Reduced percolation thresholds of immiscible conductive blends

The DC conductivity of polymer blends composed of poly(ethylene‐co‐vinyl acetate) (EVA) and high density polyethylene (HDPE), where a conductive carbon black (CB) had been preferentially blended into the HDPE, were investigated to establish the percolation characteristics. The blends exhibited reduced percolation thresholds and enhanced conductivities above that of the individually carbon filled HDPE and EVA. The percolation threshold of the EVA/HDPE/CB composites was between 3.6 and 4.2 wt % carbon black, where the volume resistivity changed by 8 orders of magnitude. This threshold is at a significantly lower carbon content than the individually filled HDPE or EVA. At a carbon black loading of 4.8 wt %, the EVA/HDPE/CB composite exhibits a volume resistivity which is approximately 14 and 11 orders of magnitude lower than the HDPE/CB and EVA/CB systems, respectively, at the same level of incorporated carbon black. The dielectric response of the ternary composites, at a temperature of 23°C and frequency of 1 kHz, exhibited an abrupt increase of ca. 252% at a carbon concentration of 4.8 wt %, suggesting that the percolation threshold is somewhat higher than the range predicted from DC conductivity measurements. Percolating composites with increasing levels of carbon black exhibit significantly greater relative permittivity and dielectric loss factors, with the composite containing 6 wt % of carbon black having a value of ϵ′ ≈ 79 and ϵ″ ≈ 14. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1899–1910, 1999     

短碳纤维填充HDPE复合体系的渗流网络与压阻行为

研究了短碳纤维(Shortcarbonfiber,SCF)填充高密度聚乙烯(HDPE)导电复合体系的渗流(Percola-tion)与压阻行为(Piezoresistivebehavior,PRB),发现SCF经物理接触而形成的导电网络是复合材料导电的根源.体系的压阻行为呈现浓度依赖性.受压时SCF间隙的减小与渗流网络的局部破坏-重建过程随填料浓度、载荷大小和力学循环次数的变化而变化,导致PRB表现为电阻负压力系数(NPC)、电阻正压力系数(PPC)或两者兼有的现象.讨论了体系PRB的稳定性,发现由于HDPE基体的塑性永久形变,电阻-时间基线随着压缩循环的进行而发生漂移,多次循环可有效提高体系的压阻稳定性.     

Changes During Storage of Electrically Conductive Blends Polyaniline–Poly(Ethylene‐co‐vinyl Acetate)

Abstract

The electrical conductivity behavior of polyaniline–poly(ethylene‐co‐vinyl acetate) (PANI–EVA) blends was variable and dynamic during their storage. It was shown that the apparent concentration of the intrinsically conductive polymer at which a conductivity jump of the blends occurs (Φ _c ) is not a constant value over time. The electrical conductivity of the films of low PANI content (below 2.5 wt.%) increased by several (ca. 5) orders of magnitude. It was found that the PANI phase undergoes a flocculation process subsequently resulting in the formation of conductive pathways and a continuous network. Besides, the shape of percolation curves was found to change during storage of the films. Decreased conductivity deviations were registered for blends of low PANI content (<2.5 wt.%), indicating that an improvement (or decreasing number of defects) of the conductive pathways took place within the bulk of the insulating EVA matrix. These results and observed phenomena are discussed by means of the interfacial model for electrically conductive polymer blends. They supported the dispersion/flocculation phase transition within similar composite materials. The phase separation and conductivity jump are attributed to the interfacial interactions between the polymeric constituents. It was shown that the microstructure of the blends consists of highly ordered PANI paths embedded in the insulating EVA matrix. Long fibrils of PANI and interconnected fractal‐like networks were observed. It was found that the sizes of the PANI domains also varied during storage of the films. Due to the spontaneous flocculation of the primary PANI particles, conductive pathways are formed at extremely low percolation threshold (Φ _c , loading level ca. 5 × 10^?3 wt. fraction). Thus, an important property of the conductive constituent, namely its solid‐state rearrangement, was proved. This PANI self‐organization is also interpreted according to the interfacial model of polymer composites. On the other hand, the competition between self‐organization of the complex of PANI with dodecylbenzenesulfonic acid and crystallization of EVA matrix has resulted in structural changes and formation of continuous conductive networks within the blends, responsible for their significantly increased conductivity.     

Percolative proton conductivity of sol-gel derived amorphous aluminosilicate thin films

The finite size effect of proton conductivity of amorphous aluminosilicate thin films, a-Al(n)Si(1-n)O(x) (n = 0.07, 0.1, 0.2, 0.3 and 0.45), prepared by a sol-gel process was investigated by experimental and numerical techniques. High-resolution TEM clarified that a-Al(n)Si(1-n)O(x) films had the heterogeneous nanoscale microstructures comprised of the ion-conducting, condensed glass microdomain and the poor-conductive, uncondensed glass microdomain. σ of the films with n≤0.1 exponentially increased upon decreasing thickness in the sub-100 nm range because the volume fraction of conductive domains was less than the percolation threshold and cluster size scaling of the conductive domain was operative. The numerical simulation suggested that conductance of the condensed domain was higher than that of the uncondensed domain by 2 orders of magnitude. Volume fractions of the condensed domain increased with increasing Al/Si molar ratio and were over the percolation threshold (24.5%) with n≥0.2. However, conductance of the condensed domain decreased with increasing Al/Si ratio with n≥0.2 because the aluminosilicate glass framework made of 4-fold-connected MO(4) tetrahedra was deformed by forming the octahedral AlO(6) moieties, as checked by Al K-edge XAS. It was found that the optimal Al/Si composition in terms of the conductance of the condensed domain is not in coincidence with that in terms of the average conductivity of the films.     

Polyaniline‐tailored electromechanical responses of the silver/epoxy conductive adhesive composites

In this article, the electromechanical properties of silver‐in‐epoxy conductive adhesives with the polyaniline (PANI) micron particles as cofillers have been investigated. PANI is a conductive polymer and has a moderate conductivity in between those of silver and epoxy. It was found that PANI can be used to tailor both the adhesive's electrical contact resistance and its relaxation behavior; however, the effects of adding PANI were complex. The addition of small amount of PANI (2 wt %) dramatically increased the contact resistance; it might block the electrical contacts among silver flakes and was not able to form a continuous path among themselves. The addition of more PANI showed a moderate increase in contact resistance, which increased with the weight fraction of PANI from 6 to 15 wt %. Interdependent behavior of compressive strain and relaxation in electrical contact resistance is characterized to evaluate the origin of this relaxation. The addition of PANI made the relaxation in electrical contact resistance more sensitive to the compressive strain and the electromechanical coupling to deviate from the linear relationship. These research findings provide insights into the way to use PANI to tailor the electromechanical properties of the adhesive bonds or joints in the development of advanced functional devices. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013, 51, 1448–1455     

聚苯胺/苯乙烯-丁二烯-苯乙烯复合材料伸长过程中的导电性变化

以简化物理模型为基础，用实测电流与理论电流之比来研究导电橡胶伸长中电导率的变化，并建立相应测试方法．聚苯胺（ＰＡｎ）／苯乙烯 丁二烯 苯乙烯三嵌段共聚物的复合物用现场乳液聚合法制备．此复合物可熔融加工（ＭＰ）和溶液加工（ＳＰ）．ＳＳＰ样为ＳＰ样用间甲酚二次掺杂处理制得．对ＰＡｎ质量分数为１１０％的复合物进行电导率随伸长变化测试结果表明：ＭＰ样随伸长增加其电导率显著提高，在伸长率为４５０％时，其电导率是初始值（σ０）的１５６倍；ＳＰ样在伸长过程中电导率随伸长而增加，在伸长率为５００％时，其电导率是σ０的５２倍；而ＳＳＰ样在整个伸长过程中电导率变化较小，σ０＜σ＜２σ０．可从复合物的聚集态结构解释以上现象．ＳＳＰ样在导电组分的含量接近逾渗域值时电导率随伸长迅速下降．而当导电组分含量高于逾渗域值时电导率随伸长表现出小的变化并具规律性，此规律性可用σσ０＝１＋ｋａ＋ｂｋ２经验公式来表达（ａ，ｂ为与含量相关的常数）．此结果是由于ＳＳＰ样中导电组分在二次掺杂过程中分子链间存在弱相互作用而自行组成导电网络     

Filler network structure in graphene nanoplatelet (GNP)-filled polymethyl methacrylate (PMMA) composites: From thermorheology to electrically and thermally conductive properties

In this work, graphene nanoplatelet (GNP) filled polymethyl methacrylate (PMMA) composites were prepared using solution method via a specially designed route and relatively high thermal conductivities of the composites were achieved at a low GNP loading. The effect of GNP content on rheological behavior, thermal and electrical conductivity of the composites was intensively investigated. Thermorheological complexity was displayed at elevated GNP loading, and the rheological percolation threshold of GNP in PMMA decreased from 7.96 wt% at 220 °C to 4.02 wt% at 260 °C according to Winter-Chambon method, suggesting that GNP was more likely to form a seepage network at higher temperature. The DMTA results showed that the increase in moduli of the composites should be ascribed to the formation of the GNP-GNP network structure. The electrical conductivity of the composites underwent a sudden jump by seven orders of magnitude, which also indicated the formation of a GNP conductive pathway in the matrix with an electrical percolation threshold of 2–4 wt%. The results of transient temperature measurement were in good consistent with the thermal conductivity versus GNP loading, which was compared with various thermal conduction models with a modified Agari model presenting an acceptable evaluation of the dispersion status of GNP in the matrix. The experimental electrical and thermal conductivities as a function of GNP content could well be interpreted by the filler network structure as observed in morphological studies.     

聚乙烯/聚并苯复合材料室温电阻率变化规律研究

以新型导电材料聚并苯替代碳黑作为导电填料制备聚乙烯/聚并苯复合物.确定了复合物渗流转变区,并解释渗流转变现象的产生机理.从聚并苯含量和热处理过程,对聚乙烯/聚并苯复合物室温电阻率变化规律进行讨论.结果表明,聚并苯质量分数在20%～40%之间是聚乙烯/聚并苯复合物渗流转变区;热处理有利于聚乙烯晶区完善排列,也有利于导电链形成;以聚并苯作为导电填料所制备的复合物具有较高的PTC强度;辐射交联可以提高聚乙烯/聚并苯复合物PTC强度,抑制NTC效应.     

Spiky nanostructured metal particles as filler of polymeric composites showing tunable electrical conductivity

This work presents a comprehensive investigation of the piezoresistive response of a composite material based on conductive nickel filler in a silicone‐insulating matrix. In the absence of a deformation, the prepared composite shows no electric conductivity, even though the metal particle content is well above the expected percolation threshold. Upon samples deformation (compressive or tensile stress), the composite exploits a variation of electrical resistance up to nine orders of magnitude. This huge variation can be explained with the quantum tunnelling mechanism where the probability of an electron to tunnel from a particle to the next one is exponentially proportional to the thickness of the insulating layer between them and strongly enhanced by the morphology of the nickel particles, showing spiky nanostructured tips. Two different conduction theoretical models are proposed and compared with the experimental results. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

HDPE/CB高分子导电复合体系的单轴形变与压阻行为

根据基体形变及其与渗流网络结构之间的关系,研究了高密度聚乙烯(HDPE)/炭黑(CB)复合材料压阻行为的发生机制及其影响稳定性的力学因素.结果表明,单轴压阻行为的产生源于材料受外力变形而导致的渗流网络微结构变化,且这种变化强烈依赖于填料含量.当填料含量较低时(渗流阈值附近),体系电阻率随压力升高而表现为电阻正压力系数行为;当填料含量较高时,体系电阻率随压力升高显现电阻负压力系数效应.完全卸载的零压力下,电阻基线随压缩循环随循环次数增大而发生漂移,这种漂移与轴向残余压缩应变有关,可以通过增加循环次数来加以稳定.交联可以减小残余压缩应变,并抑制高填充复合体系电阻基线的漂移.     

碳纳米管/炭黑混杂填充聚甲醛复合材料导电性能研究

采用在转矩流变仪中熔融混合的方法制备了多壁碳纳米管(MWCNTs)和导电炭黑(CB)共同填充的聚甲醛(POM)导电复合材料,研究了材料的体积电阻率和导电填料用量之间的关系.用该体系的实验数据点对已发表的混合导电填料体系的定量关系式进行了验证,结果表明实验数据点和理论预测略有偏差.为方便实际应用,把混合导电填料体系的定量关系式加以发展,推导出逾渗值和两种导电填料质量比之间的关系式,并绘出了工作曲线.设计实验对工作曲线进行了验证,结果表明工作曲线对实践具有指导意义.     

聚乙烯炭黑复合材料导电逾渗的蒙脱卡罗法研究

众所周知,在聚合物中加入导电粒子后可以制成导电复合材料,但是加入的导电组分的体积分数必须超过某个临界值.在这方面研究最多的体系就是聚乙烯-炭黑复合材料[1,2].当炭黑的体积分数低于该临界值时,复合材料的电导率极低.     

Percolation Transition Under Thermal Reduction of Graphene Oxide

The conductivity of thermally reduced graphene oxide (GO) measured at various reduction stages shows that the samples demonstrate transition to the conducting state within a narrow range of annealing temperatures (150-170 °С). The conductivity in this temperature region increases by about five orders of magnitude to indicate that the percolation mechanism is responsible for this transition. According to this mechanism, the above temperature range is associated with formation of a certain number of conductive channels composed of GO fragments reduced to the conducting state. Thus, it is a new type of percolation transition, since increased conductivity is a result of increased conductivity of the particles due to thermal treatment rather than a result of increased concentration of conducting particles. A further temperature growth is associated with increased number of conducting GO fragments and the number of conducting channels, and some smoother conductivity increase. The XPS spectra of partially reduced GO samples indicate correlation between increased conductivity of GO fragments and plasmon oscillations revealed as XPS peaks. The current-voltage characteristics measured for the samples of partially reduced GO indicate a non-ohmic conductivity which manifests as increased conductivity versus voltage dependence. This is due to the fact that electrical resistance of the percolation channels is the sum of resistances of conductive GO fragments and contact resistances which depend on the voltage drop at the contact and, therefore, on the voltage applied.     

Atomic force microcopy study of the mechanical and electrical properties of monolayer films of molecules with aromatic end groups

The effect of intermolecular pi-pi stacking on the electrical and mechanical properties of monolayer film molecules containing aromatic groups was studied using atomic force microscopy. Two types of aromatic molecules, (4-mercaptophenyl) anthrylacetylene (MPAA) and (4-mercaptophenyl)-phenylacetylene (MPPA), were used as model systems with different pi-pi stacking strength. Monolayer films of these molecules on Au(111) surfaces exhibited conductivities differing by more than 1 order of magnitude, with MPAA being the most conductive and MPPA being the least conductive. The response to compressive loads by the AFM tip was also found to be very different for both molecules. In MPAA films, distinct molecular conductivity changes are observed upon mechanical perturbation. This effect, however, was not observed on the MPPA film, where intermolecular pi-pi interactions are likely weaker.     

Carbon-based conductive rubber composite for 3D printed flexible strain sensors

Polymeric-based flexible electronic devices are in high demand due to its wide range of applications. Natural rubber (NR) shows a great potential as matrix phase for flexible conductive polymer composites with its high elasticity and fatigue resistance. In this study, a new 3D printable conductive NR (CNR) composite was developed for strain sensor applications. Different contents of conductive carbon black (CCB) were mixed with NR latex to investigate the effect of the filler content on electrical and mechanical properties of the composites. The best-known CNR composite with the CCB content of 12 phr was selected in order to produce the feedstock for the stereolithography process (SLA). The morphological, electrical, and mechanical properties of cast and 3D-printed samples were investigated and compared. Although the 3D-printed CNR sample had slightly lower conductivity than the cast one, it possessed comparable tensile strength and elongation at break, with values of 12.4 MPa and 703%, respectively. In addition, electrical responses of the CNR samples were investigated to demonstrate the electromechanical property of the material as a strain sensor. The 3D-printed CNR sample exhibited the highest electromechanical sensitivity with a gauge factor (GF) of 361.4 (ε = 210%–300%) and showed good repeatability for 500 cycles. In conclusion, the development of this 3D printable functional material with great sensing capability will pave the way for innovative designs of personalized sensing textiles and other smart wearable devices.     

MWCNTs与疏水纳米SiO_2填充PMMA/PS共混物的导电与动态流变行为

以多壁碳纳米管(MWCNTs)为导电填料、疏水纳米二氧化硅(SiO2)为非导电填料,填充不相容聚甲基丙烯酸甲酯(PMMA)/聚苯乙烯(PS)(1/1,V/V)共混物,制备(PMMA/SiO2)/(PS/MWCNTs)四元导电高分子复合材料(CPC),研究其导电逾渗与动态流变行为,并与PMMA/(PS/MWCNTs)三元CPC进行对比.发现三元、四元CPC具有类似的导电逾渗行为,且逾渗阈值显著低于PS/MWCNTs二元CPC.在四元CPC中,SiO2粒子可细化相区尺寸,提高熔体模量,但不影响熔体热处理过程中的依时性动态导电逾渗行为.MWCNTs与SiO2均显著影响熔体热处理中的依时性模量逾渗行为,分别缩短、延长四元CPC相粗化起始时间,但均延长相粗化时间区间.     

Effect of conductive particles on the mechanical,electrical, and thermal properties of maleated polyethylene

Inclusion of conductive particles is a convenient way for the enhancement of electrical and thermal conductivities of polymers. However, improvement of the mechanical properties of such composites has remained a challenge. In this work, maleated polyethylene is proposed as a novel matrix for the production of conductive metal–thermoplastic composites with enhanced mechanical properties. The effects of two conductive particles (iron and aluminum) on the morphological, mechanical, electrical, and thermal properties of maleated polyethylene were investigated. Morphological observations revealed that the matrix had excellent adhesion with both metal particles. Increase in particle concentration was shown to improve the tensile strength and modulus of the matrix significantly with iron being slightly more effective. Through‐plane electrical conductivity of maleated polyethylene was also substantially improved after adding iron particles, while percolation was observed at particle contents of around 20–30% vol. In the case of aluminum, no percolation was observed for particle contents of up to 50% vol., which was linked to the orientation of the particles in the in‐plane direction due to the squeezing flow. Inclusion of particles led to substantial increase (over 700%) in the thermal conductivities of both composites. The addition of high concentrations of metal particles to matrix led to the creation of two groups of materials: (i) composites with high electrical and thermal conductivities and (ii) composites with low electrical and high thermal conductivities. Such characteristics of the composites are expected to provide a unique opportunity for applications where a thermally conductive/electrically insulating material is desired. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of specimen size and inner defects on high strain rates compressive behaviors of plain woven composites

This paper reports the influence of specimen size and inner defects on high strain rates compressive behaviors of plain woven composites. The compressive behaviors of plain woven composites along out-of-plane direction were investigated from experimental and numerical approaches. In experimental, the compressive stiffness and strength decreased as the size of plain woven composite specimens increased. In finite element analysis (FEA) model, a new microstructure model with random defect distribution was established to find the influence of inner defects and specimen size effect on the compressive behaviors under high strain rates. From the numerical results, the compressive strength, modulus and fracture strain decreased obviously with the increase of volume fraction and size of defects. We found that the good agreement existed between the testing and the FEA results. The defects size and distribution were the main factors to weaken the compressive stiffness and strength.     

多壁碳纳米管对聚甲醛性能的影响

将多壁碳纳米管(MWCNTs)和聚甲醛(POM)在转矩流变仪中熔融混合得到POM/MWCNT复合材料.研究了复合材料的形态,导热性能,导电性能,流变性能和结晶性能.结果表明,MWCNTs在没有经过处理的情况下能够均匀地分散在POM基体中;当向POM中添加1.0 wt%含量MWCNTs时,复合材料的导热系数上升到0.5289 W/(K m),比纯POM的导热系数0.198 W/(K m)提高1.5倍,通过有效介质方法(EMA)验证了体系导热系数提高幅度不大的原因是MWCNTs与POM之间形成了很高的界面热阻;当MWCNTs的含量为1.0 wt%时,体系产生了导电逾渗效应,逾渗值在0.5 wt%~1.0 wt%之间;MWCNTs对POM有显著的成核作用,当向POM中添加0.5 wt%含量的MWCNTs时,POM的结晶温度提高6℃左右,但当MWCNTs的添加量进一步增加时,结晶温度几乎不再变化,成核效果呈现"饱和"状态.另外,材料的复数黏度,储能模量和损耗模量随MWCNTs含量的增加而增加.     

The development of conductive carbon nanotube network in polypropylene-based composites during simultaneous biaxial stretching

The development of electronic conducting networks during the simultaneous biaxial stretching of isotactic polypropylene/carbon nanotube (iPP/CNT) composites was investigated. During the stretching process, the electrical resistivity of the composites was found to be very sensitive to the draw ratios. This was especially true at CNT concentration close to the percolation threshold, ca. 2.2 vol.%. The resistivity–draw ratio dependence was divided into two stages. In the first stage, the stretching was taken by the amorphous zones and only led to the enlargement of the distance between CNT aggregates. This resulted in the breaking of the conductive network and, consequently, a sharp increase in resistivity. As the turning point was approached, individual nanotubes started to disentangle from CNT aggregates. Meanwhile, the resistivity of the stretched films was dramatically decreased by 7 orders of magnitude, indicating a rebuilding of the conducting network during the biaxial stretching process.