KR101325363B1 - Polymerized liquid crystal film with low retardation - Google Patents

Abstract

The present invention relates to polymerized liquid crystal (LC) films comprising photosensitive compounds and having low optical retardation, materials and methods for making such films, and decorative or security applications in LC displays or other optical or electro-optic component devices. It relates to the use thereof as an alignment layer or an optical retardation film.

Description

POLYMERIZED LIQUID CRYSTAL FILM WITH LOW RETARDATION

Figure 1 shows the phase difference (a) and the curing rate (b) of the polymer film prepared according to Example 1.

The present invention relates to a polymerized liquid crystal (LC) film comprising a photosensitive compound and having a low optical retardation. The present invention also relates to materials and methods for making such films and to their use as alignment layers, optical retardation films or optical waveguides in decorative or security applications in LC displays or other optical or electro-optic component devices. .

Polymerizable liquid crystals (LC), also known as reactive mesogens (RM), can be used to make optical quality improving films such as retardation films for LCDs, compensation films, and the like. For this function, such optical films often require a specific optical phase difference R such as

Where

Δn is the birefringence of the LC material,

d is the film thickness.

In practice, RM films are often coated by a roll-to-roll production process on plastic substrates. This process is designed to allow economical production of optical quality improving RM polymer films. Sometimes, it is desirable to coat RMs with lower birefringence to obtain specific optical effects or for processing reasons. For example, it may be desirable to coat a layer with a thickness greater than 1 μm due to its tolerances in the coating process, but it is essential to increase the phase difference of the final RM film as it is set to achieve a particular optical effect. not. If the RM layer is coated too thick (or too thin), the retardation of the polymer film becomes too large (or too small) to provide inefficient optical properties. For example, if the desired product is a quarter-wave retardation film (QWF), any region of different retardation will not be effective to switch between linear and annular polarization, but elliptical polarization will result. Since the phase difference is a product of birefringence and film thickness, it is necessary to use RM having a lower birefringence.

The prior art solves this problem in several ways. For example, low birefringence RM mixtures containing lower birefringence RM compounds have been formulated. Such compounds typically contain less aromatic groups, for example disclosed in EP 0 972 818 A1. However, these compounds are often more expensive to prepare, which makes their mixtures less commercially viable.

Another possible method disclosed in the art, which uses less expensive materials to produce low birefringence RM films, includes polymerizing RMs at elevated temperatures, for example as suggested in US Pat. No. 5,042,925. do. However, this method has the major disadvantage of being very difficult to control at temperatures close to liquid crystal / isotropic phase transitions. Only small changes in process temperature have been found to lead to large changes in the film's final birefringence. If this adjustment is not easy, it is difficult to use this method for mass production. It has also been found that in practice heating the RM film to a temperature above 90 ° C. to 100 ° C. can lead to unwanted thermal polymerization. Thus, before any photopolymerization is initiated, thermal polymerization may occur during the heating step. Another disadvantage of the heat treatment is that at elevated temperatures (eg, above 90 ° C.) the RM films may begin to dewet from their substrates. Such dewetting leads to incomplete films that are not suitable for optical film applications. Another disadvantage is that the processing temperature is limited by the thermal stability of the substrate. For this reason, the RM film processed on the plastic substrate cannot be heated to the same elevated temperature as the film processed on the glass substrate.

Another method for obtaining low birefringence RM films is to add specific non-liquid crystal materials to the RMs, for example as suggested in US Pat. No. 6,171,518. These materials are chosen to act to disrupt the liquid crystal arrangement of the RMs during polymerization.

In reducing the alignment, these monomers also reduce the effective birefringence of the RM film. In US Pat. No. 6,171,518, examples of cited birefringence are greater than 0.009 and less than 0.13. However, in order to obtain low birefringence, the polymerization process in US Pat. No. 6,171,518 is also carried out at elevated temperatures (90-110 ° C.) and then follows the method of US Pat. No. 5,042,925.

International Patent Application Publication Nos. 2004/083913 and 2004/090025 disclose polymer films having retardation patterns comprising photoisomerizable RMs and methods for their preparation. However, in the film production process disclosed in this document, isomerization and polymerization are carried out in two separate steps under different conditions (air and nitrogen atmospheres respectively).

One object of the present invention is to provide a retardation film comprising RM or LC having a low optical retardation. Another object is to provide methods and materials for making such films without the disadvantages of the prior art methods and materials as described above. Other objects of the present invention are immediately apparent to those skilled in the art from the following detailed description. Specifically, it is desirable to have an RM material that essentially includes an aromatic RM that can be polymerized to produce a low birefringence film without the need for many individual process steps or elevated temperatures. In addition, this method must be compatible with a wide range of RMs such that mixtures having a wide range of clear points can be processed on any suitable substrate, including plastic substrates. The advantage of using aromatic RMs is that they are relatively less expensive than methods using non-aromatic counterparts.

This object is achieved by using a material and a method as disclosed in the present invention, in particular a RM material comprising a photosensitive compound (e.g., a photoisomerizable compound), and a method of preparing a polymer film (e.g. Photoisomerization and photopolymerization), which occur simultaneously during film production.

The terms “photoreactive”, “photosensitive” and “photoreaction” as used above and below refer to reactions (eg, photoisomerization, light induced 2 + 2 cycloaddition, photo-prise) photo-fries) or a comparable photolysis process, but not limited thereto) to those compounds whose structure or form is modified upon irradiation with light. The photopolymerization reaction is not included in this meaning. However, the photoreactive or photosensitive compounds disclosed herein may be further polymerizable or photopolymerizable.

The present invention relates to a method for producing a polymer film comprising the following steps (a) to (d):

(a) providing a polymerizable LC material layer comprising at least one photosensitive birefringent compound on a substrate;

(b) orienting the LC material layer;

(c) exposing the LC material or selected portions thereof to conditions that cause photoreaction of the photosensitive compound and at the same time cause polymerization of the LC material; And

(d) Optionally, removing the polymerized film from the substrate.

The invention also relates to a polymer film obtainable by the method as disclosed above and below, preferably having a low optical retardation.

The invention also relates to the use of the polymer films disclosed above and below as alignment layers or optical retardation films in decorative or security applications in liquid crystal displays (LCDs) or other optical or electro-optical component devices.

The polymerizable LC material comprises at least one photosensitive compound, preferably a photoisomerizable mesogenic or LC compound, very preferably a polymerizable and photoisomerizable compound. The LC material is provided on the substrate and oriented in a uniform culture, preferably in a planar orientation, which can be achieved by known techniques. The LC material or a selected region thereof is then exposed to conditions which lead to the polymerization of its polymerizable component while at the same time causing a photoreaction of the photosensitive compound, for example EZ -or cis-trans-isomerization. This is achieved for example by actinic radiation, for example by irradiation with UV light. The photoreaction of the photosensitive compound induces a change in its birefringence and / or breakage of the uniform orientation of the LC material, leading to a decrease in the overall birefringence of the LC material. The decrease in birefringence causes a decrease in phase difference in the exposed portion of the LC material. At the same time, the phase difference of the LC material is fixed by in-situ photopolymerization at the exposed part.

Photoreaction and photopolymerization are preferably carried out in the presence of oxygen. Oxygen is prepared from the polymerization initiator or otherwise produced in the material to eliminate free radicals that interfere with the polymerization. This makes the photoreaction process better and leads to a decrease in phase difference before the film structure is fixed.

In general, different retardation values for suitable LC materials can be achieved by any factor that affects the rate of polymerization.

In a preferred embodiment of the invention, the photoreaction and polymerization are carried out in an air atmosphere.

In another preferred embodiment of the invention, the retardation is adjusted by varying the substrate used in the film making process. For example, substrates that are impermeable to oxygen or readily permeable by oxygen and / or tend to absorb water vapor can be used to make films with different phase differences. Preferably the isomerization and polymerization of the LC material is carried out on a substrate which is hydrophilic and / or permeable to oxygen. Suitable and preferred substrates are, for example, polyester, such as films of polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), polyvinyl alcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC) films, very Preferably PET or TAC film.

In another preferred embodiment of the invention, the degree of photoreaction and thus the change in birefringence in the LC material is controlled by the change in dosage, ie intensity, exposure time and / or irradiation power. For example, the birefringence can be reduced by increasing the irradiation power causing the photoreaction and polymerization.

Proper irradiation time and irradiation intensity depend on the type of LC material used, in particular the type and amount of photosensitive compound in the LC material.

The initial value of the phase difference of the LC material can be adjusted by appropriate selection of the layer thickness and the birefringence of the individual compounds of the LC material.

The thickness of the polymer film according to the invention is preferably 0.5 to 2.5 microns, very preferably 0.6 to 2 microns, most preferably 0.7 to 1.5 microns.

The axial retardation (ie at 0 ° viewing angle) of the polymer film according to the invention is preferably greater than 0 to 250 nm.

For practical applications in LCDs, it is particularly preferred that the optical retardation film exhibit a phase difference of about 0.25 hours of the wavelength of the incident light (also known as a quarter-wave retardation plate or film (QWF) or λ / 4-plate in the prior art). ). Particularly preferred phase difference for use as QWF is 90 to 250 nm, preferably 100 to 175 nm.

The polymerizable LC material is preferably a nematic or smectic LC material. It preferably comprises at least one polymerizable compound having one polymerizable group (mono-reactive) and at least one polymerizable compound having two or more polymerizable groups (di- or poly-reactive).

The polymerizable mesogenic or LC compound is preferably a monomer, very preferably a calamitic monomer. Such materials typically have good optical properties, for example reduced chromaticity, and can be easily and quickly oriented in the desired direction, which is particularly important for industrial production of large scale polymer films.

Polymerizable mesogenic mono-, di-, and poly-reactive compounds suitable for the present invention are carried out by known methods, for example the standard practice of organic chemistry disclosed in Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart. It may be prepared by a method such as.

Suitable polymerizable mesogenic or LC compounds for use as monomers or comonomers in polymerizable LC mixtures are described, for example, in International Patent Application Publication Nos. 93/22397, European Patent 0 261 712, German Patent 195 04 224, International Publication Nos. 95/22586 and 97/00600, US Pat. Nos. 5,518,652, 5,750,051, 5,770,107 and 6,514,578.

Particularly useful and preferred examples of polymerizable mesogenic or LC compounds (reactive mesogens) include the following compounds:

Where

P is a polymerizable group, preferably an acrylic, methacryl, vinyl, vinyloxy, propenyl ether, epoxy, oxetane or styrene group,

x and y are the same or different and are an integer from 1 to 12,

A and D are 1,4-phenylene optionally mono-, di- or tri-substituted with L ¹ or 1,4-cyclohexylene,

u and v are each independently 0 or 1,

Z ⁰ is -COO-, -OCO-, -CH ₂ CH ₂ -or a single bond,

Y ⁰ is F, Cl, CN, NO ₂ , OCH ₃ , OCN, SCN, optionally fluorinated alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy, mono-, Oligo- or polyfluorinated alkyl or alkoxy of 1 to 4 carbon atoms,

R ⁰ is optionally fluorinated, alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy having 1 or more, preferably 1 to 12 carbon atoms, or Y ⁰ ,

L ¹ and L ² are each independently H, F, Cl, CN or optionally halogenated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy, or alkoxycarbonyloxy having 1 to 5 carbon atoms.

Particular preference is given to mixtures comprising at least one polymerizable compound and an acetylene or tolaine group having a high birefringence, for example a compound of formula (R10). Suitable polymerizable tolanes are disclosed in British Patent No. 2,351,734.

Suitable photosensitive compounds are known in the art. It is a compound that exhibits another photolysis process, for example, photoisomerization, photo-Price rearrangement or 2 + 2 cycloaddition, or upon irradiation with light. Particular preference is given to photoisomerizable compounds. Examples of suitable and preferred photoisomerizable compounds include azobenzene, benzaldoxime, azomethine, stilbene, spiropyrane, spiroxadine, fullgid, diarylethene and cinnamates. Further examples include 2-methyleneinin-1-one as disclosed in European Patent No. 1 247 796 and (bis) -benzylidenecycloalkeneone as disclosed in European Patent No. 1 247 797. .

Particularly preferably the LC material comprises at least one cinnamate, in particular cinnamate reactive mesogens as disclosed in US Pat. No. 5,770,107 and EP 02008230.1. Very preferably the LC material comprises at least one cinnamate RM selected from the following compounds:

(III)

(III)

(IV)

(IV)

(V)

(V)

Where

P, A and v have the same meaning as described above,

L is ^one of the meanings of L ¹ as described above,

Sp is a spacer group, for example alkylene or alkyleneoxy of 1 to 12 carbon atoms or a single bond,

R is Y or R ⁰ as described above or represents P-Sp.

Particular preference is given to cinnamates RM containing a polar end group Y as described above. Very preferred are cinnamates RMs of formulas III and IV, wherein R is Y.

The radiation used to induce photoreactions in the LC material depends on the type of photosensitive compound. In general, compounds exhibiting photoisomerization induced by UV radiation are preferred. For example, for cinnamate compounds such as compounds of Formulas III, IV, and V, UV radiation is typically used having a wavelength in the UV-A range (320-400 nm) or having a wavelength of 365 nm.

Preferably the polymerizable component of the polymerizable LC material comprises at least 10 mole% of a photosensitive compound, preferably cinnamate RM, most preferably a compound selected from formulas III, IV and V.

As used above and below, the term "polymerizable component" means mesogenic and non-mesogenic compounds which are polymerizable in the entire polymerizable mixture, and other nonpolymerizable components and additives such as initiators, surfactants, stables It does not contain a topical agent, a solvent, or the like.

In a preferred embodiment of the invention, the polymerizable component of the LC material is 10 to 100 mol%, very preferably 40 to 100 mol%, in particular 60 to 100 mol%, most preferably 80 to 100 mol% of the photosensitive compound. , Preferably cinnamate RM, most preferably compounds selected from formulas III, IV and V.

In another preferred embodiment, the polymerizable component of the LC material is 20 to 99 mol%, preferably 40 to 80 mol%, most preferably 50 to 70 mol% of the photosensitive compound, preferably cinnamate RM, most Preferably a compound selected from formulas III, IV and V.

In another preferred embodiment, the polymerizable component of the LC material comprises 100 mole% of a photosensitive RM, preferably cinnamate RM, most preferably a compound selected from formulas III, IV and V.

The film according to the invention can also be used as an orientation layer for LC materials. For example, it is possible to use a polymerized LC film according to the invention to orient a continuous layer of polymerizable LC material coated on a polymerized LC film. In this way a stack of polymeric LC films can be prepared.

The polymer films of the present invention are conventional LC displays, for example displays having a vertical orientation, such as DAP (oriented phase deforming), ECB (electrically controlled birefringence), CSH (color super) Homeotropic), VA (vertically oriented), VAN or VAC (vertically oriented nematic or cholesteric), MVA (vertically oriented multidomain) or PVA (patterned and vertical) Oriented mode); Displays with bend or hybrid orientation, such as OCB (optically compensated bend cell or optically compensated birefringence), R-OCB (reflective OCB), HAN (hybrid oriented nematic) or pi-cell a display device having a (π-cell) mode; Displays having a twisted orientation, such as displays having TN (twisted nematic), HTN (highly twisted nematic), STN (super twisted nematic), AMD-TN (active matrix induced TN) modes; It can be used as a phase difference or compensation film or an alignment layer in a display device in an in-plane switching (IPS) mode or a display device having an optically isotropic phase switching as disclosed in WO 02/93244.

Particular preference is given to TN, STN, VA and IPS displays, especially active matrix types. Transflective displays are also desirable.

The polymer film according to the invention can be used as an optical retardation film between a substrate, usually a glass substrate, which forms an external LCD or switchable LC cell of the switchable LC cell of the display and contains a switchable LC medium (in-cell application). .

Unless stated otherwise, the general preparation of polymer LC films according to the invention can be carried out according to standard methods known in the literature. Typically, the polymerizable LC material is coated on the substrate or otherwise applied, where it is oriented in a uniform direction, for example by exposure to actinic radiation, preferably onto the LC by photopolymerization, very preferably UV photopolymerization. In situ polymerization, the orientation of the LC molecules is fixed. If desired, uniform orientation can be facilitated by additional methods such as shearing the LC material, surface treatment of the substrate or adding a surfactant to the LC material.

As the substrate, for example, glass or quartz sheets or plastic films can be used. It is also possible to place a second substrate on top of the coated material before and / or during the polymerization and / or after the polymerization. This substrate may or may not be removed after polymerization. If two substrates are used to cure by actinic radiation, at least one substrate should be transparent to the actinic radiation used for polymerization. Isotropic or birefringent substrates may be used. When the substrate is not removed from the polymerized film after polymerization, an isotropic substrate is preferably used.

The polymerizable material may be applied to the substrate by conventional coating techniques such as spin coating or blade coating. Conventional printing techniques known to those skilled in the art, such as screen printing, offset printing, reel to reel printing, letterpress printing, gravure printing, rotogravure printing, platen It may be applied to the substrate by flexographic printing, engraved printing, pad printing, heat seal printing, ink jet printing or printing using stamps or printing plates.

It is 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 technique, and the solvent is evaporated off prior to polymerization. In most cases it is suitable to heat the mixture to facilitate the evaporation of the solvent. As the solvent, for example, standard organic solvents can be used. Solvents are, for example, ketones such as acetone, methyl ethyl ketone, methyl propyl ketone or cyclohexane ions; 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 trimethane; Glycols or esters thereof such as PGMEA (propyl glycol monomethyl ether acetate), γ-butyrolactone and the like. It is also possible to use binary-, tertiary- or higher-mixtures of these solvents.

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

Particular preference is given to polymerizable materials comprising at least one surfactant which promotes specific surface orientation of the LC molecules. Suitable surfactants are described, for example, in J. Cognard, Mol. Cryst. Liq. Cryst. 78, Supplement 1 (1981), pages 1-77. Preferred alignment agents for planar orientation are, for example, nonionic surfactants, preferably fluorocarbon surfactants, such as commercially available Fluorad FC171 (registered trademark, 3M Company) or Zonyl. FSN®, DuPont, surfactants disclosed in British Patent No. 2 383 040, or polymerizable surfactants disclosed in European Patent No. 1 256 617.

It is also possible to apply an alignment layer to the substrate and to provide a polymerizable material on this alignment layer. Suitable alignment layers are known in the art, such as rubbed polyimide or alignment layers made by photoalignment as disclosed, for example, in US Pat. Nos. 5,602,661, 5,389,698 or 6,717,644.

Polymerization is accomplished by exposing the polymerizable material to actinic radiation. Actinic radiation refers to irradiation using light such as UV light, IR light or visible light, irradiation using X-rays or gamma-rays, or irradiation using high energy particles such as ions or electrons. In the process according to the invention, the irradiation should be chosen to induce photopolymerization of the polymerizable compound and at the same time to induce photoreaction of the photosensitive compound. Preferably the polymerization is carried out by UV irradiation. As a source of actinic radiation, for example, a single UV lamp or fixing of UV lamps can be used. Curing time can be reduced when using high ramp forces. Another possible source for actinic radiation is a laser such as a UV, IR or visible laser.

The polymerization is preferably carried out in the presence of an initiator which absorbs at the wavelength of actinic radiation. For example, when polymerizing with UV light, photoinitiators can be used that produce free radicals that decompose under UV irradiation to initiate the polymerization reaction. Radical photoinitiators are preferably used for the polymerization of acrylate or methacrylate groups. Typical radical photoinitiators include, for example, Irgacure (Irg) 907, Irgakur 651, Irgakur 184, Darocure 1173 or Darocur 4205 (Ciba Geigy AG). have.

Curing time depends on the reactivity of the polymerizable material, the thickness of the coating layer, the type of polymerization initiator, and the power of the UV lamp. The curing time is preferably 5 minutes or less, very preferably 3 minutes or less, most preferably 1 minute or less. For mass production, short cure times of up to 30 seconds are preferred.

The polymerizable material may also be used in combination with one or more dyes, in particular UV dyes, such as 4,4 "-azoxy anisole or Tinuvin® dyes (Siva Age®) having a maximum absorbance suitable for the radiation wavelength used for the polymerization. Obtained from Basel, Switzerland).

In another preferred embodiment, the polymerizable material comprises at least one mono-reactive polymerizable non-mesogenic compound in an amount of preferably 0-50%, very preferably 0-20% by weight. Typical examples include alkyl acrylates or alkyl methacrylates.

In another preferred embodiment, the polymerizable material comprises at least one di- or multi-reactive polymerizable non-mesogenic compound in an amount of preferably 0 to 50%, very preferably 0 to 20%, alternatively Or additionally include di- or multi-reactive polymerizable mesogenic compounds. Typical examples of di-reactive monomers include alkyldiacrylates or alkyldimethacrylates having alkyl groups of 1 to 20 carbon atoms. Typical examples of multi-reactive monomers include trimethylpropanetrimethacrylate or pentaerythritol tetraacrylate.

One or more chain transfer agents may be added to the polymerizable material to modify the physical properties of the polymer film. Particular preference is given to thiol compounds, for example monofunctional thiols such as dodecane thiols or multifunctional thiols such as trimethylpropane tri (3-mercaptopropionate). Very preferred are mesogenic or LC thiols as disclosed in WO 96/12209, 96/25470 or US Pat. No. 6,420,001. By using a chain transfer agent, one can control the free polymer chain length and / or the polymer chain length between two crosslinks in the polymer film. If the amount of chain transfer agent is increased, the polymer chain length of the polymer film is reduced.

The polymerizable material may also include a polymeric binder, or one or more monomers and / or one or more dispersing aids capable of forming the polymeric binder. Suitable binders and dispersing aids are disclosed, for example, in International Patent Application No. 96/02597. However, preferably the polymerizable material does not comprise a binder or a dispersion aid.

The polymerizable material may further comprise one or more additional components such as catalysts, sensitizers, stabilizers, initiators, chain transfer agents, co-reacting monomers, surface active compounds, lubricants, wetting agents, dispersants, hydrophobicizing agents, adhesives, flow improvers, antifoaming agents, Degassing agents, diluents, reactive diluents, adjuvants, colorants, dyes or pigments.

Above and below, unless stated otherwise, all temperatures mean degrees Celsius and all percentages are weight percent. The following abbreviations are used to illustrate the LC phase reactions: C, K = crystalline; N = nematic; S = smectic; N ^* , Ch = chiral nematic or cholesteric; I = isotropic. The number between these symbols indicates the phase transition temperature in degrees Celsius. In addition, mp means the melting point, cp means the clearing point (℃).

The following examples are intended to illustrate the invention, but do not limit the invention.

Example 1

Mixture Formulation

Polymerizable LC mixtures according to the invention containing polymerizable compounds (3) and (4) were formulated as disclosed in Table 1 below. For comparison purposes, a polymerizable mixture (2) containing no isomerizable compounds was formulated as shown in Table 1 below.

(1)

(One)

(2)

(2)

(3)

(3)

(4)

(4)

(5)

(5)

(6)

(6)

Compounds (1) to (6) are known in the art. Wherein Irg651 is Irgakur® 651, and Irg907 is Irgakur® 907, which is a commercially available photoinitiator (obtained from Ciba AG). FC171 is Fluoride (R) FC171, which is a commercially available nonionic fluorocarbon surfactant (obtained from 3M).

Mixture 1 was dissolved to prepare a solution. Two solutions were prepared according to the coating technique. One solution is 50% solids in PGMEA. Another solution is 38% solids in xylene.

The mixture 2 was dissolved in PGMEA to prepare a 33% solution. Mixture 2 contained an acrylate group for polymerization but was used as a control experiment because it contained no other photosensitive groups such as isomerizable groups.

Each solution was filtered to 0.2 μm through a PTFE membrane filter prior to use. Films were prepared on glass / rubbed polyimide (JSR AL 1054) or rubbed triacetyl cellulose (TAC) films.

Film manufacturing

Films of the glass / polyimide phase mixture (1) were prepared by spin coating a 50% PGMEA solution at 5,000 rpm for 30 seconds and then polymerizing by exposure to UV-A irradiation for different times at different powers.

A film of the glass / polyimide phase mixture 2 was prepared by spin coating a 33% PGMEA solution at 5,000 rpm for 30 seconds. This produces films of similar retardation allowing for easy comparison.

A film of TAC phase mixture 1 was prepared by bar coating a 38% solid / xylene solution with wire bent bars designed for applying a 4 μm wet film. This film was then polymerized by exposure to UV-A irradiation for different times at different powers.

Impact of Polymerization Environment

The spin coated films of the mixtures (1) and (2) were polymerized under air and nitrogen atmosphere (20mWcm ^-2 UV-A irradiation). The retardation of each film was measured and shown in Table 2 below. Retardation values (%) were calculated for each mixture for the values for the film polymerized under nitrogen.

Observation of the data in Table 2 shows that when polymerized in an air or nitrogen atmosphere, there is only a small difference in the final retardation value (about 3%) for the film of the mixture 2 (non-isomer). In contrast, when polymerized under a nitrogen or air atmosphere, a very large drop in the phase difference for the film of the mixture 1 (isomerizable) appeared. This is caused by different mixture formulations. Thus, compounds (3) and (4) in mixture (1) undergo photochemical isomerization of cinnamate groups due to UV exposure resulting in reduced birefringence. This process is competitive with the UV polymerization step. In a nitrogen atmosphere, the polymerization step is very efficient (as the effect of any oxygen quench is significantly reduced). In air, oxygen quenching is more important by allowing more photochemical reactions of components (3) and (4) before polymerization is fully achieved. In this way the mixture 1 can be used in an air environment to produce a film having a lower retardation value (see the same mixture polymerized in a nitrogen environment).

In order to obtain a more significant decrease in birefringence, different UVs can be used as seen in Example 2.

Example 2

Films of mixture 1 of Example 1 were prepared using different UV-A irradiation intensities in air. In each case, the exposure time is 60 seconds. The results are shown in Table 3 below.

From the data in Table 3, it can be seen that the phase difference of the polymerized films 5 to 9 decreases with the increase of the UV curing power. This decrease gradually decreases until the film reaches its maximum polymerization (about 96%). This can be explained by the fact that when the film is fully polymerized its phase difference is difficult to change by further UV exposure. These data are shown in FIG. 1, which shows the cure rate (%) of each film determined from the FTIR measurement of the acrylate peak intensity and the phase difference (%) of each film relative to film 5 in Table 3 (a).

This shows that for a suitable mixture, such as mixture (1), the polymerization environment can be used to greatly influence the range of reduction of the phase difference (birefringence) of the RM film.

Example 3

One additional way that may affect the phase difference is to use different substrates. For example, substrates that are either impermeable to oxygen or nearly impermeable to oxygen, such as glass and more easily permeable to oxygen, such as TAC, can be used to make films with different phase differences.

The film of mixture 1 is polymerized on TAC and glass / polyimide substrates as described in Example 1. The polymerization step includes exposure to 20mWcm ^-2 UV-A irradiation for 60 seconds. The retardation value of the polymer film is shown in Table 4 below.

The data in Table 4 show that the film 2 of the mixture 1 polymerized on the glass slide under nitrogen atmosphere has a relatively high retardation value (209 nm). The film 11 polymerized on TAC under a nitrogen atmosphere has a lower retardation value of 87 nm.

Polymerization of the mixture 1 in air on TAC produces an almost isotropic film 10 (phase difference = 5 nm). TAC films are known to be very polar and hydrophilic. Such substrates interfere with free radical polymerization. This effect is almost blocked in air, which can reach an isotropic state before the film is fully polymerized.

This example demonstrates that only aromatic based LC materials can be used to obtain RM films with very low retardation (birefringence) values. The exact value of the phase difference can be adjusted by changing the mixture formulation and environmental / process conditions.

Polymerized liquid crystal (LC) films comprising the photosensitive compound according to the invention and having a low optical retardation are used as alignment layers or optical retardation films in decorative or security applications in LC displays or other optical or electro-optic component devices. Useful for

Claims (15)

(a) providing a polymerizable LC material layer comprising at least one photosensitive birefringence compound on a substrate; (b) orienting the LC material layer; (c) exposing the LC material or selected portions thereof to conditions that cause a photoreaction of the photosensitive compound and at the same time cause polymerization of the LC material; And (d) optionally, removing the polymerized film from the substrate As a method for producing a polymer film comprising: The photoreaction and polymerization of step (c) is carried out in the presence of oxygen, The polymerizable LC material comprises at least one mono-reactive polymerizable mesogenic compound and at least one di-reactive polymerizable mesogenic compound, at least one of which is a photosensitive compound. Way. delete The method of claim 1, Process for producing a polymer film, characterized in that the photoreaction and polymerization of step (c) is carried out in an air atmosphere. The method of claim 1, And said substrate is a hydrophilic or oxygen permeable substrate. 5. The method of claim 4, Method for producing a polymer film, characterized in that the substrate is a TAC, PET, PVA or PE film. The method of claim 1, The photoreaction and polymerization of step (c) is caused by exposing the LC material to UV radiation. The method of claim 6, Method for producing a polymer film, characterized in that to reduce the optical retardation of the polymer film by increasing the UV irradiation power. delete The method of claim 1, Wherein said photosensitive compound is a photoisomerizable compound, and the photoreaction is a photoisomerization reaction. The method of claim 1, The polymerizable LC material may be selected from azobenzene, benzaldoxime, azomethine, stilbene, spiropyrein, spiroxadine, fulgid, diarylethene, cinnamate, 2-methyleneinyne-1-one and (bis -) A method for producing a polymer film, characterized in that it comprises at least one photoisomerizable compound selected from the group consisting of benzylidenecycloalkainone. 11. The method of claim 10, Wherein said polymerizable LC material comprises at least one photoisomerizable compound selected from polymerizable mesogenic cinnamates. The method of claim 1, The LC material and the photoreaction and polymerization conditions are selected such that the optical retardation of the polymer film is less than 200 nm. The polymer film obtainable by the manufacturing method according to any one of claims 1, 3 to 7, and 9 to 12. The polymer film according to claim 13 used in liquid crystal displays (LCDs), or other optical or electro-optical component devices, as an alignment layer or optical retardation film for decorative or security applications. An LCD comprising the polymer film of claim 14.

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