Fabrication of additive-free polyamine-based membranes by facile copolymerization of salt-functionalized poly(allylamine hydrochloride) and branched polyethyleneimine towards high performance nanofiltration

The permeability and structural stability of polyamine-based nanofiltration membranes can be significantly improved via scalable and systematic tuning of their surface chemistry. Herein, we present a rational design approach for fabricating high-performance thin-film composite (TFC) nanofiltration (NF) membranes via in situ interfacial polymerization (IP) using a novel aqueous monomer formulation comprising poly(allylamine hydrochloride) (PAH), its salt-functionalized derivative, and branched polyethyleneimine (b-PEI). By copolymerizing PAH and b-PEI in the aqueous phase, and reacting this with cyanuric chloride (CC) in the organic phase, we demonstrate a scalable and additive-free strategy to enhance membrane performance without relying on expensive nanoparticles or unstable modifiers. NaCl-functionalization significantly impacted the diffusivity and reactivity of PAH as the resulting functionalized PAH (f-PAH) membrane exhibited relaxed selective layer, enlarged pore size, as well as enhanced hydrophilicity, thereby facilitating permeability. The water permeance could reach up to 12.7 ± 0.35 L m⁻²h⁻¹ bar-¹, which is nearly twice that of the control TFC and raw PAH (r-PAH) incorporated TFC membranes, with a 2000 ppm MgCl₂ rejection of 97.1 ± 0.78 %. Molecular dynamics (MD) simulations further validated the diffusivity of PAH in the IP process. r-PAH and f-PAH TFC membranes exhibited superior antifouling properties and long-term static acid stability compare to the control TFC membrane. Specifically, the normalized rejection data of the membranes after exposure to 20 wt% H₂SO₄ solution for 744 h could still reach 0.903 and 0.908, with 2000 ppm MgCl₂ as feed solution, and could achieve flux recovery ratios of 83.8 % and 80.9 %, respectively. This work thus showcases new insights into the use of salt-tuned polyamines in IP processes and highlights the viability of polymer–polymer synergism in developing robust and efficient membranes for water purification and acid wastewater treatment.