Shear-dependent Structures of Flocculated Micro/Nanofibrillated Cellulose (MFNC) in Aqueous Suspensions.

Cellulosic nanomaterials constitute a topic of growing commercial interest for numerous applications, many of which demand a working knowledge of the rheology of the materials. We demonstrate here that aqueous suspensions of micro/nanofibrillated cellulose (MNFC) exhibit complex shear behavior governed primarily by fibrillar floc dynamics. Regimes corresponding to structure formation, persistence and breakdown are quantitatively differentiated. We assess the recovery of the network structure as a function of the applied breakdown conditions and identify critical conditions that characterize the floc dynamics as isotropic or anisotropic. A two-step yield dynamic generates persistent anisotropic flocs that effectively prohibit recovery of the initial gel structure under certain conditions. Processing within this stress window entails a risk of generating heterogenous, potentially irreproducible structures and properties. An in-depth understanding of the rheology of aqueous MNFC suspensions and their floc-dominated rather than fibril-dominated nature is critical to rationally tailoring properties through selection of judicious processing conditions.