Vertical Orientation of Nanodomains on Versatile Substrates through Self-Neutralization Induced by Star-Shaped Block Copolymers

Vertical orientation of lamellar and cylindrical nanodomains of block copolymers on substrates is one of the most promising means for developing nanopatterns of next-generation microelectronics and storage media. However, parallel orientation of lamellar and cylindrical nanodomains is generally preferred due to different affinity between two block segments in a block copolymer toward the substrate and/or air. Thus, vertical orientation of the nanodomains is only obtained under various pre- or post-treatments such as surface neutralization by random copolymers, solvent annealing, and electric or magnetic field. Here, a novel self-neutralization concept is introduced by designing molecular architecture of a block copolymer. Star-shaped 18 arm poly(methyl methacrylate)-block-polystyrene copolymers ((PMMA-b-PS)₁₈) exhibiting lamellar and PMMA cylindrical nanodomains are synthesized. When a thin film of (PMMA-b-PS)₁₈ is spin-coated on a substrate, vertically aligned lamellar and cylindrical nanodomains are obtained without any pre- or post-treatment, although thermal annealing for a short time (less than 30 min) is required to improve the spatial array of vertically aligned nanodomains. 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, SiO_x), metal (Au), PS or PMMA-brushed substrate, and a flexible polymer sheet of polyethylene naphthalate. Vertically oriented lamellar and cylindrical nanodomains are developed by employing 18 arm star-shaped (PMMA-b-PS)₁₈ through a self-neutralization concept. Without a solvent annealing or neutralization process of the substrate, vertically oriented lamellar and cylindrical nanodomains are successfully achieved on various substrates, ranging from semiconductors (Si, SiO_x) to metal (Au), polymer-brushed substrates, and flexible polymer substrates.