Effect of graphene oxide in the formation of polymeric asymmetric membranes via phase inversion

Membrane morphology of integrally skinned asymmetric membranes made by phase inversion can be tailored by precise control of the thermodynamic properties of the casting solution. In this work, the morphology of polyethersulfone (PES) and polyvinylidene fluoride (PVDF) membranes containing graphene oxide (GO) and their phase separation behavior have been studied. The Flory-Huggins theory and the Hansen solubility parameters have been used to construct theoretical binodal lines for ternary and quaternary systems, which have been validated by conducting cloud titration experiments. Moreover, the effect of different solvents on the morphology of PES membranes is discussed through the thermodynamic and kinetic properties of the casting solutions. Nontoxic solvents dimethyl sulfoxide (DMSO) and dihydrolevoglucosenone (Cyrene™) induce PES/GO membranes with finger-like pores and cellular voids, respectively. The addition of GO in the PES and the PVDF casting solutions increases their thermodynamic instability, leading to thinner selective top layers and higher porosities, up to a point where the high viscosity of the GO-containing solutions hinders the solvent-nonsolvent demixing. The highest porosity and pure water flux (PWF) occurs at 0.1 wt% GO for PES/GO/DMSO and at a loading of 0.3 wt% GO for systems PES/GO/Cyrene and PVDF/GO.