Anisotropic properties of aligned SWNT modified poly (methyl methacrylate) nanocomposites

The poly (methyl methacrylate) (PMMA)/single-walled carbon nanotube (SWNT) composites with good uniformity, dispersion and alignment of SWNT were fabricated in an improved figuration process. The semidried mixture was stretched along one direction at a drawing ratio of 50 before it was dried, and then folded along the same direction stretching repeatedly for 100 times. The transmission electron microscopic (TEM) observation demonstrated that SWNT in the PMMA/SWNT composite tends to align in the stretching direction owing to a torque exerting on it in the stretching process. The electrical and mechanical properties of PMMA/SWNT composite were studied as a function of SWNT orientation and concentration. The aligned SWNT modified PMMA/SWNT composite presented highly anisotropic properties. The experimental results showed that the electrical conductivity and mechanical properties of composite rise with the increase of SWNT concentration, and that composite films showed higher conductivity and higher mechanical draw ratios along the stretched direction than perpendicular to it. The thermogravimetric analysis (TGA) revealed that embedding the SWNTs into the PMMA matrix also improves the thermal stability of the composite.     

The effects of CNT alignment on electrical conductivity and mechanical properties of SWNT/epoxy nanocomposites

The single-walled carbon nanotubes (SWNTs) filled nanocomposite SWNT/epoxy resin composite with good uniformity, dispersion and alignment of SWNTs and with different SWNTs concentrations was produced by solution casting technique. Subsequently, the semidried mixture was stretched repeatedly along one direction at a large draw-ratio of 50 for 100 times at ambient atmosphere manually to achieve a good alignment and to promote dispersion of SWNTs in the composite matrix. Composite showed higher electrical conductivities and mechanical properties such as the Young’s modulus and tensile strength along the stretched direction than perpendicular to it, and the electrical property of composite rise with the increase of SWNT concentration. The percolation threshold value of electrical conductivity along the stretching direction is lower than the value perpendicular to the SWNTs orientation. In addition, the anisotropic electric and mechanical properties results, SEM micrograph and the polarized Raman spectra of the SWNT/epoxy composite reveal that SWNTs were well dispersed and aligned in the composites by the repeated stretching process.     

Structure and properties of highly oriented polyoxymethylene/multi-walled carbon nanotube composites produced by hot stretching

Highly-oriented polyoxymethylene (POM)/multi-walled carbon nanotube (MWCNT) composites were fabricated through solid hot stretching technology. With the draw ratio as high as 900%, the oriented composites exhibited much improved thermal conductivity and mechanical properties along the stretching direction compared with that of the isotropic samples before drawing. The thermal conductivity of the composite with 11.6 vol.% MWCNTs can reach as high as 1.2 W/m K after drawing. Microstructure observation demonstrated that the POM matrix had an ordered fibrillar bundle structure and MWCNTs in the composite tended to align parallel to the stretching direction. Wide-angle X-ray diffraction results showed that the crystal axis of the POM matrix was preferentially oriented perpendicular to the draw direction, while MWCNTs were preferentially oriented parallel to the draw direction. The strong interaction between the POM matrix and the MWCNTs hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs.     

Transparent conductive thin film synthesis based on single-walled carbon nanotubes dispersion containing polymethylmethacrylate binder

Transparent conductive hybrid thin films of single-walled carbon nanotubes (SWNTs) and polymethyl methacrylate (PMMA) are fabricated using dispersions containing SWNTs and water-borne PMMA binder. The polymer binder was used as adhesion promoter between the SWNTs and the substrate. The polymer binder content in the SWNTs dispersion is varied to obtain the optimum optical transmittance, electrical conductivity, and mechanical adhesion. The PMMA and SWNT network formed the composite over substrate. The fabricated SWNTs/PMMA hybrid films are immersed in nitric acid (HNO3) and thionyl chloride (SOCl2) to improve electrical conductivity. SWNTs films with 0.2-0.6 mg/ml polymer binder have sheet resistance of 80-140 ohms/sq at a transmittance of about 80% and a strong adhesion on glass substrate. Furthermore, the electrical stability of the films is improved via the PMMA addition. This results indicates that the SWNTs/PMMA hybrid films fabricated by this method can be used as an alternative of indium tin oxide (ITO) film on flexible substrate.     

On the texture of γFe2O3particles in magnetically and mechanically oriented magnetic tape

The degree of preferred orientation in magnetic recording tape was investigated by an X-ray pole figure technique and by measuring magnetic properties. Acicular γFe2O3particles dispersed in PVA film were oriented by mechanical stretching and magnetic orientation. The pole figures of these samples indicated the texture with [110] to the orientating direction. The degree of orientation in stretched film was much higher than in magnetically oriented film. Further, it was observed that the particles tend to align uniaxially in mechanical orientation, and biaxially in magnetic orientation.     

Processing and characterization of extruded drawn MWNT-PAN composite filaments

A process for melt mixing of carbon nanotubes in polyacrylonitrile, which is aided by addition of plasticizer is proposed and employed to extrude/draw filaments at various draw ratios and test them for a range of properties. Microscopic observations show that the nanotubes are evenly dispersed throughout the filament and that they are preferentially aligned along its axis. The extrusion/drawing process results in two fold higher draw ratios of the nanocomposite filament compared with the unfilled control, and the first is shown by wide-angle X-ray diffraction and by polarized Raman spectroscopy to possess higher crystalline and morphological orientation. Thermal analysis and measurements of electric conductivity and mechanical properties attest to the condition that the carbon nanotubes are engaged in π–π interactions with the nitrile groups and/or with their polymerized conjugated imine system. These interactions interfere with and reduce the mutual dipole interactions of the nitrile groups of the polyacrylonitrile, allowing the nanocomposite filaments to be stretched to higher draw ratios. The potential reinforcing effect of the carbon nanotubes is cancelled out by the loss of the original dipole interactions, resulting in lower mechanical properties. However, the interaction of the carbon nanotubes with the conjugated imine system offers potentially interesting electric properties.     

Elastic properties prediction of a knitted composite with inlaid yarns subjected to stretching: A coupled semi-analytical model

Knit reinforced composites cover a large range of mechanical properties thanks to the multitude of structures available. In this paper, a semi-analytical tool is presented allowing to improve the fabric design by assessing the elastic properties of a stretched knit reinforced composite with several inlays. The stretched loop geometry is obtained with a loop stretching model based on the beam theory. A geometric model of the inlays is proposed. These results are used as input data of a mesoscopic scale homogenisation model to assess the composite elastic properties. A comparison of the results to the load–strain curve of a plain knit shows that the modelled loop shape is realistic, and the homogenisation model results agree with experimental data. This tool is used to draw a map of the wale-wise versus course-wise modulus that can help to design a fabric leading to a composite with tailor-made properties.     

通过挤出-热拉伸制备CB/PET/PE导电复合材料

在前期热塑性塑料原位成纤研究基础上，尝试通过挤出-热拉伸制备原位微纤化炭黑(CB)／聚对幕二甲酸乙二醇酯(PET)／高密度聚乙烯(HDPE)导电复合材料。先将CB／PET熔融混合制成母料，再将母料和HDPE按一定的比例挤出一热拉伸。实验发现，体系成纤性能受母料的熔融粘度影响。在相对低的CB含量下，复合物能形成较好的原位微纤，从而具有较好的电性能．     

Dielectric properties of PMMA/Soot nanocomposites

Dielectric analysis (DEA) of relaxation behavior in poly(methyl methacrylate) (PMMA) soot nanocomposites is described herein. The soot, an inexpensive material, consists of carbon nanotubes, amorphous and graphitic carbon and metal particles. Results are compared to earlier studies on PMMA/multi-walled nanotube (MWNT) composites and PMMA/single-walled nanotube (SWNT) composites. The beta relaxation process appeared to be unaffected by the presence of the soot, as was noted earlier in nanotube composites. The gamma relaxation region in PMMA, normally dielectrically inactive, was "awakened" in the PMMA/soot composite. This occurrence is consistent with previously published data on nanotube composites. The dielectric permittivity, s', increased with soot content. The sample with 1% soot exhibited a permittivity (at 100 Hz and 25 degrees C) of 7.3 as compared to 5.1 for neat PMMA. Soot increased the dielectric strength, deltaE, of the composites. The 1% soot sample exhibited a dielectric strength of 6.38, while the neat PMMA had a value of 2.95 at 40 degrees C. The symmetric broadening term (alpha) was slightly higher for the 1% composite at temperatures near the secondary relaxation and near the primary relaxation, but all samples deviated from symmetrical semi-circular behavior (alpha = 1). The impact of the soot filler is seen more clearly in dielectric properties than in mechanical properties studies conducted earlier.     

Computational modeling of the thermal conductivity of single-walled carbon nanotube-polymer composites

A computational model was developed to study the thermal conductivity of single-walled carbon nanotube (SWNT)-polymer composites. A random walk simulation was used to model the effect of interfacial resistance on the heat flow in different orientations of SWNTs dispersed in the polymers. The simulation is a modification of a previous model taking into account the numerically determined thermal equilibrium factor between the SWNTs and the composite matrix material. The simulation results agreed well with reported experimental data for epoxy and polymethyl methacrylate (PMMA) composites. The effects of the SWNT orientation, weight fraction and thermal boundary resistance on the effective conductivity of composites were quantified. The present model is a useful tool for the prediction of the thermal conductivity within a wide range of volume fractions of the SWNTs, so long as the SWNTs are not in contact with each other. The developed model can be applied to other polymers and solid materials, possibly even metals.     

Enhancement in insulation and mechanical properties of PMMA nanocomposite foams infused with multi-walled carbon nanotubes

In this study, PMMA/CNTs composite materials with carboxyl-multi walled carbon nanotubes (c-MWNTs) or untreated MWNTs were prepared via in-situ bulk polymerization. The as-prepared PMMA/CNTs composite materials were then characterized by Fourier-Transformation infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM). The molecular weights of PMMA extracted from PMMA/CNTs composite materials and bulk PMMA were determined by gel permeation chromatography (GPC) with THF used as the eluant. The PMMA/CNTs composite materials were used to produce foams by a batch process in an autoclave using nitrogen as foaming agent. The cellular microstructure, insulation and compressive mechanical properties of PMMA/CNTs composite foams were also investigated in detail. Compared to neat PMMA foam, the presence of CNTs increases in cell density and reduces cell size. The insulation and compressive mechanical properties of PMMA/CNTs composite foams were found to improve substantially those of neat PMMA foam. In particular, 22.6% decrease in thermal conductivity, 19.7% decrease in dielectric constant and 160% increase in compressive modulus were observed with the addition of 0.3 wt% carboxyl-multi walled carbon nanotubes (c-MWNTs).     

超顺排碳纳米管/硅橡胶复合材料各向异性的电学性能和力学性能

采用直接浸润法制备了具有不同层数的超顺排碳纳米管(SACNT)薄膜与硅橡胶的复合材料,使碳纳米管薄膜能够在硅橡胶基体表面均匀分散。测量了SACNT薄膜/硅橡胶复合材料在各个方向的导电性能和力学性能,研究了影响复合材料导电性和力学性能的因素。实验结果表明:SACNT薄膜/硅橡胶复合材料的导电性和杨氏模量都随着碳纳米管薄膜厚度的增加而增加,且具有显著的各向异性。垂直于碳纳米管排列方向的电阻率平均比平行方向的大一个数量级。当碳纳米管层数为240层时,平行于碳纳米管排列方向的杨氏模量为116.9 MPa(比纯硅橡胶基体增加了142倍),而垂直方向的杨氏模量仅为1.23 MPa(比纯硅橡胶基体增加50%),两者之间相差近100倍。结果表明,可以通过选择不同的参数,获得具有特定导电性和杨氏模量的SACNT薄膜/硅橡胶复合材料,并在实际中加以应用。     

石墨烯纸拉花及其水泥基复合材料的制备和性能

使用石墨烯为原料,羧甲基纤维素钠(CMC)为增韧剂和优化成膜压力等工艺参数制备具有较高力学性能和导电性能的石墨烯纸薄膜,经表面切口和择向牵拉得到三维伸展的拉花式石墨烯纸网络结构,再通过高流动性水泥浆体的浇注、密实和凝结硬化制备出石墨烯拉花改性水泥基复合材料。用动态热机械分析仪(DMA Q800)测试石墨烯纸薄膜的力学性能,用四探针测试仪(RTS-8型)测试其电学性能,用Keithley 2400数字源表测试复合材料在一定压力下的电阻,研究了这种复合材料的压敏性能。结果表明,CMC掺量为50%、成膜压力为12.5 MPa的石墨烯纸薄膜其力学强度和导电能力较好,制备出的石墨烯拉花改性水泥基复合材料具有一定的压敏性能,即使在电阻循环变化率为10.29%的条件下其压敏性能仍具有良好的重复性。     

牵伸倍率对PEDOT PSS/PVA复合导电纤维结构与性能的影响

将合成的导电聚(3,4-乙撑二氧噻吩)-聚对苯乙烯磺酸(PEDOT-PSS)与聚乙烯醇(PVA)共混, 通过湿法纺丝, 得到了电导率较高、力学性能良好、可进行机织的PEDOT-PSS/PVA复合导电纤维, 研究了牵伸倍率对纤维导电性能、结构、热性能和力学性能的影响。结果表明: 随着牵伸倍率的增加, PEDOT-PSS/PVA复合导电纤维表面的微纤数量增加, 结晶性能、热稳定性及力学性能均有所提高; 当PEDOT-PSS/PVA复合导电纤维的牵伸倍率为4.0时, 其断裂强度、伸长率和初始模量分别为6.74 cN/dtex、5.95%和42.43 cN/dtex, 电导率可达到34.5 S/cm, 具有良好的应用性能。     

碳纤维/聚醚醚酮单向带各向异性导电行为的尺度效应

具有导电各向异性的高分子复合材料(ACPCs)在场发射装置及传感器设计领域具有重要应用。常规的ACPCs很难获得超大导电各向异性系数,且力学性能有限。本文采用碳纤维(CF)宽展、表面浸润与树脂复合一体化超薄热塑性单向带制备方法,制备厚度为0.04 mm和0.1 mm的CF增强聚醚醚酮(CF/PEEK)复合材料单向带,以PEEK纤维为纬线制备CF/PEEK复合材料单向编织布,采用热成型工艺制备CF/PEEK复合材料单向层合板。利用数字万用表和霍尔效应系统测试层合板面内及厚度方向的电阻率和面内的电子迁移率;采用超景深显微镜观察CF/PEEK复合材料单向层合板面内和厚度方向的纤维排列形貌。结果表明,超薄CF/PEEK复合材料单向层合板面内(纤维方向与横向)导电率之比高达377,而面内横向和厚度方向的导电率之比接近1,表明CF/PEEK复合材料获得了良好的横观各向同性;超薄化CF/PEEK复合材料的面内电子迁移行为同样具有巨大的各向异性,这一结果为CF/PEEK复合材料在场发射器件、传感器设计及其灵敏度调控方面提供了实验基础。      

导电PMMA/ATO纳米复合材料的制备及性能

以掺锑二氧化锡(ATO)粉为导电填料,聚甲基丙烯酸甲酯(PMMA)为基体,采用原位聚合法制备了导电PMMA/ATO纳米复合材料;分析了ATO粉的预处理对复合材料导电性能的影响,并对其热性能和力学性能进行了研究。结果表明,延长球磨时间,可大幅度降低复合材料的体积电阻率;ATO纳米粒子的加入使PMMA主分解温度范围变窄,残余量增大,热稳定性提高;随着纳米粒子含量的增加,复合材料的储能模量提高,玻璃化温度降低。第二分相促使ATO颗粒在基体中形成明显的导电网络结构,使导电性能得到进一步提高。     

聚苯胺导电复合膜电化学制备及其性能

采用电化学氧化聚合法,以硫酸和高氯酸为掺杂剂,制备出聚苯胺（PANI）/聚乙烯醇（PVA）导电复合膜,在此复合膜上再沉积一层很薄的银层,制备出具有高导电性的复合膜。研究了苯胺聚合时间、银沉积电流密度及银沉积时间和拉伸处理对复合膜电导率的影响。采用扫描电镜、X射线衍射对复合膜进行表征,并对复合膜导电机理进行解析。结果表明制备的PVA-PANI复合膜电导率可达4.2S.cm-1,再经沉积薄层银后,其电导可显著提高至1136 S.cm-1。最优条件下制备的PVA-PANI复合膜为纤维状,银在此复合膜上沉积呈针状;PVA-PANI复合膜具有一定的结晶度,经拉伸后,其结晶度增大,复合膜电导得到提高,PVA-PANI复合膜具有良好力学性能。复合膜导电的基本原理是PANI与PVA互穿网络,并与银形成了三维导电网络。     

DC conductivity and interfacial polarization in PMMA/nanotube and PMMA/soot composites

This work focuses on the DC conductivity and interfacial polarization of poly(methyl methacrylate) (PMMA) composites made with single wall nanotubes (SWNTs) and nanotube precursor soot. We aim to compare and contrast the behavior of the two nanofillers in an effort to determine if the precursor material imparts tunable electrical properties to PMMA. The DC activity of the polymers and composites was determined by extrapolating AC conductivity versus frequency plots to zero. Activation energies and DC conductivities were obtained through Arrhenius plots. The conductivity increased with temperature for all the samples in conjunction with viscous flow. Both nanotubes and nanosoot increased the DC conductivities. Activation energies for the SWNT decreased slightly upon the addition of nanotubes to the matrix. However, the activation energies increased with soot content in the matrix, indicating obstruction of motion or possible bridging caused by the soot. Cole-Cole plots were effectively used to determine the extent of interfacial polarization in the composite samples. Plots shifted toward the origin with the addition of carbon nanotubes or with soot concentration indicating a Maxwell-Wagner-Sillars process.     

Ni/PVDF复合薄膜电性能的拉伸改性

为了进一步研究聚合物基复合材料的拉伸效应,采用简单的物理混合法制备了Ni/PVDF复合薄膜并在120℃下进行拉伸改性研究。结果表明：镍含量远低于渗流阈值的样品,通过拉伸实现了与渗流效应类似的介电常数剧增的拉伸渗流效应;在伸长倍数为2.3时介电常数急剧增大,最大增幅17倍(12→220),同时电导率也增大了4～5个数量级,薄膜有绝缘体向导体转变的趋势。介电常数随拉伸倍数增加而大幅增加,是因为复合薄膜中的镍颗粒在外力拉伸作用下,其分布、微观结构逐步发生变化,以致形成了杆状镍颗粒群和微平行电容器结构等导电网络,发生拉伸渗流效应;同时,电导率的大幅增加也符合渗流理论的特征。     

陶瓷纤维增强氧化硅气凝胶复合材料力学性能试验

氧化硅气凝胶具有极低的热导率和密度,可作为很好的隔热材料,而脆弱的力学性能限制了其在隔热领域的应用。在不影响隔热效果的前提下,通过复合陶瓷纤维可增加氧化硅气凝胶的强度及韧性。试验探索了陶瓷纤维增强氧化硅气凝胶在室温下的拉伸、压缩和剪切等基本力学性能,分别研究了300℃、600℃和900℃下复合材料纤维铺层面方向的压缩性能,并采用扫描电子显微镜对高温试样微观结构进行了观察分析。结果表明：陶瓷纤维增强氧化硅气凝胶的性能表现出方向性,弹性模量在铺层面内方向与厚度方向的数值最大相差约28倍,强度极限亦然;在室温条件下,复合材料的拉伸和压缩弹性模量不同,X 、Y 和 Z 方向拉伸模量与对应的压缩模量之比分别为1.60、1.83和0.56;高温下复合材料沿厚度方向收缩,收缩量随温度升高而增大,900℃下的最大收缩量可达10.8%;高温下复合材料铺层面内方向压缩性能随温度升高而增强。