A composition of a cobalt chloride-modified azo dye photo-alignment layer and a preparation method thereof

Abstract

The present invention discloses a cobalt chloride modified azo dye photo-alignment layer composition, which comprises a hydrophilic azo dye compound and cobalt chloride. The composition of the present invention can maintain excellent alignment quality of the photo-alignment layer in an environment with a wide humidity range. The photo-alignment material composition prepared by this method has excellent performance, can better maintain the excellent alignment quality of the prepared alignment layer film in an environment with a wide humidity range, and at the same time realizes a more excellent photo-alignment process.

Description

Technical Field

[0001] The present invention relates to a composition for a photo-alignment layer and a preparation method thereof, and particularly relates to a composition for a cobalt chloride-modified azo dye photo-alignment layer and a preparation method thereof. Background Art

[0002] Hydrophilic azo dye compounds are excellent liquid crystal photo-alignment layer materials, and cobalt chloride compounds are used to optimize and regulate humidity problems sensitive in the hydrophilic azo dye photo-alignment process.

[0003] The photo-alignment method is a commonly used non-contact alignment method in the liquid crystal field. Among them, azo dye materials such as azobenzenesulfonic acid dye SD1 and brilliant yellow (BY) are often used as photosensitive materials in photo-alignment because they can undergo photo-induced molecular rotation. At the same time, water-soluble azo dyes such as BY and SD1 are prone to capturing moisture in the air due to hydroxyl groups, resulting in crystallization, which makes it difficult to orient the prepared thin film, significantly reducing the order parameter of the oriented molecules, or requiring a large dose of exposure intensity to obtain a comparable orientation level. Environmental humidity has a great impact on the photo-alignment quality of hydrophilic azo dye materials, both for non-composite azo dye layers and azo dye composite layers. The photo-induced phase delay, the order parameter of the alignment layer, and the alignment quality of the corresponding liquid crystal vary greatly at different humidities. For BY, the photo-stability of the alignment layer is also significantly affected by the relative humidity (RH). Rewritability can be guaranteed at extremely low RH (≤10%), but it gradually disappears as the relative humidity increases. When the relative humidity is 40% or higher, the alignment layer becomes light-resistant to further exposure. The RH range of the BY alignment layer should be below 50%. Especially when the relative humidity is below 40%, good photo-alignment quality can be guaranteed, which is recommended as the working window for BY photo-alignment applications. When the RH is greater than 50%, the photo-alignment quality is poor and should be avoided. For the SD1 material, its RH working window is recommended to be 50% - 70%. Due to this characteristic of water-soluble azo dyes, a large amount of resources are required to ensure the temperature and relative humidity of the process environment during the conventional photo-alignment thin film preparation process and exposure process. Some processes even need to be completed in a glove box filled with inert gas, which is not conducive to the large-scale production of photo-alignment technology. Summary of the Invention

[0004] Object of the Invention: The present invention aims to provide a composition for a cobalt chloride-modified azo dye photo-alignment layer that can maintain excellent alignment quality of the photo-alignment layer in an environment with a relatively large humidity range; another object of the present invention is to provide a preparation method for the photo-alignment layer composition.

[0005] Technical Solution: The cobalt chloride-modified azo dye photo-alignment layer composition described in the present invention comprises a hydrophilic azo dye compound and cobalt chloride.

[0006] Preferably, the hydrophilic azo dye compound is bright yellow or trisodium 5,5'-((1E,1'E)-(2,2'-disulfonato-[1,1'-biphenyl]-4,4'-diyl)bis(diazene-2,1-diyl))bis(2-hydroxybenzoate) (SD1), CD1 or Congo red; generally preferably a bisazo dye, which contains an azo group —N=N— structure in its molecular formula and is usually stored in a nitrogen cabinet or other moisture-proof environment.

[0007] Preferably, the molar ratio of cobalt chloride to the azo dye compound is 1:10 to 3:1.

[0008] Preferably, the composition further comprises one or more of a viscosity regulator, an acrylate polymer liquid crystal, an initiator or a photosensitizer.

[0009] The preparation method of the cobalt chloride-modified azo dye photo-alignment layer, which uses spin coating, slot coating, casting, inkjet printing or printing to dissolve the photo-alignment material composed of the composition according to any one of claims 1-4 in an organic solvent, coat the solution on a substrate, perform annealing treatment, heat on a hot stage or in an oven at 60 °C to 100 °C for 5 to 10 minutes to evaporate the residual solvent, form an amorphous photo-alignment film, and irradiate the photo-alignment layer on the substrate with polarized ultraviolet or blue light for orientation, the direction of which is perpendicular to the polarization direction of the polarized light, so as to obtain a preset photo-alignment orientation of azo dye molecules. During the annealing process, azo dye molecules and Co 2+ Metal ions form metal complexes through complexation reactions, and the formation of such complexes will further contribute to the long-term property stability of the dye molecule alignment layer.

[0010] Preferably, the organic solvent is one or more of DMF, DMSO, absolute ethanol, nitromethane, methanol, ethylene glycol, ethanolamine, aniline, isopropanol, benzyl alcohol or butyl cellosolve.

[0011] Preferably, the organic solvent is a mixed solvent of absolute ethanol and DMF with a mass ratio of 5:1 to 10:1.

[0012] Preferably, the weight ratio of the azo dye compound to the organic solvent is 0.1% to 5%, more preferably 0.5% to 1%.

[0013] Preferably, the substrate is a hard glass, plexiglass, ITO glass substrate, flexible PET or TAC film.

[0014] The composition can prepare a photo-alignment layer under the condition that the relative environmental humidity is 5 to 70%.

[0015] The optimization modification is to uniformly mix anhydrous cobalt chloride in the photo-alignment material, and the molar ratio of anhydrous cobalt chloride to the photo-alignment material is between 1:1 and 1:10. The ratio is adjusted accordingly according to the ambient humidity. The higher the humidity, the greater the ratio of anhydrous cobalt chloride.

[0016] The present invention optimizes the humidity range suitable for photo-alignment material processing. Under natural conditions, it is necessary to control the environmental humidity to meet the use conditions of azo dyes as photo-alignment materials. For the photo-alignment process, as the humidity increases, the number of water molecules inserted into the solid film of the azo dye increases, and the azo dye molecules will produce intrinsic crystals or semi-crystallizations during the film-forming process. The water molecules are combined with the hydrophilic functions of the azobenzene molecules, and the semi-crystallization interactions between the molecules become stronger, so that the azo dye molecules are firmly combined with each other before polarized light exposure. The energy of polarized light to align the azo dye molecules will be less than the interaction between the crystals or semi-crystallizations, so the photo-alignment material film loses the photo-orientation ability. According to the present invention, anhydrous cobalt chloride is mixed in the azo dye photo-alignment composition according to a certain ratio according to the environmental humidity, and the hydrophilic groups such as hydrogen bonds between the sodium sulfonate and the terminal hydroxyl groups of the azo dye molecules have a much lower affinity for water molecules than the water molecules captured by the anhydrous cobalt chloride hydrated into hexahydrate cobalt chloride, and the water molecules bound by the corresponding hydrophilic groups will be captured by the anhydrous cobalt chloride, thereby creating a relatively water-free microenvironment between the azo dye molecules. In this case, the aggregation of azo dye molecules is hindered by the intermolecular repulsive electrostatic force of the hydrophilic groups, thus forming an amorphous film that can be photo-aligned without the need to consume additional resources to control the humidity of the environment.

[0017] Furthermore, during the irradiation of the photo-aligned film with polarized light, the cobalt chloride hexahydrate produced in the aforementioned hydration process will also be exposed to light and release water, reducing to anhydrous cobalt chloride. The released crystal water will be inserted into the azo dye molecules pre-arranged by polarized light. This process will promote the further accurate arrangement of the azo dye molecules that have not been accurately arranged around the arranged molecules, thereby improving the orderliness of the film orientation.

[0018] The process of releasing crystal water and increasing the order of the photo-aligned film, i.e., the parameter, is as follows: the amorphous azo dye film is self-assembled by the molecules under polarized light to produce a one-dimensional nematic order. During the illumination process, the cobalt chloride hexahydrate generated as an optimized modification releases crystal water, which selectively hydrates around the hydrophilic functional groups of the azo dye molecules, and utilizes the soluble liquid crystal properties of the azo dye to promote molecular reordering, so that the molecular components originally having a one-dimensional nematic order are transformed into a high-order two-dimensional columnar order with greater structural anisotropy, thereby causing a sharp increase in the order parameter of the final photo-aligned film, and the corresponding parameter can be as high as 0.7 or more.

[0019] In the process of preparing the film, the cobalt chloride takes away the moisture in the microenvironment of the azo dye to form cobalt chloride hexahydrate, and the azo dye is not easy to crystallize due to the loss of moisture, so as to form an amorphous film required for the photo-orientation; subsequently, during the exposure and orientation, the cobalt chloride hexahydrate releases moisture due to the simultaneous exposure to light and is reduced to cobalt chloride. The released moisture will be inserted into the azo dye molecules that have already begun to orient, further inducing the orientation of the azo dye molecules, increasing the order parameter, which can be as high as 0.7 or more; after the photo-orientation process is completed, in the subsequent annealing process of the orientation layer film, due to the presence of cobalt ions in the cobalt chloride, under the action of the annealing temperature, BY and other dye molecules and Co 2+ Metal ions form metal complexes through complexation reactions. The formation of such complexes is beneficial to the long-term stability of the dye molecule alignment layer, thereby further solving the problem that the dye molecules as the alignment layer are easily affected by the environment such as temperature and light and cannot maintain order for a long time.

[0020] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: the composition described in the present invention can maintain the excellent alignment quality of the photo-alignment layer in an environment with a wider humidity range, the photo-alignment material composition prepared by the method has excellent performance, and can better maintain the excellent alignment quality of the prepared alignment layer film in an environment with a wider humidity range, while achieving a better photo-alignment process. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the molecular structure diagram of azo dye BY;

[0022] Figure 2 Schematic diagram of BY molecule when water molecule is inserted;

[0023] Figure 3 BY molecules and Co 2+ Diagram of the complexation reaction of metal ions;

[0024] Figure 4 BY molecules and Co 2+ Metal ion complex diagram;

[0025] Figure 5 is a BY amorphous film, and the black dots are BY images that are not fully dispersed;

[0026] Figure 6 For BY film, the crystal morphology begins to appear;

[0027] Figure 7 BY amorphous film, dots are cobalt chloride agglomeration images;

[0028] Figure 8 This is a BY film diagram. DETAILED DESCRIPTION

[0029] The technical solution of the present invention will be further described below in conjunction with embodiments.

[0030] Embodiment 1

[0031] 1) Take the dry azo dye powder, taking BY as an example, the molecular structure is as Figure 1 , anhydrous cobalt chloride, N,N-dimethylformamide (DMF), and store them in a glove box filled with argon. The glove box provides an inert environment, and the internal water vapor is controlled within 0.5 ppm. The relative humidity RH is 50%.

[0032] 2) Dissolve the BY powder at a concentration of 0.5 wt% and anhydrous cobalt chloride at a concentration of 0.05 wt% in N,N-dimethylformamide (DMF), and mix evenly.

[0033] 3) Use a spin coater to spin coat the above solution on a glass substrate at 3000 rpm for 30 seconds. The glass substrate was preliminarily treated with ozone plasma in an ultraviolet ozone cleaner for 20 minutes. Then dry it at 90 °C for 30 minutes to evaporate the solvent, and obtain an approximately 15 nm thick solid film, that is, the alignment layer film. In this process, anhydrous cobalt chloride will undergo a hydration reaction to form cobalt chloride hexahydrate.

[0034] 4) Expose the prepared alignment layer film substrate to 450 nm linearly polarized light for 15 minutes, and the light intensity at the irradiation site is 20 mW / cm 2 . The randomly distributed BY molecules will tend to align perpendicular to the light polarization. In this process, cobalt chloride hexahydrate will release crystal water due to simultaneous illumination, and water molecules will insert between the sorted BY molecules, further increasing the order parameter of the BY molecule sorting, as Figure 2 .

[0035] 5) Anneal the oriented BY film substrate at 80 °C for 15 minutes. During this process, some BY molecules and Co 2+ metal ions undergo a complexation reaction such as Figure 3 , forming metal complexes. There may be the formation of 1:1 type and 1:2 type metal complexes, such as Figure 4 , enhancing the long-term stability of the BY alignment layer.

[0036] The alignment layer film obtained in this embodiment is an amorphous film. As Figure 7 shown, no crystalline form is seen, and it can be smoothly polarized and oriented. The order parameter of the BY molecules after orientation is about 0.7.

[0037] Embodiment 2

[0038] In an environment with a relative humidity (RH) of 60%, BY powder was dissolved in DMF at a concentration of 0.5 wt% and cobalt chloride anhydrous at a concentration of 0.08 wt%, and mixed evenly. The above solution was spin-coated on a glass substrate at 3000 rpm for 30 seconds using a spin coater. The glass substrate was pre-treated with ozone plasma in an ultraviolet ozone cleaner for 20 minutes. Then it was dried at 90 °C for 30 minutes to evaporate the solvent. The remaining steps were the same as those in Example 1 to obtain an alignment layer film.

[0039] The alignment layer film obtained in this example is an amorphous film, which can be smoothly polarized and oriented. The order parameter of BY molecules after orientation is about 0.7.

[0040] Comparative Example 1

[0041] In an environment with a relative humidity (RH) of 30%, BY powder was dissolved in DMF at a concentration of 0.5 wt% and mixed evenly. The above solution was spin-coated on a glass substrate at 3000 rpm for 30 seconds using a spin coater. The glass substrate was pre-treated with ozone plasma in an ultraviolet ozone cleaner for 20 minutes. Then it was dried at 90 °C for 30 minutes to evaporate the solvent, obtaining an alignment layer film.

[0042] The alignment layer film obtained in this comparative example is an amorphous film. As Figure 5 shown, no crystalline form was seen, it can be smoothly polarized and oriented. The order parameter of BY molecules after orientation is about 0.7.

[0043] Comparative Example 2

[0044] In an environment with a relative humidity (RH) of 50%, BY powder was dissolved in DMF at a concentration of 0.5 wt% and mixed evenly. The above solution was spin-coated on a glass substrate at 3000 rpm for 30 seconds using a spin coater. The glass substrate was pre-treated with ozone plasma in an ultraviolet ozone cleaner for 20 minutes. Then it was dried at 90 °C for 30 minutes to evaporate the solvent, obtaining an alignment layer film.

[0045] The alignment layer film obtained in this comparative example began to show a crystalline form. As Figure 6 shown, the difficulty of polarization orientation increased, a larger exposure dose was required, and the order parameter of BY molecules after orientation was about 0.3.

[0046] Comparative Example 3

[0047] In an environment with a relative humidity RH of 60%, BY powder was dissolved in DMF at a concentration of 0.5 wt% and mixed evenly. The above solution was spin-coated on a glass substrate at 3000 rpm for 30 seconds using a spin coater. The glass substrate was preliminarily treated with ozone plasma in an ultraviolet ozone cleaner for 20 minutes. Then it was dried at 90°C for 30 minutes to evaporate the solvent, obtaining an alignment layer film.

[0048] For the alignment layer film obtained in this comparative example, the crystalline form of BY molecules increased. As Figure 8 shown, it was difficult to achieve polarization orientation. Even when the exposure dose was further increased and the exposure time was extended, no obvious orientation was observed.

Claims

1. A cobalt chloride-modified azo dye photo-alignment layer composition, characterized in that, The composition contains a hydrophilic azo dye compound and cobalt chloride.

2. The cobalt chloride-modified azo dye photo-alignment layer composition according to claim 1, characterized in that, The hydrophilic azo dye compound is bright yellow, SD1, CD1 or Congo red.

3. The cobalt chloride-modified azo dye photo-alignment layer composition according to claim 1, wherein The molar ratio of the cobalt chloride to the azo dye compound is 1:1 to 1:

10.

4. The cobalt chloride-modified azo dye photo-alignment layer composition according to claim 1, characterized in that, The composition further contains one or more of a viscosity regulator, an acrylate polymer liquid crystal, an initiator or a photosensitizer.

5. A method for preparing a cobalt chloride-modified azo dye photo-alignment layer according to any one of claims 1-4, characterized in that, The method uses spin coating, slot coating, casting, inkjet printing or printing to dissolve the photo-alignment material composed of the composition according to any one of claims 1-4 in an organic solvent, coat the solution on a substrate, perform annealing treatment, and irradiate the photo-alignment layer on the substrate with polarized ultraviolet or blue light for orientation.

6. The preparation method of the cobalt chloride modified azo dye photo-alignment layer according to claim 5, characterized in that, The organic solvent is one or more of DMF, absolute ethanol, ethylene glycol, benzyl alcohol or butyl cellosolve.

7. The preparation method of the cobalt chloride modified azo dye photo-alignment layer according to claim 5, characterized in that, The organic solvent is a mixed solvent of absolute ethanol and DMF with a volume ratio of 5:1 to 10:

1.

8. The preparation method of the cobalt chloride modified azo dye photo-alignment layer according to claim 5, characterized in that, The weight ratio of the azo dye compound to the organic solvent is 0.1% to 5%.

9. The preparation method of the cobalt chloride modified azo dye photo-alignment layer according to claim 5, characterized in that, The substrate is a rigid glass, plexiglass, ITO glass substrate, flexible PET or TAC film.

10. Use of the composition according to claim 1 for preparing a photo-alignment layer under the condition that the environmental relative humidity is 5-70%.

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