Structure of the nematic mesophase with cybotactic groups in liquid-crystalline poly(urethane-ester)s

The results of a structural analysis by x-ray diffraction on fiber samples of the nematic mesophase with cybotactic groups of poly(urethane-ester) TDI-C2C6, TDI-C6C4, TDI-C6C6, TDI-C6C8, and TDI-C6C10 as prepared from various mesogenic alkylene di[4-(omega-hydroxyalkyloxy-4-oxybenzoyl)oxybenzoate]s (CmCn; m = 2 or 6 and n = 4, 6, 8, or 10) and 2,4-toluenediisocyanate (TDI) are reported. Evidence is provided for the existence of two different molecular orderings within the nematic phases with cybotactic groups, to which different internal degrees of correlation correspond. The average dimension of the cybotactic clusters along the director is comprised between similar to 260 and similar to 560 Angstrom, whereas the average dimension perpendicular to the director ranges from similar to 130 to similar to 270 Angstrom. Accordingly, the cybotactic clusters present an asymmetrical ellipsoidal-like shape. On the other hand, the correlation length within the nematic liquid crystal surrounding the cybotactic clusters and along the director is between 38 and 45 Angstrom. These values are of the order of the molecular length. These findings highlight the multidomain nature of the mesophase consisting of large and asymmetric clusters surrounded by a conventional nematic endowed with a very low correlation degree.