The influence of polyamic acid molecular weight on the membrane structure and performance of polyimide solvent‐resistant nanofiltration

Abstract BACKGROUND Polyimide ( PI ) solvent‐resistant nanofiltration ( SRNF ) membranes were prepared by a two‐step method. The polyamic acid ( PAA ) precursor was synthesized first, and then PAA membranes were obtained via a non‐solvent induced phase separation method. Finally, the PAA membranes were transformed into PI membranes by the thermal imidization method. The effect of molecular weight of precursor on the structure and performance of PI membranes was investigated in detail. In addition, the influence of other processing parameters, such as evaporation time and immersion time in the coagulation bath, on the performance of PI membranes prepared from PAA with different molecular weights was also investigated. RESULTS The results showed that the structure of PI membrane varied with PAA molecular weight. The PI membranes prepared from low molecular weight PAA failed to form a defect‐free membrane with enough strength, and resulted in high flux but low rejection. Accordingly, PI membranes processed from PAA with high molecular weight retain rejection higher than 90% of FCF with decreased flux. The results also indicated that the optimized molecular weight of PAA should be between 1.23 × 10 5 g mol −1 and 2.44 × 10 5 g mol −1 to prepare defect‐free PI membrane for nanofiltration. The results also showed that the performance of the PI membranes can be controlled by adjusting the evaporation time, immersion time, and also the operating pressure. CONCLUSIONS The PAA molecular weight has a great effect on the morphology and performance of PI membranes. A suitable PAA molecular weight is necessary for preparing defect‐free PI nanofiltration membrane. © 2015 Society of Chemical Industry