A series of 6FDA/PMDA−TAB polypyrrolone copolymers has been shown to exhibit
performance lying on or above the so-called O2/N2 upper bound membrane separation tradeoff limit.
Further analysis of diffusion in these high performance materials can potentially guide future structural
enhancements, which may lead to improved separation properties. This work compares and analyzes
diffusion in this attractive family of hyperrigid polymers using entropic and energetic selectivities. When
the polymer structure and penetrant sizes are examined, possible polymer matrix configurations can be
discussed with regard to the entropy of the molecules in the normal and activated states. This analysis
extends to O2/CH4 and N2/CH4 entropic selectivities, which corroborate the trend observed with the O2/N2 gas pair, but the changes are more pronounced due to the larger difference in penetrant size.
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