Effect of Slow Cooling on a Discotic Nematic Liquid Crystal: Evidences for Nematic–Nematic Transitions

Abstract

There is considerable interest in discotic liquid crystals because of their possible applications in liquid crystal displays and in one-dimensional conductors. In view of these applications, knowledge and control of their phase behavior and transition temperatures are crucial. We have studied a discotic nematic liquid crystal, namely, hexakis(4-nonylphenylethynyl)benzene. The phase sequence reported for this sample on cooling is isotropic (I) 81.8 C discotic nematic (N D ) and 39 C crystal (Cr). The phase sequences were obtained using differential scanning calorimetry (DSC) and optical polarizing microscopy (OPM) studies while varying the sample temperature rapidly (5 C=min). However, on slowly cooling the sample (0.36 C=min), we observed anomalous optical textures and DSC peaks in the nematic region at 75 C and 52 C. We also find discontinuous changes in the optical transmission intensity and diamagnetic anisotropy in the vicinity of these temperatures on slow cooling. Further, the XRD pattern showed small but distinct shifts in the lateral (100) peaks in the vicinity of these transitions. We infer that the variations in the lateral distance between the discotic molecules may be responsible for these observed nematic-nematic (N-N) transitions.

Liquid Crystal Phase Behaviour of Attractive Disc-Like Particles

“…A similar progression from V-L-N, through I-N, to I-N 1 -N 2 phase behaviour with increasing aspect ratio has been observed for attractive rod-like LC molecules [77,93–98]. The coexistence between two nematic phases has been found experimentally in studies of hexa-alkylbenzene derivatives of discotic mesogens [99] and in solutions of the calamitic polypeptides [95], both of which are characterized by extreme oblate and prolate aspect ratios. It is also interesting to note that an analogous transition in the vapour-liquid-solid phase equilibria is exhibited by attractive spherical particles as the range of interaction is decreased: the VLE is found to become metastable relative to the isotropic fluid-solid transition, and on further decreasing the attractive range, an iso-structural coexistence between two solid phases is observed [100,101].…”

An analytical equation of state for describing isotropic-nematic phase equilibria of Lennard-Jones chain fluids with variable degree of molecular flexibility

“…1 and 3. derivatives of discotic LCs. 53 Very recently, Wu et al 42 successfully used a coarse-grained representation for PBLG in the framework of an Onsager-van der Waals theory to correlate the experimental data of the system of PBLG in dimethylformamide. 46 In Fig.…”