Compositions – Liquid crystal compositions
Patent
1993-06-25
1999-02-16
Wu, Shean C.
Compositions
Liquid crystal compositions
25229961, 25229962, 25229963, 25229966, 349183, 428 1, C09K 1952, C09K 1934, G02F 1133
Patent
active
058716653
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
This application is a 371 of PCT/EP 93/00989, filed on Apr. 23, 1993.
The invention relates to an electrooptical liquid crystal system
which between 2 electrode layers contains a PDLC film comprising a liquid crystal mixture forming microdroplets in an optically isotropic, transparent polymer matrix,
in which one of the refractive indices of the liquid crystal mixture is matched to the refractive index of the polymer matrix,
which exhibits an electrically switchable transparency which is essentially independent of the polarization of the incident light,
the precursor of the PDLC film of which comprises one or more monomers, oligomers and/or prepolymers and a photoinitiator, and is cured photographically, and
the liquid crystal mixture of which comprises one or more compounds of the formula I ##STR2## in which Z.sup.1 and Z.sup.2 independently of one another, are a single bond, --CH.sub.2 CH.sub.2 --, --COO--, --OCO-- or --C.dbd.C--, ##STR3## independently of one another, are trans-1,4-cyclohexylene, 1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene or 3,5-difluoro-1,4-phenylene and one of ##STR4## may also be pyrimidine-2,5-diyl, pyridine-2,5-diyl or trans-1,3-dioxane-2,5-diyl, bond, CH.sub.2 groups can also be replaced by --O-- and/or --CH.dbd.CH--.
The preparation of PDLC (=polymer dispersed liquid crystal) films is described, for example, in U.S. Pat. No. 4,688,900, Mol. Cryst. Liq. Cryst. Nonlin. Optic, 157, 1988, 427-441, WO 89/06264 and EP 0,272,585. In the so-called PIPS technology (=polymerization-induced phase separation) the liquid crystal mixture is first homogeneously mixed with monomers and/or oligomers of the matrix-forming material; phase-separation is then induced by polymerization. Differentiation must further be made between TIPS (temperature-induced phase separation) and SIPS (solvent-induced phase separation) Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427) both being also methods to produce PDLC films.
The process of preparation must be controlled very carefully in order to obtain systems with good electrooptical properties. F. G. Yamgishi et al., SPIE Vol. 1080, Liquid Crystal Chemistry, Physics and Applications, 1989, p.24 differentiate between a "Swiss cheese" and a "polymer ball" morphology. In the latter one, the polymer matrix consists of small polymer particles or "balls" being connected or merging into each other while in the Swiss cheese system, the polymer matrix is continuous and exhibits well defined, more or less spherical voids containing the liquid crystal. The Swiss cheese morphology is preferred because it exhibits a reversible electrooptical characteristic line while the polymer ball system shows a distinct hysteresis generally leading to a drastical deterioration of the electrooptical characteristic line when comparing the virgin and the second run.
According to Yamagishi et al., loc. cit., the Swiss cheese morphology is promoted in case the polymerization reaction runs via a step mechanism, and in WO 89/06264 it is pointed out that the step mechanism is favored in case the precursor of the polymer matrix consists of multifunctional acrylates and multifunctional mercaptanes.
In PDLC films, one of the refractive indices of the liquid crystal mixture, customarily the ordinary refractive index n.sub.o, is selected in such a way that it more or less coincides with the refractive index n.sub.p of the polymeric matrix. If no voltage is applied to the electrodes, the liquid crystal molecules in the droplets exhibit a distorted alignment, and incident light is scattered at the phase boundary between the polymeric and liquid crystal phases.
On applying a voltage, the liquid crystal molecules are aligned parallel to the field and perpendicular to the E vector of the transmitted light. Normally incident light (viewing angle .theta.=0.degree.) now sees an optically isotropic medium and appears transparent.
No polarizers are required for operating PDLC systems, as a result of which these systems have high
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Coates David
Goulding Mark John
Greenfield Simon
Nolan Patrick
Parri Owain Llyr
Merck Patent Gesellschaft mit beschrankter Haftung
Wu Shean C.
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