Mechanically tunable nanocomposite hydrogels based on functionalized cellulose nanocrystals

Nanocomposite hydrogels with cellulose nanocrystals (CNCs) as the cross-linker and reinforcement domain have been prepared using free-radical polymerization. A two-step process was carried out for the synthesis of CNC nanocomposite hydrogels. The first step was CNC surface modification with glycidyl methacrylate (GMA) to render it a suitable cross-linker, and the second formation of the network by in-situ free radical polymerization of acrylamide (AM) in the presence of different concentrations of functionalized CNCs. Tensile and compressive properties reveal a characteristic viscoelastic/plastic response relative to conventional BIS-or mixtures of CNC-BIS-cross-linked polyacrylamide-hydrogels. At inception, the response is elastic and primarily stems from the coiled polymer chains in the system. Once the coiled polymer chains are straightened out, CNC nanoparticles start to connect with each other and form a percolated network. This is the viscoplastic response, and results in a sharp increase in modulus, strength and toughness. Functionalized CNC content effects on the mechanical properties, cross-linking density, swelling, and thermal properties were investigated in detail, as well as the critical role the degree of sulphation plays in impacting the intrinsic stress-strain behaviour of these hydrogels.