Morphology Control of Polyimide-Based Membranes via Delayed Demixing for Efficient Separation of Water-in-Diesel Emulsions

Abstract Water-in-diesel emulsion separation remains challenging because finely dispersed water droplets are highly stable. Here, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)–2,4,6-trimethyl-m-phenylenediamine (TrMPD) polyimide was synthesized and formed into asymmetric membranes by nonsolvent-induced phase separation. Effects of polymer concentration and evaporation delay on morphology, surface properties, and separation were examined. NMR, FTIR, and TGA confirmed structural integrity and thermal stability. Increasing polymer concentration from 20 to 30 wt% and applying a 30 s delay transformed porous finger-like structures into denser sponge-like morphologies with thicker skins. Pentane achieved the highest pure-solvent flux, 250 L m–2 h–1 at 5 bar, while flux decreased with viscosity. During emulsion separation, diesel flux declined from ∼25 to <2.5 L m–2 h–1, whereas water rejection exceeded 99%. The 26 wt%, 0 s ET membrane best balanced efficiency, integrity, and stability during cycling, composition changes, and 8 h operation, demonstrating that controlled fabrication enables robust emulsion separation.