The Effect of LiCl and Coagulation Bath Temperature on the Structure and Performance of PVDF Membranes

Polyvinylidene fluoride (PVDF) membranes are widely used in microfiltration and ultrafiltration processes for their excellent mechanical and chemical resistance and thermal stability in comparison with other polymeric membranes. Non-solvent induced phase separation (NIPS) is the most important method by which the PVDF membranes are prepared. The structure of the membranes prepared by NIPS method depends on different parameters including the concentration of the polymer solution، polymer molecular weight، the composition and temperature of coagulation bath، type of solvent and the presence of additives in the initial solution. In the present work، the effects of coagulation bath temperature and LiCl content of the dope solution were studied with respect to the structure and performance of PVDF membranes. N-Methyl-2-pyrrolidone and water were used as solvent and coagulation bath، respectively. A set of analytical techniques including: scanning electron microscopy، mechanical test، pure water permeability and mean pore radius of pores was used to characterize the membranes. Moreover، the separation of humic acid، a main biological contaminant in surface water resources، was studied to determine membranes performance. The results show that at constant coagulation bath temperature، presence of LiCl in the dope solution increased the number as well as the mean pore radius of the pores at the surface of membranes and consequently، pure water permeability of the membranes was increased، whereas، the mechanical strength and humic acid rejection of the membranes were dropped. Moreover، at a constant content of LiCl، increasing the coagulation bath temperature decreased the size of macrovoids so that the mechanical strength as well as humic acid rejection of the membranes was increased.