Surface stability in liquid-crystalline block copolymers with semifluorinated monodendron side groups

Block copolymers with semifluorinated monodendron side groups were synthesi zed by attachment of a first generation 2- or 3-armed monodendron acid chlo ride to a hydroxylated poly(styrene-b-1,2/3,4-isoprene). A convergent growt h 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 th e bulky monodendron side groups. The resulting polymers formed a smectic B mesophase at room temperature as determined by WAXS data. The transition te mperatures, mesophase range, and enthalpy of the smectic B-isotropic transi tion were all affected by side-group structural factors such as flexible sp acer length, mesogen length, and monodendron core. The critical surface ten sions of the resulting semifluorinated polymers were as low as similar to 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 attac hment exhibited by the semifluorinated groups. The monodendron -CF2-helix w ithin 1 nm of the surfaces has a net orientation normal to the surface as m easured by near-edge X-ray absorption fine structure (NEXAFS) methods, but the orientational order parameter S-helix is much higher for the 2-armed mo nodendrons than for the 3-armed monodendrons. In both cases S-helix seems i nsensitive to monodendron attachment density along the isoprene block. We s uggest that packing frustration of the monodendron subunits produces surfac es with spontaneous curvature that differs depending on whether the monoden drons are 2- or 3-armed. The more highly curved surface topology of the 3-a rmed monodendrons may provide a partial explanation for its decreased orien tational order.