Organic compounds -- part of the class 532-570 series – Organic compounds – Sulfur containing
Reexamination Certificate
2000-06-08
2001-04-17
Kelly, C. H. (Department: 1743)
Organic compounds -- part of the class 532-570 series
Organic compounds
Sulfur containing
C568S062000, C568S067000, C568S063000, C568S064000, C568S026000
Reexamination Certificate
active
06218578
ABSTRACT:
SUMMARY OF THE INVENTION
The invention relates to a polymeric film composed of an optically anisotropic material made from a cross-linked synthetic resin composition comprising a polymer network, being obtainable by copolymerization of a mixture comprising
(a) at least one monomer or oligomer, each of said monomers or oligomers having at least two polymerizable functional groups selected from the group consisting of (meth-)acrylate ester, epoxy and vinyl ether,
(b) at least one achiral liquid crystalline monomer or oligomer, each of said monomers/oligomers having mesogenic groups and one polymerizable functional group selected from the group consisting of (meth-)acrylate ester, epoxy and vinyl ether,
(c) a photoinitiator,
(d) additives selected from inhibitors and accelerators,
(e) optionally at least one chiral component, and
(f) optionally at least one liquid-crystalline mono- or dithiol compound, and to a display device comprising such a polymeric film.
A similar display device is described in European Patent 0 246 842.
Chirality in liquid-crystalline materials leads to rotation of the molecules in a direction perpendicularly to their longitudinal axis. In the case of liquid-crystalline materials in the so-called cholesteric phase, the pitch of the rotation is 0.1 to 1 &mgr;m. For application in, for example, datagraphic displays using multiplex drive, a larger pitch of the order of magnitude of the cell thickness or even more of the display device is desirable. Such a pitch, also called cholesteric pitch referring to chiral nematic or cholesteric LCs, is obtained by adding a chiral compound which also can be a liquid-crystalline compound itself as a dopant to a nematic liquid crystal. With such materials, supertwisted nematic (STN) liquid-crystal display devices are manufactured, the total twist of the molecular axis across the cell being, for example, between 180° and 270°. Such display devices have the disadvantage that the optical properties depend to a large extent on the wavelength, of the light so that a high contrast and a colorless image (black/white instead of e.g., blue/yellow) is difficult to attain. Said disadvantage can be overcome in a known manner by using a combination of two identical cells, one of which contains left-handed liquid-crystalline material and the other contains right-handed liquid-crystalline material. When the molecular axis at the front of the second cell extends perpendicularly to the molecular axis at the rear of the first cell the wavelength dependence of the optical properties is completely compensated. However, as a result of this second cell the liquid-crystal display device becomes heavier and less compact. According to a simpler alternative, the second cell is replaced by a uniaxial foil having an adapted birefringence. In this case, the compensation of the wavelength dependence is not complete, resulting in the display device exhibiting a contrast reduction and a certain degree of color e.g. in the voltageless state. Another alternative consists in the use of a twisted stack of uniaxial foils. This solution gets closer to the ideal situation (a twist and a birefringence which are equal to the twist and birefringence of a supertwisted nematic liquid-crystal display device) as the number of foils increases. However, this leads to a considerably more complicated production process. Instead of a foil, it is alternatively possible to use a birefringent layer on a suitable substrate. In European Patent Application 91-0 007 574 a description is given of liquid-crystalline polymer materials having a chiral dopant in the form of a copolymerizable monomer. Such polymer materials are linear and have side groups which exhibit liquid-crystalline properties. A thin layer is manufactured from a solution or a melt and is oriented in the rubbery liquid-crystalline state, after which it is cooled to a temperature below the glass transition temperature. Such layers are often turbid owing to local fluctuations in the refractive index caused by a poor orientational order. Moreover, heating the material above the glass transition temperature, even when executed only once, leads to a permanent loss of order. Besides, the method does not permit the pitch and the thickness of the polymer layer to be accurately adjusted.
In the U.S. Pat. No. 5,210,630 a polymeric film for display devices is disclosed obtained from polymerization of a mixture of monomers having at least two polymerizable groups in the presence of a chiral dopant. However, it is not possible to compensate the temperature dependence of the optical pathway of the low-molecular weight liquid crystals in the display with the aid of that film.
One of the objects of the invention is to provide a polymeric film for liquid-crystal display devices being optically clear and having a large temperature resistance. Another object of the invention is to provide a supertwisted nematic liquid-crystal display device having a high contrast, the voltageless state being substantially completely dark and colorless, and the voltage on state being highly transparent. Another object of the invention is to provide a compensation film for a TN, a IPS, a ASM or a similar LC device.
A further object of the invention is to provide a film which can be manufactured with the desired accuracy in a simple manner.
Another object of the invention is to provide a material which can be suitably be used in the film.
The general object of the invention is to provide a film whose optical properties are not too dissimilar to those of the LC within the LC cell.
According to the invention, these objects are achieved by a polymeric film as described in the opening paragraph, characterized in that it is obtainable by copolymerization of a mixture comprising
(a) at least one monomer or oligomer, each of said monomers or oligomers having at least two polymerizable functional groups selected from the group consisting of (meth-)acrylate ester, epoxy and vinyl ether,
(b) at least one achiral liquid crystalline monomer or oligomer, each of said monomers/oligomers having mesogenic groups and one polymerizable functional group selected from the group consisting of (meth-)acrylate ester, epoxy and vinyl ether,
(c) a photoinitiator,
(d) additives selected from inhibitors and accelerators,
(e) optionally at least one chiral component which is preferably a liquid crystalline compound or is compatible with liquid crystalline phases, and
(f) optionally at least one liquid-crystalline mono- or dithiol compound.
The synthetic resin composition is preferably manufactured from a curable liquid-crystalline composition having a chiral dopant.
In a preferred embodiment of the polymeric film according to the invention, the synthetic resin composition is formed by curing a mixture of liquid-crystalline monomers or oligomers which consist of compounds (a) with two or more acrylate-ester groups and compounds (b) with one acrylate ester group. Instead of acrylate compounds, epoxides, vinyl ethers and thiolene compounds can alternatively and satisfactorily be used as liquid-crystalline monomers.
Further preferred embodiments are:
a) A polymeric film wherein each of said monomers or oligomers of group (a) have at least two (meth-)acrylate-ester groups and each of said monomers oroligomers of group (b) have one (meth-)acrylate-ester group.
b) A polymeric film wherein the optically anisotropic material comprises at least one mesogenic chiral additive (e).
c) A polymeric film preferably containing a thiol which is preferably a monothiol especially a liquid crystalline or liquid crystal-like thiol, whose purpose is to limit the molecular weight of the resultant polymer. These thiol compounds terminate the free radical initiated polymerization.
d) A polymeric film being obtainable by in-situ UV co-polymerization of a mixture comprising at least one monomer or oligomer of group (a), at least one monomer or oligomer of group (b), at least one UV initiator (c) and at least one chiral additive (e).
e) A polymeric film wherein the optically anisotropic material comprises
5-
Coates David
Greenfield Simon
Jolliffe Emma Jane
Kelly C. H.
Merck Patent Gesellschaft mit beschrankter Haftung
Millen White Zelano & Branigan P.C.
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