Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Patent
1994-02-28
1997-04-15
Crane, Sara W.
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
H01L 2904
Patent
active
056215525
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an electrooptical liquid crystal system optically transparent polymeric medium between two electrode layers which optionally may be attached to substrates, essentially agrees with the refractive index of the polymeric medium n.sub.m and/or in which the quotient of the mass of the liquid crystal mixture and of the mass of the polymeric medium is 1.5 or more, and frequency liquid crystal mixture and a precursor of the polymeric medium.
Depending on the mass content of the liquid crystal mixture in the system, this can be embedded in the optical transparent medium in liquid crystal microdroplets which are separated to a greater or lesser extent from one another or else form a more or less coherent, continuous phase in which the optically transparent medium is present, for example, in the form of particles. A continuous phase is also obtained, for example, if the optically transparent medium forms a sponge-like, 3-dimensional network whose pores, in which the liquid crystal is located, merge into each other to a greater or lesser extent. The expression liquid crystal microdroplets here indicates small liquid crystal compartments separated from one another which, however, in no way have to have a spherical shape, but can be irregularly shaped and/or deformed.
If the optically transparent medium contains liquid crystal microdroplets, it is described in the following as a matrix; on the other hand, if a more or less continuous phase of the liquid crystal is present, the medium is described by the expression network.
NCAP and PDLC films (NCAP=nematic curvilinear aligned phases, PDLC=polymer dispersed liquid crystal) are examples of electrooptical liquid crystal systems in which the liquid crystal is embedded in the matrix in the form of microdrops. NCAP films are usually obtained by intimately mixing the encapsulated polymericmaterial, 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. An appropriate process is described in U.S. Pat. No. 4,435,047. In contrast, the liquid crystal mixture is firstly homogeneously mixed with monomers or oligomers of the matrix-forming material in the preparation of PDLC films as 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. The mixture is then polymerized and the phase separation is induced (so-called PIPS technology; polymerization-induced phase separation). In addition, differentiations 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).
The PN system (PN=polymer network) described in EP 0,313,053 has a sponge-like network structure of the optically transparent medium. The content of the liquid crystal mixture in the material of the light-modulating layer is in general greater than 60% in systems of this type and is, in particular, between 70 and 90%. In order to prepare the PN systems, a mixture of the liquid crystal, monomers or oligomers of the material forming the 3-dimensional network and a polymerization initiator, in particular a photoinitiator, is customarily brought between 2 substrate plates provided with electrodes and then polymerized, for example by light irradiation.
The liquid crystal mixture in general has a positive dielectric anisotropy .DELTA..epsilon. but the use of dielectrically negative liquid crystal mixtures (see, for example, WO 91/01511) or two-frequency liquid crystal mixtures the dielectrical anisotropy of which depends on the frequency of the addressing voltage (see, for example, N. A. Vaz et al., J. Appl. Phys. 65, 1989, 5043) is also discussed.
In microdroplet matrix systems, one of the refractive indices of the liquid crystal, 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.
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Coates David
Nolan Patrick
Abraham Fetsum
Crane Sara W.
Merck Patent Gesellschaft mit beschrankter Haftung
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