CHAPTER 1 INTRODUCTION
1.2 L IQUID C RYSTAL P HASES
Liquid crystal is a state between crystalline solid and amorphous liquid. It can be regarded as a liquid material with ordered molecular arrangement.
Liquid crystals are generated from organic substances with anisotropic molecules. The molecules are elongated (rod-like) or flat (disc-like). The
ordering of anisotropic molecules produces mechanical, electrical, magnetic and optical properties. Liquid crystal phases are formed by several types of molecules with widely different structures. Usually, the LC molecules applied to electro-optical techniques are rod-like. Disk-like, bend-shape, board-like molecular and other shapes are not discussed in this dissertation.
Nematic
The nematic liquid crystal phase is characterized by molecules that have no positional order but tend to point toward the same direction as shown in Fig.
1-1. In the nematic phase, molecules are optically uniaxial, i.e. oriented with their long axis aligned to the rubbing direction.
Smectic
Liquid crystals in the smectic phase show an additional degree of positional order with molecules ordered in layers. In the smectic A (SmA) phase, layer spacing approximately equals the length of molecule as shown in Fig. 1-1. In the smectic C (SmC) phase, molecules are arranged in layers as in the SmA phase with the directors tilted at a preferred angle to the layer normal.
This indicates that the molecule is confined to a conical surface of which the layer normal is the axis and the cone is the so-called smectic cone. During the phase transition from SmA to SmC LC molecules tilt at an angle Ө, The layer spacing in the SmC phase is therefore smaller than that in the SmA phase. This property is called layer shrinkage.
Fig. 1-1. Molecular arrangements of liquid crystal phases [1].
1.2.1 Chirality
Chiral molecules dissolve in achiral LCs inducing twisted LC structures.
The chiral molecule itself may or may not have a liquid crystalline phase. A chiral object cannot be superimposed by any translations or rotations onto its mirror image. This is usually due to the presence of an asymmetric carbon atom bonded to four distinct groups. Chiral molecules induce chirality in nematic or smectic phases by intra- and inter-molecular transfer [2]. In intramolecular transfer the center leads to a distortion of the structure of compounds and to the formation of various conformers. Intermolecular transfer may result in a variation of the molecular orientational distribution function.
Helical twisting power (HTP) can be utilized for the induction to chiral phases.
An approximate linear function of HTP is:
1 1
HTP p c
(1.1) where p is the helical pitch length, i.e. the distance it takes the director of the LC molecules to rotate a full turn as shown in Fig. 1-2. and c is the concentration of the chiral dopant.
1.2.2 Chiral Nematic
Chiral nematic (or cholesteric) liquid crystal (CLC) phase is composed of nematic mesogenic molecules which contain a chiral center. Usually a CLC is prepared by mixing a chiral dopant with an achiral nematic liquid crystal. The chirality produces an intermolecular force that leads to a twist between each layer. As a result, the director in the chiral nematic (N* ) phase is not only oriented uniformly in individual layers but also rotates in space around a helical axis perpendicular to the layer plane, as shown in Fig. 1-2. An important characteristic of a CLC is the helical pitch. A pitch length typically varies between a few hundred nanometers and many micrometers. It relates to the properties of selective reflection.
In a homogenously aligned cell LC molecules are well oriented with their helix axis parallel to the surface normal, the so-called planar state. At the planar state, a cholesteric liquid crystal with a periodic helical structure reflects incident light in line with Bragg’s theory. The optical properties of the selective reflection wavelengths follow these equations:
(1) the center reflected wavelength c navep, where nave (ne no) / 2, and p is the pitch length. ne and no are the extraordinary and ordinary refractive indices of the cholesteric liquid crystal respectively.
(2) the reflection bandwidth (nen po)
(3) the reflected wavelength shows a blue-shift with the incident light at a certain angle, normalcos , where normal is the reflected wavelength at normal incident,
is the incident angle with respect to the layer normal.When unpolarized light is applied to right-handed cholesteric liquid crystal film, right-handed circular polarized light is reflected at the wavelengths in the reflection band and also passes other lights.
Fig. 1-2. Illustration of a cholesteric liquid crystal structure [3].
1.2.3 Chiral Smectic C
Chiral smectic C (SmC*) phase is usually formed when chiral dopants are immersed in a SmC LC. The chiral dopants induce an intermolecular force leading to a twist between each layer. The SmC* phase is a continuous rotated layer structure with tilt angle θ, as shown in Fig. 1-3. The distance the director of LC molecules takes to rotate 360oC, is called pitch length.
Ferroelectricity is a characteristic that a material exhibits when the direction of a local polarization field is switched by coupling to an external electric field. R. B. Meyer et al. first demonstrated ferroelectricity in the chiral
smectic liquid crystal in the tilted phase [4]. Ferroelectricity is produced as a result of the unusual combination of symmetrical and asymmetrical properties.
Thus, chiral smectic C liquid crystal is a ferroelectric liquid crystal. Because of the asymmetric chemical structure in the SmC* phase each LC molecule has a polarization field, called spontaneous polarization (PS) the direction of which is perpendicular to the long axis of the molecule.
Fig. 1-3. Helix layer structure of a SmC* liquid crystal.