Molecular tailoring of polystyrene-block-poly (acrylic acid) block copolymer toward additive-free asymmetric isoporous membranes via SNIPS

Polystyrene-block-poly (acrylic acid) (PS-b-PAA) diblock copolymers with different compositions were systematically synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization and used to prepare asymmetric isoporous membranes directly through self-assembly and nonsolvent induced phase separation (SNIPS) process, without using foreign additives. A simple and efficient way, for the first time, to generate additive-free asymmetric isoporous PS-b-PAA membranes was enabled just by choosing the right blocks composition. Dynamic light scattering (DLS) measurements at dilute concentrations and solution small angle X-ray scattering (solution SAXS) analysis plus rheological measurements in concentrated regime as well as field emission scanning electron microscopy (FE-SEM) were employed to investigate the relationship between properties of different block copolymer casting solutions and final membrane structures. DLS measurements and FE-SEM micrographs revealed that only micellar solutions with adequate micelle dimensions were able to produce membranes with more uniform surface pores. Micelles with short PS- lengths led to the formation of mesostructures with adequate structural characteristics in the casting solution, whereas rheological characteristics were the worst. A combination of both appropriate structural and rheological characteristics for the casting solution was necessary to achieve perfect membrane structures with ordered, high density and mono sizedispersed surface pores that all were dependent on PS/PAA blocks ratio. In addition to surface pores, membrane substructures were also influenced by the PS- block length. Sublayers with mostly finger-like structure along with higher permeability were obtained for the block copolymer with adequate PS- block length. Although dispersity (D) value of the synthesized block copolymers was acceptable (<= 1.2) to form well-defined ordered isoporous structures, a slight difference in D had a decisive role on micellization, casting solution characteristics and final membrane structures. The most appropriate membrane structure showed a approximate to 150 nm thin selectivelayer with approximate to 14 nm mean pore diameter and relatively high pore density of about 4 x 10(14) pores/m(2) at top surface, supported by the asymmetric substructure composed of a finger-like upper layer (66% of total thickness) and interconnected open pores at underneath. Considering its free-additive nature and high performances, these PS-b-PAA membranes are promising candidates for a wide range of size-based separation applications including biomedical and pharmaceutical industries.