Process of aligning liquid crystal media

Abstract: Disclosed is a process for aligning the molecules in a liquid crystal medium having a surface adjacent to a substrate, in which the liquid crystals have a molecular weight of less than 1500, and anisotropically absorbing molecules in the medium or in the substrate adjacent to it are exposed to linearly polarized light of a wavelength within the absorption band of the absorbing molecules, and alignment of the liquid crystal medium at an angle + or - ϑ with respect to the direction of the linear polarization of the incident light beam is induced by the exposed anisotropically absorbing molecules.

Description: This invention is directed to a process for orienting anisotropic molecules in a liquid crystal medium. Liquid crystal compounds are used in human- and machine-readable displays, used, for instance, in instrument controls for motor vehicles and process control devices, and in timing devices such as watches. Such display devices are primarily comprised of a liquid crystal medium sandwiched between glass or other transparent substrates to form a cell, with a transparent electrically conductive material selectively coated on both substrates to provide image-forming conductive patterns. Light transmission through these cells is conventionally controlled by changing the orientation of the liquid crystal medium so that all, some, or none of the available light passes through. The liquid crystal medium may be oriented by applying a voltage or magnetic field to the cell. It is also known to use one or two polarizers to control light transmission by the oriented liquid crystal medium. The liquid crystal medium may contain dissolved anisotropic dyes as well as the liquid crystals. Such a medium comprised of one or more liquid crystal compounds having one or more dyes dissolved therein is referred to as a "guest-host" system. The anisotropic dyes in "guest-host" systems that are best suited for displays are dichroic dyes, which absorb more light along one axis and absorb less light along a second. When the liquid crystal host composition and guest dye dissolved in it are properly matched, the orientation of the mixture can be controlled through a voltage applied so that only the desired amount of light will be transmitted. The components of these displays are referred to as anisotropic because they (a) exhibit properties with different values when measured along axes in different directions and (b) assume different positions in response to external stimuli, such as, for instance, an applied voltage that can be used to rotate the liquid crystals around a fixed axis so as to alter their optical properties. More specifically, they are birefringent. In liquid crystal cells currently in commercial use, the molecules have been oriented as to assume a homogeneous or homeotropic alignment. Without external stimuli the display will either appear opaque or transparent. Many techniques are known for aligning liquid crystal media. Typically, the inner surfaces of the transparent substrates are constituted by a transparent alignment layer, such as a film of a polyalkylsiloxanes or lecithin. For homogeneous alignment, the coating is rubbed along a given direction with a cloth before the cell is filled with the liquid crystal medium, thus preferentially altering the surface state and causing the long molecular axes of the liquid crystals to align along or nearly parallel to the rubbing direction. After the cell is filled, the alignment is transferred to the bulk of the liquid medium via intermolecular forces. A homeotropic alignment is characterized by the long axes of the liquid crystals aligning along or nearly parallel to a line perpendicular to the substrate, and is conventionally achieved by using a chemically modified alignment layer or by applying an electrical field across the cell. There is a need for a simple technique for aligning or realigning a liquid crystal medium. In addition, it is desirable to be able to provide a liquid crystal cell having two or more regions aligned in different homogeneous or homeotropic alignments, and also in some circumstances to be able to eliminate the alignment layers used in liquid crystal displays. The present invention provides a process for aligning or realigning a liquid crystal medium having a surface adjacent to a substrate, in which liquid crystals in the medium have a molecular weight of less than 1500, characterised by exposing anisotropically absorbing molecules in the medium or in or on the substrate adjacent to it to linearly polarized light of a wavelength within the absorption band of the anisotropically absorbing molecules, whereby alignment or realignment of the liquid crystal medium at an angle + and -  with respect to the direction of the linear polarization of the incident light beam and along a plane defined by the substrate is induced by the exposed anisotropically absorbing molecules, and terminating the exposure, whereby the liquid crystal medium aligned or realigned by the exposure step remains so aligned or realigned. By "anisotropically absorbing molecules" is meant compounds that exhibit light absorption properties with different values when measured along axes in different directions. The term "liquid crystal" is used to refer to molecules with anisotropic geometry, such as rod-shaped or disc-shaped, that exhibit stable states intermediate between liquid and solid, and that have a molecular weight less than about 1500, preferably 1000 or less, and most preferably 650 or less. The liquid crystal medium of this invention may contain any such liquid crystal compound that is conventionally used in liquid crystal displays and other liquid crystal devices. Thermotropic liquid crystals that exhibit nematic and smectic (including ferroelectric) phases are preferred. Nematic phases include conventional uniaxial nematics, twisted nematic, and cholesteric mesophases. The nematic liquid crystals can have either positive or negative dielectric anisotropy. As used herein the terms "positive" and "negative" refer to the net dielectric anisotropy of a mixture comprising liquid crystals. Readily available positive nematic liquid crystal materials that are suitable for the practice of this invention include: 4-cyano-4'-alkylbiphenyls, 4-cyano-4'-alkyloxybiphenyls, 4-alkyl-(4'-cyanophenyl)cyclohexanes, 4-alkyl-(4'-cyanobiphenyl)cyclohexanes, 4-cyanophenyl-4'-alkylbenzoates, 4-cyanophenyl-4'-alkyloxybenzoates, 4-alkyloxyphenyl-4'cyanobenzoates, 4-alkylphenyl-4'cyanobenzoates, 1-(4'-alkylphenyl)-4-alkylpyrimidines, 1-(4'-alkylphenyl)-4-cyanopyrimidines, 1(4-alkyloxyphenyl)-4-cyanopyrimidines and 1-(4-cyanophenyl)-4-alkylpyrimidines. Specific examples of compounds within these families are: 4-cyano-4'-pentylbiphenyl, 4-cyano-4'-hexyloxybiphenyl, trans-4-pentyl-(4'-cyanophenyl)cyclohexane, trans-4-hexyl-(4'-cyanobiphenyl)cyclohexane, 4-cyanophenyl-4' -propylbenzoate, 4-pentyloxyphenyl-4'-cyanobenzoate, 4-hexylphenyl-4'-cyanobenzoate, 1-(4'-pentylphenyl)-4-butylpyrimidine, 1-(4'-butyloxyphenyl)-4-cyanopyrimidine, 1-(4-cyanophenyl)-4-alkylpyrimidine, 4-n-hexylbenzylidene-4'-aminobenzonitrile and 4-cyanobenzylidene-4'-octyloxyaniline. Eutectic mixtures and combinations of all the above are also useful. Illustrative are eutectic mixtures of 4'-alkyl-4-cyanobiphenyls with either 4'-alkyloxy-4-cyanobiphenyls wherein the 4' substituents have 3 to 8 carbons or terphenyl liquid crystals with 4-alkyl or alkyloxy substituents containing 3 to 8 carbon atoms. Representative are the commercially available E7 mixture from BDH, Ltd., Poole, England; ROTN 404, a eutectic mixture of biphenyl pyrimidine liquid crystals from Hoffman La Roche, Nutley, New Jersey; PCH 1132, a mixture comprising the 4-alkyl-(4'cyanobiphenyl)cyclohexanes and 4-alkyl-(4'-cyanophenyl)cyclohexanes from EM Industries, Hawthorne, New York; and ZLI 1982, also available from EM Industries. Representative of nematic liquid crystals having negative dielectric anisotropy that would be useful for this invention include: 4-alkyloxy-4'-alkyloxyazoxybenzenes, 4-alkyl-4'-alkyloxyazoxybenzenes, 4-alkyl-4'-acyloxyazoxybenzenes, 4-alkyl-4'alkylazoxybenzenes and 4-alkyl-2-cyanophenyl-4'-alkylbiphenyl-1-carboxylates. Specific examples include: p-azoxyanisole, 4-butyl-4'-hexyloxyazoxybenzene, 4-butyl-4'-acetoxyazoxybenzene, 4,4'-bis(hexyl)azoxybenzene, and 4-pentyl-2-cyanophenyl-4'-heptylbiphenyl-1-carboxylate. Commercially available are Licristal S1014 from EM Industries, Hawthorne, New York; and EN-18 from Chisso Corp, Yokomoma Japan. Smectic A liquid crystals useful in this invention can be of either positive or negative dielectric anisotropy. Smectic A liquid crystals of positive anisotropy include: 4-alkyl-4'-cyanobiphenyls and 4-cyano-4'-alkylbenzylideneanilines, as well as mixtures of 4-alkyl-4"-cyano-p-terphenyls and 4-alkyl-4"-cyanobiphenyls. Also useful are smectic A mixtures derived from electron accepting and electron donating mesogenic cores, for example 4'-alkylphenyl 4-alkylbenzoates and 4'-cyanophenyl-4"-alkyloxybenxoyloxybenzoates. Specific examples of compounds useful as smectic A liquid crystals with positive dielectric anisotropy are: 4-cyano-4'-octylbenzylideneaniline, 4-decyl-4'-cyanobiphenyl, 4-dodecyl-4"-cyano-p-terphenyl, 4'-heptylphenyl-4-butylbenzoate and 4'-cyanophenyl-4"-octyloxybenzoyloxybenzoate. Eutectic mixtures and combinations of all the above smectic A materials are useful. Representative eutectic mixtures and combinations of smectic A liquid crystals are the commercially available materials S1, S2, S3, S4, S5, S6 and S7, from EM Industries, Hawthorne, New York. Representatives of smectic A liquid crystals of negative dielectric anisotropy that are useful in this invention are: 4-alkylphenyl-4-alkyloxy-3-cyanobenzoates, 4-alkyloxyphenyl-4-alkyloxy-3-cyanobenzoates and 4"-alkyloxyphenyl-4'-alkyloxybenzoyloxy-3-cyanobenzoates. Specific examples include: 4'-octylphenyl-4-decyloxy-3-cyanobenzoate, 4'-decyloxyphenyl-4-octyloxy-3-cyanobenzoate and 4'-heptyloxyphenyl-4'-decyloxybenzoyl-3-cyanobenzoate. Eutectic mixtures of these may also be used. Representative chiral smectic C liquid crystals useful in the practice of this invention include: 4'-alkyloxyphenyl-4-alkyloxybenzoates, 4'-alkyloxybiphenyl-4-alkyloxybenzoates, 4-alkyloxyphenyl-4-alkyloxybiphenylcarboxylates and terpenol esters of 4'-n-alkyloxybiphenyl-4-carboxylates. Specific examples are: 4(4-methylhexyloxy)phenyl-4-decyloxybenzoate, 4-heptyloxyphenyl-4(4-methylhexyloxy)benzoate, 4'-octyloxybiphenyl-4(2-methylbutyloxy)benzoate, 4-nonyloxyphenyl-4'-(2-methylbutyloxy)biphenyl-4-carboxylate, and menthyl 4'-n-octyloxybiphenyl-4-carboxylate. Commercially available mixtures of chiral smectic C liquid crystals include the CS 1000 series offered by Chisso Corp., Yokohoma, Japan; ZLI 4139, available from EM Industries, Hawthorne, New York; and the eutectic mixtures of laterally fluorinated esters developed by BDH, LTD., and available through EM Industries as the SCE series of eutectic mixtures SCE3 through SCE12. Single component chiral smectic C materials based on 4'-alkyloxyphenyl-4-alkyloxybenzoates, W7, W37, W81, and W82 are available from Displaytech, Inc., Boulder, Colorado. Guest-host formulations are prepared with all types of liquid crystals. Dyes useful in these mixtures include dichroic azo, diazo, triazo, tetraazo, pentaazo, anthraquinone, mericyanine, methine, 2-phenylazothiazole, 2-phenylazobenzthiazole, 4,4'-bis(arylazo)stilbenes, perylene and 4,8-diamino-1,5-naphthoquinone dyes, and other dyes that exhibit dichroic absorption bands. Preferred are those that exhibit dichroic absorption bands between about 150 nm and about 2000 nm. Some specific examples of dyes useful in this invention are listed in the Table preceding the Examples. In the drawings: Figure 1 is a schematic of a typical liquid crystal cell. Figure 2 shows an apparatus useful for aligning or realigning liquid crystal media. Figure 3 is a front view of a guest-host liquid crystal medium that has been aligned by rubbing. Figure 4 is a front view of the guest-host liquid crystal medium of Figure 3 after it has been realigned using the process of this invention. The best known liquid crystal device is a liquid crystal display. A schematic of a basic liquid crystal cell, which is the active component of such a display, is shown in Figure 1. This schematic is exemplary only, since the process of this invention is useful for aligning liquid crystal media used in all types of liquid crystal devices. The liquid crystal cell shown in Figure 1 contains a guest-host liquid crystal medium (4). The host liquid crystal molecules are represented by short lines and the guest dye molecules are represented by lines with circles on each end. In this figure, the cell comprises two substrates having their principal components (1) (e.g., glass) coated on their inward facing sides with a transparent conductive layer (e.g., indium-tin-oxide) (2) and that are in turn coated on their inward facing sides with aligning layers comprising a thin film of organic material (3) (e.g., a polyimide), which in this case has been rubbed linearly in a single direction with a buffing cloth. The coated substrates are placed in face-to-face spaced relationship with small glass fibers having diameters of from about 2-20 micrometers (not shown in the Figure) used to control the spacing. The liquid crystal layer (4) is sealed with, for example, epoxy (not shown). The ⊕ and ⊖ shown in the Figure represent applied voltage. Liquid crystal cells may be operated using AC or DC voltage, and the indicated voltage direction in this Figure is not intended to be limiting. Increased contrast may be obtained by using two or more of such cells in combination. For instance, substrates can be shared so as to form a so-called "three glass structure". The light source used in this invention must be linearly polarized. Further, the linearly polarized light must have a wavelength in the absorption band of the anisotropic medium. Here, reference is made to the absorption band of, for instance, the liquid crystals, dyes in a guest-host liquid crystal medium, or dyes forming part of a substrate (for instance, coated on a layer of the substrate adjacent to the liquid crystal medium such as the conductive coating or an aligning layer). Typically, the light will be in the ultraviolet through infrared range as the dyes and, in some instances, the liquid crystal compounds will have peak absorption in this range. Preferably, the light will have a wavelength within the range of about 150 nm to about 2000 nm. The most preferred source of light is a laser, e.g., an argon, helium neon or helium cadmium laser. With lower power lasers it may be necessary to focus the light beam onto the cell, but generally it is not necessary to focus the light beam. The process of this invention may be used to align a liquid crystal medium that is in a randomly aligned (isotropic) state or to realign a previously aligned (homogenous or homeotropic) liquid crystal medium. If the incident light beam is normal to the substrate, its aligning effect will be achieved by rotation of the geometrical projection of the longer axes of the exposed anisotropically absorbing molecules, which means that the moment of rotation of each molecule around a shorter axis is likely to be greatest for homogeneously aligned molecules and to decrease as the angle between the plane of the substrate and the longer axes increases, approaching zero at a homeotropic orientation parallel to the direction of polarization of the incident light beam. The same will likely be true in reference to the alignment of the liquid crystals induced by the exposed anisotropically absorbing molecules. Preferably, the liquid crystal medium is of the type that can readily aligned homogeneously. The linearly polarized light is applied to the entire medium that is to be aligned or realigned or to a portion thereof. The light beam may be stationary or rotated. Exposure can be in one step, in bursts or by other methods. Exposure times vary widely with the materials used, and other predictable factors, and can range from less than one second to over an hour. It will be understood by the skilled worker that the rotation of the anisotropic absorbing and liquid crystal molecules at an angle + and -  with respect to the direction of the linear polarization of the incident light beam is in two dimensions along a plane. Generally, in a liquid crystal cell this plane corresponds to the surface of a substrate of the cell defined by the substrate at an angle relative to the incident light polarization. The process of this invention should be carried out with at least one substrate contacting the liquid crystal medium. Generally, the liquid crystal medium will form part of a cell having two such substrates, e.g., as shown in Figure 1. When an alignment layer is used, rubbing generally improves performance but is not necessary. An important feature of this invention is that after the process is completed the liquid crystal medium has "memory", i.e.,it will maintain the alignment that is induced by the linearly polarized light source. The anisotropic medium can, of course, be realigned to the original or a third alignment by the process of this invention. Normally, liquid crystal media (and cells prepared therewith) aligned using the techniques of this invention perform in the same manner as displays aligned using conventional techniques. The effects of the process of this invention can be observed using polarizers. That is, when white light is applied to a cell having a single polarizer one observes that there is a change in the angular position of the maximum dye absorption (or liquid crystal absorption where the light is within the absorption band of the liquid crystal) relative to the background. Where white light is applied to a cell having a polarizer on each side one observes a color change in the exposed region relative to the background that indicates a change in the angular position of the liquid crystal molecules and, thus, a change in the birefringence of the cell. The results are sensitive to the exposure time, intensity, wavelength and direction of the linearly polarized light; sample temperature, liquid crystals and dyes used and their concentrations, the presence or absence of an aligning layer and, where present, the type of the aligning layer, and the location, amount and properties of the anisotropically absorbing molecules. Liquid crystal displays prepared using the process of this invention have conventional display configurations (e.g., two, three or more substrate structures) and may include one or more polarizers, lighting means, reflective layers, transflective layers, elastomeric connectors and circuit boards as conventionally used in the art. It is not necessary to use conventional alignment layers in the cells, such as the buffed alignment layers commonly used in the prior art. However, in many instances performance is improved with a buffed or rubbed alignment layer. Other orientation layers and techniques may also be used with this invention. The process of this invention can be used to realign layers or portions thereof that have been previously aligned by a buffed alignment layer of the process of this invention. Most notable is that one or more areas of a liquid crystal device can be aligned in a different orientation than the remainder of the device. Such cells may be prepared by realigning one or more regions of a previously aligned liquid crystal medium. Figure 3 shows a liquid crystal-host medium that has been aligned in one direction. Figure 4 shows a region of that medium that has been realigned using the process of this invention. This figure illustrates the case where the liquid crystal molecules have been rotated 90 degrees to both the incident light polarization and the original alignment direction. Another interesting feature of this invention is that the process can be carried out by adding the anistropically absorbing molecules to a substrate, for instance in an organic material coated or otherwise applied adjacent to the liquid crystal medium such as art alignment layer or the transparent conductive layer, and then exposing the anisotropically absorbing molecules. This embodiment can be carried out using a liquid crystal medium that contains anisotropically absorbing molecules or a liquid crystal medium devoid of such molecules. Further, in these embodiments exposure to the linearly polarized light may be at any time prior to or after filling the cell with the liquid crystal medium. The process of this invention may be carried out in the presence of an electric field (AC or DC). However, it is not necessary that an electric field be present and, in most instances, the process will be carried out in the absence of an electric field. Generally, this invention is best suited for aligning or realigning homeogeneous liquid crystal systems. In the presence of an electric field, however, homeotropic liquid crystals having negative dielectric anisotropy enter a homogeneous state. That homogeneous system can be aligned or realigned using polarized light per this invention. When the electric field is removed the liquid crystals generally relapse into a homeotropic state. In this case, they will align once more in the homogeneous state induced by the polarized light when an electric field is later applied. Other conventional materials such as conducting ionic dopants, chiral dopants, photostabilizers and viscosity modifiers, etc., may be used. There is no need for these materials to be present to carry out the process of this invention. Although this invention is described with respect to cells for liquid crystal displays, it should be understood that it is useful for other liquid crystal devices, e.g., all-optical light modulators, erasable read/write optical data storage media, etc. This invention is demonstrated in the following examples, which are illustrative and not intended to be limiting, wherein all percentages, are by weight. The dyes shown in the following Table were used in the Examples. Example 1 This example shows realignment of a guest-host system comprising a diazodiamine dye dissolved in a nematic liquid crystal using the process of this invention . The host nematic liquid crystal was ZLI 1982 (EM Industries, Hawthorne, New York). 0.38 weight %, based on the weight of the liquid crystal, of Dye 1 was dissolved in the host. A 25 weight % solution of polyimide in 1-methyl-2-pyrrolidinone (NMP) was diluted with reagent grade NMP to a final polyimide concentration of 0.5 weight %. This solution was placed on top of an indium tin oxide (ITO) layer of a glass substrate component and allowed to sit for 5 minutes, after which the substrate was spun for 30 seconds. The substrate was heated in an oven to cure the alignment coating and, then, the cured coating was rubbed in a linear direction with a buffing cloth. Next, glass fibers 11 micrometers thick were placed on the substrate and another substrate with similar coatings was layered on top of it, the alignment layers forming the inside surfaces of the resultant cell. The two pieces were pressed to a spacing of 11 micrometers using clamps, epoxy was applied along the edges and the epoxy was cured for 5 minutes. Two spaces on opposite edges of the cell were left unsealed. The cell was placed in a vacuum and one unsealed space was dipped into the guest-host mixture. The cell filled by capillary action. After filling, the cell was removed from the vacuum and wiped clean, and the unsealed spaces were sealed with epoxy. The rubbed polyimide caused the guest-host material to align along the rubbing direction. An apparatus as shown in Figure 2, without the optical lens, was used to realign the guest-host medium. The polarized light source was an argon laser (Model No. 2020-03, Spectra-Physics, Piscataway, New Jersey) having a maximum power of approximately 1.2 watts at 514.5 nm wavelength. The laser was polarized as indicated by the two headed arrow in Figure 2 and passed through a variable aperture camera shutter to permit control of the exposure time on the sample. The shutter was set for manual control because of the longer exposure times required for realignment. When the shutter was opened, a laser light beam of 4 mm diameter passed through. The laser beam was directed at right angles to the plane of the substrate as indicated in Figure 2. Figures 3 and 4 show an expanded front view of the cell before and after exposure with the polarized light beam. Prior to exposure, the direction of polarization of the incident light (represented by two headed arrow) was parallel to the rubbing direction of the polyimide surface and, thus, the long axis of the dye (represented by line with a circle on each end) and liquid crystal molecules (represented by a line). After exposure, the dye and liquid crystal molecules in some domains were found to have rotated an angle +  in the plane defined by the cell substrate and relative to the incident light polarization. In other domains, the dye and liquid crystal molecules had rotated an angle -  in the plane defined by the cell substrate and relative to the incident light polarization (In all subsequent references to the angle of rotation it will be understood that the rotation is in the plane defined by the cell substrate). When the magnitude of  equals ninety degrees, the two domains degenerate into one domain (i.e., all liquid crystals align in one direction), with alignment perpendicular to the incident light polarization. For any value + or -  other than zero, one observes with a single polarizer that there is a change in the angular position of the maximum dye absorption relative to the background. Also, by using two polarizers (one on each side of the cell) one observes a color change in the exposed region relative to background that indicates a change in the angular position of the liquid crystal molecule. In this example, using a laser power of 1.2 Watts at 514.5 nm and an exposure time of 45 minutes, the magnitude of + and -  was observed to be approximately 45 degrees. Example 2 This example demonstrates that liquid crystals (without dye) can be realigned using the process of this invention if the polarized light is within the absorption band of a liquid crystal compound. The liquid crystal was ZLI4139 (EM Industries, Incorporated, Hawthorne, New York) which is a ferroelectric liquid crystal (exhibits a room temperature chiral smectic C phase). ZLI4139 shows a strong absorption band in the ultraviolet region. No dye was added to the liquid crystal. A cell was fabricated as in Example 1, except that Pyrex microscope slides (available from Scientific, San Francisco, California) without ITO coatings were used to ensure low absorption in the ultraviolet and 5 micrometer fibers were used to provide the spacing. The cell was filled using capillary action at 130°C. An arrangement as in Figure 2 without the lens, was used to expose the completed cell. The light source was a polarized Helium Cadmium laser, available from Liconix, Sunnyvale, California, which emits a wavelength of 325 nm. The beam diameter was approximately 1 millimeter and the laser power was approximately 1.0 milliwatt. The cell was exposed for 1 minute and inspected. The angular position of the liquid crystal molecules in the exposed region was different from the background position. Since the absorption of the laser light was significant, the molecules on the entrance side of the cell changed orientation relative to the incident light polarization but the molecules on the exit side of the cell were unaffected (i.e., the light was absorbed and did not penetrate the liquid crystal medium so as to expose the molecules on the rear side of the cell). A twisted alignment of the liquid crystal in the cell resulted. Example 3 This example demonstrates realignment per this invention of a guest-host system using polarized light having a wavelength within the absorption band of the liquid crystal. A cell was fabricated and filled as in Example 2, except using 0.6 weight %, based on the weight of the liquid crystal, of Dye 2 dissolved in the host liquid crystal. An arrangement as in Figure 2 (without the lens) was used to expose the completed cell. The light source was a polarized Helium Cadmium laser as described in Example 2. The cell was exposed for 15 minutes and inspected. The dichroism of the illuminated region was different from the background dichroism. The angle of rotation was approximately 30 degrees to the incident light polarization. Since the absorption of the polarized light was significant, the molecules on the entrance side of the cell changed orientation relative to the incident light polarization but the molecules on the exit side of the cell were unaffected. A twisted alignment of the liquid crystal and dye molecules in the cell resulted. Example 4 This example demonstrates that guest-host systems can be realigned using polarized white light. A cell was fabricated and filled as in Example 3. An Olympus BH-2 polarization microscope (Olympus Optical Company, Tokyo, Japan) was used to expose the completed cell. The microscope uses a xenon lamp to generate white light which is then passed through a polarizer. Exposure of the cell for 2 hours at room temperature did not cause realignment. However, by using a Mettler FP52 temperature stage and FP5 temperature controller (Mettler Instrument Corporation, Princeton, New Jersey) the cell was ramped 10 degrees centigrade per minute (°C/minute) from room temperature (25°C) to 100°C (The isotropic transition temperature of ZLI4139 is 82°C). The cell was exposed with the white light polarized along the initial alignment direction of the liquid crystal at room temperature. During exposure the temperature of the cell was cooled at a rate of 1°C/minute from 100°C to room temperature. The exposed region was found to align approximately + and - 30 degrees from the incident light polarization. Example 5 Example 1 was repeated using a higher concentration of Dye 1, demonstrating that with higher concentrations of dye lower energy requirements (shorter exposure times) are required to achieve realignment. The cell was fabricated and filled as in Example 1, except using 7.0 weight %, based on the weight of the liquid crystal, of Dye 1. Realignment was carried out as in Example 1. The magnitude of + and -  was 90 degrees relative to the incident polarization after an exposure time of 20 minutes. The magnitude of + and -  did not increase beyond 90 degrees when the exposure time was increased beyond 20 minutes. Example 6 Example 1 was repeated using a different azo dye in the nematic liquid crystal host. The cell was fabricated and filled as in Example 1, except using 1.25 weight %, based on the weight of the liquid crystal, of Dye 3 dissolved in the host nematic liquid crystals. Realignment was carried out as in Example 1. The magnitude of + and -  was 90 degrees relative to the incident light polarization after an exposure time of 45 minutes. Example 7 Example 1 was repeated using an anthraquinone dye in the nematic liquid crystal host. The cell was fabricated and filled as in Example 1, except using 1.5 weight %, based on the weight of the liquid crystal, of Dye 4 dissolved in the nematic liquid crystal host. Realignment was carried out as in Example 1. The magnitude of + and -  was approximately 45 degrees relative to the incident light polarization after an exposure time of 45 minutes. Example 8 Example 1 was repeated without the polyimide aligning layer. The cell was fabricated and filled as in Example 1 except that a polyimide layer was not coated onto the ITO coated glass substrate (thus, the surfaces were not rubbed) and 10 micrometer fibers were used instead of 11 micrometer fibers. In addition, 0.28 weight %, based on the weight of the liquid crystal, of Dye 5 was dissolved in the host in place of Dye 1. Alignment was carried out using a laser as in Example 1. The magnitude of + and -  was approximately 30 degrees relative to the incident light polarization after an exposure time of 80 minutes. The uniformity of the alignment across the exposed region was acceptable, but not as good as with a rubbed polyimide aligning layer. Example 9 This example shows laser realignment of a guest-host liquid crystal system previously realigned per the process of this invention. The cell was fabricated and filled as in Example 1, except using 2.0 weight %, based on the weight of the liquid crystal, of Dye 1. The first realignment was carried out as in Example 1. The magnitude of + and -  was approximately 90 degrees relative to the incident light polarization after an exposure time of 5 minutes. Subsequently, the cell was rotated by 30 degrees (in the plane perpendicular to the incident laser beam) and exposed for 5 minutes in the same spot that was exposed previously. It was observed that the dye and liquid crystal molecules had rotated + and - 90 degrees to the incident light polarization, which was approximately 30 degrees relative to the previously realigned direction. Example 10 This example shows alignment of a guest-host liquid crystal medium wherein a dye was incorporated into the polyimide aligning layers of the cell. A cell was fabricated as in Example 1 except that 1 weight %, based on the weight of the NMP, of Dye 5 was added to the 0.5 weight % of polyimide in NMP. This mixture was spun onto the ITO coated substrate and curing was carried out as in Example 1. Before the cell was filled with the guest-host mixture described in Cell Fabrication, it was exposed with a system as described in Figure 2 (without the lens) using the laser described in Example 1. The incident light polarization was parallel to the linear rubbing direction of the cell. After an exposure for 15 seconds with 1.2 Watts laser power at 514.5 nm, the cell was filled with the guest-host mixture using the procedure described in Example 1, except 0.28 weight %, based on the weight of the liquid crystal, of Dye 5 (structure in Table 1) was dissolved in the host nematic liquid crystal. The exposed region showed a + and - 90 degree rotation from the incident laser polarization. The exposed regions of the cell aligned along the rubbing direction. By localizing the dye in the polyimide aligning layer, the exposure times required to cause the laser induced alignment were significantly reduced compared to that observed in previous Examples. Example 11 This example shows alignment of a liquid crystal medium per this invention when a dye was incorporated into the polyimide aligning layers of the cell. A cell was fabricated and aligned as in Example 10, except no dye was mixed in with the liquid crystal. The exposed region showed a + and - 90 degree rotation from the incident light polarization. The liquid crystal molecules in the unexposed regions of the cell aligned along the rubbing direction. Example 12 This example shows alignment of a guest-host system in the case where a dye was incorporated into the unrubbed polyimide aligning layers of the cell. A cell was fabricated and aligned as in Example 10, except the cured polyimide/dye aligning layer was not rubbed. The exposed region showed a + and - 90 degree rotation from the incident light polarization with uniformity as good as that of the cells aligned in Examples 10 and 11. The unexposed regions of the cell aligned along the flow direction while filling. Example 13 This example shows alignment of a liquid crystal in the case where a dye was incorporated into the unrubbed polyimide aligning layers of the cell. A cell was fabricated and aligned as in Example 12, except that no dye was dissolved in the nematic liquid crystal. Results were as in Example 12. Example 14 This example shows realignment of a cell previously aligned per the process of this invention where the aligning layer was comprised of a polyimide and dye mixture. A cell was fabricated and aligned as in Example 10, except the cured polyimide/dye aligning layer was not rubbed and the cell was exposed with a beam expanded by the lens for 5 minutes. Prior to filling the cell with the guest-host mixture (a) the beam was masked so that the beam size was smaller than the area of the beam used in the initial exposure, (b) the cell was rotated approximately 45 degrees in the plane perpendicular to the incident light beam, and (c) the cell was subsequently exposed for 5 minutes within the area previously exposed. The cell was then filled with the guest-host mixture using the procedure described in Example 1. The initially exposed region (the region that was exposed only once) showed a + and - 90 degree rotation from the incident light polarization of the first exposure and the reexposed region (the region that was exposed twice) had a + and - 90 degree rotation relative to the direction of the incident light polarization of the second exposure. The alignment of the overlapping regions was as good as the cells in Examples 10 and 11. The unexposed regions of the cell aligned along the flow direction as the cell filled. Example 15 This example shows laser alignment of a guest-host system in the case when a dye is placed directly onto the transparent conductive coating on the glass substrate component. A cell was fabricated as in Example 1 except that no polyimide was spun onto the ITO layer of each substrate. Instead, 1 weight %, based on the weight of the NMP, of Dye 4 was added to NMP and spun onto the ITO layers. The NMP was allowed to evaporate off for 1 hour. This left a thin film of dye molecules lying on top of the ITO layers of the substrates. The surface was rubbed with a buffing cloth and 10 micrometer fibers were used to provide the spacing when making the cell. Before the cell was filled with the guest-host liquid crystal mixture described in Cell Fabrication, it was exposed with a system as shown in Figure 2 (without the lens) using the laser described in Example 1. The incident light polarization was perpendicular to the linear rubbing direction of the cell. After an exposure for 10 seconds with 1.2 Watts laser power at 514.5 nm, the cell was filled with the guest-host liquid crystal material using the procedure described in Example 1. The dye on the surface was observed to dissolve into the guest-host material while the cell was filling. The exposed region showed a + and - 90 degree rotation from the incident light polarization. The unexposed regions of the cell aligned along the rubbing direction. Example 16 This example demonstrates that a ferroelectric liquid crystal host with a chiral smectic C room temperature phase can be realigned using the process of this invention. The host liquid crystal was BDH SCE-4 (BDH Limited, Poole, England) which is a ferroelectric liquid crystal. 2.0 weight %, based on the weight of the liquid crystal, of Dye 2 was dissolved in the ferroelectric liquid crystal host. A cell was fabricated and filled as in Example 2. An arrangement as in Figure 2 was used to expose the completed cell using the laser described in Example 1. The beam diameter was expanded to 1 centimeter (cm) using the lens. A laser power of 1.0 Watt at 514.5 nm was used to expose the cell for 25 minutes. Since the energy density across the 1 cm laser beam is nonuniform, the angle of rotation relative to the incident light polarization varied across the exposed spot. However, the polarized light did align the spot differently from the alignment in the unexposed regions. Example 17 This example demonstrates that a liquid crystal host with a smectic A room temperature phase can be realigned using the process of this invention. The host liquid crystal was BDH S2C (available from BDH Limited, Poole, England) which exhibits a smectic A phase at room temperature. 1.0 weight %, based on the weight of the liquid crystal, of Dye 6 was dissolved in the liquid crystal host. A cell was fabricated as in Example 1. The cell was filled using capillary action at 130°C and was sealed with epoxy. Alignment was carried out as in Example 1. A laser power of 0.9 Watts at 514.5 nm was used to expose the cell for 45 minutes. The polarized light realigned the spot approximately + and - 90 degrees from the incident laser polarization. Example 18 This example demonstrates alignment of liquid crystals using the process of this invention where one of the substrates is coated with a polyimide alignment layer containing an anisotropically absorbing dye. A cell was fabricated as in Example 10 except that the polyimide/Dye 5/NMP mixture was spun onto one ITO layer of the substrate and a polyimide/NMP mixture was spun onto the ITO layer of the other substrate. Curing for both substrates was performed as in Example 1. Both substrates were rubbed in a linear fashion with a buffing cloth and a cell was fabricated as in Example 1, except 10 micrometer fibers were used to control the spacing. The liquid crystal cell was exposed as in Example 1 using the argon laser as the polarized light source. The cell was exposed for 5 minutes at 0.8 Watts with the incident linear polarization along the linear rubbing direction. Using two polarizers, the exposed region was observed to have a twisted alignment (i.e. the orientation of the liquid crystal molecules at the polyimide/dye substrate was 90 degrees to liquid crystal molecules at the polyimide substrate).

Claims: A process for aligning or realigning a liquid crystal medium having a surface adjacent to a substrate, in which liquid crystals in the medium have a molecular weight of less than 1500, characterised by exposing anisotropically absorbing molecules in the medium or in or on the substrate adjacent to it to linearly polarized light of a wavelength within the absorption band of the anisotropically absorbing molecules, whereby alignment or realignment of the liquid crystal medium at an angle + and -  with respect to the direction of the linear polarization of the incident light beam and along a plane defined by the substrate is induced by the exposed anisotropically absorbing molecules, and terminating the exposure, whereby the liquid crystal medium aligned or realigned by the exposure step remains so aligned or realigned. A process as claimed in claim 1, further characterized in that the anisotropically absorbing molecules are thermotropic liquid crystal compounds or dichroic dyes. A process as claimed in claim 1, further characterized in that the anisotropically absorbing molecules are thermotropic liquid crystal compounds or dichroic dyes that exhibit dichroic absorption bands between about 150 nm and about 2000 nm. A process as claimed in any of claims 1-3, further characterized in that the anisotropically absorbing molecules are molecules of a dichroic dye dissolved in a thermotropic liquid crystal compound. A process as claimed in any of claims 1-4, further characterized in that the anisotropically absorbing molecules are dichroic dyes selected from the group consisting of azo, diazo, triazo, tetraazo, pentaazo, anthraquinone, mericyanine, methine, 2-phenylazothiazole, 2-phenylazobenzthiazole, 4,4'-bis(arylazo)stilbenes, perylene and 4,8-diamino-1,5-naphthoquinone dyes. A process as claimed in any of claims 1-3, further characterized in that the anisotropically absorbing molecules are nematic or smectic liquid crystal compounds. A process as claimed in any of claims 1-6, further characterized in that the substrate includes a rubbed alignment layer. A process as claimed in claim 7, further characterized in that the anisotropically absorbing molecules are part of the alignment layer. A process as claimed in any of claims 1-7, further characterized in that the liquid crystal medium contains the anisotropically absorbing molecules. A process as claimed in any of claims 1-7, further characterized in that the substrate contains the anisotropically absorbing molecules. A process as claimed in any of the preceding claims, further characterized in that the liquid crystal medium is aligned homogeneously before the anisotropically absorbing molecules are exposed to the polarized light. A process as claimed in any of the preceding claims, further characterized in that the linearly polarized light is emitted from an argon, helium-neon or helium-cadmium laser. A process as claimed in any of the preceding claims, further characterized in that it is carried out in the presence of an electric field. A process as claimed in claim 1, 2 or 3, further characterized in that the substrate comprises a top film of anisotropically absorbing dye molecules to which the said surface of the liquid crystal medium is adjacent. A process as claimed in any of the preceding claims, further characterized in that  is 90°. The use of anisotropically absorbing molecules for aligning or realigning a liquid crystal medium having a surface adjacent to a substrate, in which liquid crystals in the medium have a molecular weight of less than 1500, wherein the anisotropically absorbing molecules are disposed in the medium or in or on the substrate adjacent to it and are exposed to linearly polarized light of a wavelength within the absorption band of the anisotropically absorbing molecules, whereby alignment or realignment of the liquid crystal medium at an angle + and -  with respect to the direction of the linear polarization of the incident light beam and along a plane defined by the substrate is induced by the exposed anisotropically absorbing molecules, and the exposure is terminated, whereby the liquid crystal medium aligned or realigned by the exposure step remains so aligned or realigned. The use as claimed in claim 16, characterized in that the anisotropically absorbing molecules are thermotropic liquid crystal compounds or dichroic dyes. The use as claimed in claim 16, characterized in that the anisotropically absorbing molecules are thermotropic liquid crystal compounds or dichroic dyes that exhibit dichroic absorption bands between about 150 nm and about 2000 nm. The use as claimed in claim 16, 17 or 18, characterized in that the anisotropically absorbing molecules are molecules of a dichroic dye dissolved in a thermotropic liquid crystal compound. The use as claimed in any of claims 16 to 19, characterized in that the anisotropically absorbing molecules are dichroic dyes selected from the group consisting of azo, diazo, triazo, tetraazo, pentaazo, anthraquinone, mericyanine, methine, 2-phenylazothiazole, 2-phenylazobenzthiazole, 4,4'-bis(arylazo) stilbenes, perylene and 4,8-diamino-1,5-naphthoquinone dyes. The use as claimed in claim 16, 17 or 18, characterized in that the anisotropically absorbing molecules are nematic or smectic liquid crystal compounds. The use as claimed in any of claims 16 to 21, characterized in that the substrate includes a rubbed alignment layer. The use as claimed in claim 22, characterized in that the anisotropically absorbing molecules are part of the alignment layer. The use as claimed in any of claims 16 to 22, characterized in that the liquid crystal medium contains the anisotropically absorbing molecules. The use as claimed in any of claims 16 to 22, characterized in that the substrate contains the anisotropically absorbing molecules. The use as claimed in any of claims 16 to 25, characterized in that the liquid crystal medium is aligned homogeneously before the anisotropically absorbing molecules are exposed to the polarized light. The use as claimed in any of claims 16 to 26, characterized in that the linearly polarized light is emitted from an argon, helium-neon or helium-cadmium laser. The use as claimed in any of claims 16 to 27, characterized in that it is carried out in the presence of an electric field. The use as claimed in claim 16, 17 or 18, characterized in that the substrate comprises a top film of anisotropically absorbing dye molecules to which the said surface of the liquid crystal medium is adjacent. The use as claimed in any of claims 16 to 29, characterized in that  is 90°.

Inventors: GIBBONS WAYNE MICHAEL, SUN SHAO-TANG, SWETLIN BRIAN JOSEPH, GIBBONS, WAYNE MICHAEL, SUN, SHAO-TANG, SWETLIN, BRIAN JOSEPH
Applicants: HERCULES INC, HERCULES INCORPORATED
Application year: 1990
Main classification: G02F1
IPC codes: G11B7, G02F1, C09K19