Study of the nematic and smectic A phases of N-p-cyanobenzylidene-p-n-octyloxyaniline in tubes

Abstract

Observations on a glass tube filled with the liquid crystal N-P-cyano-benzylidene-p-n-octyloxyaniline (CBOOA) and which has been treated so that the CBOOA lies radially at the innerwall are presented. We have cooled the CBOOA tube through the nematic-smectic A transition temperature, T_c . Our results may be summarized as follows :

(1) Cooling the splay-bend mode of the nematic phase towards T_c results in the Frank planar solution.

(2) The planar solution has been observed to nucleate at disclination points.

(3) Noting the temperature at which the planar solution is the more stable enables one to make estimates of the core radius r_c . These estimates are still in the primitive stage.

(4) In the smectic phase, the smectic layers form cylinders concentric with the glass tube. A core is observed. Sometimes ‘beads’ develop along the core.

(5) Cylindrical samples of a liquid crystal which has a larger latent heat at a nematic-smectic A transition give a focal conic domain texture no matter how slowly we cool the sample.

(6) Observations on a homoeotropic square tube indicate that the simplest topological solution is probable. It also supports the notion that a critical nematic bend ‘exists’. When the bend energy of the nematic phase at T_c is smaller than this value, curl n, where n is the director field, is identically zero in the resulting smectic phase. Bends above this value lead to focal conic textures.

(7) Wanning the cylindrical sample back through T_c always results in a planar configuration. If part of a negative point disclination is available, the planar solution reverts to the splay-bend mode. If not, it is stable.