Nanocomposite Hydrogels Containing Few-Layer Graphene Sheets Prepared through Noncovalent Exfoliation Show Improved Mechanical Properties

Hydrogels show great potential as soft materials for biomedical applications and flexible devices. However, conventional hydrogels exhibit poor mechanical strengths owing to the presence of water in their polymer networks. Therefore, improving the mechanical properties of hydrogels by controlling the chemical and physical structures that affect their macroscopic behaviors is a challenging issue. In this study, we developed a nanocomposite (NC) hydrogel that harbors exfoliated few-layer graphene sheets through noncovalent interactions. The bifunctional polymer PImQ, which contains both aromatic and cationic groups, was found to enable the direct exfoliation of graphite to few-layer graphene through pi-pi interactions in 2.7% yield. The poly(acrylamide)-based NC hydrogel containing the PImQ/graphene composite as a nanofiller shows a 3.4-fold increase in tensile stress compared with the hydrogel without the nanofiller. The introduction of the PImQ/graphene nanocomposite also increases the fracture stress of the NC hydrogel through cation-pi and pi-pi interactions. The improved mechanical properties of the NC hydrogel result from the synergistic effects of the chemical crosslinking of the polymer network and the physical crosslinking of the polymer/graphene nanofiller.