Effect Of Substrate Structure On Separation Performance Of Polyamide Thin Film Composite Membrane

Polyamide(PA) composite membranes prepared by interfacial polymerization have been widely used in the reverse osmosis(RO) and nanofiltration(NF) processes. Polyamide composite membranes typically consist of two structural components:(1) top ultra-thin skin polyamide layer,(2) buttom non-woven fabric supported porous substrate. Each layer can be independently optimized to enhance the separation performance of the composite membranes. Generally, the active layer primarily determines the membrane permselectivity, while the support layer primarily serves to provide mechanical support for the membrane and also has a certain influence on the separation performance of the composite membranes. The membrane performance can be further improved by adjusting the surface properties, pore structure and mechanical strength of the substrates. This study makes attempts to optimize the performance of the polyamide composite membrane by the regulation of substrate structure and properties, and try to discuss the influence mechanism of the substrate on the formation of polyamide and the separation performance of the composite membrane.Non-woven fabric supported polyethersulfone(PES)/ polyaniline(PANI) substrates were fabricated by immersion precipitation process using PANI as the additive, and then the PA active layer was formed over the substrates via interfacial polymerization. The influence of PANI addition on the substrates structure and properties, as well as the influence of PES/PANI substrate on the active layer formation, were investigated. It was found that the addition of PANI improved the surface pore size, porosity and surface hydrophilicity of the substrates. The differences in the substrates surface properties and pore structure resulted in varing of the diffusion and reaction of the monomers during the interfacial polymerization process, leading to the changes in the cross-linking degree, thickness and surface roughness of PA layer. The PES/PANI-0.2 substrate with the increased surface pore size, surface pore density and hydrophilicity contributes to the formation of a denser and thinner PA active layer. The flux and MgSO4 rejection of the PA/PES/PANI-0.2 membrane reached to 95.3 L·m-2·h-1 and 94.9 % at 0.6 MPa TMP, respectively, which increased by about 101 % and 5.1 % compared to the PA/PES membrane.A series of sponge-like polysulfone substrates were fabricated by adjusting the content of PSf and water in the casting solutions. And then polyamide reverse osmosis composite membranes were prepared via interfacial polymerization between mphenylenediamine and trimethyl chloride over the polysulfone substrates. The resulting substrates and reverse osmosis membranes were characterized to investigate the influence of the polysulfone content on the substrates properties and the effect of the substrates properties on the polyamide reverse osmosis membranes. The results showed that with increasing PSf content, the average pore size and porosity of the substrates decreased, and the resistance ability to compaction enhanced. Besides, the difference in substrate properties resulted in the difference of the separation performance of reverse osmosis composite membranes. Considering the properties of the substrates and the polyamide reverse osmosis composite membranes, sponge-like substrate by using 15 wt.% polysulfone concentration was suitable for the preparation of reverse osmosis composite membrane.