Organic solvent-free polyelectrolyte complex membrane preparation: Effect of monomer mixing ratio and casting solution temperature

Abstract

The organic solvent-free aqueous phase separation (APS) process is a new and sustainable alternative to the conventional nonsolvent-induced phase separation (NIPS) process. However, low water permeability is a primary concern of such APS membranes. Herein, we introduced casting solution temperature and mixing ratio as two key parameters to improve the performance of poly (sodium 4-styrene sulfonate) (PSS) and branched Polyethylenimine (PEI) complex membrane prepared by pH shift-induced APS process. It was found that PSS-PEI monomer mixing ratios of 1:1.65 and 1:1.70 resulted in nanofiltration membranes with pure water permeability (PWP) of ∼4.5 and ∼8 LMHbar−1. The salt retention tests of these membranes showed higher divalent salt retention than monovalent salt following the order of MgCl2 > MgSO4 > Na2SO4 >NaCl. In comparison, PSS-PEI monomer mixing ratios of 1:1.75 and 1:1.80 led to the formation of ultrafiltration membranes with BSA retention of ∼98% and ∼36%, and PWP of ∼40 and ∼199 LMHbar−1, respectively. The variation of casting solution temperature also showed a significant influence on membrane morphology and performance. The increase in temperature from 25 to 60 °C resulted in ultrafiltration membranes with high BSA retention (>90%) and PWP in the range of ∼53–140 LMHbar−1. This sharp increase in PWP was attributed to the combined effect of the thinner skin layer and larger pore size of the membranes. Overall, this work demonstrated that casting solution temperature and monomer mixing ratio could be used as new tuning parameters for high-performance PSS-PEI membrane fabrication with excellent control over membrane structure and performance.