分子双轴性与液晶向列相的最高过热温度

Molecular Biaxiality and Highest Superheating Temperature for Nematic Liquid Crystals

向列相 (N) 各向同性相 (I)相变为弱一级相变 ,其相变潜热大约比溶化的相变潜热小2个数量级。在这种弱一级相变附近焓随温度的变化表明 :各向同性相最低过冷温度T 、N I相变温度TC 和向列相最高过热温度T 彼此十分接近。本文通过分子统计理论研究这一现象。形成单轴向列相的液晶分子并不具有D∞ 对称性。在分子场近似下和二粒子集团近似下 ,随分子双轴性参数增大 ,T /T 增大并趋近于 1。

The nematic(N)-isotropic(I) phase transition is weakly first order with quite small latent heat,about two orders of magnitude smaller than the latent heat at the melting transitions. Following this weakly first order phase transition, the temperature dependence of enthalpy near the transition indicates that the lowest supercooling temperature of the isotropic phase T~*, the N-I phase transition temperature T _C and the highest superheating temperature of the nematic phase T^(**) are very close to each other. This phenomenon was studied by means of the molecular statistical theory. The liquid crystal molecules forming uniaxial nematics do not posses D_∞ symmetry. When the molecular biaxial parameter increases, the T~*/T^(**)ratio increases and approaches to 1, in both the molecular field approximation and the two-particle cluster approximation.