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 size-dispersed 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 (Ð) value of the synthesized block copolymers was acceptable (≤1.2) to form well-defined ordered isoporous structures, a slight difference in Ð had a decisive role on micellization, casting solution characteristics and final membrane structures. The most appropriate membrane structure showed a ≈150 nm thin selective-layer with ≈14 nm mean pore diameter and relatively high pore density of about 4 × 1014 pores/m2 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.