TW202410959A - Porous membrane, composite membrane, module, fluid separation device and method of manufacturing porous membrane - Google Patents

Abstract

The porous membrane and composite membrane of the present invention contain cross-linked polymers formed by cross-linking aromatic polymers with each other, wherein the aromatic polymer is a polymer formed by copolymerizing at least one fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and anhydride monomers, and the copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer in the aromatic polymer is in the range of 25:75 to 70:30 in terms of molar ratio.

Description

Porous membrane and composite membrane

The present invention relates to a porous membrane and a composite membrane.

Membrane separation is widely used as a process for energy and resource conservation in the separation of liquid mixtures. The main application of membrane separation is water treatment, but in recent years, the application of separation for organic solvents has been expanding. Specifically, in the production of synthetic chemicals, pharmaceuticals, foods, fibers, electronic materials, etc., membrane separation is used for the concentration of wastewater containing organic solvents, the removal of solutes in organic solvents, the recovery of valuables from organic solvents, and the separation and recovery of mixed organic solvents.

These have different names depending on the size of the separation target. When separating low molecular weight products less than 200, it is called organic solvent reverse osmosis (OSRO); when removing molecules with a molecular weight of 200 to 1,000, it is called organic solvent nanofiltration (OSN); when removing molecules and impurities with a molecular weight above the above, it is called organic solvent ultrafiltration (OSU), etc.

Generally speaking, polymer membranes used for water treatment swell or dissolve in organic solvents, and therefore cannot maintain stability, which greatly reduces the separation efficiency. Therefore, the separation of liquid mixtures containing organic solvents requires the use of membranes that are resistant to organic solvents.

Various separation membranes are disclosed as separation membranes mainly for use in liquids containing organic solvents. For example, Patent Document 1 discloses a composite semipermeable membrane obtained by laminating a polyamide-based separation functional layer on a porous layer containing a polymer, wherein the polymer is selected from a fluoropolymer and an imide-containing polymer. Patent Document 1 describes that a separation membrane having excellent oil resistance and sufficient desalting performance is obtained thereby.

In non-patent document 1, an aromatic polymer is disclosed, which is obtained by polymerizing an aromatic diamine monomer or an aromatic diisocyanate monomer containing fluorine atoms and an acid anhydride monomer. It is known that the aromatic polymer has poor mechanical properties, so cracks are generated during the film formation and crosslinking of porous membranes and composite membranes, and sometimes it cannot be used as a separation membrane. [Prior art document] [Patent document]

Patent document 1: Japanese Patent Publication No. 2021-107071 [Non-patent document]

Non-patent reference 1: Zheng et al., Journal of Applied Polymer Science, 121 (2011) 702-706

[Problems to be solved by the invention] However, although the composite semipermeable membrane described in Patent Document 1 is resistant to non-polar organic solvents such as hydrocarbon oil, it has a problem of not having sufficient resistance to polar organic solvents. Furthermore, although the composite semipermeable membrane described in Patent Document 1 has desalination treatment performance due to the polyamide-based separation functional layer, there is a problem that the desalination treatment performance is significantly reduced at a high temperature above the glass transition temperature of polyamide (about 40°C to 50°C).

As described above, the aromatic polymer described in Non-Patent Document 1 has a problem that cracks may occur during the film formation and crosslinking of a porous film or a composite film, and thus the polymer cannot be used as a separation film. Therefore, it is necessary to improve the film forming property.

Therefore, the purpose of the present invention is to provide a porous membrane and composite membrane that has high film-forming properties, resistance to organic solvents, and excellent separation performance and permeability. [Means for solving the problem]

The inventors of the present invention have found that a porous membrane contains a cross-linked polymer formed by cross-linking aromatic polymers with each other, wherein the aromatic polymer is a polymer formed by copolymerizing at least one fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and anhydride monomers, and the copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer in the aromatic polymer is in the range of 25:75 to 70:30 in terms of molar ratio, so that a porous membrane and a composite membrane with improved film-forming properties, resistance to organic solvents, and excellent separation performance and permeability can be obtained, thereby completing the present invention.

In order to solve the above-mentioned problem, the present embodiment is characterized by the following (1) to (10). (1) A porous membrane comprising a crosslinked polymer formed by crosslinking aromatic polymers with each other, The aromatic polymer is a polymer formed by copolymerizing at least one fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and anhydride monomers, The copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer in the aromatic polymer is in the range of 25:75 to 70:30 in terms of molar ratio. (2) A porous membrane as described in (1), wherein the copolymerization ratio of the fluorinated aromatic monomer to the non-fluorinated aromatic monomer is in the range of 25:75 to 50:50 in terms of molar ratio. (3) A porous membrane as described in (1) or (2), wherein the aromatic polymer contains aromatic polyimide. (4) A porous membrane as described in (3), wherein the degree of imidization of the crosslinked polymer is 0.2 to 1.2. (5) A porous membrane as described in any one of (1) to (4), wherein the crosslinked polymer is a polymer obtained by crosslinking the aromatic polymers with each other using at least one crosslinking agent selected from the group consisting of epoxy crosslinking agents, hydroxymethyl crosslinking agents, alkoxymethyl crosslinking agents, and diamine crosslinking agents. (6) A porous membrane as described in any one of (1) to (5), wherein the porous membrane has at least two layers, a dense layer and a coarse layer. (7) A porous membrane as described in any one of (1) to (6), wherein the area ratio of the macropores in the cross section of the porous membrane is 3% to 60%. (8) A porous membrane as described in (7), wherein the macropores have a long axis in the thickness direction of the membrane and the aspect ratio is greater than 2.0. (9) A composite membrane, wherein the porous membrane as described in any one of (1) to (8) is laminated on at least one surface side of a substrate. (10) A composite membrane as described in (9), wherein the substrate has polyphenylene sulfide as a main component. (11) A module, comprising the porous membrane as described in any one of (1) to (8). (12) A module comprising the composite membrane as described in (9). (13) A fluid separation device comprising the module as described in (12). (14) A method for manufacturing a porous membrane comprising the following steps (i) and (ii). (i) Copolymerizing at least one fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and anhydride monomers to obtain an aromatic polymer. (ii) Crosslinking the aromatic polymer obtained in (i) by a crosslinking agent. (15) A method for producing a porous membrane as described in (14), wherein the crosslinking agent is at least one crosslinking agent selected from the group consisting of epoxy crosslinking agents, hydroxymethyl crosslinking agents, alkoxymethyl crosslinking agents, and diamine crosslinking agents. [Effect of the invention]

The present invention can provide a porous membrane and a composite membrane which can continuously and stably maintain separation performance and permeability for a treatment liquid containing an organic solvent and a high-temperature treatment liquid due to high film forming properties.

The following is a detailed description of the embodiments of the present invention, but the present invention is not limited to these embodiments. The "～" in the numerical range is a range that includes the numerical values before and after it. For example, "0～100" means a range of more than 0 and less than 100.

1. Porous membrane and composite membrane (1-1) Porous membrane The porous membrane of the present embodiment is characterized in that it contains a crosslinked polymer formed by crosslinking aromatic polymers, wherein the aromatic polymer is a polymer formed by copolymerizing at least one fluorine-containing aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-containing aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and anhydride monomers, and the copolymerization ratio of the fluorine-containing aromatic monomer to the non-fluorine-containing aromatic monomer in the aromatic polymer is in the range of 25:75 to 70:30 in terms of molar ratio.

(1-1-1) Fluorine-based aromatic monomers Examples of fluorine-containing aromatic diamine monomers include 1,4-diamino-2,3,5,6-tetrafluorobenzene, 1,3-diamino-2,4,5,6-tetrafluorobenzene, 2,2-bis(4-aminophenyl)hexafluoropropane, 2,2'-bis(trifluoromethyl)benzidine, octafluorobenzidine, 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, and 9,9-bis(3-amino-4-hydroxyphenyl)fluorene.

In addition, as the fluorine atom-containing aromatic diisocyanate monomer, for example, 2,2-bis(4-isocyanatophenyl)hexafluoropropane can be mentioned.

Among them, from the viewpoint of solubility of the aromatic polymer, etc., as the fluorinated aromatic monomer, an aromatic diamine monomer containing a fluorine atom is preferred, and among them, 2,2-bis(4-aminophenyl)hexafluoropropane or 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane is particularly preferred.

By using the fluorinated aromatic monomer, the solubility of the aromatic polymer after polymerization is improved, so that the formation of a porous membrane and a composite membrane using a polymer solution becomes easy.

(1-1-2) Non-fluorinated aromatic monomers By further copolymerizing an aromatic polymer obtained by copolymerizing a fluorinated aromatic monomer and an acid anhydride monomer with a non-fluorinated aromatic monomer, the solubility and mechanical properties of the aromatic polymer can be taken into account, and the film-forming property can be improved.

Examples of aromatic diamine monomers not containing fluorine atoms include 4,4'-diaminodiphenyl ether and 3,3'-oxydiphenylamine, and examples of aromatic diisocyanate monomers not containing fluorine atoms include bis(4-isocyanatophenyl)methane. Among them, aromatic diamine monomers not containing fluorine atoms are preferred from the viewpoints of monomer availability and mechanical properties of aromatic polymers, and 4,4'-diaminodiphenyl ether is also preferred.

In the porous membrane of the present embodiment, the copolymerization ratio of the fluorinated aromatic monomer to the non-fluorinated aromatic monomer is in the range of 25:75 to 70:30 in terms of molar ratio. By making the copolymerization ratio of the fluorinated aromatic monomer to the non-fluorinated aromatic monomer (25 or more): (75 or less), an aromatic polymer with excellent solubility can be obtained. On the other hand, by making the copolymerization ratio (70 or less): (30 or more), an aromatic polymer with excellent mechanical properties can be obtained. The copolymerization ratio is preferably in the range of 25:75 to 65:35, more preferably in the range of 25:75 to 60:40, and even more preferably in the range of 25:75 to 50:50.

(1-1-3) Acid anhydride monomers Examples of the acid anhydride monomers include 1,2,4,5-benzenetetracarboxylic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2,2'-dimethyl-3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 5,5'-dimethyl-3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2,3,3',4'-biphenyltetracarboxylic acid dianhydride, 2,2',3,3'-biphenyltetracarboxylic acid dianhydride, 3,3' ,4,4'-diphenyl ether tetracarboxylic dianhydride, 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride, 2,2',3,3'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 2,2',3,3'-benzophenone tetracarboxylic dianhydride, 2,3,3',4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, 2,3,3' ,4'-diphenylsulfone tetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfide tetracarboxylic dianhydride, 3,3',4,4'-diphenylmethylene tetracarboxylic dianhydride, 4,4'-isopropylidene diphthalic anhydride, 4,4'-(hexafluoroisopropylidene) diphthalic anhydride, 3,4,9,10-perylene tetracarboxylic dianhydride, 2,3,6,7- Naphthalene tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride, 1,2,5,6-naphthalene tetracarboxylic dianhydride, 2,3,6,7-anthracene tetracarboxylic dianhydride, 1,2,3,4-cyclobutane tetracarboxylic dianhydride, 1,2,3,4-cyclopentane tetracarboxylic dianhydride, 1,2,3,5-cyclopentane tetracarboxylic dianhydride, 1,2,4,5-dicyclohexene tetracarboxylic dianhydride, 1,2,4,5-cyclohexane tetracarboxylic dianhydride, etc. Among them, from the viewpoint of solubility or mechanical properties of aromatic polymers, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride is preferably used.

The molar ratio of the total of the fluorinated aromatic monomer and the non-fluorinated aromatic monomer to the anhydride monomer is preferably 80:100 to 100:80. By setting the total of the fluorinated aromatic monomer and the non-fluorinated aromatic monomer to the anhydride monomer to an equal molar ratio, an aromatic polymer with a high molecular weight can be obtained. On the other hand, by making the molar ratio biased to one side, an aromatic polymer with a low molecular weight can be obtained.

The fluorinated aromatic monomer, non-fluorinated aromatic monomer and anhydride monomer used in the aromatic polymer are preferably a combination of 2-bis(4-aminophenyl)hexafluoropropane or 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 4,4'-diaminodiphenyl ether, and 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride.

(1-1-4) Aromatic polymer The aromatic polymer is obtained by copolymerizing at least one fluorine-containing aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-containing aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and anhydride monomers. Examples of the aromatic polymer include aromatic polyamides, aromatic polyamide imides, aromatic polyimides, aromatic polyether imides, and aromatic polymaleimides. From the perspective of versatility, the aromatic polymer is preferably an aromatic polyimide.

The aromatic polymer may contain other components in addition to the above components within the range that does not impair the effects of the present invention. Examples of other components include polyamides, polyamines, and the like.

The production method of the aromatic polymer is not particularly limited as long as an aromatic polymer satisfying the above characteristics can be obtained, and the aromatic polymer can be produced, for example, by the production method described below.

From the perspective of separation performance, mechanical strength and film-forming properties, the weight average molecular weight (hereinafter referred to as "Mw") of the aromatic polymer is preferably 5,000 to 300,000. When the aromatic polymer is an aromatic polyimide, the Mw of the aromatic polyimide is preferably 8,000 to 200,000, and more preferably 12,000 to 100,000. When the Mw of the aromatic polyimide is 8,000 or more, better separation performance and mechanical strength can be obtained as a porous membrane and a composite membrane. On the other hand, when the Mw of the aromatic polyimide is 200,000 or less, the viscosity of the polymer solution is in an appropriate range, and good film-forming properties can be achieved. The weight average molecular weight of aromatic polymers can be measured by gel permeation chromatography and is a value converted to the molecular weight of polystyrene used as a standard substance.

(1-1-5) Crosslinked polymer The crosslinked polymer is a polymer obtained by crosslinking the aromatic polymers. The crosslinked polymer is preferably a polymer obtained by crosslinking the aromatic polymers with at least one crosslinking agent selected from the group consisting of epoxy crosslinking agents, hydroxymethyl crosslinking agents, alkoxymethyl crosslinking agents, and diamine crosslinking agents.

When the aromatic polymer is an aromatic polyimide, the imidization degree of the crosslinked polymer is preferably 0.2 to 1.2, more preferably 0.3 to 1.2, and even more preferably 0.4 to 1.2. When the imidization degree of the crosslinked polymer is 0.2 or more, a porous membrane can be obtained that can stably maintain separation performance and permeability for a treatment liquid containing an organic solvent and a high-temperature treatment liquid.

The imidization degree of the crosslinked polymer can be measured using a Fourier transform infrared spectrophotometer. The imidization degree is a value calculated using the following formula 1 based on the peak intensity A derived from the aromatic ring and the peak intensity B derived from the imide group obtained by measuring the porous membrane surface by infrared total reflection absorption measurement method (attenuated total reflectance (ATR) method). Imidization degree = B/A ･･･（Formula 1）

The degree of imidization of the crosslinked polymer can be adjusted by the progress of the heating dehydration reaction after the synthesis of polyamide. As a method for promoting imidization, for example, there are methods of distilling and removing water as a by-product of the imidization reaction, and methods of adding additives such as acetic anhydride, isoquinoline, imidazole, pyridine, etc. to the polyamide solution and then heating. On the other hand, as a method for inhibiting imidization, there are methods such as adding water to the polyamide.

(1-1-6) Porous membrane The porous membrane contains a crosslinked polymer. The aromatic polymer is dissolved in an organic solvent, but by crosslinking the aromatic polymers, a polymer having resistance to organic solvents is obtained. In this way, the porous membrane is prepared using a polymer solution, and then by crosslinking, a porous membrane that can continuously and stably maintain separation performance and permeability for a treated liquid containing an organic solvent can be obtained.

The porous membrane preferably has a three-dimensional mesh structure. Here, "three-dimensional mesh structure" refers to a structure in which the strip-shaped polymer constituting the porous membrane is expanded in a three-dimensional mesh shape. The three-dimensional mesh structure has fine pores separated by the strip-shaped solid matter forming the mesh, and has excellent separation performance.

The porous membrane preferably has at least two layers, a dense layer and a coarse layer, and more preferably has at least two layers, a dense layer and a coarse layer, in the thickness direction. Here, "dense layer" refers to a layer with an average pore size of less than 50 nm, and "coarse layer" refers to a layer with an average pore size of 50 nm or more. By having at least two layers, a dense layer and a coarse layer, in the thickness direction, a porous membrane that takes into account both separation performance and permeability can be obtained. In addition, the porous membrane is more preferably an asymmetric structure in which the pore size changes obliquely in the thickness direction.

The average surface pore size of the porous membrane can be appropriately selected according to the application. When the organic solvent is separated, such as the concentration of wastewater containing an organic solvent or the separation and recovery of a mixed organic solvent, the average surface pore size of the porous membrane is preferably 0.25 nm to 0.8 nm, and more preferably 0.4 nm to 0.7 nm. When valuable substances are recovered from organic solvents, such as molecules with a molecular weight of about 200 to 1000, the average surface pore size of the porous membrane is preferably 0.7 nm to 4 nm, and more preferably 0.75 nm to 2 nm. In the case of removing high molecular weight components in an organic solvent, for example, when molecules with a molecular weight of 1000 or more are to be separated, the average surface pore size of the porous membrane is preferably 2 nm to 100 nm, more preferably 4 nm to 50 nm. Generally speaking, separation performance and permeability are in a trade-off relationship with respect to the average surface pore size. Therefore, in order to achieve both separation performance and permeability at a high level, it is preferable to select an average surface pore size suitable for the separation target.

The thickness of the porous membrane is preferably 20 μm to 300 μm, more preferably 30 μm to 250 μm, and further preferably 40 μm to 200 μm. When the thickness of the porous membrane is 20 μm or more, a porous membrane with good separation performance can be obtained. On the other hand, when the thickness of the porous membrane is 300 μm or less, a porous membrane with good permeability can be obtained. In addition, the thickness of the porous membrane can be obtained by calculating the average value of the thickness of 20 points measured at intervals of 20 μm in a direction orthogonal to the thickness direction (the surface direction of the membrane) under cross-sectional observation.

The porosity of the porous membrane is preferably 25% to 85%, more preferably 30% to 75%. The porosity is defined as the ratio of the space volume to the total volume of the substance. A porous membrane having good permeability can be obtained by a porosity of 25% or more. On the other hand, a porous membrane having sufficient mechanical strength can be obtained by a porosity of 85% or less.

The porosity (%) of the porous membrane can be determined, for example, by using the following formula 2 based on the membrane volume (cm ³ ) and mass (g) calculated by measuring the membrane thickness of the porous membrane. Furthermore, the density used to calculate the porosity can be set according to the type of aromatic polymer. For example, when the aromatic polymer is aromatic polyimide, the density can be set to 1.42 g/cm ^3. Porosity (%) = {1-mass/(density×membrane volume)}×100 ･･･ (Formula 2)

The porous membrane of this embodiment preferably has macropores in the cross section. In addition, the area ratio of the macropores in the cross section of the porous membrane is preferably 3% to 60%, more preferably 5% to 55%, and more preferably 10% to 50%. When the area ratio of the macropores in the cross section of the porous membrane is 3% or more, liquid or gas can easily flow into the macropores, so the membrane has high permeability. On the other hand, when the area ratio of the macropores in the cross section of the porous membrane is 60% or less, the membrane shows sufficient mechanical properties to pressure, so it is easy to handle.

Here, "macropore" refers to a pore having a length of 0.5 μm or more. Typical shapes of macropores include, for example, a spherical structure having a diameter of 0.5 μm or more, a teardrop having a length of 0.5 μm or more, a pear-shaped or pear-shaped shape, a bell-shaped shape, and other structures having two or more distinct characteristic lengths, or a finger-shaped structure having a length of 0.5 μm or more and an aspect ratio of 3 or more. Here, "aspect ratio" refers to the value obtained by dividing the length in the direction perpendicular to the porous membrane surface in the macropore by the length in the direction horizontal to the porous membrane surface.

The aspect ratio of the macropores is preferably 2.0 or more, more preferably 2.5 or more, and further preferably 3.0 or more. When the aspect ratio of the macropores is 3.0 or more, the membrane exhibits sufficient mechanical properties against pressure and becomes a high permeability membrane. On the other hand, the aspect ratio is preferably 6.0 or less. When the aspect ratio is 6.0 or less, sufficient mechanical properties are also exhibited relative to the stretching direction. That is, the macropores preferably have a long axis in the thickness direction of the membrane.

In addition, the cross section refers to a cross section cut in a direction perpendicular to the surface of the porous membrane.

As a method for appropriately controlling the area ratio and aspect ratio of macropores in the cross section of the porous membrane, for example, a method of adding an additive to the membrane-forming solution can be cited. As a specific example, when the porous membrane is a polyimide membrane, macropores can be formed in the polyimide membrane by mixing an organic solvent soluble in polyimide with an organic solvent that promotes immersion of the polyimide solution in a coagulation bath. Examples of polyimide soluble organic solvents include acetone, acetonitrile, dimethyl sulfoxide, dimethylacetamide, dimethylformamide, N-methylpyrrolidone, dichloromethane, chloroform, and the like. Examples of organic solvents used to promote the solidification bath of the polyimide solution include ether solvents such as 1,4-dioxane, tetrahydrofuran, 1,3-dioxane, dimethyl ether, and diethyl ether.

The swelling degree of the porous membrane of this embodiment is preferably 100% to 200%, more preferably 105% to 150%, and further preferably 110% to 130%. By setting the swelling degree of the porous membrane to the range of 100% to 200%, the porous membrane has a moderate affinity for the solvent and exhibits high removal and high permeability. "Swelling degree" refers to the ratio of the thickness of the porous membrane after being immersed in N-methyl-2-pyrrolidone (hereinafter referred to as "NMP") to reach a balanced swelling state relative to the thickness of the porous membrane before being immersed in NMP.

(1-2) Composite membrane The porous membrane of this embodiment may be composed only of the porous membrane, or may be a composite membrane in which the porous membrane is laminated on at least one surface side of a substrate. The substrate is used to provide strength to the composite membrane as a whole by supporting the porous membrane, and it itself does not substantially have separation performance. The composite membrane in which the porous membrane is laminated on at least one surface side of the substrate only needs to have the porous membrane on at least one surface side of the substrate. The porous membrane may be laminated directly on the substrate, or may be laminated with an arbitrary layer that does not substantially have separation performance provided between the porous membrane and the substrate.

As a substrate, for example, a fabric containing polyester polymers, polyamide polymers, polyolefin polymers, polysulfide polymers, and mixtures or copolymers thereof can be cited. In terms of excellent stability relative to a treated liquid containing an organic solvent and a high-temperature treated liquid, a fabric of a polysulfide polymer is particularly preferred. As a polysulfide polymer, for example, polyphenylene sulfide (hereinafter referred to as "PPS") can be cited. As a fabric, a long-fiber nonwoven fabric or a short-fiber nonwoven fabric, or a braided fabric is preferred. Here, a short-fiber nonwoven fabric refers to a nonwoven fabric having an average fiber length of less than 300 mm and an average fiber diameter of 3 μm to 30 μm. That is, the base material preferably contains polyphenylene sulfide as a main component. The main component is preferably 50% to 100% by mass relative to the entire base material.

The thickness of the substrate affects the strength of the composite membrane and the packing density when it is made into a device. In order to obtain good mechanical strength and packing density, the thickness of the substrate is preferably 30 μm to 250 μm, and more preferably 50 μm to 180 μm. Furthermore, the thickness of the substrate can be calculated in the same way as the thickness of the porous membrane.

The air permeability of the substrate affects the separation performance and physical stability of the composite membrane. The air permeability of the substrate is preferably 0.2 cm ³ /cm ² /s to 4 cm ³ /cm ² /s, more preferably 0.25 cm ³ /cm ² /s to 3 cm ³ /cm ² /s, and further preferably 0.3 cm ³ /cm ² /s to 1 cm ³ /cm ² /s. When the air permeability of the substrate is 0.2 cm ³ /cm ² /s or more, a part of the polymer solution is impregnated into the substrate, thereby improving the adhesion between the porous membrane and the substrate, and a composite membrane with good physical stability can be obtained. On the other hand, when the air permeability of the substrate is 4 cm ³ /cm ² /s or less, defects due to leakage are less likely to occur when the polymer solution as a raw material of the porous membrane is applied to the substrate, so that a composite membrane with good separation performance can be obtained.

In order to prevent the separation target substance from penetrating into the interior of the composite membrane, the porous membrane is preferably disposed on the surface side of the composite membrane, and more preferably disposed on the primary filtration side.

Regarding the membrane permeation flux of the porous membrane or composite membrane, from the viewpoint of industrial value, when the porous membrane or composite membrane is OSRO (cutoff molecular weight less than 200), it is preferably 0.1 L/m ² /h/bar or more, when the porous membrane or composite membrane is OSN (cutoff molecular weight 200 to 1,000), it is preferably 0.5 L/m ² /h/bar or more, and when the porous membrane or composite membrane is OSU (cutoff molecular weight 1,000 or more), it is preferably 2.0 L/m ² /h/bar or more.

Membrane permeation flux can be calculated by cross-flow membrane filtration test. For example, the membrane permeation flux (L/ ^m2 /h/bar) of a porous membrane or composite membrane can be calculated using the following formula 3 based on the permeation liquid volume (L), unit membrane area ( ^m2 ), unit time (h) and operating pressure (bar). Membrane permeation flux = permeation liquid volume / (unit membrane area × unit time) / operating pressure ･･･ (Formula 3)

From the perspective of industrial value, the rejection rate of the porous membrane or composite membrane is preferably 80% or more.

The rejection rate (%) can be calculated using the following formula 4 from the solute concentration C (ppm) in the feed solution (stock solution) and the solute concentration D (ppm) in the permeate solution. Rejection rate (%) = (1-D/C) × 100 ･･･ (Formula 4)

2. Method for producing porous membranes and composite membranes The method for producing porous membranes and composite membranes of this embodiment is not particularly limited as long as a porous membrane and composite membrane satisfying the desired characteristics can be obtained. For example, the porous membrane and composite membrane can be produced by the following method.

The porous membrane and composite membrane of the present embodiment can be produced, for example, by a method comprising the following steps (i) and (ii). (i) At least one fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and anhydride monomers are copolymerized to obtain an aromatic polymer. (ii) The aromatic polymer obtained in (i) is crosslinked by a crosslinking agent.

(2-1) Polymerization of polymers First, an aromatic polymer is prepared. As monomers, at least one fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and the acid anhydride monomer are used.

The molar ratio of the total of the fluorinated aromatic monomer and the non-fluorinated aromatic monomer to the anhydride monomer is preferably 80:100 to 100:80. By setting the total of the fluorinated aromatic monomer and the non-fluorinated aromatic monomer to the anhydride monomer at an equal molar ratio, the molecular weight of the aromatic polymer can be increased. On the other hand, by making the molar ratio biased to one side, the molecular weight of the polymer can be reduced.

In addition, a capping agent may be added to control the molecular weight and molecular weight distribution of the aromatic polymer. Examples of the capping agent include anhydrides such as phthalic anhydride, 2,3-naphthalene dicarboxylic anhydride, 1,2-naphthalene dicarboxylic anhydride, 4-methylphthalic anhydride, 3-methylphthalic anhydride, 4-chlorophthalic anhydride, 4-tert-butylphthalic anhydride, and 4-fluorophthalic anhydride; amines such as aniline, 1-naphthylamine, 2-chloroaniline, 4-chloroaniline, 3-aminophenol, and 4-aminopyridine; and isocyanates such as n-butyl isocyanate, isopropyl isocyanate, phenyl isocyanate, and benzyl isocyanate.

When the aromatic polymer is an aromatic polyimide, the Mw of the aromatic polyimide is preferably 8,000 to 200,000, and more preferably 12,000 to 100,000. When the Mw of the aromatic polyimide is 8,000 or more, better separation performance and mechanical strength can be obtained as a porous membrane and a composite membrane. On the other hand, when the Mw of the aromatic polyimide is 200,000 or less, the viscosity of the polymer solution is in an appropriate range, and good film forming properties can be achieved.

The weight average molecular weight of a polymer can be measured by gel permeation chromatography and is a value converted into the molecular weight of polystyrene used as a standard substance.

The polymerization process of the polymer is described by taking the polymerization of aromatic polyimide as an example. First, a fluorinated aromatic monomer and a non-fluorinated aromatic monomer are dissolved in a solvent, an acid anhydride is added thereto, and the mixture is stirred at 0°C to 100°C for 10 minutes to 100 hours to obtain a polyamide solution.

Next, when the obtained aromatic polyimide is soluble in a solvent, after the polyamide polymerization is carried out, the temperature is raised to 120°C to 300°C in the above state, and stirred for 10 minutes to 100 hours to carry out imidization to obtain an aromatic polyimide solution. At this time, toluene, o-xylene, m-xylene, p-xylene, etc. can be added to the reaction solution to remove the water generated in the imidization reaction by azeotropy with these solvents. Furthermore, since imidization can also be carried out in the "(2-3) Cross-linking of porous membranes or composite membranes" described later, the imidization degree of the aromatic polyimide obtained by this step can be appropriately adjusted according to the polymerization temperature, polymerization time, moisture content, etc. to be suitable for the "(2-2) Formation of porous membranes or composite membranes" described later.

Examples of the solvent include dimethyl sulfoxide, dimethylformamide, dimethylacetamide, NMP, 2-pyrrolidone, gamma buthyrolactone (hereinafter referred to as "GBL"), 1,4-dioxane, 1,3-dimethylimidazolidinone, or a mixed solvent thereof.

Next, the polymer obtained by polymerization is purified. As a purification method, reprecipitation is preferred. As a poor solvent for the polymer used in the reprecipitation method, water is preferred. By drying the polymer whose purity is improved by reprecipitation, a solid aromatic polymer obtained by copolymerization of fluorinated aromatic monomers, non-fluorinated aromatic monomers, and anhydride monomers can be obtained.

(2-2) Preparation of porous membranes or composite membranes As a method for preparing porous membranes or composite membranes, nonsolvent induced phase separation (NIPS) or thermally induced phase separation (TIPS) can be used. In terms of obtaining a porous membrane or composite membrane having a three-dimensional mesh structure and an asymmetric structure that can take into account both separation performance and permeability, it is preferable to use the NIPS method. The following is a description of a membrane preparation method using the NIPS method as an example.

First, the polymer obtained in "(2-1) Polymerization of polymer" and a crosslinking agent are dissolved in a solvent to obtain a polymer solution. The solvent is preferably a good solvent for the polymer. Here, "good solvent" refers to a solvent that can dissolve 5% by mass or more of the polymer in a low temperature range below 60°C. Examples of good solvents for the polymer include dimethyl sulfoxide, dimethylformamide, dimethylacetamide, NMP, 2-pyrrolidone, GBL, 1,4-dioxane, 1,3-dimethyl-imidazolidinone, or a mixed solvent thereof.

The concentration of the polymer in the polymer solution is preferably 8 mass % to 30 mass %, more preferably 12 mass % to 26 mass %. When the concentration of the polymer in the polymer solution is 8 mass % or more, a porous membrane or composite membrane having strength or separation performance that can be used as a separation membrane can be formed. On the other hand, when the concentration of the polymer in the polymer solution is 30 mass % or less, a porous membrane or composite membrane having good permeability can be formed. Furthermore, the preferred range of the concentration of the polymer in the polymer solution can be appropriately adjusted according to the polymer, solvent, substrate, etc. used.

The polymer solution contains a polymer crosslinking agent. The polymer crosslinking agent needs to be dissolved in the polymer solution, and is preferably at least one crosslinking agent selected from the group consisting of epoxy crosslinking agents, hydroxymethyl crosslinking agents, alkoxymethyl crosslinking agents, and diamine crosslinking agents.

Examples of epoxy crosslinking agents include ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, pentanediol diglycidyl ether, hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, resorcinol diglycidyl ether, glycerol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, trihydroxymethylpropane polyglycidyl ether, sorbitol diglycidyl ether, sorbitol diglycidyl ether, and sorbitol polyglycidyl ether. Alcohol polyglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, di(2,3-epoxypropyl) ether, 1,3-butadiene diepoxide, 1,5-hexadiene diepoxide, 1,2,7,8-diepoxyoctane, 1,2,5,6-diepoxycyclooctane, 4-vinylcyclohexene diepoxide, bisphenol A diglycidyl ether, maleimide-epoxy compound.

Examples of hydroxymethyl crosslinking agents include bisphenol A-formaldehyde condensate.

Examples of the alkoxymethyl crosslinking agent include hexamethoxymethyl melamine, tetramethoxymethyl glycoluril, 3,3',5,5'-tetrakis(methoxymethyl)-[1,1'-biphenyl]-4,4'-diol, and 4,4',4''-ethylenetri[2,6-(methoxymethyl)phenol] (hereinafter referred to as "GMOM").

Examples of the diamine crosslinking agent include ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, heptamethylenediamine, octamethylenediamine, 1,8-diamino-3,6-dioxooctane, 1,4-cyclohexanediamine, 4,4'-diaminodicyclohexylmethane, and bis(aminomethyl)norbornane.

The concentration of the crosslinking agent in the polymer solution is preferably 1 mass % to 20 mass %, and more preferably 2 mass % to 15 mass %. When the concentration of the crosslinking agent in the polymer solution is 1 mass % or more, a porous membrane or composite membrane having resistance to organic solvents and good separation performance can be formed. On the other hand, when the concentration of the crosslinking agent in the polymer solution is 20 mass % or less, a porous membrane or composite membrane having good permeability can be formed.

The polymer solution may contain additives for adjusting pore size, porosity, hydrophilicity, elastic coefficient, etc. as needed. Examples of additives for adjusting pore size and porosity include water, alcohols, polyethylene glycol, polyvinyl pyrrolidone, polyvinyl alcohol, diethylene glycol, polyacrylic acid and other water-soluble polymers or their salts, and further examples include inorganic salts such as lithium chloride, sodium chloride, calcium chloride, lithium nitrate, formamide, etc. As additives for adjusting hydrophilicity or elastic coefficient, various surfactants can be listed.

Regarding the analysis of the monomers or crosslinking agents constituting the porous membrane, in the case of a composite membrane with a substrate, first, after only the porous membrane portion is obtained by stripping, various analyses can be performed. For example, in the case of an aromatic polyimide, the polyimide portion obtained by stripping is hydrolyzed with an alkali, and then analyzed by nuclear magnetic resonance, liquid chromatography mass spectrometry, or gas chromatography mass spectrometry, etc., thereby identifying the monomers constituting the aromatic polyimide. In addition, in the case of a porous membrane having chemical crosslinking using a crosslinking agent, the crosslinking agent can be identified using the above method. When the porous membrane has a cross-linked structure that is not hydrolyzed under the action of alkali, the cross-linked structure can be identified by analyzing the monomers reacting with the cross-linking agent using a nuclear magnetic resonance method or the like.

Next, the polymer solution is applied or sprayed, and immersed in a coagulation bath to solidify. When a flat film-shaped and single-layer porous membrane is formed, the polymer solution is applied on a flat metal plate or glass plate, for example. When a flat film-shaped and composite membrane having a substrate is formed, the polymer solution is applied on at least one surface side of the substrate.

In the step of applying the polymer solution in a flat film form, for example, a spin coater, a flow coater, a roll coater, a sprayer, a comma coater, a rod coater, a gravure coater, a slot die coater, a doctor blade, etc. can be used.

On the other hand, when forming a single-layer porous membrane in the form of a hollow fiber, for example, a polymer solution is ejected from the outer periphery of a double tube mold, and a core liquid is ejected from the center. When forming a composite membrane in the form of a hollow fiber and having a substrate, for example, a hollow fiber substrate is passed through a coating nozzle storing a polymer solution, and the polymer solution is applied to the outer surface of the substrate.

When the polymer solution is applied to the substrate, a part of the polymer solution is impregnated in the substrate. The amount of the polymer solution impregnated in the substrate can be appropriately adjusted according to the time from applying the polymer solution to the substrate to immersing in the coagulation bath, the viscosity of the polymer solution, the air permeability of the substrate, etc.

The time from applying the polymer solution to immersion in the coagulation bath is preferably 0.1 to 5 seconds. When the time from immersion in the coagulation bath is 0.1 seconds or longer, the polymer solution can be fully impregnated into the substrate. On the other hand, when the time from immersion in the coagulation bath is 5 seconds or shorter, the polymer solution can be prevented from solidifying by moisture in the air. Furthermore, the time from immersion in the coagulation bath can be appropriately adjusted to a preferred range according to the viscosity of the polymer solution used, etc.

The coagulation bath is preferably a non-solvent containing a polymer solution. Here, "non-solvent" refers to a solvent that does not dissolve or swell the polymer before the melting point of the polymer or the boiling point of the solvent. Examples of the non-solvent for the polymer include water, methanol, ethanol, trichloroethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butanediol, pentanediol, ethylene glycol, or a mixed solvent thereof. Generally, water is used.

Furthermore, the porous structure of the porous membrane is formed in the process of phase separation induced by a non-solvent. When a polymer solution applied in a thin layer is immersed in a non-solvent of the polymer, a porous structure such as a three-dimensional mesh structure is formed by phase separation, forming a coarse layer near the surface of the membrane and a dense layer on the outermost surface.

When the formation of a porous membrane or a composite membrane is continuously performed, in a coagulation bath in which a polymer solution is in contact with a non-solvent, the solvent of the polymer solution is mixed with the non-solvent, and the concentration of the solvent derived from the polymer solution increases. Therefore, it is preferable to appropriately replace the coagulation bath so that the composition of the liquid in the coagulation bath is maintained within a certain range. The lower the concentration of the good solvent in the coagulation bath, the faster the polymer solution coagulates, so the structure of the porous membrane or the composite membrane is homogenized, and excellent separation performance can be exhibited. In addition, since the coagulation of the polymer solution becomes faster, the membrane preparation speed can be increased, and the productivity of the porous membrane or the composite membrane can be improved. The concentration of the good solvent in the coagulation bath is preferably less than 20%, more preferably less than 15%, and further preferably less than 10%.

The obtained porous membrane or composite membrane may be washed with hot water or the like as needed to remove the residual solvent in the membrane. However, in order to crosslink the porous membrane or composite membrane in "(2-3) Crosslinking of porous membrane or composite membrane" described later, it is necessary to appropriately adjust the washing conditions so that the crosslinking agent of the polymer remaining in the porous membrane or composite membrane is not excessively eluted. In addition, the obtained porous membrane or composite membrane may be dried as needed.

(2-3) Crosslinking of porous membrane or composite membrane The obtained porous membrane or composite membrane is then crosslinked to impart resistance to organic solvents and adjust separation performance. The crosslinking uses a crosslinking agent of the polymer that is dissolved in the polymer solution in "(2-2) Formation of porous membrane or composite membrane" and remains in the porous membrane or composite membrane after solidification.

As the crosslinking method of porous membrane or composite membrane, thermal crosslinking or UV crosslinking etc. can be used, but thermal crosslinking is preferred in that the porous membrane or composite membrane can be uniformly crosslinked to the inside regardless of the presence or absence of coloring or the film thickness. Hereinafter, the crosslinking method based on thermal crosslinking is described as an example.

The thermal crosslinking is preferably performed in air. The temperature of the thermal crosslinking needs to be below the heat resistance temperature of the polymer and the substrate, preferably 90°C to 300°C, more preferably 120°C to 250°C, and further preferably 160°C to 230°C. By the thermal crosslinking temperature being above 90°C, a porous film or composite film having resistance to organic solvents and good separation performance can be formed. On the other hand, by the thermal crosslinking temperature being below 300°C, the porous structure formed by the NIPS method is maintained, thereby forming a porous film or composite film having good separation performance and permeability.

In addition, the time of thermal crosslinking is preferably 30 seconds to 20 hours, more preferably 1 minute to 10 hours, and further preferably 3 minutes to 4 hours. When the time of thermal crosslinking is more than 30 seconds, a porous membrane or composite membrane having resistance to organic solvents and good separation performance can be formed. On the other hand, when the time of thermal crosslinking is less than 20 hours, the porous structure formed by the NIPS method is maintained, thereby a porous membrane or composite membrane having good separation performance and permeability can be formed.

As described above, it is speculated that by crosslinking the obtained porous membrane, the main chain of the aromatic polymer constituting the porous membrane is crosslinked to form a three-dimensional network structure, so that it becomes stable even in an organic solvent and maintains the separation performance.

When the aromatic polymer is an aromatic polyimide, the residual polyamide in this step is imidized. Therefore, the imidization degree of the aromatic polyimide can be adjusted by the temperature and time of this step in addition to the imidization step in "(2-1) Polymerization of polymer".

The obtained porous membrane or composite membrane can also be washed with hot water or the like as needed to remove the crosslinking agent of the polymer remaining in the membrane.

In addition, the obtained porous membrane or composite membrane can be immersed in a solvent to swell it as needed. By using a solvent to swell the porous membrane or composite membrane, the permeability of the porous membrane or composite membrane that is absolutely dry and air-tight during crosslinking can be restored. In order to make the porous membrane or composite membrane swell in a balanced manner as a whole, it is better to use a good solvent of the polymer of the precursor as the solvent.

3. Utilization of porous membranes and composite membranes The porous membranes and composite membranes of this embodiment are wound around a cylindrical liquid collecting pipe with a large number of holes together with a supply liquid flow path material such as a plastic net, a permeable liquid flow path material such as a knitted product, and a membrane for improving pressure resistance as needed, and are suitable for use as a spiral element. Furthermore, the elements can also be connected in series or in parallel to form a porous membrane or composite membrane module housed in a pressure vessel. The parts of these elements or modules are preferably made of raw materials that are resistant to the supply liquid. In addition, the porous membrane and composite membrane and the elements and modules using these can be combined with a pump for supplying the supply liquid to them, a device for pre-treating the supply water, etc. to form a fluid separation device. By using the fluid separation device, the supply liquid is separated into a permeate from which solutes or impurities are removed and a concentrated liquid that has not passed through the membrane, thereby obtaining a target solution. Furthermore, the element can be of any shape, such as a flat plate type, a spiral type, a multi-pleated type, a tubular type, a hollow fiber type, etc., according to the shape of the porous membrane and the composite membrane. The module can be used in multiple stages according to the purity of the supplied liquid or the purity required after separation. [Example]

The present invention is described below with specific examples, but the present invention is not limited to these examples. The physical properties of the porous membrane and composite membrane of this embodiment are measured by the following method.

(1) Weight average molecular weight The Mw (polystyrene conversion) of the polymer was measured using a gel permeation chromatograph (Tosoh; HLC-8022). The specific measurement conditions are as follows. Column: 2 pieces of TSK gel SuperHM-H (Tosoh; inner diameter 6.0 mm, length 15 cm) Eluent: LiBr/NMP solution (10 mM) Sample concentration: 0.1 mass % Flow rate: 0.5 mL/min Temperature: 40°C

(2) Degree of imidization The degree of imidization of the crosslinked polymer was measured using a Fourier transform infrared spectrophotometer (manufactured by Shimadzu Corporation; IRTracer-100). The porous membrane or composite membrane was cut into 3 cm × 3 cm pieces, and the porous membrane surface was measured using MicromATR Vision (manufactured by ST Japan Co., Ltd.) by the infrared total reflection absorption measurement method (ATR method) (incident angle: 48 degrees, prism: diamond, resolution: 4 cm ^-1 , cumulative counts: 64 times). The degree of imidization was calculated using the following formula 1 based on the peak intensity A derived from the aromatic ring detected at 1490 cm ^-1 to 1520 cm ^-1 and the peak intensity B derived from the imide group detected at 1760 cm ^-1 to 1790 cm ^-1 . Imidization degree = B/A ･･･（Formula 1）

(3) Porosity The porosity (%) of the porous membrane is calculated using the following formula 2, based on the membrane volume (cm ³ ) and mass (g) calculated by measuring the membrane thickness of the porous membrane using the method described in "(8) Membrane Thickness" described later. In addition, the density used to calculate the porosity is the general density of the aromatic polymer according to the type of the aromatic polymer. For example, when the aromatic polymer is aromatic polyimide, the density is 1.42 g/cm ^3. Porosity (%) = {1-mass/(density×membrane volume)}×100 ･･･ (Formula 2)

(4) Membrane permeability and rejection rate The membrane permeability of the porous membrane or composite membrane was evaluated according to the membrane type. When the porous membrane or composite membrane was OSRO (cutoff molecular weight less than 200), a 20 ppm standard polystyrene (Mw162)/NMP solution was supplied at an operating pressure of 30 bar to conduct a cross-flow membrane filtration test. When the porous membrane or composite membrane was OSN (cutoff molecular weight 200-1,000), a 20 ppm standard polystyrene (Mw580)/NMP solution was supplied at an operating pressure of 15 bar to conduct a cross-flow membrane filtration test. When the porous membrane or composite membrane is OSU (cutoff molecular weight 1,000 or more), a cross-flow membrane filtration test was conducted by supplying a 20 ppm standard polystyrene (Mw 100,000)/NMP solution at an operating pressure of 2 bar. The membrane permeation flux (L/m ² /h/bar) of the porous membrane or composite membrane was calculated using the following formula 3 from the permeate volume (L), unit membrane area (m ² ) and unit time (h). In addition, the rejection rate (%) of the porous membrane or composite membrane was calculated using the following formula 4 from the standard polystyrene concentration C (ppm) in the stock solution and the standard polystyrene concentration D (ppm) in the permeate. Membrane permeability (L/m ² /h/bar) = permeate volume/(unit membrane area × unit time)/operating pressure ･･･(Formula 3) Rejection rate (%) = (1-D/C) × 100 ･･･(Formula 4)

(5) Copolymerization ratio The copolymerization ratio of the fluorinated aromatic monomer to the non-fluorinated aromatic monomer was calculated using a proton nuclear magnetic resonance method (JNM-ECZ400R (manufactured by JEOL Ltd. (JEOL))). When polyimide is synthesized using 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane as a fluorine-based aromatic monomer among diamine monomers, 4,4'-diaminodiphenyl ether as a non-fluorine-based aromatic monomer, and 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride as an acid anhydride monomer, the polyimide is dissolved in deuterated dimethyl sulfoxide as a solvent, and the area of the peak at around 8.0 ppm derived from 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride and the area of the peak at around 10.5 ppm derived from 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane in the proton nuclear magnetic resonance signal (S1) are calculated. The anhydride monomer and the diamine monomer react approximately in equimolar reaction, and the copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer is obtained using the following formula 5. Copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer = S2: (S1-S2) ･･･(Formula 5)

(6) 90-degree load test The mechanical properties of the porous membrane were evaluated using a Tensilon tester (RTG-1210 manufactured by A&D Co., Ltd.). 22% by mass of the aromatic polymer, 2% by mass of GMOM, 38% by mass of NMP, and 38% by mass of 1,4-dioxane obtained in each embodiment and comparative example were dissolved at 25°C to prepare a polymer solution. The polymer solution was applied to a glass plate at 25°C, and after 3 seconds, it was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, thereby preparing a porous membrane. The obtained porous membrane was immersed in water at 25°C for 24 hours, and then air-dried to prepare a test piece with a width of 25 mm and a length of 200 mm. Using Kapton double-sided tape (manufactured by Teraoka Seisakusho Co., Ltd.), the test piece was bonded to the ground from one end to a position of 100 mm. Then, the end of the unbonded test piece was fixed with the clamp of the tensile testing machine so that the unbonded test piece formed a fixed angle of 90 degrees with the ground, and the tensile test was performed at a tensile speed of 20 mm per minute until a load of 30 g or 50 g was applied. During this period, the test piece was bonded to the ground with Kapton double-sided tape so that the bonding surface did not peel off. This test was performed 5 times using different test pieces to confirm the presence of breaks and cracks. In all 5 tests, the case where no breaks or cracks were observed was set as "○".

(7) The area ratio of macropores and the aspect ratio of macropores in the cross section of the porous membrane The porous membrane or composite membrane was cut into 10 ^cm2 pieces, washed with distilled water at 90°C for 10 minutes, and dried. Next, the membrane was frozen with liquid nitrogen and then broken to prepare a cross-sectional observation sample of the porous membrane. After the sample was coated with platinum particles using a sputtering device, an image of the cross section of the porous membrane was taken at a magnification of 500 times using a scanning electron microscope (S-5500 manufactured by Hitachi High-technologies). Here, the cross section of the porous membrane was cut in a direction perpendicular to the surface of the porous membrane.

In the cross-sectional structure of the porous membrane obtained by observation using a scanning electron microscope, the area of the porous membrane cross-section (S3) and the sum of the areas of the macropores in the porous membrane cross-section (S4) were calculated. The area was calculated by using the image processing software "ImageJ" to depict the porous membrane part and the periphery of the macropores. At least 30 points were depicted for one macropore. The following formula 6 was used to calculate the area ratio of the macropores in the cross-section of the porous membrane. The same calculation was performed using five different images of the porous membrane cross-section, and the average value was taken as the area ratio of the macropores in the cross-section of the porous membrane. Area ratio of macropores in the cross-section of the porous membrane (%) = S4/S3×100 ･･･ (Formula 6)

In addition, regarding the aspect ratio of the macropores, 10 macropores were randomly selected from one image, and the length of each macropore in the direction perpendicular to the porous membrane surface and the length in the direction horizontal to the porous membrane surface were calculated using the image processing software "ImageJ". The calculation was performed using five images, and the average of the aspect ratios of the 50 macropores obtained was taken as the aspect ratio of the porous membrane.

(8) Film thickness The thickness of the porous film obtained, the porous film after reaching the equilibrium swelling state, and the composite film were measured using a dial thickness gauge (produced by Teclock Co., Ltd., constant pressure thickness gauge PG-01A). 10 points were measured evenly from both ends of the film, and the average value was taken as the film thickness.

(9) Swelling degree Based on the porous membrane obtained in "(8) Membrane thickness" as described above and the thickness of the porous membrane after reaching a balanced swelling state, the swelling degree of the porous membrane is calculated using the following formula 7. Swelling degree (%) = (thickness of the porous membrane after reaching a balanced swelling state/thickness of the porous membrane) × 100 ･･･(Formula 7)

The following summarizes the raw materials of porous membranes and composite membranes used in the embodiments and comparative examples. Aromatic diamine monomer containing fluorine atoms: 4,4'-dihydroxy-3,3'-diaminophenyl hexafluoropropane (manufactured by Tokyo Chemical Industry) Aromatic diamine monomer containing fluorine atoms: 2,2-bis(4-aminophenyl)hexafluoropropane (manufactured by Tokyo Chemical Industry) Aromatic diamine monomer not containing fluorine atoms: 4,4'-diaminodiphenyl ether (manufactured by Tokyo Chemical Industry) Capping agent: 3-aminophenol (manufactured by Tokyo Chemical Industry) Solvent: NMP (manufactured by Fuji Film Wako Pure Chemicals) Solvent: 1,4-dioxane (manufactured by Fuji Film Wako Pure Chemicals) Acid anhydride monomer: 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (manufactured by Tokyo Chemical Industry) Crosslinking agent: GMOM (manufactured by Qunrong Chemical Industry)

(Example 1) 4.4 mass% of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 4.8 mass% of 4,4'-diaminodiphenyl ether, 0.18 mass% of 3-aminophenol, and 82 mass% of NMP were dissolved at 20°C, 8.6 mass% of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride was added, and stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was performed by stirring at 200°C for 3 hours, and the aromatic polyimide was purified by reprecipitation using water as a poor solvent. The Mw of the obtained aromatic polyimide was 28,000.

Next, the obtained aromatic polyimide 22 mass%, GMOM 2 mass%, NMP 38 mass%, and 1,4-dioxane 38 mass% were dissolved at 25°C to prepare a polymer solution. The polymer solution was applied on a glass plate at 25°C, and after 3 seconds, it was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, and then dried to obtain a porous membrane.

Then, the obtained porous membrane was heated at 200° C. for 2 hours to crosslink the aromatic polyimide. The imidization degree of the obtained crosslinked polymer was 0.45. The thickness of the obtained porous membrane was 50 μm. The evaluation results of the obtained porous membrane are shown in Table 1.

(Example 2) The polymer solution obtained in Example 1 was applied to a PPS staple nonwoven fabric having an air permeability of 0.6 cm ³ /cm ² /s at 25°C, and after 3 seconds, was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, and dried to obtain a composite film.

Then, the obtained composite film was heated at 200°C for 2 hours to crosslink the aromatic polyimide. The imidization degree of the obtained crosslinked polymer was 0.45. The thickness of the obtained composite film was 180 μm. The evaluation results of the obtained composite film are shown in Table 1.

(Example 3) 3.9 mass% of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 4.2 mass% of 4,4'-diaminodiphenyl ether, and 82 mass% of NMP were dissolved at 20°C, 9.9 mass% of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride was added, and stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was performed by stirring at 200°C for 3 hours, and the aromatic polyimide was purified by reprecipitation using water as a poor solvent. The Mw of the obtained aromatic polyimide was 65,000, and the copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer was 33:67.

Separately, 20% by mass of the obtained aromatic polyimide, 2% by mass of GMOM, 39% by mass of NMP, and 39% by mass of 1,4-dioxane were dissolved at 25°C to prepare a polymer solution. The polymer solution was applied to a PPS short-fiber nonwoven fabric having an air permeability of 0.6 cm ³ /cm ² /s at 25°C, and after 3 seconds, the fabric was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, and then dried to obtain a composite film.

Then, the obtained composite film was heated at 200°C for 2 hours to crosslink the aromatic polyimide. The imidization degree of the obtained crosslinked polymer was 0.53. The thickness of the obtained composite film was 165 μm. The evaluation results of the obtained composite film are shown in Table 1.

(Example 4) 3.4 mass% of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 4.6 mass% of 4,4'-diaminodiphenyl ether, and 82 mass% of NMP were dissolved at 20°C, 10 mass% of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride was added, and stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was performed by stirring at 200°C for 3 hours, and the aromatic polyimide was purified by reprecipitation using water as a poor solvent. The Mw of the obtained aromatic polyimide was 54,000, and the copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer was 29:71.

Next, the obtained aromatic polyimide 22 mass%, GMOM 2 mass%, NMP 38 mass%, and 1,4-dioxane 38 mass% were dissolved at 25°C to prepare a polymer solution. The polymer solution was applied to a glass plate at 25°C, and after 3 seconds, it was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, and then dried to obtain a porous membrane.

Then, the obtained porous membrane was heated at 200° C. for 2 hours to crosslink the aromatic polyimide. The imidization degree of the obtained crosslinked polymer was 0.54. The thickness of the obtained porous membrane was 41 μm. The evaluation results of the obtained porous membrane are shown in Table 1.

(Example 5) 5.6 mass% of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 3.0 mass% of 4,4'-diaminodiphenyl ether, and 82 mass% of NMP were dissolved at 20°C, 9.4 mass% of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride was added, and stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was performed by stirring at 200°C for 3 hours, and the aromatic polyimide was purified by reprecipitation using water as a poor solvent. The Mw of the obtained aromatic polyimide was 58,000, and the copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer was 50:50.

Next, the obtained aromatic polyimide 22 mass%, GMOM 2 mass%, NMP 38 mass%, and 1,4-dioxane 38 mass% were dissolved at 25°C to prepare a polymer solution. The polymer solution was applied to a glass plate at 25°C, and after 3 seconds, it was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, and then dried to obtain a porous membrane.

Then, the obtained porous membrane was heated at 200° C. for 2 hours to crosslink the aromatic polyimide. The imidization degree of the obtained crosslinked polymer was 0.53. The thickness of the obtained porous membrane was 43 μm. The evaluation results of the obtained porous membrane are shown in Table 1.

(Example 6) 6.0 mass% of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 2.7 mass% of 4,4'-diaminodiphenyl ether, and 82 mass% of NMP were dissolved at 20°C, 9.3 mass% of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride was added, and stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was performed by stirring at 200°C for 3 hours, and the aromatic polyimide was purified by reprecipitation using water as a poor solvent. The Mw of the obtained aromatic polyimide was 56,000, and the copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer was 55:45.

Next, the obtained aromatic polyimide 22 mass%, GMOM 2 mass%, NMP 38 mass%, and 1,4-dioxane 38 mass% were dissolved at 25°C to prepare a polymer solution. The polymer solution was applied to a glass plate at 25°C, and after 3 seconds, it was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, and then dried to obtain a porous membrane.

Then, the obtained porous membrane was heated at 200° C. for 2 hours to crosslink the aromatic polyimide. The imidization degree of the obtained crosslinked polymer was 0.58. The thickness of the obtained porous membrane was 46 μm. The evaluation results of the obtained porous membrane are shown in Table 1.

(Example 7) 20 mass % of the aromatic polyimide obtained in Example 3, 2 mass % of GMOM, 54 mass % of NMP, and 24 mass % of 1,4-dioxane were dissolved at 25°C to prepare a polymer solution. The polymer solution was applied to a PPS short-fiber nonwoven fabric having an air permeability of 0.6 cm ³ /cm ² /s at 25°C, and after 3 seconds, the fabric was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, and then dried to obtain a composite membrane.

Then, the obtained composite film was heated at 200°C for 2 hours to crosslink the aromatic polyimide. The imidization degree of the obtained crosslinked polymer was 0.54. The thickness of the obtained composite film was 165 μm. The evaluation results of the obtained composite film are shown in Table 1.

(Comparative Example 1) 22 mass% of the aromatic polyimide obtained in Example 1, 39 mass% of NMP, and 39 mass% of 1,4-dioxane were dissolved at 25°C to prepare a polymer solution. Thereafter, a porous membrane was obtained by the same steps as in Example 1. The porous membrane dissolved when immersed in NMP and could not be used as a separation membrane for a treatment liquid containing an organic solvent.

(Comparative Example 2) Before heating, a composite membrane was obtained in the same manner as in Example 2. When the obtained composite membrane was heated at 80°C for 2 hours and immersed in NMP, the porous membrane dissolved and could not be used as a separation membrane for a treatment liquid containing an organic solvent.

(Comparative Example 3) 9.7% by mass of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane and 82% by mass of NMP were dissolved at 20°C, 8.3% by mass of 3,3',4,4'-diphenylethertetracarboxylic dianhydride was added, and stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was performed by stirring at 200°C for 3 hours, and the aromatic polyimide was purified by reprecipitation using water as a poor solvent. The Mw of the obtained aromatic polyimide was 72,000.

Next, the obtained aromatic polyimide 22 mass%, GMOM 2 mass%, NMP 38 mass%, and 1,4-dioxane 38 mass% were dissolved at 25°C to prepare a polymer solution. The polymer solution was applied to a glass plate at 25°C, and after 3 seconds, it was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, and then dried. As a result, cracks were generated in the porous film, and it could not be used as a separation membrane.

(Comparative Example 4) 7.1% by mass of 4,4'-diaminodiphenyl ether and 82% by mass of NMP were dissolved at 20°C, 10.9% by mass of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride was added, and stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was performed by stirring at 200°C for 3 hours, resulting in precipitation of aromatic polyimide. The obtained aromatic polyimide did not show solubility in an organic solvent, so it was not possible to form a porous membrane or a composite membrane using the polymer solution.

(Comparative Example 5) 7.5% by mass of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 1.6% by mass of 4,4'-diaminodiphenyl ether, and 82% by mass of NMP were dissolved at 20°C, 8.9% by mass of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride was added, and stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was performed by stirring at 200°C for 3 hours, and the aromatic polyimide was purified by reprecipitation using water as a poor solvent. The Mw of the obtained aromatic polyimide was 55,000, and the copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer was 72:28.

Next, the obtained aromatic polyimide 22 mass%, GMOM 2 mass%, NMP 38 mass%, and 1,4-dioxane 38 mass% were dissolved at 25°C to prepare a polymer solution. The polymer solution was applied to a glass plate at 25°C, and after 3 seconds, it was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate, and then dried to obtain a porous membrane.

Then, the obtained porous film was heated at 200°C for 2 hours to crosslink the aromatic polyimide. The imidization degree of the obtained crosslinked polymer was 0.56. The thickness of the obtained porous film was 40 μm. The evaluation results of the obtained porous film are shown in Table 1. As a result, the obtained porous film had lower mechanical properties than the porous film of the embodiment.

(Comparative Example 6) 2.4 mass% of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 5.3 mass% of 4,4'-diaminodiphenyl ether, and 82 mass% of NMP were dissolved at 20°C, 10.3 mass% of 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride was added, and stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was performed by stirring at 200°C for 3 hours, resulting in precipitation of aromatic polyimide. The obtained aromatic polyimide did not show solubility in an organic solvent, so it was impossible to form a porous membrane or a composite membrane using the polymer solution.

[Table 1] Table 1 Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Embodiment 5 Embodiment 6 Embodiment 7 Aromatic polymer Mw (g/mol) 28,000 28,000 65,000 54,000 58,000 56,000 65,000 Input ratio of fluorinated aromatic monomers: non-fluorinated aromatic monomers 33:67 33:67 34:66 29:71 51:49 55:45 34:66 Copolymerization ratio of fluorinated aromatic monomers: non-fluorinated aromatic monomers 33:67 33:67 33:67 29:71 50:50 55:45 33:67 Imidization degree 0.45 0.45 0.53 0.54 0.53 0.58 0.54 Porous membrane types (OSRO/OSN/OSU) OSU OSU OSN OSN OSN OSN OSN Void ratio (%) 62 64 55 57 57 60 53 Membrane flux (L/m ² /h/bar) 103 115 2.52 2.12 2.45 2.55 0.56 Blocking rate (%) 99 99 91 95 93 90 95 Area ratio of macropores (%) - - 29 33 27 36 14 Aspect ratio of macropores - - 2.7 2.6 3.1 3.2 1.8 90 degree load test (30 g/50 g) - - ○/○ ○/○ ○/○ ○/× ○/○ Swelling (%) 137 133 126 124 124 128 113 nd: Aromatic polymers are insoluble and cannot be measured

[Table 2] Table 2 Comparison Example 1 Comparison Example 2 Comparison Example 3 Comparison Example 4 Comparison Example 5 Comparative Example 6 Aromatic polymer Mw (g/mol) 28,000 28,000 72,000 - 55,000 - Input ratio of fluorinated aromatic monomers: non-fluorinated aromatic monomers 33:67 33:67 100:0 0:100 72:28 20:80 Copolymerization ratio of fluorinated aromatic monomers: non-fluorinated aromatic monomers 33:67 33:67 100:0 nd 72:28 nd Imidization degree 0.41 0.45 0.50 - 0.56 - Porous membrane types (OSRO/OSN/OSU) - - - - - - Void ratio (%) 66 67 - - - - Membrane flux (L/m ² /h/bar) - - - - - - Blocking rate (%) - - - - - - Area ratio of macropores (%) - - - - 44 - Aspect ratio of macropores - - - - 3.7 - 90 degree load test (30 g/50 g) - - - - ×/× - Swelling (%) Dissolve Dissolve - - 130 - nd: Aromatic polymers are insoluble and cannot be measured

Although the present invention has been described in detail with reference to a specific form, it is clear to a person skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Furthermore, this application is based on a Japanese patent application filed on September 9, 2022 (Japanese Patent Application No. 2022-144194), and the entire application is cited by reference. In addition, all references cited herein will be incorporated as a whole. [Industrial Applicability]

The present invention can provide a porous membrane and a composite membrane which can continuously and stably maintain separation performance and permeability for a treatment liquid containing an organic solvent and a high-temperature treatment liquid by improving film forming properties.

without.

Claims (15)

A porous membrane, comprising a crosslinked polymer formed by crosslinking aromatic polymers, The aromatic polymer is a polymer formed by copolymerizing at least one fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and anhydride monomers, The copolymerization ratio of the fluorine-based aromatic monomer to the non-fluorine-based aromatic monomer in the aromatic polymer is in the range of 25:75 to 70:30 in terms of molar ratio. The porous membrane according to claim 1, wherein the copolymerization ratio of the fluorinated aromatic monomer to the non-fluorinated aromatic monomer is in the range of 25:75 to 50:50 in terms of molar ratio. The porous membrane as described in claim 1 or 2, wherein the aromatic polymer contains aromatic polyimide. The porous membrane as described in claim 3, wherein the imidization degree of the crosslinking polymer is 0.2 to 1.2. The porous membrane as described in claim 1 or 2, wherein the crosslinking polymer is a polymer obtained by crosslinking the aromatic polymers with each other using at least one crosslinking agent selected from the group consisting of epoxy crosslinking agents, hydroxymethyl crosslinking agents, alkoxymethyl crosslinking agents, and diamine crosslinking agents. The porous membrane as described in claim 1 or 2, wherein the porous membrane has at least two layers: a dense layer and a coarse layer. The porous membrane as described in claim 1 or 2, wherein the area ratio of macropores in the cross section of the porous membrane is 3% to 60%. A porous membrane as described in claim 7, wherein the macropores have a major axis in the thickness direction of the membrane and an aspect ratio of 2.0 or more. A composite membrane is formed by laminating the porous membrane as described in claim 1 or 2 on at least one surface side of a substrate. A composite film as described in claim 9, wherein the substrate contains polyphenylene sulfide as a main component. A module comprising the porous membrane as described in claim 1 or 2. A module comprising the composite membrane as described in claim 9. A fluid separation device, comprising a module as described in claim 12. A method for manufacturing a porous membrane comprises the following steps (i) and (ii), (i) copolymerizing at least one fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers containing fluorine atoms and aromatic diisocyanate monomers containing fluorine atoms, at least one non-fluorine-based aromatic monomer selected from the group consisting of aromatic diamine monomers not containing fluorine atoms and aromatic diisocyanate monomers not containing fluorine atoms, and anhydride monomers to obtain an aromatic polymer; (ii) crosslinking the aromatic polymer obtained in (i) by a crosslinking agent. The method for producing a porous membrane as described in claim 14, wherein the crosslinking agent is at least one crosslinking agent selected from the group consisting of epoxy-based crosslinking agents, hydroxymethyl-based crosslinking agents, alkoxymethyl-based crosslinking agents, and diamine-based crosslinking agents.