Effect of slow cooling on a discotic nematic liquid crystal: Evidences for nematic-nematic transitions

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.