V. N. Raja

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.

A biaxial nematic liquid crystal

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

High precision density studies near the smectic A–nematic tricritical point

Abstract We have carried out very accurate density measurements (with a precision of ±5 × 10−5g cm−3) near the smectic A-nematic transition in binary mixtures of 4-n-nonyl-4′-cyanobiphenyl (9CB) and 4-n-decyl-4′-cyanobiphenyl (10CB). The transition crosses over from second to first order as the temperature range of the nematic phase decreases. For mixtures with the shortest nematic range the data deviate noticeably from a single power law behaviour. Such a deviation is an indication of the first order nature of the transition. Very good fits to a single power law have been obtained for pure 9CB and the x = 0·04 mixture where x is the mole fraction of 10CB in 9CB. The critical exponent obtained from the power law fitting has enabled us to locate the tricritical point to be very close to x=0·04, which is in agreement with the results obtained previously by high resolution calorimetric [1] and X-ray scattering studies [2].

Measurement of rotational viscosity in the Smectic Cphase

Abstract Several methods have been employed to determine the rotational viscosity γφ in the smectic C* phase associated with the molecular motion around the cone about the layer normal. We have measured this parameter using four different techniques and show that the values are in good agreement with each other. We have also studied the effect of chain length on γφ in the C* phase and γφ the viscosity associated with tilt angle fluctuations, in the A phase. The results suggest that the activation energy associated with these motions is basically controlled by the rigid core and not by the chain length of the molecule.

Phase Diagram Involving the Twist Grain Boundary Phase in the Vicinity of a Virtual Cholesteric-Smectic A-Smectic C*Point

Abstract We report the first detailed study of the phase diagram in the vicinity of a virtual Cholesteric-Smectic A-Smectic C*point in a binary liquid crystal system. The occurrence of the Twist Grain Boundary phase, which is the liquid crystal analog of the Abrikosov flux lattice in type II superconductors, is established by optical microscopy, X-ray and selective reflection measurements. The study affords a direct comparison between the experimental phase diagram and the theoretical scenario as envisaged by Renn and Lubensky.

Dielectric studies in the vicinity of the A-C ∗ transition

Abstract We have studied the soft mode and Goldstone mode relaxations in a ferroelectric liquid crystal compound (referred to as C6 for short) exhibiting a high value of spontaneous polarisation. The results show that the relaxation frequency fs as well as the dielectric strength Δϵ S can be described by a simple power law, but gives non-mean field exponents. However, fitting done to the Δϵ s data using an expression given by the generalised mean field theory, not only yields the mean field susceptibility exponent γ but also helps in determining the Landau coefficients a and C (where a is the electroclinic coefficient and C the polarisation-tilt coupling constant) as well as the soft mode viscosity. We also present the Goldstone mode viscosity and the relevant elastic torque data. Measurements on C7 and C8, the higher homologs of C6, bring out the effect of chain length on the relaxation parameters.

Experimental studies in the vicinity of the C*-I* transition

Abstract Precise layer spacing, polarisation and dielectric measurements have been performed on two compounds CE8 and 8OSI* both of which exhibit the C* and I* phases. High resolution X-ray studies confirm that the C* -I* transition is first-order in CE8, whereas the I* phase evolves continuously from the C* phase in 8OSI*. The thermal variation of the tilt order is reflected in the temperature dependence of polarisation and dielectric constants.

An Incommensurate Smectic A Phase

An incommensurate smectic A phase (Aic) existing between partially bilayer Ad and bilayer A2 phases was discovered recently by us [1]. Here we report detailed X-ray and DSC studies as well as dielectric constant measurements on this new phase.

Density, Dielectric and X-ray Studies of Smectic A-Smectic A Transitions

Abstract We report here the results of detailed density, dielectric and x-ray studies on three systems exhibiting different types of Smectic A-Smectic A transitions. It is found that although the layer spacing shows marked changes at the transitions, the corresponding density changes are extremely small. In every case studied, the dielectric anisotropy shows a pronounced decrease on going over to the lower temperature smectic A phase. This decrease can be correlated with the structural changes.

X-Ray Study of a Thermotropic Biaxial Nematic Liquid Crystal

AbstractX-ray studies have been carried out on magnetically aligned samples of nematic bis[1-(p-n-decylbiphenyl)3-(p-ethoxyphenyl) propane-1, 3-dionato] copper(II). The diffraction pattern consists of three diffuse maxima as would be expected from an orthorhombic fluid. This provides further evidence in confirmation of the biaxiality of the nematic phase of this copper complex.