Cellulose Triacetate Membranes Prepared By Thermally Induced Phase Separation And Their Interfacial Polymerization

Thin film composite forward osmosis(TFC-FO)membrane is one kind of FO membranes with good water permeability,high salt rejection and controllable structures.It generally consists of a porous support layer and a thin active layer.Among others,the pore structures of the porous support layer are directly related to the performance of TFC-FO membranes.However,the structure types and preparation methods of the porous support layer are by far limited to the traditional non-solvent induced phase separation method or electrospinning technology.Therefore,this paper aims to design cellulose triacetate(CTA)porous membranes with controllable and diverse pore structures and high porosity by combining the advantages of thermally induced phase separation(TIPS)method in pore structure modulation,and the intrinsic hydrophilicity of CTA.Then,a series of novel TFC-FO membranes can be constructed via interfacial polymerization on CTA porous membranes.The specific studies are stated as follows:First,TIPS process was established to prepare CTA porous membranes by using CTA/dimethyl sulfone(DMSO2)/polyethylene glycol(PEG400)as the membrane forming system.The phase separation behavior of CTA/DMSO2/PEG400 ternary system and the pore structures of CTA membranes were studied by hot stage polarizing microscope,DSC,WAXD and SEM,respectively.The results display that the ternary system undergoes kinetically solid-solid phase separation.Various pore structures,such as cellular,lacy,plate-,or even ellipse-shaped pores,can be modulated by changing the polymer concentration,the composition of the mixed diluent and the cooling conditions.Correspondingly,their porosity,water flux and mechanical properties are adjusted with the pore structures.Moreover,the hydrophilic CTA porous membranes present better antifouling property than polysulfone membrane.Second,the interfacial polymerization process was explored on CTA porous membranes with different thicknesses and pore structures,which were connected with the performance of TFC-FO membranes.The results of ATR-FTIR,SEM and AFM confirmed the interfacial polymerization was happened on CTA porous membranes,resulting in a series of TFC-FO membranes.Then,FO and reverse osmosis(RO)tests were used to determine the water flux,reverse salt flux,intrinsic transport properties and structural parameters of TFC-FO membranes.It shows that CTA porous membrane with small thickness,high porosity,and oriented plate-like pores can be by obtained by varying the mould thickness,CTA concentration and DMSO2/PEG400 mass ratio.Furthermore,this kind of CTA porous membranes can reduce the structural parameter,weaken internal concentration polarization effect,and improve the water flux and salt rejection of TFC-FO membrane.Finally,polydopamine/polyethyleneimine(PDA/PEI)co-deposition was proposed to improve the selectivity of TFC-FO membranes.Detailed investigation was conducted to study the effects of co-deposition time,co-deposition ratio and PEI molecular weight on the surface hydrophilicity,roughness,membrane morphology and performances of PDA/PEI modified CTA membrane and TFC-FO membranes.The results show that the modified TFC-FO membrane has higher hydrophilic,smaller surface roughness and significantly improved selectivity when the PDA/PEI co-deposition time was 1 h,PDA/PEI ratio was 2:1,and PEI molecular weight was 70,000.