Liquid crystal mixtures and devices

Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...

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25229963, 25229964, 25229965, 25229967, 25229966, 252582, 349182, C09K 1906, C09K 1920, C09K 1912, G02F 1133

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058913580

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BRIEF SUMMARY
This application is a 371 application of PCT/GB95/00018.
This invention relates to liquid crystal materials, in particular it relates to liquid crystal materials which exhibit one or more smectic phases.
Liquid crystals can exist in various phases. In essence there are three different classes of liquid crystalline material, each possessing a characteristic molecular arrangement. These classes are nematic, cholesteric and smectic. A wide range of smectic phases exists, for example smectic A and smectic C. Some liquid crystal materials possess a number of liquid crystal phases on varying the temperature, others have just one phase. For example, a liquid crystal material may show the following phases on being cooled from the isotropic phase:--isotropic--nematic--smectic A--smectic C--solid. If a material is described as being smectic A then it means that the material possesses a smectic A phase over a useful working temperature range.
Materials possessing a smectic A*(S.sub.A *) phase may exhibit an electroclinic effect. The electroclinic effect was first described by S. Garoff and R. Meyer, Phys. Rev. Lett., 38, 848 (1977). An electroclinic device has also been described in UK patent application GB 2 244 566 A. This particular device helps to overcome the poor alignment problems of electroclinic (EC) devices using a surface alignment that gives a surface tilt within a small range of angles.
When a smectic A phase is composed of chiral molecules, it may exhibit an electroclinic effect, ie a direct coupling of molecular tilt to applied field. The origin of the electroclinic effect in a smectic A* phase composed of chiral polar molecules has been described by Garoff and Meyer as follows. The application of an electric field parallel to the smectic layers of such a smectic A* phase biases the free rotation of the transverse molecular dipoles and therefore produces a non-zero average of the transverse component of the molecular polarization. When such a dipole moment is present and coupled to the molecular chirality, a tilt of the long molecular axis (the director) is induced in a plane perpendicular to the dipole moment.
In thin samples for example 1-3 .mu.m and with the smectic layers tilted or perpendicular with respect to the glass plates the electroclinic effect is detectable at low applied fields.
In an aligned smectic A sample a tilt of the director is directly related to a tilt of the optic axis. The electroclinic effect results in a linear electro-optic response. The electro-optic effect can manifest itself as a modulation of the effective birefringence of the device.
Electroclinic devices are useful, for example, in spatial light modulators having an output that varies linearly with applied voltage. A further advantage of EC devices is that they have high speed response times, much faster than nematic based devices, including twisted nematic based devices. Unlike ferroelectric devices the EC device is not bistable.
The electroclinic effect is sometimes referred to as the soft-mode effect see G. Andersson et at in Appl. Phys. Lett., 51, 9, (1987).
In general terms, regarding the electroclinic effect, it is advantageous if on applying a small voltage there results a large induced tilt.
An increase in induced tilt may result in an increase in contrast ratio or in phase modulation depth.
It is also advantageous if a large induced tilt can be obtained at as low a voltage as possible.
It is also advantageous if the relationship between molecular induced tilt and applied voltage is temperature independent. When an increase in applied voltage results in little or no change in induced tilt then the material being tested is generally referred to as exhibiting a saturation voltage effect.
Wand et al in Abstracts of the 4.sup.th International Conference of Ferroelectric Liquid Crystals Sep. 28-Oct. 1, 1993 in Tokyo, Japan: Paper P90, pages 257-58 and see references therein, report on novel electroclinic materials. They suggest that the reason for the materials they report exhibiting a large and temperature insensit

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