Negative dielectric anisotropy materials

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. 

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.

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. 

We have so far discussed only materials of negative dielectric anisotropy. Electrohydrodynamic distortions are observed even in weakly positive materials,but only when the initial orientation of the director is perpendicular to the applied field. Striations appear above a threshold voltage but vanish at still higher voltages and there is no dynamic scattering. The frequency dependence of the threshold voltage is shown in fig. 3.10.6. 

There are two types of electro-optic effects which occur with cholesteric materials. The materials which possess negative dielectric anisotropy exhibit the so-called reflective optical storage mode while positive materials undergo field-induced cholesteric-nematic phase changes. 

It was, however, found to be possible to realize a high contrast ratio from a material in the C2 state when it was combined with the r-Vmin mode [21] by using a FLC material with a negative dielectric anisotropy and/or a large positive dielectric biaxiality [22]. In addition, the C2U state has several advantages over the CIU state and bookshelf state. The C2U state provides faster response times... 

The texture change or memory effect is observed in cholesteric materials with negative dielectric anisotropy [71]. The liquid crystal layer is homogeneously oriented by boundary forces to form the planar texture which is completely transparent if the band of selective light reflection is outside the visible spectrum. The substrates are covered with conducting films that are in contact with the liquid crystal. When a d.c. or low frequency field is applied, the sample is transformed to the so-called focal conic texture. In this texture, the liquid crystal is broken up into small domains which are randomly oriented and have diameters of a few microns. Since these domains are optically anisotropic, they act as scattering centers for visible light. Therefore the focal conic texture exhibits a milky white appearance. 

In EHC, for the usual case of driving with a pure ac field of frequency to = Unf, the eigenvector Uq of Eq. 30 inherits the additional periodic time dependence and the eigenvalue 1 becomes a Floquet coefficient. Then there is an additional discrete symmetry (z, t) (-Z, t + 1/(2/)) and each component of Uq has a definite parity. Generally the conductive mode (for even parity the out-of-plane component of director and vy, for odd parity the in-plane components of velocity) destabilizes first at low frequencies f. For materials with negative dielectric anisotropy, <, < 0, there exists a cut-offfrequency fr so that the dielectric mode with the other parity destabilizes first for f>fc, where oo for 0. The existence of these two regimes was first pointed out by Orsay s group [9, 10]. For frirther details see Refs. [14, 61]. 

At low voltages (-1 V), the molecules are aligned normal to the electric field and therefore parallel to the glass plates, as expected for a material of negative dielectric anisotropy. In this regime we have a nematic single crystal. 

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