Surfactant-Assisted Orientation of Thin Diblock Copolymer Films
The precise control of morphology and orientation of block copolymers (BCPs) in thin films is crucial to fully exploit the potential of these materials for applications in nanotechnology such as nanolithographic templates for nanodevices, nanoparticle patterning, and biological applications. The control of block copolymer morphology is easily achieved by varying the molecular weight and the volume fraction of each block. The control of the orientation of block copolymer thin films is far more challenging as the BCP nanodomains spontaneously self-assemble into the configuration that minimizes the total free energy of the system. A block with a lower interfacial tension with a substrate preferentially wets the substrate whereas a block with a lower surface energy tends to locate at the surface exposed to air. These energetic fields exerted by the substrate and the free surface are strong enough to induce the parallel orientation of nanodomains.
In collaborative research between the Nanoscale Science and Engineering Center at the University of Wisconsin - Madison and Seoul National University , we report on a new approach to induce the perpendicular orientation from the top of BCP films toward the bottom substrate. Our concept is based on the basic properties of surfactants that naturally locate at any interfaces to tailor the surface properties of a material. We demonstrate that the addition of a low molecular weight surfactant, oleic acid (hereafter denoted as OA), in present case can easily create the energetically neutral conditions at the top of the polystyrene-block-poly(methylmethacylate) (PS-b-PMMA) diblock copolymer thin films, resulting in the desired perpendicular orientation.
Figure 1. (A) Schematic on the surfactant-assisted orientation of a PS-b-PMMA thin film. Cross-sectional FE-SEM images of (B) thin (~140nm) and (C) thick (~550nm) of cylinder-forming PS-b-PMMA/Oleic Acid mixture films (1wt%) after UV/acetic acid treatment. Perpendicular orientation starts from the top surface but does not propagate all the way to the bottom interface.
 Jeong Gon Son, Xavier Bulliard, Huiman Kang, Paul F. Nealey, and Kookheon Char, Advanced Materials, 20, 3643-3648, 2008.