Molecular origins of fast and selective gas transport in pentiptycene-containing polyimide membranes and their physical aging behavior

This paper reports comprehensive characterization and detailed analyses of microporous structure and fundamental gas transport properties for a series of new pentiptycene-containing polyimide gas separation membranes prepared from custom-synthesized pentiptycene-based diamines and 4,4′-hexafluoroisopropylidene bisphthalic dianhydride (6FDA) to identify the molecular origins for fast and selective gas transport. Both experimental characterizations of inter-chain spacing and microporosity and molecular modeling analysis of chain conformations and rigidity suggested that rigid H-shape pentiptycene units effectively disrupted chain packing, resulting in large fractional free volume and consequent high gas permeabilities in these membranes. Atomic-level detection of free volume architecture by positron annihilation lifetime spectroscopy (PALS) analysis revealed a bimodal microcavity size distribution with cavity sizes of d4~7–8 Å and d3~3–4 Å in this series of membranes. The microcavity size and size distribution were found to be sensitively affected by the substituent groups in the pentiptycene monomer structure based on the mechanism of "partial filling" of internal molecular cavities defined by the shape of pentiptycene units. Analysis of fundamental gas transport properties in terms of diffusivity (D) and solubility (S) coefficients demonstrated that size sieving mechanism (diffusivity contribution) dominates the gas transport in these polymers and bimodal microcavity size distribution with ultrafine microporosity is responsible for the excellent H2-related gas separation performance. Superior resistance against physical aging was observed for these high-free-volume polymers, which is ascribed to the stable, configuration-induced microcavity structure constructed by the rigid pentiptycene units.