Dimensional Scaling of Cylinders in Thin Films of Block Copolymer−Homopolymer Ternary Blends

Abstract

The cylindrical domain diameter (d), spacing (D), and uniformity in thin films of ternary blends of a cylinder-forming polystyrene-block-poly(methyl methacrylate) block copolymer and the corresponding homopolymers were investigated as a function of the molecular weight (Mn) and concentration (φH) of the homopolymers. Thin films (20−65 nm) of the blends were deposited on silicon wafers that had been modified with random copolymer brushes such that the cylindrical domains of the annealed blends were oriented perpendicular to the substrate. The best uniformity of d, at a given φH, was achieved in the blends with lower homopolymer Mns. For blends made with homopolymers that had Mn values of the same order of magnitude as those of the corresponding blocks of the copolymer, D increased with increasing φH as D = D0(1 − φH)−β, where D0 is the domain spacing of the block copolymer and β is a parameter that depends on the ratio of the homopolymer and block copolymer molecular weights. The cylindrical domains of the blends could be swollen up to 150% of the original diameter while maintaining hexagonal ordering. For most of the blends on the random copolymer brushes, uniform arrays of perpendicular cylinders were formed most frequently at a film thickness near D0. A ternary blend with homopolymers with very low Mns produced markedly different results: D decreased in size with increasing φH, measurable up to φH = 0.4, and produced uniform patterns in films with thicknesses ranging from 20 to 37 nm.