A polyimide and a method for preparing the same

Abstract

The application discloses a kind of polyimide and preparation method thereof, it is related to high polymer material technical field;High-temperature-resistant polyimide is prepared by 4,4'- azobenzene anhydride and aromatic heterocyclic diamine through polycondensation and imidization reaction, and aromatic heterocycle includes at least one of imidazole, pyrimidine, pyridine, oxazole, thiazole;The application simultaneously introduces azo bond and aromatic heterocycle into the main chain of polyimide, enhances the chain rigidity and chain interaction of polyimide, and the glass transition temperature of the prepared polyimide film is greater than 400 DEG C, meets the high-temperature processing requirement of flexible device, and greatly reduces the preparation process difficulty and production cost of high-temperature-resistant polyimide.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a polyimide and its preparation method. Background Technology

[0002] With the development of society and technology, people's demand for flexible devices such as flexible printed circuits, flexible displays, organic light-emitting diodes, and flexible solar cells is constantly increasing. Traditional rigid glass substrates are difficult to meet the requirements of flexibility and thinness. However, in industrial manufacturing, commonly used flexible polymer materials cannot withstand the high-temperature processing in industrial manufacturing. For example, in the processing of flexible display devices, the actual processing temperature can reach 400℃, but the glass transition temperatures (Tg) of polycarbonate, polyetheretherketone and polysulfone are only 135℃, 143℃ and 190℃, respectively, which cannot meet the process requirements.

[0003] Polyimide possesses excellent high-temperature resistance, chemical resistance, high dimensional stability, and insulation properties, making it an ideal substrate material for flexible devices. Polyimide refers to a class of polymers containing an imide ring (-CO-NR-CO-) in its main chain. Chemically, it can be divided into two main categories: aromatic polyimide and aliphatic polyimide. Aromatic polyimide has received more attention due to its superior performance. Although the synthesis method of aromatic polyimide is simple, its properties vary greatly with changes in monomer structure. Due to the diversity of monomers, the molecular structure of polyimide can be controlled by selecting different monomer combinations, thereby regulating the thermal stability, mechanical properties, and other properties of polyimide films to meet specific requirements. However, in existing research, the glass transition temperature of aromatic polyimide is usually below 400℃, which cannot meet the higher temperature processing requirements of some flexible devices. Summary of the Invention

[0004] This invention provides a high-temperature resistant polyimide with a glass transition temperature higher than 400°C.

[0005] The polyimide provided by this invention is prepared by polycondensation and imidization reaction of 4,4'-azobisphthalic anhydride and aromatic heterocyclic diamine. The structural formula of the high-temperature resistant polyimide is as follows: ; Wherein, R represents a molecular structure containing an aromatic heterocycle in the main chain, and A represents a polymer repeating unit; the aromatic heterocycle includes at least one of imidazole, pyrimidine, pyridine, oxazole, and thiazole.

[0006] Optionally, the aromatic heterocyclic diamine is 2-(4-aminophenyl)-5-aminobenzimidazole, and the structural formula of the high-temperature resistant polyimide is: .

[0007] The high-temperature resistant polyimide provided by this invention is prepared through the following steps: S1 reduces 4-nitrophthalic acid to 4,4'-azobenzenetetracarboxylic acid; S2, intramolecular dehydration of 4,4'-azobenzenetetracarboxylic acid to generate 4,4'-azodiphenyl anhydride; S3, 4,4'-azodiphenyl anhydride and aromatic heterocyclic diamine are added to a solvent and polycondensed to obtain polyamic acid; the polyamic acid is coated onto a substrate and heated to a preset temperature to cause the polyamic acid to undergo an imidization reaction to obtain a polyimide film; the main chain of the aromatic heterocyclic diamine contains an aromatic heterocycle, and the aromatic heterocycle includes at least one of imidazole, pyrimidine, pyridine, oxazole, and thiazole.

[0008] Optionally, step S1 specifically includes: S11, deionized water, sodium hydroxide and 4-nitrophthalic acid are added to the reaction vessel in sequence, and the mixture is stirred until the first solution is obtained. S12, add glucose aqueous solution dropwise to the first solution, and react at a constant temperature of 50~60℃ to obtain the second solution; S13, cool the second solution to room temperature, acidify it with glacial acetic acid, filter the acidified second solution and wash the precipitate with deionized water; the molar ratio of the amount of glacial acetic acid to the amount of sodium hydroxide in step S11 is (1~1.5):1; S14, the precipitate obtained in step S13 is acidified with hydrochloric acid, sulfuric acid or a mixture of hydrochloric acid and sulfuric acid and stirred, the acidified mixture is filtered and washed with deionized water to obtain a filter cake, and the filter cake is vacuum dried to obtain a yellow solid 4,4'-azobenzenetetracarboxylic acid.

[0009] Optionally, in step S14, the acid used for acidification is hydrochloric acid with pH=1.

[0010] Optionally, in step S11, the mass ratio of deionized water, sodium hydroxide and 4-nitrophthalic acid is (15~17):(3~4):1, preferably 15.6:3.1:1.

[0011] Optionally, in step S12, the molar ratio of glucose to 4-nitrophthalic acid in step S11 is (6~10):1, preferably 7.4:1.

[0012] Optionally, in step S12, the mass ratio of glucose to deionized water in the glucose aqueous solution is 1:1.

[0013] Optionally, in step S12, the temperature of the isothermal reaction is preferably 53°C.

[0014] Optionally, in step S13, the molar ratio of the amount of glacial acetic acid used to the amount of sodium hydroxide used in step S11 is 1.33:1.

[0015] Optionally, step S2 includes: S21, 4,4'-azobenzenetetracarboxylic acid, acetic anhydride and toluene are added sequentially to the reaction vessel to obtain the third solution; S22, the third solution is refluxed at a reflux temperature of 120~130℃ for a preset time, then the third solution is cooled to precipitate crystals, the third solution is filtered and the crystals are washed with toluene to obtain a filter cake, and the filter cake is dried under vacuum to obtain red 4,4'-azodiphenyl anhydride crystals.

[0016] Optionally, the molar ratio of 4,4'-azobenzenetetracarboxylic acid to acetic anhydride is 1:(60~80), and the volume ratio of acetic anhydride to toluene is 1:(1.5~3). Optionally, the molar ratio of 4,4'-azobenzenetetracarboxylic acid to acetic anhydride is 1:75, and the volume ratio of acetic anhydride to toluene is 1:2.

[0017] Optionally, in step S22, the reflux temperature is 128°C.

[0018] Optionally, the aromatic heterocyclic diamine is 2-(4-aminophenyl)-5-aminobenzimidazole, and step S3 specifically includes: S31, 2-(4-aminophenyl)-5-aminobenzimidazole and aprotic solvent are added to a reaction vessel purged with nitrogen and stirred until 2-(4-aminophenyl)-5-aminobenzimidazole is completely dissolved. Then 4,4'-azodiphenyl anhydride is gradually added to the solution and the reaction is stirred at room temperature to obtain a polyamic acid solution. S32, the polyamic acid solution after removing air bubbles is poured onto the substrate to form a thin film; the substrate with the thin film is cured sequentially through a curing process of 100℃ for 60 min, 200℃ for 60 min, 300℃ for 60 min, 400℃ for 60 min, and finally at a final temperature of 30 min; after the substrate is cooled, it is immersed in hot water to obtain a polyimide film; the final temperature is 410℃~440℃.

[0019] Optionally, in step S31, the molar ratio of 4,4'-azobenzoic anhydride and 2-(4-aminophenyl)-5-aminobenzimidazole is (1~1.1):1, preferably 1.05:1.

[0020] The total amount of 4,4'-azodiphenyl anhydride and the mass ratio of 2-(4-aminophenyl)-5-aminobenzimidazole to the mass ratio of aprotic solvent is 1:(7~12), with a preferred ratio of 1:9.

[0021] Optionally, the method for removing bubbles with polyamic acid solution is to place the polyimide solution in a vacuum oven and evacuate it for 0.5 hours.

[0022] Optionally, the aprotic solvent is at least one of N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0023] Optionally, the aprotic solvent is dimethyl sulfoxide.

[0024] Optionally, in step S31, the reaction time of 2-(4-aminophenyl)-5-aminobenzimidazole and 4,4'-azobisphthalic anhydride is 10-24 h, preferably 12 h.

[0025] Optionally, the thickness of the polyimide film prepared in step S32 is 100~500μm.

[0026] Optionally, the substrate is a metal substrate or a glass substrate.

[0027] Optionally, the final temperature is 420°C.

[0028] The present invention has the following beneficial effects: The technical solution of this invention enhances the chain rigidity and chain interaction of polyimide by simultaneously introducing azo bonds and aromatic heterocycles into the main chain of polyimide. The glass transition temperature of the prepared polyimide film is greater than 400°C, which meets the high-temperature processing requirements of flexible devices. Furthermore, the dianhydride synthesis route used in this invention to prepare polyimide is simple, and the raw materials are cheap and readily available. Diamine is also inexpensive, which greatly reduces the process difficulty and production cost of high-temperature resistant polyimide. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a synthetic route diagram of 4,4'-azobenzenetetracarboxylic acid according to the present invention; Figure 2 This is a synthetic route diagram of 4,4'-azodiphenyl anhydride of the present invention; Figure 3 This is a synthetic route diagram for preparing polyimide according to the present invention; Figure 4 Infrared spectra of 4,4'-azobenzenetetracarboxylic acid prepared in some embodiments of the present invention; Figure 5Infrared spectra of 4,4'-azodiphenyl anhydride prepared in some embodiments of the present invention; Figure 6 The images show DSC test results of polyimide films prepared in some embodiments of the present invention. Detailed Implementation

[0031] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0032] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in this specification are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0033] Experimental methods in the embodiments of this invention that do not specify specific conditions are generally performed under conventional conditions and conditions described in the manual, or under conditions recommended by the manufacturer; the general equipment, materials, reagents, etc. used are commercially available unless otherwise specified.

[0034] The high-temperature resistant polyimide proposed in this invention embodiment is prepared by polycondensation and imidization reaction of 4,4'-azodiphenyl anhydride and aromatic heterocyclic diamine. The structural formula of the high-temperature resistant polyimide is: ; Wherein, R represents a molecular structure containing an aromatic heterocycle in the main chain, and A represents a polymer repeating unit; the aromatic heterocycle includes at least one of imidazole, pyrimidine, pyridine, oxazole, and thiazole; the "N×N" bond in the structural formula represents an azo bond with cis-trans isomerism.

[0035] The embodiments of the present invention enhance the chain rigidity and chain interactions (such as inter-chain hydrogen bonds) of polyimide by simultaneously introducing azo bonds and aromatic heterocycles into the main chain of polyimide, thereby increasing the glass transition temperature of polyimide to meet the high-temperature processing requirements of flexible devices (especially greater than 400°C).

[0036] The structural formula of 4,4'-azodiphenyl anhydride is: .

[0037] In this embodiment of the invention, aromatic heterocyclic diamines refer to diamine compounds whose main chain contains aromatic heterocycles. The types of aromatic heterocycles include, but are not limited to, one or more of imidazole, pyrimidine, pyridine, oxazole, and thiazole. Among them, imidazole is a five-membered ring containing two nitrogen atoms, pyrimidine is a six-membered ring containing two nitrogen atoms, pyridine is a six-membered ring containing one nitrogen atom, oxazole is a five-membered ring containing one oxygen atom and one nitrogen atom, and thiazole is a five-membered ring containing one sulfur atom and one nitrogen atom. Aromatic heterocycles satisfy the aromatic characteristics of planar structure and the ability of π electrons in the ring to delocalize and form conjugated large π bonds. Aromatic heterocycles provide higher bond energies and chain interactions to the molecular chain of polyimide, making polyimide more thermally stable.

[0038] In some preferred embodiments, the aromatic heterocyclic diamine can be selected from a diamine monomer containing a benzimidazole heterocycle, or a diamine monomer containing a heterocycle such as benzoxazole, benzothiazole or quinazoline.

[0039] The high-temperature resistant polyimide proposed in this embodiment of the invention is prepared through the following steps S1-S3: S1 reduces 4-nitrophthalic acid to 4,4'-azobenzenetetracarboxylic acid.

[0040] For the synthetic route of 4,4'-azobenzenetetracarboxylic acid, please refer to [link / reference]. Figure 1 The specific steps include: S11, deionized water, sodium hydroxide and 4-nitrophthalic acid are added sequentially to the reaction vessel and stirred until homogeneous to obtain the first solution; the mass ratio of deionized water, sodium hydroxide and 4-nitrophthalic acid is (15~17):(3~4):1, and the preferred ratio is 15.6:3.1:1.

[0041] S12, add glucose aqueous solution dropwise to the first solution, and react at a constant temperature of 50~60℃ to obtain the second solution; the mass ratio of glucose to deionized water in the glucose aqueous solution is 1:1, and the molar ratio of the amount of glucose used in this step to the amount of 4-nitrophthalic acid used in step S11 is (6~10):1, preferably 7.4:1; the preferred temperature for the constant temperature reaction is 53℃.

[0042] S13, cool the second solution to room temperature, acidify it with glacial acetic acid, filter the acidified second solution and wash the precipitate with deionized water; the molar ratio of the amount of glacial acetic acid to the amount of sodium hydroxide in step S11 is (1~1.5):1, preferably 1.33:1.

[0043] S14, the precipitate obtained in step S13 is acidified with hydrochloric acid, sulfuric acid or a mixture of hydrochloric acid and sulfuric acid and stirred, the acidified solution is filtered and washed with deionized water to obtain a filter cake, and the filter cake is dried under vacuum to obtain a yellow solid 4,4'-azobenzenetetracarboxylic acid; the acid used for acidification in this step is preferably hydrochloric acid with pH=1.

[0044] S2, intramolecular dehydration of 4,4'-azobenzenetetracarboxylic acid to generate 4,4'-azodiphenyl anhydride.

[0045] For the synthetic route of 4,4'-azodiphenyl anhydride, please refer to [link / reference needed]. Figure 2 The specific steps include: S21, 4,4'-azobenzenetetracarboxylic acid, acetic anhydride and toluene are added sequentially to the reaction vessel to obtain a third solution; the molar ratio of 4,4'-azobenzenetetracarboxylic acid to acetic anhydride is 1:(60~80), and the volume ratio of acetic anhydride to toluene is 1:(1.5~3).

[0046] In some preferred embodiments, the molar ratio of 4,4'-azobenzenetetracarboxylic acid to acetic anhydride is 1:75, and the volume ratio of acetic anhydride to toluene is 1:2.

[0047] S22, the third solution is refluxed at a reflux temperature of 120~130℃ for a preset time, then the third solution is cooled to precipitate crystals, the third solution is filtered and the crystals are washed with toluene to obtain a filter cake, and the filter cake is dried under vacuum to obtain red 4,4'-azodiphenyl anhydride crystals.

[0048] In some preferred embodiments, the reflux temperature is selected as 128°C.

[0049] S3, the 4,4'-azodiphenyl anhydride and aromatic heterocyclic diamine prepared in the above steps are added to a solvent and polycondensed to obtain polyamic acid; the polyamic acid is coated onto a substrate (metal substrate or glass substrate), and the substrate is heated to a preset temperature to induce an imidization reaction of the polyamic acid, thereby obtaining a polyimide film; the synthesis route of polyimide is described in [reference needed]. Figure 3 .

[0050] In some preferred embodiments, the aromatic heterocyclic diamine monomer is selected as 2-(4-aminophenyl)-5-aminobenzimidazole, and step S3 specifically includes: S31, 2-(4-aminophenyl)-5-aminobenzimidazole and a protic solvent are added to a reaction vessel purged with nitrogen and stirred until the 2-(4-aminophenyl)-5-aminobenzimidazole is completely dissolved. Then, 4,4'-azobenzene anhydride is gradually added to the solution and the reaction is stirred at room temperature to obtain a polyamic acid solution.

[0051] The molar ratio of 4,4'-azodiphenyl anhydride to 2-(4-aminophenyl)-5-aminobenzimidazole is (1~1.1):1, with a preferred ratio of 1.05:1; the mass ratio of the total amount of 4,4'-azodiphenyl anhydride and 2-(4-aminophenyl)-5-aminobenzimidazole to the amount of aprotic solvent is 1:(7~12), with a preferred ratio of 1:9; the reaction time of 2-(4-aminophenyl)-5-aminobenzimidazole and 4,4'-azodiphenyl anhydride is 10~24h, with a preferred reaction time of 12h.

[0052] In this step, the aprotic solvent refers to a solvent that does not contain any protons (H+). + The core characteristic of a solvent that is a donor hydrogen atom is that it cannot form hydrogen bonds with the solute or undergo proton transfer reactions through its own hydrogen atoms; non-protic solvents can be selected from N,N-dimethylacetamide, N-methylpyrrolidone or dimethyl sulfoxide.

[0053] S32, the polyamic acid solution after removing air bubbles is poured onto the substrate to form a thin film; the substrate with the thin film is cured sequentially through a curing process of 100°C for 60 min, 200°C for 60 min, 300°C for 60 min, 400°C for 60 min, and finally at a final temperature (410°C~440°C, preferably 420°C) for 30 min; after the substrate is cooled, it is immersed in hot water to obtain a polyimide film with a thickness of 100~500 μm.

[0054] The method for removing bubbles from the polyamic acid solution is as follows: place the polyimide solution in a vacuum oven and evacuate it for a certain period of time (preferably 0.5h).

[0055] Based on the above embodiments, the present invention also proposes the following specific embodiments. It should be noted that the following specific embodiments are merely exemplary and are not intended to limit the scope of protection of the present invention in any way.

[0056] Example 1

[0057] In this embodiment, 4,4'-azobenzenetetracarboxylic acid was prepared.

[0058] Add 250ml of deionized water, 50g of NaOH, and 16g of 4-nitrophthalic acid to a 500ml three-necked flask equipped with a magnetic stirrer. Stir until the solid is completely dissolved. Heat the mixed solution to 50℃.

[0059] 100g of glucose was dissolved in 100ml of deionized water to obtain a glucose aqueous solution. Within 30 minutes of obtaining the above mixed solution, the glucose aqueous solution was added dropwise to a three-necked flask. The solution in the three-necked flask was reacted at a constant temperature of 53℃ for 8 hours and then cooled to room temperature. Then, 80ml of glacial acetic acid was added to the mixture after the reaction to acidify it. The acidified mixture was filtered and the precipitate was washed with deionized water. The solid obtained after washing was acidified with hydrochloric acid at pH=1. After hydrochloric acid acidification, the mixture was stirred for 12 hours. Then, the mixture was filtered and the precipitate was washed with deionized water. The filter cake obtained after washing was vacuum dried at 100℃ for 24 hours to obtain 11.2g of yellow solid 4,4'-azobenzenetetracarboxylic acid (yield 70.0%).

[0060] The infrared spectrum of 4,4'-azobenzenetetracarboxylic acid prepared in this embodiment is as follows: Figure 4 As shown, the C=O stretching vibration peak of carboxylic acid corresponding to wavenumbers of 1544-1486 and the N=N stretching vibration peak corresponding to wavenumber of 1370 indicate the successful preparation of 4,4'-azobenzenetetracarboxylic acid.

[0061] Example 2

[0062] In this embodiment, 4,4'-azobenzenetetracarboxylic acid was prepared.

[0063] Add 250ml of deionized water, 50g of NaOH, and 16g of 4-nitrophthalic acid to a 500ml three-necked flask equipped with a magnetic stirrer. Stir until the solid is completely dissolved. Heat the mixed solution to 50℃.

[0064] 100g of glucose was dissolved in 100ml of deionized water to obtain a glucose aqueous solution. Within 30 minutes of obtaining the above mixed solution, the glucose aqueous solution was added dropwise to a three-necked flask. The solution in the three-necked flask was reacted at a constant temperature of 60℃ for 8 hours and then cooled to room temperature. Then, 80ml of glacial acetic acid was added to the mixture after the reaction to acidify it. The acidified mixture was filtered and the precipitate was washed with deionized water. The solid obtained after washing was acidified with hydrochloric acid at pH=1. After hydrochloric acid acidification, the mixture was stirred for 12 hours. Then, the mixture was filtered and the precipitate was washed with deionized water. The filter cake obtained after washing was vacuum dried at 100℃ for 24 hours to obtain 8.1g of yellow solid 4,4'-azobenzenetetracarboxylic acid (yield 50.6%).

[0065] The infrared spectrum of the 4,4'-azobenzenetetracarboxylic acid prepared in this embodiment is also as follows. Figure 4 As shown, the C=O stretching vibration peak of carboxylic acid corresponding to wavenumbers of 1544-1486 and the N=N stretching vibration peak corresponding to wavenumber of 1370 indicate the successful preparation of 4,4'-azobenzenetetracarboxylic acid.

[0066] Example 3

[0067] In this embodiment, 4,4'-azodiphenyl anhydride was prepared.

[0068] In a 500 ml three-necked flask equipped with a water separator and a condenser, 4 g of 4,4'-azobenzenetetracarboxylic acid, 80 ml of acetic anhydride, and 160 ml of toluene were added sequentially. The mixed solution was refluxed at 128 °C for 4 h. After cooling, crystals precipitated from the solution. The cooled solution was filtered and the crystals were washed with toluene. The washed filter cake was dried under vacuum at 120 °C for 24 h to obtain 3.1 g of red 4,4'-azobenzene anhydride crystals (yield 78%).

[0069] The infrared spectrum of the 4,4'-azodiphenyl anhydride prepared in this embodiment is shown below. Figure 5 As shown, the C=O stretching vibration peaks of phthalic anhydride corresponding to wavenumbers of 1882 and 1773 and the N=N stretching vibration peak corresponding to wavenumber of 1354 indicate the successful preparation of 4,4'-azodibenzoic anhydride.

[0070] Example 4

[0071] In this embodiment, 4,4'-azodiphenyl anhydride was prepared.

[0072] In a 500 ml three-necked flask equipped with a water separator and a condenser, 4 g of 4,4'-azobenzenetetracarboxylic acid, 60 ml of acetic anhydride, and 120 ml of toluene were added sequentially. The mixed solution was refluxed at 128 °C for 4 h. After cooling, crystals precipitated from the solution. The cooled solution was filtered and the crystals were washed with toluene. The washed filter cake was dried under vacuum at 120 °C for 24 h to obtain 2.9 g of red 4,4'-azobenzene anhydride crystals (yield 73%).

[0073] The infrared spectrum of the 4,4'-azodiphenyl anhydride prepared in this embodiment is also as follows. Figure 5 As shown, the C=O stretching vibration peaks of phthalic anhydride corresponding to wavenumbers of 1882 and 1773 and the N=N stretching vibration peak corresponding to wavenumber of 1354 indicate the successful preparation of 4,4'-azodibenzoic anhydride.

[0074] Example 5

[0075] This embodiment prepares a high-temperature resistant polyimide film based on the 4,4'-azodiphenyl anhydride prepared in Examples 1-4.

[0076] Step 1: Synthesize polyamic acid.

[0077] 2.243 g of 2-(4-aminophenyl)-5-aminobenzimidazole and 46 ml of dimethyl sulfoxide were added to a 250 ml three-necked flask purged with nitrogen. The mixture was stirred until the 2-(4-aminophenyl)-5-aminobenzimidazole was completely dissolved. Then, 3.383 g of 4,4'-azobenzoic anhydride was gradually added to the 2-(4-aminophenyl)-5-aminobenzimidazole solution. The mixture was stirred at room temperature for 12 h to obtain a polyamic acid solution with a solid content of 10%.

[0078] Step 2: Prepare a polyimide film.

[0079] The polyamic acid solution prepared in step 1 was placed in a vacuum oven and evacuated for 0.5 hours to remove air bubbles. Then, the polyamic acid solution was cast onto a glass substrate at room temperature using a casting coating machine, and the thickness of the cast polyamic acid film was 200 μm. The glass substrate with cast polyamic acid was placed in a quartz boat, and the polyamic acid film was cured in a tube furnace under an argon atmosphere. The curing process adopted a stepped curing method, and the curing program was 100℃ for 60 min, 200℃ for 60 min, 300℃ for 60 min, 400℃ for 60 min, and 420℃ for 30 min in sequence. After the curing program, the glass substrate was cooled, and then the glass substrate was immersed in hot water to obtain a polyimide film.

[0080] The polyimide film prepared in this embodiment was subjected to DSC testing (test standard GB / T 19466.2-2004). The process involved a first constant-rate heating, a first constant-rate cooling, a second constant-rate heating, and a second constant-rate cooling. The DSC test results are as follows: Figure 6 As shown, when a polymer undergoes a glass transition, both its heating and cooling DSC curves exhibit small steps; the temperature corresponding to these small steps is the glass transition temperature of the polymer. Figure 6 It can be seen that the glass transition temperature of the polyimide film prepared in this embodiment is around 410°C.

[0081] Example 6

[0082] This embodiment prepares a high-temperature resistant polyimide film based on the 4,4'-azodiphenyl anhydride prepared in Examples 1-4.

[0083] Step 1: Synthesize polyamic acid.

[0084] 2.243 g of 2-(4-aminophenyl)-5-aminobenzimidazole and 45 ml of dimethyl sulfoxide were added to a 250 ml three-necked flask purged with nitrogen. The raw materials were stirred until the 2-(4-aminophenyl)-5-aminobenzimidazole was completely dissolved. Then, 3.222 g of 4,4'-azobenzoic anhydride was gradually added to the 2-(4-aminophenyl)-5-aminobenzimidazole solution. The mixture was stirred at room temperature for 12 h to obtain a polyamic acid solution with a solid content of 10%.

[0085] Step 2: Prepare a polyimide film.

[0086] The polyamic acid solution prepared in step 1 was placed in a vacuum oven and evacuated for 0.5 hours to remove air bubbles. Then, the polyamic acid solution was cast onto a glass substrate at room temperature using a casting coating machine, and the thickness of the cast polyamic acid film was 200 μm. The glass substrate with the cast polyamic acid was placed in a quartz boat, and the polyamic acid film was cured in a tube furnace under an argon atmosphere. The curing process adopted a stepped curing method, and the curing program was 100℃ for 60 min, 200℃ for 60 min, 300℃ for 60 min, 400℃ for 60 min, and 440℃ for 30 min in sequence. After the curing program, the glass substrate was cooled, and then the glass substrate was immersed in hot water to obtain a polyimide film.

[0087] As can be seen from the above embodiments, the technical solution of the present invention enhances the chain rigidity and chain interaction of polyimide by simultaneously introducing azo bonds and aromatic heterocycles into the main chain of polyimide. The glass transition temperature of the prepared polyimide film is greater than 400℃, which meets the high-temperature processing requirements of flexible devices. Moreover, the dianhydride synthesis route used in the preparation of polyimide in the present invention is simple, and raw materials such as 4-nitrophthalic acid, glucose, sodium hydroxide and glacial acetic acid are cheap and readily available. Diamine monomers such as 2-(4-aminophenyl)-5-aminobenzimidazole are inexpensive commercial raw materials. Therefore, the technical solution of the present invention also greatly reduces the difficulty of the preparation process and the production cost of high-temperature resistant polyimide.

[0088] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A polyimide, characterized in that, The high-temperature resistant polyimide is prepared by polycondensation and imidization reaction of 4,4'-azodiphenyl anhydride and aromatic heterocyclic diamine. The structural formula of the polyimide is as follows: ； Wherein, R represents a molecular structure containing an aromatic heterocycle in the main chain, and A represents a polymer repeating unit; the aromatic heterocycle includes at least one of imidazole, pyrimidine, pyridine, oxazole, and thiazole.

2. The polyimide according to claim 1, characterized in that, The aromatic heterocyclic diamine is 2-(4-aminophenyl)-5-aminobenzimidazole, and the structural formula of the high-temperature resistant polyimide is: 。 3. A method for preparing polyimide, characterized in that, Including the following steps: S1 reduces 4-nitrophthalic acid to 4,4'-azobenzenetetracarboxylic acid; S2, intramolecular dehydration of 4,4'-azobenzenetetracarboxylic acid to generate 4,4'-azodiphenyl anhydride; S3, 4,4'-azodiphenyl anhydride and aromatic heterocyclic diamine are added to a solvent and polycondensed to obtain polyamic acid; the polyamic acid is coated onto a substrate and heated to a preset temperature to cause the polyamic acid to undergo an imidization reaction to obtain a polyimide film; the main chain of the aromatic heterocyclic diamine contains an aromatic heterocycle, and the aromatic heterocycle includes at least one of imidazole, pyrimidine, pyridine, oxazole, and thiazole.

4. The method for preparing polyimide according to claim 3, characterized in that, Step S1 specifically includes: S11, deionized water, sodium hydroxide and 4-nitrophthalic acid are added to the reaction vessel in sequence, and the mixture is stirred until the first solution is obtained. S12, add glucose aqueous solution dropwise to the first solution, and react at a constant temperature of 50~60℃ to obtain the second solution; S13, cool the second solution to room temperature, acidify it with glacial acetic acid, filter the acidified second solution and wash the precipitate with deionized water; the molar ratio of the amount of glacial acetic acid to the amount of sodium hydroxide in step S11 is (1~1.5):1; S14, the precipitate obtained in step S13 is acidified with hydrochloric acid, sulfuric acid or a mixture of hydrochloric acid and sulfuric acid and stirred. The acidified mixture is filtered and washed with deionized water to obtain a filter cake. The filter cake is dried under vacuum to obtain a yellow solid 4,4'-azobenzenetetracarboxylic acid.

5. The method for preparing polyimide according to claim 4, characterized in that, In step S11, the mass ratio of deionized water, sodium hydroxide and 4-nitrophthalic acid is (15~17):(3~4):

1.

6. The method for preparing polyimide according to claim 4, characterized in that, In step S12, the molar ratio of glucose to 4-nitrophthalic acid in step S11 is (6~10):

1.

7. The method for preparing polyimide according to claim 3, characterized in that, Step S2 includes: S21, 4,4'-azobenzenetetracarboxylic acid, acetic anhydride and toluene are added sequentially to the reaction vessel to obtain the third solution; S22, the third solution is refluxed at a reflux temperature of 120~130℃ for a preset time, then the third solution is cooled to precipitate crystals, the third solution is filtered and the crystals are washed with toluene to obtain a filter cake, and the filter cake is dried under vacuum to obtain red 4,4'-azodiphenyl anhydride crystals.

8. The method for preparing polyimide according to claim 7, characterized in that, The molar ratio of 4,4'-azobenzenetetracarboxylic acid to acetic anhydride is 1:(60~80), and the volume ratio of acetic anhydride to toluene is 1:(1.5~3).

9. The method for preparing polyimide according to claim 3, characterized in that, The aromatic heterocyclic diamine is 2-(4-aminophenyl)-5-aminobenzimidazole, and step S3 specifically includes: S31, 2-(4-aminophenyl)-5-aminobenzimidazole and aprotic solvent are added to a reaction vessel purged with nitrogen and stirred until 2-(4-aminophenyl)-5-aminobenzimidazole is completely dissolved. Then, 4,4'-azodiphenyl anhydride is gradually added to the solution and stirred at room temperature to obtain a polyamic acid solution. S32, the polyamic acid solution after removing air bubbles is poured onto the substrate to form a thin film; the substrate with the thin film is cured sequentially through a curing process of 100℃ for 60 min, 200℃ for 60 min, 300℃ for 60 min, 400℃ for 60 min, and finally at a final temperature of 30 min; after the substrate is cooled, it is immersed in hot water to obtain a polyimide film; the final temperature is 410℃~440℃.

10. The method for preparing polyimide according to claim 9, characterized in that, In step S31, the molar ratio of 4,4'-azobenzoic anhydride and 2-(4-aminophenyl)-5-aminobenzimidazole is (1~1.1):1; The total amount of 4,4'-azodiphenyl anhydride and the mass ratio of 2-(4-aminophenyl)-5-aminobenzimidazole to the amount of aprotic solvent is 1:(7~12).

Images