Polypropylene Films Studied with Variable Positron Beams

Doppler broadening and positron lifetime spectra have been measured for polypropylene films of constant thickness (≈ 50 μm) and different morphology that crystallized from melt at temperature 0, 100, and 120°C. The S -parameter vs. energy curves do not change in a systematic way with crystallinity of samples. It is supposed that decrease in the fractional free volume for samples of greater crystallinity is not seen in S-values because of their increase resulting from annihilation in defects present both in the crystalline regions and on the lamellae surfaces. Intensities of the components in the positron lifetime spectra: I$\text{}_{2}$ (τ$\text{}_{2}$=1.3 ns) and I$\text{}_{3}$ (τ$\text{}_{3}$≈ 2.3÷2.8 ns) show changes with crystallinity of samples. The evident trend is observed for the longer-lived τ$\text{}_{3}$-component intensity to decrease with crystallinity while the reverse is true for the shorter, fixed τ$\text{}_{2}$-component. The decrease in the intensity of the longer-lived component can result from reduction of the fractional free volume.