The Onset of the Fast Dynamics in Glassy Polystyrene Observed by the Detrapping of Guest Molecules: a High-Field Electron Paramagnetic Resonance Study.

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 180 K the rotational times are nearly temperature independent with no apparent distribution. TEMPO is trapped. In the temperature range 180-220 K 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 1-w undergoing activated motion over an exponential distribution of barrier heights. Above 180 K a steep decrease of w is evidenced. The detrapping of TEMPO and the onset of its large increase of the rotational mobility at 180 K are interpreted as signatures of the onset of the fast motion detected by neutron scattering at T-f = 175 +/- 25 K.