Lignin-g-poly(acrylamide)-g-poly(diallyldimethyl- ammonium chloride): Synthesis, Characterization and Applications

The search for a renewable substitute to petroleum-based products has fueled increasing research on lignin, an under-utilized product from pulping processes. In this work, lignin was copolymerized with acrylamide (AM) and diallyldimethylammonium chloride (DADMAC) under acidic conditions with Na₂S₂O₈ as an initiator, generating a cationic water-soluble lignin-g-P(AM)-g-P(DADMAC) copolymer. The optimal reaction conditions, using a 5×4 factorial design experiment, were determined to be an AM/DADMAC/lignin molar ratio of 5.5:2.4:1, 90 °C, 0.26 mol L^-1 of lignin, and pH 2. Under the optimal reaction conditions, the resulting lignin-g-P(AM)-g-P(DADMAC) copolymer was 83 % soluble in an aqueous solution (at 10 g L^-1) and at neutral pH. The copolymer had a charge density of 1.27 meq g^-1, molecular weight of (1.33±0.08) ×10⁶, an AM grafting ratio of 112 wt %, and a DADMAC grafting ratio of 20 wt %. In addition, the activation energy for producing this copolymer as well as the thermal and rheological properties of the copolymer were determined. The flocculation performance of lignin-g-P(AM)-g-P(DADMAC) copolymer was evaluated in a kaolin suspension, which showed that the lignin copolymer had a comparable flocculation efficiency with the synthetic analogue of P(AM)-g-P(DADMAC) at pH 6.