Light Scattering Study of Structures in a Smectic Liquid Crystal

The effect of surface treatment, sample thickness, and cooling rate on the structural order in the smectic phase of p-n-octyl-p′-cyano-biphenyl has been investigated by light scattering, and the results have been interpreted in terms of theories developed for crystalline polymers. Fan-shaped superstructures are found when the long dimension of the liquid crystal molecules is aligned parallel to the wall of a cell. But banded two-dimensional spherulites exist when the long dimension of the molecules is aligned perpendicular at one wall and parallel at the opposite wall. Uniaxially oriented rodlike structures are observed when the wall surface is lapped. Because surfaces have a strong effect, the superstructures are less highly ordered in thick than in thin cells. With an increase in cell thickness, the size of the superstructures increases along with the degree of disorder. If a sample is cooled rapidly to the solid state and then heated to the smectic state, the order in the smectic liquid crystal is reduced, as shown by a marked increase in the effect of density fluctuations on light scattering patterns. In a sample cooled slowly from the nematic to the smectic state, light scattering results primarily from orientation fluctuations.