Effect of Primary vs Secondary Amines on the Reactivity and Dynamics of CO₂ in Polyallylamine Sorbents under Humid...

Selective facilitated transport membranes are viable technologies for separating dilute CO₂ from mixed-gas streams. In this study, we measure the reactivity and transport behavior of two polyallylamine amine polymers that bind CO₂ in humid environments. We quantify sorption of CO₂ and water components using a tandem quartz crystal microbalance mass sensor coupled with infrared spectroscopy for chemical identification of the sorbed gases and their reaction products. The two high-molecular-mass (glassy) polyallylamine materials, one containing just primary amines and the other containing isopropyl-functionalized secondary amines, react with humid CO₂ to form tethered carbamate/carbamic acid groups or bicarbonate ions, respectively. Because our technique allows for the discernment of specific water and CO₂ uptake, we also identify that the different reactivities influence the sorbed water content in the polymer films. We further quantify the local or segmental mobility of the dosed polymers with quasielastic neutron scattering measurements on the few nanoseconds to tens of picoseconds time scales, revealing that a classic jump diffusion model describes the dynamics of the polymers and their polymer-sorbate complexes. The resulting jump lengths and residence time between jumps differ based on the polymer chemistry and dosing conditions, with the hindered secondary amine systems generally presenting longer residence times and slightly longer jump distances. This combination study featuring both quasielastic neutron scattering and tandem gravimetric and chemical uptake measurements adds to the broader understanding of amine polymer dynamics and reactivity in CO₂ capture applications.