Highly permeable and selective membranes are desirable for energy-efficient gas and liquid separations.Microporous organic polymers have attracted significant attention in this respect owing to their high porosity, permeability, and molecular selectivity. However, it remains challenging to fabricate selective polymer membranes with controlled microporosity which are stable in solvents. Here we report a new approach to designing crosslinked, rigid polymer nanofilms with enhanced microporosity by manipulating the molecular structure. Ultra-thin polyarylate nanofilms with thickness down to 20 nm were formed in-situ by interfacial polymerisation. Enhanced microporosity and higher interconnectivity of intermolecular network voids, as rationalised by molecular simulations, are achieved by utilising contorted monomers for the interfacial polymerisation. Composite membranes comprising polyarylate nanofilms with enhanced microporosity fabricated in-situ on crosslinked polyimide ultrafiltration membranes show outstanding separation performance in organic solvents, with up to two orders of magnitude higher solvent permeance than membranes fabricated with nanofilms made from noncontorted planar monomers.Conventional gas and liquid separation processes such as evaporation and distillation are widely used in the oil and gas, energy, chemical, and pharmaceutical industries, but are energy-intensive. An alternative to these processes is membrane separation technology, which typically consumes an order of magnitude less energy. To enable wider deployment of membrane technology, highly permeable membranes are required to process large volumes of gas or solvent using a viable membrane area over a feasible timeframe 1-2 . There are two main strategies being followed to this end. One is to design the polymer structure at the molecular level so as to provide greater interconnected microporosity 3-10 , whilst a second approach is to reduce the thickness of the separating layer to nanometre scale [11][12][13][14][15][16] .Microporous organic materials with well-defined pore structure are excellent candidates for highly permeable and selective membranes 1 , such as metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) [17][18] , covalent organic frameworks (COFs) [19][20] , and porous organic cages 2 (POCs) [21][22][23] . However, the fabrication of these crystalline solids to form defect-free membranes is technically challenging. Recent significant progress includes fabrication of MOFs to form selective membranes by secondary crystal growth 24 , assembly of MOF nanosheets 15 , interfacial synthesis 25 , or mixed matrix membranes 10,26 . By contrast, industrial membranes are dominated by solution processing of polymers and interfacial polymerisation, for example in producing polyamide desalination membranes. Notable examples of microporous polymers are polymers of intrinsic microporosity (PIMs) [6][7][27][28][29][30][31] . Owing to the shape and rigidity of the component monomers, the polymer chains have contorted, ri...
