Kinetics of Polymer Interfacial Reactions

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TECHNICAL SUMMARY: The overall goal of the proposed research is to develop a fundamental understanding of the factors that influence interfacial reactions of long chain polymer molecules. The grafting of polymers to a surface is an important aspect in a myriad of applications ranging from biosensors, wherein bio macromolecules such as DNA are tethered to substrates to prepare microarrays, to steric stabilization, wherein a synthetic polymer is tethered to a surface to form a brush that prevents particle agglomeration. An intimate knowledge of these interfacial grafting reactions is essential to optimize these applications. Toward this end, model interfacial "click" reactions between a substrate modified with an azide function monolayer and a polymer terminated with an alkyne group are studied by infrared spectroscopy using a multiple internal reflection technique that samples the absorbance of the azide absorbance band which disappears upon reaction. Azides and alkynes react to form triazole groups in a process referred to as "click" chemistry due to its high yield and high chemoselectivity. The kinetics of solution reactions at interfaces are mediated by the effects of crowding as already grafted molecules inhibit additional polymer molecules from approaching and reacting to the surface. Factors that influence surface crowding are therefore under investigation including the nature of the solvent, the polymer molecular weight, the density of surface functional groups, the surface tension and the nature of polymer surface interactions. Interfacial reactions between the functional polymers and azide-functional nanoparticles are also monitored by transmission infrared spectroscopy in order to assess the effects of surface curvature. Interfacial melt reactions are investigated in a parallel program of research to be executed by undergraduates. The data collected are used to validate theoretical treatments of polymer interfacial reactions. NON-TECHNICAL SUMMARY:Long chain polymer molecules are often attached by their ends to a surface to meet the needs of many high-tech industrial applications. Microarray sensors, for example, used to detect biological warfare agents or to map genomes, are prepared by attaching natural biological polymers such as DNA, carbohydrates and proteins to the surface or a silicon wafer or microchip. Polymer brushes, synthetic polymers attached by their ends to solid substrates, are used in a myriad of applications including the stabilization of colloidal particles, imparting biological stealth to drug delivery vehicles and providing protein resistance in microfluidic devices to name but a few. The common feature in all of these applications is the need to covalently bond a polymer chain by its end to a substrate of interest at controlled surface density. The focus of this research proposal is to gain a fundamental understanding of such polymer reactions at surfaces and the factors that affect them. The rates of reactions of two complementary reactive functional groups are well understood for reactions in solution, however, the rates of reactions at surfaces, are not fully understood because they are influenced by a number of physical and thermodynamic parameters that are associated with the presence of a surface. The rates of polymer surface reactions are studied by an infrared spectroscopy technique that can quantitatively and directly measure the reaction rates of the surface bound reactive chemical group without disturbing the progress of the reaction. Parameters under investigation include the nature of the solvent if present, the size of the polymer molecule, the properties of the underlying surface, and the surface density of functional groups. The knowledge generated by this research will help in the design of industrial products ranging from biological sensors that can rapidly identify disease to devices that can resist biological fouling. The fundamental principles identified by the research are incorporated into several courses taught by the PI including the physical chemistry of macromolecules and polymer surface phenomenon. A diverse group of undergraduate and PhD students working on the research project receive training in materials and surface science, exposure to how intellectual property is protected, and learn how to communicate research results to the public, skills that are important in any field they decide to pursue after graduation.