Based on high-pressure pure- and mixed-gas (50:50) CO2/CH4 separation properties of two intrinsically microporous triptycene-based polyimides (TPDATMPD and TPDA6FpDA), the intrachain rigidity central to conventional PIM design principles is not a singular solution to intrinsic plasticization resistance. Despite the significant intrachain rigidity in TPDATMPD, a 300% increase in PMIX(CH4), 50% decrease in alpha(CO2/CH4) from 24 to 12, and continuous increase in PMIX(CO2) occurred from 4 to 30 bar. On the other hand, the more flexible and densely packed TPDA6FpDA exhibited a slight upturn in P-MIX(CO2) at 20 bar similar to a dense cellulose acetate (CA) film, also reported here, despite a 4-fold higher CO2 sorption capacity. Microstructural investigations suggest that the interconnected O-2- and H-2-sieving ultramicroporosity of TPDATMPD is more sensitive to slight CO2-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.