Free volume in polycarbonate studied by positron annihilation: Effects of free radicals and trapped electrons on positronium formation

Positron lifetime spectra in polycarbonate were measured as a function of t emperature and time. Ortho positronium (o-Ps) lifetime remained constant wi th prolonged measuring time, but its intensity decreased and increased with time at room temperature and low temperatures, respectively. The temperatu re dependence was measured from 30 to 375 K for two samples: one was pre-ir radiated by positrons at 30 K for 166 h. the other was a fresh sample. The variation of o-Ps intensity differed greatly between the two samples. We ex plained these results in terms of free radicals and trapped electrons. We o bserved a large decrease of I-3 at 100-130K and a change in the slope of ta u (3) at 230 K. The observations are considered to be related to the moveme nt of the methyl group and the brittle-ductile transition. respectively, wh ich were measured by other methods. By measuring the positron lifetime spec tra between 293-493 K. the glass transition temperature was determined to b e approximately 413 K.