Preparation And Properties Of Polyimide Solvent-resistant Nanofiltration Membranes Modified By Polyethyleneimine

Membrane separation technology has been widely used in desalination,environmental protection,petrochemical industry,clean production,medicine and food.Conventional polymer separation membranes are difficult to maintain stable morphology in organic solvents,which limits their application in organic systems.Crosslinking modification can significantly improve the organic solvent resistance of membranes,which can be applied to dye separation,drug concentration and refining,solvent and catalyst recovery in organic solvent systems.In this study,two kinds of organic solvent-resistant nanofiltration membranes,polyethyleneimine/polyimide（PEI/PI）and polyethersulfone-polyethyleneimine/polyimide（PES-PEI/PI）,were prepared.The effects of various single factors on the structure and properties of the membranes were investigated.The cross-sectional structure,chemical structure,surface elements,mechanical properties,hydrophilicity and surface charge of the membranes were characterized by SEM,FT-IR,XPS,UTM,CAM and zeta potential.Then the nanofiltration performance of the membranes and the application of organic solvent systems were systematically studied.The results are as follows:（1）PEI/PI membrane was prepared by blending method with PI as membrane material,PEI as crosslinking agent and trifluoroacetic acid（TFA）as inhibitor.The results of single factor experiment showed that with the increase of PEI content,the more macropores formed in the cross section of the membrane,the higher the degree of crosslinking of the membrane,the better the hydrophilicity,the higher the positive charge density on the membrane surface and the higher the tensile strength.With the increase of PI concentration,the rejection of PEI/PI membrane to PEG2000 increased and the flux decreased.With the increase of soaking time,the crosslinking effect is better,and it reaches a stable state after soaking for 4 days.When the content of PEI increased from 5wt%to 20wt%,the average effective pore size of the membrane increased from 1.14 nm to 2.25 nm,which was at the distal end of nanofiltration.Even when the content of PEI was 20wt%,the membrane was beyond the range of nanofiltration,and the rejection of salt solution was affected by the Donnan effect.The rejection of hydrochloride increased gradually and the rejection of sulfate decreased gradually.The flux of PEI/PI membrane to seven organic solvents decreased with the decrease of V_m·η^-1（Combination of molar volume（V_m）and viscosity（η））,regardless of polar,nonpolar or aprotic solvents.In ethanol solution,the rejection of dye by PEI/PI membrane increases with the increase of dye molecular weight.For the same dye,the rejection of anionic dye increases with the increase of PEI content,and with the increase of PEI content in the long-term operation of 40 h,the membrane has a stable 100%rejection of Congo red（CR）.In DMF solution,the rejection of PEI/PI membrane to dyes increased with the increase of molecular weight.For the same dye,the rejection of anionic dyes increased with the increase of PEI content,and the rejection of CR increased with the increase of PEI content in the long-term operation of 40 h.（2）The organic solvent-resistant nanofiltration membrane was prepared by blending method with PI as membrane material,PES-PEI as crosslinking agent and TFA as inhibitor.The results of single factor experiment showed that with the increase of PES-PEI content,the degree of crosslinking increased,the macropores formed in the membrane section gradually increased,the hydrophilicity was better,the positive charge density on the membrane surface was higher,the rejection of PEG2000 aqueous solution decreased,and the flux increased.With the increase of PI concentration,the rejection of PEG2000 aqueous solution increased and the flux decreased.When the PI concentration was 24wt%and the PES-PEI content was 14wt%,the PES-PEI/PI membrane had both good permeation flux and rejection performance for PEG2000.With the increase of heat treatment time,the gel fraction of the film increases.When the heat treatment time is 1h,the crosslinking reaches a stable state;with the increase of heat treatment temperature,the gel fraction increased first and then decreased.When the heat treatment temperature was 70°C,the crosslinking effect of the membrane was the best.When the content of PES-PEI was 14wt%（membrane M14b）,the average effective pore size of the membrane was 1.10 nm.The rejection of salt solution was affected by the Donnan effect,and the rejection of hydrochloride was higher than that of sulfate.The flux of PES-PEI/PI membrane to seven organic solvents decreased with the decrease of V_m·η^-1,regardless of polar,nonpolar or aprotic solvents.In ethanol solution,except for methyl red（MR）and orange yellow（OrangeⅡ）,the retention of dyes by M14b membrane was above 90%,and the retention of CR was always above 98%in the long-term operation of 40 h.In DMF solution,the dye rejection of M14b membrane increased with the increase of molecular weight,and the rejection rate of CR was more than 90%after 40 h of continuous operation.After systematic research and optimization,PEI/PI membrane and PES-PEI/PI membrane are expected to be used as nanofiltration membranes for the treatment of industrial waste solvents.