Processing-structure–property relationships of SWNT–epoxy composites prepared using ionic liquids

Experimentally achieved mechanical properties of nanotube–epoxy composites fail to match theoretical expectations; shortcomings are mainly attributed to poor dispersion. The elastic modulus of a well-dispersed single walled carbon nanotube (SWNT)-ionic liquid-epoxy composite was evaluated in tension and compared to predictions by a micromechanics homogenization model. The model takes into account the mechanical properties of the constituent phases in addition to SWNT aspect ratio, spatial distribution, dispersion, and agglomeration. These parameters were evaluated using information obtained via scanning and transmission electron microscopy. The Young’s modulus of the composite shows excellent agreement with the model at low concentrations, while discrepancies at high SWNT concentrations are possibly due to composite processing limitations. At high concentrations the uncured composite mixture is above the rheological percolation threshold. As the polymer network reaches its maximum capacity for well-dispersed SWNTs, increasing volume fraction does not result in further significant reinforcing effects.     

Molecular dynamics simulation of SWCNT–polymer nanocomposite and its constituents

Elastic and engineering properties of nanoparticle enhanced composites and their constituents (matrix, reinforcement and interface) are calculated. The nanocomposites considered in this study consist of a single-wall carbon nanotube (SWCNT) embedded in polyethylene matrix. Molecular dynamics simulations are used to estimate the elastic properties of SWCNT, interfacial bonding, polyethylene matrix and composites with aligned and randomly distributed SWCNTs. The elastic properties of bundles with 7, 9, and 19 SWCNTs are also compared using a similar approach. In all simulations, the average density of SWCNT–polymer nanocomposite was maintained in the vicinity of CNTs, to match the experimentally observed density of a similar nanocomposite. Results are found to be in good agreement with experimentally obtained values by other researchers. The interface is an important constituent of CNT–polymer composites, which has been modeled in the present research with reasonable success.     

Magnetic orientation of single-walled carbon nanotubes or their composites using polymer wrapping

Abstract

The magnetic orientation of single-walled carbon nanotubes (SWNTs) or the SWNT composites wrapped with polymer using poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEHPPV) as the conducting polymer were examined. The formation of SWNT/MEHPPV composites was confirmed by examining absorption and fluorescence spectra. The N,N-dimethylformamide solution of SWNT/MEHPPV composites or the aqueous solution of the shortened SWNTs was introduced dropwise onto a mica or glass plate. The magnetic processing of the composites or the SWNTs was carried out using a superconducting magnet with a horizontal direction (8 T). The AFM images indicated that the SWNT/MEHPPV composites or the SWNTs were oriented randomly without magnetic processing, while with magnetic processing (8 T), they were oriented with the tube axis of the composites or the SWNTs parallel to the magnetic field. In polarized absorption spectra of SWNT/MEHPPV composites on glass plates without magnetic processing, the absorbance due to semiconducting SWNT in the near-IR region in horizontal polarized light was almost the same as that in vertical polarized light. In contrast, with magnetic processing (8 T), the absorbance due to semiconducting SWNT in the horizontal polarization direction against the direction of magnetic field was stronger than that in the vertical polarization direction. Similar results were obtained from the polarized absorption spectra for the shortened SWNTs. These results of polarized absorption spectra also support the magnetic orientation of the SWNT/MEHPPV composites or the SWNTs. On the basis of a comparison of the composites and the SWNTs alone, the magnetic orientation of SWNT/MEHPPV composites is most likely ascribable to the anisotropy in susceptibilities of SWNTs.     

Characterization of interfacial stress transfer ability of particulate cellulose composite materials

Composites with cellulose reinforcements are steadily gaining increased use. The stress transfer ability between reinforcement and polymer matrix has a strong influence on mechanical properties like strength and fracture toughness. This work presents a method to assess the stress transfer ability between cellulose and polymer matrix from a model material with cellulose spheres embedded in a polymer matrix. Such a material show smaller variability compared with composites based on natural cellulose fibres, and is less cumbersome than single fibre tests with regard to interfacial characterization. Measured elastic moduli of particulate composites is compared with predicted values from a micromechanical model based on a composite sphere assembly in a self-consistent scheme with only a spring constant of an imperfect interface as fitting parameter expressed in Pa/m. This interface parameter is identified through inverse modelling and used to quantify stress-transfer ability of cellulose/polylactide and cellulose/polystyrene composite interfaces. A higher degree of interfacial interaction was found for the former. This ranking was corroborated by adhesive force measurements using a micrometre sized cellulose sphere attached to the end of a cantilever in an atomic force microscope. With the model microstructure of a cellulose-sphere composite, an interfacial efficiency parameter can be backed out from stiffness measurements to be used in e.g. ranking of different fibre surface treatments and choice of matrix in the development of stronger natural-fibre composites.     

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.     

Effect of Yarn Distortion on the Mechanical Properties of Fiber-Bar Composites Reinforced by Three-Dimensional Weaving

A meso-structure model of fiber-bar composites reinforced by three-dimensional weaving (FBCR3DW) is proposed. Optical microscopy images of the preform structure revealed that the fibers along the circumference of the yarn cross-weave were twisted randomly due to alternating yarn winding on either side of the fiber bars during the manufacturing process. Sections of the cross-woven yarn were divided into five regions based on the twist characteristics. Stochastic function theory was used to describe the twist characteristics and to calculate the compliance tensor for each twisted yarn region. The twist characteristics and compliance tensor of each region were then introduced into a finite element model to calculate the elastic properties of the twisted yarn and FBCR3DW; unidirectional tensile stress–strain curves were generated based on the Tsai–Wu failure criterion. Several FBCR3DW specimens with randomly twisted yarns inside the weave structure were used in experimental tests. Our numerical results were in good agreement with the experimental values. Yarn distortion had a significant effect on the elastic properties and axial tensile strength of the yarn; specifically, the influence of yarn distortion on the transverse elastic modulus and transverse shear modulus of FBCR3DW was severe, whereas only a slight effect occurred with regard to the other elastic constants and unidirectional tensile properties. Thus, the proposed method provides an effective reference for modeling fiber composites with a weave structure.     

The effective properties and local aggregation effect of CNT/SMP composites

A micromechanics model of the thermomechanical constitutive behavior and micro-structural inhomogeneity of carbon nanotubes (CNTs)/shape memory polymer (SMP) composites is presented. It is assumed that the CNTs are elastic and the SMP obeys a thermomechanical constitutive law. The effective properties of CNT/SMP composites are examined using a micro-mechanics method. The effect of CNT aggregation in the composite, frequently encountered in real engineering situations, is studied. The degree of aggregation is described by an aggregation coefficient, and the effective properties of SMP composites with aggregated CNTs are calculated using a stepping scheme. It is shown that the degree of CNT aggregation dramatically influences the effective properties of the CNT/SMP composites. A homogeneous microstructure leads to maximum levels of effective composite properties.     

Elastic constants and thermal expansion of aluminum-SiC metal-matrix composites

The elastic behavior and the thermal expansivity of metal-matrix composites have been investigated using ultrasonic velocity and strain gage measurements. The composites used in this study consisted of three aluminum alloys reinforced with different concentrations of SiC particles. The results show that the elastic constants increase and the coefficients of thermal expansion decrease with particle content. The results also show that the behavior of elastic constants with reinforcement can be best represented by the calculations of the upper and lower bounds of Hashin and Shtrikman. The behavior of thermal expansion, however, agrees with bounds developed by Schapery. In addition, both properties are found to be related through a model linking the strain to the elastic and thermal stresses in the composite. This relationship gives promise for the nondestructive characterization of the composites using these measurements.     

Effective reinforcement in carbon nanotube-polymer composites

Carbon nanotubes have mechanical properties that are far in excess of conventional fibrous materials used in engineering polymer composites. Effective reinforcement of polymers using carbon nanotubes is difficult due to poor dispersion and alignment of the nanotubes along the same axis as the applied force during composite loading. This paper reviews the mechanical properties of carbon nanotubes and their polymer composites to highlight how many previously prepared composites do not effectively use the excellent mechanical behaviour of the reinforcement. Nanomechanical tests using atomic force microscopy are carried out on simple uniaxially aligned carbon nanotube-reinforced polyvinyl alcohol (PVA) fibres prepared using electrospinning processes. Dispersion of the carbon nanotubes within the polymer is achieved using a surfactant. Young's modulus of these simple composites is shown to approach theoretically predicted values, indicating that the carbon nanotubes are effective reinforcements. However, the use of dispersant is also shown to lower Young's modulus of the electrospun PVA fibres.     

Mechanical Reinforcement of Polymers Using Carbon Nanotubes

Owing to their unique mechanical properties, carbon nanotubes are considered to be ideal candidates for polymer reinforcement. However, a large amount of work must be done in order to realize their full potential. Effective processing of nanotubes and polymers to fabricate new ultra‐strong composite materials is still a great challenge. This Review explores the progress that has already been made in the area of mechanical reinforcement of polymers using carbon nanotubes. First, the mechanical properties of carbon nanotubes and the system requirements to maximize reinforcement are discussed. Then, main methods described in the literature to produce and process polymer–nanotube composites are considered and analyzed. After that, mechanical properties of various nanotube–polymer composites prepared by different techniques are critically analyzed and compared. Finally, remaining problems, the achievements so far, and the research that needs to be done in the future are discussed.     

Composite electronic materials based on poly(3,4-propylenedioxythiophene) and highly charged poly(aryleneethynylene)-wrapped carbon nanotubes for supercapacitors

Supercapacitor charge storage media were fabricated using the semiconducting polymer poly(3,4-propylenedioxythiophene) (PProDOT) and single-walled carbon nanotubes (SWNTs) that were helically wrapped with ionic, conjugated poly[2,6-{1,5-bis(3-propoxysulfonicacidsodiumsalt)}naphthylene]ethynylene (PNES). These PNES-wrapped SWNTs (PNES-SWNTs) enable efficient dispersion of individualized nanotubes in a wide range of organic solvents. PNES-SWNT film-modified Pt electrodes were prepared by drop casting PNES-SWNT suspensions in MeOH; high stability, first-generation PProDOT/PNES/SWNT composites were realized via electropolymerization of the ProDOT parent monomer (3,4-propylenedioxythiophene) in a 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/propylene carbonate solution at the PNES-SWNT-modified electrode. The electrochemical properties of PProDOT and PProDOT/PNES/SWNT single electrodes and devices were examined using cyclic voltammetric methods. The hybrid composites were found to enhance key supercapacitor figures of merit (charge capacity and capacitance) by approximately a factor of 2 relative to those determined for benchmark Type I devices that exploited a classic PProDOT-based electrode material. The charge/discharge stability of the supercapacitors was probed by repeated rounds of cyclic voltammetric evaluation at a minimum depth of discharge of 73%; these experiments demonstrated that the hybrid PProDOT/PNES/SWNT composites retained ~90% of their initial charge capacity after 21,000 charge/discharge cycles, contrasting analogous data obtained for PProDOT-based devices, which showed only 84% retention of their initial charge capacity.     

Rheological behaviour and mechanical characterization of injectable poly(propylene fumarate)/single-walled carbon nanotube composites for bone tissue engineering

This work investigated the effects of the use of a surfactant or the functionalization of single-walled carbon nanotubes (SWNTs) on their dispersion in uncrosslinked poly(propylene fumarate) (PPF) and the mechanical reinforcement of crosslinked composites as a function of the SWNT concentration. Rheological measurements showed good dispersion of SWNTs in uncrosslinked PPF at low concentrations of 0.05?wt% and SWNT aggregation for higher concentrations for all formulations examined. Mechanical testing demonstrated significant reinforcement in the compressive and flexural mechanical properties of crosslinked nanocomposites which peaked for low SWNT concentrations of the order of 0.05?wt%. For example, a 74% increase was recorded for the compressive modulus and a 69% increase for the flexural modulus of nanocomposites with functionalized SWNTs at a 0.05?wt% loading. Nevertheless, this reinforcement was not related to the use of a surfactant or the functionalization of the SWNTs tested. Scanning electron microscopy examinations of fractured nanocomposite surfaces revealed the formation of SWNT aggregates at higher concentrations corroborating the rheological and mechanical data. These results suggest that the dispersion of individual SWNTs in a uncrosslinked formulation is pivotal to the development of injectable nanocomposites for bone tissue engineering applications.     

Modeling elastic properties of short flax fiber-reinforced composites by orientation averaging

Natural fibers of plant origin, used as reinforcement in polymer matrix composite materials, exhibit highly anisotropic elastic properties due to their complex internal structure. Mechanical properties can be evaluated not only by tests but also by mechanical models reflecting the principal morphological features of fibers. Such a FEM model is applied to estimate the elastic properties of a unit cell of a short-fiber-reinforced composite, an elementary flax fiber embedded in a polymer matrix. Orientation averaging approach is used for prediction of the stiffness of short flax fiber reinforced polymer matrix composite. The numerical estimates of Young’s modulus are compared to the test results of extruded flax/polypropylene composite.     

Multiscale modeling for mechanical properties of carbon nanotube reinforced nanocomposites subjected to different types of loading

Multiscale modeling was presented for the nonlinear properties of polymer/single wall carbon nanotube (SWNT) nanocomposite under tensile, bending and torsional loading conditions. To predict the mechanical properties of both armchair and zigzag SWNTs, a finite element (FE) model based on the theory of molecular mechanics was used. For reducing the computational efforts, an equivalent cylindrical beam element was proposed, which has the unique advantage of describing the mechanical properties of SWNTs considering the nonlinearity of SWNT behavior. For a direct evaluation of the rigidities of the proposed equivalent beam, the data obtained through atomistic FE analyses of SWNT were fitted to six different equations, covering the three types of loading for both armchair and zigzag configurations. The proposed equivalent beam element was then used to build a cylindrical representative volume element (RVE) using which the effects of the interphase between SWNT and the polymer on the mechanical properties of RVE could be studied. It was found that while the interphase has a small effect on the nanocomposite stiffness, the ratio of (SWNT length)/(RVE length) dramatically affects the nanocomposite stiffness.     

Processing and performance improvements of SWNT paper reinforced PEEK nanocomposites

In this letter, the sandwich-like single-walled carbon nanotube (SWNT) paper/polyetheretherketone (PEEK) composites were successfully prepared by using a hot-compress method. Based on SEM observation, it was found that the PEEK macromolecules could diffuse into the voids of SWNT paper and were able to form very good bonding to the nanotube bundles, which ensure effective stress transfer between two phases. Some typical problems in preparing conventional carbon nanotube/polymer composites, e.g. the difficulty to obtain well-dispersed high-loading nanotubes into polymer matrices, were successfully overcome. Moreover, the compact networks of SWNTs were not destroyed at the appropriate processing condition. Correspondingly, the resulting composite (with one layer of SWNT paper) exhibited about 40% increased in Young’s modulus and 4% enhanced in failure strength when comparing with that of neat PEEK, respectively. According to rule-of-mixtures of the special sandwich-like structure, the estimated Young’s modulus can reach up to about 8 GPa, which was nearly three times of that of neat matrix. Moreover, the surface electric conductivity and thermal conductivity of the PEEK films were also increased after the addition of SWNT paper. The preliminary results suggest that the SWNT paper has great potential for being used to reinforce polymers.     

A study on fiber orientation influence on the mechanical response of a short fiber composite structure

This paper deals with the structural analysis of composite materials with non-homogenous orientation of the reinforcement. During this research, a short fiber-reinforced polymer matrix composite is studied. In this case, inhomogeneity of the reinforcement orientation caused by injection molding manufacturing process is analyzed. The main objective of the paper is the investigation of an influence of process-induced orientation of the reinforcement on mechanical properties of the material in comparison with unidirectional and random reinforcement orientation. In particular, natural frequencies and transient response of an exemplary composite component are investigated. To specify effective properties of the composite, Mori–Tanaka’s micromechanical model is assumed. Orientation distribution of the reinforcement is determined by injection molding simulation. To determine elastic material properties dependent on non-homogenous orientation of the reinforcement, an orientation averaging procedure is taken into account. Therefore, during this study, effectiveness of the orientation averaging procedure and different closure approximations influence on the results are studied. Orientation averaging results are compared with numerical results obtained by finite element-based homogenization of composites with prescribed second-order orientation tensor. Finally, the obtained material parameters were applied into a macroscale finite element model, and numerical simulation with different boundary conditions was conducted.     

Effect of soy spent flakes and carbon black co-filler in rubber composites

The rubber composites that are reinforced by a mixture of soy spent flakes (SSF) and carbon black (CB) are investigated in terms of their viscoelastic properties. Soy spent flakes is a plentiful renewable material from the waste stream of commercial soy protein extraction. SSF contains mostly soy carbohydrate and dry SSF increases rubber modulus significantly. The aqueous dispersions of SSF and CB were first mixed and then blended with styrene–butadiene latex to form rubber composites by freeze-drying and compression molding method. The mixtures of SSF and CB at three different ratios are investigated as co-fillers. A 30% co-filler reinforced composite exhibits about 100 times increase in the shear elastic modulus compared with unfilled SB rubber, showing a significant reinforcement effect by the co-filler. Compared with the SSF composites, the recovery behaviors of the co-filler composites after the eight consecutive deformation cycles of dynamic strain are improved and are similar to that of the CB composites. The comparison of viscoelastic properties of the composites prepared by freeze-drying and casting methods indicates the composites prepared by freeze-drying method have a lower elastic modulus, but have a better recovery behavior due to its polymer mediated filler network structure. The co-filler composites with 50–75% substitution of CB by SSF have a greater elastic modulus than the CB reinforced composites.     

碳纤维网络增强环氧树脂基复合材料的制备与表征

为提高碳纤维/环氧树脂复合材料的刚性和热尺寸稳定性,首先利用短切碳纤维制备了碳纤维网络增强体(CFNR),并将其与环氧树脂复合制备了CFNR/环氧树脂新型复合材料。然后,分别利用扫描电镜和热机械分析仪对CFNR/环氧树脂复合材料的微观结构和热力学性能进行了表征。结果表明:CFNR/环氧树脂复合材料中有明显的网络节点,即碳质粘结点;CFNR/环氧树脂复合材料具有较好的导电性、较高的刚性和较低的热膨胀性,其弹性模量分别为常规短切碳纤维/环氧树脂复合材料及纯环氧树脂的3倍和6倍,平均热膨胀系数(60~200℃)分别为常规短切碳纤维/环氧树脂复合材料的1/15及纯环氧树脂的1/40;随着温度升高,CFNR/环氧树脂复合材料、常规短切碳纤维/环氧树脂复合材料及纯环氧树脂的弹性模量均因环氧树脂变软而降低,当温度高于80℃时,CFNR/环氧树脂复合材料的弹性模量分别约为常规短切碳纤维/环氧树脂复合材料的7倍和纯环氧树脂的近70倍。研究结论可以为开发高刚性、低膨胀聚合物基复合材料提供实验依据和理论指导。     

碳纳米管/聚合物复合材料界面结合性能的研究进展

碳纳米管(CNT)优异的力学性能使其成为复合材料优选的增强体。CNT/聚合物复合材料的力学性能主要受其界面结合性能的影响。综述了CNT/聚合物复合材料界面结合性能的研究方法和研究现状。对CNT/聚合物复合材料界面结合性能的研究,实验上采用微观表征技术、拉曼光谱分析技术和纳米力学拔出法,分子模拟方法则是通过对CNT施加位移或外力模拟CNT从聚合物基体中的抽拔过程。概述了聚合物的类型、晶态结构以及CNT的手性、功能化处理等因素对CNT/聚合物复合材料界面结合性能的影响,并展望了CNT/聚合物复合材料界面结合性能未来研究的重点方向。