Department of Chemical Engineering, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, China
Abstract:
Graphene oxide (GO) possesses a large number of oxygen-containing functional groups on its basal planes and edges, enabling it to disperse well in water and other aqueous media. This property facilitates the processing of GO by various wet-processing methods. Because of its interesting properties and useful intermediate role in preparing graphene derivatives, GO has potential applications in many fields, including composites, separators, sensors, actuators, and energy storage and conversion. At high concentrations, strong, competitive interactions occur in GO aqueous dispersions that significantly impact the rheological behavior of these dispersions. In a liquid medium, the dispersed GO nanosheets form a unique colloidal system, in which solvation, electrostatic interactions, hydrogen bonding, and the lyophilic effect play important roles. The aromatic domains preserved from precursor graphite show attractive van der Waals interaction and π–π stacking between GO sheets. In this study, the effects of pH, temperature, and different organic solvents on the rheological behavior of GO dispersions were investigated through steady and dynamic rheological tests and theoretical analysis. The results showed that enhancing acidity, increasing the temperature within a certain range, and adding organic solvents such as pyridine promote transition of the GO aqueous dispersion from a viscoelastic liquid to a gel state, which shows different rheological properties. GO sheets in dispersion interact through negative charges originating from the many ionizable groups in the nanosheets and electrical double layers. Analysis using the Deryagin-Landau-Verwey-Overbeek (DLVO) theory showed that, under the conditions described above, these interactions were remarkably altered with consequent effects on the rheological properties. Weakened electric double-layer interaction disrupted the GO colloidal dispersion state and resulted in the association of GO nanosheets to form gel. Based on the above understanding, the yield stress of the GO dispersions affected by the volume fraction was analyzed by population balance equation (PBE) modeling. Through creep and relaxation experiments, the structure and rheological properties of GO dispersions at high concentrations were found to be similar in many respects to those of polymers. Therefore, the viscoelastic behavior of GO dispersions can be well described by the Poynting-Thomson model, which can provide theoretical support and advance the study of complex GO dispersions. These results shed new light on the rheological behavior of GO dispersions and can be used to optimize the processing conditions for future applications.
