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 (ENA) over the entire concentration range. The intensity I-3 of the longest-lived component due to the annihilation of ortho-positronium (o-Ps) exhibited a complex behavior as a function of ENA concentration. The addition of up to 12% ENA 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 ENA. The effect of ENA on Ps formation was found to be strongly influenced by the application of an external electric field. For example, the o-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.