Mechanically\nViscoelastic Properties of Cellulose Nanocrystals Skeleton Reinforced\nHierarchical Composite Hydrogels

Abstract

With\ninspiration from the concept of natural dynamic materials, binary-component\ncomposite hydrogels with excellent mechanical properties and recovery\ncapability were prepared from the cellulose nanocrystal (CNC) skeleton\nreinforced covalently cross-linked polyacrylamide (PAAm) networks.\nThe hierarchical skeleton obtained by freeze-drying of CNC aqueous\nsuspension was directly impregnated into acrylamide (AAm) monomer\nsolution, and in situ polymerization occurred in the presence of hydrophilic\ncross-linker PEGDA575. Under stress, hydrogen bonds at the interface\nbetween CNC and PAAm as well as inside the CNC skeleton acted as sacrificial\nbonds to dissipate energy, while the covalently cross-linked PAAm\nchains bind the network together by providing adhesion to CNC and\nthereby suppress the catastrophic craze propagation. The above synergistic\neffects of the CNC skeleton and the elastic PAAm network enabled the\ncomposite hydrogels to withstand up to 181 kPa of tensile stress,\n1.01 MPa of compressive strength, and 1392% elongation at break with\nthe fracture energy as high as 2.82 kJ/m². Moreover, the\nhydrogels recovered more than 70% elasticity after eight loading–unloading\ncycles, revealing excellent fatigue resistance. The depth-sensing\ninstrumentation by indentation test corroborated that the CNC skeleton\ncontributed simultaneous improvements in hardness and elasticity by\nas much as 500% in comparison with the properties of the pristine\nPAAm hydrogels. This elegant strategy by using the CNC skeleton as\na reinforcing template offers a new perspective for the fabrication\nof robust hydrogels with exceptional mechanical properties that may\nbe important for biomedical applications where high strength is required,\nsuch as scaffolds for tissue engineering.