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 xray diffraction revealed that the 6FDA-1,4-triptycene polyimides exhibit two chain packing domains with d-spacing values of ~6.7 Å and ~5.3 Å, 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 ~3 Å diameter and larger interchain cavities with an average diameter of ~7 Å. 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.

Pentiptycene-based ladder polymers with configurational free volume for enhanced gas separation performance and physical aging resistance

“…The improved permeability over time observed here can potentially be credited to the opening of microcavities initially occupied by isopropoxy or n-propoxy units on neighboring pentiptycene moieties, where polymer chains undergoing their local segmental mobility during the aging process cause these substituent groups to partially or fully vacate IMFV they may have previously been occupying. This mechanism of "partial filling" has been proposed and referenced in other polymers incorporating iptycenes into polymer backbones with various substituent groups, where these groups may partially or fully block these microcavities initially (3,20,21,24,25). For the PPIM copolymers reported here, this mechanism is further supported by the observation that PPIM-np-S with linear n-propoxy substitution experienced much larger increase in permeability over time than PPIM-ip-C with the bulky isopropoxy substituent because the n-propoxy unit is more readily available to occupy or evacuate the IMFV microvoids due to its linear and more flexible nature as compared to the branched isopropoxy unit.…”

“…With iptycenes providing internal molecular free volume (IMFV) that is configuration-based and intrinsic to their superrigid molecular structure, in addition to their general bulkiness and pendant arene blades, they allow opportunity for a multipronged approach for taking on both the tradeoff relationship and physical aging challenges simultaneously. Previous research has also shown that the simultaneous incorporation of iptycenes and appropriately placed substituent groups into the polymer backbone offers a route to tunability of the polymer microstructure and separation performance due to the potential of these neighboring pendant groups to partially occupy the natural pores instilled by the molecular architecture of the iptycene unit (14,(20)(21)(22)(23)(24)(25).…”

A New Pentiptycene‐Based Dianhydride and Its High‐Free‐Volume Polymer for Carbon Dioxide Removal

“…These agree with the understanding that the polymer intersegmental distance, the FFV,a nd polymer chain packing (which affects the transport properties) can be manipulatedb yi ntroducing different moieties and groups into the polymer backbone. [53][54][55] The X-ray diffraction( XRD) patterns of 6FDA-durenea nd PPImDA-6FDA ( Figure 4) confirmt he amorphous structures of the polyimides.T he broad halos show no crystalline domain in the polymern etworks, whichw as also observed by differential scanningc alorimetry (DSC) analysis. [32] The d-spacing of 6FDAdurenei s6 .64 (Table 4), which is very close to the value of 6.7 reported in Ref.…”

Evidence for Size-Sieving Driven Vapor Sorption and Diffusion in a Glassy Polybenzoxazole Exhibiting Configurational Free Volume

“…This number, however, does not provide the size of the internal free volume of triptycene units but the average size of free volume elements, including conformational and configurational free volume. A separate study indicated that the internal size of triptycene units is <4 Å . Finally, based on purely geometric considerations, one may consider the void space between two arene blades of triptycene units as a triangular prism whose volume is 31 Å 3 .…”