Novel Methods To Fabricate High Performance Polyimide Solvent Resistance Nanofiltration Membranes

The nanofiltration membrane is a functional semipenneable membrane with a pore size of about 1 to 2 nanometers for the separation of low molecular weight solutes,solvent molecules or low valence ions.It is mainly used for the separation of desalination,reduced water hardness and waste water recovery process.However,most industrial systems use organic solvents such as solvent recovery,edible oil processing,and drug purification.Therefore,the solvent-resistant nanofiltration membrane has a wider range of applications and research value than the conventional nanofiltration membrane and has been extensively developed and studied in the past two decades.In our previous study,a polyimide film was prepared by two-step thermal imimation,exhibiting good solvent resistance and good dye rejection,but low membrane permeability.In this paper,we propose four simple but effective methods to suppress the pore collapse of the nanofiltration membrane and the increase in the thickness of the skin layer during the conversion of the polyamic acid(PAA)membrane to the polyimide(PI)membrane.Details as follows:1.Solvent exchange and chemical imidization are used in place of the vacuum drying and thennal imidization methods to prevent collapse of the pores during wet film drying.The experimental temperature(100℃)of the chemical imidization is lower than the thermal imidization temperature(300℃),avoiding the softening of the membrane at high temperatures,while still maintaining rigidity,thereby maintaining the porous structure as much as possible.2.Using a nonwoven fabric as a support layer of the membrane.It provides a support force in the horizontal direction for the membrane,and reducing the shrinkage of the membrane during the further solvent exchange and imidization,reducing the increase in skin layer thickness.3.In addition,according to the previous studies in the laboratory,it has been shown that the Polyamic acid system can form a gel structure of three-dimensional network due to the intennolecular hydrogen bonds andπ-π interaction.On the basis,the gel membrane was coated on the surface of the non-woven fabric,and the non-woven fabric was used as the support.Finally a granular pellet-like membrane structure with thin top-layer and high porosity was obtained.4.Finally,adding inorganic nano-titania sol to the casting solution can improved the crosslinking density of the gel structure,enhanced the strength of the gel skeleton.And then using sol-gel phase transformation method to prepared the membrane,permeability has also been improved further.In summary,the continuous improvenent of the experimental method,the top-layer of series insoluble polyimide membranes were gradually thinning,the membranes’ permeabilities gradually larger with a good solvent resistance,it has a huge potential for organic solvent nanofiltration application.