Study On The Solution Properties Of Polyelectrolytes, Layer-by-layer Self-assembly Multilayer Membranes And Their Pervaporation Performance

In the past decade,the layer-by-layer(LbL)self-assembly of polyelectrolytes(PE) has been considered to be the simplest and most convenient way for fabricating thin multilayer membranes with a well-defined thickness and chemical composition. However,due to its short developing history,many things are still uncertain for this novel technology which limits its application in many aspects.In order to further discover the rule of PEM during the LbL process,a series of protocols were established to investigate the properties of PEMs.Solution properties of PEs and properties of multilayer membranes were investigated to discover the relation between the surface properties,membrane thickness and separation performance.Self-assembly layer-by-layer multilayer membranes of two typical PEs,poly (dimethyl diallylammonium chloride)(PDDA)and poly(styrenesulfonate sodium salt) (PSS)were fabricated from dipping solutions with different added salt concentrations, pH,solvent compositions and temperature respectively.The absorption amount of PDDA/PSS self-assembled multilayer membranes in designed dipping solvents was measured by UV-vis-spectroscopy(UV)and quartz crystal microbalance(QCM). Intrinsic viscosities of PDDA and PSS in corresponding dipping solvents were determined by an Ubbloheode viscometer.The adsorption amount of PDDA/PSS multilayer membranes increases with decreasing the intrinsic viscosity of PDDA or PSS in different corresponding dipping solvents.That is,a negative relation between the adsorption amount of PDDA/PSS multilayer membranes and the chain dimension of PDDA and PSS in the dipping solvents exhibits for all assembly systems of PDDA and PSS except for that of PSS in the media with pH_PSS＞10.The negative relation is trusty in spite of the designed dipping solvent for only PDDA or PSS and for both of them.The high adsorption amount of PDDA/PSS multilayer membranes could be attributed to low excluded volume of the compact chain with low intrinsic viscosity of the polyelectrolyte in the dipping solvent.The AFM images show that polyelectrolyte with compact chains in solutions would form thick membranes with rough surfaces. Two sulfonated poly(sulphone)sodium salts(SPSFs)with different molecular weights and ionic exchange capacities in a N,N-dimethyl formamide/water(DMF-H₂O) mixed solvent with various DMF contents(φ_DMF)were selected as a model system for investigating the influences of solvent composition,solution properties,and charge density of polyelectrolytes on the layer-by-layer(LbL)self-assembly of water-insoluble polyelectrolytes.PDDA in aqueous solution was used as the counterpart.Intrinsic viscosity and conductivity of SPSF(ηSPSF and L_SPSF)in solutions with differentφ_DMFwere investigated.The PDDA/SPSF multilayer membranes grew nearly linearly with the layer numbers regardless of theφ_DMFin the SPSF solutions. The total absorption amount of the PDDA/SPSF multilayer membranes was strongly dependent on the charge density of the SPSF molecules and theφ_DMFvalue of the SPSF solutions.Minimum values and maximum values of absorption amount were observed atφ_DMF0.6～0.7 andφ_DMF0.8～0.9 respectively.On the other hand,the SPSF with a lower ionic exchange capacity led to thicker multilayer membranes for any givenφ_DMFvalue in the SPSF solutions.The average thickness of each bilayer was about 3.5～7.5 nm for PDDA/SPSF85 or 5.6～13.3 nm for PDDA/SPSF65,depending on the solvent composition.Surface roughness and contact angle of PDDA/SPSF increase with increasing membrane thickness.The results indicate that the molecular structures of the DMF-H₂O mixed solvent strongly affect the solution properties of SPSF,which in turn determine the growth behavior and physical properties of the PDDA/SPSF multilayer membranes.PDDA and PSS in solutions with different NaCl concentrations were self-assembled onto a polyamide reverse osmosis membrane and used for dehydration water from isopropanol.The influences of NaCl concentration of the dipping solutions, number of the bilayers and feed temperature on the pervaporation performance of PDDA/PSS multilayer membranes were investigated.Scanning electron microscope (SEM)experiment showed that the membrane surface becomes smoother with increasing NaCl concentration in dipping solutions.The results of the pervaporation tests showed that the PDDA/PSS membranes assembled in high NaCl concentration have good pervaporation performance.The separation factor is 345 and permeate flux is 2.51 kg.m^-2.h^-1for(PDDA/PSS)₆PDDA multilayer membrane assembled in 1 M NaCl used for pervaporation of 95 wt%isopropanol isopropanol-water mixtures at 35℃.With increasing operation temperature,both the flux and separation factor increase.However,the separation factor decreases with increasing operation temperature when increases the water percent in feed.An anti-"trade-off" phenomenon is found in low water percent in feed and explained in term of both solvent interaction and structure of LbL membrane.An electric field enhanced method is developed for fabricating layer-by-layer (LbL)self-assembly polyelectrolyte multilayer membranes.Three kinds of electric field enhanced polyelectrolyte multilayer membranes(EPEMs),poly(diallyl dimethylammonium chloride)/poly(styrenesulfonate sodium salt)(PDDA/PSS),poly (diallyldimethylammonium chloride)/poly(acrylic acid sodium salt)(PDDA/PAA) and polyethylenimine/poly(acrylic acid sodium salt)(PEI/PAA),were self-assembled on a polyamide reverse osmosis membrane.The pervaporation performances of EPEMs for separating isopropranol-water mixtures(90/10,w/w)are all superior to those of corresponding normal self-assembled polyelectrolytes membranes(PEMs), and the selectivity increases with PDDA/PSS,PDDA/PAA and PEI/PAA in order. The surface properties and pervaporation performance of PEI/PAA EPEM fabricated in different conditions were investigated.The results show that the electric field can improve the self-assembly process.The best dipping time of each layer is 15 min and the best applied voltage is 5 V.The PEI/PAA EPEM shows high performance and good selectivity in a large-scale temperature(40～70℃)and water feed concentration (water in feed:5～30 wt%).For(PEI/PAA)₄PEI EPEM,the separation factor is 1075 and permeation flux is 4.05 kg.m^-2·h^-1for separating isopropranol-water mixtures(90/10,w/w)at 70℃.