Compositions – Liquid crystal compositions – Containing nonchiral aligning agents
Patent
1992-09-11
1995-04-25
Wu, Shean
Compositions
Liquid crystal compositions
Containing nonchiral aligning agents
359 75, C09K 1956, G02F 113
Patent
active
054096359
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Switching and display elements containing ferroelectric liquid-crystal mixtures ("FLC light valves") are disclosed, for example, in EP-B 0 032 362. Liquid-crystal light valves are devices which modify their optical transmission properties, for example due to electrical switching, in such a way that light which is incident (and possible reflected again) is modulated in intensity. Examples are the known watch and calculator displays or liquid-crystal displays in the OA (office automation) or TV sectors. However, these also include light shutters, as employed in photocopiers, printers, welding goggles, polarizing spectacles for three-dimensional viewing, etc. So-called " spatial light modulators also fall within the area of application of liquid-crystal light valves (see Liquid Crystal Device Handbook, Nikkan Kogyo Shimbun, Tokyo, 1989; ISBN 4-526-02590-9C 3054 and the papers cited therein).
Electro-optical switching and display elements (displays) are constructed in such a way that the FLC layer is surrounded on both sides by layers which are usually, in this sequence starting from the FLC layer, at least one alignment layer, electrodes and a limiting plate (for example made of glass). In addition, they contain one polarizer if they are operated in "guest-host" mode or in reflective mode, or two polarizers if the transmissive birefrigence mode is used. Switching and display elements may, if desired, contain further auxiliary layers, such as, for example, diffusion barrier layers or insulation layers.
Together with a distance between the limiting plates which is chosen to be sufficiently small, the alignment layers, which comprise an organic (for example polyimide, polyamide or polyvinyl alcohol) or inorganic (for example SiO) material, bring the FLC molecules into a configuration in which the molecules lie with their long axes parallel to one another and the smectic planes are arranged perpendicular or inclined to the alignment layer. In this arrangement, the molecules are known to have two equivalent alignments, between which they can be switched by applying an electrical field in a pulsed manner, i.e. FLC displays are capable of bistable switching. The response times are inversely proportional to the spontaneous polarization of the FLC mixture and are in the range of microseconds.
The major advantage of FLC displays of this type over the LC displays which are still usually encountered in industrial practice is regarded as being the multiplex ratio which can be achieved, i.e. the maximum number of lines which can be addressed in the time-sequential process ("multiplex process"), which is virtually unlimited in FLC displays, in contrast to conventional LC displays. This electrical addressing is essentially based on the pulse addressing mentioned above and described in illustrative terms in SID 85 DIGEST p. 131 (1985).
Particularly important functional parameters of an FLC display are bright and dark states),
The object of the present invention is to provide alignment layers which, in ferroelectric liquid-crystal displays, result in improved brightness, lower residual transmission in the dark state and thus in a discrete improvement in the optical contrast.
In order to explain this object, the brightness (or transmission in the bright state), the transmission in the dark state, the contrast and the response time are described in more detail below.
The maximum transmission in the bright state T (bright) is, as is known, described for FLC displays to a good approximation by the equation (1): (4.theta..sub.eff)(1)
While the first of the two terms in equation (1) can be optimized relatively easily by matching .DELTA.n and d to the wavelength of visible light, the material-side optimization of sin.sup.2 (4.theta..sub.eff) causes problems since .theta..sub.eff is generally very much smaller than 22.5.degree. (optimum value).
In the so-called "chevron" geometry (see, for example, T. Rieker al., 11th Int. Liq. Cryst. Conf. Berkeley (1986)), in which the smectic layers are at an angle to o
REFERENCES:
patent: 4077900 (1978-03-01), Pohl et al.
patent: 5178792 (1993-01-01), Harada et al.
patent: 5196501 (1993-03-01), Dubal et al.
patent: 5206751 (1993-04-01), Escher et al.
Patent Abstracts of Japan, vol. 3, No. 113.
Rosch Norbert
Wegener Peter
Wingen Rainer
Hoechst Aktiengesellschaft
Wu Shean
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