TW202410958A - Porous membrane, composite membrane, module, fluid separation device and method for manufacturing porous membrane - Google Patents

Abstract

The porous membrane of the present invention contains an aromatic polymer, wherein the aromatic polymer has a plurality of units represented by the following formula (1) forming a main chain of the polymer, and a cross-linked structure in which any two or more of the units are linked via at least one of R ¹ to R ¹⁰ in the units, and the swelling degree of the porous membrane in N-methyl-2-pyrrolidone is 100% to 200%. (R ¹ to R ¹⁰ and X in formula (1) are as defined in the specification)

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 processes have different names depending on the size of the separation being targeted. 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 purposes swell or dissolve in organic solvents, so they cannot maintain stability, which greatly reduces the separation efficiency. Therefore, the separation of liquid mixtures containing organic solvents requires the use of membranes with resistance to organic solvents. In addition, heat resistance is also required when membrane separation is performed at high temperatures with high boiling point organic solvents as the target. Various separation membranes have been disclosed as separation membranes whose main purpose is to be used in liquids containing organic solvents.

For example, Koch's SelRO membrane is made of polydimethylsiloxane (hereinafter referred to as "PDMS") and polyacrylonitrile (hereinafter referred to as "PAN") as raw materials, and can be used for membrane separation in polar solvents such as methanol, ethanol, isopropanol and other alcohols, cyclohexane or pentane and other hydrocarbons, tetrahydrofuran or ethyl acetate, acetonitrile and the like. Other solvent-resistant separation membranes made of PDMS and PAN include AMS's Nanopro membrane and Ultrapro membrane. Examples of solvent-resistant membranes made of polymers as the main raw materials include polyimide membranes, polyamide imide membranes, and polyether imide membranes.

These polymer membranes are generally produced by phase separation. Phase separation is a method of intentionally changing a polymer from a solution state to a solid state. For example, a non-solvent organic phase separation method (Thermally Induced Phase Separation (TIPS)) can be cited, in which a polymer solution is mixed and precipitated by contacting a liquid that becomes a poor solvent for the polymer. They are also called phase inversion processes, etc., and the polymer forms a membrane with a porous structure. This porous structure has the permeability of liquids or gases. Separation membranes can separate and purify impurities in liquids or gases by utilizing the difference in pore size or gas solubility and diffusion.

These polymer membranes are generally cross-linked to obtain resistance to organic solvents. Since the polymer containing the imide group has a property of being stable with respect to drugs or heat, these cross-links are mainly performed by the polyamine compound represented by Patent Document 1. Here, by using the polyamine compound to cleave the imide group to form a covalent bond, the polymer is cross-linked, thereby obtaining a stable membrane even in a polar solvent as a good solvent.

For crosslinking that achieves solvent resistance in the above manner, if it is stable relative to reactive substances in the process, its application can be expected to be expanded. For example, Patent Document 2 discloses a method for ensuring stability by capping reactive terminal functional groups. In this method, a polyimide film crosslinked with polyamine is treated with an acid halide compound or an acid anhydride to convert the amine group into a functional group with low reactivity, for example, in a continuous catalytic reaction of ethylene oxide (hereinafter referred to as "EO") and carbon monoxide (hereinafter referred to as "CO") for generating β-propiolactone (hereinafter referred to as "BPL"), the effect of suppressing the ring-opening polymerization of EO and BPL is shown.

In recent years, a method using a coordination crosslinking reaction between a carboxyl group and a metal ion, as listed in Non-Patent Document 1, has been reported as a crosslinking method other than polyamine compounds. In this method, by preventing the structural change of the main chain structure of the polyimide caused by the ring opening of the imide bond, the permeability and mechanical properties are maintained, and a high-performance separation film for OSN is produced.

As a method for crosslinking a polymer film containing a polymer containing an imide group derived from a phenol group, there can be cited methods such as ultraviolet (UV) crosslinking and thermal crosslinking as listed in Patent Document 3. Here, a soluble polyimide is synthesized using a diamine monomer containing a phenol group, and the prepared polyimide solution is applied to a certain thickness and then dried to obtain a polyimide film. Furthermore, the molecules are crosslinked by UV crosslinking using a UV lamp with a wavelength of 254 nm and thermal crosslinking at a high temperature of about 450°C. [Prior Art Document] [Patent Document]

Patent document 1: Japanese Patent List No. 2010-526919 Patent document 2: Japanese Patent Publication No. 2020-189288 Patent document 3: Japanese Patent List No. 2012-521873 [Non-patent document]

Non-patent document 1: Journal of Membrane Science 621, 2021, 119002.

[Problems to be solved by the invention] Since cross-linking using the polyamine compound represented by Patent Document 1 leaves reactive functional groups such as amine groups in the membrane, there is a concern that problems may occur in some uses or processes in organic solvent separation. For example, when a catalyst containing a metal complex exists as a valuable substance in the treated liquid, an interaction with the reactive functional groups occurs. In addition, the following concerns can also be listed: when the treated liquid contains an electrophilic substance that reacts with a nucleophilic functional group, it may accumulate on the membrane surface due to interaction with the membrane or remain inside the membrane, reducing the separation efficiency and separation performance.

The polymer membrane disclosed in Patent Document 3 is not a composite membrane integrated with a substrate such as a nonwoven fabric. When manufacturing a device including a separation membrane, it is important to use a porous membrane such as a nonwoven fabric as a substrate in terms of membrane protection or device manufacturing. However, as far as the inventors know, there are almost no reports of nonwoven fabrics that can withstand a continuous use temperature below the upper limit in a heat treatment of about 450°C.

The polymer membranes with solvent resistance are mostly porous membranes, and the porous structure has a structural domain of a size that scatters light in the membrane. On the other hand, the membrane made by the method shown in Patent Document 3 is presumed to be in a film shape because it is not made by phase separation. It is presumed that the crosslinking based on UV light is only carried out on the surface of the porous membranes. When the porous membrane with only the surface crosslinked is passed through an organic solvent, it is possible to cause swelling and dissolution of the part where the crosslinking is not sufficient, and lose the function as a separation membrane.

Therefore, the subject of the present invention is to provide a porous membrane and a composite membrane for selective separation of liquid mixtures and having particularly excellent solvent resistance. [Means for solving the subject]

The inventors of the present invention have discovered that by making the porous membrane contain units having a specific structure and an aromatic polymer having a cross-linked structure that bonds between the units, the swelling degree in an organic solvent can be adjusted to obtain a porous membrane and a composite membrane having solvent resistance, thereby completing the present invention.

To solve the above problems, the present embodiment is characterized by the following (1) to (8). (1) A porous membrane comprising an aromatic polymer, wherein the aromatic polymer has a plurality of units represented by the following formula (1) forming a main chain of the polymer, and a cross-linked structure in which any two or more of the units are linked via at least one of R ¹ to R ¹⁰ in the units, wherein the porous membrane has a swelling degree of 100% to 200% in N-methyl-2-pyrrolidone.

[Chemistry 1]

(In formula (1), R ¹ to R ¹⁰ may be the same or different, and represent a group selected from the group consisting of a alkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, a fluoroalkyl group having 1 to 30 carbon atoms, an imide group, an amide group, a hydroxyl group, a hydrogen atom, a halogen atom, a carboxyl group, a carboxylate group, a phenyl group, a sulfone group, a nitro group, a cyano group, and a group bonded to the cross-linked structure; wherein at least one of R ¹ to R ⁵ and at least one of R ⁶ to R ¹⁰ is an imide group or an amide group forming the main chain of the aromatic polymer; and R ¹ to R At least one of ¹⁰ is a hydroxyl group, and at least one of the carbon atoms adjacent to the carbon atom to which the hydroxyl group is bonded is bonded to a hydrogen atom; X represents a direct bond and a bonding structure selected from the group consisting of the following formulas (2-1) to (2-11); in formulas (2-1) to (2-11), * represents a bonding bond to two aromatic rings in the unit)

[Chemistry 2]

(2) A porous membrane as described in (1), wherein the cross-linked structure is at least one structure selected from the group consisting of the following formulas (3-1) to (3-3), and the aromatic rings at both ends of the structures represented by the following formulas (3-1) to (3-3) are aromatic rings in the unit.

[Chemistry 3]

(In each of formulas (3-1) to (3-3), P, Q and R represent crosslinking agent residues) (3) A porous membrane as described in (1) or (2), wherein the aromatic polymer contains an aromatic polyimide. (4) A porous membrane as described in any one of (1) to (3), wherein the porous membrane has at least two layers, a dense layer and a coarse layer. (5) A porous membrane as described in any one of (1) to (4), wherein the area ratio of the macropores in the cross section of the porous membrane is 3% to 60%. (6) A porous membrane as described in (5), wherein the macropores have a long axis in the thickness direction of the membrane and an aspect ratio of 2.0 or more. (7) A composite membrane, which is formed by laminating the porous membrane described in any one of (1) to (6) on at least one surface side of a substrate. (8) A composite membrane as described in (7), wherein the substrate has polyphenylene sulfide as a main component. (9) A module, which includes the porous membrane described in any one of (1) to (6). (10) A module, which includes the composite membrane described in (7). (11) A fluid separation device, which includes the module described in (10). (12) A method for producing a porous membrane, which is a method for producing a porous membrane as described in any one of (1) to (6), comprising the following steps (i) and (ii). (i) Polymerizing an aromatic polymer having a unit represented by the formula (1). (ii) crosslinking the aromatic polymer obtained in (i) by a crosslinking agent. (13) A method for producing a porous membrane as described in (12), wherein the crosslinking agent is at least one crosslinking agent selected from the group consisting of epoxy crosslinking agents, hydroxymethyl crosslinking agents and polycarboxylic acid crosslinking agents. [Effect of the invention]

According to the present invention, a porous membrane and a composite semipermeable membrane having a porous structure and showing excellent solvent resistance can be obtained.

The following describes the embodiments of the present invention in detail, but the present invention is not limited to these embodiments. The "-" in a numerical range includes the numerical values before and after it. For example, "0-100" means a range of 0 or more and 100 or less.

1. Porous membrane and composite membrane (1-1) Porous membrane The porous membrane described below contains an aromatic polymer, wherein the aromatic polymer has a plurality of units represented by the following formula (1) forming a main chain of the polymer, and a cross-linked structure in which any two or more of the units are bonded via at least one of R ¹ to R ¹⁰ in the units, and the porous membrane exhibits a swelling degree of 100% to 200% in N-methyl-2-pyrrolidone (NMP).

[Chemistry 4]

In formula (1), R ¹ to R ¹⁰ may be the same or different and represent a group selected from the group consisting of a alkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, a fluoroalkyl group having 1 to 30 carbon atoms, an imide group, an amide group, a hydroxyl group, a hydrogen atom, a halogen atom, a carboxyl group, a carboxylate group, a phenyl group, a sulfonate group, a nitro group, a cyano group, and a group bonded to the cross-linked structure. At least one of R ¹ to R ⁵ and at least one of R ⁶ to R ¹⁰ is an imide group or an amide group forming the main chain of the aromatic polymer. At least one of R ¹ to R ¹⁰ is a hydroxyl group, and at least one of the carbon atoms adjacent to the carbon atom bonded to the hydroxyl group is bonded to a hydrogen atom. X represents a direct bond and a bonding structure selected from the group consisting of the following formulae (2-1) to (2-11). In formulae (2-1) to (2-11), * represents a bond with two aromatic rings in the unit.

[Chemistry 5]

It is speculated that the porous membrane of the present embodiment forms a three-dimensional network structure by containing the cross-linked aromatic polymer as described above, and thus becomes stable even in NMP, thereby maintaining separation performance. In addition, it is speculated that the porous membrane of the present embodiment functions as a membrane with high permeability and high removal performance by satisfying the above swelling. It is speculated that when the above swelling is satisfied in NMP, the separation performance is also maintained in other organic solvents, thereby having high water permeability and high removal performance.

(1-1-1) Aromatic Polymer The porous membrane of this embodiment contains an aromatic polymer having a plurality of units represented by the formula (1) forming a main chain of the polymer and a cross-linked structure in which any two or more of the units are linked via at least one of R ¹ to R ¹⁰ in the units.

In the formula (1), ^R1 and ^R6 may be the same or different. From the viewpoint of productivity of the monomer, ^R1 and ^R6 are preferably a group selected from the group consisting of a alkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, a fluoroalkyl group having 1 to 30 carbon atoms, a hydrogen atom, a halogen atom, a carboxyl group, a carboxylate group, a phenyl group, a sulfonyl group, a nitro group, and a cyano group, and are particularly preferably a hydrogen atom.

In the formula (1), ^R2 and ^R10 may be the same or different. From the viewpoint of monomer productivity or stereo hindrance, ^R2 and ^R10 are preferably an imide group or an amide group forming the main chain of the aromatic polymer, and an imide group is particularly preferred.

In the formula (1), R ³ and R ⁹ may be the same or different. From the viewpoint of productivity of the monomer, R ³ and R ⁹ are preferably hydroxyl groups.

In the formula (1), ^R4 and ^R8 may be the same or different. From the viewpoint of forming a cross-linked structure, ^R4 and ^R8 are preferably groups that bond to the cross-linked structure.

In the formula (1), R ⁵ and R ⁷ may be the same or different. From the viewpoint of monomer productivity or steric hindrance, R ⁵ and R ⁷ are preferably hydrogen atoms.

From the viewpoint of solubility of the aromatic polymer, X in the formula (1) is preferably a bonding structure selected from the group consisting of (2-1), (2-2), (2-3) and (2-5), and particularly preferably the bonding structure (2-2).

Examples of the unit represented by the formula (1) include units derived from 3,3'-dihydroxybenzidine, 4,4'-dihydroxy-3,3'-diaminophenylpropane, 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 4,4'-dihydroxy-3,3'-diaminophenylsulfone, 4,4'-dihydroxy-3,3'-diaminophenyl ether, 4,4'-dihydroxy-3,3'-diaminophenylpropanemethane, 4,4'-dihydroxy-3,3'-diaminobenzophenone, and 9,9-bis(3-amino-4-hydroxyphenyl)fluorene. Among them, from the viewpoint of solubility and film-forming property of the polymer, the unit derived from 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane or 4,4'-dihydroxy-3,3'-diaminophenylsulfone is preferred.

In the aromatic polymer, the cross-linked structure is that one unit is bonded to at least one other unit represented by the formula (1) via at least one of R ¹ to R ¹⁰ in the unit. In view of the productivity of the raw material diamine monomer, cross-linking is preferably performed at at least one of R ² and R ¹⁰ .

In the aromatic polymer, the cross-linked structure between the units represented by the formula (1) is preferably at least one structure selected from the group consisting of the following formulas (3-1) to (3-3), and the aromatic rings at both ends of the structures represented by the following formulas (3-1) to (3-3) are aromatic rings in the units represented by the formula (1).

[Chemistry 6]

(In each of formula (3-1) to formula (3-3), P, Q and R represent a crosslinking agent residue)

In each of formula (3-1) to formula (3-3), P, Q and R are crosslinking agent residues, for example, preferably a direct bond, or a alkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, a fluoroalkyl group having 1 to 30 carbon atoms, a halogen atom, a carboxylate group, a phenyl group, a sulfonyl group, an amine group, etc. The crosslinking agent residue is derived from the crosslinking agent described below, and refers to the portion after removing the bonding portion between the crosslinking agent and the unit.

Among the structures selected from the above formula (3-1) to formula (3-3), from the viewpoint of chemical resistance, the structure represented by formula (3-1) or formula (3-2) is preferred, and the structure represented by formula (3-1) is more preferred.

From the viewpoint of raw material synthesis, in formula (3-1), P is preferably a alkyl group having 1 to 30 carbon atoms including a benzyl group, in formula (3-2), Q is preferably a alkyl group having 1 to 30 carbon atoms, and in formula (3-3), R is preferably a alkyl group having 1 to 30 carbon atoms. As specific examples of formulas (3-1) to (3-3), structures in which P is a benzyl group, Q is a carbon atom, and R is a carbon atom can be cited.

The aromatic polymer may also have a unit derived from a diamine compound other than the unit represented by the formula (1). Examples of such a unit derived from a diamine compound include units derived from p-phenylenediamine, m-phenylenediamine, 2,5-diaminotoluene, 2,4-diaminotoluene, 3,5-diaminobenzoic acid, 2,6-diaminobenzoic acid, 2-methoxy-1,4-phenylenediamine, 4,4'-diaminobenzanilide, 3,4'-diaminobenzanilide, 3,3'-diaminobenzanilide, 3,3'-dimethyl-4,4'-diaminobenzanilide, 2,5-diaminophenol, 3 ,5-diaminophenol, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl methane, 4,4'-diaminodiphenyl methane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminobenzophenone, 3,4'-diaminobenzophenone, 3,3'-dimethyl-4,4'-diaminodiphenyl methane and the like.

Regarding the amount of units capable of post-crosslinking, the molar ratio of the content of the units derived from the diamine compound other than the units represented by the formula (1) in the aromatic polymer to the content of the units represented by the formula (1) is preferably 90:10 to 0:100.

The aromatic polymer may have a unit derived from an acid dianhydride. Examples of such a unit derived from an acid dianhydride 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, anhydride, 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- Units of naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 2,3,6,7-anthracenetetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1,2,3,5-cyclopentanetetracarboxylic dianhydride, 1,2,4,5-bicyclohexenetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, and the like.

From the viewpoint of the molecular weight of the obtained aromatic polymer, the molar ratio of the content of the unit derived from the acid dianhydride to the content of the unit represented by the formula (1) in the aromatic polymer is preferably 45:55 to 55:45.

The aromatic polymer may have a unit derived from a polyfunctional acid halide. Examples of the unit derived from a polyfunctional acid halide include units derived from 1,2,4-benzene trimethylene chloride, 1,2,4,5-benzene tetracarbonyl chloride, isophthalic acid chloride, terephthalic acid chloride, trimesic acid chloride, 4,4'-oxybisbenzoyl chloride, 2,2-bis(4-chloroformyloxyphenyl)propane, and the like.

The molar ratio of the unit represented by the formula (1) and/or the unit derived from other diamine compounds to the unit derived from acid dianhydride and/or the unit derived from polyfunctional acid halides is preferably 80:100 to 100:80. By making the molar ratio of the unit represented by the formula (1) and/or the unit derived from other diamine compounds to the unit derived from acid dianhydride and/or the unit derived from polyfunctional acid halides equal, 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.

Examples of the aromatic polymer having the unit represented by the formula (1) include aromatic polyamic acid, aromatic polyamide, aromatic polyamide imide, aromatic polyimide, aromatic polyetherimide, aromatic polymaleimide, etc. In terms of heat resistance and versatility, the aromatic polymer is preferably aromatic polyimide.

When the aromatic polymer is an aromatic polyimide, the weight average molecular weight (hereinafter referred to as "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, mechanical strength and heat resistance 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 moldability can be achieved.

The Mw 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.

When the aromatic polymer is an aromatic polyimide, the imidization degree of the aromatic polyimide having a cross-linked structure (also referred to as a "cross-linked 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 cross-linked 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 such as NMP and a high-temperature treatment liquid.

The degree of imidization of aromatic polyimide can be measured using a Fourier transform infrared spectrophotometer. The degree of imidization is a value calculated by dividing the peak intensity derived from the imide group by the peak intensity derived from the aromatic ring by the infrared total reflection absorption measurement method (attenuated total reflectance (ATR) method) of the porous membrane surface. 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 can be listed: a method of removing water as a by-product of the imidization reaction by distillation, and a method of adding additives such as acetic anhydride, isoquinoline, imidazole, pyridine, etc. to the polyamide solution and then heating it. On the other hand, as a method of inhibiting imidization, there is a method of adding water to polyamide.

The porous membrane may include a polymer other than an aromatic polymer having a unit represented by the formula (1) forming a main chain of the polymer, within a range not impairing the effects of the present embodiment.

(1-1-2) Physical properties of porous membrane The swelling degree of the porous membrane of this embodiment in NMP is 100% to 200%, preferably 105% to 150%, and more preferably 110% to 130%. By setting the swelling degree to the range of 100% to 200%, the porous membrane has a moderate affinity for the solvent and exhibits high removal or high permeability. The swelling degree can be calculated by dividing the thickness of the porous membrane after reaching a balanced swelling state in NMP by the thickness of the porous membrane before immersion in NMP. The swelling degree of the porous membrane can be controlled by the type and concentration of the crosslinking agent and the conditions of the crosslinking reaction.

For example, by leaving the porous membrane in NMP for more than 24 hours, the membrane is in a balanced swelling state. When the swelling degree is less than 100%, the porous membrane has low affinity with organic solvents such as NMP, and the permeability tends to be low. When the swelling degree exceeds 200%, the affinity with organic solvents such as NMP is too high, and the driving pressure in membrane separation becomes high, that is, the permeability tends to be low. The organic solvent may be a good solvent for the aromatic polymer before crosslinking.

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 having an average pore size of less than 50 nm, and "coarse layer" refers to a layer having 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 having both separation performance and permeability can be obtained. FIG. 1 is an electron microscope image of a composite membrane 4 in which a porous membrane including a dense layer 1 and a coarse layer 2 is present on a non-woven fabric (substrate) 3. In addition, as shown in FIG. 1 , the porous membrane is more preferably an asymmetric structure in which the pore size varies obliquely in the thickness direction.

The average surface pore size of the porous membrane of this embodiment can be appropriately selected according to the application. When the organic solvent is separated, such as the concentration of wastewater containing organic solvents such as NMP, or the separation and recovery of mixed organic solvents, 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 are separated, 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.

From the viewpoint of industrial value, when the porous membrane or composite membrane is OSRO (cutoff molecular weight less than 200), the membrane permeation flux of organic solvents such as NMP of the porous membrane is preferably 0.1 L/m ² /h/bar or more. When the porous membrane or composite membrane is OSN (cutoff molecular weight 200-1,000), it is preferably 0.5 L/m ² /h/bar or more, more preferably 1.0 L/m ² /h/bar or more, and further preferably 2.0 L/m ² /h/bar or more. 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, and more preferably 10.0 L/m ² /h/bar or more. Membrane flux can be calculated by measuring the permeate volume (L) by dead-end filtration or crossflow filtration, and converting it into values per unit membrane area (m ² ), per unit time (hours), and per unit pressure (bar).

From the perspective of industrial value, the rejection rate of the porous membrane is preferably 90% or more. With a rejection rate of 90% or more, sufficient solvent purification can be performed. If the rejection rate is lower than 90%, further purification is required. The rejection rate can be calculated by subtracting the solute concentration (ppm) in the permeate from 1 and dividing it by the solute concentration in the stock solution, and then multiplying the result by 100.

From the perspective of the mechanical properties and separation performance of the membrane, the porosity of the porous membrane is preferably 70% to 90%. When the porosity is above 70%, there are sufficient pores for separation, and when the porosity is below 90%, certain mechanical properties are exhibited. The porosity is defined as the ratio of the space volume to the total volume of the substance. There are many ways to determine the porosity. For example, the porosity (%) can be calculated using the following formula based on the membrane volume ( ^cm3 ) and mass (g) calculated by measuring the membrane thickness of the porous membrane. Furthermore, the density calculated from the porosity can be set to 1.42 g/ ^cm3 . Porosity = (1-mass/(density×membrane volume))×100

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 upper limit of 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.

(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 such as NMP, 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 long-fiber nonwoven fabric refers to a nonwoven fabric having an average fiber length of more 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 unit area weight of the substrate will affect the separation performance and physical stability of the composite film. The unit area weight of the substrate is preferably 70 g/m ² to 200 g/m ² , more preferably 90 g/m ² to 160 g/m ² , and even more preferably 100 g/m ² to 140 g/m ² .

When the unit area weight of the substrate is 70 g/ ^m2 or more, when the polymer solution as the raw material of the porous membrane is applied to the substrate, defects due to leakage are less likely to occur, so a composite membrane with good separation performance can be obtained. On the other hand, when the unit area weight of the substrate is 200 g/ ^m2 or less, part of the polymer solution will be impregnated into the substrate, so the adhesion between the porous membrane and the substrate is improved, and a composite membrane with good physical stability can be obtained.

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.

The preferred values of the membrane permeation flux and the rejection rate in the composite membrane are respectively the same as those in the porous membrane.

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) Polymerizing an aromatic polymer having a unit represented by the formula (1). (ii) Crosslinking the aromatic polymer obtained in (i) by a crosslinking agent.

(2-1) Polymerization of polymers First, an aromatic polymer having a unit represented by the above formula (1) is prepared. As monomers, it is preferred to use a compound having a structure represented by the above formula (1) and/or other diamines, acid dianhydrides and/or polyfunctional acid halides, etc.

Examples of the compound having the structure represented by the formula (1) include 3,3'-dihydroxybenzidine, 4,4'-dihydroxy-3,3'-diaminophenylpropane, 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 4,4'-dihydroxy-3,3'-diaminophenylsulfone, 4,4'-dihydroxy-3,3'-diaminophenyl ether, 4,4'-dihydroxy-3,3'-diaminophenylpropanemethane, 4,4'-dihydroxy-3,3'-diaminobenzophenone, and 9,9-bis(3-amino-4-hydroxyphenyl)fluorene. Among them, from the viewpoint of solubility and film-forming property of the polymer, 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane or 4,4'-dihydroxy-3,3'-diaminophenylsulfone is preferably used.

As other diamines, for example, p-phenylenediamine, m-phenylenediamine, 2,5-diaminotoluene, 2,4-diaminotoluene, 3,5-diaminobenzoic acid, 2,6-diaminobenzoic acid, 2-methoxy-1,4-phenylenediamine, 4,4'-diaminobenzanilide, 3,4'-diaminobenzanilide, 3,3'-diaminobenzanilide, 3,3'-dimethyl-4,4'-diaminobenzanilide, 2,5-diaminophenol, 3,5- Diaminophenol, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminobenzophenone, 3,4'-diaminobenzophenone, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, etc.

Examples of the acid dianhydride include 1,2,4,5-benzene tetracarboxylic 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- Naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 2,3,6,7-anthracenetetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1,2,3,5-cyclopentanetetracarboxylic dianhydride, 1,2,4,5-dicyclohexenetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, and the like.

Examples of the polyfunctional acid halide include 1,2,4-benzene trimethylene chloride, 1,2,4,5-benzene tetracarbonyl chloride, isophthaloyl chloride, terephthaloyl chloride, trimesoyl chloride, 4,4'-oxybisbenzoyl chloride, and 2,2-bis(4-chlorocarbonyloxyphenyl)propane.

The molar ratio of the compound having the structure represented by the formula (1) and/or other diamines and acid dianhydrides and/or polyfunctional acid halides is preferably 80:100 to 100:80. By making the molar ratio of the compound having the structure represented by the formula (1) and/or other diamines and acid dianhydrides and/or polyfunctional acid halides equimolar, 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, mechanical strength and heat resistance 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 formability can be achieved.

The Mw 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 step of the polymer is described by taking the polymerization of aromatic polyimide as an example. First, the compound having the structure represented by the formula (1) and/or other diamines are dissolved in a solvent, and acid dianhydride and/or polyfunctional acid halide are added thereto, and stirred at 0°C to 100°C for 10 minutes to 100 hours to obtain a polyamide solution.

Next, when the aromatic polyimide is soluble in the 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 a 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 azeotropically co-existing 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 the reprecipitation method, a solid aromatic polymer having the unit represented by the above formula (1) 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 polymer solution may contain polymers other than aromatic polymers having units represented by the formula (1) within the scope of not impairing the effect of the present embodiment. As a solvent, a good solvent for the polymer is preferred. Here, "good solvent" refers to a solvent that can still dissolve more than 5% by mass of the polymer in a low temperature range below 60°C. Examples of good solvents for polymers include: dimethyl sulfoxide, dimethylformamide, dimethylacetamide, NMP, 2-pyrrolidone, GBL, 1,4-dioxane, 1,3-dimethyl-imidazolidinone, or mixed solvents 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, which needs to be dissolved in the polymer solution, and is preferably an epoxy crosslinking agent, a hydroxymethyl crosslinking agent or a polycarboxylic acid crosslinking agent.

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 alkoxymethyl crosslinking agents 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 polycarboxylic acid crosslinking agent include aconitic acid, adipic acid, aspartic acid, acetylenedicarboxylic acid, acetone dicarboxylic acid, azelaic acid, adamantane dicarboxylic acid, A-aminoadipic acid, 2-amino-3-carboxymuconic acid semialdehyde, 2-aminomuconic acid, aldaric acid, A-ketoglutaric acid, isatonic acid, isophthalic acid, isopropyl apple acid, itaconic acid, iminodiacetic acid, oxalylacetic acid, octopine, carbocysteine, quinolinic acid, citric acid, glutaconic acid, glutaric acid, succinic acid, chorismic acid, diaminopimelic acid, citric acid, glutaconic acid, glutaric acid, succinic acid, chorismic acid, diaminopimelic acid, citric acid, citric acid, glutaconic acid, glutaric acid, succinic acid, chorismic acid, diaminopimelic acid, citric acid, citric acid, citric acid, glutaconic acid, glutaric acid, succinic acid, citric ... , 2,6-pyridinedicarboxylic acid, dibutyl succinic acid, oxalic acid, tartaric acid, suberic acid, sebacic acid, propanedioic acid, terephthalic acid, trimesic acid, puric acid, diquinolinecarboxylic acid, 2,2-bis(4-carboxyphenyl)hexafluoropropane, 2-hydroxy-3-oxosuccinic acid, biphenyldicarboxylic acid, pimelic acid, phthalic acid, 4-fumaryl acetic acid, fumaric acid, polyacrylic acid, 4-maleyl acetic acid, maleic acid, malonic acid, muconic acid, meconic acid, mesaconic acid, N-methyl-D-aspartic acid, apple acid, and its derivatives, etc.

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 obtaining only the porous membrane portion 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 the case of a porous membrane having a crosslinked structure that is not hydrolyzed under the action of an alkali, the crosslinked structure can be identified by analyzing the monomers that react with the crosslinking agent by nuclear magnetic resonance or the like.

The concentration of the crosslinking agent in the polymer solution is preferably 1 mass % to 20 mass %, 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, particularly resistance to NMP, 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.

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 unit area weight of the substrate, and the like.

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, particularly resistance to NMP, and to 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 carried out 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 membrane or composite membrane having resistance to organic solvents, particularly resistance to NMP 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 membrane or composite membrane 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. By the time of thermal crosslinking being more than 30 seconds, a porous film or composite membrane with resistance to organic solvents, particularly resistance to NMP and good separation performance can be formed. On the other hand, by the time of thermal crosslinking being less than 20 hours, the porous structure formed by the NIPS method is maintained, thereby a porous film or composite membrane with good separation performance and permeability can be formed.

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 the 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 spiral elements. 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, as well as the elements and modules using the same, can be combined with a pump for supplying a feed liquid to them, a device for pre-treating the feed water, etc. to form a fluid separation device. By using the fluid separation device, the feed liquid is separated into a permeate from which solutes or impurities are removed and a concentrated liquid that does not pass through the membrane, thereby obtaining a target solution.

Furthermore, the element can be of any shape, such as flat plate type, spiral type, multi-pleated type, tubular type, 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) Swelling degree The porous membrane and the composite membrane are immersed in NMP until a balanced swelling state is reached by standing for at least 24 hours. The swelling degree is calculated by the following formula I based on the membrane thickness measured by the method described in "(6) Membrane thickness" described later. Swelling degree (%) = B/A × 100 ･･･ (Formula I) A: Thickness of the porous membrane before immersion in NMP (μm) B: Thickness of the porous membrane after reaching a balanced swelling state in NMP (μm)

(2) Weight average molecular weight The weight average molecular weight (polystyrene conversion) of the polymer was measured using a gel permeation chromatograph (manufactured by Tosoh Co., Ltd.; HLC-8022). The specific measurement conditions are as follows. Column: 2 pieces of TSK gel SuperHM-H (manufactured by Tosoh Co., Ltd.; 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

(3) Imidization Degree The imidization degree 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 a size of 3 cm × 3 cm, and the side surface of the porous membrane 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 number of times: 64 times). The imidization degree was calculated by the following formula II based on the peak intensity C derived from the aromatic ring detected at 1490 cm ^-1 to 1520 cm ^-1 and the peak intensity D derived from the imide group detected at 1760 cm ^-1 to 1790 cm ^-1 . Imidization degree (%) = D/C ･･･ (Formula II)

(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 of 200 or more and less than 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 (molecular weight of 1,000 or more), a 20 ppm standard polystyrene (Mw 100,000)/NMP solution is supplied at an operating pressure of 2 bar to conduct a cross-flow membrane filtration test. The cross-flow membrane filtration test is a filtration method that suppresses the accumulation of impurities on the membrane surface by making the filtrate flow parallel to the membrane surface.

The membrane permeation flux (L/m ² /h/bar) of the porous membrane or composite membrane is calculated using the following formula III 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 is calculated using the following formula IV from the standard polystyrene concentration E (ppm) in the stock solution and the standard polystyrene concentration F (ppm) in the permeate. Membrane permeation flux (L/m ² /h/bar) = permeate volume / (unit membrane area × unit time) / operating pressure ･･･ (Formula III) Rejection rate = (1-F/E) × 100 ･･･ (Formula IV)

(5) Membrane structure The porous membrane or composite membrane was cut into 10 ^cm2 pieces, washed with 90°C distilled water 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 (S1) and the sum of the areas of the macropores in the porous membrane cross section (S2) 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 V 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 (%) = S2/S1×100 ･･･(Formula V)

In addition, regarding the aspect ratio of the macropores, 10 macropores were randomly selected in one image, and the length in the direction perpendicular to the porous membrane surface and the length in the direction horizontal to the porous membrane surface were calculated for each macropore using the image processing software "ImageJ". The length in the direction perpendicular to the porous membrane surface was divided by the length in the direction horizontal to the porous membrane surface to calculate the aspect ratio. 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.

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

The raw materials of the porous membrane and the composite membrane used in the examples and comparative examples are summarized below. Compound having the structure represented by the formula (1): 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane (manufactured by Tokyo Chemical Industry Co., Ltd.) End-capping agent: 3-aminophenol (manufactured by Tokyo Chemical Industry Co., Ltd.) Solvent: NMP (manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) Acid dianhydride: 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (manufactured by Tokyo Chemical Industry Co., Ltd.) Solvent: 1,4-dioxane (manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) Crosslinking agent: GMOM (manufactured by Qun-Ei Chemical Industries, Ltd.)

(Example 1) Porous membrane (without substrate) 9.2% by mass of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane, 0.18% by mass of 3-aminophenol, and 82% by mass of NMP were dissolved, and 8.6% by mass of 3,3',4,4'-diphenylethertetracarboxylic dianhydride was added. The mixture was stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization was carried out by reacting at 200°C for 3 hours, and the aromatic polyimide was purified by reprecipitation using water as a poor solvent. The weight average molecular weight of the obtained aromatic polyimide was 31,000. Next, the obtained aromatic polyimide 22 mass%, GMOM 2 mass%, NMP 38 mass%, 1,4-dioxane 38 mass% was 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 to coagulate it and dried to obtain a porous membrane. Then, the obtained porous membrane was heated at 200°C for 2 hours to crosslink the aromatic polyimide. Then, the porous membrane was immersed in NMP to make it in a balanced swelling state. The thickness of the obtained porous membrane was about 50 μm. The results of the evaluation of the obtained porous membrane are shown in Table 1.

(Example 2) A composite membrane formed by laminating a porous membrane on a substrate The polymer solution obtained in Example 1 was applied to a PPS short-fiber nonwoven fabric with an air permeability of 0.6 cm ³ /cm ² /s at 25°C, and similarly, it was immersed in a coagulation bath containing distilled water at 25°C for 30 seconds to coagulate it, and then dried to obtain a composite membrane. Subsequently, the obtained composite membrane was heated at 200°C for 2 hours to crosslink the aromatic polyimide. Thereafter, the composite membrane was immersed in NMP to bring it to a balanced swelling state. The thickness of the obtained composite membrane was 150 μm. The results of the evaluation of the obtained composite membrane are shown in Table 1.

(Example 3) A composite membrane in which a porous membrane containing a high molecular weight aromatic polyimide is laminated on a substrate (1) 9.7% by mass of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane and 82% by mass of NMP are dissolved in NMP at 20°C, and 8.3% by mass of 3,3',4,4'-diphenylethertetracarboxylic dianhydride is added. The mixture is stirred at 20°C for 3 hours to obtain an aromatic polyamide solution. Subsequently, imidization is carried out by reacting at 200°C for 3 hours, and the aromatic polyimide is purified by a reprecipitation method using water as a poor solvent. The weight average molecular weight of the obtained aromatic polyimide is 69,000. 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 PPS short-fiber nonwoven fabric with an air permeability of 0.6 cm ³ /cm ² /s at 25°C, and after 3 seconds, it was immersed in a coagulation bath containing distilled water at 25°C to coagulate it and dried to obtain a porous membrane. Subsequently, the obtained composite membrane was heated at 200°C for 2 hours to crosslink the aromatic polyimide. Thereafter, the composite membrane was immersed in NMP to be in a balanced swelling state. The thickness of the obtained composite membrane was 150 μm. The evaluation results of the obtained composite films are shown in Table 1.

(Example 4) Composite membrane in which a porous membrane containing a high molecular weight aromatic polyimide is laminated on a substrate (2) Aromatic polyimide 22 mass%, GMOM 2 mass%, NMP 58 mass%, and 1,4-dioxane 18 mass% obtained in Example 3 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.

Next, the obtained composite 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. Thereafter, the composite membrane was immersed in NMP to bring it into a balanced swelling state. The thickness of the obtained composite membrane was 165 μm. The results of the evaluation of the obtained composite membrane are shown in Table 1. Compared with the membrane obtained in Example 3, the result showed a low permeability.

(Comparative Example 1) Porous membrane produced without using a crosslinking agent Aromatic polyimide 22% by mass, NMP 39% by mass, and 1,4-dioxane 39% by mass obtained in Example 1 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 NMP.

(Comparative Example 2) Composite membrane crosslinked at low temperature Before heating, a composite membrane was obtained in the same manner as in Example 2. The obtained composite membrane was heated at 80°C for 2 hours. When the composite membrane was immersed in NMP, the porous membrane dissolved and could not be used as a separation membrane for the treated liquid containing NMP. It is believed that since the temperature during heating was much lower than the minimum reaction temperature of the crosslinking agent, the aromatic polyimide hardly formed a crosslinking structure.

(Comparative Example 3) Polyimide polymer membrane obtained by the method of the known example A polyimide membrane was prepared based on the method described in Example 1 of Japanese Patent Table 2012-521873. 13.3 g of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (APAF), 3.94 g of 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 60 mL of NMP were added to a 250 mL three-necked round-bottom flask including a nitrogen inlet and a mechanical stirrer. After APAF and HAB were completely dissolved, 18.48 g of 4,4'-dihydroxy-3,3'-diaminophenylhexafluoropropane dissolved in 60 mL of NMP was immediately added. The reaction solution was mechanically stirred at ambient temperature for 24 hours to obtain a viscous poly(amide) solution. Next, 22.2 g of acetic anhydride dissolved in 10 mL of NMP was slowly added to the reaction mixture while stirring, and then 17.3 g of pyridine mixed with 10 mL of NMP was added to the reaction mixture. The reaction mixture was further mechanically stirred at 80°C for 1 hour to obtain polyimide. The reaction mixture was recovered by slowly precipitating it in a large amount of methanol. Next, the obtained fibrous polyimide was completely washed with methanol and dried.

A polyimide film was prepared based on the method described in Example 2 of Japanese Patent Table 2012-521873. 4.0 g of polyimide obtained based on the method described in Example 1 of Japanese Patent Table 2012-521873 was dissolved in a mixed solvent of 12.0 g of NMP and 12.0 g of 1,3-dioxacyclopentane to obtain a doped solution. The doped solution was degassed and then applied to a glass plate with a thickness of 300 μm. The film and the glass plate were placed in a vacuum oven. The film was vacuum dried at 200°C for 48 hours to completely remove the residual solvent and obtain a polymer film.

(Comparative Example 4) Polymer film prepared by UV crosslinking and heat treatment using a known method A polyimide film was prepared based on the method described in Example 3 of Japanese Patent Table 2012-521873. The obtained film was UV irradiated at 50°C for 20 minutes using UV light with a wavelength of 254 nm. The UV crosslinked film was heated from 50°C to 450°C at a heating rate of 3°C/min in a nitrogen environment. The film was kept at 450°C for 1 hour and then cooled to 50°C at a rate of 3°C/min in a nitrogen flow. UV crosslinking was performed and then heat treated to obtain a UV crosslinked film.

However, the separation membranes shown in Comparative Examples 3 and 4 do not show sufficient permeability to the liquid for evaluating the separation efficiency of the membrane separation. Therefore, it is difficult to show the removal rate and the fractional molecular weight. In addition, as shown in the problem to be solved by the invention, it is estimated that it is difficult to perform UV crosslinking on the porous membrane produced by phase separation.

(Comparative Example 5) Composite membrane in which a porous membrane containing a high molecular weight aromatic polyimide is laminated on a substrate (3) Aromatic polyimide 22 mass%, GMOM 0.2 mass%, NMP 38.9 mass%, and 1,4-dioxane 38.9 mass% obtained in Example 3 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.

Next, the obtained composite membrane was heated at 200°C for 2 hours to crosslink the aromatic polyimide. The imidization degree of the obtained crosslinked polymer was 0.55. Thereafter, the composite membrane was immersed in NMP to bring it into a balanced swelling state. The thickness of the obtained composite membrane was 177 μm. The results of the evaluation of the obtained composite membrane are shown in Table 1. Compared with the membrane obtained in Example 3, the results showed a high swelling degree and a low permeability.

[Table 1] Table 1 Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Comparison Example 1 Comparison Example 2 Comparison Example 3 Comparison Example 4 Comparison Example 5 Aromatic polymer Mw (g/mol) 31,000 31,000 69,000 69,000 31,000 31,000 - - 69,000 Imidization degree 0.46 0.46 0.55 0.54 - - - - 0.55 Swelling (%) 136 136 124 122 218 Porous membrane types (OSRO/OSN/OSU) OSU OSU OSN OSN - - - - OSN Membrane flux (L/m ² /h/bar) 120.50 103.70 2.34 0.54 - - - - 0.10 Blocking rate (%) ＞99 ＞99 91 94 - - - - 92 Macropore ratio (%) 45 43 36 12 - - - - - Aspect ratio of macropores 3.3 3.5 3.1 1.9 - - - - -

The porous membrane of Example 1 and the composite membranes of Examples 2 and 3 are solvent resistant, and as shown in Table 1, they also show high membrane permeation flux and high rejection rate when used in NMP. Compared with Example 3, Example 4 has a lower permeability. As described above, the porous membrane of Comparative Example 1 and the composite membrane of Comparative Example 2 dissolve when the membrane is immersed in NMP, so they cannot be used as separation membranes for the treated liquid containing NMP. In addition, the polymer membranes of Comparative Examples 3 and 4 do not show sufficient permeability to the liquid in terms of evaluating the separation efficiency of membrane separation. The membrane of Comparative Example 5 has a high swelling degree due to the small content of the crosslinking agent, and has a lower permeability than the membrane of Example 3.

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. 2022-144195), and the entire application is cited by reference. In addition, all references cited herein will be incorporated as a whole. [Industrial Applicability]

The porous membrane and composite membrane of the present invention have a porous structure and show high solvent resistance. Thus, the porous membrane and composite membrane of the present invention can efficiently separate solutes and impurities in various organic solvents. In addition, the porous membrane and composite membrane of the present invention can be used in various applications such as gas separation or water treatment, battery separators, etc.

1:Dense layer 2:Coarse layer 3:Non-woven fabric (base material) 4:Composite film

FIG1 shows an electron microscope image of a composite film including a dense layer, a coarse layer, and a nonwoven fabric.

Claims (13)

A porous membrane comprising an aromatic polymer, wherein the aromatic polymer has a plurality of units represented by the following formula (1) forming a main chain of the polymer, and a cross-linked structure in which any two or more of the units are bonded via at least one of R ¹ to R ¹⁰ in the units, wherein the swelling degree of the porous membrane in N-methyl-2-pyrrolidone is 100% to 200%, In formula (1), R ¹ to R ¹⁰ may be the same or different, and represent a group selected from the group consisting of a alkyl group having 1 to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, a fluoroalkyl group having 1 to 30 carbon atoms, an imide group, an amide group, a hydroxyl group, a hydrogen atom, a halogen atom, a carboxyl group, a carboxylate group, a phenyl group, a sulfone group, a nitro group, a cyano group, and a group bonded to the cross-linked structure; wherein at least one of R ¹ to R ⁵ and at least one of R ⁶ to R ¹⁰ is an imide group or an amide group forming the main chain of the aromatic polymer; and R ¹ to R At least one of ¹⁰ is a hydroxyl group, and at least one of the carbon atoms adjacent to the carbon atom to which the hydroxyl group is bonded is bonded to a hydrogen atom; X represents a direct bond and a bonding structure selected from the group consisting of the following formulas (2-1) to (2-11); in formulas (2-1) to (2-11), * represents a bond to two aromatic rings in the unit, . The porous membrane according to claim 1, wherein the cross-linked structure is at least one structure selected from the group consisting of the following formulas (3-1) to (3-3), wherein the aromatic rings at both ends of the structures represented by the following formulas (3-1) to (3-3) are aromatic rings in the unit, In each of Formula (3-1) to Formula (3-3), P, Q and R represent a crosslinking agent residue. The porous membrane as described in claim 1 or 2, wherein the aromatic polymer contains aromatic polyimide. 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 5, 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 7, 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 7. A fluid separation device, comprising a module as described in claim 10. A method for producing a porous membrane is a method for producing a porous membrane as described in claim 1 or 2, comprising the following steps (i) and (ii): (i) polymerizing an aromatic polymer having a unit represented by the formula (1); (ii) crosslinking the aromatic polymer obtained in (i) by a crosslinking agent. The method for producing a porous membrane according to claim 12, wherein the crosslinking agent is at least one crosslinking agent selected from the group consisting of epoxy crosslinking agents, hydroxymethyl crosslinking agents and polycarboxylic acid crosslinking agents.