Microcosmic understanding on thickening capability of copolymers in supercritical carbon dioxide: the key role of π–π stacking

Abstract

In this study, styrene/heptadecafluorodecyl acrylate (St–HFDA) copolymers of different compositions were synthetized for the purpose of thickening supercritical carbon dioxide (SC-CO₂). The cloud point pressures of the copolymer–CO₂ mixtures and the thickening effects of these copolymers for SC-CO₂ were measured. Molecular dynamics (MD) simulations were used to evaluate the intermolecular interactions and microstructures of polymer–CO₂ systems, the copolymer–CO₂ interaction energy, cohesive energy density (CED), solubility parameter, equilibrium conformations and radial distribution functions (RDFs) were obtained, which provided useful information for microscopic understanding on the thickening capability of copolymers in SC-CO₂. It was found that all the synthesized St–HFDA copolymers induced greater viscosity enhancements of SC-CO₂ compared to poly(Heptadecafluorodecyl acrylate) (PHFDA), and π–π stacking of the Styrene (St) groups played a key role in thickening SC-CO₂. On one hand, the introduction of the St groups into PHFDA weakened the CO₂-philicity of the polymers by reducing the polymer–CO₂ interaction and increasing polymer–polymer interactions, resulting in higher cloud point pressure in SC-CO₂ compared to PHFDA. On the other hand, the increase of the polymer–polymer interaction via π–π stacking provided an associative force to thicken SC-CO₂. The subtle relationship between the copolymer composition and thickening abilities of the copolymers in SC-CO₂ were evaluated and the optimum styrene molar ratio was determined. It can be concluded that the content of the CO₂-philic HFDA groups and the CO₂-phobic St groups in the copolymers should be optimized to achieve the balance between the solubility and the thickening capability.