Decoding Interfacial Polymerization Through Steady Amine Delivery and Real‐Time QCM‐D Monitoring

Interfacial polymerization (IP) is central to the fabrication of high-performance polyamide membranes, yet the microscopic reaction pathways governing film formation remain difficult to resolve due to the strongly coupled diffusion-reaction behavior at the organic-aqueous interfaces. Here, we develop a tailored IP platform in which a solid-state piperazine source delivers amine monomers at a constant dissolution flux, thereby decoupling monomer diffusion from polymerization. Combined with in situ quartz crystal microbalance with dissipation monitoring, this system enables real‑time tracking of polyamide formation on the organic side of the reaction front. The frequency-dissipation response resolves three kinetic regimes: pre-gelation oligomerization, post-gelation aggregation, and diffusion-limited growth accompanied by late-stage densification. By systematically tuning the interfacial functional-group ratio, we reveal that the initial stoichiometry governs gelation time, nascent-network permeability, and subsequent film stiffening, which collectively determine the structure of the final polyamide selective film. Consequently, controlling interfacial reaction conditions enables systematic tuning of the membrane molecular weight cutoff, surface charge, and ion selectivity. This work provides a kinetic map of interfacial polymerization that connects monomer stoichiometry, film-formation pathways, and membrane structure, offering a mechanistic basis for rationally controlling polyamide membrane formation.