Electrospun Janus membrane hydrophobic layer, preparation method and application thereof

Abstract

The application belongs to the technical field of membrane materials, and particularly relates to an electrospun Janus membrane hydrophobic layer and a preparation method and application thereof. The electrospun Janus membrane hydrophobic layer is prepared by using water as a non-solvent induced phase separation for the first time. The method adds a certain proportion of water as a non-solvent induced phase separation in a hydrophobic polymer solution to prepare the Janus membrane hydrophobic layer, to regulate the structural characteristics of nanofibers, and to improve the porosity and surface roughness of the electrospun layer. In the membrane distillation process, the Janus membrane prepared by using the water solvent induced hydrophobic layer can effectively reduce the contact area between the hydrophobic layer and the feed liquid, alleviate the influence of the temperature difference, maintain a more stable driving force, and achieve a high water flux and a high salt retention rate. The method has the advantages of simple operation, low cost, environmental friendliness and the like, can fully balance the relationship between the permeation flux and the wet resistance, and has more excellent stability in the running process.

Description

Technical Field

[0001] This invention belongs to the field of membrane material technology. More specifically, it relates to an electrospun Janus membrane hydrophobic layer, its preparation method, and its application. Background Technology

[0002] Seawater desalination has become one of the important ways to address the current water shortage problem. Membrane distillation technology can utilize industrial waste heat to achieve efficient desalination without the need for high operating pressure and other high-energy-consuming operating conditions, and has broad application prospects in the field of seawater desalination.

[0003] With the development of interfacial solar steam power generation technology, solar direct desalination technology is considered a promising technology due to its low cost and environmental friendliness. Membrane distillation (MD), as a heat-driven membrane process, is considered a feasible and economical alternative to traditional desalination technologies (such as multi-stage flash distillation). It is easier to operate and can utilize low-grade heat (e.g., waste heat and solar energy). Compared with pressure-driven membrane desalination technologies such as reverse osmosis, MD processes have lower energy consumption and lower membrane fouling tendency. However, the lack of high-efficiency membranes with high water flux, high salt rejection rate, and good antifouling ability remains a major obstacle to the widespread practical application of MD processes in seawater desalination.

[0004] In recent years, three-dimensional porous membrane materials (Janus membranes) have attracted much attention due to their excellent water-directional transport performance and significant practical application value caused by the asymmetric properties on both sides. Janus membranes are a new type of separation membrane with single-channel liquid properties. A Janus membrane is a separation membrane material with significantly different properties on both sides, typically manifested in different chemical wetting properties. Driven by surface chemical potential, anisotropic transport of liquid can occur between the layers of the three-dimensional porous membrane material's cross-section. Janus membranes have shown great promise in many fields, particularly in seawater desalination, drinking water treatment, and wastewater treatment. However, due to the poor interfacial compatibility between the hydrophilic and hydrophobic membrane layers in Janus membranes, current Janus membranes exhibit poor structural stability, poor reusability, short effective service life (reportedly only a few hours in many cases), and low water flux, significantly limiting their large-scale application. Furthermore, existing Janus membranes and their preparation methods still have some shortcomings; membrane wetting remains a major technical bottleneck restricting the widespread application of membrane distillation materials.

[0005] Current modifications to Janus membranes often focus on chemical modification, involving complex chemical modification steps. The membrane preparation process is lengthy and difficult to scale up, especially since most low surface energy materials are fluorinated compounds, resulting in high costs, high toxicity, and severe environmental pollution. For example, Chinese invention patent application CN110273227A discloses a method for preparing a flexible Janus electrospun fiber membrane with automatic moisture-wicking function. This method uses methyl methacrylate (MMA), glycidyl methacrylate (GMA), and butyl acrylate (BA) as monomers. A random polymer matrix of PGMA-co-PMMA-co-PBA with chemically reactive and flexible polymer chains is obtained through solution polymerization. Then, a fiber membrane is obtained through electrospinning technology. Finally, chemical modification is used to perform hydrophobic and hydrophilic functional modifications on both surfaces to obtain a flexible Janus electrospun fiber membrane with automatic moisture-wicking function. However, this method is complex, requires chemical modification, consumes more chemical reagents, is costly, and is environmentally unfriendly.

[0006] Therefore, there is an urgent need to provide a green and environmentally friendly method for preparing hydrophobic layers of electrospun Janus membranes. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the shortcomings and deficiencies of existing Janus membranes, which are mainly modified by chemical modification, involving complex chemical modification steps, lengthy membrane preparation process, and difficulty in large-scale production. The present invention provides a hydrophobic layer for electrospun Janus membranes.

[0008] The purpose of this invention is to provide a method for preparing the hydrophobic layer of the electrospun Janus membrane.

[0009] Another object of the present invention is to provide the application of the preparation method or the hydrophobic layer of the electrospun Janus membrane in the field of membrane distillation.

[0010] The above-mentioned objective of this invention is achieved through the following technical solution:

[0011] This invention protects a method for preparing a hydrophobic layer of an electrospun Janus membrane, comprising the following steps:

[0012] S1. After fully dissolving the hydrophobic polymer in the organic solvent, add water and stir thoroughly to obtain the spinning solution. It is essential to ensure that the polymer is completely dissolved in the organic solvent before adding water. If the hydrophobic polymer is added after mixing water and organic solvent, the hydrophobic polymer will be difficult to dissolve.

[0013] S2. Electrospinning the spinning solution on the surface of the hydrophilic layer, the resulting hydrophobic Janus membrane layer is formed on the surface of the hydrophilic layer.

[0014] The organic solvent is soluble in water. This invention is the first to use water as a non-solvent-induced phase to prepare an electrospun Janus membrane hydrophobic layer, which is then further prepared into a Janus membrane. Compared with the membrane material prepared without water, the addition of non-solvent water reduces the number of beads on the membrane surface, but the bead surface is rougher. This means that the addition of non-solvent water induces the formation of groove structures on the bead surface, creating a new morphological structure and providing secondary roughness to the bead surface. The micro-nano-scale surface roughness morphology is beneficial for generating more cavitation between the membrane surface and the feed liquid, reducing the contact area and packing density between them, thereby improving membrane distillation performance.

[0015] Preferably, the amount of water added is 1-3 wt% of the spinning solution. By adding a certain proportion of water to the hydrophobic polymer solution as a non-solvent-induced phase separation, the structural characteristics of nanofibers can be controlled, and the porosity and surface roughness of the electrospun layer can be improved.

[0016] Preferably, the amount of water added is 2 wt% of the spinning solution.

[0017] Preferably, the water is added dropwise under stirring conditions. If added too quickly, it will cause localized coagulation of the polymer solution, which is not conducive to forming a uniform spinning solution, and once coagulated, it will be impossible to prepare a membrane.

[0018] Preferably, the hydrophobic polymer is any one or more of polystyrene, polyvinylidene fluoride, polyvinyl chloride, and polyethylene.

[0019] Preferably, the amount of the hydrophobic polymer added is 5-9 wt% of the spinning solution.

[0020] Preferably, the voltage of the electrospinning is 10-20kV.

[0021] Preferably, the electrospinning flow rate is 0.3 to 1.0 mL / h.

[0022] Preferably, the receiving distance of the electrospinning is 5 to 15 cm.

[0023] Preferably, the receiving time for electrospinning is 60–240 min.

[0024] This invention protects the Janus membrane hydrophobic layer prepared by the method described above.

[0025] This invention also protects the preparation method or the application of the Janus membrane hydrophobic layer in the field of membrane distillation.

[0026] The present invention has the following beneficial effects:

[0027] This invention is the first to use water as a non-solvent-induced phase to prepare a hydrophobic layer for electrospun Janus membranes. This method involves adding a certain proportion of water to a hydrophobic polymer solution as a non-solvent-induced phase to prepare the Janus membrane hydrophobic layer, thereby controlling the nanofiber structural characteristics and improving the porosity and surface roughness of the electrospun layer. During membrane distillation, the Janus membrane prepared with the water-induced hydrophobic layer effectively reduces the contact area between the hydrophobic layer and the feed liquid and mitigates the effects of extreme temperature differences, maintaining a more stable driving force to achieve high water flux and high salt rejection. This method has advantages such as simple operation, low cost, and environmental friendliness, and can effectively balance the relationship between permeate flux and anti-wetting properties, exhibiting superior stability during operation. Attached Figure Description

[0028] Figure 1 SEM images of membrane materials with 7PS and 7PS-2H2O as hydrophobic layers.

[0029] Figure 2 Statistical graph showing the contact angle, water flux, and salt rejection of Janus membranes prepared with different PS polymer concentrations.

[0030] Figure 3 Statistical chart showing the water flux and salt rejection rate of membrane materials with 7PS and 7PS-2H2O as hydrophobic layers after a period of use.

[0031] Figure 4 The graph shows the contact angles of membrane materials with 7PS as the hydrophobic layer and membrane materials with 7PS-2H2O as the hydrophobic layer. In the graph, (a) is the initial contact angle of the membrane material with 7PS as the hydrophobic layer, (b) is the contact angle of the membrane material with 7PS as the hydrophobic layer after filtration, (c) is the contact angle of the membrane material with 7PS as the hydrophobic layer after filtration and cleaning, (d) is the initial contact angle of the membrane material with 7PS-2H2O as the hydrophobic layer, (e) is the contact angle of the membrane material with 7PS-2H2O as the hydrophobic layer after filtration, and (f) is the contact angle of the membrane material with 7PS-2H2O as the hydrophobic layer after filtration and cleaning. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in this technical field.

[0033] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0034] Table 1. Explanation of the meanings of proper nouns and English abbreviations

[0035]

[0036] The specific methods for determining porosity, contact angle, roughness, DCMD water flux, and salt rejection rate during implementation are as follows:

[0037] 1. Porosity: First, weigh the initial membrane material mass w1, then immerse the membrane material in anhydrous ethanol at room temperature for half an hour. Remove the membrane sample immersed in ethanol, and use a rubber roller to remove excess ethanol from the sample surface. Then weigh the membrane sample after immersion in anhydrous ethanol w2.

[0038] Porosity can be calculated using the following formula:

[0039]

[0040] Where w1 is the initial dry weight of the membrane (g), w2 is the weight of the membrane in saturated state after soaking in anhydrous ethanol (g), and ρ e The density of ethanol is 0.789 g / cm³. 3 ), ρ p It is the density of the polymer used in the hydrophobic layer.

[0041] 2. Contact Angle: The contact angle (WCA) of the membrane material was measured using a dynamic contact angle meter (Attension Theta, Biolin Scientific) via the pendant drop method. A deionized water droplet was placed on the membrane sample surface for 5 seconds, and the value of each contact angle was measured. Measurements were repeated at at least three different locations for each membrane sample, and the average value was taken as the result.

[0042] 3. Roughness: The surface roughness of the film was measured using an atomic force microscope (AFM, ICON, Bruker, Germany).

[0043] 4. DCMD Water Flux: The membrane material with the determined effective membrane area was placed in a laboratory-scale direct contact membrane distillation (DCMD) apparatus. 3.5 wt% NaCl solution and deionized (DI) water were used as the feed solution and permeate, respectively. A peristaltic pump was used to regulate the flow rates of the feed solution and permeate to 0.55 L / min. The temperatures of the feed solution and permeate were maintained using an oil bath and a cooling bath, respectively. The membrane water flux was calculated based on the increase in permeate volume per unit time.

[0044] 5. Salt rejection rate: Membrane rejection efficiency is determined by measuring the permeate conductivity using a digital conductivity meter.

[0045] 6. Use scanning electron microscopy to determine the morphology and structure of the membrane material.

[0046] Preparation of hydrophilic layer of polyacrylonitrile (PAN)

[0047] PAN powder was dissolved in DMF and magnetically stirred at 50°C for 12 hours to obtain a homogeneous 10 wt% PAN solution. PAN nanofiber membrane materials were then prepared via electrospinning. Specific electrospinning parameters were: voltage 17 kV, flow rate 1.0 mL / h, receiving distance 15 cm, and receiving time 240 min. After electrospinning, the PAN nanofiber membrane was hot-pressed in a hot press to improve its mechanical strength and stability. The hot-pressing temperature was 120°C, and the pressing time was 2 min. The hot-pressed PAN nanofiber membrane served as the hydrophilic layer of the Janus membrane.

[0048] Membrane materials prepared with different PS polymer solution concentrations

[0049] 5g of PS (polystyrene) was thoroughly dried in an oven and then placed in a round-bottom flask. 47.5mL of DMF (N,N-dimethylformamide) and 47.5mL of THF (tetrahydrofuran) were added, and the mixture was stirred at 60℃ until completely dissolved, resulting in a homogeneous solution with a PS concentration of 5wt%. The homogeneous solution was then transferred to a glass bottle and allowed to stand for 12 hours to degas before being injected into a syringe. The polymer solution was electrospun onto the surface of the PAN hydrophilic layer at 16kV, with a polymer push rate of 0.5mL / h, an electrospinning spacing of 8cm, and an electrospinning time of 3 hours. The prepared membrane material was dried at room temperature and named 5PS.

[0050] 7g of PS (polystyrene) was thoroughly dried in an oven and then placed in a round-bottom flask. 46.5mL of DMF (N,N-dimethylformamide) and 46.5mL of THF (tetrahydrofuran) were added, and the mixture was stirred at 60℃ until completely dissolved, resulting in a homogeneous solution with a PS concentration of 7wt%. The homogeneous solution was then transferred to a glass bottle and allowed to stand for 12 hours to degas before being injected into a syringe. The polymer solution was electrospun onto the surface of the PAN hydrophilic layer at 16kV. The polymer push flow rate was 0.5mL / h, the electrospinning spacing was 8cm, and the electrospinning time was 3 hours. The prepared membrane material was dried at room temperature and named 7PS.

[0051] Nine g of polystyrene (PS) was thoroughly dried in an oven and then placed in a round-bottom flask. 45.5 mL of N,N-dimethylformamide (DMF) and 45.5 mL of tetrahydrofuran (THF) were added, and the mixture was stirred at 60 °C until completely dissolved, resulting in a homogeneous solution with a PS concentration of 9 wt%. The homogeneous solution was then transferred to a glass bottle and allowed to stand for 12 hours to degas before being injected into a syringe. The polymer solution was electrospun onto the surface of a PAN hydrophilic layer at 16 kV. The polymer push flow rate was 0.5 mL / h, the electrospinning spacing was 8 cm, and the electrospinning time was 3 hours. The prepared membrane material was dried at room temperature and named 9PS.

[0052] 11 g of PS (polystyrene) was thoroughly dried in an oven and then placed in a round-bottom flask. 44.5 mL of DMF (N,N-dimethylformamide) and 44.5 mL of THF (tetrahydrofuran) were added, and the mixture was stirred at 60 °C until completely dissolved, resulting in a homogeneous solution with a PS concentration of 11 wt%. The homogeneous solution was then transferred to a glass bottle and allowed to stand for 12 h for degassing before being injected into a syringe. The polymer solution was electrospun onto the surface of the PAN hydrophilic layer at 16 kV. The polymer push flow rate was 0.5 mL / h, the electrospinning spacing was 8 cm, and the electrospinning time was 3 h. The prepared membrane material was dried at room temperature and named 11PS.

[0053] Example 1: Preparation of membrane material with 7PS-1H2O as hydrophobic layer

[0054] 7g of PS was thoroughly dried in an oven and then placed in a round-bottom flask. 46.0mL of DMF and 46.0mL of THF were added, and the mixture was stirred at 60℃ until completely dissolved. Then, 1mL of deionized water was added dropwise while stirring, and the mixture was stirred until a homogeneous solution was formed. The obtained homogeneous solution contained 7wt% PS and 1wt% water. The homogeneous solution was then transferred to a glass bottle and allowed to stand for 12 hours to degas before being injected into a syringe. The polymer solution was electrospun onto the surface of the PAN hydrophilic layer at 16kV. The polymer push flow rate was 0.5mL / h, the electrospinning spacing was 8cm, and the electrospinning time was 3 hours. The prepared membrane material was dried at room temperature and named 7PS-1H2O.

[0055] Example 2: Preparation of membrane material with 7PS-2H2O as hydrophobic layer

[0056] 7g of PS was thoroughly dried in an oven and then placed in a round-bottom flask. 45.5mL of DMF and 45.5mL of THF were added, and the mixture was stirred at 60℃ until completely dissolved. Then, 2mL of deionized water was added dropwise while stirring, and the mixture was stirred until a homogeneous solution was formed. The obtained homogeneous solution contained 7wt% PS and 2wt% water. The homogeneous solution was then transferred to a glass bottle and allowed to stand for 12 hours to degas before being injected into a syringe. The polymer solution was electrospun onto the surface of the PAN hydrophilic layer at 16kV. The polymer push flow rate was 0.5mL / h, the electrospinning spacing was 8cm, and the electrospinning time was 3 hours. The prepared membrane material was dried at room temperature and named 7PS-2H2O.

[0057] Example 3: Preparation of membrane material with 7PS-3H2O as hydrophobic layer

[0058] 7g of PS was thoroughly dried in an oven and then placed in a round-bottom flask. 45mL of DMF and 45mL of THF were added, and the mixture was stirred at 60℃ until completely dissolved. 3mL of deionized water was added dropwise while stirring, and the mixture was stirred until a homogeneous solution was formed. The resulting homogeneous solution contained 7wt% PS and 3wt% water. The homogeneous solution was then transferred to a glass bottle and allowed to stand for 12 hours to degas before being injected into a syringe. The polymer solution was electrospun onto the surface of the PAN hydrophilic layer at 16kV. The polymer push flow rate was 0.5mL / h, the electrospinning spacing was 8cm, and the electrospinning time was 3 hours. The prepared membrane material was dried at room temperature and named 7PS-3H2O.

[0059] Performance testing

[0060] (1) Morphological determination

[0061] SEM results of membrane materials 7PS-2H2O and 7PS are as follows Figure 1 As shown, compared with membrane materials prepared without the addition of water ( Figure 1 a1 and Figure 1 a2), after adding non-solvent water, the number of beads on the prepared membrane surface decreased, but the surface of the beads became rougher. Figure 1 b1 and Figure 1 (b2) In other words, the addition of non-solvent water can induce the formation of groove structures on the surface of the beads, providing secondary roughness to the bead surface. The micro- and nano-scale surface roughness morphology is beneficial for generating more cavitation between the membrane surface and the feed liquid, reducing the contact area and packing density between them, thereby improving membrane distillation performance. The morphological characteristics of membrane materials 7PS-1H2O and 7PS-3H2O are basically the same as those of 7PS-2H2O.

[0062] (2) Membrane distillation performance of membrane materials

[0063] The contact angles of the prepared membrane materials 7PS, 9PS, and 11PS were measured. Figure 2 a) Water flux and salt rejection rate Figure 2 b), the result is as follows Figure 2 As shown, it can be seen that the 7PS (PS concentration of 7wt%) condition has a better effect. Therefore, water was added under the 7PS condition to prepare the membrane and test its performance.

[0064] The membrane distillation performance of the membrane materials obtained in Examples 1-3 was measured, and the results are shown in Table 2. The data in the table show that the membrane distillation performance was improved when hydrophobic layers were prepared by electrospinning with 1-3 wt% water added to a 7 wt% PS polymer solution. When the amount of water added was 2 wt%, the membrane material had better membrane distillation performance due to its high porosity and hydrophobicity. The hydrophobic layer was prepared by adding a small amount of water to initiate non-solvent-induced phase separation, which increased the permeate flux by 22.6% without reducing the salt rejection.

[0065] Table 2 Membrane distillation performance of membrane materials

[0066]

[0067] (3) Recyclability of membrane materials

[0068] The membrane distillation performance of the membrane materials 7PS-2H2O and 7PS was further measured after a period of use, and the results are as follows: Figure 3 As shown, after 24 hours of use, the water flux of the 7PS-2H2O membrane material decreased only slightly, from 100% (based on the initial measurement data) to approximately 98%, while the salt rejection rate remained almost unchanged. In contrast, the water flux of the 7PS membrane material without added water decreased from 100% to around 96% after 3 hours of use, and further decreased to approximately 92% after 24 hours. This demonstrates that the 7PS-2H2O membrane material exhibits excellent cycling stability. The membrane materials obtained in Examples 1 and 3 possess essentially the same cycling stability as the membrane material obtained in Example 2.

[0069] (4) Antifouling performance of membrane materials

[0070] The changes in contact angles of the membrane materials 7PS-2H2O and 7PS obtained in Example 2 before and after water treatment were measured, and the results are as follows: Figure 4 As shown, the contact angle is a crucial parameter for membrane distillation materials in water treatment processes, determining their resistance to fouling and wetting, and maintaining operational stability. If a high contact angle can be maintained during operation or if the original contact angle can be well restored after hydraulic cleaning, the membrane distillation application potential will be significantly enhanced. For example... Figure 4 As shown, the membrane material 7PS-2H2O with added non-solvent water exhibited a higher contact angle after operation. Furthermore, after simple water rinsing, the contact angle recovered to 99.2% of its initial value, further demonstrating the excellent recyclability of the membrane material prepared with added water. In contrast, the membrane material without added non-solvent could not recover its contact angle value after water rinsing, indicating high contaminant adhesion strength on the surface of the non-solvent-added membrane material, which is highly detrimental to later applications. The membrane materials obtained in Examples 1 and 3 exhibited essentially the same antifouling properties as the membrane material obtained in Example 2, and their contact angles after use and cleaning were very close to their initial state.

[0071] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A hydrophobic layer for an electrospun Janus membrane, characterized in that, The method for preparing the hydrophobic layer of the electrospun Janus membrane includes the following steps: S1. After fully dissolving the hydrophobic polymer in an organic solvent, add water and stir thoroughly to obtain a spinning solution; S2. Electrospinning the spinning solution on the surface of the hydrophilic layer, the resulting hydrophobic Janus membrane layer is formed on the surface of the hydrophilic layer. The organic solvent is soluble in water; The amount of water added is 1-3 wt% of the spinning solution; The water is added drop by drop under stirring conditions; The hydrophobic polymer is any one or more of polystyrene, polyvinylidene fluoride, polyvinyl chloride, and polyethylene; The amount of the hydrophobic polymer added is 5-9 wt% of the spinning solution.

2. The electrospun Janus membrane hydrophobic layer according to claim 1, characterized in that, The voltage for electrospinning is 10~20kV.

3. The electrospun Janus membrane hydrophobic layer according to claim 1, characterized in that, The electrospinning flow rate is 0.3~1.0 mL / h.

4. The hydrophobic layer of the electrospun Janus membrane according to claim 1, characterized in that, The receiving distance for the electrospinning is 5~15 cm.

5. The application of the Janus membrane hydrophobic layer according to any one of claims 1 to 4 in the field of membrane distillation.

Images