Effects of composition and external electric field on positronium formation in a polymer blend system

Positron lifetime measurements were carried out for polymer blends between polyethylene and ethylene vinyl acetate copolymer (E/VA) over the entire concentration range. The intensity $I_3$ of the longest-lived component due to the annihilation of ortho-positronium $(o-\mathrm{Ps})$ exhibited a complex behavior as a function of E/VA concentration. The addition of up to 12% E/VA to polyethylene decreased the intensity from 28.2 to 25.8 %. Further addition of E/VA, somewhat surprisingly, resulted in an enhancement of positronium (Ps) formation that continued until $I_3$ reached a value of 29.7% for pure E/VA. The effect of E/VA on Ps formation was found to be strongly influenced by the application of an external electric field. For example, the $o-\mathrm{Ps}$ intensity recorded at a field of 50 kV/cm monotonously increased from 18.8% to 29.6% with increasing E/VA concentration. These results are discussed on the basis of the spur reaction model of Ps formation.