Polysulfone Ultrafiltration Membrane: Advanced Engineering, Performance Optimization, And Industrial Applications

Molecular Composition And Structural Characteristics Of Polysulfone Ultrafiltration Membrane

Polysulfone (PSf) is an amorphous thermoplastic polymer characterized by repeating aryl-SO₂-aryl units in its backbone, conferring outstanding thermal stability (glass transition temperature Tg ≈ 185°C) and resistance to hydrolysis, oxidation, and a broad pH spectrum (1–12)19. The polymer's aromatic ether sulfone structure provides rigidity and mechanical strength, with tensile modulus typically exceeding 2.5 GPa5. However, the hydrophobic nature of unmodified polysulfone—stemming from its aromatic rings and lack of polar functional groups—results in water contact angles often above 70°, limiting wettability and promoting fouling in aqueous applications9,15.

To address hydrophobicity, researchers have developed modified polysulfone polymers substituted on phenyl rings with functional groups such as -CO-R₁ (carboxyl or ester derivatives), -CON(R₂)R₃ (amide linkages), -B(OR₂)₂ (boronate esters), -P(=O)(OR₂)₂ (phosphonate groups), and crosslinking moieties -CO-O-R₄-O-CO-1. Sulfonation—introducing -SO₃H groups—has emerged as a particularly effective strategy: sulfonated polyphenylene sulfone (sPPSU) membranes exhibit enhanced hydrophilicity, reduced protein adsorption, and improved flux without sacrificing chemical stability2,17. The degree of sulfonation (typically 0.5–8 wt% sulfonated polysulfone in blends) must be carefully controlled to balance hydrophilicity with mechanical integrity and solvent resistance17.

Polysulfone's solubility in polar aprotic solvents—dimethylformamide (DMF), dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), N-methylpyrrolidone (NMP), and γ-butyrolactone (GBL)—facilitates membrane fabrication via phase inversion3,4,5. Recent innovations include the use of γ-butyrolactone as a greener solvent alternative, reducing reliance on high-boiling, toxic solvents like NMP while maintaining membrane performance3,4. The choice of solvent, along with additives such as polyethylene glycol (PEG) or polyvinylpyrrolidone (PVP), critically influences the kinetics of phase separation, pore structure, and ultimate membrane morphology8,13.

Phase Inversion Process And Membrane Fabrication Techniques For Polysulfone Ultrafiltration Membrane

The predominant method for producing polysulfone ultrafiltration membranes is non-solvent induced phase separation (NIPS), also known as immersion precipitation4,8. In this process, a homogeneous polymer solution (dope) is cast onto a support or extruded through a spinneret, then immersed in a coagulation bath containing a non-solvent (typically water or aqueous mixtures). Rapid solvent-non-solvent exchange induces thermodynamic instability, causing the polymer-rich phase to solidify into a porous membrane structure while the polymer-lean phase forms the pore network4.

Key fabrication parameters include:

• Polymer concentration: Polysulfone dope solutions typically contain 15–25 wt% polymer. For example, a composition of 19% PSf, 19% PEG, and 62% DMAc has been optimized for turbid water treatment, yielding membranes with controlled pore size and high flux8. Higher polymer concentrations generally produce denser membranes with lower permeability but enhanced selectivity.
• Additive selection: Pore-forming agents such as PEG (molecular weight 200–20,000 Da) or PVP (molecular weight 10,000–360,000 Da) are incorporated to increase porosity and hydrophilicity8,13. PVP, in particular, is widely used in polysulfone/polyethersulfone membranes to improve water wettability and reduce protein fouling7,11. The additive concentration (typically 5–20 wt%) and molecular weight must be optimized to balance pore size, mechanical strength, and flux.
• Solvent evaporation time: A controlled evaporation step (0–10 minutes) prior to immersion allows partial solvent loss from the dope surface, influding the formation of a dense selective skin layer. For instance, a 5-minute evaporation at ambient conditions followed by coagulation at 80°C has been reported to yield polysulfone membranes with optimal turbidity rejection8.
• Coagulation bath composition and temperature: Water is the most common non-solvent, but bath temperature significantly affects phase separation kinetics. Elevated coagulation temperatures (e.g., 80°C) accelerate solvent-non-solvent exchange, promoting the formation of finger-like macrovoids and higher flux membranes8. Conversely, lower temperatures favor sponge-like structures with finer pores and higher selectivity4.
• Post-treatment (annealing): Thermal annealing (e.g., 50–80°C for 30 minutes) or autoclaving in boiling water/steam can stabilize the membrane structure, leach residual solvent and additives, and further enhance hydrophilicity8,11. Autoclaving has been shown to improve throughput (>1500 L/m²) and virus retention in polysulfone composite membranes rendered hydrophilic with hydroxyalkyl cellulose11.

For hollow fiber membranes, the dope solution is extruded through an annular spinneret while a bore fluid (center fluid) is simultaneously injected through the inner orifice to maintain the hollow geometry10,13,18. The composition of the bore fluid—typically a mixture of solvent and non-solvent (e.g., 40–50 wt% NMP and 50–60 wt% water)—critically determines the inner surface morphology and pore size13. Optimized bore fluid formulations enable the production of low molecular weight cut-off (MWCO) hollow fibers (e.g., 5–10 kDa) with high hydraulic permeability and pressure stability13.

Recent advances include the development of bicontinuous highly interconnected porous structures via spinodal decomposition-dominated phase separation, achieved by precise control of solvent/non-solvent ratios and polymer concentration. Such membranes exhibit water flux up to 500 LMH (liters per square meter per hour) for polyacrylonitrile-based ultrafiltration membranes, with polysulfone-based nanofiltration supports achieving flux >100 LMH12.

Performance Characteristics And Quantitative Metrics Of Polysulfone Ultrafiltration Membrane

The performance of polysulfone ultrafiltration membranes is evaluated through several key metrics:

• Water flux (permeability): Expressed in LMH or L/(m²·h·bar), flux quantifies the volumetric flow rate of permeate per unit membrane area and transmembrane pressure. High-flux polysulfone hollow fiber membranes prepared from poly(acrylonitrile-co-methacrylic acid) and polysulfone blends achieve fluxes of 25–200 L/(m²·h) under typical operating pressures10,18. Flat-sheet polysulfone membranes with pronounced asymmetry exhibit transmembrane flows ≥0.5 mL/(cm²·min·bar), equivalent to ≥300 LMH at 1 bar15. Sulfonated polyphenylene sulfone membranes demonstrate comparable or superior flux due to enhanced hydrophilicity2.
• Molecular weight cut-off (MWCO): MWCO defines the nominal molecular weight (in Daltons) of solutes retained at 90% rejection. Polysulfone ultrafiltration membranes span a wide MWCO range from 5 kDa to 300 kDa, depending on pore size and skin layer thickness13,14. Low cut-off membranes (5–10 kDa) are critical for hemodialysis and protein separation, requiring precise control of phase inversion kinetics to achieve sieving coefficients for albumin (66.5 kDa) below 0.01 in whole blood13.
• Rejection efficiency: Quantified as the percentage of solute retained by the membrane, rejection efficiency for polysulfone ultrafiltration membranes exceeds 99% for bacteria (>0.2 μm), viruses (20–300 nm), and colloidal particles10,18. For example, hollow fiber membranes with active layer pore sizes in the ultrafiltration range effectively reject pathogens while delivering biologically pure water at 150–300 mL/min from a 3-meter overhead tank10,18.
• Mechanical strength: Breaking strength (tensile strength at failure) for polysulfone hollow fibers typically exceeds 300 cN/mm² (≈30 MPa), ensuring durability under operational pressures and backwashing cycles15. Burst pressure—the maximum pressure before membrane rupture—often surpasses 5 bar for well-optimized hollow fibers13.
• Chemical and thermal stability: Polysulfone membranes withstand continuous exposure to pH 1–12, chlorine (up to 200 ppm for short durations), and temperatures up to 85°C in aqueous environments, with upper limits of 150–170°C in dry conditions5,19. This stability enables aggressive cleaning protocols (caustic, acidic, or enzymatic) to restore flux after fouling.
• Fouling resistance: Hydrophilic modifications (sulfonation, PVP blending, surface grafting) significantly reduce fouling by proteins, humic acids, and other organic matter. Surface-modified blend membranes with acid groups exhibit smaller pore sizes and surface charge, further improving separation efficiency and fouling resistance10,18.

Quantitative data from recent patents illustrate these metrics: polysulfone-based composite nanofiltration supports achieve water flux volume equivalents ≥2000 (dimensionless metric normalizing flux by membrane thickness and porosity) and surface equivalents ≥512. For forward osmosis applications, polysulfone supports yield water flux ≥10 LMH (active layer facing feed solution, AL-FS) or ≥20 LMH (active layer facing draw solution, AL-DS)12.

Surface Modification Strategies To Enhance Hydrophilicity And Antifouling Properties Of Polysulfone Ultrafiltration Membrane

Addressing the inherent hydrophobicity of polysulfone is paramount for aqueous applications. Several surface modification strategies have been developed:

• Blending with hydrophilic polymers: Incorporating polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), or hydroxyalkyl cellulose into the polysulfone dope solution is the most common approach7,11,13. PVP (5–20 wt%) migrates to the membrane surface during phase inversion, forming a hydrophilic coating that reduces water contact angle and protein adsorption7. However, PVP leaching over time can compromise long-term performance; crosslinking or using high-molecular-weight PVP (>360 kDa) mitigates this issue13. Hydroxyalkyl cellulose-modified polysulfone composite membranes, after autoclaving, achieve throughput >1500 L/m² and effective virus removal from protein solutions11.
• Sulfonation: Chemical sulfonation introduces -SO₃H groups onto polysulfone phenyl rings, yielding sulfonated polysulfone (sPSf) or sulfonated polyphenylene sulfone (sPPSU)2,17. Membranes containing 0.5–8 wt% sPSf blended with unmodified polysulfone exhibit enhanced hydrophilicity, reduced fouling, and maintained mechanical strength17. Sulfonated polyarylethersulfone matrices have been employed in hemodialysis membranes with improved biocompatibility17.
• Surface grafting and coating: Post-fabrication grafting of hydrophilic monomers (e.g., acrylic acid, methacrylic acid) or coating with zwitterionic polymers (e.g., poly(sulfobetaine methacrylate)) imparts antifouling properties without altering bulk membrane structure10,18. Surface-modified blend membranes with acid groups (from poly(acrylonitrile-co-methacrylic acid)) demonstrate smaller pore sizes, surface charge, and superior separation efficiency10,18.
• Incorporation of nanoadsorbents: Reverse-filling membrane pores with nanoadsorbents (e.g., functionalized nanoparticles, colloidal gold) followed by sealing with organic polymers creates multifunctional ultrafiltration membranes capable of simultaneous removal of viruses, macromolecular organics, and heavy metal ions (e.g., Pb²⁺) under low pressure16. This approach leverages the high porosity and low mass transfer resistance of polysulfone matrices while adding adsorptive functionality16.
• Nonionic surfactant treatment: Post-treatment with nonionic surfactants improves wettability and reduces protein adsorption, particularly in hemodialysis membranes13.

Each strategy involves trade-offs: blending is simple and scalable but may suffer from additive leaching; sulfonation provides permanent hydrophilicity but requires careful control to avoid excessive swelling or loss of mechanical strength; grafting offers precise surface chemistry but adds processing complexity.

Applications Of Polysulfone Ultrafiltration Membrane Across Diverse Industries

Municipal And Industrial Water Treatment

Polysulfone ultrafiltration membranes are extensively deployed in municipal water treatment plants for removal of suspended solids, turbidity, bacteria, viruses, and protozoa (e.g., Cryptosporidium, Giardia)8,10,18. Membranes with MWCO 100–300 kDa and pore sizes 20–100 nm achieve >4-log (99.99%) removal of pathogens, meeting stringent drinking water standards (e.g., US EPA Surface Water Treatment Rule, EU Drinking Water Directive)18. High-flux hollow fiber modules (25–200 L/(m²·h)) enable compact plant footprints and reduced energy consumption compared to conventional sand filtration and chlorination10,18.

Point-of-use (POU) water filtration units based on polysulfone hollow fibers provide decentralized water purification in resource-limited settings. A simple, compact device requiring no electricity—operating via gravity from a 3-meter overhead tank—delivers 150–300 mL/min of biologically pure water, addressing waterborne disease challenges in developing regions10,18. The membranes' resistance to chlorine and pH extremes facilitates periodic chemical cleaning to restore flux after fouling by natural organic matter (NOM) and particulates18.

In industrial wastewater treatment, polysulfone ultrafiltration membranes pretreat feedwater for reverse osmosis (RO) or nanofiltration (NF) systems, removing colloidal silica, oil emulsions, and macromolecular organics that would otherwise foul downstream membranes1,6. Textile, pulp and paper, and food processing industries utilize polysulfone membranes for effluent clarification and resource recovery (e.g., protein concentration, dye removal)2.

Bioprocessing And Pharmaceutical Manufacturing

Polysulfone ultrafiltration membranes play a critical role in