Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...
Patent
1989-07-27
1992-05-26
Stoll, Robert L.
Compositions
Liquid crystal compositions
Containing nonsteryl liquid crystalline compound of...
359 76, 359104, 25229961, 25229963, 25229964, 25229965, 25229966, 25229967, 25229968, C09K 1952, G02F 100
Patent
active
051165279
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
The invention relates to a display element based on the electroclinic effect containing a liquid crystal medium being a mixture of at least two chiral components and at least one non-chiral component, characterized in that said medium has a pitch compensated cholesteric phase above the optically active smectic A phase.
Ferroelectric smectics have recently entered the domain of liquid-crystal application in the so-called SSFLC (surface stabilized ferroelectric liquid crystal) device described by Clark and Lagerwall (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980); U.S. Pat. Specification No. 4,367,924).
This device utilizes the chiral smectic C phase or any chiral tilted smectic phase each of which has the characteristic of being ferroelectric.
The existence of a spontaneous polarization is possible only in chiral tilted smectic phases. However, in nontilted smectic phases, like smectic A, built up from chiral molecules one observes an induced tilt on applying an external electric field. Garoff and Meyer (S. Garoff and R. B. Meyer, Physical Review Letters 38, 848 (1977); Physical Review A 19, 338 (1979)) first described this so-called electroclinic effect (ECE).
The device they used shows only a small ECE and requires phase-sensitive methods for its detection.
Anderson et al. (G. Auderson et al., Appl. Phys. Lett. 51, 640 (1987); European Patent Application 0 263 225) describe a device having a geometry in which the samples are very thin and the smectic layers are perpendicular (instead of parallel) to the glass plates (this is the so-called bookshelf geometry typical of the SFLC-cells). In this device the ECE is detectable at conveniently low applied fields. This leads to an ECE which is also linear and gives a very fast response for a given substance.
Different compounds snowing an ECE are investigated by Nihiyama et al. (Jap. J. of Appl. Phys. 26, 1787 (1987)) or by Ch. Bahr and G. Heppke (Liquid Crystals, 1987, 825).
A great disadvantage for applications based on the electroclinic effect of the compounds currently available with smectic A phases is their low chemical, heat and light stability. Another adverse property of displays based on the ECE with compounds currently available is that they are difficult to align to an extent which is sufficient for practical application. But for many applications good alignment is vital although a smectic A phase does align over rubbed polymer (e.g. Patel, J. S., Leslie, T. M. and Goodby, J. W., Ferroelectrics 1984, 59, 137) and SiO, this process is much easier if a nematic phase precedes the smectic A phase.
The high cost of the chiral compounds used as single components in these displays is another disadvantage of the displays based on the ECE which are described until today.
Moreover, the temperature range of the smectic A phases showing an ECE is usually too small and is predominantly at temperatures being too high for commercial applications.
It now has been found that a display element based on the ECE with a liquid crystal medium which contains at least two chiral components and at least one non-chiral component and has a pitch compensated cholesteric phase above the optically active smectic A phase can substantially reduce the disadvantages mentioned.
Such a display element can particularly be used over a wide temperature range. The phases therein possess a high P.sub.s value and a compensated cholesteric phase which leads to a good alignment and a high induced tilt angle. P.sub.s is the spontaneous polarization in nC/cm.sup.2. Furthermore the costs of such a display containing a mixture of chiral and non-chiral compounds is essentially lower than those of one containing only chiral compounds.
The invention thus relates to a display element based on the electroclinic effect containing a liquid crystal medium being a mixture of at least two chiral components and at least one non-chiral component, characterized in that said medium has a pitch compensated cholesteric phase above the optically active smectic A phase.
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REFERENCES:
patent: 4725688 (1988-02-01), Taguchi et al.
patent: 4753752 (1988-06-01), Raynes et al.
patent: 4838663 (1989-06-01), Lagerwall et al.
patent: 4911863 (1990-03-01), Sage et al.
patent: 4927244 (1990-05-01), Bahr et al.
patent: 4988459 (1991-01-01), Scherowsky et al.
patent: 4997591 (1991-03-01), Heppke et al.
Physical Review A, "Electroclinic Effect at the A-C Phase Change in a Chiral Ligand Crystal", vol. 19, No. 1, pp. 338-347, 1979.
Coates David
Geelhaar Thomas
Sage Ian C.
Wachtler Andreas
Merck Patent Gesellschaft mit beschrankter Haftung
Stoll Robert L.
Wu Shean C.
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