Based on high-pressure pure- and mixed-gas (50:50) CO 2 /CH 4 separation properties of two intrinsically microporous triptycene-based polyimides (TPDA–TMPD and TPDA–6FpDA), the intrachain rigidity central to “conventional PIM” design principles is not a singular solution to intrinsic plasticization resistance. Despite the significant intrachain rigidity in TPDA–TMPD, a 300% increase in P MIX (CH 4 ), 50% decrease in α(CO 2 /CH 4 ) from 24 to 12, and continuous increase in P MIX (CO 2 ) occurred from 4 to 30 bar. On the other hand, the more flexible and densely packed TPDA–6FpDA exhibited a slight upturn in P MIX (CO 2 ) at 20 bar similar to a dense cellulose acetate (CA) film, also reported here, despite a 4-fold higher CO 2 sorption capacity. Microstructural investigations suggest that the interconnected O 2 - and H 2 -sieving ultramicroporosity of TPDA–TMPD is more sensitive to slight CO 2 -induced dilations and is the physical basis for a more extensive and accelerated plasticization. Interchain rigidity, potentially by interchain interactions, is emphasized and may be facilitated by intrachain mobility.
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Swaidan et al. (2014) studied this question.