The mechanism of light gas transport through configurational free volume in glassy polymers
2022
Box, William J. | Huang, Zihan | Guo, Ruilan | Galizia, Michele
A possible mechanism of small molecule transport in glassy polymer membranes containing configurational free volume (i.e., triptycene moieties) is proposed. To this aim, a family of triptycene-containing polybenzoxazoles (TPBOs) exhibiting systematically varied molar amount of triptycene units, ranging from 25% to 100%, was selected to perform a light gas (N₂, CH₄ and CO₂) fundamental sorption-transport study. The dual mode sorption-mobility model was used to isolate the effect of configurational free volume on penetrant transport. CO₂ and CH₄ Henry's and Langmuir's mode diffusion coefficients 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. Interestingly, while the Henry's CO₂/CH₄ diffusivity-selectivity changes little with increasing amount of configurational free volume, the Langmuir's CO₂/CH₄ diffusivity-selectivity increases up 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. In sharp contrast, pure- and mixed-gas sorption, as well as solubility-selectivity, is essentially unaffected by the triptycene molar content. According with this physical picture, the analysis of isosteric heats of CO₂ sorption indicates that TPBOs plasticization resistance is independent of triptycene content.
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