A Molecularly Impermeable Polymer from Two-Dimensional Polyaramids

All polymers exhibit gas permeability through the free volume of entangled polymer chains. However, two-dimensional (2D) materials including graphene stack densely and can exhibit molecular impermeability. Solution-synthesized 2D polymers that exhibit the latter by poly-condensation have been a longstanding goal. Herein, we demonstrate self-supporting, spin-coated 2D polyaramid nanofilms that exhibit nitrogen permeability below 3.1E-9 Barrer, roughly 6500-fold lower than existing polymers, and similar for other gases. Optical interference during the pressurization of nanofilm-coated microwells allows measurement of mechanosensitive rim opening and sealing, creating gas-filled bulges stable exceeding 3 years. This discovery enables 2D polymer resonators with high resonance frequencies (~8 MHz) and Q factors up to 537, similar to graphene. A 60-nm coating of air-sensitive perovskites reduces the lattice degradation rate 14-fold with an oxygen permeability of 3.3E-8 Barrer. Molecularly impermeable polymers promise the next generation of barrier materials that utilize exceedingly little polymer to maximize chemical rejection, ultimately contributing to sustainable materials development.