Polymer-Grafted Lignin Surfactants Prepared via Reversible Addition–Fragmentation Chain-Transfer Polymerization

Kraft lignin grafted with hydrophilic polymers has been prepared using reversible addition–fragmentation chain-transfer (RAFT) polymerization and investigated for use as a surfactant. In this preliminary study, polyacrylamide and poly­(acrylic acid) were grafted from a lignin RAFT macroinitiator at average initiator site densities estimated to be 2 per particle and 17 per particle. The target degrees of polymerization were 50 and 100, but analysis of cleaved polyacrylamide was consistent with a higher average molecular weight, suggesting not all sites were able to participate in the polymerization. All materials were readily soluble in water, and dynamic light scattering data indicate polymer-grafted lignin coexisted in isolated and aggregated forms in aqueous media. The characteristic size was 15–20 nm at low concentrations, and aggregation appeared to be a stronger function of degree of polymerization than graft density. These species were surface active, reducing the surface tension to as low as 60 dyn/cm at 1 mg/mL, and a greater decrease was observed than for polymer-grafted silica nanoparticles, suggesting that the lignin core was also surface active. While these lignin surfactants were soluble in water, they were not soluble in hexanes. Thus, it was unexpected that water-in-oil emulsions formed in all surfactant compositions and solvent ratios tested, with average droplet sizes of 10–20 μm. However, although polymer-grafted lignin has structural features similar to nanoparticles used in Pickering emulsions, its interfacial behavior was qualitatively different. While at air–water interfaces, the hydrophilic grafts promote effective reductions in surface tension, we hypothesize that the low grafting density in these lignin surfactants favors partitioning into the hexanes side of the oil–water interface because collapsed conformations of the polymer grafts improve interfacial coverage and reduce water–hexanes interactions. We propose that polymer-grafted lignin surfactants can be considered as random patchy nanoparticles with mixed hydrophilic and hydrophobic domains that result in unexpected interfacial behaviors. Further studies are necessary to clarify the molecular basis of these phenomena, but grafting of hydrophilic polymers from kraft lignin via radical polymerization could expand the use of this important biopolymer in a broad range of surfactant applications

Lignopolymer Superplasticizers for Low-CO₂ Cements

Grafting hydrophilic polyacrylamide from a kraft lignin core results in a lignopolymer that effectively plasticizes portland cement paste. Here, the potential of this sustainably sourced lignopolymer is examined for improving the workability of portland cement blended with two natural, finely divided mineral materials (kaolin clay and clinoptilolite zeolite), which can be potentially used in combination with portland cement as a means to reduce the cement clinker content and the concomitant embodied energy and greenhouse gas emissions in concrete. Both mineral materials are known to affect cement hydration reactions but can significantly reduce workability when blended with portland cement, presenting a challenge for their practical large scale use., The plastic behavior of mineral-cement combinations dosed with polyacrylamide-grafted kraft lignin was compared to the behavior of those with a lignosulfonate developed for plasticizing portland cement and a commercial polycarboxylate ether (PCE). Compared to the other admixtures, the lignopolymer was found to adsorb strongly to both kaolin and clinoptilolite and resulted in the lowest yield stresses in pastes containing 25% mineral material and 75% Type I/II portland cement at both high (5 mg/mL, 0.25 wt %) and low (0.5 mg/mL, 0.025 wt %) admixture content, but no significant retardation was observed in calorimetry measurements of hydration kinetics. In contrast, the PCE had a low affinity for both mineral admixtures at higher concentrations compared to that of both lignin superplasticizers, and the resulting tests of slump spread and viscosity were similar for these two minerals. Lignosulfonates had affinity for the mineral phases similar to that of the other superplasticizers but were ineffective at improving workability, pointing to the need for steric interactions in plasticizing mineral materials used in combination with cement. These results indicate that lignopolymers could be an effective superplasticizer for a broad range of high surface area minerals to be used in combination with portland cement