Decoupling Arrest Origins in Hydrogels of Cellulose Nanofibrils

Colloidal gels with various architectures and different types of interactions provide a unique opportunity to shed light on the interplay between microscopic structures and mechanical properties of soft glassy materials. Here, we prepare acetylated cellulose nanofibrils with 2 degrees of substitution and make a structural and rheological characterization of their hydrogels. Two-step yielding processes are observed in the shear experiments, which allow us to deduce more precise knowledge regarding localized structural changes of the fibrils. We separate the viscoelastic response into two contributions: the establishment of cross-linked clusters on a fibril level and the arrested phase separation on a cluster level. We hypothesize that with the addition of salt, the hydrogels exhibit different arrested states that are identified as unable to access the thermodynamic equilibrium. Our results highlight that the coexistence of gelation and glass transitions are experimentally recognized in the hydrogels, with a global gelation driven by a local glasslike arrest during spinodal decomposition.