A nanofiber membrane, and a method of making and use thereof

Abstract

The application discloses a nanofiber membrane, a preparation method and application thereof. The nanofiber membrane comprises a nanofiber base film and a surface layer arranged on the surface of the nanofiber base film; and the raw material of the surface layer comprises isotactic polypropylene and random polypropylene. The nanofiber membrane has excellent hydrophobic performance, a very high water contact angle, reaches a super-hydrophobic state, has good anti-wetting effect, has a porosity of 70-90%, has high air flux, high tensile strength and long-term operation stability.

Description

Technical Field

[0001] This invention relates to the field of materials engineering technology, and in particular to a nanofiber membrane, its preparation method, and its applications. Background Technology

[0002] Water scarcity and pollution are serious problems facing the world today. Membrane distillation, as a membrane separation technology, is widely used in seawater desalination, separation, purification, concentration, and wastewater treatment. It has gained increasing attention due to its advantages such as low energy consumption, environmental friendliness, high product quality, and ease of operation. Membrane distillation is a membrane separation technology driven by vapor pressure across a membrane for mass transfer. Because the high-temperature brine used in membrane distillation has a lower surface energy and a much higher wetting ability than pure water, membrane distillation technology requires membranes with high contact angles, low surface energy, and relatively small pore sizes. Currently, common methods for preparing membranes for membrane distillation include phase inversion, surface modification, blending modification, and composite membrane methods. These processes are relatively complex, and the membranes produced often suffer from problems such as low porosity, low contact angles, or surface inhomogeneity, leading to decreased flux and wetting during long-term use. Therefore, a simple technology that can prepare membranes with high gas flux and long service life has become a current research hotspot and challenge.

[0003] The key to the effectiveness of membranes used in membrane distillation lies in their pore size, porosity, surface roughness, and contact angle. The state of membrane contact with water can be represented by the Cassie-Baxter model, where when a water droplet contacts the rough surface of the membrane, air in the grooves on the membrane surface prevents the water from fully contacting the membrane surface, thus achieving a superhydrophobic state. Traditionally, the phase inversion method is commonly used to construct superhydrophobic membrane surfaces for membrane distillation. This involves preparing a homogeneous polymer solution of a certain concentration and controlling factors such as casting conditions, temperature, and humidity to induce phase separation in the homogeneous polymer solution. The polymer-depleted phase forms pores, while the polymer-rich phase forms continuous connecting parts, thus forming a capping layer on the polymer membrane surface that isolates the brine and achieves a hydrophobic effect. However, this process faces various problems during solute precipitation, such as uneven distribution of precipitated substances, insufficient pores, poor adhesion to the membrane surface, and insufficient roughness.

[0004] Therefore, the morphology of the functional layers needs to be controlled during the phase separation process in the preparation of membranes for membrane distillation. However, membranes prepared by surface modification and blending modification methods typically have low flux, and the preparation process is complex and costly. Composite membrane methods, represented by Janus membranes, use multiple functional layers to form a composite membrane for membrane distillation. While this offers good antifouling capabilities, the multilayer structure limits the flux to some extent, and the preparation process is difficult, making mass production challenging. Summary of the Invention

[0005] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a nanofiber membrane, its preparation method and uses, to solve the problems in the prior art.

[0006] To achieve the above and other related objectives, the present invention is obtained through the following technical solution.

[0007] The present invention provides a nanofiber membrane, the nanofiber membrane comprising a nanofiber base membrane and a surface layer deposited on the surface of the nanofiber base membrane; the raw material of the surface layer comprises isotactic polypropylene and atactic polypropylene.

[0008] Preferably, the raw materials for the nanofiber-based membrane include a polymer and a first solvent, and the amount of the polymer is 5 to 25 wt% based on the total mass of the first solvent and the polymer.

[0009] More preferably, the polymer is selected from one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene polymer, polymethyl methacrylate, polyvinyl chloride, polyurethane, polycarbonate, polystyrene, and polyacrylonitrile. In a more specific embodiment, the polymer has a weight-average molecular weight of 100,000 to 600,000 g / mol, such as 100,000 g / mol, 120,000 g / mol, 200,000 g / mol, 300,000 g / mol, 400,000 g / mol, 500,000 g / mol, or 600,000 g / mol.

[0010] More preferably, the first solvent is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, water, ethanol, isopropanol, n-butanol, acetone, 1,4-dioxane, dichloromethane, chloroform, tetrahydrofuran, and acetic acid.

[0011] Preferably, the raw material for the surface layer of the nanofiber membrane further includes a second solvent, and the amount of the isotactic polypropylene and the atactic polypropylene is 0.5 to 1 wt% based on the total mass of the second solvent, isotactic polypropylene and atactic polypropylene.

[0012] More preferably, the second solvent is selected from one or more of xylene, decahydronaphthalene, dibutyl phthalate and dioctyl phthalate.

[0013] Preferably, the nanofiber substrate membrane is prepared by electrospinning technology.

[0014] Preferably, the surface layer is formed with micron-sized polypropylene spherulites.

[0015] Preferably, the thickness of the surface layer is 8–20 μm.

[0016] Preferably, the weight-average molecular weight of isotactic polypropylene is 10,000–20,000 g / mol. For example, it can be 10,000 g / mol, 12,000 g / mol, 15,000 g / mol, 17,000 g / mol, or 20,000 g / mol.

[0017] Preferably, the weight-average molecular weight of atactic polypropylene is 15,000–25,000 g / mol. For example, it can be 15,000 g / mol, 20,000 g / mol, or 25,000 g / mol.

[0018] Preferably, the mass ratio of isotactic polypropylene to atactic polypropylene is 1:(0.2-3.0). For example, it can be 1:0.5, 1:1, 1:1.5, or 1:2.0. More preferably, it is 1:(0.5-2.0).

[0019] Preferably, the nanofiber base film is subjected to a pressing treatment after being formed by electrospinning technology.

[0020] More preferably, the pressure for pressing is 0.1 to 10 MPa. For example, it can be 0.1 MPa, 1 MPa, 3 MPa, 5 MPa, 7 MPa, 9 MPa, or 10 MPa.

[0021] More preferably, the pressing method can be either cold pressing or hot pressing.

[0022] More preferably, the hot-pressing temperature is 140–160°C, and the hot-pressing time is 180–900 seconds. For example, it can be 140°C, 150°C, or 160°C.

[0023] More preferably, the cold pressing temperature is room temperature.

[0024] Preferably, the nanofiber base film is further dried before pressing, and the drying temperature is 50-70°C for 10-15 hours. For example, the temperature can be 50°C, 60°C, or 70°C.

[0025] Preferably, the nanofiber base film formed by electrospinning technology has a fiber diameter of 200–600 nm.

[0026] Preferably, the porosity of the nanofiber membrane is 70-90%. For example, it can be 70%, 80%, 81%, 82%, 84%, 86%, or 90%.

[0027] Preferably, the thickness of the nanofiber membrane is 30–120 μm. For example, it can be 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 81 μm, 86 μm, 87 μm, 90 μm, 91 μm, 100 μm, 103 μm, 110 μm, or 120 μm.

[0028] Preferably, the average pore size of the nanofiber membrane is 0.4–0.9 μm. For example, it can be 0.4 μm, 0.45 μm, 0.5 μm, 0.54 μm, 0.6 μm, 0.66 μm, 0.7 μm, 0.74 μm, 0.78 μm, 0.8 μm, 0.87 μm, or 0.9 μm.

[0029] Preferably, the water contact angle of the nanofiber membrane is 140–160°. For example, it can be 140°, 141.7°, 146.5°, 150°, 152.5°, 153.6°, 154.3°, 156.6°, or 160°.

[0030] Preferably, the tensile strength of the nanofiber membrane is 13–40 MPa. For example, it can be 13 MPa, 19.7 MPa, 20 MPa, 26.6 MPa, 30 MPa, 31.7 MPa, 34 MPa, 36 MPa, or 40 MPa.

[0031] This invention also provides a method for preparing a nanofiber membrane, comprising the following steps:

[0032] The nanofiber base film is cast on one side using a casting liquid; the casting liquid is formed by mixing and dissolving the raw materials on the surface.

[0033] After standing for a while, the surface of the membrane turns white to obtain a nanofiber membrane.

[0034] Preferably, the temperature of the casting liquid during casting is 60℃ to 120℃. For example, it can be 60℃, 70℃, 80℃, 90℃, 100℃, 110℃, or 120℃. Polypropylene begins to crystallize at 60℃ to 75℃. If the temperature of the casting liquid is too low, some crystals will exist in the casting liquid, affecting the surface morphology of the prepared nanofiber membrane. Preferably, the temperature of the casting liquid during casting is 80℃ to 120℃.

[0035] Preferably, during casting, the nanofiber base film remains in a flat and stretched state.

[0036] The selection of the site involves stretching during casting to keep the nanofiber base film flat and stretched, thus preventing deformation due to heat.

[0037] Preferably, the ambient temperature is 20–25°C when the plant is left to stand.

[0038] Preferably, after standing, the membrane is dried at a temperature of 20–35°C.

[0039] The present invention also provides the use of a nanofiber membrane as a membrane for membrane distillation.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] (1) The nanofiber membrane has excellent hydrophobic properties, a very high water contact angle, reaches a superhydrophobic state, has good anti-wetting effect, and a porosity of 70-90%.

[0042] (2) The nanofiber membrane has high air flux, high tensile strength and long-term operational stability;

[0043] (3) By combining electrospinning and mold casting, a surface layer with uniform polypropylene crystal distribution is obtained on the surface of the nanofiber base film, ensuring that the obtained nanofiber film has uniform superhydrophobic properties.

[0044] (4) When the nanofiber membrane is used for membrane distillation, it can filter salt solutions with a maximum concentration of 5 wt%.

[0045] (5) Combining electrospinning, mold casting and natural infiltration methods not only enables the polypropylene distributed on the surface of the nanofiber base membrane to form complete spherical crystals with better crystallinity and higher porosity, but also allows some polypropylene to penetrate into the interior of the nanofiber base membrane, thereby improving the tensile strength of the nanofiber membrane. Attached Figure Description

[0046] Figure 1 The image shown is a scanning electron microscope image of the surface layer of the nanofiber membrane prepared in Example 1 of this invention.

[0047] Figure 2 The image shown is a scanning electron microscope image of a cross-section of the nanofiber membrane prepared in Example 1 of this invention.

[0048] Figure 3 The image shown is a scanning electron microscope image of the surface layer of the nanofiber membrane prepared in Example 2 of this invention.

[0049] Figure 4 The image shown is a scanning electron microscope image of the surface layer of the nanofiber membrane prepared in Example 3 of this invention.

[0050] Figure 5 The image shown is a scanning electron microscope image of the surface layer of the nanofiber membrane prepared in Comparative Example 1 of this invention.

[0051] Figure 6 The diagram shows the desalination test results of the nanofiber membrane prepared in Example 1 of this invention.

[0052] Figure 7 This is one of the schematic diagrams showing the results of an antifouling test on the nanofiber membrane prepared in Example 2 of the present invention.

[0053] Figure 8 The second schematic diagram shows the results of the antifouling test of the nanofiber membrane prepared in Example 2 of the present invention.

[0054] Figure 9 The diagram shows the pore size distribution of the nanofiber membrane prepared in Example 1 of this invention.

[0055] Figure 10 The diagram shows the pore size distribution of the nanofiber membrane prepared in Example 2 of this invention.

[0056] Figure 11 The diagram shown is a schematic diagram of the pore size distribution of the nanofiber membrane prepared in Example 3 of the present invention.

[0057] Figure 12 The diagram shown is a schematic diagram of the pore size distribution of the nanofiber membrane prepared in Example 4 of the present invention. Detailed Implementation

[0058] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0059] It should be noted that the process equipment or apparatus not specifically mentioned in the following embodiments are all conventional equipment or apparatus in the art.

[0060] Furthermore, it should be understood that the one or more method steps mentioned in this invention do not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated; moreover, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or limiting the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered as within the scope of the invention.

[0061] The nanofiber membranes prepared in the following examples were subjected to performance parameter tests, including thickness, tensile strength, water contact angle, porosity, average pore size, pore size distribution, and osmotic pressure. The test methods are as follows:

[0062] Thickness: Measured using a film thickness gauge.

[0063] Tensile strength: The test was conducted using an AGS-500ND (Shimadzu Corporation, Japan) universal testing machine. The technical parameters for the test were: load capacity of 50N and test speed of 10mm / min.

[0064] Water contact angle: Measured using an OCA40Micro contact angle meter, pendant drop method, at a temperature of 25℃.

[0065] Porosity: determined by gravimetric method, weighing method (immersion method), with isopropanol as the immersion reagent.

[0066] Average pore size and pore size distribution: Gas-liquid method was used with a PSDA-30M microfiltration membrane pore size analyzer. GQ-16 was used as the wetting agent, and the drilling pressure was 220 kPa.

[0067] Osmotic pressure: The osmotic pressure of the prepared nanofiber membrane was tested. The test method was as follows: First, the membrane was installed in an area of ​​1 cm². 2 In the test chamber, a container filled with deionized water is connected to the top of the measuring tank, and the container is connected to a nitrogen cylinder. Pressure is gradually applied to the container filled with deionized water at a pressure of 0.1 bar, and the pressure is maintained for 10 minutes at each pressure value. When the first drop of liquid seeps out from the bottom of the measuring tank, the pressure displayed on the pressure gauge at this time is the water osmotic pressure value of the membrane.

[0068] Example 1

[0069] This embodiment provides a method for preparing a nanofiber membrane, the specific steps of which are as follows:

[0070] (1) Preparation of spinning solution: Polyvinylidene fluoride powder with a weight average molecular weight of 500,000 g / mol was dissolved in a mixed solvent of acetone and N,N-dimethylformamide (DMF) (mass ratio 1:4) to prepare a polyvinylidene fluoride solution with a mass fraction of 15 wt%. The solution was magnetically stirred at 60 °C for 48 h to obtain a uniform and transparent spinning solution.

[0071] (2) Preparation of nanofiber-based membrane: 5 mL of spinning solution was drawn using a 5 mL syringe equipped with a 0.37 mm inner diameter needle. The syringe was fixed in a micro-pump, with a pumping speed of 8.3 μL / min, a distance of 13.5 cm between the needle and the receiving roller, a high-voltage power supply of 34 kV, and a roller rotation speed of 120 r / min. The ambient temperature and humidity during the spinning process were maintained at 30 ± 5℃ and 30 ± 5°, respectively. After spinning, the membrane was dried in a 60℃ oven for 12 h to remove residual solvent. The dried membrane was cut to a suitable size and hot-pressed at 0.1 MPa and 150℃ for 10 minutes to improve the tensile strength and dimensional stability of the membrane, thus obtaining the nanofiber-based membrane.

[0072] (3) Stretching of nanofiber base film: Fix the nanofiber base film with a thickness of 80μm after hot pressing to make the base film flat.

[0073] (4) Preparation of casting liquid: Isotactic polypropylene with Mw of 12000 and atactic polypropylene with Mw of 20000 were added to xylene at a mass ratio of 1:2 to prepare a solution with a mass fraction of 0.8wt%. The solution was magnetically stirred at 120℃ for 0.5h to obtain a clear and transparent solution.

[0074] (5) Casting: The casting liquid at 120°C is cooled to 100°C at a constant cooling rate for casting. During the casting process, the nanofiber base film is kept flat and stretched to avoid deformation due to heat. The casting volume is 5mL.

[0075] (6) Allow the membrane to stand until it turns pure white, then remove it and place it in an oven to dry for 0.5 hours to completely remove the solvent, thus obtaining the nanofiber membrane. The ambient temperature during standing is 24℃.

[0076] The prepared nanofiber membrane was tested for performance parameters, and the following results were obtained: thickness of 60 μm, average pore size of 0.45 μm, tensile strength of 36 MPa, water contact angle of 156.6°, porosity of 80%, and osmotic pressure of 2.15 bar.

[0077] The prepared nanofiber membrane was applied to direct contact membrane distillation for desalination testing. A filtration test was conducted on a 3.5 wt% NaCl aqueous solution. The temperature of the deionized water on the cold side was 20°C, and the temperature of the NaCl aqueous solution on the hot side was 60°C. Fluids circulated on both sides at a flow rate of 0.6 μL / min. The test results showed that the conductivity on the cold side remained unchanged over 24 hours, and the water vapor flux of the nanofiber membrane formed in Example 1 was 53 kg·m³. 2 During the 50-hour operation period, the desalination rate and water vapor flux of the nanofiber membrane did not decrease significantly.

[0078] The scanning electron microscope image of the surface layer of the nanofiber membrane prepared in Example 1 is shown below. Figure 1 As shown in the figure, several spherulite-like structures are formed on the surface of the nanofiber membrane. Individual spherulites exhibit a flower-like spherical shape.

[0079] Scanning electron microscope image of the cross-section of the nanofiber membrane prepared in Example 1 is shown below. Figure 2 As shown in the figure, this spherulite morphology structure is interconnected and exists not only on the surface of the nanofiber membrane but also throughout the entire surface layer; furthermore, it can be seen that at the interface between the base membrane and the surface layer, the fibers of the base membrane are mixed with the spherulite morphology structure of the surface layer.

[0080] A schematic diagram of the pore size distribution of the nanofiber membrane prepared in Example 1 is shown below. Figure 9 As shown.

[0081] Example 2

[0082] This embodiment provides a method for preparing and applying a nanofiber membrane.

[0083] The preparation method differs from that in Example 1 in that, in step (5), the polypropylene hot solution is cooled to 80°C at a constant cooling rate for casting. In step (2), the pressing method is cold pressing, which is performed at 3 MPa for 2 minutes.

[0084] The nanofiber membrane prepared in this embodiment has a porosity of 81%, an average pore size of 0.54 μm, an osmotic pressure of 2.10 Bar, a thickness of 81 μm, a tensile strength of 34 MPa, and a water contact angle of 153.6°.

[0085] The application differs from Example 1 in that the nanofiber membrane prepared in Example 2 was used for direct contact membrane distillation. During the desalination test, the water vapor flux of the nanofiber membrane was 49 kg·m³ within 20 hours. 2 ·h.

[0086] The scanning electron microscope image of the surface layer of the nanofiber membrane prepared in Example 2 is shown below. Figure 3 As shown.

[0087] A schematic diagram of the pore size distribution of the nanofiber membrane prepared in Example 2 is shown below. Figure 10 As shown.

[0088] Example 3

[0089] This embodiment provides a method for preparing and applying a nanofiber membrane.

[0090] The difference between the preparation method and Example 1 is that in step (5), the polypropylene hot solution is cooled to 60°C at a constant cooling rate for casting.

[0091] The nanofiber membrane prepared in this embodiment has a porosity of 83%, an average pore size of 0.66 μm, an osmotic pressure of 1.80 Bar, a thickness of 87 μm, a tensile strength of 31.7 MPa, and a water contact angle of 146.5°.

[0092] The difference between this application and Example 1 is that when the nanofiber membrane prepared in Example 3 was used for desalination testing in direct contact membrane distillation, the water vapor flux of the nanofiber membrane was 41.6 kg·m³ within 6 hours. 2 ·h.

[0093] The scanning electron microscope image of the surface layer of the nanofiber membrane prepared in Example 3 is shown below. Figure 4As shown.

[0094] Depend on Figure 4 and Figure 1 In comparison, it can be seen that when the casting temperature is 60℃, the crystals formed on the surface of the nanofiber membrane are not regular spherical morphology structures. This indicates that if the casting temperature is too low, it will affect the surface morphology of the nanofiber membrane.

[0095] A schematic diagram of the pore size distribution of the nanofiber membrane prepared in Example 3 is shown below. Figure 11 As shown.

[0096] Example 4

[0097] This embodiment provides a method for preparing and applying a nanofiber membrane.

[0098] The preparation method differs from that in Example 1 in that, in step (1), a molecular weight M is used. w The polyacrylonitrile is 120,000. In step (2), the pushing speed is set to 16 μL / min. The distance between the needle and the receiving roller is 15 cm. The high voltage power supply voltage is set to 20 kV. After drying, it is cold-pressed for 2 minutes under 1 MPa conditions. In step (5), the polypropylene hot solution is cooled to 80°C at a constant cooling rate for casting.

[0099] The nanofiber membrane prepared in this embodiment has a porosity of 82%, an average pore size of 0.87 μm, an osmotic pressure of 1.15 bar, a thickness of 103 μm, a tensile strength of 13 MPa, and a water contact angle of 141.7°.

[0100] The difference between this application and Example 1 is that when the nanofiber membrane prepared in Example 4 was used for desalination testing in direct contact membrane distillation, the conductivity of the permeate phase remained below 3 μS / cm for 5 hours, and the water vapor flux of the nanofiber membrane was 28 kg·m³. 2 ·h.

[0101] A schematic diagram of the pore size distribution of the nanofiber membrane prepared in Example 4 is shown below. Figure 12 As shown.

[0102] Example 5

[0103] This embodiment provides a method for preparing and applying a nanofiber membrane.

[0104] The difference between the preparation method and Example 2 is that in step (4), isotactic polypropylene and atactic polypropylene are in a mass ratio of 1:1.

[0105] The nanofiber membrane prepared in this embodiment has a porosity of 84%, an average pore size of 0.74 μm, an osmotic pressure of 1.65 bar, a thickness of 91 μm, a tensile strength of 26.6 MPa, and a water contact angle of 154.3°.

[0106] The difference between this application and Example 2 is that when the nanofiber membrane prepared in Example 5 was used for desalination testing in direct contact membrane distillation, the water vapor flux of the nanofiber membrane was 42.8 kg·m³ within 6 hours. 2 ·h.

[0107] Example 6

[0108] This embodiment provides a method for preparing and applying a nanofiber membrane.

[0109] The difference between the preparation method and Example 2 is that in step (4), isotactic polypropylene (iPP) and atactic polypropylene (aPP) are in a mass ratio of 1:0.5.

[0110] The nanofiber membrane prepared in this embodiment has a porosity of 86%, an average pore size of 0.78 μm, an osmotic pressure of 1.35 bar, a thickness of 86 μm, a tensile strength of 19.7 MPa, and a water contact angle of 152.5°.

[0111] The difference between this application and Example 2 is that when the nanofiber membrane prepared in Example 6 was used for desalination testing in direct contact membrane distillation, the water vapor flux of the nanofiber membrane was 38.4 kg·m³ within 12 hours. 2 ·h.

[0112] Comparative Example 1

[0113] Comparative Example 1 is a comparative example of Example 1. The difference in preparation method is that in step (5), the polypropylene hot solution is cooled to 40°C at a constant cooling rate for casting.

[0114] The prepared nanofiber membrane has a tensile strength of 18.3 MPa and a water contact angle of 142.3°.

[0115] The difference between this application and Example 1 is that when the nanofiber membrane prepared in Comparative Example 1 was used for desalination testing in direct contact membrane distillation, the water vapor flux of the nanofiber membrane was 27.2 kg·m³ within 3 hours. 2 ·h.

[0116] The scanning electron microscope image of the surface layer of the nanofiber membrane prepared in Comparative Example 1 is shown below. Figure 5 As shown.

[0117] Depend on Figure 5 and Figure 1 In comparison, it can be seen that when the casting temperature is 40℃, the crystals formed on the surface of the nanofiber membrane are no longer spherulitic structures. This indicates that if the casting temperature is too low, it will affect the surface morphology of the nanofiber membrane.

[0118] Comparative Example 2

[0119] Comparative Example 2 is a comparative example of Example 2, except that in step (4), isotactic polypropylene is used alone, and atactic polypropylene is not used.

[0120] The prepared nanofiber membrane has a tensile strength of 16.7 MPa and a water contact angle of 157.2°.

[0121] When the nanofiber membrane prepared in Comparative Example 2 was used for desalination testing via direct contact membrane distillation, the water vapor flux of the nanofiber membrane was 35.7 kg·m³. 2 •h, after 1 hour of testing, the conductivity of the cold-side deionized water began to increase.

[0122] Comparing Comparative Example 2 with Example 2, it can be seen that the conductivity of the cold-side deionized water of the nanofiber membrane prepared by isotactic polypropylene alone began to increase after 1 hour of desalination testing, and the rejection rate of NaCl by the nanofiber membrane decreased. This indicates that the hydrophobicity of the nanofiber membrane prepared by isotactic polypropylene alone is lower than that of the nanofiber membrane prepared by atactic polypropylene and isotactic polypropylene.

[0123] Comparative Example 3

[0124] Comparative Example 3 is a comparative example of Example 2, except that in step (4), atactic polypropylene is used alone, and isotactic polypropylene is not used.

[0125] The nanofiber membrane prepared in Comparative Example 3 was used for desalination test by direct contact membrane distillation. At the beginning of the desalination test, the conductivity of the cold-side deionized water began to increase.

[0126] Comparing Comparative Example 3 with Example 2, it can be seen that when the nanofiber membrane prepared by random polypropylene alone is used for desalination test, the conductivity of the cold-side deionized water immediately begins to rise. The nanofiber membrane has a low NaCl rejection rate, indicating that the hydrophobicity of the nanofiber membrane prepared by random polypropylene alone is much lower than that of the nanofiber membrane prepared by random polypropylene and isotactic polypropylene.

[0127] Comparative Example 4

[0128] Comparative Example 4 is a comparative example of Example 2, except that in step (6), the vacuum filtration method is used instead of the natural permeation method.

[0129] The prepared nanofiber membrane has a tensile strength of 27.4 MPa and a water contact angle of 153.8°.

[0130] When the nanofiber membrane prepared in Comparative Example 4 was used for desalination testing via direct contact membrane distillation, the water vapor flux of the nanofiber membrane was 37.6 kg·m³. 2 ·h.

[0131] Salt resistance test of nanofiber membranes:

[0132] The nanofiber membrane prepared in Example 1 was used for desalination tests in direct contact membrane distillation. Filtration tests were conducted on NaCl aqueous solutions with concentrations of 3.5 wt%, 5 wt%, 6.5 wt%, and 8 wt%. The temperature of the deionized water on the cold side was 20 °C, the temperature of the NaCl aqueous solution on the hot side was 60 °C, and the fluid flow rate on both sides was 0.6 μL / min.

[0133] The water vapor flux and rejection rate of the nanofiber membrane were tested, and the test results are shown in [the table below]. Figure 6 .

[0134] Depend on Figure 6 It can be seen that when the concentration of NaCl aqueous solution is 3.5 wt% and 5 wt%, the nanofiber membrane has a 100% rejection rate, completely filtering NaCl, and the gas flux is greater than 40 kg·m³. 2 However, when the concentration of the NaCl aqueous solution was 6.5 wt% and 8 wt%, the retention rate of the nanofiber membrane could not reach 100%, and NaCl could not be completely filtered out, and the gas flux was less than 30 kg·m⁻¹. 2 Therefore, when the nanofiber membrane prepared in Example 1 is used for desalination testing by direct contact membrane distillation, it can filter NaCl aqueous solutions with a maximum concentration of 5 wt%.

[0135] Antifouling test of nanofiber membrane: The performance of the membrane was tested using methylene blue (MB) and sunset yellow (SY), respectively.

[0136] (1) Methylene blue (MB) was used.

[0137] The nanofiber membrane prepared in Example 2 was used for direct contact membrane distillation testing. The hot-side solution was a mixture of 2 wt% NaCl and 100 ppm methylene blue (MB). All other test conditions were the same as in Example 2. The test results are shown in Table 7. Within 12 hours, the gas flux of the nanofiber membrane reached 47.66 kg / m³. 2 The conductivity remained stable, and the color of the cold-tested deionized water remained unchanged. The mass of methylene blue (MB) in the cold-side deionized water was quantitatively determined using a UV spectrophotometer (Purchip TU-1950) and the standard curve method. The results showed that the nanofiber membrane achieved a MB rejection rate of 99.86%. Furthermore, compared to the nanofiber membrane used in the desalination test in Example 2, the membrane's gas flux did not decrease significantly.

[0138] (2) Use Sunset Yellow (SY)

[0139] The nanofiber membrane prepared in Example 2 was used for direct contact membrane distillation testing. The hot-side solution was a mixed solution of 2 wt% NaCl and 100 ppm Sunset Yellow (SY). All other test conditions were the same as in Example 2. The test results are shown in Table 8. Within 12 hours, the gas flux of the nanofiber membrane reached 49.33 kg / m³. 2 The gas flux remained stable, the conductivity remained stable, and the color of the cold-tested deionized water remained unchanged. The mass of Sunset Yellow (SY) in the cold-side deionized water was quantitatively determined using a UV spectrophotometer (Purchip TU-1950) and the standard curve method. The results showed that the nanofiber membrane could completely retain SY. Furthermore, compared with the nanofiber membrane used in the desalination test in Example 2, the membrane's gas flux did not decrease significantly.

[0140] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A nanofiber membrane, characterized by, The nanofiber membrane includes a nanofiber base membrane and a surface layer disposed on the surface of the nanofiber base membrane; the raw material of the surface layer includes isotactic polypropylene and atactic polypropylene. The surface layer of the nanofiber membrane includes a second solvent; based on the total mass of the second solvent, isotactic polypropylene, and atactic polypropylene, the amount of isotactic polypropylene and atactic polypropylene is 0.5~1wt%. The mass ratio of the isotactic polypropylene to the atactic polypropylene is 1:(1.0~2.0). The weight-average molecular weight of isotactic polypropylene is 10,000 to 20,000. The weight-average molecular weight of atactic polypropylene is 15,000 to 25,000. The surface layer is formed with micron-sized polypropylene spherulites; The thickness of the surface layer is 8~20μm; The preparation method of the nanofiber membrane includes the following steps: The nanofiber base membrane is cast on one side using a casting liquid; the casting liquid is formed by mixing and dissolving the raw materials on the surface; after standing, the surface color of the membrane formed by the casting liquid turns white, and the nanofiber membrane is obtained; the temperature of the casting liquid during casting is 80℃~120℃; the ambient temperature during standing is 20~25℃. During casting, the nanofiber base film remains in a flat and stretched state; After standing, the membrane needs to be dried at a temperature of 20–35°C.

2. The nanofiber membrane of claim 1, wherein, The raw materials for the nanofiber-based membrane include polymers. And a first solvent; based on the total mass of the first solvent and the polymer, the amount of the polymer is 5~25 wt%; And / or, the nanofiber substrate film is formed by electrospinning technology.

3. The nanofiber membrane of claim 2, wherein, The polymer is selected from one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene polymer, polymethyl methacrylate, polyvinyl chloride, polyurethane, polycarbonate, polystyrene, polyacrylonitrile, and their modifiers; And / or, the first solvent is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, water, ethanol, isopropanol, n-butanol, acetone, 1,4-dioxane, dichloromethane, chloroform, tetrahydrofuran, and acetic acid; And / or, the second solvent is selected from one or more of xylene, decahydronaphthalene, dibutyl phthalate and dioctyl phthalate; And / or, the nanofiber-based membrane is further subjected to a pressing treatment after being formed by electrospinning technology; And / or, the fiber diameter of the nanofiber-based membrane formed by electrospinning technology is 200~600nm.

4. The nanofiber membrane of claim 3, wherein, The pressure for pressing is 0.1~10 MPa.

5. The nanofiber membrane of claim 1, wherein, The porosity of the nanofiber membrane is 70-90%; and / or the thickness of the nanofiber membrane is 30-120 μm. And / or, the average pore size of the nanofiber membrane is 0.4~0.9 μm; And / or, the water contact angle of the nanofiber membrane is 140~160°; And / or, the tensile strength of the nanofiber membrane is 13~40 MPa.

6. A method for preparing a nanofiber membrane as described in any one of claims 1 to 5, comprising the following steps: The nanofiber base film is cast on one side using a casting liquid; the casting liquid is formed by mixing and dissolving the raw materials on the surface. After the surface color of the film formed by the casting liquid becomes white, the nanofiber film is obtained; The temperature of the casting liquid is 80-120°C during casting; The ambient temperature is 20-25°C during standing; The nanofiber base film is kept in a flat and stretched state during casting; After standing, the film is further dried at a temperature of 20-35°C.

7. The preparation method according to claim 6, characterized in that, The nanofiber base film is kept in a flat and stretched state during casting by means of stretching, avoiding deformation caused by heat.

8. Use of the nanofiber film according to any one of claims 1-5 as a membrane for membrane distillation.

Images