Processing-microstructure-property relationship in conductive polymer nanocomposites

The processing-microstructure-property relationship in conductive polymer nanocomposites was investigated. Nanocomposites of vapor grown carbon nanofiber (VGCNF)/high density polyethylene (HDPE) with different levels of nanofiber dispersion were formulated by changing the nanocomposites’ compounding temperature. Direct (SEM and optical microscopy) and indirect methods (linear viscoelastic properties) were used to characterize the dispersion of nanofiller. VGCNF aspect ratio before and after mixing was measured. Increasing processing temperature was found to increase the nanofiller agglomeration and reduce the breakage of nanofiller because of the decrease in the mixing shear stress and energy. The electrical and electromagnetic interference (EMI) shielding properties of the VGCNF/HDPE nanocomposites decreased with increase in processing temperature from 180 °C to 220 °C because the increase in the agglomeration of VGCNF was more significant than the preservation of the VGCNF aspect ratio. This finding does not mean that the increase in processing temperature will always lead to decrease in the electrical conductivity and EMI shielding properties for all polymer composites. For some composites, it is possible to preserve the filler aspect ratio enough so that the increase in agglomeration is less of a factor.     

Electrical,rheological and electromagnetic interference shielding properties of thermoplastic polyurethane/carbon nanotube composites

Electrically conducting rubbery composites based on thermoplastic polyurethane (TPU) and carbon nanotubes (CNTs) were prepared through melt blending using a torque rheometer equipped with a mixing chamber. The electrical conductivity, morphology, rheological properties and electromagnetic interference shielding effectiveness (EMI SE) of the TPU/CNT composites were evaluated and also compared with those of carbon black (CB)‐filled TPU composites prepared under the same processing conditions. For both polymer systems, the insulator–conductor transition was very sharp and the electrical percolation threshold at room temperature was at CNT and CB contents of about 1.0 and 1.7 wt%, respectively. The EMI SE over the X‐band frequency range (8–12 GHz) for TPU/CNT and TPU/CB composites was investigated as a function of filler content. EMI SE and electrical conductivity increased with increasing amount of conductive filler, due to the formation of conductive pathways in the TPU matrix. TPU/CNT composites displayed higher electrical conductivity and EMI SE than TPU/CB composites with similar conductive filler content. EMI SE values found for TPU/CNT and TPU/CB composites containing 10 and 15 wt% conductive fillers, respectively, were in the range ?22 to ?20 dB, indicating that these composites are promising candidates for shielding applications. © 2013 Society of Chemical Industry     

Conductive polymer composites with segregated structures

Conductive polymer composites (CPCs) have generated significant academic and industrial interest for several decades. Unfortunately, ordinary CPCs with random conductive networks generally require high conductive filler loadings at the insulator/conductor transition, requiring complex processing and exhibiting inferior mechanical properties and low economic affordability. Segregated CPC (s-CPC) contains conductive fillers that are segregated in the perimeters of the polymeric granules instead of being randomly distributed throughout the bulk CPC material; these materials are overwhelmingly superior compared to normal CPCs. For example, the s-CPC materials have an ultralow percolation concentration (0.005–0.1 vol%), superior electrical conductivity (up to 10⁶ S/m), and reasonable electromagnetic interference (EMI) shielding effectiveness (above 20 dB) at low filler loadings. Therefore, considerable progress has been achieved with s-CPCs, including high-performance anti-static, EMI shielding and sensing materials. Currently, however, few systematic reviews summarizing these advances with s-CPCs are available. To understand and efficiently harness the abilities of s-CPCs, we attempted to review the major advances available in the literature. This review begins with a concise and general background on the morphology and fabrication methods of s-CPCs. Next, we investigate the ultralow percolation behaviors of and the elements exerting a relevant influence (e.g., conductive filler type, host polymers, dispersion methods, etc.) on s-CPCs. Moreover, we also briefly discussed the latest advances in the mechanical, sensing, thermoelectric and EMI shielding properties of the s-CPCs. Finally, an overview of the current challenges and tasks of s-CPC materials is provided to guide the future development of these promising materials.     

Morphological,electrical and electromagnetic interference shielding characterization of vapor grown carbon nanofiber/polystyrene nanocomposites

The influence of melt mixing conditions on the level of dispersion and the aspect ratio of vapor grown carbon nanofibers (VGCNFs) in a polystyrene (PS) matrix was studied. Final electrical and electromagnetic shielding capabilities in the 0.05–1.5 GHz frequency range are reported and discussed in the light of the composites' microstructure. The morphological study was based on analyzing scanning electron microscopy and optical microscopy micrographs and measuring the VGCNF length as a function of shear mixing conditions. The influence of mixing conditions on the microstructure was also indirectly studied by analyzing the dynamic mechanical behavior of the composites via rheology. Degradation of the VGCNF aspect ratio was found to be a function of the mixing energy. VGCNFs lost one‐third of their aspect ratio under gentle (low shear stress and mixing energy) mixing conditions. After VGCNFs had lost 40% of their aspect ratio, they had more resistance to breakage with increase in mixing energy. The dispersion of the VGCNFs was remarkably enhanced with increase in mixing energy. The percentage of area taken up by big agglomerates in the micrographs decreased from 14.1% to 5.5% when the mixing energy was increased from 100 J mL^?1 to 453 J mL^?1. The electrical and electromagnetic shielding properties of the 7.5 vol% VGCNF/PS composites were not affected by changing the processing energy because the enhancement of VGCNF dispersion with increasing mixing energy was accompanied by a loss in nanofiber aspect ratio. © 2012 Society of Chemical Industry     

Conductive composites of PVC and wood's metal with interpenetrating network structure

This work presents composite materials with interpenetrating network structure based on thermoplastic polymer and low melting metal alloy. Composites with various alloy content were prepared by PVC powder sintering to obtain polymer matrix with open pores. Then, liquid Wood's metal was intruded into the matrix using a pressure autoclave. Obtained composites have been studied with respect to microstructure, mechanical, thermal, and electrical properties. SEM micrographs revealed good dispersion of metal in the matrix but at low loading levels it is incomplete. Addition of metal improved mechanical properties, especially flexural strength. Electrical resistivity of samples varies from 10^?4 to 10^?5 Ω m and these values are typical of conductors. The measurements of electromagnetic interference shielding effectiveness (EMI SE) shows that generally PVC/Wood's metal composites have a good ability to shield electromagnetic waves. Composites containing more than 15 vol % Wood's metal exhibited EMI SE above 40 dB in the major part of frequency range. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effects of the dispersion state and aspect ratio of carbon nanotubes on their electrical percolation threshold in a polymer

The electrical percolation threshold of carbon nanotubes (CNTs) is correlated with their dispersion state and aspect ratio through modeling. An analytical percolation model based on excluded volume theory and developed for systems containing two types of fillers is used. CNTs are modeled as two types of fillers: single CNT and m‐CNT bundle, and a variable P representing the dispersion state of CNTs is introduced. An equation showing the effects of the dispersion state and aspect ratio on the electrical percolation threshold of CNTs is established and verified with some of the published experimental data. It is useful for predicting the conductive behavior of polymer/CNT composites and for the design of their processing conditions. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Nanofiber‐reinforced thermoplastic composites. I. Thermoanalytical and mechanical analyses

This article is a portion of a comprehensive study on carbon nanofiber–reinforced thermoplastic composites. The thermal behavior and dynamic and tensile mechanical properties of polypropylene–carbon nanofibers composites are discussed. Carbon nanofibers are those produced by the vapor‐grown carbon method and have an average diameter of 100 nm. These hollow‐core nanofibers are an ideal precursor system to working with multiwall and single‐wall nanotubes for composite development. Composites were prepared by conventional Banbury‐type plastic‐processing methods ideal for low‐cost composite development. Nanofiber agglomerates were eliminated because of shear working conditions, resulting in isotropic compression‐molded composites. Incorporation of carbon nanofibers raised the working temperature range of the thermoplastic by 100°C. The nanofiber additions led to an increase in the rate of polymer crystallization with no change in the nucleation mechanism, as analyzed by the Avrami method. Although the tensile strength of the composite was unaltered with increasing nanofiber composition, the dynamic modulus increased by 350%. The thermal behavior of the composites was not significantly altered by the functionalization of the nanofibers since chemical alteration is associated with the defect structure of the chemical vapor deposition (CVD) layer on the nanofibers. Composite strength was limited by the enhanced crystallization of the polymer brought on by nanofiber interaction as additional nucleation sites. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 125–133, 2001     

Rheological analysis of vapor‐grown carbon nanofiber‐reinforced polyethylene composites

The melt rheological analysis of high‐density polyethylene reinforced with vapor‐grown carbon nanofibers (VGCNFs) was performed on an oscillatory rheometer. The influence of frequency, temperature, and nanofiber concentration (up to 30 wt %) on the rheological properties of composites was investigated. Specifically, the viscosity increase is accompanied by an increase in the elastic melt properties, represented by the storage modulus G′, which is much higher than the increase in the loss modulus G″. The composites and pure PE exhibit a typical shear thinning behavior as complex viscosity decreases rapidly with the increase of shearing frequency. The shear thinning behavior is much more pronounced for the composites with high fiber concentration. The rheological threshold value for this system was found to be around 10 wt % of VGCNF. The damping factor was reduced significantly by the inclusion of nanofibers into the matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 155–162, 2004     

Electrical conductivity and EMI shielding effectiveness of polyurethane foam–conductive filler composites

The morphological, electrical, and thermal properties of polyurethane foam (PUF)/single conductive filler composites and PUF/hybrid conductive filler composites were investigated. For the PUF/single conductive filler composites, the PUF/nickel‐coated carbon fiber (NCCF) composite showed higher electrical conductivity and electromagnetic interference shielding effectiveness (EMI SE) than did the PUF/multiwall carbon nanotube (MWCNT) and PUF/graphite composites; therefore, NCCF is the most effective filler among those tested in this study. For the PUF/hybrid conductive fillers PUF/NCCF (3.0 php)/MWCNT (3.0 php) composites, the values of electrical conductivity and EMI SE were determined to be 0.171 S/cm and 24.7 dB (decibel), respectively, which were the highest among the fillers investigated in this study. NCCF and MWCNT were the most effective primary and secondary fillers, and they had a synergistic effect on the electrical conductivity and EMI SE of the PUF/NCCF/MWCNT composites. From the results of thermal conductivity and cell size of the PUF/conductive filler composites, it is suggested that a reduction in cell size lowers the thermal conductivity of the PUF/conductive filler composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44373.     

A study on nanofiber‐reinforced thermoplastic composites (II): Investigation of the mixing rheology and conduction properties

This article is portion of a comprehensive study on the development of nanofiber‐reinforced polymer composites for electrostatic discharge materials and structural composites. Vapor‐grown carbon fibers with an average diameter of 100 nm were used as a precursor and model fiber system for carbon nanotubes. These nanofibers were purified and functionalized to provide for an open network of high‐purity nanofibers. Banbury‐type mixing was used to disperse the nanofibers in the polymer matrix. Rheological and microscopic analysis showed that the high shear processing of the polymer/nanofiber mixture led to a homogeneous dispersion of nanofibers with no agglomerates present and no shortening of the nanofibers. The shear thinning behavior of polymeric materials helps in the mixing of the nanofibers to form the composites. A percolation threshold for electrical conduction of 9–18 wt % was observed for the highly dispersed nanofiber networks. The electrical behavior of these materials was not affected by changes in humidity. Microscopic analysis showed highly dispersed nanofibers with no indications of porosity. These conducting polymers are well suited for electrostatic discharge applications, and might well become multifunctional materials for strength/electrical applications. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1162–1172, 2001     

Electrical conductivity and electromagnetic interference shielding effectiveness of carbon black/sisal fiber/polyamide/polypropylene composites

The effects of hybrid fillers on the electrical conductivity and electromagnetic interference (EMI) shielding effectiveness (SE) of polyamide 6 (PA6)/polypropylene (PP) immiscible polymer blends were investigated. Carbon black (CB) and steam exploded sisal fiber (SF) were used as fillers. CB was coated on the surface of SF, and this was exploded by water steam to form carbon black modified sisal fiber (CBMSF). CB/SF/PA6/PP composites were prepared by melt compounding, and its electromagnetic SE was tested in low‐frequency and high‐frequency ranges. We observed that SF greatly contributed to the effective decrease in the percolation threshold of CB in the PA6/PP matrix and adsorbed carbon particles to form a conductive network. Furthermore, an appropriate CB/SF ratio was important for achieving the best shielding performance. The results indicate that CBMSF was suitable for use as electronic conductive fillers and the CB/SF/PA6/PP composites could be used for the purpose of EMI shielding. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42801.     

聚合物/碳纳米管导电复合材料研究进展

综述了聚合物/碳纳米管复合材料的制备方法及其电学性能的研究进展。评述了聚合物/碳纳米管复合材料常用的制备方法,包括溶液共混、熔融共混、原位聚合等,特别强调碳纳米管在聚合物基体中的分散;介绍了采用聚乙烯、环氧树脂等为基体的碳纳米管复合材料电学性能研究的两个方面:导电渗流行为和正温度系数效应;对聚合物/碳纳米管导电复合材料研究中存在的问题如工艺改进、机理解释等进行了讨论,并展望了这一类材料的发展前景。     

Mild functionalization of carbon nanotubes filled epoxy composites: Effect on electromagnetic interferences shielding effectiveness

The effect of nitric acid mild functionalized multiwalled carbon nanotubes (MWCNTs) on electromagnetic interference (EMI) shielding effectiveness (SE) of epoxy composites was examined. MWCNTs were oxidized by concentrated nitric acid under reflux conditions, with different reaction times. The dispersion of MWCNTs after functionalization was improved due to the presence of oxygen functional groups on the nanotubes surface. Functionalization at 2 h exhibits the highest EMI SE and electrical conductivity of MWCNTs filled epoxy composites. However, EMI shielding performance of MWCNTs filled epoxy composite declined when the functionalization reaction time was prolonged. This was due to extensive damage on the MWCNT structure, as verified by a Raman spectroscope. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42557.     

A representative and comprehensive review of the electrical and thermal properties of polymer composites with carbon nanotube and other nanoparticle fillers

In this review we present the results of our literature investigation into the electrical and thermal properties of carbon nanotube polymer composites. A short selection of data relating to conductive polymer composites with various fillers is provided for comparison. The effects of filler properties such as type and size, the use of hybrid fillers, fabrication methods for polymer composites and the importance of the modeling of the electronic and thermal transport mechanisms are discussed, as are more general factors influencing the properties of these composites. This review represents a comprehensive survey and constructive study and should serve as a useful reference tool for industrial and academic researchers working in this field. © 2017 Society of Chemical Industry     

Processing-structure-multi-functional property relationship in carbon nanotube/epoxy composites

The novel properties of carbon nanotubes have generated scientific and technical interest in the development of nanotube-reinforced polymer composites. In order to utilize nanotubes in multi-functional material systems it is crucial to develop processing techniques that are amenable to scale-up for high volume, high rate production. In this research we investigate a scalable calendering approach for achieving dispersion of CVD-grown multi-walled carbon nanotubes through intense shear mixing. Electron microscopy was utilized to study the micro and nanoscale structure evolution during the manufacturing process and optimize the processing conditions for producing highly-dispersed nanocomposites. After processing protocols were established, nanotube/epoxy composites were processed with varying reinforcement fractions and the fracture toughness and electrical/thermal transport properties were evaluated. The as-processed nanocomposites exhibited significantly enhanced fracture toughness at low nanotube concentrations. The high aspect ratios of the carbon nanotubes in the as-processed composites enabled the formation of a conductive percolating network at concentrations below 0.1% by weight. The thermal conductivity increased linearly with nanotube concentration to a maximum increase of 60% at 5 wt.% carbon nanotubes.     

Electromagnetic Interference Shielding Foams Based on Poly(vinylidene fluoride)/Carbon Nanotubes Composite

Polymeric electromagnetic interference (EMI) shielding foaming materials are found and applied in many frontier fields such as aerospace, transportation, and portable electronics. In this paper, a foam based on a composite system of poly(vinylidene fluoride) (PVDF) filled with carbon nanotubes (CNTs) is prepared for EMI shielding properties by using a solid-state supercritical CO₂ foaming strategy. PVDF is chosen as the matrix because of its excellent chemical resistance, thermal stability, and flame retardancy. The inclusion of CNTs renders this composite system enhanced complex viscosity and storage modulus by about two orders of magnitude. The electrical conductivity and EMI specific shielding effectiveness of obtained foams can be adjusted and reached the optimum value of 0.024 S m⁻¹ and 29.1 dB cm³ g⁻¹, respectively, originating from the gradual development of interconnected CNTs and conductive CNTs network as well as the introduction of cell structure in PVDF matrix. Interestingly, the reorientation of CNTs caused by foaming process results in electrical conductivity percolation threshold of PVDF/CNTs foams markedly decreases, in comparison to their unfoamed samples. This study provides a facile, efficient, green, and economic route for the preparation of EMI shielding foams consisted of fluorinated polymers and carbonaceous fillers.     

EMI shielding effectiveness of carbon based nanostructured polymeric materials: A comparative study

The microstructure, electromagnetic interference (EMI) shielding effectiveness (SE), DC electrical conductivity, AC electrical conductivity and complex permittivity of nanostructured polymeric materials filled with three different carbon nanofillers of different structures and intrinsic electrical properties were investigated. The nanofillers were multiwall carbon nanotubes (MWCNT), carbon nanofibers (CNF) and high structure carbon black (HS-CB) nanoparticles and the polymer was acrylonitrile-butadiene-styrene (ABS). In addition, the EMI SE mechanisms and the relation between the AC electrical conductivity in the X-band frequency range and the DC electrical conductivity were studied. The nanocomposites were fabricated by solution mixing and characterized by uniform dispersion of the nanofillers within the polymer matrix. It was found that, at the same nanofiller loading, the EMI SE, permittivity and electrical conductivity of the nanocomposites decreased in the following order: MWCNT > CNF > CB. MWCNT based nanocomposites exhibited the lowest electrical percolation threshold and the highest EMI SE owning to the higher aspect ratio and electrical conductivity of MWCNT compared to CNF and HS-CB. The AC conductivity in the X-band frequency range was found to be independent of frequency.     

聚合物/碳纳米管复合导电材料的研究进展

基于碳纳米管(CNTs)的导电性能,对以碳纳米管为导电填料的复合导电材料的制备方法及国内外研究进展进行了综述。重点介绍了几种常见聚合物/CNTs复合导电材料的研究现状。展望了此类导电材料的发展前景。     

聚合物/碳纳米管的研究进展

碳纳米管具有独特的结构，优异的力学性能、热稳定性与导电性能，与聚合物并用可开发出多种新型复合材料，评述利用直接混合法、原位聚合法与超声波处理法制备聚合物／碳纳米管材料，并讨论该复合材料的力学性能，光电性能与磨擦学性能。     

High electromagnetic interference shielding effectiveness achieved by multiple internal reflection and absorption in polybenzoxazine/graphene foams

Electromagnetic interference (EMI) is an increasingly severe issue in modern life and high-performance EMI shielding materials are in desperate need. To achieve high EMI shielding effectiveness (EMI SE), a series of polybenzoxazine/graphene composites foams are developed using a simple sol–gel method. When the graphene loading increases from 1 to 20 wt%, the density of the composites foams drops from 0.4143 g/cm³ to 0.1654 g/cm³. Meanwhile, an electrically conductive path is formed at around 7 wt% of graphene. Below the percolation threshold, the dielectric constant increases with graphene content and composite foam with 5 wt% graphene shows dielectric constant of 10.8 (1 MHz). At the highest graphene content of 20 wt%, the electric conductivity reaches 0.02 S/cm, 10 orders of magnitude higher than pure polybenzoxazine foam. Benefiting from the high electrical conductivity and lightweight porous structure, the composite foam PF/20G delivers an EMI SE of 85 dB and a specific SE of 513.9 dB·cm³/g. Importantly, the EMI shielding is dominated by absorption attenuation, with PF/20G shows absorption ratio higher than 98% in the range of 8.4–11.0 GHz, which is believed to be caused by multiple internal reflection and absorption inside the conductive foam.