Disentangling Adsorption and Absorption in Microporous Polymers

Polymers of intrinsic microporosity (PIMs) are a unique class of soft materials, which, unlike hard materials such as zeolites or carbons, are capable of both vapor adsorption (nanopore filling) and absorption (polymer plasticization/swelling). While adsorption is governed by pore structure and surface functionalization, adsorption depends on the chemical composition of the matrix material. Since both adsorption and absorption increase sorbent mass (vapor uptake), gravimetric and volumetric characterization methods exhibit severe limitations in isotherm interpretation. Thus, distinguishing between vapor adsorption and absorption remains a key challenge for understanding sub-nanometer-scale processes, which play a crucial role in many emerging applications of PIMs, including gas separation, water purification, organic solvent nanofiltration, and electrochemical energy storage/conversion. Herein, an alternative approach is presented based on in situ ellipsometric read out and concurrent optical modeling of adsorption and absorption. Ellipsometry is highly sensitive to changes in thickness and optical density of the thin film sorbents, enabling the acquisition of both adsorption and absorption isotherms. This study showcases four PIM sorbents with varied material chemistry, responding to various vapors. Their ad-/absorption contributions are studied and disentangled experimentally, with nanopore confinement and swelling models based on classical physisorption and polymer Flory-like theory.