Preparation And Performance Of Polyacrylonitrile Nanofibrous Composite Filtration Membrane

The lack of fresh water has become a big problem for all over the world since the last century.To solve this problem,a series of membrane filtration and separation processes have been developed to treat water resources for recycling in industrial production and daily life.With the development of membrane separation technology,the application and development of membrane separation materials have gained substantial attention.But these membranes are still limited in application due to the intrinsic characteristics of these functional materials,such as hydrophilicity and organic solvents resistance.Therefore,composite membranes with three-tiered structure were designed and manufactured,including a bottom supporting layer,a porous matrix and an ultrathin functional barrier layer,which could provide enough mechanical property and filtration efficiency.This three-tiered structure of composite membrane could be applied in different separating process by using different materials and fabrication methods of each layer.In recent decades,with the development of nanotechnology,electrospun nanofibrous membrane has been researched as porous matrix in composite membrane to replace the homogeneous membrane prepared by phase inversion.The as-prepared nanofibrous membranes are applied in water treatment to enhance the permeability because of its high porosity(up to over 80%),interconnected open pore structure,micro scale interstitial space,and light weight.However,it will easily cause the penetration of casting solution into the nanofiber scaffold during the dip-coating process due to its relatively large surface pore size and high porosity.Thus,various approaches have also been developed to construct a functional barrier layer on the nanofibrous substrate,such as soaking in a coagulating bath,solution induced swelling,heat and pressure induced re-melting and interfacial polymerization.In this work,a series of three-tier structure nanofibrous composite membranes were fabricated,i.e.,an intermediate transitional hydrogel layer was introduced between nanofibrous substrate and functional barrier layer.The introduction of intermediate transitional layer could not only reduce the surface pore size of nanofibrous substrates,but also introduce functional groups such as hydroxyl groups,carboxyl groups,and amino groups leading to a more hydrophilic surface for further modification.Herein,the intermediate transitional layer can build up the interconnection between functional barrier layer and nanofibrous substrate,and the integrity of composite membranes will be significantly enhanced including mechanical properties,filtration performance,and stability.Thus a series of composite membranes consisting of a PAN nanofibrous support layer,an ultrathin transitional hydrogel layer and a functional barrier layer were developed.Hydrophilic materials such as carbopol(CP)or poly(acrylonitrile-co-acrylic acid)(PAN-AA)were introduced as the transitional hydrogel layer by coating,electrospinning,and solution induced swelling.The ultrathin functional barrier layer was fabricated by interfacial polymerization and ionic cross-linking.The relationships between each layer of the nanofibrous composite membrane were studied and the effect of transitional hydrogel layer on filtration performance and stability was deeply researched.The interfacial polymerization and solution induced swelling methods were investigated systematically to optimize the structure,morphology and property of functional barrier layer.The nanofibrous composite membranes before and after introducing the transitional hydrogel layer were also analyzed in detail to characterize their filtration performance to different aqueous solution of salts or proteins.1.A novel kind of PAN/CP/PPA nanofiltration(NF)composite membrane was fabricated by interfacial polymerization,while piperazine(PIP)was used as aqueous phase monomer and trimesoyl chloride(TMC)was chosen as organic phase monomer to prepare a polypiperazine-amide(PPA)barrier layer based on polyacrylonitrile(PAN)nanofibrous substrate modified by carbopol(CP)gel(a bionic adhesive).FTIR results showed that hydrogen bonds were formed between carboxyl groups(—COOH)of CP and cyan groups(—CN)of PAN,while ionic bonds of carboxylic acid ammonium salt(+2—COONH)were obtained by neutralization reaction between carboxyl groups of CP and secondary amine groups(—NH—)of PIP.Then interactions of each layer in composite membranes were enhanced.PAN/PPA composite membranes and PAN/CP/PPA composite membranes were prepared by the same interfacial polymerization process based on PAN and PAN/CP nanofibrous substrates,respectively.Tensile test results indicated that PAN substrate and PPA barrier layer of composite membrane were firmly bonded together by CP gel to form an integrated structure of composite membrane.The resulting tensile strength of composite membrane was increased from 15.1 MPa to 24.2 MPa.Compared with PAN/PPA composite membranes,PAN/CP/PPA membranes had similar water flux but possessed better rejection rates for different kinds of nitrate salts and Mg Cl2,which may be attributed to that PAN/CP/PPA composite membrane was strongly negative charged according to a great amount of carboxyl groups of CP.2.A new method for fabrication of thin film nanofibrous composite(TFNC)ultrafiltration(UF)membrane consisting of an ultra-thin poly(acrylonitrile-co-acrylic acid)(PAN-AA)barrier layer based on polyacrylonitrile(PAN)nanofibrous support layer was proposed in this study.First,a thin PAN-AA nanofibrous layer was electrospun and deposited on a thicker PAN nanofibrous substrate.Then,the as-prepared PAN-AA nanofibers was swollen in the alkaline buffer solution and merged imperceptibly as an integrated non-porous hydrogel layer on the PAN substrate.The PAN-AA hydrogel layer was crosslinked with different bivalent metal cations(Ca2+,Mg2+)to form an ultrathin barrier layer,of which the thickness and porosity were optimized by controlling the depositing time of PAN-AA nanofibers and p H value of buffer solution.Proteins with different molecular weights were used to evaluate the ultrafiltration performance of the resultant composite membranes.Due to its hydrophilic and negative charged barrier layer,the PAN-AA-Mg and PAN-AA-Ca TFNC UF composite membranes exhibited excellent permeate flux(221.2 L/m2 h and 219.2 L/m2h)and rejection efficiency(97.8 % and 95.6 %)for bovine serum albumin(BSA)aqueous solution(1 g/L)at 0.3 MPa.According to the molecular weight cut-offs(MWCOs)of these PAN-AA TFNC UF membranes(43.5 – 47.3 k Da),the as prepared ionic cross-linked PAN-AA composite membranes could be used to retain solutes,of which the radius was larger than 4.6 nm.3.A novel kind of thin-film nanofibrous composite(TFNC)nanofiltration membrane consisting of a polypiperazine amide(PPA)barrier layer,an ultrathin electrospun poly(acrylonitrile-co-acrylic acid)(PAN-AA)transitional mid-layer and an electrospun polyacrylonitrile(PAN)nanofibrous supporting layer,was successfully fabricated by interfacial polymerization with piperazine(PIP)and trimesoyl chloride(TMC)onto the PAN-AA/PAN double-layer substrate.The PAN-AA nanofibrous mid-layer played two important roles between the PPA barrier layer and the PAN nanofibrous supporting layer.It could be swollen in the alkaline aqueous monomer(PIP)solution to form an intermediate hydrogel film,which acted as the transitional mid-layer to cover the majority of surface large pores of electrospun PAN nanofibrous substrate.On the other hand,the hydrophilic PAN-AA hydrogel film could capture and reserve abundant PIP monomers to facilitate the interfacial polymerization with TMC to form an endurable ultrathin PPA barrier layer,resulting in an integrated composite membrane confirmed by the mechanical property.The resultant TFNC membranes demonstrated high rejection rate(98.2%)and high permeate flux(64.4 L/m2h)for Mg SO4 aqueous solution(2.0 g/L),and also exhibited excellent structural stability due to the strong interactions between the barrier layer and the nanofibrous support that enhanced by the transitional PAN-AA mid-layer.To sum up,the introducing of intermedia hydrogel layer between the nanofibrous substrate and functional barrier layer could carry on aqueous monomer to improve the process of interfacial polymerization for barrier layer and obtained excellent filtration performance of the resultant nanofibrous composite membranes.While the interconnection between the nanofibrous substrate and functional barrier layer was also improved,this could enhance the structural stability of TFNC membranes.A novel method for introducing the intermedia hydrogel layer was developed through the swelling process of p H-sensitive copolymer in buffer solution,which was more suitable and easier for operating with kind conditions and strong controllability.Different functional materials could be deposited on the nanofibrous substrate.During the modification of porous substrate,the hydrogel layer would not penetrate into the large pores on the surface of nanofibrous substrate.Microstructure of the functional barrier layer could be controlled precisely by optimizing the conditions of swelling process in buffer solution.A series of nanofibrous composite membrane with different sizes of screening were prepared for ultrafiltration or nanofiltration and applied in water treatment.