Multiwalled carbon nanotube reinforced polymer composites

Due to their high stiffness and strength, as well as their electrical conductivity, carbon nanotubes are under intense investigation as fillers in polymer matrix composites. The nature of the carbon nanotube/polymer bonding and the curvature of the carbon nanotubes within the polymer have arisen as particular factors in the efficacy of the carbon nanotubes to actually provide any enhanced stiffness or strength to the composite. Here the effects of carbon nanotube curvature and interface interaction with the matrix on the composite stiffness are investigated using micromechanical analysis. In particular, the effects of poor bonding and thus poor shear lag load transfer to the carbon nanotubes are studied. In the case of poor bonding, carbon nanotubes waviness is shown to enhance the composite stiffness.     

Mechanical strength improvement of polypropylene threads modified by PVA/CNT composite coatings

Poly (vinyl alcohol)/carbon nanotube (PVA/CNT) composite was coated on the surface of polypropylene thread for toughness enhancement. Multiwall carbon nanotubes (MWNTs) were treated in acid and alkali to get water-soluble nanotubes, and then embedded into poly (vinyl alcohol) (PVA) matrix, resulting in polymer-carbon composite with homogeneous nanotube dispersion. The stress-strain measurements show that the tensile strength and toughness of the PVA/CNT coated thread increased by 117% and 560%, respectively. These results are supportive of good interfacial bonding between the carbon nanotubes (CNTs) and polymer matrix.     

Failure of carbon nanotube/polymer composites and the effect of nanotube waviness

This paper studies the failure of CNT/polymer composites by combining micromechanics model and finite element simulation. A computational model of composite of adequate size is employed so the interactions between nanotubes embedded in the matrix can be taken into account. The effects of nanotube waviness and random nanotube distribution relative to aligned straight nanotubes are investigated. The computational results indicate that the nanotube waviness tend to reduce the elastic modulus but increase the ultimate strain of a composite. The randomness of nanotube distribution tends to reduce both the composite elastic modulus and tensile strength. The damage initiation and evolution in composites with random wavy nanotubes have also been analyzed.     

Continuous carbon nanotube reinforced composites

Carbon nanotubes are considered short fibers, and polymer composites with nanotube fillers are always analogues of random, short fiber composites. The real structural carbon fiber composites, on the other hand, always contain carbon fiber reinforcements where fibers run continuously through the composite matrix. With the recent optimization in aligned nanotube growth, samples of nanotubes in macroscopic lengths have become available, and this allows the creation of composites that are similar to the continuous fiber composites with individual nanotubes running continuously through the composite body. This allows the proper utilization of the extreme high modulus and strength predicted for nanotubes in structural composites. Here, we fabricate such continuous nanotube polymer composites with continuous nanotube reinforcements and report that under compressive loadings, the nanotube composites can generate more than an order of magnitude improvement in the longitudinal modulus (up to 3,300%) as well as damping capability (up to 2,100%). It is also observed that composites with a random distribution of nanotubes of same length and similar filler fraction provide three times less effective reinforcement in composites.     

Reinforcing epoxy nanocomposites with functionalized carbon nanotubes via biotin–streptavidin interactions

We report on the preparation of nanocomposites consisting of biofunctionalized single-walled carbon nanotubes (BF-SWCNTs) reinforcing an ultraviolet curable epoxy polymer by means of biotin–streptavidin interactions. The as-produced laser ablation SWCNTs are biofunctionalized via acid oxidization based purification process and non-covalent functionalization using surfactant, followed by grafting the resulting nanotubes with biomolecules. The biotin-grafted nanotubes are capable of interacting with epoxy groups in presence of streptavidin molecules by which chemical bridges between BF-SWCNTs and epoxy matrix are formed. The biomolecules grafted to the nanotubes surface not only facilitate the load transfer, but also improve the nanotube dispersion into the epoxy matrix, as observed by optical imaging and scanning electron microscopy. Mechanical characterization on the nanocomposite microfibers demonstrates considerable enhancement in both strength (by 76%) and modulus (by 93%) with the addition of only 1 wt.% of BF-SWCNTs. The electrical measurements reveal a clear change in electrical conductivity of nanocomposite microfibers reinforced with 1 wt.% of BF-SWCNTs in comparison to the microfibers containing solely purified carbon nanotubes. These multifunctional nanocomposite materials could be used to fabricate macro and microstructures for a wide variety of applications such as high strength polymer nanocomposite and potential easily-manipulated biosensors.     

碳纳米管在聚合物基体中的分散与有序排列研究--(Ⅰ)碳纳米管在聚合物基体中的分散

聚合物/碳纳米管（CNT）纳米复合材料，可将聚合物良好的加工性能和碳纳米管（CNTs）优异的功能化性质结合起来.目前，面临的主要挑战之一是如何提高CNTs在聚合物基体中的分散性.文中综述了优化物理共混、原位聚合和化学修饰等方法在改善CNTs在聚合物基体中分散性方面的最新研究动态。     

Raman probing of adhesion loss in carbon nanotube – reinforced composite

Raman spectroscopy of a carbon nanotube – reinforced phenolic resin is used to study the interaction of nanotubes with a host matrix. The observed sublinear dependence of the Raman G-band shift on the matrix strain, accompanied by inhomogeneous broadening of the spectral line, is interpreted as a gradual loss of adhesion between nanotubes and the polymer. An approach to simulate the ensemble-averaged Raman response of the nanotubes in composite is proposed, that takes into account nanotube orientation, angular dependence of the polarized Raman response of nanotubes, and adhesion loss between the nanotubes and the polymer. The comparison of the observed Raman line shapes and Raman shifts with simulation provides interesting insights into the micromechanics of nanotube interaction with polymer.     

Effects of nanotube helical angle on mechanical properties of carbon nanotube reinforced polymer composites

Carbon nanotubes (CNTs) possess exceptional mechanical properties and are therefore suitable candidates for use as reinforcements in composite materials. The CNTs, however, form complicated shapes and do not usually appear as straight reinforcements when introduced in polymer matrices. This results in a decrease in nanotube effectiveness in enhancing the matrix mechanical properties. In this paper, theory of elasticity of anisotropic materials and finite element method (FEM) are used to investigate the effects of CNT helical angle on effective mechanical properties of nanocomposites. Helical nanotubes with different helical angles are modeled to investigate the effects of nanotube helical angle on nanocomposite effective mechanical properties. In addition, the results of models consisting of helical nanotubes are compared with the effective mechanical properties of nanocomposites reinforced with straight nanotubes. Ultimately, the effects of helical CNT volume fraction on nanocomposite longitudinal modulus are investigated.     

Computational and experimental study of interfacial bonding of single-walled nanotube reinforced composites

In the development of nanotube reinforced polymer composites, one of the fundamental issues that scientists and engineers are confronting is the nanotube/polymer interfacial bonding, which will determine load transfer capability from the polymer matrix to the nanotube. In this paper, the interfacial bonding of single-walled nanotube (SWNT) reinforced epoxy composites was investigated using a combination of computational and experimental methods. The interfacial bonding was predicted using molecular dynamics (MD) simulations based on a cured epoxy resin model, which was constructed by incorporating three-dimensional cross-links formed during curing reaction. Based on the pullout simulations, the interfacial shear strength between the nanotube and the cured epoxy resin was calculated to be up to 75 MPa, indicating that there could be an effective stress transfer from the epoxy resin to the nanotube. In the experiments, single-walled nanotube reinforced epoxy composites were fabricated, characterized and analyzed. The uniform dispersion and good interfacial bonding of the nanotubes in the epoxy resin resulted in a 250–300% increase in storage modulus with the addition of 20–30 wt% nanotubes. These experimental results provided evidence of stress transfer in agreement with the simulation results.     

Chemical functionalization of carbon nanotubes

There are many reasons why one would want to form chemical attachments to the walls or ends of carbon nanotubes: these chemical bonds might be used to tailor the interaction of the nanotube with other entities, such as a solvent, a polymer matrix, or other nanotubes. The chemically functionalized nanotube might have mechanical or electrical properties that are different from those of the unfunctionalized nanotube and thus might be used as a chemical sensor or a nanometer-scale electronic device. The challenge is to find a way to reproducibly and reliably chemically alter carbon nanotubes that, like graphite, are fairly unreactive. The various methods used to date and the possible application of the resulting functionalized nanotubes is discussed in this review paper.     

Mechanical properties of carbon nanotube reinforced polymer nanocomposites: A coarse-grained model

In this work, a coarse-grained (CG) model of carbon nanotube (CNT) reinforced polymer matrix composites is developed. A distinguishing feature of the CG model is the ability to capture interactions between polymer chains and nanotubes. The CG potentials for nanotubes and polymer chains are calibrated using the strain energy conservation between CG models and full atomistic systems. The applicability and efficiency of the CG model in predicting the elastic properties of CNT/polymer composites are evaluated through verification processes with molecular simulations. The simulation results reveal that the CG model is able to estimate the mechanical properties of the nanocomposites with high accuracy and low computational cost. The effect of the volume fraction of CNT reinforcements on the Young's modulus of the nanocomposites is investigated. The application of the method in the modeling of large unit cells with randomly distributed CNT reinforcements is examined. The established CG model will enable the simulation of reinforced polymer matrix composites across a wide range of length scales from nano to mesoscale.     

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.     

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.     

Electrospinning carbon nanotube polymer composite nanofibers

The unique and exceptional physical properties of carbon nanotubes have inspired their use as a filler within a polymeric matrix to produce carbon nanotube polymer composites with enhanced mechanical, thermal and electrical properties. A powerful method of synthesising nanofibers comprising these polymer composites is electrospinning, which utilises an applied electric stress to draw out a thin nanometer-dimension fiber from the tip of a sharp conical meniscus. The focussing of the flow due to converging streamlines at the cone vertex then ensures alignment of the carbon nanotubes along the fiber axis, thus enabling the anisotropic properties of the nanotubes to be exploited. We consider the work that has been carried out to date on various aspects encompassing preprocessing, synthesis and characterisation of these electrospun polymer composite nanofibers as well as the governing mechanisms and associated properties of such fibers. Particular attention is also dedicated to the theoretical modelling of these fiber systems, in particular to the electrohydrodynamic modelling of electrospinning polymer jets.     

Carbon nanotube-based functional materials for optical limiting

Optical limiting is an important application of nonlinear optics, useful for the protection of human eyes, optical elements, and optical sensors from intense laser pulses. An optical limiter is such a device that strongly attenuates high intensity light and potentially damaging light such as focused laser beams, whilst allowing for the high transmission of ambient light. Optical limiting properties of carbon nanotube suspensions, solubilized carbon nanotubes, small molecules doped carbon nanotubes and polymer/carbon nanotube composites have been reviewed. The optical limiting responses of carbon nanotube suspensions are shown to be dominated by nonlinear scattering as a result of thermally induced solvent-bubble formation and sublimation of the nanotubes, while the solubilized carbon nanotubes optically limit through nonlinear absorption mechanism and exhibit significant solution-concentration-dependent optical limiting responses. In the former case the optical limiting results are independent of nanotube concentrations at the same linear transmittance as that of the solubilized systems. Many efforts have been invested into the research of polymer/carbon nanotube composites in an attempt to allow for the fabrication of films required for the use of nanotubes in a real optical limiting application. The higher carbon nanotube content samples block the incident light more effectively at higher incident energy densities or intensities. The optical limiting mechanism of these composite materials is quite complicated. Besides nonlinear scattering contribution to the optical limiting, there may also be other contributions e.g., nonlinear absorption, nonlinear refraction, electronic absorption and others to the optical limiting. Further improvements in the optical limiting efficiency of the composites and in the dispersion and alignment properties of carbon nanotubes in the polymer matrix could be realized by variation of both nanostructured guest and polymer host, and by ex situ alignment and other methods. It would be very desirable, from the practical application point of view, if one can design broadband optical limiting chromophores that would function in a multimechanistic fashion.     

Numerical characterization of CNT-based polymer composites considering interface effects

Carbon nanotubes (CNTs) possess exceptional mechanical properties and are therefore suitable candidates for use as reinforcements in composite materials. Load transfer in nanocomposite materials is achieved through the CNT/matrix interface. Thus, to determine nanocomposite mechanical properties, the interface behavior must be determined. In this investigation, finite element method is used to investigate the effects of interface strength on effective CNT-based composite mechanical properties. Nanocomposite mechanical properties are evaluated using a 3D nanoscale representative volume element (RVE). A single nanotube and the surrounding polymer matrix are modeled. Two cases of perfect bonding and an elastic interface are considered. For the perfect bonding interface, the no slip conditions are applied. To better investigate the elastic interface behavior, two models are proposed for this type of interface. The first elastic interface model consists of a thin layer of an elastic material surrounding the CNT. In the second elastic interface model, a series of spring elements are used as the nanotube/matrix interface. The results of numerical models indicate the importance of adequate interface bonding for a more effective strengthening of polymer matrix by CNT’s.     

Micro orientation and anisotropy of conductivity in liquid crystalline polymer films filled with carbon nanotubes

In this communication we report the preferential orientation of single wall carbon nanotubes (SWNT) in a nematic liquid crystalline (LC) polymer matrix. The alignment of the nanotubes was characterized through anisotropy of electrical conductivity of the composite measured in directions parallel and perpendicular to the nematic director. The anisotropy of the nanocomposite films strongly depends on the nanotube concentration in the range from 1 to 10% and vanished at higher loads. The electrical conductivity of nanocomposites is related to their structural features revealed by atomic force microscopy and Raman spectroscopy experiments and is explained by a strong coupling between the nanotubes and the polymer matrix.     

Mobility of carbon nanotubes in high electric fields

The influence of electric fields on carbon nanotubes is experimentally demonstrated. Alignment of nanotubes along field lines, directed motion of nanotubes between electrodes separated by several thousand micrometers, and impressive solid-state actuation behavior of nanotube-embedded structures are demonstrated, taking into account the polarization and charging of the nanotubes. These effects are reported for long strands of nanotubes, nanotubes dispersed on substrates, and nanotube-embedded polymer strips. The relative magnitude of the field responsible for polarization and directed motion was found to be dependent on the morphology of the nanotubes used. These observations may foreshadow novel electromechanical applications for nanotube elements.     

Selective tuning of the electronic properties of coaxial nanocables through exohedral doping

The electronic properties of exohedrally doped double-walled carbon nanotubes (DWNTs) have been investigated using density functional theory and resonance Raman spectroscopy (RRS) measurements. First-principles calculations elucidate the effects of exohedral doping on the M@S and S@M systems, where a metallic (M) tube is either inside or outside a semiconducting (S) one. The results demonstrate that metallic nanotubes are extremely sensitive to doping even when they are inner tubes, in sharp contrast to semiconducting nanotubes, which are not affected by doping when the outer shell is a metallic nanotube (screening effects). The theoretical predictions are in agreement with RRS data on Br2- and H2SO4-doped DWNTs. These results pave the way to novel nanoscale electronics via exohedral doping.     

An atlas of carbon nanotube optical transitions

Electron-electron interactions are significantly enhanced in one-dimensional systems, and single-walled carbon nanotubes provide a unique opportunity for studying such interactions and the related many-body effects in one dimension. However, single-walled nanotubes can have a wide range of diameters and hundreds of different structures, each defined by its chiral index (n,m), where n and m are integers that can have values from zero up to 30 or more. Moreover, one-third of these structures are metals and two-thirds are semiconductors, and they display optical resonances at many different frequencies. Systematic studies of many-body effects in nanotubes would therefore benefit from the availability of a technique for identifying the chiral index of a nanotube based on a measurement of its optical resonances, and vice versa. Here, we report the establishment of a structure-property 'atlas' for nanotube optical transitions based on simultaneous electron diffraction measurements of the chiral index and Rayleigh scattering measurements of the optical resonances of 206 different single-walled nanotube structures. The nanotubes, which were suspended across open slit structures on silicon substrates, had diameters in the range 1.3-4.7?nm. We also use this atlas as a starting point for a systematic study of many-body effects in the excited states of single-walled nanotubes. We find that electron-electron interactions shift the optical resonance energies by the same amount for both metallic and semiconducting nanotubes, and that this shift (which corresponds to an effective Fermi velocity renormalization) increases monotonically with nanotube diameter. This behaviour arises from two sources: an intriguing cancellation of long-range electron-electron interaction effects, and the dependence of short-range electron-electron interactions on diameter.