Positron mobility in polyethylene in the 60–400 K temperature range

Abstract

We have determined the positron mobility (μ₊) in polyethylene samples (67.2% crystalline, glass transition temperatureT _g=151 K) in the 64–400 K temperature range by Doppler shift measurements. A method based on the simulataneous observation of two γ lines from¹³³Ba and¹³⁷Cs radioactive sources together with the positron annihilation γ line, was employed to measure the Doppler shift of the 511 keV γ line as a function of the electric field applied to the samples. With this method we were able to measure at the same time the drift velocity of positrons and theS parameter. This parameter is very important in the interpretation of the mobility trend in samples where the positron states change with temperature. The positron mobility was corrected for positronium formation. μ₊ at 64 K is 31.7±0.8 cm² V⁻¹ s⁻¹ then decreases up to 123 K, increases at 148 K and decreases again up to 170 K (μ₊=26.9±0.8 cm² V⁻s⁻). This sharp change in mobility is centred around the glass transition temperature of our samples. Then the mobility remains almost constant up to 230 K. From 250 K to 377 K, μ₊ increases and reaches the value of 38.4±1.0 cm² V⁻¹s⁻¹. The corrected experimental data were well fitted by a simple model taking into account scattering and a thermally activated process (hopping mechanism).