Gas sorption in polymers of intrinsic microporosity: The difference between solubility coefficients determined via time-lag and direct sorption experiments

Abstract

An understanding of gas transport in polymers of intrinsic microporosity (PIMs) is limited by a lack of directly determined (experimental) sorption data and by quantitative inaccuracies in solubilities calculated indirectly from permeation time-lag analysis. To address this, we provide a detailed gas sorption analysis for seven different PIMs and assess the influence of the non-linear steady-state CO2 concentration profiles on the apparent solubilities obtained from permeation experiments. Equilibrium sorption was analyzed with dual-mode sorption (DMS) and Guggenheim, Anderson and De Boer (GAB) models, which provided accurate descriptions of the data. The non-linear steady-state CO2 concentration profiles were calculated using the thermodynamic Fick's law based on the DMS or GAB model coefficients. Once the thermodynamic term was incorporated into the model for permeation, better agreement between directly (gravimetric sorption) and indirectly (time-lag analysis) determined solubility was observed for most membranes. The monolayer capacities were found to be linearly dependent between the models and could be used for a qualitative comparison of the apparent surface area of PIM materials. Diffusivities determined from time-lag analysis were higher (e.g., PIM-Trip-TB, CO2, 111·10−8 m2 s−1) than those from sorption measurements (24·10−8 m2 s−1), probably due to substantially low mass uptake in Henry mode (11% of total sorption capacity). Thus, a careful gas sorption analysis in combination with time lag experiments provides a deeper insight into the transport behavior of gases in PIMs than when only time lag measurements are performed.