Penetrant-Triggered Responsiveness in Carboxylate Polymer of Intrinsic Microporosity

While complex mixture fractionation has become a primary focus in membrane science, the mechanisms behind penetrant-penetrant and penetrant-polymer interactions governing molecular separations require deeper investigation. Of equal importance, how the external environment dictates these interactions remains poorly understood. To address this knowledge gap, we elucidate the molecular mechanisms that control how microporous polymers structurally adapt to their external environment. Resolving previous speculations, we show that upon reaction with carboxylic acid groups in carboxylate polymer of intrinsic microporosity (cPIM), ammonia forms ammonium carboxylate adducts. This chemical modification alters small-molecule sorption, local chain organization, free volume distribution, and, ultimately, penetrant diffusion. Notably, ammonia-modified cPIM rapidly reacts with carbon dioxide, causing in situ polymer ionic cross-linking. Using a suite of advanced experimental methods, including transport measurements, FTIR, PALS, and NMR, we uncover the mechanisms behind the observed responsiveness and its time-evolution. Ultimately, we demonstrate that accounting for coupled interactions is crucial for accurate performance prediction and optimal design of responsive membranes.