Imaging layers for the directed assembly of block copolymer films: Dependence of the physical and chemical properties of patterned polymer brushes on brush molecular weight

The directed assembly of polystyrene-block-poly(methyl methacrylate) films using chemically nanopatterned polymer brush surfaces with various molecular weights was investigated. The brushes (⁠$3–10nm$ in thickness) were made from polystyrene that was end grafted to silicon wafers and patterned to have $40–60nm$ period stripes consisting of unmodified polymer and polymer that was subjected to brief oxygen plasma. The best assembly of block copolymer films was achieved using brushes with molecular weights of $3kg∕mol$ or less. The brushes were analyzed by goniometry, infrared reflection-absorption spectroscopy, x-ray photoelectron spectroscopy, and ellipsometry. The brush thickness increased nonlinearly with molecular weight and the high molecular weight brushes reconstructed under solvent and thermal annealing. The lower molecular weight brushes form the best imaging layers due to their optimum thickness that provides uniform surface coverage and prevents reconstruction of the film surface. The thin brushes ensure contact between the block copolymer and the hydrophilic groups in the modified regions of the brush during annealing, maximizing the effective interfacial energy contrast and the driving force for perfect directed assembly.