Effect of Carbon Dioxide Absorption on the Dynamics of Polyethyleneimine under Dry Conditions

Abstract

The chemical transformations associated with carbon dioxide absorption in polyethyleneimine (PEI) have been extensively characterized. However, how these transformations impact the dynamics of PEI remains poorly understood. Through a combination of nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), broadband dielectric spectroscopy (BDS), and differential scanning calorimetry (DSC), we systematically investigate the influence of CO2 chemisorption under dry conditions at room temperature on the dynamics of PEI. PEI films containing 1.1–2.7 mmol CO2/g PEI (corresponding to [CO2]:[N] = 0.05:1 to 0.13:1) were characterized across wide time scales from ∼10–10 to 102 s, where [CO2] and [N] represent the absorbed molar numbers of CO2 and the molar numbers of amines in PEI. Spectroscopic measurements, including NMR and FT-IR, confirm ammonium carbamate formation upon CO2 sorption. Furthermore, the CO2 chemisorption induces pronounced dynamical changes, including a near-linear increase in glass transition temperature, Tg, upto 11–19 K with [CO2]:[N] = 0.05:1 to 0.13:1, and a significant broadening of the glass transition step in DSC measurements. Accordingly, BDS reveals substantial slowing down in segmental dynamics, along with the emergence of two new types of secondary relaxations associated with carbamate and ammonium ion motions. Solid-state 13C NMR further shows heterogeneous local relaxations and a reduced alkyl mobility of PEI upon CO2 absorption. Overall, CO2 sorption slows PEI dynamics from sub-nanosecond local motions to long-time chain relaxation, contributing to the often-observed slow CO2 transport and low amine efficiency in polyamine-based carbon capture systems.