Preparation Of PSf-b-PEG Separation Membrane And Study On The Regulation Mechanism Of Pore Structure

Block copolymers are developing into an important research in the field of precision separation membrane preparation due to the advantages of blocks with different physical and chemical properties,high porosity and narrow pore size distribution of the prepared membranes.At present,the trade-off effect between membrane selectivity and permeability exists in block copolymer membrane,the pore size is not easy to be controlled within the small pore size,and the mechanism of membrane and pore formation is inconclusive,which restrict the application of block copolymer membrane.Therefore,it is of great scientific value to develop a simple and easily amplified method for block copolymer membrane preparation to fabricate block copolymer membranes with tailorable pore size,narrow pore size distribution,high porosity,and to break through the trade-off effect between membrane permeability and selectivity,further broaden the application of block copolymer separation membranes in precise separation.In this work,using the polysulfone-block-polyethylene glycol（PSf-b-PEG）block copolymer,we investigated the PSf-b-PEG block copolymer membrane structure-tailoring strategy and membrane pore-formation mechanism by adjusting the block material segment ratio and casting membrane solution solvent system.The key research contents and results of this work are as follows:（1）Improving the perm-selectivity and anti-fouling property of ultrafiltration（UF）membrane through the micro-phase separation of PSf-b-PEG block copolymersPSf-b-PEG block copolymers with 11 wt%and 21 wt%PEG were used to fabricate highly perm-selective and anti-fouling UF membranes via the non-solvent induced phase separation（NIPS）process.The PSf-b-PEG block copolymer membrane exhibited the unique pore structure having a super high surface porosity and overall porosity,and also a distinctively thin top skin layer.Moreover,the higher PEG content in the PSf-b-PEG block copolymer facilitated the formation of more pores and thinner top skin layer with the slightly larger pore size and the higher molecular weight cut off（MWCO）of the fabricated membrane.Separation tests demonstrate that the PSf-b-PEG21 block copolymer membrane exhibited a high water permeation coefficient of980.0±33.0 L m^-2 h^-1 bar^-1 and a high BSA rejection above 98%.Furthermore,the membrane pore structure and perm-selectivity of PSf-b-PEG membrane could be further tailored through adjusting the PEG composition in the block copolymer materials.Antifouling experiments confirm that the PSf-b-PEG21 block copolymer membrane presented a slow and less flux decline during the fouling stage,but a fast and high flux recovery after the backwash cleaning.Because the PEG is covalently bonded with the PSf blocks in the block copolymer,the block copolymer membrane has a great potential to achieve long-term anti-fouling performances in the real applications.（2）Fine pore tailoring of PSf-b-PEG membrane below 10 nm via non-solvent induced phase separationWe have reported a simple and effective method for tailoring the structures of PSf-b-PEG membrane with the pore sizes ranging from ultrafiltration to nanofiltration simply by tuning the ratio of tetramethylene sulfone（Ts）to tetrahydrofuran（THF）in the solvent systems via the NIPS process.The surface roughness of the PSf-b-PEG membranes very low.The cross-sections of the PSf-b-PEG membranes showed very dense sponge-like pore structure from the upper to bottom surface with the entangled block copolymer nanofibers,and the higher the THF content in the casting solution,the denser the structure of the prepared membrane.As the content of THF in the casting solution increased,the MWCO of the fabricated membranes decreased from 127 k Da to 6.6 k Da,and the mean effective pore size of the membranes shrank from 15.5 nm to2.3 nm.Moreover,with the increase of the THF content in the casting solution,the diameter distribution of the block copolymer nanofibers became narrower,resulting in a narrower pore size distribution of the fabricated membranes.In addition,the PSf-b-PEG membranes not only have a pore size that can be tailored flexibly and precisely,but also exhibit excellent pure water permeance,which indicated that the great potential of the PSf-b-PEG membranes for large-scale membrane practical applications.（3）High-flux robust PSf-b-PEG nanofiltration（NF）membrane for the precise separation of dyes and saltsThe novel high-flux robust PSf-b-PEG membranes with precise separation of dyes and salts have been successfully prepared via the NIPS process.The PSf-b-PEG membrane exhibited a dense surface and smooth morphology with a sponge-like pore structure from the upper to bottom surface.The very dense structure of PSf-b-PEG membrane with rich interconnected pores and polymer entanglements contributes to its superior mechanical strength of 22.4±0.9 MPa.The mean effective pore size of PSf-b-PEG membrane was 3.8 nm in diameter,and the MWCO of PSf-b-PEG membrane towards dyes was about 655 Da.Separation mechanism of PSf-b-PEG membrane was found to be dominated by size exclusion effect.The PSf-b-PEG membrane exhibited a very high water permeance of 49.3±0.9 L m^-2 h^-1 bar^-1and precise separation efficiency of salts and dyes with high above 98%dye rejection and nearly 100%salt passage.Long-term NF results confirm the highly stable and outstanding separation performance of PSf-b-PEG membrane.The pore structure and separation performance of PSf-b-PEG membrane can be further optimized with a great potential to be used for practical applications,since the material cost of PSf-b-PEG is similar as commercial PSf and the fabrication of phase-inversion membrane can be easily scaled up with a low cost.In this work,we focus on the theme of PSf-b-PEG block copolymer membrane structure tailoring,use the NIPS method to prepare the PSf-b-PEG membranes with tailorable pore size,narrow pore size distribution,high porosity,breakthrough the trade-off effect between membrane permeability and selectivity.In addition,the mechanism of PSf-b-PEG block copolymer membrane structure and pore formation was investigated in detail.This work provided useful attempts and references for further optimization of the PSf-b-PEG membrane structure in the future.