New insights into structural evolution in carbon molecular sieve membranes during pyrolysis

Carbon molecular sieve (CMS) membranes separate penetrants using size and shape-selective pores. In this paper we report pyrolysis of a 6FDA:BPDA-DAM polyimide precursor between 500 and 800 °C and measure gas evolution during the CMS structural development. The CMS materials were then characterized using combined transport properties, porosimetry, FTIR, Raman spectroscopy, TGA-FTIR, WAXD, and elemental analysis measurements to assess their resulting physical and chemical properties. The results support a previous vision that fragmentation of the polyimide precursor occurs to form aromatic strands that provide building blocks for the overall CMS cell structure. This fact notwithstanding, these new findings indicate that constituent strands appear to be more complicated than previously suggested. An ordering process of such strands can generate a bimodal morphology comprising larger micropores with irregular cell walls containing ultramicropores. Permeability and permselectivity of the CMS for the C2H4/C2H6 pair are also correlated with CMS structures based on the above characterizations. The results of this work suggest that molecular probe-based transport measurements are by far the most useful tools to study these complex, amorphous materials for molecular separation applications.