A Literature Review on
Dispersion of Carbon Nanotubes and Their Polymer Composites
Jian Zhao
Department of Materials Science and Engineering
University of Cincinnati
October 2001
Table of Content
1. Introduction
2. Thermodynamics of Nanophase Carbon
3. Methods for dispersion of nanotubes
4. Nanotube-polymer composites
5. Summery
1. Introduction
The carbon nanotubes discovered by lijima [1] have many interesting properties such as high mechanical strength, and a remarkable electronic structure. These make it
have a wide range of potential uses. Carbon nanotubes occur in two distinct forms, single-walled nanotubes(SWNT), which are composed of a graphene sheet rolled into a cylinder (Figure 1) and multi-walled nanotubes (MWNT), which consist of multiple concentric graphene cylinders. Compared with multiwalled nanotubes, single-walled nanotubes are expensive and difficult to obtain and clean, but they have been of great interest owing to their expected novel electronic, mechanical, and gas adsorption properties [2].
Figure 1. The cylinder-like structure model of the rolled graphene sheet (Lijima, S., Brabec, C., Maiti, A. and Bernhole, J., Journal of Chemical Physics, 1996, 104, 2089)
There are several methods for preparation of carbon nanotubes. Arc-discharge and laser ablation methods have been widely used in the earlier period of time. However, the large-scale production of carbon nanotubes with controlled conformation still remains challenging. In the past few years, chemical vapour deposition(CVD) has been used as a promising solution. The process is simpler and has a higher
productivity than the arc-charge process. However, the carbon nanotubes produced by the catalytic process are usually thicker than those by the arc-discharge process and often consist of large aggregates [2-4].
Carbon nanotube based materials have inspired scientists for a range of potential applications [3-5]. The use of carbon nanotubes in polymer/carbon nanotube composites has attracted wide attention [6,7]. The carbon nanotubes have unique atomic structure, very high aspect ratio, and extraordinary mechanical properties (strength and flexibility). These properties make them ideal reinforcing fibers in nanocomposites[8]. “Carbon nanotube reinforced composites have been investigated for flame-retardant performance [9], improved electrical conductivity and electrostatic charging behavior, optical emitting devices [10,11], and in lightweight, high strength composites [9]”.
However, Colloidal materials such as carbon nanotubes do not spontaneously suspend in polymers, thus the chemistry and physics of filler dispersion become a major issue.
In the case of polymers filled with carbon nanotubes, the research challenge is particularly tremendous due to the unique character of these unusual materials. Due to strong attractive interaction, nanotubes aggregate to form bundles or “ropes” (Figure 2) that are very difficult to disrupt [12]. In case of single nanotubes, they are only 1-3 nm in diameter, however, since they like to assemble into ropes, which consist of many nanotubes, are most likely 10-200 nm in diameter. Furthermore, ropes are
tangled with one another like spaghetti or polymers. With high shear, these ropes can be untangled, but it is extremely difficult to further disperse at the single tube level.
Figure2. TEM image of partially exfoliated single-wall carbon nanotubes showing the rope-like structure. From reference 12.
Due to the low entropy of mixing, rigid molecules of high molecule weight require strong attractive interactions to disperse. Since the connectivity and rigidity of macromolecules drastically reduces the number of configurations available in the dispersed state, mixing becomes a problem. In the case of rigid fillers dispersed into stiff polymers, the problem is compounded in that neither species gains entropy on dispersion.
In this review, the origin of dispersion problem, mainly in aspect of thermodynamics is explored. Several methods have been reported to solve the problem, through functionalization of carbon nanotubes or strong acids, certain solvents and surfactants. Finally, carbon nanotube-polymer composites with a variety of polymers and fabrication methods are reviewed.
2. Thermodynamics of Nanophase Carbon
Several factors make the dispersion of nanophase carbon particularly troublesome.
These factors are dominated by strong attraction between carbon species of both enthalpic and entropic origin. In addition, the low dismensionality of carbon nanotubes leads to an enhancement of these attractive forces.
The origin of the attractive forces between graphilic structures is well known. Due to the extended pi electron system, these systems are highly polarizable, and thus subject to large attractive van der Waals forces. These forces are responsible for the secondary bonding that holds graphitic layers together. In the case of carbon nanotubes, these forces lead to so called “ropes”. Extended structures are formed by side-by-side aggregation of the nanotubes in ropes.
When suspended in a polymer, an attractive force between filler particles also arises due to pure entropic factors [13,14]. Polymer chains in the corona region of the colloidal filler suffer an entropic penalty since roughly half of their configurations are precluded. Therefore there is a depletion of polymer in the corona, thus results in an osmotic pressure forcing the filler particles together. This effective attraction is intrinsic to colloids dispersed in polymers.
Finally, the linear structure of carbon nanotubes leads to cooperatively enhanced forces because rods interact along a line.
3. Dispersion methods
Dispersion of carbon nanotubes is particularly intractable because the forces described above are especially large. The advantages of carbon nanotube fillers have not been fully realized because of the difficulty of obtaining fully dispersed nanotubes. Also the lack of affinity of the nanotubes for polymer matrix inhibits load transfer from the matrix to nanotubes [15].
The situation is improved, since strategies have been devised to overcome barriers to dispersion. A lot of methods have been reported to solve the problem of dispersion, through functionalization of carbon nanotubes or strong acids, certain solvents and surfactants.
3.1
Dispersion in solvents and surfactants
Solution-phase handling would be exceptionally useful for many of carbon nanotubes applications. To date, the best solvents reported for generating SWNT dispersions are amides, particularly N,N-dimethylformamide(DMF) and N-methylpyrrolidone(NMP)
[16,17]. However, the dispersions aggregate on a time-scale of days. Ausman et al investigated the room-temperature solubility of SWNTs in a variety of solvents. It was found that a class of non-hydrogen-bonding Lewis bases could provide better solubility [18], but they just pointed a possible way towards better solvents capable for
solvating pristine tubes. The problem that pristine SWNTs are insoluble in all solvents is still not overcome.
With aid of surfactants, carbon nanotubes have been solubilized in water. Surfactants can deposit on the surface of nanotubes [19] and help form stable colloidal dispersion
[20,21]. The surfactant acts as a coupling agent and may introduce a steric repulsive
force between the carbon nanotubes. The repulsive force overcomes the van der Waals attractive force between the carbon surfaces. A polyelectrolyte-surfactant- MWNTs complex can help micrometer-length multi-walled carbon nanotubes dissolve in organic solvents by forming a lamellar structure [22]. However, removing the surfactant afterward is problematic although the instinct structure of nanotube is not destructed.
SWNTs have been solubilized by functionalization of nanotubes. For example, nanotubes could be solubilized well by functionalizing the end-caps with long aliphatic amines [23]. Further, it has been reported that SWNTs have been solubilized by functionalizing their sidewalls with fluorine [24] and with alkanes [16]. However, the above reactions caused the opening of the nanotubes tips or detrimental damage to their sidewalls owing to the harsh conditions [25,26]. With progress of research on functionalization of nanotubes, these problems are being solved (For detail, see section 3.3: Functionalization of nanotubes).
3.2 Dispersion in strong acids
In the untreated state nanotubes may be dispersed using ultrasound, but do not remain in quiescent suspension at high concentration. And use of ultrasound has been shown to cause defects at nanotubes[27]. Appropriate chemical oxidation of their surfaces, with the mixture of concentric acid and sulfuric acid, can introduces oxygen- containing functional groups onto surface. During this process, open ends are formed in the oxidizing environment. After such treatment, the nanotubes form a well- dispersed electrostatically stabilized colloid in water and ethanol [28].
It has been indicated that through chemical reaction, SWNT bundles form intercalation compounds with HNO3 after they are immersed in nitric acid solution for a short period of time. The inter-nanotube spacing within the bundles is expanded due to intercalation. Individual SWNTs can be exfoliated from the bundles after longer exposure to HNO3. It is demonstrated that nanotubes consisting of aggregates can be easily dispersed to individual fibers by treatment with strong acids [12]
More important is that the presence of oxidizing groups helps the attachment of organic or inorganic materials to the surface that is important to soluble nanotubes, self-assembly on surfaces and chemical sensor, although destruction of structure of nanotubes makes them less intriguing for some applications.
3.3 Surface Functionalization
The intrinsic polarity of graphitic structures can be overcome by chemical means.
That is, if the pi system were disrupted through defects, the polarizability and resulting van der Waals forces would be reduced. Surface functionalization might achieve this goal. However, the chemical modification of SWNTs at the molecular level has been little studied to date. This is largely limited because it is difficult to obtain pure SWNTs, and SWNTs do not dissolve in solvents.
Several studies of chemical modification of carbon nanotubes have been reported.
Chen et al. reported on the derivatization of SWNTs dissolved in organic solutions with thioylchroride and octadecylamine[23]. Reaction of soluble SWNTs with dichlorocarbene led to functionalization of the nanotubes. They found that by solution-phase near-infrared spectroscopy, the band gaps of some types of SWNTs could be investigated directly. This type of near–infrared spectroscopy allows the study of the effects of chemical modifications on the band gaps of SWNTs. This is very important to the molecular design of new SWNT-based materials.
Wong et al [29] reported modification of multiwall carbon nanotubes via amide bond.
The amide bond forms between amine and carboxy functional groups bonded to the open ends of MWNTs. With the use of these modified nanotubes as AFM tips, the binding force between single protein-ligands pairs can be measured, based on molecular interaction.
The polymer-bound carbon nanotubes can be formed by covalently attaching nanatubes to highly soluble linear polymers, such as poly(propionylethylenimine-co- ethylenimine) (PPEI-EI) via amide linkages or poly(vinyl acetate-co-vinyl alcohol) (PVA-VA) via ester linkages. The samples of polymer bound nanotubes are soluble in both organic solvents and water, and highly colored homogenous solutions are formed [30,31].
However, the above several approaches have the drawback that acid-treated end-caps, the most convenient chemical “handles” for further modification, are tied-up.
It has been demonstrated that carbon nanotubes can be covalently fluorinated within the temperature range from 250C to 400C [24]. And fluorine can be effectively removed from the SWNTs using anhydrous hydrazine. Boul et al reported that sidewall-alkylated nanotubes can be obtained by reacting sidewall-fluorinated nanotubes with alkyl magnesium synthesis or by reaction with alkyllithium precursors[16].
When Koshio et al investigated the purification of SWNTs by utrasonication of SWNTs in a monochlorobenzene (MCB) solution of poly(methyl methacrylate) (PMMA) [32], they found that SWNTs reacted with the organic liquid during ultrasonic process. They explained that ultrasound forms hot spots in the mixture of SWNTs and organic liquids. At the hot spots, Organic molecules, such as MCB and PMMA, are decomposed. Meantime, Reactive species form and then the sidewalls of the SWNTs
are damaged. Carbon-dangling bonds form under high temperature and pressure. The reactive organic species react with the dangling bonds of SWNTs. They believed that this simple ultrasonication technique should achieve surface functionalization of SWNTs in various ways [33]. In these latter cases, the sidewall functionalization coverage is high, resulting naturally in a modification of the intrinsic SWNT properties.
For application requiring the high conductivity of carbon nanotubes, all the above methods are not attractive. Another strategy that scientists have begun to explore is to attach organic molecules to these tubular nanostructures in a noncovalent way in order to preserve the nanotuibes networks- and thus their electronic characteristics.
Dai and coworkers have found a “simple and general approach for noncovalently anchoring aromatic molecules to sidewalls of SWNTs” (Figure 3). They used a molecule containing a planar pyreny group. The pyreny group irreversibly absorbs to the surface of a SWNT with -stacking forces. The molecule’s tail is tipped with a succinimidyl ester group. While an amine group attacks the ester function, the ester group is readily displaced and an amide bond forms [34]. This may be very useful not only for immobilizing proteins or DNA, but also for solubilizing carbon nanotubes.
Figure 3. 1-Pyrenebutanoic Acid, Succinimidyl Ester 1 Irreversibly Adsorbing onto the Sidewall of a SWNT via -Stacking. From reference 34.
Staddart et al produced bundles of SWNTs that have a conjugated polymer helically wrapped around. The polymer is a poly(m-phenylenevinylene) with octyloxy chains.
After SWNTs were added to a solution of this polymer and then the mixture was sonicated, a stable suspension of nanotubes was produced. They believe that “the polymer wraps itself around the SWNT bundles, with the phenylene rings and vinyl units of the polymer backbone”. The polymer hugs the nanotube surfaces, as a result of - interactions [35]. Since the suspension can be considered as a polymer-nanotube composite, it is mentioned in more detail on next section together with other polymer solutions that have similar behaviors (Page 17). Actually, due to some analogies between polymer solution and carbon nanotube dispersion in term of an entanglement-like transition and flow-induced alignment [35], we can imagine that
molecules of high molecular weight can wrap themselves around the surfaces of nanotubes.
Scientists are enthusiastic to explore potential applications of both noncovalent modification approaches, which can manipulate nanotubes into ordered array without destroying their instinct structure.
4. Nanotube-polymer composites
Due to agglomerate structures, carbon nanotubes are very difficult to break down physically. Therefore, fabrication of homogeneous nanocomposites with carbon tubes remains a technical challenge. Generally, nanotubes-polymer composites were fabricated by direct mixing or in situ polymerization. Obviously, in situ polymerization is a better way for homogenous dispersions because nanotubes are more likely to disperse in a precursor monomer than in the polymer.
The carbon nanotubes-polymer composites were initially reported by Ajayan et al [36]. They just mechanically mixed the purified MWNTs with epoxy resin. Since then, attention has been paid to composite materials with uniform and high nanotubes loading. Carbon nanotube epoxy composites are most widely studied as nonconjugated polymer-based composites.
Sandler et al reported that untreated carbon nanotubes were dispersed in an epoxy matrix. The use of carbon nanotubes not only reduces the percolation threshold to below 0.04 wt%, but also increases the overall conductivity. Although ultrasound in ethanol and the intense stirring process improves the dispersion of the nanotubes, it is not impossible to break up all the entanglements of the carbon nanotubes. Even on the millimeter scale the distribution of nanotubes is not uniform [37]. Gong et al reported that using surfactants as wetting agents might improve dispersion and thermomechanical properties of carbon nanotubes/polymer(epoxy) composites, but even with the addition of the surfactant, complete homogeneous dispersion of the nanotubes was not obtained [38]. Better surfactants and different concentration of polymer were not investigated in this case.
Composite films of poly(vinyl alcohol) and nanotubes were prepared. Followed by casting to make films, chemically treated nanotubes were carefully mixed with aqueous poly(vinyl alcohol) solution, [CH2CH(OH)]N(PVOH) in order to prevent reaggregation. Each nanotube must be covered with a layer of polymer to form a stable mixture before it is able to interact with a number of other nanotubes. It was believed that “in colloidal terms, the absorbed polymer sterically stabilizes the nanotube dispersion and protects it against bridging flocculation and depletion aggregation. The electrical conductivies of composites show typical percolation behavior and the presence of the nanotubes stiffens the material, particularly at high temperature” [39]. However, the plausible mechanism seems too simple and was not confirmed directly.
PMMA/nanotubes composites were fabricated by in situ process. In the process treated nanotubes can be initiated by AIBN and open their -bonds. With the opened
-bonds nanotubes can link with the PMMA, thus obstruct the growth of PMMA and produce a C-C bond between the nanotubes and the PMMA. The dispersion ration of nanotubes in the PMMA matrix is improved and the properties of the composites rise due to high interfacial strength [40].
Thin film of PMMA/SWNT composite was also fabricated by spin coating. It was found that the polymers intercalated between nanotubes into bundles. At low nanotubes concentration, amorphous carbon was dispersed well, thus more uniform films were prepared [41]. A combination of solvent casting and melt mixing was used to fabricate SWNT/PMMA composites. “Melt mixing produced compositionally uniform films on the micrometer scale, while the films prepared by solvent casting were heterogeneous”. Mechanical properties and electrical conductivity of the aligned nanocomposite fibers were improved by melt spinning[42]. Drawback of this method is that melt spinning described by the authors is too complex and time-consuming.
MWNTs can be dispersed well through polystyrene matrices by an ultrasonic assisted solution-evaporation method. TEM images of the composite films indicate that the nanotubes are homogeneously distributed at ~1m length scale. Only 1% nanotube addition increases the polymer mechanical properties significantly. In situ TEM show
that the external load can be effectively transmitted from the matrix to the nanatubes.
[43].
The combination of carbon nanotubes with -conjugated polymers is of interest because their -conjugation enables the polymers to be used as an active material for light-emitting diode(LEDs), field-effect transistor(FETs), and photovoltaic device et al. Chen et al reported a novel electrochemical method for the synthesis of a nanotubes-polyppyrrole composite that has a high concentration of well-dispersed nanotubes. High-resolution TEM revealed a remarkably uniform PPy coating on individual nanotubes, indicating that nanotubes are wetted by the continuous polymer phase. Although further mechanism was not investigated, the uniform coating on individual nanotubes promises controlled modification of the surface of nanotubes.
This is “the first example of anionic nanotubes acting as a strong and conductive dopant in the polymerization of a conducting polymer” [44].
Ago et al reported that a uniform film of MWNTs and poly(p-phenylene vinylene) (PPV) widely studied was prepared by spin-coating highly concentrated MWNT dispersion[45]. An atomic force image shows that PPV covers the surface of MWNTs and forms a well-mixed composite. Results suggest that MWNT can act as a good electrode due to the formation of a complex interpenetrating network with polymer chains. Some investigation of electronic properties of the composites is beyond my knowledge.
Figure 4. Chemical structures of some polymers studied
Carbon nanotubes can be solubilized in the solution of Poly(p-phenlyenevinylene-co- 2,5-dioctoxy-m-phenylenevinylene) (PMPV), a variation on the poly(para- phenylenevinylene)(PPV). The copolymer has an added dihedral angle that makes the chain tend to spiral into helix. Polymer/toluene solution can act as a solvent for nanotubes. The suspension can be considered as a polymer-nanotube composite. It was found that the formed organic composites show an increase of the conductivity by ten orders of magnitude [46-49].
In the case of MWNT, it has been demonstrated that wrapping of polymer ropes around the tube lattice occurs in a well-ordered periodic fashion and keep them suspended in solution indefinitely. Crystalline polymer nucleates from nanotube defects and each nanotube has a uniform coating of polymer. In the case of SWNT hybrids, the pitch of the polymer coil can accommodate individual nanotubes. SWNT ropes are destroyed due to an intercalation process of polymer. At low loading fractions, the tubes are sufficiently isolated and are unable to reaggregate. “The
polymer interact strongly with nanotubes of within a diameter range of 1.35-1.55 nm and weakly with tubes of smaller and higher diameters”. However, the exact nature of polymer interaction with SWNT is not reported in these papers [46-49]. And the use of SWNT for mechanical reinforcement and thermal stabilization of the conjugated polymer were not further studied.
The optical polymer characteristics have been established for nanotube solubility, based on research on PmPV and its derivatives [50]. The combination of the - conjugation, twist allowing backbone and solubilizing side groups, results in a successful polymer conformation (a flat helix, with exposed backbone and outward pointing side groups). It was also suggested that the binding of conjugated polymers is the combination of electrostatic forces and van der Waals forces. It is not surprising that the forces are stronger than van der Walls binding for nonconjugated polymers,
Figure 5. Molecular model shows how conjugated polymer would helically wrap itself around a carbon nanotube.
From C&EN/MAY 7,2001, page 15
which, to some extent, may explain that nanotubes are generally dispersed better in conjugated polymers.
Except PmPV, Poly(hydroxy amino ethers) (PHAE)[51] and poly(fluorine-diooctane)
[50] have been reported to bind well with MWNTs in composite material. PMMA, an amorphous random-coil polymer, was not able to hold nanotubes in suspension [52]. Additionally, atactic polystyrene was found not to suspend nanotubes well.
Poly(phenylacetylene) (PPA) polymerized in situ with MWNT has been reported to dissolve short nanotubes by forming helical wrapping around them[53,54]. Based on the mechanisms of adhesion of these composites at the molecular level, it was found that helical polymer conformations are essential in determining the strength of the interface. Hydrogen bonds interactions with the -bond network of pristine carbon nanotubes improve binding to the surface. Phenyl side-groups interact strongly with the surface as well, but a regular arrangement around nanotube and good contact is necessary. These characteristics are a bit consistent with results reported by Panhuis
[50]. In addition, it was shown that isolated nanotubes are desired for dispersion in a matrix, because of weak frictional interactions between layers of MWNTs and between SWNTs in bundles.
Based on overview of literatures, it is found that interaction between pristine nanotubes and polymers is dependent on the choice of matrix polymer and also polymer conformation, thus the molecular structure may play a critical role in the interaction. Even with “best polymer”, pristine graphite may not form strong
interfaces. It have been proposed that funtionalizing nanotubes or chemical bonding might increase the interaction with polymer matrix, but it was found that some mechanical properties decrease after covalent chemical modification [55], and the structure of nanotubes would be destroyed partially. It seems that MWNTs are more suitable for chemical treatment because the inner graphene layers can be unreacted, thus the essential electronic structure can be retained, but less excellent properties than SWNT and weak interaction between layers make them not attractive for a lot of applications. Few mechanisms about adhesion, load transfer and deformation were investigated, which make it difficult to accurately predict behaviors of nanotube- polymer composites and fabricate “ideal” nanocomposites. Therefore, synthesis of more dispersible nanotubes, a more complete understanding of interfacial chemistry and dispersion mechanism and better understanding of interaction between polymer matrix and carbon nanotubes should be included in future work.
5. Summary
More and more interest exists in producing composites using carbon nanotubes as filler. Such nanocomposites would exhibit superior performance, but dispersion of nanotubes and fabrication of homogenous polymer composites remain a technical challenge. It is worth intensive investigation in order to fully exhibit the excellent properties of their polymer composites. It is proved to be feasible to disperse nanotubes well into polymer through better choices of surface modification, polymers and fabrication methods. Almost all types of nanocomposites, which are
homogenously dispersed, can be obtained with improved mechanical properties and electrical properties etc. It is fortunate that theoretical and chemical tools have just been developed that can solve the problem of dispersion.
The potential applications for these nanotubes-polymer composites are widespread and can be found both government and commercial systems. Some of these applications include thermally conductive adhesives, solar collectors, aerospace structural panels, automotive components, high stiffness-to-weight space mirror substrates or even totally new ones, which we even can not imagine.
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