Mechanically robust mesoporous membranes for enhanced ultrafiltration performance

Background

Membrane separation technology is an essential field within chemical engineering and environmental science, offering a more sustainable alternative to traditional separation methods that often require energy-intensive phase changes. The demand for advanced membrane technologies, such as isoporous membranes, is driven by their potential to achieve high selectivity and permeability, which are crucial for applications ranging from water purification to industrial filtration processes.

Isoporous membranes are promising due to their uniform pore size distribution and high pore density, which enable efficient separation at lower energy costs. Despite the advancements in self-assembly and nonsolvent-induced phase separation (SNIPS) techniques for producing isoporous membranes, several critical challenges remain.

One of the most significant issues is the mechanical integrity of the polymers used in these membranes. Many promising polymers lack the robustness needed to withstand high transmembrane pressures, limiting their practical application in industrial settings. This mechanical weakness necessitates the development of more durable materials that can maintain structural integrity under operational stresses, which is crucial for the membranes to be viable on a larger scale. Current approaches often fail to balance the need for both high selectivity and mechanical strength, highlighting the necessity for innovative solutions to address these limitations.

Technology overview

Membrane separations present significant advantages over traditional separation methods that require phase changes, such as reduced environmental impact, economic efficiency, and enhanced safety. Among these, isoporous membranes stand out due to their combination of high pore density and narrow pore size distributions, making them ideal for ultrafiltration applications. Traditional ultrafiltration membranes often suffer from a permeability-selectivity tradeoff, which is not a limitation for isoporous membranes.

Advances in self-assembly and nonsolvent-induced phase separation (SNIPS) have led to the development of highly selective isoporous membranes. However, a major challenge remains: the mechanical integrity of these membranes, which limits their operational transmembrane pressures.

To overcome this, researchers have developed mechanically robust, mesoporous membranes using novel ABAC tetrablock polymers such as 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).

These new membranes are differentiated by their enhanced mechanical toughness, achieved through the inclusion of a rubbery midblock between the polystyrene domains, which form the structural matrix. This design significantly increases the membrane's toughness compared to those made from polymers with similar or greater molecular weights that use polystyrene as the sole structural block.

The membranes exhibit average pore diameters as small as 15 nm and can be cast as freestanding membranes with strain energy densities two orders of magnitude higher than those of PS-P4VP membranes of higher molecular weight. Additionally, these membranes demonstrate pure water permeances up to approximately 2100 LMH/bar, which is an order of magnitude higher than track-etched membranes with double the pore diameter. This combination of mechanical robustness and high permeability makes these membranes highly promising for industrial applications.