Depolymerized Lignin as an Enabler for Superior Lignin–Acrylonitrile Copolymers for Sustainable Wet-spun Fibers

Abstract

The growing demand for renewable materials and the valorization of biomass by-products have positioned lignin as a promising raw material for bio-based carbon fiber (CF) production. However, its heterogeneous, branched structure, and multiple functional groups often yield polymers with irregular architectures, limiting CF mechanical performance. To address this, LignoBoost lignin (LB) was subjected to acidic oxidative depolymerization, generating a depolymerized lignin fraction (DLB) with a controlled number of reactive sites. DLB was subsequently copolymerized with non-fossil acrylonitrile (AN) through a two-step aqueous free-radical process, without prior chemical functionalization. The effect of the initiation system was evaluated using APS/NaHSO3/FeSO4, CaCl₂/H₂O₂, and a dual-initiator approach, revealing a synergistic effect that enabled the synthesis of high-molecular-weight copolymers with optimized structural properties. Copolymers were synthesized with 0 to 50 wt.% DLB and characterized alongside polyacrylonitrile (PAN) by GPC, EA, ATR-FTIR, 1H NMR, 31P NMR, TGA, and DSC. Successful copolymer formation was confirmed, with the extent of incorporation fraction and conversion quantified. Increasing DLB content resulted in improved thermal stability, reflected in higher char yield (30–36% for AN-DLB copolymers vs. 27% for PAN, at ca. 900 ºC) and a more controlled, safer cyclization pathway, which is an essential requirement for CF precursor processing. Comparison with copolymers prepared from native LB demonstrated clear advantages of depolymerization: AN-DLB copolymers exhibited higher Mn (81,868 g/mol vs. 15,588 g/mol), narrower dispersity (Ð = 1.98 vs. 2.10), thermal transitions more closely resembling PAN, and superior structural integrity when processed into wet-spun fibers. Overall, oxidative depolymerization significantly enhances lignin’s compatibility in AN-based copolymers, revealing the potential of poly(AN–DLB) to serve as a sustainable precursor for next-generation carbon fibers.