Sustainable Lignin–Acrylonitrile Copolymers via Aqueous Free Radical Polymerization for Carbon Materials Development

This study investigates a novel bio-based precursor for carbon materials by copolymerizing LignoBoost lignin (LB) with non-fossil derived acrylonitrile (AN) via free radical polymerization (FRP) in aqueous medium. Key parameters such as lignin activation without functionalization, the order of reactant addition, and the reactivity of functional groups were analyzed. Copolymers with increasing LB initial contents (3.4, 17.5, and 33 wt.%) were synthesized and characterized in comparison to polyacrylonitrile (PAN) homopolymer based on molecular weight (GPC), molecular structure (FTIR, 1H-NMR, and EA), and thermal stability (TGA and DSC). Higher LB initial content (33 vs. 3.4 wt.%) resulted in lower Mw (24 206 vs. 56 075 g/mol) and higher lignin incorporation in the copolymer (33.0 vs. 18.0 wt.%). The new copolymers exhibited enhanced thermal stability compared to PAN and LB, with char yield at 900°C in the range of 39–42 wt.% vs. 27 wt.% for PAN and 32 wt.% for LB. DSC results indicated a safer cyclization process revealed by the lower heat release per temperature interval (|ΔH| = 2.78–4.42 J/g/°C vs. 4.85 J/g/°C for PAN) and lower onset temperatures of the exotherms (177.9–208.4°C vs. 234.7°C for PAN). The lignin-AN copolymers, obtained from renewable and non-fossil derived raw materials and through a sustainable, easily scalable process, exhibited superior properties compared to PAN as precursors for carbonizable polymer systems and nitrogen-containing carbon materials.