  Cellulose nanocrystals (CNC) are naturally-derived nanostructures of growing
importance for the production of composites having attractive mechanical
properties, and offer improved sustainability over purely petroleum-based
alternatives. Fabrication of CNC composites typically involves extrusion of CNC
suspensions and gels in a variety of solvents, in the presence of additives
such as polymers and curing agents. However, most studies so far have focused
on aqueous CNC gels, yet the behavior of CNC-polymer gels in organic solvents
is important to their wider processability. 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, the polymer-CNC gels show negligible recovery upon
cessation of flow, while the presence of additives allows the gels to recover
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 universality
of the observed dynamics with respect to gel composition and flow conditions.
We therefore find that polymer-CNC composite gels display a number of the
distinctive features of colloidal glasses and, strikingly, that their response
to the flow conditions encountered during processing can be tuned by chemical
additives. These findings have implications for processing of dense CNC-polymer
composites in solvent casting, 3D printing, and other manufacturing techniques.
