Vapor grown carbon fiber composites with epoxy and poly(phenylene sulfide) matrices

Vapor grown carbon fibers (VGCF, Pyrograf III™ from Applied Sciences, Inc.), with 100–300 nm diameters and ∽10–100 μm lengths, were formulated in various fiber volume fractions into epoxy (thermoset) and into poly(phenylene sulfide) (thermoplastic) composites. Increases in stiffness were observed as with previous VGCF/organic matrix composites. Large increases in flexural strengths were achieved in both systems demonstrating for the first time that discontinuous randomly oriented Pyrograf III™ can give strength increases and has substantial potential as a reinforcement in composites. Here-to-fore, addition of VGCF caused strength decreases. Voids, residual thermal strains (as the fiber surface area is ∽35 times greater than 7 μm-diameter PAN fiber), or uncertainties about fiber strength, fiber–matrix bonding and the degree of fiber dispersion, could cause losses of strength. Thermal conductivity properties of VGCF/ABS (acrylonitrile–butadiene–styrene from GE Plastics) and VGCF/epoxy composites with various fiber volume fractions were measured. Thermal conductivity increased with an increase in fiber volume fraction. However, these increases were not significant enough to make these VGCF fiber/organic matrix composites candidates for thermally conductive materials.     

Al2O3@Y3Al5O12纳米短纤维对铝合金基复合材料的增强作用

采用一种具有芯-壳结构的复合纳米纤维增强铝合金复合材料,可以在提高抗拉强度的同时增加塑性。通过真空热压烧结技术制备了Al₂O₃@Y₃Al₅O₁₂复合纳米短纤维增强2024铝合金复合材料。研究了纤维添加质量分数对复合材料致密度、硬度、抗拉强度及延伸率的影响;并探究了芯-壳结构在复合材料增韧中的作用。结果表明:Al₂O₃@Y₃Al₅O₁₂纳米短纤维具有良好的分散性,在超声分散及机械搅拌混粉后均匀吸附在铝合金颗粒表面,无分层及团聚现象;经热压烧结后,Al₂O₃@Y₃Al₅O₁₂纳米短纤维以短纤维形态均匀分散在铝合金基体内,少量添加Al₂O₃@Y₃Al₅O₁₂纳米短纤维起到了桥联和孔洞填充作用,使复合材料致密度和硬度提高;添加质量分数为1wt%时,抗拉强度和延伸率取得最大值,由铝合金的249.3 MPa、2.9%增加到299.1 MPa、4.3%。Al₂O₃@Y₃Al₅O₁₂纳米短纤维的添加可以细化晶粒,阻碍裂纹扩展,且在拔出/断过程中Al₂O₃@Y₃Al₅O₁₂纳米短纤维芯-壳结构的塑性变形起到了增强增韧作用。      

Review of the mechanical properties of carbon nanofiber/polymer composites

In this paper, the mechanical properties of vapor grown carbon nanofiber (VGCNF)/polymer composites are reviewed. The paper starts with the structural and intrinsic mechanical properties of VGCNFs. Then the major factors (filler dispersion and distribution, filler aspect ratio, adhesion and interface between filler and polymer matrix) affecting the mechanical properties of VGCNF/polymer composites are presented. After that, VGCNF/polymer composite mechanical properties are discussed in terms of nanofibers dispersion and alignment, adhesion between the nanofiber and polymer matrix, and other factors. The influence of processing methods and processing conditions on the properties of VGCNF/polymer composite is also considered. At the end, the possible future challenges for VGCNF and VGCNF/polymer composites are highlighted.     

Recent advances in polymer nanofibers

Polymer nanofibers, with diameters in the nanometer range, possess larger surface areas per unit mass and permit easier addition of surface functionalities compared with polymer microfibers. Hence, polymer nanofiber mats are being considered for use as filters, scaffolds for tissue engineering, protective clothing, reinforcement in composite materials and sensors. Although some of these applications are in the development stage, a few have been commercially exploited. Research on polymer nanofibers, nanofiber mats, and their applications has seen a remarkable growth over the last few years. However, a review of the various issues related to these nanofibers has not been published. This article presents a review of the recent trends in the processing methods and characterization techniques for polymer nanofibers. Research challenges and future trends in the processing and characterization of polymer nanofibers are discussed in the article. Five processing methods have been examined in this review, namely drawing, template synthesis, phase separation, self-assembly, and electrospinning. Among these methods, electrospinning has been used to convert a large variety of polymers into nanofibers and may be the only process that has the potential for mass production. The structure, morphology, and geometry of nanofibers and the porosity and tensile properties of nanofiber mats can be investigated through conventional techniques and instruments. But new techniques are needed for the mechanical testing of single nanofibers. Although measurement of mechanical properties such as tensile modulus, strength, and elongation is difficult because of the small diameters of the fibers, these properties are crucial for the proper use of nanofiber mats.     

Development of vapor grown carbon fibers (VGCF) reinforced carbon/carbon composites

C/C composites are developed using vapor grown carbon fibers (VGCF) with two types of pitches as matrix precursor. The composites are carbonized at 1000°C by applying the isostatic pressure throughout the carbonization process and further heat treated at different temperatures up to 2500°C in the inert atmosphere. By applying iso-static pressure one can able to developed VGCF based C/C composites possessing the very high bulk density (1.80 g/cm³) and apparent density (2.01 g/cm³) only by heat treatment up to 2500°C without any densification cycle. This high value of density is due to the extremely strong fiber-matrix interactions and self sintering between the VGCF fibers during carbonization process under iso-static pressure. From the SEM study it reveals that, fiber-matrix interactions are strong and fiber boundaries merges with each other, also there is not a evidence of matrix shrinkage cracks in case 1500°C heat treated composites. On the other hand, in 2500°C heat treated composites, there is evidence of uniform fiber-matrix interfacial cracks and porosity in nanometer dimensions. This is due to the change in fiber morphology above HTT 1500°C. But the formation of nano width cracks does not affect on the mechanical properties of composites. The compressive strength increases from 95MPa of 1500°C to 105 MPa of 2500°C heat treated composites. However, hardness decreases due to the increase in the degree of graphitization of composites on 2500°C. The study reveals that by controlling processing condition and the uniform dispersion of VGCF fibers in the matrix phase, it can be possible to developed nano porosity at fiber-matrix interface.     

Prediction of first matrix cracking in micro/nanohybrid brittle matrix composites

The classical Aveston–Cooper–Kelly shear-lag model for predicting the first matrix cracking strength in a brittle matrix composite is extended to the case of a hybrid brittle matrix composite containing both micro-scale and nano-scale fibers. First, closed-form solutions for the stresses in the two types of fibers and the matrix are derived. These are then used along with an energy analysis to predict the matrix cracking stress as a function of relevant material parameters. The analysis is applied to a typical Nicalon-SiC/CVI-SiC ceramic matrix composite containing additional nanofibers, for a wide range of nanofiber properties. A few volume percent of small diameter, moderate-stiffness nanofibers is predicted to provide significant strengthening and reduced crack opening while maintaining acceptable post-cracking fiber stresses. Various issues in the design of such micro/nanohybrid composites are then discussed.     

The effect of flow type and chemical functionalization on the dispersion of carbon nanofiber agglomerates in polypropylene

Two melt mixing methods generating flow with different characteristics are compared to study the dispersion of 5% w/w carbon nanofibers in polypropylene, namely conventional twin-screw extrusion and the simple application of a converging/diverging flow sequence to the composite melt. The effect of nanofiber surface modification and compatibilization with polypropylene modified with maleic anhydride is also studied. The dispersion of the nanofiber agglomerates through the polymer, the electrical properties of the resulting composites and nanofiber/polymer interface achieved under the different mixing conditions are analyzed. The converging/diverging flow sequence generated a better dispersion as compared with the twin-screw extruder.     

表面改性和复合工艺对金属纤维/聚合物复合材料性能的影响

以不锈钢纤维为导电填料,分别与ABS和PP复合,制得了电磁屏蔽用导电高分子复合材料。考察了表面改性和复合工艺对金属纤维／聚合物复合材料性能的影响。结果表明,用不同表面处理剂处理不锈钢纤维后,随纤维表面张力增加,复合材料的电阻率增加。使用母料法工艺可以有效地改善金属纤维在聚合物基体中的分散,从而提高复合材料的导电性能和电磁屏蔽性能。     

Stretching-induced crystallinity and orientation to improve the mechanical properties of electrospun PAN nanocomposites

Polyacrylonitrile-based carbon fibers, embedded with single-walled carbon nanotubes have been prepared by the electrospinning technique. The as-spun nanofibers were hot-stretched in an oven to enhance the orientation and crystallinity which has been confirmed by X-ray diffraction and DSC etc. With the hot-stretched process and the introduction of SWNTs, the mechanical properties of PAN nanofibers such as the modulus and tensile strength will be enhanced correspondingly. In addition, the electrical conductivities of the PAN/SWNTs nanofiber composites were also enhanced. It was concluded that the hot-stretched nanofibers and the PAN/SWNTs nanofiber composites can be used as a potential precursor to produce high-performance carbon nanocomposites.     

含氟聚合物纳米多孔纳米纤维膜的制备

采用"电纺-相分离-沥滤"方法制备了聚(偏氟乙烯-co-六氟丙烯)(PVDF-HFP)以及聚偏氟乙烯(PVDF)纳米多孔纳米纤维膜.首先,将PVDF-HFP或PVDF和致孔剂聚乙烯吡咯烷酮(PVP)混合电纺,得到共混物纳米纤维膜.然后,将纳米纤维膜在水中沥洗出共混物中的PVP,获得纳米多孔纳米纤维膜.用场发射扫描电子显微镜(FESEM)观察水洗前后纤维表面精细结构.结果表明,纳米多孔纳米纤维表面呈多孔结构,孔径数10 nm.PVP的分子量对水洗后纤维表面结构有明显影响.致孔剂含量不同获得的PVDF-HFP纳米多孔纤维膜力学性能相近.     

VGCF的表面处理对VGCF/SMPU复合材料力学和热学性能的影响

用二步法对气相生长碳纤维(VGCF)进行表面改性处理,然后用溶液混合法制备了VGCF/形状记忆聚氨酯(SMPU)复合材料.用扫描电镜观察分析了VGCF在SMPU基体中的分散性以及与基体的界面结合情况,研究了复合材料的力学性能和热学性能.结果表明:与未经二步法处理的VGCF相比,用二步法表面处理使VGCF在基体中的分散性及与基体的界面结合能力得到较大的提高,且使其对复合材料的拉伸强度及拉伸弹性模量的增强效果更为明显;虽然SMPU与VGCF复合后的断裂伸长率有所降低,但是与未处理的VGCF制备的复合材料相比,断裂伸长率有明显增大;表面处理的VGCF更有利于提高复合材料的热稳定性.     

Alkylated graphene nanosheet composites with polyaniline nanofibers

We demonstrate a simple method to prepare alkylated graphene/polyaniline composites (a-GR/PANI) using solution mixing of exfoliated alkyl Iodododecane treated graphene oxide sheets with polyaniline nanofiber; polyaniline nanofibers (PANI) prepared by using rapid mixing polymerization significantly improve the processibility of polyaniline and its performance in many conventional applications. Also, polyaniline nanofibers exhibit excellent water dispersibility due to their uniform nanofiber morphology. Morphological study using SEM and TEM analysis showed that the fibrous PANI in the composites a-GR/PANI mainly adsorbed onto the surface or intercalated between the graphene sheets, due especially to the good interfacial interaction between the alkylated gaphene and the polyaniline nanofibers. The existence of polyaniline nanofibers on the surface of the garphene and the alkylated graphene sheets was confirmed by using FT-IR, FT-Raman and X-ray diffraction analysis. Due to the good interfacial interaction between the alkylated graphene and the polyanilines nanofibers, the composite (a-GR/PANI) exhibited excellent dispersion stability in DMF compared to the same composite (GR/PANI) without alkylation. The electrical conductivity of the (GR/PANI) composite was 9% higher than that of pure PANI and the same weight percent for the composite after alkylation was 13% higher than that of pure PANI nanofibers.     

Preparation and characterization of polystyrene-multiwalled carbon nanotube composite fibers by electrospinning

Electrospinning is a simple technique to prepare polymer fibers in nanometer diameters using very high electrostatic fields. These nanofibers are useful in many applications since the surface area to volume ratio in these fibers is very high. Here we have prepared nanofibers of composites of Polystyrene (PS) with different concentrations (0.05% to 2% w/w) of multi-walled carbon nanotubes (MWCNT) by electrospinning. The fibers are characterized by SEM, TEM, Raman spectra, and TGA. The room temperature electrical conductivity of single fibers of these composites are measured and found to show dramatic improvement in the conductivity due to the addition of MWCNT to the polymer matrix compared to the pure PS.     

Carbon-nanofiber composite electrodes for thin and flexible lithium-ion batteries

Addition of vapor-grown carbon nanofiber (VGCF) into a LiCoO₂ composite electrode increases electrode’s conductivity and adhesion strength significantly. These increases are attributed to the uniform distribution of network-like VGCF of high conductivity; VGCF not only connects the surface of the active materials, its network penetrates into and connects each active material particle. VGCF composite electrode also improves the electrochemical performance of thin and flexible lithium-ion batteries such as discharge capacity at high current densities, cycle-life stability, and low-temperature (at −20 °C) discharge capacity. These improved electrochemical properties are attributed to the well-distributed network-like carbon nanofibers, VGCF, within the cathode. The addition of VGCF reduces the electron conducting resistance and decreases the diffusion path for lithium ions, hence increases the utilization of active materials during high-current discharge and low-temperature discharge. In addition, network-like VGCF forms a more uniform cathode structure so as to have a lower deterioration rate and correspondingly better life cycle stability.     

Interfacial stress transfer and property mismatch in discontinuous nanofiber/nanotube composite materials

Novel nanotubes/nanofibers with high strength and stiffness did not lead to high failure strengths/strains of nanocomposite materials. Therefore, the interfacial stress transfer and possible stress singularities, arising at the interfacial ends of discontinuous nanofibers embedded in a matrix, subjected to tensile and shear loading, were investigated by finite element analysis. The effects of Young's moduli and volume fractions on interfacial stress distributions were studied. Round-ended nanofibers were proposed to remove the interfacial singular stresses, which were caused by high stiffness mismatch of the nanoscale reinforcement and the matrix. However, the normal stress induced in the nanofiber through interfacial stress transfer was still less than 2 times that in the matrix. This stress value is far below the high strength of the nanofiber. Therefore, the load transfer efficiency of discontinuous nanofibers or nanotube composites is very low. Hence, nanofibers or nanotubes in continuous forms, which also preclude the formation of singular interfacial stress zones, are recommended over discontinuous nanofibers to achieve high strengths in nanocomposite materials.     

Highly Sensitive,Anisotropic, and Reversible Stress/Strain‐Sensors from Mechanochromic Nanofiber Composites

Mechanochromic polymeric systems are intensively investigated for real‐time stress detection applications. However, an effective stress‐sensing material must respond to low deformation with a detectable color change that should be quickly reversible upon force unloading. In this work, mechanochromic nanofibers made by electrospinning are used to produce mechanochromic nanofiber/poly(dimethylsiloxane) (PDMS) composites with isotropic and anisoptropic response. Due to chain alignment of spiropyran copolymer chains within the nanofibers, only very small strains are required to yield a mechanochromic response. Composites with aligned and isotropic nanofibers show anisotropic and isotropic mechanochromic behavior, respectively. Due to the special substitution pattern of spiropyran in the copolymer, the mechanochromic response of these nanofiber/PDMS composites shows fast reversibility upon force unloading. The outstanding benefit of using highly sensitive mechanochromic nanofibers as filler in composite materials allows the detection of directional stress and strain, and it is a step forward in the development of smart, mechanically responsive materials.     

Carbothermal transformation of a graphitic carbon nanofiber/silica aerogel composite to a SiC/silica nanocomposite

Carbon nanofiber/silica aerogel composites are prepared by sol-gel processing of surface-enhanced herringbone graphitic carbon nanofibers (GCNF) and Si(OMe)4, followed by supercritical CO2 drying. Heating the resulting GCNF/silica aerogel composites to 1650 degrees C under a partial pressure of Ar gas initiates carbothermal reaction between the silica aerogel matrix and the carbon nanofiber component to form SiC/silica nanocomposites. The SiC phase is present as nearly spherical nanoparticles, having an average diameter of ca. 8 nm. Formation of SiC is confirmed by powder XRD and by Raman spectroscopy.     

Fabrication of Nanofiber Reinforced Protein Structures For Tissue Engineering

Extracellular matrices and degradable nanofibers are two very promising materials in the field of tissue engineering; however both of these structures face limitations as tissue engineering scaffolds. Extracellular matrices, such as collagen, gelatin, and laminin, have excellent biocompatibility and allow cell in growth and survival, but structural weakness makes them difficult to handle and greatly limits their uses. Degradable nanofibers support cell attachment and can provide structural support and directional guidance, but individual degradable nanofibers are fragile and have a tendency to form dense fiber bundles which limit cell penetration into the spaces between the nanofibers, especially in the case of aligned nanofibers. To overcome these difficulties, degradable loose nanofibers were embedded in protein matrix in an attempt to fabricate a hybrid scaffold with improved properties, such as improved strength, guidance, spacing among nanofibers, etc. Polycaprolactone (PCL) was used as a model material for degradable nanofibers. Gelatin was employed as a model protein for matrix structure formation. Thin hybrid films (average thickness = 2.78 um) were fabricated by wetting the loose aligned undirectional nanofiber arrays or loose aligned bi-directional nanofiber grids with a gelatin aqueous solution, which also allows for live cell loading into the nanofiber-protein composite if cell are premixed with protein solution or on the surface of the films. Gelatin film alone without nanofiber reinforcement is difficult to handle due to the weakness of the thin membrane. Gelatin films with a fiber density as low as 3% v/v were structurally robust enough for handling, and manipulation into complex shapes. Mechanical testing confirmed that the addition of nanofibers enhanced the strength of gelatin films, in both dry and hydrated state. In vitro testing confirmed that nanofiber reinforced films were biocompatible and provided cells with directional guidance. Results demonstrate the promise of gelatin/PCL nanofiber composites as a tissue scaffolding material.     

Controlling electrical percolation in multicomponent carbon nanotube dispersions

Carbon nanotube reinforced polymeric composites can have favourable electrical properties, which make them useful for applications such as flat-panel displays and photovoltaic devices. However, using aqueous dispersions to fabricate composites with specific physical properties requires that the processing of the nanotube dispersion be understood and controlled while in the liquid phase. Here, using a combination of experiment and theory, we study the electrical percolation of carbon nanotubes introduced into a polymer matrix, and show that the percolation threshold can be substantially lowered by adding small quantities of a conductive polymer latex. Mixing colloidal particles of different sizes and shapes (in this case, spherical latex particles and rod-like nanotubes) introduces competing length scales that can strongly influence the formation of the system-spanning networks that are needed to produce electrically conductive composites. Interplay between the different species in the dispersions leads to synergetic or antagonistic percolation, depending on the ease of charge transport between the various conductive components.     

表面改性处理对气相生长碳纤维的微观结构影响

为了使气相生长碳纤维(VGCF)能更好地作为高分子基体的增强材料,采用双氧水和硝酸(H₂O₂和HNO₃)二步处理法、硅烷偶联剂处理法及H₂O₂和HNO₃处理后再用硅烷偶联剂处理的联用改性法分别对VGCF进行表面改性处理,研究表面改性对VGCF微观结构的影响。利用AFM、FTIR、TG和XRD比较分析了改性处理对VGCF的表面微观结构、官能团、热稳定性和晶格等的影响。结果表明：3种处理方法对VGCF晶格无明显影响;H₂O₂和HNO₃二步处理法能在纤维表面接枝羧基等含氧基团;硅烷偶联剂处理法能使纤维表面接枝硅氧烷低聚物;H₂O₂和HNO₃处理后再用硅烷偶联剂处理联用改性VGCF,能使其表面接枝上更多的硅氧烷低聚物,有利于提高VGCF与高分子材料的亲和性。