Cyano Mesogens: A New Interpretation of the Structure of the Bilayer Smectic A Phase

Abstract

A new model describing the bilayer structure of the smectic A mesophases observed with cyano mesogens is proposed. Our interpretation is based upon recent experimental x-ray studies of binary mixtures containing a cyano mesogen on one hand and a non polar one, on the other, with either one or two aliphatic chains in the molecules. In this model, the smectic layers are formed of the intimate mixture of single molecules and of dimers described as being pairs of molecules associated head-to-head. As a result, the degree of dimerization is far from unity due to the steric hindrance of the aliphatic chains; it decreases notably when the average number of aliphatic chains in the system increases.