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Supramolecular Complexes of Multivalent Cholesterol-Containing Polymers to Solubilize Carbon Nanotubes in Apolar Organic Solvents
Chemistry - An Asian Journal
Abstract
Copolymers of 2-ethylhexyl acrylate (EHA) and cholesteryloxycarbonyl-2-hydroxymethacrylate (CEM) were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Supramolecular complexes of these copolymers with carbon nanotubes (CNTs) were soluble in THF, toluene, and isooctane. The colloidal solutions remained stable for months without aggregation. The rationale for the choice of CEM was based on the high adsorption energy of cholesterol on the CNT surface, as computed by DFT calculations. Adsorption isotherms were experimentally measured for copolymers of various architectures (statistical, diblock, and star copolymers), thereby demonstrating that 2-5 cholesterol groups were adsorbed per polymer chain. Once the supramolecular complex had dried, the CNTs could be easily resolubilized in isooctane without the need for high-power sonication and in the absence of added polymer. Analysis by atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) indicated that the CNTs were devoid of bundles. The supramolecular complexes could also be employed in an inverse emulsion polymerization of 2-hydroxyethylmethacrylate (HEMA) in isooctane and dodecane, thereby leading to the formation of a continuous polymeric sheath around the CNTs. Thus, this technique leads to the formation of very stable dispersions in non-polar organic solvents, without altering the fundamental properties of the CNTs.
... It was shown by us that polymers bearing such pendant cholesterol groups form strong supramolecular interactions with graphene 23 or with carbon nanotubes. 24 The RGO flakes separated from the isooctane layer are nearly devoid of surface defects, as shown by a combination of a Raman spectroscopy, Photoluminescence (PL) measurements, XPS, X-ray diffraction (XRD), Thermogravimetric Analysis (TGA), and Fourier-transform infrared (FTIR) spectroscopy. Thus, we envision that this separation process offers a promising route toward the mass-production of high-quality graphene via an economically viable process. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Aesar. ...
... Block copolymer poly(CEM11-b-EHA7) was prepared using the procedure described in reference 24. 24 In short, RAFT agent 2- Synthesis of GO and of rGO. GO was synthesized from natural graphite by using the modified Hummers method. ...
... Reduction of GO by hydrazine led to a black RGO powder which is insoluble in isooctane. However, in the presence of poly(CEM11-b-EHA7) (structure shown inFigure 1B), RGO becomes dispersible in isooctane as supramolecular interactions are formed between graphene leaflets and cholesterol units (adsorption energy = 62 kJ/mol).24 In the DLPE experiment, RGO powder is submerged by first a layer of water and then by a layer of isooctane containing poly(CEM11-b-EHA7). ...
One popular approach to prepare graphene on a large scale consists in converting Graphene Oxide (GO) into Reduced Graphene Oxide (RGO). However, this procedure yields graphene flakes with various amounts of oxygenated defects. Using a Double Liquid-Phase Extraction technique (DLPE) assisted by cholesterol-based polymers, we demonstrate that the RGO flakes of the highest quality, i.e. those with the highest π-conjugated network and with the lowest number of oxygenated defects can be selectively extracted in isooctane, while lower quality flakes remain in water. Thus, it is possible to collect single-layer graphene sheets of high-quality, as characterized by Raman spectroscopy (ID/IG below 0.2) starting from a RGO containing a heterogeneous mixture of leaflets (ID/IG ~ 1.3). The high-quality of the RGO leaflets extracted by DLPE was also confirmed by X-ray photoelectron spectroscopy, photoluminescence, Fourier transform infrared spectroscopy, X-ray diffraction, and atomic force microscopy. The conductivity of the films prepared with DLPE RGO flakes is an order of magnitude higher than the one of the films prepared with as-prepared RGO. The thermal stability of the extracted leaflets, as measured by thermal gravimetric analysis, is also greatly enhanced. Thus, sorting RGO by DLPE could become a valuable process for the large-scale production of high-quality graphene.
... 38 In order to stabilize the graphene sheets in isooctane, we exploited the fact that cholesterol forms supramolecular interactions with carbon-based materials in non-polar solvents. 39 Here, a diblock polymer containing 11 cholesterol units (CEM) and 7 ethyl-hexyl acrylate units (EHA) was used as graphene dispersant in isooctane ( Figure 1). Once stable dispersions of graphene were obtained, the printability of the resulting ink was evaluated. ...
... Cholesteryl chloroformate and hexamethyldisilazane (HMDZ) were purchased from Alfa Aesar. The diblock copolymer poly(CEM 11 -b-EHA 7 ) was prepared using the procedure outlined and described in reference 39 . ...
... In our work, exfoliation was performed in a lowpower sonicating bath, with no need for a high-power probe. 39 The polymer is constituted of two blocks, a cholesterol containing block (CEM) and a 2-ethylhexyl acrylate block (EHA) (Figure 1). The former, which is insoluble in isooctane, is adsorbed at the graphene surface (the adsorption energy of cholesterol on graphene has been reported to be -63 kJ mol -1 ) 39 while the latter, which is soluble in isooctane, forms a hairy layer responsible for steric stabilization. ...
Graphene inks are becoming widely popular. However the vast majority of these inks are formulated in polar solvents with high boiling points. Their slow evaporation is a bottleneck factor in roll to roll printing processes. Here, we developed a highly-conductive fast-drying graphene ink in isooctane, a non-polar and low boiling solvent. For this purpose, a diblock copolymer containing pendant cholesterol groups was used during the exfoliation of natural graphite in isooctane. The polymer develops non-covalent supramolecular interactions with the graphene conjugated system, resulting in the formation of stable graphene dispersions (up to c = 4 mg.mL⁻¹). These dispersions were used for direct writing on a variety of substrates, and were shown to dry instantly after application. The influence of polymer concentration on graphene characteristics, on colloidal stability and on electrochemical characteristics has been studied. The lowest sheet resistance (80 Ω/□) was obtained when 23% of the graphene surface was covered by the polymer. In this case, the flakes were constituted of 2-5 graphene layers. More extensive exfoliation, down to single layer graphene, was achieved at greater surface coverage, but led to inks with higher sheet resistance. Thus, by combining a tailored polymeric dispersant, a smooth exfoliation process and a low-boiling non-polar ink solvent, we were able to prepare highly-conductive fast-drying graphene inks which should have a high potentital for the development of roll-to-roll printed electronics
... Main concept MWCNT π-π interaction "Pluronic F38" polymeric micelles Higher dispersion in water [157] MWCNT π-π interaction Poly(maleic anhydride-co-p-acetoxystyrene)-block-poly(p-acetoxystyrene) Low sheet resistance of the electrode [158] CNT π-π interaction Pyrene-PMMA, P(MMA-b-C1 pyrene) and P(MMA-b-C4 pyrene), and P(MMA-b-4VP) Adsorption [159] CNT π-π interaction 2-Ethylhexyl acrylate and cholesteryloxycarbonyl-2-hydroxymethacrylate Solubilization [160] MWCNT π-π interaction Poly(4-azidophenyl methacrylate-co-methyl acrylate) High stability and chemical durability [161] MWCNT π-π interaction Methyl methacrylate and (1-pyrene)methyl 2-methyl-2-propenoate Solubilization in water and various organic solvents [162] MWCNT π-π interaction Poly(vinyl benzyloxy methyl naphthalene)-g-poly(t-butyl methacrylate-co-methacrylic acid) ...
... RAFT polymerization was also used for synthesis of polymers with pyrene end groups, as reported by Claverie and coworkers [160]. They synthesized copolymers of 2-ethylhexyl acrylate (EHA) and cholesteryloxycarbonyl-2-hydroxymethacrylate (CEM) with pyrene end groups via RAFT polymerization (Fig. 36). ...
This paper reviews the recent advances in non-covalent and covalent tethering of small molecules and polymer chains onto carbon nanotube (CNT) and its derivatives. The functionalized CNT has recently attracted great attention because of an increasing number of its potential applications. In non-covalent functionalization of CNT, the sp2-hybridized network plays a crucial role. The non-covalent grafting of small molecules and polymers can mainly be carried out through hydrogen bonding and π-stacking interactions. In covalent functionalization of CNT, condensation, cycloaddition, and addition reactions play a key role. Polymer modification has been reported by using three main methods of "grafting from", "grafting through", and also "grafting to". The "grafting from" and "grafting through" rely on propagation of polymer chains in the presence of CNT modified with initiator and double bond moieties, respectively. In "grafting to" method, which is the main aim of this review, the pre-fabricated polymer chains are mainly grafted onto the surface using coupling reactions. The coupling reactions are used for grafting pre-fabricated polymer chains and also small molecules onto CNT. Recent studies on grafting polymer chains onto CNT via "grafting to" method have focused on the pre-fabricated polymer chains by conventional and controlled radical polymerization (CRP) methods. CRP includes reversible activation, atom transfer, degenerative (exchange) chain transfer, and reversible chain transfer mechanisms, and could result in polymer-grafted CNT with narrow polydispersity index of the grafted polymer chains. Based on the mentioned mechanisms, nitroxide-mediated polymerization, atom transfer radical polymerization, and reversible addition-fragmentation chain transfer are known as the three commonly used CRP methods. Such polymer-modified CNT has lots of applications in batteries, biomedical fields, sensors, filtration, solar cells, etc.
... In the case of non-covalent functionalization, CNTs integrity and properties are preserved as the interaction established between the CNTs surfaces and the adsorbates is purely physical (Bilalis et al. 2013). Different approaches have been used to functionalize CNTs in a non covalent way, including π-π stacking (Haddad et al. 2009;Roquelet et al. 2010) and hydrophobic interactions (Itzhak et al. 2010;Di Crescenzo et al. 2012;Nguendia et al. 2014). In order to disperse CNTs in water, it is worth noting the use of surfactants, both ionic (Blanch et al. 2010;Yang et al. 2013;Fernandes et al. 2015), non ionic (Shim et al. 2002;Zhang et al. 2004;Vaisman et al. 2006) or even mixtures of different surfactants to achieve a synergistic effect (Tan and Resasco 2005;Madni et al. 2010). ...
Carbon nanotubes (CNTs) are allotropes of carbon with hollow, long structures, having diameters on the nanometer scale. They can be described as rolled-up graphene layers. During the last years, they have been increasingly used in the fields of pharmacy and biomedicine. However, due to their high hydrophobicity, they cannot be easily handled in most solvents of biological interest. To this end, different surfactants have been used to improve their dispersion in aqueous media. In the present work, we investigated the adsorption behavior of two surfactants, sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS), at different concentrations on a CNT surface by classical molecular dynamics (MD) simulations. Our results are presented in terms of distance between surfactant molecules and CNT surfaces, radial distribution functions and interaction energies. In all the models simulated in this work, a strong interaction of both surfactants with the CNT surface is observed, as it is demonstrated by decreasing distances between the surfactants and the CNT during simulation time, the shape of their radial distribution functions, as well as favorable adsorption processes from an energetic point of view.
... Nguenda et al. demonstrated [57] for the cholesterol moiety of a multivalent cholesterol-containing polymer (CEM-EHA, see Table 1) an interaction very similar to that between pyrene and carbon nanotubes with cholesterol laying flat above the graphene surface. Supramolecular complexes of these copolymers with CNTs were soluble in THF, toluene and isooctane. ...
Carbon nanotubes (CNTs) have been proposed and actively explored as multipurpose innovative nanoscaffolds for applications in fields such as material science, drug delivery and diagnostic applications. Their versatile physicochemical features are nonetheless limited by their scarce solubilization in both aqueous and organic solvents. In order to overcome this drawback CNTs can be easily non-covalently functionalized with different dispersants. In the present review we focus on the peculiar hydrophobic character of pristine CNTs that prevent them to easily disperse in organic solvents. We report some interesting examples of CNTs dispersants with the aim to highlight the essential features a molecule should possess in order to act as a good carbon nanotube dispersant both in water and in organic solvents. The review pinpoints also a few examples of dispersant design. The last section is devoted to the exploitation of the major quality of non-covalent functionalization that is its reversibility and the possibility to obtain stimuli-responsive precipitation or dispersion of CNTs.
... Importantly, the encapsulation method is not only applicable to TiO 2 particles, but also to a variety of other inorganic species, as we (Das et al., 2011;Das and Claverie, 2012;Zhong et al., 2012) and others (Nguyen et al., 2008(Nguyen et al., , 2013Ali et al., 2009) have demonstrated in the past. Although we and others have worked so far in water, it should also be mentioned that we recently adapted this encapsulation technique for the encapsulation of nanotubes in organic solvents (Nguendia et al., 2014). Thus, this versatile nanoencapsulation method could be used to generate carbon shells on a great number of particles, and could become an interesting alternative to the hydrothermal treatment. ...
TiO2 semiconducting nanoparticles are known to be photocatalysts of moderate activity due to their high band-gap and high rate of electron-hole recombination. The formation of a shell of carbon around the core of TiO2, i.e., the formation of TiO2@C nanoparticles, is believed to partly alleviate these problems. It is usually achieved by a hydrothermal treatment in a presence of a sugar derivative. We present here a novel method for the formation of highly uniform C shell around TiO2 nanoparticles. For this purpose, TiO2 nanoparticles were dispersed in water using an oligomeric dispersant prepared by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization. Then the nanoparticles were engaged into an emulsion polymerization of acrylonitrile, resulting in the formation of a shell of polyacrylonitrile (PAN) around each TiO2 nanoparticles. Upon pyrolysis, the PAN was transformed into carbon, resulting in the formation of TiO2@C nanoparticles. The structure of the resulting particles was elucidated by X-Ray diffraction, FTIR, UV-VIS and Raman spectroscopy as well as TEM microscopy. Preliminary results about the use of the TiO2@C particles as photocatalysts for the splitting of water are presented. They indicate that the presence of the C shell is responsible for a significant enhancement of the photocurrent.
Owning to the remarkable properties of carbon nanotube and nanofiber reinforced polymer composites such as its three dimensional morphological structural network and many more; fields in science and technology encompassing the scientific and industrial sector have paid great attention on these composite materials. Even though there are several research reports on CNTs and NFs reinforced polymer composites, there is inadequate reviews that could act as a propelling focused force for emerging researchers/researches for advanced studies towards practical implementation in high performance polymer composites/nanocomposites. Consequently, different preparation techniques, chemical modification, properties, limitations, and future prospects of CNTs, CNFs and its composite are discussed in this article.
A series of new diblock copolymers bearing rigid liquid crystal cholesterol grafts denoted as PHEMACho1-b-PBLG were designed and sucessfully prepared via a combination of reversible addition fragmentation chain transfer polymerization (RAFT), ring opening polymerization (ROP) and "click " chemistry, and their molecular structures were further characterized by NMR, FTIR, GPC and static light scattering (SLS). Then, liquid crystal (LC) phase structures and their transitions as well as thermal stabilities were characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and thermo gravimetric analysis (TGA), and secondary structures of the PBLG blocks were analyzed by FTIR, Xray diffraction (XRD) and solid-state C-13 CP/MAS NMR techniques. Moreover, their secondary molecular structures in THE were analyzed by circular dichroism spectrum (CD), and self -aggregate morphologies in THF/DMF mixed solution were investigated by transmission electron microscopy (TEM). The results demonstrated predominant a -helix secondary structures of PBLG blocks for the resulted PHEMAChol-b-PBLG in solid state, and the longer PBLG blocks led to more well-defined alpha-helix structures, and PBLG secondary structures in solution could be tuned by their block lengths and the TFA volume fraction.
Access to high-quality, easily dispersible carbon quantum dots (CQDs) is essential in order to fully exploit their desirable properties. Copolymers based on N-acryloyl-D-glucosamine and acrylic acid prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization are self-assembled into micelle-like nanoreactors. After a facile graphitization process (170 °C, atmospheric pressure), each micellar template is transformed into a CQD through a 1:1 copy process. These high-quality CQDs (quantum yield=22 %) with tunable sizes (2–5 nm) are decorated by carboxylic acid moieties and can be spontaneously redispersed in water and polar organic solvents. This preparation method renders the mass production of multifunctional CQDs possible. To demonstrate the versatility of this approach, CQDs hybridized TiO2 nanoparticles with enhanced photocatalytic activity under visible-light have been prepared.
A supramolecular artificial glutathione peroxidase (PNIPAM-CD-g-Te) was prepared based on a supramolecular graft copolymer. PNIPAM-CD-g-Te was constructed by supramolecular host–guest self-assembly. Significantly, PNIPAM-CD-g-Te displayed noticeable temperature-dependent catalytic activity and typical saturation kinetics behavior. It was also proved that the change in the self-assembled structure of PNIPAM-CD-g-Te during the temperature-dependent process played a significant role in the temperature-dependent catalytic behavior. The construction of PNIPAM-CD-g-Te based on supramolecular graft copolymer endows artificial GPx with temperature-dependent catalytic ability, enriched catalytic centers, and homogeneously distributed catalytic centers. This work bodes well for the development of other biologically related host–guest supramolecular biomaterials.
Supramolecular chemistry enabling molecules and molecular complexes binding through non-covalent bonds allows nanomedicines to serve its desirable function to deliver drugs at the right time and the right place with minimal invasiveness. Supramolecular nanomedicine is the application of nanosupramolecules to the human health and disease and its main applications include diagnosis and therapy, drug and gene delivery, and tissue engineering. Nanoparticles with different structures obtained by assembling supraamphiphiles are promising candidates for multifunctional therapeutic platforms combining imaging and therapeutic capabilities. Encapsulation in supramolecular nanocarriers such as polymeric micelles, polymeric vesciles, layer-by-layer assembly, and porphysomes has the potential to deliver imaging and therapeutic drugs to the sites of action in the body. Hybrid supramolecular nanostructures of organic and inorganic molecules show promising potential in nanomedicine. Research is progressing towards rapid development on supramolecular nanotheranostic devices. Moreover, supramolecular nanoparticles exhibit low-toxicity, low-immunogenicity, non-pathogenicity, and in-vivo degradability.
The reversible addition-fragmentation chain-transfer (RAFT) synthesis of poly(glycidyl methacrylate) (PGMA) copolymers followed by the post-polymerisation treatment with morpholine allows the rational design of amphiphilic block copolymers that are capable of efficiently dispersing single walled carbon nanotubes (SWCNTs) in aqueous media, as shown by comparative photophysical characterization and AFM analysis.
Using a RAFT polymerization process, uniform polymer coats are physically adsorbed onto the surface of a SWNT and MWNT.
Well-defined poly(lauryl methacrylate-benzyl methacrylate) (PLMA-PBzMA) diblock copolymer nanoparticles are prepared in n-heptane at 90 °C via reversible addition–fragmentation chain transfer (RAFT) polymerization. Under these conditions, the PLMA macromolecular chain transfer agent (macro-CTA) is soluble in n-heptane, whereas the growing PBzMA block quickly becomes insoluble. Thus this dispersion polymerization formulation leads to polymerization-induced self-assembly (PISA). Using a relatively long PLMA macro-CTA with a mean degree of polymerization (DP) of 37 or higher leads to the formation of well-defined spherical nanoparticles of 41 to 139 nm diameter, depending on the DP targeted for the PBzMA block. In contrast, TEM studies confirm that using a relatively short PLMA macro-CTA (DP = 17) enables both worm-like and vesicular morphologies to be produced, in addition to the spherical phase. A detailed phase diagram has been elucidated for this more asymmetric diblock copolymer formulation, which ensures that each pure phase can be targeted reproducibly. 1H NMR spectroscopy confirmed that high BzMA monomer conversions (>97%) were achieved within 5 h, while GPC studies indicated that reasonably good blocking efficiencies and relatively low diblock copolymer polydispersities (Mw/Mn < 1.30) were obtained in most cases. Compared to prior literature reports, this all-methacrylic PISA formulation is particularly novel because: (i) it is the first time that higher order morphologies (e.g. worms and vesicles) have been accessed in non-polar solvents and (ii) such diblock copolymer nano-objects are expected to have potential boundary lubrication applications for engine oils.
It has become common to reference “pi-stacking” forces or “pi–pi interactions” when describing the interactions between neighbouring aromatic rings. Here, we review experimental and theoretical literature across several fields and conclude that the terms “pi-stacking” and “pi–pi interactions” do not accurately describe the forces that drive association between aromatic molecules of the types most commonly studied in chemistry or biology laboratories. We therefore propose that these terms are misleading and should no longer be used. Even without these terms, electrostatic considerations relating to polarized pi systems, as described by Hunter and Sanders, have provided a good qualitative starting place for predicting and understanding the interactions between aromatics for almost two decades. More recent work, however, is revealing that direct electrostatic interactions between polarized atoms of substituents as well as solvation/desolvation effects in strongly interacting solvents must also be considered and even dominate in many circumstances.
Soluble single-walled carbon nanotube (SWNT) salts have been synthesized with varying negative charges, and their saturation solubilities in DMSO measured. A thermodynamic model of the Gibbs free energy of dissolution, widely used for polyelectrolytes, reproduces the experimental results with striking agreement. SWNT salts appear as a paradigm of stiff polyelectrolytes.
The effect of sonication temperature on the debundling of carbon nanotube (CNT) macro-bundles is reported and demonstrated by analysis with different particle sizing methods. The change of bundle size over time and after several comparatively gentle sonication cycles of suspensions at various temperatures is reported. A novel technique is presented that produces a more homogeneous nanotube dispersion by lowering the temperature during sonication. We produce evidence that temperature influences the suspension stability, and that low temperatures are preferable to obtain better dispersion without increasing damage to the CNT walls.
We have made simulations of the system composed of single walled carbon nanotube (SWNT) surrounded by n=9, 13, 17 cholesterol (C27H46O) molecules. The mean square displacement, radial distribution function, diffusion coefficient, rotational and translational velocity correlations functions have been calculated for several temperatures. The qualitative interpretation of dynamics of cholesterol molecule in such specific environment is presented.
Metallofullerene nanotube peapods were prepared by introducing DyC82 into the interior space of single-walled carbon nanotubes. Transport measurements show that the DyC82 molecules function as electron donors and transfer charge to the carbon nanotube host. The amount of charge transferred varies with the temperature. At room temperature, the doped nanotube shows p-type behavior as seen from the response to a back gate. As the temperature decreases, the conductance becomes n type and at T<215 K metallic behavior is observed, indicating the degenerate state by doping. Below about 75 K, single-electron charging phenomena dominate the transport and show irregular Coulomb blockade oscillation, implying that the insertion of DyC82 splits the tube into a series of several quantum dots. © 2001 American Institute of Physics.
Stable dispersions of single-walled carbon nanotubes in deionized water were prepared using six common surfactants: sodium dodecylbenzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), lithium dodecyl sulfate (LDS), tetradecyl trimethyl ammonium bromide (TTAB), sodium cholate (SC), and Fairy liquid (FL). For all nanotube dispersions (C-NT = 1 mg/mL), the optimum concentration of surfactant was found to be close to C-Surf = 10 mg/mL by measuring the fraction of nanotubes remaining after centrifugation for a range of surfactant concentrations. The aggregation state of each nanotube-surfactant dispersion was characterized as a function of nanotube concentration by AFM analysis of large numbers of nanotubes/bundles deposited onto substrates. The dispersion quality could then be quantified by four parameters: the saturation value (at low concentration) of the root-mean-square bundle diameter, the maximum value of the total number of dispersed objects (individuals and bundles) per unit volume of dispersion, the saturation value (at low concentration) of the number fraction of individual nanotubes, and the maximum value of the number of individual nanotubes per unit volume of dispersion. According to these metrics, the dispersion quality of the six surfactant-nanotube dispersions varied as SDS > LDS > SDBS > TTAB > SC > Fairy liquid. It was found that each of these dispersion-quality metrics scaled very well with the measured zeta-potential of the surfactant-nanotube dispersion. This confirms that dispersion quality is controlled by the magnitude of electrostatic repulsive forces between coated nanotubes.
Electromechanical actuators based on sheets of single-walled carbon nanotubes were shown to generate higher stresses than natural muscle and higher strains than high-modulus ferroelectrics. Like natural muscles, the macroscopic actuators are assemblies of billions of individual nanoscale actuators. The actuation mechanism (quantum chemical-based expansion due to electrochemical double-layer charging) does not require ion intercalation, which limits the rife and rate of faradaic conducting polymer actuators. Unlike conventional ferroelectric actuators, low operating voltages of a few volts generate large actuator strains. Predictions based on measurements suggest that actuators using optimized nanotube sheets may eventually provide substantially higher work densities per cycle than any previously known technology. Electromechanical actuators based on sheets of single-walled carbon nanotubes were shown to generate higher stresses than natural muscle and higher strains than high-modulus ferroelectrics. Like natural muscles, the macroscopic actuators are assemblies of billions of individual nanoscale actuators. The actuation mechanism (quantum chemical-based expansion due to electrochemical double-layer charging) does not require ion intercalation, which limits the life and rate of faradaic conducting polymer actuators. Unlike conventional ferroelectric actuators, low operating voltages of a few volts generate large actuator strains. Predictions based on measurements suggest that actuators using optimized nanotube sheets may eventually provide substantially higher work densities per cycle than any previously known technology.
Through chemical functionalization of single-walled carbon nanotubes, the prerequisites for possible applications of such nanostructures are established. The derivatized tubes differ from the crude materials in their good solubility, which enables both a more extensive characterization and subsequent chemical reactivity. Current derivatization methods include defect and covalent sidewall functionalization, as well as noncovalent exo- and endohedral functionalization. In this way, for example, a range of nanotubes can be prepared: with sidewall substituents, wrapped with polymers, or with guest molecules included. The current state of the literature is presented in this Minireview.
Carbon nanotubes (CNTs) are extensively explored in materials science due to their unique structure and consequent mystical properties. CNTs are enjoying increasing popularity as building blocks for novel drug delivery systems as well as for bioimaging and biosensing. The recent strategies to functionalize CNTs have resulted in the generation of biocompatible and water-soluble CNTs that are well suited for high treatment efficacy and minimum side effects for future cancer therapies with low drug doses. This review covers the latest advances in the strategies for the modification of CNTs (with inorganic nanoparticles, small organic molecules, polymers, or bioactive materials), with an emphasis on the development of functional biological nanointerfaces as drug vehicles, after a simple introduction of the toxicity of CNTs. The translation of these systems into clinical practice and an outlook into future approaches are also discussed.
Liquid crystal ordering is an opportunity to develop novel materials and applications with carbon nanotubes spontaneously aligned on macroscopic scales. Nevertheless, achievement of large orientational order parameter and extended monodomains remains challenging. In this work, we show that shortening nanotubes allows the formation of liquid crystals that can easily be oriented under the form of large macroscopic monodomains. The orientational order parameter of single-wall nanotube liquid crystals measured by polarized Raman spectroscopy at the isotropic-nematic transition exceeds by far the value reported in previous experiments. The presently measured order parameter approaches the value theoretically expected for liquid crystals made of rigid rods in solution. This finding suggests that the production of highly ordered nanotube-based liquid crystals was presumably limited in earlier contributions by the length and waviness of long nanotubes. Both factors increase the material viscosity, can yield some elasticity, and stabilize topological defects.
Solutions of alkali metal salts of single-walled carbon nanotubes (SWNTs) show individualization of the nanotubes without sonication-induced shortening. Bundles are fully exfoliated and length measurements show mean length over one micron, a rare case for liquid formulation of SWNTs. These concentrated solutions of individualized nanotubes in organic solvents can be freeze dried leading to ultralight, highly porous, conducting and mechanically stable carbon nanotube cryogels.
In this work, successful polymer coating of COOH-functionalized multiwalled carbon nanotubes (MWCNTs) via reversible addition fragmentation chain transfer (RAFT) mediated emulsion polymerization is reported. The method used amphiphilic macro-RAFT copolymers as stabilizers for MWCNT dispersions, followed by their subsequent coating with poly(methyl methacrylate-co-butyl acrylate). Poly(allylamine hydrochloride) was initially used to change the charge on the surface of the MWCNTs to facilitate adsorption of negatively charged macro-RAFT copolymer onto their surface via electrostatic interactions. After polymerization, the resultant latex was found to contain uniform polymer-coated MWCNTs where polymer layer thickness could be controlled by the amount of monomer fed into the reaction. The polymer-coated MWCNTs were demonstrated to be dispersible in both polar and nonpolar solvents. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013
Adsorption isotherms of four different surfactants, sodium dodecyl sulfate (SDS), sodium dodecyl benzyl sulfonate, benzethonium chloride and Triton X-100 were measured on multi-wall carbon nanotubes (MWCNT) in water. With the surfactant SDS, the isotherms were also measured on single-wall carbon nanotubes (SWCNT) as well as on MWCNT under various ionic strength and temperature conditions. The nature of the polar head had only little influence on adsorption which was mainly driven by hydrophobic interactions. However, the outcome of the dispersion experiment was dependent on the purity of the carbon nanotubes. Using these results, it was possible to prepare concentrated colloidaly stable dispersions of MWCNTs in water (c = 32 g/L). Conducting MWCNT/polymer composite films could then readily be prepared by simple formulation of the MWCNTs with a polymeric dispersion.
The goal of this article is to provide an updated and in-depth review of some of the most exciting and important developments in the processing and properties of carbon nanotubes. Nanotubes can be formed in various structures using several different processing methods. The synthesis methods used to produce specific kinds of nanotubes are discussed and a comparison is made between the methods used by researchers and industrial producers. This is followed by an overview and discussion of what makes carbon nanotubes interesting to so many: their mechanical, chemical, electrical, thermal, and optical properties. The article ends with a discussion of the future outlook for the study of carbon nanotubes.
We fabricated a piezoresistive composite using multi-walled carbon nanotubes (MWCNTs) as a conductive filler and polydimethylsiloxane (PDMS) as a polymer matrix, which operated in the extremely small pressure range required for finger-sensing. To achieve a homogeneous dispersion of MWCNTs in PDMS, the MWCNTs were modified by a polymer wrapping method using poly(3-hexylthiophene) (P3HT). The percolation threshold of the composites was significantly lowered by the presence of P3HT. The electrical conductivity and piezoresistive sensitivity of the composite were found to strongly depend on the P3HT concentration. The well-dispersed P3HT-MWCNT/PDMS composite showed good piezoresistive characteristics in the 0–0.12MPa pressure range.
The chemistry of carbon nanotubes has become an area of intense research as chemical derivatization is the only means for modifying the properties of these highly interesting and technologically promising materials. Specifically, numerous researchers have focused on improving the solubility of carbon nanotubes through chemical grafting. To this end, significant recent effort has been devoted to the attachment of polymers to the nanotube surface, as macromolecules can be more effective in modifying nanotube solubility properties than small molecules. In addition, the use of functional polymers has enabled the preparation of polymer‐nanotube composite materials that demonstrate a variety of interesting properties, such as responsiveness to environmental stimuli (solvent, temperature, pH), the ability to complex metal ions, and photoinduced electron transport. A variety of different techniques have now been developed for the functionalization of carbon nanotubes with polymers, including “grafting to”, “grafting from”, and supramolecular interactions. This review will focus on recent developments in the use of living radical polymerization methods for the functionalization of carbon nanotubes with well‐defined polymers.
The solubility of single-wall carbon nanotubes (SWCNTs) in solvents is a critical factor for the potential applications of SWCNTs. Recently, it has been reported that SWCNTs are relatively well-dispersed in organic amide solvents, such as, dimethylformamide (DMF). However, it is still a challenge to disperse and to stabilize SWCNTs in solvents for further chemical reactions. In this research, α-terpineol and Texanol were used as solvents to disperse SWCNTs. The solubility of SWCNTs in solvents was investigated by observing the sedimentation and the aggregation of SWCNTs with a digital camera, a optical microscope (OM), and a UV/vis spectroscope. Experimental results demonstrated the improved solubility of SWCNTs in α-terpineol and Texanol. SWCNTs suspensions were better dispersed and longer stabilized in α-terpineol and Texanol than in DMF after dispersing SWCNTs by 15min sonification at mild conditions. The enhanced solubility of SWCNTs in α-terpineol and Texanol may be caused by the chemical structures, such as, Lewis basicity, the branch and the ring structure, and the high viscosity.
Carbon nanotubes (CNT) are being presented as medical devices at an increasing rate. To date, they have been suggested as targets for the thermal ablation of cancers, as delivery systems for pharmaceuticals, and as bio-sensors. A common thread amongst these applications is that CNTs are used as a delivery vector for some pharmaceutical into the body. We consider here the possibility that CNTs might be used as a device to trap and remove chemicals, particularly cholesterol, from a living organism. We have performed ab-initio calculations to determine how cholesterol might interact with CNTs placed inside the body. We have found that cholesterol exhibits no particular affinity for or effect on a bare CNT; however, its binding energy can be increased by functionalizing the CNT with a Ca adatom. We found that a Ca adatom on the wall of a CNT increases the binding energy of cholesterol to a CNT by around 1.5 eV, regardless of the nanotube’s diameter. The presence of the cholesterol does not affect the band structure of the CNT, but the Ca atom does have an effect near the Fermi level. This indicates that a CNT based detector could function by detecting the alteration to the electronic structure caused by the induced adsorption of an adatom in the trinary system of CNT + cholesterol + adatom.
Previously, reduced single-walled carbon nanotube anions have been used for effective processing and functionalization. Here we report individually separate and distinct (that is, discrete) single-walled carbon nanotube cations, directly generated from a pure anode using a non-aqueous electrochemical technique. Cyclic voltammetry provides evidence for the reversibility of this nanoion electrochemisty, and can be related to the complex electronic density of states of the single-walled carbon nanotubes. Fixed potentiostatic oxidation allows spontaneous dissolution of nanotube cations ('nanotubium'); Raman spectroscopy and transmission electron microscopy show that sequential fractions are purified, separating amorphous carbon and short, defective single-walled carbon nanotubes, initially. The preparation of nanotubium, in principle, enables a new family of nucleophilic grafting reactions for single-walled carbon nanotubes, exploited here, to assemble nanotubes on amine-modified Si surfaces. Other nanoparticle polyelectrolyte cations may be anticipated.
We present a combined experimental and theoretical quantification of the adsorption enthalpies of seven organic molecules (acetone, acetonitrile, dichloromethane, ethanol, ethylacetate, hexane, and toluene) on graphene. Adsorption enthalpies were measured by inverse gas chromatography and ranged from -5.9 kcal/mol for dichloromethane to -13.5 kcal/mol for toluene. The strength of interaction between graphene and the organic molecules was estimated by density functional theory (PBE, B97D, M06-2X, optB88-vdW), wave-function theory (MP2, SCS(MI)-MP2, MP2.5, MP2.X and CCSD(T)) and empirical calculations (OPLS-AA) using two graphene models - coronene and infinite graphene. Symmetry adapted perturbation theory (SAPT) calculations indicated that the interactions were governed by London dispersive forces (amounting to ~60% of attractive interactions), even for the polar molecules. The results also showed that the adsorption enthalpies were largely controlled by the interaction energy. Adsorption enthalpies obtained from ab initio molecular dynamics employing non-local optB88-vdW functional were in excellent agreement with the experimental data, indicating that the functional can cover physical phenomena behind adsorption of organic molecules on graphene sufficiently well.
Organic photovoltaic devices based on the bulk heterojunction concept, containing a blend of single-wall carbon nanotubes (SWNTs) and soluble polythiophenes (P3OT) were studied. The open circuit voltage V-oc of the devices was found to be 0.75 V, which is larger than the theoretical limit calculated by the metal-insulator-metal (MIM) model. In order to investigate the origin of this unusually high V-oc, we have prepared P3OT-SWNT based devices with different metal negative electrodes. The V-oc measured is only very weakly dependent on the work function of the metal, suggesting that the MIM model does not apply in this case. From the analysis of the current-voltage characteristics and electron microscopy imaging of the composite structure, it is proposed that the photovoltaic response of these devices is based on the introduction of internal polymer/nanotube junctions within the polymer matrix, which due to a photoinduced electron transfer from the polymer to the nanotube contribute to enhanced charge separation and collection. The data suggest that the negligible influence of the negative electrode work function on V-oc can be explained by the metal negative electrode forming ohmic contacts to the nanotube percolation paths. (C) 2003 American Institute of Physics. [DOI: 10.1063/1.1535231].
In this article, the synthesis of well-defined, narrow polydispersity (PDI < 1.2) star polymersvia RAFT polymerisation is detailed. An ‘arm-first’ method is described using a crosslinked nanogel core. The synthetic conditions and variables were thoroughly investigated and optimised so that nearly quantitative arm incorporation was found to occur. The parameters studied included polymerization time, the arm molecular weight, and the nature of the solvent and cross-linker.
We report a simple process to solubilize high weight fraction single-wall carbon nanotubes in water by the nonspecific physical adsorption of sodium dodecylbenzene sulfonate. The diameter distribution of nanotubes in the dispersion, measured by atomic force microscopy, showed that even at 20 mg/mL 63 ± 5% of single-wall carbon nanotube bundles exfoliated into single tubes. A measure of the length distribution of the nanotubes showed that our dispersion technique reduced nanotube fragmentation.
Single-wall carbon nanotubes pack into crystalline ropes that aggregate into tangled networks due to strong van der Waals attraction. Aggregation acts as an obstacle to most applications, and diminishes the special properties of the individual tubes. We describe a simple procedure for dispersing as-produced nanotubes powder in aqueous solutions of Gum Arabic. In a single step, a stable dispersion of full-length, well separated, individual tubes is formed, apparently due to physical adsorption of the polymer.
A study was conducted to explore soluble, discrete supramolecular complexes of single-walled carbon nanotubes with fluorene-based conjugated polymers, to be used in thin-film transistors, LEDs, and photovoltaic devices. The complexes, if properly modified, can exhibit high degrees of solubility and excellent conductivity properties, making them potentially applicable for printed electronics, supercapacitors, LEDs, and photovoltaic cells. The poly(9,9-dialkylfluorene) (PF) and poly(9,9-dialkylfluorene-co-3-alkylthiophene) (PFT) polymers were synthesized by the Suzuki polycondensation from diborate and the corresponding dibromides. A dynamic equilibrium exists between bound and unbound polymer and the presence of excess polymer favors the formation of the soluble polymer-nanotube complexes. the supramolecular interaction of the polymers with SWNTs was investigated by UV/vis absorption spectroscopy.
The crystalline structures associated with melting and crystallization behaviors of monodisperse linear polyethylene confined in cylindrical nanopores were investigated by X-ray diffraction and differential scanning calorimetry. The crystalline structures, melting, and crystallization behaviors of PE under the imposed cylindrical confinement were noticeably different from those of the bulk state. The isothermal crystallization experiments showed that the overall crystallization of polyethylene in cylindrical nanopores was dominated by the nucleation rather than the growth of crystallites. The c- and a-axes of orthorhombic PE crystals developed in nanoporous alumina were preferentially oriented perpendicular to the long axis of cylindrical nanopore while the b-axis was parallel to the pore axis. The melting temperature of polyethylene in the nanopores was substantially depressed, and it was analyzed with the Thomson−Gibbs equation. The crystallinity of linear polyethylene in cylindrical nanopores was less than 50%, whereas the bulk value was 71.6%.
Reversible addition−fragmentation chain transfer (RAFT) controlled radical polymerization was employed, for the first time, to prepare well-defined (model) amphiphilic polymer co-networks based on n-butyl methacrylate (BuMA, hydrophobic monomer) and 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic ionizable monomer) cross-linked with ethylene glycol dimethacrylate (EGDMA) and bearing elastic chains having the following comonomer distributions: BuMA-b-DMAEMA-b-BuMA and DMAEMA-b-BuMA-b-DMAEMA triblock and BuMA-co-DMAEMA statistical copolymers. Two randomly cross-linked (not model) amphiphilic co-networks were also synthesized, the one by RAFT and the other by conventional free radical polymerization. The amphiphilic triblock copolymer-based model co-networks were obtained by stepwise synthesis in three stages: (1) the synthesis of linear homopolymers bearing two active ends by using a bifunctional chain transfer agent (CTA), (2) the sequential addition of the second monomer on the macro-CTAs (homopolymers) to yield linear ABA or BAB triblock copolymers, and (3) the inter-linking of the linear copolymer chains at both ends using EGDMA cross-linker to form the final model co-networks. The homopolymer and copolymer precursors to the networks were characterized using gel permeation chromatography and 1H NMR spectroscopy. The swelling behavior of the networks was investigated in tetrahydrofuran and water. The co-networks swelled more in acidic than in neutral water due to the ionization of their DMAEMA units. The low pH aqueous swelling of the statistical co-network was higher than those of its triblock counterparts due to the lack of microphase separation with the statistical copolymer chains.
Azobenzene-derived photoactive polymers (P1−P3) containing pyrene pendants were designed and synthesized (Mw 30 000) for the noncovalent functionalization of single-walled carbon nanotubes (SWNTs). P1−P3 were found to be highly effective for the solubilization of SWNTs in common organic solvents, resulting in hybrid materials with enhanced thermal stability. The solubilization process was mostly driven by the π−π stacking interactions of pyrene with SWNTs. It also brings the azobenzene chromophores to the vicinity of nanotube surface, thereby allowing the electronic interactions between them. In addition to that, stacking of the pyrene and subsequent wrapping of the polymer around CNT surface provides more volume for the photoisomerization of azobenzene. These effects eventually accelerate the kinetics of photoisomerization of azobenzene in the polymer−SWNT composite. The photoalignment property of the composite was also increased when compared to that of the parent polymer which was studied by means of photoinduced birefringence.
Stable dispersions of both as-produced (raw soot) and purified laser-generated single-wall carbon nanotubes (SWNTs) have been demonstrated with several alkyl amide solvents. Optical absorption analysis over a range of concentrations has been utilized to estimate the dispersion limits for as-produced SWNTs in N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N,N-diethylacetamide (DEA), and N,N-dimethylpropanamide (DMP). In addition, extinction coefficients have been calculated using Beer's law for each solvent at energies of 1.27 and 1.77 eV, corresponding to the electronic transitions of semiconducting and metallic SWNTs, respectively. The results imply that high polarizability and optimal geometries (appropriate bond lengths and bond angles) may account for the favorable interaction between SWNTs and the alkyl amide solvents. The successful dispersion of purified SWNTs in DMA has enabled extinction coefficients of 43.4 and 39.0 mL·mg-1·cm-1 to be calculated at the selected energies, respectively. The magnitude of the dispersion limit and extinction coefficient values has been shown to be strongly dependent on the SWNT sample purity. These findings offer the potential for solution-phase analysis of SWNTs directed at purity assessment and electrophoretic separations in a simple organic solvent.
A systematic study has been performed in order to find an appropriate medium for solubilization/dispersion of pristine single-walled carbon nanotubes (SWCNTs). Five solvents, all featuring high electron pair donicity (β) and low hydrogen bond parameter (α) have demonstrated the ability to readily form stable dispersions. The best dispersions have been characterized by UV/visible-NIR spectra, ESR spectra, and atomic force microscopy (AFM).
We present a new hybrid meta exchange-correlation functional, called M05-2X, for thermochemistry, thermochemical kinetics, and noncovalent interactions. We also provide a full discussion of the new M05 functional, previously presented in a short communication. The M05 functional was parametrized including both metals and nonmetals, whereas M05-2X is a high-nonlocality functional with double the amount of nonlocal exchange (2X) that is parametrized only for nonmetals. In particular, M05 was parametrized against 35 data values, and M05-2X is parametrized against 34 data values. Both functionals, along with 28 other functionals, have been comparatively assessed against 234 data values: the MGAE109/3 main-group atomization energy database, the IP13/3 ionization potential database, the EA13/3 electron affinity database, the HTBH38/4 database of barrier height for hydrogen-transfer reactions, five noncovalent databases, two databases involving metal−metal and metal−ligand bond energies, a dipole moment database, a database of four alkyl bond dissociation energies of alkanes and ethers, and three total energies of one-electron systems. We also tested the new functionals and 12 others for eight hydrogen-bonding and stacking interaction energies in nucleobase pairs, and we tested M05 and M05-2X and 19 other functionals for the geometry, dipole moment, and binding energy of HCN−BF3, which has recently been shown to be a very difficult case for density functional theory. We tested eight functionals for four more alkyl bond dissociation energies, and we tested 12 functionals for several additional bond energies with varying amounts of multireference character. On the basis of all the results for 256 data values in 18 databases in the present study, we recommend M05-2X, M05, PW6B95, PWB6K, and MPWB1K for general-purpose applications in thermochemistry, kinetics, and noncovalent interactions involving nonmetals and we recommend M05 for studies involving both metallic and nonmetallic elements. The M05 functional, essentially uniquely among the functionals with broad applicability to chemistry, also performs well not only for main-group thermochemistry and radical reaction barrier heights but also for transition-metal−transition-metal interactions. The M05-2X functional has the best performance for thermochemical kinetics, noncovalent interactions (especially weak interaction, hydrogen bonding, π···π stacking, and interactions energies of nucleobases), and alkyl bond dissociation energies and the best composite results for energetics, excluding metals.
Pyrene-containing polymers have been prepared for noncovalent sidewall functionalization of multiwalled carbon nanotubes (MWNTs). (1-Pyrene)methyl 2-methyl-2-propenoate (PyMMP) has been synthesized and copolymerized with methyl methacrylate (MMA). Poly(ethylene-co-butylene)-b-poly(MMA-co-PyMMP) diblocks have also been synthesized. The surface of MWNTs, produced by both the CCVD and arc discharge methods, has been modified by these copolymers for making them dispersible in a variety of organic solvents. The modified MWNTs have been characterized by thermogravimetric analysis, transmission electron microscopy, and atomic force microscopy.
Durch chemische Funktionalisierung einwandiger Kohlenstoffnanoröhren werden die Voraussetzungen für mögliche Anwendungen solcher Nanostrukturen geschaffen. Derivatisierte Röhren zeichnen sich im Unterschied zum Rohmaterial u. a. durch eine gute Löslichkeit aus und sind so einer umfangreichen Charakterisierung und Folgechemie zugänglich. Gängige Derivatisierungsmethoden umfassen die Defekt- und kovalente Seitenwandfunktionalisierung sowie die nichtkovalente exo- und endohedrale Funktionalisierung. Auf diese Weise wurde z. B. eine Reihe von seitenwandsubstituierten, polymerumwickelten oder gastmolekülhaltigen Nanoröhren erhalten. Der gegenwärtige Stand der Forschung wird in diesem Kurzaufsatz vorgestellt.
Microscale aggregate formation, resulting from high intrinsic filler attractions, is one of the major issues in nanocomposite preparation and processing. Herein, the dispersive effects achieved by a wide range of surface-active agents, as well as surface oxidation and functionalization, are investigated. The aim of our research is to form a uniform, multiwalled carbon nanotube (MWNT) distribution in water-soluble (poly(ethylene glycol)) and water-insoluble (polypropylene) polymers. In order to understand the surface-charge-related stability of the treated nanotubes solutions, zeta-potential measurements are applied. Quantification of the state of the MWNT dispersion is derived from particle-size analysis, while visual characterization is based on optical and electron microscopy. To estimate the nucleating ability of the surface-modified carbon nanotubes, the temperature of crystallization and the degree of crystallinity are calculated from differential scanning thermograms. Finally, we suggest general guidelines to produce uniform MWNT dispersions using a dispersive agent and/or surface treatment in water-soluble and water-insoluble polymers.
The effect of pyrene distribution within pyrene-functionalized random and block copolymers on noncovalent polymer/single-walled carbon nanotube (SWNT) interactions was investigated. The block copolymers served as superior solubilizing agents in comparison with the random copolymers. Also, increasing the pyrene content within a polymer, while a constant molecular weight was maintained, improved SWNT solubility and therefore had to result in stronger polymer–nanotube interactions. However, increasing the length of the pyrene-containing block diminished nanotube solubility, likely because of a lower number of polymer chains that were capable of binding to the nanotube surface. Atomic force microscopy and transmission electron microscopy indicated that the polymer–SWNT interactions were capable of partially debundling the nanotubes into individual solvated structures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1941–1951, 2006
A family of poly[(m-phenylenevinylene)-co-(p-phenylenevinylene)]s, functionalized in the synthetically accessible C-5 position of the meta-disubstituted phenylene rings have been designed and synthesized: they are essentially poly{(5-alkoxy-m-phenylenevinylene)-co-[(2,5-dioctyloxy-p-phenylene)-vinylene]} (PAmPV) derivatives. A range of these PAmPV polymers have been prepared both (1) by the polymerization of O-substituted 5-hydroxyisophthaldehydes and (2) by chemical modifications carried out on polymers bearing reactive groups at the C-5 positions. PAmPV polymers solubilize SWNT bundles in organic solvents by wrapping themselves around the nanotube bundles. PAmPV derivatives which bear tethers or rings form pseudorotaxanes with rings and threads, respectively. The formation of the polypseudorotaxanes has been investigated in solution by NMR and UV/vis spectroscopies, as well as on silicon oxide wafers in the presence of SWNTs by AFM and surface potential microscopy. Wrapping of these functionalized PAmPV polymers around SWNTs results in the grafting of pseudorotaxanes along the walls of the nanotubes in a periodic fashion. The results hold out the prospect of being able to construct arrays of molecular switches and actuators.
The incorporation of carbon nanotubes to a polymer generally improves the stiffness and strength of the polymer, but the ductility and toughness of the polymer are compromised in most cases. Here we report mechanical reinforcement of polyethlene (PE) using polyethlene grafted multiwalled carbon nanotubes (PE-g-MWNTs). The stiffness, strength, ductility and toughness of PE are all improved by the addition of PE-g-MWNTs. The grafting of PE onto MWNTs enables the well-dispersion of nanotubes in the PE matrix and improves MWNT/PE interfacial adhesion. The grafting was achieved by a reactive blending process through melt blending of PE containing 0.85 wt % of maleic anhydride and amine-functionalized MWNTs. The reaction between maleic anhydride and amine groups as evidenced by X-ray photoelectron spectroscopy and Raman spectroscopy, leads to the grafting of PE onto the nanotubes.
We have investigated a wide variety of surfactants for their efficiency in dissolving isolated single-walled carbon nanotubes (SWNTs) in water. In doing so, we have completely avoided the harsh chemical or mechanical conditions, such as acid or ultrasonic treatments, that are known to damage SWNTs. Bile salts in particular are found to be exceptionally effective in dissolving individual tubes, as evidenced by highly resolved optical absorption spectra, bright bandgap fluorescence, and the unprecedented resolution (similar to2.5 cm(-1)) of the radial breathing modes in Raman spectra. This is attributed to the formation of very regular and stable micelles around the nanotubes providing an unusually homogenous environment. Quantitative information concerning the degree of solubilization is obtained from absorption spectroscopy.
Intimate electrical contact occurs between a substituted poly(metaphenylenevinylene) (PmPv) and bundles of single-walled nanotubes (SWNT) as evidenced by atomic force microscopy, optical, and electronic measurements carried out on single, isolated SWNT/PmPv structures (see picture). PmPV may provide a useful route toward "functionalizing" the SWNT without destroying their electrical character.
We review experimental and theoretical work on electrical percolation of carbon nanotubes (CNT) in polymer composites. We give a comprehensive survey of published data together with an attempt of systematization. Parameters like CNT type, synthesis method, treatment and dimensionality as well as polymer type and dispersion method are evaluated with respect to their impact on percolation threshold, scaling law exponent and maximum conductivity of the composite. Validity as well as limitations of commonly used statistical percolation theories are discussed, in particular with respect to the recently reported existence of a lower kinetic (allowing for re-aggregation) and a higher statistical percolation threshold.
Suitable oxidative treatment of catalytically grown carbon nanotubes introduces oxygen containing surface groups. Infrared and titration studies indicate that they are predominately phenolic, carboxylic and lactonic groups. These groups stabilize dispersions of nanotubes at much higher concentrations than are possible with the raw material. Plots of the viscosity of dispersions as a function of their concentration show a dramatic increase in gradient above a critical concentration, leading to the formation of viscoelastic gels. During continued drying, the solvent mediates the formation of dense assemblies of nanotubes which then bond together through the surface groups. If the nanotubes are deposited from a dilute suspension by filtration they are able to maximize the number of intertube contacts by packing into a locally parallelized structure, reminiscent of liquid crystalline polymers.
The preparation of a new type of finite carbon structure consisting of
needlelike tubes is reported. Produced using an arc-discharge
evaporation method similar to that used for fullerene sythesis, the
needles grow at the negative end of the electrode used for the arc
discharge. Electron microscopy reveals that each needle comprises
coaxial tubes of graphitic sheets ranging in number from two up to about
50. On each tube the carbon-atom hexagons are arranged in a helical
fashion about the needle axis. The helical pitch varies from needle to
needle and from tube to tube within a single needle. It appears that
this helical structure may aid the growth process. The formation of
these needles, ranging from a few to a few tens of nanometers in
diameter, suggests that engineering of carbon structures should be
possible on scales considerably greater than those relevant to the
fullerenes.
The influence of a single walled carbon nanotube on the structure of a
cholesterol cluster (domain) developed over the surface of the endothelial
protein 1LQV has been investigated using the classical molecular dynamics (MD)
simulation technique. We have observed a substantial impact of carbon nanotube
on the arrangement of the cholesterol domain. The carbon nanotube can drag out
cholesterol molecules, remarkable reducing the volume of the domain settled
down on the protein.
Dispersions of multi-walled carbon nanotubes (MWNTs) assisted by surfactant adsorption were prepared for a number of ionic and non-ionic surfactants including sodium 4-dodecylbenzenesulfonate (NaDDBS), hexadecyl(trimethyl)azanium bromide (CTAB), sodium dodecane-1-sulfonate (SDS), Pluronic® F68, Pluronic® F127, and Triton® X-100 to examine the effects of nanotube diameter, surfactant concentration, and pH on nanotube dispersability. Nanotube diameter was found to be an important role in surfactant adsorption rendering single-walled carbon nanotube studies as unreliable in predicting MWNT dispersive behavior. Similar to other reports, increasing surfactant concentrations resulted in a solubility plateau. Quantification of nanotube solubility at these plateaus demonstrated that CTAB is the best surfactant for MWNTs at neutral pH conditions. Deviations from neutral pH demonstrated negligible influence on non-ionic surfactant adsorption. In contrast, both cationic and anionic surfactants were found to be poor dispersing aids for highly acidic solutions while, CTAB remained a good surfactant under strongly basic conditions. These pH dependent results were explained in the context of nanotube surface ionization and Debye length variation.
A method is developed to enable emulsion polymerization to be performed under RAFT control to give living character without the problems that often affect such systems: formation of an oily layer, loss of colloidal stability, or loss of molecular weight control. Trithiocarbonate RAFT agents are used to form short stabilizing blocks from a water-soluble monomer, from which diblocks can be created by the subsequent polymerization of a hydrophobic monomer. These diblocks are designed to self-assemble to form micelles. Polymerization is initially performed under conditions that avoid the presence of monomer droplets during the particle formation stage and until the hydrophobic ends of the diblocks have become sufficiently long to prevent them from desorbing from the newly formed particles. Polymerization is then continued at any desired feed rate and composition of monomer. The polymer forming in the reaction remains under RAFT control throughout the polymerization; molecular weight polydispersities are generally low. The number of RAFT-ended chains within a particle is much larger than the aggregation number at which the original micelles would have self-assembled, implying that in the early stages of the polymerization, there is aggregation of the micelles and/or migration of the diblocks. The latexes resulting from this approach are stabilized by anchored blocks of the hydrophilic monomer, e.g., acrylic acid, with no labile surfactant present. Sequential polymerization of two hydrophobic monomers gives completely novel core-shell particles where most chains extend from the core of the particles through the shell layer to the surface.
A review on the controlled/living radical polymerization (CLRP) in dispersed systems is given so as to show its use from 2007 up to early 2008. This includes: the mechanisms of CLRP; CLRP in dispersed systems; the cross-linking of CLRP in dispersed systems; and lastly its particle morphology. Regarding the CLRP in dispersed systems, the following were also discussed. The colloidal stability, the theoretical aspects of CLRP in dispersed systems, the miniemulsion polymerization as well as its emulsion polymerization, the miscellaneous miniemulsion and emulsion CLRP, the microemulsion polymerization, and finally the dispersion and precipitation polymerizations.
Radical polymerization is one of the most widely used processes for the commercial production of high-molecular-weight polymers. The main factors responsible for the preeminent position of radical polymerization are the ability to polymerize a wide array of monomers, tolerance of unprotected functionality in monomer and solvent, and compatibility with a variety of reaction conditions. Radical polymerization is simple to implement and inexpensive in relation to competitive technologies. However, conventional radical polymerization severely limits the degree of control that researchers can assert over molecular-weight distribution, copolymer composition, and macromolecular architecture.