Synthesis and Analysis of Star-Shaped Block Copolymers and Applications for Thin Film

Abstract

Development of synthetic skills, for instance, controlled radical polymerization, anionic polymerization and end-functionalization of polymers have provided opportunities for making diverse architectures of block copolymers. Synthesis of nonlinear copolymers such as grafted, miktoarm or star-shaped copolymers was attributed to the above-mentioned synthetic skills for polymers. Among them, star-shaped copolymer is one of the most interesting nonlinear block copolymers due to their different characteristics or improved properties compared with linear copolymers; higher critical micelle concentration, lower viscosity or different morphologies. Micellar architecture of star-shaped block copolymer consisted of covalently linked block copolymer arms to the junction point causes extraordinary properties derived from restricted entropy of arms. In this thesis, we studied phase behavior and analyzed the mechanism of unexpected morphologies of 18 arm star-shaped block copolymers. And we fabricated thin film of star-shaped block copolymers to explore the formation of star molecules in the film state. In chapter 2, the phase behavior of 18 arm star-shaped polystyrene-block-poly(methyl methacrylate) copolymers ((PS-b-PMMA)18) with various volume fractions of PMMA block (fPMMA) was investigated by transmission electron microscopy and small angle X-ray scattering. (PS-b-PMMA)18 was synthesized by atom transfer radical polymerization from α-cyclodextrin (α-CD) having 18 functional groups for the initiation. We also prepared the corresponding linear PS-b-PMMAs by cutting the ester groups connecting α-CD and PS chains in (PS-b-PMMA)18 through the hydrolysis. The nanodomains of (PS-b-PMMA)18 changed from body-centered cubic spheres (BCC), hexagonally packed cylinders (HEX), perforated lamellae (PL), and lamellae (LAM), with increasing fPMMA from 0.3 to 0.8. Interestingly, (PS-b-PMMA)18 with fPMMA of 0.77 showed highly asymmetric lamellar nanodomains, while the corresponding linear PS-b-PMMA with the same volume fraction should not have lamellar nanodomains. Thus, the nanodomains are highly affected by the molecular architecture of block copolymer. The experimental results are discussed with the prediction based on the self-consistent mean field theory. In chapter 3, we introduced a novel self-neutralization concept by designing molecular architecture of a block copolymer without pre- or post-treatments. We synthesized star-shaped 18 arm poly(methyl methacrylate)-block-polystyrene copolymers ((PMMA-b-PS)18) exhibiting lamellar and PMMA cylindrical nanodomains. When a thin film of (PMMA-b-PS)18 was spin-coated on a substrate, vertically aligned lamellar and cylindrical nanodomains were obtained without any pre- or post-treatment. This result is attributed to the star-shaped molecular architecture that overcomes the difference in the surface affinity between PS and PMMA chains. Moreover, vertical orientations are observed on versatile substrates, for instance, semiconductor (Si, SiOx), metal (Au), PS or PMMA-brushed substrate, and a flexible polymer sheet of polyethylene naphthalate (PEN).