GB2236759A - Laterally F-substituted biphenyl nitriles and increased anisotropy in liquid crystal compositions - Google Patents

Abstract

The invention relates to a method for increasing the dielectric anisotropy of a nematic liquid crystal composition of relatively high birefringence being based on materials selected from the following compound classes (1) to (2> <IMAGE> [wherein <IMAGE> is trans-1,4-cyclohexylene or 1,4-phenylene; and R is in every instance a C2-7-alkyl chain wherein one CH2 group may also be replaced by an oxygen atom] comprising including in said composition, as a second part, between 5 and 60% by weight of one or more compounds being selected from the classes 9(3) to (5> <IMAGE> wherein R is as defined above; n is 1 or 2; is trans-1,4-cyclohexylene or 1,4-phenylene; and denotes <IMAGE>

Description

Method for increasing the dielectric anisotropy of liquid crystals The invention relates to a method for increasing the dielectric anisotropy and/or the clearing point of a nematic liquid crystal composition of high or relatively high birefringence being based on materials selected from the following compound classes (1) to (2)

wherein

is trans-1,4-cyclohexylene or 1,4-phenylene, ana R is in every instance an alKyl cnaln containing z to 7 carbon atoms wherein one CH2 group may also replaced by an oxygen atom.

Liquid crystalline compositions being composed of compounds according to formulae (1)-(2) have been developed in the past. These mixtures usually exhibit a high or relatively high value of the birefingence bn which is for example generally required for liquid crystal scattering displays such as, for example, modified White-Taylor phase change displays described below or electrooptical devices containing nematic microdroplets embedded in a transparent matrix the latter systems being abbreviated as MENM (matrix embedded nematic microdroplets) in the following.

On the other hand, the liquid crystal compositions suited for these applications must exhibit a large or relatively large value for the dielectric anisotropy A because A and the critical voltage Vc which mirrors the threshold voltage Vth are generally correlated via Vc () 1/2 Therefore a rather low value of generally results in a high threshold voltage Vth which is of course not desired.

Vth of a TN cell, for example, is usually defined as the voltage V[10.0.20] producing at a viewing angle of 0 measured with respect to the normal axis and at a temperature of 20 OC 10 % transmission of light through the cell when the cell is placed between parallel polarisers, respectively, when the cell is placed between crossed polarisers Vth of a TN cell is usually defined as Vt90.0.20].

Especially when being used for outdoor applications the liquid crystalline compositions additionally have to exhibit a high clearing point, a wide nematic range and a high UV- and temperature stability.

The liquid crystal compositions E7 and E8, for example, manufactured by BDH, GB, are composed of 4'-alkyl-4'-cyanobiphenyls, 4-alkoxy-4'-cyanobiphenyls and 4-alkyl-4"cyanoterphenyls; these mixtures show the following physical data: Mixture birefringence dielectric threshold clearing anisotropy voltage point Vth [V] Tc [OC] E7 0.224 13.8 1.50 60.5 E8 0.246 15.6 1.44 72 E7 and E8 are characterized by high values of An but at the same time exhibit rather poor values for A resulting in rather high values for the threshold voltage Vth.

E7 has a lower optical anisotrpy An compared to that of ES.

This is advantegous, for example, for MENM application be cause a very high value of An not only results in very effective scattering in the opaque state but also in off axis haze in the transparent state which is of course not desired. Both E7 and E8 exhibit rather low values of the clearing point the value of E7 being too low for outdoor applications.

The prior art mixtures and mixture concepts do not simultaneously fulfill the requirements of a high or relatively high birefringence An on the one hand and a high or relatively high dielectric anisotropy A, a high clearing point, a wide nematic range and/or a high UV and temperature stability on the other hand.

The invention was based on the object of discovering a new mixture concept for liquid crystal compositions exhibiting high or relatively high values for An and, at the same time a high or relatively high value for A, a high clearing point, a wide nematic range and/or a high UV- and tempera ture stability.

It has now surprisingly been found that this problem can be solved by providing the method according to the invention.

The invention thus relates to a method for increasing the dielectric anisotropy and/or the clearing point of a nematic liquid crystal composition of high or relatively high birefringence being based on materials selected from the following compound classes (1) to (2)

wherein

is trans-1,4-cyclohexylene or 1,4-phenylene, ana R 1S ln every instance an alkyl chain containing 2 to 7 carbon atoms wherein one CH2 group may also replaced by an oxygen atom, taomprising including in said composition, as a second part, between 5 and 60 % by weight of one or more compounds being selected from the classes (3) to (5)

wherein R is in every instance an alkyl chain containing

2 to 7 carbon atoms wherein one CH2 group may albo replaced by an oxygen atom, n is 1 or 2, e - is trans-l,4 cyclohexylene or 1,4-phenylene, and

denotes

The invention further relates to a nematic liquid crystalline composition with a high or relatively high birefringence the dielectric anisotropy and/or the clearing point of said composition having been increased by applying the method according to the invention.

The invention further relates to an electrooptic system containing the nematic liquid crystal composition according to the invention. The system is preferably based on the principle of light scattering. Further, the system preferably contains microdroplets of the nematic liquid crystal composition according to the invention the microdroplets being embedded in a transparent matrix (MENM); in particular, the system is a PDLC or NCAP device.

By applying the method acording to the invention liquid crystal compositions are obtained being composed of the following 4 parts, part (i) and (ii) being essential and part (iii) and (iv) being optional: (i) a base mixture containing one or more carbo nitriles according to formulae (1)-(2) (ii) one or more laterally fluorinated carbonitriles according to formulae (3)-(5) (iii) additional liquid crystalline compounds being chosen to optimize with regard to the intended application the birefringence dn and/or the ordinary index of refraction nO and/or the extraordinary index of refraction ne and/or other indices of refraction like, for example, the refractive index nx the liquid crystal exhibits in a completely randomly oriented state (EP 0,272,585) or the refractive index n1 of the liquid crystal in the isotropic state and/or the viscosity ri and/or electrooptical parameters like for example the temperature dependence of the threshold voltage or the steepness of the electrooptical characteristic line and/or the mesogenic range and/or other parameters of the liquid crystal these additional compounds affecting the dielectric anisotropy A only to an acceptable and/or small or, in particular, minor degree (iv) chiral dopants in order to induce a cholesteric phase and/or pleochroic dyes and/or other additives chosen to modify other parameters of the liquid crystal like for example the conductivity.

The base mixture (i) preferably amounts to 10 %-80 %, especially to 15 %-70 % and in particular to 15 %-65 % of the liquid crystal composition. If a high birefringence of An > 0.24 of the liquid crystal composition is desired the base mixture preferably contains 4-alkyl- or alkoxy-4'cyanobiphenyls and/or 4-alkyl or alkoxy-4 " -cyanoterphenyls and/or 4-(trans-4-alkyl or alkoxycyclohexyl ) -4' -cyanobiphenyls while the base mixture of liquid crystal compositions exhibiting a relatively high birefringence of 0.13 ' An ' 0.22 preferably contains 4-(trans-4-alkyl or alkoxy-cyclohexyl)benzonitriles, especially in addition to one or more of the compounds mentioned just above.

The second part of the liquid crystal composition is composed of laterally fluorinated carbonitriles according to formula (3)-(5), the percentage of this part preferably ranging from 5 % to 60 %, especially from 10 % to 53 % and, in particular, from 10 % to 45 % of the liquid crystal composition.

In the laterally fluorinated biphenyls according.to formulae (3) and (4.1)

which are preferred, R preferably denotes ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, and heptoxy.

Futher, the laterally fluorinated compounds with 3 rings comprising terphenyls according to formulae (4.2), (5.1.1) and (5.1.2) and cyclohexylbiphenyls according to formulae (5.2.1 and (5.2.2) are preferred:

In these formulae R preferably denotes alkyl, alkoxy or alkoxyalkyl with 2-7 C atoms and especially with 2-5 C atoms.

If liquid crystal compositions with a high clearing point are to be designed the base mixture preferably contains one or more compounds with 3 rings according to formula (2) and/or one or more laterally fluorinated compounds with 3 rings according to formulae (4.2), (5.1.1), (5.1.2), (5.2.1) and/ov (5.2.2). Especially preferred are compositions containing at least one compound according to formula (2) and at least one compound of formulae (4) and/or (5) because these compositions generally exhibit a relatively high or high value of and at the same time a high clearing point.

The liquid crystalline compositions according to the invention preferably have a clearing point of 75 OC or more, especially of 80 OC or more and, in particular, of 85 OC or more.

The laterally fluroinated compounds according to formulae (4) and (5) usually exhibit a very good solubility in base mixtures comprising one or more compounds of formulae (1) and/or (2) and in modified mixtures. The solubility of laterally fluorinated terphenyls according to formulae (4.2), (5.1.1) and (5.1.2) in usual liquid crystal compositions is considerably better than that of the unfluorinated terphenyls according to formula (2.1).

Liquid crystalline compositions containing one or more laterally fluorinated compounds according to formulae (3), (4) and/or (5) and, especially, according to formulae (4) and/or (5) are characterized by a W- and temperature stability being considerably improved compared to that of the prior art mixtures E7 and E8 or similar mixtures.

The third part of the liquid crystal composition contains liquid crystalline compounds being chosen to optimize with regard to the intended application the birefringence An and/or the ordinary index of refraction nO and/or the extraordinary index of refraction ne and/or other indices of refraction and/or the viscosity 10 and/or electrooptical parameters like for example the temperature dependence of the threshold voltage or the steepness of the electrooptical characteristic line and/or the mesogenic range and/or other parameters of the liquid crystal these additional compounds, however, affecting the dielectric anisotropy A only to an acceptable and/or small or, in particular, minor degree.

These additional components are preferably chosen from the nematic or nematogenic (monotropic or isotropic) substances; in particular from the classes of azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl cyclohexanecarboxylates, phenyl or cyclohexyl cyclohexylbenzoates, phenyl or cyclohexyl cyclohexylcyclohexanecarboxylates, cyclohexylphenylbenzoates, cyclohexylphenyl cyclohexanecarboxylates, cyclohexylphenyl cyclohexylcyclohexanecarboxylates, phenylcyclohexanes, syclohexylbiphenyls, phenylcyclohexylcyclohexanes, cycghexylcyclohexanes , cyclohexylcyclohexenes, cyclohexylcyclohexylcyclohexene, 1,4-bis-cyclohexylbenzenes, 4,4'-bis-cyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexylpyridines, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyl-l, 3-dithianes, 1,2-diphenylethanes, 1,2-dicyclohexylethanes, l-phenyl2-cyclohexylethanes, 1-cyclohexyl-2-(4-phenyl-cyclohexyl) - ethanes, l-cyclohexyl-2-biphenylethanes, 1-phenyl-2-cyclohexyl-phenylethanes, optionally halogenated stilbenes, benzyl phenyl ethers, tolanes and substituted cinnamic acids.

The 1,4-phenylene groups of these compounds may be fluorinated.

The most important compounds which are possible constituents of liquid crystal media according to the invention can bevchar cterized by the formulae 1, 2, 3, 4 and 5: R'-L-U-R" 1 R'-L-COO-U-R" 2 R' -L-OOC-U-R" 3 R'-L-CN CH2-U-R" 4 R'-L-C-C-U-R" 5 In the formulae 1, 2, 3, 4 and 5 L and U may be equal or different from each other.L and U independently'from each other denote a bivalent residue selected from the group consisting of -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe-, -G-Cyc- and their mirror images; in this compilation of residues Phe denotes unsubstituted or fluorinated 1,4-phenylen, Cyc trans- 1,4cyclohexylene or 1,4-cyclohexenylen, Pyr pyrimidine-2,5diyl or pyridine-2,5-diyl, Dio 1,3-dioxane-2,5-diyl and G 2-(trans-1,4-cyclohexyl)-ethyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl.

One of the residues L and U is preferably Cyc, Phe or Pyr.

U preferably denotes Cyc, Phe or Phe-Cyc. The liquid crystal media according to the invention preferably contain one or more components selected from the compounds of formulae 1, 2, 3, 4 and 5 with L and U meaning cyc, Phe and Pyr, said liquid crystal media further containing at the same time one ore more components selected from the compounds of formulae 1, 2, 3, 4 and 5 with one of the residues L and U denoting Cyc, Phe and Pyr and the other residue being selected from the group consisting of -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Cyc-, said liquid crystal media containing in addition to this optionally one or more components selected from the compounds of formulae 1, 2, 3, 4 and 5 with L and U being selected from the group consisting of -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc.

In a preferred subgroup of the compounds of formulae 1, 2, 3, 4 and 5 (subgroup 1) R' and R" are independently from each other alkyl, alkenyl, alkoxy, alkenoxy with up to 8 carbon atoms. R' and R" differ from one another in most of these compounds, one of the residues usually being alkyl or alkenyl. In another preferred subgroup of the compounds of formulae 1, 2, 3, 4 and 5 (subgroup 2) R' denotes -CN, -CF3, -F, -C1 or -NCS while R' has the meaning indicated in subgroup 1 and is preferably alkyl or alkenyl. Other variants of the envisaged substituents in the compounds of formulae 1, 2, 3, 4 and 5 are also customary. Many such substances are commercially available.

All these substances are obtainable by methods which are known from the literature or by analogous methods.

Anyone skilled in the art can choose compounds from this large pool of nematic or nematogenic substances in order to optimize An and/or the ordinary index of refraction nO and/or the extraordinary index of refraction ne and/or other indices of refraction and/or the viscosity 10 and/or electrooptical parameters like for example the temperature dependence of the threshold voltage or the steepness of the electrooptical characteristic line and/or the mesogenic range and/or other parameters of the liquid crystal composition taking into account at the same time that the dielectric anisotropy A of the mixture being composed of compounds of part (i) and (ii) is changed only to an acceptable and/or small or, in particular, minor degree.Dielectrically positive nematics or nematogenics of this pool exhibiting a value A > 5, especially At > 10, and in particular At > 15 are preferred. It is also possible, however, to add dielectrically neutral compounds of this pool with -3 fi At < = 3. The expert being aware of the requirement for a high or relatively high dielectrical anisotropy At will adjust the concentration of such compounds so that the overall dielectric anisotropy of the composition deteriorates only to a small and/or, at any rate, acceptable degree.The range 14 5 Ae S 20 for the dielectric anisotropy is usually termed as relatively high while liquid crystal compositions with a high dielectric anisotropy generally exhibit values A8 > 20.

The viscosity of a liquid crystal composition can, for example, be raised by adding high viscous liquid crystalline compounds or, especially, by adding one or more liquid crystalline side chain polymer as is disclosed in DE 39 19 942. High viscous liquid crystalline compositions are, for example, usually required in MENM systems being operated by ac voltage in order to avoid flickering. Low viscous compositions are, for example, generally used in matrix displays with high information content. The expert can choose low viscous compounds from the great pool of nematic or nematogenic compounds like for example

with R2 and-R3 denoting independently from each other alkyl, alkoxy, alkoxycarbonyl or alkanoyloxy with 1-10 C atoms in order to reduce the viscosity of the liquid crystalline compsition.

According to the invention liquid crystalline compositions with a high clearing point Tc can be obtained by adding one or more compounds with 3 rings of formulae (4.2), (5.1.1), (5.1.2), (5.2.1) and (5.2.2) to the base mixtures containing one or more compounds of formulae (1) and/or (2) and especially of formula (2). These compositions do not only exhibit a high clearing point but are also characterized by a relatively high or high dielectrical aniostropy A. If the clearing point is to be raised even further high clearing compounds like for example

can be added the meaning of R2 and R3 being indicated above.

Optionally highly nematogenic compounds like for example

can be added the meaning of R2 and R3 being indicated above.

If liquid crystal compositions exhibiting a relatively high birefringence An are to be obtained liquid crystalline compounds of the formulae (6)-(10) are especially preferably added:

In the formulae (6)-(10) R denotes alkyl or alkoxy, preferably n-alkyl or n-alkoxy with 1-18 C atoms, - B is 1,4-phenylene or 1,4-cyclohexylene and r and s are independently from each other 1 or 2.

The third part preferably amounts to 0-25 %, in particular, however, to 0 %-20 % of the liquid crystal composition.

The liquid crystal compositions can be modified by a fourth part containing chiral dopants in order to induce a cholesteric phase and/or pleochroic dyes and/or other additives chosen to modify other parameters of the liquid crystal like for example the conductivity. Compounds of this type are known to the expert and described in detail in the literature (see for example H. Keller/R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim 1980, I. Haller et al., Mol. Cryst. Liq. Cryst., 24 (1973) 249, DE 22 09 127, DE 22 40 864, DE 23 21 632, DE 23 38 281, DE 2450 088, DE 26 37 430, DE 28 53 728, DE 29 02 177). The fourth part preferably amounts to 0-20 %, in particular, however, to 0-18 % of the liquid crystal composition.

The liquid crystal composition according to the invention preferably contains 2-40 components and in particular 3-30 components.

The liquid crystalline compositions according to the invention comprising one or more compounds chosen from the compounds classes (1) and (2) and one or more compounds of formulae (3), (4) and/or (5) can be optimized with respect to the intended application by adding compounds taken from part (iii) and/or part (iv).

The decisive point is, however, that the liquid crystalline compositions according to the invention have advantegous properties. They exhibit a high or relatively high birefringence and simultaneously a high or relatively high dielectric anisotropy and/or a high clearing point. The compounds of formulae (1)-(5) are very stable towards chemicals, heat and light and they can readily be mixed together with other compounds. The liquid crystal composition according to the invention exhibits a high UV and temperature stability and a highly nematic character, in particular also at low temperatures, and advantageous values of the viscosity.

The liquid crystal compositions according to the invention are preferably used in electrooptical displays being based on the principle of light scattering. Devices of this type require a high or relatively high birefringence An in order to minimize the contrast between the transparent and opaque state and, at the same time, re-quire a high or rather high dielectric anisotropy resulting in a low threshold voltage Vth.

Especially preferred are modified White-Taylor phase change displays and, in particular, electrooptical systems containing nematic microdroplets being embedded in a transparent matrix (MENM). The liquid crystalline compositions according to the invention are characterized by a poor solubility and the polymer used for the matrix which is especially advantgeous for the PDLC/SIPS technology (see below).

When a modified White-Taylor phase change display relaxes from the transparent field "on" state exhibiting a nematic phase to the cholesteric phase of the field off" state the cholesteric helices are often formed in a random orientation forming a scattering texture which can also be obtained by applying a low voltage to the field "off" state. The modified White-Taylor phase change display generally exhibiting a short pitch length p < 1 jim of the cholesteric phase thus relies on the scattering of light; it is not necessary that the liquid crystal used in this display contains a dye.

The liquid crystal composition forming the microdroplets in a particular and most popular embodiment of a MENM system exhibits an ordinary index of refraction nO being matched to the refractive index of the transparent and optically isotropic matrix nM. When a voltage is applied between transparent electrodes laminated onto the matrix the dielectrically positive nematic phase aligns with its director parallel to the electric field and perpendicular to the electric vector of the light traversing the MENM system. The light now sees an optically isotropic medium, that means the light detects no effective difference in refractive index of the liquid crystal composition and the supporting matrix, and the MENM system appears transparent.

When the voltage is switched off, however, light propagating through the system is scattered by the randomly aligned nematic liquid crystal molecules in the microdroplets the system thus appearing opaque Other embodiments of MENM systems partly requiring a different matching of the indices of refraction are known to those skilled in the art.

The liquid crystal composition has to exhibit a high or relative high birefringence An in order to enhance scatter ing in the opaque state and to maximize the contrast taking into account, however, that a birefrigence An being too high can result in off-axis haze in the transparent state thus deteriorating the optical performance of the display.

At the same time a high or relatively high dielectric anisotropy A is required resulting in a low threshold voltage Vth. Both requirements are fulfilled by the liquid crystal compositions according to the invention.

The birefringence bE and/or the ordinary and/or extraordinary indices of refraction and/or other indices of refraction of the composition can be optimized with respect to the intended application by varying the components as described in great detail above.

NCAP (nematic curvilinear aligned phases) and PDLC (polymer dispersed liquid crystals) are preferred embodiments of MNEM systems.

NCAP films are usually obtained by mixing together the encapsulating material for example PVA (polyvinyl alcohol), the liquid crystal composition and a carrier medium, for example water, in a colloid mill as is described in US 4,435,047. The emulsion is dried thus eliminating the carrier medium and curing the encapsulating medium.

PDLC films are prepared by thoroughly mixing the liquid crystal composition and monomers or oligomers of the matrix forming material. This mixture is polymerized and the phase separation is induced applying, for example, TIPS (temperature-induced phase separation), SIPS (solvent-induced phase separation) or PIPS (polymerizationinduced phase separation) (see, for example US 4,688,904, EP 0,272,582, Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt.

157 (1988) 427).

The following examples are to be construed as merely illustrative and not limitative. In the foregoing and in the following all parts and percentages are by weight and the temperatures are set forth in degrees Celsius.

Further are: C: crystalline-solid state, S: smectic phase (the index denoting the type of smectic phase), N: nematic phase, Ch: cholesteric phase, I: isotropic phase. The number being embraced by 2 of these symbols denotes the temperature of phase change.

Example 1 1.1 A liquid crystalline medium is prepared consisting of: 43 % 4-pentyl-4'-cyanobiphenyl 17 % 4-propoxy-4'-cyanobiphenyl 13 % 4-pentoxy-4'-cyanobiphenyl 17 % 4-octoxy-4'-cyanobiphenyl 10 % 4-pentyl-4 '-cyanoterphenyl This mixture exhibits the following physical data:: birefringence An = 0.246 threshold voltage V[90.0.20) = 1.44 V The threshold voltage was measured in a TN cell (twist angle + = ) with d = 7 pm and crossed polarisers. 4 1.2 A liquid crystalline medium is prepared consisting of 30 % 4-pentyl-4'-cyanobiphenyl 15 % 4-ethyl-4'-cyanobiphenyl 20 % 4-(propylphenyl)-3-fluoro-4'-cyanobiphenyl 10 % 4-propyl-2-fluoro-4'-cyanobiphenyl 15 % 4-pently-2-fluoro-4'-cyanobiphenyl 10 % 4-pentyl-3' '-fluoro-4' '-cyanoterphenyl This mixture exhibits the following physical data: birefringence An = 0.246 threshold voltage V[90.0.20] = 1.26 V The threshold voltage was measured in a TN cell (twist angle # = #/4) with d = 7 pm and crossed polarisers.

A comparison of the mixtures 1.1 and 1.2 clearly demonstrates the advantagenous properties of mixtures being obtained by applying the method according to the invention.

Example 2 A liquid crystalline medium consisting of 20 % 4-(trans-4-pentylcyclohexyl)-benzonitrile 15 % 4-pentyl-4'-cyanobiphenyl 15 % 4-ethyl-4'-cyanobiphenyl 5 4-pentyl-4-cyanoterphenyl 15 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 10 % 4-propyl-2-fluoro-4'-cyanobiphenyl 15 % 4-pentyl-2-fluoro-4'-cyanobiphenyl 5 % 4-(trans-4-pentylcyclohexyl)-4'-cyanobiphenyl exhibits the following physical data: birefringence An = 0.218 ordinary index of refraction nO = 1.522 dielectric anisotropy As = 16.71 threshold voltage V[90.0.20] = 1.33 V The threshold voltage was measured in a TN cell (twist angle 4, = 4) with d = 7 pm and crossed polarisers.

Example 3 A liquid crystalline medium consisting of 12 % 4-ethyl-4-cyanobiphenyl 4 % 4-propyl-4-cyanobiphenyl 29.6 % 4-pentyl-4-cyanobiphenyl 8.8 % 4-propxy-4-cyanobiphenyl 8.0 % 4-pentyl-4"-cyanoterphenyl 20.0 % 4-(B-propylphenyl)-3-fluoro-4'-cyanobiphenyl 5.6 % 4-(4-cyanophenyl)-phenyl-4-(4-heptylphenyl)- phenyl-carboxylate 12 % 4-(trans-4-pentylcyclohexyl)-4'-cyanobiphenyl exhibits the following physical data: birefringence An = 0.286 ordinary index of refraction 10o = 1.530 dielectric anisotropy As = 13.5 threshold voltage V t90.0.20] = 1.66 V extrinction coefficient y = 1.629 for # = 350 nm y = 0.130 for A = 360 nm y = 0.006 for " = 370 nm clearing point N 113 I The extinction y is determined via A 1 . CW with A denoting the absorbance, 1 the path length in cm and cw the mass of the liquid crystal in grams per 100 ml of solution.

Example 4 A liquid crystalline medium consisting of 14.0 % 4-ethyl-4-cyanobiphenyl 5.0 % 4-propyl-4-cyanobiphenyl 33.0 % 4-pentyl-4-cyanobiphenyl 12.0 % 4-propxy-4-cyanobiphenyl 10.0 % 4pentyl-41'-cyanoterphenyl 23.0 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 3.0 % 4-(4-cyanophenyl)-phenyl-4-(4-heptylphenyl)- phenyl-carboxylate exhibits the following physical data: birefringence An = 0.287 ordinary index of refraction nO = 1.533 dielectric anisotropy As = 14.1 threshold voltage V [90.0.20] = 1.60 V clearing point N 96 I Example 5 A liquid crystalline medium consisting of 35.0 % 4-pentyl-4'-cyanobiphenyl 12.0 % 4-propoxy-4'-cyanobiphenyl 14.0 % 4-pentoxy-4'-cyanobiphenyl 6.0 % 4-pentyl-4"-cyanoterphenyl 23.0 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobifenyl 3.0 % 4-(4-cyanophenyl)-phenyl-4-(4-heptylphenyl)- phenyl-carboxylate 7.0 % 4-octoxy-4l-cyanobiphenyl exhibits the following physical data: birefringence An = 0.280 ordinary index of refraction nO = 1.527 dielectric anisotropy As = 13.0 threshold voltage V [90.0.20] = 1.71 V clearing point N 97 1 Example 6 A liquid crystalline medium consisting of 32.0 % 4-pentyl-4'-cyanobiphenyl 12.0 % 4-propoxy-4'-cyanobiphenyl 13.0 % 4-pentoxy-4'-cyanobiphenyl 8.0 % 4-pentyl-4 "-cyanoterphenyl 22.0 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 3.0 % 4-(4-cyanophenyl)-phenyl-4-(4-heptylphenyl)- phenyl-carboxylate 5.0 % 4-octoxy-4'-cyanobiphenyl 5.0 % 4- (trans-4-pentylcyclohexyl ) -4 1-cyanobiphenyl exhibits the following physical data: birefringence An = 0.284 ordinary index of refraction nO = 1.526 clearing point N 108 I Example 7 A liquid crystalline medium consisting of 33.0 % 4-pentyl-4'-cyanobiphenyl 12.0 % 4-pro-poxy-4'-cyanobiphenyl 14.0 % 4-pentoxy-4'-cyanobiphenyl 10.0 % 4-pentyl-4'-cyanoterphenyl 23.0 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 3.0 % 4-(4-cyanophenyl)-phenyl-4-(4-heptylphenyl)- phenyl-carboxylate 5.0 % 4-octoxy-4'-cyanobiphenyl exhibits the following physical data: birefringence An = 0.288 ordinary index of refraction no = 1.528 clearing point N 105 I Example 8 A liquid crystalline medium consisting of 30 % 4-hexyl-4'-cyanobiphenyl 10.0 % 4-pentoxy-4-cyanobiphenyl 7.0 % 4-octoxy-4'-cyanobiphenyl 5.0 % 4-pentyl-4"-cyanoterphenyl 15.0 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 5.0 % 4-(trans-4-pentylcyclohexy)-benzonitrile 8.0 % 1-propXy-trans-4-(trans-4-propylyclohexyl)-cyclohexan 20.0 % 4- (trans-4-pentylcyclohexyl )-4 1-cyanobiphenyl exhibits the following physical data: birefringence An = 0.2293 ordinary index of refraction nO = 1.5152 clearing point N 102 I Example 9 A liquid crystalline medium consisting of 32.0 % 4-pentyl-4'-cyanobiphenyl 12.0 % 4-propoxy-4'-cyanobiphenyl 13.0 % 4-pentoxy-4'-cyanobiphenyl 6.0 % 4-pentyl-4"-cyanoterphenyl 23.0 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 3.0 % 4-(4-cyanophenyl)-phenyl-4-(4-heptylphenyl)- phenyl-carboxylate 5.0 % 4-octoxy-4'-cyanobiphenyl 6.0 % 4- (trans-4-pentylcyclohexyl ) -4 '-cyanobiphenyl exhibits the following physical data: birefringence An = 0.283 ordinary index of refraction nO = 1.564 dielectric anisotropy As = 12.8 threshold voltage V [90.0.20] = 1.73 V clearing point N 108 I Example 10 A liquid crystalline medium consisting of 10.0 % 4-pentoxy-4'-cyanobiphenyl 7.0 % 4-octoxy-4'-cyanobiphenyl 31.0 % 4-hexyl-4'-cyanobiphenyl 12.0 % 4- (trans-4-pentylcyclohexyl ) -benzonitrile 20.0 % 4-(trans-4-pentylcyclohexyl)-4'-cyanobiphenyl 15.0 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 5.0 % 4-pentyl-4"-cyanoterphenyl exhibits the following physical data: birefringence An = 0.2373 ordinary index of refraction nO = 1.5183 dielectric anisotropy As = 14.0 clearing point N 105 I Example 11 A liquid crystalline medium consisting of 20.0 L 4-(trans-4-pentylcyclohexyl)-benzonitrile 15.0 % 4-(trans-4-propylcyclohexyl)-benzonitrile 10.0 % 4-(trans-4-butylcyclohexyl)-benzonitrile 10.0 % 4-pentyl-4'-cyanobiphenyl 10.0 % 4-ethyl-4'-cyanobiphenyl 7.0 % 4-propyl-4'-cyanobiphenyl 10.0 % 4-(trans-4-pentylcyclohexyl)-4'-cyanobiphenyl 10.0 % 4- (4-propylcyclohexyl ) -3-fluoro-4'-cyanobiphenyl 8.0 % 4-pentyl-4"-cyanoterphenyl exhibits the following physical data: birefringence An = 0.2042 ordinary index of refraction nO = 1.5132 dielectric anisotropy As = 14.8 clearing point N 81 I Example 12 A liquid crystalline medium consisting of 30.0 % 4-pentyl-4'-cyanobiphenyl 15.0 % 4-ethyl-4'-cyanobiphenyl 20.0 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 10.0 % 4-propyl-2-fluoro-4'-cyanobiphenyl 15.0 % 4-pentyl-2-fluoro-4'-cyanobiphenyl 10.0 % 4-pentyl-3' '-fluoro-41 I -cyano-terphenyl exhibits the following physical data: birefringence An = 0.246 threshold voltage V [90.0.20] = 1.26 V clearing point N 62 I Example 13 A liquid crystalline medium consisting of 25.0 % 4-pentyl-4'-cyanobiphenyl 15.0 % 4-ethyl-4'-cyanobiphenyl 18.0 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 10.0 % 4-propyl-2-fluoro-4'-cyanobiphenyl 10.0 % 4-pentyl-2-fluoro-4'-cyanobiphenyl 10.0 % 4-pentyl-3",5"-difluoro-4"-cyanoterphenyl 6.0 % 4-nonyl-3"-fluoro-4"-cyanoterphenyl 6.0 % 1-[trans-4-(trans-4-propylcyclohexyl)-cyclo- hexyl3-2-[4-(4-cyanophenyl)-phenyl]-ethane ~ exhibits the following physical data: birefrigence An = 0.258 threshold voltage V [90.0.20] = 1.33 V dielectric anisotropy As = 19.7 clearing point N 82 I

Claims (10)

1. claims 1. A method for increasing the dielectric anisotropy of a nematic liquid crystal composition of relatively high birefringence being based on materials selected from the following compound classes (1) to (2)

   wherein

   is trans-1,4-cyclohexylene or 1,4phenylene, and R is in every instance an alkyl chain containing 2 to 7 carbon atoms wherein one CH2 group may also replaced by an oxygen atom, comprising including in said composition, as a second part, between S and 60 X by weight of one or more compounds being selected from the classes (3) to (5)

   wherein R is in every instance an alkyl chain containing 2 to 7 carbon atoms wherein one group may also replaced by an oxygen atom, n is 1 or 2,

   is trans-1,4-cyclohexylene or 1,4-phenylene, and
2. 2.Nematic liquid crystal composition with a high or relatively high birefringence the dielectric anisotropy of which has been increased according to the method of claim 1.
3. 3. Electrooptical system containing the nematic liquid crystal composition according to claim 2.
4. 4. Electrooptical system according to claim 3 said system being based on the principle of light scattering.
5. 5. Electrooptical system according to at least one of the claims 3 or 4 said nematic liquid crystal composition forming microdroplets being embedded in a transparent matrix.
6. 6. Electrooptical system according to claim 5 said system being a PDLC device.
7. 7. Electrooptical system according to claim 5 said system being a NCAP device.
8. 8. A method for increasing the dielectric.anisotropy of a nematic liquid crystal composition of relatively high birefringence, substantially as hereinbefore described.
9. 9. A nematic liquid crystal composition, substantially as hereinbefore described.
10. 10. An electrooptical system, substantially as hereinbefore described.