Liquid crystal medium and liquid crystal display

Abstract

The present invention relates to mesogenic media and to electro-optical displays comprising these media as light modulation media. In particular, the electro-optical displays according to the present invention are displays, which are operated at a temperature, at which the mesogenic modulation media are in an optically isotropic phase, preferably in a blue phase.

Claims

1. A mesogenic medium comprising one or more compounds of formula A,
P.sup.1A(CH.sub.2CH.sub.2O).sub.zCH.sub.2CH.sub.2P.sup.2AA wherein P.sup.1A and P.sup.2A each, independently of one another, denote a polymerisable group, and z denotes an integer between 1 and 12; and comprising one or more compounds selected from the group consisting of compounds of formulae I and II ##STR00112## wherein L.sup.1 is H or F, L.sup.21 to L.sup.23 are, independently of each other, H or F, R.sup.1 and R.sup.2 are, independently of each other, alkyl, which is straight chain or branched, is unsubstituted, mono- or poly-substituted by F, Cl or CN, and in which one or more CH.sub.2 groups are optionally replaced, in each case independently from one another, by O, S, NR.sup.01, SiR.sup.01R.sup.02, CO, COO, OCO, OCOO, SCO, COS, CY.sup.01CY.sup.02 or CC in such a manner that O and/or S atoms are not linked directly to one another, Y.sup.01 and Y.sup.02 are, independently of each other, F, Cl or CN, and alternatively one of them may be H, and R.sup.01 and R.sup.02 are, independently of each other, H or alkyl with 1 to 12 C-atoms.

2. The medium according to claim 1, further comprising one or more compounds of formula III ##STR00113## wherein R.sup.3 is alkyl, which is straight chain or branched, is unsubstituted, mono- or poly-substituted by F, Cl or CN, and in which one or more CH.sub.2 groups are optionally replaced, in each case independently from one another, by O, S, NR.sup.01, SiR.sup.01R.sup.02, CO, COO, OCO, OCOO, SCO, COS, CY.sup.01CY.sup.02 or CC in such a manner that O and/or S atoms are not linked directly to one another, Y.sup.01 and Y.sup.02 are, independently of each other, F, Cl or CN, and alternatively one of them may be H, and R.sup.01 and R.sup.02 are, independently of each other, H or alkyl with 1 to 12 C-atoms.

3. The medium according to claim 1, further comprising one or more compounds selected from the group consisting of compounds of formulae IV and V ##STR00114## wherein R.sup.4 and R.sup.5 are, independently of each other, alkyl, which is straight chain or branched, is unsubstituted, mono- or poly-substituted by F, Cl or CN, and in which one or more CH.sub.2 groups are optionally replaced, in each case independently from one another, by O, S, CO, COO, OCO, OCOO, SCO, COS or CC in such a manner that O and/or S atoms are not linked directly to one another, L.sup.5 is H or F, ##STR00115## and n and m are, independently of one another, 0 or 1.

4. The medium according to claim 1, comprising one or more compounds of formula I.

5. The medium according to claim 1, comprising one or more compounds of formula II.

6. The medium according to claim 1, which has a blue phase.

7. A method for stabilizing a mesogenic medium, comprising subjecting the medium according to claim 1 to polymerisation of its polymerisable constituents.

8. A light modulation element, comprising a medium according to claim 1.

9. An electro-optical display, comprising a medium according to claim 1.

10. A mesogenic medium comprising one or more compounds of formula A,
P.sup.1A(CH.sub.2CH.sub.2O).sub.zCH.sub.2CH.sub.2P.sup.2AA wherein P.sup.1A and P.sup.2A each, independently of one another, denote a polymerisable group, and z denotes an integer between 1 and 12; and comprising one or more compounds of formula III ##STR00116## wherein R.sup.3 is alkyl, which is straight chain or branched, is unsubstituted, mono- or poly-substituted by F, Cl or CN, and in which one or more CH.sub.2 groups are optionally replaced, in each case independently from one another, by O, S, NR.sup.01, SiR.sup.01R.sup.02, CO, COO, OCO, OCOO, SCO, COS, CY.sup.01CY.sup.02 or CC in such a manner that O and/or S atoms are not linked directly to one another, Y.sup.01 and Y.sup.02 are, independently of each other, F, Cl or CN, and alternatively one of them may be H, and R.sup.01 and R.sup.02 are, independently of each other, H or alkyl with 1 to 12 C-atoms; or comprising one or more compounds selected from the group consisting of compounds of formulae IV and V ##STR00117## wherein R.sup.4 and R.sup.5 are, independently of each other, alkyl, which is straight chain or branched, is unsubstituted, mono- or poly-substituted by F, Cl or CN, and in which one or more CH.sub.2 groups are optionally replaced, in each case independently from one another, by O, S, CO, COO, OCO, OCOO, SCO, COS or CC in such a manner that O and/or S atoms are not linked directly to one another, L.sup.5 is H or F, ##STR00118## and n and m are, independently of one another, 0 or 1.

11. The medium according to claim 10, comprising one or more compounds of formula III.

12. The medium according to claim 10, comprising one or more compounds of formula IV.

13. The medium according to claim 10, comprising one or more compounds of formula V.

14. The medium according to claim 10, which has a blue phase.

15. A method for stabilizing a mesogenic medium, comprising subjecting the medium according to claim 10 to polymerisation of its polymerisable constituents.

16. A light modulation element, comprising a medium according to claim 10.

17. An electro-optical display, comprising a medium according to claim 10.

Description

EXAMPLES

(1) The examples below illustrate the present invention without limiting it in any way.

(2) However, the physical properties show the person skilled in the art what properties can be achieved and in what ranges they can be modified. In particular, the combination of the various properties which can preferably be achieved is thus well defined for the person skilled in the art.

(3) Liquid-crystal mixtures having the composition and properties as indicated in the following tables are prepared and investigated.

(4) The so-called HTP denotes the helical twisting power of an optically active or chiral substance in an LC medium (in m.sup.1). Unless indicated otherwise, the HTP is measured in the commercially available nematic LC host mixture MLD-6260 (Merck KGaA) at a temperature of 20 C.

Example 1

(5) The following liquid crystalline mixture M-1 is prepared and investigated with respect to its general physical properties. The composition and properties are given in the following table.

(6) TABLE-US-00007 Composition and properties liquid crystal mixture M-1 Composition Compound Conc./ No. Abbreviation mass-% Physical Properties 1 PGU-4-T 4.0 T(N, I) = 71 C. 2 PGU-5-T 3.0 n(20 C., 589 nm) = 0.1929 3 DPGU-4-F 8.0 4 GUQU-3-F 7.0 (20, 1 kHz) = 201.5 5 GUQU-4-F 6.0 6 GUQGU-3-F 8.0 k.sub.11(20 C.) = 21.98 pN 7 GUQGU-4-F 6.0 8 GUQGU-5-F 4.0 9 GUQGU-2-T 12.0 10 GUQGU-3-T 12.0 11 GUQGU-4-T 12.0 12 GUQGU-5-T 12.0 13 GUUQU-3-N 6.0 100.0

(7) 3.8% of the chiral agent R-5011 and 0.2% of Irgacure 651 are solved in the achiral liquid crystal mixture M-1 and the electro-optical response of resultant mixture in an IPS-type cell is investigated. The mixture is filled into an electro optical test cell with inter-digital electrodes on one substrate side. The electrode width is 10 m, the distance between adjacent electrodes is 10 m and the cell gap is also 10 m. This test cell is evaluated electro-optically between crossed polarisers.

(8) Appropriate Concentrations

(9) a) of the reactive mesogen of the formula RM-C

(10) ##STR00109##
and
c) alternatively of one of the two reactive compounds of the formulae A

(11) ##STR00110##
or RM

(12) ##STR00111##
respectively, are added to the mixture of interest, here mixture M-1.

(13) The resultant mixture is introduced into test cells and heated to an appropriate temperature, at which the mixture is in the blue phase. Then it is exposed to UV.

(14) The mixtures are characterised as described below before the polymerisation. The reactive components are then polymerised in the blue phase by irradiation once (180 s), and the resultant media are recharacterised.

(15) Detailed Description of the Polymerisation

(16) Before the polymerisation of a sample, the phase properties of the medium are established in a test cell having a thickness of about 10 microns and an area of 22.5 cm.sup.2. The filling is carried out by capillary action at a temperature of 75 C. The measurement is carried out under a polarising microscope with heating stage with a temperature change of 1 C./min.

(17) The polymerisation of the media is carried out by irradiation with a UV lamp (Dymax, Bluewave 200, 365 nm interference filter) having an effective power of about 3.0 mW/cm.sup.2 for 180 seconds. The polymerisation is carried out directly in the electro-optical test cell.

(18) The polymerisation is carried out initially at a temperature at which the medium is in the blue phase I (BP-I). The polymerisation is carried out in a plurality of part-steps, which gradually result in complete polymerisation. The temperature range of the blue phase generally changes during the polymerisation. The temperature is therefore adapted between each part-step so that the medium is still in the blue phase. In practice, this can be carried out by observing the sample under the polarising microscope after each irradiation operation of about 5 s or longer. If the sample becomes darker, this indicates a transition into the isotropic phase. The temperature for the next part-step is reduced correspondingly.

(19) The entire irradiation time which results in maximum stabilisation is typically 180 s at the irradiation power indicated. Further polymerisations can be carried out in accordance with an optimised irradiation/temperature programme.

(20) Alternatively, the polymerisation can also be carried out in a single irradiation step, in particular if a broad blue phase is already present before the polymerisation.

(21) Electro-Optical Characterisation

(22) After the above-described polymerisation and stabilisation of the blue phase, the phase width of the blue phase is determined. The electro-optical characterisation is carried out subsequently at various temperatures within and if desired also outside this range.

(23) The test cells used are fitted on one side with interdigital electrodes on the cell surface. The cell gap, the electrode separation and the electrode width are typically each 10 microns. This uniform dimension is referred to below as the gap width. The area covered by electrodes is about 0.4 cm.sup.2. The test cells do not have an alignment layer.

(24) For the electro-optical characterisation, the cell is located between crossed polarising filters, where the longitudinal direction of the electrodes adopts an angle of 45 to the axes of the polarising filter. The measurement is carried out using a DMS301 (Autronic-Melchers) at a right angle to the cell plane, or by means of a highly sensitive camera on the polarising microscope. In the voltage-free state, the arrangement described gives an essentially dark image (definition 0% transmission).

(25) Firstly, the characteristic operating voltages and then the response times are measured on the test cell. The operating voltage is applied to the cell electrodes in the form of rectangular voltage having an alternating sign (frequency 100 Hz) and variable amplitude, as described below.

(26) The transmission is measured while the operating voltage is increased. The attainment of the maximum value of the transmission defines the characteristic quantity of the operating voltage V.sub.100. Equally, the characteristic voltage V.sub.10 is determined at 10% of the maximum transmission. These values are measured at various temperatures in the range of the blue phase.

(27) Relatively high characteristic operating voltages V.sub.100 are observed at the upper and lower end of the temperature range of the blue phase. In the region of the minimum operating voltage, V.sub.100 generally only increases slightly with increasing temperature. This temperature range, limited by T.sub.1 and T.sub.2, is referred to as the usable, flat temperature range (FR). The width of this flat range (FR) is (T.sub.2T.sub.1) and is known as the width of the flat range (WFR). The precise values of T.sub.1 and T.sub.2 are determined by the intersections of tangents on the flat curve section FR and the adjacent steep curve sections in the V.sub.100/temperature diagram.

(28) In the second part of the measurement, the response times during switching on and off (.sub.on, .sub.off) are determined. The response time .sub.on is defined by the time to achievement of 90% intensity after application of a voltage at the level of V.sub.100 at the selected temperature. The response time .sub.off is defined by the time until the decrease by 90% starting from maximum intensity at V.sub.100 after reduction of the voltage to 0 V. The response time is also determined at various temperatures in the range of the blue phase.

(29) As further characterisation, the transmission at continuously increasing and falling operating voltage between 0 V and V.sub.100 is measured at a temperature within the FR. The difference between the two curves is known as hysteresis. The difference in the transmissions at 0.5.Math.V.sub.100 and the difference in the voltages at 50% transmission are, for example, characteristic hysteresis values and are known as T.sub.50 and V.sub.50 respectively.

(30) As a further characteristic quantity, the ratio of the transmission in the voltage-free state before and after passing through a switching cycle can be measured. This transmission ratio is referred to as the memory effect. The value of the memory effect is 1.0 in the ideal state. Values above 1 mean that a certain memory effect is present in the form of excessively high residual transmission after the cell has been switched on and off. This value is also determined in the working range of the blue phase (FR).

(31) Typical concentrations of the polymer precursors are as follows.

(32) TABLE-US-00008 Mixture M-1-1 M-1-2 Constituent Concentration/% M-1 88.0 88.0 R-5011 3.8 3.8 RM-C 5.0 5.0 A 3.0 0.0 RM 0.0 3.0 IRG-651 0.2 0.2 100.0 100.0

(33) The results are summarised in the following table.

(34) TABLE-US-00009 Mixture M-1-1 M-1-2 Measurement values (20 C.) Transition point before the polymerisation Polymerisation temperature/ C. V.sub.100 (20 C.)/V 53 58 V.sub.10 (20 C.)/V V.sub.90 (20 C.)/V V.sub.50 (20 C.)/V Contrast, switching on Contrast, switching off Memory effect

(35) The polymerisable mixture is polymerised in a single irradiation step at a temperature of about 30-50 C. at the lower end of the temperature range of the blue phase (cf. above for details).

(36) The polymer-stabilised liquid-crystalline media exhibit a blue phase over a broad temperature range.

(37) The polymer-stabilised medium M-1-1, prepared using the monomer of formula A according to the invention, exhibits a reduction in the operating voltage (V.sub.op) and good contrast on switching on and on switching off compared with conventional media (M-1-2) from the prior art (WO 2012/163470 A1).

(38) It can be seen from this that the monomers according to the invention are particularly suitable for the stabilisation of blue phases, in particular in the case of media having a high concentration of chiral dopant.