Molecularly Imprinted Ionomers
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Molecularly Imprinted Ionomers George M. Murray and Glen E. Southard Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099 ABSTRACT Ionomers have been defined as copolymers that have a certain proportion of ionic groups. The ionic groups have a significant effect on the mechanical properties of the copolymers. This is generally due to aggregation of ions in a low dielectric medium. The primary result is to restrict chain motion and raise the glass transition temperature. These attributes have relevance to molecular imprinting, since restricted chain motion should help preserve the integrity of the binding site. The connection between ionomers and molecular imprinting has come from the production of metal ion imprinted resins. Metal ions are used in the production of molecularly imprinted polymer ion exchange resins and ionically permeable membranes. The polymers have applications as separations media, sequestering media and as ion selective sensors. Metal ions are also being used to form imprinted polymers based on metal mediated imprint binding. We have prepared ion exchange resins, selectively permeable polymer membranes, ion selective electrodes and ion selective optical sensors using a modified version of the molecular imprinting technique. The modification is a reduction in the amount of covalent crosslinking used to form the polymers. This reduction may be justified by the presence of residual metal ion crosslinking in the immediate region of the imprinted binding site. The effects of metal ions on the thermal and mechanical properties of the polymers, as well their impact on binding selectivity are critical variables. INTRODUCTION The production of selective metal ion sequestering and separation materials is a growing field with broad application and critical importance. Industry requires vast quantities of metals and generates tons of metal wastes. Nuclear energy production and past weapons production facilities have created unique challenges in the area of metal ions separation. Ultimately, technology must reach a point where all metal containing waste streams are treated as recoverable metal resources. Metal ion selective molecularly imprinted materials may be the means to realize this goal. An additional application of metal ion imprinted polymers is as sensors. The ability to detect a specific metal ion in a complex matrix is keenly appreciated. Several issues need to be addressed in the design of molecularly imprinted ion complexing. Of primary concern is to make a rebinding site with good affinity. The binding selectivity afforded by metal ions is starting to be appreciated [1]. The large literature database of tabulated metal to ligand affinities forms a useful guide to the selection of the correct metal for a specific target [2]. Another useful aspect of metal ions in site production is the tendency of metal ions to exhibit directional bonding. The stereo-regularity of metal coordination geometries can be exploited to form a favorable geometry for secondary interactions. Many org
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