  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.
