Rigid twisted co-monomer engineering for enhanced polyamide nanofiltration membrane performance

The pore structure characteristics of polyamide (PA) nanofiltration (NF) membranes, particularly pore size and porosity, are primarily governed by the polymer network formed through cross-linked PA chain stacking, which ultimately governs their desalination efficiency. Introducing suitable co-monomers during interfacial polymerization (IP) provides an effective approach to modulate the physicochemical properties and thereby the separation behavior of PA membranes. Herein, adamantane-1,3-diamine (Ad) was employed as an aqueous-phase co-monomer with piperazine (PIP) in the IP reaction with trimesoyl chloride (TMC) in the organic phase. The distinctive rigid and twisted structure, together with the bulky molecular dimension of Ad, regulates polymer chain stacking, producing PA membranes with enhanced porosity without markedly influencing pore size. Moreover, the relatively low reactivity of Ad reduces the cross-linking density of the PA layer, leading to increased exposure of carboxyl groups and consequently a higher negative surface charge degree. The resulting Ad/PIP-PA membrane achieved a water permeance of 19.5 L m⁻² h⁻¹ bar⁻¹, approximately 5-times higher than that of the commercial PA membrane, respectively, with an uncompromised Na₂SO₄ rejection of 99.3%. Notably, the Ad/PIP-PA membrane achieved an exceptional NaCl/Na₂SO₄ separation selectivity, reaching 86.7. This work demonstrates a straightforward and scalable co-monomer engineering approach to simultaneously enhance membrane permeability and selectivity, providing new insights for the rational design of high-performance NF materials.