Nanocomposite hydrogels: Fracture toughness and energy dissipation mechanisms

In this study, fracture toughness of nanocomposite hydrogels is quantified, and active mechanisms for dissipation of energy of nanocomposite hydrogels are ascertained. Poly(N,N-dimethylacrylamide) nanocomposite hydrogels are prepared by in situ free radical polymerization with the incorporation of Laponite, a hectorite synthetic clay. Transmission electron microscopy proves exfoliation of clay platelets that serve as multifunctional crosslinkers in the created physical network. Extraordinary high fracture energies of up to 6800 J m-2 are determined by the pure shear test approach, which shows that these soft and stretchable hydrogels are insensitive to notches. In contrast to single- and double-network hydrogels, dynamic mechanic analysis and stress relaxation experiments clarify that significant viscoelastic dissipation occurs during deformation of nanocomposite hydrogels. Similar to double-network hydrogels, crack tip blunting and plastic deformation also contribute to the observed massive fracture energies.