TEMPERATURE-DEPENDENCE OF FREE-VOLUME DISTRIBUTIONS IN POLYMERS STUDIED BY POSITRON LIFETIME SPECTROSCOPY

The temperature dependence of mean ortho-positronium lifetime (tau3BAR) and intensity (I3) was measured in the range between 60 and 293 K for polycarbonate and polyoxymethylene samples. The values of tau3BAR and I3 are a constant below T = 127 K. Above this temperature the tau3BAR as well as I3 are a linear function of T. Annihilation lifetime measurements have also been performed to deduce the free-volume hole distributions in the polycarbonate and polyoxymethylene polymer samples at temperature of 70 and 293 K. Lifetime spectra were analysed using the Laplace transform and the hole volume distributions are determined from the results of o-Ps lifetime distributions using the particle in a spherical box theory. The free-volume hole radii of polycaxbonate at 293 K are distributed between 2.1 and 3.9 angstrom with a maximum at (2.83 +/- 0.02) angstrom. Free volumes spread out between 40 and 230 angstrom3 with a maximum at (92 +/- 2) angstrom3. In the case of polyoxymethylene, the free volume hole radii at 293 K are peaked at (3.10 +/- 0.07) angstrom inside (2.0 - 4.6)angstrom interval. Free volumes are distributed between 35 and 350 angstrom3 with a maximum at (125 +/- 8) angstrom3. At approximately 75 K the distributions are quite different. They are narrower and shifted to lower values of R (V). In the case of polyoxymethylene, mean value of radius is (2.06 +/- 0.02) angstrom and mean free volume is (35 +/- 2) 3. In the case of polycarbonate, mean radius is (2.5 +/- 0.05) angstrom and the mean free volume is (70 +/- 4)