| Membrane-based separation,as an efficient and energy-saving separation technology,has show great potentials in the area of water treatment.However,the“trade-off”between permeability and selectivity is still the bottleneck that restricts the further development of membrane separation thechnology.Membrane structure decides the performance of membrane,and thus one feasible strategy to break through this limit and improve the membrane performance is to enhance the mass transfer process by optimizing the membrane structure.Since there are some uncontrolled factors and limitations in the traditional fabrication method of thin film composite（TFC）membrane,the membrane microsturcures and mesostructures are hard to control.In this thesis,by means of monomer designs and method innovations,controllable preparation of TFC membranes was achieved.And based on this,membrane performances were enhanced thanks to reduced mass transfer resistance,shortened mass transfer distance and increased mass transfer area.Firstly,in order to reduced diffusion resistance of water molecules in membrane materials,a strategy to enhance microporosity and improve microstructure of inner pore by manipulating the molecular structure was employed to prepare NF membranes.Poly（esteramide）TFC membranes with enhanced microporosity and inner-pore interconnectivity were successfully fabricated by reacting rigidly-contorted bisphenols（two phenols containing spirobisindane and Tr?ger’s base structure,respectively）and piperazine（PIP）with trimesoyl chloride（TMC）via interfacial polymerization on a porous substrate.The representative NF membranes fabricated in this work exhibited a 2.7-fold pure water flux（PWF）of the polypiperazine amide（PPA）membrane and the same multivalent salt rejections(PWF,181.1 Lm-2h-1MPa-1;rejection,99.2%for Na2SO4).Molecular simulation and N2 adsorption measurement have demonstrated that the membranes prepared according to the optimal phenol/amine ratio possess more free volumes and mostly interconnected voids,which is resulted from the shape and rigidity of the contorted monomers.Besides,owing to the micropore generation was governed at molecular level,no unselective defects were formed.As a consequence,the membranes show simultaneously high permeability and rejection.Additionally,the obtained membranes also possess great potential for long-time operation,as demonstrated by stability test.Subsequently,in order to shorten the mass transfer distance,we report a novel fabrication technique of TFC membranes,named in-situ free interfacial polymerization（IFIP）,where the IP reaction occurs at the uniform,free oil-water interface dozens of microns above the substrate,then the resulting nanofilm spontaneously assembles into TFC structure without extra manual transfer.This IFIP method not only overcomes the limitations of conventional IP,succeeding in preparing ultrathin-nanofilm composite membranes for nanofiltration and reverse osmosis application,but also enables scale membrane manufacturing that is not feasible via previously reported free-standing IP.Based on the IFIP method,the thickness of polyamide nanofilm was successfully reduced to ca.3～4 nm,which we believe is close to the ultrathin limit of polyamide nanofilm for separation application.Meanwhile,the structure-performance relationship revealed that the strategy of increasing TFC membrane permeance by reducing polyamide layer thickness also had a limit.Besides,the IP mechanisms in regard to formation of surface morphology and film growth were explored by combining experimental and molecular simulation methods.Overall,this work is expected to push forward the fundamental study and practical application of ultrathin-film composite membrane.Finally,in order to increase the mass transfer area,a new strategy was developed to fabricate NF membranes with tunable 3D surface nanostructures.The manipulation was achieved by forming aqueous template on the substrate surface during the membrane fabrication,which directly influence the consequent membrane surface morphology.Based on this,a systematic transition of surface morphology from leaf-like shapes to ridges was achieved in a facile way.The representative NF membrane with ridged nanostructures exhibited water permeance of 21.3 Lm-2h-1bar-1 and Na2SO4 rejection of 99.4%due to increased permeable area,reduced membrane thickness and low-resistance flow channel within the ridged nanostructures.The mechanism and detailed process of forming these 3D surface nanostructures were demonstrated by combining dissipative particle dynamics（DPD）simulations and experiments.In consideration of the simplicity,generality and controllability,this aqueous template-based interfacial manipulation strategy would be of importance to the fabrication of thin-film composite membranes. |
