Synthesis, characterization, and derivatization of hyperbranched polyfluorinated polymers
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A hyperbranched polyfluorinated benzyl ether polymer was prepared from the A2B monomer 3,5-bis[(pentafluorobenzyl)oxy]benzyl alcohol. The polymerization was based upon deprotonation of the benzylic alcohol (B), followed by nucleophilic substitution of the p-fluorines of the two pentafluorophenyl (A) groups to form tetrafluorophenyl benzyl ether linkages. Optimized reaction conditions for the polymerization involved the addition of sodium metal (<0.1 mm particle size, 30 wt % suspension in toluene) to a solution of monomer (0.3 M) in THF heated at reflux. The molecular weight and molecular-weight distribution of the resulting polymer were affected by the surface area of the sodium particles, the concentration of the monomer, and the polymerization temperature. An average of one pentafluorophenyl chain end per repeat unit plus one pentafluorophenyl end group was present within the hyperbranched polymer, which allowed for chemical modification by nucleophilic displacement reactions upon the p-fluorines, to alter the physical and chemical properties of the material. Reaction of lithium trifluoroethoxide and lithium 1H,1H,2H,2H-perfluorodecanoxide with the pentafluorophenyl-terminated hyperbranched polymer introduced fluoroalkyl groups. Additionally, an X-ray opaque derivative was prepared by reaction of the pentafluorophenyl-terminated hyperbranched polymer with p-iodophenol. Contact angle measurements of water (96, 99, and 120) and hexadecane (21, 14, and 62) on films of a pentafluorophenyl-terminated hyperbranched polymer and the trifluoroethoxy-substituted and 1H,1H,2H,2H-perfluorodecanoxy-substituted derivatives, respectively, indicated a high degree of hydrophobicity and lipophobicity. Surface morphologies and surface properties of films were studied with atomic force microscopy (AFM). Tapping mode AFM images suggested phase separation in partially 1H,1H,2H,2H-perfluorodecanoxy-substituted material. Lateral force AFM (LFM) demonstrated that 1H,1H,2H,2H-perfluorodecanoxy substitution of the hyperbranched polymer led to a more than 2-fold decrease in the coefficient of friction and adhesive force, as measured with a silicon nitride probe.