Low-frequency Dielectric Spectroscopy Research Articles

Analysis of Cell-Stabilizing Additives in Low-Density Polyethylene Foams Using Low-Frequency Dielectric Spectroscopy

Addition of low amounts of additives is necessary to guarantee dimensional stability of low-density flexible polyethylene foam after extrusion. It is expected that the additives form a layer on the foam cell walls. However, the physical phenomena responsible for the stability improvement are not well understood. Using low-frequency dielectric spectroscopy, we show the occurrence of interfacial polarization in these low-density polyethylene foams. We hypothesize and prove that a stabilized low-density, closed cell polyethylene foam can be regarded as a three-phase system with a polyethylene matrix forming the cell walls, a filler being the gas phase, and an intermediate additive layer. At elevated temperatures the additive layer will become conductive resulting in interfacial polarization, thereby excluding the entire filler volume from the electrical field. This is reflected in an increase of the dielectric constant of the foam from initially 1.0 to about 300 at the melting point of the additive. The frequency-independent position of the peaks suggests that the additive layer is crystalline.

Suppression of the charge-density-wave state in Sr14Cu24O41 by calcium doping.

The charge response in the spin chain and/or ladder compound Sr14-xCaxCu24O41 is characterized by dc resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x=0 to 10 K for x=9, and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the interladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure.

The charge mobility in polymer LEDs-the low frequency impedance method

The negative contribution to the capacitance found in low frequency dielectric spectroscopy provides an elegant way to measure the hole and electron mobilities in polymer devices. We present a closer look into this method. By analyzing the results of hole only and double carrier PPV devices, we argue that the reference capacitance to choose is not the zero-field capacitance but rather the one which corresponds to the fully depleted state.

Modalities of self-organized charge response in low dimensional systems

We present modalities of self-organized charge response in low dimensional systems, like diverse organic and quantum spin systems, studied by the low-frequency (10 mHz – 1 MHz) dielectric spectroscopy. Density wave structures with the order of commensurability N = 4 can be recognized as phasons in a random impurity potential, whereas those with N = 3 can be viewed as topological defects like charge domain wall pairs in the background domain structure.

Characterisation of water behaviour in cellulose ether polymers using low frequency dielectric spectroscopy.

The behaviour of water in hydroxypropylmethylcellulose (HPMC) K100LV, K4M, K15M, K100M, E4M, F4M and HPC polymers was characterised using low frequency dielectric spectroscopy (LFDS). Dielectric responses of 25% (w/w) HPMC K15M gels and deionised water were found to be similar at +22 and 0 degrees C. However, at -30 degrees C, a dielectric response typical of a solid was apparent. The melting of frozen water within gels was detected as increases in the magnitude of the dielectric response with increase in temperature. More than one phase transition was visible in the majority of gels studied which may be related to the presence of different states of water melting at different temperatures. In addition to polymer concentration, both polymer molecular weight and substitution level influenced the nature of the transitions. The magnitude of the dielectric response was increased in all HPMC gel systems in comparison to the response seen in deionised water. Drug addition affected the transitions occurring during the melting of ice in the gels. This may be related to the presence of ionic species in the systems. LFDS studies on cellulose ether gels have provided some interesting evidence for the existence of more than one state of water within such gel systems. The results are in good agreement with thermal analysis findings in similar gel systems.

Low-frequency dielectric spectroscopy of colloidal suspensions

The main difficulties related to low-frequency dielectric measurements of liquid conducting samples using impedance analyzers such as the HP 4192 A or HP 4284 A are presented and discussed. These include the small value of the phase angle and the electrode polarization impedance. We propose ways to overcome these difficulties in terms of cell design, choice of instrument settings, choice of electrode material, connection of the cell to the instrument. We also propose a new calibration method that allows a differential measurement of the dielectric properties with far higher precision than possible using the standard short–open calibration.

Low-Frequency Dielectric Properties of the Rochelle Salt and its Deuterated Analogue

The crystals of Rochelle Salt and its deuterated analogue were studied in the region of high temperature phase transition points by the low-frequency (20 - 2 2 10 4 Hz) dielectric spectroscopy method. For both crystals the influence of conductivity relaxation on the dielectric spectra was observed. The analysis of dielectric spectra was made using the fractal approach. The correlation was found between the temperature dependencies of power parameters of the universal law of dielectric response, and the characteristics of the relaxation processes.

Complex low-frequency dielectric relaxation of the charge-density wave state in the (2.5(OCH 3 ) 2 DCNQI) 2 Li

We report a detailed characterization of an unique 2- and 4-fold commensurate charge density wave (CDW) state in single crystals of the organic conductor (2.5(OCH3)2DCNQI)2Li by means of low-frequency (0.1 Hz-1 MHz) dielectric spectroscopy. We show that in addition to the narrow Debye-like relaxation mode of strength Δɛ≈ 107 with Arrhenius-like resistive decay, associated with long wavelength phason excitations, a new mode of weaker strength Δɛ≈ 106 appears below 40 K and broadens with decrease in temperature. Below 25 K the mean relaxation time of the new mode saturates, whereas the Debye mode continues gradual slowing down with weaker temperature dependence, again similar to the DC conductivity. We attribute the new process to short wavelength CDW excitations that become dominant once the free carrier density is substantially reduced at low temperatures.

Thermal aging phenomena in chitosan-related pharmaceutical systems

Results are presented of detection of thermal aging in the chitosan/ketoprofen acid (KTA) system, using low-frequency dielectric spectroscopy. Aging leads to substantial changes of the low frequency dielectric properties of the system. In order to identify the reasons for the detected aging, near infra-red (NIR) Raman spectra and distribution of molecular weights as well as direct measurements of water content were carried out. The additional measurements suggest that water desorption is mainly responsible for the detected aging phenomena. Formation of a charge transfer complex between the anionic carbonyl group of KTA and the cationic NH3/sup +/ group of chitosan, has also been detected.

Low-frequency dielectric spectroscopy of commensurate density waves

We overview complex conductivity measurements (20Hz - lMHz) in low-dimensional organic systems κ-(BEDT-TTF) 2 -Cu[N(CN) 2 ]Cl, and deuterated Cu-DCNQI and its alloys, with commensurate density wave ground states. We discuss the obtained results in comparison with those for the incommensurate density waves, like in (TMTSF) 2 PF 6 , in which the phason relaxation in a disordered background dominates. We show that in the commensurate systems with a random domain structure charged domain walls provide a more appropriate description of the observed dielectric relaxation.

Low frequency dielectric spectroscopy of the Peierls-Mott insulating state in the deuterated copper-DCNQI systems

We report a detailed characterization of an unique 3-fold commensurate insulating state in single crystals of the organic π – inorganic d hybrid Cu(DMe-DCNQI)2 systems with deuterated and partially deuterated DCNQI ring, by means of low-frequency dielectric spectroscopy. A broad relaxation mode of strength ∆e ≈ 103-104 centred at ν0 ≈ 1-10 kHz is observed in the hysteresis temperature region in which the insulating phase coexists with metallic islands. At lower temperatures, outside the nucleation range, the relaxation narrows, approaching a Debye-like form for an overdamped response of a system with a single degree of freedom. Both, the relaxation strength and the mean relaxation time (T0= 1/2πν0) are much larger than that expected for single-particle excitations. These features suggest the origin of the dielectric relaxation as an intrinsic property of the N = 3 charge density wave state.

Electrical properties of filled silicone rubber

Rubber materials for high-voltage outdoor applications have been studied. They were prepared as mixtures of silicone polymer with different concentrations of powdered aluminium trihydrate as a filler. The dielectric properties, *() = ´-i´´, were measured at different temperatures using a low-frequency dielectric spectroscopy technique. Without any filler a classical interfacial relaxation was pronounced with dominating direct-current (dc) conduction in the losses, ´´. On increasing the concentration of filler, a low-frequency dispersion mechanism started to dominate the relaxation behaviour. On adding the filler, the dc conduction first decreased and then increased again after a certain concentration level was passed. This behaviour showed that the filler particles acted like scattering centres or traps for conduction when the concentration was low. As the concentration increased, the conductivity of the filler particles and of the interface (between the filler particles and the polymer matrix) started to dominate the conduction. Master curve shifts of the data showed an Arrhenius type of activation. The data were modelled by superposition of three different processes, Havriliak-Negami dipolar relaxation, dc conduction and low-frequency dispersion (hopping conduction) contributions, using a nonlinear least-squares fitting method.

Low frequency dielectric spectroscopy of the Peierls-Mott insulating state in the deuterated copper-DCNQI systems

Combined optical and photoemission experiments on the quasi-one dimensional Bechgaard salts reveal the non-Fermi liquid character of these prototype quasi-one dimensional interacting electron systems. We show that various aspects of the exotic normal state properties along the chains are consistent with the predictions of the Tomonaga-Luttinger liquid theory. We also discuss the effect of interchain coupling on the insulator-metal transition, associated with the electron confinement-deconfinement crossover.

The influence of thermal history on the dynamic mechanical and dielectric studies of polyetheretherketone exposed to water and brine

The effects of thermal history on the absorption of distilled water and brine and their influence on the dynamic mechanical and dielectric properties of semi-crystalline polyetheretherketone (PEEK) are reported. Changes in the physical properties of the samples were monitored as a function of time of exposure using gravimetric measurement, dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC) and dielectric spectroscopy. The water and brine sorption behaviour follows a Fickian Case I process with a maximum solubility of 0.5 w/w% at 75°C. The observed sorption behaviour was independent of the salinity of the water to which the polymer was exposed. The water uptake proved to have an effect on the degree of crystallinity. It is observed that the crystallite distribution generated in the formation of the sample has a profound effect on the nature of the water uptake process and also its reversibility. During the initial stages of water uptake smaller crystallites are destroyed and these do not reform when the samples are dried. Low frequency dielectric spectroscopy exhibits a very close correlation with the gravimetric measurements and indicate that this method provides a useful non-destructive evaluation technique for the assessment of water content in filler free thermoplastics.

Magnetic anisotropy and low-frequency dielectric response of weak ferromagnetic phase in κ-(BEDT-TTF)2Cu[N(CN)2]Cl, where BEDT-TTF is Bis(ethylenedithio)tetrathiafulvalene

We report a detailed characterization of the magnetism and AC transport in single crystals of the organic conductor κ-(BEDT-TTF)2Cu[N(CN)2]Cl by means of magnetic anisotropy measurements and low-frequency dielectric spectroscopy. Magnetic anisotropy obeys Curie-Weiss law with negative Curie-Weiss temperature in the temperature range 300 K-70 K. An antiferromagnetic transition with concomitant canted antiferromagnetic state is established at 22 K. A large hysteresis in the spin-flop transition and magnetic field reversal of the weak ferromagnetic magnetization are documented for the first time. A broad dielectric relaxation mode of moderate strength (Δe ≈ 3 × 103) emerges at 32 K, and weakens with temperature. The mean relaxation time, much larger than that expected for single-particle excitations, is thermally activated in a manner similar to the DC conductivity and saturates below 22 K. These features suggest the origin of the broad relaxation as an intrinsic property of the weak ferromagnetic ground state. We propose a charged domain wall in a random ferromagnetic domain structure as the relaxation entity. We argue that the observed features might be well described if Dzyaloshinsky-Moriya interaction is taken into account. A Debye relaxation with similar temperature dependence was also observed and seems to be related to an additional ferromagnetic-like, most probably, field-induced phase. We tentatively associate this phase, whose tiny contribution was sample dependent, with a Cu2+ magnetic subsystem.

Ageing phenomena of chitosan and chitosan–diclofenac sodium system detected by low-frequency dielectric spectroscopy

The use of natural polymers for design of dosage form has received considerable attention recently, especially from the safety point of view. Among these polymers, chitosan shows very interesting biological, chemical and physical properties which makes it possible to use chitosan for various pharmaceutical applications. Microcrystalline chitosan (MCCh) is a special multifunctional polymeric material existing in the form of either of gelatinous water dispersion or a powder.Thermal ageing of chitosan and chitosan–diclofenac sodium mixture have been studied using low-frequency dielectric measurements. The ageing was carried out by annealing in ambient atmosphere in the temperature range between 25°C and 100°C. The dielectric losses in the aged samples proved to decrease by about one order of magnitude. The additional measurements of molecular weight distribution and infrared absorption were also carried out for better understanding of nature of the ageing phenomena. Partial evacuation of water, cross-linking and improvement of structural order may be suggested to be a result of thermal ageing of the investigated materials.

An investigation into the distribution of lecithins in nanosuspension systems using low frequency dielectric spectroscopy

A range of nanosuspensions comprising a model drug (RMKP 22) with varying concentrations of Phospholipon 90 were prepared using high pressure homogenization and analyzed using low frequency dielectric spectroscopy as a novel means of characterizing the distribution of Phospholipon 90 within the suspensions. A corresponding range of aqueous Phospholipon 90 suspensions were also studied for the purpose of comparison. The dielectric responses were interpreted using a modification of the Maxwell–Wagner approach, whereby the systems were considered to comprise a high frequency response corresponding to the bulk layer serially connected to a lower frequency response corresponding to an electrode barrier layer. The low frequency responses of both the Phospholipon 90 dispersions and the nanosuspensions were found to be essentially independent of phospholipid concentration, indicating the presence of a barrier layer covering the electrode surfaces. In contrast, the high frequency (bulk) loss response was found to increase with Phospholipon 90 concentration for the surfactant suspensions while a maximum in response was seen with Phospholipon 90 concentration for the nanosuspensions; this behaviour was attributed to the presence of impurities within the phospholipids. Based on this investigation, a model is proposed with which the dielectric response may be related to the surface coverage of the suspended drug particles.

Characterization of the effects of drug addition on the structure of glyceryl monoolein/water gel systems using low frequency dielectric spectroscopy

The influence of propantheline bromide incorporation on the phase structure of glyceryl monoolein/water systems has been investigated using low-frequency dielectric spectroscopy over a frequency range of 10−2 to 106 Hz at 20 °C. The responses of glyceryl monoolein systems composed of 10% and 30% w/w were measured and the spectra modeled using an equivalent circuit based on the Maxwell–Wagner theory.1,2 Marked changes in the dielectric responses of the systems were noted on addition of the propantheline bromide at concentrations up to 10% w/w. For the lamellar (10% w/w water) glyceryl monoolein systems, an increase in the imaginary permittivity was seen, corresponding to an increase in conductivity due to the presence of additional ionic species within the system. Evidence was also obtained for the incorporation of the drug directly into the lipid bilayers, particularly at higher concentrations (10% drug) at which dielectric behavior corresponding to bilayer disruption was seen. Incorporation of 3% and 5% w/w drug into the cubic phase systems (30% w/w water) resulted in a change to the lamellar phase. However, circuit modeling indicated that the system formed structures which showed features of both the lamellar and cubic phases at 3% w/w drug loadings. The study has therefore demonstrated that dielectric analysis may provide a novel means of studying the effects of drug incorporation on the phase behavior of complex gel systems.

Identification of thermotropic phase transitions of glyceryl monoolein–water systems by low frequency dielectric spectroscopy

Thermotropic phase transitions of glyceryl monoolein–water systems have been investigated using dielectric spectroscopy over a frequency range of 10 −2–10 6 Hz and a temperature range of 15–100°C, and also using differential interference contrast (DIC) microscopy fitted with a hot stage as a support technique. The frequency and temperature dependencies of the dielectric permittivity of systems undergoing transitions between lamellar, cubic, reverse hexagonal and L 2 phases are reported. It was demonstrated that the frequency dependent response is highly sensitive to changes in phase and a model is described which allows the dielectric behaviour to be interpreted in terms of an equivalent circuit diagram, thereby allowing the response to be related to specific structural features within the sample. Similarly, it was demonstrated that the phase transition temperatures and ranges could be identified using the dielectric technique, yielding results which were in good agreement with those obtained using DIC microscopy for those systems whereby the phase changes are optically identifiable. The study therefore suggests that low frequency dielectric analysis is a potentially highly useful tool for the identification of thermotropic phase changes.

Generation of charge carriers and formation of antisymmetric double electric layers in glycerine

The effect of increasing (by more than one order of magnitude) of the conductivity of glycerine under the action of a low-frequency electric field (10−4–10−3 Hz) and heating is discovered and investigated. The transition into the high-conductivity state is accompanied by formation of asymmetric double electric layers at the electrodes. Parameters of the electric barrier at the interface are determined. For example, for the sample with thickness 18 μm at the temperature 325 K, the height of the potential barrier is 0.4 V, the thickness is 2.1 nm, the concentration of ionized centers is 3×1026 m−3. The transition into the more conductive state can be induced by desorption of ions from the electrode surface. The properties of glycerine are investigated for the both low and high-conductive states by low frequency dielectric spectroscopy. The dispersion of components ε′ and ε″ of the complex dielectric permittivity for the both states is described by the Debye equation modified by Cole–Cole. The dielectric relaxation time τ and the thickness of the double electric layer WC–C, where the redistribution of charges takes place, are measured. The transition into the high-conductivity state is characterized by the decreasing (more than one order) of τ, its activation energy and WC–C. Comparison of the obtained data with the data for the liquid crystal mixture 1282 suggests that the dispersion of ε′ and ε″ is induced by redistribution of charges between the bulk and the interface.