Investigation on the formation behaviour of banded texture in thermotropic main-chain liquid crystalline polymers with linear rod-like and two-dimensional mesogenic units

Abstract

The mechanism and the formation behaviour of banded texture in non-isothermal processes have been studied for two aromatic main-chain liquid cyrstalline polyesters designated as P(2,8) and PTDT-Br which contain the X-shaped and linear rod-like mesogens along the polymer backbones, respectively. For P(2,8) regular and perfect banded textures were observed within its oriented films which were prepared by shearing in mesomorphic state and subsequent cooling down to room temperature under various conditions. Bandwidth was dependent sensitively on the cooling conditions, about 8 μm for rapid and 2 μm for slow coolings. During the cooling of an oriented film the bands were first generated around 170°C, and their regularity was improved with lowering temperature. The bandwidth as well as the extinction angle of the bands were changed drastically. In such a cooling process the birefringence Δn, the difference between the refractive indices along the shear and lateral directions, of the oriented film was decreased gradually from about 0.06 to 0.03. In the case of PTDT-Br, clear and perfect banded texture could be observed only for the rapid cooling case. The banded formation behaviour was discussed on the basis of a contraction mechanism and it could be explained as the result of zigzag rearrangement of straightforward oriented fibrils under certain contraction effects. The origins of these effects were considered to be the elastic energy stored in the specimens during shearing and the thermal contraction during cooling. The latter was more evident in the rapid cooling case. The orientational relaxation of fibrils or the formation of banded texture occurs during cooling in a small temperature range after an ‘induction stage’, while the relaxation due to free thermal motion of individual molecules may proceed in the whole temperature region before the solidification of specimens.