Novel Mixed Brushes with Tunable Internal Structures of Various Length Scales

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).TECHNICAL SUMMARY:Using a combined experimental and theoretical approach, the proposed research aims to provide fundamental understanding of the thermodynamics of novel mixed polymer brush systems, namely, the mixed brushes composed of laterally-mobile, weak polyelectrolyte and neutral polymer chains that are mutually incompatible. This novel mixed brush system offers previously unavailable opportunities (i) to produce mesoscopic surface patterns of various length scales by long-range-frustrated lateral phase separation between the two chain types and (ii) to create functional interfaces with surface properties switchable between charged and non-charged states. These properties, achievable only with this unique combination of ionizable and neutral polymers with laterally-mobile grafting points, have enormous potential to be used for advanced technological purposes in such areas as (i) functional surface patterning for advanced fabrication of two-dimensional nano/microstructures and (ii) biomedical delivery and detection. Understanding of the thermodynamics that governs the behavior of the mixed brushes is essential to furthering the development of these technological applications. The specific objectives of this research are to establish and understand the key thermodynamic properties, including (a) two-dimensional phase behavior, (b) phase-separated domain structures, (c) lateral brush compressibility and (d) chain conformations, of a model mixed brush system constructed using mixed diblock copolymers at the air-water interface. This mixed brush system will be studied using combined experimental techniques of fluorescence/AFM imaging, pressure-area isotherm and neutron/x-ray reflectivity measurements, to establish how the brush molecular characteristics (i.e., polymer molecular weights, brush composition, and chain grafting density) and the solution electrostatic environment (i.e., pH and ionic strength of the medium) influence the above-mentioned properties of the mixed brush system. These experimental investigations will be complemented by self-consistent field (SCF) theoretical studies to establish a consistent and theoretically sound understanding of the experimental results.NON-TECHNICAL SUMMARY:The intellectual merit of the proposed research is that, by using a unique combination of experiment and theory, this research will provide a useful understanding of the phase behavior and structures of this new class of mixed polyelectrolyte brush system. The knowledge gained from this research will also have great implications for developing a generalized understanding of the ?frustrated phase separation? phenomena widely observed in various materials ranging from biological cell membranes to two-dimensional electronic systems. The broader impacts of the proposed activities are far reaching. Formulating the surface of a material with a mixed polyelectrolyte and neutral polymer brush represents a new paradigm applicable, as a generic methodology, to a wide variety of advanced technologies, including nano/micro fabrication and biomedical technologies, and the proposed research will provide the fundamental materials science groundwork necessary for further technological development in these areas. The proposed research will provide integrated training for graduate and undergraduate students in a multidisciplinary, collaborative and intellectually stimulating environment to learn skills necessary for the future generation of polymer/soft materials scientists. Aspects of the proposed research will be used to enhance curricula in the areas of polymers and nanomedicine.