Liquid-Crystal Mixtures for Use in Liquid-Crystal Displays

1. Field of the Invention

This invention relates to mixtures of nematic-phase liquid-crystals, for use in light shutters and the like, that exhibit the desired nematic phase over a substantial temperature range, preferably one that includes room temperature, as well as a positive dielectric anisotropy.

2. Description of the Prior Art

There are known quite a large number of organic chemical compounds that will, within a particular temperature range, exhibit nematic-phase liquid crystals. These compounds are liquid in the sense that their molecules are neither so dissociated as in a gas nor so tightly bound within a structure as in a solid, but at the same time they are said to be crystalline, in that there is a particular ordering to the orientation of molecules, as is sometimes evidenced by peculiar optical effects. Organic materials exhibit under certain conditions what are sometimes called a mesomorphic phase, and there are different families of organic-chemical compounds that exhibit mesomorphic phases of different kinds--the cholesteric, the smectic, and the nematic. These are each characterized by a particular kind of molecular orientation. For the present, it is enough to understand that most of the known nematic-phase liquid-crystal materials are listed by W. Kast in Landolt and Bornstein, Vol. II, Part 2a, 6th Ed., Springer, Berlin (1960), pp. 266-335. The known materials are not, in the main, suitable for widespread use, since they are nematic only at above about 20.degree. Centigrade or within a temperature range somewhat too narrow to suit them for such use.

It is known that when a nematic-phase liquid-crystal material is stationed between pieces of glass that have been rubbed, each of them unidirectionally and on the surface in contact with the nematic-phase liquid-crystal material, there is thus obtained a liquid-crystal unit whose optic axis lies in a direction of unidirectional rubbing.

The techniques for placing onto flat glass a transparent electroconductive coating of tin oxide or indium oxide or the like are quite familiar to persons skilled in the art of making flat glass. Reference may be made to the patents of Tarnapol (U.S. Pat. No. 2,694,761) or Saunders (U.S. Pat. No. 2,648,752). It is appreciated that if the pieces of glass are to be of relatively large surface extent and evenly spaced substantially throughout same, it is necessary to avoid the development of any appreciable warpage in the flat glass while the transparent electroconductive coating is developed thereon. Though under the technology of about five years ago this presented a considerable problem, since the transparent electroconductive coatings then known were nearly always developed by spraying a tin-containing solution onto glass that was quite hot (about 1000.degree. F.), such that the glass was at about the point of warping, there has recently been made commercially available by a major flat-glass producer a kind of flat glass that is understood to have been produced by vacuum cathode-sputtering onto the glass, while it remains at quite a lower temperature, a composition that is principally indium oxide.

It may be taken as known that there are numerous ways of arranging patterns of luminous, transparent, or opaque material so as to constitute, with suitable selective activation, an alpha-numeric display. It may be taken as known how to supply electrical potential to, and thus activate, patches of electroconductive transparent oxide coating situated on pieces of flat glass.

It has been discovered that there exists a family of materials that exhibit nematic phase at room temperature and at the same time a positive dielectric anisotropy. Moreover, they have these properties over a substantial temperature range.

It has been discovered, moreover, that it is possible to change the known light-rotating action of nematic-phase liquid crystal materials by the action of a suitable electrical or magnetic field, and that the threshold valve for such change occurs at a conveniently low applied voltage, such as 5 volts, and that the liquid-crystal material responds rapidly and locally to such change.

A complete understanding of the invention may be obtained from the foregoing and following description thereof, taken in conjunction with the appended drawings, in which;

FIG. 1 is a schematic view, in section and not to scale, of a liquid-crystal unit made in accordance with the present invention;

FIG. 2 is a view illustrating a certain feature in the mode of preparation of the glass pieces used in a liquid-crystal unit according to the invention; and

FIG. 3 is a perspective view, partly schematic, of a structure for the selective transmission of light, made in accordance with the present invention.

In the practice of the instant invention, it is considered highly desirable to use a nematic-phase liquid-crystal composition that has a considerable temperature range within which it exhibits the nematic phase, such as minus 10.degree.C. to plus 70.degree.C., at the same time having a positive dielectric anisotropy. The importance of the latter factor is not to be overlooked. A negative dielectric anisotropy implies that with the application of a field, the molecules tend to line up parallel to the major surfaces of the pieces of flat glass between which the liquid-crystal material is contained, rather than perpendicular thereto, as is intended and required in accordance with the concept of the instant invention, which may be considered as involving the phenomenon of causing a relaxation of the twist of a twisted nematic-phase material by the application of such field. A substantial number of the known nematic-phase liquid-crystal materials have negative dielectric anisotropy.

The problem of finding a suitable mixture of organic chemicals so as to obtain a composition of matter that exhibits at, for example, minus 10.degree. Centigrade to plus 70.degree. Centigrade, the needed combination of nematic phase and positive dielectric anisotropy may be solved by compounding the following mixture: 40 parts by weight of bis-(4'-n-octyloxybenzal)-2-chlorophenylenediamine, 50 parts by weight of p-methylbenzal-p'-n-butylaniline, and 10 parts by weight of p-cyanobenzal-p'-n-butylaniline. -n-bytylaniline. In the mixture above-mentioned, it is considered that the last-named compound is the one that imparts the essential positive dielectric anisotropy; the use of it in greater amounts may be expected to lower the value of the threshold voltage that is required to obtain the needed effect. There is an equation that may be used to express the threshold voltage, namely:

where,

V equals the threshold voltage in volts;

.pi. equals 3.1416

k is an elastic constant having units of dynes, and .DELTA. .epsilon. is a difference in electrical polarizability for the material involved, parallel versus perpendicular to the long axis of the molecules, expressed in c.g.s. units.

For the mixture indicated above, perhaps the principal active ingredient is the p-cyanobenzal-p'-n-butylaniline. Its structural formula is: ##SPC1##

It may be made by refluxing p-cyanobenzaldehyde for about 6 hours with p-n-butylaniline, using methanol as solvent. The preparation is routine for those skilled in making Schiff bases. After recrystallization from methanol, the desired compound is obtained in a yield of 80 to 85 percent.

The possibility of using analogues or homologs of the particular compound discussed above in place of it is not to be ruled out, but at this time the propylaniline is not readily available, and the corresponding compounds made with methylaniline or ethylaniline are somewhat less soluble and therefore less desirable. It is considered that the effects desired may be obtained with the use of a compound of the formula: ##SPC2##

where R is a saturated aliphatic alkyl radical having 1 to 12 carbon atoms. The para relationship between the radical R and the remainder of the molecule is considered essential.

In the particular mixture mentioned above, the bis-(4'-n-octyloxybenzal)-2-chlorophenylenediamine is a low-melting solid, and the p-methylbenzal-p'n-butylaniline is a liquid at room temperature. Those skilled in the art may perceive equivalents for these compounds. It is apparent that the proportions in which they are mixed may be adjusted suitably to influence the operating temperature range of the mixture--using more of the former will give a mixture that operates (has nematic phase and positive dielectric anisotropy) at a lower temperature, while using more of the latter has the opposite effect It may be gathered from the above discussion that the two compounds discussed in this paragraph comprise the major part of the mixture, with the cyanobenzalbutylaniline comprising only a minor portion thereof. Indeed, as little as 3 parts by weight in 100 of the cyanobenzalbutylaniline appears to be a proportion effective to yield the desired positive dielectric anisotropy while being compatible with the remainder of the mixture. On the other hand, it is desirable to use greater proportions of the cyanobenzalbutylaniline, since this lowers the threshold value, but there is probably not in most cases any particular advantage in the use of more than about 40 parts by weight in 100.

Though there has been described above one suitable mixture, there are doubtless others that will be found useful and it is within the spirit and scope of the invention to use them, as hereinafter described and indicated.

In FIG. 1 of the appended drawings, there is shown a liquid-crystal unit 2 that comprises a first piece of flat glass 4 and a second piece of flat glass 6. The drawing is not to scale, and it constitutes a cross-sectional view. The major surfaces of the piece 4 are indicated at 8 and 10 and those of piece 6 at 12 and 14.

Preferably, but not absolutely necessay, there are provided upon the major surfaces 10 and 12 the coatings 16 and 18, respectively. Coatings 16 and 18 are thin, transparent, electroconductive coatings, such as the known tin-oxide or indium-oxide coatings. It should be noted that these coatings may, for the purposes of the instant invention, be quite thin and highly resistive, for example, on the order of 150 ohms per unit square or above, and possibly as high as 5,000 or 10,000 ohms per unit square. This contrasts with requirements for lower resistivity in flat-glass provided with transparent electroconductive coatings for such purposes as heated aircraft windows. If the pieces of flat glass are to be relatively large and uniformly spaced from each other, it is desirable that the transparent electroconductive coating be of the kind that is applied at relatively low temperature such as about 500.degree. Fahrenheit by a process of cathode-sputtering in vacuum, so that dangers of warpage may be safely avoided.

Completing the structure of unit 2 are the spacers 20 and a layer 22 of nematic-phase liquid-crystal material with positive dielectric anisotropy. The spacers may be made of any suitable material, such as oil-resistant double-sided adhesive tape, and have such dimensions as to separate the surfaces 10 and 12 by approximately 1/4 to 2 mils. The rapidity of the response of the liquid-crystal unit is influenced by the amount of separation between the surfaces 10 and 12. For a response time on the order of milliseconds, the separation should be small, such as 1/4 mil, whereas if a longer response time is tolerable, the separation may be correspondingly greater, being possibly as great as about 5 mils. The spacing may in some instances be as little as 0.1 or 0.05 mil.

The nematic-phase liquid-crystal material comprising the layer 22 is preferably of the kind hereinabove indicated, namely, one comprising major portions such as 20 to 80 weight percent each of bis-(4'-n-octyloxybenzal)-2-chlorophenylenediamine and p-methylbenzal-p'-n-butylaniline, these making up about 60 to 97 weight percent of the total composition and the p-cyanobenzal-p'-n-butylaniline comprising the remaining 3 to 40 weight percent. The use of substitutes in proportions the same or different is not being ruled out, but for the most part, it is difficult to find a material that exhibits the combination of properties principally desired, namely, positive dielectric anisotropy and a broad operating-temperature range that includes room temperature.

In FIG. 2, there is a view of the pieces 4 and 6 of flat glass (not to scale, the glass being about 1/8 inch thick). The lines 24 on the surface 10 indicate a direction of rubbing. The lines 26 on the surface 12 have the same significance. In the preparation of a liquid-crystal unit 2, the surfaces 10 and 12 that are to be in contact with the nematic-phase liquid-crystal material 22 are prepared by being stroked or rubbed unidirectionally with, for example, a cotton cloth. Those skilled in the art of dealing with nematic-phase, liquid-crystal materials have long known that such material tends to align with a surface that has been stroked or rubbed undirectionally. The practice for making the liquid-crystal unit 2 comprises rubbing the surfaces 10 and 12 as indicated, applying the material 22 to one of the surfaces, and then bringing the surfaces 10 and 12 together. As will be observed from FIG. 2, the direction of rubbing on the surface 10 is oriented at about 90.degree. with respect to the rubbing on the surface 12. The effect is that there is obtained a twisted nematic structure.

The molecules in a nematic-phase liquid-crystal material are each long and straight, and they tend to lie parallel, like logs in a river or straws in a broom. Their parallelism is statistical, rather than perfect and exact; they are free to move with respect to one another, and there are some that are at some or another small acute angle with respect to the "main stream", and a few others that are at any given moment in a position even less consonant with the bulk of the others. A property of the nematic-phase materials is that the molecules in the vicinity of a rubbed surface tend to align themselves with it. The molecules nearest the surface 10 are thus inclined to orient themselves parallel with the lines 24, and those nearest the surface 12 are inclined to orient themselves parallel to the lines 26. The structure is fluid and active; the molecules in between, in the various layers that are parallel to the surfaces 10 and 12, under conditions of no applied voltage, then arrange themselves in what may be considered a number of layers of suitable intermediate "main-stream" direction, ranging from one close to parallel to the lines 24 (a short distance from the surface 10) through one at about a 45.degree. angle with respect to both the lines 24 and the lines 26 (at about the midpoint of the distance between the surfaces 10 and 12) and on to one close to parallel with lines 26 (a short distance from surface 12). The effect of the liquid-crystal unit 2 upon polarized light that impinges upon its surface 8 and is polarized parallel to the lines 24 is that the unit effects a rotation of the plane of polarization of the light as it passes through the unit, so that the light emanating from the surface 14 is plane-polarized parallel to the lines 26. It would not matter if the plane-polarized light impinging upon the surface 8 were polarized in parallel planes that were at some angle with respect to the lines 24; the same effect of rotation of the plane of polarization is obtained. The extent of rotation does not need to be 90.degree.; any desired extent of rotation may be obtained, merely by orienting and corresponding properly the unidirectionally rubbed surfaces 10 and 12.

To obtain a proper understanding of the invention, it is necessary to understand that the action of an electric field may change the above-described orientation of the molecules. It is possible to classify the nematic-phase materials according to their dielectric anisotropy. Anisotropy means (in one sense) a tendency to assume a directionality and (in another sense) the property of having adopted such directionality. Theoretically, a material may have a dielectric anisotropy of zero, which means that an electric field has no influence upon the orientation of the molecules, either to make them perpendicular to it (negative) or parallel to it (positive). The particular materials with which the instant invention is practiced exhibit a positive dielectric anisotropy, which means that it is possible by the application of an electric field of sufficient strength to cause the molecules to foresake their previous orientation and align themselves parallel to the direction of the electric field. The usual elementary physics example of an electric field is that existing between the plates of a condenser, and the situation within the unit 2 of the instant invention is analogous, with the layers 16 and 18 acting like the plates of the condenser. When there is applied to the layers 16 and 18 a suitable d-c potential, the alignment of the molecules in the material 22 becomes parallel to the electric field, i.e., perpendicular to the surfaces 10 and 12. When plane-polarized light impinges upon the surface 8, there is no rotary effect, the light polarized in the same plane emanates from the surface 14.

What is surprising about the phenomena explained above is that the effect is so rapid, so local, so reversible, and so obtainable with even quite a modest impressed voltage, such as about five volts and possibly less.

Those skilled in the art will understand that in the construction of the unit 2 it is necessary to provide a way to impress the voltage across the layers 16 and 18. These means are indicated schematically in FIG. 1 as comprising a battery 28 and lead lines 30 and 32, with the line 32 preferably containing a switch 34. Of course, the desired electric field may conceivably be produced in other ways that do not require the use of the layers 16 and 18 of transparent, electroconductive material, but the use of some selectively actuable means for the generation of the required field is essential to the practice of the invention.

The required field need not necessarily be an electric one, at least in terms of the means used to produce it. With the use of a magnetic field of sufficient strength and suitable location and orientation, it is possible to obtain the same effect.

In the embodiments contemplated in the previous paragraph, the magnetic field is applied substantially similar to the layer 22 of nematic-phase liquid-crystal material. An alternative, of course, is to use a nematic-phase, liquid crystal which has positive diamagnetic anisotropy. Since most nematic-phase liquid-crystal materials exhibit negative diamagnetic anisotropy, this means that when a liquid-crystal material of positive diamagnetic anisotropy is used, the effect of the magnetic field may be reinforced by the application of an electric field.

Bearing in mind the foregoing, there will be explained below with reference to FIG. 3 how, by means of a structure 36 the light emanating from a source 38 may be allowed to reach or prevented from reaching an eye 40 or other light-sensitive device, all in accordance with the manipulation of a switch 42. The structure 36 comprises a liquid-crystal unit 2, sandwiched between crossed polarizers 44 and 46. The switch 42 corresponds to the switch 34 of FIG. 1, and the battery 48 corresponds to the battery 28. With the light source 38 operating constantly, light passes through the polarizer 44 and becomes plane-polarized. With no applied voltage, the unit 2 rotates the plane of polarization of the light passing therethrough through 90.degree., so that the light is now plane-polarized in such a manner as to be passed by the polarizer 46. Thus, the light is received by the eye 40.

When the switch 42 is closed, there is a change within the unit 2 as above described, such that its effect of rotating the plane of polarization disappears, and the light passes straight through the unit 2, coming against the polarizer 46, whose plane of polarization is crosswise of that of the light impinging upon it--hence, no transmission. The effect is rapid, requiring at most a few seconds and possibly something on the order of milliseconds. It is local; those skilled in the art will readily appreciate how suitable alphanumeric displays may be generated, merely by using suitably shaped and located and powered segments of electroconductive coating in place of the above-described layers 16 and 18.

In the foregoing description, the polarizers 44 and 46 are crossed, but it will be apparent to those skilled in the art what would happen if they were parallel. The structure would then operate in just the reverse manner; with no applied voltage, no transmission and with applied voltage, transmission.

Devices more complete can be made. For example, with three polarizers and two units, one can make an AND gate. If the polarizers are all parallel and the parts are arranged polarizer-unit-polarizer-unit-polarizer, the failure to activate one or both of the units by applying voltage will make the gate fail to transmit.

As another example, one can make an exclusive NOR gate, using two parallel polarizers and first and second units between them. This gate will not transmit if one unit or the other is activated, but it will if both or neither is activated.

The design of other and more complicated kinds of logic devices involving larger numbers of units in logical-zero or logical-one condition, is also possible.

Numerous other potential applications of the instant invention suggest themselves to those skilled in the art. Boolean-function generators may be made in this way. The devices may be used for shuttering. With a system sufficiently fast-acting, it is possible to generate three-dimensional television or movie images (though, as before, glasses polarized in different directions must be worn) by suitable rapid and repeated activation or deactivation of the electric field.

Although there has been disclosed hereinabove a practice of preparing the surfaces that are to come into contact with the liquid material by stroking or rubbing with a cotton cloth, it will be apparent to those skilled in the art that as the art develops, it is likely that various means more sophisticated and effective for preparing the surfaces involved will be developed.

In the disclosure contained hereinabove, there is contemplated only the possibility that the polarizers used may be elements external to the liquid-crystal unit. It is possible, however, to incorporate the polarizers directly into the unit. This may be done by treating the surfaces of the conductive coating, after it has been rubbed, with a solution which forms a dichroic film of the kind described by Dryer in U.S. Pat. Nos. 2,544,659, 2,524,286 and 2,400,877. A suitable solution comprises a 4 weight percent aqueous solution of methylene blue.

Those skilled in the art of constructing multipoint visual displays will perceive that it is possible with the use of the instant invention to simplify considerably the problem of addressing a plurality of points. Instead of requiring 35 separate inputs for a five-by seven alphanumeric display, the same result can be had with only 12 inputs, one for each row and one for each column.