Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...
Patent
1987-10-30
1991-07-09
Maples, John S.
Compositions
Liquid crystal compositions
Containing nonsteryl liquid crystalline compound of...
25229901, 2522995, 2522996, 25229963, 25229964, 25229965, 25229966, 25229967, 350350R, 350350S, C09K 1934
Patent
active
050303830
DESCRIPTION:
BRIEF SUMMARY
The invention relates to electrooptical display elements with a particularly low temperature-dependence of the threshold voltage.
The properties of nematic or nematic-cholesteric liquid crystal materials of modifying their optical properties, such as light absorption, light scattering, birefringence, reflectivity or color, under the influence of electrical fields are utilized for LC display elements. The functioning of such display elements is here based, for example, on the phenomena of dynamic scattering, deformation of aligned phases, the guest-host effect, the Schadt-Helfrich effect in the twisted cell, the SBE effect or the cholesteric-nematic phase transition.
LC phases which must meet a large number of requirements are needed for technical application of these effects in electronic components Chemical stability towards moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet range and towards constant and alternating electric fields, is particularly important here. LC phases which can be used industrially are also required to have a liquid crystal mesophase in a suitable temperature range, a low viscosity, a low optical anisotropy, a low temperature-dependence of the threshold voltage, a high gradient to the electrooptical characteristic line and a sufficient dissolving power for pleochroic dyestuffs.
In none of the series of compounds with a liquid crystal mesophase which are known to date is there an individual compound which meets all these requirements.
Mixtures of two to 25, preferably three to 18, compounds are therefore as a rule prepared in order to obtain substances which can be used as LC phases. However, optimum phases cannot be easily prepared in this manner, since components with high melting and clear points frequently also impart a high viscosity to the mixtures. The switching times of the electrooptical display elements produced with the mixtures are thereby modified in an undesirable manner.
Highly polar nematic compounds with terminal cyano groups are added to the LC phases known to date in order to reduce the threshold voltage. The effective dipole moment of these compounds, however, is significantly reduced by the greater or lesser antiparallel association of these molecules, so that relatively large amounts of polar compounds have to be added. This again results in many different disadvantages, such as an undesirable temperature-dependence of the threshold voltage, adverse elastic properties of the LC phases and a high viscosity. When 4-cyano-3-fluorophenyl p-alkylbenzoates were added as highly polar components to ZLI-1957/5 (commercially available mixture from E. Merck, Darmstadt, containing phenylcyclohexane, cyclohexylbiphenyl and bis-cyclohexylbiphenyl compounds and phenyl cyclohexylbenzoate), it was to be found (Hp. Schad and S. M. Kelly, J. Chem. Phys. 81 (3), 1514-15 (1984)) that the threshold voltage is reduced, which is explained by a reduced degree of association of the compounds added. However, these LC phases do not meet all the above requirements at once either. In particular, they still have a temperature-dependence of the threshold voltage which is too high for many applications and still have threshold voltages which are too high for some fields of application, and moreover the gradients of the characteristic lines are not sufficient for highly informative displays because of the relatively high ratio of the elastic constants for the bending (K.sub.3) and the spreading (K.sub.1) K.sub.3 /K.sub.1. The mixtures known to date therefore have too high a temperature-dependence of the threshold voltage and in some cases too high a threshold voltage and/or too poor a characteristic line gradient (characterized by too high a K.sub.3 /K.sub.1 value). Furthermore, mixtures known to date with a low threshold voltage have smecticnematic transition temperatures which are too high.
There is therefore still a great need for liquid crystal phases with high clear points, low melting points, a low viscosity (and therefore short switchi
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Eidenschink Rudolf
Hittich Reinhard
Scheuble Bernhard
Maples John S.
Merck Patent Gesellschaft mit beschrankter Haftung
Treanor Richard
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