Low surface energy polymeric films from solventless liquid oligoesters and partially fluorinated isocyanates

Partially fluorinated isocyanates were synthesized from hexamethylene diisocyanate (HDI) and an HDI trimer (Desmodur N3300). The perfluoroalkyl group (Rf) was C6F13 or C8F17, and the ratio between Rf and the isocyanate group (NCO) was 1/99 in the case of HDI or 1/49 in the case of N3300. Polymeric films with surface energies as low as 10 mN/m were obtained from mixtures of these partially fluorinated isocyanates and a previously reported hydroxyl-end-capped solventless liquid oligoester. Contact angles of water and hexadecane reached 120 and 80 , respectively, when less than 1 wt % of fluorine was present in the films. The surface enrichment of fluorine-containing species was confirmed by X-ray photoelectron spectroscopy (XPS) investigations. At a fluorine concentration of 0.5-1.0 wt %, the surface F/C atomic ratio (at a 15 takeoff angle) was greater than 1; the surface enrichment factor of fluorine was up to 600. The topological structures of the polymeric films were recorded by an atomic force microscope under tapping mode. While the height images indicated that the surface was smooth at the nanometer scale in a 1 m × 1 m area, the phase images revealed that fluorine-enriched domains were present at the surface. As the fluorine concentration increased, the fluorine-enriched domains grew from tiny spots (2-3 nm) to larger round domains (15-25 nm in diameter). The low surface energies of the films could be ascribed to the strong surface segregation of fluorinated species.