Performance Study Of Zwitterionic Polyimide Ultrafiltration Membranes For Protein Separation

In this paper, faced with the problem of ultrafiltration membrane fouling in protein separation processes, three diamine monomers were successfully synthesized, and then two types of zwitterionic polyimide (PI) with different chemical structures were prepared by direct copolymerization and post-modification, respectively. Two types of zwitterionic polyimide ultrafiltration membrane were prepared by the conventional immersion precipitation phase inversion method. The cross-section morphologies and surface hydrophilicities of the obtained membranes were thoroughly characterized by scanning electron microscopy and water contact angle measurements, respectively. The separation performances of bovine serum protein (BSA) and soybean protein isolate (SPI) were determined by ultrafiltration experiment. Meanwhile, static protein adsorption experiment and dynamic cycle ultrafiltration test for protein solution were carried out for studying the antifouling performances of the zwitterionic polyimide ultrafiltration membrane. The more detailed conclusions were summarized as follows:1) The new diamine monomers and zwitterionic polyimides were successfully synthesized, and the chemical structures were confirmed by Fourier transform infrared spectroscopy (FTIR) and Elemental Analyses. The thermal stability of the zwitterionic polyimides had been investigated with differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements. The results indicated that the zwitterionic polyimides have good thermal stability. The viscosity characterization showed that all obtained polymers have high molecular weight.2) Surface hydrophilicity is one of the most important factors in determining antifouling property and performance of the ultrafiltration membrane. The hydrophilicity and wettability of the zwitterionic polyimide membranes in this study was evaluated by contact angle measurement, which was also used to assess the surface (interfacial) free energies of substrate surfaces. The results indicated that the introduction of zwitterionic groups can effectively enhance hydrophilicity and surface energy, which improve antifouling property of ultrafiltration membranes. The scanning electron microscopy (SEM) was employed to investigate the cross-section morphologies of the zwitterionic polyimide membranes, and the introduction of the zwitterionic group results in some microstructure changes. This phenomenon can result from the delayed phase separation.3) Ultrafilltration experiments were carried out to investigate the protein separation performances of the zwitterionic polyimide membranes. The BSA and SPI rejections of the zwitterionic polyimide membranes are above 95%, which can meet the protein separation and concentration. The results of static protein adsorption experiment and dynamic cycle ultrafiltration test for protein solution showed that the zwitterionic polyimide membranes exhibited noticeably larger permeation fluxes during both water and protein solution flltration than the reference PI membrane. Moreover, the zwitterionic polyimide membranes have higher flux recovery ratio and lower extent of membrane fouling, especially irreversible membrane fouling, as compared to those of the reference PI membrane, which displayed a unique antifouling performance and longer operation life. In conclusion, the zwitterionic polyimide has the possibility to be a suitable ultrafiltration membrane material, which is quite beneficial for practical application.