GB2092169A

GB2092169A - Liquid crystal mixtures comprising weakly polar non-cyano components and strongly polar cyano components

Info

Publication number: GB2092169A
Application number: GB8137642A
Authority: GB
Original assignee: Brown Boveri Bbc & Co Ltd
Current assignee: Brown Boveri Bbc & Co Ltd
Priority date: 1980-12-16
Filing date: 1981-12-14
Publication date: 1982-08-11
Also published as: GB2092169B; JPS57126880A; DE3148148A1; CH645664A5

Abstract

Liquid crystal mixtures having a net positive dielectric anisotropy comprise a weakly polar ( DELTA epsilon </=3) anisotropic non-cyano component (A) and a strongly polar (( DELTA epsilon >/=5) anisotropic cyano component (B), the mixture of (A) and (B) containing from 50 to 95 mole % of component (B). Component (A) consists of not more than 5 compounds of the formula: (I) R<1>-A<1>-Z<1>[A<2>-Z<2>]m-A<3>-R<2> and component (B) consists of not more than 5 compounds of the formula R<3>-A<4>-Z<3>-[A<5>-Z<4>]p-A<6>-CN (III> in which formulae: R<1>, R<2> and R<3> are the same or are different and each is an n-alkyl, n-alkoxy, n- alkylamino, n-acyloxy or n-alkoxycarbonyl group containing not more than 12 carbon atoms in a chain (provided that component (A) consists of compounds in which one of R<1> and R<2> is a group containing not more than three atoms in a chain or component (B) consists of compounds in which R<3> is a group containing not more than three atoms in a chain); A<1>, A<2>, A<3> and A<4> is each a ring of the formula: <IMAGE> m is 0, 1 or 2; p is 0 or 1; and Z<1>, Z<2>, Z<3> and Z<4> are the same or are different and each is a covalent bond or a methylenoxy, ethylene, or carboxyl group (provided that Z<1> and Z<2> and/or Z<3> and Z<4> are not both carboxyl groups and Z<3> is not a carboxyl group when, p is zero and either A<4> and A<6> are both rings of formula (1a) or A<4> is a ring of formula (1b).

Description

SPECIFICATION Liquid crystal mixtures This invention is concerned with liquid crystal (LC) mixtures for use in twisted nematic displays i:e.

liquid crystal displays having a twisted nematic phase [see M. Schadt and W. Helfrich, Appl. Phys. Lett.

18, 127 (1971)] and which possess a positive dielectric anisotropy (hereinafter referred to as Aye).

For the operation of liquid crystal displays with the lowest possible driving voltages, the LC mixture should have a high, positive AE. This can be achieved, for example, by the use of anisotropic alkyl and alkylamino cyano compounds which have very high AE'S [see M. A. Osman, Z. Naturforschung 34b, 1092 (1 979)]. For the lowest possible operating voltages, not only AE but also the effective elastic constant kTN of the twisted nematic phase is important because the electrooptic threshold voltage is proportional to

The value of kTN of the bulk liquid crystal (liquid crystal mixture) is, however, difficult to determine, whereas the value of the splay elastic constant kr, can be determined using, for example, the procedure described by A. Saupe, Z.Naturforschung 15a, 815 (1960).The constant k" can, within a good approximation, be substituted for kTN. It can therefore be said that in order to obtain the lowest possible operating voltages, the lowest possible values of the ratio k11/AE are required.

Another parameter of an LC mixture which is important for the operation of twisted nematic displays is the switching time or the optical response time of the display field upon turning on or turning off the driving voltage. The response time should be as short as possible. The response time is related to the viscosity of the LC mixture, and in order to have the shortest possible response time it is import & nt that the mixture have the lowest possible viscosity (for a given elastic constant).

It is known from the literature [e.g. J.P. Parneix et al, Proc. Third Liquid Crystal Conference of Socia!ist Countries, Budapest 1979; M. Davies et al, J. Chem. Soc. Faraday li, 72 (1976); C. J. F.

Boettcher et al, "Theory of Electric Polarization", Second edition, Volume it (1978); and G. Weber et al 1 0. Freiburger Arbeitstagung Fluessigkristalle 1980] that the measured AE's of anisotropic cyano compounds are up to 50% smaller than would be expected from the size of their molecular dipole moments. It is also known that the viscosities of anisotropic cyano compounds are two to three times higher than those of corresponding dialkyl and alkyl/alkoxy compounds.

Various LC mixtures containing cyano and non-cyano components have been empirically discovered to be applicable to displays. These mixtures, however, turn out to have a tendency to form smectic phases, or have other disadvantages.

The investigations that led to the present invention showed that the unexpectedly low AE values as well as the unexpectedly high viscosities of anisotropic cyano compounds can, at least in part, be attributed to intermolecular association effects, e.g. an antiparallel configuration of the molecular dipoles. This antiparallel association has the consequence that the molecular dipoles of anisotropic cyano compounds are partially compensated, which makes AE smaller than expected and increases the effective molecular size, which raises the viscosity.

It was further found that the effective AE as well as the viscosity of liquid crystals based on an isotropic cyano compounds could both be favourably influenced to a surprising extent if the bulk liquid crystal containing the anisotropic cyano component is mixed with a non-cyano, and therefore at most weakly polar, component to disrupt the intermolecular association of the anisotropic cyano compounds.

This was surprising because while it was expected that the viscosity of a strongly polar, relative viscous substance could be decreased by mixing it with an at most weakly polar substance of low viscosity, it was also expected the dielectric anistropy would also be decreased since it depends upon the molecular polarity.

It is known from various investigations [see, for example, B. B. Engelen et al, Ann. Phys. 3, 403-407 (1978)] that mixing polar anisotropic compounds (e.g. cyano compounds) with nonpolar an isotropic compounds (e.g. non-cyano compounds) generally induces the formation of smectic phases in a considerable, if not predominant portion, of the mixture's mesomorphic range. Mixtures possessing smectic phases are not suitable for use in conventional LC displays.

It is the object of this invention to provide LC mixtures possessing a net positive dielectric anisotropy which on the one hand allows the intermolecular association effects of the strongly polar ( > 5) anisotropic cyano compounds to be reduced by mixing them with at least potentially anisotropic and at most weakly polar (E < 3) non-cyano compounds, while on the other hand prevents the formation of induced smectic phases. The liquid crystal mixtures of the invention can be used for the operation of twisted nematic displays having low operating voltages and short response times.

According to the invention there is provided a liquid crystal mixture having a net positive dielectric anisotropy comprising a weakly polar (43) anisotropic non-cyano component (A) and a strongly polar ( > 5) anisotropic cyano component (B) in which: (a) non-cyano component A consists of from 1 to 5 compounds of the formula: R1-A1-Z1-[A2-Z2]m-A3-R2 (1) in which: R' and R2 are the same or are different and each is an n-alkyl, n-alkylamino, n-alkoxy, n-acyloxy or n-alkoxycarbonyl group containing not more than 12 carbon atoms in a chain; A1 and A2 and A3 each represent a ring of the formula::

misO,1 or2;and Z1 and Z2 are the same or are different and each is a covalent bond (-) or a methyleneoxy (-CH2O-), ethylene (-CH2-CH2-) or carboxyl (-CO-O-) group, provided that Z1 and Z2 are not both carboxyl groups; (b) cyano component (B) consists of from 1 to 5 compounds of the formula: R3-A4-Z3-[A5-Z4-]p-A6-CN (11) in which: R3 is an n-alkyl, n-alkyl, n-alkylamino, n-alkoxy, n-acyloxy or n-alkoxycarbonyl group containing up to 12 atoms in a chain; A4, A5 and A6 are each a ring (la), (1b), (1c), (1d), (1e), (1f) or(1g) as defined above; Z3 and Z4 have the same meanings as defined for Z1 and Z2 above; and p is O or 1; provided that Z3 and Z4 are not both carboxyl groups and that Z3 is not a carboxyl group when p is zero and either (i) A4 and A6 are both rings of formula (1 a) or (ii) A4 is a ring of formula (1 b); (c) cyano component (B) forms from 50 to 95 mole % of the mixture of components (A) and (B); and (d) non-cyano component (A) consists only of compounds of formula (I) in which one of the groups R' or R2 contains not more than three atoms in a chain or cyano component (B) consists only of compounds of formula (II) in which the group R3 contains not more than three atoms in a chain.

The LC mixture of the invention consists mainly of the two components (A) and (B), i.e. at least 90 mole %, usually at least 95 mole %, and for many purposes up to 100 mole % of these two components.

The mixture preferably contains no more than a total of 6 different compounds, and typically contains a total of from 3 to 5 compounds.

With regard to the relative proportions of components (A) and (B), the surprising finding should be mentioned that the threshold voltage of a given cyano component (B) remains essentially unchanged through the addition of a considerable amount of non-cyano component (A), whereas the viscosity (response time) is significantly reduced through the addition of a much smaller amount of this component.

These surprising effects observed by mixing the components are probably due to the fact that in the given range of concentrations the decrease in the number of dipoles per unit volume caused by the addition of component (A), which breaks up the association, is compensated for by a decrease of k11 or kTN. At the same time the at least partial breaking up of the molecular association leads to smaller effective molecular sizes and correspondingly lower viscosities. The viscosities of the LC mixtures of this invention can be even further decreased by employing a component (A) having an especially low viscosity.

The non-cyano compounds of formula (I) generally have lower melting and clearing points than the comparable cyano compounds of formula (II) used for component (B). This makes it possible to obtain LC mixtures with a sufficiently wide mesomorphic range, typically from 0 to 700C, with the avoidance of smectic phases or, as the case may be, shifting them to lower temperatures.

The avoidance of smectic phases over the range of practical temperatures, obtained with the LC mixtures of the invention is assured through feature (d) above. Thus one of two possible criteria must be fulfilled. According to the first criterion, the entire non-cyano component (A) consists exclusively of such compounds of formula (I) in which one of the two substituents R1 and R2 has a very short chain, i.e.

a chain having at most a total of three atoms in the chain. The number of carbon atoms in the alkyl part of the chain will depend upon whether the short chain substituent is an alkyl, N-monoalkylamino, acyloxy, alkoxycarbonyl or alkoxy group. If the short chain substituent R1 or R2 is an n-alkyl group, then it can contain at most three carbon atoms. If the short chain substituent R' or R2 is a N-mono-nalkylamino group or an n-alkoxy group then the alkyl part can contain at most two carbon atoms because then the nitrogen or oxygen atom, as the case may be, is as an additional member of the chain.

If the short substituent chain R1 or R2 is an acyloxy or alkoxycarbonyl group then the alkyl part can contain only one carbon atom because then the carbonyl carbon atom and the oxygen atom both act as additional members of the chain. According to the second criterion, the entire cyano component (B) of the LC mixture consists exclusively of compounds of formula (II) in which the group R3 is a very short chain, i.e. one having a total of at most three atoms in the chain. As explained above for R' and R2, the atoms in the alkyl part of R3 can have three carbon atoms (n-alkyl), two carbon atoms and a nitrogen or oxygen atom (N-mono-n-alkylamino or n-alkoxy), or one carbon atom and one carbonyl carbon atom and one oxy- oxygen atom (n-acyloxy and n-alkoxycarbonyl).

Examples of short-chained substituents R', R2 and R3 are methyl, ethyl, n-propyl, methoxy, ethoxy, N-monomethylamino, N-monoethylamino, acetyl and methoxycarbonyl groups.

For many applications, LC mixtures of this invention are preferred where R' or R2 of the non-cyano component (A) satisfies the first short-chain criterion. In addition it is also preferred that the other substituents, R2 or R1, contains at most 7 atoms in the chain.

The components (A) and (B) in the mixtures of this invention are generally chosen so that the room-temperature viscosity (the bulk viscosity measured with a rotating viscorneter) of the whole mixture is less than 65 centipoise (cP = mPa.s) and preferably less than 50 cP. To this end the roomtemperature viscosity of component (A) should be at most about 60 cP and preferably lie below 40 cP, e.g. between 10 and 30 cP. The room temperature viscosity of component (B) should be at most about 70 cP and typically lies in the range of 20 to 60 cP.

it has been found that the LC mixture of the invention should contain no compounds of formulae (I) and (II) in components (A) and (B), which contain three rings and two carboxyl groups. Z' and Z2 therefore should not both be carboxyl groups in compounds of formula (I). Similarly, Z3 and Z4 should not both be carboxyl group in the compounds of formula (it).

Furthermore, for compounds of formula (II) having two rings, i.e. when p = 0, Z3 may not be a carboxyl group if both rings A4 and A6 are rings of formula (1 a) or if A4 is a ring of formula (1 b). These stipulations are based on the undesirably high viscosity of the cyano-substituted benzoic acid phenyl esters and the relatively low thermal stability of the cyano phenyl ester of cyclohexanecarboxylic acid esters. In this latter case the decomposition products cause an increase in the conductivity of the mixture (increased power consumption in the display).

Furthermore, a component (A) is preferred which has a smaller splay elastic constant k" than that of component (B).

Finally LC mixtures of the invention are preferred which possess a dielectric anisotropy of at least 10. The total number of compounds of formula (I) making up component (A) is generally at most the same and preferably smaller than the number of compounds of formula (II) making up component (B).

It is a further preferred characteristic of the LC mixtures of the invention that component (A) be chosen to make the turn-off time of the whole mixture shorter that that of component (B) alone.

Suitable compounds of formulae (I) and (II) are well known and most of them which are employed in LC displays are commercially available together with corresponding values of their material constants. Suitable synthetic procedures for preparing such compounds are known in the art. For example, DE - OS Nos. 2,344,732,2,450,088, 2,429,093,2,502,904, 2,636,684,2,701,591 and 2,752,975 relate to anisotropic compounds for (A) and (B) with rings given of formulae (1 a) and (1 b).

DE - OS 2,641,724 relates to anisotropic pyridine compounds for (A) and (B) with rings of formula (1 d).

The article by D. Demus et al in Mol. Cryst. Liq. Cryst. 56, 115-121(1979) relates to anisotropic tetrazine compounds for (A) and (B) with rings of formula (1 e). The article by H. Sorkin in Mol. Cryst. 56 (Letters), 279-281(1980) relates to anisotropic dioxan compounds for (A) and (B) with rings of formula (1f). The articles by G. Gray et al in J. Chem. Soc. 11,456 (1980) relates to anisotropic bicyclooctane compounds for (A) and (B) with rings of formula (1 g). Anisotropic pyrimidine compounds with rings of formula (1 c) are described by A. I. Pavluchenko et al in J. Physique 40 C3--1 (1 979).

If certain compounds of formulae (I) and (II) are particularly desired but are not available as such, they can be prepared by suitable modification of available compounds using well known procedures.

LC mixtures of this invention can be prepared from the components according to methods known in the art. The mixtures can be prepared, for example, by a mixing process and filled into conventional display cells using known methods.

The LC mixtures of the invention are especially suitable for conventional twisted nematic displays.

These mixtures can also be used to advantage in LC displays using the so-called "guest-host" effect.

Guest-host displays contain a dye component as a "guest" in a nematic "host" phase and, as the case may be, an optically active ("cholesteric") component as well. Such LC mixtures made for this purpose contain in addition to the non-cyarlo component (A) and the cyano component (B), a dye component and, as the case may be, a cholesteric component which can make up to 10 mole % of the total mixture.

Suitable dichloroic dyes (and dye mixtures) and cholesteric compounds are described in the literature (see for example, DE - OS 2,627,215 and 2,658,568) and are commercially available.

In order that the invention may be well understood the following examples are given by way of illustration only.

EXAMPLE 1 In this example component (A) consists of one compound of formula (I) (compound 10) in which R = methoxy, A1 = A2 = p-phenylene (ring of formula 1 a), Zr = carboxyl, R3 = n-pentyl, and m = 0 and component (B) consisted of one compound of formula (II) (Compound 20) in which R3 = n-heptyl, A3 = A4 =p-phenylene, Z3 = covalent bond and p = 0.

l A series of samples having different proportions of (A) and (B) running from 0 to 100 mole % were prepared and filled into test cells having plate separations of 40 and 9.3 microns. The dielectric anisotropy and the elastic constant k" of the samples were investigated in the 40 micron test cell using conventional measuring methods. The threshold voltages of the samples were determined in the 9.3 micron test cell. All measurements were carried out at a reduced temperature corresponding to 95% of the clearing temperature expressed in degrees Kelvin.

The composition of the samples and the test results are summarized in Table I.

TABLE I

Sample No. 1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 Compound 10 100 90 80 70 60 40 20 0 (Mol %) Compound 20 0 10 20 30 40 60 80 100 (Mol %) ## 0 1.9 3,8 5.2 6.5 8.8 10.2 10.8 k11(10-12N) 6.3 6.65 6.7 6.9 7.2 7.6 8.2 8.4 UTN (V) - 2.0 1.42 1.22 1.10 0.98 0.95 0.95 Samples 1-7 and 1-6, which correspond to mixtures according to the invention, show no noteworthy changes in threshold voltage nor a significant drop in the dielectric anisotropy AE, inspite of the increasing proportion of the nonpolar component. Since compound 1 0 used as component (A) is nonpolar (## of sample 1-1 is zero) and simple additive contribution of compound 20 used as component (B) would lead to AE's of 6.7 (instead of 8.8) for sample 1-6 and 8.9 (instead of 10.2) for sample 1-7. Accordingly, the threshold voltage and hence the driving voltage of samples 1-6 and 1-7 are practically the same as those of sample 1-8, the pure cyano component (B), even though the two samples 1-6 and 1-7 contain 40 and 20 mole % of the non-cyano component (A) which makes no significant contribution to the operating voltage since UTN = Co.

It is clear that samples 1-6 and 1-7 are not optimized with respect to temperature range (width of the mesomorphic range). This example has been included here mainly for the purpose of illustrating a characteristic feature of this invention, namely the breaking up of the association of the polar cyano component (B) by a nonpolar or at most weakly polar non-cyano component (A). None of these samples showed a tendency to form smectic phases.

EXAMPLE 2 The turn-off of a pure cyano compound (compound No. 22; Formula ll; R3 = n-heptyl, A3 = A4 = pphenylene, Z3 = covalent bond, p=O) was measured in a test call as described in Example 1 using standard techniques.

A value of 92 milliseconds (ms) was determined. A mixture containing 70 mole % of cyano compound 22 as component (B) and 30 mole % of a non-cyano compound (compound No. 12; Formula I: R' = n-heptyl, A1 = ring of formula 1 b, A2 = p-phenyiene, Z1 = carboxyl, R2 = methyl) as component (A) was then prepared and the turn-off time of this mixture was measured in the same fashion as that of the pure compound 22. The mixture of compounds 12 Rnd 22 gave a significantly shorter turn-off time of 77 ms, with no significant change in either the dielectric anisotropy or the threshold voltage compared to the pure cyano compound 22.

EXAMPLE 3-69 LC mixtures according to the invention were prepared from a non-cyano component (A) and a cyano component (B) made up of compounds given in Tables II and Ila. Component (A) in these examples consisted exclusively of compounds of formula (I) where one or the other of the groups R1 or R2 had at most three atoms in the chain, i.e. the short-chain criterion forth non-cyano component (A) was fulfilled.

The compositions of the LC mixtures in mole % as well as the melting and clearing points TF and Tc are given in Table Ill. The viscosities of the mixtures given by Examples 4-69, which contain a total of 3, 4 or 5 compounds, were all below 65 cP.

Evidence that the molecular association of the cyano component (B) of these examples is decreased is given by the observation that the viscosity of the mixture of (A) and (B) is less than would be estimated from the proportional contribution of the cyano component (B). Further evidence is given by the observation that the dielectric anisotropy of (A) and (B) is larger than would be estimated from the proportional contribution of the cyano component (B). All of these mixtures are suitable for the low voltage operation of LC displays and show no evidence of smectic phases which would disturb the normal operation of the displays.

TABLE II Compounds of formula (l)+ Compound R1 A1 Z1 m A2 Z2 A3 R2 No.

142 H3C (la) -COO- O (la) n-C7H13 144 n-H7C3 " " " " n-C5H11 156 H3C-O- " " " " " 162 n-H7C3 (1 b) ,, ,, 174 " " " " " -O-n-C5H11 175 n-H 9C4 " " ,, ,, -O-CH3 TABLE 11 (continued) Compounds of formula (I)+ Compound R1 A1 Z1 A2 Z2 A3 R2 No.

179 n-H11C5 " " " " " 187 n-H7C3 " " " (1b) n-C3H7 193 " " - 1 (1a) -coo- (1a) " 332 " " - o " -O-n-C4H9 TABLE II (a) Compounds of formula (ll)+ Compound @@. @ @ @ @ @ @ @ @ 201 H3C (1a) - o (1a) 202 H5C2 " " " " 203 n-H,C3 " ,, 204 n-H9C4 " " 205 n-H11C5 " " 206 n-H,3C6 " ,, 207 n-H15C7 " " " " 218 n-H11C5 " " 1 (1a) - " 222 n-H7C3-N(H)- " - 0 " 224 n-H11C5-N(H)- " " " " 226 n-H13C7-N(H)- " " " " 227 n-H15C6-N(H)- " " " " 301 H5C2 (1b) - " " 302 n-H,C3 ,, 303 n-H9C4 " " " " 304 n-H11C5 (1b) - 0 (1a) 305 n-H15C7 " " " 310 n-H11C5 " " 1 (1a) - " 401 " (1c) " 0 " 402 n-H,5C7 " ,, 403 n-H9C4 (1a) " 1 (1c) - " + Note covalent bonds are indicated by a dashed line " ".

TABLE 111 Example Compounds No. Nos. Mol% TF ( C) TC ( C) 3 -1.4 65.3 206 57.79 156 27.33 403 14.88 4 -1.8 70.9 206 57.03 179 28.24 403 14.73 5 -1.2 71.6 206 58.19 174 26.85 403 14.96 6 -0.8 69.8 206 59.07 175 25.80 403 15.13 7 3.5 88.9 206 68.60 403 16.95 193 14.45 8 5.2 76.7 206 72.71 193 15.20 310 12.09 9 5.7 75.8 206 74,06 193 15.44 218 10.50 TABLE III (Contd).

Example Compounds No. Nos. Mol% TF( C) TC( C) 10 -13.0 68.6 206 37.49 187 45.19 403 10.72 310 6.61 11 -12.9 69.0 206 37.68 187 45.34 403 10.76 218 6.21 12 -12.5 69.2 206 38.28 205 42.08 403 10.89 193 8.76 13 -11.7 67.6 206 39.40 304 30.43 179 19.04 403 11.13 14 -11.3 68.1 206 40.06 304 30.88 174 17.79 403 11.27 15 -11.2 66.8 206 40.28 304 31.02 175 17.38 403 11.32 TABLE Ill (Contd.) Example Compounds No. Nos.Mol% TF( C) Tc( C) 16 -8.7 79.8 206 44.26 304 33.66 403 12.16 193 9.92 17 -7.4 70.0 206 46.49 304 35.12 193 10.35 310 8.04 18 -7.2 70.2 206 46.86 304 35.37 193 10.42 218 7.36 19 --8.8 65.9 206 44.08 207 22.35 179 21.46 403 12.12 20 -8.4 66.3 206 .44.81 207 22.79 174 20.12 403 12.27 21 5.5 78.9 206 49.85 207 25.86 403 13.30 193 10.99 TABLE Ill (Contd.) Example Compounds No. Nos.Mol% TF ( C) TC (OC) 22 -4.2 68.3 206 52.26 207 27.35 193 11.44 310 8.95 23 -3.9 68.2 206 52.75 207 27.66 193 11.53 218 8.07 24 -8.7 69.0 206 44.21 305 22.12 179 21.53 403 12.14 25 -8.3 69.5 206 44.95 305 22.56 174 20.19 403 12.30 26 -8.1 68.1 206 45.27 305 22.76 175 19.60 403 12.37 27 -5.4 82.6 206 50.02 305 25.62 403 13.34 193 11.02 TABLE Ill (Contd.) Example Compounds No. Nos. Mol% TF( C) Tc( C) 28 -4.1 72.1 206 52.45 305 27.10 193 11.48 310 8.98 29 -3.8 72.0 206 52.94 305 27.40 193 11.57 218 8.09 30 -5.8 72.0 206 49.31 142 26.61 403 13.19 193 10.88 31 -5.1 72.0 206 50.55 142 27.32 403 13.45 310 8.68 32 -4.9 72.0 206 51.02 142 27.58 403 13.54 218 7.86 33 -8.1 65.3 206 45.21 162 20.38 179 22.05 403 12.35 TABLE III (Contd.) Example Compounds Mol% TF( C) Tc( C) No.Nos.

34 -7.7 65.8 206 45.98 162 20.80 174 20.70 403 12.51 35 -4,7 78.6 206 51.33 162 23.80 403 13.60 193 11.27 36 -4.0 78.8 206 52.61 162 24.52 403 13.86 310 9.00 37 3.7 78.7 206 53.12 162 24.81 403 13.96 218 8.11 38 -3.4 66.2 206 53.87 162 25.24 403 14.11 227 6.78 39 -3.3 66.7 206 53.97 162 25.30 403 14.13 222 6.60 TABLE Ill (Contd.) Example Compounds No. Nos. Mol% TF( C) Tc( C) 40 -2.6 65.2 206 55.32 162 26.07 403 14.40 226 4.20 41 -2.6 65.2 206 55.42 162 26.13 403 14.42 224 4.03 42 -3.4 67.7 206 53.84 162 25.23 193 11.74 310 9.19 43 -3.1 67.5 206 54.37 162 25.53 193 11.84 218 8.26 44 -2.4 67.6 206 55.76 162 26.33 310 9.49 218 8.42 45 -8.1 66.5 206 45.26 156 20.30 179 22.07 403 12.36 TABLE Ill (Contd.) Example Compounds No. Nos.Mol% TF( C) Tc( C) 46 -7.6 66.9 206 46.03 156 20.72 174 20.73 403 12.52 47 -7.4 65.5 206 46.39 156 20.92 175 20.10 403 12.60 48 -16.3 68.6 144 45.87 206 32.94 403 9.72 310 5.87 218 5.60 49 -17.9 68.7 144 43.85 205 36.04 403 9.25 310 5.53 218 5.32 50 -17.2 66.4 144 44.71 304 25.23 305 14.94 403 9.45 310 5.68 TABLE Ill (Contd.) Example Compounds No. Nos. Mol% TF( C) Tc( C) 51 -17.2 67.0 144 44.80 304 25.30 305 14.99 403 9.47 218 5.45 52 -14.9 66.9 144 47.61 304 27.30 402 8.80 403 10.12 310 6.17 53 -14.8 67.4 144 47.74 304 27.40 402 8.85 403 10.15 218 5.86 54 -15.0 65.4 144 47.52 304 27.24 203 9.31 403 10.10 218 5.83 55 -13.4 67.9 144 49.62 304 28.76 403 10.59 310 6.51 401 4.51 TABLE III (Contd.) Example Compounds No. Nos.Mol% TF( C) TC( C) 56 -14.0 78.8 144 48.92 304 28.25 403 10.43 310 6.39 218 6.02 57 -13.4 69.6 144 49.68 304 28.80 403 10.61 310 6.52 227 4.39 58 -13.4 70.0 144 49.60 304 28.74 403 10.58 310 6.51 222 4.57 59 -13.5 67.7 144 49.57 304 28.72 403 10.58 310 6.50 201 4.64 60 -13.3 68.3 144 49.80 304 28.89 403 10.63 401 4.54 218 6.14 TABLE 111 (Contd.) Example Compounds No. Nos. Mol% TF( C) Tc( C) 61 -13.2 70.1 144 49.86 304 28.93 403 10.65 218 6.15 227 4.41 62 -13.3 70.5 144 49.77 304 28.87 403 10.63 218 6.14 222 4.60 63 -13.3 68.2 144 49.74 304 28.84 403 10.62 218 6.13 201 4.66 64 -14.2 65.8 144 48.60 207 17.47 305 14.24 403 10.35 310 6.34 65 -14.1 66.3 144 48.74 207 17.56 305 17.32 403 10.39 218 5.99 TABLE Ill (Contd.) Example Compounds No. Nos.Mol% T ( C) Tc(0C) 66 -14.7 66.0 144 47.90 305 16.81 302 18.88 403 10.19 310 6.22 67 -14.6 66.5 144 48.04 305 16.89 302 18.95 403 10.22 218 5.90 68 -13.6 65.7 144 49.32 305 17.68 303 16.02 403 10.52 310 6.46 69 -13.5 66.2 144 49.48 305 17.77 303 16.10 403 10.56 218 6.10 Examples 70-85 The following Examples illustrate LC mixtures of this invention consisting of a non-cyano component (A) and a cyano component (B) for which the short-chain criterion is fulfilled for compounds (2) of component (B). No tendency for the formation of smectic phases was observed and, again, from the observations made earlier it is evident that the molecular association of the anisotropic cyano compound is reduced, which is advantageous for the dielectric anisotropy and the viscosity (response time).

The compounds used in these examples are listed in Tables IV and IVa, and the compositions of the mixtures in mole % and the melting (TF) and clearing (Tc) points are given in Table V.

TABLE IV Compounds given by formula (1)+ Compound No. R1 A1 Z1 m A2 Z2 A3 R2 148 n-H11C5 (1a) -COO- O (1a) n-C5H11 149 " " " " " n-C7H15 152 n-H15C7 " " " " n-C5H11 153 ,, ,, ,, ,, ,, n-C7H15 166 n-H11Cs (1b) , " n-C5H11 177 n-H9C4 " " " " -O-n-C4H9 178 " " " " " -O-n-C6H13 193 n-H7C3 " - 1 (1a) -COO- " n-C3H7 195 H5C2 " " " " " (1b) " 196 n-H9C4 " " " " " " " 321 n-H11C5 " - " " n-C5H11 Compounds given by formula (2)+ R3 A4 Z3 P A5 Z4 A6 201 H3C (1a) - O (1a) 202 H5C2 " " 203 n-H7C3 " " " 301 H5C2 (1b) " " 302 n-H7C3 " " " + Notice covalent bonds are indicated by a dashed line "-" TABLEV Example Compounds No. Nos. Mol% TF( C) Tc( C) 70 -1.1 67.8 178 37.36 302 29.87 202 20.17 193 12.60 TABLE V (Contd.) Example Compounds No. Nos. Mol% TF( C) Tc( C) 71 -0.4 67.7 177 36.11 302 30.50 202 20.53 193 12.85 72 -0.6 71.2 301 30.38 302 30.28 196 19.97 195 19.37 73 -1.5 73.3 301 32.28 302 32.44 196 21.25 193 13.63 74 -1.8 69.3 301 33.01 302 32.74 195 20.50 193 13.75 75 -3.0 67.8 301 34.31 202 22.49 196 22.14 195 21.05 76 5.4 70.4 301 37.11 202 23.95 196 23.66 193 15.28 TABLE V (Contd.) Example Compounds No. Nos. Mol% TF( C) Tc( C) 77 5.9 65.6 301 37.74 202 24.27 195 22.47 193 15.52 78 4.6 71.4 301 36.17 196 23.15 195 21.83 203 18.85 79 -5.0 68.1 178 31.70 302 26.32 202 18.11 196 17.60 201 6.28 80 -5.0 65.2 178 31.74 302 26.35 202 18.12 195 17.50 201 6.28 81 -3.1 66.6 178 34.36 302 28.01 202 19.09 193 11.84 201 6.70 TABLE V (Contd.) Example Compounds No. Nos. Mol% TF( C) Tc( C) 82 -1.8 70.1 178 36.29 302 29.22 203 15.16 193 12.33 201 7.01 83 -4.9 68.1 177 31.61 302 26.36 202 18.13 196 17.62 201 6.29 84 -4.9 65.2 177 31.65 302 26.39 202 18.15 195 17.52 201 6.29 85 -2.8 66.5 177 33.72 302 28.29 202 19.26 193 11.96 201 6.77

Claims

1. A liquid crystal mixture having a net positive dielectric anisotropy comprising a weakly polar (AE3) anisotropic non-cyano component (A) and a strongly polar (hE > 5) anisotropic cyano component (B) in which: (a) non-cyano component A consists of from 1 to 5 compounds of the formula: R1-A1-Z1-[A2-Z2]m-A3-R2 (1) in which: R1 and R2 are the same or are different and each is an n-alkyl, n-alkylamino, n-alkoxy,n-acyloxy or n-alkoxycarbonyl group containing not more than 12 carbon atoms in a chain; A1 and A2 and A3 each represent a ring of the formula:

mis isO,1 or2; and Z1 and Z2 are the same or are different and each is a covalent bond (-) or a methyleneoxy (-CH2O-), ethylene (-CH2-CH2-) or carboxyl (-CO-O-) group, provided that Z1 and Z2 are not both carboxyl groups: (b) cyano component (B) consists of from 1 to 5 compounds of the formula: R3-A4-Z3-[A5-Z4]p-A6-CN (11) in which: R3 is an n-alkyl, alkyl, n-alkylamino, n-alkoxy, n-acyloxy or n-alkoxycarbonyl group containing up to 12 atoms in a chain; A4, A6 and A6 are each a ring (1a), (1b),(1c),(1d), (1e), (if) or(1g) as defined above; Z3 and Z4 have the same meanings as defined for Z1 and Z2 above; and # is O or 1; provided that Z3 and Z4 are not both carboxyl groups and that 23 iS not a carboxyl group when p is zero and either (i) A4 and A6 are both rings of formula (1 a) or (ii) A4 iS a ring of formula (1 b); (c) cyano component (B) forms from 50 to 95 mole % of the mixture of components (A) and (B); and (d) non-cyano component (A) consists only of compounds of formula (I) in which one of the group R1 or R2 contains not more than three atoms in a chain or cyano component (B) consists only of compounds of formula (II) in which the group R3 contains not more than three atoms in a chain.

2. A liquid crystal mixture as claimed in claim 1 in which the mixture consists of not more than 6 compounds.

3. A liquid crystal mixture as claimed in claim 2 in which the mixture consists of from 3 to 5 compounds.

4. A liquid crystal mixture as claimed in any one of claims 1-3 in which the cyano component has a positive dielectric anisotropy of at least 10.

5. A liquid crystal mixture as claimed in any one of the preceding claims in which the mixture contains the non-cyano component (A) in a sufficient amount to noticeably break up the molecular association of component (B) as evidenced by a decease in the proportional contribution of the.

viscosity of component (B) to the viscosity of the mixture of (A) and (B) and/or as evidenced by an increase in the proportional contribution of the dielectric anisotropy of component (B) to the dielectric anisotropy of the mixture of (A) and (B).

6. A liquid crystal mixture as claimed in any one of the preceding claims in which the non-cyano component (A) consists solely of compounds of formula (II) in which one of the groups R1 and R2 contains not more than 3 atoms in a chain and the others of R2 and R1 contains not more than 7 atoms in the chain.

7. A liquid crystal mixture as claimed in claim 6 in which component (B) consists of compounds of formula (il) in which the group R3 contains not more than 7 atoms in a chain.

8. A liquid crystal mixture as claimed in any one of claims 1--5 in which the cyano component (B) consists solely of compounds of formula (Il) in which the group R3 contains not more than 3 atoms in a chain and the non-cyano component (A) consists of compounds of formula (I) in which both groups R1 and R2 contain not more than 7 atoms in a chain.

9. A liquid crystal mixture as claimed in any one of the preceding claims in which the total viscosity of component (A) is lower than the total viscosity of component (B).

10. A liquid crystal mixture as claimed in any one of the preceding claims in which the viscosity of the mixture at room temperature is less than 65 cP.

11. A liquid crystal mixture as claimed in claim 10 in which said viscosity is not more than 50 cP.

12. A liquid crystal mixture as claimed in any one of the preceding claims in which the room temperature viscosity of cyano component (B) is not more than 70 cP.

13. A liquid crystal mixture as claimed in any one of the preceding claims in which the room temperature viscosity of the non-cyano component (A) is not more than 60 cP.

14. A liquid crystal mixture as claimed in claim 1 3 in which the said viscosity is not more than 40 cP.

1 5. A liquid crystal mixture as claimed in any one of the preceding claims in which the splay elastic constant k" of the non-cyano component (A) is less than that of the cyano component (B).

1 6. A liquid crystal mixture as claimed in any one of the preceding claims in which the number of non-cyano compounds of formula (I) in component (A) is not more than the number of cyano compounds of formula (II) in component (B).

1 7. A liquid crystal mixture as claimed in any one of the preceding claims in which the turn-off time of the mixture is less than the turn-off time of the cyano component (B).

1 8. A liquid crystal mixture as claimed in any one of the preceding claims having a dye component and/or a cholesteric component dissolved therein.

19. A liquid crystal mixture as claimed in claim 1 substantially as hereinbefore described with reference to the examples.