Influence of temperature-induced micelle transition behavior of Pluronic additive on asymmetric ultrafiltration membrane formation and performance

Amphiphilic block copolymers are widely used as additives in ultrafiltration membranes preparation by nonsolvent-induced phase separation. During membrane-forming process, an increase in coagulation bath temperature (CBT) will accelerate the kinetics of phase inversion and also promote the micellization of amphiphilic block copolymers. Herein, polyethylene oxide-polypropylene oxide-polyethylene oxide, marketed as Pluronic, is utilized as an additive for preparing asymmetric ultrafiltration membranes. Influence of temperature-induced micelle transition behavior of Pluronic on membrane formation and performance is investigated in detail. It is found that when the CBT increases, temperature-induced micelle transition of Pluronic will lead to a dual effect: (1) The decrease in Pluronic diffusion rate results in a reduced diffusion of the solvent interacting with it, thereby slowing down the precipitation kinetics during phase inversion. (2) The decrease in particle size of Pluronic micelles brings about a reduction in the region left by Pluronic micelles extracted from the casting solution, thus decreasing the membrane pore size. The intrinsic characters of Pluronic, including molecular weight, hydrophilic and hydrophobic block ratio and concentration, determine the dual effect of micelle transition. This work provides an in-depth understanding and practical guidance for the preparation of asymmetric membranes with amphiphilic block copolymers as additive systems.