Dielectric Properties of Semicrystalline Polychlorotrifluoroethylene

Abstract

The dielectric properties of polychlorotrifluoroethylene (Tm = 224 °C, Tg = 52 °C) have been measured at temperatures between −50 and +250 °C, and at frequencies between 0.1 c/s and 8.6 kMc/s. Specimens of known crystallinities, ranging from χ = 0.80 to χ = 0.00 (pure liquid) were studied. Comprehensive tables of data are presented. The experimental techniques employed to measure the dielectric properties over these wide ranges of temperature, frequency, physical state, and sample type (disks, cylinders, and thin films), are discussed. The operation and calibration of the specimen holder, bridges, resonant circuits, and waveguide apparatus used are discussed in detail. When the dielectric loss index, ϵ″, at 1 c/s is plotted as a function of temperature for a highly crystalline specimen (χ = 0.80), where the crystallinity consists largely of lamellar spherulites, three distinct loss peaks are readily apparent. These peaks occur at about −40 °C (low-temperature process), 95 °C (intermediate-temperature process), and 150 °C (high-temperature process). The dielectric data are compared with the mechanical loss data obtained at 1 c/s by McCrum. Mechanical loss peaks at temperatures virtually identical to those in the ϵ″ versus T plot are found. The high-temperature process is attributed to the presence of well-formed chain-folded lamellar spherulites. Some evidence points to the surfaces of the lamellae as the site of the loss mechanism. The high-temperature loss peak does not appear in resolved form in non-spherulitic specimens even when the crystallinity is high. The intermediate-temperature process originates in the normal supercooled amorphous phase, and is due to the complex dipole relaxation effects involving motions of large numbers of polymer chain segments that are associated with the onset of the glass transition at Tg = 52 °C. As determined by V¯−T data, the glass transition temperature at Tg = 52 °C that is associated with this relaxation effect does not shift appreciably with increasing crystallinity. The low-temperature dielectric loss process, which is active far below Tg, originates principally in the supercooled amorphous regions, and evidently corresponds to a fairly simple motion involving a small number of chain segments. This process tends to exhibit anomalous behavior in highly crystalline specimens, particularly at low temperatures. A large dipolar contribution of the crystals to the static dielectric constant was observed. This contribution increased with increasing temperature, and corresponded to a very rapid dipole reorientation process (τ~10−11 sec at 23 °C).