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

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 degrees 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 size-controlled and sorption is enthalpy-controlled, which may create a trade-off between sorption-and 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.