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ABC 삼중블록 공중합체의 합성, 분석, 상분리 연구

ABC 삼중블록 공중합체의 합성, 분석, 상분리 연구
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Triblock block copolymers exhibit far more rich morphologies than diblock copolymers. Although many theoretical and experimental studies have been done on triblock copolymers, our understanding on the phase behavior of triblock copolymers is far less complete than diblock copolymers. One of the reasons is that the self-assembled structure of the three component system is affected by more number of variables than diblock copolymers. Another serious problem is in the difficulty in synthesis and purification of triblock copolymers. Either the solubility/compatibility of the polymers with the solvent or the reactivity difference between the monomers often limits the use of simple polymerization method such as sequential anionic polymerization. Even if a sequential polymerization is possible, inadvertent termination often occurs due to the high reactivity of carbon anions yielding unwanted side-products. Presence of the side-products is sometimes difficult to notice by size exclusion chromatography analysis, which is a method most commonly used to characterize polymer samples. In this study, a few different polymerization methods were used to synthesize PS-b-PI-b-PMMA of various compositions. As-prepared PS-b-PI-b-PMMA samples were fractionated by interaction chromatography (IC) to obtain pure triblock copolymer. Then the purified triblock copolymers were rigorously characterized for their composition and molecular weight before the investigation of self-assembled structures. In chapter 1, various synthetic methods of triblock copolymers were described and basic principles of the characterization methods used in this study were briefly reviewed. In chapter 2, end group modification of polymers to prepare macroinitiator for atom transfer radical polymerization was investigated. In this study, hydroxyl terminated polystyrene (PS-OH) was prepared using anionic polymerization of styrene and a subsequent reaction of polystyryl anions with propylene oxide. Two kinds of PS-OH with different regiochemistry at the chain end were separated using interaction chromatography (IC), and each fraction was characterized by MALDI-TOF MS and NMR. The macroinitiator for ATRP was prepared by a simple substitution reaction between end hydroxyl group and to bromine, then a second or third block was polymerized to the chain end of the macroinitiator. In chapter 3, two methods for the sequential anionic polymerization of PS-b-PI-b-PMMA triblock copolymer was introduced, and fractionation of the pure triblock copolymer was investigated. One method is to change the solvent, the other is to change the counter-cation during the polymerization process. Thus prepared PS-b-PI-b-PMMA was fractionated by IC to obtain pure triblock copolymer. We rigorously characterized the purified triblock copolymer and the byproducts by SEC with multiple detection methods, which demonstrate the possible pitfalls of the SEC characterization alone. In chapter 4, the self-assembled structures of PS-b-PI-b-PMMA in bulk and thin film were investigated. Three-domain lamellar, cylinder in lamellar, and an unidentified morphology were found. Experimentally observed structures were compared to the diagram of PS-b-PB-b-PMMA system and further expectation was tried by comparison with the SCFT diagram.
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