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SynopsisThe effects of pressure on the a (ca. 7OoC, 1 kHz) and y (ca. -lOO°C, 1 kHz) relaxations of linear polyethylene were studied dielectrically between 0 and 4 kbar. Equation of state (PVT) data were also determined in the range of interest of these relaxations. The sample was rendered dielectrically active through oxidation (0.8 C=O per 1000 CH2). The a process (which occurs in the crystalline fraction) could he studied over a much wider temperature range than heretofore possible due to the effect of pressure in increasing the melting point. Examination of relaxation strength from 50 to 15OOC showed that there must be two crystalline relaxation processes: the well-known a relaxation plus a competing one. The a relaxation is believed to be due to a chain twist-rotation-translation mechanism that results in rotation-translation of an entire chain in the crystal. The relaxation strength of the a process decreases and therefore indicates the presence of a second (faster and not directly observed) process that increases in intensity with increasing temperature. It is postulated that the second process is due to motion of defects that become more numerous through thermal injection a t higher temperatures. Analysis of the relaxation data along with the PVT data allowed the constant volume activation energy for the a relaxation to be determined. It was found to be 19.4 f 0.5 kcal/mole. The constant volume activation energy is important in modeling calculations of the crystal motions and is significantly smaller than the atmospheric constant pressure activation energy of 24.9 kcal/mole. The effect of pressure on the activation parameters and shape of the y process was also determined. There has been controversy over whether the y process occurs only in the amorphous fraction or in both the amorphous and crystalline phases. Since the two phases have quite different compressibilities, increasing the pressure should change the shape of the loss curves (versus frequency and temperature) if the process occurs in both phases. The shape (but not location) of the loss curves was found to be remarkably independent of pressure. This finding strengthens the view that the y process is entirely amorphous in origin.

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The high-temperature electrical conductivity and thermal decomposition characteristics of Sylgard@ 184 with and without hollow microspheres of glass, silica, and ceramic have been determined to 600 to 700°C in air and nitrogen environments. The materials are silicone-based dielectrics and are used as electronic encapsulants. Results show that a peak in the conductivity temperature dependence at -300°C results principally from volatilization of [Si(CH&O], with some evolution of water, that oxygen accelerates decomposition, and that the microspheres may help form a network of interconnected conductive pathways in the residual material. There is a good correlation between thermal stability and temperature-dependent electrical properties.

A recent study of the effect of pressure on the dielectric α process in slightly oxidized linear polyethylene showed that the intensity of the process decreases with increasing temperature. This phenomenon was attributed to thermal generation of conformational defects. In the present work a quantitative interpretation of that data is carried out to arrive at defect creation energy parameters. It is found that the creation energy lies in the range of 3–6 kcal/mole and that it has a high degeneracy or statistical weight (in the range 40–800). The statistical weight is interpreted as largely arising from the number of sites in a given crystalline chain for a defect to occur, that is, the number of CH2 units spanning the crystal. It is speculated that kinks are the likely conformational defect.