TECHNICAL FIELD

The present disclosure pertains to the technical field of liquid crystal display, and in particular to a liquid crystal alignment agent, a liquid crystal alignment film and a liquid crystal display element.

BACKGROUND

Liquid crystal displays have now become a mainstream technology of the information display industry, and have been widely used in various display areas such as calculator, mobile phone, digital camera, computer display, vehicle display and so on because of display advantages such as high resolution, high brightness, light weight, low energy consumption and complanation. The working principle of the liquid crystal display is to apply an external electric field to the liquid crystal, and polar molecules of the liquid crystal are twisted under an action of the external electric field, which changes the internal arrangement state of the liquid crystal molecules and changes the direction of the incident polarized light. Then the use of polarizers can control whether the light passes or not, so as to achieve purpose of display. Liquid crystal display elements usually use an alignment film to control the initial alignment state of the liquid crystal. The liquid crystal alignment film is prepared by coating a liquid crystal alignment agent on a display substrate, and then baking and aligning. The most mainstream of the alignment treatment method is rubbing alignment treatment. That is, cotton cloth or nylon is used to rub the liquid crystal alignment film on the electrode substrate in a single direction, so that the surface of the liquid crystal alignment film is grooved or the molecules on the surface of the alignment film are arranged orderly along the rubbing direction, thereby inducing the alignment of the liquid crystal molecules on the surface of the alignment film. With the development and progress of society, people have higher and higher requirements for the quality of liquid crystal displays, and various problems such as surface scratches, debris, static electricity and uneven processing in the rubbing alignment can no longer meet the existing needs.

As an alternative to the rubbing alignment, it is known that polarized ultraviolet light is used to irradiate the surface of the liquid crystal alignment film to align the liquid crystal. Photoalignment treatment can be divided into three types according to the mechanism: photodecomposition, photoisomerization and photocrosslinking. Due to its poor stability in the later stage of photoisomerization, photodecomposition and photocrosslinking are currently more popular.

The liquid crystal cell is divided into a TN (Twisted Nematic) type, a STN (Super Twisted Nematic) type, an IPS (In-Plane Switching), FFS (Fringe Field Switching) VA (Vertical Alignment) type and other liquid crystal display elements according to the change state and driving mode of the liquid crystal before and after the application of the applied electric field, where the IPS and FFS display modes are widely used in high-end display areas such as mobile phones, televisions and computer displays because of their advantages of high contrast and wide field of view. Compared with the existing rubbing alignment process, the above photoalignment treatment method helps to improve the contrast ratio and viewing angle of the IPS and FFS display mode, because the photoalignment treatment is more uniform than the rubbing alignment and is not affected by the uneven shape, scratches and static electricity of the substrate surface. However, compared with the rubbing alignment, the photo-alignment treatment has the problem of weak anchoring capacity of the liquid crystal, and residual image caused by small anisotropy will be produced after a long time of AC driving.

DC bias charges will be generated during the driving of IPS and FFS liquid crystal display elements THF (Thin Film Transistor). As the DC bias charges accumulate on the surface, the accumulated charges cannot be quickly dissipated when switching the image, which will cause the electric field intensity received by the liquid crystal to be inconsistent with the applied electric field intensity, resulting in residual image caused by the accumulation of DC charges.

When the IPS and FFS liquid crystal display elements are subjected to an alignment treatment of the mechanism of photodecomposition, a polymer on the surface of the liquid crystal alignment film will decompose along the polarization direction of the ultraviolet polarized light, resulting in small molecular decomposition products. For the decomposition products in industrial production, the liquid crystal alignment film is usually contacted with water or an organic solvent after the alignment treatment to dissolve and remove the photo-decomposition products. Insufficient removal of decomposition products will lead to poor alignment in some areas, resulting in abnormal display phenomena such as broken bright spots in the display area.

SUMMARY

In view of problems that occur in IPS or FFS mode liquid crystal display elements, such as residual image caused by weak anchoring ability of liquid crystal molecules and accumulation of DC charges, and a phenomenon of broken bright spots in the display caused by insufficient removal of photo-decomposition products, an object of the present disclosure is to provide a liquid crystal alignment agent, a liquid crystal alignment film and a liquid crystal display element with excellent residual image characteristics, good alignment effect and excellent removal of photo-decomposition products.

The technical solution to overcome the technical problems in the present disclosure is as follows:

The inventors found through a series of experiments that the above object of the present disclosure can be achieved by selecting at least one of a polyamic acid solution containing a specific structure and a polyimide solution maded by above plyamic acid.

Provided is a liquid crystal alignment agent, comprising a polymer and a solvent. The polymer comprises at least one of polyamic acid and polyimide. The polyamic acid is obtained by polymerization of diamine compound component A and dianhydride compound component B, and the polyimide is obtained by dehydration imidization of the polyamic acid. The diamine compound component A includes at least one of the diamine compounds represented by the following “Formula 1”;


wherein R1 is selected from one of —F or —CF3; R2 is selected from one of —OH and —SH; remaining hydrogen atoms on both benzene rings can be substituted.

Further, the remaining hydrogen atoms on the two benzene rings in the diamine compound represented by “Formula 1” may be substituted by methyl groups. Further, the diamine compound represented by “Formula 1” has structural formulas shown in 1-1 to 1-5:



Further, the dianhydride compound component B includes at least one of 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,3-dimethyl-1,2,3,4-cyclobutane tetracarboxylic dianhydride.

Further, the diamine compound component Ain the above 1 further includes other diamine compounds other than the “formula 1” diamine compound. The other diamine compounds are one selected from 1,2-bis(4-aminophenoxy)ethane, N-methyl-4-aminophenylethylamine, p-phenylenediamine, m-phenylenediamine, 4,4′-diaminodiphenylmethane, 2,4-diaminododecyloxybenzene, 3,5-diaminobenzoic acid, 4,4′-diaminodiphenyl ether, 1,3-bis(4-aminophenyl)urea, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene, 4-aminophenylethylamine, 4,4′-diaminodiphenylethane, 4,4′-diaminobenzophenone, 1,3-bis(4-aminophenoxy)propane, N,N′-bis(4-aminophenyl)piperazine, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,4-diaminooctadecyloxybenzene, 4,4′-diaminobenzamide, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 2,2-bis(4-aminophenyl)hexafluoropropane, 2,2′-dimethyl-4,4′-diaminobiphenyl and 4,4′-diaminodiphenylamine, or a mixture thereof.

Further, in the above diamine compound component A, a molar percentage of the diamine compound represented by Formula 1 is 5 to 80%, more preferably 10 to 50%.

Further, the dianhydride compound component B includes at least one of 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride.

Further, the above dianhydride compound component B may further include other dianhydride compounds other than 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, for example, one selected from 1,2,3,4-cyclopentane tetracarboxylic dianhydride, 2,3,5-tricarboxyl cyclopentyl acetic acid dianhydride, Pyromellitic dianhydride, 1,2,4,5-cyclohexane tetracarboxylic acid dianhydride, 3,3′,4,4′-biphenyl tetracarboxylic dianhydride, 3,3′,4,4′-biphenylsulfone tetracarboxylic dianhydride, and 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicaroxylic anhydrides, or a mixture thereof.

Further, a molar ratio of the diamine compound component A to the dianhydride compound component B is 100:80 to 120, more preferably 100:90 to 100.

Further, the solvent is one selected from N-methyl-2-pyrrolidone γ-butyrolactone, N,N-dimethylacetamide, N,N-dimethylformamide, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobuthyl ether, ethylene glycol dimethyl ether and diethylene glycol methyl ether acetate, or a mixture thereof.

A liquid crystal alignment film is made of the above liquid crystal alignment agent.

Provided is a liquid crystal display element, comprising the above liquid crystal alignment agent.

The present disclosure has the following advantageous effects:

1) The liquid crystal alignment film of the present disclosure can improve its anchoring ability to the liquid crystal and suppress residual image caused by small anisotropy after a long-term AC driving. The mechanism by which the liquid crystal alignment film of the present disclosure can improve the anchoring ability of the liquid crystal is still unclear. On the other hand, according to the following “an illustration of the beneficial effect”, when the polyamic acid containing diamine with the specific structure of “Formula 1” of the present disclosure is cured at a high temperature, the diamine of the specific structure of “Formula 1” can be dehydrated intermolecularly with the dianhydride or intramolecularly. In addition, the resulting two products are relatively stable in property, cannot be easily hydrolyzed or degraded, and carboxyl residues on the surface of the PI film, can increase the force between the PI film and the liquid crystal molecules, thereby improving the anchoring ability of the liquid crystal. In order to explain the above reaction types more effectively, the following formula example is given, but is not to limit the present disclosure:



An Illustration of the Beneficial Effect

It can be seen from the above formula that two products are generated in thermal curing, where the first product contains a large number of carboxyl groups, which can increase the force between the first product and the polar molecules of the liquid crystal.

2) The liquid crystal alignment agent and the liquid crystal alignment film of the present disclosure can quickly dissipate the accumulated DC charges, thereby suppressing the problem of residual image caused by the accumulation of the DC charges in the liquid crystal display element. The mechanism of rapid dissipation of DC charges can be considered as the reaction type in the above beneficial effect 1. The generated 1 and 2 products contain large conjugated groups, which can effectively accelerate the charge transfer.

3) After the photodecomposition alignment treatment, the liquid crystal alignment film of the present disclosure is excellent in the removal property of photo-decomposition products, and can suppress the phenomenon of poor alignment such as broken bright spots in the liquid crystal display element. According to the “benefit effect 1”, the molecules of the liquid crystal aligning film of the present disclosure contain more carboxyl solubilizing groups. After the liquid crystal alignment film is subjected to the photodecomposition alignment treatment, the resulting photo-decomposition products can be more easily removed when contacted with water or an organic solvent.

4) The method of the present disclosure is simple and has wide market prospect, and is suitable for mass production.

