Microphase-separated structure of polar polymer ultrathin films

I investigated the specific chain structure of polyurethane (PU) films, which possess strong polar groups in the main chains and multiblock structure under confinement. The PU was synthesized from poly (oxytetramethylene) glycol (PTMG), 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BD) by a prepolymer method. The hard segment contents were 20, 34 and 45 wt%. The ultrathin PU films were prepared onto a silicon wafer from a PU tetrahydrofuran solution by spin coating. The PU films exhibited microphase-separated structures, which are composed of hard segment domains surrounded by a soft segment matrix. For thicker films (∼200 nm), interdomain spacing almost corresponded to bulk value. On the other hand, it dramatically decreased for film thickness below 7 nm. This effect seems to be directly related to the decreasing film thickness. This is the first report, in which the phase-separated domain size of multiblock copolymers decrease with decreasing film thickness.