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, C3H6, and C3H8 were conducted at 35 degrees C for different upstream pressures. The Langmuir saturation constant, C-H', 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 CO2, 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 degrees C than those treated at 200 degrees 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 degrees C have almost equal or lower solubility selectivity than the original membrane for CO2/CH4 and C3H6/C3H8 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 degrees C.