Surface Stability in Liquid-Crystalline Block Copolymers with Semifluorinated Monodendron Side Groups

Abstract

Block copolymers with semifluorinated monodendron side groups were synthesized by attachment of a first generation 2- or 3-armed monodendron acid chloride to a hydroxylated poly(styrene-b-1,2/3,4-isoprene). A convergent growth strategy was developed to synthesize the monodendron groups in good yield using an approach that could be extended to higher generation monodendrons. High extents of attachment were achieved despite the steric effects of the bulky monodendron side groups. The resulting polymers formed a smectic B mesophase at room temperature as determined by WAXS data. The transition temperatures, mesophase range, and enthalpy of the smectic B−isotropic transition were all affected by side-group structural factors such as flexible spacer length, mesogen length, and monodendron core. The critical surface tensions of the resulting semifluorinated polymers were as low as 8 mN/m as determined by Zisman analysis. Surface stability of polymer films in a polar liquid environment was strongly dependent on the extent of attachment exhibited by the semifluorinated groups. The monodendron −CF2− helix within 1 nm of the surfaces has a net orientation normal to the surface as measured by near-edge X-ray absorption fine structure (NEXAFS) methods, but the orientational order parameter Shelix is much higher for the 2-armed monodendrons than for the 3-armed monodendrons. In both cases Shelix seems insensitive to monodendron attachment density along the isoprene block. We suggest that packing frustration of the monodendron subunits produces surfaces with spontaneous curvature that differs depending on whether the monodendrons are 2- or 3-armed. The more highly curved surface topology of the 3-armed monodendrons may provide a partial explanation for its decreased orientational order.