Negative dielectric anisotropy

Note 1 The nematic liquid crystal must have a negative dielectric anisotropy (Af < 0), and a positive anisotropy (Aa > 0). The optical texture corresponding to the flow pattern consists of a set of regularly spaced, black and white stripes perpendicular to the initial direction of the director. These stripes are caused by the periodicity of the change in the refractive index for the extraordinary ray due to variations in the director orientation. 

In the case of those LC materials which exhibit negative dielectric anisotropy, cells can be constracted which align vertically and twist on applying a field, exactly the converse to the twisted nematic effect from positive anisotropic LCs. Cells of this type are of interest because they can form a superior black state. 

A new class of liquid crystals with strongly negative dielectric anisotropy was explored by employing the ambivalent characteristics of the 1,3-dioxane moiety <2006EJ04819> due to both the polarity of 1,3-dioxane and axial fluorination, compounds 238-240 proved to have very useful mesogenic and electrooptical properties. 

Dynamic scattering displays, which are the first generation LCDs, make use of a nematic LC with a negative dielectric anisotropy (As = — <0) doped with an... 

Recently, the surface tensor model has been used together with the dielectric continuum model to calculate the orientational order parameters of solutes in nematic solvents [8,9,27], Figure 2.32 shows the theoretical results for anthracene and anthraquinone in nematic solvents with different dielectric anisotropy. Considering only the surface tensor contribution, positive Szz and Sxx and negative are obtained, with Szz > Sxx > Syy. This corresponds to what could be expected on the basis of the molecular shape the long axis (z) is preferentially aligned with the director, and the normal to the... 

A nematic liquid crystal of negative dielectric anisotropy is aligned with the director aligned orthogonal to the cell walls by means of a surfactant orientation layer, see Figure 3.4. One or two linear, elliptical or circular polarisers are... 

Another version of the Heilmeier and Zanoni GH-LCDs with positive contrast is essentially the inverse of the Heilmeier and Zanoni GH-LCDs with negative contrast described above. A nematic mixture of negative dielectric anisotropy incorporating a dichroic dye of negative contrast is aligned parallel... 

The application of an electric field above the threshold value results in a reorientation of the nematic liquid crystal mixture, if the nematic phase is of negative dielectric anisotropy. The optically active dopant then applies a torque to the nematic phase and causes a helical structure to be formed in the plane of the display. The guest dye molecules are also reoriented and, therefore, the display appears coloured in the activated pixels. Thus, a positive contrast display is produced of coloured information against a white background. The threshold voltage is dependent upon the elastic constants, the magnitude of the dielectric anisotropy, and the ratio of the cell gap to the chiral nematic pitch ... 

Nematic Liquid Crystals Hosts of Negative Dielectric Anisotropy... 

A binary mixture of 4-pentylphenyl 4-pentylbicyclo[2.2.2]octane and 4-heptyl-phenyl 4-pentylbicyclo[2.2.2]octane, see Table 3.8, exhibits a wide nematic phase at room temperature with a high clearing point, a low birefringence (0.08), a moderate viscosity (46 cP) and a weakly negative dielectric anisotropy (Ae —1.1) measured at 20°C. It is a surprisingly good solvent for... 

Nematic liquid crystals of negative dielectric anisotropy with a low rotational viscosity and a high value of the twist elastic constant k22 are required, see Equations 38-40. Furthermore, a high value for the resistivity and short... 

Scheme 4.11 Examples of typical super-fluorinated materials (SFM) used in the current generation of active matrix LCD. The liquid crystals 7-13 have positive dielectric anisotropy, compounds 14 and 15 have negative dielectric anisotropy. The approximate orientation of the molecular dipole moment for the two classes of material is indicated by arrows.

Since E/H appears to be independent of the magnetic field strength, an equation similar to Equation 2 should hold. With e greater than e / for liquid crystals exhibiting negative dielectric anisotropy, this equation becomes... 

Anisaldazine, which also exhibits negative dielectric anisotropy, was also investigated (1) using microwave dielectric techniques. The results in the presence of a d.c. electric field are very similar to those shown for p-azoxyanisole. 

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. 

Experimental values of the macroorder parameters S. , and (see Sect. 3.3), obtained for the various systems, are listed in Table 5. Drastic variations are observed. Apparently, a complete uniform alignment (S,., . = 1.0, Sy.. = 8., .. = —0.5) of the director axes is achieved for the LCP 2 and the monomeric analogue 5. The observed macroorder parameters are consistent with a positive diamagnetic (A > 0) and a negative dielectric anisotropy (As < 0). 

Consider a nematic film of negative dielectric anisotropy (e < 0) aligned homeotropically between glass plates. If an electric field is applied along the director axis (z axis) a distortion will set in when the field exceeds the critical Freedericksz value given by... 

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