Fabrication Of Poly(Vinylidene Fluoride) Flat Sheet Microporous Membrane Via Thermally Induced Phase Separation

Poly(vinylidene fluoride)(PVDF)membrane was most recently fabricated via immerse precipitation method with its outstanding physical and chemical properties. However microporous membrane by this method often forms finger-pore microstructure and effect the membrane strength.Therefore,it is disadvantage to extend its applied field.In this paper,PVDF flat sheet microporous membrane was prepared via thermally induced phase separation(TIPS),and attempt to put some hydrophilic polymer in PVDF for enhancing its hydrophilic abilities.The effects of thermodynamic and dynamic parameters on membrane structure and property were studied.These lay a theoretical and practical foundation for a new technology of preparing PVDF flat sheet microporous membrane with high properties.Firstly,four diluents were selected including dimethyl phthalate(DMP),dibutyl phthalate(DBP),diethylene glycol monobuthyl ether and benzophenone by miscibility experiment with PVDE The study results showed that the miscibility between polymer and diluent were controlled uneasily when a single diluent was used and the selectivity on membrane structure made in experiment is singleness,which brought disadvantage to the membrane properties.Secondly,tow diluents with different solubility parameters were mixed as combined diluent.The results showed that the solubility between polymer and combined diluent and system phase separation behaviors were controlled by changing the composition of diluents.Additionally,the addition of nucleating agent increased the nucleation density in some sense and improved the membrane structure correspondingly.On the basis of above-mentioned study,PVDF flat sheet microporous membrane was casted using the combined diluent.The study results showed that the membrane maximum pore diameter,porosity and pure water flux decreased as the increase of polymer concentration,while the membrane strength and elongation at break increased correspondingly.With the increasing of the coagulation bath temperature,the lacy structure in the membrane exhibited more obviously while the membrane maximum pore diameter and pure water flux increased gradually.The membrane disposed by long time heat treatment,its cross-section and surface pore diameter most increased correspondingly than short time disposed,and the membrane maximum pore diameter and pure water flux increased by degrees,but the membrane porosity changed unconspicuously.The influence on miscibility and hydrophilic abilities of PVDF adding some hydrophilic polymer was studied by melt-blending PVDF/HDPE-G-MAH/DMP/DOP pseudo-quadruple system.It was showed that PVDF/HDPE-G-MAH/DMP/DOP pseudo-quadruple system is partial miscibility observed by DSC and SEM,the best proportion is PVDF:HDPE-G-MAH:DMP:DOP=3:1:3:3.With the increase of hydrophilic polymer(HDPE-G-MAH)concentration,the contact angle of PVDF microporous membrane decreased gradually,pure water flux increased by degrees,as for membrane strength and elongation at break,which has a maximal numerical value at the best proportion in blending system.At last,PVDF crystalline behavior in different conditions was investigated by differential scanning calorimetry(DSC)and fourier transform infrared spectroscopy (FTIR).The results showed that PVDF presented a crystalline phase in all the studied systems;furthermore,it is irrespective to cooling condition,combined diluents,heat treatment and nucleating agent.