The viscoelastic behavior of lignin: Quantification through nanoindentation relaxation testing on hot-pressed technical lignin samples from various origins

Lignin, the second most abundant organic polymer on earth, is one of the primary causes of the viscoelastic behavior of plants. An accurate characterization of its viscoelastic properties is essential for predicting the time-dependent response of natural materials, including wood and plant fibers, and for advancing lignin-based materials and their production methods, such as 3D printing of biocomposites. To enrich the still rather sparse knowledge on the viscoelasticity of lignin, we re-evaluate nanoindentation relaxation tests performed on five hot-pressed technical lignins extracted from different feedstocks, using three different extraction methods. The viscoelastic indentation problem is addressed using the method of functional equations combined with the homogenization theory to account for the production-induced porosity. This evaluation procedure allows for quantitatively assessing the viscoelastic properties of lignin, which can be very accurately described by an isochoric four-parameter Burgers model. Remarkably, the viscoelastic properties of all tested lignins are practically identical and independent of the feedstock and the extraction processes.