Abstract:
A cholesteric display including a cholesteric liquid crystal, and a dipolar dopant having limited miscibility in the cholesteric liquid crystal is provided. In accordance with another aspect, a process is provided for forming a cholesteric liquid crystal composition by dissolving a dipolar dopant having limited solubility into a cholesteric liquid crystal. The liquid crystal composition can be utilized to produce a cholesteric display. The liquid crystal domains in the liquid crystal composition rotate away from a planar state in the presence of an applied electric field and the degree of rotation is a function of the applied electric field strength. The reflected color is then a function of the applied electric field.

Description:
BACKGROUND 
     Illustrated herein in embodiments are liquid crystal displays (LCDs) and, more particularly, cholesteric multi-color liquid crystal displays. 
     Cholesteric liquid crystal displays have attracted attention in recent years as an electronic paper (e-paper) type of display device. It would be particularly advantageous to develop an effective and inexpensive multi-color e-paper. However, successful demonstrations of cholesteric multi-color displays have so far involved a three-layer stack of cholesteric displays, with each layer reflecting a primary color, e.g. red, green or blue. Unfortunately, this type of a display is comparatively expensive and consumes a great deal of power, rendering it unsuitable for inexpensive portable displays utilizing battery power. A single-layer cholesteric color display would be less expensive to manufacture and more suitable as a multicolor electronic paper, requiring less power to operate than a three-layer design while, at the same time, being more flexible and durable than a three-layer design. 
     Color change of a single-layer cholesteric display has been previously demonstrated by switching with an electric field with voltages in the 300V range. However, such high voltage requirements are not suitable or practical for battery operated portable displays. Another known method involves a photochemical color change. Unfortunately, this involves an hours-long process to generate a display image, and the image is subject to later pollution and degradation from ambient light. Still another method produces a color change when polymeric cholesteric flakes suspended in an inert fluid are rotated by an applied electric field. However, it is very difficult to prepare the polymeric cholesteric flakes in the very narrow size distribution required, and different size flakes rotate at different voltage levels, making the display difficult to control. 
     Therefore, there is a need for a single-layer cholesteric multi-color display with a lower operating voltage than presently required, thereby also providing a more practical embodiment of multi-color electronic paper which consumes less power than present designs. 
     BRIEF DESCRIPTION 
     In accordance with one embodiment of the disclosure, there is provided a cholesteric display including a cholesteric liquid crystal and a dipolar dopant having limited solubility dissolved therein. 
     In accordance with another embodiment of the disclosure, there is provided a process for forming a cholesteric liquid crystal composition comprising dissolving a dipolar dopant having limited miscibility with the cholesteric liquid crystal material. 
     In accordance with still another embodiment of the disclosure, there is provided a device having a liquid crystal composition including a cholesteric liquid and a dipolar dopant having limited solubility dissolved therein. Liquid crystal domains in the liquid crystal composition rotate away from a planar state in the presence of an applied electric field and the degree of rotation is a function of the applied electric field strength. 
     These and other non-limiting aspects of the development are more particularly disclosed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a brief description of the drawings, which are presented for the purposes of illustrating the development disclosed herein and not for the purposes of limiting the same. 
         FIG. 1  is a schematic representation of a dipolar molecule. 
         FIG. 2  is a simplified elevational view of a liquid crystal display embodiment having liquid crystals in a planar state; 
         FIG. 3  is a simplified elevational view of a liquid crystal display embodiment having liquid crystals in a rotated state; 
         FIG. 4  depicts dipolar dopants suitable for practicing embodiments of the present disclosure; and 
         FIG. 5  is a graph of reflectance versus wavelength for different applied voltages in one embodiment of a liquid crystal device. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes introducing a dipolar dopant into a cholesteric mixture which is selected to reflect visible light. A dipolar molecule with only limited solubility in the liquid crystal is dispersed into the liquid crystal mixture. The reflected wavelength (color) changes, as a function of an applied electric field which rotates the dipolar molecules. The reflected wavelength changes due to a rotation of the cholesteric liquid crystal domains which are coerced into rotation by the rotation of the dipolar molecules. 
     A more complete understanding of the processes and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the present development, and are, therefore, not intended to indicate relative size and dimensions of the display devices or components thereof and/or to define or limit the scope of the exemplary embodiments. 
     With reference to  FIG. 2 , cholesteric liquid crystals  10  reflect color when they are arranged in the planar texture as shown in the figure, i.e. when the axis of the helix lies normal to the substrates  12 ,  14 . The reflected color is controlled by the helical pitch of the cholesteric mixture, the helical pitch being the distance it takes for the director of the cholesteric liquid crystal  10  to go through one complete rotation of 360 degrees. The director is defined as the molecular direction of preferred orientation in liquid crystalline mesophases. 
     The change in reflected color is based on a coerced rotation of small planar domains of the cholesteric liquid crystals  10  by the rotation of nearby domains  16  containing the dipolar molecule  18 . Initially, for example, the liquid crystal cell reflects red light. As shown in  FIG. 2 , for a given voltage applied to transparent electrodes  13 ,  15  for generating an applied electric field  20 , the helix is rotated through an angle θ ( 22 ), and the reflected wavelength is decreased. In this example, the cell takes on a green appearance. 
     The reflected color in a planar cholesteric display is a function of the viewing angle. In the case described above, the reflected color is a function of the angle θ ( 22 ) between the direction of the helix axis and the direction perpendicular to the display substrates  12 ,  14  given by:
 
λ reflected =λ 0 ·cos θ
 
where λ 0  is the reflected wavelength at θ=0.
 
     The dipolar molecule (see  FIG. 1 ) consists of an electron donor group and an electron acceptor group connected at the ends of a conjugated path. This results in the general structure: Donor—Conjugated Path—Acceptor. The electron donor is an atom or a group of atoms that have a negative Hammett parameter. The electron acceptor is a group of atoms having a positive Hammett parameter. Donor group is selected from an atom selected from the group consisting of N, O, S, and P, where the valence of the atom is satisfied by bonding to at least one other moiety to satisfy the valence of the atom; the other moiety or moieties to satisfy the valence of the atom selected as the electron donor moiety may be for instance a hydrogen atom, or short hydrocarbon group such as a straight alkyl chain having for example 1 to about 3 carbon atoms. 
     The conjugated bridging moiety may be any suitable group through which electrons can pass from the electron donor moiety (D) to the electron acceptor moiety (A). In embodiments, the conjugated bridging moiety (C) is a selected from the following: 
     (a) phenyl ring. 
     (b) fused aromatic rings having for instance from about 10 to about 50 carbon atoms such as 1,4-C10H6 and 1,5-C10H6. 
     Electron acceptor group is selected from: 
     The electron acceptor moiety (A) may be any suitable atom or group capable of accepting electrons. In embodiments, the electron acceptor moiety (A 1 ,A 2 ) is an electron withdrawing functional moiety which according to Hammett equation possesses a positive Hammett constant. The electron acceptor moiety may be for example the following: 
     (a) an aldehyde (—CO—H); 
     (b) a ketone (—CO—R) where R may be for example a straight chain alkyl group having for example 1 to about 3 carbon atoms, such as methyl, ethyl, propyl and isopropyl. 
     (c) an ester (—COOR) where R may be for example a straight chain alkyl group having for example 1 to about 3 carbon atoms, such as methyl, ethyl, propyl and isopropyl. 
     (d) a carboxylic acid (—COOH); 
     (e) cyano (CN); 
     (f) nitro (NO2); 
     (g) nitroso (N═O); 
     (h) a sulfur-based group (e.g., —SO2—CH3; and —SO2—CF3); 
     (i) a fluorine atom; 
     (k) a boron atom. 
     Examples of some dipolar molecules which are suitable for the described color switching method are shown in  FIG. 4 . The molecules shown are small molecules having dipole moments preferably as high as possible, but which are not colored. 
     The dipolar molecules are selected such that they melt at a temperature not higher than the temperature at which the liquid crystal starts boiling, because this results in change of the composition of liquid crystal due to evaporation. In a preferred embodiment, the dipolar dopant is completely soluble or miscible in the liquid crystal at a temperature at which the liquid crystal becomes isotropic, i.e. displays no liquid crystalline behavior. Additionally, upon cooling to room temperature, the dipolar dopant becomes phase separated from the liquid crystal material. 
     A liquid crystal is an organic compound in an intermediate or mesomorphic state between solid and liquid. These compounds are a crystalline solid until heated to become a turbid liquid, or liquid crystals. On further heating, the liquid becomes isotropic. This sequence is reversed when the substance is cooled. Many of the commercially available liquid crystals are liquid crystals at the ambient conditions. Many organic compounds exhibiting this behavior are known and used extensively in electric and electronic displays, thermometers, LCD TV sets, electronic clocks and calculators, and similar devices dependent on temperature determination. Liquid crystals have several varieties of molecular order: nematic, smectic, and cholesteric. 
     In this regard, cholesteric is a molecular structure found in some liquid crystals, so called because it was first noted in cholesteryl alcohol. It occurs in some optically active compounds and in mixtures of chiral compounds and nematic liquid crystals. 
     Suitable cholesteric liquid crystal materials for use herein include for example BL mixtures available from EM Industries, Inc., Hawthorne, N.Y. (BL088, BL 90, BL94 and BL108 as a few examples). The helical pitch is tuned to the desired range by mixing this cholesteric liquid crystal mixture with a nematic liquid crystal. Examples of nematic liquid crystal BL mixtures available at EM Industries, Inc., BL001 (E7), BL002 (E8), BL033 (version of BL002) and BL087, and 5CB (commercially available at Sigma-Aldrich). The cholesteric liquid crystal composition is adjusted in such a way as to reflect light in the visible range (from about 400 nm to about 730 nm) when is aligned in its planar state. 
     In one embodiment, a display is fabricated by melting a mixture of cholesteric liquid crystal BL088 (available from Merck Ltd.) and 17% (w/w) N,N-dimethyl-benzonitrile ( 24 , in  FIG. 4 ) between two indium-tin-oxide (ITO) coated glass slides, which act as a liquid crystal cell. When cool, e.g. room temperature, the sample embodiment reflects a red color. 
     When a DC voltage is applied, generating an electric field between the substrates, a shift of the reflected wavelength is observed. The shift becomes larger as the voltage is increased, and the shift is consistent with rotation of the helices of the cholesteric domains. With reference now to  FIG. 5 , a graph is provided showing percentage of reflectance  30  as a function of wavelength  32  at three different applied voltages when utilizing the above-described N,N-dimethyl-benzonitrile dopant ( 24 ). The shift of the peak of the reflected wavelength curve from 630 nm at 0 V ( 34 ), to 610 nm at 30 V ( 36 ), and to 580 nm at 60 V ( 38 ) is shown. 
     At higher voltages, a transparent state is obtained as the liquid crystal helices ( 10 ) rotate toward a focal-conic state, or break up in a homeotropic state at even higher voltages. Because the back of the display is coated with a black absorptive layer ( 40  in  FIG. 3 ), an observer sees black in the transparent state. Without the dipolar dopant ( 24 ), no shift is observed. Rather, only a sudden change to the homeotropic or focal-conic states is observed, both of which are transparent. 
     As it can be observed from  FIG. 5 , a relatively strong reflection is also observed in the region of 400–500 nm for each of the three reflectance curves. Because the dipolar dopant ( 24 ) is not colored, this reflection can not be attributed to the dopant. Rather, the reflection is believed to be due to the rotation of the domains ( 16  in  FIG. 3 ) containing the inserted dipolar molecule  24 . It is probable that they rotate more when compared with dopant free domains. This side effect is probably due to the fact that a small amount of the dipolar molecule  24  is miscible with the liquid crystal  10 . Use of dopants containing fewer methyl groups (for example,  42 ,  44  in  FIG. 4 ) is expected to decrease this side effect. 
     Planar alignment layers may be optionally be coated on the surface of the electrodes. This results in improved planar alignment of the helices in the color reflecting state which provides more saturated reflected color. Surface alignment layers for planar alignment are known and they include for example thermally cross-linkable polyimides as for example those produced by Nissan Chemical Ind., Ltd. Improved planar alignment is provided by rubbing the polyimide coated substrates. 
     Important factors for the fabrication of a good quality multi-color display include relatively poor miscibility of the dipolar dopant with the cholesteric liquid crystal, and the formation of essentially transparent domains containing the dipolar molecules. Formation of large crystals of dopant should be avoided so that the display does not scatter light and, as a consequence, have a white appearance. 
     It is to be appreciated that selective areas of a display and surrounding areas can be switched independently so that an image can be generated on the display. Each of selective display areas can be switched as necessary, to a selective colored state, or to a transparent (black) state. 
     While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.