Polymers have long been known to generate unique microenvironments which display unanticipated chemical behavior. Polymer microstructure strongly governs the observed physicochemical properties, and an understanding of emergent phenomena – complex behavior displayed by systems which is not displayed by the constituents – presents an exciting opportunity for the design of advanced forms of soft matter. In this work, we discuss the application of x-ray absorption spectroscopy (XAS) to investigate the effect of polymer structure on metal binding modes. An element-discriminating technique, XAS provides information on metal oxidation state and coordination environment, possesses minimal sample limitations, and affords sensitivity at even the low ppm range, making it a uniquely powerful tool for directly investigating polymer-bound metals. We apply it here to study the state of the art material for recovery of uranium from seawater, a polymer composed of polyamidoxime graft chains grown from a polyethylene trunk via radiation-induced graft polymerization. While bulk adsorption experiments are used to assess material performance, a variety of technical challenges make seawater-contacted polyamidoxime a complex system that cannot be meaningfully analyzed by traditional laboratory techniques. As a result, the conceptualization of metal binding is traditionally derived from small molecule crystallographic and computational approaches, with no assurance that such paradigms are actually representative the polymeric system. To better understand the influence of polymer structure on performance, we analyze the coordination environment of uranium in a series of small molecule crystals, uranium-containing brine and seawater-exposed polyamidoxime-<i>graft-</i>polyethylene adsorbents, porous adsorbents with well-defined pore architectures, and amidoxime polymers of controlled chain length. We observe that the binding environments favored by computational and crystallographic studies differ from what actually occurs in seawater-deployed polymeric adsorbents, and that this observed emergence is driven in part by length of the polymer graft chain. In contrast, materials with well-defined pore architectures bind uranium consistently with the traditional paradigm. These results underscore the importance of understanding complex behavior governing the observed properties of soft matter, and reveal various approaches to harnessing emergence in the development of advanced materials.

Exploration of emergence in polyamidoxime adsorbents by x-ray absorption spectroscopy

2019 Spring National Meeting

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