EVOLUTION OF NANOMETER VOIDS IN POLYCARBONATE UNDER MECHANICAL-STRESSAND THERMAL-EXPANSION USING POSITRON SPECTROSCOPY

Positron Annihilation Lifetime Spectroscopy (PALS) measurements were c onducted on polycarbonate subjected to either thermal expansion or to tensile and compressive strains. It was found that thermal expansion a ffected both the nanometer hole size and the hole number density, wher eas mechanical stress affected mainly the size of existing holes, and did not generate or eliminate holes in the quasielastic deformation re gion. The effect of stress on yield and postyield behavior of this gla ssy material was also investigated. The deduced hole volume fraction o f this polymer at 25-degrees-C was 6.8 +/- 0.5% from the thermal expan sion experiment and 7.2 +/- 1.2% from the mechanical loading experimen t. When the specimen was under compression, the hole volume fraction w as found to continuously decrease. This can be considered as evidence of the inability of the free volume concept in explaining the yield be havior of glassy polymers. (C) 1995 John Wiley & Sons, Inc.