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用准确可靠的DSC法研究了对位取代苯甲酸对苯二酯类的五个仔细纯化过的液晶化合物,得到了相变的温度、焓及熵的数据.据此给出液晶性质和分子结构关系的规律.指出不能用分子增宽导致分子间色散引力减弱的效应解释溴取代侧基的影响,相变温度的降低是熵效应的结果.用分子对称性和S_c、N相稳定性的观点解释了S_c相消失和相变温度降低的现象.从分子对称性和氰基极性观点讨论了中心苯环2,3-位氰基取代基对T_m和T_c的影响;指出烷氧基比烷基更有利于向列相的热稳定性.
Five carefully purified liquid crystal compounds of para-substituted p-phenylbenzoates were studied by an accurate and reliable DSC method, and the data of the temperature, enthalpy and entropy of the phase transition were obtained. The liquid crystal properties and molecular structure We can not explain the effect of bromine substitution on side groups and the decrease of phase transition temperature is the result of entropy effect.Based on the molecular symmetry and the stability of S_c and N phase The phenomenon of the disappearance of S_c phase and the decrease of phase transition temperature was explained.The influence of the 2,3-position cyano substituents on the central benzene ring on T_m and T_c was discussed from the viewpoint of molecular symmetry and cyano polarity. The radical is more conducive to the thermal stability of the nematic phase.