Liquid Crystal Compositions

The present invention relates generally to liquid crystal materials and devices and particularly to the liquid crystal materials described in U.S. copending application Ser. No. 413247 filed 6th Nov. 1973.

In the field of displays there is a requirement for devices having a low power consumption. Devices made from liquid crystal materials have been shown to satisfy this requirement because they have a very large electrical resistance, and at the present time a considerable amount of interest is being shown in such devices for this reason.

There are many known liquid crystal materials; some have been known for many years. Liquid crystal materials are organic materials which exhibit a liquid crystal phase in which the molecules are arranged over limited spatial ranges in an ordered structure.

There are three known kinds of liquid crystal phase. One is known as a smectic mesophase in which the ordering of the molecules is of a lamellar kind. Another is known as a nematic mesophase in which a statistical ordering of the molecules exists parallel to the long axes of the molecules. The third is known as a cholesteric mesophase, in which the ordering of the molecules is of a helical kind.

Liquid crystal materials consist of compounds having a molecular structure which is elongated. For the liquid crystal compounds which have hitherto been used in devices the structure can be generalised and represented as follows: ##SPC3##

Where Q and Q.sup.1 are terminal groups and A and B are linkage groups.

Except in the case of certain esters where the unit A-B includes the group CO.O the unit A-B generally includes a double or triple bond. For example the Schiff's bases which are widely used in commercial liquid crystal devices, such as para-methoxy-benzylidene-butylaniline (MBBA) or para-ethoxybenzylidene-butylaniline (EBBA), are compounds containing the unit C = N.

The presence of a double or triple bond in the unit A-B leads to a chemical and/or photochemical instability of the material and is therefore undesirable for this reason. For example, Schiff's bases are readily hydrolysed, even by traces of water, yielding potentially toxic amines.

It had been supposed until recently that the double or triple bond in the unit A-B is desirable to confer rigidity on the molecule and to provide a reasonably high liquid crystal to isotropic liquid transition temperature together with a reasonably low crystalline solid to liquid crystal transition temperature.

Recently, it has been shown that reasonably low crystalline solid to liquid crystal and reasonably high liquid crystal to isotropic liquid transition temperatures can be obtained whilst maintaining low chemical and/or photochemical instability by using as a liquid crystal material one of the materials defined in U.S. copending application Ser. No. 413247. This application describes material consisting of or containing at least one compound having the formula ##SPC4##

where X and Y are various selected para-substituents.

It is an object of the present invention to increase the temperature ranges over which certain individual compounds having the said formula are liquid crystal.

It has now been found that the temperature ranges over which certain individual compounds having the said formula can be significantly increased by mixing at least three of the said compounds together to form a eutectic mixture. Accordingly, the present invention provides a eutectic mixture containing at least three compounds having the formula ##SPC5##

where CN represents a cyano group and where X.sup.1 represents a group selected from the following list: an alkyl group, an alkoxy group, or a substituted phenyl group ##SPC6##

where R represents an alkyl group or an alkoxy group.

The following tables describe examples of eutectic mixtures embodying the invention. In the tables the heading M represents the molar percentage of each constituent compound in each mixture, and the headings K -- S, S -- N, N -- I, K -- N, K -- C and C -- I represent respectively the temperature in .degree.C at which the following phase transitions occur for each appropriate mixture: solid to smectic liquid crystal, smectic liquid crystal to nematic liquid crystal, nematic liquid crystal to isotropic liquid, solid to nematic liquid crystal, solid to cholesteric liquid crystal and cholesteric liquid crystal to isotropic liquid. The composition of each constituent compound in each mixture is also represented in the tables. The compositions are all of the kind ##SPC7##

as defined above. Therefore each composition is written in an abbreviated form and is represented by the atoms in its group X.sup.1 under the heading X.sup.1.

In the above mixtures the molar percentages of the stated constituent compounds may be varied by up to 3% from the stated values without significantly changing the temperature ranges over which the mixtures have their respective mesophases.

The compounds used for which the group X.sup.1 represents an n-alkyl group may be prepared by the method described in Example 1 in the said copending patent applications.

The compounds used for which the group X.sup.1 represents an alkoxy group may be prepared by the method described in Example 2 in the said copending patent applications.

The compound used for which the group X.sup.1 represents a substituted phenyl group may be prepared by the methods described in examples 12 and 13 in the said copending patent applications.

The mixtures may simply be prepared from their constituent compounds by adding the constituent compounds in the appropriate proportions in a small beaker, heating the beaker and its contents until the isotropic liquid phase (a clear liquid) is obtained, stirring the contents of the beaker in that phase for a few minutes and then allowing the beaker and its contents to cool.

The mixtures may be used in one or more of the devices described in the said copending applications.