Structural Study of the Gel Phase of a Semifluorinated Alkane in a Mixed Solvent

Semifluorinated n-alkanes, F3C(CF2)m-1(CH2)n-1CH3 (for short, FmHn) are short-chain diblock copolymers that show interesting properties when dissolved in nonaqueous media. Due to the high incompatibility between fluorocarbons and hydrocarbons that produces strong immiscibility between fluorocarbons and hydrocarbons, the semifluorinated molecules arrange themselves in peculiar ways both in the solid and in the liquid state. FmHn molecules produce micellar aggregates when dispersed in selective solvents such as perfluorooctane or toluene. They form white gel phases when the liquid solution is cooled down below certain temperatures depending on the concentration of the copolymers. These gels are described as a long network of copolymer fibers that entrap free molecules of solvent in cavities formed between different branches of fibers. In this paper we construct two plausible models for the structure of the gel phase of F8H16 dissolved in a mixture of perfluorooctane/isooctane, and we use them to analyze the small-angle X-ray-scattering (SAXS) data. From quantitative analyses of SAXS intensities in absolute scales, we are able to give the values of various structural parameters of the fiber.