Ultrathin-film composite (uTFC) membranes based on amorphous perfluoropolymers for liquid separations

Thin-film composite membranes based on ∼100 nm cross-linked polyamides (PAs) are state-of-the-art membranes for liquid separations such as desalination. However, the PA layer shows poor antifouling properties and instability in chlorine solutions. Herein, we show for the first time that glassy amorphous perfluoropolymers (PFPs, such as Teflon AF and Hyflon AD) can be fabricated into ultrathin film composite (uTFC) membranes with a selective layer of <20 nm for desalination. Increasing the polymer concentration in the coating solutions from 0.05 mass% to 0.3 mass% increases the selective layer thickness from 9 to 25 nm. When Teflon AF2400 was fabricated into 16-nm selective layers on various porous supports, the water permeance decreases from 0.37 LMH/bar for PES10k to 0.07 LMH/bar for PES1k, and the Na₂SO₄ rejection increases from 86.6% to 95.2%. While the water permeance is lower than the commercial PA-based NF membranes, the rejection in these PFP-based membranes is comparable. The effect of the porous support on the water permeance can be ascribed to the geometric restriction and satisfactorily described using an empirical model. Interestingly, the membranes also exhibit good Na⁺/Li⁺ separation performance and stable performance for organic solvent nanofiltration, showcasing the versatility of the PFP-based uTFC membranes for various liquid separations.