Rheology of amino-functionalized graphene oxide suspensions in hydrogels

This work investigates the effects of amino-functionalized graphene oxide (AFGO) suspensions on the rheological behavior of Carbopol (R) hydrogels at pHs 5, 7, and 9. The AFGO concentration and media pH were evaluated and related to the suspension's microstructure and rheology. Graphene oxide (GO) nanosheets were synthesized using the modified Hummers method and functionalized with triethylenetetramine via microwave-assisted reaction to produce AFGO. The nanosheets were characterized by different techniques, such as scanning electron microscopy (SEM), thermogravimetric analysis, Raman spectroscopy, and x-ray photoelectron spectroscopy. The suspensions were characterized by rheological tests through steady-state and dynamic flow, zeta potential, and cryo-SEM for microstructure analysis. All samples presented a viscoplastic behavior and were modeled by the Herschel-Bulkley equation. Concerning the base hydrogels, the sample prepared at pH 9 showed lower viscosity, yield stress, and elastic modulus. At all pHs, the increase in the nanosheet concentration promotes a drop in the yield stress, viscosity, storage, and loss moduli. The cryomicrographs showed the impact of pH on the base hydrogel structure. It was also possible to observe that increasing nanoadditive concentration affects the Carbopol microgel swelling and weakens the suspension microstructure.