Negatively charged polyimide nanofiltration membranes with high selectivity and performance stability by optimization of synergistic imidization

A simple strategy is proposed for high performance polyimide nanofiltration (PI-NF) membranes by optimization of a synergistic imidization reaction. Soluble poly(amic acid) polymers with partial imidization was successfully controlled and fabricated by chemical imidization. With the product as a matrix, NF membranes with an integrally skinned asymmetric architecture were obtained by phase inversion and subsequent thermal imidization. The brittle and ruptured defects caused by directly high temperature conversion for fabricating PI-NF membranes were effectively avoided. The prepared membrane showed high rejections for dye, PEG and typical salts. The fabricated PI-NF membranes were endowed with a high permeance of isopropyl alcohol [approximately 1 L/(m2 h bar)] and a high rejection of rose Bengal (greater than 92%). The molecular weight cut-off of PI-NF membrane is below 800 Da. The pure water flux of PI-NF membrane prepared under optimized conditions was approximately 20 L/(m2 h) and the rejection for Na2SO4 was 93.91% at 5 bar. The rejection for different salts followed the order: Na2SO4 > MgSO4 > NaCl > MgCl2, revealed that Donnan exclusion theory and steric hindrance effect dominated the rejection mechanism. Furthermore, the NF membranes showed excellent stability over a long-term filtration process.