Self-organizing materials with low surface energy: The synthesis and solid-state properties of semifluorinated side-chain ionenes

The synthesis of new semifluorinated alkyl side-chain ionenes (SFASI) is reported and involves reaction of semifluorinated 1-bromoalkanes with poly(N,N'-dimethyl-1,6-hexanediamine) (PDHD) which was prepared by reduction of poly(N,N'-dimethylhexamethyleneadipamide). The quaternization efficiency of the polyamine is 80-85% when carried out using a solvent mixture of DMF/ethanol (1/2 volume ratio) at 65 degrees C over long reaction times. The structure of SFASI consists of a 10 Angstrom charged layer resulting from quarternary ammonium groups with strong coulombic interactions and a smectic B-like packing of the semifluorinated side-chain layer. The perfluorocarbon segments favor self-organization in a hexagonal array to form a head-to-head bilayer between the charged layers. These ionenes exhibit a smectic to isotropic transition, with the transition temperature dominated by the length of the fluorocarbon side chain. The surface energy of a spin-coated SFASI film, as estimated from the critical surface tension, was as low as 8 dyn/cm at 20 degrees C. This result indicates that the surface of the as-spun cast film consists largely of CF3 end groups even though hydrophilic ammonium groups are present in the polymer. The surface segregation and orientation of the low surface energy mesogenic semiflorinated side groups were confirmed by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray fine structure (NEXAFS) analysis. Surface reconstruction was observed due to polymer chain defects and surface mobility.