METHOD FOR PREPARING A LIQUID CRYSTAL-BASED SWITCHING ELEMENT

Abstract

A method for preparing a switching element which is operable in and electrically switchable between an optically clear state and a scattering state, wherein one or more polymerisable mesogenic compounds provided in a layer containing a liquid-crystalline medium which comprises one or more mesogenic compounds, one or more chiral compounds and the one or more polymerisable mesogenic compounds are subjected to polymerisation in the presence of a direct current (DC) electric field in the layer. Also, a switching element obtained or respectively obtainable by carrying out the method and to the use of the switching element in a window.

Claims

1. A method for the preparation of a switching element which is operable in and electrically switchable between an optically clear state and a scattering state, comprising the steps of: (i) providing a liquid-crystalline medium which comprises one or more mesogenic compounds, one or more chiral compounds 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 presence of a direct current electric field in the layer.

2. The method according to claim 1, wherein the applied direct current electric field induces a homeotropic alignment in the layer comprising the liquid-crystalline medium, wherein the layer preferably has a thickness in the range from 4 μm to 40 μm.

3. The method according to claim 1, wherein polymerising as set forth in step (ii) is carried out by photopolymerisation.

4. The method according to claim 1, wherein the liquid-crystalline medium further comprises one or more photoinitiators.

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

6. The method according to claim 1, wherein polymerising as set forth in step (ii) is carried out for a time period 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.

7. The method according to claim 1, wherein subsequent to step (ii) a thermal treatment of the liquid-crystalline medium is carried out.

8. The method according to claim 1, wherein the electrodes are arranged as transparent conductive layers supported on the substrates and facing the liquid-crystalline medium, wherein optionally alignment layers are further provided which are in direct contact with the liquid-crystalline medium.

9. The method according to claim 1, wherein the liquid-crystalline medium comprises, based on the overall contents of the medium, at least 15% by weight of one or more mesogenic compounds of formula I ##STR00364## 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 ##STR00365## n denotes 0 or 1, and A.sup.21, A.sup.31 and A.sup.41 denote, independently of one another, ##STR00366## wherein L is on each occurrence, identically or differently, halogen selected from F, Cl and Br.

10. The method according to claim 1, wherein the liquid-crystalline medium further comprises one or more mesogenic compounds selected from the group of compounds of formulae II and III ##STR00367## wherein R.sup.3, R.sup.4, 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.1, L.sup.2, L.sup.3, L.sup.4 and L.sup.5 denote, independently of one another, H or F.

11. The method according to claim 1, 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.

12. A switching element which is operable in and electrically switchable between an optically clear state and a scattering state, wherein the switching element is obtained by or respectively obtainable from carrying out the method according to claim 1.

13. A switching element which is operable in and electrically switchable between an optically clear state and a scattering state, comprising a switching layer containing a material which comprises: a liquid-crystalline medium comprising one or more mesogenic compounds and one or more chiral compounds, 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 by or respectively 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 switching element in the optically clear state has a haze of less than 6%.

14. The switching element according to claim 12, wherein the liquid-crystalline medium exhibits a pitch of 0.55 μm or more in the scattering state.

15. A window, comprising the switching element according to claim 12.

16. The method according to claim 2, wherein the applied direct current electric field induces a homeotropic alignment in the layer comprising the liquid-crystalline medium, wherein the layer preferably has a thickness in the range from 10 μm to 25 μm.

17. The method of claim 3, wherein the photopolymerisation uses UV light.

18. The method of claim 6, wherein the light comprises UV light.

19. The method of claim 7, wherein the thermal treatment is carried out in the presence of an electric field.

20. The method of claim 19, wherein the electric field comprises a direct current.

21. The method of claim 8, wherein the transparent conductive layers are respectively embedded between two transparent dielectric layers.

22. The switching element of claim 13, wherein the haze is determined according to ASTM D 1003.

23. The switching element according to claim 13, wherein the liquid-crystalline medium exhibits a pitch of 0.55 μm or more in the scattering state.

24. A window, comprising the switching element according to claim 13.

Description

EXAMPLES

[0384] 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

[0385] 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 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%

Reference Example 2

[0386] 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%   CPG-2-F   6.00% CPG-3-F   7.00% CPG-5-F   5.00% CPU-5-F  10.00% 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-7-N  15.00% Σ 100.00%

Reference Example 3

[0387] 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 APUQU-3-F  8.00% Clearing point: 127.8° C. CPU-3-F  15.00% Δn [589 nm, 20° C.]: 0.206 CCGU-3-F  8.00% n.sub.e [589 nm, 20° C.]: 1.711 CPGP-5-2  4.00% Δε [1 kHz, 20° C.]: 42.7 CPGP-5-3  4.00% ε.sub.∥ [1 kHz, 20° C.]: 48.2 CPGU-3-OT  8.00% DPGU-4-F  4.00% PGU-2-F  10.00% PGU-3-F  11.00% PGUQU-3-F  8.00% PGUQU-4-F  10.00% PGUQU-5-F  10.00% Σ 100.00%

Reference Example 4

[0388] A liquid-crystal base mixture B-4 is prepared and characterized with respect to its general physical properties, having the composition and properties as indicated in the following table.

TABLE-US-00011 CP-3-N  14.00% Clearing point: 119.3° C. PTP-1-O2   8.00% Δn [589 nm, 20° C.]: 0.236 PTP-3-O1   6.00% n.sub.e [589 nm, 20° C.]: 1.752 CP-3-O1   8.50% Δε [1 kHz, 20° C.]: 7.2 PGP-2-2V   8.00% ε.sub.∥ [1 kHz, 20° C.]: 11.0 CPGP-4-3   5.00% CPGP-5-2   5.00% PGP-2-3   5.00% PGP-2-4   5.00% PGP-2-5  10.00% CPTP-3-O1   6.00% CPTP-3-O2   6.00% PGUQU-3-F   7.50% PGUQU-4-F   2.00% PP-1-2V1   4.00% Σ 100.00%

Reference Example 5

[0389] A liquid-crystal base mixture B-5 is prepared and characterized with respect to its general physical properties, having the composition and properties as indicated in the following table.

TABLE-US-00012 CP-1V-N  16.00% Clearing point: 110.1° C. PP-2-N   7.00% Δn [589 nm, 20° C.]: 0.218 PGUQU-3-F  10.00% n.sub.e [589 nm, 20° C.]: 1.737 CPG-2-F  10.00% Δε [1 kHz, 20° C.]: 11.1 PP-1-2V1  10.00% ε.sub.∥ [1 kHz, 20° C.]: 15.3 PGIGI-3-F  12.00% CPGP-5-2   8.00% CPGP-5-3   8.00% PGP-2-3   7.00% PGP-2-4   6.00% PGP-2-5   6.00% Σ 100.00%

Reference Example 6

[0390] A liquid-crystal base mixture B-6 is prepared and characterized with respect to its general physical properties, having the composition and properties as indicated in the following table.

TABLE-US-00013 PGUQU-3-F   6.00% Clearing point: 101.0° C. PGUQU-4-F  10.00% Δn [589 nm, 20° C.]: 0.194 PGUQU-5-F  10.00% n.sub.e [589 nm, 20° C.]: 1.699 PUQU-3-F  17.00% Δε [1 kHz, 20° C.]: 37.0 PGU-2-F  10.00% ε.sub.∥ [1 kHz, 20° C.]: 42.9 PGU-3-F  11.00% CPGU-3-OT   8.00% CCGU-3-F   8.00% CPU-3-F  12.00% CPGP-5-2   4.00% CPGP-5-3   4.00% Σ 100.00%

Reference Example 7

[0391] A liquid-crystal base mixture B-7 is prepared and characterized with respect to its general physical properties, having the composition and properties as indicated in the following table.

TABLE-US-00014 BCH-3F.F   8.00% Clearing point: 110.0° C. BCH-5F.F   8.00% Δn [589 nm, 20° C.]: 0.153 BCH-5F.F.F  11.00% n.sub.e [589 nm, 20° C.]: 1.650 CPU-7-F  11.00% Δε [1 kHz, 20° C.]: 7.8 PCH-3O2  18.00% ε.sub.∥ [1 kHz, 20° C.]: 11.4 PCH-7  13.00% CCGU-3-F   7.00% CBC-33F   4.00% CBC-53F   4.00% CBC-55F   3.00% CCZPC-3-3   3.00% PPTUI-3-2   5.00% PPTUI-3-4   5.00% Σ 100.00%

Reference Example 8

[0392] A liquid-crystal base mixture B-8 is prepared and characterized with respect to its general physical properties, having the composition and properties as indicated in the following table.

TABLE-US-00015 BCH-3F.F   8.00% Clearing point: 108.0° C. BCH-5F.F   8.00% Δn [589 nm, 20° C.]: 0.171 BCH-5F.F.F  11.00% n.sub.e [589 nm, 20° C.]: 1.670 CPU-7-F   9.00% Δε [1 kHz, 20° C.]: 8.1 PCH-3O2  18.00% ε.sub.∥ [1 kHz, 20° C.]: 11.6 PCH-7  13.00% CCGU-3-F   7.00% CBC-33F   4.00% CBC-53F   4.00% PPTUI-3-2   8.00% PPTUI-3-4  10.00% Σ 100.00%

Example 1

[0393] A cholesteric mixture C-1 is prepared by mixing 98.78% of mixture B-1 as described in Reference Example 1 above with 0.44% of chiral dopant R-5011 available from Merck KGaA, Darmstadt, Germany, 0.75% of compound of formula RM-A

##STR00359##

[0394] and 0.03% of compound of formula A-1

##STR00360##

[0395] The mixture C-1 is filled into an electro-optical cell having glass substrates (405 mm×400 mm) with ITO electrodes as well as polyimide alignment layers (AL-1254, TN-rubbed), wherein the cell gap is 25 μm.

[0396] The cell is preheated at 40° C. for 15 minutes.

[0397] Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (Philips iSOLde CLEO Performance 80 W with cut-off filter, 4 mW/cm.sup.2 light intensity) for 60 minutes while a DC voltage (70 V) is applied.

[0398] After the photopolymerisation the cell is subjected to a thermal treatment at 150° C. for 30 minutes where no voltage is applied.

[0399] The haze of the obtained cell is determined using haze-gard i from BYK-Gardner.

[0400] The obtained cell has a clear state which has 4.2% haze and a privacy (scattering) state with 95.8% haze.

[0401] The cell exhibits favourably homogeneous clear and scattering states and favourable switching and electro-optical performance. In addition, the cell exhibits a favourably low pressure sensitivity.

Comparative Example 1

[0402] A cholesteric mixture C-1 is prepared as described in Example 1 above. The mixture C-1 is filled into an electro-optical cell having glass substrates (405 mm×400 mm) with ITO electrodes as well as polyimide alignment layers (AL-1254, TN-rubbed), wherein the cell gap is 25 μm.

[0403] The cell is preheated at 40° C. for 15 minutes.

[0404] Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (Philips iSOLde CLEO Performance 80 W with cut-off filter, 4 mW/cm.sup.2 light intensity) for 30 minutes while a square-wave voltage (70 V (peak-to-peak), 20 Hz) is applied.

[0405] After the photopolymerisation the cell is subjected to a thermal treatment at 150° C. for 30 minutes where no voltage is applied.

[0406] The haze of the obtained cell is determined using haze-gard i from BYK-Gardner.

[0407] The obtained cell has a clear state which has 8.9% haze and a privacy (scattering) state with 96.9% haze.

[0408] The cell exhibits a comparatively larger residual haze in the clear state, discernible as a white cloudy appearance.

Example 2

[0409] A cholesteric mixture C-1 is prepared as described in Example 1 above. The mixture C-1 is filled into an electro-optical cell having glass substrates with ITO electrodes as well as polyimide alignment layers (AL-1254, TN-rubbed), wherein the cell gap is 25 μm.

[0410] The cell is preheated at 40° C. for 15 minutes.

[0411] Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (Philips iSOLde CLEO Performance 80 W with cut-off filter, 4 mW/cm.sup.2 light intensity) for 30 minutes while a DC voltage (70 V) is applied.

[0412] After the photopolymerisation the cell is subjected to a thermal treatment at 150° C. for 30 minutes where a DC voltage (70 V) is applied.

[0413] The haze of the obtained cell is determined using haze-gard i from BYK-Gardner.

[0414] The obtained cell has a clear state which has 2.2% haze and a privacy (scattering) state with 95.8% haze.

[0415] The cell exhibits favourably homogeneous clear and scattering states and favourable switching and electro-optical performance. In addition, the cell exhibits a favourably low pressure sensitivity.

Comparative Example 2

[0416] A cholesteric mixture C-1 is prepared as described in Example 1 above. The mixture C-1 is filled into an electro-optical cell having glass substrates (405 mm×400 mm) with ITO electrodes as well as polyimide alignment layers (AL-1254, TN-rubbed), wherein the cell gap is 25 μm.

[0417] The cell is preheated at 40° C. for 15 minutes.

[0418] Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (Philips iSOLde CLEO Performance 80 W with cut-off filter, 4 mW/cm.sup.2 light intensity) for 30 minutes while a square-wave voltage (70 V, 20 Hz) is applied.

[0419] After the photopolymerisation the cell is subjected to a thermal treatment at 150° C. for 30 minutes where a square-wave voltage (70 V, 20 Hz) is applied.

[0420] The haze of the obtained cell is determined using haze-gard i from BYK-Gardner.

[0421] The obtained cell has a clear state which has 6.2% haze and a privacy (scattering) state with 96.0% haze.

[0422] The cell exhibits a comparatively larger residual haze in the clear state, discernible as a white cloudy appearance.

Example 3

[0423] A cholesteric mixture C-2 is prepared by mixing 98.89% of mixture B-1 as described in Reference Example 1 above with 0.33% of chiral dopant R-5011 available from Merck KGaA, Darmstadt, Germany, 0.75% of compound of formula RM-A as shown in Example 1 above and 0.03% of compound of formula A-1 as shown in Example 1 above.

[0424] The mixture C-2 is filled into an electro-optical cell having glass substrates (triangular shape, side length 1200 mm) with ITO electrodes as well as polyimide alignment layers (AL-1254, TN-rubbed), wherein the cell gap is 25 μm.

[0425] The cell is preheated at 40° C. for 15 minutes.

[0426] Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (Philips iSOLde CLEO Performance 80 W with cut-off filter, 4 mW/cm.sup.2 light intensity) for 60 minutes while a DC voltage (70 V) is applied.

[0427] After the photopolymerisation the cell is subjected to a thermal treatment at 150° C. for 30 minutes where no voltage is applied.

[0428] The cell exhibits uniform privacy and clear states, wherein in the clear state no undesirable residual haze is discernible. In addition, the cell exhibits a favourably low pressure sensitivity.

Comparative Example 3

[0429] A cholesteric mixture C-1 is prepared as described in Example 1 above. The mixture C-1 is filled into an electro-optical cell having glass substrates (405 mm×400 mm) with ITO electrodes as well as polyimide alignment layers (AL-1254, TN-rubbed), wherein the cell gap is 25 μm.

[0430] The cell is preheated at 40° C. for 15 minutes.

[0431] Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (Philips iSOLde CLEO Performance 80 W with cut-off filter, 4 mW/cm.sup.2 light intensity) for 30 minutes while a square-wave voltage (70 V, 0.2 Hz) is applied.

[0432] After the photopolymerisation the cell is subjected to a thermal treatment at 150° C. for 30 minutes where no voltage is applied.

[0433] The obtained cell exhibits a non-uniform appearance in both the clear state and the privacy state.

Comparative Example 4

[0434] A cholesteric mixture C-1 is prepared as described in Example 1 above. The mixture C-1 is filled into an electro-optical cell having glass substrates (405 mm×400 mm) with ITO electrodes as well as polyimide alignment layers (AL-1254, TN-rubbed), wherein the cell gap is 25 μm.

[0435] The cell is preheated at 40° C. for 15 minutes.

[0436] Subsequently polymerisation is carried out at 40° C. by irradiating the cell with UV light (Philips iSOLde CLEO Performance 80 W with cut-off filter, 4 mW/cm.sup.2 light intensity) for 30 minutes while a square-wave voltage (70 V, dynamic frequency: non-linear logarithmic shape starting at 0.1 Hz and ramping up to 40 Hz over 5 minutes, then starting over again) is applied.

[0437] After the photopolymerisation the cell is subjected to a thermal treatment at 150° C. for 30 minutes where no voltage is applied.

[0438] The obtained cell exhibits a non-uniform appearance in both the clear state and the privacy state.

Example 4

[0439] A cholesteric mixture C-3 is prepared by mixing 98.89% of mixture B-2 as described in Reference Example 2 above with 0.33% of chiral dopant R-5011 available from Merck KGaA, Darmstadt, Germany, 0.75% of compound of formula RM-A as shown in Example 1 above and 0.03% of compound of formula A-1 as shown in Example 1 above.

[0440] The mixture C-3 is treated according to Example 1 to obtain an electro-optical cell.

[0441] The cell exhibits favourably homogeneous clear and scattering states and favourable switching and electro-optical performance. In addition, the cell exhibits a favourably low pressure sensitivity.

Example 5

[0442] A cholesteric mixture C-4 is prepared by mixing 98.64% of mixture B-1 as described in Reference Example 1 above with 0.42% of chiral dopant R-5011 available from Merck KGaA, Darmstadt, Germany, 0.45% of compound of formula RM-B

##STR00361##

[0443] 0.45% of compound of formula RM-C

##STR00362##

[0444] and 0.04% of the photoinitiator Irgacure® 651 (in the following abbreviated as IRG-651)

##STR00363##

available from Ciba, Switzerland.

[0445] The mixture C-4 is treated according to Example 1 to obtain an electro-optical cell.

[0446] The cell exhibits favourably homogeneous clear and scattering states and favourable switching and electro-optical performance. In addition, the cell exhibits a favourably low pressure sensitivity.

Example 6

[0447] A cholesteric mixture C-5 is prepared by mixing mixture B-3 as described in Reference Example 3 above with chiral dopant S-1011 available from Merck KGaA, Darmstadt, Germany such that a pitch of 2 μm is obtained, wherein 99.25% of this mixture is further mixed with 0.75% of compound of formula RM-A as shown in Example 1 above to obtain mixture C-5.

[0448] The mixture C-5 is further treated as described in Example 1.

[0449] The cell exhibits favourable clear and scattering states and favourable switching and electro-optical performance. In addition, the cell exhibits a favourably low pressure sensitivity.

Example 7

[0450] A cholesteric mixture C-6 is prepared by mixing 98.28% of mixture B-4 as described in Reference Example 4 with 0.42% of chiral dopant R-5011 available from Merck KGaA, Darmstadt, Germany, 0.63% of compound of formula RM-B as shown in Example 5 above, 0.63% of compound of formula RM-C as shown in Example 5 above and 0.04% of IRG-651.

[0451] The mixture C-6 is further treated as described in Example 1.

[0452] The cell exhibits favourable clear and scattering states and favourable switching and electro-optical performance. In addition, the cell exhibits a favourably low pressure sensitivity.

Examples 8 to 11

[0453] Cholesteric mixtures C-7, C-8, C-9 and C-10 are prepared as described for C-1 in Example 1 above, wherein instead of B-1 respectively the mixtures B-5, B-6, B-7 and B-8 as described in Reference Examples 5, 6, 7 and 8 are used.

[0454] The mixtures C-7, C-8, C-9 and C-10 are further treated as described in Example 2.

[0455] The cells exhibit favourable clear and scattering states and favourable switching and electro-optical performance. In addition, the cells exhibit a favourably low pressure sensitivity.

Example 12

[0456] A cholesteric mixture C-11 is prepared by mixing 98.60% of mixture B-2 as described in Reference Example 2 above with 0.65% of chiral dopant R-5011 available from Merck KGaA, Darmstadt, Germany and 0.75% of compound of formula RM-A as shown in Example 1 above.

[0457] The mixture C-11 is treated according to Example 1 to obtain an electro-optical cell.

[0458] The cell exhibits favourably homogeneous clear and scattering states and favourable switching and electro-optical performance. In addition, the cell exhibits a favourably low pressure sensitivity.