Controlling Morphologies of Self-Assembled Structures of Block Copolymers by Molecular Designs and External Stimuli

Department of ChemistrySelf-assembly of amphiphilic block copolymers (BCPs) in solution has received attention from polymer chemists due to their potential applications such as drug delivery system and nanoreactors for two decades. Because sorts of morphologies formed by self-assembly were so diverse, predicting and designing the desired morphologies were considered as one of the goals. The morphologies were classically controlled by modifying a block ratio of BCPs. Herein, however, I demonstrated that morphologies of self-assembled structures of BCPs can be controlled by molecular designs and external stimuli with a fixed block ratio. In this dissertation, dendritic-linear poly(ethylene glycol)-block-polystyrene (PEG-b-PS) BCPs were synthesized and their self-assembled structures were observed. Morphologies of dendritic-linear PEG-b-PS were toroids while morphologies of linear-linear PEG-b-PS with same block ratios were polymersomes. That???s because the molecular area of BCPs was expanded by branched PEG chains. To stabilize those structures, photo-cross-linkable monomers were introduced to the hydrophobic blocks. Portions of indenylstyrene that can be dimerized under UV light were copolymerized with styrene monomers by atom transfer radical polymerization (ATRP). The BCPs could be self-assembled in various morphologies and they were cross-linked under an irradiation of UV light for few hours. The enhanced stability of morphologies was demonstrated by an extreme dilution into the organic solution and the self-assembled structures survived. Furthermore, BCPs with high molecular weights (> 200 kDa) were synthesized in order to access photonic bandgap properties of polymer cubosomes because structural periodicity of polymer cubosomes was proportion to their molecular weights. However, the polymerization of BCPs with high molecular weights through living radical polymerization has been challengeable. The problem was solved by the acceleration of propagation rates with an excess loading of CuBr catalysts but it simultaneously brought out broad distributions of molecular weights. Through a gradient column chromatography, high molecular weight BCPs with narrow distribution was achieved and blue light reflections were observed from their polymer cubosomes. Finally, to control morphologies of the branched-linear PEG-b-PS BCPs by external stimuli, three pyridine groups that can be utilized as stimuli-receptors were introduced into junctions of peripheral PEG chains. As a result, self-assemblies with and without external stimuli such as low pH and metal precursors produced significantly different morphologies. From the above experiments, I demonstrated that the morphologies can be controlled by designing molecular architectures and introducing external stimuli during self-assembly processes, and the morphologies can be stabilized efficiently by the photo-cross-linking system.ope