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United States Patent m
Wu et al.
[ii] Patent Number: 4,671,618  Date of Patent: Jun. 9, 1987
 LIQUID CRYSTALLINE-PLASTIC
MATERIAL HAVING SUBMILLISECOND
SWITCH TIMES AND EXTENDED MEMORY
 Inventors: Bao-Gang Wu, 947 Allerton St.; J.
William Doane, 1618 S. Lincoln St., both of Kent, Ohio 44240
 Appl. No.: 879,327
 Filed: Jun. 27,1986
Related U.S. Application Data
 Continuation-in-part of Ser. No. 866,216, May 22, 1986, which is a continuation-in-part of Ser. No. 776,851, Sep. 17, 1985, which is a continuation-in-part of Ser. No. 590,996, Mar. 19, 1984, abandoned.
 Int. CM G02F1/13
 U.S. CI 350/347 V; 350/320;
 Field of Search 350/320, 347 V, 347 R,
350/350 R, 351
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Primary Examiner—Bruce Y. Arnold
Assistant Examiner—Richard F. Gallivan
Attorney, Agent, or Firm—Watts, Hoffmann, Fisher &
The invention provides a new liquid crystal-plastic display material having submillisecond switching times and an electrooptic bistable memory. The material comprises microdroplets of liquid crystal dispersed in a plastic matrix, the liquid crystal being present in an amount of at least 50% by weight. The material is prepared by the phase separation of a homogeneous solution of liquid crystal and synthetic polymer. Phase separation results in a liquid crystalline rich phase in the form of microdroplets and a plastic rich phase containing dissolved liquid crystal in solid solution with the plastic. The dissolved liquid crystal alters refractive index and the dielectric properties of the polymer, and lowers the temperature at which the plastic softens, making possible the fabrication of liquid crystal display materials having submillisecond switching rates and transparencies on the order of 90% and further having sufficiently high resistivity and permittivity so as to act as capacitors to maintain images for extended bistable memory.
18 Claims, 3 Drawing Figures
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LIQUID CRYSTALLINE-PLASTIC MATERIAL
HAVING SUBMILLISECOND SWITCH TIMES
AND EXTENDED MEMORY
This application is a continuation-in-part of U.S. patent application Ser. No. 866,216, filed May 22, 1986 which is a continuation-in-part of U.S. patent application Ser. No. 776,831, filed Sept. 17, 1985, which is a 10 continuation-in-part of U.S. patent application Ser. No. 590,996, filed Mar. 19, 1984, now abandoned.
This invention relates generally to liquid crystal dis- 15 play technology and, more specifically, to a new liquid crystalline-plastic material having submillisecond switch times and electrooptic memory. The material has a high electrical resistivity and permittivity and 20 comprises microdroplets of liquid crystal phase separated in situ from a homogeneous solution of liquid crystal and polymer, the liquid crystal being present in the material in an amount of at least 50% by weight
BACKGROUND ART 25
As disclosed in U.S. patent applications Ser. No. 776,831, filed Sept. 17, 1985 and Ser. No. 866,216, filed May 22, 1986, the disclosures of which are incorporated by reference, liquid crystal display devices which ex- j0 ploit the light scattering and transmitting properties of discrete quantities of liquid crystal can be fabricated by polymerization induced phase separation of homogeneous solutions of liquid crystal and various synthetic polymers. The liquid crystal-polymeric material can be 35 formed as a film or sheet and placed between transparent conducting electrodes to form an electrically switchable light shutter.
Liquid crystals useful for light scattering displays have two indices of refraction: an extraordinary index 40 of refraction ne measured along the long axis of the liquid crystals, and a smaller ordinary index of refraction n0, measured in a plane perpendicular to the long axis. The long axis of the liquid crystal defines its optic axis. 45
Entrapping liquid crystals in microdroplets in a polymeric matrix having an index of refraction n, will result in a device which either scatters or transmits incident light depending upon the relationship among the indices of refraction and dependent upon the microdroplets 50 being of a size to scatter incident light, e.g., on the order of 0.1 to 10 microns.
In the absence of an applied field, the optic axes of the microdroplets have no preferred direction in which to point, so that incident light encounters a mismatch be- 55 tween the ns of the matrix and the ne of the microdroplets. The result of the mismatch is that the light is scattered and the device appears opaque. Application of an electric field across the sheet of liquid crystallinepolymeric material causes the optic axes to align paral- 60 lei to the field and normal to the surface of the sheet. Incident light detects no mismatch between nD and ns and is transmitted so that the device appears clear.
Liquid crystal devices as disclosed in U.S. patent application Ser. No. 776,831 do not have an image mem- 65 ory. Images formed by contrasting opaque and clear areas are displayed on these devices by the constant application of a field to those areas desired to appear
clear. Once the field is removed, the clear areas switch back to opaque in about 10-100 seconds.
U.S. patent application Ser. No. 866,216 teaches liquid crystal-polymeric materials having optical memory in that light scattering devices that include the materials can be manipulated to display contrasting opaque and clear areas which are field independent. The memory materials of U.S. patent application Ser. No. 866,216 are, in general, made by forming a homogeneous solution of liquid crystal and thermoplastic polymer heated to a temperature effective to dissolve the liquid crystal, and then cooling the solution to affect phase separation of the liquid crystal and the formation of microdroplets. Memory is made possible by the selection of a liquid crystal which remains in the liquid crystalline state at temperatures above the softening temperature of the thermoplastic. Since the liquid crystal is in the liquid crystalline state while the thermoplastic is soft, the optic axes of the microdroplets can be aligned by an external field. Maintaining the external field while the thermoplastic is rehardened results in a material in which the microdroplets exposed to the field remain aligned upon removal of the field. An external field applied to the entire surface of a film during hardening of the thermoplastic will result in a wholly transparent material; a patterned field as, for instance, an alphanumeric character, applied during hardening of the thermoplastic will result in a material displaying a clear (transmitting) character in an opaque (scattering) field. The clear area or areas will remain so until the random alignment of the optic axes is restored by reheating the material and cooling it in the absence of a field.
While this type of memory achieved by the selective application of thermal and electric or magnetic fields has many useful applications, it would be advantageous in many instances to have a memory not dependent upon thermal energy and characterized by faster switching times and greater transparencies than previously achievable.
DISCLOSURE OF THE INVENTION
The invention provides a new liquid crystal-plastic display material having submillisecond switching times and an electrooptic memory. The material comprises microdroplets of liquid crystal dispersed in a plastic matrix, the liquid crystal being present in an amount of at least 50% by weight. The material is prepared by the phase separation of a homogeneous solution of liquid crystal and synthetic polymer. Phase separation results in a liquid crystalline rich phase in the form of microdroplets and a plastic rich phase containing dissolved liquid crystal in solid solution. The dissolved liquid crystal alters the refractive index and the dielectric properties of the plastic, and lowers the temperature at which the plastic softens. The resulting liquid crystal display materials ion have submillisecond switching rates and transparencies on the order of 90%. The new material is further characterized by high resistivity and permittivity so that it acts as a capacitor to retain a charge when charged between two electrodes, whereby the optic axes of the liquid crystal microdroplets will remain aligned and an image will be retained for a period of time ("memory time") after the voltage has been turned off.
As used in the specification and claims, the term "plastic" means a synthetic polymer which has been blended or plasticized with liquid crystal. The plastic materials of the invention display thermoplastic proper