GB2252977A

GB2252977A - Method for increasing the birefringence of liquid crystals

Info

Publication number: GB2252977A
Application number: GB9200765A
Authority: GB
Inventors: Bernhard Rieger; Hiroshi Kamei
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 1991-02-20
Filing date: 1992-01-15
Publication date: 1992-08-26
Anticipated expiration: 2012-01-15
Also published as: GB2252977B; GB9200765D0

Abstract

A method for increasing the birefringence of a nematic liquid crystal composition of relatively high dielectric anisotropy being based on materials selected from the following compound classes (1) to (2> <IMAGE> [wherein A is <IMAGE> and R is in every instance an alkyl chain containing 2 to 7 carbon atoms wherein one CH2 group may also be replaced by an oxygen atom,] comprising including in said composition, as a second part, in between 5 and 45% by weight of one or more compounds being selected from the classes (3) to (5). <IMAGE>

Description

Method for increasing the birefringence of liquid crvstals The invention relates to a method for increasing the birefringence of a nematic liquid crystal composition of relatively high dielectric anisotropy being based on materials selected from the following compound c-lasses (1) to (2)

wherein A is

CH2CH2 and R is in every instance an alkyl chain containing 2 to 7 carbon atoms wherein one CH2 group may also be replaced by an oxygen atom, comprising including in said composition, as a second part, in between 5 and 45 % by weight of one or more compounds being selected from the classes (3) to (5)

wherein R and A are as defined above and Ll to L5 are each H or one or two of L1 to L5 also F.

Liquid crystalline compositions comprising compounds according to formulae (1)-(2) have been developed in the past.

These mixtures usually exhibit a high or relatively high values of the dielectric anisotropy but only a medium birefringence. Compositions being based on compounds of the formulae (3) to (5) usually exhibit medium values for the dielectric anisotropy and high or relatively high values of the birefringence An 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 liquid crystal compositions suited for these applications must exhibit a large value for the dielectric anisotropy hE because tE and the critical voltage Vc which mirrors the threshold voltage Vth are generally correlated via Ve - () 1/2 Therefore a rather low value of At 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 V90,0,20)- The liquid crystal compositions E7 and E8, for example, manufactured by Merck Ltd., UK, 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 anisotropy voltage An As Vth [V] E7 0.224 13.8 1.50 E8 0.246 15.6 1.44 E7 and E8 are characterized by high values of An but at the same time exhibit rather poor values for As resulting in rather high values for the threshold voltage Vth.

The prior art mixtures and mixture concepts do not simultaneously fulfill the requirements of a high or relatively high birefringence An and a high dielectric anisotropy As.

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 As at the same time.

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 birefringence of a nematic liquid crystal composition of high dielectric anisotropy being based on materials selected from the following compound classes (1) to (2)

wherein A is

wherein R and A are as defined above and L1 to L5 are each H or one or two of L1 to L5 also F.

The invention further relates to a nematic liquid crystalline composition with a high dielectric anisotropy the birefrigence 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; 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 carbonitriles according to formulae (3)-(5), which are preferably laterally fluori nated (iii) additional liquid crystalline compounds being chosen to optimize with regard to the intended application the birefringence An and/or the ordi nary index of refraction nO and/or the extraordi nary index of refraction ne and/or other indices of refraction like, for example, the refractive index nx the liquid crystal exhibits in a com pletely randomly oriented state (EP 0,272,585) or the refractive index n1 of the liquid crystal in the isotropic state and/or the viscosity 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 As only to an acceptable and/or small or, in particu lar, minor degree (iv) chiral dopants in order to induce a cholesteric phase and/or pleochroic dyes and/or other addi tives chosen to modify other parameters of the liquid crystal like for example the conductivity.

The base mixture (i) preferably amounts to 10-90 %, especially to 30-80 % and in particular to 40-70 % of the liquid crystal composition. If a high birefringence of An > 0.22 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 carbonitriles according to formula (3)-(5), the percentage of this part preferably ranging from 5 to 40 %, especially from 10 to 35 % and, in particular, from 20 to 30 % of the liquid crystal composition.

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 these additional compounds, however, affecting the dielectric anisotropy As 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, cyclohexylbiphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclohexanes, cyclohexylcyclohexenes, cyclohexylcyclohexylcyclohexene, 1,4-bis-cyclohexylbenzenes, 4,4' -bis-cyclo- hexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexylpyridines, phenyl- or cyclohexyldioxanes, phenylor cyclohexyl-1,3-dithianes, 1,2-diphenylethanes, 1, 2-di- cyclohexylethanes, l-phenyl-2-cyclohexylethanes, 1-cyclohexyl-2-(4-phenyl-cyclohexyl)ethanes, 1-cyclohexyl-2-biphenylethanes, l-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 be characterized 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-CH2CH2-U-R'1 4 R' -L--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-phenylene, Cyc trans- 1,4-cyclohexylene or 1,4-cyclohexenylene, Pyr pyrimidine-2,5-diyl or pyridine-2,5diyl, 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, -Cl 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 taking into account at the same time that the dielectric anisotropy As 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 As > 5, especially As > 10, and in particular As > 15 are preferred. It is also possible, however, to add dielectrically neutral compounds of this pool with -3 S As < 3. The expert being aware of the requirement for a high or relatively high dielectrical anisotropy As 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 S As s 20 for the dielectric anisotropy is usually termed as relatively high while liquid crystal compositions with a high dielectric anisotropy generally exhibit values As > 40.

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,

is 1,4phenylene 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 24 50 088, 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 crystal composition according to the invention exhibits a high or relatively high birefringence and simultaneously a high or relatively high dielectric anisotropy. 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 stability and a highly nematic character, in particular also at low temperatures.

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, require 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 the latter systems being abbreviated as MENM (matrix embedded nematic microdprolets) in the following.

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 pin 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 scattering 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 As is required resulting in a low threshold voltage Vth. Both requirements are fulfilled by the liquid crystal compositions according to the invention. The birefringence 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 (polymerization-induced 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: 13.8 % of 3-fluoro-4-cyanophenyl-p-ethylbenzoate 15.4 % of 3-fluoro-4-cyanophenyl-p-propylbenzoate 23.1 % of 3-fluoro-4-cyanophenyl-p-butylbenzoate 23.1 % of 3-fluoro-4-cyanophenyl-p-pentylbenzoate 9.2 % of 3-fluoro-4-cyanophenyl-p- (trans-4-propyl- cyclohexyl)benzoate 7.7 % of 3-fluoro-4-cyanophenyl-p-(trans-4-butyl cyclohexyl)benzoate 7.7 % of 3-fluoro-4-cyanophenyl-p-(trans-4-pentyl cyclohexyl)benzoate This mixture exhibits the following physical data: birefringence An = 0.1715 threshold voltage V(1010,20) = 0.64 V The threshold voltage was measured in a TN cell (twist angle v = S with d = 7 pin and crossed polarisers.

4 1.2 A liquid crystalline medium is prepared consisting of 65 % of medium of example 1.1, 10 % of p-(trans-4-propylcyclohexyl)-cyanobenzene, 5 % of p-cyanophenyl p-ethylbenzoate, 5 % of p-cyanophenyl p-propylbenzoate, 6 % of 4'-(p-pentylphenyl)-4-cyanobiphenyl and 9 % of 3-fluoro-4' - (p-propylphenyl) -4-cyanobiphenyl This mixture exhibits the following physical data: birefringence An = 0.199 threshold voltage V(10,0,20) = 0.89 V clearing point: 74 OC The threshold voltage was measured in a TN cell (twist angle v = S with d = 7 pin and crossed polarisers.

4 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 is prepared consisting of 10 % of p-(trans-4-propylcyclohexyl)-cyanobenzene 6 % of p-cyanophenyl-ethylbenzoate 5 % of p-cyanophenyl-propylbenzoate 6 % of 3-fluoro-4-cyanophenyl-p-ethylbenzoate 7 % of 3-fluoro-4-cyanophenyl-p-propylbenzoate 12 % of 3-fluoro-4-cyanophenyl-p-butylbenzoate 15 % of 3-fluoro-4-cyanophenyl-p-pentylbenzoate 5 % of trans-l-p-methoxyphenyl-4-propylcyclohexane 8 % of trans-l-p-ethoxyphenyl-4-propylcyclohexane 9 % of 3-fluoro-4-cyanophenyl-p- (trans-4-propylcyclohexyl) - benzoate 9 % of 3-fluoro-4-cyanophenyl-p- (trans-4-butylcyclohexyl) - benzoate 8 % of 3-fluoro-4-cyanophenyl-p- (trans-4-pentylcyclohexyl) - benzoate The mixture exhibits the following physical data: birefringence An = 0.1567 threshold voltage Vet0,0,20 = 0.67 V Example 3 A liquid crystalline medium is prepared consisting of 10 % of p-(trans-4-propylcyclohexyl)-cyanobenzene 5 % of 4'-ethyl-4-cyanobiphenyl 8 % of 3-fluoro-4-cyanophenyl-p-ethylbenzoate 9 % of 3-fluoro-4-cyanophenyl-p-propylbenzoate 15 % of 3-fluoro-4-cyanophenyl-p-butylbenzoate 15 % of 3-fluoro-4-cyanophenyl-p-pentylbenzoate 5 % of 4'-(trans-4-pentylcyclohexyl)-4-cyanobiphenyl 6 % of 4' (p-pentylphenyl)-4-cyanobiphenyl 7 % of 3-fluoro-4-cyanophenyl-p- (trans-4-propylcyclohexyl) - benzoate 5 % of 3-fluoro-4-cyanophenyl-p- (trans-4-butylcyclohexyl) - benzoate 6 t of 3-fluoro-4-cyanophenyl-p- (trans-4-pentylcyclohexyl) - benzoate 9 % of 3-fluoro-4'-(p-propylphenyl)-4-cyanobiphenyl The mixture exhibits the following physical data: birefringence An = 0.2043 threshold voltage V(10,0,20) = 1.06 V clearing point: 84 C

Claims

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

wherein A is

and R is in every instance an alkyl chain containing 2 to 7 carbon atoms wherein one CH2 group may also be replaced by an oxygen atom, comprising including in said composition, as a second part, in between 5 and 45 % by weight of one or more compounds being selected from the classes (3) to (5)

wherein R and A are as defined above and L1 to Ls are each H or one or two of L1 to L5 also F.
2. Nematic liquid crystal composition with a high or rela tively high dielectric anisotropy the birefringence of which has been increased according to the method of claim 1.
3. Electrooptical system containing the nematic liquid crystal composition according to claim 2.
4. Electrooptical system according to claim 3 said system being based on the principle of light scattering.
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. Electrooptical system according to claim 5 said system being a PDLC device.
7. Electrooptical system according to claim 5 said system being a NCAP device.