Effects of Degrees of Aldehyde Modification on Molecular Structures and Properties of Cellulose Nanofiber Cross-Linked Self-Healing Hydrogels

Glycol chitosan (GC) and multifunctional cellulose nanofiber (MCNF) composite hydrogel prepared via Schiff base chemistry have demonstrated excellent self-healing, shear-thinning, and injectable properties, holding promise for various applications. MCNF, a derivative of cellulose nanofibers (CNFs), is introduced with dialdehyde groups through sequential TEMPO-mediated oxidation and periodate oxidation. These aldehyde groups form a dynamic Schiff base linkage with the amine groups of GC chains. While both cellulose and chitosan are biocompatible natural polymers, excessive aldehyde modification of MCNF can disrupt the uniformity of the hydrogel network. To elucidate the underlying molecular mechanism of aldehyde modification of MCNF on affecting the molecular structure of the hydrogel network, here, we performed molecular dynamics simulations on both a single MCNF molecule and a GC/MCNF composite hydrogel with varying degrees of aldehyde modification. For a single MCNF, increased aldehyde modification initially resulted in a more extended molecular conformation, but at higher modification levels (above 15%), the chains became more curled. Correspondingly, the Young's modulus of MCNF also decreased slightly with increased modification, consistent with the trends in experimental results suggesting reduced mechanical stiffness of the hydrogel due to a loss in intramolecular hydrogen bonding and increased chain flexibility. Among the GC/MCNF hydrogels, the aldehyde content significantly influenced the interaction and network structure. At modification levels below 15%, hydrogen bonding between GC and MCNF was enhanced and evenly distributed, supporting a homogeneous and mechanically robust hydrogel. At higher modification levels, excessive GC-MCNF interactions led to localized aggregation, reduced interconnectivity, and a decline in the tensile modulus. This study provides molecular-level insights into how the aldehyde modification of CNFs affects both the conformation of MCNF and the overall integrity of GC/MCNF hydrogels. These findings offer valuable guidance for optimizing aldehyde-modified CNF-based self-healing hydrogels for biomedical and engineering applications.