Fabrication Of Defect-free Thin Film Composite Membrane By Depositing Thin-walled Microcapsules

Compared with traditional separation technologies such as distillation,absorbtion,and adsorption,membrane separation technology has the advantages of low cost,low energy consumption,high efficiency,and easy operation.Therefore,developing membrane separation technology is necessary out of economic value.At present,the main difficulty in membrane technology is the separation for the low-molecular-weight systems,exemplified by organic solvent nanofiltration（OSN）,desalination,etc.Thin film composite membrane（TFCM）has attracted wide attention in these applications due to its high separation performance.The preparation method of TFCM mainly includes the following:the bottom-up fabrication approach（interfacial polymerization,atom transfer radical polymerization,bio-adhesive）and the top-down fabrication approach（dip coating,blade coating,spray coating,spin coating）.However,both these two approaches are difficult to prepare a defect-free selective layer on a porous support due to the penetration of casting solution into the porous structure when the film is directly prepared on the surface of porous support.This phenomenon not only increases the mass transfer resistance,but also resulting in the pinholes which will reduce the selectivity of the membrane.In recent years,laminate membranes have received widespread attention in membrane separation.It prepared by the in-situ assembly process using micro-or nanoscale aggregated materials,which avoids the penetrating of casting solution.Inspired by this,this paper prepared a defect-free composite membrane on a macro-porous support by using polymeric thin-walled microcapsules.The main results are shown as follows:（1）Thin-walled Dopa microcapsules（the thickness mostly distributing in 6-13nm and the diameter distributing in 1.5-2.5μm）with two-dimensional property were prepared by the hard-template method,and the microcapsules were filtrated on Nylon（pore size 220nm）support by vacuum filtration method.The Dopa TFC membrane exhibits excellent separation performance in OSN.The methanol permeance reaches440 L m^-2 h^-1 bar^-1,and its rejection beyond 95%for the dye molecules larger than 2nm,which far exceeds most polymer nanofiltration membranes.This is because the relatively thick Dopa microcapsules create a large number of nanovoids at the edge of the microcapsules in the inner of membrane,which allows the solvent to transport.Meanwhile,the close packing in the middle area of the microcapsules was achieved due to the strong adhesive force of Dopa,which endows the excellent rejection performance for membrane.At the same time,this strong adhesion force also gives excellent stability for Dopa composite membrane and the membrane maintains good performance in long-term operation stability.This demonstrates a defect-free TFCM can be successfully fabricated by thin-walled microcapsule deposition method.This fabrication method can fundamentally avoid the penetration of casting solution into pores,which provides a new insight for the preparation of high-performance membrane.（2）With the help of hybrid platform,MXene nanosheets are introduced into the Dopa composite membrane to regulate its microstructure.The introduction of MXene not only expands the distance between adjacent Dopa microcapsules,which increases the solvent permeance of the membrane,but also interacts with Dopa microcapsules via chelation,which strengthens the stability of the Dopa/MXene membrane.At the same time,regulating the oxidation degree of MXene can control the strength of chelation.The experimental results show that the water permeance of Dopa/MXene-5（5 wt%content）reaches the maximum and its methanol permeance is about 800 L m^-2h^-1 bar^-1,but its rejection performance is reduced,especially for the rejection of reactive black（RB,2 nm）which lower than 90%.Furthermore,in order to strengthen the rejection performance of the membrane,the oxidation degree of MXene was regulated and the chelation was achieved between MXene and Dopa microcapsules,which reduces the size of the gap between the contact surface of MXene and Dopa microcapsules.The permeance of Dopa/MXene membrane is slightly decreased and the rejection is slightly increased.The methanol permeance of Dopa/MO-MXene is750 L m^-2 h^-1 bar^-1,and the retention of RB is 91%.At the same time,The mechanical properties of the film are strengthened and the swelling characteristics are suppressed due to the presence of chelation.