Permeability, solubility, diffusivity, and PALS data of cross-linkable 6FDA-based copolyimides

The intrinsic gas transport properties of thermally treated cross-linkable 6FDA-based copolyimide membranes have been studied. Grafting various sizes of cyclodextrin (CD) to the copolyimide matrix and then thermally decomposing CD at elevated temperatures are an effective method to micromanipulate microvoids and free volume as well as gas sorption and permeation. The pressure-dependent solubility and permeability coefficients were found to follow the dual-mode sorption model and partial immobilization model, respectively. Solubility and permeability coefficients of CH₄, CO₂, C₃H ₆, and C₃H₈ were conducted at 35 C for different upstream pressures. The Langmuir saturation constant, CH′ increases with an increase in annealing temperature. On the other hand, Henry's solubility coefficient k_D and Langmuir affinity constant b do not change noticeably. The CH₄ permeability decreases with pressure, while some membranes exhibit serious plasticization with an increase in CO ₂, C₃H₆ and C₃H₈ pressures. The diffusivity coefficient of the Henry mode (D_D) and Langmuir mode (D_H) were calculated from the permeability and solubility data and their ratio, F, is higher for membranes thermally treated at 425 C than those treated at 200 C. Data from positron annihilation lifetime spectroscopy (PALS) confirm that free-volume and the number of micropores increases while the radius of pore sizes decreases during the high temperature annealing process. All CD grafted membranes thermally treated at 425 C have almost equal or lower solubility selectivity than the original membrane for CO₂/CH₄ and C₃H₆/C₃H ₈ separations but the former has much higher diffusion selectivity than the latter. As a result, diffusion selectivity plays a more important role than solubility in determining the permselectivity of the CD grafted membranes thermally treated at 425 C.