Rheological characterization of cellulose nanocrystal-laden self-healable polyvinyl alcohol hydrogels

Among various nanomaterials, cellulose nanocrystals (CNCs) are regarded as the most suitable reinforcing fillers for hydrogels owing to their high dispersibility in water and favorable hydrogen bonding with water-dispersible polymers. Herein, CNC-laden polyvinyl alcohol (PVA)/borax (P/CNC) hydrogels were prepared by solution mixing, and their mechanical and rheological properties were investigated in terms of CNC loading of 0-60 w/w%. PVA/borax hydrogels are known to exhibit self-healing ability based on the dynamic nature of the borate-diol complex, which is dependent on the rheological response because the rheological chain dynamics dominantly affect the self-healing process. In mechanical testing, the Young's modulus of the P/CNC hydrogels sharply increased above 40 w/w% CNC, indicating that the stiffening effect of CNC was enhanced above the critical loading. From a rheological perspective, the increases in the viscosity and storage modulus were further accelerated above 40 w/w%. In particular, the chain flow relaxation time (tau(f)), a quantitative parameter closely related to the self-healing performance, was observed for the P/CNC hydrogels with CNC amounts of 0-40 w/w% (1.6-97.3 s); whereas, there is no tau(f) for the P/CNC hydrogels with 45-60 w/w% CNC within a reasonable time scale we observed at 25 degrees C. Consequently, the incorporation of less than 40 w/w% CNCs affords high mechanical stiffness while maintaining self-healing ability.