Preparation method and application of hydrogen bond cross-linked polyimide energy storage material

Abstract

This invention discloses a method for preparing and applying a hydrogen-bonded crosslinked polyimide energy storage material, belonging to the field of insulating materials. The invention addresses the problems of insufficient conductivity, low utilization of active sites, and difficulty in simultaneously achieving low ionic conductivity and mechanical strength in traditional polyimides. The method involves adding 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] to N,N-dimethylacetamide and ultrasonically stirring until homogeneous. Then, pyromellitic dianhydride is added in batches and stirred until viscous to obtain solution A. Next, 2,2-bis(4-hydroxy-3-aminophenyl)propane is added to N,N-dimethylacetamide and ultrasonically stirred until homogeneous. Then, p-phenylene-bisphenyltriterpenoid dianhydride is added and stirred until viscous to obtain solution B. Finally, solution B is added to solution A, and the mixture is stirred thoroughly at room temperature to ensure complete reaction. Vacuum is then applied, followed by coating, gradient temperature curing, and peeling to obtain the final product. The material is applicable to high-frequency electronic packaging, high-temperature energy storage capacitors, flexible transparent electronic devices, and aerospace special engineering fields.

Description

Technical Field

[0001] This invention belongs to the field of insulating materials, specifically relating to aerospace membrane insulation and motor turn-to-turn insulation. More specifically, it relates to a method for preparing and applying a hydrogen-bonded crosslinked polyimide energy storage material. Background Technology

[0002] With the rapid development of renewable energy and the transformation of the energy structure, efficient and stable energy storage technologies have become key to driving the energy revolution. Polyimide, due to its excellent thermal stability, mechanical properties, and designable molecular structure, shows great promise in the field of electrochemical energy storage, especially in organic electrode materials and solid electrolytes. However, traditional polyimide still faces challenges in energy storage applications: as an electrode material, its conductivity is insufficient and the utilization rate of active sites is low; as an electrolyte, it is difficult to balance ionic conductivity and mechanical strength, limiting its practical application in high-performance energy storage devices. Summary of the Invention

[0003] To address the aforementioned issues, this invention starts with the molecular structure design of polyimide and introduces a hydrogen bonding crosslinking strategy to effectively optimize its energy storage performance. Hydrogen bonds, as reversible dynamic physical crosslinking points, can construct ordered molecular stacking structures within the polyimide matrix: on the one hand, by enhancing interchain interactions, they improve the structural stability of the material and suppress volume expansion and dissolution during cycling; on the other hand, by utilizing the dynamic characteristics of the hydrogen bond network, they promote the construction and regulation of ion transport channels, thereby potentially resolving the contradiction between ionic conductivity and mechanical properties. Furthermore, polyimide rich in nitrogen and oxygen functional groups can form multiple hydrogen bond interfaces with electrolytes or active materials, enhancing interfacial compatibility and improving charge transfer efficiency.

[0004] This invention combines molecular structure design and chain segment modification methods to conduct research on the regulation of hydrogen bond crosslinking in polyimide. It focuses on decoupling the synergistic optimization mechanism between energy storage capacity, cycle stability and rate performance, and elucidates the dynamic evolution of hydrogen bond networks in the electrochemical environment. The aim is to provide theoretical support and experimental basis for the design and preparation of high-performance polyimide-based energy storage materials.

[0005] This invention addresses the challenges that traditional polyimides still face in energy storage applications: when used as electrode materials, their conductivity is insufficient and the utilization rate of active sites is low; when used as electrolytes, it is difficult to balance ionic conductivity and mechanical strength, which limits their practical application in high-performance energy storage devices. The invention provides a method for preparing and applying a hydrogen-bonded crosslinked polyimide energy storage material.

[0006] To address the aforementioned technical problems, the present invention adopts the following technical solution: The purpose of this invention is to provide a method for preparing a hydrogen-bonded crosslinked polyimide energy storage material, characterized by comprising the following steps: Step 1: Add 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] to N,N-dimethylacetamide, and ultrasonically stir until homogeneous. Add pyromellitic dianhydride in batches and stir until viscous to obtain adhesive solution A. Step 2: Add 2,2-bis(4-hydroxy-3-aminophenyl)propane to N,N-dimethylacetamide, and ultrasonically stir until homogeneous. Add p-phenylene-bisphenyltriterpenoid dianhydride and stir until viscous to obtain adhesive B. Step 3: Then add adhesive B to adhesive A, stir thoroughly at room temperature to allow it to react completely, apply vacuum, and then coat it onto the pretreated substrate. Curing is done by gradient heating. The film on the substrate is then peeled off to obtain the energy storage material.

[0007] To further specify, in step 1, pyromellitic anhydride is added in 6 portions, with each portion spaced 2 hours apart.

[0008] To further refine the process, 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] was placed in a vacuum drying oven and dried under vacuum at 120°C for 12 h. After removing moisture, it was sealed and stored.

[0009] Further, under an anhydrous and oxygen-free environment, pyromellitic dianhydride was dissolved in acetic anhydride at 80°C. The dissolution was assisted by ultrasonication at a frequency of 40kHz and a power of 300W, with a mechanical stirring speed of 500r / min and a stirring time of 30min. After standing, at least a small amount of crystals precipitated. Then, the mixture was placed in an ice-water bath for full crystallization. The crystals were collected by filtration, dried under vacuum, and purified pyromellitic dianhydride was obtained and stored in a sealed container.

[0010] To further refine the process, 2,2-bis(4-hydroxy-3-aminophenyl)propane was dissolved in anhydrous ethanol at 60°C. The dissolution was assisted by ultrasonication at 40 kHz and 300 W, with mechanical stirring at 500 r / min for 30 min until complete dissolution. The mixture was then rapidly crystallized in an ice-water bath. The crystals were collected by filtration and vacuum dried to obtain purified 2,2-bis(4-hydroxy-3-aminophenyl)propane, which was then sealed and stored.

[0011] Further, the p-phenylene-bisphenyltriester dianhydride was placed in a vacuum drying oven and dried under vacuum at 150°C for 8 hours to remove residual solvent and moisture. Then it was cooled to room temperature in a desiccator and sealed for storage.

[0012] Further, in steps 1 and 2, ultrasonic-assisted dissolution is performed at a frequency of 40kHz and a power of 300W, with a mechanical stirring speed of 500r / min and a stirring time of 20~50min.

[0013] Further specifying, in step 1, the molar ratio of 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] to pyromellitic dianhydride is 1:1.01.

[0014] Further specifying, in step 2, the molar ratio of 2,2-bis(4-hydroxy-3-aminophenyl)propane to p-phenylene-bisphenyltriester dianhydride is 1.01:1.

[0015] Further specified, the hydroxyl polyimide obtained by reacting 2,2-bis(4-hydroxy-3-aminophenyl)propane and p-phenylene-bisphenyltriester dianhydride accounts for 10wt%-90wt% of the energy storage material, and the hydroxyl polyimide is used as a modifier for the polyimide molecular chain.

[0016] Further specifying the substrate pretreatment method: first, soak the substrate in N,N-dimethylacetamide solution for 2-3 hours, then wash it with anhydrous ethanol 3-5 times, and finally dry it at 40 ℃-60 ℃ for 8 hours to complete the process; the substrate is a high-temperature resistant silicon board.

[0017] Further specified, the coating thickness is 12 μm.

[0018] Further specifying, in step 3, the gradient temperature curing is first maintained at 80°C for 6 hours, and then the temperature is increased to 300°C at a rate of 30°C every half hour, and maintained for 1 hour.

[0019] Another object of the present invention is to provide a hydrogen-bonded crosslinked polyimide energy storage material prepared by any of the above methods.

[0020] Furthermore, the invention also provides applications of hydrogen-bonded crosslinked polyimide energy storage materials prepared by any of the methods described above, including their application in the preparation of aerospace insulating films or inter-turn insulation materials for motors. Compared with the prior art, the present invention has the following beneficial effects: Introducing nanomaterials with high dielectric constants into a polymer matrix to form polymer composites typically increases the dielectric constant at the cost of reduced breakdown field strength and mechanical properties. This invention, however, employs an all-organic system. By blending two polyamic acid precursors containing hydroxyl and carbonyl groups, a physical cross-linking network is constructed using hydrogen bonds formed between intermolecular hydroxyl and carbonyl groups. This significantly improves the structural density and mechanical toughness of the film without introducing chemical cross-linking agents. Simultaneously, the trifluoromethyl functional groups introduced into the polymer monomers create a locally electronegative microenvironment within the material, forming shallow-level charge traps that effectively capture charge carriers and suppress space charge accumulation, thereby synergistically enhancing the breakdown field strength. The all-organic system avoids interfacial defects caused by inorganic fillers, ensuring material uniformity and uniform electric field distribution. Combined with the gradient curing process's guarantee of film quality, this invention ultimately achieves a method for preparing polyimide composites with excellent insulation properties, dielectric characteristics, thermal stability, and mechanical properties.

[0021] (2) The hydrogen-bonded crosslinked polyimide energy storage material prepared by the method of the present invention has excellent dielectric and insulation properties and reduced loss. It can be widely used in advanced fields such as electrical, electronic and new energy vehicles, and in fields such as high-frequency electronic packaging, high-temperature energy storage capacitors, flexible transparent electronic devices and aerospace special engineering.

[0022] For a deeper understanding of the features and technical content of this invention, please refer to the accompanying detailed description and drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to limit the scope of the invention. Attached Figure Description

[0023] Figure 1 Atomic force microscopy (AFM) images of hydrogen-bonded crosslinked polyimide energy storage materials: (a) AFM image of the PI matrix, (b) AFM image of OH-PI, and (c) AFM image of OH-PI / PI. Figure 2 Scanning electron microscope images of hydrogen-bonded cross-linked polyimide energy storage materials; Figure 3 FTIR spectrum of hydrogen-bonded crosslinked polyimide energy storage material; Figure 4 The relative permittivity spectrum of hydrogen-bonded crosslinked polyimide energy storage material; Figure 5 Dielectric loss spectrum of hydrogen-bonded cross-linked polyimide energy storage material; Figure 6 The breakdown field strength spectra of hydrogen-bonded cross-linked polyimide energy storage materials are shown in (a) at room temperature and (b) at 150 °C. Figure 7DSC spectrum of hydrogen-bonded crosslinked polyimide energy storage material; Figure 8 The discharge energy density and charge-discharge efficiency of hydrogen-bonded cross-linked polyimide energy storage materials are shown in the graph. Figure 9 UV-Vis spectra of hydrogen-bonded crosslinked polyimide energy storage materials; Figure 10 The DMA spectrum of the hydrogen-bonded cross-linked polyimide energy storage material is shown in (a), which represents the energy storage modulus of the hydrogen-bonded cross-linked polyimide, and (b), which represents the loss factor of the hydrogen-bonded cross-linked polyimide. Detailed Implementation

[0024] The present invention will be described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but should not be considered as limiting the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0025] Example 1: A method for preparing a hydrogen-bonded crosslinked polyimide energy storage material, comprising the following steps: Step 1: 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] was placed in a vacuum drying oven and dried at 120°C for 12 h to remove moisture, then sealed and stored. In an anhydrous and oxygen-free environment, pyromellitic dianhydride was dissolved in acetic anhydride at 80°C using ultrasonic assisted dissolution at 40 kHz frequency and 300 W power, with mechanical stirring at 500 r / min for 30 min. After standing, at least a small amount of crystals precipitated, followed by crystallization in an ice-water bath. The crystals were collected by filtration, vacuum dried, and then purified pyromellitic dianhydride was obtained and sealed for storage.

[0026] In a three-necked flask, 4.92 g of pretreated 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] was dissolved in 20 mL of N,N-dimethylacetamide. The mixture was sonicated at 40 kHz and 300 W, and stirred at 500 r / min until homogeneous. Then, 2.18 mL of pretreated pyromellitic dianhydride was added in 6 portions, with an interval of 2 h between each addition. After the pyromellitic dianhydride was completely dissolved, a homogeneous and viscous gel solution A was obtained. Step 2: 2,2-bis(4-hydroxy-3-aminophenyl)propane was dissolved in anhydrous ethanol at 60°C using ultrasonic-assisted dissolution at 40kHz and 300W, with mechanical stirring at 500 rpm for 30 min until complete dissolution. The solution was then rapidly crystallized in an ice-water bath. The crystals were collected by filtration and vacuum dried to obtain purified 2,2-bis(4-hydroxy-3-aminophenyl)propane, which was then sealed and stored. p-Phenylidene-bisphenyltriptate dianhydride was placed in a vacuum drying oven and dried at 150°C for 8 h to remove residual solvent and moisture. It was then cooled to room temperature in a desiccator and sealed for later use.

[0027] In a three-necked flask, 2.53 g of pretreated hydroxyl-containing 2,2-bis(4-hydroxy-3-aminophenyl)propane was dissolved in 15 mL of N,N-dimethylacetamide, and then 3.58 g of pretreated p-phenylene-bisphenyltriptyl dianhydride was added. After the p-phenylene-bisphenyltriptyl dianhydride was completely dissolved, a homogeneous and viscous gel solution B was obtained. Step 3: Add 9g of adhesive solution A and 1g of adhesive solution B to a three-necked flask, stir thoroughly at 500r / min at room temperature to ensure complete reaction, evacuate to -0.095MPa, and then coat it onto a pretreated substrate with a coating thickness of 12μm. First, keep it at 80℃ for 6 hours, then increase the temperature to 300℃ at a rate of 30℃ every half hour, and keep it at that temperature for 1 hour. Peel off the film on the substrate to obtain the hydrogen-bonded crosslinked polyimide energy storage material.

[0028] Example 2: This example differs from Example 1 in that adhesive A accounts for 80 wt% of the polyimide energy storage material, and adhesive B accounts for 20 wt% of the polyimide energy storage material. All other steps and parameters are the same as in Example 1.

[0029] Example 3: This example differs from Example 1 in that adhesive A accounts for 70 wt% of the polyimide energy storage material, and adhesive B accounts for 30 wt% of the polyimide energy storage material. All other steps and parameters are the same as in Example 1.

[0030] Example 4: This example differs from Example 1 in that adhesive A accounts for 60 wt% of the polyimide energy storage material, and adhesive B accounts for 40 wt% of the polyimide energy storage material. All other steps and parameters are the same as in Example 1.

[0031] Example 5: This example differs from Example 1 in that adhesive A accounts for 50 wt% of the polyimide energy storage material, and adhesive B accounts for 50 wt% of the polyimide energy storage material. All other steps and parameters are the same as in Example 1.

[0032] Example 6: This example differs from Example 1 in that adhesive A accounts for 40 wt% of the polyimide energy storage material, and adhesive B accounts for 60 wt% of the polyimide energy storage material. All other steps and parameters are the same as in Example 1.

[0033] Example 7: This example differs from Example 1 in that adhesive A accounts for 30 wt% of the polyimide energy storage material, and adhesive B accounts for 70 wt% of the polyimide energy storage material. All other steps and parameters are the same as in Example 1.

[0034] Example 8: This example differs from Example 1 in that adhesive A accounts for 20 wt% of the polyimide energy storage material, and adhesive B accounts for 80 wt% of the polyimide energy storage material. All other steps and parameters are the same as in Example 1.

[0035] Example 9: This example differs from Example 1 in that adhesive A accounts for 10 wt% of the polyimide energy storage material, and adhesive B accounts for 90 wt% of the polyimide energy storage material. All other steps and parameters are the same as in Example 1.

[0036] Figure 1 The image shows an atomic force microscopy (AFM) image of a hydrogen-bonded cross-linked polyimide energy storage material according to the present invention. No obvious pores or interfacial separation defects were observed on the surface of the hydrogen-bonded cross-linked polyimide energy storage material prepared by the present invention. This further confirms that the composite film has a continuous and uniform film structure, with tight interfacial bonding between components and no obvious phase separation or filler agglomeration. This highly uniform microstructure effectively avoids the defects caused by poor interfacial compatibility and uneven dispersion in traditional inorganic filler / polyimide composite systems. It is a key guarantee for achieving high-quality blended insulating films and lays the structural foundation for their excellent electrical insulation properties.

[0037] Figure 2 The image shows a scanning electron microscope (SEM) image of a hydrogen-bonded cross-linked polyimide energy storage material according to the present invention. As shown in the figure, the cross-section of the composite material is smooth and free of pores and cracks. The brittle fracture cross-section of the composite film exhibits a continuous and dense film structure. The interface of the composite film is very uniform, with no obvious pores or defects found. This uniformity is a key indicator for the preparation of high-quality blended films, avoiding the defect problems caused by interfacial mismatch in traditional inorganic fillers.

[0038] Figure 3 The infrared spectrum of a hydrogen-bonded crosslinked polyimide energy storage material according to the present invention is shown in the figure. It can be seen from the figure that the films all have a wavelength of 1780 cm⁻¹. -1 1492cm -1 1054cm -1 862cm -1 and 725cm -1The peak at 1054 cm⁻¹ clearly shows the characteristic peak of polyimide. -1 The peak observed at 1780 cm⁻¹ corresponds to the stretching vibration of the COC bond. -1 The peak observed at 1725 cm⁻¹ corresponds to the asymmetric vibration of the C=O bond, while the peak observed at 1725 cm⁻¹ corresponds to the asymmetric vibration of the C=O bond. -1 The peak observed at 1492 cm corresponds to the symmetrical vibration of the C=O bond. -1 The reflection peak at 1054 cm⁻¹ indicates the presence of the benzene ring, proving that the PI main chain structure remains stable after blending; -1 The intensity of the CO stretching vibration peak at 3200–3500 cm⁻¹ gradually increases with increasing OH-PI content, directly confirming the successful introduction and uniform distribution of the hydroxyl functional group; (Note: The last part, "3200–3500 cm⁻¹", appears to be an unrelated fragment and is omitted from the translation.) -1 The broad OH stretching vibration peak at the point further proves that hydrogen bond interactions have formed between molecules, providing direct spectroscopic evidence for the construction of hydrogen bond cross-linking networks.

[0039] Figure 4 The dielectric constant spectrum of a hydrogen-bonded crosslinked polyimide energy storage material of the present invention is shown in Figure (a). As the OH-PI content increases, the dielectric constant of the OH-PI / PI composite film first decreases and then increases across the entire test frequency range. This phenomenon is mainly attributed to the regulatory effect of hydroxyl groups on the molecular structure and polarization behavior of polyimide. Low OH-PI content achieves tight packing of molecular chains through hydrogen bonding, effectively reducing the free volume inside the material and restricting the free orientation movement of polar groups in an alternating electric field. It also suppresses interfacial polarization and space charge accumulation, ultimately significantly weakening the polarization contribution and reducing the dielectric constant. In addition, the introduction of trifluorine groups into the hydrogen-bonded crosslinked polyimide system can synergistically reduce the dielectric constant of the material through multiple mechanisms such as suppressing polar group orientation polarization, increasing free volume, and stabilizing charge carriers. When the OH-PI content exceeds 70 wt%, the content of hydroxyl polar groups in the system increases significantly. The increased polar group density significantly enhances the dipole orientation polarization and space charge polarization effects, and the polarization contribution becomes dominant. Ultimately, the dielectric constant of the composite film increases with further increase in the OH-PI ratio.

[0040] Figure 5This diagram shows the dielectric loss spectrum of a hydrogen-bonded crosslinked polyimide energy storage material according to the present invention. The graph shows that all OH-PI / PI composite films exhibit low dielectric losses, with the lowest value achieved when 30 wt% hydroxyl-containing polyimide is added. In the low-frequency region (20–103 Hz), the dielectric loss variation is small, and the dielectric constant of the composite film is significantly lower than that of pure PI. This may be due to dipole orientation relaxation and interfacial polarization. The introduction of hydroxyl polyimide suppresses the orientation movement of polar chain segments in the alternating electric field, thereby reducing orientation polarization relaxation loss. Simultaneously, the densification of molecular chain packing and the inhibitory effect of trifluorine groups on charge migration weaken the space charge polarization effect, further reducing dielectric loss.

[0041] Figure 6 This figure shows the breakdown field strength spectrum of a hydrogen-bonded crosslinked polyimide energy storage material according to the present invention. The figure presents the Weibull distribution curves of DC breakdown strength for pure PI and OH-PI / PI composite films with different contents at 25 °C and 150 °C. It can be seen that the breakdown behavior of each sample conforms well to the Weibull statistical law at the three test temperatures, indicating that the test results have good repeatability and statistical reliability. With increasing test temperature, the characteristic breakdown strength of all samples shows a gradual decreasing trend. This is because at high temperatures, the thermal motion of the polyimide molecular chains is enhanced, the free volume increases, and the carrier concentration and migration ability are improved, thereby accelerating the charge accumulation and energy dissipation process, making the material more prone to electrical breakdown. However, compared with pure PI and OH-PI, the OH-PI / PI composite film exhibits higher breakdown strength at both temperature conditions, indicating that hydrogen bonding crosslinking has a significant enhancing effect on the electrical insulation performance of polyimide.

[0042] As shown in Figures (a) and (b), the breakdown strength of pure PI at room temperature initially increases and then decreases with increasing OH-PI content. When the mass fraction of OH-PI is 70 wt%, the breakdown strength of the OH-PI / PI composite film reaches a maximum of 658.8 kV / mm, which is significantly increased by 22.7% compared with pure PI (512.14 kV / mm). This shows that an appropriate amount of OH-PI mixing has a significant promoting effect on room temperature breakdown performance. When the temperature rises to 150℃, the characteristic breakdown strength of pure PI decreases to about 420.6 kV / mm, while the breakdown strength of the 70 wt% OH-PI / PI composite film can still be maintained at about 614.8 kV / mm, an increase of about 22.9%. This indicates that the hydrogen-bonded cross-linking structure can still effectively suppress the decay of breakdown performance under high temperature conditions and still have good electrical insulation stability under high temperature and high electric field environments. At the same time, by introducing trifluorine groups and shallow energy level traps, the carrier migration and electron acceleration processes are restricted, thus delaying the occurrence of thermo-electric synergistic breakdown. The aforementioned densification effect, combined with the reduction in polarization and conductivity loss, enables the OH-PI / PI composite film to exhibit excellent breakdown strength and stability under different temperature conditions. This demonstrates that the hydrogen-bonded crosslinked polyimide molecular structure design strategy is an effective approach that balances high-temperature adaptability and high electrical insulation performance.

[0043] Figure 7 This image shows the DSC spectrum of a hydrogen-bonded crosslinked polyimide energy storage material according to the present invention. The differential scanning calorimetry (DSC) spectrum reveals the thermal behavior of the blend film. All three curves show a gentle endothermic peak around 375 °C, corresponding to the glass transition temperature (Tg) of the material. The Tg of pure PI containing trifluoro groups is 375.4 °C, reflecting the excellent intrinsic heat resistance brought about by the steric hindrance of the aromatic backbone and trifluoromethyl groups. The Tg of OH-PI is 375.95 °C, slightly higher than that of pure PI, due to the enhanced intermolecular forces caused by the hydrogen bonds formed between hydroxyl groups. The Tg of the 70 wt% OH-PI / PI blend sample further increases to 376.12 °C. This is because weak hydrogen bonds are formed between the trifluoromethyl groups and hydroxyl groups, which synergistically construct a denser crosslinked network, significantly restricting the movement of molecular chain segments and slightly improving the heat resistance of the blend system. The low-temperature curve is stable with no obvious phase transition peak, proving that the material has excellent thermal stability below 350 ℃. The overall results show that the heat resistance is optimized by the synergistic effect of multiple hydrogen bonds after blending PI containing trifluorine groups with OH-PI, providing a reliable guarantee for high-temperature electronic packaging applications.

[0044] Figure 8This diagram illustrates the charge / discharge efficiency and discharge energy density of a hydrogen-bonded crosslinked polyimide energy storage material according to the present invention. The diagram shows the evolution of charge / discharge efficiency and discharge energy density with electric field strength for pure PI containing trifluorine groups and blends with different OH-PI contents at 150°C. Regarding charge / discharge efficiency, all samples maintained a high efficiency level close to 100% in the low electric field region (0–200 kV / cm), gradually decreasing as the electric field increased to 600 kV / cm. The decrease was more significant for pure PI and samples with low OH-PI content (10–30 wt%), with efficiency dropping to approximately 60% at high electric fields. Samples with high OH-PI content (70–90 wt%) exhibited superior efficiency stability; 70 wt% OH-PI / PI still maintained approximately 80% charge / discharge efficiency at 600 kV / cm. Regarding discharge energy density, all samples showed an approximately linear increase with increasing electric field. The energy density was low (<3 J / cm³) in the low electric field region (0–200 kV / cm) and rapidly increased in the high electric field region (400–600 kV / cm). Pure PI and samples with low OH-PI content showed significant energy density decay under high electric fields, while 70 wt% OH-PI / PI maintained a discharge energy density of approximately 8 J / cm³ at 600 kV / cm. This performance optimization can be attributed to the multiple hydrogen bond network formed between hydroxyl and trifluorine groups. Under the synergistic effect of high temperature and high electric field, this network effectively suppressed molecular chain degradation, charge leakage, and interfacial side reactions, improving energy density stability under high electric fields and reducing irreversible energy loss. This achieved a synergistic enhancement of charge / discharge efficiency and discharge energy density, providing an important basis for the design of high-temperature energy storage media.

[0045] Figure 9The UV-Vis spectrum of a hydrogen-bonded crosslinked polyimide energy storage material of the present invention is shown in Figure (a). Figure (a) shows the light absorption behavior of pure PI, OH-PI, and 70 wt% OH-PI / PI blend films containing trifluorine groups in the range of 200–1200 nm: the strong absorption peak in the ultraviolet region (200–400 nm) originates from the π–π electronic transition of the aromatic ring in the polyimide backbone. The absorption edges of pure PI and 70 wt% OH-PI / PI are both located at about 400 nm, while the absorption edge of OH-PI is significantly red-shifted to 600 nm. This is due to the hydroxyl-induced intermolecular hydrogen bonding enhancing the backbone conjugation effect and reducing the electronic transition band gap. In the visible light region (400–800 nm), the absorption of pure PI and 70 wt% OH-PI / PI quickly saturates, exhibiting good ultraviolet cutoff and visible light transmission potential, while OH-PI… Due to the enhanced conjugation effect, the light transmittance decreased significantly while maintaining continuous absorption in the visible light region. The absorption curve of the 70wt% OH-PI / PI blend sample highly overlapped with that of pure PI, without a significant red shift. This indicates that after appropriate blending, the multiple hydrogen bond network formed by the hydroxyl and trifluorine groups effectively suppressed the excessive enhancement of main chain conjugation. While retaining the excellent UV cutoff performance of pure PI, the functional properties of hydroxyl groups were introduced, providing a reliable basis for the preparation of polyimide encapsulation materials with both high transmittance and functionality.

[0046] Figure (b) shows the direct bandgap curve of (αhv)1 / m=B(hν-Eg) plotted based on UV-Vis absorption spectroscopy. It is used to calculate the optical bandgap of pure PI, OH-PI and 70 wt% OH-PI / PI blend films containing trifluorine groups. The three curves are extrapolated to obtain Eg values ​​of: pure PI 3.353 eV, 70 wt% OH-PI / PI 3.214 eV and OH-PI 2.402 eV, respectively. The band gap of pure PI conforms to the typical range of aromatic polyimides, due to the steric hindrance effect of its rigid aromatic backbone and trifluorine groups, effectively maintaining a wide electronic transition band gap. The band gap of the 70 wt% OH-PI / PI blend sample narrows slightly, attributed to the weak hydrogen bonds and hydrogen crosslinking formed between the hydroxyl and trifluorine groups, which slightly enhances the backbone conjugation effect and reduces the energy required for electronic transitions. The OH-PI band gap decreases significantly to 2.402 eV, due to the significant enhancement of backbone conjugation and band gap reduction caused by the introduction of hydroxyl groups. Overall, the results show that appropriate blending of OH-PI can introduce functional properties while maintaining a wide band gap, while pure OH-PI exhibits a significant band gap narrowing due to excessive enhancement of the conjugation effect, providing a spectroscopic basis for the structural design of polyimide optoelectronic functional materials.

[0047] Figure 10The changes in storage modulus and loss factor of pure PI and OH-PI / PI composite films with different OH-PI contents during heating are shown in the figure. As can be seen from the figure, within the test temperature range, the storage modulus of all samples gradually decreases with increasing temperature. This is because the increased temperature leads to enhanced thermal motion of the molecular chain segments, causing the material to gradually transition from a glassy state to a highly elastic state, thus gradually reducing rigidity. Between 50 and 300 °C, the storage modulus of the OH-PI / PI composite film is significantly higher than that of pure PI, indicating that hydrogen-bonded crosslinking modification significantly improves the resistance of the polyimide molecular chains to external deformation. Among them, the 70 wt% OH-PI / PI sample exhibits the highest storage modulus in the low-temperature region, with an initial storage modulus value significantly higher than that of pure PI. This suggests that hydroxyl and trifluorine groups enhance the interaction between molecular chains by regulating the molecular chain length distribution and improving the ordered arrangement of chain segments, resulting in a higher storage capacity of the system under dynamic loading. Between 250 and 350 °C, the storage modulus of all samples decreased significantly, indicating that the materials gradually approached the glass transition range and underwent large-scale chain segment movement. Figure (b) shows the loss factor (tanσ) of pure PI and OH-PI / PI composite films as a function of temperature. It can be seen that all samples have smaller tanσ values ​​in the low-temperature region, indicating that the materials mainly exhibit elastic response with low internal friction. As the temperature increases, the tanσ curve shows a significant peak in the 250–350 °C range, corresponding to the glass transition temperature (Tg) of the polyimide system. Compared with pure PI, the peak value of the loss factor of the OH-PI / PI composite film is slightly lower overall and the peak shape tends to be flatter, indicating that hydrogen bond crosslinking modification reduces the energy dissipation of molecular chain segments during the glass transition process. This phenomenon shows that this structure makes the molecular chain arrangement more regular, reduces friction and disordered movement between chain segments, and thus suppresses internal friction behavior.

[0048] The specific embodiments of the present invention have been described in detail above. It should be noted that the present invention is not limited to the specific embodiments described above. Various modifications or alterations can be made by those skilled in the art without departing from the scope of protection defined by the claims, and all such modifications or alterations fall within the scope of the present invention.

Claims

1. A method for preparing a hydrogen-bonded crosslinked polyimide energy storage material, characterized in that, Includes the following steps: Step 1: Add 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] to N,N-dimethylacetamide, and ultrasonically stir until homogeneous. Add pyromellitic dianhydride in batches and stir until viscous to obtain adhesive solution A. Step 2: Add 2,2-bis(4-hydroxy-3-aminophenyl)propane to N,N-dimethylacetamide, and ultrasonically stir until homogeneous. Add p-phenylene-bisphenyltriterpenoid dianhydride and stir until viscous to obtain adhesive B. Step 3: Then add adhesive B to adhesive A, stir thoroughly at room temperature to allow it to react completely, apply vacuum, and then coat it onto the pretreated substrate. Curing is done by gradient heating. The film on the substrate is then peeled off to obtain the energy storage material.

2. The method according to claim 1, characterized in that the pyromellitic anhydride is added in 6 portions, with an interval of 2 hours between each portion.

3. The method according to claim 1, characterized in that, in step 1, the molar ratio of 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] to pyromellitic dianhydride is 1:1.

01.

4. The method according to claim 1, characterized in that, In step 2, the molar ratio of 2,2-bis(4-hydroxy-3-aminophenyl)propane to p-phenylene-bisphenyltriester dianhydride is 1.01:

1.

5. The method according to claim 1, characterized in that... The hydroxy polyimide obtained by reacting 2,2-bis(4-hydroxy-3-aminophenyl)propane and p-phenylene-bisphenyltriester dianhydride accounts for 10 wt% to 90 wt% of the energy storage material.

6. The method according to claim 1, characterized in that... Substrate pretreatment method: First, soak the substrate in N,N-dimethylacetamide solution for 2-3 hours, then wash it with anhydrous ethanol 3-5 times, and finally dry it at 40 ℃-60 ℃ for 8 hours to complete the process; the substrate is a high-temperature resistant silicon board.

7. The method according to claim 1, characterized in that... The coating thickness is 12μm.

8. The method according to claim 1, characterized in that... For curing, first keep at 80℃ for 6 hours, then increase the temperature to 300℃ at a rate of 30℃ every half hour, and keep at that temperature for 1 hour.

9. A hydrogen-bonded crosslinked polyimide energy storage material prepared by the method of any one of claims 1-8.

10. An application of a hydrogen-bonded crosslinked polyimide energy storage material prepared by the method of any one of claims 1-8, characterized in that, Applications in the preparation of aerospace insulating films or motor turn-to-turn insulating materials.

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