Wide Neutrality Window for Block Copolymer Vertical Orientation Using Incongruent Homopolymer Blended Brushes

Distinctive polymer brushes (PBs) play a crucial role in providing a nonpreferential (neutral) surface for vertical orientation of block copolymers (BCPs). This bottom-up approach effectively aligns the formation of vertical lamellar and cylinder lattice structures from the BCP, which is crucial for nanopatterning and other applications. In conventional BCP self-assembly techniques, random copolymer brushes are commonly employed to achieve substrate neutrality. However, these methods face significant drawbacks, including batch variations, synthetic challenges for chemically incompatible monomers, and a narrow neutrality window. Blending homopolymer brushes to achieve mixed chemistry has been demonstrated to be effective for creating neutral substrates but requires precise blend formulations due to the high potential for macrophase segregation. Various approaches have been proposed to mitigate inconsistencies and enhance tunability, albeit with associated costs. Here, we propose a PB system composed of incongruent chain length homopolymers at various blend ratios, which achieves a wide neutrality window for the BCP vertical orientation. These incongruent PB blends demonstrate nonpreferential behavior with lamellar and cylinder forming BCP independent of the brush composition. A canopy effect is proposed as the mechanism that enables the responsive nature of the brush, where a sparse layer of chains will adopt conformations that minimize interfacial energy. By effectively tuning the surface energy of these PB systems, a wide neutrality window is achieved, offering a straightforward, economical template for BCP vertical orientation.