Europhys. Lett.
Volume 72, Number 4, November 2005
Page(s) 590 - 596
Section Condensed matter: structural, mechanical and thermal properties
Published online 19 October 2005
Europhys. Lett., 72 (4), pp. 590-596 (2005)
DOI: 10.1209/epl/i2005-10290-0

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

V. Bercu1, 2, M. Martinelli2, C. A. Massa2, L. A. Pardi2 and D. Leporini1, 3

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
published online 19 October 2005

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 $180\un{K}$ the rotational times are nearly temperature independent with no apparent distribution. TEMPO is trapped. In the temperature range 180-$220\un{K}$ 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 $180\un{K}$ a steep decrease of w is evidenced. The detrapping of TEMPO and the onset of its large increase of the rotational mobility at $180\un{K}$ are interpreted as signatures of the onset of the fast motion detected by neutron scattering at $T_f = 175 \pm 25\un{K}$.

64.70.Pf - Glass transitions.
76.30.-v - Electron paramagnetic resonance and relaxation.
61.25.Hq - Macromolecular and polymer solutions; polymer melts; swelling.

© EDP Sciences 2005