Polymer membranes are widely used in separation processes including desalination, organic solvent nanofiltration and crude oil fractionation. Nevertheless, direct evidence of subnanometre pores and a feasible method of manipulating their size is still challenging because of the molecular fluctuations of poorly defined voids in polymers. Macrocycles with intrinsic cavities could potentially tackle this challenge. However, unfunctionalized macrocycles with indistinguishable reactivities tend towards disordered packing in films hundreds of nanometers thick, hindering cavity interconnection and formation of through-pores. Here, we synthesized selectively functionalized macrocycles with differentiated reactivities that preferentially aligned to create well-defined pores across an ultrathin nanofilm during interfacial polymerization [1]. At the interface, the upper rim with highly reactive amines preferentially faces up into the organic phase where the crosslinking reaction occurs, and the unreactive lower rim faces downwards into the aqueous solution. Macrocycles are then preferentially aligned through the crosslinked ultrathin nanofilm to form well-defined subnanometre channels, The ordered structure was enhanced by reducing the nanofilm thickness down to several nanometers. This orientated architecture enabled direct visualization of subnanometre macrocycle pores in the nanofilm surfaces, with the size tailored to ångström precision by varying the macrocycle identity. Aligned macrocycle membranes provided twice the methanol permeance and higher selectivity compared to disordered counterparts. Used in high-value separations, exemplified here by enriching cannabidiol oil, they achieved one order of magnitude faster ethanol transport and threefold higher enrichment than commercial state-of-the-art membranes. This approach offers a feasible strategy for creating subnanometre channels in polymer membranes, and demonstrates their potential for accurate molecular separations.

Reference: 1. Jiang, Z., Dong, R., Evans, A.M., Biere, N., Ebrahim, M.A., Li, S., Anselmetti, D., Dichtel W.R., Livingston, A.G., Aligned macrocycle pores in ultrathin films for accurate molecular sieving. Nature 609, 58–64 (2022).