United States Patent
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`5,731,405
`[11] Patent Number:
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`Mar. 24, 1998
`Gibbons et al.
`[45] Date of Patent:
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`USOOS73 1405A
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`[19]
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`0539992
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`FOREIGN PATENT DOCUMENTS
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`5/1993 European Pat. Oif. .
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`OTHER PUBLICATIONS
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`Ishibashi, et al. (Mol. Cryst. Liq. Cryst., 1993, 225, 99-105).
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`Nishikawa,
`et al.,
`J. Appl. Phys.,
`1994, 33,
`(Jpn.
`L810-L812).
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`Fukuro, et al., (M01. Cryst. Liq. Cryst. 1988, 163, 157-162).
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`Dyadyusha, et aL (Mol. Cryst. Liq. Cryst. 1995, 263,
`399-413).
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`Primary Examiner—P. Hampton-Hightower
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`Attorney, Agent, or Firm—Huntley & Associates
`ABSTRACT
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`[57]
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`A process for inducing pre—tilt in alignment of a liquid
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`crystal medium comprising exposing at least one optical
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`alignment layer, comprising anisotropically absorbing mol-
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`ecules and hydrophobic moieties, to polarized light; the
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`polarized light having a wavelength within the absorption
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`band of said anisotropically absorbing molecules; wherein
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`the exposed anisotropically absorbing molecules induce
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`alignment of the liquid crystal medium at an angle + and -0
`with respect to the direction of the polarization of the
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`incident light beam and along the surface of the optical
`alignment layer, and induce a pre—tilt at an angle (1) with
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`respect to the surface of the optical alignment layer and
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`applying a liquid crystal medium to said optical alignment
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`layer, is described. The invention also is directed to liquid
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`crystal display elements made by the process of the inven-
`tion and to novel polyirnide compositions that are useful as
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`optical alignment layers in the invention.
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`24 Claims, 4 Drawing Sheets
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`[54] PROCESS AND MATERIALS FOR INDUCING
`PRE-TILT IN LIQUID CRYSTALS AND
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`LIQUID CRYSTAL DISPLAYS
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`[75]
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`Inventors: Wayne M. Gibbons, Bear, Del.; Paul
`J. Shannon, Exton, Pa.; Shao-Tang
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`Sun, Newark, Del.
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`[73] Assignee: Alliant Techsystems Inc., Hopkins,
`Minn.
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`[21] Appl. No.: 624,942
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`[22] Filed:
`Mar. 29, 1996
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`[51]
`Int. CL6 ............................ C08G 73/10; C08G 69/26
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`[52] U.S. Cl. .......................... 528/353; 528/125; 528/128;
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`528/170; 528/171; 528/172; 528/173; 528/174;
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`528/179; 528/183; 528/185; 528/188; 528/220;
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`528/229; 528/350; 428/1; 428/493.5
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`[58] Field of Search ...................,................. 528/125, 128,
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`528/170, 172, 173, 171, 174, 179, 183,
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`185, 188, 220, 229, 350, 353; 428/1, 473.5
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`[56]
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`References Cited
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`U.S. PATENT DOCUMENTS
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`4,923,960
`5/1990 Chen, Sr. et a1.
`...................... 528/353
`
`
`
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`
`.. 350/349
`12/1990 Gibbons et a1.
`4,974,941
`..
`
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`
`
`
`.. 528/353
`3/1991 Schwartz et a1.
`5,003,031
`.
`
`
`
`
`.. 350/341
`7/1991 Gibbons et a1.
`5,032,009
`..
`
`
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`5,153,304 10/1992 Nagase et al.
`
`
`
`...... 428/1
`2/1993 Estes et al.
`5,186,985
`.. 528/353
`11/1993 Auman ......
`5,260,408
`
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`8/1995 Yang ....................................... 528/353
`5,442,030
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`Page 1 of 23
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`Tianma Exhibit 1019
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`Page 1 of 23
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`Tianma Exhibit 1019
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`U.S. Patent
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`Mar. 24, 1998
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`Sheet 1 of 4
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`5,731,405
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`Fig.
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`Page 2 of 23
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`Page 2 of 23
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`U.S. Patent
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`Mar. 24, 1998
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`Sheet 2 of 4
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`5,731,405
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`/”\_/
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`Fig. 2
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`Page 3 of 23
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`Page 3 of 23
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`U.S. Patent
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`Mar. 24, 1993
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`Sheet 3 of 4
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`5,731,405
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`Fig.3
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`Page 4 of 23
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`Page 4 of 23
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`U.S. Patent
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`Mar. 24, 1998
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`Sheet 4 of 4
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`5,731,405
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`Fig.4
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`Page 5 of 23
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`Page 5 of 23
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`1
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`PROCESS AND MATERIALS FOR INDUCING
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`PRE-TILT IN LIQUID CRYSTALS AND
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`LIQUID CRYSTAL DISPLAYS
`BACKGROUND OF INVENTION
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`The present invention relates to processes for inducing
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`pre-tilt in alignment of a liquid crystal medium, composi-
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`tions useful in these processes, and liquid crystal display
`elements.
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`This invention was made with United States Government
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`support under cooperative agreement No. 70NANB4H1525
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`awarded by the United States Department of Commerce.
`The United States Government has certain rights in the
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`invention.
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`Liquid crystal compounds are used in human and machine
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`readable displays,
`finding applications in instrument
`controls, such as those in motor vehicles, avionics, medical
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`devices, process control devices and watches. Display
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`devices are primarily comprised of liquid crystal cells hav-
`ing a glass or other substrate coated with a transparent
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`conductive material in front and behind a liquid crystal
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`medium. Light transmission through these devices is con-
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`trolled through orientation of the liquid crystal compounds
`or dyes dissolved therein. In this way, a voltage or, in some
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`instances, a magnetic field may be applied to the cell so that
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`the liquid crystals are oriented in a fashion such that all,
`some or none of the light is passed through. In addition,
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`depending on the device geometry, polarizers may be used
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`in conjunction with the liquid crystal medium to control
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`light transmission.
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`Aligned liquid crystal cells in commerical use are typi-
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`cally oriented in directions suitable for controlling light
`transmission. That is, the molecules in the liquid crystal
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`composition are aligned so as to assume a homogeneous or
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`homeotropic alignment. Without external stimuli the display
`will either appear opaque or transparent. By applying an
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`electric field the molecules are rotated along a fixed axis so
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`as to alter the transmission properties in a desired fashion.
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`Current liquid crystal display elements include a product
`that utilizes a twisted nematic mode, i.e. having a structure
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`wherein the aligning direction of nematic liquid crystal
`molecules is twisted by 90° between a pair of upper and
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`lower electrode substrates, a product utilizing a supertwisted
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`nematic mode, utilizing a birefringent effect, i.e. having a
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`structure wherein the aligning direction of nematic liquid
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`crystal molecules is twisted by 180° to 300°, and a product
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`utilizing a ferroelectric liquid crystal substance or an anti-
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`ferroelectric liquid crystal substance. Common to each of
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`these products is a liquid crystal layer disposed between a
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`pair of substrates coated with a polymeric alignment layer.
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`The polymeric alignment layer controls the direction of
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`alignment of the liquid crystal medium in the absence of an
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`electric field. Usually the direction of alignment of the liquid
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`crystal medium is established in a mechanical bufling pro-
`cess wherein the polymer layer is buffed with a cloth or other
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`fiberous material. The liquid crystal medium contacting the
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`buffed surface typically aligns parallel to the mechanical
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`buffing direction. Alternatively, an alignment layer compris-
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`ing anisotropically absorbing molecules can be exposed to
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`polarized light to align a liquid crystal medium as disclosed
`in U.S. Pat. Nos. 5,032,009 and 4,974,941, both entitled
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`“Process of Aligning and Realigning Liquid Crystal Media,”
`which are hereby incorporated by reference.
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`The process for aligning liquid crystal media with polar-
`ized light is a noncontact method of alignment which can
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`reduce dust and static charge buildup on alignment layers.
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`2
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`Other advantages of the optical alignment process include
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`high resolution control of alignment direction and high
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`quality of alignment.
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`Requirements of optical alignment layers for liquid crys-
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`tal displays include low energy threshold for alignment,
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`transparency to visible light (no color), good dielectric
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`properties and voltage holding ratios, long-term thermal and
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`optical stability and in many applications a controlled uni-
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`form pre-tilt angle. Most liquid crystal devices, including
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`displays, have a finite pre-tilt angle, controlled, for instance.
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`by the mechanical bufling of selected polymeric alignment
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`layers. The liquid crystal molecules in contact with such a
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`layer aligns parallel to the bufling direction, but is not
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`exactly parallel to the substrate. The liquid crystal molecules
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`are slightly tilted from the substrate, for instance by about
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`2-15 degrees. For optimum performance in most display
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`applications a finite and uniform pre-tilt angle of the liquid
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`crystal is desirable.
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`The process for aligning liquid crystal media with polar-
`ized light has many attractive features. However, up to now,
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`controlling the pre-tilt angle of liquid crystals in contact with
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`optical alignment layers, whfle maintaining the uniformity
`of alignment, has been lacking. Furthermore to meet the
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`above stated requirements of transparency the use of aniso-
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`tropically absorbing molecules that absorb in the visible
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`region are generally not acceptable.
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`SUMMARY OF INVENTION
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`The instant invention provides a process of inducing
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`pre-tilt in alignment of a liquid crystal medium that is useful
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`in aligning liquid crystal displays and other liquid crystal
`devices; and new materials for optical alignment layers that
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`provide excellent alignment properties upon exposure to UV
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`light.
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`Specifically, the present invention provides process for
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`inducing pre-tilt in alignment of a liquid crystal medium
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`adjacent to a surface of an optical alignment layer compris-
`ing:
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`(a) exposing at least one optical alignment layer, com-
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`prising anisotropically absorbing molecules and hydro-
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`phobic moieties, to polarized light; the polarized light
`having a wavelength within the absorption band of said
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`anisotropically absorbing molecules; wherein the
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`exposed anisotropically absorbing molecules induce
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`alignment of the liquid crystal medium at an angle +
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`and -6 with respect to the direction of the polarization
`of the incident light beam and along the surface of the
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`optical alignment layer, and induce a pre-tilt at an angle
`II) with respect to the surface of the optical aligmnent
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`layer; and
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`(b) applying a liquid crystal medium to the optical align-
`ment layer.
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`The invention further pertains to a liquid crystal display
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`element derived from the process of the invention.
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`The invention further encompasses novel polyimide com-
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`positions for generating pre-tilt in alignment of a liquid
`crystal medium comprising a copolyirnide derived from at
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`least one diaryl ketone tetracarboxylic dianhydride, at least
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`one hydrophobic diamine and at least one alicyclic tetracar-
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`boxylic anhydride, which comprises at least two structural
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`elements of the formulas IV and V
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`55
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`4
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`3
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`X2
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`0
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`X1
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`0
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`0
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`0
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`O
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`O
`O
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`X X
`P
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`N—Y—
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`-N
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`IV
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`5
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`V 10
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`O
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`0
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`wherein Y is a divalent radical selected from the formulas l1
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`and III
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`15
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`E20
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`nation of layered materials that provide a useful function for
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`the final optical alignment layer or liquid crystal display. For
`example,
`the substrate can be any combination of the
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`following materials: crystalline or amorphous silicon, glass,
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`plastic, including polyester, polyethylene and polyirnide;
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`indium-tin—oxide, gold, silver, silicon dioxide,
`quartz,
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`polyimide, silicon monoxide, anti-reflective coatings. color
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`filter layers, polarizers and phase compensating films. In
`practice. some of these materials are deposited or coated
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`onto a basic supporting structure such as glass or plastic.
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`As used herein, the term “alignment layer” means the
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`layer of material on the surface of a substrate that controls
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`the alignment of a liquid crystal layer in the absence of an
`external field. A “conventional alignment layer” herein
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`refers to an alignment layer that will only align a liquid
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`crystal layer via processing other than optical means. For
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`example, mechanically bulfed polyimides, evaporated sili-
`con dioxide, Langrnuir-Blodgett films, have all been shown
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`to align liquid crystals.
`As used herein, the term “alignment of liquid crystals”
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`
`means that the long molecular axes of the liquid crystal
`
`
`
`
`
`
`
`
`
`
`molecules have a preferred local alignment direction, or
`
`
`
`
`
`
`
`director. The director is the average direction of an ensemble
`
`
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`
`
`
`
`
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`
`
`
`of liquid crystal molecules which can be quantified by order
`parameter or other measurements well known iii the art.
`
`
`
`
`
`
`
`Orientational order parameters are routinely described by
`
`
`
`
`
`
`
`
`the equation:
`S#z(3 C0520.-1)
`
`
`
`where or is the angle between the director and the long axis
`
`
`
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`
`
`
`
`
`of each molecule, the molecules being regarded as cylindri-
`
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`
`
`
`
`
`
`cally symmetric. The brackets denote an average over the
`
`
`
`
`
`
`
`
`ensemble of molecules. Order parameters range from 0 to
`
`
`
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`
`
`1.0. A 0 value indicates no long range alignment of the liquid
`
`
`
`
`
`
`
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`
`
`
`
`
`
`
`
`crystals is present. A value of 1.0 indicates the liquid crystal
`molecules are fully aligned along a director. Preferred order
`
`
`
`
`
`
`
`
`parameters resulting from the process of the instant inven-
`
`
`
`
`
`
`
`tion are in the range of about from 0.1 to 1.0.
`
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`“Optical alignment layer” herein refers to an alignment
`
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`layer that contains anisotropically absorbing molecules that
`
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`
`
`will align liquid crystals after exposure with polarized light.
`
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`
`Optical alignment layers may be processed by conventional
`means, such as mechanical rubbing, prior to or after expo-
`
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`
`
`
`sure to polarized light. The anisotropically absorbing mol-
`
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`
`
`
`
`ecules of the optical alignment layers exhibit absorption
`properties with difl°erent values when measured along axes
`
`
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`
`
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`
`
`in difl°erent directions. The anisotropic absorbing molecules
`
`
`
`
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`
`
`exhibit absorption of about from 150 nm to 2000 nm. The
`
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`anisotropically absorbing molecules of the optical alignment
`
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`layer can be covalently bonded within a main chain polymer,
`they can be covalently bonded as side groups to a main
`
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`
`polymer chain, they can be present as nonbonded solutes in
`
`
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`
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`
`
`a polymer, or they can be in the adjacent liquid crystal layer
`as a solute and adsorbed on the surface of a normal align-
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`
`
`
`ment layer to give an optical alignment layer.
`
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`
`
`Preferred optical alignment
`layers have absorbance
`maxima of about from 150 to 1600 nm. More preferable
`
`
`
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`
`
`
`
`
`
`
`
`
`
`
`optical alignment layers have absorbance maxima of about
`from 150 nm to 800 nm. Most preferable optical alignment
`
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`
`
`layers for the present invention have absorbance maxima
`
`
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`
`
`
`
`
`between 150 and 400 nm and especially between 300 and
`
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`
`400 um.
`
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`
`
`Anisotropically absorbing molecules useful in preparation
`
`
`
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`
`
`
`of optical alignment
`layers are the dichroic arylazo,
`
`
`
`di(arylazo),
`tri(arylazo),
`tetra(arylazo), penta(arylazo),
`
`
`
`anthraquinone, mericyanine, methine, 2-phenylazothiazole,
`
`
`2-phenylazobenzthiazole, stilbene, 1,4-bis(2-phenylethenyl)
`
`
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`
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`
`25
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`30
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`35
`
`
`
`40
`
`
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`45
`
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`
`50
`
`
`
`55
`
`
`
`
`/deli
`
`wherein Z is independently selected from the group con-
`
`
`
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`
`
`
`
`
`
`
`sisting of —S—, —O—, —SO2—, —CH2-, -C(CF3)2—,
`-C(O)—, -CH2CH._,—, —NR- and a covalent bond
`
`
`
`
`
`wherein R is a C1-C4 hydrocarbon chain; X is independently
`
`
`
`
`
`selected from R1, ——O—R1. —S-R1, —N(R2)-—R1;
`
`
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`
`
`wherein R1 is independently selected from C4—C2o perflu-
`
`
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`
`
`
`
`
`orinated alkyl chain, C4—C._,o partially fluorinated alkyl
`chain. and C10-C20 hydrocarbon chain; X1 is independently
`
`
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`
`
`
`selected from X and H; R2 is selected, independently, from
`
`
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`
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`
`
`H. C1-C9 hydrocarbon chain and R1; X2 is independently
`selected from the group consisting of H, CL, F, Br, R3 and
`
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`
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`
`
`
`
`R30-, wherein R3 is independently selected from the group
`
`
`
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`
`
`
`consisting of C1-C3 perflourinated alkyl chain, C1-C3 par-
`
`
`
`
`
`
`
`
`tially flourinated alkyl chain and C1-C8 hydrocarbon chain;
`In is 1 or 0; and P is a tetravalent organic radical derived
`
`
`
`
`
`
`
`
`
`
`
`
`
`from the alicyclic tetracarboxylic dianhydride containing at
`least four carbon atoms, no more than one carbonyl group of
`
`
`
`
`
`
`
`
`
`the dianhydride being attached to any one carbon atom of the
`
`
`
`
`
`
`
`
`
`tetravalent radical.
`
`
`The invention further encompasses other novel polyimide
`
`
`
`
`
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`
`
`
`
`
`
`
`compositions for generating pre-tilt in alignment of a liquid
`
`crystal medium.
`
`
`
`
`
`BRIEF DESCRIPTION OF DRAWINGS
`
`
`FIG. 1 is a cross-sectional View of a general liquid crystal
`
`
`
`
`
`
`display cell of the present invention.
`
`
`
`
`
`FIG. 2 illustrates pre-tilt angle (1).
`
`
`
`
`
`FIG. 3 shows the system used to expose coated substrates
`
`
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`
`
`to ultraviolet light.
`
`
`
`FIG. 4 is a schematic illustration of the system used to
`
`
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`
`
`expose coated substrates with ultraviolet light from a UV
`
`
`
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`
`
`
`lamp source.
`
`
`
`DETAILED DESCRIPTION
`
`
`As used herein “substrate” means the supporting structure
`
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`
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`for an alignment layer. A substrate can be any solid combi-
`
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`65
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`Page 7 of 23
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`Page 7 of 23
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`

`
`
`5,731,405
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`6
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`5
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`10
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`15
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`20
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`30
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`35
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`
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`45
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`
`
`benzene, 4,4’-bis(ary1azo)stilbenes, perylene and 4,8-
`
`
`
`
`
`diarnino-l,5-napthoquinone dyes. Other useful anisotropi-
`
`
`
`
`
`
`
`
`cally absorbing materials are the liquid crystal coupled
`
`
`
`
`
`
`
`dichroic dyes described in U.S. Pat. No. 5,389,285.
`
`
`
`
`
`
`Preparation of the anisotropically absorbing materials
`listed above are well known as shown, e.g., by Huffman et
`
`
`
`
`
`
`
`
`al, in U.S. Pat. Nos. 4,565,424, Jones et al, in 4,401,369,
`
`
`
`
`
`
`
`
`
`
`Cole, Jr. et al. in 4,122,027, Etzbach et al, in 4,667,020, and
`
`
`
`
`
`
`
`
`Shannon et al, in 5,389,285.
`
`
`
`
`Other anisotropically absorbing molecules useful in the
`
`
`
`
`
`
`
`
`
`
`
`
`
`preparation of colorless optical alignment layers are diaryl
`ketones having a ketone moiety or ketone dmivative in
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`conjugation with two aromatic rings. Specific families of
`these molecules useful in optical alignment layers are sub-
`
`
`
`
`
`
`
`
`
`
`
`
`stituted benzophenones, benzophenone imines,
`
`
`
`
`
`phenylhydrazones, and semicarbazones. Specific benzophe-
`
`
`
`
`
`
`
`
`none derivatives preferred in optical alignment layers for the
`
`
`
`
`
`
`
`process of this invention are benzophenone, 4,4’-
`
`
`diaminobenzophenone, 4,4‘-bis(trifluoromethyl)
`
`
`
`benzophenone, 3,4’-bis(trifluoromethyl)benzophenone, and
`
`
`
`3,3‘-bis(trifluoromethyl)benzophenone. The benzophenone
`and 4,4‘-trifluoromethylbenzophenone imines derived from
`
`
`
`
`
`
`
`
`n-octadecylamine, 4—hexyloxyaniline
`and
`
`
`
`
`
`
`4-octadecyloxyaniline are also preferred. The phenylhydra-
`25
`
`
`
`
`zones of benzophenone, 4,4'—bis(trifluoromethyl)
`
`
`
`benzophenone, 3,4’-bis(trifluoromethyl)benzophenone, 3,3’-
`
`
`
`bis(trifluoromethyl)benzophenone, derived from
`condensation with phenylhydrazine; and the 2,4-
`
`
`
`
`
`
`
`
`
`
`diaminophenylhydrazones of benzophenone, 4,4‘-bis
`
`
`(trifluoromethyl)benzophenone, 3,4'—bis(trifluoromethyl)
`
`
`
`benzophenone, and 3,3'-bis(trifluoromethyl)benzophenone,
`derived from the chemical reduction of the corresponding
`
`
`
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`
`
`
`
`
`
`2,4-dinitrophenylhydrazones are also preferred in optical
`
`
`
`
`
`
`alignment layers for the present invention.
`
`
`
`
`
`
`Preferred anisotropically absorbing molecules for optical
`
`
`
`
`
`
`
`alignment layers are arylazo, poly(arylazo), stilbene and
`
`
`
`
`
`
`
`
`diaryl ketone derivatives. Arylazo, stilbene and diaryl ketone
`derivatives are the most preferred dyes for optical alignment
`
`
`
`
`
`
`
`
`
`layers having absorbance maxirna of about from 150 to 400
`
`
`
`
`
`
`
`
`nm. Poly(arylazo) dyes are most preferred for optical align-
`
`
`
`
`
`
`
`
`
`ment layers having absorbance maxima of about from 400 to
`
`
`
`
`
`
`
`
`800 nm. A most preferred poly(azo) dye is diazodiamine A;
`
`
`
`
`
`
`a most preferred stilbene dye is 4,4’-diaminostilbene, B; a
`
`
`
`
`
`
`
`
`
`
`
`
`
`most preferred arylazo dye is monoazodiamine C (Table 1).
`The preparation of the dye A is described in U.S. Pat. No.
`
`
`
`
`
`
`
`
`5,389,285; synthesis of dye C is described in the examples;
`
`
`
`
`
`
`and 4,4‘-diaminostilbene is commercially available from
`
`
`
`
`
`
`Aldrich Chemical Co., Milwaukee, Wis., as the dihydro-
`
`
`
`
`
`
`
`
`chloride salt.
`
`
`
`
`
`
`
`
`Diaryl ketone tetracarboxylic dianhydrides are especially
`useful and preferred as anisotropically absorbing molecules.
`
`
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`
`
`
`
`
`
`
`
`Preferred diaryl
`lretone tetracarboxylic dianhydrides are
`further described in greater detail infra in the discussion of
`
`
`
`
`
`
`
`
`polyimides.
`
`
`
`
`
`
`
`
`Optical alignment layers used in the process of this
`invention also comprise hydrophobic moieties. “Hydropho-
`
`
`
`
`
`
`bic moieties” refer to functional groups that impart strong
`
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`
`
`water imicibility and high surface tension properties to
`
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`
`
`
`materials. The hydrophobic moieties are usually, but not
`
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`
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`
`
`exclusively, covalently bonded to a polymer that also acts as
`
`
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`
`
`
`
`a matrix or carrier for the anisotropically absorbing mol-
`
`
`
`
`
`
`
`
`ecules that make up the optical alignment
`layer. Most
`
`
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`
`
`
`
`
`notable hydrophobic moieties are fluorinated and partially
`
`
`
`
`
`
`
`. fluorinated alkyl chains and long chain aliphatic hydrocar-
`
`
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`
`
`
`
`
`
`bons. Common hydrophobic moieties containing fluorinated
`
`
`
`
`
`
`and partially fluorinated alkyl chains, for instance, are
`
`
`
`
`
`
`
`
`monovalent
`lH,1H-pentadecafluoro-l-octyloxy and HH-
`
`
`
`
`
`
`
`
`
`
`
`
`eicosafluoroundecanoyl groups, which are readily available
`from commercial materials or well known syntheses. Com-
`
`
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`
`
`
`
`
`
`
`mon hydrophobes containing long aliphatic hydrocarbon
`
`
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`
`
`
`chains are the monovalent hexadecyl, hexadecyloxy,
`
`
`
`
`
`
`
`
`octadecyl, and octadecyloxy groups and the divalent hexa-
`
`
`
`
`decylrnethylene and octadecylrnethylene groups.
`
`
`
`
`
`
`Methods for incorporating hydrophobic moieties into
`
`
`
`
`
`
`
`
`many polymer materials are well known. Examples of
`
`
`
`
`
`
`
`
`polymers having hydrophobic moieties that are useful as
`
`
`
`
`
`
`
`matrices for optical alignment layers are poly(methyl
`
`
`
`
`
`
`methacrylate) and poly(methyl acrylate)’ oopolymers con-
`
`
`
`
`
`
`
`taining various loadings of fluoroalkyl methacrylates and
`
`
`
`
`
`fluoroalkyl acryaltes
`such as
`lH,1H,l1H-
`
`
`
`
`eicosafluoroundecyl methacrylate, for example.
`Aromatic diamines substituted with hydrophobic moieties
`
`
`
`
`
`
`
`
`
`
`
`
`
`are especially useful and preferred as hydrophobic moieties
`
`
`
`
`
`
`
`
`in the synthesis of polyirnides for optical alignment layers.
`
`
`
`
`
`
`
`Preferred hydrophobic diamines are further described infra
`in the discussion of polyimides.
`
`
`
`
`
`
`
`
`
`Preferably, anisotropically absorbing molecules and
`
`
`
`
`
`
`hydrophobic moieties are covalently bonded to polymers.
`For instance, poly(arnic acid)s, which are precursors to
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`polyimides, can be prepared with anisotropic absorbing
`
`
`
`
`
`
`
`
`materials covalently bonded into the poly(a.mic. acid) poly-
`
`
`
`
`
`
`
`mer chain. This typically is accomplished by mixing of
`
`
`
`
`
`
`dianhydride and diamines,
`including the anisotropically
`absorbing molecules as one of the two reactive components.
`
`
`
`
`
`
`
`For instance, 4,4‘-diaminostilbene is an anisotropically
`
`
`
`
`
`
`absorbing molecule that also can act as a reactive diarnine in
`
`
`
`
`
`
`
`
`
`
`
`polyimide synthesis. 3,3'4,4’-benzophenonetetracarboxylic
`
`
`
`
`
`
`
`anhydride is an anisotropically absorbing molecule that also
`can act as a reactive dianhydride in polyimide synthesis.
`
`
`
`
`
`
`
`Allowing the diamines and dianhydrides to polymerize in a
`
`
`
`
`
`
`solvent such as N-methylpyrolidone or tetrahydrofuran pro-
`
`
`
`
`
`vides a prepolyrner solution that is then coated on substrates
`
`
`
`
`
`
`
`and oven baked to give the final polyimide optical alignment
`
`
`
`
`
`
`
`
`
`
`layers.
`
`
`
`
`
`
`
`Alternatively, optical alignment layers can have anisotro-
`
`
`
`
`
`
`pically absorbing molecules present as nonbonded solutes
`dissolved in a polymer containing hydrophobic moieties.
`
`
`
`
`
`These are referred to as guest—host optical alignment layers.
`
`
`
`
`
`
`
`They are prepared by coating on substrates a thin layer of
`
`
`
`
`
`
`
`
`
`
`
`
`organic material containing the anisotr'opically absorbing
`
`
`
`
`
`
`molecules. Typically the anisotropically absorbing mol-
`ecules are dissolved in solution along with a polymeric
`
`
`
`
`
`
`
`
`material. The solution is then coated on substrates using,
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`typically, a spin casting technique. ‘The coatings are then
`oven baked to remove residual solvent and perform the final
`
`
`
`
`
`
`
`
`cure.
`
`
`
`
`
`
`
`Alternatively, optical alignment layers are prepared by
`
`
`
`
`
`
`coating conventional alignment layers such as a hydropho-
`bic polyimide on the substrates. The anisotropically absorb-
`
`
`
`
`
`
`
`ing molecules are dissolved in a liquid crystal medium to
`
`
`
`
`
`
`
`
`
`
`
`
`
`give a guest-host mixture. When the guest-host mixture
`
`
`
`
`
`containing anisotropically absorbing molecules is allowed to
`contact a conventional alignment layer an optical alignment
`
`
`
`
`
`
`layer is formed.
`
`
`
`
`
`
`Alternatively, optical alignment layers are prepared by
`coating conventional alignment layers such as hydrophobic
`
`
`
`
`
`
`polyimide on the substrates and anisotropically absorbing
`
`
`
`
`
`
`molecules are dissolved in a solvent. When the solution
`
`
`
`
`
`
`containing anisotropically absorbing molecules is coated on
`
`
`
`
`
`
`
`
`
`
`
`
`the conventional alignment layer an optical alignment layer
`is formed.
`
`
`
`
`
`
`
`
`
`
`Preferred polymers for optical alignment layers of this
`invention are polyimide polymers. The preparation of poly-
`
`
`
`
`
`
`
`irnides is described in “Polyimides”, D. Wilson, H. D.
`
`
`
`
`
`
`
`
`
`
`50
`
`
`
`55
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`65
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`Page 8 of 23
`
`Page 8 of 23
`
`

`
`5,731,405
`
`
`
`8
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`chain, C4-Cm partially fluorinated alkyl chain, and C1o—C20
`
`
`
`
`
`hydrocarbon chain; X1 is independently selected fromX and
`
`
`
`
`
`
`H; R2 is independently selected from H, C1-C9 hydrocarbon
`chain and R1; and wherein M is a tetravalent organic radical
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`derived from said tetracarboxylic dianhydride containing at
`least two carbon atoms, no more than two carbonyl groups
`
`
`
`
`
`
`
`
`
`of the dianhydride being attached to any one carbon atom of
`
`
`
`
`
`
`
`
`
`the tetravalent radical.
`
`
`
`Diamines useful in this invention to induce a finite pre-tilt
`
`
`
`
`
`
`
`
`
`
`
`
`
`of a liquid crystal medium as well as provide good alignment
`
`
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`
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`uniformity are listed in Table 2. Preferred hydrophobic
`diamines have the structure
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`NH2-Y—NH2
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`10
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`15
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`20
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`25
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`7
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`Stenzenberger. and P. M. Hergenrother Eds., Chapman and
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`Hall, New York (1990). Typically polyimides are prepared
`by the condensation of one equivalent of a diamine com-
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`ponent with one equivalent of a dianhydride component in
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`a polar solvent to give a poly(amic acid) prepolymer inter-
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`mediate. Typical solvents used in the condensation reaction
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`are N-methylpyrrolidone (NMP), dimethylacetamide
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`(DMAC). dimethylformamide (DMF), dimethylsulfoxide
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`(DMSO), butyl cellosolve, ethylcarbitol,
`'y—butyrolactone,
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`etc. The poly(arnic acid) is typically formulated to give a 1
`to 30 wt % solution. The condensation reaction is usually
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`performed between room temperature and 150° C. The
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`prepolymer solution is coated onto a desired substrate and
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`thermally cured at between 180° and 300° C. to complete the
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`imidization process. Alternatively. the poly(aJnic acid) pre-
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`polymer is chemically imidized by addition of a dehydrating
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`agent to form a polyimide polymer. Examples of chemical
`imidzation reagents are organic anhydrides such as acetic
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`anhydride and trifluoroacetic anhydride in combination with
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`organic bases such as triethyl amine and pyridine. Other
`chemical imidization reagents are ethylchloroformate and
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`ttiethylamine. thionyl chloride. oxalyl chloride. acetyl chlo-
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`ride and dicyclohexylcarbodiimide. Chemical imidizations
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`are performed between room temperature and 150° C.
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`Chemical irnidization requires that the resulting polyimide
`be soluble in a solvent for further processing. Achieving
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`solubility often requires polyimides to be specially formu-
`lated for chemical imidization. The chemically imidized
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`polyimide solution is coated onto a substrate and heated to
`remove solvent. but no high temperature cure is required
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`Preferred compositions of this invention are chemically
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`imidized polyimides.
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`Especially useful in the process of the invention is a
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`polyimide polymer that is the condensation reaction product
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`of hydrophobic diamines and dianhydrides. Preferred is a
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`polyimide polymer that is a homopolymide or a copolyimide
`of at least one tetracarboxylic dianhydride and at least one
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`hydrophobic diamine. which comprises at least one struc-
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`tural element of the formula I:
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`_N/(')kM/(')K
`if if
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`N—Y—
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`wherein Y is a divalent radical selected from the formula II
`