Physics of polymers at the Italian high-field EPR facility: Heterogeneities and fast dynamics

The reorientation of the paramagnetic molecule 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) dissolved in glassy polystyrene (PS) is studied by electron paramagnetic resonance (EPR) spectroscopy in a wide range of Larmor frequencies (9-285 GHz). High-field EPR reveals that TEMPO rotates by small angular steps with a distribution of correlation times and is able to detect the onset of fast motion of PS. Differently, X-band EPR shows that TEMPO undergoes a large-angle reorientation with virtually no distribution of correlation times and largely misses the fast dynamics of PS. A unified view is proposed.