Cross-Linking Dynamics of Cellulose Nanofibrils-Based Transient Network Hydrogels: A Study of pH Dependence

Interactions between polymer chains and nanoparticles can play a dominant role in composites mechanics, yet the control of such interfacial dynamics is still a significant challenge. This paper reports the effect of pH on cellulose nanofibrils (CNFs) transient hydrogels network mechanics via interfacial ionic cross-linking bonds. For this purpose, carboxylated CNFs are incorporated with amine groups terminated 8-arm poly(ethylene glycol) (PEG-NH2) to assemble the first noncovalent network via reversible ionic interactions at the CNF surfaces, where the end-group acrylate modified linear difunctional PEG forms the second lightly covalent cross-linked network. The viscoelastic properties of the supramolecular gels are examined as a function of pH, and the unique transient mechanics resulting from CNF-PEG complexation structures show that the processing from acidic-to-alkaline pH change leads to the gel cross-links transition from covalent type to covalent-noncovalent hybrid one. This finding offers an alternative way to tune CNF gels mechanical reinforcement and sheds light on the pH dependence of network architectural changes.