Polyimide microspheres, and preparation method and application thereof

Abstract

This invention provides polyimide microspheres, their preparation method, and applications, belonging to the field of polyimide functional materials. The method includes: preparing a polyamic acid (PAA) organic solvent solution; placing the polyamic acid solution in a sealed autoclave; heating the sealed autoclave at 100℃~200℃ for 1~24 hours to induce a thermal imidization reaction in the polyamic acid, generating polyimide. During the reaction, the polyimide spontaneously forms microspheres. This method involves the complete absence of surfactants, templates, pore-forming agents, initiators, and other additives. The resulting polyimide microspheres exhibit a spherical morphology with a particle size of 1.5 μm~5 μm and a specific surface area (BET) of not less than 180 m² / g.

Description

Technical Field

[0001] This invention belongs to the field of polyimide functional materials, specifically relating to a polyimide microsphere, its preparation method, and its application. Background Technology

[0002] Polyimide (PI) microspheres, with their excellent thermal stability, chemical resistance, high mechanical strength, and tunable porous structure, have shown great application potential in fields such as battery separators, catalyst supports, gas adsorption, low-dielectric materials, and high-temperature filtration membranes. Although extensive research has been conducted on the preparation of polyimide microspheres, existing methods generally suffer from complex processes and high costs, and typically require multiple additives, templates, or specialized equipment. This not only limits large-scale production but also introduces potential contamination risks. Therefore, the preparation of porous polyimide microspheres with controllable morphology and high specific surface area is key to fully realizing their commercial value.

[0003] Electrospray ionization (ES) technology involves electrostatically atomizing a polyamic acid (PAA) solution into droplets, followed by phase separation and imidization reactions to obtain microspheres with internal pores. This technology requires specialized high-pressure equipment and precise control of multiple parameters such as solution concentration, flow rate, and applied voltage. Industrial scale-up is challenging, and the resulting microspheres exhibit a narrow particle size distribution, with a specific surface area typically difficult to consistently exceed 100 m² / g.

[0004] Water vapor induced phase separation (WVIPS) involves first exposing a polyamic acid solution to a humid environment to prepare polyamic acid microspheres with an average particle size of approximately 2 μm, followed by thermal imidization to convert them into polyimide microspheres. This method requires precise control of humidity conditions, and to achieve the target performance, it often necessitates the addition of functional fillers or surface modification, further increasing the process complexity.

[0005] A typical reprecipitation process involves injecting a polyamic acid solution containing a porogen or a secondary polymer into a poor solvent (such as cyclohexane), forming tiny droplets and causing microphase separation. Subsequent imidization yields porous polyimide nanoparticles with surface pore sizes of 20–100 nm. This method requires the use of porogens, templates, or secondary polymers, and these substances must be removed after particle formation, increasing both material costs and process complexity. To avoid product contamination, these additives must be completely removed.

[0006] CN117069939A discloses a one-step high-pressure method for preparing polyimide microspheres. This method uses amines or heterocyclic initiators (such as pyridine or triethylamine) under high pressure of 1–3 MPa to prepare microspheres. However, the BET specific surface area of ​​the microspheres obtained by this method is only 20–70 m² / g, limiting their application in high specific surface area scenarios such as adsorption and catalysis.

[0007] In summary, although significant progress has been made in the synthesis of polyimide microspheres, existing methods are still limited by many shortcomings. There is an urgent need in this field for a simpler, additive-free, and scalable method to prepare polyimide microspheres with high specific surface area (>150 m² / g), excellent thermal stability (Td5%>500°C), uniform spherical morphology, and controllable particle size, while avoiding the process complexity, cost, and pollution risks of existing technologies. Summary of the Invention

[0008] The present invention aims to overcome the shortcomings of the prior art and provide a method for preparing polyimide microspheres.

[0009] This invention represents a groundbreaking discovery: by simply placing a polyamic acid solution in a sealed autoclave and heating it under autogenous pressure, polyimide microspheres with regular morphology can be directly generated without any additives. (This discovery completely overturns the traditional understanding that controlling morphology must rely on additives.)

[0010] The reaction mechanism of this invention is as follows: Under the synergistic effect of temperature and autogenous pressure, polyamic acid chains self-assemble into microsphere structures through hydrophobic / hydrophilic interactions or imidization reactions and solvent evaporation kinetics.

[0011] This invention discloses a method for preparing polyimide microspheres, the method comprising the following steps: (1) Under a nitrogen atmosphere, one or more dianhydrides and one or more diamines are polymerized in an organic solvent to obtain a polyamic acid solution. (2) The polyamic acid solution is placed in a sealed autoclave for imidization reaction: the reaction is first carried out at 100℃~120℃ for 1~5 hours, and then the temperature is raised to 200℃~250℃ and held for 5~10 hours to convert the polyamic acid into polyimide, and the polyimide precipitates in the form of microspheres to obtain polyimide microspheres. In steps (1) and (2), no surfactants, templates, pore-forming agents, initiators, or emulsifiers are added.

[0012] Furthermore, the mass fraction of polyamic acid in the solution is 1% to 20%, preferably 3% to 10%.

[0013] Furthermore, the heating process relies on the system's own pressure and does not require external gas pressurization.

[0014] Furthermore, the imidization reaction is carried out by reacting at 100°C for 1 to 5 hours, and then raising the temperature to 200°C and holding for another 5 to 10 hours.

[0015] Furthermore, the imidization reaction is carried out by first heating at 100°C for 3 hours, and then heating to 200°C and continuing to heat for 6 hours.

[0016] Furthermore, the dianhydride is selected from one or more of pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 4,4'-oxophthalic anhydride (ODPA), and 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA); the diamine is selected from one or more of 4,4'-diaminodiphenyl ether (ODA), p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), and 4,4'-diaminodiphenylmethane (MDA); and the organic solvent is selected from one or more of N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO).

[0017] The molar ratio of the dianhydride to the diamine is 1:(1 ~ 1.02), preferably 1:1.

[0018] Furthermore, the method also includes the steps of washing and drying the prepared polyimide microspheres with a solvent.

[0019] A polyimide microsphere is prepared by any of the methods described above.

[0020] Furthermore, the average polyimide microspheres have a particle size of 1.5 μm to 5 μm, a specific surface area (BET) ≥ 181 m² / g, a thermal decomposition temperature ≥ 500 °C, and a dielectric constant ≤ 2.5 at a frequency of 10 GHz.

[0021] The above-mentioned polyimide microspheres are used in fields such as battery separator coatings, gas adsorption materials, catalyst supports, low dielectric materials for high-frequency electronic devices, or high-temperature resistant insulating components for electrical appliances.

[0022] This invention involves heating a polyamic acid (PAA) solution in a sealed autoclave without adding any additives to obtain high-quality polyimide microspheres. The autoclave is a closed system; solvent evaporation generates autogenous pressure. Under the combined effects of temperature and pressure, an imidization reaction occurs, simultaneously forming spherical particles.

[0023] The additive-free polyimide microspheres disclosed in this invention can be applied in multiple fields, including: Battery separator coating: High specific surface area (≥150m² / g) ensures excellent electrolyte wettability, while its high thermal stability (>400°C) improves battery safety, inhibits lithium dendrite growth, and prevents thermal runaway in lithium-ion batteries and lithium metal batteries.

[0024] Gas adsorption and storage: Its microporous structure and high specific surface area enable it to achieve large-capacity adsorption of gases such as CO2, H2, and CH4.

[0025] Catalyst support: The chemically resistant surface and spherical morphology provide an ideal loading substrate for catalyst nanoparticles; as a biomedical carrier, it can realize high-volume drug delivery, neural implant coating application, and can also be used to prepare tissue engineering scaffolds, effectively reducing the body's foreign body reaction.

[0026] Low dielectric material for 5G / 6G electronics: The internal porosity reduces the dielectric constant, making it suitable for high-frequency electronic substrates.

[0027] Thermal / electric insulation components: The robust polyimide skeleton can be processed into insulating pads, separators, or structural components for electric vehicle battery packs.

[0028] Because this process involves no additives and uses only conventional autoclave equipment, it is extremely easy to achieve large-scale industrial production.

[0029] The greatest advantage of this method lies in its simplicity: no additives mean lower costs and a simpler purification process; fewer steps mean higher yields and easier scaling up; and standard equipment (autoclave) means that any chemical plant can use it directly.

[0030] The reason for the high specific surface area of ​​the polyimide microspheres in this invention: High pressure can promote solvent penetration into the interior of the polymer matrix, inhibit rapid solvent evaporation, facilitate homogeneous nucleation, and ultimately form a through-hole porous framework structure. No additives: This avoids pore blockage at the source, eliminates structural damage caused by the purification process, and ensures that there is no interference from foreign molecules during the thermal imidization process; Other influencing factors include: the use of pre-synthesized polyamic acid (PAA) with controllable molecular weight; a segmented thermal imidization process (holding at 100℃ for 3 hours → holding at 200℃ for 8 hours) to achieve gradual growth and shaping of pores; the synergistic effect of the above factors ultimately produces porous flower-shaped microspheres with a BET specific surface area of ​​up to 186 m² / g.

[0031] The microspheres possess a porous, flower-like microstructure, characterized as follows: Particle size range: approximately 1.93~2.95 μm, average particle size approximately 2.5 μm; Surface characteristics: The surface is rough and porous, with clearly visible open pores distributed throughout the entire sphere; Particle shape: Spherical to slightly irregular spherical, with significant porous texture; Internal structure: The interior of the sphere is a continuous porous network, which is the core source of its high specific surface area.

[0032] This invention employs a high-pressure thermal imidization process without added additives to prepare this unique porous flower-like structure; a large number of interconnected pores are formed inside the material, maximizing the specific surface area.

[0033] Compared with the prior art, the present invention has the following beneficial effects: The entire preparation process of this invention uses only polyamic acid solution as the reaction raw material, without the need for additional surfactants, template agents, pore-forming agents, salts, and various organic and inorganic initiators. This avoids pore blockage at the source, eliminates structural damage caused by the purification process, and ensures that there is no interference from foreign molecules during the thermal imidization process, thereby increasing the specific surface area of ​​the microspheres. It also avoids the problem of residual exogenous additives, resulting in higher product purity. At the same time, the reaction system is more green and environmentally friendly, eliminating the burden of subsequent processing due to additive pollution.

[0034] This invention employs a closed high-pressure reactor to achieve a one-step (one-pot) reaction, directly converting polyamic acid into polyimide microspheres in a single high-pressure reactor reaction without the need for intermediate nucleation, gelation, or other transitional processes. This significantly shortens the overall preparation time while reducing reaction energy consumption and manual operation costs. The preparation can be completed using only a conventional simple high-pressure reactor or hydrothermal reactor, eliminating the need for electrospray devices, humidity control systems, and complex two-phase reaction systems, resulting in lower equipment investment costs.

[0035] This invention offers highly controllable processes, allowing for precise control of the microstructure and size of the microspheres. Even without adding any regulatory additives to the reaction system, the particle size and microstructure of the polyimide microspheres can still be precisely controlled by flexibly adjusting conventional process parameters such as the concentration of the polyamic acid solution, the type of organic solvent, the two-stage reaction temperature, the imidization reaction time, and the autogenous pressure of the autoclave. This results in broader process adaptability. The precise controllability is reflected in: (1) Increasing the concentration of polyamic acid increases the number of polymer chains that can participate in the growth of microspheres in the system, ultimately forming microspheres with larger particle size; (2) High-boiling-point solvents such as dimethylacetamide (DMAc) and N-methylpyrrolidone (NMP) can extend the growth cycle before microspheres are solidified, which is also beneficial for preparing large-diameter microspheres; (3) The two-stage heating process divides the nucleation stage (100℃) and the imidization stage (200℃) into two stages, which can achieve the controllable molding of spherical precursors before crosslinking and fixing the final morphology; (4) The duration of isothermal treatment at 200℃ determines the degree of imidization and structural stability. The optimal product can be obtained by isothermal treatment for 4 to 8 hours. (5) The self-generated pressure generated by the evaporation of solvent in the sealed autoclave can inhibit the premature evaporation of solvent and ensure that the polymer chains have sufficient mobility during the microsphere molding process.

[0036] In summary, the final product obtained by this invention exhibits excellent overall performance and is suitable for a variety of high-end applications. The polyimide microspheres prepared by this invention possess extremely low dielectric constant, excellent high-temperature thermal stability, and uniform particle size, making them perfectly suited for various high-end applications such as battery separator coatings, gas adsorption materials, catalyst supports, low-dielectric materials for high-frequency electronic devices, and high-temperature insulating components for electrical appliances, thus demonstrating a wide range of applications. Attached Figure Description

[0037] 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.

[0038] Figure 1 Example 1: Scanning electron microscope image of polyimide microspheres (10.5 mm × 2.00 k SE (UL)); Figure 2 Example 1: Scanning electron microscope image of polyimide microspheres (10.5 mm × 5.00 k SE (UL)); Figure 3 Schematic diagram of thermogravimetric analysis (TGA) curves of polyimide microspheres; Figure 4 Schematic diagram of BET specific surface area test data for polyimide microspheres.

[0039] Sample 4: Example 1; Sample 5: Comparative Example 1; Sample 6: Comparative Example 2. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0041] Example 1 Under a dry nitrogen atmosphere, 130.14 g of pyromellitic dianhydride (PMDA) was reacted with 24 g of 4,4'-diaminodiphenyl ether (ODA) and 51.7 g of p-phenylenediamine (p-PDA) to prepare a polyamic acid solution with a solid content of 5% (mass fraction).

[0042] Take 50 mL of the above polyamic acid solution, add it to a 100 mL stainless steel autoclave and seal it; place it in an oven and carry out the imidization reaction under the system's own pressure: first, keep it at 100℃ for 3 hours, then raise the temperature to 200℃ and keep it at that temperature for another 6 hours. After cooling, collect the product by centrifugation, wash it successively with ethanol and pure water, and finally dry it at 80℃ to obtain polyimide microspheres.

[0043] Scanning electron microscope (SEM) Figure 1 and Figure 2 Characterization results showed that the product consisted of uniformly shaped spherical particles with a particle size range of 1.5 μm to 5 μm; thermogravimetric analysis indicated that the temperature corresponding to a 5% weight loss of the sample was 530℃. Figure 3 The BET test determined its specific surface area to be 186 m² / g. Figure 4 ).

[0044] Example 2 The imidization reaction was adjusted as follows: first, the temperature was increased to 120°C for 5 hours, then the temperature was increased to 250°C and the temperature was maintained for another 10 hours. The rest was the same as in Example 1.

[0045] Performance Characterization Scanning electron microscopy results showed that the product consisted of uniformly shaped spherical particles with a particle size of 1.5 μm to 2.9 μm; the specific surface area measured by BET analysis was 186 m². 2 / g; thermogravimetric analysis showed that its 5% thermal weight loss temperature was 535℃.

[0046] Example 3 The mass fraction of polyamic acid in the solution was 20%, and the rest was the same as in Example 1.

[0047] Performance Characterization Scanning electron microscopy results showed that the product consisted of uniformly shaped spherical particles with a particle size of 1.5 μm to 2.8 μm; the specific surface area measured by BET analysis was 184 m². 2 / g; thermogravimetric analysis showed that its 5% thermal weight loss temperature was 535℃.

[0048] Comparative Example 1 Tetrahydrofuran was added as a foaming agent when preparing the polyamic acid solution, and everything else was the same as in Example 1.

[0049] Performance Characterization Scanning electron microscopy results showed that the product consisted of uniformly shaped spherical particles with a particle size of 1.5 μm to 2.9 μm; the specific surface area measured by BET analysis was 159.18 m². 2 / g; thermogravimetric analysis showed that its 5% thermal weight loss temperature was 535℃.

[0050] Results Analysis Although tetrahydrofuran (THF) can function as a hydrogen-bonding blowing agent in some polyimide systems, in this system, its addition to polyamic acid (PAA) solution not only failed to increase porosity but also interfered with the spontaneous microsphere formation process. THF molecules in the solution disrupt the self-assembly of polyamic acid molecular chains to form a flower-like porous structure; furthermore, before the polymer network fully solidifies, THF prematurely releases gas, easily causing localized microsphere structural collapse and microsphere fusion. Ultimately, the overall pore structure is poorly developed, resulting in a reduced BET specific surface area (only 159.18 m²). 2 / g). This result unexpectedly confirms that the additive-free preparation process used in this invention produces a material with a specific surface area that is even better than that obtained by deliberately adding foaming agents.

[0051] Comparative Example 2 Acetone was added as a poor solvent when preparing the polyamic acid solution, otherwise it was the same as in Example 1.

[0052] Performance Characterization Scanning electron microscopy results showed that the product consisted of uniformly shaped spherical particles with a particle size of 1.5 μm to 2.8 μm; the specific surface area measured by BET analysis was 142.93 m². 2 / g; thermogravimetric analysis showed that its 5% thermal weight loss temperature was 535℃.

[0053] Results Analysis Acetone is a poor solvent for polyamic acid (PAA). Adding it to a PAA solution leads to premature phase separation, causing the polymer to precipitate and aggregate before the controlled nucleation and growth processes in the autoclave. The PAA molecular chains cannot form structurally complete flower-like porous microspheres; instead, they rapidly shrink into dense aggregates with few pores. The resulting particles have a more compact internal structure and a significantly reduced number of contact channels, directly reflected in a significant decrease in the BET specific surface area (142.93 m² / g). This result demonstrates that an additive-free, homogeneous, and stable PAA stock solution is essential for preparing the flower-like porous material described in this invention.

[0054] Comparative Example 3 When preparing the polyamic acid solution, pyridine and triethylamine were added as catalysts, and the rest was the same as in Example 1.

[0055] Performance characterization: The product consists of uniformly shaped spherical particles with a particle size range of 0.6 μm to 2.6 μm; thermogravimetric analysis shows that the temperature corresponding to 5% thermal weight loss of the sample is 450℃; BET test shows that its specific surface area is 45 m² / g.

[0056] Results Analysis Both pyridine and triethylamine are highly reactive amine catalysts that can greatly accelerate the imidization reaction. An excessively rapid imidization reaction rate causes the polymer network to immediately cross-link and harden upon heating, preventing the polyamic acid molecular chains from having sufficient time to self-assemble into a flower-like porous structure. The polymer, after rapid hardening, solidifies into a dense, glassy structure with very few pores. Furthermore, these amine catalysts leave behind difficult-to-remove amine salts and free amino groups in the final product; these residues not only clog pores but also degrade the material's thermal stability (the 5% thermogravimetric temperature drops from 530°C to 450°C). This comparative example clearly demonstrates that strong initiators simultaneously impair the material's specific surface area and thermal stability, confirming the superiority of the additive-free preparation process of this invention.

[0057] Comparative Example 4 A one-stage heating method was used, that is, the temperature was directly raised to 200°C, and the imidization reaction was carried out for 9 hours. Other aspects were the same as in Example 1.

[0058] Performance characterization: The product consists of uniformly shaped spherical particles with a particle size range of 0.8 μm to 2.6 μm; thermogravimetric analysis shows that the temperature corresponding to 5% thermal weight loss of the sample is 400℃; BET test shows that its specific surface area is 70 m² / g.

[0059] Results Analysis In a closed autoclave system, volatile substances (mainly water generated during the imidization process) cannot escape. If the temperature is directly raised to 200°C, the imidization reaction proceeds rapidly, generating water vapor uncontrollably throughout the reaction system. This sudden and massive generation of volatile components triggers uncontrolled foaming, nucleation, and aggregation, causing the polyamic acid molecular chains to harden and solidify before forming a flower-like porous structure. The resulting material exhibits an incomplete pore structure (BET specific surface area of ​​only 70 m² / g) and incomplete imidization (5% thermogravimetric temperature of only 400°C). This invention employs a two-stage heating process: first, a constant temperature treatment at 100°C is used to control the reaction initiation and slow release of volatile substances, allowing the polyamic acid molecular chains to self-assemble into spherical precursors; then, the temperature is raised to 200°C to complete the imidization reaction. This comparative example confirms that a two-stage heating procedure is necessary to obtain a high specific surface area and achieve complete imidization.

[0060] In summary, the polyimide microspheres prepared by this invention possess extremely low dielectric constant, excellent high-temperature thermal stability, and uniform microparticle size, making them perfectly suited for a variety of high-end applications such as battery separator coatings, gas adsorption materials, catalyst supports, low-dielectric materials for high-frequency electronic devices, and high-temperature insulating components for electrical appliances, thus having a wide range of applications.

[0061] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A method for preparing polyimide microspheres, characterized in that, The preparation method includes the following steps: (1) Under a nitrogen atmosphere, one or more dianhydrides and one or more diamines are polymerized in an organic solvent to obtain a polyamic acid solution; (2) The polyamic acid solution is loaded into a sealed high-pressure reactor and subjected to an imidization reaction: the reaction is first carried out at 100℃~120℃ for 1~5 hours, and then the temperature is raised to 200℃~250℃ and kept for 5~10 hours to convert the polyamic acid into polyimide, and the polyimide is precipitated in the form of microspheres to obtain polyimide microspheres.

2. The method for preparing polyimide microspheres according to claim 1, characterized in that, The mass fraction of polyamic acid in the solution is 1% to 20%, preferably 3% to 10%.

3. The method for preparing polyimide microspheres according to claim 1, characterized in that, The imidization reaction is carried out by first holding the temperature at 100°C for 3 hours, then raising the temperature to 200°C and holding it therefore for another 6 hours.

4. The method for preparing polyimide microspheres according to claim 1, characterized in that, It also includes the steps of washing and drying the prepared polyimide microspheres with a solvent.

5. A polyimide microsphere, characterized in that, Prepared by the method described in any one of claims 1 to 4.

6. The polyimide microspheres according to claim 5, characterized in that, The polyimide microspheres have an average particle size of 1.5 μm to 5 μm, a specific surface area (BET) ≥ 181 m² / g, a thermal decomposition temperature ≥ 500℃, and a dielectric constant ≤ 2.5 at a frequency of 10 GHz.

7. The application of the polyimide microspheres according to claim 5 or 6 in battery separator coatings, gas adsorption materials, catalyst supports, low dielectric materials for high-frequency electronic devices, or high-temperature resistant insulating components for electrical appliances.

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