Effect of Polyethersulfone Molecular Weight on Structure and Performance of Ultrafiltration Membranes

Polyethersulfones (PES) with three different molecular weights were chosen to prepare ultrafiltration (UF) membranes by the nonsolvent induced phase separation method using polyvinylpyrrolidone (PVP) and N-methyl-2-pyrrolidone (NMP) as additive and solvent, respectively. The effect of PES molecular weight on the structure and performance of the prepared asymmetric membranes was investigated by means of UF experiments, measurements of membrane thickness and porosity, scanning electron microscopy, and the measurement of bursting strength, given fixed PVP or PES concentrations. It was found that increasing PES molecular weight would lead to a larger pore size in the skin layer but lower membrane porosity, and would result in membranes with higher strength, higher permeability, and lower rejection. Based on the experimental data of PES molecular weight, ternary phase diagram, and cast solution viscosity obtained from gel permeation chromatography, cloud point titration, and coaxial cylinder viscometer measurements, respectively, the underlying causes of membrane structures were accounted for from the combined perspective of thermodynamics and kinetics and polymer aggregate dimensions.