Shear melting and recovery of cellulose nanocrystal-polymer gels

Cellulose nanocrystals (CNC) are naturally-derived nanostructures of growing importance for the production of composites of attractive mechanical properties. Their fabrication involves extrusion of CNC suspensions and gels in organic solvents, in the presence of additives such as polymers and curing agents. Here, we study the rheological behavior of composite polymer-CNC gels in dimethylformamide, which include additives for both UV and thermal crosslinking. Using rheometry coupled with in-situ infrared spectroscopy, we show that under external shear, CNC-polymer gels display progressive and irreversible failure of the hydrogen bond network that is responsible for their pronounced elastic properties. In the absence of cross-linking additives, polymer-CNC gels show an instantaneous but partial recovery of their viscoelasticity upon cessation of flow, whereas, the presence of additives allows the gels to recover over much longer timescale via van der Waals interactions. By exploring a broad range of shear history and CNC concentrations, we construct master curves for the temporal evolution of the viscoelastic properties of the polymer–CNC gels, illustrating the universality of the observed dynamics with respect to gel composition and flow conditions.