Plasma-Engineered Sub-10 nm Surface Fluorination Enables Ultraselective Hollow Fiber Membranes

While polymeric hollow fiber membranes (HFMs) offer scalable solutions for gas separations, their performance is fundamentally limited by the permeance-selectivity tradeoff and imprecise microporosity regulation. Herein, we propose a surface fluorination strategy using carbon tetrafluoride (CF₄) plasma to engineer a sub-10 nm fluorinated and crosslinked layer on polydimethylsiloxane (PDMS)-coated Matrimid^® HFMs. Through precise modulation of plasma parameters, we achieved controlled substitution of PDMS methyl groups/methyl hydrogen atoms with fluorine species (up to 27.7 mol% F content), 5.3-fold enhanced chain rigidity via fluorine-induced interchain interactions and steric hindrance, as well as narrowed pore size distribution with preferential ultra-micropore filling. The optimized HFM-50W-65Pa-500s membrane exhibits record-breaking He/N₂ and He/CH₄ selectivities of 1202 ± 13 and 1790 ± 12 with 170 ± 2 GPU He permeance, surpassing perfluoropolymer upper bounds and outperforming previously reported polymeric HFMs. Remarkably, it demonstrates molecular discrimination precision (α(He/CO₂) = 56 ± 2.2, α(He/H₂) = 4.1 ± 0.2) for pure-gas and attractive ternary He/(CO₂+CH₄) selectivity of 1005 ± 20 under 40-bar mixed-gas conditions while maintaining 720-h operational stability. This plasma-engineered fluorination paradigm combines nanoscale precision with industrial scalability, opening new avenues for advanced membrane design.