Effect of electric field strength and exposure time on epoxy and high density polyethylene measured by positron annihilation lifetime

Abstract

Using the positron annihilation lifetime technique, the annihilation parameters have been measured for epoxy and high density polyethylene (HDPE) as a function of AC electric field strength and the exposure time. The lifetime spectra have been resolved into three components, the longest component (I3τ3) is attributed to the pick-off annihilation of o-Ps in the amorphous regions. The intermediate one (I2τ2) is due to the annihilation of free positrons, while the shorter component (I1τ1) stems from self annihilation of p-Ps. In HDPE, the o-Ps parameters τ3 andI3 are measured as a function of electric field strengths in the range from 10 to 100 kV/cm exposed for 24 hours. A decrease inI3 of ∼8% is observed from zero to 50 kV/cm followed by an increase of the same order from 50 to 100 kV/cm. By investigating the effect of the exposure time from 2 to 24 hours at 16 and 50 kV/cm, the effect is confirmed and is attributed to the inhibition of o-Ps formation at lower field strength. In epoxy, the effect or exposure time onI3 at 166 and 133 kV/cm shows a similar behavior as in HDPE. At 133 kV/cmI3 decreases by only 2.5%. On the other hand, the changes in τ3 occur at short exposure times. Again at large times the saturation is obtained. These effects are attributed to the expansion of free volume (increase of τ3) competing at longer exposure times with other phenomena, such as liberation of free radicals, which reduce the o-Ps intensityI3 through the conversion to p-Ps. The reactions between o-Ps and free radicals might also lead to free positrons, which could explain the increase ofI2 and the decrease of τ3 at longer exposure times.