Sequential Fractionation of Lignin for Interfacial Optimization and Enhanced Mechanical Performance in PBAT Composites

To address the inherent challenge of poor interfacial compatibility in lignin/poly(butylene adipate-co-terephthalate) (PBAT) composites, lignin was extracted from <i>Camellia oleifera</i> shells and subjected to sequential solvent fractionation using ethanol, acetone, and tetrahydrofuran (THF). Two representative fractions-acetone-soluble (ACL) and THF-soluble (THFL)-were selected for composite preparation with PBAT via solvent casting. The influence of lignin fractionation on the structural and performance characteristics of the resulting composites was systematically evaluated through Fourier-transform infrared (FTIR) spectroscopy, the water contact angle (WCA), differential scanning calorimetry (DSC), tensile testing, and scanning electron microscopy (SEM). The results revealed that the abundant hydroxyl groups and benzene rings present in both ACL and THFL facilitated hydrogen bonding and conjugation interactions with the PBAT matrix, significantly improving interfacial adhesion. Notably, the ACL fraction effectively suppressed phase separation and increased the glass transition temperature (<i>T_g</i>) by 1.9 °C, leading to a 13.9% enhancement in tensile strength compared to neat PBAT. More strikingly, the incorporation of only 7 wt% THFL resulted in a remarkable 31% improvement in tensile strength. This substantial enhancement was primarily attributed to the favorable polarity match between THFL and PBAT, as well as the nucleating effect of THFL, which increased the crystallinity of PBAT by 25.3%. This study highlights the effectiveness of sequential lignin fractionation in tailoring the interfacial properties of biodegradable polymer composites. It also provides a promising strategy for the high-value utilization of lignin toward the development of high-performance, environmentally friendly materials.