The research of rheology and thermodynamics of organic–inorganic hybrid membrane during the membrane formation

To clarify the mechanism of organic–inorganic hybrid membrane formation by phase-inversion method, the thermodynamical and rheological properties of PSF/TiO2 casting solution were investigated by the viscosity measurement and the triangle phase diagram, respectively. TiO2 introduction decreased the non-solvent tolerance of casting solution with non-solvent 20% ethanol aqueous solution, which caused thermodynamic enhancement of phase separation, and also resulted in the change of rheological properties from Newtonian fluid to non-Newtonian fluid and the viscosity increase of casting solution, which induced rheological hindrance in demixing process. On the basis of the relationship between wet membrane thickness and coagulation time, a novel method used to calculate kinetics parameter (Da) of membrane formation was founded and an equation of Da = d2/t was established. With the increase of TiO2 concentration, it was revealed that Da increased firstly and then decreased with a maximum value of 0.002204 mm2/s at certain temperature (20 °C). With the increase of coagulation bath temperature, Da increased and the viscosity decreased continuously when the composition of casting solution was constant. Da was mainly influenced by the porosity of membrane and the thermodynamic, rheological properties of casting solution. In addition, Da also had a main influence on membrane structure by SEM observation, a higher Da was apt to form a more open membrane, as consequence, it was feasible using Da as a parameter to evaluate PSF/TiO2 hybrid membrane structure.