The mechanism of light gas transport through configurational free volume in glassy polymers

Abstract

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.