Modeling the effective elastic properties of nanocomposites with circular straight CNT fibers reinforced in the epoxy matrix

In the present study, the consistent effective elastic properties of straight, circular carbon nanotube epoxy composites are derived using the micromechanics theory. The CNT composites are known to provide high stiffness and elastic properties when the shape of the fibers is cylindrical and straight. Accordingly, in the present work, the effective elastic moduli of composite are newly obtained for straight, circular CNTs aligned in the specified direction as well as distributed randomly in the matrix. In this direction, novel analytical expressions are proposed for four cases of fiber property. First, aligned, and straight CNTs are considered with transverse isotropy in fiber coordinates, and the composite properties are also transversely isotropic in global coordinates. The short comings in the earlier developments are effectively addressed by deriving the consistent form of the strain tensor and the stiffness tensor of the CNT nanocomposite. Subsequently, effective relations for composites reinforced with aligned, straight CNTs but fibers isotropic in local coordinates are newly developed under hydrostatic loading. The effect of the unsymmetric Eshelby tensor for cylindrical fibers on the overall properties of the nanocomposite is included by deriving the strain concentration tensors. Next, the random distribution of CNT fibers in the matrix is studied with fibers being transversely isotropic as well as isotropic when CNT nanocomposites are subjected to uniform loading. The corresponding relations for the effective elastic properties are newly derived. The modeling technique is validated with results reported, and the variations in the effective properties for different CNT volume fractions are presented.     

湿热环境对碳纳米管复合材料界面应力传递的影响

基于碳纳米管的热膨胀系数及弹性模量分别为温度变化的函数,基体的热、湿膨胀系数及弹性模量分别为温度变化和湿度变化的函数,应用连续介质力学的经典弹性壳理论及传统纤维拉拔模型,分析了湿热环境对碳纳米管复合材料界面应力传递的影响。数值计算表明,湿度、温度的效应及碳纳米管的层数等参数对界面应力的传递均有显著影响。     

Fabrication and characterization of carbon nanotube reinforced poly(methyl methacrylate) nanocomposites

Multiwall carbon nanotube (CNT) reinforced poly(methyl methacrylate) (PMMA) nanocomposites have been successfully fabricated with melt blending. Two melt blending approaches of batch mixing and continuous extrusion have been used and the properties of the derived nanocomposites have been compared. The interaction of PMMA and CNTs, which is crucial to greatly improve the polymer properties, has been physically enhanced by adding a third party of poly(vinylidene fluoride) (PVDF) compatibilizer. It is found that the electrical threshold for both PMMA/CNT and PMMA/PVDF/CNT nanocomposites lies between 0.5 to 1 wt% of CNTs. The thermal and mechanical properties of the nanocomposites increase with CNTs and they are further increased by the addition of PVDF For 5 wt% CNT reinforced PMMA/PVDF/CNT nanocomposite, the onset of decomposition temperature is about 17 degrees C higher and elastic modulus is about 19.5% higher than those of neat PMMA. Rheological study also shows that the CNTs incorporated in the PMMA/PVDF/CNT nanocomposites act as physical cross-linkers.     

Evaluation and visualization of the percolating networks in multi-wall carbon nanotube/epoxy composites

In this study, epoxy-based nanocomposites containing multi-wall carbon nanotubes (CNTs) were produced by a calendering approach. The electrical conductivities of these composites were investigated as a function of CNT content. The conductivity was found to obey a percolation-like power law with a percolation threshold below 0.05 vol.%. The electrical conductivity of the neat epoxy resin could be enhanced by nine orders of magnitude, with the addition of only 0.6 vol.% CNTs, suggesting the formation of a well-conducting network by the CNTs throughout the insulating polymer matrix. To characterize the dispersion and the morphology of CNTs in epoxy matrix, different microscopic techniques were applied to characterize the dispersion and the morphology of CNTs in epoxy matrix, such as atomic force microscopy, transmission electron microscopy, and scanning electron microscopy (SEM). In particular, the charge contrast imaging in SEM allows a visualization of the overall distribution of CNTs at a micro-scale, as well as the identification of CNT bundles at a nano-scale. On the basis of microscopic investigation, the electrical conduction mechanism of CNT/epoxy composites is discussed.     

Preparation, characterization and properties of acid functionalized multi-walled carbon nanotube reinforced thermoplastic polyurethane nanocomposites

The multi-walled carbon nanotube (MWNT) reinforced thermoplastic polyurethane (TPU) nanocomposites were prepared through melt compounding method followed by compression molding. The spectroscopic study indicated that a strong interfacial interaction was developed between carbon nanotube (CNT) and the TPU matrix in the nanocomposites. The microscopic observation showed that the CNTs were homogeneously dispersed throughout the TPU matrix well apart from a few clusters. The results from thermal analysis indicated that the glass transition temperature (T_g) and storage modulus (E′) of the nanocomposites were increased with increase in CNTs content and their thermal stability were also improved in comparison with pure TPU matrix. The rheological analysis showed the low frequency plateau of shear modulus and the shear thinning behavior of the nanocomposites. The electrical behaviors of the nanocomposites are increased with increase in weight percent (wt%) of CNT loading. The mechanical properties of nanocomposites were substantially improved by the incorporation of CNTs into the TPU matrix.     

Effect of surfactant treatment on thermal stability and mechanical properties of CNT/polybenzoxazine nanocomposites

The mechanical and thermo-mechanical properties of polybenzoxazine nanocomposites containing multi-walled carbon nanotubes (MWCNTs) functionalized with surfactant are studied. The results are specifically compared with the corresponding properties of epoxy-based nanocomposites. The CNTs bring about significant improvements in flexural strength, flexural modulus, storage modulus and glass transition temperature, T_g, of CNT/polybenzoxazine nanocomposites at the expense of impact fracture toughness. The surfactant treatment has a beneficial effect on the improvement of these properties, except the impact toughness, through enhanced CNT dispersion and interfacial interaction. The former four properties are in general higher for the CNT/polybenzoxazine nanocomposites than the epoxy counterparts, and vice versa for the impact toughness. The addition of CNTs has an ameliorating effect of lowering the coefficient of thermal expansion (CTE) of polybenzoxazine nanocomposites in both the regions below and above T_g, whereas the reverse is true for the epoxy nanocomposites. This observation has a particular implication of exploiting the CNT/polybenzoxazine nanocomposites in applications requiring low shrinkage and accurate dimensional control.     

Effects of carbon black-carbon nanotube complex fillers on the properties of isotactic polypropylene nanocomposites

In this study, the reinforcing effects of carbon black (CB) and carbon nanotube (CNT) complex fillers on the properties of isotactic polypropylene (iPP) nanocomposites were investigated using various methods. The surface of the CNTs was modified using a linear alkyl chain in order to create a homogeneous CNT dispersion in the iPP matrix. When the CB content that was incorporated in the iPP matrix increased, the thermal and mechanical properties of the iPP/CB nanocomposites were enhanced. Additionally these enhancements in the properties were similarly induced by introducing a small amount of alkylated CNTs (a-CNTs). In contrast, the CB/a-CNT complex filler was more effective for the iPP nanocomposites than the CB or a-CNT single filler in terms of the thermal stability and the electrical properties. However, the mechanical properties of the CB/a-CNT complex filler incorporated iPP nanocomposites were poorer than the only a-CNT incorporated iPP nanocomposites. Additionally, the complex filler did not overcome the nucleation behavior of the a-CNTs in the re-crystallization of iPP.     

Reinforcing brittle and ductile epoxy matrices using carbon nanotubes masterbatch

In this study, the mechanical and thermal properties of epoxy composites using two different forms of carbon nanotubes (powder and masterbatch) were investigated. Composites were prepared by loading the surface-modified CNT powder and/or CNT masterbatch into either ductile or brittle epoxy matrices. The results show that 3 wt.% CNT masterbatch enhances Young’s modulus by 20%, tensile strength by 30%, flexural strength by 15%, and 21.1 °C increment in the glass transition temperature (by 34%) of ductile epoxy matrix. From scanning electron microscopy images, it was observed that the CNT masterbatch was uniformly distributed indicating the pre-dispersed CNTs in the masterbatch allow an easier path for preparation of CNT-epoxy composites with reduced agglomeration of CNTs. These results demonstrate a good CNT dispersion and ductility of epoxy matrix play a key role to achieve high performance CNT-epoxy composites.     

A comprehensive study on thermal conductivities of wavy carbon nanotube-reinforced cementitious nanocomposites

The thermal conductivities of cementitious nanocomposites reinforced by wavy carbon nanotubes (CNTs) are determined by the effective medium (EM) micromechanics-based method. The nanocomposite is composed of sinusoidally wavy CNTs as reinforcement and cement paste as matrix. The interfacial region between the CNTs and cementitious material is considered in the analysis. The effects of volume fraction and waviness parameters of CNTs, interfacial thermal resistance, type of CNTs placement within the matrix including aligned or randomly oriented CNTs, cement paste properties on the thermal conductivity coefficients of the nanocomposite are studied. The estimated values of the model are in very good agreement with available experimental data. Two parameters of CNT waviness and interfacial region contributions should be included in the modeling to predict realistic results for both aligned and randomly oriented CNT-reinforced nanocomposites. The results reveal that thermal conductivities K₂₂ (transverse in-plane thermal conductivity) and K₃₃ (longitudinal in-plane thermal conductivity) of the nanocomposites are remarkably dependent on the CNT waviness. Also, it is found that the CNT waviness moderately affects the thermal conductivity of a cementitious nanocomposite containing randomly oriented CNTs. However, the non-straight shape of CNTs does not influence the value of thermal conductivity K₁₁ (transverse out of plane thermal conductivity). The achieved results can be useful to guide the design of cementitious nanocomposites with optimal thermal conductivity properties.     

Carbon nanotubes based high temperature vulcanized silicone rubber nanocomposite with excellent elasticity and electrical properties

In the present study, we have fabricated a series of high temperature vulcanized silicone rubber (HTVSR)/carbon nanotubes (CNTs) nanocomposites with different CNT contents. The CNTs were pretreated by the chitosan salt before being incorporated into the HTVSR. The nanocomposites were then characterized in terms of morphological, thermal, mechanical and electrical properties. It was found that the chitosan salt pretreated CNTs dispersed uniformly within the HTVSR matrix, improving the thermal and mechanical properties of the HTVSR. The nanocomposites could remain conductive without losing inherent properties after 100 times of repeated stretching/release cycles by 100%, 200%, and even 300%. Moreover, the nanocomposites had good response to the compressed pressures. The results obtained from this study indicate that the fabricated nanocomposites are potential to be used in many electrical fields such as the conductive elastomer or the pressure sensor.     

Microstructure and mechanical properties of CNT/Ag nanocomposites fabricated by spark plasma sintering

Carbon nanotube/silver (CNT/Ag) nanocomposites include CNT volume fraction up to 10?vol.% were prepared by chemical reduction in solution followed by spark plasma sintering. Multiwalled CNTs underwent surface modifications by acid treatments, the Fourier transform infrared spectroscopy data indicated several functional groups loaded on the CNT surface by acid functionalisation. The acid-treated CNTs were sensitised and activated. Silver was deposited on the surface of the activated CNTs by chemical reduction of alkaline silver nitrate solution at room temperature. The microstructures of the prepared CNT/Ag nanocomposite powders were investigated by high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy and X-ray powder diffraction analysis. The results indicated that the produced CNT/Ag nanocomposite powders have coated type morphology. The produced CNT/Ag nanocomposite powders were sintered by spark plasma sintering. It was observed from the microstructure investigations of the sintered materials by HRSEM that the CNTs were distributed in the silver matrix with good homogeneity. The hardness and the tensile properties of the produced CNT/Ag nanocomposites were measured. By increasing the volume fraction of CNTs in the silver matrix, the hardness values increased but the elongation values of the prepared CNT/Ag nanocomposites decreased. In addition, the tensile strength was increased by increasing the CNTs volume fraction up to 7.5?vol.%, but the sample composed of 10?vol.% CNT/Ag was fractured before yielding.     

Application of cryomilling to enhance material properties of carbon nanotube reinforced chitosan nanocomposites

Cryomilled multiwall carbon nanotube (MWCNT) reinforced chitosan nanocomposites having improved conductivity have been prepared by solution casting method. The MWCNTs were crushed to smaller particles via cryomilling, which was effective in cleaving the nanotubes regularly as well as in reducing the entanglements and agglomeration. The cryomilled CNTs were chemically oxidized by acid and base methods, where basic oxidation generated high graphitic structure. The cryomilled and oxidized CNTs were characterized by XRD, Raman spectroscopy, FTIR and SEM. The conductivity of the nanocomposites was improved by cryomilling and it was further improved by chemical oxidation. Base oxidized cryomilled CNT/chitosan nanocomposites showed large improvement in conductivity compared to all other nanocomposites having 1 wt.% CNT content. Thermal stability and tensile properties of the CNT/chitosan nanocomposites also have been improved significantly by the incorporation of acid and base oxidized cryomilled CNTs. SEM picture of the fractured surface and FTIR showed nano-level dispersion of the functionalized CNTs and good chemical interaction between chitosan and CNTs respectively.     

The relationship between microstructure and mechanical properties of carbon nanotubes/polylactic acid nanocomposites prepared by twin-screw extrusion

Twin-screw extrusion was applied to prepare the carbon nanotubes/polylactic acid (CNT/PLA) nanocomposites. Five different extruded plates were produced under variation of CNT concentrations. The internal microstructures were also observed by optical microscope to examine the distribution and dispersion of CNT in the PLA. Besides, the crystallinity of the CNT/PLA nanocomposites was investigated by differential scanning calorimetry (DSC) and density method. The effects of the CNT concentrations on the mechanical and electrical properties of the nanocomposites were investigated. Scanning electron microscope (SEM) was performed to observe the CNT dispersion in the nano-scale. These results suggested that the crystallinity was increased with the increase of CNT concentrations, demonstrating that CNT played a role as a nucleating agent in PLA. Moreover, the mechanical and electrical properties of PLA have been improved by a proper incorporation of CNTs due to a good distribution and dispersion of the CNTs.     

Self-sensing and mechanical performance of CNT/GNP/UHMWPE biocompatible nanocomposites

Ultra-high molecular weight polyethylene (UHMWPE)-based conductive nanocomposites with reduced percolation and tunable piezoresistive behavior were prepared via solution mixing followed by compression molding using carbon nanotubes (CNT) and graphene nanoplatelets (GNP). The effect of varying wt% of GNP with fixed CNT content (0.1 wt%) on the mechanical, electrical, thermal and piezoresistive properties of UHMWPE nanocomposites was evaluated. The combination of CNT and GNP enhanced the dispersion in UHMWPE matrix and lowered the probability of CNT aggregation as GNP acted as a spacer to separate the entanglement of CNT with each other. This has allowed the formation of an effective conductive path between GNP and CNT in UHMWPE matrix. The thermal conductivity, degree of crystallinity and degradation temperature of the nanocomposites increased with increasing GNP content. The elastic modulus and yield strength of the nanocomposites were improved by 37% and 33%, respectively, for 0.1/0.3 wt% of CNT/GNP compared to neat UHMWPE. The electrical conductivity was measured using four-probe method, and the lowest electrical percolation threshold was achieved at 0.1/0.1 wt% of CNT/GNP forming a nearly two-dimensional conductive network (critical value, t = 1.20). Such improvements in mechanical and electrical properties are attributed to the synergistic effect of the two-dimensional GNP and one-dimensional CNT which limits aggregation of CNTs enabling a more efficient conductive network at low wt% of fillers. These hybrid nanocomposites exhibited strong piezoresistive response with sensitivity factor of 6.2, 15.93 and 557.44 in the linear elastic, inelastic I and inelastic II regimes, respectively, for 0.1/0.5 wt% of CNT/GNP. This study demonstrates the fabrication method and the self-sensing performance of CNT/GNP/UHMWPE nanocomposites with improved properties useful for orthopedic implants.     

Effect of carbon nanotube waviness on the effective thermoelastic properties of a novel continuous fuzzy fiber reinforced composite

This paper deals with the investigation of the effect of carbon nanotube (CNT) waviness on the effective coefficient of thermal expansion (CTE) of a novel continuous fuzzy fiber reinforced composite (FFRC). This novel FFRC is composed of carbon fibers, sinusoidally wavy CNTs and epoxy matrix. The sinusoidally wavy CNTs are radially grown on the circumferential surfaces of the carbon fibers. Analytical micromechanics model based on the method of cells (MOC) approach is derived to investigate the influence of the waviness of CNTs on the effective CTEs of the FFRC. The present study reveals that if the amplitudes of the radially grown sinusoidally wavy CNTs are parallel to the axis of the carbon fiber then the thermoelastic properties of the FFRC are significantly improved over those of the FFRC being composed of straight CNTs.     

Micromechanics modeling of the electrical conductivity of carbon nanotube (CNT)–polymer nanocomposites

A mixed micromechanics model was developed to predict the overall electrical conductivity of carbon nanotube (CNT)–polymer nanocomposites. Two electrical conductivity mechanisms, electron hopping and conductive networks, were incorporated into the model by introducing an interphase layer and considering the effective aspect ratio of CNTs. It was found that the modeling results agree well with the experimental data for both single-wall carbon nanotube and multi-wall carbon nanotube based nanocomposites. Simulation results suggest that both electron hopping and conductive networks contribute to the electrical conductivity of the nanocomposites, while conductive networks become dominant as CNT volume fraction increases. It was also indicated that the sizes of CNTs have significant effects on the percolation threshold and the overall electrical conductivity of the nanocomposites. This developed model is expected to provide a more accurate prediction on the electrical conductivity of CNT–polymer nanocomposites and useful guidelines for the design and optimization of conductive polymer nanocomposites.     

Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review

Carbon nanotubes (CNTs) hold the promise of delivering exceptional mechanical properties and multi-functional characteristics. Ever-increasing interest in applying CNTs in many different fields has led to continued efforts to develop dispersion and functionalization techniques. To employ CNTs as effective reinforcement in polymer nanocomposites, proper dispersion and appropriate interfacial adhesion between the CNTs and polymer matrix have to be guaranteed. This paper reviews the current understanding of CNTs and CNT/polymer nanocomposites with two particular topics: (i) the principles and techniques for CNT dispersion and functionalization and (ii) the effects of CNT dispersion and functionalization on the properties of CNT/polymer nanocomposites. The fabrication techniques and potential applications of CNT/polymer nanocomposites are also highlighted.     

Cryogenic properties of SiO2/epoxy nanocomposites

SiO₂/epoxy nanocomposites were prepared using diglycidyl ether of bisphenol-F (DGEBF) type epoxy and tetraethylorthosilicate (TEOS) via the sol-gel process. Silica nanoparticles were collected after burning off the matrix resin and the silica nanoparticles were observed using TEM. The cryogenic tensile properties at 77 K and thermal expansion coefficient of the nanocomposites were studied. The tensile properties at room temperature were also given to compare with the cryogenic tensile properties. The fracture surfaces were examined with scanning electron microscopy (SEM). The effects of silica nanoparticle content have been studied on the cryogenic tensile and thermal properties of the nanocomposites. In addition, the dependence of the glass transition temperature on the silica nanoparticle content has also been examined.     

Macroscopic carbon nanotube assemblies: preparation, properties, and potential applications

As classical 1D nanoscale structures, carbon nanotubes (CNTs) possess remarkable mechanical, electrical, thermal, and optical properties. In the past several years, considerable attention has been paid to the use of CNTs as building blocks for novel high-performance materials. In this way, the production of macroscopic architectures based on assembled CNTs with controlled orientation and configurations is an important step towards their application. So far, various forms of macroscale CNT assemblies have been produced, such as 1D CNT fibers, 2D CNT films/sheets, and 3D aligned CNT arrays or foams. These macroarchitectures, depending on the manner in which they are assembled, display a variety of fascinating features that cannot be achieved using conventional materials. This review provides an overview of various macroscopic CNT assemblies, with a focus on their preparation and mechanical properties as well as their potential applications in practical fields.     

Effect of dispersion conditions on the thermo-mechanical and toughness properties of multi walled carbon nanotubes-reinforced epoxy

In this work, multi wall carbon nanotubes (MWCNTs) dispersed in a polymer matrix have been used to enhance the thermo-mechanical and toughness properties of the resulting nanocomposites. Dynamic mechanical analysis (DMA), tensile tests and single edge notch 3-point bending tests were performed on unfilled, 0.5 and 1 wt.% carbon nanotube (CNT)-filled epoxy to identify the effect of loading on the aforementioned properties. The effect of the dispersion conditions has been thoroughly investigated with regard to the CNT content, the sonication time and the total sonication energy input. The CNT dispersion conditions were of key importance for both the thermo-mechanical and toughness properties of the modified systems. Sonication duration of 1 h was the most effective for the storage modulus and glass transition temperature (T_g) enhancement for both 0.5 and 1 wt.% CNT loadings. The significant increase of the storage modulus and T_g under specific sonication conditions was associated with the improved dispersion and interfacial bonding between the CNTs and the epoxy matrix. Sonication energy was the influencing parameter for the toughness properties. Best results were obtained for 2 h of sonication and 50% sonication amplitude. It was suggested that this level of sonication allowed appropriate dispersion of the CNTs to the epoxy matrices without destroying the CNT’s structure.