Inhibition of positronium formation by polar groups in polymers - relation with TSL experiments

Positronium (Ps) formation in polymers is affected by polar groups, contained in polymer chains and having dipole distribution of electric charge. Oxygen, double-bonded to carbon (-C = O), is one of the examples of the group with dipole moment. Inhibition of Ps formation can be partly explained in this case by the trapping of positrons (e(+)) into negatively charged part of the dipole -C+ = O-. A proof of this explanation is based on the results of coincidence Doppler broadening spectroscopy (CDBS) and obtained in this paper for the copolymer of ethylene (E) and methyltnethacrylate (MMA). The same trapping was demonstrated also for polymethylmethacrylate (PMMA) and for one of polyimides (Kapton). However, part of the inhibition can be due to trapping of the spur electrons. A role of this process can be found from a comparison of the effects of Ps inhibition and suppression of thermostimulated luminescence (TSL). In some of the described cases, inhibition is very effective but limited: it starts at very low concentrations of inhibitors, but saturates at a certain level. This fact is interpreted as a result of incomplete slowing down of positrons inside of the spur in the moment of Ps formation. (C) 2003 Elsevier Science Ltd. All rights reserved.