The parabolic focal conic texture in a lyotropic liquid-crystalline polymer

Abstract

Hydroxypropyl cellulose (HPC) forms liquid-crystalline solutions in both water and methanol. When these solutions are sheared, the banded texture seen in many liquid-crystalline polymers is formed. Detailed optical microscopy of the bands has been carried out, showing that the bands arise from a smooth variation in the orientation of the local extinction directions, and not (as previously suggested in the literature) from sharp kinks. Over a period of time, the bands degenerate to form a new texture, which appears as a square grid pattern of extinction when specimens are viewed between crossed polars. The location of the corners of the squares depends on whether the microscope is focused near the top or bottom surface. When focus is altered to the middle of the sample, the periodicity of the pattern is halved. This texture is identified with the parabolic focal conies previously seen in small molecule smectic and cholesteric materials.