Effect of single-walled carbon nanotubes on morphology and mechanical properties of NBR/PVC blends

Dynamically vulcanized thermoplastic elastomer based on Nitrile butadiene-rubber (NBR)/PVC with functionalized single-walled carbon nanotubes (f-SWNTs) and non-functionalized single-walled carbon nanotubes (SWNTs) were prepared using a brabender internal mixer. Effects of two types of SWNTs (functionalized and non-functionalized) on morphology and mechanical properties of NBR/PVC blends were studied. Results showed that the mechanical properties of NBR/PVC/SWNTs nanocomposites improved with the increasing of SWNTs content and in particular with the increase of f-SWNTs content. Moreover, the enhancement of mechanical properties of NBR/PVC blends reinforced with functionalized SWNT was higher than that of NBR/PVC blends with non-functionalized SWNT. Dispersion of SWNTs and morphology of NBR/PVC/SWNT nanocomposites were determined by scanning electron microscopy and transmission electron microscopy (TEM) techniques. TEM images illustrated that f-SWNTs were dispersed uniformly in NBR/PVC matrix while non-functionalized SWNTs showed much aggregation. Dynamic mechanical thermal analysis of NBR/PVC/SWNTs nanocomposites was also studied. The outcomes indicated that in the case of f-SWNTs, the intensity of tan ?? peak was lower than that in the case of non-functionalized SWNTs. Meanwhile, the intensity of tan ?? peak reduced when the content of f-SWNTs was increased.     

Fabrication of pH‐ or temperature‐responsive single wall carbon nanotubes via a graft from photopolymerization

An ultraviolet light initiated “graft from” polymerization method to fabricate polymer‐functionalized single wall carbon nanotubes (SWNTs) with pendant pH‐ and temperature‐responsive polymer chains is utilized. The attached polymer chains, formed from methacrylic acid and poly(ethylene glycol) methyl ether methacrylate monomers, are well established for its pH‐responsive swelling/deswelling behavior. This special property was utilized here to control the aqueous dispersibility of the carbon nanotubes. Furthermore, poly(N‐isopropylacrylamide), a temperature‐responsive polymer, was utilized in the fabrication of SWNTs whose dispersibility was dependent on solution temperature. The morphology of the polymer‐functionalized carbon nanotubes was characterized by scanning electron microscopy (SEM) before and after functionalization. Environmental SEM was used to further characterize the morphology of the functionalized SWNTs. In addition, covalent bonding of the polymer to the carbon nanotube surface was established using Raman and Fourier transform infrared spectroscopic techniques. The physical and chemical properties of the functionalized nanotubes were further characterized by energy‐dispersive X‐ray spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2980–2986, 2012     

Spectroscopic study on the centrifugal fractionation of soluble single-walled carbon nanotubes

Optical absorption and resonant Raman spectra are proven to be convenient and effective to monitor the centrifugal fractionation and to evaluate the quality of soluble single-walled carbon nanotubes (SWNTs) achieved by inorganic oxidation and organic functionalization. Through a systemic study of a series of centrifuged solutions, we confirmed that heavily functionalized amorphous carbon was fractionated into the early centrifuged solutions, whereas lightly functionalized graphite fragments as well as polyhedral carbon and metal catalysts particles were fractionated into the late centrifuged solutions and centrifuged residue, and then highly pure and well dispersed SWNTs were collected from the middle centrifuged solutions. It is proposed that the purity, dispersibility, and aggregation state of SWNTs can be qualitatively estimated by the relative intensity of their absorption features, the fine structure and slope of their absorption curves. The Raman features of centrifuged SWNTs are found systematically up-shifted except the disorder-induced D band in comparison with those of as-prepared material, indicating that the SWNTs in centrifuged solutions are individual or in thin bundles. Two new features were identified at 1428 and 941 cm⁻¹ in the Raman spectrum of thermally annealed centrifuged SWNTs, which were assigned to achiral nanotubes and combined mode, respectively.     

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.     

A new method for the preparation of stable carbon nanotube organogels

Single-walled carbon nanotubes (SWNTs) functionalized by ferrocene-grafted poly(p-phenyleneethynylene)s can gelate common organic solvents such as chloroform to form a freestanding carbon nanotube organogel that cannot be redispersed in any organic solvents, indicating the robustness of 3D nanotube network. The drying of SWNT gel on silicon wafer in the air gives a highly convoluted film that is composed of numerous bumps and grooves at multiple length scales. In addition, we report a method for the preparation of an insoluble, homogeneous, electroactive SWNT film from a fresh SWNT solution, which may find applications in nanotube coatings and thin films that require both uniformity and solvent-resistance.     

Promotion of cell growth with magnetically enhanced single-walled carbon nanotubes

We demonstrate that purified and functionalized single walled carbon nanotubes (SWNTs) promote the growth of NIH3T3 mouse fibroblast cells under a magnetic field. The SWNTs are functionalized in acidic solutions by attaching carboxyl groups (–COOH) on their surfaces. Functionalized SWNTs (fSWNTs) exhibit a ferromagnetic property when dispersed in water. Cytotoxicity after the delivery of the fSWNTs into the cells is significantly reduced due to the complete removal of toxic metallic impurities during the functionalization process. The efficient uptake of the fSWNTs by the cells is confirmed through transmission electron microscopy (TEM). It is discovered that the growth of the NIH3T3 cells treated with the fSWNTs is enhanced by up to 25% than control cells when an external magnetic field is applied. Our findings may lead to the non-invasive and non-toxic drug delivery as well as targeted cell therapy with fSWNTs.     

Covalent addition of diethyltoluenediamines onto carbon nanotubes for composite application

Diethyltoluenediamines (DETDA) was grafted to single‐walled carbon nanotubes (SWNTs) through diazonium‐based addition for improving dispersion and interfacial bonding in SWNT/epoxy nanocomposites. Characterization results of Fourier Transformed Infrared spectroscopy and Raman spectroscopy validated covalent bonding between DETDA and carbon nanotubes. The degree of functionalization was about 4% based on thermo‐gravimetric analysis. Interfacial bonding strength was computed in the presence of chemical bonding and the computation results indicated that the interfacial shear strength in the presence of functionalized carbon nanotubes was significantly enhanced. The experimental test revealed that the tensile strength of nanocomposites was enhanced about 23% and Young's modulus about 25%, with 0.5 wt% loading of functionalized‐nanotubes. These considerable improvements further verified the load‐transfer enhancement in the functionalized‐SWNTs/epoxy nanocomposites. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Synthesis and characterization of pH-responsive single-walled carbon nanotubes with a large number of carboxy groups

A new type of pH-responsive single-walled carbon nanotubes (SWNTs) with a large number of carboxy groups was prepared by in situ grafting polymerization. Through exfoliating the SWNT bundles with sodium dodecylbenzene sulfonate (SDBS), individual SWNTs have been obtained. Due to the existence of SDBS, the individual SWNTs could be readily dispersed in water, then forming stable dispersions. Grafting polymerization of acrylonitrile was performed in micelles of SWNTs to produce polyacrylonitrile funtionalized SWNTs (PAN-SWNTs). Experimental results showed that adsorbing acrylonitrile on the SWNT surfaces plays a key role in the grafting process. After hydrolyzing PAN, polyacrylic acid functionalized SWNTs (PAA-SWNTs) were obtained. The amount of PAA grafted could be controlled by changing the feed ratio of initiator to monomer, and the maximum grafting amount could reach 40 wt%. The solubility of PAA-SWNTs in water could be adjusted by pH, with better solubility at higher pH. The large number of carboxy groups on the SWNT surfaces lends the systems convenient for further modification via amidation or esterfication.     

Selective interaction of single-walled carbon nanotubes with conducting dendrimer

We report on the transport properties of a system composed of single-wall carbon nanotubes (SWNTs) noncovalently linked to a new electrically conducting dendrimer poly(amidoamine) modified with a substituted naphthalenediimide (PAMAMC). SEM images show how the adsorption of the conducting dendrimer on SWNTs leads to the unroping of the bundles. The adsorption of PAMAMC molecules on SWNTs has been also investigated by electrical transport measurements. The electrical conductance of SWNTs drastically increases upon adsorption of conducting dendrimer. UV–Vis spectroscopy indicates that there was a modification in the electronic structure of the dendrimer as consequence of nanotube introduction while the appearance of new bands on the Raman spectra may suggest that metallic nanotubes are selectively functionalized.     

Effect of chemisorption on the interfacial bonding characteristics of carbon nanotube-polymer composites

The influence of chemical functionalization on the interfacial bonding characteristics of single-walled nanotubes (SWNTs) reinforced polymer composites was investigated using molecular mechanics and molecular dynamics simulations. The simulations show that functionalization of nanotubes at low densities of functionalized carbon atoms drastically increases their interfacial bonding and shear stress between the nanotubes and the polymer matrix, where chemisorption to as little as 5.0% of the nanotube carbon atoms increases the shear stress by about 1000%. This indicates that increasing the load transfer between SWNTs and a polymer matrix in a composite via chemisorption may be an effective way and chemical attachment of nanotubes during processing may be in part responsible for the enhanced stress transfer observed in some systems of the nanotube-polymer composites. Furthermore, this suggests the possibility to use functionalized nanotubes to effectively reinforce other kinds of polymer-based materials as well.     

Dynamics of amine functionalized nanotubes/epoxy composites by dielectric relaxation spectroscopy

An investigation was conducted to determine how the aspect ratio of single walled carbon nanotubes (SWNTs) affects the dynamics of epoxy/amine networks at various stages of cross-linking. A new insight at the molecular level was obtained by employing a novel experimental approach based on simultaneous dielectric-infrared measurements. In particular, evidence was found which supports the formation of amine functionalized SWNTs, which affords a vehicle for the migration of intrinsic charges and provides a contribution to the overall conductivity.     

Synthesis of novel single-walled carbon nanotubes/poly (<Emphasis Type="Italic">p</Emphasis>-phenylene benzobisoxazole) nanocomposite

Poly (p-phenylene benzobisoxazole) (PBO) fibers are some of the strongest organic polymer fibers. However, the introduction of single-walled carbon nanotubes (SWNT) into the PBO backbone might lead to improvements in their alignment and physical properties. Therefore, SWNT was cut and functionalized by three oxidative cutting methods. After cutting, three different types of SWNT were obtained. Furthermore, copolymerization of SWNTs with PBO polymer was successfully carried out in a mixed solvent of polyphosphoric acid and methanesulfonic acid. The SWNTs were homogeneously distributed throughout the films of copolymerized products, as determined by Raman spectroscopy. The benzoxazole moieties could be formed between the carboxyl of SWNTs and o-aminophenol derivatives of PBO polymer. The length of SWNTs affected the dispersion and reaction activity. Short SWNTs could react with the PBO polymer more easily and form more covalent bonds.     

Vibrational and photoluminescence properties of the polystyrene functionalized single-walled carbon nanotubes

Using surface enhanced Raman scattering (SERS) and FTIR spectroscopy, a covalent functionalization of single wall carbon nanotubes (SWNTs) with polystyrene (PST) is demonstrated. For this, two types PST/SWNTs composites were used: one resulting from the radical polymerization reaction of styrene achieved at 90 °C in the presence of benzene and benzoyl peroxide, latter being the initiator and another obtained by mixing the two constituents. These compounds reveal different SERS and photoluminescence spectra. The main experimental facts supporting the covalent functionalization of SWNTs with PST are provided by FTIR spectroscopy. In this frame significant is the increase of the intensities of absorption bands at 1270 and 1721 cm^− 1, which are associated with the tangential C–H bending + C–O stretching and CO stretching vibration modes, respectively. The presence of these absorption bands in the FTIR spectra of the PST/C₆₀ composites proves the formation of a compound with a similar molecular structure. in the case of polystyrene functionalized SWNTs, the appearance of a new FTIR band at ca. 1635 cm^− 1, attributed to the C–C in cyclic hydrocarbons ring di-substituted vibration mode indicates the formation of an additional reaction product.     

化学气相沉积法制备单壁碳纳米管研究进展

单壁碳纳米管的独特性能使其成为一种有着极大应用前景的新兴纳米材料,本文主要介绍了催化剂、裂解温度、载气等因素对化学气相沉积法（CVD）制备单壁碳纳米管的影响和采用化学气相沉积制备定向单壁碳纳米管方面的研究进展情况。     

Thermal properties of polypropylene nanocomposites: Effects of carbon nanomaterials and processing

Design of experiments was used to elucidate the complex interactions that determine nanocomposite properties and enable predictive models for optimization. The thermal properties of nanocomposites containingpolypropylene, single‐walled carbon nanotubes (SWNTs), dodecyl‐functionalized SWNTs, and vapor‐grown carbon fibers were investigated as a function of extrusion temperature, screw speed, and time. The effects of extruder processing conditions on thermal properties was dependent on the fraction of polymer chains stabilized in the interphase, the extent of polymer degradation, and the type of nanomaterial incorporated. Melting and crystallization temperatures were primarily affected by nanomaterial type. However, thermal decomposition temperature was affected significantly by processing conditions and the response wasdependent on the type of nanomaterial incorporated. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

New hybrid system: Poly(ethylene glycol) hydrogel with covalently bonded pegylated nanotubes

Hydrogels containing carbon nanotubes (CNTs) are expected to be promising conjugates because they might show a synergic combination of properties from both materials. Most of the hybrid materials containing CNTs only entrap them physically, and the covalent attachment has not been properly addressed yet. In this study, single‐walled carbon nanotubes (SWNTs) were successfully incorporated into a poly(ethylene glycol) (PEG) hydrogel by covalent bonds to form a hybrid material. For this purpose, SWNTs were functionalized with poly(ethylene glycol) methacrylate (PEGMA) to obtain water‐soluble pegylated SWNTs (SWNT–PEGMA). These functionalized SWNTs were covalently bonded through their PEG moieties to a PEG hydrogel. The hybrid network was obtained from the crosslinking reaction of poly(ethylene glycol) diacrylate prepolymer and the SWNT–PEGMA by dual photo‐UV and thermal initiations. The mechanical and swelling properties of the new hybrid material were studied. In addition, the material and lixiviates were analyzed to elucidate any kind of SWNT release and to evaluate a possible in vitro cytotoxic effect. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Purification process for single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) have exceptional strength and stiffness and high thermal and electrical conductivity, making them excellent candidates for aerospace structural materials. However, one of the most fundamental challenges is purifying the SWNTs. The purpose of this study was to develop a simple purification process for SWNTs, along with an understanding of the purification process. In addition, uncomplicated analytical methods were sought to screen and compare various purification methods. In this study, we demonstrate an easy method of cleaning SWNTs and evaluating their purity. The cleaning method, which employed oxidative heat treatment followed by acid reflux, was straightforward, inexpensive, and fairly effective. The purification mechanism was determined to be, first, that much of the non-nanotube carbon and iron catalyst was oxidized and, second, that the acid washing removed the iron oxide, leaving relatively pure SWNTs. Also, it was shown that a combination of thermal gravimetric analysis and Raman spectroscopy, both of which take only a few minutes and require little sample preparation, are sufficient as qualitative screening tools to determine the relative purity of SWNTs. Other analytical techniques were used to verify the validity of the screening techniques.     

Phenomenological characterization of fabrication of aligned pristine‐SWNT and COOH‐SWNT nanocomposites via dielectrophoresis under AC electric field

Dielectrophoresis under the application of AC electric fields is one of the primary fabrication techniques (DEPFT) for obtaining aligned carbon nanotube (CNT)–polymer nanocomposites, and is used here to generate data sets from which DEPFT fabrication models in terms of CNT dispersion and orientation distribution can be developed. While the general understanding of how CNTs form aligned filaments under the influence of dielectrophoretic forces and moments is well established, detailed multi‐CNT‐filament formation predictions of microstructure evolution from a random dispersion into a more ordered structure remain intractable. As such, effort here is focused towards the development of phenomenological fabrication models for controlling local CNT dispersion and orientation as a function of applied electric field magnitude, frequency, and exposure time. In this study, 0.03 wt% single‐wall nanotubes (SWNTs) and acid treated functionalized SWNTs (COOH‐SWNTs) were dispersed in a photopolymerizable monomer blend (urethane dimethacrylate (UDMA) and 1,6‐hexanediol dimethacrylate (HDDMA)). Ultrasonication techniques were used to obtain the two different acrylate solutions i.e., 0.03% SWNTs/ UDMA/ HDDMA(9/1) solution and a 0.03% COOH‐SWNTs/UDMA/HDDMA(9/1) solution, consisting of randomly oriented, well dispersed SWNTs. Pristine SWNTs and acid treated SWNTs solutions were then subjected to controlled AC electric fields in order to explore the formation of aligned SWNT‐filaments. To assess key morphological features of the as‐produced SWNT‐acrylate and SWNT‐COOH‐acrylate nanocomposite samples, such as SWNT distribution and filament thicknesses, transmission optical microscopy has been used to observe the SWNT alignment and filament formation obtained by digitally mapping individual overlapping images. The acquisition of a large field of view with high magnification allows statistically meaningful distribution functions for morphological features to be constructed. Measurements of the as‐produced nanocomposite electrical properties in the SWNT alignment direction and transverse to it were used as a macroscale measure to confirm alignment and contiguity of the SWNT‐filament structure, with polarized Raman spectroscopy used to assess the degree of SWNT alignment at the local microscale level. It is observed that a combination of exposure time to AC electric field, and its frequency, is the key driver of filament thickness and spacing and that in general, the COOH‐SWNTs align to a greater extent than the pristine SWNTs, though they do not form filaments that are as thick and contiguous for the exposure times studied. POLYM. COMPOS., 36:1266–1279, 2015. © 2014 Society of Plastics Engineers     

Photophysics of individual single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are cylindrical graphitic molecules that have remained at the forefront of nanomaterials research since 1991, largely due to their exceptional and unusual mechanical, electrical, and optical properties. The motivation for understanding how nanotubes interact with light (i.e., SWNT photophysics) is both fundamental and applied. Individual nanotubes may someday be used as superior near-infrared fluorophores, biological tags and sensors, and components for ultrahigh-speed optical communications systems. Establishing an understanding of basic nanotube photophysics is intrinsically significant and should enable the rapid development of such innovations. Unlike conventional molecules, carbon nanotubes are synthesized as heterogeneous samples, composed of molecules with different diameters, chiralities, and lengths. Because a nanotube can be either metallic or semiconducting depending on its particular molecular structure, SWNT samples are also mixtures of conductors and semiconductors. Early progress in understanding the optical characteristics of SWNTs was limited because nanotubes aggregate when synthesized, causing a mixing of the energy states of different nanotube structures. Recently, significant improvements in sample preparation have made it possible to isolate individual nanotubes, enabling many advances in characterizing their optical properties. In this Account, single-molecule confocal microscopy and spectroscopy were implemented to study the fluorescence from individual nanotubes. Single-molecule measurements naturally circumvent the difficulties associated with SWNT sample inhomogeneities. Intrinsic SWNT photoluminescence has a simple narrow Lorentzian line shape and a polarization dependence, as expected for a one-dimensional system. Although the local environment heavily influences the optical transition wavelength and intensity, single nanotubes are exceptionally photostable. In fact, they have the unique characteristic that their single molecule fluorescence intensity remains constant over time; SWNTs do not "blink" or photobleach under ambient conditions. In addition, transient absorption spectroscopy was used to examine the relaxation dynamics of photoexcited nanotubes and to elucidate the nature of the SWNT excited state. For metallic SWNTs, very fast initial recovery times (300-500 fs) corresponded to excited-state relaxation. For semiconducting SWNTs, an additional slower decay component was observed (50-100 ps) that corresponded to electron-hole recombination. As the excitation intensity was increased, multiple electron-hole pairs were generated in the SWNT; however, these e-h pairs annihilated each other completely in under 3 ps. Studying the dynamics of this annihilation process revealed the lifetimes for one, two, and three e-h pairs, which further confirmed that the photoexcitation of SWNTs produces not free electrons but rather one-dimensional bound electron-hole pairs (i.e., excitons). In summary, nanotube photophysics is a rapidly developing area of nanomaterials research. Individual SWNTs exhibit robust and unexpectedly unwavering single-molecule fluorescence in the near-infrared, show fast relaxation dynamics, and generate excitons as their optical excited states. These fundamental discoveries should enable the development of novel devices based on the impressive photophysical properties of carbon nanotubes, especially in areas like biological imaging. Many facets of nanotube photophysics still need to be better understood, but SWNTs have already proven to be an excellent starting material for future nanophotonics applications.