Synthesis and Phase Behavior of Rod-Coil Type Block Copolymers Containing Poly(3-alkylthiophene) (P3AT)
- Synthesis and Phase Behavior of Rod-Coil Type Block Copolymers Containing Poly(3-alkylthiophene) (P3AT)
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- Block copolymer has been getting great attention due to its self-assembly ability to form periodic ordered microstructures which depend on the volume fraction (f) of one of blocks, the degree of polymerization (N) and the Flory-Huggins segmental interaction parameter. While phase behavior of coil-coil block copolymers has been intensively investigated both experimentally and theoretically, there are a few reports for synthesis and self-assembly study on the rod-coil type block copolymers composed of conducting poly(3-alkylthiophene) (P3AT). Among the many classes of conducting polymers, P3AT is one of the most widely used polymers because of its high hole mobility and good solubility for various organic solvents. The synthesis for P3AT-containing block copolymers having narrow molecular weight distribution is essential for fabrication of P3AT nano-pattern with 10~20 nm length scale, because domain size is proportional to the molecular weight. In this thesis, we developed new facile synthetic method for well-defined P3AT-containing rod-coil (or coil-rod-coil) block copolymers using anionic coupling reaction. In addition, we induced several morphologies for poly(3-dodecylthiophene)-block-poly(methyl methacrylate) (P3DDT-b-PMMA) copolymers driven by competition between crystallization and block copolymer phase separation.
In chapter 2, we developed facile synthesis of coil-rod-coil type block copolymers composed of poly(3-hexylthiophene) (P3HT) by anionic coupling reaction between living coil polymer anions and aldehyde-capped P3HT. Because of stronger electrophilicity for aldehyde group compared with ally (or vinyl) group, coupling reaction was performed efficiently. This synthetic scheme was demonstrated at both of polar (THF) and non-polar (benzene) solvents. When we employed slightly excess amount of living polymer anions such as living poly(2-vinylpyridine) (P2VP) and living polyisoprene (PI) for polar and non-polar cases, respectively, all the aldehyde groups of P3HT were reacted. The crude products were purified by column chromatography. As a result, we finally obtained neat P2VP-b-P3HT-b-P2VP and PI-b-P3HT-b-PI copolymers without any byproducts.
In chapter 3, we expanded our anionic coupling reaction for synthesis of P3HT-b-poly(methyl methacrylate) (P3HT-b-PMMA) copolymer. To ensure the outstanding electrical property of P3HT for organic electronics, insulating coil part should be removed selectively. Among the various coil polymers, PMMA can be easily eliminated by UV-irradiation followed by acetic acid rinsing. Due to electron withdrawing group of carbonyl function, however, living PMMA anion is too stable to react with aldehyde-capped P3HT. Thus, we designed more reactive functionalized P3HT, phenyl acrylate (PA)-capped P3HT. When we coupled PA-capped P3HT with living PMMA anion, anionic coupling reaction was successfully conducted. Moreover, we investigated P3HT-b-PMMA thin film morphology and orientation of P3HT crystalline by tapping mode atomic force microscopy (AFM) and grazing incidence wide-angle x-ray scattering (GIWAXS), respectively.
In chapter 4, we studied self-assembled nanostructures of poly(3-dodecylthiophene)-b-PMMA (P3DDT-b-PMMA) copolymers with various block compositions. We chose P3DDT with moderate rod/rod interaction, because rod/rod interaction of P3HT is too strong to induce block copolymer phase separation. All the block copolymers having narrow molecular weight distribution were prepared by anionic coupling reaction suggested in chapter 3. When the weight fraction of P3DDT was varied from asymmetric to symmetric fraction, body centered cubic (BCC) (wP3DDT = 0.20), hexagonally packed cylinder (HEX) (wP3DDT = 0.39) and lamella (LAM) (wP3DDT = 0.56) were observed by small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). On the other hand, when the P3DDT weight fraction was 0.76, rod/rod interaction dominated block copolymer phase separation, resulting in the development of fibril structure.
Finally, in chapter 5, we investigated phase transitions such as order-to-disorder transition (ODT) for poly(3-dodecylthiophene)-b-PMMA (P3DDT-b-PMMA) copolymers. First, we prepared two symmetric P3DDT-b-PMMAs with different total molecular weight. While the larger one showed fully ordered state of lamella, another one indicated fully disordered state within experimentally accessible temperature range. To adjust total molecular weight, we blended two block copolymers with various composition. When we controlled molecular weight judiciously, order-to-disorder transition was detected by SAXS upon heating.
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