Cellulose Nanocrystals Mechanical Reinforcement in Composite Hydrogels with Multiple Cross-Links: Correlations between Dissipation Properties and Deformation Mechanisms

Nanocomposites have drawn a great interest in materials science of elastomers in recent years, and tailoring interfacial interactions between fillers and polymer matrix plays a critical role in improving their mechanical properties. The synthetic platform of tough and stretchable cellulose nanocrystal–poly(acrylamide) (CNC–PAM) composite hydrogels was proposed and applied here to unravel the role of covalent network in PAM and physical interactions by CNC surface adsorption. The attractive physical interactions in the network were considered to increase the fracture strength of the hydrogels via reversible adsorption–desorption processes on the CNC surface. Stress-sensitive characteristic shifts of the Raman peak located at 1095 cm–1 indicated an efficient load transfer across the interface, where the tensile modulus was higher than the compression modulus. In situ transmission electron microscopy observation allowed to demystify the composites deformation process and interfacial bridging between CNC and polymer matrix. A detailed comparison of strain rate effect on large strain dissipation indicated that the viscoelastic behavior of the hydrogels varied remarkably over strain rates, ranging from little hysteresis at low strain rates to highly dissipative at high strain rates, suggesting a new, slow relaxation mode, most likely due to interfacial adsorption of polymer chains on the CNC surfaces. This study showed that polymer chains desorbed from the CNC surface under periodic strains would entangle with the free chains after the rest time via conformational rearrangements, consequently triggering a recovering mechanism during multiple crazing and shear relaxation processes.