SynopsisPositron lifetime studies were performed on well-characterized annealed and quenched samples of isotactic polypropylene. The positron experiments were conducted from -20 to 110°C a s a function of both heating and cooling. Of the three decaying exponential components resolved from the lifetime spectra, only the long-lifetime ortho-positronium (o-Ps) pickofr component was affected by the changes in temperature. The behavior of both the lifetime and intensity of the o-Ps component was interpreted with the aid of x-ray diffraction, densitometry, and optical microscopy examinations and results from previously reported investigations of the thermal transition behavior of polypropylene. The present experiments demonstrate that 0-Ps lifetimes were similar for both the annealed and quenched samples, independent of thermal cycling, while the 0-Ps component intensity was significantly larger for the quenched material during heating, with both sample types exhibiting a significant hysteresis upon cooling. These results suggest that the mean free-volume cavity size is independent of prior thermal treatment, while the density of free-volume sites is a sensitive function of structure and prior thermal history. The variations of lifetime and of intensity with temperature have provided insight into polypropylene's glass transition phenomena.
SynopsisPositron annihilation lifetime spectroscopy has been used to study the isothermal relaxation response of compression molded bisphenol-A polycarbonate at temperatures of 263, 273, and 303 K. The temperature dependence of both the lifetime and intensity of the ortho-Positronium (0-Ps) pickoff component is discussed in terms of ductile-to-brittle transition behavior and free .volume theory. An additive exponential model and the Williams-Watt model were used to ;analyze the relaxation as a function of temperature and provided results consistent with the anticipated molecular mobility of polycarbonate at sub-2' ' temperatures.
A Dy-substituted Fe14Nd2B-type permanent magnet alloy (Crumax 32) has been studied by positron annihilation lifetime spectroscopy, and also magnetic coercivity and microstructural analysis, to evaluate the effects of thermal treatment of the material. The results of this study show that the room temperature lifetime spectra can be modeled accurately using a partially constrained three-component fit. The variations in lifetime spectra induced by thermal treatment of the sintered magnet can be correlated with the concomitant decreases in coercivity and with a decrease in the surface area of free α-Fe as indicated by transmission electron microscopy.
Positron annihilation lifetime spectroscopy has proven to be sensitive to glass transitions and other free volume dependent phase transitions in amorphous and semicrystalline polymers. The thermal dependence of the lifetime spectra of positrons in compression-molded poly(bisphenol-A carbonate) has been measured from 253K to 323K, then modelled using a three component fit. The longest-lived component lifetime τ3 was found to vary linearly with increasing temperature independent of thermal history. The corresponding component intensity I3 was found to vary in a non-linear fashion with increasing temperature, exhibiting a significant dependence on thermal history. The observed thermal response of τ3 and I3 is discussed in terms of both molecular relaxation and the ductile-to-brittle transition behavior of poly(bisphenol-A carbonate).
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