Missing Evidence in Interfacial Polymerization: Challenges and Opportunities for In Situ Observation of Polyamide Membrane Formation

Abstract Polyamide (PA) thin-film composite membranes are the dominant platform for reverse osmosis and nanofiltration. However, despite decades of development, the dynamic formation process of PA layers during interfacial polymerization (IP) remains insufficiently understood. Existing mechanistic interpretations are largely based on retrospective relationship analyses of formed membrane nanostructure with certain IP reaction conditions rather than direct in situ observation of transient interfacial events. The ultrafast kinetics, nanoscale reaction zone, liquid–liquid interfacial environment, and highly nonequilibrium nature of IP make the in situ observation challenging. Herein, we critically examine the major mechanistic interpretations for the nanostructure formation of fully and semiaromatic PA, including convections, interfacial degassing, monomer eruption, reaction-diffusion, and water templating. Inconsistencies and limitations of these interpretations are briefly discussed, further driving the introduction of the progress and limitations of existing in situ IP observation techniques. Despite these techniques enabling in situ spatial or temporal characterization of PA formation during IP, they still lack high-resolution spatiotemporal monitoring and excellent compatibility with the susceptible liquid–liquid interface. To address these challenges, we further envision the next-generation in situ IP observation platform combining miniaturized IP reactors, controllable reaction initiation, and ultrafast vitrification function as inspired by the advanced time-resolved cryogenic electron microscopy in biological studies. This platform is expected to allow direct in situ characterization of the transient IP reaction stages, thereby improving the fundamental understanding of the rapid and dynamic formation of diverse PA nanostructures and thus guiding the rational design of high-performance PA membranes.