The Preparation And Properties Of Polyamide Composite Membrane Based On Polyvinylidene Fluoride Support Layer

Polyamide（PA）thin film composite（TFC）membrane based on interfacial polymerization technique play a crucial role in waste water treatment and seawater desalination.PA TFC membrane consist of PA film and porous support layer,which is the mainstream for m of commercial nanofiltration and reverse osmosis membrane.Although the TFC membrane possess a good seperation performance,there still exist some faults in pratical application,such as stability in complex enviroment,the“trade-off”effect between permeate flux and rejection.In order to improve above defects,we devoted to the material selection and structure design of support layer,fabricated a series of TFC membranes with PVDF support membranes.The effect of structure and properties of support layer on the formation mechnaism and performance of PA film was systemically studied.The flux and stability of TFC membrane was improved.Details are as follow:（1）Based on blending modification method,Polyvinylidene fluoride（PVDF）support membranes containing various hydrophilic multi-walled carbon nanotubes（F-MWCNTs）were prepared via phase inversion.Polyamide（PA）films formed on the support layers by interfacial polymerization method.The characteristics（include wettability,pore size,surface roughness,and porosity）of PVDF support membranes were improved by embedding F-MWCNTs into the polymer solution.Investigated the effect of the wettibility and pore size of support layer on the formation and performance of PA film.The results revealed that,as two key parameters,pore size and surface wettability showed dominant influence on the properties of the selective layer,as well as the adhesion of them.Support layer with good wettibility and large pore size facilite to form free-defect and high rejection of PA film,as well as enhance the adhesion between PA film and support layer.Although the TFC membrane with F-MWCTs/PVDF support layer improved the water flux,the large pore size and high porosity of support layer result in the rejection decreasing（85%）.（2）The significant effect of pore structure of support layer on the performance of PA film.To improve the separation efficiency of of PA TFC membrane by adjusting the pore structure of support layer.A new type of cellulose acetate（CA）/poly（vinylidene fluoride）（PVDF）support layer was fabricated by polymer blend method.In this study,several types of CA/PVDF supports were prepared via the phase inversion method,analysis results suggested that the supports with different polymer ratio can result in various physicochemical properties.The effect of the support properties on the performance and formation mechanism of polyamide（PA）film were systematic investigated.The results show that the desired polyamide TFC membrane exhibited excellent water flux（6.56 L/（m²·h·bar））and rejection of Na₂SO₄salt solution（>97%）when the content of CA more than 30%.（3）Conventional support layer not convducive to the increase of water flux of PA TFC membrane.To improving the water flux of PA TFC membrane,in this reseach,we prepared tree-like nanofiber membrane to replace the traditional support layer.A novel thin film composite membrane（TFC）using one-step formed polyvinylidene fluoride（PVDF）tree-like electrospun nanofiber membranes（TENMs）as support layer was fabricated.Compared with conventional support membrane,TENMs possessed high surface porosity,low tortuosity,high-proportioned interconnect pore,and minimum average pore size.Therefore,it shows great potential for the preparation of high-performance TFC nanofiltration（NF）membrane.The effect of additive Tetrabutylammonium chloride（TBAC）concentration in the morphology and structure of membrane was also discussed.Results indicated that adding 0.025mol TBAC in the spinning solution facilitate to the formation of tree-like structure with the smallest pore size and narrowest pore diameter distribution,as well as dense branch structure.Furthermore,the TENMs structure and surface morphology on polyamide thin film formation and separation performance were also studied.It was found that a fraction of ultrafine branch fiber embedded into polyamide matrix formed“directed water channels”to increase water molecules transport speed.The water flux increased from 6.56 to 26.9 L/（m²·h·bar）,and the rejection above 97%.（4）The low yield and unstable structure of elecsrospun tree-like nanofiber membrane,which results in practical application difficult.To improve the production efficiency of nanofibers and enhance the stability of PA TFC film,a novel double layer support layer consisting of a PDA@MWCNTs top layer and PVDF nanofibrous membrane above layer was fabricated via vacuum filtrating assist method.PVDF nanofibrous membrane was preperaed by solution blow spinning technique.The surface pore structure,chemical and physical properties of PVDF membrane were improved by the homodispersed PDA@MWCNTs layer.PDA@MWCNTs layer provide place for interface polymerization reaction,the structure and seperation performance of PA film could be finely tailored by tuning the thickness of it.Compared with common PA TFC membrane,this triple-layered TFC membrane exhibited excellent water flux(15.8 L/（m²·h·bar）and maintain higher rejection rate（>97）.Meanwhile,it also showed a long-time stability for during a continuous operation process.