Method for preparing polymer film for CO2 / N2 separation by using interface spreading method

Abstract

The invention aims to provide a method for preparing a polymer film for CO2 / N2 separation by using an interface spreading method, and belongs to the technical field of gas separation.The method comprises the steps that a certain amount of methanol solution is doped into a polymer matrix (including polyimide, polyurethane and polysulfone), a microphase structure is introduced under the action of interface surface tension and solvent polarity, and the polymer film for CO2 / N2 separation is obtained; therefore, the construction of the polymer film is realized. Compared with a traditional method, the method can effectively relieve the mutual restriction relation between gas permeability and selectivity of the polymer membrane. The membrane prepared by the method shows significantly improved CO2 permeability coefficient and CO2 / N2 separation selectivity, breaks through the limitation of conventional material performance, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of gas separation technology, specifically relating to a method for preparing polymer films for CO2 / N2 separation using an interface spreading method. Background Technology

[0002] In recent years, with the increasing prominence of energy crises and environmental problems, efficient and low-energy-consumption gas separation technologies have gradually become a research hotspot. Traditional gas separation processes such as cryogenic distillation and pressure swing adsorption, while widely used, generally suffer from high energy consumption, complex equipment, and high operating costs. In contrast, membrane separation technology, due to its advantages such as low energy consumption, simple process, continuous operation, and environmental friendliness, has become a promising alternative in fields such as carbon dioxide capture and storage (CCS), natural gas purification, hydrogen purification, and air separation.

[0003] Among numerous membrane materials, polymers are currently the most widely used gas separation membrane materials due to their abundant sources, excellent film-forming properties, and strong designability of chemical structures. The separation mechanism of polymer gas separation membranes mainly follows the "dissolution-diffusion model," where gas molecules first dissolve in the membrane material and then diffuse across the pressure difference on both sides of the membrane to complete the separation process. Factors such as the flexibility of polymer chain segments, the introduction of rigid groups, the distribution of polar groups, and the microscopic free volume structure play a decisive role in the permeability and selectivity of gases.

[0004] Polyurethane, polysulfone, and polyimide (such as 6FDA-DAM) have been widely used in membrane separation technology in recent years as gas separation membrane materials. Polyurethane possesses excellent mechanical strength, thermal stability, and tunable permeability; 6FDA-DAM exhibits excellent thermal stability and gas separation performance due to its fluorinated groups. However, traditional polymer membranes generally suffer from a trade-off between permeability and selectivity in CO2 / N2 separation, meaning that increasing CO2 permeability often leads to a decrease in selectivity, limiting their widespread adoption in industrial applications.

[0005] To address the aforementioned issues, researchers have attempted to improve membrane performance through structural design, doping modification, or composite methods. However, conventional membrane fabrication processes still have limitations in terms of membrane thickness uniformity, defect control, and permeation / selectivity optimization. Therefore, there is an urgent need for a novel membrane fabrication method that can significantly improve CO2 permeability and CO2 / N2 selectivity while ensuring membrane integrity and stability. Summary of the Invention

[0006] This invention addresses the deficiency in existing polymer gas separation membranes where permeability and selectivity mutually restrict each other in CO2 / N2 separation. It provides a method for preparing polymer films for CO2 / N2 separation using an interfacial spreading method. This method introduces a methanol solution into a polymer matrix (such as polyurethane, polysulfone, and polyimide), inducing the formation of a microphase structure through interfacial interactions. This effectively improves the permeability and selectivity of the polymer membrane, significantly enhancing CO2 / N2 separation performance. This method can significantly improve the CO2 permeability coefficient and CO2 / N2 selectivity while maintaining membrane homogeneity and structural integrity, thereby expanding its application potential in gas separation and carbon capture.

[0007] The present invention adopts the following technical solution: A method for preparing polymer thin films for CO2 / N2 separation using an interfacial spreading method includes the following steps: S1. After drying the polymer matrix, dissolve it in an organic solvent to obtain a polymer solution; S2. Mix the polymer solution with methanol to obtain a uniformly dispersed mixed solution; S3. After the mixed solution is degassed by ultrasonic treatment, it is dropped into a horizontal glass petri dish mold pre-filled with distilled water, so that it spreads on the water surface to form a polymer film. S4. The formed polymer film is supported on a polyethersulfone substrate and dried in a vacuum oven at 100-150°C for 24-48 hours to remove organic solvents, thereby obtaining an ultra-high homogeneous polymer film.

[0008] Further, in S1, the polymer matrix includes polyimide, polysulfone, or polyurethane.

[0009] Furthermore, the polyimide is synthesized from 10 mmol of 4,4'-(hexafluoroisopropyl)diphthalic anhydride and 10 mmol of 2,4,6-trimethyl-1,3-phenylenediamine.

[0010] Further, in S1, the organic solvent includes one of N-methylpyrrolidone, N,N-dimethylformamide, and dichloromethane.

[0011] Further, in S1, the polymer solution has a mass fraction of 5-15 wt%.

[0012] Furthermore, in S2, the mass of the polymer solution is 1g.

[0013] Furthermore, in S2, the volume of methanol is 100-300 μL.

[0014] Furthermore, in S2, the methanol is added in small amounts multiple times.

[0015] Further, in S2, the mixing method is ultrasonication for 240-320 minutes or magnetic stirring at a speed of 900-1000 r / min for 30 minutes.

[0016] The mechanism of this invention is as follows: by introducing an appropriate amount of methanol solution into the polymer matrix, the film formation process and microstructure of the membrane are controlled, thereby significantly improving gas separation performance. Methanol, as a polar small-molecule solvent, can regulate the surface tension and spreading rate of the solution during interfacial film formation, making it easier for polymer segments to form a dense and uniform thin film structure during water surface spreading. During the preparation process, the introduction of methanol can induce the formation of a microphase structure in the polymer system. This microphase promotes the rearrangement of polymer segments, increasing the free volume fraction within the membrane and providing more diffusion channels for CO2 molecules. It also improves the orientation and stacking of polymer chains, effectively mitigating the "trade-off" between gas permeability and selectivity. Specifically, the presence of the microphase structure increases the amorphous region of the membrane, making it easier for small-molecule gases (such as CO2) to dissolve and diffuse within the membrane, thus improving CO2 permeability. Simultaneously, the improved polar sites and segment orientation within the membrane enhance the selective adsorption capacity for CO2 molecules, thereby improving the CO2 / N2 separation selectivity. Therefore, by introducing a methanol solution into the polymer matrix, this invention not only regulates the formation rate and morphological uniformity of the membrane, but also optimizes the arrangement of polymer chain segments and free volume at the molecular scale, thereby achieving a significant improvement in gas separation performance.

[0017] The beneficial effects of this invention are as follows: 1. By introducing an appropriate amount of methanol solution into the polymer matrix, a microphase structure is induced, which increases the free volume fraction of the membrane and significantly improves the CO2 permeability coefficient and CO2 / N2 separation selectivity, effectively breaking through the traditional trade-off between permeability and selectivity of polymer membranes.

[0018] 2. By using the interfacial spreading method to control the surface tension and spreading speed of the solution, the membrane can self-assemble on the water surface to form a uniform and dense ultrathin structure, which improves the problems of uneven membrane thickness and easy formation of defects in conventional membrane preparation methods.

[0019] 3. This method does not require high temperature and high pressure conditions, the operation steps are simple, the film preparation process is highly repeatable, and it is suitable for large-area preparation and industrial scale-up.

[0020] 4. With increasing feed pressure, the membrane prepared by this invention exhibits a continuous increase in CO2 flux, while the decrease in CO2 / N2 selectivity is significantly reduced. The presence of methanol induces a more stable microphase structure during membrane formation, increases the free volume fraction of the membrane, and improves the orientation of polymer segments, enabling the membrane to maintain a high gas diffusion rate and selectivity even under high pressure.

[0021] 5. Introducing methanol into the polymer matrix significantly improves gas separation performance. The resulting polymer membranes exhibit excellent CO2 permeability (175 GPU for polyurethane membranes, 179.30 GPU for 6FDA-DAM membranes, and 75 GPU for polysulfone membranes) and separation selectivity (CO2 / N2 = 42 for polyurethane membranes, 49.54 for 6FDA-DAM membranes, and 45 for polysulfone membranes), showing a significant improvement compared to pure polymer membranes. Attached Figure Description

[0022] Figure 1 The contact angle test diagrams for the 6FDA-DAM membrane are shown; af represents examples 1-6 respectively.

[0023] Figure 2 SEM images of cross-sections of the 6FDA-DAM membrane; af represents examples 1-6 respectively.

[0024] Figure 3 The image shows a planar SEM image and an AFM characterization image of the 6FDA-DAM membrane.

[0025] Figure 4 The graph shows the thermal stability test results for the 6FDA-DAM membrane.

[0026] Figure 5 The CO2 permeability and CO2 / N2 selectivity of 6FDA-DAM membranes with different concentrations were determined.

[0027] Figure 6 a and b show the CO2 permeability and CO2 / N2 selectivity of 6FDA-DAM membranes with different methanol additions. c and d are high-pressure test results.

[0028] Figure 7 This is a long-term test graph of 6FDA-DAM membrane (PI(10%)-M2).

[0029] Figure 8 This is a flowchart of the preparation method of the present invention.

[0030] Figure 9 The structural formula is for polyimide 6FDA-DAM.

[0031] Figure 10 Contact angle test diagram of polyurethane membrane.

[0032] Figure 11 Cross-sectional morphology of the polyurethane membrane.

[0033] Figure 12 Planar topography of the polyurethane membrane.

[0034] Figure 13 Thermal stability analysis diagram of polyurethane film.

[0035] Figure 14 Gas separation performance of polyurethane membranes with different concentrations.

[0036] Figure 15 The gas separation performance of polyurethane membranes can be adjusted by using methanol of the same content.

[0037] Figure 16 Long-term operational stability analysis diagram of polyurethane membrane. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to specific embodiments and experimental data. Obviously, the described embodiments are merely some, not all, of the embodiments of this invention. All other implementations derived by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0039] Example 1 (6FDA-DAM polyimide matrix) A method for preparing polymer thin films for CO2 / N2 separation using interfacial spreading: First, the 6FDA-DAM polymer powder was dried in a vacuum oven at 433 K for 24 hours to remove adsorbed water and impurities. Then, the polymer was dissolved in dichloromethane and stirred at room temperature for 24 hours to prepare a 5 wt% solution. 1 g of the 6FDA-DAM solution was ultrasonically degassed and dropped into a glass petri dish mold pre-filled with distilled water. 200 μL of the 6FDA-DAM solution was then added and instantly spread to form a film. The prepared film was then placed on a PES substrate and vacuum-dried at 120 °C for 24 hours to evaporate residual solvent, yielding a 5 wt% pure 6FDA-DAM polymer film, which was sealed and stored as PI(5%)-M0. The thickness of the obtained polymer film ranged from approximately 400 to 600 nm.

[0040] The hybrid matrix membrane in this embodiment was characterized using Fourier transform infrared spectroscopy (FTIR); the morphology of the hybrid matrix membrane in this embodiment was analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM); and the gas permeability coefficient and selectivity of a pure 6FDA-DAM polymer film with a mass fraction of 5 wt% were tested at 25 °C.

[0041] Example 2 (6FDA-DAM polyimide matrix) The method is similar to that in Example 1, but the polymer solution has a mass fraction of 10 wt%, denoted as PI(10%)-M0. The resulting film thickness is approximately 400-600 nm.

[0042] Characterization and testing were performed in the same manner as in Example 1.

[0043] Example 3 (6FDA-DAM polyimide matrix) The method is similar to that in Example 1, but the polymer solution mass fraction is 15 wt%, denoted as PI(15%)-M0. The resulting film thickness is approximately 400-600 nm.

[0044] Characterization and testing were performed in the same manner as in Example 1.

[0045] Example 4 (6FDA-DAM polyimide matrix, with added methanol) A method for preparing polymer thin films for CO2 / N2 separation using interfacial spreading: First, the 6FDA-DAM polymer powder was dried in a vacuum oven at 433 K for 24 hours to remove adsorbed water and impurities. Then, the polymer was dissolved in dichloromethane and stirred at room temperature for 24 hours to prepare a 10 wt% solution. 1 g of the 6FDA-DAM solution was taken, and 100 μL of methanol was added dropwise several times (while stirring). The mixture was magnetically stirred at 1000 r / min at room temperature for 12 hours to obtain a uniformly dispersed solution. After ultrasonic degassing, the solution was dropped into a glass petri dish mold pre-filled with distilled water. 200 μL of the dispersion was then added, and the mixture instantly spread to form a film. The prepared film was then placed on a PES substrate and vacuum dried at 120 °C for 24 hours to evaporate residual solvent, yielding a PI-1 polymer film, which was sealed and stored, designated PI(10%)-M1. The thickness of the obtained polymer film ranged from approximately 400 to 600 nm.

[0046] The hybrid matrix membrane in this embodiment was characterized using Fourier transform infrared spectroscopy (FTIR); the morphology of the hybrid matrix membrane in this embodiment was analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM); and the gas permeability coefficient and selectivity of the PI-1 polymer film were tested at 25°C.

[0047] Example 5 (6FDA-DAM polyimide matrix, with added methanol) The method is similar to that in Example 4, but the amount of methanol added is 150 μL, resulting in a PI-2 polymer film, denoted as PI(10%)-M2. The thickness is approximately 400-600 nm.

[0048] Characterization and testing are the same as in Example 4.

[0049] Example 6 (6FDA-DAM polyimide matrix, with added methanol) The method is similar to that in Example 4, but the amount of methanol added is 200 μL, resulting in a PI-3 polymer film, denoted as PI(10%)-M3. The thickness is approximately 400-600 nm.

[0050] Characterization and testing are the same as in Example 4.

[0051] Example 7 (6FDA-DAM polyimide matrix, large-area preparation) The method is similar to that in Example 4, but 4g of solution and 600μL of methanol are added dropwise to 1mL of dispersion to obtain a PI-4 polymer film with a diameter of 20cm and a thickness of approximately 400-600 nm.

[0052] Characterization and testing are the same as in Example 4.

[0053] Example 8 (Polyurethane matrix) A method for preparing polymer thin films for CO2 / N2 separation using interfacial spreading: First, polyurethane particles were placed in an adsorption tube and dried at 393 K in a vacuum activation station for 24 hours to remove adsorbed water and impurities. Then, the polymer was dissolved in a solution of DMF and dichloromethane, where DMF provided dissolution and dichloromethane provided film formation. The solution was stirred at room temperature for 24 hours to prepare a 5 wt% solution. 1 g of the polyurethane solution was taken, ultrasonically degassed, and then dropped into a glass petri dish mold pre-filled with distilled water. 100 μL of the polyurethane solution was then added, instantly spreading the film. The prepared film was then placed on a PES substrate and vacuum-dried at 120 °C for 24 hours to evaporate residual solvent, yielding a 5 wt% pure polyurethane polymer film, which was sealed and stored as PU(5%)-M0. The thickness of the obtained pure polyurethane polymer film ranged from approximately 400 to 600 nm.

[0054] The hybrid matrix membrane in this embodiment was characterized using Fourier transform infrared spectroscopy (FTIR); the morphology of the hybrid matrix membrane in this embodiment was analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM); and the gas permeability coefficient and selectivity of a pure polyurethane polymer film with a mass fraction of 5 wt% were tested at 25 °C.

[0055] Example 9 (Polyurethane matrix) The method is similar to that of Example 8, but the polymer solution mass fraction is 7.5 wt%, denoted as PU(7.5%)-M0. The resulting pure polyurethane polymer film has a thickness of approximately 400-600 nm.

[0056] Characterization and testing are the same as in Example 8.

[0057] Example 10 (Polyurethane matrix) The method is similar to that in Example 8, but the polymer solution has a mass fraction of 10 wt%, denoted as PU(10%)-MO. The resulting pure polyurethane polymer film has a thickness of approximately 400-600 nm.

[0058] Characterization and testing are the same as in Example 8.

[0059] Example 11 (Polyurethane matrix, with added methanol) First, polyurethane particles were placed in an adsorption tube and dried at 393 K in a vacuum activation station for 24 hours to remove adsorbed water and impurities. Then, the polymer was dissolved in a solution of DMF and dichloromethane, where DMF provided dissolution and dichloromethane provided film formation. The mixture was stirred at room temperature for 24 hours to prepare a 7.5 wt% solution. 1 g of the polyurethane solution was taken, and 100 μL of methanol was added dropwise several times while stirring. The mixture was magnetically stirred at 1000 r / min at room temperature for 12 hours to obtain a uniformly dispersed solution. After ultrasonic degassing, the solution was dropped into a glass petri dish mold pre-filled with distilled water, and then 100 μL of the dispersion was added to instantly spread the film. The prepared film was then placed on a PES substrate and vacuum dried at 120 °C for 24 hours to evaporate residual solvent, yielding a PU-1 polymer film, which was sealed and stored, designated PU(7.5%)-M1. The thickness of the obtained polymer film ranged from approximately 400-600 nm.

[0060] The hybrid matrix membrane in this embodiment was characterized using Fourier transform infrared spectroscopy (FTIR); the morphology of the hybrid matrix membrane in this embodiment was analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM); and the gas permeability coefficient and selectivity of the PU-1 polymer film were tested at 25°C.

[0061] Example 12 (Polyurethane matrix, with added methanol) The method is similar to that in Example 11, but the amount of methanol added is 200 μL, resulting in a PU-2 polymer film, denoted as PU(7.5%)-M2. The thickness is approximately 400-600 nm.

[0062] Characterization and testing are the same as in Example 11.

[0063] Example 13 (Polyurethane matrix, with added methanol) The method was similar to that in Example 11, but the amount of methanol added was 300 μL, resulting in a PU-3 polymer film, denoted as PU(7.5%)-M3. The thickness was approximately 400-600 nm.

[0064] Characterization and testing are the same as in Example 11.

[0065] Example 14 (Polyurethane matrix, large-area preparation) The method is similar to that in Example 11, but 4g of polyurethane solution and 800μL of methanol are added dropwise to 1mL of dispersion to obtain a PU-4 polymer film with a diameter of 20cm and a thickness of approximately 400-600 nm.

[0066] Characterization and testing are the same as in Example 11.

[0067] Example Conclusion: The experimental procedures for CO2 permeability coefficient and CO2 / N2 selectivity are as follows: The permeability performance of the membrane in pure gas and mixed gas was tested at room temperature using a constant pressure variable volume apparatus. Pure gas and a CO2 / N2 (13 / 87, volume fraction) mixture were used as feed gases, respectively, and helium was used as the scavenging gas (20 mL·min⁻¹). -1 The permeate-side gas was introduced into an Agilent 8890 gas chromatograph for component analysis. The gas permeability and selectivity of the membrane under steady-state conditions were calculated based on the chromatographic results. Each experiment was repeated three times, and the results are expressed as the average value, with standard deviation (error bars) provided.

[0068] The formula for calculating the gas permeability coefficient is as follows: P = Q / (ΔpA); Where P is the gas permeability coefficient, and the unit is GPU (1 GPU = 10^25). -6 cm 3 STPcm -2 s -1 cmHg -1 Q is the gas volumetric flow rate under standard conditions, in cm³. 3 (STP) s -1 A represents the effective permeation area of ​​the membrane, in cm². 2 Δp is the gas partial pressure difference across the membrane, in cmHg. -1 .

[0069] The ideal selectivity α of CO2 / N2 can be calculated by the ratio of the permeabilities of the single gases, and the formula is as follows: α = P i / P j ; Gas mixture separation selectivity α ij The calculation formula is as follows: αij =(y i / y j ) / (x i / x j ); Among them, y i / y j and x i / x j These represent the mole fraction ratios of gas i and j in the permeate side and feed side, respectively.

[0070] Depend on Figure 1 It was found that ultrathin 6FDA-DAM films with nanometer-scale thicknesses were successfully prepared using superdiffusion technology by employing casting solutions with different polymer concentrations and methanol loadings. During film formation, both the 6FDA-DAM concentration and methanol content significantly affected the diffusion behavior and film formation process. To quantitatively elucidate the role of methanol in the superdiffusion process, the top-view diffusion kinetics of the 6FDA-DAM casting solution in the aqueous substrate were recorded using a high-speed camera at 2300 frames per second. When the 6FDA-DAM concentration increased from 5% to 10% and 15%, the spreading time monotonically increased to approximately 501 ms and 683 ms, respectively. This trend indicates that high polymer concentration hinders rapid spreading, which can be attributed to increased solution viscosity and enhanced interfacial flow and deformation resistance due to increased chain entanglement. Conversely, when the polymer concentration was fixed at 10 wt%, increasing the methanol content had the opposite effect. Specifically, adding 100, 150, and 200 μL of methanol progressively shortened the spreading time to approximately 376, 293, and 188 ms, respectively. This monotonically decreasing spreading time indicates that methanol promotes the superspreading process by reducing the effective interfacial tension of the casting solution at the gas-water interface and altering the exchange kinetics between solvent and non-solvent during spreading.

[0071] Depend on Figure 2 It can be seen that cross-sectional characterization was used to assess the structural evolution of the membrane. The membrane thickness systematically increased with the concentration of 6FDA-DAM, reaching 331 nm (PI(5%)-M0), 563 nm (PI(10%)-M0), and 875 nm (PI(15%)-M0) at concentrations of 5%, 10%, and 15%, respectively, reflecting the increase in polymer content during film formation. Notably, the introduction of methanol led to a significant thinning of the selective layer: after adding 100, 150, and 200 μL of methanol, the thickness decreased to 425, 346, and 281 nm, respectively. This significant thinning phenomenon is consistent with the accelerated diffusion kinetics observed in high-speed imaging, indicating that the methanol-enhanced interfacial flow and altered exchange kinetics jointly promoted the formation of thinner, more expanded films, which is expected to translate into enhanced carbon dioxide permeability by shortening the transport distance.

[0072] Depend on Figure 3The effects of methanol on surface microstructure were further investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The methanol-free film exhibited a smooth and uniform surface, while the introduction of methanol promoted the formation of uniformly distributed nanoscale protrusions. With increasing methanol concentration, the surface morphology showed a systematic evolution: sparse, isolated protrusions appeared at 100 μL, developing into a denser and laterally continuous protrusion-rich surface at 150 μL, with significantly improved size uniformity. However, when the methanol concentration increased to 200 μL, this ordered structure was disrupted, leading to the appearance of obvious agglomeration domains. This phenomenon may be due to the fact that methanol addition simultaneously enhanced interfacial flow and altered exchange kinetics. Consistent with these observations, atomic force microscopy showed that the roughness (Rq) increased moderately at lower methanol concentrations, followed by a sharp increase at 200 μL.

[0073] Depend on Figure 4 To elucidate the effect of methanol on the segmental mobility and thermal stability of 6FDA-DAM polyimide films, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to characterize the PI(10%)-M0 to PI(10%)-M3 series samples. DSC thermograms showed that the glass transition temperature (Tg) exhibited a non-monotonic change with the amount of methanol in the casting solution. The Tg of the methanol-free PI(10%)-M0 film was 393.30℃. After the addition of methanol, the Tg of PI(10%)-M1 (100 μL) increased to 395.76℃, and the Tg of PI(10%)-M2 (150 μL) further increased to 398.23℃. This temperature increase indicates that an appropriate amount of methanol can regulate the phase separation path during film formation, promoting the formation of a more rigid chain network, thereby increasing the energy barrier for chain segment relaxation. However, when the methanol content increased to 200 μL (PI(10%)-M3), the Tg decreased to 393.23 °C, approaching the PI(10%)-M0 level. This decrease is likely due to excessive non-solvent-induced phase separation, which loosens the chain structure and may introduce defects or non-selective voids. In contrast, the TGA curves showed negligible differences in the thermal decomposition behavior of all films, indicating that the addition of methanol does not impair the inherent thermal stability of the 6FDA-DAM polyimide matrix.

[0074] Depend on Figure 5The effects of polymer concentration on intrinsic gas transport in PI(5%)-M0, PI(10%)-M0, and PI(15%)-M0 membranes were evaluated. As shown in the figure, the permeability of both gases monotonically decreased as the 6FDA-DAM concentration increased from 5% to 15% (weight percentage). This is mainly attributed to the gradual thickening of the selective layer, leading to a longer diffusion path and increased mass transfer resistance. Notably, PI(10%)-M0 and PI(15%)-M0 exhibited similar CO2 / N2 selectivity, but PI(10%)-M0 had a higher CO2 flux. Therefore, 10 wt% was selected as the baseline concentration to systematically investigate the effect of methanol addition.

[0075] Depend on Figure 6 It can be seen that, based on this concentration, PI(10%)-M1, PI(10%)-M2, and PI(10%)-M3 membranes containing 100, 150, and 200 μL of methanol were prepared, respectively. For example... Figure 6 Performance was tested under pure gas conditions. The fluxes of carbon dioxide and nitrogen increased monotonically with increasing methanol content. This is attributed to the faster interfacial diffusion induced by methanol, forming a thinner, more uniform selective layer, and its non-solvent effect modulating phase behavior, increasing interchain spacing, and reducing diffusion resistance. Actual separation performance was further evaluated under CO2 / N2 (13 / 87) mixed gas conditions. Figure 6(b) The separation behavior exhibits a clear non-monotonic dependence on methanol content. Compared to the methanol-free membrane, introducing 100 μL of methanol significantly increased CO2 flux while slightly inhibiting N2 permeation. The PI(10%)-M2 membrane showed the best performance: CO2 flux reached 187.74 GPU, N2 flux was 3.81 GPU, corresponding to a CO2 / N2 selectivity of 49.54. Compared to the PI(10%)-M0 membrane, its CO2 flux increased by approximately 61%, and its selectivity increased by approximately 178%. When the methanol content was further increased to 200 μL, the nitrogen flux rebounded (7.84 GPU), and the selectivity decreased accordingly, indicating that excessive structural relaxation led to the deterioration of molecular sieve performance. Notably, under mixed gas conditions, the CO2 / N2 selectivity of PI(10%)-M2 (49.54) significantly exceeded the intrinsic selectivity (34.16) obtained from pure gas permeation experiments, highlighting the importance of competitive adsorption effects. Carbon dioxide preferentially occupies adsorption sites and microporous channels due to stronger quadrupole interactions, further inhibiting nitrogen transport. This synergistic effect of molecular sieving and competitive adsorption enables mixed gas separation performance to surpass that of pure gas separation. To systematically investigate the effect of feed pressure, PI(10%)-M0 and PI(10%)-M2 membranes were tested in the pressure range of 1–5 bar. As shown in the figure, the CO2 flux of both membranes decreased with increasing pressure. This decrease is mainly attributed to pressure-induced polymer chain densification, a process that reduces free volume and restricts gas transport. Although both membranes exhibited pressure sensitivity, the performance degradation of PI(10%)-M2 was significantly less than that of PI(10%)-M0, indicating improved compressibility. Specifically, the CO2 flux of PI(10%)-M0 decreased from 116.30 GPU to 67.29 GPU, while the CO2 / nitrogen selectivity plummeted from 17.65 to 5.68. In contrast, PI(10%)-M2 maintained higher structural stability, with CO2 flux decreasing only slightly from 187.74 to 168.82 GPUs, and selectivity decreasing moderately from 49.54 to 43.07. These results indicate that optimizing methanol dosage promotes the formation of a more robust, pressure-resistant microstructure, enabling it to maintain effective free volume units and transport channels under high pressure. Overall, methanol incorporation not only improves the CO2 flux and CO2 / N2 separation performance of the ultrathin 6FDA-DAM membrane, but also significantly enhances its stability under high pressure conditions.

[0076] Depend on Figure 7It is evident that maintaining stable separation performance during long-term operation is equally crucial for industrial applications. Therefore, the membrane prepared in Example 7 was continuously monitored for 180 hours for its CO2 / N2 separation performance in mixed gases. During this period, the membrane exhibited excellent durability, with only minor changes in CO2 flux and CO2 / N2 selectivity, confirming its outstanding stability under mixed gas conditions.

[0077] Depend on Figure 10 To elucidate the role of methanol in the interfacial spreading process, the top-view diffusion kinetics of the PU casting solution at the aqueous interface were recorded using a high-speed camera at 2300 frames per second. When the PU concentration increased from 5 wt% to 7.5 wt% and 10 wt%, the spreading time monotonically increased from approximately 436 ms to approximately 547 ms and 674 ms, respectively. This trend indicates that higher polymer concentrations limit the spreading rate, mainly due to the increased solution viscosity and intensified molecular chain entanglement, which together increase the flow and deformation resistance at the interface. When methanol was introduced for regulation, increasing the methanol content at a fixed polymer concentration of 7.5 wt% produced the opposite effect; adding 100, 200, and 300 μL·g, respectively, produced the opposite effect. -1 The methanol spreading time gradually decreased to approximately 187, 124, and 96 ms. This monotonically decreasing spreading time indicates that methanol accelerates the spreading kinetics by reducing the effective interfacial tension of the casting solution at the air-water interface, resulting in a faster and more uniform overall spreading process.

[0078] Depend on Figure 11 To observe the microstructure of polyurethane films with different methanol contents, cross-sectional scanning electron microscopy (SEM) was used to characterize and analyze the evolution of film morphology under different conditions. As shown in the figure, the selective layer thickness monotonically increases with increasing initial PU solution concentration: when the PU concentrations are 5 wt%, 7.5 wt%, and 10 wt%, the corresponding selective layer thicknesses are 298, 368, and 575 nm, respectively. This result indicates that higher polymer content promotes an increase in selective layer thickness during interfacial spreading. In contrast, the introduction of methanol significantly reduces the film thickness. At a PU concentration of 7.5 wt%, adding 100 μL·g... -1 After methanol was added, the selective layer thickness decreased from 368 nm to 306 nm; when the methanol dosage was further increased to 200 μL·g -1 and 300 μL·g -1 At these temperatures, the film thickness decreased to 257 nm and 178 nm, respectively. Contact angle tests and film cross-sectional analysis showed that the introduction of methanol promoted the interfacial spreading process, resulting in a thinner selective layer and thus a thinner and denser separation layer structure.

[0079] Depend on Figure 12 To elucidate the regulatory effect of methanol on the surface structure of PU films, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to systematically characterize films prepared with different methanol addition amounts. SEM results showed that the PU film without added methanol had a smooth and dense surface. Figure 12 a) When 100 μL·g is added -1 After methanol treatment, dispersed nano-bulges appeared on the membrane surface. Figure 12 (b) indicates that methanol acts as a non-solvent to induce initial phase separation. As the methanol addition increases to 200 μL·g... -1 The bulge structure tends to be dense and evenly distributed. Figure 12 c) helps to form more ordered surface nanobumps; and when the methanol content is further increased to 300 μL·g -1 At that time, obvious collapse occurred on the membrane surface ( Figure 12 d), this may be related to the strong interference of excess methanol on solvent volatilization kinetics and polymer chain aggregation process. AFM results are consistent with SEM observations, such as... Figure 12 As shown in Figure ef, the unmodified PU film has a low surface roughness (5.70 nm) and a height variation of only 34.8 nm, indicating limited microphase separation. After the introduction of methanol, the film surface roughness gradually increases with increasing addition amount, reaching a maximum at 300 μL·g. -1 The wavelength increased significantly to 12.0 nm, and the height variation increased to 87.3 nm. This change is mainly attributed to the introduction of methanol, which altered the phase separation behavior of the polyurethane system, prompting a reconstruction of the microphase structure. The enhanced microphase separation not only improved the surface undulation of the membrane but also improved the internal structure and expanded the free volume, thus providing a more continuous and affinity-based diffusion channel for CO2 separation.

[0080] Depend on Figure 13 It can be seen that thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed to investigate the thermodynamic effect of methanol introduction on the membrane. For example... Figure 13 As shown, TGA results indicate that the polyurethane membrane exhibits good thermal stability both before and after the addition of methanol. Further DSC analysis revealed that the introduction of methanol significantly affects the glass transition temperatures of both the soft and hard segments of the polyurethane membrane. Methanol, as a non-solvent, participates in the phase inversion process and can induce controllable microphase separation in the polyurethane membrane during film formation. When the methanol addition amount is 100 or 200 μL·g... -1 At this time, local untangling and rearrangement of the soft segments are induced, enhancing the segment mobility and making the free volume distribution within the film more uniform, thus significantly reducing the glass transition temperature (Tg, soft) of the soft segments. However, when the methanol dosage is increased to 300 μL·g, -1At this point, excessive non-solvents induce excessive phase separation, resulting in significant plasticization of the soft segment regions and a further decrease in Tg and soft Tg. Simultaneously, the ordered stacking of hard segments and the hydrogen bond network are disrupted, causing a decrease in Tg and hard Tg. The free volume then increases significantly and becomes disordered, weakening the chain segment constraint and reducing the stability of the membrane structure.

[0081] Depend on Figure 14 It can be seen that permeation tests were conducted on PU ultrafilms prepared by the interfacial spreading method using pure CO2 and N2 gases at room temperature to evaluate the effect of polymer concentration on gas transport performance. Figure 14 As shown, membranes with different polyurethane concentrations (PU(5%)-M0, PU(7.5%)-M0, and PU(10%)-M0) were prepared to evaluate the effect of polymer concentration on membrane gas transport. Tests revealed that both CO2 permeability and CO2 / N2 selectivity changed significantly with the initial PU concentration. When the PU concentration increased from 5 wt% to 10 wt%, the permeation flux of both gases showed a monotonically decreasing trend. This is mainly attributed to the increased membrane thickness leading to a longer gas diffusion path and thus enhanced transport resistance as the polymer concentration increases. Among all samples, PU(7.5%)-M0 and PU(10%)-M0 exhibited similar CO2 / N2 selectivity, but PU(7.5%)-M0 had a higher CO2 permeability. Therefore, a PU concentration of 7.5 wt% was chosen to systematically study the effect of methanol addition.

[0082] Depend on Figure 15 It can be seen that at a polyurethane concentration of 7.5 wt%, the addition of 100, 200, and 300 μL·g -1 PU(7.5%)-M1, PU(7.5%)-M2, and PU(7.5%)-M3 membranes were prepared from methanol, and their separation performance for pure CO2, pure N2 gas, and a CO2 / N2 (13 / 87, volume fraction) mixture was tested. The pure gas separation performance is as follows: Figure 15 As shown in Figure a, with increasing methanol content, the permeability of both CO2 and N2 showed a continuous upward trend, indicating that methanol, as a non-solvent additive, not only promotes the interfacial spreading process but also regulates the phase separation process, increasing the interchain spacing and thus reducing diffusion resistance. In contrast, the CO2 / N2 selectivity showed a non-monotonic dependence on methanol content, reaching its peak in PU(7.5%)-M2. To further understand the separation performance of the polyurethane membrane, CO2 / N2 (13 / 87, volume fraction) mixed gas tests were conducted, such as... Figure 15 As shown in b, when the methanol dosage is 100 μL·g -1 At this point, CO2 permeability significantly increased, while N2 permeability slightly decreased, resulting in improved selectivity; when the methanol dosage increased to 200 μL·g -1At this point, the membrane performance reached its optimal level, with CO2 permeability and CO2 / N2 selectivity reaching 175.66 GPU and 42.11, respectively, representing improvements of approximately 40% and 83% compared to the PU(7.5%)-M0 membrane. However, when the methanol dosage was further increased to 300 μL·g... -1 While CO2 permeability continued to increase, selectivity decreased significantly with increasing N2 permeability, indicating that excessive structural relaxation weakened the molecular sieving effect. Notably, in mixed gas tests, the CO2 / N2 selectivity of the PU (7.5%)-M2 membrane (42.11) was significantly higher than the selectivity measured in pure gas permeation experiments (34.07), demonstrating the crucial role of competitive adsorption in the separation process. This is because CO2 molecules have stronger quadrupole interactions, enabling them to competitively and preferentially adsorb onto the membrane's micropores and active sites, thereby significantly inhibiting N2 transport through steric hindrance and spatial exclusion effects. This size sieving mechanism, combined with the competitive adsorption effect, allows the membrane to outperform its pure gas separation performance under mixed gas conditions.

[0083] Depend on Figure 16 It is known that, to systematically evaluate the long-term operational stability of the membranes, the effects of different pressures on separation performance and long-term stability tests were conducted. The PU(7.5%)-M0 and PU(7.5%)-M2 membranes were tested within the range of 1-5 bar. Figure 16 As shown in a and b, the CO2 permeability and selectivity of both membranes decreased with increasing pressure. This phenomenon is mainly due to the further densification of the polymer chains under high pressure, leading to a reduction in available free volume and thus inhibiting gas transport. Although both membranes exhibited a certain degree of pressure stability during the pressurization process, the performance degradation of the methanol-introduced PU(7.5%)-M2 membrane was significantly slower compared to the unmodified PU(7.5%)-M0 membrane. Specifically, the CO2 permeability of the PU(7.5%)-M0 membrane decreased from 125.02 GPU to 101.03 GPU, while the CO2 / N2 selectivity decreased significantly from 24.42 to 11.48; while the PU(7.5%)-M2 membrane exhibited superior pressure stability, with its CO2 permeability decreasing only slightly from 178.90 GPU to 170.48 GPU, and its CO2 / N2 selectivity decreasing only slightly from 43.30 to 34.81. The above results indicate that the introduction of an appropriate amount of methanol can effectively optimize the membrane's microstructure, enhance its structural integrity and free volume stability under high pressure conditions, thereby maintaining a continuous and efficient gas transport channel. To address the long-term stable operation requirements of membrane materials in industrial applications, the CO2 / N2 separation performance of the membrane prepared in Example 14 was continuously tested for 50 hours under mixed gas conditions. Figure 16As shown in Figure c, the CO2 permeability and CO2 / N2 selectivity of the membrane remained basically constant throughout the entire operation process, without significant attenuation, demonstrating excellent operational stability and durability.

Claims

1. A method for preparing polymer thin films for CO2 / N2 separation using an interfacial spreading method, characterized in that: Includes the following steps: S1. After drying the polymer matrix, dissolve it in an organic solvent to obtain a polymer solution; S2. Mix the polymer solution with methanol to obtain a uniformly dispersed mixed solution; S3. After the mixed solution is degassed by ultrasonic treatment, it is dropped into a horizontal glass petri dish mold pre-filled with distilled water, so that it spreads on the water surface to form a polymer film. S4. The formed polymer film is supported on a polyethersulfone substrate and dried in a vacuum oven at 100-150°C for 24-48 hours to remove organic solvents, thereby obtaining an ultra-high homogeneous polymer film.

2. The method for preparing a polymer thin film for CO2 / N2 separation using an interfacial spreading method according to claim 1, characterized in that: In S1, the polymer matrix includes polyimide, polysulfone, or polyurethane.

3. The method for preparing a polymer thin film for CO2 / N2 separation using an interfacial spreading method according to claim 2, characterized in that: The polyimide was synthesized from 10 mmol of 4,4'-(hexafluoroisopropyl)diphthalic anhydride and 10 mmol of 2,4,6-trimethyl-1,3-phenylenediamine.

4. The method for preparing a polymer thin film for CO2 / N2 separation using an interfacial spreading method according to claim 1, characterized in that: In S1, the organic solvent includes one of N-methylpyrrolidone, N,N-dimethylformamide, and dichloromethane.

5. The method for preparing a polymer thin film for CO2 / N2 separation using an interfacial spreading method according to claim 1, characterized in that: In S1, the polymer solution has a mass fraction of 5-15 wt%.

6. The method for preparing a polymer thin film for CO2 / N2 separation using an interfacial spreading method according to claim 1, characterized in that: In S2, the mass of the polymer solution is 1g.

7. The method for preparing a polymer thin film for CO2 / N2 separation using an interfacial spreading method according to claim 1, characterized in that: In S2, the volume of methanol is 100-300 μL.

8. The method for preparing a polymer thin film for CO2 / N2 separation using an interfacial spreading method according to claim 1, characterized in that: In S2, the methanol is added in small amounts multiple times.

9. A method for preparing a polymer thin film for CO2 / N2 separation using an interfacial spreading method according to claim 1, characterized in that: In S2, the mixing method is ultrasonication for 240-320 minutes or magnetic stirring at a speed of 900-1000 r / min for 30 minutes.

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