Preparation method of colorless polyimide film, obtained colorless polyimide film and application of colorless polyimide film

Abstract

The invention discloses a preparation method of a colorless polyimide film, the obtained polyimide film and application of the polyimide film. The preparation method comprises the following steps: (1) respectively obtaining a flexible polyamide acid solution A and a rigid polyamide acid solution B; (2) carrying out (partial) imidization treatment on the flexible polyamide acid solution A to obtain a flexible polyamide acid (partial) imidization system; (3) mixing the flexible polyamic acid (partial) imidization system obtained in the step (2) with a rigid polyamic acid solution B, and stirring to obtain a membrane casting solution; and (4) carrying out coating film formation and imidization treatment on the film casting solution to obtain the polyimide film. The development and application of the material provide feasibility for improving the comprehensive performance of the colorless transparent polyimide film.

Description

Technical Field

[0001] The present invention belongs to the field of polyimide films, and particularly relates to a preparation method of a colorless polyimide film, the obtained polyimide film and its applications. Background Art

[0002] Colorless transparent polyimide combines certain heat-resistant characteristics of polyimide and excellent visible light transmittance, providing new application feasibility for the future development of fully transparent flexible displays. However, in the fields of cover plates, touch controls, or substrates, requirements for the comprehensive performance of materials are always put forward. Compared with ordinary yellow polyimide, which has good intra-chain and inter-chain interactions such as charge transfer interactions, the optical properties of colorless transparent polyimide itself determine that charge transfer interactions need to be weakened, which requires improving the mechanical properties, thermal dimensional stability, etc. of the material from other aspects.

[0003] Conventionally, improving the performance of polyimide materials is often achieved through copolymerization. By randomly copolymerizing flexible and rigid structural monomers, or blending polyamic acids of different systems to improve the elongation at break and thermal dimensional stability of the film. However, since the polyamic acid will continuously undergo chain rearrangement reactions in solution, the orderliness of the chain segment composition is often significantly affected, making it difficult to effectively exert their respective characteristics, and even resulting in performance weaker than their respective basic structures. Summary of the Invention

[0004] In order to overcome the problems existing in the prior art, the present invention provides a preparation method of a colorless polyimide film with high elongation at break and low coefficient of thermal expansion, the obtained polyimide film and its applications. Among them, the low-molecular-weight flexible chain segment is first partially imidized, and then block-copolymerized onto the rigid polymer chain. At the same time, the rigid chain segment is induced to undergo partial orientation and in-plane stacking in the solution, thereby further reducing the coefficient of thermal expansion, improving the elongation at break of the film material, and increasing the thermal dimensional stability performance, etc. The development and application of such materials provide feasibility for improving the comprehensive performance of colorless transparent polyimide films.

[0005] One of the purposes of the present invention is to provide a preparation method of a colorless polyimide film, including: (1) respectively obtaining a flexible polyamic acid solution A and a rigid polyamic acid solution B; (2) performing (partial) imidization treatment on the flexible polyamic acid solution A to obtain a (partially) imidized flexible polyamic acid system; (3) mixing the (partially) imidized flexible polyamic acid system obtained in step (2) with the rigid polyamic acid solution B, and stirring to obtain a casting solution; (4) performing coating and film formation and imidization treatment on the casting solution to obtain the polyimide film.

[0006] In a preferred embodiment, both ends of the polyamic acid in the flexible polyamic acid solution A are capped with anhydride groups, and both ends of the polyamide in the rigid polyamic acid solution B are capped with amine groups.

[0007] Among them, the anhydride group in the present invention refers to an acid anhydride group.

[0008] In a preferred embodiment, the molecular weight of the polyamic acid in the flexible polyamic acid solution A is lower than that of the polyamic acid in the rigid polyamic acid solution B.

[0009] In a further preferred embodiment, the molecular weight of the polyamic acid in the flexible polyamic acid solution A is 2k to 40 kDa, preferably 5k to 20 kDa, such as 2 kDa, 5 kDa, 8 kDa, 10 kDa, 12 kDa, 15 kDa, 18 kDa, 20 kDa, 22 kDa, 25 kDa, 28 kDa, 30 kDa, 32 kDa, 35 kDa, 38 kDa or 40 kDa.

[0010] In a still further preferred embodiment, the molecular weight of the polyamic acid in the rigid polyamic acid solution B is 120 to 200 kDa, preferably 140 to 180 kDa, such as 120 kDa, 130 kDa, 140 kDa, 150 kDa, 160 kDa, 170 kDa, 180 kDa, 190 kDa or 200 kDa.

[0011] In a preferred embodiment, in step (1), the flexible polyamic acid solution A is prepared from raw material A including a flexible dianhydride compound A and a flexible diamine compound A.

[0012] In a further preferred embodiment, the flexible dianhydride compound A is selected from at least one of the compounds represented by formula (1), the compounds represented by formula (2), and the compounds represented by formula (3), and the flexible diamine compound A is selected from at least one of the compounds represented by formula (4) to formula (5):

[0013]

[0014] In formulas (1) to (5), R1 represents a bridging structure and is independently selected from one of oxygen, hexafluoroisopropyl, keto group, methylene, substituted methylene, and sulfone group. In formulas (2), (3), and (4), the two R1s are the same or different; in formulas (3) to (4), each R2 is selected from phenyl, substituted phenyl, and one of them, and R3 is selected from methylene or substituted methylene, etheroxy group, keto group or sulfone group.

[0015] In a further preferred embodiment, in formulas (1)-(5), R1 represents a bridging structure, and the structures of the flexible dianhydride compound A and the flexible diamine compound A contain a total of more than 3 bridging structures R1.

[0016] In a preferred embodiment, in formulas (1)-(3), the R1, R2, and R3 groups and the corresponding acid anhydride and primary amine substitution positions are in the meta or para position; in order to obtain a relatively low coefficient of thermal expansion, monomers with a para structure are more preferred.

[0017] In this way, the flexibility of the main chain of the flexible polyamic acid solution A can be further improved, especially the solubility after partial imidization, which is mainly manifested as almost unchanged solution viscosity, good solubility, and no precipitation.

[0018] Preferably, when the flexible dianhydride compound A is selected from the compounds shown in formula (1), the flexible diamine compound A is selected from the compounds shown in formula (4); when the flexible dianhydride compound A is selected from the compounds shown in formula (2) and / or formula (3), the flexible diamine compound A is selected from the compounds shown in formula (4) and / or formula (5); when the flexible diamine compound A is selected from the compounds shown in formula (4), the flexible dianhydride compound A is selected from at least one of the compounds shown in formulas (1)-(3); when the flexible diamine compound A is selected from the compounds shown in formula (5), the flexible dianhydride compound A is selected from at least one of the compounds shown in formula (2) and / or formula (3).

[0019] In a preferred embodiment, the flexible dianhydride compound shown in formula (1) containing 1 bridging structure R1 is selected from at least one of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and 3,3',4,4'-diphenylether tetracarboxylic dianhydride; and / or, the flexible diamine compound shown in formula (5) containing 1 bridging structure R1 is selected from at least one of 4,4'-diaminodiphenyl ether, 3,4`-diaminodiphenyl ether, 4,4-diaminodiphenylmethane, 4,4-diaminobenzophenone, 4,4'-diaminophenyl sulfone, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, 2,2`-bis(trifluoromethyl)-4,4`-diaminophenyl ether, bis(4-aminophenyl)hexafluoropropane (6FIP), and bis(3-aminophenyl)hexafluoropropane.

[0020] In a further preferred embodiment, the flexible dianhydride compounds of formula (2) to formula (3) containing two bridging structures R1 are selected from at least one of hydrogenated pyromellitic dianhydride, 9,9-bis(trifluoromethyl)-2,3,6,7-xanthene tetracarboxylic dianhydride, 4,4'-(4,4'-isopropyl diphenoxy) diphthalic anhydride; and / or, the flexible diamine compound of formula (4) containing two bridging structures R1 is selected from at least one of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 4,4'-(1,3-phenylene dioxy)dianiline, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 2,2'-bis(4-aminophenoxyphenyl)propane, 9,9-bis(4-aminophenyl)fluorene (fluorene diamine), 9,9-bis(3-fluoro-4-aminophenyl)fluorene.

[0021] In the most preferred embodiment, the flexible dianhydride compound A and the flexible diamine compound A are selected from at least one of compounds containing fluorine elements or compounds in which the R1 to R3 groups and the corresponding acid anhydrides and primary amines are substituted at the para positions. For example, the flexible dianhydride compound A is selected from at least one of 4,4'-(hexafluoroisopropylene) diphthalic anhydride, 4,4'-(4,4'-isopropyl diphenoxy) diphthalic anhydride; and / or, the flexible diamine compound A is selected from at least one of 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, bis(4-aminophenyl)hexafluoropropane (6FIP), 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 4,4'-(1,3-phenylene dioxy)dianiline, 1,4-bis(4-aminophenoxy)benzene.

[0022] In a preferred embodiment, when preparing the flexible polyamic acid solution A, the molar ratio of the flexible dianhydride compound A to the flexible diamine compound A is 1.2:1 to 1.06:1, preferably 1.16:1 to 1.1:1, for example 1.2:1, 1.18:1, 1.16:1, 1.14:1, 1.12:1, 1.1:1, 1.08:1, 1.06:1.

[0023] In a preferred embodiment, the flexible polyamic acid solution A is obtained as follows: the flexible diamine compound A is dissolved in solvent A, and under a protective atmosphere, the flexible dianhydride compound A is added and stirred to react to obtain the flexible polyamic acid solution A.

[0024] Among them, when preparing the flexible polyamic acid solution A, the solid content is 10 to 20 wt%, preferably 12 to 18 wt%, where the solid content refers to the proportion of the solid feed in the total weight of the solution.

[0025] In a further preferred embodiment, the solvent A is selected from aprotic polar solvents commonly used in the preparation of polyamic acid, preferably at least one of amide solvents, more preferably at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0026] In a still further preferred embodiment, the conditions of the reaction include: the temperature condition is 0 to 60°C, preferably 20 to 40°C; and / or the time is 2 to 12 h, preferably 4 to 8 h.

[0027] Among them, the protective atmosphere is selected from at least one of nitrogen and inert gases, preferably nitrogen and / or argon.

[0028] In a preferred embodiment, the imidization treatment in step (2) is carried out in the presence of an imidization reagent, and the imidization reagent includes a dehydrating agent and a catalyst.

[0029] In a further preferred embodiment, the dehydrating agent is selected from acid anhydride dehydrating agents, preferably at least one of acetic anhydride and trifluoroacetic anhydride; and / or the catalyst is selected from tertiary amine catalysts, preferably at least one of triethylamine, pyridine, quinoline, isoquinoline, triethylenediamine, methylpyrrole, methylpyrrolidine, and methylimidazole, more preferably at least one of triethylamine and quinoline.

[0030] In a still further preferred embodiment, the molar ratio of the dehydrating agent to the catalyst is 1:1 to 1.2:1 (such as 1:1, 1.02:1, 1.05:1, 1.08:1, 1.1:1, 1.12:1, 1.15:1, 1.18:1, or 1.2:1); and / or the molar ratio of the catalyst to the diamine raw material (i.e., the flexible diamine compound A) in the flexible polyamic acid solution A is 0.8:1 to 1.6:1 (such as 0.8:1, 1:1, 1.2:1, 1.4:1, or 1.6:1).

[0031] Among them, the amounts of the catalyst and the dehydrating agent can only cause partial imidization of the flexible polyamic acid solution A in step (2), and the unreacted imidization reagent or the reacted reagent is not separated in the solution, and then continues to participate in a small amount of imidization and salt formation reactions in step (4).

[0032] Preferably, in step (2), the degree of imidization is 20% to 80%, preferably 25% to 70%, such as 20%, 30%, 40%, 50%, 60%, 70%, or 80%.

[0033] In a preferred embodiment, in step (2), the conditions for the imidization treatment include: a time of 1 to 30 h, preferably 2 to 24 h; and / or a temperature of 5 to 40 °C, preferably 10 to 20 °C.

[0034] For example, in step (2), the conditions for the imidization treatment include: a time of 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 15 h, 18 h, 20 h, 22 h, 25 h, 28 h or 30 h, and a temperature of 5 °C, 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C or 40 °C.

[0035] Among them, the product obtained after the (partial) imidization of the flexible polyamic acid solution A in step (2) is still uniformly dispersed in the solvent, and the obtained flexible polyamic acid (partial) imidization system is homogeneous without precipitation. Among them, the imidization degree of step (2) can be obtained by comparison through infrared testing after precipitation, washing and drying in a non-solvent.

[0036] In the present invention, since the flexible polyamic acid solution A is partially imidized in step (2), the participation of low-molecular-weight flexible polyamic acid chain segments in the molecular chain rearrangement of the rigid polyamic acid is weakened, but the anhydride-capped characteristics of the flexible polyamic acid chain segments can form partial block characteristics with the rigid polyamic acid chain segments, thereby increasing the local flexibility characteristics of the polymer chain. And the remaining imidization reagent in the flexible polyamic acid solution A will continue to react with the polyamic acid solution B, combining with the partially ordered structure in the flexible polyamic acid chain segments. During the thermal imidization process after forming and processing, the ordered structure will induce the polyamic acid solution B to be orderly arranged in-plane at a relatively low imidization temperature, further reducing the thermal expansion coefficient of the film.

[0037] In a preferred embodiment, the rigid polyamic acid solution B is prepared from raw material B including a rigid dianhydride compound B and a rigid diamine compound B.

[0038] In a further preferred embodiment, the rigid dianhydride compound B is selected from at least one of cyclobutane tetracarboxylic dianhydride, 1,3-dimethylcyclobutane tetracarboxylic anhydride, 1,2,3,4-butanetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4-biphenyltetracarboxylic dianhydride, 6,6'-bis(methyl)-3,3',4,4'-biphenyltetracarboxylic dianhydride, 6,6'-bis(trifluoromethyl)-3,3',4,4'-biphenyltetracarboxylic dianhydride; and / or, the rigid diamine compound B is selected from at least one of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 2,2'-bis(carboxyl)-4,4'-diaminobiphenyl, 2,2'-bis(methyl)-4,4'-diaminobiphenyl, 2,2'-bis(chloro)-4,4'-diaminobiphenyl, 2,2',6,6'-tetrachloro-4,4'-diaminobiphenyl, 2,2',6,6'-tetrakis(methyl)-4,4'-diaminobiphenyl, 3,3'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 1,4-phenylenediamine, 1,3-phenylenediamine.

[0039] In a still further preferred embodiment, the molar ratio of the rigid dianhydride compound B to the rigid diamine compound B is 0.97:1 to 0.995:1, preferably 0.975:1 to 0.99:1.

[0040] In a preferred embodiment, the rigid polyamic acid solution B is obtained as follows: the rigid diamine compound B is dissolved in solvent B, and under a protective atmosphere, the rigid dianhydride compound B is added and stirred for reaction to obtain the rigid polyamic acid solution B.

[0041] Wherein, when preparing the rigid polyamic acid solution B, the solid content in the system is 10 to 20 wt%, preferably 12 to 18 wt%, wherein the solid content refers to the proportion of the solid feed in the total weight of the solution.

[0042] In the present invention, the rigid polyamic acid solution B is used to be mixed with the flexible polyamic acid (partial) imidization system, rather than using the rigid diamine compound B and the rigid dianhydride compound B to be mixed with the flexible polyamic acid (partial) imidization system.

[0043] In a further preferred embodiment, the solvent B is selected from amide solvents, preferably at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone.

[0044] In a still further preferred embodiment, the reaction conditions include: the temperature is 0 to 60 °C, preferably 20 to 40 °C; and / or, the time is 2 to 12 h, preferably 4 to 8 h.

[0045] Among them, the protective atmosphere is selected from at least one of nitrogen and inert gases, preferably nitrogen and / or argon.

[0046] In a preferred embodiment, in step (3), the weight ratio of the flexible polyamic acid imidization system to the rigid polyamic acid solution B is 20:80 to 5:95, preferably 15:85 to 10:90.

[0047] Among them, in the casting solution, the rigid polyamic acid solution B is the main component, which serves to obtain a low coefficient of thermal expansion, and the flexible polyamic acid (partially) imidization system is used as a small additive component, which serves to improve the flexibility of the film.

[0048] In a preferred embodiment, in step (3), the stirring time is 0.5 to 8 h, preferably 1 to 4 h.

[0049] In a preferred embodiment, in step (4), the film coating and forming method is preferably but not limited to one or a combination of conventional liquid film coating methods such as casting coating, spin coating, spraying, etc.

[0050] In a further preferred embodiment, the thickness of the liquid film after film coating and forming in step (4) is 100 to 500 μm, such as 100 μm, 120 μm, 140 μm, 160 μm, 180 μm, 200 μm, 220 μm, 240 μm, 260 μm, 280 μm, 300 μm, 350 μm, 400 μm, 450 μm or 500 μm.

[0051] In a preferred embodiment, the imidization treatment in step (4) is a thermal imidization treatment.

[0052] In a further preferred embodiment, the imidization treatment in step (4) includes: pre-baking in a vacuum oven and then carrying out imidization heating in an atmosphere oven; wherein, the atmosphere oven refers to an oven protected by a protective gas, and the protective gas is selected from at least one of nitrogen and inert gases, preferably nitrogen and / or argon.

[0053] Among them, pre-baking in a vacuum oven removes most of the solvent and part of the dehydrating agent and catalyst that have reacted.

[0054] In a further preferred embodiment, the conditions for pre-baking in the vacuum oven include: temperature 60°C to 90°C (such as 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C), and heat preservation for 1 to 12 h (such as 1 h, 2 h, 4 h, 6 h, 8 h, 10 h or 12 h).

[0055] In a further preferred embodiment, the imidization temperature increase in the atmosphere oven includes: first heating to a low temperature stage of 100-140°C for heat preservation treatment; then heating to a medium temperature stage of 220-280°C (preferably 230-270°C) for heat preservation treatment; and finally heating to a high temperature stage of 300-400°C (preferably 320-380°C) for heat preservation treatment.

[0056] Among them, the solvent is removed in the low temperature stage, the dehydration imidization reaction is carried out in the medium temperature stage, and complete imidization and stress release are carried out in the high temperature stage.

[0057] Preferably, the heating rate for each temperature increase is independently 0.5-5°C / min, preferably 1-3°C / min, such as 0.5°C / min, 1°C / min, 1.5°C / min, 2°C / min, 2.5°C / min, 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min or 5°C / min.

[0058] More preferably, the heat preservation treatment in the low temperature stage is carried out for 0.25-2 h, the heat preservation treatment in the medium temperature stage is carried out for 0.5-1 h, and the heat preservation treatment in the high temperature stage is carried out for 0.25-2 h (such as 0.25 h, 0.4 h, 0.6 h, 0.8 h, 1 h, 1.2 h, 1.4 h, 1.6 h, 1.8 h or 2 h).

[0059] The second object of the present invention is to provide a colorless polyimide film obtained by using the preparation method described in the first object of the present invention, which has the characteristics of high elongation at break and low thermal expansion.

[0060] In the colorless polyimide film system with high elongation at break and low thermal expansion, the low molecular weight polymer effectively improves the flexibility of the polyimide and increases its elongation at break in the form of partial block. At the same time, the high molecular weight rigid structure combines with the partial imidization-induced chain segment stacking and orientation process before heat treatment, which can further reduce the thermal expansion coefficient of the film, and the film finally has good flexibility and thermal dimensional stability.

[0061] In a preferred embodiment, it can be applied in the flexible display field, such as used as a transparent cover plate, a transparent substrate, etc.

[0062] The third object of the present invention is to provide the application of the colorless polyimide film obtained by using the preparation method described in the first object of the present invention in flexible display devices, preferably in transparent materials, and more preferably used as a transparent cover plate or a transparent substrate, etc.

[0063] In the ranges and any values disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. In the following text, in principle, various technical solutions can be combined with each other to obtain new technical solutions, and this should also be regarded as specifically disclosed herein.

[0064] Compared with the prior art, the present invention has the following beneficial effects: The present invention preferably solves the problem that it is impossible to take into account both the flexibility and the thermal dimensional stability of the colorless polyimide film, combines the respective advantages of a partially blocky flexible low molecular weight polymer and a rigid high molecular weight polymer, and avoids the deficiency of random copolymerization caused by chain exchange during the conventional polyamic acid blending process. Detailed implementation manners

[0065] The present invention will be specifically described below in conjunction with specific embodiments. It is necessary to point out here that the following embodiments are only used for further illustration of the present invention and cannot be understood as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art to the present invention based on the content of the present invention still fall within the protection scope of the present invention.

[0066] In addition, it should be noted that the various specific technical features described in the following detailed implementation manners can be combined in any suitable manner without conflict. To avoid unnecessary repetition, the present invention will not separately describe various possible combination manners.

[0067] In addition, any combination can be made between various different implementation manners of the present invention as long as it does not violate the idea of the present invention. The technical solutions formed thereby belong to a part of the original disclosure content of this specification and also fall within the protection scope of the present invention.

[0068] If there is no special limitation on the raw materials used in the examples and comparative examples, they are all disclosed in the prior art. For example, they can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0069]

Example 1

[0070] (1) Preparation of polyamic acid A solution: Dissolve 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then add 7.278 g of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.4 mmol). Stir and react at 25 °C for 4 h to obtain a polyamic acid A solution. The molecular weight of the polyamic acid in the flexible polyamic acid solution A is 18.1 KDa.

[0071] (2) Partial imidization of polyamic acid A solution: Mix 1.508 g (14.9 mmol) of triethylamine and 1.672 g

[0072] (16.4 mmol) of acetic anhydride, and then gradually add them to the polyamic acid A solution. Stir under the protection of a nitrogen atmosphere and react at 25 °C for 4 h to obtain a partially imidized polyamic acid solution A.

[0073] Dropwise add the partially imidized solution into pure water to obtain a polymer precipitate. After washing and filtering, freeze-dry to obtain a polymer powder. By infrared test comparison, the imidization degree of the polyamic acid solution A can be obtained as 40%.

[0074] (3) Preparation of polyamic acid B solution: Dissolve 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then gradually add 5.662 g (28.9 mmol) of cyclobutanetetracarboxylic dianhydride. Stir and react at 40 °C for 24 h to obtain a polyamic acid B solution. The molecular weight of the polyamic acid in the polyamic acid B solution is 169.5 KDa.

[0075] (4) Blend the partially imidized polyamic acid solution A and the polyamic acid B solution in a mass ratio of 10:90 and stir for 4 h until evenly dispersed to obtain a casting solution for coating.

[0076] (5) By the method of casting coating, coat the casting solution on a glass plate, and set the gap between the blade and the glass plate to 300 μm. Place the liquid film in a vacuum oven with a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere. Set the program for temperature increase, and the temperature increase conditions are as follows: Heat from room temperature to 140 °C at a rate of 1 °C / min, stay at 140 °C for 1 h, then heat to 250 °C at a rate of 3 °C / min, stay at this temperature for 1 h, and then heat to 350 °C at a rate of 3 °C / min and stay at this temperature for 1 h. Finally, stop heating and let it cool naturally to obtain a colorless and transparent polyimide film.

[0077] Finally, the obtained colorless polyimide film has an elongation at break of 25%, a coefficient of thermal expansion of 13.5 ppm / K, a film yellowness value of YI 3.9, and an average light transmittance of 91.2%.

[0078]

Example 2

[0079] (1) Preparation of polyamic acid A solution: 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.8 mmol) was added. The reaction was stirred at 25 °C for 4 h to obtain a polyamic acid A solution. The molecular weight of the polyamic acid in the flexible polyamic acid solution A was 10.6 Kda.

[0080] (2) Partial imidization of polyamic acid A solution: 1.508 g (14.9 mmol) of triethylamine and 1.672 g

[0081] (16.4 mmol) of acetic anhydride were mixed and then gradually added to the polyamic acid A solution. Stirring was carried out under nitrogen atmosphere protection, and the reaction was stirred at 25 °C for 4 h to obtain a partially imidized polyamic acid solution A.

[0082] The partially imidized solution was added dropwise to pure water to obtain a polymer precipitate. After washing and filtering, the polymer powder was obtained by freeze-drying. The imidization degree result of the polymer A solution was 42% by infrared test comparison.

[0083] (3) Preparation of polyamic acid B solution: 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride was gradually added. The reaction was stirred at 40 °C for 24 h to obtain a polyamic acid B solution. The molecular weight of the polyamic acid in the polyamic acid B solution was 169.5 KDa.

[0084] (4) The partially imidized polyamic acid solution A and the polyamic acid B solution were blended and stirred at a mass ratio of 10:90 for 4 h until uniformly dispersed to obtain a casting solution for coating.

[0085] (5) By means of casting coating, the casting solution was coated on a glass plate, and the gap between the doctor blade and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven with a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere, and the temperature was programmed to rise. The heating conditions were as follows: heating from room temperature to 140 °C at 1 °C / min, staying at 140 °C for 1 h, then heating to 250 °C at 3 °C / min, staying at this temperature for 1 h, then heating to 350 °C at 3 °C / min, and staying at this temperature for 1 h. Finally, the heating was stopped and it was allowed to cool naturally to obtain a colorless and transparent polyimide film.

[0086] Finally, the obtained colorless polyimide film had an elongation at break of 22.4%, a coefficient of thermal expansion of 13.2 ppm / K, a yellowness index of the film of YI 4.0, and an average light transmittance of 91.0%.

[0087]

Example 3

[0088] (1) Preparation of polyamic acid A solution: 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (17.3 mmol) was added. The reaction was stirred at 25 °C for 4 h to obtain a polyamic acid A solution. The molecular weight of the polyamic acid in the flexible polyamic acid solution A was 5.4 KDa.

[0089] (2) Partial imidization of polyamic acid A solution: After mixing 1.508 g (14.9 mmol) of triethylamine and 1.672 g

[0090] (16.4 mmol) of acetic anhydride, it was gradually added to the polyamic acid A solution, stirred under a nitrogen atmosphere, and the reaction was stirred at 25 °C for 4 h to obtain a partially imidized polyamic acid solution A.

[0091] The partially imidized solution was added dropwise to pure water to obtain a polymer precipitate. After washing and filtering, the polymer powder could be obtained by freeze-drying. By infrared test comparison, the imidization degree result of the polymer A solution was 45%.

[0092] (3) Preparation of polyamic acid B solution: 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride was gradually added. The reaction was stirred at 40 °C for 24 h to obtain a polyamic acid B solution. The molecular weight of the polyamic acid in the polyamic acid B solution was 169.5 KDa.

[0093] (4) Blend the partially imidized polyamic acid solution A and the polyamic acid B solution at a mass ratio of 10:90 and stir for 4 h until uniformly dispersed to obtain a casting solution for coating.

[0094] (5) By means of casting coating, coat the casting solution on a glass plate, and set the gap between the doctor blade and the glass plate to 300 μm. Place the liquid film in a vacuum oven with a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere, set the program to increase the temperature, and the temperature increase conditions are as follows: increase the temperature from room temperature to 140 °C at 1 °C / min, stay at 140 °C for 1 h, then increase the temperature to 250 °C at 3 °C / min, stay at this temperature for 1 h, then increase the temperature to 350 °C at 3 °C / min, and stay at this temperature for 1 h. Finally, stop heating and cool naturally to obtain a colorless and transparent polyimide film.

[0095] Finally, the obtained colorless polyimide film has an elongation at break of 19.8%, a coefficient of thermal expansion of 12.8 ppm / K, a film yellowness index of YI 4.0, and an average light transmittance of 90.7%.

[0096]

Example 4

[0097] (1) Preparation of polyamic acid A solution: Dissolve 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then add 7.278 g of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.4 mmol), and stir and react at 25 °C for 4 h to obtain polyamic acid A solution. The molecular weight of the polyamic acid in the flexible polyamic acid solution A is 18.1 KDa.

[0098] (2) Partial imidization of polyamic acid A solution: After mixing 11.9 mmol of triethylamine and 13.1 mmol of acetic anhydride, gradually add them to the polyamic acid A solution, stir under the protection of a nitrogen atmosphere, and stir and react at 25 °C for 4 h to obtain a partially imidized polyamic acid solution A.

[0099] Dropwise add the partially imidized solution into pure water to obtain a polymer precipitate. After washing and filtering, the polymer powder can be obtained by freeze-drying. By infrared test and comparison, the imidization degree result of the polymer A solution is 29%.

[0100] (3) Preparation of polyamic acid B solution: Dissolve 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then gradually add 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride. Stir and react at 40 °C for 24 h to obtain a polyamic acid B solution. The molecular weight of the polyamic acid in the polyamic acid B solution is 169.5 KDa.

[0101] (4) Blend and stir a partially imidized polyamic acid solution A and a polyamic acid B solution in a mass ratio of 10:90 for 4 h until uniformly dispersed to obtain a casting solution for coating.

[0102] (5) By means of casting coating, coat the casting solution on a glass plate, and set the gap between the scraper and the glass plate to 300 μm. Place the liquid film in a vacuum oven at a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere, set the program to increase the temperature, and the temperature increase conditions are as follows: increase the temperature from room temperature to 140 °C at 1 °C / min, stay at 140 °C for 1 h, then increase the temperature to 250 °C at 3 °C / min, stay at this temperature for 1 h, then increase the temperature to 350 °C at 3 °C / min, and stay at this temperature for 1 h. Finally, stop heating and cool naturally to obtain a colorless and transparent polyimide film.

[0103] Finally, the elongation at break of the obtained colorless polyimide film is 24.3%, the coefficient of thermal expansion is 16.3 ppm / K, the yellowness index of the film is YI 4.6, and the average light transmittance is 90.1%.

[0104]

Example 5

[0105] (1) Preparation of polyamic acid A solution: Dissolve 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then add 7.278 g of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.4 mmol). Stir and react at 25 °C for 4 h to obtain a polyamic acid A solution. The molecular weight of the polyamic acid in the flexible polyamic acid solution A is 18.1 KDa.

[0106] (2) Partial imidization of polyamic acid A solution: After mixing 23.8 mmol of triethylamine and 26.2 mmol of acetic anhydride, gradually add them to the polyamic acid A solution, stir under the protection of a nitrogen atmosphere, and stir and react at 25 °C for 4 h to obtain a partially imidized polyamic acid solution A.

[0107] A polymer precipitate can be obtained by dropwise adding a partially imidized solution to pure water. After washing and filtering, the polymer powder can be obtained by freeze-drying. The imidization degree of the polymer A solution can be obtained as 66% by infrared test comparison.

[0108] (3) Preparation of polyamic acid B solution: Dissolve 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then gradually add 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride. Stir and react at 40 °C for 24 h to obtain a polyamic acid B solution. The molecular weight of the polyamic acid in the polyamic acid B solution is 169.5 KDa.

[0109] (4) Blend and stir the partially imidized polyamic acid solution A and the polyamic acid B solution in a mass ratio of 10:90 for 4 h until uniformly dispersed to obtain a casting solution for coating.

[0110] (5) By means of casting coating, coat the casting solution on a glass plate, and set the gap between the scraper and the glass plate to 300 μm. Place the liquid film in a vacuum oven at a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere, set the program to increase the temperature, and the temperature increase conditions are as follows: increase the temperature from room temperature to 140 °C at 1 °C / min, stay at 140 °C for 1 h, then increase the temperature to 250 °C at 3 °C / min, stay at this temperature for 1 h, then increase the temperature to 350 °C at 3 °C / min, and stay at this temperature for 1 h. Finally, stop heating and let it cool naturally to obtain a colorless and transparent polyimide film.

[0111] Finally, the elongation at break of the obtained colorless polyimide film is 20.8%, the coefficient of thermal expansion is 12.6 ppm / K, the yellowness index of the film is YI 3.7, and the average light transmittance is 91.3%.

[0112]

Example 6

[0113] (1) Preparation of polyamic acid A solution: Dissolve 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then add 7.278 g of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.4 mmol). Stir and react at 25 °C for 4 h to obtain a polyamic acid A solution. The molecular weight of the polyamic acid in the flexible polyamic acid solution A is 18.1 KDa.

[0114] (2) Partial imidization of polyamic acid A solution: Add 1.508 g (14.9 mmol) of triethylamine and 1.672 g

[0115] After mixing with acetic anhydride (16.4 mmol), it was gradually added to the polyamic acid A solution, stirred under a nitrogen atmosphere, and reacted with stirring at 25 °C for 4 h to obtain a partially imidized polyamic acid solution A.

[0116] The partially imidized solution was added dropwise to pure water to obtain a polymer precipitate. After washing and filtering, the polymer powder could be obtained by freeze-drying. By infrared test comparison, the imidization degree of the polyamic acid solution A was 40%.

[0117] (3) Preparation of polyamic acid B solution: 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 28.8 mmol of cyclobutane tetracarboxylic dianhydride was gradually added, and reacted with stirring at 40 °C for 24 h to obtain a polyamic acid B solution. The molecular weight of the polyamic acid in the polyamic acid B solution was 165.1 KDa.

[0118] (4) The partially imidized polyamic acid solution A and the polyamic acid B solution were blended and stirred for 4 h until uniformly dispersed according to a mass ratio of 10:90 to obtain a casting solution for coating.

[0119] (5) By means of casting coating, the casting solution was coated on a glass plate, and the gap between the scraper and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven at a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere, and the program temperature was set. The temperature rising conditions were as follows: rising from room temperature to 140 °C at 1 °C / min, staying at 140 °C for 1 h, then rising to 250 °C at 3 °C / min, staying at this temperature for 1 h, and then rising to 350 °C at 3 °C / min, staying at this temperature for 1 h. Finally, the heating was stopped and it was naturally cooled to obtain a colorless and transparent polyimide film.

[0120] Finally, the obtained colorless polyimide film had an elongation at break of 25.1%, a coefficient of thermal expansion of 14.7 ppm / K, a film yellowness index of YI 3.9, and an average light transmittance of 91.1%.

[0121]

Example 7

[0122] (1) Preparation of polyamic acid A solution: 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 7.278 g of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.4 mmol) was added, and reacted with stirring at 25 °C for 4 h to obtain a polyamic acid A solution. The molecular weight of the polyamic acid in the flexible polyamic acid solution A was 18.1 KDa.

[0123] (2) Partial imidization of the polyamic acid A solution. After mixing 1.508 g (14.9 mmol) of triethylamine with 1.672 g

[0124] (16.4 mmol) of acetic anhydride, it was gradually added to the polyamic acid A solution, and stirred under a nitrogen atmosphere. The reaction was stirred at 25 °C for 4 h to obtain a partially imidized polyamic acid solution A.

[0125] The partially imidized solution was added dropwise to pure water to obtain a polymer precipitate. After washing and filtering, the polymer powder could be obtained by freeze-drying. By infrared test comparison, the imidization degree of the polyamic acid A solution was 40%.

[0126] (3) Preparation of the polyamic acid B solution. Dissolve 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then gradually add 28.5 mmol of cyclobutane tetracarboxylic dianhydride. Stir and react at 40 °C for 24 h to obtain the polyamic acid B solution. The molecular weight of the polyamic acid in the polyamic acid B solution was 144.6 KDa.

[0127] (4) The partially imidized polyamic acid solution A and the polyamic acid B solution were blended and stirred at a mass ratio of 10:90 for 4 h until uniformly dispersed to obtain a casting solution for coating.

[0128] (5) By the method of casting coating, the casting solution was coated on a glass plate, and the gap between the blade and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven at a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h. Subsequently, in a hot oven under a nitrogen atmosphere, a programmed temperature rise was set, and the temperature rise conditions were as follows: heated from room temperature to 140 °C at 1 °C / min, stayed at 140 °C for 1 h, then heated to 250 °C at 3 °C / min, stayed at this temperature for 1 h, then heated to 350 °C at 3 °C / min, and stayed at this temperature for 1 h. Finally, the heating was stopped and it was naturally cooled to obtain a colorless and transparent polyimide film.

[0129] Finally, the obtained colorless polyimide film had an elongation at break of 25.4%, a coefficient of thermal expansion of 15.1 ppm / K, a film yellowness index of YI 4.0, and an average light transmittance of 91.0%.

[0130]

Example 8

[0131] Solutions A and B were obtained in the same steps (1), (2), and (3) as in Example 1.

[0132] (4) The partially imidized polyamic acid solution A and the polyamic acid B solution were blended and stirred at a mass ratio of 13:87 for 4 h until uniformly dispersed to obtain a casting solution for coating.

[0133] (5) By means of casting coating, the casting solution was coated on a glass plate, and the gap between the doctor blade and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven with a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere. The program temperature increase was set, and the temperature increase conditions were as follows: It was heated from room temperature to 140 °C at 1 °C / min, stayed at 140 °C for 1 h, then heated to 250 °C at 3 °C / min, stayed at this temperature for 1 h, then heated to 350 °C at 3 °C / min, and stayed at this temperature for 1 h. Finally, the heating was stopped and it was naturally cooled to obtain a colorless and transparent polyimide film.

[0134] Finally, the obtained colorless polyimide film had an elongation at break of 27.7%, a coefficient of thermal expansion of 15.5 ppm / K, a film yellowness value of YI 4.0, and an average light transmittance of 91.0%.

[0135]

Example 9

[0136] The same steps (1), (2), and (3) of Example 1 were used to obtain Solution A and Solution B.

[0137] (4) Partially imidized polyamic acid Solution A and polyamic acid Solution B were blended and stirred at a mass ratio of 15:85 for 4 h until uniformly dispersed to obtain a casting solution for coating.

[0138] (5) By means of casting coating, the casting solution was coated on a glass plate, and the gap between the doctor blade and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven with a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere. The program temperature increase was set, and the temperature increase conditions were as follows: It was heated from room temperature to 140 °C at 1 °C / min, stayed at 140 °C for 1 h, then heated to 250 °C at 3 °C / min, stayed at this temperature for 1 h, then heated to 350 °C at 3 °C / min, and stayed at this temperature for 1 h. Finally, the heating was stopped and it was naturally cooled to obtain a colorless and transparent polyimide film.

[0139] Finally, the obtained colorless polyimide film had an elongation at break of 27.8%, a coefficient of thermal expansion of 15.7 ppm / K, a film yellowness value of YI 4.1, and an average light transmittance of 90.9%.

[0140]

Example 10

[0141] (1) Preparation of polyamic acid Solution A: 14.9 mmol of 1,4-bis(4-aminophenoxy)benzene was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 16.4 mmol of 4,4'-(4,4'-isopropyl diphenoxy)diphthalic anhydride was added. It was stirred and reacted at 25 °C for 4 h to obtain polyamic acid Solution A. The molecular weight of the polyamic acid in the flexible polyamic acid Solution A was 17.2 KDa.

[0142] (2) Partial imidization of the polyamic acid A solution. After mixing 1.508 g (14.9 mmol) of triethylamine with 1.672 g

[0143] (16.4 mmol) of acetic anhydride, it was gradually added to the polyamic acid A solution and stirred under a nitrogen atmosphere. The reaction was stirred at 25 °C for 4 h to obtain a partially imidized polyamic acid solution A.

[0144] The partially imidized solution was added dropwise to pure water to obtain a polymer precipitate. After washing and filtering, the polymer powder could be obtained by freeze-drying. By infrared test comparison, the imidization degree result of the polyamic acid A solution was 41%.

[0145] (3) Preparation of the polyamic acid B solution. 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was dissolved in 85 g of N,N-dimethylacetamide and stirred under a nitrogen atmosphere. Subsequently, 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride was gradually added, and the reaction was stirred at 40 °C for 24 h to obtain the polyamic acid B solution. The molecular weight of the polyamic acid in the polyamic acid B solution was 169.5 KDa.

[0146] (4) The partially imidized polyamic acid solution A and the polyamic acid B solution were blended and stirred at a mass ratio of 10:90 for 4 h until uniformly dispersed to obtain a casting solution for coating.

[0147] (5) By the method of casting coating, the casting solution was coated on a glass plate, and the gap between the doctor blade and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven at a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h. Subsequently, in a hot oven under a nitrogen atmosphere, the temperature was programmed to rise. The temperature rising conditions were as follows: rising from room temperature to 140 °C at 1 °C / min, staying at 140 °C for 1 h, then rising to 250 °C at 3 °C / min, staying at this temperature for 1 h, then rising to 350 °C at 3 °C / min, and staying at this temperature for 1 h. Finally, the heating was stopped and it was allowed to cool naturally to obtain a colorless and transparent polyimide film.

[0148] Finally, the elongation at break of the obtained colorless polyimide film was 29.6%, the coefficient of thermal expansion was 19.8 ppm / K, the film yellowness index was YI 4.2, and the average light transmittance was 90.5%.

[0149]

Example 11

[0150] (1) Preparation of polyamic acid A solution: Dissolve 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then add 7.278 g of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.4 mmol). Stir and react at 25 °C for 4 h to obtain a polyamic acid A solution. The molecular weight of the polyamic acid in the flexible polyamic acid solution A is 18.1 kDa.

[0151] (2) Partial imidization of polyamic acid A solution: Mix 1.508 g (14.9 mmol) of triethylamine and 1.672 g

[0152] (16.4 mmol) of acetic anhydride, and then gradually add them to the polyamic acid A solution. Stir under the protection of a nitrogen atmosphere and react at 25 °C for 4 h to obtain a partially imidized polyamic acid solution A.

[0153] Dropwise add the partially imidized solution into pure water to obtain a polymer precipitate. After washing and filtering, freeze-dry to obtain a polymer powder. By infrared test comparison, the imidization degree result of the polyamic acid solution A is 40%.

[0154] (3) Preparation of polyamic acid B solution: Dissolve 29.2 mmol of p-phenylenediamine in 85 g of N,N-dimethylacetamide, stir under a nitrogen atmosphere, and then gradually add 28.9 mmol of 3,3',4,4'-biphenyltetracarboxylic dianhydride. Stir and react at 40 °C for 24 h to obtain a polyamic acid B solution. The molecular weight of the polyamic acid in the polyamic acid B solution is 178.2 kDa.

[0155] (4) Blend the partially imidized polyamic acid solution A and the polyamic acid B solution in a mass ratio of 10:90 and stir for 4 h until evenly dispersed to obtain a casting solution for coating.

[0156] (5) By means of casting coating, coat the casting solution on a glass plate, and set the gap between the blade and the glass plate to 300 μm. Place the liquid film in a vacuum oven at a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere. Set the program for temperature rise, and the temperature rise conditions are as follows: heat from room temperature to 140 °C at a rate of 1 °C / min, stay at 140 °C for 1 h, then heat to 250 °C at a rate of 3 °C / min, stay at this temperature for 1 h, and then heat to 350 °C at a rate of 3 °C / min and stay at this temperature for 1 h. Finally, stop heating and let it cool naturally to obtain a colorless and transparent polyimide film.

[0157] Finally, the obtained colorless polyimide film has an elongation at break of 21.4%, a coefficient of thermal expansion of 9.6 ppm / K, a film yellowness index of YI 9.6, and an average light transmittance of 87.2%.

[0158] Comparative Example 1

[0159] Preparation of polyamic acid B solution: 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride was gradually added, and the mixture was stirred at 40 °C for 24 h to obtain a polyamic acid B solution.

[0160] The polyamic acid B solution was coated on a glass plate by casting coating, and the gap between the doctor blade and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven at a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere, and the temperature was programmed to rise. The temperature rising conditions were as follows: rising from room temperature to 140 °C at 1 °C / min, staying at 140 °C for 1 h, then rising to 250 °C at 3 °C / min, staying at this temperature for 1 h, and then rising to 350 °C at 3 °C / min, staying at this temperature for 1 h. Finally, the heating was stopped and the film was cooled naturally to obtain a colorless transparent polyimide film.

[0161] Finally, the obtained colorless polyimide film was relatively brittle, and due to the large sample size selected, stable test data could not be obtained for its mechanical properties. The thermal expansion coefficient was 18.7 ppm / K, the film yellowness value was YI 3.7, and the average light transmittance was 90.5%.

[0162] Comparative Example 2

[0163] (1) Preparation of polyamic acid A solution: 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 7.278 g of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.4 mmol) was added, and the mixture was stirred at 25 °C for 4 h to obtain a polyamic acid A solution.

[0164] (2) Preparation of polyamic acid B solution: 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride was gradually added, and the mixture was stirred at 40 °C for 24 h to obtain a polyamic acid B solution.

[0165] (3) The untreated polyamic acid solution A and solution B were blended and stirred at a mass ratio of 10:90 for 12 h until uniformly dispersed to obtain a casting solution for coating.

[0166] (4) By means of casting coating, the casting solution was coated on a glass plate, and the gap between the doctor blade and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven with a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere, and the programmed temperature rise was set. The temperature rise conditions were as follows: heating from room temperature to 140 °C at 1 °C / min, staying at 140 °C for 1 h, then heating to 250 °C at 3 °C / min, staying at this temperature for 1 h, and then heating to 350 °C at 3 °C / min, staying at this temperature for 1 h. Finally, the heating was stopped and the film was cooled naturally to obtain a colorless and transparent polyimide film.

[0167] Finally, the obtained colorless polyimide film had an elongation at break of 16.9%, a coefficient of thermal expansion of 24.5 ppm / K, a yellowness index value of YI 4.0, and an average light transmittance of 91.6%.

[0168]

Comparative Example 3

[0169] (1) Preparation of polyamic acid A solution: 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 7.278 g of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.4 mmol) was added, and the reaction was stirred at 25 °C for 4 h to obtain a polyamic acid A solution.

[0170] (2) Preparation of polyamic acid B solution: 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride was gradually added, and the reaction was stirred at 40 °C for 24 h to obtain a polyamic acid B solution.

[0171] (3) The untreated polyamic acid solution A and solution B were blended and stirred at a mass ratio of 10:90 for 4 h until uniformly dispersed.

[0172] (4) Subsequently, partial imidization of the blended solution was carried out. After mixing 3.015 g (29.8 mmol) of triethylamine and 3.343 g (32.8 mmol) of acetic anhydride, they were gradually added to the blended solution, stirred under a nitrogen atmosphere, and the reaction was stirred at 25 °C for 4 h to obtain a solution for subsequent coating.

[0173] (5) By means of casting coating, the casting solution was coated on a glass plate, and the gap between the doctor blade and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven with a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h, and then in a hot oven under a nitrogen atmosphere, and the program temperature was set. The temperature rising conditions were as follows: rising from room temperature to 140 °C at 1 °C / min, staying at 140 °C for 1 h, then rising to 250 °C at 3 °C / min, staying at this temperature for 1 h, and then rising to 350 °C at 3 °C / min, staying at this temperature for 1 h. Finally, the heating was stopped and it was naturally cooled to obtain a colorless and transparent polyimide film.

[0174] Finally, the obtained colorless polyimide film had an elongation at break of 12.4%, a coefficient of thermal expansion of 17.5 ppm / K, a film yellowness index of YI 4.0, and an average light transmittance of 90%.

[0175]

Comparative Example 4

[0176] The partially imidized solution was obtained according to the method of steps (1) to (2) of Example 1.

[0177] Preparation of a mixture of rigid diamine and rigid dianhydride: 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride, and 85 g of N,N-dimethylacetamide were mixed to obtain polymer solution B.

[0178] The partially imidized polyamic acid solution A and the mixture of rigid diamine and rigid dianhydride were blended and stirred for 4 h until uniformly dispersed according to a mass ratio of 10:90 to obtain a casting solution for coating.

[0179] Both solution A and solution B had a polymer structure with an excess of anhydride.

[0180] Coating and complete imidization treatment were carried out according to the method of step (5) of Example 1. A colorless and transparent polyimide film was obtained.

[0181] Finally, the obtained colorless polyimide film was brittle, and due to the large sample size selected, stable test data could not be obtained for its mechanical properties. Its coefficient of thermal expansion was 21.6 ppm / K, the film yellowness index was 4.1, and the average light transmittance was 90.2%.

[0182]

Comparative Example 5

[0183] (1) Preparation of polyamic acid B solution: 9.338 g (29.2 mmol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was dissolved in 85 g of N,N-dimethylacetamide, stirred under a nitrogen atmosphere, and then 5.662 g (28.9 mmol) of cyclobutane tetracarboxylic dianhydride was gradually added, and the reaction was stirred at 40 °C for 24 h to obtain polyamic acid B solution.

[0184] (2) Partial imidization of the polyamic acid B solution. After mixing 3.015 g (29.8 mmol) of triethylamine and 3.343 g (32.8 mmol) of acetic anhydride, it was gradually added to the polyamic acid B solution, and stirred under a nitrogen atmosphere. The reaction was stirred at 25 °C for 4 h. The viscosity of the polymer solution increased sharply, and the rod climbing phenomenon occurred. The zero-shear viscosity increased from 10.9 Pa·s to 320 Pa·s, which brought great difficulties to the subsequent mixing and forming processes.

[0185] (3) Preparation of the polyamic acid A solution. 7.722 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (14.9 mmol) was dissolved in 85 g of N,N-dimethylacetamide, and stirred under a nitrogen atmosphere. Subsequently, 7.278 g of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (16.4 mmol) was added, and the reaction was stirred at 25 °C for 4 h to obtain the polyamic acid A solution.

[0186] (4) The partially imidized polyamic acid solution B and solution A were blended and stirred at a mass ratio of 90:10 for 12 h until uniformly dispersed to obtain a casting solution for coating.

[0187] (5) By means of casting coating, the casting solution was coated on a glass plate, and the gap between the doctor blade and the glass plate was set to 300 μm. The liquid film was placed in a vacuum oven at a vacuum degree of 25 Pa and a temperature of 85 °C for 2 h. Subsequently, in a hot oven under a nitrogen atmosphere, a programmed temperature increase was set, and the temperature increase conditions were as follows: heated from room temperature to 140 °C at 1 °C / min, held at 140 °C for 1 h, then heated to 250 °C at 3 °C / min, held at this temperature for 1 h, then heated to 350 °C at 3 °C / min, and held at this temperature for 1 h. Finally, the heating was stopped and it was allowed to cool naturally to obtain a colorless and transparent polyimide film.

[0188] Finally, the obtained colorless polyimide film had an elongation at break of 13.6%, a coefficient of thermal expansion of 18.8 ppm / K, a yellowness index of the film of YI 4.3, and an average light transmittance of 90.2%. Due to the relatively high viscosity of the slurry for coating, more corrugated structures were formed on the film surface, which was caused by the viscoelasticity of the liquid film during the coating process.

[0189] The present invention has been described in detail above in combination with specific embodiments and exemplary examples, but these descriptions should not be construed as limiting the present invention. Those skilled in the art understand that without departing from the spirit and scope of the present invention, various equivalent substitutions, modifications or improvements can be made to the technical solutions of the present invention and their implementation manners, and these all fall within the scope of the present invention. The protection scope of the present invention is subject to the appended claims.

Claims

1. A method for preparing a colorless polyimide film, comprising: (1) The flexible polyamic acid solution A and the rigid polyamic acid solution B are obtained separately; (2) The flexible polyamic acid solution A is imidized to obtain a flexible polyamic acid imidized system; (3) The flexible polyamic acid imidized system obtained in step (2) is mixed with the rigid polyamic acid solution B and stirred to obtain a casting solution; (4) The casting solution is coated into a film and imidized to obtain the polyimide film.

2. The preparation method according to claim 1, characterized in that, The molecular weight of the polyamic acid in the flexible polyamic acid solution A is lower than that of the polyamic acid in the rigid polyamic acid solution B; Preferably, the molecular weight of the polyamic acid in the flexible polyamic acid solution A is 2k to 40 kDa, preferably 5k to 20 kDa; More preferably, the molecular weight of the polyamic acid in the rigid polyamic acid solution B is 120 to 200 kDa, preferably 140 to 180 kDa.

3. The preparation method according to claim 1, characterized in that, In step (1), the flexible polyamic acid solution A is prepared from raw material A including a flexible dianhydride compound A and a flexible diamine compound A; preferably, the molar ratio of the flexible dianhydride compound A to the flexible diamine compound A is 1.2:1 to 1.06:1, preferably 1.16:1 to 1.1:1; More preferably, the flexible dianhydride compound A is selected from at least one of the compounds shown in formula (1), the compounds shown in formula (2), and the compounds shown in formula (3), and the flexible diamine compound A is selected from at least one of the compounds shown in formula (4) to formula (5): In formulas (1) to (5), R1 represents a bridging structure and is independently selected from the group consisting of oxygen, hexafluoroisopropyl, keto group, methylene group, substituted methylene group, and sulfone group. In formulas (2), (3), and (4), the two R1s are the same or different; in formulas (3) to (4), each R2 is selected from the group consisting of phenyl, substituted phenyl, and one of them, and R3 is selected from methylene or substituted methylene, etheroxy group, keto group, or sulfone group.

4. The preparation method according to claim 3, characterized in that, In formulas (1) to (5), R1 represents a bridging structure, and the structures of the flexible dianhydride compound A and the flexible diamine compound A contain more than 3 bridging structures R1 in total; preferably: The flexible dianhydride compound shown in formula (1) containing 1 bridging structure R1 is selected from at least one of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and 3,3',4,4'-diphenylether tetracarboxylic dianhydride; and / or, The flexible diamine compound shown in formula (5) containing 1 bridging structure R1 is selected from at least one of 4,4'-diaminodiphenyl ether, 3,4`-diaminodiphenyl ether, 4,4-diaminodiphenylmethane, 4,4-diaminobenzophenone, 4,4'-diaminophenyl sulfone, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, 2,2`-bis(trifluoromethyl)-4,4`-diaminophenyl ether, bis(4-aminophenyl)hexafluoropropane (6FIP), and bis(3-aminophenyl)hexafluoropropane; and / or, The flexible dianhydride compounds shown in formulas (2) to (3) containing 2 bridging structures R1 are selected from at least one of hydrogenated pyromellitic dianhydride, 9,9-bis(trifluoromethyl)-2,3,6,7-xanthene tetracarboxylic dianhydride, and 4,4'-(4,4'-isopropyl diphenoxy) diphthalic anhydride; and / or, The flexible diamine compound of formula (4) containing two bridging structures R1 is selected from at least one of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 4,4'-(1,3-phenylenedioxy)dianiline, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 2,2'-bis(4-aminophenoxyphenyl)propane, 9,9-bis(4-aminophenyl)fluorene (fluorene diamine), 9,9-bis(3-fluoro-4-aminophenyl)fluorene.

5. The preparation method according to claim 3, characterized in that, The flexible polyamic acid solution A is obtained as follows: The flexible diamine compound A is dissolved in solvent A, and under a protective atmosphere, the flexible dianhydride compound A is added and stirred for reaction to obtain the flexible polyamic acid solution A; Preferably, the solvent A is selected from aprotic polar solvents, preferably at least one of amide solvents, more preferably at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone; Preferably, the reaction conditions include: the temperature condition is 0-60°C, preferably 20-40°C; and / or, the time is 2-12 h, preferably 4-8 h.

6. The preparation method according to claim 1, characterized in that, The imidization treatment in step (2) is carried out in the presence of an imidization reagent, and the imidization reagent includes a dehydrating agent and a catalyst; Preferably: the dehydrating agent is selected from acid anhydride dehydrating agents, preferably at least one of acetic anhydride and trifluoroacetic anhydride; and / or, the catalyst is selected from tertiary amine catalysts, preferably at least one of triethylamine, pyridine, quinoline, isoquinoline, triethylenediamine, methylpyrrole, methylpyrrolidine, methylimidazole, more preferably at least one of triethylamine and quinoline; Preferably: the molar ratio of the dehydrating agent to the catalyst is 1:1-1.2:1; and / or, the molar ratio of the catalyst to the flexible diamine compound A in the flexible polyamic acid solution A is 0.8:1-1.6:1; More preferably, in step (2), the conditions of the imidization treatment include: the time is 1-30 h, preferably 2-24 h; and / or, the temperature is 5-40°C, preferably 10-20°C.

7. The preparation method according to claim 1, characterized in that, The rigid polyamic acid solution B is prepared from raw material B including a rigid dianhydride compound B and a rigid diamine compound B; Preferably, the rigid dianhydride compound B is selected from at least one of cyclobutane tetracarboxylic dianhydride, 1,3-dimethylcyclobutane tetracarboxylic anhydride, 1,2,3,4-butanetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4-biphenyltetracarboxylic dianhydride, 6,6'-bis(methyl)-3,3',4,4'-biphenyltetracarboxylic dianhydride, 6,6'-bis(trifluoromethyl)-3,3',4,4'-biphenyltetracarboxylic dianhydride; and / or, the rigid diamine compound B is selected from at least one of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 2,2'-bis(carboxyl)-4,4'-diaminobiphenyl, 2,2'-bis(methyl)-4,4'-diaminobiphenyl, 2,2'-bis(chloro)-4,4'-diaminobiphenyl, 2,2',6,6'-tetrachloro-4,4'-diaminobiphenyl, 2,2',6,6'-tetrakis(methyl)-4,4'-diaminobiphenyl, 3,3'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 1,4-phenylenediamine, 1,3-phenylenediamine; More preferably, the molar ratio of the rigid dianhydride compound B to the rigid diamine compound B is 0.97:1 to 0.995:1, preferably 0.975:1 to 0.99:

1.

8. The preparation method according to claim 7, characterized in that, The rigid polyamic acid solution B is obtained as follows: The rigid diamine compound B is dissolved in the solvent B, and under a protective atmosphere, the rigid dianhydride compound B is added and stirred for reaction to obtain the rigid polyamic acid solution B; Preferably, the solvent B is selected from amide solvents, preferably at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone; More preferably, the reaction conditions include: the temperature is 0 to 60°C, preferably 20 to 40°C; and / or, the time is 2 to 12 h, preferably 4 to 8 h.

9. The preparation method according to claim 1, wherein in step (3), the weight ratio of the flexible polyamic acid imidization system to the rigid polyamic acid solution B is 20:80 to 5:95, preferably 15:85 to 10:90; and / or, in step (3), the stirring time is 0.5 to 8 h, preferably 1 to 4 h.

10. The preparation method according to any one of claims 1 to 9, characterized in that, The imidization treatment in step (4) is thermal imidization treatment; Preferably, the imidization treatment in step (4) includes: pre-drying in a vacuum oven and then performing imidization heating in an atmosphere oven; wherein, the atmosphere oven refers to an oven protected by a protective gas, and the protective gas is selected from at least one of nitrogen and inert gases, preferably nitrogen and / or argon; More preferably, the conditions for pre-drying in the vacuum oven include: the temperature is 60°C to 90°C and the heat preservation time is 1 to 12 h; and / or, the imidization heating in the atmosphere oven includes: first heating to a low temperature stage of 100 to 140°C for heat preservation treatment; then heating to a medium temperature stage of 220 to 280°C for heat preservation treatment; and finally heating to a high temperature stage of 300 to 400°C for heat preservation treatment.

11. The colorless polyimide film obtained by the preparation method according to any one of claims 1 to 10.

12. Application of the colorless polyimide film obtained by the preparation method according to any one of claims 1 to 10 in a flexible display device, preferably in a transparent material, and more preferably used as a transparent cover plate or a transparent substrate.

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