(595a) Investigating the Chemical and Morphological Structure of TR Polymers

Thermally rearranged (TR) polymers have shown outstanding separation performance for a number of current and emerging applications for membrane-based gas separations.  In certain cases, these polymers surpass the best combinations of permeability and selectivity currently known for polymers.  It is quite surprising, therefore, that a significant and outstanding question remaining in the field relates to the identification of their chemical structure.

TR polymers are traditionally formed from aromatic polyimides that contain nucleophilic chemical functionality ortho-position to their precursor diamine.  Upon heating, these polyimides undergo a chemical reaction, whereby they become insoluble in all known solvents.  This insolubility endows these polymers with outstanding chemical resistance to aggressive feed gas streams; however, it also  precludes the use of traditional characterization techniques such as solution-state NMR for identifying their structure.

Herein, we seek to identify the structure of TR polymers by designing a partially soluble TR polymer.  This polymer, prepared from a polyamide, which we will refer to as PA-TR, was characterized through a suite of 2-D solution-state NMR experiments to confirm its chemical structure, after which, we prepared a commonly studied polyimide-based TR polymer, PI-TR, with a nearly identical proposed chemical structure to that of PA-TR.  Cross-polarization magic-angle spinning NMR reveals that both of these TR polymers are polybenzoxazoles.

Interestingly, despite having nearly identical chemical structure, these TR polymers have different gas transport property sets.  Using positron annihilation lifetime spectroscopy to estimate the fractional free volume in these polymers, differences in transport properties are attributed to unique morphologies between each sample.  Therefore, the outstanding transport properties for TR polymers are achieved not simply from their chemical functionality, but also their physical nature.