(562c) Tailoring Backbone Rigidity of Triptycene-Containing Polymers for Enhanced Gas Transport Propertie

Guo, R., University of Notre Dame
Weidman, J., University of Notre Dame
Luo, S., University of Notre Dame
Gas transport in dense polymer membranes is a pressure driven, solution-diffusion process, which involves sorption of a species into the membrane surface, diffusion across the membrane through a series of ‘jumps’ through the free volume elements, and desorption at the downstream. In this process, the transient free volume spaces are largely generated by inefficient chain packing and dictated by the segmental motions of polymer chains to produce gaps that open and close within the polymer. Therefore, polymer backbone rigidity, which governs the local segmental motions and chain packing plays a critical role in constructing low-barrier diffusion pathways enabling fast and selective gas transport. In this talk, we report two new strategies in tailoring polymer backbone rigidity of triptycene-containing polymers to further improve their overall gas separation performance. Specifically, the first strategy is to incorporate a super rigid triptycene-based diamine as a comonomer in polyimide synthesis. The resulting copolymers are composed of a rigid element with the triptycene skeleton directly linked to imide rings and a relatively flexible element containing ether linkage. The ratio of the two components was systematically varied to finely tune the chain rigidity and thus gas transport properties. It is found that the copolymer composition of 50% of the rigid structure caused a 42 °C increase in glass transition temperature and a more than 2-fold increase in gas permeability. Moreover, the much improved gas permeability was accompanied with almost constant gas selectivities due to the unique internal free volume of the triptycene moiety, leading to the overall separation performance laterally approaching the upper bound. A second strategy is to investigate the integration of highly size-sieving triptycene moieties in polybenzoxazole (PBO) structure, which is arguably a more rigid backbone structure with heterocyclic ring than polyimides. A series of triptycene-containing PBOs with systematically varied backbone rigidity were prepared, all of which displayed superior gas separation performance that is far beyond the 2008 upper bound. Strikingly, via increasing the backbone rigidity by adjusting the content of triptycene units, the highly permeable triptycene-PBO copolymers showed a vertically increasing trend in permselectivities, leading to the overall separation performance of several films surpassing the most recently reported 2015 upper bounds for H2/CH4 separations. In this talk, synthesis and characterization of these new highly rigid triptycene-containing polymers will be presented. Fundamental physical and transport properties will be discussed to illustrate the fundamental relationship between microscopic structures with macroscopic transport properties for these new triptycene-containing polymer membranes.