Partially Carbonized Polyimide Membranes with High Permeability for Air Separation.

Partially carbonized polyimide (CPI) membranes were prepared from a solution of 30 wt% polyamic acid in N, N-dimethylacetamide. The polymer membranes formed on an alumina support were thermally treated, involving imidization in air at 180°C and carbonization in N2 at relatively low temperature (400–500°C). The cross-sectional views of the supported CPI membranes show that the membranes consist of a top layer (thickness, 10 μm) on the support and a CPI/alumina thin layer in the support. The CPI membranes carbonized at 500°C showed high permeability for O2 of 1000–30000 barrer and permselectivity for O2/N2 of 3–6. The permeability of the CPI membranes was much higher than that of the reported polymer membranes and the carbon membranes. The pores formed under carbonization at 500°C and 400°C were effective for separating O2/N2 and CO2/CH4 mixtures, respectively. TG analysis indicated that the carbonization proceeds even at a constant temperature of 500°C. The successive generation of flexible pores before the formation of graphite structure with rigid pores seems to contribute to the higher permeability of the CPI membranes.