Membranes derived from ultrathin polymeric films are promising to meet fast separations, but currently available approaches to produce polymer films with greatly reduced thicknesses on porous supports still faces challenges. Here, defect-free ultrathin polymer covering films (UPCFs) are realized by a facile general approach of rapid solvent evaporation. By fast evaporating dilute polymer solutions, we realize ultrathin coating (~30 nm) of porous substrates exclusively on the top surface, forming UPCFs with a block copolymer of polystyrene-block-poly(2-vinyl pyridine) at room temperature or a homopolymer of poly(vinyl alcohol) (PVA) at elevated temperatures. With subsequent selective swelling to the block copolymer and crosslinking to PVA, the resulting bi-layered composite structures serve as highly permeable membranes delivering ~2-10 times higher permeability in ultrafiltration and pervaporation applications than state-of-the-art separation membranes with similar rejections and selectivities. This work opens up a new, facile avenue for the controllable fabrication of ultrathin coatings on porous substrates, which shows great potentials in membrane-based separations and other areas.
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