Design And Construction Of PPTA/PSF In-situ Blend Membrane And Its Polyamine Composite Membrane

Conventional desalination membranes are fabricated via interfacial polymerization between an aqueous phase of diamine solution and an organic phase of trimesoyl chloride/n-hexane solution to deposite an ultra-thin functional polyamide layer on the surface of polysulfone（PSf）ultrafiltration membrane,which have been widely used in the field of wastewater reuse,brackish water and seawater desalination.But,there are two potential problems in the integrity of TFC membranes that are the improvement of the adhesion strength between the functional layer and the substrate and the compaction resistance of the substrate.However,the methods which have been developed are focusing on either the improvement the compaction resistance,or the adhesion strength.This study will provide a new idea to fabricate high stable composite membranes with enhanced adhesion strength between the functional layer and the substrate as well as compaction resistance of the substrate by synthesizing rigid macromolecule poly（p-phenylene terephthamide）（PPTA）in a PSf membrane-casting solution,and via the-NH₂ of the terminal group of PPTA reacted during the interfacial polymerization to anchor the function layer to the substrate.Firstly,the PPTA/PSf casting solution had been fabricated by low temperature solution polycondensation using p-phenylenediamine and paraphthaloyl chloride as monomers.FTIR,XRD analysis indicated that the PPTA had been synthesized successfully.Taking the maximum weight average molecular weight as the standard,the optimization in-situ synthesis condition of PPTA were:Polymer concentration:18%,PPTA/PSf in-situ blending ratio:8/92,CLiCl/C-NH-=80%,Npy/N-NH-=100%,.Secondly,The rheological analysis of PPTA/PSf in-situ blending casting solution indicated that the non-Newtonian index was less than 1,which was belonging to pseudoplastic fluid.The viscosity was increasing with PPTA content as well as the flow activation energy decrease firstly and then increase,which indicated that the viscosity was sensitive with the temperature under high PPTA content because the PPTA aggregated.Besides,Orthogonal polarized microscope and TEM were used to observe the PPTA aggregation structure,and the results indicated that the PPTA excited as spindle structure which was synthesized in solution,but excited as bundle structure when synthesized in-situ condition,and in situ synthesis of PPTA aggregates was about 10 μm in length,and was only about 500 nm in width,and the forming mechanical of bundle structure was described in detail.Thirdly,the PPTA/PSf in-situ blending membrane was prepared by immersion precipitation phase inversion.ATR-FTIR.XPS,water contact angle（WCA）,Zeta potential,DSC,FESEM and AFM testing indicated that incorporating PPTA into the membrane resulted in more open porous structures and thinner dense layer,higher thermal mechanical stability and rougher and more hydrophilic surface.And the compaction resistance was improved which was proved by the curve of the pressure-flux and hydraulic resistance.Lastly,the polyamide thin film composite membrane had been prepared by interfacial polymerization.The results of back-flush test indicated that the strength of the polyamide skin layer adhesion onto the substrates was improved after in-situ blended with PPTA because the JMgSO₄ of PA-PPTA/PSf was remaining over 90%after back-flush testing,and in contrast,the JMgSO₄ of PA-PSf was only about 60%.By the way.the mechanism of improvement in adhesion strength was analyzed in detail.The TFC membranes exhibited a typical nanofiltration performance with salt rejections in the order of Na₂SO₄>MgSO₄>MgCl₂>NaCl,and the rejection of Na₂SO₄ and MgSO₄ was both about 99%.