A polyamide composite membrane based on a nanobubble-assisted free interface polymerization strategy, its preparation method and application

Abstract

This invention discloses a polyamide composite membrane based on a nanobubble-assisted free interface polymerization strategy, its preparation method, and its application. By loading different types and concentrations of nanobubble precursors (chemical method) into an aqueous solution, nanobubbles are pre-constructed in the aqueous solution through the reaction between the precursors and water or the self-decomposition of the precursors. Under the free interface polymerization process, the upward migration of nanobubbles at the interface in the aqueous solution and the phases achieves hierarchical control of the polyamide separation layer structure from bottom to top. This effectively improves the bottom opening characteristics, internal nanocavity structure, and surface morphology of the polyamide separation layer, significantly enhances the transmembrane transport rate of water molecules, and reduces hydraulic resistance. Without sacrificing the salt rejection rate of the polyamide composite membrane, the water flux of the membrane is significantly improved.

Description

Technical Field

[0001] This invention discloses a polyamide composite membrane based on a nanobubble-assisted free interface polymerization strategy, its preparation method, and its application, belonging to the field of separation membrane preparation and application technology. Background Technology

[0002] Polyamide composite membranes are widely used in water purification, wastewater reuse, and seawater desalination. The permeability and selectivity of the membrane are typically determined by the physicochemical properties of the dense polyamide separation layer. Studies have shown that beneath the typical "ridge-valley," strip-like, and leaf-like morphologies on the surface of the polyamide separation layer, there may exist nanocavity structures that improve the water-permeable area and reduce hydraulic resistance of the polyamide composite membrane. In recent years, based on in-depth research on the reaction heat and byproduct acids during interfacial polymerization, researchers have innovatively proposed the concepts of interfacial vaporization and interfacial degassing, providing a reasonable explanation for the formation of nanocavities. These concepts have, to some extent, promoted the development of nanobubble strategies and opened up a new path for the design of nanocavities within membranes.

[0003] Currently, the generation of nanobubbles is generally divided into in-situ reaction and pre-dispersion of nanobubbles in aqueous solution through physical or chemical means, and both are ultimately applied to in-situ interfacial polymerization processes. Patent CN 110545903B describes a membrane composite membrane with enhanced permeability and nano-sized bubbles, its preparation method, and its uses. It generates nanobubbles by using reactive additives or soluble gases present in the aqueous solution, or by using a nanobubble generator or ultrasound in the aqueous solution, and then forms a polyamide separation layer containing nanocavities through in-situ interfacial polymerization. Patent CN 116920637B describes a dense membrane and its preparation method that simultaneously improves flux and rejection rate through chemical bubbles. It uses carbamate as an aqueous monomer, utilizing the unique structure of carbamate to release a large number of nanobubbles under alkaline conditions. Simultaneously, it polymerizes with trimesoyl chloride to form a dense polyamide separation layer. This results in the polyamide separation layer forming a loose structure rich in nanocavities within the continuously generated bubbles, increasing the effective filtration area of ​​the membrane. However, in-situ reactions cannot guarantee the effectiveness of nanobubble generation. Furthermore, during the instantaneous reaction of the aqueous amine monomer and the organic acyl chloride monomer, nanobubbles formed in-situ by reactive additives may be difficult for polyamide to capture, thus affecting the formation of nanocavity structures. In addition, under in-situ interfacial polymerization, rapidly released nanobubbles are often confined by the support layer and cannot escape, accumulating at the interface and causing strong interfacial fluctuations. This ultimately affects the crosslinking process and fine structure of the polyamide separation layer, impairing its selectivity. Physical methods such as pre-injecting soluble gas into the aqueous solution, generating nanobubbles in the aqueous solution using a bubble generator, and ultrasonically treating the aqueous solution can effectively avoid these problems. However, these methods of pre-dispersing nanobubbles in the aqueous solution are also affected by the in-situ interfacial polymerization process. During in-situ interfacial polymerization, the aqueous solution on the surface of the support layer is usually removed by natural air drying or roller drying, which may cause a large number of nanobubbles to disappear, thus weakening its ability to control the nanocavity structure within the polyamide separation layer. These problems restrict the development of nanobubble-controlled polyamide composite membrane structures. Summary of the Invention

[0004] Driven by the urgent need to improve the performance of polyamide composite membranes and recognizing the limitations of current nanobubble strategies for controlling polyamide composite membrane structure, this invention proposes a method for preparing polyamide composite membranes based on a nanobubble-assisted free interface polymerization strategy. This method involves introducing a nanobubble precursor into an aqueous solution. Nanobubbles are pre-constructed in the aqueous solution through the reaction between the precursor and water or through the self-decomposition of the precursor (chemical method). During free interface polymerization, the upward migration of nanobubbles at the interface in the aqueous solution allows for the sequential adjustment of the bottom opening characteristics (pore size, opening density, etc.), the size and distribution of internal nanocavities, and the surface morphology of the polyamide separation layer from bottom to top. By controlling these key structural parameters, the overall structure of the polyamide separation layer is optimized, significantly improving its water permeability and enhancing the water molecule transport rate, ultimately improving the performance of the polyamide composite membrane.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A polyamide composite membrane based on a nanobubble-assisted free interface polymerization strategy is disclosed. This involves introducing a nanobubble precursor into an aqueous solution. Nanobubbles are pre-constructed in the aqueous solution through reaction between the precursor and water or through the self-decomposition of the precursor. During free interface polymerization, the upward migration of nanobubbles at the interface in the aqueous solution allows for hierarchical control of the polyamide separation layer structure, including bottom opening characteristics (pore size and opening density), internal nanocavities, and surface morphology. Specifically, the average pore size of the bottom openings is 86-183 nm, and the opening density is 1.6-7.4 openings / μm. 2 The average lateral dimension of the nanocavities is 123-212 nm, and the average longitudinal dimension is 81-125 nm.

[0007] A method for preparing polyamide composite films based on a nanobubble-assisted free interface polymerization strategy includes the following steps:

[0008] (1) Dissolve the polyamine in water to form an aqueous solution with a polyamine concentration of 1.0-3.0 wt%; dissolve the polyacryl chloride in an organic solvent to form an organic solution with a polyacryl chloride concentration of 0.05-0.2 wt%.

[0009] (2) The interface polymerization reaction was carried out using a free interface polymerization strategy. First, a porous support layer was placed at the bottom of the reaction vessel. Then, a nanobubble precursor was added to the aqueous solution and sonicated to preconstruct nanobubbles in the aqueous solution. The concentration of the nanobubble precursor in the aqueous solution was 0.025-1.0 wt%. Subsequently, the sonicated aqueous solution, organic buffer solution and organic solution were added to the reaction vessel in sequence to carry out the interface polymerization reaction. The reaction vessel consisted of a porous support layer, an aqueous solution, an organic buffer solution and an organic solution from bottom to top. The polyamine in the aqueous solution and the polyacrylamide in the organic solution migrated to the organic buffer solution to carry out the interface polymerization reaction. At the same time, the nanobubbles in the aqueous solution also migrated upward and were captured during the interface polymerization reaction.

[0010] (3) After the polymerization reaction is completed, the remaining solution between the polyamide separation layer and the support layer is extracted through the porous support layer through the outlet at the bottom of the reaction vessel, so that the prepared polyamide separation layer is deposited on the surface of the polysulfone support layer to form a primary composite film. Then the primary composite film is taken out from the reaction vessel, cleaned, and heat-treated to form a stable polyamide composite film.

[0011] In step (1), the polyamine and polyacrylamide chloride are raw materials containing amine groups and acrylamide chlorides that are well known to those skilled in the art for use in the preparation of polyamides. The polyamines include, but are not limited to, piperazine, ethylenediamine, hydrazine carbonate, pyridineamine, β-cyclodextrin, 4-aminopiperazine, p-phenylenediamine, o-phenylenediamine, m-phenylenediamine, polyethyleneimine, diethylenetriamine, triethylenetetramine, 1,4-diaminocyclohexane, 3,5-diaminobenzoic acid, 2-aminophenol-4-sulfonamide, bis(2-hydroxyethyl)dimethylammonium chloride, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane. Or multiple types; polyacryl chlorides include, but are not limited to, any one or more of phthaloyl chloride, terephthaloyl chloride, trimesoyl chloride, 1,3,5-benzenetrisulfonyl chloride, 1,3,6-naphthalenetrisulfonyl chloride, biphenyl dicarboxylate chloride, cyclohexanetriacryl chloride, trimesoyl trioxide chloride, 1,2,4,5-benzenetetracarbonyl chloride, 2,2',4,4'-biphenyltetramethyl chloride, 3,3',5,5'-biphenyltetramethyl chloride, 5-nitrobenzene-1,3-diacyl chloride, 5-tert-butyl-1,3-benzenediacol chloride, 5-(1-pyrrolidinyl)-isophthaloyl chloride, and 5-sulfinylaminoisophthaloyl chloride.

[0012] The porous support layer material in step (2) includes, but is not limited to, any one of polysulfone, polyester, polyethersulfone, polypropylene, polycarbonate, and polyetherimide.

[0013] In one implementation method, in step (2), the nanobubble precursor reacts with water to form nanobubbles in an aqueous solution. The nanobubble precursor includes, but is not limited to, any one or more of sodium borohydride, sodium hydride, calcium hydride, calcium carbide, aluminum carbide, and potassium superoxide. After adding the nanobubble precursor to the aqueous solution, it is immediately subjected to ultrasonic treatment for a total of 2-4 times, each time for 1-6 seconds, to promote the rapid reaction between the precursor and water to generate nanobubbles, that is, to pre-construct nanobubbles in the aqueous solution, thereby controlling the interfacial polymerization reaction. The ultrasonic power is 200-300W and the frequency is 20-40kHz. The types of gas inside the nanobubbles include, but are not limited to, any one or more of H2, C2H2, CH4, and O2.

[0014] As another implementation method, in step (2), the nanobubble precursor self-decomposes (chemical method) to form nanobubbles in an aqueous solution. The nanobubble precursor in step (2) includes, but is not limited to, any one or more of sodium peroxide, calcium peroxide, sodium percarbonate, hydrogen peroxide, ammonium bicarbonate, and sodium bicarbonate. After adding the nanobubble precursor to the aqueous solution, it is immediately subjected to ultrasonic treatment, 2-4 times in total, 1-6 seconds each time, to promote the self-decomposition of the precursor to generate nanobubbles. That is, the interfacial polymerization reaction is controlled by pre-constructing nanobubbles in the aqueous solution. The ultrasonic power is 200-300W, and the frequency is 20-40kHz. The gas types inside the nanobubbles include, but are not limited to, any one or two of O2 and CO2.

[0015] The interfacial polymerization reaction time in step (2) is 0.5-5 min, preferably 1-3 min.

[0016] In step (2), the organic buffer solution is an organic phase in an organic phase solution, including but not limited to any one of toluene, n-pentane, n-hexane, n-heptane, n-octane, n-decane, cyclohexane, and isoparaffins.

[0017] In step (3), the outlet at the bottom of the reaction vessel is connected to a vacuum pump or a blower. The remaining aqueous solution is discharged through the outlet at the bottom of the reaction vessel by the vacuum pump or blower, while promoting the deposition of the polyamide separation layer on the surface of the polysulfone support layer.

[0018] In step (3), the height of the aqueous solution is 3-6 mm, the height of the organic buffer solution is 0.1-0.2 mm, and the height of the organic solution is 2-4 mm.

[0019] In step (3), the heat treatment temperature is 50-90℃ and the time is 1-5min to remove the organic solvent remaining on the surface and inside of the polyamide separation layer and to promote further cross-linking between amine groups and acyl chlorides to form a polyamide composite film.

[0020] The polyamide composite membrane is used for desalination in solution.

[0021] Compared with the prior art, the present invention has the following beneficial effects: (1) Compared with the strategy of generating nanocavities by additional acid etching, the present invention preconstructs nanobubbles in the aqueous solution through the reaction between the precursor and water or the self-decomposition of the precursor, and uses nanobubbles to construct nanocavities in the polyamide separation layer, avoiding the negative impact that the acid etching process may have on the physicochemical structure of the separation layer, which is conducive to ensuring the compactness of the separation layer; (2) Compared with the control strategy of generating nanobubbles through in-situ chemical reaction in the in-situ interface polymerization process, the present invention uses the strategy of preconstructing nanobubbles before the interface reaction, which avoids the problem that the nanobubbles generated in situ are difficult to be captured by the polyamide separation layer, and ensures the effectiveness of nanobubbles in controlling the nanocavity structure in the separation layer; (3) Compared with the strategy of pre-dispersing nanobubbles by physical means in the in-situ interface polymerization process, the present invention adopts the strategy of preconstructing nanobubbles to assist free interface polymerization, which effectively avoids the phenomenon of nanobubbles disappearing due to the drying or rolling of the aqueous solution during the in-situ interface polymerization process. (4) Compared with the in-situ interfacial polymerization strategy controlled by nanobubbles, the present invention adopts the nanobubble-controlled free interfacial polymerization strategy to avoid the adsorption of nanobubbles on the surface of the support layer and allows nanobubbles at the interface of the aqueous solution to migrate freely to the organic solution, thereby realizing the hierarchical control of the bottom, interior and surface structure of the polyamide separation layer, laying a solid foundation for the improvement of the performance of the polyamide composite membrane; (5) The present invention adds a buffer between the aqueous solution and the organic solution to control the reaction rate of interfacial polymerization, avoid the interfacial polymerization reaction being too fast to capture the bubbles, and the rising of the bubbles is conducive to the hierarchical control of the polyamide separation layer structure; (6) The nanobubble control strategy proposed in the present invention has strong universality and can be widely applied to a variety of precursors that can react with water or can self-decompose to generate nanobubbles. Moreover, this strategy can be flexibly applied to a variety of interfacial polymerization systems and has high practical application value. Attached Figure Description

[0022] Figure 1 For Comparative Example 1, SEM images of the surface, cross-section, and back side of the polyamide layer prepared without the introduction of nanobubbles are shown.

[0023] Figure 2 Example 2 shows SEM images of the surface, cross-section, and back side of the polyamide layer prepared based on the H2 nanobubble control strategy.

[0024] Figure 3 Examples 1, 3, and 4 show surface SEM images of polyamide layers prepared using a strategy to control H2 nanobubbles at different concentrations.

[0025] Figure 4 Example 5 shows SEM images of the surface, cross-section, and back side of the polyamide layer prepared based on the O2 nanobubble control strategy.

[0026] Figure 5 Example 6 shows SEM images of the surface, cross-section, and back side of the polyamide layer prepared based on the CO2 nanobubble control strategy. Detailed Implementation

[0027] This invention provides a method for preparing polyamide composite membranes based on a nanobubble-assisted free interface polymerization strategy. By introducing a nanobubble precursor into an aqueous solution, nanobubbles are pre-constructed in the aqueous solution through the reaction between the precursor and water or the self-decomposition of the precursor. Under the free interface polymerization process, the overall structure of the polyamide separation layer is hierarchically controlled by utilizing the upward migration of nanobubbles at the neutralization interface in the aqueous solution, significantly improving the performance of the polyamide composite membrane. The technical solution will be described in detail below with reference to embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention and do not represent all embodiments.

[0028] Comparative Example 1:

[0029] A method for preparing a polyamide reverse osmosis membrane using a free interface polymerization process is as follows:

[0030] First, a polysulfone support layer was placed at the bottom of a reaction vessel (40 mm radius). An aqueous solution containing 2.0 wt% m-phenylenediamine was poured into the reaction vessel, with the aqueous solution reaching a height of approximately 3 mm. Subsequently, 0.5 mL of n-hexane buffer (approximately 0.1 mm in height) and an organic solution containing 0.1 wt% trimesoyl chloride were added sequentially to the surface of the aqueous solution, with the organic solution reaching a height of approximately 2 mm. After reacting for approximately 3 minutes, a vacuum pump was connected to the bottom outlet of the reaction vessel. The reaction vessel was then filtered using the vacuum pump, and the solution between the polysulfone support layer and the polyamide separation layer was extracted through the polysulfone support layer, allowing the prepared polyamide separation layer to deposit on the surface of the polysulfone support layer. Finally, the prepared composite reverse osmosis membrane was removed by lifting, rinsed with n-hexane, and transferred to an 80°C oven for heat treatment for 3 minutes.

[0031] Example 1:

[0032] A method for preparing polyamide reverse osmosis membranes based on an H2 nanobubble control strategy using a free interface polymerization process is as follows:

[0033] First, a polysulfone support layer was placed at the bottom of a reaction vessel (40 mm in radius). 0.025 wt% sodium borohydride was added to an aqueous solution containing 2.0 wt% m-phenylenediamine. The mixture was sonicated three times for 2 seconds each time to promote a vigorous reaction between sodium borohydride and water, thereby rapidly generating H2 nanobubbles. The sonication power was 300 W and the frequency was 40 kHz. Subsequently, an aqueous solution containing nanobubbles and m-phenylenediamine was poured into the reaction vessel, with the aqueous solution reaching a height of approximately 3 mm. Subsequent experimental steps and conditions were the same as in Comparative Example 1.

[0034] Examples 2-4:

[0035] The sodium borohydride concentrations were 0.05 wt%, 0.075 wt%, and 0.1 wt%, respectively, and the other experimental conditions were the same as in Example 1.

[0036] Comparative Example 2:

[0037] Compared with Example 1, the aqueous solution of this comparative example was not treated with sodium borohydride but was subjected to ultrasonic treatment; all other aspects were the same.

[0038] Comparative Example 3:

[0039] Compared with Example 1, the aqueous solution of this comparative example only had 0.05 wt% sodium borohydride added and was not subjected to ultrasonic treatment; all other aspects were the same.

[0040] Example 5:

[0041] A method for preparing polyamide reverse osmosis membranes based on an O2 nanobubble control strategy using a free interface polymerization process is as follows:

[0042] Add 0.05 wt% sodium percarbonate to an aqueous solution containing 2.0 wt% m-phenylenediamine, and sonicate three times for 2 seconds each time to promote the self-decomposition of sodium percarbonate, thereby generating O2 nanobubbles. The ultrasonic power is 300 W and the frequency is 40 kHz. The subsequent experimental steps and conditions are the same as in Example 1.

[0043] Example 6:

[0044] A method for preparing polyamide reverse osmosis membranes based on a CO2 nanobubble control strategy using a free interface polymerization process is as follows:

[0045] Add 0.46 wt% sodium bicarbonate to an aqueous solution containing 2.0 wt% m-phenylenediamine, and sonicate three times for 2 seconds each time to promote the self-decomposition of sodium bicarbonate, thereby generating CO2 nanobubbles. The ultrasonic power is 300 W and the frequency is 40 kHz. The subsequent experimental steps and conditions are the same as in Example 1.

[0046] Comparative Example 4:

[0047] Compared to Example 6, this comparative example aqueous solution only had 0.46 wt% sodium bicarbonate added and was not subjected to ultrasonic treatment; all other aspects were the same.

[0048] Test Example 1:

[0049] The performance testing process for polyamide reverse osmosis membranes is as follows:

[0050] The water flux and salt rejection ratio of the reverse osmosis membrane were tested in a cross-flow filtration unit. The test conditions were: feed solution of 2000 ppm sodium chloride solution, operating temperature of 25℃, pressure of 2 MPa, and effective filtration area of ​​the membrane of 12.56 cm². 2 The running time should be at least 1 hour to maintain a stable water flux.

[0051] Table 1 shows the water flux and salt rejection rate of polyamide reverse osmosis membranes prepared based on H2, O2, or CO2 nanobubbles under the free interface polymerization process. During the process of controlling the polyamide separation layer structure using H2 nanobubbles, the size of the H2 nanobubbles increased with the increase of sodium borohydride concentration. This increase in size not only led to the continuous expansion of the nanocavity size within the polyamide separation layer but also promoted the continuous increase in the apparent thickness of the separation layer. Theoretically, the expanded nanocavities are beneficial for improving the effective permeable area of ​​the polyamide separation layer and reducing the transport resistance of water molecules, ultimately inducing an increase in water flux. However, the performance test results of Comparative Example 1 and Examples 1-4 show that the salt rejection rates of the various reverse osmosis membranes are relatively similar, while the water flux shows a trend of first increasing and then decreasing, with Example 2 exhibiting the highest water flux. This result may be related to the change in the thickness of the polyamide separation layer; the continuously expanding nanocavities lead to an excessive increase in the apparent thickness of the polyamide separation layer, thereby weakening the advantages brought by the nanocavities to some extent. The above results indicate that by adjusting the concentration of the nanobubble precursor and coordinating the changes between the nanocavity size and the polyamide separation layer thickness, optimal membrane performance can be obtained.

[0052] The nanobubble-controlled free interface polymerization strategy proposed in this invention has a certain degree of universality. By adjusting the way the precursor generates nanobubbles, that is, by utilizing their self-decomposition to pre-construct nanobubbles in the aqueous solution, the structure and performance of reverse osmosis membranes can also be improved. In Examples 5 and 6, by changing the nanobubble precursors to sodium percarbonate and sodium bicarbonate, the reverse osmosis membranes prepared accordingly showed varying degrees of improvement in water flux while retaining a relatively reasonable salt rejection rate. Comparative Example 2, which only underwent ultrasonic treatment, showed performance close to that of Comparative Example 1. This is mainly because relying solely on ultrasound is insufficient to generate a large number of nanobubbles in the aqueous solution to regulate the structure of the polyamide layer (the opening density and pore size at the bottom of the polyamide layer, and the size of the nanocavities inside the polyamide layer). Comparative Example 3, which added sodium borohydride but did not undergo ultrasonic treatment, showed a significant increase in water flux but a marked decrease in salt rejection rate. This is because the vigorous reaction between sodium borohydride and the aqueous solution generated a large number of large H2 bubbles, forming large and unevenly distributed nanocavities, which increased the thickness of the polyamide layer and disrupted the stability of the interfacial polymerization process, thereby impairing the compactness of the polyamide separation layer. It can be seen that ultrasound can eliminate large bubbles, increase the bubble generation rate, and improve bubble uniformity. The results of Comparative Examples 2 and 3 indicate that ultrasonic treatment of the aqueous solution alone, or the introduction of sodium borohydride precursor alone, cannot effectively regulate the structure and performance of the polyamide composite membrane. Comparative Example 4, with only sodium bicarbonate added to the aqueous solution without ultrasonic treatment, showed a decrease in water flux and a slight decrease in salt rejection rate compared to Example 6. The self-decomposition of sodium bicarbonate in the aqueous phase to generate carbon dioxide bubbles is slow, and ultrasonic treatment can accelerate the generation of bubbles. Omitting ultrasonic treatment affects the structure of the separation layer and reduces the overall separation performance. Compared with Comparative Example 1, the water flux is improved, and the salt rejection rate is not significantly different. This is mainly attributed to the reaction between the interfacial polymerization byproduct acid and the undecomposed sodium bicarbonate. The in-situ generated nanobubbles effectively regulate the overall structure of the polyamide layer, which can also be confirmed by the average density of the bottom openings and the size of the nanocavities in Table 2.

[0053] Table 1 shows the water flux and salt rejection of the reverse osmosis membranes prepared in Comparative Examples 1–4 and Examples 1–6.

[0054]

[0055]

[0056] Figure 1 The polyamide reverse osmosis membrane in Comparative Example 1 shows a ring-shaped structure on its surface, with fewer nanocavities inside the membrane and a large number of incompletely open pores dispersed on the back side of the membrane. Figure 2 a and Figure 3For the membrane surface morphology characteristics of Examples 1-4, with the increase of H2 nanobubble precursor concentration, it can be seen that the membrane surface exhibits an evolution from rough to smooth, and then to a bubble ring structure. The smooth surface of the H2-0.05% membrane is mainly attributed to the high heat dissipation rate of the free interface polymerization process and the appropriate amount of nanobubbles generated by this precursor concentration, which provides a template for the growth of the polyamide prepolymer, thereby enhancing the interfacial stability during the interfacial polymerization process. Subsequently, as the H2 nanobubble precursor concentration increases to 0.075 wt%, the nanobubble content in the aqueous solution increases accordingly, leading to an increase in the amount of nanobubbles migrating into the organic phase solution. During the bottom-up cross-interfacial transport of nanobubbles, a bubble ring structure is finally formed on the surface of the polyamide separation layer. When the precursor concentration is further increased to 0.1 wt%, the size of the bubble rings on the membrane surface increases significantly. A more in-depth analysis of Example 2 is as follows... Figure 2 As shown in figures b and c, the H2-0.05% membrane exhibits a distinct nanocavity structure. Compared to Comparative Example 1, the pore size and opening density on the back side of the H2-0.05% membrane are significantly increased. The introduction of H2 nanobubbles effectively improves the bottom opening characteristics of the polyamide separation layer and optimizes the size and distribution of the internal nanocavities. The synergistic effect of these two factors promotes the transmembrane transport rate of water molecules and enhances the performance of the reverse osmosis membrane.

[0057] Figure 4 a and 5a represent the surface morphologies of the reverse osmosis membranes in Examples 5 and 6, respectively. It can be seen that the presence of O2 and CO2 nanobubbles promotes the formation of annular or flat membrane surfaces. The above analysis shows that even small changes in nanobubble concentration have a significant impact on membrane surface morphology. Therefore, the difference in the surface morphology of the O2 and CO2 membranes may be attributed to the different contents of nanobubbles generated by the self-decomposition of the two precursors. Compared to Comparative Example 1, the polyamide separation layer regulated by O2 and CO2 nanobubbles exhibits clearly dispersed nanocavities, and the pore size of the back opening is significantly increased.

[0058] Table 2 shows the structural parameters of the polyamide release layers prepared in Comparative Examples 1–4 and Examples 1–6.

[0059]

[0060] The above results indicate that by adding nanobubble precursors to an aqueous solution, relying on the reaction between the precursor and water or the self-decomposition of the precursor to pre-construct nanobubbles, and utilizing the migration of nanobubbles from the aqueous phase to the organic phase solution, the bottom opening characteristics, internal nanocavity structure, and surface morphology of the polyamide separation layer can be hierarchically controlled, thereby effectively improving the performance of the polyamide composite membrane.

Claims

1. A method for preparing polyamide composite films based on a nanobubble-assisted free interface polymerization strategy, characterized in that, Includes the following steps: (1) Dissolve the polyamine in water to form an aqueous solution with a polyamine concentration of 1.0–3.0 wt%; dissolve the polyacryl chloride in an organic solvent to form an organic solution with a polyacryl chloride concentration of 0.05–0.2 wt%. (2) The interface polymerization reaction was carried out using a free interface polymerization strategy. First, the porous support layer was placed at the bottom of the reaction vessel. Then, the nanobubble precursor was added to the aqueous solution and ultrasonically treated to preconstruct nanobubbles in the aqueous solution. The concentration of the nanobubble precursor in the aqueous solution was 0.025-1.0 wt%. Subsequently, the ultrasonically treated aqueous solution, organic buffer solution and organic solution were added to the reaction vessel in sequence to carry out the interface polymerization reaction. The porous support layer, aqueous solution, organic buffer solution and organic solution were arranged from bottom to top in the reaction vessel. (3) After the polymerization reaction is completed, the remaining solution between the polyamide separation layer and the support layer is extracted through the porous support layer through the outlet at the bottom of the reaction vessel, so that the prepared polyamide separation layer is deposited on the surface of the polysulfone support layer to form a nascent composite film. Then the nascent composite film is taken out from the reaction vessel, cleaned, and heat-treated to form a stable polyamide composite film. The nanobubble precursor is any one or more of sodium borohydride, sodium hydride, calcium hydride, calcium carbide, aluminum carbide, potassium superoxide, sodium peroxide, calcium peroxide, sodium percarbonate, hydrogen peroxide, ammonium bicarbonate, and sodium bicarbonate. The polyamide separation layer forms a bottom open layer and an internal nanocavity layer, wherein the average pore size of the bottom opening is 86–183 nm and the average density is 1.6–7.4 pores / μm. 2 The average lateral dimension of the nanocavities is 123–212 nm, the average longitudinal dimension is 81–125 nm, and the average thickness of the polyamide separation layer is 168 nm–346 nm.

2. The method for preparing polyamide composite films based on nanobubble-assisted free interface polymerization strategy according to claim 1, characterized in that, The polyamine is any one or more of piperazine, ethylenediamine, hydrazine carbonate, pyridineamine, β-cyclodextrin, 4-aminopiperazine, p-phenylenediamine, o-phenylenediamine, m-phenylenediamine, polyethyleneimine, diethylenetriamine, triethylenetetramine, 1,4-diaminocyclohexane, 3,5-diaminobenzoic acid, 2-aminophenol-4-sulfonamide, bis(2-hydroxyethyl)dimethylammonium chloride, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane; the polyacrylamide is phthaloyl chloride, terephthaloyl chloride, etc. Any one or more of the following: benzotrisulfonyl chloride, 1,3,5-benzenetrisulfonyl chloride, 1,3,6-naphthalenetrisulfonyl chloride, biphenyl dicarboxylate chloride, cyclohexanetriyl chloride, trimellitic anhydride chloride, 1,2,4,5-benzenetetracarbonyl chloride, 2,2',4,4'-biphenyltetramethyl chloride, 3,3',5,5'-biphenyltetramethyl chloride, 5-nitrobenzene-1,3-diacyl chloride, 5-tert-butyl-1,3-benzenediayl chloride, 5-(1-pyrrolidinyl)-isophthaloyl chloride, and 5-sulfinylaminoisophthaloyl chloride.

3. The method for preparing polyamide composite films based on nanobubble-assisted free interface polymerization strategy according to claim 1, characterized in that, The porous support layer material is any one of polysulfone, polyester, polyethersulfone, polypropylene, polycarbonate, and polyetherimide.

4. The method for preparing polyamide composite films based on nanobubble-assisted free interface polymerization strategy according to claim 1, characterized in that, In step (2), the nanobubble precursor reacts with water to form nanobubbles in the aqueous solution, or in step (2), the nanobubble precursor decomposes itself to form nanobubbles in the aqueous solution.

5. The method for preparing polyamide composite films based on nanobubble-assisted free interface polymerization strategy according to claim 1, characterized in that, After the nanobubble precursor was added to the aqueous solution, it was immediately subjected to ultrasonic treatment for a total of 2–4 times, each time for 1–6 seconds, with an ultrasonic power of 200–300 W and a frequency of 20–40 kHz.

6. The method for preparing polyamide composite films based on nanobubble-assisted free interface polymerization strategy according to claim 1, characterized in that, In step (2), the height of the aqueous solution is 3–6 mm, the height of the organic buffer solution is 0.1–0.2 mm, and the height of the organic solution is 2–4 mm.

7. The polyamide composite film prepared by the method according to any one of claims 1-6.

8. The application of the polyamide composite membrane of claim 7, or the polyamide composite membrane prepared by the method of any one of claims 1-6, in the solution desalination process.

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