Volume‐exclusion effects in polyethylene blends filled with carbon black,graphite, or carbon fiber

Conductive polymer composites possessing a low percolation‐threshold concentration as a result of double percolation of a conductive filler and its host phase in an immiscible polymer blend afford a desirable alternative to conventional composites. In this work, blends of high‐density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE) were used to produce ternary composites containing either carbon black (CB), graphite (G), or carbon fiber (CF). Blend composition had a synergistic effect on electrical conductivity, with pronounced conductivity maxima observed at about 70–80 wt % UHMWPE in the CB and G composites. A much broader maximum occurred at about 25 wt % UHMWPE in composites prepared with CF. Optical and electron microscopies were used to ascertain the extent to which the polymers, and hence filler particles, are segregated. Differential scanning calorimetry of the composites confirmed that the constituent polymers are indistinguishable in terms of their thermal signatures and virtually unaffected by the presence of any of the fillers examined here. Dynamic mechanical analysis revealed that CF imparts the greatest stiffness and thermal stability to the composites. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1013–1023, 2002     

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.     

Rheology,electrical conductivity,and the phase behavior of cocontinuous PA6/ABS blends with MWNT: Correlating the aspect ratio of MWNT with the percolation threshold

Multiwalled carbon nanotubes (purified, p‐MWNT and ～ NH₂ functionalized, f‐MWNT) were melt‐mixed with 50/50 cocontinuous blends of polyamide 6 (PA6) and acrylonitrile–butadiene–styrene in a conical twin‐screw microcompounder to obtain conductive polymer blends utilizing the conceptual approach of double‐percolation. The state of dispersion of the tubes was assessed using AC electrical conductivity measurements and melt‐rheology. The rheological and the electrical percolation threshold was observed to be ～ 1–2 wt % and ～ 3–4 wt %, respectively, for blends with p‐MWNT. In case of blends with f‐MWNT, the rheological percolation threshold was observed to be higher (2–3 wt %) than p‐MWNT but the electrical percolation threshold remained almost same. However, the absolute values were significantly lower than blends with p‐MWNT. In addition, significant refinement in the cocontinuous morphology of the blends with increasing concentration of MWNT was observed in both the cases. Further, an attempt was made to understand the underlying concepts in relation to cocontinuous morphologies that how the geometrical percolation threshold which adversely suffered because of the attrition of tubes under prolonged shear contributed further in retaining the rheological percolation threshold. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1619–1631, 2008     

Influence of multiwall carbon nanotubes on morphology and electrical conductivity of PP/ABS blends

Polypropylene (PP) and acrylonitrile‐butadiene‐styrene (ABS) blends with multiwall carbon nanotubes (MWNT) were prepared by melt mixing. PP/ABS blends without MWNT revealed coarse co continuous structures on varying the ABS content from 40 to 70 wt %. Bulk electrical conductivity of the blends showed lower percolation threshold (0.4–0.5 wt %) in the 45/55 co continuous blends whereas the percolation threshold was between 2 and 3 wt % in matrix‐particle dispersed morphology of 80/20 blends. Interestingly, droplet size was observed to decrease with addition of MWNT above percolation threshold in 80/20 blends. Further, the bulk electrical conductivity was found to be dependent on the melt flow index of PP. The non‐polar or weakly polar nature of blends constituents resulted in the temperature independent dielectric constant, dielectric loss, and DC electrical conductivity. Rheological analysis revealed the formation of 3D network‐like structure in 80/20 PP/ABS blends at 3 wt % MWNT. An attempt was made to understand the relationship between rheology, morphology, and electrical conductivity of these blends. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2286–2295, 2008     

Positive Temperature Coefficient Characteristics of Multi-walled Carbon Nanotube Filled Polyvinylidene Fluoride Nanocomposites

Composites of polyvinylidene fluoride (PVDF) and multi-wall carbon nanotubes (MWNT) were prepared by a melt mixing process. Temperature dependence of electrical properties of the nanocomposites was investigated for composites containing different amounts of MWNT. An obvious positive temperature coefficient was observed. It was found that resistivity of the composites was decreased with increasing MWNT content and the electrical percolation threshold was formed at 3 wt% MWNT, which were caused by the formation of conductive chains in the composites. The mechanism of the positive temperature coefficient behavior of the nanocomposites is discussed. The rheological results showed that the materials experience a fluid–solid transition at the composition of 2 wt%, beyond which a continuous MWNT network forms throughout the matrix leading to a percolated network structure, which further indictes the nanotubes were dispersed uniformaly, in the PVDF matrix.     

Electrical conductivity of poly(vinylidene fluoride)/polyaniline blends under oscillatory and steady shear conditions

Blends of poly(vinylidene fluoride) (PVDF) and polyaniline (PAni) were prepared through melt blending in a batch mixer. The morphology, rheological behavior and electrical conductivity were investigated through transmission electron microscopy (TEM) and combined electro-rheological measurements. Through TEM analysis, it was possible to observe that all blends showed typical phase separation with the presence of conductive polymer aggregates. Deformations imposed during a strain sweep caused, not only disturbance of the linear viscoelastic behavior, but also changes in electrical conductivity. The oscillatory shear altered the morphology, breaking the PAni domains into smaller ones. This effect increases the distance between them and, consequently, resulted in a decrease of the electrical conductivity. The measurements under quiescent conditions and steady shear proved that the disturbance in morphology for PVDF/PAni system is non-recoverable. Through combined electrical and rheological measurements, it was possible to achieve good correlation between the electrical and flow behavior of PVDF/PAni blends.     

Functionalized graphene sheet—Poly(vinylidene fluoride) conductive nanocomposites

PVDF nanocomposites based on functionalized graphene sheets, FGS prepared from graphite oxide, and exfoliated graphite, EG, were prepared by solution processing and compression molding. FGS remains well dispersed in the PVDF composites as evidenced by the lack of the characteristic graphite reflection in the composites. Although the α‐phase of PVDF is seen in the EG‐based composites, a mixture of α‐ and β‐phases is present in the FGS analogs. SEM and TEM imaging show smooth fractured surfaces with oriented platelets of graphite stacks and obvious debonding from the matrix in the EG‐PVDF composites. In contrast, the FGS‐PVDF composites show a wrinkled topography of relatively thin graphene sheets bonded well to the matrix. Storage modulus of the composites was increased with FGS and EG concentration. A lower percolation threshold (2 wt %) was obtained for FGS‐PVDF composites compared to EG‐PVDF composites (above 5 wt %). Lastly, the FGS‐PVDF composites show an unusual resistance/temperature behavior. The resistance decreases with temperature, indicating an NTC behavior, whereas EG‐PVDF composites show a PTC behavior (e.g., the resistance increases with temperature). We attribute the NTC behavior of the FGS based composites to the higher aspect ratio of FGS which leads to contact resistance predominating over tunneling resistance. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 888–897, 2009     

Thermal conductive PS/graphite composites

Polystyrene (PS) was compounded with graphite that possesses high thermal conductivity and layer structures, and the PS/graphite thermal conductive nano‐composites were prepared. Thermal conductivity of PS improved remarkably in the presence of the graphite, and a much higher thermal conductivity of 1.95 W/m K can be achieved for the composite with 34 vol% of colloidal graphite. The Maxwell‐Eucken model and the Agari model were used to evaluate the thermal conductivity of the composites. For the purpose of improving the interfacial compatibility of PS/graphite, realizing the exfoliation and nano‐dispersion of graphite in the PS matrix, three intercalation methods, including rolling intercalation, solvent intercalation, and pan milling intercalation, were applied to prepare the composites, and the morphologies, thermal conductivities, and mechanical properties of the composites were investigated. It should be noted that the one prepared by pan milling intercalation not only had excellent thermal conductivity but also much higher mechanical properties, resulting from a high degree of layer exfoliation of the graphite, the formation of the chain structure agglomerates of the graphite, and the creation of more conductive paths under the strong shear stress of pan milling. Copyright © 2008 John Wiley & Sons, Ltd.     

Polylactide/exfoliated graphite nanocomposites with enhanced thermal stability,mechanical modulus,and electrical conductivity

We have prepared a series of polylactide/exfoliated graphite (PLA/EG) nanocomposites by melt‐compounding and investigated their morphology, structures, thermal stability, mechanical, and electrical properties. For PLA/EG nanocomposites, EG was prepared by the acid treatment and following rapid thermal expansion of micron‐sized crystalline natural graphite (NG), and it was characterized to be composed of disordered graphite nanoplatelets. It was revealed that graphite nanoplatelets of PLA/EG nanocomposites were dispersed homogeneously in the PLA matrix without forming the crystalline aggregates, unlike PLA/NG composites. Thermal degradation temperatures of PLA/EG nanocomposites increased substantially with the increment of EG content up to ～3 wt %, whereas those of PLA/NG composites remained constant regardless of the NG content. For instance, thermal degradation temperature of PLA/EG nanocomposite with only 0.5 wt % EG was improved by ～10 K over PLA homopolymer. Young's moduli of PLA/EG nanocomposites increased noticeably with the increment of EG content up to ～3 wt %, compared with PLA/NG composites. The percolation threshold for electrical conduction of PLA/EG nanocomposites was found to be at 3–5 wt % EG, which is far lower graphite content than that (10–15 wt % NG) of PLA/NG composites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 850–858, 2010     

Thermal conductivity of exfoliated graphite nanocomposites

Since the late 1990’s, research has been reported where intercalated, expanded, and/or exfoliated graphite nanoflakes could also be used as reinforcements in polymer systems. The key point to utilizing graphite as a platelet nanoreinforcement is in the ability to exfoliate graphite using Graphite Intercalated Compounds (GICs). Natural graphite is still abundant and its cost is quite low compared to the other nano–size carbon materials, the cost of producing graphite nanoplatelets is expected to be ~$5/lb. This is significantly less expensive than single wall nanotubes (SWNT) (>$45000/lb) or vapor grown carbon fiber (VGCF) ($40–50/lb), yet the mechanical, electrical, and thermal properties of crystalline graphite flakes are comparable to those of SWNT and VGCF. The use of exfoliated graphite flakes (xGnP) opens up many new applications where electromagnetic shielding, high thermal conductivity, gas barrier resistance or low flammability are required. A special thermal treatment was developed to exfoliate graphite flakes for the production of nylon and high density polypropylene nanocomposites. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to assess the degree of exfoliation of the graphite platelets and the morphology of the nanocomposites. The thermal conductivity of these composites was investigated by three different methods, namely, by DSC, modified hot wire, and halogen flash lamp methods. The addition of small amounts of exfoliated graphite flakes showed a marked improvement in thermal and electrical conductivity of the composites.     

聚丙烯/石墨纳米复合材料的导电性能研究

磨盘碾磨固相剪切复合技术(S3C)是制备聚合物 石墨导电复合材料的有效途径,所得聚丙烯 膨胀石墨复合材料具有纳米插层复合结构,石墨纳米片层的相互搭接可形成导电网络,具有纳米间隙的石墨插层结构可形成隧道电流,从而大幅度降低复合体系的导电逾渗阈值,在低填充量实现聚合物复合材料高电导性,与熔体共混相比,导电逾渗阈值由4 .3vol%降低到0 . 5 5vol% ,在石墨含量为4 .0 1vol%时,电导率提高10个数量级.     

Synergic effect in electrical conductivity using a combination of two fillers in PVDF hybrids composites

The objective of this work was to prepare novel conductive blends of poly(vinylidene fluoride) (PVDF) with polypyrrole (PPy) and to compare their performance with PVDF/multiwall carbon nanotube (MWCNT) composites and novel PVDF/PPy/MWCNT hybrid systems. All the compositions were prepared by melt mixing using a miniature mixer. The mixtures were characterized by Fourier transformed infrared (FTIR), wide angle X-ray diffraction (WAXD), thermogravimetric analyses (TGA), scanning and transmission electron microscopy (SEM and TEM, respectively) and volume electrical resistivity. For the binary PVDF/PPy and PVDF/MWCNT systems, percolation thresholds of 10 and 0.3 wt%, respectively, were found. In the hybrid systems, however, the percolation threshold for each filler was lower than in the binary systems, but the electrical conductivities were always much higher at all concentrations than the conductivities of the binary systems. Therefore, the addition of both fillers had a synergistic effect on the hybrid system conductivity, which was attributed to its morphology: the PPy increased the homogeneity of the MWCNT distribution and decreased the available free volume for the MWCNT; as a result the MWCNT rolled around the PPy particles bridging them through the PVDF matrix, increasing the quantum tunneling effect and thus, the electrical conductivity of the system.     

高分子导热复合材料结构设计及性能研究进展

近年来,电子设备的需求逐渐向集成化、微型化发展,随之带来了愈发严重的发热问题已经成为了阻碍电子设备发展的重要因素之一。作为电子设备重要组成材料之一的高分子材料对优良导热性能的要求也越来越高,导热高分子复合材料的研究已经成为当前功能复合材料的重要发展方向。本文综述了高分子导热复合材料的发展趋势,介绍了当前选用填料法来制备单一填料、混杂填料高分子导热复合材料以及双逾渗结构、隔离结构等复杂多相结构的高分子导热复合材料的研究进展。重点介绍了通过多种导热填料的组合利用来制备高性能导热高分子复合材料。最后,对填料法高导热高分子复合材料的发展方向做出了简要展望。     

Conductive mechanism of polymer/graphite conducting composites with low percolation threshold

Preparation of the conducting composites of polystyrene/expanded graphite via in situ polymerization and investigation of the conductive mechanism were carried out. They are characterized by high conductivity and a low percolation threshold. The electrical conductivity reached 10^?2 S/cm with 3.0 vol % expanded graphite content, whereas the percolation threshold was 1.0 vol %. Optical micrographs revealed the heterogeneous distribution of the graphite particles and the formation of a conductive network in the polymer matrix. A model of primary particle was proposed to interpret the conductive phenomena. The primary particle is the basic conductive unit in the composites that comprises three of the following parts: the graphite particle, the compact‐adsorbed layer, and the wrapping shell. Our model was also used to explain the experimental data in our previous studies on nylon‐6/expanded graphite composites. A low percolation threshold of conducting composites can be also explained according to the model of the primary particle. Furthermore, the theoretical line of conductivity versus primary particle content calculated from the revised Flory's theory fits the experimental data well. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 954–963, 2002     

Significant enhancement of electrical and thermal conductivities of polyethylene carbon nanotube composites by the addition of a low amount of silver nanoparticles

Electrically and thermally conductive high‐density polyethylene composites filled with hybrid fillers, multiwall carbon nanotubes (MWCNTs) and silver nanoparticles (Ag‐NPs), have been prepared in the melt state. The investigation of their electrical and thermal conductivities while comparing with high‐density polyethylene/MWCNT binary composites shows that the addition of only 3 vol% of Ag‐NPs does not reduce the electrical percolation threshold (P_c) that remains as low as 0.40 vol% of MWCNTs but leads to an increase in the maximum dc electrical conductivity of PE/MWCNT composites by two orders of magnitudes. Moreover, the association of both Ag‐NPs and carbon nanotube particles improved our composite's thermal conductivity. Copyright © 2014 John Wiley & Sons, Ltd.     

Electrical conducting behaviors in polymeric composites with carbonaceous fillers

Several kinds of polymer composites with carbonaceous fillers such as carbon black (CB), vapor‐grown carbon fiber (CF), and carbon nanotube (CNT) are prepared by a gelation/crystallization process or a melt mixing method. The electrical phenomena, changes of electrical conductivities with different filler's type, filler's concentration and temperatures, and the mechanism of electron transport in these carbon‐filled polymer composites are directly influenced by the geometric grain shape and aggregating morphology of the fillers dispersed in the polymer matrix. For the composites of CB and CF, long‐range macroscopic conduction are governed by the percolation phenomenon, the conduction is behaved through the conductive path formed by the conductors' contacting, and the thermal expansion changes the physical dimensions of the entire electrical network and leads to the changes in the electrical phenomenon. Microscopic conduction between conductive elements is influenced by the tunneling barrier or tunneling voltage, which varies with the temperature change, explaining the apparent observation of the temperature dependence of the composites. In comparison with fillers of CB and CF, the CNT performs unique electric properties for their nonspherical geometry and morphology as a three‐dimensional network (high structures), which has been visually proved by SEM photos in our former research, leading to the percolation threshold lower than 1% in the volume fraction and much less temperature dependence in its composites. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1037–1044, 2007     

The role of interfacial compatibilization upon the microstructure and electrical conductivity threshold in polypropylene/expanded graphite nanocomposites

Attempts have been made to evaluate the effect of interface and degree of interfacial interaction upon electrical conductivity threshold in polypropylene/expanded graphite (PP/EG) nanocomposites, and dispersion state of graphite nanosheets. For this purpose, maleic anhydride grafted polypropylene (PPgMA) and maleic anhydride grafted EPDM (EPDMgMA) were used as compatibilizer. Nanocomposite samples containing 1–5 vol% of EG were prepared by melt mixing method using laboratory scale internal mixer. Characterization was carried out by using X‐ray diffraction (XRD), differential scanning calorimeter (DSC), thermo‐gravimetric analysis (TGA), scanning electron microscope (SEM), transmission electron microscope (TEM), and rheo‐mechanical spectroscopy (RMS). The conductivity measurements were carried out by using four point probe method according to ASTM D991. Results showed that the conductivity threshold is controlled by the extent of interfacial interaction between PP and EG. So, better conductivity was obtained using PPgMA as compatibilizer which causes higher level of interaction between PP and EG, and therefore better dispersion of the EG nanolayers in the polymer matrix. On the other hand, high levels of compatibilizers, especially EPDMgMA, caused formation of separated aggregates of EG shelled with the compatibilizer, which results in the reduction of conductivity of the nanocomposites. This finding has been verified by SEM, RMS, and conductivity measurements. Effects of EG nanolayers on crystalline structure and thermal decomposition temperature of the nanocomposites have also been investigated by DSC and TGA, respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

Electrical properties of conductive polymer composites prepared by an innovational method

<正>An innovational method that poly(styrene-co-maleic anhydride)(SMA),a compatibilizer of immiscible nylon6/polystyrene(PA6/PS) blends,was first reacted with carbon black(CB) and then blended with PA6/PS,has been employed to prepare the PA6/PS/(SMA-CB) composites of which CB localized at the interface.In PA6/PS/CB blends,CB was found to preferentially localize in the PA6 phase.However,in the PA6/PS/(SMA-CB) blends,it was found that CB particles can be induced by SMA to localize at the interface.The electrical porperties of PA6/PS/(SMA-CB) composites were investigated.The results showed that the composites exhibited distinct triple percolation behavior,i.e.the percolation is governed by the percolation of CB in SMA phase,the continuity of SMA-CB at the interface and the continuity of PA6/PS interface.The percolation threshold of PA6/PS/(SMA-CB) was only 0.15 wt%,which is much lower than that of PA6/PS/CB.Moreover,the PTC(positive temperature coefficient) intensity of PA6/PS/(SMA-CB) composites was stronger than that of PA6/PS/CB and the negative temperature coefficient(NTC) effect was eliminated.The electrical properties of PA6/PS/(SMA-CB) were explained in terms of its special interface morphology:SMA and CB localize at interphase to form the conductive pathways.     

马来酸酐接枝聚丙烯/石墨导电纳米复合材料的研究

用溶液插层 (SI)法制备了马来酸酐接枝聚丙烯 (gPP) 膨胀石墨 (EG)导电纳米复合材料 ,以熔体混合(MM)法作对照 ,通过室温体积电导率 (σ)测试和OM、SEM、TEM观察 ,研究了复合材料的制备方法、微观结构和导电性能关系 .结果表明 ,SI法制得纳米复合材料的室温逾渗阈值c=0 6 7vol% ,远低于MM法制得复合材料的c=2 96vol% ;3 90vol%EG含量下 ,前者的σ达 2 4 9× 10 - 3S cm ,而后者的σ仅 6 85× 10 - 9S cm .产生上述差异的原因 ,与两种方法制得复合材料中EG分散相的形态及其内部微结构直接相关 .     

Sensing of liquids by electrically conductive immiscible polypropylene/thermoplastic polyurethane blends containing carbon black

Immiscible polymer blends based on polypropylene/thermoplastic polyurethane (PP/TPU) are interesting host multiphase systems for the incorporation of low concentrations of conductive carbon black (CB) particles. The enhancement of conductivity (and the lower critical CB content for percolation) in the PP/TPU blend is achieved via double percolation, that is, structural and electrical. The CB particles form chainlike network structures within the TPU phase, which exhibit phase continuity of elongated particles within the PP matrix. Moreover, scanning electron microscopy and dynamic mechanical thermal analysis studies indicated that the incorporation of CB particles into the PP/TPU blend has a “compatibilizing” effect, resulting in an enhanced interaction between the two polymers. Extruded PP/TPU/CB filaments produced by a capillary rheometer process at various shear rates were examined as sensing materials for a homologous series of alcohols, that is, methanol, ethanol, and 1‐propanol. All filaments displayed increasing resistance upon exposure to the various alcohols combined with excellent reproducibility and recovery behavior. An attempt is made to identify the dominant mechanisms controlling the sensing process in a CB‐containing immiscible polymer blend characterized by a double‐continuity structure. The interphase region, its quantity, and continuity played a significant role in the liquid‐transport process. Blend composition, filaments' extrusion temperature, and production shear rate level were considered as significant parameters determining the structure and the resultant sensing properties. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1428–1440, 2003