Structural engineering on 6FDA-Durene based polyimide membranes for highly selective gas separation

•Engineering polyimide membrane structure based on optimization of diamine monomer.•The free volume property and pore size distribution was regulated by introducing second amino monomer.•Crown ether structure monomer granted the membrane better size sieving effect and diffusion selectivity enhancement of 170%.•Inclusion of crown ether group into 6FDA-Durene showed the highest gas selectivity.