(557d) Synthesis of Abac Multiblock Polymers That Form Mesoporous Membranes with Enhanced Mechanical Properties Via Self-Assembly and Nonsolvent-Induced Phase Separation

Membrane separations offer significant environmental, economic, and safety benefits over traditional separation methods that require phase changes. Isoporous membranes that combine high pore density with narrow pore size distributions are one of 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 make these membranes an industrially viable technology. One of the largest drawbacks of many of the polymers that have shown great promise with SNIPS processing is their lack of mechanical integrity, which severely limits the transmembrane pressures at which they can operate. To address this critical issue, mechanically robust, mesoporous 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) (PS-PI-PS-P4VP) (SISV). The polymers were synthesized via sequential anionic polymerization to molecular weights between 65 and 110 kg/mol. Small angle X-ray scattering (SAXS) was used to verify polymer morphology in the casting solutions, while scanning electron microscopy (SEM) was used to verify the structure 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. Membranes with average pore diameters of as low as 15 nm were obtained. 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. Our membranes also exhibited pure water permeances up to ca. 2100 LMH/bar, an order of magnitude higher than a track etched membrane with double the pore diameter.