A halogenated fluorene-containing polyimide, a dielectric film material and a preparation method and application thereof

Abstract

The application discloses a kind of polyimides containing halogenated fluorene structure, dielectric film material and preparation method and application thereof, belong to energy storage material technical field.Polyimides containing halogenated fluorene structure has the following repeating structural unit:;Wherein, X is halogen substituent, it is introduced by main chain simultaneously trifluoromethyl side group and halogenated fluorene structure unit, give it good high-temperature dielectric property and high-temperature energy storage performance, and its solubility and film-forming performance are good, easy to process into shape, meet the high energy density and high efficiency energy storage demand of new generation power electronics, aerospace and deep space exploration system under harsh environment.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of polyimides, especially to a kind of polyimides containing halogenated fluorene structure, also relates to a kind of polyimides containing halogenated fluorene structure preparation method, and relates to a kind of dielectric film material based on polyimides containing halogenated fluorene structure and its preparation method and as high temperature energy storage material application, belong to energy storage material technical field. BACKGROUND

[0002] In the core components of electric 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 large-area film forming characteristics. Currently, the most mature and widely used polymer dielectric in industry is biaxially oriented polypropylene (BOPP) film. BOPP has high transparency, good surface gloss, excellent gas barrier properties, good impact toughness, and low temperature resistance, and has high insulation reliability, mature processing technology, and low cost, thus long-term occupying the dominant position in the market of medium and low temperature capacitors. However, the lack of thermal stability of BOPP limits its wider application, with a melting point of only about 160-170°C, and a long-term safe working temperature generally not exceeding 105°C. Above this temperature, BOPP film will exhibit significant thermal shrinkage and size instability, and the relaxation of molecular chain segments will intensify, resulting in a sharp decrease in dielectric insulation and mechanical strength, and even an 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. In addition to BOPP, commonly used polymer dielectrics such as polyethylene terephthalate (PET) and polyphenylene sulfide (PPS) have improved temperature resistance, but their continuous use temperature is generally limited to about 120-150°C. In more severe working conditions, such as environments above 150°C or even above 200°C, PET and PPS will also fail due to thermal deformation and increased dielectric loss. With the increasing demand for high-temperature stable energy storage in new energy vehicles, advanced power electronics, deep space probes, and high-power pulse systems, existing polyolefins and conventional engineering plastic dielectrics have been unable to meet the requirements. 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) is a typical high-performance engineering plastic with high molecular designability, excellent thermal stability (glass transition temperature can exceed 300°C), excellent dielectric insulation, and chemical stability. However, to date, there have been no reports of using halogen atoms to design polyimide dielectric materials for high-temperature energy storage materials. SUMMARY

[0004] In view of the defects of the prior art, a first object of the present application is to provide a halogenated fluorene structure-containing polyimide, the key of which is to introduce trifluoromethyl side groups and halogenated fluorene structure units into the main chain of the polyimide at the same time, so as to endow the polyimide with good high-temperature dielectric properties and high-temperature energy storage properties, and good solubility and film-forming properties, and facilitate processing and molding.

[0005] A second object of the present application is to provide a preparation method of the halogenated fluorene structure-containing polyimide, the raw materials of which are easy to obtain, and the method can be synthesized by using the existing mature polyimide polymerization process, which is conducive to industrial production.

[0006] A third object of the present application is to provide a dielectric film material, the main material of which is the halogenated fluorene structure-containing polyimide, which still has high energy efficiency and dielectric stability at high temperature, and meets the application requirements of high-temperature energy storage devices.

[0007] A fourth object of the present application is to provide a preparation method of the dielectric film material, the method of which adopts the existing conventional polyimide film casting process, and is simple to operate, which is conducive to industrial production.

[0008] A fifth object of the present application is to provide an application of the dielectric film material, which has good high-temperature dielectric properties and high-temperature energy storage properties, and can meet the high-energy density and high-efficiency energy storage requirements of the new generation of power electronics, aerospace and deep space exploration systems in harsh environments.

[0009] In order to achieve the above technical objects, the present application provides a halogenated fluorene structure-containing polyimide, which has a repeating structural unit shown in formula 1:

[0010] Formula 1;

[0011] In formula 1, X is a halogen substituent.

[0012] The structural design of the halogenated fluorene structure-containing polyimide of the present application is to introduce halogenated fluorene structure units and a large number of trifluoromethyl groups into the polymer main chain at the same time: on the one hand, the fluorene structure unit is a bulky, twisted non-planar structure, which can reduce the regularity of the polymer main chain, effectively prevent the close packing of the polymer main chain, reduce the crystallization ability, and reduce the intermolecular interaction force, which is conducive to improving the solubility and facilitating the processing and film forming; on the other hand, the halogen substituent is introduced on the fluorene structure unit, which can improve the dielectric properties and energy storage properties of the polymer at high temperature by using the electron localization and vertical steric hindrance effect of the halogen substituent; thirdly, a large number of trifluoromethyl side groups are introduced, which not only endow the polyimide with lower dielectric loss, but also can inhibit the conduction loss at high temperature, and exhibit excellent dielectric reliability at high temperature, and the trifluoromethyl groups can improve the solubility of the polyimide, which is conducive to the processing and film forming.

[0013] As a preferred scheme, X is a fluorine substituent, a chlorine substituent or a bromine substituent. By introducing a halogen substituent such as a fluorine substituent, a chlorine substituent or a bromine substituent on the fluorene structural unit, the high-temperature dielectric properties and high-temperature energy storage properties of the polyimide can be improved. On the one hand, the halogen substituent can increase the dielectric constant of the polyimide, and in particular, the fluorine substituent is introduced by a C-F high-polar bond, which is conducive to improving the dielectric constant. On the other hand, the halogen substituent can effectively inhibit the carrier migration and space charge accumulation, thereby improving the dielectric stability and breakdown reliability of the polyimide. On the third aspect, the halogen substitution makes the polarization reversibility of the polyimide at high temperature increase and the energy loss decrease, thereby realizing higher energy efficiency and dielectric stability at high temperature, and effectively maintaining the polarization stability and energy release capacity under high temperature conditions.

[0014] The application also provides a preparation method of the halogenated fluorene structure-containing polyimide, which comprises the following steps:

[0015] 1) obtaining a polyamic acid intermediate by subjecting a diamine monomer to a polycondensation reaction with a hexafluoro dianhydride monomer;

[0016] 2) obtaining the halogenated fluorene structure-containing polyimide by subjecting the polyamic acid intermediate to an imidization reaction;

[0017] The diamine monomer has a structure of formula 2.

[0018] Formula 2;

[0019] X is a halogen substituent.

[0020] As a preferred scheme, the polycondensation reaction is carried out at room temperature for 18-30 h in the presence of an organic promoter.

[0021] As a preferred scheme, the organic promoter is benzoic acid, isoquinoline, pyridine-acetic anhydride or pyridine-triethylamine. Preferably, the amount of the organic promoter is 0.3-0.8 equivalent of the diamine monomer.

[0022] As a preferred scheme, the imidization reaction is carried out at a temperature of 150-170 ℃ for 8-16 h.

[0023] The organic solvent used in the polycondensation reaction of the application is one of high-boiling-point anhydrous organic solvents such as m-cresol, N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF) and m-cresol.

[0024] The synthesis route of the halogenated fluorene structure-containing polyimide of the application is as follows:

[0025] ;

[0026] The application also provides a dielectric film material obtained by casting the halogenated fluorene structure-containing polyimide film.

[0027] The dielectric film material of the application has the characteristics of high-temperature resistance and low conductive loss, and has a glass transition temperature higher than 350 DEG C. The introduction of the trifluoromethyl and halogenated fluorene structure unit can effectively inhibit the formation of charge complex transfer substances at high temperature, and reduce the conductive loss at high temperature and high field.

[0028] The application also provides a preparation method of the dielectric film material. The method is to dissolve the halogenated fluorene structure-containing polyimide in a solvent to form a solution, cast the solution on a plane substrate, and heat treat the obtained film to obtain the dielectric film material. The plane substrate may be glass.

[0029] As a preferred scheme, the heat treatment conditions are as follows: first, volatilize the solvent at a temperature of 60-80 DEG C, and then heat solidify by gradient temperature rising; the gradient temperature rising process is as follows: heat at a temperature of 140-160 DEG C for 0.5-1.5 h, heat at a temperature of 190-210 DEG C for 0.5-1.5 h, and heat at a temperature of 240-260 DEG C for 3.0-5.0 h.

[0030] The solvent involved in the application is a polar solvent such as NMP.

[0031] The application also provides an application of the dielectric film material, which is used as a high-temperature energy storage material for energy storage devices.

[0032] The high-temperature energy storage material involved in the application refers to a dielectric film material suitable for an environment temperature of 150 DEG C or higher.

[0033] Compared with the prior art, the technical scheme of the application has the following beneficial technical effects:

[0034] The thermal decomposition temperature and the glass transition temperature of the conventional polyimide can be higher than 350 DEG C, and theoretically, the polyimide has excellent heat resistance. However, the electrical insulation performance of the polyimide will sharply decay with the increase of temperature, and especially, the insulation retention rate of the polyimide is greatly discounted at high temperature, which seriously limits the application of the polyimide in high-temperature capacitors and power electronics. The key of the application is to design and optimize the molecular structure of the polyimide to significantly improve the insulation reliability and energy storage performance of the polyimide at high temperature. The halogenated fluorene structure-containing polyimide of the application can obviously improve the high-temperature dielectric performance and high-temperature energy storage performance of the polyimide by introducing a large number of trifluoromethyl units and introducing imide group ortho halogen substituents, and meet the application requirements of high-temperature energy storage devices in new-generation aerospace, power electronics and deep space exploration systems.

[0035] The polyimide containing halogenated fluorene structure has good solubility, is easy to be processed into a film, and is beneficial to application.

[0036] The preparation method of the polyimide containing halogenated fluorene structure is easy to obtain raw materials, and can be synthesized by using existing mature polymerization process, which is beneficial to industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 The Fourier transform infrared spectrum of the halogenated polyimide containing fluorene structure.

[0038] Figure 2 The DSC curve of the halogenated polyimide containing fluorene structure.

[0039] Figure 3 The dielectric spectrum of the halogenated polyimide containing fluorene structure; Figure 3 a is the dielectric spectrum at room temperature, Figure 3 b is the dielectric spectrum at 150℃.

[0040] Figure 4 The Weibull distribution graph of the halogenated polyimide containing fluorene structure; Figure 4 a is the Weibull distribution graph at room temperature, Figure 4 b is the Weibull distribution graph at 150℃.

[0041] Figure 5 The potential displacement-electric field (D-E) curve at room temperature; Figure 5 a is the D-E curve of FPI at different electric fields, Figure 5 b is the D-E curve of FFPI at different electric fields, Figure 5 c is the D-E curve of CFPI at different electric fields, Figure 5 d is the D-E curve of BFPI at different electric fields.

[0042] Figure 6 The potential displacement-electric field (D-E) curve of the halogenated polyimide containing fluorene structure at 150℃.

[0043] Figure 7 The energy storage performance of the halogenated polyimide containing fluorene structure at room temperature and 150℃; Figure 7 a is the energy storage performance at room temperature, Figure 7 b is the energy storage performance at 150℃. DETAILED DESCRIPTION

[0044] The following specific examples are intended to further illustrate the content of the present application, but not to limit the protection scope of the claims of the present application.

[0045] Example 1

[0046] Synthesis of halogenated polyimides containing fluorene structure:

[0047] In a 25 mL Schlenk flask purged with nitrogen, diamine monomer (2 mmol) (9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-fluorophenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene, 9,9-bis(4-amino-3-bromophenyl)fluorene, respectively) and freshly distilled m-phenol (5 mL) were added and stirred until the diamine monomer was completely dissolved. Then, an equimolar amount of hexafluorodiphthalic anhydride (6FDA) and 0.5 equivalent of isoquinoline catalyst were added, and the solution was purged with nitrogen three times to remove residual oxygen and moisture in the solution, ensuring that the polycondensation reaction was carried out in a nitrogen atmosphere, and then sealed. The system was then stirred at room temperature for 24 h, and then heated to 160 °C for 12 h to promote the formation of polyamic acid (PAA) and imidization. After the reaction was completed, the solution was cooled to room temperature, diluted with an excess of NMP to reduce the viscosity of the system, and then slowly poured into a large amount of methanol to precipitate the polymer. The obtained precipitate was dried at 80 °C under vacuum for 24 h to remove residual solvents and low molecular weight impurities. To further improve the purity and dielectric stability of the polymer, the product was repeatedly subjected to dissolution-precipitation cycles (redissolved in NMP and precipitated in methanol), a total of 4 times, and finally obtained a high-purity polyimide solid. The polyimides obtained from different diamine monomers are denoted as FPI, FFPI, CFPI, and BFPI, respectively.

[0048] Example 2

[0049] Preparation of halogenated polyimide film materials containing fluorene structure:

[0050] Example 1, FFPI, CFPI, or BFPI was prepared. 0.1 g of each was dissolved in about 3 mL of NMP to obtain a uniform, transparent, and flowable viscous liquid. The viscous liquid was then filtered with a nylon filter, and the bubbles in the viscous liquid were removed by placing it in the refrigerator. A clean 5 cm x 5 cm square glass plate was placed on a constant temperature heating platform at 70 °C, and when the temperature reached a stable state, the polyimide viscous liquid was cast from the middle of the glass plate using the flow casting method, evenly spread on the glass plate, and dried at 70 °C for 10 h under infrared lamp irradiation to slowly evaporate most of the solvent. Then it was transferred to a constant temperature vacuum drying oven at 150 °C and dried for 1 h, at 200 °C for 1 h, and at 250 °C for 4 h. After cooling to room temperature, a polyimide film with a thickness of about 10 μm was obtained under a 200 μm doctor blade.

[0051] Performance test of halogenated polyimide film materials containing fluorene structure prepared in Example 2:

[0052] Thermal stability test:

[0053] Figure 1 For the infrared spectrum of the halogenated polyimide film material containing fluorene structure, C-F, C-Cl, C-Br characteristic bonds can be found in the corresponding polymer, which shows that the corresponding elements have been successfully introduced.

[0054] Figure 2 For the DSC curve of the halogenated polyimide film material containing fluorene structure at 100-350℃, it can be seen that the glass transition of the three kinds of polyimides at 350℃ is not obvious, which shows that their heat resistance can be higher than 350℃.

[0055] Electric property test: design a metal mask plate with a circular hole diameter of 3mm, clamp the prepared film in the middle of the two metal mask plates, and symmetrically sputter gold electrodes on the upper and lower surfaces, and sputter for 300s under a sputtering current of 30mA. Use a ferroelectric workstation and an impedance analyzer to test the performance.

[0056] Figure 3 The dielectric spectrum characteristics of the halogenated polyimide film material containing fluorene structure. Comparing the film materials of FPI, FFPI, CFPI and BFPI, it can be seen that at room temperature, due to the introduction of high-polar C-F bond in FFPI, it shows a significantly enhanced polarization response, and its dielectric constant reaches 3.25 at 1kHz. With the halogen atoms changing from F to Cl and Br, due to the decrease of electronegativity and the increase of atomic radius, the intramolecular dipole density and dipole moment decrease, and the steric hindrance increases, so that the orientation polarization of the polyimide segment is limited. Therefore, the dielectric constants of CFPI and BFPI are reduced to 3.02 and 2.85 (1kHz), respectively, and the dielectric constant of BFPI is even slightly lower than that of CFPI (3.01 at 1kHz). At the same time, the introduction of high-polar bond is beneficial to the increase of dielectric constant, but also leads to larger dielectric loss, so FFPI shows the highest dielectric loss value; in comparison, the loss of CFPI and BFPI is obviously lower. The test results at 150℃ also show the same rule, and the dielectric constant does not show obvious decrease, which shows that the polymer of the application has good dielectric stability, thermal stability and repeatability.

[0057] Figure 4 The breakdown field strength Weibull distribution of the halogenated polyimide film material containing fluorene structure. At room temperature, the average breakdown field strength of FFPI, CFPI and BFPI is 702.02MVm -1 , 716.16MVm -1 and 680.24MVm -1 , respectively, which are significantly higher than 642.43MVm -1. Even under high temperature condition (150℃), the three halogen-substituted systems still maintain high breakdown strength, 635.60MVm -1 , 645.75MVm -1 and 618.82MVm -1 , which are obviously superior to FPI’s 568.98MVm -1 . This result indicates that halogen substitution can effectively inhibit carrier migration and space charge accumulation, thus improving the dielectric stability and breakdown reliability of the system. With the decrease of electronegativity and the increase of atomic volume of halogen atoms, the molecular polarization environment and electric field distribution are also regulated, making the charge transport path more limited, which provides an important molecular design basis for the improvement of the insulating properties of polyimide films under high temperature and high field.

[0058] Figure 5 D-E loop characteristics of halogenated polyimide film materials containing fluorene structure at room temperature. Compared with unsubstituted FPI, FFPI, CFPI and BFPI after halogen substitution all show significantly narrowed D-E loop, indicating that the polarization reversibility is enhanced and the energy loss is reduced. With the halogen atom changing from F to Cl and Br, the D-E curve is further narrowed, reflecting that the polarization process of the system is more limited and the charge hysteresis effect is gradually weakened, thus realizing higher energy efficiency and dielectric stability.

[0059] Figure 6 D-E loop characteristics of halogenated polyimide film materials containing fluorene structure at 150℃, which has the same trend as the D-E curve at room temperature, further supporting the conclusion at room temperature.

[0060] Figure 7 The energy storage performance of halogenated polyimide film materials containing fluorene structure at room temperature and 150℃ and 100Hz, the test electrode diameter is 3mm. At room temperature, halogen-substituted systems all show high discharge energy density, compared with unsubstituted FPI (5.27Jcm -3 ), the discharge energy density of FFPI, CFPI and BFPI is increased to 7.39Jcm -3 , 6.95Jcm -3 and 7.04Jcm -3 , respectively, and the charge and discharge efficiency is more than 90%. Under high temperature environment (150℃), although the molecular chain thermal motion is intensified, the discharge energy density of FFPI, CFPI and BFPI still remains at 4.75Jcm -3 , 5.49Jcm -3 and 5.07Jcm -3 , respectively, which is significantly higher than FPI’s 3.45Jcm -3The results show that halogen substitution can effectively maintain the polarization stability and energy release ability under high temperature conditions.

[0061] In addition, the halogenated polyimide containing fluorene structure has good film-forming property and high temperature resistance, indicating that it has high molecular weight. At the same time, the electrical property test shows that it can be used as a dielectric film material for high temperature energy storage devices.

Claims

1. An application of a dielectric film material, characterized in that: Used as a high-temperature energy storage material in energy storage devices; The dielectric film material is obtained by casting a polyimide film containing a halogenated fluorene structure. The halogenated fluorene polyimide has repeating structural units as shown in Formula 1: Formula 1; Wherein, X is a fluorine substituent, a chlorine substituent, or a bromine substituent.

2. The application of the dielectric film material according to claim 1, characterized in that: The halogenated fluorene-structured polyimide casting film is prepared by the following method, which 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; Wherein, X is a fluorine substituent, a chlorine substituent, or a bromine substituent.

3. The application of the dielectric film material according to claim 2, characterized in that: The conditions for the polycondensation reaction are: under the action of an organic accelerator, at room temperature, for 18-30 hours.

4. The application of the dielectric film material according to claim 3, characterized in that: The organic accelerator is benzoic acid, isoquinoline, pyridine-acetic anhydride, or pyridine-triethylamine.

5. The application of the dielectric film material according to claim 2, characterized in that: The conditions for the imidization reaction are: reaction at 150~170°C for 8~16 hours.

6. The application of the dielectric film material according to claim 1, characterized in that: The dielectric film material is prepared by the following method: a halogenated fluorene polyimide is dissolved in a solvent to form a solution, the solution is cast onto a planar substrate, and the resulting film is heat-treated to obtain the dielectric film material.

7. The application of the dielectric film material according to claim 6, characterized in that: The heat treatment conditions are as follows: first, the solvent is evaporated at a temperature of 60~80°C, and then a gradient temperature thermosetting treatment is performed; the gradient temperature process is as follows: hold at a temperature of 140~160°C for 0.5~1.5h; hold at a temperature of 190~210°C for 0.5~1.5h; and hold at a temperature of 240~260°C for 3.0~5.0h.

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