GB2295397A - Liquid crystal mixture embedded in a polymeric medium having specified birefringence and refractive index ranges - Google Patents

Abstract

The invention relates to an electrooptical liquid crystal system - which contains a liquid crystal mixture with a birefringence DELTA n = ne-nO and an optically transparent and isotropic polymeric medium with a refractive index of nM between two electrode layers which optionally may be attached to substrates with the liquid crystal mixture being embedded in form of microdroplets in the polymeric medium or being present as a continuous phase in a 3-dimensional network formed by the polymeric medium, and - which, independently of the polarization of the incident light, has a reduced transmission in the off-state compared to the on-state, characterized in that in order to optimize contrast and off-haxis haze of the systen, the liquid crystal mixture exhibits the following properties: - birefringence of the liquid crystal mixture ( DELTA n = ne - nO) is between 0.080 and 0.130, preferably between 0.100 and 0.130 - nM-nO < 0.04 - the clearing point of the liquid crystal mixture is higher than 80 DEG C wherein all refractive indices are given for 589 nm and 20 DEG C.

Description

Electroaptical liquid crystal system The invention relates to an electrooptical liquid crystal system - which contains a dielectrically positive liquid crystal mixture with a birefringence An = ne-nO and an optically transparent and isotropic medium with a refractive index of nM between two electrode layers which optionally may be attached to substrates with the liquid crystal mixture being embedded in form of microdroplets in the polymeric medium or being present as a continuous phase in a 3 dimensional network formed by the polymeric medium, and - which, independently of the polarization of the incident light, has a reduced transmission in the on-state com pared to the off-state.

Depending on the mass content of the liquid crystal mixture in the system, this can be embedded in the optical transparent medium in liquid crystal microdroplets which are separated to a greater or lesser extent from one another or else form a more or less coherent, continuous phase in which the optically transparent medium is present, for example, in the form of particles. A continuous phase is also obtained, for example, if the optically transparent medium forms a sprongelike, 3-dimensional network whose pores, in which the liquid crystal is located, merge into each other to a greater or lesser extent. The expression liquid compartments separated from one another which, however, in no way have to have a spherical shape, but can be irregularly shaped and/or deformed.

If the optically transparent medium contains liquid crystal microdroplets, it is described in the following as a matrix; on the other hand, if a more or less continuous phase of the liquid crystal is present, the medium is described by the expression network.

NCAP and PDLC films (NCAP = nematic curvilinear aligned phases, PDLC = polymer dispersed liquid crystal) are examples of electrooptical liquid crystal systems in which the liquid crystal is embedded in the matrix in the form of microdrops.

NCAP films are usually obtained by intimately mixing the encapsulated polymeric material, such as, for example, polyvinyl alcohol, the liquid crystal mixture and a carrier material, such as, for example, water, in a colloid mill. The carrier material is then removed, for example by drying. An appropriate process is described in US 4,435,047. In contrast, the liquid crystal mixture us first homogeneously mixed with monomers or oligomers of the matrix-forming material in the preparation of PDLC films as is described, for example, in US 4,688,900. Mol. Crystl. Liq. Crystl. Nonlin.

Optic, 157, 1988, 427-441, WO 89/06264 and EP 0,272,585. The mixture is then polymerized and the phase separation is induced (so-called PIPS technology; polymerization-induced phase separation). In addition, differentiation must further be made between TIPS (temperature-induced phase separation) and SIPS (solvent-induced phase separation) (Mol. Cryst. Liq.

Cryst. Inc. Nonlin. opt. 157 (188) 427).

The PN system (PN = polymer network) described in EP 0,313,053 has a sponge-like network structure of the optically transparent medium. The content of the liquid crystal mixture in the material of the light-modulating layer is in general greater than 60 % in systems of this type and is, in particular, between 70 and 90 %. In order to prepare the PN systems, a mixture of the liquid crystal, monomers or oligomers of the material forming the 3-dimensional network and a polymerization initiator, in particular a photoinitiator, is customarily brought between 2 substrate plates provided with electrodes and then polymerized, for example by light irradation.

The liquid crystal mixture in general has a positive dielectric anisotropy but the use of dielectrically negative liquid crystal mixtures (see, for example, WO 91/01511) or two-frequency liquid crystal mixtures the dielectric anisotropy of which depends on the frequency of the addressing voltage (see, for example, N.A. Vaz et al., J. Appl. Phys. 65, 1989, 5043) is also discussed. The present invention, however, is restricted to the use of dielectrically positive liquid crystal mixtures.

In microdroplet matrix systems, one of the refractive indices of the liquid crystal, customarily the ordinary refractive index no, is selected in such a way that it more or less coincides with the refractive index nH of the polymeric matrix. If no voltage is applied to the electrodes, the liquid crystal molecules in the droplets exhibit a distorted alignment, and incident light is scattered at the phase boundary between the polymeric and liquid crystal phases.

On applying a voltage, the liquid crystal molecules are aligned parallel to the field and perpendicular to the E vector of the transmitted light. Due to the matching of nO and nM, normally incident light (viewing angle o = 00, measured from the normal), therefore sees an optically isotropic medium and appears transparent.Light impinging obliquely under an angle of 8, however, has a co:ponent of its E vector in the direction of the long axis of the liquid crystal molecules, and the effective refractive index of the liquid crystal mixture neff is greater than n; neff is given via = = ne ne (ne2 cos2 0 + no2 sin2 #) 1/2 (1) wherein ne and nO are the extraordinary and the ordinary refractive index of the liquid crystal mixture and o is the viewing angle measured with respect to the normal onto the electrode surface.

Obliquely incident light therefore sees in the on-state a mismatch between the refractive index of the matrix nM and the effective refractive index neff of the liquid crystal mixture, and the system cloudy when viewed at obliquely.

Transparency decreases giving rise to increasing "haze" at increasing oblique angles until an essentially opaque is detected at an angle being oblique enough. This phenomenon which is generally termed off-axis haze, is the less pronounced the lower the birefringence of the mixture and vice versa. On the other hand, the contrast between the opaque off-state and the on-state with normal viewing angles increases with increasing values of the birefringence An because the scattering in the opaque off-state is the stronger the higher the birefringence.

The same discussion in principal holds also for network systems, however, to a less degree. In the case of network systems, an adjustment of the refractive indices owing to the customarily very much higher crystal content in the lightmodulating layer is not absolutely necessary and therefore off-axis haze problems are considerably reduced. On the other hand, matching of the refractive index of the polymer network and the ordinary refractive index is advantageously carried out in order to increase light transmission and contrast so that for sophisticated network systems, the same dilemma pointed out above arises according to which improving of the contrast will lead to a deterioration of off-axis haze and vice versa.

Several proposals for microdroplets systems with reduced off-axis haze can be found in literature. In WO 89/09807 a haze-free microdroplet system is described employing a liquid crystal polymer matrix. The viewing angle characteristic of this system is indeed excellent; on the other hand, the use of a liquid crystal polymer matrix is somewhat complicated from a technical point of view and the devices are rather expensive. In EP 0,409,442 it is proposed to reduce off-axis haze in microdroplets system with an isotropic matrix material by using a liquid crystal mixture with a low birefringence An < 0.115 and by choosing the polymeric medium in such a way that its reactive index is greater than the ordinary refractive index of the liquid crystal mixture.It is demonstrated that a liquid crystal mixture with a rather low birefringence of An = 0.080 produces acceptable haze-levels up to viewing angles o of about plus or minus 600 from the normal, while liquid crystal material with birefringence values of 0.123 and 0.143 result in unacceptable haze levels of more than 20 % at viewing angles between 30 and 400. On the other hand, these prior art systems and in particular the system with very low values of the birefringence exhibit a contrast which is insufficient for many applications.

The problem to optimize contrast and off-axis haze for electrooptical liquid crystal system according to the preamble of claim 1 at the same time, has not been solved satisfactorily so far. The systems described in the literature are either too expensive or complicated or concentrate on the improvement of one of these properties and neglect and thus deteriorate at the same time the other property.

The invention was thus based on the aim of making available electrooptical liquid crystal systems according to the preamble of claim 1 which do not have the disadvantages mentioned of conventional systems and exhibit higher clearing points or only have them to a smaller extent and exhibit values for the contrast and the off-axis haze which are both optimized and acceptable. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.

It has been found that these aims can be realized by the electrooptical liquid crystal systems according to the present invention.

The invention thus relates to an electrooptical liquid crystal system, in particular to a system based on an NCAP film - which contains a liquid crystal mixture with a birefrin gence An = ne-nO and an optically transparent and isotro pic polymeric medium with a refractive index of n, between two electrode layers which optionally may be attached to substrates with the liquid crystal mixture being embedded in form of microdroplets in the polymeric medium or being present as a continuous phase in a 3-dimensional network formed by the polymeric medium, and - which, independently of the polarization of the incident light, has a reduced transmission in the off-state com pared to the on-state, characterized in that in order to optimize contrast and off-haxis haze of the system, the liquid crystal mixture exhibits the following properties: - birefringence of the liquid crystal mixture An = ne-nO is between 0.08 and 0.130, preferably between 0.080 and 0.130, preferably between 0.100 and 0.130, - nM-nO < 0.04, - the clearing point of the liquid crystal mixture is higher than 80 OC, wherein all refractive indices are given for 589 nm and 20 C.

The construction of the electrooptical system according to the present invention corresponds to the customary mode of construction for systems of this type. The term customary mode of construction is in this case broadly interpretated and includes all adaptions and modifications.

Thus, for example, in the case of PDLC and NCAP films, the matrix formed by the transparent medium in which the liquid crystal mixture is microdispersed or microencapsulated is arranged between conducting electrodes like a sandwich.

The electrodes are applied, inter alia, to substrate sheets of, for example, glass, plastic or the like; if desired, however, the matrix can also be provided directly with electrodes so that the use of substrates can be avoided. The electrodes can form a passive matrix or an active matrix, for example a transistor switch matrix, in order to enable multiplex driving schemes as is described in E. Kaneko, Liquid Crystals TV displays, Tokyo, 1987.

In the case of network systems, the liquid crystal is located in the pores of the sponge-like, 3-dimensional network or the optically transparent medium is located in the form of small, for example spherical, particles in the liquid crystal. The network is customarily arranged between substrates provided with electrodes in order to prevent escape of the liquid crystal.

Both network systems and microdroplets matrix systems can be operated reflectively or transmissively so that at least one electrode and, if present, the associated substrate are transparent. Both systems customarily contain no polarizers, as a result of which a distinctly higher light transmission results. Furthermore, no orientation layers are necessary, which is a considerable technological simplification in the production of these systems compared with conventional liquid crystal systems such as, for example, TN or STN cells.

The matrix or the 3-dimensional network are based, in particular, on isotropic thermoplastics, thermoset plastics and elastomers. Depending on the intended application, the systems obtained can be flexible, elastic or rigid.

A system based on a thermoplastic polymer and/or an elastomer can easily be deformed by the action of a mechanical stress at temperatures which are greater than the glass temperature of the matrix. This can be used, for example, in microdroplets matrix systems in order to freeze a specifically deformed shape of the droplets by cooling the matrix to temperature below the glass temperature. Furthermore, for example, the matrix can be mechanically stretched at temperatures above the glass temperature or orientated by the action of electrical or magnetic fields, this orientation, which is maintained at temperatures below the glass temperature, causing optically anisotropic properties of the matrix.

While flexible and/or elastic systems are preferably based on the thermoplastic and/or elastomers, thermoset polymers are preferably used for the production of rigid systems. These can be deformed mechanically, for example, during hardening, the shape and arrangement of the microdroplets, for example, being fixed in the hardened matrix.

In the literature, there are various details about materials particularly suitable for the production of the matrix or of the network. Thus, for example, in US 4,435,047 or in Liquid Crystal, 3, (1988) 1543, water-soluble polymers are proposed, such as, for example, polyvinyl alcohol PVA or latex-like emulsions.

In US 4,672,618, US 4,673,255, US 4,688,900, WO 85/04262 and in Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427, on the other hand, synthetic resins such as, for example, epoxy resins and polyurethanes which, for example, are thermally cured, are mentioned as suitable matrix materials.

EP 0,272,585 describes matrix or network materials based on photocurable vinyl compounds and WO 89/06264 proposed copolymers of multi functional acrylates containing multi functional mercaptans. Other details about polymers which are suitable, in particular, for matrix systems are found, for example, in EP 0,165,063, EP 0,345,029, EP 0,357,234 or EP 0,205,261.

For the production of network system, a number of 3-dimensional crosslinkable monomers such as, for example, di- and triacrylates are mentioned in EP 0,313,053.

In addition, however, other transparent materials such as, for example, inorganic glass monoliths (US 4,814,211), other inorganic materials (see, for example, Japanese Laid-Open Specification 303325/1988) or, alternatively, other materials can also be used for matrix and network systems.

The materials mentioned are intended to illustrate the invention only by way of example, but should in no case limit it.

In principle, all transparent materials can be used which permit the production of the matrix or network structures described according to the preamble of Claim 1.

Preferred embodiments of the electrooptical liquid crystal systems according to the invention are NCAP films, PDLC films and microdroplets matrix systems produced by modified processes. Processes for the production of these films are described, for example, in US 4,688,900, US 4,673,255, US 4,671,618, WO 85/0426, US 4,435,047, EP 0,272,595, Mol.

Cryst. Liq. Cryst, Inc. Nonlin. Opt. 157 (1988) 427, Liquid Crystals, 3 (1988) 1543, EP 0,165,063, EP 0,345,029, EP 0,357,234 and EP 0,205,261.

A further preferred embodiment of the electrooptical systems according to the invention are the network systems whose production is described in EP 0,313,053. Included in the network systems here are also arrangements in which the transparent medium is dispersed in the form of individual, for example, spherical, particles in the liquid crystal, such as is described, for example, in GB 1,442, 360.

However, in addition also those embodiments of the invention are included in which the transparent medium has a structure which lies between the network structure on the one side and the microdroplets matrix configuration on the other side.

In addition, other embodiments of the invention not explicitly mentioned here are also included.

The thickness d of the electrooptical systems is customarily chosen to be small in order to achieve a threshold voltage Vth which is as low as possible. Thus, for example, layer thicknesses of 0.8 and 1.6 mm are reported in US 4,435,047, while values for the layer thickness between 10 and 300 Rm are given in US 4,688,900 and between 5 and 30 Wm in EP 0,313,053. The electrooptical systems according to the invention only have layer thicknesses d greater than a few mm in exceptional cases; layer thicknesses below 200 Zm and especially below 100 Am are preferred.

The threshold voltage is also influenced by the size of the microdroplets or the mesh width of the network. Generally, relatively small microdroplets cause a relatively high threshold voltage Vth, but relatively short switching times ton or toff (US 4,673,255). Experimental methods for incluencing the average droplet size are described, for example, in US 4,673,255 and in J.L. West, Mol. Cryst. Liq. Cryst. Inc.

Nonlin. Opt., 157, 1988, 427. In US 4,673,255, average drop diameters between 0.1 pin and 8 Zm are given, while, for example, a matrix which is based on a glass monolith has pores having a diameter between 15 and 2,000 A. For the mesh width of the network of PN systems, a preferred range between 0.5 and 2 pn is given in EP 0,313,053.

An essential difference between the electrooptical liquid crystal system according to the present invention and those customary hitherto exists, however, in that the refractive indices ne and nO of the liquid crystal mixture and the refractive index nM of the isotropic polymeric medium fulfill the following conditions: An = ne-nO is between 0.080 and 0.130, preferably between 0.100 and 0.130, nm-n < 0.04, and the clearing point of the liquid crystal mixture higher than 80 OC wherein all refractive indices are given for 589 nm and 20 OC.

The present invention starts with the same observation as was pointed out in EP 0,409,422: the effective viewing angle range which is characterized by a high transmission and an acceptable haze level, can be enlarged if the refractive index of the matrix nM is not matched to nO but is made greater than nO- In EP 0,409,422 it is concluded from these considerations which are given there, however, in a less pointed form, that the birefringence has to be chosen very low and at any rate smaller than 0.100 in order to realize a large effective range of the viewing angle. All the Lc-mixtures described there exhibit clearing points below 70 OC. These mixtures are not suitable for outdoor application, in particular not for sun-roofs.

It was recognized in the present invention that such a low birefringence indeed results in a low haze level over a wide range of the viewing angle but at the same time leads to a very low contrast which is not acceptable for many applications.

It was found that systems with optimized properties cannot be obtained by adjusting the birefringence only, but that several parameters have to be considered and optimized. Specifically it was found that optimized systems are obtained of the following conditions are fulfilled at the same time: - birefringence of the liquid crystal mixture An = ne - nO is between 0.08 and 0.130, preferably between 0.100 and 0.130 - nM - nO < 0.04, and the clearing point of the liquid crystal mixture is higher than 80 OC wherein all refractive indices are given for 589 nm and 20 OC.

It is the combination of these conditions and the simultaneous consideration of 4 parameters, i.e. nO, n,, An and clearing point which produces optimized electrooptical systems according to the preamble of claim 1 which exhibit a low haze level or a wide range of e and the same time an acceptable or even high contrast, even at high temperatures which is essential in particular for sun-roof applications.

Specifically it was found that the birefringence of the liquid crystal must not be chosen too low. The birefringence has to be greater than 0.08 in order to obtain an acceptable contrast but values of An which are larger than 0.102 and especially not smaller than 0.125 are preferred in particular An is between 0.1020 and 0.1080. The higher the birefringence the lower the specific value #max of the viewing angle for which maximum transmission is observed has to be chosen. It was found that this value of > x is preferably selected to be lower than 250 and especially lower than 200 if the birefringence is larger than 0.10.If a very wide range of the viewing angle 6 is required the birefringence can be chosen to be within 0.080 < An < 0.130, preferably 0.1020 < An < 0.1080 and OmaX can be chosen between 25 < 6 # 300. This embodiment of the electrooptical system according to the invention exhibits on the other hand a rather low contrast which may be termed as just acceptable. At any rate, this specific embodiment is possible but not preferred.

Another important condition is that the difference nM - nO must not exceed 0.03 in order to obtain a reasonable transmission around 6 = 00. High values of nM - nO correspond to high values of + oman, and for values of EmaX being too high the overlapping of the 2 symmetrical curves in Fig. 2 is not sufficient. This is especially true for high values of the birefringence An. The difference between n, - nO preferably is lower than 0.025 and especially lower than 0.020.

For given values EmaXf nM and An the ordinary index of refraction of the liquid crystal mixture is selected in such a way that the condition nO = nM nO sin#/ (no2-nM2 cos2 9 =) - Ab with 00 < o < 300 is fulfilled. Small deviations of nO from the optimum value given by this equation are possible but this deviation is preferably small than 10 % and especially not more than 7.5 %. The same consideration hold for nM in case no, An and OmaX are given.

It is evident from the preceding detailed discussion that several parameters have t be considered if systems are to be obtained exhibiting at the same time a low haze level and a good contrast. The core of the present invention consists in giving relations which have to be fulfilled at the same if a high quality system according to the preamble of claim 1 is to be obtained. The mere consideration of the birefringence as was done in EP 0,409,442 is not adequate with respect to the complex physical situation and leads to systems the optical properties of which often fulfill the requirements for various applications only to an insufficient degree.

In particular the mixtures described in EP 0,409,442 exhibit very low clearing points which are insufficient for outdoor applications.

The clearing point of the liquid crystal mixture is preferably higher than 100 OC in particular between 1100 and 140 OC.

The liquid crystal mixtures exhibit preferably a nematic phase range between < -100C and > +110 OC, most preferred between < -20 OC and > +115 OC.

The liquid crystal mixture used in the electrooptical systems according to the present invention is preferably based on compounds of the group A imparting to high values of the dielectric ansisotropy of formula I,

of the group B imparting to broad nematic phase ranges of formula II,

and of the group C imparting to high clearing points of formula III,

wherein R1, R2 and R3 are independently from each other an alkyl group with 1-15 C atoms wherein one or two non-ad jacent CH2 groups may be replaced by -O-, -CO-, -COO-, O-, -HC=CH- and/or -CiC, Z1 is -COO- or -OCO, Z2 is a single bond, -COO- or -OCO-,

A and B are independently 4 or .

from each other

Preferred embodiments are: a. Electrooptical systems consisting essentially of: 20-50 % by weight of at least one compound of formula I, 40-70 % by weight of at least one compound of formula II, and 3-10 % by weight of at least one compound of formula III.

b. Electrooptical system wherein component B comprises at least one compound of formula Ia, formula Ib, or formula Ic and formula Id

wherein n and m are each integers between 1 and 7, preferably between 3 and 5.

A further aspect of the invention is the use of the electrooptical liquid crystal system according to the invention for architectural applications and for sunroofs in automobils.

The liquid crystal mixtures used in the electrooptical systems according to the present invention preferably contain at least one or more compounds of this smaller group of carbonitriles.

In the compounds according to formulae I-III and especially in the compounds of the preferred smaller group, R1 preferably is methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonly, decyl, methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, octoxy, methoxymethyl, ethoxymethyl, propoxymethyl, butoxymethyl, methoxyethyl, ethoxyethyl or propoxyethyl.

The compounds of formulae I-III are known, and they can be prepared by methods known per se, as are described in literature (for example in the standard works, such as Houben-Weyl, Methoden der organischen Chemie, Georg-Thieme-Verlag, Stuttgart, Vol. IX, pp. 867 ff), to be precise under reaction conditions which are known and suitable for the reactions mentioned.

The liquid crystalline mixtures used in the electrooptical systems according to the invention preferably contain 2 to 40, especially 3 to 30 and in particular 4 to 25 compounds.

The liquid crystalline media preferably contain 1-20 compounds which do not correspond to formulae I-III, in addition to one or more compounds selected from the group of compounds of formulae I-IV and especially from the smaller subgroup listed above.

These further constituents are preferably selected from nematic or nematogenic (monotropic or isotropic) substances, in particular substances from the classes of the azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl esters of cyclohexanecarboxylic acid, phenyl or cylcohexyl esters of cyclohexylbenzoic acid, phenyl or cyclohexyl esters of cyclohexylcyclohexanecarboxylic acid, cyclohexylphenyl esters of benzoic acid, of cyclohexanecarboxylic acid and of cyclohexylcyclohexanecarboxylic acid, phenylcyclohexanes, cyclohexylbiphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclohexenes, 1,4-bis-cyclohexylbenzenes, 4,4' -bis-cyclohexylbi- phenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexylpyridines, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyl-1,3-dithianes, 1,2-diphenylethanes, 1, 2-di- cyclohexylethanes, l-phenyl-2-cyclohexylethanes, l-cyclohexyl-2-(4-phenylcyclohexyl)ethanes, l-cyclohexyl-2-biphenylylethanes, l-phenyl-2-cyclohexylphenylethanes, optionally halogenated stilbenes, benzyl phenyl ethers, tolans and substituted cinnamic acids. The 1,4-phenylene groups in these compounds may also be fluorinated.

The most important compounds suitable as further constituents of mixtures used in the systems according to the invention can be characterized by the formulae 1, 2, 3, 4 and 5: R'-L-E-R" 1 R' -L-COO-E-R" 2 R'-L-OOC-E-R" 3 R'-L-CH2CH2-E-R" 4 R'-L-C3C-E-R" 5 In the formulae 1, 2, 3, 4 and 5, L and E, which may be identical or different, are, in each case independently of one another, a bivalent radical from the group formed by -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe- and -G-Cyc- and their mirror images, where Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cyc is trans-1,4-cyclohexylene or 1,4-cyclohexenylene, Pyr is pyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3dioxane-2,5-diyl and G is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5diyl.

One of the radicals L and E is preferably Cyc, Phe or Pyr. E is preferably Cyc, Phe or Phe-Cyc. The media according to the invention preferably contain one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which L and E are selected from the group comprising Cyc, Phe and Pyr and simultaneously one or more components selected from the group comprising Cyc, Phe and Pyr and the other radical is selected from the group comprising -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-, and optionally one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which the radicals L and E are selected from the group comprising -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and G-Cyc-.

In the compounds of the sub-formulae la, 2a, 3a, 4a and 5a, R' and R" are in each case independently of one another alkyl, alkenyl, alkoxy, alkenyloxy or having up to 8 carbon atoms. In most of these compounds, R' and R" are different from one another, on of these radicals usually being alkyl or alkenyl. In the compound of the sub-formulae lb, 2b, 3b, 4b and 5b, R" is -CN, -CF3, -OCF3, -OCHF2,-F, -Cl- or -NCS; in this case, R has the meaning given for the compounds of the sub-formulae la to 5a and is preferably alkyl or alkenyl.

However, other variants of the proposed substituents in the compounds of the formulae 1, 2, 3, 4 and 5 are common. Many such substances or alternatively mixtures thereof are commercially available. All these substances can be obtained by methods which are known from the literature or analogously thereto.

Besides components from the group comprising the compounds la, 2a, 3a, 4a and 5a (Group 1), the mixtures used in the electrooptical systems, according to the invention also preferably contain components from the group comprising the compounds lb, 2b, 3b, 4b and 5b (Group 2), whose proportions are preferably as follows: Group 1:20 to 90 %, in particular 30 to 90 %, Group 2:10 to 80 %, in particular 10 to 50 %, the sum of the proportions of the compounds according to the invention and of the compounds from Groups 1 and 2 adding up to 100 %.

The mixtures used in the electrooptical systems according to the invention preferably contain 1 to 40 %, in particular preferably 5 to 30 %, of one or more compounds according to formulae I-IV. Further preferred mixtures are those which contain more than 40 %, in particular 45 to 90 %, of compounds according to formulae I-IV. The liquid crystal mixture may contain additional compounds such as one or more pleochroic dyes, one or more chiral compounds or other customary additives.The precursor of the polymeric medium, i.e. the low molecular mass or high molecular mass material which when cured gives the optically transparent and isotropic polymeric medium, is conventional and can be selected from the materials enumerated above; especially preferred are usually photoradically curable precursors being based on vinylchloride, vinylidenchloride, acrylnitriles, methacrylnitriles, acrylamides, methacrylamides, methyl-, ethyl-, n- or tert.

butyl-, cyclohexyl, 2-ethylhexyl-, phenyloxyethyl-, hydroxyethyl-, hydroxypropyl, 2-5 C-alkoxyethyl-, tetrahydrofurfurylacrylates or methacrylates, vinylacetates, -propionates, -acrylates, -succinates, N-vinylpyrrolidones, N-vinylcarbazoles, styroles, divinylbenzenes, ethylendiacrylates, 1,6-hexandiolacrylates, bisphenol-A-diacrylates and -dimethacrylates, trimethylylpropandiacrylates, pentaerythrittriacrylates, triethylenglycoldiacrylates, ethylenglycoldimethacrylates, tripropylenglycoltriacrylates, pentaerythritoltrioacrylates, pentaerythritoltetraacrylates, ditrimethylpropantetraacrylates or dipentaerythritolpenta- or hexaacrylates.

The NCAP films preferably are obtained using the methods described in the examples of EP 0,409,442, US 4,435,047, US 4,579,423 and US 4,950,052 which are hereby incorporated by reference.

Also thiol-enes are preferred like, for example, the commercially available product Norland 65 (Norland Products).

This enumeration is intended to be illustrative without limiting the scope of the invention. Suitable photoinitiators are described, for example, in DE 41 02 215. Further preferred are photo-cationically curable precursors, Xnd suitable photoinitiators of this type are described, for example, in JP 90-409225.

In principal, one can distinguish between two routes of preparing electrooptical systems according to the present invention.

The first route starts with selecting a suitable precursor of the isotropic medium, thus fixing the refractive index of the cured medium nM. In the next step a suitable liquid crystal mixture is selected which exhibits a birefringe An being closely by the desired value of An, and which furthermore exhibits other desired properties like, for example, clearing point, viscosity, dielectrical anisotropy etc.; the liquid crystal mixture chosen is preferably based on one or more compounds according to formulae I-IV.The choice of the liquid crystal mixture and a certain value of An is relevant with respect to the selection of Oman, the viewing angle with the maximum transmission, as was pointed out above, lower values of emar are preferred for high birefringence mixtures while for mixtures with a lower value of the birefringence, higher values of OmaX are possible.

After these pre-selections the fine-tuning begins. Inserting n,, Omax and An of the selected mixture into equation (4) the ideal value of no can be calculated. If no of the liquid crystal mixture differs from the ideal value, the liquid crystal mixture will be modified by adding a rather small percentage, generally less than 10 %, of one or more further compounds. If no is smaller than the ideal value one can try to add a compound with more or less the same An as is exhibited by the liquid crystal mixture, but a somewhat larger value of no. Unfortunately, the An-, nO- and ne-values often do not add up linearly so that this procedure is straightforward in some cases only. In general, after the addition of one or more further compounds, both An and and no of the liquid crystal mixture will be changed and one has to check once again if condition (4) is fulfilled. If the situation has improved with respect to the starting mixture but is not yet satisfactory, another step will follow etc., until the desired parameters are obtained; if the situation has deteriorated with respect to the starting mixture, the added compound will be eliminated and one starts again at the beginning. Small deviations of the realized nO compared to the ideal no according to (4) can of course be accepted but generally they should be not more than 10 %, in particular less than 7.5 % and especially not more than 5 %. Systems with optimum properties are obtained if the difference is not more than 2.5 %.

The procedure sounds somewhat tedious but in practice it became evident that an ideal mixture can be obtained by performing four steps of the kind described.

The second route starts with selecting a liquid crystal medium thus fixing An and no of the liquid crystal medium.

Considering the value of An, an appropriate value of twx is selected. Inserting these fixed values of An, nO and 0x into equation (4) one obtains the optimum value for the refractive index of the isotropic cured medium. One starts with a polymer medium responding a precursor of the polymer medium which exhibits in the cured state a value for nM closely by the ideal one and fulfills other desired properties such as, for example, the miscibility with the precursor, thermal stability, resistivity etc. If the value of nu deviates from the ideal one to an inacceptable degree, one will blend the initial polymeric medium responding the initial precursor with another polymer responding with other monomers, oligomers and/or prepolymers.The amount of the additional components being usually less than 10 % and in particular not more than 7.5 %. Further blending steps will follow until the difference between the actual value of nM and the ideal value according to (4) is acceptable. In general, the deviation should be less than 10 %, in particular less than 7.5 % and especially not more than 5 %. Systems with optimum properties are obtained if the difference is not more than 2.5 %.

In the first route described the liquid crystal mixture is optimized while the isotropic polymeric medium is fixed, and the situation is vice versa for the second route.

It goes without saying that intermediate routes during which both the liquid crystal mixture and the isotropic medium are changed or other routes being a slight modification of the routes described above are also possible and the invention is by no means restricted to the routes described.

After this process of selection is finished, the chosen liquid crystal mixture, the chosen precursor if the isotropic, transparent polymeric medium and, optionally, further components such as surfactants etc. are thoroughly mixed to form a complete isotropic solution or, particularly in the NCAP case, a dispersion by application of ultrasonic, by using a colloid mill or by other methods. The resulting solution responding dispersion is subsequently injected between the electrode layers which are usually attached to glass substrates and cured.

The electrooptical liquid crystal systems according to the invention exhibit advantageous properties and in particular, a low off-axis haze and simultaneously a good contrast. The systems according to the present invention are especially suited for large-surface area indicating systems such as billboards etc., for architectural applications like windows, room dividers, sunroofs etc. and for motor vehicles (windows, sunroofs etc.). The systems according to the invention do not exhibit the shortcomings of the systems known hitherto or exhibit them only to a lesser extent, and they are therefore of considerable economic importance.

The compounds used in the liquid crystalline mixtures of the inventive electrooptical systems are described by aeronyms as follows:

The following examples are intended to illustrate the invention without limiting it.

The percentages given above and below are percentages by weight.

ExamDle 1 A liquid crystal mixture, which consists of PCH-2 15 % PCH-3 15 % PCH-4 10 % CP-33 6 % CP-35 6 % CP-43 6 % CP-45 6 % HD-34 6 % HD-35 6 % CBC-33F 3 % CBC-53F 3 % CH-45 9 g CH-33 9 % and exhibits the following physical properties: S ON < -20 C Clearing point 120.4 C Viscosity + 20 C 32 cSt Dielectric ansiotropy ## 1 kHz, 20 0 C 6.9 l 1 kHz, 20 C 11.1 #l 1 kHz, 20 C 4.2 ##/#l 1 kHz, 20 C 1.64 Optical anisotropy An 0.1022 (20 OC, 589 nm) no 1.4912 nc 1.5934 The liquid crystalline mixture is encapsulated with polymer having nM 1.50 as described by EP 0 409 442.

Example 2 A liquid crystal mixture, which consists of PCH-5 15 % PCH-7 15 % PCH-4 10 % CP-33F 9 % CP-55F 9 % CP-33 6 % CP-35 6 % CP-43 6 % CP-45 6 % HD-34 6 % HD-35 6 % CBC-33F 3 % CBC-53F 3 % and exhibits the following physical properties: S #N < -20 0C Clearing point 130.3 C Viscosity + 20 C 61 cSt Dielectric ansiotropy AE 1 kHz, 20 C 6.0 #l 1 kHz, 20 C 9.9 t1 1 kHz, 20 C 3.9 ##/#l 1 kHz, 20 C 1.54 Optical anisotropy An 0.1062 (20 OC, 589 nm) rio 1.4899 rio 1.5961 The thickness of the system amounts to 15 Am. The system is then exposed for 2 min to an Argon lamp (0.5 mW/cm2).

The liquid crystalline mixture is encapsulated by a polymer having nM 1.50 as described by EP 0 409 442.

Claims (5)

Patent Claims

1. Electrooptical liquid crystal system - which contains a liquid crystal mixture with a bire fringence An = ne-nO and an optically transparent and isotropic polymeric medium with a refractive index of n, between two electrode layers which optionally may be attached to substrates with the liquid crystal mixture being embedded in form of microdroplets in the polymeric medium or being present as a continuous phase in a 3-dimensional network formed by the poly meric medium, and - which, independently of the polarization of the inci dent light, has a reduced transmission in the off state compared to the on-state, characterized in that in order to optimize contrast and off-haxis haze of the system, the liquid crystal mixture exhibits the following properties:: - birefringence of the liquid crystal mixture An = ne-ne is between 0.08 and 0.130, preferably between 0.100 and 0.130, - rk-no < 0.04, - the clearing point of the liquid crystal mixture is higher than 80 OC, wherein all refractive indices are given for 589 nm and 20 OC.

2. Electrooptical system according to claim 1 wherein the liquid crystal mixture is based on compounds of the group A imparting to high values of the dielectric anisotropy of formula I,

of the group B imparting to broad nematic phase ranges of formula II,

and of the group C imparting to high clearing points of formula III,

wherein R1, R2 and R3 are independently from each other an alkyl group with 1-15 C atoms wherein one or two non-ad jacent CH2 groups may be replaced by -O-, -CO-, -COO-, -OCO-, -HC=CH- and/or -C3C, Z1 is -COO- or -OCO, Z2 is a single bond, -COO- or -OCO-,

and are independently from each other

3. Electrooptical system according to claim 2, characterized in that is essentially consists of 20-50 % by weight of at least one compound of formula I, 40-70 % by weight of at least one compound of formula II, and 3-10 % by weight of at least one compound of formula III.

4. Electrooptical system according to claim 2 or 3 charac terized in that component B comprises at least one com pound of formula Ia, formula Ib or formula Ic and formula Id

wherein n and m are each integers between 1 and 7, preferably between 3 and 5.

5. Use of the electrooptical liquid crystal system according to the invention for architectural applications and for sunroofs in automobils.