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.     

Dielectric properties of moist CaCO3 filled polyethylene composites

The capacitance and the dielectric loss tangents of CaCO₃ filled polyethylene composites were studied. Composite samples, prepared by polymerizing ethylene on the surfaces of fillers, pre-treated by polymerization catalysts, were compared to normal mechanical mixtures. Dielectric dispersion,, and loss, , proved to be sensitive to heating or vacuum treatment. Investigation of samples under conditions of different relative humidities showed that the dielectric dispersion is due to adsorbed water. Both and increased with decreasing frequency and the ratio of loss and dispersion was nearly constant. Dielectric data measured at different relative humidities could be represented by a single Cole-Cole plot. Samples soaked in water for different periods yielded qualitatively similar but quantitatively different Cole-Cole plots. Composite samples showed higher losses at similar humidities.Possible interpretations in terms of a molecular relaxation model, an interfacial relaxation model, including a charged double layer mechanism, percolation theory and the universal response theory were examined, but none was able to fully explain the observed phenomena.     

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.     

Analysis of dielectric observations of KCl-charged poly(methyl methacrylate) microcapsules using a two-component model consisting of KCl-permeable and KCl-impermeable capsules

Dielectric measurements were carried out on aqueous suspensions of poly(methyl methacrylate) microcapsules charged with KCl solutions to examine the KCl permeability of the microcapsules. The specimens exhibited three kinds of dielectric relaxation. Two kinds of dielectric relaxation were observed immediately after washing the specimens with distilled water. These overlapped each other when the specimen was left standing in the measuring cell for a long time. Their relaxation frequencies were affected by the KCl concentration of the suspending medium in which the specimen was kept before washing. Another relaxation was observed distinctly when the spepcimens were washed thoroughly with distilled water. Its relaxation frequency was affected by the KCl concentration of the charged solution. These results were explained by assuming that the specimens were mixtures of KCl-permeable and KCl-impermeable capsules. The theoretical analysis procedure was devised in the light of dielectric theory of interfacial polarization to estimate the parameters which characterize the structure of the specimens.     

Influence of crosslinking on mechanical and dielectric properties of nitrile-butadiene-rubber

Several copolymers of acrylonitrile with butadiene (different acrylonitrile content, different molecular weight), uncrosslinked and crosslinked by tetramethylthiuramdisulfide or radiation, are investigated by dynamic mechanical measurements (10^–4 Hz to 100 Hz). The viscoelastic behaviour at very low frequencies is strongly influenced by molecular weight and crosslinking whereas the main relaxation (glass process) remains nearly the same. Stress strain curves (Mullins effect) and some dielectric measurements are also reported.     

Recent development in dielectric relaxation spectroscopy of polymers

An evaluation strategy for dielectric measurements in the frequency and/or time domain is presented which provides complete information about a relaxation mechanism (intensity, position, and shape of the relaxation function) independent of overlapping with neighboring mechanisms. As an example results on poly-(ethyleneterephthalate) are given.     

Study of the polymer latex coalescence by dielectric measurements at microwave frequency—feasibility of the method

The measurement of the complex dielectric constant ( ^*=–j) in the dipolar absorption domain of the free water molecule (microwave region) permits us to follow quasi-specifically and precisely the water circulation during the coalescence process of a latex. Weight losses and dielectric constants variations have been simultaneously recorded upon latex drying, in a resonant cavity at 5 and 9 GHz and under controlled atmosphere and temperature. Two different latexes were studied (polybutylacrylate (PBuA) and polystyrene (PS)) from which the glass transition temperature effect was investigated. It is found that the harder the polymer particles, the more clearly evidenced the flocculation and packing points are. This method appears to be quite reliable for discriminating the various steps in the film formation process of latexes.     

Broadband dielectric spectroscopy on a discotic liquid crystalline polymer

Broadband dielectric spectroscopy is employed for the first time to study the molecular dynamics in discotic liquid crystalline polymers. One dielectric relaxation process is found which is strongly broadened and asymmetric. It is assigned a local hindered rotation of the ester groups attaching the spacer to the discotic mesogen. This assignment is supported by NMR measurements on the identical substance.Dedicated to Prof. E. W. Fischer on the occasion of his 60th birthday.     

Dielectric relaxation in polymeric solids Part 1. A new model for the interpretation of the shape of the dielectric relaxation function

A model is proposed which explains the shape of the dielectric relaxation function at the glass transition of polymers. The model is based on the idea that the molecular mobility at the glass transition is controlled by intra- and intermolecular interaction. In addition, a specific model for the local chain dynamics in amorphous polymer systems is used. According to the scaling hypothesis of molecular dynamics the model relates the high frequency dependence of the dielectric loss curve to the local chain dynamics and the low frequency dependence to the intermolecular correlation.     

On the observation of micellar structures by dielectric spectroscopy

Many cationic surfactants form, in the presence of certain organic acids, large supramolecular micellar structures in water. The dielectric response of one such system (cetyl trimethyl ammonium bromide-salicylic acid, CTMAS) has been studied as a function of frequency, concentration and temperature. The results are compared with dynamic mechanical data on the same system, which has been published in the literature.The dielectric response shows that the micelles form a rigid, open network structure, which does not impede ionic transport in the bulk liquid. However, the response also shows the presence of a frequency dispersive barrier capacitance. From the variation of the properties of this capacitance with CTMAS concentration and applied voltage over a range of frequencies, it is deduced that the barrier originates in an ordered micelle structure at each electrode.     

Theoretical study of dielectric behavior of microcapsule suspensions with distributed drug release rate

Theoretical consideration is made concerning the dielectric behavior of aqueous suspensions of W/O-type microcapsules containing aqueous electrolyte solutions, especially concerning distribution effects of the electrolyte release rate on the dielectric behavior.It is shown at first that the distribution of the release rate causes electrical conductivity of the capsule interior. Next, it is deduced, in the light of a theory of the interfacial polarization, that the distribution characteristics of the capsule interior's electrical conductivity are closely related to those of the relaxation time for a dielectric relaxation exhibited by the suspensions. Several formulas are derived to describe the relation between the distribution characteristics of the release rate and those of the relaxation time.     

Network properties of mixtures of protonated and deuterated polyethylene

Mixtures of protonated and deuterated polyethylene were irradiated in the melt. The degree of crystallinity, the degree of crosslinking, as well as the enthalpyH and the melting pointT _M were determined. No significant differences in the degree of crosslinking between protonated and deuterated chains were found. The mass specific entropyS of the uncrosslinked samples remained constant and independent of the deuterium concentration. For the crosslinked samples, a netpoint entropy was postulated. A weaker Van der Waals interaction could explain the decrease in melting temperature by deuteration (for weakly crosslinked samples).     

The exact shear modulus of particulates based on the concept of mesophase

An accurate relationship for the shear modulus of particulates is derived based on the Kerner model, but not using its approximate relations. Furthermore, the model takes into account the existence of the mesophase layer between the inclusions and the matrix, which acts as a smooth transition boundary layer between constituent materials. By applying Christensen's field to the Kerner model, modified by introducing the mesophase, the new model is liberated from any inconsistencies. Experimental evidence and application to a glass particle-epoxy resin-matrix composite indicated the superiority of the model over previous ones.     

Deformation and fracture behavior of polystyrene ionomer and ionomer blends

The deformation and fracture behavior of sulphonated polystyrene ionomers, and of blends of these with polystyrene have been investigated. The microstructure of the ionomer, which varies with ion content, appears to have a significant effect on mechanical properties. Both tensile strength and toughness increase appreciably at ion contents near 5 mol%, where clusters become dominant over ion pairs and multiplets. In blends of the ionomers and polystyrene, phase separation occurs and the ionomer component appears in the form of fine particles dispersed in the polystyrene matrix. These particles possess a greater effective entanglement density than the matrix, as a result of ionic crosslinking, and they provide reinforcement against early craze breakdown and fracture. Tensile strength and fracture energy increase rapidly as the ionomer concentration in the blend is increased and they become essentially independent of blend ratio above about 10 wt% of the ionomer. Tests carried out on thin film specimens of the blends show that the dispersed ionomer particles adhere well to the matrix and contribute to the fracture energy both by inducing matrix crazing and by internal fibrillation within the particles.Dedicated to Professor Hans-Henning Kausch on the occasion of his 60th birthday.     

Time domain dielectric spectroscopy. A new effective tool for physical chemistry investigation

The general principles of time domain dielectric spectroscopy (TDS) are summarized. The methods of data treatment and presentation, and different TDS methods which enable one to obtain the permanent spectrum of ^* () in the frequency range of 10⁵–10¹⁰ are given. The examples of TDS application for the investigation of dielectric properties in samples of different nature and structure are considered in this review.     

Dielectric relaxation of rod-like labels in polyisoprene

Rod-like molecules with a strong dipole in the long axis were mixed with synthetic polyisoprene (IR305) in low concentrations (0.1–5 wt%) and measured dielectrically in the frequency range 10^–2–10⁶ Hz and the temperature range –60°–0°C (glass relaxation range). The measurements showed an additional symmetrical relaxation process slower than the glass process by a factor ranging from 10 to 100 times, depending on the length and structure of the label. The label relaxation process was characterized by a narrow distribution of relaxation times and its strong coupling with the segmental motion. For the labels with only a longitudinal dipole moment, no relevant part of the label relaxation was observed in the relaxation frequencies of the glass process; this may be attributed to the parallelism and compactness of the polyisoprene chains. In contrast, the labels with an additional vertical dipole component showed an additional relaxation process superimposed on the glass relaxation with also almost the same distribution of relaxation times. The slow label relaxation could be attributed to a diffusion or rotational diffusion relaxation process through a multistep relaxation mechanism determined by the segmental motion of the matrix.     

A DSC study of PVAC-PMMA two-stage emulsion polymers

Core-shell polymer colloids were produced by a two-stage emulsion polymerization technique. The polymers thus formed were investigated by means of differential scanning calorimetry (DSC). The DSC thermograms show two separate glass transition regions of the core and the shell component with positions unchanged regardless of chemical composition. Increases in specific heat capacity at the glass transition temperature of both components are lower than theoretical values calculated from the net chemical composition. From these results, it is possible to estimate i) the chemical composition of the interfacial layer and ii) the interfacial layer thickness. This interfacial layer thickness is in the order of 2–7 nm.Part 7 of Polymerizations in the Presence of Seeds     

Counterion binding on mixed anionic/cationic micelles

Measurements of counterion binding in mixtures of surfactant aqueous solutions have been performed to study the structure of the anionic/cationic mixed micelle/solution interface. The mixtures studied were SDS/DDAC and STS/TDPC. The binding of chloride and sodium ions to mixed anionic/cationic micelles was measured using ion-specific electrodes. Counterion binding was found to be strongly dependent on the molar ratio of surfactants present. The mixed micelle/solution interface includes the headgroups of both surfactants and counterions of surfactant in excess. The addition of oppositely charged surfactant caused an increasing dissociation of counterions.     

Comment on the “dielectric properties of LDPE/Ny6 blends”

Data published by La Mantia et al. [1] on dielectric dispersion and loss in polyethylene/nylon 6 blends are analyzed in terms of dielectric mixture formulae. It is shown that an ohmic interfacial polarization process can not be responsible for the unexpected increase of and values observed in these blends at high temperatures. The observed phenomena are tentatively attributed to space charge processes at the electrodes or to other defects dipole mechanisms.     

Complex polarizability as used to analyze dielectric relaxation measurements

The effect of representing dielectric properties in terms of the complex polarizability ^c = – i is examined. Loss curves ( and tan ) are shifted towards higher frequencies, revealing the existence of new relaxations and allowing the clarifications of ones already known. We have calculated the shift ratios (at maximum or tan )/ (at maximum or tan ) from the more conventional empirical equations representing the dielectric behavior. Some examples are given.