Diamine compound, liquid crystal alignment agent, liquid crystal alignment film and liquid crystal display element

Abstract

The invention provides a diamine compound which has a structure as shown in formula (I), and each symbol in the formula (I) is as defined in the specification. Therefore, due to the fact that the diamine compound contains the nitrogen-containing heterocyclic ring and the hydroxyl at the tail end of the structure, the voltage holding ratio of the liquid crystal display element can be increased, charge release can be accelerated, and then the problem of ghost shadow is solved. # imgabs0 #

Description

Technical Field

[0001] The present invention relates to a diamine compound, a liquid crystal alignment agent, a liquid crystal alignment film and a liquid crystal display element, and in particular to a diamine compound that enables a liquid crystal display element to have a good voltage retention rate and can improve the afterimage phenomenon, a liquid crystal alignment agent containing the diamine compound, a liquid crystal alignment film containing the liquid crystal alignment agent and a liquid crystal display element made of the liquid crystal alignment film. Background Art

[0002] Liquid crystal displays use electric fields to change the direction of the liquid crystal molecules in the display, so as to adjust the degree of penetration of the light source and make each pixel bright or dark or change color, and then combine to form the image seen by the naked eye. In order to avoid the rotation angle of the liquid crystal molecules being chaotic, when no electric field is applied, the direction of the liquid crystal molecules must be consistent and maintain a specific pretilt angle (Pretilt angle), so it is necessary to use an alignment film to control the direction of the liquid crystal molecules.

[0003] However, when voltage is applied to a liquid crystal display, electric charges are easily adsorbed on the alignment film and are not easy to be detached, resulting in deterioration of the electrical properties of the alignment film and the problem of image sticking in the liquid crystal display after long-term use. Therefore, how to improve the electrical properties of the alignment film after long-term power-on and reduce the occurrence of image sticking remains a problem to be solved. Summary of the invention

[0004] In order to solve the above problems, the purpose of the present invention is to provide a diamine compound, the liquid crystal alignment agent and liquid crystal alignment film prepared by the diamine compound can provide good voltage holding rate and reduce charge residue, further improve the afterimage phenomenon of liquid crystal display elements under long-term use, and thus enhance the performance of liquid crystal display elements.

[0005] A diamine compound according to one embodiment of the present invention has a structure as shown in formula (I):

[0006]

[0008] Among them, R 1 , R 2 , R 3 , R 4 With R 5 Two of them are primary amine groups and the other three are each independently a hydrogen atom or a monovalent organic group that is not a primary amine group, X 1 is a single bond or a divalent group, the divalent group is an ether group, an ester group or an amide group, X 2 is a carbon atom or a nitrogen atom, and X 3 is a single bond, a phenylene group, a cyclohexylene group, an alkylene group, a phenylene group containing a substituent, or an alkylene group containing a substituent.2 With R 3 When both are primary amine groups, X 1 Not a single bond.

[0009] Thus, the diamine compound of the present invention can improve the voltage holding ratio of the liquid crystal display element and accelerate the charge release, thereby reducing the problem of image sticking, because it contains a nitrogen-containing heterocyclic ring and a hydroxyl group at the end of the structure. Furthermore, the liquid crystal alignment agent or liquid crystal display element containing the diamine compound of the present invention has a good vertical alignment performance, and when used in the horizontal alignment, because the chemical structure of the diamine compound introduces a hydroxyl group with a high dielectric constant, it can not only maintain a high voltage holding ratio, but also its highly polar structure tends to be distributed on the glass substrate side of the liquid crystal display element, so it will not have an adverse effect on the horizontal alignment between the liquid crystal alignment film and the liquid crystal layer.

[0010] According to the diamine compound of the aforementioned embodiment, wherein X 1 Can be a single bond.

[0011] According to the diamine compound of the aforementioned embodiment, the diamine compound may have a structure as shown in formula (I-1):

[0012]

[0013] According to the diamine compound of the aforementioned embodiment, the diamine compound may have a structure as shown in formula (I-2):

[0014]

[0015] According to another embodiment of the present invention, a liquid crystal alignment agent comprises a first polymer, wherein the first polymer is obtained by polymerization of a diamine reactant and a tetracarboxylic dianhydride reactant, wherein the diamine reactant comprises a first diamine compound, and the first diamine compound has a structure as shown in formula (I):

[0016]

[0017] Among them, R 1 , R 2 , R 3 , R 4 With R 5 Two of them are primary amine groups and the other three are each independently a hydrogen atom or a monovalent organic group that is not a primary amine group, X 1 is a single bond or a divalent group, wherein the divalent group is an ether group, an ester group, a ketone group or an amide group, and X 2 is a carbon atom or a nitrogen atom, and X 3 It is a single bond, a phenylene group, a cyclohexylene group, an alkylene group, a phenylene group having a substituent, or an alkylene group having a substituent.

[0018] According to the liquid crystal alignment agent of the aforementioned embodiment, wherein X 1 Can be a single bond.

[0019] According to the liquid crystal alignment agent of the aforementioned embodiment, the molar fraction of the first diamine compound in the total diamine reactants of the liquid crystal alignment agent may be greater than or equal to 2.5 mol %.

[0020] According to the liquid crystal alignment agent of the aforementioned embodiment, the molar fraction of the first diamine compound in the whole diamine reactants of the liquid crystal alignment agent may be greater than or equal to 25 mol % and less than or equal to 100 mol %.

[0021] According to the liquid crystal alignment agent of the aforementioned embodiment, the tetracarboxylic dianhydride reactant may be at least one selected from the group consisting of aromatic tetracarboxylic dianhydride, alicyclic tetracarboxylic dianhydride and non-cyclic aliphatic tetracarboxylic dianhydride.

[0022] According to the liquid crystal alignment agent of the aforementioned embodiment, the first diamine compound may have a structure as shown in Formula (I-1) or Formula (I-2):

[0023]

[0024] According to another embodiment of the present invention, a liquid crystal alignment film is made of the liquid crystal alignment agent described in the above embodiment.

[0025] A liquid crystal display element according to yet another embodiment of the present invention includes the liquid crystal alignment film as described in the above embodiment. DETAILED DESCRIPTION

[0026] The following will discuss various embodiments of the present invention in more detail. However, this embodiment can be an application of various inventive concepts and can be specifically implemented in various specific scopes. The specific embodiments are for illustrative purposes only and are not limited to the scope of the disclosure.

[0027] A diamine compound according to one embodiment of the present invention has a structure as shown in formula (I):

[0028]

[0029] Among them, R 1 , R 2 , R 3 , R 4 With R 5 Two of them are primary amine groups and the other three are each independently a hydrogen atom or a monovalent organic group that is not a primary amine group, X 1 is a single bond or a divalent group, the divalent group is an ether group, an ester group or an amide group, X 2 is a carbon atom or a nitrogen atom, and X3 is a single bond, a phenylene group, a cyclohexylene group, an alkylene group, a phenylene group containing a substituent, or an alkylene group containing a substituent. 2 With R 3 When both are primary amine groups, X 1 Not a single bond.

[0030] In detail, X 1 It may be a single bond, and the diamine compound may have a structure as shown in formula (I-1) or formula (I-2):

[0031]

[0032] From the above chemical structure, it can be seen that the diamine compound contains a nitrogen-containing heterocyclic ring and a hydroxyl group at the end of the structure, which can improve the voltage holding rate of the liquid crystal display element and accelerate the charge release, thereby reducing the problem of image sticking. In addition, the diamine compound has a hydroxyl group with a high dielectric constant, which can not only maintain a high voltage holding rate, but also if the diamine compound is applied to a liquid crystal alignment agent or a liquid crystal display element, the high polarity structure of the diamine compound tends to be distributed on the glass substrate side of the liquid crystal display element, so it will not have a negative impact on the alignment between the liquid crystal alignment film and the liquid crystal layer.

[0033] According to another embodiment of the present invention, a liquid crystal alignment agent comprises a first polymer, wherein the first polymer is obtained by polymerization of a diamine reactant and a tetracarboxylic dianhydride reactant, wherein the diamine reactant comprises a first diamine compound, and the first diamine compound has a structure as shown in formula (I):

[0034]

[0035] Among them, R 1 , R 2 , R 3 , R 4 With R 5 Two of them are primary amine groups and the other three are each independently a hydrogen atom or a monovalent organic group that is not a primary amine group, X 1 is a single bond or a divalent group, wherein the divalent group is an ether group, an ester group, a ketone group or an amide group, and X 2 is a carbon atom or a nitrogen atom, and X 3 It is a single bond, a phenylene group, a cyclohexylene group, an alkylene group, a phenylene group having a substituent, or an alkylene group having a substituent.

[0036] X of the first diamine compound 1It can be a single bond, and the first diamine compound can also have a structure as shown in the above formula (I-1) or formula (I-2). The molar fraction of the first diamine compound in the total diamine reactants of the liquid crystal alignment agent can be greater than or equal to 2.5 mol%, preferably greater than or equal to 25 mol% and less than or equal to 100 mol%, and most preferably greater than or equal to 25 mol% and less than or equal to 35 mol%.

[0037] Furthermore, the diamine reactant may further include a second diamine compound, and the second diamine compound may be, for example, p-phenylenediamine (PPDA), 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl (TBHG), 4,4'-diaminodiphenylmethane (MDA), 4,4'-diaminodiphenyl ether (ODA), 5(6)-amino-1-(4-aminophenyl)-1,3,3-triazine, Methyl indane (1-(4-Aminophenyl)-1,3,3-trimethyl-5-indanamine; TMDA), 4,4'-(4,4'-Isopropylidenediphenyl-1,1'-diyldioxy)dianiline, 4,4'-(1,1'-Biphenyl-4,4'-diyldioxy)dianiline, 3,5-Diaminobenzoic acid At least one of 3,3'-dicarboxy-4,4'-diaminodiphenylmethane (3,3'-Methylenebis(6-aminobenzoic acid; MBAA), a diamine compound as shown in the following formula (a), a diamine compound as shown in the following formula (b), and a diamine compound as shown in the following formula (c) (1,2-bis(4-aminophenoxy)ethane; 1,2-Bis(4-aminophenoxy)ethane). The chemical formula of the second diamine compound is shown in Table 1 below, but the present invention is not limited thereto.

[0038]

[0039]

[0040]

[0041] The tetracarboxylic dianhydride reactant may be selected from at least one of the group consisting of aromatic tetracarboxylic dianhydride, alicyclic tetracarboxylic dianhydride and non-cyclic aliphatic tetracarboxylic dianhydride. For example, the tetracarboxylic dianhydride reactant may include 1,2,4,5-Benzenetetracarboxylic-1,2:4,5-dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 4,4'-oxydiphthalic anhydride (ODDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPA ... dianhydride; BTDA), 3-(Carboxymethyl)-1,2,4-cyclopentanetricarboxylicacid-1,4:2,3-dianhydride; TCA), 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxyl ic anhydride; TDA), 1,2,3,4-butanetetracarboxylic dianhydride; dianhydride; BT-100) or 1,2,3,4-cyclobutanetetracarboxylic dianhydride (Cyclobutane-1,2,3,4-tetracarboxylic dianhydride; CBDA), the chemical formula of the tetracarboxylic dianhydride reactant is shown in Table 2 below, but the present invention is not limited thereto.

[0042]

[0043]

[0044] In some embodiments of the present invention, the liquid crystal alignment agent may further include a polymer other than the first polymer, for example: a second polymer obtained by reacting the second diamine compound with a tetracarboxylic dianhydride reactant, wherein the second diamine compound included in the second polymer does not include the diamine compound shown in the above formula (I).

[0045] According to another embodiment of the present invention, a liquid crystal alignment film is made of the liquid crystal alignment agent as described in the above embodiment. Specifically, the liquid crystal alignment film is prepared by coating the liquid crystal alignment agent on a substrate to form a layer of liquid crystal alignment film on the substrate.

[0046] The above-mentioned liquid crystal alignment agent can be coated on the substrate by roller coating, spin coating or printing, and heated by an oven or a heating plate to form a liquid crystal alignment film. The pre-bake temperature during heating can be 50°C to 120°C, and the hard-bake temperature can be 120°C to 240°C.

[0047] According to another embodiment of the present invention, a liquid crystal display element comprises the liquid crystal alignment film as described in the above embodiment. Specifically, the preparation method of the liquid crystal display element is to take two substrates with the liquid crystal alignment film, spray a gap material on the liquid crystal alignment film of one of the substrates, and assemble the two substrates so that the two layers of liquid crystal alignment film are located between the two substrates, and then inject liquid crystal into the gap between the two substrates and seal it to form a liquid crystal display element.

[0048] The present invention is further illustrated by the following specific embodiments, which are used to facilitate those skilled in the art to which the present invention belongs, so that they can fully utilize and practice the present invention without excessive interpretation. These embodiments should not be regarded as limiting the scope of the present invention, but are used to illustrate how to implement the materials and methods of the present invention.

[0049] <First Synthesis Example>

[0050] The first synthesis example is to perform a reaction through the following reaction formula 1 to synthesize the first diamine compound as shown in the above formula (I-1).

[0051] In detail, 400 grams of 1-(4-hydroxyphenyl)piperazine and 900 grams of isopropanol (IPA) are placed in a reactor, stirred and dissolved, and then 450 grams of 1,4-dinitrofluorobenzene are slowly dripped in. After the dripping is completed, the solution is heated to 90°C and refluxed for 6 hours, and then the reaction is confirmed to be complete by thin-layer chromatography (TLC). Then, the solution after the reaction is vacuum filtered, the solid is collected, and it is dried under high vacuum for one day to obtain a nitro intermediate (bright yellow powder). The nitro intermediate and 6000 grams of tetrahydrofuran (THF) are placed in a reactor, stirred and dissolved evenly, 60 grams of 5% palladium carbon are added, and then 200 grams of hydrazine hydrate are slowly dripped in. After the dripping is completed, the solution is heated to 70°C and refluxed for 2 hours. After the hydrogenation is completed, it is filtered and the solvent is removed by vacuum concentration to obtain a crude product. Next, the crude product was stirred and washed, and then vacuum filtered to collect the solid, which was then dried under high vacuum for one day to obtain the first diamine compound (off-white powder) represented by formula (I-1).

[0052]

[0053]

[0054] <Second Synthesis Example>

[0055] The second synthesis example is to perform a reaction through the following reaction formula 2 to synthesize the first diamine compound as shown in the above formula (I-2).

[0056] In detail, 200 grams of 4-hydroxypiperidine and 900 grams of IPA are placed in a reactor and stirred to dissolve, and then 450 grams of 1,4-dinitrofluorobenzene are slowly dripped in. After the dropwise addition is completed, the solution is heated to 90°C and refluxed for 6 hours, and then the reaction is confirmed to be complete by TLC. Then, the solution after the reaction is completed is vacuum filtered, the solid is collected, and it is dried under high vacuum for one day to obtain a nitro intermediate (bright yellow powder). The nitro intermediate and 6000 grams of THF are placed in a reactor, stirred and dissolved evenly, 60 grams of 5% palladium carbon are added, and then 200 grams of hydrazine hydrate are slowly dripped in. After the dropwise addition is completed, the solution is heated to 70°C and refluxed for 2 hours. After the hydrogenation is completed, it is filtered and the solvent is removed by vacuum concentration to obtain a crude product. Then, the crude product is stirred and washed, and then vacuum filtered, the solid is collected, and it is dried under high vacuum for one day to obtain the first diamine compound (off-white powder) shown in formula (I-2).

[0057]

[0058] <First Comparative Synthesis Example>

[0059] The first comparative synthesis example is to perform a reaction through the following reaction formula 3 to synthesize a diamine compound as shown in the following formula (a).

[0060] In detail, 200 grams of 4-hydroxypiperidine and 900 grams of IPA are placed in a reactor, stirred and dissolved, and then 450 grams of 1,4-dinitrofluorobenzene are slowly dripped in. After the dropwise addition is completed, the solution is heated to 90°C and refluxed for 6 hours, and then the reaction is confirmed to be complete by TLC. Then, the solution after the reaction is completed is vacuum filtered, the solid is collected, and it is dried under high vacuum for one day to obtain a nitro intermediate (bright yellow powder). 160 grams of sodium hydride and 3000 grams of THF are placed in a reactor under an ice bath, and after the mixed solution of the nitro intermediate and THF is dripped in, 900 grams of iodomethane is slowly dripped in. Then, the solution is naturally returned to room temperature, and after confirming the completion of the reaction by TLC, water and ethyl acetate are added under an ice bath, and the product is purified by column chromatography after extraction. The product and THF are placed in a reaction kettle, stirred and dissolved evenly, 60 grams of 5% palladium carbon are added, and then 200 grams of hydrazine hydrate is slowly dripped in. After the addition is completed, the solution is heated to 70°C and refluxed for 2 hours. After the hydrogenation is completed, the solvent is removed by filtration and concentration under reduced pressure to obtain a crude product. Then, the crude product is stirred and washed, and then vacuum filtered, the solid is collected, and dried under high vacuum for one day to obtain a second diamine compound (purple powder) as shown in formula (a).

[0061]

[0062] <Polymer 1 to Polymer 15>

[0063] Polymers 1 to 15 were prepared by the following method: A 100 ml three-mouth reaction kettle was used, wherein a thermometer was inserted into the first inlet, the second inlet was a feed inlet, and the second inlet was closed after the feed was added, and nitrogen was introduced into the third inlet. Next, a diamine reactant was placed in the three-mouth reaction kettle and dissolved in N-methyl-2-pyrrolidone (1-Methyl-2-pyrrolidone; NMP) to prepare a diamine reactant solution with a solid content of 20 wt%.

[0064] Next, after the tetracarboxylic dianhydride reactant was added to the diamine reactant solution, NMP was added again to adjust the solid content to 15 wt %, and then a polymerization reaction was carried out at 30° C. After the polymerization reaction lasted for 6 hours, a polyamic acid solution with a solid content of 15 wt % was obtained. The types and molar fractions of the diamine reactant and the tetracarboxylic dianhydride reactant used in each polymer are listed in Table 3 below.

[0065]

[0066] <First to Seventh Embodiments and First to Seventh Comparative Examples>

[0067] According to the molar fractions shown in Table 4 below, polymers 1 to 15 are dissolved in a solvent of N-methyl-2-pyrrolidone and diethylene glycol monobutyl ether in a weight ratio of 7:3, and the polymer solution is diluted to a solid content of 5.3 wt % to obtain the liquid crystal alignment agents of the first to seventh embodiments and the first to seventh comparative examples.

[0068]

[0069] Next, the liquid crystal alignment agents of the first to seventh embodiments and the first to seventh comparative examples are respectively coated on the substrate with electrodes by spin coating, and then placed on a heating plate at 80°C for drying for 1 minute. If the application is with vertical liquid crystal, it is placed in an oven at 230°C for 30 minutes for curing to form a film with a thickness of If it is used with horizontal liquid crystal, it will be placed in an oven at 230°C for 30 minutes for curing. The cured film is photo-aligned with linear polarized light with a wavelength of 254nm and an exposure energy of 250mJ as the light source, and then baked for a second time at 230°C for 30 minutes to form a film with a thickness of The application of vertical liquid crystal or horizontal liquid crystal will be listed in the subsequent Table 5.

[0070] Next, a frame glue is applied on the substrate with the aforementioned liquid crystal alignment film, and a spacer is sprayed on another substrate with the aforementioned liquid crystal alignment film. The two substrates are then paired together, liquid crystal is injected into the gap between the two substrates, and the liquid crystal injection hole between the two substrates is sealed to preliminarily form a liquid crystal display element (liquid crystal box).

[0071] The following will be conducted on the electrical test and alignment test of the liquid crystal display element, the liquid crystal display element respectively comprises the liquid crystal alignment film prepared by the liquid crystal alignment agent of the first embodiment to the seventh embodiment and the first comparative example to the seventh comparative example, the test results are listed in the following Table 5, and the test method of each test is as follows:

[0072] (1) Voltage holding ratio (VHR): The liquid crystal display element is placed in an oven at 60°C and an initial voltage is applied to the liquid crystal display element. The initial voltage is a rectangular wave with a frequency of 0.6 Hz and a wave height of ±1 V. Then, the measured voltage is measured 1667 milliseconds after the circuit is opened, and the voltage holding ratio is calculated using the following formula:

[0073] VHR=(V2 / V1)×100%;

[0074] Wherein, V1 is the initial voltage and V2 is the measured voltage.

[0075] (2) Residual direct current (RDC): The liquid crystal display element was placed in an oven at 60°C and a 5V direct current was applied to the liquid crystal display element for 1 hour. The element was then grounded for 1 second and the residual direct current was measured 10 minutes after the circuit was opened.

[0076] (3) Alignment evaluation: Place the liquid crystal display element under the polarizing film and observe the uniformity of the display screen of the liquid crystal display element. If the brightness is uniform, the result is judged as "OK". If the alignment is bad (i.e. there are liquid crystal washout marks), the result is judged as "NG".

[0077]

[0078]

[0079] Please compare the first embodiment with the first comparative example. When the tetracarboxylic dianhydride reactant is the same, for a vertical liquid crystal liquid crystal display element, if a liquid crystal alignment agent prepared by using the diamine compound of the present invention as a reactant is used, RDC can be reduced and charge is not easily retained, thereby improving the afterimage phenomenon of the liquid crystal display element.

[0080] Please compare the third embodiment with the second comparative example. When the tetracarboxylic dianhydride reactant is the same, for a horizontal liquid crystal display element, if a liquid crystal alignment agent prepared using the diamine compound of the present invention as a reactant is used, RDC can be reduced and a better alignment evaluation result can be obtained.

[0081] Please compare the third embodiment with the third comparative example. When the tetracarboxylic dianhydride reactant is the same, for a horizontal liquid crystal display element, if a liquid crystal alignment agent prepared using the diamine compound of the present invention as a reactant is used, VHR can be increased, RDC can be reduced, and better alignment evaluation results can be obtained.

[0082] Please compare the third embodiment with the fourth comparison example or the fifth comparison example. When the tetracarboxylic dianhydride reactant is the same, for a horizontal liquid crystal display element, if a liquid crystal alignment agent prepared using the diamine compound of the present invention as a reactant is used, VHR can be increased or RDC can be reduced, and a better alignment evaluation result can be obtained.

[0083] Comparing the fourth embodiment with the sixth comparative example, when the tetracarboxylic dianhydride reactant is the same, for a horizontal liquid crystal display element, if a liquid crystal alignment agent prepared using the diamine compound of the present invention as a reactant is used, VHR can be increased and RDC can be reduced.

[0084] Comparing the fifth embodiment with the seventh comparative example, when the tetracarboxylic dianhydride reactant is the same, for a horizontal liquid crystal display element, if a liquid crystal alignment agent prepared by using the diamine compound of the present invention as a reactant is used, VHR can be increased and RDC can be reduced.

[0085] Please pay special attention to the sixth embodiment, which includes 50 mol% of the first polymer (polymer 13) and 50 mol% of the second polymer (polymer 15), wherein the molar fraction of the first diamine compound represented by formula (I-2) in the total diamine reactants of the first polymer is 5 mol%. From the test results of the sixth embodiment, it can be seen that the liquid crystal alignment agent of the sixth embodiment has good VHR and alignment evaluation results. In other words, when the molar fraction of the first diamine compound contained in the liquid crystal alignment agent in the total diamine reactants of the liquid crystal alignment agent is greater than or equal to 2.5 mol%, better alignment evaluation results can be obtained, and it is helpful to improve VHR.

[0086] In summary, the diamine compound of the present invention can improve the voltage holding ratio of the liquid crystal display element and accelerate the charge release, thereby reducing the problem of image sticking due to the presence of a nitrogen-containing heterocyclic ring and a hydroxyl group at the end of the structure. Furthermore, the liquid crystal alignment agent or liquid crystal display element containing the diamine compound of the present invention has a good vertical alignment performance, and when applied in the horizontal alignment, because the chemical structure of the diamine compound introduces a hydroxyl group with a high dielectric constant, it can not only maintain a high voltage holding ratio, but also its highly polar structure tends to be distributed on the glass substrate side of the liquid crystal display element, so it will not have an adverse effect on the horizontal alignment between the liquid crystal alignment film and the liquid crystal layer.

[0087] Although the present invention has been disclosed as above by way of embodiments, it is not intended to limit the present invention. Anyone skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be determined by the definition of the attached claims.

Claims

1. A diamine compound having a structure as shown in formula (I): in, Two of R1, R2, R3, R4 and R5 are primary amine groups and the other three are each independently a hydrogen atom or a non-primary amine monovalent organic group, X1 is a single bond or a divalent group, the divalent group is an ether group, an ester group or an amide group, X2 is a carbon atom or a nitrogen atom, and X3 is a single bond, a phenylene group, a cyclohexylene group, an alkylene group, a phenylene group containing a substituent, or an alkylene group containing a substituent; When R2 and R3 are both primary amine groups, X1 is not a single bond.

2. The diamine compound according to claim 1, wherein X1 is a single bond.

3. The diamine compound according to claim 1, wherein the diamine compound has a structure as shown in formula (I-1):

4. The diamine compound as claimed in claim 1, wherein the diamine compound has a structure as shown in formula (I-2):

5. A liquid crystal alignment agent, comprising: The first polymer is obtained by polymerization of a diamine reactant and a tetracarboxylic dianhydride reactant. The diamine reactant comprises a first diamine compound, and the first diamine compound has a structure as shown in formula (I): in, Two of R1, R2, R3, R4 and R5 are primary amine groups and the other three are each independently a hydrogen atom or a non-primary amine monovalent organic group, X1 is a single bond or a divalent group, the divalent group is an ether group, an ester group, a ketone group or an amide group, X2 is a carbon atom or a nitrogen atom, and X3 is a single bond, a phenylene group, a cyclohexylene group, an alkylene group, a phenylene group containing a substituent, or an alkylene group containing a substituent. The liquid crystal alignment agent according to claim 5 , wherein X1 is a single bond. 7 . The liquid crystal alignment agent as claimed in claim 5 , wherein the molar fraction of the first diamine compound to the whole diamine reactants of the liquid crystal alignment agent is greater than or equal to 2.5 mol %. 8 . The liquid crystal alignment agent as claimed in claim 7 , wherein the molar fraction of the first diamine compound to the whole diamine reactants of the liquid crystal alignment agent is greater than or equal to 25 mol % and less than or equal to 100 mol %. 9 . The liquid crystal alignment agent as claimed in claim 5 , wherein the tetracarboxylic dianhydride reactant is at least one selected from the group consisting of aromatic tetracarboxylic dianhydride, alicyclic tetracarboxylic dianhydride and non-cyclic aliphatic tetracarboxylic dianhydride.

10. The liquid crystal alignment agent according to claim 5, wherein the first diamine compound has a structure as shown in Formula (I-1) or Formula (I-2): 11 . A liquid crystal alignment film, made of the liquid crystal alignment agent according to any one of claim 5 to claim 10 . 12 . A liquid crystal display element, comprising the liquid crystal alignment film according to claim 11 .

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