Polyamic acid nanofiber membrane having three-dimensionally entangled curved nanofibers and manufacturing method therefor

Abstract

A polyamic acid nanofiber membrane having three-dimensionally entangled curved nanofibers, according to the present invention, is manufactured by: immersing an electrospun polyamic acid nanofiber membrane for 10 seconds to 2 minutes in a solution for shrinkage treatment in which (a) one type of hydrogen-bond-promoting compound selected from among an ammonium compound containing multiple hydrogen atoms, an interhalogen ion compound, a peroxide, a polysalt, and a polymer ion compound, and (b) a hydrogen-bond-induced polymer consisting of one type of polymer selected from among a polymer of polyethylene glycol diacrylate having double bonds attached to ends thereof, and an organic polymer having an OH group attached thereto are dissolved in distilled water and ethanol; and shrinking the polyamic acid nanofiber membrane simultaneously in all three axial directions at a uniform shrinkage ratio. Since the nanofibers constituting the polyamic acid nanofiber membrane of the present invention are entangled in three axial directions while maintaining curved shapes, by using the polyamic acid nanofiber membrane, a polyimide nanofiber membrane, a carbon nanofiber membrane, and a graphite nanofiber membrane, having superb flexibility, tensile strength, and electrical properties, a high dielectric constant, easily controllable porosity, and the like, can be manufactured.

Description

Polyamic acid nanofiber membrane in which curved nanofibers are entangled three-dimensionally and method for manufacturing the same

[0001] The present invention relates to a polyamic acid nanofiber membrane in which curved nanofibers are entangled three-dimensionally and a method for manufacturing the same. More specifically, the invention relates to a polyamic acid nanofiber membrane composed of curved nanofibers and having a structure in which the curved nanofibers are entangled in three axial directions, and a method for manufacturing the same, so as to be useful as a precursor for manufacturing polyimide nanofiber membranes, carbon nanofiber membranes, and graphite nanofiber membranes having excellent flexibility and dielectric constant.

[0002] The present invention produces a polyamic acid nanofiber membrane having a very ideal structure in which nanofibers are intertwined in all directions (up, down, left, and right) and has a very high nanofiber density, by performing electrospinning using polyamic acid and copolymers thereof, and immersing the resulting material in a solution in which a hydrogen bond-promoting compound containing ammonium groups and a hydrogen bond-inducing polymer such as polyethylene glycol diacrylate are dissolved, thereby shrinking the material in three axial directions within 2 minutes. The polyamic acid nanofiber membrane having a very ideal structure in which nanofibers are intertwined in all directions (hereinafter referred to as PAA-S) is used to perform imidization by thermal or chemical methods to produce a polyimide nanofiber membrane having a structure in which nanofibers are intertwined in all directions (hereinafter referred to as PI-S). Next, carbonization is performed using PI-S at over 900°C to produce a carbon nanofiber membrane (hereinafter referred to as CNF-E) that has excellent flexibility as well as tensile strength, with the nanofibers maintaining a curved shape and having a structure in which the nanofibers are intertwined in all directions. CNF-E can be used in heating elements, electrode materials for secondary batteries, gas diffusion layers for fuel cells, etc. by utilizing properties such as high conductivity, high heat resistance, lightweight and chemical resistance, and a high dielectric constant.

[0003] In addition, graphitizing CNF-E at over 2000°C produces a graphite nanofiber membrane (hereinafter referred to as GNF-E), which has a structure in which nanofibers are intertwined in all directions (up, down, left, and right) of CNF-E. Since GNF-E is formed through a graphitization process at high temperatures above 2000°C, it possesses excellent thermal stability, mechanical strength, electrical conductivity, oxidation resistance, and a very high dielectric constant. Due to these characteristics, it can be applied as a core material in the aerospace industry, including radar absorption materials for supersonic aircraft.

[0004] Shrinkage in polyamic acid nanofiber membranes can occur through a wide variety of mechanisms. When electrospun nanofiber membranes are immersed in a solvent, shrinkage occurs due to factors such as polymer-solvent interactions, capillary effects, internal stress relaxation, polymer rearrangement, and changes in crystallization. Regarding polymer-solvent interactions, when a solvent penetrates the nanofiber membrane, the intermolecular bonds between polymer chains (e.g., hydrogen bonds, van der Waals forces) weaken. Some solvents partially swell the polymers, inducing structural instability. When the solvent evaporates, the polymer chains rearrange and tend to return to their original state, resulting in shrinkage. In terms of capillary effects and surface tension, when a solvent penetrates the nanofiber network, a capillary effect occurs, causing the liquid to move along the micropores. Upon solvent evaporation, surface tension acts to pull the fiber structure inward, inducing overall shrinkage. In particular, polar solvents can enhance capillary condensation, further increasing the degree of shrinkage. Internal stress relief occurs because electrospun nanofibers retain residual mechanical stress within the fiber as they are stretched by high voltage. When immersed in a solvent, these stresses are relieved as polymer chains interact with solvent molecules. This leads to a denser structure, which can result in overall shrinkage. The higher the tension applied during the electrospinning process, the greater the likelihood of shrinkage upon solvent exposure. Due to polymer chain rearrangement and crystallization changes, some polymers may undergo structural rearrangement by the solvent, transforming into a denser form. As polymers in the amorphous regions rearrange through interactions with the solvent, changes in intermolecular forces cause the membrane to shrink. Conversely, highly crystalline polymers (e.g., polyimide (PI), polytetrafluoroethylene (PTFE)) exhibit relatively less shrinkage.Regarding the effect of solvent on shrinkage, good solvents interact strongly with the polymer, causing significant swelling, and extreme shrinkage may occur after solvent evaporation (e.g., DMF, DMSO, THF, acetone). Moderate solvents partially swell the polymer and exhibit moderate shrinkage (e.g., ethanol, isopropanol). Poor solvents interact weakly with the polymer, resulting in minimal swelling and almost no shrinkage (e.g., hexane, toluene).

[0005] In summary, solvent shrinkage of electrospun nanofiber membranes is caused by factors such as weakening of polymer chain interactions, capillary effects, relaxation of mechanical stress, polymer rearrangement, and changes in crystallization. The degree of shrinkage varies depending on the type of solvent, the physical properties of the polymer, and electrospinning conditions.

[0006] For example, a polyvinylidene fluoride (PVDF)-acetone system is a case of shrinkage in electrospun nanofiber membranes. PVDF nanofibers are primarily bound by van der Waals forces and dipole-dipole interactions; however, when immersed in acetone, the interaction between PVDF and acetone becomes stronger than the interaction between PVDF and PVDF. Consequently, the nanofiber structure becomes less rigid and more flexible. Solvent molecules penetrate the nanofiber network, separating the polymer chains and causing slight swelling. Although acetone does not effectively dissolve PVDF, it causes partial swelling. As the nanofibers absorb the solvent, the arrangement of the polymer matrix is ​​disrupted and plasticized. This weakens structural support and makes the structure more susceptible to deformation. As the solvent evaporates, the polymer chains attempt to return to their original state, but the chains that migrated during the swelling process rearrange into a denser structure. Capillary forces generated by the evaporation of the solvent within the nanofiber compress the fibers more densely. As a result, the nanofiber membrane shrinks. PVDF nanofiber membranes shrink in both the transverse and thickness directions. Regarding the effect of residual charge on electrospun nanofiber membranes immersed in a solvent, electrical loads may remain on the electrospun nanofibers, and this residual charge can affect the fiber structure and degree of shrinkage within the solvent. When residual electrostatic charge is present in electrospun nanofibers, applying a high voltage (10–50 kV) during the electrospinning process induces a strong charge on the nanofibers. In insulating polymers (e.g., PI, PVDF, PS), residual charge is not easily released and can persist for a long time. Residual charge creates electrostatic repulsion between fibers, affecting fiber alignment and pore structure. During solvent immersion, the influence of residual electrostatic charge is that the repulsion between fibers decreases due to solvent penetration, causing the fibers to move closer together. If charge remains, there is a high probability that the fibers will shrink rapidly upon solvent evaporation. The more polar the solvent (DMF, DMSO, water), the more severe the shrinkage becomes as charge rearrangement is promoted.Methods to reduce the effects of residual charge include removing the charge through heat treatment (100–200°C) and promoting charge release by adding carbon nanotubes (CNT) or graphene. Surface charge can also be neutralized through plasma treatment.

[0007] Residual electrostatic charge can alter the structural stability and degree of shrinkage of nanofiber membranes after solvent immersion. Controlling these effects can improve the stability of nanofiber membranes in the solvent. Polyamic acid nanofiber membranes are utilized to induce simultaneous shrinkage in both axial directions. Simultaneous shrinkage can be achieved through uniform heat treatment. Polyamic acid nanofiber membranes are converted into polyimide (PI) through heat treatment, and shrinkage occurs during this process. However, since shrinkage generally occurs randomly when heat-treated in a free state, it is difficult to induce uniform mutual entanglement in three axial directions.

[0008] Polyamic acid nanofiber membranes are fixed under isotropic tension and then heat-treated at a constant temperature. For example, if heat-treated at 200–350°C after mounting on a frame, uniform biaxial shrinkage can be induced. Glass plates, metal frames, silicone rubber jigs, etc., can be used as the frame material. There is also a method of performing heat treatment after applying pre-tension. If the membrane is fixed in an initial state with equal tensile force applied in both axial directions and then heated at a constant rate, shrinkage proceeds uniformly. Generally, at 250–300°C, polyamic acid (hereinafter referred to as PAA) is converted into polyimide (PI), and a specific shrinkage rate occurs during this process. Another method involves inducing uniform shrinkage through solvent treatment. PAA nanofibers are partially plasticized by specific solvents, and shrinkage may occur as the solvent is removed. By exposing PAA nanofiber membranes to NMP (N-methyl-2-pyrrolidone) vapor or DMF (dimethylformamide) vapor using the solvent vapor annealing method, the fibers can soften and shrink uniformly in the biaxial direction. Subsequently, the drying process is controlled to maintain isotropic shrinkage. A consistent shrinkage effect can be obtained by uniformly spraying solvents such as isopropanol (IPA), acetone, or DMF, followed by natural drying. During this process, external mechanical tension must be minimized to induce natural biaxial shrinkage.

[0009] Biaxial simultaneous shrinkage can also be achieved using mechanical processing. By utilizing mechanical methods, shrinkage can be induced by applying a uniform force in two axial directions rather than in a specific direction. By performing biaxial mechanical pressing, a nanofiber membrane is placed on an elastomer (silicone sheet), and a press applying the same force in two axial directions is applied for a certain period of time, followed by heat treatment, to induce uniform shrinkage in two axial directions. Alternatively, uniform shrinkage can be induced by initially stretching the PAA nanofiber membrane in two axial directions and then allowing it to naturally release.

[0010] One method of inducing shrinkage through nanostructure control is to pre-modify the structure of PAA nanofibers, thereby controlling the direction of shrinkage. By utilizing a core-shell structure, it is possible to design a bilayer structure (core-shell nanofibers) during PAA electrospinning to ensure uniform deformation during shrinkage. For example, fabricating with a PAA / polyurethane (PU) core-shell structure allows for the induction of shrinkage in a uniform direction after heat treatment. Another method involves control through polymer blending. By blending PAA with other polymers (e.g., PU, ​​PEO, etc.) and utilizing their different thermal behaviors, shrinkage can be induced in a specific direction.

[0011] However, no method has yet been reported to shrink polyamic acid nanofiber membranes in a very short time, namely within one minute, through the various methods mentioned above. In the present invention, when PAA nanofiber membranes are manufactured by electrospinning using polyamic acid and copolymers, which are precursors of polyimide nanofiber membranes, the nanofibers exhibit a linear shape; however, carbon nanofibers produced by carbonizing PI nanofibers having such a linear structure cannot secure flexibility. Furthermore, PI nanofiber membranes possessing a linear nanofiber shape also act as many limiting factors when applied to battery separators, such as those for secondary batteries, in terms of securing uniformity of pores.

[0012] Although shrinkage in three axial directions can be performed through the various methods mentioned above, mechanical methods, solvent vapor methods, and even methods using nanostructures can induce shrinkage in three axial directions simultaneously through very complex equipment and complex processes. In addition, there is a disadvantage in that precise control of shrinkage in three axial directions is difficult.

[0013] The present invention is characterized by the production of a PAA nanofiber membrane (PAA-S) that is easier and more uniform than the various methods mentioned above, and in which nanofibers instantaneously contract and intertwine in the up, down, left, and right directions and in three axial directions. The reason this is possible is that when a hydrogen bond-promoting compound containing ammonium groups and hydrogen bond-inducing polymers formed with double bonds of polyethylene glycol diacrylate (PEGDA) at their ends are dissolved together in a solution of distilled water and ethanol to prepare a shrinkage treatment solution, ammonium chloride is ionized and the acrylates located at both ends of PEGDA are separated, thereby inducing strong hydrogen bonds with oxygen and nitrogen located in PAA, and the hydrogen in the ammonium groups also contributes significantly to the hydrogen bonding, thereby creating a mechanism that instantaneously shrinks the PAA nanofiber membrane.

[0014] Nanofiber membranes manufactured by electrospinning using PAA polymers possess a linear nanofiber structure. Consequently, when these PAA nanofiber membranes are subsequently subjected to imidization followed by carbonization (900–1500°C) or graphitization (above 2000°C), industrial application is highly difficult due to the excessively low brittleness of the resulting carbon or graphite nanofiber membranes. This is because, when manufacturing polyimide nanofiber membranes, the fibers maintain linearity rather than curvature, leading to issues regarding pore size, porosity, and tensile strength. Furthermore, carbon or graphite nanofiber membranes produced after carbonization or graphitization using such polyimide nanofiber membranes are prone to crumbling due to their extremely low brittleness. Therefore, if industrially applicable fibers are formed into a curved structure that is tangled up, down, left, and right, many problems such as tensile strength, flexibility, electrical properties, dielectric constant, and porosity control of the aforementioned polyimide nanofibers, carbon nanofibers, and graphite nanofibers can be solved.

[0015] The sample in which a PAA nanofiber membrane is immersed in the previously described shrinkage treatment solution to induce triaxial shrinkage—in other words, the shrunken PAA nanofiber membrane—will hereafter be referred to as PAA-S (shrinkage PAA). PAA-S is imidized by heat at over 200°C under a nitrogen atmosphere or through chemical methods. The chemical method is explained in more detail as follows.

[0016] PAA exists in the forms of amide (-NH-) and carboxylic acid (-COOH) groups and is converted into polyimide (PI) through a cyclization reaction. In this process, chemical imidization is performed by imidizing solid-state PAA at relatively low temperatures using a dehydrating agent and a catalyst (or accelerator). Acetic anhydride (Ac₂O) is used to remove water generated during the imidization reaction, and basic organic bases such as pyridine, isoquinoline, or triethylamine (TEA) are used as catalysts. Very dilute solutions such as NMP, DMF, and DMAc are utilized as solvents, but care must be taken to avoid dissolving the nanofibers. The reaction is carried out in such an immersion solution at 40°C to 80°C for 30 minutes to 1 hour. Residual reagents are removed by washing with methanol or ethanol. The PI nanofiber membrane is then prepared by drying.

[0017] The imidation method of PAA nanofiber membranes is facilitated by heating at about 200 to 400°C under a nitrogen atmosphere.

[0018] The PI nanofiber membrane itself, which is uniform and simultaneously intertwined with each other in all directions using PAA-S, has excellent tensile strength and also has a uniform pore distribution as well as pore size and porosity, so it can be utilized in various industrial applications such as Li-S all-solid separation membranes, energy storage separation membranes, and filters for air and water treatment.

[0019] PI-S has a structure in which nanofibers are intertwined in all directions (up, down, left, and right). Carbon nanofiber membranes (CNF) produced by imidizing nanofibers manufactured by conventional electrospinning and then carbonizing them have low flexibility and break easily, which poses a significant obstacle to the development of industrial applications. However, when carbon nanofiber membranes are manufactured by carbonizing at over 900°C using the PI-S of the present invention, the nanofibers have an intertwined structure in a curved shape. Although conventionally manufactured carbon nanofibers possess excellent properties such as excellent conductivity, heat resistance, and chemical resistance, the biggest obstacle to industrial application is their lack of brittleness, which makes them prone to crumbling. Carbon nanofibers (CNF-E) with an intertwined structure that solves these problems can utilize flexibility and high-performance characteristics, and can satisfy the essential requirements of major industries and various other industries. It is very important that a three-dimensional structure composed of nanofibers entangled in all directions can maintain excellent softness and tensile strength.

[0020] Using PAA-S (referring to a PAA nanofiber membrane that has been instantaneously shrunk after immersing PAA in a hydrogen bond-inducing solution), an imidized polyimide nanofiber membrane (PI-S) entangled in all directions via physical or chemical methods is subjected to a carbonization process at 900-1500°C under an inert gas. The resulting carbon nanofiber membrane, which maintains a curved shape while exhibiting excellent flexibility, possesses not only excellent flexibility but also consistent tensile strength. This highly flexible carbon nanofiber membrane has numerous industrial applications. From now on, this entangled carbon nanofiber membrane will be referred to as CNF-E. Generally, carbon nanofibers produced by imidization and carbonization following conventional electrospinning—rather than PAA-S—have very low flexibility and are prone to breakage, making industrial applications impossible.

[0021] To enhance the electrical conductivity or flexibility of CNF-E, graphitization is performed at 2000°C or higher to produce a graphite nanofiber membrane in which nanofibers entangled in all directions are formed into a curved shape. Subsequently, the graphite nanofiber membrane in which nanofibers entangled in all directions are formed into a curved shape is called GNF-E.

[0022] The polyamic acid nanofiber membrane produced by the present invention has a structure in which the nanofibers constituting it maintain a curved shape and are intertwined in three axial directions, so it is possible to produce polyimide nanofiber membranes, carbon nanofiber membranes, and graphite nanofiber membranes with excellent flexibility, tensile strength, electrical properties, dielectric constant, heat resistance, and porosity by using this.

[0023] Figure 1 is an electron microscope image of a polyamic acid nanofiber membrane in an unshrinked state prepared in Example 1.

[0024] Figure 2 is a low-magnification electron microscope image of a polyamic acid nanofiber membrane prepared in Example 1 and in a shrunken state.

[0025] Figure 3 is a high-magnification electron microscope image of a polyamic acid nanofiber membrane prepared in Example 1 and in a shrunken state.

[0026] Figure 4 is an electron microscope image of a polyamic acid nanofiber membrane in a shrunken state prepared according to Example 2.

[0027] Figure 5 is an electron microscope image of a carbon nanofiber membrane prepared in Example 3.

[0028] Figure 6 is a photograph showing the flexibility measurement of the carbon nanofiber membrane prepared in Example 3.

[0029] Figure 7 is an electron microscope image of a polyamic acid nanofiber membrane in a shrunken state prepared according to Comparative Example 1.

[0030] Figure 8 is a figure showing the mechanism of shrinkage of the polyamic acid nanofiber membrane in Example 1.

[0031] Figure 9 is a figure showing the form in which PEGDA dissolved in the shrinkage treatment solution in Example 1 is hydrolyzed to separate into PEG and acrylic acid, and the reaction mechanism in which ammonium chloride is separated into an ionic state by hydrolysis.

[0032] Figure 10 is a figure that further comprehensively explains, at the molecular chain level, the mechanism by which shrinkage occurs within a very short time when the polyamic acid nanofiber membrane in Example 1 is immersed in a shrinkage treatment solution.

[0033] The present invention will be described in detail below.

[0034] The polyamic acid nanofiber membrane according to the present invention is characterized by curved nanofibers being entangled in three dimensions.

[0035] A method for manufacturing a polyamic acid nanofiber membrane according to the present invention, more specifically a polyamic acid nanofiber membrane in which curved nanofibers are three-dimensionally intertwined, comprises: (i) a process of manufacturing a polyamic acid nanofiber membrane by electrospinning a polymer solution selected from a polyamic acid solution and a polyamic acid copolymer solution; and (ii) a process of immersing the polyamic acid nanofiber membrane for 10 seconds to 2 minutes in a shrinkage treatment solution in which (a) a hydrogen bond promoting compound selected from an ammonium compound containing multiple hydrogens, a halogen ion compound, a peroxide, a polysalt, and a polymer ion compound, and (b) a hydrogen bond inducing polymer selected from a polymer having a double bond of polyethylene glycol diacrylate attached to its end and an organic polymer having an OH group attached are dissolved together in distilled water and ethanol, thereby shrinking the polyamic acid nanofiber membrane simultaneously in three axial directions at a constant shrinkage rate. Includes.

[0036] The above ammonium compound is one selected from ammonium chloride, ammonium nitrate, and ammonium sulfate; the halogen ion compound is one selected from antimonium hexafluoride tetrafluoride hydrochloride, bromine tetrafluoride, and dichloriodide; the peroxide is one selected from hydrogen peroxide and ammonium peroxide; and the polymer ion compound is one selected from cyanide ion, thiocyanate ion, tetrafluoride boron ion, nitrogen ion, and sulfate ion.

[0037] The polymer having a double bond of the above-mentioned polyethylene glycol diacrylate attached to the end is one selected from polyethylene glycol dimethacrylate, polyethylene glycol diacrylamide, polypropylene glycol diacrylate, pentaerythritate tetraacrylate, trimethylolpropane triacrylate, urethane dimethacrylate, acrylated polyvinyl alcohol, gelatin methacryloyl, hyaluronic acid methacrylate, dextran methacrylate, Pluronic F127 diacrylate, PEO-PPO-PEO triblock copolymer, polyethylene glycol-fumarate copolymer, and polyethylene glycol-norbornene copolymer.

[0038] The organic polymer with the above-mentioned OH group attached is one selected from polypropylene glycol, polyvinyl alcohol, poly(N-vinylfinolidone), poly(2-hydroxyethyl methacrylate), polyacrylamide, polycarboxybetaine acrylamide, and polysarcosine.

[0039] The present invention uses a mixture of a polymer with a double bond of polyethylene glycol diacrylate attached and an organic polymer with an OH group attached as the hydrogen bond-inducing polymer.

[0040] A compound is required to promote hydrogen bonding between the carboxyl groups present in polyamic acid and oxygen and nitrogen, and to induce strong hydrogen bonding with hydrogen-rich compounds such as ammonium chloride. Examples of compounds similar to ammonium chloride include ammonium hydroxide, hydrizine, ethylene diamine, hydroxyl amine, and formamidine.

[0041] The principle is that organic polymers, such as the previously mentioned hydrogen bond-inducing polymer and polyethylene glycol diacrylate (PEGDA) with double bonds attached to the ends, undergo hydrolysis, causing the double bonds to dissociate and separate. Subsequently, ammonium chloride separates into ions, and the hydrogen from these ions forms strong hydrogen bonds with the oxygen or nitrogen contained in PAA. This results in instantaneous and uniform shrinkage, transforming linear nanofibers into curved shapes and causing entanglement among the nanofibers to form a highly ideal structure. Polymers that promote hydrogen bonding through the dissociation of terminal diacrylates include not only PEGDA but also polyethylene glycol (PEG). These are typical crosslinking agents with terminal double bonds (C=C) that are generally capable of photopolymerization or radical polymerization. Materials with terminal double bonds similar to PEGDA include the following. Polyethylene glycol dimethacrylate (PEGDMA) is similar to PEGDA, but the terminal group is methacrylate. Polyethylene glycol diacrylamide (PEGDAAm) has acrylamide instead of acrylate at the terminal. Polypropylene glycol diacrylate (PPGDA) uses PPG instead of PEG, pentaerythritate tetraacrylate (PETA) is a compound with four acrylate groups, and trimethylolpropane triacrylate (TMPTA) is an organic polymer with three acrylate groups.

[0042] In addition, acrylated polyvinyl alcohol (PVA-Ac) is produced by introducing acrylate groups into the hydrophilic polymer PVA. Naturally derived polymers such as gelatin methacryloyl (GelMA), in which methacryloyl groups are introduced into gelatin, can also be utilized. Hyaluronic acid methacrylate (HAMA) is hyaluronic acid with methacryloyl groups attached, and dextran methacrylate is dextran-based methacrylate attached.

[0043] Figure 1 shows the shape of a PAA nanofiber membrane before and after immersion in the aforementioned shrinkage treatment solution. Figures 2 and 3 are electron microscope images of PAA-S having a structure in which curved nanofibers are intertwined after instantaneous shrinkage occurs following immersion of the PAA nanofiber membrane in the shrinkage treatment solution.

[0044] Figure 8 is a figure showing the mechanism by which an ammonium compound and a PEGDA compound dissolved in a shrinkage treatment solution form chemical hydrogen bonds with oxygen and nitrogen capable of forming hydrogen bonds contained in PAA, thereby causing the PAA nanofiber membrane to shrink.

[0045] Figure 9 shows the reaction mechanism in which PEGDA dissolved in a shrinkage treatment solution is hydrolyzed into PEG and acrylic acid, and ammonium chloride is separated into an ionic state by hydrolysis.

[0046] Figure 10 is a figure that comprehensively explains, at the molecular chain level, the mechanism by which shrinkage occurs within a very short time when a polyamic acid nanofiber membrane is immersed in a shrinkage treatment solution.

[0047] Figure 7 shows the results of a shrinkage experiment using DEG (diethylene glycol) in a single-molecule form, which can be seen to show that the linear nanofibers remain intact and almost no shrinkage occurs. This is because DEG cannot exist in an ionic state, so its contribution to hydrogen bonding is low.

[0048] Figure 4 shows the results of shrinkage using PEG. Although the degree of curvature of the nanofiber is lower than when using PEGDA, more than 50% shrinkage occurred in a very short time. The reason for the lower degree of shrinkage compared to PEGDA is that the diacrylate at the end of PEGDA is hydrolyzed into acrylic acid, and this acrylic acid contributes to hydrogen bonding.

[0049] The reason PAA nanofiber membranes shrink rapidly is that when ammonium chloride or PEGDA is used alone, almost no shrinkage occurs. However, when hydrogen bond-inducing compounds are mixed with hydrogen bond-promoting polymers such as PGEDA, which is a hydrophilic polymer with C=C bonds attached to the ends to impart hydrogen bonds, very rapid shrinkage occurs in the three-dimensional direction, forming a structure in which the nanofibers become entangled in all directions.

[0050] Shrinkage also occurs due to OH groups attached to organic polymers such as PEG, which lack double bonds at the ends. For example, when using PEGDA, a shrinkage of about 70% occurs instantaneously at room temperature, whereas when using only PEG, a shrinkage of 52.8% occurred under the same conditions. However, when using the monomeric ethylene glycol, a shrinkage of about 3% occurs, which is considered to contribute very weakly to forming a strong hydro-bond with PAA. Controlling instantaneous shrinkage to occur more uniformly, transforming linear nanofibers into curved ones, and thereby enabling the nanofibers to form a three-dimensional entangled structure is a highly effective method for realizing an ideal structure in various aspects, such as controlling the weak mechanical properties and porosity of nanofibers produced by electrospinning.

[0051] Hydrophilic polymers, such as polyethylene glycol (PEG), can also be used as hydrogen bond-inducing polymers. The following are examples of polymer types that have properties similar to PEG. Polypropylene glycol (PPG) is similar to PEG but possesses slight hydrophobicity due to the presence of methyl groups; polyvinyl alcohol (PVA) is hydrophilic, and poly N-vinylpyrrolidone (PVP) is both hydrophilic and water-soluble. Poly 2-hydroxyethyl methacrylate (PHEMA) is a hydrophilic polymer useful for hydrogel formation, and polyacrylamide (PAAm) as well as polycarboxybetaine acrylate (PCBAA) can be used. Natural polymers such as polysarcosine can also be utilized.

[0052] Various applications can be found in the filter field utilizing PI nanofiber membranes, which feature a nanofiber structure intertwined in all directions. Polyimide (PI) nanofiber membranes possess excellent properties such as high heat resistance (>400°C), chemical resistance, mechanical strength, and an ultrafine pore structure, making them suitable for use as filtering materials that maintain stable performance in extreme environments compared to conventional membrane materials. Since polyimide is one of the few polymer materials that maintains stability even at high temperatures, it is used in environments where general nylon or polypropylene filters cannot withstand. Applications include industrial high-temperature gas filters; because they can be used without deformation even at temperatures exceeding 400°C, their porous structure allows them to maintain high filtration efficiency (>99.9%) even at high temperatures, and they exhibit strong resistance to harsh chemicals such as sulfuric acid, nitric acid, and organic solvents. Additionally, they are applicable to high-temperature dust removal filters, where they offer excellent wear resistance and superior lightness compared to conventional ceramic filters. As a filter for ultrafine dust and nanoparticles, the pore size of polyimide nanofibers can be controlled to the level of tens to hundreds of nanometers (nm), making it effective in removing ultrafine dust and viruses smaller than PM2.5.

[0053] Figure 2 is a figure showing the surface of PEGDA after shrinkage as a result of shrinkage using PEGDA, showing the nanofibers forming an ideally intertwined structure. This figure shows that when a hydrogen bond-inducing polymer material such as PEGDA, which has double bonds at the ends in a polymer form, is combined with a hydrogen bond-promoting compound having ammonium groups, the nanofibers show a more favorable three-dimensional intertwined structure.

[0054] With high temperature resistance (≥ 300-400°C), it can be used as a high-temperature exhaust gas filter in HVAC systems. Its excellent chemical and moisture resistance makes it stable against acidic gases, organic solvents, and moisture, making it suitable for industrial HVAC applications. It can maintain low pressure loss while removing fine dust through its porous structure that maintains airflow. Its strong thermal and mechanical durability over long-term use ensures a long lifespan, and it is washable, allowing for reuse. It is also promising as a solvent and chemical purification filter; since polyimide does not decompose even in environments with organic solvents, strong acids, and strong bases, it is suitable for use as a purification filter in chemical processes and semiconductor manufacturing.

[0055] It can be utilized in membranes for the semiconductor and pharmaceutical industries (Ultrafiltration & Nanofiltration), as well as in organic solvent filtration filters (Solvent Filtration). Since the intertwined nanofiber membrane has a unique structure that allows for uniformity while simultaneously maintaining high uniformity in the depth direction, it can achieve excellent results in industrial applications where it is difficult to apply conventional PI nanofiber membranes.

[0056] I would like to explain this by providing examples of utilizing polyimide nanofiber membranes in seawater desalination R / O membranes. Polyimide nanofiber membranes, which possess properties highly suitable for three-layer R / O membranes, exhibit not only heat resistance, chemical resistance, and mechanical properties but also an ideal structure in which nanofibers are entangled three-dimensionally. Polyimide nanofiber membranes are utilized as a single layer (base layer, support), and they exhibit excellent uniformity of porosity. This ensures mechanical strength and durability while maintaining high porosity to minimize resistance to water permeation. The pore size is controlled to be 10–100 nm, and the porosity is maintained at 60–80%. For the intermediate layer, the degree of shrinkage of the polyimide nanofibers is controlled to adjust the pore size to 1–10 nm and the porosity to 40–60%. In the process of shrinking PAA nanofibers by immersing them in a solution containing a hydrogen bonding promoter and a crosslinking polymer, the pore size can be controlled by adding various pore-forming materials such as LiCl, NaCl, and CaCO3 to form pores smaller than 1 nm. The intermediate layer is coated with PDA (polydopamine) or PEI (polyethyleneimine) to increase hydrophilicity.

[0057] An interfacial polymerization method using a combination of M-phenylenediamine (MPD) and trimethol chloride (TMC) is utilized. The previously mentioned basal layer and intermediate layer polyimide nanofiber membranes are prepared, and an MPD concentration of 2.0 wt% is prepared. As additives, 0.3 wt% of PVP (polyvinylpyrrolidone) is prepared for pore homogenization, and 0.3 wt% of TEA (triethylamine) is prepared to strengthen cross-linking. This solution is uniformly coated onto the nanofiber membrane. Next, a TMC solution composition is prepared.

[0058] Prepare a solution of TMC at a concentration of 0.15 wt% in isooctane solvent and perform dip coating (10–30 seconds). To complete interfacial polymerization, crosslinking is performed at a temperature of 60–90°C for 5–10 minutes. Wash to remove unreacted materials.

[0059] In addition, a structure in which PI nanofibers are densely intertwined in all directions can be applied as an R / O membrane by utilizing a membrane. It is also possible to form a membrane layer with a pore size of about 0.1-1 nm on the upper layer by dissolving PAA, a PI precursor, in NMP or DMF on a PI membrane and then gradually adding methylene diphenyl diisocyanate (MDI) and triazine as crosslinking agents in a certain ratio to perform crosslinking.

[0060] Artificial hemodialysis membranes with a multilayer structure can also be applied by utilizing a structure that is tangled up, down, left, and right. For example, the layer that comes into direct contact with blood can be used as an artificial hemodialysis membrane by utilizing a PI nanofiber membrane that is oxidized with hydrogen peroxide to introduce C=O carbonyl groups and binds to the NH2 groups of heparin to provide long-term hydrophilicity, thereby preventing blood coagulation and controlling the pore size to about 5-10 nm; the middle layer is a layer that removes low-molecular-weight toxins such as urea and prevents the loss of blood cells or albumin, and similarly introduces hydrophilic groups to bind to heparin, with the pore size controlled to about 10-80 nm; and the final layer, which is a supporting layer that maintains membrane strength and allows for rapid blood flow, with the pore size controlled to about 1-10 microns. Polyimide nanofibers intertwine in all directions (up, down, left, and right) and possess many advantages when manufacturing not only electrical conductivity but also mechanical properties, carbon nanofiber membranes, and graphite nanofiber membranes. Fundamentally, the experiment of an ideal structure by introducing carbon nanofibers through a simple solvent shrinkage process enables applications in various industrial fields.

[0061] Polyimide nanofiber membranes are a promising material capable of improving battery life and performance when applied as separators for Li-S all-solid-state batteries, as they possess advantages such as excellent mechanical strength, chemical stability, polysulfide blocking effects, and compatibility with solid electrolytes. The key is to block the flow of conductive electrons (holes and electrons) while ensuring the smooth movement of lithium ions within the battery. To achieve this, methods such as composite formation with solid electrolytes, surface modification, and nanostructure control are required.

[0062] To facilitate the transport of lithium ions, the composite of ion-conducting polymers and solid electrolytes is necessary. It is required to introduce ion-conducting polymers (PEO, PVDF-HFP, PAN) onto the surface or inside polyimide nanofibers. In the case of PEO (polyethylene oxide), ion conductivity is enhanced by including ether bonds that bind with lithium ions. To reduce interfacial resistance with the solid electrolyte, the solid electrolyte can be coated or infiltrated onto the surface of the polyimide nanofibers. Solid electrolyte microparticles (tens to hundreds of nm) are uniformly dispersed between the nanofibers to increase the transport pathways for lithium ions. To achieve this objective, a lithium-friendly design is required, which can be accomplished by introducing lithium-friendly functional groups such as -SO₃H (sulfonic acid), -PO₄H (phosphate), and -COOH (carboxyl group).

[0063] On the other hand, a modification method is required to block electron flow using polyimide nanofiber membranes as separators. Although polyimide is inherently a polymer with strong insulating properties, additional modification is necessary to further block electron conductivity. To block electron flow while maintaining high porosity, a structure with high porosity and low density is advantageous. As porosity increases, ion movement becomes smoother but electron conductivity decreases, thereby increasing electrical insulation. By reducing the nanofiber diameter or shaping the nanofiber structure into a curved form to create a continuous nanofiber network along three axes, the structure possesses excellent suitability for blocking the direct path of electron movement.

[0064] It is necessary to reduce conductivity by removing conductive carbon / metal nanoparticles. Carbon or metal impurities within the polyimide increase electronic conductivity, so their removal is required. Electron movement can be restricted by coating the surface with insulating polymers such as PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene) or PMMA (polymethyl methacrylate). Electron flow can be suppressed by thinly coating the surface of polyimide nanofibers with insulating ceramics such as Al₂O₃, SiO₂, or ZrO₂. Excellent effects can be obtained by adding a layer that blocks electron flow while maintaining lithium ion conductivity. Solid electrolytes such as LLZO, LATP, and LGPS are utilized as a coating layer to prevent direct electron movement.

[0065] The next important factor is the need to block the shuttle effect of polyimide nanofiber membranes. In Li-S batteries, the shuttle effect (polysulfide migration) is a major cause of battery performance degradation. This phenomenon involves lithium polysulfide moving through the electrolyte and traversing the anode and cathode during charging and discharging, resulting in reduced capacity and shortened lifespan. To prevent this, polyimide (PI) nanofiber membranes must be modified to maximize their polysulfide blocking capabilities. This can be achieved by applying a high-insulating ceramic coating; this is accomplished by coating the surface of polyimide nanofibers with ceramic nanoparticles such as Al₂O₃, SiO₂, ZrO₂, and TiO₂. By blocking electron transport pathways, the reduction of polysulfides is inhibited. Alternatively, the shuttle effect can be blocked by applying an insulating polymer coating; this can be achieved by coating the polyimide membrane with high-insulating polymers such as PVDF-HFP, PEO, PAN, and PMMA. Polysulfides can be selectively blocked using metal-organic frameworks (MOFs) such as ZIF-8 (Zeolitic Imidazolate Framework-8), MOF-808, and UiO-66. MOFs form a porous structure and can be used to suppress polysulfides while allowing lithium ions to move freely. Polysulfide migration is inhibited by impregnating polyimide nanofibers with solid electrolytes (LLZO, LATP, LGPS).

[0066] The pore size of polyimide nanofibers is optimized (~7 nm or less). -NH₂, -COOH, and -SO₃H functional groups are introduced through surface modification. An adsorption layer of polysulfide-affinity metal oxides, such as MnO₂ and Co₃O₄, is applied. Interfacial stability with solid electrolytes is ensured. Impregnation with solid electrolytes (LLZO, LATP, LGPS) facilitates ion transport while preventing the shuttle effect. Transition metal catalysts are introduced to add Co, Mo, and Ti-based catalyst layers, thereby promoting the Li₂S conversion of polysulfides.

[0067] To apply solid electrolytes at the cathode (lithium metal contact surface) and anode (sulfur contact surface), it is important to customize the design to match the characteristics of the lithium metal-containing cathode side and the sulfur-containing anode side when adding the solid electrolyte to the polyimide nanofiber membrane. When applying a solid electrolyte at the cathode (lithium metal contact surface), it is crucial to suppress lithium dendrite growth while maintaining interfacial stability with the lithium metal. An LLZO or LATP-based solid electrolyte is introduced on the cathode side, utilizing LLZO or LATP which possess high lithium affinity and excellent lithium ion conductivity. To maintain high ion conductivity while suppressing lithium dendrite penetration, the formation of a solid electrolyte-polyimide composite layer is necessary. LLZO or LATP is mixed with the polyimide nanofiber membrane to form a porous protective film, and combined with flexible ion-conducting polymers such as PEO / PVDF-HFP to maintain smooth contact with the lithium metal. In addition, an insulating ceramic coating (Al₂O₃, SiO₂, HfO₂) is introduced. By coating the surface in direct contact with the lithium metal with insulating ceramics such as Al₂O₃, SiO₂, and HfO₂, electron transfer is blocked. This suppresses the growth of lithium dendrites, thereby improving battery life.

[0068] As mentioned earlier, it is efficient to perform composites using mutually different types of polyimide nanofiber membranes at the surfaces in contact with the cathode and anode surfaces, but the interfacial resistance between these two surfaces may increase. To solve this, it is possible to introduce an ion-conducting solid electrolyte between the polyimide nanofiber membranes (formation of an interfacial electrolyte layer) by forming a solid electrolyte layer with high ion conductivity (LLZO, LATP, LGPS) between the two polyimide nanofiber membranes.

[0069] As such, conventional linear nanofiber membranes facilitate uniform pore size, mechanical properties, and uniform impregnation of solid electrolytes; therefore, a structure in which curved nanofibers are intertwined in all directions (up, down, left, and right) possesses an optimal structure, particularly for use as a separator in secondary batteries. It forms a structure that is highly ideal for use in the aforementioned Li-S all-solid-state secondary battery, as well as various Li-S ion secondary batteries and various ESSs.

[0070] Another application example involves utilizing a structure in which PI nanofibers are entangled in various directions (up, down, left, and right) to serve as a nanofiber waterproof membrane for devices such as mobile phone cameras. A key requirement for this is water entry pressure. A water entry pressure of ≥ 30 kPa is required. To ensure transparency, the pore size should be < 100 nm, ideally between 10 and 50 nm, and the film thickness must be within 1 to 5 μm. Additionally, a transmittance of ≥ 90% in visible light and a superhydrophobic contact angle of ≥ 150° are required; these requirements can be met by introducing silane coatings.

[0071] This relates to a heating element based on CNF-E. The objective is to manufacture a carbon nanofiber heating element with the lowest possible thermal conductivity to enable heating with low power consumption, thereby allowing for simple attachment or detachment of the heating element to clothing or wallpaper. Conventional carbon fibers require high power consumption due to their very high heat transfer coefficients.

[0072] P=U x A x △T

[0073] Here, P represents power consumption (W), and U is the heat transfer coefficient (W / (m²)). 2 K) and A is the area (m² 2 It represents ), and △T(K) represents the temperature difference.

[0074] P=IxV

[0075] Here, P represents power consumption (W), I (A) is current, and V (V) is voltage.

[0076] Low power consumption is essential when intending to create a heating element attached to clothing using a standard battery. For example, if a lithium-ion rechargeable battery with a capacity of 10,000 mAh is used and the device measures 10 cm x 10 cm, and the device is attached to clothing, with a starting temperature of 20°C and the heating element temperature of 50°C, then ΔT becomes 30K. To maintain a 10W output for 8 hours using a rechargeable battery (10,000 mAh, 5V output) and to maintain a 30°C temperature difference using a 10 cm × 10 cm carbon nanofiber heating element, the resistance of the carbon nanofiber heating element must be maintained at a constant level. For the rechargeable battery to generate heat, the output is 10W at 5V and 2A. In the case of a 10,000 mAh battery, the total energy of the rechargeable electrons is 37Wh because the lithium-ion battery operates at 3.7V. Using it for 8 hours requires 4.6W per hour. Therefore, to design the carbon nanofiber heating element to meet this 4.6W power consumption, a current of 0.92A is required since the heater is supplied with 5V. Consequently, it is necessary to fabricate a carbon nanofiber membrane with a constant resistance, i.e., R=V 2 Since it is / P, if we calculate the resistance, 5.4 Ω is needed.

[0077] Gas Diffusion Layers (GDLs) for fuel cells require mechanical strength and compressive durability; to satisfy these requirements, mechanical properties such as tensile strength, compressive strength, and compressive modulus are critical. Generally, the mechanical property standards for GDLs used in PEMFCs and other types of fuel cells are as follows. Since shape retention is required during fuel cell assembly and operation, tensile strength of approximately 5–10 MPa is necessary. Compressive strength of approximately 3–10 MPa is required to maintain the pore structure under compressive stress. Elastic recovery, which refers to the ability to return to the original thickness after compression, must be at least 70%. The porosity must be at least 70%, and the thickness should be approximately 100–400 micrometers; if the thickness is too thin, mechanical strength is insufficient, while if it is too thick, gas diffusion is hindered. Carbon nanofiber membranes that possess the necessary conditions to sufficiently meet these physical property requirements, as well as electrical conductivity, can be applied to GDLs.

[0078]

[0079] The production and separation of hydrogen (H₂) through ammonia (NH3) cracking is an important technology in the hydrogen economy.

[0080] CNF-E-based membranes offer advantages such as high-temperature stability, high porosity, and gas selectivity, making them suitable for application as hydrogen separation membranes. High-temperature stability (above 900°C) allows for use under ammonia cracking reaction conditions, and adjustable pore size (0.3–1.0 nm) enables selective hydrogen permeation. Excellent chemical resistance ensures stability in ammonia and nitrogen environments. Palladium (Pd) membranes exhibit excellent selectivity but are very expensive. Regarding gas molecule size, the pore size of a CNF membrane is H₂ (0.29 nm); adjusting the pore size to 0.3–0.5 nm (selective permeability) allows for the passage of only hydrogen.

[0081] Graphite nanofiber membranes (GNF-E) are advanced materials possessing high conductivity, high heat resistance, excellent mechanical strength, and chemical stability, making them applicable in various industries. Carbon nanofibers, known for their excellent ductility, can be utilized in diverse sectors such as energy, biotechnology, environment, aviation, and wearables. In particular, the development of more innovative high-performance materials is possible when technologies such as porous structure design, electrochemical functionalization, and mechanical strength control are combined. They can be effectively used as electrodes for lithium-ion batteries (Li-ion), lithium-sulfur batteries (Li-S), all-solid-state batteries (ASSB), and supercapacitors. GNF-E can be utilized in various industries including energy, environment, biotechnology, aviation, and electronic devices. Specifically, applications such as porous electrodes for lithium-sulfur batteries, conductive membranes for all-solid-state batteries, biosensor electrodes, and flexible heat dissipation materials are possible. The aviation sector is a field with high potential for the application of GNF-E. GNF-E is formed through a graphitization process at high temperatures of over 2000°C, and thus possesses excellent thermal stability, mechanical strength, electrical conductivity, and oxidation resistance. Due to these characteristics, it has high potential for application in the following key areas of the aerospace industry. These include thermal shielding structures for spacecraft, hypersonic aircraft (Mach 5 or higher), and reusable rockets (RLVs); internal components of aircraft engines (turbine blades, nozzle liners, heat dissipation systems); and anti-friction and high-temperature protective coatings (spacecraft return capsules, aircraft materials for extreme environments).

[0082] One application is in the aerospace field, where three-dimensional nanofiber structures can be excellently imparted to evade enemy radar detection or reduce signal reflection by absorbing radio waves. Polyimide nanofiber-based RAM (Radar Absorber) can be utilized. PI nanofibers have a low dielectric constant ε rIt has a value of approximately 2.2 to 3.0. It can be applied as the outer layer (electromagnetic wave penetration layer) of the frequency RAM in the L-band (1-2 GHz) and S-band (2-4 GHz) frequency bands, and transmits electromagnetic waves through the internal multilayer structure. As an intermediate layer, it is a carbon nanofiber layer (εr = 10~50) frequency band: X-band (812 GHz). As an absorption layer of the layer, it functions as the absorption layer (electromagnetic wave attenuation layer) of the RAM and causes electromagnetic wave absorption and internal multiple reflection.

[0083] Graphite nanofiber (GNF) membranes possess a very high dielectric constant ranging from 50 to 200 and act as a highly conductive layer, functioning as an absorption layer in the frequency range of the Ku and Ka bands (12–40 GHz). When a three-layer structure based on PI nanofibers is applied as a RAM material, it can be used for broadband RAM in the 1–40 GHz range. However, general PI nanofiber membrane structures have linearity in their fiber structure and lack entanglement in all directions, which limits the frequency attenuation through reflection and absorption functions when applied as a high-performance RAM material.

[0084] However, the present invention exhibits an optimal structure for reflecting or absorbing light of the input wavelength, as the nanofibers exhibit excellent mutual entanglement in the up, down, left, and right directions. Generally, even if PI nanofibers are formed into a partially curved shape, residual stress is restored after the carbonization process, causing the nanofibers to straighten out into a straight line, making them unsuitable for use as RAM materials.

[0085] As previously mentioned, the nanofiber membrane based on PAA-S possesses more uniform pore characteristics because the nanofibers are entangled and held together in a three-dimensional manner. Consequently, PI-E obtained by imidizing PAA-S maintains a structure in which the nanofibers are intertwined in a curved shape. Furthermore, CNF-E, produced by carbonizing PI-E, also maintains a structure in which the nanofibers are intertwined in a curved shape, thereby producing a membrane with superior flexibility. Moreover, GNF-E, which is obtained by graphitizing CNF-E, not only exhibits high conductivity but also maintains excellent flexibility as the graphitized nanofibers retain their curved shape and are intertwined in all directions.

[0086] The industrial application of each of the aforementioned materials, namely PI-E, CNF-E, and GNF-E, is not limited to that.

[0087] The present invention will be examined in more detail below through examples.

[0088] Example 1

[0089] A polyamic acid solution with a concentration of 12 wt% was prepared by dissolving a polyamic acid polymer in a dimethylformamide solvent, and then the solution was electrospun to produce a polyamic acid nanofiber membrane.

[0090] At this time, the radiation voltage was set to 20 kV and the radiation range to 25 cm.

[0091] Next, a shrinkage treatment solution was prepared by mixing a first solution in which polyethylene glycol diacrylate was dissolved at a concentration of 3% in ethanol and distilled water and a second solution in which ammonium chloride was dissolved at a concentration of 3% in ethanol and distilled water, and then the polyamic acid nanofiber membrane was immersed in this solution for 20 seconds to produce a polyamic acid nanofiber membrane that was shrunk by 70% in three axial directions.

[0092] The electron microscope image of the unshrinked polyamic acid nanofiber membrane was as shown in Fig. 1, and the low-magnification and high-magnification electron microscope images of the shrink-treated polyamic acid nanofiber membrane were as shown in Figs. 2 and 3, respectively.

[0093] Example 2

[0094] A shrink-treated polyamic acid nanofiber membrane was prepared using the same process and conditions as in Example 1, except that the polyethylene glycol diacrylate dissolved in the shrink-treatment solution of Example 1 was changed to polyethylene glycol, and the immersion time of the polyamic acid nanofiber membrane in the shrink-treatment solution was changed to 2 minutes. An electron microscope image of the shrink-treated polyamic acid nanofiber membrane was shown in Figure 4.

[0095] Example 3

[0096] A polyimide nanofiber membrane was prepared by imidizing the shrinkage polyamic acid nanofiber membrane prepared in Example 1 at 350 °C for 1 hour under a nitrogen atmosphere, and then a carbon nanofiber membrane was prepared by carbonizing the polyimide nanofibers at 1,250 °C under a nitrogen atmosphere. An electron microscope image of the carbon nanofiber membrane is shown in Fig. 5, and a photograph of its flexibility measurement is shown in Fig. 6.

[0097] Comparative Example 1

[0098] A shrink-treated polyamic acid nanofiber membrane was prepared using the same process and conditions as in Example 1, except that the polyethylene glycol diacrylate dissolved in the shrink-treatment solution of Example 1 was changed to monomeric diethylene glycol, and the immersion time of the polyamic acid nanofiber membrane in the shrink-treatment solution was changed to 5 minutes. An electron microscope image of the shrink-treated polyamic acid nanofiber membrane was shown in Fig. 7.

Claims

1. A polyamic acid nanofiber membrane characterized by curved nanofibers being entangled with each other in three dimensions.

2. (i) A process for manufacturing a polyamic acid nanofiber membrane by electrospinning a polymer solution selected from a polyamic acid solution and a polyamic acid copolymer solution: and (ii) a process of immersing the polyamic acid nanofiber membrane for 10 seconds to 2 minutes in a shrinkage treatment solution in which (a) one hydrogen bond-promoting compound selected from ammonium compounds containing multiple hydrogens, halogen ion compounds, peroxides, polysalts, and polymer ion compounds, and (b) one polymer selected from a polymer having a double bond of polyethylene glycol diacrylate attached to its end and an organic polymer having an OH group attached, together with the polyamic acid nanofiber membrane, thereby shrinking the polyamic acid nanofiber membrane simultaneously in three axial directions at a constant shrinkage rate; characterized by comprising the above steps.

3. A method for manufacturing a polyamic acid nanofiber membrane in which curved nanofibers are entangled three-dimensionally, wherein, in paragraph 2, the ammonium compound is one selected from ammonium chloride, ammonium nitrate, and ammonium sulfate; the halogen ion compound is one selected from antimonium hexafluoride tetrafluoride hydrochloride, bromine ion tetrafluoride, and dichloriodine ion; the peroxide is one selected from hydrogen peroxide and ammonium peroxide; and the polymer ion compound is one selected from cyanide ion, thiocyanate ion, tetrafluoride boron ion, nitrogen ion, and sulfate ion.

4. A method for manufacturing a polyamic acid nanofiber membrane in which curved nanofibers are entangled three-dimensionally, wherein, in paragraph 2, the polymer having a double bond of polyethylene glycol diacrylate attached to the end is one selected from polyethylene glycol dimethacrylate, polyethylene glycol diacrylamide, polypropylene glycol diacrylate, pentaerythritate tetraacrylate, trimethylolpropane triacrylate, urethane dimethacrylate, acrylated polyvinyl alcohol, gelatin methacryloyl, hyaluronic acid methacrylate, dextran methacrylate, Pluronic F127 diacrylate, PEO-PPO-PEO triblock copolymer, polyethylene glycol-fumarate copolymer, and polyethylene glycol-norbornene copolymer.

5. A method for manufacturing a polyamic acid nanofiber membrane in which curved nanofibers are entangled three-dimensionally, characterized in that, in claim 2, the organic polymer with attached OH groups is one selected from polypropylene glycol, polyvinyl alcohol, poly(N-vinylpinolidone), poly(2-hydroxyethyl methacrylate), polyacrylamide, polycarboxybetaine acrylamide, and polysarcosine.

6. In paragraph 2, using a mixture of a polymer with a double bond attached to polyethylene glycol diacrylate and an organic polymer with an attached OH group as a hydrogen bond-inducing polymer. A method for manufacturing a polyamic acid nanofiber membrane in which curved nanofibers characterized by being entangled with each other in three dimensions.

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