Dielectric Study of Nematic LC Built with Bent-Core Molecules

Here we report results of the study of bent-core (BC) cybotactic nematic LC by dielectric spectroscopy and electro-optic measurements. In the absence of electric field, this material is uniaxial and exhibits only one relaxation process corresponding to the fluctuations of the polar clusters. The application of the external electric field induces biaxialityandthesecondrelaxationprocessappearsinthedielectricspectra.Thisisassigned to the collective mode. Possible models for cluster transformation under electric field are discussed

Occurrence of Five Different Orthogonal Smectic Phases in a Bent-Core (BC) Liquid Crystal

Five orthogonal smectic phases (Sm-A, Sm-Ab, Sm-APR, Sm-APAR, and Sm-APA) were observed in a 4-cyanoresorcinol bisbenzoate with twoterephthalate–based wings achiral bent-core(BC) compound. The phase behavior was investigated by polarising optical microscopy, second harmonic electro-optic response(EO2) and dielectric spectroscopy. The field dependent biaxiality of polar and non-polar Sm-A phases was studied. It is obviously of enormous scientific significance and practical interest to find five different orthogonal smectic phases in a low molecular weight bent-core (BC) compound at relatively low temperatures

Flexoelectric Polarization Studies in Bent-Core Nematic Liquid Crystals

The flexoelectric polarization (Pf) of four bent-core nematic liquid crystals (LCs) has been measured using the pyroelectric effect. Hybrid aligned nematic cells are fabricated for measuring the pyroelectric response over the entire range of the nematic phase. It is found that the magnitude of flexoelectric polarization Pf and the sum of the flexoelectric coefficients |e1 +e3| for the bent-core LCs studied here are three to six times higher than for the calamitics. Pf is found to depend on the transverse dipole moment of LC molecules. However,|e1 +e3| values are by no means giant as |e3| alone had been reported for a bent-core nematic system previously. The dependence of the sum of “splay and bend flexoelectric coefficients” is discussed in terms of the shape of the molecule and of the dipole moment directed normal to the molecular axis

Properties of Non-Tilted Bent-Core Orthogonal Smectic Liquid Crystal

We present the properties of different achiral orthogonal polar smectic phases observed in a homologous series of bent-core molecular compounds. The macroscopically uniaxial SmAPR phase transforms to biaxial state by the application of higher electric fields and biaxial SmAPA phase undergoes biaxial-uniaxial-biaxial texture transformation with increasing electric fields,i.e.,initial antiferroelectric structure transforms into ferroelectric state under higher electric fields and it shows three different optically distinguishable states. The SmAPAR phase shows an entirely different response compared to other polar smectic phases under study. The dielectric measurements in the SmAPAR phase is the evidence of antiferroelectric behavior of uniaxial state. The polarizing microscopy texture observation supports the corresponding biaxiality measurements

Structure and Polymorphism of Biaxial Bent-Core Smectic Liquid Crystal

The mesomorphic properties of a homologous series of achiral bent-core compounds are studied by polarizing optical microscopy, electro-optics and polarization measurements. Induction of a new orthogonal smectic phases with the increase of alkyl side chain length was observed. One of the compounds exhibits a unique phase transition between four non-tilted smectic phases (SmAPA–SmAPAR-SmAPR-SmA). The uniaxial but antiferroelectric nature of SmAPAR phase was confirmed by POM, current response, the 2nd harmonic electro-optic response and polarization measurements. The structure of SmAPAR phase was studied theoretically by Next-Nearest-Neighbor model and was identified as SmAPα

Biaxial order parameter in the homologous series of orthogonal bent-core smectic liquid crystals

The fundamental parameter of the uniaxial liquid crystalline state that governs nearly all of its physical properties is the primary orientational order parameter (S) for the long axes of molecules with respect to the director. The biaxial liquid crystals (LCs) possess biaxial order parameters depending on the phase symmetry of the system. In this paper we show that in the first approximation a biaxial orthogonal smectic phase can be described by two primary order parameters: S for the long axes and C for the ordering of the short axes of molecules. The temperature dependencies of S and C are obtained by the Haller's extrapolation technique through measurements of the optical birefringence and biaxiality on a nontilted polar antiferroelectric (Sm-APA) phase of a homologous series of LCs built from the bent-core achiral molecules. For such a biaxial smectic phase both S and C, particularly the temperature dependency of the latter, are being experimentally determined. Results show that S in the orthogonal smectic phase composed of bent cores is higher than in Sm-A calamatic LCs and C is also significantly large

Dielectric and Optical Study of Biaxial Bent-Core Nematic Liquid Crystal

We report the observation of biaxial nematic phase in a bent-core molecular system in which the biaxiality exists on a microscopic scale using polarizing microscopy, electro-optics and dielectric spectroscopy. An application of electric field induces a macroscopic biaxiality and therefore electro-optic switching. The observed electro-optic switching is explained in terms of the interaction of the ferroelectric clusters with the electric field. Dielectric spectra of the sample shows existence of low-frequency collective processes related to the microscopic polar clusters. The dielectric constant changes its sign from positive to negative and again to positive values

Dielectric properties of liquid crystalline dimer mixtures exhibiting the nematic and twist-bend nematic phases

A detailed investigation of the thermal and dielectric properties of a series of binary mixtures exhibiting the nematic ( N ) and twist-bend nematic ( N TB ) liquid crystal phases is presented. The mixtures consist of an achiral, dimeric liquid crystal CB7CB, which forms the nematic and twist-bend nematic phases, and a calamitic liquid crystal 5CB, which shows the nematic phase. As the concentration of the calamitic liquid crystal is increased, the transition temperatures decrease linearly, and the width of the nematic phase increases. The enthalpies of phase transitions obtained from DSC measurements show that on increasing the concentration of 5CB in the binary mixtures, the enthalpy associated with the N − N TB phase transitions reduces considerably compared to a clear first-order N − N TB transition in pure CB7CB. The real and imaginary parts of the dielectric permittivity are measured as a function of frequency from 100 Hz to 2 MHz in the nematic and twist-bend nematic phases in planar and homeotropic devices. A significant decrease in the average dielectric permittivity as a function of temperature for mixtures forming the N TB phase is observed. Measurements of the imaginary part of the dielectric permittivity show a relaxation peak in the measured frequency window for all of the mixtures exhibiting the N TB phase. The activation energy associated with this relaxation process is calculated and is shown to remain constant irrespective of the composition of the mixtures