Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...
Patent
1989-01-27
1991-10-08
Maples, John S.
Compositions
Liquid crystal compositions
Containing nonsteryl liquid crystalline compound of...
2522996, 25229901, 25229961, 359102, 359106, C09K 1930, G02F 113
Patent
active
050552244
DESCRIPTION:
BRIEF SUMMARY
The invention relates to liquid crystal phases (LC phases) having elastic and dielectric properties appropriate for use in supertwisted nematic cells.
Liquid crystal display cells can be made to support more than 90.degree. of twist in the liquid crystal layer by a combination of offset alignment directions at the surfaces, and incorporation of chiral additives. If the twist angle is increased in this way, the slope of the threshold characteristic increases and eventually becomes vertical. Further increase of the twist angle will result in bistable behaviour, but at the onset of bistability the very steep switching curve allows high level multiplex drive using conventional RMS waveforms. The limit to the increase of twist angle is set by a scattering texture which becomes stabilised as the cholesteric pitch is shortened; the effect can be alleviated but not eliminated by increasing the surface tilt in the cell.
The term "supertwisted nematic cell" includes all cells with clearly more than 90.degree. of twist, e.g. the cells using a Guest-Host mode (C. M. Waters, V. Brimmell and E. P. Raynes, Proc. 3. Int. Display Research Conference, Kobe (1983) 396; C. M. Waters, V. Brimmell and E. P. Raynes, Proc. SID 25/4 (1984) 261), a birefringence mode (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021; T. J. Scheffer and J. Nehring, J. Appl. Phys. 58 (1985) 3022), Black & White STN-LCD's (optical mode interference) (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987) 236; K. Kawasaki, K. Yamada, R. Watanabe and K. Mizunoya, Proc. SID '87 Digest (1987) 391), HBELCD's (K. Kinugawa, Y. Kando, M. Kanasaki, H. Kawakami and E. Kaneko), Double Layer Black & White STN LCD's (K. Katoh, Y. Endo, M. Akatsuka, M. Ohgawara and K. Sawada).
By comparison with the standard twisted nematic display, the supertwist nematic cell has important differences in behaviour. Because light guiding through the highly twisted structure is imperfect, the cells are commonly operated in a variable birefringence mode and this, together with a strong dependence of threshold voltage on cell thickness, means that a high accuracy of cell spacing is required. In order to minimise the change in materials properties with temperature, a relatively high clearing point is desirable in the mixture. A near optimal optical performance can be obtained, e.g. by setting d.DELTA.n.congruent.0.73 for the 270.degree. twist cell and adjusting the polariser orientation if necessary to obtain the best contrast.
Because of the large number of interacting cell and material properties which contribute to the performance of the supertwisted nematic display, it is highly difficult to select combinations of materials to suit the parameters of the cell for best performance.
There is thus still a great need for liquid crystal phases having suitable material properties for use in supertwisted nematic display cells.
The invention has for its object to prepare liquid crystal phases which have a nematic phase including room temperature and exhibit elastic and dielectric properties appropriate for use in supertwist cells.
It has now been found that the liquid crystal phases having particularly favorable combinations of material properties are obtained when they consist of at least three components A, B, and C, each of which contains one or more compounds, wherein component A has a viscosity of at most about 25 mPa.s at 20.degree. , a clearing point of at most 130.degree. and a dielectric anisotropy between about -2 and about +2, component B has a dielectric anisotropy above about +8, component C has a clearing point of more than 130.degree. and a dielectric anisotropy between about -2 and about +2, characterized in that component A contains at least one compound of the formula III ##STR4## in which R.sup.1 is alkyl of 1 to 12 C atoms, in which in addition one or two nonadjacent CH.sub.2 groups can be replaced by 0 atoms, --CO--, --O--CO--, --CO--O-- and/or --CH=CH-- groups, ##STR5## Z.sup.1 is --CO--O--, --O--CO-- or --CH.sub.2 CH.sub.2 --, o is 1 or, if A.sup.3 is
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Chambers Michael K.
Sage Ian C.
Maples John S.
Merck Patent Gesellschaft mit beschrankter Haftung
Treanor Richard
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