The evolution of physicochemical and transport properties of 6FDA-durene toward carbon membranes; from polymer, intermediate to carbon

A high Tg material, which is 6FDA-durene (Tg: 425 °C) polyimide, is applied to study the Tg's effect on the evolution of physicochemical and transport properties of membranes from polymeric, intermediate to carbon stages. Tg greatly affects the configurational organization of membranes during pyrolysis especially at the intermediate stage (400-500 °C) and finally influences the pore size and pore size distribution of carbon membranes, which has been mostly ignored by the previous studies. The physicochemical and transport properties in various membrane stages are thoroughly characterized by TGA-FTIR, FTIR-ATR, XRD, thickness and gas transport tests. It is found that the membranes derived from the high Tg polyimides show a significant increase in chain mobility and physicochemical development in the intermediate stage. Interestingly, the gas permeability with annealing temperature show double peaks for medium-size gases such as O2, N2 and CH4, and single peaks for light gases such as He, H2 and CO2. In addition, the ideal selectivity generally shows double minimum values with annealing temperature. These phenomena may arise from the fact that significant physicochemical changes take place around the Tg, while the maximum degradation rate occurs at temperatures 70-100 °C above Tg. As compared with the trade-off line [L.M. Robeson, J. Membr. Sci. 62 (1991) 165], the temperature for membranes derived from the high Tg polyimide to surpass the trade-off line occurs around 450 °C. This temperature is just fall on the intermediate-stage temperature range. The resultant carbon membranes pyrolyzed with 1 °C/min heating rate show better transport performance at low pyrolysis temperatures than those pyrolyzed with 3 °C/min heating rate. However, when the pyrolysis temperature is elevated to 800 °C, the resultant carbon membranes pyrolyzed by different protocols all show similar and superior performance for gas separation.