The first stage of the modification targets the fundamental chemistry of the membrane itself. The researchers synthesized poly(sodium p-styrene sulfonate), or PSS, a polymer densely decorated with negatively charged sulfonate groups, and wove it into the PVDF matrix during membrane fabrication. Rather than simply blending the two polymers, which risks the hydrophilic component washing away over time, the team used a one-pot approach combining free-radical polymerization with non-solvent induced phase separation, the standard industrial casting technique. The result is a semi-interpenetrating polymer network, in which linear PVDF chains are physically entangled with a crosslinked PSS network at the molecular level. This interlocking creates what polymer scientists call a forced compatibility effect: the two dissimilar phases cannot easily separate, component leaching is suppressed, and the sulfonate groups remain permanently anchored where they can attract water molecules.

The consequences of this structural change ripple through the entire architecture of the membrane. During phase inversion, the sulfonate groups draw water into the casting film, accelerating the exchange between solvent and non-solvent and promoting the formation of finger-like pores in the supporting layer, which replaced the sponge-like structure of the unmodified PVDF membrane. Scanning electron microscopy revealed fewer large surface defects and a far more efficient pore network for water transport. The water contact angle, a measure of surface wettability, dropped from 81.3 degrees for the pristine membrane to 63.3 degrees after the first modification. Pure water flux surged more than fivefold, from 141.54 to 737.20 liters per square meter per hour, though the protein rejection rate of 86.83 percent still fell short of the 90 percent benchmark demanded of practical ultrafiltration membranes. That shortfall set the stage for the second modification.

The team explored two distinct routes to grow a polyaniline layer on the membrane surface. In the first, the membrane was soaked in an aniline solution so that protonated monomers adsorbed electrostatically onto the negatively charged sulfonate sites, then transferred to an oxidizing solution to trigger polymerization. In the second, a custom reactor delivered monomer and oxidant simultaneously across a liquid-liquid interface at the membrane surface, enabling polymerization to proceed directly where it was wanted. Both routes exploit the same anchoring chemistry: the protonated imine nitrogen atoms of the growing polyaniline chains form ionic bonds with the sulfonate groups of PSS, reinforced by hydrogen bonding, effectively locking the functional layer in place. The films announced their presence visually, turning one membrane dark brown and the other green depending on the oxidation state of the polyaniline.

The choice between the two polymerization methods proved decisive. With adsorption followed by polymerization, aniline monomers penetrated into the membrane pores before reacting, and the polyaniline formed inside partially blocked the channels, throttling flux to 265.39 liters per square meter per hour. Synchronous polymerization, by contrast, confined the reaction largely to the surface, producing a thin, uniform and dense sieving layer that left the transport channels open. The resulting membrane, designated M3, achieved a pure water flux of 401.03 liters per square meter per hour while rejecting 99.02 percent of bovine serum albumin, a standard protein foulant. Its water contact angle fell to 42.9 degrees, and its surface carried a stronger negative charge at neutral pH than either precursor membrane, sharpening the electrostatic repulsion that keeps negatively charged proteins at bay.

Antifouling performance, the metric that ultimately determines whether a membrane survives real-world service, improved dramatically. After a cycle of protein filtration and a simple water rinse, the pristine PVDF membrane recovered only 54.30 percent of its original flux, while the dual-modified M3 membrane recovered 95.59 percent. Its irreversible fouling ratio, the portion of fouling that physical cleaning cannot remove, dropped to just 4.41 percent, compared with 45.69 percent for the unmodified control. Static adsorption experiments told the same story: the M3 membrane bound the least protein of all four samples tested. The researchers attribute this resilience to a stable hydration layer, in which hydrophilic PSS and PANI chains hold water molecules at the surface as a sacrificial barrier, combined with steric hindrance from the anchored polymer layer that physically blocks proteins from entering the pores.

The membrane also showed unexpected muscle against biological fouling, one of the most expensive problems in water treatment. In plate-counting assays, the M3 membrane inhibited 88.00 percent of Escherichia coli and 87.38 percent of Staphylococcus aureus colonies, far outperforming the modest effects of the PSS-only membrane. The mechanism, according to the study, hinges on the positively charged imino sites of polyaniline, which bind to the negatively charged bacterial cell membranes and disrupt the potential gradient across bacterial ion channels, destroying the cells. Because the polyaniline is anchored ionically rather than merely coated onto the surface, this antibacterial activity should persist through cleaning cycles rather than being lost, addressing a common failure mode of antimicrobial membrane coatings.

Chemical robustness tests reinforced the picture of a membrane built for harsh duty. After 28 days of immersion in acidic solution at pH 2, the dual-modified membrane still rejected 86.83 percent of albumin, and after 28 days in alkaline solution at pH 10, it retained 84.67 percent rejection, with flux increases far smaller than those of the unmodified control. The semi-interpenetrating network raises the thermal degradation temperature of the matrix as well, limiting the motion of PVDF chains under heat. Stability against component loss is equally important: unlike conventional blended membranes, in which hydrophilic additives gradually leach into the treated water, the interlocked PSS network and ionically bonded polyaniline layer resist dissolution, a property the authors link directly to the high and repeatable flux recovery observed across filtration cycles.

Perhaps most striking is the membrane’s performance against dye pollution. Treating simulated wastewater containing Methylene Blue and Congo Red at 10 milligrams per liter, the M3 membrane rejected 99.99 percent of both dyes, relying on a combination of surface adsorption, size exclusion, electrostatic repulsion or attraction, and steric hindrance from the PANI layer. Cationic Methylene Blue was initially captured by the negatively charged surface, forming a concentration-polarization layer that then repelled further deposition, while anionic Congo Red was largely deflected electrostatically and blocked by the aggregation of its molecules in solution. After extended filtration and cleaning, dye fluxes remained stable above 200 liters per square meter per hour, with recovery rates near 97 percent. An economic analysis estimated production costs at roughly 3 to 3.5 dollars per square meter, far below typical commercial PVDF membrane costs, suggesting that this laboratory-scale dual-modification strategy could, if it survives scale-up, offer an affordable route to treating protein-rich and dye-laden industrial wastewater.

Subject of Research: Dual hydrophilic and antibacterial modification of PVDF ultrafiltration membranes for wastewater treatment

Article Title: Dual-modified PVDF ultrafiltration membranes via semi-interpenetrating polymer network construction and PANI surface polymerization

Article References: Zhang, J., Han, Y., Wu, M., Zhang, D., Zhao, W., Shi, C., Xu, J., & Cui, H. (2026). Dual-modified PVDF ultrafiltration membranes via semi-interpenetrating polymer network construction and PANI surface polymerization. Journal of Saudi Chemical Society, 30(4), Article 48. https://doi.org/10.1007/s44442-026-00099-1

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00099-1

Keywords: PVDF, ultrafiltration membrane, semi-interpenetrating polymer network, polyaniline, poly(sodium p-styrene sulfonate), antifouling, antibacterial, dye removal, wastewater treatment, NIPS, membrane fouling, BSA rejection