New insights into the mechanism of interaction between CO2 and polymers from thermodynamic parameters obtained by in situ ATR-FTIR spectroscopy.

This work reports new physical insights of the thermodynamic parameters and mechanisms of possible interactions occurring in polymers subjected to high-pressure CO2. ATR-FTIR spectroscopy has been used in situ to determine the thermodynamic parameters of the intermolecular interactions between CO2 and different functional groups of the polymers capable of specific interactions with sorbed CO2 molecules. Based on the measured ATR-FTIR spectra of the polymer samples subjected to high-pressure CO2 (30 bar) at different temperatures (300–340 K), it was possible to characterize polymer–polymer and CO2–polymer interactions. Particularly, the enthalpy and entropy of the formation of the specific non-covalent complexes between CO2 and the hydroxy (–OH), carbonyl (CO) and hydroxyimino (N–OH) functional groups of the polymer samples have been measured. Furthermore, the obtained spectroscopic results have provided an opportunity for the structure of these complexes to be proposed. An interesting phenomenon regarding the behavior of CO2/polymer systems has also been observed. It has been found that only for the polyketone, the value of enthalpy was negative indicating an exothermic process during the formation of the CO2–polymer non-covalent complexes. Conversely, for the polyoxime and polyalcohol samples there is a positive enthalpy determined. This is a result of the initial polymer–polymer interactions requiring more energy to break than is released during the formation of the CO2–polymer complex. The effect of increasing temperature to facilitate the breaking of the polymer–polymer interactions has also been observed. Hence, a mechanism for the formation of CO2–polymer complexes was suggested based on these results, which occurs via a two-step process: (1) the breaking of the existing polymer–polymer interactions followed by (2) the formation of new CO2–polymer non-covalent interactions.