Non axisymmetric LCD system by the isotropic phase to the biaxial nematic phase transition
|School||Nanjing Normal University|
|Course||Condensed Matter Physics|
|Keywords||biaxial nematic liquid crystal phase transition molecular interaction|
The research for liquid crystal phase transition of a system composed of traditional non-cylindrical liquid crystal molecules shows that the system could go from isotropic to uniaxial, and then from uniaxial to biaxial nematics when temperature decreases. The peak temperature of phase transition appears in a system when it goes directly from isotropic to biaxial nematics (Landau point). At this Landau point, system can most easily enter into biaxial phase. In such sense, the molecular structure satisfying Landau point requirement is the optimal structure. In order to study a wider molecular structural range in which system can enter directly from isotropic to biaxial, i.e., the Landau curve instead of a Landau point, we did the following work:(1) a system composed of cross-like nematic liquid crystal molecules is considered in this paper. 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 parameter 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, occurrence of Landau curve depend on the values of molecular interaction strengths.(2)Based on a microscopic molecular model consisted by three perpendicular oscillators, molecular interaction form and interaction coefficients are derived, and the coefficients are classified into four groups according to their variation range. Discussions are focused on the second and third group. Using mean field approximation and numerical calculations, order parameters in equilibrium state changing with temperature and molecular parameter are obtained, and four types of phase diagrams in the plane of temperature and molecular parameter are drawn. In the first type of phase diagram, the range of Landau curve where system enters from isotropic to biaxial phase directly is the largest, while in the second type, the range of Landau curve decreases. In both types, there exist two critical points connecting first to second order phase transitions. Yet in the second type, uniaxial phase region is larger than that in the first type. Besides, at two sides of Landau curve, two different uniaxial phases occur. In the third type, Landau curve shrinks to a point, while in the forth type, Landau point disappears. The above phase diagrams show that the phase transition types, appearance and the range of Landau curve depend strongly on the choice and range of interaction coefficients.