Extraction the homopolymers pre-mixed in the aligned block copolymer films by rinsing with selective solvents has long been used for the preparation of membranes with uniform straight pores (homoporous membranes). The rinsing operation is typically carried out with little care as it is generally assumed that rinsing will not change the pore sizes as long as the incorporated homopolymers are completely extracted. However, in this work, we demonstrate that the effect of swelling play a significant role in determining the pore sizes. We prepare blended films of block copolymer of polystyrene-<i>block</i>-poly (2-vinyl pyridine) (PS-<i>b</i>-P2VP) and P2VP homopolymers with low molecular weight, and anneal the films in chloroform 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 the P2VP homopolymers. Interestingly, the pore sizes can also be effectively tuned by changing the rinsing temperatures and/or durations. Rinsing at room temperature for short durations leads to small pores down to 15 nm as only the dissolution of the homopolymer contributes the formation of pores; in contrast, a stronger rinsing condition, e.g., 50°C for 3 h, produces much larger pores reaching 50 nm because the significant contribution of the selective swelling of P2VP blocks in addition to the contribution of the dissolution of P2VP homopolymers. By comparing the sizes of the pores obtained at these different conditions, we differentiate the portion of contribution from these two factors, suggesting that controlling the rinsing operation by selective solvents allows us to flexibly tune the pore sizes of homoporous membranes within a wider range at no expense of pore ordering and uniformity.

Effect of swelling in the tailoring of homoporous membranes by soaking block copolymer/homopolymer blends in selective solvents

251st National Meeting (2016)

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