Preparation method of poly (oxindole biphenyl) microfiltration membrane, microfiltration membrane and application of microfiltration membrane in bacterial separation

Abstract

The invention discloses a preparation method of a poly (oxindole biphenyl) microfiltration membrane, the microfiltration membrane and application of the microfiltration membrane in bacterial separation. The preparation method comprises the following steps: dissolving a poly (oxindole biphenyl) polymer in an aprotic solvent to obtain a homogeneous solution, adding the aprotic solvent into the homogeneous solution, and adding a plasticizer to obtain a membrane casting solution; uniformly casting the membrane casting solution on a substrate on a casting machine to obtain a membrane casting sheet; the casting membrane is placed in a gas phase environment for phase separation, and the relative humidity of the gas phase environment is controlled to range from 40% to 100%; the temperature is controlled to be more than 20 DEG C and less than 60 DEG C, and the retention time is 0-10 minutes; and after phase inversion is completed, immersing the casting membrane sheet into a non-solvent coagulating bath for coagulating, and washing and drying to obtain the poly (oxindole biphenyl) microfiltration membrane. The method has the advantages that repeated sterilization circulation can be tolerated, the stability of a pore structure is kept, and the intrinsic anti-pollution characteristic is achieved.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and in particular to a method for preparing a poly(hydroxyindole biphenyl) microfiltration membrane, the microfiltration membrane, and its application in bacterial isolation. Background Technology

[0002] Membrane separation technology, due to its high efficiency, ease of operation, low energy consumption, and environmental compatibility, is widely used in water resource management, environmental remediation, food processing, and biomedicine. Among these, polyethersulfone (PES) microfiltration membranes, with their excellent chemical resistance and mechanical strength (tensile strength approaching 70 MPa), have become the benchmark material for bacterial separation. However, this technology faces a critical challenge of continuously declining separation efficiency during operation, leading to a significant reduction in membrane lifespan. This performance degradation stems from two synergistic mechanisms: first, thermal degradation during sterilization—the heat energy during 121°C autoclaving disrupts the metastable chain stacking of the polymer matrix, accelerating physical aging and causing irreversible pore structure collapse and morphological changes, ultimately resulting in a continuous decrease in flux and loss of separation selectivity; second, irreversible biofouling—the persistent adhesion of proteins, bacteria, and organic pollutants to the membrane surface. Pollutant accumulation not only reduces separation efficiency but also significantly increases operating costs due to frequent chemical cleaning and premature membrane module replacement. The combined effect of thermal and chemical degradation pathways constitutes a fundamental obstacle to the sustainable development of membrane applications. Therefore, developing long-life bacterial separation membranes that can withstand repeated sterilization cycles, maintain pore structure stability, and possess intrinsic anti-fouling properties has become an urgent task for materials innovation. Summary of the Invention

[0003] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is: how to provide a method for preparing a poly(hydroxyindole biphenyl) microfiltration membrane that can withstand repeated sterilization cycles, maintain pore structure stability and have intrinsic antifouling properties, as well as the microfiltration membrane and its application in bacterial isolation.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] A method for preparing a poly(hydroxyindole biphenyl) microfiltration membrane includes the following steps:

[0006] S1. Preparation of casting solution: Dissolve poly(hydroxyindole biphenyl) polymer in an aprotic solvent to obtain a homogeneous solution, add 25wt% to 40wt% of a non-solvent and 10wt% to 25wt% of a plasticizer to the homogeneous solution to obtain the casting solution;

[0007] S2. Casting film: The casting liquid is uniformly cast onto the substrate on a casting machine to obtain a cast film sheet;

[0008] S3. Gas-phase induced phase separation: The cast film is placed in a gas-phase environment for phase separation. The relative humidity of the gas-phase environment is controlled at 40% to 100%; the temperature is controlled at above 20°C and below 60°C; and the residence time is 0 to 10 minutes.

[0009] S4. Coagulation and washing: After the phase transformation is complete, the cast membrane is immersed in a non-solvent coagulation bath for coagulation. After washing and drying, a poly(hydroxyindole biphenyl) microfiltration membrane is obtained.

[0010] Furthermore, in step S1, the aprotic solvent is one of NMP, DMAc, DMSO, and DMF.

[0011] Furthermore, in step S1, the non-solvent is triethylene glycol, and the plasticizer is dibutyl phthalate.

[0012] Furthermore, the concentration of the poly(hydroxyindole biphenyl) polymer in the homogeneous solution is 10wt% to 30wt%.

[0013] Furthermore, the mass ratio of the plasticizer to the non-solvent is 15 / 35 to 20 / 30.

[0014] Furthermore, the non-solvent coagulation bath is ultrapure water, the washing includes immersing the membrane in ultrapure water for 24 hours, and the drying method is vacuum drying at 50°C.

[0015] Furthermore, the preparation steps of the poly(hydroxyindole biphenyl) polymer are as follows: under conditions of -5 to 0°C, biphenyl and indigo are dissolved in dichloromethane, and trifluoromethanesulfonic acid is added dropwise through a funnel under controlled local temperature rise; the mixture is heated to 10°C and stirred for 2 hours to obtain the poly(hydroxyindole biphenyl) polymer.

[0016] A poly(hydroxyindolebiphenyl) microfiltration membrane, said microfiltration membrane being prepared from a poly(hydroxyindolebiphenyl) polymer according to the preparation method described above.

[0017] Furthermore, the average pore size of the microfiltration membrane is 0.2–0.25 μm, and the thickness is 85 ± 5 μm.

[0018] Application of a poly(hydroxyindolebiphenyl) microfiltration membrane prepared by the method described above in bacterial isolation or microbial removal.

[0019] In summary, the present invention has the advantages of being able to withstand repeated sterilization cycles, maintaining pore structure stability, and possessing intrinsic anti-fouling properties. Attached Figure Description

[0020] Figure 1 For the performance characterization of POBP membrane.

[0021] Figure 2 SEM images of the surface of films with different additive systems.

[0022] Figure 3 SEM images of the surface and cross-section of films with different DBP / TEG mass ratios.

[0023] Figure 4 In this context, 'a' represents the average pore size and the pure water flux.

[0024] Figure 4 In the figure, b represents the tensile strength at different DBP / TEG mass ratios.

[0025] Figure 5 SEM images under different temperature and relative humidity conditions.

[0026] Figure 6 SEM images of the top surface, bottom surface, and cross-section.

[0027] Figure 7 The pore size distributions of PES and POBP membranes are shown.

[0028] Figure 8 The flux retention rate and flux recovery rate of PES and POBP membranes after multiple sterilization cycles are given.

[0029] Figure 9 The pore size distribution of PES membrane and POBM membrane after multiple sterilization cycles.

[0030] Figure 10 The images show cross-sectional SEM images of the POBP membrane and PES membrane before and after multiple sterilization cycles.

[0031] Figure 11 The BSA adsorption capacity of each membrane is represented by different BSA concentrations.

[0032] Figure 12 Photographs of bacterial suspension filtrate cultured on agar plates before and 48 hours after incubation.

[0033] Figure 13 The image shows a SEM image of the membrane surface and an agar plate photograph 24 hours after bacterial adhesion. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to the embodiments.

[0035] Preparation of Escherichia coli suspension: Resuscitate frozen E. coli culture in LB liquid medium and incubate overnight at 37°C with shaking at 200 rpm. Take 0.5 mL of the bacterial suspension, centrifuge at 5000 × g for 5 minutes, discard the supernatant, and resuspend the bacteria in 2 mL of PBS buffer (pH 7.4). Repeat this washing step twice to thoroughly remove any culture medium residue. Serially dilute the washed bacterial suspension with PBS (10⁻⁶ ppm). 4 Up to 10 6 For each dilution, 50 μL of bacterial suspension was taken and spread in triplicate onto LB agar plates. After incubation at 37°C for 24 hours, the colony-forming units (CFU) were counted to calculate the original bacterial concentration (CFU / mL). Finally, the bacterial suspension was adjusted to 10⁻¹⁰ CFU / mL with PBS. 7 CFU / mL available for use.

[0036] Synthesis of POBP: A 100 mL three-necked flask was filled with biphenyl (1.542 g, 10.0 mmol), indigo (1.618 g, 11.0 mmol), and dichloromethane (DCM, 4 mL). The system temperature was maintained at -5 to 0 °C using a low-temperature circulating bath. Trifluoromethanesulfonic acid (TFSA, 8.9 mL, 100 mmol) was slowly added dropwise through a uniform pressure funnel, with strict control over the addition rate to prevent localized overheating (to be completed within 30 min). The reaction mixture was heated to 10 °C and stirred continuously for 2 hours. Then, under vigorous stirring, the highly viscous solution was added dropwise to 200 mL of ethanol. The precipitated polymer was filtered under vacuum through a Büchner funnel and then washed continuously with 1 M Na₂CO₃ solution and deionized water at 60 °C until a neutral pH of (7.0 ± 0.5) was reached. The mixture was then vacuum dried at 120 °C for 24 h to obtain the target white POBP polymer as a free-flowing powder.

[0037] Membrane fabrication: POBP was dissolved at a concentration of 10 wt%–30 wt% in four aprotic solvents (NMP, DMAc, DMSO, and DMF), preferably 15 wt%, and mechanically stirred at 80°C for 24 hours, followed by standing for degassing for 12 hours. 25–40 wt% TEG was systematically added to the resulting homogeneous solution as a non-solvent to induce the thermodynamic instabilities required for phase separation in subsequent steps by precisely controlling the amount of non-solvent added. Simultaneously, 10–25 wt% dibutyl phthalate (DBP) was added as a plasticizer to regulate polymer chain mobility and influence phase separation and pore formation kinetics. The prepared casting solution was uniformly coated onto an ultra-flat glass substrate using a 100 μm precision doctor blade, and then transferred to a temperature and humidity controlled chamber (temperature 20–80°C, relative humidity 40–100%) for gas-phase induced phase separation (VIPS), with a residence time of 0–10 minutes. After the phase transformation is complete, the membrane is immersed in ultrapure water for 24 hours to remove residual solvent, and finally vacuum dried at 50°C to obtain a microporous membrane with a thickness controlled at 85 ± 5 μm.

[0038] Material Characterization and Testing Methods: Preparation of Specific-Dimensional Specimens: Tensile test specimens were 20 × 5 mm, dynamic mechanical analysis (DMA) and dimensional stability test specimens were 15 × 5 × 0.2 mm (length × width × thickness), and wettability study specimens were 20 × 20 mm. Thickness was determined by averaging five random measurements using a digital micrometer (accuracy ±1 μm). Tensile testing was performed using a universal testing machine (Shimadzu) at a crosshead speed of 2 mm / min, and tensile strength and elongation at break were recorded. DMA was performed using a TA Instruments Q800 analyzer with a double cantilever clamp. Dry specimens were scanned from -50 to 500°C under nitrogen protection at a heating rate of 3°C / min. Test parameters included an oscillation frequency of 1 Hz, dynamic strain amplitude of 0.1%, and static force of 0.01 N. Storage modulus (E′), loss modulus (E″), and loss factor (tan δ) were continuously monitored.

[0039] Wettability was characterized by static contact angle testing and water absorption rate experiments: The dried film sample was placed on a contact angle measuring instrument (JC2000D1, Shanghai Zhongchen Digital Technology Equipment Co., Ltd.) platform for water droplet deposition measurement; for the water absorption rate test, the dried film was immersed in deionized water (room temperature) for 24 hours until saturation, then removed and the surface moisture was gently wiped with absorbent paper and weighed. The water absorption rate was calculated based on the mass change. All samples were vacuum dried at 50°C for 24 hours before dimensional measurement, pore structure analysis, and SEM observation. The cross-sectional morphology was observed using a scanning electron microscope (XL 30ESME): the film samples underwent liquid nitrogen embrittlement followed by gold sputtering. Based on the capillary flow principle, the pore size distribution, maximum pore size, average pore size, and porosity were determined using a capillary flow pore size analyzer (CFP-1500AE, Porous Materials Inc., USA) via wetting liquid extrusion method. The data were processed with computer assistance. Hydrogen nuclear magnetic resonance (¹H NMR) spectra were acquired using a Bruker AVII-400 spectrometer (400 MHz, Bruker GmbH, Germany): the sample was dissolved in deuterated dimethyl sulfoxide (DMSO-d6) and analyzed at room temperature with tetramethylsilane (TMS) as an internal standard.

[0040] Pure water flux measurement and thermal stability assessment were performed using a circulating steam sterilization method. Before testing, a membrane sample with an effective area of ​​3.14 cm² was equilibrated in ultrapure water (18.2 MΩ·cm) for 24 hours to remove residual components. The membrane sample was then placed in a custom-designed dead-end filter (200 mL capacity) using a silicone sealing ring to ensure leak-free operation. Nitrogen gas was introduced to pressurize the membrane at 25°C and a transmembrane pressure of 0.1 bar. The permeate mass was recorded every 5 minutes using an analytical balance (accuracy 0.001 g), and converted to volumetric flux at a water density of 1.0 g / mL. Each pressure condition was tested in triplicate after reaching steady-state flux. The pure water flux (J) is calculated using the following formula:

[0041]

[0042] In the formula, V is the permeate volume (L), S is the effective membrane area (m²), and t is the permeate collection time (h).

[0043] To assess thermal stability, hydrated membrane samples were subjected to cyclic steam sterilization (121°C, 0.15 MPa, 30 minutes per cycle). After each sterilization, the membrane samples were first placed in ultrapure water for reequilibration at room temperature for 24 hours, and then vacuum dried at 40°C for 6 hours until completely dry. Subsequently, the flux of treated pure water was measured at the same transmembrane pressure (0.1 bar), and the degree of structural degradation was quantified by calculating the flux retention rate (FRR) after each sterilization cycle.

[0044]

[0045] Where J1 represents the initial pure water flux measured before the first sterilization, J n This represents the pure water flux measured after n sterilization cycles under the same test conditions.

[0046] Protein antifouling: The prepared POBP microfiltration membranes were cut into uniform thin sheets and fixed to the bottom of 24-well cell culture plates. Bovine serum albumin (BSA) solutions with concentration gradients of 1–5 g / L were prepared using phosphate-buffered saline (PBS, pH 7.4). Before testing, POBP microfiltration membrane samples (1 × 1 cm²) were pretreated in PBS for 1 hour to equilibrate surface properties. Each membrane was then immersed in 10 mL of the corresponding concentration of BSA solution and incubated at 37°C and 100 rpm for 4 hours to simulate protein adsorption under physiological conditions. After incubation, the residual BSA concentration in the supernatant was detected at 280 nm using a UV-Vis spectrophotometer (Shimadzu UV-2600, Japan), and the BSA adsorption capacity per unit membrane area (μg / cm²) was calculated using the following formula:

[0047]

[0048] Where Q is the adsorption capacity (μg / cm²), and C0 and C2 are... e Let V represent the initial concentration and equilibrium concentration (mg / mL), respectively, V be the solution volume (mL), and A be the membrane surface area (cm²).

[0049] Bacterial Removal Test and Antimicrobial Efficacy Assessment: The bacterial retention efficiency of the prepared POBP membrane was assessed using the syringe filtration method: using the membrane sample as the filtration medium, 10 mL of Escherichia coli suspension (10 6 -10 7 The bacterial solution was filtered (CFU / mL). Before testing, all filtration devices and connections were autoclaved at 121°C, 1.2 bar for 20 minutes. Experiments were conducted at room temperature (22–24°C) in triplicate. After complete filtration by manual pressure, 100 μL of the permeate was transferred to a nutrient agar plate and spread evenly using a spreader. The plate was incubated at 37°C for 48 hours to promote the growth of residual bacteria.

[0050] To assess bacterial adhesion, 100 μL of E. coli suspension (10 μL / L) was pipetted. 7 (CFU / mL) was precisely added to each membrane surface to ensure complete coverage. After incubation at room temperature under static conditions for 24 hours, the membrane surface was gently rinsed three times with PBS buffer (pH 7.4) to remove any unadhered bacteria.

[0051] Synthesis and characterization of POBP:

[0052]

[0053] like Figure 1 As shown, Figure 1 Performance characterization of POBP membranes: (a) ¹H NMR spectrum of POBP, (b) hydrophilicity assessment of dense POBP membrane and PES membrane by water contact angle and water absorption rate, (c) dynamic mechanical analysis (DMA) of PES membrane, (d) dynamic mechanical analysis (DMA) of POBP membrane.

[0054] POBP was successfully synthesized via superacid-catalyzed polymerization. ¹H NMR spectrum ( Figure 1 The molecular structure was confirmed, with the characteristic peak at 10.85 ppm corresponding to the NH proton of the isoindole backbone, verifying the expected polymer configuration. The rigid main chain structure of this polymer was clearly confirmed by two key thermal properties: (1) an extremely high glass transition temperature (Tg>400°C). Figure 1 d); (2) No β-transition behavior was observed in the dynamic mechanical analysis. This thermal characteristic contrasts sharply with traditional polyethersulfone (PES)—PES exhibits a significant β-transition at approximately 120°C, corresponding to local molecular motion in its more compliant segments. Furthermore, POBP exhibits excellent solubility in common organic solvents such as dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc). Compared to the original PES membrane, the solution-cast POBP membrane has superior hydrophilicity, directly reflected in a lower dynamic contact angle (77.73° vs. 87.99° for PES). Figure 1 (b) and significantly higher water absorption (6% vs 0%). A lower contact angle implies stronger hydrophilicity, thus promoting greater water absorption. This combination of high thermal stability and excellent hydrophilicity makes POBP a highly promising candidate material in the field of bacterial separation membranes.

[0055] Preparation and characterization of POBP porous membranes: The preparation of phase-separated membranes involves a polymer-solvent-nonsolvent ternary system, where the solvent-polymer interaction and solvent / nonsolvent compatibility jointly determine the thermodynamic stability of the casting solution. To evaluate the solubility of four organic solvents (DMSO, DMF, DMAc, and NMP) in POBP and the stability of the corresponding casting solutions, this example uses intrinsic viscosity testing and cloud point titration for analysis. As shown in Table 1:

[0056] Table 1. Effect of solvent type on casting solution viscosity and cloud point time

[0057]

[0058] The results show that the solvent dissolution ability order is: DMSO < DMF < DMAc < NMP. Among them, NMP shows the strongest dissolution ability for POBP, and the prepared casting solution has the best thermodynamic stability, which is crucial for achieving precise control of the membrane structure.

[0059] Further comparative analysis of the casting solutions containing DMF, DMAc, and NMP (Table 1) shows that the viscosity order is NMP > DMAc > DMF > DMSO; the cloud point time order during the gas-phase induced phase separation process is: NMP > DMAc > DMF > DMSO. The higher viscosity of the NMP system enhances the solution kinetic stability and delays the phase separation process; correspondingly, the longer cloud point time confirms that the NMP-based solution undergoes phase separation at the slowest rate during the VIPS process, which is consistent with the viscosity change law. This delayed phase separation behavior helps to effectively regulate the membrane morphology uniformity. Therefore, in this example, NMP is selected as the optimal solvent for preparing POBP membranes by the VIPS method, which achieves a good balance between thermodynamic stability and controllable phase separation kinetics.

[0060] Figure 2 SEM images of the surface of membranes with different additive systems: (a1) top surface of pure TEG, (b1) top surface of TEG / DBP, (c1) top surface of TEG / PVP, (d1) top surface of TEG / PEG; (a2) bottom surface of pure TEG, (b2) bottom surface of TEG / DBP, (c2) bottom surface of TEG / PVP, (d2) bottom surface of TEG / PEG.

[0061] In this example, poly(oxyindole biphenyl) (POBP) is selected as the membrane-making substrate, and its advantages are a super-high glass transition temperature (Tg > 400 °C) and no β-transition peak detected in dynamic mechanical analysis, as Figure 2 shown in b. More importantly, the POBP main chain is rich in oxyindole groups and has intrinsic persistent hydrophilicity without post-modification. This unique combination of thermal stability and hydrophilicity not only ensures the structural integrity of the membrane during repeated high-temperature sterilization cycles but also meets the stringent requirements of anti-fouling performance. In addition, to explore the structure-property relationship and optimize the membrane performance, in this example, POBP microfiltration membranes are prepared by the gas-phase induced phase separation method, and the influence laws of parameters such as non-solvent type and concentration, pore-forming agent formulation, etc. on the membrane structure characteristics are systematically studied.

[0062] Triethylene glycol (TEG), as a hydrophilic non-solvent, accelerates liquid-liquid phase separation through a solvent / non-solvent exchange mechanism, thereby effectively improving the overall porosity of the membrane. During gas-phase induced phase separation, solvent evaporation and non-solvent infiltration jointly trigger phase separation in the polymer solution. The top surface, directly exposed to air, easily forms an open-pore structure due to rapid solvent evaporation and free non-solvent infiltration; while the bottom surface, due to contact with the glass substrate, suffers severe mass transfer restriction, hindering both solvent evaporation and non-solvent diffusion, resulting in a significant delay in bottom phase separation kinetics, ultimately forming a dense layer. Figure 2 a).

[0063] While introducing hydrophilic additives such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) can alter the thermodynamic properties of the system, they cannot overcome the kinetic constraints imposed by the glass substrate on the bottom, even though they promote pore formation on the top free surface. Even with increased hydrophilicity, limited solvent / non-solvent exchange still hinders effective phase separation at the glass interface. Furthermore, PVP and PEG may increase solution viscosity, both impeding downward solvent diffusion and accelerating the top phase separation process. This premature "solidification" of the top structure further reduces the solvent supply to the bottom, exacerbating the formation of a dense layer. Their hydrophilic properties may also cause them to preferentially accumulate on the top surface, amplifying the differences between the two surfaces.

[0064] Conversely, the hydrophobic plasticizer dibutyl phthalate (DBP) directly addresses the core kinetics issue: by weakening inter-chain interactions and enhancing chain mobility, DBP can delay the onset of phase separation (especially at the top surface), extending the time window for solvent redistribution. Simultaneously, it may reduce the overall viscosity of the casting solution, promoting downward solvent migration. Solvent accumulation in the bottom region further delays the local phase separation process. When the non-solvent finally overcomes the mass transfer barrier during this extended period, the lower local viscosity and higher solvent content environment induce more intense phase separation at the bottom, resulting in an open-pore structure. DBP's small molecular size and hydrophobic properties also facilitate its migration to the bottom; this "top-suppressing, bottom-promoting" mechanism precisely counteracts surface differences, rather than exacerbating the phenomenon as hydrophilic additives might.

[0065] Figure 3SEM images of the surface and cross-section of films with different DBP / TEG mass ratios: (a1) Top surface of 10 / 40 mass ratio film, (b1) Top surface of 15 / 35 mass ratio film, (c1) Top surface of 20 / 30 mass ratio film, (d1) Top surface of 25 / 25 mass ratio film; (a2) Bottom surface of 10 / 40 mass ratio film, (b2) Bottom surface of 15 / 35 mass ratio film, (c2) Bottom surface of 20 / 30 mass ratio film, (d2) Bottom surface of 25 / 25 mass ratio film; (a3) ​​Cross-sectional morphology of 10 / 40 mass ratio film, (b3) Cross-sectional morphology of 15 / 35 mass ratio film, (c3) Cross-sectional morphology of 20 / 30 mass ratio film, (d3) Cross-sectional morphology of 25 / 25 mass ratio film.

[0066] Figure 4 a represents the average pore size and pure water flux; Figure 4 b represents the tensile strength at different DBP / TEG mass ratios.

[0067] This embodiment systematically studied the effect of pore-forming agent (DBP / TEG) content on the structure and performance of POBP membranes: casting solutions containing 10% PES, 40% NMP, and different DBP / TEG mass ratios were placed in a steam environment (25°C, 40% RH) for 1 minute (VIPS process), and then immersed in a pure water coagulation bath. The results showed that the membrane pore size decreased with decreasing DBP / TEG mass ratio. When the DBP / TEG ratio was 25 / 25, the POBP membrane exhibited a relatively dense top surface, a large pore bottom surface, and a macroporous cross-section. This morphology stemmed from a rapid phase transformation process—water vapor-induced surface solidification hindered further permeation, and insufficient internal steam action led to rapid coagulation in water, forming macroporous structures, resulting in poor rejection rate and mechanical strength.

[0068] In contrast, a DBP / TEG ratio below 20 / 30 forms a sponge-like cross-section with high pore connectivity, which is beneficial for achieving high porosity and excellent filtration performance, making it suitable for aqueous microfiltration; however, an excessively low ratio (10 / 40) inhibits pore formation on the bottom surface. A comprehensive analysis of average pore size, pure water flux, and tensile strength (…) Figure 4 This study confirmed that increasing the DBP / TEG ratio gradually increases the average pore size, and the corresponding increase in water flux reflects this structural change. However, the high flux at a high DBP / TEG ratio stems from the macroscopic pore structure, which not only provides a non-uniform permeation path but also, according to tensile strength tests, has mechanical strength that is insufficient for practical applications. Therefore, a DBP / TEG mass ratio of 15 / 35 or higher was ultimately selected as the optimized casting solution formulation.

[0069] Figure 5 In the image, a to d are SEM images taken at different temperatures. Figure 5In the image, e to h represent SEM images under relative humidity conditions.

[0070] Besides formulation parameters, VIPS process conditions have a decisive influence on membrane structure. Using a fixed casting solution (10 wt% PES, 40 wt% NMP, and a total TEG / DBP content of 50 wt% at a mass ratio of 15 / 35), this example investigated the effect of VIPS conditions on the structure and performance of the PES membrane. When the relative humidity was below 50%, a completely dense skin formed on the surface of the resulting PES membrane, leading to a loss of permeability. SEM characterization results (…) Figure 6 This confirmed the formation of a dense surface layer under 30% RH conditions. This is because the low humidity environment provides insufficient vapor partial pressure (driving force) to induce surface gelation or phase separation; instead, the gradual evaporation of the solvent into the air becomes dominant, thus hindering the formation of the pore structure. Therefore, for this formulation system, maintaining a relative humidity above 50% is crucial for obtaining the ideal bicontinuous pore structure.

[0071] Furthermore, at a constant relative humidity (90%), changing the vapor temperature significantly affects the membrane morphology. SEM results ( Figure 5 The results show that higher temperatures lead to significant changes in the vapor contact surface: membranes formed at 20°C exhibit a highly porous, open surface structure, while membranes treated at 60°C show gel aggregation, resulting in a significant reduction in surface porosity. This temperature dependence stems from two mechanisms: firstly, increased temperature reduces surface viscosity, promoting polymer chain aggregation during phase separation; secondly, rising temperature weakens the system's solvation capacity, enhances thermodynamic instability, and accelerates solvent-nonsolvent exchange rates. The synergistic effect of these effects crucially alters the phase separation kinetics, ultimately determining the membrane's surface morphology.

[0072] Figure 6 In the image, a1, b1, and c1 are SEM images of the top surface, bottom surface, and cross-section of the POBP membrane, respectively. Figure 6 In the image, a2, b2, and c2 are SEM images of the top surface, bottom surface, and cross-section of the PES membrane, respectively.

[0073] Figure 7 In Figure 1, a is a statistical diagram of the pore size distribution of the PES membrane, and b is a statistical diagram of the pore size distribution of the POBP membrane.

[0074] In this embodiment, a microfiltration membrane was successfully prepared using gas-induced phase separation (VIPS) technology at 50% relative humidity and 20°C. The POBP membrane optimized by this method has an average pore size of 0.21 μm. Contact angle testing confirmed that this unique structural feature directly enhances the membrane's hydrophilicity, thus laying the structural foundation for its anti-biofouling performance in subsequent bacterial isolation tests.

[0075] Thermal stability assessment: A systematic comparison of POBP and conventional PES microfiltration membranes before and after steam sterilization at 121°C confirmed that POBP exhibits superior structural and functional stability under high-temperature sterilization conditions. This stability stems from POBP's unique molecular design: its high glass transition temperature (Tg>400°C) contrasts sharply with its lack of β-transition. In contrast, PES membranes, with a Tg (approximately 220°C) close to the sterilization temperature and exhibiting a significant β-transition, show pore structure collapse after sterilization. Figure 10 The high Tg and rigid molecular chains of POBP effectively suppress chain segment movement, avoiding observable changes in pore structure.

[0076] DMA data directly revealed the underlying mechanism: at a sterilization temperature of 121°C, the PES membrane was in the β-transition range (as evidenced by the broadening of the tanδ peak), leading to a sharp 35% drop in its storage modulus. This rapid decrease in modulus spurred segment migration and plastic deformation. Conversely, the POBP membrane maintained >91% of its modulus and showed no β-relaxation peak, confirming that it remained in a rigid glassy state throughout the sterilization process. After three sterilization cycles, the POBP membrane's pure water flux reduction rate (<12%) was significantly lower than that of the PES membrane (flux loss of approximately 29%), highlighting its potential for extended service life.

[0077] Figure 8 In the figure, a represents the flux retention rate of PES and POBP membranes after multiple sterilization cycles; b represents the flux recovery rate (FRR%) of PES and POBP membranes after multiple sterilization cycles.

[0078] Figure 9 Figure a shows the pore size distribution of the PES membrane after multiple sterilization cycles; Figure b shows the pore size distribution of the POBM membrane after multiple sterilization cycles.

[0079] Figure 10 In the image, a1 and a2 are cross-sectional SEM images of the POBP membrane before and after multiple sterilization cycles; b1 and b2 are cross-sectional SEM images of the PES membrane before and after multiple sterilization cycles.

[0080] Antifouling properties of POBP membrane: Figure 11 To assess the BSA adsorption capacity of each membrane at different BSA concentrations, membrane performance was evaluated using a static adsorption fouling experiment: samples were statically incubated in bovine serum albumin (BSA) solutions (1 / 3 / 5 g / L, pH 7.4) for 3 hours to evaluate protein adsorption tendency. The BSA content bound to the membrane surface was determined by gravimetric analysis. Consistent with the inherent hydrophilicity of the POBP polymer backbone, the POBP membrane exhibited a significantly reduced BSA adsorption capacity compared to the hydrophobic PES microfiltration membrane. This correlation clearly confirms that enhanced surface hydrophilicity can directly reduce fouling tendency by weakening the hydrophobic interaction between proteins and the membrane.

[0081] Sterilization performance: Figure 12 a1, b1, and c1 are photographs of agar plates cultured from bacterial suspension filtrate before culture. Figure 12 Photographs a2, b2, and c2 show agar plates of bacterial suspension filtrate after 48 hours of incubation. In this example, the initial concentration was 10... 6 -10 7 A bacterial suspension of CFU / mL was prepared and the filtrate was cultured on agar plates. The bacterial retention performance of the prepared POBP membrane and the commercial PES microfiltration membrane was systematically evaluated. The presence or absence of bacterial colonies on the agar plates after a specific culture period reflects the bacterial retention capacity of the respective membranes. Figure 3 Photographs of different membrane samples used for bacterial rejection testing and a control (+) sample (the control sample was a feed solution containing 100% of the original bacterial concentration) are shown. No colonies were observed on the culture plates of either the POBP or PES membranes, indicating that both membrane filters can achieve complete bacterial rejection. This phenomenon is directly related to the morphology and pore size characteristics of the membrane samples: both the POBP and PES membranes, which have 100% bacterial rejection, possess a surface structure with uniform pore size distribution and smaller pore sizes.

[0082] Further static adsorption experiments using E. coli solution were conducted. Figure 13 The biofouling resistance of the membranes was investigated. Results showed that a large number of *E. coli* bacteria adhered to the surface of the original PES membrane, and the bacteria gradually aggregated into colonies, indicating that it had entered the initial stage of biofilm formation, fully demonstrating the poor biofouling resistance of the PES membrane. In contrast, no significant bacterial adhesion was observed on the surface of the POBP membrane, indicating its excellent resistance to bacterial adhesion. This enhanced biofouling resistance is mainly attributed to the increased hydrophilicity of the membrane surface.

[0083] Concurrent bacterial adhesion and fouling tests showed that although the initial water flux of POBP and PES membranes were similar (28,700 mL / m²·h and 28,800 mL / m²·h, respectively), the POBP membrane exhibited stronger antibacterial adhesion capabilities due to its significantly enhanced surface hydrophilicity. This advantage was further validated in the post-fouling flux recovery test: the flux recovery value of the POBP membrane reached 21,065 mL / m²·h, significantly higher than the 17,280 mL / m²·h of the PES membrane. Therefore, the flux retention rate of the POBP membrane reached 73.4%, significantly better than the 60.1% of the PES membrane. These findings collectively confirm that the enhanced hydrophilicity of the POBP membrane effectively improves its antibacterial adhesion capability, thereby ensuring more stable flux performance and superior antifouling characteristics during long-term operation. Figure 13 As shown in Table 2.

[0084] Table 2 Water flux retention rate after bacterial adsorption

[0085]

[0086] In summary, based on the combined results of this embodiment, the novel polyoxyindole-biphenyl (POBP) microfiltration membrane demonstrates great potential as a next-generation sustainable bacterial separation material. Its excellent intrinsic thermal stability (Tg > 400°C, no β-transition) ensures structural integrity during repeated high-temperature sterilization, effectively preventing irreversible pore structure collapse and achieving a stable high flux recovery rate (FRR > 95%). Simultaneously, the abundant oxyindole groups in the POBP framework endow it with permanent hydrophilicity (water contact angle < 50°), significantly inhibiting the adsorption of biomolecules such as proteins, mitigating biofouling at its source, and overcoming the core limitations of traditional hydrophobic polyethersulfone (PES) membranes. It should be noted that although the POBP membrane did not show a significant improvement in antibacterial adhesion, this is mainly attributed to the rough surface morphology formed during the phase inversion membrane fabrication process, rather than the chemical properties of the material itself. These results indicate that by further optimizing the membrane fabrication process to control the surface topology, the antifouling performance of the POBP membrane still has significant room for improvement.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a poly(hydroxyindole biphenyl) microfiltration membrane, characterized in that, Includes the following steps: S1. Preparation of casting solution: Dissolve poly(hydroxyindole biphenyl) polymer in an aprotic solvent to obtain a homogeneous solution, add 25wt% to 40wt% of a non-solvent and 10wt% to 25wt% of a plasticizer to the homogeneous solution to obtain the casting solution; S2. Casting film: The casting liquid is uniformly cast onto the substrate on a casting machine to obtain a cast film sheet; S3. Gas-phase induced phase separation: The cast film is placed in a gas-phase environment for phase separation. The relative humidity of the gas-phase environment is controlled at 40% to 100%; the temperature is controlled at above 20°C and below 60°C; and the residence time is 0 to 10 minutes. S4. Coagulation and washing: After the phase transformation is complete, the cast membrane is immersed in a non-solvent coagulation bath for coagulation. After washing and drying, a poly(hydroxyindole biphenyl) microfiltration membrane is obtained.

2. The method for preparing the poly(hydroxyindole biphenyl) microfiltration membrane as described in claim 1, characterized in that, In step S1, the aprotic solvent is one of NMP, DMAc, DMSO, and DMF.

3. The method for preparing the poly(hydroxyindole biphenyl) microfiltration membrane as described in claim 1, characterized in that, In step S1, the non-solvent is triethylene glycol, and the plasticizer is dibutyl phthalate.

4. The method for preparing the poly(hydroxyindolebiphenyl) microfiltration membrane according to any one of claims 1 to 3, characterized in that, In the homogeneous solution, the concentration of poly(hydroxyindole biphenyl) polymer is 10wt% to 30wt%.

5. The method for preparing the poly(hydroxyindolebiphenyl) microfiltration membrane as described in claim 3, characterized in that, The mass ratio of the plasticizer to the non-solvent is 15 / 35 to 20 / 30.

6. The method for preparing the poly(hydroxyindole biphenyl) microfiltration membrane as described in claim 1, characterized in that, The non-solvent coagulation bath is ultrapure water, the washing includes immersing the membrane in ultrapure water for 24 hours, and the drying method is vacuum drying at 50°C.

7. The method for preparing the poly(hydroxyindole biphenyl) microfiltration membrane as described in claim 1, characterized in that, The preparation steps of the poly(hydroxyindole biphenyl) polymer are as follows: under conditions of -5 to 0°C, biphenyl and indigo are dissolved in dichloromethane, and trifluoromethanesulfonic acid is added dropwise through a funnel under controlled local temperature rise; the mixture is heated to 10°C and stirred for 2 hours to obtain the poly(hydroxyindole biphenyl) polymer.

8. A poly(hydroxyindole biphenyl) microfiltration membrane, characterized in that, The microfiltration membrane is prepared from poly(hydroxyindole biphenyl) polymer according to any one of claims 1 to 7.

9. The poly(hydroxyindole) microfiltration membrane as described in claim 8, characterized in that, The microfiltration membrane has an average pore size of 0.2–0.25 μm and a thickness of 85 ± 5 μm.

10. The application of a poly(hydroxyindolebiphenyl) microfiltration membrane prepared by any one of claims 1 to 7 in bacterial isolation or microbial removal.

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