Herein, a novel rod-coil type polyhedral oligomeric silsesquioxane (POSS)-containing diblock copolymer was designed to enable the self-assembly of hexagonally packed cylinders of the POSS-containing domain in a poly(n-butyl methacrylate) (PnBMA) matrix. When POSS-containing diblock copolymers were synthesized with polyisoprene or poly(methyl methacrylate), cylindrical structures could not be obtained as POSS-containing polymers form stretched rigid rods. This makes the formation of cylindrical structures with the POSS-containing domain entropically unfavorable. Therefore, to obtain the cylindrical structures, we constructed a novel diblock copolymer using PnBMA to increase the steric bulk and segment volume of the flexible coil. Steric crowding of the butyl groups reduces the entropic free stretching energy of the PnBMA chains, which in turn encourages the formation of a POSS-containing hexagonally packed cylindrical structure within the PnBMA matrix as the system minimizes the total free energy of the thermodynamically stable nanostructure. Small angle X-ray scattering and transmission electron microscopy analyses indicated that cylinders of the POSS domain had formed. Oxygen plasma etching was then used on the thin film to selectively remove the PnBMA domain to yield line and space structures with a high degree of long-range order and a 14 nm feature size. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2234-2242