WO2006039980A1

WO2006039980A1 - Polymerisable liquid crystal mixture

Info

Publication number: WO2006039980A1
Application number: PCT/EP2005/010073
Authority: WO
Inventors: Richard Harding; Matthew Francis
Original assignee: Merck Patent Gmbh
Priority date: 2004-10-14
Filing date: 2005-09-19
Publication date: 2006-04-20
Also published as: TWI498416B; JP2008517077A; TW200619362A; KR20070064341A; JP5255278B2; KR101356857B1

Abstract

The invention relates to a polymerisable mixture, its use for preparing polymer films with improved adhesion to a substrate, methods for preparing such polymer films, multilayer films comprising such polymer films, and to the use of the mixture and films for optical, electrooptical, decorative or security devices and applications.

Description

Polymerisable Liquid Crystal Mixture

Field of Invention

Background and Prior Art

Polymerisable liquid crystal (LC) materials are commonly used for the preparation of optical films in liquid crystal displays. These materials usually contain a certain amount of compounds with two or more polymerisable groups (di- or multifunctional), which are crosslinked to give a hard film. However, these hard films often do not readily adhere to the surface of the plastic substrates commonly used in the manufacturing process, because the polymerisation and crosslinking process causes the film to shrink. Films made from such LC materials are therefore usually delaminated and re-attached to another substrate, such as glass or the plastic of a polariser, for use in a liquid crystal display. However, the processes of delamination and application to an alternative substrate are costly in time and materials. It also gives two potential points at which loss of film may occur by damage during the delamination or relamination, leading to a lower overall yield of product.

In prior art it has also been reported to use adhesion and aligning layers to bond an LC polymer film to plastic substrates. For example, US 5,631 ,051 discloses a method of preparing an optical compensation sheet on a transparent substrate of triacetyl cellullose (TAC), by first providing an adhesion layer of gelatine on the TAC film. Then an aligning layer is formed by coating a solution of denaturated polyvinyl alcohol (PVA), which was chemically modified by addition of polymerizable groups, onto the gelatine layer, evaporating the solvent and rubbing the surface of the polymerised PVA layer unidirectionally, Finally an optically anisotropic layer comprising discotic LC material is coated onto the rubbed surface of the PVA layer and polymerised.

US 5,747,121 discloses a method of preparing an optical compensation sheet by coating a solution of denaturated polyvinyl alcohol (PVA), which was chemically modified by addition of polymerizable groups, onto a transparent substrate, evaporating the solvent and rubbing the surface of the PVA layer unidirectionally.

Then an optically anisotropic layer comprising discotic LC material is coated onto the rubbed surface of the PVA layer and polymerised. Afterwards the film is subjected to heat treatment whereby the PVA layer and the discotic LC layer are reported to be chemically bonded to each other via free, crosslinkable groups.

However, the above methods require many separate coating steps. Furthermore, the use of several intermediate layers, such as adhesion or aligning layers, comprising isotropic materials like gelatine or PVA can negatively influence the optical performance of the optical film.

GB 2 398 077 discloses a method of providing a crosslinked LC film with good adhesion by using a polymerisable LC material comprising not more than 7 % by weight of crosslinkable compounds having two or more polymerisable groups. Such a film does better adhere to plastic substrates as commonly used in the optical films industry. However, since the low crosslinked LC film is soft and can be sensitive against mechanical stress, it is preferably covered by a hardcoat or by another polymerised LC film comprising a high degree of crosslinking.

Therefore, there is a need for an advantageous method to provide a film of crosslinked LC material that does well adhere to a plastic substrate, whilst saving processing time and allowing to reduce losses due to processing damage of the LC film, and wherein the film has a harder surface and higher stability against mechanical stress or damage.

It was an aim of the present invention to provide crosslinked LC films, and methods and materials for their preparation, which do not have the drawbacks mentioned above. Other aims of the invention are immediately evident to the expert from the following description.

The above-mentioned aims were solved by providing films, methods and materials according to the present invention as described above and below.

Definition of Terms

The term "film" includes rigid or flexible, self-supporting or free¬ standing films with mechanical stability, as well as coatings or layers on a supporting substrate or between two substrates.

The term "liquid crystal or mesogenic material" or "liquid crystal or mesogenic compound" means materials or compounds comprising one or more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal (LC) phase behaviour. LC compounds with rod-shaped or board-shaped groups are also known in the art as "calamitic" liquid crystals. LC compounds with a disk-shaped group are also known in the art as "discotic" liquid crystals. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit an LC phase themselves. It is also possible that they show LC phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerized.

For the sake of simplicity, the term "liquid crystal material" is used hereinafter for both mesogenic and LC materials.

Polymerizable compounds with one polymerizable group are also referred to as "monoreactive" compounds, compounds with two polymerizable groups as "direactive" compounds, and compounds with more than two polymerizable groups as "multireactive" compounds. Compounds without a polymerizable group are also referred to as "non-reactive" compounds. Multireactive compounds are also referred to as "crosslinkable" compounds.

The term "reactive mesogen" (RM) means a polymerizable mesogenic or liquid crystal compound.

Unless stated otherwise, in the following the percentages by weight (wt.%) of individual solid or liquid crystalline compounds in a polymerisable liquid crystal material relate to the total amount of solid or liquid crystalline compounds in said material.

The term "director" means the preferred orientation direction of the long molecular axes (in case of calamitic compounds) or short molecular axis (in case of discotic compounds) of the mesogenic groups in an LC material.

In films comprising uniaxially positive birefringent LC material the optical axis is given by the director.

The term "cholesteric structure" or "helically twisted structure" refers to a film comprising LC molecules wherein the director is parallel to the film plane and is helically twisted around an axis perpendicular to the film plane.

The term "homeotropic structure" or "homeotropic orientation" refers to a film wherein the optical axis is substantially perpendicular to the film plane.

The term "planar structure" or "planar orientation" refers to a film wherein the optical axis is substantially parallel to the film plane. The term "tilted structure" or "tilted orientation" refers to a film wherein the optical axis is tilted at an angle θ between 0 and 90° relative to the film plane.

The term "splayed structure" or "splayed orientation" means a tilted orientation as defined above, wherein the tilt angle varies in the direction perpendicular to the film plane, preferably from a minimum to a maximum value.

Summary of the Invention

The invention relates to a mixture comprising a) > 0 to 12 % of one or more crosslinkable compounds, preferably crosslinkable mesogenic compounds, and b) > 50 % of one or more compounds of formula I

wherein

P is a polymerisable group,

Sp is a spacer group or a single bond,

A is an aromatic or aliphatic 5- or 6-ring, or a group comprising two or three fused aromatic or aliphatic 5- or 6-rings, these rings optionally containing one or more hetero atoms selected from N, O and S, and optionally being substituted by one or more identical or different groups L¹,

Z is in case of multiple occurrence independently of one another -O-, -S-, -CO-, -COO-, -OCO-, -S-CO-, -CO-

S-, -O-CO-O-, -CO-NR⁰-, -NR⁰-CO-, -OCH₂-, -CH₂O-, -SCH₂-, -CH₂S-, -CF₂O-, -OCF₂-, -CF₂S-, -SCF₂-_, - CH₂CH₂-, -CF₂CH₂-, -CH₂CF₂-, -CF₂CF₂-, -CH=CR -, - CH=CH-, -CH=CF-, -CY¹=CY¹-, -OC-, -CH=CH- COO-, -OCO-CH=CH- or a single bond,

Z¹ has one of the meanings of Z,

R⁰ and R⁰⁰ are independently of each other H or alkyl with 1 to 12

C-atoms,

Y¹ and Y² are independently of each other H, F, Cl or CN,

m is O or 1 ,

L¹, L² are independently of each other F, Cl, Br, I, CN, NO₂,

P-Sp- or alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonlyoxy or alkoxycarbonyloxy with 1 to 12 C atoms, wherein one or more H atoms are optionally replaced by F or Cl,

r1 , r2 are independently of each other O, 1 , 2, 3 or 4.

The invention further relates to a polymer film obtainable by crosslinking a mixture according to the present invention.

The invention further relates to the use of a mixture or film according to the present invention in optical, electrooptical, information storage, decorative and security applications.

The invention further relates to an optical component or device comprising a mixture or film according to the present invention.

The invention further relates to a liquid crystal display comprising a mixture or film according to the present invention. The invention further relates to an authentification, verification or security marking or a coloured image comprising a mixture or film according to the present invention.

The invention further relates to an object or document of value comprising an authentification, verification or security marking or an image as described above and below.

Detailed Description of the Invention

In the formulae above and below, A is preferably selected from 1 ,4- phenylene that is optionally substituted by 1 to 4 groups L¹ as defined above, or trans-1 ,4-cyclohexylene.

Z¹ is preferably selected from -COO-, -OCO-, -CH₂CH₂-, -CH=CH- or a single bond, especially preferably -COO- or -OCO-.

Z is preferably selected from -COO-, -OCO-, -CH₂CH₂-, -CH=CH- or a single bond.

L¹ and L² are preferably selected from F, Cl, CN, NO₂, CH₃, C₂H₅, OCH₃, OC₂H₅, COCH₃, COC₂H₅, COOCH₃, COOC₂H₅, CF₃, OCF₃, OCHF₂ or OC₂F₅, in particular F, Cl, CN, CH₃, C₂H₅, OCH₃, COCH₃ or OCF₃, most preferably F, Cl, CH₃, OCH₃ or COCH₃.

r1 and r2 are preferably O, 1 or 2.

Substituted phenylene is preferably selected from

-

with L having one of the meanings of L¹ of formula I.

Halogen is preferably F or Cl. Y¹ and Y² are preferably H or F.

The reactive or polymerisable group P is a group that is capable of participating in a polymerisation reaction, like radicalic or ionic chain polymerisation, polyaddition or polycondensation, or capable of being grafted, for example by condensation or addition, to a polymer backbone in a polymeranaloguous reaction. Especially preferred are polymerisable groups for chain polymerisation reactions, like radicalic, cationic or anionic polymerisation. Very preferred are polymerisable groups comprising a C-C double or triple bond, and polymerisable groups capable of polymerisation by a ring-opening reaction, like oxetanes or epoxides.

Very preferably the polymerisable or reactive group P is selected from

CH₂=CW¹-COO-, W²HC-CH - , W² (^CH ₂)_k-O- , CH₂=CW²- (O)_k1, CH₃-CH=CH-O-, (CH₂=CH)₂CH-OCO-, (CH₂=CH-CH₂)₂CH- OCO-, (CH₂=CH)₂CH-O-, (CH₂=CH-CH₂)₂N-, (CH₂=CH-CH₂)₂N-CO-, HO-CW²W³-, HS-CW²W³-, HW²N-, HO-CW²W³-NH-, CH₂=CW¹ -CO- NH-, CH₂=CH-(COO)_k1-Phe-(O)_k2-, Phe-CH=CH-, HOOC-, OCN-, and W⁴W⁵W⁶Si-, with W¹ being H, Cl, CN, phenyl or alkyl with 1 to 5 C- atoms, in particular H, Cl or CH₃, W² and W³ being independently of each other H or alkyl with 1 to 5 C-atoms, in particular methyl, ethyl or n-propyl, W⁴, W⁵ and W⁶ being independently of each other Cl, oxaalkyl or oxacarbonylalkyl with 1 to 5 C-atoms, Phe being 1 ,4- phenylene that is optionally substituted by one or more groups L as defined above, and k-i and k₂ being independently of each other O or 1.

Especially preferably P is a vinyl group, an acrylate group, a methacrylate group, an oxetane group or an epoxy group, especially preferably an acrylate or methacrylate group.

As for the spacer group Sp all groups can be used that are known for this purpose to the skilled in the art. The spacer group Sp is preferably of formula Sp'-X, such that P-Sp- is P-Sp'-X-, wherein Sp¹ is alkylene with 1 to 20 C atoms, preferably 1 to 12 C-atoms, which is optionally mono- or polysubstituted by F, Cl, Br₁ I or CN, and wherein one or more non-adjacent CH₂ groups are optionally replaced, in each case independently from one another, by -O-, -S-, -NH-, -NR⁰-, -SiR⁰R⁰⁰-, -CO-, -COO-, - OCO-, -OCO-O-, -S-CO-, -CO-S-, -NR⁰-CO-O-, -O-CO-NR⁰-, - NR⁰-CO-NR⁰-, -CH=CH- or -C≡C- in such a manner that O and/or S atoms are not linked directly to one another,

X is -O-, -S-, -CO-, -COO-, -OCO-, -O-COO-, -CO-NR⁰-, -NR⁰-CO- , -NR⁰-CO-NR⁰-, -OCH₂-, -CH₂O-, -SCH₂-, -CH₂S-, -CF₂O-, - OCF₂-, -CF₂S-, -SCF₂-, -CF₂CH₂-, -CH₂CF₂-, -CF₂CF₂-, -CH=N-, -N=CH-, -N=N-, -CH=CR⁰-, -CY¹=CY²-, -C≡C-, -CH=CH-COO-, - OCO-CH=CH- or a single bond,

R⁰ and R⁰⁰ are independently of each other H or alkyl with 1 to 12 C-atoms, and

Y¹ and Y² are independently of each other H, F, Cl or CN.

X is preferably -O-, -S-, -OCH₂-, -CH₂O-, -SCH₂-, -CH₂S-, -CF₂O-, - OCF₂-, -CF₂S-, -SCF₂-_, -CH₂CH₂-, -CF₂CH₂-, -CH₂CF₂-, -CF₂CF₂-, - CH=N-, -N=CH-, -N=N-, -CH=CR⁰-, -CY¹=CY²-, -C≡C- or a single bond, in particular -O-, -S-, -C≡C-, -CY¹=CY²- or a single bond, very preferably a group that is able to from a conjugated system, such as -C≡C- or -CY¹=CY²-, or a single bond.

Typical groups Sp¹ are, for example, -(CH₂)_P-, -(CH₂CH₂O)_q -CH₂CH₂-, - CH₂CH₂-S-CH₂CH₂- or -CH₂CH₂-NH-CH₂CH₂- or -(SiR⁰R⁰⁰-O)_p-, with p being an integer from 2 to 12, q being an integer from 1 to 3 and R⁰ and R⁰⁰ having the meanings given above.

Preferred groups Sp' are ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, octadecylene, ethyleneoxyethylene, methyleneoxybutylene, ethylene-thioethylene, ethylene-N-methyl- iminoethylene, 1 -methylalkylene, ethenylene, propenylene and butenylene for example.

Further preferred are compounds wherein Sp is a single bond.

Preferred mixtures comprise

- one or more compounds of formula Ia

wherein P, Sp, L¹, L², r1 and r2 have the meanings given in formula I,

- one or more compounds of formula I or Ia wherein r1 and r2 are 0,

- one or more compounds of formula I or Ia wherein r1 and r2 are 1 ,

- one or more compounds of formula I or Ia wherein one of r1 and r2 is 0 and the other is 1 ,

- one or more crosslinkable mesogenic compounds of formula Il

wherein P and Sp, L¹, L², r1 and r2 are as defined in formula I₁ L³ has one of the meanings of L¹ in formula I, r3 is 0, 1 , 2, 3 or 4, and Z¹ and Z² have one of the meanings of Z in formula I, - one or more compounds of formula Il wherein r2 and r3 are 0 and r1 is 0, 1 or 2,

- one or more compounds of formula Il wherein Z¹ and Z² are selected from -COO-, -OCO-, -CH₂CH₂- and a single bond,

- one or more crosslinkable mesogenic compounds of formula Ha

wherein P, Sp, L¹ and r1 are as defined in formula II, r1 is preferably O or 1 and L¹ is preferably Cl, CH₃ or OCH₃.

- additional component c) comprising one or more compounds of formula III

wherein P and Sp are as defined in formula I and R is alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonlyoxy or alkoxycarbonyloxy with 1 to 12 C atoms, wherein one or more H atoms are optionally replaced by F or Cl,

- 5 to 12 % of one or more crosslinkable compounds,

- > 7 to 12 % of one or more crosslinkable compounds,

- 50 to 80 %, preferably 55 to 75 % of one or more compounds of formula I or Ia,

- 5 to 50 %, preferably 10 to 40 % of one or more compounds of formula III, - a) > 0 to 12 % of one or more crosslinkable compounds, b) > 50 % of one or more compounds of formula I or Ia, c) optionally one or more compounds of formula III, d) one or more photoinitiators, preferably in a concentration from

0.5 to 8 %, e) optionally one or more surfactants, preferably in a concentration from 0.1 to 2 %, f) optionally one or more stabilizers.

The clearing point of the polymerisable LC mixture is preferably at least 50°C, very preferably at least 6O°C.

The invention also relates to a method of preparing a polymer film by

- providing a layer of a mixture according to the present invention onto a substrate,

- optionally aligning the mixture so that the mesogenic or LC compounds adopt a uniform orientation, - crosslinking the mixture to form a polymer film, and

- optionally removing the polymer film from the substrate.

The thickness of the polymer film according to the present invention is preferably from 0.5 to 5 μm, very preferably from 1 to 2 μm.

The LC molecules in the polymer film preferably have planar orientation.

A polymer film according to the present invention has limited crosslink density due to the low amount of crosslinkable compounds, and shows good adhesion in particular to plastic substrates. It can therefore can be used as adhesive or base coating for subsequent LC layers which otherwise would not well adhere to the substrates. At the same time, the polymer film has a harder surface which is less tacky, and shows improved durability compared to low crosslinked films from prior art. A polymer film according to the present invention can also promote alignment in LC layers that are coated on top of it. It can therefore be used as alignment layer for LC materials. By variations in the thickness of the polymer film it is possible to influence the orientation, in particular the tilt angle, of subsequent LC layers. For example, a polymer film according to the invention with planar orientation can be used to induce planar orientation in a subsequent LC layer. A polymer film according to the invention with tilted or splayed orientation can be used to induce tilted or splayed orientation in a subsequent LC layer.

Thus, another preferred embodiment of the present invention relates to a multilayer comprising as base layer a low crosslinked polymer film according to the present invention, and further comprising one or more further polymerised or crosslinked LC films. Either the base polymer film or the additional LC polymer film or both films can act for example as optical layer.

The invention further relates to a method of preparing a multilayer by

- providing a layer of a polymerisable LC mixture according to the present invention onto a substrate,

- optionally aligning the mixture so that the mesogenic or LC compounds adopt a uniform orientation,

- polymerising said LC mixture to form a polymer film,

- providing a layer of a second polymerisable LC material onto the free surface of the polymer film, - optionally aligning said second LC material so that the mesogenic or LC compounds adopt a uniform orientation, and

- polymerising said second LC material to form a second polymer film.

The second polymer film has for example planar, tilted or splayed orientation. The polymerizable LC mixture according to the present invention comprises at least one polymerizable compound having one polymerizable group (monoreactive) and at least one polymerizable compound having two or more polymerizable groups (di- or multireactive).

The polymerizable LC mixture can also comprise one or more chiral compounds, which can in addition be polymerizable and/or mesogenic or liquid crystalline.

The polymerizable LC mixture preferably has a nematic or smectic phase or a cholesteric phase, very preferably a nematic phase.

The polymerizable mesogenic or LC compounds are preferably monomers, very preferably calamitic monomers. These materials typically have good optical properties, like reduced chromaticity, and can be easily and quickly aligned into the desired orientation, which is especially important for the industrial production of polymer films at large scale.

Polymerisable mesogenic mono-, di- and multireactive compounds suitable for the present invention can be prepared by methods which are known per se and which are described in standard works of organic chemistry like for example Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart.

The polymerisable LC mixture according to the present invention may comprise further chiral or achiral polymerizable mesogenic or LC compounds. Suitable compounds of this type are disclosed for example in WO 93/22397, EP 0 261 712, DE 195 04224, WO 95/22586, WO 97/00600, US 5,518,652, US 5,750,051 , US 5,770,107 and US 6,514,578.

Examples of especially useful and preferred polymerizable mesogenic or LC compounds are shown in the following list.

wherein

P is a polymerizable group, preferably an acryl, methacryl, vinyl, vinyloxy, propenyl ether, epoxy or styrene group, x and y are identical or different integers from 1 to 12 ,

A and D are 1 ,4-phenylene that is optionally mono- di or trisubstituted by L¹ or 1 ,4-cyclohexylene, u and v are independently of each other O or 1 ,

Z⁰ is -COO-, -OCO-, -CH₂CH₂- or a single bond,

Y is F, Cl, CN, NO₂, OCH₃, OCN, SCN, optionally fluorinated alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy with 1 to 4 C atoms, or mono- oligo- or polyfluorinated alkyl or alkoxy with 1 to 4 C atoms, R⁰ is alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy with 1 or more, preferably 1 to 12 C atoms which is optionally fluorinated,

Ter is a terpenoid radical like for example menthyl,

Choi is cholesteryl,

L¹ and L² are independently of each other H, F, Cl, CN or optionally halogenated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy with 1 to 5 C atoms.

Suitable chiral compounds are those shown in the list above, furthermore commercially available chiral dopants like R- or S-811 , R- or S-101 1 , R- or S-2011 , R- or S-301 1 , R- or S-4011 , R- or S- 5011 , or CB 15 (all from Merck KGaA, Darmstadt, Germany). Very preferred are chiral compounds with a high helical twisting power (HTP), in particular compounds comprising a sorbitol group as described in WO 98/00428, compounds comprising a hydrobenzoin group as described in GB 2,328,207, chiral binaphthyl derivatives as described in WO 02/94805, chiral binaphthol acetal derivatives as described in WO 02/34739, chiral TADDOL derivatives as described in WO 02/06265, and chiral compounds having at least one fluorinated linkage group and a terminal or central chiral group as described in WO 02/06196 and WO 02/06195.

Unless stated otherwise, the general preparation of polymer LC films according to this invention can be carried out according to standard methods known from the literature. Typically a polymerizable LC material is coated or otherwise applied onto a substrate where it aligns into uniform orientation, and polymerized in situ in its LC phase for example by exposure to heat or actinic radiation, preferably by photo- polymerization, very preferably by UV-photopolymerization, to fix the alignment of the LC molecules. If necessary, uniform alignment can promoted by additional means like shearing the LC material, surface treatment of the substrate, or adding surfactants to the LC material. As substrate for example glass or quarz sheets or plastic films can be used. It is also possible to put a second substrate on top of the coated material prior to and/or during and/or after polymerization. The substrates can be removed after polymerization or not. When using two substrates in case of curing by actinic radiation, at least one substrate has to be transmissive for the actinic radiation used for the polymerisation. Isotropic or birefringent substrates can be used. In case the substrate is not removed from the polymerized film after polymerisation, preferably isotropic substrates are used.

Suitable and preferred plastic substrates are for example films of polyester such as polyethyleneterephthalate (PET) or polyethylene- naphthalate (PEN), polyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC), very preferably PET or TAC films. As birefringent substrates for example uniaxially stretched plastics film can be used. PET films are commercially available for example from DuPont Teijin Films under the trade name Melinex ®.

The polymerizable material can be applied onto the substrate by conventional coating techniques like spin-coating or blade coating. It can also be applied to the substrate by conventional printing techniques which are known to the expert, like for example screen printing, offset printing, reel-to-reel printing, letter press printing, gravure printing, rotogravure printing, flexographic printing, intaglio printing, pad printing, heat-seal printing, ink-jet printing or printing by means of a stamp or printing plate.

It is also possible to dissolve the polymerizable material in a suitable solvent. This solution is then coated or printed onto the substrate, for example by spin-coating or printing or other known techniques, and the solvent is evaporated off before polymerization. In most cases it is suitable to heat the mixture in order to facilitate the evaporation of the solvent. As solvents for example standard organic solvents can be used. The solvents can be selected for example from ketones such as acetone, methyl ethyl ketone, methyl propyl ketone or cyclohexanone; acetates such as methyl, ethyl or butyl acetate or methyl acetoacetate; alcohols such as methanol, ethanol or isopropyl alcohol; aromatic solvents such as toluene or xylene; halogenated hydrocarbons such as di- or trichloromethane; glycols or their esters such as PGMEA (propyl glycol monomethyl ether acetate), D- butyrolactone, and the like. It is also possible to use binary, ternary or higher mixtures of the above solvents.

The initial alignment (e.g. planar alignment) of the polymerizable LC material can be achieved for example by rubbing treatment of the substrate, by shearing the material during or after coating, by application of an alignment layer, by applying a magnetic or electric field to the coated material, or by the addition of surface-active compounds to the material. Reviews of alignment techniques are given for example by I. Sage in Thermotropic Liquid Crystals", edited by G. W. Gray, John Wiley & Sons, 1987, pages 75-77; and by T. Uchida and H. Seki in "Liquid Crystals - Applications and Uses Vol. 3", edited by B. Bahadur, World Scientific Publishing, Singapore 1992, pages 1 -63. A review of alignment materials and techniques is given by J. Cognard, MoI. Cryst. Liq. Cryst. 78, Supplement 1 (1981), pages 1 -77.

Especially preferred is a polymerizable material comprising one or more surfactants that promote a specific surface alignment of the LC molecules. Suitable surfactants are described for example in J. Cognard, MoI. Cryst. Liq. Cryst. 78, Supplement 1 , 1-77 (1981).

Preferred aligning agents for planar alignment are for example non- ionic surfactants, preferably fluorocarbon surfactants such as the commercially available Fluorad FC-171® (from 3M Co.) or Zonyl FSN ® (from DuPont), the surfactants described in GB 2 383 040 or polymerizable surfactants as described in EP 1 256 617.

It is also possible to apply an alignment layer onto the substrate and provide the polymerizable material onto this alignment layer. Suitable alignment layers are known in the art, like for example rubbed polyimide or alignment layers prepared by photoalignment as described in US 5,602,661 , US 5,389,698 or US 6,717,644. Polymerization is achieved for example by exposing the polymerizable material to heat or actinic radiation. Actinic radiation means irradiation with light, like UV light, IR light or visible light, irradiation with X-rays or gamma rays or irradiation with high energy particles, such as ions or electrons. Preferably polymerization is carried out by UV irradiation. As a source for actinic radiation for example a single UV lamp or a set of UV lamps can be used. When using a high lamp power the curing time can be reduced. Another possible source for actinic radiation is a laser, like for example a UV, IR or visible laser.

Polymerization is preferably carried out in the presence of an initiator absorbing at the wavelength of the actinic radiation. For example, when polymerizing by means of UV light, a photoinitiator can be used that decomposes under UV irradiation to produce free radicals or ions that start the polymerization reaction. For polymerizing acrylate or methacrylate groups preferably a radical photoinitiator is used. For polymerizing vinyl, epoxide or oxetane groups preferably a cationic photoinitiator is used. It is also possible to use a thermal polymerization initiator that decomposes when heated to produce free radicals or ions that start the polymerization. Typical radicalic photoinitiators are for example lrgacure 907, lrgacure 651 , lrgacure 184, Darocure 1173 or Darocure 4205 (Ciba Geigy AG), a typical cationic photoinitiators is for example UVI 6974 (Union Carbide).

The curing time is dependent, inter alia, on the reactivity of the polymerizable material, the thickness of the coated layer, the type of polymerization initiator and the power of the UV lamp. The curing time is preferably < 5 minutes, very preferably < 3 minutes, most preferably < 1 minute. For mass production short curing times of < 30 seconds are preferred.

The polymerizable material may also comprise one or more dyes having an absorption maximum adjusted to the wavelength of the radiation used for polymerization, in particular UV dyes like e.g. 4,4"- azoxy anisole or Tinuvin ® dyes (from Ciba AG, Basel, Switzerland). In another preferred embodiment the polymerizable material comprises one or more monoreactive polymerizable non-mesogenic compounds, preferably in an amount of 0 to 50 %, very preferably 0 to 20 %. Typical examples are alkylacrylates or alkylmethacrylates.

It is also possible to add one or more chain transfer agents to the polymerizable material in order to modify the physical properties of the polymer film. Especially preferred are thiol compounds, for example monofunctional thiols like dodecane thiol or multifunctional thiols like trimethylpropane tri(3-mercaptopropionate). Very preferred are mesogenic or LC thiols as disclosed for example in WO 96/12209, WO 96/25470 or US 6,420,001. By using chain transfer agents the length of the free polymer chains and/or the length of the polymer chains between two crosslinks in the polymer film can be controlled. When the amount of the chain transfer agent is increased, the polymer chain length in the polymer film decreases.

The polymerizable material can additionally comprise one or more additional components like for example catalysts, sensitizers, stabilizers, inhibitors, chain-transfer agents, co-reacting monomers, surface-active compounds, lubricating agents, wetting agents, dispersing agents, hydrophobing agents, adhesive agents, flow improvers, defoaming agents, deaerators, diluents, reactive diluents, auxiliaries, colourants, dyes or pigments.

The polymer films of the present invention can be used as alignment layers, retardation or compensation films in conventional LC displays, for example displays with vertical alignment like the DAP (deformation of aligned phases), ECB (electrically controlled birefringence), CSH (colour super homeotropic), VA (vertically aligned), VAN or VAC (vertically aligned nematic or cholesteric), MVA (multi-domain vertically aligned) or PVA (patterned vertically aligned) mode; displays with bend or hybrid alignment like the OCB (optically compensated bend cell or optically compensated birefringence), R-OCB (reflective OCB), HAN (hybrid aligned nematic) or pi-cell (π-cell) mode; displays with twisted alignment like the TN (twisted nematic), HTN (highly twisted nematic), STN (super twisted nematic), AMD-TN (active matrix driven TN) mode; displays of the IPS (in plane switching) mode, or displays with switching in an optically isotropic phase like those described in WO 02/93244.

In the foregoing and the following, all temperatures are given in degrees Celsius, and all percentages are by weight, unless stated otherwise. The following abbreviations are used to illustrate the LC phase behaviour: C, K = crystalline; N = nematic; S = smectic; N*, Ch = chiral nematic or cholesteric; I = isotropic. The numbers between these symbols indicate the phase transition temperatures in degree Celsius. Furthermore, m.p. is the melting point and c.p. is the clearing point (in °C).

The examples below shall illustrate the invention without limiting it.

Example 1

Polymerisable LC mixtures are formulated from compounds (1)-(8) in different concentrations as shown in Table 1.

(1) = crosslinkable (direactive) mesogenic compound (2), (4) = monoreactive mesogenic compound (3), (5) = monoreactive mesogenic compound of formula (I)

(6) = lrgacure 907 ® (photoinitiator, from Ciba AG)

(7) = Irganox 1076 ® (stabilizer, from Ciba AG)

(8) = Fluorad FC171 ® (surfactant, from 3M)

(1 )

(2)

(3)

(4)

LC polymer films with planar alignment are prepared from these mixtures by dissolving them in xylene or toluene at a concentration of 30% or 50%. The solutions are bar-coated onto a substrate using a wire wound bar to deposit approx. 4 μm wet film thickness (bar #0 from RK Coating, England). Immediately after coating, the samples are annealed at 60°C for 30s, and polymerised using a medium pressure mercury lamp at a power of 20mW/cm² for 60s in an air environment. The substrate is triacetyl cellulose (TAC) from LoFo Germany. Before coating the TAC film is rubbed with a velvet cloth to provide planar alignment for the polymerisable LC mixture.

The pencil hardness of the surface of the polymer films is measured according to ASTM D3363-00. The extent of cure (EoC) is defined as the percentage of the acrylate groups that have reacted and is measured by FTIR taking the ratio of the area under the acrylate peak (at 810 cm^-1) before and after polymerisation. The adhesion of the polymer films to TAC substrates is measured using the Crosshatch method (of 25 squares) with both Scotch 610 (3M) ("Tape 1 ") and Sekisui no. 252 tapes ("Tape 2") averaged for at least 3 tests. The Crosshatch method involves scoring the film (i.e. cutting to the substrate) with a device that creates grids of 25 squares, to which the adhesive tape is then applied and pressed on before removal. The number of the squares remaining for each grid are then measured and then typically quoted in an overall percentage for a given film. This methos is described in more detail in the British Standard IS 2409:1992(E).

The composition and clearing point of the polymerisable LC mixtures, and the EoC, pencil hardness and adhesion of the polymer films P1- P12 prepared thereof are shown in Table 1 below.

Table 1 for 50% solutions

Table 1 (continued)

Mixtures 1-5 comprise low amounts of crosslinking compounds and of monoreactive compounds of formula I. The resulting polymer films P1-P5 have partially good adhesion to TAC, but after polymerisation still have a tacky surface. Such films tend to stick to itself for example when wound up on a roller, as occurs e.g. in a production process when preparing optical films or multilayer components, thus causing damage to the film.

Mixture 6 comprises higher amounts of monoreactive compounds of formula I and more than 12 % crosslinkable compound. Resulting polymer film P6 has a harder surface but a low adhesion to TAC.

Mixtures 7-12 according to the present invention comprise up to 12 % of crosslinking compounds and high amounts of monoreactive compounds of formula I. The resulting polymer films P7-P12 have good or sufficient adhesion to TAC, and at the same time have a harder surface which is less tacky and more stable against mechanical stress, for example when being further processed or used for preparing optical multilayers.

Claims

Patent Claims

1. Mixture comprising a) > 0 to 12 % of one or more crosslinkable compounds, and b) > 50 % of one or more compounds of formula I

wherein

P is a polymerisable group,

Sp is a spacer group or a single bond,

Z is in case of multiple occurrence independently of one another -O-, -S-, -CO-, -COO-, -OCO-, -S-CO-,

-CO-S-, -O-CO-O-, -CO-NR⁰-, -NR⁰-CO-, -OCH₂-, - CH₂O-, -SCH₂-, -CH₂S-, -CF₂O-, -OCF₂-, -CF₂S-, - SCF₂- -CH₂CH₂-, -CF₂CH₂-, -CH₂CF₂-, -CF₂CF₂-, - CH=CR⁰-, -CH=CH-, -CH=CF-, -CY¹=CY¹-, -C≡C-, - CH=CH-COO-, -OCO-CH=CH- or a single bond,

Z¹ has one of the meanings of Z,

R⁰ and R⁰⁰ are independently of each other H or alkyl with 1 to 12 C-atoms, Y¹ and Y² are independently of each other H, F, Cl or CN,

m is O or i ,

L¹, L² are independently of each other F, Cl, Br, I, CN,

NO₂, P-Sp- or alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonlyoxy or alkoxycarbonyloxy with 1 to 12 C atoms, wherein one or more H atoms are optionally replaced by F or Cl,

r1 , r2 are independently of each other 0, 1 , 2, 3 or 4.

2. Mixture according to claim 1 , characterized in that it comprises one or more compounds of formula Ia

wherein P, Sp, L¹, L², r1 and r2 have the meanings given in formula I,

3. Mixture according to at least one of the preceding claims, characterized in that it comprises one or more crosslinkable mesogenic compounds of formula Il

wherein P and Sp, L¹, L², r1 and r2 are as defined in formula I, L³ has one of the meanings of L¹ in formula I, r3 is 0, 1 , 2, 3 or 4, and Z¹ and Z² have one of the meanings of Z in formula I,

4. Mixture according to the preceding claim, characterized in that it comprises one or more crosslinkable mesogenic compounds of formula Ha

wherein P, Sp, L¹ and r1 are as defined in formula II.

5. Mixture according to at least one of the preceding claims, characterized in that it comprises one or more compounds of formula III

6. Mixture according to at least one of the preceding claims, characterized in that it comprises 5 to 12 % of one or more crosslinkable compounds,

7. Mixture according to at least one of the preceding claims, characterized in that it comprises 50 to 80 % of one or more compounds of formula I or Ia and 5 to 50 % of one or more compounds of formula III.

8. Mixture according to at least one of the preceding claims, characterized in that it additionally comprises d) one or more photoinitiators, e) optionally one or more surfactants, f) optionally one or more stabilizers.

9. Polymer film obtainable by crosslinking a mixture according to at least one of the preceding claims.

10. Use of a mixture or film according to at least one of the preceding claims in optical, electrooptical, information storage, decorative and security applications.

11. Optical component or device comprising a mixture or film according to at least one of the preceding claims.

12. Liquid crystal display comprising a mixture or film according to at least one of the preceding claims.

13. Authentification, verification or security marking or coloured image comprising a mixture or film according to at least one of the preceding claims.

14. Object or document of value comprising an authentification, verification or security marking or an image according, to the preceding claim.