Effect of temperature on ortho-positronium in poly(methyl methacrylate)

Positron annihilation lifetimes (PAL) were measured for two viscosity-avera ge molecular weights of poly(methyl methacrylate), PMMA, as a function of t emperature. The PAL measurements were performed under vacuum in the tempera ture range from 22 to 150 degreesC with interval of 10 degreesC. The lifeti me spectra were analysed using two methods: (1) average results of the orth o-positronium (o-Ps) lifetime and its intensity obtained by PATFIT program, and (2) the o-Ps lifetime and o-Ps hole volume distributions given by Baye s' theorem and the maximum entropy principle using MELT program. The value of the o-Ps lifetime in the sample with the lower viscosity-average molecul ar weight is higher than that with the higher viscosity-average molecular w eight while it increases with increasing temperature. On the other hand, th e o-Ps intensity as well as the relative fractional of the o-Ps hole volume shows behaviour in contrast to the o-Ps lifetime with the viscosity-averag e molecular weight. Within the temperature range two different transitions are observed. The first is due to the reduction of non-equilibrium states t hat are frozen below this temperature. The other one is in agreement with t he glass transition temperature of PMMA. (C) 2000 Elsevier Science Ltd. All rights reserved.