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.