Homogeneous nanofiber string crystal oil-water separation membrane and application thereof

Abstract

The application discloses a homogeneous nanofiber string crystal oil-water separation membrane and application thereof, and belongs to the technical field of oil-water separation membranes. The membrane comprises polymer nanofibers, the fiber surface has periodic lamellae, the lamellae and the fibers are composed of the same polymer, and the membrane has the characteristics of liquid-submerged super-amphiphobicity, that is, the underwater oil contact angle is greater than or equal to 150 degrees and the oil-underwater water contact angle is greater than or equal to 150 degrees. The preparation method comprises the following steps: preparing a nanofiber base film by electrospinning of a crystallizable polymer; immersing the base film into a second solvent for pre-swelling treatment, so that the fiber surface layer molecular chains have migration ability; immersing the treated base film into a third solution containing the same polymer, and performing solvent evaporation crystallization or poor solvent incubation to make the polymer molecules epitaxially crystallize on the fiber surface to form a string crystal structure; and obtaining the membrane through heat annealing, washing and drying. The membrane can switch the separation mode through pre-wetting liquid, and can be respectively used for high-efficiency separation of oil-in-water emulsion and water-in-oil emulsion.

Description

Technical Field

[0001] This invention belongs to the field of oil-water separation membrane technology, specifically relating to a homogeneous nanofiber crystalline oil-water separation membrane and its applications. Background Technology

[0002] With the acceleration of industrialization and urbanization, the discharge of oily wastewater has been increasing year by year, mainly originating from petrochemical, machinery processing, metallurgy, textile, food processing, and catering industries. Untreated oily wastewater not only seriously pollutes the aquatic environment and damages aquatic ecosystems but also wastes water resources. Traditional oil-water separation technologies (such as skimmers, flotation, centrifugation, and coarse granulation) generally suffer from low separation efficiency, high energy consumption, high cost, or the potential for secondary pollution, and are particularly ineffective in treating stable emulsified oil systems. Membrane separation technology, due to its advantages such as high separation precision, no phase change, relatively low energy consumption, and compact equipment, is considered one of the advanced technologies for treating oily wastewater, especially emulsified oil wastewater.

[0003] However, membrane separation technology faces a long-standing technical problem in oil-water treatment applications: emulsified oil droplets easily accumulate on the membrane surface and cause membrane fouling, leading to a significant decrease in flux and severely limiting the long-term operational stability and service life of the membrane. Membrane fouling behavior is closely related to the chemical properties and pore structure of the membrane surface; therefore, there is an urgent need to develop oil-water separation membrane materials with enhanced hydrophilicity and antifouling properties.

[0004] To address the aforementioned challenges, researchers primarily employ the following modification strategies: first, grafting hydrophilic or hydrophobic polymer chains onto the membrane surface; second, introducing inorganic nanoparticles (such as titanium dioxide (TiO2), silicon dioxide (SiO2), and graphene oxide) to construct micro-nano composite structures; and third, controlling the surface energy and roughness of the membrane through blending modification to form a stable hydration layer and reduce oil adhesion. For example, patent application number CN202610020703.3 discloses an oil-water separation membrane and its preparation method. This method first grafts polyethyleneimine onto the surface of zeolite powder, then uses a hydrothermal reaction to deposit magnesium, aluminum, and cerium metal salts with urea onto the zeolite powder surface to generate a composite metal hydroxide. Finally, the resulting composite is coated onto the surface of a polyethylene terephthalate (PET) base membrane to obtain an oil-water separation membrane with superhydrophilic properties, capable of efficiently separating oil-in-water emulsions. Another patent application with application number 202511207567.0 discloses a method for preparing graphene carbon nanofiber films for oil-water separation. The steps include: firstly, preparing a polyacrylonitrile fiber membrane by electrospinning; then coating the surface of the fiber membrane with a pre-prepared graphene oxide dispersion; and finally pre-oxidizing, activating, and carbonizing the composite membrane to obtain a graphene carbon nanofiber film with superhydrophobic and superoleophilic properties, with an oil-water separation efficiency of up to 99.21%.

[0005] While the aforementioned methods can improve membrane wettability and antifouling capabilities to some extent, they still suffer from the following inherent drawbacks: First, the heterogeneous interface exhibits poor compatibility, lacking chemical bonds or strong physical adhesion between the functional layer and the substrate material. This leads to the functional layer easily detaching during long-term use, resulting in decreased separation performance. Second, introduced inorganic nanoparticles or coatings can easily clog the original pores of the base membrane, causing a significant decrease in permeation flux. Third, some modification processes involve multi-step reactions or high-temperature carbonization, resulting in high costs and complex processes, which are not conducive to large-scale production. These drawbacks have not yet been fundamentally resolved.

[0006] In recent years, inspired by the "chain crystal" structure in polymer crystallography, researchers have discovered that epitaxial growth of polymer lamellars on the surface of nanofibers can form hierarchical structures, significantly increasing specific surface area, constructing micro / nano rough surfaces, and improving mechanical properties. Among these, the "homogeneous chain crystal" structure composed of the same polymer exhibits unique application potential because it completely avoids the heterogeneous interface problem. However, current research on homogeneous chain crystal structures is limited to a few polymer systems, and the preparation methods suffer from problems such as stringent solvent selection and narrow process windows. Therefore, developing a homogeneous nanofiber chain crystal oil-water separation membrane suitable for various polymers, with controllable processes, and capable of achieving high-performance oil-water separation has significant industrial value and innovative significance. Summary of the Invention

[0007] To address the technical shortcomings of existing oil-water separation membranes, such as poor interfacial compatibility, easy functional layer detachment, mutual constraints between flux and rejection rate, and poor process universality, this invention provides a homogeneous nanofiber skein crystal oil-water separation membrane and its applications. This invention employs a homogeneous skein crystal structure, where both the polymer nanofibers and surface lamellar crystals are composed of the same polymer, fundamentally avoiding the problems of poor interfacial compatibility and easy functional layer detachment caused by heterogeneous modification. Simultaneously, the periodic lamellar crystals create a micro-nano composite rough morphology on the surface of the polymer nanofibers, endowing the membrane with superhydrophobic properties underwater (oil contact angle ≥150° underwater, water contact angle ≥150° oil underwater), effectively reducing oil droplet adhesion and significantly improving antifouling ability and long-term operational stability. Furthermore, this membrane can flexibly switch between oil-in-water emulsion separation and water-in-oil emulsion separation through a simple pre-wetting operation, showing broad industrial application prospects.

[0008] To achieve the above objectives, the specific solution adopted by the present invention is as follows: In a first aspect, the present invention provides a homogeneous nanofiber crystalline oil-water separation membrane, the membrane comprising polymer nanofibers, the surface of which has periodic lamellar crystals; the membrane has underwater superhydrophobic properties, i.e., an underwater oil contact angle ≥150° and an oil-water contact angle ≥150°. The preparation method of this membrane includes the following steps: Step (1): Dissolve the crystallizable polymer in the first solvent and prepare a nanofiber base film by electrospinning; Step (2): Immerse the nanofiber base film in a second solvent for pre-swelling treatment, so that the surface molecular chains of the polymer nanofibers in the nanofiber base film can acquire migration ability; the temperature of the pre-swelling treatment is between room temperature and (t-15)℃, and the time is 5~30min, where t is the melting point of the crystallizable polymer; Step (3): Prepare a third solution containing a solute and a third solvent. The solute is the same crystallizable polymer as in step (1). Immerse the nanofiber substrate film treated in step (2) in the third solution and induce the crystallizable polymer molecules to epitaxially crystallize on the surface of the polymer nanofiber treated in step (2) by solvent evaporation crystallization or incubation with a poor solvent. This forms a tandem crystal structure composed of a central polymer nanofiber and periodic lamellar crystals. The lamellar crystals and the polymer nanofibers are composed of the same polymer, thus obtaining a tandem crystal film. Step (4): Perform thermal annealing on the crystal film to stabilize the crystal morphology; Step (5): Wash and dry the crystal membrane obtained in step (4) to obtain the homogeneous nanofiber crystal oil-water separation membrane. The second solvent and the third solvent are each independently a mixture of a good solvent and a bad solvent for the crystallizable polymer, or composed of at least one bad solvent.

[0009] Further, in step (3), the concentration of the third solution is 0.005~0.5wt%.

[0010] Further, the solvent evaporation crystallization in step (3) is as follows: the nanofiber substrate membrane treated in step (2) is immersed in the third solution, and after immersion for 1 to 10 minutes, it is taken out and slowly evaporated at a temperature of room temperature to (t-15)℃ for 1 to 24 hours.

[0011] Further, the poor solvent incubation in step (3) is as follows: the nanofiber base film treated in step (2) is immersed in the third solution and incubated at room temperature to (t-15)℃ for 0.5~12h.

[0012] Further, the specific method of step (1) is as follows: dissolve the crystallizable polymer in the first solvent, stir at 20~80℃ for 6~12h until completely dissolved, let stand to remove bubbles, and obtain a spinning solution with a concentration of 8~20wt%; inject the spinning solution into a syringe connected to a high voltage power supply, use a metal needle with an inner diameter of 0.5~1.0mm as the spinneret, and perform electrospinning under the conditions of voltage 5~30kV, receiving distance 10~25cm, propulsion rate 0.3~1.0mL / h, ambient temperature 20~35℃, and relative humidity 30~60% for 2~10h, and collect the nanofiber base film.

[0013] Furthermore, the crystallizable polymer is selected from polyamide, polylactic acid, or polycaprolactone; the polyamide is selected from PA6, PA66, PA11, or PA12.

[0014] Furthermore, when the crystallizable polymer is polyamide, the good solvent is selected from one of formic acid, hexafluoroisopropanol, trifluoroacetic acid, and m-cresol, and the poor solvent is selected from one of toluene, glacial acetic acid, chlorobenzene, N,N-dimethylacetamide, water, ethanol, butanediol, ethylene glycol, N-methylpyrrolidone, polydimethylsiloxane, dimethyl sulfoxide, glycerol, and ethyl acetate. When the crystallizable polymer is polylactic acid, the good solvent is selected from one of dichloromethane, N,N-dimethylformamide, chloroform, tetrahydrofuran, acetone, and dimethyl sulfoxide, and the poor solvent is selected from one of ethanol, ethylene glycol, glycerol, carbon tetrachloride, ethylbenzene, xylene, o-dichlorobenzene, tetrachloroethylene, diethyl ether, and water. When the crystallizable polymer is polycaprolactone, the good solvent is selected from one of dichloromethane, chloroform, carbon tetrachloride, acetone, cyclohexanone, ethyl acetate, N,N-dimethylformamide, acetonitrile, tetrahydrofuran, and toluene, and the poor solvent is selected from one of methanol, ethanol, isopropanol, ethylene glycol, petroleum ether, n-hexane, acetic acid, water, isooctane, amyl acetate, and butyl acetate.

[0015] Furthermore, when the second solvent or the third solvent is composed of a poor solvent, the poor solvent is selected from substances other than ethanol, ethylene glycol, or water.

[0016] Furthermore, in step (4), the temperature of the heat annealing treatment is between 30°C and (t-15)°C, and the time is 1~5h; In step (5), the washing process involves alternating between deionized water and ethanol, with a drying temperature of 30-80°C and a drying time of 12-24 hours.

[0017] Secondly, the present invention provides an application of the above-mentioned homogeneous nanofiber crystalline oil-water separation membrane in the treatment of oily wastewater. When the membrane is used for the treatment of oily wastewater, different liquids are used to pre-wet the membrane to switch the separation mode: when the membrane is pre-wetted with water, it is used to separate oil-in-water emulsions; when the membrane is pre-wetted with oil, it is used to separate water-in-oil emulsions.

[0018] Beneficial effects Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention adopts a homogeneous lattice structure, in which the polymer nanofibers and the surface lamellar crystals are both composed of the same polymer, fundamentally avoiding the problems of poor interfacial compatibility and easy detachment of functional layers caused by heterogeneous modification. The periodic lamellar crystals form a micro-nano composite rough morphology on the surface of the polymer nanofibers, giving the membrane superhydrophobic properties (oil contact angle ≥150° underwater, water contact angle ≥150° oil), effectively reducing oil droplet adhesion and significantly improving anti-fouling ability and long-term operational stability.

[0019] (2) In this invention, the nanofiber substrate film is pre-swollen before the construction of the skeletal crystals. The selected second solvent can moderately weaken the interaction between the molecular chains on the surface of the polymer nanofibers, enabling the chain segments to acquire the necessary migration ability, while avoiding the overall dissolution or structural collapse of the polymer nanofibers. This treatment significantly increases the density of nucleation sites in the subsequent crystallization process, promotes the uniform and dense growth of skeletal crystals, thereby obtaining a more regular skeletal crystal morphology, shortening the skeletal crystal growth time, and improving process efficiency.

[0020] (3) The extended chain crystals and folded chain crystals in the tandem crystal structure form a synergistic enhancement effect at the nanoscale, which can improve the mechanical strength of the membrane; at the same time, the lamellar structure increases the specific surface area and fluid eddy disturbance, reduces the mass transfer resistance, and achieves the improvement of permeation flux while ensuring high rejection rate, thus breaking through the bottleneck of mutual constraint between flux and rejection rate.

[0021] (4) This invention provides a crystallization construction path of solvent evaporation crystallization and poor solvent incubation. It has a wide process window and good repeatability, and is applicable to crystallizable polymer systems such as polyamide, polylactic acid, and polycaprolactone. It has broad industrialization prospects.

[0022] (5) The homogeneous nanofiber skein crystal oil-water separation membrane prepared by this invention has excellent underwater superhydrophobic properties (underwater oil contact angle ≥150° and oil-water contact angle ≥150°). This property originates from the micro-nano composite rough morphology formed on the surface of polymer nanofibers by periodic lamellar crystals. In practical applications, the separation mode of the membrane can be switched by a simple pre-wetting operation according to the actual type of oily wastewater: when the membrane is pre-wetted with water, it exhibits an underwater superhydrophobic state, which is suitable for the separation of oil-in-water emulsions; when the membrane is pre-wetted with oil, it exhibits an oil-underwater superhydrophobic state, which is suitable for the separation of water-in-oil emulsions. This switching separation characteristic allows the same membrane to flexibly handle emulsions of different continuous phases without changing the membrane material, which greatly improves the applicability and economy of the separation device, especially suitable for industrial scenarios where the composition varies and oil-in-water and water-in-oil emulsions are generated alternately. Attached Figure Description

[0023] Figure 1 This is a scanning electron microscope (SEM) image of the membrane prepared in Example 1 of the present invention.

[0024] Figure 2 This is a photograph of the underwater oil contact angle of the membrane prepared in Example 1 of the present invention.

[0025] Figure 3 This is a photograph of the oil-water contact angle of the membrane prepared in Example 1 of the present invention.

[0026] Figure 4 This is a scanning electron microscope (SEM) image of the membrane prepared in Comparative Example 1 of the present invention.

[0027] Figure 5 This is a scanning electron microscope (SEM) image of the membrane prepared in Comparative Example 2 of the present invention.

[0028] Figure 6 This is a scanning electron microscope (SEM) image of the membrane prepared in Comparative Example 4 of the present invention.

[0029] Figure 7 This is a scanning electron microscope (SEM) image of the membrane prepared in Comparative Example 5 of the present invention. Detailed Implementation

[0030] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be described in detail below with reference to specific embodiments and comparative examples. The following embodiments are implemented based on the technical solutions of the present invention, providing detailed implementation methods and specific operating procedures. However, the present invention can also be implemented in other ways different from those described herein; therefore, the scope of protection of the present invention is not limited to the following embodiments.

[0031] This invention provides a homogeneous nanofiber crystalline oil-water separation membrane, comprising polymer nanofibers with periodic lamellar crystals on their surface; the membrane exhibits underwater superhydrophobic properties, i.e., an underwater oil contact angle ≥150° and an oil-water contact angle ≥150°. In this invention, the "periodic lamellar crystals" refer to lamellar crystal structures formed by epitaxial crystallization of the same polymer molecular chains along the fiber axial direction on the surface of the polymer nanofibers, arranged at approximately equal intervals. These periodically arranged lamellar crystals on the surface of the polymer nanofibers create a micro-nano composite rough morphology.

[0032] The preparation method of this membrane includes the following steps: Step (1): The crystallizable polymer is dissolved in a first solvent and electrospinned to form a nanofiber-based membrane. The specific method for step (1) is as follows: The crystallizable polymer is dissolved in the first solvent and stirred at 20-80℃ for 6-12 hours until completely dissolved. After standing to remove bubbles, a spinning solution with a concentration of 8-20 wt% is obtained. The spinning solution is injected into a syringe connected to a high-voltage power supply. A metal needle with an inner diameter of 0.5-1.0 mm is used as the spinneret. Electrospinning is performed under conditions of 5-30 kV voltage, 10-25 cm receiving distance, 0.3-1.0 mL / h propulsion rate, 20-35℃ ambient temperature, and 30-60% relative humidity for 2-10 hours. The nanofiber-based membrane is then collected. Electrospinning is a mature nanofiber preparation technology. It involves spraying the spinning solution into nanofibers under a high-voltage electric field, which are then deposited to form a fiber membrane. The nanofiber-based membrane serves as the substrate for tandem crystal growth, and the diameter, porosity, and surface morphology of its polymer nanofibers directly affect the nucleation density and growth uniformity of subsequent tandem crystals. Step (2): The nanofiber substrate membrane is immersed in a second solvent for pre-swelling treatment, which enables the surface molecular chains of the polymer nanofibers in the nanofiber substrate membrane to acquire migration ability. The temperature of the pre-swelling treatment is between room temperature and (t-15)℃, and the time is 5~30min, where t is the melting point of the crystallizable polymer in℃. The pre-swelling treatment is a key step in achieving uniform crystalline growth in this invention. Its mechanism is that the selected second solvent (a mixture of a good solvent and a poor solvent for the crystallizable polymer, or composed of at least one poor solvent) can moderately weaken the interaction between the surface molecular chains of the polymer nanofibers, enabling the chain segments to acquire the necessary migration ability, while avoiding the overall dissolution or structural collapse of the polymer nanofibers. After the pre-swelling treatment, the polymer molecular chains on the surface of the polymer nanofibers are in a "semi-activated" state, which has sufficient mobility to participate in the subsequent crystallization arrangement, while maintaining the overall morphological integrity of the polymer nanofibers. This treatment significantly increases the density of nucleation sites during the subsequent crystallization process, providing a uniform and dense growth starting point for subsequent epitaxial crystallization, thereby ensuring that the crystals grow regularly and uniformly on the surface of the polymer nanofibers. Step (3): Immerse the nanofiber substrate film treated in step (2) in a third solution. Induce epitaxial crystallization of the crystallizable polymer molecules on the surface of the polymer nanofibers treated in step (2) by solvent evaporation crystallization or incubation with a poor solvent. This allows the polymer nanofibers and the periodically growing lamellar crystals on their surface to form a tandem crystal structure, resulting in a tandem crystal film. The third solution contains a solute and a third solvent. The solute is the same crystallizable polymer as in step (1), and the concentration of the third solution is 0.005~0.5wt%. The second solvent and the third solvent are each independently a mixture of a good solvent and a poor solvent for the crystallizable polymer, or composed of at least one poor solvent. Epitaxial crystallization is the core step in the formation of the tandem crystal structure. The principle is that the crystallizable polymer molecules in the third solution undergo heterogeneous nucleation on the surface of the polymer nanofibers, using the activated molecular chain segments on the surface of the polymer nanofibers as nucleation induction sites, and periodically growing lamellar crystals outward on the surface of the polymer nanofibers. Since the solute and the polymer nanofiber matrix are the same polymer with identical chemical compositions, there is no lattice mismatch or interfacial energy difference between the epitaxially grown lamellar crystals and the surface of the polymer nanofibers. The lamellar crystals can firmly bond to the surface of the polymer nanofibers, forming a stable "homogeneous crystal" structure. This homogeneous epitaxy method fundamentally avoids the problems of poor interfacial compatibility and easy detachment of functional layers caused by heterogeneous modification. The specific operation of solvent evaporation crystallization in step (3) is as follows: the nanofiber substrate film treated in step (2) is immersed in the third solution for 1-10 min and then taken out. It is then slowly evaporated at a temperature from room temperature to (t-15)℃ for 1-24 h. During the slow evaporation of the solvent, the concentration of the third solution gradually increases, and the polymer molecular chains gradually arrange and stack. Under the induction of the existing crystal nuclei on the surface of the polymer nanofibers, the lamellar crystals are epitaxially grown. The solvent evaporation rate is controlled by temperature and ventilation conditions. A slower evaporation rate is conducive to the full arrangement of molecular chains and the formation of a regular lamellar crystal structure. The specific operation of the poor solvent incubation in step (3) is as follows: the nanofiber substrate film treated in step (2) is immersed in the third solution and incubated at room temperature to (t-15)℃ for 0.5~12h. The principle of poor solvent incubation is that the good solvent in the third solution can dissolve polymer molecules, while the poor solvent reduces the solubility of the polymer in the solution, causing the polymer molecules to gradually precipitate from the solution and nucleate and grow on the surface of the polymer nanofibers. By adjusting the ratio of good solvent to poor solvent and the incubation temperature and time, the precipitation rate and crystallization kinetics of polymer molecules can be precisely controlled, thereby obtaining a uniformly sized, periodically arranged lamellar structure; Step (4): The crystalline film is subjected to thermal annealing to stabilize its morphology. The thermal annealing temperature is between 30℃ and (t-15)℃, and the time is 1~5h. The purpose of thermal annealing is to further arrange the molecular chains inside the lamellae in an orderly manner by keeping the crystalline film at an appropriate temperature for a certain time, eliminating the internal stress and structural defects generated during crystallization, improving the crystallinity and structural integrity of the lamellae, thereby stabilizing the morphology of the crystalline film and preventing deformation or detachment of the lamellae during subsequent use. The selection of annealing temperature needs to take into account two factors: if the temperature is too low, the molecular chain mobility is insufficient and defects cannot be effectively eliminated; if the temperature is too high, the formed lamellae may partially melt. In this invention, the annealing temperature is controlled between 30℃ and (t-15)℃ (t is the melting point of the polymer). Within this temperature range, the molecular chains have sufficient mobility to adjust the structure, but it is below the melting point temperature so as not to destroy the existing crystal structure. Step (5): Wash and dry the crystalline membrane obtained in step (4) to obtain the homogeneous nanofiber crystalline oil-water separation membrane. The washing in step (5) involves alternating washing with deionized water and ethanol, and the drying temperature is 30~80℃ for 12~24h. The purpose of washing is to remove uncrystallized polymers, solvents, and other impurities remaining in the crystalline membrane, ensuring the purity of the crystalline membrane.

[0033] In this invention, a "good solvent" refers to a solvent that, within the range of room temperature to the solvent's boiling point, has a solubility for the target polymer greater than 5 g / 100 mL of solvent and can form a homogeneous, transparent solution without significant swelling or gelation. A "bad solvent" refers to a solvent that, under the same conditions, has a solubility for the target polymer less than 0.1 g / 100 mL of solvent, or a solvent that only causes limited swelling of the polymer without forming a true solution.

[0034] The crystallizable polymer is selected from one of polyamide (PA), polylactic acid (PLA), or polycaprolactone (PCL); the polyamide is selected from one of PA6, PA66, PA11, or PA12.

[0035] When the crystallizable polymer is polyamide, the good solvent is selected from one of formic acid, hexafluoroisopropanol, trifluoroacetic acid, and m-cresol, and the poor solvent is selected from one of toluene, glacial acetic acid, chlorobenzene, N,N-dimethylacetamide, water, ethanol, butanediol, ethylene glycol, N-methylpyrrolidone, polydimethylsiloxane, dimethyl sulfoxide, glycerol, and ethyl acetate.

[0036] When the crystallizable polymer is polylactic acid, the good solvent is selected from one of dichloromethane, N,N-dimethylformamide, chloroform, tetrahydrofuran, acetone, and dimethyl sulfoxide, and the poor solvent is selected from one of ethanol, ethylene glycol, glycerol, carbon tetrachloride, ethylbenzene, xylene, o-dichlorobenzene, tetrachloroethylene, diethyl ether, and water.

[0037] When the crystallizable polymer is polycaprolactone, the good solvent is selected from one of dichloromethane, chloroform, carbon tetrachloride, acetone, cyclohexanone, ethyl acetate, N,N-dimethylformamide, acetonitrile, tetrahydrofuran, and toluene, and the poor solvent is selected from one of methanol, ethanol, isopropanol, ethylene glycol, petroleum ether, n-hexane, acetic acid, water, isooctane, amyl acetate, and butyl acetate.

[0038] It should be noted that when the second solvent or the third solvent is composed of a single unsuitable solvent, the unsuitable solvent is selected from substances other than ethanol, ethylene glycol, or water. That is, when the second solvent and the third solvent are selected independently, if a single unsuitable solvent is used as the second or third solvent, that unsuitable solvent cannot be ethanol, ethylene glycol, or water. The purpose of this limitation is that, as a single unsuitable solvent, ethanol, ethylene glycol, or water are difficult to precisely control the migration ability of the fiber surface molecular chains (when used as the second solvent) or cannot provide a suitable crystallization environment (when used as the third solvent), making it difficult to obtain a lamellar structure with the required size and density. By excluding these three substances, it is ensured that the selected single unsuitable solvent can appropriately weaken the interactions between the fiber surface molecular chains, allowing the chain segments to obtain the necessary migration ability while avoiding overall fiber dissolution or structural collapse, and simultaneously providing a suitable solution environment for epitaxial crystallization.

[0039] The above steps (1) to (5) constitute the complete preparation process of this invention. The core innovation of this process is as follows: the surface molecular chains of polymer nanofibers are activated by pre-swelling treatment, providing sufficient nucleation sites for subsequent epitaxial crystallization; a tandem crystal structure is constructed by epitaxial crystallization of the same polymer molecules on the surface of polymer nanofibers, resulting in a micro-nano composite rough morphology on the surface of polymer nanofibers; and internal defects of lamellar crystals are eliminated by thermal annealing treatment, ensuring the long-term structural stability of the tandem crystals. The synergistic effect of these three factors results in a membrane material that possesses underwater superhydrophobic properties, high separation efficiency, high permeation flux, and excellent cycling stability.

[0040] The homogeneous nanofiber skein crystal oil-water separation membrane prepared by this invention exhibits excellent underwater superhydrophobic properties (underwater oil contact angle ≥150° and oil-water contact angle ≥150°). The "underwater superhydrophobic" property described in this invention refers to a contact angle ≥150° for oil droplets underwater (i.e., underwater superoleophobic) and a contact angle ≥150° for water droplets underwater (i.e., oil-water superhydrophobic). This property is attributed to the dual effect of the skein crystal structure: on the one hand, the periodically arranged lamellar crystals create a micro-nano composite rough morphology on the surface of the polymer nanofibers. This micro-nano composite rough morphology can capture a large number of liquid molecules, forming a stable hydration layer (underwater) or oil layer (under oil) on the membrane surface, thereby effectively reducing the adhesion of oil or water droplets; on the other hand, the skein crystal structure increases the specific surface area of ​​the membrane, providing more solid-liquid contact sites and enhancing the wettability between the surface and the pre-wetting liquid, making it difficult for oil or water droplets to penetrate the hydration layer or oil layer and contact the membrane surface. It is this synergistic effect of physical structure (micro-nano roughness) and chemical composition (homogeneous polymer surface) that endows the membrane with dual properties of superoleophobicity underwater and superhydrophobicity under oil.

[0041] Based on the aforementioned underwater superhydrophobic properties, the homogeneous nanofiber skein crystal oil-water separation membrane prepared in this invention exhibits unique application advantages in oily wastewater treatment. In practical operation, users only need to select the appropriate liquid to pre-wet the membrane according to the type of wastewater to achieve the switching of separation modes: When the oily wastewater to be treated is an oil-in-water emulsion (i.e., an emulsion in which water is the continuous phase and oil is the dispersed phase), the membrane is first pre-wetted with water. Due to the underwater superoleophobic properties of the membrane surface, water can pass smoothly through the membrane pores, while oil droplets are retained, thus achieving the separation of the oil-in-water emulsion. In this operating mode, the membrane exhibits selective permeability, allowing water to pass through while retaining oil. When the oily wastewater to be treated is a water-in-oil emulsion (i.e., an emulsion in which oil is the continuous phase and water is the dispersed phase), the membrane is first pre-wetted with oil. Due to the superhydrophobic properties of the membrane surface under oil, oil can pass smoothly through the membrane pores, while water droplets are retained, thereby achieving the separation of the water-in-oil emulsion. In this operating mode, the membrane exhibits selective permeability, allowing oil to pass through while retaining water.

[0042] This switchable separation characteristic allows the same membrane to flexibly switch separation modes according to changes in the actual wastewater type through a simple pre-wetting operation, without the need to replace the membrane material or perform any chemical treatment or physical modification. This feature is particularly suitable for industrial scenarios involving the treatment of variable compositions and the alternating generation of oil-in-water and water-in-oil emulsions. For example, in oil extraction, the wastewater types generated at different stages of the operation may differ; or in chemical production, the wastewater properties discharged from different production batches may alternate. The membrane material of this invention can quickly adapt to such changes through a simple pre-wetting operation, significantly improving the applicability and economy of the separation device and reducing equipment investment and operation and maintenance costs.

[0043] The technical solution of the present invention will be further described below with reference to specific embodiments and comparative examples. It should be noted that after the membrane is prepared, its underwater superhydrophobic properties and oil-water separation performance are tested using the following test methods.

[0044] Underwater superhydrophobic performance test: Immerse the prepared sample in oil or water, and after it is completely wetted, drop 5 microliters of liquid (water drop or oil drop) onto the membrane surface. When the droplet is stable, record the contact angle at this time, which is the oil-water contact angle and the water-oil contact angle.

[0045] Oil-water separation performance test: An oil-water emulsion separation experiment was conducted at 0.01 MPa using terminal filtration. The flux and efficiency of the oil-water separation were calculated using formulas (1) and (2). (1) (2) In equations (1) and (2), J w V, A, Δt, ΔP, c0, and c represent the membrane flux (L / m³). 2 (·h·bar), filtrate volume (L), effective membrane area (m²) 2 The parameters included running time (h), transmembrane pressure (bar), the content of dispersed phase in the emulsion, and the content of dispersed phase in the corresponding filtrate. For water-in-oil and oil-in-water emulsions, the water and oil content in the emulsions were measured using a Karl Fischer titrator and a total organic carbon analyzer (Shimadzu TOC-L CPN), respectively.

[0046] Example 1 (PA66 system, solvent evaporation crystallization) A homogeneous PA66 nanofiber crystalline oil-water separation membrane was prepared by the following method: (1) Electrospinning preparation of nanofiber-based membrane: PA66 was dissolved in formic acid (first solvent) and stirred at 60℃ for 12h until completely dissolved. After standing to remove bubbles, a spinning solution with a concentration of 20wt% was obtained. The spinning solution was injected into a syringe connected to a high-voltage power supply. A metal needle with an inner diameter of 1.0mm was used as the spinneret. Electrospinning was carried out under the conditions of voltage of 20kV, receiving distance of 20cm, feed rate of 0.5mL / h, roller collector speed of 500r / min, ambient temperature of 20℃ and relative humidity of 30% for 10h to obtain PA66 nanofiber-based membrane; (2) Pre-swelling treatment: The PA66 nanofiber substrate film is immersed in a glycerol / ethylene glycol (1:1, v / v) mixed solution (second solvent) at 90°C for 15 min; (3) Solvent evaporation crystallization: A 0.1 wt% PA66 / formic acid and glycerol mixed solution (formic acid: glycerol = 1:9, v / v) was prepared as the third solution. The pre-swollen nanofiber base film was immersed in the third solution for 5 min, and then suspended in a 30℃ oven for slow evaporation for 12 h, so that PA66 lamellar crystals could be epitaxially grown on the surface of polymer nanofibers to form a tandem crystal structure, thus obtaining a tandem crystal film; (4) Heat annealing treatment: The crystal film is annealed in a vacuum oven at 80°C for 2 hours to stabilize the crystal morphology; (5) Washing and drying: Wash three times each with anhydrous ethanol and deionized water, and vacuum dry at 60°C for 24 hours to obtain a homogeneous PA66 nanofiber crystalline oil-water separation membrane.

[0047] The homogeneous PA66 nanofiber crystalline oil-water separation membrane prepared in Example 1 was tested, and its fiber surface had a periodically arranged lamellar structure. Figure 1 The underwater oil contact angle (dichloromethane) of the sample was 154.3°. Figure 2 The oil-water contact angle is 152.7°. Figure 3 This indicates that the sample possesses underwater superhydrophobicity. The separation flux and separation efficiency for oil-in-water and water-in-oil emulsions (dichloromethane / water, dispersed phase concentration 1000 ppm) without emulsifiers were 6286 L·m⁻¹. -2 ·h -1 ·bar -1 98.8% and 8478 L·m -2 ·h -1 ·bar -1 The separation flux and separation efficiency for oil-in-water and water-in-oil emulsions containing emulsifiers (dichloromethane / water, dispersed phase concentration 1000 ppm, surfactants sodium dodecyl sulfate and Span 80, respectively) were 97.2% and 2852 L·m⁻², respectively. -2 ·h -1 ·bar-1 95.4% and 3042 L·m -2 ·h -1 ·bar -1 The sample's tensile strength was 11.5 MPa. After 50 cycles of separation, the membrane's flux recovery rate was 93.5%.

[0048] The superior underwater superhydrophobic and separation performance of the homogeneous PA66 nanofiber skein crystal oil-water separation membrane prepared in this invention is fundamentally due to the micro-nano composite rough morphology formed on the surface of the polymer nanofibers by the skein crystal structure, and the stable interfacial bonding provided by the homogeneous polymer. The periodically arranged lamellar crystals create a micro-nano composite rough morphology on the surface of the polymer nanofibers (…). Figure 1 This morphology, after prewetting, can capture a large number of liquid molecules, forming a stable liquid layer on the membrane surface (a hydrated layer is formed when water is prewetting, and an oil-based layer is formed when oil is prewetting), thus endowing the membrane with underwater superoleophobicity. Figure 2 ) and superhydrophobic under oil ( Figure 3 The membrane exhibits a dual characteristic. When the oil-water emulsion contacts the membrane surface, the stable liquid layer effectively blocks dispersed phase droplets (oil or water droplets) from penetrating the membrane pores, achieving high-efficiency retention. Simultaneously, the lamellar structure increases fluid eddy current disturbance, reducing mass transfer resistance and thus ensuring high permeation flux. Furthermore, since the lamellar structure and the polymer nanofiber matrix are made of the same polymer, there are no interfacial compatibility issues between them, and the lamellar structure is firmly bonded, resulting in excellent cycling stability.

[0049] Example 2 (PA11 system, incubation in poor solvent) A homogeneous PA11 nanofiber crystalline oil-water separation membrane was prepared by the following method: (1) Electrospinning preparation of nanofiber-based membrane: PA11 was dissolved in m-cresol (first solvent) and stirred at 50℃ for 10h until completely dissolved. After standing to remove bubbles, a spinning solution with a concentration of 15wt% was prepared. The spinning solution was injected into a syringe connected to a high-voltage power supply. A metal needle with an inner diameter of 0.8mm was used as the spinneret. Electrospinning was carried out under the conditions of voltage 20kV, receiving distance 15cm, feed rate 0.6mL / h, roller collector speed 300r / min, ambient temperature 20℃, and relative humidity 40% for 8h to obtain PA11 nanofiber-based membrane; (2) Pre-swelling treatment: The PA11 nanofiber substrate film was immersed in a glycerol / N-methylpyrrolidone (1:1, v / v) mixed solution (second solvent) at 60°C for 30 min; (3) Incubation in poor solvent: A PA11 / N-methylpyrrolidone solution with a concentration of 0.3 wt% was prepared as the third solution. The pre-swollen nanofiber substrate film was immersed in the third solution and incubated at 140 °C for 2 h to allow PA11 lamellar epitaxial growth to form a tandem crystal structure, thus obtaining a tandem crystal film; (4) Heat annealing treatment: The crystal film is annealed in a vacuum oven at 100°C for 1 hour; (5) Washing and drying: Wash 5 times each with anhydrous ethanol and deionized water, and then vacuum dry at 60°C for 24 hours to obtain the product.

[0050] The homogeneous PA11 nanofiber crystalline oil-water separation membrane prepared in Example 2 was tested, and its fiber surface exhibited a periodically arranged lamellar structure. The sample showed an underwater oil contact angle (petroleum ether) of 157.3° and an oil-water contact angle of 153.2°, demonstrating underwater superhydrophobicity. The separation flux and separation efficiency for emulsions (petroleum ether / water, dispersed phase concentration 1000 ppm) without emulsifiers were 6791 L·m⁻¹. -2 ·h -1 ·bar -1 99.1% and 8457 L·m -2 ·h -1 ·bar -1 The separation flux and separation efficiency for oil-in-water and water-in-oil emulsions containing emulsifiers (petroleum ether / water, dispersed phase concentration 1000 ppm, surfactants sodium dodecyl sulfate and Span 80, respectively) were 2476 L·m⁻¹. -2 ·h -1 ·bar -1 94.4% and 3452 L·m -2 ·h -1 ·bar -1 The tensile strength of the sample was 13.5 MPa. After 50 cycles of separation, the flux recovery rate of the membrane was 94.5%.

[0051] Example 3 (PLA system, solvent evaporation and crystallization) A homogeneous PLA nanofiber crystalline oil-water separation membrane is prepared by the following method: (1) Preparation of nanofiber-based membrane by electrospinning: PLA was dissolved in dichloromethane (the first solvent) and stirred at 35°C for 6 hours until completely dissolved. After standing to remove bubbles, a 16wt% spinning solution was prepared. The spinning solution was injected into a syringe connected to a high-voltage power supply. A metal needle with an inner diameter of 0.6 mm was used as the spinneret. Electrospinning was carried out under the conditions of 5 kV voltage, 20 cm receiving distance, 0.7 mL / h feed rate, 600 r / min rotation speed of the roller collector, 35°C ambient temperature, and 35% relative humidity for 6 hours to obtain PLA nanofiber-based membrane. (2) Pre-swelling treatment: The PLA nanofiber substrate film is immersed in an ethanol / o-dichlorobenzene (1:2, v / v) mixed solution (second solvent) at 80°C for 30 min; (3) Solvent evaporation crystallization: A 0.2 wt% PLA / acetonitrile and ethanol mixed solution (acetonitrile:ethanol = 1:4, v / v) was prepared as the third solution. The pre-swollen nanofiber substrate film was immersed in the third solution for 3 min, and then suspended in a fume hood at room temperature for slow evaporation for 8 h, so that PLA sheet crystals could be epitaxially grown to form a tandem crystal structure, thus obtaining a tandem crystal film; (4) Heat annealing treatment: The crystal film is annealed in a vacuum oven at 50°C for 5 hours; (5) Washing and drying: Wash three times each with anhydrous ethanol and deionized water, and then vacuum dry at 50°C for 24 hours to obtain the product.

[0052] The homogeneous PLA nanofiber crystalline oil-water separation membrane prepared in Example 3 was tested, and its fiber surface exhibited a periodically arranged lamellar structure. The sample showed an underwater oil contact angle (toluene) of 151.7° and an oil-water contact angle of 150.8°, demonstrating underwater superhydrophobicity. The separation flux and separation efficiency for emulsions (toluene / water, dispersed phase concentration 1000 ppm) without emulsifiers were 6047 L·m⁻¹. -2 ·h -1 ·bar -1 98.6% and 7834 L·m -2 ·h -1 ·bar -1 97.4%; the separation flux and separation efficiency for oil-in-water and water-in-oil emulsions containing emulsifiers (toluene / water, dispersed phase concentration 1000 ppm, surfactants sodium dodecyl sulfate and Span 80, respectively) were 2043 L·m⁻¹. -2 ·h -1 ·bar -1 95.6% and 2671 L·m -2 ·h -1 ·bar -1The tensile strength of the sample was 10.9 MPa. After 50 cycles of separation, the flux recovery rate of the membrane was 90.8%.

[0053] Example 4 (PLA system, incubation in unsuitable solvents) A homogeneous PLA nanofiber crystalline oil-water separation membrane is prepared by the following method: (1) Preparation of nanofiber-based membrane by electrospinning: PLA was dissolved in a mixed solvent of dichloromethane / N,N-dimethylformamide (9:1, v / v) (first solvent), and stirred at 25°C for 12 h until completely dissolved. After standing to remove bubbles, a 10 wt% spinning solution was prepared. The spinning solution was injected into a syringe connected to a high-voltage power supply. A metal needle with an inner diameter of 0.5 mm was used as the spinneret. Electrospinning was carried out under the conditions of 18 kV voltage, 10 cm receiving distance, 0.8 mL / h feed rate, 200 r / min rotation speed of the roller collector, 25°C ambient temperature, and 40% relative humidity for 4 h to obtain PLA nanofiber-based membrane; (2) Pre-swelling treatment: Immerse the PLA nanofiber substrate film in ethanol (second solvent) at 40°C for 10 min; (3) Incubation in poor solvent: A 0.05 wt% PLA / dichloromethane and ethanol mixed solution (dichloromethane:ethanol = 1:6, v / v) was prepared as the third solution. The pre-swollen nanofiber substrate film was immersed in the third solution and incubated at 40 °C for 2 h to allow the PLA lamellar epitaxial growth to form a tandem crystal structure, thus obtaining the tandem crystal film; (4) Heat annealing: The crystal film is annealed in a vacuum oven at 40°C for 3 hours; (5) Washing and drying: Wash three times each with anhydrous ethanol and deionized water, and then vacuum dry at 50°C for 24 hours.

[0054] The homogeneous PLA nanofiber crystalline oil-water separation membrane prepared in Example 4 was tested, and its fiber surface exhibited a periodically arranged lamellar structure. The sample had an underwater oil contact angle (carbon tetrachloride) of 152.3° and an oil-water contact angle of 151.9°, demonstrating underwater superhydrophobicity. The separation flux and separation efficiency for emulsions (carbon tetrachloride / water, dispersed phase concentration 1000 ppm) without emulsifiers were 5762 L·m⁻¹. -2 ·h -1 ·bar -1 97.8% and 7495 L·m -2 ·h -1 ·bar -197.1%; the separation flux and separation efficiency for oil-in-water and water-in-oil emulsions containing emulsifiers (carbon tetrachloride / water, dispersed phase concentration 1000 ppm, surfactants sodium dodecyl sulfate and Span 80, respectively) were 2004 L·m⁻¹. -2 ·h -1 ·bar -1 93.5% and 2450 L·m -2 ·h -1 ·bar -1 The tensile strength of the sample was 10.2 MPa. After 50 cycles of separation, the flux recovery rate of the membrane was 93.4%.

[0055] Example 5 (PCL system, solvent evaporation and crystallization) A homogeneous PCL nanofiber crystalline oil-water separation membrane is prepared by the following method: (1) Electrospinning preparation of nanofiber-based membrane: PCL was dissolved in a dichloromethane / acetone (7:3, v / v) mixed solvent (first solvent), stirred at 25°C for 6 h until completely dissolved, allowed to stand to remove bubbles, and prepared as a 12wt% spinning solution. The spinning solution was injected into a syringe connected to a high-voltage power supply, and a metal needle with an inner diameter of 0.7 mm was used as the spinneret. Electrospinning was carried out under the conditions of 15 kV voltage, 15 cm receiving distance, 0.8 mL / h feed rate, 250 r / min drum collector speed, 25°C ambient temperature, and 50% relative humidity for 3 h to obtain PCL nanofiber-based membrane; (2) Pre-swelling treatment: Immerse the PCL nanofiber substrate film in ethanol (second solvent) at 35°C for 15 min; (3) Solvent evaporation crystallization: A 0.2 wt% PCL / ethanol and water mixed solution (ethanol:water = 3:1, v / v) was prepared as the third solution. The pre-swollen nanofiber substrate film was immersed in the third solution for 10 min, and then suspended in a 35℃ oven for slow evaporation for 8 h to allow PCL sheet epitaxial growth to form a tandem crystal structure, thus obtaining a tandem crystal film; (4) Heat annealing: The crystal film is annealed in a vacuum oven at 30°C for 4 hours; (5) Washing and drying: Wash with deionized water 3 times, wash with ethanol 2 times, and vacuum dry at 30℃ for 24 hours.

[0056] The homogeneous PCL nanofiber crystalline oil-water separation membrane prepared in Example 5 was tested, and its fiber surface exhibited a periodically arranged lamellar structure. The sample showed an underwater oil contact angle (n-hexane) of 158.1° and an oil-water contact angle of 154.7°, demonstrating underwater superhydrophobicity. The separation flux and separation efficiency for emulsions (n-hexane / water, dispersed phase concentration 1000 ppm) without emulsifiers were 4977 L·m⁻¹, respectively. -2 ·h -1 ·bar -1 98.3% and 6548 L·m -2 ·h -1 ·bar -1 The separation flux and separation efficiency for oil-in-water and water-in-oil emulsions containing emulsifiers (n-hexane / water, dispersed phase concentration 1000 ppm, surfactants sodium dodecyl sulfate and Span 80, respectively) were 2345 L·m⁻¹. -2 ·h -1 ·bar -1 94.8% and 3072 L·m -2 ·h -1 ·bar -1 The sample's tensile strength was 10.4 MPa. After 50 cycles of separation, the membrane's flux recovery rate was 92.7%.

[0057] Example 6 (PCL system, incubation with poor solvent) A homogeneous PCL nanofiber crystalline oil-water separation membrane is prepared by the following method: (1) Electrospinning preparation of nanofiber-based membrane: PCL was dissolved in dichloromethane (first solvent) to prepare a spinning solution of 18wt%, and stirred at 40℃ for 8h until completely dissolved, and allowed to stand to remove bubbles. The spinning solution was injected into a syringe connected to a high voltage power supply, and a metal needle with an inner diameter of 0.5mm was used as the spinneret. Electrospinning was carried out under the conditions of voltage of 18kV, receiving distance of 20cm, feed rate of 1.0mL / h, roller collector speed of 400r / min, ambient temperature of 30℃ and relative humidity of 40% for 5h to obtain polycaprolactone (PCL) nanofiber-based membrane; (2) Pre-swelling treatment: Immerse the PCL nanofiber substrate film in isopropanol (second solvent) at 30°C for 10 min; (3) Incubation in poor solvents: A 0.1 wt% PCL / dichloromethane and isopropanol mixed solution (dichloromethane:isopropanol = 1:8, v / v) was prepared as the third solution. The pre-swollen nanofiber substrate film was immersed in the third solution and incubated at 35°C for 3 h to allow PCL lamellar epitaxial growth to form a tandem crystal structure, thus obtaining a tandem crystal film; (4) Heat annealing treatment: The crystal film is annealed in a vacuum oven at 40°C for 2 hours; (5) Washing and drying: Wash three times each with anhydrous ethanol and deionized water, and then vacuum dry at 40°C for 24 hours to obtain the product.

[0058] The homogeneous PCL nanofiber crystalline oil-water separation membrane prepared in Example 6 was tested, and its fiber surface exhibited a periodically arranged lamellar structure. The sample showed an underwater oil contact angle (n-hexane) of 151.5° and an oil-water contact angle of 150.3°, demonstrating underwater superhydrophobicity. The separation flux and separation efficiency for emulsions (n-hexane / water, dispersed phase concentration 1000 ppm) without emulsifiers were 5635 L·m⁻¹. -2 ·h -1 ·bar -1 97.4% and 6820 L·m -2 ·h -1 ·bar -1 The separation flux and separation efficiency for oil-in-water and water-in-oil emulsions containing emulsifiers (n-hexane / water, dispersed phase concentration 1000 ppm, surfactants sodium dodecyl sulfate and Span 80, respectively) were 2638 L·m⁻¹. -2 ·h -1 ·bar -1 92.9% and 3338 L·m -2 ·h -1 ·bar -1 The tensile strength of the sample was 11.2 MPa. After 50 cycles of separation, the flux recovery rate of the membrane was 94.5%.

[0059] Comparative Example 1 (PA66 nanofiber membrane without crystallization treatment) The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not include step (2) pre-swelling and step (3) solvent evaporation crystallization treatment.

[0060] The PA66 nanofiber membrane prepared in Comparative Example 1 has a smooth fiber surface and no cross-crystal structure. Figure 4 The underwater oil contact angle (dichloromethane) of the sample was 128.2°, and the underwater water contact angle was 115.0°, indicating a lack of underwater superhydrophobicity. The separation flux and efficiency for emulsions without emulsifiers (dichloromethane / water, dispersed phase concentration 1000 ppm) were 6547 L·m⁻¹. -2 ·h -1 ·bar -1 42.3% and 8278 L·m -2 ·h -1 ·bar -159.7%; the separation flux and separation efficiency for oil-in-water and water-in-oil emulsions containing emulsifiers (dichloromethane / water, dispersed phase concentration 1000 ppm, surfactants sodium dodecyl sulfate and Span 80, respectively) were 2070 L·m⁻¹. -2 ·h -1 ·bar -1 23.7% and 2342 L·m -2 ·h -1 ·bar -1 35.7%. The tensile strength of the sample was 10.2 MPa.

[0061] Comparing Comparative Example 1 with Example 1, it can be seen that although the PA66 nanofiber membrane without crystallization treatment has a permeation flux comparable to the crystallization membrane of the present invention, its separation efficiency is much lower than that of Example 1 (oil-in-water type without emulsifier: 42.3% vs 98.8%). This is because the fiber surface without crystallization structure is smooth ( Figure 4 Lacking a micro-nano composite rough morphology, it is impossible to form a stable hydration or oil layer on the surface, and dispersed phase droplets are easily penetrated or trapped in the membrane pores. This comparative result demonstrates that the tandem crystal structure is the structural basis for achieving underwater superhydrophobicity and high separation performance.

[0062] Comparative Example 2 (no pre-swelling treatment, direct solvent evaporation crystallization) The difference between Comparative Example 2 and Example 1 is that step (2) pre-swelling treatment was not performed.

[0063] Scanning electron microscope images of the homogeneous PA66 nanofiber skein oil-water separation membrane prepared in Comparative Example 2 without pre-swelling treatment are shown. Figure 5 The fiber surface lacks obvious lamellar structures, with a large amount of PA66 encapsulated on the fiber surface and within the fiber pores. The underwater oil contact angle (dichloromethane) of the sample is 127.2°, and the oil-water contact angle is 112.1°, indicating a lack of underwater superhydrophobicity. The separation flux and separation efficiency for emulsions without emulsifiers (dichloromethane / water, dispersed phase concentration 1000 ppm) are 1578 L·m⁻¹, respectively. -2 ·h -1 ·bar -1 52.4% and 1074 L·m -2 ·h -1 ·bar -1 43.2%; the separation flux and separation efficiency for oil-in-water and water-in-oil emulsions containing emulsifiers (dichloromethane / water, dispersed phase concentration 1000 ppm, surfactants sodium dodecyl sulfate and Span 80, respectively) were 547 L·m⁻¹. -2 ·h -1 ·bar -133.2% and 247 L·m -2 ·h -1 ·bar -1 34.7%. After 5 cycles of separation, the flux recovery rate of the membrane was only 13%.

[0064] Comparing Comparative Example 2 with Example 1, it can be seen that without pre-swelling treatment, the molecular chains on the fiber surface lack migration ability and cannot provide sufficient nucleation sites for epitaxial crystallization, resulting in the inability of polymer molecules to grow regularly on the fiber surface. Figure 5 Instead of forming a crystalline structure, the crystals deposit randomly on the fiber surface and in the pores, clogging the fiber pores and causing a significant decrease in flux. This comparative result shows that pre-swelling treatment is a key prerequisite for forming a uniform and regular crystalline structure—it significantly increases the density of nucleation sites by enabling the molecular chains on the fiber surface to migrate, thereby ensuring the regular and uniform growth of the crystals on the fiber surface.

[0065] Comparative Example 3 (using only a good solvent during solvent evaporation and crystallization) The difference between Comparative Example 3 and Example 1 is that step (3) uses a good solvent to prepare a PA66 solution. Specifically, the third solution is a PA66 / formic acid solution with a concentration of 0.1wt%.

[0066] After immersing the pre-swollen nanofiber substrate film in the solution for 5 minutes, the PA66 nanofiber substrate film completely dissolved in the solution, making subsequent experimental operations impossible. This is because formic acid has an excessively strong dissolving ability for PA66. When the solvent of the third solution consists entirely of a good solvent, it not only dissolves the polymer solute in the third solution but also continuously erodes the already fabricated nanofiber substrate film, leading to the destruction of the fiber structure. This comparative result indicates that the third solvent must contain a poor solvent, because a poor solvent can limit excessive erosion of the substrate film, thereby achieving epitaxial crystallization while maintaining the integrity of the fiber morphology.

[0067] Comparative Example 4 (using only a single, unsuitable solvent, ethylene glycol, during the solvent evaporation and crystallization process) The difference between Comparative Example 4 and Example 1 is that step (3) uses ethylene glycol, a single poor solvent, to prepare the PA66 solution. Specifically, the third solution is a PA66 / ethylene glycol solution with a concentration of 0.1 wt%.

[0068] The membrane prepared in Comparative Example 4 has a periodically arranged lamellar structure on its fiber surface, but the lamellar structure is small, and the surface roughness of the fiber membrane is low. Figure 6The underwater oil contact angle (dichloromethane) of the sample was 136.7°, and the underwater water contact angle was 122.4°, indicating that it did not exhibit underwater superhydrophobicity. The separation flux and separation efficiency for emulsions without emulsifiers (dichloromethane / water, dispersed phase concentration 1000 ppm) were 6247 L·m⁻¹. -2 ·h -1 ·bar -1 90.4% and 8523 L·m -2 ·h -1 ·bar -1 The separation flux and separation efficiency for oil-in-water and water-in-oil emulsions containing emulsifiers (dichloromethane / water, dispersed phase concentration 1000 ppm, surfactants sodium dodecyl sulfate and Span 80, respectively) were 2945 L·m⁻¹. -2 ·h -1 ·bar -1 73.4% and 2997 L·m -2 ·h -1 ·bar -1 The sample's tensile strength was 10.8 MPa. After 10 cycles of separation, the membrane's flux recovery rate was only 63.3%.

[0069] Comparing Comparative Example 4 with Example 1, it can be seen that when the solvent of the third solution consists of only a single undesirable solvent (ethylene glycol), although a lamellar structure can also be obtained, the lamellar size is small and the density is low. Figure 6 The inability to form a sufficiently rough micro / nano composite rough morphology on the surface of polymer nanofibers results in insufficient surface roughness of the film, failing to achieve the ≥150° contact angle required for underwater superhydrophobicity. This is because ethylene glycol, a single poor solvent, has extremely low solubility for the polymer solute, and the number of polymer molecules in the third solution that can participate in crystallization is insufficient, limiting the growth size and density of lamellar crystals. This comparative result indicates that the synergistic use of a good solvent and a poor solvent, or the use of at least one poor solvent (when only one poor solvent is used, it cannot be ethylene glycol), is necessary to provide a suitable polymer concentration and crystallization driving force, obtaining lamellar crystals of sufficient size and density, thereby constructing a micro / nano composite rough morphology that meets the requirements of underwater superhydrophobicity.

[0070] Comparative Example 5 (Heterogeneous String Crystal Structure) The difference between Comparative Example 5 and Example 1 is that step (3) uses heteropolymers for skeletal construction. Specifically, the third solution is a 0.1 wt% polycaprolactone (PCL) / ethanol and water mixture (ethanol:water = 3:1, v / v).

[0071] Although the film obtained in Comparative Example 5 has PCL lamellar crystals on its surface, the lamellar crystals are easily detached due to the poor interfacial compatibility between PA66 and PCL. Figure 7Initially, the underwater oil contact angle (dichloromethane) of the sample was 148.5°, and the oil-water contact angle was 142.5°. The separation flux and separation efficiency for emulsions without emulsifiers (dichloromethane / water, dispersed phase concentration 1000 ppm) were 5785 L·m⁻¹. -2 ·h -1 ·bar -1 97.5% and 8014 L·m -2 ·h -1 ·bar -1 96.7%. After 10 cycles of separation, a large amount of crystal flakes detached, the underwater oil contact angle decreased to 105.7°, and the oil-water contact angle decreased to 112.4°. The separation flux and separation efficiency for oil-in-water and water-in-oil emulsions without emulsifiers decreased to 3482 L·m⁻¹, respectively. -2 ·h -1 ·bar -1 75.3% and 3834 L·m -2 ·h -1 ·bar -1 77.1%.

[0072] Comparing Comparative Example 5 with Example 1, it can be seen that although the heterostructure has a certain degree of separation performance initially, due to the fact that the lamellar crystals (polycaprolactone) and the fiber matrix (polyamide) are different polymers, their chemical compositions and cell parameters are different, resulting in lattice mismatch and interfacial energy differences between the lamellar crystals and the fiber surface. Figure 7 The interfacial bonding is weak, and the lamellar crystals are prone to detachment during repeated use, leading to a significant performance degradation. In contrast, in the homogeneous crystalline structure of this invention, the lamellar crystals and the fiber matrix are made of the same polymer, with identical chemical composition and cell parameters. There is no lattice mismatch or interfacial energy difference, allowing the lamellar crystals to firmly bond to the fiber surface, thus exhibiting excellent long-term cycling stability. This comparative result fully demonstrates the significant advantages of the homogeneous crystalline structure over the heterogeneous crystalline structure.

[0073] Comparative Example 6 (without heat annealing) The difference between Comparative Example 6 and Example 1 is that step (4) heat annealing is not performed.

[0074] The membrane prepared in Comparative Example 6 had a periodically arranged lamellar structure on its fiber surface, and its initial performance was similar to that of Example 1. However, after 20 cycles of separation, the lamellar structures deformed and detached, and the underwater oil contact angle (dichloromethane) decreased to 128.3°, and the oil-water contact angle decreased to 114.6°. The separation flux and separation efficiency for emulsions without emulsifiers (dichloromethane / water, dispersed phase concentration 1000 ppm) decreased to 3872 L·m⁻¹, respectively. -2 ·h -1 ·bar-1 78.0% and 4083 L·m -2 ·h -1 ·bar -1 79.6%.

[0075] Comparing Example 6 with Example 1, it can be seen that although the initial performance of the untreated crystalline membrane is comparable to that of the treated membrane, its long-term cycling stability is much lower than that of Example 1 (the flux recovery rate decreases significantly after 20 cycles). This is because the untreated lamellae contain crystallization defects and internal stress. Under the mechanical force and liquid scouring during use, these defects and stresses cause the lamellae to gradually deform or even detach. Thermal annealing, by further arranging the molecular chains within the lamellae in an orderly manner, eliminates the internal stress and crystal defects generated during crystallization, improves the crystallinity and structural integrity of the lamellae, thereby stabilizing the crystalline morphology and preventing deformation or detachment of the lamellae during subsequent use. This comparative result demonstrates that thermal annealing is a key post-processing step for improving the long-term service stability of crystalline membranes.

[0076] Examples 1-6 demonstrate that this method is applicable to various crystallizable polymer systems (polyamide, polylactic acid, polycaprolactone), and both solvent evaporation crystallization and incubation in poor solvents can achieve ideal crystal morphology and separation performance. The membranes prepared in all examples exhibit underwater superhydrophobic properties (oil contact angle ≥150° underwater and water contact angle ≥150° oil underwater), and demonstrate a separation efficiency of ≥92% and a separation capacity of ≥2000 L·m⁻¹ for various oil-water emulsions. -2 ·h -1 ·bar -1 The flux was high, and the flux recovery rate was ≥90% after 50 cycles of separation. Comparative Examples 1-6, on the other hand, verified the necessity of each key step in the technical solution of the present invention from the opposite perspective: Comparative Example 1 shows that the fiber surface without the crystalline structure is smooth and does not have the rough morphology of micro-nano composites, and the separation efficiency is much lower than that of Example 1 (oil-in-water type without emulsifier: 42.3% vs 98.8%), which confirms that the crystalline structure is the structural basis for achieving superhydrophobicity and high separation performance under liquid. Figure 4 Comparative Example 2 shows that regular lamellar crystals cannot grow on the fiber surface without pre-swelling treatment. Figure 5 This confirms that pre-swelling treatment is a key prerequisite for forming a uniform and regular tandem crystal structure; Comparative Example 3 shows that the fiber-based membrane completely dissolves when the third solution contains only a good solvent, confirming that the mixed use of a good solvent and a bad solvent is a necessary condition for ensuring the integrity of the fiber morphology; Comparative Example 4 shows that when only a single bad solvent is used, the lamellar size is small and the density is low ( Figure 6 The inability to form a sufficiently rough micro-nano composite rough morphology on the fiber surface confirms the necessity of using good and bad solvents in synergy; Comparative Example 5 shows that the lamellar crystals of the heterostructure are prone to detachment ( Figure 7This demonstrates the crucial role of the homogeneous crystalline structure in long-term stability. Comparative Example 6 shows that the crystalline membrane without thermal annealing has poor long-term cycling stability, confirming that thermal annealing is a key post-processing step for stabilizing the crystalline morphology and improving long-term service stability. In summary, the above examples and comparative examples show that this invention successfully prepared a homogeneous nanofiber crystalline oil-water separation membrane with underwater superhydrophobic properties through the technical route of "electrospinning to prepare nanofiber-based membrane - pre-swelling treatment to activate the surface molecular chains of polymer nanofibers - epitaxial crystallization to construct a homogeneous crystalline structure - thermal annealing to stabilize the crystalline morphology". The absence or deviation of any step will lead to the loss of the underwater superhydrophobic properties of the membrane material or a significant decrease in long-term operational stability.

[0077] In summary, the homogeneous nanofiber crystalline oil-water separation membrane and its application provided by this invention have advantages such as controllable process, wide applicability, high separation efficiency, large throughput, and good cycle stability, and have important industrial application value in the field of oily wastewater treatment.

[0078] The foregoing has shown and described the main features, usage methods, basic principles, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention based on actual circumstances without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A homogeneous nanofiber crystalline oil-water separation membrane, characterized in that, The membrane comprises polymer nanofibers, the surface of which has periodic lamellar crystals; the membrane has underwater superhydrophobic properties, i.e., an underwater oil contact angle ≥150° and an oil-water contact angle ≥150°. The preparation method of this membrane includes the following steps: Step (1): Dissolve the crystallizable polymer in the first solvent and prepare a nanofiber base film by electrospinning; Step (2): Immerse the nanofiber base film in a second solvent for pre-swelling treatment, so that the surface molecular chains of the polymer nanofibers in the nanofiber base film can acquire migration ability; the temperature of the pre-swelling treatment is between room temperature and (t-15)℃, and the time is 5~30min, where t is the melting point of the crystallizable polymer; Step (3): Prepare a third solution containing a solute and a third solvent. The solute is the same crystallizable polymer as in step (1). Immerse the nanofiber substrate film treated in step (2) in the third solution and induce the crystallizable polymer molecules to epitaxially crystallize on the surface of the polymer nanofiber treated in step (2) by solvent evaporation crystallization or incubation with a poor solvent. This forms a tandem crystal structure composed of a central polymer nanofiber and periodic lamellar crystals. The lamellar crystals and the polymer nanofibers are composed of the same polymer, thus obtaining a tandem crystal film. Step (4): Perform thermal annealing on the crystal film to stabilize the crystal morphology; Step (5): Wash and dry the crystal membrane obtained in step (4) to obtain the homogeneous nanofiber crystal oil-water separation membrane. The second solvent and the third solvent are each independently a mixture of a good solvent and a bad solvent for the crystallizable polymer, or composed of at least one bad solvent.

2. The homogeneous nanofiber crystalline oil-water separation membrane according to claim 1, characterized in that, In step (3), the concentration of the third solution is 0.005~0.5wt%.

3. The homogeneous nanofiber crystalline oil-water separation membrane according to claim 1, characterized in that, The solvent evaporation crystallization in step (3) is as follows: the nanofiber substrate membrane treated in step (2) is immersed in the third solution, and after immersion for 1 to 10 minutes, it is taken out and slowly evaporated at a temperature from room temperature to (t-15)℃ for 1 to 24 hours.

4. The homogeneous nanofiber crystalline oil-water separation membrane according to claim 1, characterized in that, The poor solvent incubation in step (3) is as follows: the nanofiber base film treated in step (2) is immersed in the third solution and incubated at room temperature to (t-15)℃ for 0.5~12h.

5. The homogeneous nanofiber crystalline oil-water separation membrane according to claim 1, characterized in that, The specific method of step (1) is as follows: the crystallizable polymer is dissolved in the first solvent and stirred at 20~80℃ for 6~12h until completely dissolved. After standing to remove bubbles, a spinning solution with a concentration of 8~20wt% is obtained. The spinning solution is injected into a syringe connected to a high-voltage power supply. A metal needle with an inner diameter of 0.5~1.0mm is used as the spinneret. Electrospinning is carried out under the conditions of voltage 5~30kV, receiving distance 10~25cm, feed rate 0.3~1.0mL / h, ambient temperature 20~35℃, and relative humidity 30~60%. The spinning time is 2~10h, and the nanofiber base film is collected.

6. The homogeneous nanofiber crystalline oil-water separation membrane according to any one of claims 1 to 5, characterized in that, The crystallizable polymer is selected from polyamide, polylactic acid, or polycaprolactone; the polyamide is selected from PA6, PA66, PA11, or PA12.

7. The homogeneous nanofiber crystalline oil-water separation membrane according to claim 6, characterized in that, When the crystallizable polymer is polyamide, the good solvent is selected from one of formic acid, hexafluoroisopropanol, trifluoroacetic acid, and m-cresol, and the poor solvent is selected from one of toluene, glacial acetic acid, chlorobenzene, N,N-dimethylacetamide, water, ethanol, butanediol, ethylene glycol, N-methylpyrrolidone, polydimethylsiloxane, dimethyl sulfoxide, glycerol, and ethyl acetate. When the crystallizable polymer is polylactic acid, the good solvent is selected from one of dichloromethane, N,N-dimethylformamide, chloroform, tetrahydrofuran, acetone, and dimethyl sulfoxide, and the poor solvent is selected from one of ethanol, ethylene glycol, glycerol, carbon tetrachloride, ethylbenzene, xylene, o-dichlorobenzene, tetrachloroethylene, diethyl ether, and water. When the crystallizable polymer is polycaprolactone, the good solvent is selected from one of dichloromethane, chloroform, carbon tetrachloride, acetone, cyclohexanone, ethyl acetate, N,N-dimethylformamide, acetonitrile, tetrahydrofuran, and toluene, and the poor solvent is selected from one of methanol, ethanol, isopropanol, ethylene glycol, petroleum ether, n-hexane, acetic acid, water, isooctane, amyl acetate, and butyl acetate.

8. The homogeneous nanofiber crystalline oil-water separation membrane according to claim 7, characterized in that, When the second solvent or the third solvent is composed of a poor solvent, the poor solvent is selected from substances other than ethanol, ethylene glycol, or water.

9. The homogeneous nanofiber crystalline oil-water separation membrane according to claim 1, characterized in that, In step (4), the temperature of the heat annealing treatment is between 30°C and (t-15)°C, and the time is 1~5h; In step (5), the washing process involves alternating between deionized water and ethanol, and the drying temperature is 30~80℃, with a drying time of 12~24h.

10. The application of a homogeneous nanofiber crystalline oil-water separation membrane as described in any one of claims 1 to 9 in the treatment of oily wastewater, characterized in that, When the membrane is used for treating oily wastewater, different liquids are used to pre-wet the membrane to switch the separation mode: when the membrane is pre-wetted with water, it is used to separate oil-in-water emulsions; when the membrane is pre-wetted with oil, it is used to separate water-in-oil emulsions.

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