Structural and electro-optical studies of the smectic polymorphism of a homologous series of chiral molecules

(Received 28 October 1992, accepted in final form 5 February 1993)

Abstract
The molecular organization in bulk samples of the smectic O mesophase of 2-heptyl and 2-octyl 1,4-terephthalydene-bis-4-aminocinnamates mixtures is studied by various methods. The phase diagrams are obtained by thermo-optical and microcalorimetric analysis. The structures are investigated by X-ray diffraction and the electrical properties (spontaneous polarization, dielectric constants) are studied in detail. The smectic O phase has a lamellar structure in which the molecules are tilted in the layers. The order inside the layers is liquid-like but, contrariwise to the smectic C phase, where it is uniform, the azimuth turns between two adjacent layers with an angle of nearly 180°. We observe the smectic O-smectic C transition by varying temperature or by applying an electric field. Moreover, the smectic C-smectic O-smectic C sequence is obtained in a narrow composition range. Our results are in good agreement with those previously reported obtained on thin films growing at the isotropic liquid-smectic interface.