Phase transition studies in mesogenic dimers

Schiff base liquid crystal dimers, both symmetric and non‐symmetric, in which two anisotropic groups are linked by a flexible spacer, exhibit a rich variety of smectic mesomorphism. The interest in this class of mesogens stems not only from their ability to act as model compounds for semi‐flexible mainchain liquid crystal polymers but also from their quite different properties compared to conventional low molar mass liquid crystals (monomers). We report here the phase transition studies on two examples of these schiff base liquid crystal dimers using the Differential Scanning Calorimetry and density measurements as a function of temperature. The symmetric liquid crystal dimer, α,ω‐bis (4‐n‐dodecylaniline benzylidene‐4′‐oxy)decane (10.O12O.10) exhibits a very rare Isotropic to G transition. Where as, the non‐symmetric dimer, α‐(4‐cyano biphenyl‐4′‐yloxy)‐ω‐(4‐n‐decylanilinebenzylidene‐4′‐oxy)decane (CB.O10O.10) exhibits a rare nematic to intercalated smectic A phase transition. The transitions studied, isotropic to nematic and isotropic to G exhibit a large density jump at the transition confirming their first order nature. Nematic to intercalated smectic A transition is found to be a second order transition whose behavior is similar to nematic to smectic A transition. Calculated values of pressure dependence of transition temperature and thermal expansion coefficient are also reported.     

Ultrasonic relaxation studies in the liquid crystal 80CB

The variation of the ultrasonic velocity with temperature in the liquid crystal 4′-n-octyloxy-4-cyanobiphenyl (80CB) at four different fixed ultrasonic frequencies is studied. From these studies various relaxation parameters like the adiabatic relaxation time, velocity at zero frequency, in the vicinity of isotropic-nematic phase transition and velocity at infinite frequency are estimated. From these parameters the relaxation mechanism in the vicinity of isotropic-nematic phase transition is studied and the results are discussed.     

Investigation of thermo-electro-optic properties of some mixed nematic liquid crystals

The 6CB/8CB/8OCB liquid crystal mixture has been studied by Differential Scanning Calorimetry (DSC), polarised optic microscopy (POM), Semiconductor Characterization System, and Ultra-violet spectrophotometry (UV). DSC and POM results indicate that the 6CB/8CB/8OCB mixture exhibits liquid crystalline properties. The capacitance-voltage and conductance-voltage measurements were performed in the frequency range of 200-500 kHz and in the temperature range of 30oC–50oC. The 6CB/8CB/8OCB mixture showed an extremely large positive dielectric anisotropy. The molar absorptivity ? for the 6CB/8CB/8OCB mixture was calculated and found to be higher than the absorptivity values of the binary mixtures due to the alkyl chain length with H-aggregation.     

Self-assembled PEDOT nanoropes: Materials for improving electro optical properties of liquid crystal

A ropelike conducting polymer nanostructure was synthesized in this work. 3,4-ethylenedioxythiophene (EDOT) was polymerized in presence of a nematic liquid crystal (5CB) giving rise to the PEDOT nanoropes with diameter 10–20 nm. UV–Vis spectra of the nanoropes was comparable with the lightly doped bulk PEDOT. However, unlike the bulk polymer, the nanoropes or helical nanofibres, separated from the medium could be re-dispersed in chloroform and could be blended with 5CB. Electro optical properties of 5CB were remarkably modified after incorporation of PEDOT nanoropes. The residual current for optical transmission, threshold voltage for Freedericksz transition and capacitance hysteresis of 5CB were substantially lowered after nanorope incorporation.     

Determination of Nematic Liquid Crystal Elastic and Dielectric Properties from the Shape of a Capacitance-Voltage Curve

This paper shows how capacitance-voltage curves can be used to determine nematic liquid crystal dielectric and elastic constants and surface tilt angles. Careful analysis of the C-V curves reveals that distortion free energy theory is useful up to a V/V_c of about two. A detailed curve of tilt angle as a function of SiO deposition angle is also presented along with temperature variation in liquid crystal properties. Pentylcyanobiphenyl (5CB) was used throughout the study.     

Low-temperature luminescence and phase structure of 5CB liquid crystal under different cooling regimes

Abstract

The effects of the 5CB liquid crystal (LC) cooling regimes on its luminescence spectra at the low temperature (T?=?4.2 K) have been found out and investigated. Experiments have been carried out at two cooling rates: 2 and 150?deg/min. Differential scanning calorimetry (DSC) data allow us to establish that the 5CB glass-like state (GLS) is formed during the rapid cooling of the sample. Peculiarities of the LC dimer formation under the different cooling conditions are discussed.     

The nematic reentrant phase in binary mixtures composed of a 4-(trans-4-alkylcyclohexyl)-1-(2-cyanoethenyl)-benzene and 60CB

It is shown that the smectic A phase in the 4-(trans-4-n-alkylcyclohexyl)-1-(2-cyanoethenyl)-benzene EPCH homologous series has a partially dimolecular structure of A_d type. The EPCH compounds show a virtual S_Ad → N_re transition revealed in mixtures of 60CB with members of the homologous series with five or more carbon atoms in the alkyl chain. The temperature dependences of electric conductivity and electric permittivity of the 8EPCH-60CB mixture have been tested in the three successive phases, N_re, S_Ad and N.     

Electrical conductivity and thermoelectric power of liquid AgSb Te2

The electrical conductivity and thermoelectric power of liquid AgSb Te₂ have been investigated as a function of temperature. Experimental data are analyzed in terms of a recent model proposed by Mott. The activation energy for electrical conductivity and thermoelectric power is found to be approximately 0.50 eV with a large temperature coefficient γ ～ 7 × 10^?4 eV/deg K. The gradual transition from a semiconducting to a metallic behaviour has been observed at high temperature.     

Photo Controlled Surfaces in Rheology of Liquid Crystals

The modified method for shear viscosity measurements of nematic liquid crystals (NLC) oriented by surfaces is described. The main modification is concerned with usage of photoalignment technique for preparation of mono-domain samples with an orientation controlled by a polarization state of the secondary light irradiation. It provides the azimuthal rotation of a sample at any desirable angle. Thus different geometries of shear flows can be realized after filling the measuring cell with a liquid crystal. The method is experimentally approved at investigation of Poiseuille decay flows of 4-pentyl-4-cyanobiphenyl (5CB) through the channels with a rectangular cross section. The proposed construction of the cell with different surface treatment provides measurements of the three principal viscosities coefficients (Miesowicz’ viscosities) with additional usage of electric field. A small amount (less than 0.2 ml) of a liquid crystal needed for measurements and simple measuring procedure makes the method to be useful for rheological studies of newly synthesised liquid crystal materials.     

Coefficient of Volume Expansion and Thermoacoustic Parameters of Alkyl-Cyano-Biphenyl Liquid Crystals

Dilatometric studies were performed on the nematic and isotropic phases of the homologous series of alkyl-cyano-biphenyl liquid crystals C_nH_2n+1C₆H₄C₆H₄. CN for n = 5 to 9. From these studies the coefficient of volume expansion (a) of the liquid crystal is estimated. Using this data a number of thermoacoustic and an harmonic parameters of the liquid crystal are evaluated. The temperature dependence of these parameters have also been investigated. All these thermoacoustic and an harmonic parameters evaluated in this case show characteristic change in the immediate vicinity of the phase transition temperature.     

Polarization mechanism of LiNbO3 crystal due to thermoelectric power and current-induced electric fields during growth via micro-pulling-down method

The mechanism of polarization due to thermoelectric power and current-induced electric fields during the growth of LiNbO₃ crystals was studied using a micro-pulling-down method. With no applied electric current, a +c single-domain crystal was grown regardless of the domain orientation of the seed crystal. This +c domain growth was consistent with the direction of the electric field caused by the thermoelectric power in the liquid, despite an opposing electric field in the solid due to the opposite sign of the Seebeck coefficient. Thus, it was the electric field in the liquid that determined the domain structure of the growing crystal. On the other hand, when a current was applied from the melt to the crystal, a −c domain crystal was grown. The electric current required for this domain inversion to occur became larger as the temperature gradient in the solid phase decreased. This shows that the electric field in the solid phase became large enough to induce domain inversion from +c to −c through a combination of the thermoelectric power in the solid phase and current-induced electric field.     

On the Dielectric Behavior of Unaligned Samples of 4-n-Octyl-4′-cyanobiphenyl (8CB)

Data on the dielectric properties of the liquid crystal 4-octyl-4′-cyanobiphenyl (8CB) have been determined onto unaligned samples, for a large scale of frequency, as function of temperature. The components of the complex dielectric permittivity as well as the corresponding dielectric parameters were evaluated using Havriliak-Negami (HN) functions. The dielectric characteristics were discussed and were compared with the literature, a good agreement being observed. The method allows to obtain for the first time the dielectric strength for a large range of the temperature and gives the two functions describing the relaxation processes in the isotropic range.     

Molecular orientation and dielectric anisotropy properties of 4-cyano-4′-n-heptylbiphenyl-TiO2 liquid crystal composite

The molecular orientation and the dielectric anisotropy of the nematic liquid crystal (LC) 4-cyano-4′-n-heptylbiphenyl (7CB) and of TiO₂-doped 7CB have been investigated. The dielectric properties of the LCs exhibit a relaxation peak that shifts to lower frequencies with increasing voltages. The relaxation frequencies of 7CB and 7CB/TiO₂ liquid crystals were calculated and found to decrease as the bias voltage increases. This is attributed to molecular reorientation. The dielectric anisotropy of the LCs changes from the positive type to negative type and the static electric permittivity and dielectric anisotropy values were found to be lower for the 7CB/TiO₂ system.     

Electrical Conductivity and Thermoelectric Power of Lithium?Zinc Ferrites

Electrical conductivity (σ) and thermoelectric (Q) of polycrystalline lithium-zinc ferrites of different compositions was investigated as a function of composition and temperature. Plots of log (σT) versus 10³/T are almost linear and show a transition near the Curie temperature. The activation energy. in the ferrimagnetic region is in general less than that in the paramagnetic region. The carrier concentration and mobility of charge carriers has been discussed as a function of composition and temperature.     

Studies on induced smectic phases (III). Fourier transform infrared and laser-raman spectral differences between isotropic and induced smectic phase of binary mixtures of nematogens with contrasting dielectric anisotropies

The S_E, S_B, S_A induced phase sequence was detected in binary mixtures of two nematogens, 4,4′-di-n-heptyl azobenzene (7AB) with 4-n-pentyl-4′-cyano biphenyl (5CB). The mixtures as well as the single components were studied by Fourier transform IR and laser-Raman spectroscopy at different temperatures. Charge redistribution of 5CB is demonstrated by the spectral differences found by the ratio-recorded IR spectra and also by integrating the intensities of some Raman lines. The spectra of the different phases of the 7AB—5CB mixture, built up from components of different length, indicate a population increase of the gauche conformers of 7AB in the smectic A phase.     

Measurement of the Refractive Index and Estimate of the Order Parameter in Oriented Liquid Crystals

The refractive indices of a cholesteric liquid crystal, cholesteryl dodecyl carbonate (CDC), and a smectic liquid crystal, p-n-octoxybenzilidene-p-n-butylaniline (OOBBA), have been measured as a function of temperature in the transition temperature regions. The measurements were made on oriented liquid crystals with an Abbe refractometer. By combining the data on ordinary and extraordinary refractive indices, the relative order parameter, α_aS/α, is plotted as a function of temperature. The behavior of the order parameter near the cholesteric to isotropic transition of CDC is similar to a nematic liquid crystal, whereas, the order parameter in OOBBA shows only a slight temperature variation like many solids.     

Electrical conductivity and thermoelectric power of nickel-zinc ferrites

Electrical conductivity (σ) and thermoelectric power (Q) of polycrystalline nickel-zinc ferrites of different compositions was investigated as a function of composition and temperature. The electrical conductivity in these ferrites is explained on the basis of the hopping mechanism. Plots of log (σT) versus 10³/T are almost linear and show a transition near the curie temperature. The activation energy in the ferrimagnetic region is in general less than that in the paramagnetic region. The carrier concentration and mobility of charge carriers has been discussed as a function of composition and temperature.     

Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series

Experimental results for the temperature dependence of the density of propyl – cyanobiphenyl (3CB), butyl – cyanobiphenyl (4CB) and hexyl – cyanobiphenyl (6CB) are presented. The results are compared with previous results for temperature dependence of other members of the alkyl cyanobiphenyl series (nCB). The deviation of the density from linear temperature dependence is discussed in terms of the recently discovered corresponding rule for nematic liquid crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature and Pressure Study of 8S5

We have studied the liquid crystal phase transitions of 4-n-pentylbenzenethio.4′-n-octyloxybenzoate (8S5) as a function of pressure and temperature. There were no pressure induced anomalies. We measured the latent heat at the N-1 transition (.5±.1 calories/gram) but were not able to observe any evidence of a latent heat for both the A-N and C-A transitions. We speculate that the continuous nature of these transitions may be linked to the flexibility of the polar part of 8S5.     

Quasi-elastic neutron scattering of cyanobiphenyl compounds with different terminal chains

Quasi-elastic neutron scattering measurements were performed for five cyanobiphenyl compounds with different terminal chains (5CB, 5*CB, 8OCB, 8*OCB and 6O2OCB) in their liquid and liquid-crystalline phases to investigate the molecular dynamics. The spectra were successfully decomposed into two components, indicating that two types of molecular motions are involved. For the broad (or fast) component, the temperature dependence as well as the momentum transfer dependence did not differ very much among those compounds. For the narrow (or slow) component, on the other hand, the compounds with a branched alkyl-chain (5*CB and 8*OCB) gave significantly smaller diffusion constants than the others. A molecular dynamics simulation for 5*CB and 5CB suggests that the reorientational motion of the whole molecule around the long molecular axis is responsible for the broad component, whereas a diffusive motion of the terminal chain is responsible for the narrow one.