Triple percolation behavior and positive temperature coefficient effect of conductive polymer composites with especial interface morphology

Selective localization of carbon black (CB) at the interface of polymer blends was achieved by the method that poly(styrene-co-maleic anhydride) (SMA) was first reacted with CB, and then blended with nylon6/polystyrene (PA6/PS). In the PA6/PS blends, CB was localized in PA6 phase and typical double percolation was exhibited. In the PA6/PS/(SMA–CB) blends, TEM results showed that CB particles were induced by SMA to localize at the interface, resulting in the especial interface morphology fabricated by SMA and CB. The especial interface morphology of PA6/PS/(SMA–CB) caused distinct triple percolation behavior and very low percolation threshold. The positive temperature coefficient (PTC) 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 elimination of NTC effect was arisen from the especial interface morphology. A stronger PTC intensity was attributed to the low percolation threshold and the morphology.     

Designs of conductive polymer composites with exceptional reproducibility of positive temperature coefficient effect: A review

Conductive polymer composites with positive temperature coefficient (PTC) effect have gained intensive attention for the potential application in the smart heating field. The PTC reproducibility is significantly essential to guarantee the security and utility of PTC composites. Regrettably, during the repeated temperature cycles, the irreversible self-aggregation of conductive filler and the random reconstruction of conductive network lead to unsatisfactory performance of PTC reproducibility. Extensive efforts have been conducted to address this issue by strategies, including modification of fillers, cross-linking of a polymer matrix, hybrids of fillers, and application of binary polymer matrix. Nevertheless, there are very limited reviews about this issue. In this review, the recent advances in fabricating PTC composites with the enhanced PTC reproducibility have been systematically summarized. Meanwhile, the current challenges and future prospects of PTC composite are also presented. We hope that this review will provide some inspirations for designing PTC materials of long-term performance for commercial applications.     

Room temperature resistance relaxation behavior for carbon black filled conductive polymer composites

Room temperature resistance relaxation was studied with respect to carbon black (CB) volume fraction, the type of polymer matrix, and the environment. It was found that resistance of CB filled poly(methylvinylsiloxane) and polypropylene (PP) conductive composites changed at room temperature with different directions and amplitudes, depending on the filler volume fraction and the environment. The room temperature resistance relaxation was ascribed to the local Joule heat at the tunneling junction or the swelling effect of the solvents. On the other hand, CB filled immiscible PP/Nylon 1212 blends exhibited a stable electrical conduction due to the selective distribution of CB aggregates along the interface between polymer matrices. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effects of thermal ageing treatment and antioxidants on the positive temperature coefficient characteristics of carbon black/polyethylene conductive composites

Polyethylene (PE)‐filled with carbon black (CB) is a prototypical composite that displays resistance switching. These materials can exhibit either a positive temperature coefficient (PTC) or negative temperature coefficient (NTC). The CB‐filled semicrystalline polymer composites ideally need antioxidants, which stabilize the composites against thermooxidative degradation, because they should be resistant to the severe conditions of high temperature. The characterization of PTC materials is affected by the crystallinity of the polymer, and the crystallinity of the polymer is changed with thermal ageing treatment. Thermal ageing of PTC samples was conducted in an oven in the range 50–140°C, in air. The composites, containing 0.5–3% (by weight) Irganox 1076 (Ciba‐Geigy), were irradiated under nitrogen at room temperature with different doses of gamma rays from a ⁶⁰Co source. The resulting composites were analyzed by differential scanning calorimetry, gel fractionation, X‐ray diffraction, and dynamic mechanical analysis. The conductivity of the composites depended on the amounts of antioxidants and the duration of the thermal ageing treatment. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2316–2322, 2003     

Percolation conductivity of polymer composites filled with dispersed conductive filler

This paper deals with the conductivity of binary polymer composites filled with an electronically conductive material. A “dynamic cluster model” is offered to describe the conductivity of such polymer composites in the highly filled region, i.e. above the percolation threshold. The model is based on the following assumptions:

• 1 a modification of the basic statistical percolation equation, i.e. σ (φ−φ_c)^t, where t = 1.6 to 1.9, should be applied for all systems in the highly filled region, although application is limited to the range φ = φ_c + Δφ, Δφ ⟹ 0 in the strict statistical percolation approach;
• 2 the most important modifications with respect to the basic equation of the statistical percolation theory are
  • (a) the use of a constant t_eff, including a constant part t₁ (resembling “t” in the basic statistical percolation approach) and a variable part t₂ (depending on the filler concentration φ of the specific mixture) and
  • (b) the definition of φ_c as the filler concentration where a perfect three-dimensional network of the conductive phase has been established. This idea has been adopted from the bond-percolation approach of Aharoni;
• 3 the resulting equation should include parameters of specific polymer composites.

The generalized equation σ = f(φ) is used to calculate the maximum possible conductivity of a certain mixture as well as the dependence of σ on the filler content.     

Economical conductive graphite-filled polymer composites via adjustable segregated structures: Construction,low percolation threshold,and positive temperature coefficient effect

The economical graphite-filled thermoplastic urethane/ultra-high molecular weight polyethylene (TPU/UHMWPE) composites with the segregated structure were constructed by the combination of mechanical crushing and melt blending method. The low percolation threshold of 1.89 wt% graphite in the adjustable segregated composites was obtained and high electrical conductivity was about 10⁻¹ S m⁻¹ at 10 wt% graphite loadings owing to the formation of three-dimensional conductive networks. Moreover, when the graphite loadings were over the percolation threshold, the remarkable positive temperature coefficient (PTC) effect of electrical resistivity for TPU/UHMWPE-Graphite composites were achieved, originating from the combined thermal motion of TPU and UHMWPE. Meanwhile, the outstanding repeatability of PTC effects was obtained after 5-time cycles. Therefore, economical conductive polymer composites were still the promising field in the practical application of PTC materials.     

The positive temperature coefficient phenomenon of vinyl polymer/CB composites

In this paper, the results of a systematic study of carbon black (CB)-filled conducting polymer positive and negative temperature coefficient (PTC/NTC) effects are report. The conductivity of the composites jumps by several orders of magnitude at the critical value of carbon black. This critical value, ø_c, decreases with the increase of melt index and degree of crystallinity of the polymer. The crystalline lamellae just “modify” the distribution of carbon black and make the dispersion heterogeneous. Radiation cross-linking enhances the PTC intensity and decreases the NTC effect of the materials. The electrical reproducibility of compounds is improved by the cross-linking structure that reduces the freedom of carbon black movement at high temperature. The NTC phenomenon is related to the carbon black coagulation that facilitates electrical conduction at high temperature. The larger the melt index, the more easily carbon black coagulates. A new model was set up to explain the results successfully. © 1993 John Wiley & Sons, Inc.     

Developing electrically conductive polypropylene/polyamide6/carbon black composites with microfibrillar morphology

We have established that the PP/PA6/CB composite with 3D microfibrillar conducting network can be prepared in situ using melt spinning process. CB particles preferably were localized at the interface between polypropylene as the matrix and PA6 microfibrils, which act as the conducting paths inside the matrix. The percolation threshold of the system reduced when aspect ratio of the conducting phase was increased by developing microfibrillar morphology. The effect of annealing process on the conductivity of PP/PA6/CB composite with co‐ continuous and microfibrillar morphologies was studied. It was observed that, annealing process forces CB particles towards the interface (2D space) of PP and PA6 co‐continuous phases, and percolation threshold and critical exponent of classical percolation theory will be decreased, while the conductivity of conducting composite with microfibrillar morphology was not affected considerably by annealing process at temperatures either higher or lower than the melting point of the PA6 microfibrils. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Conductive polymer blends filled with carbon black: Positive temperature coefficient behavior

Electrical conductivity and positive temperature coefficient (PTC) behavior of carbon black (CB) filled incompatible polyblends of ethylene-vinyl acetate copolymer/low density polyethylene (EVA/LDPE) were investigated. In comparison with single polymer systems, more possibilities for tailoring composite performance were brought about with the employment of polymer blends as matrix resins in conductive composites. Based on the concepts of double percolation and two-step percolation, PTC-type composites with balanced performance, improved processability, and reproducibility can be made. Thermodynamical and kinetic factors including interfacial energy, melt viscosity, blending ratio, melt mixing time, sequence of blending as well as CB concentration were shown to be closely related to the ultimate properties obtained.     

Effect of surface treatment on electrical conductivity of carbon black filled conductive polymer composites

Two different types of surface modifiers, 3‐aminopropyltriethoxysilane and formamide, were applied to carbon black (CB) particles to lower electrical resistivity of polymer composites prepared by treated CB. Two different matrices, low‐density polyethylene and nylon 6, were chosen to compound with surface modified CB. Surface energy of CB was increased by adding amine or amide functional groups during surface treatment of CB. According to electron spectroscopy for chemical analysis (ESCA), chemical modification in surface chemistry of CB was obtained with the chemicals used for the treatment due to the nitrogen atoms in their structures, which may act as dopant atom. As a result of this, electrical resistivity of composites prepared by treated CB decreased. In addition, there was not any significant change in tensile strength and tensile modulus of the composites with the surface treatment. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Influence of morphology on PTC effect for poly (ethylene-co-butyl acrylate)/nylon6 blends with multiwall carbon nanotubes dispersed at interface and in matrix

Multiwall carbon nanotubes (MWCNTs) filled poly (ethylene-co-butyl acrylate)/nylon6 (EBA/PA6) blends were prepared by melt-mixing method. MWCNTs were localized in PA6 phase and the percolation threshold was 6 wt%. A weak PTC (positive temperature coefficient) effect was observed. The method that EBA-g-MAH was first reacted with MWCNTs, and then blended with EBA/PA6 was employed to prepare EBA/PA6/EBA-g-MAH/MWCNTs composites. TEM results showed that MWCNTs were localized both at the interface and in PA6 phase, resulting in the sharp decrease of the percolation threshold. Influence of morphology on the PTC effect of EBA/PA6/EBA-g-MAH/MWCNTs composites was studied. In composites with dispersed PA6 phase, the conductive pathways were fabricated by the contact of dispersed PA6 phase and MWCNTs in PA6 phase. The melt of polyethylene segment crystals in EBA and PA6 phase interrupted the contact of dispersed phases and conductive network formed by MWCNTs in PA6 phase, resulting in the double PTC effect. For composites with dispersed EBA phase, although the conductive pathways were similar with the composites with dispersed PA6 phase, the single PTC effect was observed. And the PTC effect was attributed to the melt of PA6 phase. The conductive pathways of composites with co-continuous morphology were fabricated by MWCNTs at the interface and in continuous PA6 phase. Two strong and a weak PTC effect were observed. PTC effects appeared at the melting temperature of PA6 crystals, polyethylene segment crystals and viscous flow temperature of butyl acrylate units in EBA.     

Dielectric effects on positive temperature coefficient composites of polyethylene and short carbon fibers

By adding the short carbon fibers to the polyethylene matrix, excellent positive temperature coefficient (PTC) effect was achieved. Alternating current (AC) electrical properties of this PTC composite were studied as a function of frequency. The analysis of AC electrical conductivity and dielectric permittivity was done by using a micro-morphology model, which included conductive carbon fiber-aggregates in series with an equivalent circuit of resistor-capacitor parallel that represent the blends at these contact regions. The observed electrical properties of PTC composites were due to the breakage of the conduction networks caused by thermal expansion. The dielectric behaviors of the interfacial polarization between polyethylene matrix and carbon fibers could be described by Maxwell-Wagner-Sillars relaxation when the composite was heated above 116 °C. The analysis of the electric modulus in the frequency range from 100 Hz to 10 MHz revealed that the interfacial relaxation followed the Cole-Davidson distribution of relaxation time.     

Dielectric properties and morphology of polymer composites filled with dispersed iron

The dielectric properties and the structure of various metal–polymer composites, based on a polymer matrix of polyamide (PA), polyethylene (PE), polyoxymethylene (POM), or blend PE/POM filled with dispersed iron (Fe) particles, have been investigated in this work. In PE–Fe, PA–Fe, and POM–Fe composites the filler spatial distribution is random. In the PE/POM–Fe composites, the polymer matrix is two‐phase and the filler particles are localized only in the POM phase, resulting in an ordered distribution of the dispersed filler particles within the blend. The concentration and frequency dependence of the dielectric permittivity, ε′, and the dielectric loss tangent, tanδ, are described in terms of the percolation theory. The experimental values of the critical exponents (namely, s, r, and y) are in good agreement with those predicted by the theory for the composites with random filler distribution. The PE/POM–Fe composites demonstrate low value of the percolation threshold, P_C, and high values of the critical exponents r and y. This is attributed to the specific structure of these composites. A schematic model for the morphology of the composites studied has been proposed. This model explains the peculiar behavior of the PE/POM–Fe composites by assuming ordered distribution of the filler particles in a binary polymer matrix. The proposed model is in good agreement with the results of optical microscopy. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3013–3020, 2003     

碳系填料在聚合物基导电复合材料中的应用

介绍碳系填料在聚合物基导电复合材料中的应用以及导电复合材料渗流阈值的影响因素。碳系填料主要包括炭黑、石墨、碳纤维和碳纳米管等,对各类碳系填料的研究应侧重于提高其本征导电性能,并解决填料在聚合物基体中的分散问题。开发复合填料是拓宽碳系填料在电磁屏蔽领域应用的有效途径。采用混合填料、复合填料和多种聚合物材料并用可显著降低复合材料的电阻率,提高电磁屏蔽性能,降低导电渗流阈值。     

Control of carbon nanotubes at the interface of a co-continuous immiscible polymer blend to fabricate conductive composites with ultralow percolation thresholds

The concept of “double percolation”, i.e., conductive fillers are selectively located in one phase of a co-continuous polymer blend to form a percolated network in the selected phase, is widely used to reduce the percolation thresholds of conductive polymer composites to a fraction of their original values. However, it is expected that the percolation threshold can be significantly reduced further if the conductive fillers are only selectively distributed at the continuous interface of the co-continuous polymer blend, where only a very small amount of fillers are needed to build up the conductive percolated network. Multiwalled carbon nanotubes (MWCNTs) with very high aspect ratio (ca. 1000) are selectively distributed at a continuous interface of a co-continuous immiscible poly(lactic acid)/poly(ε-caprolactone) (PLA/PCL) blend at a weight ratio of 50/50 by controlling the migration process of MWCNTs from the unfavorable PLA to the favorable PCL phase. Compared to the PLA/PCL/MWCNTs composites by the traditional double percolation method (percolation threshold is ca. 0.97 wt%), the percolation threshold of PLA/MWCNTs/PCL composites (ca. 0.025 wt%) drops 2 orders of magnitude due to controlling the MWCNTs at the continuous interface between the PLA and PCL phases.     

丙烯酸树脂基导电油墨正/负升温系数协同效应的研究

采用乳液聚合的方法,利用Fox方程进行设计合成了玻璃化温度可控型聚(甲基丙烯酸甲酯-丙烯酸丁酯)[P(MMA-BA)]。借助激光粒度分布仪得到了控制乳液的粒径分布与大小的工艺条件;利用差热扫描量热仪(DSC)测定了聚合物的玻璃化温度(Tg)。实验发现所测的Tg与实验设计值有一定的偏差,为此对Fox公式进行了适用性的修正,理论设计和实测结果达到了很好的吻合。重点探讨了丙烯酸粘结料树脂玻璃化温度与导电填料的配比对电热膜电阻值稳定性的影响。利用扫描电子显微镜观察分析导电网络。结果表明,非晶态丙烯酸高聚物的PTC效应显著,消除了导电填料在温度升高过程中的NTC效应。     

Influence of irradiation conditions on the electrical behavior of polyethylene–carbon black conductive composites

The temperature‐dependent resistivity behavior of carbon black–loaded polyethylene (PE) composites irradiated both at room temperature and 170°C above the PE melting point was studied. The irradiation doses were varied. At a given loading level, irradiation at room temperature corresponded to an energy treatment on a low‐resistive, solid, three‐phase composite system, while at a high temperature it corresponded to a treatment on high‐resistive, viscous, two‐phase system. The irradiation condition had a complicated influence on the electrical response to temperature. The resulting composite structure was analyzed by using differential scanning calorimetry, gel fraction, and wide‐angle X‐ray diffraction. The results were then discussed by comparing them with those of the unirradiated sample. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 494–499, 2000     

LDPE/carbon black conductive composites: Influence of radiation crosslinking on PTC and NTC properties

The electrical resistivity of low‐density polyethylene/carbon black composites irradiated by ⁶⁰Co γ‐rays was investigated as a function of temperature. The experimental results obtained by scanning electron microscopy, solvent extraction techniques, and pressure‐specific volume‐temperature analysis techniques showed that the positive temperature coefficient (PTC) and negative temperature coefficient (NTC) effects of the composites were influenced by the irradiation dose, network forming (gel), and soluble fractions (sol). The NTC effect was effectively eliminated when the radiation dose reached 400 kGy. The results showed that the elimination of the NTC effect was related to the difference in the thermal expansion of the gel and sol regions. The thermal expansion of the sol played an important role in both increasing the PTC intensity and decreasing the NTC intensity at 400 kGy. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2742–2749, 2002     

Highly conductive polymer composites based on controlled agglomeration of carbon nanotubes

This paper reports an innovative approach to enhance electrical conductivity of fiber composites based on non-conductive fiber and polymer matrix. The dispersion of carbon nanotubes (CNTs) is carried out using a fiber sizing agent which contains uniformly distributed CNTs. The infusion of the sizing agent into the fiber preform prior to resin infusion gives rise to high agglomeration of CNTs on the fiber surface and results in electrical conductivities of 2-3 orders of magnitude higher than those of specimens prepared by a calendering approach.