Domain orientation of block copolymer films is critical for a number of potential applications including lithography, membrane separations, and surface patterning. Perpendicular orientation of block copolymers is generally challenging with techniques developed based on surface treatments to produce a neutral substrate or solvent vapor annealing. In most block copolymers, the surface energies of the blocks are generally different with a narrow temperature range where the surface energies are matched to favor perpendicular orientation of domains (no wetting layer). This range is further limited as it must be less than the degradation temperature, but greater than the glass transition temperature for the block copolymer. Polystyrene-b-poly(methyl acrylate) is a unique system as both surface free energies are similar with a small difference in the temperature coefficient of the surface free energy. These characteristics lead to a perpendicular cylinder structure at the surface from simple oven annealing of films spun cast on silicon wafers. From analysis of AFM micrographs, a predominately perpendicular cylinder structure is obtained over large temperature range from 125 oC to 170 oC. For films thicker than approximately 150 nm, the fraction of perpendicular orientation is nearly thickness invariant for a fixed annealing temperature. For thinner films, the fraction of perpendicularly oriented domains decreases when the film thickness becomes commensurate with the d-spacing to yield an oscillatory thickness dependence to the perpendicular:parallel ratio. A comparison in the orientation between the surface and bulk of the films is obtained using GISAXS. This system of polystyrene-b-poly(methyl acrylate) provides a simple route to perpendicular orientation.

Facile perpendicular orientation in polystyrene-block-poly(methyl acrylate) films

246th National Meeting (2013)

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