Positron annihilation investigations on poly(methyl methacrylate)

Positron lifetime and Doppler broadened annihilation radiation were measured for seven different samples of poly(methyl methacrylate) at room temperature in vacuum. The polymerisation of methyl methacrylate was carried out as a bulk polymerisation in the presence of benzoyl peroxide as an initiator. The effect of the amount of the initiator on the viscosity-average molecular weight was studied. It was found that the viscosity-average molecular weight decreased with increasing amount of the initiator. The average lifetime and intensity of ortho-positronium (o-Ps) increased with increasing viscosity-average molecular weight up to 6.85 x 10(4) and remained constant after that. The S-parameter showed a similar behaviour as that of the o-Ps intensity. (C) 2000 Elsevier Science Ltd. All rights reserved.