Light modulating material comprising polymer dispersed liquid cr

Details Light modulating material comprising polymer dispersed liquid cr Light modulating material comprising polymer dispersed liquid cr

1992-06-12

1996-05-21

Geist, Gary L.

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

25229901, C09K 1912, C09K 1952

Patent

active

055186548

DESCRIPTION:

BRIEF SUMMARY
This invention relates to novel light modulating materials comprising a solid, light transmissive polymeric matrix having dispersed therein droplets of a liquid crystalline composition. The invention also relates to novel compositions suitable for use in such materials.
Liquid crystalline materials are widely used in liquid crystal displays such as watches, calculators etc. Most displays of this type consist of a thin film of a liquid crystalline composition sandwiched in a cell between two substrates, at least one of which is transparent, and having transparent electrodes on their inner surfaces. On applying a potential difference across the electrodes the alignment of the molecules of the liquid crystalline composition is altered, resulting in an electro-optical effect in the material, which is exploited by the display. Most often electro-optical effects in the nematic liquid crystal phase are exploited in such displays. Examples of types of such displays include the twisted nematic device, the Freedericksz effect device, cholesteric memory mode device, cholesteric to nematic phase change effect device, dynamic scattering effect device, two frequency switching effect device and the "supertwist" effect device.
There are problems in constructing a large, e.g. several square metres area, liquid crystal display device of the above type. For example, there are substantial engineering problems in simply constructing a large cell with a constant substrate spacing of typically 1-10 .mu.m over its entire area. Moreover, as liquid crystal materials are fluid they may flow causing variations in the film thickness in the sandwich. Both of these problems result in variations in the appearance or quality of the display over a large area. Consequently most present liquid crystal displays are at most a few square centimetres in area.
Recently a new type of liquid crystal display material has been developed, in which droplets of a suitable liquid crystalline composition are dispersed in a solid, light transmissive polymeric matrix. In the art such materials are known generally as polymer dispersed liquid crystal (abbreviated "PDLC") materials and/or as non curvilinear aligned phase (abbreviated "NCAP") materials because of their construction and physics respectively. Light is scattered from such materials because of a mismatch between the refractive index of the droplets and the matrix. The director of the liquid crystals within the droplets has no preferred orientation but varies randomly from droplet to droplet in the absence of an electric field. The liquid crystal has a positive dielectric anisotropy and therefore aligns parallel to an applied electric field. When no field is applied incident light is efficiently scattered because of the mismatch of the refractive index of the droplets and the matrix. When a field is applied the alignment of the liquid crystal results in the droplets having an effective refractive index closer to the ordinary refractive index of the liquid crystal, n.sub.o, for light incident normal to the material. The matrix has a refractive index, n.sub.p, essentially equal to n.sub.o and therefore the material becomes transparent to incident light.
Such materials may be made into large area display devices, such as windows, privacy screens etc, without the problems mentioned above. The construction of such a device is described below and illustrated in FIG. 1 of the accompanying drawings. Further references to such materials and devices are found in Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. (1988) 157 427-441, Liquid Crystals (1988) 3(11) 1543-1559, FR 2.139.537, U.S. Pat. No. 4,435,047 and U.S. Pat. No. 4,688,900.
To date, only a limited range of compounds have been used in compositions for PDLC materials. The references mentioned above only refer to the use of benzylidineanilines, 4-alkyl or alkoxy 4'-cyanobiphenyls or 4"-cyanoterphenyls, alkyl or alkoxy cyanophenylcyclohexanes or cyanobiphenylcyclohexanes, and aromatic esters in these compositions.
There is clearly scope for development of new

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