Study On Preparation And Formation Mechanism Of PVDF Ultrafiltration Membranes

In this work, poly(vinylidene fluoride), which is a kind of semi-crystalline polymer with excellent physical-chemical characteristics, was used as membrane material. Based on Flory-Huggin's theory and Fick's law, Multi-component phase diagrams and coagulation paths were obtained. The calculation results were examined with the experimental results.The effects of different membrane forming conditions on the morphology and performance of the membranes were investigated. These membrane forming conditions includes solvent, additive, composition and temperature of coagulation bath, and the time of pre-evaporation. The different solvents include Dimethyl Formamide(DMF), Dimethyl Acetamide(DMAc), Dimethyl Sulphoxide(DMSO) and Triethyl Phosphate(TEP). The different additives include Polyvinyl Pyrrolidone(PVP), Lithium Chloride(LiCl), glycerol, phosphoric acid and water.The mathematics models for thermodynamic phase diagrams and dynamical coagulation paths were presented. The bi-nodal lines and spin-nodal lines were calculated from the phase balances for ternary and quaternary systems. Based on Matlab 6.5 software, the equations for the systems were solved, and the results agree well with experimental data. The inter-diffusion of solvent and non-solvent occurs during membrane forming process. Concerning the convection induced by diffusion, a series of expressions were deduced. These expressions include the expression for polymer concentration distribution in membrane thickness direction, the expression for the interface diffusion flux vs. time, and the expression for membrane thickness vs. time.Some conclusions were drawn in this work. With the increasing of the polymer concentration in the casting solution, the skin of the membrane becomes denser. It was also found that cross-section of membranes was finger-like pores when DMF, DMAc and DMSO were used as solvent, but network pores were found using TEP. When LiCl and phosphoric acid were used as additive, the skin of the membrane was much denser. Uniform porous surface of membrane could be obtained using water as additive. The porous cross-section of membrane was formed with PVP, and the connectivity of pores could be enhanced when Glycerol was used. When soft solvent was added into nonsolvent, the formation of fingerlike structure wassuppressed, and the resultant membrane has more porous on the surface. The similar effect can be found when an evaporation step was introduced before immersion in a nonsolvent bath. It was noted that when coagulation bath temperature was increased, crystallization could be suppressed, and more porous structure was obtained on the surface of the membrane.In addition, the membrane surface became more porous and the formation of finger-like pores was restrained when soft non-solvent was added to coagulation bath.When composition of casting solution was 17 wt.% PVDF, 5wt.% glycerol and 78wt.% DMAc, respectively, the ultrafiltration membrane has dense surface, a network cross-section andpossesses relative high performance. Which flux for pure water was 83L/m2 /h, and rejection to BSA was 93% under 0.1 MPa.