Polysiloxanes

Details Polysiloxanes Polysiloxanes

1997-10-06

2000-05-09

Wu, Shean C.

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With charge transferring layer of specified composition

25229961, 526 10, C09K 1934, C09K 1936, C09K 1940, C08G 7704

Patent

active

060601337

DESCRIPTION:

BRIEF SUMMARY
This invention relates to liquid crystal polysiloxanes suitable for use in liquid crystal mixtures and their inclusion in liquid crystal devices.


BACKGROUND OF THE INVENTION

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, (chiral nematic) 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.
Ferroelectric smectic liquid crystal materials, which can be produced by mixing an achiral host and a chiral dopant, use the ferroelectric properties of the tilted chiral smectic C, F, G, H, I, J and K phases. The chiral smectic C phase is denoted S.sub.C * with the asterisk denoting chirality. The S.sub.C phase is generally considered to be the most useful as it is the least viscous. Ferroelectric smectic liquid crystal materials should ideally possess the following characteristics: low viscosity, controllable spontaneous polarisation (Ps) and an S.sub.C phase that persists over a broad temperature range, which should include ambient temperature and exhibits chemical and photochemical stability. Materials which possess these characteristics offer the prospect of very fast switching liquid crystal containing devices. Some applications of ferroelectric liquid crystals are described by J. S. Patel and J. W. Goodby in Opt. Eng., 1987, 26, 273.
In ferroelectric liquid crystal devices the molecules switch between different alignment directions depending on the polarity of an applied electric field. These devices can be arranged to exhibit bistability where the molecules tend to remain in one of two states until switched to the other switched state. Such devices are termed surface stabilised ferroelectric devices, eg as described in U.S. Pat. No. 5,061,047 and U.S. Pat. No. 4,367,924 and U.S. Pat. No. 4,563,059. This bistability allows the multiplex addressing of quite large and complex devices.
It is well known in the field of ferroelectric liquid crystal device technology that in order to achieve the highest performance from devices, it is important to use mixtures of compounds which give materials possessing the most suitable ferroelectric smectic characteristics for particular types of device. The thermal and physical properties of the device mixture may be finely tuned by adjusting the concentrations and nature of the components in the mixture.
Cholesteric or chiral nematic liquid crystals possess a twisted helical structure which is capable of responding to a temperature change through a change in the helical pitch length. Therefore as the temperature is changed then the wavelength of the light reflected from the planar cholesteric structure will change and if the reflected light covers the visible range then distinct changes in colour occur as the temperature varies. This means that there are many possible applications including the areas of thermography and thermooptics.
The cholesteric mesophase differs from the nematic phase in that in the cholesteric phase the director is not constant in space but undergoes a helical distortion. The pitchlength for the helix is a measure of the distance for the director to turn through 360.degree..
By definition a cholesteric material is a material which contains a chiral centre. Cholesteric materials may also be used in electrooptical displays as dopants, for example in twisted nematic displays where they may be used to remove reverse twist defects. They may also be used in cholesteric to nematic dyed phase change displays where they may be used to enha

REFERENCES:
patent: 5138010 (1992-08-01), Keller et al.
patent: 5304667 (1994-04-01), Haeberle et al.
patent: 5610258 (1997-03-01), Weitzel et al.
patent: 5688436 (1997-11-01), Ohnishi et al.

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