Structure Manipulation in Triptycene-Based Polyimides through Main Chain Geometry Variation and Its Effect on Gas Transport Properties

Two new triptycene-based polyimides, 6FDA-1,4-trip_ortho and 6FDA-2,6-trip_para, were synthesized to investigate the effect of varying polymer backbone geometry on chain packing and gas transport properties. Changing the imide linkage geometry from para to ortho reduced gas permeabilities by ∼48% due to more efficient chain packing of the asymmetric ortho structure, which is demonstrated by decreased d-spacing and fractional free volume. Varying the triptycene orientation from the 1,4- to 2,6-connection also caused a decrease in permeability (e.g., 29% decrease for PCO2). This is likely the result of reduced chain mobility, as evidenced by increased Tg, and a shift in free volume distribution toward smaller cavities, as supported by smaller d-spacing. Physical aging studies show that the equilibrium specific volume of these isomeric polymers is similar, as evidenced by nearly identical gas transport properties exhibited by all aged samples.