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

Abstract

This paper reports, for the first time, the effect of configurational free volume (i.e., triptycene units) on condensable vapor transport in polymers. Alcohol and water vapor solubility and diffusivity isotherms at 25 °C in a triptycene-containing polybenzoxazole (TPBO) exhibiting configurational free volume are presented as a function of vapor activity, discussed, and used to develop fundamental structure−property correlations. This study provides evidence that while in conventional glassy polymers alcohol diffusion is sizecontrolled and sorption is enthalpy-controlled, which may create a trade-off between sorptionand diffusion-selectivity, alcohol sorption and diffusion in TPBO are both size-controlled, which makes it potentially easier to simultaneously tune sorption-and diffusion-selectivity to achieve highly selective separations. To put these results in a broad perspective, alcohol sorption and diffusion properties of TPBO were compared with those of conventional glassy polymers exhibiting conformational free volume, such as PIM-1, Teflon AF2400, polynorbornene, polysulfone, as well as rubbery PDMS. Finally, new exciting opportunities to exploit these unique TPBO's features for large scale molecular separations are discussed.

Polymer Morphological Effect on Gas Transport within Triptycene-Based Polysulfones

“…The ability of a gas to permeate through a dense, glassy polymer membrane is dependent on the free volume architecture created by the polymer’s structure and the interaction between the polymer chains. Combinations of the presence of bulky groups protruding off the polymer backbone, the incorporation of natural free volume microcavities (such as is observed in iptycenes ,,,,− ), and the overall rigidity of the polymer chains helps create the gas transport pathways that determine a polymer’s overall gas separation performance. Generally, increasing the amount of free volume in a polymer leads to increased permeability, and narrowing the size distribution of these free volume spaces around the size range between the gases that are being separated provides increased selectivity.…”

Cross-Linked Polymer Membranes for Energy-Efficient Gas Separation: Innovations and Perspectives

“…In this regard, macromolecular strategies that can instill more permanent and better-defined free volume architecture in polymers hold the key to advancing polymer gas separation membranes to a new level of sophistication. In this front, new strategies focusing on incorporating structure units with configuration-based intrinsic free volume elements, ,− constructing super-rigid ladder polymers, , or creating cascaded microcavities via postsynthesis modification are highly promising, though most of them involve sophisticated and complex synthesis. On the other hand, chemical cross-linking remains as a less synthetically challenging yet highly effective approach that can suppress physical aging and condensable gas-induced plasticization by restricting the mobility of polymer chains and improving polymer rigidity. − However, random cross-link processes in most existing studies result in reduced gas permeabilities due to tightened chain packing and consequent reduced free volume caused by cross-linked network structures.…”