Production and Application of Triblock Hydrolysis Lignin-Based Anionic Copolymers in Aqueous Systems

Although lignin is currently an under-utilized biopolymer, it has the potential to be valorized through different modification pathways to yield alternative products to petroleum-based ones. In this work, hydrolysis lignin (HL) was copolymerized with acrylamide (AM) and acrylic acid (AA) under acidic conditions to generate the lignin/AM polymer (HM), lignin/AA polymer (HA), and lignin/AM/AA copolymer (HAM) with different negative charge densities and molecular weights. Lignin-based polymers characterized by advanced tools, such as proton nuclear magnetic resonance (¹H NMR), gel permission chromatography (GPC), and elemental analysis confirmed the successful polymerization of HL with AM, AA, or AM/AA monomers. The adsorption analysis using a quartz crystal microbalance (QCM) revealed that compared to diblock HM and HA, the triblock copolymers of HAM adsorbed more on the Al₂O₃ surface and generated a bulkier adsorbed layer, which is important for lignin-based coating formulation. HAM1 with a lower charge density yielded a higher surface excess density, while HAM2 with a larger R _h occupied more space (153.7 Ų) at the interface of water and Al₂O₃. In suspension systems, because of the higher M _w, R _h, and adsorption affinity, the bridging performance of HAM2 was more remarkable than that of the other lignin derivatives for Al₂O₃ particles via forming stronger flocs (with a deflocculation parameter, T _df, of 80.6 s). However, the diblock lignin-AA (HA1) polymer showed the fastest floc regrowth capability after reducing the shear forces (with a reflocculation parameter, T _rf, of 62.5 s). The high thermal stability, T _g, and rheological characteristics of the HAM copolymer proved that it can be an excellent material for coating formulations and flocculants for wastewater treatment systems.