Aqueous Dispersion of Graphene Sheets Stabilized by Pluronic Copolymers: Formation of Supramolecular Hydrogel

A facile approach to disperse graphene in aqueous solution is described. Triblock copolymers (PEO-b-PPO-b-PEO) were employed as the solubilizing agent for chemically exfoliated graphite oxide, and graphene formed through in situ reduction by hydrazine. The formation of the stable aqueous copolymer-coated graphene solution is due to the noncovalent interaction between the hydrophobic PPO segments of the triblock copolymer and the hydrophobic graphene surface, whereas the hydrophilic PEO chains extend into water. It was characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Utilizing the dual roles of Pluronic copolymer in dispersing graphene in aqueous solution and forming supramolecular hydrogel with alpha-cyclodextrin through the penetration of PEO chains into the cyclodextrin cavities, we further developed a facile and effective method to hybridize the well-dispersed graphene into a supramolecular hydrogel, which was investigated by a variety of techniques, such as X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and rheometer. The supramolecular hybrid hydrogel possessed a cross-linked network and showed shear-thinning properties. The viscosity and strength of the supramolecular hybrid hydrogels decreased significantly as compared to those of the native hydrogel resulting from the lamellar structure of the hybrid hydrogel because of the presence of the graphene sheets. Furthermore, the supramolecular hybrid hydrogel transformed into a sol upon increase in temperature. All these features made our supramolecular hybrid hydrogel a nice candidate in a drug delivery and controlled release system.