Effect of postcasting heat‐treatment on the structure and properties of semicrystalline phase‐inversion poly(vinylidene fluoride) membranes

Abstract

Microporous PVDF membranes were prepared by immersion‐precipitation in 1‐octanol of casting dopes dissolved at different temperatures, with dissolution temperature affecting strongly the membrane microstructure. The effect of postcoagulation thermal annealing, which is an additional thermal parameter, on membrane microstructure and properties is probed herein. Membranes obtained were annealed at temperatures up to 160 °C, which is close to the melting point of PVDF polymer. Annealing leads to a substantial modification of the nano‐scale fine structure of the membranes, while the overall‐microporous structure is preserved. At elevated annealing temperatures, nano‐grains, fibrils, and stick‐like crystalline entities gradually eclipse, while globules develop more robust connections based on wide bands of crystal elements. Probing by X‐ray diffraction and dynamic scanning calorimetry shows that crystallinity increases when annealing temperature and time are increased. As regards mechanical properties, the tensile strength of the membranes can be enhanced substantially, up to about 10 times, upon appropriate high temperature prolonged annealing. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1880–1893, 2009

Crystalline polymorphism in poly(vinylidenefluoride) membranes

“…The annealing of a family of globular microporous PVDF membranes at temperatures near the melting point of PVDF significantly affects the fine structure of themembrane [155]. Upon proper annealing, nano-scale features such as nano-grains, fibrils, and stick-like entities disappear, whereas globules become connected to each other by wide bands of crystal elements.…”

Mixed Matrix PVDF/Graphene and Composite‐Skin PVDF/Graphene Oxide Membranes Applied in Membrane Distillation

“…Most of the asymmetric PVDF membranes are produced by the immersion‐precipitation (IP) method due to the simplicity of the process and the ease of PVDF dissolution in a range of organic solvents, while nonsolvent‐induced phase separation processes are also applied . The major attempts to fine tune the morphology and pore structure of asymmetric PVDF membranes and make them appropriate for DCMD processes, had initially focused on the preparation conditions including the choice of solvent, the evaporation period between casting and coagulation, the air gap distance , the effect of coagulation bath medium and temperature (i.e., sequence of liquid–liquid demixing and crystallization, and/or dominance of crystallization or liquid–liquid demixing), the postcasting heat treatment , and the effect of low and high MW nonsolvent additives that are usually hydrophilic ones and can function as pore formers. The aforementioned processes conclude to the formation of various types of pores with the major distinguishable categories being short finger like pores with sponge‐like structure, or sponge‐like pores, or even macro‐voids with high connectivity .…”

Modification on crystallization of poly(vinylidene fluoride) (PVDF) by solvent extraction of poly(methyl methacrylate) (PMMA) in PVDF/PMMA blends

“…Because of its excellent piezoelectric, chemical and physical properties, poly(vinylidene fluoride) (PVDF) has attracted more and more attention in recent years [1][2][3][4] and has been widely applied in many industrial fields, such as sensor devices [5], battery separators [6][7][8], and biomedicine [9]. According to previous studies, PVDF has five crystal phases, which involve three different chain conformations, namely, β-phase (TTTT), α-and δ-phase (TGTG′), γ-and ε-phase (TTTGTTTG′) [10][11][12].…”