Metal-Crosslinked-Polyimide-based Thin-fim Composite (TFC) Membrane For Organic Solvent Nanofiltration

Organic solvent nanofiltration（OSN）,a membrane-based pressure-driven process,is aimed for the separation of solutes with molecular weights of 200-1000 g/mol from the organic solvent.The typical thin film composite（TFC）membrane with polyamide（PA）selective layer is the most common type for OSN applications.For TFC OSN membrane,the good solvent resistance to guarantee the high operation stability in the harsh condition is a prerequisite.The polyamide selective layer generally has good resistance in most solvents to ensure the adequate stability in OSN applications.At the same time,the solvent resistance of substrate is another important factor,which is the key issue to determine the practical OSN applications.Common chemical crosslinking of polymeric substrates can improve the solvent resistance and performance stability of polymeric membranes in OSN process,by building covalent bonding among polymeric chains.Nevertheless,the sacrifice of the permeance is often resulted because of the tightened polymeric chains and denser microstructure.So far,no work has been reported to address the permeability-selectivity trade-off of OSN membranes effectively by chemical crosslinking.Therefore,in this thesis,a novel metal-coordination crosslinking of the polyimide（PI）-based TFC membranes is proposed to improve its solvent resistance for its OSN appliatons.The effects on permeability and selectivity of PI-based TFC OSN membranes are investigated.It is believed to pave a way for the development of high performance composite membranes for OSN applications.Firstly,a Ca²⁺-coordination crosslinked PI substrate of TFC membrane is developed.Polyimide chains can be partially hydrolyzed and converted into polyamate salt in the alkaline coagulation bath,while the etching of hydrolyzed PI layer tunes the pore size of the substrate effectively,resulting in an optimized micro-morphology and therefore the higher permeability.The following Ca²⁺ion exchange results in the metal-coordination crosslinked PI film with enhanced solvent resistance and superior hydrophilicity achieved.The effects of the alkaline coagulation bath,Ca²⁺ion exchange,and further solvent activation are investigated.The resultant TFC membrane breaks through the typical permeability-selectivity trade-off of OSN membranes successfully,exhibiting an astonishingly superior solvent permeance.Later,metal-coordination crosslinking is further applied to improve the interface stability between the substrate and selective layer.Because Fe³⁺usually has a strong complexing ability,it can form stable complexes with tannic acid,phytic acid and other monomers,so in this part,Fe³⁺ion is selected as the metal ion crosslinker for further research.The alkali-treated PI-based TFC membrane is post-treated and crosslinked with Fe Cl₃ ethanol solution,which can overcome the strong acidity of Fe Cl₃ aqueous solution and avoid the damage to the membrane.Characteizations confirmed that Fe³⁺ions can interact with both PI substrate and PA layer to bind two layers via coordination interaction.Compared with the TFC membrane with Fe³⁺-crosslinked PI substrate,the integrally Fe³⁺-crosslinked TFC membrane has better stability and OSN performance.The effect of Fe³⁺concentration is also studied and found to have a significant impact on the membrane hydrophilicity and morphology,resulting in different OSN performance.The optimized solvent-activated TFC membrane has good stability in most solvents,and exhibits good OSN performance in alcohol solvents.