Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase

Abstract

The physical picture of the nematic conformation has been discussed on the basis of the rotational isomeric state analysis for segmented liquid-crystal (LC)-forming molecules, comprising mesogenic units on both sides of a flexible spacer. The disorientation angles (θ) of the two terminal mesogenic units are calculated for given spatial configurations of the spacer. The observed odd-even character of the thermodynamic quantities at the nematic (N)-isotropic (I) phase transition T NI ¼ ΔH NI /ΔS NI has been interpreted in terms of the profiles of the calculated distribution curves P(θ) À θ. Combined use of rotational isomeric state and 2 H NMR techniques has led to an estimate of the conformer fraction in the nematic state. The transition entropies derived on this basis are favorably compared with the constant-volume transition entropies obtained from the pressure-volume-temperature measurement. The observed V-T relation indicates that the expansivity of the nematic LC phase is higher relative to that of the isotropic melt. It has been pointed out that the nematic conformation might possibly gain extra stability from the free volume effect in the LC state. These considerations offer an explanation why amazingly long flexible chain segments can be accommodated in the nematic fluid.