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Schematic model of liquid crystals

Liquid Crystals for LCD Technology

The nematic phase is generally used in LC displays. The molecular requirement is the calamitic structure to form the nematic phase. The other properties the molecules need to have depend on the application and function of the liquid crystal display.

For example, a display should work at particularly high or low temperatures, depending on the application. This means that the melting point should be low and the clearing point high.
The image formation is based on changes and reformation of the order of liquid crystal phases. For this to happen, the molecules are modulated in an electric field. Therefore, the molecules must show sufficient polarity (De), which results from uneven charge distribution in the molecule. Depending on the display technology, the polarity can be oriented both in the direction of the molecular axis (positive) or perpendicular to it (negative).
The molecules are moved within the electric field. The lower the viscosity (g1) of the material, the easier it is and the faster an image can be formed.

Side chains with double bonds

The size of the molecules and the design of the side chain can be used to influence the clearing point. Another important property is birefringence (Dn). Birefringence is the ability of an anisotropic medium to split a ray of light into two polarized rays at right angles to each other. The birefringence can be used to influence the thickness of the liquid crystal layer in a display, for example. The extent of the birefringence depends on the structural elements of the molecule. Unsaturated rings such as phenyl rings (see above) or side chains with double bonds (see left) cause a high level of birefringence value.

A single substance is not sufficient to meet the complex requirements of the liquid crystal layer in a display. Therefore, mixtures of substances are used, in which the individual components take on specific roles. Polar substances and those with low viscosity and high clearing points are combined to achieve the best possible result. Up to twenty different substances are used. Some molecules are pictured below.