Effect of uniaxial stretching on gas separation performance of polyimide membranes

Fine-tuning of the physical stacking structure for microporous polyimide (PI) membranes via the stretching processing is a practical way of achieving advanced membrane materials with improved aging resistance as well as high gas separation performance. Here, a fluorinated polyimide 6FDA:BPDA-TFDB(1:1:2) is used as a prototype and a series of stretched membranes X%-PI are fabricated by uniaxially stretching the membrane to different elongation ratio and then annealing. Based on birefringence and wide-angle X-ray diffraction measurements, it's found that regular orientation of polymer chains along the stretching direction and fine-tuning of d-spaces from interchain packing and π-π stacking happen during the uniaxial stretching process. As such, compared to pristine PI membrane, the stretched membranes show better mechanical and thermal stability, especially for the one with higher stretching ratio. In addition, owing to the increased packing density of polymer chains, the highly stretched membrane of 40%-PI shows significantly improved gas perm-selectivity by 32.8%, 24.3%, and 12.6% for He/CH4, H2/CH4, and CO2/CH4 gas pairs, respectively, in comparison to pristine membrane. Furthermore, 40%-PI exhibits much higher gas permeability retention rate than that of other membranes after three months' aging. We expect that this work can offer polymer materials processing guidance for the preparation of high-performance gas separation membrane in future.