Molecular Structure and Mesomorphic Properties of Thermotropic Liquid Crystals. III. Lateral Substituents

Abstract

The effect of lateral substituents on the type and thermodynamic stability of the mesophases shown by terminally polar and nonpolar rod-like mesogens is described. Lateral substituents both sterically force the interacting molecules apart and also hinder the packing of the molecules in a layered structure, thus suppressing smectic order and favouring the nematic phase. The nematic → isotropic transition temperature, ν_m decreases with increasing van der Waals volume, V_w of the lateral substituent, irrespective of its dipole moment, due to the increase in intermolecular distance r. The relationship between ν_m and V_w is non-linear because the repulsive and attractive forces are different functions of r. The depression in ν_ni depends not only on V_w of the substituent, but also on its position and on the structure of the rigid core. In aromatic nitriles, lateral substituents with + M effect in an ortho position with respect to the terminal cyano group, hinder the molecular association and therefore, lower the clearing point and increase ε||.