Trade-off between thermodynamic enhancement and kinetic hindrance during phase inversion in the preparation of polysulfone membranes Original Research Article

Polysulfone membranes were prepared via the diffusion-induced phase inversion process from casting solutions consisting of polysulfone, dimethyformamide, and polyvinyl pyrrolidone as a polymeric additive. The effect of PVP added in casting solutions was analyzed by measuring the prepared membranesʹ morphology and water permeability. Variations in a casting solutionʹs thermodynamic and kinetic properties caused by PVP addition suggest that the thermodynamic variation works in favor of the enhancement of demixing in the casting solution, but the rheological variation induces the opposite trend, or the delay of demixing. When prepared by the immersion coagulation into a water bath, the solidified membranesʹ structural and functional properties indicate that the coagulation of cast solutions was affected by the trade-offrelationship between thermodynamic enhancement and kinetic hindrance. With a small amount of PVP in the casting solution, the thermodynamic driving force played a major role on solution demixing, inducing the demixing enhancement, corresponding to the acceleration of phase separation due to the PVPʹs nonsolvent effect. Consequently, the PVP acts as a phase separation enhancer, resulting in both macropore enlargement and permeate flux increase. With more addition of PVP, however, the macropore structure and the water permeability were suppressed rather than enlarged or increased. These phenomena reflect that the demixing of the cast solution was delayed, with the kinetic hindrance offsetting the thermodynamic effect for phase separation enhancement.