Materials with Positive Dielectric Anisotropy

The dielectric anisotropy Ae of LC-materi-als is defined by Ae= j -ex, where and ej. are the dielectric constants parallel and perpendicular to the director. From the Maier and Meier theory it can be seen that both the polarizability anisotropy Aa and the permanent dipole movement /i of the LC molecule determine the dielectric anisotropy 

Currently available materials with highly positive Ae are mainly based on benzo-nitrile structures (Tables 4 and 5). Introduction of heterocyclic ring systems (Structures 2.5 and2.6)or ester links(Structures2.7 and 3.5) lead to an increase of Ae compared with the corresponding non-heterocyclic and directly connected systems. Unfortunately, the effectivity regarding Ae of the benzoni-trile structures is decreased by a local antiparallel ordering of the dipole moments 

Ihble 8. Commercially used materials with large elastic constant ratio K IKn. 

Compared with the corresponding neutral structures the rotational viscosity, /j, of polar substituted LC - structures is increased (compare Structures 2.2Z7.4, 3.3/S.3 and 3.5/S.6). As given in Eqs. (19) and (20) this leads to an unwanted increase of the switching time. Therefore it is practice [89] in the design of mixtures for TN cells to include compounds with both large and small positive Ae. The use of polar three ring materi- 

Fluoroaromatic polar LC-materials (Table 6) with low to moderate Ae show a better ratio of /j to clearing point. Owing to their high resistivity even at higher temperature these materials are especially useful in active matrix devices (TFT). 

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In a liquid crystal cell based on the TN mode, a homogeneously aligned layer of a nematic liquid crystalline material with positive dielectric anisotropy (Ac), helically twisted by 90°, is placed in an ITO-lined glass cell between crossed polarizers... 

Typical temperature dependences of the dielectric constants and q, are shown in Figure 8.1a for a nematic material with positive dielectric anisotropy. 

For a nematic material with positive dielectric anisotropy, induced birefringence can also be observed. However, the liquid crystal must be in the uniform parallel orientation at zero volts.Above the threshold voltage, the director aligns itself parallel to the applied field. With crossed polarizer and analyzer, the voltage dependence of the light intensity is reversed from that described previously for a fluid of negative dielectric anisotropy. ... 

The critical cutoff frequency for both conduction-induced and field effect phenomena is exponentially dependent on temperature. In the former case, the temperature dependence occurs because the cutoff frequency is proportional to the conductivity, whereas for field effect materials with positive dielectric anisotropy the exponential temperature dependence is a property of the molecular relaxation process which is responsible for the cutoff frequency. 

TABLE 1 Standard materials with positive dielectric anisotropy. 

The threshold in equation (6.143) above has an analogous formulation in terms of an electric field. Using the substitution fioAxH sw 6 CoeaE sm O replacing the magnetic field by an electric field for a material with positive dielectric anisotropy gives the critical threshold ... 

Case A planar alignment, Ca <0 0. This is the most studied, classical case, since the conductivity anisotropy of usual nematics (substances without a smectic phase) is typically positive. As for , there is a wide range of materials with negative dielectric anisotropy. 

Several cases of dielectric, hydrodynamic, and flexoelectric instabilities and domain structures have been observed and extensively studied in CLCs. Their appearance depends on the initial orientation of molecules, the physical parameters of the material, and the applied electric field. In CLCs with positive dielectric anisotropy Ae > 0, an electric field applied along the helix axis of a planar (Grandjean) texture can induce a two-dimensional spatially periodic deformation which has the form of a square grid [96], The period and threshold voltage of this field-induced instability depend on the elastie constants, the dieleetric anisotropy, and the sample thickness [97],... 

An alternative write/erase mechanism that may be exploited to provide a scattering display in smectic liquid crystals [60] relies on the electrothermal reorientation of a material of positive dielectric anisotropy (Fig. 14). The thermal input to the material can be provided either by ohmic heating of stripe electrodes [61] in the cell, or by scanning a visible or infrared laser spot across the device. A dye can with profit be incorporated into the liquid crystal [62] to improve the absorption of the incident light. Typically, the liquid crystal used will pos-... 

Fig. 1—Field effect (lateral electrode geometry). The change in optical orientation properties with frequency is a result of changing field penetration. The dielectric relaxation frequency is 80 Hz. This material of positive dielectric anisotropy does not show any turbulent flow (R.A. SoreP).

In materials of positive dielectric anisotropy, most electro-optic phenomena are frequency independent field effects hydrodynamic effects, occurring with certain boundary conditions result in stable (laminar) flow, and no turbulent-flow reorientation is observed. ... 

Table 4. Commercially used two-ring materials with high positive dielectric anisotropy.

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