MORPHOLOGICAL-STUDIES OF ORIENTED SPECIMENS OF THERMOTROPIC LIQUID-CRYSTALLINE POLYESTERS WITH 2-DIMENSIONAL MESOGENIC UNITS

Morphological studies have been carried out in detail by means of diff erential scanning calorimetry, polarizing and scanning electron micros copies, small-angle light scattering, X-ray diffraction, infra-red spe ctroscopy and refractive-index measurements for oriented specimens of thermotropic liquid-crystalline polyesters with so-called two-dimensio nal mesogenic units along the polymer chains, which were synthesized b y solution polycondensation of the monomers 4,4'-(alkylene dioyldioxy) dibenzoyl dichloride and 2,5-dihydroxyl-1,4-phenylene di-p-alkyloxyben zoate at low temperatures. All the studied polyesters, designated resp ectively as P(2,6), P(2,8) and P(5,8), were found to exhibit nematic l iquid-crystalline behaviour above their melting points. Mat-like or ba nded textures have been observed in oriented films of these specimens, which were prepared by shearing in the mesomorphic state and subseque nt quenching to room temperature. More regular and perfect bands were exhibited in specimen P(2,8) than in P(5,8) and P(2,6), and their aver age bandwidths evaluated optically were around 6, 8 and 12 mum, respec tively. These three specimens had different molecular weights, and suc h differences were found to influence strongly the features and ease o f formation of the banded structure: the bandwidth was decreased and b and quality was improved with increasing molecular weight. Furthermore , bandwidth could also be changed sensitively on varying the shearing rate during preparation of oriented specimens. In the case of P(2,8), the orientation direction of fibrils aggregated by polymer chains was found to deviate from the shearing direction alternately by a deviatio n angle of about +/- 30-degrees, on going from band to band. Based on the scanning electron microscope observations and X-ray diffraction da ta, it was understood that the zigzag planes of oriented fibrils, whic h have average diameter ranging from 50 to 100 nm, are parallel or nea rly parallel to the specimen surface, and they are stacked layer-like along the specimen thickness direction. Moreover, it was suggested fro m the three-dimensional infra-red spectra and refractive-index data th at the planes including the intersected X-shaped mesogens may be orien ted parallel or nearly parallel to the zigzag plane or film surface, a nd all the phenylene rings tend to orient their planes parallel to the zigzag plane as a whole. The banded texture and its interior structur e were found to be quite thermally stable even at temperatures somewha t higher than the isotropization point of the unoriented polymer, sugg esting that the thermal motions of individual polymer chains are sever ely limited and do not easily destroy the fibrillar structure and othe r supermolecular structures formed in the process of preparing oriente d films.