Simultaneous Improvement of Melt Processability and Ductility in Poly(butylene succinate-co-terephthalate) via In-Situ Esterification with Kraft Lignin

Controlling polymer chain mobility, which dictates the glass transition temperature (T_g), remains a critical challenge in designing biodegradable polyesters with an optimal balance of thermal stability and mechanical performance. However, conventional blending approaches offer limited control over local chain dynamics due to pronounced macro-phase separation. In this work, kraft lignin was introduced as a biomass-based rigid aromatic modifier to systematically modulate the segmental mobility of poly(butylene succinate-co-terephthalate) (PBST) via in-situ polymerization. The formation of covalent or strong physical ester linkages between the PBST matrix and lignin was indicated by a distinct ester carbonyl (C=O) stretching peak at 1720-1730 cm^-1 in the FTIR analysis of the isolated lignin fraction. Notably, despite the incorporation of rigid aromatic structures, the in-situ synthesized PBST-kraft lignin composites successfully overcame the conventional trade-off between stiffness and ductility. At an optimized lignin content of 0.5 wt% (PBST_L0.5), the composite exhibited an outstanding elongation at break of 733.7% (compared to 708.2% for neat PBST) and a maximum toughness of 159.8 J·m^-3. This synergistic mechanical enhancement was well preserved even after processing, where the injection-molded PBST_L0.5 specimen demonstrated a superior tensile stress of 36.0 MPa and an increased elongation of 700.2% (compared to 31.2 MPa and 603.9% for neat PBST). Furthermore, rheological and thermal analyses demonstrated that the modified chain architectures lowered flow resistance, leading to enhanced melt processability. This study provides critical molecular insights into the structure-mobility relationships of lignin-incorporated polyesters and demonstrates a scalable, molecularly informed strategy for tuning the physical profiles of sustainable materials.