Morphological studies of oriented specimens of thermotropic liquid-crystalline polyesters with two-dimensional mesogenic units

Abstract

Morphological studies have been carried out in detail by means of differential scanning calorimetry, polarizing and scanning electron microscopies, small-angle light scattering, X-ray diffraction, infra-red spectroscopy and refractive-index measurements for oriented specimens of thermotropic liquid-crystalline polyesters with so-called two-dimensional mesogenic units along the polymer chains, which were synthesized by solution polycondensation of the monomers 4,4′-(alkylene dioyldioxy)dibenzoyl dichloride and 2,5-dihydroxyl-1,4-phenylene di- p-alkyloxybenzoate at low temperatures. All the studied polyesters, designated respectively as P(2,6), P(2,8) and P(5,8), were found to exhibit nematic liquid-crystalline behaviour above their melting points. Mat-like or banded textures have been observed in oriented films of these specimens, which were prepared by shearing in the mesomorphic state and subsequent 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 average bandwidths evaluated optically were around 6, 8 and 12 μm, respectively. These three specimens had different molecular weights, and such differences were found to influence strongly the features and ease of formation of the banded structure: the bandwidth was decreased and band 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 deviation angle of about ±30°, on going from band to band. Based on the scanning electron microscope observations and X-ray diffraction data, it was understood that the zigzag planes of oriented fibrils, which have average diameter ranging from 50 to 100 nm, are parallel or nearly parallel to the specimen surface, and they are stacked layer-like along the specimen thickness direction. Moreover, it was suggested from the three-dimensional infra-red spectra and refractive-index data that the planes including the intersected X-shaped mesogens may be oriented parallel or nearly parallel to the zigzag plane or film surface, and all the phenylene rings tend to orient their planes parallel to the zigzag plane as a whole. The banded texture and its interior structure were found to be quite thermally stable even at temperatures somewhat higher than the isotropization point of the unoriented polymer, suggesting that the thermal motions of individual polymer chains are severely limited and do not easily destroy the fibrillar structure and other supermolecular structures formed in the process of preparing oriented films.