Solvent‐Free Mechanochemical Activation of Lignin for Highly Bio‐Based Polymer Composites With Enhanced Photothermal Performance

ABSTRACT As the most abundant renewable aromatic biopolymer, lignin holds great promise for sustainable photothermal composites. However, its practical application is constrained by inefficient photothermal conversion, mainly due to strong aggregation and inherent structural features. To improve its photothermal efficiency and compatibility with polymer matrices, chemical modification is often required. Nevertheless, conventional modification methods typically rely on hazardous organic solvents and reagents, impeding the green and scalable production of lignin‐based materials. Herein, we report highly bio‐based photothermal polymer composites (denoted as GMLEC) fabricated via the solvent‐free co‐ball milling of lignin and epoxidized soybean oil (ESO), followed by thermal curing with citric acid (CA). This strategy avoids conventional petroleum‐based polymer matrices and effectively addresses the phase separation challenges. Benefiting from the mechanochemical activation of lignin and improved network homogeneity, the optimized GM 6 L 20 EC exhibits a tensile strength of 6.91 MPa and an apparent photothermal conversion efficiency of 48.12%. Furthermore, a photothermal‐driven thermoelectric generator assembled with GMLEC delivers a stable voltage output of 345.2 mV for over 12 h. Given its cost‐effective fabrication process and favorable screening‐level carbon‐footprint estimate, GMLEC represents a promising and more sustainable alternative to conventional petroleum‐based photothermal polymer materials for green energy harvesting.