Ordered isoporous membranes from ionic diblock copolymers via SNIPS: Optimizing effective factors with a structural survey

Abstract

The membrane formation directly through the self-assembly and nonsolvent induced phase separation (SNIPS) method without the aid of foreign additives has been more challenging in the case of ionic block copolymers, like polystyrene-block-poly (acrylic acid) (PS-b-PAA), in comparison with non-ionic amphiphilic systems. Herein, the influence of governing factors on the structure tuning, including physicochemical characteristics of polymer, solvent and nonsolvent, casting parameters and process conditions to prepare additive-free and ordered isoporous membranes was studied. Due to the high degree of ionization and hydrophilicity of PAA blocks, micellization in the form of “reverse micelles” was aimed to induce more stable micelles assembly. The well-ordered micelles assembly was generated by the suitable selection of solvent (1,4-dioxane), strongly segregated characteristic of PS-b-PAA (high χN) and proper adjustment of casting parameters including polymer concentration and solvent evaporation rate and time. The uniformity and regularity of the micelles assembly during phase inversion step was preserved owing to the high freezing point of solvent. Highly permeable asymmetric isoporous membranes prepared over a wide range of evaporation times (40–120 s) using relatively low polymer concentrations (15 wt%) with adjustable and monodispersed pore-size in the range of sub-10 up to 22 nm. FESEM and AFM images in addition to DLS measurements and solution SAXS analysis were applied to demonstrate the hexagonal close-packed (HCP) structure of micelles assembly and partially hexagonal arrangement of pores at the membrane selective-layer. It was also demonstrated that corona (PS) of adjacent micelles on the membrane surface fused together to form a continuous membrane matrix, while surface-pores were the nanoscale voids, formed between the densely packed spherical reverse micelles practically immobilized by immersion in low-temperature water bath. Such additive-free, asymmetric, iso- and nano-porous membranes have attractive potential practical applications in biomedical and pharmaceutical industries for size-based separations, in addition to their conventional applications in water purification.