Signatures of the fast dynamics in glassy polystyrene by multi-frequency, high-field Electron Paramagnetic Resonance of molecular guests

The reorientation of one small paramagnetic molecule ( spin probe ) in glassy polystyrene ( PS ) is studied by high-field Electron Paramagnetic Resonance spectroscopy at three different Larmor frequencies ( 95, 190 and 285 GHz ). Two different regimes separated by a crossover region are evidenced. Below 180K the rotational times are nearly temperature-independent with no apparent distribution. In the temperature range 180− 220K 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 spin probes w which persist in the low-temperature dynamics; ii) a strongly Preprint submitted to Elsevier Science 18 January 2006 temperature-dependent component, to be described by a power-distribution, representing the fraction of spin probes 1 − w undergoing activated motion over an exponential distribution of barrier heights g(E). Above 180K a steep decrease of w is evidenced. The shape and the width of g(E) do not differ from the reported ones for PS within the errors. The large increase of the rotational mobility of the spin probe at 180K is ascribed to the onset of the fast dynamics detected by neutron scattering at Tf = 175± 25K.