Microdomain Orientation of Star-Shaped Block Copolymer Thin Film Depending on Molecular Weight

The vertical orientation of microdomains of a block copolymer (BCP) on a substrate is essential to applying nano-lithography. Thin films of star-shaped block copolymers have shown vertical orientation of microdomains without using pre- or post-treatment 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 small-angle 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), six-arm 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 (greater than or similar to 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.