HIGH-PRESSURE DIFFERENTIAL THERMAL-ANALYSIS OF LATERALLY SUBSTITUTED NEMATIC LIQUID-CRYSTALLINE COMPOUNDS

By means of high pressure thermal analysis we measured the pressure de pendence of the clearing temperatures of nematic, 2-substituted hydroq uinone-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 clea ring lines decrease strongly with increasing pressure, indicating a pr onounced decrease of the transition volume with increasing pressure. C ompounds with long chain lateral substituents exhibit liquid crystalli ne behaviour because the lateral substituents tend to have an orientat ion parallel to the molecular long axes. We expected that the preferre d orientation of the lateral substituents would be strengthened by hig h pressure, demonstrated by enhanced dT/dp values. However, despite th e strong deviation of the compounds from the ideal rod-like shape, the magnitudes and the trend of the data are similar to those of classica l nematics.