Using fluorinated junctions to bias orientation of block copolymers*

We synthesized hydrophobic-hydrophobic block copolymers (BCPs) of poly(solketal methacrylate)-block-polystyrene, PSM-b-PS, with various volume ratios of the two blocks, with and without a fluorinated junction point. After hydrolysis of the PSM block into poly(glycerol monomethacrylate), the hydrophilic-hydrophobic block copolymers are within their strong segregation limit. In the BCPs with fluorinated junctions, the junctions have lower surface energy than both blocks. The fluorinated junctions tend to segregate to the polymer-air and polymer-substrate interfaces, promoting the orientation of the microdomain normal to the surface. For the BCPs with fluorinated junctions, we observed a mixture of perpendicular and parallel lamellae. When the thickness of the film is commensurate with the full pitch of the lamellae, the lamellae will tend to form a parallel orientation. Otherwise, when the film thickness is not commensurate with the full pitch, the lamellae are oriented normal to the interface. This perpendicular orientation is driven by the enthalpic gain of placing the fluorinated junctions at the interface. The lamellae are parallel to the substrate for BCPs without the fluorinated junctions for the same molecular weight. With the island-hole morphologies, a transition from parallel to perpendicular lamellae was observed at the edge of the islands. The transition is caused by the gradual change in film thickness at the edge of the islands, where the film thickness is no longer commensurate with the full pitch of the lamellae. The self-assembled morphologies were characterized by atomic force microscopy and grazing-incidence small-angle x-ray scattering.