GB2260141A - Electrooptical liquid crystal system - Google Patents

Abstract

An electrooptical liquid crystal system, comprising a liquid crystal material, with a birefringence DELTA n = ne-no, and a polymeric medium, which is optically substantially transparent and isotropic and has a refractive index of nm when between two electrode layers, the liquid crystal material being embedded in microdroplets in the polymeric medium, or present as a continuous phase in a 3-dimensional network formed by or with the polymeric medium, and the system allowing reduced transmission of incident light in an off-state, compared to an on-state, independently of the polarization of the incident light, characterized in that the birefringence of the liquid crystal material DELTA n = ne-no > 0.13 (2), nm-no < 0.03 (3), and no = nmnosin theta /(no <2>-nm<2>cos<2> theta ) 1 DIVIDED 2- DELTA n (4) for absolute values of the viewing angle ranging between 0 DEG < theta < 30 DEG , wherein all refractive indices are given for 589 nm radiation and at 20 DEG C and the values of no, nm and DELTA can vary up to 10% from those derived from equation (4) for a particular value of theta . Such a liquid crystal system exhibits low haze level over a wide range of viewing angles theta and an acceptable level of contrast. <IMAGE>

Description

ELECTROOPTICAL LIQUID CRYSTAL SYSTEM DESCRIPTION The invention relates to an electrooptical liquid crystal system, comprising a liquid crystal material (usually a mixture), with a birefringence is n=ne-nOt and a polymeric medium, which is optically substantially transparent and isotropic and has a refractive index of nm when between two electrode layers which, optionally, can each be attached to a substrate, the liquid crystal material being embedded in microdroplets in the polymeric medium, or present as a continuous phase in a 3-dimensional network formed by or with the polymeric medium, and the system allowing reduced transmission of incident light in an off-state, compared to an on-state, independently of the polarization of the incident light.

Depending on the quantity of the liquid crystal material in the system, either microdroplets thereof can be embedded in the optically transparent medium, or else the liquid crystal material can 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 system can also be obtained when the optically transparent medium forms a sponge-like, 3-dimensional network whose pores, in which the liquid crystal material is located, are interconnected, to a greater or lesser extent. The expression liquid crystal microdoplets will be used hereinafter to refer to small drops or packets of liquid crystal material which are separated from one another but which, however, need not be spherical and can be irregularly shaped and/or deformed.

If the optically transparent medium contains liquid crystal microdoplets, it will be hereinafter referred to as a matrix, however, if a more or less continuous phase of the liquid crystal material is present, the medium will be hereinafter referred to as a network.

NCAP and PDLC films (NCAP = nematic curvilinear aligned phases, PDLC = polymer dispersed liquid crystal) areexamples of electrooptical liquid crystal systems in which the liquid crystal is embedded in a matrix in the form of microdroplets. NCAP films are usually obtained by intimately mixing an encapsulating polymeric material, such as a polyvinyl alcohol, a liquid crystal mixture and a carrier material, such as 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, in the preparation of PDLC films, a liquid crystal mixture is first homogenously mixed with monomers or oligomers of a matrix-forming material as is described, for example, in US 4,688,900, Mol. Cryst. Liq. Cryst.

Nonlin. Optic, 157 , 1988, 427-441, WO 89/06264 and EP 0,272,585. The mixture is then polymerized, inducing phase separation (so-called PIPS technology; polymerization-induced phase separation). Phase separation can also be induced by the TIPS (temperature-induced phase separation) and SIPS (solvent-induced phase separation) methods. (Mol.

Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427).

The PN system (PN = polymer network) described in EP 0,313,053 provides an optically transparent medium with a sponge-like network structure. When the material in - a light modulating layer is formed from a system of this type, it, in general, will comprise greater than 60% and, preferably, between 70 and 90% liquid crystal material, the remainder being made up by the network.

In order to prepare such a PN system, a mixture of the liquid crystal material, monomers or oligomers of the material forming the 3-dimensional network and a polymerization initiator, preferably a photoinitiator, is sandwiched between 2 substrate plates, provided with electrodes, and then polymerized, for example by light irradiation.

Liquid crystal mixtures in general have 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 dielectrical 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 known. The present invention, however, is preferably restricted to the use of dielectrically positive liquid crystal mixtures.

In microdroplet matrix systems, one of the refractive indices of the liquid crystal material, customarily the ordinary refractive index nO, is selected so that it is substantially coincident with the refractive index nm of the polymeric matrix. If no voltage is applied to the electrodes, the molecules of the liquid crystal material in the droplets are out of 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 incident light travelling normal to the electrode layers (i.e. parallel to the field). Due to the matching of nO and nm, normal incident light (viewing angle e = 00, measured from the normal to the electrode layers), therefore sees an optically isotropic medium which appears transparent thereto.

Light impinging at an oblique angle of 8, however, has a component 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 no; neff is give by formual (I): neff = no ne (ne cos o + no2 sin2 (I) wherein ne and no are the extraordinary and the ordinary refractive indices of the liquid crystal mixture and o is the viewing angle measured with respect to the normal onto the electrode surface.

In the on-state, obliquely incident light, therefore, sees a mismatch between the refractibe index of the matrix nm and the effective refractive index neff of the liquid crystal material, and the system, therefore, appears cloudy when viewed obliquely. Transparency decreases, giving rise to increasing "haze", at increasingly oblique angles, until an essentially opaque appearance is detected at a sufficiently oblique angle. This phenomenon, which is generally termed off-axis haze, is less pronounced the lower the birefringence of the liquid crystal material and vice versa. However, the contrast between the opaque off-state and the substantially transparent (at normal viewing angles) on-state increases as the birefringence tin is increased because the scattering in the opaque off-state becomes more extensive.

The same situation, in principle, holds also for network systems, although, to a lesser degree. In network systems, because they customarily have a very much higher liquid crystal content, it is not absolutely necessary to match the refractive indices of the liquid crystal material and polymer and, therefore, off-axis haze problems are considerably reduced. On the other hand, matching the refractive index of the polymer network to the ordinary refractive index of the liquid crystal material allows increased light transmission and contrast to be achieved. Thus, for sophisticated network systems, the same dilemma, as pointed out above, arises whereby improvements to the contrast will lead to an increased off-axis haze and vice versa.

Several proposals for microdroplet systems with reduced off-axis haze can be found in the literature. In WO 89/09807 a haze-free microdroplet system is described employing a liquid crystal polymer matrix. The viewing angle characteristics of this system are indeed excellent. However, the use of a liquid crystal polymer matrix is somewhat complicated from a technical point of view and the resulting devices are rather expensive. In EP 0,409,442 it is proposed to reduce off-axis haze in a microdroplet/matrix system by using a liquid crystal mixture with a rather low birefringence ss n < 0.115 and by choosing the polymeric medium in such a way that its refractive index is greater than the ordinary refractive index of the liquid crystal mixture.In this document, it is demonstrated that a liquid crystal mixture with a rather low birefringence of An = 0.080 can produce acceptable haze-levels up to viewing angles 8 of about plus or minus 600 from the normal, while liquid crystal materials 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. However, these prior art systems and, in particular, the system using the liquid crystal material with the lowest value birefringence have insufficient contrast for many applications.

Up until the present invention, it has not proven possible to optimize contrast and off-axis haze for electrooptical liquid crystal systems of the type to which the invention relates. The systems described in the literature are either too expensive or complicated, or concentrate on the improvement of one of these properties and neglect the other property, to its detriment.

An object of the invention, therefore, is to provide an electrooptical liquid crystal system which does not have the aforementioned disadvantages of conventional systems or, at least, suffers them to a diminished extent and exhibits values for contrast and off-axis haze which are preferably optimized and acceptable.

Other aims of the present invention will become immediately evident to the person skilled in the art from the following detailed description.

In accordance with the present invention, there is provided an electrooptical liquid crystal system, comprising a liquid crystal material, with a birefringence an n = ne-nO, and a polymeric medium, which is optically substantially transparent and isotropic and has a refractive index of nm when between two electrode layers, the liquid crystal material being embedded in microdroplets in the polymeric medium, or present as a continuous phase in a 3-dimensional network formed by or with the polymeric medium, and the system allowing reduced transmission of incident light in an off-state, compared to an on-state, independently of the polarization of the incident light, characterized in that the birefringence of the liquid cyrstal material #n = ne-no < 0.13 (2), nm-nO < 0.03 (3), and no = nmnosin#/(no2-nm2cos2#)-#n (4) for absolute values of the viewing angle ranging between oo < 6 < 30o, wherein all refractive indices are given for 589 nm radiation and at 200C and the values of no, nm and An can vary by up to 10% from those derived from equation (4) for a particular value of 6.

Preferably, 6 = 6max(the angle e at which transmittance is at a maximum) and the values of no, nm, An and #max can vary by up to 10% from those derived from equation (4). In an embodiment, An n > 0.15 and, preferably, the variation in the values of no, nm,n and #max can be up to 7.5%, more preferably 5% and most preferably 2.5%.

In a preferred embodiment, the liquid crystal material (which is preferably a mixture) and polymeric medium are located between two electrode layers, at least one of which, preferably, is attached to a substrate.

In an embodiment the liquid crystal material and polymeric medium are located between two electrode layers. Preferably, at least one of said electrodes is attached to a substrate.

An electrooptical system according to the present invention can be constructed in a conventional manner for systems of this type. The term conventional manner of construction is, in this case, to be broadly interpretated and includes all known and future adaptions and modifications.

Thus, for example, in the case of PDLC or NCAP films, the matrix formed by the transparent medium in which the liquid crystal material 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 of the type described in E. Kaneko, Liquid Crystals TV Displays, Tokyo, 1987.

In network systems, the liquid crystal material 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 material. The network is customarily arranged between substrates provided with electrodes in order to prevent escape of the liquid crystal material.

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 and, therefore, allow the transmission of more light than other systems.

Furthermore, because no orientation layers are necessary, the production of these systems is considerably more simple compared with conventional liquid crystal systems such as, for example, TN or STN cells.

The matrix or the 3-dimensional network, preferably, is based on an isotropic thermoplastic, thermoset plastic or an elastomer. 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 mechanical stress at temperatures which are greater than the glass temperature of the matrix. This property can be used, for example, in microdroplet matrix systems in order to freeze the droplets in a specifically deformed shape by cooling the matrix to a temperature below its glass temperature. Furthermore, for example, the matrix can be mechanically stretched at temperatures above its glass temperature or orientated by the action of electrical or magnetic fields. This orientation, which is maintained at temperatures below the glass temperature, can cause optically anisotropic properties in the matrix.

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

Materials particularly suitable for the production of a matrix or network are disclosed in the literature.

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, 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 proposes copolymers of multifunctional acrylates containing multifunctional mercaptans.

Further polymers which are suitable, in particular, for matrix systems are discussed in EP 0,165,063, EP 0,345,029, EP 0,357,234 and EP 0,205,261.

A number of 3-dimensional crosslinkable monomers such as, for example, di- and triacrylates, which are useful in the preparation of network systems, are mentioned in EP 0,313,053.

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

The materials mentioned are intended to illustrate the invention only by way of example and its scope is in no way limited to their use. In principle, the matrix or network can be formed from any transparent material so long as the requirements of the invention are met.

Preferred embodiments of the electrooptical liquid crystal system according to the invention are NCAP films, PDLC films and microdroplet 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.

Further preferred embodiments of the electrooptical system according to the invention can be in the form of the network systems of the type described in EP 0,313,053. Among suitable network systems are also those arrangements in which the transparent medium is dispersed in the form of individual, for example, spherical, particles in the liquid crystal material, such as is described, for example, in GB 1,442,360.

However, the invention includes within its ambit materials in which the transparent medium has a structure which lies between a network structure and a microdroplet/matrix configuration.

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

The thickness d of 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 layer thicknesses of between 10 and 300 > m are given in US 4,688,900 and of between 5 and 30 > m 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 200Rm and especially below 100 m 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 influencing 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 Cun and 8 gm are disclosed, while, for example, a matrix based on a glass monolith can have pores with a diameter between 15 and 2,000 & The mesh width of the network in PN systems is, preferably, between 0.5 and 2 zm, as disclosed in EP 0,313,053.

The present invention starts from the realisation, as set out in EP 0,409,422, that the range, within which the viewing angle must remain for light transmission and haze levels to be acceptable (the effective viewing angle range) can be enlarged, if the refractive index of the matrix nm is not matched to nO but is made greater than no.

This effect is illustrated schematically in Fig. 1 and 2 in which the transmission is plotted against the viewing angle 6. Fig. 1 gives the diagram for a conventional electrooptical system, of the type to which the invention relates, wherein nm is matched to no.

Maximum transmission is obtained for 6 = 0 and the transmission decreases with increasing e substantially symmetrically on either side of 6 = 00.

Fig. 2 shows an improved system wherein nm matches with neff at #max, wherein #max # 0 , and transmission is at a maximum at +6maxand -#max The overall transmission characteristic is obtained by superimposing two conventional curves which have been shifted so as to be centered on +6maxand 8max and the result is an increase in the effective viewing angle range, for the viewing angle #.

The optimum conditions can be derived as follows.

Light entering the system obliquely at an angle 6 from the normal sees an effective refractive index of neff given by neff = no ne (no2 cos2# + no2 sin2 # ) (1) wherein ne and no are the extraordinary and the oridnary refractive indices of the liquid crystal material and 6 is the viewing angle measured with respect to the normal to the electrode surface.

Maximum transmission is obtained for a specific value of 8( #max) for which the equation nm neff(0max) holds. Combining this with formula (I) and rearranging gives nm no sin#/(no2 - nm2 cos2 #) = no + #n (4) Thus, for a given polymeric medium with a given nm and a particular selected value for emax, the liquid crystal material has to be selected to exhibit sufficient birefringence (#n) in order to provide acceptable or good contrast, and have a value of nO which satisfies formula (4) on the basis of the aforesaid values of nm, #max and #n.

Alternatively, the system can be optimized by firstly selecting a liquid crystal material which exhibits a desired birefringence Inn and a specific value of no and, then, adjusting the refractive index of the matrix, by blending one or more further polymer materials into the polymer forming the matrix, so that equation (4) is fulfilled for the values of no and n of the liquid crystal material and for a selected value of #max.

Similar, although less detailed, considerations led the authors of EP 0,409,422 to conclude that the liquid crystal material should exhibit very low birefringence, preferably of less than 0.115, in order to achieve a large effective viewing angle range.

However, although such a low birefringence indeed results in a low haze level over a wide range of viewing angles it also leads to very low contrast between the on-state and off-state, which is not acceptable for many applications.

Indeed, it has now been learned 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, in accordance with the invention, optimized systems are obtained if the following conditions are fulfilled at the same time: birefringence of the liquid crystal mixture #n = ne - no < 0.13; (2) nm ~ nO < 0.03; (3) no = nm no sin#/(no2-nm2 cos2#) - #n (4); with the absolute value of the viewing angle of maximum transmission ranging between 00 < e < 300 wherein all refractive indices are given for 589 nm and 200C.

It is the combination of these conditions and the simultaneous consideration of 4 parameters, i.e. no, nm, n n and the value of e with maximum transmission (#max), which produces optimized electrooptical systems of the type to which this invention relates and which exhibit a low haze level over a wide range of viewing angles e and, at the same time, an acceptable or even high contrast.

Specifically it was found that the birefringence of the liquid crystal must not be too low. The birefringence is probably greater than 0.15 in order to obtain an acceptable contrast but values of L!n which are larger than 0.16 and, especially, those greater than 0.18 are preferred. The higher the birefringence the lower the specific value emax of the viewing angle for which maximum transmission is observed should be. It was found that the value of emax is preferably lower than 250 and more preferably lower than 200, if the birefringence is larger than 0.17. When the birefringence is larger than 0.195, Emax preferably should not exceed 22.50 and is preferably lower than 17.50.Another preferred combination is: birefringence greater than 0.15 and emax lower than 300 and more preferably lower than 250. If a very wide range for the effective viewing angle e is required the birefringence can be chosen to be within 0.13 < n < 0.15 and emax can be chosen between 250 S 0 6 < 300. This embodiment of the electrooptical system according to the invention, however, exhibits a rather low contrast, which is only just acceptable.

Another important condition is that the difference nm-nO must not exceed 0.03 in order to obtain a reasonable transmission around e = 00. High values of nm-nO can correspond to high values of ##max, and when the values of #max are too high, the overlapping of the 2 symmetrical curves in Fig. 2 is not sufficient. This is especially true where birefringence An is high. The difference between nm-nO preferably is lower than 0.025 and more preferably lower than 0.020. When the birefringence is higher than 0.1975, the difference nm-nO is preferably lower than 0.0200 and more preferably no more than 0.0185.If Lin is more than 0.21, nm-nO is preferably smaller than 0.0175 and more preferably no greater than 0.0150.

For given values of emax, nm and # n the ordinary index of refraction of the liquid crystal material is selected in such a way that the condition no = nm no sin#/(no2-nm2 cos2 #=) - #n with 0 < e < 300 is fulfilled. Small deviations of nO from the optimum value given by this equation are possible but this deviation is preferably smaller than 10% and more preferably not more than 7.5%. The same considerations hold for nm in cases where no, 4 n and tmax are known.

It is evident from the preceding detailed discussion that several parameters have to 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 defining the relationships which have to be simultaneously complied with if a high quality system of the type to which the - invention relates 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, existing in these systems, and leads to systems having inadequate optical properties for many applications.

The liquid crystal mixture used in the electrooptical systems according to the present invention is preferably based on compounds which are selected from the group consisting of compounds of formulae I-IV

wherein (independently in each compound) R1 is an alkyl group with 1-15 C atoms, in which one or two non-adjacent CH2 groups may be replaced by -O-, -CO-, -COO-, -OCO-, -HC=CH- and/or -CC-, Y is a single bond, -COO- or -OCO-,

- is a 1,4-phenylene group which is laterally unsubstituted or may be substituted in 2- and/or 3-position,

The liquid crystal mixture preferably contains at least 10 %, in particular more than 15 % and especially at least 20 % of one or more compounds of formulae I-IV.

Especially preferred is the following smaller group of com pounds according to formulae I-TV; in this list the following abbreviations are used: Phe 1,4-phenylene Cyc trans-1,4-cyclohexylene Phe .2F 2-fluoro-1 , 4-phenylene Phe.3F 3-fluoro-1,4-phenylene Pyd pyridine-2,5-diyl R1-Phe-Phe-CN R1-Phe.2F-Phe-CN Rl-Phe.3F-Phe-CN R1-Phe-Phe.2F-CN R1-Phe-Ph3.3F-CN Rl-Phe.2F-Phe.3F-CN R1-Phe.3F-Phe.3F-CN R1-Cyc-Phe-CN R1-Cyc-Phe.3F-CN R1-Cyc-Phe.2F-CN R1-Phe-COO-Phe-CN R1-Phe.2F-COO-Phe-CN R1-Phe.3F-COO-Phe-CN R1-Phe-COO-Phe.2F-CN R1-Phe-COO-Phe.3F-CN R1-Cyc-Cyc-CN R1-Phe-Phe-Phe-CN R1-Phe-Phe.2F-Phe-CN R1-Phe-Phe.3F-Phe-CN R1-Phe-Phe-Phe.2F-CN R1-Phe-Phe-Phe.3F-CN R1-Phe.2F-Phe-Phe-CN R1-Phe.3F-Phe-Phe-CN R1-Phe-Phe.2F-Phe.3F-CN R1-Phe-Phe.3F-Phe.3F-CN R1-Cyc-Phe-Phe-CN R1-Cyc-Phe.2F-Phe-CN R1-Cyc-Phe.3F-Phe-CN R1-Cyc-Phe-Phe.2F-CN R1-Cyc-Phe-Phe.3F-CN R1-Cyc-Phe.3F-Phe.3F-CN R1-Phe-Pyd-Phe-CN R1-Phe-Pyd-Phe.2F-CN R1-Phe-Pyd-Phe.3F-CN R1-Phe.2F-Pyd-Phe-CN R1-Cyc-Pyd-Phe-CN R1-Cyc-Pyd-Phe.2F-CN R1-Cyc-Pyd-Phe.3F-CN 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-IV 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-IV 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-IV, 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 cyclo- hexanecarboxylic acid, phenyl or cylcohexyl esters of cyclohexylbenzoic acid, phenyl or cyclohexyl esters of cyclohexylcyclohexanecarboxylic acid, cyclohexylchenyl esters of benzoic acid, of cyclohexanecarboxylic acid and of cyclohexylcyclohexanecarboxylic acid, phenylcyclohexanes, cyclohexylbiphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclo- hexenes, 1,4-bis-cyclohexylbenzenes, ,4'-bis-cyclohexyl- biphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexylpyridines, phenyl- or cyclohexyldioxanes, phenylor cyclohexyl-1,3-dithianes, 1,2-diphenylethanes, 1,2-dicyclohexylethanes, l-phenyl-2-cyclohexylethanes, l-cyclohexyl-2-(4-phenylcyclohexyl)ethanes, l-cyclohexyl-2-biphenyl- ylethanes, 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-COOE-R" 2 R' -L-OOC-E-R" 3 R' -LCH2CH2-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 i 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 compounds1 R'and R" are different from one another, one of these radicals usually being alkyl or alkenyl. In the compounds of the sub-formulae lb, 2b, 3b, 4b and Sb, R" is -CN, -CF3, -OCF3, -OCHF2,-F, -C1- 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 ib, 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-TV. 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, trimethylolpropandiacrylates, pentaerythrittriacrylates, triethylenglycoldiacrylates, ethylenglycoldimethacrylates, tripropylenglycoltriacrylates, pentaerythritoltrioacrylates, pentaerythritoltetraacrylates, ditrimethylpropantetraacrylates or dipentaerythritolpenta- or hexaacrylates.

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, and suitable photoinitiators of this type are described, for example, in JP 90-409225.

In principle, there are two routes for preparing electrooptical systems according to the present invention.

The first route starts with the selection of a suitable precursor for the isotropic medium, thus fixing the refractive index of the cured medium nm. In the next step a suitable liquid crystal material, usually a mixture, is selected which exhibits a birefringence nn close to the desired value of t n, 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 n n is relevant with respect to the selection of emax, the viewing angle with the maximum transmission.As was pointed out above, lower values of e maxare preferred for high birefringence mixtures, while for mixtures with lower birefringence, higher values of emax are possible.

After these pre-selections the fine-tuning begins.

Inserting nm, e max and iXn of the selected mixture into equation (4) allows the ideal value of nO to be calculated. If nO of the liquid crystal material differs from the ideal value, the liquid crystal material should be modified by adding a small percentage, generally less than 10%, of one or more further compounds. If nO is smaller than the ideal value, one can try adding a compound with substantially the same An as is exhibited by the existing liquid crystal mixture, but with a somewhat larger value of no.Unfortunately, the n-, nO- and ne-values for a liquid crystal mixture cannot always be accurately derived by summing the values for these parameters exhibited by the component compounds and, therefore, the aforementioned procedure is rarely entirely straightforward. In general, after addition of one or more further compounds, both the # n and the nO of the resulting liquid crystal mixture will change and one has to check, once again, if the conditions of equation (4) are 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 should be eliminated and one must start the procedure again. Small deviations of nO compared to the ideal value for nO derived from (4), are acceptable but generally these should not be more than 10%, preferably less than 7.5% and more preferably 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 is usual for an ideal mixture to be obtained by performing four steps of the kind described.

The second route starts with the selection of a liquid crystal material or mixture which, thus, fixes LXn and nO. Having regard to the value of # n, an appropriate value of emax is then selected. By inserting these fixed values of An, nO and 8max into equation (4), the optimum value for the refractive index nm of the isotropic cured medium can be derived. The next step is to select a precursor to a polymer medium which exhibits, in the cured state, a value for nm that is close to this calculated ideal value and which has other desirable properties such as, for example, miscibility with other precursors, thermal stability, resistivity etc. If the value of nm deviates from the ideal to an unacceptable degree, the initially selected precursor is blended with other monomers, oligomers and/or prepolymers. The amount of these additional components is usually less than 10% and, preferably 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%, preferably less than 7.5% and, more preferably, 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 medium 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, in which both the liquid crystal mixture and the isotropic medium are changed, or other slightly modified routes, are also useable and the invention is by no means restricted to the routes described above.

After the selection process is finished, the chosen liquid crystal mixture, precursor for the 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 is formed by subjecting the mixture to ultrasonic vibration, by using a colloid mill or by other methods. The resulting solution or dispersion is then 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 to a lesser extent, and, therefore, are of considerable potential economic importance.

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 8.75 % of 4-ethyl-4'-cyanobiphenyl 10.01 % of 4-(trans-4-pentylcyclohexyl)-4'-cyanobiphenyl 4.97 % of 4-pentyl-4"-cyanoterphenyl 18.76 % 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 25.00 % of 4-pentyl-4'-cyanobiphenyl 15.10 % of 4- (trans-4-propylcyclohexyl) -benzonitrile 17.41 % of 4- (trans-4-pentylcyclohexyl) -benzonitrile and exhibits the following physical properties: birefringence An = 0.2238 (589 nm, 20 C) ordinary refractive index no = 1.5194 (589 nm, 20 C) clearing point Tc = 89.5 C) viscosity TI = 42 cSt (20 C) dielectrical anisotropy ## = 15.5 (1 kHz, 20 C), is optimized with respect to an angle of emax 100 and a refractive index for the cured isotropic transparent medium of nm = 1.528, which corresponds to that of the adhesive Norland 65 (Norland Products).

This liquid mixture is mixed with Norland 65, which can be cured by UV irradiation, in the ratio of 1:1 to give a homogenous isotropic solution. The solution is capillary filled between 2 transparent glass substrates which are provided with electrode layers made from indium tin oxide. The thickness of the system was 15 m. The system is then exposed for 2 min to an Argon lamp (0.5 mW/cm2).

The angular transmission of this system which was measured via a photodetector is shown in Fig. 3; the transmission measured without inserting an electrooptical system is set to be 100%.

Example 2 A liquid crystal mixture, which consists of 8.75 % of 4-ethyl-4t-cyanobiphenyl 10.01 % of 4-(trans-4-pentylcyclohexyl)-4'-cyanobiphenyl 4.97 % of 4-pentyl-4"-cyanoterphenyl 18.76 % of 4-(4-propylphenyl)-3-fluoro-4'-cyanobiphenyl 16.0 % of 4-pentyl-4'-cyanobiphenyl 13.0 % of 4-(trans-4-propylcyclohexyl)-benzonitrile 14.51 % of 4-(trans-4-pentylcyclohexyl)-benzonitrile 14.0 % of trans-4- (trans-4-butylcyclohexyl) -cyclohexyicarDo- nitrile and exhibits the following physical properties:: birefringence An = 0.2041 (20 OC, 589 nm) ordinary index of refraction no = 1.5129 (20 OC, 589 nm) clearing point Tc = 92 C viscosity TI = 49 cSt (20 C dielectrical anisotropy ## = 13.77 (20 OC, 1 kHz) is optimized with respect to an angle of emax = 150 and a refractive index for the cured isatropic transparent medium of nm = 1.528, which corresponds to that of the adhesive Norland 65 (Norland Products).

This liquid crystal mixture is mixed with Norland 65 in the ratio of 1:1 to give a homogenous istropic solution. The solution is capillary filled between 2 transparent glass substrates which are provided with electrode layers made from indium tin oxide. The thickness of the system was 15 m. The system is then exposed for 2 min to an Argon lamp (0.5 mW.cm2).

The angular transmission of this system which was measured via a photodetector is shown in Fig. 3; the transmission measured without inserting an electrooptical system is set to be 100%.

Claims (11)

CLAMS

1. An electrooptical liquid crystal system, comprising a liquid crystal material, with a birefringence #n = ne-nO, and a polymeric medium, which is optically substantially transparent and isotropic and has a refractive index of nm when between two electrode alyers, the liquid crystal material being embedded in microdroplets in the polymeric medium, or present as a continuous phase in a 3-dimensional network formed by or with the polymeric medium, and the system allowing reduced transmission of incident light in an off-state, compared to an on-state, independently of the polarization of the incident light, characterized in that the birefringence of the liquid cyrstal material #n = ne-no > 0.13 (2), nm-nO < 0.03 (3), and no = nmnosin#/(no2-nm2cos2#)-#n (4) for absolute values of the viewing angle ranging between oo < e < 300, wherein all refractive indices are given for 589 nm radiation and at 200C and the values of nO, nm and can vary by up to 10% from those derived from equation (4) for a particular value of 0.

2. An electrooptical liquid crystal system, as claimed in claim 1, wherein 8 = emax and the value of no, nm, #n and emax can vary by up to 10% from those derived from equation (4).

3. An electrooptical liquid crystal system, as claimed in either of claims 1 and 2, wherein > 0.15.

4. An electrooptical liquid crystal system, as claimed in any of the preceding claims, wherein the values of no, nm, n and #max can vary by up to 7.5%, preferably 5% and most preferably 2.5%, from those derived from equation (4).

5. An electrooptical liquid crystal system as claimed in any of the preceding claims, wherein the liquid crystal material and polymeric medium are located between two electrode layers.

6. An electrooptical liquid crystal system as claimed in any of the preceding claims, wherein at least one of said electrode layers is attached to a substrate.

7. An electrooptical system according to claim 1 wherein the liquid crystal material is based on a mixture of compounds selected from the group comprising compounds of formulae I, II, III and IV

wherein (independently in each compound) R1is an alkyl group with 1-15 C atoms in which one or two non-adjacent CH2 groups can be replaced by -O-, -CO-, -COO-, -OCO-, -HC=CH- and/or -CrC-, Y is a single bond, -COO- or -OCO-,

is a 1,4-phenylene group, which is laterally unsubstituted or can be substituted in the 2- and/or 3-position,

8.A process for the preparation of an electrooptical liquid crystal system according to any of claims 1-7, characterized in that for a given liquid crystal material within > 0.15 and for a specific value of emax, the angle with maximum transmission, a polymeric transparent medium with a refractive index nm is selected in such a way that following conditions are fulfilled: nm~no < 0.03 no = nmnosin#/(no2-nm2cos2#) - #n, with Oo < 6 < 300 (4) wherein 6 = 6max' the value of nm can be up to 10% greater or less than that derived from equation (4) and all refractive indices are given for 589 nm and 200C.

9. Use of an electrooptical liquid crystal system a.

claimed in any of claims 1-7 for an architectural application or for a sunroof in an automobile.

10. A pane or panel, for use in a window, sunroof, mirror or other like application in an automobile, or for use as a glazing material in an architectural application, characterized by comprising an electrooptical liquid crystal system as claimed in any of claims 1-7.

11. An electrooptical liquid crystal system substantially as hereinbefore described as being in accordance with the invention.