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_<i>ortho</i> and 6FDA-2,6-trip_<i>para</i>, were synthesized to investigate the effect of varying polymer backbone geometry on chain packing and gas transport properties. Changing the imide linkage geometry from <i>para</i> to <i>ortho</i> reduced gas permeabilities by ∼48% due to more efficient chain packing of the asymmetric <i>ortho</i> structure, which is demonstrated by decreased <i>d</i>-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 <i>P</i><sub>CO2</sub>). This is likely the result of reduced chain mobility, as evidenced by increased <i>T</i><sub>g</sub>, and a shift in free volume distribution toward smaller cavities, as supported by smaller <i>d</i>-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.