Investigation of swelling kinetic behavior of polylactic acid in supercritical CO₂

As a renewable bio-based polymer, polylactic acid (PLA) is commonly processed and modified using supercritical CO₂ (scCO₂) and is widely used in food packaging and biomedical fields. In this study, molecular dynamics (MD) simulations combined with density functional theory (DFT) calculations were employed to systematically investigate the microscopic swelling dynamics of PLA under different thermodynamic conditions and solvent environments, revealing a cooperative mechanism of “CO₂ adsorption–chain relaxation–structural expansion” during scCO₂-induced swelling. The results demonstrated that, compared with ambient-pressure CO₂ or neat PLA systems, the introduction of scCO₂ reduced the glass transition temperature (T_g) of PLA, and the pressure-dependent increase in T_g depression was consistent with experimentally observed trends. Within the pressure range of 10–25 MPa, the T_g reduction reached 47.58–57.26 K. As swelling proceeded, the mobility of PLA chain segments increased, and the polymer structure gradually evolved from a densely packed state into a porous network. Contrary to the conventional expectation that higher temperatures are more favorable for polymer chain mobility and swelling, the strongest swelling response was observed under low-temperature, high-pressure conditions (323 K, 25 MPa), where the solvent-accessible surface area (SASA) increased by up to 85.14%. Further DFT analysis revealed that the carbonyl oxygen in PLA acts as a Lewis base and donates electrons to the carbon atom of CO₂ (Lewis acid), forming weak electron donor–acceptor (EDA) interactions. These intermolecular interactions facilitated the adsorption and distribution of CO₂ within the polymer matrix and partially modulated the mobility of polymer chain segments.