Preparation of isoporous membranes from low χ block copolymers via co-assembly with H-bond interacting homopolymers

Self-assembly and non-solvent induced phase separation (SNIPS) was extensively used in the fabrication of isoporous membranes for its compatibility with massive production. However, for the well-defined SNIPS, preparation of isoporous membranes from low chi block copolymers is challenging because self-assembly of these polymers is usually hard to induce. In this work, we showed that such a problem could be effectively overcome through the co-assembly strategy by blending with homopolymers that interacted with block copolymers (BCPs). For the polystyrene-block-poly(N-isopropylacrylamide) (PS-b-PNIPAM) which has relatively small Flory-Huggins interaction parameter (chi approximate to 0.05), the formation of the isopores was readily induced when polyacrylic acid (PAA) was added as an additive. Three PS-b-PNIPAM with the PNIPAM mass fractions ranging from 5.9 to 36.5 wt% were used as membrane-forming materials, and three PAA with different molecular weights (MWs) were used to investigate how MWs of homopolymer affected the isopore formation. The selective H-bonding of PAA with PNIPAM was in-situ determined by infrared spectroscopy, which was further supported by the small angle X-ray scattering that indicated the microphase separation of PS-b-PNIPAM in solution was remarkably enhanced by PAA. The contribution of PAA on isopore formation was also found for the casting solutions of a wide range of concentrations and different solvent compositions. Our finding showed herein actually provided a facile route to prepare isoporous membranes from low chi BCPs under general conditions.