DNA-assisted dispersion and separation of carbon nanotubes

Carbon nanotubes are man-made one-dimensional carbon crystals with different diameters and chiralities. Owing to their superb mechanical and electrical properties, many potential applications have been proposed for them. However, polydispersity and poor solubility in both aqueous and non-aqueous solution impose a considerable challenge for their separation and assembly, which is required for many applications. Here we report our finding of DNA-assisted dispersion and separation of carbon nanotubes. Bundled single-walled carbon nanotubes are effectively dispersed in water by their sonication in the presence of single-stranded DNA (ssDNA). Optical absorption and fluorescence spectroscopy and atomic force microscopy measurements provide evidence for individually dispersed carbon nanotubes. Molecular modelling suggests that ssDNA can bind to carbon nanotubes through pi-stacking, resulting in helical wrapping to the surface. The binding free energy of ssDNA to carbon nanotubes rivals that of two nanotubes for each other. We also demonstrate that DNA-coated carbon nanotubes can be separated into fractions with different electronic structures by ion-exchange chromatography. This finding links one of the central molecules in biology to a technologically very important nanomaterial, and opens the door to carbon-nanotube-based applications in biotechnology.     

Spectral density matrix description of polarization mode dispersion

We introduce a power spectral density matrix formalism that incorporates both the pulse shape and the field polarization and can therefore easily describe averages over random fluctuations of the local birefringence vector. We demonstrate that quantities such as the differential time delay, power diffusion, and decoherence effects can be obtained directly from the equations of motion for the power density matrix. This approach can be applied to pulses with arbitrary frequency-dependent polarization and intensity distributions and in particular makes possible the minimization of the eye-opening penalty through the proper choice of the initial pulse profile.     

Evolution of dispersion of carbon nanotubes in Polyamide 11 matrix composites as determined by DC conductivity

Double-walled Carbon NanoTubes (DWCNTs) have been dispersed in a Polyamide 11 (PA11) matrix by two routes: in the solvent way, Polyamide 11 was first dissolved in its solvent to ensure a liquid state dispersion of carbon nanotubes by ultrasonic way; in the melt mixing way, an optimization of the extrusion parameters, such as mixing time, mixing speed, mixing temperature and screw rotation direction allow to reach satisfactory dispersion. Dispersion and percolation threshold have been compared thanks to the evolution of DC conductivity with carbon nanotubes weight fraction in Polyamide 11.     

Quantifying the dispersion of carbon nanotubes in thermoplastic-toughened epoxy polymers

The distribution of particles within modern materials must be defined to understand the change in properties attained by their addition. Two methods of analysis, which use different size scales, are presented here. These methods are applied to characterise the dispersion of multi-walled carbon nanotubes in a thermoplastic-toughened epoxy polymer. First, the greyscale method uses transmission optical micrographs, and calculates the ratio of the variance/mean of the greyscale values. Higher values indicate a greater degree of clustering; lower values may be described as showing a ‘better’ distribution of nanotubes, hence allowing the results to be ranked. This method is relatively easier to carry out, but care must be taken to use a consistent small thickness of sample. Secondly, the quadrat analysis uses transmission electron micrographs of the same materials, after identifying the centre of each nanotube observed. This defines the distribution on the scale of the nanotubes. Peaks in the relationship between the ratio of the variance/mean and cell size are related to microstructural features such as agglomeration. This scale is expected to be related to the scale of microstructural deformation mechanisms which determine global material properties.     

Ceramic matrix composites containing carbon nanotubes

Due to the remarkable physical and mechanical properties of individual, perfect carbon nanotubes (CNTs), they are considered to be one of the most promising new reinforcements for structural composites. Their impressive electrical and thermal properties also suggest opportunities for multifunctional applications. In the context of inorganic matrix composites, researchers have particularly focussed on CNTs as toughening elements to overcome the intrinsic brittleness of the ceramic or glass material. Although there are now a number of studies published in the literature, these inorganic systems have received much less attention than CNT/polymer matrix composites. This paper reviews the current status of the research and development of CNT-loaded ceramic matrix composite (CMC) materials. It includes a summary of the key issues related to the optimisation of CNT-based composites, with particular reference to brittle matrices and provides an overview of the processing techniques developed to optimise dispersion quality, interfaces, and density. The properties of the various composite systems are discussed, with an emphasis on toughness; a comprehensive comparative summary is provided, together with a discussion of the possible toughening mechanism that may operate. Last, a range of potential applications are discussed, concluding with a discussion of the scope for future developments in the field.     

Graphene oxides for homogeneous dispersion of carbon nanotubes

Graphene oxides (GOs) in terms of both structure and property are essentially polyelectrolytes in a two-dimensional sheet configuration. As is well-established in the literature, polyelectrolytes are, in general, good dispersion agents for single-walled carbon nanotubes (SWNTs), which are otherwise in bundles because of strong van der Waals interactions. We report here a study in which GOs were used to disperse SWNTs, both as-purified and separated semiconducting SWNTs, for solution-like homogeneous suspensions. As a demonstration for their potentials, the optically transparent dispersions were used in a more accurate determination of the absorptivities for the band-gap transitions in semiconducting SWNTs. Results on exploration of the use of the GO-dispersed SWNTs in the development of unique carbon nanocomposite materials are also presented and discussed.     

Non-adiabatic phonon dispersion of metallic single-walled carbon nanotubes

Non-adiabatic effects can considerably modify the phonon dispersion of low-dimensional metallic systems. Here, these effects are studied for the case of metallic single-walled carbon nanotubes using a perturbative approach within a density-functional-based non-orthogonal tight-binding model. The adiabatic phonon dispersion was found to have logarithmic Kohn anomalies at the Brillouin zone center and at two mirror points inside the zone. The obtained dynamic corrections to the adiabatic phonon dispersion essentially modify and shift the Kohn anomalies as exemplified in the case of nanotube (8, 5). Large corrections have the longitudinal optical phonon, which gives rise to the so-called G- band in the Raman spectra, and the carbon hexagon breathing phonon. The results obtained for the G- band for all nanotubes in the diameter range from 0.8 to 3.0 nm can be used for assignment of the high-frequency features in the Raman spectra of nanotube samples.     

Light-scattering and dispersion behavior of multiwalled carbon nanotubes

Elliptically polarized light-scattering measurements were performed to investigate the dispersion behavior of multiwalled carbon nanotubes (MWNT). Xylene- and pyridine-derived MWNT powders were dispersed in water and ethanol in separate optic cells and allowed to sit undisturbed over a two-week time period after probe sonication. Continuous light-scattering measurements taken between scattering angles of 10-170 deg and repeated over several days showed that the nanotubes formed fractal-like networks. The pyridine-derived MWNTs showed greater dispersion variation over time, tending to aggregate and clump much faster than the xylene-derived tubes. The water suspensions appeared much more stable than the ethanol suspensions, which transformed into nonfractal morphology after a few hours. We relate the dispersion stability to size and fringe patterns on the outer surface of the nanotubes. Measured values of fractal dimension were distinctly lower than those in previous studies of single-walled carbon nanotubes. Profiles of both diagonal and off-diagonal scattering matrix elements are presented.     

Chemical stability of carbon nanotubes in the 2024Al matrix

Five volume percent of carbon nanotubes and 2024Al alloy powder were mixed with ball milling method, and then the composite was fabricated at 873 K by hot pressing sintering technique. The microstructure of the composite was investigated using optical microscopy, transmission electron microscopy, and X-ray diffraction. The experimental results showed that carbon nanotubes are reacted and changed into Al₄C₃. Nano-Al₄C₃ phases with needle shape are distributed mainly on Al grain boundaries; meanwhile some of them exist within Al grains. The reaction mechanism of carbon nanotubes-Al is discussed.     

Monitoring dispersion of carbon nanotubes in a thermosetting polyester resin

The paper concerns the initial steps in the preparation of carbon nanotube containing nanocomposites of an isophthalic unsaturated polyester resin, prior to cure. Developments in the nature of the rheology of the liquid samples were monitored as a function of the level of energy introduced via ultrasonic horn mixing and related to microscopic observations. On-line sampling, coupled with off-line viscosity measurements, is compared with on-line measurements of electrical resistivity of the mixture, in terms of the relative suitability of these techniques for real-time monitoring of nanofiller dispersion in the liquid mixtures. The shear thinning parameter, N, derived from fitting Carreau model to the shear viscosity data, appears to provide a good qualitative indicator of the state of nanotube dispersion in the sample.     

Effect of dispersion method on the deterioration,interfacial interactions and re-agglomeration of carbon nanotubes in titanium metal matrix composites

The influence of various synthesis techniques on the dispersion and evolution of multi-walled carbon nanotubes (MWCNTs) in titanium (Ti) metal matrix composites (TMCs) prepared via powder metallurgy routes has been investigated. The synthesis techniques included sonication, high energy ball milling (HEBM), cold compaction, high temperature vacuum sintering and spark plasma sintering (SPS). Powder mixtures of Ti and MWCNTs (0.5 wt.%) were processed by HEBM in two batches: (i) ball milling of the mixtures (Batch 1) and (ii) ball milling of Ti powder alone, followed by a further ball milling with sonicated MWCNTs (Batch 2). Both batches of the powder mixtures were pressed at 40 MPa into green compacts and then sintered in vacuum. Batch 2 powder mixtures were also consolidated using SPS. The crystallinity and sp² carbon network of the MWCNTs were characterized through analyzing the characteristic Raman peak ratio (I_D/I_G) of each processed sample. X-ray diffraction (XRD) was used for phase identification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the morphology of the MWCNTs in the powder mixtures. The evolution of MWCNTs during the fabrication process and mechanical properties of the sintered compacts were discussed in conjunction with the formation of nano-crystalline TiC.     

A comparison of melt and solution mixing on the dispersion of carbon nanotubes in a poly(vinylidene fluoride) matrix

Multi-walled carbon nanotube (MWCNT)/poly(vinylidene fluoride) (PVDF) composites were prepared by two frequently-used approaches, melt and solution mixing. The dispersion state of MWCNTs in the PVDF matrix was evaluated by rheological properties and confirmed by optical microscope, scanning electron microscope and transmission electron microscope images. It was found that the dispersion state of MWCNTs was dependent not only on the concentration of MWCNTs but also the mixing method. Specifically, solution mixing led to a much better dispersion of MWCNTs than melt mixing at low concentrations of MWCNTs (less than 5 wt.%), while the situation was reversed at higher concentrations. The dispersion mechanisms of MWCNTs for the two mixing methods were also discussed.     

多壁碳纳米管的表面修饰及其在溶剂中的分散性

利用高温催化裂解生长多壁碳纳米管,用硝酸氧化使其表面羧酸化,并经酰氯化后与十二烷基胺反应形成表面酰胺化,通过红外、核磁、微量热天平等方法进行表征.结果表明:硝酸氧化后的碳纳米管在水等强极性溶剂中有良好的分散性;酰胺化后,十二烷基脂肪链使碳纳米管表面极性大为降低,因此在氟仿等弱极性溶剂中有良好的分散性.     

Surfactant assisted dispersion of functionalized multi-walled carbon nanotubes in aqueous media

The dispersibility of unfunctionalized and three differently functionalized multi-walled carbon nanotubes (MWNTs) in the presence of anionic, cationic, and non-ionic surfactants was investigated. Significant differences in their dispersibility were revealed by UV–vis spectroscopy of the dispersed MWNTs. X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, streaming potential measurements as well as pH measurements were conducted to characterize the MWNTs and their dispersions. Based on this information, possible interaction mechanisms between the surfactants and the differently functionalized MWNTs are proposed in order to account for the distinct dispersibility observed. Aqueous sizings containing low weight fractions of MWNTs were used for on-line modification of glass fibre (GF) sizings and preparation of GF/polypropylene composites, resulting in enhanced mechanical properties.     

Highly selective dispersion of single-walled carbon nanotubes using aromatic polymers

Solubilizing and purifying carbon nanotubes remains one of the foremost technological hurdles in their investigation and application. We report a dramatic improvement in the preparation of single-walled carbon nanotube solutions based on the ability of specific aromatic polymers to efficiently disperse certain nanotube species with a high degree of selectivity. Evidence of this is provided by optical absorbance and photoluminescence excitation spectra, which show suspensions corresponding to up to approximately 60% relative concentration of a single species of isolated nanotubes with fluorescence quantum yields of up to 1.5%. Different polymers show the ability to discriminate between nanotube species in terms of either diameter or chiral angle. Modelling suggests that rigid-backbone polymers form ordered molecular structures surrounding the nanotubes with n-fold symmetry determined by the tube diameter.     

多壁碳纳米管的表面功能化及分散性研究

多壁碳纳米管(MWCNTs)分别经混合、强酸氧化浸泡和酰氯化处理后,再与9,10-二氢-9-氧杂-10-磷杂菲-10-氧化物(DOPO)的衍生物(DHDOPO)进行接枝反应得到表面功能化的MWCNTs。利用傅里叶变换红外光谱仪(FT-IR)、透射电子显微镜(TEM)、扫描电子显微镜(SEM)、热重分析仪(TGA)、紫外-可见分光光度仪(UV-Vis)和沉降实验等分别表征改性前后MWCNTs的结构和表面形态,估算DHDOPO在MWCNTs表面的相对接枝率,研究改性前后MWCNTs在乙醇中的分散性。结果表明,MWCNTs经混合强酸氧化后表面出现羧基;DHDOPO在MWCNTs上的相对接枝率为51%;混合强酸氧化和表面接枝DHDOPO的MWCNTs在无水乙醇中具有良好的分散性。