Mesogenicity drives fractionation in lyotropic aqueous suspensions of multiwall carbon nanotubes

We describe a simple method for separating carbon nanotubes on the basis of their mesogenicity by fractionating biphasic aqueous suspensions within the Flory chimney of the lyotropic phase diagram. Macroscopic phase separation occurs on centrifuging the biphasic nanotube suspension or allowing it to stand. Long, straight nanotubes with higher mesogenicity (liquid crystalline forming ability) segregate preferentially to the liquid crystalline phase, whereas shorter nanotubes and impurities with lower mesogenicity segregate preferentially to the isotropic phase.     

Percolation threshold of carbon nanotubes filled unsaturated polyesters

This paper reports on the development of electrically conductive nanocomposites containing multi-walled carbon nanotubes in an unsaturated polyester matrix. The resistivity of the liquid suspension during processing is used to evaluate the quality of the filler dispersion, which is also studied using optical microscopy. The electrical properties of the cured composites are analysed by AC impedance spectroscopy and DC conductivity measurements. The conductivity of the cured nanocomposite follows a statistical percolation model, with percolation threshold at 0.026 wt.% loading of nanotubes. The results obtained show that unsaturated polyesters are a matrix suitable for the preparation of electrically conductive thermosetting nanocomposites at low nanotube concentrations. The effect of carbon nanotubes reaggregation on the electrical properties of the spatial structure generated is discussed.     

碳纳米管掺杂影响液晶物理参数与显示性能的实验及第一性原理计算

将4"-正戊基-4-氰基联苯(5CB)液晶与液晶4-[反式-4-[(E)-1-丙烯基]环己基]苯腈以5∶1的比例混合,并将预处理的碳纳米管分别与5CB单晶和混晶复合,测试两种掺杂碳纳米管的液晶的光电和介电性能。结果表明:碳纳米管的掺入影响了液晶体系的阈值电压和介电各向异性,其中介电各向异性的增幅最高达到4.671%,并且展曲弹性常数也有所增大;碳纳米管的掺入也影响液晶体系的响应时间和粘滞系数,其中粘滞系数的降幅最高达到25.131%。实验还表明,混晶的介电各向异性高于单晶,且其响应时间和粘滞系数的降幅均比5CB单晶明显。混晶的更大优势是,与碳纳米管复合、改善复合体系的物理参数和显示性能。同时,理论研究结果表明,碳纳米管与液晶分子的结合能介于液晶分子与液晶分子、碳纳米管与碳纳米管之间,并且在液晶分子的诱导下碳纳米管产生诱导偶极矩,印证了碳纳米管掺杂可提高液晶的介电各向异性、降低其响应时间的实验结果。     

Progress on mechanics of carbon nanotubes and derived materials

This review focuses on the most recent progress in understanding mechanical properties of individual carbon nanotubes (CNT), carbon nanotube arrays, random networks, and polymer matrix composites. The key factors that influence the mechanical properties of these new (nano)materials are identified and discussed. The critical issue appears to be the load transfer efficiency; between nanotubes when organized in bundles, ropes, and networks; between matrix and nanotubes in composites. Among the different paths used to increase load transfer, cross-linking by irradiation is emphasized. A particular attention is paid on the role of nanotubes as nucleating agents in polymer composites, initiating the formation of a crystalline polymer sheath that has important consequence on the mechanical properties. The reinforcing element to be considered in that case is not CNT alone but CNT covered with a cylinder of crystalline polymer. Whereas a lot of effort has been focused on the problem of dispersion, it appears that the problem of nanotube-matrix interphase is almost as important. Recent works show that appropriate surface functionalization can be used both to improve dispersion and tailor the interphase. Nanotube surface engineering combined with methods producing oriented nanocomposites should bring exceptional materials in the near future.     

Polyethylene crystallization nucleated by carbon nanotubes under shear

Polyethylene crystallization under shear has been studied in the presence of single-wall, few-wall, and multiwall carbon nanotubes (SWNT, FWNT, and MWNT). Polyethylene crystal d-spacings for (110) and (200) planes in polyethylene/carbon nanotubes (CNT) are smaller than in the control polyethylene without CNT and the polymer chain is oriented along the CNT axis. The single-wall carbon nanotube templated polyethylene crystals do not redissolve in boiling xylenes; instead, the chain morphology transforms to an amorphous conformation but remains oriented along the nanotube axis. SWNT crystal peaks were also observed in polyethylene/SWNT fibers.     

碳纳米管改性聚丙烯复合材料的研究进展

综述了碳纳米管对聚丙烯力学性能改性、电学性能改性、结晶行为改性、阻燃和热稳定性能改性等方面的研究现状,包括单壁碳纳米管和多壁碳纳米管对聚丙烯改性的影响,展望了碳纳米管改性聚丙烯复合材料的发展前景。     

pH-controlled carbon nanotube aggregation/dispersion based on intermolecular i-motif DNA formation

In this work, we report a new strategy to manipulate the aggregation and dispersion of carbon nanotube in solution via formation of intermolecular i-motif (four-stranded C-quadruplex) structures in a pH dependent manner. Firstly, single-stranded (ss) DNAs containing two stretches of cytosine (C)-rich domains are covalently linked to carbon nanotubes. At pH 8.0, DNAs are at random coil state, which enhance the dispersion of multi-wall carbon nanotubes (MWNTs) in water; after changing pH to 5.0, the intermolecular i-motif structures formed by the C-rich ssDNAs on neighboring carbon nanotube could drive the MWNTs aggregate. This process is reversible and the transition process has been verified by circular dichroism (CD) spectroscopy, gel electrophoresis and transmission electron microscopy (TEM). Considering the mechanical properties of carbon nanotube, this finding will benefit many application research fields, such as artificial muscle, functional nano-devices and so on.     

Dispersion study of long and aligned multi-walled carbon nanotubes in water

Dispersion of nanotubes is a crucial step for many applications. The properties of the final nanotube-based material are strongly dependent on the quality of nanotube suspensions. In this study, long and aligned multi-walled carbon nanotubes obtained by catalytic chemical vapour deposition were dispersed in water with different dispersing agents using high intensity ultrasounds. Among different additives, we selected sodium dodecyl sulfate (SDS) as dispersing agent to prepare suspensions of nanotubes. UV-Visible spectrometry method was used to measure the influence of dispersion parameters (power and duration of sonication) on dispersion state and suspension stability. Therefore, we demonstrated that, even if high intensity ultra-sounds are breaking nanotubes, it is possible to obtain stable water-based suspensions containing MWNTs which exhibit length up to 20 microm.     

A biocompatible chitosan composite containing phosphotungstic acid modified single-walled carbon nanotubes

Surface modification of carbon nanotubes is crucial for the dispersion and interfacial adhesion of carbon nanotubes in polymer composites. Here we present a novel method to construct single-walled carbon nanotube/chitosan composites using phosphotungstic acid as an anchor reagent to modify single-walled carbon nanotubes. The most direct benefit from this method is that this modification is mild but effective: the induced defects on single-walled carbon nanotubes are negligible based on Raman and transmission electron microscopy observations; and homogeneous dispersion of single-walled carbon nanotubes in chitosan matrices and strong binding between single-walled carbon nanotubes and chitosan are achieved. Moreover, according to the results of tetrazolium-based colorimetric assays in vitro, we demonstrate that the produced phosphotungstic-acid-modified single-walled carbon nanotube/chitosan composites have good biocompatibility. Thus, our study provides a feasible route to fabricate biocompatible composites containing single-walled carbon nanotubes for potential application in bone tissue engineering.     

Carbon nanotubes for reinforcement of plastics? A case study with poly(vinyl alcohol)

This paper reports a 4.5 fold increase in the Young’s modulus of a semi-crystalline polymer, poly(vinyl alcohol), with the addition of carbon nanotubes. The importance of nanotube diameter is addressed with the production of four composite solutions with each solution containing carbon nanotubes of a specific diameter. Thin films were cast for tensile tests while Differential Scanning Calorimetry (DSC) measured the crystallinity of the composite material. Nanotubes were observed to increase polymer crystallinity such that the Young’s modulus of the composite also increased. Thus it is suggested here that in semi-crystalline polymer/nanotube systems, with non-covalent bonding between the filler and matrix, the formation of nanotube induced crystalline polymer domains is the dominant reinforcement mechanism and not stress transfer to the nanotube: the role of the nanotube lies in nucleating crystallization. In addition, the calorimetry data indicates that nanotube diameter and quality of dispersion determines the increase in crystallinity which in turn determines the mechanical properties of the composite. Furthermore, Transmission Electron Microscopy (TEM) images confirm an excellent dispersion and wetting of the various nanotube types in the polymer solution.     

The influence of carbon nanotube functionalization route on the efficiency of dispersion in polypropylene by twin-screw extrusion

The joint effect of chemical functionalization and polymer melt blending conditions on carbon nanotube dispersion in polypropylene, as well as its influence on the electrical and mechanical properties of the resulting composites were investigated. Melt blending was performed using a prototype twin screw extruder enabling sampling along the barrel. The carbon nanotube dispersion was assessed by optical and scanning electron microscopy. The functionalization reaction was tailored for compatibility with the polymer, and characterized by X-ray photoelectron spectroscopy. In particular, nanotubes covalently bonded to polypropylene showed distinctive dispersion ability, while the carbon nanotube dispersion remained stable even after re-melting. However, the polypropylene-functionalized nanotubes produced composites with higher electrical resistivity, possibly due to the insulating effect of the polymer bonded to the nanotubes surface.     

碳纳米管增强铝基复合材料分散方法研究进展

碳纳米管与铝基体的结合,可以获得导电和导热性良好及综合力学性能优异的复合材料,有望成为新一代轻质高强、结构功能一体化的复合材料.在制备碳纳米管增强铝基复合材料过程中,碳纳米管的团聚将降低界面结合,诱发缺陷产生,导致性能大幅下降,因此,调控优化碳纳米管的分散状态、含量成为获取良好界面结合,获得高性能碳纳米管增强铝基复合材...     

Enhancement of β phase crystals formation with the use of nanofillers in PVDF films and fibres

Nanoclay and carbon nanotubes (CNT) have been in focus recently as means of enhancing β phase crystals formation in poly(vinylidene fluoride)(PVDF). Dominantly, the so-far work has been carried out on films/thin sheets filled with nanoclay. It has been found, mainly from combined XRD and DSC data, that nanoclay influences the PVDF structure, and particularly the β phase crystals formation is enhanced. Results published by various groups are in fairly good agreement. There are no results for nanoclay filled melt-spun PVDF fibres.The influence of CNT on PVDF structure has been less studied. XRD data indicating an enhancing role of multi-wall carbon nanotubes (MWNT) on β phase crystals formation in solution compounded PVDF films are available. Published results for MWNT/PVDF films are not in good agreement. The only study into single-wall carbon nanotube (SWNT)/PVDF has been made on electrospun nanofibres.We explore above findings towards melt-spun nanofilled PVDF fibres. We present new results obtained by us for melt-spun PVDF fibres containing non-functionalized and amino-functionalized double-wall carbon nanotubes (DWNT). The key finding is that amino-DWNT can influence the β to α polymorphic balance.     

Effects of gamma irradiation for sterilization on aqueous dispersions of length sorted carbon nanotubes

There is currently great interest in the potential use of carbon nanotubes as delivery vessels for nanotherapeutics and other medical applications. However, no data are available on the effects of sterilization methods on the properties of nanotube dispersions, the form in which most medical applications will be processed. Here we show the effects of gamma irradiation from a ⁶⁰Co source on the dispersion and optical properties of single-wall carbon nanotubes in aqueous dispersion. Samples of different length-refined populations were sealed in ampoules and exposed to a dose of approximately 28 kGy, a level sufficient to ensure sterility of the dispersions. In contrast to literature results for solid-phase nanotube samples, the effects of gamma irradiation on the dispersion and optical properties of the nanotube samples were found to be minimal. Based on these results, gamma irradiation appears sufficiently non-destructive to be industrially useful for the sterilization of nanotube dispersions.     

PA6／碳纳米管复合材料的复合方法的研究

ＣＶＤ法生产的碳纳米管经处理后呈纳米级分散，且表面产生大量的－ＯＨ官能团。碳纳米管与ＰＡ６（尼龙６）复合时，将通过其表面的－ＯＨ参加ＰＡ６的聚合反应，并阻碍ＰＡ６分子链的长大，削弱基体的强度。选择适当的复合方法，在尽量减少碳纳米管对ＰＡ６分子长大阻碍的基础上，改善碳纳米管在基体ＰＡ６中的分散，并进行后处理，可获得机能性能较好的ＰＡ６／碳纳米管复合材料。     

Optical transmission of polymer dispersed liquid crystals doped with carbon nanotubes

The optical transmission of polymer-dispersed liquid crystals (PDLCs) doped with multiwall carbon nanotubes (MWNTs) has been studied in comparison to the analogous system without MWNTs. The transmission of MWNT-doped PDLC cells in crossed polarizers exhibits an anomalous dependence on the control voltage. The anomalous behavior is attributed to a preliminary partial orientation of molecules in the presence of carbon nanotubes in the dispersed liquid crystal droplets.     

Polyisoprene-multi wall carbon nanotube composite structure for flexible pressure sensor application

The major problem of conventional rigid sensor materials is difficulty to integer them into soft flexible structures. Piezoresistive polyisoprene/nanostructured carbon composite appears as promising materials for such application. Previous research approved high structure carbon black and carbon nanotube filled composites as finger pressure sensitive piezoresistive materials. Carbon nanotubes originate with variable length to width ratio and high electric conductivity in longitudinal direction of the tubes, which theoretically should make it possible to obtain electric percolation in polymercarbon nanotube composites at very low loads of filler. However recent experience with mechanically dispersed carbon nanotubes shows quite high values of percolation threshold and specific sensing properties. In this work we present an attempt to use ultrasound for improved dispersion of the filler in a piezoresistive polyisoprene-multi wall carbon nanotube composite as well polyisoprene-high structure carbon black composite. The noticeable shift of percolation threshold for both types of composites have been achieved. The piezoresistive behavior of sonicated composites have been determined and compared with mechanically mixed ones. The differences have been evaluated and explained.     

The impact of fluorinated MWCNT additives on the enhanced dynamic mechanical properties of e-beam-cured epoxy

Multi-walled carbon nanotubes were embedded into e-beam-cured epoxy resin to improve the mechanical properties of epoxy resin. The surfaces of these carbon nanotubes were modified using a fluorination treatment to improve their dispersion and adhesion in epoxy resin. The dynamic mechanical properties of epoxy/carbon nanotube composites were investigated at various heating rates and frequencies. As an effect of fluorination treatment, the semi-ionic bond of C–F on the surface of multi-walled carbon nanotubes played an important role in the improved dispersion and adhesion of carbon nanotubes into the epoxy resin. The storage modulus and loss modulus of the composites increased with higher applied frequency. The activation energy of the composites was increased by the effects of a higher heating rate due to the slow heat transfer in the epoxy/carbon nanotube composites. Eventually, the dynamic mechanical properties of the investigated epoxy were significantly improved by the carbon nanotubes dispersed therein via the fluorination treatment.     

The effect of carbon nanotube dimensions and dispersion on the fatigue behavior of epoxy nanocomposites

Fatigue is one of the primary reasons for failure in structural materials. It has been demonstrated that carbon nanotubes can suppress fatigue in polymer composites via crack-bridging and a frictional pull-out mechanism. However, a detailed study of the effects of nanotube dimensions and dispersion on the fatigue behavior of nanocomposites has not been performed. In this work, we show the strong effect of carbon nanotube dimensions (i.e.?length, diameter) and dispersion quality on fatigue crack growth suppression in epoxy nanocomposites. We observe that the fatigue crack growth rates can be significantly reduced by (1) reducing the nanotube diameter, (2) increasing the nanotube length and (3) improving the nanotube dispersion. We qualitatively explain these observations by using a fracture mechanics model based on crack-bridging and pull-out of the nanotubes. By optimizing the above parameters (tube length, diameter and dispersion) we demonstrate an over 20-fold reduction in the fatigue crack propagation rate for the nanocomposite epoxy compared to the baseline (unfilled) epoxy.     

Improvement of the properties of PC/LCP blends in the presence of carbon nanotubes

Polycarbonate (PC)/liquid crystalline polymer (LCP)/Multiwall carbon nanotube (MWNTs) nanocomposites containing as-received and modified (COOH-MWNT) carbon nanotubes were prepared through melt process in extruder and then compression molded. The chemical modification of MWCNT enhances the compatibility as well as miscibility between Polycarbonate and LCP in the concerned composites. Incorporation of functionalized MWCNTs improves the thermal, mechanical dynamic mechanical and electrical properties of the composites. Storage modulus and glass transition temperature increases in a remarkable fashion due to the enhanced interfacial adhesion between the PC/LCP/COOH-MWCNT. Field emission microscopy (FESEM) and HRTEM exhibit LCP fibrillation and fine dispersion of COOH-MWCNT throughout the matrix highlighting the miscibility between LCP and PC matrix.