Positronium formation in PE/EVA polymer blends at low temperatures

Positronium (Ps) formation in a polymer blend system consisting of polar and non-polar components has been investigated. We measured positron lifetime spectra for a series of polymer blends between polyethylene (PE) and ethylene-vinyl acetate (EVA) as functions of temperature (T = 100-300 K) and composition (EVA weight content Phi = 0-100%) as well as measurement time (t = 0-100 h) at T = 100 and 250 K. The glass transition temperatures for blends, around 227 K as determined by the ortho-positronium (o-Ps) lifetime tau (3) versus temperature, were invariant, within the experimental error, as a function of EVA concentration. Large variations of the o-Ps formation probability I-3 versus temperature and time, observed for pure LDPE and blends with relatively low concentrations of EVA, were interpreted on the basis of the spur reaction model of Ps formation, with reference to the effects of localized electrons and trapping centers produced by positron irradiation. In the blends with higher EVA concentrations, the effects of positron irradiation were appreciably weakened, as the result of positron trapping on the polar acetate group of EVA. (C) 2000 Elsevier Science Ltd. All rights reserved.