Synchrotron X-ray study of the phase transitions in liquid crystal polyesters derived from p,p '-bibenzoic acid and racemic- and (R)-3-methyl-1,6-hexanediol

The phase transitions in two liquid crystalline polyesters have been studie d by diffraction techniques employing synchrotron radiation. The two polyes ters, derived from p,p'-bibenzoic acid and 3-methyl-1,6-hexanediol, only di ffer on the optical activity of the chiral spacer: both the active and race mic polymers have been investigated. Time-resolved experiments at variable temperature were performed, analyzing the cooling from the isotropic melt a nd the subsequent melting. The analysis of the different diffraction profil es (covering WAXS, MAXS, and SAXS regions) shows important differences in t he phase behavior of the two polyesters, more important than those anticipa ted from the analysis of the DSC curves. Nevertheless, a common feature is the obtention of Sc mesophases in both polymers, characterized by a continu ous increase of the tilting angle as the temperature is decreased (before t he appearance of high-order mesophases).