Electrospinning of poly(trimethylene terephthalate)/carbon nanotube composites

Poly(trimethylene terephthalate) (PTT) nanocomposites containing carbon nanotubes (CNTs) with different surface structure and aspect ratio were prepared by melt compounding for electrospinning. The dispersion state of the CNTs in the composites was then examined utilizing rheology tools. The results show that carboxylic surface functionalized CNTs present better dispersion in the matrix than hydroxy surface functionalized CNTs because the former has stronger affinity to the PTT. Besides surface functionalization, the aspect ratio of CNTs is also vital to their final dispersion. The CNTs with lower aspect ratio are dispersed as individuals or small bundles while those with higher aspect ratio are dispersed mainly as flocs with large hydrodynamic radius, showing higher effective volume fraction. The presence of CNTs has a large influence on the morphologies of electrospun fiber and on the appearances of CNTs in the fibers. In the presence of CNTs with lower aspect ratio, continuous composite fibers are obtained. But the structure of those continuous fibers highly depends on the surface group of CNTs. Carboxylic surface functionalized CNTs are well embedded by the PTT and oriented along the fiber axis during electrospinning, leading to bead-free and uniform fiber morphology; while hydroxy surface functionalized CNTs show tortuous conformations with less orientation in the fibers, and as a result, the obtained fibers show beaded and misshaped morphologies. In the case of higher aspect ratio, however, the CNTs prefer to exist as entanglements or knots in the streamlines, and thereby only beaded or even uncontinuous fibers are obtained. Therefore, the formation and fiber morphology of PTT/CNT composite fibers obtained by electrospinning strongly depend on the surface functional groups of the CNTs, as well as on the CNT structure.     

Flexible supercapacitors based on carbon nanotubes

This review provides an overview of recent progress towards the development of flexible supercapacitors based on macroscopic carbon nanotubes-based electrodes, including one-dimensional (1D) fibers, 2D films, and 3D foams, with a focus on electrode preparation and configuration design as well as their integration with other multifunctional devices.     

DNA mediated assembly of single walled carbon nanotubes: role of DNA linkers and annealing

With the high demand for nanoelectronic devices, extensive research has focused on the use of single walled carbon nanotubes (CNTs) due to their high electron carrier mobility, large tensile strength, and single nanometer dimensions. Despite their promise, however, their applicability has been greatly hindered by the inherent difficulties of both separating nanotubes of different chiralities and diameters and positioning them from metallic tubes and positioning them in a precise location on a surface. In recent years, single stranded DNA (ssDNA) has been identified as a potential solution for both of these problems since DNA can be used to both separate the different types of CNTs as well as direct their organization. We demonstrate here the first principles on how to guide CNT assembly directly on surfaces from solution by specific DNA hybridization. It was found that the specific DNA sequence used to disperse the carbon nanotubes greatly influences the adsorption and specificity of nanotube binding to the surface. Furthermore, we demonstrate here that thermal annealing can correct misaligned tubes or incorrect binding. These studies provide an excellent foundation for employing two-dimensional DNA templates for CNT organization for nanoelectronic logic and memory based applications. Furthermore, using a single biomaterial to both sort and place CNTs in minimal steps would greatly help the throughput, manufacturability, and cost of such devices.     

Flow cytometry-based evaluation and enrichment of multiwalled carbon nanotube dispersions

The uniform aqueous dispersion of carbon nanotubes (CNTs) is a vital but challenging task required for their utilization in most technologies. We propose and demonstrate a technique based on forward- and side-scatter analysis on a flow cytometer to characterize the components in a dispersion of multiwalled CNTs (MWCNTs). The method simultaneously distinguishes various MWCNT components such as short and long CNTs, nanotube bundles, and particulates. It also detects the emergence of new CNT populations as a result of centrifugation. We use this method, together with classical methods such as UV and Raman spectroscopy, to observe and study the multistep MWCNT dispersion process in various surfactants (Pluronic, Triton X-100, sodium dodecyl sulfate, and cetyl trimethylammonium bromide). On the basis of the distinct scatter patterns obtained, we confirm and elaborate the surfactant-assisted unzipping mechanism of MWCNT dispersion. We also show that the ultrasonic energy spent after MWCNT unbundling and unwinding can be minimized and the process optimized for each surfactant by correct end point detection through scatter analysis. The ability to enrich nanotube population in dispersion by using the sorting mode of a flow cytometer is confirmed by electron microscopy and Raman spectroscopy. This method can thus be used for observing and enriching MWCNT components and as a complementary technique to UV spectroscopy for studying and optimizing MWCNT dispersion in surfactants.     

Effect of nanotube-length on the transport properties of single-file water molecules: transition from bidirectional to unidirectional

We use molecular dynamics (MD) simulations to study the transport of single-file water molecules through carbon nanotubes (CNTs) with various lengths in an electric field. Most importantly, we find that even the water dipoles inside the CNT are maintained along the field direction, a large amount of water molecules can still transport against the field direction for short CNTs, leading to a low unidirectional transport efficiency (η). As the CNT length increases, the efficiency η will increase remarkably, and achieves the maximum value of 1.0 at or exceeding a critical CNT length. Consequently, the transition from bidirectional to unidirectional transport is observed and is found to be relevant to thermal fluctuations of the two reservoirs, which is explored by the interaction between water molecules inside and outside the CNT. We also find that the water flow vs CNT length follows an exponential decay of f ～ exp?(- L/L(0)), and the average translocation time of individual water molecules yields to a power law of τ(trans) ～ L(υ), where L(0) and ν are constant and slightly depend on the field strength. We further compare our results with the continuous-time random-walk (CTRW) model and find that the water flow can also be described by a power law of f ～ L(-μ) modified from CTRW. Our results provide some new physical insights into the biased transport of single-file water molecules, which show the feasibility of using CNTs with any length to pump water in an electric field. The mechanism is important for designing efficient nanofluidic apparatuses.     

Spectroscopic evidence for pi-pi interaction between poly(diallyl dimethylammonium) chloride and multiwalled carbon nanotubes

The interaction between multiwalled carbon nanotubes (MWCNTs) and aqueous poly(diallyl dimethylammonium) chloride (PDDA) was studied by X-ray photoelectron (XPS) and photoacoustic Fourier transform infrared (PA-FTIR) spectroscopies. We have found that the mild sonication of MWCNTs in aqueous PDDA results in a significant improvement of CNT dispersibility and greatly enhances their adhesion to Au and Si substrates. The MWCNT-PDDA interaction is due to the presence of an unsaturated contaminant in the PDDA chain, as confirmed by both XPS and PA-FTIR, which enters into a pi-pi interaction with the CNTs. Electrostatic group repulsions of the coated CNTs then provide the dispersibility and adhesion.     

Critical aspects related to processing of carbon nanotube/unsaturated thermoset polyester nanocomposites

Carbon nanotubes (CNTs) have outstanding mechanical, thermal and electrical properties. As a result, particular interest has been recently given in exploiting these properties by incorporating carbon nanotubes into some form of matrix. Although unsaturated polyesters with styrene have widespread use in the industrial applications, surprisingly there is no study in the literature about CNT/thermoset polyester nanocomposite systems. In the present paper, we underline some important issues and limitations during the processing of unsaturated polyester resins with different types of carbon nanotubes. In that manner, 3-roll mill and sonication techniques were comparatively evaluated to process nanocomposites made of CNTs with and without amine (NH₂) functional groups and polyesters. It was found that styrene evaporation from the polyester resin system was a critical issue for nanocomposite processing. Rheological behaviour of the suspensions containing CNTs and tensile strengths of their resulting nanocomposites were characterized. CNT/polyester suspensions exhibited a shear thinning behaviour, while polyester resin blends act as a Newtonian fluid. It was also found that nanotubes with amine functional groups have better tensile strength, as compared to those with untreated CNTs. Transmission electron microscopy (TEM) was also employed to reveal the degree of dispersion of CNTs in the matrix.     

Polymer crystallization-driven, periodic patterning on carbon nanotubes

We report herein a unique means to periodically pattern polymeric materials on individual carbon nanotubes (CNTs) using a controlled polymer crystallization method. One-dimensional (1D) CNTs were periodically decorated with polymer lamellar crystals, resulting in nano-hybrid shish-kebab (NHSK) structures. The periodicity of the polymer lamellae varies from 20 to 150 nm. The kebabs are approximately 5-10 nm thick (along CNT direction) with a lateral size of approximately 20 nm to micrometers, which can be readily controlled by varying crystallization conditions. Both polyethylene and Nylon 66 were successfully decorated on single-walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes (MWNTs), as well as vapor grown carbon nanofibers (CNFs). The formation mechanism was attributed to "size-dependent soft epitaxy". Because NHSK formation conditions depend on CNT structures, it further provides a unique opportunity for CNT separation. The reported method opens a gateway to periodically patterning polymers and different functional groups on individual CNTs in an ordered and controlled manner, an attractive research field that is yet to be explored.     

Molecular dynamics simulation of pressure-driven water flow in silicon-carbide nanotubes

Many properties of silicon carbide (SiC) nanotubes, such as their high mechanical strength and resistance to corrosive environments, are superior to those of their carboneous counterparts, namely, carbon nanotubes (CNTs) and, therefore, SiC nanotubes can be a viable alternative to CNTs in a variety of applications. We employ molecular dynamics simulations to examine flow of water in SiC nanotubes and to study the differences and similarities with the same phenomenon in the CNTs. The simulations indicate that SiC nanotubes always provide larger flow enhancements than those reported for the CNTs. Moreover, a given flow enhancement in SiC nanotubes requires an applied pressure gradient that is at least an order of magnitude smaller than the corresponding value in a CNT of the same size.     

聚碳酸酯/碳纳米管复合热电材料的制备与性能研究

本文首先通过溶液混合法将碳纳米管(CNTs)分散到聚碳酸酯(PC)基体中,然后将获得的PC/CNT絮状物通过高温模压的方法制备了一种柔性的高热电性能PC/CNT复合材料。PC/CNT复合材料断面形貌分析表明,CNTs均匀地分散在PC基体中。此外,PC/CNT复合材料的导电性随着CNTs含量的增加而急剧增加,而Seebeck系数几乎保持恒定,使得材料的功率因子随着CNTs含量的增加而快速增加,最大功率因子达到4.6μW·m^-1·K^-2。     

On the Origin of Water Flow through Carbon Nanotubes

The transport of water molecules through carbon nanotubes (CNTs) is of primary importance for understanding water‐mediated biological activities as well as for the design of novel nanoporous materials. Herein, we analyze the water flow through CNTs by using molecular dynamics simulations with the hope of finding basic parameters determining the flow value. Of particular interest is that a simple equation as a function of water diffusion, occupancy and CNT size, can well describe the water flow through CNTs with different sizes. Specifically, both the simulation and equation flow exhibit power law relations with the CNT diameter and length, where the two exponents are close to each other. The water occupancy and translocation time also demonstrate interesting relations with the CNT size. The water dipole orientations and density profiles are also sensitive to the change of CNT size. These results greatly enhance our knowledge on the nature of water flow through CNTs and are helpful in predicting the water flow of CNTs up to the experimental length scale.     

Nanoparticle‐Assisted Alignment of Carbon Nanotubes on DNA Origami

Aligning carbon nanotubes (CNTs) is a key challenge for fabricating CNT‐based electronic devices. Herein, we report a spherical nucleic acid (SNA) mediated approach for the highly precise alignment of CNTs at prescribed sites on DNA origami. We find that the cooperative DNA hybridization occurring at the interface of SNA and DNA‐coated CNTs leads to an approximately five‐fold improvement of the positioning efficiency. By combining this with the intrinsic positioning addressability of DNA origami, CNTs can be aligned in parallel with an extremely small angular variation of within 10°. Moreover, we demonstrate that the parallel alignment of CNTs prevents incorrect logic functionality originating from stray conducting paths formed by misaligned CNTs. This SNA‐mediated method thus holds great potential for fabricating scalable CNT arrays for nanoelectronics.     

Effects of processing conditions on rheological,thermal, and electrical properties of multiwall carbon nanotube/epoxy resin composites

We report on the effect of processing conditions on rheology, thermal and electrical properties of nanocomposites containing 0.02–0.3 wt % multiwall carbon nanotubes in an epoxy resin. The influence of the sonication, the surface functionalization during mixing, as well as the application of external magnetic field (EMF) throughout the curing process was examined. Rheological tests combined with optical microscopy visualization are proved as a very useful methodology to determine the optimal processing conditions for the preparation of the nanocomposites. The Raman spectra provide evidence for more pronounced effect on the functionalized with hardener compositions, particularly by curing upon application of EMF. Different chain morphology of CNTs is created depending of the preparation conditions, which induced different effects on the thermal and electrical properties of the nanocomposites. The thermal degradation peak is significantly shifted towards higher temperatures by increasing the nanotube content, this confirming that even the small amount of carbon nanotubes produces a strong barrier effect for the volatile products during the degradation. The ac conductivity measurements revealed lower values of the percolation threshold (pc) in the range of 0.03–0.05 wt %. CNTs for the nanocomposites produced by preliminary dispersing of nanotubes in the epoxy resin, compared to those prepared by preliminary functionalization of the nanotubes in the amine hardener. This is attributed to the higher viscosity and stronger interfacial interactions of the amine hardener/CNT dispersion which restricts the reorganization of the nanotubes. The application of the EMF does not influence the pc value but the dc conductivity values (σ_dc) of the nanocomposites increased at about one order of magnitude due to the development of the aforementioned chain structure. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Dynamics and density profile of water in nanotubes as one-dimensional fluid

The transport and structural properties of water confined in nanotubes with different diameters were studied by molecular dynamics (MD) simulation. The effects of pore size, molecule-wall interaction, and the helicity of CNT on the diffusivity, thermal conductivity, and shear viscosity as well as density profile were analyzed. For diffusivity, in model NT > in armchair CNT > in zigzag CNT at similar conditions. However in contrast to the diffusivity, the thermal conductivity and the shear viscosity increase as the pore size decreases, in zigzag CNT > in armchair CNT > (or approximately ) in model NT. The ordered layer distribution of water molecules in nanotubes is clear. It suggests the structure of fluid in the zigzag CNTs is more ordered, and more solidlike. In the nanotubes, where the molecule and the pore dimensions are of similar order of magnitude, the nature of water-water and water-wall interactions, the confinement effect of space, and the helicity of CNT become more significant.     

Atomic force microscopy studies of DNA-wrapped carbon nanotube structure and binding to quantum dots

Single-stranded DNA is an effective noncovalent dispersant for individual single-walled carbon nanotubes (CNTs) in aqueous solution, forming a CNT-DNA hybrid material that has advantages for CNT separations and applications. Atomic force microscopy (AFM) reveals a regular pattern on the surface of CNT-DNA. We found this pattern to be independent of the length and sequence of the wrapping DNA, yet different from the structures observed for CNTs dispersed with sodium dodecyl sulfate in the absence of DNA. We wrapped CNTs with thiol-modified DNA to form stable conjugates of CNT-DNA and core/shell CdSe/ZnS quantum dots; AFM imaging of these conjugates identified for the first time the location of DNA on the CNT-DNA nanomaterial. Our results suggest that the AFM pattern of CNT-DNA is formed by helical turns (approximately 14-nm pitch) of wrapped DNA strands that are closely arranged end-to-end in a single layer along the CNT. This work demonstrates the useful functionalization of CNTs with quantum dots in a manner that avoids direct, destructive modification of the CNT surface and suggests nearly complete surface coverage of the nanotubes with DNA.     

Constructing carbon nanotube junctions by Ar ion beam irradiation

Carbon nanotubes (CNTs) irradiated by Ar ion beams at elevated temperature were studied. The irradiation-induced defects in CNTs are greatly reduced by elevated temperature. Moreover, the two types of CNT junctions, the crossing junction and the parallel junction, were formed. And the CNT networks may be fabricated by the two types of CNT junctions. The formation process and the corresponding mechanism of CNT networks are discussed.     

碳纳米管/聚丙烯腈原液的制备及可纺性

通过原位聚合的方法制备了碳纳米管／聚丙烯腈复合材料原液,并采用湿纺成型工艺制得碳纳米管／聚丙烯腈复合材料纤维。与共混工艺相比较,采用该方法制得的碳纳米管／聚丙烯腈复合材料碳纳米管在聚丙烯腈基中分散均匀,具有较好的可纺性。     

Comparative study of the electrochemical behavior and analytical applications of (bio)sensing platforms based on the use of multi-walled carbon nanotubes dispersed in different polymers

This review present a critical comparison of the electrochemical behavior and analytical performance of glassy carbon electrodes (GCE) modified with carbon nanotubes (CNTs) dispersed in different polymers: polyethylenimine (PEI), PEI functionalized with dopamine (PEI-Do), polyhistidine (Polyhis), polylysine (Polylys), glucose oxidase (GOx) and double stranded calf-thymus DNA (dsDNA). The comparison is focused on the analysis of the influence of the sonication time, solvent, polymer/CNT ratio, and nature of the polymer on the efficiency of the dispersions and on the electrochemical behavior of the resulting modified electrodes. The results allow to conclude that an adequate selection of the polymers makes possible not only an efficient dispersion of CNTs but also, and even more important, the building of successful analytical platforms for the detection of different bioanalytes like NADH, glucose, DNA and dopamine.     

Stable superhydrophobic surface via carbon nanotubes coated with a ZnO thin film

We report the formation of a stable superhydrophobic surface via aligned carbon nanotubes (CNTs) coated with a zinc oxide (ZnO) thin film. The CNT template was synthesized by chemical vapor deposition on an Fe-N catalyst layer. The ZnO film, with a low surface energy, was deposited on the CNT template by the filtered cathodic vacuum arc technique. Contact angle measurement reveals that the surface of the ZnO-coated CNTs is superhydrophobic with water contact angle of 159 degrees . Unlike the uncoated CNTs surface, the ZnO-coated CNTs surface shows no sign of water seepage even after a prolonged period of time. The wettability of the surface can be reversibly changed from superhydrophobicity to hydrophilicity by alternation of ultraviolet (UV) irradiation and dark storage.     

Enhanced Nucleation and Crystallization in PLA/CNT Composites via Disperse Orange 3 with Corresponding Improvement in Nanomechanical Properties

The potential for the material property improvement through the addition of carbon nanotubes (CNTs) in composite materials is often limited due to CNT agglomeration. In this work, Disperse Orange 3 (DO3) was investigated to determine its effectiveness in dispersing CNTs in a poly (lactic acid) (PLA) matrix. First, adsorption studies of DO3 onto CNTs were performed to determine the appropriate amount of DO3 to add so that the CNT surface will be nearly saturated with DO3 while limiting the excess DO3 dissolved in the polymer. The resultant improvements in the mechanical properties were determined via nanoindentation. Highly stable dispersion of CNTs in tetrahydrofuran with DO3 was observed 72 hours after sonication. Scanning electron microscopy confirmed that DO3‐functionalized CNTs were able to separate and disperse well inside of the PLA matrix. Addition of DO3 to the nanocomposite resulted in an increase in the glass transition temperature and crystallinity of the composite due to the more effective dispersion of the nanofiller which serves as a nucleation agent. The CNTs treated with DO3 also increased the elastic modulus and hardness of the composite compared to neat PLA and untreated PLA‐CNT composites. From this study, DO3 was demonstrated to be an effective dispersing agent in the solvent and the PLA matrix which allowed for enhanced crystallization and improved nanomechanical properties in the resultant composite.