Semi-flexible liquid crystalline polyesters based on twin di(p-oxybenzoyl) units x-ray study on smectic mesophase structures

An X-ray study is performed of oriented smectic mesophases of several polyesters consisting of two p-oxybenzoyl diads interconnected by a pentamethylene segment and a polymethylene segment of variable length. The X-ray diffraction patterns are very similar for all the samples and are constituted by one or more inner reflections, lying on the meridional line, and by two broad outer haloes split into two intense portions symmetrically placed above and below the equatorial line. The former reflections are attributed to the regular piling of the smectic layers; the latter are associated with the presence of rigid mesogenic groups that are tilted with respect to the layer normal, while the average chain axis lies perpendicular to the layer planes. Accordingly, the analysed smectic mesophases can be classified as unconventional smectic C. A structural model is proposed to account for the intensity profile of the X-ray patterns.