Molecular motions at low temperature observed by positron annihilation

Positron annihilation lifetime spectroscopy (PALS) was applied to low-density polyethylene (LDPE) and ethylene-methylmethacrylate (EMMA) copolymers with various MMA contents. The experiments were conducted in the dark as well as under visible light condition. In the dark at a low temperature of about 30 K, the o-Ps intensity was observed to increase with the elapsed time, and the increasing rate depended on the MMA content. After being kept for a long time at low temperature, the sample temperature was increased. The o-Ps intensity and its lifetime were found to change as a function of temperature. In the light, no significant change was found for the o-Ps intensity below the glass transition temperature. However, a dramatic variation of the o-Ps intensity was found to correspond to a change of its lifetime in the dark. The observations showed that not only the variation of the o-Ps lifetime could be related to the structural change and molecular motions of polymers, but also the change of the o-Ps intensity, particularly at low temperature in the dark, could be related to the activation of molecular motions. The results indicated that PALS could be used as a novel probe for molecular motions at low temperature