Tuning Free Volume via Diamine Structure in 6FDA-Based Polyimide Membranes for He/Ne Separation

Abstract

Helium and neon are strategically important noble gases in semiconductor manufacturing and cryogenic technologies, yet their similar molecular sizes and inertness make separation challenging. Although membrane-based separation offers an energy-efficient alternative to cryogenic processes, polymer membranes and theoretical designs for He/Ne separation remain limited. In this study, the free volume of polyimides based on 6FDA was tailored by incorporating different diamine structures, and the resulting effects on He/Ne separation performance were systematically investigated. Two classes of diamines were selected: monoaromatic diamines affecting chain symmetry and stacking, and diaromatic bridged diamines introducing varying degrees of chain rigidity. Structure-dependent differences in chain packing and free volume were analyzed using PALS and MD simulations. A quantitative correlation between free volume parameters and gas transport properties established a clear structure–property relationship. Among the studied membranes, 6FDA-ODA polyimide membrane prepared from 6FDA and ODA exhibited a favorable permeability–selectivity balance with helium permeability of 134.78 barrer and an ideal He/Ne selectivity of 6.18. The membrane maintained stable permeation at 200 psi and exhibited increased selectivity after physical aging. These results demonstrate that tuning diamine structures effectively regulates chain packing and free volume, enabling the development of high performance 6FDA-based polyimide membranes for He/Ne separation.