Membrane-based separations offer significant economic, environmental, and safety benefits over traditional unit operations that typically require phase changes to achieve separations. One of the main factors limiting the deployment of membranes more broadly has been the inherent trade-off between a membrane’s permeability and selectivity. For ultrafiltration specifically, the flux of a membrane is often limited by the smaller pores and suboptimal pore density of the selective layer while the selectivity is limited by the broadness of the pore size distribution. Isoporous membranes that combine high pore density with a narrow pore size distribution are among the most promising candidates for the next generation of ultrafiltration membranes because they are not limited by the same permeability-selectivity tradeoff more traditional ultrafiltration membranes exhibit. Although significant advances have been made since the discovery of self-assembly and nonsolvent-induced phase separation (SNIPS) as a method for producing highly selective isoporous membranes, significant work remains to fully understand the underlying thermodynamics of this methodology as well as how to improve the mechanical properties of the polymers that are amenable to SNIPS processing. To address this critical issue, mechanically robust, isoporous membranes were cast from solutions of novel ABAC tetrablock polymers polystyrene-b-poly(ethylene-alt-propylene)-b-polystyrene-b-poly(ethylene oxide) (SESO) and polystyrene-b-polyisoprene-b-polystyrene-b-poly(4-vinylpyridine) (SISV). The polymers were synthesized with narrow polydispersity via sequential anionic polymerization up to molecular weights of 120 kDa, which allowed for good control over relative block fractions – a variable that can greatly impact the solution self-assembly behavior of the polymers as well as the mechanical properties of the resultant membranes. The rubbery midblock between the two polystyrene domains that make up the structural matrix of the membrane substantially increased the toughness of the membrane compared to membranes fabricated from polymers of similar and greater molecular weight that use polystyrene as the only structural block. The inclusion of this midblock also greatly impacted both the solution thermodynamics as well as the self-assembly of the polymer prior to phase inversion, complicating the selection of optimal casting conditions. The added toughness of the material allowed for the casting freestanding membranes with strain energy densities two orders of magnitude higher than a poly(styrene-b-4-vinylpyridine) membrane of significantly higher molecular weight. Membrane rejection and permeance could be tuned in the case of the SISV polymer through changes in the feed solution pH, allowing for dynamic tuning of transport properties of the membrane. Ultimately, this study highlights important considerations in multiblock polymer design and how block identities and interaction parameters in more complicated tetrablock polymers can affect the solution self-assembly and phase inversion of amphiphilic polymers.
