Synthesis and Phase Behavior of Star Copolymer with Complex Architecture

Abstract

Block copolymer (BCP) has received great attention because of its ability to generate various nano-sized structures depending on Flory−Huggins segmental interaction parameter (χ), degree of polymerization (N) and volume fraction of one block (fA). BCPs can form a variety of microdomains such as spheres, cylinders, gyorids and lamellae, but have a limitation that those microdomains are strongly restricted by the volume fraction of each block. In other words, the volume fraction range for each nanostructure are fixed and only a single microdomain is organized at a specific volume fraction. To increase the applicability of BCP in future nanotechnology, it is necessary to form various structures regardless the volume fraction. Many studies have been done to obtain different phase behavior from that of a typical linear BCP. One method is to newly design the architecture of BCP. When BCP has a complicated chain architecture, there is a big change in the configurational entropy arising from the chain stretching. Because star copolymer has an architecture where several linear chains link at a junction point, entropy penalty from chain stretching or packing frustration becomes large, which generates new nanostructures which have not been observed in linear BCP. In this thesis, I synthesized several star-shaped block copolymers and investigated their phase behavior and found that the phase behavior is quite different from that of copolymer and linear BCP. In chapter 2, I investigated, via amall-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), the phase diagram of 18-arm star-shaped polystyrene-block-poly(methyl methacrylate) copolymers [(PS-b-PMMA)18], depending on the degree of initiation of the second PMMA block. (PS-b-PMMA)18 was synthesized by atom transfer radical polymerization (ATRP) from α -cyclodextrin having 18 functional groups. The core PS block was first synthesized by using copper(I) bromide as a catalyst. Then, two different catalysts were used for the synthesis of the second PMMA block. When copper(I) bromide was used as a catalyst for ATRP, only 75% arms were initiated to polymerize MMA; thus 25% PS arms did not contain any PMMA block. However, when copper(I) chloride was used, 100% arms were initiated. The phase behavior of (PS-b-PMMA)18 with 100% arms of block copolymer chains is significantly different from that with only 75% of PS-b-PMMA and 25 % of PS. For instance, at a volume of PMMA block is 0.77, the former exhibited body-centered cubic spherical microdomains, while the latter showed asymmetric lamellar microdomains. In chapter 3, I investigated the morphology in the binary blend of six-arm star-shaped copolymer [(PMMA-b-PS)6] and PMMA-b-PS linear diblock copolymer by varying their molecular weights as well as volume fractions of the blocks. When the molecular weight of PMMA-b-PS is much larger (>∼4) than that of one arm of (PMMA-b-PS)6, PMMA-cylindrical microdomains are formed even though the volume fraction of PMMA (fPMMA) in both (PMMA-b-PS)6 and PMMA-b-PS is nearly symmetric (fPMMA∼0.5). On the other hand, when the ratio of molecular weights between these two copolymers is not large, lamellar morphology is observed in the blend as expected. Very interestingly, we found that even for a binary blend with the overall volume fraction of the PMMA block (f ̅PMMA) as large as 0.71, the major PMMA blocks still aggregate into cylindrical microdomains, and thus, “ inverted cylinders ” are formed, although PS-cylinders are observed in the neat (PMMA-b-PS)6 and PMMA-b-PS melts. The experimental results as well as the formation of the inverted cylinders have been verified by self-consistent field theory (SCFT). In chapter 4, I synthesized PS-[PS-b-poly(2-vinylpyridine)]3 miktoarm star copolymer [PS(PS-b-P2VP)3]. This miktoarm star copolymers were prepared varying the volume fraction of PS (fPS) and chain asymmetry (τ) of the PS. Asymmetric lamellae were observed at fPS = 0.82, while inverted gyroid with minor blocks matrix at fPS = 0.64 were observed when τ is 0.79 and 0.65, respectively. Interestingly, different morphologies were observed depending on τ value at a fixed fPS. At a fixed fPS (0.64), , lamellar structure was observed at τ = 0.46, where inverted gyroids were formed at τ = 0.65. The experimental results are consistent with predictions by self-consistent field theory.