B. R. Ratna

A Thermotropic Biaxial Nematic Liquid Crystal

Abstract Optical studies have been carried out on a nematogenic copper complex, which incorporates the features of both rod-like and disk-like molecules. Conoscopic figures are presented demonstrating (i) the occurrence of a biaxial nematic phase in the pure complex, (ii) the uniaxial-biaxial (Na-Nb) transition in binary mixtures, and (iii) the temperature variation of the biaxiality near this transition. The I-Na-Nb phase diagram has been studied for the binary system.

Dielectric Studies on Liquid Crystals of Strong Positive Dielectric Anisotropy

Abstract The static dielectric constants of the following compounds have been measured as functions of temperature in their nematic and isotropic phases: (i) 4′-n-alkyl-4-cyanobiphyenyls, (ii) trans-p-n-alkoxy-α-methyl [pacute]-cyanophenyl cinnamates and (iii) CBOOA. All these exhibit a very large positive dielectric anisotropy because of the C≡N end group. For all the compounds the average dielectric constant ϵ evaluated from the measured values of ϵ ∥ and ϵ∥ is, throughout the nematic phase, less than the extrapolated isotropic value. Further, ϵ decreases with decreasing temperature, the effect being greter in compounds whose nematic phase is preceded by smectic phase. These facts are in conformity wiht the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar. For the two homologous series (i) and (ii), the dielectric anistropy does not show an alternation with increasing chain length, but decreases continuously.

Dielectric properties of 4′-n-alkyl-4-cyanobiphenyls in their nematic phases

AbstractThe principal dielectric constants of the pentyl to octyl derivatives of 4′-n-alkyl-4-cyanobiphenyl have been measured as functions of temperature in their nematic and isotropic phases. All the compounds exhibit a strong positive dielectric anisotropy due to the presence of a large dipole moment along the major molecular axis. The principal dielectric constantsε‖ andε⊥ as well as the anisotropy Δε decrease with increasing alkyl chain length. The experimental value ofn

Dielectric Dispersion in 4′-n-Alkyl-4-Cyanobiphyenyls

bar in = frac{1}{3}

Evidence of continuous evolution of smectic A2 from smectic Ad

n (ε‖+2ε⊥) decreases with decreasing temperature, and is throughout less than the extrapolated isotropic value, in conformity with the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar.

Experimental Studies on a Terminally Nitro Substituted Compound with a Latent Reentrant Nematic Phase

We report a biaxial nematic liquid crystal in a simple thermotropic system.

A New Kind of A-A Transition: Studies on Binary Mixtures of Terminally Substituted Cyano and Nitro Compounds

Abstract The dispersion of ϵ∥ in the frequency range of 1-10 MHz has been studied for 4′-n-alkyl-4-cyanobiphenyls (pentyl to octyl) at different temperatures in teh nematic phase. It is established for the first time that the relaxation frequency at a common relative temperature (TNI - T) for the successive homologues shows an alternation similar to that exhibited by the nematic-isotropic transition point or the orientational order parameter. The activation energy evaluated from the Arrhenius plot is found to be the same (∼ 0.56 eV) for the compounds.

Dielectric Properties of Some Nematics of Positive Dielectric Anisotropy

Abstract Static dielectric constants, ∊∥ and ∊⊥ as well as the low frequency dielectric dispersion of ∊∥ have been studied in the vicinity of the hexatic B-smectic A transition in n-hexyl-4-pentyloxybiphenyl-4-carboxylate (65OBC) and n-butyl-4-hexyloxybiphenyl-4-carboxylate (46OBC) and also near the crystal B-smectic A transition in n-(4-n-butyloxybenzylidene)-4-n-octylaniline (4O. 8). While the behaviour of the dielectric anisotropy is essentially similar for both transitions, the dispersion results show significant differences.