Bistable polymer dispersed cholesteric liquid crystal displays

• US 6,061,107 A
• Filed: 05/07/1997
• Issued: 05/09/2000
• Est. Priority Date: 05/10/1996
• Status: Expired due to Term

• Current Assignee: Kent State University
• Original Assignee: Kent State University
• Inventors: Lu, Zhijian, Yang, Deng-Ke, Doane, J. William
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): Chowdhury, Tarifur R.
• Application Number:
  US08/852,589
• Time in Patent Office: 1,098 Days
• Field of Search: 349/86, 349/92, 349/93, 349/156, 349/35
• US Class Current: 349/86
• CPC Class Codes: G02F 1/1334 based on polymer dispersed liquid crystals, e.g. microencapsulated liquid crystals
• CPC Class Codes: G02F 1/13718 based on a change of the texture state of a cholesteric liquid crystal

Abstract

Polymer/cholesteric liquid crystal dispersions are provided in which the liquid crystal phase separated from the polymer matrix to form droplets. The cholesteric liquid crystals were positive dielectric anisotropic. At a zero field condition, the liquid crystal in the droplets was bistable, that is, the liquid crystal can be in either the reflecting planar state or the scattering focal conic state. When the liquid crystal 101 was in the planar texture, the helical axis of the liquid crystal was more or less perpendicular to the cell surface; colored light 105 was Bragg reflected. When the liquid crystal 101 was in the focal conic texture, the helical axis was more or less parallel to the cell surface, incident light was scattering 106 in the forward direction.

Claims

1. A bistable polymer dispersed cholesteric liquid crystal display comprising cells containing cholesteric liquid crystals having positive dielectric anisotropy, confined in domains which have a major axis and a minor axis, and wherein the major axis is larger in size than the cell thickness, wherein solid polymer walls are disposed between regions of liquid crystals.

2. The liquid crystal display of claim 1 wherein the domains comprise droplets.

3. The liquid crystal display of claim 2 having a black backplate, wherein the cell appears weakly scattering when a low electric field is applied perpendicular to the surface of the cell, and after the removal of the field the cell appears black.

4. The liquid crystal display of claim 3, wherein the liquid crystal within the droplet is transformed into a configuration where its helical axes are substantially parallel to the surface of the cell, and after removal of the field, the liquid crystal remains in this configuration.

5. The liquid crystal display of claim 3, wherein the cell appears color reflective when a high voltage is applied to the cell and is thereafter removed.

6. The liquid crystal display of claim 5, wherein the liquid crystal is switched into a homeotropic state with its director parallel to the applied high field, and after removal of the field, the liquid crystal relaxes back to a configuration where the helical axes are substantially perpendicular to the surface of the cell.

7. The liquid crystal display of claim 2 wherein droplets have an oval or oblong shape and droplet diameter between about 10 to about 50 μ

m.

8. The liquid crystal display of claim 1 having planar and focal conic textures, wherein both the planar and focal conic textures of the liquid crystals are stable under zero field.

9. The liquid crystal display of claim 1 wherein the cell is reflective when the cholesteric liquid crystals are in planar texture and is black when the cholesteric liquid crystals are in focal conic texture.

10. The liquid crystal of claim 1 wherein the liquid crystal is a liquid crystal that reflects visible light.

11. A method of making a multicolor reflective, bistable polymer dispersed cholesteric liquid crystal display comprising, providing a mixture comprising a photo tunable chiral material, a liquid crystal and polymer;

dissolving the mixture in a solvent to form a solution;

introducing the solution into a cell and heating;

cooling the mixture to obtain droplets of liquid crystal material in polymer that forms domains that have a major axis and a minor axis, and wherein the major axis is larger in size than the cell thickness for a majority of the domains;

masking and exposing with UV light, selected portions of the cell to localize cholesteric liquid crystal material of differing pitches in predetermined pixels within the cell.

12. The method of claim 11 wherein the polymer is a thermoplastic.

13. The method of claim 12 including controlling droplet size by the rate of cooling the mixture.

14. The method of claim 11 including forming a high droplet density in the polymer.

15. The method of claim 11 including forming pixels of four different colors by varying UV exposure times to various portions of the cell.