Polyimide containing halogenated fluorene structure, dielectric film material as well as preparation method and application of dielectric film material

Abstract

The invention discloses polyimide containing a halogenated fluorene structure, a dielectric film material as well as a preparation method and application of the dielectric film material, and belongs to the technical field of energy storage materials. The polyimide containing the halogenated fluorene structure has the following repetitive structural units, wherein X is a halogen substituent group, a trifluoromethyl side group and a halogenated fluorene-containing structural unit are introduced into a main chain at the same time, so that good high-temperature dielectric property and high-temperature energy storage property are achieved, the solubility and film-forming property are good, and processing and forming are easy; and the requirements of high energy density and high-efficiency energy storage of a new generation of power electronics, aerospace and deep space exploration systems in a severe environment are met.

Description

Technical Field

[0001] This invention relates to a polyimide, particularly to a polyimide with a halogenated fluorene structure, a method for preparing a polyimide with a halogenated fluorene structure, and a dielectric membrane material based on a polyimide with a halogenated fluorene structure, its preparation method, and its application as a high-temperature energy storage material, belonging to the field of energy storage material technology. Background Technology

[0002] In core components of electrical energy storage and transmission, such as capacitors, organic polymer dielectrics are widely used due to their excellent insulation, high dielectric strength, low dielectric loss, and ability to be formed into large-area films. Currently, the most mature and widely used polymer dielectric in industry is biaxially oriented polypropylene (BOPP) film. BOPP also possesses advantages such as high transparency, good surface gloss, excellent gas barrier properties, good impact toughness, and low-temperature resistance. Furthermore, it boasts advantages such as high insulation reliability, mature processing technology, and low cost, thus maintaining its dominant position in the medium- and low-temperature capacitor market for a long time. However, the insufficient thermal stability of BOPP limits its wider application. Its melting point is only about 160-170℃, and its long-term safe operating temperature generally does not exceed 105℃. Above this temperature, BOPP film exhibits significant thermal shrinkage and dimensional instability, while the relaxation of molecular chain segments intensifies, leading to a sharp decline in dielectric insulation and mechanical strength, and even instantaneous insulation breakdown. This makes it difficult for BOPP to maintain stable operation in high-temperature and high-field scenarios such as new energy vehicle drive inverters, rail transit power supplies, aerospace power electronics, and high-voltage direct current transmission. Besides BOPP, industrially used polymer dielectrics such as polyethylene terephthalate (PET) and polyphenylene sulfide (PPS) have improved in temperature resistance, but their continuous operating temperature is generally still limited to around 120-150℃. Under more demanding conditions, such as temperatures above 150℃ or even above 200℃, PET and PPS will also fail due to increased thermal deformation and dielectric loss. With the continuously rising demand for high-temperature stable energy storage from new energy vehicles, advanced power electronics, deep space probes, and high-power pulse systems, existing polyolefin and conventional engineering plastic dielectrics are no longer sufficient. Therefore, there is an urgent need to develop polymer dielectrics with high heat resistance, excellent electrical insulation properties, and reliable mechanical strength.

[0003] Polyimide (PI), as a typical high-performance engineering plastic, has high molecular designability, excellent thermal stability (glass transition temperature can exceed 300℃), excellent dielectric insulation and chemical stability. However, to date, there are no reports on the use of halogen atoms for ortho-positioning design of polyimide dielectric materials for high-temperature energy storage materials. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the first objective of this invention is to provide a polyimide with a halogenated fluorene structure. The key is to simultaneously introduce trifluoromethyl side groups and halogenated fluorene structural units into the main chain of the polyimide, thereby endowing it with good high-temperature dielectric properties and high-temperature energy storage properties, as well as good solubility and film-forming properties, and easy processing and molding.

[0005] The second objective of this invention is to provide a method for preparing polyimides with halogenated fluorene structures. This method uses readily available raw materials and can be synthesized using existing mature polyimide polymerization processes, which is beneficial for industrial production.

[0006] The third objective of this invention is to provide a dielectric film material, based on which the main material is a halogenated fluorene polyimide, which still has high energy efficiency and dielectric stability at high temperatures, meeting the application requirements of high-temperature energy storage devices.

[0007] The fourth objective of this invention is to provide a method for preparing dielectric film materials, which employs existing conventional polyimide casting processes, is simple to operate, and is conducive to industrial production.

[0008] The fifth objective of this invention is to provide an application of a dielectric film material that has excellent high-temperature dielectric properties and high-temperature energy storage properties, which can meet the high energy density and high-efficiency energy storage requirements of next-generation power electronics, aerospace and deep space exploration systems in harsh environments.

[0009] To achieve the above-mentioned technical objectives, the present invention provides a halogenated fluorene-containing polyimide having repeating structural units as shown in Formula 1:

[0010] Formula 1;

[0011] Where X is a halogen substituent.

[0012] The key to the structural design of the halogenated fluorene polyimide of this invention lies in simultaneously introducing halogenated fluorene structural units and a large number of trifluoromethyl groups into the polymer backbone. Firstly, the fluorene structural units are large-volume, twisted, and non-coplanar structures, which can reduce the regularity of the polymer backbone, effectively prevent the close packing of the polymer backbone, weaken its crystallization ability, reduce intermolecular interaction forces, and improve its solubility, making it easier to process into films. Secondly, halogen substituents are introduced onto the fluorene structural units, utilizing the electronic localization and vertical steric hindrance effects of the halogen substituents to improve the dielectric properties and energy storage performance of the polymer at high temperatures. Thirdly, the introduction of a large number of trifluoromethyl side groups not only endows the polyimide with lower dielectric loss but also suppresses its conductive loss at high temperatures, exhibiting excellent dielectric reliability at high temperatures. Simultaneously, the trifluoromethyl groups can improve the solubility of the polyimide, which is beneficial for film processing.

[0013] As a preferred embodiment, X is a fluorine-, chlorine-, or bromine-substituent. Introducing halogen substituents such as fluorine, chlorine, or bromine substituents into the fluorene structural unit can improve the high-temperature dielectric properties and high-temperature energy storage performance of polyimides. Firstly, halogen substituents can increase the dielectric constant of polyimides, especially fluorine substituents introduced through the CF high-polarization bond, which is beneficial for improving the dielectric constant. Secondly, halogen substituents can effectively suppress carrier migration and space charge accumulation, thereby improving the dielectric stability and breakdown reliability of polyimides. Thirdly, halogen substitution enhances the polarization reversibility and reduces energy loss of polyimides at high temperatures, thus achieving higher energy efficiency and dielectric stability at high temperatures, and effectively maintaining polarization stability and energy release capability under high-temperature conditions.

[0014] This invention also provides a method for preparing a polyimide containing a halogenated fluorene structure, which includes the following steps:

[0015] 1) A polyamic acid intermediate is obtained by polycondensation reaction of diamine monomer and hexafluorodianhydride monomer;

[0016] 2) The polyamic acid intermediate is subjected to an imidization reaction to obtain a polyimide with a halogenated fluorene structure;

[0017] The diamine monomer has the structure of Formula 2:

[0018] Formula 2;

[0019] Where X is a halogen substituent.

[0020] As a preferred embodiment, the polycondensation reaction is carried out under the following conditions: at room temperature for 18-30 hours with the aid of an organic accelerator.

[0021] As a preferred embodiment, the organic accelerator is benzoic acid, isoquinoline, pyridine-acetic anhydride, or pyridine-triethylamine. A preferred organic accelerator is used at 0.3 to 0.8 equivalents of the diamine monomer.

[0022] As a preferred embodiment, the imidization reaction is carried out at a temperature of 150-170°C for 8-16 hours.

[0023] The organic solvent used in the polycondensation reaction of the present invention is one of the following high-boiling-point anhydrous organic solvents: resorcinol, N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), and resorcinol.

[0024] The synthetic route for the halogenated fluorene-structured polyimide of this invention is as follows:

[0025] ;

[0026] The present invention also provides a dielectric film material obtained by casting the aforementioned halogenated fluorene-structured polyimide film.

[0027] The dielectric film material of the present invention has the characteristics of high temperature resistance and low conductivity loss. Its glass transition temperature is higher than 350°C. At the same time, the introduction of trifluoromethyl and halogenated fluorene structural units can effectively suppress the formation of charge complex transfer products at high temperature and reduce conductivity loss under high temperature and high field.

[0028] This invention also provides a method for preparing a dielectric film material. The method involves dissolving a halogenated fluorene-structured polyimide in a solvent to form a solution, casting the solution onto a planar substrate, and then heat-treating the resulting film to obtain the dielectric film material. The planar substrate involved is, for example, glass.

[0029] As a preferred embodiment, the heat treatment conditions are as follows: first, the solvent is evaporated at a temperature of 60~80℃, and then a gradient temperature thermosetting treatment is performed; the gradient temperature process is as follows: holding at a temperature of 140~160℃ for 0.5~1.5h; holding at a temperature of 190~210℃ for 0.5~1.5h; and holding at a temperature of 240~260℃ for 3.0~5.0h.

[0030] The solvents involved in this invention are polar solvents such as NMP.

[0031] The present invention also provides an application of a dielectric membrane material, which is used as a high-temperature energy storage material in energy storage devices.

[0032] The high-temperature energy storage material involved in this invention refers to a dielectric membrane material that is adapted to an ambient temperature of 150°C or higher.

[0033] Compared with existing technologies, the beneficial technical effects of the present invention are as follows:

[0034] Traditional polyimides have thermal decomposition and glass transition temperatures exceeding 350°C, theoretically exhibiting excellent heat resistance. However, their electrical insulation properties degrade sharply with increasing temperature, especially at high temperatures where their insulation retention rate is significantly reduced, severely limiting their application in high-temperature capacitors and power electronics. The key to this invention lies in designing and optimizing the molecular structure of polyimides to significantly improve their insulation reliability and energy storage performance under high-temperature conditions. The halogenated fluorene-structured polyimide of this invention, through the introduction of numerous trifluoromethyl units and ortho-halogen substituents at the imide group, significantly improves the high-temperature dielectric and energy storage properties of polyimides, meeting the application requirements of high-temperature energy storage devices in next-generation aerospace, power electronics, and deep space exploration systems.

[0035] The halogenated fluorene polyimide of the present invention has good solubility, is easy to process into films, and is beneficial to its application.

[0036] The method for preparing the halogenated fluorene polyimide of the present invention uses readily available raw materials and can be synthesized using existing mature polymerization processes, which is beneficial for industrial production. Attached Figure Description

[0037] Figure 1 The Fourier transform infrared spectrum of the halogenated fluorene-containing polyimide of the present invention is shown.

[0038] Figure 2 This is a DSC curve of the halogenated fluorene-containing polyimide of the present invention.

[0039] Figure 3 The dielectric spectrum of the halogenated fluorene-containing polyimide of the present invention; Figure 3 In the figure, 'a' represents the dielectric spectrum at room temperature. Figure 3 b represents the dielectric spectrum at 150℃.

[0040] Figure 4 This is a Weibull distribution diagram of the halogenated fluorene-containing polyimide of the present invention; Figure 4 In the diagram, 'a' represents the Weibull distribution at room temperature. Figure 4 Figure b is the Weibull distribution at 150℃.

[0041] Figure 5 The electric displacement-electric field (DE) curves are shown at room temperature. Figure 5 In the figure, 'a' represents the DE curves under different electric fields of FPI. Figure 5 In the figure, b represents the DE curves under different electric fields of FFPI. Figure 5 c represents the DE curve under different electric fields of CFPI. Figure 5 In the figure, d represents the DE curves of BFPI under different electric fields.

[0042] Figure 6 The electric displacement-electric field (DE) curves of the halogenated fluorene-containing polyimide of the present invention at 150°C are shown.

[0043] Figure 7 The energy storage performance of the halogenated fluorene-containing polyimide of the present invention at room temperature and 150°C; Figure 7 In this context, 'a' represents the energy storage performance at room temperature. Figure 7 In the figure, b represents the energy storage performance at 150℃. Detailed Implementation

[0044] The following specific embodiments are intended to further illustrate the content of the present invention, rather than to limit the scope of protection of the claims of the present invention.

[0045] Example 1

[0046] Synthesis of halogenated fluorene-containing polyimides:

[0047] In a 25 mL Schlenk flask purged with nitrogen, 2 mmol of diamine monomers (using 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-fluorophenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene, or 9,9-bis(4-amino-3-bromophenyl)fluorene, and 5 mL of redistilled resorcinol were added. The mixture was stirred until the diamine monomers were completely dissolved. Then, an equimolar amount of hexafluorodianhydride (6FDA) and 0.5 equivalents of isoquinoline catalyst were added. The mixture was then subjected to three cycles of vacuuming and nitrogen purging to thoroughly remove residual oxygen and moisture from the solution, ensuring that the polycondensation reaction was carried out under a nitrogen atmosphere. The flask was then sealed. The system was then stirred at room temperature for 24 h, followed by heating to 160 °C and holding for 12 h to promote the formation of polyamic acid (PAA) and achieve imidization. After the reaction was completed, the solution was cooled to room temperature and diluted appropriately with excess NMP to reduce the viscosity of the system. Then, the reaction solution was slowly poured into a large amount of methanol to precipitate the polymer. The resulting precipitate was dried under vacuum at 80°C for 24 hours to remove residual solvent and low-molecular-weight impurities. To further improve the polymer purity and dielectric stability, the product was repeatedly subjected to a dissolution-precipitation cycle (redissolving in NMP and then precipitating in methanol) for a total of 4 times, finally obtaining high-purity polyimide solids. The polyimides obtained from different diamine monomers were designated as FPI, FFPI, CFPI, and BFPI, respectively.

[0048] Example 2

[0049] Preparation of halogenated fluorene-containing polyimide membrane materials:

[0050] In Example 1, 0.1 g of each of the prepared FPI, FFPI, CFPI, or BFPI was dissolved in approximately 3 mL of NMP to obtain a homogeneous, transparent, and highly fluid viscous liquid. The viscous liquid was then filtered through a nylon filter and placed in a refrigerator to remove air bubbles. A cleaned 5 cm × 5 cm square glass plate was placed on a 70°C constant-temperature heating stage. Once the temperature stabilized, the polyimide viscous liquid was spread evenly on the glass plate, starting from the center, using a flow-casting method. The plate was then dried at 70°C for 10 hours under infrared light to slowly evaporate most of the solvent. It was then transferred to a 150°C constant-temperature vacuum drying oven for 1 hour, followed by drying at 200°C for 1 hour, and finally at 250°C for 4 hours. After cooling to room temperature, a polyimide film with a thickness of approximately 10 μm was obtained using a 200 μm doctor blade.

[0051] Performance testing of the halogenated fluorene-containing polyimide membrane material prepared in Example 2:

[0052] Thermal stability test:

[0053] Figure 1 The infrared spectrum of the halogenated fluorene-containing polyimide film material shows that the characteristic bonds of CF, C-Cl, and C-Br can be found in the corresponding polymers, indicating that the corresponding elements have been successfully introduced.

[0054] Figure 2 The DSC curves of fluorene-containing polyimide film materials at temperatures ranging from 100 to 350°C show that these polyimides do not exhibit a significant glass transition at 350°C, indicating that their heat resistance can exceed 350°C.

[0055] Electrical performance testing: A metal mask with a circular aperture diameter of 3 mm was designed. The prepared thin film was sandwiched between two metal masks, and gold electrodes were symmetrically sputtered onto the upper and lower surfaces at a sputtering current of 30 mA for 300 s. Performance testing was then performed using a ferroelectric workstation and an impedance analyzer.

[0056] Figure 3 The dielectric spectrum characteristics of halogenated fluorene-containing polyimide films are presented. Comparison of FPI, FFPI, CFPI, and BFPI films reveals that at room temperature, FFPI exhibits a significantly enhanced polarization response due to the introduction of highly polarized C–F bonds, with its dielectric constant reaching 3.25 at 1 kHz. However, as the halogen atoms transition from F to Cl and Br, the decrease in electronegativity and increase in atomic radius lead to a reduction in intramolecular dipole density and dipole moment, while steric hindrance increases, thus restricting the orientation polarization of the polyimide segments. Consequently, the dielectric constants of CFPI and BFPI decrease to 3.02 and 2.85 (1 kHz), respectively, with BFPI's dielectric constant even slightly lower than CFPI's (3.01 at 1 kHz). While the introduction of highly polarized bonds is beneficial for increasing the dielectric constant, it also leads to greater dielectric loss; therefore, FFPI exhibits the highest dielectric loss value. In contrast, CFPI and BFPI show significantly lower losses. The test results at 150°C also showed the same pattern, and the dielectric constant did not decrease significantly, indicating that the polymer of the present invention has good dielectric stability, thermal stability and repeatability.

[0057] Figure 4 The Weibull distribution of breakdown field strength for halogenated fluorene-containing polyimide film materials is shown. At room temperature, the average breakdown field strengths of FFPI, CFPI, and BFPI reach 702.02 MVm. -1 716.16MVm -1 and 680.24MVm -1 Both were significantly higher than the 642.43MVm of the unsubstituted FPI system. -1Even under high temperature conditions (150℃), the three halogen-substituted systems still maintain high breakdown strengths of 635.60 MVm. -1 645.75MVm -1 and 618.82MVm -1 This is significantly better than FPI's 568.98MVm. -1 This result demonstrates that halogen substitution can effectively suppress carrier migration and space charge accumulation, thereby improving the dielectric stability and breakdown reliability of the system. As the electronegativity of halogen atoms decreases and their atomic volume increases, the molecular polarization environment and electric field distribution are also regulated, further restricting charge transport paths. This provides important molecular design basis for improving the insulation performance of polyimide films under high temperature and high field conditions.

[0058] Figure 5 The D-E hysteresis loops of halogenated fluorene-containing polyimide films at room temperature are shown. Compared to unsubstituted FPI, halogen-substituted FFPI, CFPI, and BFPI all exhibit significantly narrower D-E hysteresis loops, indicating enhanced polarization reversibility and reduced energy loss. As the halogen atom transitions from F to Cl and Br, the D-E curves narrow further, reflecting a more constrained polarization process and a gradual weakening of charge hysteresis, thus achieving higher energy efficiency and dielectric stability.

[0059] Figure 6 The D-E curve characteristics of the halogenated fluorene-containing polyimide film material at 150°C show the same trend as the DE curve at room temperature, further supporting the conclusion at room temperature.

[0060] Figure 7 To assess the energy storage performance of halogenated fluorene-containing polyimide membrane materials at room temperature, 150°C, and 100Hz, an electrode diameter of 3 mm was used for testing. At room temperature, the halogen-substituted systems all exhibited higher discharge energy densities compared to the unsubstituted FPI (5.27 J / cm²). -3 The discharge energy densities of FFPI, CFPI, and BFPI were increased to 7.39 J / cm². -3 6.95 Jcm -3 and 7.04 Jcm -3 Furthermore, the charge / discharge efficiencies all exceeded 90%. At high temperatures (150℃), although the increased thermal motion of the molecular chains led to a slight decrease in energy density, the discharge energy densities of FFPI, CFPI, and BFPI remained at 4.75 J / cm². -3 5.49 Jcm -3 and 5.07 Jcm -3 It is significantly higher than the 3.45 Jcm of FPI. -3This result indicates that halogen substitution can effectively maintain polarization stability and energy release capacity under high-temperature conditions.

[0061] Furthermore, the good film-forming properties and high-temperature resistance of halogenated fluorene-containing polyimides indicate their high molecular weight. Simultaneously, electrical performance tests demonstrate their suitability as dielectric film materials for high-temperature energy storage devices.

Claims

1. A polyimide with a halogenated fluorene structure, characterized in that: It has the repeating structural unit shown in Equation 1: Formula 1; Where X is a halogen substituent.

2. The halogenated fluorene-containing polyimide according to claim 1, characterized in that: X is a fluorine substituent, a chlorine substituent, or a bromine substituent.

3. The method for preparing a halogenated fluorene-structured polyimide according to claim 1, characterized in that: Includes the following steps: 1) A polyamic acid intermediate is obtained by polycondensation reaction of diamine monomer and hexafluorodianhydride monomer; 2) The polyamic acid intermediate is subjected to an imidization reaction to obtain a polyimide with a halogenated fluorene structure; The diamine monomer has the structure of Formula 2: Formula 2; Where X is a halogen substituent.

4. The method for preparing a halogenated fluorene-structured polyimide according to claim 3, characterized in that: The conditions for the polycondensation reaction are: under the action of an organic accelerator, at room temperature, for 18-30 hours.

5. The method for preparing a halogenated fluorene-structured polyimide according to claim 4, characterized in that: The organic accelerator is benzoic acid, isoquinoline, pyridine-acetic anhydride, or pyridine-triethylamine.

6. The method for preparing a halogenated fluorene-structured polyimide according to claim 3, characterized in that: The conditions for the imidization reaction are: reaction at 150~170℃ for 8~16h.

7. A dielectric film material, characterized in that: It is obtained by casting a polyimide film with a halogenated fluorene structure as described in claim 1 or 2.

8. The method for preparing a dielectric film material according to claim 7, characterized in that: The halogenated fluorene polyimide of claim 1 or 2 is dissolved in a solvent to form a solution, and the solution is cast onto a planar substrate. The resulting film is then heat-treated to obtain a dielectric film material.

9. The method for preparing a dielectric film material according to claim 8, characterized in that: The heat treatment conditions are as follows: first, the solvent is evaporated at a temperature of 60~80℃, and then a gradient temperature thermosetting treatment is performed; the gradient temperature process is as follows: hold at a temperature of 140~160℃ for 0.5~1.5h; hold at a temperature of 190~210℃ for 0.5~1.5h; and hold at a temperature of 240~260℃ for 3.0~5.0h.

10. The application of the dielectric film material according to claim 7, characterized in that: It is used as a high-temperature energy storage material in energy storage devices.

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