Compositions – Liquid crystal compositions
Patent
1991-04-10
1993-12-21
Stoll, Robert L.
Compositions
Liquid crystal compositions
25229961, 25229963, 25229966, 25229967, 359103, C09K 1952, C09K 1930, C09K 1920, G02F 113
Patent
active
052718654
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a liquid-crystalline mixture of low viscosity, and to the use thereof for electrooptical purposes.
BACKGROUND OF THE INVENTION
Liquid crystals are used, in particular, as dielectrics in display devices since the optical properties of such substances can be affected by an applied voltage. Electrooptical devices based on liquid crystals are extremely well known to those skilled in the art and may be based on various effects Devices of this type are, for example, cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN cells having a twisted nematic structure, STN (supertwisted nematic) cells, SBE (super-birefringence effect) cells and OMI (optical mode interference) cells. The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability toward electrical fields and electromagnetic radiation, especially in the visible and ultraviolet spectral range. Furthermore, the liquid-crystal materials should have low viscosity and give short addressing times, low threshold voltages and high contrast in the cells. Furthermore, they should have a suitable mesophase, for example, for the abovementioned cells, a nematic or cholesteric mesophase, at customary operating temperatures, i.e. generally in the broadest possible range above and below room temperature. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as electrical conductivity, dielectric anisotropy and optical anisotropy, must meet various requirements depending on the cell type and the area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For display devices addressed with low multiplex ratios (common use displays), which are the preferred subject-matter of the present invention, nematic mixtures having clearing points above 55.degree. (preferably above 65.degree.), birefringence in the range from 0.04 to 0.33 (preferably from 0.07 to 0.23) and dielectric anisotropy in the range from 1.5 to 39 (preferably from 4 to 19) are desired.
A known mixture from the prior art is E7 (BDH, Poole, Great Britain), which contains cyanobiphenyls and has the following parameters
For many applications, however, mixtures of this type have undesired high viscosity and/or excessively low dielectric anisotropy, which results in undesirably long switching times for excessive threshold voltages.
SUMMARY OF THE INVENTION
The invention on relates to a liquid-crystalline mixture containing one or more compounds of the general formula ##STR5## in which R.sup.1 is straight-chain alkyl having 2 to 12 carbon atoms in which, in addition, one CH.sub.2 group may be replaced by --O-- or --CH.dbd.CH--, the ring A is 1,4-phenylene or trans-1,4-cyclohexylene, and r is 0 or 1.
DESCRIPTION OF THE INVENTION
It has been found that even a relatively small proportion of compounds of the formula I mixed with one or more compounds of the formula II, III or IV results in a considerable improvement in the addressing times and in low threshold voltages The compounds of the formula II have high optical anisotropy. However, the optical anisotropy of the mixture in the mixing range indicated is increased only relatively little by the compounds of the formula II, while the threshold voltage is significantly improved. Larger amounts of a compound of formula II generally only cause a slight improvement in the threshold voltage, but a comparatively large increase in the viscosity. The invention thus facilitates liquid-crystal mixtures of low viscosity and short switching times.
Furthermore, the addition of a compound of the formula II frequently lowers the melting point of the mixture. The compounds of the formula I and II are colorless, stable and readily misci
REFERENCES:
patent: 4759870 (1988-07-01), Fujimura
patent: 4917818 (1990-04-01), Sawada et al.
Sage, I "Materials requirements for nematic of chiral nematic electrooptical display" edited by Gray in Themstropic Liquid Crystal, pp. 64-66, 1987.
Hittich Reinhard
Plach Herbert
Reiffenrath Volker
Merck Patent Gesellschaft mit beschrankter Haftung
Stoll Robert L.
Wu Shean C.
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