation of devices and instruments with smaller volume, lesser power consumption but greater performance. The progression relies upon the searching out new desirable materials and the ability of making tiny structures with high precission. However, the development is not so smooth and easy. For instance, current electronic industry based on silicon is very mature and reliable, but it is reaching an unsurmountable barrier of quantum effects as nanoscale approaching. Search for new suitable materials and fabrication methods therefore are being urgent tasks for the near coming development.
Carbon nanotubes (CNTs) and conducting polymers (CPs), newfound materials have shown exceptional characteristics. The coupling of CPs and CNTs furthermore reveals synergistic effects which offer an attractive route to create new multifunctional materials with greater potential in application. Envisioned applications from CPs/CNTs systems involve mechanical, thermal, electrical, electrochemical features such as supercapacitors, sensors, organic light emitting diodes (OLEDs), solar cells, electromagnetic absorbers, and, last but not the least, advanced electronic devices.
Conducting polymers commonly are classified as conjugated polymers which consist of alternating single and double bonds along its linear chains (sp2 hybridized structure). The conductivity of the CPs relies on these double bonds which are sensitive to physical or chemical interactions [1-3]. 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 CPs [4-6]. For instance, CNTs have a very long mean free path, ultrahigh carrier mobility, and can be either very good conductors or narrow bandgap semiconductors depending on the chirality and diameter. Mixing up together, both materials show a strong interfacial coupling via dono-acceptor binding and pi-pi interaction [7, 8]. The coupling of CNTs and CPs in a composite has been found to affect their chemical and electronic structure. Beyond a simple combination of their properties, some synergistic effects and new features appear and can develop into applications [9-14]. From a chemical viewpoint of consideration, two possible impacts may take place in a CPs/CNTs system: either the CNTs are functionalized by the CPs or the CPs are modified (doped) with the CNTs. Therefore, either the morphological modifications, electronic interactions, charge transfers or a combination of these effects may occur between the two constituents in the system [15-18]. Due to the nanoscale confinement in the system, the interaction via interfacial bonding is considered to play an essential role in the impacts [19-23]. Morphologically, the interfacial interaction sites on CNTs surface are:
defect sites at the tube ends and side walls
covalent side wall bindings
non-covalent exohedral side wall bindings and
endohedral filling (Fig. 1).
Three routes have been commonly used for preparation of CPs/CNTs composite:
chemical polymerization and
Back to the development, the study made on the composites of CPs and CNTs relatives started in early 90s of the last century. Since 1992, Heerger
Studies on CPs/CNTs 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 strong polymer-CNT interaction, nucleation ability of CNTs, CNTs templating of polymer orientation and crystallinity are all features that one can build on to develop high performance composites. With that goal, this chapter tries to provide a look on the development and trend in future research of CPs/CNTs systems.
2. Composite preparation
Generally, the composite properties are governed by variety of factors such as the preparation conditions, quantity, contents as well as the nature of its components. With respectto those of CPs/CNTs composite, the purity of carbon nanotubes, the dispersion and the interfacial interaction strength between components are considered to be the essential factors [10, 13]. Carbon nanotubes are created and recombined from carbon sublimation, then naturally accompany with variety of contaminants such as residual catalysts and amorphous carbon phases. In order to obtain purer CNTs, many purification procedures have been proposed to remove these inherent contaminants. Most techniques have been based on chemical and thermal treatment. In the first stage,, strong chemical oxidants, surfactants or burning the unpurified carbon nanotubes were commonly used. However, these techniques have shown some disadvantages such as adding some defects on CNTs surface and poor yield. Later, Davey
For thermoplastic polymers, melt processing is a common technique. As the dispersion of CNTs in polymer melt is much more difficult than in solution, additive techniques such as mechanical stirring, ultrasonic vibration, melt blending, extrusion and melt spinning are used to enhance the dispersion. For insoluble and thermally unstable polymers, chemical processing is often chosen, which involves
Electrochemical polymerization is another way for the preparation of polymer CNTs composites. A direct electrochemical deposition of monomer on CNTs layers acting as an electrode or an electropolymerization from an aqueous dispersion of monomer and CNTs are common approaches to realize the composites.
Normally, the composites have been characterized by microscopic, spectral, conductivity, mechanical and thermal measurement. Scanning electron microscope (SEM), scanning tunneling microscope (STM), atomic force microscope (AFM), and transmission electron microscope (TEM) are the main techniques used for morphological characterization of the composite. For example, the morphology of the composites could reveal the wrapping of CNTs by conducting polymers in core-shell structure with several modes. UV-Vis, photoluminescence spectra, Raman and FTIR are complementary means used to investigate the chemical structure of the composites. X-ray diffraction, X-ray photoelectron spectral studies made on the composites show the crystallinity, intrinsic oxidation states.
4.1. Electrically conductive composite
The incorporation of CNTs in polymer matrices is made to design electrical properties suitable for variety of applications such as circuit components, electronic products or electrostatic shields. For example, to protect against electrostatic discharge a lower level of conductivity is enough whereas to prevent electromagnetic interference a higher conductivity is needed and for circuit components the level of conductivity must be comparable to those of metals. However, most of composites, although a high weight percentage of CNTs has been used, have only a moderate conductivity below the required level in electronic applications, except for a composite developed by Blanchet-Fincher
In principle, the conductivity of a conducting polymer can be largely tuned by “doping”, however highly conductive polymers have been limited in practical applications as they are chemically unstable in use and unsuited for solution or melt processing. Among the conductive polymers, polyaniline (PANi) is known to be chemically stable and readily soluble in environmentally friendly solvents. Protonic acid doped PANi emeradine is the most highly conductive form with conductivity about 5 S/cm, which is well below the 102 S/cm threshold conductivity required for widespread utility in electronics. Blanchet-Fincher
4.2. Mechanically reinforced composite
With respect to mechanically effective reinforcement, basically required features for composite components are purity, aspect ratio, dispersion, alignment and interfacial stress transfer. Among these, the interfacial stress transfer, characterized by the applied shear stress at which the interface fails, is the most important. However, CNTs have atomically smooth non-reactive surfaces, and then lack of interfacial bonding between the CNTs and polymer. In order to increase bonding sites to the polymer chain, chemical modification and functionalization of CNTs such as solution processing, melt processing, and chemical processing were proposed. Quantum mechanics and molecular dynamics calculations supported this approach [35-38]. By chemical functionalizing and integrating CNTs into epoxy polymer, Margrave
A composite formed by
By combining the uniformly dispersed nanotube with polypropylene matrix/solvent mixture and then heating the nanotube/matrix mixture to a temperature above the melting point of the matrix materials, Shambaugh
On the other hand, in some composites, CNTs act as a nucleating agent for polymer crystallization [9, 42-44] and reorientation. For example, SWNTs induced oriented crystallization is observed in PET/SWNTs composites . Two samples, PET and PET/ SWNTs (1wt% loading) are prepared under identical conditions by melt compounding at 270 °C. Both PET and PET/SWNTs samples were made into 0.5 mm thick, 0.4 mm wide strips and subsequently stretched to a draw ratio of four to induce PET and SWNTs orientation. As can be seen from Fig.3, wide-angle X-ray diffraction (WAXD) of both materials shows that PET is oriented in the samples (Fig. 3A and Fig. 3C). However, when the train PET and PET/SWNTs were heated and were recrystallized by cooling down, only PET/SWNTs shows the orientation but not the neat PET (Fig. 3B and Fig. 3D). Individual CNTs also have found to promote polypropylene (PP) crystallization, as a result, thick PP interfacial layer is formed on the nanotube surface . Polypropylene transcrystals were observed on the carbon nanotube surface (Figure 2E), when polymer melt was isothermally crystallized in the temperature range of 118 to 132 °C. In PP/CNTs fiber samples melted and recrystallized spherulite growth is influenced by the presence of CNTs, where the presence of CNTs increase the number of nucleation sites for the polymer and smaller spherulites are observed (Figure 2F to I) even a low CNTs loading (0.1 wt %) . PP/CNTs fibers heated at above polypropylene melting temperature and then cooled down shows that PP/CNTs fibers retain polymer orientation and the average degree of crystallinity is about 80 %. Such a high level of crystallinity is attributed to more complete PP crystallization in dilute solution and PP-CNTs interaction.
In addition, some proteins have been shown to crystallize in an ordered helical fashion on the surfaces of MWNTs. The MWNTs that induce protein crystallization are of a specific size, and protein crystallization occurred consistently throughout the system. The SWNTs have also been shown to induce crystallization and orientation in the sheared polymer melt, and polymer melt containing aligned SWNTs. These observations are important evidences indicating that CNTs can be used as nucleating agents in polymer processing to promote polymer crystallization and orientation.
4.3. Electrochemical capacitors
The requirement for energy store and conversion systems has drawn much attention on development of effective energy store devices. Electrochemical capacitor (EC) (or so called supercapacitor) is device which can store the electric field energy in high density by using an electrochemical double-layer with high specific surface-area. With respect to classification, supercapacitors may be distinguished by criteria such as the electrode materials, the electrolyte, or the cell design. Traditionally, the electrode materials are divided into three main groups: carbon based, metal oxides (mainly ruthenium dioxide) and polymeric materials such as PPy, PANi and PTh [45-49]. Polymer-based electrochemical capacitors in particular have attracted much attention due to the polymeric materials possess high capacitive energy density and low cost. However, these kinds of capacitors have shown some disadvantages, namely, lower cycle-life and slow ion transport kinetics because the redox sites in the polymer backbone are not sufficiently stable. As an alternation, the composites of CPs and CNTs (both SWNTs and MWNTs) have been investigated as an electrode materials for supercapacitors [50-52]. The combination of CNTs into a polymer matrix provides an additional active materials for capacitive energy storage. For example, using MWNTs coated with PPy as the active electrode for a supercapacitor assembly, Frackowiak
4.4. Solar cell
Solarcells are specific devices which transform light into electricity. Depending on the materials and techniques, solarcell may be classified into inorganic or organic (including conducting polymer) based devices. Currently, most solarcells in the market are based on inorganic materials such as Si or CuInGaS(Se) or CdTe. However, due to the production cost and shortage of rare materials organic based solarcell emerges as potential alternative. In principle, photon absorption in the organic-based materials produces primarily bound-state excitons. Some of these charge pairs eventually are dissociated generating free carriers (electrons and holes) which give rise to a current in the materials. The dissociation is faciliated by the potential difference across a polymer-metal junction, provided by agglomeration of excitons near the interface. The dissociation can be further accomplished via electron acceptor impurities . Under illumination, a transfer of electrons to the acceptors will take place and the holes will be preferentially transported through the CPs. This process is known as photo-induced charge transfer. Since the discovery of photo-induced charge transfer, a variety of acceptor materials have been introduced into CPs to produce photovoltaic devices. Amongst of these materials, CNTs have shown to be one of the most effective acceptor materials [24, 25, 55, 56]. Since the efficiency of photo-induced charge generation is dependent on the interface between the two components, the extremely high surface area of CNTs (for purified SWNTs, ~ 1600 m2/g), offers a tremendous opportunity for exciton dissociation. The primary step in these polymer photovoltaic devices is an ultrafast photo-induced electron transfer reaction at the donor-acceptor interface, which results in a metastable charge-separated state. In the case of the oligo(phenylenevinylene)/C60 composite, the quantum efficiency of this step is assumed to be close to one [57, 58]. However, the overall conversion efficiency of these solarcells is limited by the carrier collection efficiency, which is greatly influenced by the morphology of the active film.
SWNT/poly(3-octylthiophene) (P3OT) composites have been used for the fabrication of new photovoltaic devices [25, 26]. P3OT, acting as the photoexcited electron donor, is blended with SWNTs which act as the electron acceptors. In such devices the transferred electrons are transported by percolation paths provided by the addition of SWNTs. It was shown that the internal polymer/nanotube junctions act as dissociation centers, which are able to split up the excitons and also create a continuous pathway for the electrons to be efficiently transported to the negative electrode. This results in an increase of electron mobility, and hence, balances the charge carrier transport to the electrodes. The CP/SWNTs composite represents an alternative class of hybrid organic semiconducting materials that is promising for organic photovoltaic cells with improved performance. Other beneficial properties of SWNTs relevant to polymeric photovoltaic development include composite reinforcement and thermal management. The high Young’s modulus and strength/weight ratio of SWNTs could help provide much-needed mechanical stability to large-area thin-film arrays. SWNTs, on the other hand, may provide assistance in thermal management for such arrays, too. Polymer composites doped with as little as 1% wt SWNTs have shown a 70% increase in the thermal conductivity at 40 K .
Optical and photovoltaic properties of a composite based on MWNTs and PPV was studied by Curran
4.5. Optical limiting devices
Organic light emitting diodes (OLEDs) are one of most interest in recent years for their potential applications in electronic informatic, lighting, display technology. The simplest version of OLED consists of an electroluminescent organic material layer sandwiched between two electrodes. The luminescent emission of OLEDs is due to the radiative recombination of excitons, a process is somewhat opposite to that of solar cell. Fabrication of high efficient OLEDs depends not only on the electronic and the optical properties of the pure organic materials but also on the control of charge transport, holes or electrons through the electroluminescent layers and on the enhancement of charges migration by doping the emissive materials [61, 62]. A proper layer combination in OLEDs can also balance the injected charges in an emissive layer thus increasing the external efficiency. The buffer layer leads to a reduction of the charge injection barrier and an even charge distribution with a large contact area at the electrodes/organics interface. A recent work shows that the dispersion of SWNTs in a host polymer (PmPV, hole conducting) traps the holes in a double emitting organic light emitting diode (DE-OLED) . The device fabricated without SWNTs dispersed in the PmPV has shown a dominant emission near red at 600 nm, which is in the range of the characteristic emission of Nile Red-doped Alq3, while the addition of a small amount of SWNTs enhances a green emission, In addition, the devices fabricated with the polymer/SWNTs composite have shown an increase in the oscillator strength of the green emission with a dominant emission peak near 500 nm, the characteristic emission of PmPV. The shift in the emission indicates that the SWNTs in the PmPV matrix act as a hole-blocking materials that causes a shifting of the recombination region from the Nile Red-doped Alq3 layer to the PmPV composite layer. The addition of CNTs in conducting polymer also has found to modify the electronics properties of polymer composite. For example, OLEDs fabricated with a hole conducting polymers composite dispersed with SWNTs such as PEDOT, poly-carbazole (PVK)),… show a change in EL, PL and I-V data. The modification in electronic structure of the composite originates from the hole trapping nature of SWNTs and SWNT-CP interaction.
4.6. Sensors and actuators
Both CPs and CNTs inherit a delocalized
Materials such as shape-memory alloys or liquid crystal elastomers exhibit a latent ability to actuate under the right conditions whereas other systems require the blending of two or more materials to impart a new physical response leading to the actuation process. Recently, polymer nanocomposites appeared as the subject of mechanical actuation studies, but most of them concentrated on accentuating the already present features of the host matrix by adding nanotubes. In 2003, Courty
4.7. Thermal conductivity
Carbon based composites with high thermal conductivity have had a number of potential applications, particularly in thermal management such as heat sinking for electronics and motors. In particular, SWNTs are superior to carbon black and carbon fibers because their nano-scale diameter and larger aspect ratio facilitate the formation of extensive network at the same weight loading. Theoretical calculation and measurement show that the thermal conductivity CNTs is much more than that of best thermal conductive metals such as Ag, Cu [83-85]. The thermal conductivity of CP/CNTs nanocomposites although has received lesser attention, initial studies shows that the presence of CNTs in some polymer matrixes has improved the thermal conductivity of the polymer. For example, the thermal conductivity of epoxy/SWNTs composites with only 1 wt% SWNTs enhanced more than 120 % and 70% at room temperature and 40°C, respectively, as compared to epoxy filled with carbon fiber [60, 85]. However, the thermal conductivity of CNTs/epoxy composites seems to be unaffected by increasing CNTs loading in samples [86, 87]. The impact of CNTs on thermal conductivity of polymer/CNTs systems has not been explored as far as the study on the other areas of the composites and needs to be further exploited.
4.8. Fuel cell
Fuel cell is an efficient way transforming chemical energy of hydrogen rich compounds to electrical energy. The research in this area has gained momentum since the 80s due to the increased awareness of energy and environmental concerns. Fuel cell are usually characterized by their electrolyte, temperature of operation, transported ion and fuel. The center of the fuel cell is the electrolyte membrane, as it determines the properties needed for the other components. Electrolyte membrance based on conducting polymer has shown some advantages over the other materials due to low operating temperature, high energy density and easy handling of the fuel other than hydrogen. The PANi/CNTs composites can be used as efficient electrocatalytic materials in fuel cell reactions like oxygen reduction and methanol oxidation. The CNTs provides higher surface area and better electronic conductivity while PANi facilitates the electron transfer through the conducting matrix. A PANi-grafted MWNTs composite has shown a 610 mV more positive current onset potential for the two-electron oxygen reduction with a 20-fold enhancement in amperometric current . A poly(
4.9. Electromagnetic absorbers
Electromagnetic (EM) shielding by absorption rather than reflection is presently more important for many applications from electronics to military use. Even though metals or metal-coated materials exhibit very high EM shielding efficiency ranging from 40 to 100 dB, they cannot be used as an electromagnetic wave absorbent since their shallow skin depth makes them shield electromagnetic waves mainly through surface reflection. On the other hand, electrically conducting polymers are capable of not only reflecting but also absorbing the electromagnetic waves and therefore exhibit a significant advantage over the metallic materials. Currently, commercial and military applications require high performance absorbing materials with light weight and high strength over a broad frequency band . This could be carried out if one could design and optimize a combination of different CPs components based on their dielectric properties and random scattering effects present due to their respective geometry. For example, EM absorbers with different dielectric properties and thickness were carried out on the base of the polyurethane composite containing carbon nanotubes, carbon fibers and microballoons along with polypyrrole fabric having different surface resistances. It has been shown that both the surface resistance of the PPy fabric and the order in which the composite layers are stacked are critical for the reflection. A PPy fabric composite gave greater than 15 dB reflection loss in the 4–18 GHz frequency range. With proper arrangement, the required bandwidth and better performance can be achieved by using a combination of PPy fabric and composite layer stacks [95, 96].
4.10. Other applications
Organic electronics have been a field of most research interests since it exhibits some advantages over inorganic including low-cost and flexible. The development relies on CPs with suitable properties in conductivity, proccecibility, charge mobility, etc. The addition of CNTs in CPs has shown to enhance the conductivity of the nanocomposites and furthermore improve the processing. For example, poly (p-phenlyenevinlene-co-2, 5-dicotoxy-m-phenylenevinylene) (PmPV) with CNTs form a hybrid composite whose conductivity is increased by ten orders of magnitude due to the introduction of CNTs conducting path to the polymer [59, 97]. The addition of SWNTs in PANi doped with dinonylnaphtalene sulfonic acid (DNNSA) creates a highly conducting  and the composites show to be a high-resolution printable conductor. Transparent SWNTs film on a polyethylene terephthalate substrate can be used to replace ITO for PANi-based electrochromic devices . Field effect transistors (FETs) based on P3HT using SWNT-contact exhibit three orders of magnitude higher current modulation (
On the other aspect, radiation shielding materials have been developed to protect personnel and equipment from the damaging effects of radiation including galactic cosmic radiation (GCR). Polyethylene (PE) is a CP has been used in space applications for shielding GCR in the low temperature applications. However, transparent composites composed of SWNTs and poly-4-methyl-1-pentene (PMP) exhibit superior strength, optical and thermal properties, and has a melt temperature of 235°C (compared to 136°C for PE) . The PMP is also transparent in the visible region of electromagnetic spectrum and can be modified by doping with an organic dye having phenyl ring. These composites can be employed in thermo-luminescent detection where high energy radiation excite pi-electrons in the phenyl rings and on relaxation to ground state emit photons which can be transported to photodetectors and counted. By this way, the radiation environment of the shielding materials can be continuously monitored.
This chapter presents a brief summary of the preparative methods, characterization data, and possible applications of conducting polymer/carbon nanotube composites. The electrical, thermal, mechanical and electrochemical properties of the composites in general are intermediate between pure polymer and CNTs but vary depending on the method of preparation, type, purity, content and the dispersion of CNTs in the polymer matrix. In particular, the composite reveals synergistic effects and new properties which account for the interaction between CPs and CNTs at nanoscale. The effect offers an attractive route to create new multifunctional materials with great potential inuses involving mechanical, thermal, electrical, electrochemical features. However, the nature of the CP/CNT interaction and its effect on overall properties of the system still are unclear and need to further exploit and develop into practical application.
The work is carried on thanks to the support from Basic Research Project 103 02 103 09 Grant in Aid by National Foundation for Science and Technology Development (Nafosted).