Compositions – Liquid crystal compositions
Patent
1991-08-16
1993-09-07
Stoll, Robert L.
Compositions
Liquid crystal compositions
C09K 1952
Patent
active
052426165
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
The invention relates to an electrooptical liquid crystal system containing nematic microdroplets which are embedded in a transparent matrix and consist of a liquid crystal mixture, characterized in that, to increase the flow viscosity to values >40 mm.sup.2 /s at 20.degree. C., the liquid crystal mixture contains one or more side-chain polymers containing monomer units of the formula I ##STR2## wherein P is a main polymer chain unit, non-adjacent CH.sub.2 groups can also be replaced by --O--, --S-- and/or --NR.sup.2 --, and
Such electrooptical liquid crystal systems contain nematic microdroplets which are embedded in a transparent matrix. The matrix is located sandwich-like between conductive electrodes. Examples of these systems are NCAP and PDLC films (NCAP=nematic curvilinear aligned phases, PDLC=polymer dispersed liquid crystal). NCAP films are usually obtained by intimately mixing the encapsulating polymeric material, such as, for example, polyvinyl alcohol, the liquid crystal mixture and a carrier material, such as, for example, water, in a colloid mill. The carrier material is then removed, for example by drying. A corresponding process is described in U.S. Pat. No. 4,435,047. In contrast, in the preparation of PDLC films which is described, for example, in EP 0,272,582 and U.S. Pat. No. 4,688,900, the liquid crystal mixture is first mixed homogeneously with monomers or oligomers of the matrix-forming material. The mixture is then polymerized and phase separation is induced, a distinction being made between TIPS (temperature-induced phase separation), SIPS (solvent-induced phase separation) and PIPS (polymerization-induced phase separation) (Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427).
The liquid crystal mixture in general has a positive dielectric anisotropy .DELTA..epsilon. and a relatively high optical anisotropy .DELTA.n, and the ordinary refractive index n.sub.o is usually chosen so that it more or less coincides with the refractive index n.sub.M of the polymeric matrix.
A light scattering effect which can be switched electrically is observed on these electrooptical liquid crystal systems. If no voltage is applied to the electrodes, impinging light is scattered widely at the randomly aligned liquid crystal molecules and the system is opaque. When a voltage is applied, the liquid crystal molecules are aligned parallel to the field and perpendicular to the E vector of the light passing through. Because n.sub.M is adjusted to n.sub.o, the impinging light now sees an optically isotropic medium and the system appears transparent. EP 0,272,585 describes another embodiment in which the refractive index n.sub.x which the liquid crystal has in completely random orientation is adjusted to the refractive index of the matrix n.sub.M. In this case, the system is transparent in the fieldless state and is converted into the opaque state by application of a voltage.
Such systems have been proposed above all for large-area display systems, for architectonic uses (windows, room dividers, sun-roofs and the like) and for motor vehicles (windows, sun-roofs and the like). They can be switched by applying a direct or alternating voltage.
Since these systems are also intended in particular for "outdoor" uses, liquid crystal mixtures which are characterized by a high clear point, a wide nematic range and a high stability to UV and temperature are required.
Conventional electrooptical liquid crystal systems usually employ liquid crystal mixtures which consist of alkyl- or alkoxycyanobiphenyls and -terphenyls. The use of the liquid crystal mixture E8 (prepared by BDH, Poole, Great Britain) is thus described, for example, in U.S. Pat. No. 4,688,900 and in EP 0,272,585. This liquid crystal mixture is distinguished by a relatively high value for the flow viscosity of 54 mm.sup.2 /s and a very high value for the optical anisotropy .DELTA.n of 0.247, but at the same time has a relatively low clear point of only 72.degree. C. If polynuclear polyphenyl compounds are added to the mixtu
REFERENCES:
patent: 4671618 (1987-06-01), Wu et al.
patent: 4688900 (1987-08-01), Doane et al.
patent: 4944576 (1990-07-01), Lacher et al.
patent: 4994204 (1991-02-01), Doane et al.
patent: 5093471 (1992-03-01), West
Dorsch Dieter
Finkenzeller Ulrich
Merck Patent Gesellschaft mit beschrankter Haftung
Stoll Robert L.
Wu Shean C.
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