Weak Interactions between Poly(ether imide) and Carbon Dioxide: A Multiscale Investigation Combining Experiments, Theory, and Simulations

This contribution aims at an understanding, at a molecular scale, of the state of CO2 molecules absorbed in glassy poly(ether imide) (PEI). This issue has been challenged by combining different approaches that tackle the problem from both the experimental and the theoretical viewpoints and providing a comprehensive physicochemical picture. In situ FTIR spectroscopy and gravimetry were exploited to gather relevant experimental information, while molecular dynamics (MD), density functional theory (DFT), and statistical thermodynamics approaches were used to model the behavior of the binary system at different scales. Based on the findings of FTIR spectroscopy and on DFT and MD calculations, it was determined that, among the possible interaction configurations, some are prevailing. In particular, the carbon atom of carbon dioxide molecules establishes relatively weak interactions prevalently with the carbonyl groups of PEI. A quantitative estimate of such interaction has been provided by MD calculations. The system was also analyzed using a lattice fluid model, specifically developed to deal with sorption of low molecular weight compounds in glassy polymers, that is rooted on statistical thermodynamics, determining the values of the isosteric heat of sorption and carbon-dioxide-polymer interaction energy. Finally, experimental data of CO2-PEI mutual diffusivity have been interpreted using a semiempirical theoretical model accounting for the effects of the penetrant concentration, of energy barriers associated with the occurrence of an effective diffusive jump, and of a thermodynamic factor.