The Role of Solution and Coagulation Temperatures in Crystalline Structure, Morphology, Roughness, Pore Diameter Distribution, and Separation Properties of Nanoporous Membranes Fabricated Via Phase Inversion

Abstract

In this study, fabrication of poly(vinylidene fluoride) (PVDF) flat sheet membranes and correlation of membrane morphology, roughness, crystalline structure, and pore diameter distribution as functions of coagulant temperature (T_C) and precursor-solution preparation temperature (T_S) in the phase inversion (PI) process of PVDF-DMF-H₂O mixture with 20%wt of PVDF concentration was investigated. The results demonstrated that membranes have a typical asymmetric structure with a dense skin top layer and a porous substructure. An increasing amount of macrovoids was observed in the membrane substructure when T_S is decreased. It was found that at lower T_C, the membrane solution precipitated into a uniform morphology composed of spherical crystallites that exhibited the β-form crystal structure. By contrast, when PVDF was precipitated at higher temperatures, the formed membrane became largely in the α-form crystal structure. The pore size was estimated by Barett-Joyner-Halenda (BJH) method, ranging from 15 nm to tenth of hundred nm, depending on T_C. The performance of the prepared membranes has been tested by the measure the effects of T_C and T_S on the separation characteristic of nanoporous PVDF membranes. We observed that the removal of Acid Yellow 23 (AY23) decreases as T_S increases and extremely high dye removal efficiency of 99.37% was achieved.

Keywords:

• α, β, γ type crystallites
• filtration
• membranes
• phase inversion
• PVDF

ACKNOWLEDGEMENTS

This study was financially supported by a research grant from the government of Iran. Also, we acknowledge the scholarship, equipment, and material support, and study fee waiver provided for A. Akbari by the Institute of Nanoscience & Nanotechnology of the University of Kashan.