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

Abstract

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.