Dielectric investigations on a binary system showing a “filled” SA Phase

A binary system of a double swallow-tailed compound and a substance with a strong polar terminal group shows a pronounced induced S_A phase. Investigations of the static dielectric anisotropy indicated a deviation from the simple mixing rule which was empirically introduced. We derived a better approximation taking into account the different molecular volumes of the substances. An unexpected increasing mobility of the strong polar molecules could be detected by relaxation measurements. This may be connected with a change in the short range order of the strong polar compound which is caused by the repulsive forces of the molecules.     

Dielectric Relaxation in Liquid Crystals

We present recent progress in the study of the liquid crystals by dielectric relaxation. First, we discuss the possible mechanism controlling the rotation of the molecules around a short axis: rotational diffusion or free volume effect. In the case of phase transition between Sm A phases, dielectric relaxation can bring information about the short range order and also the long range order when a dipole ordered structure is established.     

Dielectric relaxation in nematic,smectic A and C phases in the MHz region

The dielectric properties of four 4-n-alkyloxyphenyl 4-n-alkyloxybenzoates with nematic, smectic C and partly additional S_A phases were investigated. In 2 substances for ϵ′ reproducible values also in the smectic phases resulted allowing to conclude on a good orientation in the samples. In all 4 substances the dielectric relaxation in the MHz region was measured and the respective activation energies were determined. The molecular mobility in the S_C phases is higher than in the S_A phases, however comparable with that of the nematic phases. In all 3 phase types a single relaxation mechanism exists, which is explained as rotation of the molecules around a short axis.     

Importance of Steric Effects in the Isotropic Phase of Nematogenic Compounds

The dielectric spectra of some mesomorphic substances in their isotropic phase are reported. The measurements are carried out from the radio frequencies up to the far infrared region. We have investigated several substances such as azoxybenzenes, cyanophenylcyclohexanes, azomethines, tolanes and biphenyls. Two special cases appear with the last two series where the dipole moment is directed either perpendicular or parallel to the long molecular axis respectively. The dielectric spectra show different absorption connected with rotational movements around or perpendicular to the long axis. The influence of the molecular surrounding on the dielectric properties is also studied by investigating the influence of dilution. The results have been discussed by using a computer simulation of molecular dynamics of fluid systems containing ellipsoidal particles which interact pair-wisely, assuming the interactions to be governed mainly by steric effects.     

Change of the Prenematic Order in the Isotropic Phase Close to the Isotropic–Nematic Phase Transition with Increasing Aerosil Nanoparticles Density

The influence of the hydrophilic and hydrophobic aerosil nanoparticles density on the formation of the prenematic order close to the isotropic – nematic (I–N) phase transition is presented. The lowering of the activation energy, with the increase of aerosil nanoparticles density, suggests a change of the prenematic order in the isotropic phase. The temperature dependence of the static dielectric permittivity for 4–n–pentyl–4′–cyanobiphenyl (5CB) exhibits an anomaly near the I–N phase transition. With the increase of aerosil density, a substantial modification in the anomaly of the static permittivity was observed in the isotropic phase. Indicated changes are different for hydrophilic and hydrophobic aerosil as a result of different surface interaction between nematic molecules and aerosil nanoparticles.     

Dielectric relaxation investigations of the isotropic and nematic phases of 4-n-Butyloxyphenyl-4-n-hexylbenzoate (BOHB)

Experimental values of the complex dielectric permittivity in a wide frequency range are analized for the isotropic and nematic phase of BOHB. The results are discussed from the point of view of different of molecular reorientation mechanisms which are related to the structure of the molecule. It can be concluded that the molecules in the nematic phase exhibit a stronger tendency to reorient as “rigid bodies”.     

Anisotropy of Electric Properties in Binary Cholesteric Mixtures

Dielectric permittivity and electric conductivity anisotropies were determined for cholesterol laurate, cholesterol caprilate as well as for their mixtures 25:75%, 50:50% and 75:25% (by weight). Connection between anisotropies of pure substances and their mixtures were found. The mixtures 75:25% and 25:75% have positive conductivities anisotropies within the cholesteric range as for the initial esters; the sign of the dielectric anisotropy is the same as for the prevailing ester. Within the smectic range the dielectric anisotropy is negative for both mixtures and pure compounds, in accordance to Carr's hypothesis with reference to the conduction mechanism in smectics.

The phase transitions are revealed in most cases by maxima or minima of the dielectric anisotropies.     

The influence of the boundary conditions on the character of the phase transition nematic-smectic C

The possibility is investigated for inducing the intermediate smectic A structure in substances with nematic and smectic C phases by the boundary forces. Different geometries are used: homeotropic, homogeneous and tilted under a different angle toward the supporting substrates of the liquid crystal cell. At homeotropic and at tilted orientation where the molecules tilt toward the supporting substrates is different from the molecule tilt in the smectic layer, an intermediate smectic A structure is observed at the transition nematicsmectic C in a narrow temperature interval. In this case the phase transition nematicsmectic C extends and transforms into phase transition of second order. At homogeneous orientation and at tilted orientation of an angle equal to the molecule tilt in the smectic layer, the transition nematic-smetic C carried out in two stages, as partially it begins on the surface still in the nematic phase and terminates at the temperature of the phase transition N-Sc. The influence of the boundary forces depends essentially on the energy of the phase transition N-S_C of the substances.     

The development of a nematic two frequency addressing mixture

An example is given for the development of a nematic two frequency addressing mixture based on the equations for the threshold voltages, the switching times, and the dielectric relaxation. New substances have been synthesized with relaxation frequencies in the kHz range and big values of the static dielectric anisotropy at room temperature. The isotropy frequency and the relation between the high and low frequency dielectric anisotropy, respectively, are adjustable by adding of substances without a relaxation mechanism in the kHz range and with a big negative static dielectric anisotropy. The produced mixture was dielectrically investigated. Calculated and experimentally found data are in a good agreement.     

Dielectric investigations of a binary system with strong correlation of the molecules

Dielectric measurements on oriented liquid crystalline substances in the frequency range from 10 Hz to 100 kHz have been carried out. The static dielectric constant parallel to the director of both single components shows a strong decrease in relation to the expected value. In the case of the swallow-tailed compound this effect can be described by assuming an equilibrium between statistically and antiparallel oriented molecules. The second component has an extremly strong tendency to be antiparallel oriented. In the mixture of both a continuous change of the correlation was observed.     

Dielectric properties of natural rubber/chitosan blends: Effects of blend ratio and compatibilization

The electrical properties of blends from natural rubber and chitosan have been studied in the frequency range of 10²-10⁶ Hz. Blending of chitosan with a non-polar polymer such as natural rubber is expected to improve the dielectric performance of chitosan, by reducing the polarity. This paper presents the results of dielectric properties of natural rubber/chitosan blends. Special attention has been paid to analyze the effect of blend composition, vulcanization of the major phase and the influence of compatibilization using maleic anhydride on the dielectric properties of natural rubber/chitosan blends. The temperature dependence of dielectric properties on the blends has also been studied.     

Refractive Indices and Dielectric Studies of Three Phenylcyclohexane Liquid Crystals in the Nematic Phase

The refractive indices n_e, n_o and densities and the dielectric primittivities ε_∥ and ε_⊥ of three P-cyano-P-alkylcyclohexanes (PHC-3, PCH-5, PCH-7) have been measured at different temperatures. The order parameters calculated by using Neugebauer and Vuks' methods were comparable. A comparison of order parameters of the PCH with the corresponding alkylcyanobiphenyls at reduced temperatures τ (= T-T_NI/T_NI) shows that the replacement of a phenyl ring with a cyclohexyl ring decreases the order parameter appreciably. The experimental values of dielectric permittivities ε_⊥ in each of the PCHs are found to be less than half of the ε_⊥-value calculated from Maier and Meier equations using the polarizability and the order parameter obtained from refactive index measurements. This was explained as due to short range antiparallel ordering of strongly polar molecules in the nematic phase, as proposed by Chandrasekhar and Madhusudan, which was not taken into consideration in Maier and Meiers' theory     

Nematic Behavior of 5OCB in a Dielectric Medium at Phase Transition Temperature ‐A Statistical Analysis

A statistical analysis has been carried out to determine the configurationally preference of a pair of 5OCB (4′‐n‐pentyloxy‐4‐biphenylcarbonitrile) molecules with respect to translatory and orientational motions. The CNDO/2 method has been employed to evaluate the net atomic charge and atomic dipole components at each atomic centre of the molecule. Modified Rayleigh‐Schrodinger perturbation theory along with multicentered‐multipole expansion method has been employed to evaluate long‐range intermolecular interactions, while a ‘6‐exp’ potential function has been assumed for short‐range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration in vacuum and in a dielectric medium at phase transition temperature using Maxwell‐Boltzmann formula. It has been observed that the probabilities are redistributed in dielectric medium and there is considerable rise in the probabilities of interactions, although the order of preference remains the same. On the basis of stacking, in‐plane and terminal interaction energy calculations, all possible geometrical arrangements of the molecular pairs have been considered. The results are discussed in the light of experimental as well as other theoretical observations.     

Internal bias field in TGS crystals doped with different impurities

The influence of an internal bias field formed by impurity ions of heavy metals and alpha-alanine molecules on the dielectric nonlinearity and emission properties of ferroelectric triglycine sulfate crystals has been studied. It is shown that a role of defects increases near the phase transition, which manifests itself in a decrease in the maximum values of the effective permittivity and an increase in the emission current with increasing impurity concentration.     

Liquid crystalline swallow-tailed compounds (II) dielectric relaxation

Dielectric investigations in the kHz-range on swallow-tailed liquid crystals show a strong decrease of the dielectric constant as well as of the dielectric loss parallel to the nematic director with decreasing temperature. In the perpendicular direction no strong anomaly has been observed. By consideration of X-ray data of the S_A phase the dielectric results can be interpreted by the dipole-dipole interaction which is forced by the anisotropy of repulsive forces of the respective molecules.     

Dielectric Relaxation in Low Temperature Smectic Phases of 4-n-Alkyloxy-benzylidene-amino-4′-cyanobiphenyls

The dielectric behaviour of five homologous 4-n-alkyloxy-benzylidene-amino-4′-cyanobiphenyls was investigated. Especially, we were interested in the dielectric relaxation frequencies of the smectic low temperature phases S_B and S_E as well as the dielectric behaviour in the solid state. We have found in all liquid crystalline phases a low frequency relaxation region explained by a hindered rotation of the molecules around their short axes. By supercooling of the hexyl and heptyl derivatives a new liquid crystalline phase can be obtained. A dielectric relaxation in the solid phase could not be found.     

Dielectric Studies of Three Liquid Crystals HBT,E8 and PCH-1132

The dielectric behavior of a Schiff base N-(P-Hexyloxybenzylidene)-P-toluidine (HBT) and two technologically important mixtures, E₈ and PCH-1132, are reported in the radio frequency region. The value of the mean dielectric constant increases by ～5% on going from the nematic to the isotropic phase in the case of E₈ and PCH-1132. This may perhaps be due to antiferroelectric arrangement of the molecules in the nematic phase. These mesogens show unusual dielectric behavior and exhibit positive anisotropy at lower frequencies and negative dielectric anisotropy above a critical frequency termed as isotropic frequency. The isotropic frequency is found to be temperature dependent. All these nematogens exhibit single relaxation in the radio frequency region which is due to hindered rotation of the molecules under nematic potential. The relaxation frequencies are 3.98 MHz (at 61.4°C), 2.27 MHz (at 34.3°C), 1.87 MHz (at 33°C) in case of HBT, E₈ and PCH-1132 respectively. The activation energy is found to be 16.7 KCal/mole, 14.6 KCal/mole and 16.1 KCal/mole for HBT, Ea and PCH-1132 respectively. In smectic B phase of HBT, the dipolar orientations are found to be somewhat “locked” similar to that in solids. The inversion of the sign of dielectric anisotropy in S_B phase of HBT is also discussed.     

Dielectric reorientation of smectic C,F and I phases

As shown for 4 liquid crystalline substances also smectic F and smectic I phases may be reoriented by an electric field similar to a smectic C phase. The threshold voltage of the effect was determined approximately. For one substance the tilt angle of the smectic C and I phase could be measured by means of the dielectric reorientation.     

Topological solitons in branched aliphatic molecules

Polymorphic transitions are typical for molecular crystals of long chain aliphatic compounds. In the present work we studied the possibility of polymorphic transitions via the mechanism of topological solitons. The solitons correspond to stretching/compression of aliphatic chain by a half of the period, accompanied by 180° turn in branched molecules. Triacyglycerides (TAG) were chosen as an object of study; temperature dependencies of the specific heat capacity at constant pressure, complex dielectric permittivity, spectral (IR) and structural (powder XRD) properties of TAGs were characterized performed at ?170 ÷ 70°C. Following model was proposed to explain the experimental results. Thermal transformations of TAGs started from initiation of topological solitons on the methyl groups. The solitons passed along the alkyl chains and reoriented the ester groups, resulted in thermal dipole-relaxation polarization defrost observed as a relaxation process in the range (?80÷0)°C. Reaching the stoichiometric obstruction (branching), the solitons reflect from it. When the temperature increases up to 60°C, melting of TAG polycrystals takes place.     

La3+掺杂Ca1-xLaxCu3Ti4O12陶瓷的制备及性能研究

采用固相反应法制备了La3+掺杂的CaCu3Ti4O12(CCTO)陶瓷,研究了La3+掺杂量对Ca1-xLaxCu3Ti4O12(x=0;,1;,3;,5;,7;)陶瓷物相结构、微观形貌和介电性能影响,对La3+掺杂影响CCTO陶瓷介电性能的机理进行了分析.结果表明:x为3;时,开始出现杂相;x高于5;时,陶瓷晶粒开始细化;La3+掺杂可以显著提高CaCu3Ti4O12陶瓷的介电常数,同时介电损耗在高频段也相应降低,从而有助于CCTO陶瓷的综合介电性能的提升.