Mechanical Performance of Macrofibers of Cellulose and Chitin Nanofibrils Aligned by Wet-Stretching: A Critical Comparison

Renewable nanofibrillated cellulose (NFC) and nanofibrillated chitin (NFCh) are attractive fibrillar bionanoparticles due to their remarkable properties such as outstanding mechanical stiffness and strength, thermostability, barrier properties, and also for their global availability from renewable resources and food waste. One major bottleneck to maximize the mechanical properties of materials based on these bionanoparticles (e.g., nanopapers and macroscale fibers) is to find pathways to control their direction of alignment and understand how preferred alignment correlates with macroscale properties. Herein, we will demonstrate how strain-rate controlled wet-stretching of rehydrated macroscale fibers composed of nanofibrillated chitin and cellulose (NFCh, NFC) induces a high degree of orientation and how the degree of alignment scales with macroscale mechanical stiffness. We find similar degrees of alignment in both types of nanofibril-based macrofibers, yet substantially different macroscale stiffness, with the NEC-based fibers (E-NFC = 33 GPa) outperforming the NFCh-based ones (E-NFCh = 12 GPa) considerably. These differences can be correlated to the mechanical properties of the underlying cellulose I and alpha-chitin crystals and the degree of crystallinity of the nanofibrils, which both govern the stiffness of an individual nanofibril. Our study likely demonstrates the maximum performance in terms of stiffness of materials prepared by NEC and NFCh and reveals a critical difference in the performance of both classes of bionanoparticles.