Surface Stability in Liquid-Crystalline Block Copolymers with Semifluorinated Monodendron Side Groups Maoliang Xiang, † Xuefa Li, † Christopher K. Ober,* ,† Kookheon Char, † Jan Genzer, ‡ Easan Sivaniah, ‡ Edward J. Kramer, ‡,§ and Daniel A. Fischer ⊥,# Materials Science & Engineering, Bard Hall, Cornell University, Ithaca, New York 14853-1501; Department of Materials, University of California at Santa Barbara, Santa Barbara, California 93106; Department of Chemical Engineering, University of California at Santa Barbara, Santa Barbara, California 93106; Materials Science & Engineering Laboratory, National Institute for Standards and Technology, Gaithersburg, Maryland 20899; and NSLS, Brookhaven National Lab, Upton, New York 11973 Received December 16, 1999; Revised Manuscript Received April 24, 2000 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 -CF 2- 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. Introduction This paper describes the synthesis and surface prop- erties of a series of block copolymers containing pendent semifluorinated groups with a monodendron structure. The synthetic procedure involved monodendron group attachment to the isoprene block of a presynthesized block copolymer of styrene and isoprene. This strategy aided processing and organic solvent solubility of these new fluorinated polymers. The resulting materials were examined using a variety of methods including near edge X-ray absorption fine structure (NEXAFS) and contact angle studies and were shown to have stable, nonreconstructing surfaces in polar aqueous environ- ments. These polymers possess stable nonreconstructing surfaces that depend on the mesomorphic character of these side groups and their extent of attachment. Creating a compliant (elastomeric) polymer surface that does not reconstruct in changing environments is normally a difficult challenge. Such behavior is, how- ever, of great interest in many applications of synthetic polymers ranging from simple protective coatings to release agents to biologically stable surfaces. In par- ticular, the materials with the most interest, those with low-energy surfaces, often have the greatest thermody- namic driving force for reconstruction when in contact with a polar liquid as such as water. In the early 1960s, Zisman et al. 1 showed that the critical surface energy of fluoropolymers such as perfluorooctyl methacrylate polymers can reach as low as 11 mJ/m 2 . Lindner 2 has, for example, recently synthesized a fluorinated silicone via the hydrosilylation reaction of fluorinated 1-olefins with poly(hydromethylsiloxane) and formed a material with a surface energy approaching 11 mJ/m 2 and at the same time demonstrated the feasibility of its nonwet- ting, low-energy surface to resist fouling by marine organisms. Thomas et al. 3 reported that water- and oil- repellent surfaces could be obtained even though the incorporation of fluorinated monomer in a methacrylate copolymer was as low as 1.5 wt %. To the best of our knowledge, there is little information on long-term surface stability reported in these studies. Silicone- and fluorine-containing polymers are usually quite susceptible to rapid surface rearrangement. Since our goal is the creation of low surface energy materials for long-term use in a polar environment, a critical question is how the surface properties of polymeric materials can remain stable. Several approaches to creating stable surfaces have been successfully exam- ined previously and include the creation of a network scaffolding immediately below the low-energy surface 4 and the stabilization of the surface using liquid crystal- line groups. 5 Of special interest are polymers with semifluorinated (SF) side groups, that is, short segments of alkyl and perfluoroalkyl groups of 5-10 carbons each. 5,6 † Cornell University. ‡ Department of Materials, UCSB. § Department of Chemical Engineering, UCSB. ⊥ National Institute for Standards and Technology. # Brookhaven National Lab. 6106 Macromolecules 2000, 33, 6106-6119 10.1021/ma992111s CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000