Leveraging Liquid–Liquid Interfaces in a 3D-Printable Reactor to Form Sub-Micron Freestanding Membrane Selective Layers

Fabricating ultrathin, defect-free polymer nanofilms on varying substrates remains a persistent challenge in thin-film composite (TFC) membrane development, particularly when substrate interference, solution intrusion, and film transfer limit reproducibility and performance. We introduce a modular, cost-effective, solvent-compatible, 3D-printed reactor─FIPzR (Freestanding Interfacial Polymerization Reactor)─designed to fabricate thin, defect-free polymer films at liquid-liquid interfaces. Using iterative CAD-based design and additive manufacturing (AM), the device is engineered to decouple film formation from the underlying substrate, enabling reproducible fabrication of high-quality films with controlled morphologies and direct transfer onto both porous and nonporous substrates using a floating guide ring. The reactor accommodates multiple fabrication strategies, demonstrated here through interfacial polymerization to synthesize polyamide (PA) membranes of varying morphologies, drop casting of a preformed polysulfone (PSU) solution, and curing of a reactive polydimethylsiloxane (PDMS) mixture─with thicknesses spanning ultrathin (<20 nm) to submicron scales. The desalination performance of smooth and rough PA membranes was evaluated in a custom-built crossflow setup under standard brackish water reverse osmosis (RO) conditions, exhibiting water permeance and salt rejection characteristics in line with standard RO membranes. PSU and PDMS membranes were tested in a custom-built gas separation setup to verify structural integrity and defect-free film quality, showing CO₂/N₂ selectivity consistent with reported literature benchmarks. The FIPzR offers a reproducible and substrate-independent polymer nanofilm fabrication, with potential utility in membrane separations, coatings, flexible electronics, and sensing technologies.