Ordering of lamellar block copolymers on oxidized silane coatings

Abstract

Thin films of lamellar poly(styrene-b-methyl methacrylate) (PS–PMMA) block copolymers are widely investigated for surface patterning. These materials can generate dense arrays of nanoscale lines when the lamellar domains are oriented perpendicular to the substrate. To stabilize this preferred domain orientation, we tuned the substrate surface energy using oxidation of hydrophobic silane coatings. This simple approach is effective for a broad range of PS–PMMA film thicknesses when the oxidation time is optimized, which demonstrates that the substrate coating is energetically neutral with respect to PS and PMMA segments. The lamellar films are characterized by high densities of defects that exhibit a strong dependence on film thickness: in-plane topological defects disrupt the lateral order in ultrathin films, while lamellar domains in thick films can bend and tilt to large misorientation angles. The types and densities of these defects are similar to those observed with other classes of neutral substrate coatings, such as random copolymer brushes, which demonstrates that oxidized silanes can be used to control PS–PMMA self assembly in thin films.