Dielectric relaxation spectroscopy and electro-optical studies of phase behaviour of a chiral smectic liquid crystal

Dielectric relaxation processes in various phases of a chiral smectic liquid crystal compound have been investigated over a wide range of frequencies from 10 to 300 kHz. Dielectric spectroscopy reveals the existence of two relaxation frequencies corresponding to the ferroelectric and ferrielectric Goldstone modes in the range of temperature where two different phases coexist. This phenomenon of coexistence is examined with respect to the cell thickness and reversal temperature.     

Dielectric properties of four room temperature ferroelectric and antiferroelectric multi-component liquid crystalline mixtures

Dielectric properties of four recently formulated room temperature multi-component liquid crystalline mixtures with paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmC*_A) phases have been studied as a function of temperature and frequency. Under planer anchoring condition, dielectric spectroscopy revealed all the characteristic modes: low frequency P_L and high frequency P_H mode in SmC*_A phase, Goldstone mode (GM) in SmC* phase and soft mode (SM) in SmA* phase. Dielectric behaviour has also been studied under the application of DC bias electric field. With bias electric field, we have been able to study the soft mode dielectric behaviour in the SmC* phase. An unknown high frequency mode (X-mode) with and without bias is also observed in SmC* phase. Dielectric results are explained in the light of generalised Landau theory. The mixtures show very high soft mode electroclinic coefficient in the SmA* phase in addition to fast switching in SmC*_A and SmC* phases [30].     

Dielectric properties of polytetrafluoroethylene films modified by direct current discharge

Dielectric characteristics of polytetrafluoroethylene (PTFE) films modified by direct current discharge at the anode and cathode have been studied as a function of frequency and temperature. It has been found that the dielectric constant increases slightly as a result of plasma treatment and weakly depends on temperature. It has been shown that the frequency dependences of conductivity at 20°C differ somewhat for the original and treated films, but the closer resemble one another at 80°C, with the difference in the critical conductivity indices becoming more significant.     

Enhancement of the stability and temperature range of the mesophases in the binary mixtures of cholesteryl myristate and 4-n-decyloxy benzoic acid

Thermodynamical, optical, and electrical properties of the binary mixtures of cholesteryl myristate (ChM) and 4-n-decyloxybenzoic acid (DOBA) have been carried out by the differential scanning calorimeter, polarized optical microscopy (POM), and impedance spectroscopy. Through thermodynamic study, various phase transition temperatures, transition enthalpies, and transition entropies have been determined to investigate temperature range and stability of the mesophases of the mixtures. Phase diagrams in the heating and cooling cycles have been drawn for the ChM–DOBA binary system. Optical textures of different mesophases have identified with the help of POM. Dielectric permittivity has been determined in planar and homeotropic alignments of the mixtures having DOBA concentrations 30.0 and 92.3 mol %. The experimental dielectric data in the frequency range of 1 Hz to 35 MHz do not show any relaxation mode in both the alignments of these mixtures. Dielectric anisotropy has been determined for the various observed phases of the mixtures.     

Natural weathering of low density polyethylene: Part 2—Dielectric properties

Dielectric properties of naturally weathered low density polyethylenes have been measured as a function of temperature, frequency and weathering time. The results show that dielectric loss, dielectric constant and the β transition increase with the weathering time. However, the deterioration of the dielectric properties is strongly reduced by the presence of a uv stabilizer system.The changes in the dielectric properties can be qualitatively explained by the formation of polar groups and crosslinks.     

Influence of low concentration silver nanoparticles on the electrical and electro-optical parameters of nematic liquid crystals

Thermodynamical, optical, dielectric and electro-optical characterisation of nematic liquid crystals (LCs) and silver nanoparticle (NP) composites have been carried out. Transition temperatures of pure and composites systems have been measured. Thermodynamical studies suggest increase of clearing temperature of the composite material as compared to the pure material. Threshold voltage for switching from bright to dark state and splay elastic constant of the pure and composite materials have been determined. From frequency dependence of dielectric measurements, permittivity, loss, relaxation frequency and dielectric strength of flip-flop mechanism of LC molecules in the nematic phase have been calculated. Dielectric properties of composites have been explained in reference of Maier and Meier theory. The effects of doping of NPs on dielectric and electro-optic properties of LC-NP composites have been discussed.     

A terahertz spectroscopic study of chitosan-based bionanocomposites containing clay nanoparticles

Dielectric properties of bionanocomposites resulting from regulated self-organization of chitosan and nanoparticles of synthetic saponite clay have been investigated by terahertz pulsed spectroscopy. Spectral characteristics of the composites considered in correlation with their structural features, which have been characterized by atomic force microscopy and scanning electron microscopy, depend on the concentration ratio of the components. The study of the effect of temperature on terahertz absorption spectra has led to the conclusion that hydrogen bonding may be involved in the formation of the bionanocomposites.     

Sn-containing liquid crystalline side group homopolymers with two glass transitions

Three homologous tin-containing homopolymers with a terminal CN-dipole in the side group have been synthesized and characterized by dynamical calorimetry, polarization microscopy, X-ray and dielectric methods. AFM was used to evaluate the texture at room temperature. Four different phase transitions were detected by DSC. The high temperature phases were identified by polarization microscopy as SmA and SmC. AFM-measurements show focal-conic domains at room temperature and confirm so the smectic nature of all phases. X-ray measurements on nonoriented samples give hints to a phase segregation on nanometer scale. Dielectric investigation and temperature-modulated DSC (TMDSC) confirm clearly phase separation by appearance of two glass transitions related to the liquid order of the main chains and the liquid crystalline of the side groups.     

A study of thermal and dielectric behavior of manganese malonate dihydrate single crystals

Thermal decomposition of manganese malonate dihydrate single crystals grown by gel method has been studied using the TG-DTA and DSC techniques. The presence of water molecules and the dehydration stages are discussed. Dielectric constant, dielectric loss, and AC conductivity have been estimated as a function of temperature in the range of 40–120 °C for four different frequencies. Thermal studies reveal that the material is thermally stable up to123 °C. The dielectric measurements indicate that the dielectric parameters increase with the increase in temperature. Also, the dielectric constant and dielectric loss factor values decrease whereas the electrical conductivities increase with the increase in frequency of the AC applied.     

Sub-Tg molecular motions in glassy poly(vinyl chloride). Influence of the nucleophilic substitution

Dielectric local molecular motions in the glass transition region of poly(vinyl chloride) (PVC) have been investigated by means of the isothermal depolarization currents technique. Attention is paid to the influence of the modification of the polymer by the substitution of chlorine atoms of PVC chain by the benzenethiolate group. The results are well described by stretched exponential laws. According to the Ngai's coupling model, the coupling between moving units as well as the activation energy responsible for the motions occurring at primitive level have been calculated for each sample at each temperature. All these parameters are noticeably altered by the modification of PVC. © 1994 John Wiley & Sons, Inc.     

Dielectric relaxation of a nematic eutectic mixture with a high glass temperature

Dielectric relaxation measurements in the frequency range from 2 Hz to 100 kHz have been carried out on an oriented binary nematic mixture of low molecular liquid crystals with a glass temperature of 307 K. For both relaxation frequencies, f_R‖ and f_R⊥, a glass behaviour according to the Vogel-Fulcher law could be detected. The Cole-Cole distribution parameter for reorientation around the long molecular axis increases strongly with decreasing temperature. This effect is discussed with regard to the optical biaxiality of the mixture.     

Thermally stimulated dielectric properties of polyvinylidenefluoride–zinc oxide nanocomposites

Thirty-micrometer thick polyvinylidenefluoride (PVDF)–zinc oxide (ZnO) nanocomposite samples in the mass ratio of ZnO (1–6% (w/w)) have been prepared by solution mixing method. The nano- and microstructures of PVDF–ZnO nanocomposite of different mass ratios were characterized by using high-resolution techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM). The SEM and AFM images show the presence of different components such as nanoparticles, amorphous and crystalline phases in nanocomposite samples. Dielectric properties of polymer nanocomposite based on PVDF and ZnO of different mass/% compositions have been studied to understand the molecular motion at different frequencies in the temperature range from 300 to 500 K. The permittivity of the nanocomposites decreases with frequency, while increases with the increasing temperature and ZnO content. The loss peak that disappeared at higher frequency is the remarkable result of this study.     

Dielectric studies under high pressure on strongly polar liquid crystals exhibiting monolayer smectic A phase

Dielectric studies have been performed at elevated pressures on two compounds of a homologous series possessing a strongly polar terminal group and three phenyl rings in their molecular structure. This is the first high pressure dielectric study on such systems that exhibit a monolayer smectic A phase. Also notable is the fact that these compounds show a dual frequency response, a promising feature for fast liquid crystal display devices. The low frequency relaxation recorded in the nematic and smectic A phases is attributed to the reorientation of the molecules about the short axis. The temperature and pressure dependence of the relaxation frequency of this mode as well as the involved activation parameters are discussed. At a given relative temperature the relaxation frequency decreases as the pressure is increased. However, the parameter that characterizes the temperature dependences of the relaxation frequency, viz., the activation enthalpy, has a very weak dependence on pressure. In contrast, the activation volume shows a linear decrease with temperature.     

Dielectric studies of different phases of a thin-layered ferroelectric liquid crystal with a small pitch and high spontaneous polarization

Dielectric spectroscopy of a ferroelectric liquid crystal with planar texture in the frequency range 100 Hz to 1 MHz has been carried out as a function of temperature from 25°C to 60°C at different DC fields varying from 0 to 12 kVcm^-1. From the measured dielectric strengths and relaxation frequencies, it is observed that the material has a S_c*-S_A* transition at 56·1°C and a S_A*-I transition at 58°C. The data of the S_c* phase have been assigned to the Goldstone mode. The rotational viscosity and elastic constant have been calculated from the observed data. The capacitance shows an abrupt drop at the critical DC field where unwinding of the helix occurs. The measured value of the critical field decreases with temperature and agrees to literature data from electrooptical experiments. On applying a DC field greater than 6·5 kV cm^-1 at room temperature, the liquid crystal shows the unwound S_c* and S_A* phases.     

Dielectric study of hot-pressed nanocomposite polymer electrolytes

Ion-conducting nanocomposite polymer electrolyte films based on poly(ethylene oxide)-NaPO₃ 3: 1 with up to 15 wt % of SiO₂ have been prepared using recently developed hot-press technique instead of conventional solution cast method. With 7 wt % of SiO₂, the film conductivity has been enhanced by an order of magnitude. The materials have been characterized by Fourier transform infrared spectrometry and thermogravimetric analysis. For the composition with the highest conductivity, the temperature dependences of ionic mobility, mobile ions concentration, ionic transference number, and ionic drift velocity have been determined. Dielectric constant and dielectric loss have been measured. The conductivity enhancement has been discussed on the basis of existing theories of dielectrics.     

Dielectric spectroscopy analysis in employing liquid crystal phthalonitrile derivative in nematic liquid crystals

Dielectric anizotropy and relaxation properties of 2,3-dicyano-1,4-di[3,4,5-tri(dodecyloxy)phenylcarbonyloxy] benzene (DCDPB)-doped E7 and E7 liquid crystal have been investigated by the dielectric spectroscopy method. Dielectric anisotropy property of the LCs changes from the positive type to negative type. Dielectric relaxation properties suggest that LCs exhibit a monodispersive dielectric property. The relaxation frequency of E7 and E7/DCDPB liquid crystals was calculated by means of Cole-Cole plots. Consequently, DCDPB dopant changes the dielectric anizotropy and relaxation parameters of E7 LC.     

Dielectric relaxation of acetonitrile and propionitrile in benzene and carbon tetrachloride solutions

Dielectric permittivities and losses have been measured at wavelengths of 3 × 10⁴, 10.0, 3.23, 1.95, 1.25, and 0.40 cm at three temperatures 24, 40, and 60°C for benzene and carbon tetrachloride solutions of acetonitrile and propionitrile at a fixed concentration of 25 mole per cent. The far-infrared spectra in the range 10–150 cm⁻¹ of dilute solutions of both the nitriles have been investigated. A single relaxation time has been obtained for acetonitrile at each temperature whereas the dielectric data of propionitrile indicate the presence of multiple dispersion regions at microwave frequencies.     

Li+3 ion beam irradiation induced changes in the thermodynamic and electrical parameters of 4-n-(nonyloxy) benzoic acid

Li⁺³ ion beam irradiation studies on 4-n-(nonyloxy) benzoic acid (NOBA) have been carried out. Thermodynamic measurements demonstrate that all the phase transition temperatures are depressed for low value of irradiation fluence as compared to those for the pure NOBA and thereafter increase linearly with increase in irradiation fluence. It has also been observed that the temperature range of N and SmC phases in the heating and cooling cycles is improved due to irradiation. Dielectric investigations show that both the transverse as well as the longitudinal components of the dielectric permittivity are increased as compared with those of the pure sample due to irradiation. The observed change in the thermodynamic and dielectric parameters is attributed to the increased concentration of NOBA monomers over the NOBA dimers due to fragmentation of the hydrogen bonding in some of the NOBA dimers.     

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.     

Antiferroelectric phase in liquid crystalline compounds containing an azo group in their molecular core

Novel lactic acid derivatives containing an azo group in their molecular core have been synthesised and their mesomorphic properties established. Compounds with a short aliphatic non-chiral chain exhibited the SmA* phase and ferroelectric (FE) SmC* phase. The monotropic antiferroelectric (AF) phase was found in the homologues with a longer terminal chain. Spontaneous polarisation and tilt angle were measured for the FE and AF phases. Dielectric spectroscopy in the frequency range 1 Hz–10 MHz was carried out and dielectric properties established within the temperature range of the smectic phases. Two high frequency modes were detected in the AF phase. Small-angle X-ray measurements were conducted, and the temperature dependence of layer spacing values is established and discussed.