Nodular structure and crystallinity of poly(vinylidene fluoride) membranes: Impact on the performance of direct-contact membrane distillation for nutrient isolation

The main objective of this study was to prepare a low-surface-energy poly(vinylidene fluoride) (PVDF) membrane distillation via changes in its surface morphology and crystallinity. We found that membranes prepared at different polymer concentrations with a dual coagulation bath showed distinctive differences in terms of their physical and chemical properties. We found that when the polymer concentration was increased, both the pore sizes and porosities of the membranes decreased. In addition, the polymer concentration also resulted in significant changes to the surface roughness and morphology of the membrane, which were manifested through varied nodule shapes and sizes. Higher polymer concentrations gave better wetting resistance (higher liquid entry pressure) because of the increasing hydrophobic α-phase crystallinity but suffered from a lower porosity (lower vapor permeability). At a higher PVDF concentration, we expected that the thermal efficiency would be lower (because of the lower porosity); however, we observed an appreciable contrast phenomenon due to the increasing nonconductive α-phase, and this lowered the thermal conductivity. These observations showed that the wettability of the membrane very much depended on the chemical properties of PVDF. The optimum membrane (20% PVDF) showed a consistent nutrient rejection of more than 99% with a membrane flux of 9.491 kg m2 h ± 0.1 at a feed temperature of 90 °C. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46866.