Homoporous Membranes with Tailored Pores by Soaking Block Copolymer/Homopolymer Blends in Selective Solvents: Dissolution versus Swelling

Extraction homopolymers premixed in aligned films of block copolymers by rinsing with selective solvents has long been used for the preparation of membranes with uniform straight pores (homoporous membranes). It is frequently assumed that only the dissolution of homopolymers contributes to the pore formation. However, in this work, we demonstrate that the effect of swelling plays a significant role in determining the pore sizes. We prepare blended films of block copolymers of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) and P2VP homopolymers with low molecular weight and anneal the films to perpendicularly align the P2VP microdomains. Rinsing the aligned films in ethanol results in homoporous membranes, and the pore sizes can be tuned by the dosages of P2VP homopolymers. Interestingly, the pore sizes can also be effectively tailored by changing the rinsing temperatures and/or durations because of the significant contribution of the selective swelling of P2VP blocks under strong rinsing conditions in addition to the contribution of the dissolution of P2VP homopolymers. We identify the portion of the contribution from dissolution and from swelling and demonstrate that the pore sizes can be flexibly tuned within a wider range at no expense of pore ordering and uniformity by balancing the effect of dissolution and swelling.