A method for the preparation of porous symmetric polymer membranes by delayed phase inversion and its application in membrane distillation

Abstract

The application discloses a method for preparing a porous symmetrical polymer membrane by a delayed phase inversion method and membrane distillation application of the porous symmetrical polymer membrane. The method is characterized in that a non-woven fabric is covered on a membrane surface to establish a barrier layer to slow down the exchange rate of a solvent and a non-solvent, instead of changing the components of the solvent / non-solvent or adding a pore-forming agent, so that the polymer membrane has a symmetrical structure with an overall sponge-like hole. The prepared porous symmetrical polymer membrane has high surface porosity and overall hole permeability, and can greatly improve the permeation performance of the membrane.

Description

Technical Field

[0001] This invention relates to a method for preparing porous symmetrical polymer membranes by delayed phase inversion and its application in membrane distillation, belonging to the field of membrane material preparation technology. Background Technology

[0002] Membrane distillation, as a novel membrane separation technology, shows promising potential in the treatment of high-concentration brine. Polymer membranes have attracted widespread attention due to their excellent performance and are widely used in membrane-based water treatment, making them a promising material for both industry and academic research. Polymer membranes are chosen as the "chip" in the membrane distillation process, utilizing their hydrophobic surfaces to resist brine wetting and fouling, and their micro-nano pore structures as channels for water vapor transport. However, membranes prepared by traditional phase inversion processes suffer from low mechanical strength and dense surface, significantly limiting their effectiveness in membrane distillation applications. On the one hand, mechanical strength is crucial for membrane performance; membranes with insufficient mechanical strength will deform due to wrinkling and are unsuitable for membrane distillation systems, especially in vacuum membrane distillation. On the other hand, the rapid exchange between solvent and non-solvent during NIPS (Neuro-Induced Polymerization) significantly impacts the permeability of polymer membranes, severely reducing the separation efficiency of seawater desalination via membrane distillation.

[0003] Phase inversion methods can be used to prepare membranes of various forms and have wide industrial applications. Typically, flat sheet membranes are formed by coating a porous mechanical support with a polymer solution. The polymer solution consists of at least one polymer, at least one suitable solvent, and may contain additives. The membrane and support are immersed together in a coagulation bath composed of a non-solvent. The polymer membrane solidifies through the exchange of solvent and non-solvent. Much research focuses on modifying the composition of the solvent or coagulation bath to adjust the membrane structure. Other approaches include adjusting the casting solution, including adding non-solvent small molecules, inorganic materials, macromolecules, and combinations thereof.

[0004] Selvaraj Munirasu et al. [Desalination. 417(2015): 77-86.] prepared intrinsically superhydrophobic PVDF membranes using methanol as the non-solvent via a phase inversion method. This method investigated the compatibility properties of the solvent and non-solvent, significantly improving the hydrophobic properties of the membrane. Zou et al. [J Membr Sci. 655(2022): 120577] prepared porous PVDF membranes using a simple co-casting method. During the coating process, ATBC or PEG was used as a separating layer to avoid rapid exchange between the solvent and non-solvent, thereby obtaining a highly porous surface. Tian et al. [J Membr Sci. 655(2021): 119299] co-cast polyethersulfone (PES) membranes onto PVDF membranes, reducing the solvent / non-solvent exchange rate of the lower membrane during phase inversion, causing delayed liquid-liquid stratification and crystallization of the polymer solution, thereby controlling the membrane surface morphology. However, the above methods are all quite cumbersome. Therefore, it is essential to establish a separator as an efficient and simple method to slow down the exchange rate, control the pore size, and increase the surface porosity to enhance the performance of membrane distillation. Summary of the Invention

[0005] The present invention aims to provide a method for preparing porous symmetrical polymer membranes using a delayed phase inversion method and its application in membrane distillation. This method achieves a symmetrical polymer membrane with an overall sponge-like pore structure by covering the membrane surface with nonwoven fabric to create a separator that slows down the exchange rate between solvent and non-solvent, rather than changing the solvent / non-solvent composition or adding pore-forming agents. The prepared porous symmetrical polymer membrane exhibits high surface porosity and overall pore permeability, significantly improving the membrane's permeability.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing porous symmetrical polymer membranes using a delayed phase inversion method involves dissolving a polymer to obtain a casting solution; after coating the membrane, covering the membrane surface with nonwoven fabric, and then immersing it in a water coagulation bath; after completion, the membrane is cleaned, protected, and dried to obtain the polymer membrane.

[0008] Preferably, the casting solution is prepared by adding the polymer to a solvent, stirring at 25-160°C for 1-10 hours, and then allowing it to stand to remove bubbles.

[0009] Preferably, the nonwoven fabric is made of any one of PET, PA, or PP, with a pore size of 1-10μm, a porosity of 30-60%, and a thickness of 80-200μm.

[0010] Preferably, the polymer is any one of PVDF, PES, and PSF, and the solid content of the polymer is 18-34 wt%.

[0011] Preferably, the solvent is any one of DMAC, DMF, NMP, and Polarclean.

[0012] Preferably, during the static degassing process, the temperature is 25-160℃, and the standing time is 1-24 hours.

[0013] Preferably, in the film coating process, the coating speed is 10-50 mm / s and the coating thickness is 100-500 μm;

[0014] The medium of the water coagulation bath is water, the temperature is 20-80℃, and the soaking time is 3-24h.

[0015] Preferably, the cleaning, protection, and drying processes are as follows: soaking in ethanol at room temperature for 12-72 hours; soaking in n-hexane at room temperature for 6-24 hours; and air drying at room temperature for 1-24 hours.

[0016] A polymer membrane, prepared by any of the methods described above; the polymer membrane has a symmetrical structure with integral sponge-like pores, and the membrane thickness is 50-300 μm, the average pore size is 30-1000 nm, and the membrane distillation flux is 5-50 kg·m³. -2 ·h -1 ·bar -1 .

[0017] The above-mentioned polymer membranes are used in membrane distillation.

[0018] The beneficial effects of this invention are as follows:

[0019] This invention utilizes a simple physical method involving nonwoven fabric covering to prepare porous symmetrical polymer membranes in a one-step process based on a solvent-induced phase inversion. This method eliminates the need for any pore-forming agents or alterations to the coagulation bath composition, overcoming the influence of the solvent-nonsolvent interaction coefficient on the overall membrane structure in traditional phase inversion processes. It offers advantages such as simple coagulation bath composition and convenient method. The prepared porous symmetrical polymer membrane exhibits high surface porosity and overall pore permeability, significantly improving membrane permeability. Attached Figure Description

[0020] Figure 1 A flowchart illustrating the preparation of porous polymer membranes using the delayed phase inversion method;

[0021] Figure 2 The images show the PET nonwoven fabric (c) used in Example 1's delayed phase inversion method, and its upper surface structure (a) and bottom surface structure (b).

[0022] Figure 3 This is a diagram of the upper surface structure of the porous polymer membrane prepared in Example 1;

[0023] Figure 4 This is a diagram of the upper surface structure of the porous polymer membrane prepared in Example 8;

[0024] Figure 5 The images show the upper surface structure (a), bottom surface structure (b), cross-sectional structure (c), and pore size distribution (d) of the porous polymer membrane prepared in the comparative case of Example 10.

[0025] Figure 6 The images show the upper surface structure (a) and (b), cross-sectional structure (c), and pore size distribution (d) of the porous polymer membrane prepared in Example 2.

[0026] Figure 7 The graph shows the membrane distillation performance test results of the porous symmetrical polymer membrane prepared in Example 1. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0028] This invention provides a one-step method for preparing symmetrical polymer membranes based on nonsolvent-induced phase separation (NIPS). Specifically, a certain amount of polymer is dissolved in a solvent by heating and stirring to obtain a uniform casting solution; subsequently, the casting solution is allowed to stand for degassing; after casting on a glass plate, a nonwoven fabric is covered on the surface of the nascent membrane, and the membrane is immersed in a water coagulation bath to achieve the phase transformation process. Figure 1 Finally, the membrane is cleaned, protected, and dried. The preparation method is based on a solvent-inducible phase inversion process, using polyvinylidene fluoride (PVDF), polyethersulfone (PES), or polysulfone (PSF) as raw materials, and dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), or methyl 5-dimethylamino-2-methyl-5-oxovalerate (Polarclean) as solvents, while water is used as the non-solvent. This method uses a simple physical approach, rather than changing the solvent / non-solvent composition or adding pore-forming agents, to achieve a symmetrical, sponge-like pore structure by covering the membrane surface with nonwoven fabric, creating a barrier to slow down the exchange rate between the solvent and non-solvent. The prepared symmetrical PVDF membrane has high surface porosity and overall pore permeability, significantly improving the membrane's permeability.

[0029] Example 1

[0030] First, PVDF with a solid content of 24wt% was completely dissolved in NMP under mechanical stirring at 50℃ for 6 hours. Then, it was allowed to stand at 25℃ for 12 hours to remove bubbles, obtaining a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 30 mm / s, creating a uniform film layer of 350 μm thickness. Next, a PET nonwoven fabric with pore size, porosity, and thickness of 6 μm, 40%, and 100 μm (structure shown in...) was used. Figure 2 A glass plate was applied to the surface of the nascent membrane, and then the glass plate and the nascent membrane were immersed in a 20°C water coagulation bath for 24 hours according to a traditional phase inversion process to completely release the solvent. Finally, the membrane was immersed in ethanol for 12 hours at room temperature, followed by immersion in n-hexane for 24 hours at room temperature to prevent pore shrinkage, and then air-dried for 12 hours at room temperature. The upper surface structure diagram of the prepared porous symmetrical polymer membrane is shown below. Figure 3 As shown.

[0031] The prepared porous symmetrical PVDF membrane with a thickness of 128 μm and an average pore size of 390 nm was used to treat a 3.5 wt% sodium chloride solution. The feed temperature was 70 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum degree of -0.98 MPa. The membrane distillation performance test results are shown in the figure below. Figure 7 As shown, the results indicate a sodium chloride rejection rate higher than 99.99%, and a membrane distillation flux of 36 kg·m³. -2 ·h -1 ·bar -1 .

[0032] Example 2

[0033] First, PVDF with a solid content of 34 wt% was completely dissolved in NMP under mechanical stirring at 70℃ for 3 hours. Then, it was allowed to stand at 70℃ for 24 hours to remove bubbles, obtaining a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 20 mm / s, creating a uniform film layer of 300 μm thickness. Next, a PET nonwoven fabric with pore size, porosity, and thickness of 5 μm, 40%, and 100 μm was placed on the surface of the nascent membrane. Then, following a traditional phase inversion process, the glass plate and the nascent membrane were immersed in a 50℃ water coagulation bath for 12 hours to completely release the solvent. Finally, the membrane was immersed in ethanol at room temperature for 24 hours, then in n-hexane at room temperature for 12 hours to prevent pore shrinkage, and finally air-dried at room temperature for 12 hours. The upper surface of the prepared porous symmetrical polymer membrane is shown below. Figure 6 As shown in region ab, the cross-sectional structure diagram and aperture distribution diagram are as follows: Figure 6 The cd region is shown.

[0034] A porous symmetrical PVDF membrane with a thickness of 133 μm and an average pore size of 310 nm was used to treat a 3.5 wt% sodium chloride solution. The feed temperature was 70 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum level of -0.98 MPa. The results showed that the sodium chloride rejection rate was higher than 99.99%, and the membrane distillation flux was 27 kg·m³. -2 ·h -1 ·bar -1 .

[0035] Example 3

[0036] First, PES with a solid content of 22 wt% was completely dissolved in NMP under mechanical stirring at 50℃ for 10 h. Then, it was allowed to stand at 25℃ for 18 h to degas and obtain a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 40 mm / s, creating a uniform film layer of 350 μm thickness. Next, a PA nonwoven fabric with a pore size of 7 μm, a porosity of 45%, and a thickness of 90 μm was placed on the surface of the nascent film. Then, following a traditional phase inversion process, the glass plate and the nascent film were immersed in a 20℃ water coagulation bath for 24 h to completely release the solvent. Finally, the film was immersed in ethanol at room temperature for 36 h, followed by immersion in n-hexane at room temperature for 12 h to prevent pore shrinkage, and then air-dried at room temperature for 6 h.

[0037] A porous symmetrical PVDF membrane with a thickness of 115 μm and an average pore size of 230 nm was used to treat a 3.5 wt% sodium chloride solution. The feed temperature was 70 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum level of -0.98 MPa. The results showed that the sodium chloride rejection rate was higher than 99.99%, and the membrane distillation flux was 33 kg·m³. -2 ·h -1 ·bar -1 .

[0038] Example 4

[0039] First, PES with a solid content of 18 wt% was completely dissolved in DMF under mechanical stirring at 80℃ for 1 hour. Then, it was allowed to stand at 25℃ for 1 hour to remove bubbles, obtaining a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 50 mm / s, creating a film layer with a uniform thickness of 500 μm. Next, a piece of PA nonwoven fabric with a pore size of 2 μm, a porosity of 36%, and a thickness of 150 μm was placed on the surface of the nascent film. Then, following a traditional phase inversion process, the glass plate and the nascent film were immersed in a 20℃ water coagulation bath for 3 hours to completely release the solvent. Finally, the film was immersed in ethanol at room temperature for 72 hours, followed by immersion in n-hexane at room temperature for 24 hours to prevent pore shrinkage, and then air-dried at room temperature for 24 hours.

[0040] A porous symmetrical PVDF membrane with a thickness of 100 μm and an average pore size of 210 nm was used to treat a 10 wt% sodium chloride solution. The feed temperature was 70 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum level of -0.98 MPa. The results showed that the sodium chloride rejection rate was higher than 99.99%, and the membrane distillation flux was 15 kg·m³. -2 ·h -1 ·bar -1 .

[0041] Example 5

[0042] First, PSF with a solid content of 25 wt% was completely dissolved in DMF using mechanical stirring at 160℃ for 3 hours. Then, it was allowed to stand at 140℃ for 3 hours to remove bubbles, obtaining a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 30 mm / s, creating a uniform film layer of 500 μm thickness. Next, a piece of PP nonwoven fabric with a pore size of 1 μm, a porosity of 30%, and a thickness of 200 μm was placed on the surface of the nascent film. Then, following a traditional phase inversion process, the glass plate and the nascent film were immersed in a 20℃ water coagulation bath for 12 hours to completely release the solvent. Finally, the film was immersed in ethanol at room temperature for 12 hours, followed by immersion in n-hexane at room temperature for 24 hours to prevent pore shrinkage, and then air-dried at room temperature for 12 hours.

[0043] A porous symmetrical PVDF membrane with a thickness of 254 μm and an average pore size of 30 nm was used to treat a 10 wt% sodium chloride solution. The feed temperature was 50 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum level of -0.98 MPa. The results showed that the sodium chloride rejection rate was higher than 99.99%, and the membrane distillation flux was 5 kg·m³. -2 ·h -1 ·bar -1 .

[0044] Example 6

[0045] First, PVDF with a solid content of 25 wt% was completely dissolved in Polarclean under mechanical stirring at 160℃ for 6 hours. Then, it was allowed to stand at 120℃ for 12 hours to remove bubbles, obtaining a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 10 mm / s, creating a uniform film layer of 300 μm thickness. Next, a PET nonwoven fabric with a pore size of 10 μm, a porosity of 60%, and a thickness of 150 μm was placed on the surface of the nascent film. Then, following a traditional phase inversion process, the glass plate and the nascent film were immersed in a 20℃ water coagulation bath for 12 hours to completely release the solvent. Finally, the film was immersed in ethanol at room temperature for 12 hours, followed by immersion in n-hexane at room temperature for 24 hours to prevent pore shrinkage, and then air-dried at room temperature for 12 hours.

[0046] A porous symmetrical PVDF membrane with a thickness of 120 μm and an average pore size of 1000 nm was used to treat a 3.5 wt% sodium chloride solution. The feed temperature was 70 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum level of -0.98 MPa. The results showed that the sodium chloride rejection rate was higher than 99.99%, and the membrane distillation flux was 50 kg·m³. -2 ·h -1 ·bar -1 .

[0047] Example 7

[0048] First, PVDF with a solid content of 30wt% was completely dissolved in DMF under mechanical stirring at 90℃ for 8 hours. Then, the solution was allowed to stand at 90℃ for 6 hours to remove bubbles, resulting in a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 20 mm / s, creating a uniform film layer of 270 μm thickness. Next, a piece of PP nonwoven fabric with a pore size of 8 μm, a porosity of 50%, and a thickness of 120 μm was placed on the surface of the nascent film. Then, following a traditional phase inversion process, the glass plate and the nascent film were immersed in a 20℃ water coagulation bath for 16 hours to completely release the solvent. Finally, the film was immersed in ethanol at room temperature for 24 hours, followed by immersion in n-hexane at room temperature for 18 hours to prevent pore shrinkage, and then air-dried at room temperature for 10 hours.

[0049] A porous symmetrical PVDF membrane with a thickness of 118 μm and an average pore size of 470 nm was used to treat a 3.5 wt% sodium chloride solution. The feed temperature was 60 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum level of -0.98 MPa. The results showed that the sodium chloride rejection rate was higher than 99.99%, and the membrane distillation flux was 29 kg·m³. -2 ·h -1 ·bar -1 .

[0050] Example 8

[0051] First, PVDF with a solid content of 22 wt% was completely dissolved in Polarclean under mechanical stirring at 140℃ for 2 hours. Then, it was allowed to stand at 100℃ for 7 hours to remove bubbles, obtaining a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 30 mm / s, creating a uniform film layer of 400 μm thickness. Next, a PET nonwoven fabric with a pore size of 4 μm, a porosity of 40%, and a thickness of 80 μm was placed on the surface of the nascent film. Then, the glass plate and the nascent film were immersed in a 20℃ water coagulation bath for 12 hours according to a traditional phase inversion process to completely release the solvent. Finally, the film was immersed in ethanol at room temperature for 12 hours, followed by immersion in n-hexane at room temperature for 24 hours to prevent pore shrinkage, and then air-dried at room temperature for 12 hours. The upper surface structure of the prepared porous symmetrical polymer film is shown in the figure below. Figure 4 As shown.

[0052] A porous symmetrical PVDF membrane with a thickness of 128 μm and an average pore size of 390 nm was used to treat a 3.5 wt% sodium chloride solution. The feed temperature was 70 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum level of -0.98 MPa. The results showed that the sodium chloride rejection rate was higher than 99.99%, and the membrane distillation flux was 36 kg·m³. -2 ·h -1 ·bar -1 .

[0053] Example 9

[0054] First, PSF with a solid content of 20 wt% was completely dissolved in DMF under mechanical stirring at 80℃ for 6 hours. Then, it was allowed to stand at 80℃ for 4 hours to remove bubbles, obtaining a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 50 mm / s, creating a uniform film layer of 250 μm thickness. Next, a piece of PA nonwoven fabric with a pore size of 9 μm, a porosity of 50%, and a thickness of 130 μm was placed on the surface of the nascent film. Then, following a traditional phase inversion process, the glass plate and the nascent film were immersed in a 20℃ water coagulation bath for 24 hours to completely release the solvent. Finally, the film was immersed in ethanol at room temperature for 12 hours, followed by immersion in n-hexane at room temperature for 24 hours to prevent pore shrinkage, and then air-dried at room temperature for 12 hours.

[0055] A porous symmetrical PVDF membrane with a thickness of 70 μm and an average pore size of 350 nm was used to treat a 3.5 wt% sodium chloride solution. The feed temperature was 70 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum level of -0.98 MPa. The results showed that the sodium chloride rejection rate was higher than 99.99%, and the membrane distillation flux was 25 kg·m³. -2 ·h -1 ·bar -1 .

[0056] Example 10

[0057] Example 10 serves as a control case for Example 1 without the use of nonwoven fabric covering. First, PVDF with a solid content of 24 wt% was completely dissolved in NMP under mechanical stirring at 50°C for 6 hours. Then, it was allowed to stand at 25°C for 12 hours to degas and obtain a uniform casting solution. The casting solution was dripped onto a glass plate, and a doctor blade was used to cast a flat film at a speed of 30 mm / s, creating a uniform film layer of 350 μm thickness. Following a conventional phase inversion process, the (glass plate and) nascent film were immersed in a 20°C water coagulation bath for 24 hours to completely release the solvent. Finally, the film was immersed in ethanol at room temperature for 12 hours, then in n-hexane at room temperature for 24 hours to prevent pore shrinkage, and finally air-dried at room temperature for 12 hours. The upper surface structure, bottom surface structure, cross-sectional structure, and pore size distribution diagrams of the prepared porous symmetrical polymer film are shown below. Figure 5 As shown in region ad. It can be observed that without the use of nonwoven fabric, i.e., without the effect of delaying phase inversion to slow down the exchange rate between solvent and non-solvent, as... Figure 5 The cross-sectional structure diagram shown in region c reveals an asymmetric structure of the membrane and a dense layer on the upper surface, while the upper surface structure... Figure 5 a region and bottom surface structure Figure 5 Region b demonstrates the degree of surface density, combined with the pore size distribution. Figure 5Region d shows that only small pores of about 140 nm exist, and their number is relatively small. The low surface porosity significantly reduces the permeation performance of this membrane.

[0058] A porous symmetrical PVDF membrane with a thickness of 131 μm and an average pore size of 140 nm was used to treat a 3.5 wt% sodium chloride solution. The feed temperature was 70 °C, the feed flow rate was 200 ml / min, and the operation was vacuum membrane distillation at a vacuum level of -0.98 MPa. The results showed that the sodium chloride rejection rate was higher than 99.99%, and the membrane distillation flux was 10 kg·m³. -2 ·h -1 ·bar -1 .

[0059] Table 1 compares the microfiltration membrane prepared in Example 1 with that in the literature. As can be seen from the table, the microfiltration membrane prepared by this method (with an average pore size of about 390 nm) has better permeability than that in the literature.

[0060] Table 1. Performance comparison of the microfiltration membrane prepared in Example 1 with literature (pore size unit: nm, permeability unit: kg·m³) -2 ·h -1 ·bar -1 Salt rejection rate (unit: %)

[0061]

[0062]

[0063] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing porous symmetrical polymer membranes using a delayed phase inversion method, characterized in that, The polymer is dissolved in a solvent to prepare a casting solution. The solvent used is any one of DMAC, DMF, NMP, or Polarclean. After the film is scraped, a non-woven fabric is covered on the film surface. The non-woven fabric can be made of any one of PET, PA, or PP. Then soak it in a water coagulation bath; After completion, the membrane is cleaned, protected, and dried to obtain a polymer membrane.

2. The method for preparing porous symmetrical polymer membranes by delayed phase inversion according to claim 1, characterized in that, The casting solution is prepared by adding the polymer to the solvent, stirring at 25-160℃ for 1-10 hours, and then allowing it to stand to remove bubbles.

3. The method for preparing porous symmetrical polymer membranes by delayed phase inversion according to claim 2, characterized in that, The nonwoven fabric has a pore size of 1-10 μm, a porosity of 30-60%, and a thickness of 80-200 μm.

4. The method for preparing porous symmetrical polymer membranes by delayed phase inversion according to claim 2, characterized in that, The polymer is any one of PVDF, PES, and PSF, and the solid content of the polymer is 18-34 wt%.

5. The method for preparing porous symmetrical polymer membranes by delayed phase inversion according to claim 2, characterized in that, During the static degassing process, the temperature is 25-160℃, and the standing time is 1-24 hours.

6. The method for preparing porous symmetrical polymer membranes by delayed phase inversion according to claim 1, characterized in that, In the film coating process, the coating speed is 10-50 mm / s, and the coating thickness is 100-500 μm; The medium for the water coagulation bath is water, the temperature is 20-80℃, and the soaking time is 3-24 hours.

7. The method for preparing porous symmetrical polymer membranes by delayed phase inversion according to claim 1, characterized in that, The cleaning, protection, and drying processes are as follows: soaking in ethanol at room temperature for 12-72 h; soaking in n-hexane at room temperature for 6-24 h; and air drying at room temperature for 1-24 h.

8. A polymer film, prepared by the method according to any one of claims 1-7, characterized in that, The polymer membrane has a symmetrical structure with integral sponge-like pores, and the membrane thickness is 50-300 μm, the average pore size is 30-1000 nm, and the membrane distillation flux is 5-50 kg·m³. -2 ·h -1 ·bar -1 .

9. The application of the polymer membrane according to claim 8 in membrane distillation.

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