Effects of dope sonication and hydrophilic polymer addition on the properties of low pressure PVDF mixed matrix membranes

Mixed matrix membranes (MMM) made from hydrophilic fillers incorporated into poly (vinylidene fluoride) (PVDF) are well known to have improved membrane properties and durability. However, the contributions from two key steps in MMM fabrication: dope sonication and the addition of a hydrophilic polymer like poly (vinylpyrrolidone) (PVP), have not been sufficiently highlighted. In this work, a range of MMM were made by extended dope sonication and stepwise PVP addition. The results showed that PVP, popularly referred to as a ‘pore former’, was surprisingly not responsible for pore formation but functioned rather as a ‘pore connector’ and as an aid to particle dispersion. While dope sonication resulted in efficient particle dispersion as may be expected, it did not improve membrane surface wettability as the hydrophilic nanoparticles were completely locked within the hydrophobic PVDF matrix. Sonication resulted in significant reduction in membrane porosity but did not change the membrane thermal stability. The addition of PVP, however, significantly reduced thermal stability as well as the PVDF β-phase mass fraction. The results of this study provide valuable insights to the effects of dope sonication and PVP addition for manufacturers and researchers developing low pressure membranes for water treatment applications.