Disclination and molecular director studies on bowlic columnar nematic phase using mosaic-like morphology decoration method

Abstract

Two bowlic cyclotriveratrylene CTV-1 and CTV-2, with different peripheral groups of -OCH 3 and -OCH 2 CH 3 for CTV-1 and -OCH 3 and -OCH 2 COOCH 3 for CTV-2, respectively, were synthesized by typical trimerization via a multistep sequence from vanillin. Both bowlic CTV molecules were thermotropic liquid crystals, and presented typical grainy textures of the nematic phase and homogeneous texture of the single domain nematic phase. It is of interest to observe the regular and beautiful mosaic-like morphologies after cooling from liquid crystalline phases, which appeared and vanished repeatedly in several circles of cooling and heating. The size of each mosaic was several dozens of micron. In nature, the mosaic-like morphologies are the optical pattern of cracks formed by the shrinking, due to the crystallization of frozen texture of nematic phases. By means of scanning electron microscopy, the mosaic-like morphologies were observed to consist of lamellae, and each mosaic is a rectangular multi-layer lamella, which is composed of packed single-layered lamellae. The fibrils in the diameter of about 1 μm were observed, which are the structural units of lamellae and would be the bundles of the bowlic molecular columns. The mosaic-like morphologies decorate the bowlic molecular columnar nematic phase, therefore, a novel mosaic-like morphologies decoration method was applied to reveal the director distribution of several kinds of point disclinations, such as s = +1(δ =0°and δ =90°) and s = ±1/2, and Nèel domain walls. It was shown that the bowlic molecular columnar nematic phase behaved as normal nematic phases; however, the basic structural units ordered were the bowlic molecular column or the bundles of bowlic molecular column (i.e. fibrils), but not the bowlic molecules themselves. The bowlic molecular columns acted as the rod-like molecules in a normal nematic phase. Therefore, a new term B CN (bowlic columnar nematic phase) is used to describe the anomalous nematic phase in this paper.

Bowlics: history, advances and applications

“…Interestingly, these mesophases were found to exhibit an uncommon feature in that their optical anisotropy changed sign as function of temperature within the mesophase region. They assumed that these phases had columnar structures similar to those occurring in the conventional columnar discotics, which were confirmed by X-ray characterisations [43,59,60].…”