Preparation and characterization of highly micro-porous PVDF membranes for desalination of saline water through vacuum membrane distillation

Improving Liquid Entry Pressure of Polyvinylidene Fluoride (PVDF) Membranes by Exploiting the Role of Fabrication Parameters in Vapor-Induced Phase Separation VIPS and Non-Solvent-Induced Phase Separation (NIPS) Processes

“…All of these have been applied to prepare PVDF-based MD membranes [2][3][4]. The NIPS process is the most popular and has been used to prepare different types of membranes, including reverse osmosis, nanofiltration, ultrafiltration, microfiltration and MD [5,6]. When used to produce PVDF membranes in a strong solvent/non-solvent system, typically, the NIPS process is characterized by a relatively fast mass transfer rate (instantaneous demixing, relative to VIPS) leading to asymmetric structure (dense top layer, supported by a more porous layer) [5], narrow pore size distribution (PSD), relatively low mean flow pore size (MPS) and maximum pore size (BP) very close to MPS; all are desired properties of an MD membrane.…”

Effect of the exposure time on the structure and performance of hydrophobic polydimethylsiloxane–poly(vinylidene fluoride) membranes via a non‐solvent‐induced phase separation process in a clean room

“…With increasing exposure time, more solvent evaporated from the casting film; this induced the crystallization of the PVDF polymer in the coagulation bath. 2,9,16,28 From the cross-sectional pictures of MT1 to MT6, we observed that with increasing exposure time, demixing changed from liquid/liquid to solid/liquid, and the pore structure changed from cellular type to globule type. When the exposure time was 10 s, that is, when the polymer concentration was the lowest, droplet coalescence took place easily before the solidification of the polymer-rich phase, and this resulted in the large tear-drop macrovoids.…”

“…According to the poreformation mechanism, 9,22,24,25 a fast precipitation will result in a liquid/liquid demixing; this can lead to an interconnected cellulartype structure, whereas low precipitation may result in solid/liquid demixing, which could cause a spherical globule-type structure. When the cast film was exposed to humid air, the solvents in the dope (DMAc 1 THF) evaporated, especially THF, which caused an increase in the polymer concentration in the casting film.…”

“…Drioli et al 2 confirmed that the transmembrane flux was related to the spinning conditions and polymeric dope compositions. Studies of Devi et al 9 showed that the membranes prepared with low-concentration polymer casting solutions were much more porous compared to those prepared with high-concentration polymer casting solutions. Chen et al 34 fabricated microporous membranes with a blend of highmolecular-weight PVDF and low-molecular-weight PVDF and found that the water vapor flux was highest when the highmolecular-weight PVDF/low-molecular-weight PVDF ratio was 4:6.…”

“…During the precipitation, the polymer demixing rate and phase-separation patterns (liquid-liquid and liquid-solid) can affect the structure and properties of the microporous membranes. 21,[25][26][27][28] The factors that may influence precipitation rate include the choice of polymer, 23 For the preparation of PVDF microporous membranes used for MD processes, the usual solvents used to dissolve PVDF include N,N-dimethylformamide, 2 N,N-dimethylacetamide (DMAc), 9,16,22,23 and N-methylpyrrolydone; 27 and the usual coagulation baths include water, ethanol, and their mixtures. 8,23 The rapid precipitation rate of casing solution creates a fingerlike macrovoid structure, whereas the low precipitation rate creates a spongelike structure or spherulite formation.…”