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 rotating mandrel in a tubing die produces multiaxial orientation in the extruded melt. If the polymer melt is quenched rapidly, some multiaxial orientation is retained in the resultant tubing. Multiaxially oriented tubing exhibits reduced crack propagation and some enhancement of yield and fracture strengths. During the processing of multiaxially oriented tubing, relaxation occurs after the melt leaves the die and gives rise to wall thickening and reduction in the lumen of the tubing; the magnitude of these effects is a function of the rate of mandrel rotation.
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