Effect of solvent on the dipole rotation of poly(vinylidene fluoride) during porous membrane formation by precipitation in alcohol baths

Publication date: 21 April 2017
Source:Polymer, Volume 115
Author(s): Hsu-Hsien Chang, Liang-Kuei Chang, Cheng-Dau Yang, Dar-Jong Lin, Liao-Ping Cheng
Microporous PVDF membranes were prepared by isothermal immersion precipitation of PVDF/DMSO and PVDF/TEP casting dopes in different alcohol baths. The formed membranes exhibited the so-called particulate morphology, being packed by interconnected crystalline particles of a similar size. For the alcohol/TEP/PVDF system, the particles appeared as sheaf-like spherulites consisting of curled lamellae with thickness of ∼15 nm. In contrast, for the alcohol/DMSO/PVDF system, the formed membranes comprise relatively compact globules whose surface was covered with granular objects of ca. 30 nm dia. Wide angle X-ray diffraction patterns of the membranes indicate that the sheaf-like crystals were of the α-type structure, while the compact globules the γ-type structure, as were confirmed by FTIR-ATR analyses. ¹⁹F NMR spectroscopy indicated that PVDF chains exhibited TG⁺TG⁻ conformation in TEP and T3G⁺T3G⁻ conformation in DMSO. These conformations remained unchanged during immersion-precipitation to yield the α- and γ-type crystal structures. The crystallinity of the membranes, as determined by X-ray diffraction and differential scanning calorimetry, was found to be ∼68%. Furthermore, desalination by membrane-distillation of 3.5% NaCl(aq) feed at 50 °C were carried to see the desalination capability of the membranes. High fluxes (10–13 LMH) and rejections (∼99.8%) were obtained for both types of membranes.

Graphical abstract

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