Tailoring Pore Size of Graded Mesoporous Block Copolymer Membranes: Moving from Ultrafiltration toward Nanofiltration

Abstract

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.

Polyanionic pH-responsive polystyrene-b-poly(4-vinyl pyridine-N-oxide) isoporous membranes

“…1). The average pore size of PS-b-P4VP and PS-bP4VPNoxide membranes were calculated from their surface SEM images using Image J software [26]. Minimum 250 pores were selected to obtain the average pore size of the membranes and the standard deviation (σ) values.…”

Fit-for-purpose block polymer membranes molecularly engineered for water treatment

“…Strategies to control the gutter layer structure have been studied extensively for the PI-PS-P4VP system with the chemical composition and temperature of non-solvent bath and the introduction of additives in the casting solution being examined systematically. 72,73 For example, the inclusion of additives that modify the rate of solvent exchange (i.e., TiO 2 75 and glycerol 76 ) in the casting solution drove the formation of a highly-porous support layer. In these cases, the cross-sectional structures do not contribute significantly to the overall hydraulic resistance and the potential of the self-assembled block polymer membranes has been realized.…”

“…For example, the implementation of polyelectrolyte brushes that are covalently attached within the nanopores of a block polymer membrane have been shown to swell upon hydration and lead to pores smaller than 5 nm in diameter from mechanically stable building blocks. 76 A membrane with a pore diameter of 2 nm that exhibited sub-nanometer selectivity during filtration experiments was achieved through the use of pores lined by a high density of poly(2-acrylamido-ethane-1,1-disulfonic acid) (PADSA) brushes. A PI-PS-PDMA material served as a precursor for these membranes.…”

Structure Formation of Binary Blends of Amphiphilic Block Copolymers in Solution and in Bulk

“…Indeed, it is beneficial to find a more facile approach to prepare membranes with well‐defined pore sizes rather than to synthesize the corresponding block copolymer for a desired pore size. Blending of block copolymers with homopolymers or low molecular weight additives has not led to membranes with predictable, narrow pore size distribution so far . In addition, homopolymer or low molecular weight additives may leach out over the time of usage.…”