Diamine and polyimide containing bipyridine structure as well as preparation method and application of diamine and polyimide

Abstract

The invention discloses diamine and polyimide containing a bipyridine structure as well as a preparation method and application of the diamine and the polyimide, and the diamine containing the bipyridine structure is characterized in that the bipyridine structure is introduced into a chemical structure of a diamine monomer, so that a conjugated system of the bipyridine structure is utilized to react with tetracarboxy dianhydride with an ether bond to prepare a class of novel polyimide; the polyimide utilizes a conjugated system of a bipyridine structure and ether bonds in tetracarboxy dianhydride to balance the rigidity and flexibility of a polyimide main chain, so that the polyimide material has good thermodynamic properties, the molecular stability and the light absorption capability are improved, the fluorescence property of a polyimide film is improved, and the application prospect is broad. The dissolvability of the material is improved, the processability is high, and the market application and generalization performance of the fluorescent film are achieved.

Description

Technical Field

[0001] This invention relates to the field of polyimide synthesis technology, and particularly to a diamine containing a bipyridine structure, a polyimide, its preparation method, and its application. Background Technology

[0002] Polyimides are renowned for their excellent thermal stability, mechanical properties, chemical stability, and electrical insulation properties, and have been widely used in films, composites, adhesives, fibers, and electronic products. Fully aromatic polyimides, in particular, possess a strong rigid structure and strong interactions from intra- and inter-chain charge transfer complexes. This results in relatively good solubility in high-boiling-point polar solvents (such as DMF, DMAc, and NMP), but poor solubility in low-boiling-point polar solvents such as chloroform, leading to poor processability and limiting their applications. Therefore, current research has focused on incorporating aliphatic, symmetrical, flexible, and large, curved substituents into the polyimide chain to improve the solubility of fully aromatic polyimides, thereby enhancing their processability. However, introducing flexible groups and alicyclic structures into the polyimide backbone inevitably leads to the sacrifice of other beneficial properties of polyimides, such as thermal stability and mechanical properties. Based on this, it is necessary to design and explore modifications to existing aromatic polyimides in order to synthesize a class of polyimide materials that can improve their processability without affecting their corresponding beneficial properties. Summary of the Invention

[0003] The purpose of this invention is to provide a diamine containing a bipyridine structure that can be used to synthesize polyimides and improve their optical properties, and a method for preparing the same.

[0004] Another object of the present invention is to provide a polyimide film synthesized using the above-mentioned diamine containing a bipyridine structure and a method thereof for preparation.

[0005] Another object of the present invention is to provide an application of the above-described polyimide film.

[0006] Therefore, the technical solution of the present invention is as follows:

[0007] A diamine containing a bipyridine structure has the following chemical structural formula:

[0008]

[0009] In the formula, R1 is

[0010] This diamine containing a bipyridine structure introduces a bipyridine structure into the chemical structure of the diamine monomer. In this structure, the pyridine ring, compared to the benzene ring, exhibits high molecular symmetry and aromaticity, and possesses a lone pair of electrons in the sp2 orbital of the nitrogen atom. Therefore, the derived polymer exhibits higher electron affinity, improving the overall electron transport performance of the molecular chain and enabling the protonation or alkylation of the lone pair. Thus, by introducing a pyridine ring onto the diamine monomer, and by introducing pyridine units at appropriate positions in the polymer molecular chain, it is expected that the polar interactions between molecular chains or between the molecular chain and the solvent can be increased, thereby overcoming common polymer dissolution problems. Furthermore… Pyridine rings possess strong rigidity and polarization capabilities, effectively balancing the conflict between the thermal stability and processing performance of polymer materials. Specifically, in addition to exhibiting the properties of monopyridine heterocyclic polymers, bipyridine heterocyclic polymers also form a large conjugated system due to the carbon-carbon bond connecting the two pyridine rings. Within this conjugated system, electrons can move freely between the two pyridine rings, and this electron delocalization enhances molecular stability and light absorption. Furthermore, the conjugated structure and electron delocalization of bipyridine enable it to undergo π-π* transitions upon photoexcitation and emit fluorescence when electrons return from the excited state to the ground state, thus predictably endowing the polymer with luminescent properties.

[0011] A method for preparing the above-mentioned diamine containing a bipyridine structure, comprising the following steps:

[0012] S1. Dissolve 4-nitroacetophenone and pyridine-4-carboxaldehyde, or 4-nitroacetophenone and pyridine-2-carboxaldehyde, in organic solvent I to form a reaction system; dissolve NaOH in a mixed solution of organic solvent I and water to obtain an alkaline mixed solvent, and add it dropwise to the reaction system under an ice-alcohol bath. React for 1 h to 6 h to obtain intermediate I;

[0013] The chemical reaction formula for step S1 is as follows:

[0014]

[0015] S2,4-nitroacetophenone was dispersed in organic solvent II, then iodine was added, and the mixture was placed at 75℃~85℃ and reacted for 12h~14h to obtain intermediate II;

[0016] The chemical reaction formula for step S2 is as follows:

[0017]

[0018] S3. Intermediate I and Intermediate II are dispersed in organic solvent III and reacted at 80℃~85℃ for 12h~16h under the action of ammonium acetate to obtain Intermediate III;

[0019] S4. Disperse intermediate III and catalyst in organic solvent IV, add reducing agent and catalyst, and react at 50℃~80℃ for 24h~48h to obtain diamine containing bipyridine structure.

[0020] The chemical reaction formulas for steps S3 and S4 above are shown below:

[0021]

[0022] Preferably, in step S1, the molar ratio of 4-nitroacetophenone to pyridine-4-carboxaldehyde, or 4-nitroacetophenone to pyridine-2-carboxaldehyde, is 1:1.

[0023] Preferably, in step S1, the organic solvent I is methanol, ethanol, or N,N-dimethylformamide, more preferably ethanol; the amount of organic solvent I added is 4 to 6 times the weight of 4-nitroacetophenone.

[0024] Preferably, in step S1, the molar ratio of 4-nitroacetophenone to NaOH is 1:(0.2 to 0.5), more preferably 1:0.3; specifically, the yield of intermediate 1 is highest when the molar ratio of 4-nitroacetophenone to NaOH is 1:0.3.

[0025] Preferably, in step S1, the volume ratio of organic solvent I to water in the alkaline mixed solvent is 1:1; the amount of alkaline mixed solvent added is 15 to 30 times the weight of NaOH.

[0026] Preferably, in step S2, the molar ratio of 4-nitroacetophenone to iodine is 1:1.

[0027] Preferably, in step S2, the organic solvent II is pyridine, and the amount added is 10 to 15 times the weight of 4-nitroacetophenone.

[0028] Preferably, in step S3, the molar ratio of intermediate I to ammonium acetate is 1:(5-15), more preferably 1:(8-12).

[0029] Preferably, in step S3, organic solvent III is methanol, ethanol, N,N-dimethylformamide, more preferably ethanol; the amount of organic solvent III added is 15 to 20 times the total weight of intermediate I and intermediate II.

[0030] Preferably, in step S4, the reducing agent is hydrogen, hydrazine hydrate, formic acid, ammonium formate, or sodium formate; wherein, when pyridine-4-carboxaldehyde is used in step S1, the reducing agent is preferably hydrazine hydrate; when pyridine-2-carboxaldehyde is used in step S1, the reducing agent is preferably hydrogen, in order to obtain the maximum yield.

[0031] Preferably, in step S4, the catalyst is a metal carbon catalyst, specifically a palladium-carbon catalyst, a nickel-carbon catalyst, or a platinum-carbon catalyst, preferably 10% Pd / C.

[0032] Preferably, in step S4, the mass ratio of intermediate III to catalyst is 1:(0.1 to 0.3), more preferably 1:(0.2 to 0.3).

[0033] Preferably, in step S4, the organic solvent IV is ethylene glycol, methanol, ethanol, or N,N-dimethylformamide; wherein, when pyridine-4-carboxaldehyde is used in step S1, N,N-dimethylformamide is preferably used as the organic solvent IV; when pyridine-2-carboxaldehyde is used in step S1, methanol is preferably used as the organic solvent IV. The amount of organic solvent IV added is 15 to 60 times the weight of intermediate III.

[0034] A polyimide is obtained by polymerizing the above-mentioned diamine containing a bipyridine structure with a tetracarboxylic acid dianhydride, followed by imidization; wherein the tetracarboxylic acid dianhydride is a bisphenol a type diether dianhydride or 4,4'-oxobisphthalic anhydride.

[0035] A polyimide film, which is prepared from the polyimide as described in claim 8.

[0036] Preferably, a specific preparation method for the polyimide film is as follows: Under a nitrogen atmosphere, a diamine containing a bipyridine structure is dissolved in a solvent, tetracarboxylic acid dianhydride is added in batches and the solvent is replenished appropriately, and the mixture is stirred at room temperature for 12-24 hours to obtain a viscous precursor polyamic acid solution with a solid content generally of 20-30 wt%. The polyamic acid solution is cast onto a clean glass substrate, and most of the solvent is removed by drying. Subsequently, the glass substrate is transferred to a muffle furnace, and thermal imidization is performed according to a high-temperature program setting, following the steps of 100℃ for 1 hour, 150℃ for 1 hour, 200℃ for 1 hour, 250℃ for 1 hour, and 280℃ for 1 hour to obtain a polyimide film. After the temperature is cooled to room temperature, the film is immersed in deionized water and peeled off from the glass plate. The peeled film is placed in a vacuum oven at 100℃ and dried for 6 hours to obtain the polyimide film.

[0037] One application of a polyimide film is as a fluorescent labeling film, for example, used on product packaging to enhance anti-counterfeiting properties with specific fluorescence; or, in hazardous environments such as laboratories, it can be made into a safety warning sign to remind passersby to pay attention to safety.

[0038] Compared with existing technologies, this bipyridine-containing diamine introduces a bipyridine structure into the chemical structure of the diamine monomer. Utilizing the conjugated system of the bipyridine structure, it reacts with a tetracarboxylic dianhydride containing ether bonds to prepare a novel type of polyimide. This type of polyimide balances the rigidity and flexibility of the polyimide backbone by leveraging the conjugated system of the bipyridine structure and the ether bonds in the tetracarboxylic dianhydride. While ensuring good thermodynamic properties, it enhances molecular stability and light absorption, improves the fluorescence performance of the polyimide film, and increases the material's solubility and processing performance, making it suitable for market application and widespread use as a fluorescent film material. Attached Figure Description

[0039] Figure 1 The NMR spectrum of 2,6-bis(4-aminophenyl)-4,4'-bipyridine prepared in Example 1 of this invention;

[0040] Figure 2 The NMR spectrum of 2,6-bis(4-aminophenyl)-2,4'-bipyridine prepared in Example 3 of this invention;

[0041] Figure 3 Thermogravimetric curves of the polyimide films prepared in Examples 5 to 8 of this invention;

[0042] Figure 4 Differential scanning calorimetry (DSC) curves of the polyimide films prepared in Examples 5 to 8 of this invention;

[0043] Figure 5 The fluorescence emission spectrum curves of the polyimide films prepared in Examples 5 to 8 of this invention are shown. Detailed Implementation

[0044] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention.

[0045] Example 1

[0046] At room temperature, 8.26 g (50 mmol) of 4-nitroacetophenone and 5.36 g (50 mmol) of pyridine-4-carboxaldehyde were dissolved in 50 mL of ethanol solution, and the solution was transferred to an ice-cold alcohol bath and stirred for 15 min; 1.0 g (25 mmol) of NaOH was dissolved in 20 mL of ethanol-water solution (V 乙醇 V 水 =1:1) and added dropwise to the reaction system. After the addition was completed, the reaction was carried out for 1.5 h. The reaction was monitored by TLC until it ended. The reaction solution was filtered, and the filter cake was washed with ethanol in small amounts several times. After vacuum drying, 6.12 g of 4-nitrophenyl-3-(pyridin-4-yl)propenone was obtained.

[0047] At room temperature, 8.26 g (50 mmol) of 4-nitroacetophenone was added to a round-bottom flask, followed by 85 mL of pyridine. Then, 12.69 g (50 mmol) of elemental iodine was added under light-protected conditions. The mixture was heated to 80 °C and reacted for 12 h. The reaction was stopped by TLC. 250 mL of ethyl acetate was added directly to the reaction system to precipitate a solid. The solid was filtered, slurried three times with ethyl acetate, and dried under vacuum to obtain 10.37 g of 1-(2-(4-nitrophenyl)-2-oxoethyl)pyridine-1-ammonium iodide.

[0048] At room temperature, 3.81 g (15 mmol) of 4-nitrophenyl-3-(pyridin-4-yl)propenone and 5.55 g (50 mmol) of 1-(2-(4-nitrophenyl)-2-oxoethyl)pyridine-1-iodine ammonium salt were dispersed in 200 mL of ethanol, and 9.25 g (120 mmol) of ammonium acetate were added. The reaction was carried out at 80 °C for 12 hours. The reaction was monitored by TLC until it ended. The filter cake was filtered, slurried three times with methanol, and dried under vacuum to obtain 3.43 g of 2,6-bis(4-nitrophenyl)-4,4'-bipyridine.

[0049] At room temperature, 3.98 g (10 mmol) of 2,6-bis(4-nitrophenyl)-4,4'-bipyridine and 1.0 g of 10% Pd / C were added to 80 mL of LDM and dispersed. Hydrogen gas was introduced for reduction, and the reaction was carried out at 50 °C for 48 h. The reaction was monitored by TLC until it ended. Pd / C was filtered off, the reaction solution was evaporated to dryness, and tetrahydrofuran was slurried once to obtain 1.09 g of 2,6-bis(4-aminophenyl)-4,4'-bipyridine.

[0050] The chemical structural formula of the bipyridine-containing diamine prepared in Example 1 is as follows:

[0051]

[0052] Example 2

[0053] At room temperature, 8.26 g (50 mmol) of 4-nitroacetophenone and 5.36 g (50 mmol) of pyridine-4-carboxaldehyde were dissolved in 50 mL of ethanol solution, and the solution was transferred to an ice-cold alcohol bath and stirred for 15 min; 1.0 g (25 mmol) of NaOH was dissolved in 20 mL of ethanol-water solution (V 乙醇 V 水 =1:1) and added dropwise to the reaction system. After the addition was completed, the reaction was carried out for 1.5 h. The reaction was monitored by TLC until it ended. The reaction solution was filtered, and the filter cake was washed with ethanol in small amounts several times. After vacuum drying, 6.12 g of 4-nitrophenyl-3-(pyridin-4-yl)propenone was obtained.

[0054] At room temperature, 8.26 g (50 mmol) of 4-nitroacetophenone was added to a round-bottom flask, followed by 85 mL of pyridine. Then, 12.69 g (50 mmol) of elemental iodine was added under light-protected conditions. The mixture was heated to 80 °C and reacted for 12 h. The reaction was stopped by TLC. 250 mL of ethyl acetate was added directly to the reaction system to precipitate a solid. The solid was filtered, slurried three times with ethyl acetate, and dried under vacuum to obtain 10.37 g of 1-(2-(4-nitrophenyl)-2-oxoethyl)pyridine-1-ammonium iodide.

[0055] At room temperature, 3.81 g (15 mmol) of 4-nitrophenyl-3-(pyridin-4-yl)propenone and 5.55 g (50 mmol) of 1-(2-(4-nitrophenyl)-2-oxoethyl)pyridine-1-iodine ammonium salt were dispersed in 200 mL of ethanol, and 9.25 g (120 mmol) of ammonium acetate were added. The reaction was carried out at 80 °C for 12 hours. The reaction was monitored by TLC until it ended. The filter cake was filtered, slurried three times with methanol, and dried under vacuum to obtain 3.43 g of 2,6-bis(4-nitrophenyl)-4,4'-bipyridine.

[0056] At room temperature, 3.98 g (10 mmol) of 2,6-bis(4-nitrophenyl)-4,4'-bipyridine and 1.0 g of 10% Pd / C were added to 80 mL of LDM for dispersion. Hydrazine hydrate was added for reduction, and the reaction was carried out at 80 °C for 48 h. The reaction was monitored by TLC until it ended. Pd / C was filtered off, the reaction solution was evaporated to dryness, and tetrahydrofuran was slurried once to obtain 1.85 g of 2,6-bis(4-aminophenyl)-4,4'-bipyridine.

[0057] Example 3

[0058] At room temperature, 8.26 g (50 mmol) of 4-nitroacetophenone and 5.36 g (50 mmol) of pyridine-2-carboxaldehyde were dissolved in 50 mL of ethanol solution, and the solution was transferred to an ice-cold alcohol bath and stirred for 15 min. 0.6 g (15 mmol) of NaOH was added to 20 mL of the ethanol-water solution (V... 乙醇 V 水 =1:1), and then the prepared NaOH ethanol aqueous solution was added dropwise to the reaction system. The reaction was carried out for 5 hours, and the reaction was monitored by TLC until it ended. The reaction solution was filtered, and the filter cake was washed with ethanol in small amounts several times. After vacuum drying, 7.53 g of 4-nitrophenyl-3-(pyridin-2-yl)propenone was obtained.

[0059] At room temperature, 8.26 g (50 mmol) of 4-nitroacetophenone was added to a round-bottom flask, followed by 85 mL of pyridine. Then, 12.69 g (50 mmol) of elemental iodine was added under light-protected conditions. The mixture was heated to 80 °C and reacted for 12 h. The reaction was stopped by TLC. 250 mL of ethyl acetate was added directly to the reaction system to precipitate a solid. The solid was filtered, slurried three times with ethyl acetate, and dried under vacuum to obtain 10.37 g of 1-(2-(4-nitrophenyl)-2-oxoethyl)pyridine-1-ammonium iodide.

[0060] At room temperature, 3.81 g (15 mmol) of 4-nitrophenyl-3-(pyridin-2-yl)propenone and 5.55 g (50 mmol) of 1-(2-(4-nitrophenyl)-2-oxoethyl)pyridine-1-iodine ammonium salt were dispersed in 200 mL of ethanol, and 13.87 g (180 mmol) of ammonium acetate was added. The reaction was carried out at 80 °C for 12 hours. The reaction was monitored by TLC until it ended. The filter cake was filtered, slurried three times with methanol, and dried under vacuum to obtain 3.70 g of 2,6-bis(4-nitrophenyl)-2,4'-bipyridine.

[0061] At room temperature, 3.98 g (10 mmol) of 2,6-bis(4-nitrophenyl)-2,4'-bipyridine and 1.0 g of 10% Pd / C were added to 80 mL of LDM and dispersed. Hydrogen gas was introduced for reduction, and the reaction was carried out at 60 °C for 24 h. The reaction was monitored by TLC until it ended. Pd / C was filtered off, the reaction solution was evaporated to dryness, and the solution was slurried in tetrahydrofuran once to obtain 1.33 g of 2,6-bis(4-aminophenyl)-2,4'-bipyridine, with a yield of 39.3%.

[0062] The chemical structural formula of the bipyridine-containing diamine prepared in Example 2 is as follows:

[0063]

[0064] Example 4

[0065] At room temperature, 8.26 g (50 mmol) of 4-nitroacetophenone and 5.36 g (50 mmol) of pyridine-2-carboxaldehyde were dissolved in 50 mL of ethanol solution, and the solution was transferred to an ice-cold alcohol bath and stirred for 15 min. 0.6 g (15 mmol) of NaOH was added to 20 mL of the ethanol-water solution (V... 乙醇 V 水 =1:1), and then the prepared NaOH ethanol aqueous solution was added dropwise to the reaction system. The reaction was carried out for 5 hours, and the reaction was monitored by TLC until it ended. The reaction solution was filtered, and the filter cake was washed with ethanol in small amounts several times. After vacuum drying, 7.53 g of 4-nitrophenyl-3-(pyridin-2-yl)propenone was obtained.

[0066] At room temperature, 8.26 g (50 mmol) of 4-nitroacetophenone was added to a round-bottom flask, followed by 85 mL of pyridine. Then, 12.69 g (50 mmol) of elemental iodine was added under light-protected conditions. The mixture was heated to 80 °C and reacted for 12 h. The reaction was stopped by TLC. 250 mL of ethyl acetate was added directly to the reaction system to precipitate a solid. The solid was filtered, slurried three times with ethyl acetate, and dried under vacuum to obtain 10.37 g of 1-(2-(4-nitrophenyl)-2-oxoethyl)pyridine-1-ammonium iodide.

[0067] At room temperature, 3.81 g (15 mmol) of 4-nitrophenyl-3-(pyridin-2-yl)propenone and 5.55 g (50 mmol) of 1-(2-(4-nitrophenyl)-2-oxoethyl)pyridine-1-iodine ammonium salt were dispersed in 200 mL of ethanol, and 13.87 g (180 mmol) of ammonium acetate was added. The reaction was carried out at 80 °C for 12 hours. The reaction was monitored by TLC until it ended. The filter cake was filtered, slurried three times with methanol, and dried under vacuum to obtain 3.70 g of 2,6-bis(4-nitrophenyl)-2,4'-bipyridine.

[0068] At room temperature, 3.98 g (10 mmol) of 2,6-bis(4-nitrophenyl)-2,4'-bipyridine and 1.0 g of 10% Pd / C were added to 280 mL of methanol for dispersion, and hydrogen gas was introduced for reduction. The reaction was carried out at 60 °C for 24 h. The reaction was monitored by TLC until it ended. Pd / C was filtered off, the reaction solution was evaporated to dryness, and tetrahydrofuran was slurried once to obtain 1.77 g of 2,6-bis(4-aminophenyl)-2,4'-bipyridine.

[0069] Example 5

[0070] Under a nitrogen atmosphere, 0.338 g (1 mmol) of 2,6-bis(4-aminophenyl)-4,4'-bipyridine prepared in Example 1 was dissolved in 1 mL. In DMAC, 0.531 g of bisphenol A diether dianhydride was added in batches, and 2.8 mL of DMAC was added in batches during the addition of bisphenol A diether dianhydride. The mixture was stirred at room temperature for 12 h to obtain a viscous precursor polyamic acid solution. The polyamic acid solution was cast onto a clean glass substrate and placed in an 80 °C oven for 6 h to remove most of the solvent. The glass substrate was then transferred to a muffle furnace and subjected to thermal imidization at 100 °C / 1 h, 150 °C / 1 h, 200 °C / 1 h, 250 °C / 1 h, and 280 °C / 1 h to obtain a polyimide film. After cooling to room temperature, the film was immersed in deionized water and peeled off from the glass plate. The peeled film was dried in a 100 °C vacuum oven for 6 h to obtain a polyimide film prepared from 2,6-bis(4-aminophenyl)-4,4'-bipyridine with a thickness of approximately 25 μm.

[0071] The structural formula of the polyimide film prepared in Example 5 is as follows:

[0072]

[0073] Example 6

[0074] Under a nitrogen atmosphere, 0.338 g (1 mmol) of 2,6-bis(4-aminophenyl)-4,4'-bipyridine prepared in Example 1 was dissolved in 1 mL of DMAC. 0.316 g of 4,4'-oxophthalic anhydride was added in portions, with 1.8 mL of DMAC added in portions during each addition. The mixture was stirred at room temperature for 12 h to obtain a viscous polyamic acid precursor solution. The polyamic acid solution was cast onto a clean glass substrate and placed in an 80°C oven for 6 h to remove impurities. Most of the solvent was used; then the glass substrate was transferred to a muffle furnace, and thermal imidization was carried out according to the following steps: 100℃ / 1h, 150℃ / 1h, 200℃ / 1h, 250℃ / 1h, 280℃ / 1h, to obtain a polyimide film; after cooling to room temperature, the film was immersed in deionized water and peeled off from the glass plate; the peeled film was placed in a vacuum oven at 100℃ and dried for 6h to obtain a polyimide film prepared from 2,6-bis(4-aminophenyl)-4,4'-bipyridine with a thickness of about 25μm.

[0075] Example 7

[0076] Under a nitrogen atmosphere, 0.338 g (1 mmol) of 2,6-bis(4-aminophenyl)-2,4'-bipyridine prepared in Example 3 was dissolved in 1 mL of DMAC. 0.531 g of bisphenol a diether dianhydride was added in portions, with 2.8 mL of DMAC added in portions during each addition. The mixture was stirred at room temperature for 12 h to obtain a viscous polyamic acid precursor solution. The polyamic acid solution was cast onto a clean glass substrate and placed in an 80°C oven for 6 h to remove most of the solvent. The glass substrate was then transferred to a muffle furnace and subjected to thermal imidization at 100℃ / 1h, 150℃ / 1h, 200℃ / 1h, 250℃ / 1h, and 280℃ / 1h to obtain a polyimide film. After cooling to room temperature, the film was immersed in deionized water and peeled off from the glass plate. The peeled film was then dried in a vacuum oven at 100℃ for 6h to obtain a polyimide film prepared from 2,6-bis(4-aminophenyl)-2,4'-bipyridine with a thickness of approximately 25μm.

[0077] The structural formula of the polyimide film prepared in Example 7 is as follows:

[0078]

[0079] Example 8

[0080] Under a nitrogen atmosphere, 0.338 g (1 mmol) of 2,6-bis(4-aminophenyl)-2,4'-bipyridine prepared in Example 3 was dissolved in 1 mL of DMAC. 0.316 g of 4,4'-oxophthalic anhydride was added in portions, with 1.8 mL of DMAC added in portions during each addition. The mixture was stirred at room temperature for 12 h to obtain a viscous polyamic acid precursor solution. The polyamic acid solution was cast onto a clean glass substrate and placed in an 80°C oven for 6 h to remove impurities. Most of the solvent was used; then the glass substrate was transferred to a muffle furnace, and thermal imidization was carried out according to the following steps: 100℃ / 1h, 150℃ / 1h, 200℃ / 1h, 250℃ / 1h, 280℃ / 1h, to obtain a polyimide film; after cooling to room temperature, the film was immersed in deionized water and peeled off from the glass plate; the peeled film was placed in a vacuum oven at 100℃ and dried for 6h to obtain a polyimide film prepared from 2,6-bis(4-aminophenyl)-2,4'-bipyridine with a thickness of about 25μm.

[0081] Comparative Example 1

[0082] Under a nitrogen atmosphere, 0.338 g (1 mmol) of 2,6-bis(4-aminophenyl)-4,4'-bipyridine prepared in Example 1 was dissolved in 1 mL of DMAC. 0.453 g of hexafluorodianhydride was added in portions, with 1 mL of DMAC added in each portion during the addition of hexafluorodianhydride. The mixture was stirred for 12 h to obtain a viscous polyamic acid precursor solution. The resulting polyamic acid solution was then cast onto a clean glass substrate and placed in an 80°C oven for 6 hours. h, to remove most of the solvent; then the glass substrate was transferred to a muffle furnace, and thermal imidization was performed according to the program steps of 100℃ / 1h, 150℃ / 1h, 200℃ / 1h, 250℃ / 1h, 280℃ / 1h to prepare 2,6-bis(4-aminophenyl)-4,4'-bipyridine to prepare a polyimide film; after the temperature was cooled to room temperature, the glass substrate was removed and it was found that the polyimide film was broken and a complete polyimide film could not be obtained.

[0083] Comparative Example 2

[0084] The difference from Comparative Example 1 is that the total amount of DMAC added in the hexafluorodianhydride stage was increased to 2.4 mL. The polyimide film prepared using the method in Comparative Example 2 also broke after cooling to room temperature, making it impossible to obtain a complete polyimide film. This indicates that the poor film-forming properties of the diamine monomer and hexafluorodianhydride are not related to the control of the solid content of the polyamic acid solution.

[0085] Comparative Example 3

[0086] The difference from Comparative Example 1 is that the maximum temperature in the thermal amidation reaction, 280℃ / 1h, was changed to 260℃ / 1h. The polyimide film prepared using the method in Comparative Example 3 also broke upon cooling to room temperature, failing to yield a complete polyimide film. This indicates that the poor film-forming properties of the diamine monomer and hexafluorodianhydride are not due to an excessively high maximum temperature setting during the thermal amidation process.

[0087] Comparative Example 4

[0088] The difference from Comparative Example 1 is that the 0.338 g (1 mmol) of 2,6-bis(4-aminophenyl)-4,4'-bipyridine prepared in Example 1 was replaced with the 0.338 g (1 mmol) of 2,6-bis(4-aminophenyl)-2,4'-bipyridine prepared in Example 3. The polyimide film prepared using the method in Comparative Example 4 also broke upon cooling to room temperature, resulting in an incomplete polyimide film, indicating poor film-forming properties of this diamine monomer and hexafluorodianhydride.

[0089] Comparative Example 5

[0090] The difference from Comparative Example 4 is that the total amount of DMAC added in the hexafluorodianhydride stage was increased to 2.4 mL. The polyimide film prepared using the method in Comparative Example 5 also broke after cooling to room temperature, failing to yield a complete polyimide film. This indicates that the poor film-forming properties of the diamine monomer and hexafluorodianhydride are not related to the amount of solvent added during the reaction.

[0091] Comparative Example 6

[0092] The difference from Comparative Example 4 is that the maximum temperature in the thermal amidation reaction, 280℃ / 1h, was changed to 260℃ / 1h. The polyimide film prepared using the method in Comparative Example 6 also broke after cooling to room temperature, failing to yield a complete polyimide film. This indicates that the poor film-forming properties of the diamine monomer and hexafluorodianhydride are not due to an excessively high maximum temperature setting during the thermal amidation process.

[0093] Performance testing

[0094] (I) Thermogravimetric analysis and differential scanning calorimetry of polyimide films:

[0095] The polyimide film prepared in the examples was tested using a thermogravimetric analyzer. The experimental conditions were as follows: the temperature was increased from 40°C to 800°C at a rate of 10°C / min under a nitrogen atmosphere. The mass of the sample was 5 mg. The temperatures at which thermal weight loss of 5% and 10% occurred were recorded as T. d5％ and T d10％ .

[0096] The glass transition temperature (Tg) of the polyimide film prepared in the examples was tested by differential scanning calorimetry. The experimental conditions were as follows: sample mass 5 mg to 10 mg, nitrogen atmosphere, nitrogen flow rate 50 mL / min, heating from room temperature to 400 °C at a heating rate of 20 °C / min, holding for 5 min, then cooling to 40 °C, and then heating again to 400 °C at a heating rate of 10 °C / min, with a nitrogen flow rate of 50 mL / min. The Tg of the polymer was measured by the second heating curve.

[0097] The specific test results are shown in Table 1 below.

[0098] Table 1:

[0099] Test case Tg (°C) <![CDATA[T d5％ (℃)]]> <![CDATA[R w800 (%)]]> Example 5 294 450.7 59.8 Example 6 328 482 65.4 Example 7 280 490 59.5 Example 8 341 563 71.5

[0100] As can be seen from Table 1, the polyimide films prepared in the examples exhibited excellent thermal properties in the above tests. The glass transition temperatures of the four polyimide films were 294℃, 328℃, 280℃ and 341℃, respectively; the temperatures at which the weight loss was 5% were 450.7℃, 482℃, 490℃ and 563℃, respectively; and the residual mass percentages at 800℃ were 59.8%, 65.4%, 59.5% and 71.5%, respectively.

[0101] like Figure 3 The figures shown are the thermogravimetric curves of the polyimide films prepared in Examples 5 to 8; Figure 4 The differential scanning calorimetry (DSC) curves of the polyimide films prepared in Examples 5 to 8 are shown.

[0102] Combining Table 1 and Figures 1-2 The polyimide films prepared using the method of this invention all exhibit excellent thermal properties. The reason for this is that polyimides contain imide rings connected by rigid ring structures (such as benzene rings). These rigid chain segments hinder chain movement, thus improving the material's thermal stability. Simultaneously, in conjunction with Comparative Examples 1-6, the polyimides prepared using the method of this invention contain ether bonds in their chemical structure. The addition ratio achieved by tetracarboxylic dianhydride in the polymer precisely adjusts the flexibility of the polyimide chains, allowing the molecular chains to have an appropriate range of motion. This ensures that the material possesses the required thermal properties while also guaranteeing the film-forming properties of the polyimide material, preventing chain segment movement and low-temperature degradation during heat treatment.

[0103] The polyimide film prepared by the method of this invention has a highly conjugated and planar rigid structure in its imide rings. Furthermore, the planarity of the imide rings promotes π-π stacking, resulting in a more compact molecular chain arrangement, reduced free volume, and increased glass transition temperature. The proportion of ether bonds in the polyimide macromolecular chain continuously increases, causing the polyimide macromolecular chain to gradually transform from a completely rigid segment structure to a state where rigid and flexible segments coexist. In the test results in Table 1, the glass transition temperature and 5% thermogravimetric temperature of the polyimide prepared in Example 5 are lower than those in Example 7, and the glass transition temperature and 5% thermogravimetric temperature of Example 6 are lower than those in Example 8. This shows that when the proportion of ether bonds in the polyimide molecular chain increases, the material may begin to transform from a completely thermosetting material to a thermoplastic material. Therefore, it can be inferred that if the ether bond content in the polyimide main chain is further increased, its thermal properties will decrease significantly, failing to meet application requirements.

[0104] (II) Emission fluorescence spectroscopy analysis of polyimide films:

[0105] The fluorescence emission spectra of the polyimide films prepared in Examples 5-8 were measured using a Hitachi F-7100FL spectrophotometer. Figure 5 The figures show the fluorescence emission spectra of the polyimide films prepared in Examples 5 to 8. It can be seen from the figures that the polyimide in Example 5 has a lower fluorescence intensity than the polyimide in Example 7, and the polyimide in Example 6 has a lower fluorescence intensity than the polyimide in Example 8.

[0106] The performance characterization results, analyzed from the chemical structure of polyimide, are due to the fact that the polyimide molecular chain has a relatively long conjugated system, which effectively enhances fluorescence intensity. Specifically, the increase in the conjugated system generally improves the excited-state lifetime and fluorescence quantum yield, thereby enhancing fluorescence intensity. The introduction of functional groups affects the stacking pattern of the molecular chain within the polyimide, thus affecting the fluorescence intensity of the polyimide film. However, because the ether bond ratio of 4,4'-oxyphthalic anhydride is lower than that of bisphenol A diether dianhydride, meaning that as the ether bond ratio in the polyimide backbone increases, the chain stacking within the molecular chain weakens the dipole-dipole interactions, resulting in an increase in van der Waals volume and thus reducing fluorescence intensity. Therefore, it can be inferred that if the ether bond content in the polyimide backbone is further increased, its fluorescence performance will decrease significantly, failing to meet application requirements.

[0107] (III) Analysis of the solubility of polyimide films:

[0108] The polyimide films prepared in Examples 5 to 8 were subjected to solubility tests. Specifically, a 1cm × 1cm polyimide film sample was immersed in 15mL of a reagent, which included DMF, DMAc, NMP, THF, chloroform, concentrated nitric acid, and concentrated hydrochloric acid. The solubility of the polyimide film was observed after one week of immersion. The specific test results are shown in Table 2 below.

[0109] Table 2:

[0110] Test case DMF DMAC NMP THF chloroform concentrated nitric acid concentrated hydrochloric acid Example 5 +- + + - + - - Example 6 +- + + - +- - - Example 7 +- + + - + - - Example 8 +- + + - +- - -

[0111] Note: "+" indicates soluble at room temperature, "+-" indicates partially soluble, and "-" indicates insoluble.

[0112] As can be seen from the test results in Table 2, the polyimide films prepared in Examples 5 to 8 have significantly improved solubility. Specifically, the polyimide films have good solubility in high-boiling-point polar solvents such as DMF, DMAc, and NMP, good solubility in chloroform, and are insoluble in THF, concentrated nitric acid, and concentrated hydrochloric acid.

[0113] The performance test results indicate that the solubility of polyimide is related to the size of its functional groups. Specifically, introducing large functional groups into the polyimide molecular chain can effectively reduce the interaction forces between molecular chains without reducing the rigidity of the molecular chain. This can improve the solubility of polyimide in organic solvents while ensuring its thermal properties. In addition, the solubility of polyimide is also related to the proportion of ether bonds in the polyimide backbone. Specifically, as the proportion of ether bonds in the molecular chain increases, the polarity and segment flexibility of polyimide increase, thereby enhancing its solubility in polar solvents and giving it a certain degree of solubility in low-boiling-point polar solvents.

[0114] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A diamine containing a bipyridine structure, characterized in that, The chemical structural formula is: In the formula, R1is 2. A method for preparing a diamine containing a bipyridine structure as described in claim 1, characterized in that, The steps are as follows: S1, 4-nitroacetophenone and pyridine-4-carboxaldehyde, or 4-nitroacetophenone and pyridine-2-carboxaldehyde are dissolved in an organic solvent I to form a reaction system; NaOH is dissolved in a mixed solution of the organic solvent I and water to obtain a basic mixed solvent, and the basic mixed solvent is added dropwise into the reaction system under an ice alcohol bath, and reacted for 1-6 hours to obtain an intermediate I; S2, 4-nitroacetophenone is dispersed in an organic solvent II, and then iodine is added, and the mixture is placed at 75-85°C for 12-14 hours to obtain an intermediate II; S3, the intermediate I and the intermediate II are dispersed in an organic solvent III, and reacted at 80-85°C for 12-16 hours under the action of ammonium acetate to obtain an intermediate III; S4, the intermediate III and a catalyst are dispersed in an organic solvent IV, a reducing agent and a catalyst are added, and the mixture is placed at 50-80°C for 24-48 hours to obtain a bipyridine structure-containing diamine by reduction.

3. The preparation method according to claim 1, characterized in that, In step S1, the molar ratio of 4-nitroacetophenone to pyridine-4-carboxaldehyde, or 4-nitroacetophenone to pyridine-2-carboxaldehyde is 1:1; the organic solvent I is methanol, ethanol or N,N-dimethylformamide; and the molar ratio of 4-nitroacetophenone to NaOH is 1:(0.2-0.5).

4. The production method according to claim 1, characterized by, In step S2, the molar ratio of 4-nitroacetophenone to iodine is 1:1; and the organic solvent II is pyridine.

5. The method of claim 1, wherein, In step S3, the molar ratio of the intermediate I to ammonium acetate is 1:(5-15); and the organic solvent III is methanol, ethanol or N,N-dimethylformamide.

6. The method of claim 1, wherein, In step S4, the reducing agent is hydrogen, hydrazine hydrate, formic acid, ammonium formate or sodium formate; the catalyst is a metal carbon catalyst, and the mass ratio of the intermediate III to the catalyst is 1:(0.1-0.3); and the organic solvent IV is ethylene glycol, methanol, ethanol or N,N-dimethylformamide.

7. A polyimide, characterized by, The bipyridine structure-containing diamine is obtained by polymerization of the bipyridine structure-containing diamine of claim 1 with a tetracarboxylic dianhydride, and then imidization.

8. A polyimide film characterized by comprising: The film is prepared from the polyimide of claim 8.

9. Use of a polyimide film, characterized in that Used as a fluorescent identification film.

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