Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-02-04
2001-01-23
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S141000, C349S093000
Reexamination Certificate
active
06177972
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to liquid crystal display (LCD) devices. In particular, the invention relates to display panels for use in LCD devices, such as handheld personal organizers, flat-panel desktop displays, portable computers, and handheld personal communication devices.
BACKGROUND OF THE INVENTION
U.S. Pat. No 4,345,249 to S. Togashi describes a LCD display panel in which the electrodes of each display element of the LCD panel comprise a pair of intermeshed, or interdigitated, electrodes in substantially a single plane. U.S. Pat. No. 4,345,249 discloses a first comb-shaped electrode pattern, to which a display signal voltage is applied through a switching element, and a second comb-shaped electrode pattern which is held at a reference potential. The first and second comb-shaped patterns are mutually intermeshed and formed upon a single substrate of the panel, such that application of a display signal voltage causes an electric field to be produced in a direction substantially parallel to the substrate plane. Such a display has come to be referred to as an in-plane switched (IPS) LCD. This is in contrast to a conventional twisted nematic (TN) LCD, in which the electric field which acts upon the liquid crystal is produced in a direction normal to the substrate plane.
IPS LCDs typically have the advantage of much wider viewing angles than TN LCDs ( see M. Oh-e and K. Kondo, Appl. Phys. Lett. 67, 3895 (1995)) exhibiting less angle-dependent contrast and color variation. However, IPS displays typically have the disadvantage of slower switching times than conventional TN LCDs following the removal of the applied display signal voltage. These slower switching times have been one of the factors hindering the widespread adoption of IPS liquid crystal display panels despite the important advantage of much wider viewing angle.
SUMMARY OF THE INVENTION
It is an objective of the present invention to reduce the switching time of an IPS liquid crystal display device following removal of the applied display signal voltage. This is done by forming a network of polymer strands within the nematic liquid crystal. The switching of display elements of the IPS LCD panel or cell when the applied field is removed is believed to be aided by any reorientation force which may be applied by the display panel itself to the liquid crystal material. Typically in current IPS LC displays, this is done by alignment layers applied on the two substrates of the cell. According to the present invention, the formation of a network of polymer strands throughout the entire liquid crystal material is believed to provide an elastic orienting force which permeates the entire volume of the liquid crystal within the display panel, resulting in a faster switching time after the applied field is removed. Polymer networks have been shown to stabilize desired textures in cholesteric-based displays, as described in I. Dierking, L. L. Kosbar, A. Afzali-Ardakani, A. C. Lowe and G. A. Held, J. Appl. Phys. 81, 3007 (1997), the results being known as polymer stabilized cholesteric textures (PSCTs). U.S. Pat. No. 5,691,795 to J. William Doane et al. for “Polymer Stabilized Liquid Crystalline Light Modulating Device and Material” discusses the use of PSCT's in reflective displays and other light modulating devices.
Broadly, the present invention provides an in-plane switched liquid crystal display panel comprising:
a) a pair of substrates,
b) a layer of liquid crystal material disposed between said pair of substrates, said liquid crystal material having a nematic orientation in the absence of an applied electric field (i.e., in the “field-off” state),
c) a plurality of display elements, each display element comprising a pair of mutually intermeshed electrodes disposed in a substantially single plane, and
d) a network of phase-separated strands of mesogenic polymer distributed throughout said liquid crystal material, said polymer strands having an average orientation which substantially conforms with the aforesaid nematic orientation in the absence of an applied electric field.
The invention further provides a method of fabricating an in-plane switched liquid crystal display device panel comprising the steps of:
a) preparing a mixture of a nematic liquid crystal material (having a nematic orientation in the absence of an applied electric field) and a polymerizable material, preferably selected from mesogenic polymers and mesogenic polymer precursors (e.g. mesogenic monomers) having appropriate chain-growing (polymerizing) and cross-linking functional groups, together with photoinitiators and curing agents, admixed therewith, as necessary, to support polymerization,
b) introducing said mixture into an empty in-plane switched liquid crystal display (IPS LCD) panel comprising a pair of substrates, one of said substrates comprising a plurality of display elements formed thereon, each display element comprising a pair of mutually intermeshed electrodes disposed in a substantially single plane, and
c) initiating polymerization of said polymerizable material (e.g. thermally, by UV radiation) to cause it to form phase-separated strands of cross-linked polymer, said polymer strands having an average orientation which substantially conforms with said nematic orientation in the absence of an applied field.
The mesogenic polymeric material may be selected from UV-curable, thermoplastic, and thermosetting polymers and their monomeric precursors, including polymers formed from monomers having at least one chain-growing functional group (e.g. double bond) with or without the presence of cross-linking agents. By way of specific example, the liquid crystal material may be nematic liquid crystal E-48 (Merck Industries), the polymeric material may be formed from the monomer RM206 (Merck Industries) with benzoin methyl ether (BME) used as a photoinitiator responsive to UV (ultraviolet) radiation.
REFERENCES:
patent: 4345249 (1982-08-01), Togashi
patent: 4925708 (1990-05-01), Waters et al.
patent: 5305126 (1994-04-01), Kobayashi et al.
patent: 5422036 (1995-06-01), Kawakami et al.
patent: 5686017 (1997-11-01), Kobayashi et al.
patent: 5814378 (1998-09-01), Onishi et al.
Fung et al “Polymer networks formed in liquid crystals” Liquid Crystals 1995, vol. 19 No. 6, p. 797, May 1995.
Held Glenn Allen
Lien Shui-Chih Alan
Yoon Do Yeung
International Business Machines - Corporation
McGinn & Gibb, P.C.
Sikes William L.
Ton Toan
Underweiser, Esq. Marian
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