Improving the Degree of Functionalization and Solubility of Single-Walled Carbon Nanotubes via Covalent Multiple Functionalization

The synthesis of covalent multi-functionalized single-walled carbon nanotubes (SWNT) via the combination of two diazonium salt addition reactions with tetrabutylammonium hydroxide to form carbon nanotube salts was carried out. The formation of carbon nanotube salts allows SWNT to be easily dissolved in common solvents, which increases the processing capability of the SWNT. The dissolutions of some of the obtained products can be increased three-fold through multi-functionalization. Thus tetrabutylammonium cation plays an extraordinarily important role; i.e., balancing the charges on the functionalized SWNT surface and de-bundling of the functionalized SWNT. This approach provides a useful way of preparing some SWNT-containing biomaterials. The multi-functionalized SWNT were confirmed by standard techniques used for carbon nanotube characterization.     

In situ grafting of carboxylic acid-terminated hyperbranched poly(ether-ketone) to the surface of carbon nanotubes

Carboxylic acid-terminated hyperbranched poly(ether-ketone)s (HPEKs) were successfully grafted onto the surfaces of single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT) to afford HPEK-g-SWNT and HPEK-g-MWNT nanocomposites. They were prepared via in situ polymerization of 5-phenoxyisophthalic acid as an AB₂ monomer for the HPEK in the presence of SWNT or MWNT in polyphosphoric acid (PPA)/phosphorous pentoxide (P₂O₅) medium. The resultant nanocomposites were homogeneously dispersed in various common polar aprotic solvents as well as in concentrated ammonium hydroxide. The experimental results from Soxhlet extraction, solubility enhancement, elemental analysis (EA), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) provided clear evidences for grafting of hyperbranched polymers onto the surfaces of corresponding CNT's. Achieving enhanced solubility of CNT's in common organic solvents via the functionalization of CNT's is a key step for CNT's to be used in various application-specific purposes. The results could potentially envision to the area of CNT researches via the efficient introduction of three-dimensional globular dendritic macromolecules as increasing solubility, available multi-functionality, reactivity, processability, and also biocompatibility.     

Electrochemical properties of double wall carbon nanotube electrodes

Electrochemical properties of double wall carbon nanotubes (DWNT) were assessed and compared to their single wall (SWNT) counterparts. The double and single wall carbon nanotube materials were characterized by Raman spectroscopy, scanning and transmission electron microscopy and electrochemistry. The electrochemical behavior of DWNT film electrodes was characterized by using cyclic voltammetry of ferricyanide and NADH. It is shown that while both DWNT and SWNT were significantly functionalized with oxygen containing groups, double wall carbon nanotube film electrodes show a fast electron transfer and substantial decrease of overpotential of NADH when compared to the same way treated single wall carbon nanotubes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Interfacial in situ polymerization of single wall carbon nanotube/nylon 6,6 nanocomposites

An interfacial polymerization method for nylon 6,6 was adapted to produce nanocomposites with single wall carbon nanotubes (SWNT) via in situ polymerization. SWNT were incorporated in purified, functionalized or surfactant stabilized forms. The functionalization of SWNT was characterized by FTIR, Raman spectroscopy and TGA and the SWNT dispersion was characterized by optical microscopy before and after the in situ polymerization. SWNT functionalization and surfactant stabilization improved the nanotube dispersion in solvents but only functionalized SWNT showed a good dispersion in composites, whereas purified and surfactant stabilized SWNT resulted in poor dispersion and nanotube agglomeration. Weak shear flow induced SWNT flocculation in these nanocomposites. The electrical and mechanical properties of the SWNT/nylon nanocomposites are briefly discussed in terms of SWNT loading, dispersion, length and type of functionalization.     

Structure and properties of polyacrylonitrile/single wall carbon nanotube composite films

Polyacrylonitrile (PAN)/single wall carbon nanotube (SWNT) composite films have been processed with unique combination of tensile strength (103 MPa), modulus (10.9 GPa), electrical conductivity (1.5×10⁴ S/m), dimensional stability (coefficient of thermal expansion 1.7×10⁻⁶/°C), low density (1.08 g/cm³), solvent resistance, and thermal stability. PAN molecular motion above the glass transition temperature (T_g) in the composite film is significantly suppressed, resulting in high PAN/SWNT storage modulus above T_g (40 times the PAN storage modulus). Rope diameter in the SWNT powder was 26 nm, while in 60/40 PAN/SWNT film, the rope diameter was 40 nm. PAN crystallite size from (110) plane in PAN and PAN/SWNT films was 5.3 and 2.9 nm, respectively. This study suggests good interaction between PAN and SWNT.     

Polymer conformation-assisted wrapping of single-walled carbon nanotube: The impact of cis-vinylene linkage

A soluble π-conjugated polymer cis-PmPV is found to be twice as effective as its trans-PmPV isomer in dispersing SWNTs into organic solvents. The improved efficiency is related to the specific conformation of cis-vinylene-enriched PmPV, which facilitates a planar π-π interaction with SWNT surface and leads to improved nanotube dispersion. ¹H NMR spectra indicate that the cis-CHCH bonds are partially converted to the trans-CHCH, thereby providing necessary conformational cavity for SWNT wrapping. Irradiation triggers a precipitation from SWNT dispersion, providing a purified SWNT/conjugated polymer composite.     

(Co-)solvent selection for single-wall carbon nanotubes: best solvents, acids, superacids and guest-host inclusion complexes

Analysis of 1-octanol-water, cyclohexane-water and chloroform (CHCl(3))-water partition coefficients P(o-ch-cf) allows calculation of molecular lipophilicity patterns, which show that for a given atom log P(o-ch-cf) is sensitive to the presence of functional groups. Program CDHI does not properly differentiate between non-equivalent atoms. The most abundant single-wall carbon nanotube (SWNT), (10,10), presents a relatively small aqueous solubility and large elementary polarizability, P(o-ch-cf) and kinetic stability. The SWNT solubility is studied in various solvents, finding a class of non-hydrogen-bonding Lewis bases with good solubility. Solvents group into three classes. The SWNTs in some organic solvents are cationic while in water/Triton X mixture are anionic. Categorized solubility is semiquantitatively correlated with solvent parameters. The coefficient of term β is positive while the ones of ε and V negative. The electron affinity of d-glucopyranoses (d-Glcp(n)) suggests the formation of colloids of anionic SWNTs in water. Dipole moment for d-Glcp(n)-linear increases with n until four in agreement with 18-fold helix. The I(n)(z-) and SWNT(-) are proposed to form inclusion complexes with cyclodextrin (CD) and amylose (Amy). Starch, d-Glcp, CD and Amy are proposed as SWNT co-solvents. Guests-hosts are unperturbed. A central channel expansion is suggested.     

Optimization of single-walled carbon nanotube growth and study of the hysteresis of random network carbon nanotube thin film transistors

Random network single-walled carbon nanotube (SWNT)-based thin film transistors show excellent properties in sensors, electronic circuits, and flexible devices. However, they exhibit a significant amount of hysteresis behavior, which should be solved prior to use in industrial applications. This paper provides optimum conditions for the growth of random network SWNTs and reveals that the observed hysteresis behavior originates from the charge exchange between the SWNTs and the dielectric layer rather than from changes in the intrinsic properties of the SWNTs. This was proven by studying the conditions of stepwise gate sweep experiments and time measurements. This paper also shows that top gate SWNT thin film transistors (TFTs) with an SU-8 dielectric layer could provide a practical solution to the hysteresis problem for SWNT TFTs in electronic circuit applications.     

Carbon nanotube core–polymer shell nanofibers

Single‐walled carbon nanotube (SWNT)/poly(methyl methacrylate) and SWNT/polyacrylonitrile composite nanofibers were electrospun with SWNT bundles as the cores and the polymers as the shells. This was a novel approach for processing core (carbon nanotube)–shell (polymer) nanofibers. Raman spectroscopy results show strain‐induced intensity variations in the SWNT radial breathing mode and an upshift in the tangential (G) and overtone of the disorder (G′) bands, suggesting compressive forces on the SWNTs in the electrospun composite fibers. Such fibers may find applications as conducting nanowires and as atomic force microscopy tips. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1992–1995, 2005     

Noncovalent solubilization of multi-walled carbon nanotubes in common low-polarity organic solvents with branched Pd–diimine polyethylenes: Effects of polymer chain topology,molecular weight and terminal pyrene group

Noncovalent nonspecific solubilization of carbon nanotubes with common polymers without having any specific functionality is an important strategy for rendering debundled nanotube solutions for their processing and technological applications. Among the various polymers investigated thus far for noncovalent nonspecific nanotube solubilization, hyperbranched polyethylene (HBPE) featured with distinct highly compact dendritic chain architecture has been discovered to show outstanding performance in rendering stable nanotube solutions in common low-polarity organic solvents (including tetrahydrofuran (THF) and chloroform) at surprisingly high concentrations. To understand the mechanism of the nanotube solubilization with this unique class of polymers and to elucidate the effects of various macromolecular structural parameters, we have designed and synthesized in this work four sets of highly branched polyethylenes varying in chain topology, molecular weight, and end group. With these polymers, we have systematically investigated and compared their performance for the solubilization of multi-walled carbon nanotubes in common solvents including THF, chloroform, n-heptane, and toluene. We have found that these macromolecular structural parameters as well as the solvent play complex but sensitive roles in this noncovalent solubilization system. This work thus provides some valuable guidelines towards the design of optimum polymers for efficient noncovalent nonspecific solubilization of carbon nanotubes.     

Single wall carbon nanotube templated oriented crystallization of poly(vinyl alcohol)

Shearing of poly(vinyl alcohol) (PVA)/single wall carbon nanotube (SWNT) dispersions result in the formation of self-assembled oriented PVA/SWNT fibers or ribbons, while PVA solution results in the formation of unoriented fibers. Diameter/width and length of these self-assembled fibers was 5-45 μm and 0.5-3 mm, respectively. High-resolution transmission electron micrographs showed well resolved PVA (200) lattice with molecules oriented parallel to the nanotube axis. Nanotube orientation in the self-assembled fibers was also determined from Raman spectroscopy. PVA fibers exhibited about 48% crystallinity, while crystallinity in PVA/SWNT fibers was 84% as determined by wide angle X-ray diffraction. PVA and carbon nanotubes were at an angle of 25-40° to the self-assembled fiber axis. In comparison to PVA, PVA/SWNT samples exhibited significantly enhanced electron beam radiation resistance. This study shows that single wall carbon nanotubes not only nucleate polymer crystallization, but also act as a template for polymer orientation.     

Improvement of Vickers hardness measurement on SWNT/Al2O3 composites consolidated by spark plasma sintering

Dense alumina composites with different carbon nanotube content were prepared by colloidal processing and consolidated by Spark Plasma Sintering (SPS). Single-wall carbon nanotubes (SWNTs) were distributed at grain boundaries and also into agglomerates homogeneously dispersed. Carrying out Vickers hardness tests on the cross-section surfaces instead of top (or bottom) surfaces has shown a noticeable increase in the reliability of the hardness measurements. This improvement has been mainly attributed to the different morphology of carbon nanotube agglomerates, which however does not seem to affect the Vickers hardness value. Composites with lower SWNT content maintain the Vickers hardness of monolithic alumina, whereas it significantly decreases for the rest of compositions. The decreasing trend with increasing SWNT content has been explained by the presence of higher SWNT quantities at grain boundaries. Based on the results obtained, a method for optimizing Vickers hardness tests performance on SWNT/Al₂O₃ composites sintered by SPS is proposed.     

Chemical modification of carbon nanotubes with organic hydrazines

A reaction of single-wall carbon nanotubes with an organic hydrazine proceeds in an aqueous surfactant solution. Raman spectrum of the product shows the typical disorder band, indicating the occurrence of sidewall functionalization of nanotubes. Elemental analyses of the products suggest that C-N bonds are formed on the nanotube surface. The functionalized nanotubes are soluble in organic solvents up to 100 mg/L. The attached groups can be removed by heating.     

Processing and properties of carbon nanotube/poly(methyl methacrylate) composite films

A systematic study of the reinforcement of single‐walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes, and vapor‐grown carbon nanofibers (VGCNFs) in poly(methyl methacrylate) (PMMA) is reported. SWNT/PMMA composite films with various SWNT concentrations (from 0.5 to 50 wt % with respect to the weight of PMMA) were processed from nitromethane. Two types of SWNTs were used: SWNT‐A, which contained 35 wt % metal catalyst, and SWNT‐B, which contained about 2.4 wt % metal catalyst. Properties of different nanotubes containing composites were compared with 15 wt % carbon nanotubes (CNTs). Property enhancement included electrical conductivity, mechanical properties, and solvent resistance. The thermal degradation of PMMA in the presence of CNTs in air and nitrogen environments was studied. No variation in the thermal degradation behavior of PMMA/CNT was observed in nitrogen. The peak degradation temperature increased for the composites in air at low CNT loadings. Dynamic and thermomechanical properties were also studied. At a 35 wt % SWNT loading, a composite film exhibited good mechanical and electrical properties, good chemical resistance, and a very low coefficient of thermal expansion. Property improvements were rationalized in terms of the nanotube surface area. Composite films were also characterized with Raman spectroscopy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Poly(N-vinyl carbazole)-functionalized single-walled carbon nanotubes: Synthesis, characterization, and nanocomposite thin films

A poly(N-vinyl carbazole) (PVK) copolymer containing pendant hydroxyl groups was synthesized for the functionalization of single-walled carbon nanotubes (SWNTs) under typical reaction conditions for the esterification of the nanotube-bound carboxylic acids. The functionalized nanotube samples, soluble in common organic solvents, were characterized by using optical absorption, Raman, and several microscopy techniques. The presence of ester linkages was supported by the results from chemical defunctionalization in hydrolysis that recovered insoluble SWNTs. The shared solubility of the functionalized nanotube samples with PVK enabled the wet-casting of high-quality PVK-SWNT nanocomposite thin films for an evaluation of their enhanced charge dissipation under illumination.     

Transparent and flexible electrodes and supercapacitors using polyaniline/single-walled carbon nanotube composite thin films

Large-scale transparent and flexible electronic devices have been pursued for potential applications such as those in touch sensors and display technologies. These applications require that the power source of these devices must also comply with transparent and flexible features. Here we present transparent and flexible supercapacitors assembled from polyaniline (PANI)/single-walled carbon nanotube (SWNT) composite thin film electrodes. The ultrathin, optically homogeneous and transparent, electrically conducting films of the PANI/SWNT composite show a large specific capacitance due to combined double-layer capacitance and pseudo-capacitance mechanisms. A supercapacitor assembled using electrodes with a SWNT density of 10.0 μg cm(-2) and 59 wt% PANI gives a specific capacitance of 55.0 F g(-1) at a current density of 2.6 A g(-1), showing its possibility for transparent and flexible energy storage.     

Stabilization and carbonization of gel spun polyacrylonitrile/single wall carbon nanotube composite fibers

Gel spun polyacrylonitrile (PAN) and PAN/single wall carbon nanotube (SWNT) composite fibers have been stabilized in air and subsequently carbonized in argon at 1100 °C. Differential scanning calorimetry (DSC) and infrared spectroscopy suggests that the presence of single wall carbon nanotube affects PAN stabilization. Carbonized PAN/SWNT fibers exhibited 10-30 nm diameter fibrils embedded in brittle carbon matrix, while the control PAN carbonized under the same conditions exhibited brittle fracture with no fibrils. High resolution transmission electron microscopy and Raman spectroscopy suggest the existence of well developed graphitic regions in carbonized PAN/SWNT and mostly disordered carbon in carbonized PAN. Tensile modulus and strength of the carbonized fibers were as high as 250 N/tex and 1.8 N/tex for the composite fibers and 168 N/tex and 1.1 N/tex for the control PAN based carbon fibers, respectively. The addition of 1 wt% carbon nanotubes enhanced the carbon fiber modulus by 49% and strength by 64%.     

PEDOT‐PSS/singlewall carbon nanotubes composites

Poly(3,4‐ethylenedioxythiophene)‐single wall carbon nanotube (SWNT) composites were prepared via coating methods for improving electrical conductivity and flexibility, minimizing loss of transparency. Three types of surface modified SWNTs were prepared through different modification methods: carboxylated SWNT treated with nitric acid to form carboxylic acid group on their surfaces, 1‐pyrenebutyric acid wrapped SWNT, and 1‐pyrenebutyric acid, lithium salt wrapped SWNT. The surface modifiers had influence on the dispersion states of the SWNTs in 2‐propanol. The dispersion states had influence of aggregate concentrations of surface modified SWNTs in polymer matrix, showing lower aggregate concentration for the carboxylated SWNT than those of the other surface modified SWNTs. The dispersion behaviors of SWNTs were also related with transparency and electrical resistance, and flexibility of composite films. Based on the layer‐by‐layer coating method, SWNT composite film properties such as thickness, transparency, and electrical conductivity could be controlled and would be a good example for improving contradictory properties. POLYM. ENG. SCI., 48:1–10, 2008. © 2007 Society of Plastics Engineers     

Electrical conductivity of single-wall carbon nanotube film deposited by electrostatic layer-by-layer assembly with the aid of polyelectrolytes

A single-wall carbon nanotube film was deposited by the sequential deposition of positively charged single-wall carbon nanotube with poly diallyldimethylammonium and negatively charged single-wall nanotube with poly sodium 4-styrenesulfonate. The sequential deposition was repeated 10 times and Al was used as an electrode to measure the electrical conductivity of the film. Electrochemical impedance spectroscopy showed that the film had two semicircles with the resistances of 300 and 1700 Ω, representing single-wall carbon nanotube and polyelectrolyte parts, respectively. The conductivity of the film was enhanced when the water molecules were removed by heat treatment. The analysis of the J-V characteristics indicates that the mechanism of the charge transfer through the single-wall carbon nanotube-polyelectrolyte film is quantum mechanical tunneling.     

Graft Polymerization of Styrene from Single-Walled Carbon Nanotube using Atom Transfer Radical Polymerization

Summary We grafted polystyrene from the surface of single-walled carbon nanotube (SWNT) via Atom Transfer Radical Polymerization (ATRP). Direct electrophilic addition of chloroform and subsequent hydrolysis introduced hydroxyl groups to the surface of SWNT. ATRP initiators were attached to the SWNT by esterification of 2-chrolopropyl chloride with hydroxyl groups. Subsequent polymerization of styrene with CuCl / N,N,N’,N’,N”-pentamethyldiethylenetriamine catalyst yielded polystyrene grafted SWNT. Transmission electron microscopy analysis clearly showed that approximately 6 nm-thick polystyrene layer was uniformly formed on 1.2 nm diameter SWNT.