Lignin-based polymers and nanomaterials.

Approximately 50 million tons of lignin are currently produced annually as a by-product of the pulp- and paper industry, and this amount is likely to double in the future with the anticipated production of renewable fuels and chemicals from lignocellulosic biomass, as a sustainable alternative to petroleum. The latter process can be expedited by valorizing lignin, which entails making products from lignin that generate additional revenues for biorefineries so that the production of biofuels becomes more competitive with gasoline. Industrially produced lignin is considered a low-value material that is used as a boiler fuel to generate heat and electricity, and as an ingredient of adhesives, cement, and drilling fluids for underwater oil wells. The aromatic nature of lignin, its ability to participate in radical-mediated cross-linking reactions, the many functional groups available for derivatization or chemical reactions, and its amenability to existing procedures for making thermoplastics, make it attractive as an additive to polymers to enhance UV-tolerance and/or other physico-chemical properties. Lignin can also be used as the basis for various nanomaterials, either per se or in combination with other polymers. This review summarizes recent developments in the synthesis of lignin-containing polymers and nanomaterials, whereby inherent variation in lignin subunit composition and structure, as a function of plant species and lignin extraction method, offer unique opportunities for fine-tuning material properties (e.g. tensile strength, hardness, elasticity) to match specific applications.