Solvent-evaporation-induced kinetic trapping tunes chain packing and CO2/CH4 separation in polyimide membranes

Evaporation-rate control regulates kinetic trapping during polyimide membrane formation. • Stronger kinetic trapping preserves loosely packed nonequilibrium chain configurations with increased free volume. • Decarboxylative cross-linking amplifies structural gains, enhancing CO 2 permeability with modest selectivity loss. • This simple processing strategy improves CO 2 /CH 4 separation without polymer-backbone redesign or additional reagents. Kinetic trapping, the arrest of polymer chains in non-equilibrium configurations during rapid solvent evaporation, provides a deliberate route to engineer free-volume architecture in gas separation membranes. By controlling the solvent evaporation rate through casting temperature, faster evaporation from two solvents of contrasting volatility (DMAc and THF) systematically traps 6FDA-TFDB-DABA polyimide chains in more loosely packed configurations. This produces enlarged free volume, evidenced by expanded interchain spacing (5.33 to 5.49 Å, DMAc; 5.40 to 5.58 Å, THF), decreased density, enhanced birefringence, and increased surface roughness. This looser chain packing contributes to a substantial increase in CO₂ permeability, from 25.7 to 41.1 Barrer for DMAc-cast membranes and from 33.8 to 49.0 Barrer for THF-cast membranes, corresponding to increases of approximately 60% and 45%, respectively. These permeability gains arise from concurrent improvements in both CO₂ diffusivity and solubility, while the CO₂/CH₄ selectivity decreases only modestly. After thermal decarboxylation cross-linking at 340   °C, the rapidly evaporated THF-based membrane reaches 89.9 Barrer and surpasses the 1991 Robeson upper bound, withstanding feed pressures up to 600 psi without plasticization. This kinetic-trapping strategy requires no new reagents, monomer synthesis, or complex processing, offering a scalable, industrially compatible route to high-performance polyimide gas-separation membranes. Download: Download high-res image (280KB) Download: Download full-size image