(647c) Unveiling the Mechanism of Light Gas and Vapor Transport through Configurational Free Volume in Glassy Polymers

Conventional glassy polymers suffer from three major issues, namely the permeability-selectivity trade-off, physical aging and plasticization, which represent a roadblock to progress in separation science and technology. Polymers exhibiting iptycene units, that is, configurational free volume, help overcome this limitation. The internal free volume of triptycenes is not related to the molecular conformation, such as the excess free volume in conventional glassy polymers, but to the molecular configuration. Moreover, the size of the internal volume of iptycene units is extremely regular and comparable to the size of one single molecule, which makes iptycene-based polymers highly selective. Despite this, the fundamental mechanism of small molecules transport in polymers exhibiting configurational free volume is still poorly understood. To fill this knowledge gap, a class of polybenzoxazoles (TPBOs) with systematically varied molar amount of triptycene units was selected to perform a fundamental transport study considering a large variety of penetrants, spanning from light gases to bulky vapors and propose a possible mechanism of small molecule transport in glassy polymers exhibiting configurational free volume.

The dual mode sorption-mobility model was used to isolate and elucidate the specific effect of configurational free volume on small molecule transport. For light gases, solubility and solubility-selectivity were found to be virtually unaffected by configurational free volume. In contrast, configurational free volume affects diffusivity and diffusivity-selectivity to a significant extent. Specifically, the Henry’s and Langmuir’s mode diffusion coefficients, D_D and D_H, were estimated and used to quantify the Henry’s and Langmuir’s contributions to CO₂/CH₄ diffusivity-selectivity as a function of the triptycene molar content in TPBOs. Remarkably, while the Henry’s CO₂/CH₄ diffusivity-selectivity is fairly constant with increasing amount of configurational free volume, the Langmuir’s CO₂/CH₄ diffusivity-selectivity increases to infinity when the molar triptycene concentration in the polymer is 75% or higher, showing that the diffusion of larger molecules (i.e., CH₄) is increasingly hindered relative to smaller molecules (i.e. CO₂) diffusion when more configurational free volume is incorporated in the polymer. Therefore, our study provides the final proof of the superior size-sieving ability exhibited by iptycene-based polymers, by unveiling, for the first time, its molecular origin.

In contrast, when considering bulky vapor sorption and transport, both solubility and diffusion coefficients are affected by the presence of configurational free volume. Remarkably, vapor sorption coefficients decrease with increasing critical temperature, which represents a deviation from a well-established rule. The molecular origin of this behavior is discussed.