Biaxial Liquid Crystals

Abstract

Convergent light observations have been made on three liquid crystalline substances with nematic and smectic phases. The nematic phase was observed to be uniaxial as expected, but smectic C was found to be biaxial. The optic axial angle 2V for smectic C was found to be on the order of 20° for all three compounds. Also, the tilt angle for smectic C is nearly 45° for all three liquid-crystal systems. Anisotropy of the degrees of order is suggested as a partial explanation for the biaxial character of smectic C.We have studied three compounds which exhibit a nematic phase and a single smectic phase of the type classified by Sackmann and Demus 1 as type C. The smectic phases classified by Sackmann and Demus correspond to different molecular arrangements and are separated from each other by first-order phase transitions.The nematic phase has a long-range order such that the long axes of the molecules are nearly parallel and the parallelism of the long axis generally varies continuously throughout the bulk of the liquid. Assuming a random arrangement of the center of mass and free rotation about the long molecular axis, nematic liquids would be uniaxial. All observations do show the nematic phase to be uniaxial. Smectic A is a system in which the molecules are arranged in layers with the long molecular axis perpendicular to the layer. 2 If the centers of mass within the layers are random and free rotation is assumed, then smectic A would be uniaxial. Experimentally, all smectic -A phases have been shown, in general, to be uniaxial. Smectic C is generally considered to have a structure such that the molecules are arranged in layers but with the long molecular axis tilted with respect to the layer normal. 2 In general, it has been assumed that all smectic phases including smectic C would show uniaxial character and, in fact, Friedel 3 argued that all smectic phases would be uniaxial.

Proton NMR Investigations of the Smectic-C Phases of Three Alkyloxy-Azoxybenzenes

“…Especially the smectic-C phase of Hept-OAB has been studied fairly thoroughly by different methods as, for example. X-ray [1][2][3][4], optical conoscopy [5], EPR [6,7], NMR [8. 9], electric conductivity [10], with the aim of elucidating the structure and the dynamic molecular organisation of this phase.…”

On the biaxiality of smectic C and ferroelectric liquid crystals

“…As explained previously, the biaxial nature of the S C phase had been realised from the outset, [18] but measurements showed that the optical biaxiality is negligible [19] and it had been assumed that the dielectric biaxiality was also insignificant. It was Roy Sambles who first realised that the AC stabilising effect of negative Δε S C (*) mixtures in the chevron geometry had to have a significant dielectric biaxiality, in discussions with Peter Raynes and Frank Leslie.…”

“…Given that the optical biaxiality is negligible [18,19] and the materials may be considered optically uniaxial with the optic axis parallel to the director, then this choice seems sensible. Hence, we choose three principal axes ( !…”

Optical, Thermal Studies on Binary and Ternary Hydrogen-Bonded Liquid Crystal Complexes

“…To explain this, the direction perpendicular to the layer plane in the smectic phase is taken as the reference axis. The tilt angle of the smectic C* phase in the given LC complex is experimentally determined by optical extinction method [39,40]. The temperature variation is evaluated by applying the measured data of temperature θ (T) in the relation given below:…”