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

Abstract

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.

Lignin from Micro- to Nanosize: Production Methods

“…Therefore, this method was used for polymer-grafted lignin NPs synthesis by Gupta and Washburn [78] and Silmore et al [79] from KL. The identical procedure was used in both publications where [78] is more focused on the NPs and [79] on the behavior of the NPs in emulsions. The synthesis was comprised of two steps: (1) Preparation of RAFT Macroinitiator; (2) grafting.…”

Mussel‐inspired bio‐based water‐resistant soy adhesives with low‐cost dopamine analogue‐modified silkworm silk Fiber

“…As is well known, a certain number of phenols and catechol structures were found in alkali lignin. These structural features make alkali lignin a natural polymeric surfactant with strong reactivity with other functional groups, such as aldehyde groups and isocyanate moieties . Zhang et al .…”

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

“…Lignin's hydroxyl groups can initiate ring‐opening polymerization (ROP) of ϵ ‐caprolactone, yielding lignin‐poly(caprolactone) . Lignin copolymers can also be generated using free radical polymerization, grafting vinylic monomers onto the lignin backbone using a chemical initiator . This technique has been used to generate lignin‐polystyrene, lignin‐polyacrylamide, lignin‐poly(acrylic acid), and lignin‐poly(vinyl acetate) .…”

“…[15] Lignin copolymers can also be generated using free radicalp olymerization, grafting vinylic monomers onto the lignin backboneu sing ac hemical initiator. [16][17][18][19] This technique has been used to generate lignin-polystyrene, [20] lignin-polyacrylamide, [21] lignin-poly(acrylic acid), [19] and lignin-poly(vinyl acetate). [20] Both lignin-poly(acrylic acid) and lignin-poly(acrylamide) are water soluble products,w hereas lignin-poly(styrene) is water insoluble and has been used as ab iodegradable wood coating to reduce water sensitivity and increase the binding strengtho fp oly(1-phenylethylene) plastic coatingso n wood.…”