Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified... – Alignment layer of specified composition
Reexamination Certificate
1999-01-25
2001-03-13
Wu, Shean C. (Department: 1256)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
Alignment layer of specified composition
C428S001250, C428S001260, C349S124000, C349S123000, C349S135000, C252S299400
Reexamination Certificate
active
06200655
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to processes for aligning liquid crystals, compositions useful for generating alignment of liquid crystals and liquid crystal optical elements.
Liquid crystal compounds are used in human and machine readable displays, fining applications in instrument controls, such as those in motor vehicles, avionics, medical devices, process control devices and watches. Display devices are primarily comprised of liquid crystal cells having a glass or other substrate coated with a transparent conductive material in front and behind a liquid crystal medium. Light transmission through these devices is controlled through orientation of the liquid crystal compounds or dyes dissolved therein. In this way, a voltage or, in some instances, a magnetic field may be applied to the cell so that the liquid crystals are oriented in a fashion such that all, some or none of the light is passed through. In addition, depending on the device geometry, polarizers may be used in conjunction with the liquid crystal medium to control light transmission.
Aligned liquid crystal cells in commercial use are typically oriented in directions suitable for controlling light transmission. That is, the molecules in the liquid crystal composition are aligned so as to assume a homogeneous or homeotropic alignment. Without external stimuli the display will either appear opaque or transparent. By applying an electric field the molecules are rotated along a fixed axis so as to alter the transmission properties in a desired fashion.
Current liquid crystal display elements include a product that utilizes a twisted nematic mode, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 90° between a pair of upper and lower electrode substrates, a product utilizing a supertwisted nematic mode, utilizing a birefringent effect, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 180° to 300°, a product utilizing a ferroelectric liquid crystal substance or an antiferroelectric liquid crystal substance. Common to each of these products is a liquid crystal layer disposed between a pair of substrate coated with a polymeric alignment layer. The polymeric alignment layer controls the direction of alignment of the liquid crystal medium in the absence of an electric field. Usually the direction of alignment of the liquid crystal medium is established in a mechanical buffing process wherein the polymer layer is buffed with a cloth or other fibrous material. The liquid crystal medium contacting the buffed surface typically aligns parallel to the mechanical buffing direction. Alternatively, an alignment layer comprising anisotropically absorbing molecules may be exposed to polarized light to align a liquid crystal medium as disclosed in U.S. Pat. Nos. 5,032,009 and 4,974,941, both entitled “Process of Aligning and Realigning Liquid Crystal Media.” both of which are hereby incorporated by reference.
The process for aligning liquid crystal media with polarized light is a noncontact method of alignment which can reduce dust and static charge buildup on alignment layers. Other advantages of the optical alignment process include high resolution control of alignment direction and high quality of alignment.
Requirements of optical alignment layers for liquid crystal displays include low energy threshold for alignment, transparency to visible light (no color), good dielectric properties and voltage holding ratios, long-term thermal and optical stability and in many, but not all, applications, a controlled uniform pre-tilt angle.
The process for aligning liquid crystal media with polarized light has many attractive features. To exploit this process for use in many liquid crystal device applications, anisotropically absorbing molecules that absorb in the ultraviolet (UV) region are desirable because they can be transparent in the visible region. Schadt, et al (Jpn. J. Appl. Phys., 1992, 31, 2155), for instance, has described polyvinyl cinnamates as a useful material for optical alignment of liquid crystals; and Hasegawa, et al (J. Photopolymer Sci. & Tech., 1995, 8, 241) has described UV exposure of a commercial polyimide and shown it to align liquid crystals.
SUMMARY OF INVENTION
The instant invention provides a process for aligning a liquid crystal medium that is useful in aligning liquid crystal displays and other liquid crystal devices. New materials for optical alignment layers are also disclosed that provide excellent alignment properties upon exposure to UV light.
Specifically, the present invention provides a process for aligning liquid crystal adjacent to a surface of an optical alignment layer comprising: (a) exposing at least one optical alignment layer, comprising anisotropically absorbing molecules, to polarized light; the polarized light having a wavelength within the absorption band of said anisotropically absorbing molecules; wherein the exposed anisotropically absorbing molecules induce alignment of a liquid crystal medium at an angle + and −&thgr; with respect to the direction of the polarization of the incident light beam and along the surface of the optical alignment layer; and (b) applying a liquid crystal medium to said optical alignment layer; wherein the anisotropically absorbing molecules consist essentially of at least one diaryl ketone.
The invention also provides a liquid crystal optical storage medium, liquid crystal display element and a diffractive optical element preferably made by the process of the invention.
The invention further provides novel polyimide compositions for generating alignment of liquid crystals, consisting essentially of a polyimide polymer that is a copolyimide of at least one diaryl ketone tetracarboxylic dianhydride and at least three diamines, consisting essentially of at least three structural elements of the formula:
wherein Y is a divalent radical selected from the formulas:
wherein Z and Z
1
are independently selected from the group consisting of —S—, —SO
2
—, —O—, —CH
2
CH
2
—, —CH
2
—, —NR—, —C(CF
3
)
2
—, —C(O)— and a covalent bond, wherein R is a C
1
-C
4
hydrocarbon chain; X
2
is independently selected from —R
3
, —OR
3
, —SR
3
, and —N(R
4
)R
3
; wherein R
3
is selected from C
1
-C
3
perfluorinated alkyl chain and partially fluorinated alkyl chain and R
4
is independently selected from R
3
and H; X
3
is independently selected from X
2
and H; X is independently selected from the group consisting of H, Cl, F, and Br, and m is 1 or 0.
The invention further encompasses several other novel polyimide compositions for generating alignment of liquid crystals.
REFERENCES:
patent: 5166304 (1992-11-01), Dubal et al.
patent: 5807498 (1998-09-01), Gibbons et al.
patent: 5808716 (1998-09-01), Gass et al.
patent: 5965691 (1999-10-01), Gibbons et al.
Gibbons Wayne M.
Shannon Paul J.
Sun Shao-Tang
Elsicon Inc.
Huntley & Associates
Wu Shean C.
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