Exploring the Concentrated Lyotropic and Dilute Solution Assembly of Linear-Dendritic Block Copolymers

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Dendrimers are unique and relatively new macromolecular systems whose exponentially branched interiors and dense exterior functional groups offer a number of advantageous properties. The interior or exterior regions of dendrimers can be functionalized with specific binding groups, ranging from crown ethers to hydrogen bonded complexes, to selectively adsorb a desired guest system, and the high density of functional groups in the dendrimer molecule allows for highly efficient targeted binding of biological substrates. Dendritic materials systems have also been shown to act as nanoreactors for the formation of metals and metal oxides, and as agents in solubilization and separations applications. Dendritic architectures can be tailored for specific interactions, or for electrical, optical or biological functionality. For these reasons, dendrimers represent a unique macromolecular system with novel and interesting properties. In recent years, a number of important synthetic developments have led to the creation of novel linear-dendritic hybrid block copolymers with a wide variety of chemical and structural properties. Linear-dendritic hybrid copolymers offer a clear opportunity to take advantage of the properties of dendrimers and the phase segregated morphological self-assembly of traditional block copolymers, which have been shown to self-assemble in solution to form concentrated lyotropic phases such as lamellae and columnar phases, and micellar phases consisting of spherical and cylindrical micellar systems. Although the amphiphilic behavior of linear block copolymers has been studied extensively, resulting in an understanding of the lyotropic phases presented by these systems, there has not been an extensive study of amphiphilic linear-dendritic hybrid block copolymers, and their assembly behavior in solution. It is the objective and intellectual merit of this work to undertake the first systematic and inclusive examination of linear-dendritic block copolymer self-assembly in the solution state, from concentrated to dilute polymer solution, and the investigation of the stabilized nanostructured materials resulting from this assembly. The architecture and topology of the highly branched dendritic block is expected to lead to changes in the preferred curvature at the block copolymer interface, and variations and shifts in the solution phase behavior of these systems, as has been observed in studies of the bulk morphology of these systems in funded by the previoius related grant creativity extension. Linear-dendritic block copolymers represent a new form of macromolecular amphiphile which may exhibit novel phase behavior; this work will contribute significantly to a fundamental understanding of these macroamphiphiles, and the effects of generation, linear block length and solvent-polymer interactions on the assembly in solution. It is anticipated that the phase behavior of these systems will introduce a new paradigm to the area of macromolecular amphiphiles . rather than the traditional hydrodynamic radius of a coil-coil block copolymer, it is possible to examine the arrangement of a block consisting of the coil and a much more geometrically constrained polymer block. The broader impacts of this study include the creation of novel self-assembled dendritic nanostructures ranging from continuous phase nanoporous media to highly functional dendritic nanoparticles and other nanoscale objects. The potential use of such linear-dendritic hybrid block copolymers as templates for the formation of nanostructured materials and nanoscale objects is yet to be explored, and is expected to impact the design of nanostructured materials systems. Utilizing the principles of amphiphilic block copolymer assembly, micellar systems containing dendritic exteriors and linear chain interiors will be created and stabilized with crosslinking, resulting in nanoparticles for which the exterior functionality is 10 to 100 times greater than that of the original dendron. The presence of the dendritic block will be advantageous for a number of applications, including the creation of nanostructured porous materials formed from the lyotropic phase in which, for example, the dendrimer end groups are exposed within the interiors of nanopores that provide a highly reactive surface region for catalytic or interactive pores. A number of new and novel materials structures can result from the careful design and synthesis of functional linear-dendritic systems which may undergo crosslinking, encourage or support metal oxide synthesis, or serve as a template for a second organic material. Finally, the research described here is an integral part of the investigator's research and teaching plan, and includes the education and training of undergraduate and graduate students in the laboratory environment, the integration of concepts of self-assembly in the teaching of polymer science in under-graduate and graduate courses. The mentorship of students takes place on every level, including academic and career issues, work and family concerns, and includes a number of women and minority students. Outreach of the PI includes teaching of students aged 9-12 in a Math and Science Saturday Program in Cambridge.