Correlation of molecular weight diversity and colloidal disability performance of anionic lignin derived polymer

Abstract

Polymerization is considered one of the valorization techniques of lignin for various applications. Generally, polymerization techniques affect the molecular structure of polymers. In this work, we evaluated a hypothesis that the polymerization strategy and, thus, the molecular weight distribution of lignin-derived polymers would impact their effectiveness as a colloidal disability agent (i.e., flocculant) for a suspension system. To achieve our goal, different lignin-polymethacrylic acid (PMAA) polymers were synthesized via free radical polymerization (FRP) and atom transfer radical polymerization (ATRP) techniques. The experiments were designed such that two sets of polymers with different molecular polydispersities but similar average molecular weights, charge densities, radii of gyration, and hydrodynamic diameters were obtained. Both systems developed colloidal destabilization mainly via bridging phenomenon. The flocculation performance of the ATRP-synthesized samples with narrower molecular weight distribution was notably better than that of FRP-induced samples. The rheological and flocculation analyses under different shear rates confirmed that the ATRP-induced sample promoted the entanglement of particles and had a higher flocculation efficiency. The flocculant prepared using the ATRP technique demonstrated a significantly lower optimal dosage, requiring only 0.4 mg per gram, compared to 25.6 mg per gram for the samples produced using the FRP method. Also, this sample produced stronger flocs with more re-flocculation tendency under different shear rates, while it generated larger flocs under stirring conditions. Such ATRP-derived flocs had an average chord length of approximately 60 μm, and those of FRP-based ones had a 30 μm chord length, while both FRP and ATRP polymers had 500 Da molecular weight. The results of this work confirmed that, while lignin-derived polymers can be an effective flocculant, strategies must be taken into account for generating lignin-derived polymers with a controlled and preferably narrow molecular weight distribution.