Mild nonsolvent treatment enables efficient gas separation in glassy polyetherimide membranes

Glassy polymer membranes are attractive for energy-efficient gas separation, yet their intrinsically disordered free-volume structures limit precise molecular-size discrimination. Existing approaches to engineer subnanometer transport pathways, such as thermal rearrangement, crosslinking or rigid backbone design, often require complex synthesis or harsh processing. Here, we establish a mild nonsolvent-treating strategy that overcomes these limitations by restructuring the free-volume architecture of glassy polyetherimide (PEI) hollow fiber membranes. Nonsolvent treatment redistributes local chain packing and free-volume populations below the bulk glass transition temperature. Multiscale characterization indicates a redistribution of local packing/free-volume states, while gas transport exhibits a pronounced size-dependent transition in the approximately 3.5–3.7 Å range. This tailored free-volume environment suppresses CH 4 transport while maintaining favorable CO 2 permeation, resulting in up to a 2.4-fold enhancement in CO 2 /CH 4 ideal selectivity relative to pristine PEI and a competitive selectivity–permeance combination among reported polymer hollow-fiber membranes. Techno-economic analysis reveals that integration of the PEI-based membrane module with amine absorption reduces energy consumption by 47.0% compared with standalone amine absorption. This simple, scalable nonsolvent strategy provides a practical tool for engineering subnanometer transport pathways in glassy polymers, advancing high-performance membranes for industrial gas separation applications.