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Tailoring pore size of ultrafiltration membranes all the way down toward the nanofiltration regime in a predictable manner from molecular design principles is highly desirable. Here we present a way to achieve this in surface separation layers of nonsolvent induced phase separation (NIPS) derived graded block copolymer (BCP) membranes by means of an organic additive. Glycerol, a nontoxic organic molecule, is incorporated at varying amounts into poly(isoprene- b -styrene- b -4-vinylpyridine) (ISV) triblock terpolymer casting solutions. Employing scanning electron microscopy image analysis and solute rejection tests on resulting membranes, the relationship between the amount of additives and membrane performance (permeability, selectivity) is established. Pore size increases from 23 to 48 nm are achieved by moving from membranes cast from pure ISV solutions to those cast from up to 40% weight (relative to ISV) glycerol containing solutions. It is then demonstrated how a combination of additive driven pore expansion in conjunction with P4VP chain stretching via charge repulsion can be used to reduce pore sizes down to 5 nm under acidic (pH 3.6) conditions. This provides a path to move from ultrafiltration toward nanofiltration applications for asymmetric BCP membranes without compromising membrane mechanical properties. It also enables production of advanced membranes with wide tunability, low cost, and high performance.
Gu et al. (Mon,) studied this question.