The onset of the fast dynamics in glassy polystyrene observed by the detrapping of guest molecules: A high-field Electron Paramagnetic Resonance study

¹ Dipartimento di Fisica “Enrico Fermi”, Universitá di Pisa Largo B. Pontecorvo 3, I-56127 Pisa, Italy
² IPCF-CNR - via G. Moruzzi 1, I-56124 Pisa, Italy
³ SOFT-INFM-CNR - P.zza A. Moro 2, I-00185, Roma, Italy

Received: 22 July 2005
Accepted: 27 September 2005

Abstract

The reorientation of the paramagnetic molecule TEMPO dissolved in glassy polystyrene (PS) is studied by high-field Electron Paramagnetic Resonance spectroscopy. Two different regimes separated by a crossover region are evidenced. Below the rotational times are nearly temperature independent with no apparent distribution. TEMPO is trapped. In the temperature range 180– a large increase of the rotational mobility is observed with widening of the distribution of correlation times which exhibits two components: i) a delta-like, temperature-independent component representing the fraction of TEMPO w still trapped; ii) a strongly temperature-dependent component representing the fraction of untrapped TEMPO undergoing activated motion over an exponential distribution of barrier heights. Above a steep decrease of w is evidenced. The detrapping of TEMPO and the onset of its large increase of the rotational mobility at are interpreted as signatures of the onset of the fast motion detected by neutron scattering at .