Fine structure and formation mechanism of particulate phase‐inversion poly(vinylidene fluoride) membranes

Abstract

The structure and formation mechanism of a microporous phase‐inversion poly(vinylidene fluoride) (PVDF) membrane exhibiting a relatively loosely packed agglomerate of semicrystalline globules are explored. The membrane has been prepared by the coagulation of a solution of PVDF in dimethylformamide by the action of 1‐octanol, which is a soft nonsolvent. Experimental observations pertain to the globule surface, which is dominated by a grainy nanostructure; the globular interior, which exhibits a range of fine structures (e.g., twisted sheets and treelike branches); and the globule–globule connections, which exhibit a sheetlike or ropelike structure. On the basis of the observed structural details and phase diagram considerations, it is proposed that the membrane structure is the result of a unique combination of a polymer crystallization and a liquid–liquid phase‐separation process, with end‐result globular structural features of remarkable uniformity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1578–1588, 2003

Strong effect of precursor preparation on the morphology of semicrystalline phase inversion poly(vinylidene fluoride) membranes

“…PVDF membranes were prepared by the phase inversion method [11]. In brief, PVDF was dissolved at 50, 60, 70, 90 or 110 • C in DMF in a sealed glass bottle to form a 23 wt.% homogeneous dope solution.…”

“…Recently, we employed a poly(vinylidene fluoride) (PVDF)/dimethyl formamide (DMF) dope coagulated in 1-octanol in order to explore a PI case where both types of equilibria have a substantial effect on membrane morphology [11].…”

“…In the case of PI, it is well established that the membrane morphology can change dramatically by a substantial change in the composition and/or temperature of the dope, the coagulant or both [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. On the other hand, the exact route of preparation of the PI dope is usually overlooked as a parameter capable of affecting membrane morphology; pertinent concerns are usually limited to the filtering of the dope to avoid defective structures as a result of the presence of sizable inclusions.…”

Hydrophobic asymmetric ultrafiltration PVDF membranes: an alternative separator for VFB with excellent stability

“…Normally two mechanisms, namely liquid-liquid demixing and crystallization, will determine the final morphology of the PVDF membranes. 18,19 As reported by Akbari et al, 19 the totally crystallinity increases with the PVDF concentration, since the mass exchange rate in the more viscous casting solutions is slower than in the lower concentration case, allowing crystallization to commence much earlier to yield higher crystallinity. This possibly induces the formation of spherical particles, which can provide obstacles for ion transportation.…”

“…Similarly, with the membranes prepared from casting solutions with different concentrations, an increased thickness slows down the phase inversion process at the bottom side of the membrane. 19 The magnified sponge structures of the membranes with different thicknesses clearly indicate that with increasing thickness, some spherical particles, generally referred to as nodules, 18 become formed, most probably induced by some crystallization of the PVDF. Moreover, a sandwich-like structure with two skin layers was found for the membrane with a thickness of 125 mm, which possibly creates an additional ion selectivity in the VFB.…”