Study On The Preparation Process And Chemical Modification Of Poly (Vinylidene Fluoride) Flat Membrane

Poly(vinylidene fluoride) (PVDF) is a kind of membrane material with excellent characteristics. In this thesis, flat PVDF membranes were prepared by phase inversion method. The effects of preparation conditions on the structures and properties of PVDF membranes were studied. Besides, PVDF flat membrane was modified by chemical treatment in KOH solution in methanol.It was found that with the increasing of PVDF concentration in casting solution, the pore density and size at the upper surface of the membrane, the average pore size, and porosity inside the membrane decreased, while water flux of membrane also decreased gradually. When hydrophilic substance including polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG) and inorganic salt of LiCl were used as additives in casting solution, the water flux and porosity of the obtained membranes is higher than that of membranes prepared from additive-free casting solution. Also both of PVP and PEG resulted in a better hydrophilic surface of membrane. With the increasing of LiCl content in casting solution, the finger-like macro-void became larger inside the membranes. The studies of the effect of PEG molecular weight on the structures and properties of membrane showed that the pores on the upper surface of the membrane and the water flux increased while the molecular weight of PEG additives increased. Among the membrane made from casting solutions with solvents of N,N-dimethylacetamide(DMAc), N,N-dimethylformamide (DMF ), N-methyl-2- pyrrolidone ( NMP ) and dimethyl sulfoxide (DMSO) , the water flux of the membrane increased in sequence of DMF < DMAc < NMP < DMSO. The results were explained by kinetics of phase inversion process. Addition of ethanol into water coagulation bath would slow diffusion rate of non-solvent into casting solution, and PVDF precipitation rate is reduced. The formation of the finger-like macrovoid is suppressed, and the finger-like structure near the upper surface changed to a sponge-like structure. When NaCl was added into water coagulation bath, the long finger-like structures inside the membrane also changed to a short finger-like structure. While the temperature of coagulation bath composed of ethanol and water (20/80 in volume) was increased, the water flux and porosity of obtained membranes became higher,whereas the change of the average pore size was not found. Rising temperature of casting solution will increase the water flux, porosity and average pore size of PVDF membrane.In modification processes, KOH concentration, the reaction temperature and the reaction time influenced the structures and properties of the membrane surface. FT-IR spectra revealed that the groups of -OH, C=O, C=C were introdued onto the surface of PVDF membrane. The contact angle at the surface of the modified membrane was reduced, but water flux of membrane changed slightly after the modification.