An interlayer model for the complex dielectric constant of composites

The complex dielectric constant of a composite with an interlayer was studied as a function of the volume fractions and the properties of the filler, the interlayer, and the matrix. The theoretical approach is analogous to the calculation of the shear modulus, the bulk modulus, and the termal expansivity of particulate filled polymers using the interlayer model (IM).An analytical expression describing the influence of an interlayer on the generalized dielectric constant of the composite as a function of the volume fraction and interlayer properties is derived.In the case of a composite with non-conductive constituents, the equations for static and oscillatory electric fields are similar. When conductive constituents are present, the complex dielectric constants have to be replaced by the generalized complex dielectric constants.For a composite of non-conductive materials, without interlayer, the obtained relation reduces to the classical Rayleigh equation. In the case of a composite with conductive constituents, also without interlayer, the complete solution of Wagner's theory is found. Special attention has been paid to the case of a water interlayer in a glass-bead filled non-conductive matrix material.     

Electrorheological characterization of polyaniline-coated poly(methyl methacrylate) suspensions

Surface-conductive particles consisting of a poly(methyl methacrylate) (PMMA) core and a polyaniline (PA)-coated shell were synthesized and adopted as suspended particles for electrorheological (ER) fluids. The PA-PMMA composite particles synthesized were monodisperse and spherical in shape. The PA-PMMA suspensions in silicone oil showed typical ER characteristics under an applied electric field. The PA-PMMA composite particles possess a higher dielectric constant and conductivity than the pure PA particle, within an acceptable conductivity range for ER fluids, but the PA-based ER fluid showed larger shear-stress enhancement than the PA-PMMA-based systems. This phenomena can be explained by the interfacial polarizability of PA-based ER fluids, which is the difference between the ER fluid's dielectric constant and loss factor - this polarizability was higher than that of PA-PMMA-based ER fluids, as shown by the dielectric spectrum of each fluid. The insulating PMMA core suppressed the interfacial polarization in ER fluids, resulting in reduced interaction among particles under an imposed electric field.     

Fabrication of thermoplastic polyurethane composites with a high dielectric constant and thermal conductivity using a hybrid filler of CNT@BaTiO3

Thermoplastic polyurethane composites with an excellent dielectric constant and high thermal conductivity were obtained using CNT@BaTiO₃ as a filler through a low-speed melt extrusion method. Before preparing the hybrid filler for the composite, the filler particles were surface modified to ensure that the outer surfaces could facilitate the reaction among particles to form the hybrid and ensure complete dispersion in the thermoplastic polyurethane matrix. After confirming the proper surface treatment of the filler particles using infrared spectroscopy, thermal degradation analysis and field emission scanning electron microscopy, they were used to prepare the composite materials at a processing temperature of 200 °C. The thermal stability, thermomechanical properties, mechanical properties, thermal conductivity, and dielectric properties of the composites were investigated. Compared to the neat thermoplastic polyurethane matrix, the prepared composite exhibited a higher thermal stability, approximately 300% higher storage modulus, higher tensile strength and elongation at break values, approximately three times higher thermal conductivity (improved from 0.19 W/(m.K) to 0.38 W/(m.K), and approximately five times larger dielectric constant at high frequencies (at 1 MHz a dielectric constant of 19.2 was obtained).     

Improved dielectric and mechanical properties of polystyrene-hybrid silica sphere composite induced through bifunctionalization at the interface

Hybrid silica spheres (HS) of size 270-350 nm with vinyl and aminopropyl surface groups were incorporated in polystyrene (PS), and its effect on dielectric properties, coefficient of thermal expansion (CTE), and strength of PS-HS composite was studied. Incorporation of HS in PS followed a decrease in the dielectric constant from 3.2 for PS to 2.6 for composite with 7.5 vol % HS. The decrease in the dielectric constant was attributed to (i) increased interfacial porosity, (ii) formation of anhydrous HS having low dielectric constant, during hot processing of the composites, and (iii) dispersion and preservation of the anhydrous HS in the hydrophobic matrix. The dielectric constant of the composites with HS content up to 7.5 vol % does not vary much with temperature in the range from -20 to 65 °C. These composites also exhibited reduced CTE and improved flexural strength/stiffness due to good interfacial bonding through HS vinyl groups and dispersion of the filler in the matrix. The dielectric loss increased with HS content, and the loss measured for 7.5 vol % PS-HS composite was 6 × 10(-3), as compared to 10(-4) for PS. At HS loading above 7.5 vol %, the tendency of HS to agglomerate and form percolated structure lead to an increase in the dielectric constant and decrease in the mechanical properties of the composites.     

Permeation models for mixed matrix membranes

Permeation models for mixed matrix membranes (MMMs) are discussed. A new model is proposed for the effective permeability of a species in MMMs. The model takes into account the presence of interfacial layer (shell) at the surface of the core filler particles. According to the proposed model, the relative permeability (Pr) of a species in MMM, defined as permeability in MMM divided by matrix permeability, is a function of five variables, namely: ratio of interfacial shell-to-core particle radii (delta), ratio of interfacial shell-to-matrix permeabilities (lambdaIm), ratio of core particle-to-interfacial shell permeabilities (lambdadI), volume fraction of composite core-shell particles (phi), and maximum packing volume fraction of particles (phim). The predictions of the model are discussed and compared with available experimental data on permeability and selectivity of mixed matrix membranes.     

Insulator conductor transition in low-density polyethylene-graphite composites

The present paper reports the electrical conductivity of graphite filled low-density polyethylene composite in a broad range of composition. The observed non-percolation insulator conductor transition could well be explained in terms of the random resistor network of Miller and Abrahams model by considering the concentration of sites and size of the filler particle. The variation of the dielectric constant and dissipation factor has also been investigated. The dielectric constant and dissipation factor of low-density polyethylene-graphite composite increase with the increase in graphite volume fraction and significant changes were observed near the transition region. High values of dielectric constant and dissipation factor were observed in the low frequency region due to the Maxwell-Wagner interfacial polarization. A non-linear to linear variation of current density with electric field has been observed as the polymer composite transform to conducting region.     

Achieving high electric energy storage in a polymer nanocomposite at low filling ratios using a highly polarizable phthalocyanine interphase

Polymer nanodielectrics have become attractive for practical applications such as electric energy storage and electromechanical actuation. However, to enhance the apparent dielectric constant of polymer nanodielectrics, a significant amount (>30 vol %) of spherical particles needs to be incorporated into the polymer matrix. As a consequence, melt-processing of polymer nanodielectrics into uniform thin films becomes difficult at such a high filler content, and electric breakdown strength will greatly decrease. In this work, we describe a three-phase composite approach towards high energy density nanodielectrics at low filling ratios. In this approach, a highly polarizable tetrameric metallophthalocyanine (TMPc) initiator is coated onto 68 nm BaTiO₃ nanoparticles, from which poly(methyl methacrylate) (PMMA) brushes are grafted using atom transfer radical polymerization for the nanoparticles to be uniformly dispersed in a poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] matrix. For comparison, two-phase P(VDF-HFP)/BaTiO₃ composites without the TMPc interfacial layer are also prepared. Owing to the high polarizability of the TMPc interface layer, the three-phase composite films exhibit higher dielectric constant and thus higher energy density than the two-phase composite films at volume-filling ratios below 5 vol %. Therefore, these high energy density three-phase nanodielectrics with a low filling ratio are promising for melt-processing into thin dielectric films. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1669–1680     

Role,effect, and influences of micro and nano‐fillers on various properties of polymer matrix composites for microelectronics: A review

Ever since the discovery of polymer composites, its potential has been anticipated for numerous applications in various fields such as microelectronics, automobiles, and industrial applications. In this paper, we review filler reinforced polymer composites for its enormous potential in microelectronic applications. The interface and compatibility between matrix and filler have a significant role in property alteration of a polymer nanocomposites. Ceramic reinforced polymeric nanocomposites are promising candidate dielectric materials for several micro‐ and nano‐electronic devices. Because of its synergistic effect like high thermal conductivity, low thermal expansion, and dielectric constant of ceramic fillers with the polymer matrix, the resultant nanocomposites have high dielectric breakdown strength. The thermal and dielectric properties are discussed in the view of filler alignment techniques and its effect on the composites. Furthermore, the effect of various surface modified filler materials in polymer matrix, concepts of network forming using filler, and benefits of filler alignment are also discussed in this work. As a whole, this review article addresses the overall view to novice researchers on various properties such as thermal and dielectric properties of polymer matrix composites and direction for future research to be carried out.     

Interfacial polarization in composite materials with spherical fillers: characteristic frequencies and scaling laws

The dielectric properties of composite materials consisting of a host matrix filled with spherical particles are investigated as a function of frequency by means of numerical calculations. Two different cases are analyzed: (a) composites with a conductive matrix and insulating fillers and (b) composites with an insulating matrix and conductive fillers. In both situations, dielectric dispersions due to interfacial polarization effects are observed in the dielectric spectra. In the present contribution, the characteristic frequencies of interfacial polarization effects are systematically analyzed in dependence on the volume fraction of the spherical fillers and on the conductivity values of the composite phases. The resulting scaling laws are discussed in detail.     

浓差极化的介电模型——复合膜/溶液体系的数值模拟

提出了具有电导率和介电常数线性分布的介质的介电模型, 并导出了其内部电的和结构性质的参数与宏观测量的电容和电导之间定量关系的理论表达式, 以模拟复合膜中的多孔层部分的介电弛豫行为. 大量的模拟计算描述并解释了多孔层介电谱随介电常数分布、厚度等性质而变化的规律. 进一步对具有层状构造的复合膜以及复合膜和溶液相组成的多层体系的弛豫行为进行了数值模拟, 比较了三个体系(多孔层、复合膜、复合膜/液相层状体系)的介电谱, 结果揭示了介电谱对各层性质的依赖关系. 所提出的电导率和介电常数线性分布的多孔层的介电模型, 也可用于具有其他电导率、介电常数分布规律的体系.     

Effect of water sorption on the dielectric behavior of calcium chondroitin-4-sulfate

The dielectric properties of calcium chondroitin-4-sulfate were studied as a function of frequency, temperature, and water content. The dielectric constant changes very little with water content below “monolayer” coverage, and after that the dielectric constant follows the shape of the adsorption isotherm. The dielectric behavior is discussed as affected by interfacial polarization and by the increase in the rotational freedom of the polymer side chains.     

Effect of the shape of human erythrocytes on the evaluation of the passive electrical properties of the cell membrane

The possible influence of the cell shape on the derivation of the passive electrical parameters of a biological cell membrane is discussed in light of two different models which describe the cell as a shelled ellipsoidal particle and as a biconcave disk obtained by the revolution of the Cassini oval, respectively. Whereas within the first model, the Laplace equation can be solved analytically, in the second one a numerical algorithm based on the boundary element method has been employed. We have compared the results obtained by these two different models in the case of normal human erythrocyte cell membrane, using radiowave dielectric spectroscopy measurements. Our findings show that, although in principle the cell shape might deeply affect the evaluation of the passive electrical parameters of the cell membrane, in the case of the erythrocyte shape modelled by the Cassini curve, only small deviations are evidenced in comparison to the values derived, as usually done in the dielectric spectroscopy of biological cell suspensions, from an ellipsoidal model analysis. This result gives further support to the reliability of the data reported in the literature based on an ellipsoidal shape erythrocyte model.     

The analysis of electric properties of thin film composite reverse osmosis membrane with wet impedance method

A study on the electric properties of reverse osmosis composite membrane was conducted by wet impedance method. The thin film of composite membrane was prepared by interfacial polycondensation with 1,3-phenylene diamine and 1,3,5-benzene tricarbonyl chloride. The electric resistance and capacitance of polyamide skin layer of composite membrane was analyzed by alternating current. The dielectric constant of membrane was also analyzed and compared with theoretical value. Concentration and dipping time of aqueous phase play major role in electrical properties of membrane. The effect of acid treatment on membrane properties was also investigated.     

Physicochemical and Electrophysical Properties of Metal/Semiconductor Containing Nanostructured Composites

The properties of nanostructured composites based on metal oxides and metal–polymer materials are analyzed, along with ways of preparing them. The effect the interaction between metal and semiconductor nanoparticles has on the conductivity, photoconductivity, catalytic activity, and magnetic, dielectric, and sensor properties of nanocomposites is discussed. It is shown that as a result of this interaction, a material can acquire properties that do not exist in systems of isolated particles. The transfer of electrons between metal particles of different sizes in polymeric matrices leads to specific dielectric losses, and to an increase in the rate and a change in the direction of chemical reactions catalyzed by these particles. The interaction between metal-oxide semiconductor particles results in the electronic and chemical sensitization of sensor effects in nanostructured composite materials. Studies on creating molecular machines (Brownian motors), devices for magnetic recording of information, and high-temperature superconductors based on nanostructured systems are reviewed.     

Enhanced dielectric response in P(VDF‐TrFE) based all‐organic nanocomposites

A nanocomposite with enhanced dielectric response is developed using poly(vinylidene fluoride‐trifluoroethylene) [P(VDF‐TrFE)] as matrix and Chemically modified high dielectric constant organic semiconductor—copper phthalocyanine oligomer (CuPc)—as filler. Transmission electron microscope (TEM)‐observed morphologies reveal that in the nanocomposite the average size of CuPc particles is about 25 nm [1/24 of that of CuPc in physical blend of P(VDF‐TrFE) and CuPc]. The hot‐press nanocomposite film with 15 wt % CuPc can realize a dielectric constant of 540 at 100 Hz. The enhanced dielectric response in the nanocomposite demonstrates the significance of the interface effect in raising the material responses far beyond that expected by simple mixing rules when there is a large dielectric contrast between the polymer matrix and the dielectric filler in the composite. It is also interesting to note that at high frequencies (such as 100 MHz) the nanocomposite has a dielectric constant of ～100 and this value is comparable to those of current materials used in microwave applications. At 105 °C that is near the ferroelectric‐to‐paraelectric phase transition temperature of the P(VDF‐TrFE) ferroelectric, a much higher dielectric constant (about 1200 at 100 Hz) is obtained. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 490–495, 2010     

DC dielectric study of polyethylene/CaCO3 composites

The dielectric properties of composite samples prepared by polymerizing ethylene on the surface of filler are compared to those of mechanical mixtures consisting of CaCO₃ and ultra high molecular weight polyethylene. After presenting the normalized master curves of AC dispersion and loss measured at different relative humidities, the field strength dependence of the 50 Hz AC and DC responses were studied. With one exception, the effect is small. Thermally stimulated polarization (TSP) and depolarization (TSD) curves are presented; the peak appearing on the TSP curves of the samples stored under ambient conditions is interpreted as a result of water desorption. The high temperature DC conductivity and the depolarization current density are higher in the composites and mechanical mixtures than in the matrix. The dielectric properties of the wet filler particles were calculated from the measured composite and matrix data using various mixture formulae. The results can be understood and interpreted if the dielectric properties of adsorbed water are described by the cluster theory of dielectric relaxation.     

Frequency dependence of the dielectric and electro-optic response in suspensions of charged rod-like colloidal particles

We have performed an experimental investigation on the electrokinetic properties of charged rod-like fluorinated latex colloids. Systematic measurements of electrophoretic mobility, dielectric constant and electric birefringence have been performed as a function of the concentration of added nonionic surfactant and salt. In the investigated range of parameters, the zeta potential is a strongly decreasing function of the concentration of nonionic surfactant, while it is basically independent from ionic strength. We have obtained the frequency dependence of dielectric constant and Kerr constant as a function of zeta-potential and ionic strength. We observe the transition from a low frequency behavior, where both the dielectric constant and the Kerr constant are enhanced by the presence of the double layer, to a high frequency behavior, where both quantities take the value expected for unchanged particles in an insulating medium. The shape of the frequency dispersion of the Kerr constant coincides with that of the dielectric constant, but the cut-off frequencies are the same only when the zeta-potential of the particles is low.     

Finite Element Based Micro-mechanical Modeling of Interphase in Filler Reinforced Elastomers

Characteristic properties of elastomers can be tailored by embedding them with filler particles. Along with enhancing the overall properties of the composite, filler particles also induce some inelastic effects. In this paper, a finite element computational model is used to study the effect of microstructure morphology in filled elastomers, on its macroscopic large deformation behavior. A multiphase material model that accounts for the hypothesis of shift in glass transition temperature in the vicinity of the filler particle is developed to simulate the interphase between the fillers and the matrix. It also accounts for the breakdown and re-aggregation of filler networks under cyclic loading. Examples at the microstructural level, demonstrating the dynamics of the interphase using the developed multiphase model have been successfully simulated. The obtained results are in good qualitative agreement with the Mullins effect. Therefore, computational experiments using this methodology enable the prediction of the experimentally observed softening behavior in filled elastomers based on its microstructure evolution.     

Role of the interface in multicomponent materials

Studies of the relationship between interfacial structure and mechanical properties in multicomponent materials are reviewed in this article. The following categories are considered for role of the interface in multicomponent systems: Interpenetrating polymer network(IPN), catalytic effect of silane coupling agent, morphological differences of filler surface, particle-particle interaction and particle size of the filler. The interfacial role in terms of the reinforcement mechanism of the composite and the behavior in the melt state is also discussed in the multicomponent system.     

Dielectric Relaxation Phenomena and Electric Properties of Conductive Composite Polyurethane/Polyaniline Films

Summary: Composite polyurethane/polyaniline (PU/PANI) films have been chemically prepared by oxidative in-situ polymerization of aniline inside the previously swelled PU film. Swelling kinetic studies have shown that for PU films the swelling degree of aniline is 25 wt.%. The dielectric and electrical properties of the composite films were measured using dielectric relaxation spectroscopy and four-probe method. Dielectric measurements as a function of temperature and frequency revealed the presence of a relaxation process for the composite PU/PANI-HCl film. This relaxation was explained in terms of interfacial polarization due to the double-layered structure of the composite film. The activation energy values found by dielectric and electrical measurements are close and this result confirms the conducting character of the PANI containing layer.