Molecular tilt in the smectic C phase : a zigzag model

Abstract

We have collected data concerning the molecular tilt angle of 4 smectic C compounds, presenting a second order transition to a smectic A phase. We have completed these data by direct measurements, using the conoscopic technique, for the tilt angle of the optical axis, and the X-ray technique, to follow the varying thickness of smectic layers. The comparison of X-rays and optical data shows a discrepancy which becomes very large for the molecules which possess long aliphatic end chains. Resuming an idea from Guillon and Skoulios, we propose a molecular model for the C phase derived from lyotropic systems: the rigid central core, optically anisotropic, imposes the tilt of the optical axis. The melted end chains are on the average closer to the normal to the layers. The rigidity parameter, ratio of the rigid core length to total length of the molecule, will be important to describe the mechanical properties of smectics.