WO2017123828A1 - Method for preparing liquid crystal alignment layer - Google Patents

Abstract

A method for preparing an alignment layer for a liquid crystal device includes: treating a substrate surface with a particle beam to impart an alignment function; depositing an auxiliary layer made of a cross-linkable material (e.g., a reactive mesogen-containing composition) such that the auxiliary layer acquires the alignment function; and cross-linking the auxiliary layer.

Description

METHOD FOR PREPARING LIQUID CRYSTAL ALIGNMENT LAYER

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Ukrainian Patent Application No. a 2016 00283, filed January 14, 2016 and titled "THE METHOD FOR PREPARATION OF LIQUID CRYSTAL ALIGNMENT LAYER", which is incorporated by reference herein in its entirety.

BACKGROUND

[0002] The present disclosure relates to liquid crystal displays, namely, methods of liquid crystal (LC) alignment. It is known that, for use in electro-optic devices LC layers, except devices based on amorphous LC alignment [K. Kobayashi et al., Jpn. J. of Appl. Phys. 43 (4A), 1464-1468 (2004)] and LC and polymer composites [P. Drzaic, Liquid Crystal Dispersions. World Scientific, Singapore, 429 p (1995)], should be properly aligned. This alignment is provided by boundary substrates, which usually are appropriately processed. Both substrate material(s) and the process influence LC alignment.

[0003] The treatment process used in practically all modern LCD industry is rubbing. To achieve low-pretilt LC alignment (θ<5°), polyimides with hydrophilic or slightly hydrophobic properties may be used. The layers of these polymers are quite strongly rubbed to impart alignment function. On the other hand, high-pretilt angle LC alignment (θ>85°) is usually realized by using strongly hydrophobic polyimides whose films are weakly rubbed before contact with the LC layer.

[0004] The rubbing technique has number of well-known disadvantages, such as mechanical damage, electric charging, dusting and microscopic non-uniformity. Additionally, it is difficult to apply it to curved substrates, closed volumes and to provide patterned alignment. These drawbacks are mainly caused by direct mechanical contact of a brusher with the aligning surface.

[0005] The above mentioned intrinsic drawbacks of the rubbing alignment technique stimulated development of alternative methods of LC alignment. Among the most promising of them are the so called particle beam alignment (PBA) methods. In the most preferred embodiment of this method, the directed beam of particles impinges upon the substrate obliquely and causes modification of the aligning surface. Depending on the type of particles (e.g., their mass, reactivity, etc.) and their energy, either particle etching/milling or particle deposition process can prevail (FIG. 1). In the first case, energetic particles eject atoms from the substrate causing anisotropic breaking of the molecular bonds on the substrate's surface. In the second case, the particles condense on the substrate, forming new alignment layer. In both cases, because of oblique incidence of the particle beam, the surface layer of the substrate becomes anisotropic and thus acquires the ability to align LCs.

[0006] The sputtering and deposition type particle beam processes adopted for LC alignment are schematically presented in FIG. 2. In the sputtering (etching/milling) process (FIG. 2a), energetic particles (E=100-10000 eV), accelerated in an electric field, are commonly used. To form the beams of accelerated particles, special ion and plasma beam sources are developed. The sources most frequently used for the LC alignment purpose are Kaufman source and anode layer source. The corresponding alignment processes are called ion beam alignment [Callegari et al., US patent 6,020,946A, 2/2000; Chaudhari et al., US patent 6,124,914A, 9/2000] and plasma beam alignment [O. Yaroshchuk, R. Kravchuk, A. Dobrovolskyy, L. Qiu, O. Lavrentovich, Liq.Cryst., 31 , No6, 859-869 (2004)]. The processes of physical and chemical etching can be used for LC alignment. Usually non-reactive etching regime is utilized in the alignment process. For this purpose, noble gases, often argon, are most commonly used.

[0007] The deposition of aligning layers is usually realized by evaporative deposition processes [J. Janning, Appl.Phys.Lett., 21 , 173 (1972)] (FIG. 2b), sputter deposition [Motohiro, Y. Taga, Thin Solid Films, 85, 137 (1990)] (FIG. 2c), and direct particle beam deposition [WO 2009/106208A1 ] (FIG. 2d). In the evaporative process, the material to be deposited is heated to a high vapor pressure electrically or via bombardment with an electron beam. The vapor diffuses to the substrate and condenses on it. The vapor particles in this process have low energy (E<0.01 eV). In the sputter deposition process, the primary beam of energetic particles (E=500-10000 eV) bombards a target and causes its sputtering. The ejected particles have much higher energy (E<10 eV) than the particles in the evaporative process. They reach the substrate and travel on it filling available vacancies and providing more dense coatings. The magnetron [P.K. Son et al. Thin Solid Films 515, 3102 (2007)] and ion/plasma beam sputter deposition based on Kaufman and anode layer sources [A. Khakhlou et al., J. SID 14/3, 257 (2006)] was adopted for deposition of LC aligning layers. In the direct deposition process, the aligning coating is formed by the particles generated in a glow discharge and weakly accelerated in an electric field (E=30-100 eV). This process can be particularly realized by using end Hall source [WO 2009/106208A1]. The deposition processes can be physical and chemical depending on reactivity of condensing particles, temperature of substrate, etc.

[0008] The PBA techniques according to the present disclosure have number of advantages compared to alignment method based on rubbing:

[0009] Because they avoid mechanical contact with the substrates, the above mentioned drawbacks of rubbing technique are, to a large extent, eliminated.

[0010] They provide much better microscopic uniformity of planar (tilted planar, pretilt angle θ<10°) and homeotropic (tilted homeotropic, 80° <θ< 90°) alignment.

[0011 ] They are applicable for a wider variety of substrates.

[0012] In the case of ion/plasma beam processing, high-quality alignment of LCs can be attained directly on the substrate without the use of additional aligning layers.

[0013] The deposition techniques extend the class of alignment films to a large number of inorganic materials

[0014] Compared with rubbing, the PBA techniques provide a much simpler alignment patterning procedure.

[0015] The major problem which hampers practical application of PBA methods is insufficiently high stability of LC alignment. The desirable alignment, achieved in the freshly prepared cells, starts to gradually degrade with the cell storage and/or operation. Causes and manifestations of this phenomenon, hereinafter referred to as "alignment aging", depend on the type of processing. In the case of sputtering (etching) alignment process, the major problem is gradual degradation of LC alignment in the whole cell, which accelerates at elevated temperature. As earlier demonstrated [O. Yaroshchuk, R. Kravchuk, A. Dobrovolskyy, L. Qiu, O. Lavrentovich, Liq.Cryst., 31 , No6, 859-869 (2004); O. Yaroshchuk et al., Proceed. Eurodisplay'05, 521], at room temperature this degradation occurs slowly (for several months), first of all affecting LC pretilt angle, which can be considered as an indicator of this process. As a result of these changes, one can observe pronounced changes in the electro-optic response. The most likely reason for this kind of etching is the generation of free radicals and ions on the treated surface. These species may chemically react with LC changing boundary conditions and thus LC alignment [Katoh, Y., Nakagawa, Y., Odahara, S., & Samant, M., US Patent 2002/0063055 A1]. The other reason, in case of polymeric aligning films, might be a scission of polymeric chains resulting in decrease of surface viscosity and thus slow alteration of boundary layers [O. Yaroshchuk et al., Mol. Cryst. Liq. Cryst., 479, 1 11-120 (2007)].

[0016] To overcome alignment aging, US Patent 2002/0063055 A1 suggested introducing a post-etching treatment of the aligning surface. The idea was to passivate the aligning surface with the chemically active atoms such as atomic hydrogen. The hydrogen atoms may react easily with the destructive radicals from the aligning surface transferring this surface from the chemically active to the inactive state. In the following, to simplify industrial implication of this method, it was suggested to combine alignment and passivation steps using Ar/H₂ gaseous feed [O.V. Yaroshchuk et al., Proc. IMID'05, 768-773 (Seoul, Korea, July 19-23, 2005)]. However, efficiency of surface passivation with hydrogen was not sufficiently high.

[0017] In the case of inorganic aligning layers produced by deposition techniques, other problems occur associated with a destructive effect of adhesive material(s) on LC alignment. Very often, homeotropic LC alignment in the cell begins to deteriorate in the areas close to the glue paths at the cell edges. With increasing storage time, the front of the degradation moves towards the center of the cell and, eventually, the alignment in the cell can be completely deteriorated (see FIG. 8a and FIG. 9a). This process is apparently due to the diffusion of some components of the adhesive in the LC and subsequent adsorption on the aligning surfaces. One way of solving this problem is associated with the careful selection of an adhesive and its curing conditions for a particular alignment material [D. Cuypers, G. Van Doorselaer, J. Van Den Steen, H. De Smet and Andre Van Calster, Assembly of an XGA 0.9" LCOS display using inorganic alignment layers for VAN LC. Conference Record of the International Display Research Conference, pp.551 -554 (2002)]. However, it is not universal and, according to testing, does not provide a complete solution to the problem.

BRIEF DESCRIPTION

[0018] It is an object of the present disclosure to provide a method for improving stability (e.g., phot and thermal stability) of LC alignment on the particle beam processed surfaces and alignment aging resistance. Another object of the present disclosure is to provide a method for reducing destructive chemical interaction of particle beam processed surfaces with liquid crystal. A further object of the present disclosure is to eliminate the destructive effect on LC alignment of glue used in assembling of LC cells.

[0019] To stabilize LC alignment on the aligning surface processed by a particle beam, it is proposed to passivate the surface by the thin orientationally ordered layer of polymer, preferably an aligned and polymerized layer of a reactive mesogen (RM). Within this concept, the particle beam treated substrate serves as an aligning layer for the RM. The RM layer, in turn, acts as alignment layer for the LC (FIG. 1). To minimize undesirable retardation, the RM layer is sufficiently thin (e.g., d<100 nm).

[0020] Applying this approach the following advantages are obtained:

[0021] (1) The alignment function of the treated substrate is required only for the short period of time needed to align and solidify the RM layer.

[0022] (2) The reactive species generated on the treated surface do not affect LCs, since they have no direct contact with this surface. The alignment of LCs is determined by the well aligned and cross-linked layer of reactive mesogen. This results in radical improvement of alignment stability.

[0023] (3) Passivation of inorganic coatings with thin organic layers of RM prevents destructive effect of glue on homeotropic LC alignment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same. [0025] FIG. 1 schematically illustrates non-limiting examples of methods in accordance with some embodiments of the present disclosure: (a) substrate optionally containing alignment coating; (b) particle beam processing of the alignment substrate; (c) particle beam processed substrate with etched (A) or deposited (B) aligning layer (prior art); (d) and (e) particle beam processed substrates containing auxiliary passivation layers (embodiments according to present disclosure), (d) and (e) correspond to continuous and discontinuous (patterned) passivation coating, respectively.

[0026] FIGS. 2a-d schematically illustrate particle beam processes for LC alignment: (a) Particle beam sputtering (etching/milling) process: 1 - particle beam source, 2 - accelerated particles, 3 - substrate, 4 - ejected from the substrate particles; (b) Evaporative deposition processes; 1 - crucible with evaporating materials, 2 - evaporated particles, 3 - substrate, (c) Particle beam sputter deposition; 1 - particle beam source, 2 - accelerated particles (primary particle beam), 3 - target, 4 - ejected particles (secondary particle beam), 5 - substrate, (d) Direct particle beam deposition; 1 - particle beam source, 2 - generated beam of weakly accelerated particles, 3 - substrate.

[0027] FIGS. 3a-d are photographs of symmetric antiparallel cells based on (a, b) bare plasma beam treated polyimide substrates (prior art substrates, see Comparative Example 3) and (c, d) plasma beam treated polyimide substrates passivated by RM layers in accordance with some embodiments of the present disclosure (see Example 3). The cells are filled with LC ZLI2293. The cells are viewed between crossed polarizer and analyzer so that the angle between the alignment axis in the cell and polarizer axis is 0° (a, c) and 45° (b, d).

[0028] FIG 4 is a graph illustrating pretilt angle vs. aging time curves for the cells based on prior art alignment layers (Comparative Example 3) and alignment layers in accordance with some embodiments of the present disclosure (Example 3), curves 1 and 2, respectively. The cells are filled with LC ZLI2293 and aged at room temperature.

[0029] FIG. 5 is a graph illustrating pretilt angle vs. aging time curves for cells based on prior art alignment layers (Comparative Example 4) and alignment layers in accordance with some embodiments of the present disclosure (Example 4), curves 1 and 2, respectively. The cells are filled with LC ZLI4801-000 and aged at room temperature.

[0030] FIG. 6 is a graph illustrating pretilt angle vs. aging time curves for cells based on prior art alignment layers (Comparative Example 5) and some embodiments of the present disclosure (Example 5), curves 1 and 2, respectively. The cells are filled with LC ZLI4801-000 and aged at 90°C.

[0031] FIGS. 7a-d are photographs of symmetric 90° twist cells with a thickness of 5.5 μιτι based on (a, b) bare plasma beam treated polyimide substrates (prior art substrates, see Comparative Example 6) and (c, d) plasma beam treated polyimide substrates passivated by RM layers in accordance with some embodiments of the present disclosure (see Example 6). The cells are filled with LC ZLI2293 and viewed between crossed polarizer and analyzer. Cases (a, c) and (b, d) correspond to field on and off states, respectively. In the latter case sin-like voltage 10 V (f = 1 kHz) is applied.

[0032] FIG. 8 is a graph illustrating transmittance vs. voltage curves corresponding to twisted cells based on the prior art aligning substrates (Comparative Example 6) and aligning substrates in accordance with some embodiments of the present disclosure (Example 6) filled with LC ZLI2293, curves 1 and 2, respectively.

[0033] FIGS. 9a and 9b are photographs of symmetric antiparallel cells with a thickness of 16 μηη containing (a) bare Si0₂ aligning films obtained by plasma beam sputtering deposition (prior art substrates, see Comparative Example 7) and (b) SiC½ aligning films passivated by RM layers in accordance with some embodiments of the present disclosure (see Example 7). The cells are glued with the NOA65 glue from Norland and filled with nematic LC MLC6609 (Δε<0). The cells are viewed between crossed polarizer and analyzer. Labels 1 , 2, 3, and 4 correspond to the room temperature storage of the cells for 1 , 14, 30 and 60 days, respectively.

[0034] FIGS. 10a and 10b are photographs of symmetric antiparallel cells with a thickness of 16 μηη containing (a) bare S1O2 aligning films obtained by sputter deposition (prior art substrates, see Comparative Example 8) and (b) Si0₂ aligning films passivated by RM layers in accordance with some embodiments of the present disclosure (see Example 8). The cells are glued with the fast hardening epoxy glue and filled with nematic LC MJ961180 (Δε<0). The cells are viewed between crossed polarizer and analyzer. Labels 1 , 2, 3, and 4 correspond to the room temperature storage of the cells for 1 , 14, 30 and 60 days, respectively.

DETAILED DESCRIPTION

[0035] A more complete understanding of the devices and methods disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the existing art and/or the present development, and are, therefore, not intended to indicate relative size and dimensions of the assemblies or components thereof.

[0036] Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function. In the following specification and the claims which follow, reference will be made to a number of terms, which shall be defined to have the following meanings.

[0037] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0038] As used in the specification and in the claims, the term "comprising" may include the embodiments "consisting of" and "consisting essentially of." The terms "comprise(s)," "include(s)," "having," "has," "can," "contain(s)," and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that require the presence of the named components/steps and permit the presence of other components/steps. However, such description should be construed as also describing devices or methods as "consisting of" and "consisting essentially of" the enumerated components/steps, which allows the presence of only the named components/steps, and excludes other components/steps.

[0039] Numerical values in the specification and claims of this application should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

[0040] Numerical values should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

[0041] All ranges disclosed herein are inclusive of the recited endpoint and independently combinable.

[0042] The term "substrate" or "aligning substrate" means a LC medium limiting layer. It can be of any material, form, bare or covered with functional layers like electrode, compensating, polarizing and/or aligning films, rigid or flexible, isotropic or anisotropic. Non-limiting examples of the substrates are glass slides and plastic strips optionally equipped with some functional films.

[0043] The term "particle beam" means a beam of ions, neutrals, radicals, electrons, or mixtures thereof such as plasma. Depending of kind of particles and their energy, the particles can either sputter the substrate (break chemical bonds and ejects atoms from the aligning substrate) or condense on it forming an additional film. The corresponding processes are defined as "particle beam sputtering" and "particle beam deposition".

[0044] The sputtering process realized by an inert gas is usually called "ion/plasma milling" or "ion/plasma etching". The sputtering realized by involving chemically active ions and/or radicals is called "reactive sputtering" or "reactive etching".

[0045] The deposition process implemented by condensation of particles obtained by heating of the source material (by resistive heating or electron beam bombardment) is called "evaporative deposition" or "thermal deposition".

[0046] The deposition process implemented by condensation of particles obtained in the course of sputtering of source material (sputter target) is called a "sputtering deposition".

[0047] The deposition process implemented by condensation of particles generated immediately in a gaseous discharge is called a "direct deposition". [0048] The deposition process which does not involve chemical bonding between the condensed particles is "physical deposition", while the process involving chemical bonding between the particles is "chemical deposition".

[0049] The term "mesomorphic material" or "liquid crystalline material" relates to materials having liquid crystalline phases in some ranges of temperatures and/or concentrations.

[0050] The term "reactive mesogen" (RM) means a polymerizable mesogenic or liquid crystalline compound, preferably a monomeric compound. These compounds can be used as pure compounds or as mixtures of RMs with other compounds such as photoinitiators, inhibitors, surfactants, stabilizers, chain transfer agents, non- polymerisable compounds, etc. Reactive mesogens with one polymerisable group are also referred to as "monoreactive" compounds, compounds with two polymerisable groups as "direactive" compounds, and compounds with more than two polymerisable groups as "multireactive" compounds.

[0051] Liquid crystalline compounds without polymerisable groups are also referred to as "non-reactive" compounds. To distinguish from the reactive LC compounds or RMs, only the non-reactive LC compounds, usually filled in LC cells and controlled with electric field, will be hereinafter called "liquid crystals" (LCs).

[0052] The term "alignment" or "orientation" relates to alignment of anisotropic molecular units such as small molecules or fragments of big molecules in a common direction named the "alignment direction". In an aligned layer of LC or RM material, the

LC director and optic axis coincides with the alignment direction.

[0053] The terms "uniform alignment" of an LC or RM material means that director field is uniform (i.e., the lines of LC director are parallel). Throughout this application, the alignment of LC or RM layers, unless specified otherwise, is uniform alignment.

[0054] The term "homeotropic alignment" or "vertical alignment" defines alignment of an LC or RM with director oriented normally to the aligning substrates.

[0055] The term "planar alignment" defines alignment of an LC or RM with director oriented parallel to the aligning substrates.

[0056] The term "tilted alignment" defines alignment of an LC or RM with director oriented obliquely to the aligning substrates. [0057] The term "alignment stability" characterizes preservation of parameters of LC alignment over time of sample storage and/or operation, at normal conditions or abnormal conditions (e.g., elevated temperature, high humidity, exposure to intensive UV or visible light).

[0058] The term "alignment aging" defines the alignment degradation process in LC cells when parameters of LC alignment change with a time of cell storage and/or operation.

[0059] The term "passivation" hereinafter means covering aligning surface with an auxiliary layer to neutralize chemically active species on this surface and/or to isolate these species from LC.

[0060] To stabilize LC alignment on the particle beam processed surfaces, the passivation principle may be used. However, in contrast to the passivation method described in US Patent 2002/0063055 A1 , the passivation layers may be made of organic anisotropic material. Preferably, this material is a reactive mesogen.

[0061] Within this concept the aligning surface processed by particle beam is used to align a layer of a reactive mesogen. In turn, an auxiliary layer of RM aligned and subsequently cross-linked is used as the aligning film for the LC. This provides the following advantages:

[0062] (1) The alignment function of the treated substrate is required only for the short period of time needed to align and solidify RM layer.

[0063] (2) The reactive species generated on the treated surface does not affect LC, which has no direct contact with this surface. On the contrary, the alignment of LC is determined by the chemically neutral layer of reactive mesogen aligned and subsequently cross-linked. This results in radically improved alignment stability.

[0064] (3) In parallel with alignment stability, auxiliary RM layers allow adjusting alignment parameters, such as pretilt angle and anchoring coefficient.

[0065] (4) Passivation of inorganic coatings with thin organic layers of RM prevents the destructive effect of glue on homeotropic LC alignment.

[0066] As previously reported [O. Yaroshchuk et al., J. SID, 16/9, 905 (2008)], surfaces processed by PBA methods provide excellent alignment of RMs on both macroscopic and microscopic scale. This makes these techniques very promising for technologies of retardation films and polarizers based on RM layers.

[0067] US Patent Application 2005/0153274A1 and US Patent 6,157,427 (2000) disclose the capability of RM layers to align liquid crystals and suggests combining retardation and alignment function of RM layers in in-cell applications. In this case, however, LC alignment is predetermined by alignment of reactive mesogen that is not desirable for many applications. To decouple molecular alignment in adjacent RM and LC layers, the surface of the RM film should be properly treated.

[0068] US Patent 6,201 ,588 B1 suggests using auxiliary layers of reactive mesogens above the aligning layers to control the pretilt angle of liquid crystal. In the disclosed method a mixture of two reactive mesogens is used in which the component with two reactive groups effects planar alignment, whereas the component with only one terminal reactive group effects homeotropic alignment. The employed RM layers are thin so that their intrinsic retardation is minimized and thus only alignment function of RM films is utilized.

[0069] A recently disclosed technique using a fluorinated reactive monomer of liquid crystal as "a passivation layer" deposited on top of the rubbed poiyimide alignment layer has been reported to prevent drop Mura defect occurred during implementing the one- drop-fill technique for display panel fabrication. [Chung-Ching Hsieh and Chengming He, Reactive Monomer of Liquid Crystal Panel, US Patent No. 2013/0021568, Jan. 24, 2013] The interaction between poiyimide alignment layer and reactive monomer molecules was reduced so that good alignment for this filling method has been achieved.

[0070] The methods of the present disclosure may utilize RM passivation layers for stabilization of LC alignment on the surfaces subjected to PBA treatment. By using RM auxiliary aligning layers in the LC cells (FIG. 1) one can isolate the LC layer from the chemically active boundary surfaces processed by the particle beam. The aligning function of this surface is used to align an auxiliary layer of RM. In turn, the well-aligned and cross-linked layer of RM (layer with a firmly fixed LC order) contacts the LC layer and causes its alignment. This method radically improves stability of LC alignment on the photoaligning layers [O. Yaroshchuk et al., Appl. Phys. Lett., 95, 021902 (2009)]. The present application shows that the same stabilization principle is effective for PBA treatments. Furthermore, along with improvement of thermal stability this approach allows varying pretilt angle and anchoring energy of LC on the surfaces treated by particle beams.

[0071] The structure of the aligning layers according to some embodiments of the present disclosure is illustrated in FIG. 1 (d, e). Compared to prior art PBA layers, it contains an additional layer coated above that is subjected to PBA process. This top layer is the anisotropic polymer layer, preferably a layer of reactive mesogen aligned on PBA film and then solidified via polymerization. Subsequently, this film is used for LC alignment.

[0072] The substrates are made of any suitable organic or inorganic material. The glass slides, silicon wafers or plastic strips, bare ones or covered by ITO electrodes or surface electronic elements can be used as boundary substrates. The substrates can be bare or covered with some aligning layer. All sites of the aligning surface can be processed by particle beam at the same conditions (particle beam composition, particle flux density, particle energy, incidence angle, exposure time) or at different conditions using suitable masks. The latter principle can be used to realize a patterned alignment.

[0073] A suitable reactive mesogen material is a single RM compound doped with photoinitiator or a complex mixture containing, along with reactive mesogen(s) and photoinitiator, thermal inhibitors, mesogenic non-polymerizable compounds, etc. A preferable RM material has a nematic mesophase. A very preferable material has a broad temperature range of nematic mesophase (at least 5°) and a low transition temperature to this mesophase (within 10°C and 80°C). A reactive mesogen is deposited on PBA layer in any suitable manner: e.g., from solution, for example, with centrifugation, dipping or printing, but with certain way of dry deposition such as deposition from vapor phase. To eliminate the light phase delay by RM layer, a layer with a thickness of less than 100 nm may be selected.

[0074] In a preferred embodiment of the present disclosure, the reactive mesogen film is deposited on the surface treated by PBA method before bringing this surface in contact with LC and, preferably, before assembling LC cell. The RM layer is deposited in this case by any technique suitable for film deposition: e.g., solvent cast technique like spin coating, deep coating, or printing, or dry cast technique like vapor deposition. The RM layer formed is polymerized by any suitable method, for example photopolymerization, thermal polymerization or e-beam polymerization. The preferable polymerization technique is photopolymerization. In this case the RM material necessarily contains suitable photoinitiator. Depending on photoinitiator, the photopolymerization is realizable in air or other atmosphere excluding oxygen (for instance, in nitrogen). The photopolymerization of RM layer can be realized before assembling liquid crystal cell or immediately in the cell, before or after filling LC. This approach allows for various thicknesses of the RM layers. For low thicknesses (less than 100 nm) their phase retardation may be inessential. For higher thicknesses the retardation they introduce can be compensated by appropriate compensation films. To simplify design of LC devices, the thickness of the films may be small.

[0075] Alternatively, a passivating RM film may be formed on the aligning surface during polymerization of RM dissolved in a LC host. The polymerization may occur by any suitable method, but photopolymerization is preferable. This deposition method of RM films was previously proposed for many purposes, particularly, for speeding up response of VA-LCDs [C-H. Pai et al., J.SID, 18(11), 960 (2010)], and improvement of electro-optic characteristics [Y.J. Lim et al., Appl.Phys. Express, 5, 081701 (2012)] and thermal stability [Y-J. Lee et al., J.Phys. D: Appl.Phys., 46, 145305 (20130] of LC cells based on photoalignment layers.

[0076] In a preferred embodiment of the present disclosure the RM layer is entirely photopolymerized so that alignment layer has a structure presented in FIG. 1d. The direction of LC alignment on this aligning film can be non-patterned or patterned. The latter can be realized by using several particle beam exposures in different directions, involving appropriate masks. This embodiment allows improving the stability of LC alignment on the PBA layers as well as controlling parameters of LC alignment on these layers.

[0077] In other embodiments, the RM layer is set only in desired sites of the PBA substrate. This can be realized by illuminating the RM layer through a suitable mask. The non-exposed (i.e., non-polymerized) RM can be easily removed, for instance, by washing out in a suitable solvent so that the parts of the aligning film with only PBA layer and both PBA and RM layers can be obtained (FIG. 1 e). This principle is useful to realize alignment patterns with various pretilt angles and anchoring energy.

[0078] A thin auxiliary layer of reactive mesogen on the top of inorganic aligning layer may also prevent deterioration of homeotropic or tilted homeotropic alignment caused by adhesive material. For this purpose, special RMs designed for homeotropic alignment are effective.

[0079] The following examples are provided to illustrate the devices and methods of the present disclosure. The examples are merely illustrative and are not intended to limit the disclosure to the materials, conditions, or process parameters set forth therein.

[0080] COMPARATIVE EXAMPLES

[0081 ] Comparative Example 1.

[0082] Films of polyimide AL-3064 from JSR were spin coated (2500 rpm, 1 min) on two rectangular glass substrates (2 x 3 cm) containing ITO electrode. Then the substrates were maintained for 1 hour at 200°C for imidization. One of the PI films was unidirectionally rubbed using an in-house fabricated rubbing machine. The other substrate film was subjected to a plasma beam treatment.

[0083] The treatment setup was based on an anode layer source of linear type generating two sheet-like plasma beams. This setup is described elsewhere [O. Yaroshchuk, R. Kravchuk, A. Dobrovolskyy, L. Qiu, O. Lavrentovich, Liq.Cryst., 31 , No6, 859-869 (2004); O. Yaroshchuk et al., JSID, 13/4, 289 (2005)]. The feed gas was argon. It was let immediately in a vacuum chamber. The residual pressure in the chamber was 3 x 10^"5 Torr, while the working pressure was 3 x 1 ο^-4 Torr. The anode potential determining maximal particle energy in the beam was 600 V. At these conditions, the ion current density in the sample position was 6-8 μΑ/cm². The angle of plasma beam incidence, accounted from the normal to the substrate, was 70° and the exposure time was 3 min. The substrate was translated in the direction normal to the plasma "sheet" with a speed 2 mm/s to improve treatment uniformity.

[0084] The asymmetric cell was prepared in which the plasma treated polyimide layer (object substrate) and the rubbed polyimide layer (reference substrate) were combined such a way that the rubbing direction on reference substrate and projection of plasma beam on the object substrate are antiparallel. The gap between the substrates was maintained with the 16 μηι spacers. The cell was glued with an epoxy glue and filled with nematic LC ZLI 2293 from Merck Taiwan Ltd. The LC in the cell was uniformly aligned in the direction determined by rubbing and plasma beam projection on the substrate. This implies that the easy axis of LC alignment on the object substrate is generated in the incidence plane of the plasma beam.

[0085] Comparative Example 2.

[0086] LC cell was prepared as in Comparative Example 1 and filled with nematic LC ZLI 4801-000 from Merck. The cell demonstrated high-quality alignment with an easy axis on the object substrate in the incidence plane of plasma beam.

[0087] Comparative Example 3.

[0088] Two polyimide layers were deposited on the glass/ITO substrates and processed by plasma beam as in Comparative Example 1. The symmetric cell was prepared in which the plasma-processed substrates were combined in such a way that plasma processing directions were antiparallel. The cell with a gap of 16 μιη was filled with LC ZLI2293. The photographs of this cell are shown in FIGS. 3a and 3b. It is evident that the quality of LC alignment in the cell is high. The pretilt angle in the cell estimated by crystal rotation method is about 2.5° (Table 1).

[0089] The cell was stored at ambient conditions and the pretilt angle in this cell was monitored over several months. The curve 1 in FIG. 4 demonstrates that the pretilt angle decays with a time of storage approaching 0.

[0090] Table 1. Values of pretilt angle obtained for different LCs on the prior-art and alignment substrates in accordance with some embodiments of the present disclosure.

[0091] Comparative Example 4. [0092] The symmetric antiparallel LC cell was prepared as in Comparative Example

3 and filled with LC ZLI 4801-000. High-quality alignment is observed in the cell. The pretilt angle in the cell estimated by crystal rotation technique to be about 0.9° (Table 1). The cell was stored at ambient conditions and the pretilt angle in this cell was monitored over several months. The curve 1 in FIG. 5 demonstrates that the pretilt angle decays with a time of storage approaching 0.

[0093] Comparative Example 5.

[0094] The symmetric antiparallel LC cell was prepared as in Comparative Example

4 and filled with LC ZLI 4801-000. The cell was kept at 90°C Oust below clearing point of LC ZLI 4801-000, T_C=96°C). Periodically, it was cooled down to room temperature to measure the value of pretilt angle. The pretilt angle as a function of time of backing at 90°C is presented in FIG. 6 (curve 1). It is evident that pretilt angle decays with the backing time approaching zero. The decay rate is much higher than the decay rate at room temperature (see Comparative Example 4).

[0095] Comparative Example 6.

[0096] Two polyimide layers were deposited on two glass/ITO substrates and processed by a plasma beam as in Comparative Example 1. The symmetric cell was prepared in which the plasma processed substrates were combined in such a way that plasma processing directions were perpendicular (twist cell). The cell gap was 5.5 μητι. The cell was filled with LCZLI2293. Pictures of this cell are presented in FIGS. 7a and 7b. They demonstrate a high quality of LC alignment in the cell. The azimuthal anchoring energy was determined to be 7.2 x 10^"5 J/cm². The transmittance-voltage curve for this cell is presented in FIG. 8 (curve 1).

[0097] Comparative Example 7.

[0098] Two glass substrates containing ITO electrodes were coated by Si0₂ layers using a sputter deposition method. The deposition device was previously described [A. Khakhlou et al., J. SID 14/3, 257 (2006)]. It was based on the linear anode-layer source using argon as a working gas. Two sheet-like fluxes of accelerated plasma generated by this source were focused on the surface of a lengthy target, which was a vitreous silica slab. The incidence angle of the fluxes onto the target, accounted from the target's normal, was about 60°. This angle corresponded to maximal efficiency of sputtering at the same power consumption. The following operation parameters were used: residual pressure 2 x 10^~5 Torr, working pressure 5 x 10^"4 Torr, anode potential 4 kV, and discharge current about 100 mA. To neutralize the charges generated on the nonconducting surface of the target under the action of the intensive flux of Ar+ ions, a thermoionic compensator made of wolfram filament was placed near the target. The average incidence angle of the particles ejected from the target and impinging upon the aligning substrate was 75°. The thickness of the SiOx coatings, measured by a quartz- crystal controller, was about 20 nm.

[0099] The symmetric cell was prepared in which the substrates with the deposited Si0₂ layers were combined in such a way that the directions of S1O2 deposition were antiparallel. The cell with a gap of 16 μιη was assembled and glued using optically curable adhesive NOA65 from Norland. The adhesive paths were cured over 20 min with a UV light of intensity 35 mW/cm² from a high-pressure mercury lamp. The cell was filled with a nematic LC MLC6609 from Merck designed for VA mode. The pretilt angle in the cell was about 89.5°.

[00100] The alignment aging in this cell was investigated. FIG. 9a shows photographs of the cell taken after passing of different time after preparation. It is evident that the LC alignment gradually deteriorates starting from the glue paths. The pretilt angle did not change noticeably with storage time.

[00101] Comparative Example 8.

[00102] The cell was prepared as in Comparative Example 7 except that gluing was made by using fast hardening epoxy glue. One hour after the gluing, the cell was filled with a nematic LC MJ961180 from Merck Japan designed for VA mode. The pretilt angle in the cell was about 88.6°. The alignment aging in this cell was investigated. FIG. 10a shows photographs of the cell taken at different times after preparation. It is evident that, similarly to previous case, LC alignment deteriorates. This destructive process starts from the glue paths and gradually approaches middle of the cell. The pretilt angle did not change noticeably with time of storage.

[00103] EXAMPLES

[00104] Example 1. [00105] Polyimide layers were deposited on the glass/ITO substrates and treated as in Comparative Example 1. The planar reactive mesogen mixture RMM 256C purchased from Merck was dissolved in toluene at concentration 2 wt. % and filtered to 0.2 μιη. The prepared solution was spin coated onto the plasma beam treated polyimide layer at a spinning velocity of 3000 rpm and a spinning time of 30 s. After that, the RM film was polymerized by irradiation with a non-polarized UV light from the high pressure mercury lamp (at integral intensity 60 mW/cm² and exposure time 3 min).

[00106] An asymmetric LC cell was fabricated as in Comparative Example 1 and filled with LC ZLI2293. The quality of LC alignment in this cell was high. The LC in the cell was uniformly aligned in the direction determined by rubbing and plasma beam projection on the substrate. This implies that the easy axis of LC alignment on the object substrate (plasma beam treated PI substrate passivated by RM film) is generated in the incidence plane of the plasma beam. This in turn implies that RM passivation layer does not affect in-plane direction of the easy alignment axis generated by the plasma beam.

[00107] Example 2.

[00108] An asymmetric LC cell was prepared as in Example 1 and filled with LC ZLI4801-000. The cell demonstrated high quality parallel alignment. The RM passivation layer on the object substrate did not affect the in-plane direction of the easy alignment axis generated by the plasma beam.

[00109] Example 3.

[00110] The symmetric antiparallel LC cell was prepared as in Comparative Example 3, except that the plasma beam processed PI layers were passivated by RM layers as described in Example 1. The cell was filled with LC ZLI2293. The photographs of this cell, presented in FIGS. 3c and 3d, demonstrate high alignment of LCs in the cell. The pretilt angle in the cell was estimated by crystal rotation method to be 13° (Table 1).

[00111] The cell was stored at ambient conditions and pretilt angle in this cell was monitored over several months. The curve 2 in FIG. 4 exhibits that the pretilt angle initially decays and then stabilizes at the level of about 4°. Comparing with a Comparative Example 3 one can conclude that passivation of the aligning layer with RM stabilizes pretilt angle.

[00112] Example 4. [00113] The symmetric antiparallel LC cell was prepared as in Comparative Example 4, except that the plasma beam processed PI layers were passivated by RM layers as described in Example 1. The cell was filled with LC ZLI4801-000. High quality LC alignment is observed. The pretilt angle in the cell estimated by crystal rotation method was 6° (Table 1).

[00114] The curve 2 in FIG. 5 shows that the pretilt angle initially decays and then stabilizes at the level of about 4°. Compared to Comparative Example 4, one can conclude that aligning layers passivation with RM stabilizes pretilt angle.

[00115] Example 5.

[00116] A symmetric antiparallel LC cell was prepared as in Comparative Example 5, except that the plasma beam processed PI layers were passivated by RM layers as described in Example 1. The cell was filled with LC ZLI 4801-000 and kept at 90°C (just below clearing point of LC ZLI 4801-000, T_C=96°C). Periodically, the cell was cooled down to room temperature to measure value of pretilt angle. The pretilt angle as a function of time of backing at 90°C is ploted in FIG. 6 (curve 2). It is evident that pretilt angle practically does not change with a time of backing. Compared to Comparative Example 5, one can conclude that passivation of aligning layers with RM radically improves thermal stability of LC alignment.

[00117] Example 6.

[00118] The twist LC cell was fabricated as in Comparative Example 6, except that the plasma beam-processed PI layers were passivated by RM layers as described in Example 1. The cell was filled with LC ZLI2293. The pictures of this cell are presented in FIGS. 7c and 7d. They demonstrate high quality of LC alignment in the cell. The transmittance-voltage curve for this cell is presented in FIG. 8 (curve 2). The azimuthal anchoring energy was determined to be 8.9 x 10^"5 J/cm². The comparison with Comparative Example 6 leads to conclusion that the RM passivation does not change essentially the transmittance-voltage curve.

[00119] Example 7.

[00120] A symmetric antiparallel LC cell was prepared as in Comparative Example 3. The cell was filled with LC ZLI 4801-000, which contained 1.5 wt. % of reactive mesogen RMM 257 purchased from Merck and 0.1 wt. % of photoinitiator Irgacure 651. The cell was kept overnight for diffusion of RM to the aligning substrates and then irradiated with UV light from a high pressure mercury lamp to polymerize the RM. The light intensity and polymerization time were 8 mW/cm² and 10 min, respectively. After each 15 s of irradiation the cell was rotated in 180° so that the cell substrates faced to UV light alternately. The quality of LC alignment in the cell was the same as in Comparative Example 3. The pretilt angle in the cell estimated by crystal rotation method was 3.4°. The cell was kept at 90°C Qust below clearing point of LC ZLI 4801- 000, T_C=96°C). Periodically, it was cooled down to room temperature to measure value of pretilt angle. Within 1.5 h of cell annealing the pretilt angle partially dropped and stabilized on the level 2.2°. This level was stable over the next 24 hours of the annealing.

[00121] Example 8.

[00122] The Si0₂ aligning layers were deposited on glass/ITO substrates as in Comparative Example 7. The surface of Si0₂ layers was in turn coated by reactive mesogen mixture RMS006 from Merck designed for positive C optical films (for homeotropic alignment). For this purpose, commercial solution RMS006 was diluted by toluene in proportion 1 :30 and spin coated on the S1O2 layers at 3000 rpm over 30 s. After that the RM film was polymerized by irradiation with a non-polarized UV light from the high-pressure mercury lamp (at integral intensity of 60 mW/cm² and exposure time of 3 min).

[00123] Using these substrates the symmetric LC cell was fabricated as in Comparative Example 7 and filled with LCMLC6609. The quality of LC alignment was as high as for the reference cell (Comparative Example 7). The pretilt angle in the cell was about 89.0°.

[00124] The alignment aging in this cell was investigated. FIG. 9b shows photographs of the cell taken after the passing of different times after preparation. It is evident that, in contrast to the reference cell (FIG. 9a), LC alignment does not damage in the cell, even in the vicinity of the glue paths. The pretilt angle in the cell did not change noticeably for the time of aging monitoring.

[00125] Example 9. [00126] The cell was prepared as in Example 8 except that the cell gluing was provided by using fast hardening epoxy glue as in Comparative Example 8. The cell was filled with LC MJ961180. The quality of LC alignment was high, and the pretilt angle was about 88.2°. The pictures of the cell taken different time after preparation are shown in FIG. 10b. It is evident that, in contrast to reference cell (FIG. 10a), LC alignment does not damage in the cell, even in vicinity of the glue paths. The pretilt angle in the cell did not change noticeably for the time of aging monitoring.

[00127] The examples above serve to illustrate some embodiments of the present disclosure without limiting it. Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this present disclosure. For example, the disclosed method is applicable for stabilization of LC alignment on the substrates treated by atmospheric plasma flux [E. Jang, H. Song, S.D. Lee, Jpn.J.Appl.Phys., 46, L.1238 (2006)] or combination of several alignment processes, such as ion/plasma beam and rubbing, glow discharge plasma and rubbing [J. Fonseca, P. Charue, Y. Galerne, Mol.Cryst.Liq.Cryst., 329, 597 (1999)] atmospheric plasma and rubbing [O. Yaroshchuk et al., Appl. Surf. Science 257, 2443 (201 )], ion/plasma beam alignment and photoalignment, etc. Also, besides nematic LCs used in Examples, the method can be applied for other classes of LCs.

Claims

CLAIMS:

1. A method for preparing an alignment layer for a liquid crystal device comprising the following steps: (1) treating a substrate optionally containing a functional coating on the top by a particle beam treatment or a combination of treatments including the particle beam treatment thereby imparting to the substrate an alignment function; (2) depositing an auxiliary layer made of cross-linkable material so that this auxiliary layer aligns with the treated surface acquiring the alignment function; and (3) cross-linking the auxiliary layer.

2. The method of claim 1 , wherein the particle beam is an ion beam or a plasma beam providing anisotropic etching of the aligning substrate.

3. The method of claim 1 , wherein the particle beam is an atomic beam, an ion beam or a plasma beam providing deposition of an alignment coating on the top of the substrate.

4. The method of claim 1 , wherein the cross-linkable material is a reactive mesogen.

5. The method of claim 1 , wherein the auxiliary layer has uniform thickness.

6. The method of claim 1 , wherein the thickness of the auxiliary layer is in the range of from 1 nm to 1000 nm.

7. The method of claim 1 , wherein the thickness of the auxiliary layer is in the range of from 3 nm to 100 nm.

8. The method of claim 1 , wherein the thickness of the auxiliary layer is in the range of from 5 nm to 20 nm.

9. The method of claim 1 , wherein the thickness of auxiliary layer is patterned to be continuously or discretely varied between 0 and a maximum value in the range of from 1 nm to 1000 nm.

10. The method of claim 1 , wherein molecular alignment in the auxiliary layer is uniform.

11. The method of claim 1 , wherein molecular alignment in the auxiliary layer is patterned.

12. The method of claim 1 , wherein the auxiliary layer of cross-linkable materials is deposited on the aligning surface before placing liquid crystals in contact with the alignment surface.

13. The method of claim 1 , wherein the auxiliary layer of cross-linkable material is deposited on the aligning surface after placing liquid crystals in contact with this surface.

14. An alignment layer prepared according to any one of the preceding claims.

15. A method for improving stability of liquid crystal alignment on a particle beam-processed surface based on the use of an alignment layer prepared according to any one of claims 1-13.

16. A method for controlling pretilt angle and anchoring on a particle beam- processed surface combined with improvement of alignment stability, based on the use of an alignment substrate prepared according to one or more claims 1-13.