Determination of the helical pitch in the smectic C*α phase by means of gyrotropy measurements

Abstract

The existence of a short pitch helical structure in the smectic C^* _α phase has been confirmed in two compounds. This conclusion has been reached by using optical activity measurements along the helix axis and along a direction perpendicular to it. No measurable gyrotropy is detected along the helix, in agreement with earlier results. However, a rather large optical activity appears in the perpendicular direction. This optical rotation is difficult to detect, because it coexists with birefringence, and their separation requires the use of non-standard polarimetric techniques. The optical activity data, together with additional birefringence measurements performed simultaneously, have allowed us to obtain the helical pitch and the molecular tilt of the materials. The pitch is in the range 80-40 nm in the compounds studied, and its variation with temperature shows some irreproducible features on heating. On cooling from the smectic A phase, however, the pitch behaviour is perfectly reproducible.