Banded and torsad textures of sheared liquid crystalline polymers and their anisotropic mechanical properties.

摘要

The periodic structures formed in liquid-crystalline polymers after the cessation of shear flow in some cases consist of a primary set of bands perpendicular to the shear direction and a second set of bands (called "torsads") that are slightly inclined to that direction.; In the first chapter, models are given for the formation of bands and torsads. Because it has been suggested that the torsads might be related to the rewinding of the cholesteric structure, the relationship between the pitch of the cholesteric array and the torsad spacing is investigated thoroughly and reported in chapter 2. It is found, however, that the torsads occur as a consequence of the hydrodynamic flow associated with evaporation of the solvent. The apparent dependence of torsad spacing on pitch is a consequence of the related changes in the materials parameter of the system.; In the third chapter, variations in the banded texture of sheared liquid crystal polymer films are shown to control the anisotropy of their mechanical properties. Films, prepared by shearing lyotropic liquid crystal polymer solutions of 2-hydroxypropyl cellulose, were submitted to elongation both along and normal to the shear direction. The films were found to be brittle along the shear direction and ductile in the transverse direction. Stretch along the shear direction does not significantly affect the molecular orientation for any of the films. However, in the transverse direction, profound molecular reorientation occurs on stretching the film, resulting in a fibrillar structure, except at very small band wavelengths. After the disappearance of the original bands due to the deformation in this transverse direction, new bands appear which are of a much smaller wavelength.; In the Appendix, preliminary investigations of the molecular order in HPC films and the diffusion properties of these films are reported.