Structural Regulation And Performance Of Poly(m-Phenylene Isophthalamide) Based Solvent-Resistant Composite Nanofiltration Membranes

Nanofiltration（NF）is a new membrane separation technology between ultrafiltration and reverse osmosis,which is driven by pressure.It has a good effect on the retention of salt ions with divalent or above divalent and substances with molecular weight between 200 Da and1000 Da.At present,many separation processes are in the organic solvent system,which has a certain requirement on the solvent resistance of membrane,so the solvent resistance of nanofiltration came into being.The preparation of membrane is the core of membrane separation technology.Therefore,it is of practical significance to develop a nanofiltration membrane with relatively stable separation effect in organic solvent system.At present,the main types of commercial nanofiltration membranes are composite polyamide nanofiltration membranes,and interfacial polymerization is the most widely used method to prepare composite separation layer.In order to solve the problems of low flux and poor solvent resistance of composite nanofiltration membrane in practical application,researchers devote themselves to preparing composite nanofiltration membrane with good performance and strong stability.The composite nanofiltration membrane is usually composed of a large pore size ultrafiltration membrane substrate and the active separation layer on the surface.In this work,an aromatic poly（m-phenylene isophthalamide）（PMIA）with good thermal stability and chemical stability and strong solvent resistance was selected as the base membrane material.Calcium chloride（Ca Cl₂）and water（H₂O）were used as mixed additives.N-N,dimethylacetamide（DMAC）was used as solvent to prepare the casting solution.PMIA ultrafiltration membrane was prepared by phase conversion method as substrate.The effects of the composition of casting solution,membrane scraping conditions and phase conversion process on the structure and properties of ultrafiltration membrane were investigated.Piperazine（PIP）was selected as the aqueous monomer on the surface of PMIA ultrafiltration membrane substrate,and tribenzoyl chloride（TMC）was used as the organic monomer.The polyamide separation layer was prepared by interface polymerization method,and the solvent resistant PMIA composite nanofiltration membrane was successfully prepared.The effects of interface polymerization conditions and post-treatment conditions on the performance of nanofiltration membrane were investigated.The results are as follows:（1）PMIA ultrafiltration membrane as a substrate,need to have appropriate pore size and good cross-section structure（spongy pores and more vertical finger-like pores）,so that it as a substrate at the same time with good compressive capacity to maintain stable performance.During the phase conversion process,the composition of casting solution and the phase conversion process have great influence on the structure and properties of the casting solution.With the increase of PMIA content,the macropores of ultrafiltration membrane were inhibited,the thickness of spongy pores increased,the water flux decreased rapidly,and the rejection rate increased.With the increase of Ca Cl₂ content,the fingerlike pores in the membrane section grow and become more vertical and uniform.With the increase of H₂O content,the thickness and density of the spongy pores in the ultrafiltration membrane also increase In the phase conversion process,by extending the air retention time,reducing the thickness of the film,increasing the temperature of the gel bath,and changing the composition of the gel bath,the spongy pores in the cross section of the membrane were increased and thickened,and the fingerlike pores were grown,so as to effectively regulate the structure of the ultrafiltration membrane.（2）A polyamide separation layer was successfully prepared on the surface of ultrafiltration membrane by interfacial polymerization.When PIP concentration is 0.6 w/w%,TMC concentration is 0.15 w/v%,interfacial polymerization time is 40 s,and post-treatment temperature is 70℃for 3 min,the nanofiltration membrane has excellent performance.The flux of nanofiltration membrane is about 33.08 L/（m²·h）,and the retention of sodium sulfate is about 95.6%.The surface of nanofiltration membrane is electronegative,and the repulsive order of four typical salt ions is R（Na₂SO₄）>R（Mg SO₄）>R（Na Cl）>R（Mg Cl₂）.The prepared composite nanofiltration membrane can be used in alkanes,alcohols,ketones,esters,benzene and acid and base solvents,and shows good solvent resistance.