Decoupling Arrest Origins in Hydrogels of Cellulose Nanofibrils

Abstract

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.

Yielding and bifurcated aging in nanofibrillar networks

“…1). The rheology of the suspensions originates due to physical interactions akin to entanglements, and to weak association involving Van der Waals interactions and hydrogen bonding [18,19]. At low volume fractions φ < ∼ 0.01, the CNF form viscoelastic fluids in which the storage modulus scales as G ∼ φ 11/5 in good agreement with predictions for suspensions of semiflexible fibers that account for stretching entropy and fibrillar bending rigidity [20].…”

Driving Forces for Accumulation of Cellulose Nanofibrils at the Oil/Water Interface

“…It should be noted that CNF1.32 and CNF2.01 have very similar DIT values in spite of having different DSs. With the DS increasing from 1.32 to 2.01, higher amount of charge groups are introduced to the fibrils (the zeta potential of CNF1.32 and CNF2.01 is −35.70 and −42.78 mV, respectively) . The interfacial activity of the fibrils thus depends on the amount of hydrophobic groups as well as that of charge groups on their surface.…”

“…As the fibril coverage increases, the adsorbed fibrils at the interface create an energy barrier via electrostatic repulsion for the new fibrils to adsorb at the interface from the bulk. The salt addition greatly weakens the electrostatic repulsion relative to the van der Waals attraction, thereby facilitating the formation of interconnected fibrils as well as triggering a local density fluctuation in the aqueous phase . The density fluctuation is related to the formation of locally dense clusters and voids by spinodal decomposition.…”

“…The acetylated CNFs were prepared and characterized according to the method previously described . Cotton microcrystalline cellulose (ALDRTCH Chemistry, Shanghai, China) was dispersed into a mixture of acetic acid (1:10, w/v) and sulfuric acid (5%, w/w cellulose), and then a desired amount of acetic anhydride was added to obtain different acetylated samples.…”

“…Our recent work has showed that acetylated CNFs exhibited different aggregated states in aqueous solutions when their electrostatic repulsions were screened by salt addition . The occurrence of two-step yielding observed in the shear experiments was related to the establishment of cross-linked clusters on a fibril level and the arrested phase separation on a cluster level.…”