The Effect Of PVDF Resin On The Film-forming Properties Of The Spin-fabrication By Thermally Induced Phase Separation

Polyvinylidene fluoride(PVDF)is semicrystalline polymer,and it has aroused much attention because of its excellent thermal stability,mechanical properties and chemical properties.As membrane material,PVDF has been widely used in preparing ultrafiltration and microfiltration membranes,and has become promising membrane material for membrane contactor and membrane distillation process.Currently,the thermally induced phase separation(TIPS)is widely used to research PVDF membranes,especially in membrane preparation process.But it lacks systematic discussion about the effect of PVDF on the memebrane structure and performance.In this paper,PVDF hollow fiber membranes were prepared by TIPS method using various kinds of PVDF resins as raw materials.The influence of PVDF resin on the structure and properties of the membranes was investigated.The performance differences of PVDF raw materials in molecular weight and melting index were compared by studying the properties of raw materials.PVDF hollow membrane was prepared by TIPS with di-butyl phthalate and di-octyl phthalate as mixed diluent.The possibility of preparing PVDF hollow membrane was studied.The cloud point of the casting solution was observed by thermally polarizing microscope.The crystalline properties and pore structure of membranes were analyzed.Meanwhile,the permeability and mechanical properties of membranes were tested by the test of differential scanning calorimetry,field emission scanning electron microscope,the water flux,porosity and tensile test.It was systematically studied that the relationship between the molecular weight of PVDF and the phase separation behavior of the casting solution and the preparation technology and the effect of them on structure and properties of the membrane.The results showed that the cloud point of casting solutions and preparation temperature increased while melt index decreased and melt liquidity became worse with the increase of the molecular weight.In the process of membrane preparation by TIPS,changing the molecular weight of polymer can affect both the dynamics and thermodynamics of phase separation,and the synergies of them have a significant impact on the structure and properties of membranes.The crystallinity of the membrane increased with the increase of molecular weight.However,it led to the decrease of the crystallinity when the molecular weight is too large.Membrane morphology was changed from spongy pore to the combination of spongy pore and deformed spherulites.And the average pore size,water flux and porosity of the membrane decreased at first,and then increased.The axial orientation and tensile strength of the membrane first increased with the increase of molecular weight.After reducing the molecular weight of domestic PVDF,the hollow fiber membrane showed homogeneous sponge structure.,the pore size distribution of the hollow fiber membrane was more uniform,the pure water flux and average pore diameter of the membrane increased,and the permeability was better compared with PVDF2.PVDF hollow fiber membranes with different DBP/DOP wt%were prepared by TIPS method.The effects of DBP/DOP wt%on the structure and properties of membranes were investigated.The results showed that with the increase of DOP content in mixed diluent,the cloud point temperature of PVDF/DBP/DOP casting liquid system increased,the crystallization temperature was almost unchanged,and the area between cloud point temperature and crystallization temperature became larger.DBP/DOP wt%decreased from 1:2 to 1:2.6,the structure of membrane changed from cell pore structure to spongy pore structure,average pore diameter increased,water flux and porosity increased,and the rejection rate decreased.