High pressure differential thermal analysis of laterally substituted nematic liquid crystalline compounds

Abstract

By means of high pressure thermal analysis we measured the pressure dependence of the clearing temperatures of nematic, 2-substituted hydroquinone-bis-(4-substituted benzoates) up to pressures of about 500 MPa. The clearing lines are strongly non-linear and can be well reproduced by a modified Simon-Glatzel equation. The gradients dT/dp of the clearing lines decrease strongly with increasing pressure, indicating a pronounced decrease of the transition volume with increasing pressure. Compounds with long chain lateral substituents exhibit liquid crystalline behaviour because the lateral substituents tend to have an orientation parallel to the molecular long axes. We expected that the preferred orientation of the lateral substituents would be strengthened by high pressure, demonstrated by enhanced dT/dp values. However, despite the strong deviation of the compounds from the ideal rod-like shape, the magnitudes and the trend of the data are similar to those of classical nematics.