Graft-modified polyethylene dielectric material, method of making and use

Abstract

This invention relates to the field of energy storage materials technology, specifically disclosing a graft-modified polyethylene dielectric material, its preparation method, and its applications. The material is prepared by chemically grafting 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV) into the polyethylene backbone. The pyridine rings in the SPV side groups, through their conjugated electron effect, introduce localized energy level structures into the nonpolar polyethylene matrix, forming distributed deep-trap charge-capturing regions. This significantly suppresses the migration and collisional ionization of charge carriers under high electric fields, effectively improving the material's breakdown field strength and high-field service stability. Simultaneously, the grafting modification method does not disrupt the regular arrangement of the polyethylene backbone, thus maintaining the inherent low dielectric loss, good flexibility, and processing performance of polyethylene. It is particularly suitable for the preparation of high-energy-density film capacitors and has broad application prospects in electrical equipment operating under extreme conditions such as new energy vehicles, aerospace, and smart grids.

Description

Technical Field

[0001] This invention relates to the field of energy storage materials technology, and in particular to a grafted modified polyethylene dielectric material, its preparation method, and its application. Background Technology

[0002] With the rapid development of high-end industries such as new energy vehicles, aerospace, flexible electronics, and smart grids, the demand for high-performance energy storage devices is increasing. Polymer films, due to their lightweight, flexibility, ease of processing, and tunable dielectric properties, have become a key basic material for components such as thin-film capacitors, flexible sensors, and insulating encapsulations.

[0003] In the field of thin-film capacitors, the overall performance of polymer dielectric films directly determines the energy storage and pulse power output capabilities of the devices. Currently, biaxially oriented polypropylene (BOPP) film is the most widely used commercial dielectric material, exhibiting high breakdown strength and low dielectric loss at room temperature, which meets the requirements of low-voltage civilian applications. However, in high-temperature, high-electric-field operating environments such as aerospace engine nacelles and new energy vehicle motor control systems, the maximum operating temperature of BOPP film is only 105°C. When the ambient temperature exceeds this threshold, the carrier mobility inside the material increases significantly, the leakage current density rises sharply, leading to increased conduction losses and a significant decrease in capacitor charging and discharging efficiency and energy density, making it difficult to meet the actual needs of high-power, high-reliability systems.

[0004] Polyethylene, as a non-polar polyolefin material, has molecular chains mainly composed of carbon-carbon single bonds and carbon-hydrogen bonds, exhibiting characteristics such as low dielectric loss and stable insulation performance over a wide frequency range. Compared to BOPP, polyethylene has certain advantages in medium- and high-frequency electrical applications, and its raw material sources are widely available and its costs are lower, which is conducive to large-scale production. However, polyethylene dielectric films have relatively low breakdown field strength and insufficient resistance to strong electric fields, resulting in a narrow range of electric fields that allow for safe operation, thus limiting its application in high-energy-density dielectric energy storage.

[0005] To improve the breakdown field strength of polyethylene films and broaden their safe service electric field range, existing technologies often employ methods such as cross-linking modification, high density, or the introduction of high-dielectric inorganic nanofillers. However, cross-linking modification or the introduction of inorganic fillers can disrupt the regular arrangement of polyethylene molecular chains to some extent, leading to a decrease in the film's flexibility and processability.

[0006] Therefore, how to effectively improve the breakdown field strength of polyethylene while maintaining its original advantages such as low loss and easy processing remains a problem to be solved in this field. Summary of the Invention

[0007] To address the problems of insufficient breakdown field strength and weak resistance to strong electric fields in existing polyethylene films, and the tendency of existing modification methods to sacrifice flexibility and processability, this invention provides a grafted modified polyethylene dielectric material, its preparation method, and its applications. This invention introduces zwitterionic groups with specific structures into the polyethylene backbone, utilizing the electric field modulation effect and dipole polarization suppression effect of the grafted side groups. While essentially maintaining the original low dielectric loss and good flexibility of polyethylene, it significantly improves the material's breakdown field strength and high electric field tolerance. The grafted modified polyethylene dielectric material provided by this invention possesses both excellent dielectric stability and machinability, and has broad application prospects in the field of film capacitors.

[0008] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows: In a first aspect, the present invention provides a grafted modified polyethylene dielectric material comprising a polyethylene derivative represented by formula (I);

[0009] Equation (Ⅰ).

[0010] Polyethylene is a type of nonpolar polyolefin material. Although it has low dielectric loss, its molecular chain has no polar groups, resulting in weak charge transport regulation ability. Furthermore, under high electric fields, the breakdown field strength is easily reduced due to the migration of free charges.

[0011] Compared to existing technologies, the grafted modified polyethylene dielectric material provided by this invention introduces 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV) into the polyethylene backbone via chemical grafting, constructing a zwitterionic modified polyolefin dielectric material that combines high breakdown field strength and excellent mechanical flexibility. The SPV molecule contains both pyridinium cations and sulfonate anions, and its molecular orbital energy levels differ significantly from those of the polyethylene matrix. When the SPV side groups are covalently grafted onto the polyethylene backbone, the pyridine rings, through their conjugated electron effect, introduce localized energy level structures into the nonpolar polyethylene matrix, forming distributed charge trap regions. These trap regions have deeper trap depths and higher trap densities, effectively confining injected space charges under high electric field conditions. Specifically, the electron cloud distribution of the pyridine rings can construct energy barriers higher than the intrinsic energy levels of polyethylene between molecular chains. Charge carriers, lacking sufficient energy to overcome these barriers, are confined within the trap regions, thus significantly suppressing long-range migration and collisional ionization of free charges.

[0012] By utilizing the energy level difference and charge scattering effect between the polyethylene matrix and the pyridine rings in the SPV side chain, this invention effectively reduces the leakage current density and conduction loss of polyethylene materials under high electric field conditions, ultimately significantly improving the breakdown field strength and high electric field service reliability of polyethylene dielectric materials, while maintaining the inherent low dielectric loss and good mechanical flexibility of polyethylene, laying a material foundation for the practical application of high-end dielectric films.

[0013] Furthermore, the method for preparing the polyethylene derivative includes the following steps: S1, polyethylene is added to an alkaline alcohol solution and heated for pretreatment to obtain alkaline-treated polyethylene; S2, under an inert atmosphere, alkali-treated polyethylene is dissolved in an organic solvent, and a photo-initiated free radical reaction is carried out under ultraviolet light to obtain a free radicalized polyethylene solution; 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV) and an initiator are added to the free radicalized polyethylene solution, and a graft polymerization reaction is carried out under an inert atmosphere to obtain the polyethylene derivative. The specific reaction process is as follows:

[0014] The method for preparing polyethylene derivatives provided by this invention first involves pretreatment under alkaline conditions to remove surface impurities and enhance surface reactivity. Then, a photoinitiated free radical reaction is performed under ultraviolet light, generating free radicals on the polyethylene main chain to form a free radical-containing polyethylene intermediate. This significantly enhances the reactivity of the polyethylene main chain, providing abundant and controllable reaction sites for subsequent grafting of SPV monomers. Subsequently, the free radicals in the polyethylene intermediate copolymerize with the double bonds of the SPV monomers in the presence of an initiator, achieving efficient grafting of SPV side groups onto the polyethylene main chain. This process not only significantly improves grafting efficiency and reaction controllability but also effectively increases the grafting density and uniformity of SPV side groups in the polyolefin matrix. This ensures that the modified material retains the excellent flexibility and processing properties of the original polyethylene main chain while significantly improving the breakdown field strength. In summary, the preparation method of this invention has outstanding advantages such as high reactivity, simple process route, controllable product structure, and stable performance, providing a practical and feasible technical path for the large-scale preparation of high-performance graft-modified polyolefin dielectric materials.

[0015] Further, in S1, the alkaline alcohol solution is a mixed solution of potassium hydroxide solution and anhydrous ethanol, wherein the molar ratio of polyethylene, KOH and anhydrous ethanol is 100:71:25 to 100:81:25; and the concentration of the potassium hydroxide solution is 1 mol / L to 5 mol / L.

[0016] The above-mentioned ratio and concentration range allow polyethylene to undergo alkali treatment in a fully swollen state, which can effectively improve the subsequent grafting activity and avoid molecular chain breakage and degradation caused by excessive alkali concentration or excessive reaction. Thus, while ensuring pretreatment efficiency, the structural integrity and mechanical properties of the polyethylene main chain are maintained.

[0017] Furthermore, in S1, the pretreatment temperature is 40℃~70℃, and the time is 15min~20min.

[0018] Within the aforementioned temperature and time range, polyethylene can undergo controllable pretreatment, resulting in polyethylene intermediates that possess both high reactivity and good main chain structure retention.

[0019] Specifically, after the reaction in S1 is completed, a post-processing procedure is also included: the reaction liquid is separated into solid and liquid phases, then washed with deionized water, dried, and alkali-treated polyethylene is obtained.

[0020] Furthermore, in S2, the wavelength of the photo-initiated free radical reaction is 240nm~260nm, and the illumination time is 8min~12min.

[0021] Furthermore, in S2, the mass-to-volume ratio of the alkali-treated polyethylene to the organic solvent is 1g:20mL to 1g:30mL.

[0022] Further, in S2, the molar ratio of the alkali-treated polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine and the initiator is 10000:100:1 to 2500:100:1.

[0023] The above-mentioned ratio range ensures that the SPV monomer undergoes graft copolymerization with the free radicals on the polyethylene main chain at an appropriate rate under the action of the initiator, and effectively suppresses the homopolymerization side reaction of the monomer itself and the tendency of crosslinking between polymer chains. This allows the SPV side groups to achieve uniform and controllable chemical grafting on the polyethylene main chain, thereby balancing the improvement of the high breakdown field strength of the modified material with the maintenance of the main chain flexibility.

[0024] Furthermore, in S2, the organic solvent is 1,2,4-trichlorobenzene (TCB).

[0025] Furthermore, in S2, the initiator is azobisisobutyronitrile.

[0026] Furthermore, in S2, the temperature of the graft polymerization reaction is 60℃~100℃, and the time is 2h~6h.

[0027] The above-mentioned reaction temperature and time can ensure that the grafting reaction proceeds at a reasonable rate, and can also avoid side reactions such as monomer homopolymerization, polymer crosslinking, or thermal degradation of the main chain.

[0028] Specifically, after the reaction in S2 is completed, the following post-processing steps are also included: the reaction solution is added to deionized water for precipitation, solid-liquid separation is performed, the precipitate is dissolved in N-methylpyrrolidone, and then deionized water is added again for precipitation. This process is repeated 2 to 3 times for purification. The purified product is then placed in a vacuum drying oven for drying to obtain the polyethylene derivative.

[0029] Secondly, the present invention also provides the application of the above-mentioned grafted modified polyethylene dielectric material in the preparation of dielectric films.

[0030] Thirdly, the present invention also provides a dielectric film prepared from the grafted modified polyethylene dielectric material described in any of the preceding claims.

[0031] The grafted modified polyethylene dielectric material of this invention can be prepared into dielectric films through conventional processes such as melt extrusion casting, solution casting, or hot pressing. The thickness of the resulting film can be controlled at the micrometer to sub-millimeter level according to actual application requirements. In this invention, the SPV is chemically grafted onto the polyethylene molecular chain, and no phase separation occurs during film formation. The dielectric film prepared from it has a smooth and uniform surface and exhibits excellent film-forming processing performance.

[0032] Furthermore, the thickness of the dielectric film is 20μm~30μm.

[0033] Fourthly, the present invention provides a method for preparing a dielectric thin film, comprising the following steps: The grafted modified polyethylene dielectric material is dissolved in an organic solvent, cast into a film, and then the film is subjected to thermal annealing and peeled off from the substrate to obtain a dielectric film.

[0034] As a specific embodiment of the present invention, the method for preparing the dielectric thin film specifically includes the following steps: The grafted modified polyethylene dielectric material is added to an organic solvent, heated and stirred until homogeneous, then cast onto a glass plate, vacuum dried, and then subjected to heat annealing treatment to peel off from the glass plate to obtain a dielectric film.

[0035] Furthermore, the organic solvent is N,N-dimethylformamide.

[0036] Furthermore, the mass-to-volume ratio of the grafted modified polyethylene dielectric material to the organic solvent is 1g:10mL to 1g:50mL.

[0037] Furthermore, the temperature for heating and stirring is 40℃~70℃, the temperature for vacuum drying is 40℃~70℃, and the drying time is 8h~14h.

[0038] Furthermore, the temperature of the heat annealing treatment is 100℃~150℃, and the time is 2h~5h.

[0039] Specifically, after the heat annealing process, the temperature is reduced to room temperature at a rate of 1℃ / min to 5℃ / min.

[0040] The above-mentioned thermal annealing process can effectively eliminate the internal stress and micro-pore defects caused by the difference in solvent evaporation rate during solution casting, making the prepared dielectric film smoother and denser.

[0041] Fifthly, the present invention provides an energy storage capacitor comprising the dielectric thin film described above.

[0042] The energy storage capacitor of this invention uses the aforementioned grafted modified polyethylene dielectric film as the core dielectric layer, with metal electrodes deposited or bonded to both sides of the film to form a wound or stacked capacitor element. Due to the distributed charge trap structure formed by the introduction of SPV side groups and the high breakdown field strength of this dielectric film, the leakage current of the manufactured capacitor is significantly reduced under high electric field conditions, effectively improving charge / discharge efficiency and energy storage density. Simultaneously, the film's excellent flexibility and thermal stability enable it to withstand the mechanical stress during capacitor winding and allow for long-term stable operation in high-temperature, high-ripple-current scenarios such as new energy vehicle motor drives, photovoltaic inverters, and pulse power supplies, significantly broadening the operating temperature window and safe service electric field range of polyethylene-based dielectric capacitors.

[0043] Compared to traditional BOPP film capacitors, the energy storage capacitor provided by this invention maintains the advantages of low loss and low cost while having higher power density and reliability, making it particularly suitable for energy storage and filtering units in high power density power electronic systems.

[0044] In summary, this invention provides a grafted modified polyethylene dielectric material, which is prepared by chemically grafting 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV) into the polyethylene backbone. The pyridine rings in the SPV side groups, through their conjugated electron effect, introduce localized energy level structures into the nonpolar polyethylene matrix, forming distributed deep-trap charge-capturing regions. This significantly suppresses the migration and collisional ionization of charge carriers under high electric fields, effectively improving the material's breakdown field strength and high-field service stability. Simultaneously, the grafting modification method does not disrupt the regular arrangement of the polyethylene backbone, thus maintaining the inherent low dielectric loss, good flexibility, and processing performance of polyethylene. It is particularly suitable for the fabrication of high-energy-density film capacitors and has broad application prospects in electrical equipment operating under extreme conditions such as new energy vehicles, aerospace, and smart grids. Attached Figure Description

[0045] Figure 1 The following are DSC images of the dielectric thin films prepared in Examples 1-4, Comparative Examples 5 and 7 of this invention; Figure 2 The TG images are of the dielectric thin films prepared in Examples 1-4, Comparative Examples 5 and 7 of this invention. Figure 3 The graph shows the change of dielectric constant as a function of frequency for the dielectric thin films prepared in Examples 1-4, Comparative Examples 5 and 7 of this invention. Figure 4 The graph shows the change of dielectric constant as a function of frequency for the dielectric thin films prepared in Comparative Examples 1-4, Comparative Example 6 and Comparative Example 8 of this invention. Figure 5 The breakdown field strength diagrams are for the dielectric thin films prepared in Examples 1-4 and Comparative Examples 5 and 7 of this invention. Figure 6 The breakdown field strength diagrams are for the dielectric thin films prepared by Comparative Examples 1-4, 6, and 8 of this invention.

[0046] Figure 7 The images show physical images of the dielectric thin films prepared in Comparative Example 8 and Example 3. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0048] This invention provides a grafted modified polyethylene dielectric material, prepared by the following method: S1, polyethylene is added to an alkaline alcohol solution, heated for pretreatment, and after the reaction is completed, it is filtered, washed with deionized water, and dried under vacuum to obtain alkaline-treated polyethylene. S2, under an inert atmosphere, alkali-treated polyethylene is dissolved in an organic solvent, and a photo-initiated free radical reaction is carried out under ultraviolet light to obtain a free radicalized polyethylene solution. 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV) and an initiator are added to the free radicalized polyethylene solution, and a graft polymerization reaction is carried out under an inert atmosphere. After the reaction is complete, deionized water is added to the reaction solution to precipitate the product. The precipitate is redissolved in an organic solvent and then precipitated again with water. This purification process is repeated 2-4 times. Finally, the product is dried under vacuum to obtain the polyethylene derivative. The specific reaction process is as follows:

[0049] The grafted modified polyethylene dielectric material obtained by the above method can be directly used in the preparation of dielectric films. For example, the material can be dissolved in an organic solvent to prepare a solution of a certain concentration, and then cast onto a flat substrate by casting. After heat annealing, the film is peeled off from the substrate to obtain a self-supporting dielectric film.

[0050] To better illustrate the present invention, further examples are provided below.

[0051] Example 1 This embodiment provides a method for preparing a grafted modified polyethylene dielectric material, comprising the following steps: S1. Weigh polyethylene powder, KOH solution and anhydrous ethanol according to a molar ratio of 100:81:25. Add polyethylene powder to KOH solution with a concentration of 2.5 mol / L, add anhydrous ethanol while stirring, then heat to 60℃ and heat for 15 min. Filter, wash with deionized water, and dry in a vacuum drying oven at 60℃ for 12 h to obtain alkali-treated polyethylene powder. S2, alkali-treated polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV), and azobisisobutyronitrile (AIBN) were weighed according to a molar ratio of 10000:100:1; alkali-treated polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) at a ratio of 1 g:25 mL, heated to 120 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 240 nm UV light irradiation for 12 min to obtain free radicals. The basic polyethylene solution was cooled and then 1-(3-thiopropyl)-2-vinylpyridine betaine and an initiator were added. The mixture was then cooled to 80°C under an inert atmosphere and reacted for 4 hours. After the reaction was completed, deionized water was added to the reaction solution to precipitate the product. The product was filtered and the precipitate was dissolved in N-methylpyrrolidone. Then, deionized water was added to precipitate the product again. This process was repeated 3 times. The product was then dried in a vacuum drying oven at 60°C for 12 hours to obtain the grafted modified polyethylene dielectric material, denoted as PE-g-SPV.

[0052] The above-mentioned grafted modified polyethylene dielectric material PE-g-SPV was used to prepare dielectric films: The PE-g-SPV powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:25mL, heated to 60℃ and stirred for 6h, then poured onto a glass plate, placed in a vacuum drying oven at 60℃ for 12h, then heated to 100℃ and kept at that temperature for 5h, and then cooled to room temperature at a rate of 1℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0053] Example 2 This embodiment provides a method for preparing a grafted modified polyethylene dielectric material, comprising the following steps: S1. Weigh polyethylene powder, KOH solution and anhydrous ethanol according to a molar ratio of 100:71:25. Add polyethylene powder to KOH solution with a concentration of 1 mol / L, add anhydrous ethanol while stirring, then heat to 40℃ and heat for 20 min. Filter, wash with deionized water, and dry in a vacuum drying oven at 50℃ for 10 h to obtain alkali-treated polyethylene powder. S2, alkali-treated polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV), and azobisisobutyronitrile (AIBN) were weighed according to a molar ratio of 7500:100:1; alkali-treated polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) at a ratio of 1 g:30 mL, heated to 125 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 245 nm ultraviolet light irradiation for 10 min to obtain free radicals. A polyethylene solution was cooled, and 1-(3-thiopropyl)-2-vinylpyridine betaine and an initiator were added. The mixture was then cooled to 100°C under an inert atmosphere and reacted for 2 hours. After the reaction was complete, deionized water was added to the reaction solution to precipitate the product. The product was filtered, and the precipitate was dissolved in N-methylpyrrolidone. Then, deionized water was added to precipitate the product again. This process was repeated three times. The product was then dried in a vacuum drying oven at 50°C for 10 hours to obtain the grafted modified polyethylene dielectric material, denoted as PE-g-SPV.

[0054] The above-mentioned grafted modified polyethylene dielectric material PE-g-SPV was used to prepare dielectric films: The PE-g-SPV powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:10mL, heated to 50℃ and stirred for 5h, then poured onto a glass plate, placed in a vacuum drying oven at 40℃ for 14h, then heated to 130℃ and held for 3h, and then cooled to room temperature at a rate of 4℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0055] Example 3 This embodiment provides a method for preparing a grafted modified polyethylene dielectric material, comprising the following steps: S1. Weigh polyethylene powder, KOH solution and anhydrous ethanol according to a molar ratio of 4:3:1. Add polyethylene powder to KOH solution with a concentration of 4 mol / L, add anhydrous ethanol while stirring, then heat to 50℃ and heat for 20 min. Filter, wash with deionized water, and dry in a vacuum drying oven at 40℃ for 14 h to obtain alkali-treated polyethylene powder. S2, alkali-treated polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV), and azobisisobutyronitrile (AIBN) were weighed according to a molar ratio of 5000:100:1; alkali-treated polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) at a ratio of 1 g:25 mL, heated to 130 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 254 nm ultraviolet light irradiation for 9 min to obtain free radicals. A polyethylene solution was cooled, and 1-(3-thiopropyl)-2-vinylpyridine betaine and an initiator were added. The mixture was then cooled to 60°C under an inert atmosphere and reacted for 6 hours. After the reaction was complete, deionized water was added to the reaction solution to precipitate the product. The product was filtered, and the precipitate was dissolved in N-methylpyrrolidone. Then, deionized water was added to precipitate the product again. This process was repeated three times. The product was then dried in a vacuum drying oven at 40°C for 14 hours to obtain the grafted modified polyethylene dielectric material, denoted as PE-g-SPV.

[0056] The above-mentioned grafted modified polyethylene dielectric material PE-g-SPV was used to prepare dielectric films: The PE-g-SPV powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:50mL, heated to 70℃ and stirred for 4h, then poured onto a glass plate, placed in a vacuum drying oven at 40℃ for 8h, then heated to 115℃ and held for 3h, and then cooled to room temperature at a rate of 2℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0057] Example 4 This embodiment provides a method for preparing a grafted modified polyethylene dielectric material, comprising the following steps: S1. Weigh polyethylene powder, KOH solution and anhydrous ethanol according to a molar ratio of 100:78:25. Add polyethylene powder to a 5 mol / L KOH solution, add anhydrous ethanol while stirring, then heat to 70℃ and heat for 15 min. Filter, wash with deionized water, and dry in a vacuum drying oven at 70℃ for 8 h to obtain alkali-treated polyethylene powder. S2, alkali-treated polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV), and azobisisobutyronitrile (AIBN) were weighed according to a molar ratio of 2500:100:1; alkali-treated polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) at a ratio of 1 g:20 mL, heated to 140 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 260 nm ultraviolet light irradiation for 8 min to obtain free radicals. A polyethylene solution was cooled, and 1-(3-thiopropyl)-2-vinylpyridine betaine and an initiator were added. The mixture was then heated to 70°C under an inert atmosphere and reacted for 5 hours. After the reaction was complete, deionized water was added to the reaction solution to precipitate the product. The product was filtered, and the precipitate was dissolved in N-methylpyrrolidone. Then, deionized water was added to precipitate the product again. This process was repeated three times. The product was then dried in a vacuum drying oven at 70°C for 8 hours to obtain the grafted modified polyethylene dielectric material, denoted as PE-g-SPV.

[0058] The above-mentioned grafted modified polyethylene dielectric material PE-g-SPV was used to prepare dielectric films: The PE-g-SPV powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:35mL, heated to 40℃ and stirred for 6h, then poured onto a glass plate, placed in a vacuum drying oven at 50℃ for 10h, then heated to 150℃ and held for 2h, and then cooled to room temperature at a rate of 5℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0059] Comparative Example 1 This comparative example provides a dielectric film, which differs from Example 1 only in that the polyethylene powder is not treated with alkali and is directly subjected to the grafting reaction, that is, step S1 is omitted. The rest are exactly the same, and the specific steps are as follows: Polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV), and azobisisobutyronitrile were weighed in a molar ratio of 10000:100:1. Polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) in a ratio of 1 g:25 mL. The solution was heated to 120 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 240 nm UV light for 12 min to obtain a free radicalized polyethylene solution. After cooling, 1-(3-thiopropyl)-2-vinylpyridine betaine and an initiator were added. The reaction was continued under an inert atmosphere, cooled to 80 °C, and carried out for 4 h. After the reaction was completed, deionized water was added to the reaction solution to precipitate the product. The product was filtered and dissolved in N-methylpyrrolidone. Then, deionized water was added to precipitate the product again. This process was repeated 3 times. The product was dried in a vacuum dryer at 60 °C for 12 h to obtain the grafted modified polyethylene dielectric material, denoted as PE-g-SPV.

[0060] The PE-g-SPV powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:25mL, heated to 60℃ and stirred for 6h, then poured onto a glass plate, placed in a vacuum drying oven at 60℃ for 12h, then heated to 100℃ and kept at that temperature for 5h, and then cooled to room temperature at a rate of 1℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0061] Comparative Example 2 This comparative example provides a dielectric film, which differs from Example 2 only in that the polyethylene powder is not treated with alkali and the grafting reaction is carried out directly, that is, step S1 is omitted. The rest are exactly the same, and the specific steps are as follows: Polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV), and azobisisobutyronitrile were weighed in a molar ratio of 7500:100:1. Polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) at a ratio of 1 g:30 mL. The solution was heated to 125 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 245 nm UV light for 10 min to obtain a free radicalized polyethylene solution. After cooling, 1-(3-thiopropyl)-2-vinylpyridine betaine and an initiator were added. The reaction was continued under an inert atmosphere, cooled to 100 °C, and carried out for 2 h. After the reaction was completed, deionized water was added to the reaction solution to precipitate the product. The product was filtered and dissolved in N-methylpyrrolidone. Then, deionized water was added to precipitate the product again. This process was repeated 3 times. The product was dried in a vacuum dryer at 50 °C for 10 h to obtain the grafted modified polyethylene dielectric material, denoted as PE-g-SPV.

[0062] The PE-g-SPV powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:10mL, heated to 50℃ and stirred for 5h, then poured onto a glass plate, placed in a vacuum drying oven at 40℃ for 14h, then heated to 130℃ and held for 3h, and then cooled to room temperature at a rate of 4℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0063] Comparative Example 3 This comparative example provides a dielectric film, which differs from Example 3 only in that the polyethylene powder is not treated with alkali and the grafting reaction is carried out directly, that is, step S1 is omitted. The rest are exactly the same, and the specific steps are as follows: Polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV), and azobisisobutyronitrile were weighed in a molar ratio of 5000:100:1. Polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) at a ratio of 1 g:25 mL. The solution was heated to 130 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 254 nm UV light for 9 min to obtain a free radicalized polyethylene solution. After cooling, 1-(3-thiopropyl)-2-vinylpyridine betaine and an initiator were added. The reaction was continued under an inert atmosphere, cooled to 60 °C, and carried out for 6 h. After the reaction was completed, deionized water was added to the reaction solution to precipitate the product. The product was filtered and dissolved in N-methylpyrrolidone. Then, deionized water was added to precipitate the product again. This process was repeated 3 times. The product was dried in a vacuum dryer at 40 °C for 14 h to obtain the grafted modified polyethylene dielectric material, denoted as PE-g-SPV.

[0064] The PE-g-SPV powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:50mL, heated to 70℃ and stirred for 4h, then poured onto a glass plate, placed in a vacuum drying oven at 40℃ for 8h, then heated to 115℃ and held for 3h, and then cooled to room temperature at a rate of 2℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0065] Comparative Example 4 This comparative example provides a dielectric film, which differs from Example 4 only in that the polyethylene powder is not treated with alkali and the grafting reaction is carried out directly, that is, step S1 is omitted. The rest are exactly the same, and the specific steps are as follows: Polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine (SPV), and azobisisobutyronitrile were weighed in a molar ratio of 2500:100:1. Polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) at a ratio of 1 g:20 mL. The solution was heated to 140 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 260 nm UV light for 8 min to obtain a free radicalized polyethylene solution. After cooling, 1-(3-thiopropyl)-2-vinylpyridine betaine and an initiator were added. The reaction was continued under an inert atmosphere, cooled to 70 °C, and allowed to proceed for 5 h. After the reaction was completed, deionized water was added to the reaction solution to precipitate the product. The product was filtered and dissolved in N-methylpyrrolidone. Then, deionized water was added to precipitate the product again. This process was repeated 3 times. The product was then dried in a vacuum dryer at 70 °C for 8 h to obtain the grafted modified polyethylene dielectric material, denoted as PE-g-SPV.

[0066] The PE-g-SPV powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:35mL, heated to 40℃ and stirred for 6h, then poured onto a glass plate, placed in a vacuum drying oven at 50℃ for 10h, then heated to 150℃ and held for 2h, and then cooled to room temperature at a rate of 5℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0067] Comparative Example 5 This comparative example provides a method for preparing a dielectric thin film, which differs from Example 3 only in that the SPV is replaced with an equimolar amount of sulfobetaine methacrylate (SBMA). The specific steps are as follows: S1. Weigh polyethylene powder, KOH solution and anhydrous ethanol according to a molar ratio of 4:3:1. Add polyethylene powder to KOH solution with a concentration of 4 mol / L, add anhydrous ethanol while stirring, then heat to 50℃ and heat for 20 min. Filter, wash with deionized water, and dry in a vacuum drying oven at 40℃ for 14 h to obtain alkali-treated polyethylene powder. S2, alkali-treated polyethylene, sulfobetaine methacrylate (SBMA), and azobisisobutyronitrile (AIBN) were weighed according to a molar ratio of 5000:100:1; alkali-treated polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) at a ratio of 1 g:25 mL, heated to 130 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 254 nm UV light for 9 min to obtain a free radicalized polyethylene solution. After cooling, SBMA and an initiator were added, and the reaction was continued under an inert atmosphere, cooled to 60 °C, and reacted for 6 h. After the reaction was completed, deionized water was added to the reaction solution to precipitate the product, which was then filtered. The precipitate was dissolved in N-methylpyrrolidone, and then deionized water was added to precipitate it again. This process was repeated 3 times. The product was then dried in a vacuum dryer at 40 °C for 14 h to obtain a grafted modified polyethylene dielectric material, denoted as PE-g-SBMA. S3. Dissolve the PE-g-SBMA powder prepared above in N,N-dimethylformamide at a ratio of 1g:50mL, heat to 70℃ and stir for 4h, then pour it onto a glass plate, place it in a vacuum drying oven at 40℃ for 8h, then heat to 115℃ and keep warm for 3h, then cool to room temperature at a rate of 2℃ / min, peel it off from the substrate, and obtain a dielectric film.

[0068] Comparative Example 6 The only difference between this comparative example and comparative example 5 is that the alkali treatment is omitted, that is, step S1 is omitted. The specific steps are as follows: S1. Polyethylene, sulfobetaine methacrylate (SBMA), and azobisisobutyronitrile (AIBN) were weighed in a molar ratio of 5000:100:1. Polyethylene powder was dissolved in 1,2,4-trichlorobenzene (TCB) in a ratio of 1 g:25 mL. The mixture was heated to 130 °C and stirred until completely dissolved. Under an inert atmosphere, a photoinitiated free radical reaction was carried out under 254 nm UV light for 9 min to obtain a free radicalized polyethylene solution. After cooling, SBMA and an initiator were added. The mixture was then heated to 60 °C under an inert atmosphere and reacted for 6 h. After the reaction was completed, deionized water was added to the reaction solution to precipitate the product. The product was filtered and dissolved in N-methylpyrrolidone. Then, deionized water was added to precipitate the product again. This process was repeated 3 times. The product was then dried in a vacuum dryer at 40 °C for 14 h to obtain a grafted modified polyethylene dielectric material, denoted as PE-g-SBMA. S2, the PE-g-SBMA powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:50mL, heated to 70℃ and stirred for 4h, then poured onto a glass plate, placed in a vacuum drying oven at 40℃ for 8h, then heated to 115℃ and held for 3h, and then cooled to room temperature at a rate of 2℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0069] Comparative Example 7 This comparative example provides a method for preparing a dielectric thin film, which differs from Example 3 only in that the polyethylene is only treated with alkali, without ultraviolet light irradiation, and without grafting SPV. The specific steps are as follows: S1. Weigh polyethylene powder, KOH solution and anhydrous ethanol according to a molar ratio of 4:3:1. Add polyethylene powder to KOH solution with a concentration of 4 mol / L, add anhydrous ethanol while stirring, then heat to 50℃ and heat for 20 min. Filter, wash with deionized water, and dry in a vacuum drying oven at 40℃ for 14 h to obtain alkali-treated polyethylene powder. S2, the alkali-treated polyethylene powder prepared above was dissolved in N,N-dimethylformamide at a ratio of 1g:50mL, heated to 70℃ and stirred for 4h, then poured onto a glass plate, placed in a vacuum drying oven at 40℃ for 8h, then heated to 115℃ and kept at that temperature for 3h, and then cooled to room temperature at a rate of 2℃ / min, and peeled off from the substrate to obtain a dielectric film.

[0070] Comparative Example 8 This comparative example provides a method for preparing a dielectric thin film, the specific steps of which are as follows: Polyethylene powder was dissolved in N,N-dimethylformamide at a ratio of 1g:50mL, heated to 70℃ and stirred for 4h, then poured onto a glass plate and dried in a vacuum drying oven at 40℃ for 8h. The temperature was then raised to 115℃ and held for 3h, and then cooled to room temperature at a rate of 2℃ / min to peel off from the substrate, thus obtaining a dielectric film.

[0071] Characterization Figure 1 The figures show the DSC diagrams of the PE-g-SPV films prepared in Examples 1-4, the PE-g-SBMA film prepared in Comparative Example 5, and the PE film in Comparative Example 7 that underwent only alkali treatment. As shown, compared to the PE film samples treated only with alkali, the melting temperatures of the PE-g-SPV film samples in Examples 1-4 and the PE-g-SBMA film in Comparative Example 5 all decreased to varying degrees. Specifically, with the increase of SPV grafting content, the melting temperature of the PE-g-SPV film showed a systematic decreasing trend. This phenomenon can be attributed to the steric hindrance effect of the SPV side chains: after the SPV side groups are introduced into the polyethylene backbone through covalent bonds, their larger molecular volume hinders the orderly arrangement and close packing of the polyethylene molecular chains, interfering with the orderly folding and crystallization process of the chain segments, resulting in a reduction in the size of the crystalline regions or a decrease in the degree of crystallization perfection, thereby causing a decrease in the melting temperature.

[0072] Figure 2 The figures show the TG curves of the PE-g-SPV films prepared in Examples 1-4, the PE-g-SBMA film prepared in Comparative Example 5, and the PE film in Comparative Example 7 that underwent only alkali treatment. As shown, the PE film sample treated only with alkali began to thermally degrade at approximately 400°C, while the initial decomposition temperatures of the PE-g-SPV film samples in Examples 1-4 and the PE-g-SBMA film sample in Comparative Example 5 all increased to around 430°C, indicating that the introduction of SPV side groups improved the thermal stability of the material to some extent. Notably, the PE-g-SPV film sample obtained in Example 4 exhibited a clear two-stage degradation characteristic: the first stage began at approximately 300°C, corresponding to the thermal decomposition of the SPV side groups; the second stage began at approximately 410°C, attributed to the degradation of the polyethylene backbone. The overall thermal stability of this sample was lower than that of Examples 1-3. In addition, the residual mass of each sample at 800°C showed an increasing trend with the increase of SPV grafting content. The residual mass of the PE-g-SPV film samples in Examples 1 to 4 was approximately 25%, 27%, 30%, and 32%, respectively, which were all higher than those of the PE film samples treated only with alkali.

[0073] Figure 3The graph shows the dielectric constant of the PE-g-SPV films prepared in Examples 1-4, the PE-g-SBMA film prepared in Comparative Example 5, and the PE film treated with only alkali in Comparative Example 7 as a function of frequency. The dielectric constant test results of the PE-g-SPV films in Examples 1-4 show that the dielectric constant of the films first increases and then decreases with the increase of SPV grafting content. At a test frequency of 100Hz, the dielectric constant of the film obtained in Example 3 is 3.55, which is 50.42% higher than the 2.36 of the PE film treated with only alkali in Comparative Example 7, indicating that the introduction of an appropriate amount of SPV side groups can effectively enhance the polarization response of the polyethylene matrix. However, in Example 4, when the SPV grafting content was further increased, the dielectric constant decreased instead. This is attributed to the excessive SPV content causing local agglomeration in the polyethylene matrix, reducing grafting efficiency and dispersion uniformity, and weakening the contribution of zwitterionic side groups to the polarization response. The dielectric constant of the PE-g-SBMA film in Comparative Example 5 is 3.11 at 100 Hz, which is lower than that of the PE-g-SPV film in Example 3. This is mainly because the zwitterions of SBMA are prone to form ion cluster structures, the molecular chains are more tightly packed, the free volume is smaller, and the polarization response capability of the dipole is limited.

[0074] Furthermore, with increasing testing frequency, the dielectric constants of all film samples showed a significant decreasing trend, which is a typical characteristic of the difficulty in the dipole relaxation process of polymer materials to follow changes in high-frequency electric fields. It is noteworthy that even under high-frequency conditions of 1 MHz, the dielectric constant of the grafted modified polyethylene film prepared in the embodiments of this invention is still higher than that of the PE film of Comparative Example 7 that has only undergone alkali treatment, exhibiting good broadband dielectric stability.

[0075] Figure 4 The graph shows the dielectric constant of the dielectric films prepared in Comparative Examples 1-4, 6, and 8 as a function of frequency. The test results of the PE-g-SPV films obtained in Comparative Examples 1-4 show that the dielectric constant of the films also exhibits a trend of first increasing and then decreasing with the increase of SPV grafting content. At a test frequency of 100Hz, the dielectric constant of the film obtained in Comparative Example 3 is 2.88, which is 30.32% higher than the 2.21 of the pure PE film in Comparative Example 8. The dielectric constant of the untreated PE-g-SBMA film obtained in Comparative Example 6 is 2.43 at 100Hz. Figure 3Compared with the test results of alkali-treated PE-g-SBMA films (Comparative Example 5), the dielectric constants of all untreated film samples showed a significant decrease. This difference is mainly attributed to the following two reasons: First, the surface of untreated polyethylene lacks sufficient active sites, resulting in a low grafting rate of polar functional monomers and an insufficient number of polar groups in the system that can participate in electric field polarization; second, the interfacial compatibility between the polyethylene matrix without activated grafting sites and the polar monomers is poor, easily forming tiny interfacial voids and structural defects, further weakening the polarization response capability of the material. The above results indicate that the alkali treatment process plays a crucial role in improving the grafting efficiency of polar monomers, improving the interfacial bonding quality, and enhancing the dielectric properties of the material.

[0076] Figure 5 The diagram shows the breakdown field strength of the dielectric films prepared in Examples 1-4 and Comparative Examples 5 and 7 of this invention. The breakdown field strength of the PE film of Comparative Example 7, which was only treated with alkali, is 161 kV / mm. The test results of the PE-g-SPV films obtained in Examples 1-4 show that the breakdown field strength first increases and then decreases with the increase of SPV grafting content. The film obtained in Example 3 has a breakdown field strength of 317 kV / mm, which is 96.89% higher than that of the PE film in Comparative Example 7, demonstrating a significant improvement. This performance improvement is mainly attributed to the following two mechanisms: First, after SPV, as a zwitterionic monomer, is grafted onto the polyethylene backbone, it simultaneously introduces deep electron traps and deep hole traps into the matrix, effectively suppressing the injection and long-range migration of space charge under high electric fields, thereby reducing the conduction current and delaying the breakdown process; Second, the electrostatic interaction between positive and negative charged groups in the SPV side groups can form a physical cross-linked network structure, further hindering the migration path of charge carriers between molecular chains, synergistically improving the breakdown field strength of the material.

[0077] However, when the SPV grafting content was further increased in Example 4, the breakdown field strength decreased. This was because the excessively high SPV content led to local aggregation in the polyethylene matrix, reducing grafting efficiency and decreasing dispersion uniformity, thus weakening the distribution density and trapping effect of charge traps. The breakdown field strength of the PE-g-SBMA film obtained in Comparative Example 5 was 238 kV / mm, significantly lower than that of the PE-g-SPV film in Example 3. This was mainly attributed to the excessively polarity of the sulfobetaine methacrylate groups, which easily aggregated in the non-polar polyethylene matrix to form ion clusters, causing interface defects and local electric field distortion, thereby limiting further improvement in the material's breakdown field strength.

[0078] Figure 6The breakdown field strength diagrams are shown for the dielectric films prepared in Comparative Examples 1-4, 6, and 8. The breakdown field strength of the pure PE film in Comparative Example 8 is only 97 kV / mm. The test results of the PE-g-SPV films obtained in Comparative Examples 1-4 show that the breakdown field strength initially increases and then decreases with increasing SPV grafting content. The film obtained in Comparative Example 3 achieves a breakdown field strength of 253 kV / mm, an increase of 160.82% compared to Comparative Example 8. The breakdown field strength of the untreated PE-g-SBMA film in Comparative Example 6 is 165 kV / mm. Figure 5 Compared to the alkali-treated samples, the breakdown field strength of all untreated film samples showed a significant decrease. This is mainly because the untreated polyethylene surface lacks active sites, failing to provide a basis for subsequent grafting and indirectly leading to insufficient carrier trap density. Furthermore, the unactivated polyethylene matrix has poor interfacial compatibility with the polar monomers, easily forming tiny interfacial voids and structural defects. These defects become concentrated areas of electric field distortion under high electric field conditions, evolving into preferential channels for carrier breakdown, thus significantly reducing the material's breakdown field strength. These comparative results further confirm that the alkali treatment process plays an indispensable and crucial role in achieving high grafting rates, constructing efficient charge trap networks, and improving the material's high electric field tolerance.

[0079] Figure 7 The figures show physical images of the dielectric films prepared for Comparative Example 8 and Example 3 of this invention. As can be seen from the figures, both films retain good flexibility after folding, indicating that grafting has no significant impact on the flexibility of the dielectric films.

[0080] Performance testing To better demonstrate the performance of the dielectric thin film samples prepared in Examples 1-4 and Comparative Examples 1-7 of the present invention, the following performance tests were conducted: (1) Dielectric constant test: The dielectric constant of the thin film was tested at room temperature using a precision impedance analyzer, with a test frequency range of 10 Hz. 2 ~10 6 Hz. The test sample was cut into a square film with a side length of 10 mm, and a circular silver electrode with a diameter of 6 mm was coated in the center of the sample surface as the test electrode.

[0081] (2) Breakdown field strength test: The breakdown strength of the thin film sample was characterized by a breakdown voltage tester. The test was carried out at room temperature. A cylindrical electrode with a diameter of 8 mm was used. A DC electric field was applied at a voltage increase rate of 200 V / s until the sample underwent dielectric breakdown. The breakdown voltage value was recorded and converted into the breakdown field strength.

[0082] (3) Energy storage density test: The ferroelectric properties of the thin film were characterized using a ferroelectric analyzer (Huace FE-2000). A triangular unipolar pulse voltage waveform with a frequency of 100 Hz was applied. During the test, the instrument recorded the relationship between the electric field (E) and the electric displacement (D), thereby plotting the DE hysteresis loop. The test was conducted at room temperature, and the discharge energy storage density (Ue) of the thin film was calculated based on the integral of the obtained DE hysteresis loop.

[0083] (4) The elongation at break was obtained by tensile testing at a rate of 200 mm / min using a universal testing machine in accordance with GB / T 1040.3-2006. The results are shown in Table 1.

[0084] Table 1

[0085] As can be seen from the data in Table 1, the polyethylene-grafted SPV dielectric films prepared in Examples 1-4 exhibit higher breakdown field strength and energy storage density at room temperature. The SPV molecule contains both pyridinium cations and sulfonate anions; its zwitterionic structure can construct efficient charge trap centers within the polyethylene matrix, effectively regulating the migration behavior of charge carriers under high electric fields, thereby improving the polarization response characteristics and enhancing the dielectric strength of the material.

[0086] In Comparative Examples 1-4, the polyethylene was grafted directly without alkali treatment, and the resulting films showed significantly lower breakdown field strength and storage energy density compared to Examples 1-4. This indicates that the alkali treatment step is crucial for introducing reactive sites onto the polyethylene surface backbone. The untreated polyethylene surface lacks sufficient unsaturated bond anchoring sites, resulting in a low SPV grafting rate and insufficient charge trap density, which in turn limits the improvement of the material's dielectric properties.

[0087] Comparative Examples 5 and 6 used sulfobetaine methacrylate instead of SPV for grafting. Regardless of whether alkali treatment was performed, the breakdown field strength and energy storage density of the grafted systems were lower than those of the SPV-grafted systems. This indicates that the pyridine aromatic ring structure contained in SPV has better interfacial compatibility with the polyethylene matrix, making it easier to achieve uniform molecular-level dispersion, effectively reducing interfacial defects and local electric field concentration, and thus outperforming other betaine monomers in suppressing the formation of charge carrier breakdown channels.

[0088] Comparative Examples 7 and 8 are polyethylene films without SPV grafting and untreated and ungrafted control samples, respectively. Both have low breakdown field strength and energy storage density, further confirming the key role of SPV grafting modification in improving the energy storage performance of polyethylene dielectric materials.

[0089] In summary, the introduction of zwitterionic structure in SPV is the core means to construct efficient charge traps, enhance carrier capture capability, and improve the breakdown field strength and energy storage density of polyethylene dielectric films, while alkali treatment process is a necessary prerequisite for achieving high grafting rate and excellent dielectric properties.

[0090] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A grafted modified polyethylene dielectric material, characterized in that, It includes polyethylene derivatives represented by formula (I); Equation (Ⅰ).

2. The grafted modified polyethylene dielectric material as described in claim 1, characterized in that, The preparation method of the polyethylene derivative includes the following steps: S1, polyethylene is added to an alkaline alcohol solution and heated for pretreatment to obtain alkaline-treated polyethylene; S2, Under an inert atmosphere, alkali-treated polyethylene is dissolved in an organic solvent and subjected to a photo-initiated free radical reaction under ultraviolet light to obtain a free radicalized polyethylene solution; 1-(3-thiopropyl)-2-vinylpyridine betaine and an initiator are added to the free radicalized polyethylene solution, and a graft polymerization reaction is carried out under an inert atmosphere to obtain the polyethylene derivative.

3. The grafted modified polyethylene dielectric material as described in claim 2, characterized in that, In S1, the alkaline alcohol solution is a mixture of potassium hydroxide solution and anhydrous ethanol, wherein the molar ratio of polyethylene, KOH, and anhydrous ethanol is 100:71:25 to 100:81:25; the concentration of the potassium hydroxide solution is 1 mol / L to 5 mol / L; and / or In S1, the pretreatment temperature is 40℃~70℃ and the time is 15min~20min. In S2, the wavelength of the light-induced free radical reaction is 240nm~260nm, and the illumination time is 8min~12min.

4. The grafted modified polyethylene dielectric material as described in claim 2, characterized in that, In S2, the mass-to-volume ratio of the alkali-treated polyethylene to the organic solvent is 1g:20mL to 1g:30mL; and / or In S2, the molar ratio of the alkali-treated polyethylene, 1-(3-thiopropyl)-2-vinylpyridine betaine, and initiator is 10000:100:1 to 2500:100:

1.

5. The grafted modified polyethylene dielectric material as described in claim 2 or 4, characterized in that, In S2, the organic solvent is 1,2,4-trichlorobenzene; and / or In S2, the initiator is azobisisobutyronitrile; and / or In S2, the graft polymerization reaction is carried out at a temperature of 60℃~100℃ for a time of 2h~6h.

6. The application of the grafted modified polyethylene dielectric material according to any one of claims 1 to 5 in the preparation of dielectric films.

7. A dielectric thin film, characterized in that, It is prepared from the grafted modified polyethylene dielectric material according to any one of claims 1 to 5.

8. The method for preparing the dielectric thin film according to claim 7, characterized in that, Includes the following steps: The grafted modified polyethylene dielectric material is dissolved in an organic solvent, cast into a film, and then the film is subjected to thermal annealing and peeled off from the substrate to obtain a dielectric film.

9. The method for preparing a dielectric thin film as described in claim 8, characterized in that, The mass-to-volume ratio of the grafted modified polyethylene dielectric material to the organic solvent is 1g:10mL to 1g:50mL; and / or The heat annealing process is performed at a temperature of 100℃ to 150℃ for 2 hours to 5 hours.

10. An energy storage capacitor, characterized in that, Includes the dielectric thin film as described in claim 7.

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