Configurational Aspects of the Odd-Even Effect in Thermotropic Liquid Crystalline Polyesters

Conformational analysis has been performed on semiflexible polyesters having repeatunits such as-[X-O-CO(CH2)nCO-O]-and-[X-CO-O(CH2)nO-CO]-. These polymers are known to exhibit thermotropic mesophases when (aromatic) rigid cores X are sufficiently anisotropic. Spatial orientations of a given core have been elucidated in a Cartesian coordinate system fixed to the preceding core. The angle θ defined by unit vectors attached to two successive rigid cores has been evaluated for each conformation of the intervening flexible segment. When the number of methylene units n in the flexible segment is even, the angle is θ found to be distributed in the range 0-30° (30-40%) and 85-130° (60-70%). For polymers with = odd, the major portion of the calculated angle θ is located in the region 50-90°, and to a varying degree (0-20%), orientations are also permitted in the range θ > 160°. Only the = even polymers conform to the concept of an ordinary nematic ordering. Based on these results, an explanation has been offered for the observed odd-even oscillation in the entropy change ASni at the isotropization temperature. For the n = even polymers, the orientational and combinatorial contributions to the entropy change ∆Sni at the nematicisotropic transition have been estimated from the calculated distribution curves within the framework of the modified Flory-Ronca theory. The results are compared with experimentaldata reported in the literature. © 1984, American Chemical Society. All rights reserved.