Hierarchical Layered Heterogeneous Graphene-poly(N-isopropylacrylamide)-clay Hydrogels with Superior Modulus, Strength, and Toughness

Biological composites are renowned for their elaborate heterogeneous architectures at multiple scales, which lead to a unique combination of modulus, strength, and toughness. Inspired by biological composites, mimicking the heterogeneous structural design principles of biological composites is a powerful strategy to construct high-performance structural composites. Here, we creatively transfer some heterogeneous principles of biological composites to the structural design of nanocomposite hydrogels. Unique heterogeneous conductive graphene-PNIPAM-clay hydrogels are prepared through a combination of inhomogeneous water removal processes, in situ free-radical polymerization, and chemical reduction of graphene oxide. The nanocomposite hydrogels exhibit hierarchical layered heterogeneous architectures with alternate stacking of dense laminated layers and loose porous layers. Under tensile load, the stiff dense laminated layers serve as sacrificial layers that fracture at a relatively low strain, while the stretchable loose porous layers serve as energy dissipation layers by large extension afterward. Such local inhomogeneous deformation of the two heterogeneous layers enables the nanocomposite hydrogels to integrate superior modulus, strength, and toughness (9.69 MPa, 0.97 MPa, and 5.60 MJ/m(3), respectively). The study might provide meaningful enlightenments for rational structural design of future high-performance nanocomposite hydrogels.