Amorphous perfluoropolymers

The main application of amorphous perfluoropolymers is as cladding of optical fibers, antireflective coatings, low dielectric coatings, and in the electronic industry (e.g., photoresists)1012 and as a low-dielectric-constant insulator for high-performance interconnects.13... 

The unusual solubility of gases and vapors in perfluoropolymers has several applications relevant to membrane gas separations. Perfluoropolymers have solubility selectivities that are significantly different from those of hydrocarbon-based polymers. The amorphous perfluoropolymers can be fabricated into thin, high-flux composite membranes, which possess the excellent chemical and thermal stability. Typical reported pure gas permeabilities and selectivities of these fine amorphous perfluoropolymers are shown in Table 16.8 [33]. 

These materials cover a broad permeability range from Teflon AF 2400, one of the most permeable polymers, to Cytop , the least permeable among these amorphous perfluoropolymers. However, even Cytop is about ten times more permeable than conventional glassy polymers such as polycarbonates. The relative permeability of the perfluoropolymers is well described by their fractional free volume (FFV), a common measure of the free space in a polymer matrix available for molecular transport. For instance, both gas permeability and FFV exhibit the following order Teflon AF 2400 > Teflon AF 1600 > Hyflon AD SOX Hyflon AD 60X > Cytop . As the amount of bulky perfluorodioxole increases, the permeability and diffusivity increases. 

Y, and Okamoto, Y. (2011) Characterization and properties of semicrystalline and amorphous perfluoropolymer poly(perfluoro-2-methylene-l,3-dioxolane). Polym. Adv. Technol, 22 (8), 1272-1277. 

Table 14.1. Molecular Structure and Properties of Amorphous and Semicrystalline Perfluoropolymers...

Coatings with Amorphous Cyclic Perfluoropolymers. Recently, novel cyclic per-fluoropolymers have been reported [2.24a]. 

Defect-free membranes comprising zeolites and amorphous glassy perfluoropolymers can be prepared by modifying the surface of the filler. The pure gas permeation experiments of a series of Teflon AF 1600 membranes with various amounts of 80 and 350nm silicalite-1 crystals cannot be interpreted on the basis of the Maxwell model, but are compatible with a model in which a barrier to transport exists on the zeolite surface and a lower density polymer layer surrounds the crystals. With a small zeolite size (80nm) the low density layers around the crystals may coalesce and form percolation paths of lesser resistance and less selectivity. Silicalite-1 crystals improve the CO2/CH4 selectivity of Hyflon AD60X, and drive the N2/CH4 selectivity beyond the Robeson s upper bound. It also turns out that the presence of silicaUte-l crystals, like fumed silica, promote the inversion of the methane/butane selectivity of Teflon AF2400 in mixed gas experiments. 

The physical properties of these perfluoro polymers are summarized in Table 16.2. These perfluoropolymers are completely amorphous and contain no hydrogen atoms, show excellent chemical and thermal stability, and are soluble in fluorinated solvents such as hexafluorobenzene (HFB) and perfluorohexane. The dielectric constants of these fluoropolymers are considerably low, and they are almost unaffected by humidity. 

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