Textural Analysis of a Mesophase with Banana Shaped Molecules

TL;DR

This paper investigates the complex textures and defect structures of the B7 mesophase in banana-shaped liquid crystal compounds, revealing the coexistence of different states and their relation to the phase's unique columnar and smectic nature.

Contribution

It provides a detailed analysis of defect textures in the B7 phase, linking them to its dual columnar and smectic structure, and distinguishes between metastable and ground states.

Findings

Identification of two defect types: helical ribbons and developable domains.

Helical ribbons are linked to a metastable state, while developable domains correspond to the ground state.

Textures are consistent with a model involving screw dislocations with large Burgers vectors.

Abstract

Observed under the polarizing microscope, the B7bis phase in the banana compound D14F3 [J.P. Bedel et al, Liq. Cryst., 27, 1411 (2000)] displays two types of textures of defects, namely (a): helical ribbons, that nucleate in large quantities when the samples are quenched from a sufficiently high temperature in the isotropic phase (b)- shapes with no helicity having the structure of developable domains much akin to those observed in columnar phases, either resulting from the annealing of the helical ribbons or nucleating under slow cooling processes. The existence of these two kinds of defects points toward the complex nature of the structure of the B7 phase, which is at the same time a columnar and a smectic phase. Our observations fit the model [M. Kleman, J. de Phys., 46, 1193 (1985)] according to which the geometry of a helical ribbon is that one of the central region of a screw dislocation with a giant Burgers vector, split into two helical disclination lines of strength k = 1/2 which bound the ribbon. Textures and defects, already partly documented, and growth features and annealing processes, not yet reported in the literature, are analyzed. We conclude that the helical ribbons and the developable domains with no helicity are textures of two different B7 states, namely a metastable state and the ground state respectively. Comparative textural analysis is performed for two other banana compounds exhibiting B2 phases.