This paper is focused on investigating the effect of various parameters on the phase separation step on the structure and morphology of membranes fabricated from polyvinylidene fluoride. The studied parameters are PVDF polymer concentration (15-30 wt.%), solvent types including dimethyl acetamide, dimethylformamide, and normal methyl pyrrolidine, different additives such as lithium chloride, polyethylene glycol, acetic acid, methanol, and glycerol, and coagulation bath temperature (5-65°C). Scanning electron microscopy is used to examine membrane morphology. For investigating the effect of additive, three-phase diagrams are shown in the presence of dimethyl acetamide solvent. Various tests including pure water flux, effective surface porosity, maximum pore at the membrane surface, mean membrane pore size, and nitrogen penetration, have been performed on each membrane with additive and dimethyl acetamide solvent. The results proved that coagulation bath temperature, solvent type, and polymer concentration, as well as the presence of additives in the membrane solution, leads to considerable change in the membrane morphology, as clearly shown by SEM images. Adding additives to the dense layer of the membrane increases the porosity and the size of the pores, which increases the tensile strength of the membranes. Among the considered additives, glycerol and polyethylene glycol lead to the highest increase in the viscosity of polymer solution. Furthermore, the results substantiated that the presence of the additive leads to an increase in the porosity of the membrane and a decrease in the contact angle. From the pure water flux test, it was concluded that with increasing the additives, the membrane structure has changed towards higher degree of hydrophilicity. Also, nitrogen penetration test showed that the membrane porosity increases with increasing the additive, and this increment in porosity is more significant for acetic acid compared to other additives.
Published Date: 2022-01-03;