V-Shaped Electro-Optic Response Observed in a Chiral Ferroelectric Smectic Liquid Crystal

We report on the observation of V-shaped switching in a ferroelectric liquid crystal cell over a wide range of temperatures. Results of the optical transmittance in the visible region give us the helical pitch for various temperatures of the ferroelectric liquid crystalline compound used. We show that the helical pitch, in addition to the spontaneous polarization (PS) and thickness of the alignment layer of the cell, is an important factor in giving V-shaped switching. A longer or helical compensated mixture gives a better V-shaped switching. © 2008 American Institute of Physics

Two-dimensional films of discotic molecules at an air-water interface

The possibility of forming Langmuir monolayers with disc-like molecules of benzene-hexa-alkanoates has been investigated for the first time. Surface pressure measurements show that these monolayers behave in many respects as the well-known films of fatty acids. The benzene ring plays the role of the polar head group while the alkanoates chains are the long aliphatic tails. Molecular dimensions can be derived from the surface pressure-concentration isotherms. The results indicate that the benzene rings lie flat at the interface. The projection of the molecular diameter onto the interface is practically identical to the lattice spacing measured in the liquid-crystalline columnar mesophases formed by benzene-hexa-alkanoates in bulk. These experiments allow to refute the interpretation of the liquid expanded-liquid condensed transition in terms of a second order isotropic → nematic phase transition, as recently proposed by several theoretical groups.On montre pour la première fois que des molécules discotiques de benzène-hexa-alkanoates peuvent être étalées en monocouches à un interface eau-air. Les mesures de pression superficielles donnent des résultats très analogues à ceux des films d'acides gras. Le noyau benzénique joue le rôle de la tête polaire grâce à six groupes carboxyliques périphériques, et se met à plat sur l'interface. Les chaines alkanoates jouent le rôle des chaines aliphatiques. Les isothermes de pression permettent une estimation des dimensions moléculaires dans le plan de l'interface. Ces dimensions sont pratiquement identiques à celles déduites des mesures de rayons X dans les phases à colonnes formées par les benzène-hexa-alkanoates (cristaux liquides thermotropes). Ces expériences permettent de réfuter l'interprétation de la transition liquide expansé-liquide condensé comme une transition du second ordre entre une phase isotrope et une phase cristal liquide nématique