(232b) Material Property Targets for Nanostructured Adsorptive Membranes in Residential Water Purification Applications

Access to clean water is an ever-growing concern for modern society as it is critical to ensure human health, to protect threatened ecosystems, and to promote economic growth and prosperity. Yet the significant advances promised by nanotechnology for water purification have been slow to manifest because the connections between fundamental scientific research and technology development are often overlooked. There is a critical need to systematically identify the most promising applications for novel membrane materials and identify the gaps in scientific knowledge that most inhibit their translation to scalable water treatment technologies.

Self-assembled block polymers are one such promising class of material for novel water purification systems. Specifically, recent work has explored using A-B-C triblock polymers composed of polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI-PS-PDMA) to generate membranes with molecularly-engineered performance profiles.^1-4 In these systems, the chemically-distinct nature of the A, B, and C moieties in the triblock polymer together with the self-assembly and non-solvent induced phase separation (SNIPS) casting technique used in membrane fabrication allow for well-ordered, uniform nanoscale pores to form on the surface of the membranes. Moreover, these small pores quickly taper into larger pores which allows for the simultaneous attainment of both high size-selectivity and high throughput separations. In addition, because the PDMA moiety that lines the pore walls is covalently bound to the robust matrix of the nanoporous thin film, a remarkably high number of binding sites are generated within the membrane structure.⁵ Importantly, the chemistry of the C-block, which lines the pore walls, can be tuned in a remarkably facile manner after the creation of the nanoporous thin film. Hence, high capacity adsorptive membranes, which contain ligands spanning a broad range of specific binding affinities, can be generated in a straightforward manner. Despite the excellent control over material properties provided by the block polymer platform, the nanostructures and chemistries that optimize performance at a module or system level have not been identified in a systematic manner.

In this presentation, we explore the feasibility of triblock polymers for residential water purification systems to remove heavy metal contaminants. We propose a device-scale model for batch and semi-batch adsorptive separation systems governed by a Langmuir isotherm. Using the model, we explore the necessary material properties (e.g., isotherm parameters, fluxes, etc.) to achieve system energy and size goals. Preliminary analysis reveals simultaneous 2 to 3 order magnitude improvement in both biding affinity and capacity (material properties) is required for modular residential systems relative to existing membrane materials³. Guided by this finding, we discuss molecular and nanoscale design strategies to meet these materials performance targets. Finally, we generalize the systems-scale modeling to arbitrary adsorptive separation systems.

References:

[1] Mulvenna, R. A.; Weidman, J. L.; Jing, B.; Pople, J. A.; Zhu, Y.; Boudouris, B. W.; Phillip, W. A. Tunable Nanoporous Membranes with Chemically-Tailored Pore Walls from Triblock Polymer Templates. Journal of Membrane Science 2014, 470, 246-256.

[2] Weidman, J. L.; Mulvenna, R. A.; Boudouris, B. W.; Phillip, W. A. Unusually Stable Hysteresis in the pH-Response of Poly(Acrylic Acid) Brushes Confined within Nanoporous Block Polymer Thin Films. Journal of the American Chemical Society 2016, 138, 7030-7039.

[3] Weidman, J. L.; Mulvenna, R. A.; Boudouris, B. W.; Phillip, W. A. Nanoporous Block Polymer Thin Films Functionalized with Bio-Inspired Ligands for the Efficient Capture of Heavy Metal Ions from Water. ACS Applied Materials and Interfaces 2017, 9, 19152-19160

[4] Zhang, Y. Z.; Sargent, J. L.; Boudouris, B. W.; Phillip, W. A. Nanoporous Membranes Generated from Self-assembled Block Polymer Precursors: Quo Vadis? Journal of Applied Polymer Science 2015, 132, 41683.

[5] Weidman, J. L.; Mulvenna, R. A.; Boudouris, B. W.; Phillip, W. A. Nanostructured Membranes from Triblock Polymer Precursors as High Capacity Copper Adsorbents. Langmuir 2015, 31, 11113-11123