Method for preparing a liquid crystal-based switching element
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.
1. A method for the preparation of a window element which is operable in and electrically switchable between an optically clear state and a scattering state, the method 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;
(ii) polymerising the one or more polymerisable mesogenic compounds via photopolymerisation the presence of a direct current electric field in the layer, wherein the direct current electric field has a DC voltage in a range from 30 V to 150 V for a time period of 1 minute to 240 minutes, further comprising polymerising the one or more polymerisable mesogenic compounds in the presence of a direct current electric field in the layer, the applied direct current electric field inducing a homeotropic alignment in the layer comprising the liquid-crystalline medium; and
after performing step (ii), (iii) performing a thermal treatment of the liquid-crystalline medium in the presence of the direct current electric field.
2. The method according to claim 1 , wherein the layer comprising the liquid-crystalline medium preferably has a thickness in the range from 4 μm to 40 μm.
3. The method according to claim 1 , wherein the liquid-crystalline medium further comprises one or more photoinitiators.
4. 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.
5. The method according to claim 1 , wherein polymerising as set forth in step (ii) uses light having an intensity in the range of from 0.1 mW/cm 2 to 100 mW/cm 2 .
6. 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.
7. 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
wherein
R 1 and R 2 denote, independently of one another, a group selected from F, Cl, CF 3 , OCF 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 3 or mono- or polysubstituted by halogen and wherein one or more CH 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 11 denotes
n denotes 0 or 1, and
A 21 , A 31 and A 41 denote, independently of one another,
wherein L is on each occurrence, identically or differently, halogen selected from F, Cl and Br.
8. 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
wherein
R 3 , R 4 , R 5 and R 6 denote, independently of one another, a group selected from F, CF 3 , OCF 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 3 or mono- or polysubstituted by halogen and wherein one or more CH 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 1 , L 2 , L 3 , L 4 and L 5 denote, independently of one another, H or F.
9. 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-1 or more.
10. 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.
11. The method of claim 1 , wherein the photopolymerisation uses UV light.
12. The method of claim 5 , wherein the light comprises UV light.
13. The method of claim 6 , wherein the transparent conductive layers are respectively embedded between two transparent dielectric layers.