LIQUID CRYSTAL-BASED LIGHT VALVE

Abstract

A light valve that is operable in and electrically switchable between an optically clear and transparent state and an opaque state, wherein one or more polymerisable mesogenic compounds provided in a layer containing a liquid-crystalline medium that comprises one or more mesogenic compounds, one or more chiral compounds, one or more dichroic dyes and the one or more polymerisable mesogenic compounds are subjected to photopolymerisation. Further, a liquid-crystalline medium used in the method, a light valve obtained or respectively obtainable by carrying out the method, and the use of the light valve in mobile devices.

Claims

1. A method for the preparation of a light valve which is operable in and electrically switchable between an optically transparent state and an opaque state, comprising, the steps of: (i) providing a liquid-crystalline medium which comprises one or more mesogenic compounds, one or more chiral compounds, one or more dichroic dyes and one or more polymerisable mesogenic compounds as a layer interposed between two opposing transparent substrates which are each provided with an electrode, wherein the liquid-crystalline medium has a clearing point of 70° C. or more, and wherein the one or more polymerisable mesogenic compounds are contained in the medium in an amount, based on the overall contents of the medium, of 4% by weight or less; and (ii) polymerising the one or more polymerisable mesogenic compounds in the layer using photopolymerisation.

2. The method according to claim 1, wherein the photopolymerisation of the one or more polymerisable mesogenic compounds is carried out in the presence of an electric field in the layer.

3. The method according to claim 1, wherein the one or more polymerisable mesogenic compounds comprise one, two or more acrylate and/or methacrylate groups.

4. The method according to claim 1, wherein the photopolymerisation is carried out for a period of from 1 minute to 240 minutes using light having an intensity in the range of from 0.1 mW/cm.sup.2 to 100 mW/cm.sup.2.

5. A light valve that is operable in and electrically switchable between an optically transparent state and an opaque state, wherein the light valve is obtained or obtainable from carrying out the method according to claim 1.

6. A light valve that is operable in and electrically switchable between an optically transparent state and an opaque state and that comprises a switching layer containing a material that comprises a liquid-crystalline medium comprising one or more mesogenic compounds, one or more chiral compounds and one or more dichroic dyes, wherein the liquid-crystalline medium has a clearing point of 70° C. or more, and a polymeric component comprising one or more polymeric structures obtained or obtainable from polymerisation of one or more polymerisable mesogenic compounds, wherein the polymeric component is contained in the material in an amount, based on the overall contents of the material, of 4% by weight or less, wherein the light valve in the optically transparent state has a haze of less than 15% and a degree of light transmission of more than 45% and in the opaque state has a haze of more than 65% and a degree of light transmission of less than 35%.

7. The light valve according to claim 6, wherein the liquid-crystalline medium exhibits a pitch of 0.55 μm or more in the opaque state.

8. A liquid-crystalline medium for use in the method according to claim 1, comprising: one or more polymerisable mesogenic compounds in an amount, based on the overall contents of the medium, of 4% by weight or less, one or more mesogenic compounds, one or more chiral compounds; and one or more dichroic compounds.

9. The liquid-crystalline medium according to claim 8, wherein the liquid-crystalline medium exhibits a positive dielectric anisotropy Δε and an optical anisotropy Δn, determined at 20° C. and 589 nm, of 0.13 or more, and wherein the one or more chiral compounds contained in the liquid-crystalline medium have an absolute value of the helical twisting power of 5 μm.sup.−1 or more.

10. The liquid-crystalline medium according to claim 8, wherein the liquid-crystalline medium comprises one or more mesogenic compounds selected from the compounds of formulae I and II ##STR00377## wherein R.sup.1 and R.sup.2 denote, independently of one another, a group selected from F, Cl, CF.sub.3, OCF.sub.3, and straight-chain or branched alkyl or alkoxy having 1 to 15 carbon atoms or straight-chain or branched alkenyl having 2 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF.sub.3 or mono- or polysubstituted by halogen and wherein one or more CH.sub.2 groups may be, in each case independently of one another, replaced by —O—, —S—, —CO—, —COO—, —OCO—, —OCOO— or —C≡C— in such a manner that oxygen atoms are not linked directly to one another, A.sup.11 denotes ##STR00378## n denotes 0 or 1, and A.sup.21, A.sup.31 and A.sup.41 denote, independently of one another, ##STR00379## wherein L is on each occurrence, identically or differently, halogen selected from F, Cl and Br, and R.sup.3 and R.sup.4 denote, independently of one another, a group selected from F, CF.sub.3, OCF.sub.3, CN, and straight-chain or branched alkyl or alkoxy having 1 to 15 carbon atoms or straight-chain or branched alkenyl having 2 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF.sub.3 or mono- or polysubstituted by halogen and wherein one or more CH.sub.2 groups may be, in each case independently of one another, replaced by —O—, —S—, —CO—, —COO—, —OCO—, —OCOO— or —C≡C— in such a manner that oxygen atoms are not linked directly to one another, and L.sup.1, L.sup.2 and L.sup.3 denote, independently of one another, H or F.

11. The liquid-crystalline medium according to claim 8, wherein the liquid-crystalline medium comprises one or more mesogenic compounds selected from the compounds of formulae III, IV and V ##STR00380## wherein R.sup.5 and R.sup.6 denote, independently of one another, a group selected from F, CF.sub.3, OCF.sub.3, CN, and straight-chain or branched alkyl or alkoxy having 1 to 15 carbon atoms or straight-chain or branched alkenyl having 2 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF.sub.3 or mono- or polysubstituted by halogen and wherein one or more CH.sub.2 groups may be, in each case independently of one another, replaced by —O—, —S—, —CO—, —COO—, —OCO—, —OCOO— or —C≡C— in such a manner that oxygen atoms are not linked directly to one another, and L.sup.4, L.sup.5, L.sup.6 and L.sup.7 denote, independently of one another, H or F, R.sup.7 and R.sup.8 denote, independently of one another, straight-chain or branched alkyl or alkoxy having 1 to 15 carbon atoms, preferably 1 to 7 carbon atoms, or straight-chain or branched alkenyl having 2 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF.sub.3 or mono- or polysubstituted by halogen and wherein one or more CH.sub.2 groups may be, in each case independently of one another, replaced by —O—, —S—, —CO—, —COO—, —OCO—, —OCOO— or —C≡C— in such a manner that oxygen atoms are not linked directly to one another, is 0, 1 or 2, and X.sup.1 and X.sup.2 denote, independently of one another, F, CF.sub.3, OCF.sub.3 or CN.

12. The liquid-crystalline medium according to claim 8, wherein the liquid-crystalline medium comprises at least three different dichroic dyes, preferably in an amount, based on the overall contents of the medium, of 3% by weight or less.

13. The liquid-crystalline medium according to claim 8, wherein the liquid-crystalline medium comprises at least one di- or multireactive polymerisable mesogenic compound.

14. A modulation material for use in the light valve according to claim 5, comprising: a liquid-crystalline medium comprising one or more mesogenic compounds, one or more chiral compounds and one or more dichroic dyes, wherein the liquid-crystalline medium has a clearing point of 70° C. or more, and a polymeric component comprising one or more polymeric structures obtained or obtainable from polymerisation of one or more polymerisable mesogenic compounds, wherein the polymeric component is contained in the material in an amount, based on the overall contents of the material, of 4% by weight or less.

15. A mobile or a portable device comprising the light valve according to claim 5.

16. The method according to claim 2, wherein the electric field comprises an alternating current electric field, the layer has a thickness of from 2 μm to 50 μm, and the applied electric field induces a homeotropic alignment in the layer comprising the liquid-crystalline medium.

17. The method according to claim 16, wherein the layer has a thickness of from 5 μm to 25 μm.

18. The method of claim 4, wherein the light is UV light.

19. The light valve according to claim 6, wherein the haze is determined according to ASTM D 1003, and wherein the degree of light transmission is determined in accordance with DIN EN410.

20. The light valve according to claim 5, wherein the liquid-crystalline medium exhibits a pitch of 0.55 μm or more in the opaque state.

21. The liquid-crystalline medium according to claim 12, comprising 3% by weight or less of the dichroic dyes, based on the overall contents of the medium.

Description

EXAMPLES

[0377] In the Examples, [0378] V.sub.o denotes threshold voltage, capacitive [V] at 20° C., [0379] n.sub.e denotes extraordinary refractive index at 20° C. and 589 nm, [0380] n.sub.o denotes ordinary refractive index at 20° C. and 589 nm, [0381] Δn denotes optical anisotropy at 20° C. and 589 nm, [0382] ε∥ denotes dielectric permittivity parallel to the director at 20° C. and 1 kHz, [0383] ε⊥ denotes dielectric permittivity perpendicular to the director at 20° C. and 1 kHz, [0384] Δε denotes dielectric anisotropy at 20° C. and 1 kHz, [0385] cl.p., T(N,I) denotes clearing point [° C.], [0386] γ.sub.1 denotes rotational viscosity measured at 20° C. [mPa.Math.s], determined by the rotation method in a magnetic field, [0387] K.sub.1 denotes elastic constant, “splay” deformation at 20° C. [pN], [0388] K.sub.2 denotes elastic constant, “twist” deformation at 20° C. [pN], [0389] K.sub.3 denotes elastic constant, “bend” deformation at 20° C. [pN].

[0390] The term “threshold voltage” for the present invention relates to the capacitive threshold (V.sub.0), unless explicitly indicated otherwise. In the Examples, as is generally usual, the optical threshold can also be indicated for 10% relative contrast (V.sub.10).

[0391] Liquid crystal mixtures and composite systems are realized with the compositions and properties as given in the following. Their properties and optical performance are investigated.

Reference Example 1

[0392] A liquid-crystal base mixture B-1 is prepared and characterized with respect to its general physical properties, having the composition and properties as indicated in the following table.

TABLE-US-00008 PPZPP-7-N    7.00% Clearing point: 100.0° C. PP-3-N    5.00% Δn [589 nm, 20° C.] 0.263 PP-2-N   15.00% n.sub.e [589 nm, 20° C.] 1.790 PPP-5-N   10.00% Δε [1 kHz, 20° C.] 17.2 CPP-5-N   15.00% ε.sub.|| [1 kHz, 20° C.] 22.4 PP-30-N   11.00% PP-5-N   37.00% Σ 100.00%

Reference Example 2

[0393] A liquid-crystal base mixture B-2 is prepared and characterized with respect to its general physical properties, having the composition and properties as indicated in the following table.

TABLE-US-00009 PGIGI-3-F   10.00% Clearing point: 105.0° C. CPG-2-F    6.00% Δn [589 nm, 20° C.] 0.160 CPG-3-F    7.00% n.sub.e [589 nm, 20° C.] 1.663 CPG-5-F    5.00% Δε [1 kHz, 20° C.] 11.4 CPU-5-F   10.00% ε.sub.|| [1 kHz, 20° C.]: 15.7 CPU-7-F   10.00% PGU-3-F    4.00% PGU-5-F    7.00% CCGU-3-F    8.00% CPP-3-2    4.00% CGPC-3-3    3.00% CGPC-5-3    3.00% CGPC-5-5    3.00% CPGU-3-OT    5.00% CP-5-N   15.00% Σ 100.00%

[0394] A cholesteric mixture C-2 is prepared by mixing 99.00% of mixture B-2 with 1.00% of chiral dopant R-5011 available from Merck KGaA, Darmstadt, Germany.

Reference Example 3

[0395] A liquid-crystal base mixture B-3 is prepared and characterized with respect to its general physical properties, having the composition and properties as indicated in the following table.

TABLE-US-00010 CPP-3-2    4.00% Clearing point: 104.4° C. CPG-3-F    9.00% Δn [589 nm, 20° C.]: 0.160 CPG-5-F    9.00% n.sub.e [589 nm, 20° C.]: 1.654 CCGU-3-F    9.00% Δε [1 kHz, 20° C.]: 18.8 CC-3-V1    8.00% ε.sub.|| [1 kHz, 20° C.] 22.9 CC-4-V1    6.00% CPGU-3-OT    6.00% CPU-7-F    8.00% PGU-3-F    6.00% PGU-5-F    6.00% PGUQU-3-F    9.00% PGUQU-5-F    8.00% PUQU-3-F    6.00% CLP-3-T    6.00% Σ 100.00%

[0396] A cholesteric mixture C-3 is prepared by mixing 99.54% of mixture B-3 with 0.46% of chiral dopant R-5011 available from Merck KGaA, Darmstadt, Germany.

Comparative Example 1

[0397] A cholesteric mixture CC-1 is prepared by mixing 91.125% of mixture B-1 as described in Reference Example 1 above with 6.493% of chiral dopant CB15 available from Merck KGaA, Darmstadt, 2.370% of ethylene glycol dimethacrylate, 0.010% of polydimethylsiloxane and 0.002% of 2,6-di-tert-butyl-4-methylphenol.

[0398] The mixture CC-1 is filled into an electro-optical cell having glass substrates (0.7 mm alkali-free glass) with ITO electrodes as well as polyimide alignment layers (AL-1054 from Japan Synthetic Rubber, planar, TN-rubbed), wherein the cell gap is 25 μm.

[0399] The cell is preheated to 40° C. Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (facial tanner with intensity cut-off below 340 nm, 3 mW/cm.sup.2 light intensity) for 60 minutes while a voltage (100 V rms) is applied.

[0400] The obtained cell has an opaque state with a white appearance and a haze of 96%, where the haze is determined according to ASTM 1003-92 using a spectrophotometer (Lambda 1050, Perkin Elmer) and a 150 mm Ulbricht's sphere. The obtained cell has an optically clear state at 50 V with a haze of 5.2%, having a τ.sub.on of 99 ms and τ.sub.off of 32 ms at 20° C. At 0° C. the cell has a τ.sub.on of 750 ms and τ.sub.off of 134 ms.

Example 1

[0401] A dye-doped mixture M-1 is prepared by mixing 99.50% of cholesteric mixture C-2 described in Reference Example 2 above with 0.50% of Irgaphor Black X11 DC (from BASF). A mixture M-1-1 is prepared by mixing 99.225% of mixture M-1 with 0.75% of compound of formula RM-A

##STR00365##

[0402] and 0.025% of the photoinitiator Irgacure® 651 (in the following abbreviated as IRG-651).

##STR00366##

available from Ciba, Switzerland.

[0403] The mixture M-1-1 is filled into an electro-optical cell having glass substrates (0.7 mm alkali-free glass) with ITO electrodes as well as polyimide alignment layers (AL-1054 from Japan Synthetic Rubber, planar, TN-rubbed), wherein the cell gap is 25 μm.

[0404] The cell is preheated to 40° C. Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (facial tanner with intensity cut-off below 340 nm, 3 mW/cm.sup.2 light intensity) for 60 minutes while a voltage (100 V rms) is applied.

[0405] The obtained cell in the opaque state has a haze of 67%. The obtained cell has an optically clear state at 100 V with a haze of 2.0%, having a τ.sub.on of 65 ms and τ.sub.off of 2.8 ms at 20° C. At 0° C. the cell has a τ.sub.on of 236 ms and τ.sub.off of 10.0 ms.

Example 2

[0406] A dye-doped mixture M-2 is prepared by adding to cholesteric mixture C-2 described in Reference Example 2 above 0.118% of compound of formula DD-1

##STR00367##

[0407] 0.142% of compound of formula DD-2

##STR00368##

[0408] and 0.242% of compound of formula DD-3

##STR00369##

[0409] A mixture M-2-1 is prepared by mixing 99.225% of mixture M-2 with 0.750% of compound of formula RM-A as described in Example 1 above and 0.025% of IRG-651 as described in Example 1 above.

[0410] The mixture M-2-1 is filled into an electro-optical cell having glass substrates (0.7 mm alkali-free glass) with ITO electrodes as well as polyimide alignment layers (AL-1054 from Japan Synthetic Rubber, planar, TN-rubbed), wherein the cell gap is 25 μm.

[0411] The cell is preheated to 40° C. Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (facial tanner with intensity cut-off below 340 nm, 3 mW/cm.sup.2 light intensity) for 60 minutes while a voltage (100 V rms) is applied.

[0412] The obtained cell has an opaque state with black appearance and a haze of 84%. The obtained cell has an optically clear state at 100 V with a haze of 2.5%, having a τ.sub.on of 101 ms and τ.sub.off of 2.8 ms at 20° C. At 0° C. the cell has a τ.sub.on of 368 ms and τ.sub.off of 9.8 ms.

Example 3

[0413] A mixture M-3 is prepared by mixing 98.713% of mixture M-2 as prepared in Example 2 above with 0.037% of IRG-651, 0.625% of compound of formula RM-B

##STR00370##

[0414] and 0.625% of compound of formula RM-C

##STR00371##

[0415] The mixture M-3 is treated and measured as described for mixture M-2-1 in Example 2 above. The obtained cell exhibits a favourable electro-optical performance in the opaque state and the optically transparent state.

Example 4

[0416] A mixture M-4 is prepared by adding to cholesteric mixture C-3 described in Reference Example 3 above 0.750% of compound of formula RM-A as described in Example 1 above, 0.025% of IRG-651, 0.109% of compound of formula DD-1 as described in Example 2 above, 0.132% of compound of formula DD-2 as described in Example 2 above and 0.254% of compound of formula DD-3 as described in Example 2 above.

[0417] The mixture M-4 is filled into an electro-optical cell having glass substrates (0.7 mm alkali-free glass) with ITO electrodes as well as polyimide alignment layers (AL-1054 from Japan Synthetic Rubber, planar, TN-rubbed), wherein the cell gap is 25 μm.

[0418] The cell is preheated to 40° C. Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (facial tanner with intensity cut-off below 340 nm, 3 mW/cm.sup.2 light intensity) for 60 minutes while a voltage (100 V rms) is applied.

[0419] The obtained cell has an opaque state with black appearance and a haze of 93%. The obtained cell has an optically clear state at 50 V with a haze of 5.0%, having a τ.sub.on of 68 ms and τ.sub.off of 2.8 ms at 20° C. At 0° C. the cell has a τ.sub.on of 126 ms and τ.sub.off of 9.0 ms.

Example 5

[0420] A mixture M-5 is prepared by mixing 98.298% of mixture B-3 described in Reference Example 3 above with 0.458% of chiral dopant R-5011, 0.750% of compound of formula RM-A as described in Example 1 above, 0.109% of compound of formula DD-1 as described in Example 2 above, 0.132% of compound of formula DD-2 as described in Example 2 above and 0.253% of compound of formula DD-3 as described in Example 2 above.

[0421] The mixture M-5 is filled into an electro-optical cell having glass substrates (0.7 mm alkali-free glass) with ITO electrodes as well as polyimide alignment layers (AL-1054 from Japan Synthetic Rubber, planar, TN-rubbed), wherein the cell gap is 25 μm.

[0422] The cell is preheated to 40° C. Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (facial tanner with intensity cut-off below 340 nm, 3 mW/cm.sup.2 light intensity) for 60 minutes while a voltage (100 V rms) is applied.

[0423] The obtained cell has an opaque state with black appearance and a haze of 83%. The obtained cell has an optically clear state at 50 V with a haze of 1.5%, having a τ.sub.on of 56 ms and τ.sub.off of 7.7 ms at 20° C. At 0° C. the cell has a τ.sub.on of 116 ms and τ.sub.off of 25 ms.

Example 6

[0424] A mixture M-6 is prepared by adding to mixture B-3 described in Reference Example 3 above 0.458% of chiral dopant R-5011, 0.750% of compound of formula RM-A as described in Example 1 above, 0.025% of IRG-651, 0.123% of compound of formula DD-4

##STR00372##

[0425] wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 denote, independently of one another,

##STR00373##

[0426] 0.337% of a mixture of compounds of formulae DD-5 and DD-6

##STR00374##

[0427] 0.117% of compound of formula DD-7

##STR00375##

[0428] and 0.173% of compound of formula DD-8

##STR00376##

[0429] The mixture M-6 is filled into an electro-optical cell having glass substrates (0.7 mm alkali-free glass) with ITO electrodes as well as polyimide alignment layers (AL-1054 from Japan Synthetic Rubber, planar, TN-rubbed), wherein the cell gap is 25 μm.

[0430] The cell is preheated to 40° C. Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (facial tanner with intensity cut-off below 340 nm, 3 mW/cm.sup.2 light intensity) for 60 minutes while a voltage (100 V rms) is applied.

[0431] The obtained cell has an opaque state with black appearance and a haze of 94%. The obtained cell has an optically clear state at 100 V with a haze of 7.6%, having a τ.sub.on of 15 ms and τ.sub.off of 3.3 ms at 20° C. At 0° C. the cell has a τ.sub.on of 42 ms and τ.sub.off of 9.8 ms.

Example 7

[0432] A mixture M-7 is prepared by adding to mixture C-3 as described in Reference Example 3 above 0.037% of IRG-651, 0.625% of compound of formula RM-B as described in Example 3 above, 0.625% of compound of formula RM-C as described in Example 3 above, 0.109% of compound of formula DD-1 as described in Example 2 above, 0.132% of compound of formula DD-2 as described in Example 2 above and 0.254% of compound of formula DD-3 as described in Example 2 above.

[0433] The mixture M-7 is treated and measured as described for mixture M-2-1 in Example 2 above. The obtained cell exhibits a favourable electro-optical performance in the opaque state and the optically transparent state.