Study On The Preparation Of Solvent-Resistant Polyimide Nanofiltration Membranes

Polyimides(PIs)are widely used as high-performance engineering plastics due to their excellent resistance on heat,chemicals,and impressive mechanical strength.PIs had been successfully processed to membranes including ultrafiltration,nanofiltration,gas separation and pervaporation.Owning to the high rigidity of backbones,PI membranes possess higher resistibility to heat and chemicals than membranes made from other traditional materials such as polyacrylonitrile(PAN),polyether sulfone(PES),polysulfone(PS)or polyvinylidene fluoride(PVDF).However,the high chemical resistance of PIs led to high difficulty on membrane processing.Generally,there are two methods to prepare PI membranes:(1)using organo-soluable PIs to fabricate membranes via the phase inversion method;(2)using their precursor,polyamic acid(PAA),to fabricate membranes,and then converting them to PI membranes by thermal or chemical imidization.The 1st method can be easily carried out,but the resulted PI membranes have poor thermal and chemical stabilities.The only way to solve this problem is to crosslink the PI membranes.However,crosslinking is a time-consuming and complex process,the permeability of PI membranes may decline as well.On the contrary,the 2nd method is very suitable for preparation of organo-insoluble PI membranes,which subtly avoid crosslinking.Nevertheless,there are still two challenges to overcome when implementing the 2nd method.First,the phase inversion rate of PAA in water is very low because of the PAA’s hydrophility which often leads to deformation of the PAA hollow fiber membranes during spinning.Second,PIs may become brittle after thermal imidization.Thus,Taking PMDA-ODA PI as an example,our research is to develop suitable methodology to address these problems:1.Phase separation between PAA and water is slow,since PAA has abundant carboxyl groups.Thus,the nascent PAA hollow fibers was too soft to stand the drag force generated from the take-up godet,resulting in the filamentary deformation or break.In this paper,19 spinning conditions were investigated to systematically study the fabrication methods of PAA.The results showed that by decreasing the PAA concentration,and ethanol content of dope solution,increasing water content of the bore solution,lowering the dope and bore flow rates,reducing the take-up speed,shortening air gap,adopting longer coagulation bath with higher temperature,a mechanically strong PAA nascent fiber would be obtained,which could stand from deformation or break.Finally,the resultant PI hollow fiber imidized by chemical imidization showed good separation performances on OSN.The optimal permeability and rejection in DMF solution was 2.51 L m-2 h-1 bar-1 and 90.6%for Fast Green(MW=808 Da)dye.2.The imidization conversion and the pore structure of the PI membrane play key roles on membrane properties,especially for the mechanical strength and separation performance.When chemically-imidized PI membranes were kept in water for a long time,both the mechanical strength and separation performance would decrease due to the decompositioin of the PI molecular by hydrolysis.However,the thermally-imidized PI membranes were stable in aqueous system.The reason is that thermal treatment leads to higher imidization degree than chemical imidization.In addition,it had been vertified that membrane with macrovoid-free structure had higher mechanical strength than that with finger-like pore structure.This is because the finger-like pores are mechanical weak point and easily generate stress concentration,leading to membrane failure.Therefore,PI membranes with both thermal imidization and macrovoid-free structure would show excellent mechanical performance and outstanding separation properties,which were suitable for OSN relevant applications.