Amino-terminated hyperbranched polyamide acid as well as preparation method and application thereof

Abstract

The invention discloses amino-terminated hyperbranched polyamide acid as well as a preparation method and application thereof. The amino-terminated hyperbranched polyamide acid is formed by polymerizing an aromatic anhydride monomer, an aromatic triamine monomer and an aromatic diamine monomer; the mole numbers of the aromatic anhydride monomer, the aromatic triamine monomer and the aromatic diamine monomer are a, b and c respectively, a: (b + c) = 1: 1-1: 1.5, and the proportion of b to c is any proportion. The amino-terminated hyperbranched polyamide acid disclosed by the invention not only has abundant terminal amino groups, but also contains a large number of polar groups such as ether bonds, imidazole rings, amide groups and carboxyl groups in molecular chains. Therefore, a stable water-based colloid emulsion can be formed in the presence of water and a small amount of organic solvent, other auxiliaries are not needed, and the interface performance and the mechanical performance of the composite material can be greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of polymers and composite materials, specifically relating to an amino-terminated hyperbranched polyamic acid, its preparation method, and its application. Background Technology

[0002] Aramid fiber, also known as aromatic polyamide fiber, possesses excellent properties such as ultra-high strength, high modulus, high temperature resistance, acid and alkali resistance, and light weight. Its most important application area is as a reinforcement in the preparation of resin-based high-performance composite materials. Currently, aramid fiber composites have numerous applications in aerospace, construction, and transportation. The large benzene rings in the aramid fiber molecular chain hinder the reaction of the amide groups with other atoms or groups, resulting in the chemical inertness of aramid fibers. Furthermore, the high crystallinity and smooth surface of aramid fibers prevent the formation of a stable interfacial phase with the matrix resin, leading to low interfacial bonding strength in aramid fiber composites, making them prone to delamination under load. Currently, surface treatment methods for aramid fibers include chemical modification, plasma treatment, radiation therapy, and surface sizing. The first three surface treatment methods sacrifice the mechanical properties of the aramid fiber itself to increase the surface polarity and roughness. However, surface sizing involves coating the fiber surface with a thin layer of resin without damaging the aramid fiber's structure. However, commercial sizing agents that can be used for aramid fibers are still in their infancy. Therefore, it is urgent to develop a sizing agent for aramid fibers that can be used in industrial production. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide an amino-terminated hyperbranched polyamic acid, which, as a sizing agent, can significantly improve the interfacial bonding strength between aramid fibers and matrix resins. The invention also provides a method for preparing and applying this amino-terminated hyperbranched polyamic acid.

[0004] The first aspect of the present invention provides an amino-terminated hyperbranched polyamic acid, wherein the amino-terminated hyperbranched polyamic acid is polymerized from an aromatic anhydride monomer, an aromatic triamine monomer and an aromatic diamine monomer; wherein the molar amounts of the aromatic anhydride monomer, the aromatic triamine monomer and the aromatic diamine monomer are a, b and c, respectively, wherein a:(b+c)=1:1~1:1.5 and the ratio of b to c is arbitrary.

[0005] According to one embodiment of the present invention, the aromatic anhydride monomer is one or more of 4,4′-(hexafluoroisopropene)phthalic anhydride, 4,4′-biphenyl ether dianhydride, 3,3′4,4′-benzophenone tetracarboxylic dianhydride, 3,3′,4,4′-diphenyl sulfone tetracarboxylic dianhydride, and 4,4′-(4,4′-isopropyldiphenoxy)phthalic anhydride; the aromatic triamine monomer is one or more of 2,4,6-triaminopyrimidine, tris(4-aminophenyl)amine, and 1,3,5-tris(4-aminophenyl)benzene; and the aromatic diamine monomer is one or more of 1,4-bis(4-aminophenoxy)benzene, 1,3-di(4-aminophenoxy)benzene, 4,4′-diaminodiphenyl ether, p-phenylenediamine, anthracene-2,6-diamine, 4-chloro-1,3-phenylenediamine, and 5-amino-2-(4-aminophenyl)benzimidazole.

[0006] A second aspect of the present invention provides a method for preparing amino-terminated hyperbranched polyamic acid, comprising: dissolving the aromatic triamine monomer and the aromatic diamine monomer in a first organic solvent to form a first solution; dissolving the aromatic anhydride monomer in a second organic solvent to form a second solution; adding the second solution dropwise to the first solution over a period of 1 to 2 hours; and continuing a polymerization reaction for 6 to 24 hours at a temperature of -10 to 10°C with mechanical stirring to obtain an organic solution of amino-terminated hyperbranched polyamic acid.

[0007] According to one embodiment of the present invention, the total mass of the aromatic anhydride monomer, the aromatic triamine monomer, and the aromatic diamine monomer is m4, and the total mass of the first organic solvent and the second organic solvent is m5, wherein m4:m5 = 1:15 to 1:34; the first organic solvent and the second organic solvent are each independently one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc).

[0008] A third aspect of the present invention provides a sizing agent comprising the above-mentioned amino-terminated hyperbranched polyamic acid, a third organic solvent, and water.

[0009] According to one embodiment of the present invention, the masses of the amino-terminated hyperbranched polyamic acid, the third organic solvent and water are m1, m2 and m3, respectively, wherein m1:m2 = 1:30 to 1:62 and m1:(m1+m2+m3) = 1:100 to 1:100000.

[0010] A fourth aspect of the present invention provides a method for preparing a sizing agent, comprising: diluting an organic solution of the amino-terminated hyperbranched polyamic acid obtained in the above-mentioned method for preparing amino-terminated hyperbranched polyamic acid with a fourth organic solvent, and then adding deionized water to obtain the sizing agent; or adding deionized water to an organic solution of the amino-terminated hyperbranched polyamic acid obtained in the above-mentioned method for preparing amino-terminated hyperbranched polyamic acid to obtain the sizing agent.

[0011] The fifth aspect of the present invention provides a method for modifying aramid fibers, comprising: applying the above-mentioned aqueous sizing agent to perform surface sizing treatment on the aramid fibers, followed by drying to cause the amino-terminated hyperbranched polyamic acid in the sizing agent to undergo thermal imidization to form amino-terminated hyperbranched polyimide, which is then coated on the surface of the aramid fibers.

[0012] In one embodiment of the present invention, the mass ratio of the amino-terminated hyperbranched polyimide coating the surface of the aramid fiber to the aramid fiber is 0.005 to 0.015.

[0013] A sixth aspect of the present invention provides a modified aramid fiber obtained by the above-described modification method.

[0014] The amino-terminated hyperbranched polyamic acid of this invention not only possesses abundant terminal amino groups, but also contains a large number of polar groups in its molecular chain, such as ether bonds, imidazole rings, amide groups, and carboxyl groups. The hyperbranched polyamic acid provided by this invention forms a stable aqueous colloidal emulsion in the presence of water and a small amount of organic solvent, requiring no other additives, and can significantly improve the interfacial and mechanical properties of composite materials. The sizing agent of this invention has advantages such as small particle size, good storage stability, and good heat resistance, and has significant industrial application value. Using the sizing agent of this invention to modify aramid fibers can improve the tensile strength of aramid fiber impregnated multifilaments, while simultaneously solving the difficulty of incompatibility between aramid fibers and thermosetting resin composites, and is also suitable for aramid fiber reinforced thermoplastic resin composites. Attached Figure Description

[0015] Figure 1 These are scanning electron microscope images of the aramid fibers in Example 1 before (a) and after (b) sizing.

[0016] Figure 2 The tensile strength (a) and elongation at break (b) of the aramid fiber impregnated multifilament in Example 1 are shown. Detailed Implementation

[0017] The present invention will now be described in detail with reference to specific embodiments.

[0018] This invention relates to amino-terminated hyperbranched polyamic acid, which is polymerized from aromatic anhydride monomers, aromatic triamine monomers, and aromatic diamine monomers. The molar amounts of the aromatic anhydride monomer, aromatic triamine monomer, and aromatic diamine monomer are a, b, and c, respectively, where a:(b+c) = 1:1 to 1:1.5, and the ratio of b to c is arbitrary. Since this invention's amino-terminated hyperbranched polyamic acid is polymerized from three monomers, both b and c are greater than 0. The amino-terminated hyperbranched polyamic acid formed from the above three monomers not only has abundant terminal amino groups but also contains a large number of polar groups in its molecular chain, such as ether bonds, imidazole rings, amide groups, and carboxyl groups. Therefore, this hyperbranched polyamic acid forms a stable aqueous colloidal emulsion in the presence of water and a small amount of organic solvent, requiring no other additives, and can significantly improve the interfacial and mechanical properties of the composite material. Simultaneously, the amino-terminated hyperbranched polyamic acid sizing agent of this invention has advantages such as small particle size, good storage stability, and good heat resistance, and has significant industrial application value. The amino-terminated hyperbranched polyamic acid of the present invention can be used as a sizing agent to modify aramid fibers, thereby improving the tensile strength of aramid fiber impregnation multifilaments. At the same time, it solves the problem of incompatibility between aramid fibers and thermosetting resin composites, and is also applicable to aramid fiber reinforced thermoplastic resin composites.

[0019] In optional embodiments, the aromatic anhydride monomer may be one or more of 4,4′-(hexafluoroisopropene)phthalic anhydride, 4,4′-biphenyl ether dianhydride, 3,3′4,4′-benzophenone tetracarboxylic acid dianhydride, 3,3′,4,4′-diphenyl sulfone tetracarboxylic acid dianhydride, and 4,4′-(4,4′-isopropyldiphenoxy)phthalic anhydride. The aromatic triamine monomer may be one or more of 2,4,6-triaminopyrimidine, tris(4-aminophenyl)amine, and 1,3,5-tris(4-aminophenyl)benzene. The aromatic diamine monomer may be one or more of 1,4-bis(4-aminophenoxy)benzene, 1,3-di(4-aminophenoxy)benzene, 4,4′-diaminodiphenyl ether, p-phenylenediamine, anthracene-2,6-diamine, 4-chloro-1,3-phenylenediamine, and 5-amino-2-(4-aminophenyl)benzimidazole. The amino-terminated hyperbranched polyamic acid of this invention, depending on the monomer used, also contains corresponding polar groups in its molecular chain, such as ether bonds, imidazole rings, amide groups, and carboxyl groups. By introducing these structural features, the polarity of the aramid fiber surface can be significantly enhanced, thereby effectively promoting the wettability of the matrix resin to the aramid fiber, and thus greatly improving the interfacial adhesion between the two, thereby improving the overall performance of the composite material.

[0020] The method for preparing amino-terminated hyperbranched polyamic acid of the present invention involves dissolving an aromatic triamine monomer and an aromatic diamine monomer in a first organic solvent to form a first solution; dissolving an aromatic anhydride monomer in a second organic solvent to form a second solution; adding the second solution dropwise to the first solution over 1-2 hours; and continuing the polymerization reaction at -10 to 10°C with mechanical stirring for 6-24 hours to obtain an organic solution of amino-terminated hyperbranched polyamic acid. The method for preparing amino-terminated hyperbranched polyamic acid of the present invention has low cost and therefore can be mass-produced.

[0021] In an optional embodiment, the total mass of the aromatic anhydride monomer, the aromatic triamine monomer, and the aromatic diamine monomer is m4, and the total mass of the first organic solvent and the second organic solvent is m5, wherein m4:m5 = 1:15 to 1:34. The first and second organic solvents are independently any one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc). For the polymerization reaction, if the solid content of the polymerization liquid is too low (i.e., m4:m5 is less than 1:34), the polymerization reaction may be incomplete; if the solid content of the polymerization liquid is too high (i.e., m4:m5 is greater than 1:15), premature gelation will occur. Therefore, the solid content of the polymerization liquid must be controlled within the above-mentioned range.

[0022] The amino-terminated hyperbranched polyamic acid of this invention is a highly branched three-dimensional macromolecule. This special molecular structure endows it with unique structure and properties, such as a large number of active terminal groups, intramolecular cavities, high solubility, multifunctionality, and excellent flowability and processing characteristics due to less intermolecular or intramolecular entanglement. Compared with linear polymers of similar molecular weight, hyperbranched polymers have lower solution viscosity, which is crucial for the development of coating materials. Sizing agents, as "bridges" connecting fibers and resins, can enhance intermolecular interactions (chemical bonds, hydrogen bonds, van der Waals forces) between the sizing agent and the fiber surface, and between the sizing agent and the resin through molecular design, thereby improving the interfacial strength of the composite material. Amino-terminated hyperbranched polyamic acid contains abundant reactive groups, providing a large number of crosslinking sites, which is beneficial to increasing the probability of physical and chemical interactions between the fiber and the matrix. Therefore, this type of polymer is suitable as a raw material for sizing agents. Specifically, the sizing agent provided by this invention includes the above-mentioned amino-terminated hyperbranched polyamic acid, a third organic solvent, and water. The hyperbranched polyamic acid provided by this invention forms a stable aqueous colloidal emulsion in the presence of water and a small amount of organic solvent, without the need for other additives, and can significantly improve the interfacial and mechanical properties of composite materials.

[0023] The masses of amino-terminated hyperbranched polyamic acid, the third organic solvent, and water are m1, m2, and m3, respectively, where m1:m2 = 1:30 to 1:62 and m1:(m1+m2+m3) = 1:100 to 1:100000.

[0024] The preparation method of the above-mentioned sizing agent includes: diluting the organic solution of amino-terminated hyperbranched polyamic acid obtained by the above preparation method with a fourth organic solvent, and then adding deionized water to obtain the aqueous sizing agent of the present invention (hereinafter referred to as Scheme 1). Alternatively, adding deionized water to the organic solution of amino-terminated hyperbranched polyamic acid obtained in the preparation method of the above-mentioned amino-terminated hyperbranched polyamic acid to obtain the aqueous sizing agent of the present invention (hereinafter referred to as Scheme 2). In the sizing agent obtained in Scheme 1, the third organic solvent is the sum of the first organic solvent, the second organic solvent, and the fourth organic solvent. In the sizing agent obtained in Scheme 2, the third organic solvent is the sum of the first organic solvent and the second organic solvent. The fourth organic solvent may be the same as or different from the first and second organic solvents. The fourth organic solvent may also be any one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc).

[0025] The sizing agent prepared by the method of this invention has advantages such as small particle size, good storage stability, and good heat resistance, and has significant industrial application value. Of course, the sizing agent of this invention can also be prepared by other methods. For example, the prepared amino-terminated hyperbranched polyamic acid can be directly dissolved in an organic solvent, and the sizing agent can be prepared by adding deionized water dropwise while stirring.

[0026] The sizing agent of this invention can modify aramid fibers to improve their mechanical properties and solve the problem of incompatibility between aramid fibers and thermosetting resin composites. Furthermore, the modified aramid fibers are also suitable for composite with thermoplastic resins. Specifically, the modification method includes: applying the above-mentioned sizing agent to the surface of the aramid fibers, followed by drying to allow the amino-terminated hyperbranched polyamic acid in the sizing agent to undergo thermal imidization to form amino-terminated hyperbranched polyimide, which coats the surface of the aramid fibers.

[0027] In an optional embodiment, the mass ratio of amino-terminated hyperbranched polyimide coated on the surface of aramid fibers to aramid fibers is 0.005 to 0.015.

[0028] The present invention also protects a modified aramid fiber obtained by the above-described modification method.

[0029] The present invention is further described below through specific examples. However, these examples are merely exemplary and do not constitute any limitation on the scope of protection of the present invention. Unless otherwise specified, the reagents, materials, and instruments used in the following embodiments and comparative examples are commercially available.

[0030] Example 1

[0031] (1) Under nitrogen protection, 450g of N-methylpyrrolidone was weighed, and 5.0g of 2,4,6-triaminopyrimidine was added with mechanical stirring. After complete dissolution, 9.0g of 5-amino-2-(4-aminophenyl)benzimidazole was added. Simultaneously, 41.6g of 4,4′-(4,4′-isopropyldiphenoxy)phthalic anhydride was weighed and rapidly dissolved in 450g of N-methylpyrrolidone. Finally, the organic solution of the anhydride was added dropwise to the solution containing the triamine monomer over a period of 1 hour. The polycondensation reaction was then carried out at 0°C with mechanical stirring for 8 hours to obtain an organic solution of amino-terminated hyperbranched polyamic acid.

[0032] (2) First, dilute the above-mentioned amino-terminated hyperbranched polyamic acid organic solution with N-methylpyrrolidone so that the mass ratio of amino-terminated hyperbranched polyamic acid m1 to organic solvent N-methylpyrrolidone m2 is 1:50. Before sizing, add deionized water dropwise to the diluted polyamic acid organic solution under the condition of stirring speed of 1000 rpm for 1 hour until the system changes from brown to milky white, so that the final concentration of amino-terminated hyperbranched polyamic acid in the sizing agent is 0.04 wt%.

[0033] (3) Pour the prepared sizing agent into the sizing tank, and immerse the aramid fiber III produced by our unit in the sizing tank at a speed of 2.5 m / min. Then scrape off the excess sizing liquid with a scraper roller, and finally dry it with hot air at 160℃ and then take it out. The mass ratio of the hyperbranched polyimide coating the surface of the aramid fiber to the aramid fiber is 0.0098.

[0034] (4) Completely dissolve E-51 epoxy resin and curing agent (4,4′-diaminodiphenylmethane) in acetone, pour the prepared adhesive solution into the impregnation tank, and control the temperature between 23 and 38°C. After sizing, the aramid fiber is introduced into the impregnation tank through a tension controller, yarn guide head, and guide roller, then scraped by a scraper roller, and finally wound onto a winding frame. Then, it is placed at room temperature for 24 hours to allow the acetone to evaporate, and then the winding frame with the aramid fiber impregnated multifilament is placed in a forced-air oven for curing to obtain an aramid fiber impregnated multifilament sample. According to GJB348A-2018, the aramid fiber impregnated multifilament was tested. After sizing with amino-terminated hyperbranched polyamic acid, the tensile strength and elongation at break of the impregnated multifilament were 6.41 GPa and 4.20%, respectively, which were increased by 19.14% and 22.09% compared with the unsizing (e.g., ...). Figure 2 (As shown).

[0035] Figure 1 Electron micrographs of the fibers before and after sizing in this embodiment are shown. As can be seen from the figures, the sizing agent coats the fiber surface.

[0036] Example 2

[0037] (1) Under argon protection, 450g of N,N-dimethylformamide was weighed, and 0.3g of tris(4-aminophenyl)amine was added with mechanical stirring. After complete dissolution, 17.7g of 5-amino-2-(4-aminophenyl)benzimidazole was added. Simultaneously, 41.6g of 4,4′-(4,4′-isopropyldiphenoxy)phthalic anhydride was weighed and rapidly dissolved in 450g of N,N-dimethylformamide. Finally, the organic solution of the anhydride was added dropwise to the solution containing the triamine monomer over a period of 1 hour. The polycondensation reaction was then carried out at 10°C with mechanical stirring for 24 hours to obtain an organic solution of amino-terminated hyperbranched polyamic acid.

[0038] (2) First, dilute the above-mentioned amino-terminated hyperbranched polyamic acid organic solution with N,N-dimethylformamide so that the mass ratio of amino-terminated hyperbranched polyamic acid m1 to organic solvent N,N-dimethylformamide m2 is 1:40. Before sizing, add deionized water dropwise to the diluted polyamic acid organic solution under the condition of stirring speed of 1000 rpm for 1 hour until the system turns milky white, so that the final concentration of amino-terminated hyperbranched polyamic acid in the sizing agent is 0.03 wt%.

[0039] (3) Pour the prepared sizing agent into the sizing tank, and immerse the same aramid fiber III produced by our unit as in Example 1 through the sizing tank at a speed of 2.0 m / min. Then scrape off the excess sizing liquid with a scraper roller, and finally dry it with hot air at 140°C and then take it out. The mass ratio of hyperbranched polyimide coated on the surface of the aramid fiber to the aramid fiber is 0.0076.

[0040] (4) Completely dissolve E-51 epoxy resin and curing agent (4,4′-diaminodiphenylmethane) in acetone, pour the prepared adhesive solution into the impregnation tank, and control the temperature between 23 and 38°C. After sizing, the aramid fibers are introduced into the impregnation tank through a tension controller, yarn guide, and guide roller, then scraped by a scraper roller, and finally wound onto a winding frame. Then, place at room temperature for 24 hours to allow the acetone to evaporate, and then place the winding frame with the aramid fiber impregnated multifilament in a forced-air oven for curing to obtain the aramid fiber impregnated multifilament sample. According to GJB348A-2018, the aramid fiber impregnated multifilament was tested. After sizing with amino-terminated hyperbranched polyamic acid, the tensile strength and elongation at break of the impregnated multifilament were 5.49 GPa and 3.56%, respectively.

[0041] Example 3

[0042] (1) Under nitrogen protection, 870 g of N,N-dimethylacetamide was weighed, and 27.8 g of 1,3,5-tris(4-aminophenyl)benzene was added with mechanical stirring. After complete dissolution, 0.2 g of 5-amino-2-(4-aminophenyl)benzimidazole was added. At the same time, 25.8 g of 3,3′4,4′-benzophenone tetracarboxylic acid dianhydride was weighed and rapidly dissolved in 870 g of N,N-dimethylacetamide. Finally, the organic solution of the anhydride was added dropwise to the solution containing the triamine monomer over 2 hours. The polycondensation reaction was then carried out at 5 °C with mechanical stirring for 12 hours to obtain an organic solution of amino-terminated hyperbranched polyamic acid.

[0043] (2) Before coating, add deionized water dropwise to the organic solution of amino-terminated hyperbranched polyamic acid at a stirring speed of 1000 rpm for 1 hour, so that the final concentration of amino-terminated hyperbranched polyamic acid in the coating agent is 0.05 wt%.

[0044] (3) Pour the prepared sizing agent into the sizing tank, and immerse the same aramid fiber III produced by our unit as in Example 1 through the sizing tank at a speed of 2.5 m / min. Then scrape off the excess sizing liquid with a scraper roller, and finally dry it with hot air at 150°C and then take it out. The mass ratio of the hyperbranched polyimide coating the surface of the aramid fiber to the aramid fiber is 0.012.

[0045] (4) Completely dissolve E-51 epoxy resin and curing agent (4,4′-diaminodiphenylmethane) in acetone, pour the prepared adhesive solution into the impregnation tank, and control the temperature between 23 and 38°C. After sizing, the aramid fibers are introduced into the impregnation tank through a tension controller, yarn guide, and guide roller, then scraped by a scraper roller, and finally wound onto a winding frame. Then, place it at room temperature for 24 hours to allow the acetone to evaporate, and then place the winding frame with the aramid fiber impregnated multifilament in a forced-air oven for curing to obtain the aramid fiber impregnated multifilament sample. According to GJB348A-2018, the aramid fiber impregnated multifilament was tested. After sizing with amino-terminated hyperbranched polyamic acid, the tensile strength and elongation at break of the impregnated multifilament were 5.73 GPa and 3.63%, respectively.

[0046] Comparative Example 1

[0047] E-51 epoxy resin and curing agent (4,4′-diaminodiphenylmethane) were completely dissolved in acetone. The prepared adhesive solution was poured into an impregnation tank, and the temperature was controlled between 23 and 38°C. Unsized aramid fiber III, produced in-house and identical to that in Example 1, was introduced into the impregnation tank via a tension controller, yarn guide, and guide roller. It was then sized by a sizing roller and finally wound onto a sample winding frame. The sample was then left at room temperature for 24 hours to allow the acetone to evaporate. The winding frame containing the aramid fiber impregnated multifilament was then placed in a forced-air drying oven for curing, yielding an aramid fiber impregnated multifilament sample. The aramid fiber impregnated multifilament was tested according to GJB348A-2018, and the tensile strength and elongation at break were 5.38 GPa and 3.44%, respectively.

[0048] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. An amino-terminated hyperbranched polyamic acid, characterized in that, The amino-terminated hyperbranched polyamic acid is polymerized from aromatic anhydride monomers, aromatic triamine monomers and aromatic diamine monomers; the molar amounts of the aromatic anhydride monomers, aromatic triamine monomers and aromatic diamine monomers are a, b and c, respectively, wherein a:(b+c)=1:1~1:1.5, and the ratio of b to c is arbitrary.

2. The amino-terminated hyperbranched polyamic acid according to claim 1, characterized in that, The aromatic anhydride monomer is one or more of the following: 4,4′-(hexafluoroisopropene) phthalic anhydride, 4,4′-biphenyl ether phthalic anhydride, 3,3′4,4′-benzophenone tetracarboxylic phthalic anhydride, 3,3′,4,4′-diphenyl sulfone tetracarboxylic phthalic anhydride, and 4,4′-(4,4′-isopropyldiphenoxy) phthalic anhydride. The aromatic triamine monomer is one or more selected from 2,4,6-triaminopyrimidine, tris(4-aminophenyl)amine, and 1,3,5-tris(4-aminophenyl)benzene; The aromatic diamine monomer is one or more of 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 4,4′-diaminodiphenyl ether, p-phenylenediamine, anthracene-2,6-diamine, 4-chloro-1,3-phenylenediamine, and 5-amino-2-(4-aminophenyl)benzimidazole.

3. A method for preparing amino-terminated hyperbranched polyamic acid according to claim 1 or 2, characterized in that, include: The aromatic triamine monomer and the aromatic diamine monomer are dissolved in a first organic solvent to form a first solution; The aromatic anhydride monomer is dissolved in a second organic solvent to form a second solution; The second solution is added dropwise to the first solution over a period of 1 to 2 hours; The polymerization reaction was carried out continuously for 6 to 24 hours at -10 to 10°C with mechanical stirring to obtain an organic solution of amino-terminated hyperbranched polyamic acid.

4. The preparation method according to claim 3, characterized in that, The total mass of the aromatic anhydride monomer, the aromatic triamine monomer, and the aromatic diamine monomer is m4, and the total mass of the first organic solvent and the second organic solvent is m5, wherein m4:m5 = 1:15 to 1:34; The first organic solvent and the second organic solvent are each independently one or more of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

5. A sizing agent, characterized in that, It includes the amino-terminated hyperbranched polyamic acid as described in claim 1 or 2, a third organic solvent, and water.

6. The sizing agent according to claim 5, characterized in that, The amino-terminated hyperbranched polyamic acid, the third organic solvent, and water have masses of m1, m2, and m3, respectively, where m1:m2 = 1:30 to 1:62 and m1:(m1+m2+m3) = 1:100 to 1:100000.

7. A method for preparing the sizing agent according to claim 5 or 6, characterized in that, include: The sizing agent is obtained by adding a fourth organic solvent to the organic solution of the amino-terminated hyperbranched polyamic acid obtained according to claim 3 or 4, and then adding deionized water. Alternatively, deionized water can be added to the organic solution of the amino-terminated hyperbranched polyamic acid obtained according to claim 3 or 4 to obtain the sizing agent.

8. A method for modifying aramid fibers, characterized in that, include: The sizing agent described in claim 5 or 6 is used to perform surface sizing treatment on aramid fibers, followed by drying to cause the amino-terminated hyperbranched polyamic acid in the sizing agent to undergo thermal imidization to form amino-terminated hyperbranched polyimide, which coats the surface of the aramid fibers.

9. The method for modifying aramid fibers according to claim 8, characterized in that, The mass ratio of the amino-terminated hyperbranched polyimide coating the surface of the aramid fiber to the aramid fiber is 0.005 to 0.

015.

10. A modified aramid fiber, characterized in that, Obtained by the modification method described in claim 8 or 9.

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