Large Graphene Oxide Flakes: From Isotopic to Nematic Liquid Crystal Phase Transition

Abstract

Herein, graphene oxide (GO) with large flakes of average size 38 μm is studied, following the isotropic–nematic phase transition over time and determining the critical concentration, i.e., the minimum value for the formation of the pure liquid crystal (LC) phase. The GO flakes in aqueous suspensions can form LC phases, which are attractive for various applications because of their exceptional properties. Stability over time is very important for their applications, especially, if the GO suspension is in the biphase concentration region in which a crucial change can occur—the isotropic LC phase separation. GO LCs have a wide biphase region, and even very low concentration suspensions appear purely nematic when prepared. However, over time, they appear to be biphasic and higher concentrations are needed for pure LC phase. The changes in the critical concentration were followed for over 4 months, and the findings indicate that it increases with time until it reaches a stable value that is, nevertheless, very low in concentration, 0.7 mg mL−1 (0.035 vol%). From the stable threshold volume fraction, the effective flake thickness, relevant in the LC phase formation behavior, is estimated. This value is almost double than what is measured by atomic force microscopy.

Mechanically strong and highly ion conductive graphene oxide liquid crystal film containing the poly(amic acid) salt

“…Figure 1A shows that the LC phase is well established even at a low concentration. The bright area represents the nematic phase of the LC, and a suspension with isotropic properties appears as the black area 36 . Here, we added poly(amic acid) salt (PAAS) to the GO solution to disperse it.…”

“…The bright area represents the nematic phase of the LC, and a suspension with isotropic properties appears as the black area. 36 Here, we added poly(amic acid) salt (PAAS) to the GO solution to disperse it. PAAS has an ionized structure, which gives it higher solubility in various solvents than conventional poly(amic acid).…”

Clay nanolayer encapsulation, evolving from origins of life to future technologies

“…Very much like exfoliated GO in aqueous suspension [67,68], exfoliated clay nanolayers in aqueous suspension form entropically driven nematic phases [13,33,34,[69][70][71][72][73]] merely due to their individual platelet shape and thus loss of rotational degrees of freedom. It has been found that also clay particle stacks form nematic phases [14,[74][75][76][77][78] independently of delamination and exfoliation, since also the full particles are platelet shaped, although much thicker than the individual nanolayers.…”

Dynamic light scattering analysis of size-selected graphene oxide 2D colloids fractionedvialiquid crystal phase separation

“…Since GO flakes are strongly negatively charged, their effective hydrodynamic dimensions – including the hydration shell – should be significantly larger in the aqueous suspension than in the dried state. 19,20 In addition, it was found that GO sheets are not completely rigid, rather they can be considered super-flexible like a 2D polymer and can form folded and wrinkled structures, in particular when dried. 21–24…”

“…Since GO flakes are strongly negatively charged, their effective hydrodynamic dimensions -including the hydration shell -should be significantly larger in the aqueous suspension than in the dried state. 19,20 In addition, it was found that GO sheets are not completely rigid, rather they can be considered super-flexible like a 2D polymer and can form folded and wrinkled structures, in particular when dried. [21][22][23][24] A widely applicable tool to investigate the size of colloidal particles in fluid dispersions is dynamic light scattering (DLS), which at first provides information about the diffusion coefficients of the particles.…”