(496f) Synthesis of Tetrablock Polymers That Form Isoporous Membranes with Enhanced Mechanical Properties Via Self- Assembly and Nonsolvent-Induced Phase Separation

Membrane-based separations offer significant economic, environmental, and safety benefits over traditional methods 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 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 distributions 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 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 via sequential anionic polymerization up to molecular weights of 120 kDa, which allowed for good control over molecular weight and relative block fractions – two variables that greatly impact the 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 over membranes fabricated from polymers of similar and greater molecular weight that use polystyrene as the only structural block. The added toughness of the material allowed for the casting freestanding membranes with strain energy densities two orders of magnitude higher than a PS-P4VP 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. The added stability of the tetrablock polymers allowed for free-standing membrane testing up to a transmembrane pressure of 4.0 bar, which is significantly higher than is typically seen in reports of isoporous membranes. The rubber toughening approach taken in this work helps increase the ease with which this class of membrane can be manufactured and eventually deployed in a wide range of applications from waste water treatment to pharmaceutical separations.