Large Graphene Oxide Flakes: Isotopic to Nematic Liquid Crystal Phase Transition
Résumé
Here we study large GO flakes with an average size of ≈38 µm, following the isotropic-nematic phase transition over time and determining the critical concentration i.e. the minimum value for the formation of pure LC phase. Graphene oxide (GO) flakes in aqueous suspensions can form liquid crystal (LC) phases, attractive for various applications because of their exceptional properties. Stability over time is very important for their application especially if the GO suspension is in the bi-phase concentration region in which a crucial change that can happen, is the isotropic – liquid crystal phase separation. GO LCs have a wide bi-phase region, and even very low concentration suspensions appeared purely nematic when prepared. However, by passing enough time they showed to be biphasic and higher concentrations are needed for pure LC phase. We followed the changes of the critical concentration over four months, finding it increases by time until it reaches a stable value that is nevertheless, very low in concentration, 0.7 mg mL−1 (0.035 vol%) and truly stable over time. From the stable threshold volume fraction, we estimated the effective flake thickness, relevant in the LC phase formation behavior. This value is almost double than what is measured by atomic force microscopy.