Analysis of governing factors controlling gas transport through fresh and aged triptycene-based polyimide films

Abstract

The fundamental gas transport properties and physical aging behavior of a series of triptycene-based polyimides with various substitution groups were investigated. Wide-angle x-ray diffraction revealed that the 6FDA-1,4-triptycene polyimides exhibit two chain packing domains with d-spacing values of ~6.7Å and ~5.3Å, corresponding to triptycene-induced chain packing disruption and chain segments lacking the triptycene moiety, respectively. Positron annihilation lifetime spectroscopy (PALS) showed a bimodal distribution of microcavity size made up of triptycene cleft cavities that are ~3Å diameter and larger interchain cavities with an average diameter of ~7Å. Additionally, PALS confirmed 6FDA-1,4-trip_CH3 has the lowest fractional free volume, primarily due to smaller contributions from the interchain cavities, and 6FDA-1,4-trip_CF3 has the largest fractional free volume. It was also found that the addition of substituent groups had a unique and positive effect on the aging properties over nine months, in which the addition of CH3 and CF3 groups led to permeability increases of ~20% and ~40%, respectively. This is caused by the combination of the lowered driving force for physical aging, the substituent groups sterically hindering tighter packing with time, and the interconnectivity and/or unblocking of free volume over time through small-scale, local chain motion.