Liquid crystalline-plastic material having submillisecond switch times and extended memory

• US 4,671,618 A
• Filed: 06/27/1986
• Issued: 06/09/1987
• Est. Priority Date: 05/22/1986
• Status: Expired due to Term

• Current Assignee: Kent State University
• Original Assignee: Kent State University
• Inventors: Doane, J. William, Wu, Bao-Gang
• Primary Examiner(s): Arnold, Bruce Y.
• Assistant Examiner(s): Gallivan, Richard F.
• Application Number:
  US06/879,327
• Time in Patent Office: 347 Days
• Field of Search: 350/320, 350/347 V, 350/347 R, 350/350 R, 350/351
• US Class Current: 349/92
• CPC Class Codes: C09K 19/544 as dispersing or encapsulating medium around the liquid crystal
• CPC Class Codes: G02F 1/1334 based on polymer dispersed liquid crystals, e.g. microencapsulated liquid crystals
• CPC Class Codes: Y10S 359/90 Methods

Abstract

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.

Claims

1. A liquid crystalline-plastic material wherein liquid crystal is dispersed in set plastic as microdroplets having a size effective to scatter incident light, the liquid crystal being present in the material in an amount of at least 50% by weight and wherein the plastic has an index of refraction n_s matching the ordinary index of refraction n.sub. o of the liquid crystal so that the material will transmit on the order of 90% of incident light when a voltage is applied, and said material being further characterized by a resistivity and a permittivity such that the product of the two is on the order of a few seconds or longer, whereby the material will act as a capacitor to retain a charge when charged between two electrodes.

5. A liquid crystalline-plastic material as claimed in claims 1, 2, 3, or 4 wherein the liquid crystal comprises a nematic liquid crystal.

6. A liquid crystalline-plastic material as claimed in claim 5 wherein the nematic liquid crystal comprises a cyanobiphenyl.

7. A liquid crystalline-plastic material as claimed in claims 1, 2, 3 or 4 wherein the plastic is selected from the group consisting of poly(vinyl butyral), poly(vinyl acetate), poly(vinyl formal), polycarbonate, poly(vinyl methyl ketone), poly(methyl acrylate), poly(cyclohexyl methacrylate) poly(isobutyl methacrylate) and poly(methyl methacrylate).

2. A liquid crystalline-plastic material wherein liquid crystal is dispersed in set plastic as microdroplets having a size effective to scatter incident light, the liquid crystal being present in the material in an amount of at least 50% by weight and wherein the plastic has an index of refraction n_s matching the ordinary index of refraction n_o of the liquid crystal so that the material will transmit on the order of 90% of incident light when a voltage is applied, and said material being further characterized by a switching time on the order of less than about 1.0 millisecond.

3. A liquid crystalline-plastic material wherein liquid crystal is dispersed in set plastic as microdroplets having a size effective to scatter incident light, the liquid crystal being present is an amount of at least 50% by weight and wherein the plastic has an index of refraction n_s sufficiently greater than the ordinary index of refraction n_o of the liquid crystal so that the material will have maximum transparency in the presence of an applied field at an angle of less then 90°

to the surface of the material.

4. A liquid crystalline-plastic material wherein the liquid crystal is dispersed in the plastic as microdroplets having a size effective to scatter incident light, the liquid crystal being present in the material in an amount of at least 50% by weight, wherein the plastic has an index of refraction n_s matching the ordinary index of refraction n_o of the liquid crystal so that the material will transmit on the order of 90% of incident light when a voltage is applied, and wherein the plastic has the characteristic of forming a homogeneous solution with the liquid crystal and of undergoing phase separation from the liquid crystal to form a coherent matrix surrounding microdroplets of liquid crystal, said material being characterized by a resistivity and a permittivity such that the product of the two is on the order of a few seconds or longer, whereby the material will act as a capacitor to retain a charge when charged between two electrodes and said material being further characterized by a switching time on the order of less than about 1.0 milliseconds.

8. A method of making a liquid-crystalline plastic material comprising the steps of forming a homogeneous solution of plastic and at least 50% by weight liquid crystal and phase separating the solution to form microdroplets of liquid crystal dispersed in a coherent closedcell-like matrix of plastic.

9. The method of claim 8 wherein the step of forming the homogeneous solution includes heating the plastic to a temperature effective to dissolve the liquid crystal and wherein the step of phase separating is carried out by cooling.

10. The method as claimed in claim 8 wherein the step of forming the homogeneous solution includes dissolving the plastic and the liquid crystal in about 5 parts solvent and wherein the step of phase separation is carried out by evaporating the solvent.

11. The method as claimed in claim 8 wherein the liquid crystal comprises a nematic liquid crystal.

12. The method as claimed in claim 11 wherein the nematic liquid crystals comprises a cyanobiphenyl.

13. The method as claimed in claim 8 wherein the plastic is selected from the group consisting of poly(vinyl butyral), poly(vinyl acetate), poly(vinyl formal), polycarbonate, poly(vinyl methyl ketone), poly(methyl acrylate), poly(cyclohexyl methacrylate) poly(isobutyl methacrylate) and poly(methyl methacrylate).

14. An electro-optic device capable of transmitting and scattering incident light and having memory comprising:

a pair of transparent conducting electrodes;

a sheet of liquid crystalline-plastic material sandwiched between the electrodes, said material containing liquid crystal in the form of microdroplets and further containing plastic in the form of a closed cell-like structure surrounding the microdroplets, said liquid crystal being present in the material in an amount of at least 50% by weight, said material characterized by a resistivity and a permittivity such that the product of the two is on the order of a few seconds or longer whereby the material will act as a capacitor to retain a charge between the electrodes and further characterized by a switching time on the order of less than about 1.0 microseconds upon shortcircuiting of the charge.

15. An electrooptic device as claimed in claim 14 wherein the liquid crystal comprises a nematic liquid crystal.

16. An electrooptic device as claimed in claim 15 wherein the liquid crystal comprises a cyanobiphenyl.

17. An electrooptic device as claimed in claim 14 wherein the plastic has an index of refraction n_s matching the ordinary index n_o of the liquid crystal so that at least about 90% of incident light is transmitted when the device is on.

18. An electrooptic device as claimed in claim 14 wherein the plastic is selected from the group consisting of poly(vinyl butyral), poly(vinyl acetate), poly(vinyl formal), polycarbonate, poly(vinyl methyl ketone), poly(methyl acrylate), poly(cyclohexyl methacrylate) poly(isobutyl methacrylate) and poly(methyl methacrylate).