Lignin is the third most abundant biopolymer on Earth (behind cellulose and chitosan). The chemistry of lignin is based on three monolignols, coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which are polymerized in situ to form a complex, three-dimensional material.
Schematic of native lignin structure showing a diversity of linkages based on ether and alkane groups. The only significantly reactive groups in native lignin are aliphatic and aromatic hydroxyls.
Purified lignin is obtained through various biomass-processing strategies (usually referred to as “delignification”!), primarily for pulp and paper production, but more recently in cellulosic ethanol production as well as the production of other biomass-derived chemicals. Depending on the source (hardwood, softwood, etc.) and processing conditions, the final product can vary significantly in terms of monomer composition, molecular weight, and the concentration of additional functional groups, such as carboxylates (found in kraft lignin) or sulfonates (found in lignosulfonates). Most commercial lignin forms have molecular weights below 30 kDa, making them organic nanoparticles that can have a diversity of potential uses.
In plants, lignin serves several critical functions, including interfacial binding of cellulose fibers, contributing to mechanical strength, and facilitating water transport. These native functions suggest numerous technological applications of lignin, but lignin finds limited use, primarily as a dispersant in cement and other multi-phase systems. The hypothesis in the Washburn Lab is that grafting synthetic polymers onto a lignin nanoparticle core can leverage these native functions in many applications. Synthesis, characterization, and testing of a broad range of polymer-grafted lignin nanoparticles is a central focus of the Washburn Lab.
Schematic of a polymer-grafted lignin nanoparticle based on a discrete lignin core with a polymer corona.
Lignin Biosynthesis and Structure. Ruben Vanholme, Brecht Demedts, Kris Morreel, John Ralph, and Wout Boerjan. Plant Physiology. 2010; 153: 895–905.
Lignin valorization: improving lignin processing in the biorefinery. A. J. Ragauskas et al. Science. 2014; 344: 1246843.