Crosslinked polyimide membranes prepared via controlled activation of polyethyleneimine for dye/salt separation with organic solvent

Organic solvent nanofiltration (OSN) has emerged as a promising separation technology for product purification and solvent recycling in organic solution. Crosslinked polyimide (PI) membranes were widely used in the OSN system. However, the existence of some drawbacks, such as low utilization of crosslinkers and generation of profuse organic waste, hindered the development. Furthermore, when crosslinkers of large molecular weight were applied, the crosslinking degree of membranes was usually found to be insufficient, restricting their performance in harsh solvent systems. In this work, PEI with specific protonation degree, performing as a crosslinker, was directly introduced into PI solution for the preparation of OSN membranes. The partially activated PEI would graft onto PI chains, which could improve the retention of PEI chains during the membrane forming process, while preventing the gelation of PI solution. The as-prepared PEI/PI membrane showed enhanced hydrophilicity and highly positive charge with the isoelectric point as high as 9.1. The PEI/PI membranes were post-treated by K 2 CO 3 solution, aiming to further improve the crosslinking degree. The PEI/PI membranes exhibited good organic solvent resistance even in N, N-dimethylformamide (DMF), with the tensile strength of 0.28 MPa and permeance of 2.38 L m −2  h −1 bar −1 . Meanwhile, the dye/salt separation performance of the membrane was carried out by using DMF/H 2 O (1:9 w/w) as the solvent, and the rejection for several dyes was above 97 %, with the rejection for salts below 8 %. After 80 h test, the membrane maintained high rejection for Congo Red and Methyl Blue and low rejection for salt, with the permeation retained at 20 L m −2  h −1 bar −1 , indicating excellent anti-fouling performance. The controlled activation method takes the advantage of high utilization and reaction efficiency, as well as simple procedure, rendering it valuable for fabrication of OSN membranes.