Synthesis and Phase Behavior of Miktoarm Star Copolymers Containing Poly(3-alkylthiophene) (P3AT)

Abstract

Block copolymer has been received great attention due to its ability to form periodic microstructures which depend on the Flory-Huggins segmental interaction parameter (), the volume fraction (f) of one of blocks and the degree of polymerization (N). In addition, microstructures of block copolymers are strongly affected by macromolecular architecture. For example, miktoarm star copolymer, which consists of heteroarms with different chemical compositions and/or molecular weights emanated from a central core, has attracted great interest for its distinct phase behavior compared with linear block copolymer. While phase behavior of rod-coil linear block copolymers has been investigated for the past 10 years both experimentally and theoretically, there are a few reports for synthesis and phase behavior on the miktoarm star copolymers containing rod-type polymers due to synthetic difficulty. Therefore, the synthesis for miktoarm star copolymers containing rod-type polymers having narrow molecular weight distribution (PDI) is essential for systemic study of phase behavior. Among the various kinds of rod-like conjugated polymers, poly(3-alkylthiophene) (P3AT) is one of the most attractive polymers because of good solubility for various organic solvents and its high charge carrier mobility. In this thesis, I developed new synthetic methods for well-defined P3AT-containing miktoarm star copolymers. Also, I investigated phase behavior and morphologies for three different miktoarm star copolymers: (1) miktoarm star copolymer composed of two poly(3-hexylthiophene) (P3HT) arms and one poly(methyl methacrylate) (PMMA) arm (P3HT2PMMA), (2) miktoarm star copolymer composed of two poly(3-dodecylthiophene) (P3DDT) arms and one PMMA arm (P3DDT2PMMA), and another (3) miktoarm star copolymer composed of one P3HT arm and two PMMA arms (PMMA2P3HT). In chapter 2, I developed facile synthesis of P3HT2PMMA by copper (I)-catalyzed Huisgen 1,3-dipolar cycloaddition click reaction between ethynyl end-functionalized P3HT and PMMA functionalized with two azides at one chain end (PMMA-(N3)2). Because of high reactivity between azides and terminal alkynes, coupling reaction between end-functionalized polymers was performed efficiently. When I employed slightly excess amount of ethynyl end-functionalized P3HT respectively, all the end-functional groups of PMMA-(N3)2 were reacted. The crude products were purified by column chromatography. As a result, I obtained neat P3HT2PMMA miktoarm star copolymers without any homopolymers. In chapter 3, I studied self-assembled microstructures of P3DDT2PMMAs with various weight fractions of P3DDT block (wP3DDT). P3DDT was chosen because P3DDT has moderate rod/rod interaction to induce phase separation of block copolymer. All P3DDT2PMMAs prepared by click reaction suggested in chapter 2 showed narrow PDI. The morphologies of P3DDT2PMMAs were observed by small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) and compared with that of linear diblock copolymers (P3DDT-b-PMMA). P3DDT2PMMA with wP3DDT of 0.33 showed lamellae (LAM), while P3DDT-b-PMMA with wP3DDT of 0.37 had hexagonally packed cylinder (HEX) of P3DDT. Also, P3DDT2PMMA with wP3DDT of 0.59 showed the HEX of PMMA in the P3DDT matrix, whereas P3DDT-b-PMMA with wP3DDT of 0.56 exhibited LAM. Interestingly, P3DDT2PMMA with wP3DDT of 0.76 had HEX of PMMA in the P3DDT matrix at molten state, and this morphology was maintained even after P3DDT crystallization. This is quite different from P3DDT-b-PMMA with the same wP3DDT because the latter has fibril morphology after P3DDT crystallization. In chapter 4, I investigated phase behavior of PMMA2P3HTs with various weight fractions of P3HT block (wP3HT). PMMA2P3HTs were synthesized via anionic coupling reaction. For this purpose, P3HT with two bromine groups (P3HT-Br2) was synthesized by Williamson reaction between hydroxyl-terminated P3HT and excess amount of tris(bromomethyl)benzene. Anionic coupling reaction between P3HT-Br2 and living PMMA anions gives PMMA2P3HTs having narrow PDI. The miktoarm architecture was chosen because its chain overcrowding at interface could induce microphase separation. The morphologies were investigated via TEM and SAXS. Sphere, HEX and LAM were observed depending on wP3HT, and this morphology was maintained even after P3HT crystallization. These results imply that macromolecular architecture is important to control the morphology of P3HT-containing block copolymers.