Inelastic behaviour of cellulose microfibril networks.

Cellulose microfibrils (CMF) are a unique class of shape anisotropic bio-nanomaterials, already finding many applications in diverse fields owing to their advantageous material properties and abundant availability. The rich non-linear mechanical behaviour of CMF networks has been under-studied due to the complex nature of this system, being influenced by many factors such as strong inter-fibril interactions, a heterogeneous microstructure, and process conditions. In this work, we systematically explore the non-linear rheological behaviour of these networks using a CMF model system with controlled process conditions and fibril interactions. The microfibrils were dispersed in dimethyl sulfoxide to minimise the attractive van der Waals interactions and thereby also the network heterogeneity. We show that the networks exhibit a transition with increasing shear stress from a predominantly elastic to a plastic deformation where they undergo softening. We find that the network stiffness and plasticity are dependent on the loading rate. Finally, we observed that the networks regain their original viscoelastic moduli on cessation of shear. These findings form a basis towards understanding and ultimately modelling the mechanics of CMF networks, which is a prerequisite for the rational design of novel bio-based materials.

Revealing and Quantifying the Three-Dimensional Nano- and Microscale Structures in Self-Assembled Cellulose Microfibrils in Dispersions.

Cellulose microfibrils (CMFs) are an important nanoscale building block in many novel biobased functional materials. The spatial nano- and microscale organization of the CMFs is a crucial factor for defining the properties of these materials. Here, we report for the first time a direct three-dimensional (3D) real-space analysis of individual CMFs and their networks formed after ultrahigh-shear-induced transient deagglomeration and self-assembly in a solvent. Using point-scanning confocal microscopy combined with tracking the centerlines of the fibrils and their junctions by a stretching open active contours method, we reveal that dispersions of the native CMFs assemble into highly heterogeneous networks of individual fibrils and bundles. The average network mesh size decreases with increasing CMF volume fraction. The cross-sectional width and the average length between the twists in the ribbon-shaped CMFs are directly determined and compared well with that of fibrils in the dried state. Finally, the generality of the fluorescent labeling and imaging approach on other CMF sources is illustrated. The unique ability to quantify in situ the multiscale structure in CMF dispersions provides a powerful tool for the correlation of process-structure-property relationship in cellulose-containing composites and dispersions.