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

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 beta-form crystal structure. By contrast, when PVDF was precipitated at higher temperatures, the formed membrane became largely in the alpha-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.