十字形液晶分子系统各向同性相至双轴向列相的相变

Phase Transitions From Isotropic to Biaxial Phase of Cross-Like Nematic Liquid Crystal Molecules

本文考虑十字形液晶分子组成的系统,假设分子间相互作用为棒与棒相互作用的叠加,且相互作用强度系数彼此独立.利用平均场理论,得到系统在温度和分子结构参数平面内的3类相图:第一类相图中只有一个直接由各向同性相进入双轴相的Landau点,相变为二级相变.第二类相图中Landau点扩展为Landau曲线,为一级相变.第三类相图中,Landau点同样扩展为Landau曲线,但范围缩小.上述3类相图表明,相变类型,Landau曲线的出现与分子间相互作用强度系数的选取有关.

A system composed of cross-like nematic liquid crystal molecules is considered.Suppose the molecular interaction is the superposition of interactions among molecular rods,and the strengths of interactions are independent.Within the mean field theory,three types of phase diagrams in the plane of temperature and the molecular structure parameters are obtained.In the first type of phase diagram,there appears only one Landau point at which system enter directly from isotropic to biaxial phase by second order phase transition.In the second type,Landau point becomes a Landau curve where system enters from isotropic to biaxial phase by first order phase transition.In the third type,the Landau curve still exists,but within a smaller range.The above phase diagrams show that the types of phase transitions and occurrence of Landau curve depend on the values of molecular interaction strengths.