Polymer, Vol.115, 164-175, 2017
Effect of solvent on the dipole rotation of poly(vinylidene fluoride) during porous membrane formation by precipitation in alcohol baths
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 similar to 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 a-type structure, while the compact globules the g-type structure, as were confirmed by FTIR-ATR analyses. F-19 NMR spectroscopy indicated that PVDF chains exhibited TG(+)TG(-) conformation in TEP and T(3)G(+)T(3)G(-) conformation in DMSO. These conformations remained unchanged during immersion-precipitation to yield the a-and g-type crystal structures. The crystallinity of the membranes, as determined by X-ray diffraction and differential scanning calorimetry, was found to be similar to 68%. Furthermore, desalination by membrane-distillation of 3.5% NaCl(aq) feed at 50 degrees C were carried to see the desalination capability of the membranes. High fluxes (10-13 LMH) and rejections (similar to 99.8%) were obtained for both types of membranes. (C) 2017 Elsevier Ltd. All rights reserved.