Beaded electrospun polyvinylidene fluoride (PVDF) membranes for membrane distillation (MD)

A new class of PVDF-based membranes suitable for membrane distillation were prepared via electrospinning. The classical fiber morphology of PVDF electrospun membranes was changed to generate a beaded structure by tuning the solvent ratio and operating parameters during membrane fabrication. Results showed that by increasing the N, N-Dimethylacetamide (DMAC) to acetone ratio used to prepare the PVDF solution, PVDF micron-sized beads can be created within the fiber structure of the electrospun membrane. This is mainly ascribed to the lower saturated vapor pressure of the DMAC, which is slowing the polymer solidification rate, resulting in a fiber-to-bead morphology. An increase in the bead density induces an increase of the WCA of the resulting membrane (from 134° to 148°), with a concomitant reduction of the membrane thickness. Large bead-to-fiber ratios may be obtained by working with low PVDF concentration (17 wt%) but at the expense of membrane mechanical stability. The latter can be enhanced by increasing the PVDF solution concentration. The results show that simultaneous deployment of 17 and 25 wt% of PVDF solutions in a double-needle electrospinning device enables the fabrication of electrospun PVDF-beaded membranes with high hydrophobicity, low thickness and good mechanical stability for membrane distillation. The best membrane showed a 34% increase in membrane permeability and maintained a more stable water flux compared to a commercial PVDF membrane when filtering hypersaline solution in high-recovery direct contact membrane distillation. Thus beaded PVDF structured electrospun structures may represent a facile method to improve PVDF membrane performance in MD applications.