Comparison of antifouling behaviours of modified PVDF membranes by TiO2 sols with different nanoparticle size: Implications of casting solution stability

Widespread applications of membrane technology call for the development of antifouling membranes, and preparation of mixed matrix membranes (MMMs) by mixing nanoparticles (NPs) into polymeric membrane matrix has attracted much attention. In this work, polyvinylidene fluoride (PVDF) MMMs were prepared by two TiO2 sols with different NPs size (one from hydrolysis and condensation with an average size of 5 nm, and the other from dispersion of TiO2 nanoparticles with a particle size of 21 nm), and comparison of their antifouling behaviours was conducted. Compared to the pristine PVDF membrane (NO) and membrane (N1) modified by TiO2 sol from dispersion of TiO2 NPs, the surface pore size, porosity and permeability of modified membrane (N2) by TiO2 sol from hydrolysis and condensation were slightly decreased, while the hydrophilicity and the absolute value of Zeta potential were obviously improved. Moreover, the N2 membrane showed a higher surface energy barrier, smaller frequency shift in quartz crystal microbalance with dissipation monitoring, and higher relative flux than N1, indicating a better antifouling performance. The abundant hydroxyl groups and the increased Ti and 0 elements on the membrane surface accounted for the enhanced hydrophilicity and antifouling ability. Scanning electron microscope (SEM) images clearly show that TiO2 particles were uniformly distributed for N2; however, the severe agglomeration and sedimentation were observed for N1. Multiple light scattering spectroscopy (MLiSSP) analysis showed that N2 casting solution had a higher stability as revealed by the negligible backscattering change, decreased demixing thickness and low turbiscan stability index (TSI) value. The different stability of casting solutions might result in the different NPs' behaviours during membrane formation process, accounting for the discrepancy in the physicochemical properties and antifouling ability between N1 and N2 membranes.