Fine-tuning the microporosity of PIM-1 by heat treatment was applied to develop a suitable carbon molecular sieve membrane for ethylene/ethane separation. Pristine PIM-1 films were heated from 400 to 800 degrees C under inert N-2 atmosphere (< 2 ppm O-2). At 400 degrees C, PIM-1 self-cross-linked and developed polar carbonyl and hydroxyl groups due to partial dioxane splitting in the polymer backbone. Significant degradation occurred at 600 degrees C due to carbonization of PIM-1 and resulted in 30% increase in cumulative surface area compared to its cross-linked predecessor. In addition, PIM-1-based CMS developed smaller ultramicropores with increasing pyrolysis temperature, which enhanced their molecular sieving capability by restricted diffusion of ethylene and ethane through the matrix due to microstructural carbon densification. Consequently, the pure-gas ethylene permeability (measured at 35 degrees C and 2 bar) decreased from 1600 Barrer for the pristine PIM-1 to 1.3 Barrer for the amorphous carbon generated at 800 degrees C, whereas the ethylene/ethane pure-gas selectivity increased significantly from 1.8 to 13. (C) 2016 Elsevier B.V. All rights reserved.