On understanding the optical extinction behaviour of uniaxially sheared liquid crystalline polymers

Publisher Website
Google Scholar
Abstract
When fibres or thin unidirectionally sheared films of a liquid crystalline polymer are viewed between crossed polars in the optical microscope, a characteristic banded texture is often seen. We have observed that the appearance of this texture may depend on whether the shear direction is parallel to the transmission direction of the polarizer or the analyser, and also on the orientation of the crossed polars relative to the microscope. We examine possible factors which could account for these apparently anomalous results for specimens with a one-dimensional periodic microstructure. Anisotropy of the transmission coefficient for diffracted light, at interfaces in the space between the specimen and the objective, is particularly important. The effect of such anisotropy is thickness dependent in specimens which are optically dichroic. Similar polarization effects may be introduced by beam splitters in the microscope. It is also significant that the transmittance of the analyser depends on the azimuth and obliquity at which light of a given polarization enters it. One finally has to take into account that the longitudinal and transverse refractive indices may not vary in the same way with position along the shear direction.