PH-responsive size- and charge-selective block copolymer membrane for the separation of small proteins

Abstract

Blue-light-induced RAFT polymerization was utilized to synthesize poly(N,N-dimethylacrylamide)–block–polystyrene (PDMA–b–PS) diblock copolymers to obtain 40 g per batch for membrane fabrication. Using a combination of self-assembly and non-solvent induced phase separation (SNIPS), larger isoporous membranes (≈ 0.5 m · 0.3 m) were fabricated with pore diameters of ≈ 25 nm in the dry state and around 4 nm in the wet state. These membranes were post-modified with hydrochloric acid to impart negatively charged, pH-responsive poly(acrylic acid)–block–polystyrene (PAA–b–PS) membranes. While all proteins like cytochrome C, insulin, lysozyme, myoglobin, and ovalbumin were fully rejected by the pristine PDMA–b–PS membrane, the PAA–b–PS membrane demonstrated selective rejection based on protein size and charge, effectively rejecting the proteins with a hydrodynamic radius Rh ≥ 1.9 nm. The PAA–b–PS membrane showed a real selectivity of 4.1 for the positively charged protein cytochrome C compared to the similarly sized, but negatively charged insulin, as well as a real selectivity of 16.1 towards the negatively charged ovalbumin. This illustrates the size- and charge-selective characteristics of the PAA–b–PS membrane for protein separation.