Liquid crystalline mixture

Details Liquid crystalline mixture Liquid crystalline mixture

1988-09-30

1993-03-02

Stoll, Robert L.

Compositions

Liquid crystal compositions

2522996, 25229961, 25229963, 25229965, 25229966, 25229967, 2522992, 25229968, C09K 1952, C09K 1934, C09K 1906, C09K 1930

Patent

active

051906880

DESCRIPTION:

BRIEF SUMMARY
This invention relates to liquid crystalline mixtures with at least two liquid-crystalline compounds which are free of perfluoro alkylene groups and at least one partially fluorinated additive of formula I group of formula non-adjacent CH.sub.2 groups to be replaced by --O--, --CO--, --O--CO--, --CO--O--, --O--CO--O--, --C.tbd.C-- and/or --CH.dbd.CH--, polysubstituted 1,4-cyclohexylene group, it also being possible for one or two non-adjacent CH.sub.2 groups to be replaced by --O-- and/or --S--, or denote a 1,4-bicyclo-[2,2,2]octylene group, or a 1,4-phenylene group which is unsubstituted or substituted by one or two F and/or Cl atoms and/or CH.sub.3 groups and/or CN groups, it also being possible for one or two CH groups to be replaced by N, --CHCN--CH.sub.2 --, --CH.sub.2 --CHCN--, --CH.dbd.CH--, --C.tbd.C--, --OCH.sub.2 --, --CH.sub.2 O--, --CH.dbd.N--, --N.dbd.CH--, --N.dbd.N--, --NO.dbd.N--, --N.dbd.NO--or a single bond, --CHCN--CH.sub.2 --, --CH.sub.2 --CHCN--, --CH.dbd.CH--, --C.tbd.C--, --OCH.sub.2 --, --CH.sub.2 O--, --CH.dbd.N--, --N.dbd.CH--, --N.dbd.N--, --NO.dbd.N--, --N.dbd.NO--, --(CH.sub.2).sub.k --CO--O-- or a single bond, that the sum of m+p+q equals 1 to 15,
and in particular means whereby liquid crystalline mixtures can be provided having refractive indices tailored to optimal values for practical applications.
Liquid crystal phases are commonly exhibited by organic compounds having extended rod-like molecules, and are characterised in their liquid crystalline state by a degree of order intermediate between those of a crystalline solid and of an isotropic liquid respectively. The wide spread use of liquid crystalline materials in electrooptic devices arises from their combination of fluid like flow with an anisotropy of their physical properties that is typical of a crystalline material. When a liquid crystal material is utilised in an electro-optic device, optimal performance can only be obtained when the physical properties of the material are adjusted to extreme or optimal values to suit the particular application and device geometry in use. Examples of the physical properties which may beneficially be changed to improve the utility of a liquid crystalline material for a particular application include such properties as the mesogenic phase range, the dielectric constants, the elastic constants, the viscosity coefficients, and the refractive indices of the material.
Means is now provided by the present invention for the adjustment of the refractive indices of a liquid crystal by addition of one or more partially fluorinated additives which are compatible with a liquid crystalline base mixture. In particular the invention provides means for the reduction of the absolute refractive indices and/or the reduction of the birefringence of a liquid crystalline host mixture. It is well known that the performance of known optical and electro-optic devices can be improved by the reduction of the refractive indices or birefringence of the liquid crystalline material contained therein. For example, in the electro-optic display based upon the twisted nematic mode of operation, the off-state transmission of light only achieves its ideal value for discreet values of the parameter U where U is defined by the relation of the liquid crystal and .lambda. is the average wavelength of visible light and optimal performance in the display cell is obtained for values of U equal to .sqroot.3, .sqroot.15, etc. To allow use of a liquid crystalline material in a display cell it is therefore desirable that its birefringence should be adjusted so that the above equation is satisfied for the particular cell thickness which is chosen. In liquid crystal display cells containing dichroic dyed liquid crystal materials and operating in the cholesteric to nematic phase-change mode otherwise known as the White Taylor mode of operation, the birefringence of the liquid crystalline host mixture leads to the undesirable propagation of eliptically polarised light rays in the display cell- which- diminishes the optical efficienc

REFERENCES:
patent: 4346235 (1982-08-01), Sonoda et al.
patent: 4686289 (1987-08-01), Huynh-Ba et al.
patent: 4871469 (1989-10-01), Reiffenrath et al.
patent: 4871472 (1989-10-01), Krause et al.
patent: 4886619 (1989-12-01), Janulis

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