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Orientational Order

  • Ronald Y. Dong
Part of the Partially Ordered Systems book series (PARTIAL.ORDERED)

Abstract

One of the characteristic features of liquid crystals is the existence of longrange orientational order in a mesomorphic phase. Molecules are preferentially aligned along a particular direction in space labeled by a unit vector n known as the local director. A set of microscopic order parameters was introduced in Chapter 2 for rigid molecules in a uniaxial phase. They may be determined from the splitting or shift of spectral lines in NMR spectra of liquid crystalline molecules. A typical difference between the isotropic phase and the nematic phase is that, in the latter, the measurement of all macroscopic tensor properties is possible. Thus, a macroscopic property can be used to define order parameters without requiring prior knowledge of molecular symmetry or invoking molecular rigidity. Given certain models for the molecules, the macroscopic order parameters may be related to the microscopic order parameters. The macroscopic order parameters are introduced in Section 3.1. The ordering of molecules in mesophases may be described by an orientational distribution function (Section 3.2), which depends in general on three Eulerian angles (Æ, θ, ψ) as illustrated in Fig. 3.1. It is often difficult to experimentally determine the full distribution function. An expansion of this distribution function in terms of microscopic order parameters is possible. Therefore, it is important to measure these orientational order parameters in order to determine the orientational distribution function. In Section 3.3 mean-field theories for molecules in uniaxial phases are outlined. These molecular theories can predict the temperature dependences of order parameters for rigid solutes and solvent molecules. NMR studies of orientational order in biaxial mesophases are presented in Section 3.4. This chapter ends with the NMR study of orientational order of rigid solutes in liquid crystalline solvents and of liquid crystalline molecules in uniaxial mesophases (Section 3.5).

Keywords

Liquid Crystal Dipolar Coupling Nematic Phase Orientational Order Local Order Parameter 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer Science+Business Media New York 1997

Authors and Affiliations

  • Ronald Y. Dong
    • 1
  1. 1.Department of Physics and AstronomyBrandon UniversityBrandonCanada

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