Natural Deep Eutectic Solvents as Sustainable Modifiers for Tailoring the End-of-Life Degradation of Polylactic Acid: A Combined Experimental and Computational Study

This study presents a combined experimental and computational investigation to elucidate the degradation mechanisms of polylactic acid (PLA) in the presence of specific natural deep eutectic solvents (NADES): choline chloride/glycerol, betaine/glycerol, and choline chloride/lactic acid during processing, with the aim of promoting PLA molecular degradation during processing, as a first step toward facilitating its subsequent biodegradation after use (at end-of-life). The degradation processes were experimentally monitored through thermal, rheological, and physicochemical characterizations, while density functional theory (DFT) modeling provided atomic-scale insights into the reaction pathways and interactions between the polymer chain and NADES at the atomic scale. Experimental and computational results indicate that NADES significantly enhance PLA degradation rates after thermomechanical solicitation, particularly the acidic NADES3 (choline chloride/lactic acid), the only formulation containing a carboxylic acid component. Furthermore, computational analyses revealed key intermediate states and energy barriers associated with bond cleavage in the polymer chain. To establish a correlation between the experimental and theoretical studies, MALDI-TOF mass spectrometry analyses were performed, along with QTAIM analyses, enabling a clear and precise explanation of the interaction between the polymer and NADES, as well as the formation of degradation products resulting from this interaction. This approach represents a strong and innovative aspect of this multidisciplinary study. This work highlights the potential of NADES as eco-friendly promoters of polymer degradation after use and provides a fundamental understanding of the mechanisms involved, thereby contributing to the development of sustainable waste management strategies. The biological biodegradation and mineralization of the resulting PLA/NADES materials were not assessed here and are the subject of an ongoing complementary study.