We investigated, via atomic force microscopy and fieldemission scanning electron microscopy, the thin-film morphology of six-arm star-shaped poly(methyl methacrylate)-b-polystyrene [(PMMA-b-PS) 6 ] with the volume fraction of PS block of ∼0.5. At smaller molecular weight (M) but higher than the critical molecular weight (M crit ) above which block copolymers start to microphase-separate, the thin-film morphology greatly depends on the surface tension at the air side when a substrate has preferential interaction with one block (PMMA). For a near-neutral air surface, interesting tube-like nanostructures, instead of vertically oriented lamellae, were formed at the top of the film, while a PMMA layer was formed on the bottom film, contacting the silicon substrate with a native oxide. This is because the combination of vertical and parallel lamellae generates a huge energy penalty at the T-junction connecting these two different lamellar orientations. Tube-like nanostructures were also formed on other substrates that are preferential to one block, for instance, gold or a substrate grafted by a PS brush, when the film thickness does not meet the commensurability. On the other hand, when M is much higher than M crit , vertical lamellae were formed throughout the entire film thickness. The self-consistent field theory supported the experimental findings.

Nucleobase-containing polymers have received great attention for their complementary multiple hydrogen bonding between nucleobases. However, their polymerization is difficult due to poor solubility in a solvent. In this study, we successfully synthesized adenine-containing block copolymers, poly­(9-(4-vinylbenzyl)­adenine)-block-polystyrene (PVBA-b-PS), using reversible addition–fragmentation chain transfer (RAFT) polymerization in polar solvents of dimethyl sulfoxide and N , N-dimethyl­formamide and characterized them by size exclusion chromatography and nuclear magnetic resonance spectroscopy. We measured the temperature dependence of the Flory–Huggins interaction parameter (χ) between PVBA and PS as χ = 0.3847 + 55.763/T. The χ was very large (∼0.5 at 200 °C). The phase behavior of PVBA-b-PS with various volume fractions of PS block (f PS) was investigated via small-angle X-ray scattering and transmission electron microscopy. With increasing f PS from 0.1 to 0.8, body-centered-cubic spheres of PS, hexagonally packed (HEX) cylinders of PS, lamellae, and HEX cylinders of PVBA were observed. Interestingly, PVBA-b-PS with f PS = 0.75 showed asymmetric lamellar microdomains. We also prepared a thin film of PVBA-b-PS on a substrate as a template for spatial arrangement of gold nanoparticles (AuNPs). When the surface of AuNPs was modified with thymine-containing polymer chains, AuNPs were selectively sequestered into PVBA microdomains through the complementary hydrogen bonding between thymine and adenine units.

The vertical orientation of microdomains of a block copolymer (BCP) on a substrate is essential to applying nanolithography. Thin films of star-shaped block copolymers have shown vertical orientation of microdomains without using pre-or posttreatment of a substrate because of the huge entropic penalty for parallel orientation of microdomains arising from their chain architecture. Because entropy depends on the total molecular weight (M) of a star-shaped block copolymer, we investigated, via smallangle X-ray scattering and atomic force microscopy, the effect of M on the orientation of microdomains on various substrates. We synthesized, by atom transfer radical polymerizations (ATRP), sixarm star-shaped poly(methyl methacrylate)-block-polystyrene copolymer [(PMMA-b-PS) 6 ] exhibiting lamellar and PMMA cylindrical microdomains. When M is close to the critical molecular weight (M crit ) above which a block copolymer starts to microphase separate, a parallel orientation of lamellae (or cylinders) was obtained. However, even at a small increase in M (≳1.5M crit ), thin films showed vertically oriented microdomains regardless of substrates. Thus, star-shaped block copolymers are very effective to obtain vertical oriented cylindrical and lamellar microdomains on versatile substrates.

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The self-assembly of block copolymers has potential applications for next-generation lithography with a small feature size of less than 10 nm. Though a feature size of ∼5 nm has been reported, the fabrication of ultra-small feature sizes (less than 5 nm) still remains a great challenge. In this study, we utilized a compound with three hydroxyl groups named gallol, which shows strong intermolecular interactions. We synthesized, via a reversible addition–fragmentation chain-transfer polymerization, a polytrihydroxystyrene-block-polystyrene copolymer (PTHS-b-PS) having lamellar and cylindrical microdomains by adjusting the volume fraction of the PS block (f PS). We found that the Flory–Huggins interaction parameter (χ) between the PTHS and PS segments was extremely large, 1.24 at 220 °C. By using a PTHS-b-PS with a total molecular weight of 1.4 kg mol–1 and f PS = 0.53, we obtained a lamellar domain spacing (L 0) as small as 4.5 nm. Also, the diameter of hexagonally packed cylinders was reduced to 4.0 nm by employing another PTHS-b-PS with a molecular weight of 2.9 kg mol–1 and f PS = 0.74. This ultra-small feature size can be used for next-generation lithography.