A method for preparing a membrane based on the dynamic network reconstruction of polyimide gel

Abstract

A method for preparing a membrane based on the dynamic network reconstruction of polyimide gel belongs to the field of functional materials technology. The method involves: dispersing a diamine monomer (fluorine-free) in an aprotic polar solvent under ice-water bath conditions, then slowly adding a dianhydride monomer and stirring for 18-24 hours to obtain a PAA solution; adding a triamine crosslinking agent to the PAA solution, ensuring that the total molar amounts of amino and anhydride functional groups are roughly equal during the addition process; placing the thermosetting PAA gel in an oven and heating it at 120℃-150℃ for 15-30 minutes (the higher the crosslinking agent content, the higher the required temperature and the longer the heating time), until the gel transforms into a solution state, which is then coated onto a glass plate; placing the thermosetting PAA film in a muffle furnace for gradient heating treatment, naturally cooling to room temperature, and then immersing it in 60℃ deionized water to detach the film, obtaining a thermosetting polyimide membrane. The film prepared by this invention exhibits good thermal stability and excellent shape memory performance, maintaining a shape fixation rate of 99.77% and a recovery rate of 99.95% after ten shape memory cycles.

Description

Technical Field

[0001] This invention belongs to the field of functional materials technology, specifically relating to a method for preparing a membrane based on the dynamic network reconstruction of polyimide gel. Background Technology

[0002] Polyimide (PI), as a high-performance polymer material, is widely used in aerospace, microelectronics, and flexible devices due to its excellent thermal stability, mechanical strength, and chemical resistance. In recent years, with the increasing demand for smart materials, endowing polyimide with shape memory properties has become a research hotspot to meet the lightweight and functional integration requirements of space-deployable structures and adaptive devices. However, the shape memory performance of polyimide materials is closely related to the mobility of its molecular chains, and the linear molecular chain-dominated topology in traditional preparation methods often limits its shape retention rate and recovery efficiency.

[0003] Currently, mainstream methods for preparing polyimide films (such as solution coating and casting) rely on the physical entanglement of linear molecular chains or weak hydrogen bonding to form films. These films are prone to molecular chain slippage under high temperature or stress conditions, leading to a degradation of shape memory properties. To improve performance stability, researchers have attempted to construct three-dimensional network structures through chemical crosslinking to enhance the cooperative motion of molecular chains. However, highly crosslinked polyimides are prone to forming irreversible gels during film formation due to excessive crosslinking, resulting in difficulty in solvent evaporation, stress accumulation within the film layer, and ultimately, cracking or wrinkling defects. This contradiction makes it difficult to simultaneously achieve the processing performance and functional properties of highly crosslinked polyimides, severely restricting their engineering applications.

[0004] Therefore, developing a polyimide film-forming method that combines high cross-linking network stability with process controllability, overcoming the bottleneck of irreversible gel solidification, and achieving synergistic optimization of shape memory performance and process feasibility has become a key technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to address the problem that excessive cross-linking of polyimide can lead to gel formation, making film preparation difficult. This invention provides a method for preparing films based on the dynamic network reconstruction of polyimide gel. This method utilizes the principle of heating, dissociating, cooling, and reconstructing the dynamic cross-linked network to heat the gel into a fluid sol, which is then rapidly coated onto a film, significantly improving its shape memory properties. Through molecular network design, a new approach is provided to resolving the contradiction between the processing performance and functional properties of highly cross-linked polymers.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a membrane based on a dynamic network of polyimide gel, the method comprising the following steps:

[0008] Step 1: Preparation of polyamic acid (PAA) solution: Under ice-water bath conditions, the diamine monomer (fluorine-free) is dispersed in an aprotic polar solvent, and then the dianhydride monomer is slowly added. The reaction is stirred for 18-24 hours to obtain the polyamic acid solution.

[0009] Step 2: Preparation of thermosetting PAA gel: Add a triamine crosslinking agent to the PAA solution, ensuring that the total molar amounts of amino and anhydride functional groups are equal during the addition process. After adding the crosslinking agent for 5-30 minutes, if the mixture is in gel form, proceed to step 3; if the mixture is in solution form, proceed to step 4. For example, if the diamine is X mol, the dianhydride is Y mol, and the triamine is Z mol, and the number of amino functional groups equals the number of anhydride functional groups, then the ratio between the three monomers is 2Y = 2X + 3Z. When the molar ratio of dianhydride monomer to diamine monomer in step 1 is 1:0.9625, the required crosslinking agent content is the critical crosslinking agent content.

[0010] Step 3: Gel-sol transition: Place the thermosetting PAA gel in an oven and heat it at 120℃-150℃ for 15-30 minutes. The higher the crosslinking agent content, the higher the required temperature and the longer the heating time. The gel will then transition to a solution state.

[0011] Step 4: Pour the fluid solution onto a glass plate and apply it by scraping.

[0012] Step 5: Imidification: The thermosetting PAA film was placed in a muffle furnace and heated using a gradient heating method: from 25°C to 100°C for 100 min and held at 100°C for 2 h; from 100°C to 150°C for 80 min and held at 150°C for 2 h; from 150°C to 175°C for 50 min and held at 175°C for 1 h; from 175°C to 200°C for 50 min and held at 200°C for 1 h; from 200°C to 250°C for 80 min and held at 250°C for 1 h; after cooling naturally to room temperature, the film was removed and immersed in 60°C deionized water to detach the film; the sample film was washed with distilled water and dried in a 120°C oven for 3 h to obtain the thermosetting polyimide film.

[0013] Further, in step one, the aprotic polar solvent is one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, or dimethyl sulfoxide.

[0014] Further, in step one, the diamine monomer is one of 4,4'-diaminodiphenyl ether, 1,3-bis(3-aminophenoxy)benzene, or 5-amino-2-(4-aminophenyl)benzimidazole; the dianhydride monomer is one of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride), pyromellitic dianhydride, or 4,4'-biphenyl ether dianhydride.

[0015] Further, in step one, the molar ratio of the dianhydride monomer to the diamine monomer is 1:0.7-1, and the mass concentration of the polyamic acid solution is 10%-20%.

[0016] Further, in step two, the triamine crosslinking agent is 1,3,5-tris(4-aminophenoxy)benzene or 1,3,5-tris(4-aminophenyl)benzene.

[0017] This invention addresses the limitation of shape memory properties in polyimide films prepared by traditional solution coating and casting methods due to the dominance of linear molecular chains. Performance optimization is achieved by introducing a controllable crosslinking network. Specifically, a crosslinking agent (0-20 wt%) is added to a polyamic acid precursor solution to construct a dynamically responsive gel network. This system achieves high-performance film preparation through the following innovative steps:

[0018] (1) Gel network construction: Crosslinking agent induces the formation of a three-dimensional network structure, and the sol-gel transformation is achieved through solvent encapsulation effect.

[0019] (2) Dynamic network regulation: Heat treatment induces reversible breakage of the dynamic cross-linked network, releasing the encapsulated solvent and rearranging the gel network into a processable sol-like state. During this process, the covalent cross-linking points maintain structural integrity, ensuring the stability of the network topology.

[0020] (3) Gradient curing: A scraping process is performed during the dynamic network dissociation, and the molecular chain orientation is fixed through the substrate interface constraint effect and rapid cooling. After cooling, the hydrogen bond network is reconstructed to form an interlocking structure, which effectively suppresses film wrinkling.

[0021] The invention has the following advantages: 1. Structure-performance synergistic regulation: covalent cross-linking points enhance shape recovery ability, and network reconstruction gives processing fluidity; 2. Process compatibility: it breaks through the limitations of traditional irreversible gel solidification, realizes dynamic gel-sol transformation, and is suitable for large-area continuous production; 3. Application potential: the obtained film exhibits excellent shape memory performance, meeting the needs of aerospace flexible deployment mechanisms for lightweight smart materials.

[0022] The thin film prepared by this invention has good thermal stability and excellent shape memory performance. After ten shape memory cycles, the shape fixation rate is 99.77% and the recovery rate is 99.95%. Attached Figure Description

[0023] Figure 1 This is a shape memory cycle diagram of thermoplastic polyimide (PI);

[0024] Figure 2 Thermosetting polyimide (TCPI-O) 2.5％Shape memory cycle diagram (2.5% is the critical crosslinking agent content, at which point the thermosetting PAA is in liquid state);

[0025] Figure 3 Thermosetting polyimide (TCPI-O) 10％ Shape memory cycle diagram (high cross-linking agent content, at which point thermosetting PAA is in a gel state);

[0026] Figure 4 Thermosetting polyimide (TCPI-O) 10％ ) 10-cycle shape memory diagram (high cross-linking agent content, at which point the thermosetting PAA is in a gel state);

[0027] Figure 5 This is a diagram of the gel-sol transition.

[0028] Figure 6 This is a schematic diagram of gel aging.

[0029] Figure 7 This is a schematic diagram of a polyimide film.

[0030] Figure 8 This is a schematic diagram of the reaction equation for thermosetting polyimide.

[0031] Figure 9 This is a shape memory cycle diagram of the thermosetting polyimide in Example 3;

[0032] Figure 10 This is a shape memory cycle diagram of thermosetting polyimide from Example 4. Detailed Implementation

[0033] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0034] Comparative Example 1: PI Preparation

[0035] 1. Preparation of polyamic acid

[0036] Weigh 2.9234 g of 1,3-bis(3-aminophenoxy)benzene (BAB) (10 mmol) and add it to a 100 mL three-necked flask equipped with a magnetic stirrer, a nitrogen inlet and outlet. Seal the top of the three-necked flask with a rubber stopper, evacuate the flask, and replace the nitrogen gas through a three-way valve to make the reaction apparatus water-free and oxygen-free. Inject 10 ml of DMAc into a three-necked flask using a syringe and stir at room temperature for 30 min under dry nitrogen. After BAB is completely dissolved, weigh 5.2049 g of 4,4'-(4,4'-isopropylidene diphenoxy)bis(phthalic anhydride) (BPADA) and dissolve it in 35 ml of DMAc. Sonicate for 30 min to completely dissolve it. Under nitrogen atmosphere and ice-water bath conditions, slowly add the solution in 4 batches over 1 hour using a syringe. Stir the solution at room temperature for 24 hours. After stirring, remove the PAA solution, pour it onto a clean glass plate (15×25 cm), and scrape it with a coating applicator (500 μm facing down).

[0037] 2. Imine

[0038] The prepared PAA membrane was placed in a muffle furnace and heated using a gradient heating method: from 25°C to 100°C for 100 min, and held at 100°C for 2 h; from 100°C to 150°C for 80 min, and held at 150°C for 2 h; from 150°C to 175°C for 50 min, and held at 175°C for 1 h; from 175°C to 200°C for 50 min, and held at 200°C for 1 h; from 200°C to 250°C for 80 min, and held at 250°C for 1 h; then allowed to cool naturally to room temperature and removed. The membrane was then immersed in 60°C deionized water to detach. The sample membrane was washed with distilled water and dried in a 120°C oven for 3 h to obtain the polyimide membrane. The measured shape memory cycle is as follows: Figure 1 As shown in the figure, the corresponding fixed rate and the response rate are shown in Table 1.

[0039] Example 1: TCPI-O (2.5％) preparation

[0040] 1. Preparation of PAA

[0041] Weigh 2.8138 g of 1,3-bis(3-aminophenoxy)benzene (BAB) (10 mmol) and add it to a 100 mL three-necked flask equipped with a magnetic stirrer, a nitrogen inlet and outlet. Seal the top of the three-necked flask with a rubber stopper, evacuate the flask, and replace the nitrogen gas through a three-way valve to ensure that the reaction apparatus is anhydrous and oxygen-free. Inject 10 mL of DMAc into the three-necked flask using a syringe and stir at room temperature for 30 min under dry nitrogen. After BAB is completely dissolved, weigh 5.2049 g of 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA) and dissolve it in 35 mL of DMAc. Sonicate for 30 min until completely dissolved. Add the DMAc slowly in 4 batches over 1 hour under a nitrogen atmosphere and an ice-water bath. Stir the solution at room temperature for 24 hours.

[0042] 2. Preparation of thermosetting PAA

[0043] Weigh 0.0999 g of 1,3,5-tris(4-aminophenoxy)benzene (TAPO) and dissolve it in 10 mL of anhydrous DMAc. Inject the solution into a three-necked flask using a syringe and continue stirring for 30 min. (Experiments have confirmed that this crosslinking agent content is critical. When the TAPO content > 0.0999 g, a stable gel state is achieved; when the TAPO content < 0.0999 g, a stable liquid state is achieved; and when the TAPO content = 0.0999 g, a probabilistic gel state is achieved. The corresponding critical crosslinking density Xc = 2.8490 × 10⁻⁶.) 20 As shown in Table 3, the thermosetting PAA prepared this time is in liquid form. The thermosetting PAA solution was taken out and poured onto a clean glass plate (15×25cm), and then scraped with a coating tool (500μm facing down).

[0044] 3. Imine

[0045] The prepared thermosetting PAA film was placed in a muffle furnace and heated using a gradient heating method: 25℃ to 100℃ for 100 min, held at 100℃ for 2 h; 100℃ to 150℃ for 80 min, held at 150℃ for 2 h; 150℃ to 175℃ for 50 min, held at 175℃ for 1 h; 175℃ to 200℃ for 50 min, held at 200℃ for 1 h; 200℃ to 250℃ for 80 min, held at 250℃ for 1 h; and then allowed to cool naturally to room temperature before removal. The film was then immersed in 60℃ deionized water to detach. The sample film was washed with distilled water and dried in a 120℃ oven for 3 h to obtain the polyimide film. The measured shape memory cycle is as follows: Figure 2 As shown in Table 1, the corresponding fixation rate and response rate are as follows. Compared with PI, TCPI-O... (2.5％) The creep resistance is significantly enhanced, and the shape memory performance is significantly improved.

[0046] Example 2: TCPI-O (10％) preparation

[0047] 1. Preparation of PAA

[0048] Weigh 2.4849 g of 1,3-bis(3-aminophenoxy)benzene (BAB) (10 mmol) and add it to a 100 mL three-necked flask equipped with a magnetic stirrer, a nitrogen inlet and outlet, and seal the top of the flask with a rubber stopper. Evacuate the flask and replace the nitrogen gas through a three-way valve to ensure that the reaction apparatus is anhydrous and oxygen-free. Inject 10 mL of DMAc into the three-necked flask using a syringe and stir at room temperature for 30 min under dry nitrogen. After BAB is completely dissolved, weigh 5.2049 g of 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA) and dissolve it in 35 mL of DMAc. Sonicate for 30 min until completely dissolved. Add the DMAc slowly in 4 batches over 1 hour under a nitrogen atmosphere and an ice-water bath. Stir the solution at room temperature for 24 hours.

[0049] 2. Preparation of thermosetting PAA

[0050] Weigh 0.3996 g of 1,3,5-tris(4-aminophenoxy)benzene (TAPO) and dissolve it in 10 mL of anhydrous DMAc. Inject the solution using a syringe and continue stirring for 10 min. At this point, the PAA solution becomes a gel (crosslinking density X > Xc, as shown in Table 3).

[0051] 3. Gel-sol transition

[0052] Place the thermosetting PAA gel in an oven and heat at 150°C for 30 minutes; at this point, the gel has transformed into a flowable sol, as shown below. Figure 5 As shown, quickly remove it, pour it onto a glass plate, and scrape it with a coating applicator (500 μm side down). If the gel is left to stand overnight (24 h), the gel will age and will not dissolve upon reheating, as... Figure 6 As shown.

[0053] 4. Imine

[0054] The prepared thermosetting PAA film was placed in a muffle furnace and heated using a gradient heating method: 25℃ to 100℃ for 100 min, held at 100℃ for 2 h; 100℃ to 150℃ for 80 min, held at 150℃ for 2 h; 150℃ to 175℃ for 50 min, held at 175℃ for 1 h; 175℃ to 200℃ for 50 min, held at 200℃ for 1 h; 200℃ to 250℃ for 80 min, held at 250℃ for 1 h; and then allowed to cool naturally to room temperature before removal. The film was then immersed in 60℃ deionized water to detach. The sample film was washed with distilled water and dried in a 120℃ oven for 3 h to obtain the polyimide film. The measured shape memory cycle is as follows: Figure 3 As shown, the corresponding fixed rate and response rate are shown in Table 1. (Compared to TCP / IP-O) (2.5％) The comparison shows that TCPI-O (10％) The significantly improved cyclic stability of shape memory demonstrates the feasibility of this method. Furthermore, the TCPI-O... (10％) Ten shape memory cycle tests were conducted, such as Figure 4 As shown in the table, it is excellent in both creep resistance and cycle stability. Furthermore, the shape recovery rate still reaches 99.95% after ten cycles. The fixation rate and recovery rate data after ten cycles are shown in Table 2.

[0055] Table 1. Shape Memory Data from Three Cycles

[0056]

[0057] Table 2. Data from ten cycles of shape memory practice.

[0058]

[0059]

[0060] Table 3. Data related to polyimide

[0061] sample <![CDATA[M n ]]> <![CDATA[M w ]]> <![CDATA[T g ]]> X PI 125223 1899797 195 ---- <![CDATA[TCPI-O (2.5％) ]]> 49656 576885 200 <![CDATA[2.8490×10 20 <!-- 5 -->]]> <![CDATA[TCPI-O (5％) ]]> 14357 117714 208 <![CDATA[61.9133×10 20 ]]>

[0062] M n : Corresponding thermoplastic number-average molecular weight; M w : Corresponding to the weight-average molecular weight of thermoplastics.

[0063] T g : Glass transition temperature; X: Crosslinking density.

[0064] Example 3:

[0065] 1. Preparation of PAA

[0066] Weigh 2.7041 g of 1,3-bis(3-aminophenoxy)benzene (BAB) (10 mmol) and add it to a 100 mL three-necked flask equipped with a magnetic stirrer, a nitrogen inlet and outlet, and seal the top of the flask with a rubber stopper. Evacuate the flask and replace the nitrogen gas through a three-way valve to ensure that the reaction apparatus is anhydrous and oxygen-free. Inject 10 mL of DMAc into the three-necked flask using a syringe and stir at room temperature for 30 min under dry nitrogen. After BAB is completely dissolved, weigh 5.2049 g of 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA) and dissolve it in 35 mL of DMAc. The solution will be completely dissolved by sonication for 30 min. Add the solution slowly in 4 batches over 1 h under a nitrogen atmosphere and an ice-water bath. Stir the solution at room temperature for 24 h.

[0067] 2. Preparation of thermosetting PAA

[0068] Weigh 0.3996 g of 1,3,5-tris(4-aminophenyl)benzene (TAB) and dissolve it in 10 mL of anhydrous DMAc. Inject the solution using a syringe and continue stirring for 10 min. At this point, the PAA solution becomes a gel (crosslinking density X > Xc).

[0069] 3. Gel-sol transition

[0070] Place the thermosetting PAA gel in an oven and heat it at 150°C for 20 minutes. At this point, the gel has transformed into a free-flowing sol. Quickly remove it, pour it onto a glass plate, and scrape it with a coating applicator (500 μm facing down).

[0071] 4. Imine

[0072] The prepared thermosetting PAA film was placed in a muffle furnace and heated using a gradient heating method: 25℃ to 100℃ for 100 min, held at 100℃ for 2 h; 100℃ to 150℃ for 80 min, held at 150℃ for 2 h; 150℃ to 175℃ for 50 min, held at 175℃ for 1 h; 175℃ to 200℃ for 50 min, held at 200℃ for 1 h; 200℃ to 250℃ for 80 min, held at 250℃ for 1 h; and then allowed to cool naturally to room temperature before removal. The film was then immersed in 60℃ deionized water to detach. The sample film was washed with distilled water and dried in a 120℃ oven for 3 h to obtain the polyimide film. The measured shape memory cycle is as follows: Figure 9 As shown, compared with PI, TCPI-B (5％) The creep resistance is significantly enhanced, and the shape memory performance is significantly improved.

[0073] Example 4:

[0074] 1. Preparation of PAA

[0075] Weigh 2.7041 g of 1,3-bis(3-aminophenoxy)benzene (BAB) (10 mmol) and add it to a 100 mL three-necked flask equipped with a magnetic stirrer, a nitrogen inlet and outlet, and seal the top of the flask with a rubber stopper. Evacuate the flask and replace the nitrogen gas through a three-way valve to ensure that the reaction apparatus is anhydrous and oxygen-free. Inject 10 mL of DMAc into the three-necked flask using a syringe and stir at room temperature for 30 min under dry nitrogen. After BAB is completely dissolved, weigh 3.1021 g of 4,4'-diphenyl ether dianhydride (ODA) and dissolve it in 30 mL of DMAc. The solution will be completely dissolved by sonication for 30 min. Add the solution slowly in 4 batches over 1 hour using a syringe under nitrogen atmosphere and ice-water bath conditions. Stir the solution at room temperature for 24 hours.

[0076] 2. Preparation of thermosetting PAA

[0077] Weigh 0.1997 g of 1,3,5-tris(4-aminophenoxy)benzene (TAPO) and dissolve it in 10 mL of anhydrous DMAc. Inject the solution using a syringe and continue stirring for 10 min. At this point, the PAA solution becomes a gel state (crosslinking density X > Xc).

[0078] 3. Gel-sol transition

[0079] Place the thermosetting PAA gel in an oven and heat it at 150°C for 20 minutes. At this point, the gel has transformed into a free-flowing sol. Quickly remove it, pour it onto a glass plate, and scrape it with a coating applicator (500 μm facing down).

[0080] 4. Imine

[0081] The prepared thermosetting PAA film was placed in a muffle furnace and heated using a gradient heating method: 25℃ to 100℃ for 100 min, held at 100℃ for 2 h; 100℃ to 150℃ for 80 min, held at 150℃ for 2 h; 150℃ to 175℃ for 50 min, held at 175℃ for 1 h; 175℃ to 200℃ for 50 min, held at 200℃ for 1 h; 200℃ to 250℃ for 80 min, held at 250℃ for 1 h; and then allowed to cool naturally to room temperature before removal. The film was then immersed in 60℃ deionized water to detach. The sample film was washed with distilled water and dried in a 120℃ oven for 3 h to obtain the polyimide film. The measured shape memory cycle is as follows: Figure 10 As shown, compared with PI, it can be seen that PI-O (5％) The creep resistance is significantly enhanced, and the shape memory performance is significantly improved.

Claims

1. A method for preparing a membrane based on the reconstruction of a polyimide gel dynamic network, characterized in that: The method includes the following steps: Step 1: Preparation of polyamic acid solution: Under ice-water bath conditions, the diamine monomer is dispersed in an aprotic polar solvent, and then the dianhydride monomer is slowly added. The mixture is stirred and reacted for 18-24 hours to obtain a polyamic acid solution. The diamine monomer is one of 4,4'-diaminodiphenyl ether, 1,3-bis(3-aminophenoxy)benzene, or 5-amino-2-(4-aminophenyl)benzimidazole. The dianhydride monomer is one of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride), pyromellitic dianhydride, or 4,4'-biphenyl ether dianhydride. The molar ratio of the dianhydride monomer to the diamine monomer is 1:0.85-0.9625, and the mass concentration of the polyamic acid solution is 10%-20%. The aprotic polar solvent is one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, or dimethyl sulfoxide. Step 2: Preparation of thermosetting polyamic acid gel: Add a triamine crosslinking agent to the polyamic acid solution, ensuring that the total molar amounts of amino and anhydride functional groups are equal during the addition process. After adding the crosslinking agent for 5-30 minutes, if the mixture is in gel form, proceed to Step 3; if the mixture is in solution form, proceed to Step 4. The triamine crosslinking agent is 1,3,5-tris(4-aminophenoxy)benzene or 1,3,5-tris(4-aminophenyl)benzene. Step 3: Gel-sol transition: Place the thermosetting polyamic acid gel in an oven and heat it at 120℃~150℃ for 15~30 minutes until the gel transitions to a solution state; Step 4: Pour the fluid solution onto a glass plate and apply it by scraping. Step 5: Imidification: The thermosetting polyamic acid film was placed in a muffle furnace and heated using a gradient heating method: from 25 ℃ to 100 ℃ for 100 min, and held at 100 ℃ for 2 h; from 100 ℃ to 150 ℃ for 80 min, and held at 150 ℃ for 2 h; from 150 ℃ to 175 ℃ for 50 min, and held at 175 ℃ for 1 h; from 175 ℃ to 200 ℃ for 50 min, and held at 200 ℃ for 1 h; from 200 ℃ to 250 ℃ for 80 min, and held at 250 ℃ for 1 h; after natural cooling to room temperature, the film was removed and immersed in 60 ℃ deionized water to detach the film; the sample film was washed with distilled water and dried in a 120 ℃ oven for 3 h to obtain the thermosetting polyimide film.

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