Surface structure of asymmetric fluorinated block copolymers

Surface structures of asymmetric poly [styrene-block-4-(perfluorooctylpropyloxy)styrene] block copolymers (PS-PF) with the shorter PF block than the PS block have been investigated using X-ray photoelectron spectroscopy (XPS) and dynamic secondary ion mass spectrometry (SIMS). Fluorooctyl side groups (C8F17) of the block copolymers segregate to the surface due to their low surface energy. Asymmetry of the block remarkably influences the structure of the C8F17 side groups and the backbone of PS-PF copolymers at the surface. When a weight fraction of the PF block, f(PF)(wt), is 0.09, the surface is partially covered by the C8F17 side groups with a thickness thinner than the length of the C8F17 side group. The C8F17 side groups hence lie parallel to the surface. When f(PF)(wt) is approximately 0.25, the backbone of the PF block lies parallel to the surface, and the C8F17 side groups stand perpendicular to the surface. Such a conformation results in a domain thickness of the PF block comparable to the length of the C8F17 side group. However, as f(PF)(wt) goes beyond 0.3, the backbone of the PF block orients toward the surface while the top surface region, i.e., 1.5 nm from the surface, is still occupied by the C8F17 side groups standing perpendicular to the surface.