Synergistic Reinforcing Mechanisms in Cellulose Nanofibrils Composite Hydrogels: Interfacial Dynamics, Energy Dissipation, and Damage Resistance.

Engineering reversible cross-links between nanoparticles and polymer matrix is a promising avenue to reinforce the mechanical properties of elastomers and in particular soft hydrogels. In this work, we study a model system of composite hydrogel reinforced with cellulose nanofibrils (CNFs), where the integration of reversible hydrogen bonds into a lightly covalently cross-linked polyacrylamide (PAAm) matrix. This approach yields the dual cross-linked networks with synergistically improved strength, modulus, and toughness. The reversible nature of the hydrogen-bonded cross-links manifests a strong strain rate (έ) dependent dynamics properties. The CNF-PAAm interaction among physically adsorbed chains on the surface of CNF is examined as a function of CNF fraction by sum frequency generation spectroscopy. The results indicate a decrease of the number of free -OH groups on the CNF surface. Moreover, the deformation-resting experiments show a unique interface stiffening mechanism where the polymer chains desorbed from the CNF surface under oscillatory shear become entangled during resting time. The bending micromechanics test reveals that the CNF interfacial slip imparts the capability to strengthen the composites during deformation. The fibril pull-out process activates a series of dissipation mechanisms that increase the crack propagation resistance. These findings advance our understanding the role of interfacial layer in microscopic reinforcement mechanism and provide a constitutive foundation for exploring the deformation behaviors of the cellulosic hydrogels.