Effect Of Water-soluble Polyester On The Structure And Properties Of Polyvinylidene Fluoride Porous Membrane

The rapid development of modern industry has brought about many problems,such as industrial pollution,the bottleneck of production efficiency,the recycling of resources and so on.In the context of sustainable development at home and abroad,membrane separation technology is emerging in response to demand and plays an increasingly important role in the field of purification and separation.Polyvinylidene fluoride（PVDF）porous membrane with its unique processability and excellent chemical stability and weather resistance in many separation membranes by the majority of scientific research workers and industrial production,has been widely used in chemical,electronic,medical,food and environmental protection and other fields.Polyvinylidene fluoride（PVDF）membrane is prepared by immersion precipitation phase transformation.Many technological parameters,such as additive type,solvent selection,solidification bath temperature and so on,will affect the structure and properties of the membrane.The addition of additives to polyvinylidene fluoride（PVDF）solution can affect the thermodynamic stability and film forming kinetics of the solution during the process of polymer phase separation,so that different membrane structures can be obtained.In this paper,PVDF porous membrane was prepared by immersion precipitation phase transformation using water-soluble polyester（WSPT）as new additive.The effects of additive content,solvent type and additive type on the thermodynamic stability,film structure and properties of casting liquid were analyzed.The research work in this paper is divided into the following three aspects:（1）With WSPT as additive and N,N-dimethylacetamide（DMAc）as solvent,the casting solution of PVDF/WSPT/DMAc was prepared.Turbidity method was used to study the influence of WSPT content on the thermodynamic stability of PVDF/WSPT/DMAc/H2O film forming system,and water was used as coagulation bath.PVDF porous membrane was prepared by immersion precipitation phase transformation.The effect of WSPT content on the structure and properties of PVDF porous membrane was studied.The results found that:With the increase of WSPT content in the casting liquid,the gel value of the casting liquid system gradually decreases,indicating that the addition of WSPT reduces the thermodynamic stability of the casting liquid system.The higher the WSPT content is,the more unstable the casting liquid system is.The instability of the casting liquid system speeds up the phase separation process,so that the porosity and pore size of the film are improved.XRD（X-ray diffraction）and DSC（differential scanning calorimetry）test results have shown WSPT content is higher,the film crystallinity decreased on the whole,combined with the results of ir analysis,PVDF membraneβ-crystal percentage content with the increase of the WSPT content in the casting solution;SEM（scanning electron microscope）showed that after addition of WSPT,the upper surface of the membrane showed porous structure,the cross section showed finger-like macropore structure,and the finger-like macropore structure made the lower surface of the membrane porous.The water flux and anti-pollution tests of the membranes showed that the water flux of PVDF membranes increased with the increase of WSPT content in the casting solution,but the membrane interception rate decreased,the membrane water flux recovery rate increased,and the degree of irreversible pollution decreased.These results indicate that WSPT can be used as a pore-making agent for PVDF porous membranes prepared by NIPS method.Overall,when the WSPT/PVDF ratio is 5/90,the overall performance of Mws5 membrane is relatively the best,with large membrane porosity（88.9%）and pore size（43.0 nm）,crystallinity of 42.6%,membrane water flux(J_W1)of 123.3 L/h·m²,and BSA retention rate of 83.9%.Water flux recovery rate（FRR）was 94.5%,and irreversible contamination degree(R_ir)was 5.5%.（2）Choose four solvents:N,N-dimethylformamide（DMF）,N,N-dimethylacetamide（DMAc）,N-methylpyrrolidone（NMP）and methylsulfoxide（DMSO）were measured by turbidity method.The gel values of PVDF/WSPT/solvent/H₂O membrane forming system were studied.The influence of thermodynamic stability changes in different solvents on membrane structure and properties was investigated.The results show that the thermodynamic stability of the casting solution in different solvents is as follows:DMAc system>NMP system>DMF system>DMSO system,in the ternary phase diagram,the phase separation line is farther and farther away from the polymer-solvent axis,and the homogeneous region decreases successively.By combining the four membrane structures,it is found that the pore structure and aggregation structure of PVDF membrane are mainly controlled by the thermodynamic properties of DMAc,NMP and DMF systems.The higher the thermodynamic stability of the cast liquid system,the slower the phase separation rate,and the more the liquid-liquid delayed phase separation occurs,and the pore size and porosity of the generated membrane tend to decrease.In DMAc,NMP and DMF systems,DMAc and NMP have good thermodynamic stability,so the average pore size and porosity of the generated films are low,the porosity is90.7%and 90.8%,and the average pore size is 53.8 nm and 59.3 nm,respectively.However,the DMF system has relatively poor thermodynamic stability,and the film forming speed is fast.The porosity and average pore diameter of the obtained membranes are high,and the porosity and average pore diameter of the membranes are 91.5%and 85.3 nm.Due to its very high viscosity,the film forming process of DMSO casting system is mainly controlled by film dynamics,namely diffusion rate.Due to high viscosity,in the process of phase separation,the mutual expansion rate of solvent DMSO and solidifying bath water is slow,resulting in low phase separation rate and a tendency to delay phase separation.The resulting membranes exhibited the lowest porosity（89.6%）and average pore size（39.8 nm）.FTIR,XRD and DSC test results showed that the crystallinity of four kinds of membrane with falls with a decrease in the stability of the system,on the membrane surface by SEM observation to spherical crystal is less and less,especially the DMSO system,due to high viscosity,PVDF molecular motion is restricted,the crystallization ability weaker,the highestβ-crystal percentage content in the membrane.The experimental results of membrane water flux and anti-pollution showed that the size of membrane water flux(J_W1)was directly proportional to the pore size and porosity of the membrane.The membrane M_DMF prepared by WSPT/PVDF/DMF casting system had the largest pore size and porosity,and also had the largest water flux of 336.7 L/h·m².However,the adsorption rate of BSA in this membrane was the lowest（64.4%）,and the irreversible contamination degree(R_ir)of the membrane was the highest（7.9%）.Large pores would cause more BSA molecules to enter the membrane and fail to elution,resulting in permanent membrane pollution.By comparing the performance parameters of different membranes,it was found that M_DMAc had good comprehensive performance,including water flux of 196.1 L/h·m²,BSA retention rate of 77.2%,reversible pollution rate（R_r）of 5.6%,and irreversible pollution degree(R_ir)of 7.2%.（3）PVDF porous membranes with PVP-k30,PEG2000 and WSPT as additives were prepared respectively,and the structure and properties of the three membranes were compared.Test values to characterize the gel casting membrane system of different additives thermodynamic stability,and the results found that WSPT as additive of minimum value of the gel casting solution 0.3 g,the thermodynamic properties is the most unstable,membrane M_WSPTrelative maximum average pore size（63.0 nm）and water flux（215.2 L/h·m²）,The crystallinity was the lowest（29.1%）.The solution system with PEG as additive had the highest gel value（1.13g）,the most stable thermodynamic properties,the lowest porosity（89.2%）and pore size（58.1nm）,and the highest crystallinity（46.2%）.It can also be observed in SEM that more spherical crystals can be observed on the upper surface of the Membrane prepared by the more thermodynamically stable casting liquid system.With the decrease of the thermodynamic stability of the casting liquid,the cross section of the Membrane shows more and more large interconnected fingerlike pores.In addition,Gergorio method was used to calculate the percentage content ofβcrystals in the three kinds of Membranes,which is close to but not more than 40%.The surface crystals of the Membranes are mainlyαcrystals.In the DSC test,the melting point of different Membranes is similar to about 165.5℃,and the variation of crystallinity calculated by enthalpy is consistent with XRD.The results show that the MPEG with the smallest pore diameter has the largest BSA retention rate（83.7%）,and the M_WSPT has the same interception rate as the M_PVP.According to the above comparative study,the PVDF membrane prepared with WSPT as pore-causing agent can be comparable to PVDF membrane prepared with common pore-causing agents PVP and PEG2000 in terms of structure and performance,further indicating that WSPT can be used as pore-causing agent for the preparation of porous membrane by immersed precipitation phase transformation method,and according to the requirements of membrane structure and performance,The content of WSPT or solvent type can be adjusted to achieve the purpose.