INVESTIGATION ON THE FORMATION BEHAVIOR OF BANDED TEXTURE IN THERMOTROPIC MAIN-CHAIN LIQUID-CRYSTALLINE POLYMERS WITH LINEAR ROD-LIKE AND 2-DIMENSIONAL MESOGENIC UNITS

The mechanism and the formation behaviour of banded texture in non-iso thermal processes have been studied for two aromatic main-chain liquid crystalline 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 ob served within its oriented films which were prepared by shearing in me somorphic state and subsequent cooling down to room temperature under various conditions. Bandwidth was dependent sensitively on the cooling conditions, about 8 mu m for rapid and 2 mu m for slow coolings. Duri ng the cooling of an oriented film the bands were first generated arou nd 170 degrees C, and their regularity was improved with lowering temp erature. The bandwidth as well as the extinction angle of the bands we re changed drastically. In such a cooling process the birefringence an , the difference between the refractive indices along the shear and la teral directions, of the oriented film was decreased gradually from ab out 0.06 to 0.03. In the case of PTDT-Br, clear and perfect banded tex ture could be observed only for the rapid cooling case. The banded for mation behaviour was discussed on the basis of a contraction mechanism and it could be explained as the result of zigzag rearrangement of st raightforward oriented fibrils under certain contraction effects. The origins of these effects were considered to be the elastic energy stor ed in the specimens during shearing and the thermal contraction during cooling. The latter was more evident in the rapid cooling case. The o rientational 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 soli dification of specimens. Copyright (C) 1996 Elsevier Science Ltd.