Polymers were dissolved in THF to form 5 w/v % solutions. PTFE syringe filters (pore size 0.45 mm) were used to filter the solutions before casting membranes on glass plates at room temperature. The glass plates were placed in a dry clean chamber to avoid the contamination of dusts and to slow the evaporation of the solvent THF. Because of the worse solubility of 6FDA-ODA in THF, the THF solution 5 w/v % could not be obtained for 6FDA-ODA. In this case, pyridine, as an alternative solvent, was used as the solvent instead. The glass plate cast with 6FDA-ODA/pyridine was kept in a clean oven
with a temperature of 40 C. Transparent light brown membranes were prepared from the procedures above and were easily peeled off from the glass plates. The thicknesses of the dense membranes were 8-15 mm.